4:00 PM - 5:00 PM
Southern California is a critical component in paleotectonic models for: (1) the evolution of the USA-Mexico Cordillera; (2) the interaction of continental and oceanic plates; and (3) 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 broader region. The unique geologic history of southern California can be described in terms of distinct phases of tectonic development, which resulted in corresponding distinct tectonostratigraphic sequences. Steep-slab subduction of the Farallon plate characterized most of the Cretaceous history of the southwestern USA, whereas flat-slab subduction characterized the Laramide orogenic event (80-40 Ma). As the subducting Farallon plate rolled back following the Laramide orogeny, the Pacific plate first came into contact with the North American plate in southern California soon after 30 Ma. Two triple junctions then traveled in opposite directions along the continental margin. The southern triple junction had a complex history, including three distinct stages of capture of Farallon microplates and contiguous parts of the North American margin by the Pacific plate. These three microplate-capture events resulted in transrotation (18-12 Ma), transtension (12-6) and transpression (6-0 Ma) in the Los Angeles region. As the rigid Sierra Nevada and Peninsular Ranges batholiths have converged along opposite sides of the San Andreas fault (6-0 Ma), the intervening San Andreas fault has rotated counterclockwise, thus tightening restraining double bends and resulting in clockwise rotation of crustal blocks moving through these bends. Reversal of 240 km of dextral slip on the San Andreas fault, 70 km of dextral slip on the San Gabriel/Canton fault and varied clockwise rotations results in a coherent set of SW-NE-trending normal faults, basins and antiformal structures. Reconstructing this complex history is an extraordinarily challenging and rewarding endeavor.