Timing, rate, and magnitude of slip on the Buckskin‐Rawhide detachment fault, west central Arizona

Abstract

AbstractWe 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 postdetachment volcanic rocks, indicate that large‐magnitude extension associated with the detachment fault initiated at ~21–20 Ma and continued until ~12–11 Ma in the southwestern portion of the Buckskin‐Rawhide metamorphic core complex. (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 3 + 1.5/−1 km/Myr 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 6–11 km northeast in the middle Miocene by the Buckskin‐Rawhide detachment fault at a slip rate of 1.2–2.7 km/Myr. The total displacement across the detachment fault in the southwestern portion of the core complex is 24 ± 10 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. Based on these data and new observations, we propose that total displacement on the Buckskin‐Rawhide detachment fault system increases in the slip direction to ~40–50 km at the northeastern end of the exposed footwall, corresponding to time‐averaged slip rates that ranged from ~2 km/Myr to ≤6 km/Myr across the entire core complex.