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Concept
Midcontinent Rift System
Summary
The Midcontinent Rift System (MRS) or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the Precambrian, about 1.1 billion years ago. The rift failed, leaving behind thick layers of igneous rock that are exposed in its northern reaches, but buried beneath later sedimentary formations along most of its western and eastern arms. Those arms meet at Lake Superior, which is contained within the rift valley. The lake's north shore in Ontario and Minnesota defines the northern arc of the rift. From the lake, the rift's eastern arm trends south to central lower Michigan, and possibly into Indiana, Ohio, Kentucky, Tennessee, and Alabama. The western arm runs from Lake Superior southwest through portions of Wisconsin, Minnesota, Iowa, and Nebraska to northeastern Kansas, and possibly into Oklahoma.
The rock formations created by the rift included gabbro and granite intrusive rocks and basalt lavas. In the Lake Superior region, the upwelling of this molten rock may have been the result of a hotspot which produced a triple junction. The hotspot domed the rocks of the Lake Superior area. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the present-day rifting underway in the Afar Depression of the East African Rift system.
The southwest and southeast extensions represent two arms of the triple junction while a third failed arm extends north into Ontario as the Nipigon Embayment. This failed arm includes Lake Nipigon, Ontario.
The rift system may have been the result of extensional forces behind the continental collision of the Grenville Orogeny to the east which in part overlaps the timing of the rift development. Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure.
Official source
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