Data from more than 400 magnetotelluric soundings, made since the early 1980s in the Canadian Cordillera over a 300,000 sq km area between 49 degrees N and 53.5 degrees N, are used to image qualitatively regional threedimensional crustal variation in electrical conductivity by means of phase maps, phasefrequency sections, and maps of resistivity at depth. Two hundred of the soundings were acquired as part of Lithoprobe Southern Cordilleran transect activities, and their locations were coordinated with the seismic reflection and refraction experiments. The lower crust has a generally pervasive, low resistivity (1-100 Ohm.m) throughout the Cordillera west of the Foreland Belt. Within this "Canadian Cordilleran Regional" conductor, the magnetotelluric data reveal both 2D structures, with highest conductivities along the Coast Belt and Omineca Belt, and 3D variation along geological strike. This conductor, mapped over a volume in excess of 10 million cubic kilometres, is most probably caused by fluids saline waters and silicate melts in fractures and along interconnected grain boundaries. The observed lateral variations in conductivity may result from variations in fracture density, in temperature, and in the sources of hot fluid such as the subducting Juan de Fuca plate under the Coast Belt, and mantle upflow under the Omineca Belt. In addition, we report a major eastwest trending geophysical discontinuity in the upper and midcrust of the Omineca Belt at a latitude of 500N, with highly resistive rocks (>1,000 Ohm.m) to the south, and more conductive rocks to the north (30 - 300 Ohm.m). Seismic refraction models, residual gravity and filtered magnetic maps correlate changes in compressionalwave velocity, density and magnetization along this crossstrike discontinuity.
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