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The Hikurangi margin has been an important global focus for subduction zone research for the last decade. International Ocean Discovery Program drilling and geophysical investigations have advanced our understanding of megathrust slip behavior. Along and across the margin, detailed imaging reveals that the megathrust structure varies spatially and evolves over time. Heterogeneous properties of the plate boundary zone and overriding plate are impacted by the evolving nature of regional tectonics and inherited overriding plate structure. Along-strike variability in thickness of subducting sediment and northward increasing influence of seamount subduction strongly influence mega-thrust lithologies, fluid pressure, and permeability structure. Together, these exert strong control on spatial variations in coupling, slow slip, and seismicity distribution. Thicker incoming sediment, combined with a compressional upper plate, influences deeper coupling at southern Hikurangi, where paleoseismic investigations reveal recurring great (Mw> 8.0) earthquakes.▪The Hikurangi Subduction Zone is marked by large-scale changes in the subducting Pacific Plate and the overlying plate, with varied tectonic stress, crustal thickness, and sediment cover.▪The roughness of the lower plate influences the variability in megathrust slip behavior, particularly where seamounts enhance subduction of fluid-rich sediments.▪Variations in sediment composition impact the strength of the subduction interface, with the southern Hikurangi Subduction Zone exhibiting a more uniform megathrust fault.▪Properties of the upper plate influence fluid pressures and contribute to the observed along-strike variations in Hikurangi plate coupling and slip behavior.
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Anomalous Sediment Consolidation and Alteration From Buried Incoming Plate Seamounts Along the Cascadia Margin
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 (Vp) 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 highVpsediment 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 attributeVpanomalies to sediment cementation linked primarily to smectite dehydration. Signatures of fluid seepage above seamounts are also identified offshore Vancouver Island, but anomalously lowVpsediment 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.
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- PAR ID:
- 10571402
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 26
- Issue:
- 2
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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