DETAILED MAPPING OF THE SOUTHERN TERMINUS OF THE BIG AND LITTLE CHINO FAULT ZONES, NORTH-CENTRAL ARIZONA

Recent detailed geologic mapping completed north of Prescott in central Arizona identifies multiple fault scarps extending about 15 km south of the previously mapped terminus of Big Chino Fault. The strike of the fault zone changes from SE along the north side of Big Chino Valley to S along the east side of Little Chino Valley. The pattern of faulting is complex in detail, with orientations ranging from E to SW, but primary fault strikes are SE, SSE, and S. Faults cut primarily Paleozoic carbonate rocks and extensive late Miocene to Pliocene volcanic rocks, but locally, Quaternary alluvial deposits are faulted as well. Surface displacement is variable but generally down to the SW, with the possibility of substantial right-lateral displacement across 2 fault strands. Fault scarps are <10 m high and are fairly gentle, yet appear to have re-oriented local drainage networks and may have affected regional drainage patterns as well. A fortuitous roadcut through a complex fault zone in Little Chino Valley provides insights into the late Quaternary rupture history of these faults. The road cuts entirely through a 10m-high fault scarp and alluvial ridge. A team of AZGS geologists and volunteers mapped the 80-m long exposure, where we identified 14 individual faults forming a SE-trending graben on the crest of the fault scarp. A stacked sequence of five moderately to weakly developed buried Pleistocene soils records at least 3 individual surface-faulting events with up to 3 m of cumulative vertical displacement across individual faults. A radiocarbon date of 6.5 ka was obtained from charcoal associated with deposits that filled the youngest fault-related graben, suggesting a latest Pleistocene to early Holocene age of youngest rupture. This age estimate is similar to the estimated age of the youngest event for the main Big Chino fault zone, suggesting that the Big and Little Chino fault zones, and the splays between them, may have ruptured in the most recent large earthquake. This increases the length of the fault zone from 50 km to at least 65 km, suggesting that the most recent large earthquake was at least M7.