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Research Paper THEMED ISSUE: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara (“Silicon”) Valley

GEOSPHERE The Evergreen basin and the role of the Silver Creek fault in the San Andreas fault system, San Francisco Bay region, California GEOSPHERE; v. 13, no. 2 R.C. Jachens1, C.M. Wentworth1, R.W. Graymer1, R.A. Williams2, D.A. Ponce1, E.A. Mankinen1, W.J. Stephenson2, and V.E. Langenheim1 doi:10.1130/GES01385.1 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA 2U.S. Geological Survey, 1711 Illinois St., Golden, Colorado 80401, USA 9 figures

CORRESPONDENCE: zulanger@usgs.gov ABSTRACT Silver Creek fault has had minor ongoing slip over the past few hundred thou- sand years. Two earthquakes with ~M6 occurred in A.D. 1903 in the vicinity of CITATION: Jachens, R.C., Wentworth, C.M., Gray- The Evergreen basin is a 40-km-long, 8-km-wide Cenozoic sedimentary the Silver Creek fault, but the available information is not sufficient to reliably mer, R.W., Williams, R.A., Ponce, D.A., Mankinen, E.A., Stephenson, W.J., and Langenheim, V.E., 2017, basin that lies mostly concealed beneath the northeastern margin of the identify them as Silver Creek fault events. The Evergreen basin and the role of the Silver Creek Santa Clara Valley near the south end of San Francisco Bay (California, USA). fault in the San Andreas fault system, San Francisco The basin is bounded on the northeast by the strike-slip Hayward fault and Bay region, California: Geosphere, v. 13, no. 2, p. 269–286, doi:10.1130/GES01385.1. an approximately parallel subsurface fault that is structurally overlain by a INTRODUCTION set of west-verging reverse-oblique faults which form the present-day south- Received 22 June 2016 eastward extension of the Hayward fault. It is bounded on the southwest by The San Andreas fault zone in central California (USA) south of the San Revision received 9 November 2016 the Silver Creek fault, a largely dormant or abandoned fault that splays from Francisco Bay region (Fig. 1 inset) is a regionally simple structure (Jennings, Accepted 14 December 2016 the active southern Calaveras fault. We propose that the Evergreen basin 1994) composed of a single strand or, more likely, a set of subparallel but Published online 27 January 2017 formed as a strike-slip pull-apart basin in the right step from the Silver Creek closely spaced strands (e.g., Sims, 1993; Simpson et al., 2006) that took turns fault to the Hayward fault during a time when the Silver Creek fault served as accommodating slip following inception of movement on the system. For the a segment of the main route by which slip was transferred from the central past few million years, the San Andreas fault zone between the Garlock fault California San Andreas fault to the Hayward and other East Bay faults. The di- in the Transverse Ranges and roughly the latitude of Monterey, California, has mensions and shape of the Evergreen basin, together with palinspastic recon- been a zone at most a few kilometers wide (Jennings, 1994). structions of geologic and geophysical features surrounding it, suggest that The morphology of the San Andreas fault system is dramatically different during its lifetime, the Silver Creek fault transferred a significant portion of the in the greater San Francisco Bay region and to the north (Fig. 1) compared ~100 km of total offset accommodated by the Hayward fault, and of the 175 km to that in central California to the south. East of Monterey, the active Cala of total San Andreas system offset thought to have been accommodated by veras fault splays northward from the San Andreas fault, feeding slip onto the the entire East Bay fault system. As shown previously, at ca. 1.5–2.5 Ma the East Bay fault system, a system of 13 major faults forming six fault systems Hayward-Calaveras connection changed from a right-step, releasing regime to (three active and three abandoned) along with dozens of secondary faults that a left-step, restraining regime, with the consequent effective abandonment of are currently or recently active (Graymer et al., 2006). Inclusion of the East the Silver Creek fault. This reorganization was, perhaps, preceded by develop- Bay fault system widens the total San Andreas fault system to at least 80 km ment of the previously proposed basin-bisecting Mount Misery fault, a fault and adds such active members as the Hayward, Calaveras, Greenville, and that directly linked the southern end of the Hayward fault with the southern Rodgers Creek faults (Fig. 1), which together accommodate as much or more Calaveras fault during extinction of pull-apart activity. Historic seismicity indi- slip today than does the stretch of the San Andreas fault between San Fran cates that slip below a depth of 5 km is mostly transferred from the Calaveras cisco and Hollister (i.e., Evans et al., 2012). fault to the Hayward fault across the Mission seismic trend northeast of the The San Andreas fault, sensu strictu, between its intersections with the Evergreen basin, whereas slip above a depth of 5 km is transferred through San Gregorio fault to the north and the Calaveras fault to the south, is, like its a complex zone of oblique-reverse faults along and over the northeast basin counterpart in central California, relatively simple, with most of its 125 km of margin. However, a prominent groundwater flow barrier and related land-sub- total slip (Jachens et al., 1998) accommodated on a few closely spaced faults. sidence discontinuity coincident with the concealed Silver Creek fault, a dis- In contrast, the ~175 km of total slip accommodated by the East Bay fault sys continuity in the pattern of seismicity on the Calaveras fault at the Silver Creek tem (McLaughlin et al., 1996; Jachens et al., 1998) was partitioned onto many For permission to copy, contact Copyright fault intersection, and a structural sag indicative of a negative flower structure different strands and initiated ca. 12 Ma as required by the correlation of vol Permissions, GSA, or [email protected]. in Quaternary sediments along the southwest basin margin indicate that the canic rocks across the East Bay system at Burdell Mountain and Quien Sabe

GEOSPHERE | Volume 13 | Number 2 Jachens et al. | Evergreen basin and Silver Creek fault Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/269/1000918/269.pdf 269 by guest on 24 September 2021 Research Paper

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36° Ha Parkfield yw Mt. Diablo ar Gr Figure 1. (A) Map showing the San Andreas San # d F. GF fault system in the greater San Fran- een Francisco Cala cisco Bay region (California, USA) (after ville F. Jennings, 1994). Major Holocene active veras faults are labeled thick gray lines. Red Fremont line shows the area of Figures 2, 3, and 6. Yellow line is the Mission seismic trend. # 34° F. (B) Map showing the San Andreas fault system between the Transverse Ranges Si and Point Arena, where it passes out to lv sea, and area of A (green line). SAF—San er Cr Andreas fault; HF—Hayward fault; SGF— San Gr # San Gregorio fault; CF—Calaveras fault; San eek F. GF—Garlock fault. Jose

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(Jones and Curtis, 1991; Ford, 2007; Fig. 1). The details of some of this par paper, at least for the partition of central California San Andreas fault slip be titioning have been revealed (Graymer et al., 2002; McLaughlin et al., 1996), tween the San Andreas fault, sensu strictu, and the southernmost Calaveras but the analysis is far from complete. Other models have been proposed for fault in the San Francisco Bay region. The partitioning of slip from the southern the partitioning of slip on the San Andreas system faults in the San Francisco Calaveras fault onto all other faults of the East Bay fault system (e.g., Graymer Bay region (e.g., Powell, 1993; Wakabayashi, 1999). More recent work (Gray et al., 2002), the details of which do not affect the results presented here, is not mer et al., 2002; Wentworth et al., 2010) supports the model adopted in this discussed in this paper.

In this study we address one of the lesser-known members of the East Bay et al. (1964) crop out within the low along a reach ~10 km long and extend fault system, the Silver Creek fault1 (Figs. 1, 2)—one that we will assert has ing across its full width. These geologic observations led Robbins (1971) to played a major role in the development of the entire system, but that appears interpret the source of the low to be rocks in a graben that likely affected the to be largely dormant today. Our analysis provides a quasi–four-dimensional entire crust. In this inferred graben, Great Valley sequence rocks were dropped picture of how fault strands evolve in a large strike-slip system and documents down into denser Franciscan Complex basement, Franciscan Complex base the transition from an extensional stepover to a contractional stepover. But the ment rocks were down-dropped into lower crustal rocks, and perhaps lower nature and role of the Silver Creek fault comes later in our analysis. This story crustal rocks were dropped down into the upper mantle. The proposed graben- begins with the discovery of a large, deep sedimentary basin, the Evergreen bounding structures were the Silver Creek fault on the southwest and the Hay basin, concealed beneath the Santa Clara Valley at the south end of San Fran ward (and its structural continuation) and Calaveras faults on the northeast. cisco Bay. Seismic Velocity Model EVERGREEN BASIN SETTING For nearly two decades, the gravity observations were the only data that Normally, we would begin the description of the setting of the Evergreen suggested the presence of a basin or other major structural feature beneath basin with a discussion of the local geology. However, much of the area is the northeastern edge of the Santa Clara Valley. In 1988, a three-dimensional covered by Quaternary deposits that conceal the basin, or by Cenozoic and (3-D) seismic velocity model (Michael, 1988) of the volume of crust surround Mesozoic rocks that variously reveal or conceal the basin. Thus we find it more ing part of the Calaveras fault covered the southern half of the Evergreen grav useful to begin the discussion with a view of the geophysical data that suggest ity low, including the section with outcropping Mesozoic rocks. This analysis the presence of a concealed basin, and defer the discussion of the geology revealed materials with low seismic velocities (relative to surrounding crust until the geophysical setting has been established. at comparable depth) to depths of at least 6 km, and perhaps deeper (Fig. 3), in the area of the gravity low. The low velocities of these materials, although Gravity Surveys mostly confined between the traces of the Silver Creek fault and the Calaveras fault including the reach with outcropping Mesozoic rocks, were not suffi Gravity surveys of the Santa Clara Valley during the late 1950s and 1960s ciently diagnostic to permit differentiation between different types of Mesozoic revealed prominent gravity lows along the southwest and the northeast sides rocks, or even between Mesozoic rocks and younger rocks. The place where of the valley, separated by a northwest-trending gravity ridge (Taylor, 1956; the low velocity material penetrates the deepest is where late Cenozoic Silver Robbins, 1971) that extends from an outcrop of Franciscan Complex basement Creek gravels crop out within the gravity low, south of the outcropping Meso with interleaved serpentinite at Communication Hill (CH in Fig. 2; also known zoic Great Valley sequence (Fig. 3). as Oak Hill) to the southern tip of San Francisco Bay. Subsequent gravity ob servations (Roberts et al., 2004) have added detail to the image of the gravity Geology field but show the same basic gravity features as the early surveys (Fig. 2). The Evergreen gravity low, lying along the northeast side of the valley, is a regular Geologic mapping and compilations in the 1990s (Wagner et al., 1991; Gray elongate feature ~10 km wide at its widest, nearly 40 km long, and as much as mer and DeVito, 1993; Jones et al., 1994; Page et al., 1999; Wentworth et al., 40 mGal in anomaly amplitude. 1998) around the Evergreen gravity low clarified the nature and importance Gravity lows in the California Coast Ranges that are similar to the Ever of its surrounding faults and provided new information about the Mesozoic green low typically reflect thick sections of low-density Cenozoic deposits rocks that extend across the low. New geophysical data also contributed to a (Chapman and Griscom, 1980), but comparing the Evergreen low with the better understanding of the geology. The Silver Creek fault, the steep structure surface geology indicated that the cause of the low might be more compli bounding the southwest side of the gravity low beneath Yerba Buena Ridge, is cated (Robbins, 1971). Although the low coincides predominantly with Ceno largely concealed beneath thin sheets of Mesozoic rock thrust northeast across zoic deposits at the surface (Fig. 2), denser Mesozoic Coast Range ophiolitic the margin along the Silver Creek thrust. The Silver Creek thrust at the surface and forearc marine sedimentary rocks of the Great Valley sequence of Bailey is observed to be a zone of reverse faults that places Mesozoic rocks of Yerba Buena Ridge on top of the late Cenozoic gravels (Crittenden, 1951; Jones et al.,

1Two nearly collinear structures of very different age and origin both have been called the Silver 1994). A reverse-fault interpretation is supported by the gravity data (Fig. 2), Creek fault. The younger structure (Silver Creek fault of Crittenden [1951]) is a reverse fault that which show that the fault trace lies well down toward the bottom of the gravity places Mesozoic rocks on top of Cenozoic basin deposits, whereas the older structure is an low, a strong indicator that the Mesozoic rocks lie on top of less-dense late Ceno oblique normal fault that bounds the Evergreen basin on the southwest (Silver Creek fault of Wentworth et al. [2010]). Here we follow the usage of Wentworth et al. (2010) and refer to the zoic deposits. However, the gravity data imply a steeper southwest dip for the younger thrust fault as the Silver Creek thrust (Fig. 2). margin of the gravity low than for the reverse fault shown by Jones et al. (1994).

Principal Geologic Units

Tsl Quaternary sediment 122° QhbQQha pa late Ha 122° Holocene Holocene Pleistocene yw bay mud alluvium alluvium ar Quaternary and Tert iary rocks d F Tss QTc Tsl Tss Tv . Plio-Quater- siliceous sandstonevolcanics 37°30′ 37°30′N nary gravel rock A –10 Qhb –1 B Qpa Great Valley sequence 0 gvm gvs gvc mudstone sandstone sandstone & –20 121°45′ San Francisco 121°45′W & mudstone conglomerate Bay Coast Range ophiolite

–20 –30 sp bg Ev serpentinite basalt to –4 Cala –1 er gabbro 0 0 MOFT gr ver fm Franciscan Complex een Lo a’ as F Qha fm fmb fms fc Silv . melange meta- meta- chert Cuper er Creek F w GUAD basalt sandstone Tss tino Lo a fms Isostatic gravity, CCOC . –1 EVGR w 0 in mGal –1 Qha 0 Qpa 16 gvc 12 CH 8 –30 gvm 4 37°15′ 37 15¬ 37°15′ fc 0 Silv Clayto –20 121°30′ 121°30′ –4 er Creek n F. bg –8 –1 0 YBR QTc –12 Thrust –16 –20 sp –24 0 fms

–28 gvs –32 N fmb –36 –40 Tv

0 10 20 km

Figure 2. Gravity anomaly maps over the eastern Santa Clara Valley (California, USA). (A) Isostatic residual gravity anomaly shown as color bands with a contour interval of 2 mGal. Thick black lines are major faults from geologic maps and geophysical inference (see text). Thin black lines are other faults from Graymer et al. (2006). Yellow lines show the location of seismic reflection profiles (Williams et al., 2006; Wentworth et al., 2010); line labeled a-a′ is shown in Figure 5. White dashed line shows the minimum southwest extent of denser flaps of Mesozoic and/or Cenozoic rock. (B) Isostatic residual gravity anomaly contours (red lines) on geology after Graymer et al. (1996) and Wentworth et al. (1998). Contour interval is 2 mGal. Note the deep gravity low between the Silver Creek fault and the Hayward and Calaveras faults, variously over Quaternary alluvium, Tertiary rocks, and Mesozoic rocks at the surface. CCOC—Coyote Creek Outdoor Classroom drill hole; CH—Communication Hill; EVGR—Evergreen drill hole; GUAD—Guadalupe River drill hole; MOFT—Moffett Field drill hole (Beyer, 1980); YBR—Yerba Buena Ridge. Locations of drill holes are from Newhouse et al. (2004) unless otherwise noted.

Tsl Principal Geologic Units

122° Quaternary sediment QhbQQha pa late Holocene Holocene Pleistocene Tss bay mud alluvium alluvium Quaternary and Tert iary rocks 37°30′N QTc Tsl Tss Tv Qpa Plio-Quater- siliceous sandstone volcanics B nary gravel rock Qhb San Francisco 121°45′W Great Valley sequence Bay gvm gvs gvc mudstone sandstone sandstone & & mudstone conglomerate Coast Range ophiolite

sp bg fm serpentinite basalt to Qha gabbro Franciscan Complex Figure 3. Map showing depth to the 4.8 km/s isovelocity surface over the eastern fms fc fm fmb Santa Clara Valley (California, USA) (after Tss –1 melange meta- meta- chert Michael, 1988). (A) Depths shown as color A basalt sandstone bands with contour interval of 0.5 km. fms Thick black lines are major faults from Qha geologic maps and geophysical inference Qpa –4 (see text). (B) Depth contours on geology Ca CH (brown lines, contour interval 0.5 km). Sil la –3 gvc ver Cr veras –2 Note that velocities of 4.8 km/s extend to –2 >4 km depth in areas with both Cenozoic eek F. F. rocks and Mesozoic rocks exposed at the gvm 37°15′ ground surface. Velocities of 4.8 km/s at fc 4 km depth are typical of Cenozoic rocks Depth to 4.8 km/s 121°30′ bg in the San Francisco Bay region (Brocher, isovelcity surface, –1 2008). CH—Communication Hill; YBR— –1 in km YBR QTc Yerba Buena Ridge.

–0.5 sp –1.0 –2 –1 –3 fms –1.5 –2 Si lv gvs –2.0 er Cr –4 –2 –2.5 fmb e –1 –3.0 ek Th –3.5 –1 ru Tv st –4.0 –4.5 N

0 10 20 km

Likely here the gravity data reflect the superposition of the Silver Creek thrust tion encountered in the Evergreen drill hole (EVGR in Fig. 2; Wentworth et al., onto the Silver Creek fault. The northern half of the southwest basin margin is 2015). The minimum southwest extent of these inferred flaps is shown by the concealed by alluvium, but we have extended the Silver Creek fault into this white dotted line in Figure 2A. This interpretation has a Cenozoic basin as the area on the basis of InSAR satellite imaging data (Fig. 4), which delineate a primary source of the huge Evergreen gravity low rather than a graben that discontinuity in land subsidence attributed to a sharp groundwater flow barrier dropped Mesozoic sedimentary rocks down into denser crust. Furthermore, as interpreted to be caused by the concealed fault zone (Galloway et al., 2000; discussed later, this basin formed in response to a releasing stepover within Schmidt and Bürgmann, 2003; Hanson et al., 2004; Choussard et al., 2014), and the East Bay fault system. on the basis of sharp gravity and magnetic anomaly gradients (Fig. 2; Went worth et al., 2010). Seismic Reflection Profiles Multiple faults roughly coincide with the northeast margin of the Ever green low (Fig. 2). The active creeping strand of the Hayward fault (Lien Two seismic reflection profiles (Williams et al., 2005b, 2006; Wentworth kaemper, 1992; thick black line in Fig. 2) ends within the northern end of et al., 2010, 2015) and a well drilled along one of them (Guadalupe hole; the gravity low, but a system of northeast-dipping reverse-oblique faults Newhouse et al., 2004) provide strong support for the Cenozoic basin inter continues toward the southeast along the northeast half of the low (Jones pretation (Williams et al., 2005b). One 10-km-long profile lies west of and et al., 1994; Wentworth et al., 1998). The active Calaveras fault at the surface subparallel to the concealed Silver Creek fault, along the axis of the mid- does not mark the northeast boundary of the low, but instead cuts across valley gravity ridge that separates the Cupertino and Evergreen gravity lows the southeast end of the low. At the southeast end of the low, the Calaveras (Fig. 2). This profile imaged 300–500 m of relatively flat-lying, prominent re fault likely dips east as it does both to the northwest and southeast based on flectors above a strong reflector interpreted to be the top of basement (Went historic seismicity (Jachens et al., 2002; Williams et al., 2005a; Simpson et al., worth et al., 2015). The Guadalupe drill hole, located adjacent to this profile 2004; Graymer et al., 2007). Thus the fault probably lies above the source (GUAD in Fig. 2), penetrated ~410 m of unconsolidated material before bot of the low. toming in sandstone and argillite of the Franciscan Complex (Wentworth Because much of the northeastern boundary of the Evergreen gravity low et al., 2015). Paleomagnetic measurements on cores from the upper 290 m is characterized by northeast-dipping reverse faults, we interpret the presence of this drill hole indicate that the unconsolidated sediments all are normally of the Great Valley sequence rocks and Coast Range ophiolite extending all the polarized, implying that they were deposited during the Brunhes normal po way across the low to the Silver Creek fault to reflect a thrust flap of Mesozoic larity chron and thus are <780,000 yr old (Mankinen and Wentworth, 2016). rocks, rooted to the northeast and lying on top of younger Cenozoic depos Similar ages based on paleomagnetic measurements have been found for its that produce the gravity low (Jones et al., 1994). Additional geologic and the upper 250–300 m in other drill holes in the valley (Mankinen and Went gravity data support this interpretation. Near the south end of the Mesozoic worth, 2003, 2016). flap, the fault that separates Jurassic Knoxville Formation (Great Valley se The second seismic reflection profile (Fig. 5; a-a′ in Fig. 2) starts on the quence) and underlying gabbro of the Coast Range ophiolite from adjacent axial gravity high, crosses the inferred Silver Creek fault, extends across late Cenozoic Silver Creek gravels (the Clayton fault, not to be confused with the Evergreen gravity low, and terminates near the eastern edge of the grav the right-lateral fault of the same name that lies north of Mount Diablo) is rec ity low in the vicinity of the west-verging thrust faults that place Great Valley ognized as a low-angle, north-dipping thrust fault that places the older rocks sequence rocks on top of late Cenozoic deposits. On its west end, this profile on top of the late Cenozoic deposits (Jones et al., 1994). Gravity profiles across images the same package of relatively flat-lying reflectors above the strong this fault also indicate that the fault has a very low dip (the fault lies at the very basement reflector that is seen in the axial profile. At the Silver Creek fault, bottom of the gravity gradient that reflects the transition from a gravity low the basement reflector terminates abruptly whereas the reflectors above it over the low-density Silver Creek gravels to a gravity high over the higher- continue eastward into the area of the Evergreen gravity low (Fig. 5). East density Mesozoic rocks; Fig. 2). of the Silver Creek fault, the package of overlying reflectors associated with Somewhat more speculatively, we interpret the gravity data in combina Cenozoic strata is imaged down to at least 1.5 km, the depth limit of reli tion with the structural style shown by the thrust faults northeast of the grav able data from this seismic reflection survey (Wentworth et al., 2010). The ity low to indicate that more than one flap of Mesozoic rock extends into the reflection data image the Silver Creek fault in the unconsolidated deposits gravity low, but that the deeper one (or more) is mostly concealed beneath the to within ~300 m of the surface. A structural sag lies above the concealed alluvium northwest of the exposed flap. We suggest that the Evergreen gravity tip of the Silver Creek fault, suggesting to Wentworth et al. (2010) that con low is mostly unperturbed at both its northwest and southeast ends and that tinuing minor strike-slip movement with some cross-fault extension affected the decreased amplitude of the low in the central 15–20 km reflects denser sediments as young as ca. 140 ka. No actual dip-slip displacement across flaps of Mesozoic and/or older Cenozoic rock rooted to the east. Indirect sup the sag is discernable in the section, so the sag is attributed to transtension port for this interpretation is thinning and uplift of part of the Quaternary sec (Wentworth et al., 2010).

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Figure 4. Map showing seasonal ground deformation detected by InSAR satellite imaging (Galloway et al., 2000). This seasonal ground deformation has been related to the seasonal pumping and recharge of groundwater (Schmidt and Bürgmann, 2003; Hanson et al., 2004). Warm colors (yellows and oranges) indicate high peak-to-peak amplitudes of seasonal ground deformation; cool colors (greens and blues) indicate low peak-to-peak amplitudes of seasonal ground deformation. Note the sharp lateral transition from high to low amplitudes of seasonal ground deformation across the northwest projection of the Silver Creek fault. Green lines show locations of seismic-reflection profiles (Williams et al., 2006; Wentworth et al., 2010). Heavy black lines are major strike-slip faults; thin black lines, other faults within the stepovers between the Silver Creek, Hayward, and Calaveras faults. Beige and yellow areas show mountainous and valley areas, respectively.

a a′ SW NE Highway Interstate Capitol Penitencia Noble 1st St InSAR 101 680 Avenue Creek Road Avenue CCOC 0.0 0.0 0.2 0.2 Quaternary 0.4 0.4

0.6 S 0.6 ) Franciscan C F km 0.8 0.8 h( Complex Pliocene and older? 1.0 1.0 Dept 1.2 1.2 Evergreen basin ? 1.4 1.4 Topofbasement 1.6 from gravity 1km 1.6

Figure 5. Interpreted seismic reflection profile across the inferred Evergreen basin (Santa Clara Valley, California, USA) (modified from Williams et al., 2006); see location in Figure 2. Vertical exag- geration 1.25. Note the absence of horizontal reflectors below 0.5 km at the west end of the profile (over the mid-valley basement ridge inferred from the gravity data) compared to the reflectors at >1.5 km farther east, over the center of the gravity low. Horizontal lines in the Quaternary part of the section highlight the generally undisrupted and flat-lying nature of the basin, except where truncated by the Silver Creek fault (SCF) and warped by inferred thrust faults (red arrow) at depth. Red lines are faults interpreted from the seismic-reflection profile. CCOC—Coyote Creek Outdoor Classroom drill hole (Newhouse et al., 2004); InSAR—location of sharp lateral transition from high to low amplitudes of seasonal ground deformation.

This seismic reflection profile also reveals features interpreted by Williams (<5 km/s) to depths of 5 km beneath the southern part of the Evergreen gravity et al. (2006) as two west-verging thrust faults within the Cenozoic section near low, although their model does not show low velocities at depth beneath the the east end of the reflection profile (thick red lines in Fig. 5). Their interpreta deepest part of the gravity low near its northwest end. Hartzell et al. (2006) tion combined with the recognition of thrust flaps of Mesozoic rock discussed presented results compatible with low seismic velocities extending to depth in the previous section and the identification of thrust faults in the mapped in the vicinity of the gravity low. Finally, Dolenc and Dreger (2005) and Dolenc geology indicate that thrust faults are broadly distributed along the northeast et al. (2005) detected seismic wave delays, amplifications, and other character margin of the Evergreen basin and extend concealed beneath the Quaternary istics at stations over the Evergreen gravity low that are consistent with a thick deposits of the Evergreen basin. Concealed thrust flaps can explain the up section of low-velocity material. Given that seismic velocity and density are lifted and thinned section encountered in the Evergreen drill hole. related properties, all of these studies provide added support for the presence Other recent seismic investigations detected materials with low seismic ve and distribution of low-density materials that are the source of the gravity low. locities beneath the Evergreen gravity low. A seismic refraction profile that in part followed the same path as the seismic reflection profile discussed above EVERGREEN BASIN MODEL (Catchings et al., 2003; Boatwright et al., 2004) detected materials with low seismic velocities characteristic of Cenozoic basin fill to depths of at least 3 km Qualitative Interpretation below the center of the gravity low. Materials with much higher velocities were detected at considerably shallower depths both beneath the mid-valley gravity The geologic and geophysical evidence discussed in the previous sections ridge southwest of the gravity low and also to the northeast of the gravity is all compatible with an interpretation of the Evergreen gravity low as being low. Thurber et al. (2007) constructed a 3-D model of seismic P-wave veloci caused by an 8-km-wide, 40-km-long basin filled mostly with late Cenozoic ties in the greater San Francisco Bay region that reveals low-velocity materials deposits that has had Mesozoic rocks derived from the Coast Range ophio

lite and its overlying Great Valley sequence and Franciscan Complex (in (Newhouse et al., 2004). We prefer the density based on the borehole gravity cluding interleaved serpentine mélange with enclosed blueschist tectonic measurements because that method continuously samples a greater volume blocks) thrust over its central reach. In addition to the gravity anomaly, the of sediment around the borehole than do measurements on core. Densities strongest evidence supporting this interpretation comes from the combina for deeper parts of the basin fill section were based on the compilations of tion of the seismic reflection profiling, the data from the Guadalupe drill hole, Brocher et al. (1997) and Smith (1992), and are as follows: between 0.3 and and the geologic relations revealed near the southeastern end of the gravity 1.3 km, 2220 kg/m3; 1.3–2.3 km, 2350 kg/m3; 2.3–3.3 km, 2470 kg/m3; >3 km, low. The seismic reflection and well data show Quaternary deposits above the 2570 kg/m3. We assumed a density of 2670 kg/m3 for the basement rocks (Irwin, mid-valley basement ridge continuing northeastward into the area of the low 1961), inferred to be predominantly greywacke of the Franciscan Complex. The est gravity (deepest part of the basin) whereas the basement reflector termi change in density of the basement rock with depth is considered small relative nates abruptly at the inferred location of the Silver Creek fault. Near the center to that of the overlying basin fill because basement rock in general is consid of the gravity low, horizontal reflections continue at least to a depth of ~1.5 km erably less porous than the basin fill and its density would be less affected by (the depth where the seismic data lose resolution) without any noticeable in compaction as a function of depth. terruption, but no strong reflections are seen that are similar to the basement reflection over the mid-valley ridge. The seismic reflection profile samples the Evergreen Basin Geometry basin at a place where the gravity data suggest that the thrust flaps of Meso zoic rock are present only along the northeast margin and so give a relatively The result of the constrained gravity inversion is shown in Figure 6. The clear view of the upper 1.5 km of the basin. Evergreen basin is seen to be a deep, highly elongate basin ~40 km long by 8 km wide. It is deepest near its northwest end (~5.5 km) and progressively Basin Shape Calculation shallows toward its southeast end. Its plan-view shape is relatively simple, although this simplicity is partly due to the simple way in which the gravity We calculated the 3-D shape of the Evergreen basin by inverting the gravity effects of the overthrust flaps were removed. As was apparent from the gravity data following the method of Jachens and Moring (1990) (see also Saltus and anomaly itself, the Evergreen basin lies mostly between the Silver Creek fault Jachens, 1995), constrained by outcrop geology, known depths to basement (the mapped traces and their extension to the northwest based on InSAR data from seismic and drill hole data, and an assumed distribution of density versus and gravity anomalies) and the southeastward projection of the active Hay depth for the basin-filling deposits. In this procedure, the gravity field is itera ward fault (in part coincident in map view with the oblique-reverse faults that tively partitioned into a component caused by the low-density deposits that fill today characterize the northeast edge of the basin). the basin and a regional component caused by lateral density variations within Both Taylor (1956) and Dr. Rodger H. Chapman (California Department of the surrounding bedrock. We then interpret the “basin” gravity component in Water Resources, 1967) carried the Silver Creek fault farther northwest than terms of the 3-D distribution of Cenozoic deposits. We force the basin thickness the northwest end of the Evergreen gravity low, and Catchings et al. (2006) to be zero in places where Mesozoic rocks (e.g., Franciscan Complex and Great suggested that it extends >35 km northwestward east of San Francisco Bay Valley ophiolite) crop out and are considered to be in place (e.g., not lying past the Coyote Hills (Fig. 4). Hanson et al. (2004) and Hanson (2015) also on top of younger, less-dense Cenozoic deposits). We compensated for the adopted the linear northwest extension of the Silver Creek fault by Catch distortion of the basin gravity anomaly caused by the overthrust flaps of older, ings et al. (2006) as an element in their groundwater model, but the model denser rock by smoothly joining the contours from the northwestern end of did not test the extension of the fault because they only used wells south the anomaly with those from the extreme southeastern end, those locations of San Francisco Bay. Wentworth et al. (2010) examined the possibility of where our interpretation predicts that the anomaly is least perturbed by the the northwest fault extension in detail and concluded that no large, well- overthrust flaps. integrated late Cenozoic strike-slip fault continues northwestward from the We have few direct measurements of density in the deeper parts of the northwest end of the Silver Creek fault bounding the Evergreen basin. They Evergreen basin and so have assumed a density-depth function based on mea found no evidence for either lateral or vertical offset in the basement surface surements in deep drill holes in other parts of the San Francisco Bay region as defined by gravity, well, and seismic refraction control across the pro (Beyer, 1980; Brocher et al., 1997; Tiballi and Brocher, 1998). The density of the jection of the fault (their figure 8). Furthermore, they found no evidence of upper 300 m, 2120 kg/m3, was based on the borehole gravity measurements of a hydrologic barrier along the projection of the fault, as saltwater intrusion Beyer (1980) taken in the Moffett Field drill hole near the southern end of San in the shallowest aquifer (Newark) extends across the northwest extension Francisco Bay (MOFT in Fig. 2). This value is virtually identical to the average of the fault north and south of the Coyote Hills (California Department of of density measurements (2124 ± 128 kg/m3) taken on core from the upper Water Resources, 1968) and pump-testing indicates hydraulic continuity in 300 m of the Guadalupe drill hole and somewhat less than the average of the next deeper aquifer south of the hills (California Department of Water density measurements (2235 ± 66 kg/m3) measured in the Evergreen drill hole Resources, 1967).

Principal Geologic Units Quaternary sediment QhbQQha pa

Tsl late Holocene Holocene Pleistocene bay mud alluvium alluvium 122° HA Quaternary and Tertiary rocks Ha 122° yw YW QTc Tsl Tss Tv ar ARD FA Plio-Quater- siliceous sandstonevolcanics d Fa nary gravel rock Tss ult Great Valley sequence UL 37°30′N gvm gvs gvc T 37°30′ mudstone sandstone sandstone & Qpa & mudstone conglomerate Qhb Coast Range ophiolite EVERGREEN BASIN San Francisco 121°45′W –2 121°45′ Bay sp bg serpentinite basalt to –4 gabbro Franciscan Complex

fm fmb fms fc fm melange meta- meta- chert Qha basalt sandstone C f Cala l ALA Silv a

er Cr e veVERras d g Tss eek F e Qpa ( FaAS FA c SI o ult LV ault n c fms ER CREEK e UL a l Qha ed T Basin depth, ) in km Mesozoic CH FA gvc 0 UL bedrock T –2 –1 gvm 37°15′ 37°15′ fc –2 121°30′ 121°30′ bg –3 YBR Mesozoic –4 bedrock QTc sp –5 fms

gvs N fmb

0 10 20 km

Figure 6. Subsurface configuration of the Evergreen basin (Santa Clara Valley, California, USA), inferred from constrained inversion of the gravity data. (A) Modeled depth to Mesozoic bedrock shown as color bands with contour interval of 1 km. Heavy black lines are major strike-slip faults; thin black lines, other faults. Major faults are from geologic maps and geophysical inference (see text). Green lines show location of seismic-reflection profiles (Williams et al., 2006; Wentworth et al., 2010). (B) Depth to Mesozoic bedrock shown as contours on geology (red lines, contour interval 1 km). CH—Communication Hill; YBR—Yerba Buena Ridge.

STRUCTURAL AND TECTONIC INTERPRETATION 122°

HA Evergreen Basin YW ARD FA We propose, on the basis of the inferred shape of the Evergreen basin and its position relative to the active Hayward fault (with as much as 100 km of total

UL strike-slip offset [Graymer et al., 2002]) and the subparallel, largely dormant T CALA Silver Creek fault, that the Evergreen basin formed as a pull-apart basin in the wake of a right step from the Silver Creek fault to the Hayward fault (Fig. 7). The VERA length of the basin and its apparent along-strike simplicity suggest that at least 40 km of strike-slip offset has been accommodated on the Silver Creek fault. It MT EVERGREEN S FA -2 . MISERY FA would be possible to generate a pull-apart basin of 40 km length with <40 km BASIN of slip by merging nearby basins resulting from multiple right steps. However, UL -4 121°45′ we do not see the type of irregular plan-view geometry of the basin that we T SIL would expect from such a process. VER CREEK FA UL A minimum of 40 km of offset accommodated on the Silver Creek fault is a T significant portion of the total slip of ~100 km accommodated by the Hayward -1 fault and suggests that the Silver Creek fault played an important role in the 37°30′N transfer of slip from the central California San Andreas fault to the Hayward fault. In contrast, recent seismicity precisely relocated using the double-differ UL T ence method indicates that today almost all the slip is transferred to the Hay 121°30′W ward fault via the central Calaveras fault and across the Mission seismic trend (yellow line in Fig. 1; Waldhauser and Ellsworth, 2002; Ponce et al., 2004; Simp son et al., 2004), all northeast of the Evergreen basin. In the rest of this paper, we will examine whether our proposed interpretation of the origin of the Ever green basin is consistent with current knowledge of the East Bay fault sys tem, and if it is, what the implications of the Silver Creek fault as an important element in the San Andreas fault system in the San Francisco Bay region are. SILVER 0 N 20 km CREEK THRUST Constraints on Palinspastic Reconstruction: San Andreas Fault System 37°15′ In order to evaluate the possible importance of the Silver Creek fault in the San Andreas system, a review of the San Andreas system offset and the partitioning of that offset is warranted. The San Andreas fault in central Cali fornia (south of the latitude of Monterey) has accommodated ~300 km of total offset (Ross, 1970; Matthews, 1976, Jachens et al., 1998). In the San Francisco 1 km contours Bay region, this total offset is distributed onto a number of splays including from Figure 6 (1) the San Andreas fault proper lying west of the bay and (2) faults of the East Bay fault system (e.g., Hayward fault, Calaveras fault, Greenville fault, and others) lying east of the bay. McLaughlin et al. (1996) identified a critical cross-fault correlation between (1) rocks of the Franciscan Permanente terrane in the southern San Francisco Bay region west of the Calaveras fault but east of the San Andreas fault and (2) equivalent rocks east of the San Andreas fault in the Parkfield area of central California (Fig. 1). This correlation indicated that Figure 7. Schematic map view of the Evergreen basin (yellow) and vicinity (Santa Clara Valley, California, USA), showing important bounding faults and other thrust faults. Mount Misery between 160 km and 190 km of the central California San Andreas total off fault, shown in blue, is completely concealed. set was partitioned onto the East Bay fault system along the southernmost

Calaveras fault. Jachens et al. (1998) used prominent gravity highs produced The palinspastic restoration of 174 km of right-lateral offset on the central by the Permanente terrane rocks in both locations and strong magnetic anom California San Andreas fault and the southernmost Calaveras fault not only alies bracketing both Permanente terrane bodies to refine this offset estimate brings the Permanente terrane rocks (and their gravity anomalies) and the Palo to 174 ± 3 km (Fig. 8), which is consistent with the correlation of Miocene vol Prieto Pass (Hanna et al., 1972) and Hollister Valley (Robbins, 1982) magnetic canic rocks and related strata at Quien Sabe and Burdell Mountain (Jones and bodies into alignment, but to the north also places two large 50–60-km-long Curtis, 1991; Ford, 2007). Graymer et al. (2002) analyzed the distribution of this magnetic bodies (as reflected by the magnetic anomalies they produce) adja offset among the various East Bay faults and concluded that at least 100 km of cent to and facing each other across fault boundaries (Fig. 8). The western body offset was transferred from the southern Calaveras fault to the Hayward fault is the long serpentinite body exposed on Yerba Buena Ridge that lies south over the past 12 m.y. west of the Silver Creek fault, on the southwestern lip of the Evergreen basin.

123° 122° 121° 120°W Magnetic FIeld, in nT

200

175

150

Yerba 125 Buena Ridge 37°N 100 anomaly Figure 8. Shaded-relief aeromagnetic map of central California, USA (modified from 75 Roberts and Jachens, 1999) showing the Hollister magnetic anomalies associated with the Priest Valley and Palo Prieto Pass ophio 50 Valley lites and their cross-fault counterparts, anomaly the Yerba Buena Ridge and Hollister Valley 25 ophiolites. Warm colors indicate magnetic highs; cool colors indicate magnetic lows. Note that the Yerba Buena Ridge and Hol- 0 lister Valley anomalies are generally lower Priest Valley in amplitude than their cross-fault counter anomaly –25 parts, likely the result of the post-separation uplift and erosion of the Yerba Buena 36° Ridge and Hollister Valley ophiolites, as –50 Palo Prieto Pass discussed in text. anomaly –75

–100

–125

–150 N

35°

050100 150 km

As expressed by its associated magnetic anomaly, this serpentinite body has Creek fault). Second, the along-fault dimensions of both bodies are very similar a roughly linear northeast edge nearly 50 km long, the southern half of which (~40 km). Third, both bodies are likely subhorizontal and sheet-like in shape lies along a trace of the Silver Creek fault. Much of the northern half of this based on the mapping of Page et al. (1999) for the Yerba Buena Ridge body and edge is concealed beneath alluvium, but its linearity suggests fault control the modeling of Griscom and Jachens (1990) for the Priest Valley body. Fourth, even though we cannot establish this directly. The linearity of the southern both bodies extend significant distances from their respective bounding faults, most edge of this body is affected by movement on the Silver Creek thrust. as much as 10 km or more, making it improbable that conditions within the fault The eastern body, north of Parkfield and roughly centered over Priest Valley, is zone would affect the entire bodies. Finally, both bodies are likely Coast Range a mostly concealed, generally flat-lying tabular body cut by the San Andreas ophiolite: Page et al. (1999) described the Yerba Buena Ridge body as such, fault and inferred to be composed largely of serpentinite (Griscom and and, although the Priest Valley body is concealed, the deep drill hole at the Jachens, 1990; Eberhart-Phillips and Michael, 1993). Based on the width of its San Andreas Fault Observatory at Depth (SAFOD) penetrated rocks of the Great magnetic gradient, this body is estimated to lie at a depth of a few kilometers, Valley sequence northeast of the San Andreas fault (Zoback et al., 2011) above although precisely estimating the depth of flat-lying magnetic bodies based on the Priest Valley body. Thus, Coast Range ophiolite, the depositional basement their magnetic anomalies is difficult. The magnetic data indicate that this mag of the Great Valley sequence, would likely exist at depth below the drill hole. netic body has a linear southwest boundary ~50 km long, more than half (and The higher amplitude of the magnetic anomaly over the Priest Valley body perhaps most) of which coincides closely with the location of the San Andreas compared to that over Yerba Buena Ridge is likely the result of the Yerba Buena fault. At the south end of the Priest Valley magnetic anomaly, serpentinite is Ridge body being thinner than its cross-fault counterpart at Priest Valley. The being extruded or thrust to the surface at Table Mountain (Dickinson, 1966), Yerba Buena body is exposed and has been subject to erosion. Page et al. and smaller bodies of serpentinite crop out in many parts of the area enclosed (1999) concluded that on Yerba Buena Ridge, uplift and deep erosion removed by the magnetic anomaly. In this reconstruction, the Priest Valley anomaly Great Valley sequence rocks and the upper units of the Coast Range ophio lines up closely with the Yerba Buena Ridge magnetic anomaly (Figs. 8, 9). lite, thus exposing the basal serpentinite, which now probably is less than a Correlating magnetic bodies likely composed of serpentinite on oppo kilometer thick. The Priest Valley body, present beneath Great Valley sequence site sides of the San Andreas fault to estimate fault offset is not necessarily and thus never having been exposed, would likely be much thicker. Thus straightforward, because serpentinite can behave diapirically and be subject the Yerba Buena Ridge magnetic sheet should produce a lower-amplitude to further serpentinization while being transported within fault zones. Thus magnetic anomaly, just as we observe. Alternatively, the presence of upper counterpart bodies could change shape and degree of magnetization follow ophiolitic rocks on Yerba Buena Ridge suggests that the body may have been ing dismemberment. Also, the counterpart bodies may experience different originally thinner than its offset counterpart at Priest Valley. In any case, differ tectonic histories following separation. ential deformation, before and/or after separation, resulted in the difference in Nevertheless, we feel that the evidence supports the identification of the thickness and depth. Furthermore, the Priest Valley body has magnetic sand Priest Valley and Yerba Buena Ridge magnetic bodies as offset counterparts stone of the Etchegoin Formation above it (Dibblee, 1971) that would tend to of a once-continuous serpentinite body, even though the amplitudes of the augment the amplitude of that anomaly. magnetic anomalies produced by the two are quite different. First, both bodies On the basis of the geophysical observations and the palinspastic recon extend up to their adjacent fault faces (the San Andreas fault and the Silver struction, together with our interpretation of the Evergreen basin as a strike-

Yerba Buena Ridge Figure 9. Schematic representation of the opening of HF Evergreen SCF HF the Evergreen basin as a pull-apart basin in the wake HF HF basin ophiolite H F of the right step from the Silver Creek fault to the Hay- MMF CC ward fault (Santa Clara Valley, California, USA). The five al panels (A–E) show successive stages of basin formation SA SA SAF SA SAF F F (and overprinting by transpression) and the concurrent F

SC dismemberment and dispersion of the once-continu-

al ous ophiolite sheet that now exists as the Yerba Buena Ridge ophiolite in the Santa Clara Valley and its cross- fault counterpart at Priest Valley near Parkfield, Califor- Priest Valley nia. Yellow areas are basins; active faults are shown in black; abandoned or dormant faults are shown in gray. SA SAF ophiolite SAF SA SAF F Faults that are about to become active are shown as F dotted gray lines. CCal—central Calaveras fault; HF— Hayward fault; MMF—Mount Misery fault; SAF—San Andreas fault; SCal—southern Calaveras fault; SCF— A ca. 12 Ma B ca. 10 Ma C ca. 6 Ma D ca. 3 Ma E Present day Silver Creek fault.

slip pull-apart basin associated with right slip on the Silver Creek fault, we Silver Creek fault and the central Calaveras fault is small enough that move propose that the Yerba Buena Ridge and Priest Valley serpentinite sheets were ment on the southern Calaveras fault could probably feed onto either fault. An once a single sheet. Offset on the San Andreas system dismembered this sheet alternate scenario (Graymer et al., 2015; Langenheim et al., 2015, their figure 4) and dispersed the two pieces by movement on the Silver Creek, southern Cala posits that an active central (and northern) Calaveras fault zone was translated veras, and central California San Andreas faults. In the following section, we south by movement on the Hayward fault to its present location east of the expand on this proposal and examine some of its implications regarding offset Evergreen basin. Regardless, slip on some combination of the Silver Creek on faults of the East Bay fault system. fault and the southern Calaveras fault and other East Bay faults eventually re sulted in the total separation of 174 km (Jachens et al., 1998) between the Priest Scenario Valley and Yerba Buena Ridge ophiolite bodies seen today. A variant of the scenario presented above has been proposed by Went Our proposed model for the development of the Evergreen basin and the worth et al. (2010; also adopted in Langenheim et al., 2015) and is shown in southern East Bay fault system is shown schematically in Figure 9; for a recon Fig. 9D. They suggested that sometime after sufficient offset had occurred to struction that adopts much of what is presented here, but encompasses the completely separate the Yerba Buena Ridge and Priest Valley magnetic bod broader Santa Clara Valley, see Langenheim et al. (2015). Prior to the initiation ies, a new fault, the Mount Misery fault, broke across the basin, directly con of the East Bay fault system ca. 12 Ma, a large flat-lying sheet of serpentinite necting the southern Hayward fault with the southern Calaveras fault, thus and associated ophiolitic rocks (at least 25 km wide and >50 km long; Fig. 9A) bisecting the basin. Such faults are common in the extinction phase of some lay concealed beneath Mesozoic and Paleogene rocks. At that time, faults de pull-apart basins (Zhang et al., 1989; Anderson et al., 2004). Wentworth et al. veloped that were later to become the southern Calaveras, Silver Creek, and (2010) argued that this scenario accounts for the shape of the present basin Hayward faults, among others. The proto–Silver Creek and proto–southern (significantly narrower at its southeast end) and the fact that the present-day Calaveras faults were a single, continuous fault, whereas the proto–Hayward southern part of the central Calaveras fault truncates the Evergreen basin at its fault was subparallel to the proto–Silver Creek fault but displaced to the east southeast end rather than forming the northeast boundary of the basin. The (right) ~10 km. The proto–Silver Creek fault probably extended north of the proposed Mount Misery fault is everywhere concealed today so that obtaining south end of the proto–Hayward fault, and to the southeast, it cut across further information about its existence (or more accurately, whether the pres the concealed serpentinite sheet. The central and northern Calaveras faults ent-day northeastern boundary of the basin is a basin-bisecting fault) is diffi would develop from faults that were far to the north at this time. cult. We favor this proposed scenario, but whether it or the previous scenario As right-lateral offset began to accumulate on the southern Calaveras, Sil characterizes the later stages of development of the Evergreen basin does not ver Creek, and Hayward faults (Fig. 9B), the eastern part of the serpentinite change the basic arguments and conclusions presented here. sheet, which eventually would become the Priest Valley serpentinite sheet, Today, seismicity indicates that most of the slip on the central California moved southeastward with respect to the western part, later to become the San Andreas fault is transferred to the East Bay fault system along the south Yerba Buena Ridge body. Right slip on the Silver Creek fault–southern Cala ern and central Calaveras fault, through a left bend (Fig. 9E; Waldhauser and veras fault was transferred to the Hayward fault across a right separation be Ellsworth, 2002; Manaker et al., 2005; Ponce et al., 2004; Simpson et al., 2004) tween them, which may originally have taken the form of a diffuse extensional to the Hayward fault. This path bypasses the Silver Creek fault and largely zone. Eventually, as the slip transfer became tightly organized near the south passes northeast of the Evergreen basin (Graymer et al., 2015). The change end of the active Hayward fault, the Evergreen basin began to form as a pull- from a right step to a left step would have been accompanied by a change apart basin in the right step from the Silver Creek fault to the Hayward fault. from an extensional regime to one of compression. Cross-basin compression Some time after enough offset had been taken up by the Silver Creek fault provides an explanation for the reverse and thrust faults that today character to cause the complete separation of the two parts of the ophiolitic serpentinite ize both margins of the Evergreen basin. Graymer et al. (2003, 2015) proposed sheet (Fig. 9C), the central (and probably northern) Calaveras faults formed that the reorganization of the regional Calaveras fault–Hayward fault junction and connected with the southern part of the fault. Restoration of magnetic from a releasing to a restraining geometry began ca. 2.5 Ma and was largely bodies places no tight constraints on how much total offset took place on the completed by ca. 1.5 Ma and that since then, most of the slip on the southern Silver Creek fault prior to the initiation of the central Calaveras fault zone, but it Calaveras fault probably bypassed the Silver Creek fault. Total offset during must have been at least the 50 km necessary to effect the complete separation that period probably amounted to no more than ~20 km, and likely much less of the two parts of the serpentinite sheet. This minimum estimate is in accord on the present-day Calaveras fault north of its junction with the Silver Creek with the minimum estimate of 40 km of right offset across the Silver Creek fault fault (Jachens et al., 2002). implied by the length of the Evergreen basin. The initiation of movement on According to our proposed scenario, the geometric aspects of the Ever the central Calaveras fault zone would not necessarily have caused the Silver green basin and the dismembered serpentinite sheet predict that, at a min Creek fault to be abandoned because, at least today, the angle between the imum, half of the roughly 100 km of offset across the Hayward fault (Gray

mer et al., 2002) reached the Hayward fault via the Silver Creek fault. Our data eament that lies along the west margin of Yerba Buena Ridge, parallel to and moreover suggest that the bulk of the 174 km of East Bay fault system offset 3 km southwest of the Silver Creek fault; however, these events cannot at this was channeled along the Silver Creek fault and the Mount Misery fault. Uplift time be attributed uniquely to the Silver Creek fault, but could also be related and unroofing of the Yerba Buena Ridge serpentinite sheet must have taken to the Silver Creek thrust or faults mapped along the west margin of Yerba place after complete separation of the two parts of the original sheet, because Buena Ridge. the Priest Valley body remains buried. This uplift might be a manifestation of the same apparent cross-basin compression that resulted in the thrusting of the flaps of Mesozoic rock across the top of the central Evergreen basin, and CONCLUSIONS in the near-surface southwest dip of the southern Silver Creek thrust. Combined analysis of geologic and geophysical information from the Duration of Activity on the Silver Creek Fault Santa Clara Valley reveals a large Cenozoic basin, the Evergreen basin, along the northeastern edge of the valley. The basin is 40 km long, 8 km wide, and If our identification of the Yerba Buena Ridge serpentinite sheet and the ~5.5 km deep at its deepest. It is fault bounded, with the Silver Creek fault de inferred Priest Valley serpentinite sheet as cross-fault counterparts is correct, fining its southwest margin and the Hayward fault and its probable on-strike then movement on the Silver Creek fault must have begun very close to the continuations defining the northeast margin. Reverse faults exist along both time of inception of the East Bay fault system ca. 12 Ma. Such early initiation margins, and in the central section, Mesozoic rocks rooted to the northeast is required by the fact that the two serpentinite sheets now are separated by are thrust entirely across the Cenozoic deposits that fill the basin. We propose 174 km, the total offset believed to be accommodated on the East Bay fault that the Evergreen basin formed as a pull-apart basin in the wake of the right system. Furthermore, the Silver Creek fault had to have accommodated at step from the Silver Creek fault to the Hayward fault and that >40 km of San least 40 km of offset prior to initiation of movement on the Mount Misery fault Andreas fault system offset was transferred across this right step. Later in the and/or the central Calaveras fault, thus implying that the Silver Creek fault was evolution of the basin, the stress regime changed, resulting in cross-basin active during a significant part of the 12 m.y. lifetime of the Hayward fault. The compression manifest as reverse and thrust fault movement along the bound presence of basalts with K-Ar ages of 2.5 and 3.6 Ma (Nakata et al., 1993) and ing faults. The timing of this change is not obviously tied to changes in relative containing upper mantle and lower crustal xenoliths (Jové and Coleman, 1998) motion between the Pacific and North American plates, as discussed by Gray adjacent to the southern Evergreen basin near the southeastern end of the mer et al. (2015) and Langenheim et al. (2015), but may be more related to local Silver Creek fault suggests that the extensional regime associated with move fault reorganization. ment on the Silver Creek fault existed in Pliocene time. Jové and Coleman We elaborate on this proposal by searching for the offset counterpart of the (1998) pointed out that some of these basalts are located near local right steps 50-km-long, fault-bounded serpentinite sheet that crops out on Yerba Buena in the Calaveras fault, however, which could have produced local extension Ridge and occupies the southwest rim of the Evergreen basin. Within the con favorable for the movement of magmas from great depth. Thus, not all of the straints of known offset on the central California San Andreas fault and the basalts necessarily indicate active formation of the Evergreen basin. However, amount of offset believed to have been partitioned onto the East Bay fault Graymer et al. (2007) showed that the right steps in the Calaveras system prob system, the only viable candidate for an offset counterpart of the Yerba Buena ably do not extend >5 km deep. Ridge serpentinite is a comparable-length, mostly concealed serpentinite sheet As shown earlier, three lines of evidence suggest minor late Quaternary east of the San Andreas fault in central California roughly centered over Priest activity on the Silver Creek fault: a small structural sag and negative flower Valley. If this identification is correct, then the two parts of the serpentinite structure in the late Quaternary strata above the fault (Wentworth et al., 2010), sheet dismembered by the Silver Creek fault are separated by the full 174 km InSAR results that point to a groundwater barrier (Hanson et al., 2004) above of offset believed to be accommodated by the East Bay fault system. This, in the deeper fault (Schmidt and Bürgmann, 2003; Choussard et al., 2014), and turn, would suggest that the Silver Creek fault and its stepover to the Hayward relocated seismicity on the deep Calaveras fault (Simpson et al., 2004) that fault was active at the initiation of movement on the East Bay fault system, shows a discontinuity at depth at its intersection with the Silver Creek fault. and that the central Calaveras fault north of its junction with the Silver Creek In addition, two ~M6 earthquakes occurred in A.D. 1903 in the Santa Clara fault zone formed later. Additional information such as the presence around Valley. Estimates of the locations of the epicenters of these earthquakes based the Evergreen basin of Pliocene basalts interpreted to have been erupted in an on historical reports interpreted in terms of intensities (Bakun, 1999) place extensional setting, a groundwater barrier in Pleistocene deposits coincident the earthquakes near the Silver Creek fault. However, the data are not suffi with the Silver Creek fault zone, and a marked kink in the pattern of current ciently diagnostic to permit unequivocal identification of these earthquakes seismicity on the Calaveras fault at its junction with the Silver Creek fault all as Silver Creek fault events. Precisely relocated microseismicity (Waldhauser suggest that the Silver Creek fault has been active much of the 12 m.y. lifespan and Schaff, 2016) indicates a rather diffuse, steeply dipping, ~10-km-long lin of the East Bay fault system.

ACKNOWLEDGMENTS Dolenc, D., Dreger, D., and Larsen, S., 2005, Basin structure influence on the propagation of This work was supported by funding from the National Cooperative Geologic Mapping and Earth teleseismic waves in the Santa Clara Valley, California: Bulletin of the Seismological Society quake Hazards Programs of the U.S. Geological Survey and from the Santa Clara Valley Water of America, v. 95, p. 1120–1136, doi:10.1785/0120040059. District. Reviews by Robert Coleman, Richard Stanley, and John Wakabayashi improved the manu Eberhart-Phillips, D., and Michael, A., 1993, Three-dimensional velocity structure, seismicity, and script. We also appreciate the comments by guest editor Randy Hanson. fault structure in the Parkfield region, central California: Journal of Geophysical Research, v. 98, p. 15,737–15,758, doi:10.1029/93JB01029. 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