THE CALIFORNIA TRANSVERSE RANGES AS A COMPRESSIONAL WEDGE: EVIDENCE FOR A THIN SKINNED ORIGIN

3D Seismo-Tectonic Imaging (STI) is used to study the regional tectonics of a portion of the California Transverse Ranges (TR) in the vicinity of Los Angeles. STI is a methodology for structural investigation using independent but related data sets (structural and earthquake). Consistent with the transpressional nature of the SAF in the vicinity of Los Angles, we find evidence for a major crustal scale detachment interpreted as the base of a compressional wedge encompassing the Transverse Range. Further, earthquake-defined surfaces indicating ramp/flat geometry internal to the wedge are conformal with structures independently imaged from geologic and seismic reflection and refraction data. Other elements of the regional geology, e.g. a fundamental tectonic vergence whose direction flips across the crystalline core, structural zonation, etc., also support this interpretation. In contrast in order to fit these data a “thick skinned” Laramide style model requires unnecessary multiplication of hypotheses. Of particular interest is what we term the Los Angeles Seismic Zone (LASZ). The LASZ is a “cloud” of background thrust earthquakes whose geometry and structural location suggesting a major frontal ramp and flat in the form of a “proto-mountain front” developing at depths of 10 - 15 kms. beneath the city of Los Angeles. The minimum size of the LASZ is approximately 100 x 50 kms. Extending the wedge model to the entire TR predicts a basal detachment that may form a single (segmented) seismogenic surface some 350 x 100 kms in size. If the model we propose for the TR is correct, then the potential seismic hazard to southern California may be considerably greater than that presently postulated.