Post-Keweenawan compressional faults in the eastern Lake Superior region and their tectonic significance
GLIMPCE aeromagnetic data in eastern Lake Superior are characterized by a series of strong easterly- and northeasterly-oriented gradients that relate to mapped post-Keweenawan faults occurring along the eastern shore. The reversed nature of three of the faults is established through field observations and potential field modelling. Middle Keweenawan volcanic rocks at Mamainse Point are in fault contact on their south side with upper Keweenawan sandstone of Bayfield–Jacobsville type. Gravity modelling suggests that the fault is a low angle thrust dipping to the north. Field observations and high-resolution aeromagnetic data show that it extends inland along the southern margin of the Batchawana Greenstone Belt for at least 17 km. To the west, the Mamainse Point fault may extend across eastern Lake Superior to the Keweenaw Peninsula, linking several offsets in the seismic data that are consistent with the same attitude and sense of displacement. Along the south side of Batchawana Bay at Havilland, sandstones of Bayfield–Jacobsville type are isoclinally folded against a package of upthrust older rocks that include drag-folded middle Keweenawan volcanics. At Grindstone Point, north of Cape Gargantua, a reverse fault separating isoclinally-folded upper Keweenawan sandstones from Archean basement may, on aeromagnetic evidence, be an eastward extension of the Michipicoten Island fault.These faults mark a significant change in the style of late compressional tectonism observed within the Midcontinent Rift. All cut Keweenawan rocks across strike. The inference is that broad north–south or northwest–southeast compression, consistent in timing and orientation with the Grenville Orogeny, led to a reversal of movement along the major graben faults in western Lake Superior and was taken up in the eastern region by reverse faults oriented normal to the extensional axis of the rift.