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 permissionGeosphere, to copy, contact February [email protected] 2015 63 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/1/63/3334559/63.pdf by guest on 02 October 2021 Graymer et al. 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 BayHayward 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. 64 Geosphere, February 2015 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/1/63/3334559/63.pdf by guest on 02 October 2021 Structural superposition bounding Santa Clara Valley 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.