INITIAL MAPPING ALONG THE CENTRAL REACH OF THE PINTO MOUNTAIN FAULT ZONE, SOUTHERN CALIFORNIA, REVEALS NEW INSIGHTS INTO TECTONICS AND PALEOHYDROGRAPHY

The Pinto Mountain fault zone (PMfz) is an EW-trending, sinistral fault zone that forms a structural boundary between NW-trending dextral faults of the Eastern California Shear Zone (ECSZ) and the EW-trending sinistral faults of the eastern Transverse Ranges. Initial detailed mapping along the central reach of the PMfz reveals previously unrecognized faults and fault orientations at the intersection between the two structural domains. Detailed mapping at the intersection of the NW-trending Copper Mountain fault and the PMfz reveals NW- and N-trending Quaternary faults delineated by moderately-to well-defined geomorphic evidence for up-to-the east normal dip-slip displacement. Features include scarps on Pleistocene alluvium, offset drainages, beheaded alluvial fans, and steep linear bedrock scarps. To the west at the intersection of Bartlett Mountain, the PMfz, and a previously mapped NW-trending fault, new mapping presents a series of NE-trending normal Quaternary faults. Geomorphic evidence includes a series of offset Holocene(?) debris-fan lobes, scarps on Pleistocene alluvium, offset drainages, and steep linear bedrock scarps. Recently collected lidar has led to improved definition and understanding of the tectono-geomorphic features. These newly identified faults and fault orientations give insight into fault geometry, kinematics, and rupture patterns at intersections between faults of the ECSZ and PMfz.

New mapping has also led to the discovery of a paleo-wetland deposit that may bear on PMfz movement history and landscape evolution. The deposit occupies ~0.5 km² and consists of biotite-rich olive-green to pale-brown packages of very fine-grained sand and clay-rich mud capped by a carbonate-rich unit. The deposit lies entirely south of and abuts the PMfz and displays a gentle west-southwest dip. Based on soil development and stratigraphic position, the deposit likely is late Pleistocene, which correlates well to previously recognized and dated paleo-wetlands within the Mojave Desert. More conclusive ages are pending based on sample collection for IRSL dating. The paleo-wetland deposit may provide clues to landscape evolution of the PMfz corridor, to changing hydrologic conditions related to movement along the PMfz, and (or) to climate changes across glacial-interglacial cycles.