Shear wave splitting and the pattern of mantle flow beneath eastern Oregon

The tectonic and geologic setting of eastern Oregon includes the volcanically active High Lava Plains (HLP) province and the accreted terrains of the Blue and Wallowa Mountains and is bounded by the Columbia River flood basalts to the north, Basin and Range extension to the south, the Cascade arc to the west, and stable North America to the east. Several models have been proposed to explain the tectonic evolution of eastern Oregon and, in particular, the voluminous volcanic activity in the HLP, but a consensus on which model fully describes the complex range of processes remains elusive. Measurements of the seismic anisotropy that results from active mantle flow beneath the region can provide a crucial test of such models. To constrain this anisotropy, here we present new SKS splitting results obtained at approximately 200 broadband seismic stations in eastern Oregon and the surrounding region. Data come from the USArray Transportable Array (TA) and two temporary experiments carried out in the HLP and in the Wallowa Mountains. Our splitting data set includes ~ 2900 individual splitting measurements from SKS phases recorded between 2006 and 2008. Stations in eastern Oregon exhibit significant shear wave splitting, with average delay times at individual stations between ~ 0.8 s and ~ 2.7 s. In the HLP, nearly all observed fast directions are approximately E-W, while to the north in the Blue and Wallowa Mountains there is more variability in the splitting patterns. The average delay time observed at stations located in the heart of the HLP province is ~ 2 s, well above the global average of ~ 1 s for continental regions. We infer from the large split times and homogeneous fast directions that there must be significant active flow in a roughly E-W direction in the asthenosphere beneath the HLP; this inferred flow field places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region. In the Wallowa region, the anisotropic signature is more complicated and there may be a significant contribution from fossil fabrics in the crust or mantle lithosphere.