DEVELOPMENT OF THE BUCKSKIN-RAWHIDE DETACHMENT FAULT SYSTEM, WEST-CENTRAL ARIZONA: EVIDENCE FROM THE SEDIMENTARY RECORD

We present a new synthesis of stratigraphic constraints on the spatial and temporal development of the large magnitude displacement (~40-50 km) Buckskin-Rawhide detachment fault in west-central Arizona. Tertiary inception of sedimentation in the core complex is marked by a basal arkose that grades up into tuffaceous clastic strata and minor lacustrine limestone. Geochronometry of basal tuffs indicate sedimentation began at ~23 Ma near the detachment breakaway in the western Bouse Hills and at ~27 Ma in the Lincoln Ranch supradetachment basin. The absence of growth strata in basal deposits suggests minimal synextensional deposition. Previous studies demonstrated rapid footwall cooling and slip along the Buckskin-Rawhide detachment system began at ~21-20 Ma and ~17 Ma along a secondary breakaway in the Bouse Hills, west of the mylonitic front. New geologic mapping of the Lincoln Ranch basin and geochronometric constraints on hanging wall strata indicate tertiary breakaway development in the central Buckskin Mountains at ~14.5 Ma. The >1.2 km-thick Sandtrap Conglomerate (SC) records tertiary breakaway formation and the first significant input of footwall mylonite clasts in the hanging wall basin (~30-65% of clasts). Paleocurrent data record an abrupt shift from SSE-directed transport within the Chapin Wash Formation sandstone to NNW-directed transport during deposition of the overlying SC. The absence of extension-parallel sediment transport, clast frequencies, and (U-Th)/He cooling ages (~16-13 Ma) of mylonitic boulders in the SC indicate deposition ~10-20 km SW of the current exposure in Lincoln Ranch basin. These depositional constraints indicate time-averaged slip rates on the tertiary breakaway of 3-6 km/Myr from ~14.5 Ma until slip cessation at ~11 Ma. These slip rates are similar to the ~3 km/Myr slip rates determined for the western portion of the Buckskin-Rawhide detachment fault system, contradicting previous interpretations of rapidly accelerated slip starting at ~15 Ma within the region. Steeply dipping bedding (40-70° SW) in the lower ~1 km of the SC shallows to ≤30° in the upper SC, indicating late-stage tilting may have resulted from a relatively brief period of more rapid extension, although average slip rates during the final ~3.5 Myrs of core complex extension were not significantly higher.