Geophysical mapping of the eastern arm of the Midcontinent Rift in Upper Michigan

The western arm of the Midcontinent Rift system (MRS), extending from western Lake Superior into Kansas, is somewhat understood due to exposures of MRS igneous and sedimentary rocks, scattered drillholes, and seismic imaging. By comparison, the eastern rift arm (ERA), extending from eastern Lake Superior through lower Michigan is poorly understood, with almost no outcropping volcanic rocks south of Michipicoten and Caribou Islands in Lake Superior, few boreholes that intercept MRS rocks, and the majority of the rift concealed beneath the Michigan basin. Seismic reflection sections in the eastern Lake Superior basin (Behrendt et al., 1990; Mariano and Hinze, 1994) and in Lake Michigan (Cannon et al., 1991) provide the primary constraints on the gross structure of the ERA. Unlike the western rift arm, which has undergone considerable post-rift shortening (Cannon et al., 1993), the ERA appears less deformed, with perhaps as much as 30 km of rift clastic and volcanic rocks preserved within a broadly symmetric basin (Behrendt et al., 1988) and about 5 km of anticlinal relief on the top of the volcanics (Mariano and Hinze, 1994). Under Paleozoic cover in the eastern part of Michigan’s Upper Peninsula, the ERA western margin is inferred from linear northwest-trending potential-field anomalies (Fig. 1a, b) and the occurrence of MRS basalt and rhyolite beneath ~2 km of Paleozoic and MRS clastic rocks in the St. Amour drillhole (Ojakangas and Dickas, 2002).