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INTERPRETATION OF THE 

GRAVITY MAP OF CALIFORNIA 

AND ITS CONTINENTAL MARGIN 




PHYSICAL SCIENCES 
LIBRARY 



BULLETIN 205 

California Division of Mines and Geology 
Sacramento, California, 1980 




STATE OF CALIFORNIA 

EDMUND G BROWN JR. 
GOVERNOR 

THE RESOURCES AGENCY 

HUEY D JOHNSON 
SECRETARY FOR RESOURCES 

DEPARTMENT OF CONSERVATION 

PRISCILLA C GREW 
DIRECTOR 



DIVISION OF MINES AND GEOLOGY 

JAMES F DAVIS 
STATE GEOLOGIST 



INTERPRETATION OF THE 

GRAVITY MAP OF CALIFORNIA 

AND ITS CONTINENTAL MARGIN 

By 
H.W. Oliver, Editor 

1980 




BULLETIN 205 



.VER3ir» ALIFORNIA 

DAVIS 

MAR 5 1981 

CALIF. DEPOS. 
G*. i^OCS.- LIBRARY 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 
1416 Ninth Street, Room 1341 
Sacramento, California 95814 



•< 

•' 



, M 



TABLE OF CONTENTS 

Page 

UNITS AND ABBREVIATIONS USED IN THIS REPORT vii 

ABSTRACT 'x 

GENERAL INTRODUCTION, by H.W. Oliver 1 

OFFSHORE SOUTHERN CALIFORNIA, by L.A. Beyer 

Physiography 8 

Geology 8 

Previous and Present Gravity Studies 10 

Regional Gravity and Crust-Mantle Structure 10 

Santo Barbara Channel 12 

Northern Channel Islands 13 

Inner Basins and Ridges 13 

Outer Banks and Ridges 14 

TRANSVERSE RANGES, by H.W. Oliver 

Physiography and Geologic Setting 15 

Regional Gravity 15 

Basin Anomalies 16 

Relation to Faults 16 

PENINSULAR RANGES, by H.W. Oliver 

Topography and General Geology 17 

Regional Gravity 18 

Local Basement Anomalies 18 

Basin Anomalies 19 

Relation to Major Faults 19 

SALTON TROUGH, by Andrew Griscom 

Physiography and Geologic Setting 20 

Regional Bouguer Gravity Field and Basin Anomalies 20 

Offset on the San Andreas Fault 21 

MOJAVE DESERT, by R.H. Chapman 21 

OFFSHORE CENTRAL AND NORTHERN CALIFORNIA, by E.A. Silver 22 

COAST RANGES, by R.H. Chapman and Andrew Griscom 

Physiography and Geologic Setting 24 

Bouguer Anomalies North of Latitude 39°N 24 

Sooth of Latitude 39°N 26 

GREAT VALLEY, by H.W. Oliver and Andrew Griscom 

General Geology 27 

Densities 27 

Gravity Anomalies 28 

The West Side Gravity Low 28 

The Great Valley and Dinuba Gravity Highs 28 

Seml-Locol Anomalies 29 

Relation to Faults 29 



SIERRA NEVADA, by H.W. Oliver 

Physiogrohy, General Geology, and Densities 30 

The Gravity Field 30 

Isostasy 31 

Interpretation of Local Anomalies 31 

Relation of Gravity to Major Faults 32 

GREAT BASIN, by H.W. Oliver 

Physiography and General Geology 32 

Regional Gravity 33 

Basin Anomalies 34 

Relation to Major Faults 34 

KLAMATH MOUNTAINS PROVINCE, by Andrew Griscom 34 

CASCADE RANGE AND MODOC PLATEAU, by Andrev^ Griscom 36 

REFERENCES CITED 39 



APPENDIX 

Gravity Measurements, Reductions, and Conversion 

Formulas to IGSN 71 and GRS 67 47 



FIGURES 

Figure 1. Index map of California shov^ing sources of data 1 

Figure 2. Index to gravity anomaly maps and published data used in the compilation 2 

Figure 3. Gravity anomaly map of California with a contour interval of 30 mgal 5 

Figure 4. Generalized topography of California 6 

Figure 5. Relief map of California showing physiographic provinces 7 

Figure 6. Generalized geologic mop of the California continental margin off southern 

California 9 

Figure 7. Bouguer anomaly map of the California continental margin off southern 

California 12 

Figure 8. New gravity base network in east— central California 48 

Figure 9. Gravity differences between measurements made with LaCoste and Romberg 

meters G17 and G22 at 33 base stations A9 



TABLES 



Table 1. Gravity map sheets and data 3 

Table 2. Structural regions of the California Continental Borderland 10 

Table 3. Selected Neogene depositional basins 11 

Table 4. Comparison between average elevations and Bouguer anomalies 15 

Table 5. Relation between average elevations, Bouguer anomalies, and type of 

basement rocks 33 

Table 6. Estimated thicknesses of fill in the major basins within the Great 

Basin sector of California 35 

Table 7. Prime base stations in east— central California 47 

Table 8. Gravity meter correction factors 50 

Table 9. Comparison between ISGN 71 and Chapman's (1966) observed 

gravity values 51 

Table 10. Changes in the scale values for IGSN 71 relative to that for Woollord and 

Rose (1963) and Chapman (1966) 51 

Table 11. Changes in Bouguer anomalies resulting from adoption of GRS 1967 and 

IGSN 71 ^2 

Layout by Louise Huckoby 



UNITS AND ABBREVIATIONS USED IN THIS REPORT 



Units 



cm/s' - centimeters per second per second 

mgal - milligals = 10"'cm/s^ 

km - kilometer 

m - meter 

mm - millimeters 

m.y. - million years 

g/cm' - grams per cubic centimeter 

(used for density contrast Ap) 



Abbreviations 



USGS - U.S. Geological Survey 
CDMG - California Division of Mines and Geology 
NOAA - U.S. National Oceanic and Atmospheric 
Administration 
DMA/TC - U.S. Defense Mapping Agency, 

Topwgraphic Command, Washington, D.C. 
DMA/ AC - U.S. Defense Mapping Agency, 
Aerospace Center, St. Louis, Mo. 
U.C. - University of California 
N. - north 
W. - west 
Ap - density contrast (delta rho) 



v« 



-f 
^ 



ABSTRACT 

A gravity map of California has been compiled and overprinted on the Fault map of 
California, scale 1:750,000. The gravity overlay consists of Bouguer anomaly contours onshore 
and free-air anomaly contours offshore at intervals of 5 mgal and 10 mgal, respectively. The 
compilation is based on over 50,000 gravity measurements on land and 30,000 measurements at 
sea. Both land and sea measurements were made relative to the WooUard and Rose (1963) 
gravity datum and reduced using the International Gravity formula of 1930. The land data were 
further reduced using a Bouguer reduction density of 2.67 g/cm', and include curvature and 
terrain corrections to a distance of 166.7 km for all 50,000 stations. 

Bouguer anomalies range from about -280 mgal in Long Valley on the east side of the Sierra 
Nevada to about + 30 mgal along several sections of the California coastline, although they 
increase further on the offshore islands to as much as + 80 mgal near the center of Santa Cruz 
Island west of Santa Barbara. Free-air anomalies on the continental margin range from -110 
mgal in the Santa Monica Basin to +60 mgal on the south shore of Santa Cruz Island and over 
San Juan Seamount about 300 km west of San Diego. 

A generalized topographic map of California based on averaging elevations to a distance of 
about 40 km shows a striking correlation with Bouguer anomahes. The ratio at low elevations 
is about -1 mgal/10 m increase in average elevation, but at average elevations over 2 km this 
decreases to about -0.8 mgal/10 m. Local departures of Bouguer anomalies from those predicted 
by average elevations range up to ± 50 mgal and are discussed by provinces from southwest to 
northeast. 

In offshore southern California, regional Bouguer gravity decreases toward the northeast due 
to a northeastward thickening of the crust. Less f)ositive free-air gravity anomalies usually occur 
over basins and more positive free-air anomalies usually occur over submarine ridges, knolls, and 
banks because these anomalies are uncorrected for topography. Bouguer anomahes and topogra- 
phy show a similar though less strong correlation because sequences of relatively young lower 
density rocks usually underlie basins whereas relatively old, higher density rocks usually underlie 
submarine ridges, knolls, and banks. Bouguer gradients and anomaly trends conform to the 
general northwest-southeast structural grain and in some places express the offshore extensions 
of major faults. 

The San Gabriel Mountains are characterized by a general northeast decrease in Bouguer 
anomahes from -60 to -90 mgal matching a northeast increase in average elevation from 600 
to 900 m. Bouguer anomalies in the San Bernardino Mountains are stronger, about -120 mgal 
over their northern sector, and this low corresponds to an average elevation of 1200 m, indicating 
that the ranges are in regional isostatic balance. Northeast of the San Gabriel Mountains, average 
elevation continues to rise well out into the Mojave Desert in spite of the sharp decrease in local 
elevation. Bouguer anomalies similarly decrease to a minimum value of about -105 mgal over 
bedrock in the southwestern part of the Mojave Desert before starting to increase farther north 
with decreasing average elevation. The location and shape of this regional gravity low corre- 
sponds closely with a reported region of aseismic uplift and may be related to it. 

In the Peninsular Ranges, Bouguer anomalies measure about -20 mgal along the coast, 
decrease eastward to a minimum value of -90 mgal at the maximum average elevation of about 
1000 m, and increase farther east to -25 mgal at the lower eastern edge of the province, in general 
accordance with isostasy. However, gravity is abnormally high and benchlike over the western 
part of the southern California bathoUth, and the gravity bench extends to the eastern limit of 
exposed gabbroic plutons within the bathohth. A north-striking gravity gradient along this eastern 
limit serves to divide the batholith into two parts and is not offset where it crosses the Elsinore 
fault near Lake Henshaw. Another gravity gradient is coincident with the north end of the San 
Jacinto fault and becomes very steep near San Bernardino, producing a gravity step of 20 mgal 
down to the northeast. Local gravity highs of about 25 mgal occur over structural highs in the 
Palos Verdes and San Joaquin Hills; others of 5 to 10 mgal occur over several bodies of gabbro. 
A major gravity low of -75 mgal occurs over Los Angeles basin. 

Bouguer anomaly values are high (average level -35 mgal) over the southern half of the Salton 
Trough where a sedimentary basin about 5.9 km deep (drill hole and seismic-refraction data) 
fails to display an associated gravity low that should exceed -50 mgal. The high regional values 
are probably caused by extensional thinning of the earth's crust beneath the basin to values 8-10 
km thinner than adjacent areas and also possibly by density increase of the basement due to 



2—81713 



basaltic intrusions. The extension is related to the spreading centers in the Gulf of California and 
the associated transform faults striking northwest into southern California. A gravity high over 
the Orocopia Schist on the northeast side of the San Andreas fault may correlate with other 
gravity highs over the Pelona Schist on the opposite side of the fault 90 and 300 km to the 
northwest. These three highs may reflect uplifted former oceanic crust at a shallow depth beneath 
the schists. Other gravity highs up to 20 mgal in amplitude associated with geothermal areas 
probably reflect density increases due to metamorphism of near-surface sedimentary rocks. In 
the northern part of the Salton Trough a gravity low defines a valley basin containing a maximum 
interpreted sediment thickness of at least 4.7 km. 

Bouguer anomalies in the Mojave Desert Province range from more than -25 mgal in some 
of the mountain ranges to less than -145 mgal in Ivanpah Valley. On a regional scale, the gravity 
field may reflect crustal thickness; seismic-refraction measurements indicate that the thickness 
of the crust ranges from about 20 km in the Salton Sea Province to the south to 27 km or more 
in the northern part of the Mojave. The province is characterized by a general random pattern 
of local anomalies. In general, positive anomalies tend to follow mountain ranges and negative 
anomalies follow the intervening valleys. The strongest positive anomalies are related to relatively 
dense Precambrian igneous and metamorphic rocks and Mesozoic mafic rocks. The strongest 
negative anomalies are related to Cenozoic sedimentary deposits. 

Free-air gravity anomahes outline major structural ridges and basins on the central California 
continental margin, a region of large translational tectonic movements. The Farallon ridge, from 
Point Arena to Pigeon Point, is underlain in large part by granitic rocks and is truncated on the 
southeast by the San Gregorio fault, although the associated anomaly continues east of the fault 
over the Ben Lomond batholith. This extension may rule out large right-slip offset on the fault, 
but it may also be fortuitous. Gravity anomahes over Santa Lucia bank parallel the northwest- 
trending bank morphology, but those over Santa Maria basin to the east trend northeast. The 
anomalies change trend abruptly across the Santa Lucia Bank fault and may indicate structural 
trends below the basin that are difficult to map by other techniques. Gravity interpretation has 
not yet provided new insights for the Bodega, outer Santa Cruz, or Point Arena basins. On the 
margin of subduction, north of Cap)e Mendocino, a free-air gravity low of -80 mgal occurs at 
the base of the continental slope just north of the Mendocino fault. Seismic data have not revealed 
a very thick section of sediment here. The extreme low may represent some effect of the tectonic 
intersection of the Mendocino fault and Cascadia subduction zone. 

Bouguer anomalies in the Coast Ranges Province decrease, and crustal thickness increases in 
general both north and south of San Francisco. The Bouguer anomalies in this province also 
decrease inland from the coastline largely because of the transition from thin oceanic to thick 
continental crust. In the northern part of the Coast Ranges this gradient over the imbricated 
Franciscan assemblage and melange is relatively smooth; to the south the gradient is interrupted 
by a complex pattern of local anomalies reflecting more complex geology. The striking contrast 
between the extreme geological complexity of the southern part of the Coast Ranges and the 
relatively regular geology of the northern part may be attributable to differences between the 
length of time since the two parts were subject to eastward subduction. A northward migration 
of the triple junction now located offshore at latitude 4O°20'N terminated eastward subduction 
in the northern part much more recently than it did eastward subduction in the southern part. 
Local gravity features in the southern province trend northwest or north, paralleling regional 
geologic structure. Major positive anomalies to the south of latitude 39°N are caused by granitic, 
mafic, and Franciscan rocks. Major negative anomalies are related to bodies of Tertiary and 
Quaternary sedimentary rocks. Unusual anomalies are: (a) a gravity high of 50 mgal over a 
diabase body (Mt. Diablo, lat 37°55'N) having the shape either of a piercement structure or, 
more likely, an antiformal sheet, (2) a gravity low of 30 mgal associated with a possible magma 
chamber (The Geysers, lat 38°55'N), and (3) a gravity low of 20 mgal caused by a graben (east 
of San Jose, lat 3T20'N) extending into the lower crust or upper mantle. 

Connected gravity lows of 20 to 60 mgal occur over thicknesses of 6 to II km of Cretaceous 
and Cenozoic sedimentary rocks along the west sideoftheGreat Valley. The axis of the connected 
lows determines the average axis of the asymmetrical syncline, which has shifted 20 km to the 
east since Cretaceous time. Connected gravity highs of 10 to 50 mgal extend from Red Bluff to 
Fresno near the center of the valley. A sharper group of connected highs of 10 to 30 mgal extends 
along the southeast side of the valley and into the Sierra Nevada near Porterville, where they 
are a.ssociated with remnants of 300 million-year-old oceanic crust. The similanty of the gravity 
highs in the central part of the valley suggests that they reflect buried fragments of oceanic crust 
younger than those exposed in the Sierra Nevada and older than the 151-160 million year old 
Coast Range ophiolite. Gravity lows of about 40 mgal reveal two salients of the Sierra Nevada 
batholith that extend westward under the Great Valley near Sacramento and Fresno. 



The Sierra Nevada is characterized by an eastward decrease in Bouguer anomalies from a high 
value of about -50 mgal at the west edge to a gravity low whose axis is located near and generally 
parallel to the Sierra crest. Bouguer anomalies along the axis range from -130 mgal east of 
Bakersfleld to -240 mgal west of Mammoth. East of the Sierra crest, gravity generally rises 10 
to 20 mgal to the east edge of the mountains. The general form and magnitude of the Bouguer 
anomalies are similar to average elevations, but the incremental ratio between the two (about 
-80 mgal/km) is smaller in the Sierra Nevada than in other parts of the California because the 
corresponding compensating mass is deeper (40 to 55 km) and the solid angle subtended at the 
surface is less. Local gravity highs of 20 to 40 mgal are associated with ophiolites at several 
locahties along the Melones and Bear Mountain faults. Unconnected local lows of as much as 
-30 mgal over isolated felsic plutons in the northwest Sierra Nevada indicate that the plutons 
are not connected at depth. A gravity low of a least -15 mgal over Lake Tahoe indicates the 
presence of at least 800 m of sediment. 

Regional gravity in the Great Basin increases east of the Sierra Nevada with decreasing average 
elevation according to the ratio -1 mgal/10 m. Within the Great Basin positive residuals are 
associated with Precambrian metamorphic and Tertiary volcanic rocks and negative residuals 
with Mesozoic granitic rocks. Residual gravity lows of -15 to -50 mgal over nine major basins 
reflect sedimentary thicknesses of 0.6 to 3.0 km. The average density contrast between sediments 
in the basins and surrounding bedrock ranges from 0.35 g/cm' for Indian Wells Valley to 0.95 
g/cm' for Honey Lake Valley. Steep gravity gradients reveal the locations of many buried normal 
faults and indicate that some fault zones consist of a series of step faults combined with warping. 

Gravity highs in the Klamath Mountains are generally associated with sheets of ultramafic 
rocks that are probably parts of ophiolite complexes. On the east side of the province the major 
gravity highs extend north-south along the Trinity ophiolite assemblage, but locally ophiolite 
is not indicated where extensive serpentinization has reduced the density of the ultramafic rocks. 
From gravity and aeromagnetic data the interpreted extent of the ophiolite is over 170 km in 
a north-south direction, but it may consist of three different ophiolite masses now tectonically 
juxtaposed along northeast-striking faults. 

In the Cascade Range, subcircular gravity minima 50 to 70 km in diameter are associated with 
the major volcanoes Lassen and Shasta and probably show the combined effects of low-density 
volcanic rocks and concealed batholiths. The form and location of the oblong Lassen anomaly, 
which is centered on the gap between the Klamath Mountains and the Sierra Nevada, offer 
support for the proposed rift separating these provinces. The northeast-sloping regional gravity 
gradient in this area is related to the topography, which probably postdates the rifting. Gravity 
trends in the Modoc Plateau east of the Cascade Range strike north and northwest, representing 
faults or steep downwarps associated with structural trends paralleling those in the Basin and 
Range Province. A Une of closed gravity highs trends northeast across the plateau and may 
represent a basement ridge below the volcanic rocks, having an elevation of about 2.4 km if the 
density contrast is 0.2 g/cm'. 



GENERAL INTRODUCTION 

by H. W. Oliver' 

The gravity map of California is the result of a combined 
10-year effort by the California Division of Mines and Geology 
(CDMG), the U.S. Geology Survey (USGS), several campuses 
of the University of California (U.C.), and the University of 
Oregon. The U.S. Defense Mapping Agency provided help in 
instrumentation and financial assistance without which the map 
in its present form would not have been possible. Figure 1 shows 
the various areas of responsibility, and Figure 2 shows the pubh- 
cation status, as of 1979, of the l:250,0OO-scale gravity maps in 
California and the data on which they are based. Table 1 keys 
the report numbers in Figure 2 to corresponding references. 



124" 



120° 



EXPLANATION 

Colitornio Oivtsion of 
J Mines and Geology 

U S Geological Survey 
and NOAA 

University of ColiforniQ 
at Riverside 

University of California 
at Santa Ouz and NOAA 

University of Colifornio 
at Santa Barbara 

University of Oregon 




116° 



)00 KILOMETERS 

_l 



Figure 1. Index map of California and its continental margin showing 
areas for which various State and Federal agencies and universities were 
responsible for obtaining and compiling grovity data. 



As indicated on the map itself, this l:750,00O-scale compila- 
tion is primarily a reduction to one-third of the published size 
and mosaic of the 1:250,000 maps, but it also includes data for 
the seven unpublished sheets in southern California and for un- 
published sheets in offshore California between latitudes 35°N 
and -WN. It also incorporates considerable ocean-bottom data 
along the inner shelf between latitudes 35°N and 42°N made 
available by the National Oceanic and Atmospheric Administra- 
tion (NOAA) (A. Bilik, written communication, 1973), and, in 
the area between latitudes 36°N and 3TN, from theses done at 
the U.S. Naval Postgraduate School in Monterey, California 
(Brooks, 1973; Cronyn, 1973; Souto, 1973; Spikes, 1973; and 



' U.S. Geological Survey, Menlo Park, CA. 94025. 



Woodson, 1973). These data were particularly helpful in resolv- 
ing problems in continuity between the free-air anomalies ob- 
tained with surface ships along the outer shelf and the Bouguer 
anomalies on land. Some of these problems are unresolved, and 
we have dashed and queried such areas along the California 
coastline. Free-air gravity data are not presently available for the 
inner shelf between about 34 '/;°N and 35 '/,°N. 

The map is based on over 50,000 land stations and 30,000 sea 
stations, which are unevenly distributed (inset 1). Many of the 
areas with the greatest concentration of stations are from Ph.D. 
theses such as those by Corbato (1963) in San Fernando Valley, 
Biehler (1964) in the Salton trough, von Huene (1960) in In- 
dian Wells Valley, and Greve (1962) on the San Francisco 
Peninsula. Data concentrated over Cenozoic basins have been 
obtained largely for commercial purposes. Although the south- 
em San Joaquin Valley appears poorly controlled on the index 
to gravity coverage (inset 1), additional control in the form of 
one-mgal contour maps for much of this area was made avail- 
able to us by oil companies, and we have used our own control 
to adjust the datum of such maps and have incorporated them 
into the state gravity map. (See Hanna and others, 1975a, for a 
more detailed discusson of the Bakersfield area. ) The areas of 
poorest control on the map are the eastern San Bernardino 
Mountains, the Peninsular Ranges midway between Lx3s Angeles 
and San Diego, parts of the southeastern Mojave Desert, and the 
east slope of the Sierra Nevada. Most of the gravity contours in 
northern California are controlled by a station spacing of 5 km 
or less. 

Gravity Datum 

Both the land and sea gravity data are on the WooUard and 
Rose (1963) gravity datum, which is based on an observed 
gravity value of 980118.8 mgal at the National Reference Base 
Station 0165-0 in Washington, D.C. (see Jablonski, 1974, p. 618, 
for a description of the National Base) . 

This datum is 0.8 mgal higherlhan the datum used until 1973 
by the U.S. Departments of Commerce and Defense (Duerksen, 
1949; Schwimmer and Rice, 1969; D.M. Scheibe, personal com- 
munication, 1978). The Woollard and Rose datum is 14.3 mgal 
to 14.7 mgal higher in California than the recently adopted 
International Gravity Standardization Net 1971 (IGSN 71) of 
Morelh (1974). The variation in datum is due chiefly to a differ- 
ence in the fundamental calibration standard of gravity meters 
used to carry absolute gravity to California from the 1906 meas- 
urement in Potsdam, Germany, used by Woollard and Rose, and 
from eight 1965-1970 measurements in the United States, 
United Kingdom, France, and Colombia used by Morelli ( 1974, 
p. 97), the closest of which was at Denver, Colorado. 

In November 1977, the first absolute measurement of gravity 
in California was made at San Francisco at Woollard and Rose's 
(1963, p. 41) Pendulum station GW54 in Golden Gate Park. 
The preliminary value there, corrected to floor level, is 
979972.05 ±.02 mgal (Marson and Alasia, in press), which is 
14.65 mgal lowerthan Woollard and Rose's pendulum measure- 
ment and 0.08 mgal lower than the adopted IGSN 71 value 
(Morelli, 1974, p. 48, station 12172-A). 

The 1977 absolute measurement has been carried to the prime 
gravity base station A in Menlo Park (see appendix and figure 
5) using one closed tie with three LaCoste and Romberg meters 
(R.C. Jachens, written communication, 1978). The average 
gravity difference (A - GW54) is -27.83 ±0.01 (s.e.) mgal 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



exPlANATlON 



NTIS 232 728 

Son Joie 

1976 

USGS OF 74 184 



Colifornia groviiy map sheets (scale t 250 000) published by the Coliformo 
Division of Mines and Geology showing yeor of publication. If no dote 
oppeors under the name, the mop sheet has not been published os of 
1978. Mop sheets for which preliminary grovity mops hove been released 
scale 1 250 000) ore morked "USGS OF" ond give the year that the mop 
was releosed to the open files by the U.S. Geologtcol Survey ond report 
number below the name ot the map sheet. Mop sheets for which the 
gravity doto hove been releosed through the Notional Technical 
Information Service, California Division of Mines and Geology or U.S. 
Geological Survey are marked "NTIS," "CDMG, ' or"USGS OF" respectively 
and the report number given obove the mop nome. 

II90 



Kilometers 




Figure 2. Index to gravity mops ond published data used for compiling Gravity Map of California and Its Continental Margin. Unpublished preliminary 
compilotion of five unpublished map sheets in southern California were provided by Shawn Biehler (written communication, 1977). An advance compilation 
of the Sonto Maria sheet wos mode ovoilable by Jan Rietmon (written communication, 1977). Gravity doto for the San Francisco sheet ore ovoiloble from 
R.M. Chopmon. Grovity date for the Weed &heet .were compiled by Kim [1974, appendices 1 ond 2) .Toble 1 keys map sheets ond data indexed here to reports 
cited in references. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



Table I. Gravity map sheets and data indexed by area in Figure 2 and the corresponding author 
citations of reports in the references list or sources of unpublished maps. 



QUADRANGLE 



REPORT 
NUMBER 
IN FIG. 2 



CITATION 



Published Gravity Maps 

Alturas 1%8 Chapman and Bishop (1968a) 

Bakersfield 1975 Hanna and others (1975a) 

Death Valley 1973 Chapman and others (1973) 

Fresno 1980 Oliver and Robbins (1980) 

Kingman 1973 Healey (1973) 

Los Angeles 1975 Hanna and others (1975b) 

Needles 1978 Chapman and Rietman (1978) 

Redding 1973 Griscom (1973a) 

San Francisco 1968 Chapman and Bishop (1968b) 

San Jose 1976 Robbins and others (1976) 

San Luis Obispo 1971 Burch and others (1971) 

Santa Cruz 1967 Bishop and Chapman (1967) 

Santa Rosa 1974 Chapman and Bishop (1974) 

Trona 1974 Nilsen and Chapman (1974) 

Ukiah 1975 Chapman and others (1975) 

Weed 1973 Kim and Blank (1973) 

Offshore 40°-42°N MF 852 Kososki and others (1977) 

Offshore 38°-40°N MF 854 Kososki and others (1979) 

Offshore 32 '/i°-35°N MF 024 Vedder and others (1974) 

Preliminary Gravity Maps 

Chico 74-182 Oliver and others (1974) 

Fresno 74-177 Oliver and Robbins (1974a) 

Mariposa 73-210 Oliver and Robbins (1973) 

Sacramento 74-183 Oliver and Robbins (1974b) 

San Jose 74-184 Robbins and Oliver (1974) 

Susanville 75-534 Oliver and others (1975b) 

Walker Lake 73-211 Oliver and others (1973) 

Published Gravity Data 

Alturas OFR 77-17 SAC Chapman and others (1977a) 

Bakersfield 243036 Robbins and others (1975a) 

238122 Hanna and Sikora (1974a) 

Death Valley OFR 79-9 SAC Chapman (1979) 

Fresno 241577 Robbins and others (1975a) 

Kingman 70-158 Healey (1970) 

Los Angeles 231909 Hanna and Sikora (1974b) 

Mariposa 241469 Robbins and others (1975b) 

Needles OFR 77-18 SAC Chapman and others (1977b) 

Redding 70-143 Griscom (1970) 

Sacramento 258470 Robbins and others (1976a) 

San Jose 232728 Robbins and others (1974) 

San Luis Obispo 226057 Burch and others (1974) 

Santa Cruz OFR 78-14 SAC Chapman and Bishop (1978b) 

Santa Rosa OFR 78-7 SAC Chapman (1978a) 

Susanville 254061 Robbins and others (1976) 

Trona 242459 Nilsen and Chapman (1975) 

Ukiah OFR 78-8 SAC Chapman and Bishop (1978a) 

Walker Lake 251249 Robbins and Ohver (1976) 

Weed Ph.D. thesis Kim (1974) 



Unpublished Maps 

San Bernardino 

Santa Ana 

Salton Sea 

San Diego-El Centre 

Long Beach 

Offshore 35"'-38°N 



Source 

Shawn Biehler, 1978 

Shawn Biehler, 1978 

Shawn Biehler, 1978 

Shawn Biehler, 1978 

L.A. Beyer and Shawn Biehler, 1978 

E.A. Silver, 1978 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



whicli provides an absolute value at Menlo Park A of 
979944.22 ±0.03 mgal at bench mark level. This value is 14.52 
mgal lower than the value of 979958.74 mgal determined by 
Chapman ( 1966) relative to the Woollard and Rose datum. The 
difference between the comparisons at Golden Gate Park (- 
14.65 mgal) and Menlo Park (-14.52 mgal) of 0.13 mgal more 
likely represents an error in Woollard and Rose's (1963) pendu- 
lum measurement than an error in the later gravity meter ties. 

Gravity Measurements and Reductions 

Measurements of gravity differences in California have been 
made relative to 388 base stations established throughout the 
state relative to Woollard and Rose's ( 1963, p. 94) main control 
base WA 86 at San Francisco airport (Chapman, 1966; appen- 
dix). Most of the land measurements were made with LaCoste 
and Romberg gravity meters and are accurate to 0.1 mgal. Most 
of the offshore data were obtained with LaCoste and Romberg 
or Bell surface-ship gravity meters and are accurate to about 3 
mgal. It was necessary to establish a number of mountain cali- 
bration loops to ensure the 0. 1 mgal accuracy of the land data 
over the 1500-mgal gravity range in California (978.8 to 980.3 
gals) . For detailed discussions of the base stations, gravity me- 
ters used, and calibration problems, see the App>endix. 

The approximately 50,000 gravity measurements on land were 
reduced to Bouguer anomalies assuming an average density of 
rocks above sea level of 2.67 g/cm'. The reductions include 
terrain corrections to a distance of 166.7 km from nearly all 
gravity stations. New techniques developed to expedite these 
reductions are summarized in the Appendix. The approximately 
30,000 measurements at sea were reduced to free-air anomalies, 
and the formulas used are also given in the Appendix. The 
Bouguer anomalies on land are generally accurate to about 0.3 
mgal but may be in error as much as 2 mgal in mountainous 
parts of the state. The offshore free-air anomalies have the same 
accuracy as the measurements, that is, about 3 mgal. 

Reductions of both land and sea data are based on the Wool- 
lard and Rose (1963) gravity datum discussed above and on the 
1930 International Gravity Formula (Swick, 1942, p. 61). For- 
mulas for converting these data to the recently adopted IGSN 
71 datum and the 1967 Gravity Reference System are developed 
in the Appendix. The conversion effect on free-air and Bouguer 
gravity anomalies within California and its continental margin 
varies gradually from -1.5 mgal at San Diego to about -3.2 mgal 
near the Oregon border (appendix, table 11). Thus, the net effect 
on the California gravity map would be to shift the contours by 
about half a contour interval. 

Bouguer Gravity Anomalies 
and Average Elevations 

As Bouguer himself recognized in about 1790, there is 
generally an inverse correlation between Bouguer gravity 
anomalies and topography. The correlation is improved if the 
topography is averaged over some radius in the range of 30 to 
100 km, the radius for best correlation varying from province to 
province (Putnam, 1895; Mabey, 1960; Ohver, 1977). A com- 
parison of a simplified version of the California gravity map 
(figure 3) and topography averaged to a radius of about 41 km 
(figure 4) shows a strong inverse correlation of approximately 
-1 mgal/ 10m or a little less than the attraction of an infinite 
sheet of 1 . 1 1 mgal/ 1 m of thickness (the simple Bouguer reduc- 
tion factor). Thus the -30 mgal contour (figure 3) roughly 



correlates with the 300 meter contour (figure 4), the -60 mgal 
contour with the 600 meter contour, and so on. The incremental 
ratio of Bouguer anomalies to average elevations is not a con- 
stant but decreases at higher elevations above about 2000 m to 
about 0.8 mgal/10 because the corresponding compensating 
mass is deef>er and the solid angle subtended is less (see section 
on the Sierra Nevada). 

Departures of the Bouguer anomaly contours from those pre- 
dicted by the average elevation contours are caused by in- 
homogeneities in the earth's crust and upper mantle. The 
interpretations of these inhomogeneities in terms of geologic 
structures make up the main body of this report. 



Geomorphic Provinces and Scope of Report 

Interpretations of the Bouguer gravity contours on land have 
previously been published for 16 of the 27 map sheets in Califor- 
nia (figure 2). The major anomahes are here discussed by physi- 
ographic provinces with particular reference to the relation 
between Bouguer anomalies, average elevation (figure 4) and 
the major faults shown on the base map. Figure 5 shows the 
physiographic provinces used in this rep)ort. 

Only minimal geologic summaries of the provinces are includ- 
ed in this report, and they are pointed toward possible variations 
in rock densities that might be expected to produce gravity ano- 
mahes. For more detailed geologic expositions of southern Cali- 
fornia, the reader is referred to Jahns (1954), and for northern 
California to Bowen (1962) and Bailey (1966). Oakeshott 
(1978) has most recently summarized the geology of the whole 
State, and Hamilton (1978) has summarized the outstanding 
structural problems from the point of view of plate tectonics. A 
simple Bouguer gravity map of the western United States west 
of 109''W (Eaton and others, 1978, plate 1) provides areal per- 
spective for the major gravity features in California, but the map 
suffers from the lack of terrain corrections. 

No attempt in this overview has been made to interpret all 
anomalies. All previously published gravity work has been sum- 
marized, and some new interpretations have been made to the 
extent possible without computer modelling. Some attempt has 
been made to call attention to anomalies of particular geologic 
interest where further work should be rewarding. 



Acknowledgements 

In addition to the authors who contnbuted various sections of 
this report, I wish to thank Francis Birch, G.P. Woollard and 
L.C. Pakiser, whose vision and initial encouragement in the early 
1950s helped get regional gravity studies started in California; 
Ian Campbell and D.R. Mabey, who along with R.H. Chapman 
and myself in 1962 formulated the concept of and started plan- 
ning toward the production of a gravity map for the whole state 
of California; P.M. Schwimmer, E.J. Hauer and Bob Iverson, 
who helped suppwrt and accelerate the California gravity pro- 
gram in 1968; and Shawn Biehler. J.D. Rietman, H.R. Blank, Jr., 
W.F. Hanna, D L Healey. S.H Burch, J.F. Evemden, and Ed- 
ward Byerly, all of whom later contributed considerable gravity 
data. The following staff members of the California Division of 
Mines and Geology helped obtain and reduce gravity data: 
Charles C. Bishop, Gordon W. Chase, Gary C. Taylor. Lydia 
Lofgren, and Gordon L. Campbell. Similarly, members of the 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



EXPLANATION 



dD 



Bouguer onomGly contours on lond and free-oir onomaly contours at sea 

Both the lond ond sea data are referenced to the Woollord 
ond Rose (1963) gravity datum ond reduced by the tnternottonol Gravity 
Formulo of 1930 (Swick,(I942, p6U Land doto ore further reduced 
for rock density above seo level of 2 67 g/cm' and include terrain 
and curvature corrections to a distance of 166 7 km (see text) Gravity 
highs and lows ore indicated by "+"and "— ", respectively, within 
closed contours The contour intervol is 30 mgol. 



300 Kilometers 




-- ^^...^" 



Figure 3. Graviry anomaly mop of Californio with a contour interval of 30 mgal. This mop is o reduction in both size and contour interval of the Gravity 
Mop of Colifornra ond Its Continentol Morgin (Oliver and others, 1980). 



3—81713 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



EXPLANATION 



Average elevation contours in meters. 
Contour interval 150 meters 



300 Kilometers 
J 




Figure 4. Generalized topography of Colifornia. Elevations have been overaged over rectangular blocks of three by three 15-fninute quadrangles, the central 
quadrangle being weighted double. Dinnensions of blocks ore about 66 km by 81 km and hove on area equal to that of a circle with a radius of about 41 
km. Averoge elevotioni determined in this way hove been plotted at the centers of each of the approximately 600 15-minute quodrangles in California and 
the dota contoured After Jomes Gilluly (written communication, 1966) and Gilluly and others (1968, figures 10-15). Gilluly's original contour values in feet 
have been converted to meters using the opproximotion 1000 ft — 300 m. Thii opproximotion introduces an error of less than 2% to the contour values, which 
is within the uncertainty in their estimated values ( ~ 100 m). 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 




Mi 

it 



^-v. 



Figure 5. Relief mop of California showing names and boundaries of ph/siogrophic provinces used in this report. From Bailey (1966, figure 1|. 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



U.S. Geological Survey who contributed significantly are: R.F. 
Sikora. Victor McAllister, W L. Rambo, Carter Roberts, R.C. 
Farewell, and Annabelle Kook. Field measurements were sup- 
ported by the Defense Mapping Agency/Topographic Com- 
mand (DMA/TC) under Project 3-68 and coordinated through 
T.H. Nilsen. Terrain corrections were supp<irted by DMA/AC. 

All the sections of this report have benefitted by review of J.E. 
Case and Warren Hamilton. Additional reviews of the offshore 
sections by David McCullough and Jack Vedder were most help- 
ful. I wish to especially thank Andrew Griscom for helping edit 
the first draft of the manuscript and R.H. Chapman, who coor- 
dinated the contribution to the program by State agencies and 
several universities. 



OFFSHORE SOUTHERN CALIFORNIA 
by L.A. Beyer' 

Physiography 

The dominantly subsea geomorphic province between lati- 
tudes 32.5°N and 34.5°N and between the mainland and the 
Patton Escarpment is characterized by rugged and irregular 
topography. This is the northern part of the California Continen- 
tal Borderland (Vedder, 1976). It is composed primarily of large 
submarine ridges and basins and smaller islands, banks, sea 
knolls, and ridges and valleys (Shepard and Emery, 1941; Em- 
ery, 1960). Slopes that connect basins and ridges are cut by 
numerous submarine canyons and gullies and range from gently 
convex upward to very steep (Moore, 1969). Elevations range 
from + 746 m on Santa Cruz Island to -2, 100 m in San Clemente 
Basin. Topographic relief from the highest elevation on Santa 
Cruz Island to the bottom of Santa Cruz Basin is 2,713 m (Ved- 
der and others, 1974). The slope of the seafloor exceeds 20° in 
many locations, and late Cenozoic folds and faults frequently are 
expressed physiographically. 

Geology 

Vedder and others (1976b) described the complex late Ceno- 
zoic tectonic history of the California Continental Borderland as 
follows: 

The geologic evolution of the region is attributed to 
tectonic instability of the continental margin along the 
boundary between the Pacific and North American 
plates. As a result of nght-lateral shear which began 
along the plate boundary about 30 my. ago, a network 
of ridge-and-basin structures developed. Rapid ero- 
sion of the ndges and thick accumulation of sediment 
in the basins accompanied by volcanism began about 
20 my. ago. Subsequent deformation in response to 
continued right shear, which resulted in the formation 
of local en echelon zones of folds and faults, began 
about 12 my. ago and is continuing today. 

Because prc-Miocene rocks have been subjected to this late Ce- 
nozoic tectonism and because mappmg and sampling of sub- 
merged terrain is difficult, the geology of the borderland is 
poorly understood, especially south of the northern Channel 
Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa). 

' us Ctologicai Survey. 343 Middldield Road, Menio Park. CA 94023. 



The part of the borderland that is north of the westward 
extension of the Santa Monica fault zone has a dominant east- 
west structural grain and is included in the Transverse Ranges 
Province (figure 5). The Santa Barbara Channel is the westward 
extension of the onshore Ventura Basin and is underlain by 
Cretaceous to Holocene sedimentary rocks and lesser thick- 
nesses of Miocene volcanic rocks. The northern Channel Islands 
customarily are included in the Transverse Ranges Province, 
although J.G. Vedder (1978, personal communication) jxiinted 
out that late Cenozoic displacement along a westward extension 
of the Santa Monica fault zone may be spread among many small 
faults that curve northwestward and either die out or mostly lie 
north of the westernmost island. Howell and others (1978) also 
beheve that pre-Miocene structures on the northern Channel 
Islands are similar to those south of the islands but that post- 
Miocene geologic features and geomorphology are analogous to 
those of the Transverse Ranges Province. 

South of the northern Channel Islands, the borderland is cus- 
tomanly included in the Peninsular Ranges province because of 
its predominantly northwest-southeast structural grain. Al- 
though this region remains poorly understood, recent studies 
have greatly expanded our knowledge of subbottom structure, 
distribution of rocks on the seafloor, island geology, and areal 
distribution of gravity and magnetic anomalies, heat flow, and 
earthquakes (Vedder and others, 1974, 1976a,b,c; Greene and 
others, 1975; Howell, 1976; Taylor, 1976, Junger and Wagner, 
1977; Nardin and Henyey, 1978; Blake and others, 1978). A 
generalized geologic map of the borderland is given in Figure 6, 
and a recently proposed subdivision of the area south of the 
northern Channel Islands into three structural blocks, based on 
distinctive types of basement and sedimentary cover, is summa- 
rized in Table 2. Table 3 summarizes characteristics of the Neo- 
gene depositional basins in the borderland and adjacent 
mainland. 



Vedder and others (1974) described the general rock types 
included in the stratigraphic subdivisions of the generalized geo- 
logic map (figure 6). Basement rocks include: (I) the zeolite- 
bearing, Franciscan-like metasedimentary rocks and sepentinite 
dredged from localities west of the Santa Rosa-Cortes Ridge; 
(2) blueschist- and greenschist-facies rocks exposed on Santa 
Catalina Island and dredged from widely spaced localities; (3) 
metamorphosed mafic igneous rocks, mainly amphibolite and 
pyroxenite, exposed on Santa Catalina Island and dredged from 
the Patton Escarpment and the ridge between Santa Barbara and 
San Clemente Islands; (4) metamorphosed volcanic, sedimen- 
tary, and hypabyssal rocks exposed on Santa Cruz Island; and 
(5) silicic plutonic rocks exposed on Santa Cruz and Santa 
Catalina Islands. In some cases, dense Miocene volcanic rocks 
are effective basement in gravity and seismic interpretations. 

Upper Cretaceous and lower Tertiary sedimentary rocks are 
mostly sandstone, siltstone, and claystone that in general have 
low porosities. These rocks are present along the Santa Rosa- 
Cortes Ridge, along the shelf just offshore from San Diego and 
presumably beneath the Santa Barbara Channel. The thickness 
of those strata is not well know, and their occurrence elsewhere 
in the borderland is uncertain. 



Miocene volcanic rocks consist chiefly of andesitic and basal- 
tic flows, flow breccia, tuff, and volcaniclastic rocks and are 
widely distnbuted over the borderland. Density varies widely 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



• 




■o 



"S 



O 



10 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



Table 2. — Tentative structural regions of California Continental Borderland south of the northern Channel Islands (Howell and 
others, 1978] . Divisions based on samples taken from seafloor, seismic-reflection profiling, and geology of islands and mainland. 



REGION 


BOUNDARIES 


BASEMENT ROCKS 


SUPRABASEMENT ROCKS 


I 


Eastward from base of Patton Es- 
carpment to northwest-trending 
hneament that approximately par- 
allels the west slope of Santa Rosa 
-Cortes Ridge from Cortes Bank 
to near San Miguel Island. 


Zeolite-bearing arenite and argillite 
with blocks of schist, mafic volcanic, 
and ultramafic rocks. 


Late Oligocene and younger clastic and 
volcanic rocks fill sedimentary basins. 


11 


Eastward from Region I to a line 
that extends approximately along 
the west margin of San Clemente 
Island to the eastern part of the 
Santa Cruz Island fault. 


Unknown except for basic plutonic 
rocks and greenstone exposed on Santa 
Cruz Island. 


Thick sections of Cretaceous to Eocene 
clastic rocks are widespread and, local- 
ly, are overlain by manne and non-ma- 
rine Oligocene sedimentary rocks, 
Miocene volcanic and volcaniclastic 
rocks, and correlative and younger 
rocks. 


III 


Eastward from Region II to the 
onshore part of the Newport-In- 
glewood fault zone; southeast of 
Newport Beach the eastern margin 
is uncertain. 


Catalina Schist (blueschist-and greens- 
chist-facies rocks with amphibolite and 
serpentinite) intruded and overlain by 
Miocene plutonic and volcanic rocks. 


Locally thick sections of late Cenozoic 
clastic rocks overlie and butt against 
basement and volcanic rocks. 



among these diverse volcanic rocks, making it difTicult to evalu- 
ate their effect on the gravity field. 

Miocene sedimentary rocks, chiefly claystone and siltstone, 
are widely distributed over the borderland. Pliocene and Quater- 
nary rocks consist chiefly of semiconsolidated clay, silt, sand, 
and gravel and form relatively thick deposits on shelves and 
slopes near the mainland and in the basins. Miocene and younger 
sedimentary rocks are less dense than older sedimentary and 
basement rocks in the borderland, and low gravity field values 
usually are associated with significant accumulations of these 
younger rocks. 



Previous and Present Gravity Studies 

Early gravity mapping of the California Continental Border- 
land was a pioneering effort to evaluate and improve the per- 
formance of the surface ship gravity meter and to compile and 
analyze gravity measurements of variable precision (Caputo and 
others, 1963; von Huene and Ridlon, 1966; Harrison and La 
Coste, 1968). Harrison and others (1966) presented a regional 
Bouguer gravity map for much of the borderland and used a 
spatial filtering technique to separate anomalies into short-, in- 
termediate-, and long-wavelength compx)nents. They concluded 
that short-wavelength anomalies due to upper crustal structure 
show a pronounced northwest-southeast strike, that intermedi- 
ate-wavelength anomalies due to deeper structure are aligned 
cast-west, and that long-wave length or regional Bouguer grav- 
ity decreases toward the northeast in response to a thickening of 
the crust. Von Huene and Ridlon (1966) presented regional 
free-air and Bouguer gravity anomaly maps of the Santa Barbara 
Channel and northern Channel Islands. They described a gravity 
maximum over the Channel Islands and an elongate gravity 
minimum that coincides with the westward extension of the 
Ventura Basin Rietman and Aldrich (1969) discussed Bouguer 
anomalies of the north Channel Islands in terms of geologic 
structure of the islands and their platform. 



The present free-air gravity anomaly map of the California 
Continental Borderland is an adaptation and update of the map 
published by Vedder and others (1974, sheet 7). This map is 
based almost exclusively on stable platform shipboard gravity 
surveys made since 1970 by the National Ocean Surveys of the 
National Oceanic and Atmospheric Administration (NOAA) 
and by the U.S. Geological Survey. Because of the strong influ- 
ence of the rugged topography on the free-air anomalies in the 
borderland, parts of the discussion that follows are based on a 
Bouguer gravity map (figure 7) and unpubUshed regional and 
residual gravity maps prepared by me. 



Regional Gravity and Crust-Mantle Structure 

Regional Bouguer gravity in the borderland decreases toward 
the northeast at a rate of about 0.55 mgalAm. Harrison and 
others (1966) attributed this trend to a general thickening of the 
crust toward the northeast. The rate of decrease of Bouguer 
gravity appears to be slightly greater near the mainland than 
near the Patton Escarpment, possibly indicating that the crust 
is thickening more rapidly near the mainland or that crustal or 
upper mantle rocks become less dense toward the mainland. 
Seismic-refraction measurements suggest that the top of the 
mantle is at depths of about 18 to 21 km beneath Patton Ridge, 
24 km beneath Catalina Basin, 29 km beneath Santa Monica 
Bay, and 30 to 32 km beneath the Los Angeles area and the 
Peninsular Ranges (Shor and Raitt, 1958; Roller and Healy, 
1963). 

The regional gravity trend is satisfied by a model of thickening 
crust toward the northeast based on these refraction data when 
densities of 2.95 g/cm' and 3.43 g/cm" are assumed for lower 
crustal and upper mantle rocks (Beyer and others, 1975). This 
model undoubtedly is too simple, but insufficient geophysical 
data exist to determine with confidence the finer structures of the 
lower cnjst and upper mantle beneath the borderland. The upper 
mantle velocity structure may be anomalous beneath the Trans- 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



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12 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 




Figure 7. Bouguer anomaly mop of the Californio Continental Borderland off southern California. Terrain corrections for marine gravity stations were mode 
for ocean bottom topograph/ extending from 3.5 to 99 km from eoch station. The seofloor closer than 3.5 km was assumed to be level and at the water depth 
of the stoton. Terrain corrections for island gravity stations were determined to distance of 99 km ond include the effect of submarine topogrophy. 



verse Ranges, according to P-delay time studies by Hadley and 
Kanamori (1977). Their P-delay time measurements made on 
borderland islands show intriguing variations of uncertain ori- 
gin. 

Short-wavelength free-air gravity anomahes generally corre- 
late with the basin and ridge physiography of the borderland 
because they have not been adjusted to minimize the effects of 
topography. As a consequence, low free-air gravity anomalies 
usually occur over the basins and high free-air anomalies are 
associated with ridges and knolls. Bouguer gravity anomalies 
also partly reflect the basin and ndge physiography because the 
thicker accumulations of young, low-density rocks are found in 
the basins. 

Santa Barbara Channel 

The pronounced Bouguer gravity low that extends from Cas- 
taic through Fillmore and Santa Paula to the coast at Ventura 
IS located over the eastern part of the Ventura Basm. This basin 
IS estimated to contain more than 1 6,000 m of Cretaceous and 
Cenozoic sedimentary rocks near the town of Fillmore (Nagle 
and Parker, 1971). The elongate east-west trending Bouguer 
gravity low over the Santa Barbara Channel reflects the tectonic 
depression that forms the westward extension of the Ventura 
Basin The deepest part of the Santa Barbara Channel gravity 
low IS adjacent to the town of Ventura, where Cretaceous and 
Cenozoic rocks arc estimated to be 1 1 ,000 to 1 3,000 m thick. The 



axis of the Bouguer gravity low which extends westward from 
Ventura, close to the mainland coast, presumably indicates the 
axis of maximum accumulation of sedimentary rocks in the 
channel. Bouguer gravity values increase gradually westward 
along the axis of the channel low, indicating a gradual decrease 
in the thickness of the sedimentary sequence toward the west. 
The closed free-air gravity low north of Santa Rosa Island corre- 
sponds to the bathymetric low in that area. 

The Santa Barbara Channel is bounded on the north by the 
Santa Ynez Mountains, a homocline dipping steeply south that 
incorporates Cretaceous to Miocene rocks west of Santa Barbara 
and includes strata as young as Pleistocene east of Santa Barbara 
(Vedder and others, 1969). Bouguer gravity values increase 
northward from the channel into the Santa Ynez Mountains in 
response to the overall thinning of the sedimentary sequence in 
that direction and decrease eastward within the Santa Ynez 
Mountains primarily in response to an eastward increase in the 
thickness of young low-density sedimentary rocks. 

Southward from the eastern part of the channel low, gravity 
increases toward the gravity high over Anacapa and Santa Cruz 
Islands. The westerly trend of this gradient is interrupted north 
of the western part of Santa Cruz Island by a ndge of high 
free-air and Bouguer gravity (lat 34°12'N, long 1 19°48'W) that 
extends north-northwest into the channel. A ndge in the base- 
ment probably is responsible for this anomaly, although Miocene 
volcanic rocks are known to occur at the edge of the island 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



13 



platform north of the west end of Santa Cruz Island. The north- 
west-southeast trend of this anomaly distinguishes it from the 
east-west structural grain of the channel. Farther west the south 
flank of the channel low bends toward the northwest and appears 
to merge smoothly with the northwest-trending structural grain 
west of Point Arguello. 

Northern Channel Islands 

The general free-air and Bouguer gravity pattern over the 
northern Channel Islands and their platform results from the 
interaction of (1) the Santa Cruz Island gravity high, (2) the 
ridge of high gravity that extends from southwest of Santa Rosa 
Island to west of San Miguel Island, (3) the east-west-trending 
gradient that extends onto the island platform from the Santa 
Barbara Channel gravity low, and (4) the protrusion of the 
Santa Cruz Basin gravity low between Santa Cruz and Santa 
Rosa Islands (Rietman and Aldrich, 1969). 

The elongate Bouguer gravity high over Santa Cruz Island is 
one of three large Bouguer gravity highs in the northern border- 
land. The others are over Santa Catalina Island and over San 
Clemente Ridge southeast of Osbom Bank. The Santa Cruz 
Island high is centered over Jurassic schist exposed on the south 
side of the Santa Cruz Island fault (Hill, 1976; Rietman and 
Aldrich, 1969). This gravity high extends west-northwest of the 
island for about 1 5 to 20 km and to the east of the island where, 
following bathymetric ridges, it sphts into two lobes or ridges. 
One ridge extends east, paralleling mapped faults, through 
Anacapa Island and east-northeast to join the gravity high of the 
western Santa Monica Mountains. The other ridge, which has 
more positive Bouguer gravity, turns southeast along the north 
end of the Santa Cruz-Catalina Ridge. Although Miocene vol- 
canic rocks, especially volcanic centers with pipes, also contrib- 
ute to the Santa Cruz Island gravity high, the narrow band of 
highest gravity that extends offshore reflects Jurassic basement 
that abuts the south side of the Santa Cruz Island fault. Relative- 
ly high Bouguer gravity values that extend from Santa Cruz 
Island over the north end of Santa Cruz-Catalina Ridge suggest 
that basement also remains relatively shallow at the nonh end 
of this ndge. 

High pressure-low temperature rocks of the blue amphibole 
facies, similar to some of the metamorphic rocks exposed on 
Santa Catalina Island, have been recovered from the submarine 
ridge south of Santa Rosa Island (Vedder, 1976; Piatt, 1976). 
This submarine ridge (lat 33°50'N, long 120°1 rW)is near the 
southeast end of the elongate free-air and Bouguer gravity high 
that extends northwest to west of San Miguel Island. This elon- 
gate high presumably reflects a ridge of Mesozoic basement 
rocks, possibly with Miocene volcanic rocks. The north east- 
ward decrease of gravity from this ridge through San Miguel and 
Santa Rosa Islands suggests that basement dips gently northeast 
toward the Santa Barbara Channel. Basement rocks presumably 
are at a greater depth beneath Santa Rosa Island than beneath 
San Miguel Island. Immediately south and southwest of Santa 
Rosa Island and north of the ridge of high gravity, a trough of 
low Bouguer gravity (lat 33°53'N, long 120°10'W) coincides 
with a narrow faulted syncline that appears from seismic profiles 
to contain late Cenozoic rocks. Another poorly controlled Bou- 
guer gravity low (not shown on map) immediately south of the 
channel between Sjmta Rosa and San Miguel Islands and a 
trough of slightly lower gravity (lat 34°N, long 120°25"W) im- 
mediately south of San Miguel Island may be related to the 
projected westward extension of the Santa Rosa Island fault. 



North of Santa Rosa Island and northeast of San Miguel 
Island, the slight southward embayment of lower gravity onto 
the island platform suggests that a thicker sequence of relatively 
low density pre-Pliocene strata may be present there. This area, 
together with Santa Rosa Island, may be the location of a north- 
west-trending basement trough between the basement high 
southwest of Santa Rosa and San Miguel Islands and the base- 
ment high of Santa Cruz Island. North and west of San Miguel 
Island, the trend of the gravity contours follows the northwester- 
ly strike of mapjjed faults. The regional gravity high over the 
northern Channel Islands may extend northwest to Arguello 
Canyon. 

The Santa Cruz Basin gravity low, which extends over the 
extreme east end of Santa Rosa Island and into the passage 
between Santa Rosa and Santa Cruz Islands, is apparently due 
to a thicker sequence of sedimentary rocks in this area; it also 
appears to be structurally controlled by faulting along its eastern 
margin. Small gravity features on the Channel Islands that re- 
flect the local geology are discussed by Rietman and Aldrich 
(1969). 

Inner Basins and Ridges 

The Bouguer gravity high centered over the Palos Verdes 
Peninsula extends northwest to a point about 20 km west of 
Manhattan Beach, indicating a northwestward extension of the 
Palos Verdes uplift to the east edge of Santa Monica Canyon. 
North of Santa Monica Canyon the saddle between the gravity 
lows over Los Angeles and Santa Monica Basins coincides with 
the Dume embayment (Junger and Wagner, 1977). The Dume 
embayment, one of the main routes for sediment transport into 
Santa Monica Basin during Pliocene time, may be underlain by 
as much as 500 to 1 ,000 m of post-Miocene strata. 

The Bouguer gravity high over the Palos Verdes Peninsula 
appears to be the north end of a ridge of high gravity that extends 
southeast beyond Crespi Knoll toward Coronado Bank, and 
presumably reflects the presence of a basement ridge. There are 
several Bouguer gravity lows between this ridge of high gravity 
and the mainland. The oval gravity low east of Lausen Knoll, 
situated principally between the southeast extensions of the 
Palos Verdes and Newport-Inglewood fault zones, is located 
over a small basin or graben. Seismic-reflection profiles suggest 
' that this basin has at least 1,500 m of post-Miocene strata (Jung- 
er and Wagner, 1977). Steep gravity gradients along the 
northeast and southwest margins of this low coincide with 
mapped faults. Vedder and others (1976b) believe that as much 
as 5,500 m of late Mesozoic and Cenozoic rocks may be present 
on the inner shelf east of the Newport-Inglewood fault zone near 
Dana Point, but these rocks probably do not extend offshore far 
enough to contribute to this gravity low. 

Another Bouguer gravity low extends from west of Oceanside 
south through the lower reaches of La Jolla Canyon to the 
structurally controlled submarine valley between the Coronado 
Bank and San Diego shelf gravity highs. Cretaceous and (or) 
Cenozoic strata up to several thousand meters thick may be 
present beneath parts of this gravity low, especially straddling 
the inner shelf break west of Oceanside and immediately beyond 
the inner shelf break west of La Jolla Canyon. As much as 1,500 
m of Upper Cretaceous and Paleogene sedimentary rocks are 
thought to be present on the San Diego shelf (Vedder and others, 
1976b) and to extend a short distance offshore between San 
Diego and Dana Point. Miocene and younger sedimentary rocks 



4— ani3 



14 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



are exposed on Coronado Bank, which is a broad, nearly sym- 
metncal anticlinal structure (Vedder and others, 1976b). 

The free-air and Bouguer gravity lows over the Santa Monica 
Ba.sin are centered over the thickest accumlation of post-Mio- 
cene sediinents in the basin. Junger and Wagner ( 1977) estimat- 
ed from seismic profiles that this post-Miocene sequence is 3,500 
m thick. Steep gravity gradients along the north margin of the 
basin coincide with the offshore extension of the Santa Monica 
fault and the Malibu Coast fault. A long linear gradient coincides 
with the southwest margin of the Santa Monica basin. Seismic 
profiles indicate that the southwest margin of the basin also is 
faulted. The free-air and Bouguer gravity lows over the San 
Pedro Basin also coincide with the thickest accumulation of 
post-Miocene sediments in the basin; Junger and Wagner esti- 
mate that there is 1,800 m of post-Miocene rocks in the San 
Pedro Basin. 

Free-air and Bouguer gravity lows over the San Diego Trough 
are located over the eastern part of the trough against the slope 
that leads up to Coronado Bank. Maximum accumulation of 
post-Miocene rocks is about 1,000 m in the northwest end of the 
trough and about 600 m in the southeast end (J.G. Vedder, 
written communication, 1978). This distribution of post-Mio- 
cene rocks cannot fully account for the Bouguer anomaly, which 
in part must be due to an eastward-thickening sequence of rela- 
tively less dense older sedimentary or basement rocks. The elon- 
gate Bouguer gravity low over the San Diego Trough extends 
northwest and joins an arm of low Bouguer gravity that extends 
southeast from the Catalina Basin. A basement trough probably 
coincides with this trend of low Bouguer gravity. 

The free-air and Bouguer gravity lows in the Catalina Basin 
correspond to about 600 m of post-Miocene sedimentary rocks. 
Faults bounding the Catalina Basin are expressed as steep free- 
air and Bouguer gravity gradients although there is not a steep 
Bouguer gravity gradient associated with the San Clemente fault 
adjacent to San Clemente Island and the San Clemente Basin. 
The San Clemente fault may not juxtapose rocks of significantly 
different densities in this area. The Bouguer gravity high over 
Emery Knoll supports the contention that the knoll is either a 
local basement high underlain by a shallow intrusive body or a 
volcanic dome with a volcanic pipe or shallow intrusive body. 

The narrow elongate Bouguer gravity high over Thirtymile 
Bank, where Miocene volcanic and basement rocks have been 
dredged, supports the contention that this ridge is a fault-bound- 
ed basement high. A relatively thin sequence of Miocene sedi- 
mentary rocks cover Fortymile Bank and Boundary Bank. 

TTie Bouguer gravity high over Santa Catalina Island is cen- 
tered over Jurassic basement rocks that, judging from the shape 
of the gravity high, extend northwest and southeast of the island. 
Saddles in the Bouguer gravity high along Santa Cruz-Catalina 
Ridge between Santa Cruz Island and Pilgram Banks and 
between Pilgram Banks and Santa Catalina Island presumably 
indicate basement lows and may indicate greater accumulations 
of sedimentary rocks. 

A pronounced Bouguer gravity high extends southeast from 
Osbom Bank to west of San Clemente Island. Lobes of this 
gravity high extend north, northwest, and west of Osborn Bank 
to include Santa Barbara Island, the southern part of the Santa 
Cruz Basin, and the San Nicolas Island platform. Metamor- 
phosed mafic Igneous rocks have been dredged from the San 



Clemente Ridge near the center of this high, and basement rocks 
presumably occur at relatively shallow depths over much of the 
surrounding region. 

Outer Banks and Ridges 

The gravity lows over the Santa Cruz and San Nicolas Basins 
are located over the northwest ends of the basins on the free-air 
gravity map and, on Bouguer maps, are located even farther 
northwest. In the San Nicolas Basin the greatest accumulation 
of post-Miocene strata is in the northwest part of the basin, 
where seismic profiles suggest that 1,200 m of sediment are 
present. Pre-Miocene sedimentary rocks dip beneath the basins 
from the Santa Rosa-Cortes Ridge and presumably contribute 
to the displacement of the gravity lows from the bathymetric 
centers. These pre-Miocene rocks are believed to wedge out near 
the bases of the slopes that form the eastern margins of these 
basins. 

The Santa Rosa-Cortes Ridge is believed to be underlain by 
as much as 5,000 m of Cretaceous to Holocene sedimentary 
rocks in the north and as much as 3,500 m of Cretaceous to 
Miocene rocks at its south end on Cortes Bank. An outer conti- 
nental shelf stratigraphic test (OCS-CAL 75-70 No. 1) drilled 
at latitude 32°26'05"N, longitude 118°59'49"W on Cortes Bank 
to a depth of 3,328 m penetrated mostly shale and sandstone 
ranging in age from Upper Cretaceous (Cenomanian) to middle 
Miocene (Luisian) (Paul and others, 1976). Basalt flows 183 m 
thick were penetrated at a depth of 695 m. Eocene sedimentary 
rocks are exposed on San Nicolas Island. Embayments of low 
gravity extend onto the ridge from the Santa Cruz and San 
Nicolas Basins north and south of San Nicolas Island, possibly 
indicating greater thicknesses of sedimentary rocks in these 
areas. 

The area west of the Santa Rosa-Cortes Ridge is very poorly 
understood. Gravity lows over the Tanner Basin correspond well 
with accumulations of post-Miocene sedimentary rocks. A 
thickness of about 1,000 m of these rocks is believed to be present 
in the southern part of the Tanner Basin from seismic profile 
estimates, and gravity data suggest that more than this thickness 
may be present in the northern pari of the Tanner Basin. Jurassic 
basement has been dredged from Albatross Knoll, and Bouguer 
gravity suggests shallow basement on Nidever Bank also. To the 
southeast, lower Bouguer values over Garrett Ridge and Han- 
cock Bank suggest that these features are underlain by rocks less 
dense than the Jurassic basement; this also appears to be true for 
Trask Knoll to the northwest. 

Patton Ridge is bounded on the west by the long straight 
free-air and Bouguer gravity gradient associated with the Patton 
Escarpment. On the east, Patton Ridge is bounded by a straight 
free-air and Bouguer gradient that extends about 100 km south- 
southeast from Trask Knoll and coincides with a topographic 
lineament and structural downwarp. Vedder and others (1976b) 
believe that Patton Ridge may be underlain by pre-late Creta- 
ceous igneous, sedimentary and metamorphic rocks, partly in- 
truded and overlain by Miocene volcanic rocks. Pliocene(?), 
Miocene, and some Oligocene sedimentary rocks have been 
dredged and cored from Patton Ridge. Embayments of low Bou- 
guer gravity extend over Patton Ridge west of Albatross Knoll 
and west and southwest of Trask Knoll. These areas of lower 
Bouguer gravity values coincide with regions that have a thin 
cover of fxjst-Miocene sediment; these lower values may in part 
indicate relatively thicker sections of pre-Pliocene strata 
beneath. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



IS 



Patton Ridge is bounded on its east side by a downwarp that 
leads into an elongate north-northwest-trending basin informal- 
ly called Patton basin. A thick section (> 1.200 m) of post- 
Miocene and Miocene rocks appears to underlie the north end 
of this basin immediately south of Trask Knoll. At the north end 
of Patton Ridge, the free-air and Bouguer gravity lows directly 
east of San Miguel Gap coincide with a small basin that, from 
seismic profiles, appears to contain late Cenozoic sediment. 

Harrison and others (1966) conclude that the long straight 
gravity gradient associated with the Patton Escarpment reflects 
the rapid eastward thickening of the crust from about 8 km west 
of Patton Escarpment to about 20 km at the top of the escarp- 
ment. They believe that the boundary between the mantle and 
crust dips steeper than 45° beneath Patton Escarpment. The 
free-air gravity lows at the base of Patton Escarpment are large- 
ly artifacts of the gravitational effects of the slope and the rapidly 
thinning crust and do not necessarily indicate significant ac- 
cumulations of low-density material at the base of the slope. 

TRANSVERSE RANGES 

by H. W. Oliver' 

Physiography and Geologic Setting 

The Transverse Ranges trend west, transverse to the north to 
northwest tectonic grain of California (figure 5) and, indeed, of 
the entire west coast of North America. The main ranges are the 
San Gabriel Mountains north of Los Angeles, the San Bernar- 
dino and Little San Bernardino Mountains to their east, the 
Santa Monica Mountains west of Los Angeles, and the Santa 
Ynez Mountains north of Santa Barbara. The San Bernardino 
Mountains are the highest range and culminate in Mount San 
Gorgonio (3502 m, 11,502 ft), the highest point in southern 
California. The highest point in the San Gabriel Mountains, 
Mount San Antonio, is almost as high (3067 m, 10,064 ft) 
although the average elevation of these mountains is about 300 
m (1000 ft) less than the 1200 m (4000 ft) value for the San 
Bernardino Mountains (figure 4). The Transverse Ranges in- 
clude two major onshore basins: the San Fernando Valley north- 
west of Los Angeles, and the Ventura Basin just south and east 
of Ventura. Santa Cruz, Santa Rosa, and San Miguel Islands 
constitute an offshore extension of the Transverse Ranges, but 
the discussion of their gravity features is included in the section 
on Offshore Southern California. 

Both the San Gabriel and the San Bernardino Mountains are 
made of Precambrian and Paleozoic metamorphic and plutonic 
rocks and of Mesozoic granitic rocks (Jennings and others, 
1977). The San Gabriel Mountains include an extensive 
anorthosite complex south of Palmdale. The ranges west of Los 
Angeles consist of folded Cretaceous and Cenozoic strata. 

Structurally, the San Gabriel Mountains form part of what 
Bailey and Jahns (1954) regard as a "gigantic horst," although 
the structure is not a simple extensional feature like those in the 
Great Basin (see section on that area). The mountains are 
bounded on the south by the Sierra Madre fault zone, which is 
a high-angle reverse fault that dips north beneath the mountains. 
The San Andreas fault bounds the San Gabriel Mountains on the 
north and passes obliquely between the San Gabriel and San 
Bernardino Mountains without apparent offset of the east-west 



topography, in spite of its horizontal dislocation of 250 km since 
Cretaceous time (Crowell, 1973). 

Regional Gravity 

Aside from the effect of sediments and other unusually high 
or low density rock units, Bouguer gravity contours over the 
Transverse Ranges are similar to the average elevation contours 
(figure 4), indicating that the ranges are in regional isostatic 
balance. The ratio between the change in Bouguer gravity ano- 
malies to change in regional elevation is about 1 mgal/ 10 m, the 
same as that for the Great Basin (see table 5) . The data in Table 
4 are critical to this argument and were obtained by placing a 
1 :750,0OO clear film enlargement of Figure 4 on a 1 :750,000 clear 
film copy of the gravity overlay, and both overlays on the 1:750,- 
000 scale geologic map of California (Jennings and others, 
1977). The geologic base is better than the fault base for this 
purpose because it shows which gravity contours are clearly 
associated with some unusual rock unit and therefore are not 
representative of regional gravity. For example, the range of 
Bouguer anomalies of -80 to -110 mgal along the 900-meter 
average elevation contour (table 4) is taken primarily on Meso- 
zoic granite, and the local anomalous values associated with 
Precambrian anorthosite and schist have been avoided. Gravity 
values do appear to decrease slightly to the southeast along the 
average 900-meter contour toward the San Bernardino Moun- 
tains, where the -110 mgal value occurs. 



Table 4. Comparison between average elevations and 
regional Bouguer anomalies in the Transverse Ranges. 



A verage 
elevation 
(meters) 




Range in 


A verage 


Location 


Bouguer 

anomalies 

(mgal) 


Bouguer 

anomaly 

(mgal) 


300 


Santa Monica Mountains 


-10 to -50 


-30 


600 


South boundary of San 
Gabriel Mountains 


-60 to -70 


-65 


900 


Arcadia to Saugus, 6 km 
south of and parallel 
to north boundary of San 
Gabriel and southwest 
boundary of San 
Bernardino Mountains 


-80 to -110 


-95 


1200 


North-central San 
Bernardino Mountains 


-115 to -125 


-120 



U.S. Geological Survey, Menio Park. CA. 94025. 



The conclusion that the Transverse Ranges are in regional 
isostatic equilibrium is seemingly contradicted by seismic evi- 
dence that there is no distinct "isostatic" crustal root beneath the 
Transverse Ranges (Roller and Healy, 1963; Mellman, 1972; 
Hadley and Kanamori, 1977). Actually, the average elevation, 
as defined by the method used to derive Figure 4, along the 
highest part of the San Gabriel Mountains near Mount San 
Antonio is only 700 m (3000 ft), and the average elevation 
continues to rise well out into the Mojave Desert in spite of the 
local decrease in elevation at the northern boundary of the San 
Gabriel block. Bouguer anomalies also continue to decrease 
northward from about -90 mgal over the north flank of the San 
Gabriel Mountains to about -105 mgal over bedrock near Hi 
Vista in the Mojave Desert before starting to increase again in 



16 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



accordance with the decreasing elevation (figure 4) . Thus a local 
root under the San Gabnel Mountains is not required for re- 
gional isostatic equilibrium. However, a regional mass deficiency 
under the higher southwestern part of the Mojave Desert and 
higher northern edge of the Transverse Ranges is defined by the 
region of gravity anomalies at stations on bedrock with values 
less than -90 mgal. This area includes the San Andreas fault and 
IS difficult to visualize on the gravity map but corresponds ap- 
proximately to the elongate area with an average elevation great- 
er than 900 m (figure 4). The mass-deficient area is very similar 
to the elliptical area centered near Palmdale (lat 34°42'N, long 
118TW), which has sustained a historic rise of 30 to 45 cm 
(Castle and others, 1976; U.S. Geological Survey, 1977) and 
may be related to it. 

The gravity and regional elevation data suggest that the thick- 
ness of the crust is about 3 km thicker under the northern part 
of the San Bernardino Mountains than under the San Gabriel 
Mountains within the closure of the -125 mgal contour, or per- 
haps more accurately the 1200 m average elevation closure. This 
calculation assumes a crust-mantle density contrast of 0.3 g/ 
cm', the same value which was required to reconcile gravity and 
seismic data in the Sierra Nevada (Oliver, 1977, figure 4). The 
expected delay in P„ arrivals caused by a 3-km crustal thickening 
is about 0.2 second, and such delays were recorded at two sta- 
tions on the east side of the San Bernardino Mountains from a 
magnitude 4.5 earthquake (Hadley and Kanamori, 1977 figure 
3). 

According to the most recent seismic evidence (Hadley and 
Kanamori, 1977, figure 3), the crust-mantle interface, as defined 
by the transition of 6.7 to 7.8 km/s material, rises slightly from 
a depth of about 32 km under the San Gabriel Mountains to a 
depth of about 30 km under the San Bernardino Mountains, 
the reverse of the apparent Bouguer anomaly trends. However, 
the eastward rise is accompanied by a 3-km thickening of a 6.2 
km/s upper layer at the expense of the 6.7 km/s lower crustal 
layer. The seismic evidence is poorly controlled in the vicinity of 
the San Bernardino Mountains, but it suggests that the extra 300 
m of average elevation there may be partly compensated by an 
unusually great thickness of light rocks (6.2 km/s — granite?) 
within the upper crust. 

Compensation of the San Gabriel Mountains is further com- 
plicated by the presence of a high-velocity ridgelike structure 
within the upper mantle directly beneath the mountains inter- 
preted from early p-wave arrivals (Hadley and Kanamori, 1977, 
figure 4). The ndge rises from a depth of over 100 km under the 
northern Mojave Desert to a depth of 40 km under the San 
Gabriel Mountains and then drops off southward to depths of 
about 70 km under the Peninsular Ranges. The velocity within 
the proposed ridge is 8.3 km/s or 0.5 km/s higher than the 
surrounding upper mantle. Hadley and Kanamori estimate that 
the associated density contrast is in the range 0.03-0.15 g/cm' 
and that the expected gravity effect of the proposed upper mantle 
structure is a 30- to 1 50-mgal high centered near Cajon Pass and 
stnking about N45°E. The gravity map does not contain such a 
feature, although a broad lower amplitude anomaly of less than 
1 5 mgal might be difficult to recognize. 

Basin Anomalies 

Very strong gravity lows occur over both the Ventura Basin 
(lat 34*2rN, long 119*15'W) and San Fernando Valley (lat 



34°20'N, long I18°27°W) and have been assessed previously by 
Hanna and others (1975b) and by Corbato (1963), respectively. 

The series of gravity lows between Ventura and Castaic (lat 
34°30'N, long 1 18°36'W) marks the axis of the elongate Ventura 
basin, which is a highly folded synclinorium containing an estima- 
ted 15 km of Cenozoic sedimentary rocks (Bailey and Jahns, 
1954). The basin extends westward into the continental border- 
land as marked by the closures of the -80 mgal and -70 mgal 
contours in the Santa Barbara Channel (see section on Offshore 
Southern California). The decrease in Bouguer anomalies of -80 
to -105 mgal along the axis of the gravity minimum between 
Ventura and Castaic should not be interpreted as indicating 
greater thicknesses of sediment toward Castaic. A regional east- 
ward decrease in gravity affects values in the ranges as well, and 
this can be estimated from the State average elevation map (fig- 
ure 4). The average elevation of Castaic is about 750 m and that 
of Ventura is about 400 m. Using the ratio of gravity difference 
to average elevation difference of -0.1 mgal/m (table 4), the 
estimated regional gravity difference is (0.1) (750-400) = 35 
mgal lower at Castaic. Correcting the -105 mgal gravity values 
for regional gravity indicates that the residual gravity level at the 
gravity minimum near Castaic is really about -70 mgal, 10 mgal 
higher than the gravity minimum of -80 mgal at Ventura. The 
maximum gravity closure associated with the Ventura basin is 
difficult to determine because gravity values in the Santa Monica 
Mountains to the south ( + 10 mgal) are higher than in the Santa 
Ynez Mountains to the north (-60 to -40 mgal). By again using 
the elevation information in Figure 4, the isostatically corrected 
residual anomaly at Ventura is -80 + -(-0.1(400) _ -40 
mgal, and this value represents the net perturbation in "normal 
gravity" caused by the Ventura basin. Similarly, the average 
elevation at the center of the Santa Monica Mountains gravity 
high is about 200 m, so the departure from normal gravity there 
is about +30 mgal. 

Bouguer gravity anomalies over San Fernando Valley are as 
low as about -90 mgal just south of the San Fernando fault zone, 
along which movement occurred in 1971. The residual closure 
associated with the valley is about 45 mgal and has been attribut- 
ed to upper Cenozoic sedimentary rocks about 4.5 km thick 
(Corbato, 1963). Remeasurements of Corbato's gravity meas- 
urements after the 1971 earthquake indicated changes of up to 
-(-0.45 mgal north of the fault zone, reflecting the uplift of as 
much as 2 m there (Oliver and others, 1975a). 

Relation to Faults 

The gravity expression of the San Andreas fault is small where 
it passes through the Transverse Ranges. No anomalies or pat- 
terns are obviously offset by the fault. Of course, if Crowell's 
(1962) figure of 250 km right-lateral movement since Creta- 
ceous time is approximately correct, the pre-Cenozoic rocks 
which were once continuous with those on the northeast side of 
the fault in the Mojave Desert and San Bernardino Mountains 
are now located on the southwest side of the fault east of San 
Luis ObisfK). The low gravity relief in the southwest Mojave 
Desert is similar to low relief patterns east of San Luis Obispo, 
but it is difficult to single out any particular anomaly that mat- 
ches up on both sides of the fault. The effects of crustal structure 
would be quite different in the two areas and would tend to 
camoufiage the respective gravity effects of offset geologic units. 

Similarly, the rocks exposed on the southwest side of the San 
Andreas fault where it passes through the northeast flank of the 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



17 



San Gabriel Mountains are located on the northeast side of the 
fault northeast of the Salton Sea. Specifically, the northern part 
of the gravity high (-60 mgal contour) over the Sierra Pelona 
(lat 34°33'N, long 118°26'W) occurs over the type locality of the 
Pelona Schist. The local amplitude of the anomaly is about 20 
mgal. Discussion of a similar gravity high on the opposite side 
of the San Andreas fault in the Salton Trough is included in that 
section along with an analysis of its relevance to offset on the San 
Andreas fault. 

Another gravity high appears to be associated with the Pelona 
Schist about 20 km east of Redlands (lat 34°3'N, long 
116°55'W). The center of the high is over rocks mapped as 
mylonitic gneiss by Dibblee (1964), and this gneiss has been 
thrust over the Pelona Schist at two nearby localities both west 
and east of the central outcrop and within the gravity high 
(Rogers, 1969). Thus, the Pelona Schist is autochthonous and 
probably underlies much of the gravity anomaly. The anomaly 
has an amplitude of about 30 mgal and its shape is poorly deter- 
mined (inset 1; Oliver and others, 1980). Apparently, the schis- 
tose bodies east of Redlands and at Sierra Pelona are either 
much thicker or contain a denser facies than other bodies that 
crop out in the Transverse Ranges (see Haxel and Dillon, 1978, 
figure 2; Rogers, 1969). The gravity high east of Redlands also 
occurs directly on the South Branch of the San Andreas fault. 
Because the sources of gravity anomalies are directly below the 
anomalies, this feature raises questions concerning the likelihood 
of extensive strikeslip movement on the South Branch. Detailed 
gravity measurements along the South Branch west of Redlands 
indicate that the buried fault scarp is located 200 to 300 m south 
of the exposed mountain front (Dana, 1968, 1970). 

The gravity expression of the San Jacinto fault is particularly 
impressive near San Bernardino (lat 34°5'N, long 117°22"W) 
where a gravity step of 30 to 40 mgal occurs across the fault, the 
high side being on the southwest. Another gravity step of about 
30 mgal lies across the Santa Monica fault between San Monica 
and Glendale. East of Glendale, the linear gravity gradient bends 
to the south and seems to reflect the northwest-striking pre- 
Quatemary faults shown on the base map. The Raymond fault 
between South Pasadena and Arcadia does not have a gravity 
expression, but the Sierra Madre fault zone east of Arcadia to 
Upland has a step of 5 to 10 mgal, the gravity field being down 
on the south side of the fault. 

The San Gabriel fault slices through bedrock between San 
Fernando and Pyramid Lake(lat 34°40'N, long 118°40'W)and a 
small eastward rise in gravity signals the eastern terminus of the 
Ventura Basin anomaly; but north of Pyramid Lake, the correla- 
tion is excellent between the San Gabriel fault and a gravity 
gradient that reaches 8 mgalAm. Where the fault approaches 
the Frazier Mountain area and the San Andreas fault, the gravity 
gradient bends toward the north away from the surface fault 
trace, indicating the probable location of the major displacement 
at depth. 

The Santa Ynez fault is a reverse fault that is steep near the 
surface and that may have some left-lateral motion (Bailey and 
Jahns, 1954). The fault dips steeply south and the rocks on the 
south side have been upthrown 1 '/, to 3 km on the north flank 
of a west-trending anticline that controls the topography of the 
Santa Ynez Mountains. An east-plunging gravity nose with a 
residual amplitude of 20 to 30 mgal is associated with the anti- 
cline, and a significant gravity gradient is coincident with the 
fault northwest of Santa Barbara in the vicinity of Lake Ca- 



chume (lat 34°35'N, long 119°55'W). About 10 km west of the 
lake, the fault branches, and the south branch has apparently 
been active in Quaternary time (Jennings, 1975). However, the 
major gravity gradient marks the north branch of the fault which 
continues west (Jennings and others, 1977). East of Lake Ca- 
chume, the associated gravity step decreases in magnitude, sug- 
gesting that the throw on the fault decreases proportionately. 

PENINSULAR RANGES 

by H.V/. Oliver' 

Topography and General Geology 

The Peninsular Ranges of southwestern California (figure 5) 
make up the northern part of a larger geologic province that 
extends 1100 km south to the tip of Baja California (Jahns, 
1954). The ranges form the backbone of southern and Baja 
Cahfomia and culminate in Mount San Jacinto (3293 m, 10,805 
ft.) located in the northeast comer of the province (3 3°49'N, 
1 16°39"W). The highest mountains are along the east side of the 
province and form a ridge that is nearly in alignment with the 
crest of the Sierra Nevada (figure 5). Spectacular scarps 2 to 3 
km high are disposed en echelon along the east face of the 
Peninsular Ranges and bear a striking resemblance to the eastern 
scarps of the Sierra Nevada. 

The ranges are made chiefly of Cretaceous granitic rocks that 
constitute the southern California bathohth (Larsen, 1948; Mor- 
ton and Gray, 1971; Budnik, 1972). The age of the batholith 
ranges from about 120 million years on the west to about 105 
million years on the east based on concordant biotite and horn- 
blende K-Ar dates and limited control with zircon ages (Banks 
and Silver, 1966; Evemden and Kistler, 1970; Krummenacher 
and others, 1975). Initial Sr'VSr" ratios increase from west to 
east, reaching continental values of 0.706 near Mount San Ja- 
cinto (Kistler and Peterman, 1973; Kistler and others, 1973). 
Thus, the southern California batholith is similar in some re- 
spects to the Sierra Nevada bathohth, but it is slightly more 
mafic, the average composition being tonalite (Larsen, 1948; see 
Bateman and others, 1963, for comparative data). About 15 
percent of the batholith west of the Elsinore fault consists of a 
myriad of gabbroic bodies (Jennings and others, 1977) whereas 
the area east of the San Jacinto fault is free of mafic bodies and 
generally more felsic. Density measurements of the southern 
California batholith are not available but, judging from the pe- 
trographic descriptions (Larsen, 1948), must vary from about 
3.0 g/cm' for the gabbros to 2.6 g/cm' for the leucogranites (see 
Oliver and Robbins, 1980, for comprehensive density data of 
granitic rocks). 

Paleozoic and Mesozoic metamorphic rocks make up the wall- 
rocks and the roof pendants that project into the batholith. The 
western wallrocks are chiefly low-giade slate and greenstone 
whereas the roof pendants include graywacke quartzite, marble, 
and some andalusite-sillimanite-facies rocks. Densities of these 
rocks probably range from 2.6 g/cm' for the quartzite to 2.9 
g/cm' for sillimanite schist. 

Cenozoic rocks lap unconformably over the pre-Cenozoic 
rocks along the coast and fill the Los Angeles basin (33°58'N, 
II8°20'W) and smaller basins between the ranges. Over 2000 



US Geological Survey, Menlo Park. CA 94025. 



18 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



density measurements have been made of core samples of the Los 
Angeles Basin (McCuUoh, 1960) and these densities range from 
2.1 g/cm' for saturated Holocene alluvium to 2.6 g/cm' for 
lower Cenozoic sand and shale at depths greater than about 3 
lun. 

The Peninsular Ranges are divided into three structural 
blocks by the Elsinore and San Jacinto faults. The San Jacinto 
fault IS the more active of the two, having historic breaks near 
Hemet and west of the Salton Sea caused by five earthquakes of 
magnitude 6.0 to 6.8 between 1915 and 1954 (Hileman and 
others, 1973, figure 4). No historic movement has been discov- 
ered along the Elsinore fault, but a number of small earthquakes 
with magnitudes less than 4 occurred along it in 1949 and 1970- 
1972 (Hileman and others. 1973, figures 22 and 54). 

The San Jacinto fault is primarily a young strike-slip fault 
with a cumulative right-lateral movement of about 25 km since 
Miocene time (Sharp, 1967). The Elsinore fault is more of an 
enigma. Because of its arrangement parallel to the San Jacinto 
and San Andreas faults, one would suspect that it also would be 
dominantly a strike-slip right-lateral fault. However, the most 
detailed study of the fault indicates that the motion has been 
chiefly dip-shp in the vicinity of Temecula, the east side having 
moved down about 1 km (Mann, 1955). Other studies did not 
reach conclusions on the horizontal sense of movement but cited 
geologic evidence for limiting either right- or left-lateral strike- 
slip displacements to "small" (Sharp. 1968, p. 292) or "on the 
order of a few miles" (Morton and Gray. 1971, p. 73). New 
evidence from biotite isochrons show a prejudice for small right- 
lateral displacement, but the data control is inadequate to resolve 
the question (Krummenacher and others. 1975, figure 1). 

Earthquakes have also occurred along the Newport-Ingle- 
wood fault zone (Hileman and others, 1973), which is largely 
concealed by the Holocene alluvium of the Los Angeles Basin. 
About 1 '/j km of right-lateral motion is inferred to have oc- 
curred along the fault since early Pliocene time (Yerkes and 
others, 1965, p. A48). 

Regional Gravity 

Bouguer anomalies are about -20 mgal along the coast 
between San Juan Capistrano and San Diego and gradually de- 
crease eastward, reaching a low of about -90 mgal along an axis 
that parallels the coast but not the major faults. Farther east, 
gravity rises toward the Salton Trough, reaching values as high 
as -25 mgal near the eastern edge of the province west of Braw- 
ley. This east-west profile is complicated by a northward de- 
crease in regional gravity, causing the -80 to -90 mgal contours 
to open in successively wider parabolic forms. Gravity is relative- 
ly fiat over the San Jacinto Mountains but decreases gradually 
northward from about -75 mgal west of Palm Desert to -95 mgal 
at Mount San Jacinto. 

These regional gravity variations bear a striking resemblance 
to the average elevation contours (figure 4). The average eleva- 
tion along the coast is 1 50 m, which corresponds to the - 1 5 mgal 
gravity contour (sec Introduction and sections on the Great 
Basin and Transverse Ranges). Similarly, the average elevation 
at Mount San Jacinto is 900 m yielding a computed gravity 
contour of (-0.1) (900) or -90 mgal, which is only 5 mgal 
greater than the observed value there. Mount San Jacinto is 
located about 10 km east of the maximum average elevation of 



about 1000 m at that latitude, and the gravity behaves according- 
ly, although the data are sparse in that area (see index to gravity 
coverage and Oliver and others, 1980). 

A local lack of correlation between regional gravity and aver- 
age elevation indicates areas underlain by rocks in the upper 
crust with densities different from about 2.7 g/cm'. Aside from 
the basin areas, to be discussed in the next section, the major area 
of departure from normal gravity-elevation relations is in the 
western part of the southern California batholith. This area ex- 
tends from the coast inland for about 50 km and is marked by 
a nearly benchlike gravity expression with a level of -20 i 10 
mgal, despite the average elevation increase to 600 m and the 
values of -60 mgal that would therefore be predicted for this 
distance from the coast. This area is underlain by old, mafic 
batholithic rocks which have mantle-type initial Sr'VSr" ratios 
of 0.704. The rocks are quartz diorite with a probable average 
density of about 2.8 g/cm'. The gravity bench is also coincident 
with the area within the batholith containing numerous gabbroic 
bodies discussed above. Thus, the southern California batholith 
can be divided on the basis of the associated gravity field into two 
parts; an abnormally dense western half and a normal eastern 
half The division between the two halves — the eastern edge of 
the gravity bench — is marked by the increased gradient, which 
reaches a maximum value of 10 mgal/km along the -55 mgal 
contour between Camj>o (lat 32°36'N. long 1 16°28'W). near the 
Mexican border, and the Winchester-Homeland area about 10 
km west of Hemet (lat 33°45'N, long 116°58'W). The westward 
divergence of the -20 to -30 mgal contours to the west of the 
main gradient along the Elsinore fault zone is a separate problem 
discussed below and does not obscure the fact that the major 
gravity gradient crosses the Elsinore fault and is primarily relat- 
ed to the bathohth. 

Shawn Biehler (in Elders and others. 1972, figure 3) has 
modeled the crustal structure of the Peninsular Ranges along an 
approximately east-west profile through San Diego, using a den- 
sity contrast of 0.35 g/cm' between the crust and upf)er mantle. 
The model is controlled by offshore seismic data and shows an 
eastward crustal thickening from about 29 km under San Diego 
to about 32 km under the highest part of the Peninsular Ranges. 
Farther east, the model shows the crust thinning rapidly to 21 
km at the center of the Salton Trough. 

Local Basement Anomalies 

Two types of basement rocks in this area are associated with 
gravity highs. One is the Catalina Schist, a glaucophanitic schist 
which crops out in the Palos Verdes Hills near Point Fermin (lat 
33*44 N, long 11 8°I8'W) about 20 km west of Long Beach (see 
McCulloh, 1957, for a detailed gravity and geologic map of this 
area); the other is gabbro, which makes up a number of bodies 
up to 10 km in size throughout the western half of the batholith. 
The 5- to lO-mgal residual gravity highs north of Fallbrook (lat 
33°26'N, long liri5'W). 4 km east of Lake Elsinore (lat 
33''4rN, long 1 17''20'W), and 40 km east of the San Diego at 
Alpine (lat 32°52'N, long 116°45'W) are directly over gabbro 
bodies (Jennings and others, 1977). 

The large horseshoe-shaped anomaly centered at Laguna 
Beach (lat 33°33'N, long 1 IT'48'W) is harder to explain because 
no gabbro, glaucophane schist or other dense basement rocks 
crop out in the vicinity of the anomaly. However, Paleocene 
marine sedimentary rocks crop out at the north edge of the San 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



19 



Joaquin Hills near the center of the anomaly (Jennings and 
others, 1977). Moreover, these hills are known from drilhng to 
be the surface manifestation of a faulted anticline with 2 '/; km 
of structural relief (Vedder, 1975). The anticline and associated 
faults — the latter are shown on the base map — strike northwest 
roughly parallel to the coastline and the elongate axis of the 
closed 0-mgal contour. The northern part of the anomaly 
between East Irvine and Tustin is associated with another north- 
west-striking anticline (Yerkes and others, 1965, p. A49; J.G. 
Vedder, 1978, personal communication). Thus, the regional 
anomaly is at least partially the result of the superposition and 
coalescence of the gravity effects of two or more anticlines. The 
large size (about 25 mgal) and extent of the gravity high suggest 
that basement rocks were also involved in the San Joaquin Hills 
uplift. For example, a density contrast of about 0.4 g/cm' is 
required for a semi-infinite block 2 '/; km high and 6 km wide 
to cause a 25-mgal anomaly at a height of about 2 km above the 
block. A 0.4 g/cm' density contrast is larger than normally 
occurs between Paleocene and upper Tertiary rocks and usually 
indicates tectonic juxtaposition of pre-Cenozoic crystalline base- 
ment rocks with much lighter Cenozoic sedimentary deposits in 
California. The Laguna Beach gravity high is similar in ampli- 
tude and extent to the gravity high west of Long Beach and does 
not resemble the smaller gravity highs over gabbro in the Penin- 
sular Ranges. 

Basin Anomalies 

The largest and deepest basin in the Peninsular Ranges is the 
Los Angeles Basin, which is both a physiographic alluviated 
lowland and a structural depression covering the area of Los 
Angeles and its suburbs (Yerkes and others, 1965). A simple 
Bouguer anomaly map with a contour interval of 1 mgal is 
available for the northwestern part of the basin (McCulloh, 
1957). Complete Bouguer and regional gravity maps with an 
interval of 5 mgal are also available for the entire basin (McCul- 
loh, 1960, figures 150.2 and 150.4). McCuUoh's Figure 150.2 is 
nearly identical with the Los Angeles area of the State Map 
presented here except for details around the edges of the basin 
based on more recent data (Hanna and others, 1975b). 

Bouguer gravity anomalies decrease from about + 20 mgal on 
the coast west of Long Beach to a minimum closure of -80 mgal 
encircling the city of South Gate. North of the basin, Bouguer 
gravity rises to about -30 mgal in the Santa Monica Mountains, 
whereas on the northeast flank it reaches only about -60 mgal 
over crystalhne rocks. A regional gradient of about 2 mgal/km, 
dipping northeast, is superimposed on the effect of the low- 
density sediments in the basin. The regional gravity gradient is 
consistent with the northeast increase in average elevation of 
about 450 m across the basin (figure 4) . After removing regional 
gravity, the residual anomaly is about -75 mgal. McCulloh 
(I960) modeled the residual anomaly and showed that it is 
caused by a maximum thickness of about 10 km of Upper Creta- 
ceous and Cenozoic sedimentary rocks. 

Detailed gravity surveys have also been made in two small 
inland basins in support of hydrologic studies. One of these is 
Gamer Valley (Durbin, 1975), which is not named as such on 
the State Map but is located under the closure of the -90 mgal 
contour 25 km south of the San Jacinto Mountains and just 
south of Lake Hemet at latitude 33°35'N, longitude 116°40'W. 
According to Durbin's one-mgal contour map, the gravity mini- 
mum is located on the San Jacinto fault zone and has a closure 
of about 5 mgal. 



Another detailed gravity map of a Cenozoic basin is available 
for the gravity low at latitude 33°30'N, longitude 117°7'W just 
east of Temecula, which is 47 km east of San Clemente, The map 
is unpublished but available for inspection at the Geological 
Survey Water Resources Division office in San Clemente (Rich- 
ard Moyles, oral communication, 1978). 

Relation to Major Faults 

The faults in the Peninsular Ranges have a more pronounced 
effect on gravity than in other parts of California. The types of 
effects include coincident gradients, associated lows, and pattern 
offsets. 

The north end of the San Jacinto fault near San Bernardino 
has a coincident gravity gradient so steep that it is almost a step 
down to the northeast from about -75 to about -95 mgal. The 
line of maximum gradient is about 2 km southwest of the surface 
trace as shown on the base map, suggesting that the fault plane 
dips to the southwest in this area. Farther southeast, in the 
vicinity of Hemet, the Hot Springs fault splits off from the main 
San Jacinto fault zone, leaving a downfaulted valley that is filled 
with at least 2 km of sediment, judging from the amplitude of 
the gravity low over it. The gravity step into the valley is larger 
on the southwest side, indicating a larger vertical displacement 
there than along the Hot Springs fault. 

South of Lake Hemet, there are several small gravity lows over 
the fault zone, but no convincing gravity evidence of right- 
lateral offset. The small 10 to 15 mgal highs on opposite sides 
of the Superstition Mountain fault are of similar aspect; if they 
were once continuous, their right-lateral offset would be about 
16 km. 

The gravity character of the Whittier-Elsinore fault system is 
somewhat different and is generally more supportive of dip-shp 
movements. There is a sharp gravity rise of 5 to 10 mgal across 
the Whittier fault, which indicates that denser rocks are nearer 
the surface on the northeast than they are on the southwest side 
of the fault. On the basis of seismic-reflection data, McCulloh 
(1960, figure 50.3) showed the Whittier fault as a high-angle 
reverse fault with a basement scarp on the north side about 0.6 
km high buried by about 2 km of late Cenozoic sediments. 

The north end of the Elsinore fault is marked by a sharp 
gravity low of about 1 5 mgal that probably indicates a buried 
narrow graben located on the fault line. The fault bedrock on 
both sides of a graben is exposed at Lake Elsinore, where it has 
been described by Mann (1955). The gravity low extends with 
varying amplitude as far south as Temecula, beyond which no 
significant areas of Cenozoic fill are associated with the Elsinore 
fault. 

The gradient that divides the southern California batholith as 
described above crosses the Elsinore fault in the vicinity of Lake 
Henshaw without an obvious offset. South of the fault, the locus 
of maximum gradient, approximated by the -55 mgal contour, 
does bend toward the west and almost parallels the fault before 
turning more to the north right at the crossing. The bend to the 
west seems to curve around a group of gabbro bodies that dis- 
place the -30 to -45 mgal contours eastward near Cuyamaca 
Peak and may not be related to the fault. It is worth noting in 
this context that apparent left-lateral offset can be caused by 
vertical offset of a density discontinuity that dips to the east. In 



20 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



any case, this throughgoing gradient seems to put a significant 
constraint on possible lateral movement of the Elsinore fault. 

Gravity contours in the vicinity of Long Beach cross the New- 
port-Inglewood fault zone obliquely without disturbance. 
McCulloh (1960, figure 150.1) shows the section of the fault 
near Bellflower to be a reverse fault with about 0.3 km of up- 
throw on the northeast side of the fault. However, there are 
difTerent types of basement on opposite sides of the fault, which 
suggests major strikeslip displacement (Warren Hamilton, writ- 
ten communication, 1978). For a density contrast of 0.3 g/cm', 
the ma.ximum gravity effect of a 0.3-km vertical displacement is 
about 4 mgal or less than one contour interval on the State map. 
Wiggles of about half a contour interval occur near the fault, but 
the effect is small. 

The rise in gravity southwest of the Palos Verdes fault is much 
more pronounced. McCulloh ( 1957, section AB) has interpreted 
the gravity step there as indicating nearly 2 km of structural 
relief as the result of both anticlinal folding and thrusting of the 
Catalina Schist to the northeast over Miocene strata. 

SALTON TROUGH 

by Andrew Griscom' 

Physiography and Geologic Setting 

The Salton Trough is the northern structural extension of the 
Gulf of California and is topKsgraphically low, having an average 
regional elevation of less than 150 m in the southern part and 
valley floors at or below sea level. The east part of the trough 
southeast of the Salton Sea is termed the Imp>erial Valley, and 
at the opposite end of the Salton Sea the Coachella Valley ex- 
tends 70 km to the northwest. 

Three major right-lateral strike-slip faults extend along the 
trough and have a total strike-slip displacement perhaps as great 
as 300 km (Crowell, 1975). The San Andreas fault is located on 
the northeast side of the trough, and the southwest side of the 
trough is occupied successively by the Elsinore fault and the San 
Jacinto fault zone. The bordering ranges expose crystalline meta- 
morphic and igneous rocks that within the trough are generally 
covered by great thicknesses of Cenozoic sedimentary deposits. 
The eastern half of the trough is an elongate structural depres- 
sion occupying the Imperial Valley, Salton Sea, and Coachella 
Valley. Drill holes and geophysical data show that maximum 
depths to crystalline rocks exceed 4 km in the Coachella Valley 
and are over 6 km near Brawley south of the Salton Sea. West 
of this large structural depression, the trough is composed of 
smaller structural depressions separated by faults and occasional 
outcrops of uplifted crystalline rocks. At the south end of the 
Salton Sea five small rhyolite domes (Kelley and Soske, 1936; 
Muffler and White, 1969) intrude Pleistocene sediments. These 
domes contain abundant inclusions of basalt (Elders and others, 
1972) and are associated with a more extensive magnetic anom- 
aly (Griscom and Muffler, 1971 ) interpreted to be caused by an 
intrusion parallel to the trough and at least 30 km long, 5-8 km 
wide, and 2-4 km thick with its upper surface about 3 km below 
sea level. This intrusion may be mostly composed of basaltic 
rocks. 

Associated with the rhyolite domes and the magnetic anomaly 
at the south end of the Salton Sea is the Salton Sea geothermal 



' U.S. GeologiciJ Survey, Menio P«rk. CA 94025. 



area (White and others, 1963; Muffler and White, 1969), where 
wells record the near-surface metamorphism of former sedi- 
ments to greenschist-facies mineral assemblages of relatively 
high density. 



Regional Bouguer Gravity Field 
and Basin Anomalies 

The Bouguer gravity anomalies of the Salton Trough have 
been studied by Kovach (Kovach and others, 1962) and in the 
greater detail by Biehler (Biehler and others, 1964; Biehler, 
1964; Elders and others, 1972). Gravity values range from a 
closed low of-1 15 mgal in the Coachella Valley to -20 mgal near 
the Mexican border and at the south end of the Salton Sea. The 
northwest-trending gravity contours reflect the similar tectonic 
trends. A regional gravity high (Elders and others, 1972, figure 
3) over the southern part of the Salton Trough (the Imperial 
Valley and the Salton Sear) trends northwest and ranges from a 
high of -30 mgal near the Mexican border to -60 mgal at the 
north end of the Salton Sea. The regional gravity high extends 
northwest of the Salton Sea, as is demonstrated by the highs 
flanking the more local intense gravity low associated with the 
Coachella Valley. 

The gravity anomaly over the eastern structural depression in 
the Salton Trough varies from a low in the Coachella Valley to 
a high south of the Salton Sea. In the Coachella Valley, gravity 
analysis of the major gravity low suggested at least 4.7 km of 
sediments (Biehler, 1964) . The asymmetry of the low shows that 
the sediment is thickest on the northeast side of the valley near 
the San Andreas fault. The steep gravity gradient across the San 
Andreas fault east of Indio (lat 33°45'N) is the result of a steep 
contact between crystalline rock and sediments that here exceed 
4 km in thickness (Biehler, 1964). Southeast of the Coachella 
Valley, where the Salton Trough widens at the Salton Sea, the 
Bouguer anomalies rise to a local high of -20 mgal centered over 
the trough, and continue at a level of about -35 mgal southeast 
along the Imperial Valley to the Mexican border. These high 
values are unexpected because an east-west seismic-refraction 
profile across the valley at Westmorland, 8 km south of the 
Salton Sea, indicates that the interface between material having 
a longitudinal wave velocity of 4.7 km/s and basement having 
a velocity of 6.4 km/s is at most about 5.9 km deep (Biehler, 
1964). Biehler also reported that in this same area the average 
density of sedimentary rocks from well samples to depths of 3 
km is 2.40 g/cm'. Given a basement depth of only 3.5 km at the 
Westmorland profile, a residual gravity low of -40 mgal would 
be predicted assuming basement rock densities of about 2.67 
g/cm' (Biehler, 1964), but such a residual low is not evident on 
the gravity map. Biehler (1964) offered two explanations, both 
involving density decreases beneath the main structural trough, 
to account for the absence of the gravity low: (1) a thinning of 
the crust under the Salton Trough and (2) a local increase of 
crustal density beneath the trough. Biehler's thin-crust model 
shows that the gravity data require a crust only about 20-22 km 
thick, given a density contrast of 0.35 g/cm' between the crust 
and mantle. The crust here is about 8-10 km thinner than in the 
adjacent areas of his model. Seismic-refraction data at the north 
end of the Gulf of California do not define the base of the crust, 
but it is "probably at a depth no less than 24 km below sea level" 
(Phillips, 1964). This result is comparable to Biehler's calcula- 
tion. A denser crust under the Salton Sea and Imr>erial Valley 
is suggested by the large inferred basalt intrusion beneath the 
sediments of the Salton Sea area. Such mafic intrusions may 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



21 



indeed be expected if the Salton Trough, like the Gulf of Califor- 
nia, is the result of tensional thinning and spreading between the 
North America and the Pacific plates (Hamilton, 1961; Elders 
and others, 1972). 

The local Bouguer gravity high at the south end of the Salton 
Sea has a maximum contour of -20 mgal and is 10-15 mgal 
above the regional background level. This anomaly probably 
reflects mostly the local increase in density of the former sedi- 
mentary rocks due to their metamorphism in this active geother- 
mal area, but the inferred mafic intrusion could also contribute 
to the high. The metamorphism is most likely caused by the 
concealed mafic intrusion. Other known geothermal areas in the 
Salton Trough, such as the one near Brawley (lat 33°00'N, long 
115°30'W), also coincide with local gravity highs 2-22 mgal in 
amplitude (Elders and others, 1972). 

Southwest of the main structural trough of the Salton Sea and 
Imperial Valley, the various closed gravity lows and gravity 
highs are respectively the expression of structural depressions 
filled with low-density sediments and of structural elevations 
where high-density crystalline bedrock is commonly exposed. 

Offset on the San Andreas Fault 

On the northeast side of the Salton Sea northwest of the San 
Andreas fault is a gravity high with a maximum contour of -35 
mgal. This anomaly is associated with the Orocopia Schist, one 
of a series of similar outcrops, the cores of crudely domical 
structures, forming a disconnected northwest-trending belt of 
schist in southern California and southwestemmost Arizona 
(Haxel and Dillon, 1978). Overlying the schist in thrust contact 
is a distinctive series of plutonic rocks which include Precambri- 
an gneiss, norite, anorthosite, and syenite plus Triassic granodi- 
orite and quartz diorite (Crowell, 1962; summarized in 
Hamilton, 1978). These distinctive rock assemblages together 
with the schist and their mutual thrust contacts have been used 
to demonstrate approximately 300 km of offset on the San An- 
dreas fault since Miocene time by correlation of the Orocopia 
Schist with the similar Pelona Schist and associated rocks of the 
Sierra Pelona (lat 34°30'N, long llg'lS'W) on the west side of 
the San Andreas fault in the Transverse Ranges (Crowell, 1962, 
1975). 

A local gravity high is associated with the Pelona Schist of the 
Sierra Pelona and is similar to the high over the Orocopia Schist 
in the Salton Trough. The source of the gravity highs is not 
obvious and is probably not the rocks exposed at the surface. The 
protoliths of the schist were predominantly quartzose graywacke 
with subordinate amounts of chert, basalt, thin limestone beds, 
and small pods and lenses of ultramafic rocks. Most are now 
metamorphosed to the greenschist or epidote-amphibolite facies 
and therefore are unlikely to have an average density in excess 
of about 2.72 g/cm'. The surrounding and superimposed pluton- 
ic assemblages may well be denser than the schist; hence these 
structures, cored by schist, would not be expected to display a 
gravity high. The unknown basement rocks beneath the schist 
are considered most probably oceanic crust on the basis of the 
associated metabasalt and ultramafic rocks (Haxel and Dillon, 
1978). Perhaps the gravity highs indicate relatively uplifted oce- 
anic crust beneath the exposures of the schist. A second large 
area of Pelona Schist is in the eastern San Gabriel Mountains 
(lat 34°20' N, long 11T40'W); it does not display a gravity 
high and has not been directly correlated with the Orocopia 
Schist to determine offset on the San Andreas fault because of 



the absence of distinctive associated Precambrian anorthosite 
and syenite. 

A strong gravity high with a maximum contour of -80 mgal 
and a local amplitude of about 30 mgal is located about 30 km 
northwest of the Coachella Valley (lat 34°05'N, long 1 16°55'W). 
The source of this anomaly is associated with an outcrop of 
mylonitized gneiss (Dibblee, 1964) that lies halfway between 
two small exposures of Pelona Schist, 15 km apart, each overlain 
by mylonitized gtieiss (Dibblee, 1964; Rogers, 1969). The Pelo- 
na Schist probably also underlies the central area of mylonitized 
gneiss, which suggests that this gravity high is probably associat- 
ed with Pelona Schist. As mentioned by Oliver in the section on 
the Transverse Ranges, this inferred belt of Pelona Schist and the 
associated gravity high both straddle the south branch of the San 
Andreas fault and preclude major offset here. Although there is 
no associated anorthosite and syenite, the gravity high suggests 
that this belt of rocks may correlate directly with the Orocopia 
Schist (and its gravity high) and thus may represent displace- 
ment of about 90 km within the fault zone as suggested by 
Crowell (1962, p. 39). 

MOJAVE DESERT 
by R.H. Chapman' 

The Mojave Desert Province in CaUfomia is a large wedge- 
shaped tectonic block bordered on the north and northwest by 
the Garlock fault zone and on the southwest by the San Andreas 
fault zone and the Salton Trough. Gravity values in the province 
range from above -25 mgal in some of the mountain ranges in 
the southeastern part of the area to below -145 mgal in Invanpah 
Valley (lat 35°25'N, long 115°20'W) in the northeastern part of 
the area. 

Gravity values decrease both to the east and west of a regional 
gravity high that extends from the southeastern part of the area 
across the province to near Death Valley. This high follows the 
topographic low (figure 4) containing a 600 m closed depres- 
sion. Hunt and Mabey (1966, p. A78) in their discussion of the 
Death Valley area suggested that the regional high may reflect 
a thinning of the earth's crust. The negative anomaly in the 
western part of the province may be a southern extension of the 
minimum which to the north has been attributed largely to an 
isostatic effect related to the Sierra Nevada (Ohver and Mabey, 
1963). 

Gibbs and Roller (1966) reported a crustal thickness of about 
27 km at Ludlow (lat 34''44'N, long 1 16°09'W), near the central 
part of the province, based on seismic-refraction measurements. 
In apparent agreement with the gravity data, crustal thickness 
increases to the north in Nevada and decreases to the south 
toward the Salton Trough, where a crustal thickness of about 21 
km was reported. Elders and others (1972) have estimated a 
crustal thickness of about 21 km in the Salton Trough using 
gravity data. A seismic-refraction profile from Santa Monica 
Bay to Lake Mead reported by Roller and Healy ( 1963), howev- 
er, indicates that the base of the crust across nearly the entire 
northern part of the province is at a depth of about 26 km and 
is essentially flat, which is in apparent disagreement with the 
interpretation based on gravity data. 

Local gravity anomalies in the Mojave Desert are not general- 
ly characterized by any one principal orientation (Mabey, 

' California Division of Mines and Geology, Sacramento, C A 95816 



22 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



1960). The relatively random pattern of the anomalies in much 
of the area distinguishes this province from the surrounding 
areas. Anomalies near the Oarlock fault tend to parallel it (Ma- 
bey, 1960) Similarly, on the southwest, anomalies tend to paral- 
lel the San Andreas fault and the Salton Trough. In the northeast 
part of the area, anomalies tend to have north and northwest 
trends similar to those in the Great Basin Province to the north. 

Many positive anomalies tend to follow the mountain ranges, 
and negative anomalies follow the intervening basins. Some of 
the strong positive anomalies are related to Precambrian igneous 
and metamorphic rocks that have average densities of at least 2.7 
g/cm' (Healey, 1973; Mabey, 1960) in a series of mountain 
ranges just west of the Colorado River from the Dead Mountains 
(lat 35°00'N, long 114°45'W) on the north to the Big Maria 
Mountams (lat 33°50'N, long 114°40'W) on the south. Other 
positive anomalies evidently reflect Mesozoic mafic intrusive 
rocks that may range in density from 2.8 to 3.0 g/cm' (Chapman 
and Rietman, 1978). 

Areas of Mesozoic granitic rocks are commonly characterized 
by anomalies that range from near zero to slightly positive (up 
to 10 mgal), in agreement with the range of measured density 
values for these rocks (between 2.60 g/cm' and 2.70 g/cm') 
(Nilsen and Chapman, 1974; Mabey, 1960), which is close to the 
value used for reduction of the gravity data. Some broad negative 
anomalies, however, such as those centered in Superior Valley 
near Goldstone Lake (lat 35°25'W, long 115°37'W), with ampli- 
tudes of at least 30 mgal, suggest the presence of batholiths of 
granitic rocks that are, on the whole, less dense than the sur- 
rounding rocks (Nilsen and Chapman, 1974; Healey, 1973). 
Assuming a density contrast of -0.10 g/cm' with the surround- 
ing rocks, a mass at least 8 km thick is required to account for 
the Goldstone Lake anomaly (Nilsen and Chapman, 1974). 

Negative anomalies are associated with Cenozoic sedimentary 
deposits including alluvium and lake beds, and with some areas 
of Tertiary volcanic rocks. Because of the pronounced density 
differences between Cenozoic sedimentary rocks and most of the 
older rocks (an average difference of perhaps about 0.4 g/cm'), 
the gravity data are particularly useful in the Mojave Desert for 
indicating the thicknesses of the younger rocks in the valleys. A 
number of deep basins or troughs are suggested in the Mojave 
Desert area; for example, a northwest-trending negative anom- 
aly with an amplitude of about 35 mgal, located north and 
northwest of Blythe (lat 33°40'N, long 114°40'W), indicates a 
basin more than 2 km deep (Peterson and others, 1967; Rotstein 
and others, 1976). Also in the far northwest comer of the Mo- 
jave Desert, in Antelope Valley (lat 34°50'W, long II8°30'W) 
and south of Rosamond Lake(lat 34°45'N,longl 18°05'W), anom- 
alies with amplitudes of more than 25 mgal each indicate other 
major basins (Mabey, I960). These data have been used in 
exploration for borate deposits in the western Mojave Desert 
(Mabey, I960) and for evaluation of water resources in some 
areas such as the Picacho-Bard basin in the southeast part of the 
area, southwest of the Chocolate Mountains (lat 32°50'N, long 
1 14'35'W) (Mattick and others, 1973). In contrast to the areas 
with major negative anomalies, the lack of a significant anomaly 
in a valley area, such as between the Providence Mountains (lat 
34*55'N, long 115°35'W) and the Clipper Mountains (lat 
34"50'N, long 1 15*25'W), suggests that bedrock is near the sur- 
face. 

Faults, particularly those that bound some of the mountain 
ranges, are commonly indicated either by relatively steep gravity 



gradients or by negative anomalies. For example, the southwest 
side of the Sacramento Mountains and the Saw Tooth Range (lat 
34°35N, long 1 14°40'W) (Chapman and Rietman, 1978), the 
southwest side of the Big Maria Mountains, and the west side of 
the Palen Mountains (lat 33''45'N, long I15°10'W) (Rotstein 
and others, 1976), both the northwest and southeast sides of 
Cantil Valley (lat 35°20'N, long IWSO'W) (Mabey, I960; Nils- 
en and Chapman, 1974), and near Needles (lat 34°10'N, long 
1 14°36'W) (Peterson. 1969; Chapman and Rietman, 1978) show 
steep gradients that are almost certainly related to faults. Three 
strong east-trending negative anomalies located in the south- 
central part of the Mojave Desert evidently represent major fault 
zones: the Pinto Mountain fault zone (lat 34°I0'N, long 
1 16°I5'W), the Blue Cut fault zone (lat 33°55'N, long 1 15°45'W) 
and the Orocopia lineament (lat 33°40'N, long 115°45'W) 
(Biehler, 1964; Rotstein and others, 1976). 

Some other major gravity features in the Mojave Desert area 
include: (1) a northeast-trending negative anomaly associated 
with Cantil Valley, (2) a relatively sharp, deep gravity low in 
southwestern Lanfair Valley (lat 35°05'N, long 1I5°I5'W), (3) 
a north- to northwest-trending positive anomaly near Emerson 
Lake (lat 34°30'N, long 116°25'W). and (4) an east-trending 
positive anomaly near Barstow (lat 34°55'N. long IITOO'W). 

The negative anomaly associated with Cantil Valley has an 
amplitude of at least 30 mgal. Mabey (1960) has estimated that 
Cenozoic deposits beneath the anomaly are more than 3.2 km 
thick in a tectonically depressed block between the Garlock and 
El Paso faults on the north and the Cantil Valley fault on the 
south. 

The relatively sharp gravity low in southwestern Lanfair Val- 
ley, near Hackberry Mountain, is situated over Tertiary intrusive 
and extrusive rocks. The magnitude (more than 20 mgal) and 
subcircular form of this anomaly suggest that it is situated over 
a caldera-like structure from which the local volcanic rocks were 
erupted (Healey, 1973). 

The positive anomaly with an amplitude of about 20 mgal 
near Emerson Lake is located between the Calico fault on the 
northeast and the Taylor Valley fault on the southwest. Because 
part of the area is characterized by numerous exposures of rela- 
tively dense Mesozoic mafic intrusive rocks (Rogers, 1969), the 
anomaly may indicate the presence of a relatively large mass of 
these rocks. The Barstow positive anomaly with an amplitude of 
about 1 5 mgal is also associated, at least in part, with a number 
of exposures of mafic rocks. 

OFFSHORE CENTRAL AND 

NORTHERN CALIFORNIA 

by E.A. Silver' 

Free-air gravity anomalies over the continental margin off 
northern and central California result from the combined effects 
of ridges, basins, canyons, large faults, and the crustal thinning 
at the edge of the continent. South of the Mendocino escarpment 
major structural ridges reflected in the gravity field are Farallon 
Ridge, Santa Cruz high, and Santa Lucia bank. Major basins 
include the Bodega, Outer Santa Cruz. Sur. and Santa Maria 
basins. The Point Arena Basin does not produce a significant 
gravity effect. 

' Botrd of Euth Sciences. Univeraily of Caliroraia, Sant* Cnii, CA <)J064 



1980 



I^4TERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



23 



Of the ridges, Farallon Ridge is the longest and shows the 
largest gravity effect. The maximum free-air anomaly over the 
ridge is 50 mgal, 35 km west of Point Reyes. The anomaly 
dwindles rapidly to the north but continues with reduced ampli- 
tude to the west of Point Arena. South of the 50 mgal maximum 
the anomaly bends southeastward and trends toward Pigeon 
Point. Forty kilometers northwest of Pigeon Point a pronounced 
saddle in the anomaly field may correspond to a buried erosional 
notch or fault. Pioneer submarine canyon heads just seaward of 
this saddle. The basement rocks underlying the Farallon Ridge 
are granitic. Quartz diorite is exposed on the Farallon Islands 
and was dredged from Cordell Bank north of the islands (Han- 
na, 1952). Hoskins and Griffiths (1971) inferred its presence 
just offshore from Pigeon Point, but direct recovery of granitic 
rock there has not been reported. It is not known whether these 
rocks continue as part of the Farallon Ridge as far north as Point 
Arena, but the decreased gravity effect over this northern seg- 
ment implies either an increase in depth to the top of these rocks 
or a decrese in density (and presumably in type) of the underly- 
ing rocks. 

A gravity high of 30 mgal overlies the northwest end of the 
Santa Cruz structural high, but most of this structural high is 
characterized by an anomaly of less than 20 mgal. The anomaly, 
and presumably therefore the ridge, does not extend to the coast. 
Volcanic rocks have been sampled from the southwest side of 
this ridge, and Miocene and younger sediments cover the ridge 
(Hoskins and Griffiths, 1971). However, Uttle is known of the 
geology of this feature. 

Seismic-reflection profiles show several kilometers of late Mi- 
ocene and younger strata on the east side of both the Santa Cruz 
high and Farallon Ridge, but strata are upturned only against 
the latter (Silver and others, 1971). This difference in structure 
suggests recent uplift only of the Farallon Ridge and may partly 
explain the stronger gravity effect of that feature. 

The gravity lows of as much as -70 to -80 mgal along the 
lower continental slope between latitudes 37°40' and 39°20'N and 
between latitudes 34°20' and 36°00'N result from the effects of 
increasing water depth down the continental slope, the rapidly 
decreasing crustal thickness westward beneath the continental 
margin, and possibly a thick mass of sedimentary rocks under 
the continental slope. 

Bodega Basin lies just east of Farallon Ridge and is nearly 
enclosed by the 0-mgal contour. The basin is divided by a struc- 
tural high of low relief off Point Reyes. Two small areas in the 
northern part and one in the southern part have values as low 
as -20 mgal. The basin overlies granitic basement and contains 
2.5 to 3 km of sediment, most of which is late middle Miocene 
and younger (Hoskins and Griffiths, 1971). The eastern margin 
of the basin is bounded in the southern part by the San Andreas 
and Seal Cove faults and in the northern part by the Point Reyes 
fault. The 0-mgal anomaly near the coast trends subparallel to 
these faults and outUnes the ridges and basins between latitudes 
3r and 39*N. 

The granitic rocks underlying the Farallon Ridge and Bodega 
Basin are part of the Salinian block, a long sHver of dominantly 
granitic basement in central California, bounded on the east by 
the San Andreas fault and on the west, at least from Monterey 
south, by the Sur-Nacimiento fault. The latter fault is probably 
offset across Monterey Bay in a right-lateral sense by the San 
Gregorio fault zone. The amount of offset of a variety of geologic 



features across this fault has been estimated at 1 1 5 km (Graham 
and Dickinson, 1978b). The Farallon Ridge is granitic, but gra- 
nitic rocks have not been recovered from the Santa Cruz high; 
hence it is possible that the west margin of the Salinian block 
passes between these ridges and lies along the west side of 
the Farallon Ridge. 



The gravity data apparently conflict with the interpretation of 
large lateral offset on the San Gregorio fault. The gravity high 
that overlies the Farallon Ridge intersects the coast at Pigeon 
Point and appears to continue onshore into the Santa Cruz 
Mountains over the Ben Lomond batholith. This continuity 
across the fault may be fortuitous. Many granitic masses occur 
in the Salinian block, and the San Gregorio fault cuts the block 
at a low angle, so the chance of such a juxtaposition is significant. 
Alternatively, the continuity of the anomaly may indicate very 
little horizontal offset along the fault. A saddle in the gravity 
high and deflection of the contours in a right-lateral sense occur 
where the fault crosses the gravity high, as expected from the 
presence of such a fault, but these effects do not require large 
offset. Ironically, large right-slip offset on the San Gregorio was 
suggestsed initially on the basis of the offshore gravity map and 
the onshore geology, matching the Farallon Ridge at Pigeon 
Point with granitic rocks just north of Point Sur (Silver, 1974). 
The geologic studies of Graham and Dickinson (1978a and b) 
supf>ort large right-slip offset, but the new gravity map present- 
ed here raises doubts about such an interpretation. I beheve the 
continuity of the gravity anomaly is fortuitous and that the fault 
has undergone large Neogene right-sUp. But a more vigorous 
study of regional geologic relations and an attempt to establish 
offset lines is clearly needed. 

Outer Santa Cruz Basin is outlined partly by the 0-mgal con- 
tour. Its gravity effect is less than -10 mgal in the southeast 
where sediment thickness exceeds 1.5 km (Hoskins and Grif- 
fiths, 1971). Sediment thickness in the northern part of Bodega 
Basin, by comparison, is 3 km (Hoskins and Griffiths, 1971), 
and the gravity effect there is greater (more negative) than at 
Outer Santa Cruz Basin. 

Free-air gravity highs with local closures of about 30 mgal are 
associated with both Guide and Pioneer seamounts, but the 
anomalies are displaced to the west of the seamounts by 7 km 
and 4 km, respectively. Magnetic data obtained at the same time 
as the gravity survey (National Oceanic and Atmospheric Ad- 
ministration, 1974a) show magnetic highs of 500 to 600 gammas 
offset to the west of both seamounts by comparable distances 
when compared with detailed bathymetry (National Oceanic 
and Atmospheric Administration, 1974b). These offsets may be 
partially due to navigation errors, but the character and location 
of the gravity and magnetic anomalies suggest that they are not 
simply reflections of the seamounts but caused by dense mag- 
netic plate-like masses that extend southwestward from the sea- 
mounts below the ocean floor. 



Free-air gravity over Monterey Bay and the continental slope 
west of the bay is dominated by the topographic effect of Monte- 
rey Canyon. Free-air anomalies as low as -90 mgal are found 
along the canyon axis and thus obscure possible structural effects 
of deeper origin. The west-trending free-air gravity high west of 
Point Sur also is largely topographic, separating gravity lows 
over Monterey Canyon to the north and the Sur Basin to the 
south. 



24 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



The Sur Basin has a free-air gravity low of -70 mgal. Al- 
though maximum sediment thickness (3 km on the east side of 
the basm) is no greater than in the Bodega Basin, water depth 
below the anomaly minimum is 1 km, compared to less than 100 
m m Bodega basin. The steep gradient along the east side of the 
Sur Basin marks the location and trend of a large fault with a ' 
vertical basement relief of over 3 km. This fault probably con- 
nects with the San Gregorio fault to the north and the Hosgri 
fault to the south (Silver, 1978; Graham and Dickmson, 1978a; 
1978b). 

Santa Maria Basin, in contrast to the Sur, Bodega, and Outer 
Santa Cruz basins, shows an irregular set of local lows and highs, 
the lows reaching -30 to -40 mgal. This pattern reflects the very 
irregular underlying structure. The basin is bounded on the east 
by the Hosgn fault and on the west by the Santa Lucia Bank 
fault, which shows a gravity effect and is nearly on the -lOmgal 
contour approximately between latitudes 34°40' and 
35°10'N. Nearshore gravity data are insufficient to evaluate the 
effect of the Hosgri fault. Four of the five gravity lows mapped 
over the basin lie near the Santa Lucia Bank fault, indicating 
greater sediment thickness adjacent to the fault than in the cen- 
tral part of the basin; the one exception is a centrally disposed 
gravity low at latitude 35''10'N. Seismic-reflection profiles show 
complex basement structure beneath this basin. 

Free-air gravity anomalies differ in trend on either side of the 
Santa Lucia Bank fault. To the west, a broad gravity high trends 
slightly west of north, parallel to the bank and to the fault. To 
the east, anomalies over the Santa Maria Basin trend northeast. 
West of the bank, a northwest-trending gravity high follows the 
top of the Santa Lucia escarpment, reaching -i- 20 mgal over a 
topographic high. The gravity high plunges to the northwest. 
Ohvme basalt was recovered by dredging the continental slope 
at the north end of this gravity high, and Cretaceous sandstone 
was dredged near the top of Santa Lucia Bank, just below the 
topographic high marked by the closed +20 mgal contour. 
Between this outer gravity high and that overlying Santa Lucia 
Bank is a gravity low, also lessening northwestward, that overlies 
a sedimentary basin — as seen in reflection profiles — containing 
nearly 1 km of sediments. 

Northwest of Santa Lucia bank a free-air gravity high reach- 
ing -10 mgal bounds the gravity low over Sur Basin on the west. 
This gravity high overlies a structural ridge that does not appear 
to be separated from the adjacent basin by a fault, as is the case 
to the south. Rounded boulders of quartz monzonite were 
dredged from the southern part of this structural ridge, at about 
latitude 35°30'N. The gravity high over this ridge terminates on 
the north at Sur Canyon, just south of a west-trending anomaly 
off Point Sur. Elongate free-air gravity highs on the lower part 
of the continental slope between latitudes 35°30' and 37°30'N are 
associated with Davidson, Guide, Pioneer and Mulberry sea- 
mounts. 

The gravity field between Point Arena and the Mendocino 
fault is very fiat, and free-air anomalies closely record the topo- 
graphic effect of Viscaino and Noyo submarine canyons. The 
lower slope negative anomaly is not well develojied in this region, 
reficcting the very gentle bathymetric slope from the coastline to 
the deep scafloor and pwssibly a lower than normal rate of 
change of crustal thickness as well. 

The geology and gravity field of the continental margin 
changes abruptly at the Mendocino fault. A west-trending grav- 



ity high reaching -t- 30 mgal over the Mendocino Ridge marks 
both high topography and dense basaltic rocks there. Based on 
gravity modeling, Talwani and others (1959) suggest thicker 
crust south of the fault but lower density mantle to the north. 
This interpretation is consistent with the presence of younger 
oceanic crust ( < 7 my.) north of the fault and older crust 
(25-30 m.y.) to the south (Atwater, 1970). 

The oceanic lithosphere north of the fault is undergoing sub- 
duction beneath the continental margin (Silver, 1969, 1971). 
The slope-base gravity low is a function not only of surface slope 
and increasing depth to mantle eastward but also of thick sedi- 
ments ponded in a buried trench at the base of the slope and 
deformed on the lower part of the slope (Silver, 1971). The 
gravity low at the head of Trinidad Seavalley exceeds -60 mgal 
and coincides with a thick section of upper Cenozoic sediments 
trapped in a basin behind an upper slope structural ridge (Silver, 
1971). The ridge is associated with a long gravity high that 
extends, parallel to the gravity low over the basin, into the mar- 
gin offshore of southern Oregon. The slope-base low just north 
of the Mendocino fault exceeds -80 mgal and may be a combined 
effect of the gravity low over the lower part of the continental 
slope and the north side of the Mendocino escarpment, or it may 
indicate thicker sediments in this comer of the subducting plate. 

COAST RANGES 

by R.H. Chapman' and Andrew Griscom* 

Physiography and Geologic Setting 

The Coast Ranges Province is composed of northwest-trend- 
ing ranges and intervening valleys, reaching maximum eleva- 
tions of about 2700 m near latitude 40°N but rarely exceeding 
2(X)0 m. The maximum average elevation (figure 4) is about 
1 100 m at latitude 40°N. 

Two major faults, the San Andreas fault and the Nacimiento 
fault zone, strike northwest through the province and subdivide 
it into three major areas of distinctive basement rocks. East of 
the San Andreas fault and west of the Nacimiento fault zone, the 
basement is composed of the Franciscan assemblage, a mass of 
melange and imbricated rocks that are predominantly gray- 
wacke, siltstone, and shale, subordinate volcanic rocks and chert, 
and minor amounts of serpentinite and mafic intrusive rocks 
(Bailey and others, 1964; Hamilton, 1978). These rocks range 
in age from late Jurassic to Eocene. The assemblage resulted 
from the accumulation of materials scarped off the oceanic crust 
during eastward subduction beneath California. Between the San 
Andreas fault and the Nacimiento fault zone is the Salinian 
terrane, where basement is composed of granitic and meta- 
morphic rocks. Overlying these various basement materials are 
Cretaceous and Tertiary sedimentary deposits as well as lesser 
amounts of Tertiary and Quaternary volcanic rocks. Cenozoic 
volcanic rocks are especially abundant north of San Francisco as 
far as Clear Lake (lat 39°N), where several young volcancoes are 
shown on the map. The Cenozoic sedimentary basins are gener- 
ally elongated northwest, may be deeper than 30(X) m (Smith, 
1964), and are bounded locally by steep-dipping faults. 

Bouguer Anomalies North of Latitude 39°N 

North of latitude 39°N the Bouguer gravity anomalies over the 
Coast Ranges Province are characterized by a smooth gradient 



California Division of Mines and Geology, Sacramento, CA *)58I6. 
U.S. Geological Survey, Menio Park. CA 9402; 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



25 



sloping from values of + 20 mgal near the shore to a closed low 
of-1 15 mgal centered at Shell Mountain near latitude 40°N. The 
closed low also corresponds in location with a generalized topo- 
graphic high of 1050 m (figure 4) so that the gravity anomaly 
in part can be regarded as the isostatic effect of a root of low- 
density crustal rocks (Griscom, 1973a; Chapman and others, 
1975). The gradient is produced by a combination of three ef- 
fects: ( 1 ) the transition from thin high-density oceanic crust to 
thick low-density continental crust, (2) the transition from high 
-density oceanic mantle to low-density continental mantle, and 
(3) thickening of the total section of the Franciscan assemblage 
to a maximum of perhaps 10 km (Griscom, 1973a, figure 1) in 
the vicnity of the closed low. 

Gravity levels over the Coast Ranges Province increase south- 
ward from the closed minimum of-1 15 mgal at latitude 40°N to 
about mgal near San Francisco. This change reflects the change 
in crustal thickness from about 33 km at the gravity minimum 
(Griscom, 1973a, figure 1) to 22-25 km at San Francisco (Ea- 
ton, 1966) and also perhaps an increase in upper mantle density 
at San Francisco. Stewart (1968) from seismic-refraction data 
placed the base of t-he Franciscan at a depth of 10-16 km in the 
Diablo Range 60 km east of the San Francisco Peninsula, and 
placed the base of the crust at about 30-32 km. These dimensions 
are similar to the crustal model calculated from gravity and 
seismic-refraction data at latitude 40°N (Griscom, 1973a). Simi- 
la (1978) reported apparent upper mantle P-wave velocities of 
7.5-7.7 km/s in the northern Coast Ranges as compared to the 
value of 8.18 km/s obtained by Stewart (1968) for the Diablo 
Range east of San Francisco. The velocity differences, if not an 
azimuthal effect (Peselnick and others, 1977), support the con- 
clusion from gravity data that upper mantle densities are greater 
in the San Francisco area than in the northern Coast Ranges. 



The smoothness of the gravity field over the Coast Ranges 
north of latitude 39°N contrasts strongly with the irregularity of 
the gravity field east of the Coast Range thrust and the inferred 
deep structure of this province. The sedimentary rocks of the 
Franciscan were deposited in a deep oceanic environment upon 
oceanic crust, the mafic rocks of which should show a density 
contrast of perhaps 0.3 g/cm' with the Franciscan sedimentary 
rocks. Thus the smoothness of the gravity field indicates that the 
bottom surface of the Franciscan is smooth on a regional scale, 
probably with no local relief in excess of 1.5 km. This deduced 
smoothness contrasts strongly with the intricate structure of the 
Franciscan rocks themselves and leads to the inference (Gris- 
com, 1973a, 1973b) of a relatively smooth surface of decollement 
at or a short distance above the bottom of the Franciscan in this 
area. This decollement surface is to be expected if the Franciscan 
assemblage is composed of materials scraped off an oceanic plate 
against the inner wall of a trench during subduction. Calcula- 
tions from the steep gravity gradient at Point Delgada (lat 40°N) 
indicate a maximum dip of about 20° northeast for the formerly 
active decollement surface (Griscom, 1973a, figure 1). North of 
the Mendocino fault zone (lat 40° 20'N) the subduction 
beneath the continental margin is still active (Silver, 1971 ), and 
here the dip on the decollement surface is probably less than 10°, 
judging by the gravity gradient in the vicinity of the -50 mgal 
contour. 



The northward motion of the Mendocino triple junction (the 
junction of the Mendocino fault zone, the San Andreas fault, 
and the subduction fault) along the coast of California has 



progressively terminated subduction beneath the Franciscan 
south of the Mendocino fault zone (Atwater, 1970). Cenozoic 
tectonism increases southward within the Franciscan south of 
the fault zone, perhaps due to the temporal and physical adjust- 
ments to cessation of subduction. The increasing complexity 
south of latitude 39°N of the gravity field associated with the 
Coast Ranges Province may be indirectly a result of a northward 
migration of the triple junction and hence a result of the south- 
ward increase of the time span since local subduction ceased. To 
the south, increased normal faulting and volcanism produce lo- 
cal concentrations of low-density material and associated local 
gravity features. 

Two major local gravity features are observed within the 
broader regional pattern north of latitude 39°N. The first is a 
gravity low at the mouth of the Eel River (lat 40°35'N) superim- 
posed on the regional gradient. This low is caused by the low 
density Tertiary sedimentary rocks (Ogle, 1953) of the Eel River 
Basin. The northeast side of the basin is bounded by the Little 
Salmon and Yaeger faults, which show as a steep linear gravity 
gradient. A second gravity feature is the irregularly linear grav- 
ity high on the east side of the Coast Ranges Province in the 
general vicinity of the Coast Range thrust and extending as far 
north as the east side of the closed -115 mgal low, for a total 
length of about 125 km. The high has an ampliltude of 10-25 
mgal. Directly east of the thrust is the ophiolite sequence (Bailey 
and others, 1970) of serpentinized ultramafic and mafic rocks 
that forms the oceanic crust upon which the Great Valley se- 
quence was deposited. These rocks, where now predominantly 
serpentinite, are not expected to display significant gravity ex- 
pression because of the lack of density contrast with the Francis- 
can. Nevertheless, five local closures or near-closures are found 
in the gravity contours along this high, all of which can be 
ascribed to high-density rocks in the ophiolite above the Coast 
Range thrust (Chapman and others, 1975). From south to north 
these local features include: ( 1 ) a -40 mgal closure at latitude 
39°N; (2) a -50 mgal closure 20 km to the north; (3) a neariy 
closed nose in the -45 mgal contour about 10 km northwest of 
the previous feature; (4) a -45 mgal closure 10 km farther 
northwest over a local thrust sheet of Great Valley volcanic 
rocks (Brown, 1964) in an outlier of the Coast Range thrust; and 
(5) a -75 mgal closure at latitude 40°N. Regardless of these local 
features, the crest of the linear gravity anomaly is commonly at 
or west of the Coast Range thrust, which from geologic and 
aeromagnetic data (Griscom, 1973a) is known to dip steeply 
east. Thus the bulk of the high cannot reflect the ophiolite. 
Griscom (1973a) suggests two explanations for the anomaly. 
Directly below the Coast Range thrust, the Franciscan assem- 
blage is metamorphosed to higher density rocks of blueschist 
facies (Blake and others, 1967). The blueschists can cause the 
anomaly with a density contrast of only 0. 1 g/cm'. Alternatively, 
aeromagnetic evidence (Griscom, 1966) indicates that a mass of 
magnetic material, the crest of a gently folded thick magnetic 
sheet of rock, possibly serpentinitized ultramafic rocks, occurs at 
a depth of 1.5 to 3 km below the surface along the axis of the 
gravity high. This concealed mass may also be a source for the 
gravity high. 

Various small negative closures or flexures within the area 
from latitude 39°N to latitude 40°N with amplitudes of about -5 
mgal are associated with intermontane basins containing alluvial 
fill (Chapman and others, 1975). Some of the anomalies are 
larger in areal extent than the valleys, suggesting that not only 
the fill but also structural displacements of the layers in the 
upper crust affect the anomalies. 



26 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



South of Latitude 39°N 

Bouguer anomalies in the Coast Ranges Province decrease 
southeastward from approximately -10 mgal in the area just 
north of San Francisco to about -25 mgal near the latitude of 
Monterey (36°35'N) and to -50 mgal or less east of Point Con- 
ception (lat 34°30'N). The decrease in anomaly magnitude cor- 
responds in a general way to a decrease in the proportion of 
exposed Franciscan and granitic rocks relative to exposures of 
Tertiary and Quaternary sedimentary rocks, at least south of 
Monterey. Also, the average elevation of the Coast Ranges de- 
creases to the southeast (figure 4) . Nevertheless, seismic-refrac- 
tion data indicate a crustal thickness of 22 to 25 km at San 
Francisco and about the same thickness at Camp Roberts (lat 
35°48'N. long 120°44'W), about 260 km to the southeast (Eaton, 
1966, figure 3). From Camp Roberts southward, however, crus- 
tal thickness may increase as suggested by the gravity data be- 
cause seismic data indicate a thickness of about 35 km near Los 
Angeles, south of the Transverse Range Province (Healy, 1963). 

The general level of the Bouguer gravity field decreases inland 
in the Coast Ranges south of latitude 39°N. The smooth regional 
gradient noted north of latitude 39''N is partially obscured in the 
southern area by local anomalies related to the complex geologic 
features in this area. Most of these local anomalies trend north- 
west or north parallel to numerous faults and the regional geo- 
logic structure. 

Much of the coastline from about San Luis Obispo (lat 
35*20'N) to about Fort Bragg (lat 39° 27'N) is marked by linear 
positive anomalies approximately parallel to the coastline. These 
px)sitive anomalies are mostly related to exposures of granitic, 
mafic, and melamorphic rocks in the Salinian terrane west of the 
San Andreas fault and to exposures of Franciscan rocks south 
of Monterey and west of the Nacimiento fault zone. Although 
these positive anomalies primarily reflect the relatively dense 
rocks along the coast, they are accentuated by a combination of 
the regional gravity gradient and the presence on the continental 
shelf of gravity lows related to basins of Tertiary sedimentary 
rocks. Examples of coastal gravity highs a few tens of milligals 
in amphtude include those related to exposed granitic rocks from 
Point Reyes (lat 38°00'N) to Bodega Head (lat 38°20'N) north 
of San Francisco; Montara Mountain (lat 37°32'N) and Ben 
Lomond Mountain (lat 37°05'N) south of San Francisco; the 
granitic and meuimorphic rocks exposed in the San Lucia Range 
south of Monterey Bay (lat 36°20'N), and Franciscan assem- 
blage rocks along the coast from about latitude 36°N to near San 
Luis Obispo. 

Other positive anomalies southwest of the San Andreas fault 
are related to granitic rocks northwest of Paso Robles (lat 
35°40'N. long 120°45'W), and east of Santa Margarita (lat 
35°25'N. long 120°35'W) and Franciscan assemblage rocks near 
Stanley Mountain (lat 35''05'N, long 120°13'W). 

Near Stewart's Point (lat 38°39'N) north of Point Reyes, a 
sharp positive anomaly apparently is associated with outcrops of 
basalt that may represent the floor of Gualala Basin (Silver and 
others, 1971; Chapman and Bishop, 1974). The aeromagnetic 
map of this area (U.S. Geological Survey, 1976) suggests that 
this basalt may extend westward offshore for at least 10 km. 

The coastal gravity highs are separated by negative anomalies 
in a few areas where sedimentary basins cross the coa.stline at, 
for example, the Santa Cruz (La Honda) Basin (lat 37*22'N) 35 
km south of San Francisco (Chapman and Bishop, 1968b), at 
the offshore Salinas basin (lat 36'45'N). at a thick section of 



Tertiary sedimentary rocks south of Estero Bay (lat 35°20'N) 
(Burch and others, 1971). and at the Santa Maria basin (lat 
34°57'N) in the southern part of the Coast Ranges (Rietman and 
Beyer, 1980). 

The northwest-trending positive anomaly associated with Ben 
Lomond Mountain appears to be a shoreward continuation of 
the offshore Farallon Ridge-Pigeon Point anomaly. Northeast of 
the Farallon Ridge-Pigeon Point anomaly trend, negative ano- 
malies also suggest continuity in the offshore area between the 
Bodega and Santa Cruz basins. Similarly, on the southeast, nega- 
tive anomalies associated with the Outer Santa Cruz and the 
Salinas basins are in approximate alignment. According to Gra- 
ham and Dickinson (1978), however, the San Gregorio fault, 
which crosses these anomalies, has an estimated 1 15 km of offset 
in a right lateral sense in the Monterey Bay area. If this figure 
for offset is approximately correct, the apparent alignment of 
gravity anomalies must be fortuitous. This is discussed in more 
detail by Silver (this report). 

The northeast boundary of the Coast Ranges is characterized 
in many places by steep gravity gradients; these gradients proba- 
bly represent the contact between Franciscan rocks or rocks of 
the Great Valley sequence and lower density Tertiary and Qua- 
ternary rocks in the Sacramento and San Joaquin Valleys. The 
linear steep gradients along the northeast side of the Coast 
Ranges are interrupted in a few places such as east of Clear Lake 
(lat 39°00'N, long 122°30'W), Suisun Bay (lat 38°08'N, long 
122°03'W), and south of Panoche Valley (lat 36°30'N, long 
120°47'W), where negative anomalies represent synclines or 
other structures superimjjosed on the margin of the Great Val- 
ley. Strong positive anomalies are associated with some areas of 
Franciscan rocks that are close to the overlying Coast Range 
thrust along the east side of the Coast Ranges, such as in the 
Diablo Range north of Panoche Valley (lat 36°43'N), between 
Cholameand Parkfield (lat 35'"50'N) (Hanna and others, 1972), 
west of Lake Berryessa (lat 38°32'N), and near Vallejo (lat 
38''08'N). These anomalies might reflect metamorphic Francis- 
can rocks, possibly in combination with ultramafic and mafic 
rocks in some places, as postulated for the positive anomalies 
west of the ophiolite sequence north of latitude 39°N. 

Many of the major faults in the Coast Ranges are marked by 
relatively steep gravity gradients, but only in those areas where 
the faults bound rock units that are characterized by distinct 
density differences. For example, the San Andreas fault north of 
San Francisco forms the boundary between granitic rocks of the 
Salinian block on the southwest and rocks of the Franciscan 
assemblage on the northeast (Clement, 1965; Chapman and 
Bishop, 1968b). Because the average densities of granitic and 
Franciscan rocks are similar, there is no density contrast, and no 
apparent gravity anomaly. The lack of a gravity anomaly also 
suggests that deeper layers of contrasting density are not signifi- 
cantly offset in the vertical direction at the fault. Farther south 
along the San Andreas fault (near lat 36°40'N), however, the 
fault forms the boundary between granitic rocks in the Gabilan 
Range on the southwest and lower density Tertiary sedimentary 
rocks on the northeast. In this area, a steep gravity gradient 
marks the fault zone, separating a high over the granitic rocks 
from a low over the Tertiary sedimentary rocks (Pavoni, 1973; 
Bishop and Chapman, 1967). Byerly (1966), however, found 
that a Bouguer anomaly profile in this area corrected for near- 
surface geology shows no evident anomaly associated with the 
fault. This result is in apparent agreement with the lack of an 
anomaly for the San Andreas fault north of San Francisco noted 
above. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



27 



Southwest of the San Andreas fault, strong negative Bouguer 
gravity anomalies of a few tens of milligals amplitude are found 
in association with numerous irregularly spaced valleys and ba- 
sins containing Tertiary and Quaternary sedimentary rocks from 
Point Arena on the north to the Santa Maria basin (lat 34°40'N, 
long 120°05"W) and Cuyama Valley (lat 34°55'N, long 
1 19°37'W) on the south. These include the prominent long Unear 
anomahes over Salinas Valley (centered near lat 36°25'N, long 
12nO'W) and the Carrizo Plain (centered near lat 35°23'N, 
long 120°05'W). 

Bouguer gravity values northeast of the San Andreas fault in 
the Coast Ranges are similarly generally high in areas of Francis- 
can rocks and low in areas of Tertiary and Quaternary sedimen- 
tary rocks. Locally strong lows include those associated with the 
Santa Clara Valley near San Jose (lat 37°23'N, long 12r52"W), 
a linear anomaly extending southeastward from HoUister (lat 
36°50'N, long 121°25'W) along the San Andreas fault, and an 
anomaly trend east of the Hayward fault that includes closures 
at Livermore Valley (lat 3r40'N, long 12r53'W) and San Pablo 
Bay (lat 38°08'N, long 122°20'W). Distinct highs are related to 
the Diablo Range (lat 3r35'N, long 121°40"W)and,in general.to 
the exposed Franciscan rocks north of San Francisco. A strong 
gravity high is located over the San Emigdio Mountains near the 
south end of the San Joaquin Valley (lat 34°53'N, long 
119''12'W) (Hanna and" others, 1975a). This anomaly is primar- 
ily the reflection of metamorphic and granitic rocks that are 
present in this area on both sides of the San Andreas fault. 

Exposures of relatively dense "greenstone" show shiirp local 
gravity highs of 5 to 10 mgal in many areas in Franciscan ter- 
rane. Large masses of ultramafic rocks may be characterized by 
either local highs or lows, depending in part on the relative 
proportion of serpentinite and unaltered ultramafic rocks 
present. Lows are found over ultramafic rocks near Cuesta Pass, 
north of San Luis Obispo (lat 35°23'N, long 120°38'W) (Burch 
and others, 1971), Joaquin Ridge (lat 36°38'N, long 120°35'W) 
(Byerly, 1966; Bishop and Chapman, 1967), and The Cedars 
(lat 38°38'N. long 123°08'W) (Thompson and Robinson, 1975; 
Chapman and Bishop, 1974). Small highs are associated with 
Burro Mountain (lat 35°52'N, long 12ri6'W) (Burch and oth- 
ers, 1971), east of Cape San Martin, and possibly with the Point 
Sal ophiolite (lat 34°54'N, long 120°37'W). 

Noteworthy Bouguer gravity anomalies in the Coast Ranges 
include a 40-mgal high over Mount Diablo (lat 37°55'N, long 
12r57'W).Thegravity interpretation by Wood (1964) support- 
ed a piercement structure hypothesis for the origin of this strong 
anomaly where diabase was forcefully emplaced. However, an 
analysis by Andrew Griscom (unpublished data, 1978) of the 
Mount Diablo magnetic anomaly (Griscom, 1966) indicates 
that an antiformal folded tabular mass of mafic rocks fits the 
data at least as well as does a deep-rooted piercement structure. 

Of particular interest also are the Bouguer gravity lows related 
to ( 1 ) the east side of the north end of Santa Clara Valley near 
San Jose (Evergreen low), (2) Livermore Valley, and (3) the 
Clear Lake area. Analysis of the Evergreen gravity low suggests 
that a graben extends into the lower crust and possibly into the 
upper mantle (Robbins, 1971), as there is no density contrast at 
the surface adequate to explain this anomaly. According to the 
interpretation by Robbins and others (1977), gravity data at 
Livermore Valley suggest not only a great thickness of Creta- 
ceous and Tertiary rocks but also a thinner crust north of the 
valley than to the south in the Diablo Range. The negative 



anomaly with an amplitude of more than 25 mgal south of Clear 
Lake in the vicinity of the Clear Lake volcanic field has been 
interpreted as a possible magma body at a depth of 10 km or less 
(Chapman. 1975, 1978b; Isherwood, 1976). This anomaly is 
associated with Pleistocene and Holocene volcanic rocks, the 
Geysers steam field, numerous hot springs, and a region of high 
heat flow (Urban and others, 1976). 



GREAT VALLEY 
by H.W. Oliver' and Andrev/ Griscom' 

General Geology 

The Great Valley of California is about 700 km long and 100 
km wide, and ranges in elevation from about 10 m in the west- 
central area to about 150 m at the north and south ends (figure 
5 ) . Elevations averaged to a distance of about 40 km are some- 
what greater and reach about 300 m around the periphery of the 
valley, except where it opens westward toward San Francisco 
Bay (figure 4) . The south half of the Great Valley is called the 
San Joaquin Valley and drains to the north, except the south end, 
parts of which have closed drainage; the north half is the Sacra- 
mento Valley and drains southward. 

The Great Valley is a very large asymmetric syncline with 5 
to 10 km of uppermost Jurassic to Quaternary sedimentary 
deposits along the structural axis defined by the configuration of 
older basement rocks (Kilkenny, 1951; Ingersoll, 1978). This 
axis is located near the western edge of the valley about 20 km 
west of the present axis of deposition marked by the Sacramento 
and San Joaquin Rivers. Drilling and seismic data indicate that 
the eastward shift in the axis of deposition has been progressive 
and began with the uplift of the west side at the end of the 
Cretaceous (Safonov, 1962). The most severe period of deforma- 
tion was in the middle Pleistocene, when extensive folding and 
faulting affected the upturned west valley sedimentary strata 
(Hackel, 1966). Some of these faults-for example, the Quater- 
nary faults in the Elk Hills west of Bakersfield and the Midland 
fault zone west of Sacramento-are shown on the base map. 

The major structure along the strike of the valley is a tilting 
of all the pre-Pleistocene beds to the south and an accompanying 
southward thickening of late Cenozoic formations (Safonov, 
1962). This general pattern is interrupted by arching of the 
pre-Cretaceous basement rocks near Stockton and Bakersfield 
(Repenning, I960, figure 2). 

Densities 

The density of the surficial alluvial deposits is known from 
gravity measurements over local tof)ography to average about 
1.9 g/cm'. Density and sonic logs in deep wells indicate that 
densities of older sediments increase approximately linearly with 
depth to about 2.6 g/cm' at a depth of about 5 km (Byerly, 1966; 
R.O. Hovey, personal communication, 1970). 

The densities of the pre-Cretaceous basement rocks beneath 
the valley sediments are not well known. Density measurements 
of 41 basement cores in the vicinity of Madera (Bayoumi, 1961, 
appendix 1) ranged from 2.43 g/cm* for serpentinite to 3.11 
g/cm' for mafic meta-igneous rocks. Petrographic studies of 



'U.S. Geological Survey. Menlo Park. CA 94025. 



28 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



basement cores suggest that the same general range of basement 
rock types and corresponding densities occurs within the Great 
Valley basement as in the Sierra Nevada (May and Hewitt, 1948; 
Thompson and Talwani, 1964). 

Gravity Anomalies 

Bouguer gravity anomalies range from about + 25 mgal over 
Sutter Buttes to -1 10 mgal west of Red Bluff. The closure of the 
-1 10 mgal contour in this area is the northernmost of a series of 
gravity lows that extends south along the west side of the Great 
Valley all the way to the White Wolf fault south of Bakersfield. 
This feature is referred to here as the west side gravity low. 

A series of gravity highs extends both north and south from 
the closure over Sutter Buttes at least as far north as Red Bluff 
and as far south as Fresno. Several gravity highs farther north 
form an elbow that strikes into crystalline rocks northwest of 
Redding, and are thought to be caused by older rocks of the 
Klamath Mountains Province. South of Fresno the gravity ridge 
is interrupted by a broad negative closure of the -45 mgal con- 
tour, and continuity of the anomaly is uncertain. One possibility 
is that it diminishes greatly in amplitude and continues to the 
west of the negative closure near Raisin City, being manifest as 
south-pointing flexures in the -45 to -60 mgal gravity contours 
south of Stratford over the Tulare Lake Bed. Farther south, an 
extension of the gravity anomaly connects it with the gravity 
high centered about 1 5 km northwest of Balcersfield, although 
this connection is obscure west of Delano. The 700-km-long axis 
of the gravity high is thus broadly arcuate, being slightly convex 
to the west-southwest. Ivanhoe (1957, figure 2) suggested an- 
other possible extension of the major gravity high south of 
Fresno by bending the axis to the east of the -45 mgal negative 
closure, passing through the saddle near Easton, and connecting 
with the highs near Delano and Bakersfield. A third possibility 
is that the series of highs does not extend south of Fresno. 
Whatever the case, the distinctive anomaly north of Fresno will 
be referred to as the Great Valley gravity high in this report. 

Another nearly linear gravity high occurs along the east side 
of the San Joaquin Valley between Clovis and Porterville. The 
amplitude of the anomaly near Dinuba, 50 km southeast of 
Fresno, is about 25 mgal. The gravity high was referred to as the 
Dinuba gravity lineament by OUver and Robbins (1980). 

Other Bouguer gravity highs include the East Valley gravity 
anomaly (Cady, 1975, figure 4) about 30 km southeast of Sacra- 
mento, the anomaly near Hanford (lat 36°23'N, long 1 I9°37'W), 
and the circular high at Sutter Buttes. Strong gravity lows occur 
at Rocklin (30 km northeast of Sacramento) and north of Ma- 
dera. The double low near Madera was termed the Madera 
doublet by Ahmed (1965). 

The West Side Gravity Low 

The connected Bouguer gravity lows along the west side of the 
Great Valley occur over the thickest part of the section of Creta- 
ceous and Cenozoic sediments, which here ranges in thickness 
from 6 to 1 1 km. Although the minimum Bouguer anomalies are 
similar in both the north and south parts of the Valley — closure 
of -1 10 mgal at Red Bluff, and a southern closure of -100 mgal 
south of Bakersfield — the relative magnitude of the anomalies as 
compared with values in the adjacent Coast Ranges varies widely 
(15-20 mgal at Red Bluff, 50-60 mgal south of Bakersfield). 
The anomaly south of Bakersfield delineates the southern basin 



which is filled with an estimated 10 km of Cenozoic sediments 
(Repenning, 1960, figure 2). The residual anomaly is significant- 
ly larger in this area than near Red Bluff in the Sacramento 
Valley, where great thicknesses of sediments are also known to 
occur, because sediments at the south end of the San Joaquin 
Valley are mostly unconsolidated late Tertiary deposits that have 
a larger density contrast with the Franciscan rocks in the Coast 
Ranges than the Sacramento Valley sediments, which arc mostly 
more-indurated Cretaceous deposits. The deepest well in the 
Great Valley is located in the southern basin 30 km south of 
Bakersfield and bottoms in Miocene sedimentary rocks at a depth 
of 6.9 km (Munger Oilogram, 1977). 

The closure of the gravity low over the southern basin nearly 
pinches out near Fellows where the Bakersfield arch has brought 
basement rocks within about 3 km of the surface (Repenning, 
1960). North of McKittrick, the valley sediments thicken to 
about 7 km within the closure of the -85 mgal contour and 
include low-density diatomaceous sediments beneath local clo- 
sures west of Lost Hills (Barton, 1948). 

In general, the deeper parts of the Great Valley fill coincide 
with negative closures 70 km west of Fresno, 50 km west of 
Merced, and at Rio Vista, the last anomaly overlying 1 1 km of 
sediments (Safonov, 1962, figure 5). The axis of the west side 
gravity low does not directly overlie the synclinal basement axis 
but coincides with the average axis as integrated over the multi- 
tude of horizons and associated density contrasts between the 
numerous Cretaceous and Cenozoic formations. The late Ceno- 
zoic formations have the most easterly axis, the lowest densities, 
and the largest influence on the integrated effect (Byerly, 1966). 



The Great Valley and Dinuba Gravity Highs 

The source of the Great Valley gravity high has been the 
subject of speculation since Woollard (1943, plate 3) first trav- 
ersed it about 25 km north of Bakersfield as part of his transcon- 
tinental gravity and magnetic profile of North America. With 
some insight from other areas, Woollard proposed that the 
source of the anomaly was a buried gabbro body, although he 
had no idea of the dimensions from the single profile. About 10 
years later, Ivanhoe (1957) released a small-scale gravity map 
of the Great Valley with a contour interval of 20 mgal based on 
Standard Oil of California data that showed the great extent of 
the gravity high. Ivanhoe did not have magnetic coverage of the 
Valley at that time and interpreted the gravity feature as an 
"isostatic hinge line," that is, a relative maximum separating the 
effects of a great thickness of low-density sediments on the west 
side of the valley and a low-density mountain root beneath the 
Sierra Nevada (see next section). Although Ivanhoe's reasoning 
had some validity, the gravity high was later found to have a 
substantial magnetic anomaly associated with it at least as far 
south as Fresno (Grantz and Zietz, 1960; Meuschke and others, 
1966; Zietz and Kirby, 1968; Cady, 1975). South of Fresno, the 
only non-proprietary magnetic data across the valley consist of 
a few high-level aeromagnetic traverses, which show that the 
magnetic anomaly continues down the center of the valley but 
with significantly diminished amplitude and increased breadth. 
Depth estimates of the magnetic high indicate that the dense 
magnetic mass causing both the gravity and magnetic anomalies 
crops out on the buried basement surface as far south as Fresno 
and perhaps plunges below the ba.semeni surface in the southern 
San Joaquin Valley (Griscom, 1966). The breadth of the gravity 
anomaly near Tulare Lake Bed is about 18 km and is suitable for 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



29 



a source at the top of the basement, which is buried about 4 Vj 
km in this area (Smith, 1964). 

As the Great Valley gravity and magnetic highs terminate 
short of basement outcrops at both the north and south ends of 
the Great Valley, it is of interest to look for similar anomalies 
that extend into basement rocks. The Dinuba gravity lineament 
along the east side of the San Joaquin Valley, noted above, is 
similar in both amplitude and direction, and would look more 
like the Great Valley gravity high were it similarly buried by 2 
to 4 km of sediments (Oliver and Hanna, 1970). The Dinuba 
feature is associated with mafic and ultramafic rocks that crop 
out at Smith Mountain near Dinuba 5 km north of Dinuba at 
latitude 36°35'N, longitude 1 19°22'W) and in the Sierra Nevada 
south of Porterville (Oliver and Robbins, 1980). These rocks 
have been dated and determined to be remnants of late Paleozoic 
(30 m.y.) oceanic crust that have been sutured to older continen- 
tal rocks of the Sierra Nevada (Saleeby, 1975, 1977). The dens- 
est, most magnetic rocks along the suture are olivine gabbro of 
Early Cretaceous age, which was regarded by Saleeby (1975, p. 
vii) to have been "preferentially emplaced into the structurally 
weakened zones provided by the disrupted ophiolite belt." 

The source of the Great Valley gravity and magnetic anomal- 
ies is also generally considered to be a tectonically emplaced 
fragment of oceanic crust (Griscom, 1973; Cady, 1975; Jones 
and others, 1976). In one computer model of the anomalies near 
Sacramento, the average density of the fragment is 2.98 g/cm' 
and the average magnetization is 3.8x10^ emu/cm', properties 
that are reasonable for a gabbroic lower crustal layer (Cady, 
1975, figure 7). The form of the anomalous body is like a gable 
with a more gently dipping western slope, and the body extends 
to a computed depth of about 10 km below the basement surface. 
The age of the interpreted oceanic crustal fragment is unknown, 
but it is presumably the same as or between the ages of the Coast 
Range ophiolite (151-160 my.) and the ophiolites in the Sierra 
foothills (250 to 300 m.y.) (Irwin, 1978, figure 2). 

Semi-Local Anomalies 

Both the elongate gravity high southeast of Sacramento and 
the gravity high at Hanford have associated magnetic highs of 
several thousand gammas, and they are also thought to reflect 
buried gabbroic rocks cropping out at the buried basement sur- 
face (Griscom, 1966; Cady, 1975; R.M. Hovey, personal com- 
munication, 1970). The small gravity high located 5 km 
southeast of Dinuba has been explored by magnetic, seismic, and 
drilling methods and found to be associated with gabbro as well 
as related basement topography (Bom, 1956). 

The highest gravity value in the Great Valley, more than -)-25 
mgal, is within a circular gravity high centered over the Sutter 
Buttes, an eroded Pliocene volcano (Garrison, 1962) in the 
Sacramento Valley. This circular high is also centered on the axis 
of the Great Valley gravity high, perhaps by coincidence, and its 
approximate residual amplitude is -1-25 mgal as determined 
from a north-south profile taken along the axis of the linear 
high. The circular anomaly is the result of two effects: (1) the 
relatively higher density of the intrusions of porphyritic andesite 
and rhyolite and (2) the updoming of the surrounding older and 
more dense sedimentary rocks from which large volumes of gas 
have been obtained. 

The gravity lows on the east side of the valley at Rocklin (lat 
39°47'N, long 121°10'W) and near Madera (lat 3r4'N, long 



12O°02'W) indicate low-density granitic rocks within the base- 
ment (Cady, 1975; Ahmed, 1965; Robbins and others, 1977). 
The Madera doublet represents a 40-km westward salient of the 
Sierra Nevada batholith, most of the salient being buried under 
valley fill. The northernmost of the two lows of the gravity 
doublet has been drilled and the basement core found to be 
garnet-bearing leucocratic trondhjemite (sodic granite) with an 
average density of 2.67 g/cm' (F.C. Dodge, written communica- 
tion, 1972). 

Relation to Faults 

Historic movement has occurred along the White Wolf fault 
at the south end of the Great Valley, the Kern Front fault north 
of Bakersfield, the Cleveland Hill fault near Oroville (lat 
39°27'N, long 12r25'W), and a fault on the west side of the 
valley near Antioch (lat 38°rN, long 12r48'W). The signifi- 
cance of the decrease in Bouguer gravity anomalies to the north- 
west across the White Wolf fault near Bear Mountain is 
discussed in the next section. In the Great Valley, the White 
Wolf fault cuts across the southern basin at approximately the 
-85 mgal contour and causes sharp bends in the -90 to -95 mgal 
contours near Mettler. The correlation with these contours ter- 
minates short of the intersection with the Pleito fault to the west, 
suggesting that the two faults are not continuous. The Pleito 
fault cuts across gravity contours farther west but is reflected by 
the sharp gravity gradient at its east end near Grapevine. 

The Kem Front fault is an active normal fault, the west side 
moving down relative to the east side (Manning, 1973). The 
fault plane dips about 70°, and the blocks are creeping relative 
to each other at the average rate of 1.1 cm/year, according to 
releveling data. The stratigraphic throw of upper Miocene beds 
is about 60 m. The eastward decrease in gravity in this area is 
generally considered to parallel an eastward decrease in the den- 
sity of basement rocks (Hanna and others, 1975a), so it is not 
surprising that the southern segment of the fault having historic 
movement cuts across the gravity contours. The northern, short- 
er segment, just east of Famoso, is parallel to and near the 
maximum gravity gradient, suggesting that movement there may 
be related to basement lithologic contrasts. Farther north along 
the east side of the valley, mapping of the concealed faults of 
pre-Quatemary age between Porterville and Clovis ( 1 3 km 
northwest of Fresno) is based largely on vertical offsets in the 
water table. The fault at Clovis seems to be related to the north 
end of the Dinuba gravity lineament, and detailed gravity studies 
of that section of the fault are in progress (Braun, Skaggs, 
Kevorkian and Simons, Inc., Fresno, CA, written communica- 
tion, 1978). 

The historic fault, marked "1975," about 1 5 km southeast of 
Oroville is known as the Cleveland Hill fault and is the surface 
rupture that occurred at the time of the 5.7-magnitude earth- 
quake of August 1, 1975 (Hart and Rapp, 1975; Clark and 
others, 1976). The fault is a normal fault dipping 60° to the west, 
and the west side moved down 0.36 m relative to the east side 
at the time of the earthquake (Savage and others, 1977). Ac- 
cording to aftershock data, the extent of rupture along the strike 
of the fault was 7.5 km, and no movement occurred along the 
fault just west of Lake Oroville to the north or along the "shear 
zone" to the south (Lahr and others, 1976). There is no obvious 
relation between the 1975 rupture and regional gravity contours, 
which are associated primarily with known lithologic contrasts 
within the basement rocks (see next section). The 1975 surface 
rupture is on a sharp bend in the -40 to -65 mgal Bouguer 



30 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



anomaly contours and at the southern edge of an east-west 
gradient that extends to the northern edge of Lake Oroville. The 
rupture is also on the east limb of a gravity high that trends 
N30°W through Oroville and that appears to be an extension of 
the shear zone. The regional anomalies to the east are clearly 
associated with the Bald Rock batholith and the Smartville ophi- 
olite (see next section), but the rupture area and the area farther 
west are covered with Holocene alluvium, and the sources of the 
anomalies in that area are presently unknown. 



The concealed northeast-trending fault through Red Bluff is 
of some geophysical mterest because its existence was interpreted 
from magnetic data (Griscom, 1473, figure 2) independently of 
its later inclusion on this fault map base by Jennings (1975). 
According to C.W. Jennings (oral communication, 1978), the 
fault shown on the base map is known as the Red Bluff fault 
within private industry and has been mapped by seismic meth- 
ods. According to the interpretation of seismic data, the block 
to the northwest of the fault has moved up relative to the south- 
em block, but the direction of strike-slip movement, if any, is 
unknown. The Great Valley gravity high, discussed above, to- 
gether with the associated magnetic anomaly, appears to be cut 
off by the Red Bluff fault; so strike-slip offset may have oc- 
curred. The gravity ridge that strikes northwest between Byrnes 
Creek and Redding is probably not an extension of the Great 
Valley high because it extends into the Trinity ultramafic sheet 
north of Redding, and this sheet is much older than the likely 
age of the source of the Great Valley gravity high (Griscom, 
1973). Zircon ages of the Trinity sheet are in the range 440-480 
m.y. whereas the plausible age of the source of the main Great 
Valley anomaly is within the range 160 to 300 m.y. (Irwin, 1978, 
figure 2). 



The major concealed Midland fault about 40 km southwest of 
Sacramento has been studied by subsurface methods in connec- 
tion with exploration of the Rio Vista gas field. The gas is pri- 
manly west of the fault within the thick section of Cenozoic 
sediments marked by the closure of the -55 mgal gravity con- 
tour. The vertical offset along the Midland fault zone is down to 
the west and ranges from zero for Pliocene strata to '/2 km for 
Eocene strata and about 1 km for pre-Cretaceous crystalline 
rocks (Safonov, 1962, figure 5). An average density contrast 
across the fault of 0.1 g/cm' with an average offset of Vi km 
would produce a gravity step of 6 mgal. This gravity effect is on 
the order of the observed westward decrease in gravity across the 
fault near Rio Vista. The local wiggle in the -45 mgal contour 
west of the Midland fault near Brentwood reflects the anticlinal 
structure of the gas field. 



Perhaps the second most important fault in the Great Valley 
after the White Wolf fault is the Stockton fault, which cuts 
across the Great Valley between Tracy and Stockton to a point 
near Peters. The Stockton fault consists of a zone of three paral- 
lel reverse faults with a total throw of about I km upward on the 
south side (Hoffman, 1972); these three faults form the Stockton 
arch, which separates the San Joaquin structural basin from the 
Sacramento ba.sin to the north (Safonov, 1968). The Stockton 
arch is reflected in the gravity contours as an interruption in the 
west side low at Tracy and as an offshoot of the Great Valley 
gravity high to the southwest of Stockton that is not associated 
with any known magnetic feature (Robbins and others, 1977, 
figure 2). 



SIERRA NEVADA 

by H.W. Oliver' 

Physiography, General Geology, and Densities 

The Sierra Nevada is a competent fault block that has been 
tilted up to the east during late Cenozoic time (Bateman and 
Wahrhaftig, 1966). The mountains culminate in a nearly con- 
tinuous crest along the east side of the range. On the base map 
the range is marked by long and short green dashes because it 
coincides with county boundaries over most of its length. The 
Sierra crest includes most of the highest peaks, such as Mount 
Whitney (4756 m, 14,496 ft) just west of Lone Pine and North 
Palisade Peak (4673 m, 14,242 ft) about 30 km southwest of 
Bishop. 

The fault block is made up chiefly of granitic rocks of the 
Sierra Nevada batholith, and they have an average density of 
2.68 g/cm', very close to the Bouguer reduction density of 2.67 
g/cm'. The wallrocks on both sides of the batholith and roof 
pendants and septa within it consist of Paleozoic and Mesozoic 
metamorphic rocks having on the average slightly higher densi- 
ties than the batholith (Oliver, 1977). The densest rocks are 
olivine-homblende gabbros which range in density up to 3.2 
g/cm'; they are associated with, but not part of, an ophiolite belt 
in the western foothills (Saleeby, 1978). The least dense rocks 
having significant volume are pre-Cretaceous shale and slate, 
which crop out in the western Sierra foothills and average about 
2.5 g/cm' (Oliver and Robbins, 1980, table 3). Proglacial depos- 
its with densities of about 2.0 g/cm' occur in some of the west- 
draining valleys. In Yosemite Valley these deposits are as thick 
as 600 m according to the interpretation of seismic data (Guten- 
berg and others, 1956) and recent drilling by the National Park 
Service (G. Witucke, written communication, 1975). 

The major faults within the Sierra Nevada as shown on the 
base map are the pre-Quatemary Kern Canyon, Melones, and 
Bear Mountain faults; but the latter two may not be faults in the 
classic sense but rather tectonic zones of polymict melange and 
presumably old sutures (Hamilton, 1978). Quaternary and his- 
toric movements have taken place along the Sierra Nevada fault 
zone, which bounds the province on the east, and along the 
White Wolf and Kern Front faults near Bakersfield. 

The Gravity Field 

Bouguer gravity anomalies decrease to the east across the 
western Sierra Nevada foothills from a high value of about -50 
mgal at the east edge of the Great Valley to a regional gravity 
low whose axis is generally located just west of the Sierra crest. 
Bouguer anomalies along the gravity low vary, being about -130 
mgal east of Bakersfield, decreasing to a minimum of about -240 
mgal west of Mammoth (somewhat north of the highest topog- 
raphy), and rising gradually to about -190 mgal near Lake 
Tahoe. The gravity low passes to the east of the Sierra crest and 
back west again near Bishop and is modified by the negative 
effect of sediments in Owens Valley, Long Valley, and Mono 
basin (see section on the Great Basin). The axis of the gravity 
low is shown in Figure 4 relative to the generalized topography. 

Superimposed on this east-dipping regional gradient are a 
scries of elongate gravity highs over the western foothills with 

' U.S. Geological Survey, Menio Park. CA 9402J. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



31 



amplitudes of 10 to 30 mgal. Two major lows in the foothills 
reach Bouguer anomaly values of -95 mgal about 35 km 
northeast of Sacramento and -135 mgal about 40 km east of 
Chico. 

Isostasy 

The main source of the gravity low over the eastern Sierra 
Nevada is the Sierra Nevada root, which supports the excess 
mass of the mountain range in approximate isostatic balance 
(Byerly, 1938). According to interpretation of seismic data 
(Bateman and Eaton, 1967), the root consists of low-velocity 
low-density crustal material and thickens at the expense of high- 
er velocity mantle rocks from a normal thickness of about 20 to 
25 km along the western edge of the Sierra Nevada to about 55 
km under the Sierra crest. Farther east, the crust thins gradually 
to about 30 km beneath the Great Basin. 

There is some disagreement regarding the form and thickness 
of the seismically determined root (Carder and others, 1970; 
Carder, 1973), but the analysis of the gravity data shown on the 
map taken along Eaton's seismic profile through Bishop tends to 
confirm the major features of Battman and Eaton's crustal mod- 
el (Ohver, 1977, figure 4). More recent gravity analysis along an 
east-west profile through Mount Whitney (Oliver and Robbins, 
1980) suggests that there may be two separate mountain roots 
under the Sierra Nevada: one beneath the Sierra crest and the 
other beneath the Great Western Divide marked on the base map 
just west of the Kern Canyon faults. However, the computations 
show that the gravity effects of the two roots coalesce into a 
single gravity feature that is difficult to distinguish from the 
effect of a single root located midway between them. 

The elevations averaged to a radius of 41 km (E41, figure 4) 
correlate with Bouguer anomalies (BA) over normal crystalUne 
rocks having densities near 2.67 g/craK The approximate empiri- 
cal relation is 

BA = a -H b E^i 

where both a and b vary somewhat, but are about -10 mgal and 
-80 mgalAm respectively for the Mount Whitney region (Oliver 
and Robbins, 1980). Thus the 1500-m contour (figure 4) ap- 
proximately correlates with the -130 mgal contour (gravity 
map). 

The reason for the correlation is that the form of the average 
elevation contours approximates the gravity effect of the Sierra 
Nevada root (see Introduction). I have tested this hypothesis 
against calculations of the gravity effect of both the seismically 
determined root and 12 different hypothetical models that as- 
sume perfect isostasy. The Airy-Heiskanen isostatic model with 
T = 20 km (Heiskanen, 1938) produces the best fit to observed 
Bouguer anomalies, but computations of this effect are presently 
Umited to three profiles across the Sierra Nevada (Oliver, 1973, 
appendix 3). 

Interpretation of Local Anomalies 

Although the axis of the regional gravity low closely follows 
the axis of maximum average elevation (figure 4), there are some 
local perturbations associated with rock masses of unusually low 
density. One such perturbation in the vicinity of Mount Whitney 
occurs where the axis of the gravity low is displaced to the east 
of the maximum average elevation by the negative effect of the 



Whitney pluton. This pluton is rich in potassium, has an unusu- 
ally low average density of about 2.64 g/cm', and extends for a 
distance of 60 km along the Sierra crest (Moore and du Bray, 
1978). After removal of regional gravity, the residual gravity 
anomaly is about -11 mgal (Oliver and Robbins, 1980). 

A similar perturbation occurs west of Mount Dana, the sec- 
ond highest peak in Yosemite National Park (figure 4) . Here the 
regional gravity low is displaced to the west of the maximum 
average elevation by the negative effect of the Cathedral Peak 
Granodiorite (Batemen and Chappell, in press; Oliver, 1977, 
figure 5). 

The gravity lows northeast of Sacramento and east of Chico 
also are the result of low-density granitic plutons. These foothill 
plutons are surrounded by high-density metamorphic rocks in- 
stead of average-density granitic rocks, and the gravity effect is 
therefore more pronounced. The anomaly northeast of Sacra- 
mento lies directly over the Rocklin pluton (Strand and Koenig, 
1965; Swanson; 1978), for which density measurements of se- 
lected samples are as low as 2.55 g/cm' (F.C. Dodge, written 
communication, 1977). The extension of the circular gravity low 
into the Great Valley suggests that the quartz diorite extends 
westward beneath the valley sediments. On the basis of magnetic 
data, Cady (1975, p. 16) believed that the granitic rocks crop out 
on the buried basement surface as far west eis the -70 mgal 
contour and that its contact with metamorphic rock dips west. 

The strong gravity low east of Chico is nearly coincident with 
the Bald Rock batholith studied by Compton (1955). The clo- 
sure is over 30 mgal, indicating a minimum thickness of 8 km 
for the bathohth if there is a density contrast of -0. 1 g/cm' with 
the surrounding metamorphic rocks. Compton (1955, p. 44) 
wondered whether or not the Bald Rock batholith and several 
adjacent plutons are "merely large cupolas of an extensive elon- 
gate pluton that underlies the northwest Sierra." The gravity 
map indicates that this is not the case, but that the main thick- 
ness of granitic rocks lies directly under the Bald Rock batholith. 

The positives anomalies along the west edge of the Sierra 
Nevada occur primarily over mafic and ultramafic rocks, the 
greenstone belt of Thompson and Talwani (1964), now recog- 
nized as ophiolitic sequences of late Paleozoic through middle 
Mesozoic oceanic crustal rocks (Cady, 1975; Saleeby, 1977, 
1978). The large gravity high about 40 km east of Marysville, 
culminating in a closed -15 mgal contour, lies directly over the 
Smartville ophiolite complex of gabbro, sheeted diabase, and 
pillowed metabasalt with some pyroclastic andesite (Bond and 
others, 1977). The gravity high 40 km east of Sacramento denot- 
ed by a " + " within the closed -25 mgal contour occurs over the 
Pine Hill Intrusive Complex of Springer (1974), which is com- 
posed largely of olivine gabbro and clinopyroxenite. Fifty sam- 
ples of the gabbro have an average density of about 3.1 g/cm', 
a value which contrasts sufficiently with the surrounding meta- 
volcanic rocks (2.7 to 2.9 g/cm' density) to account for the 
anomaly (Andrew Griscom, personal communication, 1978). 
Smaller positive anomalies, 10 to 20 mgal in amplitude, occur 
over probable ophiolites near Sonora (lat 3T58'N, long 
120'^rW), Coulterville (lat 3r26'N, long 120°15'W), Dinuba 
(lat 36°33'N, long 119°24'W), and Porterville (lat 36°4'N, long 
119°2rW). The 25-mgal anomaly near Dinuba has been mod- 
eled by Saleeby ( 1975, figure Al-3), who attributed it to a 9-km 
thickness of gabbro with an average density of 3.1 g/cm'. 



32 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



Residual gravity lows occur over low-density sediments along 
the shore of Lake Tahoe and in Sierra Valley (55 km N20°W of 
the north end of Lake Tahoe). The Lake Tahoe anomaly has a 
local amplitude of at least -15 mgal and is probably larger over 
the lake itself where no gravity data have been obtained. The 
Bouguer anomalies have been corrected for the effect of water 
in the lake, which is about 450 m deep; so the gravity low there 
is caused by sediments below the lake bottom that are known 
from seismic-reflection (air gun) data to extend to at least 400 
m below the bottom or at least 850 m below lake level (Hyne 
and others, 1972). The minimum amplitude of the gravity anom- 
aly (15 mgal) suggests that the sediments may be as much as 800 
m thick (assuming Ag = 0.5 g/cm'). The better-determined 
Bouguer gravity field over Sierra Valley has a local depression 
of about -15 mgal relative to an ambient level of about -165 
mgal. This gravity low has been interpreted by Jackson and 
others (1961) to represent a thickness of 750 to 900 m of Ceno- 
zoic deposits. 

A residual gravity low of 9 mgal occurs over the section of 
low-density sediments in Yosemite Valley and causes the wig- 
gles pointed downstream in the -160 mgal to -195 mgal Bouguer 
anomaly contours. Similar wiggles occur in the -195 mgal to 
-210 contours where they cross the South and Middle Forks of 
the Kings River. Both of these Kings River valleys have been 
glaciated, and these data suggest that they contain comparable 
thicknesses of stream and glacial deposits, perhaps as much as 
600 m (Oliver and Robbins, 1980). 

Relation of Gravity to Major Faults 

The Sierra Nevada fault zone along the east margin of the 
southern Sierra is characterized by a steep gravity gradient that 
produces an eastward decrease in Bouguer anomalies of 10 to 20 
mgal along the range front. In the vicinity of Lone Pine, the fault 
zone is split into two major segments, and the gravity decrease 
is much greater across the Owens Valley segment, along which 
histonc movement has occurred. By contrast, there is almost no 
change in Bouguer anomalies across the Independence segment 
of the Sierra Nevada fault zone west of Lone Pine (see Pakiser 
and others, 1964, for a detailed interpretation of this area). 

The Sierra Nevada fault zone north of Bishop consists of north- 
striking discontinuous segments arranged en echelon with an 
overall strike of about N30°W. These segments are not reflected 
significantly in the Bouguer anomalies, but some of the offsets 
are reflected, such as along the south side of Long Valley. Here, 
the maximum gravity gradient coincides approximately with the 
-245 mgal contour and a concealed fault (Pakiser and others, 
1964; Kane and others, 1976). 

The Kern Canyon fault strikes north and nearly bisects the 
southern part of the mountain range. It is a right lateral fault, 
subparallel to the Sierra Nevada fault, but it does not appear to 
offset an overlying 3.5 million-year-old basalt flow near latitude 
36*1 5'N. The amount of nght-lateral offset of the contacts of 
Late Cretaceous granitic bodies increases to the south from 
about 2 km at the north end of the inferred fault (shown with 
dashes on the map) to about 13 km at latitude 36°00'N (Moore 
and du Bray, 1978). The gravity field seems to be related to the 
fault in two ways: ( 1 ) a gravity low of 5 to 10 mgal is manifest 
as broad south-pointed wiggles in the -165 mgal to -185 mgal 
contours and (2) the -190 mgal contour appears to be offset in 
a right-lateral sense by about 8 km The gravity low is not 
related to isostasy (Oliver and Robbins, 1980). 



The White Wolf fault is not continuous with the Kern Canyon 
fault, and indeed the historic movement in 1952 was primarily 
left-lateral. However, in the vicinity of Bear Mountain the 
southeastern block has been thrust over the San Joaquin Valley 
sediments, and the fault exhibits a small component of nght- 
lateral movement (Buwalda and St. Amand, 1955). The total 
amount of Cenozoic vertical offset at Bear Mountain is about 3 
km, and this offset is reflected as a gravity step of 10 to 15 mgal 
(Hanna and others, 1975). The gravity step widens to the south- 
west as the basement scarp plunges to sediment depths of as 
much as 10 km due south of Bakersfield. Northeast of Bear 
Mountain, plutonic rocks have been thrust over other plutonic 
rocks, and the gravity step disappears because of the absence of 
a density contrast across the fault. 

The Kern Gorge fault northeast of Bakersfield is related to a 
maximum gravity step of 7 mgal, up to the east, where the fault 
has brought Sierra granitic rocks into juxtaposition with Ceno- 
zoic sediments, but there is little or no gravity relief associated 
with faults within the southwestern Sierra foothills. 

Serpenlinite occurs from Mariposa (about 37 "/N) nearly to 
Lake Almanor (about 40°N) within melanges associated with 
both the Melones and Bear Mountain fault zones (Jennings and 
others, 1977), and generally underlies intermittent gravity lows 
along the zones because the serpentinite is significantly lower in 
density (~2.5 g/cm') than the surrounding granitic and meta- 
morphic rocks (2.7-2.8 g/cm'). This effect produces local chev- 
roning of the -65 and -70 mgal contours over the Melones fault 
southwest of Sonora (see Robbins and others, 1977). A gravity 
gradient is associated with the Melones fault between Baxter and 
Downieville (Oliver and others, 1974; Oliver and Robbins, 
1974b) and is accounted for primarily by the contrast between 
dense Mesozoic metavolcanic rocks on the west of the fault 
(average density about 2.8 g/cm') and lower density Paleozoic 
metasedimentary rocks to the east (average density about 2.6 
g/cm') (Jennings and others, 1977; Oliver, 1977). The eastward 
displacement of the maximum gravity gradient over the north- 
ward extension of the Bear Mountain fault in the vicinity of New 
Bullards Bar Reservoir (about 39'/i°N) indicates that the fault 
plane dips to the east. 

GREAT BASIN 
by H.W. Oliver' 

Physiography and General Geology 

The Great Basin sector in California is bounded on the south 
by the Garlock fault, on the west by the Sierra Nevada and 
Modoc Plateau, on the north by the Oregon border and on the 
east by the Nevada border (figure 5). The Great Basin is that 
part of the Basin and Range province having a closed drainage 
system, and it extends across Nevada into western Utah (Fenne- 
man, 1946). 

The province is characterized by north-trending mountain 
ranges separated by elongate basins as long as several hundred 
kilometers. The most prominent ranges are the Panamint Moun- 
tains (elevation 3 3 km; lat 36 ' ..°N), the White Mountains (ele- 
vation 4.4 km; lat 37'.,°N), and the Warner Mountains 
(elevation 3.1 km; lat 41 'CN). The series of basins to the east 
of the Sierra Nevada are, from south to north: Owens Valley 



U.S. Geologic*! Survey, Menlo Park. CA '*A02^ 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



33 



(elevation 1.2 km). Long Valley (2.1 km), Mono Basin (2.0 
km), Bridgeport Valley (2.1 km), and Honey Lake Valley (1.2 
km). Panamint Valley (0.5 km) and Death Valley (-0.1 km) lie 
respectively west and east of the Panamint Range. Surprise Val- 
ley (1.5 km) is located east of the Warner Mountains. The 
greatest local relief in the California sector is between the Pana- 
mint Range and Death Valley (3.4 km). Saline Valley (0.3 km), 
located 40 km northeast of Owens Lake, has the greatest topo- 
graphic closure (1.3 km). 

Most of the ranges consist of Precambrian to Cretaceous crys- 
talline rocks. Paleozoic sedimentary rocks, and Tertiary volcanic 
rocks. The valleys are filled with late Cenozoic nonmarine sedi- 
mentary deposits and extrusive igneous rocks. The Precambrian 
rocks crop out over a considerable area in the vicinity of Death 
Valley (Jennings and others, 1977), and they have a higher 
average density range (2.76 - 2.86 g/cm') than Mesozoic plu- 
tonic rocks (2.60 - 2.67 g/cm") (Chapman and others, 1973). 
The Cenozoic deposits are significantly lower in density, varying 
in density from 1.7 g/cm' for tuffaceous deposits in Mono Basin 
(L.C. Pakiser, written communication, 1975) to about 2.5 g/cm' 
for indurated middle Tertiary sedimentary and fiow rocks (Nils- 
en and Chapman, 1974; Chapman and others, 1973). 

The major structures in the Great Basin are normal faults 
bounding most of the ranges which are tilted as much as 30° 
(Stewart, 1978). The ranges east of the Sierra Nevada typically 
are tilted to the east whereas the Warner Mountains in northern 
California are tilted to the west. In addition to the major normal 
faults, which strike approximately north, strike-slip faults strike 
approximately northwest through some of the basins and have 
major right-lateral displacements. Examples of Quartemary 
northwest-trending strike-slip faults in eastern California in- 
clude the Death Valley-Furnace Creek fault zone, the unnamed 
fault zone along the west side of Saline Valley, the Honey Lake 
fault, and the Likely fault. The strike of the Sierra Nevada fault 
zone is more northerly, but the trend of the zone of en echelon 
faults north of Bishop is northwest and nearly parallel to the 
Death Valley fault. 

The structural development of the basin and range structures 
is generally considered to have begun in eastern California about 
17 million years ago and to have been largely completed by 7 
million years ago, although there has been continuing movement 
along many of the fault zones during Quaternary and even his- 
toric time (Stewart, 1978; Jennings, 1975). The 17 million year 



date marks the transition from predominantly compressive tec- 
tonics (related perhaps to a subduction zone) to extensional 
tectonics (related to wrench faulting, back-arc spreading, or 
some other factor) (Stewart, 1978). Estimates of the amount of 
extension in an east-west direction across the Great Basin in 
Nevada range from 10 to 100 percent, but most fall in the range 
20 to 30 percent (Stewart, 1978). Wright and Troxel (1973) 
concluded that the extension in the Death Valley region was 
about 40 percent. 

Regional Gravity 

Bouguer gravity anomalies over the ranges generally vary in- 
versely with the average topographic elevation (figure 4). Al- 
though a detailed study has not been made, the constant of 
proportionality for the Great Basin in California may have a 
higher value (about -1(X) mgalAm) than the constant deter- 
mined for the Sierra Nevada (about -80 mgal/km) (see section 
on the Sierra Nevada, this report). That is, the regional Bouguer 
anomalies (BA) are related to average elevation averaged to a 
distance of 41 km by the approximate relation 

BA — 100 E„ (1) 

where E., in kilometers yields BA in milligals. The constant 
-100 mgalAm compares with a value of -105 mgalAm (0.032 
mgal/ft) determined by Mabey (1966) from data in Nevada 
where elevations were averaged to a radius of 64 km. 

The estimated constant -1(X) mgalAm is based on simple trial 
and error testing of the coefficients -85, -100, and -115 mgal/ 
km along the California-Nevada border from the Garlock fault 
to the Warner Mountains (Table 5). 

The largest positive residuals occur over Precambrian meta- 
morphic rocks and Tertiary volcanic rocks whereas negative 
residuals occur over Mesozoic granitic rocks within the ranges. 

The similarity between Bouguer anomalies and regional eleva- 
tion (figure 4) is not obvious on the 5-mgal contour map be- 
cause of the myriad of relative gravity lows over the basins. The 
relation is more apparent on gravity maps with 10-mgal contour 
intervals (Diment and others, 1961, figure 2; Oliver, 1977, figure 
1), and obvious by comparing the 30-mgal interval map (figure 
3) with Figure 4. The general eastward increase in gravity and 
decrease in elevation is illustrated in a section from the southern 
Sierra Nevada to Death Valley by Oliver and Mabey (1963, 
figure 1). 



Table 5. Relation between 
average elevations, Bouguer 
anomalies, and type of base- 
ment rocks along California- 
Nevada border from the 
Garlock fault to the Oregon 
border. 



North 
latitude 


A verage 

elevation 

£•« (km) 

(Fig 4) 


Ag~ 

-100 £•« 

(mgal) 


Bouguer 
anomaly 

BA 
(mgal) 


Residual 
gravity 
BA-Ag 
(mgal) 


Exposed basement 
rock type' 


35'/,° 


1.05 


-105 


-110 


-5 


Mesozoic 

granitic rocks 


36'/,° 


.90 


- 90 


- 81 


+ 9 


Precambrian 
metamorphic 
rocks 


38° 
40° 


2.10 
1.50 


-210 
-150 


-220 
-145 


-10 

+ 15 


Mesozoic 

granitic rocks 
Tertiary 

volcanic rocks 


41'/,° 


1.65 


-165 


-155 


+ 10 


Tertiary 

volcanic rocks 



' From Jennings and others (1977), Lydon and othen (1960). and Duffield and Weldin (1976). 



34 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



Basin Anomalies 

After removing regional gravity, the residual anomalies as- 
sociated with the various basms within the Great Basin range 
from 15 mgal in Panamint Valley to 50 mgal in Death Valley and 
Honey Lake Valley (table 6). The calculated thicknesses of 
sedimentary and volcanic fill within the basins are not only 
dependent on the size of the gravity anomalies but also on the 
density contrast between the average density of the fill and that 
of the enclosing bedrock. Thus, the fill in Death Valley is es- 
timated to be twice as deep as Honey Lake Valley although they 
both have the same gravity closure (50 mgal), because the den- 
sity contrast for Death Valley ( -0.45 g/cm') is approximately 
half of that for Honey Lake Valley (0.95 g/cm'). Similarly, 
Indian Wells Valley (0.35 g/cm") is nearly as deep as Mono 
Basin (0.8 g/cm') although its gravity closure is only about 60 
percent that of Mono Basin (table 6). 

There is a large uncertainity in most of the density contrasts 
used to estimate basin depths. The best determined values are 
those for Indian Wells Valley and Mono Basin, which are based 
on both seismic control and well data. Formation-density logs, 
also known as "gamma-gamma," were run by Schlumberger 
Corporation in two wells in Mono Basin to depths of 1253 m on 
the south side and 744 m on the north side of Mono Lake. The 
south-side log indicates a density range of 1.7 to 1.8 g/cm' over 
both the upper 400 m and again in the lower part of the basin 
over the interval 900-1200 m separated by higher densities (2.0 
- 2.3 g/cm') between 400 and 900 m (Geothermal Resources 
International, 1971; Getty Oil Company, 1971). Density con- 
trasts for most of the other basins listed in Table 6 are uncertain 
by at least ± 25 percent. 

One of the unresolved problems in the analysis of basin ano- 
malies is that many of the gravity gradients associated with the 
faulted edges of the basins extend onto bedrock outcrops and 
cannot be fully explained by any configuration of low-density 
matenal underlying the valley (Mabey, 1963; Chapman and 
others, 1973; Kane and others, 1976). 

Relation to Major Faults 

Strong gravity gradients occur along most of the major histor- 
ic and Quaternary faults, and the gradients' points of inflection 
have been used to locate buried scarps along the Garlock, Sierra 
Nevada and Death Valley fault zones (Pakiser and others, 1964; 
Mabey, 1956). Concealed faults were revealed by the steep grav- 
ity gradients surrounding Long Valley and Mono Basin (Pakis- 
er, 1961; Pakiser and others, 1960). In Indian Wells and Owens 
Valleys, the locus of gravity inflection points (and thus the 
interpreted location for the main Sierra fault) is displaced into 
the basin about 1 '/, km from the contact between basin sedi- 
ments and basement rock, suggesting that some exposed scarps 
have eroded back a considerable distance (Healy and Press, 
1 964; Pakiser and Kane, 1 962 ) . However, detailed gravity analy- 
ses in Indian Wells Valley. Owens Valley, Carson Valley, and 
Surpnse Valley indicate that many parts of these fault zones 
consist of a senes of step faults combined with warping as op- 
posed to displacement along a single fault (Healy and Press, 
1964, figure 10; Pakiser and Kane, 1962; Tabor and Ellen, 1976; 
Gnscom and Conradi, 1976). The linear gravity high over the 
Warner Mountains on the west side of the Surprise Valley fault 
is probably associated with local uplift and westward tilting of 
a dense core of crystalline rocks draped by the exposed Tertiary 
volcanic and sedimentary rocks (table 5). Cobbles and boulders 



of granitic rcKk near the base of the oldest (Oligocene) sedimen- 
tary rocks now exposed in the Warner Mountains (Dufiield and 
Weldin, 1976, p. D8) lends credence to the existence nearby of 
exposed crystalline rocks at the time of deposition. 

Gravity gradients do not occur everywhere along the major 
faults, and places without gradients are areas where the displace- 
ments either are small or juxtapose rocks differing little in den- 
sity. A number of Quaternary faults within the ranges 
themselves have no gravity expressions but have significant dis- 
placements (Stewart, 1978). 

Some of the Surprise Valley frontal faults, along which hot 
springs are present, have small associated gravity highs of ", to 
1 '/, mgal. The anomalies presumably reflect local hydrothermal 
alteration and induration of the sediments (Griscom and Con- 
radi, 1976). 

Geologic and seismic evidence in Nevada suggests that the 
normal faults there probably do not penetrate deeply into the 
continental lithosphere but flatten instead with depth (Eaton 
and others, 1978). There have not been any gravity studies in 
eastern California of the configuration of normal faults at depths 
greater than the base of the sediments, and this avenue of re- 
search represents perhaps the greatest remaining challenge in 
this area. 

KLAMATH MOUNTAINS PROVINCE 
by Andrew Griscom' 

The Klamath Mountains have maximum elevations of 2500 to 
3000 m and average elevations (figure 4) of 750 to 1350 m. In 
general the rocks of this province (Irwin. 1977; Hamilton, 1978) 
are a tectonic assemblage of fragments from Mesozoic and Pale- 
ozoic island arcs, melange belts, and ophiolite masses, and are 
separated from each other by major thrust faults dipping gener- 
ally to the east. These rocks, particularly the eastern ones, are 
variably metamorphosed and are intruded by granitic and diont- 
ic plutons, chiefly of Mesozoic age. Beneath the layered Paleo- 
zoic rocks of the eastern part of the province is exposed the 
Trinity assemblage of ultramafic and mafic rocks, considered to 
be the lower portion of a large ophiolite sequence of early Paleo- 
zoic age (Lindsley-Griffin, 1973). 

The Bouguer gravity anomalies of the Klamath Mountains 
slope down to the east from values of about -50 mgal in the 
northwest comer to a low of -130 mgal near the north border 
of the state at longitude 1 23°W. This gradient is mostly the result 
of two effects: ( 1 ) the eastward transition from oceanic to conti- 
nental crust and mantle and (2) the isostatic effect of the thicker 
crust eastward as inferred from the increase in average altitude 
(figure 4). Increased crustal thickness seems an especially likely 
explanation here because the rock densities at the surface and 
near the surface (as inferred from local gravity anomalies) are 
relatively high. 

Over the central part of the province, the Bouguer gravity 
pattern is generally an irregular senes of closed highs and lows, 
many of which are equidimensional and others of which trend 
north or northwest. The average background gravity level in this 
central area is about -100 mgal; only local anomalies are 
markedly above or below this level. The background level is 



US Geologic*! Survey. Menio Park. CA 9402S. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



35 



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36 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



consistent with an isostatic factor of about 25 mgal/300 m con- 
sidenng that the average altitude is 1050 to 1200 m (figure 4). 
The relative abundance of gravity highs suggests that the average 
density of the upper crust in the Klamath Mountains is consider- 
ably higher than that of the Coast Ranges, where a gravity 
minimum of -11 5 mgals is associated with an average elevation 
of only 1050 m (figure 4). Correlations of individual anomalies 
with geology in this central area are only partly clear. The series 
of central gravity highs with maximum closures of -70 and -75 
mgal has no obvious source, although there appears to be a 
general correlation with higher grade metamorphic rocks of the 
amphibolite facies (Griscom, 1973a; Kim and Blank, 1973). 
Perhaps fault slices of dense ophiolitic rocks are present in the 
subsurface. 



Certain batholiths in the western part of the province are 
associated with gravity highs, probably because the rocks are 
mafic dionte (Lanphere and others, 1968, p. 1038). Other 
quartz diorite plutons in the eastern part of the province are 
associated with gravity lows(closure of -110 mgal, lat 40°40'N, 
long 122°45'W west of Redding, and closure of -1 15 mgal at lat 
41'20'N, long 123°00'W). 

In the northwestern part of the Klamath Mountains is a trian- 
gular area above -70 mgal, separated from the rest of the prov- 
ince by a northeast-trending gravity step of about 30 mgal 
amplitude (Kim and Blank, 1973, p. 6). Within this triangular 
area, four closed highs with peak amplitudes of -40, -50, and -55 
mgal are all associated with masses of ultramafic rocks. The 
gravity step corresponds with a series of thrust faults, and the 
high gravity values northwest of the step may indicate thick 
masses of ultramafic rocks, probably flat-lying fault slices (Kim 
and Blank, 1973) with associated volcanic rocks, and probably 
concealed in large part by the overlying sedimentary rocks. 

Extending up the east side of the Klamath Mountains prov- 
ince at longitude 122°40' is a row of gravity highs with maximum 
closures from south to north of -65, -85, -70, and -85 mgal, plus 
associated highs of -95 and -80 mgal a few kilometers to the east 
and west respectively. These anomalies are probably all caused 
by the relatively dense rocks of the Trinity assemblage, a proba- 
ble ophiolite sequence. The most extensive gravity high, which 
has a maximum closure of -70 mgal, was discussed by LaFehr 
( 1966), who pointed out the association of the south half of the 
feature with ultramafic rocks of the Tnnity assemblage and 
calculated that a sheet about 2 km thick with a density contrast 
of 0.6 g/cm' could account for the anomaly. The analysis indi- 
cates that the sheet extends in the subsurface north of the ex- 
posed Trinity assemblage. I have shown (Griscom, 1977) that 
the maximum gravity closures within the southern part of the 
exposed Trinity assemblage are associated with the mafic parts 
of the ophiolite rather than the ultramafic rocks. Kim and Blank 
(1973) suggested that the absence of a gravity high over the 
southern part of the ophiolite between the -85 and -95 mgal 
closed gravity highs indicates that the sheet must be thin. In 
1977 I showed by analysis of aeromagnetic data that here near 
the -110 mgal closed gravity low the sheet may actually have its 
maximum thickness, f)ossibly more than 6 km (Griscom, 1977). 
The scrpcntinization of the ultramafic rocks has reduced their 
density to a value similar to that of the country rocks, so there 
is no gravity anomaly. The Trinity assemblage extends in the 
subsurface (Griscom, 1973) below the associated magnetic and 
gravity highs (-60 mgal closure) at Redding (lat 48*35'N, long 
12270'W). 



The north end of a seismic-refraction profile (Eaton, 1966) 
is located at Shasta Lake, 20 km north of Redding. Here the 
upper crust is comjxjsed of matenal in which longitudinzd waves 
have a velocity of 5.9 km/s (density 2.67 g/cm") down to 6 km 
below sea level, and other matenal with a velocity of 6.8 km/s 
(density 2.99 g/cm') lies below that depth. The details below 6 
km are obscure, but assuming a simple crust, then the base of the 
crust should be at a depth of about 32 km. The proposed north- 
em extension of the Trinity assemblage beneath the northern- 
most gravity high (-85 mgal closure at lat 4r50'N) is 
problematical because the south-dipping basal thrust fault at the 
base of the assemblage crops out at Yreka (lat 4r45'N) and 
trends northeast across the gravity saddle in this location. I 
believe, however, that the northern extension of the gravity fea- 
ture is too compelling to disregard, and I suggest that there are 
structural complications, perhaps including repetition of the 
complex by thrust faults, such that the assemblage extends in the 
subsurface north of Yreka to underlie the -85 mgal gravity 
closure. The interpreted extent of the Trinity assemblage from 
south to north in California is over 170 km and the maximum 
outcrop width from west to east is more than 50 km. Alternative- 
ly, the gravity feature may consist of three different ophiolite 
masses now tectonically juxtaposed (Hamilton, 1978), with the 
discontinuities approximately located on the west margin of the 
assemblage at longitudes 122°15'W and 122°45'W, and with both 
discontinuities striking northeast. 

CASCADE RANGE AND MODOC PLATEAU 

by Andrev/ Griscom' 

The Cascade Range physiographic province in northern Cali- 
fornia is dominated by two irregular areas of high topography 
centered around two major volcanic centers, Lassen Peak (lat 
4O°30'N, long 12r30'W), and Mount Shasta (lat 4r25'N, long 
122°10'W) plus the nearby Medicine Lake Highlands to the 
east (lat 4r35'N, long 12I°35'W). South of Mount Shasta 
the province is nearly disconnected by an eastward pro- 
jection of the Klamath Mountains Province, the boundary of 
which here approximately follows the -115 mgal gravity con- 
tour. The Tertiary flows and pyroclastic rocks of the western 
Cascade Range are exposed only north of Mount Shasta (Mac- 
Donald, 1966) and were eroded to rolling hills before renewed 
volcanism beginning in Pliocene time built the large volcanoes 
of the High Cascades, predominantly composed of andesite, ba- 
salt, and dacite. 

East of the Cascade Range Province is the Modoc Plateau, a 
region of young volcanic landforms separated by broad basalt 
plains (MacDonald, 1966). The plateau is characterized by 
block faulting, and the locations of its physiographic boundaries 
with the Cascade Range to the west and the Great Basin to the 
east are indefinite. Structural depressions commonly contain 
Quaternary lake beds. The total thickness of the Tertiary and 
younger volcanic rocks is unknown but is at least several kilome- 
ters. The age and lithology of the underlying older rocks are also 
unknown. 



A regional Bouguer gravity gradient between about -110 to 
-140 mgal slopes down from west to east across the Cascade 
Range (LaFehr, 1965) and may reflect either thickening of the 
entire crust to the cast or thickening to the east of the low- 



' U.S. Geological Survey, Menlo Park. CA 94025 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



37 



density upper part while the total crustal thickness remains con- 
stant (LaFehr, 1965; Griscom, 1973). The isostatic effects in 
either case will cause the observed increase in average altitude 
to the east (figure 4). Simila (1978) reported an average crustal 
thickness for the Cascade Range of 35 km from seismic-refrac- 
tion data. 

LaFehr (1965) removed the regional gravity field from a 
Bouguer gravity map of the Cascade Range in California and 
demonstrated two similar subcircular residual gravity minima 
with amplitudes about -50 mgal and diameters of about 50 to 70 
km, associated with the Lassen and the Shasta-Medicine Lake 
volcanic areas. These lows are obvious on the State gravity map. 
Earlier, Pakiser (1964) showed that the Lassen gravity low 
could be accounted for by a near-surface slab of low-density 
rocks with its bottom about 8 km below sea level, if the density 
contrast were -0.2 g/cm'. Pakiser believed that low-density 
deposits of volcanic origin in a large subcircular volcano-tecton- 
ic depression were the most likely source of the gravity anomaly, 
although a batholith was another possibility, and that the mass 
excess of the Cascade Range was in approximate isostatic equi- 
librium with the buried low-density mass. LaFehr (1965) came 
to similar conclusions for the Shasta-Medicine Lake gravity low, 
pointing out that the association of the two minima with major 
volcanoes was strong support for an igneous source. He deduced 
that the mass of Mount Shasta was itself compensatesd by a 
residual gravity low of about -35 mgal in the immediate vicinity 
of the mountain. 

Kim and Blank (1973) showed that the west side of the Shasta 
gravity low was composed of two steep gradients separated by 
a flatter bench 10-20 km wide. The western gradient marked the 
contact between the Klamath Mountains Province and the Terti- 
ary volcanic rocks of the western Cascade Range and was proba- 
bly the expression of concealed high-angle marginal faults. The 
eastern gravity gradient was considered to correspond in turn 
with the western limit of younger (Pliocene and Pleistocene) 
volcanic rocks of the High Cascades associated with the circular 
Shasta gravity low. My calculations on aeromagnetic data over 
the western gravity gradient northwest of Mount Shasta (Gris- 
com, 1977) showed that the top of the Trinity assemblage may 
be offset steeply downward to the east and may there extend 
beneath the west side of the Shasta gravity depression, thus 
offering independent support for the interpreted faults. 

Chapman and Bishop (1968a) noted the gravity high (max- 
imum contour -140 mgal) associated with the Medicine Lake 
Highland on the east border of the Shasta gravity minimum and 
stated that the anomaly may be caused either by the volcanic 
rocks of this large shield volcano with central collapse caldera 
(Anderson, 1941) or by an underlying intrusive mass. They 
favored the latter explanation because most Cascade Range vol- 
canoes show associated gravity minima. 

Calculations by LaFehr (1965) showed that the steep gravity 
gradients on the west sides of the two major gravity minima 
indicate steep west contacts for the underlying low-density 
rocks. Tliese steep contacts may be a series of normal faults 
bordering the two volcano-tectonic depressions. However, La- 
Fehr did not take into account the fact that the westernmost part 
of his residual anomaly for the Shasta feature is Jissociated with 
older Tertiary volcanic rocks rather than the younger ones of 
Mount Shasta, and thus his Shasta anomaly is not fully isolated. 
Some of his inferred border faults must border the Cascade 
Range province itself rather than the inferred Shasta volcano 
tectonic depression. 



Interpretation of the gravity minima depends strongly upon 
the assumed density contrast between the younger low-density 
mass and the older basement rocks. LaFehr (1965) assumed that 
the basement rocks beneath Mount Shasta were similar to those 
exposed near the bottom of the mountain on the north (a small 
steptoe), south, and southeast sides. His density measurements 
on representative rock samples from this area gave results of 2.72 
±0.18 g/cm' (50 samples) for the "Paleozoic basement" and 
2.52 ± .13 g/cm' for Tertiary volcanic rocks (58 samples). 
These data suggest that the density contrast of -0.2 g/cm' used 
in the calculations is reasonable, but the density of pyroclastic 
rock units is difficult to measure accurately. Furthermore, if the 
Trinity assemblage lies beneath Mount Shasta, the density con- 
trast may be larger. The nature of the basement rocks beneath 
Lassen Peak is unknown, but the similarity of its gravity expres- 
sion with that of Shasta suggests basement of similar density. 1 
believe that low-density volcanic deposits may not be a sufficient 
explanation for these features because the calculated models of 
LaFehr (1965) and Pakiser (1964) are slabs with relatively 
constant thickness and steep margins. Such configurations sug- 
gest on tectonic grounds the presence of concealed plutons of 
similar or larger diameter beneath the gravity minima. The cal- 
culated models (LaFehr, 1965) also show roots with diameters 
of about 15 km centered under the volcanoes and extending 
down about 10 km. These roots may represent great thicknesses 
of volcanic rocks but more probably represent the stocks that fed 
the eruptions. The arcuate topographic scarp cutting the Klam- 
ath Mountains around the south and west side of the Shasta 
gravity low was interpreted by Heiken (1976) as evidence for a 
tectonic depression formed by collapse over a batholith. North 
of Lassen Peak a small gravity low with a minimum contour of 
-155 mgal (lat 40°40'N, long 12r30'W) is associated with sev- 
eral major andesitic pyroclastic cones having local heights of 800 
to 1 100 m. This anomaly may be caused by a small near-surface 
stock 10-20 km in diameter, and the local topography suggests 
a caldera. 



The various tectonic assemblages of the Klamath Mountains 
disappear to the southeast beneath the younger cover of the 
Great Valley and Cascade Range, reappearing in the western 
part of the northern Sierra Nevada (Davis, 1969). The continu- 
ity of these assemblages, where concealed, is a matter of some 
interest. I have interpreted from gravity and magnetic data a 
major concealed northeast-trending fault, possibly of Creta- 
ceous age, passing across the north end of the Great Valley 
(Griscom, 1973). If extended to the northeast, this fault lies 
about 20 km south of Lassen. I have also interpreted aeromag- 
netic data (Isidore Zietz, unpublished map) over the southern 
Cascades and northern Sierra Nevada to indicate that Sierra 
northwest structural trends can be traced into the Cascade 
Range Province at least as far as a point about 25 km south of 
Lassen Peak. Blake and Jones (1977) suggested that a rift zone 
may have extended northeast from the vicinity of Red Bluff (lat 
40°10'N, long 122°15'W), presumably near the aforementioned 
fault, resulting in northwest movement of the Klamath block 
relative to the Sierra block. Hamilton (1978, figure 4 and p. 60), 
in an extension of ideas develof)ed by Hamilton and Myers 
(1966), suggested that the Klamath Mountains were rifted and 
rotated away from the Sierra Nevada leaving a gap about 50-75 
km wide in the area of the southern Cascades, the gap having 
been filled with Cenozoic volcanic rocks and sediments. The 
shortest distance between outcrops of Klamath and Sierra base- 
ment is about 62 km (Lydon and others, 1960). The suggested 
location for the rift zone is supported by the rather rectangular 



38 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



outline of the Lassen gravity low, the southeast edge of the 
proposed rift being located approximately on the -160 mgal 
contour at the southeast side of the low, and the northwest edge 
of the rift on the -145 mgal contour at the northwest side of the 
low. The steep regional gravity gradient extending from the 
Sierra Nevada northwest across the proposed rift zone does not 
necessarily contradict the existence of the rift because this gradi- 
ent is isostatic in origin and related to the topography (figure 4), 
which probably postdates the rifting. The close relation between 
regional gravity and regional topography in California implies 
that the regional gravity field has the same age as the topogra- 
phy, predominantly late Cenozoic. 

The Lassen and Shasta gravity lows are separated by a 
northeast-trending gravity ridge with maximum contours of 
-105 and -125 mgal. This gravity ridge, described below, extends 
farther northeast across the entire Modoc Plateau as a series of 
gravity highs, and the composite gravity ridge suggests some 
fundamental tectonic division of the Cascade Range and Modoc 
Plateau into separate halves in California. The southeast side of 
this ridge may be the northwest side of the proposed rift zone 
described above. The gravity ndge probably represents a struc- 
tural high of the basement underlying the Cenozoic volcanic 
rocks. The volcanic rocks are presumed to have a lower density 
than the basement rocks. 

The gravity data do not always indicate agreement with the 
defined physiographic boundaries between the Cascade Range 
Province and the Modoc Plateau. The east side of the Lassen 
gravity low corresponds with the local boundary between the 
physiographic provinces, but the north half of the Shasta-Medi- 
cine Lake low lies in the Modoc Plateau, including the topo- 
graphically low area of Butte Valley and Meiss Lake. Evidently 
in this regon the boundary of the geophysical data transgresses 
the physiographic boundary. 

The regional gravity field over the Modoc Plateau ranges from 
about -140 mgal at the west side to about -175 mgal on the east 
side. Just as in the Cascade Range, this gradient may be caused 
either by thickening of the entire crust to the east or by a thicken- 
ing to the east of the low-density upper part while holding the 
total crustal thickness constant (LaFehr, 1965; Griscom, 1973). 
A third alternative may be the best explanation for the regional 
gravity field. Because of the structural similarities and indefinite 
boundaries between the Modoc Plateau and the Great Basin, the 
Modoc Plateau may have the same thinned crust (Hamilton, 
1978) and anomalous low-density mantle as is present in the 
Great Basin (Pakiser, 1963; Pakiser and Steinhart, 1964; Pro- 
dehl, 1970). The presence of low-density upper mantle can ex- 



plain the otherwise unusual association of thinned crust, low 
gravity, and moderate elevation. Thus the eastward decrease in 
gravity (in the Cascade Range as well as the Modoc Plateau) 
may represent a transition from more normal crust and mantle 
to anomalous crust and mantle, directly related to the extension- 
al tectonics of the Great Basin Province. 

A line of closed gravity highs trends northeast across the 
MtxlcK Plateau from the northeast-trending gravity ridge sepa- 
rating the two gravity depressions of the Cascade Range. The 
row of highs was described by Chapman and Bishop (1968a), 
who suggested as possible sources near-surface basement rocks, 
intrusive rocks underlying the volcanic rocks, and lateral density 
changes within the volcanic rocks. The on-trend gravity ridge of 
the Cascade Range reflects shallow basement between the struc- 
tural depressions of Mount Lassen and Shasta-Medicine Lake, 
and the row of gravity highs across the Modoc Plateau probably 
has a similar source. The Modoc anomalies have maximum am- 
plitudes of about 20 mgal, so that if the density contrast between 
the volcanic rocks and the basement rocks is 0.2 g/cm' (LaFehr, 
1965, p. 5584), then the relative elevation of the basement at the 
highs is approximately 2.4 km. Approximately 25 km north of 
the prominent high (long 12rW) with a closed contour of-130 
mgal is a gravity low with a closed contour of -165 mgal. More 
detailed gravity data in the vicinity of this gravity low (Chapman 
and others, 1978) reveal a large sub-circular fault-bounded ba- 
sin or possible caldera filled with low-density sedimentary or 
volcanic rocks. 

Chapman and Bishop ( 1968a) described various local gravity 
lows caused by Quaternary lake sediments filling structural 
depressions in the volcanic rocks. All major lakes and dry lakes 
have associated gravity lows of this sort, ranging in amplitude 
from -10 to -20 mgal. Density contrasts between sediments and 
volcanic rocks are probably at least 0.5 g/cm', so that the max- 
imum thicknesses of sediments may be no greater than about I 
km unless concealed sediments of higher density underlie them. 
These gravity lows are found at the following locations, from 
southeast to northwest (lat 40°30' to 42°N): liagle Lake, Made- 
line Plains, Big Valley, Big Lake. Goose Lake, Clear Lake Reser- 
voir, Tule Lake Sump, and Lower Klamath Lake. Many of the 
lows are bordered by local steep gravity gradients trending 
northwest or, less commonly, north. The gradients probably 
represent faults or steep downwarps related to similar structural 
trends in the Great Basin Province to the east and southeast. 

The Likely fault is the major known Quaternary fault within 
the Modoc Plateau. The fault has little if any influence on the 
gravity field (Chapman and Bishop, 1968a). 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



39 



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40 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



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shipboard gravity meters in 1963 ond 1964: Journol of Geophysical 
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petroleum appraisal, and noture of environmentol hazards, eastern 
Pacific shelf, latitude 28° to 38° north: U.S. Geological Survey Circular 
(in press). 



42 



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Howell. D.G., ond Vedder, J.C, 1978, Strotigrophic and stnicturo! frome- 
work of the southern Colifornio borderland, in Ernjt, W.G., editor, Geo- 
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Weed sheet: Californio Division of Mines and Geology, scale 1:250,000, 

8 p. te«t. 
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Krummenocher, Doniel, Gostil, R.G., Bushee, Jonothon, ond Doupont, Joan, 
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Mobey, D.R , 1966, Relotion between Bouguer grovity onomolies and re 
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Division of Mines ond Geology Speciol Report 43, 22 p. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



43 



Manning, J.C., 1973, Field trip to oreos of active faulting and shallow 
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Meuschke, J.L., Pitkin, J.A., ond Smith, CVi/., 1966, Aeromognetic mop of 
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Moore, D.G., 1969, Reflection profiling studies of the California Continentol 
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Moore, J.G., and du Broy, Edward, 1978, Mapped offset on the right-loteral 
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Morelli, C, editor, 1974, The Internotional gravity standardization net 1971: 
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Morton, D.M., and Gray, C.H., 1971, Field trip No. 4, Geology of the 
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Munger, Oilogrom, 1977, Annual California - Alaska oil and gos review: Los 
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Notional Oceanic ond Atmospheric Administration, 1974a, Bothymetric mop, 
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Notional Oceanic and Atmospheric Administrotion, 1974b, Magnetic mop, 
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Nilsen, T.H., and Chapman, R.H., 1974, Bouguer gravity mop of Colifornio, 
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Nilsen, T.H., ond Chapman, R.H., 1975, Principal facts, accuracies, ond 
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P- 

Oliver, H.W., 1973, Principal facts, plots, ond reduction programs for 1753 
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Oliver, H.W., 1977, Gravity ond mognetic investigations of the Sierro Ne- 
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Oliver, H.W., Chapman, R.H., and Biehler, Shown, 1979, Mop showing 
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Oliver, H.W., and Robbins, S.L., 1973, Preliminary complete Bouguer onom- 
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Oliver, H.W., and Robbins, S.L., 1974a, Preliminary Bouguer gravity mop of 
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Oliver, H.W., and Robbins, S.L., 1974b, Preliminary Bouguer gravity mop of 
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Oliver, H.W., and Robbins, S.L., 1980, Bouguer gravity mop of Colifornio, 
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65 p. text (in press) . 
Oliver, H.W., Robbins, S.L., Gronnell, R.B., Alewine, R.W., and Biehler, 
Shown, 1975a, Surface and subsurfoce movements determined by 
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quoke of February 9, 1971; California Division of Mines ond Geology 
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Oliver, H.W., Robbins, S.L., and Griscom, Andrew, 1975b, Preliminary Bou- 
guer gravity mop of the Susonville T by 2° quadrangle, Colifornio; U.S. 
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Bouguer anomaly mop of port of the Wolker Lake 1° by 2° quodrongle, 
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Oliver, H.W., Robbins, S.L., Rombo, W.L., ond Evernden, J.F., 1974, Prelimi- 
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Pokiser, L.C., ond Kane, M.F., 1962, Geophysicol study of Cenozoic geologic 
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Pokiser, L.C., Kone, M.F., and Jackson, W.H., 1964, Structural geology ond 
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Pokiser, L.C., Press, Fronk, and Kane, M.F., 1960, Geophysical investigation 
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J.C, Tudor, R.B., ond Webster, F.L., 1976, Geological and operotionol 
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Cortes Bonk oreo offshore southern Colifornio; U.S. Geological Survey 
Open-File Report 76-232, 65 p. 



44 



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Povoni, Naxorio, 1973, A structural model for the San Andreas fault zone 
olong the northeast side of the Gobilon Range, in Kovach, R.L., ond Nur, 
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Peselnick, Louis, Lockwood, J.P,, and Stewort, Roger, 1977, Anisotropic 
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Peterson, D.L., Conradi, Arthur, Zohdy, A.A.R., 1967, Principol facts for 
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PB212-998, 34 p. 

Peterson, D.L., 1969, Bouguer gravity map of the Needles area, Son Bernar- 
dino County, Californio, Mojove County, Arizona, and Clark County, 
Nevodo: U.S. Geological Survey open-file mop, scale 1:125,000. 

Phillips, R.P., 1964, Seismic refraction investigations in Gulf of Colifornia, in 
van Andel, T.H., and Shor, G.G., Jr., editors, Marine geology of the Gulf 
of California — A symposium: Americon Association of Petroleum Geolo- 
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Piatt, J. P., 1976, The petrology, structure, and geologic history of the Catali- 
no Schist terrain, southern California: University of Colifornia Publico- 
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Plouff, Donald, 1977, Preliminary documentotion for a FORTRAN progrom 
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P 

Prodehl, Clous, 1970, Seismic refraction study of crustol structure in the 
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P- 

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Rietmon, J.D., ond Beyer, L.A.. 1980, Gravity map of California, Sonto Mario 
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Robbins, S.L., 1971, Gravity and mognetic doto in the vicinity of the Colo- 
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Robbins, S.L., Oliver.H.W., ond Huber, D.F., 1975a, Principol facts, bose 
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Robbins, S.L., Oliver, H.W., and Huber, D.F., 1975b, Principal focts, accura- 
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Robbins, S.L., Oliver, H.W,, and Sikoro, R.F., 1975c, Principal focts, occuro- 
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1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



45 



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1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



47 



APPENDIX 

Gravity Measurements, Reductions, and 
Conversion Fornfiulas to IGSN 71 and GRS 67 

by 
H.W. Oliver', S.L. Robbins^ and R.H. Chapman^ 



Base Stations, Gravity Meters, and Calibration 

The prime gravity base station to which all the measurements 
of gravity differences at land stations in California have been 
referenced is Woollard's main control station WA 86 at San 
Francisco Airport (Behrendt and Woollard, 1961, table 2; 
WooUard and Rose, 1963, p. 94). Using Behrendt's (written 
communication, 1963) value at WA 86 of 979988.33 mgal. 
Chapman (1966) estabUshed 360 base stations in California with 
LaCoste and Romberg gravity meter G22. A correction factor 
of 1 .0009 was applied to the factory calibration based on recom- 
mendations by the manufacturer at that time and on tests along 
the Yosemite Calibration Loop (Barnes and others, 1969). Most 
of the gravity data in southern California and northwestern 
California are tied to these bases, and their observed gravity 
values have been determined relative to the published base sta- 
tion values given by Chapman (1966, Supplement 1). 

In northern Califomia.additional work was done in 1968-1970 
to strengthen the existing base network, and 28 new base stations 
were added in the Central Valley, Sierra Nevada, and northern 
Coast Ranges. The new data for the four prime bases in these 
areas are listed in Table 7, and the new base network is shown 



US. Geological Survey, Menlo Park. CA 94025. 

U.S. Geological Survey. Box 25046, Federal Center. Mail Stop 964, 

Denver. CO 80225. 

California Division of Mines and Geology, 2815 O Street. 

Sacramento. CA 95816. 



in Figure 8. Station A at the U.S. Geological Survey office in 
Menlo Park (Chapman, 1966, p. 36, station 173) was used as the 
reference base for the new work, station WA 86 at San Francisco 
Airport having been made unoccupiable by new airport con- 
struction in 1966. Five ties were made between Menlo Park and 
San Francisco Airport before the 1966 construction which yield- 
ed the gravity difference of 29.59 ± .01 mgal (s.e.). The prime 
bases at Porterville and Sonora are particularly well established 
relative to Menlo Park, so that possible relative vertical move- 
ments between the coast of California and the Sierra Nevada 
greater than 5 cm should be detectable by repeating measure- 
ments of gravity differences between these base stations. 

The 28 new base stations in northern Cahfomia (figure 8) and 
most of the gravity data in northern California were obtained 
using calibrations of LaCoste and Romberg gravity meters that 
are 3 parts in 10,000 lower than the calibration standard used to 
establish the California base station network (Chapman, 1966). 
The calibration standard was primarily the Yosemite Calibration 
Loop for which a correction factor of 1.0009 had been deter- 
mined for LaCoste and Romberg meter G22 to bring its data in 
line with meter G17, which had been calibrated on the North 
American Calibration Range from Costa Rica to Point Barrow, 
Alaska (Chapman, 1966, figure 1). However, more comparisons 
were made in 1968 between G22 and G17 as well as Defense 
Mapping Agency meter Gl 15 (factor 1.00012). The results sug- 
gested that the correction factor for G22 should be reduced from 
1.0009 to 1.0006. This change was verified in 1971 by a direct 
comparison of gravity differences of base station values in east- 



Table 7. Observed gravity values, number of ties to Menlo Park, and gravity meters used for establishing 

the four prime base stations in east-central California. [Locations of the base stations are shown in Figure 8 by both 

name and number. Prime bases are those which hove 14 or more ties to station A in Menlo Pork and standard errors of ±.01 mgal or less.] 



Station 
Number 


Place names 


Observed 
gravity 
(mgal) 


Standard 

deviation 

(mgal) 


Standard 

error 

(mgal) 


Ties (one way) 
to Menlo Park 


Gravity meters 

used in decreasing 

number of ties 


A2 

A7A 
CHI 22 

CHI 86 


Fresno' 

Porterville' 
Auburn^ 

Sonora^ 


979,832.95 

979,746.33 
979,953.28 

979,832.11 


.025 

.028 
.038 

.025 


±.006 

±.005 
±.009 

±.007 


20 

28 
18 

14 


-.G17,G115,G8.G58, 
G65,G129,G143,G22 

GI7,G8,G58,G67 

G17,G8,G58,G65, 
G67,G22.G143 

G17,G8,G58,G67, 
G22,G143 



' See Robbins and others ( 1975a, p '%, 30) for descnptions and pictures of these bases 

'The onginal descriptions of these st.tions (Chapman. 1966. p 31. 37) have been updated and photographs have been taken (Robbins and others, 1976a, 
p 20. Robbins and others, 1974. p 19) 



48 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



EXPLANATION 
Pnme base station 

Isi, 2r>d. or 3rd order base station 



Number of one way ties between 
base stations 



CalibratKXi loop ties for parts of the 
« Mt Hamilton, Mt Pinos. and Yosemite 
Calibration Ranges 



50 Mites 



50 K ilometers 




Figure 8. New gravity bote jtotion network in eo»t-centrol Colifornio. Observed gravity values for the prime bose stations ore listed in Toble 7. Volues for 
the descriptions of lovi^er-order boses ore provided in the local NTIS reports shown in Figure 2. 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



49 



central California obtained with G17 (figure 9). The revised 
observed gravity values of the base stations plotted in Figure 9 
as well as the pictures, descriptions, and gravity values of the 28 
new base stations are presented in the NTIS publications listed 
by area in Figure 2. Revised values of other base stations in the 
state as reported by Chapman (1966) can be obtained from the 
following formula: 

G, = 0.9997 (G, -979988.33) -(- 979988.33 (1) 

or more simply 
G, = 0.9997 Gj + 294.00 

where G, = Revised observed gravity value in mgal for a given 
base station whose published value is G, in mgal (Chapman, 
1966). For example, Gj at base station number 103 at Bridgeport 
is 979395.61 mgal (Chapman, 1966, Supplement). The calculat- 
ed revised value is 979395.80 mgal or 0.19 mgal higher (see plot 
of "CH 103", figure 9). The correction ranges from -0.09 mgal 
at Crescent City to +0.20 mgal at Desconso Valley in the moun- 
tains east of San Diego (Base station numbers 1 and 341, Chap- 



man, 1966, p. 36 and 34, respectively). Thus, the calibration 
problem is not serious, but these corrections should be made to 
gravity data referenced to 1966 base station values for which the 
desired accuracy relative to other parts of California is 0. 1 mgal 
or better. A considerable effort was made to hold this accuracy 
throughout California. Table 8 shows the 17 LaCoste and Rom- 
berg meters that were used in obtaining about 30,000 new sta- 
tions during 1968 to 1971 and their correction factors as 
determined over the various calibration loops in California 
(Barnes and others, 1969). 

The base stations used by the U.S. Naval Hydrographic Office 
as references for offshore data were also established with La- 
Coste and Romberg meters relative to 980118.8 mgal at the 
National Reference Base in Washington, D.C. (Cliff Gray, per- 
sonal communication, 1978). The bottom meter data off north- 
em California referred to above were tied to bases at Humboldt 
Bay Pier B in Eureka and at the foot of Pier 14, Treasure Island, 
using base values of 980223.36 mgal and 979991.86 mgal, respec- 
tively (N.A. Prahl and G.B. Mills, written communication, 
1971). Descriptions and observed gravity values at other bases 



-t-0.3 - 



+0.2 - 



< 
o 

5 



+0.1- 



> 
< 

O 



- 


1 


1 1 

^Lee Vining 
•224 


1 1 1 1 1 1 1 


^. 


Tuolumne 
•Meadows 
218 


,_„ Lone Pine 
\^ .lOS ,236 
^^ 234» 

^~~- ^•Bishop 
•l02 ^226^ 


• Big Trees Quincy 
116 'as 

•^ ■ .Pulga 
•78 
^^ ,93 ,42 


" 




• 279 


.120^\^ -2° 

•qq-^^ #19 »94 Eureka 
89 ^v. 46» 






207* ^^-^ 00=979,988 33 Of San ^5 
Carson City iqr«^ Francisco AP(Prlme base) 
351* ^'^''•a2-\ / 




G22( 1.0009) 




•24 ^v. / ,39 Chico 
21- 12>--f,62 .oi* ^76 








117* A "^«»83 50 
(1731 ^!r-^,52 
Menio Park 82 ^^x5 








• Placerviile Redding .^^.49 
108 35^ •^ 








-Chico AP^^ 
75 




1 


1 1 


1 1 1 1 1 1 1 



-0.1- 



979,200 



979,400 



979,600 979,800 

OBSERVED GRAVITY IN MGAL 



980,000 



980,200 



Figure 9. Gravity differences between measurements made at 33 base stations in California with LaCoste and Romberg gravity meter G17 using a correction 
factor of 1.0009 during 1968-1970 and those mode with meter G22 using the some factor (Chopmon, 1966) . The grovity differences ore plotted as o function 
of observed gravity values and tend to decrease with increosing grovity. This general dependence is removed by reducing the correction factor of meter G22 
to 1.0006 (dashed line}. The average scatter for G17 (1.0009} - G22 (1.0006) is about ±0.03 mgal, and this variance is a measure of the repeatability 
of the gravity measurements. 



50 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



Table 8. Correction factors to factory calibration tables of LaCoste— Romberg gravity 
meters used in the California gravity program. The following calibration loops are used 
to determine the factors (see Barnes and others, 1969) : 



LaCoste-Romberg 


Correction factor 
relative to 1 00016 


Range or loop and 




merer and owner 


for GIISB 


number of runs 




G8 U.S. Geological Survey 


1.0006' 


N2. H8. P8, Y5 




GIO Defense Mapping Agency 


.9997 


H2 




G12 Defense Mapping Agency 


1.0001 


H2, C4 




G17 U.S. Geological Survey 


1.0009 


H7, H22, P5, Y9. S3, L3 


, C3' 


G22 Univ. Calif. Riverside 


1.0006 


Yl, HI 




G58 Defense Mapping Agency 


.9998 


HI, P3 




G62 Defense Mapping Agency 


1.0004 


H4, Y3 




G65 Defense Mapping Agency 


1.0003 


H5, Y2 




G102 Stanford Univ. 


1.0003 


H3, Y2 




G 1 1 5 Defense Mapping Agency 


(1.00016) 


El, H5, P4, Y4, S3, L3, 


C2 


G129 Calif Div. Mines Geology 


1.0003 


H6, PI 




G130 Defense Mapping Agency 


1.0003 


H4, Al 




G143 Defense Mapping Agency 


1.0003 


H6, PI 




G159 U.S. Geological Survey 


1.0004- 


LI. M2 




G161 U.S. Geological Survey 


1.0002" 


H6, PI 




G172 U.S. Geological Survey 


1.0002 


H3 




G198 U.S. Bur. Mines 


1.0005 


H3 





W = North Americon Western Calibration Range 
(Costo Rica to Point Borrow, Ak.) 
E = North American Eastern Calibration Range 
(Key West, Flo. to Woshington, D.C.) 



H 


= 


Ml. Homilton, Co. 




P 


= 


Ml. Pinos, Co. 




Y 


= 


Yosemile, Co. 




S 


= 


Palm Springs, Co. 




L 


= 


Ml. Lassen, Co. 




C 


= 


Croter Lake, Ore. 




LM 


= 


Looltout Mountain, 


Colo 


M 


= 


Ml. Evans, Colo. 




A 


= 


Anchorage, Ak. 





' This IS the factor determined in 1968 lo 1971 dunng its use in California The main spnng in G8 has since been replaced 

and the meter converted to electronic readout for measunng microgravity changes. 
' Relative to LaCostc-Romberg meter G-IA calibrated on the North American Range (Behrendt, 1952. p 889) As of 1978, 

more data on the Mt Evans loop indicate that the "best" correction faclor is 1.0002 for G159 (D.L. Peterson, wntten 
.communication, 1978). 
^ The scatter in the 7 runs with this meter during 1967-1973 was 1.0001 to 1.0005. In 1976. the meter was converted to 

electronic readout, and this process apparently caused a decrease in the factory calibration by 4 parts in 10.000 based 

on factory tests in Cloudcroft, New Mexico 



used along the California coast by the Hydrographic Office have 
not been published, but specific base information can be obtained 
from their National Standards and Testing Laboratory Branch 
in Bay St. Louis, Mississippi 39522. 

Reduction of Data 

All the gravity measurements on land have been reduced to 
Bouguer anomalies using first a computer program that trans- 
forms meter readings in scale divisions to simple Bouguer ano- 
malies (Oliver, 1973, appendix 2) and a second program that 
makes terrain and curvature corrections and adds them to the 
simple Bouguer anomalies (Plouff, 1977). The basic procedures 
and formulas of the reduction are as follows: 

( 1 ) The meter readings in scale divisions of both the base 
and field stations are converted to milligals using 
stored factory calibration tables and the correction 
factor for the particular gravity meter (table 8). The 
meter readings in milligals are then corrected for 
tidal variations using LB. Slichter"s (written comm- 
munication, 1969) program and an elasticity faclor 
for earth tides of 1.16. The residual dnft is generally 
removed linearly, although if it is in excess of 0.1 
mgal for any given traverse the data are studied for 
possible tares and non-linear drift is removed. If the 
residual drift is greater than 0.2 mgal. the data are 
usually discarded The gravity difference (A g) in 
milligals IS then oblained by taking the difference 
between the corrected base and corrected field sta- 
tion. 



(2) Observed gravity (go) = Previously determined 
Gravity Base Value -I- Ag. 

(3) Theoretical gravity g, =978049 (1 -|- 5.229 x 10^ 
sin^8-5.9 x ICT* sin^), where 9 = latitude. 

(4) Free-air anomaly (FAA) =&, - g, + (9.411549 x 
10^ - 1.37789xl0^sin=e) E -6.7 x 10* E% where E 
is the elevation in feet. 

(5) Simple Bouguer anomaly (BA) = FAA-(1.2774 x 
10 "* pE) where p = reduction density in g/cm'. 

(6) Curvature correction (CC) = 4.462 x IQ-^E - 3.28 
X 10-" E' X 1.27 X 10"" E' where E is the elevation in 
feet. 

(7) Complete Bouguer anomaly (CBA) = BA - CC -h 
TC where TC = terrain correction for p. The terrain 
correction is generally made manually to a distance 
of 2.29 km and extended to 166.7 km using the digi- 
tal model of California and adjacent regions (Rob- 
bins and others, 1973). 

The offshore data obtained with surface shipboard meas- 
urements were reduced to free-air anomalies using the sim- 
pler formula: 

FAA = go - gt + 9.406 X 10-* E 

where E is the elevation of the gravity meter above the sea 
surface in feet. Reduction of the ocean-bottom meter data is 



1980 



INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA 



51 



Table 9. Comparison between ICSN 71 (Morelli, 1974) and Chapman 's ( 1966) observed gravity values in California and Nevada. 



Name and location 


Station numbers 


Observed gravities 


Gravity difference 


IGSN' 


DMA' 


Chapman 


IGSN 
(mgal) 


Chapman 
(mgal) 


(IGSN-Chapman) 
(mgaJ) 


San Diego-Lindbergh Field 
Los Angeles-UCLA 
Reno-Airport Weather 
San Francisco-Airport 

(upper AF disk) 
Potsdam, Germany 


12027-J 
12038-B 
12099-K 
12172-0 

A 


187-0 
420-3 
454-2 
133-0 


343 
311 
349 

1 


979522.32 
979583.10 
979675.88 
979972.37 

981260.19 


979537.05 
979597.68 
979690.39 
979986.81' 

981274.20^ 


-14.73 
-14.58 
-14.51 
-14.44 

-14.0^ 



CiUforau Bum Net value detemiined by H W Oliver (in 1970) relative to Chapman's stations 156, 159. 164. 165, and 173 
' See MorelU (1974, p 18) and WooUard (1963, p 33) 
' These are the station designations as given by MoreUi (1974, p 48-49) for IGSN and Hauer (1974) for DMA 



based on the following equation (afer Prahl and Mills, writ- 
ten communication, 1971): 



FAA = go - [g, + F, (D-T) - F, (2D-T)] 



where 



go = observed gravity on the ocean floor 



g, = theoretical gravity at the surface (see above formula) 
F, = free-air gradient taken to be 9.406 x 10^ mgal/ft. 
Fj = water slab coefficient of 1.315 x 10^ mgal/ft. 
D = water depth in feet 
T = Height of tide above datum in feet. 



The equation consists of a free-air and a double Bouguer slab 
correction for the depth of water and a single Bouguer slab 
correction for the tide. Inserting the constants and simplifying, 
the above equation reduces to 

FAA = go - g, -I- 0.06776 D - 0.08091 T 

Conversion to IGSN 71 and GRS 67 

As most of the onshore and offshore gravity data in California 
were obtained during 1966-1971, it was reasonable to use the 
Woollard and Rose (1963) gravity datum and reduce the data 



on the basis of the 1930 International Gravity formula (Swick, 
1942, p. 61). Since the adoption of the new absolute gravity 
standard "IGSN 71" (MorelU, 1974) discussed above and the 
"Geodetic Reference System 1967" (GRS 67) (International 
Association of Geodesy, 1971), new gravity data in California are 
being processed using these combined systems (see for example 
Isherwood and Plouff, 1978). Also, some other State gravity 
maps such as Alaska's (Barnes, 1977) are being compiled with 
these updated systems. Therefore, it is of interest to set forth 
what would be involved in making these changes to the approxi- 
mately 80,000 stations in California and estimating the effect on 
onshore Bouguer anomalies and offshore free-air anomalies. 

Table 9 hsts observed gravity values on IGSN 71 and Chap- 
man datums at three stations in California, at one in Nevada, and 
at Potsdam. The differences (last column table 9) are a function 
of observed gravity (go) and are approximated by the linear re-' 
lation 



Ago (IGSN 71 -Chapman) = 
980000) 



-14.4 + A (go Chapman 



(1) 



where go (IGSN) and g„ (Chapman) are in milligals and A is 
the change in gravity scale, which appears to average about 4 x 
10 "* for successive differences in gravity south of San Francisco 
(table 10). 





Difference in 


Difference' 


Apparent' 


Locations 


observed gra vity' 


in IGSN-Chapman 


change in 




(mgal) 


(mgal) 


scale vaJue 


Woollard and Rose 








Potsdam - San Francisco 


-1-1287.2 


-1-0.44 


+ 3.4 X 10-' 


Potsdam - Reno 


-1-1588.1 


-t-0.51 


+ 3.2 X 10-' 


Potsdam - San Diego 


-1-1736.9 


-1-0.73 


+4.2 X 10-' 


Chapman 








San Francisco - San Diego 


-1-450.05 


-t-0.29 


+ 6.4 X 10-' 


San Francisco - Los Angeles 


-1-389.13 


+0.14 


+ 3.6 X 10-' 


San Francisco - Reno 


-1-296.42 


+ 0.07 


+ 2.4 X 10-' 



Based on IGSN 71 values. 

From table 9. 

For example: +.44/+ 1287.2 = +3.4 x 10-' 



Table 10. Changes in the scale 
values for IGSN 71 relative to 
that for Woollard and Rose 
(1963) and Chapman (1966). 



S2 



CALIFORNIA DIVISION OF MINES AND GEOLOGY 



BULL. 205 



The effect of converting from the 1930 to the 1967 reference 
ellipsoids is 



Ag, (1967-1930) = -17.2 + 13.6 sin^ 



(2) 



where 6 is the latitude and Ag, is in milligals (International 
Association of Geodesy, 1971, p. 60). 

Thus, the efTect on free-air and Bouguer gravity anomalies 



(Ag.) of adopting IGSN 71 and GRS 67 is 

(go -980000) 



Ag. = Ago 
Ag. = 2.8 - 



Ag, 
13.6sin'e + 4 X 10- 



where g„ and Ag, are in milligals. 

For values of 6 and go in California, Ag. varies from about 
— 1.5 mgal at San Diego to about -3.2 mgal near the Oregon 
border and is about -2 mgal in central California (table 11). 










Change in 


Change in 


Bouguer 


Location 


Latitude 


theoretical 


observed 


anomaly 






gravity (mgal) 


gra vity (mgal) 


(mgal) 


San Diego 


32°44' 


-13.2 


-14.7 


-1.5 


Los Angeles 


33-57' 


-13.0 


-14.6 


-1.6 


San Francisco 


3r37' 


-12.1 


-14.4 


-2.3 


Reno. Nev. 


39-31' 


-11.7 


-14.5 


-2.8 


Medford, Ore. 


42.22' 


-11.1 


(-14.4)' 


-3.3 



Table 1 1. Changes in Bouguer anomalies 
resulting from adoption of GRS 1967 and 
IGSN 71. 



Based on a comparison of observed gravity values at stations at Medford Airport which are not 
exactly at the same location but very close This difference may be 1 too large, in which case 
the resulting change in Bouguer anomaly at Medford would be -3 2 mgal. 



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