SYNEXTENSIONAL SEDIMENTATION IN THE HANGING WALL OF THE BUCKSKIN DETACHMENT FAULT, WEST-CENTRAL ARIZONA: EVIDENCE FOR ACCELERATED EXTENSION RATES IN THE MIDDLE MIOCENE?

New geologic mapping of the southern Lincoln Ranch area in the Buckskin Mountains, west-central Arizona, documents the stratigraphic evolution of a synextensional basin in the hanging wall of the Buckskin detachment fault. The Buckskin-Rawhide metamorphic core complex underwent large-magnitude crustal extension during the early to middle Miocene (~21-11 Ma), resulting in exhumation of mid-crustal mylonites in the footwall of the low-angle Buckskin detachment fault. In the Lincoln Ranch basin, early to middle Miocene strata overlie intensely fractured Proterozoic granitoid. The base of the synextensional section consists of arkose and tuffaceous sandstone, volcanic breccia, and a lower conglomerate unit dominated by granitic and volcanic clasts. The middle portion of the section consists primarily of lacustrine sandstone and interbedded limestone. A reworked ash-fall tuff within this lacustrine section yielded zircons with U-Pb ages as young as ~14.5 Ma. The uppermost synextensional unit in the basin is the “Sandtrap Conglomerate”, a steeply tilted alluvial fan deposit containing abundant clasts of crystalline mylonite. The thickness of the conglomerate (>650 meters) is substantially greater than other upper plate units, suggesting that accelerated extension towards the end of detachment faulting (~14-11 Ma) increased sedimentation rates. This hypothesis is consistent with previous interpretations that suggest detachment fault slip rates accelerated after ~15 Ma due to the passage of a slab-free window beneath the region. Alternatively, deposition of the Sandtrap Conglomerate could be related to increased erosion rates due to isostatic uplift of the lower plate.