GEOPHYSICAL INSIGHTS INTO A PALEOPROTEROZOIC WILSON CYCLE ALONG THE SOUTHERN MARGIN OF THE SUPERIOR PROVINCE, CENTRAL UPPER PENINSULA, MICHIGAN

The southern margin of the Archean Superior Province in the Upper Peninsula of Michigan was a nexus for Paleoproterozoic tectonic events involved in the breakup of proposed supercraton Superia and subsequent assembly of Laurentia. A Wilson cycle began with ~2.1 Ga rifting, proposed to involve separation of the current Wyoming Province from the Superior Province, and was completed with the ~1.87-1.83 Ga Penokean orogeny that accreted the Wisconsin magmatic terranes to the southern margin of the Superior Province. The Penokean orogeny was among the earliest Paleoproterozoic orogenies that progressively assembled Laurentia. These events and later overprints left behind geologic effects that are readily mappable with aeromagnetic and gravity geophysical methods through glacial and Paleozoic sedimentary cover, allowing development of a richer perspective on all these events, and permitting projection of features from areas of exposure to areas of cover. Direct evidence and derived interpretations for ~2.1 Ga rifting is expressed as ~2.1 Ga granitic magmatism; the Dickinson Group, a major tectonic element in the region is perhaps a unique record of the progression of rift-related sedimentation and magmatism; metamorphism and unroofing of lower crustal rocks; proposed tilting of Archean rocks along a continentward-dipping listric normal fault; and northeast-striking dikes cutting Archean rocks that are plausibly related to ~2.1 Ga plume magmatism. The Penokean orogeny is expressed locally as formation of the Niagara fault zone suture, thin-skinned faulting in the Menominee iron district, and proposed emplacement of an allochthonous Calumet trough. Numerous east-west trending structures are proposed to have originated, or been significantly reactivated by, post-Penokean deformation during ~1.76 Ga and ~1.65 Ga metamorphic events that may correspond to the proposed Yavapai and Mazatzal orogenies. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is a major structure in the region, and is speculatively related to ~1.76 Ga thick-skinned deformation. Oblique disruptions of east-west striking structures have strong geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga.