Hydrothermal circulation cools continental crust under exhumation

Abstract

Abstract The formation of continental crust in magmatic arcs involves cooling of hot magmas to a relatively colder crust enhanced by exhumation and hydrothermal circulation in the upper crust. To quantify the influence of these processes on the thermal and rheological states of the crust, we developed a one-dimensional thermal evolution model, which invokes conductive cooling, advection of crust by erosion-driven exhumation, and cooling by hydrothermal circulation. We parameterized hydrothermal cooling by adopting depth-dependent effective thermal conductivity, which is determined by the crustal permeability structure and the prescribed Nusselt number at the surface. Different combinations of erosion rate and Nusselt number were tested to study the evolution of crustal geotherms, surface heat flux, and cooling rate. Simulations and scaling analyses quantify how erosion and hydrothermal circulation promote cooling via increasing total surface heat flux compared to pure conductive cooling. Hydrothermal circulation imposes intense short-term and persistent long-term cooling effects. Thinner, warmer, fast exhuming crust, with higher permeability and more vigorous hydrothermal circulation, leads to higher steady-state total surface heat flux. Hydrothermal cooling at steady state is more effective when the Peclet number is small. Hydrothermal cooling also changes crustal rheological state and thickens the brittle crust. This in turn promotes the initiation of brittle deformation in the upper crust in magmatic arcs or in regions undergoing exhumation. Interpretation of low-temperature thermochronological data could overestimate average cooling rates if hydrothermal cooling is not considered.