Heat Flow Evidence for Hydrothermal Circulation in Oceanic Crust Offshore Grays Harbor, Washington

We report 30 new conductive heat flow measurements collected on the Juan de Fuca plate, offshore of Grays Harbor, Washington, just seaward of the deformation front. The data were collected along Cascadia Open-Access Seismic Transect Lines 4 and 5. A prominent basement high associated with a rift propagation pseudofault is imaged near the western end of Line 4. Lithospheric conductive cooling models, for 9 Ma oceanic crust, predict that heat flow should be similar to 175 mW m(-2). Just seaward of the deformation front heat flow values corrected for the impact of sedimentation are similar to 200 mW m(-2) and rapidly rise to a value of similar to 750 mW m(-2) over the basement high. We find that (1) hydrothermal circulation redistributes heat in the Juan de Fuca plate offshore Grays Harbor, (2) heat in addition to the basal heat flux is required to fit the data, and (3) this heat likely results from a combination of fluid flow associated from ongoing hydrothermal circulation within the subducted oceanic crust and possibly with the pseudofault. Our data and modeling support previous inferences that hydrothermal circulation within the subducting oceanic crust plays an important control on plate interface temperatures. Plain Language Summary Fluid circulation within subducted oceanic crust can have a large impact on temperatures along the subduction plate boundary. Knowledge of these temperatures is important for understanding the zone in which earthquakes originate. In general, it is thought that the thermal impact of fluid circulation within oceanic crust is suppressed by laterally continuous and thick sediments. Here we use heat flow data over sediment-buried basement relief to unambiguously identify ongoing hydrothermal circulation. We also show that an added component of heat is necessary to explain our data and suggest that this extra heat is likely coming from subducted oceanic crust and possibly deep fluid circulation associated with a zone of faulted oceanic crust.