The Central North American Rift System (1100 Ma old)
The Central North American Rift System, sometimes called the Midcontinent Rift System or the Keeweenawan Rift System, extends from Lake Superior to Kansas. It is only exposed on the surface in the Lake Superior region but is clearly evident on gravity and magnetic maps and from subsurface drilling. Other buried rift basins, probably related to this system, extend from Michigan into Ohio and Indiana. The cause of the rifting was probably the initial collision of a continent with eastern North America. This event, the Grenville Orogeny, deformed and metamorphosed rocks from Labrador to Lake Huron and south beneath Paleozoic rocks to Kentucky. The collision began about 1100 Ma ago and metamorphism peaked about 950 Ma ago. The later stages of the collision compressed the crust far inland and resulted in thrust faulting in the former rift.
South of Lake Superior the rocks of the rift consist of basaltic volcanic rocks and red conglomerates and sandstones. The Mellen Gabbro, a mafic intrusion in northwestern Wisconsin, is also part of the rift complex. On the north shore of Lake Superior there are basalts, rhyolites, minor sedimentary rocks, and a vast gabbro intrusion, the Duluth Gabbro. The Duluth Gabbro is a lopolith, effectively a mega-sill kilometers thick. Around Lake Nipigon in Ontario, there are great sills also associated with the rifting event. These form dramatic mesa-like hills with cliffs hundreds of meters high.
Created 11 Sep 1997, Last Update 02 April 2013
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