Magnetotelluric investigations of the lithosphere beneath the central Rae craton, mainland

Abstract New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of> 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45–53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15–35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3Ga) and Trans-Hudson orogenies (1.85Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from ~3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to ~10,000 Ω m beneath the central Rae craton, to>50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of ~500 Ωm at depths of 180–220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary. 1