Landward vergence in accretionary wedges is an uncommon phenomenon not readily explained by classical Mohr-Coulomb critical wedge theory. Predominantly landward- vergent thrust faults are observed along the Cascadia convergent margin from 45°N to 48°N. We present depth-migrated multichannel seismic images of the internal structure of the accretionary wedge offshore Washington collected during the ORWELL project in 1996. These reveal a high p-wave velocity (≥4 km/s) basal layer that thickens landward and serves as a décollement for a series of overlying landward-vergent thrust faults. Analog modeling using a ductile basal layer consisting of silicone putty produces an array of trenchward-propagating, landward-vergent thrusts and offers a plausible mechanical model for the evolution of these structures. The rheological properties of a basal calcareous mudstone layer offshore Cascadia are discussed in relation to the mechanics of landward vergence and to rapid loading due to the prograding Nitinat and Astoria deep-sea fans. A viscoelastic layer beneath the accretionary wedge is considered to be capable of rupturing during great interplate earthquakes and thus represents an increased risk for both the maximum size of such an event and for the generation of tsunamis.

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