Two major phases of early Proterozoic Penokean folding are now recognized in the Thomson Formation of east-central Minnesota. Effects of the first deformation are found only in the southern two-thirds of the exposed Thomson Formation; the second deformation affected the entire region of outcrop. Several lines of evidence indicate that northward-directed nappes developed during the early phase of folding. This evidence includes lithologic differences between the areas of one and two deformations, the pervasive nature of an S1 foliation in the area of its occurrence, facing directions of F2 folds, and the refraction pattern of the S1 foliation in graded metagraywacke-slate beds. Taken together, the data show that an area of several hundred square kilometres is on the upper limb or limbs of a large, northward-directed recumbent fold or folds (nappes) of the early phase of Penokean deformation.

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