PALEOTECTONIC EVOLUTION OF SOUTHERN CALIFORNIA

Southern California is a critical component in paleotectonic models for the evolution of the USA-Mexico Cordillera, the interaction of continental and oceanic plates, and relations between subduction and transform processes during the Mesozoic and Cenozoic. Detailed palinspastic reconstruction of both offshore and onshore components of the diverse and complex settings of southern California is essential in order to test paleotectonic models for the evolution of the region. The following events must be understood and their effects must be removed in order to achieve the desired reconstruction: Late Jurassic “Nevadan” orogeny, followed by initiation of Cretaceous Franciscan-Great Valley-Sierran arc-trench system (and Baja equivalents); Cretaceous “normal” subduction, followed by Laramide flat-slab subduction of Farallon plate (80-40 Ma); rapid rollback of slab and return of magmatic arc to its “normal” position (40-30 Ma); initiation of Pacific-North American plate interaction (soon after 30 Ma), followed by NW movement of the unstable Mendocino triple junction and SE jumping of the Rivera triple junction during three microplate captures, resulting ultimately in the transfer of Baja and coastal southern California to the Pacific plate.

These paleotectonic events are represented by the following tectonostratigraphic units: Jurassic “Nevadan” orogeny (Santa Monica Fm.), Cretaceous forearc (Chatsworth Fm.), Laramide forearc (San Francisquito Fm.), transitional forearc (Sespe Fm.), triple-junction extension (Vasquez Fm.), transrotation (Topanga Fm.), transtension (Puente Fm.) and transpression (Fernando Fm.). Systematic retrodeformation of each event (using all available data sets), from youngest to oldest, should provide important constraints for paleotectonic models.