Midcontinent Rift

The Midcontinent Rift System (MRS) or Keweenawan Rift is a 2,000-kilometer 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 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 (illustration) 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]

Formation and failure

The rock formations created by the rift included gabbro and granites from magma and basalts from lava.^[2] The upwelling of this molten rock may have been the result of a hotspot which produced a triple junction in the vicinity of Lake Superior.^[3] 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.^[4]^[5] This failed arm now forms Lake Nipigon.

It is possible that the rift was 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.^[4] Later compressive forces from the Grenville Orogeny likely played a major role in the rift's eventual failure and closure.^[6]^[4] 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.^[7] But after about 15–22 million years the rift failed.^[8]^[6] The Midcontinent Rift is the deepest closed or healed rift yet discovered; no deeper rift ever failed to become an ocean.^[7]

The rift today

Lake Superior occupies a basin created by the rift.^[4] Near the lake rocks produced by the rift can be found on the surface in the Keweenaw Peninsula of Upper Michigan, northwest Wisconsin,^[9] and on the North Shore of the lake in Minnesota and Ontario.^[2] Similar rocks are exposed as far south as Interstate Park near Saint Paul, Minnesota,^[10] but otherwise the rift is buried beneath more recent sedimentary rocks up to 9,000 meters thick.^[11] Where buried, the rift has been mapped by gravity anomalies (its dense basaltic rock increases gravity locally),^[12] aeromagnetic surveys,^[13] and seismic data.^[14]

Natural resources

The Proterozoic Nonesuch Shale formation in the Keweenaw Rift contains enough organic carbon to be considered a potential source rock for petroleum. A few deep wells have been drilled to explore for oil and gas (so far unsuccessfully), in rift rocks as far southwest as Kansas, making some deep rock samples available.^[11] These include two "dry holes" drilled by Amoco: a 7,238 feet (2,206 m) well in Alger County, Michigan in 1987 and 1988, and one in Bayfield County, Wisconsin to a depth of 4,966 feet (1,514 m) in 1992.^[15] In 1987 Amoco also drilled a 17,851-foot dry hole that penetrated rift sediments in Iowa.^[16]

Contents

Formation and failure

The rift today

Natural resources

See also

References