POST-KEWEENAWAN COMPRESSIONAL FAULTS IN THE EASTERN LAKE-SUPERIOR REGION AND THEIR TECTONIC SIGNIFICANCE

GLIMPCE aeromagnetic data in eastern Lake Superior are characterized b y a series of strong easterly- and northeasterly-oriented gradients th at relate to mapped post-Keweenawan faults occurring along the eastern shore. The reversed nature of three of the faults is established thro ugh field observations and potential field modelling. Middle Keweenawa n 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 dippin g to the north. Field observations and high-resolution aeromagnetic da ta show that it extends inland along the southern margin of the Batcha wana Greenstone Belt for at least 17 km. To the west, the Mamainse Poi nt fault may extend across eastern Lake Superior to the Keweenaw Penin sula, 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 t ype are isoclinally folded against a package of upthrust older rocks t hat include drag-folded middle Keweenawan volcanics. At Grindstone Poi nt, north of Cape Gargantua, a reverse fault separating isoclinally-fo lded upper Keweenawan sandstones from Archean basement may, on aeromag netic evidence, be an eastward extension of the Michipicoten Island fa ult.These faults mark a significant change in the style of late compre ssional tectonism observed within the Midcontinent Rift. Atl cut Kewee nawan rocks across strike. The inference is that broad north - south o r northwest - southeast compression, consistent in timing and orientat ion with the Grenville Orogeny, led to a reversal of movement along th e major graben faults in western Lake Superior and was taken up in the eastern region by reverse faults oriented normal to the extensional a xis of the rift.