A MODEL OF LOWER CRUSTAL ELECTRICAL ANISOTROPY FOR THE PONTIAC SUBPROVINCE OF THE CANADIAN SHIELD

Magnetotelluric data from the Pontiac Subprovince of the Canadian Shield indicate that the crustal electrical conductivity structure of the region is very uniform. Because the data are distorted by near-surface electrical heterogeneities, the exact parameters of the resistivity structure may vary for different static shift corrections but the same sequence of layers is retained. An approximate one-dimensional model of the Pontiac, based on inversion of the effective impedance recorded at each site, consists of a thin conductive weathered layer, underlain by at least 12 km of resistive upper crust at 5000 OMEGAm or more. A mid-crustal layer of 200 OMEGAm extends to a depth of 25 km or more, below which there is evidence of an increase in the bulk resistivity in the lower crust and/or upper mantle. However, this last layer displays marked non-one-dimensional magnetotelluric responses which appear to be quite uniform along the profile. Similar responses have been seen in other parts of the Canadian Shield and the Baltic Shield. This feature in the MT data can be explained by azimuthal electrical anisotropy or by a two-dimensional conductive structure. The absence of any large vertical magnetic field responses suggests that anisotropy is the more likely cause. Modelling the anisotropy shows that a ratio of horizontal resistivities between 1 : 6 and 1 : 13 is representative of the lower crust and/or the upper mantle in the area. The higher conductivity is found for electric fields measured in the east-west direction subparallel to the tectonic fabric of the shield in this region.