SEISMIC STRUCTURE OF THE HIGH LAVA PLAINS AND SURROUNDING REGIONS
Seismic stations located in the HLP region between 2006 and 2010 include 118 broadband stations deployed as part of the High Lava Plains Project, augmented by the uniform coverage provided by EarthScope’s USArray Transportable Array. Results from receiver function imaging of the crust-mantle boundary show a transition from thicker crust at the edges of the HLP to thinner crust within the HLP proper. Zones of abnormally high Poisson’s ratios and low-velocity zones in the crust beneath north‐central and southern Oregon mark the likely presence of partial melt bordering the HLP trend, suggesting that the HLP is a central zone where crustal melts have drained to the surface. Body and surface wave tomography yield interesting patterns of lower than average seismic wavespeeds in the uppermost mantle beneath the region that correlate well with areas of more recent (<5 Ma) volcanism. At greater depths, the Juan de Fuca slab is clearly evident in many regions of the upper mantle, but is segmented beneath the HLP, consistent with the notion of stranded Farallon slab fragments beneath the area. Shear wave splitting results exhibit a uniform pattern of ~E-W fast polarization directions and very large (2.0-2.5+ sec) splitting times. These results match well with a regional surface wave inversion showing nearly the same fast direction and a maximum strength of anisotropy located in the uppermost mantle, perhaps at the base of very thin regional lithosphere, suggesting uniform and well-organized mantle flow. Taken together, this range of seismic results is consistent with a model in which dramatic dismantling of the Juan de Fuca slab has induced regional mantle upwelling that may be responsible for the range of tectonomagmatic events that flooded the region.