LATE PALEOPROTEROZOIC (1900-1600 MA) TECTONIC HISTORY PROPOSED FOR THE SOUTHERN LAKE SUPERIOR REGION, USA

We propose that the complex assembly of Paleoproterozoic metamorphic, structural and geochronometric patterns in the southern Lake Superior region is the result of northwest-directed convergence during and following geon 18 Penokean accretion. Oblique Penokean convergence during south-directed subduction ca. 1870 Ma resulted in volcanic filled footwall collapse basins within the south-facing continental margin of Laurentia. Midcrustal amphibolite facies metamorphism (M1) peaked around 1835 Ma and was concurrent with late Penokean plutonism in the upper crust. Post-Penokean magmatism began ca. 1800 Ma and generally migrated southeastward across the newly accreted terrane. Magmatic pulses at 1800, 1775, and 1750 Ma may correlate with northwest-directed subduction associated with southward growth of the North American mid-continent. We suggest that geon 17 Yavapai-age slab rollback of a northward subducting oceanic plate caused continental arc magmatism to step southeastward between 1800 and 1750 Ma. As the slab steepened, it would have disengaged from the overlying, overthickened crust. The reduced compressional stresses and increased thermal input allowed for collapse of the Penokean orogen. Geon 17 amphibolite facies (M2) metamorphism and further structural modification of the deformed continental margin occurred during collapse. Post-collapse crustal stabilization (the 1750-1650 Ma “Baraboo Interval”) was relatively short-lived as geon 16 Mazatzal arc accretion from the south caused widespread reheating and deformation of much of the southern Lake Superior region. The 1900 to 1600 Ma geologic history of the north-central United States is thus, not surprisingly, a well-preserved record of the southward growth and tectonic development of the southern Laurentian margin.