Shear-Wave Anisotropy at the Yellowstone Hotspot

Several features of the 800-km long Yellowstone-Snake River Plain volcanic system can be explained by the steady motion of the North American lithosphere over a hotspot source, often thought to originate from a mantle plume. A 10 m geoid high centered on Yellowstone, a series of progressively older calderas that track the southwestern motion of the North America Plate, and a parabolic pattern of high topography, seismicity, and active faulting around the eastern Snake River Plain are consistent with a model in which buoyant plume material flattens against the moving lithosphere. We examine the upper mantle strain field in the Yellowstone region using shear-wave splitting measurements of teleseismic core phases (SKS, SKKS, and PKS). The teleseismic data were recorded at six permanent USNSN stations and a temporary array of 47 IRIS-PASSCAL seismographs deployed in a 300 km by 500 km area centered on Yellowstone. The shear-wave splitting fast polarization directions are dominantly aligned parallel to the direction of North America Plate motion. While the fast polarization directions are not as coherent as those observed farther down the Snake River Plain to the southwest, there is no clear evidence for the divergent mantle flow commonly expected to be produced as a plume interacts with the lithosphere. It is possible that near the plume top, the overturn of material from the plume-plate interaction would not align olivine, and thus produce a region of no anisotropy. However, the largest split times of more than 2 sec are observed at the stations located within the present-day Yellowstone caldera, implying no cornerflow. The average delay time at stations outside the caldera is 0.9 sec. The splitting data are consistent with deformation from shearing by the North America Plate. In addition, upper mantle flow velocity may be parallel to the direction of plate motion as indicated by some global-scale models. There may also be significant lithospheric anisotropy related to several large-scale Precambrian tectonic features which trend northeast-southwest roughly parallel to the direction of plate motion and fast shear-wave polarization.