Kinematics of the Snake River Plain and Centennial Shear Zone, Idaho, from GPS and earthquatte data

New horizontal Global Positioning System (GPS) velocities at 405 sites using GPS phase data collected from 1994 to 2010 along with earthquakes, faults, and volcanic features reveal how contemporary strain is accommodated in the Northern Basin and Range Province. The 1994-2010 velocity field has observable gradients arising from both rotation and strain. Kinematic interpretations are guided by using a block-model approach and inverting velocities, earthquake slip vector azimuths, and dike-opening rates to simultaneously solve for angular velocities of the blocks and uniform horizontal strain rate tensors within selected blocks. The Northern Basin and Range block model has thirteen blocks representing tectonic provinces based on knowledge of geology, seismicity, volcanism, active tectonic faults, and regions with differences in observed velocities. Ten variations of the thirteen blocks are tested to assess the statistical significance of boundaries for tectonic provinces, motions along those boundaries, and estimates of long-term deformation within the provinces. From these tests, a preferred model with seven tectonic provinces is determined by applying a maximum confidence level of ≥99% probability to F-distribution tests between two models to indicate one model with added boundaries has a better fit to the data over a second model. The preferred model is varied to test hypotheses of post-seismic viscoelastic relaxation, significance of dikes in accommodating extension, and bookshelf faulting in accommodating shear. Six variations of the preferred model indicate time-varying components due to viscoelastic relaxation from the 1959 Hebgen Lake, Montana and 1983 Borah Peak, Idaho earthquakes have either ceased as of 2002 or are too small to be evident in the observed velocities. Inversions with dike-opening models indicate that the previously hypothesized rapid extension by dike intrusion in volcanic rift zones to keep pace with normal faulting is not currently occurring in the Snake River Plain. Alternatively, the preferred model reveals a low deforming region (-0.1 +/- 0.4 x 10-9 yr -1, which is not discernable from zero) covering 125 km x 650 km within the Snake River Plain and Owyhee-Oregon Plateau that is separated from the actively extending adjacent Basin and Range regions by narrow belts of localized shear. Velocities reveal rapid extension occurs to the north of the Snake River Plain in the Centennial Tectonic Belt (5.6 +/- 0.7 x 10 -9 yr-1) and to the south in the Intermountain Seismic Belt and Great Basin (3.5 +/- 0.2 x 10-9 yr-1). The "Centennial Shear Zone" is a NE-trending zone of up to 1.5 mm yr -1 of right-lateral shear and is the result of rapid extension in the Centennial Tectonic Belt adjacent to the low deforming region of the Snake River Plain. Variations of the preferred model that test the hypothesis of bookshelf faulting demonstrate shear does not drive Basin and Range extension in the Centennial Tectonic Belt. Instead, the velocity gradient across the Centennial Shear Zone indicates that shear is distributed and deformation is due to strike-slip faulting, distributed simple shear, regional-scale rotation, or any combination of these. Near the fastest rates of right-lateral slip, focal mechanisms are observed with strike-slip components of motion consistent with right-lateral shear. Here also, the segment boundary between two E-trending Basin and Range faults, which are oriented subparallel to the NE-trending shear zone, provides supporting Holocene to mid-Pleistocene geologic evidence for accommodation of right-lateral shear in the Centennial Shear Zone. The southernmost ends of NW-trending Basin and Range faults in the Centennial Tectonic Belt at their juncture with the eastern Snake River Plain could accommodate right-lateral shear through components of left-lateral oblique slip. Right-lateral shear may be accommodated by components of strike-slip motion on multiple NE-trending faults since geologic evidence does not support slip along one continuous NE-trending fault along the boundary between the eastern Snake River Plain and Centennial Tectonic Belt. Regional velocity gradients are best fit by nearby poles of rotation for the Centennial Tectonic Belt, Snake River Plain, Owyhee-Oregon Plateau, and eastern Oregon, indicating that clockwise rotation is driven by extension to the south in the Great Basin and not by Yellowstone hotspot volcanism or from localized extension in the Centennial Tectonic Belt. The velocity field may reveal long-term motions of the Northern Basin and Range Province. GPS-derived clockwise rotation rates are consistent with paleomagnetic rotation rates in 15--12 Ma basalts in eastern Oregon and in Eocene volcanic rocks (∼48 Ma) within the Centennial Tectonic Belt.