LARGE-MAGNITUDE MIOCENE EXTENSION IN WEST-CENTRAL ARIZONA: NEW INSIGHTS FROM GEOLOGIC MAPPING IN THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX

Recent 1:12,000-scale geologic mapping in the central and southeastern portion of the Buckskin-Rawhide metamorphic core complex provides insights into the Miocene structural evolution of west-central Arizona. This mapping builds on a significant amount of work by the Arizona Geological Survey, which has made major contributions to our understanding of both the region’s complex geologic history and the processes involved in large-magnitude extension. The geologic mapping and structural analysis presented in this study document the following Miocene structural history: 1) early Miocene mylonitization and emplacement of the Swansea Plutonic Suite in a top-NE directed shear zone, 2) NE-directed slip on the Buckskin detachment fault system, coeval with higher angle normal faulting in both the upper plate and lower plate, 3) extension-perpendicular shortening during detachment faulting, resulting in amplification of prominent NE-SW trending corrugations, and 4) postdetachment (post-middle Miocene) dextral faulting. In the Swansea-Clara Peak area in the central Buckskin Mountains, most lower plate mylonitic fabrics have been tilted gently to moderately SW by NE-dipping, syndetachment normal faults. The amount of this back-tilting is similar to the amount of tilting in upper plate Miocene strata, suggesting that at least locally the amount of lower plate and upper plate extension are comparable. Portions of the Buckskin detachment fault system have incised into the lower plate, truncating mylonitic fabrics and resulting in variable thicknesses of detachment-related cataclasite. Evidence for extension-perpendicular shortening during detachment faulting includes constrictional fabrics in Swansea Plutonic Suite mylonites, NW- and SE-trending slip lineations on the Buckskin detachment fault and lower plate foliation surfaces, broad fold patterns in upper plate bedding, and m- to km-scale upright folds in well-layered portions of the lower plate. Geologic mapping in the Little Buckskin Mountains indicates that dextral slip on NW-striking postdetachment faults has resulted in the misalignment between the dominant core complex extension direction and the topographic trend of the corrugations.