Geomagnetic induction soundings over the Michigan Basin

Abstract

Geomagnetic variation sounding and magnetotelluric results from six sites on an east-west long-period profile across the Michigan Basin are presented. A strongly conductive zone on the western flank of the presumed underlying Keweenawan rift system in the Precambrian basement is shown to exist near Cadillac, Michigan. This zone is perhaps similar in character to the Flambeau anomaly exposed at the surface in Wisconsin, also on the flank of the Keweenawan rift, whose conductivity resides in iron formations and in massive and disseminated graphite and sulphides. No clear expression of the Michigan Basin itself, or of the presumed Keweenawan rift in the basement is seen in the results. The limited magnetotelluric sounding results correspond to a surface conducting layer over a resistive deeper crust over a rise in conductivity at mantle depths. They show several hundred seimens of crustal conductance in excess of that provided by the Phanerozoic sediments of the basin, but do not determine whether the excess crustal conductivity is associated with the underlying Keweenawan clastics and volcanics or whether some of that conductivity is in a deeper crustal layer like that found under the Adirondack dome by Connerney et al.