The Keweenaw fault system along Lake Superior’s south shore in Michigan, USA is one of the most significant fault systems associated with North America’s Midcontinent Rift System. Reverse slip has thrust Portage Lake Volcanics (ca. 1.1 Ga) southeastward over Jacobsville Sandstone (ca. 1.0 Ga). Growing consensus in the 1970s about a major rift beneath Lake Superior led to the idea that reverse movement on the Keweenaw fault involved inversion of a rift-bounding normal fault. To test this idea, we integrated bedrock geology maps, subsurface mining data, and geophysical data to build a wellconstrained cross-section from the Minnesota, USA-Ontario, Canada border to central Upper Michigan. The cross-section shows the Keweenaw and Hancock faults to be parts of a reverse, listric system detached deep within the layered volcanic section. Correlation of offshore seismic units with onshore stratigraphic units defines a gradual change in stratal dip from 55° NW near the faults to horizontal near the center of Lake Superior. This stratal geometry implies that onshore listric fault geometry continues offshore and that the fault system soles into a basal detachment at 12–15 km depth. Absence of second-order folds northwest of the peninsula indicates that the fault system does not ramp downward from the basal detachment through the lower crust. The observations and model preclude ideas that the Keweenaw fault was a rift-bounding normal fault inverted by post-rift crustal shortening, or that it formed by upward propagation of a deeper normal fault undergoing inversion. Instead, our results imply that the fault initiated as a detached thrust in relatively brittle upper crust and that shortening of relatively ductile lower crust occurred on other structures.

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