CRYSTALLINE BEDROCK AND CRUSTAL TECTONICS IN THE WESTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Situated just east of the LARSE II line, the uplifted crystalline rocks of the San Gabriel Mts (SGM) and Verdugo Mts (VM) are key to interpreting the evolving crustal structure of the western Transverse Ranges. The crystalline rocks constrain tectonic evolution of the region in three ways: (1) Reassembly of the SGM massif into its pre-Late Cenozoic paleogeologic position in the crystalline terrane of southern California provides measurements of overall displacement on strike-slip faults of the San Andreas system. In the western SGM, right-lateral displacements are demonstrable for the San Gabriel (22-23 km) and Vasquez Creek (5-15 km) faults. Displacement indicated for all strands of the San Gabriel fault system in and NW of the SGM is 42-43 km. The Vasquez Creek fault is conjugate to the left-lateral Santa Monica-Raymond fault. Bedrock patterns neither require nor preclude a component of lateral slip on the Verdugo fault; they do show that there is no significant strike-slip displacement between the SGM and VM. In reconstructing the SGM-VM and Frazier Mtn-Whitaker Pk crystalline blocks into their pre-San Andreas positions against the North American plate, it is not possible to both restore displaced geologic patterns and incorporate large (> 20-30°) vertical-axis rotation of major crystalline blocks. (2) The distribution of crystalline rocks in the western Transverse Ranges also constrains vertical tectonics in the region. The widths of gaps that open in reconstructing the bedrock terrane provide estimates of the magnitude of contraction in the region that are consistent with magnitudes calculated by analyzing fold- and thrust-belt geometry. Moreover, the mapped pattern of crystalline terranes, the distribution of deeply weathered bedrock, and the dip of flanking Tertiary sedimentary strata along the range-front of the SGM suggest that folding—in addition to reverse faulting—has contributed to uplift of the range. (3) The presence of bedrock clasts in Cenozoic sedimentary deposits of various ages constrains the timing of horizontal displacement and vertical uplift of crystalline blocks.