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 by David Dexter Frydenlund September 1974 Thesis Advisors: J. J. von Schwind R. S, Andrews Approved for public release; distribution unlimited. T163256 UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (Whtn Dmim En(»r#dJ REPORT DOCUMENTATION PAGE READ INSTRUCTTONS BEFORE COMPLETING FORM 1. REPORT NUMBER 2. GOVT ACCESSION NO 3. RECIPIENT'S CATALOG NUMBER 4. J\Tl.E (and Subline) Structural Geology of the Continental Margin off Pt . Ano Nuevo , California S. TYPE OF REPORT & PERIOD COVERED Master's Thesis; September 1974 6. PERFORMING ORG. REPORT NUMBER 7. AUTHORft; David Dexter Frydenlund *. CONTRACT OR GRANT NUMBERffJ 9. PERFORMING ORGANIZATION NAME AND ADDRESS Naval Postgraduate School Monterey, California 93940 10. PROGRAM ELEMENT, PROJECT TASK AREA ft WORK UNIT NUMBERS II. CONTROLLING OFFICE NAME AND ADDRESS Naval Postgraduate School Monterey, California 93940 12. REPORT DATE September 1974 13. NUMBER OF PAGES 55 U. MONITORING AGENCY NAME 4 ADDRESSf// d(//er»n( /rom Conlrolllng Olllce) Naval Postgraduate School Monterey, California 93940 15. SECURITY CLASS, (ol Ihim riport) Unclassified I5«. DECLASSIFICATION/ DOWNGRADING SCHEDULE 16. DISTRIBUTION STATEMENT fo/ l/i/« R»por(; Approved for public release; distribution unlimited 17. DISTRIBUTION STATEMENT (ol Iht mburact entered In Block 30, II dllltrmnt from Rmport) 18. SUPPLEMENTARY NOTES 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 DD 1 JAN 73 1473 EDITION OF 1 NOV 65 IS OBSOLETE (Page 1) S/N 0102-OM-6601 I .. UNCLASSTFIFD SECURITY CLASSIFICATION OF THIS PAGE (When Date Kniered) UNCLASSIFIED tVCUltlTY CLASSIFICATION OF THIS PAOEfHTi.n £)»(• Enl.r.cf. Block #19 Continued Palo Colorado-San Gregorio Fault Zone DD ^ yovm^ 1473 (BACK) UNCLASSIFIED S/N 0102-014-6601 2 security classification of this PAGE(T»T.»n D»(« em.r.dj Structural Geology of the Continental Margin Off Pt. Ano Nuevo, California by 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 F' 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 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 10 [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]. 11 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. 12 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 13 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 14 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, 15 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. 16 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. 17 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 18 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. 19 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 20 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. 21 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 22 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 23 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 24 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. 25 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 26 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 27 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 28 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 29 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. 30 E >^ _E O CO (U *^ a E "i CL C < C o c _o o u o D 0) Q) > to 0) 31 Figure 2. Regional Map of Central California Showing Major Fault Zones. After Brov\/n and Lee (1971), Silver et at. (1971), Cooper (1971). 32 Cordel Bank -38 N\\ Farallon % ^ ,, , , I % Farallons Islands ^ Pioneer Seamount Q •••«^ » * 1 Guide Seamount ,123 W 122 W Figure 3. Regional Map of Central California Showing Place Names . 33 late Crefaceous Present .f^' 4 ^ ^ k ^ \ ■JrSan Andreas Fault '•* .Gualala Basin i Franciscan Rocks iForallon Islands :o Block \\'.'''.7'/\'/^ iSIerra Nevada "A wM ■■/ • : /tV^ / Basement ,•, \ ','V/ / iFarallon Islands V'"', i' •/'•"/ / /San Francisc ''■•.V\/' '•/'"■ / /Salinian B ••,' \ \ '< ' i ' ' I / ■ -1 \'\ '•# ••',' '/ K ifMonter ^^ \- ->. K«^ 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] 34 35 36 E ^ -^ n o. CM CN - z K o- CO J. in 0) <4-l o u a, •xi u o •H 37 u «» ^ I- ;:'>?:; 1 ( '11 j>'!|. ISi'i 11" 'Mi y •4-i'iv lipase'-: I pic;::. : , ||, . I'll, I il

•H .J P! nJ •d o o 0) o V) •H Q> CO < O •H m o 00 bO •H 38 Ifc.-vv >'/>:^,.■x^ ■(■ '.vJ=^ ■v:&i' -., ]\i'. t *• ■■-•••; ifr^A^ !"■•■)'''•» liliv^-'.r. a •H ■H hJ PI t:! J-l O o o oi o ■H 6 to •H 0) CO w m o rH •H o f1 * 3 bO •H V-, 39 " i \%M::^::-.'&^'MM>^!^^>i:$i-.^& nil': I- ■Ifil' ] i ■' " ' ' ilii.u:'%iia!!;;?^''-.i.;4' '•■:^;; • i.V''^-^:-i^;f ^^V^^ • H J1 o o ■H •-:] P! oi •r> o o PS o •H (A ■H 0) W u u •H m o ^^ •H 40 / 1 = ' ■ ^■f-^;• v; :;t;::ju.;;- r^~-j. vA ^ ";\^iAV--^>iVv^ • •-•- - ' '^ Ci .• -Js--'-* ''U'"-^'- '^"v- -" -V>>^^w-;•^■^••^^ ^:^s:i^^i:[^''a^ 5t;j'-^-'',-.^ :■' o 41 00 to •H O -f.^ I. >r'?E=S 13 o o o ■H S •H CO W 0 O Oh to •H 43 00 ■o ir> n CN •H (U p; w o (1> •H 44 t3 O U O u •H e to •H (U C/0 SO •H PL, 45 00 hv o lO CO CN •H 12 Q CD U. 0 rH •H m o u vO (D u •H 46 o o '^\mrAf/ ♦*■ a\i\- -•. , iJ u ) ( I i } il' 1. 1; n!isi>uv,^ \\-;,hU'A\'-v' v.v\' ...'>• -{;:■;•.■■ .-.-•'. :V:^ MH - :.".T; '.-.i-^ I T^-i^- I 'c 'VA'- ''','?»'•''.■>• I ' f. -'y-w.' ''•. :■ --:-:;C-" -■^.-i-■ •-■■.if •-•- o o a> 6 0) CO rH •H m o bO •H 47 o <> VI i r CN // / / / / .^.-^ 1 ^ ^ / E o CN °'/ / .^ CM " "" — u-> ^ >. -^ • / «^ O '^ /• • • K- Z io j:5 O - T -JL 49 50 "•57 N A M \ \ % \ \ Pt, Ano Nuevo •X \ \\ \ \ ^ A \^ ^ ^ ^ ^^ Fault Location Inferred Location Outline of Structural Features ""40' Q Possible Radial Point 123 W I 40' 20 JL. Figure 21. Suminary Map Showing Relationship of Faults to Possible Radial Point and Structural Features 51 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) . 52 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. 53 INITIAL DISTRIBUTION LIST No. Copies 1. Defense Documentation Center 2 Cameron Station Alexandria, Virginia 22314 2. Library, Code 0212 2 Naval Postgraduate School Monterey, California 93940 3. COMPT (PTP) 2 U. S. Coast Guard Washington, D. C. 20590 4. Oceanography Department 3 Naval Postgraduate School Monterey, California 93940 5. Dr. R. S. Andrews 10 Department of Oceanography Naval Postgraduate School Monterey, California 93940 6. Dr. J. J. von Schwind 3 Department of Oceanography Naval Postgraduate School Monterey, California 93940 7. Oceanographer of the Navy 1 Hoffman Building Number-2 2461 Eisenhower Avenue Alexandria, Virginia 22314 8. Office of Naval Research 1 Code 480-D Arlington, Virginia 22217 9. Dr. Robert E. Stevenson 1 Scientific Liaison Office Scripps Institute of Oceanography La Jolla, California 92307 10. Professor Warren Thompson 1 Department of Oceanography Naval Postgraduate School Monterey, California 93940 54 11. U. S. Coast Guard Oceanographic Unit Bldg. 159E, Washington Navy Yard Annex Washington, D. C. 20390 12. CDR R. Kollmeyer Department o£ Oceanography U.S. Coast Guard Academy New London, Connecticut 06320 13. LT D. D. Frydenlund U.S.C.G. Oceanographic Unit Bldg. 159E, Washington Navy Yard Annex Washington, D. C. 20390 14. Mr. T, James Lewis Point Reyes Bird Observatory Mesa Road Bolinas, California 94924 15. Mr. George Schaeffer NAVOCEANO Pacific Support Group San Diego, California 92147 16. Library, Code 3330 Naval Oceanographic Office Washington, D. C. 20373 17. SIO Library University of California, San Diego P.O. Box 2 36 7 _La Jolla, California 92037 18. Department of Oceanography Library University of Washington Seattle, Washington 98105 19. Department of Oceanography Library Oregon State University Corvallis, Oregon 97331 20. Commanding Officer Fleet Numerical Weather Control Monterey, California 93940 21. Commanding Officer Environmental Prediction Research Facility Monterey, California 93940 22. Department of the Navy Commander Oceanographic System Pacific Box 1390 FPO San Francisco 96610 55 m 1« "^'Sl IIWERUBMrn LOAI\r Thesis F898 c.l '^ ^- '^' -r-L,S..«V .0.. f « Thesis F898 c.l 1 "^r 1^9 Frydenlund Structural geology of the continental margin off Pt. Ano Nuevo, Cali- fornia. thesF898 Structural geology of the continental ma 3 2768 001 90069 9 DUDLEY KNOX LIBRARY