Upper Mantle Origin of the Newberry Hotspot Track: Evidence From Shear-Wave Splitting

Mei Xue and Richard M. Allen
University of Wisconsin-Madison

AGU Fall Meeting 2004

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 probably the most popular cause of the Yellowstone track, the Newberry track cannot be the product of plate motion over a stationary mantle source. Instead proposed causal mechanisms include upper mantle process where melt buoyancy driven convection is directed west-northwest by subduction-driven corner flow or alternatively a westward-spreading plume head. In this SKS splitting study, we collected data from the OATS (Oregon Array for Teleseismic Study) array, a deployment of the University of Wisconsin Broadband Network (UWBN) along the Newberry track from NW to SE Oregon, which was installed in May 2003 and will operate until September 2005. Measurements were made for 23 events at 12 OATS stations using Wolfe and Silver (1998)’s multi-event stacking procedure. A gradual rotation of fast polarization direction is observed from NE-SW at the northwest end of the array to E-W to the southeast. 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 indicating that either the splits are not sensitive to mantle flow oriented along the track or the track is not the product of asthenospheric flow. We prefer the second explanation as our null splitting observations strongly argue for one layer of anisotropy. If our continuing analysis confirms this conclusion, then the alignment of the Yellowstone track with plate motion and anisotropy may be coincidental rather than representative of the causal mechanism.