IMPLICATIONS OF THE PELONA-OROCOPIA-RAND SCHISTS FOR EVOLUTION OF THE NACIMIENTO FAULT, CALIFORNIA

The ca. 75-62 Ma Nacimiento fault of central coastal California separates the Salinian block, a fragment of the central to eastern Sierran arc, from the Nacimiento block, which is underlain primarily by the Franciscan Complex. The Nacimiento fault thus marks a locus along which a substantial part of the western Sierran arc and much of the Great Valley forearc basin has been excised, either by thrust faulting or left-lateral strike-slip faulting. Prior to Neogene offset on the San Andreas fault, the Salinian and Nacimiento blocks, and intervening Nacimiento fault, were situated outboard of southern California and the Pelona-Orocopia-Rand (POR) Schists. The POR schists are metamorphosed graywacke-basalt-chert assemblages that sit structurally beneath North American continental basement and were most likely emplaced by low-angle, east-dipping subduction of the Farallon plate. The schists crop out in a northwest-southeast-trending linear array extending from the southwestern Sierran tail to southwesternmost Arizona. Emplacement age ranges from 90 Ma in the northwesternmost part of the belt to 60 Ma or younger in the southeast. Thus, underplating of the POR schists began before, but in the end coincided with, movement on the Nacimiento fault. Whether the Nacimiento fault is a strike-slip or thrust fault, the required magnitude of displacement is such that this structure must in some way have been kinematically linked with the subduction thrust responsible for underplating of the schists. However, if the Nacimiento fault is a thrust, then the evidence for such a linkage is cryptic. Wherever the base of southern California crust is exposed, it is underlain by POR schists. Nowhere are rocks of the Great Valley Group found to intervene as a structural slice between arc crust and POR schists, as would be predicted by the thrust model. On the other hand, if the Nacimiento fault is a left-lateral strike-slip fault, as proposed by Dickinson (1983), then the POR schists could have been underplated as a continuous sheet beneath North America, even as the latter was undergoing syn-subduction strike-slip. Also, because the proposed left-lateral fault cuts inboard to the southeast, it will progressively transport inboard locations closer to the margin and can thus explain the southeast younging of the POR schists.