Anisotropy of the Yellowstone Hot Spot Wake, Eastern Snake River Plain, Idaho

Over the last 10 million years, the Yellowstone hot spot has passed beneath the eastern Snake River Plain, both magmatically modifying the Snake River Plain crust and creating a wider, wake-like "tectonic parabola" of seismicity and uplift. Analysis of SKS arrivals to a line array of 55 mostly broadband stations, distribution across the tectonic parabola, reveals a nearly uniform orienta tion of anisotropy, with an average fast axis orientation of N64E. The back azimuth of null splitting events is parallel to the measured fast axis, suggesting that anisotropic material consists of a single layer. Splitting parameters are independent of backazimuth, suggesting that anisotropy is constant beneath each station. Thus station-averaged split parameters are representative of the anisotropy beneath the station. Station-averaged split times range from 0.6–1.5 s, and define a pronounced depression in split times centered about 80 km southeast of the axis of the Snake River Plain.¶Assuming the degree of anisotropy (averaged over the ray path) to be no more than 10%, the split times are far too great for the anisotropy to be confined solely to the lithosphere. The simplest way to explain the observed anisotropy structure is to attribute it to simple shear strain caused by the absolute motion of North America. Because anisotropy is different in nearby Colorado and Nevada, we hypothesize that fossil anisotropy created in past orogens and continent-building events in the Snake River Plain area has been reset or erased by the passage of the hot spot, and that subsequent strain of the hot spot-related asthenospheric wake created a uniformly oriented fast axis. If this is true, then our array constrains the minimum of the hot spot’s asthenospheric wake.