IMPLICATIONS OF EOCENE SUBDUCTION REFRIGERATION OF SOUTHERN BAJA FOR THE LATE MIOCENE GEOTHERM DURING INITIAL RIFTING OF THE GULF OF CALIFORNIA

The southern margin of Baja California records a middle to Late Miocene (12.5-5.0 Ma) transition from subduction of youthful oceanic crust to highly oblique oceanic rifting. Significant extensional faulting of basement rocks in southernmost Baja California along the San Jose del Cabo (SJDC) normal fault is constrained by integrated K-feldspar multi-diffusion and zircon and apatite fission track and (U-Th)-He thermal history results to have begun no earlier than ~ 8 Ma. Present-day thermal gradients indicate that the vertical component of exhumation would be no more than ~5 km. However, the time dependant geotherm throughout the Cenozoic and leading up to Late Miocene rifting remains uncertain because subduction affected the margin as late a 12.5 Ma. Basement rocks representing the deepest exposures of the Los Cabos block have been sampled from the topographic base of SJDC footwall. These rocks all record Significant Early Cenozoic cooling that could be consistent with either Eocene (60-45 Ma) erosional denudation and/or subduction refrigeration during shallow subduction that relocated the magmatic arc well east of the convergent margin into formerly adjacent mainland Mexico. We have modeled the effects of both processes. In the extreme case in which geothermal gradients were significantly depressed, the vertical component of tectonic denudation along the SJDC fault could have been as great as 10 km. Overall, our results highlight the importance of highly integrated mid-to-low thermochronologic studies with numerical simulations of crustal deformation.