THE 1.1 GA MIDCONTINENT RIFT IN THE LAKE SUPERIOR REGION

Lake Superior lies at the junction of three arms of the Midcontitnent Rift. The southwestern arm extends 1,000 km from Lake Superior to Kansas. The southeastern arm extends 400 km to southern Michigan and the failed Nipigon arm extends 200 km to the north. The rift developed between 1108 and 1094 Ma. Older crust beneath the lake was nearly totally separated and replaced by a volcanic section, mostly flood basalts as much as 25 km thick, and 10 km of overlying clastic sediments deposited in a successor basin. Accompanying volcanism, great volumes of mafic magma, most notably the Duluth Complex in Minnesota, were intruded into the growing volcanic pile and older rocks. Regional diabase dike swarms and large gabbro and diabase sheets, such as the Nipigon sills in Ontario, were also intruded. The enormous volume of basaltic magma that erupted in a geologically short period and the chemical and isotopic composition of the igneous rocks suggest that the rifting and volcanism were caused by the arrival of a new mantle plume beneath the present Lake Superior basin. Crustal underplating by mafic magma produced anomalously thick crust, still as much as 55 km thick beneath the Lake Superior region. The rift was partly inverted between 1060 and 1040 Ma when the central graben was uplifted along major thrust faults such as the Keweenaw fault in Michigan and Isle Royale fault in northern Lake Superior. Tectonic, magmatic, and thermal events during rifting resulted in a diverse suite of mineral resources. These include the classic native copper deposits of the Keweenaw Peninsula of Michigan, large sedimentary copper deposits such as the White Pine deposit, and magmatic Cu-Ni-PGE deposits, such as the very large deposits of the Duluth Complex and others in smaller intrusions. The results of the rifting have a strong influence on the present and future Lake Superior whose present basin was created mostly by glacial erosion of sedimentary rocks along the rift axis. Continued isostatic uplift of the region, a result of overthickened crust, creates a remarkably small watershed for the lake. A wealth of mineral deposits in the rift has influenced human activity in the region. These deposits continue to be actively explored with new mines being developed and planned, which will require diligence in avoiding potential adverse effects on future Lake Superior.