Spatial and Temporal Distribution of Strain in the Sevier Desert Region from a Decade of BARGEN Continuous GPS Observations
A transect of four BARGEN GPS sites from 1997-2000 at latitude 39°N revealed a linear increase in velocity from 0 mm/yr on the Colorado Plateau to ~4 mm/yr in east-central Nevada. These geodetic data, when combined with paleoseismic and neotectonic observations, were used to argue for present-day strain accumulation on the Sevier Desert detachment, a seismically-imaged, low-angle (12°) normal fault that underlies a significant portion of west central Utah [Niemi et al., 2004]. A new GPS velocity solution [Davis et al., 2006], including 5 additional years of observations and a new GPS station in the eastern half of the transect, serves to both clarify and complicate our understanding of the spatial and temporal distribution of strain in this region. Geodetic velocity gradients among the three original eastern sites (from east to west, CAST, SMEL, and FOOT), and including the new site, SPIC, located between CAST and SMEL, continue to suggest a linear strain gradient from the Colorado Plateau to westernmost Utah, on a transect spanning the Wasatch, Sevier Desert, and House Range normal faults. In contrast, the baseline between FOOT and EGAN, which underwent extension from 1997-2000, began to contract in 2000, as site EGAN slowed with respect to the Colorado Plateau. This deviation in velocity continued until 2003, when EGAN began moving westward, and has, as of 2006, returned to near its pre-2000 velocity with respect to the Colorado Plateau. The eastward excursion of GPS site EGAN is not unique, and similar excursions are observed in the time series of all BARGEN GPS sites that lie west of ~114.25°W over the time period 2000-2003 [Davis et al., 2006]. The origin of this velocity anomaly is uncertain, but the magnitude and spatial extent of the excursion, as well as the observation of a deep earthquake swarm (~30-40 km depth) coincident with dramatic motion of GPS site SLID, near Lake Tahoe, in 2003 [Smith et al., 2004] suggest a deep crustal or mantle influence on the observed GPS velocities. Bright, mid-crustal horizontal reflections observed in COCORP reflection seismic data west of 114°W have been postulated to represent a detachment that could accommodate shearing along the base of the crust in Nevada [Hauser et al., 1987], a hypothesis broadly consistent with the observed GPS velocities, with the crust shearing relatively east over the subcrustal lithosphere. A deep observatory in the Sevier Desert region, transecting the Sevier Desert detachment, would complement existing paleoseismic and neotectonic studies, and the decade of BARGEN continuous GPS observations, as well as new GPS data from 8 Earthscope Plate Boundary Observatory sites installed along this transect in the past few years. Key questions that could be assessed with a deep observatory are how slip, and strain, at depth on an inclined fault are reflected in geodetic observations of strain at the surface; whether the state of stress and strain rate on the Sevier Desert detachment change through time, as suggested by long-term periodicity in strain release as recorded in the geologic record, and what effect long-lived velocity excursions, such as observed from 2000-2003, have on the regional stress state, perhaps leading to a clearer understanding of the source of these anomalies, and the aseismic tectonic behavior of the lithosphere.
