MIDDLE MIOCENE EXHUMATION OF THE AVAWATZ MOUNTAINS, SOUTHERN CALIFORNIA: A TERMINATION THRUST BELT AT THE EASTERN TIP OF THE GARLOCK FAULT

The Avawatz Mountains are located at the eastern tip of the left-slip Garlock fault where the fault intersects the NW-striking right-slip Southern Death Valley fault (SDVF). The Avawatz Mountains were constructed via late Cenozoic contractional deformation involving active thrust and/or oblique-slip faults. This zone may represent a termination thrust belt of the Garlock fault or a restraining bend along the SDVF. Here we present detailed geologic mapping, structural analysis, and low-temperature thermochronology to resolve the timing and kinematics of deformation as they relate to either fault system. Field observations show numerous subvertical west-striking strike-slip faults in the western portion of the range and a prominent west-dipping reverse fault along the eastern range front. Zircon (U-Th)/He thermochronology from a ~1 km vertical transect in the hanging wall of the eastern range-front fault yielded ages ranging from 79 ± 11 Ma in the structurally highest samples to 12.3 ± 3.1 Ma in the structurally lowest samples. Across the vertical sampling transect, most samples yield average ages that cluster around ca. 15 Ma. This age distribution is consistent with exhumation initiating in the middle Miocene (ca. 15 Ma), synchronous with the initiation of slip on the Garlock fault. Vertical exhumation rates calculated from zircon thermochronology are < 1 mm/yr, which corresponds to a horizontal shortening rate of ~1 mm/yr based on the observed dip of the primary west-dipping reverse fault structure. These estimates are consistent with published geologic slip rates for the eastern segment of the Garlock fault. Our observations suggest a complex fault and exhumation history that may reflect transfer of Garlock left-slip displacement onto an east-directed termination thrust system that was later overprinted by slip on the SDVF and the development of a restraining bend.