A geophysical profile of the southern margin of the midcontinent rift system in western Lake Superior

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Abstract
A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region.