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Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California

Malin, P. E. and Goodman, E. D. and Henyey, T. L. and Li, Y. G. and Okaya, D. A. and Saleeby, J. B. (1995) Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California. Journal of Geophysical Research B, 100 (B2). pp. 2069-2087. ISSN 0148-0227. doi:10.1029/94JB02127.

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The focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface, with the Tehachapi Mountains in California being the case in point. As a prime example of the lower crust, these mountains expose Cretaceous gneisses that formed 25–30 km down in the Sierra Nevada batholith and appear to be underlain by the ensimatic Rand schist. Integrated geophysical and geological studies by the CALCRUST program have produced a cross section through this post-Mid-Cretaceous structure and suggest a general model for its development. Seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the San Joaquin Basin's Tejon embayment. Two south dipping reverse faults on the rim of the Tejon embayment were discovered in the reflection data and verified in the field. The faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the Rand schist. The upper and lower crusts are separated by a zone of laterally discontinuous reflectors. Reflections from the lower crust form a wedge, the base of which is a nearly flat Moho at 33 km. Regional geological relations and gravity models both suggest that the reflective zone corresponds to the Rand schist and the newly recognized faults account for its Neogene exposure. Alternatively, the reflective zone maybe part of the gneiss complex, suggesting that the schist either lies deeper or is not present under the gneisses. If the Rand schist underlies the Tehachapi Mountains and Mojave region to their south, a model for their evolution can be constructed from regional geological relations. It seems that during Late Cretaceous Laramide subduction the protolith of the schist was thrust eastward beneath the Mojave. Along this portion of the Cordilleran batholithic belt the subduction was evidently at very low angles. The bottom of the batholith was removed and replaced by a thick section of schist, fluids from which weakened the overlying batholith. This thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat-lying slabs. When emplacement of the schist ended in latest Cretaceous/earliest Paleocene, the underlying mantle rose, compensating for the extension and providing material for magmatic underplating. In the Neogene, transpression and rotation of the upper crust along the San Andreas and Garlock faults resulted in the exposure of the schist.

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Additional Information:© 1995 American Geophysical Union. Received May 10, 1993; Revised August 3, 1994. Paper number 94JB02127. This work was supported under NSF grants EAR 83-19254 to CALCRUST, EAR87-08266 and 89-04063 to J. Saleeby, and EAR91-19263 and 91-19263 to P. Malin. J. Plescia of the Jet Propulsion Laboratory, Pasadena, California, provided gravity data and several early models from which Figure 3 was developed. We are grateful for his open and kind help. The authors have benefited from conversations with J. Crowell, T. McEvilly, S. Richard, and J. Sharry and many others. We would like to thank ARCO for providing their "vibrator buster" test equipment to check the vibrators before the survey and the Tejon Ranch Company for access to their property in the Tehachapi Mountains. We thank E. Karageorgi for generating the filtered field data at the Earth Science Division, Lawrence Berkeley Laboratory. Comments on the original manuscript by L. Serpa, T. Broecker, and anonymous reviewer are gratefully acknowledged. Further constructive reviews, resulting in the present manuscript, were provided by E. Hauser, G. Fuis, and T. Pratt. Three thoughtful and knowledgeable reviews by G. Fuis were especially appreciated.
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Issue or Number:B2
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Official Citation:Malin, P. E., E. D. Goodman, T. L. Henyey, Y. G. Li, D. A. Okaya, and J. B. Saleeby (1995), Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California, J. Geophys. Res., 100(B2), 2069–2087, doi:10.1029/94JB02127.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49001
Deposited On:28 Aug 2014 17:40
Last Modified:10 Nov 2021 18:38

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