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; firstname.lastname@example.org
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.
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.
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.
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).
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.
|Spring 1990||Spring 1991||Spring 1992*|
|University||Penn State||Penn State||U. Nevada, Reno|
|Profiles (Fig. 2)||SHO OST||THS||AR CCV+|
|Seismic source points||34||29||107|
|Results||600 m deep basin, steep range-front fault||Deep basin is continuous||Early Lake Tecopa stratigraphy, geometric differences of basins|
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.
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.
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.
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.
Fleck, R. J., 1970, Age and tectonic significance of volcanic rocks, Death Valley area, California: Geological Society of America Bulletin, v. 81, p. 2807-2816.
Gross, M. R., and Louie, J. N., 1992, Geometry of normal faulting in Tecopa Valley, California from magnetic Surveys: California Geology, v. 45, no. 4 (July/August), p. 110-117.
Gross, M. R., Louie, J., Laird, R., Nichols, S., Verdonck, D., Yonkers, N., and Zhang Jie, 1990, Geometry of normal faulting in Tecopa Valley, California, from small-scale geophysical surveys [abs.]: EOS (Transactions, American Geophysical Union), v. 71 , no. 43 (Oct. 23), p. 1585.
Hillhouse, J. W., 1987, Late Tertiary and Quaternary geology of the Tecopa basin, southeastern California: U.S. Geological Survey Miscellaneous Investigations Map I-1728, scale 1:48,000, 1 sheet, 16 p. text.
Jannick, N. O., Phillips, F. M., Smith, G. I., and Elmore, D., 1991, A 36Cl chronology of lacustrine sedimentation in the Pleistocene Owens River system: Geological Society of America Bulletin, v. 103, p. 1146-1159.
King, G., and Ellis, M., 1990, Origin of large local uplift in extensional regions: Nature, v. 348, p. 689-693.
Mabey, D. R., 1963, Complete Bouguer anomaly map of the Death Valley region California: U.S. Geological Survey Geophysical Investigations Map GP-305, scale 1:250,000, 1 sheet.
Morrison, R. B., 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa, in Morrison, R. B., ed., Quaternary non-glacial geology, conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. K-2, p. 283-320.
Nilsen, T. H., and Chapman, R. H., 1974, Bouguer gravity map of California, Trona sheet: California Division of Mines and Geology, scale 1:250,000, 1 sheet, 9 p. text.
Serpa, L., de Voogd, B., Wright, L., Willemin, J., Oliver, J., Hauser, E., and Troxel, B., 1988, Structure of the central Death Valley pull-apart basin and vicinity from COCORP profiles in the southern Great Basin: Geological Society of America Bulletin, v. 100, p. 1437-1450.
Snow, J. K., and Wernicke, B., 1989, Uniqueness of geological correlations: an example from the Death Valley terrain: Geological Society of America Bulletin, v. 101, p. 1351-1362.
Stewart, J. H., 1971, Basin and Range structure: a system of horsts and grabens produced by deep-seated extension: Geological Society of America Bulletin, v. 82, p. 1019-1044.
Wernicke, B., 1981, Low-angle normal faults in the Basin and Range province: nappe tectonics in an extending orogen: Nature, v. 291, p. 645-648.
Wernicke, B., 1992, Cenozoic extensional tectonics of the U.S. Cordillera, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. D., eds., The Cordilleran Orogen; conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. G3, p. 553-581.
Wernicke, B., and Axen, G. J., 1988, On the role of isostasy in the evolution of normal fault systems: Geology, v. 16, p. 848-851.
Wernicke, B., Axen, G. J., and Snow, J. K., 1988, Basin and Range extensional tectonics at the latitude of Las Vegas, Nevada: Geological Society of America Bulletin, v. 100, p. 1738-1757.
Wernicke, B., Spencer, J. E., Burchfiel, B. C., and Guth, P. L., 1982, Magnitude of crustal extension in the southern Great Basin: Geology, v. 10, p. 499-502.
Wright, L. A., 1973, Geology of the southeast quarter of the Tecopa quadrangle, San Bernardino and Inyo Counties, California: California Division of Mines and Geology Map Sheet 20, scale 1:24,000, 1 sheet.
Wright, L. A., 1976, Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block: Geology, v. 4, p. 489-494.
Wright, L. A., and Troxel, B. W., 1973, Shallow-fault interpretation of Basin and Range structure, southwestern Great Basin: in De Jong, K. A., and Scholten, R., eds., Gravity and tectonics, New York, Wiley.