Geophysical evidence for the evolution of the California Inner ContinentalBorderland as a metamorphic core complex

We use new seismic and gravity data collected during the 1994 Los Angeles R egion Seismic Experiment (LARSE) to discuss the origin of the California In ner Continental Borderland (ICB) as an extended terrain possibly in a metam orphic core complex mode. The data provide detailed crustal structure of th e Borderland and its transition to mainland southern California. Using tomo graphic inversion as well as traditional forward ray tracing to model the w ide-angle seismic data, we find little or no sediments, low (less than or e qual to 6.6 km/s) P wave velocity extending down to the crust-mantle bounda ry, and a thin crust (19 to 23 km thick). Coincident multichannel seismic r eflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an under plated fossil oceanic layer at the base of the crust. Coincident gravity da ta suggest an abrupt increase in crustal thickness under the shelf edge, wh ich represents the transition to the western Transverse Ranges. On the shel f the Pales Verdes Fault merges downward into a landward dipping surface wh ich separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity s tructure is interpreted to represent Catalina Schist rocks extending from t op to bottom of the crust. This interpretation is compatible with a model f or the origin of the ICE as an autochthonous formerly hot highly extended r egion that was filled with the exhumed metamorphic rocks. The basin and rid ge topography and the protracted volcanism probably represent continued ext ension as a wide rift until similar to 13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland , followed by rotation and translation of the western Transverse Ranges, ma y have provided the necessary thermomechanical conditions for this extensio n and crustal inflow.