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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 R m or more. A mid-crustal layer of 200 Q m 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 €or electric fields measured in the east-west direction subparallel to the tectonic fabric of the shield in this region.

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Teleseismic shear-wave splitting and magnetotelluric experiments across the Grenville front, between the Archean craton and the Proterozoic Grenville province in the regions of the Pontiac subprovince and northwestern Grenville province (Canada), show a consistent obliquity between the polarization direction of the fast split shear wave () and the most electrically conductive direction (MT) in the upper mantle transcurrent shear zones. At all wellrecorded stations, is nearly N103؇E, and MT is approximately N80؇E. The obliquity may be considered a potential kinematic indicator, because the seismic and electrical anisotropies are thought to be controlled by lattice-preferred and shape-preferred orientations of mantle minerals (mainly olivine), respectively. The dextral movement sense of the transcurrent shear zones in the mantle, inferred from the observed obliquity, is consistent with that inferred from surface geology of the crustal shear zones. This consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region.