Magnetotellurics is used in two geologic settings on scales ranging from 1000-km tectonic structures to local features hundreds of meters wide. These areas are the Midcontinent Rift System (MRS) and its related mantle plume in the northern Midwestern United States and Newberry Volcano in central Oregon.
The MRS study uses MT data to image structures of the midcrust to the upper mantle. In addition to the rift, an elongated conductor is present at the base of the lithosphere, interpreted as the hot spot track from the mantle plume that initiated rifting. This track terminates in southern Wisconsin at the 1.8-1.7 Ga Yavapai-Mazatzal accretion that was contemporaneous with a major change in the chemistry of subducted material. During this time, conductive sediment rich in metallic sulfide minerals that recorded a change in global ocean chemistry from iron-saturated to sulfur-saturated during the Proterozoic Great Oxygenation Event was deposited on the seafloor. Melting of sediment rich in metallic sulfide minerals during the onset of the Keweenaw mantle plume that initiated the MRS in western Lake Superior enabled interconnection of disseminated sulfide crystals to produce a hot spot track at upper lithospheric depths that are in agreement with melting depths and temperatures inferred from petrologic studies of syn-rift flood basalts. The hot spot’s location shows that pre-Yavapai accretion subduction was north-dipping and low-angle.
The Newberry work shows that magma within the caldera is unusually resistive and therefore must be felsic and dry if it is to agree with seismic models and with the petrology of recent eruptions. Using seismic and petrological constraints, we find that the magma reservoir has no more than 8.7% partial melt. A major component of the volcano’s hydrothermal system is expressed along the caldera’s south rim as a vertical conductive anomaly reaching from the magma chamber to the vent that produced the most recent eruption.