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Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data

Li, Yong-Gang and Henyey, Thomas L. and Silver, Leon T. (1992) Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data. Journal of Geophysical Research B, 97 (B6). pp. 8805-8816. ISSN 0148-0227. doi:10.1029/91JB02119.

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Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N-S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two-way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross-section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment-basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present-day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E-W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

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Additional Information:© 1992 by the American Geophysical Union. Received December 27, 1990; revised July 18, 1991; accepted August 12, 1991. The Mojave seismic reflection data reprocessed in this study were kindly provided by COCORP. We would like to acknowledge discussions regarding the processing and interpretation with David Okaya, Peter Malin, Eric Frost, and Gregory Davis. A. Levander, J. Namson, and an associate editor provided helpful comments during the review process. In particular we are indebted to J. Namson for his suggestion that the Hitchbrook is an older, down-to-the north normal fault. The gravity data were obtained with the aid of John Scott. Susan Turnbow and Desser Motion typed the final version of the manuscript. The study was supported by NSF grants EAR-8319254 and EAR-8609247 to CALCRUST.
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Issue or Number:B6
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Official Citation:Li, Y.-G., T. L. Henyey, and L. T. Silver (1992), Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data, J. Geophys. Res., 97(B6), 8805–8816, doi:10.1029/91JB02119
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43384
Deposited By: Tony Diaz
Deposited On:30 Jan 2014 18:00
Last Modified:10 Nov 2021 16:36

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