Structural constraints on Laramide shortening and Rio Grande rift extension in the central Franklin Mountains, El Paso County, Texas.

摘要

The Franklin Mountains, a north-south trending mountain range located in west Texas and southern New Mexico, exhibit deformation associated with both the Laramide orogeny and the Rio Grande rift. Detailed geologic mapping of the Tom Mays State Park area of the central Franklin Mountains was completed in order to better refine the structural geology, to determine the incremental strain history of the major fault systems in the area, and to determine the origin of prevalent low-angle normal faults.; Three major fault systems were found in the Tom Mays State Park area, each associated with a separate period of deformation. These are the Western Boundary fault zone (WBFZ) reverse fault, the Avispa detachment, and the WBFZ normal fault. The WBFZ reverse fault was produced by northeast southwest oriented Laramide shortening, and deformed by dip-slip displacements. This single period of Laramide shortening in the central Franklin Mountains created map- and small-scale fault propagation folds in the area which plunge gently to the northwest. The WBFZ reverse fault also shows structures that supports the idea that the Franklin Mountains were a transitional area between thick- and thin-deformation during the Laramide orogeny.; The low-angle Avispa detachment was produced by northeast-southwest oriented extension, interpreted to have formed during an early period of Rio Grande rift extension. Structural analysis of low-angle faults associated with the Avispa detachment shows that they have been progressively rotated to their present shallow dip during northeast directed extension demonstrating large horizontal axis rotations during rifting. Finally, the WBFZ normal fault was produced by east-west oriented extension during the most recent period of Rio Grande rifting.