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Abstract Sediment thermal history controls the progress of diagenetic reactions that can alter the mechanical behavior of material entering a subduction zone that then: accretes to the margin, hosts the plate boundary interface, or is carried deeper within the Earth. On the Cascadia margin offshore Oregon (USA), hydrothermal circulation in the oceanic crust affects thermally controlled processes, enhancing sediment alteration above the MARGIN seamount, which is buried by the Astoria Fan. Hydrothermal circulation increases temperatures at the summit of the seamount and in the overlying sediment by up to ∼100°C. We use sediment thermal history constrained by heat flux observations to model the expected progress of the smectite‐to‐illite reaction around the MARGIN seamount. Above the seamount, the smectite‐to‐illite reaction is expected to progress to completion ∼250 m below the seafloor; away from the seamount, smectite is likely unaltered to a burial depth of ∼800 m. The altered sediment above the seamount has higher rigidity and p‐wave velocity than the surrounding sediment. Spatial variability in sediment alteration may be present around other buried seamounts. We use vertical gravity gradient anomalies to estimate the locations and heights of additional seamounts. Each of these seamounts may have altered sediment around it, which could affect deformation and seismicity in the margin wedge. Because cemented sediment with greater elastic strength is better able to store elastic strain energy, enhanced sediment alteration and cementation above seamounts entering the subduction zone could facilitate earthquake nucleation for material in the margin wedge that was above a seamount prior to subduction. 

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.