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Early Proterozoic tectonic style in central Wisconsin

R. S. Maass
R. S. Maass
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January 01, 1983

The Penokean orogeny (1,900 to 1,800 m.y. ago) was a major early Proterozoic tectonic event in central Wisconsin, involving deformation, metamorphism, volcanism, and plutonism. Penokean isoclinal F1 folding affected both Archean and lower Proterozoic rocks, producing a penetrative axial planar foliation that dips steeply throughout the terrane, implying significant horizontal compression during the Penokean orogeny. Foliation has been complexly reoriented by open to tight F3 folding. Fold axes and penetrative mineral lineation are generally colinear, plunging steeply at all but a few localities. Regional metamorphism accompanying deformation is predominantly lower or middle amphibolite facies.

Penokean tonalite to granite plutons are synkinematic to postkinematic; the earliest plutons intruded during the late stages of F1 folding. F1, F2, and F3 folding is estimated to have occurred over the interval 1,880 to 1,810 m.y. ago. Although Archean deformation and metamorphism may have affected the terrane, no unequivocal evidence for such events has been recognized. The existence of large-scale Proterozoic shear zones in central Wisconsin is contradicted by evidence which indicates that the proposed zones consist of metavolcanic rocks rather than mylonites.

Spatial separation, differing ages, and contrasting tectonic histories suggest that the Archean gneisses of central Wisconsin do not correlate with the Archean gneisses of Minnesota and northern Michigan.

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GSA Memoirs

Early Proterozoic Geology of the Great Lakes Region

L. G. Medaris, Jr.
L. G. Medaris, Jr.
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Geological Society of America
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Publication date:
January 01, 1983




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