Geothermal gradients in continental magmatic arcs; constraints from the eastern Peninsular Ranges Batholith, Baja California, Mexico

Abstract

In continental arcs the extension of geothermal gradients derived from shallow crustal levels to depth predicts widespread melting at pressures that are inconsistent with seismic studies. Numerical models of low-pressure metamorphism in continental arcs suggest that these extrapolations are problematic because magmatic advection is the dominant mechanism of heat transport in these terranes rather than conduction from the base of the lithosphere. Metamorphic thermobarometry data from the middle crust of the eastern Peninsular Ranges batholith in Baja California, Mexico, provide a useful field test of these models. Graphite-bearing pelitic and semi-pelitic schists record peak metamorphic temperatures of 475-720 °C at pressures of 3-6 kbar. These data bridge a gap between shallow and deep crustal levels of continental magmatic arcs in the southwestern United States and Baja California, Mexico. Recognition of the transient, isobaric heating that accompanies contact metamorphism allows the definition of a gradient in minimum wall-rock temperatures of ∼22 °C/km from 10 to 25 km with thermobarometric data from the eastern Peninsular Ranges batholith and other continental arcs that have relatively simple thermal histories. This gradient defines a maximum background geotherm that reconciles the results of geophysical and numerical models with wall-rock thermobarometry and is consistent with the formation of granulites in the lower crust of sub-arc regions and numerical models of the thermal effects of nested plutons. Recognition of the proposed geotherm may lower estimates of the depth to the seismic Moho and increase strain and unroofing rates inferred from structural and thermochronologic studies, respectively.