IDENTIFYING THE SIGNATURE AND MECHANISM OF LONG-TERM, PERMANENT STRAIN ALONG THE CASCADIA SUBDUCTION ZONE COASTLINE IN SOUTHWESTERN WASHINGTON

Deformation of the overriding plate at subduction zones denotes interseismic strain accumulation. Although most interseismic strain is thought to be released elastically during a subduction zone earthquake, coastal mountain ranges and uplifted marine or coastal deposits express long-term, permanent strain at many subduction zones. In coastal southwestern Washington, late Pleistocene estuarine deposits record long-term coastal uplift. The uplift rate around Willapa Bay and Grays Harbor nears the rates associated with folds or faults at other coastal uplift sites in Cascadia. We employ 2-D geophysical modeling and map-view interpretation using observations from gravity and aeromagnetic data to investigate the possible underlying mechanisms for observed coastal uplift. These reveal N- and NW-trending faults underlying onshore Quaternary deposits and Willapa Bay. Modeling suggests two 20-25° E-dipping reverse faults, one of which aligns with the previously mapped Willapa Bay fault zone. The Willapa Bay fault zone likely accommodates the observed uplift of estuarine deposits. Longitudinal stream profiles at the projected trace of the nearby Raymond fault show knickpoints. The stream profile knickpoints are at approximately similar elevations and may reflect a relict fluvial system once graded to a base level 85-150 m higher than current sea level. Since late Pleistocene sea-level high stands were not that high, the base level change likely reflects regional rock uplift accommodated by the Raymond fault. Combined with previous geologic mapping, the geophysical modeling suggests oblique motion on many regional N- and NW-trending faults. Coastal southwestern Washington may be located between deformational domains, with faults accommodating both east-directed, subduction-related strain and north-directed strain related to intraplate tectonic block rotation.