The research in this proposal will improve our understanding of the High Lava Plains in eastern Oregon. The High Lava Plains are a series of silica-rich volcanic eruptions which began some 15 Ma at the south eastern corner of Oregon, and have been progressing to the north and east since then. While volcanism along the Cascades is easily explained by the subduction of the Juan de Fuca plate beneath Oregon and Washington, volcanism further inland which later progresses towards the arc is very difficult to explain. Possible explanations may include the presence of a deep mantle upwelling of hot material called a plume, differences in the thickness of the rigid portion of the upper plate which could control flow patterns beneath it, and/or dynamic mantle flow patterns caused by the subducting Juan de Fuca plate further to the east. The research in this proposal will develop a new method for analyzing seismic data from earthquakes around the world collected at seismic stations located within the High Lava Plains to better image the uppermost 150 - 200 km of the earth. This imaging will help constrain which of these factors may be causing this volcanism, and how the various factors may relate to each other.
The new methodology is a joint inversion of surface waves and teleseismic body waves. It combines the complementary strengths of surface wave tomography and teleseismic body wave tomography into a single inversion for 3-D shear wave velocity structure, thereby reducing the limitations that exist when using either type of approach alone. Once developed, this methodology will be readily applicable to existing and future datasets. Preliminary resolution tests indicate that this methodology promises to greatly improve our understanding, not only of the High Lava Plains volcanism, but of tectonic processes in the upper mantle in many other settings where dense regional datasets are available.
