NEW INSIGHTS ON THE AGE, DISTRIBUTION, AND STRUCTURE OF BASEMENT ROCKS ALONG THE SOUTHWEST SIDE OF THE SAN ANDREAS FAULT IN THE BURNT PEAK 7.5’ QUADRANGLE, SOUTHERN CALIFORNIA

New geologic mapping of the Burnt Peak 7.5’ Quadrangle by the California Geological Survey and U-Pb dating of zircons from basement rocks by the CSUN Laser Lab has revealed new insights into the geologic history of the Liebre Mountain block of the northwestern San Gabriel Mountains (SGM), south of the San Andreas Fault (SAF). Previously published work suggested the basement rocks are composed of undated gneiss complexly intruded by extensive quartz diorite and lesser granite. Much of the gneiss was found to be Early Proterozoic paragneiss, with upper-intercept ages of 1,718, 1,749, and 1,775 +/-35 Ma (2% uncertainty), for hornblende, biotite, and quartzofeldspathic gneisses respectively, similar to previously reported ages for Mendenhall Gneiss of the southwestern SGM. However, upper-intercept ages of 1,954 +/-39 Ma and 1,897 +/-38 Ma were produced for biotite paragneiss and local granitic orthogneiss, respectively, which are spatially associated with intermixed marble bodies along the SW side of the SAF; these ages may be the oldest known for rocks exposed in the Transverse Ranges. Marble-bearing gneiss is present on the NE side of the SAF in the upper Mill Creek watershed in the San Bernardino Mountains; correlation of the marble-bearing gneiss units at Burnt Peak and Mill Creek would support a total right-lateral displacement of about 160 km on the Mojave Strand of the SAF, as previously reported for a Triassic mega-porphyry, exposed 6-7 km west of both gneiss outcrops. Foliation within the gneiss forms a large-scale, east-west-trending synform not previously mapped, which is cut by eastern strands of the Liebre Mountain Fault; remnants of deformed Tertiary sedimentary rock are preserved near the core of this fold. A band of gneiss on the south fold limb also contains interleaved marble and U-Pb dating is in progress to assess if both marble-bearing gneisses are the same age. Mapping indicates that weakly to moderately foliated quartz diorite is much less extensive than previously mapped and has a weighted-average ²⁰⁶Pb/²³⁸U age of 76.6 +/-1.5 Ma. Granite on the south side of the SAF returned two weighted-average ²⁰⁶Pb/²³⁸U ages of 73.1 and 76.3 +/-1.5 Ma. In contrast, quartz monzonite on the north side of the SAF associated with the Sierra Nevada Batholith returned an older weighted-average ²⁰⁶Pb/²³⁸U age of 94.6 +/-1.9 Ma.