CONSTRAINTS ON THE REORGANIZATION OF THE SAN ANDREAS FAULT SYSTEM IN THE LATE MIOCENE BASED ON DRAINAGE DEVELOPMENT AND SYN-TECTONIC DEPOSITION OF THE HUNGRY VALLEY FORMATION, TEJON PASS, CA

Fault reorganization along transform plate boundaries is a fundamental part of the evolution of active margins. For example, slip along the San Andreas Fault (SAF) system was transferred eastward from the San Gabriel Fault (SGF) onto the SAF in the late Miocene to early Pliocene. The Hungry Valley Formation (HVF) south of Tejon Pass is wedged between the SGF and the SAF and preserves sedimentologic evidence of waning slip on the SGF and the onset of slip along the SAF. The HVF is approximately 1,500 m thick and is divided into three members based on changes in sedimentology, paleocurrent directions, and conglomerate clast counts. The lowest member is bright white medium-grained sandstone with large channels and cross beds that contains dominantly granitic and metamorphic clasts considered to be basement from the San Gabriel Mountains (SGM). This member represents a braided fluvial-deltaic system that deposited local material from the southeast while the SGF was still active. The middle member is yellow-white coarse to very coarse sandstone with some channels that dominantly contains metamorphic basement and Mojave Desert clasts in addition to SGM clasts. We interpret the middle member to represents a braided stream system and defines the onset of dextral slip on the SAF when major drainage reorganizations allowed Mojave clasts to penetrate the HVF from the northeast in addition to contribution from the SGM. The upper member is white to brown very coarse sandstone to pebble conglomerate that is dominated by clasts of basalt, tuff, and tuffaceous sandstone likely sourced from the Mojave rather than the SGM. During deposition of the alluvial upper member, rivers sourced in the Mojave Desert flowed southwest into the HVF, and sedimentation likely terminated as the region was uplifted due to transpression in the Pliocene. Previous studies suggest the HVF was deposited between 5.5 m.y. and 4 m.y. As the first appearance of Mojave clasts occur in the middle member, and the lower member overlays the top of the SGF, it is likely that the eastward migration of slip may have occurred in less than 0.5 m.y.. Our results imply that fault reorganization may occur rapidly and have profound influence on sedimentary basin architecture.