THE SPATIAL DISTRIBUTION AND LONGEVITY OF ONGOING PERMANENT FOREARC DEFORMATION SURROUNDING THE OLYMPIC MOUNTAINS

A combination of geodetic, paleomagnetic, geologic and paleoseismic datasets provide new insights into the spatial distribution and longevity of permanent deformation in the forearc of the Cascadia subduction zone surrounding the Olympic Mountains. A growing number of paleoseismic trenching studies, primarily utilizing lidar topography and the deformation of post-glacial (<12 ka) sedimentary units, indicate opposing senses of late Quaternary fault kinematics across the northern and southern flanks of the Olympic Mountains. North of the Olympic Mountains, prominent east-west striking faults have primarily hosted right-lateral transpression throughout the late Quaternary, whereas similarly oriented Quaternary-active faults south of the Olympic Mountains have accommodated left-lateral transpression. Our analyses of GNSS data suggest that these contrasting fault kinematics serve to accommodate opposing senses of vertical axis rotation and folding by flexural slip related to active oroclinal bending centered on the Olympic Mountains. The GNSS data reveal clockwise vertical axis rotations south of the Olympic Mountains that abruptly switch to counterclockwise rotations along a NE-trending axis that bisects the map-view bend in the Eocene Crescent-Siletz terrane where it flanks the Olympic Mountains. The spatial correlation between the sense of GNSS-derived rotations and the sign of paleomagnetic vertical axis rotations recorded within Eocene basalts suggests that forearc deformation due to oroclinal bending has persisted since at least the Miocene growth of the Olympic Mountains. These results imply that the style and kinematics of late Quaternary deformation recorded by paleoseismic trenching studies across the northern Cascadia forearc reflect long-lived oroclinal bending processes that have persisted throughout this region of the Pacific Northwest since the Neogene.