GEODYNAMICS OF THE WESTERN U.S. INTERIOR

Western U.S. interior deformation, as measured by GPS, shows westward extension in the Basin-Range province coupled with a clockwise rotation in the direction of motion. The rotation is characterized by southwest motion of the Yellowstone-Snake River Plain, westward extension at the Wasatch fault, northwest oblique-shear in Nevada, and northeast motion in Washington-Oregon. The observed velocity field is modeled using the hypothesis that most deformation is accommodated on existing fault zones that divide the western interior into blocks of uniform motion. We identify fault-bounded blocks and compare block velocities with known fault slip rates. Viscoelastic deformation following historic, M>7 earthquakes was examined and found to contribute significantly to the observed velocity field. These postseismic velocities were subtracted from the observed data to produce a modified velocity field that reflects long-term deformation. Having analyzed the deformation of the western interior, we turn to geodynamic modeling of the stresses associated with gravitational potential energy variations that drive the observed deformation. A crustal thickness and density model from seismic and topographic data is used to compute the internal forces. The volcanically reworked crust of the Yellowstone-Snake River Plain system is specifically incorporated in order to assess the effects of the Yellowstone hotspot volcanism in addition to the stresses from variations in total crustal thickness. Upper mantle densities and horizontal lithospheric stresses are calculated from the crustal structure model. The resulting geodynamic model shows that low-density upper mantle correlates with the thinned crust of the Basin and Range, and high-stress regions correlate to transitional zones between different crustal thicknesses and upper mantle densities. Much of the Basin-Range also experiences high stress, suggesting that the buoyancy forces are driving extension across the region. At the topographically high Yellowstone Plateau, high buoyancy stresses arise from a combination of low-density mantle and a crustal magmatic system, leading to some of the largest stresses in the western interior.