Numerical model of heat flow in back-arc regions

Abstract

We numerically modeled mantle wedge flow driven by subducting plates and its consequences to surface heat flow in back-arc regions, where an increase in surface heat flow has been observed in almost all subduction zones. In order to calculate the steady state temperature and velocity field in a generic subduction zone model we utilized a new technique which does not require a priori assumptions about the position of the slab–wedge boundary. We demonstrated that calculated surface heat flow is very sensitive to the thermal boundary condition prescribed at the base of the model for a simple temperature-dependent viscosity. Since an increase of surface heat flow is a common feature of many back-arc regions, it should be rather independent on various thermal conditions applied at the bottom. We showed that such a robust behavior can be reproduced if, moreover, a strong pressure-dependence of viscosity is taken into account. The pattern of increased surface heat flow is then stable for a wide range of settings including various dip angles and age of the subducting slab. The temperature- and pressure-dependent viscosity resulted in a nearly isothermal mantle wedge, where temperatures exceeded 1200 °C even in relatively shallow depths, and, consequently, elevated surface heat flow is consistent with recent hot back-arcs evidence. Circulation in the wedge was obtained since the employed viscosity law resulted in creation of a zone of substantially reduced viscosity in shallow depths below the continental crust.