Two phases of deformation are needed to describe the Cenozoic tectonic evolution of the Pahrump and Mesquite basins in the southern Great Basin and eastern Mojave Desert, United States. By interpreting seismic reflection and gravity observations along with bedrock and surficial mapping, we infer an extensional phase of basin formation followed by a transtensional phase, in this area straddling the border of southern Nevada and southeastern California. We reprocessed ∼220 line km of industry seismic reflection data from the Pahrump and Mesquite Valleys to emphasize reflections in the basin fill, and combined these results with analysis of gravity data. The seismic lines portray the complex geometry of the Stateline fault system, a major Neogene dextral strike-slip system that passes through these valleys, and provide evidence for multiple ages of faulting along structures that bound the Pahrump basin. Locally thick sequences of preextensional Tertiary sedimentary rocks are cut by large-offset, relatively high-angle normal faults that record a phase of extensional basin formation that preceded transtension. The existence of preextensional basins beneath the Pahrump and Mesquite Valleys bears on tectonic reconstruction of the region and suggests that tilted ranges blocks to the west of these valleys need not restore to positions immediately adjacent to the Spring Mountains to the east. Subsequent dextral offset on the Stateline fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fill and bedrock density, and they confirm that the Paleozoic outcrop of Black Butte, a topographic high separating the Pahrump and Mesquite Valleys, is unrooted to underlying bedrock.
The Pahrump and Mesquite Valleys (western United States) and the Cenozoic basins that underlie them (Fig. 1) are at a transition between domains characterized by predominant extension in the Basin and Range province and transtension in the eastern California shear zone (Dixon et al., 2003; Wesnousky, 2005; Guest et al., 2007). The geologic setting of these basins and the surrounding region is complex, consisting of Proterozoic to Mesozoic rocks deformed in a Mesozoic thrust belt that was disrupted by Cenozoic extensional and strike-slip faulting and concomitant sedimentation (Armstrong, 1968; Wernicke et al., 1988; Caskey and Schweickert, 1992; Serpa and Pavlis, 1996; Snow and Wernicke, 2000). Cenozoic deformation of the region surrounding the Pahrump and Mesquite Valleys is characterized by a variety of structural patterns that overlap in space and time, including local extreme extension along detachment faults that currently have gentle dips, vertical-axis and horizontal-axis rotation of major range blocks, and development of discrete strike-slip faults and transtensional basins in the Walker Lane belt and eastern California shear zone (Serpa and Pavlis, 1996; Wright et al., 1999; Guest et al., 2003; Miller and Pavlis, 2005). The relative magnitudes of late Cenozoic extensional, strike-slip, and transtensional deformation in the region have been controversial, and the role of each of these deformation patterns on the evolution of the Pahrump and Mesquite basins has not been fully explored.
Relatively continuous thrust faults exposed to the east and northeast of the Pahrump and Mesquite Valleys represent the southern end of the Mesozoic Cordilleran fold-and-thrust belt (Armstrong, 1968; Allmendinger, 1992) (Fig. 2). These thrusts are preserved in the Spring Mountains, a range that has been relatively undisturbed by subsequent Cenozoic tectonism. In contrast, thrust faults are also exposed in ranges to the northwest, west, and southwest of the Pahrump and Mesquite Valleys, but these thrust faults are exposed as discontinuous slivers within tilted range blocks (Fig. 2) as a result of Cenozoic tectonism (e.g., Wernicke et al., 1988; Serpa and Pavlis, 1996). As such, the Pahrump and Mesquite basins occupy a transition zone between relatively unextended and highly extended crust.
Regional tectonic models that call on large-magnitude extension infer that the once-continuous Mesozoic thrust belt was disrupted by major, low-angle normal faults associated with a regional detachment surface (Stewart, 1983; Wernicke et al., 1988; Davis et al., 1993; Fowler and Calzia, 1999; Snow and Wernicke, 2000; McQuarrie and Wernicke, 2005). In these models, large-scale offset along the detachment surface is inferred to be the cause of the opening of the basins and tilting of major range blocks, such as the Nopah and Resting Spring Ranges to the west of the Pahrump Valley (Fig. 1). Restoration of detachment fault offset was interpreted to have the effect of closing the Pahrump basin and restoring the Nopah Range to a position immediately west of the present Spring Mountains (Wernicke et al., 1988; Snow and Wernicke, 2000). Such a restoration would imply that basin-fill deposits would be synchronous with, or postdate, the regional extension.
Gravity maps and analyses indicate that the Pahrump and Mesquite Valleys are underlain by deep, steep-sided basins (Bates, 1965; Hoffard, 1991; Blakely et al., 1998, 1999; Langenheim et al., 2005). The Mesquite basin appears to be a single deep depression, whereas the Pahrump basin appears to consist of two deep basins separated by a buried ridge along the axis of the Pahrump Valley. Both valleys are traversed by the northwest-southeast–oriented Stateline fault system (e.g., Guest et al., 2007), a major dextral strike-slip system at the northeastern edge of the eastern California shear zone. Basin formation in the Pahrump and Mesquite Valleys has been interpreted as the result of oblique pull apart related to dextral slip along the Stateline fault system (Liggett and Childs, 1973; Wright, 1989; Hoffard, 1991; Schweickert and Lahren, 1997; Blakely et al., 1998, 1999).
Some regional reconstructions have been proposed that require smaller amounts of extension and emphasize the role of strike-slip faults in accommodating regional strain (Serpa and Pavlis, 1996; Wright et al., 1999; Miller and Pavlis, 2005). To the west of the study area in the vicinity of Death Valley, ranges that expose detachment faults and metamorphic core complexes are separated by major strike-slip faults such as the Furnace Creek fault and the Death Valley fault system. Upper-plate transport directions and lower-plate stretching lineations in the core complexes trend northwest, parallel to the bounding strike-slip faults. Wright et al. (1991, 1999) suggested that the 4-km-thick sequence of middle Miocene–Pliocene sedimentary and volcanic rock package of the Furnace Creek basin was deposited in a tectonic setting where both strike-slip faults and the extensional detachments may have evolved coevally, as the result of northwest-directed extension. The regional tectonic models of Serpa and Pavlis (1996) and Guest et al. (2003) emphasize the role that strike-slip faults and transtension may play in accommodating regional strain and also call upon multiple extensional faults that need not be linked to an underlying master regional detachment surface. However, none of these conceptualizations has explicitly addressed the relative role of regional extension, transtension, and dextral strike-slip offset in the Pahrump and Mesquite Valleys.
Our study of the Pahrump and Mesquite basins uses seismic reflection (Fig. 3) and gravity (Fig. 4) observations to shed light on the relative importance of extensional versus strike-slip faulting in the opening and filling of these basins. Seismic reflection observations illuminate sequences of basin-fill units that record the style of deposition and deformation of these basins through time along individual seismic profiles. Gravity observations can define the gross basin geometry, which is especially helpful in tying widely spaced seismic reflection lines to one another. Geophysical methods are needed to define the basin geometry and history of this area because there are only very limited surface exposures of Miocene and older rocks within these basins, no deep wells to constrain basin stratigraphy, and few dated units and structures to constrain the history of basin formation.
Basin depths derived from analyses of basin gravity anomalies are uncertain because the size of a modeled basin trades off with the density contrast of its infill material with respect to surrounding rocks: for a given basin gravity anomaly, as the density contrast decreases toward zero, basin depths must increase greatly. In addition, short-wavelength variations of deep basin floors cannot be constrained by gravity measured at the land surface because gravity anomalies are progressively smoothed for deeper and deeper sources. Basin depths derived from seismic reflection analysis are also uncertain, but the degree of uncertainty is less than in the gravity case, because seismic depths are a function of seismic velocity, which has a smaller relative uncertainty than does density contrast. Our analysis of basin depths proceeded in an iterative fashion: we adjusted basin-floor picks and basin-fill densities until we achieved a model that fit both seismic and gravity observations adequately. In addition, by defining sedimentary packages in the basins from the seismic sections and by assigning them separate densities, we relax a common assumption that density of the basin fill varies only with depth and not laterally, and we thus generate geophysical models that are more consistent with both seismic and gravity observations.
The Pahrump Valley covers an area of ∼1800 km2 and is surrounded by the Spring Mountains to the east and north, the Montgomery Mountains (Burchfiel et al., 1982, 1983) to the northwest, the Nopah Range to the west, and the Kingston Range to the south (Fig. 1). The Mesquite Valley to the southeast covers ∼450 km2 and is bordered by the Mesquite Mountains to the west, the southernmost Spring Mountains to the east, and the northern Clark Mountains to the south. The two valleys are both topographically closed and thus have no surface-water drainage outlet, and they are separated from each other by low topographic divide that includes the small topographic high named Black Butte (Fig. 1).
The ranges that surround the Pahrump and Mesquite Valleys are composed of Late Proterozoic, Paleozoic, and minor Mesozoic sedimentary rocks (Fig. 2; Longwell et al., 1965; Burchfiel et al., 1974, 1982, 1983; Page et al., 2005). In addition, the Kingston Range contains a large middle Miocene (12.4 Ma) granitic pluton (Davis et al., 1993; Calzia et al., 2000).
Cenozoic rocks older than Pliocene are rarely exposed in the valleys or along their margins. Between the northern Nopah Range and the Kingston Range, on the west side of the Pahrump Valley, a northeast-dipping Pliocene–Miocene (Workman et al., 2002a) sedimentary section is exposed (unit Mio; Fig. 2); it is several hundred meters thick and consists primarily of sandstone with minor interbedded conglomerate and limestone. A second occurrence of this unit is in an anticlinal fold along the Stateline fault system, just to the northwest of Black Butte (Fig. 2). A northwest-aligned chain of hills near the center of the Pahrump basin, and along the trace of the inferred Stateline fault system, is formed by outcrops of northeast-dipping alluvial gravels (unit Plio; Fig. 2) that are the oldest units in the central part of the basin (Lundstrom et al., 2003). These units were described by Lundstrom et al. (2003) as early Pleistocene? to late Miocene; McMackin (1999) reported an Ar/Ar date of 10.76 Ma from a lapilli tuff in correlative units farther south along this line of hills.
Surface geologic maps and drill-hole data show that the uppermost basin fill consists of unconsolidated coarse-grained alluvial materials that were deposited near the sides of the valleys, and fine-grained playa, spring, and associated marsh or wetland deposits in the central parts of the valleys (Malmberg, 1967; Lundstrom et al., 2003; Page et al., 2005; Schmidt and McMackin, 2006; Sweetkind et al., 2003). The northeastern side of the Pahrump basin is characterized by large alluvial fans, the most prominent of which are the coalesced Pahrump and Manse fans (Malmberg, 1967; Harrill, 1986).
The Stateline fault was defined by Hewett (1956) on the Ivanpah quadrangle to the southeast of the study area. Following Guest et al. (2007), we refer to the northwest continuation of this fault system through and beyond our study area as the Stateline fault system; this continuation has been given various names, such as the Pahrump Valley fault zone, by other investigators. Guest et al. (2007) provided a summary of names used in the literature for this fault system. The Stateline fault system has right-lateral strike-slip offset on the Ivanpah quadrangle, based on bedrock offsets (Hewett, 1956). The fault has been inferred to have 20 and 30 km of right-lateral offset in the Pahrump Valley and in the Amargosa Desert, respectively, immediately to the northwest of the study area, based on offset Proterozoic and Paleozoic rocks (Stewart et al., 1968), interpreted correlations of thrust sheets (Snow, 1992; Snow and Wernicke, 2000), and offsets in regional facies trends (Stevens et al., 1991; Cooper et al., 1982). Neogene sedimentary deposits almost everywhere cover the bedrock expression of the fault zone in the study area (Hoffard, 1991; Workman et al., 2002a; Schmidt and McMackin, 2006). The fault zone is exposed in bedrock only in the southern Montgomery Mountains, where it juxtaposes parts of the Late Proterozoic Johnnie Formation and Stirling Quartzite (Burchfiel et al., 1983), and at Stateline Pass southeast of the Mesquite Valley (Hewett, 1956; Fig. 2).
Scarps and lineaments within young alluvial deposits define a zone of late Quaternary deformation associated with the Stateline fault system in the Pahrump Valley (Hoffard, 1991; Piety, 1996; Anderson et al., 1995). Many of these features parallel and partially bound exposures of early Pleistocene? to late Miocene alluvium (Lundstrom et al., 2003) that are the oldest exposed units in the central part of the basin. Extensive paleospring discharge deposits are localized northeast of the zone of faulting, presumably the result of local damming of groundwater moving southwest from recharge sources in the Spring Mountains (Quade et al., 1995).
Low-density, unconsolidated to semiconsolidated basin-fill deposits are juxtaposed against high-density bedrock units beneath many of the valleys in the Great Basin, and this configuration produces significant gravity anomalies that can be used to infer the shape of the basins. Basin-fill deposits are often bedded, with seismic impedance contrasts that allow them to be imaged with seismic reflection surveys. By combining gravity analysis, which allows estimation of the volume and shape of basin-fill deposits, with seismic reflection analysis, which illuminates the depositional and deformational architecture, we gain insights into the history and present configuration of these basins that complement insights from surface mapping.
Prior gravity studies of the Pahrump and Mesquite Valleys (Bates, 1965; MIT Field Geophysics Course, 1985; Blakely et al., 1998, 1999; Langenheim et al., 2005) defined the underlying basins as having relatively steep sided edges bounding range-front pediments. Aeromagnetic data across these valleys show only small anomalies (Langenheim et al., 2005) and are not as useful for defining subsurface structures as in other areas where volcanic or Precambrian basement units are shallow. The geophysical study by the MIT Field Geophysics Course (1985) collected gravity, seismic refraction (Fig. A1), and magnetotelluric observations, which were used to develop a model of the geometry of the Mesquite basin and to test the hypothesis that Black Butte is an allochthonous feature. With regards to Black Butte, they concluded that the bedrock outcrop is separated from underlying basement by several hundred meters.
SEISMIC REFLECTION DATA AND PROCESSING
The seismic reflection data consist of seven multichannel lines purchased by the U.S. Geological Survey (USGS) from Geophysical Pursuit, Inc., in 2002. The data were originally acquired in February–May 1980 by Seisdata Services for Anschutz Petroleum. The lines form a loose network covering a large part of the Pahrump and Mesquite basins (Fig. 3; labeled SSN). In total, 221 line km of data were purchased. The acquisition agreement of these data by the USGS included limited publication rights. Portions of these lines were interpreted by Hoffard (1991). Hoffard's (1991) interpretations were limited by two factors: the display of the sections was in time rather than in depth, and data from the upper 1 s of two-way traveltime were omitted (corresponding to the uppermost ∼1.2 km of the basin fill, using typical seismic velocities) by the data sharing agreement at the time.
The seismic data were acquired by the USGS to obtain constraints on the thickness of the Neogene basin fill to aid in the interpretation of gravity anomalies, and to image reflections within the upper part of the basin fill to investigate structure and basin-filling processes. The seismic reprocessing was conducted to emphasize reflections in the shallow section and to convert the seismic features into depth coordinates.
In the Appendix we describe the seismic reflection processing steps, present the seismic depth sections with and without interpretations superimposed, and provide general descriptions of the reflection lines and our interpretations of reflections in terms of stratigraphic and structural features. In the Results section we describe aspects of the seismic reflection observations that relate to the gravity models and to the formation of the Pahrump and Mesquite basins.
GRAVITY DATA AND ANALYSIS
Gravity data within the study area (Fig. 3) include ∼2500 stations collected on both basin fill (∼80%) and bedrock (∼20%). Gravity calculations were performed in a region extending at least 50 km beyond the study area boundaries, to displace edge artifacts away from the gravity effects of the Pahrump and Mesquite basins. In this broader calculation region, an order of magnitude greater number of gravity stations were utilized, many within the Nevada Test Site, the southern border of which is ∼20 km north of the study area. Gravity data were obtained from the Nevada statewide compilation of Ponce (1997), supplemented by other surveys (e.g., Blakely et al., 1998; Morin et al., 1999; Langenheim et al., 1999).
The average gravity station spacing across the floors of the Pahrump and Mesquite Valleys is 1.6 km, with denser coverage along many roadways and less-dense coverage in undeveloped areas. In the surrounding ranges, gravity coverage is sparse, with stations concentrated along roads and tracks. We used the same gravity stations as in the studies of Blakely et al. (1998, 1999) and Langenheim et al. (2005), with the addition of 71 stations in Stewart Valley (R. Blakely, 2008, personal commun.) and with the omission of ∼10 stations that appeared to be in error because either their tabulated elevations differed greatly from corresponding digital elevation model elevations, or their gravity values differed significantly from those of adjacent stations.
The gravity data were reduced to isostatic residual gravity anomalies using a series of calculations accounting for the predictable gravity effects of the global gravity field, the reduction in gravity with increasing elevation (free-air correction), the effect of mass between the station and the geoid (simple Bouguer correction), the effect of topographic variation near the station (terrain correction), and the effect of mass near the base of the crust that compensates topographic loads (isostatic correction). The density of the rocks forming the topography is assumed to be 2670 kg/m3 (2.67 g/cm3). For the isostatic correction, the crust is assumed to be 25 km thick where the ground elevation is at sea level, and a density contrast from crust to mantle is assumed to be 400 kg/m3 (0.4 g/cm3). See Blakely et al. (1998) and Langenheim et al. (1999) for a complete description of these gravity reduction steps. The final gravity anomaly after application of these effects is termed the isostatic residual gravity anomaly and is useful for interpretation because it primarily reflects the density variations in the upper crust and mid-crust (Simpson et al., 1986).
Isostatic residual gravity values from station data were gridded with a 500 m spacing, which is somewhat finer than the average station spacing in the valleys and significantly finer that the spacing in the ranges. Isostatic gravity anomalies were analyzed using two approaches: a grid-based inversion technique (Jachens and Moring, 1990) to estimate the depth variation of Cenozoic basins across the study area, and a forward-model technique (Talwani et al., 1959) to analyze gravity variations along select seismic reflection lines.
The depth-to-basement inversion technique endeavors (1) to separate contributions to the isostatic residual gravity anomaly that arise from Cenozoic deposits and from pre-Cenozoic rocks and (2) to convert the low-density contributions from the Cenozoic deposits into a model of basin depth (Jachens and Moring, 1990; Saltus and Jachens, 1995). This is an inverse geophysical approach because it solves for model geometry based on observations of gravity, constrained by outcrop patterns and by a priori assumptions about the density contrast of basin fill relative to surrounding rocks. This method is iterative and has been successfully applied to the entire Great Basin (Saltus and Jachens, 1995) and to individual basins and groups of basins in southern Nevada (e.g., Blakely et al., 1998). The depth-to-basement method first separates those gravity stations that are on Cenozoic deposits (termed basin) from those that are on pre-Cenozoic rocks. These pre-Cenozoic units are defined as “basement,” a usage that differs from a common description of crystalline rocks as basement in many areas. The isostatic residual gravity anomalies at basement stations are then interpolated across the intervening basins, and differences between the interpolated basement gravity values and those measured at basin stations are referred to as the basin gravity anomaly, which is attributed to the low-density basin fill. Using a one-dimensional approximation, the depth of the basin fill is estimated from the size of the basin gravity anomaly at each grid point, and then the gravitational attraction of these interpreted basins is calculated. Where a basement gravity station is close to basin material, some of the attraction of the low-density fill will influence its gravity value; thus, the calculated basin attraction must be removed from the gravity value at each basement station. This process yields estimates of basement gravity, basin gravity, and depth to basement beneath the basins. This analysis sequence is then repeated for multiple iterations with successively refined basement and basin gravity estimates, until the estimates of basin depth converge.
In this study gravity stations are classified by their placement on two generalized geological units, pre-Cenozoic basement and Cenozoic deposits. The distribution of these units is based on digitized polygons of the California and Nevada state geologic maps (Jennings, 1977; Stewart and Carlson, 1978). In many depth-to-basement inversions of areas across the Basin and Range, low-density Cenozoic deposits are divided into two groups, sedimentary and volcanic, with different density properties. In our study area, volcanic outcrops cover <1% of the study area; the volcanic units are thin, and their gravity effects are not measurable with the existing gravity station coverage. Thus, these units are bundled with the low-density Cenozoic sedimentary deposits in our depth-to-basement analysis. One important exception to the classification of pre-Cenozoic basement gravity stations is the omission from this group of the stations on Paleozoic carbonate outcrop on Black Butte; this exception represents the greatest difference between our depth-to-basement analysis and prior analyses in this area (Blakely et al., 1998; Langenheim et al., 2005). The reason for this exception is that unrealistic changes in basement and basin gravity are required by assuming that Black Butte is rooted to dense, pre-Cenozoic basement. An unrooted geometry for Black Butte was suggested by field mapping (Hewett, 1956) and field geophysics (MIT Field Geophysics Course, 1985) and is confirmed by our analyses.
A critical input to the depth-to-basement method is the depth variation of the density contrast of basin-fill material relative to surrounding basement. This density contrast profile is the link that allows conversion of basin gravity anomalies to basin depth estimates. Measured density-depth functions are available from gravity logging of deep boreholes in some parts of the Great Basin (e.g., Healey et al., 1984), but not from the study area. Density-depth functions may be estimated from velocities determined by seismic refraction or by borehole logging, but such logs are not available in the study area. Thus, for our analysis, we considered a variety of density-depth functions, including one that worked well in prior studies and other functions that incrementally modified the prior density contrasts, to explore the range of permissible gravity-derived models of basin depth.
In the depth-to-basement technique, independently measured depths to the base of Cenozoic deposits provide important constraints to the estimation of basin depths. These measurements are available typically from deep boreholes and from seismic reflection observations with velocity control. Because no oil and gas exploration wells are located on basin fill in the study area (Hess, 2004), deep borehole constraints are not available. Water wells are abundant in developed areas of the valleys, but they are shallow and do not penetrate bedrock, and thus contribute little to the gravity analysis. Nonetheless, lithologic data from well drillers’ logs (State of Nevada, Department of Conservation and Natural Resources, Division of Water Resources: http://water.nv.gov/engineering/wlog/wlog.cfm) were compiled for wells along each seismic line (see Appendix figures). The deepest wells along each line are 150–300 m deep. These wells penetrate depths corresponding to the uppermost reflections on the seismic lines, but mostly serve to emphasize the lack of deep control throughout the basins.
In this study we develop models of the basin structure that are consistent both with seismic reflections and with gravity observations. When comparing the seismic and gravity interpretations, we see discrepancies that are both small and large. Small differences between the gravity-derived depth-to-basement surface and that defined by seismic reflections may arise from the intrinsically lower resolution of the gravity inversion. This lower resolution stems from three factors: first, the gravity inversion is conducted using a 500 m grid spacing while the seismic data have a lateral resolution several times finer; second, the gravity observations generally have a greater and more irregular spacing than the gridded solution; and third, gravity anomalies due to sources at depth are smoother laterally than the underlying density structure (Blakely, 1995). Thus, basin depth results from gravity inversion provide a smoothed model of the actual basin depth, with increased smoothing at greater basin depths. Larger differences between the seismic and inverted gravity basin geometries appear to stem from uncertainties in our ability to separate basin fill from basement contributions to the isostatic residual gravity anomaly and from violations of the assumption of constant densities at given depths across the basin. The exposures of well-cemented Pleistocene and older sedimentary rocks near the center of the Pahrump Valley, surrounded by younger and less-indurated sediments, motivates further analysis of the gravity by relaxing the one-dimensional density contrast structure adopted for the depth-to-basement analysis. Limitations aside, gravity observations provide important constraints on interpolating basin structures between the widely spaced seismic reflection lines and on extrapolating them beyond the seismic survey.
The isostatic residual gravity field of the study area (Fig. 4) reflects the major Basin and Range physiography: positive anomalies are associated with the ranges and negative anomalies are associated with the basins. The exception to this relationship is the southern Kingston Range, which coincides with an isostatic residual gravity low created by low-density plutonic rocks intruded into a supradetachment Cenozoic basin (Davis et al., 1993; Calzia et al., 2000). Areas with positive gravity anomalies have densities in the shallow mid-crust that are generally greater than the 2670 kg/m3 gravity reduction density, and areas with negative gravity anomalies are associated with lower densities. The most positive anomalies, >15 mGal, correspond to the Spring Mountains and the Resting Spring Range, where dense Neoproterozoic siliciclastic rocks and dense lower Paleozoic carbonate rocks occur near the surface. The most negative anomalies, <–25 mGal, are beneath the Pahrump and Mesquite Valleys, and beneath the Shadow Valley west of the Clark Mountains. The variable magnitudes of positive and negative isostatic residual gravity anomalies among the ranges and the basins indicate both that basement densities vary across the study area and that basin-fill thickness and density vary among the basins. The greatest gradients of the isostatic residual gravity anomalies beneath the Pahrump and Mesquite Valleys are distinctly basinward of the adjacent range fronts, indicating that pediments covered by relatively thin sedimentary deposits surround the deep basins.
Using the depth-to-basement technique, we separate the isostatic residual gravity anomaly (Fig. 4) into basement (Fig. 5) and basin components (Fig. 6). Basement gravity varies smoothly across the study area, and its variation is largely independent of the geometry of the basins. In particular, no distinctive basement gravity signature is associated with Black Butte, a consequence of prescribing that Paleozoic rocks of Black Butte overlie a layer of low-density fill, as noted above. If these Paleozoic rocks were assumed to root to basement at depth, then an ∼10 mGal negative isostatic residual gravity anomaly would center on Black Butte, implying unreasonably low density basement rocks at that site. Basement gravity varies by >50 mGal, from a high at the center of Spring Mountains to a low in the Kingston Range and northern Clark Mountains (Fig. 5). Across the Pahrump and Mesquite Valleys, the basement gravity contours are generally oriented northwest-southeast. The basement gravity low beneath the Shadow Valley (Fig. 5) is not expected unless the valley is underlain by low-density material such as the Kingston Peak pluton. Alternatively, this may be a case where there are insufficient gravity data in the ranges surrounding the Shadow Valley to interpolate a valid basement gravity field across that basin.
The basin gravity field (Fig. 6) exhibits negative anomalies as great as −40 mGal, with most basins in excess of −15 mGal. The Shadow Valley has an unusually small gravity anomaly (>–10 mGal), which may reflect poor separation of basement from basin gravity components. Converting these basin gravity anomalies to depth yields the basin depth model (Fig. 7). Although basin depths were calculated across the study area and the surrounding calculation border, we have clipped the solution to the watershed areas for the Pahrump and Mesquite Valleys, where we have greatest confidence in the basin depth results. To test the suitability of this model, we calculated a residual gravity anomaly by subtracting from the input isostatic residual gravity field the combined output gravity effects of the basement gravity variations and of the basin depth model. Gravity residuals (Fig. 8) at the vast majority of stations have magnitudes <0.5 mGal (>91% of the basement gravity stations; >97% of the basin gravity stations), demonstrating the excellent model fit, given that the basin gravity anomalies range from −30 to −5 mGal (Fig. 6).
To convert the basin gravity anomaly into depth, we tested a number of basin-fill density-depth functions in our calculations. One function (Table 1; Fig. 9) was devised by Jachens and Moring (1990) and was deemed appropriate for the entire Great Basin (Saltus and Jachens, 1995); this function was also used by Blakely et al. (1998, 1999) and Langenheim et al. (2005) in study areas overlapping ours. We adopted a function with slightly lower basin-fill densities (50 kg/m3 lower at all depths; Table 1; Fig. 9) to generate modeled basin depths that are slightly shallower (∼10% on average) than those using the prior density-depth function; this provides better agreement as a whole with basin depths interpreted from the seismic reflection lines that cross the basins. The magnitude of gravity residuals at gravity stations (Fig. 8) does not vary significantly between models derived from the density-depth functions adopted in this and the prior studies; varying the densities by ∼100 kg/m3 about these functions does not yield significantly different gravity misfits, although the derived basin depths vary significantly. This behavior illustrates the intrinsic tradeoff between the density contrast and volume of viable sources of a given gravity anomaly.
The depth-to-basement results (Fig. 7) are similar in geometry to those of prior gravity studies (Blakely et al., 1998, 1999; Langenheim et al., 2005), with the exception of the behavior near Black Butte and at sites where gravity stations were in error. The Pahrump Valley is subdivided into two main subbasins, separated by shallow pre-Cenozoic basement beneath the California-Nevada state line, whereas the Mesquite Valley hosts a single basin. Much of the basin beneath the Pahrump Valley is thin (<500 m) and is interpreted to be buried pediment from the adjacent ranges; a similar but narrower pediment is northeast of the deep basin in the Mesquite Valley (Fig. 7).
The northeastern subbasin of the Pahrump Valley reaches the deepest depths, nearly 4 km, in the study area (Fig. 7), and the southwestern subbasin of the Pahrump Valley reaches only about half of that depth. The basin beneath the Mesquite Valley reaches nearly 3 km depth. The seismic lines that cross the valleys do not cross the deepest portions of the basins.
Seismic Interpretations of the Pahrump Valley Reflection Lines
A systematic presentation of the seismic reflection data and our interpretations for each line is contained in the Appendix. In this section, we summarize the interpretations from the Appendix that bear upon the Neogene tectonics of the Pahrump and Mesquite Valleys. The Appendix includes a full description of the interpretation of each section, including the criteria used for the selection of the tops and bottoms of each reflection package and for fault interpretation. In the following discussion, distances along the horizontal axes are relative either to the California-Nevada border (SSN-11, SSN-13, SSN-15, SSN-17, SSN-19) or to the SSN-19 crossing (SSN-10, SSN-12). We abbreviate the along-profile distances in kilometers as “kmAP” to distinguish these values from depths or relative distances.
Seismic lines SSN-19, SSN-17, and SSN-15 are oriented southwest-northeast and cross the northern, central, and south-central parts, respectively, of the Pahrump Valley (Figs. 2 and 3). As defined by the gravity inversion (Fig. 7), lines SSN-17 and SSN-15 cross the mid-basin high and the flanking southwestern and northeastern subbasins, whereas SSN-19 crosses the valley where bedrock is shallow to the northwest of both subbasins. Lines SSN-10 and SSN-12 are oriented northwest-southeast (Fig. 2). Line SSN-10 parallels the northeastern subbasin at the edge of the pediment flanking the western side of the Spring Mountains (Fig. 7); SSN-12 is nearly along the mid-basin high separating the subbasins. Neither SSN-17 nor SSN-15 crosses the deepest part of the northeastern subbasin as defined by the gravity inversion (Fig. 7). The thickness of basin fill interpreted from the seismic data (Fig. 10) is ∼2.5 km on line SSN-17 and ∼1.2 km on line SSN-15, both in general agreement with the depth-to-basement solution.
Nature of the Stateline Fault System
The mid-basin high associated with the Stateline fault system on seismic lines SSN-17, SSN-15, and SSN-19 is characterized by the termination of packages of reflections and abrupt changes in reflection amplitude and orientation (Fig. 10). Numerous small reflection offsets suggest that the Stateline fault system has the shallow multistrand nature of flower structures associated with strike-slip systems elsewhere.
On SSN-19 (Fig. 10A), the Stateline fault system is interpreted as an upward-splaying zone of faults between −0.5 and 2.5 kmAP. These faults appear to cut pre-Cenozoic rocks and the lower part of the Cenozoic sequence; no similar amount of offset is observed in the overlying sequence. In this vicinity, offset of known Quaternary faults is not resolved from the seismic data. On SSN-17, the Stateline fault system appears primarily as a single subvertical fault at 0.5 kmAP; this fault may splay upward into faults with gentler dips (Fig. 10B). On SSN-15, the mid-basin high along the Stateline fault system is broader and more complex than on SSN-17, taking the form of a broad, faulted antiform (Fig. 10C). The western side of the mid-basin high at ∼−1 kmAP is relatively steep and the interpreted top of pre-Cenozoic rocks has as much as 1.5 km of structural relief, whereas the eastern margin is more diffuse and is defined by several faults.
Juxtaposition of Basins of Different Character
The Stateline fault system appears to juxtapose basins that have very different character, supporting previous interpretations that the Stateline fault system has significant amount of strike-slip offset. In her analysis of the northeastern and southwestern subbasins in the Pahrump Valley, Hoffard (1991, p. 111) stated that these were two parts of a once continuous basin that had been split by the mid-basin strike-slip fault. However, on both SSN-17 and SSN-15, reflection packages in the southwestern subbasin differ in number, amplitude, thickness, and overall reflectivity character from packages in the northeastern subbasin; they do not appear to correlate across the mid-basin high (Fig. 10). On lines SSN-15 and SSN-17, the southwestern subbasin is characterized by strong reflections near the top of the sections and weaker, less continuous reflections deeper in the subbasin, which is generally the inverse of the patterns seen in the northeastern subbasin, where stronger reflections dominate the deeper part of the section. Thus, the northeastern and southwestern subbasins in the Pahrump Valley appear to be two separate basins that record different histories and that are now juxtaposed by offset along the Stateline fault system.
Defining the Base of the Cenozoic Basins
One of the principal requirements and challenges of the seismic interpretation was to interpret, on the basis of seismic reflections, the base of the Cenozoic section in order to compare with the gravity-derived depth-to-basement solution. Seismic reflections interpreted as arising within Cenozoic basin fill are generally continuous, subparallel, relatively high amplitude reflections, especially in the shallow parts of the seismic images near basin centers (reflection packages 1 and 3, SSN-15; Fig. 10C; reflection packages 3, SSN-11; Fig. 11B). Deeper parts of the inferred Cenozoic section are often characterized by reflections that have sigmoidal geometry and display distinct onlap relations with underlying reflections (reflection package 4, SSN-15; Fig. 10C; reflection packages 2 and 3, SSN-12; Fig. 11C). In contrast, reflections from pre-Cenozoic rocks are often moderate- to high-amplitude reflections that are generally discontinuous and variably oriented, such as at deeper levels of SSN-17 and SSN-15 (Fig. 10).
The change in reflection character at the interpreted contact between the Cenozoic section and consolidated pre-Cenozoic rocks is somewhat variable. In places the contact must have a sharp impedance contrast that produces a good reflection (horizon B, SSN-15; Fig. 10C). In many places reflections interpreted as being within the deepest parts of the Cenozoic basin fill have high-amplitude reflections similar to the underlying pre-Cenozoic rocks, complicating the interpretation (reflection package 4, SSN-17; Fig. 10B; reflection package 2, SSN-15; Fig. 10C; reflection package 4, SSN-11; Fig. 11B). In some places, the depth-to-basement solution helped guide the interpretation of seismic reflections at the base of the Cenozoic section (e.g., the southwestern basin on the west side of SSN-17; Fig. 10B). In general, the picked location of the base of the Cenozoic basins was interpreted iteratively by considering reasonable velocity and density variations, coupled with allowable interpretations of reflection character and geometry.
Reflections from the interpreted pre-Cenozoic section are variable. In places there appear to be coherent reflections from the pre-Cenozoic section (west side of SSN-19; Fig. 10A; west side of SSN-15; Fig. 10C); elsewhere there are almost no reflections (east side of SSN-15; Fig. 10C; much of SSN-13; Fig. 11A). We used a variety of methods to deduce the nature of the pre-Cenozoic section, including using computed variations in bedrock density from the gravity inversion, computed variations in seismic velocity along each seismic section, and attempting to rigorously interpret deep reflections on the seismic images. In the end, we chose to make very conservative and limited interpretations of bedrock reflections.
Evidence for Multiple Periods of Fault Motion
On line SSN-17 and to a lesser extent, on lines SSN-15 and SSN-12, rotated and faulted sedimentary packages deep in the Pahrump basin are covered by considerable thicknesses of less-deformed sediments (Fig. 10). On line SSN-17, the northeastern subbasin contains reflection packages deep in the basin (reflection packages 3 and 4; Fig. 10B) that define a northeast-dipping half graben associated with a southwest-dipping normal fault that is buried by the youngest basin fill (Fig. 10B). Small folds and reflection offsets suggest that this fault has normal offset. Overlying these reflection packages is a wedge-shaped zone of weak reflectivity between horizons B and A, which thickens toward the normal fault (reflection package 2; Fig. 10B). Overlying this package is a variably thick group of moderate-amplitude reflections that overlaps the updip extent of the normal fault (reflection package 1; Fig. 10B). On line SSN-17, the continuity and parallelism of the deepest basin-fill reflection packages (reflection packages 3 and 4; Fig. 10B) suggest that these packages predate motion on the buried eastern basin-bounding normal fault. The overlying reflection package (reflection package 2; Fig. 10B) largely postdates major motion on these deep faults, yet this package thins against the mid-basin high and is clearly influenced by uplift related to the Stateline fault system. This implies the existence of an early basin that was disrupted by normal faulting, followed by uplift and offset along the Stateline strike-slip fault system.
On line SSN-19, reflections from Cenozoic rocks of the uppermost sequence may be traced across the top of the Stateline fault zone, where they appear to be folded into an upright anticline (reflection package 2; Fig. 10A). This section records a period of sediment deposition, followed by faulting with a vertical component that folded the sediment package, followed by renewed sedimentation in Stewart Valley.
Evidence for Older, Pre–Normal Faulting Sediments
On line SSN-17, the lowest reflection packages, between horizons B and D (Fig. 10B), have an approximately constant thickness and a consistent apparent 20° dip to the northeast in this section, and reflections are terminated against the steep, southwest-dipping fault centered at a distance of 8 kmAP. These packages with continuous, parallel reflections are interpreted to indicate a quiescent depositional setting not influenced by nearby faulting. On SSN-15, high-amplitude and parallel reflections in the lower half of reflection package 2 (Fig. 10C) may correspond to the similar but thicker section seen at depth on SSN-17.
Seismic Interpretations of the Mesquite Valley Reflection Lines
Three seismic lines cross the Mesquite Valley (Figs. 2 and 3). Seismic line SSN-11 is oriented southwest-northeast, roughly normal to the trace of the Stateline fault system, and it crosses just south of the deepest portion of the basin as defined by the gravity inversion (Fig. 7). Line SSN-12 is oriented northwest-southeast, generally to the west of the inferred trace of the Stateline fault system. SSN-12 crosses the deepest portion of the Mesquite basin and, to the northwest, the structural boundary with the Pahrump basin. Line SSN-13 is oriented southwest-northeast and generally parallels the topographic and structural high between the Pahrump and Mesquite basins (Fig. 9).
Lack of a Mid-Basin High
The Mesquite basin has a half-graben geometry. Basin-fill sediments have apparent dips to the northeast and thicken to the northeast, where their reflections diminish to the west of a near-vertical basin-bounding fault (Fig. 11B). Unlike in the Pahrump basin, there is no mid-basin high associated with the Stateline fault system because the fault zone is on the east side of the Mesquite basin. Based on the consistent apparent northeast dips and the consistent direction of thickening, the depocenter in the Mesquite basin appears to have been skewed to the east side of basin, adjacent to the bounding fault, over most of the basin history. The fault that bounds the northeast side of the Mesquite basin is expressed as a moderately steep gravity gradient, but is not imaged well on the seismic line (Fig. 11B). Rocks to the east of the fault have almost no reflections to aid in locating the fault, but reflections within the basin and at the basin floor preclude a west-dipping fault that would be coincident with the depth to basement solution. The fault is portrayed with a very steeply west dipping geometry, on the basis of truncation of deep basin reflections. Within the basin immediately to the west of the fault, reflections are of poor quality and are short and variably oriented. This area (reflection package 5; Fig. 11B) is inferred to be dominated by acoustically transparent, coarse-grained material shed off of the evolving fault, which interfingers with basin-fill sediments within the basin. The presence of this interpreted coarse-grained fill adjacent to three of the reflection packages (Fig. 11B) implies a long-lived period of fault motion that spans much of the basin history.
Early Episode of Faulting
In the deepest portions of the interpreted Cenozoic sections on SSN-11 and the southeast portion of SSN-12, there is a reflection package characterized by very high amplitude, parallel reflections (reflection package 4; Fig. 11B; reflection package 5; Fig. 11C). This package is much more strongly deformed than the overlying packages, and most of the faults that cut this deep reflection package cannot be traced into the overlying reflection packages. In contrast to the Pahrump basin, the early faulting appears as a number of small faults, rather than a single master basin-bounding fault. On SSN-11, the deep basin reflection package is also strongly folded against the basin-bounding fault into a broad syncline with an amplitude of ∼0.5 km (reflection package 4; Fig. 11B).
Long-Lived Depositional Setting
Line SSN-12 appears to record a lengthy record of sediment deposition within the Mesquite basin. On this line it appears that the maximum thickness of the reflection packages is farther to the southeast for the shallower packages and farther northwest for the deeper packages (Fig. 11C). This is likely the consequence of continued sediment deposition at a relatively fixed depocenter in the axis of the Mesquite basin during continued motion on the Stateline fault system. As the sediments move to the northwest, they are folded, uplifted, and possibly eroded to contribute to younger deposition to the southeast. As such, SSN-12 portrays the continuum of how the deposited rocks have migrated northwestward along the fault relative to a relatively fixed depocenter in the Mesquite basin. Unconformities within the section (horizons A, B, and C; Fig. 11C) may represent discrete periods of motion along the Stateline fault system where sediment deposition was interrupted.
Joint Gravity and Seismic Modeling
Systematic differences between basin depths interpreted from the seismic reflection sections and those inferred from the depth-to-basement gravity inversion show areas where the assumptions of the depth-to-basement technique are violated. We have conducted forward gravity analysis (Talwani et al., 1959) along the three seismic lines that cross the basin gravity anomalies in a near-perpendicular manner, SSN-11, SSN-15, and SSN-17 (Fig. 6). Although the gravity anomalies are not strictly two-dimensional and the seismic lines are not perfectly straight, this analysis allows us to determine the first-order gravity signatures of the basin geometry defined by the seismic reflection images and to determine the average density of basin-fill material in the seismically defined blocks. To facilitate this analysis, we projected the crooked seismic plan-view geometry to best-fit lines prior to gravity calculations for lines SSN-11, SSN-15, and SSN-17 (Figs. 12, 13, and 14, respectively).
Seismic line SSN-11, which crosses the relatively simple structure of the Mesquite basin near its center, illustrates our approach. We digitized a polygon corresponding to the reflection interpretations, in this case a half-graben with thin sediment extending to the northeast over a pediment surface (Fig. 12). Using this geometry, we calculated gravity anomalies assuming a wide range of density contrasts of the basin fill relative to the surrounding basement until we determined the density contrast that best predicts the observed gravity anomaly (Fig. 12, top panel). The misfit of the gravity anomalies as a function of the assumed density contrasts (Fig. 15) shows that for the Mesquite basin, an average density contrast of −470 kg/m3 fits the observed gravity best. This value is approximately the average density contrast of the adopted density versus depth function over the depth range of the basin (0–2 km; Fig. 9).
Below seismic line SSN-15, the southwestern subbasin of the Pahrump Valley is somewhat thicker than the northeastern subbasin, and it has a correspondingly greater gravity anomaly (Fig. 13). By allowing the two subbasins to have constant but different densities, the derived density values are slightly different statistically, −370 kg/m3 and −430 kg/m3 for the southwest and northeast subbasins, respectively. Figure 15B illustrates the tradeoff of gravity misfit as a function of the densities of the two subbasins; the southwestern subbasin, with the greater gravity anomaly, has a smaller range of acceptable densities than the northeastern subbasin.
The crossing of the two subbasins beneath SSN-17 was the prime example where the gravity-derived basement surface disagreed with the seismic reflection picks of the basin floor (Fig. 10B). In particular, the floor of the northeastern subbasin is marked by constant-tilt and constant-thickness packages of reflections (labeled 3 and 4 in Fig. 10B) at a place where the gravity inversion implies a much steeper floor (dashed green line in Fig. 10B). The updip coincidence of this reflection package with the outcrop of dipping Pliocene and older sedimentary rocks (Lundstrom et al., 2003) suggests that this package likely has a greater density than younger, less consolidated units. The forward gravity model results for SSN-17 are shown in Figure 14. The southwestern subbasin, with a floor picked along a weak reflection at ∼0.5 km below sea level, rather than at the base of stronger and more continuous reflections, yields a density contrast of −350 kg/m3. Assuming the shallower basin floor would require a near-doubling of the density contrast of its sedimentary infill, to a level (−750 kg/m3) reasonable only for the very shallowest sediments in the depth-to-basement technique (Fig. 9). More notably, the gravity anomaly associated with the northeastern subbasin is well matched by a two-layer model, where the average density contrast of the dipping, underlying layer is −250 kg/m3 and the average density contrast of the overlying layer is −410 kg/m3 (Fig. 14), values that are statistically different from one another (Fig. 15C). The density contrast of the underlying layer implies that the rocks of that sedimentary package are slightly denser than what is typically assumed for depths <1.2 km in the depth-to-basement analysis (Fig. 9).
The interpretation that denser, more indurated rocks are present on the western margin of the northeastern subbasin than elsewhere is supported by interval velocities determined for SSN-17 (Fig. A2). Velocities from the center of the subbasin (5–7 kmAP in Fig. 14) are everywhere less than those at the same depths at the western edge of the subbasin (1–2 kmAP; Fig. 14), in the uppermost 2 km.
Combined analyses of gravity and seismic data have allowed us to better define the location of the Stateline fault system in the Pahrump and Mesquite basins, to understand the three-dimensional geometry of the fault system and associated basins, and to tie our observations of the Pahrump and Mesquite basins to the regional extensional and strike-slip history.
Location of the Stateline Fault System
Previous studies of the Stateline fault system have portrayed the location of the fault in a generalized fashion or have focused on a single type of geologic data to constrain interpretations. Schweickert and Lahren (1997) and Guest et al. (2007) portrayed the fault system with a generalized trace at a regional scale. Previous geophysical studies (Blakely et al., 1998, 1999) relied heavily on the results of gravity analysis to define the fault geometry. Recent regional fault compilations (Workman et al., 2002b; Potter et al., 2002) used a maximum gradient analysis of a modeled depth-to-basement surface as a guide in selecting fault locations. While this latter method identifies structures, it does not distinguish between different generations of high-angle faults, nor does it account for lateral variations in basin-fill density that might produce gravity anomalies that mimic those from faults. Other compilations (Anderson et al., 1995; McKittrick, 1998; Schmidt and McMackin, 2006) emphasize recent faulting. We have tried to combine all data sources, including geologic map data, recent faulting, gravity analysis, aeromagnetic observations, and seismic reflection data, to pick the definitive location of the Stateline fault system in the study area (Fig. 16) and to support our interpretations of the strike-slip system.
The results of this study are consistent with previous geophysical studies indicating that the Stateline fault system in the Pahrump Valley is generally coincident with a buried ridge along the axis of the Pahrump Valley (Bates, 1965; Hoffard, 1991; Blakely et al., 1998, 1999; Langenheim et al., 2005). The depth-to-basement solution (Fig. 16) indicates that both the Pahrump and Mesquite Valleys are underlain by deep, steep-sided basins. The Mesquite basin appears to be a single deep depression, whereas the Pahrump basin appears to consist of two deep basins separated by a buried ridge along the axis of the Pahrump Valley (Fig. 16). Based on the combined analysis of gravity and seismic data, the Stateline fault system throughout much of the Pahrump Valley is parallel to, and just east of, seismic line SSN-12 (Fig. 16). The generalized trace of the fault is nearly a straight line between SSN-19 and SSN-15. The zone that includes all splays inferred to be related to the Stateline fault system is narrow in this area (Fig. 16). In this area, the fault coincides with outcrops of coarse-grained Pliocene gravelly basin fill that are the oldest mapped units found in the central part of the Pahrump Valley (Page et al., 2005); these units are presumably uplifted along this part of the fault system. The fault is also coincident with a zone of late Quaternary scarps and lineaments in the Pahrump Valley (Hoffard, 1991; Piety, 1996; Anderson et al., 1995; Shields et al., 1998) and with the bedrock high that separates the northeastern and southwestern subbasins (Fig. 16).
Although beyond the limits of our combined gravity and seismic analysis, we infer that to the north of line SSN-19, the Stateline fault system follows a more northward-trending trace along the eastern margin of Stewart Valley, coincident with mapped young faults (Piety, 1996; Anderson et al., 1995; Shields et al., 1998). An older, inactive strand of the fault is exposed in the southern Montgomery Mountains where it cuts bedrock exposures of Neoproterozoic Stirling Quartzite (Burchfiel et al., 1983). This fault has a more western strike than the currently active strand (Burchfiel et al., 1983; Potter et al., 2002). This fault projects southeastward into the northern Pahrump Valley and potentially intersects the eastern portion of line SSN-19. However, given rather poor data along line SSN-19 we chose not to interpret the location of this fault. At the northern part of Stewart Valley, there appears to be complex accommodation along several inferred fault strands (Workman et al., 2002b; Potter et al., 2002) as the fault bends back to the northwest in the Amargosa Desert (Fig. 16).
We infer that the Stateline fault system jogs to the east at a position south of SSN-15, placing uplifted Pliocene and Miocene sediments and the Paleozoic outcrops at Black Butte on the southwestern side of the fault system (Fig. 16). Such an alignment is consistent with the interpreted offset of Black Butte from a similar lithologic sequence in the southern Spring Mountains at Devil Peak, ∼30 km to the southeast (Guest et al., 2007). This location is also consistent with the trace of known Quaternary faults (Schmidt and McMackin, 2006) and with the seismic reflection data presented here. The zone that includes all splays inferred to be related to the Stateline fault system is very wide in the area of this bend in the fault system. This broad zone includes Black Butte and anticlinally folded Pliocene–Miocene sediments (Workman et al., 2002a) to the north of SSN-13 (Fig. 16).
In the Mesquite Valley, the steep gravity gradient on the east side of the basin (Fig. 16) and the steeply dipping fault apparent on SSN-11 (Fig. 11B) support the interpreted trace of the Stateline fault system along the east side of the valley. We interpret this fault segment to be a straight, near-vertical segment that extends southeast to bedrock exposures of the fault at Stateline Pass (Hewett, 1956). In the northeastern part of the Mesquite Valley, the interpreted fault trace is aligned with outcrops of Pliocene and Miocene units (Fig. 16; Workman et al., 2002a) that are presumably uplifted along the fault in a fashion similar to outcrops in the Pahrump Valley.
Geometry of the Stateline Fault System
The above-described trace of the Stateline fault system includes generally straight, northwest-striking segments in the Pahrump and Mesquite basins that are linked by a left step or bend near the buried high between the two basins. A right step in the fault system links the straight segment in the Pahrump Valley with the Stewart Valley. There are natural geometric consequences that follow, given a nonlinear fault with an interpreted 30 km of right-lateral offset (Guest et al., 2007). The seismic reflection character of the Stateline fault system differs distinctly between sections where the fault is straight, such as SSN-17 and SSN-11, and areas near a restraining bend, such as SSN-15, SSN-13, or the middle part of SSN-12. In the former case, the mid-basin high is bound by a single subvertical fault with minimal splaying, and there is an abrupt structural offset between the adjacent subbasins. In the latter, the mid-basin high is broad and structurally complex with numerous fault strands.
The pre-Cenozoic high that separates the Pahrump and Mesquite basins near the intersection of lines SSN-13 and SSN-12 (Fig. 2) is interpreted to be a consequence of local transpression at a left step in the fault system. This culmination, imaged on SSN-12 (Fig. 11C), coincides with an anticline mapped at the surface (Workman et al., 2002a) that exposes Pliocene–Miocene sediments in its core and that is along the Stateline fault system, just to the north of Black Butte (Fig. 16). At the surface, the rocks are strongly deformed, with dips as great as 50° (McMackin, 1999).
An additional geometric consideration along the Stateline fault system is the presence of significant uplift and structural relief along even straight segments of the fault and the presence of flower-type structures in the seismic lines crossing the Pahrump Valley. We suggest that these features are the result of a small oblique component of relative motion along a system that is dominantly strike slip. Fault motion at a restraining bend, coupled with continual basin sedimentation, yields evidence of an evolution in structural style. On line SSN-15, there appear to be multiple episodes of faulting along the mid-basin high. The deeper parts of the section, particularly to the southwest of the mid-basin high (horizon D and adjacent packages; Fig. 10C), are folded and disrupted by faults, which we interpret as thrust faulting as a result of transpression at the restraining bend on the fault. However, the shallowest sediments (reflection package 3; Fig. 10C) are subhorizontal and are truncated by a nearly vertical fault. We infer that deformation within the deeper reflection packages in the southwestern subbasin is related to local transpression at a restraining bend and that the uppermost sediments were deposited after this area moved to the northwest of the bend, where they are breached by a straight fault with little compressional component.
Geophysical Data Bearing on the Origin of Black Butte
The small topographic high of Black Butte (Fig. 16) forms the only outcrop of Paleozoic rock within either the Pahrump or Mesquite Valleys, and it is located directly above the pronounced structural high that separates the Pahrump and Mesquite basins (Fig. 16). Previous gravity, seismic refraction, and magnetotelluric observations (MIT Field Geophysics Course, 1985) were used to conclude that the Paleozoic rock outcrops at Black Butte were separated from underlying basement by several hundred meters. Our models of the gravity data are consistent with this finding and suggest that Black Butte is essentially rootless: the gravity signature at Black Butte is best fit if the butte is underlain by several hundred meters of basin fill, rather than being rooted to the pre-Cenozoic bedrock. If these Paleozoic rocks were assumed to root to basement at depth, then an ∼10 mGal negative isostatic residual gravity anomaly would center on Black Butte, implying unreasonably low density basement rocks at that site. Seismic line SSN-13 crosses just to the south of Black Butte (Fig. 16), and reflection data suggest that basin-fill sediments are very thin along the line, consistent with the line's location along the structural high between the Pahrump and Mesquite basins (Fig. 9). The seismic quality is poor where the line crosses to the south of Black Butte, but there is no evidence of a distinctly uplifted area of higher amplitude reflections that are generally typical of pre-Cenozoic section. Instead, a shallow Cenozoic basin is interpreted near the center of the line (Fig. 11A), consistent with the depth-to-basement solution.
The Paleozoic carbonate rocks exposed at Black Butte have been previously interpreted to be a megabreccia deposit, not intact bedrock (Calzia et al., 2000; Guest et al., 2007). On the basis of geologic mapping, similarity in stratigraphic sequence, and the age, type, and geochemistry of associated Miocene volcanic rocks, Guest et al. (2007) interpreted that the rocks at Black Butte can be correlated to carbonate-rock and volcanic rock exposures at Devil Peak, ∼30 km to the southeast (Fig. 16). This correlation was used to establish a piercing point along the Stateline fault system, where the megabreccia deposit and underlying volcanic rocks were interpreted as localized deposits that have been offset by 30 km of right slip on the Stateline fault system since 13.1 Ma (Guest et al., 2007). In this scenario, breccia sheets were deposited in an actively tilting basin and buried. Subsequently, they were offset and transported northward on the west side of the fault; their present exposure appears to be a consequence of uplift and tilting near the restraining bend of the fault. While recognizing the existence of some breccia bodies at Black Butte, McMackin (2001) emphasized the continuously bedded nature of the Paleozoic rocks at Black Butte and their thrust contact with underlying Miocene and Pliocene volcanic and sedimentary rocks. On this basis, McMackin (2001) suggested that the central part of Black Butte is a fault block uplifted on a high-angle reverse fault in the strike-slip zone. However, if Black Butte represents a locally derived bedrock sliver that has been caught up in the Stateline fault system, a complex fault geometry would need to exist beneath Black Butte such that the sliver was separated from underlying bedrock, as suggested by the geophysical data.
Interpretation of Subsurface Data in the Context of Regional Extension
Regional tectonic reconstructions of the Death Valley region, including the vicinity of the Pahrump and Mesquite basins, differ in the interpreted magnitude of regional extension and the relative importance of different fault types. The most extreme estimates of regional extension derive from reconstructions that have relied primarily on map-view reassembly of faulted range blocks. Such reconstructions have been made on the basis of restoration of interpreted piercing points such as correlation of Mesozoic thrust faults and correlation of distinctive Neogene basin deposits (Wernicke et al., 1988; Snow and Wernicke, 2000; Brady et al., 2000; Niemi et al., 2001; McQuarrie and Wernicke, 2005). In this conceptualization, carbonate-rock mountain ranges of the Pahrump–Death Valley region are in a zone of extreme crustal extension, implying that these ranges are thin slivers that detached above a migrating flexure in highly thinned crust (Holm et al., 1992; Wernicke, 1992). Restoration of detachment fault offset results in a very narrow Mesozoic fold-and-thrust belt, closing the Pahrump basin and restoring the Nopah Range to a position immediately west of the present Spring Mountains (Wernicke et al., 1988; Snow and Wernicke, 2000; McQuarrie and Wernicke, 2005). Alternative regional models require smaller amounts of extension and emphasize the role of strike-slip faults and transtension in accommodating regional strain (Serpa and Pavlis, 1996; Wright et al., 1999; Guest et al., 2003; Miller and Pavlis, 2005). These models call upon multiple extensional faults that need not be linked to an underlying master regional detachment surface and allow for a wider preextensional Mesozoic fold-and-thrust belt. Although we lack deep wells or age control within the Pahrump and Mesquite basins, the seismic reflection packages and basin geometry identified in this study allow us to comment on these reconstruction alternatives.
Regional studies document Lower to Middle Miocene alluvial fan, lacustrine, and fluvial deposits deposited in the southern Death Valley region prior to the onset of regional extension ca. 13.5 Ma (Fig. 17A). These deposits include the 16–15 Ma Panuga Formation and overlying Bat Mountain Formation (Snow and Lux, 1999), which form a preextensional sequence exposed at the southern end of the Funeral Mountains, and the 15–11 Ma Eagle Mountain Formation (Niemi et al., 2001; Renik et al., 2008), which was apparently deposited early in the extension history of the southern Death Valley region and subsequently dismembered. Seismic sections in the Pahrump and Mesquite basins feature reflection packages at the bottom of the basins that have regular parallel reflections, the amplitudes of which do not vary laterally, suggestive of units that were deposited in a relatively quiescent environment over a wide area, with minimal lateral facies changes. These deep reflection packages are abruptly cut by small- and large-offset faults and appear to predate the oldest faults that can be identified on the sections. On SSN-17 and SSN-15, the stratigraphically highest part of the reflection package with regular parallel reflections appears to correlate with outcrops that contain a 10.76 Ma crystal lapilli tuff (McMackin, 1999), giving a minimum age to the tilted, bedded sedimentary sections represented by these reflection packages. We suggest that these deep sediments may correlate with preextensional sequences elsewhere in the Pahrump–Death Valley region (Fig. 17A). As such, the Pahrump basin may have had a significant width prior to regional extension, at odds with regional reconstructions that would restore the Nopah Range to a position immediately west of the present Spring Mountains (Wernicke et al., 1988; Snow and Wernicke, 2000; McQuarrie and Wernicke, 2005).
Timing of Basin Opening and Slip Rates along the Eastern California Shear Zone
The gravity-defined basins in the Pahrump Valley that are aligned with the Stateline fault system have previously been interpreted as the result of transtensional pull apart associated with the Stateline fault system (Wright, 1989; Hoffard, 1991; Blakely et al., 1999). A significant result of this study is that the Pahrump basin has likely had a multistage history of opening, and that not all of the basin sedimentation seen in the gravity data was a result of strike-slip-related transtension. Instead, a portion of the gravity anomaly is created by sedimentary deposits that predate strike-slip offset. We infer that the gravity anomaly is the summed response of a preexisting, extensional basin overprinted by the effects of strike-slip faulting.
Regional studies document a major tectonic transition between 14 and 9 Ma as major extension in the region swept westward from the western side of the Spring Mountains to Death Valley, to be succeeded by regional transtension and ultimately to transcurrent deformation along strike-slip faults (Serpa and Pavlis, 1996; Snow and Wernicke, 2000). The magnitude of extension and fault offset in the area surrounding the Pahrump and Mesquite basins decreased through time following the Middle Miocene pulse of maximum extension, as the locus of deformation shifted westward (Snow and Wernicke, 2000).
The Kingston Range detachment fault, immediately west of the Mesquite Valley, was the site of the earliest documented large-magnitude extension in the southern vicinity of the Pahrump and Mesquite basins (Fig. 17B). Motion on the detachment is interpreted to have occurred between 13.4 and 12.4 Ma (Fowler and Calzia, 1999). Many of the seismic lines from the Pahrump and Mesquite basins show evidence of an early period of faulting that affects the deepest reflection packages, but not the younger overlying packages. Examples of such faults include numerous small-offset faults in the Mesquite basin interpreted on SSN-11 (Fig. 11B) and the southeast end of SSN-12 (Fig. 11C), and the large-offset fault interpreted on SSN-17 in the Pahrump basin (Fig. 10B). We suggest that these early faults may be related to the early extension in the Pahrump–Death Valley region. Such extension would have disrupted the preextensional sedimentary sequence and appears to have been responsible for the northeastward tilting of the northeastern subbasin in the Pahrump Valley (Fig. 10B). These faults, especially the large-offset fault interpreted on SSN-17, may correlate generally with extension along the Spring Mountains breakaway, the easternmost extent of significant extension in the greater Pahrump Valley, and to the Point of Rocks detachment along the northwest end of the Spring Mountains (Snow and Wernicke, 2000) (Fig. 17B). Tilting and uplift associated with the emplacement of the Devil Peak rhyolites are inferred have resulted in the formation of megabreccia deposits at this time (Guest et al., 2007).
Slip on the Kingston Range detachment terminated at 12.4 Ma with the intrusion of the Kingston Range pluton (Fowler and Calzia, 1999). The Pahrump and Mesquite basins transitioned into a broadly transtensional opening mode in response to northwest-directed extension as peak extension shifted westward to the west side of the Kingston Range and the Death Valley region (Davis et al., 1993; Calzia et al., 2000; Serpa and Pavlis, 1996; Wright et al., 1999; Miller and Pavlis, 2005) (Fig. 17C). Based on the seismic reflection data in the Pahrump and Mesquite basins, the rate and magnitude of extension appear to have waned with time. Faults that had an early history of movement and disrupt the deepest reflection packages were subsequently buried by younger sediment. On line SSN-17 and, to a lesser extent, on lines SSN-15 and SSN-12, we interpret an older sequence of rotated, faulted sedimentary packages deep in the basin that are covered by considerable thicknesses of less-deformed sediments.
The reflection packages within the Pahrump basin appear to document the tectonic transition from extension to transcurrent motion. On line SSN-17, the deepest reflection packages (3 and 4; Fig. 10B) predate motion on the eastern basin-bounding normal fault. The overlying reflection package (2; Fig. 10B) largely postdates major motion on the deep normal fault, yet this package thins against the mid-basin high and is clearly influenced by uplift related to the Stateline fault system. This implies the existence of an early basin that was disrupted by normal faulting, followed by uplift and strike-slip offset along the Stateline fault system (Fig. 17). If this is the case, it would imply a two-stage history, with extension followed by strike-slip offset. Blakely et al. (1999) proposed a similar type of two-stage history to explain the basin configuration beneath Death Valley.
The proposed correlation of the lithologic sequence at Black Butte with a similar section near Devil Peak (Fig. 1) implies ∼30 km of right-lateral offset on the Stateline fault system since Middle Miocene time (Guest et al., 2007). On SSN-17 and SSN-15, the upper part of reflection packages interpreted as a tilted, parallel-bedded sedimentary section correlates with outcrops that contain a 10.76 Ma crystal lapilli tuff (McMackin, 1999). Deposition of this section predates offset by the Stateline fault system, constraining the timing of inception of transcurrent motion on the Stateline fault system. If the proposed 30 km of right-lateral offset (Guest et al., 2007) on the Stateline fault system is correct, the constraints imposed by our seismic interpretation and the dated tuff (McMackin, 1999) would imply a long-term geologic slip rate of ∼2.8 mm/a, a rate slightly higher than suggested by Guest et al. (2007). Without additional age constraints, we cannot generalize our results regarding the overall evolution of the eastern California shear zone, but we concur with Guest et al. (2007) that the Stateline fault system appears to have been a geologically significant component on the eastern margin of the shear zone.
Seismic reflection profiles in the Pahrump and Mesquite Valleys define a multiphase opening history of the Pahrump basin, i.e., an extensional phase followed by a strike-slip–dominated phase, that was not apparent by investigations of bedrock geology or potential field geophysics alone. This multiphase history has important implications to the timing and magnitude of crustal extension in this area and to the role of the Stateline fault system as a major crustal boundary. Restraining bends in the trace of the Stateline fault system correlate with uplifted sections of early deposited basin fill and with multiple fault strands at shallow levels. The presence of young basins along straight-line sections of the fault system suggests that other faults, not seen in outcrop or in the seismic lines, and/or an oblique component of extension, are operative in the Mesquite and Pahrump Valleys. Our geophysical results demonstrate that the analysis of gravity data alone, without independent constraints on basin geometry, can produce misleading basin solutions unless lateral variations in infill density are considered. Seismic reflection images of these basins can be used to define density packages that readily explain the gravity anomalies in a manner consistent with the seismically defined basin architecture.
APPENDIX. SEISMIC RESULTS
SEISMIC DATA AND PROCESSING
Seismic reflection data across the Pahrump and Mesquite Valleys were collected in 1980 with a nominal 24-fold subsurface coverage using a Primacord explosive source with a charge length of 220 ft (67 m) buried at 30 in (76 cm) depth. The shot spacing and the geophone spacing were also 67 m. There were 48 recording channels with a near offset of 440 ft (134 m) and a far offset of 5500 ft (1676 m); the data were recorded with a sample interval of 2 ms and a record length of 6 s.
Seismic processing by the U.S. Geological Survey consisted of a sequence of automatic gain control scaling, single design window spiking deconvolution, datum statics using smoothed surface elevations, velocity analysis, surface-consistent residual statics, a second velocity analysis, normal move-out and stack, shift to a horizontal datum equal to the average elevation of the line, migration using a smoothed stacking velocity field, automatic gain control scaling, second-zero-crossing predictive deconvolution, bandpass filter between 10 and 50 Hz, and shift to an elevation datum of 2000 m. Seismic sections were created both in time and in depth. Because of the ∼130 m separation between the shots and the nearest geophone, the seismic sections do not image the shallowest ∼100 m of the subsurface. Representative interval velocities from the stacking analysis are presented for SSN-11 and SSN-17 in Figures A1 and A2, respectively.
The depth values of reflections, including the interpreted basin floors, have an uncertainty related to the uncertainty of the velocity field utilized in the conversion from traveltime to depth. The proportional uncertainty of the average velocity to a reflection at a particular two-way traveltime translates into the same proportional uncertainty of its depth; if the actual velocity is 10% faster than an assumed value, then the actual reflection depth will be 10% shallower (if 10% slower, then the actual depth will be 10% deeper) than calculated. Because there are no deep wells with sonic logs nor are there coincident seismic refraction studies, we used interval velocities determined from smoothed stacking velocities for migration and depth conversion. The stacking velocity field was smoothed both horizontally (100 common midpoints, or ∼3.4 km) and vertically (200 ms), and then interval velocities were calculated with the Dix (1955) equation.
Within the sedimentary basins crossed by the SSN lines, the stacking velocities determined directly from the seismic data can be considered accurate to ±5%, based on velocity semblance and constant-velocity-stack analysis. Modifying the stacking velocities beyond that percentage noticeably degrades the image quality of stacked basin reflections. The factors necessary for good velocity analysis are present within most areas of the basins: subhorizontally layered section, strong impedance contrasts between rock units, high signal to noise ratio, and broad frequency content (10–50 Hz) in the reflected arrivals. In addition, the maximum source to receiver offset of 1676 m was sufficient to give good velocity resolution at the maximum basin depth of 3 km; the traveltime difference between seismic arrivals from the same reflector at the near offset (134 m) and the far offset (1676 m) was greater than the time-width of the seismic pulse at all depths. Nevertheless, there was some noise in the final stacking velocity fields, which motivated using smoothed versions of the stacking velocity field for seismic migration and depth conversion. In addition, with manual inspection we made small changes to the velocity fields in localized areas to avoid overmigrating or undermigrating individual reflections.
Because the proportional uncertainties of seismic velocities are generally smaller than the proportional uncertainties of density contrasts, we assume that the seismically determined basin depths are more accurate than gravity-determined estimates. In one case in the southwest portion of SSN-17 (Fig. 10B), where our original seismic pick of a shallow basin floor implied an unreasonably large density contrast (low basin-fill density), we adjusted the seismic pick to a deeper but weaker reflection.
Although interval velocities derived from stacking analysis are intrinsically less accurate than velocities measured by seismic refraction or deep borehole sonic logging, the interval velocity profiles show systematic changes that correspond with surface geology, gravity anomalies, and with results from a short seismic refraction line that crosses one of the seismic profiles in the Mesquite Valley (Fig. 3). This correspondence increases our confidence that the velocities within the basins used in converting reflection times to depths have small error. The seismic refraction line collected and analyzed by the MIT Field Geophysics Course (1985) is presented as a one-dimensional model (Fig. A1) with two constant-velocity layers <3000 m/s in the uppermost ∼500 m, an increasing velocity to a depth of ∼2000 m, then 6500 m/s below (Fig. A1). The shallowest layer has a very low velocity (∼1500 m/s) over a depth of ∼25 m; it is difficult to distinguish in Figure A1 and would not be resolvable in the reflection velocity analysis, where stackable reflections must be ∼100 m or deeper. The actual velocity and depth of the deep portion of the refraction velocity profile (Fig. A1) depend on the unknown shape of the surface giving rise to the refracted arrival, but the elevated velocities >6000 m/s clearly indicate that consolidated bedrock is present at those depths.
The three stacking velocity analyses of SSN-11 at the common midpoint gathers closest to the location where SSN-11 crosses the northern end of the MIT refraction line (Fig. 3) are displayed in Figure A1. In a general sense, there is good agreement between the average interval velocities at this site with the uppermost 1000–1500 m of the refraction profile, even though the seismic refraction results were generated assuming constant-velocity layers and the seismic reflection results allow steadily increasing velocities. Deeper than 1000–1500 m, SSN-11 does not exhibit clear reflections at this location, so it is not surprising that the interval velocities do not converge toward the refraction velocities at deeper depths.
SEISMIC SECTIONS AND INTERPRETATION
We present plates containing each seismic section, both with and without seismic interpretations superimposed. These migrated depth sections are displayed at a horizontal scale of 1:80,000 and with a vertical exaggeration of 2. Distances along the horizontal axes are relative either to the California-Nevada state border (SSN-11, SSN-13, SSN-15, SSN-17, SSN-19) or to the SSN-19 crossing (SSN-10, SSN-12). We abbreviate the along-profile distances in kilometers as “kmAP” to distinguish these values from depths or relative distances. Note that the seismic image does not extend to the surface topography; rather there is a ∼100 m gap due to the source and receiver geometry.
We describe the seismic interpretations in terms of stratigraphic and structural features. Seismic picks were based on the reflections observed in both depth sections and time sections; reflection quality and continuity varied somewhat between these different image types. In addition, we compare the seismic results with our gravity-derived model of the depth to pre-Cenozoic basement for this area, which we term the depth-to-basement solution. In some lines, the gravity-modeled basin depth disagrees with interpretations based on seismic reflection, and we provide possible explanations of these differences. The obvious disagreement of the gravity-modeled depth and the pattern of seismic reflections in the Pahrump Valley led us to devise the more complicated density model of valley fill that is described herein.
Water wells are abundant in developed areas of the valleys; lithologic data from well drillers’ logs (State of Nevada, Division of Water Resources: http://water.nv.gov/engineering/wlog/wlog.cfm) were compiled for wells along each seismic line. The deepest wells along each seismic line are 150–300 m deep and are shown projected onto the line. These wells penetrate depths corresponding to only the uppermost reflections on the seismic lines. Wells are annotated with a description of the predominant lithologic types described for the hole.
Line SSN-19 is oriented southwest-northeast and crosses the northern part of the Pahrump Valley (Fig. 3 and Plate A1). The line extends northeastward from Chicago Pass at the northern limit of the Nopah Range, passing just south of the southern limit of the Montgomery Mountains, to the base of the Pahrump fan on the east.
SSN-19 reflection interpretation.
East of 3 kmAP (Plate A1), the line is characterized by fairly continuous, high-amplitude reflections between 0.2 and 0.5 km elevation. Deeper reflections are much less continuous and are variable in amplitude, and poor data quality along this line hinders detailed interpretation of the reflections. A generalized boundary (horizon C; Plate A1) is interpreted to represent the contact between Cenozoic rocks and underlying pre-Cenozoic rocks, but poor data quality makes this pick only an approximation. This horizon was picked on the basis of continuous, high-amplitude reflections in the upper package (package 3) that show onlap onto poorly imaged reflections in the lower section.
Between 0 and 2 kmAP (Plate A1), the rocks are disrupted by the Stateline fault system. The subsurface configuration, and location, of the fault differs from that expected based on surface mapping. Mapped Quaternary scarps cross the line at ∼0 kmAP (Hoffard, 1991; Piety, 1996; Anderson et al., 1995) and the fault system was expected to be encountered along the east side of Stewart Valley and west of the low bedrock hills of the southern Montgomery Mountains (Fig. 2). However, no fault is obvious in the data from the shallowest part of the section near 0 kmAP.
On the basis of offset reflections, the fault zone is interpreted to occur as an upward-splaying zone of faults between −0.5 and 2.5 kmAP (Plate A1). These faults appear to cut the pre-Cenozoic rocks and the lowest Cenozoic sequence (package 3); no similar amount of offset is observed in the overlying sequence (package 2). Reflections from the overlying sequence (package 2) may generally be traced across the top of this complex fault zone (e.g., horizon B; Plate A1). Small amounts of vertical offset that might correspond to the mapped surficial Quaternary faults cannot be resolved from the seismic data. Reflections in package 2 show onlap relations on the undulating top of deformed reflection package 3 between 2 and 6 kmAP. On the west side of the line, reflection package 2 extends westward beneath the southernmost part of Stewart Valley, where the reflections compose a synform centered at −1 kmAP.
The highest reflection package (package 1) overlies horizon A and occupies the shallow subsurface beneath Stewart Valley (Plate A1). Reflections here are subhorizontal and clearly lap onto the underlying folded section of package 2. These highest reflections of package 1 are interpreted to be related to the recent basin-fill and playa deposition in Stewart Valley.
Comparison with gravity-derived depth to basement.
The depth-to-basement solution appears to be ∼300 m too shallow compared with the seismic reflection data along SSN-19 east of 2 kmAP, given the interpretation that the high-amplitude, regularly spaced, subparallel reflections extending across the upper part of the line represent the Cenozoic section. In places, the shape of the depth-to-basement solution mimics the shape of the base of the highly reflective package (Plate A1).
Two factors may explain the consistently too shallow depth-to-basement solution along much of SSN-19. First, if the basin-fill deposits at this northern end of the Pahrump Valley are denser than assumed in the gravity inversion, then the actual basement depths would be greater than modeled by gravity. Alternatively, or additionally, the gravity effect of the pre-Cenozoic basement rocks may not be adequately defined along this line. Errors in this predominantly long-wavelength basement gravity field, derived from sparse availability of gravity stations in the adjacent mountain ranges, may introduce long-wavelength errors in the depth-to-basement solution.
Line SSN-17 is oriented southwest-northeast and crosses the north-central part of the Pahrump Valley (Fig. 2 and Plate A2). As defined by the gravity inversion (Fig. 9), the line crosses the southwestern subbasin to the west of 1 kmAP, the mid-basin high at ∼1 kmAP, and the northeastern subbasin to the east of 1 kmAP. SSN-17 crosses SSN-12 over the mid-basin high, and it crosses SSN-10 near the base of alluvial fans on the northeastern side of the valley.
Interpretation of reflections within the northeastern subbasin (east of 1 kmAP).
On line SSN-17, the northeastern subbasin defines a northeast-dipping half-graben; basin-fill sediments have apparent dips to the northeast and truncate against a major buried normal fault and interpreted splays. Seismic reflections interpreted as Cenozoic basin fill within the northeastern subbasin are defined for four reflection packages (1, 2, 3, and 4 in Plate A2) that are bound by distinct reflections (horizons A, B, C, and D in Plate A2).
Reflection package 1.
This uppermost package (1 in Plate A2) consists of relatively continuous, moderate-amplitude reflections. Reflections are subparallel, wavy to sinusoidal, and dip gently (the apparent dip in this section is <10°) to the northeast. Reflections in the uppermost part of package appear to have gentler dips than those lower in the package. This interval is bound at the base by horizon A, a reflection that is strong and continuous between 2 and 7 kmAP, but that loses amplitude eastward near the buried normal faults. Reflection package 1 extends from the mid-basin high to ∼14 kmAP near the projected trace of the West Spring Mountains fault. Reflection package 1 extends over the top of the buried normal faults at depth and does not appear to be faulted by them. We have interpreted reflection package 1 to exist east of the buried normal faults as a way of explaining low densities implied by the gravity anomaly, but low reflection amplitudes and lack of continuous reflections makes this package difficult to delineate east of the buried faults. Strong reflections in the shallow part of package 1 between 8 and 9 kmAP appear to correspond to clay-rich horizons in two nearby wells.
At the eastern end of line SSN-17, the depth-to-basement solution shallows between 12 and 13 kmAP, slightly west of where the basin reflections truncate near 13.5 kmAP (Plate A2). We interpret these features as the West Spring Mountains fault. The mapped trace of this fault is sinuous (Anderson et al., 1995; Ramelli et al., 2003), suggestive of a gently dipping fault; as such, a small (<1 km) difference in the updip extension of the interpreted fault on SSN-17 and its mapped trace may not be too significant.
Reflection package 2.
Horizon A marks the top of a package of comparatively low amplitude reflections; the package is distinctly wedge shaped (Plate A2). This reflection package is thickest just to the west of the major buried normal fault between 8 and 10 kmAP, and it thins markedly in the direction of the mid-basin high. The base of this reflection package is chosen as the uppermost high-amplitude reflection in the underlying package (horizon B; Plate A2).
Reflection package 3.
This package has roughly constant thickness and is characterized by several strong reflections that are parallel and have an apparent dip of ∼20° to the northeast. The package extends from a shallow point at the mid-basin high near 1 kmAP to its abrupt truncation against the buried normal faults near 8 kmAP (Plate A2). Reflections have the highest amplitudes near the buried normal faults; they are traceable but lose amplitude toward the mid-basin high, This reflection package is not present to the east of the buried normal faults. Adjacent to the buried faults, the reflections form a hanging-wall syncline that may be the result of drag along the fault surface or the result of normal fault offset on a hanging-wall splay to the west of the interpreted faults. The interpreted location of the buried normal faults at ∼8 kmAP is consistent between the seismic reflection data and the depth-to-basement model. The fault zone is relatively steep (70° or steeper) and has ∼2 km of normal offset. The fault is buried by younger, largely unfaulted sediments above horizon A. The base of package 3 is chosen at the top of a series of high-amplitude reflections (horizon C; Fig. A2).
Reflection package 4.
This package has roughly constant thickness and is characterized by high-amplitude reflections that have wavy to sigmoidal patterns (Plate A2). The reflection package has an apparent dip similar to that of the overlying package 3, ∼20° to the northeast. The reflections in this package abruptly truncate against the buried normal faults at 8 kmAP; this reflection package is not present to the east of the buried normal faults. We interpret reflection package 4 as part of the Cenozoic basin fill based on the parallelism of the reflections with those in the overlying section and on the truncation of the reflections at the buried normal fault. From the seismic data alone, we would interpret horizon D as the base of the Cenozoic section.
The depth-to-basement solution is inconsistent with the deepest observed strong and continuous reflections (horizon D) at the base of the interpreted Cenozoic section between 1 and 4 kmAP. While the reflectivity indicates a linear base to the Cenozoic section of the northeastern subbasin, gravity implies sinusoidal variation in depth (Plate A2). This inconsistency led us to pursue a more complicated gravity model that incorporates lateral density variations in the basin-fill section described herein.
Discontinuous strong reflections (shown with dashed lines and lowercase d; Plate A2), below and subparallel to horizon D could be interpreted as: (1) reflections arising from the Paleozoic section; (2) a continuation of the Cenozoic section; or (3) multiples from the strong reflections higher in the section. We prefer the interpretation that these reflections originate in the Paleozoic section in which bedding is subparallel to the overlying Cenozoic section. We do not favor the explanation of seismic multiples because several steps in the seismic processing sequence were designed to diminish multiples in the processed image and because the apparent dip of the deeper reflections in time sections does not increase substantially, as would be expected for an origin as seismic multiples. We do not favor the interpretation that these reflections come from deeply buried Cenozoic section because these reflections appear 1–2 km below the modeled depth-to-basement solution, implying densities of basin fill considerably higher than typically assumed.
Mid-basin high reflection interpretation.
The mid-basin high is characterized by the termination of packages of reflections and abrupt changes in reflection amplitude and orientation. Most of these changes are centered around an interpreted subvertical fault at 0.5 kmAP; this fault appears to splay upward into faults with gentler dips (Plate A2). Reflections between the fault strands occasionally define small folds. As expected for vertical-incidence reflection imaging, the steeply dipping faults do not have seismic reflections from their fault surfaces; rather, we identify these faults by how they truncate subhorizontal reflections and separate regions of differing reflectivity character.
Interpretation of reflections within the southwestern subbasin (west of 0.5 kmAP).
To the west of 0.5 kmAP, the southwestern subbasin is characterized by two reflection packages (5 and 6; Plate A2). Neither of the two reflection packages is easily correlated to any of the packages northeast of the mid-basin high. The two basins have generally opposite reflectivity characters: the northeastern subbasin has high-amplitude, continuous reflections deep in the section and discontinuous reflections at shallow depths; the opposite is true for the southwestern basin.
Reflection package 5 within the southwestern subbasin consists of a number of moderate-amplitude, subparallel reflections (Plate A2). Reflections at the base of the package are broadly folded, particularly against the mid-basin high; reflections higher in the package appear subhorizontal. The base of the strong continuous reflections (horizon E) was our preferred pick for the base of the Cenozoic section, given the relative lack of coherent reflections beneath. However, this results in a major discrepancy in interpretation between the seismic reflection data and the depth-to-basement model; horizon E is ∼1 km shallower than the modeled depth-to-basement solution, which would require extremely low densities (<2000 kg/m3) for deposits filling the southwestern subbasin. Instead, we used the depth-to-basement solution to guide our selection of a weak but reasonably continuous reflection (horizon F; Plate A2) as the base of the Cenozoic section in the southwestern subbasin.
Line SSN-15 is oriented southwest-northeast and crosses the central part of the Pahrump Valley, extending northeastward from the northern part of the California Valley, passing Stump Spring, to a northeastern endpoint on the Manse fan near the base of the Spring Mountains (Fig. 2 and Plate A3). As defined by the gravity inversion (Fig. 9), the line crosses the southwestern subbasin to the west of −1 kmAP, the mid-basin high between ∼−1 kmAP and 3 kmAP, and crosses the northeastern subbasin to the east of 3 kmAP. In comparison to line SSN-17, the southwestern subbasin is deeper and the northeastern subbasin is shallower on line SSN-15. The only seismic tie point is with SSN-12, directly over faults associated with the mid-basin high; neither seismic line shows any useful reflections near this tie point.
Interpretation of reflections within the northeastern subbasin (east of 3 kmAP).
On line SSN-15, the northeastern subbasin has sag-basin geometry in which reflection packages define a broad, basin-wide syncline. Basin-bounding faults are relatively difficult to define on the basis of truncated reflections. Seismic reflections interpreted as arising within Cenozoic basin fill within the northeastern subbasin are defined as two reflection packages (1 and 2; Plate A3) that are bound by unconformities (horizons A and B; Plate A3). Both reflection packages are thin on the basin margins and thickest in the basin center. Both packages contain continuous, relatively high amplitude reflections, and the basinward thickening of each package is accomplished by the thickening of reflection cycles and the appearance of additional reflections. Horizon A marks the base of reflection package 1, where reflections show subtle onlap relations with the underlying package (Plate A3). Horizon B marks the base of reflection package 2 and is interpreted to represent the base of the Cenozoic section. Reflections beneath horizon B are less continuous and have more variable dips than the overlying reflections. High-amplitude, parallel reflections in the lower half of reflection package 2 may correspond to a similar, but thicker section seen at depth on the northeast subbasin of SSN-17.
The existence and location of basin-bounding faults is difficult to establish from either the seismic reflections or the depth-to-basement solution. We have located a fault bounding the northeastern side of the basin at ∼9.5 kmAP where there appears to be a change in reflection character at depth and some truncation of reflections in the shallower part of the section (Plate A3). A thin section of discontinuous reflections to the east of the fault probably represents coarse alluvial gravels overlying a range-front pediment.
As on line SSN-17, the depth-to-basement solution is inconsistent in detail with both the observed reflections interpreted as part of the Cenozoic section (reflection package 2) and with the interpreted base of the Cenozoic section (horizon B). This inconsistency led us to pursue a more complicated gravity model that incorporates lateral density variations in the basin-fill section, described herein.
Mid-basin high reflection interpretation.
The mid-basin high along the Stateline fault system is broader and more complex on line SSN-15 than on SSN-17. Here it takes the form of a broad, faulted antiform. The depth-to-basement solution along the western margin of the mid-basin high at ∼−1 kmAP indicates almost 1.5 km of structural relief, consistent with the rapid thinning and disappearance of reflection packages to the southwest of the mid-basin high (Plate A3). The eastern margin of the mid-basin high is more diffuse and not so obviously bound by a large-offset fault. The interpreted top of pre-Cenozoic rocks (horizon B; Plate A3) is interrupted by several faults.
Interpretation of reflections within the southwestern subbasin (west of −1 kmAP).
The southwestern subbasin on line SSN-15 is almost twice as deep as the northeastern subbasin, and it contains three distinct reflection packages. These packages are distinguished by reflection character, apparent dip, and degree of faulting.
Reflection package 3 is the uppermost package (Plate A3) in the southwestern subbasin, and it consists of moderate-amplitude, regular, subparallel, reflections that are broadly folded. Reflections have their highest amplitude and continuity near the center of the subbasin, and they lose definition toward the southwestern margin and the mid-basin high. This change in reflectivity character likely represents a transition from fine-grained sediments along the basin axis to coarser-grained alluvial sediments at the basin margins. Fold amplitude appears to increase downward; high-amplitude reflections at the base of the package at ∼−1.5 kmAP define an anticline that terminates against the mid-basin high. The base of this package (horizon C; Plate A3) is defined by the base of the continuous, parallel reflections.
Reflection package 4 (Plate A3) extends from ∼200 m elevation to a prominent reflection pair at ∼−750 m elevation below −3 kmAP (horizon D; Plate A3). Reflections in this package are less continuous and of lower amplitude than those of the overlying package. The reflections appear to be disrupted by small-offset faults (not interpreted in Plate A3) and to be folded against the mid-basin high, although some of the changes in apparent dip are likely the result of bends in the section. This package thins to the southwest; reflections are truncated by faulting along the southwest basin margin.
Reflection package 5 (Plate A3) underlies the reflection pair at ∼−750 m elevation (horizon D; Plate A3); the reflection pair decreases in amplitude approaching the mid-basin high. In this package, reflections are internally faulted, some of the faults appearing to have a reverse sense of offset. The base of this reflection package was chosen at a series of very strong and evenly spaced reflections (horizon E; Plate A3) interpreted as pre-Cenozoic bedrock.
Line SSN-12 is oriented northwest-southeast and generally follows the Nevada-California state line along the axis of the Pahrump and Mesquite Valleys (Fig. 2 and Plate A4). As defined by the gravity inversion (Fig. 9), the line generally parallels the trace of the Stateline fault system in the Pahrump Valley and generally follows the structural high associated with the fault system. As a result, this seismic section, more than any other in this study, is prone to reflections that arise from structures to the side of the seismic line, even though the processing sequence was designed to minimize such sideswipe.
The northwestern part of SSN-12 in the Pahrump Valley extends northwestward from Black Butte (Fig. 1); the line ties with lines SSN-15, SSN-17, and SSN-19 as it crosses the Pahrump Valley. The interpretation of seismic reflections from the northwestern part of SSN-12 in the Pahrump Valley is presented in three parts as follows: from the southwest end of this part of SSN-12 near Black Butte to the tie point with SSN-15 (from 31 to 46 kmAP); from the tie point with SSN-15 to the tie point with SSN-17 (from 18 to 31 kmAP); and northwest of the tie point with SSN-17 to the northwest end of line SSN-12 (from −1 to 18 kmAP).
Interpretation from the tie point with SSN-15 to the southwest end of this part of SSN-12 near Black Butte (31–46 kmAP).
This part of line SSN-12 is interpreted to display two reflection packages (3 and 4; Plate A4) that overlie a broadly undulating surface (horizon D; Plate A4) that is interpreted to represent the top of the pre-Cenozoic section (reflection package 5; Plate A4). Reflection package 3 is characterized by moderate-amplitude reflections that are only locally continuous; reflection package 4 is characterized by distinctly lower amplitude, short reflections. The boundary between these two reflection packages (horizon C; Plate A4) is primarily defined on this basis of change in amplitude. The top of the pre-Cenozoic section (horizon D; Plate A4) is interpreted at the top of a series of high-amplitude, variably oriented reflections. Broad undulations of this surface may be fault controlled, but poor reflection quality limits the degree to which fault offset can be confidently interpreted. The depth-to-basement profile is in general agreement with the seismic interpretation; elevation differences of 100–200 m between the two surfaces probably result from unaccounted-for lateral variations in density of the underlying units.
Between 36 and 40 kmAP, reflections within reflection package 3 are folded into an antiform, as shown by reflections within the package dipping away from each other on either side of the interpreted structure (Plate A4). This culmination coincides with an anticline mapped at the surface (Workman et al., 2002a) that exposes Pliocene–Miocene sediments in its core and that is along the Stateline fault system, just to the north of Black Butte (Fig. 2). At the surface, the rocks are strongly deformed, with dips to 50° (McMackin, 1999).
Interpretation between the tie point with SSN-15 and the tie point with SSN-17 (18–31 kmAP).
On this part of line SSN-12, three reflection packages are defined on the basis of reflection dip, amplitude, and reflection terminations. The uppermost reflection package (1; Plate A4) is characterized by moderate-amplitude reflections that are generally short. The base of this package (horizon A; Plate A4) is interpreted at a high-amplitude, continuous reflection pair; reflections in package 1 display onlap onto this horizon. Reflection package 1 is thickest to the southeast (toward the tie point with SSN-15), where it is as thick as 700 m; the package thins to the northwest on the flanks of a structural high near 18 kmAP. The middle reflection package (2; Plate A4) displays reflections that are continuous and show pronounced internal convergence that results in a sigmoid shape of reflections and a thickening toward the southeast. Reflection package 2 is bound above by horizon A and below by horizon B. Reflections in this package show pronounced onlap onto the underlying unconformity (horizon B; Plate A4). The deepest reflection package that is interpreted to be part of the Cenozoic section (reflection package 3; Plate A4) consists of a series of high-amplitude, generally parallel reflections that are also generally parallel with the interpreted base of the pre-Cenozoic section (horizon C; Plate A4). Package 3 has a near-constant thickness of ∼200 m.
On the basis of the gravity inversion (Fig. 9) and reflection packages on the southwestern parts of lines SSN-15 and SSN-17, this part of line SSN-12 is inferred to be just west of the Stateline fault system and to portray reflections within the southwestern subbasin. From the SSN-17 tie, reflection packages thicken to the southeast, reaching a maximum thickness of ∼1.5 km beneath 25 kmAP (Plate A4). The gravity-derived depth-to-basement solution does not correspond well to the reflection interpretation in this part of SSN-12, likely the result of the near parallelism of the SSN-12 line with the Stateline fault system. The three reflection packages come to an apparent termination to the southeast against an ill-defined structure as the line becomes coincident with the mid-basin high (Fig. 9); a dashed fault schematically represents this transition.
Interpretation to the northeast of the tie with SSN-17 (−1–18 kmAP).
Just to the north of the tie point with SSN-17, the three reflection packages described above are draped over a structural high. The highest reflection package (1; Plate A4) appears to have been removed by erosion on the crest of the structural high. On the basis of the seismic reflections, the structural high appears to be related to a fault-bound uplift of pre-Cenozoic rocks, yet the depth-to-basement solution does not correspond well to the reflection interpretation in this part of SSN-12. The map pattern of the southwestern subbasin in relation to the trace of SSN-12 (Fig. 9) suggests that the line traverses the irregular northeastern edge of the basin and is probably obliquely recording the presence of the mid-basin high. Two shallow wells near this part of the line are dominated by clay-rich sediments, consistent with the parallel, moderately high amplitude reflections seen in the shallow subsurface of this portion of SSN-12.
North of 12 kmAP, the depth-to-basement solution generally follows an undulating surface, horizon C, marked by discontinuous reflections that are slightly higher amplitude than those above and below (Plate A4). The interpreted Cenozoic section lacks continuous reflections, perhaps the result of proximity to the topographic front of the Nopah Range, where an increase in the amount of alluvial gravels is expected in the section. However, there are no water wells along this portion of the line to confirm this suggestion.
SSN-10 reflection interpretation.
The Cenozoic basin fill on SSN-10 is interpreted to consist of two reflection packages (packages 1 and 2; Plate A5). Reflection package 1 consists of short, discontinuous, variably oriented reflections. Package 2 consists of slightly higher amplitude reflections that, at least in the center of the section, are more continuous than the overlying reflections. The boundary between the two reflection packages (horizon A; Plate A5) was selected at the change in reflection character. The distinction between the two reflection packages becomes less clear at either end of the section, in part due to thinning of the lower reflection package. The base of the Cenozoic basin fill is chosen at a horizon (horizon B; Plate A5) where there is as much as 20° of apparent angular discordance between reflections in package 2 and underlying reflections (best seen between 2 and 3 kmAP). Relatively continuous and high-amplitude reflections below this horizon are interpreted to come from the pre-Cenozoic section (reflection package 3; Plate A5). Reflections in package 3 may arise from rapid changes in acoustic impedance associated with thin-bedded carbonate rocks of the upper Mississippian–Lower Permian Bird Spring Formation, exposed in outcrop in the Spring Mountains to the northeast of SSN-10 (Page et al., 2005).
In the interpreted Cenozoic basin fill, geologic mapping of surficial materials and water-well control down to ∼300 m suggest that the variably oriented reflections in package 1 probably arise from recent alluvial fans. The more continuous reflections within package 2 suggest deposition in a more quiescent depositional environment and suggest that basin fill corresponding to reflection package 2 may be significantly older than the overlying fan material.
Line SSN-11 is the single southwest-northeast line that crosses the Mesquite basin (Fig. 2 and Plate A6). Both ends of the line cross bedrock that is covered by very thin alluvium; the center portion of the line crosses the southern end of the deepest part of the basin (Fig. 9).
Interpretation of reflections within basin fill.
The Mesquite basin has a half-graben geometry; basin-fill sediments have apparent dips to the northeast and thicken to the northeast, where they lose definition to the west of the interpreted basin-bounding fault (Plate A6). On the southwestern side of the basin, the reflection packages interpreted to arise from the basin-fill sediments generally appear to lap onto interpreted pre-Cenozoic rocks. However, reflections are also truncated by a number of relatively small-offset faults along the southwest side of the basin. Seismic reflections interpreted as arising within the basin fill are defined as four reflection packages (1, 2, 3, and 4; Plate A6) that are bound by unconformities (horizons A, B, and C; Plate A6).
Reflection package 1.
This uppermost package (1; Plate A6) consists of parallel, moderate-amplitude continuous reflections that can generally be traced across the entire basin (from −4–2 kmAP). This package is a roughly constant 300–400 m in thickness and it consists of reflections that are nearly horizontal in this image. On the basis of reflection continuity, shallow depth, and location at the basin axis, this reflection package is interpreted to be mostly thin-bedded playa and basin-axis deposits, although the package is displaced 1–2 km eastward relative to the modern playa mapped at the surface. The base of this package is an unconformity (horizon A; Plate A6); reflections beneath this horizon show distinct low-angle erosional truncation at this horizon, whereas reflections above this horizon are continuous and parallel with the underlying unconformity. This package is consistent with the uppermost package seen on the southeast end of SSN-12 and is similarly bound at the base by a low-angle unconformity.
Reflection package 2.
This package (2; Plate A6) consists of high-amplitude reflections that are gently sigmoidal in shape and divergent to the northeast. Reflection package 2 is ∼400 m thick and is bound above and below by unconformities (horizons A and B, respectively; Plate A6). The upper boundary of this reflection sequence (horizon A; Plate A6) appears to be an erosional surface; reflections abruptly truncate at this horizon. The base of this package appears to be another slight angular unconformity (horizon B; Plate A6); reflections of package 2 appear to lap onto this horizon. Both bounding horizons are also apparent on the crossing line SSN-12. This reflection package thickens to the northeast primarily by addition of reflections; the basin depocenter was apparently skewed to the northeast side of the basin adjacent to the main bounding fault. Reflection quality is degraded near the interpreted basin-bounding fault on the northeast side of the basin. This degradation in seismic signal is interpreted to be the result of coarse debris associated with the fault (reflection package 5; Plate A6) that has much poorer reflection character than the fine-grained sediments in the basin.
Reflection package 3.
This package (3; Plate A6) consists of high-amplitude, moderately continuous reflections in a package that is distinctly wedge shaped. Reflections are divergent, such that the wedge-shaped geometry of the package is accomplished by northeastward thickening of reflection cycles and the appearance of additional reflections. The upper boundary of this reflection sequence (horizon B; Plate A6) appears to be an erosional surface; reflections truncate at this horizon. The base of this package appears to be an angular unconformity (horizon C; Plate A6); reflections of package 3 appear to lap onto this horizon. There are no obvious faults within this lower sequence, although it is folded, with apparent dips on the eastern limb of the fold to 35°. On the southwest margin of the basin, reflections within package 3 lap onto horizon C along a distinct angular unconformity.
Reflection package 4.
We originally interpreted the base of the Cenozoic section at the top of the high-amplitude, upward-curved reflections (horizon C; Plate A6) that are as deep as −800 m elevation beneath the basin axis (0.5 kmAP). The depth-to-basement inversion suggests that there may be an additional 250 m of section that overlies pre-Cenozoic basement. This interval, defined as reflection package 4, is characterized by very high amplitude, parallel reflections (Plate A6). The package is bound above by horizon C, where reflections in the overlying package lap onto this horizon with a distinct angularity. The base of the package, and our preferred choice for the base of the pre-Cenozoic section, is defined by a sharp drop-off in reflection amplitude and continuity (horizon D; Plate A6), although these underlying reflections remain subparallel to reflections within package 4.
Reflection package 4 is much more strongly deformed than the overlying packages (Plate A6). Normal faults disrupt the reflections on the southwest basin margin; in general most of these faults cannot be traced into the overlying reflection packages. The package is also strongly folded against the basin-bounding fault into a broad syncline with an amplitude of ∼0.5 km.
On the southwest margin of the basin, the modeled depth-to-basement profile is generally coincident with the interpreted top of pre-Cenozoic rocks as it is downdropped basinward along the series of stepped faults. In detail, the depth-to-basement solution diverges from the interpreted top of pre-Cenozoic rocks based on the seismic image (Plate A6). The gravity inversion from −5 to −4 kmAP is slightly shallower than the continuous reflections that underlie this horizon that are interpreted to be part of the pre-Cenozoic section, but the mismatch is only ∼200 m in thickness. Basinward, the modeled top of depth-to-basement diverges from horizon D, the interpreted top of pre-Cenozoic basement, and the along-profile positions of greatest basin depths differ by ∼1 km.
Fault location on the northeastern side of the Mesquite basin.
The fault that bounds the northeast side of the Mesquite basin is expressed as a moderately steep gravity gradient, but is not imaged well on the seismic line. Rocks to the east of the fault have almost no reflections to aid in locating the fault, but reflections within the basin and at the basin floor preclude a <60° west-dipping fault that would be coincident with the depth to basement solution. The fault is portrayed with a steeply west dipping geometry on the basis of truncation of deep basin reflections. Within the basin immediately to the west of the fault, reflections are of poor quality and are short and variably oriented. This area (package 5; Plate A6) is inferred to be dominated by acoustically quiet, coarse-grained material shed off the evolving fault that interfingers with basin-fill sediments within the basin. The presence of this interpreted coarse-grained fill adjacent to three of the reflection packages implies a long-lived period of fault motion that spans much of the basin history.
Reflections interpreted to be within the pre-Cenozoic bedrock and within the deepest part of the overlying basin fill appear to be folded against and dragged upward along the southwest side of the basin-bounding fault. This is a major upwarp with an amplitude of ∼500 m (Plate A6).
The gravity-derived depth-to-basement inversion has a maximum gradient somewhat to the southwest of the fault interpreted from the seismic data, at ∼1 kmAP, and the depth-to-basement inversion surface crosses continuous reflections in the basin fill (Plate A6). These differences are permissible given the intrinsic smoothing of gravity anomalies that arise from depth, the irregular spacing of gravity observations, and the grid cell size, 500 m, of the depth-to-basement gravity solution.
Interpretation of bedrock reflections.
For stretches of >8 km, both the northeastern and southwestern ends of line SSN-11 cross bedrock that is covered by very thin alluvium. In these areas, reflection quality is generally poor, but we have attempted to relate mapped geology at the surface to broad changes in reflection character.
Northeast end of line SSN-11.
From ∼5 kmAP to the east end of the line, SSN-11 is close to bedrock outcrops in the southern Spring Mountains (Fig. 2). Two thrusts are exposed in the southern Spring Mountains; the Green Monster thrust and the Keystone thrust (Hewett, 1956; Carr, 1983; Burchfiel et al., 1998), from west to east at ∼8 kmAP and ∼12.5 kmAP, respectively. Both thrusts carry rocks as old as Cambrian Bonanza King dolomite over thick limestone of the Mississippian Monte Cristo Group and Pennsylvanian–Permian Bird Spring Formation. We interpret the Green Monster thrust as separating reflections with slightly higher amplitude and continuity in the upper plate from lower amplitude and less-continuous reflections below (Plate A6). The Keystone thrust is interpreted to separate discontinuous, moderate-amplitude reflections in the upper plate from lower plate reflections that are slightly more continuous and higher amplitude (such as at 12.5 kmAP at 1.0 km elevation; Plate A6).
Southwest end of line SSN-11.
From ∼−21 to −9 kmAP, line SSN-11 is close to bedrock outcrops in the Mesquite Mountains (Fig. 2). The line is approximately coincident with, but generally south of, the trace of the Winters Pass thrust. Reflections near the top of the reflection section with higher amplitudes and slightly greater continuity (between −21 and −13 kmAP, between 0.8 and 1.0 km elevation; Plate A6) than underlying reflections may represent late Proterozoic siliciclastic rocks in the upper plate of the thrust; these rocks overlie early Paleozoic carbonate rocks within the Mesquite thrust plate, beneath the Winters Pass thrust (Burchfiel and Davis, 1971).
The southeastern part of SSN-12 in the Pahrump Valley extends southeastward from Black Butte (Fig. 2) and ties with SSN-13 and SSN-11 as it crosses the Mesquite Valley. The near-parallel orientation of SSN-12 with respect to the predominant structural grain introduces complexities in interpreting reflections in terms of basin evolution and in comparisons with gravity models of basin structure.
Plate A7 shows the southeastern part of SSN-12, where the line crosses the Mesquite basin to the south of Black Butte. The Mesquite basin displays at least four reflection packages that are bound by unconformities. To the northwest, most of the packages lap onto a structurally controlled high. Line SSN-12 ties to the SSN-11 near the axis of the Mesquite basin.
Reflection package 1.
The uppermost reflection package (1; Plate A7) consists of parallel, continuous reflections that can be traced across the entire basin. The package is as much as 600 m thick at the south end of the basin, but it thins to zero toward the northwest (52 kmAP) as the unit laps onto underlying reflection packages that are uplifted on the northern edge of the basin. Reflections within this package have gently fanning dips; reflections are subhorizontal high in the section, whereas the lower reflections in this package have apparent dips of ∼5°. This sequence shows pronounced onlap onto the underlying unconformity (horizon A; Plate A7). This unconformity ties well with SSN-11 (Plate A6), where a similar reflection package with subhorizontal, continuous reflections laps onto a major unconformity at an elevation of ∼300 m. The upper reflection package is interpreted to be thin-bedded playa and basin axis deposits that are thought to have been deposited after tilting.
Reflection packages 2 and 3.
Beneath reflection package 1 are two reflection packages (2 and 3; Plate A7) that are bound above and below by angular unconformities. Reflections within both packages have southward apparent dips and show pronounced internal convergence that result in a sigmoid shape of reflections and thickening toward the basin axis. Reflection package 2 is ∼400 m thick and is bound above by horizon A and below by horizon B. This reflection package shows pronounced onlap onto the underlying unconformity (horizon B; Plate A7); this unconformity ties well with SSN-11 (Plate A6). Reflections within package 2 appear to be folded, although some of this may result from southward depositional progradation. Reflection package 3 is 300–400 m thick and is bound above by horizon B and below by horizon C. This reflection package shows pronounced onlap onto the underlying unconformity (horizon C; Plate A7). This unconformity ties well with SSN-11 (Plate A6), where reflections clearly onlap onto this surface. Reflections within this package have a sigmoidal pattern similar to those in the overlying package.
Reflection package 4.
This relatively thin (100–250 m) package is bound above horizon C and below by horizon D (Plate A7). Reflection quality within this package is generally poor, and reflections are subparallel to the underlying unconformity.
Reflection package 5.
This package consists of high-amplitude reflections that are bound at the top by horizon D (Plate A7). Dips within this interval are highly variable, and there are numerous faults that do not appear to affect the overlying sequences. The base of this older package is not easily picked from the seismic data. Based on similar reflection character on other seismic lines in the Mesquite and Pahrump basins, including elsewhere on SSN-12, this package might reasonably be interpreted as pre-Cenozoic rocks, and horizon D would thus be chosen as the base of the Cenozoic section. However, this creates a large discrepancy with the depth-to-basement solution between 54 and 62 kmAP (Plate A7). Alternatively, this package of high-amplitude reflections may be interpreted as a tilted, faulted interval of older Tertiary strata, the contact with underlying basement being difficult to discern. In either alternative, the depth-to-basin solution crosses reflections and interpreted horizons in the deep portion of the southeast end of the basin.
Interpretation of basin-bounding structures.
The northeastern bounding structure of the Mesquite basin is not well expressed on line SSN-12 because the fault is nearly parallel to the seismic line. As such, the location of the bounding fault shown (∼66 kmAP; Plate A7) is approximate, but in general agreement with the depth-to-basement profile.
The northwestern end of the Mesquite basin at 50–51 kmAP is bound by a structurally controlled high. The structural high may be bound on its southern edge by a northwest-dipping reverse fault that offsets basement reflections but folds the overlying reflection packages (51 kmAP; Plate A7).
The northwestern end of this part of line SSN-12 is interpreted to display two reflection packages (3 and 4; Plate A7) that overlie a broadly undulating surface (horizon D; Plate A5) interpreted to represent the top of the pre-Cenozoic section (reflection package 5; Plate A5). Reflection package 3 is characterized by moderate-amplitude reflections that are generally short; reflection package 4 is characterized by distinctly lower amplitude, short, variably oriented reflections. The boundary between these two reflection packages (horizon C; Plate A7) is primarily defined by the change in amplitude, rather than on reflection terminations at the boundary between the two packages. The top of the pre-Cenozoic section (horizon D; Plate A7) is interpreted at the top of a series of high-amplitude, variably oriented reflections. Broad undulations of this surface may be fault controlled, but poor reflection quality limits the degree to which fault offset can be confidently interpreted. The depth-to-basement profile is in general agreement with the seismic interpretation; elevation differences of 100–300 m between the two surfaces probably result from unaccounted-for lateral variations in the density of the underlying units.
Line SSN-13 is oriented southwest-northeast; the southwest end of the line is at the base of the Kingston Range, the line passes just south of Black Butte, and continues northeast to the base of the Spring Mountains (Fig. 2 and Plate A8). The line generally parallels the modest topographic high and, based on the gravity inversion, the pronounced structural high between the Pahrump and Mesquite basins (Fig. 9).
For most of the northeastern and southwestern parts of this line, the depth-to-basement solution suggests that basin-fill sediments are very thin; on the eastern part of the line, the solution predicts that basin-fill sediments are at depths shallower than the seismic acquisition can image (Plate A8). The seismic character beneath the northeastern and southwestern parts of the line is typical of reflections from pre-Cenozoic rocks: moderate- to high-amplitude reflections that are generally discontinuous and variably oriented. The interpreted contact between basin fill and pre-Cenozoic bedrock is not shown on the northeastern part of the line, where the location is inferred to be at depths shallower than the seismic experiment could image. No water-well data are available in the vicinity of this line to aid in the interpretation of basin-fill materials.
Interpretation of reflections within the basin.
In the small basin near the center of the line, reflection quality is poor and the interpretation of the top of the pre-Cenozoic section (horizon B; Plate A8) is mostly guided by the depth-to-basement solution. The interpreted Cenozoic basin fill (reflection package 2; Plate A8) consists of short, discontinuous, variably oriented reflections. The interpreted underlying pre-Cenozoic section has generally higher amplitude reflections with occasional longer, more continuous reflections.
A thin package of higher-amplitude, more continuous reflections (reflection package 1; Plate A8) is at the top of the interpreted Cenozoic basin fill, separated by horizon A from underlying lower amplitude, discontinuous, variably oriented reflections (Plate A8). Line SSN-13 crosses just to the south of Black Butte, which projects onto the line between 1 and 3 kmAP, in the vicinity of the high-amplitude reflection package. This reflection package may be a buried portion of the carbonate megabreccia deposits that crop out at Black Butte (Guest et al., 2007), or may represent bedded lacustrine deposits known to underlie the megabreccia.
Interpretation of bedrock reflections.
On the basis of projecting surface geology and faults interpreted from the shape of the depth-to-basement solution (Blakely et al., 1998; Potter et al., 2002b), the Stateline fault system is interpreted to cross SSN-13 near 5 kmAP (fault shown as vertical dashed line; Plate A8). At this location, there is no great inflection in the depth-to-basement solution; however, there is a cutoff of relatively continuous, high-amplitude bedrock reflections in the shallowest section that are common on the northeast end of line from 5 to 13 kmAP. These reflections appear disrupted and ultimately truncated at ∼5 kmAP. A second fault, perhaps associated with the Stateline system, is interpreted at ∼0.5 kmAP, where reflections in the bedrock beneath horizon B are truncated (vertical dashed line; Plate A8). However, because of the poor reflection quality, it is difficult to project the faults to depth or to characterize the Stateline fault system as a single or multiple strand system where crossed by SSN-13.
On the west end of SSN-13, the line passes very close to outcrops of pre-Cenozoic rocks in the Kingston Range (Fig. 2). Continuous, high-amplitude, broadly folded reflections in the upper 300–400 m of the section may represent pre-Cenozoic section in the upper plate of the Wheeler Pass thrust (Wernicke et al., 1988; Snow, 1992); the dashed line on the southwestern part of the seismic line separates packages of distinctly different reflection character and may represent the base of the thrust plate.
We thank Rick Blakely, Chris Potter, Terry Pavlis, and an anonymous reviewer for constructive reviews of this manuscript. We thank Geophysical Pursuit, Inc., for permission to publish the seismic reflection sections.