Geophysical constraints on the cessation of extension and thickness of basin fill in Tecopa Valley, California

John N. Louie, Arif Cetintas, Vijay Chekuri, William D. Corchuelo, Yutian Lei, Li Li, Govardhan Mekala, Serdar Ozalaybey, John Raskulinecz
Seismological Laboratory and Department of Geological Sciences, The University of Nevada, Reno, NV 89557-0141; (775) 784-4219; fax (775) 784-1833; louie@seismo.unr.edu

Roger B. Morrison
Morrison and Assoc., 13150 W. Ninth Ave., Golden, CO 80401

submitted to L. A. Wright and B. W. Troxel, eds., Cenozoic Basins of the Death Valley Region: Geol. Soc. Amer. Spec. Pap., revised 1 June.

ABSTRACT

Studies of the geometry of basin fill in Tecopa Valley using shallow (<500 m) seismic, magnetic, and gravity exploration provide clues to the tectonic history of the basin. Tecopa Valley is the first major structural basin and late-Tertiary depocenter east of southern Death Valley. It lies just north of the Grand View fault zone, a regional structural boundary. The geophysical studies show that the structural basin below Tecopa Valley formed during an extensional episode between 11 and 7 Ma lasting 4 m.y. or less. Extension nearly ceased after 7 Ma. The post-11 Ma sedimentary fill in south-central Tecopa Valley is more than 500 m thick, of which the upper 134+-20 m below the 2 Ma Huckleberry Ridge tephra layer (``Tuff C'') are fine-grained Lake Tecopa beds showing little deformation.

INTRODUCTION

Mechanisms of late Cenozoic extension of the southern Great Basin near Death Valley in California and Nevada are reflected in the geometry of fluvial and lacustrine basins at shallow depths. Tecopa Valley contains one such basin, the first major structural basin east of southern Death Valley. Small-scale geophysical techniques, demonstrated here, give insights into this geometry and into the timing and rates of late Cenozoic extension.

Tectonic Setting

The Great Basin province consists of many north-trending grabens separated from surrounding horsts by normal faults. Stewart (1971) characterized the normal faulting as a product of crustal extension during the Cenozoic of as much as 20% across the province. Wright and Troxel (1973) and Wright (1976) modified this model for the southern Great Basin near Death Valley, California (Fig. 1). Interaction between shallow-dipping normal faults and complementary pairs of strike-slip faults (``pull-aparts'') would allow extensions of up to 50% in this area.
Figure 1
Figure 1- Map of the Death Valley region of eastern California and Nevada, with major fault lines and geography. The shading indicates relative elevation. ``CC8,'' ``CC9,'' and ``CC10'' refer to COCORP survey line numbers (Serpa et al., 1988). (Click on image for viewable Adobe Acrobat PDF version.)

Wernicke (1992), Wernicke et al. (1988), and Snow and Wernicke (1989) challenge the traditional models of the region by observing correlations between widely separated Devonian to Mesozoic folds and thrust sheets that deform a late Proterozoic to Paleozoic miogeoclinal sedimentary sequence. Using the offsets of marker structures within the orogen along the Northern Death Valley-Furnace Creek fault zone, Snow and Wernicke (1989) and Wernicke et al. (1988) derive extensions for the Death Valley region far in excess of 100%. Wernicke et al. (1988), in the manner of Wernicke (1981), suggest that this extension is accommodated in the upper crust below Tecopa Valley by a shallow, gently dipping regional detachment extending from a breakaway zone in Pahrump Valley west to the Sierra Nevada (Fig. 1). If this detachment conforms to the ``rolling hinge'' model of isostatic rebound in response to tectonic denudation (Wernicke and Axen, 1988), then the Pahrump, California, Chicago, and Tecopa Valleys should have opened in that order near the beginning of extensional activity. Now the hinge and active extension have progressed westward to central Death Valley.

An alternative extensional model offered by King and Ellis (1990) would constrain the extension due to normal faulting to less than 50%. In their model the flow of inviscid mid-crustal material into the footwall of steep normal faults would provide the deep levels of exposure observed in the Black Mountains and at the edge of the Colorado Plateau east of Las Vegas (Fig. 1). King and Ellis (1990) imply that lateral displacement along intersecting, complementary strike-slip faults such as the Northern Death Valley-Furnace Creek, Southern Death Valley, and Garlock fault zones (Wernicke et al., 1982) would have to explain the displacement of structural features without large net extensions.

Previous geophysical research

Previously published geophysical work east of Death Valley includes regional gravity, magnetic, and seismic reflection surveys. Mabey (1963) and Nilsen and Chapman (1974) report Bouguer anomalies at a total of six gravity stations in northern Tecopa Valley and eight in the southern part of the Valley. Blake et al. (1978) compiled total-field aeromagnetic surveys over the Tecopa Valley region; five-mile intervals separated the high-altitude flight lines.

The Consortium for Continental Reflection Profiling (COCORP) collected 250 km of deep-crustal seismic surveys in the Death Valley region (Serpa et al., 1988). Two of the five surveys crossed Tecopa Valley (Figs. 1, 2). The COCORP consortium designed the survey parameters of these lines to give high resolution in the deep crust, using low frequencies and source and receiver stations spaced 100 m apart, thus limiting resolution of the shallow crust. Serpa et al. (1988) published stacked sections and line drawings from these surveys at small scale. These suggest the presence of reflections below southern Tecopa Valley at between 0.5 and 1.2 s. Using reasonable ranges of velocity in the uppermost section (Serpa et al., 1988), these reflections could arise at depths between 300 and 2400 m below the surface. On line 10 the reflections deepen near the center of Tecopa Valley. If they represent a reflective basin fill sequence and the basement interface, then Tecopa Valley is probably several hundred meters deep.

Tecopa Valley

Tecopa Valley contains a sequence of Recent to Miocene mudstones, tufas, and volcanic ashes deposited in Pleistocene Lake Tecopa, ringed by conglomerates on its margin (Hillhouse, 1987; Morrison, 1991). Morrison (1991) shows that the 72 m of exposed Lake Tecopa sequence dates back at least 5 m.y. to the late Miocene and is relatively undeformed. Underlying parts of the lacustrine sequence are late-Tertiary basalts possibly as young as 4 Ma (Hillhouse, 1987). At the south end of Tecopa Valley the Pleistocene Lake Tecopa sequence onlaps to the mudstones, fanglomerates, and pumice of the China Ranch Beds. The large slump blocks and megabreccias that comprise the fanglomerates suggest a history of uplift and rapid erosion in the latest Tertiary, as they include clasts of Tertiary volcanics (Wright, 1973).

In the Dublin Hills and the Resting Spring Range (Fig. 2) pyroclastic tuffs outcrop with ages from 5.3 to 11 Ma (Fleck, 1970; and Wright, 1991 unpub. data). While normal faulting has broken and exposed the tuffs and older rocks in the mountains around the basin, the Lake Tecopa sediments show much less deformation and only subtle faulting. Morrison (1991) attributes them to an early phase of slow deposition at a rate of 5 m/m.y., increasing after about 0.9 Ma to 100 m/m.y., all in a quiet tectonic environment. Holocene alluvial fan deposits overlying the Pleistocene sequence likewise show little folding or faulting (Hillhouse, 1987; R.B.M. field observations). These observations suggest the tectonism that formed Tecopa Valley largely ceased between 11 and 5 Ma. The draining of Lake Tecopa and erosion of Tecopa Valley by the Amargosa River (Fig. 2) since 0.16 Ma is probably related to the subsidence of the central Death Valley graben and the lowering of hydrographic base level to the west (Morrison, 1991).

Figure 2
Figure 2- Map of Tecopa Valley showing the general extent of exposures of the Lake Tecopa sediments (Hillhouse, 1987). The medium shading indicates mountains and hills exposing pre-Neogene rocks (pre-Cambrian to mid-Miocene, ~10 Ma and older). State highway numbers, the locations of COCORP lines, and the profiles listed in Table 1 are labeled. Distances along the section in Fig. 4 are indicated. (Click on image for viewable Adobe Acrobat PDF version.)

OUR GEOPHYSICAL WORK

The geologic and geophysical observations outlined above suggested the use of shallow geophysical techniques to probe the structure of Tecopa Valley. Given the age constraints on 72 m of the Lake Tecopa sequence (Hillhouse, 1987; Morrison, 1991), simply finding the true depth of basin fill in contrast to older rocks could constrain the amount and rapidity of Miocene extension. We sought to distinguish Miocene-Pleistocene fluvial and lacustrine sediments as basin fill, in contrast to a basement that includes 5.3 Ma volcanics and all rocks older than 10 Ma.

Small-scale geophysical methods are particularly suited to the search for basin geometry within 1 km of the surface. In the Basin and Range and Mojave Desert, Plio-Pleistocene fluvial and lacustrine basin fill often has physical properties that contrast sharply with older-rock basement. Basin fill typically has significantly lower densities, magnetic susceptibilities, and seismic velocities. They usually also show much higher permeabilities, creating large ground water permeability contrasts and electrical resistivity variations in this arid climate.

Three years of one-week annual geophysical field camps operated out of courses at Penn State and UNR have conducted gravity, magnetic, seismic, and electromagnetic surveys in Tecopa Valley. Each year the application of several geophysical methods to the same profile provided constraints on the interpretations that would not be available from one or two methods alone. Table 1 and the Acknowledgments summarize the work done and its locations.

TABLE 1. WORK OF PSU AND UNR FIELD CAMPS IN TECOPA VALLEY
Spring 1990Spring 1991Spring 1992*
UniversityPenn StatePenn StateU. Nevada, Reno
Profiles (Fig. 2)SHO OSTTHSAR CCV+
Gravity stations7775+
Magnetic stations74150+
Seismic source points3429107
EM measurements548204
Results 600 m deep basin, steep range-front fault Deep basin is continuous Early Lake Tecopa stratigraphy, geometric differences of basins
* Additional exercises in 1994 and 1996 focused on Pahrump Valley and will be reported elsewhere.
+ The 1992 field camp conducted potential field surveys outside Tecopa Valley in Chicago Valley. Some of these locations are outside the area shown on Fig. 2.

RESULTS

The principal results from the Spring 1990 exercise were the location of unexpectedly thick Neogene basin fill in Tecopa Valley, and the characterization of the Resting Spring Range-front fault as a deeply penetrating, basalt-intruded break. Previous, widely spaced gravity data in Tecopa Valley (Mabey, 1963) had missed the presence of a 13 mGal negative Bouguer anomaly along the Amargosa River (Figs. 2 and 3) that our 100 m station spacing along profile SHO picked up easily. A similar though broader anomaly also appeared at the southern end of Tecopa Valley on profile OST (Fig. 3). We estimated the total thickness of Plio-Pleistocene basin fill to be ~580 m, using a conservative estimate for the density contrast between the basin fill and the older substrate (Gross et al., 1990). This unexpectedly large thickness indicates a long history of strong extension and sediment deposition before Tecopa Valley was breached and drained.
Figure 3 SHO Figure 3 OST
Figure 3- Potential-field data and model cross sections for the profiles SHO and OST (Table 1, Fig. 2), from Gross and Louie (1992). Vertical exaggeration varies between the two sections; elevations are above sea level. Differences between the density and magnetic-susceptibility models are due to the possible presence within the basin of pyroclastic tuff having low density but high magnetic susceptibility. (Click on image for viewable Adobe Acrobat PDF version.)

On the east side of Tecopa Valley, a 700 nT magnetic anomaly and seismic refraction measurements along the trace of the Resting Spring Range-front fault at profile SHO demanded a 200-500 m thick sliver or intrusion of basalt within the fault zone (Gross and Louie, 1992; Fig. 3). Tiny exposures of this basalt crop out along the trace of this fault zone. Additional magnetic profiles along the fault south of profile SHO (C. Langston, 1992 unpub. data) show similar anomalies. It is likely that the pyroclastic dacites that crop out east of the fault atop the Resting Spring Range have been faulted down into Tecopa Valley, helping to account for the differences between the gravity and magnetic models of Fig. 3. In any case the thick basalt suggests intrusion up a deeply penetrating and relatively steeply dipping fault plane during an extensional episode. To produce the >1 km down drop of basement or volcanics, the Resting Spring Range-front fault must have been highly active during the formation of Tecopa Valley.

The Spring 1991 field exercise confirmed the existence of a 600 m deep trough along the west side of Tecopa Valley with gravity and magnetic measurements along profile THS (Figs. 2 and 4). This profile was anchored to the substrate of the basin fill at both ends. Preliminary seismic reflection work near the east end of the profile indicated the feasibility of imaging the Lake Tecopa sedimentary stratigraphy, at least in wet weather where the lake beds are not buried by alluvium.

Figure 4
Figure 4- Interpreted cross section for profiles THS and AR of Table 1 and Fig. 2. The gravity data and model (top) suggest the thickness of Miocene to Holocene rocks above pre-Cambrian to Tertiary basement (pC-T). An unknown sequence of Miocene alluvium and volcanics (Tm), perhaps similar to the China Ranch Beds, fills most of the basin. Above is a 170+-20 m section of Tecopa lacustrine mudstone (Qtlm), as shown by the 1992 seismic reflection work (from Fig. 5) and a locally reported borehole that hit fractured volcanics (Tv?) at ~100 m depth. The westernmost end of profile THS crosses recent alluvium (Qa) and an outcrop of basalt (Tb). Vertical exaggeration 2.9 times. (Click on image for viewable Adobe Acrobat PDF version.)

The Spring 1992 field camp was able to image Lake Tecopa stratigraphy with a high-resolution seismic reflection survey along profile AR at the Amargosa River (Fig. 2). Electromagnetic profiles showed where relatively non-conductive alluvium did not cover the conductive Lake Tecopa sediments, allowing us to locate the survey in the most advantageous area. Ideal conditions after wet weather allowed us to record reflections from depths as great as 180 m using a sledgehammer source.

The stacked seismic section (Fig. 5) shows that the regular and undeformed Plio-Pleistocene lake-bed sequence continues to a depth of 134+-20 m below the 2.01 Ma Huckleberry Ridge tephra (Morrison, 1991; Tuff C of Hillhouse, 1987) , extending the 72 m thick exposed lacustrine section by at least 100 m. Angular unconformities and sequences marked by lack of continuity underlie the lacustrine sequence.

Figure 5
Figure 5- Unmigrated stacked seismic section from profile AR (Table 1; Fig. 2) near the Amargosa River produced by the 1992 field exercise, after extensive velocity analysis and automatic gain control for display. The previously unexposed and evenly bedded lower portion of the Lake Tecopa sequence (L) is underlain at ~120 m depth below the profile by a strongly reflecting unconformity (between arrows). The apparent dips are actually velocity pull-downs due to variable conditions at the surface. Below the unconformity is an alluvial sequence (A) showing short, discontinuous reflections and evidence of diffractions and dipping strata. This sequence is of unknown origin, but its discontinuous, alluvial nature suggests rapid deposition during active basin subsidence. Stratigraphic imaging was lost in the disturbed area (D) where a layer of alluvium at the surface scatters reflections incoherently. (Click on image for viewable Adobe Acrobat PDF version.)

Age of Lake Tecopa sequence

Morrison (1991) finds that the pre-0.9 Ma deposition rate at Lake Tecopa was a slow 5 m/m.y., in the Greenwater Fan alloformation above the 2.01 Ma Huckleberry Ridge tephra. Below the Huckleberry Ridge tephra, the 17 m of exposed Spanish Trail alloformation appear lithologically similar, giving a possible 5+ Ma age of the Lake Tecopa sediments at the top of the seismic section (Fig. 5). If we were to assume the 5 m/m.y. Greenwater Fan alloformation deposition rate holds for at least an additional 80 m down to the base of the lacustrine sequence in the seismic section, the oldest Lake Tecopa deposits would take on an insupportable age of more than 20 Ma. If on the other hand the 80 m of lacustrine sequence in the seismic section were deposited at Morrison's higher post-0.9 Ma rate of 100 m/m.y., then we arrive at a minimum age estimate of 6.2 Ma for the base of the sequence. Using Hillhouse's (1987) post-Huckleberry Ridge average rate of 27 m/m.y., the seismic section pushes the base of the sequence back to 7 Ma. Given Morrison's (1991) evidence for substantially slower early deposition rates, we consider a 7 Ma age for the base of the Lake Tecopa sequence to be a minimum.

The stratigraphy exhibited by the lacustrine sequence in Fig. 5 shows no internal evidence of large changes in depositional environment or rate, suggesting that the 134+-20 m of section below the 2.01 Ma Huckleberry Ridge tephra are very similar to the immediately post-2.01 Ma section above the tephra. Regionally, detailed 36Cl chronostratigraphy by Jannick et al. (1991) of a core from Searles Lake shows stable sedimentation rates there between 2 and 0.5 Ma, increasing only since 0.5 Ma. Searles Lake, like Lake Tecopa, was one of many Pluvial-period lakes that occasionally drained to Manly Lake in Death Valley (Fig. 1). The detailed chronostratigraphy of Searles Lake does not exhibit any early, high sedimentation rates that would suggest any age for the base of the Lake Tecopa sequence younger than 7 Ma. This interpretation suggests that high rates of extension in Tecopa Valley ceased prior to 7 Ma.

CONCLUSION

The surprisingly great age of the relatively undeformed Lake Tecopa sequence constrains the time for rapid, basin-forming Cenozoic extension to a period between 11 and 7 Ma. Any tectonic model must accomplish the development of Tecopa Valley within the 4 m.y. period. Wernicke's (1992) model for the Death Valley region might allow an extensional hinge to have passed quickly through Tecopa Valley. However, isostatic compensation within his viscous ``fluid'' crustal layer must have taken place over a long enough time to allow mid-crustal material to flow to the surface, driven only by temperature and density gradients. The simple but dynamic model of King and Ellis (1990) must also accommodate buoyant crustal flow within the 4 m.y. period.

In sum, the application of simple, shallow geophysical survey techniques by university students over a period of a few years was able to put physical constraints on regional extensional models. These results depended on high-resolution characterizations of sedimentary basin geometry and stratigraphy. Constraint of the basin models required the use of several geophysical methods.

ACKNOWLEDGMENTS

Drs. Douglas R. Schmitt of the University of Alberta, Charles A. Langston and Lauren A. Wright of Penn State, Chris Sanders of Arizona State, Laurie Serpa and Terry Pavlis of the University of New Orleans; and Bennie W. Troxel have generously contributed to this work. Undergraduate and graduate students from Penn State and UNR carried out most of the survey planning, fieldwork, and analysis. Participants included Abu Asad, Dr. Michael Gross, John Hammer, Greg Jablunovsky, Raymond Laird, Brian Lassige, Steve Nichols, Jordi Prims, Dr. David Verdonck, Nancy Yonkers, and Jie Zhang, as well as the authors. Supported in part by the National Science Foundation under grant EAR-9405534.

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