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ItemStructural and thermal evolution of the Gulf Extensional Province in Baja California, Mexico: implications for Neogene rifting and opening of the Gulf of California( 2009)The Gulf of California in western Mexico is a prime example of a young passive margin that is currently undergoing the transition from continental rifting to seafloor spreading. With less than ~25 km of the width of the original continental surface area submerged, the northern Gulf Extensional Province represents a key area to assess the history of strain localisation during the early stages of continental extension. Geological mapping revealed that the basins and ranges of the Sierra San Felipe, located in the hanging wall of the Main Gulf Escarpment, are bounded to the east by an en-echelon array of left-stepping moderate- to low-angle normal faults that represent the next dominant set of normal faults from the break-away fault in direction of transport. Structural displacement estimates suggest up to ~4.5–9 km of broadly east-directed extension on the Las Cuevitas, Santa Rosa and Huatamote detachments. Fault kinematics suggest a transtensional stress regime with NE- to SE-directed extension and permutating vertical and N–S subhorizontal shortening. Clockwise vertical-axis block rotations and constrictional folding of the detachments were an integral part of the late Miocene to Pleistocene deformation history of the San Felipe fault array. This overall constrictional strain regime is indistinguishable from the present-day deformation in the Gulf Extensional Province and indicates that the fault array formed during a single phase of integrated transtensional shearing since rifting began in the late Miocene. Apatite fission track (AFT) and (U-Th)/He results of Cretaceous crystalline basement samples from the Sierra San Felipe record a three-stage Cenozoic cooling history. Moderate cooling (~4–7ºC/m.y.) during late Paleocene to Eocene times is attributed to progressive down-wearing and bevelling of the ancestral Peninsular Ranges. Beginning at ~45–35 Ma, a period of tectonic quiescence with cooling rates of ≤1ºC/m.y. marks final unroofing of the basement and the development of a regional Oligocene to Miocene peneplain. Thermal modelling of samples from the footwall of the Las Cuevitas and Santa Rosa fault systems indicates that accelerated cooling began at ~9–8 Ma. This cooling pulse is attributed to tectonic denudation of the footwall and implies that faulting initiated synchronously on both detachments at ~9–8 Ma. Late Miocene deformation occurred distributed throughout the Sierra San Felipe, but started waning after the Pacific-North America plate boundary had localised into the Gulf of California by ~4.7 Ma. During a late Pliocene structural reorganisation in the northern Gulf, the locus of extension shifted from the Tiburón to the Delfín basins, thereby initiating strike-slip faulting on the Ballenas fracture zone, a transform fault located approximately 1.5–4.5 km offshore in central Baja California. This is consistent with low-temperature thermochronometric data from two horizontal transects perpendicular to the strike of the transform, which document a pronounced late Pliocene to Pleistocene heating event that is related to the structural and/or magmatic evolution of the transform fault. During reheating, maximum paleotemperatures reached >100–120ºC near the coast, but did not exceed ~60ºC some 5–8 km further inland. Highly non-systematic overprinting patterns are best explained by circulating hydrothermal fluids, which are most likely associated with magmatic leaking along the transform fault. AFT and (U-Th)/He ages from a vertical profile collected on the Libertad escarpment, which forms part of the Main Gulf Escarpment in central Baja, pre-date Neogene extension and indicate that rift-related denudation was insufficient to expose samples from temperatures higher than the sensitivity zones of the two systems. One sample from the base of the escarpment however, records a middle to late Miocene hydrothermal overprint and suggests that extension in central Baja California likely initiated before ~10–8 Ma.