FATE OF THE EASTERN CALIFORNIA SHEAR ZONE AT THE CASCADIA MARGIN AND ENTRAINMENT OF THE CASCADIA FORE-ARC IN OBLIQUE SUBDUCTION

High-precision, continuous GPS geodesy in the Pacific Northwest provides a synoptic view of the along-strike variation in Cascadia margin kinematics. Here, we present a version 2 velocity field that uses a different period of data collection, improvement in the processing of daily solutions, a North America-based approach to daily network stabilization, and only 50% overlap in the set of stations to define the North America reference frame in version 1 [Miller et al., 2001]. Yet version 2 yields a velocity field and stable continent reference frame that agree at better than the 95% confidence level, detailed structure of the time series are realized and precision is improved. These results constrain interfering deformation fields. Coastal stations in the northern and central parts of the margin are strongly entrained in the Juan de Fuca–North America convergence direction. The magnitude of northward forearc motion relative to North America increases southward from Vancouver Island (2 mm/yr), to western Washington (5-7 mm/yr) to northern and central Oregon (~7-8 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin-parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. The California-southern Oregon boundary reflects a composite velocity that includes the San Andreas transform system, Mendocino triple junction migration, and interaction between North America and the south Gorda plate. The Klamath Mountains are partially entrained with the impinging Sierra Nevada block, and Eastern California shear zone deformation previously believed to penetrate the arc and back arc is at least partly distributed back out to the plate boundary. Inland stations have lesser motions, consistent with their structural domains from south to north: the Canada and northern Washington back arc experiences slow convergence parallel motion, the Yakima fold belt actively contracts, and southeastern Oregon shows integrated Basin and Range extension.