A Summary of the Late Cenozoic Stratigraphic and Tectonic History of the Santa Clara Valley, California

Home , Calaveras Fault, California Coast Ranges, San Andreas Fault, Silver Creek Fault

A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara (“Silicon”) Valley Langenheim et al. A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

V.E. Langenheim1, R.C. Jachens1, C.M. Wentworth1, R.W. Graymer1, R.G. Stanley1, R.J. McLaughlin1, R.W. Simpson1, R.A. Williams2, D.W. Andersen3, and D.A. Ponce1 1U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA 2U.S. Geological Survey, P.O. Box 25046, Denver, Colorado 80225, USA 3Geology Department, One Washington Square, San Jose State University, San Jose, California 95192, USA

ABSTRACT Creek fault. Sometime between 9 and 4 Ma concealed basins that can amplify and prolong (9 and 1 Ma for the central block), the area shaking from local and distant earthquakes. The late Cenozoic stratigraphic and rose above sea level, and a regional surface This paper summarizes the late Cenozoic tectonic history of the Santa Clara Valley of erosion was carved into the Mesozoic and stratigraphic and tectonic history of Santa Clara illustrates the dynamic nature of the North Tertiary rocks. Alluvial gravels were depos- Valley as inferred from geologic, stratigraphic, American–Pacifi c plate boundary and its ited on this surface along the margins of the and geophysical data. This summary builds on effect on basin and landscape develop- valley beginning ca. 4 Ma, but they may not the more detailed and comprehensive papers ment. Prior to early Miocene time, the area have prograded onto the central block until in this theme issue and is illustrated by a series that became Santa Clara Valley consisted ca. 1 Ma, because no older equivalents of the of schematic cross sections (Fig. 3). Interpreta- of eroding Franciscan complex basement Pliocene–Quaternary Santa Clara gravels tions of the multiple geologic and geophysical structurally interleaved in places with Coast have been found there. Thus, either the cen- data sets indicate that the history of Santa Clara Range ophiolite and Mesozoic Great Valley tral block was high enough relative to the Valley is a tale of three sedimentary basins. The sequence, and locally overlapped by Paleo- surrounding areas that Santa Clara gravels youngest basin, called the Santa Clara Basin, gene strata. During early to middle Mio- were never deposited on it, or any Santa forms the present-day shape of the valley fl oor cene time, this landscape was fl ooded by the Clara gravels deposited there were stripped and during the past 1–1.5 m.y. has been accu- sea and was deformed locally into deeper away before ca. 1 Ma. Analysis of alluvium mulating debris from the Santa Cruz Moun- depressions such as the Cupertino Basin in on the central block implies a remarkably tains on the west and the Diablo Range on the the southwestern part of the valley. Marine uniform, piston-like, subsidence of the valley east. This alluvial basin conceals two deep late deposition during the middle and late Mio- of ~0.4 mm/yr since ca. 0.8 Ma, possibly Cenozoic basins that are revealed by analysis of cene laid down thin deposits in shallow extending north to northern San Francisco geophysical, primarily gravity, data. One is the water and thick deeper-water deposits in the Bay. Today, the central block continues to Cupertino Basin in the southwestern part of the Cupertino Basin. During this sedimentation, subside, the range-front reverse faults are valley, which records transtension associated the San Andreas fault system encroached active, and the major active faults of the San with the passage of the Mendocino triple junc- into the valley, with most offset partitioned Andreas system are mostly outside the valley. tion and development of the San Andreas fault. onto the San Andreas fault southwest of the The other is the Evergreen Basin in the east- valley and the southern Calaveras–Silver INTRODUCTION ern part of the valley, which is the product of a Creek–Hayward fault system in the north- right step within the East Bay fault system. The eastern part of the valley. A 6-km-wide right Santa Clara Valley, located within the broad margins of these concealed basins have been step between the Hayward and Silver Creek San Andreas fault system in northern Califor- largely overridden by thrust and reverse faults faults formed the 40-km-long Evergreen pull- nia (Figs. 1 and 2), hosts a population of nearly along the western and eastern edges of the val- apart basin along the northeastern margin of 2,000,000 people (U.S. Census Bureau, 2010). ley. The history of these three basins beneath the the valley, leaving a basement ridge between Groundwater from the valley provides this pop- urbanized Santa Clara Valley provides insight it and the Cupertino Basin. The Silver Creek ulation with nearly half of its water supply. The into the way in which the San Andreas fault fault was largely abandoned ca. 2.5 Ma in stratigraphy and structure beneath this highly system has developed in this region, illustrating favor of a compressional left step between urbanized valley provide a foundation for defi n- the dynamic nature of the plate boundary and the Calaveras and Hayward fault, although ing the boundaries of the groundwater fl ow sys- its effect on basin and landscape development. some slip continued to at least mid-Quater- tem because aquifer systems are a product of the nary time. Gravity, seismic, stratigraphic, interplay of depositional and deformational pro- REGIONAL GEOLOGIC SETTING and interferometric synthetic aperture radar cesses through time. The stratigraphy and struc- (InSAR) data indicate no other major San ture of the valley also provide a framework for Santa Clara Valley, a broad, mostly fl at allu- Andreas system faults within the central assessing seismic hazard by mapping locations vial plain extending southward from San Fran- block between the present-day range-front of faults concealed beneath urban development cisco Bay, is situated within the San Francisco faults bounding the valley and the Silver and young surfi cial deposits, and by defi ning Bay block (blue shaded region in Fig. 2). We

Geosphere; February 2015; v. 11; no. 1; p. 50–62; doi:10.1130/GES01093.1; 4 ﬁ gures. Received 19 June 2014 ♦ Accepted 8 December 2014 ♦ Published online 14 January 2015

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124°W 123°W 122°W 121°W 120°W offset determined from analysis of gravity and LEGEND magnetic anomalies suggests even less offset COAST RANGES on the Peninsula segment, ~22 km (Jachens Vizcaino Quaternary Block and Zoback, 2000). The discrepancy between Tertiary sedimentary rocks 300 and 330 km of displacement on the San Tertiary volcanic rocks 39°N Andreas fault documented along the central Point Franciscan Complex part of the San Andreas fault and 124 km of Arena San Andreas Fault Mesozoic sedimentary rocks displacement along the Santa Cruz Mountains (Great Valley sequence east of segment is reconciled by adding displacement the San Andreas fault) taken up on faults east of the San Francisco Serpentinite and ophiolite Bay block. The Hayward and Calaveras faults Granitic and related rocks are part of the East Bay fault system, and they (Salinia) have 160–190 km of cumulative right-lateral BM displacement (McLaughlin et al., 1996) since Hayward Fault 38°N ca. 12 Ma (Graymer et al., 2002). This dis- GREAT VALLEY placement is further reﬁ ned by offset gravity and magnetic anomalies to ~174 km (Jachens Calaveras Fault et al., 1998). The early development of the San Andreas SCV fault system was accompanied by slab-window volcanism in California and northern Mexico

San Gregorio-Hosgri Fault YBR starting about 28–27 Ma that resulted from the 37°N deaths of a series of spreading ridge segments HV QS during piecewise destruction of an older sub- PACIFIC duction regime (Atwater, 1970; Wilson et al., OCEAN 2005; McCrory et al., 2009). In the vicinity of San Andreas Fault the Santa Clara Valley, the development of dis- crete faults and northwestward-younging volcanism may have begun about 18–15 Ma in the COAST wake of a slab window that accompanied the PV northwestward-migrating Mendocino Triple 36°N Junction (Dickinson and Snyder, 1979; John- son and O’Neil, 1984; Fox et al., 1985; Stanley, Parkfield 1987b; McLaughlin et al., 1996). The distribution of basement beneath Santa RANGES PPP Clara Valley is not just the result of strike-slip faulting and transtension, but also earlier subduction. The basement within the San Francisco 0 50 100 150 200 KM Bay block consists of two main, often com- plexly interleaved coeval Mesozoic packages, 35°N the Franciscan complex (a subduction complex) Figure 1. Index map showing simplifi ed geology (modifi ed from Jennings, 2010). Box shows and the Coast Range ophiolite with its overlying location of Figure 2 centered on Santa Clara Valley (SCV). Black lines are Quaternary faults sedimentary section, the Great Valley sequence modifi ed from Jennings (1994). Abbreviations are for signifi cant localities discussed in text. (forearc-basin complex) of Bailey et al. (1964). BM—Burdell Mountain; HV—Hollister Valley; PPP—Palo Prieto Pass; PV—Priest Valley; Along the east side of the Coast Ranges for a QS—Quien Sabe volcanic fi eld; YBR—Yerba Buena Ridge. Thick red line near Point Arena distance of 600 km, these two packages are jux- marks the southern extent of the Vizcaino block. taposed by the Coast Range fault, a subduction boundary (Jayko et al., 1987) highly modifi ed by the roof thrust of a tectonic wedge inboard of the fossil subduction megathrust (Wentworth restrict our discussion to that part of Santa Matthews, 1976; Stanley, 1987a; Graham et al., and Zoback, 1990; Jachens et al., 1995) and/or Clara Valley northwest of Coyote Narrows 1989; Sharman et al., 2013). However, the seg- by attenuation faulting related to collapse of the (CN in Fig. 2). The San Francisco Bay block is ment of the San Andreas fault immediately west accretionary prism and unroofi ng of the low- bounded by major right-lateral faults, the San of Santa Clara Valley (Santa Cruz segment) temperature–high-pressure metamorphic rocks Andreas fault on the southwest and the Hay- has signifi cantly less offset, ~124 km (Jachens in the Early Tertiary (Harms et al., 1992). About ward and Calaveras faults on the northeast. The et al., 1998), and the Peninsula segment north 5 km south of Los Gatos, the Aldercroft fault San Andreas fault has ~300–330 km of right- of the intersection with the Pilarcitos fault has is the principal boundary between rocks of the lateral displacement since 23 Ma based on off- even less displacement, 28–30 km of right- Coast Range ophiolite (called at this location the sets of volcanic fi elds, shorelines, and deep-sea lateral displacement since ca. 3.3 Ma (Dibblee, Sierra Azul ophiolite) and the Franciscan com- fans (Hill and Dibblee, 1953; Huffman, 1972; 1966; McLaughlin et al., 1996, 2007). Bedrock plex, occupying the same structural position as

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122°30′W 122°15′W 122°W 121°45′W 121°30′W

Livermore Basin SAN ANDREAS FAULT San Leandro SAN FRANCISCOSynform BAY Hayward Fault

Coyote Calaveras Fault Point Pilarcitos Fault Coyote Hills

37°30′N

San Gregorio-Hosgri Fault Palo Alto

A T CLARA SANTA Evergreen Basin Calaveras Fault DIABLO Silv er Creek Fault RANGE MVF Cupertino GUAD Basin Mt. Misery (?) Fault SCF WLLO SaratogaNCF Yerba Buena Ridge MGCY OH 37°15′N Los BF Gatos CN SF Calaveras Fault SAN ANDREAS FAULT Morgan Hill VALLEY PACIFIC OCEAN LP SANTA CRUZ Gilroy MOUNTAINS

0 5 10 KM SaF

Figure 2. Shaded-relief topographic map of Santa Clara Valley and vicinity. Faults from Graymer et al. (2006) are shown in red. Other possible faults (discussed in text) shown in white: NCF—New Cascade fault of Hanson et al. (2004); SCF—Santa Clara fault. Blue shaded region denotes extent of the San Francisco Bay block; the central San Francisco bay block is between the range-front fault system on the southwest (BF—Berrocal fault; MVF—Monte Vista fault; SF—Shannon fault) and the Silver Creek fault on the northeast. Yellow shaded areas are basins as defi ned by gravity lows. Purple shaded area is the Sierra Azul block. Green lines are seismic-refl ection profi les from Williams et al. (2004; north-south profi le) and Catchings et al. (2006). Circles are selected drill holes that provide constraints on the depth of the alluvial Santa Clara Basin (GUAD—Guadalupe; MGCY—McGlincy; WLLO—Willow). Other abbreviations: OH—Oak Hill (also known as Communications Hill); CN—Coyote Narrows; LP—Loma Prieta; SaF—Sargent fault.

the Coast Range fault (McLaughlin and Clark, Pre–18 Ma are the exhumed equivalents of eastward- 2004). In parts of the San Francisco Bay block, tapering tectonic wedges that underlie the Coast Range ophiolite and Great Valley sequence In early Miocene time, the area that became western parts of the Great Valley sequence rocks are known to be interleaved with Francis- Santa Clara Valley (the area currently between and are imaged by seismic-refl ection data can complex rocks, suggesting that the simple the San Andreas fault and the East Bay fault along the Coast Ranges–Great Valley margin relations displayed by the Coast Range fault system) consisted of uplands of Franciscan (Wentworth and Zoback, 1990). The Oligo- have been modifi ed by subsequent deformation. complex basement structurally overlain in cene to early Miocene Temblor Formation, Next, we summarize our reconstruction of the places across the Coast Range fault by Coast exposed along the southwest margin of the tectonic and stratigraphic history of Santa Clara Range ophiolite and Mesozoic Great Val- Santa Clara Valley, contains locally derived Valley, aided by schematic cross sections shown ley sequence and overlying Lower to Middle angular detritus from uplifted Franciscan com- in Figure 3, which show the geometry of basins Eocene strata. Magnetic modeling (Jachens plex and Coast Range ophiolite and from the and faults at various time intervals and by palin- and Griscom, 2004) suggests that slices of the Eocene sedimentary sequence in the Sierra spastically restored map views (Fig. 4). Coast Range ophiolite (simplifi ed on Fig. 3) Azul block (McLaughlin et al., 1991a, 1991b).

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/1/50/3334215/50.pdf by guest on 30 September 2021 Stratigraphic and tectonic history of Santa Clara Valley, California Diablo Range Diablo Range Diablo Range T T A A Jurassic Coast Range Ophiolite Cretaceous and Jurassic Franciscan Complex Fault Fault Calaveras Calaveras Fault Calaveras T Proto-Hayward A A (Mt. Misery (?) Fault) A T T A T basin basin T T T basin T Evergreen Evergreen A A Evergreen AT A Thrust Fault Silver Creek Paleogene strata Cretaceous and Jurassic Sequence Great Valley basin Miocene to early Pliocene (?) strata Silver Creek Santa Clara Santa Clara Valley Santa Clara Valley ? ? basin basin ? basin Cupertino Cupertino 10 km 10 km Cupertino 10 km ? ? Mtns Mtns ? ? Mtns Santa Cruz Santa Cruz Santa Cruz T T T Fault T Fault T T San Andreas Holocene and Pleistocene (<1Ma) strata of the Santa Clara basin Pliocene and Pleistocene strata (continental deposits) Water Water San Andreas Fault –5 km +2 km –5 km +2 km San Andreas –5 km +2 km Sea level Sea level Sea level 4–1 Ma 1–0 Ma 9–4 Ma NE Fault Fault Proto-Hayward Proto-Hayward AT AT ? ? basin basin Evergreen Evergreen AT AT Fault Fault ? ? Silver Creek Silver Creek 10 km Figure 3. Schematic cross sections through time across Santa Clara Valley. A—away; T—toward. A—away; Valley. Santa Clara time across sections through 3. Schematic cross Figure Santa Clara Valley Santa Clara Valley Santa Clara Valley 10 km Santa Clara Valley basin ? ? Cupertino basin basin ? Cupertino Cupertino Fault Fault San Andreas Fault San Andreas T T San Andreas –5 km +2 km –5 km +2 km –5 km –5 km +2 km +2 km SW Sea level Sea level Sea level Sea level 12–9 Ma pre- 18–15 Ma 12 Ma 18–15 Ma

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A SF Bay 37°30′N San Andreas Fault Zone, including PA Sargent and Paicines Faults East Bay Fault System, including v Calaveras and Hayward Faults EB West Santa Clara Valley thrust/reverse MtH CB SJ faults East Santa Clara Valley thrust/reverse faults West Santa Clara Valley concealed v (oblique right-lateral?) normal San Calaveras fault, wide-spaced where projected south of gravity low v Approximate eastern margin of Cupertino Basin, wide-spaced PacP where projected south of gravity v low Sargent Faul G v 37°N East Santa Clara Valley concealed v oblique right-lateral normal faults, Fault t including Silver Creek Fault QSV Quaternary surficial deposits of Santa Clara Valley HB Other Quaternary surficial deposits Monterey Bay v Pleistocene and Neogene strata, pink where volcanic Pre-Neogene rocks

Andreas 01020 30 40 50 km PanP Vall ecitos S yncline 36°30′N

SB Mtn

122°W

PV Co Coalinga Co G Gilroy MtH Mount Hamilton SMtn Pa Parkfield PacP Pacheco Pass 36°N PanP Panoche Pass PA Palo Alto PPP Palo Prieto Pass Pa PV Priest Valley SBMtn San Benito Mtn SJ San Jose SMtn Smith Mountain Fault

CB Cupertino Basin EB Evergreen Basin HB Hollister Basin 121°30′W QSV Quien Sabe Volcanics PPP

35°30′N

121°W 120°30′W 120°W

Figure 4 (on this and following two pages). Paleogeography and palinspastic reconstruction of the Santa Clara Valley block. (A) Present- day geography and simpliﬁ ed geology of the California Coast Ranges east of the San Andreas fault between Palo Prieto Pass (PPP) and southern San Francisco Bay (SF Bay), including the Santa Clara Valley. Geology is modiﬁ ed from Jennings (2010). Santa Clara Valley faults, including possible extension of Cupertino Basin south of the related gravity low, under overthrust pre-Neogene rocks, are from Graymer et al. (2015).

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B ~18 Ma SB ~16 Ma SB Mtn Mtn

PV PV Co Co SMtn SMtn Temblor SJ SJ deposition Dacitic volcanics

Temblor Pa Pa deposition (Cupertino?) Basin San Andreas Fault very early San Andreas Fault early (Cupertino?) Basin Saucesian Monterey deposition G G Relizian Co Coalinga Monterey G Gilroy deposition Pa Parkfield PA Palo Alto PPP PPP PPP Palo Prieto Pass [HB] [HB] PV Priest Valley SBMtn San Benito Mtn SJ San Jose SMtn Smith Mountain HB Hollister Basin

Hayward F. ~14Ma SB ~12 Ma SB Mtn Mtn

Big Santa Basaltic Blue Margarita volcanics deposition deposition PV PV Co Co SMtn Cupertino Cupertino SMtn Basin SJ Basin SJ Connection Connection uncertain uncertain Luisian Pa Monterey Pa Mohnian deposition Monterey Cupertino? Cupertino? deposition San Andreas Fault Basin San Andreas FaultBasin Calaveras F.

G G

PPP PPP [HB] [HB]

0102030 40 50 km

Figure 4 (continued). (B) Sequential schematic paleogeographic maps of the Santa Clara Valley block and areas directly east at the time after restoring offset on the San Andreas fault system.

Fault reconstructions of the San Francisco Bay Palo Prieto Pass and Priest Valley (Figs. 1 and topography of the area that now is Santa Clara block (Jachens et al., 1998) place Santa Clara 4). Local uplift and erosion during this time may Valley probably resembled the present-day Valley east of the Vizcaino block before 15 Ma have been associated with low-angle attenuation Santa Cruz Mountains or Diablo Range to the (today the Vizcaino block lies north of Point faulting northeast of the San Andreas fault, fol- northwest, with broad areas of shallow marine Arena; Fig. 1) and opposite that part of the Great lowed by reverse faulting along the Sargent fault deposition, and local areas of deep marine depo- Valley block to the east that is today between 10–17 Ma (McLaughlin and Clark, 2004). The sition (Fig. 4B).

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B cont. ~10 Ma ~8 Ma Hayward Fault Santa Oro Loma Margarita Oro Loma nonmarine deposition nonmarine deposition deposition Santa Cruz PanP Mudstone-age Vallecitos Syncline deposition Santa SB PA Margarita Mtn PA deposition

EB Silver Creek Fa EB SJ CB SJ SB CB PV Mtn D elmontian Co ult

Early SMtn Early San Andreas Fault Jacalitos compression San Andreascompression Fault Etchegoin and uplift Mohnian and uplift Ree PV deposition Monterey f Ridge deposition

G Pa SMtn G Gilroy deposition

MtH Mount Hamilton Calave Pa Parkfield PacP Pacheco Pass PanP Panoche Pass ras F. PA Palo Alto [HB] [HB] PV Priest Valley Pa SBMtn San Benito Mtn CB Cupertino Basin SJ San Jose EB Evergreen Basin SMtn Smith Mountain HB Hollister Basin

Hayward Fault H MtH ~6 Ma ~3 Ma ayward Fau

lt Silver Creek PA Oro Loma nonmarine PanP nonmarine PA deposition deposition EB PacP EB Vallecitos Silver Creek FaultMt Misery(?) Fault Syncline Silver Cree Mt Misery(?) Fault CB SJ CB SJ k Fault QSV

SB Santa Clara Compression and uplift Compression Mtn nonmarine Olivine-basaltic San And and uplift deposition San Andreas Fault volcanics Etchegoin Etchegoin deposition deposition reas Fault

Calaveras Fault PV Calaveras F. G G

offset PanP Val Syn

eastern HB HB “Purisima” deposition “Purisima” EB deposition

01020304050 km

Figure 4 (continued).

Circa 18–15 Ma in the wake of the Mendocino triple junction, sibly the advent of faulting to the west of the because the timing of transfer of transform valley and displacement of valley rocks relative Exactly when the San Andreas fault reached plate-boundary slip inboard to the San Andreas to rocks west of the San Andreas fault. as far north as the Santa Clara region is only fault is not well known. In the hills east of the During early to middle Miocene time, this loosely constrained by the age of volcanism and San Andreas fault, ca. 16–14 Ma volcanic rocks landscape further subsided beneath a shal- hydrothermal alteration, presumably formed mark the passage of the triple junction, and pos- low sea and was deformed locally into deeper

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depressions such as the Cupertino Basin in the deposition also appears to be transgressive, with marine environments. Concurrent development southwestern part of the valley. Miocene strata basal Monterey strata younging to the north- of the Cupertino and Evergreen Basins begin- exposed along the southwest margin of the west from Gilroy to Los Gatos. The Cupertino ning at ca. 12 Ma led to formation of a basement valley indicate marine deposition at depths of Basin may have extended southeast of its cur- ridge that now bisects the Santa Clara Valley ~150–1500 m (Stanley et al., 2002), and deep- rent confi guration as defi ned by its gravity low; (Fig. 3). ening of that basin migrated northward between Graymer et al. (2015) speculates that a bench Gilroy and Cupertino between the early and in the gravity fi eld southeast of San Jose may 12–9 Ma middle Miocene (McLaughlin et al., 1999; refl ect Miocene basin deposits that are now Fig. 4B). structurally concealed (Fig. 4B). The Monterey During this time, slip across the right step Multiple lines of evidence point to the exis- Formation (and its equivalent, the Claremont between the Silver Creek–Calaveras and Hay- tence of the Cupertino Basin. Driller’s logs from Shale, east of the Hayward fault) may also be ward faults produced ongoing lengthening a minor oil fi eld in the Los Gatos area indicate a present elsewhere within the San Francisco Bay and subsidence of the Evergreen Basin (Fig. thick (780 m) section of brown shale interpreted block, such as in the east-dipping sedimentary 4B). We suspect that apparent thinning of the to be Monterey Formation; geochemistry from package below an unconformity seen in seis- ophiolite section and Franciscan complex rocks an oil sample from this fi eld indicates deriva- mic-refl ection profi les in the San Leandro area beneath the Evergreen Basin resulted because tion from a Monterey Formation source rock (Marlow et al., 1999). of translation of more southerly rocks into the generated at a depth of ~2.1–2.7 km (Stanley line of section by right-lateral faults (Fig. 3). et al., 2002). Gravity data indicate that the basin Circa 12 Ma Concurrently, the Cupertino Basin continued to is ~3 km deep, 30 km long, and ~10 km wide, subside and fi ll with marine sediments. Marine with a steep southwest margin and gently slop- Marine sedimentation continued even as sedimentation continued until at least 8 Ma, ing northeast margin (Langenheim et al., 1997; the faults of the San Andreas system displaced the age of the sediments at the bottom (260 m Stanley et al., 2002). A seismic-refraction pro- the block containing Santa Clara Valley at ca. depth) of the McGlincy drill hole based on dia- fi le (Catchings et al., 2006) confi rms the pres- 12 Ma. By 12 Ma, the San Andreas fault (sensu toms (MGCY, Fig. 2; Lisa White, San Francisco ence of a deep basin in the Los Gatos area, with stricto) cut through the ophiolite in the Sierra State University, 2004, written commun.). Tilt- velocities of <5 km/s extending to depths of Azul block, displacing to the northwest a slice of ing of the Miocene section in the Cupertino 3 km. Although Catchings et al. (2006) inter- the ophiolite that currently resides north of Point Basin must have ended by 8 Ma, because the preted the base of the basin at 1.6–1.8 km depth Arena in the Vizcaino block (Fig. 1; Jachens sediments found at the bottom of the McGlincy based on the isovelocity contour of 3.5 km/s, et al., 1998) on the west side of the fault. On the drill hole project onto the fl at-lying section velocity-depth relationships for the various Bay east side of the valley, slip on the East Bay fault imaged in the seismic-refl ection profi le of Wil- Area rock types indicate that Franciscan com- system began some time later, based on correla- liams et al. (2004). plex basement rocks should have velocities of tion of the 12–10 Ma Quien Sabe volcanic fi eld 4.5 km/s or higher at these depths (Brocher, east of Hollister with equivalent-aged volcanics 9–4 Ma 2008). A north-south–oriented seismic-refl ec- on Burdell Mountain near Novato (McLaughlin tion profi le through Santa Clara and Cupertino et al., 1996; Graymer et al., 2002; Ford, 2007). During this time, Salinian basement and its (Williams et al., 2004) indicates an apparent At the time of initiation of East Bay fault- overlying sedimentary cover were translated south dip to the basin sequence, which is consis- ing, the present Santa Clara Valley region was northwestward to a position adjacent to the San tent with the sense of tilting in the hanging wall ~100–110 km south of the Quien Sabe volcanic Francisco Bay block by the San Andreas–Pilar- of a northeast-dipping master normal fault. The fi eld. Slip on the east side of Santa Clara Valley citos fault, according to the fault reconstructions asymmetry of the basin (Fig. 3) suggests that it appears to have begun on the Silver Creek fault in Jachens et al. (1998). Slip continued along formed as a half graben. (also referred to as the proto–Calaveras fault by the Silver Creek and proto–Calaveras-Hayward Initial subsidence and sedimentation in the Graymer et al., 2005), stepping right 6–8 km faults in the eastern part of the Santa Clara Cupertino Basin were likely the result of trans- onto the proto–Hayward fault. The Silver Creek Valley, with accompanying subsidence in the tension accompanied by northward-younging fault, as defi ned by Wentworth et al. (2010), is Evergreen Basin. Some slip may have also been volcanism and hydrothermal activity. Along the the older right-lateral fault that was later locally taken up by the Mount Misery (?) fault, a com- western margin of Santa Clara Valley, there are obscured by the younger Silver Creek thrust in pletely concealed fault along the east margin of the 14.8 Ma Page Mill Basalt near Palo Alto, the Yerba Buena Ridge (also known as Coyote the Evergreen Basin (Wentworth et al., 2010) a 15.6 Ma dacite tuff and local intrusive rocks Ridge) area. These faults began to dissect a large that is considered by Graymer et al. (2015) as in the New Almaden area, and 17.6 Ma hydro- tabular fl at-lying ophiolite, displacing the Priest probable, but not required. Early compression thermal veining near Gilroy (McLaughlin et al., Valley ophiolite, mostly concealed, but exposed took place along the western range front, based 1996). These ages suggest that the formation along its southernmost extent near Parkfi eld, on a signifi cant angular unconformity at the base of the basin initiated 15–18 Ma. Graymer et al. southeastward from its cross-fault counter- of the Pliocene–Quaternary Santa Clara gravels (2015) favor an alternate interpretation in which part, the ophiolite on Yerba Buena Ridge (R.C. (Graymer et al., 2015). Sometime during this the Cupertino Basin formed later after eruption Jachens, 2013, personal commun.). The area time interval, probably toward the end, however, of the dacitic volcanics, based on regional corre- within the stepover region began to subside, the San Francisco Bay block and its surround- lation of the Temblor Formation with equivalent forming the Evergreen Basin. Although there ing terrain emerged above sea level. Uplift of rocks to the east after palinspastic restoration are no deep wells within the Evergreen Basin, the central part of the San Francisco Bay block and the presence of Monterey petroleum source it is likely that marine sediments were deposited and erosion of the intervening bedrock ridge rocks near the base of the basin fi ll. in this basin at this time because Miocene sedi- between the Cupertino and Evergreen Basins Deposits of the Monterey Formation may mentary rocks of this age throughout the Cali- likely began during this time frame, eventually have been widespread in Santa Clara Valley. Its fornia Coast Ranges were mostly deposited in exposing the serpentinite at Yerba Buena Ridge,

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which provided serpentinite detritus near the .gov/WellSearch /WellSearch .aspx, accessed refl ection profi les and various drill holes in base of the Santa Clara Basin fi ll in the Guada- June 2014), penetrates more than 2.3 km of Santa Clara Valley may extend at least as far lupe drill hole (GUAD, Fig. 2; Oze et al., 2003). late Miocene (?) and Pliocene Purisima For- north as the San Leandro synform (Fig. 2; Uplift also continued in the central Diablo mation (California Division of Oil and Gas, Wentworth et al., 2010), a structure revealed Range, stripping Coast Range ophiolite and 1982; Robbins, 1982; Rogers, 1993). Other by seismic-refl ection and gravity data (Marlow any overlying Great Valley and Tertiary section deep drill holes west of the Calaveras fault in et al., 1999). The western, east-dipping limb that had not already been thinned or removed Hollister Valley (Texaco Recht No. 1, API of the synform consists of ~1–1.5 km of Ter- by attenuation faulting during or before early 06900290; Trico Breen #1, 06900062; Chevron tiary(?) marine (?) sedimentary rocks truncated Miocene time (Harms et al., 1992), while shal- O’Connell B-2, 06900061; Chevron O’Connell by a horizontal, angular unconformity overlain low marine and nonmarine deposition continued B 1, 069000289; http:// owr .conservation .ca .gov by ~300 m of fl at-lying strata. The projection around the perimeter of the range and in the Val- /WellSearch /WellSearch .aspx, accessed June of the east-dipping limb to the west lies above lecitos syncline region (Fig. 4B). 2014) encounter as much as 2.2 km of Purisima the angular unconformity imaged in seismic- The emergence of the Santa Clara Valley Formation overlying Mesozoic basement rocks refl ection data, suggesting that ~1–1.5 km of block at this time may imply that a signifi - that include serpentinite. Note that all of these section were stripped or eroded away (Marlow cant volume of the Miocene section outside of wells lie south of or near the projection of the et al., 1999). The age of the development of this the Cupertino Basin (and possibly Evergreen Sargent fault (Figs. 2 and 4B), which has an unconformity is poorly constrained; the devel- Basin) was also removed by erosion, forming unknown amount of strike slip during this time opment of the surface may have initiated during the unconformity that forms the base of the frame but is likely to be on the order of several this time frame or initiated later (as discussed by alluvial Santa Clara Basin. The evidence that kilometers (which restores ophiolitic and Fran- Wentworth et al., 2010). this section was originally laid down and sub- ciscan complex rocks into reasonable proxim- sequently eroded can be found in sections that ity; McLaughlin et al., 1999). Robbins (1982) 4–1 Ma have been displaced out of Santa Clara Valley by modeled the gravity and magnetic anomalies the San Andreas fault system. The offset equiva- in this area as refl ecting a substantial section of Deposition of alluvial gravels began on both lent of the ophiolite on Yerba Buena Ridge late Miocene (?) to Pliocene sedimentary rocks sides of Santa Clara Valley. At ca. 4 Ma, the (YBR in Fig. 1) east of the San Andreas fault overlying basement that must include serpen- margins of the Cupertino and Evergreen Basins system, near Parkfi eld, coincides with a large, tinite and related ophiolitic rocks. This mag- began to receive nonmarine sediments, such as prominent aeromagnetic anomaly produced netic basement produces a magnetic anomaly the Silver Creek, Santa Clara, and Irvington/ by the ophiolite in Priest Valley (PV in Fig. 1; that is truncated at the Calaveras fault along a Packwood gravels (Page, 1992). During this R.C. Jachens, 2013, personal commun.). This reach of ~25 km (R.C. Jachens, 2013, personal time interval, the range-front thrust systems ophiolite is buried where it is truncated by the commun.) and can be followed northwestward along both margins of the valley were active San Andreas fault, and modeling of its resultant into the Santa Cruz Mountains. Restoring (Figs. 3 and 4B), overriding the Miocene– magnetic anomaly places the top of the source 174 km of displacement on the Calaveras and Pliocene (?) margins of the Cupertino and at ~2–3 km (Griscom and Jachens, 1990). Seis- central San Andreas faults places the Hollister Evergreen Basins. Fission-track data indicate mic velocity models of this area also indicate magnetic anomaly against a higher-amplitude that rocks in the Santa Cruz Mountains in the a strong velocity gradient at about the same magnetic anomaly near Palo Prieto Pass (PPP area of Loma Prieta (LP on Fig. 2) have been depth above a low-velocity zone interpreted to on Fig. 1). This magnetic anomaly, truncated uplifted 3.5–4 km since ca. 4 Ma (Bürgmann indicate high pore pressures (Eberhart-Phillips, along a reach of 25 km along the San Andreas et al., 1994). Gravels of the Santa Clara For- 1989). A drill hole, the Philips Petroleum Com- fault, refl ects a large concealed magnetic body, mation were folded, uplifted, and faulted in the pany Varian A1, indicates that the source of this likely composed at least partly of serpentinite hanging-wall blocks of the Monte Vista and anomaly is buried beneath a 1.5-km-thick sec- (R.C. Jachens, 2013, personal commun.; Hanna Berrocal thrust faults. Average shortening rates tion of Miocene and younger sedimentary rocks et al., 1972). Because Mesozoic rocks crop out of these faults during the past 5 m.y. are simi- (John Sims, U.S. Geological Survey, 1987, writ- above the Palo Prieto Pass body, this body has lar to those accommodated by folding, 0.5–0.6 ten commun.). The Monterey (middle Miocene) remained buried since separation; Hanna et al. mm/yr (McLaughlin et al., 1999). During this and Temblor Formations in this drill hole were (1972) modeled the top of the magnetic body time interval, the San Andreas fault reorganized, likely also present in Santa Clara Valley, indicat- at 4–4.5 km depth. The Hollister Valley mag- abandoning the Pilarcitos strand and straighten- ing that more than 1.5 km of section may have netic body, on the other hand, probably forms ing its trace (McLaughlin et al., 2007). As a result been removed by erosion outside the Cupertino the fl oor of the mostly Pliocene deposits that fi ll of this reorganization, a slice of Permanente ter- and Evergreen Basins. The amplitude of the Hollister Valley (Robbins, 1982), indicating that rane bounded by the Pilarcitos and San Andreas Yerba Buena aeromagnetic anomaly is smaller the serpentinite body was exposed after separa- faults was offset 22 km to the northwest rela- than at Priest Valley, suggesting that the serpen- tion from the Palo Prieto Pass body and subject tive to the belt of the Permanente terrane along tinite source is thinner in Santa Clara Valley, and to erosion before being reburied beneath Hol- the southwestern margin of Santa Clara Valley thus a deeper level of erosion. lister Valley. Note also that the amplitude of the (Jachens and Zoback, 2000). Timing of this Another inference for erosion and removal of magnetic anomaly in the Palo Prieto Pass area slight reorganization in the San Andreas fault part of the Neogene sedimentary section comes is much higher than that in the Hollister area, is provided by Pliocene–Pleistocene gravels of from southern Santa Clara Valley near Hollister suggesting uplift, exposure, and erosion of the the Santa Clara Formation that are offset from and highlights the lateral variation of deposi- serpentinite beneath Hollister Valley as it was a distinctive clast source across the San Andreas tion within the San Francisco Bay block. A drill translated right-laterally by the Calaveras– fault by ~28–30 km (Cummings, 1968; Dibblee, hole in Hollister Valley (HV in Fig. 1), west of Silver Creek–Hayward faults (Fig. 4B). 1966). The discrepancy between the bedrock the Calaveras fault (Texaco Nutting No. 1, API The erosional surface at the base of the offset based on matching magnetic anomalies number 069000288; http:// owr .conservation .ca Quaternary deposits documented by seismic- and offset of the Santa Clara Formation can be

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resolved if the offset of the Santa Clara gravels faults (including the Silver Creek thrust of San Francisco Bay, despite sea-level highstands did not solely occur on the Peninsula segment of Went worth et al., 2010) that placed serpentinite during the past 1–1.5 m.y. the San Andreas fault, but also partly occurred over the 2–4 Ma Silver Creek gravels. Strike slip Seismic-refl ection data in Santa Clara Valley on the Pilarcitos fault (McLaughlin et al., 2007). on the central Calaveras fault took over at ca. indicate that the strata within this <1–1.5 Ma This would argue for a period of time during 3.5 Ma (Page, 1992), or even later at ca. 2 Ma alluvial basin are generally fl at-lying and paral- which slip occurred on both the Pilarcitos fault (Wentworth et al., 2010; Graymer et al., 2015). lel to the ground surface (Williams et al., 2004). and the Peninsula segment. On the eastern side of the Evergreen Basin, the The alluvial strata are only locally deformed, for The distribution of Santa Clara gravels in basin margin formed by the southern Hayward example, by the Silver Creek fault (Wentworth and around Santa Clara Valley is puzzling. fault is also overprinted by a stacked sequence et al., 2010) and prominently at the eastern side Santa Clara gravels crop out on the hanging- of thrust faults. A fl ap of Cretaceous Great Val- of the Evergreen Basin. In the seismic-refl ection wall block of the Monte Vista fault, which is at ley sequence is thrust over the basin as a result profi les, the alluvial section is at most 400 m a higher elevation than the valley fl oor, where of oblique right slip within the restraining bend thick (at least 468 m thick according to a well at least the older part of the section (>1 Ma) between the Calaveras and Hayward faults. This on the west rim of the Evergreen Basin; Crit- has not been identifi ed in any of the drill holes fault reorganization on the east side of the valley tenden, 1951) and lies on top of an irregular (Wentworth and Tinsley, 2005; Andersen et al., postdated the eruption of the basalts of Ander- Mesozoic basement or over a horizontal Mio- 2005). This suggests one of two possibilities. son–Coyote Lake Reservoir (Graymer et al., cene (Cupertino) or Pliocene (Evergreen) sedi- (1) The basin block may have been at a higher 2015) dated at 2.5–4 Ma (Nakata et al., 1993). mentary section. In the Cupertino Basin, the elevation than that of the hanging-wall block of The reorganization of the fault systems that Miocene sediment in turn overlies a dipping, the Monte Vista fault during the deposition of bound Santa Clara Valley within this time inter- presumably Miocene sedimentary section. The these gravels; thus, gravels were never deposited val may have resulted from a change in plate extent of subsidence and sedimentation of this on the valley fl oor. (2) Santa Clara and other motion that produced as much as 10 mm/yr young basin may have been widespread, if these gravels may have been deposited out in the val- plate-normal convergence beginning between basin sediments are the same as those imaged ley, but then they were eroded to remove all 3.9 and 3.4 Ma (Harbert, 1991), timing that in the seismic-refl ection profi les across the San trace of the Santa Clara gravels. Subsequently, overlaps with the onset of uplift and transpres- Leandro synform, as suggested by Wentworth other gravels were deposited across the valley sion throughout the central and southern Coast et al. (2010). Only in scattered areas, such as fl oor. Some support for the second scenario Ranges (Page et al., 1998). Later plate recon- Coyote Hills and Coyote Point in the north, and comes from distinctive chert clasts derived from structions, however, indicate signifi cantly less Oak Hill (also known as Communications Hill) the Claremont Shale (Andersen et al., 2005) and plate-normal convergence (DeMets et al., 1994) and Yerba Buena Ridge in the south, is the relief “exotic” siliceous felsite and volcanic porphyry or plate-normal convergence initiating much on Mesozoic bedrock large enough that bedrock clasts (Vanderhurst et al., 1982) in the oldest earlier at about 8 Ma (Atwater and Stock, 1998). crops out above the alluvial deposits and San Santa Clara gravels exposed in the hanging-wall Furthermore, refi ned estimates of the onset of Francisco Bay mud. It is this alluvial basin that block of the range-front system near Saratoga. transpression along the margins of the Santa hosts the groundwater for the valley. The most likely source of these clasts is to the Clara Valley suggest that transpression may east and south of the valley; paleocurrent indi- not be synchronous (Graymer et al., 2015), as DISCUSSION AND CONCLUSIONS cators suggest that the older part of the section would be expected if the plate-motion changes apparently was deposited to the north across the are solely responsible for onset of uplift and Several models have been proposed for the valley ca. 2 Ma. The mechanism to accomplish transpression. Alternatively, the reorganization formation of Santa Clara Valley and can be clas- this history of deposition, erosion, and deposi- of these faults may be related to more local fault sifi ed in terms of extensional, compressional, tion of gravels in and around the Santa Clara interactions and strain partitioning. or strike-slip tectonics (Sedlock, 1995). Exten- Valley may have involved two periods of slip on sional tectonics predict thick, late Cenozoic the Monte Vista fault system or other unknown 1–0 Ma basin fi ll in the present-day San Francisco Bay basin-bounding faults. Alternatively, this history depression. The tectonic and stratigraphic his- could have resulted from changes in base level After widespread uplift and erosion across tory summarized here suggests that the exten- that were not structurally controlled. Santa Clara Valley, the area began to subside and sional model is applicable to the San Francisco On the east side of Santa Clara Valley, a reor- receive sediment to form the Santa Clara Basin Bay block locally during the Miocene, but ganization of the East Bay fault system led to ca. 1–1.5 Ma, while adjacent uplands continued certainly not during the late Pliocene. Over- abandonment of the right step between the to rise. Analysis of the sediment from the vari- printed on this Miocene history is basin forma- proto–Calaveras-Hayward faults, displacement ous deep drill holes in Santa Clara Valley indi- tion caused by stepovers within the East Bay on the Mount Misery (?) fault, and development cates a uniform subsidence rate of ~0.4 mm/yr strike-slip fault system. Compressional tectonic of a restraining bend between the active Cala- during the past 750 k.y. (Wentworth and Tinsley, mechanisms, expressed both in terms of fold- veras and Hayward faults. The Silver Creek fault 2005). Sedimentation rates, on average, appar- ing and thrust faults, clearly were predominant (or central proto–Calaveras fault of Graymer ently were equal to the subsidence rates, as sedi- during the Pliocene and Quaternary. However, et al., 2005) no longer transferred right-lateral mentary environments within Santa Clara Val- no one tectonic model completely explains the slip to the Calaveras-Hayward faults. Instead, ley have not migrated much in time and space history of Santa Clara Valley, which illustrates Wentworth et al. (2010) have argued that the during the past 1–1.5 m.y. Clast compositions the dynamic and complex nature of deforma- pull-apart basin between the Silver Creek and throughout the coreholes indicate that the major tion here. It is not clear which model applies to proto–Hayward faults was bisected by the com- present-day alluvial fans on the west side of the the formation of the Santa Clara Valley alluvial pletely concealed Mount Misery fault. Also valley were also prevalent in the past (Andersen basin, a very widespread depression with only during this time, the southern part of the Silver et al., 2005; Locke, 2011), and estuarine sedi- 150–500 m of sediment. Clearly, the forma- Creek fault was modifi ed by reverse and thrust ments are not found far from the current shore of tion of the depression within the San Francisco

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Bay block is not the result of a huge pull-apart and management of groundwater resources. The the 1906 San Francisco and 1989 Loma Prieta basin under San Francisco Bay, given that the old basin-bounding faults of the Cupertino and earthquakes also show elevated ground motions margins of the valley are being overriden by Evergreen Basins may not be slipping at rates as within Cupertino Basin (Aagaard et al., 2008; reverse faults. high, or in the same manner, as they did during Graves and Pitarka, 2010). The deep, V-shaped From the data presented here, it appears that basin formation, but there is evidence for reac- geometry of Evergreen Basin can also amplify Santa Clara Valley is relatively free of young tivation and Quaternary movement. The Monte ground motions by as much as a factor of 3 intravalley faults. Seismic profi ling (Williams Vista fault zone bounding the Cupertino Basin by trapping long-period waves from sources et al., 2004) does not image signifi cant structures and the Silver Creek fault bounding the Ever- along the long axis of the basin (Hartzell et al., projecting up into the uppermost 100 m other green Basin may be the sources of the M6.5 2010). Although most of the formation of deep than the Hayward fault, Hayward-Calaveras October 1865 earthquake and the two 1903 sedimentary basins occurred before 4 Ma, the connecting thrusts, and the Silver Creek fault. M6 earthquakes, respectively. The locations superposition of thrusting on the basin margins Williams et al. (2004) did not fi nd evidence of for these events based on analysis of historical and the geometry of these old basins empha- the Cascade and Shannon faults, faults that have accounts of damage (Bakun, 1999) are tantaliz- size the ongoing relevance of the tectonic and been previously postulated to lie basinward of ingly close to the Monte Vista fault zone and stratigraphic history beneath Santa Clara Val- the Monte Vista fault, along their Cupertino– the Silver Creek fault, respectively. Damage ley in terms of seismic hazard and groundwater Santa Clara seismic-reflection profile. The with evidence of contraction was concentrated resources to the inhabitants and infrastructure Cascade, Shannon (north of Los Gatos), and along the inferred extent of the Monte Vista fault that reside within the valley.

Santa Clara (also called the Stanford) faults zone (as well as along the Berrocal and Shannon ACKNOWLEDGMENTS were located on the basis of truncated stream faults) during the 1989 Loma Prieta earthquake channel sands inferred from interpretations of (Schmidt et al., 1995, 2014; Langenheim et al., This work would not have been possible without funding from the National Cooperative Geologic driller’s logs (California Department of Water 1997). Geodetic and surveying data suggest Mapping and Earthquake Hazards Programs of the Resources, 1975), a fi nding inconsistent with that northeast-directed contraction continued U.S. Geological Survey and from the Santa Clara Val- the analysis of driller’s logs and stratigraphy by after the earthquake along the southwest margin ley Water District. Reviews by Joseph Clark, Andrei Wentworth et al. (2010, their p. 8), although the of Santa Clara Valley (Bürgmann et al., 1997; Sarna-Wojcicki, and Richard Sedlock improved the spatial resolution of their data may not be suffi - Schmidt et al., 2014). The Silver Creek fault manuscript. We also appreciate the comments by guest editor Randy Hanson. cient to rule out faults with small displacement. (proto–central Calaveras fault of Graymer et al., The Cascade and Shannon faults, however, 2005) beneath Santa Clara Valley coincides with REFERENCES CITED coincide with changes in stream sinuosity and a sharp deformation gradient in interferometric Aagaard, B., Brocher, T.M., Dolenc, D., Dreger, D., Graves, stream-channel gradients interpreted to result synthetic aperture radar (InSAR) data (Gallo- R.W., Harmsen, S., Hartzell, S., Larsen, S., McCand- less, K., Nilsson, S., Petersson, N.A., Rodgers, A., from zones of localized uplift (Hitchcock and way et al., 2000; Schmidt and Bürgmann, 2003). Sjögreen, B., and Zoback, M.L., 2008, Ground motion Kelson, 1999). Furthermore, the Cascade fault, The deformation results from partitioning of the modeling of the 1906 San Francisco earthquake, part as defi ned, relocated, and renamed as New Cas- basin aquifer by the fault, rather than by tectonic II: Ground-motion estimates for the 1906 earthquake and scenario events: Bulletin of the Seismological cade fault by Hanson et al. (2004), appears to slip (Schmidt, 2002; Schmidt and Bürgmann, Society of America, v. 98, p. 1012–1046, doi:10 .1785 form a groundwater barrier, except where tra- 2003) and is accentuated by coarser-grained /0120060410 . Andersen, D.W., Metzger, E.P., Ramstetter, N.P., and versed by coarse-grained channels, within the facies present on the west side of the fault (Han- Shostak, N.C., 2005, Composition of sediment from basin. The location of this barrier is constrained son, 2015). Hydrologic modeling indicates that deep wells in Quaternary alluvium: Geological Society by a pair of wells and is drawn to merge into the the fault acts as a groundwater barrier, except of America Abstracts with Programs, v. 37, no. 4, p. 91. Atwater, T., 1970, Implications of plate tectonics for the range-front faults to the northwest and south- where traversed by coarse-grained, permeable Pacifi c and North American plates deduced from sea- east. Perhaps these faults, if they are faults, are stream channels (Hanson et al., 2004; Hanson, fl oor spreading in the Atlantic, Indian and South Pacifi c so young or have such low slip rates that they do 2015). The documented latest movement on the Oceans, in Kovach, R.L., and Nur, A., eds., Proceed- ings of the Conference on Tectonic Problems of the not have enough displacement (<20 m according Silver Creek fault is 140 ka, the maximum age San Andreas Fault System: Stanford University Publi- to William et al., 2004) to be imaged by seismic- of the sediments within the groundwater basin cations in Geological Sciences 13, p. 136–148. Atwater, T., and Stock, J.M., 1998, Pacific–North refl ection or stratigraphic data. In contrast, Catch- that show evidence of deformation on a seismic- America plate tectonics of the Neogene south- ings et al. (2006) interpreted extensive faulting refl ection profi le (Williams et al., 2004; Went- western United States: An update: International and folding along a profi le ~10 km to the south- worth et al., 2010). Holocene displacement is Geology Review, v. 40, p. 375–402, doi:10 .1080 /00206819809465216 . east of the Williams et al. 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