On the Origin of the High Lava Plains Volcanic Track: A Detailed Tomographic Study and Multidisciplinary Interpretation

The High Lava Plains (HLP) of central and southeastern Oregon is a time-progressive SE-NW volcanic track originating roughly at the same time (~10-12 Ma) and in the same region (corner of Oregon, Nevada, and Idaho) as the Snake River Plain/Yellowstone (SRP/Y) hotspot track, but with an opposing trend in the direction of Newberry volcano in the eastern Cascades. The volcanic lineament is thus highly oblique to plate motion, and the underlying cause of the volcanism and its temporal migration remains a puzzle. As with SRP/Y, only the rhyolite eruptions are temporally and spatially progressive; subsequent basaltic eruptions that continue almost to the present day are not. In both time and space the HLP volcanism is closely associated with the earlier Steens Mountain and Columbia River flood basalts and the coeval SRP/Y volcanism, yet the inter-relationships (if any) are very poorly understood and remain a topic of active debate. The issue is further muddied by the proximity of the HLP to the northern terminus of Basin and Range extension and the apparent association of the Cascade Newberry volcano with the terminus of the HLP lineament. Here we present new high-resolution seismic images of uppermost mantle structure beneath the region based on data from the 118 stations of the high density (~12-15 km grid) HLP broadband array, augmented by data from EarthScope's USArray Transportable Array (TA). The inversion is parameterized to maximize resolution at uppermost mantle depths, where results from preliminary body wave tomography indicated lower than average seismic velocity perturbations that correlate approximately with areas of active Holocene volcanism. We further constrain the new P-wave and S-wave tomographic analyses by incorporating comprehensive crustal velocity and thickness determinations drawn from published crustal receiver function measurements of crustal velocities and Moho depths (Eagar et al., EPSL, vol. 297, pp. 141-153, 2010; H. Gilbert, Geosphere, submitted, 2011). We interpret the new body wave tomography in relationship to both crustal structure and recently published regional 3-D surface wave inversions of the crust and uppermost mantle (e.g., Wagner et al., EPSL, vol. 299, 273-284, 2010): The crustal studies reveal zones of abnormally high Poisson's ratios and low seismic velocities in the crust beneath northcentral and southern Oregon, marking the likely presence of partial melt adjacent to, but not within, the main HLP lineament. The surface wave results likewise show an apparent lack of correlation between low S-wave velocities in the uppermost mantle and the track of the HLP lineament itself. We will present detailed cross-sectional and map images that help better define the relationship between uppermost mantle structure and the on-going HLP volcanism. Finally, we will present and assess arguments as to subduction vs. plume origins of the HLP and the surrounding tectonic terranes.