Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province?

The Sierra San Felipe, located in the Gulf Extensional Province of northeastern Baja California, experienced a complex deformation history of integrated normal and strike-slip faulting, block rotations and extension-parallel folding as a result of Neogene transtensional plate margin shear between the Pacific and North American plates. The eastern range-front of the Sierra San Felipe is defined by three left-stepping, en-echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones. The Las Cuevitas, Santa Rosa and Huatamote detachments comprise NE- to SE-dipping, moderate- to low-angle normal faults that accommodate between similar to 1.5 and 9 km of broadly E- to SE-directed extension and show strong along-strike displacement gradients. Hanging wall half-grabens are infilled with northwest tilted Miocene-Pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement. Stratigraphic relationships indicate that faulting on the Las Cuevitas and Santa Rosa detachment faults initiated synchronously as a kinematically linked fault system before similar to 7 Ma (similar to 9-8 Ma based on the timing of footwall exhumation), during the so-called 'proto-gulf' phase of rifting. Paleostress calculations suggest a transtensional stress regime with NE- to SE-directed extension and permutating vertical to N-S trending, subhorizontal contraction. Fault kinematics and paleostress orientations of the fault array do not vary through time, but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments. Our data indicate that clockwise vertical-axis block rotations and constrictional folding were an integral part of the deformation history in the Sierra San Felipe since rifting began in the late Miocene, and may have played an important role in facilitating the transfer of deformation between the Main Gulf Escarpment and offshore faults in the Gulf of California. These observations support the hypothesis that middle Miocene to present oblique-divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the Gulf Extensional Province. (C) 2009 Elsevier B.V. All rights reserved.