Shear-wave splitting beneath the Snake River Plain suggests a mantle upwelling beneath eastern Nevada, USA

Abstract

The Snake River Plain (SRP), a 90-km-wide topographic depression in southern Idaho, is a topographically anomalous feature in the western U.S. Previous seismic studies focused on the northeastern SRP to study its relationship with the Yellowstone hotspot. We present new teleseismic shear-wave splitting data from six broadband seismic stations deployed along the axis of the SRP from June 2000 to September 2001. We also analyze splitting at HLID, a permanent station of the National Seismic Network located ∼100 km north of the plain. Splitting of individual teleseismic phases is consistent at all stations within 2σ errors, and we favor the interpretation of anisotropy with a single horizontal fast axis, although a dipping-axis interpretation is statistically permitted at two of the stations. Our station fast directions, as well as shear-wave splitting data from numerous other stations throughout the Basin and Range, are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume-like upwelling material beneath eastern Nevada into a southwestward flowing asthenosphere (with respect to a fixed hotspot reference frame). This parabolic asthenospheric flow (PAF) model for the Great Basin is attractive because it explains the observed high elevations, high mantle buoyancy, low-velocity anomaly beneath eastern Nevada, high heat flow, and depleted geochemistry of some erupted basalts. The lack of Pliocene–Recent major volcanism in eastern Nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy. Our splitting station delay times vary in a way not predicted by the PAF model, and can be explained by: a zone of aligned magma-filled lenses and/or partially molten dikes beneath the SRP lithosphere, a depleted olivine-rich residuum underneath the sides of the eastern SRP, and/or the effect of lateral lower crustal flow from beneath the eSRP toward its adjacent flanks.