Contemporary tectonic motion of the eastern Snake River Plain: A campaign global positioning system study

A comparison of precision campaign GPS data from 1995 and 2004 from ten benchmarks on the eastern Snake River Plain (eSRP) has revealed that the province moved 2.8 ± 0.3 mm/yr to the SW (232.4 ± 6.3°) relative to a fixed North American reference frame. The benchmarks had no measurable displacement relative to one another at the resolution of the GPS during the nine-year study, evidence that the province moves as a rigid, non-extending block. This scenario is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes. However, an additional small component of intra-plain extension must also be invoked to account for the observed NW-trending volcanic rift zones that transect the eSRP. We suggest that intra-plain extension is too slow (<1 mm/yr) to measure using our campaign GPS methods, but may be sufficient over millennial time scales to accommodate rift zone formation. Slower velocities measured on three benchmarks within the neighboring Basin and Range ‘seismic parabola’ are consistent with this region serving as a zone of detachment between the North American craton and the faster-moving eSRP.