MIOCENE STRUCTURAL EVOLUTION OF THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

New geologic mapping and structural analysis document the Miocene kinematic and geometric evolution of the Buckskin-Rawhide metamorphic core complex in west-central Arizona. Early Miocene mylonitization in the footwall of the Buckskin detachment fault was characterized by consistent top-NE-directed shear and ~450-500°C deformation temperatures that varied by ≤50°C across a distance of ~35 km in the extension direction. The relatively uniform deformation conditions and strain recorded in mylonitized ~22-21 Ma granitoids indicate that footwall mylonites initiated as a subhorizontal shear zone that was captured and rapidly exhumed by the Buckskin detachment fault.

Dominant NE-directed slip on the detachment fault was progressively overprinted by NW- and SE-directed slip associated with folding of extension-parallel corrugations. Extension-perpendicular shortening is also recorded by constriction during the late stages of mylonitization and folding of hanging wall bedding and footwall fabrics. Upright m-scale and km-scale footwall folds parallel the detachment fault corrugations and developed primarily by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the footwall is ~10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ~1%.

Following exhumation to brittle conditions, footwall mylonites were extended up to ~20-30% by NE-dipping, syndetachment normal faults. Towards the end of detachment faulting, the extension direction rotated clockwise, and some portions of the Buckskin detachment fault record a transition from dominant top-NE slip to ENE- and E-directed slip. After detachment faulting ceased, E-W extension was accommodated primarily by steeply NE-dipping, dextral and oblique dextral-normal faults. 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. Postdetachment dextral/transtensional faulting across the Buckskin-Rawhide metamorphic core complex reflects the increasing influence of the Pacific-North American transform plate boundary towards the end of the middle Miocene.