The origin of hotspot volcanism in the Pacific Northwest

Richard M. Allen and Mei Xue
University of California Berkeley

IRIS/UNAVCO Annual Meeting

In the northwestern United States there are two hotspot tracks: the Newberry track and the Yellowstone track. Both are located on the North American Plate with the Yellowstone track parallel to plate motion and the Newberry track oblique to it. While a mantle plume is often cited as the cause of the Yellowstone track, the Newberry track cannot be the product of plate motion over a stationary mantle source. Instead proposed causal mechanisms for the Newberry track include upper mantle process where melt buoyancy driven convection is directed west-northwest, parallel to the hotspot track, by subduction-driven corner flow or alternatively by topography of the base of the lithosphere. In this SKS splitting study, we collected data from the OATS (Oregon Array for Teleseismic Study) array currently deployed along the Newberry track from NW to SE Oregon. Measurements were made for 23 events at 12 OATS stations and show a gradual rotation of the fast polarization direction from NE-SW at the northwest end of the array to E-W to the southeast, consistent with regional observations. Most stations also exhibit null results when the event back azimuth was parallel or perpendicular to the fast direction determined from other events, strongly indicating a single layer of anisotropy. The first order observation is that the SKS splits are not aligned with the Newberry hotspot track. This suggests there is little or no mantle flow oriented along the track and the track is not the product of asthenospheric flow. It therefore seems likely that the Newberry hotspot track is the product of lithospheric processes. Tomographic imaging of the region currently underway and will provide additional constraints mantle processes operating in the region.