A BOTTOM GRAVITY SURVEY OF THE CONTINENTAL SHELF BETWEEN POINT LOBOS AND POINT SUR, CALIFORNIA Walter Browne Woodson Na^Pos^raauate Scnoo^ Monterey, California 93940 1 1 1 II I D II n ? a jppp si onterey, California PC i asm ^saaB*> varans ^*mer A BOTTOM GRAVITY SURVEY OF THE CONTINENTAL SHELF BETWEEN POINT LOBOS AND POINT SUR, CALIFORNIA by Walter Browne Woodson, III Thesis Advisor R. S. Andrews J. J. von Schwind September 1973 kppnovzd ion. puhLLc. fintzji^z; dlit/ubiitioyi untimLtzd. T156< A Bottom Gravity Survey of the Continental Shelf Between Point Lobos and Point Sur, California by Walter Browne Woodson, III Lieutenant, United States Navy B.A., University of Mississippi, 1966 Submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN OCEANOGRAPHY from the NAVAL POSTGRADUATE SCHOOL September 1973 Libra Havai ry Postgr, toonter^, lfaauate ,ere* C»»°Z%\°°' ^3940 ABSTRACT From an occupation of 68 ocean bottom and 38 land gravity stations between Pt. Lobos and Pt. Sur, California, a complete Bouguer anomaly map was produced and analyzed. The steps in data reduction leading to the complete Bouguer anomaly field is presented, unique features of which are associated with bottom gravimetry. The geological interpretation of the gravity data shows excellent correlation with earlier seismic records of the proposed offshore extension of the Serra Hill fault, a structure long associated with the Sur-Nacimiento fault zone. Two dimensional models of gravity anomaly profiles were constructed across this fault and another fault located several kilometers to the northwest and extending into the western tributary of the Carmel Canyon. The results indicate a minimum vertical displacement of the basement of approximately 2 km on the southwest sides, It was concluded that these two faults are one in the same. Evidence is presented which indicates that the Palo Colorado fault zone, located approximately 2 km to the east, parallels the Serra Hill fault and subsequently leads into the eastern tributary of the Carmel Canyon. TABLE OF CONTENTS I. INTRODUCTION 10 A. OBJECTIVES 10 B. LOCATION AND TOPOGRAPHY ■ 10 C. PREVIOUS WORK 14 II. SURVEY PROCEDURES 25 A. UNDERWATER GRAVIMETRY 25 1. Equipment 25 2. Calibration 29 B. STATION SELECTION AND LOCATION 33 C. SURVEY OPERATIONS 34 1. Measurement Procedures 34 2. Navigation 37 3. Equipment Reliability 3 8 D. COASTAL SURVEY 38 III. DATA REDUCTION 42 A. OBSERVED GRAVITY 42 1. Instrument Drift Correction 44 2. Earth Tide Correction 45 3. Curvature Correction 45 B. THEORETICAL GRAVITY 46 C. TOTAL UNDERWATER REDUCTION 46 1. Corrections 47 a. Initial Bouguer Correction 47 b. Free-Air Correction 48 c. Secondary Bouguer Correction 49 d. Terrain Correction 50 2. Gravity Anomalies 56 a. Free-Air Anomaly 56 b. Simple Bouguer Anomaly 57 c. Complete Bouguer Anomaly 57 D. PARTIAL UNDERWATER REDUCTION TO COMPARE WITH SEA SURFACE GRAVIMETRY 58 1. Elevation Correction 58 2. Mass Adjusted Free-Air Anomaly 59 E. LAND REDUCTION 59 1. Corrections 59 a. Bouguer Correction 59 b. Free-Air Correction 60 c. Terrain Correction 60 2. Gravity Anomalies 62 IV. DATA PRESENTATION AND ANALYSIS 63 A. GENERAL DISCUSSION 63 B. CBA ANALYSIS 65 C. TWO-DIMENSIONAL PROFILES 71 D. CONCLUSIONS _- 89 4 V. FUTURE WORK 92 APPENDIX A: DATA REDUCTION CORRECTION VALUES FOR INDIVIDUAL STATIONS 93 APPENDIX B: VARIOUS GRAVITY ANOMALIES AND LOCATIONS FOR INDIVIDUAL STATIONS 98 APPENDIX C: COMPUTER PROGRAM 103 REFERENCES CITED 106 INITIAL DISTRIBUTION LIST 109 FORM DD 1473 ppp LIST OF TABLES Table Page I Explanation of Geologic Abbreviations 21 II Possible Errors in Complete Bouguer Anomaly Calculation (values in milligals) 66 LIST OF FIGURES Figure 1 Bathymetry of the Survey Area 11 2 Survey Area Limits and Location 13 3 Geological Boundaries of the Salinian Block (after Greene et al. , 1973) 16 4 Regional Fault Map (after Greene et al. , 1973) 19 5 Regional Geology and Fault Location (after Dohrewend, 1971) (geologic abbreviations listed in Table I) 20 6 Simplified Diagram of the LaCoste and Romberg Gravimeter (after LaCoste, 1967) 26 7 Model HG6 Gravimeter Ready for Use 27 8 Internal View of the Model HG6 Gravimeter 28 9 Schematic Diagram of the Auxiliary Equipment 30 10 Auxiliary Equipment Installed Aboard the R/V ACANIA 31 11 Naval Postgraduate School's Oceanographic Research Vessel R/V ACANIA 32 12 Ocean Bottom and Land Station Density and Location 35 13 LaCoste and Romberg Model G-08 Geodetic Land Gravimeter 40 14 Schematic Representation of the Steps Necessary to Compute the Elevation Correction ( O = density in grams per cubic centimeters) 51 15 Schematic Diagram Showing Areas Involved in Terrain Corrections for Ocean Bottom Stations 54 16 CBA Distribution for the Continental Shelf and Adjacent Coastline Between Pt. Lobos and Pt. Sur (values in milligals, contour interval 2 mgal) 64 17 PDR Profile Locations and Fault Scarp Position from a Bathymetric Study 72 18 PDR Profile A_A' 73 19 .PDR Profile B-B» 74 20 PDR Profile D-D' 75 21 PDR Profile E-E' 76 22 PDR Profile F_F' 77 23 PDR Profile H_H' 78 24 Location of Two-Dimensional Modeling Profiles 79 25 Depth to Basement of Profile Model A_A* 82 26 Calculated and Observed Gravity for Profile A-A1 83 27 Depth to Basement of Profile Model B-B1 84 28 Calculated and Observed Gravity for Profile B-B1 85 29 Depth to Basement of Profile Model C-C 86 30 Calculated and Observed Gravity for Profile C-C 87 31 Summary Fault Map Indicating Proposed Locations 90 ACKNOWLEDGEMENTS The author wishes to express his appreciation for the professional guidance and cheerful support provided by his thesis advisors, Dr. Robert S. Andrews and Dr. J. J. von Schwind of the Naval Postgraduate School (NPS) Department of Oceanography. To Capt. "Woody" Reynolds and the crew of the R/V ACANIA go a heartfelt thanks for their timely assistance in all aspects of the gravity survey operations and for their never-ending good sense of humor. Dr. Howard Oliver, Dr. S. L. Robbins, and Mr. Richard Farwell of the U. S. Geological Survey (USGS) provided the land gravimeter, computer programs, and much helpful information needed to conduct the survey. Mr. H. Gary Greene, USGS marine geologist, made available much unpublished material pertaining to the local geology of the survey area. Acknowledgements are also made to Mr. 'Yogi' Parks and Mr. Joe Bighorse of LaCoste and Romberg for their patience and guidance in the maintenance of the underwater gravimeter. ST1 Rick Desgrange and Mr. Dana May- berry of the NPS Oceanography Department were essential in maintain- ing the underwater gravimeter in working order. The NPS Public Works Department and Mr. Pete Wisler provided necessary assistance on a number of occasions. Finally, this work would not have been possible without the assistance of Lt. Henry Spikes, co-worker and good friend. Partial funding for this project was provided by NPS Research Foundation from Office of Naval Research Resources. I. INTRODUCTION A. OBJECTIVES The continental shelf between Point Lobos and Point Sur, Cali- fornia (Fig. 1), is an area in whicb the geology has only been super- ficially examined. In contrast, the coastal region of this part of the California coast has been studied extensively in the past, and continues to be in the present, due to increased public awareness of probable future earthquakes and proposed residential and commercial construction. Seismically active fault zones exist along the continental shelf of Central California. The area between Pt. Lobos and Pt. Sur must be included as one of these zones. Although no major earthquakes have occurred in this region throughout the period of record (since 1926), there is a good probability for future seismicity based on the recent mapping of offshore structures by Greene et al. [1973]. The present gravity survey was undertaken to add to the sparse geological data that presently exists and in hopes that the composite will, in the future, produce a detailed and complete understanding of the structure of the continental shelf and surrounding area. B. LOCATION AND TOPOGRAPHY The area of study encompassed in this research is that of the continental shelf, from the coastline west to the 100 fathom (183 m) contour and from Pt. Sur north to Pt. Lobos, California. This area 10 Figure 1. Bathymetry of the Survey Area (depth contours in fathoms) 11 is bounded by latitude 36°18.5' N and 36°31.5'N and by longitude 121°54' W and 122°58. 8" W (Fig. 2). In this region the shelf break occurs at approximately the 70 fathom (128 m) contour at an approxi- mate distance of 2 km from the shoreline in the north to over 11 km to the south off of the Pt. Sur tombolo. The shelf is interrupted in the north by two tributaries of the Carmel Canyon located approximately 3.7 km southwest of Pt. Lobos, by an extension of the Monterey Canyon to the west of the central portion of the area, and by the Sur Canyon almost due south of Pt. Sur. The area generally exhibits flat terrain over the continental shelf with the exception of one or two northwest trending fault scarps of approxi- mately 20 m relief and numerous rock outcroppings marking the sea- ward extensions of the many rocky headlands. Dohrewend [1971], from bathymetric and seismic profiling, determined the slope of the shelf o to be approximately 1.5 over 90% of the area. Immediately to the east of the area, the Santa Lucia Range rises abruptly attaining frontal heights of over 800 m. This rugged range extends the entire length of the area attaining a maximum separation from the coastline of 16 km at Pt. Sur. Two major streams, the Big Sur and Little Sur Rivers, are found on the western half of the range. Both have only seasonal flow, and like all streams in the area, are separated from the eastern side of the mountains. Two well defined marine terraces are evident along the coast- line, one averaging 25-30 m above present sea level, and the other, 12 122 u00 •30 f36°24' OB OS ? km f nmi OBERANES PT ASLAR PT OCKY PT IXBY LDG RRICANE PT Little Sur River Figure 2. Survey Area Limits and Location 13 approximately 65 m [Trask, 1926; Phifer, 1972], These well defined marine terraces are probable evidence of the recent emergence of the northwest portion of the Santa Lucia Range. This uplift is most likely- continuing at the present time [Phifer, 1972]. C. PREVIOUS WORK It has only been within the past 3 or 4 years that any geological work has been carried out within the study area. Dohrewend [1971] and Ellsworth [1971] utilized seismic reflection profile records, precision depth recording (PDR) traces, and core and grab samples to describe the geology of the continental shelf between Pt. Lobos and Pt. Sur. Greene et al. [1973] conducted a study of faults and earth- quakes in the Monterey Bay region. Most of their work within the region appears to be a compilation of previous onshore studies, coupled with some offshore seismic reflection profiles and dredge hauls used to approximate offshore extensions of onshore faults. Colomb [1973] made a study of recent sediments on the shelf between Pt. Lobos and Pt. Sur, and in conjunction with this, conducted two bathymetric and seismic profiling cruises through the area. These studies represent, as far as can be ascertained, the extent of the scientific investigation of the continental shelf between Pt. Lobos and Pt. Sur. Before attempting to understand any of the offshore geology it is first necessary to have a thorough appreciation of the onshore geology of the surrounding area. There exist several geological investigations 14 of the adjacent coastal regions as well as some offshore studies to the north. Trask [1926] mapped and studied the geology of the Pt. Sur Quadrangle. Shepard [1948], Martin [1964], Martin and Emery [ 1967], and Greene [1970] investigated the geology of Monterey Bay and the Monterey Bay Submarine Canyon. Martin and Emery's work contains a brief description of the continental shelf just to the south of the Carmel Canyon. Page [1970] describes the geology of the area sur- rounding the Sur-Nacimiento fault zone at the southern edge of the study area including a probable time sequence of events in the formation of the present day geology. The following geological summary of the area is based on the above works. Three major fault zones that exist in the vicinity of the area of study influence the local geology. These include the San Andreas, the Palo Colorado-San Gregario, and the Sur-Nacimiento fault zones. It is generally believed that the boundary of the Salinian Block is the San Andreas fault to the northeast and the Sur-Nacimiento fault to the southwest. The Salinian Block is comprised chiefly of Cretaceous granitic- metamorphic rocks with oceanic crust of Franciscan assem- blage to either side (Fig. 3). Overlying the granitic basement rocks of the Salinian Block is a layer of Tertiary strata, primarily sedimen- tary rocks. The Sur-Nacimiento fault zone extends northwest through the southern and central Coast Ranges of California and presumably extends 15 TT 122*00' ~W &. TNueyo. -V — - --' r~'oo — o 0 K i— i O V ^ O • r-l 1—1 d CO 0) pi 28 comparable 7000 mgal range. The modification of a land meter to permit underwater measurements includes the mounting of the meter within two thick aluminum hemispheres, the inclusion of an automatic leveling system and depth sensing unit, and an insulated multi-conductor shielded by an armored cable. The cable enables measurements to be taken remotely at a control box. Remote operation through the control box consists of various step functions which include nulling of the pressure sensor depth unit, high and low speed meter leveling, flood and tilt indication, clamping/unclamping of the gravimeter mass, nulling of the mass position, and the gravity counter display giving the measuring screw adjustment to null the mass. Auxiliary equipment includes a specialized winch with a secondary termination at the bitter end of the armored cable through a set of slip rings. A marine gasoline engine is coupled to a hydraulic pump which is used to position an A-frame and to run the hydraulic winch. Power to the meter itself is supplied by the ship's 115 vac system through a rectifier and an isolation transformer to the control box and the meter. Figure 9 is a schematic diagram of the auxiliary equipment while Fig. 10 illustrates the equipment as installed on NPS R/V ACANIA (Fig. 11). The engine, winch, and A-frame assembly were mounted to the after upper deck of the ACANIA. 2. Calibration The meter itself was calibrated by utilizing two standard- ization bases located in the immediate area. The first base, WA-84, 29 Directional TO GRAVIMETER Isolation Transformer Ship's 115vac Power Figure 9 . Schematic Diagram of the Auxiliary Equipment 30 ro O < > CD x: O CD Q Oh O H CD x: C o +-> o +-> CD 6 a '3 cr w CD u 60 31 32 is located at the Monterey County Airport, and the second, WH-29, at the base of the steel tower at the end of the Monterey Coast Guard pier [Wollard and Rose, 1963]. The published gravity difference between the two stations is 22.5 mgal. The readings obtained by the author showed a 22. 1 mgal difference. The discrepancy of 0. 4 mgal is probably due to the recent construction of a new terminal at the airport and in the inability to locate the exact position of the bench- mark. Since the absolute gravity reading at the airport is less than that at the pier, and since the addition of a new concrete foundation on top of the airport benchmark would introduce a greater gravity reading and therefore a decreased difference between the two stations, it is felt that the readings were within the tolerance of 0. 1 mgal. Upon completion of the calibration check, the gravity meter was connected to the armored cable onboard the ACANIA. A trial run was conducted and successfully completed in the shallow waters adjacent to the Coast Guard pier. B. STATION SELECTION AND LOCATION Station selection within the area between Pt. Lobos and Pt. Sur was originally based on a grid of 0. 5 nmi (0. 9 km) interval between stations with the constraints of the 100 fm (183 m) contour to the west and the practical depth limitation of the research vessel of not less than 5 fm (9 m) to the east. It was intended to bracket the fault scarp 33 identified by Dohrewend [1971] and by the author from shipboard PDR traces. It was felt that this particular scarp runs into the western tributary of the Carmel Canyon as previously mentioned. It later became necessary to modify the 0. 5 nmi grid due to equipment malfunctions and decreased ship availability. A final grid of approximately 0. 5 nmi spacing in the northern portion of the area, and approximately 1. 0 nmi (1. 9 km) spacing in the southern portion resulted in a total of 68 ocean bottom stations. It is believed that the fault scarp was bracketed a total of seven times (i. e. , seven stations on each side for a total of 14 stations). Figure 12 illustrates the ocean bottom and land station densities; the coordinates of each station are given in Appendix B. C. SURVEY OPERATIONS 1. Measurement Procedures The survey included a total of four 12_hour ship-days over a 2 month period from 29 May 1973 to 27 July 1973. Prior to getting underway each day, a base station reading was taken at the ship's mooring location approximately 25 m south of the Coast Guard pier in Monterey Harbor, Monterey, California. Conditions permitting, this check was repeated at the conclusion of the day's operation in order to determine meter drift. Upon arrival at a particular station the meter was lowered directly to the bottom at an approximate rate of 30 m/min. Bottom 34 •30' .36°25' -20' 122°00' f nmt f km Figure 12. Ocean Bottom and Land Station Density and Location 35 arrival was monitored through the use of the depth nulling galvanometer on the control box. Originally the operating procedure called for obtain- ing a pressure sensor depth reading while the meter was suspended at the sea surface. This was found to be impractical due to motion in- duced by sea and swell. Subsequently, a check of the depth reading was taken when the meter was in the two-block position at the A-frame. The depth reading here was essentially the same as that obtained just prior to the meter entering the water. This value was used as the surface depth counter reading. Once the meter reached the bottom it was necessary for the winch operator to continually pay out cable to allow for ship drift and preclude dragging the meter on the bottom. The control box operator would obtain an ocean bottom pressure depth sensor reading and, con- currently, a reading of the ship's fathometer was recorded. Next, a check of the flood and tilt indicators was required. If the meter was o tilted more than 15 from the horizontal it was necessary to reposition it at a different location. This occurred a number of times throughout the survey, particularly in shallow rocky areas. No flood indication ever occurred. After the meter was leveled and the mass undamped, a gravity counter reading was taken. It is worth noting here that the time involved in taking a reading as well as the accuracy of the reading was a function of meter depth and sea state. Even at depths as great as 90 m, the influence of the water motion above the meter could be 36 discerned. This effect varied directly with the sea state and inversely with depth and required, in some cases, an averaging procedure to obtain a reading. Once the gravity reading was recorded and the mass clamped, the meter was raised to the surface. During periods of strong winds it became apparent that the ship had drifted a noticeable distance from the original lowering position and it was necessary to maneuver the ship back as near as possible to this position in order to attempt to lift the meter off the bottom vertically to prevent dragging it. Because of various difficulties, this procedure was not always successful. It is felt that the meter was dragged over the bottom for short distance on a few occasions, apparently with no damage. Under ideal conditions of calm, windless seas, the entire operation of obtaining a reading took approximately 15 min. ; under adverse conditions the time increased to 30 min. 2. Navigation All navigation was conducted by the crew of the research vessel. This was necessary since it required both the author and a co-worker to operate the gravity equipment. Visual navigation was utilized throughout the entire survey for bearing information, while radar provided a check of range on all fixes. Normally, a three-point fix was obtained at each station from three lines of bearing. In areas where this was not possible, radar 37 ranges were used to supplement bearing information. Examination of the station charts indicated that three-line fix triangles were accurate to within a 0. 1 nmi (0. 19 km) or less on a side. In the north-south direction this would introduce a maximum error of 0. 14 mgal to the final complete Bouquer anomaly (CBA) values. 3. Equipment Reliability On seven separate occasions it became necessary to re- terminate the electrical connections located at the bitter end of the armored cable. A rubber boot with two watertight clamps at either was designed to prevent salt-water leakage to the electrical terminals. It is believed that at operational depths increased pressures caused a leakage through the clamps at the ends of the boot. Various salt-water seals were attempted but none proved entirely satisfactory. After each retermination a re_occupation of either base station WH 29 or the ship's mooring location was accomplished to determine if the absolute base counter reading had changed. No such change was ever observed. D. COASTAL SURVEY A coastal gravimetric survey was conducted concurrently with the ocean bottom survey for the purpose of tieing-in the complete Bouguer anomaly (CBA) values obtained on the continental shelf with those obtained by the U. S. Geological Survey (USGS) along the Cali- fornia coast. It was also felt that the gravity readings along the coast 38 would provide a verification of the ocean bottom readings and facilitate in the identification of any trends. A total of 38 stations were occupied using a LaCoste and Romberg Model G-08 gravimeter on loan from USGS at Menlo Park, California. The meter has a 7000 mgal range and an optimum accuracy of + 0. 01 mgal [LaCoste and Romberg, 1970] (Fig. 13). Station selection was based on the ability to accurately define the elevation of any given location and on the accessibility of that location. For this reason the majority of the stations selected were located at USGS monumented benchmarks as noted on USGS topographical charts. The remainder of the stations were located at road intersections where elevation was noted on the chart or along the beach as close as possible to the water level. The beach locations were selected so as to bracket the location of the Palo Colorado fault. Station elevation relative to mean sea level at these locations was later calculated from tidal information. Prior to use within the study area, a calibration run was made from USGS headquarters in Menlo Park (USGS 1 JD) to Skeggs Point (USGS B-388), spanning a range of 137.2 mgal [Chapman, 1966a], Reduction of the data from the calibration run yielded a 137. 13 mgal range for a difference of -0.07 mgal. A subsequent tie-in was made at base station WH-29 at the Coast Guard pier to determine a counter reading on the meter corresponding to the known absolute gravity value at WH-29. This value was determined to be 3405. 63 counter units. 39 Figure 13. LaCoste and Romberg Model G-08 Geodetic Land Gravimeter 40 This process was necessary since readings prior to and subsequent to each day's survey were difficult to accurately obtain. Water motion around the Coast Guard pier coupled with the sensitivity of the instru- ment caused considerable oscillation of the spirit level of the meter and only average values could be obtained. This factor made the determination of any meter drift impossible. Counter readings varied randomly about 3405. 63 with a maximum deviation of 0. 11 counter units. Therefore, meter drift was not considered measurable and 3405.63 was used as a base reading for all land gravimetry. 41 III. DATA REDUCTION The data obtained from both the underwater survey and the coastal survey had to be converted from counter units to milligals. Corrections were then made for elevation, topography, earth curvature, earth tides, and latitude. This process is necessary in order to obtain gravity values which can be compared with measurements obtained at any location on the surface of the earth. These values are used to calculate the CBA. There are intermediate anomalies which may be useful within a particular region but the CBA is the ultimate goal for most surveys, this one included. The following section is devoted to the methods used to obtain it. A. OBSERVED GRAVITY Observed gravity (OG), for the purposes of this paper, is defined as the value of gravity at a given location corrected for earth tide, meter drift, and curvature of the earth. In order to convert counter readings from the meter to observed gravity, it is first necessary to convert counter readings to equivalent milligal readings. This is done through the use of a conversion table for each individual meter which gives calibration factors for each 100-increment counter reading [LaCoste and Romberg, Inc. , 1970]. Since all of the author's counter values fell within the range of 3300- 42 3400, only the one calibration factor of 1.03985 was required. The next step involves transforming the equivalent milligal reading to an uncorrected observed gravity (OGfi). In order to do this a base value of absolute gravity and the corresponding counter reading must be known. For the entire survey the absolute gravity at base station WH-29 of 979891. 7 mgal [Wollard and Rose, 1963] was used as a reference. The counter reading corresponding to this value was 3323.66 as measured prior to the start of the survey. The above steps can be combined to produce the formula for obtaining uncorrected observed gravity: OG - 979891.7 + (CV - 3323. 66) (1. 03985) mgal f (1) 0 0 where CV is the control box counter reading recorded at each station. It became apparent at the conclusion of one day's work that a tare had occurred at some point during the day. A tare is defined as a sudden jump in the readings between observations. Scrutiny of the data worksheets lead to the conclusion that the tare occurred when the gravity meter struck the A-frame during a heavy roll of the ship. This conclusion was verified when the original data for that day was reduced to CBA values and a sudden jump in the values was noted. Subsequently, the counter reading corresponding to the absolute gravity of WH-29 had to be modified. Measurement at WH-29 resulted in a new value of 3327. 67, a difference of 4. 01 units from the original 3323.66. The value of 3327.67 was utilized as the base reading from 43 the time when the tare occurred until the conclusion of the survey. Equation (1) was modified accordingly. 1. Instrument Drift Correction On only one of the 4 days was it possible to obtain meter drift readings. Instrument malfunction on two of the days precluded obtain- ing a final reading at the ship's mooring location, and the tare, dis- cussed previously, occurred on one of the other days. Calculation of meter drift (D) is possible when two readings taken at the same location (the ship's mooring location in this case) over a time span are corrected for the variables of earth and ocean tides. On the one day, meter drift was calculated to be 0. 07 mgal over a 12 hour period; this value was assumed to be negligible. To determine the drift over the entire 4 days of the survey, all readings taken at the mooring location were corrected for earth and ocean tides. Between the first and third days, a drift of +0. 18 mgal was calculated. However for the interval from the beginning of the third day to the end of the fourth day, the drift was determined to be -0. 19 mgal. This resulted in a net drift of -0. 01 mgal for the 4 day interval. Since the reading precision of the gravity counter on the control box is 0. 10 units, it was felt that the non-linear jumps which occurred within the 4 day period were probably a result of reading error. For this reason, coupled with the fact that the 4 day variation was only -0.01 mgal, meter drift corrections were neglected. 44 2. Earth Tide Correction Since the earth is a non-rigid body, gravitational forces, primarily as a result of attractional forces from the moon and the sun, act to deform its shape. The adjustment made for this deform- ation is known as the earth tide correction (ET). The variation is cyclic with a range of 0. 3 mgal encompassing the tidal period. All calculation of earth tides were computed using a USGS computer program modified to be compatible with the NPS IBM 360 computer system. 3. Curvature Correction A discussion of the Bouguer correction is found in a later section of this paper. This correction assumes that gravity measure- ments were taken over a flat surface. This is valid only in those cases where terrain effects on the gravity are computed out to short dis- tances. However, for this survey, the large variations in topography necessitated considering the terrain effects as far distant as 167 km from the station. At these distances it becomes necessary to com- pensate for the curvature of the earth. The following USGS equation for curvature correction (CC) was utilized: CC=-1. 376X10"4(Z_Zt) +3. 049X10'9(Z_Zt) -1. 110X10"17 (Z_Zt)mgal, (2) where Z is gravimeter depth in meters (measured positively downward), and Z is the height of the tide (measured positively upward) relative 45 to mean sea level in meters. For ocean bottom stations (Z > 0) this correction is negative; for land stations (Z-Z < 0) the correction is positive. Observed gravity (OG) then is given by: OG = OG +D+ET+CC . (3) 0 B. THEORETICAL GRAVITY The reference spheroid utilized for this survey was chosen so as to be compatible with the work of the USGS and the California State Division of Mines and Geology. The constants used for the equation of the ellipsoid were those of the 1930 International Spheroid [Dobrin, I960]. The formula for the theoretical gravity (GTH) with the constants incorporated is: 2 2 GTH = 978049. 0(1+0. 0052884 sin L - 0. 0000059 sin 2L) mgal, (4) where L is the latitude. This formula points out the necessity of accurate navigation since a north-south variation of 1 km results in a difference in GTH of 0. 81 mgal. C. TOTAL UNDERWATER REDUCTION Up to this point the reduction of data for either a land station or an ocean bottom station is similar with the one exception of the sign of the curvature correction (plus for land, minus for underwater). The remainder of the data reduction exhibits some unique differences 46 depending upon whether the measurement was made on land or on the ocean bottom. This is particularly true for the terrain correction which not only is the most time consuming, but also the most difficult to fully comprehend. The majority of the literature dealing with the reduction of gravity data is written from the standpoint of a land reduction. Although this survey involved both land and ocean bottom environments, it was primarily an ocean bottom survey, and the following sections are written from that viewpoint. Land reduction methods are included in a separate section. Many of the methods and concepts described were derived from a paper by Andrews [1973]. 1. Corrections a. Initial Bouguer Correction The Bouguer correction assumes that measurements were made on an infinitely flat surface with no regional terrain irregularities The initial Bouguer correction (BC^) has the effect of removing the gravitational attraction of the water above the meter and replacing it with air. The 'Bouguer plate' of water above the meter is assumed to be of uniform composition and thickness with infinite length. Its gravitational effect is given by: BC1 - 27TG OwZ , (5) - 8 3 where G is the universal gravitational constant (6.67 x 10 cm / 2 g-sec ), a is the density of the water, and Z is the meter depth. w 47 3 For water of density 1. 027 gm/cm and Z in meters, equation (5) reduces to: BCj = (0.0430) Z mgal. (6) This correction is positive since the water is attracting the gravimeter mass upward. b. Free-Air Correction The free-air correction (FAC) is the vertical gradient of gravity at MSL as determined in free space multiplied by some change in elevation. The general formula for this correction is: FAC = 2 GM (Z _ Z ) / R3 , (7) where M is the mass of the Earth, R is the radius of the Earth, and 27 Z is the tidal height. For an Earth of mass of 5. 976 x 10 gm and radius 6371 km [MacDonald, 1966] equation (7) reduces to: FAC = -0.3083 (Z - Z ) mgal, (8) where both Z and Z are in meters. The correction is negative (Z > 0 for bottom stations) since, in essence, the meter is being positioned further from the center of the Earth. The FAC is the largest single correction to be applied, and from equation (8) it can be seen that accurate measurements of station depth and ocean tides are of prime importance. A pressure transducer, mounted on the inside of the bottom hemisphere of the gravimeter and with an external opening to the out- side, provides an indication of depth in the form of counter units on the 48 control box. The counter reading at the sea surface is subtracted from the reading at the bottom, the difference being directly propor- tional to depth. The proportionality constant is determined at the time of calibration of the pressure sensor unit and was provided by the manufacturer. Tidal information was based on tidal heights at Carmel Bay with Los Angeles as the reference station [U. S. Department of Commerce, 1973]. The tide tables are based on a reference datum of mean lower low water (MLLW), and in order to relate tidal heights to MSL, it was necessary to determine the difference in height between the two. This value was found to be 0. 884 m (Coast and Geodetic Sur- vey Nautical Chart 5476). c. Secondary Bouguer Correction The gravimeter at this point may be envisioned as being positioned at MSL directly above its original position. The Bouguer plate directly below, originally filled with water, now consists of air. In order to be compatible with land measurements, the Bouguer plate must be filled with rock. The general formula for this secondary Bouguer correction (BC?) is: BC = 2 7TGO (Z - Z ) , (9) 2 r t where O is the density of the rock. Using a common value of 2. 67 3 gm/cm [Dobrin, I960] for the density of the crustal rock, equation (9) reduces to: 49 BC2 = 0. 119 (Z - Zt) mgal, (10) where Z and Z are again in meters. This correction is positive since mass is being added beneath the reference ellipsoid. The initial Bouguer correction (BCJ, the free-air correc tion (FAC), and the secondary Bouguer correction (BC?) are often combined to produce the elevation correction (EC) [Nettleton, 1971]: EC = BC + FAC + BC2 . (11) 3 3 Using densities of O = 1.027 gm/cm and o =2.67 gm/cm , equations (6), (8), and (10) combine and reduce to: EC = (0. 1964 Z- 0. 1534 Z) mgal, (12) where Z and Z are in meters. Figure 14illustrates the corrections t & necessary to determine EC. d. Terrain Correction In applying the preceding corrections it was assumed that gravity measurements were made on an infinitely flat bottom with an overlying Bouguer plate of uniform composition and thickness and infinite length. Regional terrain irregularities were neglected. This assumption may be valid at some location such as certain areas of the continental shelf in the northern Gulf of Mexico. However, it does not hold true in the area between Pt. Lobos and Pt. Sur where the con- tinental shelf is transected by deep submarine canyons to the north, south, and west, and where, to the east, the Santa Lucia Range rises abruptly. Therefore, a topographic or terrain correction (TC) is necessary. 50 LAND STATIONS UNDERWATER STATIONS air ( 0) Using a density of 2.67 gm/cm , equation (26) reduces to: BC = -0. 1119h mgal , (27) where h is in meters. This correction is now negative since it removes mass below the meter. b. Free-Air Correction The free-air correction repositions the gravimeter from station elevation to MSL. Modification of equation (8), the free-air correction for underwater stations, gives the correction: FAC = 0.3083h mgal , (28) where h is in meters. This correction is now positive since the meter is being repositioned closer to the center of the earth. Combining equations (27) and (28) produces the elevation correction for land stations: EC = 0. 1964h mgal , (29) where h is in meters. c. Terrain Correction As in the terrain correction (TC) for underwater stations, for land stations the concept is to reduce the surrounding topography 60 to produce a flat, infinite surface on the same horizontal plane as the station. For compartments which overlie land alone, the modified Hayford-Bowie tables can be used intact. This correction is positive. For compartments which overlie water, it is necessary to modify the correction due to existing density differentials. Calculation of the attraction of each compartment involves two steps. First, it is nec- essary to fill in the depression between station level and the ocean bottom with rock. This correction is positive. However, after this is carried out, a portion of the rock will have displaced a wedge of water from MSL to the bottom; the previous correction will have been too large. The next step requires removing the effect of this displaced 3 water. Assuming a density for water of 1.027 gm/cm , the modifica- tion to the tables is again in the form of a constant multiplication factor: °™ = i-027 = 0.385 . (30) a 2. 67 r This correction must be subtracted as the attraction of the water below station elevation is being removed. In actual practice, since some of the land stations were relatively close to MSL, and the depth of the ocean water within the compartment was far greater (especially in the outer zones) than the station elevation, the following multiplication factor was applied to the values read from the tables, assuming standard densities: 61 vr - w 2.67 - 1.027 =o.615 . (31) 0W 2.67 The error introduced using this simplication results from the assump- tion that a small wedge of water extends from MSL to station elevation when, in fact, it is actually air. This error was determined to be negligible. For stations of high elevation and for compartments in shallow waters, equation (30) was utilized. 2. Gravity Anomalies Employing the correct signs land station gravity anomalies are calculated from equations (15), (16), and (18), using equations (28) and (29) to determine FAC and EC. 62 IV. DATA PRESENTATION AND ANALYSIS The procedures and discussion thus far have dealt primarily with obtaining and reducing of gravity values. The interpretation of results in terms of the geologic structure below the surface, a more qualitative analysis, is discussed in this section. A. GENERAL DISCUSSION The CBA gravity values were plotted on a USGS topographic map of the area (scale 1 : 24, 000) and CBA isolines were drawn by hand at a 2 mgal interval. The decision to adopt a 2 mgal interval was based upon a desire to depict the greatest detail while still maintaining the general trend. Transference and reduction of the gravity map on the topographic chart to a larger scale map was accomplished through the use of a pantograph on a 3/16-scale reduction. This proved to be satisfactory in maintaining the detail and the general trends of the gravity map (Fig. 16). The accuracy of this map is dependent upon two factors: (1) the accuracy of the CBA values themselves, and (2) the contouring of the gravity field in the construction of the isoline pattern. From former surveys utilizing the HG6 Model gravimeter [Brooks, 1973; Cronyn, 1973; Souto, 1973] and from the author's own analysis, the accuracy in determination of CBA values is + 1. 04 mgal for sea stations and 63 Figure 16. CBA Distribution for the Continental Shelf and Adjacent Coastline Between Pt. Lobos and Pt. Sur (values in milligals, contour interval 2 mgal) 64 +0.72 mgal for land stations. Table II delineates possible error sources and the reduction step to which they relate. The contouring of the iso- lines is the author's interpretation of the 'best fit' of the gravity values and is therefore subject to error since the process is partly subjective. B. CBA ANALYSIS Assuming that the effects of elevation, topography, and latitude have been correctly removed, the anomalies in the map of the gravity field are caused by horizontal variations of the density within the crust and upper mantle of the earth. The magnitude of the anomaly is depend- ent upon the density contrasts, the location of the contrast relative to the gravity station, and the form or sharpness of the geologic discon- tinuity. If there is no density contrast, no anomaly will exist. The interpretation of a CBA map in terms of subsurface geologic structure is not unique from utilization of gravity data alone. Other sources of information such as seismic data, drill core data, or specific geologic data from outcrops must be available. From this information density values can be inferred and actual depths to a density contrast can be obtained and subsequently related to the subsurface structure of the surrounding area. The degree of uniqueness is dependent upon the validity of the accepted geological data. In the case of this survey specific geological data was not available; therefore, it was neces- sary, when making a final analysis, to generalize. 65 INITIAL ERROR SOURCE DATA REDUCTION STEP ERROR IN COASTAL SURVEY (Land Gravimetry ERROR IN CONTINENTAL SHELF SURVEY ) (Bottom Gravimetry) GRAVIMETER ACCURACY OBSERVED GRAVITY +0.04 + 0. 10 OPERATOR READING ACCURACY OBSERVED GRAVITY +0.01 +0. 10 NAVIGATION THEORETICAL GRAVITY + 0. 05 +0. 14 ELEVATION/ DEPTH CALCULATIONS FREE-AIR AND BOUGUER CORRECTIONS +0. 12 +0. 20 ELEVATION/ DEPTH CALCULATIONS TERRAIN CORRECTION + 0.50 +0. 50 TOTAL COMPLETE BOUGUER ANOMALY +0. 72 + 1. 04 TABLE II. POSSIBLE ERRORS IN COMPLETE BOUGUER ANOMALY CALCULATION (Values in Milligals) 66 The CBA values along the California coast usually do not range far from a value of about 0 mgal. They decrease eastward to high negative values of -200 mgal or less in the vicinity of the Sierra Nevada and Great Basin provinces [Chapman, 1 966]. From a gravity profile analysis conducted by Thompson and Talwani [1964] for a cross-section approximately 160 km to the northwest of the survey area, the regional trend of the gravity anomaly along the profile from the Pacific Basin to the continental shelf is highly negative, going from approximately 250 mgal to less than 50 mgal. Thompson and Talwani relate this strong negative gradient to an increase in thickness of the crust as the continental margin is approached. Although this particular profile was to the north of the author's area of interest, the regional trend which it exhibits could be expected to be similar to the one within the study area. However, from the author's CBA map, the trend of the gravity anomaly is opposite, showing a strong positive west to east gradient. This apparent contradiction can be related to local anomalies and is believed to be caused, at least in part, by a rapid rise of the granitic basement just offshore from Pt. Lobos to Pt. Sur. From the CBA map partially closed gravity highs are evident in the vicinity of Soberanes Pt. , Kaslar Pt. , and Pt. Sur (location of these points is shown in Fig. 2). These highs are generally oriented northwest-southeast and are congruent with the trend of the local geology. They may represent granitic outcroppings, particularly the 67 most northern high which parallels the geographic trend of Pt. Lobos. An alternative explanation is that these highs may be associated with the fault zones which transect the area. This is especially true for the 34 mgal high at Kaslar Pt. , the axis of which is nearly parallel to the Palo Colorado fault. The position of this high coincides with the 30 mgal high plotted on the Santa Cruz Sheet of Bishop and Chapman [1967], The 30 mgal high at Pt. Sur exhibits generally lower gravity values within the semi-closed isoline system in comparison to the other highs and is possibly an indication of the lower density Franciscan assem- blage which is reported to exist in this area on the southwest side of the Sur-Nacimiento fault zone. Offshore of Hurricane Pt. a definite ridging of the isolines is evident. The orientation of the isolines to the north of this ridge is northwest-southeast while to the south they exhibit a northeast- south- west trend. The location of this ridge coincides with the offshore extension of the Serra Hill fault as reported by Greene et al. [1973] (Fig. 4). Judging from the gravity data alone, no further faulting occurs between this ridge south to Pt. Sur. Since it is generally accepted that the Sur-Nacimiento fault zone leaves the coast north of Pt. Sur, the structure associated with this gravity ridge may be related to its offshore extension. The conclusion that this isoline configuration is the reflection of a fault is supported by both the gradient and the orientation of the isolines which appear to indicate 68 either strike-slip or dip-slip motion where the southwestern side has descended relative to the northeastern side. Gilbert [1971] states that the fault which leaves the coast near Hurricane Pt. has a dip o 50-60 NE with an estimated 300 m vertical displacement. This would call for a normal fault where the Sur-Nacimiento fault zone is usually associated with thrusting. Page [1970] contends that the formation and movements of the Sur fault are more complex and that it cannot be described as a simple thrust fault. Other mechanisms must be introduced to understand its present state. He contends that thrust faulting was only the initial stage in its development. Subsequent normal faulting is believed to have occurred as a result of the collision of the East Pacific Rise crest and the westward moving North American Continent. Possible strike-slip motion followed when Pacific spreading became oriented to a northwest-southeast direction. This occurred at approximately the same time that the San Andreas fault became active. Further evidence for strike- slip motion can be obtained from an exam- ination of the bathymetry in the Pt. Sur area (Fig. 1). The continental shelf is at its widest at a point almost due west of Pt. Sur. The ridg- ing of the depth contours could have been influenced by compressional forces that existed when the oceanic crust came into contact with the Salinian Block, or it could be viewed as an elongation associated with right-lateral strike- slip motion where the southwestern block has moved northwesterly in relation to the northeastern block. 69 From the CBA map it is seen that the Hurricane Pt. ridging abruptly terminates as the coastline is reached. No explanation can be put forth as to why this is so. Serra Hill rises sharply at this point and gravity measurements were taken only along its western edge. More data are needed in this area, particularly along the eastern side and top of Serra Hill. Southwest of Pt. Lobos there is strong evidence from the gravity field of a partially closed 10 mgal low. The gradient to the east of this low is very steep, attaining maximum values of 12 _ 1 3 mgal/km. This is a strong indication of a rapid rise in the basement, most probably associated with faulting. The direction of ridging of the iso- lines to the north and orientation of the partial low can be projected into the western tributary of the Carmel Canyon. The form of the 10 through 20 mgal isolines to the west of this low is unknown beyond the limits shown on the map. It is possible that these isolines continue south and connect with the open isolines west of Pt. Sur. If this is the case, a large low of 8 mgal or less would result. Insufficient data precluded projection of the open isolines. Just to the east of the 10 mgal low and coincident with its orienta- tion, the extension of either the Palo Colorado fault zone [Dohrewend, 1971] or the Sur fault zone [Greene et al. , 1973] leads into the west- ern tributary of the Carmel Canyon. Dohrewend obtained PDR traces of a fault scarp extending for 6 km of approximate 20 m relief at a 70 depth of 82-101 m. He correlates this scarp with the offshore extension of the Palo Colorado fault and projects it onshore just north of Kaslar Pt. (Fig. 5). A bathymetric profile survey was conducted from the ACANIA in an attempt to substantiate the location of this fault scarp. Profile locations and the plotted position of the fault scarp are shown in Fig. 17. The profiles are reproduced in Fig. 18-23. An attempt to locate the position of the scarp south of profile A_A' was unsuccessful and lack of time prevented a more thorough search. The fault scarp is easily recognizable in profiles A-A', B-B', and D_D'. In profiles F-F' and H-H1 the original scarp has trended into the western tributary of the Carmel Canyon while a new scarp emerges on the shoreward side of profile F-F'. This scarp is barely recognizable in profile H-H' as it leads into the eastern tributary of the Carmel Canyon. In both profiles granitic outcroppings are evident between the two canyons. C. TWO DIMENSIONAL PROFILES The evidence of faults leading into the tributaries of the Carmel Canyon lead to the construction of three separate two-dimensional modeling profiles, the locations of which are shown in Fig. 24. It was hoped that the depth to the basement could be determined. The computer program used in this effort, based on an earlier model by Talwani, was developed by Cady [1972] of the USGS. The model requires that the regional trend be filtered out of the total gravity 71 -30' N 36°25' 0 i 122°00' Sobercsnes Pt Kaslar Pt Figure 17. PDR Profile Locations and Fault Scarp Position from a Bathymetric Study 72 40 .-., Depth, m 80 ^ 120 57/h€T i~M£ A JL. {-oSa's. &,ck:S lo/o r I w 1. .-.— \ "1 4 160 T 200 to < K, -7 m a»er- Jd Uu€ Pi'(\ 1024 Zo^n 50 i'p 100 150 One-woy travel time, msec 200 *) • UJ 250 Figure 18. PDR Profile A-A' 73 40 Depth, m 80 120 St-*^- irxie 6 '3 14XSL - 6- ... »■*£ ■U C«f 160 C*3 t^^s £ Z.< * Arf 200 **£B?' ; ' > : -X. SB5J jii«jli '-.- O'j '.' , , i'gLii , ,...., ,. -• ! v i (ii ■._: .■..•■<■■■■ -■' ■ ■ • ,' ■I •' " ■nt. -i "mtfivr -:■;■■ V - -_' .. p ■ ' i ■ ■ '•-I ,-f. . .. 2Ch" 0 50 ;• . ,.; ■ ■".'.. ill f . ■ ■: ;; l 100 150~ ' 200 ' One-way travel timey msec Figure 19. PDR Profile B-B' ■ 250 74 Depth, m 80 120 160 200 V i i '■'-'• m- i__$! JU& — ; . 100 150 One-way travel time, msec Figure 20. PDR Profile D-D" 75 Depth, m LU*^"5 100 150 One-way travel time, msec Figure 21. PDR Profile E-E' 76 L ° I .i*?- 40 i \ Depth, m 80 120 160 9;q 200 - CO < LU . -i i^C- S 1 - s?-;- S>e 50 Eg ■w. 100 150 One- wry travel time,, msec Figure 22. PDR Profile F-F' 200 oo LU 250 77 Depth, i - 3Ei i&; .- » « ■*■■ ..-»■ - ■--- ^avi 0 SB*? £«;■> t.«/«; /v//' -m < IXI 50 o 10° * I ,• 150 One-way travel time, msec Figure 23. PDR Profile H-H' 78 200 250 Figure 24. Location of Two-Dimensional Modeling Profiles 79 anomaly. In this case, however, the length of the profiles ranged from 3. 48 to 3. 90 km and for such short distances it is impossible to determine a regional trend. If a regional trend, such as the one determined by Thompson and Talwani [1964] is assumed, the gradient of the gravity anomaly input to the model would be increased slightly but would not appreciably alter the end result. In order to successfully run the computer program it is necessary to input the depth to the basement at one or more points along the profile. In all three cases the basement, assumed to be quartz diorite with a mean density of 3 2. 806 gm/cm [Daly, Manger, and Clark, 1966], was located at sur- face level at the eastern boundary of each profile. This is in agreement with actual coastal conditions found between Pt. Lobos and Pt. Sur [Trask, 1926]. Dohrewend [1971] reports that the fault brings at least 200 m of Plio-Pleistocene sedimentary rocks into high angle contact with the Cretaceous Santa Lucia quartz diorite. He also states that Miocene marine rocks outcrop in the southern portion of the study area and consist of claystone and shale. This unit is characteristically overlain unconformably by marine mud and siltstones. Because Mio- cene marine sedimentary rock was the deepest unit that Dohrewend could distinguish seismically, he was unable to measure its thickness. The Plio-Pleistocene marine rocks which are found on the south- western side of the fault probably have a composition similar to that of the Miocene marine unit found to the south (i. e. , claystone and 80 shale). An approximation of the density of nearly horizontal and un- 3 disturbed Miocene shale is 2.06 gm/cm [Daly et al. , 1 966] - This was the second density value used in the model. The difference between the two assumed density values, 0.746 gm/cm , constitutes the density contrast parameter input. The results of the computer- run models are illustrated in Fig. 25, 27, and 29, while Fig. 26, 28, and 30 give the respective model-calculated and the observed gravity profiles of each cross section. Although by no means the only solution, these models represent the best fit utilizing the above assumptions and existing data. Various 3 density contrasts ranging from 0. 0 to 0. 840 gm/cm were tried but all attempts introduced errors greater than that which resulted from 3 the 0. 746 gm/cm contrast. The RMS errors for each model are given in Fig. 26, 28, and 30. It is apparent that the greatest errors for each profile occurred at the boundaries. The program was designed for much longer profiles where the errors occurring at the boundaries would be averaged out over the length of the profile. For short distances these errors become readily apparent. The position of the fault break for each section is shown on Fig. 24. The correlation between these points and the location of the off- shore fault scarp as mapped by the author from the PDR profiles is quite good. This is also true for the proposed offshore extension of the Serra Hill fault across model profile C-C The computed minimum Figure 25. Depth to Basement of Profile Model A-A1 82 1 > i 1 -- • = 0.2649 mgal = 1.0953 mgal *> o 6 T Mean Error RMS Error : -*- _o o • o Gravity V\. CO. - t3 I 0) o E cs- "( \ "5 O) E -. \$\ CN o CN » • o Figure 26. Calculated and Observed Gravity for Profile A-A1 83 t r T ~T co E < E O O oo CM E CM CO E CN4 Figure 27. Depth to Basement of Profile Model B-B' 84 Figure 28. Calculated and Observed Gravity for Profile B-B 85 Figure 29. Depth to Basement of Profile Model C-C 86 Figure 30. Calculated and Observed Gravity for Profile C-Cl 87 displacement of the basement ranges from 1. 5 km for model profile C-C to 2. 1 km for model profile A_A'. These values are much greater than the ones proposed by Dohrewend [1971] and Greene et al. [1973] whose conclusions were based on seismic data alone. However, Thompson and Talwani [1964] for a 40 mgal negative residual anomaly in the vicinity of the continental slope suggest that this relative low is caused by a thickness of about 3 km of sedimentary rocks. The west- ern most stations of model profile A_A' were located on the continental slope and the deep depth to basement could at least be partially related to the similar relative low southwest of Pt. Lobos. The fault which separates this sedimentary boundary is believed to be the primary cause of the deep basement. The model itself gives the appearance of dip- slip motion where the southwestern block has dropped relative to the northeastern block. Strike-slip motion could also be a cause of the vertical separation; most likely, as with the Sur fault, it is a combination of both dip-slip and strike-slip motion. The most eastern scarp shown on PDR profiles F-F' and H-H' (Fig. 17) is probably the northern extension of the fault running into the eastern tributary of the Carmel Canyon in the north which Greene et al. , [1973] believe to connect with the onland Palo Colorado fault. It was hoped that some indication of this structure would be found in the profile models; however, no such structure was evident. One explanation is that a fault may indeed exist in this location but lacks the density contrast necessary to bring it out in the model. 88 D. CONCLUSIONS From interpretation of the two-dimensional profiles and the gravity anomaly pattern, the author concludes that the offshore extension of the Serra Hill fault as mapped by Greene et al. [1973] (Fig. 4) and the fault trending into the western tributary of the Carmel Canyon are one in the same (Fig. 31). This conclusion is based on similar minimum displace, ments of the basement on the southwest side of each fault. Dohrewend [1971] projects the offshore Palo Colorado fault into the western trib- utary of the Carmel Canyon (Fig. 5). If this were true it could be expected that the isoline pattern in the vicinity of Kaslar Pt. , the location of the onland Palo Colorado fault, would indicate a basement displacement closely approximating the one found just south of the western tributary. No such correlation was found. Instead, the iso- lines indicate a continuation further south to the vicinity of Hurricane Pt. and the Serra Hill fault. The fault scarp leading into the eastern tributary of the Carmel Canyon is probably associated with the offshore extension of the Palo Colorado fault. The close proximity of this scarp to the shoreline precluded locating its position south of PDR profile F- F' and the gravity data offered no additional information. The decreased CBA gradient in the vicinity of the onland Palo Colorado fault is also evident near the eastern tributary of the Carmel Canyon and may be indicative of a small density contrast across this fault. 89 Position located from v i._. nmi km Position located from CBA isoline ridging and model profile Hurricane Pt SERRA HILL FAULT Figure 31. Summary Fault Map Indicating Proposed Locations 90 If the Sur-Nacimiento fault zone leaves the California coast north of Pt. Sur, there is no indication of this trend from an analysis of the gravity data. However, onshore on the southern side of Serra Hill there is an outcrop of Franciscan assemblage reported by Page [1970] to be found only to the southwest of the Sur-Nacimiento fault zone. It is concluded that this fault zone leaves the coast in the vicinity of this outcrop and may in part be correlated with the gravity isoline ridging just to the north. What becomes of the Sur-Nacimiento fault zone after it leaves the coast is still unknown. In the author's viewpoint there are two possibilities: (1) the Sur-Nacimiento fault zone connects at some location to the north with the Palo Colorado-Serra Hill fault complex, or (2) it proceeds out to the west undetected from the gravity data. There is a gap in the gravity data in the central portion of the survey area which may possibly hold the answer to this question. 91 V. FUTURE WORK It is suggested that additional gravity measurements be made to the west of Pt. Lobos and in particular on the ridge located between the two tributaries of the Carmel Canyon. This would perhaps result in a better determination of the orientation of the gravity isolines in this area. More measurements are also required in the central portion of the area to perhaps tie-in the Hurricane Pt. ridging with the partially enclosed low to the north. Land gravity measurements are also required on the top and eastern side of Serra Hill to attempt to explain why the Hurricane Pt. ridging is terminated at the coast. It is also recommended that a detailed bathymetric survey be conducted within the entire area. In conjunction with this, either deep penetration coring or dredging should be accomplished particularly on the widened shelf area west of Pt. Sur to determine the boundaries of the Franciscan assemblage and thus the Sur-Nacimiento fault zone. A correlative magnetic study within the area would be helpful and of particular use in the two-dimensional profile studies. Finally, sea- surface gravimetry would be of use in those areas too deep for bottom gravimetry. Particular attention should be given to the western exten- sion of the Monterey Canyon located 12 km to the west of Soberanes Pt. This would be a logical location for the Sur-Nacimiento fault zone if it does proceed out to the west after leaving the coast. 92 APPENDIX A DATA REDUCTION CORRECTION VALUES FOR INDIVIDUAL STATIONS (values in mgal) STA G CESEfcVEC G THEORETICAL BC FAG EC TC 1 9799C7.24C 2 979906.068 2 9799C6.cec 4 979909. IS4 5 9799C6.1C2 6 9 7 99C2.91C •7 9799C4. 961 8 9799C<.3C9 9 9799C5.756 1C 9799C4.591 11 9799C7.719 12 9799C2.C13 13 9799C2.369 14 9799C6.C29 15 9799C9.C36 16 979902.465 17 9799C3.C99 18 979699. £53 19 9799C4.C22 2C 979904.027 21 979894.717 22 979901.659 23 979 9 C3. 928 24 979901.628 25 979897.348 26 979897.442 27 979899. C36 2Q 979898.243 29 979892.664 979680.359 6.56 -17.02 -8.47 6 .94 979876.132 9.91 -19.71 -9.80 5 .84 979875.097 7.46 -14.84 -7.38 5.67 979874.580 11.42 -22.73 -11.31 5.41 979874.493 7.27 -14.47 -7.19 5 .44 979874.235 4.3C -8.54 -4.24 6 .26 979873.976 9. 54 -18.98 -9.44 5.6«= 979873.890 12. CI -23.90 -11.89 5 .3C 979873.028 6.63 -17.57 -6.74 5.16 979873.286 6. 30 -12.53 -6.23 5.4 9 979E72.338 11.98 -23.84 -11.87 5.5S 979873.200 17. ce -33.99 -16.91 5.7 3 979873.545 15.33 -30.52 -15.19 5 .69 979872.769 15.27 -30.39 -15. 12 5.62 979872.597 25.87 -51 .50 -25.63 5.9C 979871.993 4.76 -9.47 -4.71 5 ,S 1 979871.907 -30.49 -15.17 5.7C 979871.907 15.93 -31.68 -15.75 5.7 6 979871.476 7.28 -14.49 -7.21 5.75 979871 .390 10. C2 -20. G2 -10.00 5.75 979871.304 18.87 -37.56 -16.69 5.7 1 979870.873 5.5C -10.94 -5.44 6.29 979870.787 1C.91 -21.78 -10.88 5.66 97987 I. 131 14.25 -28.35 -14.10 5.42 979870.614 14.24 -28 .42 -14.18 5 .54 979870.614 18. 2C -36.23 -18.03 5.37 979870. 183 3.38 -6.73 -3.35 6 .74 979870.097 12.60 -25.07 -12.47 5.2C 979669.839 16.04 -35.92 -17.88 5 .4<= 93 STA G CESEPVEC G ThECRETICAL BC FAG EC 7C 2C 3L 32 33 24 35 36 37 38 29 40 41 42 43 44 45 46 47 4£ 49 50 51 52 53 54 55 56 57 58 59 979899. 12C 979898.626 979892. 4C7 979892.416 979897. 92C 979899. 7C3 979891. 6C1 979897.622 979896.495 979895.472 979895. 2C6 979895. 92C 979891.997 9798 9 4. 399 9 79898.215 979888.067 979892.686 9798 9 2.CC4 97989C.CC2 9 7 9 8 8 7.994 979882.970 979882.677 979887.547 979892. C22 979881.227 979879.915 979882.422 97988 1.895 9 79879.872 979879.272 979869.752 6.64 -17.19 -8.55 5.65 979869. 149 7.92 -15.78 -7.86 6.C2 979869.149 12. 22 -24.35 -12.12 5.29 979869.063 18.95 -37.73 -18.78 5.9 6 9798fc8.460 6.44 -12.83 -6.39 6.C4 979866.632 9.32 -18.57 -9.24 5.51 979868.029 15. 03 -29.93 -14.90 5.76 979868.029 11.64 -23.19 -11.55 6 .C5 979868.029 3.68 -7.34 -3.65 7 .C5 979867.684 7. 12 -14. 19 -7. 08 6.2C 979867.254 3.62 -7.20 -3.59 6 .85 979867. 081 7.45 -14.87 -7.42 6.CC 979867.031 14.35 -28.61 -14.26 5.7 9 979866.476 5.44 -10.84 -5.40 6.42 979866.306 8. 05 -ie.C7 -8. 01 5 .74 979866.048 14.40 -28.70 -14.30 5.5C 979665.875 4.67 -9.20 -4.64 6 .56 979865.273 5.93 -11.82 -5.89 6.27 979865.359 8.33 -16.61 -8.28 5.82 979865.531 11.27 -22.67 -11.30 5.44 979865.273 15.15 -30.18 -15.04 5.28 979864.842 12.28 -26.47 -12.19 5.27 979864.584 11. 12 -22. 15 -11.03 5.54 979864.153 6.23 -12.38 -6. 15 6 .61 979864.41 1 14.29 -28.66 -14.27 5.17 979863.895 1C.19 -20.29 -10.10 5.26 979662.689 6.11 -12.15 -6.04 6.C7 979862.689 11.29 -22.49 -11.20 5.27 979862.948 13.85 -27.59 -13.74 5.C2 979861.570 12.62 -25. 16 -12.53 4.97 94 STA G CESEPVEC G THECRETICAL EC FAC EC TC 60 979682. £27 979861 .570 8.89 -17.73 -8.84 5.22 61 979884.277 979861.484 5.71 -11.25 -5.65 5.76 62 979363. 186 979860.623 10. 1C -20. 14 -1C.04 5 .C4 63 979879.234 979860.365 14.68 -29.26 -14.53 4.92 64 97968 1.429 979859.849 1C.82 -21.58 -IC.75 4.9 I 65 979C87.692 979859.676 7.26 -14.47 -7.21 5.24 66 9 7 9 6 8 5 . 7 9 C 979858.644 8.57 -17.07 -8.50 5.C4 67 979862.676 979859.504 3.61 -7.18 -3.57 6 .C8 68 979665.156 979857.525 5.55 -11.06 -5.51 5.16 95 STA 6 OBSERVED G THEORETICAL BC FAC EC TC A 979896.381 979875.873 ^0.85 2.35 1.50 5.56 B 979893.355 979875.356 -2.73 7.52 4.80 5.41 C 979882. 1C1 979875.528 -7.84 21.63 13 .79 5.97 D 97989C.C88 979874.235 -4.50 12.42 7.9 1 5.9C E 979884.599 979873.114 -7.67 21.16 13.49 6. 14 F 9796S1.C76 979873.028 -3.58 9.88 6.30 5.95 G 979889.591 979871.321 -3.96 10.91 6.95 6. 1C H 979893.724 979871.390 -1 .86 5.17 3 .30 6.14 1 979887.258 979870.700 -4.50 12.42 7.91 7.36 J 979889.407 979870.011 -2.83 7.81 4.98 7.78 K 979885.427 979869.322 -4.40 12.13 7.73 6.78 L 979891.470 979869.322 -1.53 4.23 2.70 6.66 M 979889.242 979868.632 -1 .71 4.70 3 .00 7.28 N 979832.151 979867.857 -5.49 15.14 9.65 7.41 O 979891.090 979867.340 -0.03 0.03 C.05 8.74 P 979 891.140 979867.254 -0.03 0.08 •CO 5 8.74 Q 979891. 120 979867.163 -0.03 0.08 0.05 7. 11 R 979891.350 979367.081 0.0 0.0 CO 7.11 S 979891.270 979866.995 0.0 0.0 0.0 7.CC T 979891.250 979866.323 -0.C3 0.09 C.06 6.8C U 979885.603 979866.651 -3.27 9.03 5.76 6.46 V 979888.332 979860.651 -1.67 4.61 2.94 6.47 W 979878.993 979865.962 -5.63 15.52 9.89 6.51 X 979878.185 979865.273 -5. 80 15.99 1C.19 7.46 Y 979855.742 979864.756 -18.45 50.89 32.44 11. AC Z 979878.836 97986-+. 842 -5.12 14.11 8.99 9.15 A' 979873.428 979864.153 -7.64 21.07 13.43 7.91 B1 9 798 36.2 31 979863.464 -0.04 0.11 C.07 7.29 e 979867.251 979862.775 -9.41 25.96 16.55 5.78 96 STA G CESEPVEC G ThECRETICAL i ec FAC EC TC D' 979851. C79 E' 979649.451 F' 979864.466 G' 979669.456 H' 97987C.201 I' 979668.197 J' 979669.655 K' 97967C.745 L' 979855.117 979861.742 979860.795 979859.935 979859.332 979858.988 979857.955 979857.353 979856.665 979857.095 -16.54 45.62 29.08 8.16 -15.07 41.57 26.50 11.62 -7.44 20.51 12.07 7.5C -4.20 11.85 7.55 -4.71 12.98 8.27 -5.93 16.37 1C.43 -5. 01 13.83 8.81 -4.26 11.76 7.49 6.76 5.79 6.92 6.28 5.78 -12.31 33.96 21.64 10.89 97 APPENDIX B VARIOUS GRAVITY ANOMALIES AND LOCATIONS FOR INDIVIDUAL STATIONS ST/> L/HITICE LONGITUDE DEPTh (m) FAA MF££ se/> CBA I 26 31.57 121 57.53 55.2 9.86 14.6 18.4 I 25.35 2 26 31. 1C 121 57.80 63.9 12.22 17.7 22. 12 27.97 Q 36 3C .88 121 57.22 48. 1 16.14 20. 3 23. 60 29.27 4 26 2C .52 121 57.67 73.7 11.84 18.2 23.26 28.67 5 36 3C.5C 121 57.05 46.9 17.14 21 .2 24.42 29.86 6 36 2C.28 121 56.47 27.7 20.13 22.5 24.42 20.69 7 36 2C.12 121 57.08 61.5 12.00 17.2 21.54 27.22 8 36 2C.C5 121 57.72 77.4 11.52 18.2 23.52 28.82 9 36 29 .48 121 57.45 56.9 15.16 20. 1 23 .99 29. 17 10 36 29.67 121 56.83 40.6 16.76 22 .3 25.06 20.57 11 26 29. CC 121 57.50 7 7.2 11.54 16.2 23.51 29.10 12 26 2 9.58 121 58.12 110.2 -5.18 4.3 11 .90 17.62 13 36 29. ac 121 58.68 93.9 -C.70 7.6 14.64 20.52 14 26 29.28 121 57.98 93.5 2.87 11.4 13. 14 23.76 15 26 2 9.15 121 58.87 166.9 -15.06 -C .7 10.8 1 16.71 16 26 2i.ll 121 56.50 30. 7 21.02 2 2.7 25.78 21 .69 17 26 26 .65 121 57.87 98.8 0.70 9.2 16. C2 21.72 18 26 28.72 121 56.80 10 2.7 -3.73 5 .1 12. 20 17.96 19 26 28.27 121 56.53 47. C 18.06 22.1 25 .34 21.09 2C 26 28 .33 121 57.17 t4 .9 12.63 18.2 22.65 28.40 21 36 2 8.22 121 58.65 121.7 -14.14 -2.7 4.72 10.44 22 36 27 .97 121 56. 13 35.4 19.85 22.9 25.24 21.62 22 36 27 .9C 121 56.78 70.6 11.36 17 .4 22,21 27.92 24 36 26.12 121 5 7.67 91.9 2.15 10. 1 16.2 9 21.8 1 25 36 27 .80 121 57.55 92. 1 -1.69 6 .2 12.55 16.09 26 36 27 .78 121 58.63 117.4 -9.40 0.7 8.80 14.17 27 36 27 .48 121 5 5.80 21.8 22. 12 24 .0 25.50 22.24 28 36 27 .40 121 5 7.08 81.2 3.18 10 .2 15.78 21.06 29 36 27.22 121 56.22 116.4 -13.10 -2.1 4.95 10.44 98 ST/i LATITICE LONGITUDE DEPTH FAA MF£A SB£ CSA 3C 36 27, ,18 121 31 36 26 , ,78 121 32 36 26 .78 121 3 a 36 26 . .68 121 34 36 26, ,3C 121 35 36 26 , ,4C 121 36 36 25 ,95 121 37 36 25, .98 121 36 36 25, .98 121 39 36 25 ,72 121 4C 36 25, ,47 121 41 36 25, ,2£ 121 42 36 25. ,3C 121 43 36 24. 92 121 44 36 24, ,6C 121 45 36 24, ,6C 121 46 36 24, ,45 121 47 3t 24 ,05 121 46 36 24, ,1C 121 49 36 24 .25 121 5C 36 24 ,C5 121 51 36 23 .78 121 52 36 22 .63 121 53 36 23. ,26 121 54 36 23 .4 7 121 55 36 23 .ce 121 56 36 22 .75 121 5 7 36 22 ,3C 121 58 36 2 2 .43 121 59 36 2 1 .5C 121 56.53 55.7 12.17 17. C 20.61 26.46 56. IC 51. 1 13.70 18.1 21 .63 27.65 56.95 78.9 -1.09 5 .7 11 . 14 16.43 56.22 122.3 -14.37 -3.8 4.5 7 IC . 53 55. 9C 41.6 16.63 2C.2 23. C 7 29.11 56.25 60.2 12.50 17.7 21.83 2 7.34 57. C5 97. C -6.36 2.C 8.6 7 14.45 56.30 75. 1 6.41 12.9 18. CS 24. IC 55. 4C 23.8 21.13 23 .2 24.81 31.86 55.85 4 6 . C 13.60 17.5 20.7 1 27.01 55. 3C 23.3 20.85 22 .9 24.46 31.31 55.67 48. 2 13.97 18. 1 21 .42 27.42 56.62 92.7 -3.70 4.3 10. tt 16.45 CC -3-2 35.1 17.08 20.1 22.52 28.95 55.82 52. 1 15.94 2C.4 24. CC 2 9.74 56.67 93. C -6.68 1 .3 7.72 13. 22 55.05 30. 1 18.51 21.1 23. 17 29.73 55. CO 38.3 14.91 18.2 20.84 27. 11 55.63 53.8 8.03 12.7 16. 3£ 22.19 56.25 73.5 -C.21 6. 1 11.17 16.6 1 56.83 97.8 -12.49 -4 .1 2.66 7.94 56.3 0 85.8 -8.63 -1.3 4.65 10.0 2 55.72 71.8 C.61 7.C 11 .93 17.47 54.52 40. 1 15.50 19. C 21.73 26.34 56.92 92.9 -11.73 -3 .7 2.66 7.83 55.72 65.7 -4.27 1.4 5.92 11.2-6 54.72 39.4 7.58 11. C 13.69 19.76 >t • Lu 72.9 -3.28 3.C 8.CC 13.27 57.27 69.4 -10.66 -3.C 3. 16 8. 20 5 7. C3 81.5 -7.36 -C.3 5.27 10.24 99 Slfi L^TITLCE LCNGITUCE. DEPTH FAA MF££ SE£ CBA 6C 26 2 1.45 121 55.77 57.5 4.54 9.5 13.4 2 18.66 61 26 2 1 .42 121 54.87 36. e 11.44 14.6 17. 14 22.92 62 26 2C.82 121 56.15 65.3 2.42 8 .C 12.52 17.57 63 26 2C .62 121 5 7.75 94.6 -IC.39 -2.2 4.2 9 9.22 64 36 2C.25 121 56.58 69.9 0.01 6 .C 10.64 15.75 65 26 2C.12 121 55. 2C 46.9 13.55 17.6 20.61 26. C5 66 26 19.42 121 55.67 55.3 10.07 14.8 18.64 22.68 67 36 2C .C7 121 54. CC 23.3 16.00 18 .C 19. 6C 25.66 66 36 18.68 121 54.57 35. 8 16.57 19.7 22.12 27.28 100 STA LATITIDE LGNGITUCE ELEV FAA SEA CBA A 26 2 1. 48 12L 55.37 7.6 22.86 22. Ci 27.57 B 26 31. C5 121 56.08 24.4 25.52 22 .6C 28.21 C 26 21. 2C 121 56.72 7C.1 28.21 20.36 26.33 D 36 2C, "2 "2 121 56.10 40.2 28.27 23.77 29.67 E 26 29, C -5 121 56.25 68 .6 32.65 24.97 31.11 F 26 29. 4 7 121 56.60 32.0 27.93 24.24 30.29 G 36 28. ,6C 121 56.12 35.4 28.66 24.7 2 30.83 H 36 26, ,32 121 56.12 16.8 27.51 25.62 31.77 1 36 21, 82 121 55.53 40.2 28.97 24.47 31.83 J 36 27. ,25 121 55.33 25.2 27.20 24.27 22.15 K 36 26. 8"? 121 5 5.47 39.3 28.24 23.84 30.62 L 36 26, 9C 121 5 5.63 13.7 26.38 24.6 5 31.51 M 36 26, 4C 121 55.27 15.2 25.31 23.61 30.89 N 36 21, ,82* 121 54.92 49.1 29.44 23.95 31.36 O 36 25. ,5C 121. 54.75 0.2 23.84 23. ec 22.54 P 36 25, ,42 121 54.77 0.3 23.97 23 .94 32.68 Q 36 25, ,28 121 54.78 0.2 24.04 24. CI 31.12 R 36 25 , 2 2 121 54.77 0.0 24.27 24.27 31.38 S 36 25, ,22 121 54.82 CO 24.27 24 .27 31.27 T 36 25 . 12 121 54.83 0.2 24.52 24.49 31.29 U 36 25. ,CC 121 54.72 29.3 27.98 24 .71 31.17 V 36 25 ,C2 121 54.83 14.9 26.29 24.62 31.09 w 36 24 ,57 121 54.78 50.2 28.55 22.92 29.43 X 36 24 ,ce 121 54.33 51.6 28.90 23. 1C 30.56 Y 36 22 .12 121 52.68 164.9 41.87 23.42 34.82 z 36 22 .72 121 54.08 45.7 28. 10 22.99 22.14 A' 26 2 2 .25 121 53.98 68.2 30.34 22. 7C 20.61 B' 36 2 2 ,8C 121 54.08 0.4 22.68 22.64 20. 12 C 36 22 .23 121 54.07 84 .1 20.44 21 .02 26.80 101 ST£ LATITUDE LONGITUCE ELEV FAA SEA CBA D' 36 21.58 121 53.92 147.8 34.96 18.41 26.57 E' 36 2C.97 121 53.71 134.7 3C.23 15.16 26.79 F' 36 2C.32 121 53.42 66.4 25.06 17.62 25.12 G' 36 15.92 121 53.38 38.4 21.98 17 .68 24.44 H' 36 19.65 121 53.62 42.1 24.29 19.59 25.38 1' 36 16.98 121 53.43 53.0 26.61 20.67 27.60 J' 36 18.57 121 53.10 44.8 26.13 21 .12 27.40 K' 36 ifi.07 121 52.62 38.1 25.84 21.57 27.35 L« 36 16.33 121 53.85 110.0 31.98 19.67 30.56 102 < ti 0 o >- ri IU 0, (X H > D H- ft »— i ;> s — ■• o o«-o— o ooc^o o wro*-^ o X «.<<— o I .U<1 —I OOWU "v. —. — « *-0"> ►Qi CO— 2 — 2 ~ — < ii •-* it ►-»-. ii .» _ ». 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Xh- rocM I--* I ■ - - X X X —►-« I— ■-« >tLUl/l OCM LO 00 ^H- »0 h-lC\ O O •vx „!— < ► • fc>-. ^^-i ^xo »-iL o o ^^^•^CMsO — *-» — — v^in n CM •. * «, >v ••k~,v •-"• i— tsO — •— • — »-^> \» — »>X) •• •> — * CQ —"CO V." » '*■«■ •■•O •— "O mZ x<-o-Ooci wo —o \o— cMni— win »-^ V»*J •■— JvQ ► •> Ns»vLL — -^1— I— -^-*-» — ~—— *V.LL ••(— CM— . 0~ O •-» r-4>— i •• ••O'-*'— iO-h>— < CM •>» -*-^ •• • c\)— »r0tNj-^ O • »-o— '- -jcjo-uowCj. ••m ii - o*-?^! ii om n » >3" •■cQ" m *• •-co •■ »-cq -^ -jt*^ ••«• •■ •~r^' *■ ••r>- •» <«* »->ot/>»^LL>OsOt/iNOvoi/)-o—' •> ••-•^»a>^ •.— »nu ».— » • «-» •■ *»mw »-~-' «— •»~- » vC>— i in*— nO*— <— »o •— < I— X — h- O — — I— Uw-'h I— — — -* — -<— '0OLU — I — I — I -H r-t — I , — I —J „( ,^ o — i cm en cr> O O O O rp 105 REFERENCES CITED 1. Andrews, R. S. , 1973. Corrections for Underwater Gravimetry. Naval Postgraduate School, Department of Oceanography, Monterey, Ca. (paper submitted for publication) 2. Brooks, R. A. , 1973. 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America Bull. , v. 81, p. 667-690. 25. Phifer, D. W. , 1972. Recent Marine and Thrust Faulting in the Santa Lucia Mountains of Soberanes Point Quadrangle Monterey County, California. Student Report, Naval Postgraduate School, Monterey, Ca. 35 p. (unpublished report) 26. Shepard, F. P. , 1948. Investigation of the Head of Monterey Submarine Canyon. Scripps Institute of Oceanography. Submarine Geology Report 1. 15 p. 27. Souto, A. P. D. , 1973. A Bottom Gravity Survey of Carmel Bay, California. M.S. Thesis, Naval Postgraduate School, Monterey, Ca. 57 p. (unptiblished report) 2 8. Swick, D. H. , 1942. Pendulum Gravity Measurements and Iso- static Reductions (U.S.C. k G.S. Spec. Pub. No. 232). U. S. Government Printing Office, Washington, D. C. 82 p. 29. Thompson, G. A. , and M. Talwani, 1964. Crustal Structure from Pacific Basin to Central Nevada. Jour. Geophys. Research, v. 69, no. 22, p. 4813-4837. 30. Trask, P. D. , 1926. Geology of Point Sur Quadrangle, California. University of California Publications Bulletin of the Department of Geological Sciences, v. 16, no. 6, 186 p. 31. U. S. Department of Commerce. 1973. Tide Tables, High and Low Water Predictions, 1973, West Coast of North and South America Including The Hawaiian Islands. U. S. Government Printing Office, Washington, D. C. 226 p. 32. Wollard, G. P. and J. C. Rose, 1963. Internation Gravity Measurements. George Banta Co. , Inc. , Menasha, Wise. 518 p. 108 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. Department of Oceanography 3 Naval Postgraduate School Monterey, California 93940 4. Oceanographer of the Navy 1 Hoffman Bldg #2 2461 Eisenhower Ave. Alexandria, Virginia 22314 5. Office of Naval Research 1 Code 480-D Arlington, Virginia 22217 6. Professor Robert S. Andrews 10 Department of Oceanography, Code 58Ad Naval Postgraduate School Monterey, California 93940 7. Professor Joseph J. von Schwind 3 Department of Oceanography, Code 58Vs Naval Postgraduate School Monterey, California 8. Lieutenant Walter B. Woodson, USN 3 USS Gallant (MSO-489) Fleet Post Office San Francisco, California 9. Dr. Howard Oliver 1 United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 109 10. Dr. S. L. Robbins United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 11. Mr. H. Gary Greene United States Geological Survey 345 Middlefield Road Menlo Park, California 94025 12. Gravity Section Naval Oceanographic Office Washington, D. C. 20390 13. Mr. H. B. Parks LaCoste and Romberg, Inc. 6606 North Lamar Austin, Texas 78752 14. Master R/V ACANIA Department of Oceanography Naval Postgraduate School Monterey, California 93940 15. Lieutenant Clayton H. Spikes 3303 Sycamore Place Carmel, California 93921 16. Dr. Robert E. Stevenson Scientific Liaison Office of ONR Scripps Institution of Oceanography La Jolla, California 92037 17. Captain W. B. Woodson, Jr., USN (Ret) Paradise Avenue Middletown, Rhode Island 18. Library, Code 3330 Naval Oceanographic Office Washington, D. C. 20370 19. Dr. Gary Griggs University of California, Santa Cruz Division of Natural Sciences Santa Cruz, California 95060 110 Unclassified SECURITY CL ASMFIC ATION OF THIS PAGE (When Data Entered) REPORT DOCUMENTATION PAGE READ INSTRUCTIONS BEFORE COMPLETING FORM 1. REPORT NUMBER 2. GOVT ACCESSION NO 3. RECIPIENT'S CATALOG NUMBER 4. TITLE (and Subtitle) A Bottom Gravity Survey of the Continental Shelf Between Point Lobos and Point Sur, California 5. TYPE OF REPORT ft PERIOD COVERED Master's Thesis September 197 3 6. PERFORMING ORG. REPORT NUMBER 7. AUTHORf*,) Walter Browne Woodson, III 8. CONTRACT OR GRANT NUMBERft; 9. PERFORMING ORGANIZATION NAME AND ADDRESS Naval Postgraduate School Monterey, California 93940 10. PROGRAM ELEMENT. PROJECT, TASK AREA 4 WORK UNIT NUMBERS II. CONTROLLING OFFICE NAME AND ADDRESS Naval Postgraduate School Monterey, California 93940 12. REPORT DATE September 1Q73 13. NUMBER OF PAGES 112 14. MONITORING AGENCY NAME ft ADDRESSf// dttterent from Controlling Otllce) 15. SECURITY CLASS, (ot thia report) Unclas sified 15«. DECLASSIFI CATION/ DOWN GRADING SCHEDULE 16. DISTRIBUTION STATEMENT (ol this Report) Approved for public release; distribution unlimited 17. DISTRIBUTION STATEMENT (ol the abatract entered In Block 30, It dlllerent from Report) 18. SUPPLEMENTARY NOTES 19. KEY WORDS (Continue on reverie aide If neceeaary and Identify by block number) 20. ABSTRACT (Continue on reveree tide If necaaaary and Identity by block number) From an occupation of 68 ocean bottom and 38 land gravity stations between Pt. Lobos and Pt. Sur, California, a complete Bouguer anomaly map was produced and analyzed. The steps in data reduction leading to the complete Bouguer anomaly field is presented, unique features of which are associated with bottom gravimetry. The geological interpretation of the gravity data shows excellent correlation with earlier seismic records of the proposed offshore extension of the Serra Hill fault, a structure long associated with DD .^s 1473 (Page 1) EDITION OF 1 NOV 65 IS OBSOLETE S/N 0102-014- 6601 | 111 Unclas sified SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered) Unclassified i'UCU^ITY CLASSIFICATION OF THIS PAGEfH'hen Data Ente/sd) 20. the Sur-Nacimiento fault zone. Two dimensional models of gravity anomaly profiles were constructed across this fault and another fault located several kilometers to the northwest and extending into the western tributary of the Carmel Canyon. The results indicate a minimum vertical displacement of the basement of approximately 2 km on the southwest sides. It was con- cluded that these two faults are one in the same. Evidence is presented which indicates that the Palo Colorado fault zone, located approximately 2 km to the east, parallels the Serra Hill fault and subsequently leads into the eastern tributary of the Carmel Canyon. DD Form 1473 , 1 Jan 73 S/N 0102-014-6601 (BACK) Unclassified SECURITY CLASSIFICATION OF THIS PAGE(Wi»n Dae Enffd) 112 146318 Thesis W8436 Woodson c i A bottom gravity sur- vey of the continental shelf between Point Lo- bos and Point Sur, Cal- ifornia. Thesis 146318 V/8436 Woodson c,l A bottom gravity sur- vey of the continental shelf between Point Lo- bos and Point Sur, Cal- i fornia. thesW8436 A bottom gravity survey of the continent 3 2768 001 90624 1 DUDLEY KNOX LIBRARY