Santa Clara Valley is bounded on the southwest and northeast by active strike-slip and reverse-oblique faults of the San Andreas fault system. On both sides of the valley, these faults are superposed on older normal and/or right-lateral normal oblique faults. The older faults comprised early components of the San Andreas fault system as it formed in the wake of the northward passage of the Mendocino Triple Junction. On the east side of the valley, the great majority of fault displacement was accommodated by the older faults, which were almost entirely abandoned when the presently active faults became active after ca. 2.5 Ma. On the west side of the valley, the older faults were abandoned earlier, before ca. 8 Ma and probably accumulated only a small amount, if any, of the total right-lateral offset accommodated by the fault zone as a whole. Apparent contradictions in observations of fault offset and the relation of the gravity field to the distribution of dense rocks at the surface are explained by recognition of superposed structures in the Santa Clara Valley region.

Santa Clara Valley, which extends southeastward from the south end of San Francisco Bay, is bounded on the southwest and northeast by well-studied Quaternary-active structures of the San Andreas fault system (Fig. 1; e.g., U.S. Geological Survey and California Geological Survey, 2006; Field et al., 2013). As described below, on both sides, the active bounding faults are reverse or right-lateral reverse-oblique faults that dip toward and merge at depth with (or root in) major strike-slip faults, which themselves have a slight reverse obliquity. However, as further described below, both the strike-slip and reverse-oblique fault zones are geologically recent structures that are structurally superposed on preexisting fault zones that represent early phases of deformation on the San Andreas fault system in the region.

We use the term “structural superposition” to emphasize that younger structural features are on top of older structural features as a result of later tectonic deformation, such that they now conceal or obscure the older features. We use the term in contrast to structural reactivation, where preexisting structures accommodate additional deformation, commonly in a different sense from the original deformation (e.g., a normal fault reactivated as a reverse fault), and in contrast to structural overprinting, where preexisting structures are themselves deformed by younger structures (Fig. 2). Structural superposition has been observed elsewhere, though not named as such, for example, in the Montana fold and thrust belt (e.g., Reynolds and Brandt, 2005) and southern California (e.g., Davis et al., 1996).

Although the earlier faults were responsible for much of the total right-lateral offset, as well as deformation perpendicular to the faults, throughout the Neogene history of the San Andreas fault system in the region, they are largely concealed by the presently active structures. The superposition by later structures has led to apparent contradictions among various geologic and geophysical observations.

In this paper, we summarize the geometry and timing of initiation of the present valley-bounding faults and discuss the evidence for the location and nature of earlier structures and the timing of their initiation and abandonment. We also address apparent contradictions in geological observations that result from structural superposition and explain the observations in light of the detection of obscured earlier structures. This is important because seismotectonic models apparently excluded by geologic observations can be explained by structural superposition and deformation on older structures.

The Santa Clara Valley is bounded on the northeast by the dominantly right-lateral Calaveras and Hayward faults and a series of reverse and/or reverse-oblique faults, including the Silver Creek Thrust1 (Fig. 3). The reverse and/or reverse-oblique faults are generated by a combination of regional fault-normal compression (Page, 1982; Page and Engebretson, 1984) combined with the restraining left-step transfer of slip between the central Calaveras fault and the southern Hayward fault (Aydin and Page, 1984; Andrews et al., 1993; Kelson et al., 1993). Approximately two-thirds of present-day right-lateral slip on the southern part of the Calaveras fault, ∼15 mm/yr, is transferred onto the Hayward fault, ∼9 mm/yr at this left step (Dawson and Weldon, 2013). The Calaveras and Hayward faults also have small (∼10%) reverse components along the full length of the Santa Clara Valley margin, resulting in differential uplift on the east side of steeply east-dipping faults (Simpson et al., 2004; Williams et al., 2005; Bürgmann et al., 2006).

The present fault system is structurally superposed on a rhombochasm revealed by gravity data (the Evergreen Basin; Brocher et al., 1997; Jachens et al., 2002; Roberts et al., 2004) that indicates the presence of an earlier fault zone (Fig. 4). The earlier zone consisted of normal and normal oblique right-lateral faults forming a releasing right-step that transferred slip from the Silver Creek fault (a proto–central Calaveras fault) to a proto–southern Hayward fault (Jachens et al., 2002; Wentworth et al., 2010). The Silver Creek fault has been imaged in a seismic-reflection profile (Wentworth et al., 2010) that shows it to be a steeply east-northeast–dipping fault with large apparent down-to-the-east throw, juxtaposing Mesozoic rocks in the footwall with Pleistocene and Miocene rocks in the hanging wall. Footwall rocks have been observed in two wells (Wentworth and Tinsley, 2005); hanging-wall rocks are in part extrapolated from nearby outcrops of Pleistocene and Pliocene strata, while regional considerations suggest the basin also contains Miocene strata (Stanley et al., 2005). The strike-slip component ascribed to this fault is inferred from both the elongate rhomboid shape of the basin and the regional strike-slip deformation prevalent in the late Miocene and younger period (McLaughlin et al., 1996; Graymer et al., 2002). The proto–southern Hayward fault is entirely concealed by the structural 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 the Evergreen Basin, which suggests a transtensional 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 upon the observation that pull-apart basins elsewhere have been dissected by subsequent strike-slip faults. In that case, the Mount Misery fault would be the proto–southern Hayward fault forming the eastern boundary of the Evergreen Basin, and the eastern part of the original basin would be offset southward by slip on the Mount 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 subsidence continued through the Pliocene based on the age of basin fill as discussed below. Note that the proto–southern Hayward fault (Figs. 4 and 5) is drawn directly from the maximum horizontal gravity gradient, rather than the depth to basement interpretation, and so differs in shape and position somewhat from the Mount Misery fault of Wentworth et al. (2010). Because the Mount Misery fault is nowhere exposed, and because we interpret the shape of the eastern boundary of the present basin somewhat differently than Wentworth et al. (2010), we suggest that the idea that the eastern margin of the present basin is a strike-slip fault that dissected the original basin is probably true, but not proven.

The East Bay fault system, which includes the faults of this earlier system (Silver Creek fault, proto–Hayward right-lateral normal oblique fault, and probably Mount Misery fault), became active ca. 12 Ma, as shown by 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 later. Graymer et al. (2002) estimate ∼20 km of right-lateral offset transferred from the central Calaveras fault to the Hayward fault between 10 and 12 Ma, strongly suggesting that the normal oblique right-lateral fault zone and pull-apart basin was initiated along with the earliest faults of the East Bay fault system, although a more complex history including a now obscured additional connection is possible.

Because the pull-apart Evergreen Basin was formed by fault slip through the proto–Calaveras/Hayward releasing step over, the timing of fault reorganization to form the presently active compressional system must postdate the youngest basin-fill sediments. Compressional deformation in the active fault zone has exhumed basin-fill sediments in several places (Fig. 5), the youngest of which are the Pliocene gravels of Silver Creek. These gravels are interbedded with tuff that has yielded 40Ar/39Ar dates of 3–4 Ma (Wills, 1995; Wentworth et al., 1998). In addition, the uppermost gravels of Silver Creek adjacent to the Evergreen Basin interfinger with alkali basalt (basalt of Anderson Reservoir, Fig. 5) that has yielded conventional K/Ar whole-rock ages in two clusters around 3.6 Ma and 2.5 Ma (Nakata et al., 1993). The alkali basalt includes xenoliths derived from the lower crust and upper mantle (Nakata, 1980; Wilshire et al., 1988) and is distributed along the Silver Creek fault and the southern part of the central Calaveras fault (Fig. 5A), suggesting that the fault zone served as a conduit for lava rising from great depth prior to overthrusting by the younger faults. Development of the present transpressional Hayward-Calaveras fault zone therefore took place no earlier than ca. 2.5 Ma.

The timing of fault zone reorganization can also be constrained based on evidence of the oldest offsets on the active zone. In the area east of Silver Creek Valley, a thrust fault that places Mesozoic rocks over basin-fill sediments (gravels of Silver Creek, Tsg, Fig. 5B) is in turn overlain by the younger Packwood gravels (QTp), thought to be early Pleistocene (early Irvingtonian North American Land Mammal Age) based on similarity in lithification and deformation with the Irvington Gravels (Wentworth et al., 1998); the exposures of Irvington Gravels in Fremont (Fig. 3) are a reference locality for the early Irvingtonian (Bell et al., 2004). The early Irvingtonian is constrained at ca. 1.8 Ma to 0.85 Ma (Bell et al., 2004); therefore, the earliest parts of the presently active compressional system probably formed prior to 0.85–1.8 Ma. Altogether, this evidence shows that the reorganization of the fault zone from transtensional to transpressive occurred roughly between ca. 1.5 and 2.5 Ma.

Despite the reorganization in the larger fault zone, some displacement continued on the northern half of the Silver Creek fault into the Holocene. The seismic-reflection line across the Silver Creek fault shown in Wentworth et al. (2010) is interpreted to show ∼200 m of down-to-the-east offset on the base of the Quaternary section. A minor negative flower structure shown in the seismic-reflection line suggests late Quaternary offset (Wentworth et al., 2010), but, as they point out, that offset must be relatively minor because there is little or no offset of late Quaternary layers shown in the reflection line or geomorphic expression of a surface rupture. Minor late Quaternary fault offset was also suggested by the stream gradient and fluvial terrace analysis of Hitchcock and Brankman (2002), which indicated some broad Holocene deformation above the inferred buried fault.

Graymer et al. (2002) suggest 160 km of cumulative post–12 Ma offset along the southern part of the central Calaveras fault, including any earlier components such as the Silver Creek fault, more than half (100 km) of which has been transferred onto the Hayward fault. Jachens et al. (2002) point out that at least 40 km of the total central Calaveras offset must have taken place on the earlier releasing step over between the Silver Creek fault and the Hayward fault given the length of the “pull-apart” Evergreen Basin, but combining the timing of the reorganization of the faults described above with the control on timing and amount of offset described in Graymer et al. (2002) gives a more complete account. Graymer et al. (2002) show that prior to ca. 3.5 Ma, the earlier system accommodated ∼130 km of right-lateral offset, of which ∼75 km was transferred to the Hayward fault via the releasing step over and, probably, the Mount Misery fault, while ∼55 km was partitioned eastward, primarily onto the now largely inactive Palomares–Miller Creek–Moraga-Pinole faults. After ca. 3.5 Ma, a portion of the remaining ∼30 km of right-lateral offset (∼25 km transferred to the Hayward fault) was accommodated by the earlier system, but the bulk of that was probably taken up by the later fault system after fault reorganization ca. 2.5 Ma.

The Santa Clara Valley is bounded on the west side by a set of thrust, reverse, and reverse-oblique faults known as the Foothills Thrust Belt (Fig. 3; Graymer et al., 2006a), including the Shannon, Monte Vista, and Berrocal faults, that root in the largely right-lateral San Andreas fault and Sargent fault (Schwartz et al., 1990; McLaughlin et al., 1999). The reverse and/or reverse-oblique faults are generated by a combination of regional compression normal to the San Andreas fault (Page, 1982; Sébrier et al., 1992) and a left-restraining bend in the San Andreas fault (Fig. 1; Schwartz et al., 1990; Horsman et al., 2009). The San Andreas fault dips steeply southwest and probably has a minor west-up reverse component (Waldhauser and Schaff, 2008).

Like the active faults on the east side of Santa Clara Valley, the Foothills Thrust Belt faults are superposed on an older fault zone. Regional gravity (Roberts et al., 2004) shows a deep sedimentary basin (the Cupertino Basin) with a steep west side and a more gently sloped east side (Fig. 6A; Langenheim et al., 1997; Stanley et al., 2002, 2005). The steep west side is interpreted herein (see below) as a normal or right-lateral oblique normal fault concealed by superposition of the active fault zone (for other possible interpretations, see Stanley et al., 2002). Unlike the Evergreen Basin, the Cupertino Basin is not a pull-apart basin. The gently sloping northeast side suggests a half-graben structure (Fig. 7). Strata deposited during basin formation would have been progressively down tilted toward the bounding fault along the southwest basin margin. The interpreted progressive southwestward tilting of strata is supported by the seismic-reflection profile across the eastern Cupertino Basin margin (Fig. 7B), which shows basin-fill strata with an apparent southward dip (interpreted as southwestward) and truncated by an angular unconformity at the base of the overlying subhorizontal deposits. It is unknown if any cumulative San Andreas fault right-lateral offset was accommodated by the half-graben bounding fault.

The timing of development of the Cupertino Basin is not tightly constrained. Some workers (Stanley et al., 2002; Langenheim et al., 2015) suggest that deposition in the basin began as early as 15–18 Ma, implying basin formation in early to middle Miocene time associated with deposition of the adjacent Temblor Formation, the passing of the Mendocino Triple Junction, and the propagation of the San Andreas fault system. However, herein we propose a model of somewhat later faulting, suggesting basin formation in middle and possibly late Miocene time (<15 Ma). We infer this timing because of the following line of reasoning:

  1. The basin is at least in part filled with petroleum source rocks that have been correlated with the Miocene Monterey Formation (Stanley et al., 2002). The evidence from petroleum sourced in the basin suggests that the Monterey Formation is present down to depths of more than 2.1–2.5 km (Stanley et al., 2002).

  2. If the total depth of the basin is ∼3 km (Langenheim et al., 2015), the basin fill is dominantly Monterey Formation, suggesting the bulk of basin formation occurred during deposition of that unit. Monterey Formation deposition in the basin was probably ca. 15 Ma and younger because the Monterey Formation overlies Temblor Formation in the uplifted blocks along the western margin of the basin and the Temblor Formation there includes 15.6 Ma volcanic strata (McLaughlin et al., 1996).

  3. Deepening of depositional environment ca. 15 Ma indicated by the Temblor to Monterey transition in the basin margin strata may mark the advent of regional extension, including normal offset on the basin-bounding fault.

  4. When post–15 Ma strike-slip offset on the East Bay fault system is restored, the Cupertino Basin lies ∼175 km to the southeast relative to rocks across the faults, adjacent to rocks now in the region of Smith Mountain–Parkfield, west-southwest of Coalinga. The stratigraphy there includes a relatively thin layer of Temblor Formation underlying Monterey Formation (Dibblee, 1971; Richardson et al., 1972; Sims, 1990). The age of the 0- to 300-m-thick Temblor Formation in the Smith Mountain area is not well controlled, but in the Parkfield area, the 0- to 600-m-thick Temblor Formation contains middle Miocene mollusks (Dickinson, 1963) and lies below Monterey Formation that includes Relizian and Luisian foraminifers (Sims, 1988), quite similar to the stratigraphic sequence in the uplifted blocks along the western basin margin (McLaughlin et al., 2001). We interpret this to suggest regional deposition of a thin layer of shallow marine Temblor Formation prior to basin formation. Presumably the bottom of the basin would also include this thin layer of shallow marine strata beneath the Monterey Formation.

In this interpretation, the passage of the triple junction in this region was followed first by the eruption of volcanic rocks around 15.6 Ma, part of the suite of the northward-younging volcanic rocks related to a locus of melting that followed in the wake of the triple junction (Fox et al., 1985; McLaughlin et al., 1996), and then the initiation of faulting. Note that this sequence of events is the same as that shown by the timing of offset of some, but not all, of the northward- younging volcanic centers (e.g., ca. 12 Ma volcanics on the East Bay fault system; Graymer et al., 2002; Ford, 2007).

The upper part of the basin-fill sedimentary rocks may be, at least in part, somewhat coarser grained strata of late middle and early late Miocene age (Margaritan California Provincial Molluscan Stage or ca. 8.5–12 Ma; Powell, 2008) that were deposited during late stages of basin formation. These rocks overlie the Monterey Formation within the imbricate thrust belt (Sorg and McLaughlin, 1975) and so may be present in the basin as well. An apparent erosional unconformity in the uppermost part of the dipping basin-fill strata seen in the seismic-reflection profile (Fig. 7; Williams et al., 2004; R. Williams, 2014, written commun.) may represent the basal contact of the Margaritan sandstones.

Younger late Miocene (ca. 8 Ma, Lisa White, California State University, San Francisco, 2004, written commun.) diatoms have been identified from silty sandstone extracted from the bottom (∼250 m [830 ft] depth, within the subhorizontal layers shown in Fig. 7) of a research well drilled within the basin (McGlincy well, Figs. 6B and 7A; Stanley et al., 2005; Wentworth and Tinsley, 2005). Because the younger late Miocene deposits are horizontal, the interpreted southwestward tilting and associated early faulting must have ended prior to deposition of them, or prior to ca. 8 Ma.

In summary, the normal or normal right-lateral oblique fault bounding the Cupertino Basin was probably active between ca. 15 Ma and somewhat more than 8 Ma. Although we suggest this chronology best fits all the available evidence, there is uncertainty. Because the strata at the bottom of the Cupertino Basin (below the oil-producing level within the Monterey Formation below 2.1–2.5 km) are unobserved, it is possible that there is a slightly thickened section of Temblor Formation in the basin, which would suggest basin formation began somewhat earlier, during Temblor deposition. In addition, Monterey Formation farther south near Gilroy has fossils as old as early Miocene (McLaughlin et al., 1999); so it is possible to postulate early Monterey deposition within the basin simultaneous with Temblor deposition adjacent to the basin. Likewise, the sediments at the base of the subhorizontal late Miocene strata are not sampled; so the subhorizontal package could also contain Margaritan sandstone, which would suggest southwestward tilting related to normal or oblique right-lateral normal faulting could have ended somewhat earlier than ca. 8 Ma.

An alternative basin geometry, similar in some respects to that proposed by McLaughlin et al. (1999), is that the Cupertino Basin as expressed by the gravity low is just the northwestern part of an elongate Miocene basin bounded on the southwest by a normal or right-lateral normal oblique fault that extended as far southeast as Gilroy (Fig. 6). As they point out, the early Miocene (Saucesian or 17.5–23 Ma; McDougall, 2007) age of the lower part of the Monterey Formation in the Gilroy area suggests a northwestward progression of basin formation, so that the fault bounding the basin on the west would have become progressively active from south to north starting between 17.5 and 23 Ma and reaching the Cupertino area around 15 Ma. Some part of the extended basin could be preserved in the subsurface west of southern Santa Clara Valley, associated with a bench in the gravity gradient between Santa Teresa Hills and Morgan Hill (Fig. 6). If present, these basin-fill sediments are completely concealed by overthrust Mesozoic rocks.

Initiation of the presently active reverse and/or reverse-oblique faults adjacent to the Cupertino Basin (Foothills Thrust Belt) clearly postdates the cessation of normal faulting there in the late Miocene and is further constrained by the contrast in deformation between the late Miocene strata and the overlying Pliocene and Pleistocene Santa Clara Formation in the uplifted blocks along the western margin of the basin. The late Miocene and older strata have undergone significantly more compression than the Santa Clara Formation, as shown in geologic maps of the region (e.g., Sorg and McLaughlin, 1975; McLaughlin et al., 2001); so deformation along the western basin margin must have switched from extension to compression prior to deposition of the Santa Clara Formation. This differs from the interpretation of McLaughlin et al. (1999) that compression initiated during or after deposition of the Santa Clara Formation, but subsequent geologic mapping (McLaughlin et al., 2001) has clearly documented the ubiquity of the angular unconformity at the basal contact of Santa Clara Formation over more deformed Miocene strata. Fossils of Pliocene or early Pleistocene age (Blancan North American Land Mammal Age) have been found in the lower parts of the Santa Clara Formation (Sorg and McLaughlin, 1975; Adam et al., 1983). As presently understood, the Blancan extends from ca. 1.8 to 4.9 Ma (Alroy, 2000), so reverse and/or reverse-oblique right-lateral faulting began during or prior to that interval.

In summary, the initiation of compression took place between ca. 12 Ma (very early Margaritan sandstone in the subhorizontal strata and very rapid change from extension to compression) and ca. 2 Ma (assuming deposition of Santa Clara Formation only at the very end of the Blancan and rapid deformation of late Miocene strata prior to Santa Clara Formation deposition), but most likely took place between ca. 7.5 Ma (after deposition of ca. 8 Ma subhorizontal strata that marks the end of extension) and ca. 3.5 Ma (less constrained timing of Santa Clara Formation deposition and prior compression).

If Santa Clara Formation is limited to the very latest part of the Blancan, it is possible that initiation of reverse deformation on the west side of the Santa Clara Valley was coeval with that on the east side. That is unlikely, however, given the tightness of the timing and the amount of pre–Santa Clara compression shown in the late Miocene and older strata in the uplifted blocks (McLaughlin et al., 2001). It is more likely that reverse deformation on the west side predates that on the east.

Several apparent geologic contradictions associated with the faults bounding Santa Clara Valley can be explained by the structural superposition of two generations of fault zones as described above. Two examples of such contradictions are described here:

Miocene geologic units (Tbr/Tcc) east of Santa Clara Valley are offset ∼3 km by the Quaternary-active central Calaveras fault (Fig. 8). Although these units (Tbr—Briones Sandstone and Tcc—Claremont Chert) are widespread in the east San Francisco Bay region (e.g., Graymer et al., 1994, 1996; Wentworth et al., 1998), and so the apparent offset might be a coincidence, it is echoed by a similarly small offset on the western margin of the Mesozoic Franciscan Complex rocks (Fig. 8) and by the presence of a very small strike-slip basin in San Felipe Valley (Chuang et al., 2002; Fig. 8). These small offsets contradict the large (60 km) right-lateral offset required by the correlation of Eocene and Paleocene strata (unit Tpe) there with similar strata in the Oakland Hills described by Graymer et al. (2002). However, this contradiction can be explained by superposed structures with little right-lateral offset on the Quaternary-active fault and large offset on the fault zone as a whole that is largely attributed to offset accumulated on the earlier, now buried faults. In addition, 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 Tpe) are now on both sides of the upward projection of the buried fault.

Another apparent contradiction is the observation of widespread dense Mesozoic sedimentary, volcanic, and plutonic rock outcrops in the center of the gravity low associated with the Evergreen Basin (Fig. 4). Mesozoic rocks in this area are significantly denser than Cenozoic basin fill; so surface outcrops of Mesozoic rock are usually associated with gravity highs, such as the extensive high related to Franciscan Complex rocks east of the Calaveras fault (Fig. 4). In our present model, these exposures are interpreted to be part of a thin flap of Mesozoic rocks that has been thrust over the basin-fill sedimentary rocks that cause the gravity low. A similar, but less pronounced, gravity “contradiction” is observed on the west side of the southern Santa Clara Valley where a flat in the gravity gradient is not reflected by different units in the surface geology but may also result from basin-fill sedimentary rocks overthrust by the Mesozoic rocks seen at the surface.

Santa Clara Valley is bounded on either side by active strike-slip and reverse faults that are superposed on earlier normal and normal oblique strike-slip faults. Recognizing this structural superposition explains apparent contradictions in geologic and geophysical observations in the region.

Structural superposition occurs where a fault system changes to compression or transpression from a previous translational, extensional, or transtensional regime. Older structures are not reactivated, probably because their orientation is unfavorable to accommodate the new stress regime but instead are overridden and concealed by the formation of new structures and the emplacement of new structural blocks.

Recognition of superposed structures often requires the simultaneous application of varied geological disciplines such as geologic mapping, potential field geophysics, and reflection seismology, along with data from several other disciplines. We have found that collaboration of discipline experts is far more effective in this respect than attempts by single scientists or scientists of a single discipline to apply data from outside their area of expertise.

It is likely that structural superposition is common in tectonically active areas worldwide. Many apparent contradictions in geologic and geophysical observations might be explained by recognition, and unravelling the history, of structures hidden by superposition of later structures.

This work would not have been possible without funding from the National Cooperative Geologic Mapping and Earthquake Hazards Programs of the U.S. Geological Survey and from the Santa Clara Valley Water District. Reviews by Paul Stone, Darcy McPhee, Victoria Langenheim, Keith Kelson, and Scott Minor greatly improved the manuscript.

1A note on fault nomenclature: Because they are largely colinear in map view, two different structures have been called Silver Creek fault—the older normal oblique right-lateral fault that bounds the Evergreen Basin on the southwest and a younger thrust fault that emplaces Mesozoic rocks onto Evergreen Basin sedimentary fill. Because the name Silver Creek fault was originally designated for the younger thrust fault (Crittenden, 1951), Graymer previously followed that prior usage (Graymer, 1995; Graymer et al., 2005). However, the use of the name Silver Creek fault for the normal oblique right-lateral fault in the bulk of the previous work (e.g., U.S. Geological Survey and California Geological Survey, 2006) led us in Wentworth et al. (2010) to use the term Silver Creek Thrust for the younger thrust and Silver Creek fault for the older normal oblique right-lateral fault. We follow that convention herein.