Electromagnetic images of crustal structures in southern and central Canadian Cordillera

Abstract

Data from more than 400 magnetotelluric soundings, made since the early 1980's in the Canadian Cordillera over a 300 000 km2area between 49 and 53.5°N, are used to image qualitatively regional three-dimensional crustal variation in electrical conductivity by means of phase maps, phase–frequency sections, and maps of resistivity at depth. Two hundred of the soundings were acquired as part of Lithoprobe Southern Canadian Cordillera 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 Ω·m) throughout the Cordillera west of the Foreland Belt. Within this "Canadian Cordilleran Regional" conductor, the magnetotelluric data reveal both two-dimensional structures, with highest conductivities along the Coast Belt and Omineca Belt, and three-dimensional variation along geological strike. This conductor, mapped over a volume in excess of 106 km3, 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, 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 east–west-trending geophysical discontinuity in the upper and middle crust of the Omineca Belt at a latitude of 50°N, with highly resistive rocks (>1000 Ω·m) to the south and more conductive rocks to the north (30–300 Ω·m). Seismic refraction models, residual gravity, and filtered magnetic maps correlate changes in compressional-wave velocity, density, and magnetization along this cross-strike discontinuity.