NEW GEOLOGIC MAPPING IN NORTHEASTERN LOS ANGELES COUNTY, CALIFORNIA
Initial work included digitizing and compiling existing geologic maps, ranging in scale from 1:12,000 to 1:62,500. Additions and modifications were made to these data based on new field mapping and supported by interpretations using digital orthophoto layers and stereo-pair aerial photographs. The area is traversed by the northwest-trending San Andreas Fault Zone, with distinctive rock units found north and south of the fault, as well as within the fault zone. North of the fault, the area is primarily underlain by Mesozoic age Portal Schist, Cretaceous Holcomb Quartz Monzonite, and old and young alluvium. Exposed within the fault zone are Tertiary Ritter Formation overlain by fluvial deposits of the Pliocene Anaverde Formation. South of the fault, basement rocks include a Proterozoic quartzo-feldspathic and amphibolite gneiss complex, the San Gabriel Mountains anorthosite-gabbro-syenite complex, and Triassic Mt. Lowe Intrusive Suite. Also exposed are Mesozoic rocks that include diorite, Pelona Schist, and mylonite. Draped over the intrusive and basement rocks are volcanic rocks of the Oligocene to Miocene Vasquez Formation. Structural components include Sierra Pelona, a westward-plunging antiform comprised of Pelona Schist. Mylonitic rocks mapped at the contact between the Pelona Schist and the basement complex, have been interpreted as a segment of the south-dipping Vincent Thrust Fault, described as the most important structural feature in the basement rocks of the San Gabriel Mountains.