A Late Cenozoic Kinematic Model for Fault Motion Within Greater Cascadia

Widely accepted tectonic reconstructions indicate at least 100 km of coast-parallel northwestward translation of the Sierra Nevada block of California and 15-20 degrees clockwise rotation of most of Oregon since the current phase of Basin and Range extension began similar to 17 Ma. These reconstructions require at least 100 km of convergence between the central Coast Range of Oregon and rigid North America in mainland British Columbia, yet there is little discussion of how such convergence might be distributed. This study offers a kinematic model of the distribution of such deformation, constrained by geodesy, paleomagnetism, and fault offsets in Nevada, California and Oregon. The model includes differential rotation across the thrust faults of the Yakima fold and thrust belt (YFTB), compressive right-lateral faulting in the Washington Cascade Range, substantial thrust faulting within the Puget Lowland, and oroclinal bending and doming in the Olympic Mountains. Shortening across YTFB along 120 degrees W longitude is modeled as 47 km, across Puget Lowland at 123 degrees W (Olympia-Bellingham) is 94 km, and total shortening between the central Oregon Coast Range and northern Washington (Corvallis-Bellingham) is 125 km. Current motion of the coastal regions above the Cascadia subduction zone results from both permanent deformation of the continent and elastic coupling to the subducting plate. Permanent deformation in the model is based on extrapolating geodesy from east of 120 degrees W or south of 40 degrees N, indicating a very uniform convergence velocity with the Juan de Fuca plate for northernmost California and Oregon near 31 mm/yr at N61 degrees E.