CONTEMPORARY TECTONIC MOTION OF THE EASTERN SNAKE RIVER PLAIN, IDAHO: A GLOBAL POSITIONING SYSTEM STUDY, 1995-2004

A precision campaign GPS study of 10 benchmarks on the eastern Snake River Plain (ESRP) has revealed that from 1995-2004 the province moved 2.8 ± 0.3 mm/yr to the SW (232° ± 6°) relative to a fixed North American reference frame. The benchmarks had no measurable displacement relative to one another during the nine-year study, evidence that the ESRP behaves as a rigid, non-extending block at the resolution of the GPS. This conclusion is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes. Motion of the ESRP away from the stable craton is apparently accommodated by extension of the Yellowstone Plateau, which is part of a parabolic belt of high seismicity that extends into parts of the Basin and Range and surrounds the ESRP. The velocity of the ESRP measured in this study, combined with the North American plate velocity, is sufficient at first order to account for the ~290 km chain of silicic calderas created by the Yellowstone hotspot over the past 10 my. However, an additional small component of intra-plain extension must also be invoked to account for the NW-trending volcanic rift zones that transect the plain. We suggest that intra-plain extensional elastic strain accumulates at a rate that is too slow (<10-16 s-1) to measure using GPS over decadal time scales, but is sufficient over millennial time scales to require dike injection and consequent rift zone formation. Three additional GPS benchmarks in the Basin and Range province immediately north of the ESRP moved in a similar direction to ESRP stations but at slightly slower rates (1.5 to 2.3 mm/yr). If these limited data are representative, the velocity difference between the two provinces may be accommodated by a previously unrecognized diffuse transfer zone.