Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ont. K1A 0Y3, Canada

In order to map better the trend of the NACP in the Province of Saskatchewan, Canada, two magnetotelluric (MT) profiles were conducted over the NACP previously defined by magnetometer arrays and profiles. Data from these two profiles, along with an earlier MT profile just north of the U.S./Canadian border, suggest that the NACP is not a continuous feature, but rather that it exhibits a definite break at latitude 51° N. Other geophysical evidence examined herein is concordant with this characteristic. If a second MT anomaly mapped to the northwest of this break is, in fact, a manifestation of the same geological structure, then one possible interpretation is of a major NW-SE trending sinistral fault in the deep crust, previously undetected, with a movement of some 100–150 km along strike.

The MT data from the southernmost profile, after correction for static shift, are modelled in a 2D manner, and it is shown that the NACP is consistent with an arcuate structure in vertical section of high conductivity (> 2 S m^?1) beginning at a depth of 10 km centred on 103° W dipping down to the west reaching possibly the base of the crust. It is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal. Such high conductivities cannot be explained in terms of connected fluids, as it would require implausibly high porosities (12–20%). A comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region; which are, high electrical conductivity, positive density contrast, no magnetization, and high heat flow in the basement, in terms of a single causative body. One possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low-density differentia from a mantle-derived body. An alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite. However, difficulties exist with both of these explanations and further data are required.