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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 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.