The Origin of Hotspot Volcanism in the Pacific Northwest
The Origin of Hotspot Volcanism in the Pacific Northwest
Credit:
Richard M. Allen, Mei Xue • University of California, Berkeley/IRIS Consortium
Description
Variation in shear-wave splitting fast directions across the Pacific Northwest. Our shear wave splitting results are show in red along with those from other studies (Fabritius, 1995; Ozalaybey and Savage, 1995; Barruol et al., 1997; Polet and Kanamori, 1997; Schutt et al., 1998; Savage and Sheehan, 2000; Schutt and Humphreys, 2001; Savage, 2002; Currie et al., 2004; Waite et al., submitted) in black with sticks indicating the fast direction. Our results show a WNW-ESE fast direction adjacent to the coast which is consistent with all splitting observations in the Cascadian forearc and parallel to absolute plate motion of the Juan de Fuca plate. The fast direction rotates to E-W inland along the OATS array consistent with observations in the western Snake River Plain.
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 processes 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. Alternatively, lithospheric processes such as fracture propagation have been proposed as the cause of the volcanism.
In this SKS splitting study, we collected data from the OATS (Oregon Array for Teleseismic Study) array 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 (see figure). 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 implying that the hotspot is not the product of asthenospheric flow. It therefore seems likely that the Newberry hotspot track is the product of lithospheric processes.
Photographer / Contributor: Richard M. Allen, Mei Xue • University of California, Berkeley
Request a Hi-Res Version of this Image / Feedback on this Image
