Thermal evolution of Monte Blanco dome: Low-angle normal faulting during Gulf of California rifting and late Eocene denudation of the eastern Peninsular Ranges

Footwall rocks of the Cañada David detachment fault, northeastern Baja California, record late Eocene-earliest Oligocene and late Neogene cooling events previously unrecognized in the region. Biotite 40Ar/39Ar ages of ∼ 65 Ma reflect slow cooling through ∼ 350°C, 5 to 10 m.y. later than is typical in the region. Multiple diffusion domain modeling of K feldspar 40Ar/39Ar release spectra shows very slow cooling (∼ 1°C/m.y.) from ∼ 65 to ∼ 45 Ma. Accelerated cooling from ∼ 315°C to ∼ 215°C between ∼ 45 and ∼ 33 Ma records at least 3-4 km of denudation that we relate to cast-side-up reactivation of late Cretaceous structures that generally follow the oceanic-continental suture. Previously established rivers flowing west from mainland Mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast. Ultimately, surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m.y. before the cooling event ended. Footwall rocks remained nearly isothermal from ∼30 to ∼15-10 Ma, when renewed rapid cooling (33° ± 17°C/m.y.) began in response to footwall exhumation by top-to-the-west low-angle normal faulting that accommodated rift-related extension in what finally became the Gulf of California. Apatite fission track and (U-Th)/He ages of ∼5 Ma and ∼4 Ma, respectively, record final detachment-related cooling through ∼ 110°C to ∼ 70°C. Thermal-kinematic modeling suggests that 5-7 km of late Neogene tectonic denudation and 10-12 km of horizontal extension were necessary in order to unroof the samples by 2 Ma. Additional extension, of unknown magnitude, has probably occurred subsequently. Geodetically measured horizontal extension rates are considerably higher than the long-term extension rates that can be attributed to detachment faulting.