A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara (“Silicon”) Valley themed issue Structural superposition bounding Santa Clara Valley Structural superposition in fault systems bounding Santa Clara Valley, California

R.W. Graymer, R.G. Stanley, D.A. Ponce, R.C. Jachens, R.W. Simpson, and C.M. Wentworth U.S. Geological Survey, 345 Middlefi eld Road, MS 973, Menlo Park, California 94025, USA

ABSTRACT We use the term “structural superposition” to and/or reverse-oblique faults, including the emphasize that younger structural features are Silver Creek Thrust1 (Fig. 3). The reverse and/or Santa Clara Valley is bounded on the on top of older structural features as a result of reverse-oblique faults are generated by a com- southwest and northeast by active strike-slip later tectonic deformation, such that they now bination of regional fault-normal compression and reverse-oblique faults of the San Andreas conceal or obscure the older features. We use the (Page, 1982; Page and Engebretson, 1984) fault system. On both sides of the valley, these term in contrast to structural reactivation, where combined with the restraining left-step transfer faults are superposed on older normal and/or pre existing structures accommodate additional of slip between the central Calaveras fault and right-lateral normal oblique faults. The older deformation, commonly in a different sense the southern Hayward fault (Aydin and Page, faults comprised early components of the San from the original deformation (e.g., a normal 1984; Andrews et al., 1993; Kelson et al., 1993). Andreas fault system as it formed in the wake fault reactivated as a reverse fault), and in con- Approximately two-thirds of present-day right- of the northward passage of the Mendocino trast to structural overprinting, where preexisting lateral slip on the southern part of the Calaveras Triple Junction. On the east side of the val- structures are themselves deformed by younger fault, ~15 mm/yr, is transferred onto the Hay- ley, the great majority of fault displacement structures (Fig. 2). Structural superposition has ward fault, ~9 mm/yr at this left step (Dawson was accommodated by the older faults, which been observed elsewhere, though not named and Weldon, 2013). The Calaveras and Hay- were almost entirely abandoned when the as such, for example, in the Montana fold and ward faults also have small (~10%) reverse presently active faults became active after thrust belt (e.g., Reynolds and Brandt, 2005) and components along the full length of the Santa ca. 2.5 Ma. On the west side of the valley, the southern California (e.g., Davis et al., 1996). Clara Valley margin, resulting in differential older faults were abandoned earlier, before Although the earlier faults were respon- uplift on the east side of steeply east-dipping ca. 8 Ma and probably accumulated only a sible for much of the total right-lateral offset, faults (Simpson et al., 2004; Williams et al., small amount, if any, of the total right-lateral as well as deformation perpendicular to the 2005; Bürgmann et al., 2006). offset accommodated by the fault zone as a faults, throughout the Neogene history of the The present fault system is structurally super- whole. Apparent contradictions in observa- San Andreas fault system in the region, they are posed on a rhombochasm revealed by gravity tions of fault offset and the relation of the largely concealed by the presently active struc- data (the Evergreen Basin; Brocher et al., 1997; gravity fi eld to the distribution of dense rocks tures. The superposition by later structures has Jachens et al., 2002; Roberts et al., 2004) that at the surface are explained by recognition led to apparent contradictions among various indicates the presence of an earlier fault zone of superposed structures in the Santa Clara geologic and geophysical observations. (Fig. 4). The earlier zone consisted of normal Valley region. In this paper, we summarize the geometry and normal oblique right-lateral faults forming and timing of initiation of the present valley- a releasing right-step that transferred slip from INTRODUCTION bounding faults and discuss the evidence for the Silver Creek fault (a proto–central Calaveras the location and nature of earlier structures and fault) to a proto–southern Hayward fault (Jachens Santa Clara Valley, which extends southeast- the timing of their initiation and abandonment. et al., 2002; Wentworth et al., 2010). The Silver ward from the south end of San Francisco Bay, is We also address apparent contradictions in geo- bounded on the southwest and northeast by well- logical observations that result from structural 1A note on fault nomenclature: Because they are studied Quaternary-active structures of the San superposition and explain the observations in largely colinear in map view, two different structures Andreas fault system (Fig. 1; e.g., U.S. Geologi- light of the detection of obscured earlier struc- have been called Silver Creek fault—the older nor- mal oblique right-lateral fault that bounds the Ever- cal Survey and California Geological Survey, tures. This is important because seismotectonic green Basin on the southwest and a younger thrust 2006; Field et al., 2013). As described below, on models apparently excluded by geologic obser- fault that emplaces Mesozoic rocks onto Evergreen both sides, the active bounding faults are reverse vations can be explained by structural super- Basin sedimentary fi ll. Because the name Silver or right-lateral reverse-oblique faults that dip position and deformation on older structures. Creek fault was originally designated for the younger toward and merge at depth with (or root in) major thrust fault (Crittenden, 1951), Graymer previously followed that prior usage (Graymer, 1995; Graymer strike-slip faults, which themselves have a slight STRUCTURAL SUPERPOSITION et al., 2005). However, the use of the name Silver reverse obliquity. However, as further described ON THE EASTERN MARGIN OF Creek fault for the normal oblique right-lateral fault below, both the strike-slip and reverse-oblique SANTA CLARA VALLEY in the bulk of the previous work (e.g., U.S. Geologi- fault zones are geologically recent structures that cal Survey and California Geological Survey, 2006) led us in Wentworth et al. (2010) to use the term Sil- are structurally superposed on preexisting fault The Santa Clara Valley is bounded on the ver Creek Thrust for the younger thrust and Silver zones that represent early phases of deformation northeast by the dominantly right-lateral Cala- Creek fault for the older normal oblique right-lateral on the San Andreas fault system in the region. veras and Hayward faults and a series of reverse fault. We follow that convention herein.

Geosphere; February 2015; v. 11; no. 1; p. 63–75; doi:10.1130/GES01100.1; 8 fi gures. Received 9 July 2014 ♦ Revision received 12 November 2014 ♦ Accepted 8 December 2014 ♦ Published online 14 January 2015

For permission Geosphere, to copy, contact February [email protected] 2015 63 © 2015 Geological Society of America

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Maacama Fault

San Andreas Fault Berryessa Healdsburg Fault Mt St Helena

Fault

Santa Rosa Rodgers Creek Fault West Napa Faul

Cor Green Valley Fault

Napa delia

t Fa

Sonoma ult

San Pablo Concord Fault Bay Pinole Fa 38°00′N Point ult Reyes Mt. Mt. Tamalpais Diablo Berkeley Green

San Gregorio Fault Calaveras OAK- ville Fault SAN PACIFIC LAND FRAN- San Francisco HaywardBay Fa OCEAN CISCO Fault

u lt San 123°00′ Mateo Fremont

Palo Alto Santa SAN Mt. JOSE Hamilton Butano Clara Fault

Valley C a la v e Morgan r Za a yante Hill s 5 0510 15 20 MILES Fault F a Area of u 550 15 25 KILOMETERS l t Figure 3 Gilroy

1 14 /2 ° 37°00′ Santa LRB Cruz Quien S

CALIF. abe Fault C NORTH Monterey Hollister

TRUE NORTH Bay

MAGNETI

APPROXIMATE MEAN MAP LOCATION DECLINATION, 2006 San Andreas Fault

Salinas Monterey

122°00′W 121°00′

Figure 1. Quaternary-active faults in the San Francisco Bay region, with main predominantly strike-slip faults emphasized in heavy black lines (modifi ed from Graymer et al., 2006a; Berryessa fault from Lienkaemper, 2012). Red polygon shows the approximate area of Figure 3. Orange bracket shows the extent of the left-restraining bend (LRB) in the San Andreas fault.

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Map View Cross-section View ward fault is entirely concealed by the structural A A′ superposition but is inferred to be a steep down- to-the-west fault based on the gravity gradient (Fig. 4). We further infer right-lateral oblique normal offset based on the rhomboid shape of A′ the Evergreen Basin, which suggests a trans- Structural Superposition A tensional origin. Alternatively, Wentworth et al. (2010) proposed that the original pull-apart basin has been dissected by a subsequent near-vertical, largely strike-slip fault (the Mount Misery fault) forming a more direct connection between the Calaveras and Hayward faults (Fig. 4). They noted that the eastern margin of the Evergreen Basin in the depth to basement interpretation of geological and gravity data was somewhat straighter than the Silver Creek fault and drew B B′ upon the observation that pull-apart basins else- Q Q where have been dissected by subsequent strike- slip faults. In that case, the Mount Misery fault T A T would be the proto–southern Hayward fault Structural T B forming the eastern boundary of the Evergreen Reactivation B′ Basin, and the eastern part of the original basin K would be offset southward by slip on the Mount K Misery fault. As they point out, although the Mount Misery fault is largely strike-slip, coeval normal or right-lateral normal oblique offset on the Silver Creek fault and related basin sub- sidence continued through the Pliocene based on the age of basin fi ll as discussed below. Note that C C′ the proto–southern Hayward fault (Figs. 4 and 5) is drawn directly from the maximum horizontal T C′ A gravity gradient, rather than the depth to base- ment interpretation, and so differs in shape and A T AT position somewhat from the Mount Misery fault Structural C T of Wentworth et al. (2010). Because the Mount Overprinting A Misery fault is nowhere exposed, and because we interpret the shape of the eastern boundary of the present basin somewhat differently than Figure 2. Schematic maps and cross sections demonstrating the differences between struc- Wentworth et al. (2010), we suggest that the idea tural superposition (thrust fault superposed on a normal fault), structural reactivation that the eastern margin of the present basin is a (normal fault reactivated as a right-lateral fault), and structural overprinting (roughly strike-slip fault that dissected the original basin east-west–trending reverse fault overprinted by a set of north-northwest–trending right- is probably true, but not proven. lateral faults). Thick lines are faults, dashed where concealed, teeth on the upthrown block The East Bay fault system, which includes of reverse faults, tics on the downthrown block of normal faults; arrows show direction of the faults of this earlier system (Silver Creek relative movement; A/T pairs show direction (Away/Toward) of out-of-plane motion in cross fault, proto–Hayward right-lateral normal section. Thin black lines in cross section represent ground surface and depositional contacts. oblique fault, and probably Mount Misery Thin black lines in map view show schematic lines of cross section. Thin blue dash-dot line fault), became active ca. 12 Ma, as shown by represents offset stream along the active right-lateral fault. the equivalent ~175 km offset of correlated ca. 12 Ma volcanics and Mesozoic Franciscan units (McLaughlin et al., 1996; Graymer et al., 2002); so the releasing step over initiated ca. 12 Ma or Creek fault has been imaged in a seismic-refl ec- crops of Pleistocene and Pliocene strata, while later. Graymer et al. (2002) estimate ~20 km of tion profi le (Wentworth et al., 2010) that shows it regional considerations suggest the basin also right-lateral offset transferred from the central to be a steeply east-northeast–dipping fault with contains Miocene strata (Stanley et al., 2005). Calaveras fault to the Hayward fault between 10 large apparent down-to-the-east throw, juxtapos- The strike-slip component ascribed to this fault and 12 Ma, strongly suggesting that the normal ing Mesozoic rocks in the footwall with Pleisto- is inferred from both the elongate rhomboid oblique right-lateral fault zone and pull-apart cene and Miocene rocks in the hanging wall. shape of the basin and the regional strike-slip basin was initiated along with the earliest faults Footwall rocks have been observed in two wells deformation prevalent in the late Miocene of the East Bay fault system, although a more (Wentworth and Tinsley, 2005); hanging-wall and younger period (McLaughlin et al., 1996; complex history including a now obscured addi- rocks are in part extrapolated from nearby out- Graymer et al., 2002). The proto–southern Hay- tional connection is possible.

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′ Fault

37°15 Fault

N Coyote Creek ? 6b). Faults modifi 012345 km 012345 er (1995). er

? emphasized (domi- lt zones are ll blocks, dotted where concealed ll blocks, dotted where Valley Calaveras Thrust ? k

Cree

Fault oblique right-lateral Steeply dipping reverse Andreas faults of the San Reverse and thrust faults, dotted where and East Bay Fault Systems concealed by Quaternary units, side teeth on upthrown Silver N ′ 37°30 W ′ Central

Hayward F. 121°45 Fault

? non Fault extension Clara

Shan Sargent Belt

Southeast intrusive contact Depositional or Fault Fault active in the Holocene years) (within the last 11,500

Fault ? I ′ Santa Figure 7B 122°00

Andreas Berrocal Thrust

Monte Vista Fault BASEMENT COMPLEX ROCKS Franciscan Complex mélange (Eocene, complex sedimentary rocks Valley Great Paleocene,and (or) Late Cretaceous) (Cretaceous) Franciscan Complex sedimentary rocks (Cretaceous) Franciscan Complex volcanic rocks (Cretaceous) Franciscan Complex metamorphic rocks (Cretaceous) (Early complex sedimentary rocks Valley Great (Early Franciscan Complex sedimentary rocks and (or) Late Jurassic) Cretaceous Franciscan Complex chert (Early Cretaceous and (or) Late Jurassic) Cretaceous (Early Franciscan Complex volcanic rocks and (or) Late Jurassic) (Jurassic) complex volcanic rocks Valley Great and (or) Late Jurassic) Cretaceous (Jurassic) complex plutonic rocks Valley Great complex serpentinite (Jurassic) Valley Great Ji Jv fsr Ks Kfs Kfv Jsp KJs Kfm Fault San KJfs KJfc KJfv

Fault Pulgas Foothills ′ Pilarcitos 122°15 SURFICIAL SEDIMENTS Artificial fill Mud deposits (late Holocene) Alluvium (Holocene) Hillslope deposits (Quaternary) Alluvium (Pleistocene) Alluvium (early Pleistocene) OVERLYING ROCKS OVERLYING Sediments (early Pleistocene (Pliocene) Sedimentary rocks and (or) Pliocene) (Pliocene) rocks Volcanic (Pliocene Sedimentary rocks (Miocene) Sedimentary rocks and early Miocene) (Miocene) rocks Volcanic (Miocene Sedimentary rocks (Miocene rocks Volcanic and (or) Oligocene) (Oligocene Sedimentary rocks and (or) Oligocene) (Eocene) Sedimentary rocks and (or) Eocene) (Eocene Sedimentary rocks and (or) Paleocene) af Tes Qsl Tps Tpv QTs Tms Tmv Qha Qpa Qoa Toes Tpms Tmos Tmov Tepas Qhym by Quaternary units). The Irvington Gravels in Fremont, part of unit QTs, are marked “I.” Geologic map from Graymer et al. (200 Graymer marked “I.” Geologic map from are part of unit QTs, The Irvington Gravels in Fremont, by Quaternary units). Figure 3. Geologic map of Santa Clara Valley and adjacent hills. The primary faults of the presently active valley bounding fau The primary faults of the presently and adjacent hills. Valley 3. Geologic map of Santa Clara Figure nantly strike-slip faults in heavy black lines, dominantly reverse or thrust faults in heavy red lines, teeth on the hanging-wa thrust faults in heavy red or nantly strike-slip faults in heavy black lines, dominantly reverse Hitchcock et al. (1994), McLaughlin (1999, 2001), Schmidt (1995), and Graym et al. (2006a) with data from Graymer

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Calaveras Thrust Valley

Creek

Central Silver re 3 for geologic map key. 3 for re m Roberts et al. (2004). (B) Geologic map of the asin–bounding fault, the Mount Misery fault of Gal) showing the elongate gravity low associated tive to the later fault system (black). Area of Fig- Area fault system (black). tive to the later 121°45´W

121°45´ Hayward F.

Clara extension Fig 5B

utheast So

Santa 37°00´ 37°15´ 37°30´N 0 5 10 15 20 25 km Valley N 121°45´W

Proto southern HaywardEvergreen Fault Basin

Clara

Silver Creek Fault Mt. Misery Fault of Wentworth et al., 2010

Santa AB ure 8 is shown by black outline; area of Figure 5B is shown by pink outline. Geologic map from Graymer et al. (2006b); see Figu Graymer 5B is shown by pink outline. Geologic map from of Figure 8 is shown by black outline; area ure northeast part of Santa Clara Valley showing the position of the earlier fault system from (A) (red and blue dotted lines) rela (A) (red fault system from showing the position of earlier Valley northeast part of Santa Clara Figure 4. (A) Gravity map of the northeast part of Santa Clara Valley (isostatic residual gravity contours in red, interval 2 m gravity contours in red, (isostatic residual Valley 4. (A) Gravity map of the northeast part Santa Clara Figure The alternate eastern b bounding faults (black dotted lines). position of the earlier Basin and the inferred with the Evergreen Gravity map fro locations of gravity measurement. et al. (2010), is also shown (blue dotted line). Red dots represent Wentworth

Geosphere, February 2015 67

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SE ext. QTp

Hayward Fault Mz Mz Q Thrust Tsg

Mz k QTp

Cree QTp rust fault in the later fault zone that rust fault in the later Creek (black) and the earlier ts of the later later zone in black, and the overlapping later

Silver Mz Mz ed from Wentworth et al. (1998). Wentworth ed from Evergreen Surficial deposits (Quaternary) Packwood gravels (Pleistocene and Pliocene?) Gravels of Silver Creek (Pliocene) Undifferentiated basement (Mesozoic) Dominantly right-lateral faults of the later fault zone Dominantly thrust/reverse faults of the later fault zone, teeth on upthrown block, dotted where concealed by Quaternary deposits Basal contact of Packwood gravels that overlaps an early thrust of the later fault zone ll sediments (Pliocene gravels of Silver Creek), correlative correlative Creek), ll sediments (Pliocene gravels of Silver

r Q Mz

e Tsg QTp Q 012345 km 012345 v

l

i S ed from Wentworth et al. (1998). (B) Geologic map of the Silver Creek Creek et al. (1998). (B) Geologic map of the Silver Wentworth ed from Mz B ′ ′ N ′ ′ 37°00 ′ 37°15 37°30 Fault 121°30 121°30 N

hrust Calaveras T Valley

Creek

Silver W ′ W ′ Central Hayward F. 121°45 121°45

Clara extension ed version of Figure 4B showing the distribution of exposed Evergreen basin-fi 4B showing the distribution of exposed Evergreen ed version of Figure

Area of Figure 5B Fault

Creek

Southeast 0 5 10 15 20 25 km

Silver Basalt of Anderson Reservoir Gravels of Silver Creek originally adjacent to Evergreen Basin Evergreen Basin fill, including gravels of Silver Creek Dominantly right-lateral faults of the later fault zone Dominantly thrust/reverse faults of the later fault zone, teeth on upthrown block, dotted where concealed by Quaternary deposits Normal oblique right-lateral faults of the structurally concealed earlier zone SAN JOSE SAN

A Santa Figure 5. (A) Simplifi Figure gravels of Silver Creek adjacent to the basin, and interlayered 2.5–3.5 Ma basalt of Anderson Reservoir in relation to the faul in relation Anderson Reservoir 2.5–3.5 Ma basalt of adjacent to the basin, and interlayered Creek gravels of Silver 5B. Geologic mapping modifi fault zones. Gray box shows the extent of map in Figure (red) the faults of in green, (Tsg) Creek location), showing the surface extent of gravels Silver (see Fig. 4B for area Valley depositional contact at the base of early Pleistocene and Pliocene(?) Packwood gravels (QTp) in dark blue. Note that th on the northeast is in turn overlain by unfaulted Packwood gravels. Modifi Creek bounds the gravels of Silver

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Because the pull-apart Evergreen Basin was minor because there is little or no offset of late Like the active faults on the east side of Santa formed by fault slip through the proto–Cala- Quaternary layers shown in the refl ection line Clara Valley, the Foothills Thrust Belt faults are veras/Hayward releasing step over, the timing or geomorphic expression of a surface rupture. superposed on an older fault zone. Regional of fault reorganization to form the presently Minor late Quaternary fault offset was also sug- gravity (Roberts et al., 2004) shows a deep sedi- active compressional system must postdate gested by the stream gradient and fl uvial terrace mentary basin (the Cupertino Basin) with a steep the youngest basin-fi ll sediments. Compres- analysis of Hitchcock and Brankman (2002), west side and a more gently sloped east side (Fig. sional deformation in the active fault zone has which indicated some broad Holocene deforma- 6A; Langenheim et al., 1997; Stanley et al., 2002, exhumed basin-fi ll sediments in several places tion above the inferred buried fault. 2005). The steep west side is interpreted herein (Fig. 5), the youngest of which are the Plio- Graymer et al. (2002) suggest 160 km of (see below) as a normal or right-lateral oblique cene gravels of Silver Creek. These gravels are cumulative post–12 Ma offset along the south- normal fault concealed by superposition of the interbedded with tuff that has yielded 40Ar/39Ar ern part of the central Calaveras fault, includ- active fault zone (for other possible interpreta- dates of 3–4 Ma (Wills, 1995; Wentworth et al., ing any earlier components such as the Silver tions, see Stanley et al., 2002). Unlike the Ever- 1998). In addition, the uppermost gravels of Creek fault, more than half (100 km) of which green Basin, the Cupertino Basin is not a pull- Silver Creek adjacent to the Evergreen Basin has been transferred onto the Hayward fault. apart basin. The gently sloping northeast side interfi nger with alkali basalt (basalt of Anderson Jachens et al. (2002) point out that at least 40 km suggests a half-graben structure (Fig. 7). Strata Reservoir, Fig. 5) that has yielded conventional of the total central Calaveras offset must have deposited during basin formation would have K/Ar whole-rock ages in two clusters around taken place on the earlier releasing step over been progressively down tilted toward the bound- 3.6 Ma and 2.5 Ma (Nakata et al., 1993). The between the Silver Creek fault and the Hay- ing fault along the southwest basin margin. The alkali basalt includes xenoliths derived from the ward fault given the length of the “pull-apart” interpreted progressive southwestward tilting of lower crust and upper mantle (Nakata, 1980; Evergreen Basin, but combining the timing of strata is supported by the seismic-refl ection pro- Wilshire et al., 1988) and is distributed along the reorganization of the faults described above fi le across the eastern Cupertino Basin margin the Silver Creek fault and the southern part of with the control on timing and amount of off- (Fig. 7B), which shows basin-fi ll strata with an the central Calaveras fault (Fig. 5A), suggesting set described in Graymer et al. (2002) gives a apparent southward dip (interpreted as south- that the fault zone served as a conduit for lava more complete account. Graymer et al. (2002) westward) and truncated by an angular uncon- rising from great depth prior to overthrusting by show that prior to ca. 3.5 Ma, the earlier sys- formity at the base of the overlying subhorizontal the younger faults. Development of the present tem accommodated ~130 km of right-lateral deposits. It is unknown if any cumulative San transpressional Hayward-Calaveras fault zone offset, of which ~75 km was transferred to the Andreas fault right-lateral offset was accommo- therefore took place no earlier than ca. 2.5 Ma. Hayward fault via the releasing step over and, dated by the half-graben bounding fault. The timing of fault zone reorganization can probably, the Mount Misery fault, while ~55 km The timing of development of the Cupertino also be constrained based on evidence of the was partitioned eastward, primarily onto the Basin is not tightly constrained. Some workers oldest offsets on the active zone. In the area east now largely inactive Palomares–Miller Creek– (Stanley et al., 2002; Langenheim et al., 2015) of Silver Creek Valley, a thrust fault that places Moraga-Pinole faults. After ca. 3.5 Ma, a por- suggest that deposition in the basin began as Mesozoic rocks over basin-fi ll sediments (gravels tion of the remaining ~30 km of right-lateral early as 15–18 Ma, implying basin formation of Silver Creek, Tsg, Fig. 5B) is in turn overlain offset (~25 km transferred to the Hayward in early to middle Miocene time associated with by the younger Packwood gravels (QTp), thought fault) was accommodated by the earlier system, deposition of the adjacent Temblor Formation, to be early Pleistocene (early Irvingtonian North but the bulk of that was probably taken up by the passing of the Mendocino Triple Junction, American Land Mammal Age) based on simi- the later fault system after fault reorganization and the propagation of the San Andreas fault larity in lithifi cation and deformation with the ca. 2.5 Ma. system. However, herein we propose a model Irvington Gravels (Wentworth et al., 1998); of somewhat later faulting, suggesting basin the exposures of Irvington Gravels in Fremont STRUCTURAL SUPERPOSITION formation in middle and possibly late Miocene (Fig. 3) are a reference locality for the early ON THE WESTERN MARGIN OF time (<15 Ma). We infer this timing because of Irvingtonian (Bell et al., 2004). The early Irving- SANTA CLARA VALLEY the following line of reasoning: tonian is constrained at ca. 1.8 Ma to 0.85 Ma 1. The basin is at least in part fi lled with (Bell et al., 2004); therefore, the earliest parts of The Santa Clara Valley is bounded on the petroleum source rocks that have been cor- the presently active compressional system prob- west side by a set of thrust, reverse, and reverse- related with the Miocene Monterey Forma- ably formed prior to 0.85–1.8 Ma. Altogether, oblique faults known as the Foothills Thrust tion (Stanley et al., 2002). The evidence from this evidence shows that the reorganization of the Belt (Fig. 3; Graymer et al., 2006a), including petroleum sourced in the basin suggests that the fault zone from transtensional to transpressive the Shannon, Monte Vista, and Berrocal faults, Monterey Formation is present down to depths occurred roughly between ca. 1.5 and 2.5 Ma. that root in the largely right-lateral San Andreas of more than 2.1–2.5 km (Stanley et al., 2002). Despite the reorganization in the larger fault fault and Sargent fault (Schwartz et al., 1990; 2. If the total depth of the basin is ~3 km zone, some displacement continued on the McLaughlin et al., 1999). The reverse and/or (Langenheim et al., 2015), the basin fi ll is domi- northern half of the Silver Creek fault into the reverse-oblique faults are generated by a com- nantly Monterey Formation, suggesting the bulk Holocene. The seismic-refl ection line across bination of regional compression normal to the of basin formation occurred during deposition the Silver Creek fault shown in Wentworth et al. San Andreas fault (Page, 1982; Sébrier et al., of that unit. Monterey Formation deposition in (2010) is interpreted to show ~200 m of down- 1992) and a left-restraining bend in the San the basin was probably ca. 15 Ma and younger to-the-east offset on the base of the Quaternary Andreas fault (Fig. 1; Schwartz et al., 1990; because the Monterey Formation overlies Tem- section. A minor negative fl ower structure shown Horsman et al., 2009). The San Andreas fault blor Formation in the uplifted blocks along the in the seismic-refl ection line suggests late Qua- dips steeply southwest and probably has a minor western margin of the basin and the Temblor ternary offset (Wentworth et al., 2010), but, as west-up reverse component (Waldhauser and Formation there includes 15.6 Ma volcanic they point out, that offset must be relatively Schaff, 2008). strata (McLaughlin et al., 1996).

Geosphere, February 2015 69

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?

? ′ 121°45 s. Red 6b); see of Santa t inferred to t inferred s the area of s the area ? Gal) showing ′ The general posi-

37°00 ? M Figure 7A ′ 7B is shown ection line in Figure 122°00 Figure 7B ′ 37°15 ′ W ′ 37°00 122°15 N ′ 37°30 ′ 37°15 0 5 10 15 20 25 km W ′ 121°45 N

ructurally covered extended basin

Possible st

N Southeast extension of basin-bounding fault ′ 37°30 ′

Fault 122°00

Cupertino Basin Oblique

lateral Figure 3 for geologic map key. 3 for Figure by the purple line; the approximate position of the McGlincy well is shown by the bold M. Geologic map from Graymer et al. (200 Graymer position of the McGlincy well is shown by bold M. Geologic map from by the purple line; approximate dots represent locations of gravity measurement. Gravity map from Roberts et al. (2004). (B) Geologic map of the southwest part Gravity map from locations of gravity measurement. dots represent position of the seismic-refl line; the approximate is shown by the green 7A section in Figure tion of the cross Right- interval 2 m gravity contours in red, (isostatic residual Valley 6. (A) Gravity map of the southwest part Santa Clara Figure right-lateral oblique normal faul normal or the gravity low associated with Cupertino Basin and position of earlier Also shown is the possible southeastward extension of fault as well a bound the basin on southwest (black dotted line). the stepped gravity gradient possibly denoting subsurface extension of Cupertino Basin beneath overthrust Mesozoic rock faults (black). to the later relative fault and possible extended basin (red) showing the position of earlier Valley Clara or

Normal ′ Earlier AB 122°15

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A

Relative location of SW Tmu NE Tsm QTsc Tms Tsm the McGlincy well QTsc Qa 0 Sea level Mz Tsm?

Tt 1 Mz 1000 Explanation meters Qa Alluvium (Quaternary) Tms QTsc Santa Clara Formation (early Pleistocene and Pliocene) 2 Tmu Post-basin-tilting strata (late Approximate Depth Approximate Mz Miocene) Tsm Sandstone with Margaritan CPMS Mz Tt? fossils (late and(or) middle Miocene) 3 km Tms Monterey Formation (middle and 0123 km early? Miocene) Tt Temblor Formation (early and Approximate horizontal scale middle Miocene) Vertical exaggeration ~1.75X Mz Basement rock (Mesozoic)

B S N

0 Qa

Tmu

0.5 Tsm?

Tms

1.0 Approximate Depth Approximate Two-way travel time, ms time, travel Two-way

1.5

~500 m

2.0 km Figure 7. (A) Schematic SW–NE cross section showing the west-tilted half graben of the Cupertino Basin, the superposed reverse faults of the active zone, the distribution of west-tilted strata including Temblor Formation (sandstone and volcanics) that predate normal faulting, fanning beds of Monterey Formation and possibly “Margaritan” sandstone that are coeval with normal faulting, and overlying subhori- zontal beds that postdate normal faulting. Green box shows the interpreted relative location of the seismic-refl ection line shown in B. The relative location of the McGlincy well, which bottoms in subhorizontal late Miocene (ca. 8 Ma) silty sandstone, is also shown (see Fig. 6B for actual location). (B) Interpreted seismic-refl ection profi le of part of Cupertino Basin (see Figs. 3 and 6B for location). Note the shal- low subhorizontal refl ectors interpreted as Quaternary alluvium and underlying late Miocene strata and fanning south-dipping refl ectors interpreted as layers of basin-fi ll rock. Also note the erosional unconformity (brown line) near the top of the dipping layers, which might represent the base of the “Margaritan” sandstone. Modifi ed from Williams et al. (2004); R. Williams, 2014, written commun.

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3. Deepening of depositional environment ca. from silty sandstone extracted from the bottom cessation of normal faulting there in the late Mio- 15 Ma indicated by the Temblor to Monterey (~250 m [830 ft] depth, within the subhorizon- cene and is further constrained by the contrast transition in the basin margin strata may mark tal layers shown in Fig. 7) of a research well in deformation between the late Miocene strata the advent of regional extension, including nor- drilled within the basin (McGlincy well, Figs. and the overlying Pliocene and Pleistocene Santa mal offset on the basin-bounding fault. 6B and 7A; Stanley et al., 2005; Wentworth Clara Formation in the uplifted blocks along the 4. When post–15 Ma strike-slip offset on the and Tinsley, 2005). Because the younger late western margin of the basin. The late Miocene East Bay fault system is restored, the Cuper- Miocene deposits are horizontal, the interpreted and older strata have undergone signifi cantly tino Basin lies ~175 km to the southeast rela- southwestward tilting and associated early fault- more compression than the Santa Clara Forma- tive to rocks across the faults, adjacent to rocks ing must have ended prior to deposition of them, tion, as shown in geologic maps of the region now in the region of Smith Mountain–Parkfi eld, or prior to ca. 8 Ma. (e.g., Sorg and McLaughlin, 1975; McLaughlin west-southwest of Coalinga. The stratigraphy In summary, the normal or normal right- et al., 2001); so deformation along the western there includes a relatively thin layer of Temblor lateral oblique fault bounding the Cupertino basin margin must have switched from extension Formation underlying Monterey Formation Basin was probably active between ca. 15 Ma to compression prior to deposition of the Santa (Dibblee, 1971; Richardson et al., 1972; Sims, and somewhat more than 8 Ma. Although we Clara Formation. This differs from the interpreta- 1990). The age of the 0- to 300-m-thick Tem- suggest this chronology best fi ts all the available tion of McLaughlin et al. (1999) that compres- blor Formation in the Smith Mountain area is evidence, there is uncertainty. Because the strata sion initiated during or after deposition of the not well controlled, but in the Parkfi eld area, the at the bottom of the Cupertino Basin (below the Santa Clara Formation, but subsequent geologic 0- to 600-m-thick Temblor Formation contains oil-producing level within the Monterey Forma- mapping (McLaughlin et al., 2001) has clearly middle Miocene mollusks (Dickinson, 1963) tion below 2.1–2.5 km) are unobserved, it is documented the ubiquity of the angular uncon- and lies below Monterey Formation that includes possible that there is a slightly thickened sec- formity at the basal contact of Santa Clara For- Relizian and Luisian foraminifers (Sims, 1988), tion of Temblor Formation in the basin, which mation over more deformed Miocene strata. Fos- quite similar to the stratigraphic sequence in the would suggest basin formation began somewhat sils of Pliocene or early Pleistocene age (Blancan uplifted blocks along the western basin mar- earlier, during Temblor deposition. In addition, North American Land Mammal Age) have been gin (McLaughlin et al., 2001). We interpret this Monterey Formation farther south near Gilroy found in the lower parts of the Santa Clara For- to suggest regional deposition of a thin layer of has fossils as old as early Miocene (McLaughlin mation (Sorg and McLaughlin, 1975; Adam shallow marine Temblor Formation prior to basin et al., 1999); so it is possible to postulate early et al., 1983). As presently understood, the Blan- formation. Presumably the bottom of the basin Monterey deposition within the basin simulta- can extends from ca. 1.8 to 4.9 Ma (Alroy, 2000), would also include this thin layer of shallow neous with Temblor deposition adjacent to the so reverse and/or reverse-oblique right-lateral marine strata beneath the Monterey Formation. basin. Likewise, the sediments at the base of faulting began during or prior to that interval. In this interpretation, the passage of the triple the subhorizontal late Miocene strata are not In summary, the initiation of compression junction in this region was followed fi rst by the sampled; so the subhorizontal package could took place between ca. 12 Ma (very early Mar- eruption of volcanic rocks around 15.6 Ma, part also contain Margaritan sandstone, which would garitan sandstone in the subhorizontal strata of the suite of the northward-younging volcanic suggest southwestward tilting related to normal and very rapid change from extension to com- rocks related to a locus of melting that followed or oblique right-lateral normal faulting could pression) and ca. 2 Ma (assuming deposition of in the wake of the triple junction (Fox et al., have ended somewhat earlier than ca. 8 Ma. Santa Clara Formation only at the very end of 1985; McLaughlin et al., 1996), and then the An alternative basin geometry, similar in some the Blancan and rapid deformation of late Mio- initiation of faulting. Note that this sequence of respects to that proposed by McLaughlin et al. cene strata prior to Santa Clara Formation depo- events is the same as that shown by the timing (1999), is that the Cupertino Basin as expressed sition), but most likely took place between ca. of offset of some, but not all, of the northward- by the gravity low is just the northwestern part 7.5 Ma (after deposition of ca. 8 Ma subhorizon- younging volcanic centers (e.g., ca. 12 Ma vol- of an elongate Miocene basin bounded on the tal strata that marks the end of extension) and ca. canics on the East Bay fault system; Graymer southwest by a normal or right-lateral normal 3.5 Ma (less constrained timing of Santa Clara et al., 2002; Ford, 2007). oblique fault that extended as far southeast as Formation deposition and prior compression). The upper part of the basin-fi ll sedimentary Gilroy (Fig. 6). As they point out, the early Mio- If Santa Clara Formation is limited to the very rocks may be, at least in part, somewhat coarser cene (Sauce sian or 17.5–23 Ma; McDougall, latest part of the Blancan, it is possible that initia- grained strata of late middle and early late Mio- 2007) age of the lower part of the Monterey For- tion of reverse deformation on the west side of the cene age (Margaritan California Provincial Mol- mation in the Gilroy area suggests a northwest- Santa Clara Valley was coeval with that on the east luscan Stage or ca. 8.5–12 Ma; Powell, 2008) ward progression of basin formation, so that the side. That is unlikely, however, given the tightness that were deposited during late stages of basin fault bounding the basin on the west would have of the timing and the amount of pre–Santa Clara formation. These rocks overlie the Monterey become progressively active from south to north compression shown in the late Miocene and older Formation within the imbricate thrust belt (Sorg starting between 17.5 and 23 Ma and reaching the strata in the uplifted blocks (McLaughlin et al., and McLaughlin, 1975) and so may be present in Cupertino area around 15 Ma. Some part of the 2001). It is more likely that reverse deformation the basin as well. An apparent erosional uncon- extended basin could be preserved in the subsur- on the west side predates that on the east. formity in the uppermost part of the dipping face west of southern Santa Clara Valley, associ- basin-fi ll strata seen in the seismic-refl ection pro- ated with a bench in the gravity gradient between APPARENT CONTRADICTIONS fi le (Fig. 7; Williams et al., 2004; R. Williams, Santa Teresa Hills and Morgan Hill (Fig. 6). If EXPLAINED BY STRUCTURAL 2014, written commun.) may represent the basal present, these basin-fi ll sediments are completely SUPERPOSITION contact of the Margaritan sandstones. concealed by overthrust Mesozoic rocks. Younger late Miocene (ca. 8 Ma, Lisa White, Initiation of the presently active reverse and/or Several apparent geologic contradictions California State University, San Francisco, 2004, reverse-oblique faults adjacent to the Cupertino associated with the faults bounding Santa Clara written commun.) diatoms have been identifi ed Basin (Foothills Thrust Belt) clearly postdates the Valley can be explained by the structural super-

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position of two generations of fault zones as Proto southern HaywardCentral Fault

described above. Two examples of such contra- SE extension Hayward Fault dictions are described here:

Miocene geologic units (Tbr/Tcc) east of Calaveras Santa Clara Valley are offset ~3 km by the Qua- ternary-active central Calaveras fault (Fig. 8). Although these units (Tbr—Briones Sandstone Franciscan Complex rocks and Tcc—Claremont Chert) are widespread in Tbr the east San Francisco Bay region (e.g., Graymer et al., 1994, 1996; Wentworth et al., 1998), and Offset so the apparent offset might be a coincidence, Franciscan margin it is echoed by a similarly small offset on the Tcc western margin of the Mesozoic Franciscan N Offset Tbr Offset Complex rocks (Fig. 8) and by the presence of Tbr/ Tbr/ a very small strike-slip basin in San Felipe Val- Tcc Tcc ley (Chuang et al., 2002; Fig. 8). These small contact contact offsets contradict the large (60 km) right-lateral San Felipe offset required by the correlation of Eocene Valley strike- and Paleocene strata (unit Tpe) there with Silver Creek Fault slip basin Tpe similar strata in the Oakland Hills described by Graymer et al. (2002). However, this contradic- tion can be explained by superposed structures Tpe with little right-lateral offset on the Quaternary- Tcc Fault active fault and large offset on the fault zone as a whole that is largely attributed to offset accumu- lated on the earlier, now buried faults. In addi- Silver Creek Thrust Tpe tion, the westward transport of the upper plate on the superposed reverse-oblique right-lateral faults explains why rocks interpreted to be part

of the eastern side of the older fault zone (unit Zone Tpe) are now on both sides of the upward pro- Tpe jection of the buried fault. Another apparent contradiction is the obser- vation of widespread dense Mesozoic sedimen- 012345 km tary, volcanic, and plutonic rock outcrops in the center of the gravity low associated with the Figure 8. Simplifi ed geologic map of the hills northeast of Santa Clara Valley (see Fig. 4B for Evergreen Basin (Fig. 4). Mesozoic rocks in location). Map unit Tpe, highlighted in orange, consists of Eocene and Paleocene strata cor- this area are signifi cantly denser than Cenozoic related with coeval strata in the Oakland Hills far to the northwest (Graymer, 2000; Graymer basin fi ll; so surface outcrops of Mesozoic rock et al., 2002), suggesting ~60 km offset on the central Calaveras fault (shown as a thick black are usually associated with gravity highs, such line). Tbr/Tcc marks the contact between Miocene fossiliferous sandstone (Briones Forma- as the extensive high related to Franciscan Com- tion) and underlying Miocene chert and siliceous shale (Claremont Shale), apparently offset plex rocks east of the Calaveras fault (Fig. 4). only a few kilometers by the Calaveras fault. The apparent contradiction is explained by In our present model, these exposures are inter- understanding that most offset of Tpe occurred on the earlier fault system (shown as dotted preted to be part of a thin fl ap of Mesozoic rocks red lines). Note that unit Tpe crosses a buried trace of the earlier fault zone because it has that has been thrust over the basin-fi ll sedimen- been transported westward by thrust/reverse offset on the later fault zone. Modifi ed from tary rocks that cause the gravity low. A similar, Wentworth et al. (1998). San Felipe Valley strike-slip basin from Chuang et al. (2002). but less pronounced, gravity “contradiction” is observed on the west side of the southern Santa Clara Valley where a fl at in the gravity gradient structural superposition explains apparent con- Recognition of superposed structures often is not refl ected by different units in the surface tradictions in geologic and geophysical observa- requires the simultaneous application of varied geology but may also result from basin-fi ll sedi- tions in the region. geological disciplines such as geologic map- mentary rocks overthrust by the Mesozoic rocks Structural superposition occurs where a fault ping, potential fi eld geophysics, and refl ection seen at the surface. system changes to compression or transpression seismology, along with data from several other from a previous translational, extensional, or disciplines. We have found that collaboration of CONCLUSIONS transtensional regime. Older structures are not discipline experts is far more effective in this reactivated, probably because their orientation respect than attempts by single scientists or sci- Santa Clara Valley is bounded on either is unfavorable to accommodate the new stress entists of a single discipline to apply data from side by active strike-slip and reverse faults regime but instead are overridden and concealed outside their area of expertise. that are superposed on earlier normal and nor- by the formation of new structures and the It is likely that structural superposition is mal oblique strike-slip faults. Recognizing this emplacement of new structural blocks. common in tectonically active areas worldwide.

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