Identifying the signature and mechanism of long-term permanent strain along the Cascadia coastline, southwestern Washington

Abstract

Long-term deformation at a subduction margin (i.e., over many earthquake cycles) implies permanent strain accumulation. This complicates regional strain budgets as well as expectations of earthquake cyclicity based on the elastic rebound model because not all interseismic strain is released during an earthquake. Characterizing the extent, timing, and rate of forearc deformation at a subduction zone is crucial to understanding the strain budget and subduction zone earthquake cycle. Characterizing coastal uplift is also necessary for assessing the possible mechanisms accommodating permanent deformation in the forearc, such as crustal folding or faulting on the overriding plate. Finally, constraining permanent strain accumulation informs tectonic models and provides key information for estimating seismic hazards. Pleistocene coastal uplift is observed at the Cascadia subduction zone, although no prior studies constrained long-term uplift in coastal southwestern Washington. This dissertation presents new mapping and luminescence dating of Quaternary deposits near Grays Harbor and Willapa Bay to show that estuarine deposits record a late Pleistocene average uplift rate of 0.4 ± 0.1 mm/yr. Uplift rates of this magnitude are consistent with other Pleistocene uplift and incision rates in Cascadia, and when compared to observed interseismic vertical deformation, the rates suggest that about one-tenth of interseismic strain may become permanent. Other locations in Cascadia with similar uplift rates are characterized by crustal folds or faults, but no faults are evident in the onshore Quaternary deposits near Grays Harbor and Willapa Bay. Map-view interpretation and two-dimensional modeling with gravity and magnetic data indicate north- and northwest-trending faults underlie the Quaternary deposits. The modeling suggests two 20-25° east-dipping reverse faults. One fault aligns with the active Willapa Bay fault zone, identified previously from offshore seismic-reflection studies, and the other fault aligns with the Raymond fault, previously inferred from geophysical modeling. Uplift recorded in the estuarine deposits is likely accommodated by the Willapa Bay fault zone. The modeling also suggests that faults mapped in regional bedrock with small lateral offsets must also have a significant vertical component of slip. The geophysical modeling combined with previous geologic mapping thus suggests that regional faults may be oblique. Coastal southwestern Washington may be transitional between deformational domains, with faults accommodating both east-directed, subduction-related strain and north-directed strain related to tectonic block rotation. Active subsurface faults may leave a geomorphic signature of deformation. Where geophysical modeling locates the Raymond fault, geomorphic analyses indicate knickpoints in stream profiles that may reflect a relict fluvial system once graded to a base level 85-150m higher than current sea level. Because Quaternary sea level fluctuations cannot account for 85-150 m of base level fall, the base level change likely reflects regional rock uplift, possibly accommodated by the Raymond fault. The Willapa Bay fault zone, however, does not produce surficial lineaments nor measurable differences in stream profile steepness where it projects ashore nor where geophysical maps indicate a change in strike. The geomorphic analyses of longitudinal stream profiles indicate a close relationship between steepened channels, bedrock faults, and lithologic contacts, an observation that supports structural and geophysical models for uplifted fault-bound blocks of basalt. Taken together, long-term permanent uplift in coastal southwestern Washington is at least partially accommodated by long-lived, active crustal faults.