Geologic observations from the Resting Spring and Nopah Ranges (California, USA) together with a synthesis of regional data indicate that previous reconstructions of the Death Valley extensional terrane need to be revised because they do not account for three-dimensional, preextensional structures that complicate structural markers used in these reconstructions. This conclusion arises from detailed mapping that indicates structural overprinting of early northeast-trending fold-thrust systems of the Sevier orogenic belt by younger northwest-trending structures. The northwest-trending fold-thrust system includes a series of folds and a major thrust that includes the Nopah Peak and the Gerstley faults. This thrust fully involved crystalline basement, and there are extensive basement exposures in the hanging wall of the thrust in the southern Nopah Range. Basement involvement presumably produced a large ramp anticline to the southwest of the Nopah Range in what is now the Death Valley extensional terrane. The existence of this ramp anticline is supported by occurrences of successively older rocks to the southwest, beneath the Tertiary unconformity. The northwest-trending fold-thrust system is also recognizable to the north and east of the Gerstley–Nopah Peak fault as a series of west-northwest– to northwest-trending folds that have been known for more than 30 years, but were overlooked in earlier reconstructions.
We use our observations together with recently published data on the State Line fault and extensional structure in Pahrump Valley to consider two preliminary map-view restorations of the Resting Spring and Nopah Ranges relative to the Spring Mountains, a relatively unextended block to the east. These restorations place the northwest-trending thrust system along strike from the southern Spring Mountains, where a similar overprinting is observed, supporting the basic restoration. One restoration that strictly adheres to published estimates for motion across the Pahrump Valley suggests, however, that the widely cited correlation of the Wheeler Pass and Chicago Pass thrust system is highly suspect. Our preferred interpretation concludes that the Chicago Pass–Shaw thrust system of the Resting Spring–Nopah Ranges is an along-strike equivalent of the Lee Canyon thrust; the Wheeler Pass thrust correlates to the Montgomery Mountains thrust; and both thrust systems have an uncertain continuation into the Death Valley extensional terrane.
The Resting Spring–Nopah–Spring Mountains restoration provides a template for future restorations. We emphasize that the three-dimensional preextensional geometry, together with other markers like Mesozoic magmatic belts and older sedimentary facies trends, provides an opportunity for using modern visualization and database systems to develop high-precision reconstructions using the abundance of crosscutting markers. Thus, although this study, along with other recent studies, indicates that previous reconstructions are not workable, future studies that include the full three-dimensional data could lead to a nearly unique solution to the preextensional paleogeography.
The Death Valley region (California and Nevada, USA) has been the centerpiece of many studies of extensional tectonics because the hyperarid climate produces superb bedrock exposure, the area has a rich record of synextensional surficial deposits, and the system is sufficiently young that active features can be projected into the near geologic past to infer processes. These characteristics have led to a number of concepts that were developed from studies in this region: (1) the term “pull-apart basin” was first coined in the context of the active Death Valley system (Burchfiel and Stewart, 1966); (2) one of the original papers on low-angle normal fault systems was developed from Death Valley geology (Wright and Troxel, 1973); and (3) many of the modern concepts of detachment fault systems were developed in the Death Valley extensional terrane (e.g., Wernicke et al., 1988, 1989).
Although many studies have focused on extension in the Death Valley region, the system has long been recognized as an area of both extension and dextral strike slip where the active Walker Lane belt passes through the region (e.g., Hamilton and Myers, 1966) and connects to the San Andreas fault system through the eastern California shear zone (e.g., Dokka and Travis, 1990). Transtensional processes complicate application of purely extensional models and have led to controversy on the fundamental driver for the system over time. Although all agree that the modern system is transtensional and has been for the past 5–6 m.y. (e.g., Norton, 2011), the extension began in late Miocene time, between ca. 12 and 14 Ma (e.g., Snow and Wernicke, 2000; Fridrich and Thompson, 2011). North America–Pacific plate interactions in the late Miocene span the transition from a short transform boundary when Pacific plate first interacted with North America to the modern long transform boundary (e.g., Atwater and Stock, 1998). Late Miocene plate motions were, however, characterized by oblique transtension across the margin (e.g., Bohannon and Parsons, 1995) suggesting that plate-driven transtension could have driven the Miocene extension. This conclusion was recently supported by Busby (2013), who presented extensive evidence for onset of plate-driven transtension across this region beginning ca. 12 Ma. Nonetheless, because the plate boundary system was not yet fully formed in the late Miocene, gravity collapse has long been suggested for the initial phases of the extension (e.g., Sonder and Jones, 1999; Wernicke, 1992). Thus, although active strike-slip systems are known to have been active throughout the extensional history (e.g., Snow and Wernicke, 1989) the driving process for extension has included a gravity collapse hypothesis (e.g., Wernicke, 1992) and a plate-driven transtensional system hypothesis (e.g., Serpa and Pavlis, 1996; Wright et al., 1991).
Various attempts to restore the extensional system to its preextensional configuration have been fundamental to this analysis of processes. Wernicke et al. (1988) opened the debate on this problem with their initial work that used thrust faults as piercing lines, and a series of studies have tested this hypothesis with several alternative reconstructions using a plethora of markers (e.g., Fridrich and Thompson, 2011; McQuarrie and Wernicke, 2005; Norton, 2011; Renik and Christie-Blick, 2013; Serpa and Pavlis, 1996; Snow and Wernicke, 2000; Wright et al., 1991). Attempts at reconstructing the system generally have fallen into two schools of thought: (1) the large-magnitude extension models that generally adhere to the principal markers used in the original analysis of Wernicke et al. (1988) but include many other features; and (2) models that assume alternative correlations of preextensional structure, Cenozoic features, or both, and typically infer smaller amounts of extension within the Death Valley extended terrane. The large-magnitude extension models have been expanded to orogen scales (McQuarrie and Wernicke, 2005; Wernicke and Snow, 1998) in a regional, internally consistent synthesis, and this analysis has led to a widespread acceptance of this model (e.g., Norton, 2011). Nonetheless, it is important to recognize that map reconstructions are subject to the same issues as cross-section restorations. That is, in cross-section construction the ability to restore the section indicates it is allowable, yet because an infinite number of sections are typically allowable, the ability to restore it does not necessarily mean it is correct (e.g., Elliott, 1983).
In the past 10 years new data have accumulated on the Death Valley region that challenge all of the previous reconstructions for the region (e.g., Renik and Christie-Blick, 2013). We previously acknowledged that the reconstruction in Serpa and Pavlis (1996) was flawed by not properly restoring the eastern margins of the region, and considered alternatives for parts of the reconstruction (e.g., Guest et al., 2003; Luckow et al., 2005). Similarly, however, the large-magnitude extension models of Snow and Wernicke (2000) are also no longer workable for several reasons. (1) Evidence from detailed mapping in the Owlshead and southern Panamint Mountains (Guest et al., 2003; Luckow et al., 2005; Pavlis et al., 2012) shows that the current map area of this region must restore to approximately its present size, contradicting the restorations for this region that restore it to a thin sliver (e.g., Snow and Wernicke, 2000). (2) Studies of Tertiary deposits that seemingly tied the Panamint Mountains closely to the Resting Spring Range (Niemi et al., 2001) were negated by evidence of a fluvial rather than alluvial fan origin for the deposits (Renik et al., 2008), although the controversy has recently been reopened (Niemi, 2013). (3) A reanalysis of the Mesozoic piercing points of the Funeral and Grapevine Mountains relative to the Cottonwood and Panamint Mountains indicates that the correlations across this region (Snow and Wernicke, 1989) may not be unique (Renik and Christie-Blick, 2013).
In this paper we report on new discoveries on relative chronologies of preextensional structures in the Death Valley region that suggest an entirely new family of reconstructions needs to be considered. The key observations arise from crosscutting relationships in the Resting Spring and Nopah Ranges that indicate that two kinematically distinct generations of thrust systems are present in the Death Valley region, and many of the complexities of the Death Valley region are probably associated with the overprinting. In one sense, this paper is a rediscovery of an old concept, because many of the data were already in place (work by Burchfiel et al., 1983), but important details were overlooked in later syntheses. We use these data and other recent work to produce a restoration template from the Spring Mountains, Nopah Range, and Resting Spring Range and speculate on how this template forces a rethinking of the preextensional paleogeography.
The Death Valley region is famous for its complex Neogene extensional structures, but this extension was superimposed on a region that was already complexly deformed by Mesozoic contraction. Mesozoic contractional structures compose a classic thin-skinned fold-thrust belt developed in miogeoclinal strata deposited on the Neoproterozoic to Paleozoic passive margin of western North America. The regional details of this fold-thrust belt in the Death Valley region, however, are commonly overlooked and are critical to the thesis of this paper.
From northern Canada to eastern California (Fig. 1), the Cordilleran fold-thrust belt is developed in miogeoclinal strata, but in eastern California the miogeocline was truncated through late Paleozoic and early Mesozoic events that probably record a strike-slip event (e.g., Dickinson, 2004; Walker, 1988). The thrust belt continues eastward beyond eastern California, but changes radically in structural style, age, and trend. From northern Utah to eastern California, the fold-thrust belt has long been referred to as the Sevier orogenic belt (Fig. 1) and is characterized by a northeast-trending thrust system developed between Paleozoic and middle Cretaceous time (DeCelles, 2004). From Arizona to northern Mexico, however, Cordilleran fold-thrust systems are developed in entirely different rocks and deformation is Late Cretaceous to Paleogene age (Fig. 1). These structures are coeval with the Laramide foreland deformation of the Rocky Mountains, but are of a different style (e.g., Burchfiel et al., 1992; Dickinson, 2004). From northeastern Mexico to southern Arizona fold-thrust structures typically follow, and invert, a Jurassic–Cretaceous rift system that Dickinson and Lawton (2001) referred to as the Border Rift. These structures trend northwest from northern Mexico through Arizona, and continue into eastern California (Border fold-thrust belt in Fig. 1). From northern Mexico to New Mexico, the thrusts are predominantly thin-skinned thrusts that inverted the Border rift basin (e.g., Dickinson and Lawton, 2001; Haenggi, 2002), but from eastern Arizona to California these thrust systems fully involve Proterozoic crystalline basement and Mesozoic plutonic rocks (Burchfiel et al., 1992; DeCelles, 2004). The involvement of Mesozoic plutonic rocks indicates that the contractional belt merges with the late Mesozoic magmatic arc in this region (Fig. 1) and presumably records significantly greater contraction given widespread metamorphism in what are now the eastern California and Arizona metamorphic core complexes (Burchfiel et al., 1992; DeCelles, 2004). This Arizona contractional belt somehow links to metamorphic structures exposed from the Mojave Desert into the Sierra Nevada, but complexities appear as these structures interact with the Sevier thrust belt (DeCelles, 2004).
Immediately west and south of Death Valley (Figs. 1 and 2) a series of west-northwest– to northwest-trending contractional structures parallel Laramide structures that are seen from southern Arizona eastward. However, field relationships and geochronology establish that these northwest-trending structures include Jurassic–Cretaceous structures that overlap in age with the Sevier orogenic belt to the north (e.g., Burchfiel et al., 1992; DeCelles, 2004). The most well known of these northwest-trending structures is the eastern Sierran thrust system (Dunne and Walker, 2004; Stevens et al., 1997) and its correlatives in the Mojave Desert; well-documented crosscutting relationships indicate a Late Jurassic age for the thrust system (Walker et al., 1990). Farther east, west-northwest–trending fold-thrust systems involve basement and Paleozoic platform strata just south and east of the Death Valley region from the Clark Mountains (Fig. 2) to the New York Mountains (Burchfiel and Davis, 1971, 1977). The Clark Mountain thrust system is particularly important for this study because it is the westernmost known limit of the deformation front for these northwest-trending structures. Walker et al. (1995) showed that this thrust system spanned a contractional period that included Late Jurassic ductile deformation and ended with Late Cretaceous thrust systems that were no older than ca. 100 Ma and probably no younger than ca. 70 Ma. The Late Jurassic deformation is broadly correlated through a large region from the eastern Sierra Nevada into the Mojave Desert and is closely associated with syntectonic plutons that place age constraints on at least part of the contraction (Walker et al., 1990).
Traditionally the Clark Mountain thrust complex has been correlated to the thrusts of the Sevier orogenic belt to the north because of inferred continuity of structures through the region (e.g., Burchfiel and Davis, 1971). Moreover, there is no doubt that some of these structures overlap in age. However, the southeast-vergent thrust systems of the Sevier orogenic belt moved at nearly 90° to the northeast-vergent structures seen from the Clark Mountains eastward; this is difficult to rationalize as a single deformational event, particularly given clear evidence of younger deformation to the east. We show here that in the Death Valley region, the northwest-trending fold-thrust systems overprint northeast-trending structures, raising fundamental questions on the interpretation of a Sevier age for all of the northwest-trending structures. Thus, we consider here an alternative hypothesis that earlier Sevier thrust systems are overprinted by younger structures in this region, and these younger structures are probably Laramide in age.
GENERAL STRUCTURE OF THE RESTING SPRING AND NOPAH RANGES, EASTERN DEATH VALLEY REGION
The Resting Spring and Nopah Ranges are the westernmost, relatively intact range blocks between the Spring Mountains and the Panamint Mountains (Fig. 2). These ranges have been mapped by several generations of geologists, although the most recent published geologic maps are from the 1980s (Burchfiel et al., 1982, 1983) and a smaller scale U.S. Geological Survey compilation (Workman et al., 2002). The stratigraphic section is well known (Fig. 2B) and distinctive units aid routine mapping. The ranges have been in the center of Death Valley controversies because one of the thrusts in the ranges, the Chicago Pass thrust, is commonly correlated to the Panamint thrust in the northern Panamint Mountains (e.g., Snow and Wernicke, 2000). This correlation is the foundation of the large-magnitude extension models, but the correlation has been questioned (e.g., Wright et al., 1991; Renik and Christie-Blick, 2013).
The Resting Spring and Nopah Ranges (Fig. 3) today are a pair of east-tilted half-horst blocks separated by a half-graben, the Chicago Valley (e.g., Serpa et al., 1988). The base of the Neogene section is intermittently exposed along the eastern margin of the Resting Spring Range, overlying with moderate angular unconformity on more steeply east tilted Cambrian rocks (Fig. 3A). The Tertiary rocks show a relatively uniform east dip of 30°–40°. Tertiary rocks are also exposed along the eastern side of the Nopah Range (Fig. 3A), although less extensively, and show similar, but slightly lower dips (20°–30°) with angular discordance similar to that of the Resting Spring Range, but overlying rocks as young as late Paleozoic (e.g., Burchfiel et al., 1983). The angular unconformity is important because it demonstrates that prior to deposition of Neogene rocks across the unconformity, rocks exposed at the mid-Neogene surface dipped eastward at 10°–60° throughout what is now these ranges, exposing successively older rocks from east to west. Thus, there was a paleo–structural high, presumably an anticline, to the west of these ranges that is a remnant of Mesozoic structure. The range blocks were internally faulted during development of the extensional structures (Burchfiel et al., 1983), but these faults have low slip relative to range-bounding structures. This is evident in Figure 3A, where these faults are not plotted for clarity, yet the map pattern is not perturbed significantly at this scale (e.g., Fig. 3B). Given the range tilts of 20°–40°, initial west-dipping normal faults responsible for the range tilt should now dip 5°–50° west, assuming original 45°–70° fault dips, consistent with structures we have seen in both ranges.
East tilting has exposed the Mesozoic thrust system within both range blocks such that the map view is an oblique cross section through the fold-thrust systems (Fig. 3). With modern imagery (Fig. 3C), these thrust systems are conspicuous because of nearly 100% rock outcrop throughout both ranges. Thus, the Mesozoic geology is best analyzed in map view because cross sections can be misleading in this context. As a result, the bulk of our descriptions focuses on map-view relationships. Several observations are important from careful inspection of regional map-scale structure (Fig. 3).
The low saddle that separates the northern Nopah Range from the Resting Spring Range, Chicago Pass, exposes a mismatch in Mesozoic structure across the pass (Fig. 3). To the south, a pair of thrusts place rocks as old as the Neoproterozoic Stirling Quartzite on rocks as young as the Pennsylvanian Bird Spring Formation (Burchfiel et al., 1983). The structurally higher thrust is the Chicago Pass thrust and the structurally lower fault is the Shaw thrust (Fig. 3), the former placing Neoproterozoic on Cambrian and the latter placing Cambrian on a large, overturned, southeast-vergent footwall syncline in Paleozoic rocks as young as Pennsylvanian (Burchfiel et al., 1983). However, to the north, across a small area of Quaternary cover, only the Cambrian Bonanza King Formation is exposed (Fig. 3). Thus, Stirling Quartzite is juxtaposed against Bonanza King Formation, a stratigraphic separation of ∼2 km that requires a fault contact beneath the Quaternary cover. There is no disagreement that there is a fault at the pass, but a variety of interpretations have been given for this fault.
Most of the northern Nopah Range exposes the upright limb of the overturned syncline below the Chicago Pass–Shaw thrust system, forming a faulted, east-dipping homoclinal succession (Fig. 3A). In the central Nopah Range, however, this homocline is truncated by a shallow, south- to southwest-dipping fault, the Nopah Peak fault (Fig. 3A). The Nopah Peak fault is presumably a thrust fault because it places older rocks over younger, but has been shown on some maps as a normal fault reactivation of a thrust (e.g., Snow and Wernicke, 2000). The Nopah Peak fault carries a full stratigraphic sequence (Fig. 2B) from Proterozoic to Cambrian in its hanging wall, including extensive exposures of crystalline basement (Fig. 3) in the southern Nopah Range (Wright, 1974). Thus, the Nopah Peak fault is not a thin-skinned thrust like the Chicago Pass–Shaw thrust system, but fully involves crystalline basement. This structural style is typical of thrust systems to the east, but is atypical of the Sevier belt to the north. Thus, although the Chicago Pass–Shaw thrust system has been correlated with the Nopah Peak fault (e.g., Snow and Wernicke, 2000), this correlation seem highly unlikely because they contain grossly different structural styles. This conclusion is further supported by structural details described in the following sections.
The Resting Spring Range (Fig. 3C) exposes three major thrust sheets that include, from north to south (Burchfiel et al., 1983), the Baxter, Resting, and Gerstley thrusts. The Baxter and Resting thrusts show a map pattern normally seen in cross section for thrust systems, due to the exposures of the east-tilted section (Fig. 3). The Baxter thrust cuts upsection to the south in the hanging wall (hanging-wall ramp cutoff), carrying Neoproterozoic to Cambrian strata on Cambrian. The Baxter thrust splays to a footwall imbricate, the Resting thrust, which shows a ramp-flat geometry with a long hanging-wall flat in the Cambrian Carrara Formation along with a minor hanging-wall backthrust (Canyon thrust of Burchfiel et al., 1983) and a less prominent footwall flat in the Cambrian Bonanza King Formation (Fig. 3C). In contrast, the third fault, the Gerstley fault, exhibits an approximately east-west strike and south dip (Heydari Laibidi, 1981). The hanging wall of the Gerstley fault carries through to the southern tip of the Resting Spring Range, exposing primarily Cambrian strata, but with rocks as old as the Neoproterozoic Stirling Quartzite (Fig. 3).
We suggest that the Gerstley fault (Fig. 3) is the “Rosetta Stone” of the preextensional geology of these ranges. Moreover, because the Chicago Pass–Shaw system has played so heavily in the regional reconstructions, its role in the system is also critical. We analyze this problem further based on detailed mapping within key areas in the Resting Spring and Nopah Ranges.
STRUCTURAL DETAILS IN THE RESTING SPRING–NOPAH RANGES: RESULTS FROM NEW GEOLOGIC MAPPING
Background and Methods
In the past 15 years, we have used the Resting Spring and Nopah Ranges as field sites for undergraduate and graduate field geology classes. The central Resting Spring Range was also the site of thesis work by one of us (Guerrero, 2008) and undergraduate student research by another (Rutkofske). All of the geologic mapping described here is multiyear, repeat mapping that evolved during different field geology classes and field research efforts. Early mapping was traditional paper-based, fixed-scale mapping, but since 2003 the data were acquired through digital mapping techniques (summarized in Pavlis et al., 2010). As a result of repeated mapping together with use of global positioning system, the geologic maps described here are generally accurate to within 2–10 m for all depositional contacts, and most faults are comparable. Faults are of variable precision, however, ranging from high-precision features shown on all map generations (as traditional solid and dashed lines) to low-precision inferred or cryptic features (dotted or queried lines). The maps were produced by compiling multiple map sheets into a geographic information system (GIS; both georeferenced, scanned paper maps and digital line files) overlain on ∼1-m-resolution orthorectified imagery that became available in this area in 2010. We used the maps as the basic observations and refined the line work using the aerial photography as multiple data layers in GIS software (both ArcGIS and QGIS; https://www.arcgis.com, http://www.qgis.org/en/site/) to produce the maps discussed here (Figs. 4 and 6). All regional map figures were generated from U.S. Geological Survey data files using QGIS software. Our detailed maps also were imported into Move software (Midland Valley Ltd., ww.mve.com) where they were draped onto a digital elevation model (DEM) to produce three-dimensional (3D) visualizations (Supplemental File1) to aid analysis of the map interpretations and to allow rapid spatial queries to generate stereonet plots (Fig. 5) and cross sections (Figs. 6 and 7; Supplemental File [see footnote 1]). Final figures and our reconstruction figures were drawn in Adobe Illustrator after export from other applications.
We consider three detailed field study areas (Fig. 3): field area 1 focuses on the nature of the Chicago Pass thrust and its relationship to other structures; field area 2 contains key data indicating crosscutting relationships between structures that provide a structural chronology that is the foundation of the basic thesis of this paper; and field area 3 describes structures in the southern Nopah Range that we interpret as part of the younger deformation.
Field Area 1: Northern Chicago Valley–Resting Spring Range
The mapped area of this site covers a northeast-trending strip along the mountain front from the Baxter thrust to just west of Chicago Pass (Figs. 3 and 4). The geologic map is most accurate in the central part of the mapped area, but is accurate to a nominal scale of ∼1:10,000 throughout the mapped area. The greatest map uncertainties are in the northwest corner of the map, where complex fault arrays are difficult to interpret unequivocally. Wernicke et al. (1993) described structural features in this area with interpretations of extensive normal-slip reactivation of the Baxter and Resting thrusts, consistent with the observed complexity. Nonetheless, the details are difficult to evaluate because Wernicke et al. (1993) did not present any data other than descriptions. Similarly, although the Burchfiel et al. (1982, 1983) maps covered the mapped area, the scale of their maps obscures several critical features, and a seemingly minor rock unit identification error on the map has led to much of the confusion for this region. These issues are clarified by the work described here.
The structure of the mapped area is dominated by a northeast-trending fold-thrust system that has been decapitated by a low-angle, south-dipping, normal fault system (Fig. 4). The fold-thrust system was recognized by Wilhelms (1963), who referred to the thrust as the Burro thrust, but our mapping clarifies the nature of the Burro thrust relative to adjacent structures. In the central part of the mapped area (Fig. 4), the Burro thrust is well exposed, dips southeast, and places the middle Wood Canyon Formation on the Carrara Formation. The footwall Carrara Formation together with the underlying Zabriskie Quartzite form a close, upright syncline that plunges gently to the northeast (Figs. 4 and 5). To the west, the Burro thrust cuts downsection in the footwall across the syncline into successively older rocks. In the hanging wall the thrust also cuts downsection to the west, into middle Stirling Quartzite before merging with the Baxter thrust to the west (Fig. 4). Although more work is needed to clarify the complex fault relationships at the intersection of the Baxter and Burro thrusts, the general map-scale relationships suggest strongly that the Burro thrust is a backthrust within the hanging wall of the Baxter thrust, analogous to the Canyon backthrust on the Resting thrust system (Fig. 3C).
A well-exposed, low-angle, south-dipping fault is structurally above the Burro thrust system, exposing low-angle fault contacts and local klippen of the hanging wall of this fault (Fig. 4). Wilhelms (1963) interpreted this feature as a landslide, but our mapping indicates that it is a low-angle normal fault with a top-to-the-southeast sense of offset indicated by multiple repetitions of the underlying structure (Fig. 4), notably three displaced fragments (labeled A, B, C in Fig. 4) of the Burro thrust in the hanging walls of klippen, but structurally higher level equivalents of the thrust with Zabriskie Quartzite and Wood Canyon Formation on upper Carrara Formation to Bonanza King Formation; and remnants of the footwall syncline carried in the hanging wall of the fault, below the displaced Burro thrust (B in Fig. 4). The coherence of this assemblage seems inconsistent with a landslide, but is easily explained by a normal fault with a southeast displacement. Using geometric parameters and reasonable projections of the major structures, we constructed a 3D model of the footwall of this fault system to determine the cutoff positions that match the cutoffs observed in the klippen (Supplemental File [see footnote 1]). This analysis indicates that the fault slip is ∼500–700 m along an azimuth of 320–350, hanging wall toward the southeast. It is not clear how this low-angle normal fault connects westward to other structures because it is difficult to trace beyond an area where it places brecciated Zabriskie Quartzite on Zabriskie Quartzite, but we show a speculative trace through lower Carrara Formation shales that appear to be thinned along the projected fault trace (southern edge of the map). Scattered exposures of Tertiary rocks are recognized along this mountain front (Niemi et al., 2001), but extensive Quaternary deposits obscure contact relationships. Thus, it is not clear if this south-dipping fault is younger than these Tertiary deposits or older.
This pair of fault systems is important to this paper because both can be traced eastward where they relate to the Chicago Pass thrust system (Figs. 3 and 4). The Burro thrust is recognizable in two sets of low hills along the mountain front, where it places upper Wood Canyon Formation and Zabriskie Quartzite on upper Carrara Formation to lower Bonanza King Formation (near C in Fig. 4). Wilhelms (1963) misidentified (or mislabeled) these Zabriskie Quartzite exposures as Stirling Quartzite, and this error carried through to the maps by Burchfiel et al. (1982), and complicated structural interpretations in this area. Stratigraphic continuity with Wood Canyon Formation in the hills (near C in Fig. 4) and the presence of Scolithos fossils in the quartzite exposures leave no doubt that these outcrops are Zabriskie Quartzite. A low-angle normal fault that presumably is the same low-angle normal fault recognized to the west, as well as high-angle faults, complicates the geometry in the area (near C in Fig. 4). Nonetheless, these exposures, together with the geology to the west, solve the problem of the juxtaposition of rock units across Chicago Pass. Specifically, the presence of older rocks south of Chicago Pass (Stirling Quartzite to Zabriskie Quartzite) relative to the Cambrian Bonanza King Formation to the north (Fig. 3) is the composite effect of a south-dipping backthrust (Burro thrust) and a low-angle, south-dipping normal fault system that cuts across the thrust at a low angle. That is, the thrust produced the basic juxtaposition of older rocks to the south against younger to the north, but the normal fault and cover disrupt the pattern to obscure the details without mapping at this scale.
This correlation is important because by association the rocks south of Chicago Pass (Fig. 3), which were carried in the hanging wall of the Chicago Pass–Shaw Pass thrust system, are also in the hanging wall of the Burro thrust system. Thus, assuming our interpretation that the Burro thrust is a backthrust within the Baxter thrust hanging wall, the Chicago Pass thrust and the Baxter thrust are the same thrust. This conclusion is consistent with the interpretations of Burchfiel et al. (1983) and Wernicke et al. (1988) of a Chicago Pass–Baxter thrust correlation. However, because some (e.g., Serpa and Pavlis, 1996) have questioned this correlation, it is important to clarify that our more detailed work supports the original correlation by Burchfiel et al. (1983).
In the southwest corner of the mapped area (section 15, Fig. 4), a large open syncline and smaller folds have distinctive northwest-southeast trends. These folds have axes nearly 90° from the footwall syncline of the Burro thrust (Fig. 4), indicating that they are distinct structures. The large syncline was described by Burchfiel et al. (1983) together with other northwest-trending folds in this region, but the full significance is best shown just to the south, in our second field area.
Field Area 2: Resting Thrust–Gerstley Fault Intersection
In the central Resting Spring Range (Fig. 3), the Resting thrust intersects the Gerstley fault (Heydari Laibidi, 1981) and the crosscutting relationships between these faults are a key observation. Burchfiel et al. (1983) recognized the importance of this problem, but concluded that field relationships were ambiguous on relative timing and kinematics of the thrust systems in this region. The greater resolution in our mapping provided by modern mapping methods resolves this ambiguity (Fig. 6).
Although the structure of this area is complex, the following observations indicate that the Gerstley thrust is a younger, northeast-vergent thrust that is kinematically distinct from the southeast-vergent Resting thrust (Fig. 6).
The Resting thrust can be easily traced from the north edge of the mapped area to a point where it intersects other fault systems (Fig. 6). North of the intersection, the fault displays a hanging-wall flat on footwall cutoff geometry. That is, the fault is nearly bed parallel in the upper Carrara Formation hanging wall, but cuts upsection in the footwall from a Bonanza King Formation–Nopah Formation footwall cutoff just to the north of the mapped area. A Bonanza King Formation footwall flat is apparent farther north on imagery and there are also significant small-scale folds in the Carrara Formation hanging wall. This geometry indicates that the Resting thrust is kinematically indistinguishable from the Baxter thrust to the north (Figs. 3B and 4).
In the western part of the mapped area, the Gerstley fault strikes northeast and dips ∼15°–30° southeast, placing Wood Canyon Formation and younger rocks on the Bonanza King and Nopah Formations (Fig. 6). This older on younger relationship and fault geometry indicate that the Gerstley fault is a thrust, but not a typical ramp-flat thrust. From southwest to northeast the fault cuts obliquely across an east-northeast–trending hanging-wall anticline (Gerstley anticline, Fig. 6), cutting upsection from Wood Canyon Formation into rocks as young as Bonanza King Formation. Hanging-wall cutoff angles on this fault are as high as 70°, and cutoff lines trend approximately east-west. This geometry cannot be reconciled with the geometry of the Resting thrust and the high cutoff angles would require a steeply dipping fault if the fault formed in initially flat-lying strata. This geometry is easily explained, however, if the Gerstley anticline formed first and was later truncated by a younger thrust, the Gerstley thrust.
Where the Gerstley thrust meets the Resting thrust, the basic map pattern indicates that the Gerstley thrust cuts the Resting thrust (Fig. 6). Nonetheless, this map pattern is an interpretation because faults and brecciation at the intersection obscure the structure. This complexity is the principal reason earlier studies were unable to resolve the geometry. The key observation at the intersection, however, is the occurrence of a third fault system, a low-angle, west-dipping, normal fault system that cuts both the Gerstley and Resting thrusts. The footwall of this low-angle normal fault system exhumes a half-window in the Resting thrust with Nopah Formation in the footwall and folded Carrara and Bonanza King Formations in the hanging wall (Resting thrust window, Fig. 6). Fold hinge lines and an axial planar pressure-solution cleavage associated with the folds clearly establish that the Resting thrust here is southeast vergent (Guerrero, 2008) and consistent with other structures to the north and east. A continuous exposure of the Resting thrust hanging wall can be traced to the paleochannel of the Gerstley arroyo in the central part of the mapped area (Fig. 6), where it forms a large, steeply northeast-plunging ramp anticline (Figs. 5 and 6), which is also southeast vergent. Because the Gerstley thrust dips southeast, however, the low-angle normal fault shifts the Gerstley thrust contact south, into the arroyo (Fig. 6). This interpretation that the Gerstley thrust is in the arroyo is supported by exposures of older rocks (Zabriskie Quartzite and Carrara Formation) on the southern side of the paleochannel of the Gerstley arroyo (Fig. 6). The low-angle normal fault is difficult to trace along the north canyon wall of the arroyo because it places brecciated Bonanza King Formation in the hanging wall of the Resting thrust against Bonanza King Formation in the hanging wall of the Gerstley thrust, but the fault trace can be approximated by abrupt changes in bedding orientations. To the south of the arroyo, the low-angle normal fault reappears as a west-dipping fault contact between the Carrara and Bonanza King Formations.
Consistent with the map-scale observation that the Resting thrust is cut by the Gerstley fault, we observed overprinted cleavages within the Carrara Formation in the central part of the mapped area (cleavage overprints, Fig. 6): an older, steeply dipping, northeast-striking slaty cleavage that is axial planar to the Gerstley anticline, and a younger, steeply dipping, northwest-striking crenulation cleavage. The observation suggests strongly that the second cleavage is associated with the Gerstley thrust and, accordingly, that the Gerstley thrust is northeast vergent.
Although at regional map scales (Fig. 3) the rocks south of the Gerstley anticline appear homoclinal, these rocks are actually warped into open, northwest-trending folds (blue axial traces, Fig. 6) consistent with the overprinting cleavages seen in the Carrara Formation.
Field Area 3: Southern Nopah Range
The southern Nopah Range reveals that the northwest-trending folds observed in the hanging wall of the Gerstley thrust continue to the southern end of these ranges and compose the main fold system in this area. The southern Nopah Range is in the hanging wall of the Nopah Peak fault, and appears to be a simple east-tilted homocline at a regional scale (Fig. 3). However, careful analysis reveals that this map pattern masks fold systems within the range, seen primarily as strike changes.
This relationship is most easily seen in the Emigrant Pass area, where the highway crosses the southern Nopah Range (Fig. 3). The most conspicuous structures are a series of cross-faults and northwest-dipping normal-fault systems that are the primary focus of numerous field geology classes that visit this site. Nonetheless, at map scale there are clear strike changes as well as small-scale exposures of a thrust fault with a hanging-wall anticline that hint at the larger scale structure, i.e., an anticline-syncline fold pair that that plunges steeply southeast (Fig. 5). This fold system is most easily recognized in cross section (Fig. 7). This section was prepared using Move software by directly projecting our geologic map contacts onto a vertical section perpendicular to the best estimate for the fold axis trend determined from stereonet analysis of bedding data (fold axis at 130/29, Fig. 5). Because the section is a simple projection of the map, the location of the section is irrelevant other than the orientation of the section and the topographic profile. The important observation is that this structure, which is relatively obscure in map view, is a very conspicuous anticline-syncline pair on the cross section. Note that when the section is corrected for east tilt of the range (right panel, Fig. 7), the apparent northeast vergence disappears, indicating an upright, northwest-trending fold pair, cored by an originally northeast-dipping thrust (lower panel, Fig. 7). Thus, this fold system is presumably a blind, southwest-vergent, minor backthrust carried in the hanging wall of the Nopah Peak fault (Fig. 3).
Similar structures are seen in the southern end of the Nopah Range. This area was mapped at a scale of 1:24,000 by Wright (1974), and like Emigrant Pass, this classic map is used to display tilt-block extensional structures with a system of low-angle west-dipping normal faults in east-tilted strata. Nonetheless, within these apparently homoclinal strata are numerous folds, mostly too small to map at scales of 1:24,000, but locally well displayed at larger map scales. A stereonet plot of poles to bedding within a part of this region (Fig. 5) indicates that these folds, like the Emigrant pass fold, plunge southeast (135/10). When Neogene east tilt is restored, these folds originally plunged gently northwest and represent part of a northwest-southeast–trending fold-thrust system in the hanging wall of the Nopah Peak thrust.
Synthesis of Structure in the Nopah–Resting Spring Ranges
These observations indicate that the Nopah and Resting Spring Ranges are comprised of three distinct thrust sheets (Fig. 3B):
At the top of the local structural stack is the Baxter–Chicago Pass thrust that carries a backthrust, the Burro thrust, in its hanging wall. This thrust sheet is relatively intact aside from disruption by a south-dipping, low-angle normal fault system that drops the Nopah Range and Chicago Valley downward relative to the Resting Spring Range. Slip on this normal fault is well constrained by the offsets of the Burro thrust of ∼500–700 m. Although it is possible that a second, north-dipping normal fault reactivates the Chicago Pass thrust in the northern Nopah Range (as suggested by Snow and Wernicke, 2000), that interpretation would not change this inferred correlation.
Below the Baxter–Chicago Pass thrust are the Shaw thrust and its associated southeast-vergent footwall syncline in the Nopah Range. This fold-thrust pair presumably is correlated to the southeast-vergent Resting thrust, based on structural position. Given the apparent small amount of Neogene structural relief across Chicago Pass, the simplest interpretation of this fold-thrust complex is that the Resting thrust and Shaw thrust are the same thrust, but this thrust system is tipping out to the east, probably forming a lateral ramp in the Shaw thrust at the stratigraphic level of the Cambrian carbonates (Bonanza King and Nopah Formations). That is, the Shaw thrust probably merges with the Chicago Pass thrust to the east along a branch line related to a lateral ramp at depth, with the Resting–Shaw thrust presumably increasing in slip westward. Initially this interpretation may seem counterintuitive because the stratigraphic throw is much larger on the Shaw thrust than the Resting thrust (Fig. 3). However, the Shaw thrust cuts upsection in the footwall throughout its exposures in the Nopah Range, with the footwall cutoff in the Ordovician along the western escarpment of the Nopah Range (Fig. 3). Thus, continuity of this footwall cutoff beneath Quaternary cover of the Chicago Valley can allow for the deeper stratigraphic levels of the Resting thrust footwall at the level of the upper Cambrian Nopah Formation (Fig. 6). Moreover, because the Resting-Shaw system is northeast trending, the fault is climbing upsection in the footwall parallel to structural trend, which is consistent with a northwest-trending lateral ramp in the footwall. Thus, this conclusion is a simple, although admittedly nonunique, solution to the apparent discrepancy along the Shaw thrust.
This southeast-vergent fold-thrust system is truncated by a northwest-trending, northeast-vergent fold-thrust system composed of the Gerstley and Nopah Peak thrusts. In the Resting Spring Range, this overprinting produces a complex geologic relationship along the Gerstley thrust (Fig. 6) and is almost certainly the origin of a series of northwest-trending folds in the Resting Spring Range (Figs. 3B and 4). These folds were recognized by Burchfiel et al. (1983) as overprinting structures, but their association with the Gerstley thrust was not recognized. The structure is more straightforward in the Nopah Range because the Nopah Peak thrust cuts obliquely across the footwall of the Shaw thrust in relatively homoclinal strata (Fig. 3B). The Nopah Peak thrust carries northwest-trending fold-thrust systems in its hanging wall (Fig. 7), as does the Gerstley thrust (Fig. 6), and is undoubtedly the same thrust separated by normal faults that form the Chicago Valley (Fig. 3). The Gerstley–Nopah Peak thrust system must be a major thrust because it carries the entire stratigraphic section from basement to Cambrian Bonanza King Formation: at least 6–7 km of section plus an unknown amount of basement (Fig. 2B). Moreover, the presence of basement in the hanging wall indicates that somewhere to the southwest there was a subsurface footwall cutoff where the thrust cut into basement. The Gerstley–Nopah Peak thrust is clearly northeast vergent based on folds and cleavages within the thrust sheet and in areas to the northeast of the thrust (Fig. 3B).
Because the Gerstley–Nopah Peak thrust system carries basement in its hanging wall, the thrust must have produced a large, basement-cored anticlinorium to the southwest of the present Nopah–Resting Spring region. The area of this inferred fold, however, is now occupied by the complexly deformed Death Valley extensional terrane (Fig. 2), where it is doubtful a structure of this type could be recognized without insight from the structures in the Nopah–Resting Spring Ranges. This fundamental observation is critical to reconstructing the Neogene extension but has not been recognized in previous reconstructions (Fig. 8). For example, although previous studies recognized basement-involved deformation, the assumption of continuity of structure through a smooth change in strike (Fig. 8) leads to a very different restoration template from a system where the northwest-trending structures are an overprinting structure with a very different geometry (cf. upper and lower halves, Fig. 8). We suggest that this relationship is at the heart of the controversy on reconstructing the Death Valley extensional terrane, and that previous reconstructions (e.g., Fig. 8), including our own (Serpa and Pavlis, 1996), are almost certainly wrong because they typically have compared structures that are different in age and kinematics as well as geometrically much more complex than simple fold-thrust structures.
Regional Evidence for a Younger, Northwest-Trending Fold-Thrust Belt in the Death Valley Extensional Terrane
Our interpretation that the Gerstley–Nopah Peak thrust system is part of a system of northwest-trending, younger structures makes three simple predictions for the area to the west, in the Death Valley extensional terrane. (1) A deep erosion surface developed on this region prior to deposition of Neogene cover (e.g., Fridrich and Thompson, 2011) and thus, our hypothesis predicts that we should see variations in stratigraphic levels immediately beneath the Tertiary unconformity consistent with a northwest-trending ramp anticline in the Gerstley–Nopah Peak thrust system. (2) Fold-thrust overprints should be recognizable through a large region where these fold-thrust systems interacted. (3) Low-temperature cooling ages should be distinctly different in the hanging wall versus footwall of the Gerstley–Nopah Peak thrust system. More work is needed on these issues, but our review of regional observations is consistent with these predictions, which are described in the following by area.
Southern Death Valley
Here we use the term southern Death Valley area to include all rocks south of a structural contact that Wright and Troxel (1984) referred to as the Sheephead fault and east of the southern Death Valley fault (Fig. 9). The Sheephead fault separates an area dominated by pre-Cenozoic rocks to the south from an area with predominantly Cenozoic volcanic rocks to the north (Wright and Troxel, 1984). Wright et al. (1991) considered the Sheephead fault as the southern margin of a Miocene pull-apart basin filled by the volcanics to the north, but the fault has been interpreted as both dextral and sinistral (Renik and Christie-Blick, 2013; Serpa and Pavlis, 1996). In any case, the fault merges with complex low-angle normal faults of the Amargosa Chaos detachment system to the northwest, where other complexities occur (Holm and Wernicke, 1990; Serpa and Pavlis, 1996; Wright et al., 1991).
This region is important in the context of this study in that regardless of large versus smaller magnitude extensional models for the system, this region now occupies the area directly west and south of what was the preextensional surface trace of the Nopah-Gerstley thrust system. If there is significant right-lateral motion on the Sheephead fault, the Grandview fault, or both (Fig. 9), this area would have been well south of the Nopah-Gerstley thrust. Nonetheless, this region should have exposed the Nopah-Gerstley hanging wall and the structural high associated with the basement-involved ramp anticline (Fig. 8) it carried. This hypothesis is supported by three observations.
Along the eastern edge of the Nopah Range the angular unconformity cuts to deeper levels to the south, from Cambrian to Neoproterozoic (Fig. 9), which is consistent with the hypothesis of a structural high to the south. In the southern Black Mountains the angular unconformity between Tertiary and older rocks is exposed extensively but dispersed by extension (e.g., Topping, 1993; Wright et al., 1991; Wright and Troxel, 1984). In this area the angular unconformity is typically at stratigraphic levels ranging from Crystal Springs Formation in the Ibex Hills and Buckwheat Wash, to rocks as young as the Wood Canyon Formation and Zabriskie Quartzite in the Sperry Hills (Fig. 9). This pattern indicates that the unconformity cuts successively deeper to the west, consistent with our hypothesis.
Giallorenzo (2013) conducted a low-temperature thermochronology study of rocks in the Spring Mountains and the Nopah and Resting Spring Ranges, and reported zircon U-Th-He dates across the southern Nopah Range and the northern Resting Spring Range, which represent the hanging wall and footwall, respectively, of the Gerstley–Nopah Peak thrust system. Our hypothesis would predict much younger cooling ages for the southern Nopah Range, and Giallorenzo’s (2013) data clearly support this hypothesis. Specifically, Giallorenzo (2013) recognized only Jurassic to earliest Cretaceous cooling ages from the northern Resting Spring Range and ages as young as Paleocene in the southern Nopah Range. Moreover, age versus paleodepth analyses of these data indicate that the northern Resting Spring Range cooled entirely in the Jurassic, whereas the southern Nopah Range cooled relatively rapidly between 98 and 85 Ma and more slowly into the Paleogene (Giallorenzo, 2013). These data suggest strongly that the main deformation-related exhumation in the southern Nopah Range was Late Cretaceous and younger than Jurassic exhumation of the northern Resting Spring Range.
Although our studies are incomplete in the Ibex Hills and Saratoga Hills, regional mapping and our work in these areas show clear evidence of both older southeast- and northeast-directed contraction (green and black lines in Fig. 9). One manifestation of northeast-directed contraction is visible at the scale of Figure 9 in a large-scale southeast-plunging syncline in the northern Ibex Hills (below Sheephead fault in Fig. 9), yet elsewhere in this area northeast-trending folds and cleavage are observed.
Southern Death Valley also exposes the northeast fringes of Mesozoic arc rocks (dotted blue line, Fig. 9). This observation is based on a series of Mesozoic plutons exposed just south of the Saratoga and Saddle Peak Hills and more extensive plutonic exposures in the Salt Spring Hills (Figs. 1 and 9). It is significant, however, that these plutons are absent throughout the remainder of this region (Miller and Pavlis, 2005; Wright et al., 1991). Thus, any reconstruction must deal with this jagged piercing line, including the apparent absence of Mesozoic plutonic rocks in the footwall of the Amargosa detachment system. In addition, the spatial association between this plutonic front and the hanging wall of the Nopah-Gerstley thrust is similar to all thrusts with this trend to the east. Thus, this association is broadly consistent with continuity of a distinct, younger thrust system through this area.
Amargosa Chaos–Amargosa Valley
To the north of the Sheephead fault (Fig. 9) Neogene extension dominates the geology of the Amargosa Chaos with Miocene volcanics covering much of the older rock assemblage (Wright and Troxel, 1984). In addition, deeply exhumed rocks are exposed along the western edge of the Black Mountains, beneath the detachment (e.g., Miller and Pavlis, 2005). The complexity of the Neogene geology makes it difficult to evaluate the preextensional paleogeography, but a few observations are relevant to our hypothesis.
Tertiary rocks in the hanging wall of the detachment system overlie relatively old preextensional rocks in this area. Although many of these Neogene cover rocks are younger than the onset of extension, Topping (1993) showed that in many areas the basal units are 13–14 Ma volcanic rocks that predate the major extension, consistent with conclusions of Fridrich and Thompson (2011). In localities with this known preextensional cover above the unconformity (Fig. 9), the Tertiary overlies Neoproterozoic (Noonday-Johnnie-Stirling) formations (Fig. 9). Thus, like southern Death Valley, this observation supports the hypothesis of a preextensional anticline.
Along the Amargosa Valley, preextensional bedrock exposures are limited to Eagle Mountain and the Dublin Hills (Figs. 3 and 8). Eagle Mountain carries an obvious preextensional fold visible from the highway that is part of the older southeast-vergent system, consistent with a restored position close to the northern Resting Spring Range (Niemi et al., 2001; Renik et al., 2008). However, from our field observations the Dublin Hills contain clear evidence of the younger northwest-trending structures recorded as folds and a slaty cleavage in shales of the Wood Canyon and Carrara Formations (Figs. 3 and 9). At both Eagle Mountain and the Dublin Hills, the angular unconformity below the Tertiary is at the stratigraphic level of the Cambrian Bonanza King and Carrara Formations (Fig. 9), consistent with close correlation to rocks immediately to the east in the Resting Spring Range. Together these observations indicate that the preextensional Gerstley–Nopah Peak thrust system had a surface trace between Eagle Mountain and the Dublin Hills (Fig. 9), as would be expected from the present geometry.
The northern Resting Spring Range and the low range of hills just east of Stewart Valley (Fig. 9) show structures indicating that the northwest-trending fold overprints also extend northeast of the Gerstley–Nopah Peak thrust system. Specifically, the hills east of Stewart Valley expose a prominent west-northwest–trending anticline in Neoproterozoic to Cambrian rocks, and map patterns across Stewart Valley show clear evidence for an accompanying southeast-plunging syncline in the northern Resting Spring Range (Figs. 3B and 9). The anticline is demonstrably younger than a prominent slaty cleavage in the middle Johnnie Formation exposed in the core of the anticline, indicating an overprinting similar to that seen in the Gerstley Mine area (Fig. 6), but limited to fold overprints. Burchfiel et al. (1983) interpreted these folds as Cenozoic structures because of their spatial association with Tertiary strike-slip systems; nonetheless, they showed that the folds predated Neogene strata in this area, consistent with our observations. Thus, we infer the folds are late Mesozoic or Paleogene in age and part of the northwest-trending fold systems observed to the south.
Below the Amargosa detachment fault system, most of the structures in the Black Mountains are ductile, synmetamorphic structures, including the Death Valley turtlebacks and associated Miocene plutonic rocks (Fig. 9). Much of this ductile structure is Neogene and synextensional (e.g., Holm et al., 1992; Miller and Pavlis, 2005; Miller and Friedman, 1999; Miller, 2003; Pavlis, 1996; Serpa and Pavlis, 1996), but work by Miller and Friedman (1999), Miller (2003), and Miller and Pavlis (2005) demonstrated that the main foliation in these rocks originated in the Mesozoic to earliest Cenozoic and is associated with a northwest-trending fold-thrust system that duplicated Mesoproterozoic crystalline basement along thrust systems. Because northwest trends are also the general direction of Neogene extension, it is difficult to parse Mesozoic versus Cenozoic structure (Miller and Pavlis, 2005). Thus, although Miller (2003) recognized top-to-the-southeast transport for one of these systems, Neogene overprinting could contaminate this interpretation. Geometrically, the thrusts recognized in the Black Mountains core are in a reasonable position for basement ramps, or footwall basement imbrication, beneath a northeast-directed Nopah–Gerstley thrust system. That is, the ductile thrust systems preserved in these rocks could represent mid-crustal levels of the northwest-trending Mesozoic thrust belt. Moreover, the syntectonic plutons dated by Miller and Friedman (1999) indicate that these structures are as young as Eocene. This Eocene age has long been problematic, but is consistent with our hypothesis that the northwest-trending fold-thrust systems are younger, possibly Laramide, structures.
Observations to the West of Death Valley: A Key for Future Reconstructions
The geology across Death Valley shows dramatic differences that are related to the exhumation of the Black Mountains along low-angle normal fault systems as well as strike slip across the region (e.g., Serpa et al., 1988; Wernicke et al., 1988; Wright et al., 1991). Most notably, the mountain blocks west of Death Valley are more coherent structural blocks, albeit with complex tilting and internal disruption. Nonetheless, all restorations (e.g., Fig. 8) place these rocks west and south of the Nopah and Resting Spring Ranges, and their internal geologic structure is the western template that needs to be matched in any restoration. Wernicke et al. (1988) and later studies relied heavily on correlation of the Panamint thrust in the northern Panamint Mountains to the Chicago Pass thrust system in the Nopah–Resting Spring Ranges, yet in light of our studies reported here and the work of Renik and Christie-Blick (2013), this hypothesis is suspect. Unfortunately, we believe it is premature to clearly resolve these issues because of uncertainties on the complex structure of the Panamint Mountains. Nonetheless, several observations are important that need to be considered as we move toward the next generation of reconstructions.
From just north of Butte Valley southward to the Garlock fault (Figs. 2 and 9), Neogene extension is relatively insignificant, whereas strike-slip–related deformation is intense (Guest et al., 2003; Luckow et al., 2005; Pavlis et al., 2012). Snow and Wernicke (2000) restored this region to a narrow preextensional sliver (Fig. 8) and Fridrich and Thompson (2011) inferred that the Amargosa detachment system originally passed through this region in the area now occupied by Wingate Wash (Fig. 9). Neither of these hypotheses is possible from the known surface geology (Guest et al., 2003; Luckow et al., 2005; Pavlis et al., 2012). This area may have been carried in the hanging wall of a detachment system, but the Cenozoic geology cannot be reconciled with large amounts of extension within this region based on several lines of evidence.
All of the southern Panamint Mountains between Butte Valley and Wingate Wash are essentially an intact, 13–14 Ma volcanic field that includes a large volcanic center just north of Wingate Wash (Luckow et al., 2005; Pavlis et al., 2012). These rocks are essentially in depositional position aside from a few minor faults and show only modest tilts of ∼20° to the southeast along Wingate Wash, consistent with extensional structures imaged in southernmost Death Valley (Serpa et al., 1988).
The unconformity below the Cenozoic is extensively exposed throughout the Owlshead Mountains with dips <30° in most of the region, and with no ranges containing exposures that do not include extensive exposures of this unconformity. Thus, there are no exposures comparable to the core-complex–type exposure in the central Black Mountains indicative of large-scale detachment faulting within this region.
All of the structures in this region show a mixed history that includes normal, strike-slip, and fold-thrust fault development during the Neogene, a structural style indicative of a strike-slip–related system (Luckow et al., 2005). In addition to the well-known sinistral Garlock system, other strike-slip systems include sinistral-oblique motion on the Wingate Wash and Owl Lake fault systems as well as dextral motion in the southwestern Owlshead Mountains related to the Panamint Valley fault system (Andrew and Walker, 2009; Guest et al., 2003; Luckow et al., 2005).
One of the most enigmatic structures of this region is the Butte Valley fault (Fig. 9). The Butte Valley fault is intermittently exposed in the southern Panamint Mountains and separates Neoproterozoic rocks with Mesozoic plutonic sheets from late Paleozoic rocks and Mesozoic arc-related volcanic rocks (Wrucke et al., 1995). This structure has typically been described as a thrust fault, but is not a typical thrust because fault cutoff angles vary along strike and intrusions complicate the feature (Wrucke et al., 1995). Wernicke et al. (1988) correlated the structure to the Chicago Pass thrust system because of similar large stratigraphic throw and structural position; i.e., the fault juxtaposes Pahrump Group and Mesozoic plutonic rocks against Permian to Triassic rocks (Fig. 9). However, other interpretations have been suggested. For example, Davis and Burchfiel (1997) inferred that the fault may record a Mesozoic volcanic caldera rim, and Andrew and Walker (2009) considered it a composite structure with a complex history. Recognition of the overprinting structures described here led to yet another hypothesis consistent with Andrew and Walker’s (2009) work, that the structure is some composite feature generated by the overprinting of the southeast-vergent and northwest-vergent contractional structures.
The Mesozoic structures on which the Cenozoic rocks were deposited in this region have significant implications for preextensional paleogeography. Throughout the Owlshead Mountains, the dominant rock type below the Tertiary unconformity is Mesozoic granitic rock (Fig. 9). Marbles that are probably derived from Paleozoic rocks are present within parts of this assemblage, but these rocks are very different from the weakly metamorphosed rocks to the east. The only exception in the Owlshead Mountains is a large exposure of weakly metamorphosed Pahrump Group strata in intrusive contact with Mesozoic granitoids in the extreme northeast corner of the Owlshead Mountains (Wright and Troxel, 1984) and an exposure of similar rocks beneath the Tertiary strata just north of Wingate Wash (Luckow et al., 2005; Pavlis et al., 2012). Any restoration of these rocks must consider the basic observation that this region carries the full Mesozoic arc assemblage with it, and cannot restore above any rocks devoid of Mesozoic plutonic rock. Moreover, the observation that the northeast corner of the Owlshead Mountains carries unmetamorphosed rocks represents an important piercing line that should be useful in reconstruction (blue dotted line, Fig. 9). The unconformity along the eastern front of the Panamint Mountains (Fig. 9) cuts upsection to the north from Pahrump Group to Paleozoic rocks in the northern Panamint Mountains. This pattern is similar to the eastern Nopah Range (Fig. 9), albeit larger scale and complicated by Neogene structure. Nonetheless, this observation also is broadly consistent with a paleo–structural high across what is now the Panamint Mountains that supports a hypothesis that the Panamint Mountains were carried in the hanging wall of the Gerstley–Nopah Peak thrust system.
Resting Spring–Nopah Restoration Template
There is widespread agreement that the Spring Mountains compose a reasonably undeformed template against which the Death Valley extensional terrane must restore. The geology of this range has been well known for some time (Burchfiel et al., 1974, 1983; Carr, 1983) and has a distinctive thrust stack that carries successively more distal miogeoclinal strata onto cratonal strata through the thrust stack. The combination of the structural succession and stratigraphic variations among thrust sheets is the foundation for all of the reconstructions of the Death Valley extensional terrane. The original restorations of the system primarily treated the Spring Mountains as a cross-sectional template where the western escarpment of the Spring Mountains represents an oblique section through the southeast-vergent thrust system (e.g., Wernicke et al., 1988). This thrust succession has remained unquestioned through subsequent restorations (e.g., Snow and Wernicke, 2000; Wernicke and Snow, 1998). Snow and Wernicke (2000) emphasized stratigraphic variations in conjunction with the structural stack to further constrain reconstructions. However, the structure in the Spring Mountains is also three-dimensional, and this variation generally has not been exploited. Here we recognize two observations that suggest the original thrust correlations need to be critically evaluated.
The Wheeler Pass thrust (Fig. 10) has been featured in all reconstructions of the Death Valley extensional terrane. Wernicke et al. (1988) and later works correlated this thrust with the Montgomery Mountains thrust and Chicago Pass–Baxter thrust system because of similar juxtapositions of Neoproterozoic clastics atop Paleozoic strata with a footwall syncline. Historically, however, Burchfiel et al. (1983) considered this correlation unlikely and made a strong case for correlation of the Montgomery Mountains thrust to the Kwichup Springs thrust in the northern Spring Mountains. Similarly, a major thrust below the Wheeler Pass thrust, the Lee Canyon thrust, is often ignored in regional structural correlations, yet this is also a major thrust system that places the Cambrian Bonanza King and Nopah Formations on the Pennsylvanian Bird Spring Formation (Fig. 10). This juxtaposition involves ∼5 km of stratigraphic throw and is the same stratigraphic shift seen along the Shaw thrust at Chicago Pass. Burchfiel et al. (1974) clearly showed that the Lee Canyon thrust decreases in slip to the east and tips out into a fold complex at the eastern edge of the Spring Mountains. Using Elliott’s (1976) classic bow and arrow rule for thrusts, this geometry suggests that the Lee Canyon thrust should be increasing in slip to the southwest, not disappearing. To illustrate the importance of this issue, we used the empirical displacement-length relationship developed by Bergen and Shaw (2010) for thrust systems to determine a minimum strike length for the Lee Canyon thrust. Assuming that the maximum observed throw (∼5 km) was transferred to the simplest thrust possible, a 30° fault with no ramp-flat complications, the net slip is at least 10 km. Assuming that this slip is Dmax (Where Dmax is the maximum displacement) in the Bergen and Shaw (2010) formulation, their equations lead to strike length estimates that range from 55 to 238 km; their best estimate equation yields 147 km. This suggests strongly that the Lee Canyon thrust should be recognizable in ranges now to the west, and the only question is which fault is equivalent.
The southern Spring Mountains expose a complex structural knot that was described in detail by Carr (1983). Particularly notable is that early northeast-trending structures in this region are crosscut and folded by northwest-trending structures with a pattern very similar to what we describe here along the Gerstley–Nopah Peak fault system. Carr (1983) interpreted this deformation pattern in the context of a major recess in the thrust front of the Keystone thrust, the principal northeast-trending structure to the north in the Spring Mountains. We see no unequivocal evidence for this interpretation, however, and an easier interpretation based on our work is that the northwest-trending structures simply are younger, and overprint an older northeast-trending Keystone thrust system. Carr (1983) documented several examples of northwest-trending folds deforming older northeast-trending structures, including folding of a northeast-trending segment of the Keystone thrust.
Fleck and Carr (1990) and Fleck et al. (1994) dated the youngest rocks cut by the thrust as ca. 100 Ma, indicating a Late Cretaceous maximum age for the thrusting. How much younger is debatable, although crosscutting relationships to the south in the Clark Mountains suggest that pre–77 Ma ages are likely (Walker et al., 1995). Nonetheless, even this minimum age is suspect because overprinting is very difficult to date using plutonic rocks that cut faults with multiple movement histories. For example, when a fault is reactivated or folded, a crosscutting relationship at one locality need not be indicative of the entire fault. This issue is displayed well in the Border Ranges and Contact faults of southern Alaska. These faults have segments with well-documented minimum ages from crosscutting plutons at specific localities, yet along strike other fault segments cut even younger rocks than the crosscutting pluton, or even show evidence that they remain active today (e.g., Pavlis et al., 2004; Pavlis and Roeske, 2007). This arises because of variable overprinting or reactivation along strike as well as fault stepovers that preserve older segments. Clearly the southern Spring Mountains are not of the scale of these southern Alaskan structures, but the point is that dating a segment of a fault by crosscutting relationships does not necessarily date the motion of the entire fault system when overprinting is involved. Thus, we speculate that the conventional wisdom that all of these structures in the southern Spring Mountains are pre–77 Ma should be critically evaluated, because folding of thrusts and motion on some fault segments could be younger. We suspect that this deformation is latest Cretaceous to Paleogene, Laramide age, based on correlation to structures just to the east (Fig. 1).
Based on these observations we interpret the overprints of the southern Spring Mountains as the same thrust system we recognize as the Gerstley–Nopah Peak fault system. Similarly, although we recognize that correlation of the Wheeler Pass, Montgomery, and Chicago Pass thrust systems is possible, we consider that correlation questionable. Thus, alternative hypotheses need to be considered.
Given these interpretations we present a preliminary reconstruction for the Nopah–Resting Spring Ranges (Fig. 10). Ultimately this is a 4D problem of restoring a 3D system in time. Nonetheless, a preliminary assessment is possible using a map reconstruction constrained by cross-section and subsurface data. Here we present two alternative 2D map reconstruction scenarios for the Resting Spring–Nopah Range relative to the Spring Mountains that differ primarily in assumptions of displacements across the Pahrump Valley (Figs. 9B, 9C).
The Pahrump Valley is not a simple extensional basin, but rather is an extensional basin overprinted by a transtensional strike-slip system (Scheirer et al., 2010). Components of this system include the dextral State Line fault and numerous normal faults within and surrounding the Pahrump Valley. This fault system is well known from both surface geology (e.g., Guest et al., 2007) and subsurface imaging (Scheirer et al., 2010). The strike-slip component of the deformation has been well documented as 30 ± 4 km of post–middle Miocene dextral slip on the State Line fault in the southern Pahrump Valley with uncertainties of how this slip is partitioned into the northern Pahrump Valley (Guest et al., 2007). The extensional component is constrained by seismic images that demonstrate early preextensional Tertiary sediments overlain by east-dipping growth strata with maximum dips of 20°–30° (Scheirer et al., 2010). These basin floor dips are consistent with the eastward, domino-style tilting of the Resting Spring and Nopah Ranges to the west of Pahrump Valley (Fig. 10A); a simple range tilt model can account for the extensional component of the deformation in the Pahrump Valley. The Scheirer et al. (2010) seismic images also show high cutoff angles between faults and growth strata, which imply relatively modest extension consistent with adjacent range blocks.
In our analysis we restored extension across the Nopah–Chicago Valley–Resting Spring Range system by assuming simple range blocks tilted 30° by parallel normal faults with initial 60° dips. This assumption is consistent with the high cutoff angles observed by Scheirer et al. (2010) in Pahrump Valley structures just to the east and observed in low-angle normal faults in the range blocks (e.g., Fig. 6). From simple trigonometry this equates to a total stretch of 1.73, or to restore, the reciprocal stretch 0.58 along the extension direction. To illustrate this component of the restoration we applied a map-view strain on these ranges (Fig. 10) that crudely restores map dimensions but does not restore rotation or original shape. To restore motion across the Pahrump Valley we moved this block relative to the Spring Mountains (Fig. 10) with two simple rigid-body components: (1) dextral strike slip along the State Line fault ± vertical axis rotation, and (2) extension comparable in magnitude to the extension within the Nopah and Resting Spring Ranges, given that the basin floor and range tilts are similar (Scheirer et al., 2010). We explored several options, but summarize this analysis with two alternative scenarios.
Scenario A (Fig. 10B) shows a reconstruction that strictly adheres to the Guest et al. (2007) strike-slip estimate and a realistic estimate of the extension from Scheirer et al. (2010) of ∼12 km across the Pahrump Valley with no rigid-body, vertical-axis rotations. This scenario places the Gerstley–Nopah Peak thrust system directly along strike from the northwest-trending structures in the southern Spring Mountains, providing support for our hypothesis that these structures are equivalents. However, we note that this reconstruction essentially precludes a correlation of the Wheeler Pass thrust and the Chicago Pass–Baxter–Shaw thrust system because the Chicago Pass thrust system reconstructs far to the south of the Wheeler Pass thrust. Instead, in this scenario the Chicago Pass thrust system is directly along strike from the Lee Canyon thrust (Fig. 10B), suggesting these faults are the same thrust complex. This correlation is reasonable because the increase in slip to the southwest along the Lee Canyon thrust (Burchfiel et al., 1974) is compatible with the total stratigraphic throw across the Chicago Pass–Shaw system. Moreover, this scenario is also compatible with our interpretation that the Shaw thrust tips out to the east, linking to a large thrust system (Fig. 10B). In the reconstruction (Fig. 10B), we also diagrammatically show the traces of these thrusts folded by the younger northwest-trending fold systems in the northern Resting Spring Range and the hills east of Steward Valley. The Montgomery Mountains thrust cannot be restored as part of the Chicago Pass thrust system in this scenario because it is northeast of the State Line fault. Thus, in this scenario we assume that the Montgomery Mountains thrust is part of the Wheeler Pass thrust system, displaced to the northwest during early phases of the extension along low-angle normal fault systems like the Six Mile fault, a low-angle younger-on-older fault in the Montgomery Mountains (Burchfiel et al., 1983).
Scenario B (Fig. 10C) shows a reconstruction that forces an inferred correlation of the Wheeler Pass thrust system with the Chicago Pass–Baxter–Shaw thrust system of the Nopah–Resting Spring Ranges, within the broad limits of the displacements indicated by the studies of Guest et al. (2007) and Scheirer et al. (2010). This reconstruction (Fig. 10C) is only allowable if the Nopah–Resting Spring block rotated ∼20° clockwise (shown as 20° counterclockwise in the reconstruction). Assuming this rotation, the model predicts pure strike-slip motion at the northern tip of the Nopah–Resting Spring block, but transtension to the south between the Nopah–Resting Spring block and the Spring Mountains. The modeled extension is within error of the extension recognized by Scheirer et al. (2010) in the southern Pahrump Valley (Fig. 10C), but the strike slip is well below the minimum estimated by Guest et al. (2007). This scenario leaves sufficient map area between the Spring Mountains and the Nopah–Resting Spring block to restore the Montgomery Mountains thrust along strike to the Wheeler Pass thrust and, projected southwest through inferred northwest-trending folds, to connect to the Chicago Pass–Baxter–Shaw system (Fig. 10C). However, this scenario not only underestimates the strike slip indicated by Guest et al. (2007), but also provides no correlative for the Lee Canyon thrust (Fig. 10C). Thus, scenario B violates the bow and arrow rule analysis of the Lee Canyon thrust described above because the Lee Canyon thrust should continue at least another ∼20–30 km to the southwest, yet in the restoration (Fig. 10C), there is no thrust in the predicted position of the Lee Canyon thrust. To be allowable, this scenario requires that the strike-slip estimates for the State Line fault are too high and that the Lee Canyon thrust somehow merges with the Shaw thrust beneath the Pahrump Valley. Although allowable, this scenario seems unlikely.
Although both scenarios are broadly allowable, scenario A most closely fits the available data. This suggests that previous correlations of the Wheeler Pass thrust to the Chicago Pass thrust system need critical evaluation and future work needs to consider these alternatives in reconstructing the Death Valley extensional system.
Working Model for Paleogeography of Southern Death Valley
Using scenario A, we speculate that the preextensional paleogeography of what is now southern Death Valley was broadly similar to that illustrated in Figure 11. The eastern half of Figure 11 is simply scenario A with crude traces of inferred surface geology prior to deposition of the Tertiary cover, and the western half is extrapolation of these structures westward. This figure is not meant as a reconstruction, but as a template for consideration against which future studies need to consider reconstructions. We emphasize several features in Figure 11.
The correlation of southern Spring Mountains geology and the northwest-trending fold-thrust system in the Nopah–Resting Spring Ranges is a robust correlation supported by the juxtaposition of these similar structural trends using independent slip estimates for the State Line fault (Guest et al., 2007). The thrust correlations in scenario A are subject to debate, however, and deserve further consideration, particularly the complex association of the Shaw, Resting, and Lee Canyon thrusts in some as yet poorly constrained tangle of faults. Nonetheless, at present a correlation of the Lee Canyon thrust and the Chicago Pass system seems most likely, and future studies need to consider this alternative, which scrambles the classic template for restorations.
The northwest-trending fold-thrust systems complicate simple interpretation of the paleogeography, but a robust prediction of the model is a requirement that the Gerstley–Nopah Peak–southern Spring Mountains fault system involves crystalline basement. The hanging wall of that thrust system must possess a major ramp anticline behind the hanging-wall cutoff (Fig. 8), and the anticline was almost certainly located in what is now the southern Black Mountains, where Miocene rocks overlie much older rocks than in the Resting Spring Range (shown in Fig. 10 as Neoproterozoic labels). The details of this paleogeography, however, must have been complex, with northeast-trending fold-thrust systems refolded about northwest-trending axes. Thus, we have not even attempted a diagrammatic illustration of the paleogeography in Figure 10 other than labels of approximate sub-Tertiary unconformity rock ages.
An important, but blunt, marker for restoration is the northeastern limit of Mesozoic arc magmatism, which includes both Mesozoic volcanic rocks and Mesozoic intrusives. The marker line is blunt because it is irregular today, and because it could suffer from unrecognized rocks; e.g., given that Miller and Friedman (1999) recognized Eocene deformed plutonic rocks in the Black Mountains, it is possible that there are undated Mesozoic plutonic rocks in the Black Mountains metamorphic complex or the southern Black Mountains basement complex. With the ease of modern geochronology, a systematic dating scheme might recognize temporal and spatial variations in the magmatic system that could allow this marker to be less bunt. Nonetheless, in the meantime, the lack of Mesozoic plutonic rocks in the Black Mountains requires that reconstructions of the Mesozoic plutonic rocks must crudely fit the Figure 10 template, wherein the plutonic rocks are all south of the present position of the Black Mountains crystalline core.
The contractional structures of the Panamint Mountains are critical for future reconstructions, which ultimately must match the template to the east. Most important may be the Butte Valley fault, which could represent one of the older northeast-trending thrust systems that was refolded by the ramp anticline in the hanging wall of the Gerstley–Nopah Peak system (Fig. 10). Similarly, the Panamint thrust in the northern Panamint Mountains has featured heavily in previous restorations (e.g., Wernicke et al., 1988), yet our analysis casts doubt on all previous correlations of this structure. The Panamint thrust carried in the hanging wall of the Tucki Wash fault (Wernicke et al., 1993) could correlate to the Wheeler Pass system, as long suggested (Wernicke et al., 1988), but would require large transport on the detachment. However, the so-called ductile Panamint thrust below the Tucki detachment could be any of a number of structures, given our prediction that these rocks should be in the hanging wall of the Gerstley–Nopah Peak thrust and therefore should carry an overprint of both structural generations. Thus, we suggest that this issue can only be unraveled through a better understanding of structural details in the Panamint Mountains, and studies in progress will help resolve this issue.
Geologic observations from the Resting Spring and Nopah Ranges indicate that thrust systems of the Sevier orogenic belt that project into the Death Valley region are crosscut, and folded, by younger northwest-trending fold-thrust systems. This study confirms previous suggestions that the Baxter thrust and Chicago Pass thrust system are the same thrust, and the Resting thrust is part of this system that is connected to Sevier thrust structures to the north and east. The Gerstley thrust and Nopah Peak fault are part of a younger northeast-vergent thrust system that is presumably a late Mesozoic to Paleogene structure that is tied in age and kinematics to younger Mesozoic contraction that continues into Arizona and southeastward to eastern Mexico. The Gerstley–Nopah Peak thrust carries crystalline basement in its hanging wall and has a throw >6 km and unknown heave. The thrust geometry of the Gerstley–Nopah Peak thrust suggests that a basement-cored anticlinorium was to the south and west of the Resting Spring Range prior to Neogene extension; this is supported by increasing age of strata below the angular unconformity beneath Miocene cover as well as confusing structural geometries recognized in the southern Panamint Mountains.
Recognition of this overprinting in the Resting Spring and Nopah Ranges region indicates that previous reconstructions that use pre-Cenozoic markers are almost certainly wrong because they do not account for the structural geometries produced by this overprinting. However, the occurrence of crosscutting structures and 3D preextensional geometry implies that if we can resolve the Mesozoic structure, the Death Valley region can probably be reconstructed to a very high precision. Reconstruction of the Nopah and Resting Spring Ranges to the Spring Mountains template using new data on slip estimates for the State Line fault (Guest et al., 2007) and extension in Pahrump Valley (Scheirer et al., 2010) suggests strongly that previous correlations of the Wheeler Pass thrust and the Chicago Pass system are not likely, and a Lee Canyon thrust correlation is more likely. Future studies need to consider this template, including predictions on structural details in what is now southern Death Valley and the Panamint Mountains. Modern digital map database capabilities together with visualization may provide the tools to use this structure for high-precision reconstructions and should be a goal of future tectonic studies for this region.
This project is an outgrowth of years of work during field geology classes taught at the University of New Orleans and the University of Texas at El Paso, and we thank all of the students who worked in the areas described here during those years. Early work in this effort was also supported by research grants from the National Science Foundation to Pavlis and Serpa, including EAR-9706233 and EAR-9304715. The latest phases of the work were supported by grant EAR-1250388. We also thank the Louisiana Education support fund and Keck Foundation for early equipment purchases and later equipment purchases through the Texas STARS (Science and Technology Acquisition and Retention) program that led to development of digital mapping techniques used in this project. We thank Bennie Troxel and acknowledge the late Lauren Wright for extensive discussions on the geology of this complex region. Rutkofske thanks Jamie Bernstein, Skylar Primm, Pam Rein, Jeff Olman, and John Burwalk for assistance in the field as well as for discussions about the local geologic problems. We thank Marli Miller and an anonymous reviewer for thoughtful reviews of the manuscript that greatly improved the paper.