We have inverted magneto telluric (MT) data collected in nine states of the northwestern United States as a part of the EarthScope project for 3D imaging of electrical resistivity to a depth of 500 km using recent advances in extremely largescale electromagnetic modeling and inversion. The results of our mega-cell 3D inversion reveal multi-scale geo electrical in homogeneities in the upper mantle, which are closely related to major known tectonic features. Our geoelectrical model clearly shows a resistive structure associated with the Juan de Fuca slab subducting beneath the northwestern United States, and the conductive zone of partially melted material above the subducting slab due to the release of fluids from the down going slab. We observe extensive areas of moderate-to-high conductive asthenosphere below 100 to 200 km. The geoelectrical model also shows a prominent conductive feature associated with the partially melted mantle plumelike layer of the Yellowstone hotspot.
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