Abstract

The Great Lakes tectonic zone is a major Precambrian crustal feature more than 1,200 km long extending eastward from Minnesota into Ontario, Canada. It is a zone of distinctive tectonism, affecting both Archean and early Proterozoic rocks, along the northern margin of the early Proterozoic Penokean fold belt adjacent to the Archean Superior province. The zone coincides with the boundary between two Archean crustal segments recognized in the region: a greenstone-granite terrane (∼2,700 m.y. old) to the north (Superior province) and an older (in part 3,500 m.y. old) gneiss terrane to the south. Tectonism along the zone began in the late Archean, during the joining together of the two terranes into a single continental mass, and culminated in the early Proterozoic, when steep or northward-facing overturned folds were formed in the supracrustal rocks, and intense cataclasis and a penetrative cleavage developed in subjacent basement rocks of the greenstone-granite terrane. The Proterozoic deformation took place under low to intermediate pressures.

Movement occurred along the Great Lakes tectonic zone through much of the Precambrian time recorded in the region. In the early Proterozoic, crustal foundering, which was parallel to the zone and was diachronous, initiated the structural basins in which the early Proterozoic sequences of the Lake Superior and Lake Huron regions were deposited. Later, during the Penokean orogeny (∼1,850 to 1,900 m.y. ago), compression deformed the sequences in both regions. Still later, intermittent (∼1,850 to 1,100 m.y. ago) crustal extension provided sites for emplacement of abundant mafic igneous rocks. There is no definite evidence that any of the extensional events progressed to the stage of development of oceanic crust; probably the zone has been wholly intracratonal since its inception in late Archean time.

During the Phanerozoic, minor differential movements occurred locally in the Great Lakes tectonic zone, as recorded by the thinning of Cretaceous strata and their subsequent tilting and by historic earthquakes in Minnesota.

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