Evidence for nappe development during the early Proterozoic Penokean orogeny, Minnesota

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.