PALEOTECTONICS OF CALIFORNIA AND MEXICO 75-60 MA, WITH EMPHASIS ON THE SINISTRAL NACIMIENTO FAULT

Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains and rotations, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate and stress direction at 6my intervals. The input data derive from literature from 20° to 49° N and from 0 to 90 Ma. The computed paleogeologic map at 60 Ma shows the west edge of the North American belt of Cretaceous granodiorite (Kg) plutons offset 370-480 km in a sinistral sense along southeast trends in southern California. This offset corresponds with restored traces of the Nacimiento fault in central and southern California at 60 Ma. Two models were calculated using alternate traces for the continuation of this fault trend into Mexico during 75-60 Ma. A separate talk at this meeting champions a proposed Nacimiento-Sonoyta-Fronteras-San Marcos (NSFSM) fault trace defined by offsets of both east and west edges of the Kg belt. The present talk champions the Nacimiento-Caborca-Durango-Zacatecas (NCDZ) fault trace, which partly follows the California-Coahuila transform. Both models successfully restore the western edge of the Kg belt; restoration of the eastern edge is successful with the NCDZ model if the presence of Kg beneath younger volcanic cover in the Sierra Madre Occidental is assumed. The northwestern continuation of the Nacimiento fault is unconstrained in the Franciscan Complex, and is, therefore, not part of the finite-element grid of Restore. The result is an implausible restoration at the western edge of the model. The Franciscan trench can be manually restored by extending the NCDZ fault northwestward through coastal Franciscan terrane, where it was part of a TTF triple junction that initiated at ~75 Ma in central California. Sinistral slip on the NCDZ would have induced canyon cutting along the newly exposed continental margin (consisting of subduction complex-forearc-magmatic arc from northwest to southeast), followed by subsidence and infill by abundant arkosic sediment derived from the eroding arc. Some of the sediment would have been trapped in canyons and along the new shelves, but much of it would have flowed into the new trench, where it would have been subducted, deformed and metamorphosed to form the Pelona-Orocopia-Rand schist.