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The Penokean orogen of Michigan's Upper Peninsula includes a belt of dome-and-keel structure presently defined by deep troughs, or “keels,” of Paleoproterozoic Marquette Range Supergroup strata between gneiss domes composed of Archean basement rock. Structural, metamorphic, and geochronological data from the Southern Complex indicates that dome-and-keel structure developed in two stages. The first stage involved rise (intrusion, possibly diapirically) of the 2.6 Ga Bell Creek Assemblage (a gneissic megacrystic granite) into the Twin Lake Assemblage (migmatitic mafic to felsic gneiss). Flow folding in gneisses and migmatites indicate that this Archean event involved plastic flow of basement. The second stage occurred after the ca. 1.8 Ga Penokean orogeny, subsequent to the formation of a fold-thrust belt involving Paleoproterozoic Marquette Range Supergroup strata. During this stage, deep, narrow troughs developed in the region that had been the fold-thrust belt. Analysis of structures bordering the Republic Trough indicates that Paleoproterozoic keel borders are shear zones; keel rocks moved down relative to dome rocks. In effect, the Paleoproterozoic keels are steep- to vertical-sided grabens, suggesting that the dome-and-keel architecture is a consequence of extensional faulting. Amphibolite facies metamorphism occurred in Paleoproterozoic keel strata along dome-keel borders. Peak-metamorphism developed adjacent to dome borders at the time keel-bounding shear zones were active. The relative timing of Paleoproterozoic keel formation supports the model that this stage reflects collapse of the Penokean orogen. Our results show that the present dome-and-keel structure of the Southern Complex region represents superposition of Paleoproterozoic collapse structures on preexisting Archean gneiss domes.

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