Geologic Mapping and Fracture Analysis of the Northeastern Rosillos Mountains, Brewster County, Texas

The long and complex geologic history of the Trans-Pecos region in west Texas is highlighted by Cretaceous and Tertiary tectonism and magmatism. Upper Cretaceous sedimentary deposits in the Rosillos Mountains were deformed during the Late Cretaceous to Oligocene Laramide orogeny. Intrusion of an Oligocene quartz syenitie laccolith was followed by Basin and Range extension starting ∼32 Ma.;Geologic mapping was completed using a revised stratigraphic column (Lehman, 1985a), which allowed for reinterpretation of the geology of the field area. The Upper Cretaceous Boquillos, Pen, and Aguja Formations are identified in the field area and are intruded by the Rosillos Mounatins laccolith. Several contacts previously mapped as faults are now identified as sedimentary contacts.;A detailed fracture analysis revealed fractures associated with Laramide and Basin and Range deformation. The maximum stress direction during the Laramide orogeny was oriented southwest-northeast, resulting in low-angle northwest-southeast-oriented fractures. During Basin and Range extension, the maximum stress direction was vertical, resulting in lateral failure and fundamentally different structures. The intermediate and minimum stress directions rotated approximately 90° around 17 Ma. The dynamic stress regime can be traced through high-angle normal faults and shear fractures trending northwest to southeast and northeast-southwest.