Long period (10-20,000s) magnetotelluric (MT) data are being acquired in a series of temporary arrays deployed across the continental USA through the EMScope component of EarthScope. The MT data are highly sensitive to fluids and melt, and thus provide a valuable complement to other observational components of EarthScope. In this presentation we will review basics of the MT method, and then discuss 3D inversion and interpretation of EMScope data from the Northwestern US, acquired in 2006-2011. For the inversion we use full impedances and vertical field TFs from 325 sites on a quasi-regular grid (nominal ~70km spacing of the seismic TA) covering a rectangular area from NW Washington to NW Colorado. The inverse solutions reveal extensive areas of high conductivity in the lower crust and uppermost mantle beneath the extensional Basin and Range, High Lava Plains, and Snake River Plain provinces, as well as beneath the Cascade volcanic arc. These high conductivities can only be explained by partial melt and/or magmatic or subduction related saline fluids. Stable Proterozoic lithosphere in the northeastern part of the domain is generally much more resistive, with the thickest resistive sections coinciding with the Wyoming and Medicine Hat Cratons. Oceanic lithosphere of the subducting Juan de Fuca Plate is clearly imaged as a zone of very high resistivity beneath the Coast Ranges. Other prominent resistive zones in the NW part of the domain may represent relict oceanic lithosphere: the accreted “Siletzia” terrane beneath the Coast ranges and Columbia Embayment, and a deep vertical resistive feature just to the east—the seismically fast “slab curtain” beneath Eastern Idaho that others have interpreted to be stranded Farallon lithosphere. Aesthenospheric conductivities are generally consistent with laboratory results for moderately hydrated (~200-300 ppm) olivine, with a potential temperature of ~1300C. Higher aesthenospheric conductivities occur east of the Rocky Mountain front, where greater hydration or higher temperatures are required, and in the back arc, where broad fingers of high conductivity rise to very shallow depths. The most prominent of these features occurs in Washington State, where anomalously high conductivities dip to the S-E, suggesting aesthenospheric flow around the slab curtain. There are also some areas of reduced aesthenospheric conductivity (beneath the slab curtain, and south of the Yellowstone hot spot), suggesting lower temperatures and/or depletion of volatiles in these areas.
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