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The Midcontinent Rift System (MRS) or Keweenawan Rift is a 2,000 km (1,200 mi) 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 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. The western arm runs from Lake Superior southwest through portions of Wisconsin, Minnesota, Iowa, and Nebraska to northeastern Kansas,[1] and possibly into Oklahoma.[2]

Formation and failure

Rift rocks are exposed in the buff-colored areas around Lake Superior (black). The buff area on the north margin is the Lake Nipigon area.

The rock formations created by the rift included gabbro and granite intrusive rocks and basalt lavas.[3] In the Lake Superior region, the upwelling of this molten rock may have been the result of a hotspot which produced a triple junction.[1] 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 present-day rifting under way 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.[2][4] 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.[2] Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure.[2][5] 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.[6] But after about 15–22 million years the rift failed.[5][7] The Midcontinent Rift is the deepest closed or healed rift yet discovered; no known deeper rift ever failed to become an ocean.[6]

The rift today

Volcanic strata protrude at Isle Royale and the Keweenaw Peninsula[8]

Lake Superior occupies a basin created by the rift.[2] Near the lake, rocks produced by the rift can be found on the surface of Isle Royale and the Keweenaw Peninsula of the Upper Peninsula of Michigan,[8] northwest Wisconsin,[9] and on the North Shore of the lake in Minnesota and Ontario.[3] Similar rocks are exposed as far south as Interstate Park near Saint Paul, Minnesota,[2][10] but elsewhere the rift is buried beneath more recent sedimentary rocks up to 9 km (6 mi) thick.[11] Where buried, the rift has been mapped by gravity anomalies (its dense basaltic rock increases gravity locally),[12] aeromagnetic surveys, and seismic data.[13]

A slightly older but possibly related geologic feature is the 2,700,000 km2 (1,000,000 sq mi) Mackenzie Large Igneous Province in Canada, which extends from the Arctic in Nunavut to near the Great Lakes in Northwestern Ontario.[14]

Natural resources

Iowa magnetic anomaly map showing the rift curving from the north center to the southwest of the state.[15]

The Proterozoic Nonesuch Shale formation in the Keweenawan Rift contains enough organic carbon (greater than 0.5%) to be considered a potential source rock for petroleum. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan.[16] A few deep wells were drilled to explore for oil and gas in rift rocks as far southwest as Kansas. No oil and gas were found, but the explorations did make some deep rock samples available.[11] These include two "dry holes" drilled by Amoco: a 7,238 ft (2,206 m) well in Alger County, Michigan in 1987 and 1988, and one in Bayfield County, Wisconsin to a depth of 4,966 ft (1,514 m) in 1992.[17] In 1987 Amoco also drilled a 17,851 ft (5,441 m) dry hole that penetrated rift sediments in Iowa.[18]

The Michigan Copper Country in the Upper Peninsula and Isle Royale contains major native copper deposits in Keweenawan-age rocks associated with the rift. A copper mining industry was developed in Precolumbian times, reactivated in the 1840s and continued for more than a century. Low-grade copper and nickel deposits, among the largest in the world, also exist in the Duluth Complex north of the lake.[19] Once thought uneconomic to mine,[20] they have attracted renewed interest from resource companies.[21]

Gallery

See also

References

  1. ^ a b "Middle Proterozoic and The Mid-continent Rift". Minnesota's Rocks and Waters. Winona State University. Retrieved 2008-04-13. (Powerpoint presentation)
  2. ^ a b c d e f Van Schmus, W. R.; Hinze, W. J. (May 1985). "The Midcontinent Rift System" (PDF). Annual Review of Earth and Planetary Sciences. 13 (1): 345–383. Bibcode:1985AREPS..13..345V. doi:10.1146/annurev.ea.13.050185.002021. Retrieved June 10, 2007. 
  3. ^ a b Ojakangas, Richard W.; Charles L. Matsch (1982). Minnesota's Geology. Illus. Dan Breedy. Minneapolis, Minnesota: University of Minnesota Press. ISBN 0-8166-0953-5. 
  4. ^ Kean, William F. (24 November 2000). "Keweenawan Rift System". Field Trips, Northern Wisconsin and Upper Michigan. University of Wisconsin-Milwaukee. Archived from the original on 6 July 2008. Retrieved 8 June 2007. 
  5. ^ a b Ojakangas, R. W.; Morey, G. B.; Green, J. C. (June 1, 2001). "The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA". Sedimentary Geology. 141–142: 421–442. Bibcode:2001SedG..141..421O. doi:10.1016/S0037-0738(01)00085-9. Retrieved April 15, 2008.  (Abstract)
  6. ^ a b Reeves, T. K.; Carroll, Herbert B. (April 1999). "Geologic Analysis of Priority Basins for Exploration and Drilling" (PDF). U.S. Department of Energy, Office of Scientific and Technical Information. Retrieved 2007-06-10. 
  7. ^ Schneider, D. A.; Holm, D. K.; Chandler, V. (Fall 2006). A Superior Swath: Proterozoic Geology of the North American Midcontinent. American Geophysical Union. Abstract #T42A-06. Retrieved April 15, 2008. 
  8. ^ a b Huber, Norman King (1975). "The story of Isle Royale National Park". Bulletin 1309. United States Geological Survey. 
  9. ^ Anderson, Raymond R. (2014-11-25). "Introduction to the Midcontinental Rift". Iowa Department of Natural Resources Geological Survey. 
  10. ^ Jol, Harry M. (2006). "Interstate State Park, A Brief Geologic History". University of Wisconsin at Eau Claire. Archived from the original on 15 December 2012. Retrieved 10 June 2007. 
  11. ^ a b Palacas, James G. (1995). "Superior Province (051)" (PDF). National Oil and Gas Assessment. United States Geological Survey. Retrieved 2007-06-10. 
  12. ^ Daniels, David L.; Snyder, Stephen L. (1991). "Bouguer gravity anomaly map of north-central United States". Minnesota Geological Survey. Retrieved 2016-05-27. 
  13. ^ Hinze, W.; Kellogg, D.; Merritt, D. (1971). "Gravity and Aeromagnetic Anomaly Maps of the Southern Peninsula of Michigan" (PDF). Report of Investigation 14. Michigan State Geological Survey. Retrieved 2007-06-10. 
  14. ^ National Research Council; Division on Engineering and Physical Sciences; Commission on Physical Sciences, Mathematics, and Applications; Geophysics Research Board, Assembly of Mathematical and Physical Sciences, Geophysics Study Committee (1980). Continental Tectonics. National Academies Press. p. 191. ISBN 978-0-309-02928-5. 
  15. ^ Kucks, Robert P.; Hill, Patricia L. (2005). "Iowa magnetic and gravity maps and data". U.S. Geological Survey. Retrieved 2009-09-03. 
  16. ^ J.A. Seglund, "Midcontinent rift continues to show promise as petroleum prospect," Oil and Gas Journal, 15 May 1989, p.55-58.
  17. ^ Albert B. Dickas, Results of the Middle Proterozoic Midcontinent rift frontier play along Lake Superior's south shore, Oil & Gas Journal, 18 September 1995, pp. 80–82.
  18. ^ "Exploring the Midcontinent Rift". Iowa Geological Survey Resource Information Fact Sheet 2006-2.
  19. ^ Kraker, Dan (2013-12-02). "FAQ: Everything you need to know about PolyMet". Retrieved 2013-12-10. 
  20. ^ Jira, Mark; David Southwick. "Mineral potential and geology of the Duluth Complex". Mineral Potential and Geology of Minnesota. Minnesota Geological Survey, University of Minnesota. Retrieved 2008-04-13. 
  21. ^ Kraker, Dan (December 5, 2013). "PolyMet's copper-nickel mine rekindles decades-old environmental debate". Minnesota Public Radio. Retrieved December 10, 2013. 
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