INTERPRETATION OF THE
GRAVITY MAP OF CALIFORNIA
AND ITS CONTINENTAL MARGIN
California Division of Mines and Geology
Sacramento, California, 1980
STATE OF CALIFORNIA
EDMUND G BROWN JR.
THE RESOURCES AGENCY
HUEY D JOHNSON
SECRETARY FOR RESOURCES
DEPARTMENT OF CONSERVATION
PRISCILLA C GREW
DIVISION OF MINES AND GEOLOGY
JAMES F DAVIS
INTERPRETATION OF THE
GRAVITY MAP OF CALIFORNIA
AND ITS CONTINENTAL MARGIN
H.W. Oliver, Editor
MAR 5 1981
G*. i^OCS.- LIBRARY
CALIFORNIA DIVISION OF MINES AND GEOLOGY
1416 Ninth Street, Room 1341
Sacramento, California 95814
TABLE OF CONTENTS
UNITS AND ABBREVIATIONS USED IN THIS REPORT vii
GENERAL INTRODUCTION, by H.W. Oliver 1
OFFSHORE SOUTHERN CALIFORNIA, by L.A. Beyer
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
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
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
Gravity Measurements, Reductions, and Conversion
Formulas to IGSN 71 and GRS 67 47
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
Figure 7. Bouguer anomaly map of the California continental margin off southern
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
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
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)
USGS - U.S. Geological Survey
CDMG - California Division of Mines and Geology
NOAA - U.S. National Oceanic and Atmospheric
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)
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
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
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'.
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.
Colitornio Oivtsion of
J Mines and Geology
U S Geological Survey
University of ColiforniQ
University of California
at Santa Ouz and NOAA
University of Colifornio
at Santa Barbara
University of Oregon
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
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
NTIS 232 728
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.
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.
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.
IN FIG. 2
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)
San Diego-El Centre
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
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.
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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.
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).
CALIFORNIA DIVISION OF MINES AND GEOLOGY
Average elevation contours in meters.
Contour interval 150 meters
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).
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
OFFSHORE SOUTHERN CALIFORNIA
by L.A. Beyer'
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
Vedder and others (1976b) described the complex late Ceno-
zoic tectonic history of the California Continental Borderland as
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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.
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.
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
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
Eastward from Region II to the
onshore part of the Newport-In-
glewood fault zone; southeast of
Newport Beach the eastern margin
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
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,
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-
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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CALIFORNIA DIVISION OF MINES AND GEOLOGY
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-
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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
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
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
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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.
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).
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.
Santa Monica Mountains
-10 to -50
South boundary of San
-60 to -70
Arcadia to Saugus, 6 km
south of and parallel
to north boundary of San
Gabriel and southwest
boundary of San
-80 to -110
-115 to -125
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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
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.
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
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.
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
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.
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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
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).
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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-
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
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
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
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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.
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
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
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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,
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
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).
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
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,
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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-
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
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
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
' Botrd of Euth Sciences. Univeraily of Caliroraia, Sant* Cnii, CA <)J064
I^4TERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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-
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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;
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-
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.
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;
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
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
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.
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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-
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
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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,
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).
by H.W. Oliver' and Andrev/ Griscom'
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).
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.
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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).
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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).
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-
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-
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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,
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,
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
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.
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
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,
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,
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'.
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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-
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.
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-
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^
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
(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.
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,
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
' From Jennings and others (1977), Lydon and othen (1960). and Duffield and Weldin (1976).
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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-
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
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
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.
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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CALIFORNIA DIVISION OF MINES AND GEOLOGY
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
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
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
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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-
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
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
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
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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
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).
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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CALIFORNIA DIVISION OF MINES AND GEOLOGY
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INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
Gravity Measurements, Reductions, and
Conversion Fornfiulas to IGSN 71 and GRS 67
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.]
Ties (one way)
to Menlo Park
used in decreasing
number of ties
' 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)
CALIFORNIA DIVISION OF MINES AND GEOLOGY
Pnme base station
Isi, 2r>d. or 3rd order base station
Number of one way ties between
CalibratKXi loop ties for parts of the
« Mt Hamilton, Mt Pinos. and Yosemite
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.
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
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
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
1 1 1 1 1 1 1
,_„ Lone Pine
\^ .lOS ,236
• Big Trees Quincy
•^ ■ .Pulga
^^ ,93 ,42
•qq-^^ #19 »94 Eureka
89 ^v. 46»
207* ^^-^ 00=979,988 33 Of San ^5
Carson City iqr«^ Francisco AP(Prlme base)
351* ^'^''•a2-\ /
•24 ^v. / ,39 Chico
21- 12>--f,62 .oi* ^76
117* A "^«»83 50
Menio Park 82 ^^x5
• Placerviile Redding .^^.49
108 35^ •^
1 1 1 1 1 1 1
OBSERVED GRAVITY IN MGAL
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.
CALIFORNIA DIVISION OF MINES AND GEOLOGY
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) :
relative to 1 00016
Range or loop and
merer and owner
number of runs
G8 U.S. Geological Survey
N2. H8. P8, Y5
GIO Defense Mapping Agency
G12 Defense Mapping Agency
G17 U.S. Geological Survey
H7, H22, P5, Y9. S3, L3
G22 Univ. Calif. Riverside
G58 Defense Mapping Agency
G62 Defense Mapping Agency
G65 Defense Mapping Agency
G102 Stanford Univ.
G 1 1 5 Defense Mapping Agency
El, H5, P4, Y4, S3, L3,
G129 Calif Div. Mines Geology
G130 Defense Mapping Agency
G143 Defense Mapping Agency
G159 U.S. Geological Survey
G161 U.S. Geological Survey
G172 U.S. Geological Survey
G198 U.S. Bur. Mines
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.)
Ml. Homilton, Co.
Ml. Pinos, Co.
Palm Springs, Co.
Ml. Lassen, Co.
Croter Lake, Ore.
Ml. Evans, Colo.
' 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
^ 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-
(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
(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-
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
INTERPRETATION OF THE GRAVITY MAP OF CALIFORNIA
Table 9. Comparison between ICSN 71 (Morelli, 1974) and Chapman 's ( 1966) observed gravity values in California and Nevada.
Name and location
San Diego-Lindbergh Field
(upper AF disk)
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)]
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-'
Ago (IGSN 71 -Chapman) =
-14.4 + A (go Chapman
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
observed gra vity'
Woollard and Rose
Potsdam - San Francisco
+ 3.4 X 10-'
Potsdam - Reno
+ 3.2 X 10-'
Potsdam - San Diego
+4.2 X 10-'
San Francisco - San Diego
+ 6.4 X 10-'
San Francisco - Los Angeles
+ 3.6 X 10-'
San Francisco - Reno
+ 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).
CALIFORNIA DIVISION OF MINES AND GEOLOGY
The effect of converting from the 1930 to the 1967 reference
Ag, (1967-1930) = -17.2 + 13.6 sin^
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
Ag. = Ago
Ag. = 2.8 -
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).
gra vity (mgal)
Table 1 1. Changes in Bouguer anomalies
resulting from adoption of GRS 1967 and
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.
eiyiT-e^ 12-eo osp 1290 uda
H UIW IE '3;
PHYS SCI UBHARY
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APR 1 8 1983
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