The Olympic subduction complex is the exposed subaerial Cascadia accretionary wedge in the Olympic Mountains of Washington State. Uplift of the mountains has been attributed to two competing models: margin-normal deformation from frontal accretion and underplating, and margin-parallel deformation from the clockwise rotation and northward movement of the Oregon Coast Range block compressing the Olympic Mountains block against the Canadian Coast Range. East-northeast−oriented folds and Quaternary thrust faults and paleostress analysis of faults in the Coastal Olympic subduction complex, west of the subduction complex massif, provide new evidence for north-south shortening in the Coastal Olympic subduction complex that fills a large spatial gap in the north-south shortening documented in prior studies, substantially strengthening the block rotation model. These new data, together with previous studies that document north-south shortening in the subduction complex and at numerous locations in the Coast Range terrane peripheral to the complex, indicate that margin-parallel deformation of the Cascadia forearc has contributed significantly to uplift of the Olympic Mountains. Coastal Olympic subduction complex shallow-level fold structural style and deformation mechanisms provide a template for analyzing folding processes in other accretionary wedges.
Similar-shaped folds in shallow-level Miocene turbidite sediments of the Coastal Olympic subduction complex formed in two shortening phases not previously recognized in accretionary wedges. Folds began forming by bed-parallel flow of sediment into developing hinges. When the strata could no longer accommodate shortening by flexural flow, further shortening was taken up by flexural slip. Similar-shaped folds in the deeper accretionary wedge rocks of the subduction complex massif have a well-developed axial-surface cleavage that facilitated shear folding with sediment moving parallel to the axial surface into the hinges, a structural style that is common to accretionary wedges. The pressure-temperature conditions and depth at which the formation of similar folds transitions from bed-parallel to axial-surface−parallel deformation are bracketed.