THE PACIFIC-NORTH AMERICA PLATE BOUNDARY IN THE WESTERN UNITED STATES: DYNAMICS OF THE SIERRA NEVADA BLOCK AND IMPLICATIONS FOR WESTERN U.S. TECTONICS

We report results of modeling the dynamics of the Sierra Nevada block (SNB) at the Pacific-North American plate boundary in the western United States. The SNB is a non-deforming crustal block that is bounded by the dextral San Andreas fault on the west and the nascent Eastern California Shear Zone/Walker Lane Belt on the east. The SNB is translating to the northwest at ~12 mm/yr with respect to stable North America (e.g., Dixon et al., 2000) and rotating about a vertical axis, though the sense of rotation is debated (e.g., Dixon et al., 2000; McCaffrey, 2005). We estimate the loads acting on the block and determine their relative values by enforcing the zero net torque constraint. Loads include shear tractions transmitted across faults, normal tractions from the excess potential energy of the elevated continental interior, and normal and shear tractions at the block's northern end where it impinges on the Oregon Coast block (e.g., Wells et al., 1998). Absolute stress estimates are available by referencing our results to the recently estimated plate-scale stress modeling results of Humphreys and Coblentz (in review). Preliminary results suggest clockwise-torque on the block caused by dextral shear tractions are balanced, at least in part, by shear and normal tractions on the block's northern end, implying that interactions between crustal blocks is an important component of the transfer and support of stress in the lithosphere.