The Penokean Orogeny, a major event that occurred at 1850-1900Ma in the Great Lakes region, involved deformation, metamorphism, and igneous activity. In Minnesota, the effects of the Penokean deformation are most clearly seen in the Thomson Formation. Here, two main Penokean deformational events are recognized. The first involved isoclinal recumbent folds (F1) with a well developed axial-planar foliation (S1). The second deformation involved open, upright and subhorizontal folds (F2), which folded bedding (So) and the first foliation (S,), and refolded the early folds. A crenulation cleavage (S2) developed axial planar to the Fz folds. Post-Penokean structural features include joints, kink bands, and basalt dikes.Evidence for the first Penokean deformation is found in the southern region of Thomson Formation outcrop, whereas the features associated with the second Penokean deformation are found throughout its outcrop area. Thus, the only Penokean-age structural features seen at the type locality of the Thomson Formation are from the second Penokean deformation.L'orogtnbse du P6nok6en constitue un Cvtnement trks important dans la r6gion des Grands Lacs, elle s'est produite entre 1850 et 1900Ma et elle fut accompagn6e de dtformation, de mCtarnorphisme et d'activit6 ignCe. Au Minnesota, les effets de la dBformation p 6 n o k 6 e~e sont facilement observables dans la formation de Thompson. Nous reconnaissons ici deux phases principales de d6formation pknokknne. La premikre s'est manifestke par la formation de plis couch6s isoclinaux (FI) avec un plan de foliation axial bien dtveloppk (S,). La deuxikme phase de dtformation a produit des plis ouverts, droits B subhorizontaux (F2), laquelle a pliss6 les strates (So) et la premibre foliation (S1) et a repliss6 les premiers plis. Un clivage avec crknulation (Sz) s'est d6veloppk le long du plan axial des plis F2. Les d6formations structurales post-pCnok6ennes se manifestent par des diaclases, des associations de plis en chevrons et des dykes de basalte.La partie sud de I'aflleurement de la formation de Thompson montre avec Bvidence la premikre dkformation pknokbenne, tandis que les caractkristiques structurales associ6es avec la deuxikme d6formation pCnok6enne s'observent sur toute la superficie de I'affleurement. Donc, les seuls ph6nomknes structuraux d'lge p6nokCen observCs h la localit6 type de la formation de Thompson relbvent de la deuxikme dkformation pknoktenne.[Traduit par le journal]Can.
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