Origin of the Newberry Hotspot Track: Evidence from shear-wave splitting

Located in the northwestern United States, the Newberry Hotspot Track consists of a sequence of age-progressive silicic volcanic domes and lava flows, showing a monotonic age progression from east to west ending at the Newberry Caldera. While mantle plumes are often called upon to explain hotspot tracks, the Newberry track cannot be the direct product of plate motion over a stationary mantle source as its orientation is ∼120° to plate motion, making it a good case study for alternative causal mechanisms of hotspot tracks. Four end-member tectonic models have been proposed: (1) subduction counterflow, (2) gravitational flow along the base of the lithosphere, (3) lithospheric faulting, and (4) extension of the Basin and Range. To get fabric information about the asthenosphere and constrain the possible flow fields beneath the Newberry track, SKS splitting measurements were made for 27 events at 12 stations of the Oregon Array for Teleseismic Study (OATS) along the track. A gradual rotation of the fast direction is observed from ENE–WSW at the northwest end of the array to E–W to the southeast and the delay times average 1.65 s. We infer that the SKS splits are the product of anisotropy in the asthenosphere and the anisotropy orientation does not vary with depth beneath the track. The average fast directions ENE–WSW to the northwest are consistent with generation by mantle shear parallel to the subduction of the Juan de Fuca Plate, and the more E–W fast directions to the east are perhaps due to shear caused by the Basin and Range extension. Since the observed fast directions are not parallel to the Newberry track, as the subduction counterflow model or the gravitational flow model suggests, the age-progressive volcanism is unlikely a direct product of asthenospheric flow. Instead, we propose that the Newberry track is the product of lithosphere-controlled processes.