The Midcontinent Rift

The Midcontinent Rift

Mark Krippner

April 19, 2013

Emporia State University

Abstract

The scenic North Shore of Lake Superior is home to evidence of a once active geological past. Due to a magma plume in the mantle, a rift developed and threatened the cohesiveness of Proterozoic North America. The rifting process would eventually fail, leaving Laurentia whole. The rift would be appropriately named the Midcontinent Rift or the Keweenawan Rift after the Keweenawa Pennisula in Michigan. Besides the volcanic rock along North Shore, the Midcontinent Rift would leave little surficial evidence. It was not until the mid-1900s when magnetic gravity mapping would show the scar of ancient rifting. Since the mid-1900s, scientists have been working to unlock the secrets of the failed rift.

Magnetic Anomalies

Some of the clearest evidence of early continental rifting was discovered from magnetic gravity anomaly profiling of North America in the mid-1900s. This profile showed a strongly positive magnetic anomaly located in the upper Midwest (Ojakangas, 1982). The igneous rock of the Midcontinent Rift contains a large amount of iron creating the positive magnetic anomaly throughout the Midwest.

The Midcontinent Rift can be thought of as having two well developed arms. The western arm extends northeast from east-central Kansas through the southeast tip of Nebraska, central Iowa, and eastern Minnesota to Lake Superior. The eastern arm extends southeast from Lake Superior through Michigan and into parts of Ohio. The eastern arm of the Midcontinent Rift crops out due to uplift and orogenies related to the Grenville province. Evidence for a third arm to the north of Lake Superior is not nearly as defined as the two arms to the south (Van Schmus & Hinze, 1985).

This narrow band of igneous rock extending through the Midwest was not interpreted for many years due to the fact that most of the rock is buried and hidden by younger sedimentary rock (Van Schmus & Hinze, 1985). The Lake Superior region is one of the few locations where igneous rock outcrops from the Midcontinent Rift are exposed at the surface and can be easily studied.

Bouguer map of eastern United StatesBouguer map showing the magnetic anomaly in the upper Midwest (in magenta). Taken from USGS

Midcontinent Rifting

The Midcontinent Rift occurred between two minor orogenies, both relating to the Grenville province in the east. The Elzevirian orogeny occurred between 1240 and 1160 Ma and the Ottawan orogeny occurred between 1090 and 1025 Ma (Miller, 2007). These two orogenies gave rise to the Grenville Mountains being formed from compression on the eastern coast of Laurentia. Between these two orogenies, continental extension on Laurentia was thought to occur to due to upwelling of a magmatic plume near present day Lake Superior. The plume, along with its diverging convective currents, would force the land apart causing the rift. There is evidence of four stages of magmatic activity along the Lake Superior shoreline (Miller, 2007):

  1. The early magmatic stage occurred from 1115-1107 Ma. This stage likely saw the magmatic plume head interacting with the relatively cool lithosphere. This rifting left the area with various magma intrusions of ultra-mafic and mafic rock. This stage occurred during reversed magnetic polarity.
  2. The latent magmatic stage occurred from 1107-1102 Ma. With the exception of a few rhyolitic eruptions, this stage was largely dormant. The lack of volcanic activity was possibly due to a felsic shift in magma composition. The magnetic polarity, back to normal polarity, switch likely occurred near the end of the latent magmatic stage.
  3. The main magmatic stage occurred during 1102-1092 Ma. This stage is thought to be the beginning of the upper crust separation. The magma of this time varied in composition. This stage would deposit the North Shore Volcanic Group and the Duluth Complex. The igneous rocks of this stage show normal magnetic polarity.
  4. The late magmatic stage occurred during 1094-1086 Ma. This stage would see the end of the volcanism. Due to the approach of the Grenville province to the east, the rifting would cease. The Grenville province would eventually cause minor convergence along the rift as evidenced by reverse faulting in the region.

Questions remain as to the southern extent of the rifting in both arms of the Midcontinent Rift. The western arm could extend as far south as Oklahoma, while the eastern arm could extend through Tennessee (Van Schmus & Hinze, 1985). Evidence of this is not well known due to the fact that the rifting is buried by younger sedimentary rock and data comes exclusively from the magnetic anomaly maps and sparse deep drill-hole data.

The Midcontinent Rift has its own unique characteristics. The rift itself has similarities to the present-day rifting of Iceland and the Great Rift Valley of Africa. Differences remain between the outcome, or failure, of the Midcontinent Rift and modern locations of rifting today. The failure of the Midcontinent Rift was caused by the compressional pushing in the direction of the northwest (Miller, 2007). The Grenville province in the southeast was likely responsible for compressional pushing on Laurentia and the associated reverse faulting. This compression would leave the western arm of the Midcontinent Rift much narrower than the eastern arm (Miller, 2007).

The extent of the western armThe western extent of the Midcontinent Rift. Image taken from the Iowa DNR
Faulting along the North ShoreAnorthosite (left) and a diabase (right) fault found along the scenic North Shore. Photo date 6/12; by Mark Krippner

The Rocks of Lake Superior

The Midcontinent Rift left behind various igneous rocks as evidence of crustal extension and the subsequent fissure eruptions. Two sets of rocks that are well studied in the Lake Superior region are the North Shore Volcanic Group and the Duluth Complex. Both the North Shore Volcanic Group and the Duluth Complex tend to be mafic in nature, but they differ in texture. Several different formations of sedimentary rock throughout the Midwest are associated with covering the rift basin.

North Shore Volcanic Group

The composition of the North Shore Volcanic Group is mostly basaltic. There are a few lava flows that have an intermediate or even rhyolitic composition within the North Shore Volcanic Group. The fissure eruptions likely originated in what is now the Lake Superior basin (Ojakangas, 2009). The viscosity of the lava was very low and created many flat lava lakes and ponds when the lava spread out in the low areas. Along the North Shore of Lake Superior, there are hundreds of individual lava flows that vary in size and thickness. These flows are not necessarily stacked on top of each other but vary in locations all along the North Shore.

The tops of the North Shore Volcanic Group lava flows can be determined by observing air pockets in the individual lava flow tops. These vesicles in tops of the North Shore Volcanic Group lava flows often contain amygdules. The well known Lake Superior agates developed in these amygdules. Native copper is also known to develop in some amygdules, especially in the Upper Peninsula of Michigan (Ojakangas & Mastch, 1982).

Air bubbles in the tops of lava flowsAir bubble indentations created at the top of a lava flow. Photo date 6/12; by Lee Schmidt, used with permission.
Amygdules in well weathered lava rockHighly weathered amygdule-filled rock collected on the shore of Lake Superior. Photo date 4/13; by Mark Krippner
Lake Superior agates found in Central MinnesotaLake Superior agates transported by glacial activity and collected in Central Minnesota. Photo date 4/13; by Mark Krippner
Pipe vesicles in lava flowsPipe amygdules created from escaping gases in lava flows, later filled with minerals. Photo date 6/12; by Lee Schmidt, used with permission.
A columnar joint under Jim Miller at Gooseberry FallsJim Miller kneeling on a large columnar joint at Gooseberry Falls State Park. Photo date 6/12; by Lee Schmidt, used with permission.

Duluth Complex

Covering much of the northeast corner of Minnesota, the Duluth Complex crystallized under some of the aforementioned North Shore Volcanic Group. The course grained mafic rock was thought to be one large gabbro intrusion, but upon further study was determined to be a variety of intrusive igneous rocks from a number of intrusions. Despite being found below the North Shore Volcanic Group, the Duluth Complex is younger than some of the lava flows in the North Shore Volcanic Group. Evidence of this age relationship can be seen where the Duluth Complex intrudes the North Shore Volcanic Group lava flows (Ojakangas, 2009).

Anorthosite from the Duluth ComplexVarying tints of green in the rock anorthosite. Sample collected along the North Shore. Photo date 4/13; by Mark Krippner

Lava Flow Syncline & Sedimentary Rock

Lying under present day Lake Superior is over 18,000 meters of lava flows (Ojakangas, 2009). The weight of these high-density lava flows has created a syncline that tilts toward the center of the lake. This syncline can be viewed in many of the gradual tilting lava flows and rock formations along both the north and south shores of Lake Superior.

A gradual tilting of the lava flowsGently sloping lava flows towards the Lake Superior basin at Sugarloaf Scientific & Natural Area. Photo date 6/12; by Lee Schmidt, used with permission.

Running water over the course of millions of years would allow the rift basin to be filled in with sedimentary rock. Three different sedimentary formations are found covering Minnesota’s Midcontinent Rift south of Lake Superior. The oldest formation is the Solar Church Formation which was found only by drilling deep holes during natural gas exploration (Ojakangas & Mastch, 1982). The two other sedimentary rock formations are much better known. The Fond du Lac Formation and the Hinckley Sandstone sit atop the Solar Church Formation. These two layers are fluvial in nature and said to be the youngest Precambrian rocks in Minnesota (Ojakangas, 2009).

Conclusion

The drive along the North Shore in northeast Minnesota allows one to travel up and down the scars of a once violent past. Lava flows are stacked on top of each other as evidence of Laurentia pulling apart. With the exception of the Lake Superior region, evidence of the Midcontinent Rift remains hidden from scientist’s vision. While there are several examples of continental rifting occurring today, these examples do not completely model the complexities of this ancient rift. Many scientists today are working hard to unlock the secrets of the failed Midcontinent Rift.

Sources

  1. Miller, J. (2007). The Midcontinent Rift in the Lake Superior region. Large Igneous Provinces Comission, November 2007. Retrieved from http://www.largeigneousprovinces.org/07nov
  2. Ojakangas, R. (2009). Roadside geology of Minnesota. Missoula, MT: Mountain Press Publishing Company.
  3. Ojakangas, R. & Matsch, C. (1982). Minnesota’s geology. Minneapolis, MN: University of Minnesota Press.
  4. Van Schmus, W. & Hinze, W. (1985). The Midcontinent Rift system. Annual Review: Earth and Planetary Science, 13, 345-383.