Accretionary wedge structure, porosity loss, and relative timing of fault activity in the landward vergent region, Cascadia subduction zone

Abstract:

The Cascadia subduction zone has a history of large magnitude earthquakes, but the offshore extent of the locked seismogenic zone is unknown. To address this, we have performed a detailed structural interpretation of 2D time-migrated multichannel seismic lines shot as part of the Cascadia Open Source Seismic Transect (COAST) offshore Grays Harbor, Washington. Objectives of this study are to document (1) the relative timing of major thrusts and attendant super-wedge sedimentation, (2) the geometry of faulting, particularly the basal decollement and evidence, if any, for underthrust sediments, and (3) the porosity distribution based on interval velocity, all of which bear on stress state and therefore likelihood of large coseismic slip. Key features of the central Cascadia accretionary margin are a thick incoming sediment section, a shallowly dipping lower plate, an extremely low overall wedge taper angle, landward vergent faults in the outer wedge, and substantial sedimentation on the growing wedge. Relative timing of fault activity, revealed by detailed interpretation of deformation of slope basins, indicates out-of-sequence faulting in the outer wedge, with the majority of recent motion along faults in the central portion of the wedge. In the lower-slope terrace, relatively undisturbed basin sediments overly structures that appear to be landward vergent thrust sheets that are less developed than their outer-wedge counterparts. The deformation of lower packages of overlying sediment show that these inactive thrust sheets, which have incoherent internal reflections, may show evidence for (partial) diapiric intrusion. Velocity models and structural and sedimentary history as interpreted from 2D seismic sections are used to calculate porosity through the wedge down to the subducting oceanic crust. These calculated porosities are used to estimate the effective stress within the wedge and at depth to shed light on the stress-state in the Cascadia margin.