Geoelectric response of Archean lithosphere in the western Superior Province, central Canada

Magnetotelluric (MT) soundings were made at more than 230 sites in the western Superior Province, in central Canada, as part of the Lithoprobe Western Superior Transect investigations. The Superior Province forms the nucleus of the North American continent. Its western part contains a regional pattern of east-west trending 100-200 km wide granite-greenstone and metasedimentary belts or subprovinces. The objectives of the MT study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries, and to determine if there exists a signature in the upper mantle related to the formation of the craton. The exceptionally large MT database for the western Superior Province provides a three-dimensional sampling of the resistivity structure. In this paper, the primary NIT responses from the data set, including geoelectric strike, apparent resistivity and phase, are used to define first-order geoelectrical structures in the region. Apparent resistivity responses indicate the ca 3.0 Ga North Caribou terrane contains extremely resistive crust. Crust to the south, accreted at ca. 2.7 Ga, is significantly more conductive, with resistivities similar to in the southeast Superior Province. The results suggest that the supracrustal component of the North Caribou terrane provides minimal enhancement of the crustal conductivity. At the southern margin of the former North Caribou superterrane, induction arrows define a conductivity anomaly that extends in an east-west direction for at least 600 km and causes a reversal of arrows in a region 100-200 km wide. This anomaly is interpreted to be caused by a conductive component of the 2.7 Ga metasedimentary rocks of the English River subprovince. The observations demonstrate the ability of electromagnetic methods to map some Neoarchean continental margins. In the southwest Superior Province, geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere. This result provides support to the interpretation that the lithospheric mantle of the western Superior Province has remained intact since the 2.7 Ga accretion of the subprovinces. The NIT responses from the western Superior Province have thus defined a number of large-scale resistivity structures providing information on tectonic structure and history, and confirming the value of electromagnetic imaging of Archean lithosphere. (c) 2004 Elsevier B.V. All rights reserved.