Northward migration of the Cascadia forearc in the northwestern US and implications for subduction deformation

Geologic and paleomagnetic data from the Cascadia forearc indicate long-ter m northward migration and clockwise rotation of an Oregon coastal block wit h respect to North America. Paleomagnetic rotation of coastal Oregon is lin ked by a Klamath Mountains pole to geodetically and geologically determined motion of the Sierra Nevada block to derive a new Oregon Coast-North Ameri ca (OC-NA) pole of rotation and velocity field. This long-term velocity fie ld, which is independent of Pacific Northwest GPS data, is interpreted to b e the result of Basin-Range extension and Pacific-North America dextral she ar. The resulting Oregon Coast pole compares favorably to those derived sol ely from GPS data, although uncertainties are large. Subtracting the long-t erm motion from forearc GPS velocities reveals ENE motion with respect to a n OC reference frame that is parallel to the direction of Juan de Fuca-OC c onvergence and decreases inland. We interpret this to be largely the result of subduction-related deformation. The adjusted mean GPS velocities are ge nerally subparallel to those predicted from elastic dislocation models fur Cascadia, but more definitive interpretations await refinement of the prese nt large uncertainty in the Sierra Nevada block motion.