U.S. Geological Survey Bulletin
Gene L. LaBerge ¡ David Leroy Southwick ¡ J. S. Klasner ¡ J. Warren Beck ¡ Karin M. Barovich ¡ Paul Kibler Sims ¡ Rodney C. Johnson ¡ John Patchett ¡ K. J. Schulz ¡ R. W. Ojakangas ¡ Theodore J. Bornhorst ¡ V. Rama Murthy ¡ Ed DeWitt ¡ Paul E. Myers ¡ Zell E. Peterman ¡ Bruce Brasaemle
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The Felch and Calumet troughs area of northern Michigan is part of the Penokean fold-thrust belt of the continental foreland of the Superior craton. The area lies immediately north of the Niagara fault zone, the north-verging suture between the continental foreland and the Early Proterozoic Wisconsin magmatic terranes to the south. Accretion of the magmatic terranes to the continental margin approximately equals 1,850 Ma produced south-verging backthrusting and backfolding in this region involving both Archean basement and Early Proterozoic supracrustal strata. Evidence for backthrusting exists throughoutthe Felch and Calumet troughs area. The backthrusting is characterized by southward-overturned bedding and small-scale, south-verging asymmetric folds with a subhorizontal axial-planar foliation. The Carney Lake Archean block appears to be a crystalline-core nappe wherein the Sturgeon Quartzite of the Chocolay Group forms the lower overturned limb. The deformation probably started as a north-verging foreland thrust event, but out-of- sequence south-verging backthrusts and backfolds developed to accommodate abrupt changes in crustal thickness along the continental margin. The backthrusting in the Penokean orogen resembles that in the younger rocks of the southern Alps. Proceeding inward from the continental margin, both orogens have accreted oceanic crust, indicated by the presence of ophiolite, that is thrust onto the continental margin; a zone of thick-skinned complex deformation characterized by backthrusting and backfolding; a marginal basement arch; and, inboard of the arch, a fold-thrust belt that mainly involves thin-skinned deformation.
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