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STRUCTURAL GEOLOGY, OF THE CONTINENTAL
MARGIN OFF PT. ANO NUEVO, CALIFORNIA

David Dexter Frydenlund

nilDLEY KNOX LIBR'^RY
NAvit POSTGRADUATE SCHOOL

MONTEREY. CAL.FORNU^ 93940

lAlL POSTGE.IOUATE SCliSQL

Monterey, California

l»l I ■III— — IBII II

STRUCTURAL GEOLOGY OF THE CONTINENTAL
. MARGIN OFF PT . ANO NUEVO , CALIFORNIA

David Dexter Frydenlund

September 1974

Thesis Advisors:

J. J. von Schwind
R. S, Andrews

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Structural Geology of the Continental
Margin off Pt . Ano Nuevo , California

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Master's Thesis;
September 1974

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David Dexter Frydenlund

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Naval Postgraduate School
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IS. KEY WORDS (Continue on reveree e/de II nmcfmry and Identity by block number)

Faults

Sur-Naciemento Fault Zone

Structural Geology

Salinian Block

Seismic Survev

Geophysics
Geology
Marine Geology

20. ABSTRACT (Continue on reveree tide II neceetery end Identity by block number)

Nine fault zones, including several possible offshore
extensions of the Sur-Naciemento fault, v\fere located and
traced on the Continental Margin off Point Ano Nuevo, California
by seismic reflection profiling. Plate tectonic theory
was combined with regional geology to arrive at the most
plausible choice for the Sur-Naciemento fault zone and to
generate a brief geologic history of the area.

DD 1 JAN 73 1473 EDITION OF 1 NOV 65 IS OBSOLETE

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Palo Colorado-San Gregorio Fault Zone

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Structural Geology of the Continental
Margin Off Pt. Ano Nuevo, California

David Dexter ^Frydenlund

Lieutenant, United States Coast Guard

B.S., United States Coast Guard Academy, 1969

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY

from the

NAVAL POSTGRADUATE SCHOOL
September 1974

DUDLEY KNOX LIBR'^RY
NAVAL POSTGRADUATE SCHOOL
MONTEREY. CALIFORNIA 93940

ABSTRACT

Nine fault zones, including several possible offshore
extensions of the Sur-Naciemento fault, were located and
traced on the Continental Margin off Point Ano Nuevo ,
California by seismic reflection profiling. Plate tectonic
theory was combined with regional geology to arrive at the
most plausible choice for the Sur-Naciemento fault zone and
to generate a brief geologic history of the area.

TABLE OF CONTENTS

I. INTRODUCTION 9

A. OBJECTIVE 9

B. AREA DESCRIPTION 9

C . REGIONAL GEOLOGY 10

D. TECTONIC DEVELOPMENT 13

E. PREVIOUS AREA INVESTIGATION 13

II . COLLECTION OF DATA 16

A . SURVEY PROCEDURE 16

B. NAVIGATION 17

III. ANALYSIS OF DATA 18

A. INTERPRETATION NOTES 18

B . GENERAL STRUCTURE 19

1. The Eastern Ridge 19

2. The Sedimentary Basin 20

3. The Western Ridge 20

C . FAULT ZONES 21

1. Fault No. 1 22

2. Fault No. 2 2 2

3. Fault No. 3 23

4. Fault No. 4 2 3

5. Fault No. 5 24

6. Fault No, 6 24

7. Fault No. 7 24

8. Fault No. 8 25

9. Fault No. 9 2 5

IV. DISCUSSION 26

A. SUR NACIEMENTO FAULT ZONE 26

B. PIGEON POINT AREA 28

C . SUMMARY 28

D. FUTURE WORK 29

BIBLIOGRAPHY 52

INITIAL DISTRIBUTION LIST 54

LIST OF FIGURES

1. Survey Area Location and Limits with Approximate
Bathymetry 31

2. Regional Map of Central California Showing Major
Fault Zones 32

3. Regional Map of Central California Showing Place
Names 33

4. Hypothetical Regional Development 34

5. Location of Ships Tracks Used in the Investigation. 35

6. Map of Structural Provinces in the Survey Area ^6

7. Location of Selected Profiles 37

8. Profile A A' Seismic Record and Line Drawing 38

9. Profile B B' Seismic Record and Line Drawing 39

10. Profile C C Seismic Record and Line Drawing 40

11. Profile D D' Seismic Record 41

12. Profile D D' Line Drawing 42

13. Profile E E' Seismic Record 43

14. Profile E E' Line Drawing 44

15. Profile F F' Seismic Record 45

16. Profile F F' Line Drawing 46

17. Profile G G' Seismic Record 47

18. Profile G G' Line Drawing 48

19. Structure Contour Map of Apparent Basement 49

20. Location of Faults 50

21. Summary Map Showing Relationship of Faults to
Possible Radial Point and Structural Features 51

ACKNOWLEDGEMENTS

The author wishes to express his appreciation to his
thesis advisors, Dr. Robert S. Andrews and Dr. J, J. von
Schwind of the Naval Postgraduate School, Department of
Oceanography for their professional guidance, provoking
questions, cheerful support, and, especially for allowing me
to conduct the work in a highly independent manner. Thanks
are extended to the Naval West Coast AGOR Pool and especially
to the officers and crews of the USNS BARTLETT (T-AGOR 13)
and the USNS DE STEIGER (T-AGOR 12) without whose co-opera-
tion this work would not have been possible. Funding for
the use of these vessels was provided through the Oceano-
grapher of the Navy. The U. S. Geologic Survey (USGS)
Marine Geology Division, Menlo Park, California, provided
excellent library, equipment, and manpower assistance.
Special thanks go to Mr. H. Gary Green, USGS marine geolo-
gist, who helped co-ordinate inter-agency co-operation and
provided unpublished material relating to the area. The
Naval Postgraduate School Educational Media Department pro-
vided excellent photographic support.

I. INTRODUCTION

A. OBJECTIVE

The objective of this study was to conduct a seismic
survey of the continental margin off Pt. Ano Nuevo , California.
This survey, in combination with previous reconnaisance sur-
veys, future surveys of a similar nature, and other geological
and geophysical information should allow the accurate delinea-
tion of the fault zones in the area and provide some clue to
their interrelationships. Of particular interest is the lo-
cation of the offshore extension of the Sur-Naciemento Fault
Zone, corresponding to the western boundary of the Salinian
Block. The realization of this information sliould prove use-
ful in helping to explain the genesis and evolution of the
gross geologic structure of the region. This survey is a
complement to an ongoing study of the structure and origins
of the continental margin from Point Sur to the Farallon
Islands.

B. AREA DESCRIPTION

The area surveyed extends from the Monterey Submarine
Canyon in the south to approximately 80 Km north at latitude
37°20'N and laterally from the coast to a line approximately
70 Km offshore (Fig. 1) .

In general, the area is a sedimentary basin on the
Salinian Block. The basin is a post-middle Miocene syncline
[Hoskins and Griffiths, 1971] with its major axis plunging

nearly continuously to the northeast. The proximity of
granite onshore and offshore, particularly on the Farallon
Ridge, lead to the tenative conclusion that the basement is
granite throughout.

The basin is marked by evidence of a severe erosional
history. There are generally marked angular unconformities
between the Cretaceous and younger rocks. Middle Miocrene
and older strata in the basin show much more evidence of
faulting and folding than do younger strata which were com-
paratively free of evidence of such tectonic activity.

C. REGIONAL GEOLOGY

The region is probably part of the transform fault sys-
tem along which the Pacific and North T^jnerican Plates are
slipping relative to each other (Fig. 2). About 30 Km to
the east of the survey area is the San Andreas Fault which
generally marks the boundary between the continental crust
of the Salinian Block to the west and the oceanic crust of
the Franciscan Assembage to the east.

The eastern edge of the survey area is dominated by the
relatively well defined Palo Colorado-San Gregorio fault
zone. This zone appears to extend to the south from the
San Gregorio fault on land to the Carmel Canyon fault off-
shore and then to the Palo Colorado fault near Kaslar Point
[Green et at., 1973]. In the survey area it is approximately
4 Km wide. The main feature is a reverse fault which marks
where the Miocene Monterey Formation has been thrust to the
southwest over the Pleistocene marine terrace deposits

[Clark, 1970] . Offshore it separates the Pliocene Purisma
Formation and the Miocene Monterey mudstone. Analysis of
earthquake motion along the fault since 1969 indicates
right lateral slip motion along nearly vertical fault planes
[Green et al . , 1973].

Similar orientation and motion is noted in the numerous
faults of the Monterey Bay fault zone. All these faults
generally have a north west-southeast trend and many of them
show disturbance of sediments up to within 6m below the sea-
floor indicating that they have been active in the recent
past [Green et at. 3 1973].

The southern edge of the survey area is dominated by the
Monterey Bay Canyon Systems (Fig. 3). The highly irregular
and steep topography in the area makes seismic reflection
profiling difficult at best and has prevented the tracing
of even major faults across its axis.

In the western edge of the survey area is the offshore
extension of the Sur-Naciemento fault zone marking the bound-
ary between the Salinian Block and an offshore Franciscan
Assemblage. The 1000m isobath passes through the western
edge at the area in an irregular line extending northwest
and southeast and roughly represents the offshore edge of
a ridge which forms the western boundary of the sedimentary
basin (Fig. 1). Just to the west of the survey area are
several seamounts (Fig. 3). The most prominant of these.
Pioneer and Guide, have been dredged and appear to be pri-
marily basalt [Chesterman, 1952; Andrews, Personal Communica-
tion, 1974].

At the northern edge of the survey area is the Pioneer
Canyon. To the north of that is the Gulf of the Farallons.
In the Gulf, the continental shelf is divided into two struc-
tural platforms by the offshore extension of the Seal Cove
Fault. To the east is the Golden Gate Platform marked by
the San Andreas and Pilarcitos faults. To the west is the
Farallon Platform which is characterized by thick Tertiary
sediments bounded on their western edge by a ridge of Creta-
ceous granite rock which outcrops in the Farallon Islands
[Cooper, 1971]. This ridge, which extends northward to the
Cordell Bank, is thought also to extend southward into the
survey area [Curray, 1965].

The granite basements, assumed to underlie the entire
Salinian Block, imparts rigidity to the region and leads to
a block-faulting structure in the over-lying sediments
[Hoskins and Griffiths, 1971].

Most of the continental margin in and around the survey
area has an unconsolidated sediment overburden. This sedi-
ment is mostly green and grey sands and muds [Uchupi and
Emery, 1963] .

Two regions of magnetic anomaly are found in the survey
area. The Pigeon Pt. High along the edge of the San Gregario
fault zone and the Santa Cruz High over the ridge along the
upper edge of the continental slope. The Farallon Islands to
the north are also a region of high magnetic anomaly.

D. TECTONIC DEVELOPMENT

The Sur-Naciemento fault zone which lies on the western
edge of the survey area has been interpreted by Page [1970]
as being a former subduction zone. This fault zone and a
similar fault and subduction zone separating Franciscan and
Siefran granitic rocks under the Great Valley may represent
an initially continuous plain which was offset by the San
Andreas fault. Wentworth [1968] has suggested a minimum off-
set of approximately 600 Km beginning in the late Cretaceous.

It has been suggested by Silver, Curray and Cooper [1971],
that the first stage in the late Cretaceous involved lateral
movement along the ancestral trend of the San Andreas fault
(Fig, 4). Atwater [1970] suggests under thrusting beneath
the Central California Continental Margin was occurring from
the early Tertiary until the early Miocene when the Pacific
and North American Plates came into contact at which time
the under thrusting ceased. The remaining offset of the San
Andreas fault system has occurred since the early Miocene,
possibly in two stages as suggested by Suppe [1970]. The
sedimentary basin of interest probably formed during this
period of time.

E. PREVIOUS AREA INVESTIGATION

The area surveyed has been of interest, for various rea-
sons, for a long period of time. Soon after California be-
came part of the United States of America, the Coast and
Geodetic Survey commenced bathymetric work which has con-
tinued to the present. In 1891, the U.S.S. ALBATROSS made a

transect while surveying for a submarine cable route. The
ALBATROSS returned to the area in 1904 and surveyed sporad-
ically until 1920 concerning itself mostly with biology,
but gathering general geological data as well.

Investigation was renewed by the Scripps Institution of
Oceanography [Shepard and Emery, 1941; Shepard, 1948] and
the California Academy of Sciences [Hanna, 1952; Chesterman,
1952].

The major effort began in the 1960 's [Uchupi and Emery,
1963; Martin, 1964; Curray, 1965; Rusnak, 1966; Martin
and Emery, 1967; Hoskins and Griffiths , 1971; Silver et at.
1971; Green et at. 1973; Spikes, 1973]. To date, the in-
vestigations have been essentially reconnaisance studies
concerned with the major regional structures of the central
California shelf.

The land geology of adjacent and related structures has
been studied at great length, starting with the initial
mapping by Johnson in 1855. Of particular interest is the
work of Clark (1970) who described the onshore geology in
the region near Pt. Ano Nuevo. The Sur-Naciemento fault
system has been studied at some length most notably by Trask
(1926) and Page (1970) .

Offshore, the most notable surveys have been the fairly
general works of Shepard, Martin, Emery, and Rusnak, who
concerned themselves with the topography of the continental
margin; Curray, who concerned himself with the general
geologic structure of the region; Hoskins and Griffiths, who

looked at the stratigraphy of the area in question; Green
et at. J who depicted the crustal structure of Monterey Bay
in some detail, giving special attention to offshore faults;
and Spikes, who did a gravimetric survey of the near shore
region from Pt. Ano Nuevo to Santa Cruz,

II. COLLECTION OF DATA

A. SURVEY PROCEDURE

The data analyzed in this paper was gathered in three
separate cruises using U. S. Navy AGOR class oceanographic
research vessels operated by civilian crews (Fig, 5).

The first o£ these cruises was in November o£ 1972 on
the USNS BARTLETT (T-AGOR 13). This cruise was a co-opera-
tive effort of the U. S. Geologic Survey (USGS) and the
Naval Postgraduate School (NPS) . It was primarily a recon-
naisance survey of the continental margin from the Farallon
Islands to Pt. Sur. Segments of three tracks from this
cruise were in the area of interest. The primary seismic
information utilized from this cruise was obtained using a
USGS Marine Geology Division 160 KJ arcer system. Hydro-
phone signals were processed with a 25-98 Hz bandpass filter.
A 4-sec firing rate was used throughout.

The second cruise was in November of 1973 on the USNS
DE STEIGER (T-AGOR 12). This cruise was an NPS continental
margin study from Pt. Sur to Half Moon Bay. The third cruise
was on the USNS BARTLETT in April of 1974 and was undertaken
to augment data from the previous cruises.

On the second and third cruises the primary seismic in-
formation utilized came from a 30 KJ Teledyne Arcer Seismic
System. The system was operated with a 4-sec firing rate at
16 KJ. Hydroplione signals were processed with a 63-125 Hz
bandpass filter.

On all three cruises, a 3.5 KHz normal incidence sonar
system was run for high resolution of surface sediment and
a proton precession magnetometer was towed. Ship speed
varied from 5 to 8 knots depending on weather conditions.

B. NAVIGATION

In the area surveyed, accurate navigation was a problem.
A combination of factors served to degrade the accuracy of
position finding. Visual navigation was hampered by the
prevelance of coastal fog and the paucity, especially at
night, of prominent landmarks. LORAN coverage in the area
is limited to one dependable line. Much of the survey was
conducted outside effective radar navigation range. During
the third cruise, tlie Navigational Sattelite (NAVSAT) equip-
ment installed on the AGOR was erratic in its performance.

In general, the navigation was handled by using NAVSAT
as the primary source. This was used to correct a plot
which usually consisted of dead reckoning and one LORAN line,
When possible, visual and radar lines were .added.

The navigation was checked and hand corrected in the
laboratory using all available inputs to rectify errors.
Within 10 Km of shore, the rectified navigation is generally
accurate to ±h^m. This accuracy degrades steadily offshore
to ± 3-4 Km in the western edge of the survey area.

III. ANALYSIS OF DATA

A. INTERPRETATION NOTES

Sound speed in the water column was determined from an
XBT trace. The average speed for the water column was ap-
proximately 1.5 Km/sec. No sound speed data was available
for the sub-bottom layers in the sedimentary basin in ques-
tion. After considering the age, depth, and probable com-
position of the sedimentary rock, it was decided to use a
value of 3.0 Km/sec as the mean speed. Any depths given in
meters will be based on these two assumptions. All travel
times given in the body of this paper are oneway travel
times .

Insufficient data was available to determine whether
faults in the area were dip-slip or strike-slip. In all
cases where up or down is indicated along a fault it is a
best estimate of the present relationship of the bedding on
either side. The mechanism by which the juxtaposition took
place is not implied. Faults in the area are probably mainly
strike-slip [Green et at. 1973] with some possible dip-slip
motion.

In analyzing the major faults, the assumption was made
that they would form continuous, near vertical traces, paral-
lel or nearly parallel to the Palo Colorado-San Grcgorio
fault. This assumption was based on the nature of previously
identified faults in the region and the expected block fault-
ing effect of the under- lying Salinian Block. When no

evidence to the contrary was available, this assumption was
used to infer the location of fault zones.

High resolution data was used only occasionally to verify
the locations of surficial evidence of faulting such as
scarps. Magnetometer data was used to aid in the identifica-
tion of faults from one trackline to the next by comparing
anomalies at fault boundaries.

B. GENERAL STRUCTURE

The area studied can be divided into three basic struc-
tures; a ridge on the eastern edge, a central sedimentary
basin, and a ridge on the western edge (Fig. 6). The entire
area has been subjected to moderate to complex folding and
faulting. Although they are all inter-related, each of the
structures and individual major fault zones will be discussed
separately. Features discussed can be seen in Fig. 7 thru
Fig. 18.

1. The Eastern Ridge

The eastern ridge is probably an extension of the
Farallon Ridge and is, therefore, probably granitic rock.
In the survey area, its boundaries correspond well to those
of the Pigeon Point region of high magnetic anomaly. The
ridge is complexly folded and faulted, specially in the
first 10 Km on the western side of the Palo Colorado-San
Gregorio fault zone. The sediment over- lying the ridge is
thin. Generally the deepest identifiable sedimentary layers
varied from 0.1 to 0.2 sec of one-way travel time (300 to
600m in thickness) depending on the degree of folding.

These sedimentary layers are probably of the same nature as
the Pleistocene, Pliocene, Miocene, Upper Cretaceous Marine
series seen on land to the west of the San Gregario fault
north of Point Aiio Nuevo. The complex folding and faulting
of the ridge precludes any realistic extension of the land
structure to the sub-bottom based on available profiles.
Most of the faults on the ridge penetrate to the basement.
The western edge of the ridge drops off relatively steeply
(10 to 15° slope) into the sedimentary basin.

2 . The Sedimentary Basin

A post-Middle Miocene syncline dominates the center
of the survey area. A structure contour map of the deepest
identifiable horizon (believed to be basement) (Fig. 19) shows
the major axis plunging fairly regularly (approximately 3°
slope) to the northeast. On the syncline axis, sediment
thickness increases from approximately 0.1 sec (300m) near
Santa Cruz in the south to 1.0 sec (3 Km) at 37°20'N. As it
deepens, the basin broadens from its 20 Km width in the south
to slightly more than 60 Km at 37°20'N. Evidence of faulting
is found throughout the basin. Most of the faults in the
sedimentary basin appear to be inactive and few penetrate to
the surface layers. The slope of the western side of the
basin rises at a more moderate angle (6 to 10°) to the western
ridge. A good description of the most probable basin strati-
graphy is available in Hoskins and Griffiths (1971).

3 . The Western Ridge

• Over the western ridge the sediment thins until the
sedimentary layer is thinner than the seismic source bubble

pulse. Sediments there are probably less than 100m. The
under-lying rock is lacking in structural detail and is
probably high grade metamorphic or granitic. Hoskins and
Griffiths (1971) indicate that it is probably Cretaceous.
Curray (1966) indicates that it is probably granite.

The ridge has an irregular surface with several
small sedimentary basins in evidence (Fig. 11). Several
faults of varying ages are in evidence along the ridge. The
ridge averages 15 to 20 Km in width and terminates at, or is
truncated by, the Ascension Canyon and the Pioneer Canyon in
the south and the north, respectively. The ridge corresponds
generally to the region of magnetic anomaly known as the
Santa Cruz High (Fig. 6).

C. FAULT ZONES

Nine fault zones were located and traced in the survey
area. Each zone was numbered starting from shore (Fig. 20).
The faults divide the area into two regions. Those to the
east of fault 4 are in a region of complex folding and
faulting and seem to be approximately parallel to the Palo
Colarado San Gregario fault. From fault 4 to the west,
the faulting and folding are less complex. The faults here
seem to radiate from a common point in Monterey Bay. In
the following sections each of the faults will be discussed
separately. The locations of the designated faults can be
seen in the seismic profiles in Fig. 8 through 18.

1. Fault No. 1

Fault 1 is the previously known and charted Palo
Colorado-San Gregario fault zone in the southern part of
the survey area. It is a series of high angle faults spread
over a 4 Km width. To the north of Pescadero Point and
into Half Moon Bay it is a wider series of faults. Fault 1,
as shown on Figs. 8, 9 and 18, is slightly to the west of
the charted location of the Seal Cove fault and may be the
Seal Cove fault or a related parallel fault. The turn points
on the track lines were at the fault line and the exact na-
ture of the interrelationships of the faults near shore in
Half Moon Bay was obscured.

2. Fault No. 2

In the southern portion of the survey area, fault 2
is a zone of numerous high angle faults approximately 5 Km
wide. Generally, reflectors on the west side are lower than
their counterparts on the east. In the complex folding off
Pigeon Point the location of the fault is obscure. Assuming
a linear trace, it is thought to pass approximately 3 Km off
Pigeon Point and connect to a less complex fault zone to the
north.

The northern portion of the fault zone generally ap-
pears as one or two distinctive faults. The vertical dis-
placement in the north seems to be the opposite of that to
the south with the western side being generally elevated.
The irregularity from track to track of the relationship of
strata on opposite sides of the fault, with first one side

then the other in an elevated position, suggests that the
apparent vertical displacement of strata is probably a re-
sult of the horizontal displacement of strata of irregular
contour by strike-slip rather than a result of vertical dis-
placement by dip-slip. Throughout the survey .area, the
fault zone penetrates from near surface to the apparent base-
ment. No evidence was seen of recent activity of the fault.

3. Fault No. 5

Throughout the survey area. Fault 3 appears as a pair
of nearly parallel faults with separation varying from 0.5
to 5 Km. Generally, the area between the faults is elevated
with respect to the surrounding strata. The trace of the
fault is distinct except in the area of complex folding and
faulting off Pigeon Point. The faulting extends from ap-
proximately 300 m below the sea floor into the apparent base-
ment and approximately parallels the western edge of the
crust of the eastern ridge.

4. Fault No. 4

Fault 4 marks the western extreme of the area of com-
plex folding and faulting off Pigeon Point. It is readily
identified throughout the survey area except at the extreme
southern end where it becomes obscure near the Ascension
Canyon. The fault generally shows as a single high angle
fracture with the west side depressed and the east side
elevated. In the north it extends from approximately 300m
below the sea floor into the apparent basement while in the
south it reaches the surface. It appears to form an eastern

boundary for two small branches of Ascension Canyon. For
a short distance it appears to break the surface with a
definite scarp in evidence (Fig. 14).

5. Fault No. 5

Fault 5 is a distinct trace throughout the central
section of the survey area being less distinct in the south
and north. The fault shows as a single fracture with the
western side depressed. In the north it extends from about
200m below the sea floor to the apparent basement. In the
south it appears to reach the surface but there is no evidence
of scarp. This fault appears to merge with fault 4 near
Ascension Canyon.

6. Fault No. 6

Fault 6 appears as a very definite double fault in
the north, extending from very near the sea floor to the
basement (Fig. 11). To the south it is less distinct and
generally appears as a single fracture. It appears to reach
basement throughout. To the south it is generally found 200
to 400m below the sea floor. Throughout its length, it is
depressed on the western side. Its location is obscure in
the central section of the survey area.

7. Fault No. 7

Fault 7 is not easily seen in any of the profiles;
its trace is faint but discernible in all of the profiles.
This fault appears as a single fracture which is confined
to the middle of the sedimentary layers. It generally ex-
tends from 500m below the sea floor to 200 to 1500m above

the basement. The western side is generally elevated through
several lines indicated a reverse orientation.

8. Fault No. 8

This fault lies near the crest of the western ridge.
Its vertical extent is from near surface or surface into
the basement. It appears as one to three fractures with the
west predominantly depressed. Occasionally a surface scarp
is in evidence (Fig. 10). The trace is very distinct in the
north becoming obscure as Ascension Canyon is approached.

9. Fault No. 9

Appearing on the western slope of the western ridge
only a short section of this fault was seen. It seems to
extend from the sea floor to the basement. From one to
three distinct fractures are seen with the west predominantly
elevated. Possible surface scarps were noted in the high
resolution profile corresponding to Fig. 9.

IV. DISCUSSION

A. SUR-NACIEMENTO FAULT ZONE

It has been suggested, based on the granitic composition
of the Farallon Ridge, that the Sur-Naciemento fault zone
passes to the west of the Farallon Islands. If the Santa
Cruz High represents a granitic composition of the western
ridge in the survey area, following the same logic, the
Sur-Naciemento fault should pass to the west of the ridge.
This would place the fault much further offshore than has
been previously suggested. Verification of the nature of
the basement material on the western ridge would be useful
in determining its relationship to Llie regional geology.
Such verification might be obtained through dredging the
sourth wall of the Pioneer Canyon or the northwest walls
of Ascension Canyon.

If the Sur-Naciemento fault passes to the west of the
Santa Cruz High, it is outside the area of this survey.
If the ridge is made up of high grade Cretaceous metamor-
phic rock, or, if the presence of Mesozoic granitic rocks
to the west of the Sur-Naciemento fault zone is accepted,
one of the nine fault zones noted in the survey is probably
the Sur-Naciemento.

Fault 4 appears to be amongst the most important faults
in the area. As was previously noted, it marks the dividing
line between the complex folding and faulting to the east
and the simpler basin to tlic west. If extended straight to

the soutk it meets the Sur-Naciemento fault at Point Sur,
In the survey area, it generally lies along the west edge
of the eastern ridge which is probably the southern exten-
sion of the Farallon Ridge. If faults 4, 6, 7, 8 and 9 are
all extended in straight lines they converge in a small area
centered at 36°42'N, 122°12'W (Fig. 21). This area is ap-
proximately 5 Km north of the axis of the Monterey Submarine
Canyon and 6 to 8 Km to the west of an earthquake epicenter
cluster reported by Green et al. (1973). Several of the
faults show evidence of possible recent activity.

Fault 4 seems to be a high angle reverse fault with the
western side depressed. Faults 5 and 6 show similar rela-
tive displacement. These could represent a subduction zone.
The suggestion by Page (1970) that the Sur-Naciemento is a
subduction zone with successive movements in several dif-
ferent epochs makes faults 4, 5 and 6 attractive candidates
for the Sur-Naciemento. Page (1970) noted that the Sur-
Naciemento fault on land shows modest dip separation in the
Miocene and younger formations. Similar apparent displace-
ment is noted in faults 4, 5 and 6.

The hypothesis that one or more of these faults is an
extension of the Sur-Naciemento would be reinforced by the
determination that the western ridge is an upper Jurrasic
to Mid-Cretaceous Franciscan eugosynclinal assemblage rather
than Mesozoic granite.

If the western ridge is metamorphic rock, faults 8 and
9 also fit the general pattern of the Sur-Naciemento fault

as described by Page (1970). To match the Sur-Naciemento
on land a missing sedimentary zone would be required to the
west and the apparent orientation of fault 9 is the reverse
of that expected.

B. PIGEON POINT AREA

The region of complex faulting and folding off Pigeon
Point is important in the overall regional structure. If
fault 4 has been a region of compression and subduction
through the late Cenozoic, it is plausible that these folds
and faults are a product of this compressional stress. If
fault 1 or faults 1 and 4 have been experiencing strike
slip motion, the folds may be drag-folds. The features
found off Pigeon Point are probably a product of both these
mechanisms.

C . SUMMARY

If the western ridge is granite, the Sur-Naciemento
fault probably passes to the west of the survey area. On the
other hand, if the western ridge is high grade metamorphic
rock, the Sur-Naciemento fault zone is probably a combination
of faults 4 and 5. The existence of the synclinal basin
could then be attributed to a depression caused by the sub-
duction of oceanic crust at the edge of the continental plate
from the Tertiary or late Cretaceous until tlie early Miocene.
At this point subduction ceased and strike-slip began along
the plate boundaries. From then until the present, the area
moved northward and filled with sediment. It probably

experienced alternate deposition and erosion during periods
of uplift in the Pliocene and mid-Pleistocene. Some of the
high angle faults were probably formed during these periods.

Strike-slip motion while centered in the San Andreas
fault system, also occurred in subsidiary faults including
those in the survey area. Fault 1, the Palo Colorado-San
Gregorio, is known to be right-lateral strike-slip.

The folding and faulting in the Pigeon Point fault zone
probably occurred in the mid-Pleistocene as a function of
uplift, compression, and drag folding.

D. FUTURE WORK

The location and orientation of fault zones on the con-
tinental margin to the south of Monterey Canyon should be
analyzed and projected through the canyon and on to the north.
A seismic profiling program carried out from 37°20'N to 37°40'N
and from the coast to 123° 25 'W would provide a connection be-
tween the survey in this report and that of the Gulf of the
Farallons [Cooper, 1971]. Such a survey would allow confir-
mation of the location of the Farallon Ridge and indicate
whether the western ridge continues beyond Pioneer Canyon,
The northern reaches of the basin could be charted and the
fault zones traced further to the north. Of particular in-
terest are the traces of faults 4, 5,7 and 8 and an answer
as to whether faults 4 and 5 pass to the east or the west
of the Farallon Islands. Dredging should be conducted as
previously noted. The area shows signs of recent seismic
activity, though no reference to earthquake epicenters in

the area was found. Bottom seismographs in the survey area
specially near faults 4, 5 and 6 would indicate v\^hether the
area is still seismically active.

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Figure 2. Regional Map of Central California Showing Major
Fault Zones. After Brov\/n and Lee (1971), Silver
et at. (1971), Cooper (1971).

Cordel
Bank
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Farallon % ^ ,,
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Islands ^

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122 W

Figure 3. Regional Map of Central California Showing Place
Names .

late Crefaceous

Present

.f^'

^ ^ k ^ \

■JrSan Andreas Fault
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i Franciscan Rocks
iForallon Islands
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Figure 4. Hypothetical Regional Development as a Basalt
Floored Rift Resulted From Right Slip Movement
Between Echelon I-aults in Late Cretaceous Time,
After Silver et al . [1971]

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Fault Location
Inferred Location

Outline of Structural Features

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Figure 21. Suminary Map Showing Relationship of Faults to
Possible Radial Point and Structural Features

BIBLIOGRAPHY

Atwater, T,, 1970, Implications of Plate Tectonics for
the Cenozoic Tectonic Evolution of Western North America:
Geol. Soc. America Bull. v. 81, no. 12.

Brown, R. D. and Lee, W. H. K., 1971, Active Faults and
Preliminary Earthquake Epicenters (1969-1970) in the Southern
Part of the San Francisco Bay Region: San Francisco Bay
Region Environment and Resources Planning Study, BDC #30,
(Unpublished Report) .

Chesterman, C, W. , 1952, Descriptive Petrology of Rocks
Dredged off the Coast of Central California: Proc. Calif.
Acad. Sci,, v. 27, no. 10.

Clark, J. C, 1970, Preliminary Geologic and Gravity
Maps of the Santa Cruz --San Juan Bautista Area, Santa Cruz,
Santa Clara, Monterey, and San Benito Counties, California:
U.S. Geol. Survey open File Report (Unpublished Report).

Cooper, A., 1971, Structure of the Continental Shelf West
of San Francisco, California: MS Thesis, San Juse State
College, San Jose, California, p. 65, (Unpublished Report).

Curray, J. R. , 1965, Structure of the Continental Margin
off Central California: New York Acad. Sci. Trans., sec. II,
v. 27 , no. 7.

Dobrin, M. B, 1960, Introduction to Geophysical Prospect-
ing, 2nd ed. , McGraw-Hill Book Co., Inc., New York, p. 446.

Green, H. G., Lee, W. H. K., McCulloch, D. S., and Brabb,
E. E., 1973, Faults and Earthquakes in the Monterey Bay Region
California: San Francisco Bay Region Environment and Resources
Planning Study, BDC #58, (Unpublished Report).

Hanna, G. D., 1952, Geology of the Continental Slope off
Central California: Proc. Calif. Acad. Sci., v. 27, no. 9

Hoskins , E. G., and Griffiths, J. R. , 1971, Hydrocarbon
Potential of Northern and Central California Offshore, in
Possible Future Petroleum Provinces of the United States -
Their Geology and Potential": Am. Assoc . Petroleum Geologists
Memoir 15, v. 1.

Martin, B. D., 1964, Monterey Submarine Canyon, California:
Genesis and Relationship to Continental Geology, Ph.D Disserra-
tion. University of Southern California, Los Angeles, p. 249,
(Unpublislied Report) .

Martin, B. D. and Emery, K. 0., 1967, Geology of Monterey '
Canyon, California: Am. Assoc. Petroleum Geologists Bull.,
V. 51.

Page, B. M. , 1970, Sur-Naciemento Fault Zone of California:
Continental Margin Tectonics: Geo. Soc. America Bull., v. 81,
no. 3.

Rusnak, G. A., 1966, The Continental Margin of Northern
and Central California, in Geology of Northern California:
Calif. Div. of Mines and Geology Bull. 190.

Shepard, F. P., 1948, Investigation of the Head of Monterey
Submarine Canyon: Scripps Inst. Oceanography, Submarine Canyon
Report 1, p. 15.

Shepard F. P. and Emery, K. 0., 1941, Submarine Topography
Off the California Coast: Canyons and Tectonic Interpretations;
Geo. Soc. America Spec. Paper No. 31.

- Silver, E. A., Curray , J. R. and Cooper, A. K. , 1971, Tec-
tonic Development of the Continental Margin Off Central Cali-
fornia, Geol. Soc. of Sacremento, Calif. Guidebook for 1971.

Spikes ; C. H.. 1973, A Gravimetric Survey of the Santa
Cruz-Aiio Nuevo Point Continental Slielf and Adjacent Coastline:
MS Thesis, Naval Postgraduate School, Monterey, California,
p. 114 (Unpublished Report).

Suppe, J., 1970, Offset of Late Mesozoic Basement Terrain
by the San Andreas Fault System: Geo. Soc. America Bull.
V. 81, no. 11.

Trask, P. D., 1926, Geology of the Point Sur Quadrangle,
California: Univ. Calif. Publ. Bull. Dept. Geol. Sci., v. 16,
no . 6 .

Tucker, P. M. and Yorston, H. J,, 1973, Pitfalls in Seismic
Interpretation, Soc. of Exploration Geophys. Monoyram Series #2,

Uchupi, E., and Emery, K. 0., 1963, The Continental Slope
Betv\/een San Francisco, Calif, and Cedros Island, Mexico: Deep
Sea Research, v. 10, no. 4.

Wcntworth, C. M. , 1968, Upper Cretaceous and Lower Ter-
tiary Strata near Gualala, California and Jnfered Large Right
Slip on the San Andreas Fault in Dickinson, W. R. and Grantz,
A., Editors, Proceedings of the Conference on Geologic Problems
of the San Andreas Fault System: Stanford Univ. Pubs. Geol.
Sci. V. 11.

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Structural geology of
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fornia.

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Structural geology of the continental ma

3 2768 001 90069 9

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