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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. 

The MARGIN site is at a buried basement high (likely a buried seamount) offshore Oregon, ~25 km seaward of the Cascadia subduction zone deformation front. The buried basement high rises ~1300 m above the surrounding basaltic basement. The MARGIN basement high was first identified on the north-south-trending MARGIN transect, collected as part of the Juan de Fuca Ridge-to-Trench project; that seismic transect crossed the flank of the MARGIN basement high. Subsequently, the east-west-trending Line PD11 collected as part of the MGL2104 CASIE21 project conducted in 2021 crossed a higher and more substantial portion of the MARGIN basement high. In 2022, in-situ measurements of thermal gradient were collected crossing and around the MARGIN basement high using a 3.5-m violin-style heat flow probe. This multi-penetration heat flow probe was loaned from the U.S. Marine Heat Flow Capability. The thermistor string houses 11 thermistors. The in-situ thermal gradient is combined with an assumed thermal conductivity versus depth trend to determine the heat flow. The data was processed using SlugHeat (https://marine-heatflow.ceoas.oregonstate.edu/software/). The data file is in ASCII tab-separated format, with column headers. Funding was provided by National Science Foundation awards OCE19-24331, OCE20-34872, and OCE20-34896. 

Abstract Oceanic plate seamounts are believed to play an important role in megathrust rupture at subduction zones, although consistent relationships between subducting seamounts and plate interface seismicity patterns are not found. While most studies focus on impacts linked to their topography, seamounts are also sites of heterogeneity in incoming plate sediments that may contribute to megathrust properties. Here, we characterize incoming plate sediments along the Cascadia subduction zone using new high‐resolution seismic images and compressional wave (V_p) models from the CASIE21 multi‐channel‐seismic experiment. Nine fully‐to‐partially buried seamounts are identified seaward of the deformation front within a region of thick Plio‐Pleistocene sediment where the Juan de Fuca plate is bending into the subduction zone. Anomalously highV_psediment blankets two seamounts offshore Washington‐Central Oregon, with wavespeeds reaching 36% and 20% higher than adjacent sediment. Fluid seepage and temperatures warm enough for smectite diagenesis extending to shallow depths are inferred from heat flow studies and we attributeV_panomalies to sediment cementation linked primarily to smectite dehydration. Signatures of fluid seepage above seamounts are also identified offshore Vancouver Island, but anomalously lowV_psediment below distinct reverse polarity reflections are found, indicating trapped fluids, and cooler basement temperatures are inferred. Landward of one seamount, a zone of enhanced sediment compaction is found, consistent with the predicted stress modulating effects of seamount subduction. These new findings of variations in sediment diagenesis and strength around seamounts prior to subduction may contribute to the diverse megathrust frictional properties and seismicity patterns evident at subducting seamounts.