ALMOST AN OCEAN–THE EVOLUTION OF THE 1.1 GA MIDCONTINENT RIFT (Invited Presentation)

The Midcontinent Rift (MCR) is a 2,200 km long Mesoproterozoic rift system that transects the middle of North America, extending from Kansas north to Lake Superior and then south through Michigan to the Grenville Front. The MCR is structurally similar to intracratonic rifts such as the East African Rift with a series of basins and opposed half grabens separated by accommodation structures. However, with >2 million km³ of basalt, the evolution of the MCR is most similar to large igneous provinces formed at volcanic rifted margins like East Greenland. The large volume of basalt with enriched trace element compositions and distinctive isotopic signatures supports involvement of a mantle plume in the evolution of the MCR. The MCR evolved in two magmatic stages followed by a sedimentary stage. The first magmatic stage, during a period of magnetic reversal between ~1110 to 1105 Ma, produced extensive subaerial flood basalts in the Lake Superior region that formed a volcanic plateau ≤10 km thick. There is little evidence of rifting during this stage, although the basalt sequence thickens toward the eventual rift axis. Some basal basalts in western Lake Superior have compositions similar to recent ocean island basalts and were likely derived from >120 km depth, whereas other basal basalts have less enriched compositions and were derived from much shallower depth (<80 km). This suggests that subcrustal lithosphere was already anomalously thin locally in the Lake Superior region prior to development of the MCR. Following a short hiatus of ~3-5 Myr, the second magmatic stage took place during a period of normal magnetism between ~1102 to 1094 Ma. Large-volume subaerial tholeiitic basalts now erupted along the length of the MCR and major mafic intrusions of the Duluth Complex in Minnesota were emplaced. This stage was characterized by rapid extension, with normal faulting and graben development resulting in a rift ~100 km wide in the area of Lake Superior and about half that width in Kansas and central Michigan. At ~1094 Ma extension and magmatism began to wane, being succeeded by ~20 Myr of thermal subsidence during which clastic sediments accumulated to as much as 10 km thick along the former rift axis. By ~1060 Ma the rift responded to stresses transmitted from the adjacent Grenville orogen resulting in large scale inversion of the rift grabens.