Interpretation of Seismic Reflection, Gravity, and Magnetic Data Across Middle Proterozoic Mid-Continent Rift System, Northwestern Wisconsin, Eastern Minnesota, and Central Iowa

Abstract

Seismic reflection, gravity, and magnetic data were used along four profiles to study the Mid-Continent rift system between northwestern Wisconsin and central Iowa. Mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick. Mafic roots extending beneath the reflective sequence to mid-crustal depths were defined from gravity modeling in northwestern Wisconsin, southeastern Minnesota, and central Iowa. A strong normally directed remanent magnetization was modeled for most lavas, although some lavas deep in the section of northwestern Wisconsin and central Iowa were modeled as magnetically reversed. The horsts are apparently bounded by faults that are vertical or dip moderately inwar . Several of these faults originated as growth faults in fault-bounded grabens, but these faults were transposed into reverse faults by a late-stage compressional event. The medial horsts are flanked by several half-graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the Oronto and Bayfield Groups of Michigan and Wisconsin. Several synclinal basins characterized by 2-4 km of sedimentary rocks are developed atop the medial horsts. When viewed regionally, the four interpretive profiles show a marked asymmetry, with regard to the off-axis placement of the deep mafic roots and the positions of postulated growth faults. This asymmetry is consistent with the rift propagating as a series of northeast-striking, en echelon half grabens, separated by northwest-striking scissor faults or accommodation zones.