Structure and Mechanics of the Outer Accretionary Wedge and Shallow Seismogenic Zone off Southern Washington from new Seismic Reflection and Multibeam Imaging

At the Cascadia subduction zone convergent margin off southern Washington and northern Oregon, the young, warm incoming Juan de Fuca plate is blanketed by a 2 to 3 kilometer-thick incoming plate sediment cover. Best available estimates for the extent of the potential seismogenic zone during megathrust earthquakes suggest that it lies entirely offshore and is the basal décollement fault to the accretionary wedge. It extends from a position beneath the outer continental shelf all the way to the deformation front (trench). In July 2012, a grid of 100-120 km long lines of new multi-channel seismic reflection imaging was carried out off Grays Harbor, Washington on the R/V Marcus Langseth, known as the COAST (Cascadia Open-Access Seismic Transects) project. The seafloor was simultaneously mapped with multibeam swath bathymetry and sonar backscatter imaging. We present and interpret a combination of bathymetry and both prestack-depth-migrated and poststack-time-migrated reflection images to address the structure, evolution, inferred mechanics, state of stress and pore fluid conditions at and around the plate boundary decollement. Off southern Washington, the lower continental slope and outer wedge is marked by a zone of dominantly landward-vergent thrust imbrication and shortening of the incoming sedimentary section, the result of rapid build-out of the accretionary complex into thick Pleistocene sediment accumulations. The landward-vergent zone is marked by a near-zero surface taper angle, and in some cases negative (landward deepening) surface slope. The basal plate boundary also dips very shallowly beneath the outer wedge, producing a nearly taperless wedge 40-80 kilometers wide. Individual thrust sheets and hanging wall fault-bend folds are widely separated and nearly buried in piggyback slope basin turbidite sediments and mass transport complexes. Between some of the thrusts, kilometer-wide zones of nearly undeformed strata are preserved. Virtually all of the incoming section is involved in frontal thrusting, with very little or no sediment initially underthrust. Previous workers have addressed the origin of the landward vergent wedge system, and arrived at contrasting conclusions, involving various combinations of (a) near-lithostatic pore fluid pressure, (b) triangle-zone wedging against older accreted material, and (c) bulk plastic flow of the lowermost sedimentary section producing fluidized cores of the folds and mud diapirism. With improved imaging and velocity analysis on the COAST lines, we address these hypotheses. We also present a new hypothesis that the outer wedge architecture is controlled to a substantial degree by supra-wedge sedimentation suppressing taper angle. We conclude by addressing the implications for the state of stress and fault locking in the potential tsunamigenic slip zone.