Abstract

Extension in Death Valley is usually interpreted as a combination of low-angle Basin and Range–style extension and strike slip associated with the developing Pacific-North America plate boundary in western North America, with these two tectonic regimes operating synchronously in Death Valley. Examination of structural, stratigraphic, and timing relationships in the region suggests that this interpretation needs revision. Evolution of Death Valley is best described as a two-stage process. In the first stage, lasting from ca. 18 to 5 Ma, low-angle Basin and Range extension transported allochthons consisting of Late Proterozoic through Early Paleozoic miogeoclinal section along detachment surfaces that, as extension continued, were exhumed from mid-lower crustal levels to the surface. Near the end of this extensional phase and lasting until ca. 3 Ma, deposition of a thick sequence of volcanics, clastics, and some lacustrine carbonates signaled a period of relative tectonic quiescence, with sediments in some areas covering the exhumed detachment surfaces. At ca. 3 Ma, initiation of the East California Shear Zone started development of the present-day topographic depression of Death Valley, formed as a pull-apart basin associated with this strike slip. Faulting broke the older, inactive, Basin and Range detachment surfaces, with high-angle transtensional faulting along the Black Mountains front. The Black Mountains were elevated as a result of footwall uplift, with the well-known turtleback structures being megamullions along these bounding faults. These megamullions are similar to those seen at oceanic spreading centers. The Panamint Range has previously been interpreted as an extensional allochthon, with the entire range transported from on top of or east of the Black Mountains. A new interpretation presented here is that the range is a large core complex similar to the core complex at Tucki Mountain, at the northern end of the range. The Basin and Range extensional detachment tracks over the top of the range, with extensional allochthons perched on the eastern flanks of the range. This modified model for evolution of Death Valley suggests a strong link between timing and style of deformation in the basin with the developing Pacific-North America plate boundary, particularly eastward propagation of this boundary.

INTRODUCTION

Interpretations of the geology of Death Valley have played an important role in the development of models of continental extension, particularly for models that incorporate large-magnitude extension accommodated by low-angle detachment faults (Wright and Troxel, 1973; Hamilton, 1988; Wernicke et al., 1988a; Snow and Wernicke, 2000; Hayman et al., 2003). Detachment fault surfaces are exposed in several places in the Death Valley region, with the best studied being the antiformal structures known as turtlebacks in the Black Mountains on the east side of the valley (Figs. 1 and 2; Miller and Pavlis, 2005). The detachment fault in the Black Mountains is known as the Amargosa detachment (Wright et al., 1974). Another important detachment surface is exposed at Tucki Mountain, located at the north end of the Panamint Range on the west side of Death Valley (TM on Fig. 2; Hodges et al., 1987; Wernicke et al., 1988b). Stewart (1983) proposed that the Panamint Range forms the hanging wall of an extensional system that transported the Panamint Range westward from on top or east of the Black Mountains, with the latter forming the footwall of this extensional system. In this interpretation, motion was accommodated along the Amargosa detachment fault. A prediction of this model is that there is a detachment fault underneath the Panamint Range; this will be addressed later in this paper.

Prior to recognition of low-angle extensional faulting in Death Valley, Burchfiel and Stewart (1966), on the basis of the morphology of the valley and occurrence of strike-slip faults, suggested that Death Valley was formed as a strike-slip pull-apart basin, with the basin forming between the Northern Death Valley–Furnace Creek fault to the north and the Southern Death Valley fault zone to the south (Fig. 2). This idea has been incorporated into several models for the structural evolution of Death Valley. Miller and Pavlis (2005) divided these models into two categories, depending on how strike-slip and low-angle detachment faulting are combined. The first category described by Miller and Pavlis (2005) is the “Rolling Hinge” model as proposed by Stewart (1983), Hamilton (1988), Wernicke et al. (1988a), and Snow and Wernicke (2000). In these models, the low-angle detachment faults are dominant, with the Panamint Range moving 80 km west from an original position east of the Black Mountains and strike-slip faults as upper crustal edges of the detachment system. In the second category, based on the pull-apart model developed by Burchfiel and Stewart (1966), strike-slip faults penetrate deeply into the crust and drive extension between their terminations (Wright and Troxel, 1984; Topping, 1993; Serpa and Pavlis, 1996; Miller and Prave, 2002). The low-angle detachment surfaces in the latter category are normal faults linking the strike-slip faults. The pull-apart concept has also been applied to the Panamint Valley, the basin on the west side of the Panamint Range, which links via the Hunter Mountain strike-slip fault northwards into Saline Valley (Burchfiel et al., 1987; Lee et al., 2009). In all current models, strike-slip and low-angle extensional faults are regarded as synchronous.

To understand more about structural evolution of Death Valley, it is useful to consider the area's position in relation to structural provinces that have been defined in western North America. Death Valley is located in three partly overlapping structural provinces (Fig. 1). It is located in the Basin and Range extensional province (Sonder and Jones, 1999; Burchfiel et al., 1992), in the Walker Lane belt (Stewart, 1988), and also in the Eastern California Shear Zone (ECSZ; Dokka and Travis, 1990a). The Basin and Range is characterized by block faulting and, in areas of large-magnitude extension like Death Valley, by exhumation of middle and lower crustal level rocks in core complexes (Davis, 1980; Armstrong, 1982; Stewart, 1998; Dickinson, 2002). Both the Walker Lane and ECSZ are characterized by north- to northwest-trending topography with active dextral strike-slip and normal faults (Oldow et al., 1994, 2008; Henry et al., 2007; Lee et al., 2009; Dokka and Travis, 1990a, 1990b). The Walker Lane belt was originally defined by Stewart (1988) as the triangular zone of high topographic relief separating the Sierra Nevada from the Great Basin, with its southern limit the Garlock fault. The ECSZ was originally defined by Dokka and Travis (1990a) based on mapping of strike-slip faults in the Mojave Desert, and included the Death Valley area (i.e., north of the Garlock fault) as its northern limit. Since definition of these domains, satellite geodesy (GPS) has shown that the Walker Lane belt and ECSZ are together accommodating ∼25% of present-day plate motion between the Pacific and North American plates (Miller et al., 2001; Hammond and Thatcher, 2007). The Walker Lane belt and ECSZ therefore together form a single strike-slip zone, which will be referred to as the ECSZ in this paper. This shear zone developed as a result of the evolving tectonics of western North America, particularly eastward propagation of the boundary between the Pacific and North America plates. A recent phase of this propagation was opening of the Gulf of California and initiation of the San Andreas fault as the plate boundary at 5–6 Ma (Lonsdale, 1989; Atwater and Stock, 1998). This eastward propagation continues today with strike slip in the ECSZ, which will become the full plate boundary if the San Andreas fault ceases activity in western California and the plate boundary continues its eastward migration (Faulds et al., 2005).

Although the ECSZ today accommodates 25% of Pacific-North America relative motion, timing of initiation of this shear zone is poorly known. Dokka and Travis (1990b) favored an initiation age of 10–6 Ma for the Mojave portion of the ECSZ. In the White Mountains northwest of Death Valley, Stockli et al. (2003) found two main phases of tectonism. In the first phase, up to 8 km of uplift of the White Mountains (WM, Fig. 1) occurred in the Middle Miocene as a result of footwall uplift associated with east-west extension. In the second phase, transtensional faulting began east of the White Mountains at 6 Ma, followed at 3 Ma by initiation of strike slip in the Owens Valley (Fig. 1) on the west side of the White Mountains. The Inyo Mountains, south of the White Mountains, are bounded to the east by the East Inyo fault zone, which links via the Hunter Mountain fault to the Panamint Valley. Lee et al. (2009), in a detailed analysis of the Inyo Mountains, report a phase of normal faulting starting at 15.6 Ma and initiation of strike-slip faulting on the Hunter Mountain fault at 2.8 Ma. The Hunter Mountain fault connects to strike-slip faults on the west side of the Panamint Range; Burchfiel et al. (1987) document 8–10 km of offset on this fault system.

These recent studies indicate that strike-slip faulting and extension occurred in separate phases. In this paper, I suggest that Death Valley also formed in two phases, with low-angle Basin and Range–style extension in the Miocene followed by pull-apart basin development during strike-slip deformation in the last three million years. This inference is based on a compilation of age data for the Death Valley region, combined with a new interpretation of the geometry of detachment surfaces. In this new interpretation, a detachment surface tracks over the crest of the Panamint Range, rather than underneath the range, and the Black Mountains turtlebacks are megamullion structures formed as a result of strike slip. The rocks now exposed in the turtlebacks were exhumed from deep crustal levels as a result of Basin and Range extension; a thick series of Pliocene and younger sediments were deposited on these surfaces before renewed uplift and exhumation to the present-day surface as a result of formation of the Death Valley pull-apart basin.

GEOLOGIC FRAMEWORK OF THE DEATH VALLEY REGION

When Burchfiel and Stewart (1966) first suggested that Death Valley was formed as a pull-apart basin, little was known about timing of tectonic events in the region. Their paper was published in the same year as Hunt and Mabey's (1966) seminal paper that mapped the geology of Death Valley. This paper established a robust stratigraphy, including detailed relative stratigraphy of Neogene sediments. These sediments are important recorders of tectonic development of Death Valley, but their chronostratigraphy was poorly known in 1966 because of the mostly endemic fauna in these deposits and limited radiometric age control. Recent work, especially tephrochronology (Sarna-Wojcicki et al., 2001) and argon-argon radiometric dating, has greatly increased our knowledge of the ages of these deposits. Summaries have been published by Snow and Lux (1999), Calzia and Rämö (2000), and Knott et al. (2005). Also in the last few years, several important data sets relevant to a regional understanding of the geology of the Death Valley area have become available. These data sets include digital geologic maps published by the U.S. Geological Survey (USGS) (Workman et al., 2002) and the California Geologic Survey (Saucedo et al., 2000), digital elevation model (DEM) topography (USGS DEM, 2009), and tectonic and sediment fill compilations (Blakely and Ponce, 2002).

Figure 3 is a tectonostratigraphic chart showing a compilation of Neogene geologic events in the Death Valley region. The area covered ranges from Kingston Peak to the southeast of Death Valley, northeast to the Southwest Nevada Volcanic Field (SWNVF), and as far west as the Darwin Plateau on the west side of Panamint Valley. For the SWNVF, Sawyer et al. (1994) provide an interpretation of eruption volume versus time, which the sawtooth pattern in Figure 3 schematically follows. Ash layers, important for age control on sediments, are included in the igneous activity column, although the volcanism that produced these ash beds was usually well outside the Death Valley area (Knott et al., 2005).

The last two columns in Figure 3 show tectonic events and interpreted tectonic regimes. Events associated with Basin and Range extension occurred in the Miocene to very early Pliocene. These events include exhumation of the detachment surfaces now exposed in the Black Mountains (Holm et al., 1992; Holm and Dokka, 1993) and, outside Death Valley, extension in the Cottonwood Mountains (Snow and Lux, 1999), the Inyo Range (Lee et al., 2009), and the Kingston Peak–Tecopa Basin area (Calzia and Rämö, 2000). Neogene tectonic activity reported by Lee et al. (2009) consists of exhumation and normal faulting initiated along the eastern Inyo fault zone (IFZ in Fig. 2) at 15.6 Ma, followed by a period of slow uplift then renewed normal slip at 2.8 Ma. The event that commenced at 15.6 Ma included 16° westward tilt of the Inyo Range and 5300 m of uplift of the eastern flank, at a rapid rate of 1.9 mm/a. Lee et al. (2009) suggest that the later event at 2.8 Ma dates onset of strike-slip faulting along the Hunter Mountain fault zone and initiation of strike slip in this portion of the ECSZ. This two-stage structural evolution of extensional faulting followed by strike slip is also seen in the Panamint Valley (Andrew and Walker, 2009) and, north of the Inyo Range, in the White Mountains (Stockli et al., 2003) and central Walker Lane belt (Oldow et al., 2008).

Interpretations of tectonic settings in Figure 3 come from Snow and Lux (1999) for formations in the Cottonwood Mountains (Tub, Tpn, Tnv), from Knott et al. (2005) for the Ubehebe–Lake Rogers Formation (U-LR), also in the Cottonwoods area, and from Snow and Lux (1999) for the Bat Mountain Formation. The Bat Mountain Formation is an interlayered conglomerate and sandstone section found at the east end of the Funeral Mountains (Fig. 2; Cemen et al., 1999). It overlies Mississippian carbonates and has not been directly dated. It overlies a 19.8 ± 0.2 Ma (K/Ar age) tuff and is in turn overlain by a 13.7 ± 0.4 Ma (K/Ar age) tuff (Cemen et al., 1999). This age is an important constraint on initiation of Basin and Range extension, as the Bat Mountain Formation is interpreted to have been deposited in a basin formed during the early stages of extension (Cemen et al., 1999). A lithologic correlation with a similar conglomerate-sandstone succession at Tucki Mountain has also been used to constrain both the age and amount of extension along the Furnace Creek fault zone (Wernicke et al., 1988b). Cemen et al. (1999) question the correlation because the Funeral Mountains section is underlain by a Miocene conglomerate unit that does not occur at Tucki Mountain and there are differences in structures in the underlying Paleozoic section (a thrust fault in the Mississippian strata in the Funeral Mountains is not seen at Tucki Mountain). These observations imply that details of the structural connection between Tucki Mountain and the east end of the Funeral Mountains must be reexamined, although a probable Early Miocene age of the Tucki Mountain section can still be used to date extension at Tucki Mountain (Cemen et al., 1999).

Neogene sediments preserved within Death Valley record the latest phases of structural evolution of the basin. In the northern Black Mountains and Furnace Creek Basin, the Artist Drive, Greenwater, Furnace Creek, and Funeral Formations reach a cumulative 4000 m (Hunt and Mabey, 1966; Greene, 1997; Wright et al., 1999). Naming conventions for these formations vary among different authors (e.g., Knott et al. [2005] include the Artist Drive and Greenwater in the Furnace Creek), but these differences are not substantial to this paper. Deposition of these formations initiated near the end of events associated with Basin and Range extension, starting with the predominantly volcanic Artist Drive Formation (Greene, 1997). Structural inferences from younger formations include: (1) two correlated outcrops of the Greenwater Formation in the Natural Bridge area are separated by a normal fault with 2000 m of throw (Greene, 1997); (2) conglomerates in the lower Furnace Creek Formation include granite boulders derived from the northwest, possibly Hunter Mountain (Hunt and Mabey, 1966; Wright et al., 1999), suggesting that the Death Valley basin did not yet exist at 5 Ma; (3) fan-deltas in the upper part of the Funeral Formation are offset ∼8 km by dextral strike slip along the Furnace Creek fault zone, implying this much strike slip in the last 2 m.y. (Blair and Raynolds, 1999); (4) although there are angular unconformities within these formations, there are no data such as progressive tilt (Snow and Lux, 1999) to suggest that they were deposited in active grabens. This relative timing can be seen in Figure 3, where events associated with Basin and Range extension predate deposition of most of the Artist Drive through Funeral Formations. Today, however, these formations all show significant structural dip, such as the 30° NE dip of the Furnace Creek Formation seen at Zabriskie Point (Greene, 1997). The depositional base of the oldest of these northern Black Mountains formations, the Artist Drive, is not seen. The Artist Drive Formation is in fault contact with Proterozoic gneiss and Miocene intrusives of the Badwater turtleback structure. Rocks along the highest points of the northern Black Mountains are volcanics of the Artist Drive formation, including 6.5-m.y.-old rhyolite at the popular tourist overlook, Dantes View (Greene, 1997).

Other sedimentary formations that contain structurally relevant data are found along the Black Mountains. North of Copper Canyon turtleback (CC, Fig. 4), on the flanks of the Black Mountains, the Copper Canyon formation preserves 1800 m of mostly fluvial to lacustrine sediments, including some distinctive tufa mounds (Nyborg and Buchheim, 2009). Ages of basalt within the succession constrain the age of the formation to between 5 and 3.3 Ma (Nyborg and Buchheim, 2009). The younger age is further confirmed by occurrence of one of the Mesquite Spring tuffs (3.1–3.35 Ma; numbers 7 and 8 in Fig. 3) near the top of the beds (Knott et al., 2005). Presumably deposited in a basin like present-day Death Valley, these sediments are today located up to 800 m above the valley floor. They are in fault contact with the Copper Canyon turtleback (Drewes, 1963; Holm et al., 1994). Further south, between the Copper Canyon and Mormon Point turtlebacks, the Mormon Point Formation is also in fault contact with basement rocks, in this case the Mormon Point turtleback (Hayman, 2006; MP, Fig. 4). These sediments are younger than the Copper Canyon Formation, with the 0.77 Ma Bishop tuff (number 3 in Fig. 3) found near the top of the section (Knott et al., 2005). South of Mormon Point, another important data point for structural evolution is in the Confidence Hills (CH, Fig. 4). These low hills are composed of clastic sediments of the Confidence Hills Formation (Wright and Troxel, 1984), with an age range of 1.7–2.2 Ma (Knott et al., 2005). Like the Black Mountains sections, these sediments show little evidence of syndepositional tectonism, yet are now highly deformed, with almost vertical dips in places (Beratan et al., 1999). This deformation is associated with the Southern Death Valley strike-slip fault (Dooley and McClay, 1996) and gives a qualitative indication of the large amount of post-1.7 Ma motion there has been on this fault.

East of Death Valley, the Tecopa Basin (TB, Fig. 4) contains some useful timing information. The oldest outcropping basin deposit consists of flat-lying basalt as old as 5.12 Ma (Calzia and Rämö, 2000). These are overlain by sediments of ancestral Lake Tecopa, which include the Lava Creek B tuff (0.62 Ma; number 2 in Fig. 3) near the top of the section (Hillhouse, 1987). Seismic reflection data (Louie et al., 1992) show ∼140 m of flat-lying section below the 5.12 Ma basalts, this section lying unconformably on tilted and faulted sediments. This older faulting is interpreted as the final phase of Basin and Range extension in the area (Louie et al., 1992; Calzia and Rämö, 2000). Extrapolation of measured sedimentary rates in the upper flat-lying section yields an age of between 5 and 7 Ma for the unconformity above the fault blocks, suggesting that Basin and Range extension ceased in the Late Miocene in the Tecopa Basin (Calzia and Rämö, 2000).

The above summary of younger deposits in the Death Valley region highlights some of the structurally important inferences that can be drawn from these sections. These will be used to build a structural model for evolution of the basin, but before doing that, it is useful to examine the geometry of Basin and Range detachment faults in the basin.

BASIN AND RANGE DETACHMENT FAULTS

Figure 4 shows surface geology (Workman et al., 2002) and Figure 5 the thickness of Cenozoic sediments (Blakely and Ponce, 2002) in the Death Valley area. Rocks exposed in the Panamint Range include Proterozoic metamorphic basement, a Neoproterozoic through Paleozoic sedimentary section, and younger Tertiary sedimentary and volcanic rocks. The northern nose of the Panamints is formed by an extensional core complex, Tucki Mountain (Hodges et al., 1987; Wernicke et al., 1988b). Along the east side of Tucki Mountain, extension is marked by Paleozoic sedimentary rocks in fault contact with Proterozoic rocks that have been tectonically exhumed from mid-crustal (greenschist; Hodges et al., 1987) levels. This extension occurred in the Miocene (Cemen et al., 1999; Wernicke et al., 1988b). The main extensional detachment surface is gently folded into the arched topographic surface of Tucki Mountain (Wernicke et al., 1988b). Hunt and Mabey (1966) recognized this structure and correlated it with the “Amargosa Thrust” of Noble (1941). This unfortunate thrust fault term was used by Hunt and Mabey, even though they recognized the extensional origin of the younger-over-older structural relationship at Tucki Mountain (Hunt and Mabey, 1966, p. A99). Their cross section across Tucki Mountain nevertheless remains a classic illustration of the geometry of a core complex. Figure 6 is a cross section constructed from the data in Figures 2 and 4, approximately along the line illustrated by Hunt and Mabey (1966), profile A in Figures 4 and 5. The cross section was made by extracting topography from the DEM data (Fig. 2) and geology from the digital map of Workman et al. (2002; Fig. 4). Dips of the Paleozoic strata at the eastern end of the section are from Hunt and Mabey (1966), as is the location of the detachment below these strata; Figure 7 is a summary of stratigraphic nomenclature for these older sediments. Along the western side of Tucki Mountain, rocks in the hanging wall of the detachment are Miocene–Pliocene sediments and volcanics of the Nova Basin (Tn in Fig. 3; Snyder and Hodges, 2000).

As summarized in Figure 6, interpretations of the extensional structures at Tucki Mountain show that the footwall of the detachment surface consists of Proterozoic basement and metasediments. At Mosaic Canyon, for instance, where the detachment surface is exposed (TD, Fig. 4), the canyon cuts through mylonized dolomite. This dolomite has been correlated to the Noonday Dolomite (Hunt and Mabey, 1966; Williams et al., 1976), which is also found in the footwall of the Black Mountains detachment fault at Copper Canyon and Mormon Point (Williams et al., 1976), although Miller and Pavlis (2005) question details of this correlation. The Noonday Dolomite (Fig. 7) is a platformal carbonate deposited unconformably across a Neoproterozoic rift basin that marks the onset of Proterozoic through Paleozoic miogeoclinal deposits of the western North American passive margin (Wright et al., 1978). The Proterozoic–Cambrian boundary is within the Wood Canyon Formation (Corsetti and Hagadorn, 2000). The Wood Canyon also marks the base of the Paleozoic succession that, in the Death Valley region, commonly forms Basin and Range extensional allochthons like the Funeral Mountains and Nopah Range (McAllister, 1976; Burchfiel et al., 1983; Hamilton, 1988). As shown in Figure 6, this section is also seen in the extensional allochthons on the east flank of Tucki Mountain. Detachment surfaces in the Death Valley region are commonly located in the upper Proterozoic section (Fig. 7), probably because this section is the oldest that covers the entire area. The Pahrump Group is found in the western Panamint Range and southern Black Mountains region, in a configuration that Wright et al. (1976) relate to a Proterozoic aulacogen.

As mentioned in the Introduction, Stewart (1983) suggested that the Panamint Range forms the hanging wall of a fault system that transported the Panamints westward from a position on top or east of the Black Mountains. This implies that there should be a detachment fault underneath the Panamint Range. Along the eastern base of the Panamint Range, between Trail and Hanaupah Canyons (TC and HC, Fig. 4), several outcrops of Proterozoic gneissic basement were mapped by Hunt and Mabey (1966) as the footwall of the “Amargosa Thrust.” This Proterozoic gneiss is overlain by east-tilted Basin and Range fault blocks consisting of Wood Canyon Formation through Ordovician section, like other extensional allochthons in the region and also structurally similar to the east flank of Tucki Mountain (Fig. 6). The gneiss displays mylonitic fabric consistent with westward displacement of the extensional allochthons (McKenna and Hodges, 1990). Hamilton (1988) examined the structures at Hanaupah Canyon and correlated this Hanaupah detachment (Hamilton's more appropriate term; HD, Fig. 4) with that at Tucki Mountain. McKenna and Hodges (1990) mapped extensional structures at Trail Canyon. McKenna and Hodges (1990) use the name “Eastern Panamint fault system” for a series of faults that include a basal detachment and higher faults that collectively form an extensional duplex. The Paleozoic extensional allochthons are located above this duplex. Also in this area, McKenna and Hodges (1990) mapped the 8.6–9.4 Ma Trail Canyon volcanic sequence (Fig. 3), which yields some important constraints on extensional geometry. These volcanics correlate across Death Valley with the Rhodes Tuff and Sheepshead Andesite in the southern Black Mountains (TC+RT, Fig. 3). This correlation implies 25–55 km of post-eruption offset between the two locations, the distance depending on details of how the intervening faults are restored (McKenna and Hodges, 1990).

If the detachment system along the east side of the Panamints is the one along which the Panamints moved, as in the model of Stewart (1983), then there is a correlation problem. At Tucki Mountain, the detachment tracks over the top of the anticline that forms Tucki Mountain (Fig. 6). Rocks in the footwall of the detachment are Late Proterozoic Pahrump Group and Johnnie Formation. These rocks are found along the full length of the Panamint Range (Fig. 4) and form the crest of the range, just like at Tucki Mountain. There is no apparent reason for these rocks to be in the footwall of the detachment at Tucki Mountain and in the hanging wall further south along the range, as they would need to be if the entire Panamint Range is allochthonous. A better interpretation is that the Hanaupah detachment does indeed correlate with the Tucki detachment and that this detachment tracks up the flank and over the top of the Panamint Range, just like the Tucki detachment tracks over the top of Tucki Mountain. This is illustrated in Figure 8, a cross section across the Panamint Range near Trail Canyon. This cross section, like Figure 6, uses topographic data from the DEM and outcrop geology from Workman et al. (2002), in this case supplemented from the map of Hunt and Mabey (1966). Bedding dips are from Hunt and Mabey (1966). In this figure, the detachment surface is shown schematically tracking across the top of the Panamint Range, rather than underneath it. It lies underneath the extensional allochthons on the east side of the range, surfacing along the west side of these allochthons in the east-dipping Harrisburg fault zone mapped by Hodges et al. (1990). This interpretation is similar to one shown by Hunt and Mabey (1996) in a cross section near Hanaupah Canyon (their fig. 108). As noted above, Late Proterozoic rocks make up the crest of the Panamint Range. These consist of sediments of the Pahrump Group, Noonday Dolomite, and Johnnie Formation, along with some outcrops of Proterozoic basement (Fig. 4; the Noonday Dolomite is included in the Johnnie Formation in this digital compilation), with the highest peak of the range at Telescope Peak consisting of argillites of the Johnnie Formation (Albee et al., 1981). As shown in Figure 7, these rocks are found in the footwall of detachments elsewhere in the Death Valley region and their greenschist-grade metamorphism in the Panamint Range (Labotka and Albee, 1990) is also consistent with deep burial and subsequent exhumation like at these other detachments. As Figure 4 shows, outcrops of Paleozoic sediments along the eastern side of the range are continuous from Tucki Mountain southward along the eastern slope of the Panamints, and these Paleozoic sediments are interpreted to be Basin and Range–style extensional allochthons. Further justification for the detachment geometry shown in Figure 7 comes from the southwestern flanks of the Panamint Range. Here, detachment faults that dip to the west, like the Emigrant fault at Tucki Mountain, have been mapped by Cichanski (2000). So both flanks of the Panamint Range display fault geometries that indicate detachment surfaces tracking over the top of the range, rather than underneath it.

The structural connection between the Panamint Range and Black Mountains is illustrated in Figure 9. This cross section includes the thickness of Cenozoic sediment fill as determined by Blakely and Ponce (2002; Fig. 4). Basement geometry below Death Valley shows a steep-sided basin nearly 5000 m deep. The dip on the eastern side of the basin is 50°. This matches well with fault dips of 45°–54° mapped by Miller (1991) along the base of the Badwater turtleback, so it is logical to assume that the eastern flank of the basin is a high-angle fault. The west side of the basin shows a steep segment bordering the main basin and a gently dipping surface that joins to the Panamint Range. The dip of the steeper portion of the west side of the basin is ∼30°, and the surface projection of this portion is close to faults at the base of the alluvial fans on the west side of Death Valley (Hunt and Mabey, 1966; USGS Quaternary faults database), again suggesting that the basin is fault-bounded, as shown in Figure 9. Along this profile, then, Death Valley is an asymmetric graben with a steep (50°) eastern fault and less steep (30°) western fault. This fault pattern is not consistent with a low-angle detachment fault underlying Death Valley that would link the Black Mountains and Panamint Range, with the Black Mountains the footwall of the detachment and the Panamints the hanging wall. Also not consistent with this model is the fact that the Panamint Range is much higher in elevation (highest point is Telescope Peak, 3368 m) than the Black Mountains (highest point is Funeral Peak, 1946 m); this would mean that the hanging wall of the detachment would be higher than the footwall, a structurally unlikely scenario.

The Black Mountains display an asymmetric east-tilted geometry that is typical of a rift-flank uplift (Watts, 2001). The Panamint Range is more symmetric, with perhaps a slightly steeper western flank. Along the profile of Figure 9, the west flank of the range averages a 10° dip and the east flank 9.5°. There does not appear to be a high-angle fault along the west side of the Panamints. Basement mapped by Blakely and Ponce (2002) shows only a shallow basin in Panamint Valley, west of the Panamint Range. As mentioned above in the discussion on Tucki Mountain, the west side of Tucki Mountain is today a low-angle fault. Further south, a strike-slip fault has been mapped by Zhang et al. (1990) along the west side of the range, but the dip of this fault is not known. Cichanski (2000) mapped the southern portion of the range front and found generally low-angle faults (dips 15°–34°), with indications that these fault surfaces have been cut by steeper faults, but the vertical offset on these faults is minor. Cichanski (2000) suggests that the higher-angle faults are part of the Panamint Valley strike-slip fault.

Although the cross section in Figure 9 shows a simple graben structure, the three-dimensional basement structure of Death Valley is complex. Blakely et al. (1999), using an earlier version of the gravity-derived sediment thickness map of Blakely and Ponce (2002), showed a system of northwest-striking strike-slip faults and northeast-striking normal faults that is illustrated in Figure 5. Blakely et al. (1999) relate this fault system to the pull-apart fault system that Burchfiel and Stewart (1966) postulated in their model for formation of Death Valley. Pull-apart basins are characterized by steeply dipping bounding faults (e.g., the Dead Sea graben, Garfunkel and Ben-Avraham, 1996). The steeply dipping faults seen in Figure 9 make interpretation of Death Valley as a pull-apart basin more realistic than the low-angle fault interpretation.

The age of fill in Death Valley is not directly observed. Hunt and Mabey (1966) summarize data from three borax exploration wells, the deepest reaching 300 m, drilled in Death Valley. From these data, Hunt and Mabey (1966) “assume that about a third of the fill [of Death Valley] is Quaternary and that the rest is Tertiary” (Hunt and Mabey, 1966, p. A72). Using this very rough estimate of 1.81 Ma (base of Quaternary) for a third of the section and assuming a constant sedimentation rate, the base of the section would be at 5.4 Ma. A more recent drill hole near Badwater reached a total depth of 187 m (Lowenstein et al., 1999). Sediment age at the base of this well was estimated by Lowenstein et al. (1999) as 200 ± 10 kA. Extrapolating this age to the 4.7 km maximum depth in the Badwater Basin (Blakely and Ponce, 2002) yields an age of 4.8–5.3 Ma for the base of the section. These age estimates are very uncertain, but do indicate that the bulk of the fill of Death Valley could be Pliocene and younger, i.e., post–Basin and Range extension (Fig. 3).

FORMATION OF DEATH VALLEY

This model, using the preceding data and interpretations, includes two tectonic phases separated by a short interval of relative tectonic quiescence, as summarized in the timing chart in Figure 3. The first phase, lasting from ca. 18 to 5 Ma, involved Basin and Range–style low-angle extension with fault blocks made up predominantly of Paleozoic sediments moving on detachment faults that exhumed rocks from mid crustal levels. The second phase was the development of Death Valley as a pull-apart basin, with higher-angle faults and transtensional motion.

Figure 10 is an illustration of the evolution of Death Valley. Figure 10A, “Present day,” shows the geometry of the Basin and Range detachment surface. This surface tracks over the top of the Panamint Range, with extensional allochthons consisting of an Early Paleozoic section perched on the eastern flank of the range. In the Black Mountains, the detachment surface tracks over the top of the Badwater turtleback and is covered by Late Miocene to Pliocene sediments of the Artist Drive Formation. The 3 Ma restoration is shown in two phases for convenience. The first restoration at 3 Ma removes the graben formed as a result of pull-apart basin formation; strike slip, which would be perpendicular to the page (Topping, 1993), is not incorporated. The concept illustrated here is that faulting due to strike slip broke and separated the original Basin and Range detachment surface. In the second 3 Ma restoration, the topography that formed as a result of young transtensional strike slip is removed. The suggestion illustrated here is that the Black Mountains, with their present-day asymmetric structure typical of footwall uplift geometry, formed as a result of footwall uplift associated with the Death Valley pull-apart graben. As discussed earlier, the present-day contact between the Artist Drive Formation and basement of the Badwater surface is faulted, but the geometry shown in this restoration shows that the Artist Drive Formation could have been originally deposited on basement, with faulting seen today a result of postdeposition emergence of the Badwater turtleback. As also discussed earlier, the Panamint Range is a large, almost symmetric anticlinal structure, with no high-angle faults along the western side of the range. Dips of the flanks of the Panamint Range are shallow, averaging ∼10°. Tucki Mountain at the north end of the range (Fig. 6) is an antiformal structure formed by arching of the exposed detachment surface; I suggest that this arching also forms the rest of the Panamint Range, making the entire range a core complex. The scale of this core complex, 30–40 km wide by 60 km long with an amplitude of ∼4 km, is larger than other well-known core complexes like the Snake Range and Whipple core complexes (Block and Royden, 1990). Uplift history of the range is uncertain with present data. Rocks now in the crest of the range were metamorphosed to greenschist facies, representing ∼10 km burial, in the Jurassic, with retrograde metamorphism associated with granitic intrusions in the Cretaceous (Labotka and Albee, 1990). Extension along the Harrisburg fault (HF, Fig. 4) commenced prior to intrusion of the 10.6 Ma Little Chief stock, which was intruded to ∼3 km below the contemporary surface. This implies ∼7 km of unroofing between the Cretaceous and 10.6 Ma, with some of this unroofing due to the extension (Labotka and Albee, 1990). Lack of preserved basinal section equivalent to the Black Mountains’ Artist Drive Formation and younger formations across the top of the Panamint Range suggests that the range was a high by 8 Ma, although the northwestern flanks were low enough at 10 Ma to begin accumulation of Nova Basin sediments (Fig. 3).

TURTLEBACK STRUCTURES IN THE BLACK MOUNTAINS

The three turtleback structures in the Black Mountains, as pointed out by Miller and Pavlis (2005), are keys to understanding tectonic evolution of the area. Miller and Pavlis (2005) give an excellent overview of current data and understanding of the turtlebacks. In summary, the turtlebacks are antiformal structures that strike and dip toward the northwest, exposed along the range front bordering Death Valley. They are made up of Proterozoic basement gneiss, early Tertiary pegmatitic intrusives, and Late Miocene plutonic rocks. The Late Miocene intrusives consist of the mafic Willow Springs pluton and the felsic Smith Mountain granite. A feature of these intrusives is that they are intruded along the structural top of each turtleback, with the Willow Springs diorite structurally beneath the younger (Fig. 3) Smith Mountain granite. As shown in the tectonostratigraphic summary of Figure 3, the Miocene Black Mountain intrusives were formed during Basin and Range extension, as indicated by their ages relative to age ranges of exhumation associated with extensional tectonic denudation. Toward the end of this episode of tectonic denudation, deposition of the predominantly volcanic Artist Drive Formation began. In Pliocene time, the Furnace Creek and Funeral Formations were deposited in the northern Black Mountains, these sections being predominantly clastic with little evidence for syndepositional tectonism. At the same time and further south along the Black Mountains, the Copper Canyon and Mormon Point Formations were deposited. All these formations were deposited in basins formed by Basin and Range low-angle extension; in the model presented here, the floor of these basins was in some areas the exhumed Basin and Range detachment surface. When the Death Valley pull-apart basin started to form ca. 3 Ma, these basins were uplifted in the footwall of the Black Mountains fault block, which is why they now lie high above the valley floor.

With the northwest strike of the turtleback structures, parallel to the inferred direction of strike slip, an origin of these features as a result of the strike slip is suggested. Wright et al. (1974) showed that the turtlebacks include many linear structures parallel to the direction of strike slip, ranging from “minute slickenslides to fault mullions tens or hundreds of meters in amplitude” (Wright et al., 1974, p. 54). The slickenslides are on the southwest flanks of the turtleback surfaces and trend northwest with a plunge of 10° to 15° (Troxel and Wright, 1987); these structures imply both normal and strike-slip components of motion along the flanks of the turtlebacks. Wright et al. (1974) do not suggest that the turtlebacks themselves are mullion structures, but a comparison with oceanic megamullions suggests that this is what they are. Megamullions in oceanic crust have been mapped since the development of multibeam bathymetry in the 1980s. The term is applied to large domed structures formed in inside-corner settings at the intersections of spreading centers and transform faults (Tucholke and Lin, 1998). The structures are usually corrugated with mullion structures parallel to spreading direction and Tucholke and Lin (1998) relate them to continental metamorphic core complexes. Tucholke and Lin (1998) compiled dimensional data on 17 oceanic megamullions. A simple average of their compilation yields dimensions of 15.2 km wide by 1550 m relief. In Figure 11 a sinusoidal function with these dimensions is included in the cross section across Badwater. It is intriguing that the ocean megamullion matches fairly well with the shape of the basement surface of the Badwater turtleback. In map view, Figure 5, the turtlebacks are seen to be adjacent to inferred strike-slip faults and in an “inside corner” position similar to that of oceanic megamullions (Tucholke and Lin, 1998). Based on these comparisons, I suggest that the turtlebacks are megamullions formed as a result of strike-slip faulting in Death Valley. This means that the structures are young, created in the last 3 m.y., although the rocks that make up the turtleback megamullions formed the footwall to the Basin and Range extensional system and were tectonically exhumed in the Miocene. The turtlebacks are therefore formed of rocks that were exhumed due to extension and at the same time were intruded by a variety of igneous rocks. They were then cut by strike-slip faults, forming the flanks of the Death Valley pull-apart basin. They also lie on the flank of a Proterozoic rift basin filled with Pahrump Group sediments, as mapped by Wright et al. (1976). This multiphase history of the structures plus their location along a preexisting rift may explain why the turtlebacks are unique features of the western North America extensional province.

CONCLUSIONS

The interpretations presented above offer some new ideas for the evolution of Death Valley. The Panamint Range, rather than being a large fault block detached from above the Black Mountains, is a core complex like the northern end of the range at Tucki Mountain. Basin and Range extensional allochthons occur along the east flank of the range; these formed during Miocene–Early Pliocene Basin and Range extension. Death Valley is a pull-apart basin, as originally proposed by Burchfiel and Stewart (1966), formed in the last 3 m.y. Prior to this time, a thick sedimentary sequence was deposited in what is now the northern Black Mountains, with early sedimentation being mostly volcanics. These volcanics were deposited partly on the exhumed detachment surface from the earlier phase of Basin and Range extension. The turtlebacks are megamullions very similar to megamullions adjacent to seafloor spreading centers. The turtleback structures formed in the last 3 m.y., although the rocks making up the turtlebacks were exhumed from mid-crustal levels during Basin and Range extension. Fault dips along the southwest flanks of the turtlebacks are steep, e.g., 50° at Badwater as shown in Figure 9.

The Death Valley region has played a key role in development of models of Basin and Range extension. One of the data points used in structural restorations comes from a thrust complex, preserved in one of the Basin and Range allochthons in the northern Panamint Range, which correlates with a thrust complex of similar geometry in the Nopah Range (Snow and Wernicke, 2000). In Snow and Wernicke's (2000) reconstruction, this correlation is used to restore the Panamint Range to a position adjacent to the Nopah Range. The suggestion in this paper that the Panamints are not a far-traveled extensional allochthon means that this reconstruction must be reexamined. The correlation between thrust complexes is still valid, but instead of the entire Panamint Range being moved, only the extensional allochthons need to be moved. Further implications of this update will be discussed elsewhere.

The change from Basin and Range extension to strike slip between 8 and 3 Ma seen in the Death Valley region is very close to age ranges of changes in similar tectonic settings seen in several other localities in the ECSZ (Stockli et al., 2003; Faulds et al., 2005; Oldow et al., 2008; Lee et al., 2009; Andrew and Walker, 2009). It also coincides with final opening of the northern Gulf of California and initiation of the southern portion of the San Andreas fault (Dorsey et al., 2007). The strong link between Pacific-North America plate motion and structural evolution of the ECSZ is emphasized by this synchronous timing. It would appear that, as the southern portion of the San Andreas fault joined up to the northern Gulf of California, the large-scale restraining bend in the San Andreas fault formed by the Mojave Block (Fig. 1) developed and has forced deformation to the east, into the ECSZ. This eastward propagation is likely to continue with the Pacific-North America plate boundary ultimately moving to the ECSZ, with motion on the San Andreas fault ceasing (Faulds et al., 2005).

This paper was sponsored by the PLATES project at UTIG. Dave Reynolds, Larry Lawver, and especially an anonymous reviewer provided excellent suggestions for improving the manuscript. I would like to thank Dave Reynolds and Stefan Boettcher for introducing me to Death Valley geology.