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Abstract: 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 ~1.5 and 9km 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 ~7Ma (~9–8Ma 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. [Copyright &y& Elsevier]

Abstract: The Ballenas transform margin in central Baja California offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution. Apatite fission track and (U–Th)/He results from two transects perpendicular to the coast reveal a pronounced latest Pliocene to Pleistocene (~1.8 Ma) heating event related to the Neogene opening of the Gulf of California. Proximity to a regional pre-rift unconformity indicates that samples remained at near-surface levels since Paleogene unroofing, despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone. In general, maximum paleotemperatures during overprinting decrease from >100–120 °C near the coast to below 60 °C ca. 5–8 km further inland, suggesting lateral heat flow from a source within the Gulf of California. Heat sources related to the structural development of the Ballenas transform fault, located approximately 1.5–4.5 km offshore from the two sample transects, most likely controlled the observed reheating. Overprinting patterns do not support conductive reheating due to reburial, magmatism or frictional shear. Instead, a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process. Hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or, more likely, magmatic leaking along the transform fault. Latest Pliocene to Pleistocene (~1.8 Ma) activity on the Ballenas transform fault is closely linked to extension in the Lower and Upper Delfín basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern Gulf of California. This study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre. Paleotemperatures from the Ballenas transform are similar to those of other transform margins after passage of a spreading centre, which suggests that hydrothermal fluids may have an important thermal buffering effect, moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin. [Copyright &y& Elsevier]