Near-isothermal conditions in the middle and lower crust induced by melt migration.

The thermal structure of the crust strongly influences deformation, metamorphism and plutonism. Models for the geothermal gradient in stable crust predict a steady increase of temperature with depth. This thermal structure, however, is incompatible with observations from high-temperature metamorphic terranes exhumed in orogens. Global compilations of peak conditions in high-temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure, for both isothermal decompression and isobaric cooling paths. Here we develop simple one-dimensional thermal models that include the effects of melt migration. These models show that long-lived plutonism results in a quasi-steady-state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust. The models also predict that the upward advection of heat by melt generates granulite facies metamorphism, and widespread andalusite-sillimanite metamorphism in the upper crust. Once the quasi-steady-state thermal profile is reached, the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions, thus setting the stage for gravitational collapse, exhumation and isothermal decompression after the onset of plutonism. Near-isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that, in part, control the shape of the geotherm.