TIMING, RATE, AND MAGNITUDE OF SLIP ON THE BUCKSKIN-RAWHIDE DETACHMENT FAULT, WEST-CENTRAL ARIZONA

We present thermochronologic and geochronologic data that constrain the slip history of the Buckskin-Rawhide detachment fault in west-central Arizona, one of the largest extensional fault systems in the North American Cordillera. (U-Th)/He zircon and apatite thermochronology, integrated with ⁴⁰Ar/³⁹Ar geochronology of syn- and postdetachment volcanic rocks, indicate that large-magnitude extension associated with the detachment fault initiated ~22 ± 1 Ma and continued until ~12-11 Ma in the southwestern portion of the Buckskin-Rawhide metamorphic core complex. The inception of detachment faulting was approximately coeval with widespread plutonism in the footwall. Apatite (U-Th)/He footwall cooling ages from the breakaway zone in the western Bouse Hills to upper greenschist-facies mylonites in the southern Buckskin Mountains indicate that the slip rate on the detachment fault was 2.8 +0.7/-0.5 km/m.y. during the early Miocene. Space-time patterns of hanging wall tilting suggest that at 17-16 Ma a secondary detachment fault breakaway developed ~12 km northeast of the primary detachment fault breakaway. Proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 8-10.5 km northeast in the middle Miocene by the Buckskin-Rawhide detachment fault at a slip rate of ~2.1 ± 0.5 km/m.y. The total displacement across the detachment fault system in the southwestern portion of the core complex is 26 ± 5 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. We propose that total displacement on the Buckskin-Rawhide detachment fault system increases in the slip direction to ~35-40 km at the northeastern end of the exposed footwall, correlating to time-averaged slip rates no greater than ~4 km/m.y.