On the Causes of Continental Intraplate Volcanism: An Example From the High Lava Plains of Eastern Oregon

Fluid-fluxing at convergent margins, adiabatic ascent beneath extending crust, and thermo-chemically driven mantle upwelling are obvious causes of the mantle melting that leads to volcanism along convergent margins, ocean ridges, and intraplate hotspots, respectively. Each of these mechanisms has at one time or another been used to explain the cause of the widely dispersed late-Cenozoic volcanism of the High Lava Plains (HLP) of eastern Oregon. Covering an area of nearly 105 km3 east of the Cascade arc, the exposed HLP upper-crustal stratigraphy is dominated by volcanic rocks: evolved mafic calc-alkaline rocks older than 20 Ma, high-Fe differentiated flood basalts between 12-17 Ma, and scattered bimodal large volume siliceous and small-volume primitive basalt eruptions for the last 10 Ma. Post-17 Ma HLP volcanism shows only hints of the compositional signatures expected for convergent margin volcanism. Both the composition and the orthogonal to plate-motion propagation of HLP volcanism argue against the type of hotspot that may be responsible for Snake River Plain volcanism to the east. The HLP form the northern limit of the Basin and Range province, but most workers consider that extension in the HLP has been relatively limited, and hence unlikely to be driving the volcanism through lithospheric thinning. What process then explains the fact that the HLP is among the most volcanically active areas in western North America in the late-Cenozoic? This is the question at the heart of the on-going HLP project where a multitude of collaborators are combining volcanology, geochronology, geochemistry, experimental petrology, seismology, and geodynamic modeling investigations to better define the history and character of volcanism in the HLP, whether or not the volcanism can be connected to structures imaged in the upper mantle, and whether a common dynamic cause to the volcanism can be deciphered. Preliminary results highlight the complexities of mantle flow in a continental area bordered on the west by a westward retreating slab that has a nearby southern terminus, and on the north and east by the thick lithosphere of Precambrian North America. Petrologic and geochemical characteristics of primitive basalts argue for a shallow origin for the mantle- derived volcanism, consistent with seismic tomography and anisotropy that show most coherence with surface geology only at depths shallower than circa 100 km. These early results seem most consistent with geodynamic models of a mantle wedge affected by a combination of flow into a retreating slab and around the southern terminus of the shrinking Juan-de-Fuca plate.