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The 'Midcontinent Rift System' (MRS) or 'Keweenawan Rift' is a 2000 km. long geological rift in the center of the North American continent and south-central part of the North American plate. It trends north from central lower Michigan, turns west through Lake Superior with its outer arc following the line of the north shore of that lake in Minnesota and Ontario, then turns southwest through portions of Wisconsin, Minnesota, Iowa, Nebraska and Kansas. Explore for Minnesota Gas Map of Midcontinent Rift area. It is approximately 1,100 million years old, from the Precambrian Mesoproterozoic era.

Contents

Formation and failure

The rift today

Natural resources

References

Formation and failure

The lava flows created by the rift were formed from basaltic magma. The upwelling of this magma may have been the result of a hotspot which produced a triple junction in the vicinity of Lake Superior. The hotspot made a dome that covered the Lake Superior area. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the rifting of 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. The Midcontinent Rift System, , W. R., Van Schmus, Annual Review of Earth and Planetary Sciences, Keweenawan Rift System This failed arm now forms Lake Nipigon. It is also possible that the rift is 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.
It is likely that later compressive forces from the Grenville Orogeny also played a major role in the rift's eventual failure and closure. Had the rifting process continued, the eventual result would have been sundering of the North American craton and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded. Geologic Analysis of Priority Basins for Exploration and Drilling But after about 10-20 million years the rift failed. Post-rift deformation of the Midcontinent Rift under Grenville tectonism, , Muhammad A., Soofi, Tectonophysics, The Midcontinent Rift is the deepest closed or healed rift yet discovered; no deeper rift ever failed to become an ocean.

The rift today

Rift rocks are exposed in the lighter area around Lake Superior in this geological map.

Near Lake Superior, rocks produced by this rift can be found on the surface along the shores of the Keweenaw Peninsula of Upper Michigan, Midcontinent Rift System in Iowa and on the North Shore of the lake in Minnesota. Minnesota's Geology, , Richard W., Ojakangas, University of Minnesota Press, 1982, ISBN 0-8166-0953-5 The lake itself lies in the rift valley formed by the rifting. Similar rocks are exposed as far south as Interstate Park near Saint Paul, Minnesota, Interstate State Park, A Brief Geologic History but otherwise the rift is buried thousands of feet below the surface. Where buried, it has been mapped by gravity anomalies (its dense basaltic rock increases gravity locally), Bouguer gravity anomaly map of north-central United States
aeromagnetic surveys, Shaded-relief map, total magnetic intensity anomoly and seismic data. Gravity and Aeromagnetic Anomaly Maps of the Southern Penninsula of Michigan

Natural resources

A few deep wells have been drilled into the rift strata to explore for oil and gas (so far unsuccessfully), making some deep rock samples available. Superior Province (051)
The Michigan Copper Country contains native copper deposits in Keweenawan-age rocks associated with the rift.

References