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Abstract Vigorous hydrothermal circulation in the basement aquifer of the oceanic crust homogenizes temperatures within the aquifer and generates fluid overpressures at the tops of buried basement highs. At a site ∼25 km seaward of the Cascadia subduction zone deformation front, fluid overpressure at the top of the buried MARGIN seamount drives vertical fluid seepage through sediment overlying the seamount and results in anomalously high heat flux at the seafloor. In this study, we use numerical models of coupled heat and fluid transport to investigate the sensitivity of fluid overpressures to sediment thickness and basement relief for a 2D buried basement ridge. For ∼8 Ma oceanic crust buried by low permeability sediment, we find that the overpressure at the summit of a basement ridge increases by ∼0.10 kPa per meter of burial depth and by ∼0.71 kPa per meter of basement relief. For a 3D system with a geometry similar to the MARGIN seamount buried by low permeability sediment, the modeled fluid overpressure at the top of the seamount is ∼996 kPa. However, the Astoria Fan sediment above the MARGIN seamount likely has relatively high permeability, permitting rapid vertical seepage, thereby reducing fluid overpressure maintained at the top of the seamount. An overpressure of 492 kPa at the summit of the buried seamount at the MARGIN site and a bulk permeability of the Astoria Fan sediments of 4 × 10⁻¹⁵ m²are consistent with the seepage rate of 5.4 cm yr⁻¹estimated from the elevated heat flux. 

Abstract 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⁻¹in 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.