Crustal Rotation and Deformation In the Snake River Plain and Northern Basin and Range Province

We integrate geodetic, geologic, volcanic, and seismologic observations to assess rotation and strain in the Snake River Plain (SRP) and surrounding Northern Basin and Range (NBR). Crustal deformation in the NBR results from extension overprinted by bimodal volcanism in the SRP associated with the Yellowstone Hotspot. The SRP is a seismically quiet, low-relief physiographic feature that extends from eastern Oregon through southern Idaho and into northwestern Wyoming, USA. The NBR surrounds the SRP and is distinguished by its higher elevations, higher rates of seismicity, and active normal faulting. From 2006 to 2008, we reoccupied 110 survey-mode GPS sites in the SRP and NBR. The new regional velocity field includes these data merged with GPS phase data collected previously throughout the Pacific Northwest. Velocity gradients perpendicular to the directions of the velocities reveal that the SRP and NBR are part of the regional-scale clockwise rotation observed over the Pacific Northwest. The SRP is extending at a rate that is an order of magnitude lower than the NBR, explaining its low seismicity. Between these two regions we identify a NE-trending zone of right-lateral shear. We put together block regions based on GPS velocities, seismicity, and geology and invert simultaneously for angular velocities of the blocks, locking on faults between blocks, and horizontal strain rate tensors within blocks. Most of the velocity gradients are due to rotation with smaller components due to strain rates. The SRP and eastern Oregon rotate clockwise with a common angular velocity. Such large-scale regional rotations of the crust require that stresses be transmitted horizontally for long distances through the lithosphere and are probably not reflecting circular flow about a vertical axis in the mantle.