A new interpretation of deformation rates in the Snake River Plain and adjacent basin and range regions based on GPS measurements

Within the Northern Basin and Range Province, USA, we estimate horizontal velocities for 405 sites using Global Positioning System (GPS) phase data collected from 1994 to 2010. The velocities, together with geologic, volcanic, and earthquake data, reveal a slowly deforming region within the Snake River Plain in Idaho and OwyheeOregon Plateau in Oregon separated from the actively extending adjacent Basin and Range regions by shear. Our results show a NE-oriented extensional strain rate of 5.6 +/- 0.7 x 10-9 yr-1 in the Centennial Tectonic Belt and an similar to E-oriented extensional strain rate of 3.5 +/- 0.2 x 10-9 yr-1 in the Great Basin. These extensional rates contrast with the very low strain rate within the 125 km x 650 km region of the Snake River Plain and OwyheeOregon Plateau, which is indistinguishable from zero (-0.1 +/- 0.4 x 10-9 yr-1). Inversions of the velocities with dyke-opening models indicate that rapid extension by dyke intrusion in volcanic rift zones, as previously hypothesized, is not currently occurring in the Snake River Plain. This slow internal deformation, in contrast to the rapidly extending adjacent Basin and Range regions, indicates shear along the boundaries of the Snake River Plain. We estimate right-lateral shear with slip rates of 0.31.4 mm yr-1 along the northwestern boundary adjacent to the Centennial Tectonic Belt and left-lateral oblique extension with slip rates of 0.51.5 mm yr-1 along the southeastern boundary adjacent to the Intermountain Seismic Belt. The fastest lateral shearing evident in the GPS occurs near the Yellowstone Plateau where strike-slip focal mechanisms and faults with observed strike-slip components of motion are documented. The regional velocity gradients are best fit by nearby poles of rotation for the Centennial Tectonic Belt, Snake River Plain, OwyheeOregon Plateau, and eastern Oregon, indicating that clockwise rotation is not locally driven by Yellowstone hotspot volcanism, but instead by extension to the south across the Wasatch fault due to gravitational collapse and by shear in the Walker Lane belt resulting from PacificNorthern America relative plate motion.