Abstract

COCORP deep seismic reflection profiles in the vicinity of the central Death Valley pull-apart basin in southeastern California provide three-dimensional information on the subsurface of an active extensional terrane. Variations in the orientation and density of reflectors indicate that the crust and upper mantle of the region is divisible into three seismic zones which may represent regions of differing lithology, rheology, or both. The reflections in the upper ∼5 s (15 km) of the data have gentle to moderate dips; between 5 and 10 s, reflections are predominantly subhorizontal; and below ∼ 10 s (30 km), there are no notable reflections. The boundaries between the above reflecting zones are marked by prominent reflecting horizons which are continuous throughout the survey region.

The observed reflection geometries resemble those predicted by the crustal model of the region proposed by Wright and Troxel (1973) on the basis of geological studies. In addition, many of the upper-crustal reflectors can be traced directly to mapped features. Based on those correlations, the upper reflecting zone (0-5 s) is interpreted to be a region of brittle deformation with the various upper-crustal reflectors interpreted as faults and basin sediments. The reflecting horizon at the base of the upper zone appears to be the lower boundary of the faulted upper crustal blocks and, it has been suggested that it locally includes partially molten rock. The observed geometries and amplitudes of reflections from the lower crust (15-30 km depth or 5-10 s two-way traveltime) are consistent with the model of Wright and Troxel for a ductilely deformed and intruded lower crust. The prominent reflecting horizon at the base of that zone is designated the reflection Moho and the seismically transparent lowest zone appears to correspond to the upper mantle.

The seismic data define a zone of faults (referred to here as the "Wingate Wash fault zone") which appears to form the southern boundary of the central Death Valley basin and also may have provided a conduit for the migration of magma from a mid-crustal magma body to the surface. The Wingate Wash fault zone appears to intersect the southern Death Valley fault zone and the frontal faults of the Black Mountains in the subsurface beneath the youngest volcanic edifice in the region. Those three fault zones appear to separate the Panamint, Owlshead, and Black Mountain upper-crustal fault blocks. From the available data, a reconstruction of the possible fault-block movements during the time of basin subsidence is presented. That reconstruction suggests that the central Death Valley basin formed as a result of the combined down-to-the-east rotation and northwest translation of the fault blocks in manner similar to that proposed by Reches for other parts of the Basin and Range.

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