The Onshore-Offshore LARSE I Transect: San Clemente Island to the Mojave Desert--Crustal blocks and the Moho

The 1994 seismic transect of the Los Angeles Region Seismic Experiment (LARSE I) crosses the Inner California Continental Borderland, Los Angeles basin, San Gabriel Mountains, and Mojave Desert. In addition to LARSE I seismic refraction and reflection data, we have used gravity, geologic, petrophysical, oil-test well, earthquake, and receiver-function data and models to constrain our geologic/tectonic model of this transect. Several blocks can be defined along the transect and are described from south to north: (1) The Borderland block, seaward of the Palos Verdes fault, is characterized by generally thin Cenozoic sedimentary rocks (< 1 km), and a basement (2.8-5.9 km/s in the upper few km, increasing to 6.4 km/s at depth) that is interpreted as chiefly Catalina Schist. A lower-crustal layer (6.6-6.8 km/s), 4-5 km thick, is 18 km deep, and Moho appears relatively flat at ~22-km depth. (2) A transitional block, between the Palos Verdes and Newport-Inglewood faults, is characterized by moderately thick Cenozoic sedimentary rocks (3 to 4 km), and a basement of chiefly Catalina Schist (velocity poorly constrained). A lower-crustal layer thickens landward from ~5 to ~10 km thickness above a Moho that deepens in a sharp ramp from ~23 km, slightly west of the Palos Verdes fault, at the shelf edge to ~28 km at the Newport-Inglewood fault. (3) The Los Angeles basin block, between the Newport-Inglewood and Sierra Madre faults, is characterized by thick Cenozoic sedimentary rocks (10 km maximum in Los Angeles basin). Basement is Peninsular Ranges igneous and metamorphic rocks with velocities ranging from 6.1 to 6.8 km/s at or near the sedimentary contact to 6.8 to 6.9 km/s in the lower crust. Moho is flat at 28-km depth. (4) The San Gabriel block, extending from the Sierra Madre fault to the San Andreas fault (SAF), consists of 3 distinct rock units. Mylonitic lower-crustal rocks, ranging in thickness along the transect from 0 to 5 km, overlie Pelona Schist along the inactive Vincent thrust fault. The Pelona Schist (similar to the Catalina Schist) is characterized by a relatively low velocity (~6.0 km/s) to ~20-km depth. A bright reflective zone, interpreted as a fluid-lubricated, active decollement, is observed at the interpreted base of the Pelona Schist. The lower crust (6.7 km/s ?) has a maximum thickness of 14 km. Moho deepens in a sharp ramp in the southern block from 28-km depth to 36-km depth. Moho depth remains at ~36 km depth from 20-km south of the SAF to 10-km north of the SAF. (5) The Mojave block is characterized by very thin Cenozoic sediments (0 to ~1 km, within a few km of the transect) overlying a chiefly granitic basement (2.5 to 5 km/s near the surface). This block has no high-velocity lower crust (> 6.5 km/s), and Moho is flat at ~32 km, except in the southern Mojave Desert, where it deepens in a sharp ramp to ~36 km at the SAF. The shape of the Moho as given above has been established by raytracing of PmP; further constraints and/or alternate models are being developed from a receiver-function study.