Evidence for multiple deformation during the Penokean Orogeny in the Middle Precambrian Thomson Formation, Minnesota

Abstract

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.

Tectonic imbrication and foredeep development in the Penokean orogen, east-central Minnesota: An interpretation based on regional geophysics and the results of test-drilling. The Penokean orogeny in Minnesota and upper Michigan; a comparison of structural geology

“…The Animikie and Nimrod probably were once joined. Long-dashed line demarks northern limit of recumbent deformational style as mapped by Hoist (1982Hoist ( , 1984 and extended in this study.…”

“…(See Holm and others, 1988, for discussion.) The discontinuities are inferred to dip mainly toward the south, consistent with the sense of northward overturning of folds in the region (Schmidt, 1963;Morey, 1983;Hoist, 1982Hoist, , 1984Hoist, , 1985, although local reversals of dip are likely. The Werner deconvolution studies of Ferderer (1988a, b) indirectly confirm the general southward (or southeastward) dip; south-dipping slabs or lithologic contacts give the best calculated fits for most of the aeromagnetic profiles analyzed.…”

“…Structural studies by Hoist (1982Hoist ( ,1984Hoist ( ,1985 and Holm and others (1988) have shown conclusively that the southeastern part of the Penokean orogen in Minnesota has undergone an early phase of deformation that was of recumbent style, and that the early, subhorizontal structures of that phase were refolded into upright second-generation folds. These observations are consistent with the presence of nappe structures in the southern terranes, and Hoist (1984) has postulated that the brow of a major north-verging nappe lies about 2 km south of Carlton, at a position shown by the long-dashed line on figure 2.…”

“…In addition, they contain abundant metapelite, metasiltite, and graphitic argillite, many thin, lensoidal units of iron-formation (Morey and Morey, 1986), and poorly known amounts of quartzite and related arenaceous rocks. All these rocks are closely folded and cleaved, and locally show evidence of multiple fold generations (Hoist, 1982(Hoist, , 1984(Hoist, , 1985. Taken as a whole, the Moose Lake-Glen Township and Cuyuna South range terranes have the attributes of a medial tectonic zone dominated by fold-and-thrust deformation.…”

“…Folding and cleavage are lacking along the Mesabi iron range; broad, open folds with incipient slaty cleavage appear about 15 km south of the range ( fig. 2) and folding becomes progressively tighter south of there (Southwick, 1987;Hoist, 1982Hoist, , 1984.…”