STRUCTURAL ACCOMMODATION OF NORTH-DIRECTED COMPRESSION IN CASCADIA ACCRETIONARY WEDGE MIOCENE TURBIDITES, COASTAL OLYMPIC MOUNTAINS, WASHINGTON

The Cascadia accretionary wedge, which forms the Olympic Mountains of Washington, is the result of subduction of the Juan de Fuca Plate beneath the North American Plate. Detailed mapping and analysis of structures in the lower to middle Miocene Hoh rock assemblage of the Coastal unit of the Olympic Structural Complex (OSC) is being done to better understand the structural development and tectonics of the wedge. The Hoh rock assemblage in the study area consists of northeast-striking beds of thick to thinly bedded turbidite sandstone with a lesser amount of siltstone and shale. Where the formation consists primarily of thick (generally a meter of more) sandstone beds, folding at the outcrop scale is absent and faults uncommon, and bedding dips uniformly. Thinly bedded Hoe strata contain significant amounts of siltstone and shale and are moderately to highly deformed. Boudinage, folds, faults, and joints are common. Structural relationships indicate boudins developed early in thin sandstone beds followed by shortening and duplication of bedding along subparallel bedding plane faults. Stress analyses of faults and a train of flexural-flow/flexural-slip folds, with northeast-striking axial surfaces and hinge lines with an average plunge 23/N54E, indicate north-to northwest-directed compression has had an important role in the deformation process. The beds in the fold train, which bottoms on a thrust fault with two distinct sets of slickenlines, have shortened ~40%. Northeast striking faults tend to have wider damage zones, some more than a meter across. Analysis of slip-surfaces on these faults indicates that displacements on them are the result of both northeast- and northerly-directed compression. Northwest-striking faults appear to be the youngest structures and have narrow damage zones that typically are no more than a few centimeters across with offsets less than a meter. The results of the investigation indicate that with the onset of block rotations in the Pacific Northwest at ~15 Ma, north- to northwest-directed compression took on a significant role in development of the extant Coastal OSC structures along with the northeast-directed compression associated with the subducting Juan de Fuca Plate.