Thermal effects of fluid circulation in the basement aquifer of subducting ocean crust

Abstract

Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the plate boundary fault zone, and margin wedge likely transports only a minor amount of heat. We model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can greatly influence subduction zone temperatures. Fractured basaltic basement is several orders of magnitude more permeable than a typical plate boundary fault zone or marine sediments, allowing fluid circulation to redistribute and extract heat from a subduction zone. Fluid circulation within the basement aquifer suppresses temperatures along the subducting slab relative to cases with no fluid transport. Heat is extracted from under the margin wedge and transported into the ocean crust near the trench. This circulation has a large effect on subduction zone temperatures when topographic wavelength-dominated convection occurs (>5 times the critical Rayleigh number). With the exception of very cold subduction zones (i.e., those with very fast convergence or very small taper) topographic wavelength-dominated convection occurs with aquifer permeability ≥ 10-11 m2 (in the range typically determined for upper ocean crust suggesting such fluid circulation may be important in many subduction zones. Because fluid circulation more effectively transports heat in warmer systems, hydrothermal circulation moderates the effects of convergence rate as a control on subduction zone temperatures. Copyright 2009 by the American Geophysical Union.