REGIONAL AND STRUCTURAL EVOLUTION OF THE REGION, CENTRAL

H. Gary Greene Branch of Pacific Marine U.S. Geological Survey 345 Middlefield Road, MS-999 Menlo Park, California 94025

ABSTRACT INTRODUCTION

The tectonic and structural evolution of the Monterey The coastal region of from Mon­ Bay region of central California is complex and diverse. terey Bay to , encompasses a geologically Onshore and offshore geologic investigations during the complex part of the of North America, past two decades indicate that the region has been subjected the Province (Fig. .1). Interest and to at least two different types of tectonic forces; to a pre­ knowledge of the geology of this region has increased sig­ Neogene orthogonal converging plate () and a nificantly over the past two decades due to detailed map­ Neogene-Quaternary obliquely converging plate (transform) ping and the recasting of the Tertiary tectonic history of the tectonic influence. Present-day structural fabric, however, area in light of plate tectonic concepts (McKenzie and appears to have formed during the transition from a sub­ Parker, 1967; Morgan, 1968; Atwater, 1970; Atwater and ducting regime to transform regime and since has been Molnar, 1973; McWilliams and Howell, 1982; Howell et al., modified by both strike-slip and thrust movement. 198Sa; McCulloch, 1987). The purpose of this paper is to review the structure and tectonics of the region, with empha­ Monterey Bay region is part of an exotic allocthonous sis on the Monterey Bay area. structural feature known as the or Salinia tectonostratigraphic . This block is proposed to have Monterey Bay is a nearly crescentic bay that lies along originated as part of a a considerable distance the central California coast approximately 115 km south of south of its present location, somewhere between the Trans­ San Francisco (Fig. 1). It was named by the Spanish explorer verse Range (being the displaced segment of the southern Sebastian Vizcaino who in 1602-1603 voyaged up the coast of Sierra-Nevada Mountain Range) and the latitude of Central California to as far as , and named many America. It consist of Cretaceous granodiorite basement prominent land features with the familiar names we know with an incomplete cover of Tertiary strata. Paleogene rocks and use today. The bay measures 37 km across its mouth are scarce, evidently stripped from the block during a time between Point Santa Cruz a::ld Point Pif\os. The towns of of emergence in the Oligocene time. Santa Cruz and Monterey are located on the bay's north and south shores, and Moss Landing is positioned approximately The Salinian block is presently located on the Pacific in the middle of the east shore (Fig. 1). plate at the Pacific and North American plates' active tec­ tonic boundary. This boundary shifted to a transform mar­ Topography is varied along the coast between San gin approximately 21 Ma when the Mendocino triple-junc­ Francisco and Point Sur. Steep bluffs with flat-topped ter­ tion passed through the Monterey Bay region. Since that races interspersed with sandy pocket beaches, including the time the Salinian block has been moving northward along half crescent-shaped embayment of Half Moon Bay, are the San Andreas zone and basin and ridge topography characteristic of the coastal region from San Francisco to was generated within the strike-slip faults of the San An­ Sequel Point. From Sequel Point southward almost to Moss dreas fault system. Sometime between 5 and 3.5 Ma, due to Landing, cliffs fronted by sandy beaches are prevalent. the shift in the direction of motion and the de­ Broad sandy beaches backed by large dune fields stretch velopment of a more orthogonal convergence between the southward in an unbroken line from Moss Landing to the Pacific and North American plates, compressional forces rocky headland of the Monterey Peninsula, and from this became more pronounced in the region. The 1979 Loma point southward to Point Sur steep, rocky cliffs predomi­ Prieta and recently reprocesses multichannel nate. seismic-reflection data offshore indicate that the the Mon­ terey Bay region is presently being subjected to both strike­ Low to high relief mountain ranges and broad, flat­ slip (wrench) and thrust (compressional) type tectonic floored valleys are prevalent farther inland. The Santa Cruz forces. and Gabilan mountain ranges dominate the topography in the northern and central half of the region. Two major rivers enter Monterey Bay from these highlands through well defined valleys. The San Lorenzo River enters Monterey Day

31 32 at the town of Santa Cruz, and the empties into Monterey Bay 8 km south of Moss Landing. South of Mon­ the bay 6 km north of Moss Landing (Fig. 1). Elkhorn terey, the west flank of the drops Slough, an old river estuary that today is occupied only by abruptly into the . The valley of the Carmel River, tidal salt marshes, extends inland from Moss Landing for which empties into the Pacific Ocean in Carmel Bay, and the more than 10 km. The broad, extensive and , which enters the ocean 6.5 km north of Point the northern Santa Lucia Range are the dominant topo­ Sur, are dominant topographic features in the southern part graphic features in the southern half of the region, where the of the region (Fig. 1). Salinas River is the major drainage system and empties into

122° 380 i 380

CONTOURS IN METERS

0 25

KILOMETERS

37° 37°

3500

Figure 1. Generalized regional index map of the central California coastal region showing locations of major cultural and gco­ morphic features. Cabrillo Ano Nuevo Canyon Canyon

MONTEREY PENINSULA Carmel ~·. - ---- Canyon

Shepard Meander

Figure 2. Submarine physiographic diagram generated from SeaBeam data collected by NOAA. Diagram courtesy of NOAA, National Ocean Services, Rockville, Maryland. w 34

SUBMARINE PHYSIOGRAPHY nes make up the Santa Lucia-Orocopia allocthon. The Sur­ Obispo composite terrane is composed of the Stanley Moun­ The undersea topography of the Monterey Bay region tain and San Simeon which lie west of the Sur­ is as diverse and even more spectacular than that onshore. Nacimiento and the zones (Fig. 3). The The submarine Ascension-Monterey Canyon system that Stanley Mountain terrane is composed of a Middle Jurassic comprises Monterey , fan-valley, and fan ophiolite overlying thick Upper Jurassic to Upper Creta­ dominates the submarine topography, exhibiting much ceous basin strata that resemble, and are coeval with, greater relief than similar features onshore (Fig. 2; see the . Lying structurally beneath the Greene, this volume). Monterey Canyon was first described Stanley Mountain terrane is the San Simeon terrane, a Fran­ by Davidson (1897), who noted that ciscan melange formed approximately 72 Ma.

" ... at Monterey Bay there is a complete The Monterey Bay region is located on the major breaking down of the Coast Ranges from 25 structural element known as the Salinian block of Page miles (40 km); with the n1ountains receding (1970), or Sa.Jinia terrane of Howell et a1. (1985a, b). The well inland. Into this broad and deep bight Salinian block consists of composed princi­ heads the finest of submerged valleys." pally of Cretaceous granitic rocks and metamorphic rocks of indeterminate age, it is flanked on either side by a heteroge­ Monterey Canyon cuts deeply into the flat shelf of Monterey neous aggregation of Jurassic and Cretaceous eugeosynclinal Bay, heads less than 2 km seaward of the mouth of Elkhorn rocks assigned to the Franciscan assemblage. The northeast Slough, extends westward for over 90 km, and bisects the and southwest boundaries of this block are formed by the bay along its trend (Fig. 2). It is one of the world's widest, San Andreas and Sur-Nacimiento fault zones, and it extends deepest, and longest submarine canyons; its overall dimen­ from the northward almost 800 km to sions are comparable to those of the Grand Canyon of the Cape Mendocino (Page, 1970; Silver et at., 1971). It was (Martin, 1%4; Shepard and Dill, 1966). initially proposed that the Salinian block was a mass of Sierran granitic basement displaced northward during Other canyons of the system, Ascension (named for Fray Ascension, a Spanish priest who accompanied the Vizcaino expedition), Af\o Nuevo and Cabrillo Canyons, cut 126• 123• into the continental slope about 35 km northwest of Santa Cruz. Several of the eleven branches that compose the head­ ward parts of these canyons cut the distal edge of the conti­ nental shelf, but do not continue to the shoreline. These canyons converge at a depth of about 2,200 m and connect 31' EXPLANATION 311• with Monterey Canyon at a depth of 3,290 m. Monterey ____....._ P-~clicn ..., Canyon is joined directly with two smaller canyons, Soquel ..... and Carmel Canyons (Fig. 1). Soqucl Canyon heads far out , TEARAHES on the , in the northern part of Monterey .. 38' Bay. The head of Carmel Canyon lies in Carmel Bay, 58 km south of the head of Monterey Canyon, only 30 m offshore. M A Both Carmel and Soquel Canyons steepen in gradient as 37• they approach Monterey Canyon.

Further north along the coast, between Af\o Nucvo 36• Point and San Francisco, two other submarine canyons arc present (Fig. 2). One canyon, located off Pescadero Point, is unnamed and is restricted to the continental slope (Fig. 1). 35' North of this canyon, offshore and west of Half Moon Bay, ... Pioneer Canyon is located. This canyon also does not cut the continental shelf, but is restricted to the slope where it gener­ •M "+"-:!:,.,...... ::!;:-'--:i,,.!:-'--:tt-1--'--­.. TlloE "8f'I ally trends in a straight line between Pioneer and Guide Scamounts, two volcanic located at the base of Figure 3. Map of generalized boundaries of tectonostratigra­ the continental slope. phic tcrranes of coastal central California (modified after Howell ct al., 1985a) showing major related faults, and a dis­ TECTONIC SEITING assemblage diagram of tcrrane collision and trajectory as proposed by V cddcr ct at. (1983). Faults: MFZ = Mendocino The geology of the coastal central California region is Zone, N = Nacimicnto, S = Sur, SA = San Andreas, diverse and tectonically defined by allochthonous blocks or SC =San Gregorio, SLB = Santa Lucia Bank. Place names: tectonostratigraphic terranes of Coney et al. (1990), Jones CM =Cape Mendocino, FR = Fort Ross, PA =Point Arena, (1983) and Howell et al. (1983, 1985a). McWilliams and PD = Point Dclgada, SF= San Francisco. (After McCulloch, Howell (1982) describe two major tectonic components for 1989.) the coastal central California region, the Sur-Obispo rompos­ itc tcrrane and the Salinian terranc

Notth A.

50'

BL8 Ben Lomond block 411' GB GtbiM block MHB ML Hllmillon block Cl> 68T S... 9-nilo tn>ugh 'g 30' SFBB SM FranciM:o Bay bbdt ::J SG s.inMO,.._, SLB SarULuc:labMln = 20' 68 Surblodc i TMB T°"' Ml. block see SarU Cruz--. 2 MB Mo,...19)' block CG 1(1' 10' Q.

NI., Ci.ti<, 1 ll30. 10'

South B. 160 140 120 100 8() 60 40 20

Age (Myr BP) Bl.H Ben Lomond hiol> BLF e.n l.Mnond f..,I Figure f. Paleolatitude of the Salinlan and Stanley Mountain tcmmcs as BB ~buin CCF cwm.. c ..yon f ...1 a function of time, compared with paleolatitude of "target area" on North CFA C.,,tr.. Franciocan.,.. America for present-day position, 35-39° north latitude. Stippled path GF o.bilan fail represents the two terranes after suturing. The North American and GSA Gobi., l1oble area MF Monlerey f..,I Salinlan/Stanley Mountain tracks intersect at -SO.SS Myr BP. MG Monlereygnben • Data from Sur-Obispo terrane.. Paleomagnetic data from Salinian MSA Monlel9)' ..m._ area NFA Nolthem Francilcan.,.. terrane. Arrows In lower right give northward velocity In cm/yr for PCF Palo Color.do f..,I reference. The dashed and solid lines represent altenative paleolatitudcs see Sanla Cruz bMln for the Stanley Mountain terrane depending on the choice of magnetic SF Surf..,I 6GF San G199orio tail polarity. (AfterMcWilliams and Howell, 1982.) TF T ulercloa tail VF Verve1t1 taia

Tertiary time by movement along the , - Martin. , 064. with the Sur-Nacimiento fault zone representing a displaced segment of the boundary between Sierran and Franciscan c. basement rocks (Hill and Dibblee, 1953; King, 1959; Page, 1970). However, McWilliams and Howell (1982), based on paleomagnetic data from upper Cretaceous strata on the BF 9"'-f..,I Salinian block, proposed that this exotic terrane was dis­ BlH e.n L.Mnond high placed from a location 2,500 km to the south. They show BLF e.n Lomond fail CF Chupine1tail that the Stanley Mountain and Salinian terranes collided in GB GobNn block KCF King Cily f..,I Cretaceous time (-72 Ma), in the latitude of Central America, LHB La Honda buin to form a new amalgamated terrane that then moved north­ MB Monlerey blodc SAF San Andre• f .... ward (Fig. 4). Accretion of the Sur-Obispo composite terrane SCF Sanla Cruz f..a SNF Su<·Nacirniento fail to North America occurred in late Paleocene to early Eocene TF Tulardoe fail VF Vergolt1 tail time (McWilliams and Howell, 1982). ZF z.y.,t• f..,I ZV8 Zay.,t•V•~ block Tectonic Blocks of the Salinian Terrane Aft., Roea and Bt8bb, 1072. Beginning with Lawson in 1914, many geologists have described the structure of the Coast Ranges in terms of D. orographic or tectonic blocks (Clark and Rietman, 1973). Likewise the Salinian block in the Monterey Bay region was described by Clark (1930) as comprising a series of smaller, BLB e.n Lomond block elongate, northwest-trending, uplifted blocks and basins. MB Monlerey block 68 8UnM block The basins-Santa Cruz basin (La Honda basin of Oark and &LB Santa Lucia block ZV8 Zayant•V•~ block Rietman, 1973 and Stanley, this volume), Salinas , and BLF e.n Lomond faul Santa Lucia basin-are separated from each other by faults V: Zay_,lt F..,I SS SurSiwr and by structural highs known as the Ben Lomond, Gabilan, AMH ~naior>-Monlerey high SH Sur high and Toro Mountain blocks (Fig. Sa). The most northerly ML Monlerey low PC-SGFZ Palo Coloredo-San Gregorio fault zone MBFZ Monlerey Bey fault 2•ne MF Mon1erey _c.,,on 1... a Figure 5. Historical development of subdivision of the Salinian tcrrane Nier G..-. 1077. in the Monterey Bay region into tectonics, "orographic~ blocks (modified after Greene, 19?7). 36 basin delineated in this region by Clark (1930) is the Santa offshore area east of the Palo Colorado-San Gregorio fault Cruz basin (La Honda basin), a synclinorium in a thick zone (Fig. Sd). sequence of Tertiary sedimentary rocks located between the central and the northern Gabilan The southern edge of the uplifted Ben Lomond block Range. This basin (described in greater detail by Stanley, this abruptly plunges to the southwest in northern Monterey Bay volume) is separated from the Ben Lomond block to the (Fig. Sd}. Strata of the Monterey block to the south lap onto southwest by the Ben Lomond fault. Evidence from gravity and wedge out against this . The Monterey data tends to confirm Clark's (1930) belief that the Santa block forms a relatively flat, shallow basin that extends from Cruz basin (La Honda basin) extended southeastward to the the Zayante fault westward to the Palo Colorado-San Grego­ San Juan Bautista area during early Tertiary time (Oark and rio fault zone offshore, where the block may be truncated Rietman, 1973). The largest structural depres.sion of the (Figs. 5d and 6). In the southwestern part of Monterey Bay region, variously termed the Salinas graben (Oark, 1930), the northern edge of the uplifted Santa Lucia block, which Salinas trough (Starke and Howard, 1968), and King City forms the northern Santa Lucia Range and the Monterey flexure (Fairborn, 1963), extends from Monterey Bay down Peninsula. is separated from the Monterey block to the north the Salinas Valley to King City, and is bounded on the north­ by Monterey Canyon. To the east, faults within the Monterey east and southwest by the Cabilan and Toro Mountain Bay fault zone mark the boundary between the Santa Lucia blocks. Southwest of the Toro Mountain block is an irregular and Salinas blocks. The Salinas block contains sediments of area named the Santa Lucia basin by Clark (1930). the Salinas basin that lap onto the Monterey high (Fig. Sd). The Monterey high separates the Monterey and Salinas Martin and Emery (1%7/ divided that part of the blocks (Fig. 6). Salinian block underlying the Monterey Bay area into three tectonic blocks: the Gabilan stable area, the Monterey semi­ Regional structure and stratigraphy are different and stable area, and the Monterey graben (Fig. Sb). The Gabilan less well understood west of the Palo Colorado-San Gregorio stable area is bounded on the northeast by the San Andreas fault zone than to the east. Identification of tectonic blocks fault zone, and is separated from the Monterey graben and west of the Palo Colorado-San Gregorio fault zone is difficult Monterey semi-stable area to the southwest by the Gabilan­ because of the absence of data, and because true and acousti­ King City fault of Martin and Emery (1967) . The Monterey cal basement are not correlative. However, a discrete zone of graben and Monterey semi-stable area, in tum, are separated complexly deformed rocks termed the Sur Sliver by Greene from the central Franciscan area to the west by the Carmel (1977), lying between the Palo Colorado-San Gregorio fault Canyon fault (later extended and named the Palo Colorado­ and the Ascension fault of Greene (1977), separates the San Gregorio fault zone by Greene et al.,1973), and the San contrasting tectonic regimes of the Monterey Bay area and Gregorio- zone by Graham (1978), and from the continental slope (Figs. 6 and Sd). Because of right-slip each other by the so-called Monterey fault located in lower and compression (thrusting) along the Ascension Fault and Monterey Canyon (Martin and Emery, 1967) . faults within the Palo Colorado-San Gregorio and Monterey Bay fault zones, differing structural units are juxtaposed on Based on their petrologic analysis of basement rocks, either side of the Sur Sliver, which is characterized by nei­ Ross and Brabb (1972) have divided the Salinian block in the ther positive nor negative displacement. Monterey Bay region into four tectonic sub-blocks: the Zay­ ante-Vergeles block, Ben Lomond block, Gabilan block, and Assuming acoustical basement west of the Palo Colo­ the Monterey block (Fig. Sc) . The Zayante-Vergeles block is rado-San Gregorio fault zone to either directly represent true bounded on the northeast by the San Andreas fault, and is basement or to indirectly express basement relief, three separated from the Ben Lomond and Gabilan blocks to the tectonic blocks were delineated along the continental slope southwest by the Zayante-Vergelcs fault. The Ben Lomond seaward of the Monterey Bay area (Greene, 1977). These are and Gabilan blocks, in tum, are separated from each other the Ascension high, the Monterey low (Monterey basin}, and by the Santa Cruz fault of Ross and Brabb (1972), which the Sur high (Figs. 5d and 6). The Ascension high is an since has been dropped because no evidence of it has been acoustical basement supporting the submarine upland found. The Monterey block is bounded on the southwest by between the Ascension and Monterey submarine canyon the seaward projection of the Sur-Nacimiento fault zone, and systems (Greene, 1977; Nagel et al., 1986). It is bounded both is separated from the Ben Lomond and Gabilan blocks to the on the cast and west by faults that appear to displace only northeast by the northwestward extension of the King City Miocene and older strata. To the southeast these same faults fault. Oark and Rietman (1973) consider the Ben Lomond bound the Monterey basin, which is bisected by the lower and Gabilan blocks of Clark (1930) to be a single block, Monterey Canyon. The Sur high lies in fault contact along terming it the Ben Lomond-Gabilan block. This block sepa­ the southwest side of the Monterey basin. This is an acousti­ rates the Santa Cruz basin of Oark (1930) from the Santa cal basement high extending northwestward from Point Sur. Lucia basin (Fig. 5) • Tectonic Movement The "orographic" and "tectonic" blocks of the various authors mentioned above were redefined by Greene (1977) The central California coastal region records a com­ on the basis of subsurface geophysical (seismic-reflection) plex late Cenozoic sedimentary history. and data collected in Monterey Bay. Four blocks-the Ben Lo­ depression have produced a succession of regressive and mond, Monterey, Salinas, and f.:mta Lucia-appear to have transgressive sedimentary units (Fig. 7), while contempora­ had a major influence on the Tertiary stratigr.:iphy of the neous right-slip along faults of the San Andreas system has 37

Figure 6. Offshore basement contour map of the Monterey Bay region, central California (after Greene, 1977; Greene and Oark, 1979).

EXPL ANATION

------F'AULT?----- Solid w rier e well Oehne d. dosnN wher e oppro.1u m otely loca t ed or p oori;, oe'lined, Que rie d where Q~st1ono bly 1oc oted or obscure

8ASEMENl-+-- RIDGE CRE ST Arrow tnd1cot e'° 01rect1on ot ph.inge --+ -7 BASEMENT TROUGH AX IS A r-row 1nd1cotes C11rect1on o t plunQe ----.----- ·-....__..,,,. PAL[OORA INAGE LI NES

(;:~ CON1AC1 · 8ASE MENT EXPOSURE -.._. ... o~6o0o-- BASEMENT CONTOUR Two d 1g1 t numbers (.. O> represent u ism1c two·w oy travel ttme 1n seconds; foul" d 1g1t numbers (4000) reoresent depth 1n meters bose

~ -733 EXPlORATORV OIL ANO GAS WELl S.ng1e to two c1tg1t numbers ore wt!'ll numbers; tN"ee dtQrt numbers o r e depth 1n m e t e rs b elON l e ltfel t o surface of boseme"t rocks

Figure 7. Generalized stratigraphic cross section showing transgressive­ rcgresslve relationships of Neogene sedimentary units In the onland re­ gions adjacent to Monterey Bay (after Greene and Clark, 1979).

N s scons VALLEY early Pliocene Tp i Tse

Oar - Aromas Sand; eolian - Pleistocene OTP - Paso Robles Formation; non-marine, locally marine at base · late Pliocene (?) to early Plelstocene Tp - Purisima Fonnation; marine, brackish Tac - Santa Cruz M.udstone; ~arine. ~If deposits } marine nearshore ID shelf deposits Tsm - Santa Marganta Fonnaton; manne, nearshore ' Tm • Monterey Fonnation Tio - Lompico Sandstone gr - Granitic rocks 38 offset major structural and lithologic elements (Hill and during Neogene time. However, Howell (1975) and Clarke et Dibblee, 1953; King, 1959; Hamilton, 1969; Page, 1970; al. (1975) argue for a Late Ct'etaceous episode of slivering Suppe, 1970). In addition, diastrophism resulting in the south of the Monterey Bay region in order to create the deformation or removal of some sedimentary units has borderland physiography inferred for Late Cretaceous and produced three regional and several local unconformities Paleogene rocks of west central California. The style of within the upper Tertiary rocks (Fig. 7). In the Monterey Bay faulting observed in the Monterey Bay offshore area sug­ offshore area mainly the Neogene history is recorded in the gests that the Salinian block in this region has a history of sedimentary rocks, whereas onshore the Neogene and lo­ tectonic "slivering'' that continues into the present (Greene, cally the Paleogene histories are recorded. Paleogene units 1977). Elongation of the block appears to be resulting from appear to have been present offshore, but were extensively strike-slip along faults inside the block that are within the eroded by pre-middle Miocene erosion, possible being San Andreas fault system, such as the Palo Colorado-San preserved in small wedges on the Monterey shelf and in the Gregorio and Hayward fault zones (Greene et al., 1973; deeper parts of offshore sedimentary basins. Greene, 1977; Greene and Clark. 1979; Clark et al., 1984). Also, stresses built up in the San Andreas fault system are During Late Cretaceous and early Tertiary time, released along faults within the Monterey Bay fault zone, subduction and underthrusting of an oceanic (Farallon) plate considered part of the San Andreas fault system. Burford occurred along the continental margin off central and south­ (1971) noted that a sequence of fault creep and small ­ ern California (Atwater, 1970; Atwater and Molnar, 1973). quakes on the San Andreas fault near Pinnacles National Monterey Bay at this time was located in the north part of Monument abruptly ceased with a magnitude 4. 3 earth­ the Continental Borderland, near the quake (August 3, 1970) in Monterey Bay, probably within present location of the Transverse Ranges. Subduction the Monterey Bay fault zone. ceased in the Monterey Bay in early Tertiary time (-21 Ma), coincident with the northward migration through the region One probable effect of slivering the Salinian block is of the Mendocino and the local subduction of to produce a serrated, rather than a straight, western bound­ the . Thereafter the relative motion between ary for the block. Fragmentation of this margin as the Salin­ the Pacific and North American plates was expressed as ian block moves northward along the San Andreas fault strike-slip along the San Andreas fault system; in the Mon­ seems likely; in this case fragments and slivers of basement terey Bay region this system is composed of parallel to sub­ rocks may have been pushed ahead of the main block or parallel faults located between Ascension fault offshore and shoved out alongside the block, where they exist as isolated the Hayward- zone onshore (Fig. 8) (McKen­ pods west of the Sur-Nacimiento and Palo Colorado-San zie and Parker, 1967; Morgan, 1968; Atwater, 1970; Atwater Gregorio fault zones (Greene, 1977). and Molnar, 1973). Although the boundary between these two rigid plates was commonly portrayed as a single The recent 1989 7.1 magnitude Loma Prieta earth­ through-going , several workers think that quake exhibited both horizontal (right-lateral slip) and some motion was taken up on other faults of the system vertical (thrust) motion (Plafker and Galloway, 1989; see (Atwater, 1970; Johnson and Ncrmark, 1974; Graham, 1976; below). Although the San Andreas fault is Greene, 1977; Greene and Clark. 1979). Movement on the primarily a straight, right-slip fault zone, a prominent bend San Andreas fault itself only accounts for about 40 percent of in the fault at the location of the epicenter and slight conver­ the relative plate motion (Plafker and Galloway, 1989). gence in relative plate motion favor both horizontal and vertical offsets (Donna Eberhart-Phillips, written comm., Cox and Engebretson (198.5), based on shifts in the 1989). The Salinian block, being carried along by the Pacific alignments of volcanic centers along the Hawaiian hotspot plate, was thrust up onto the , as well trace, proposed that many of the compressional features as pushed right-laterally to the north. Reprocessed multi­ formed parallel to the San Andre.as fault in the last 5-3 M.y. channel seismic-reflection profiles collected by the uses in result from a shift in Pacific plate motion with an increase in 1983 along the western margin of the Salinian block also plate convergence continuing from the Pliocene to Pleisto­ show evidence of thrusting along this margin (Fig. 9). Sev­ cene time. Pollitz (1986), however, based on correlations of eral lines across the southern offshore extension of the Palo computed hotspot tracks within the Pacific and the Nazca Colorado-San Gregorio fault zone exhibit thrust faults verg­ hotspot, places the time of plate motion change at between 5 ing to the east that may be the result of the shift in plate and 3.2 Ma. fie suggests, based on work by Epp (1978), that motion proposed to have occurred 8-3 Ma (Cox and Enge­ the slip rate along the San Andreas fault decreased by about bretson, 198.5; Pollitz, 1986). Plafker and Galloway (1989) 22 mm/yr since 3.2 Myr ago. This is also the time that the state that compression associated with oblique convergence divergence within the started (Mammer­ between the Pacific and North American plates, which is ickx, 1980). Both Cox and Engebretson (198.5) and Pollitz perpendicular to the San Andreas fault, caused uplift of the (1986) relate the shift in plate motion to plate torque brought mountains and deformation of younger rocks within the about from slab detachments within the western Pacific. Santa Cruz Mountains. Mclaughlin (1974) indicates that thrust faults that bound the northeastern margin of the Johnson and Normark (1974) and Howell (1976) have southern Santa Cruz Mountains are active with the moun­ suggested that slivering and northwestward extension of the tain side moving upward and northeastward relative to the Salinian block occurred along faults west of the San Andreas valley side. 39

Therefore, it appears that there may be a series of gated and pulled apart by northward transcurrent fault inclined fault surfaces along which the Santa Cruz Moun­ movement, but is also being sheared by thrust movement tains, and perhaps the Gabilan and Santa Lucia Ranges, are resulting from compression and pushed further eastward being thrusted. The Salinian block is not only being elon- onto the North American plate.

~ N j

0 10 20 30

0 10 20 30 ~ so Kilomel..

I I I I I '' ! ' ' \ ' \ '' \ '\ \ ' \ 1'' 'I \ '\ I \ ' \ '.

Figure 8. Fault map of the central coastal region of California. 40

A A' Ascension Fault San Gregorio, Fault Zone Monterey Bay Fault Zone NW ' SE '"" CDP

Figure 9. Preliminary Interpretation of multichannel seismic-reflection profile across the Palo Colorado-San Gregorio fault zone near Santa Cruz showing thrust faults. See Figure 1 for location. US. Geological Survey unpublished data.

SfRUCTURE represented by a fault (the Ascension fault of Greene, 1977) located along the edge of the continental shelf north of The structure of the Monterey Bay region is complex Monterey Canyon. and is produced largely by post-Miocene tectonic events. Major structures in the Monterey Bay region include faults, Faults folds, and fault-bounded basement ridges associated with sedimentary basins (compressed horsts and ). The San Andreas Fault Zone structural grain of the region is variable in trend, and may chronicle shifts in or changes in the Pacific­ The San Andreas fault zone is the longest, most con­ American plate boundary since Miocene time. Structural tinuous fault zone in California, extending from the Salton trend is generally northwest-southeast for the region as a Sea at the extreme southern end of the state for nearly 1,300 whole. However, the Palo Colorado-San Gregorio fault zone km to Cape Mendocino in the north. It is part of a seismi­ trends more north-south than other major faults in the re­ cally active system, a transform fault system (the San An­ gion, obliquely truncating structures in Monterey Bay (Fig. dreas fault system) that marks the boundary between the 8). Structures west of the Palo Colorado-San Gregorio fault Pacific and North American plates. Relative plate motion zone fan out westward away from the fault zone, with a along this right-lateral strike-slip boundary is 5.6 cm/yr pivot point somewhere south of Point Sur. Faults farthest (Circum-Pacific Map Project, 1985). Since its inception (- 30 west are oriented more nearly east-west than structures Ma), over 300 km of displacement has occurred (Plafker and nearer the fault zone. South and west of the pivot point, Galloway, 1989). The San Andreas fault system not only structures are oriented nearly north-south. The two major includes the San Andreas fault zone, but many other faults fault zones present along the coastal region of central Cali­ that lie up to 50 km away from the San Andreas fault zone; fornia, the San Andreas and the Palo Colorado-San Gregorio for example the Palo Colorado-San Gregorio, Monterey Bay fault zones of the San Andreas fault system, appear to con­ and Hayward-Calaveras fault zones are all considered part trol structural development of the basins and ranges, al­ of this system of faults. though recent (last 3 m.y.) plate convergence has imprinted compressional structures onto !"trike-slip features (Cox and In the Monterey Bay region the San Andreas fault Engebretson, 1985). Faults in the Monterey Day region lie zone is composed of several faults in addition to the San primarily within two major, northwest-trending, converging Andreas, including the Sargent, Dutano, and Zayante-Verge­ fault zones, the Palo Colorado-San Gregorio and Monterey les faults (Fig. 10). All of these faults generally trend parallel Bay fault zones (Fig. 10). The probable offshore extension of or sub-parallel (oriented northwest-southeast) to the San the Sur-Nacimiento fault zone may connoct with, and be­ Andreas fault. An exception occurs where the Zayante fault come part of the Palo Colorado-San Gregorio fault zone or, swings westward and becomes east-west trending near as proposed by McCulloch (1989), could have been displaced Scotts Valley. In this area faulting is complex with the Ben along the Palo Colorado-San Gregorio fault zone and now be Lomond fault extending away to the south from its intersec- 41

'

~ 36°45' Cabillm 36°45'

··... ·· ..

Vall~

36°30' 36°30'

Salinas ' .... ,, ... ,, s,,, ... High ...... ; i ... I s.. r. I . ~ l I QI I I I 36115' I I 36°15' 122°15' 122°00' 121 °45' 121°30' 121°15'

Figure 10. Structural sketch map of the Monterey Bay region showing locations of faults, folds and major physiographic foatures. Outline of submarine canyon shown In broad, gray lines; Monterey canyon axis shown by broad, gray, dashed line. 42 tion with the Zayante fault. The Ben Lomond fault is an Silver (1977) first proposed 80 to 90 km of right-lateral arcuate fault that marks the eastern face of Ben Lomond offset on the offshore San Gregorio fault. Others followed Mountain. It appears to be quiet seismically and may not be with estimates of differing amounts of offset; Greene (1977), an within the San Andreas fault system. based on proposed offset of submarine canyons, proposed 70 km as did Howell and Vedder (1978); Graham and Dick­ Palo Colorado - San G~orio Fault Zone inson (1978) proposed 115 km based on matching pairs of onland geologic features. However, Oark et al. (1984), The Palo Colorado-San Gregorio fault zone is a nar­ matching up the lithologies of the area with the row (approximately 3 km. wide) feature defined by at least area (Fig. 1) and making a detailed biostratigra­ two fairly continuous faults (Fig. 10). It appears to merge to phic analysis of the region proposed that 150 km of right-slip the south with the Serra Hill and Palo Colorado faults on­ has occurred along the Palo Colorado-San Gregorio fault shore (Trask, 1926; Jennings and Strand, 1958; Gilbert, 1971), zone since late Miocene time. and to the north with the San Gregorio fault zone, composed of the Coastways and Frijoles (thrust) strands (Weber, 1980, Monterey Bay Fault Zone Griggs and Weber, 1990). The length of this fault zone, including its onland segments, is at least 125 km; however, The Monterey Bay fault zone is located in Monterey its total length may be considerably greater, for it appears to Bay between the cities of Monterey and Santa Cruz and join faults at Half Moon Bay () that in tum forms a diffuse zone, 10 to 15 km wide, of short, en echelon, join the San Andreas fault northwest of the Golden Gate northwest-trending faults (Fig. 10). This zone may comprise (Cooper, 1971). It may also join the Coast Ridge fault to the the offshore extensions of northwest-trending faults in the south of the Monterey Bay region (Ross, 1976). Salinas Valley and the to the southeast. To the north, this zone appears to terminate against the Palo The Palo Colorado fault onshore, south of Monterey Colorado-San Gregorio fault zone. Bay, has been described as a southeast-trending thrust that parallels the coast for approximately 2 km, ultimately bend­ Most faults in the Monterey Bay fault zone displace ing to the east, with Mesozoic quartz diorite thrust over late Tertiary and Pleistocene sediments. North of Monterey Cretaceous sandstone (Trask, 1926). According to Trask Canyon, faults rise to within 6 m of the ocean floor; most (1926), the fault plane probably dips 70°NE, with separation displace upper Pliocene strata, some displace Pleistocene of about 1,000 m. deposits, and a few may displace deposits (Greene et al., 1973; Greene, 1977). South of Monterey Canyon, most Offshore, south of Monterey Bay, the Palo Colorado­ faults close to shore near Monterey also rise to within 6 m of San Gregorio fault zone consists of two faults that trend the ocean floor and cut Pleistocene deposits. Some cut Holo­ northwestward from the coast. This fault zone cene deposits as well. Farther offshore in southern Monterey trends northwest across the Carmel-Sur coastline and one of Bay, faults appear to be older because they displace upper the faults trends N25°W down the axis of the western tribu­ Tertiary and older strata and are covered by about 100 m of tary of Carmel Canyon, and may have controlled the loca­ Quaternary sediment. A few faults cut only Cretaceous tion of this canyon. Faults in the rone here appear to be and strata of early to middle Tertiary age. continuous for more than 26 km, and bound a zone of de­ formed, steeply dipping rocks. Onshore, near Point Sur, one Previous mapping has demonstrated the onshore of the faults in this fault zone dips 50°-60°N. Mesozoic continuity of many of the offshore faults of the Monterey granodiorite northeast of this fault is thrust over upper Bay fault zone (Clark and others, 1974). Three relatively Miocene sandstone. Gilbert (1971) estimates that vertical continuous faults in southern Monterey Bay appear to ex­ separation on the fault is at least 300 m. tend onshore between the southern part of Fort Ord and Monterey; two are about 9 km long and the third is approxi­ Continuations of the offshore Palo Colorado-San mately 3 km (Fig. 10). One of the 9 km-long faults may be Gregorio fault zone to the north on the northern Santa Cruz the offshore extension of the Chupines fault, which appears shelf show evidence of relatively recent displacement (Fig. to enter the bay north of Monterey near Seaside. Seismic­ 10; Greene, 1977). The easternmost fault lies directly on the reflection profiles from the offshore area and geophysical trend with the main strand of the San Gregorio fault (Coast­ data on land indicate that the two 9 km-long faults exhibit ways strand of Weber, 1980), which, as mapped by Oark the same sense of separation as the Chupines and Tularcitos­ (1970), has juxtaposed Pliocene strata of the Purisima Forma­ Navy faults on land (Greene et al., 1973; Greene, 1977). tion to the west against the upper Miocene Santa Cruz These faults have Tertiary sedimentary rocks downthrown Mudstone. The westernmost fault lies on trend with a fault on the northeast against Mesozoic on the southwest. on Aflo Nuevo Point, where the Miocene Monterey Forma­ However, high resolution seismic-reflection profiles across tion is thrust (northeast side up) over Pleistocene marine the nearshore part of the southernmost fault offshore show terrace deposits; alternatively, it may connect to the Frijoles drag folds indicating the opposite sense of displacement, i.e., strand (Clark, 1970; Weber, 1980). Recently re-processed northeast side upthrown. This drag folding could be the multi-channel seismic-reflection data across the Palo Colo­ result of recent fault motion, which might differ from the rado-San Gregorio fault zone shows high-angle reverse predominant displacement, or may indicate that the fault faults with the east block upthrown (Fig. 9). has a component of strike slip (Greene, 1977). 43

On land, the Chupines fault trends northwestward The Reliz fault was first mapped as a branch of the from the Sierra de Salinas and extends beneath an area of King City fault (Schombcl, 1943); however, Durham (1970) alluvial deposits several kilometers wide near the coast. The terminates the Reliz fault about 4 km south of the King City fault is well defined in the mountains of the Siem de Sali­ fault near Olsen Ranch <.-ene Aromas Sand (Jennings ham (1978), believe that the Reliz fault extends northwest­ and Strand, 1958; Bowen, 1969; California State Dept. of ward along the northern base of Sierra de Salinas into the Water Resources, 1970), and with granitic basement rocks at King City fault, and refer to both faults as the Reliz fault. depth (Qark and others, 1974). Sieck (1964) has interpreted Dibblee (1972) concurred with the latter interpretation on the gravity data to indicate that the Chupines fault lies beneath basis of the linearity and near alignment of the steep front of the alluvium at the foot of the mountains, and has suggested the Sierra de Salinas with the Reliz fault south of Olsen that dip-slip movement along the fault may have displaced Ranch. Dibblee (1972) also ronsiders the Reliz fault to be part the Monterey Formation near Canyon del Rey. The fault of a system including the Rinconada fault, which extends for exhibits a vertical separation of about 300 m, upthrown to more than 110 km farther to the southeast. the southwest. The Chupines fault is more than 26 km long if it is an onland continuation of one of the more continuous Continuation of the King City fault beneath the allu­ faults in the Monterey Bay fault zone. vial cover of the lower Salinas Valley near the Monterey coast is inferred from water well data. These data also sug­ The Navy fault, a southwest-dipping reverse fault gest that the fault has a probable post-Pleistocene vertical where exposed, is a serond major onshore fault that may be separation of 30 m, south side up, near the town of Marina continuous with faults offshore in southern Monterey Day. about 3 km from the roast (California State Dept. of Water The Navy fault has been mapped in a northerly direction Resources, 1970, Sheet 5 of Pl. 2). Movement along the fault across Carmel Valley and the Meadow Tract area of the has juxtaposed the Miocene Monterey Formation, on the Monterey Peninsula. It then passes beneath alluvium at the sbuthwest, with the upper Pliocene to lower Pleistocene base of the mountains, and finally trends out to sea near the Paso Robles Formation (see Figure 11 for stratigraphic rela­ U.S. Navy Postgraduate School (Clark et al., 1974). The Navy tionship). Farther southeast, at Fort Ord, the Paso Robles fault may represent the northwestward continuation of the Formation southwest of the fault occurs in the subsurface at Tularcitos fault. If the Tularcitos-Navy fault continues into an elevation of 30 m above . The Paso Robles is not Monterey Bay and joins the southernmost of the relatively encountered near the surface to the northeast, across the continuous faults offshore, its length would be more than 42 fault, but deposits northeast of the fault occur at a depth of km. more than 160 m below sea level (Greene, 1977). The south­ west limit of the Monterey Bay fault zone i~ southern Mon­ The northeastern boundary of the Monterey Bay fault terey Bay is formed by a series of parallel faults that trend zone in southern Monterey Bay is gradational and is formed northwestward from Cypre5s Point (Fig. 10). Three of these by a relatively continuous fault and several discontinuous en faults displace the sea floor by 1 to 5 m; two show relative echelon faults (Fig. 10). This boundary may be the offshore uplift on the southwest and the other shows relative uplift extension of the King City fault (also called Gabilan fault) on the northeast (Greene ct al., 1973; Greene, 1977). The most (Clark, 1930; Reed, 1933). This continuous offshore fault is continuous fault in this boundary zone may connect with the aligned with the inferred trace of the King City fault as onland Cypress Point fault that extends southeastward from projected from the northern base of the Sierra de Salinas to Cypress Point across Carmel Bay to ronnect with a fault just the ocean just south of the town of Marina. north of the mouth of Carmel River (Bowen, 1969; Clark et al., 1974). On land, the King City fault is a high-angle reverse fault along which granitic rocks of the Sierra de Salinas are Monterey Canyon Fault uplifted to form the south-western border of the Salinas Valley (Reed, 1925, 1933; Clark, 1930; Sieck, 1964). Vertical Deep penetration (160 kj) seismic-reflection profiles separation along this fault decreases to the north, toward provide evidence for the Monterey Canyon fault (Monterey Monterey Bay, where it may die out. The King City fault is fault of Martin, 1964), previously inferred to lie beneath, and presumed to extend beneath the southwest margin of the parallel to, the head ward axis of Monterey Canyon (Martin, Salinas Valley (Figs. 8 and 10) at least as far southeastward 1964; Greene, 1970, 1977). Cretaceous granites forming the as the area west of Greenfield (Clark, 1930; Reed, 1933; basement complex occur at shallower depths on the south Schombel, 1943; D.L. Durham, oral commun., 1972) . Grav­ side of the canyon than on the north; the apparent vertical ity data from the southern margin of the Salinas Valley separation is approximately 60 to 150 m, south side up, and suggest about 2,500 m of vertical separation (Fairborn, 1963), increases in amount farther offshore. whereas gravity data from the west end of the Sierra de Salinas indicate between 900 and 1,200 m of separation The Monterey Canyon fault is approximately 10 km (Sieck, 1964). If the King City fault and faults forming the long and follows the axis of the canyon from the easternmost northeast boundary of the Monterey Bay fault zone are meander to the mouth of Elkhorn Slough (Fig. 10). The fault continuous, the vertical separation decreases to about 240 m may extend onshore, and could be responsible for the trough offshore (Greene, 1977). in basement rocks beneath Elkhorn Slough (Starke and Howard, 1968). Shallow reflectors are poorly defined in the seismic-reflection profiles, and it is difficult to determine the 44

offset of reflectors in the overlying strata of probable Plio­ cene age, in seismic-reflection profiles (Greene, 1977). HICK AGE @ FORtMTION LITH. NESS DESCRIPTION Sur-Nacimiento Fault Z.One Cl) ...... Several faults identified on the shelf between Point Alonw SlnCI Sur and Cypress Point are aligned with faults onshore in the Sur-Nacimiento fault zone, a major structural feature in the 6 ? Pao Rabies F°"'*'On ~ cdonld&Mds Ind pa; southern Monterey Bay region (Fig. 8). This zone is a belt of i2~ marine ? ~ faults of various kinds and ages that extends onshore for 8 Whllll alkoslc 11nd; minor p ll; about 300 km southeastward from the Sur fault zone (Page, i2 5-118~ mart111 and lnddsh madne, localr g Sandslolle toulfllQll 1970) and includes the Sur thrust zone, the Nacimiento fault, 0 and several smaller faults. w s Whlle llllomll8 In l4ll* pe~; z IC lgl'a brown 5lceollS shale In ~ lower part !!s =: Other Offshore Faults ~ Bull cdonld Wllllherlng alkoglc sand and blown sandstone, gnMll, ll'inor 0 lhlle . mart111. lnlelbeCkllld ciYint Two other faults extend N50°W from the Point Sur :s bl5M1, ~Of amtgdalddll shelf. These linear faults are the northeastern and southwest­ :s0 Marine Slndslone 1 Red Beds ern margins of the basement ridge that forms the Sur plat­ PALEO- CalllllloFomdon form (Figs. 6 and 8). The northeastern fault i~ at least 80 km CENE of Bowen (1965) long; it passes about 11 km seaward of Point Sur and proba­ CRETA· bly continues to the south, paralleling the southern Sur CEOOS Clysllllllne 0( coast. To the north McCulloch and Greene (in press) show JUR- ~ l ASSIC this fault connecting to a that swings eastward to merge near Guide with a paleo-subduction zone SurSerlls ()Ilk blown ~llt!·mlca sclllst wld PALEO- ollruk gneiss; mlftOf crySlali111 time&I0111 of McCulloch (1989), a remnant zone or thrust repre­ ZOIC (1926) senting a former convergent boundary (Fig. 12). 1 Terrtior Fonndon of Trask, 1926. 2 Red bods cC AoHnson Canyon. A third fault is located along the east side of Surveyor sea knoll on the continental slope due west of Point Sur (Fig. 10). This fault is oriented nc rth-south, as are structures to the Fig11tt 11. Composite stratlgraphlc column of Uppc!r Salinas Valley­ south. It appears to be about 60 km long, but may extend a northem Santa Lucia Range (modified after Beal, 1915; Bowen, 1965, greater distance to the south (Greene et al., 1989). This fault 1969; California State Department of Water Resources, 1970; Clark et al., 1974). has a distinct seafloor expression as shown by SeaBeam data (Fig. 2). The southern ends of the most nearshore faults appear to approach a common juncture south of Point Sur. youngest strata cut by the Monterey Canyon fault. However, McCulloch (1989) and McCulloch and Greene (in press) this fault probably does not extend up to the base of the show that faults seaward of the Sur coast trend northwest­ modern canyon fill (Greene, 1977). The fault is not active southeast until they approach Point Sur, where their trend today and probably records an earlier stress field as its appears to change abruptly to a nearly cast-west orientation. orientation is nearly perpendicular to present-day structural Along the base of the continental slope McCulloch (1987; trend; it most likely was associated with a pre-transform 1989) has mapped the palco-subduction zone (Fig. 12). west to east subduction stress field. Folds Ascension Fault Bedding of Tertiary sedimentary rocks is generally Offshore, the Ascension fault (questionable Sur­ flat-lying or homoclinal in the Monterey Day region. The Nacimiento fault of McCulloch, 1987, 1989) generally ex­ most complex structures occur in the Monterey Bay fault tends northwestward for over 180 km from the Palo Colo­ zone, where Tertiary rocks have been severely contorted and rado-San Gregorio fault zone, where it appears to be trun­ compressed into tight synclinal and anticlinal folds (Greene, cated near the southernmost head of Cabrillo Canyon (Fig. 1977). Most well-defined folds lie offshore and onshore in 8). However, continuation of this fault to the south for 60 the sou~hcrn Santa Cruz Mountains. Onshore, synclinal km, where it may tie into the Sur thrust fault zone, has been basins of Eocene age rocks have been mapped between the proposed by Greene (1977) and is questionably mapped by San Andreas and Z"1yante faults (Fig. 10). In this area sev­ McCulloch and Greene (in press). Seismic reflection data eral northwest-southeast oriented and collected by the U.S. Geological Survey to the north indicate are mapped (Drabb, 1989). the fault to be a thrust, west over east (McCulloch and Greene, in press), and recently reprocessed USGS multichan­ East of Palo Colorado Fault Zone nel seismic-reflection profiles suggest the fault exhibits both strike-slip and thrust movement (Fig. 9). The youngest rocks Between Den Lomond and the San Andreas faults in displaced by this fault appear to be Miocene, based on the the southern Santa Cruz Mountains several major north­ offset of reflectors of probable Miocene age and the lack of west-southeast oriented folds are mapped (Fig 10). Less 45

Figure 12. Major structural features and faults offshore central Califor­ nia. Teeth on upthrown side of high-angle reverse faults or upper plate of thrust faults. Shaded offshore areas are late Tertiary basins. Faults: HF= Hosgri, MFZ = Mendocino , NF= Nacimiento, PF= Pilardtos, PRF = Point Reyes, SAF = San Andreas, SF= Sur, SGF = San Gregorio, SLBF = Santa Lucia Banlt. Structural highs: ANB = AJ\o Nuevo, PPB= Pigeon Point (Modified after McCulloch, 1989.)

continuous folds are exposed along the western flank of Ben Lomond Mountain near Afl.o Nuevo Point (Brabb, 1989).

Anticlines and synclines in the Monterey Bay fault zone are short and have orientations that appear to range from about N700W to N80°W (Fig. 10). Major folds near shore, in the Monterey bight area, have orientations that parallel faulting (NSO°W); some folds extend onshore and join folds mapped in the subsurface by Oark et al. (1974). Folding and shearing within the northern part of the Palo Colorado-San Gregorio fault zone have severely distorted the sedimentary strata between the two major faults of the zone. Most folds within this zone have flank dips greater than 35 degrees. These folds may have formed by compres­ sional forces associated with strike-slip along faults within the Monterey Bay fault zone, or they may represent the recent (last 8-3 m.y.) plate convergence compressional event.

The only other folded structures east of the Palo Colorado-San Gregorio fault zone are located in northern Monterey Bay. Five short, discontinuous folds in gently dipping Pliocene strata of the Purisima Formation are lo­ cated between the head of Soquel Canyon and the Capitola­ Aptos coastline (Fig. 10). In addition, several discontinuous folds occur in Miocene marine strata and upper Tertiary to Quaternary deltaic deposits offshore from Santa Cruz. These structures are oriented approximately N80°W.

West of the Palo Colorado-San Gregorio Fault Zone

West of the Palo Colorado-San Gregorio fault zone, Tertiary sedimentary rocks are strongly folded along the northwest part of the Point Sur shelf and slope, between .:!>· faults fanning outward from the Sur platform (Figs. 6 and :!6• -0 10). Orientation of these folds ranges from east-west to about !; N80°W. These folds appear to be the result of motion along \?.- .,. -I m nearby faults, and are limited in occurrence to areas of shallow basement. In addition, a faulted has been . mapped in the area between Monterey Canyon and the Sur ,?.-' platform. The only other significant folds in this area are a large syncline located on the slope between Monterey and Ascension Canyons, and an folded between the S' :!6• converging thrust fault and paleo-subduction zone southeast ss. ~ CD of Guide Searnount (Fig. 12). A small north-south syncline ...... lies just west of Surveyor Seamount .

Sedimentary Basins and Basement Ridges

Several basins and basement ridges have been identi­ fied in the submerged continental borderland of central California (Curray, 1%5, 1966; Hoskins and Griffiths, 1971; Silver et al., 1971; Silver, 1974; McCulloch et al, 1977, 1980; Howell et al., 1980; Nagel and Mullins, 1983; McCulloch, 1987, 1989; see Fig. 13). The Santa Cruz and Pigeon Point highs are individual basement ridges delineated by Hoskins 46

125° 124° 121°

40° ~ 40° ~'.. . NOR1H ...... 0 so 100 ...... , ICllorntltns Point Arena \ basin 39° 39°

Cordell baain

38° Bodega basin _.,rl--...l.,.,.-~~

FaraJlon Is. _ __...,,,...... ,~>:;

. · GuH of the Farallones

outer Santa Cruz basin 37° Guide Smt.

122° 121°

Figure 13. Map showing generalized boundaries of ccntr;il California contincnt;il shelf, slope and coastal basins and offshore exploratory drill holes (modified after MtCulloch, 1989). 47 and Griffiths (1971) and Silver et al. (1971). The outer ridge in deep penetration seismic-reflection (Santa Cruz high) lies northwest of Monterey Bay and south profiles collected along the continental slope between Ascen­ of the (Fig. 14). This ridge was first pro­ sion and Monterey Canyons (Greene, 1977; Nagel and posed by Silver et al. (1971) to be composed of rocks of the Mullins, 1983). Tertiary sediments have an aggregate thick­ Franciscan assemblage, and has been mapped as three dis­ ness of more than 2,000 m in this basin. This sedimentary tinct segments that are believed to be continuous at depth. sequence thins abruptly to the south and west where the Greene (1977), based on offshore seismic-reflection data, sequence laps onto the basement ridges. For a complete, proposed that the offshore Palo Colorado-San Gregorio fault detailed description of this basin see Heck et al. (this vol­ zone marked the boundary between Franciscan and the ume). granitic rocks of the Salinian block in the Monterey Bay region where this ridge is truncated and speculated that A smaller basin exists southwest of Monterey Bay and Franciscan rocks crop out in Ascension Canyon. Mullins and is a fault bounded basement depression termed the Mon­ Nagel (1981) confirmed from dredge samples that Franciscan terey low by Greene (1977), or Monterey basin (Figs. 6 and rocks are exposed in the lower head ward parts of Ascension 13). This basin trends northwest-southeast and is bounded Canyon and represent basement rocks of the southern Santa to the north by the Ascension high of Greene (1977), to the Cruz high. Shoreward of the Santa Cruz high is another south by the eastern faulted flank of the Pt. Sur high, and to ridge (Pigeon Point high) composed of granitic rocks that the east by the Ascension fault (Fig. 10). Monterey Canyon crop out as quartz diorite in the Farallon Islands (Silver et bisects the basin and exposes the metamorphic basement al., 1971). rocks here. The Sur high is a northwest-plunging basement ridge that appears to be the buried seaward extension of the The Santa Cruz and Pigeon Point highs bound a Sur platform. major sedimentary basin seaward and northwest of Mon­ terey Bay which is described by Hoskins and Griffiths (1971) No significant sedimentary basins are present within as follows: Monterey Bay proper (Fig. 6). A relatively flat-topped granitic basement complex supports a late Tertiary sedimen­ 'The outer Santa Cruz basin extends northwest tary cover of less than 1,000 m (Greene, 1977). A subtle from Monterey for 80 miles (130 km) to about basement ridge, the Monterey high, trends northward from 37°30'N lat., where it appears to merge with the Monterey Peninsula to the Soquel-Aptos area and proba­ the continental slope. It is a shallow, post­ bly limits the seaward extension of the onland Salinas basin Miocene syncline which encompasses approxi­ (Fig. 6 and 13). mately 1,400 sq. mi. (3,630 km1) of Miocene and younger marine beds." EARTHQUAKES

These authors note that the basin axis plunges continuously Earthquakes in the Monterey Bay region reflect the toward the northwest and that structural trends within the tectonic movement and deformation that is presently occur­ basin generally parallel this axis (Fig. 14). They conclude that ring along the Pacific-North American plates' boundary. By the basement of the basin is granite, although basement studying earthquakes and their associated effects such as rocks are not exposed on either Pigeon Point and Santa Cruz surface rupturing and tectonic forces actively high. The extreme southeast end of the outer Santa Cruz deforming the region can be determined. The Monterey Bay basin of Hoskins and Griffiths (1971) appears as a synclinal region is a very active seismic area and in 1989 was subjected to an earthquake that resulted from release of tectonic stress built up between the Pacific and North American plates.

The largest recorded earthquakes in the Monterey Day region occurred in 1926 and 1989. Steinbrugge (1968) has described the 1926 earthquakes as follows:

"1926, October 22, 4:35 A.M. Center on the continental shelf off Monterey Bay. Intensity VIII at Santa Cruz, where many chimneys were thrown down: VII at Capitola, Monterey, Sali­ nas and Soquel. Felt from Healdsburg to Lom­ poc (a distance of 250 miles (450 km)) and cast to the Sierra, an area of nearly 100,000 square miles (180,000 km1). Another shock one hour later was similar to the first in almost every respect."

Detection of earthquakes and determination of their Figure H . Generalil.ed sbucture map showing Outer Santa Cruz basin locations and focal depths (hypocenters) in the Monterey on the continental margin between Monterey and S;m Francisco (S.F.). Day region has been difficult because the area lies largely (Modified after Hoskins and Griffiths, 1971; Nagel and Mullins, 1983.) outside of the network of permanently located scismo- 48

122• 0

t:AUttOUAKt. fOC.\l MKN4MISM$ """""""""' ~ ...... ~

~ ...... 0 M•M"M O '-'-u....---C"-­""- ... ,... ,.... ,,..,..._

3 7•

0 • 0 O .@ , ..,. •

0 ...... e> ',.... ML<1.0 ' ...... 0 Ul

0 0 ! 3'6°30' 0 - -i Cb • 0 IS> 0

0

0 0 0

""0 0

0

0 25

Kilometers

36°

Figure 15. Map showing faults, earthquakes, and fault plane solutions in the Monterey Bay region. Heavy dotted line shows approximate locations of aftershocks from the October 17, 1969 Loma Prieta earthquake; mainshock shown with large circle and leadered first motion ball. Epicenter data from 1926 to 1986 with the exception of 1969 earthquake mainshock (sec Figure 17 for distribution). (After Cockerham et al., In press.) 49

graphic stations. Nevertheless, refinement and expansion of these epicenters probably are not known with sufficient the California Seismographic Network (CALNET) has re­ accuracy for certain assignment to a specific fault zone. sulted in more accurate location and analysis of earthquakes These were the largest earthquakes recorded in the region in the region. until 1989.

First motion studies of magnitude 3.5 and greater On October 17, 1989 the 7.1 M Loma Prieta earth­ earthquakes occurring between 1969 and 1976 were inter­ quake occurred along the San Andreas fault within the Santa preted by Greene et al. (1973) indicate that right-lateral Cruz Mountains (37°02'N' N latitude, 121°53'W longitude), strike-slip was occurring on no1thwest-trending faults in the approximately 15 km cast-northeast of the city of Santa Cruz bay (Greene et al., 1974). Analyses of seismic events that and about 95 km southeast of San Francisco (Fig. 17) Al­ occurred in the Monterey Bay region between 1January1969 though this earthquake was not of a great magnitude the and 30 April 1986 (Fig.15) indicate that the fault planes destruction and damage in the Monterey Bay region was within the Palo Colorado-San Gregorio and Monterey Bay similar to that which occurred during the great San Fran­ fault zones vary from nearly vertical to high-angle reverse cisco earthquake of 1906 (Lawson et al., 1908). The initial and that strike-slip movement as well as thrust motion has fault rupture (focal point) occurred at a depth of 18.5 km and occurred along these northeast or northwest trending faults propagated along strike for nearly 40 km. In contrast the (Cockerham et al., in press). Recent re-processing of multi­ 1906 earthquake rupture length was about 450 km. During channel seismic-reflection profiles in the area of the southern the 1989 event, a subsurface area of over 300 km2 ruptured Monterey Bay fault zone (Fig. 16) and high-resolution seis­ along a fault plane striking N50°W ± 8° and dipping 70° ± mic-reflection profiles and side-scan sonographs indicate 10° to the southwest; direction of slip was 130° ± 15° (Plafker that the zone may be a thrust. and Calloway, 1989).

The two large earthquakes (magnitude 6.1) that oc­ Plafker and Galloway (1989), from whom most of the curred in 1926 also have been located within the Monterey following description is taken, describe the area of rupture Bay fault zone by Richter (1958). However, the locations of accompanying the 1989 earthquake as being along a locked

B B' Palo Colorado - San Gregorio Fault Zo ne Mon terey Bay Faul t Zone SW NE

COP

I I I I I I I J I l I l l L

Figure 16. Preliminary interpretation of multichannel scismic·reflcction profil e across the southern part of the Monterey Bay fault zone showing thrust faults. 50

20' t•' • \,, I\ \ \ 10' \ "\ ·. \ \ \ ,,'\ \ \ \ \ \ \ 37° \ \ '\ \ \ \\ \ \ ... \ \\ \ ':...'\ \ '~ ''-<..' '' \ ~~"''' .-( \ \ ' ' \ \ ''' ' ... , '"' ,,,,\ ',,,, ' \ 50' \ \ ' ·-<. '' ' ', ' ' '·• \ \ \' Moss \ \ ' \ ' Landing \ \ ' \ ' ' 20 KM \ ,. ' \ ' '· ''"'""""""''" '\ ' '' 20' 10' 122° 50' 40' 30' 20'

Figure 17. Generalized al'Clll distribuftershodc sequence and location of mainshodc from October 17, 1989 Loma Pr".eta-Santa Cruz Mountains earthquake for the period October 17 to October 31, 1989 (after Donna Eberhllrt-Phillips and Lynn D. Deitz, Branch of , US. Geological Survey, written communication, 1989).

segment of the San Andreas fault that last ruptured in 1906. southwest block and about-0.2 m of subsidence on the In contrast, south of this area accumulated elastic strain is northeast block (Fig. 20). No surface ruptures were identi­ absent and numerous small earthquakes occur, indicating fied; however, local cracks and crack sets were found along continuous movement on that segment of the San Andreas the trace of the San Andreas fault (Plafker and Calloway, fault. The 1989 Loma Prieta earthquake occurred in a seismic 1989). Plafkcr and Calloway (1989) descnbe a combination gap that existed between Los Gatos and Watsonville (Fig. of extension and left-laterally or right-laterally offset cracks 18). After the initial event a complex pattern of aftershocks along Summit Road, just south of Hwy 17 (see road log, this began that appear to have resulted from movement on volume; Canby, 1990) that uppear to have formed by move­ multiple branching faults, including vertical segments of the ment along the fault at depth. Older, local slump cracks also San Andreas and $argent faults (Plafker and Calloway, opened up in the Summit Road area; many of these cracks 1989). A subparallel cluster of secondary aftershocks trig­ follow the trend of bedding, and are interpreted by Plafker gered by a 5.0 magnitude event 33 hours after the main and Calloway (1989) to be concentrated over weaker shale shock is believed by Plafker anl.l Calloway (1989) to be beds. Severe shaking along ridge tops occurred and Plafker associated with possible movement along the Zayante fault, and Calloway (1989) speculate that the this may have caused even though no clear evidence of surface rupture was found. cracking when the tops spread laterally as the slopes moved outward and downward to•.vard the valleys below under the Accurate geodetic measurements made shortly after influence of gravity. Severe shaking of the ridges can also be the Loma Prieta earthquake and reported by Plafker and explained by the enhancement of seismic waves travelling Calloway (1989) established that over 2 m of northward upward from the focal point by reflecting off the air-earth (right-lateral) horizontal movement and nearly 1.5 m of interface. on the upthrown (southwest) block resulted from the earthquake (Fig. 19). These data suggest Seismic intensities in the Monterey Bay region result­ that a broad area of uplift occurred with about +0.5 m on the ing from the Loma Prieta earthquake were very high, rang- 51

-123°45' -121°30' -123°20' -123° .-122°40' ·122°20' -122° -121°40' 38°10' r------r------.-....----.-----..------nr:n-----,---, 38°1 O' rf

38°00' 0 38°00'

LEGEND

Symbol Magnitude

0 37°40' 0 OiML

LOMA PRIETA GAP •

36°40' 36°40' O-t....-:--...... ,...---..--__.rr-:r.------...-.rm"'."'T""T- • 5 ~ 1~~~ ·• • ...... a.~~.~- t 't...... ,,,,;. 4!~RIQll- o. "f'6":"~ ..~ . -....,..- ..... "'• ;~,\;,.. p ••••,.~ ~ 10 •• • • ...,.. '" : 0 0 • ~ 15-+------~----+- • 0 ~ A 36°20' 36°20' ~~- ~o~:~------.------=------~:.----~~j 'l'.~~· :·, ~ B 50 100 DISTANCE, IN MILES 36°00' .______--'------l-----1.-----L-----L.---...... __---.AZS 36°00' -123°45' -123°20' -122°40' -122°20' -122° -121040· -121030·

Figure 18. Map showing location of earthquakes occurring from 1 May 1986to 16October1989. The epicenters arc well located, but the magnitudes are preliminary. Note lack of activity (seismic gap) In area where October 17, 1989 occurred. Inset cross sections show seismicity along San Andrcas fault from north of San Francisco to south of San Juan Bautista. A, B3ckground seismicity rccordcd during 20-year period before October 17, 1989 earthquake. On Loma Prieta segment, what little seismicity has occurred has effectively outlined a U-shapcd area (Loma Prieta Gap) that has bccn virtually aseismlc over the past 20 years. 8, Aftershocks and mainshock (largest circle) r.'.:nost completely filled former quiet zone of Loma Prieta Gap. Data provicled by the Branch of Seismology, US. Geological Survey (modified after Plafker i;:td Galloway, 1989; Cockerham et al., in press). 52

Surface cracks In general, seismicity of the Monterey Bay region showing direction SAN ANDREAS FAULT indicates that both compressional and strike-slip motion is of movement occurring along the plate boundary there. Recurrent earth­ ...... quakes, although not rupturing the ground surface, cause significant uplift in the coas~al mountains and stimulate erosion through landslides. In the valleys and coastal areas liquefaction and associated subsidence accelerates erosion locally. One tectonic event such as the 1989 Loma Prieta earthquake can significantly alter the geology.

CONCLUSIONS

The geologic structure and active seismicity of the Monterey Bay region indicate that the central California >.~ coast has been, and is today, subjected to complex tectonic ~ processes. The region was subjected to nearly orthogonal '•1~ collision and subduction until about 21 Ma when a trans- ~ Fault

'·~ " ~~"'I 'l Nptu~• Fault slip • • Hypocenter at depth in 122"15' 121"30' this earthquake 37"40'

Figure 19. Schematic diagram showing inferred motion on San Andreas fault during the October 17, 1989 Loma Prieta earthquake as determined from geodetic data. (After Plafker and Galloway, 1989.)

ing from VI to VIII on the modified Mercalli (MM) scale. In the general area around the epicenter, from Santa Cruz to Watsonville to Los Gatos, MM VII intensity occurred (Fig. 21). From Moss Landing to Castroville to Salinas and north­ ward from Davenport intensity of MM VII occurred. A MM VI intensity was felt in the Monterey-Carmel area (Plafker and Calloway, 1989).

Strong ground accelerations were recorded from the Loma Pricta earthquake with a 64 percent gravity uplift reported within the focal region, south of the Summit Road area (Plafker and Galloway, 1989). McNally ct al. (1989) reported a 100 percent gravity cplift in the same area. Strong ground motion triggered landslides within a large area around the epicenter where rock falls, rock slides and soil slides occurred in road cuts, near-vertical mountain cliffs, and coastal bluffs. Rotational slumps and transla­ tional block slides were also reported (Plafker and Calloway, 1989). Liquefaction was also widespread in the Monterey Bay region, occurring in the flood planes of the Pajaro and Salinas Rivers, as well as near the mouth of the San Lorenzo River in Santa Cruz. During shaking sand boils formed in 36°40' ..__ .....___ ...._ ____.._..._ ___ ._ __ _ the fl<>tsunami or was made (Schwing et al., 1990). locations are dashed and inferred faults are dotted (foults after Jennings et al., 1975. (Modified after Plafker and Galloway, 1989.) 53

Stanley, and Gribi, this volume). All of these fields are located along structures that resulted from Neogene and Quaternary tectonic movement.

ACKNOWLEDGEMENTS

This paper is distilled and updated from a longer one (Greene, 1977). The manuscript was substantially improved by the reviews of Robert Carrison and Florence Wong. Karen Hicks' involvement in research, computer processing of the manuscript, and construction of the illustrations was invaluable in the preparation of this paper.

REFERENCES CITED

Atwater, T., 1970, Implications of for the Cenozoic tectonic evolution of western North Amer­ ica: Ccol. Soc. America Bull., v. 81, no. 12, p. 3513- 3536. Atwater, T., and Molnar, P., 1973, Relative motion of the 0 10 20 30 Pacific and North American plates deducted from MILES sea-floor spreading in the Atlantic, Indian, and South Pacific : Stanford Univ. Puhl. Geol. Sci., v. 13, Figme 21. Map showing preliminary distribution of Modified Mercalli p.136-148. (MM) intensity for the October 17, 19119 Loma Prieta earthquake in the Beal, C.H., 1915, The geology of the Monterey quadrangle, Santa Cruz Mountains. Numbers indicate intensity values for localities where observations were made; Roman numerals represent intensity California: M.A. Thesis, Stanford Univ., Stanford, level between isoseismlcal lines. Star shows location of mainshock. California, 87 p . (Modified after Plafker and Galloway, 1989.) Bowen, O .E., Jr., 1959, Geology and economic possibilities of the limestone and solomite deposits of the northern form margin formed. Transtensional and transpressional : California Div. Mines Spec. Rept. 56, structures resulted from widespread transcurrent movement 40p. along faults of the San Andreas fault system and these struc­ ____ 1%5, Stratigraphy, structure and oil possibilities in tures were either overprinted or altered and deformed Monterey and Salinas quadrangles, California (abs.): during and after a shift in the stress field. Change in direc­ Am. Assoc. Petroleum Geologists Bull.,v. 49, n. 7, p. tion of the Pacific plate in relation to the North American 1081. plate some 5 -3 Ma produced a more orthogonal conver­ ____ 1969, Geologic map of the Monterey quadrangle: gence that initiated formation of compressional structures California Division Mines and Geology Open-File parallel to the San Andreas fault. The 1989 Loma Prieta Map, scale 1:62,500. earthquake exhibited the complex deformation that is occur­ Brabb, E.E., 1989, Geologic map of Santa Cruz County, ring along this central California boundary. Both thrusting California: U.S. Geological Survey Misc. Investiga­ and strike-slip displacement occurred indicating that the tions Map 1-1905, 1:62,500. structural pattern of the region is the result of both trans­ Burford, R. 0 ., 1971, Fault creep and related seismicity along form movement and compression between the two plates. the San Andreas fault system between Pinnacles National Monument and San Juan Bautista (abs.), in The sedimentary basins, both onshore and offshore, Cordilleran Sec. 67th Ann. Mtg., Hayward, Calif: including the Outer Santa Cruz basin (sec Heck et al., this Gcol. Soc. America, Abs. with Program, v. 2, no. 2, p. volume), were formed along the California margin during 90-91. the transition period between subduction and transforma­ Canby, T.Y., 1990, Earthquake, prelude to the big one?: tion. These basins were then enlarged, elongated, filled and National Geographic, v. 177, no. 5, p. 76-105 (May, deformed during the transform movement, and transported 1990). on the Pacific plate to their present location. During their California Department of Water Resources, 1970, Sea-water evolution many of these basins underwent a depositional intrusion into the lower Salinas Valley, a progress and thermal history that favored the generation and concen­ report: Calif. Dept. Water Resources Open-File Re­ tration of hydrocarbons. Today, onshore seeps of tar sands port, 28 p. are present along the coast between Santa Cruz and Daven­ Circum-Pacific Map Project, 1985, Pacific basin sheet: plate­ port (see Phillips, this volume). Offshore, the concentration tectonic map of the Circum-Pacific region, ed. 3, of shallow hydrocarbons, including natural seepage of Circum-Pacific Council for Energy and Mineral Re­ thermogenic hydrocarbons into the oceanic , sources, Houston, Texas, scale 1:17,000,000. near the southern terminus of the outer Santa Cruz basin is Clark, B. L., 1930, Tectonics of the Coast Ranges of middle reported by Mullins and Nagel (this volume). Several oil California: Geol. Soc. America Bull., v. 41, p. 747-828 fields have been developed in the region including the Half Clark, J.C., 1970, Preliminary geologic and gravity maps of Moon Bay, La Honda and Moody Gulch fields (see Wright, the Santa Cruz-San Juan Bautista area, Santa Cruz, 54

Santa Clara, Monterey and San Benito Counties, Gilbert, W. G., 1971, Sur fault zone, Monterey County, Cali­ California: U.S. Geol. Survey open-file map, scale fornia: Ph.D. thesis, Dept. Geol., Stanford Univ., 1:125,000. Stanford, Calif., 80 p. Clark, J.C., and Rietman, J. D., 1973, Oligocene stratigraphy, Graham, S. A., 1976, Tertiary sedimentary tectonics of the tectonics, and paleogeography southwest of the San central Salinian block of California: Ph.D. thesis, Andreas fault, Santa Cruz Mountains and Gabilan Stanford Univ., Stanford, Calif., 510 p. Range, California Coast Ranges: U.S. Geo!. Survey Graham, S.A., 1978, Role of Salinian block evolution of San Prof. Paper 783, 18 p. Andreas fault system, California: Amer. Assoc. Petro­ Clark, J.C., Dibblee, T. W., Jr., Greene, H. G., and Bowen, 0. leum Geologists Bull., v. 6, n. 11, p. 2214-2231. E., Jr., 1974, Preliminary geologic map of the Mon­ Graham, S.A. and Dickinson, W.R., 1978, Apparent offsets terey and Seaside 7 .5-minute quadrangles, Monterey of onland geologic features across the San Gregorio­ County, California: U. S. Geol. Survey Misc. Field Hosgri fault trend: California Division of Mines and Studies Map MF-577, scale 1:24,000. Geology Special Report 137, p. 13-23. Clark, J.C., Brabb, E.E., Greene, H.G., and Ross, D.C., 1984, Greene, H.G., 1970, Geology of southern Monterey Bay and Geology of Point Reyes Peninsula and implications its relationship to groundwater basin and sea water for San Gregorio fault history, in Crouch, J .K. and intrusion, U.S. Geological Survey Open-File Report, Bachman, S.B., eds., Tectonics and sedimentation SOp. along the California margin: Pacific Section SEPM, v. ____, 1977, Geology of the Monterey Bay region: U.S. 38, p. 67-86. Geological Survey Open-File Report 77-718, 347 p., 9 Clarke, S. H., Jr., Howell, D. G., and Nilsen, T. H., 1975, pl. Paleogene : Paleogene Sym­ Greene, H.G., Lee, W.H.K., McCulloch, D.S., and Brabb, E.E., posium, Pac. Section, Soc. Econ. Paleontologists and 1973, Faults and earthquakes in the Monterey Bay Mineralogists, p. 121-15~. region, California: U.S. Geol. Survey Misc. Field Cockerham, McCulloch, D.S., and Greene, H.G. (in press), Invest. Map MF-515. Earthquake epicenters and selected fault plane solu­ Greene, H. G., McCrory, P.A., Lajoie, K. R., Ellsworth, W. L., tions of the central California continental margin, in and Gawthrop, W. H., 1974, Map showing faults and Greene, H.G. and Kennedy, M.P., eds., Geologic Map seismicity, San Mateo, Santa Cruz, and Monterey Series of the California Division of Mines and Geol­ counties, California: U.S. Geol. Survey open-file map, ogy, scale 1:250,000. scale 1:250,000. Coney, P.J., Jones, D.C., and Monger, J.W.H., 1980, Cordille­ Greene, H.G., and Oark, J.C., 1979, Neogene paleogeogra­ ran suspect terranes: , v. 288, p. 329-333. phy of the Monterey Bay area, California, in Cole, Cooper, A., 1971, Structure of the continental shelf west of M.R., TcrBest, H., and Armentrout, J., eds., Cenozoic San Francisco, California: U.S. Gcol. Survey Open­ paleogeography of the western : Pacific File Report, 65 p. Section SEPM, Pacific Coast Paleogeography Sympo­ Cox, A. and Engebretson, D., 1985, Change in motion of sium 3, p. 277-296. Pacific plate at 5 Myr 8.P.: Nature, v. 313, p. 472-474. Greene, H.G., Stubblefield, W.L. and Theberge, H.E., Jr., Curray, J. R., 1965, Structure of the continental margin off 1989, Geology of the Monterey submarine canyon central California: Trans. New York Acad. Sci., ser. II, system and adjacent areas, offshore central California, v. 27, no. 7, p. 794-801. Results of NOAA SeaBeam survey, descriptive report ____, 1966, Geologic structure of the continental margin for Surveyor cruise: U.S. Geological Survey Open-File from subbottom profiles, northern and central Report 89-221, 33p., 4 maps. California: in H. Bailey (ed.), Geology of northern Greene, H.G., Ledbetter, M.T., Gardner-Taggart, J., Chase, California: E. Calif. Div. Mines and Geology Bull. 190, T.E., Baxter, C., and Pilskaln, C.H., 1990, Liquefaction p. 337-342. in the Moss Landing area - result of the October 17, Davidson, G., 1897, The submerged valleys of the coast of 1989 Santa Cruz Earthquake: EOS, v. 71, n. 8, Febru­ California U.S.A., and of Lower California, : ary 20, 1990, p. 288. Calif. Acad. Sci. Proc., 3rd ser., v. 1, p. 73-103, rev. by Gribi, E. A., Jr., 1967, The &tlinas basin oil province, in W.S.T. Smith, 1897, Jour. Geology, v. 5, p. 533-534. Payne, M. B., Guidebook to the geology of Salinas Dibblee, T. W., Jr., 1972, The Rinconada fault in the southern Valley and the San Andreas fault, p. 16-27: , California, and its significance: Geo!. Secs., Am. Assoc. Petroleum Geologists and Soc. Soc. America Abs. with Programs, Cordilleran Sec., v. Econ. Paleontologists and Mineralogists, 168 p. 4, no. 3, p. 145-146. Griggs, G.B. and Weber, G.E., 1990, Field Trip Guide, Coastal Durham, D.L., 1970, Geology of the Sycamore Flat and geologic hazards and coastal tectonics, Northern Paraiso Springs quadrangles, Monterey County, Monterey Bay and Santa Cruz/San Mateo County California: U.S. Gcol. Survey Bull. 1285, 34 p. coastlines: Association Engineering Geologists, 94 p. Epp, D.J., 1978, Possible perturbations to hotspot traces and Hamilton, W., 1969, Mesozoic California and the underflow implications for the origin and structure of the Line of Pacific mantle: Gcol. Soc. America Bull:, v. 80, no. Islands: Journal Gcophys. Research, v. 89, n. 813, p. 12, p. 2409-2430. 11273-11286. Hill, M. L., and Dibblee, T. W., Jr., 1953, San Andreas, Gar­ Fairborn, J. W., 1963, Gravity survey and interpretation of lock, and Big faults, California - a study of the the northern portion of the Salinas Valley: student character, history and tectonic significance of their rcpt., Stanford Univ., Stanford, Calif., 33 p. displacements: Geol. Soc. America Bull., v. 64, no. 4. SS

p.443-458. Lawson, A.C. et al., 1908, The California earthquake of April Hoskins, E.G., and Griffiths, J. t<., 1971, Hydrocarbon poten­ 18, 1906, Report of the State Earthquake Investigation tial of northern and central California offshore, in Commission: Carnegie Institute of Washington, Pub. Cram, I. H. (ed.), Future petroleum provinces of the 87, Washington, D.C., 2 vols., 1 atlas. U.S.· their geology and potential: Amer. Assoc. Mammerickx, J., 1980, Neogene reorganization of spreading Petroleum Geologists Mem. 15, v. 1, p. 212-228. between the Tamayo and the Rivera fracture zone: Howell, D. C., 1975, Hypothesis suggesting 700 kilometers of Journ. Marine Geophysical Research, v. 4, p. 305-318. right slip in California along northwest oriented Martin, B. D., 1964, Monterey submarine canyon, California: faults: Geology, v. 3, no. 2, p. 81-83. Genesis and relation~hip to continental geology: Ph.D. ____, 1976, Aspects of the geologic history of the Califor­ thesis, Univ. Southern California, , Calif., nia Continental Borderland, in D. C. Howell (ed.), 249p. Aspects of the geologic history of the California Martin, B. D., and Emery, K. 0., 1967, Geology of Monterey Continental Borderland: Pac. Sec. American Assoc. Canyon, California: Am. Assoc. Petroleum Geologists Pet. Ceol., Misc. Puhl. 24, p. 2-5. Bull., v. 51, p. 2281-2304. Howell, D.C. and Vedqer, J.C., 1978, Late Cretaceous pale­ McCulloch, D.S., 1987, Regional geology and hydrocarbon ogeography of the Salinian block, California, in How­ potential of offshore central California, in Scholl, ell, D.C. and McDougall, K.A., eds., Mesozoic pale­ D.W., Crantz, A., and Vedder, J.C., Geology and ogeography of the western United States: Society of resource potential of the continental margin of west­ Economic Paleontologists and Mineralogists, Pacific ern North America and adjacent ocean basins - Section, Pacific Coast Paleogeography Symposium 2, Beaufort Sea to : Circum-Pacific Coun­ p . 523-534. cil for Energy and Mineral Resources, Earth Science Howell, D.C., Crouch, J.K., Greene, H.C., McCulloch, D.S., Series, v. 6, p. 353-401. and Vedder, J.C., 1980, Basin development along the ____, 1989, Evolution of the offshore central California plate tectonic margin of subduction, oblique subduc­ margin, in Winterer, E.L., Hussong, D.M., and Decker, tion and transform tectonics, in Ballance, P.F. and R.W., eds., The Eastern Pacific Ocean and Hawaii: Reading, H.C., eds., Sedimentation in oblique-slip Boulder, Colorado, Geological Society of America, mobile zones: International Association of Sedimen­ The , v., n., p . 439-470. tologists Special Publication Number 4, Blackwell McCulloch, D.S., Clarke, S.H., Jr., Field, M.E., Scott, E.W., Scientific Publications, p. 43-62. and Utter, P., 1977, A summary report on the regional Howell, D.G., Jones, D.L., and Schermer, E.R., 1983, Tecton­ geology, petroleum potential and environmental ostratigraphic terranes of the frontier circum-Pacific geology of the southern proposed lease sale 53, cen­ region: AAPC Bulletin, v. 67, p. 485-486. tral and outer continental shelf: ____,1985a, Tectonostratigraphic terranes of the Circum­ U.S. Geological Survey Open-File Report 77-593, 57 p. Pacific region, in Howell, D.G., ed., Tectonostratigra­ McCulloch, D.S., Greene, H.G., Heston, K.S. and Rubin, phic terranes of the Circum-Pacific region: Circum­ D.M., 1980, A summary report of the geology and Pacific Council for Energy and Mineral Resources, geologic hazards in proposed lease sale 53, central Earth Science Series, n. 1, p. 3-30. California outer continental shelf: U.S. Geological Howell, D.G., Schermer, E.R., Jones, D.L., Ben-Avraham, Z., Survey Open-File Report 80-1095, 76 p., 12 maps, and Scheibner, E., 1985b, Preliminary tectonostratigra­ scale 1 :25,000. phic terrane map of the circum-Pacific region: Cir­ McCulloch, D.S. and Greene, H.G., (in press), Geologic map cum-Pacific Council for Energy and Mineral Re­ of the central California margin, in Greene, H.G. and sources, Circum-Pacific Map Project, Houston, Texas, Kennedy, M.P., eds., Geologic Map Series of the scale 1:17,000,000. California Division of Mines and Geology, scale Jennings, C. W., and Strand, R. G., 1958, Geologic map of 1:250,000. California, in 0. P. Jenkins, ed., Santa Cruz sheet: McKenzie, D. P., and Parker, R. L., 1967, The North Pacific: Calif. Div. Mines and Geology, scale 1: 250, 000. an example of tecton:cs on a sphere: Nature, v. 216, p. Jennings, C.W., Strand, R.G., Rogers, T.H., Stonson, M.C., 1276-1280. Burnett, J .L., Kahle, J.E., Streitz, R., Switzer, R.A., and McLaughlin, R.J., 1974, The Sargent-Berrocal fault zone Moar, R.R., 1975, Fault map of California with loca­ and its relation to the San Andreas fault system in the tions of volcanoes, thermal springs, and thermal southern San Francis-:o Bay region and Santa Clara wells: California Division of Mines and Geology, Valley, California: U.S. Geological Survey Journal of Geologic Data Map No. 1. Research, v. 2, n. 5, p. 593-598. Johnson, J. D., and Normark, W.R., 1974, Neogene tectonic McNally, K.C., Lay, T., Protti-Quesada, M., Valensise, G., evolution of the Salinian block, west-central Califor­ Orange, D., and Anderson, R.S., 1989, Santa Cruz nia: Geology, v. 2, p . 11-16. Mountains (Loma Prieta) earthquake: EOS Trans., Jones, D.L., 1983, Recognition, character, and analysis of American Geophysical Union, v. 70, n. 45, November tectonostratigraphic terranes in western North 7, 1989, p. 1463-1467. America, in Hashimoto, N. and Uyeda, S., eds., Accre­ McWilliams, M.O. and Howell, D.G., 1982, Exotic terranes tion tectonics in the Circum-Pacific regions: Boston, of western California: Nature, v. 297, 20 May, 1982, p. D. Reidel Publishing Co., p. 21-36. 215-217. . King, P. B., 1959, The Evolution of North America: Princeton Morgan, W. J ., 1968, Rises, trenches, great faults and crustal Univ. Press, Princeton, New Jersey, 189 p. blocks: }our. Gcophys. Research, v. 73, p. 1959-1982. 56

Nagel, D.K. and Mullins, H .T., J983, Late Cenozoic offset Steinbrugge, K. V., 1968, Earthquake hazard in the San and uplift along the San Gregorio fault zone, central Francisco Bay area: a continuing problem in public California margin, in Andersen, D.W. and Rymer, policy: Institute of Governmental Studies, University M.J., eds., Tectonics and sedimentation along faults of of California, Berkeley, Calif., 80 p. the San Andreas system: Soc. Economic Paleontolo­ Suppe, J.E., 1970, Offset of Late Mesozoic basement terrains gists and Mineralogists Symp., Pacific Section., p. 91- by the San Andreas fault system: Geol. Soc. America 103. Dull., v. 81, p. 3253-3258. Nagel, D.K., Mullins, H.T., and Greene, H.G., 1986, Ascen­ Tinsley, J. C.111, 1975, Quaternary geology of northern Sali­ sion Submarine Canyon, California - Evolution of a nas Valley, Monterey County, California: Ph.D. thesis, multi-headed canyon system along a strike-slip conti­ Stanford Univ., Stanford, Calif., 189 p. nental margin: , v. 73, p. 285-310. Trask, P. D., 1926, Geology of Point Sur Quadrangle, Califor­ Page, B.M., 1970, Sur-Nacimiento fault zone of California: nia: Univ. Calif. Publ. Bull. Dept. Geol. Sci., v. 16, no. Continental margin tectonics: Geol. Soc. America 6, p. 119-186. Bull., v. 81, no. 3, p. 667-690. Vedder, J.C., Howell, D.G., and McLean, 1983, Stratigraphy, Plafker, G., and Galloway, J.P., eds., 1989, Lessons learned sedimentation, and tectonic accretion of exotic terra· from the Loma Prieta, California, earthquake of Octo­ nes, southern Coast Ranges, California, in W~tkins, ber 17, 1989. U.S.G.S. Circular 1045, 48 pp. . J.S. and Drake, C.L., eds., Studies in continental mar­ Pollitz, F.F., 1986, Pliocene change in Pacific plate motion: gin geology: Amer. Assoc. Petroleum Geologists Nature, v. 320, 24 April, 1986, p. 738-741. Memoir 34, p. 471-496. Reed, R. D., 1925, The post-Monterey in the Walrond, H., Thorup, R.R., Gribi, E. H ., Jr., and Rogers, T. Salinas Valley, California: Jour. Geology, v. 33, p. 588- H., 1967, Geologic map of the Gabilan uplift and 607. adjacent areas, Monterey, San Benito, Fresno and --~ 1933, Geology of California: Amer. Assoc. Petro­ Santa Cruz Counties, in Guidebook, Gabilan Range leum Geologists, Tulsa, Okla., 355p. and adjacent San Andreas fault: Pacific Sec., AAPC­ Richter, C.F., 1958, Elementary seisl'l)Ology: W.H. Freeman SEPM Annual Field Trip. and Company, San Francisco, Calif., 768 p. Weber, G.E., 1980, Recurrence intervals and recency of Ross, D.C., 1976, Reconnaissance geologic map of pre-Ceno­ faulting along the San Gregorio fault zone, San Mateo zoic basement rocks, northern Santa Lucia Range, County, California: Ph.D. Dissertation, Univ. Califor­ Monterey County, California: U.S. Ceol. Survey, Misc. nia Santa Cruz, 204 p. Field Studies Map, MF-750, scale 1 :25,000. Ross, D. C., and Drabb, E. E., 1972, Pctrography and struc­ tural relations of granitic basement rocks in the Mon­ terey Bay area, California: Jour. Res. U.S. Ceol. Sur­ vey, v. 1, p. 273-282. Schombel, L. F., 1943, Soledad quadrangle {Calif.): Calif. Div. Mines and Geology Bull. 118, p. 467-470. Schwing, F.B., Norton, J.C., and Pilskaln, C.H., 1990, Earth­ quake and Day: Response of Monterey Day to the Loma Prieta Earthquake of October 17, 1989: EOS Trans. ACU, v. 71, n. 6, February 6, 1990, p. 250-252. Shepard, F. P., and Dill, R. F., 1966, Submarine canyons and other sea valleys; Rand McNally, Chicago, Ill., 381 p. Sieck, H. C., 1964, A gravity investigation of the Monterey­ Salinas area, California: student rcpt., Dept. Geophys­ ics, Stanford Univ., Stanford, Calif., 40 p. Silver, E. A., 1974, Basin development along translational continental margins, in Dickinson, W.R. (ed.), Geo­ logic interpretations from global tectonics with appli­ cations for California geology and petroleum explora­ tion: San Joaquin Geologic Society Short Course Notes. --~ 1977, Arc the San Gregorio and Hosgri fault zones a single fault system?: Gcol. Soc. America {Abs. with Programs), Cordilleran Sec., 73rd Annual Meeting, vol. 9, no. 4, p. 500. Silver, E. A., Curray, J. R., and Cooper, A. K., 1971, Tectonic development of the continental margin off central California: Geological Society of Sacramento, Annual Field Trip Guidebook. p. 1-10. Starke, G. W., and Howard, A. D., 1968, Polygenetic origin of Monterey Submarine Canyon: Geo!. Soc. America Bull., v. 79, no. 7., p. 813-826.