Regional electrical resistivity structure of the southern Canadian Cordillera and its physical interpretation

Abstract

The regional geoelectric crustal structure of the southern and central Canadian Cordillera of western Canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts, with emphasis on the Intermontane and Omineca Belts. Decomposition of the tensor impedance response estimates demonstrates that large-scale regional structures can be reasonably approximated along each profile as two-dimensional, with dominant geoelectric strikes of either -30° or +15°, depending on profile location. These profile-specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern Intermontane and Omineca Belts suggested by others based on paleomagnetic data and palinspastic reconstructions. Comparing the resistivity models derived from two-dimensional inversions of the distortion-corrected data along each profile allows us to construct an orogen-scale three-dimensional resistivity model for southern British Columbia. Generally, the model shows a resistive upper crust overlying a conductive lower crust. The resistivity of the lower crust beneath the Intermontane Belt is independent of latitude and is similar for all profiles. In stark contrast, a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the Omineca Belt, with higher conductivities to the south in the region of Eocene extension and lower conductivities to the north in the unextended part of the belt. Such spatial association has noteworthy implications for the cause of lower crustal resistivity in active, or recently active, young regions. Our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids, with brines dominating for the most part but partial melt possible at the base of the crust in specific localities. We attribute the along-strike variation in the Omineca Belt mostly to variation in fluid content and interconnectivity, with the lowermost crust of the southern Omineca Belt being partially molten. This physical state difference is a consequence of degree of extension and implies that mantle-derived fluids are important for lower crustal resistivity. Copyright 2001 by the American Geophysical Union.