Style of Laramide Deformation, Southern Eagle Mountains, Trans-Pecos Texas

Brian S. Brister

Laramide (75-50 ma) thin-skinned deformation associated with the Chihuahua tectonic belt is exposed in the southern Eagle Mountains 28 km southwest of Van Horn, Texas. Deformed rocks are unmetamorphosed Comanchean clastics and carbonates of the Yucca, Bluff Mesa, and Cox formations that were originally deposited near the eastern margin of the Mesozoic Chihuahua trough. Thrusting of these strata eastward against the structurally high Diablo platform led to the formation of a leading imbricate-fan thrust system. The Devil Ridge fault is the surface expression of the sole fault, which underwent maximum horizontal slip within the imbricate fan. Associated features include the north-northwest-oriented Pinon, Horse Peak, and Bramblett Ridge thrusts; broad, axially faulted, asym etrical folds with north-northwest axial trends; and east-northeast-trending tear faults. These major structures, and mesostructures such as joints and en echelon veins, suggest compressive stress oriented east-northeast. Post-Laramide normal faults related to the formation of the nearby Eagle Mountains caldera and later Basin and Range tectonics complicate structural analysis through reactivation of the Laramide faults.

AAPG Search and Discovery Article #91034©1988 AAPG Southwest Section, El Paso, Texas, 21-23 February 1988.