Midcontinent Rift extension ceased and the rift inverted due to the Grenvillian orogeny

Nicholas Swanson-Hysell, University of Minnesota
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Abstract

The cessation of rifting within the Midcontinent Rift was a key event in the evolution of the Lake Superior region. Had rifting continued to form an ocean basin, the subsequent geologic and paleogeographic history would have been profoundly different. Using emerging geochronology from the Midcontinent Rift and the Grenville orogen, Cannon (1994) concluded that closure of the rift was a far-field effect of compression associated with the Grenvillian orogeny. An alternative was proposed by Stein et al. (2014), who interpreted the rift as an abandoned segment associated with successful rifting along Laurentia's margin. Here we revisit this question by leveraging improved chronostratigraphy within the volcanic and sedimentary rocks of the Midcontinent Rift (e.g. Fairchild et al., 2017; Hodgin et al., 2024) together with rich records of metamorphic chronology associated with the Grenvillian orogeny.

The transition from active rift extension to post-rift thermal subsidence is recorded by the Brownstone Falls angular unconformity in northern Wisconsin. The thinning of the Copper Harbor Conglomerate from >2,200 m thick on the Keweenaw Peninsula to pinching out against the unconformity implies topographic relief at the onset of post-rift sedimentation comparable to the modern-day East African rift. The end of active extension (ca. 1090 to 1085 Ma) is coincident with early prograde Grenvillian metamorphism, whose imprint extends from the Blue Ridge inliers up through the Grenville Province of eastern Canada—consistent with the onset of continent-continent collision driving the cessation of extension. Following the end of extension, deposition of the Oronto Group continued until ca. 1045 Ma (Hodgin et al., 2024; Fuentes et al., 2025) through post-rift thermal subsidence. Paleomagnetic records from the Oronto Group, including data from the Nonesuch Formation (Slotznick et al., 2024) and new data from the upper Freda Formation, reveal that Laurentia's plate motion dramatically slowed coincident with the onset of Grenvillian orogenesis, consistent with ocean basin closure and continent-continent collision changing the force balance on the plate.

Oronto Group deposition ended as Grenvillian contractional deformation propagated into the Midcontinent in two phases: major exhumation during the peak Ottawan phase and a more minor ca. 1000 to 980 Ma Rigolet phase, the latter associated with ca. 990 Ma deposition of the Jacobsville-Bayfield Group (Hodgin et al., 2022; Alemu et al., 2023). Following 130 Myr of tectonic excitement from ca. 1110 to 980 Ma, stability returned to Laurentia's Midcontinent, with only minor tectonism since.

Session

Large-scale tectonics