CONTINENTAL ARCHITECTURE AND MAGMATIC/HYDROTHERMAL PROCESSES OF SUBDUCTION AND EXTENSIONAL ENVIRONMENTS IN THE WESTERN U.S. VIEWED FROM EARTHSCOPE MT TA AND FOCUSED MT DATA SETS

Tectonic processes observed near the Earth’s surface can have origins at depths up to hundreds of kilometers. Earth electrical conductivity can be influenced by factors in these processes, such as very small amounts of fluids or melts, that are difficult to evaluate by other means. The broad bandwidth of magnetotelluric (MT) EM wavefields allows unified views of conductivity structure from scales of 100’s of m to 100’s of km. However, interpretation in terms of fluid/melt, or even possible non-fluid causes, is non-unique and requires constraints. An important constraint in active terranes is temperature, for example considering a geotherm near an average current mantle adiabat (ACMA) for much of the upper mantle, or utilizing thermal models of subduction zones. Inversion of MT TA data over the western US reveals extensive areas of high, horizontally-anisotropic conductivity (low resistivity) in the lower crust and uppermost mantle beneath active extensional regions that are best explained by underplated, flow-aligned hybridized magmas and associated exsolved, highly saline fluids. Focused MT profiling reveals common high-angle, fluidized crustal fault zones connecting these magmas with high-temperature geothermal systems, explaining anomalous ³He and CO₂ emissions. The upper mantle beneath the underplating to 150-200 km depth is rather resistive, interpreted to signify dehydration and melt depletion in forming the underplated melts above. The more conductive, deeper upper mantle, perhaps relatively fertile still, possesses apparent hydration of several hundred ppm. The subducting Juan de Fuca plate is resistive, with overlying conductive plumes dipping eastward from the Cascadia arc suggestive of upwelling hydrated and perhaps melted asthenospheric mantle. Denser profiling shows conductive lower crust extending 10’s of km into the forearc, denoting fluids risen from the dewatering slab (eclogitization), and coincides with loss of seismic receiver function coherency of the plate interface and onset of slow-slip events and tremor. To the east in the cratonic domains, high resistivity lithosphere extends to ~200 km depth, but is segmented by conductive fossil suture zones containing deeply underthrust graphite or sulfides.