Thermally Significant Fluid Seepage Through Thick Sediment on the Juan de Fuca Plate Entering the Cascadia Subduction Zone.

We use heat flux measurements colocated with seismic reflection profiles over a buried basement high on the Juan de Fuca plate ∼25 km seaward of the deformation front offshore Oregon to test for the presence of hydrothermal circulation in the oceanic crust. We also revisit heat flux data crossing a buried basement high ∼25 km seaward of the deformation front ∼150 km north, offshore Washington. Seafloor heat flux is inversely correlated with sediment thickness, consistent with vigorous hydrothermal circulation in the basement aquifer homogenizing temperatures at the top of the basement. Heat flux immediately above the summit of the basement highs is greater than expected solely from conduction. Fluid seepage at rates of ∼2.6–5.4 cm yr−1 in a 1–1.5 km‐wide conduit through ∼800–1,300 m thick sediment sections above these basement highs can explain these observations. Observations of thermally significant fluid seepage through sediment >225 m thick on oceanic crust are unprecedented. High sediment permeability, high fluid overpressure in the basement, or a combination of both is required to drive fluid seepage at the observed rates. We infer that rapid seepage occurs because the basement highs rise above the low permeability basal sediment with their tops protruding into the base of high permeability Nitinat or Astoria Fan sediment. Seepage from basement highs penetrating into the submarine fans can affect the thermal state of crust entering the subduction zone. Plain Language Summary: Fluid circulation in the oceanic crust can transport large amounts of heat, influencing the heat content and the distribution of heat within the crust. This fluid circulation primarily occurs in a high permeability aquifer composed of rapidly cooled basalt flows that comprise the upper ∼300 m of oceanic crustal rocks. Most marine sediments that accumulate on the basaltic aquifer have very low permeability and inhibit fluid flow; for sediment covered oceanic crust, fluid recharge/discharge from/to the ocean only occurs at sediment‐free basement outcrops. For most sediment types, ∼40 m of sediment accumulation is sufficient to slow fluid flow rates through them, so that fluid flow is not thermally significant. For sites offshore Oregon and Washington, we document thermally significant fluid seepage through sediment ∼800–1,300 m thick. This seepage occurs over buried basaltic hills that penetrate through low permeability sediment into a thick layer of high permeability sandy sediment that hosts the seepage. The fluid seepage, at ∼2.6–5.4 cm yr−1, is fast enough to remove heat from the underlying crust. This can affect the temperature of the crust entering the subduction zone offshore North America. Key Points: We discovered rapid fluid seepage through 1 km thick sediment on the Juan de Fuca plate 25 km seaward of the Cascadia subduction zoneBuried basement highs penetrating through basal sediments are high permeability conduits to the base of sandy submarine fan sedimentsThis previously unknown thermally significant seepage can affect the thermal state of the crust entering the subduction zone [ABSTRACT FROM AUTHOR]