RECONCILING CASCADIA FOREARC LONG-TERM DEFORMATION MODELS

The two dominant models for long-term deformation of the Cascadia forearc associate widely different contraction orientations for the Olympic Mountains. The “wedge” model interprets development of the Olympics to be the result of a growing subduction wedge, named “Olympic Structural Complex” (OSC) (Stewart and Brandon, 2004), accompanied by NE-SW shortening approximately parallel to the convergence direction (e.g. Brandon et al., 1998). Finite-strain data from pressure-solution cleavage in metamorphosed Lower OSC rocks support this model as they show no evidence for N-S shortening (Brandon and Thissen, 2016). The “block” model, in contrast, infers N-S shortening in the Olympic Mountains due to Neogene clockwise rotation and margin-parallel transport of the Oregon forearc block pushing the Olympic Peninsula northward against a Canadian Coast Range buttress (e.g. Wells et al, 1998; Wells and Simpson, 2001).

Quaternary thrust faults and folds in Pleistocene sediments in the western Washington Cascadia forearc were mapped by McCory, 1996, as changing from NNW-trending structures in the OSC to ENE-trending south of the OSC. She attributed the NNW-trending structures to subduction-related contraction, and the ENE-trending structures to N-S contraction driven by northward translation of Siletzia terrane. Recently mapped ENE-trending Late Quaternary (<122 ka) thrust faults and folds, 20 km north of McCory’s northern-most ENE-trending structures, reflect N-S shortening in the Coastal OSC, consistent with the block model.

Extant deformation data for the Olympic Mountains cannot all be accounted for by the wedge or block model alone, indicating Cascadia forearc deformation on the Olympic Peninsula involves components of both models. Factors that contribute to data sets on various OSC structures being inconsistent with either the wedge or block model may include, but are not limited to, the proximity of OSC structures to Siletzia terrane, and an apparent extreme difference between the Cascadia forearc's faster plate convergence (margin-normal) contraction rate and slower N-S (margin-parallel) contraction rate.