MID-CONTINENT RIFT: RIFT, LIP, OR BOTH AND IMPLICATIONS FOR RODINIA RECONSTRUCTIONS

The Midcontinent Rift (MCR) is traditionally considered to have formed by midplate extension and volcanism ~1.1 Ga that ended due to compression from the Grenville orogeny, the assembly of Amazonia (Precambrian NE South America), Laurentia (Precambrian North America), and other continents into the supercontinent of Rodinia. Reappraisal of age and paleomagnetic data suggests instead that it formed as part of the rifting of Amazonia from Laurentia 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 MCR’s extension to northwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent identification of contemporaneous large igneous provinces in Amazonia. The history of sedimentation and faulting in the Lake Superior region shows that rifting and volcanism ended long before compression. There is no evidence of compression before volcanism ended or in the following ~25 million years as ~6 km thick post-rift sediments were deposited in a subsiding basin. The earliest unambiguous structural evidence for compression is faulting and folding of the Jacobsville/Bayfield sandstones. Paleomagnetic and recent detrital zircon data indicate that these units are much younger than previously thought, ~760 - ~615 Ma or ~565 - ~542 Ma, so much of the deformation previously thought to have occurred at ~1.06 ± 0.020 Ga is probably post-Grenville. The MCR has aspects both of a continental rift - a segmented linear depression filled with sedimentary and igneous rocks - and a large igneous province (LIP) - an enormous volume of flood basalts. Comparison of areas and volumes for a range of continental LIPS shows that the MCR volcanic rocks are significantly thicker than in other LIPs. The MCR flood basalts have steeper dips and thicker overlying sediments than other continental flood basalts, and were deposited in a bowl-shaped subsiding basin after most extension ended, indicating that they are better viewed as post-rift than syn-rift rocks. Hence we view the MCR as a LIP deposited in crust weakened by rifting, and thus first a rift and then a LIP.