In-person Lecture: A Midcontinent Rift nearly split N.Amer. apart—& then stopped

Presenter: Nick Swanson-Hysell, Ph.D., Associate Professor, Earth & Environmental Sciences, University of Minnesota

Link to Presenter: https://cse.umn.edu/esci/nicholas-swanson-hysell

Full Title of Lecture: How the Midcontinent Rift nearly split ancient North America apart 1.1 billion years ago—and why it stopped

Summary: The cessation of rifting within the 1.1 billion-year-old Midcontinent Rift was a key event in the evolution of Minnesota and the Lake Superior region—if rifting had continued and led to the formation of an ocean basin, the subsequent geologic and paleogeographic history of our region would have been profoundly different. We have been able to use new age constraints to test the hypothesis that the rift ended due to the continent-continent collision of the Grenvillian orogeny. The transition from active extension to post-rift subsidence is recorded by the Brownstone Falls angular unconformity in northern Wisconsin, with thickness variations in the Copper Harbor Conglomerate implying topographic relief comparable to the modern East African rift. The end of active extension (ca. 1090–1085 Ma) coincides with the onset of Grenvillian metamorphism, consistent with continent-continent collision causing rifting to cease. Post-rift sedimentation of the Oronto Group continued until ca. 1045 Ma, and paleomagnetic data from these rocks reveal that Laurentia’s plate motion dramatically slowed as collision progressed and changed the force balance on the plate. Sedimentation ended when contractional deformation from the Grenvillian orogeny propagated into the continental interior in two phases: major uplift during the Ottawan phase and a secondary ca. 1000–980 Ma phase associated with the Rigolet stage. This final phase of contraction is associated with ca. 990 Ma deposition of the Jacobsville-Bayfield Group. Following 130 Myr of tectonic excitement from ca. 1110 to 980 Ma, stability returned to the midcontinent region. While the comings and goings of inland seas and the occasional impact crater have left their mark on the geological record, there has been only very minor tectonism over the past billion years.

This lecture will be recorded for GSM's YouTube Channel.

Biography: Nick Swanson-Hysell is an Associate Professor of Earth and Environmental Sciences at the University of Minnesota. He received his B.A. in Geology from Carleton College, his Ph.D. from Princeton University, and was an NSF Postdoctoral Fellow at the University of Minnesota before joining the faculty at UC Berkeley where he spent over a decade before returning to Minnesota in 2024. His research integrates paleomagnetic, rock magnetic, and geochemical data within rigorous geologic contexts to constrain the long-term evolution of Earth, including the paleogeographic history of continents, major climate transitions, and the behavior of Earth’s magnetic field. He has a long-term research interest in using the geology of the Lake Superior region to advance understanding of the ancient Earth. His research group includes undergraduate students, graduate students, and postdoctoral researchers who benefit from close affiliation with the Institute for Rock Magnetism and its state-of-the-art instrumentation.

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