Upper mantle structure of the Yellowstone hotspot from teleseismic body-wave velocity tomography and shear-wave anisotropy

The question of whether the Yellowstone hotspot volcanic system is the result of a mantle plume, or shallower convection processes, is addressed by examining the upper mantle seismic isotropic and anisotropic velocity structure with data from temporary and permanent seismographs. Key features of the system can be explained by the motion of the lithosphere over a stationary mantle plume: a series of progressively older calderas that track plate motion for the last 16 million years; a parabolic pattern of high topography, seismicity, and active faulting around the axis of the Snake River Plain (SRP---the hotspot track); and high 3He/4He ratios found at Yellowstone. Alternatively, models involving upper mantle processes may explain these observations. The VP and VS tomography reveals a strong low-velocity anomaly (up to -2.3% for VP and -5.5% for VS) from ∼50 to 200 km beneath the Yellowstone caldera and SRP. A weaker, smaller-volume anomaly (-1.0% VP and -2.5% VS) is imaged from about 250 to at least 350 km depth at a position ∼100 km WNW of the caldera. A prominent high-velocity anomaly (1.2% VP and 1.9% VS) is located at ∼50 to 200 km depth to the SE of Yellowstone. The fast polarization directions measured from splitting of teleseismic shear-wave core phases are generally aligned parallel to the direction of plate motion. This suggests asthenosphere flow is primarily due to shear of the lithosphere. There is little evidence for radial flow away from Yellowstone that is expected for buoyant plume material flattening against a plate. The buoyancy flux of a Yellowstone plume, calculated from the excess temperature and radius inferred from the tomography, is an order of magnitude lower than previous estimates for Yellowstone. The tomography and anisotropy results are inconsistent with a cylindrical, vertical mantle plume. However, the seismic results could be interpreted as a weak plume dipping to the WNW. This upwelling may work in conjunction with lithosphere extension to enhance convection in the uppermost mantle.