UPDATED GEOLOGIC MAPPING OF THE FRAZIER MOUNTAIN 7.5' QUADRANGLE, SOUTHERN CALIFORNIA

The California Geological Survey currently is preparing a geologic map of the Frazier Mountain 7.5’ quadrangle as part of a larger STATEMAP project to prepare a uniform geologic map of the Lancaster 30’ x 60’ quadrangle in southern California. The Frazier Mountain quadrangle spans a geologically complex area at the intersection of the southern Tehachapi Mountains, Coast Ranges, and Transverse Ranges that is bisected by the Mojave Segment of the San Andreas Fault (SAF), which last ruptured through the area during the M_W7.9 Fort Tejon Earthquake of 1857. The mountainous terrain north of the SAF consists largely of Cretaceous igneous plutons related to the Sierra Nevada Batholith, Paleozoic(?) metamorphic roof pendants, and local orthogneiss, which are cut by the regionally significant Garlock and Pastoria Faults.

Frazier Mountain dominates the landscape south of the SAF. It is underlain by Proterozoic augen gneiss, variable quartzofeldspathic gneisses, and migmatite that were uplifted along the Frazier Mountain Fault and East Frazier Fault (EFF), and are related to crystalline rocks of the San Gabriel Mountains. Both the SAF and Frazier Mountain Fault truncate the Pliocene nonmarine Hungry Valley Fm., which is the youngest unit of Ridge Basin. The San Gabriel Fault (SGF) has been interpreted to intersect/join the SAF within the quadrangle, concealed below younger strata of the Hungry Valley Fm., and the EFF may represent a displaced and reactivated segment of the SGF. Strata northwest of Frazier Mountain include Oligocene to lower Miocene basalt and interbedded lacustrine deposits of the Plush Ranch Fm., and upper Miocene nonmarine strata of the Caliente Fm., both of which are correlated with similar strata in Soledad Basin to support large, right-lateral displacement on the SGF. Frazier Mountain is flanked by a series of Pleistocene alluvial fan deposits that form prominent terrace surfaces, and by several large landslides.

New mapping has so far identified: 1) fossil-bearing strata northeast of Frazier Mountain that appear to correlate with the basal unit of the San Francisquito Fm., extending the range of this unit by 15 miles; 2) east-northeast-trending, south-vergent reverse faults and drag folding east of the EFF; 3) augen gneiss on Frazier Mountain is more interleaved with other gneisses than previously mapped.