ELECTROMAGNETIC IMAGES OF CRUSTAL STRUCTURES IN SOUTHERN AND CENTRAL CANADIAN CORDILLERA

Data from more than 400 magnetotelluric soundings, made since the earl y 1980's in the Canadian Cordillera over a 300 000 km(2) area between 49 and 53.5 degrees N, are used to image qualitatively regional three- dimensional crustal variation in electrical conductivity by means of p hase maps, phase-frequency sections, and maps of resistivity at depth. Two hundred of the soundings were acquired as part of Lithoprobe Sout hern Canadian Cordillera Transect activities, and their locations were coordinated with the seismic reflection and refraction experiments. T he 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 r eveal both two-dimensional structures, with highest conductivities alo ng the Coast Belt and Omineca Belt, and three-dimensional variation al ong geological strike. This conductor, mapped over a volume in excess of 10(6) km(3), is most probably caused by fluids - saline waters and silicate melts - in fractures and along interconnected grain boundarie s. The observed lateral variations in conductivity may result from var iations in fracture density, temperature, and 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 e ast-west-trending geophysical discontinuity in the upper and middle cr ust of the Omineca Belt at a latitude of 50 degrees N, with highly res istive rocks (> 1000 Ohm . m) to the south and more conductive rocks t o the north (30-300 Ohm . m). Seismic refraction models, residual grav ity, and filtered magnetic maps correlate changes in compressional-wav e velocity, density, and magnetization along this cross-strike discont inuity.