GEOLOGICAL CONSEQUENCES OF MIOCENE INITIATION OF MICROPLATE CAPTURE AND TRANSROTATION IN SOUTHERN CALIFORNIA, USA

Following initial contact between the Pacific and North American plates at 28 Ma, intervening parts of the Farallon plate broke into several microplates (Nicholson et al., 1994). These microplates were sequentially captured by the Pacific plate, meaning that they began moving northwestward along the North American margin. The onland geological consequences of these microplate-capture events are becoming clearer as the intricate paleotectonic evolution of southern California is unraveled.

Regional NE-SW extension dominated southern California following initiation of capture of the Monterey microplate (mp) at 24 Ma. The Simmler, Plush Ranch,Vasquez and Diligencia basins, along with core-complex extension of the Mojave Desert, represent this period. It is significant that none of these basins contain clasts of Pelona-Orocopia schist, even though thermochronology indicates cooling of these schists at 24-22 Ma (Jacobson et al., 2007). The Monterey mp was fully captured by the Pacific plate by 18 Ma; the overlying Western Transverse Ranges block began transrotation at this time because its northern boundary prevented it from moving directly to the NW. Present exposures of the P-O schist palinspastically reconstruct to locations near the NE corner of the WTR and the oldest known clasts of P-O schist are found in associated basins (e.g., Tick Canyon Formation), suggesting a causal relation between the initiation of transrotation of the WTR and final unroofing of the P-O schist. Reconstruction of subsequent deformation is essential to clarify these relations.