POST-MIDDLE MIOCENE DEXTRAL FAULTING IN THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR: INSIGHTS FROM THE BUCKSKIN-RAWHIDE CORE COMPLEX, WEST-CENTRAL ARIZONA

Structural data from the Buckskin-Rawhide metamorphic core complex in west-central Arizona document the Miocene transition from large-magnitude, NE-directed extension to distributed dextral shear along NW-striking faults. The Buckskin detachment fault locally records a clockwise rotation of the slip direction from dominant top-NE-directed slip to ENE- and E-directed slip during the late stages of activity. New (U-Th)/He thermochronology of lower plate mylonites and ⁴⁰Ar/³⁹Ar geochronology of postdetachment volcanic rocks indicate that detachment faulting ceased ca. 12-11 Ma. Postdetachment deformation was dominated by E-W extension and associated dextral-normal faulting. At least ten NE-dipping postdetachment faults record ~0.1-1 km dextral or oblique dextral displacement. The cumulative amount of dextral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Geologic mapping confirms that the misalignment between the lower plate topography and the corrugation axis in the Little Buckskin Mountains is due to dextral faulting. Postdetachment dextral/transtensional faulting across the core complex reflects the increasing influence of the diffuse Pacific-North American transform plate boundary towards the end of the middle Miocene. Although strike-slip faulting associated with this plate boundary is not well recognized in the lower Colorado River extensional corridor, post-middle Miocene dextral faults appear to be widespread across the region and most likely account for at least 15 km of dextral shear.