Wet roots of high elevation in the western United States

Abstract

Lithospheric heat transfer strongly modulates the distributions of melt, rock strength, and buoyancy that are responsible for volcanism, seismicity, and elevation. Geotherm models often extrapolate shallow (<3 km) heat flow measurements but are complicated by near-surface hydrology, poorly-known crustal thermal properties, and deep advective transfer by melts and volatiles. Here we compare temperatures estimated from P-wave velocities in the uppermost mantle to those modeled from surface heat flow in the western United States. We show that U.S. Cordilleran regions of high heat flow and high elevation have deep temperatures much lower than predicted by steady-state conductive cooling models. We hypothesize that the discrepancy reflects reaction thermodynamics and advection by migration of volatiles and melts up the lithospheric column. Hydration of the mantle and lower crust by Farallon subduction consumes garnet into melts that absorb latent heat in the lower crust and upper mantle, while hydration reaction enthalpy heats the subduction back-arc mantle, and advection amplifies surface heat flow. This process would increase elevation both by raising temperatures and by converting dense constituents of the mineral assemblage to more buoyant intrusions in the middle and upper crust. The results imply hydration reaction enthalpy changes play a significant role in the dynamics of Cordilleran regions.