HYDROTHERMAL CIRCULATION IN SUBDUCTING CRUST REDUCES SUBDUCTION ZONE TEMPERATURES

Hydrothermal circulation plays an important role in redistributing heat within high-permeability ocean crust. The thermal state of subducting ocean crust is an important control on subduction zone temperature. Knowledge of subduction zone temperatures is important for understanding processes controlling the updip and downdip limits of seismicity and characterizing subduction zone fluid flow systems. Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the décollement, 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 temperature. Fractured basaltic basement has permeability several orders of magnitude higher than a typical décollement, allowing fluid circulation to redistribute and extract heat from a subduction zone. Models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport. With continuous sediment cover on the incoming plate, heat is extracted from under the margin wedge to the trench. In models where faulted ocean crust exposes high permeability basement to the ocean floor, cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models. Permeability reduction with depth limits how far into a subduction zone significant hydrothermal circulation can persist.