Nature and style of deformation in the foreland of the early Proterozoic Penokean orogen, northern Michigan

Evidence for Early Proterozoic north-verging folding and thrusting in the continental foreland of the Penokean orogen is present in two widely separated areas in northern Michigan. In the eastern part of the exposed orogen, asymmetric to over-turned folds in the Early Proterozoic Michigamme Formation suggest an initial (D₁) phase of possibly north verging thin-skinned deformation. A second phase (D₂ ) is characterized by a more thick skinned deformation consisting of northward thrusting of Archean gneiss and overlying Early Proterozoic quartzite along ductile thrust faults. Crosscutting shear zones indicate a third phase (D₃ ) of deformation involving east- northeast thrusting of Archean gneiss. Thick-skinned deformation D₂ and D₃ phases include spaced fracture cleavage, kink folds, and lineations in D₁, foliation surfaces. Near the west end of the exposed orogen in Michigan, bedding-cleavage relationships in the Early Proterozoic Tyler Formation also indicate northward tectonic transport. These structures, which are interpreted as D₁ lie along the north edge of the Archean Puritan batholith and associated Archean greenstone. The north-verging structures in the northern Michigan segment of the Penokean orogen were formed approximately 1,850 Ma during collision of the Wisconsin magmatic terranes with the continental foreland of Michigan, Wisconsin, and Minnesota.