Abstract

Geologic studies of Precambrian rocks in northern Michigan indicate that the Penokean orogeny, which occurred about 1.9 b.y. ago, consisted of four stages of deformation, three of which occurred during a prolonged period of regional metamorphism. The first two stages of deformation were possibly caused by gravity sliding northward off an ancestral Penokean range located in central Wisconsin. The deformation probably started while the sediments were still soft, and it produced a pervasive west-northwest–trending foliation in the middle Precambrian rocks. The third and fourth stages of deformation were caused by uplift of rigid blocks of lower Precambrian basement rocks; this uplift produced prominent grabens such as the Marquette and Republic troughs. Metamorphism began very early in the deformational sequence, peaked during the third stage of deformation, and ended in a period of retrograde metamorphism.

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