Magnetotelluric (MT) data are being acquired in a series of temporary arrays deployed across the continental US as a component of USArray (http://www.iris.edu/USArray/researchers/mf.html). Long-period (10-20,000 s) data were collected from 110 sites covering the Pacific Northwest in 2006-2007 and are being used to constrain the electrical conductivity structure of the crust and upper mantle to depths exceeding 100 km. In contrast to traditional long period MT surveys in which sites are concentrated along a few profiles, these MT arrays have occupied sites on a quasi-uniform 70 km grid, allowing regional scale coverage. The large station spacing and the wide range of geologic environments traversed demand three-dimensional (3-D) inversion and interpretation approaches, which are just now becoming available. 3-D inversion was carried out using WSINV3D to fit 4 complex impedance tensor components for the 109 sites with data of acceptable quality. I have experimented with several variants on the model covariance. The smoothest inverse solution, computed with larger horizontal decorrelation length scales. The results provide an unprecedented regional scale view of conductivity variations in the lower crust and upper mantle beneath the Pacific Northwest, including the Juan de Fuca subduction zone along the west coast, the Cascade volcanoes, the Columbia Plateau, and the high desert transitioning into the Basin and Range (BR). The most striking and robust feature revealed by the inversion is an extensive, triangular-shaped area of high conductivity in the lower crust beneath the southeastern part of the array. The conductance of this layer, which is about 15 km thick with a top at roughly 20 km depth, exceeds 3000 S beneath the BR. The high conductivity in this region probably results from fluids - including possibly partial melt at depth - associated with magmatic underplating and BR extension.

For details see: Patro, P. K. and Egbert, G. D., Regional conductivity structure of Cascadia: Preliminary results from 3D inversion of USArray transportable array magnetotelluric data, Geophysical Research Letters, 35, L20311, doi:10.1029/2008GL035326, 2008