Integrated crustal structure across the south central California Margin: Santa Lucia escarpment to the San Andreas Fault

Abstract

The continental margin of south central California has undergone different styles of deformation over the past 30 m.y. as the North America‐Pacific plate boundary has evolved from subduction to transform tectonics. Results presented here identify strike‐slip and compressional structures in the lower crust that probably developed during this period of tectonic transition. An integrated model of crustal structure along deep seismic line PG&E‐3 crosses the entire continental margin west of the San Andreas fault near the latitude of San Luis Obispo. A compressional wave velocity model was developed using coincident marine multi‐channel seismic and onshore/offshore wide‐angle reflection/refraction seismic data and was tested independently with gravity modeling. A high‐velocity layer (6.8–7.0 km/s, about 6 km thick) in the lower crust that extends east possibly as far as the San Andreas fault is interpreted as tectonically underplated oceanic crust. The upper and middle crust of the Patton and Sur‐Obispo Franciscan terranes extends to depths of 14 to 20 km. The velocity and density modeling indicate lower average velocities and densities for the Patton terrane, supporting earlier interpretations that it is a distinct terrane within the Franciscan complex. Abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the Santa Lucia Escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 Ma following the subduction of the Pacific‐Arguello ridge. An imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust. Convergence between the Monterey and Arguello microplates during the final stages of Farallon subduction (about 24 to 16 Ma) probably caused the imbrication.