A Superior Swath: Proterozoic Geology of the North American Midcontinent

Over fifty years of geologic investigations in the Upper Great Lakes region of southern Laurentia reveal a dynamic tectonic history of recurring Paleoproterozoic rifting and juvenile-crust accretionary events, and a final transformation to stable, cratonic lithosphere. A modern and revised understanding of the geologic evolution of this important Proterozoic margin is reported in a Precambrian Research special volume (in press), including a new regional tectonic province map that is based primarily on the integration of aeromagnetic and geochronologic data. Contrary to earlier models, ca. 1870-1830 Ma Penokean-interval crust is primarily constrained to northern Wisconsin, in a ~2100 Ma tectonic embayment of the rift-margin of the Archean Superior craton. The newly recognized Spirit Lake tectonic zone, a sharp magnetic discontinuity that marks the southern limit of Archean and Penokean-interval rocks, likely represents the eastward continuation of the Cheyenne belt suture zone in southern Wyoming. South of this boundary, 1800-1760 Ma Yavapai-interval rocks form the basement upon which 1750 Ma rhyolite and quartzite sequences were deposited. Yavapai- interval suturing was simultaneous with exhumation of a corridor of Archean-cored gneiss domes and significant magmatism and metamorphism along the main Penokean suture to the north. Much of the newly accreted Proterozoic terrane was strongly deformed during the 1650-1630 Ma Mazatzal orogeny, the dominant effect of which is represented by south-verging folds in 1700 Ma red quartzites; metamorphism is overall less intense and magmatism is absent. The northern boundary of Mazatzal terrane is obscured by abundant 1470- 1430 Ma "anorogenic" plutons that stitched the suture. These data reveal a successive accretion of juvenile arc terranes along the southern margin of Laurentia from ca. 1900-1600 Ma and an overall waning of Proterozoic metamorphism in this region. The final significant crustal modification represented in the area is the Mesoproterozoic Midcontinent Rift System, a mafic-dominated continental rift that lasted 15 m.y. and produced pronounced geophysical anomalies but only a weak thermal overprinting. This series of events reflects a changing tectonic environment from active margin to continental interior, as Laurentia grew southward and stabilized during the Proterozoic. In comparison to the western U.S., little tectonism has occurred here in the last one billion years, providing a uniquely unaltered perspective into the Precambrian evolution of the continental U.S. lithosphere.