CONSTRAINTS ON TIMING AND MAGNITUDE OF CONTINENTAL RIFTING IN THE GULF EXTENSIONAL PROVINCE, BAJA CALIFORNIA PENINSULA, MEXICO, FROM LOW TEMPERATURE THERMOCHRONOLOGY

The 1300 km long Baja California (BC) peninsula has been undergoing transfer from the North American to the Pacific plate since the Miocene. This motion has mainly occurred across the Gulf Extensional Province (GEP), which is spatially and kinematically partitioned into three distinct fault systems; embryonic spreading centres linked by an en-echelon array of right stepping transform faults and a broader network of N-NW striking normal faults along the margins of the Gulf of California (GC). Based on low-temperature thermochronology of apatites by the fission track and (U-Th)/He methods, our study characterizes the 3-D cooling and unroofing patterns in two of the best developed east-directed normal faults on the rifted margin of BC: the escarpment of the Sierra San Pedro Matir, and the fault array cutting the Los Cabos Block.

The San José del Cabo fault (SJC) in the Los Cabos block in the south, a massif of Mesozoic crystalline basement, is an east-dipping fault with a strike length of ~150 km. The fault forms a topographic escarpment >1 km high and defines the eastern limit of the block. The SJC footwall records rapid cooling related to tectonic exhumation between ~10-5 Ma. The fault accommodated ~5-6.5 km of exhumation at rates as high as ~1.5-2 mm/yr, but much less since the late Pliocene. A similar exhumation history is discerned to the east of the SJC for the clockwise-rotated Sierra La Trinidad block, the deepest levels of which are inferred to lie offshore. Late Miocene cooling ages from a N-trending fault on the Isla Santa Catalina, some 135 km NNW of the Los Cabos block, confirms rapid exhumation at that time on a different strand of the normal fault system.

The Sierra San Pedro Martír block in northern BC, comprises middle Cretaceous granitoids and forms a prominent escarpment rising to 3 km. AFT ages vary from 59-65 Ma along the range crest to 35-38 Ma at the base, suggesting that erosion and uplift of the block was less than ~2 km since ~37 Ma. Samples from the base of tilted blocks to the east of the escarpment reveal that they cooled from slightly deeper crustal levels. Extended tectonic blocks in both areas show progressively deeper crustal levels and more rotation (about both vertical and horizontal axes) toward the east in the direction of transport.