Lake Superior and the spectacular scenery around it, as seen in places like Isle Royale National Park, Pictured Rocks and Apostle Islands National Lakeshores, Tettegouche and Interstate State Parks, and Porcupine Mountain Wilderness State Park, result from one of the more important processes shaping our planet – the breaking apart of continents to form new oceans. Throughout earth’s history, continents have split along rifts, long zones of volcanism and stretching, which become spreading centers along which new ocean basins form and can grow to the size of the Atlantic and Pacific. It even looks like similar rifting may have happened on other planets.
Understanding how rifting works is important for both science and society, because rifting provides conditions for the deposition of hydrocarbons and other mineral resources. The problem is that when rifting succeeds in forming a new ocean, not much remains from its early stages so it’s hard to see what happened.
To explore these issues, I and other researchers are studying the spectacular 1.1 billion year old Mid-Continent Rift (MCR). The rift failed, leaving a 3000 km long belt of volcanic and sedimentary rocks left from the early rifting stage. This fossil rift, which is similar in length to the present East African and Baikal rifts, has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as Oklahoma, and the other extends southeastward through Michigan to Alabama. These arms are identified using the large gravity and magnetic anomalies resulting from dense and highly magnetic igneous rocks. They are largely covered by younger sediments but outcrop near Lake Superior and along the St. Croix river (see picture above) and can be followed further south in drill cores.
Our studies combine seismic data, some acquired by a project called SPREE (Superior Province Rifting EarthScope Experiment) as part of NSF’s EarthScope program, with gravity, magnetic, and geologic data to develop an integrated three-dimensional image of the rift system and constrain its evolution. We are learning more about how the rift started, how rifting proceeded, why it ended, and what it tells us about other rifts in the US and worldwide. The video below gives an overview of the MCR, what we know about its geology and evolution, and how it affected the area’s growth.
Reflection seismic data across Lake Superior show that the MCR is a basin between dipping faults that contains basaltic volcanic rocks up to 20 kilometers thick, overlain by about 5–8 kilometers of sedimentary rocks. Working backward from the geometry of the volcanic and sedimentary rock layers seen today provides an evolutionary model of the MCR. It began as a half graben filled by flood basalts. After rifting (extension) stopped, the basin further subsided, accommodating more flood basalts. After volcanism ended, subsidence continued, accompanied by sedimentation. The crust depressed and flexed under the load of the basalt and sedimentary rocks. Long after subsidence ended, the area was compressed— a process called basin inversion—which reversed motion on the faults and activated new ones. This brought the volcanic rocks to the surface, where we see them today.
These results give a new view of the MCR’s evolution. Traditionally, the MCR was viewed as a failed rift formed by isolated midplate volcanism and extension within a stable continent. However, the MCR appears to have been a boundary of a relatively rigid microplate within a larger rift system formed during a short extensional phase within the longer ~1.3 – 098 Ga Grenville orogeny associated with the assembly of the supercontinent of Rodinia. The MCR formed as part of a larger plate boundary evolution process in which rifting succeeded. Although the MCR arms themselves failed, they are better viewed as part of a larger system that evolved successfully into seafloor spreading.
A good analogy is the way present-day continental extension in the East African Rift (EAR) and seafloor spreading in the Red Sea and Gulf of Aden form a classic three-arm rift geometry as Africa splits into Nubia, Somalia, and Arabia. GPS and earthquake data show that the opening involves several microplates between the large Nubian and Somalian plates. If the EAR does not evolve to seafloor spreading and dies, in a billion years it would appear as an isolated intracontinental failed rift similar to the MCR.
The MCR probably formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentia’s apparent polar wander path near the onset of MCR volcanism, recorded by the MCR’s volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the geometry and timing of Precambrian rifting events including the MCR’s extension to southwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent interpretation of large igneous provinces in Amazonia.
In addition to our research, we are developing material to help tell the MCR’s story. Some is to help national and state park interpreters present the story of the MCR to park visitors, so they can see how what one sees at a site fits into an exciting regional picture spanning much of the Midwest. This effort is part of an EarthScope program for educators.
For more information:
Articles about our work:
New Insights into North America’s Midcontinent Rift
Solving the Midwest’s Biggest Geologic Mystery
Mysterious Midcontinent Rift is a Geological Hybrid
Sharing Lake Superior’s Secrets
North America’s broken heart (Nature)
A real rift in the midcontinent (Earth magazine)
A webpage about the MCR as the “LIP (Large Igneous Province) of the month” with more discussion and references here.
A non-technical video about hiking Isle Royale National Park, including a quick “hikers geology” explaining the spectacular Midcontinent Rift rocks
Scientific Papers:
Stein, C.A., Stein, S., Gallahue, M., and Elling, R., Revisiting hotspots and continental breakup – Updating the classical three-arm model, GSA Special paper “In the Footsteps of Warren B. Hamilton: New Ideas in Earth Science,” 2021. Click here for pdf.
Gallahue, M., Stein, S., Stein, C.A., Jurdy, D., Barklage, M., and Rooney, T., A compilation of igneous rock volumes at volcanic passive continental margins, Marine and Petroleum Geology, 2020. Click here for link. Click here for pdf.
Elling, R., Stein, S., Stein, C.A., and Keller, R., Tectonic implications of the gravity signatures of the Midcontinent Rift and Grenville Front. Tectonophysics, 2020. Click here for pdf. Click here for link
Malone, D., Stein., C., Craddock, J., Stein, S., and Malone, J., Neoproterozoic sedimentation and tectonics of the Laurentian midcontinent: Detrital zircon provenance of the Jacobsville Sandstone, Lake Superior Basin, USA and Canada, Terra Nova, 2020. Click here for pdf
Stein, C.A., Stein, S., Elling, R., Keller, G.R. and Kley, J. Is the “Grenville Front” in the central United States really the Midcontinent Rift?. GSA Today, 28(5, 2018). Click here for link
Stein, S., Stein, C.A., Elling, R., Kley, J., Keller, R., Wysession, M., Rooney, T., Frederiksen, A. and Moucha, R.. Insights from North America’s failed Midcontinent Rift into the evolution of continental rifts and passive continental margins. Tectonophysics, July 2018. Click here for pdf. Click here for link
Stein, C.A., Stein, S., Elling, R., Keller, G.R. and Kley, J. Is the “Grenville Front” in the central United States really the Midcontinent Rift?. GSA Today, 28(5, 2018). Click here for link
Stein, S., C. Stein, J. Kley, R. Keller, M. Merino, E. Wolin, D. Jurdy, D. Wiens, M. Wysession, G. Al-Equabi, W. Shen, A. Frederiksen, F. Darbyshire, G. Waite, W. Rose, E. Vye, T. Rooney, R. Moucha, and E. Brown, New insights into North America’s Midcontinent Rift, EOS, 97, doi:10.1029/2016EO056659. Published on 04 August 2016 2016. Click here for EOS story with links or here for pdf.
D. Malone, C. Stein, J. Craddock, J. Kley, S. Stein, and J. Malone, Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift Geosphere, doi:10.1130/GES01302.1., 12, 4, 2016. For pdf click here.
Stein, C. A., S. Stein, J. Kley, D. Hindle, and G.R. Keller, North America’s Midcontinent Rift: when rift met LIP, Geosphere, 11, 5, 1-10, 2015. For pdf click here.
Stein, C. A., S. Stein, M. Merino, G. Randy Keller, L. M. Flesch, and D. M. Jurdy, Was the Mid-Continent Rift part of a successful seafloor-spreading episode?, Geophys. Res. Lett., 41, doi:10.1002/2013GL059176, 2014. For pdf click here.
Variations in Mid-Continent Rift magma volumes consistent with microplate evolution, by M. Merino, G. R. Keller, S. Stein, and C. Stein, Geophys. Res. Lett. DOI: 10.1002/grl.50295, 2013. For pdf click here.
Learning from failure: the SPREE Mid-Continent Rift Experiment, by S. Stein, S. van der Lee, D. Jurdy, C. Stein, D. Wiens, M. Wysession, J. Revenaugh, A. Frederiksen, F. Darbyshire, T. Bollmann, J. Lodewyk, E. Wolin, M. Merino, and K. Tekverk, GSA Today, 21, 9, pp 5-7, September 2011. For pdf click here.
Overview talk:
Stein, C. A,. et al., New Insights into North America’s Midcontinent Rift (2020). For ppt click here.
Information for national and state park interpreters:
Stein, S., C. Stein, E. Blavascunas, and J. Kley, Using Lake Superior parks to explain the Midcontinent Rift, Park Science, 32,1, 19-29, 2015. For pdf click here.
Over the years, our collaborators in studying the MCRS included Carol Stein (University of Illinois at Chicago), Randy Keller (University of Oklahoma), Jonas Kley and David Hindle (University of Gottingen), Miguel Merino, Reece Elling, Molly Gallahue, Suzan van der Lee and Donna Jurdy (Northwestern), Douglas Wiens, Michael Wysession, Ghassan Al-Equabi, and Weisen Shen (Washington University in St. Louis), Dave Malone (Illinois State University), Gregory Waite, Erika Vye, and Bill Rose (Michigan Technological University), Tyrone Rooney (Michigan State), Andrew Frederiksen (University of Manitoba), Fiona Darbyshire (University of Quebec), Eric Brown (Aarhaus University), Rob Moucha (Syracuse University), and many others.