ACTIVE DEFORMATION AND LITHOSPHERE RHEOLOGY OF THE NORTH AMERICAN CORDILLERA

Space geodetic measurements show that most of the relative motion between the North American and Pacific plates is taken up by deformation across the San Andreas fault system, while convergence between the Juan de Fuca and North American plates involves subduction. However, significant tectonic strain occurs to the east of the San Andreas and Cascadia fault systems. Using cluster analysis of the velocities of >1700 GPS stations (from the National Seismic Hazard Mapping project, NSHM 2014 velocity field), we confirm that this distributed intra-continental deformation is localized along the Eastern California Shear Zone (~8 mm/yr), the Central Nevada Seismic Belt (~1.5 mm/yr) and the Wasatch fault zone (~3 mm/yr). Elastic strain from the strongly coupled subduction zone affects the velocity field deep into North America. If we remove a model of the deformation associated with elastic coupling (McCaffrey et al., 2013 JGR), we can better resolve the pattern of intra-plate deformation in the Pacific Northwest. When this is done, we clearly see the rotation of the Oregon Coast block, oblique right-lateral slip and extension across central Oregon, and N-S convergence of ~5 mm/yr across NW Washington.

Active tectonic deformation is not steady, but rates vary in the aftermath of large earthquakes. This postseismic deformation is due to time-dependent relaxation of coseismic stress changes by viscous flow of rocks in the lower crust and upper mantle and aseismic fault slip. Thus, analysis of postseismic deformation reveals information about the rheologic properties of the lithosphere. Investigations of postseismic deformation in the Western Cordillera show that the upper mantle below ~50 km depth has a low steady-state viscosity of order 1 x 10¹⁹ Pas, underlying a lower crust with much higher viscosities. This is in contrast to findings from ice unloading studies in the cratonic interior of North America where elastic plate thicknesses and underlying viscosity values are several times greater. The pattern of active deformation can be reconciled with elastic plate-thickness estimates from gravity and topographic data and seismic tomography studies that show substantially greater thickness and strength of the North American plate in its interior compared to the thinned and weakened Western North American Cordillera.