--> Abstract: Stratigraphic and Sedimentologic Response to the Evolution of a Metamorphic Core Complex: an Example from the Early Miocene Mojave Extensional Belt, California, by S. M. Jones, C. J. Travis, and R. K. Dokka; #90987 (1993).

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JONES, STEVEN M., and CHRISTOPHER J. TRAVIS, BP Exploration, Houston, TX; and ROY K. DOKKA, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA

Stratigraphic and Sedimentologic Response to the Evolution of a Metamorphic Core Complex: an Example from the Early Miocene Mojave Extensional Belt, California

Detailed thermochronologic and stratigraphic/sedimentologic analyses of the Mojave Extensional Belt reveal the depositional systems response to tectonic unroofing of middle-crustal rocks by detachment faulting and provide important new constraints for models of the tectonic evolution of the area.

Unroofing caused by extension along the detachment is reflected in the concordant fission track ages on apatite, zircon and sphene, and {40}Ar/{39}Ar ages on biotite. Cooling curves derived from samples at several localities show a rapid cooling from >300 degrees C to <70 degrees C at -20 Ma, indicating the timing of detachment faulting. PT estimates from metamorphic petrology suggest 3-5 kbar and 300-350 degrees C for pre- or syn extension conditions. Apatite fission track data and numerical modeling of annealing in apatite suggest that rapid cooling continued below (approx.) 70 degrees C. This degree of exhumation (>300 degrees C to below 70 degrees C) is comparable to that observed in other detachment faulted terranes.

Topography generated during detachment faulting resulted in deposition of coarse-grained sediments exposed in the Mud Hills near Barstow. Syntectonic breccias of the Mud Hills Formation (approx. 19.8-18.1 Ma) are unconformably overlain by finer-grained sedimentary rocks of the mainly post-tectonic Barstow Formation. Stratigraphic, sedimentologic, and structural analyses indicate that the Mud Hills Formation probably accumulated in a NW-trending half graben with northerly and southerly upper plate crystalline basement sediment sources. Southerly-derived metamorphic clasts present in the lower Barstow Formation herald the final unroofing of the detachment in the Waterman Hills/Mitchel Range (approx. 16.5 Ma.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.