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A Model for the Penokean Orogeny in East-Central Minnesota

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A Model for the Penokean Orogeny in East-Central Minnesota Oblique subduction, footwall deformation, and imbrication: A model for the Penokean orogeny in east-central Minnesota DANIEL K. HOLM* | TIMOTHY B. HOLST \ Department of Geology, University of Minnesota Duluth, Duluth, Minnesota 55812 MICHAEL ELLIS* ) ABSTRACT during uplift associated with continued com- which they called the "Great Lakes tectonic pression and isostatic rebound. Later-formed zone" (Sims and others, 1980). They noted that The Penokean orogeny was a major early structures associated with imbrication and rocks which overlie the granite-greenstone ter- Proterozoic (1875-1825 Ma) tectonic event deformation within the hanging wall consist rane (Animikie Group) are less deformed and in the Great Lakes region. In east-central of folding of the foliation and development of metamorphosed than those which overlie the Minnesota, it is marked by multiply deformed shear zones in the McGrath Gneiss and open Great Lakes tectonic zone and gneissic terrane. and highly metamorphosed supracrustal to close, upright-to-overturned folds in the Because of this, these authors suggested that this rocks of the early Proterozoic Denham and Denham and Thomson Formations. The peak early Precambrian boundary acted as a locus for Thomson Formations. Structural features metamorphic event (represented by stauro- limited intracontinental tectonic movement and similar to those in the supracrustal rocks also lite) occurred after the later deformation at rising geothermal gradients (Morey, 1970). exist in the basement Archean (2700 Ma) temperatures of about 470-530 °C and a min- Recent structural investigations in the early McGrath Gneiss. Such features are here ex- imum pressure of 3.4 kbar (minimum depth of Proterozoic Thomson Formation in east-central plained in a tectonic model consisting of 12.4 km). Increasing temperature associated Minnesota (Hoist, 1982,1984c) reveal evidence southward-directed oblique subduction along with decreasing pressure (uplift) is explained for multiple deformation and document the ex- the Great Lakes tectonic zone. Intense de- by conductive relaxation caused by crustal istence of northward-directed nappes during the formation occurred in the footwall of the thickening and erosion. This tectonic model Penokean orogeny. One of the possible models major thrust, which marked the boundary may have more widespread implications for of nappe emplacement in Minnesota would be between downgoing and overriding plates explaining similar structural features found in gravity gliding off a rising diapir, following the during A-type (continental) subduction. Sedi- many Precambrian terranes worldwide. suggestions of Morey (1979) and Sims and oth- mentary rocks (Thomson Formation) depos- ers (1980) for intracratonic deformation. The ited on the footwall during loading caused by INTRODUCTION high strains associated with nappe emplacement thrusting eventually became incorporated (Hoist, 1985a), however, do not support this into the deformation zone. Early-formed The Penokean orogeny, which occurred near idea (see the values of strain above a rising dia- structures related to footwall deformation are the close of early Proterozoic time (1875-1825 pir in Dixon, 1975). Such strains are more con- a dominantly well-developed foliation in the Ma; Van Schmus, 1976, 1980, 1981) involved sistent with a plate-tectonic model (Hoist, gneiss and isoclinal, recumbent folds with a deformation and metamorphism of rocks in 1985a, 1985b). The presence of nappes has also bedding-subparallel foliation in the Denham Minnesota, Wisconsin, upper Michigan, and the recently been reported farther to the east in the and lower Thomson Formations. Progressive Superior, Southern, and Grenville provinces in Penokean orogenic belt (Sims and others, 1987), metamorphism during subduction reached Canada (Hoist, 1982; Maass and others, 1980; and Penokean volcanic rocks have been shown the garnet zone of the amphibolite facies. Var- Cannon, 1973; Brocoum and Dalziel, 1974). to be of island-arc affinity (Schulz and others, ious deformation inversions show that this Until recently, the Penokean orogeny in Minne- 1984), resulting in plate-tectonic syntheses for early phase of deformation involved ex- sota was usually interpreted as intracratonic several areas of the Penokean orogenic belt. treme flattening (with Z vertical) and large (Morey and Sims, 1976; Sims, 1976; Sims and The growing body of structural and petro- amounts of extension in both the north-south others, 1980) with an emphasis on the role of logic evidence from Wisconsin (LaBerge and and east-west directions. basement rock involving "vertical remobiliza- others, 1984; Sims and others, 1985) and upper Footwall deformation was followed by im- tion" (Morey, 1979). Of fundamental impor- Michigan (Cambray, 1977, 1978) is consistent brication and accretion onto the hanging wall tance in this interpretation is the boundary, with the structural evidence in east-central Min- in west-central Minnesota, between an ancient nesota for a convergent plate-boundary model *Present addresses: (Holm) Department of Earth (in part, 3550 Ma) gneissic terrane and a (Hoist, 1984a, 1984b). It seems from recent and Planetary Sciences, Harvard University, 24 Ox- younger (ca. 2700 Ma) granite-greenstone ter- publications that there is large agreement in ford St., Cambridge, Massachusetts 02138; (Ellis) rane (Fig. 1). Morey and Sims (1976) suggested favor of a plate-tectonic model for the Penokean Center for Neotectonic Studies, Mackay School of that this boundary is part of a major Precam- Mines, University of Nevada Reno, Reno, Nevada orogeny, although details are far from clear and 89557. brian crustal feature more than 1,200 km long complete. In this paper, we describe the struc- Geological Society of America Bulletin, v. 100, p. 1811-1818, 5 figs., 1 table, November 1988. 1811 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/11/1811/3380093/i0016-7606-100-11-1811.pdf by guest on 30 September 2021 1812 HOLM AND OTHERS southern part of the basin, particularly over the Great Lakes tectonic zone and the gneissic ter- rane. Ojakangas (1983) has inferred the extent of the Animikie basin on the basis of sedimento- logical and lithological similarities in Minnesota, Wisconsin, and Michigan. Because rocks of the Mid-continent Rift system (middle Proterozoic igneous and sedimentary rocks, Fig. 1) separate the Animikie basin into two physically isolated segments, the strata in the northwestern segment are assigned to the Animikie and Mille Lacs Groups, whereas those in the southeastern seg- Middle Proterozoic ment are assigned to the Marquette Range Su- igneous and sedimentary rocks pergroup. Correlations have been made among Early Proterozoic the early Proterozoic bedded rocks in Minne- ,vi intrusive rocks sota, Michigan, and Wisconsin (Morey, 1983). LA Early Proterozoic The rocks of east-central Minnesota studied ipn here include the early Proterozoic Thomson with iron formation Formation (Animikie Group), the early Proter- Archean granite- greenstone terrane ozoic Denham Formation (Mille Lacs Group), and the Archean McGrath Gneiss (terrane 1 Archean gneissic terrane above). These are part of the supracrustal se- quence and gneiss terrane outlined in Figure 1. 100 km . DESCRIPTIVE STRUCTURAL GEOLOGY The early Proterozoic Thomson Formation consists of a thick sequence of interbedded slate, slaty graywacke, and metagraywacke that strati- graphically overlies the Denham Formation. Figure 1. Generalized geologic map of the Precambrian geology of Minnesota (after Sims The southern two-thirds of the Thomson For- and others, 1980; Morey and others, 1982) with study area shown. GLTZ is the Great Lakes mation has a pervasive, nearly bedding-parallel tectonic zone. foliation. It ranges from a slaty cleavage in the north to a schistosity in the south near the Den- ham Formation (Hoist, 1982). Strain analysis by tural and metamorphic features of both base- part >3550 Ma), and a central sheared, schistose Hoist (1985a) has firmly established the tectonic ment and cover rocks in east-central Minnesota segment (Morey, 1978) also known as the Great nature of the foliation in the Thomson Forma- that were deformed during the Penokean orog- Lakes tectonic zone (Sims and others, 1980). tion. Also present in the southern terrane of the eny, From this we infer a tectonic history and Along this zone, granitic plutons (2600 Ma, Thomson Formation are isoclinal recumbent present a plate-tectonic model (not inconsistent Sims and others, 1980) acted as a weld between folds in scales ranging from centimeters to with that given by Sims and Peterman, 1983, for the northern and southern segments forming a kilometers (nappes), with east-west fold axes. the entire Lake Superior region) which best ex- relatively stable craton by the end of the Ar- The entire area of exposed Thomson Formation plains these features. chean (Morey, 1978). has been folded into gentle-to-open upright Sedimentation into a large basin on this folds. Fold axial surfaces strike east-west and GEOLOGIC SETTING craton, the Animikie basin, began at ca. 2100 fold axes have horizontal to gentle plunges either Ma (Van Schmus, 1976). Depositional patterns east or west. In the southern terrane, these up- The Precambrian rocks of east-central Minne- reflect contrasting tectonic conditions in the right folds (F2) refold the earlier isoclinal re- sota can be divided into four distinct terranes northern and southern segments of the Animikie cumbent folds (Fl). A well-developed
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