Oblique subduction, footwall deformation, and imbrication: A model for the Penokean 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- (1875-1825 Ma) tectonic event deformation within the hanging wall consist rane () 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, , upper , 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 (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 region) which best ex- relatively stable 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 axial- (Fig. 1): (1) Archean rocks, (2) early Protero- basin. A relatively thin succession (2-3 km) of planar cleavage, vertical or dipping steeply to zoic stratified rocks, (3) early Proterozoic predominantly sedimentary rocks (labeled the the south, is present in the slaty beds in the plutonic rocks, and (4) middle Proterozoic Animikie Group; Keighin and others, 1972) was north. In the south, this foliation is a well- (Keweenawan) sedimentary and volcanic rocks deposited north of the northern front of the developed crenulation cleavage, axial planar to (Morey, 1978). A stratigraphic column for east- Great Lakes tectonic zone, whereas a much F2 folds. Detailed mapping has allowed a central Minnesota and detailed descriptions of thicker and more heterogeneous succession boundary to be drawn between the area of a the different rock types have been presented by (>6 km) of sedimentary and volcanic rocks single Penokean deformation in the north and Morey (1978,1979). (Animikie and Mille Lacs Groups; Morey, the area of two Penokean-aged deformations in the south. This was interpreted by Hoist (1984c) The Archean terrane consists of a northern 1978) was deposited south of this front (Morey, as a nappe front because of the abrupt nature of granite-greenstone belt terrane (ca. 2700 Ma), a 1983; Morey and South wick, 1984). Appar- the change across this structural boundary, and southern highly deformed gneissic terrane (in ently, subsidence was relatively greater in the

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 MODEL FOR PENOKEAN OROGENY, MINNESOTA 1813

Figure 2. Equal-area projections of: A. 75 poles to bed- ding in the Denham Formation, contours at 2%, 8%, and 12% of data per 1% area. B. 133 poles to axial surfaces in the Denham Formation, contours at 1%, 3%, and 8% of data per 1% area. C. 75 mineral and crenulation lineations in the Denham Formation, contours at 1%, 5%, 7%, and 13% of data per 1% area. D. 266 poles to foliation in the McGrath Gneiss, contours at 1%, 2%, 4%, 7%, and 15% of data per 1% area.

because the refraction pattern in the early folia- tion just south of this boundary suggests that a nappe front must be located in the immediate vicinity to the north. This boundary between the two tectonostratigraphic terranes of the Thom- son Formation corresponds fairly well with a mappable break in aeromagnetic data for east- central Minnesota (Chandler and others, 1984). Underlying the southern part of the Thomson Formation is the early Proterozoic Denham Formation which has been multiply deformed and metamorphosed in a fashion similar to the of augen, but many are euhedral and oriented feldspar grains. Brittle deformation is indicated southern Thomson Formation and is here con- obliquely to the foliation, whereas still others in the coarser feldspar by fractures along which sidered part of the same nappe terrane. The have a sigmoidal shape and are aligned in the new grains have recrystallized. Simultaneous Denham Formation is a sequence of primarily foliation. Sense of shear from the sigmoidal por- ductile deformation of the finer grained feldspars quartz-rich metasedimentary rocks (metaarkose, phyroclasts (after Simpson and Schmid, 1983) is indicated by undulatory extinction, recovery, quartzite, mica schist, and garnet-staurolite indicates that the foliation developed in a and recrystallization commonly resulting in a schist) with minor amounts of marble and vol- dominantly dextral shear regime. The foliation granoblastic polygonal texture. Mica grains are canic rocks. It has been postulated to be strati- is commonly well developed, strikes east-west, relatively strain free in both basement and cover graphically equivalent to the Chocolay Group of and varies in dip from horizontal to vertical rocks, probably because of recrystallization. The the Marquette Range Supergroup in Michigan (Fig. 2D). A nearly horizontal east-west mineral textures described here indicate dominantly duc- (Larue, 1981; Morey, 1983). Bedding strikes lineation ranging from crude to locally well tile deformation under moderate to high tem- east-west and dips dominantly steeply (Fig. 2A). developed is also present. The McGrath Gneiss perature and moderate pressure conditions. A nearly bedding-parallel foliation is present is locally folded and is crosscut by nearly verti- everywhere in the Denham Formation. The cal, commonly anastomosing shear zones that METAMORPHISM foliation is refracted at a higher angle to bedding strike generally east-west. in the more competent arkosic and quartzitic Metamorphism of the early Proterozoic sedi- units. The foliation and bedding have been CONDITIONS OF DEFORMATION mentary rocks in this region increases from folded with the development, locally, of a crenu- north to south (Keighin and others, 1972). At lation cleavage. Orientations of axial surfaces to Petrographic analysis indicates that the base- the type locality near Thomson in the northern these folds vary from horizontal to vertical and ment and cover rocks have undergone similar terrane, the Thomson Formation is metamor- strike east-west (Fig. 2B). conditions of deformation related to the Peno- phosed to lower greenschist fades (chlorite The Denham Formation also contains a very kean orogeny. In all rocks analyzed, the predom- zone). Within the Thomson Formation, there is well-developed, nearly horizontal, east-west inant deformational processes appear to be a progressive increase in metamorphic grade to mineral and crenulation lineation (Fig. 2C). normal crystal-plastic type, involving dynamic lower-amphibolite facies (garnet zone) in the Chocolate-tablet boudinage of quartz veins oc- recovery and recrystallization. In both basement south (Morey, 1979). Farther south, the Den- curs parallel to bedding throughout the area. and cover rocks, quartz has undergone ductile ham Formation has been metamorphosed to the The basement to this terrane is the Archean deformation. Bending of the crystal lattice has staurolite zone of the amphibolite facies (indi- (2700 Ma) McGrath Gneiss which contains a produced undulatory extinction and, locally, de- cated by the presence of a coarse-grained schist poorly developed to well-developed foliation formation bands. Recovery and recrystallization containing quartz-muscovite-biotite-garnet-stau- and a number of crosscutting shear zones. It is a are indicated by the development of subgrains rolite). Morey (1983) has mapped the biotite, coarse-grained, pinkish-gray, biotite gneiss con- and strain-free new grains in and along the mar- garnet, and staurolite isograds for early Protero- taining megacrysts of microcline. Some of the gins of quartz aggregates. Both brittle and ductile zoic stratified rocks in Minnesota, illustrating megacrysts are rounded, giving the appearance deformation processes have occurred in the this progressive increase in metamorphic grade

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

TABLE 1. FINITE DEFORMATION ANALYSES OF THOMSON both deformations, D is the model D2 deforma- from north to south and the similarity of the FORMATION INCORPORATING VARIABLE (ASSUMED) VALUES trend of metamorphic isograds and structural OF VOLUME CHANGE, AV, EITHER FOR EARLY tion, and E0 is the finite strain of the Dt DEFORMATION ONLY (Dl) OR FOR THE LATER features (see Morey, 1983, fig. 7). DEFORMATION ONLY (D2) OR FOR BOTH DEFORMATIONS deformation. (Dl AND D2), AND RIGID-BODY ROTATION, a In order to invert the deformation (or part of Petrographic analysis reveals further informa- BASED ON INITIAL STRAIN ANALYSIS BY HOLST (1985a). tion about the timing of metamorphism and de- it) we need information about all of its compo- formation. Progressive metamorphism during Deformation condition Results nents, including rotation and volume change for the early phase of deformation in the Denham each deformation. As this information is not Formation reached the garnet zone of the am- NS% EW% Vertical % available, we have simulated a variety of "rea- 1. AV = 0, n = 0 (Hoist, 1985a) 330 275 -89 sonable" geological models (Table 1). For exam- phibolite facies as indicated by the presence of 2. D1AV = -30%, (1 = 0 270 228 -89 syntectonic garnet porphyroblasts (Fig. 3A). 3. AV = 0,!) = 30°N 271 261 -86 ple, because the second-phase folds are common- 4. D1AV = -30%, fl = 30°N 240 228 -88 The peak of metamorphism, however, occurred 5. Dl & D2AV = -30%, fi = 0 305 255 -88 ly overturned to the north, models 3, 4, and 7 6. D2AV = -30%, n = 0 348 293 -84 incorporated a rigid-body rotation toward the during or after the later deformation as indicated 7. D2AV = -30%, il = 30°N 301 293 -84 by staurolite overgrowing both the schistosity north and about a horizontal east-west axis. We and the crenulation cleavage (Fig. 3B). also have no information on volume change dur- Microprobe analysis of garnet shows only ing either of the deformational phases, but it has slight compositional variation and no systematic mations, whereas the strain in the northern ter- been shown that volume losses of up to 50% zonation (Holm, 1986a). Temperature estimates rane is that associated with only the later may occur during the formation of some slates were obtained using the garnet-biotite geother- deformation. To infer the strain of the early de- (Wright and Piatt, 1982). Thus, a number of the mometry techniques of Ferry and Spear (1978) formation in the southern terrane of the Thom- models do incorporate a volume loss for either and Perchuk and Lavrent'eva (1983) on 15 dif- son Formation, Hoist used the assumptions of Dj or D2 or both (Table 1). ferent garnet-biotite pairs. With these tech- no volume loss and coaxiality of the two defor- The results from these models (Table 1) con- niques, a peak metamorphic temperature of mations to perform a deformation inversion (un- firm the findings of Hoist (1985a) that the early about 470-530 °C was obtained for the later straining), removing the effect of the second deformation involved extreme flattening strains deformation, assuming that equilibration of deformation. He obtained a resultant strain ellip- during the development of a tectonic foliation garnet occurred then. The temperature range ob- soid for the early deformation of extreme flatten- with Z vertical and in the range -84% to -89%. tained using specific geothermometry techniques ing with Z, the minimum extension direction, In addition, it is important to note that large is consistent with the thermal stability range of vertical. This model, assuming no volume amounts of extension have occurred in both the the assemblage quartz+muscovite+staurolite change, gives a north-south extension of 330%, north-south (orogen-perpendicular) direction given by Carmichael (1978, Fig. 2, reaction 6). an east-west extension of 275%, and a vertical (minimum of 240%) and east-west (orogen- Assemblages useful for detailed geobarometry shortening of 89% (Table 1). parallel) direction (minimum of 228%). These are lacking, but a minimum pressure for the We have taken the initial strain determination data represent important constraints on any tec- peak metamorphism can be inferred from the results of Hoist (1985a) for the Thomson For- tonic model which must account for the basic mineral assemblage in the Denham Formation. mation and have applied various other deforma- structural morphology. Experimental and inferred pressure and temper- tion inversion models (Table 1) in order to From the above models of deformation inver- ature limits of staurolite occurrence by Richard- obtain other possible realistic variations in ex- sion, it is also clear that the minimum extension son (1968) give a minimum pressure estimate tension and shortening for the early deforma- direction, Z, of the strain associated with the first for staurolite coexisting with garnet and quartz tion. The models take into account a geologi- deformation in the southern terrane (the defor- of 3.4 kbar (minimum depth of 12.4 km). cally reasonable volume loss (either during the mation that involved nappe emplacement) was early deformation or the later deformation or vertical. It is likely that this is the result of shear STRAIN ANALYSIS AND both) and/or a component of rotation in the in a subhorizontal plane rather than a vertical DEFORMATION INVERSION later deformation (Table 1) (Holm, 1986a, flattening. Lateral confinement of the rocks 1986b; Holm and Ellis, 1986). makes vertical flattening unlikely, whereas large Hoist (1985a) used deformed mud chips, car- A deformation may be decomposed by: D = flattening strains are possible with moderate bonate concretions, and conglomerate clasts to Si) (Malvern, 1969; Flinn, 1979) and a progres- values of shear accompanied by volume loss

determine the strain in the multiply deformed sive deformation by: Df = DnD„_i ... D2D1 (Ramsay, 1980). Hoist (1985b) used strain es- Thomson Formation at 23 localities: 13 in the where D is the deformation gradient tensor, S timates for the early deformation in the southern northern terrane and 10 in the southern terrane. the stretch tensor, and ft is the rigid-body rota- terrane to produce models of this deformation He found the strain in the northern terrane to be tion tensor (all written in matrix form). Transla- factorized into simple shear and pure shear typical of results from many slates (Wood, tion is neglected. Volume change information is components, or simple shear and constant vol- 1974). There is a large flattening strain with the stored within the stretch tensor. ume nonpure shear components, in the manner of Coward and Kim (1981) and Sanderson minimum extension, Z, oriented horizontal and The post-deformation ellipsoid (Er) that re- north-south, perpendicular to the slaty cleavage. sults from the application of the deformation (1982). The results of that exercise showed that Maximum extension, X, is most commonly ver- gradient tensor to some predeformation state (el- shear indeed played a large role in the develop- 1 T 1 ment of the strain during nappe emplacement tical in this terrane. In the southern terrane, Z is lipsoid E„) is given by: Er = [D" ] E0[D- ]. (Hoist, 1985b), as suggested in other studies of vertical, and X is horizontal and east-west. This Similarly, E0 may be obtained by inverting the T strain within nappes (Ramsay and others, 1983; finite strain in the southern terrane is the result deformation: E0 = D ErD. In the examples de- Siddans, 1983; and Coward and Kim, 1981). of the superposition of strains from both defor- scribed here, Er represents the finite strain after

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 MODEL FOR PENOKEAN OROGENY, MINNESOTA 1813

of the continental material is significantly less than that of the oceanic material. Modern ex- amples may be the north Africa boundary in Algeria, and the western Japan Sea where un- derthrusting appears to be connected to the initi- ation of subduction (Ellis, 1988). Thrusting is interpreted to be north directed to maintain compatibility with the northward- verging nappe(s) of Hoist (1984c) and the re- gional pattern of overturned folds to the north (Sims and Peterman, 1983) and also to be con- sistent with the asymmetry of the Animikie basin, which deepens to the south (Morey, 1983), although geophysical evidence for a north-dipping thrust fault across the Great Lakes tectonic zone exists (Smithson and others, 1985). We envisage the convergence zone to com- prise a major segment of continental material on the downgoing plate (the footwall) and a major section of continental material on the overriding plate (the hanging wall), separated by a major thrust fault (Fig. 4B), such as the situation on either side of the Main Central Thrust in the Himalayas. Within each of these sections of con- Figure 3. A. Photomicrograph (plane light) of garnet porphyroblasts from the Denham tinental material, there are probably a number of Formation with internal schistosity made up of ilmenite and quartz grains. S2 foliation is minor thrust faults. In the ensuing discussion, the approximately vertical. Scale bar is 0.5 mm. B. Photomicrograph (plane light) of staurolite terms "footwall" and "hanging wall" are used to (center) overgrowing both the Si schistosity (left to right) and the S2 crenulation cleavage refer to these major sections of continental mate- (vertical) in the garnet-staurolite schist unit of the Denham Formation. Scale bar is 0.5 mm. rial on either side of the major thrust contact between them. Continued north-directed thrusting caused se- vere loading of the granite-greenstone terrane INFERRED TECTONIC HISTORY east is best explained by the existence of a rifted directly to the north and created a linear asym- passive margin (Larue and Sloss, 1980; Cam- metric basin (the Animikie basin) which acted The Denham Formation, and its correlative, bray, 1978). Sims and others (1980) suggested as a sediment trap for the deposition of the the Chocolay Group in Michigan (Larue, 1981), that rifting started soon after 2500 Ma in the Thomson Formation. Hoffman (1987) has dis- were deposited onto Archean basement rock Lake Huron region and migrated westward with cussed the development of foreland basins in the during rifting (Fig. 4A). Sims and Peterman time, beginning in the Lake Superior region Proterozoic, and identified the Animikie Group (1983) inferred that rifting occurred along the about 2100 Ma. The distribution of early Prot- as foredeep sediments deposited in front of the Great Lakes tectonic zone, a pre-existing zone of erozoic bedded rocks (Sims and Peterman, developing Penokean orogenic belt. Sediment weakness, at the beginning of the early Protero- 1983; Ojakangas, 1983) implies that rifting deposition into foreland basins reflect a transi- zoic. A reconstruction of early Proterozoic ended to the west in the Lake Superior region. tion from shelf or cratonic conditions to orog- sedimentation and volcanism in the Lake Super- Our analysis indicates that rifting was fol- eny. Such sediments consist mostly of immature ior region (Sims and Peterman, 1983) shows lowed by convergence with the initiation of a clastic debris derived either from the fold-and- that principal depocenters were along the Great southward-dipping intracontinental subduction thrust belt or from the craton. The Thomson Lakes tectonic zone and, in eastern upper Mich- zone (A-type subduction; Bally, 1981) in the Formation is the result of turbidity-current igan, in diversely oriented basins bounded by McGrath Gneiss of the southern continent. This deposition into the Animikie basin mainly from Archean rocks. Larue and Sloss (1980) delin- is suggested by the lack of a known typical the craton to the north (Morey, 1973; Morey eated the extent and discussed the style of oceanic suture zone farther to the south. and Ojakangas, 1970) but also from local Ar- basinal sedimentation during the early Protero- Initiation of subduction within continental chean highlands within the basin and on its zoic in the Lake Superior region. They contend material rather than oceanic may be expected southern flank (Peterman, 1966). that, at least in upper Michigan, sedimentary along the boundaries of relatively small ocean Deposition of the Thomson Formation oc- basin development is associated with extensional basins where gravitational instabilities (due to curred simultaneously with subduction and tectonics. Although unable to characterize with excessive sediment loading or an old, cold oce- loading until it eventually became incorporated certainty the style of Animikie basin develop- anic plate) are negligible (Ellis, 1986a, 1988). In within the zone of deformation. Hoffman (1987, ment, they noted that basin development to the such an environment, the compressive strength fig. 3) has shown diagrammatically the progres-

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

RIFTING tion, in association with a prograde metamor- phism and isoclinal recumbent folds, suggests that the relative convergence during the Peno- N DENHAM kean orogeny was oblique.

Î * » Continued underthrusting of the McGrath ineissic^;;-'' 4 Gneiss and overlying metasedimentary rocks granite- ^^-•- basemeñtjV; ~ ^ greenston was ultimately opposed by buoyancy forces due MCGRATH to the low density of the granitic material. A Moho future new zone of thrusting related to the imbrication subduction zone became active to the north, and what was pre- viously the major thrust (separating downgoing SUBDUCTION and overriding material) became part of the new hanging wall. Early structures, formed during tectonic burial, were then overprinted by a later suite of structures related to imbrication Cover Basement £ of the footwall into the hanging wall during uplift (Fig. 4C). These later structures were B caused by continued compression and isostatic Early deformation rebound of the thickened crust, and they reflect the increasing rheological and structural anisot- ropics (due to decreasing pressure) associated with crustal emergence. Tectonic surface It is important to realize in this tectonic model that the two phases of deformation are occurring simultaneously in different tectonic environ- ments. That is, early structures are forming in the footwall during subduction and progressive Later deformation metamorphism while later structures are form- Figure 4. Diagrammatic interpretation of early Proterozoic rifting (A) and collision in ing in the hanging wall. east-central Minnesota with early deformation structures forming in the footwall (B) and later Imbrication of the deformed footwall during deformation structures forming after imbrication into the hanging wall (C) of the major thrust. emergence (the later deformation) produced folding of the foliation (Fig. 2D), and east- west-striking, subvertical anastomosing shear sion of foredeep migration and incorporation north-south and east-west), however, must be zones in the basement McGrath Gneiss. In the into the actively deforming orogenic belt. In- accounted for in any tectonic model. overlying early Proterozoic rocks open to close tense deformation occurred in the footwall dur- Orogens are usually modeled as two-dimen- upright-to-overturned folds, with subhorizontal ing subduction (the early deformation), forming sional phenomena comprising normally directed east-west fold axes, and variably dipping, east- a dominantly well-developed east-west foliation subduction and collision involving kinematics west-striking axial surfaces developed. Orienta- in the McGrath Gneiss and isoclinal, recumbent invariant out of the plane of the cross section. A tions of folds in the Thomson Formation are folds with a bedding-subparallel foliation in the significant problem concerned with the devel- much more uniform than in the Denham For- Denham and lower Thomson Formations. opment of a collisional orogen is the observation mation (Fig. 2B). Participation of the basement Rocks located stratigraphically higher in the that many of these belts exhibit an early phase of rock during deformation, together with deeper Thomson Formation were deposited late in the deformation accompanied by a large amount of tectonic burial, may account for contrasting deformational history and therefore were not in- extension subparallel to the orogen length (Ellis, orientation of folds and differing intensity of corporated into the deformation zone during 1986b; Ellis and Watkinson, 1987). In concert metamorphism in the Denham and Thomson subduction. This explains the presence of two with this early extension is a prograde metamor- Formations. Basement shortening during defor- tectonostratigraphic terranes in the Thomson phism often up to the amphibolite facies; in mation is similar to that interpreted by Sims and Formation (mentioned above) with highly de- other words, the early deformation probably oc- Peterman (1983) for the basement deformation formed rocks to the south and less-deformed curs during significant tectonic burial. The origin in northeastern Wisconsin. rocks to the north (Hoist, 1984c). of this extension may be connected to oblique Peak metamorphic conditions associated with The east-west subhorizontal mineral lineation A-type (continental) subduction. Deformation uplift during the later deformation are thought in the McGrath Gneiss and Denham Formation in the downgoing footwall produces a major ex- to be due to conductive relaxation caused by is most readily explained by superposition of the tension and associated isoclinal folds that are crustal thickening and erosion (England and two deformations and cannot be used as evi- subparallel to the length of the belt (Ellis and Richardson, 1977; England and Thompson, dence of the maximum extension direction for Watkinson, 1987). The large amount of orogen 1984). If a continental crust is thickened by the orogeny (Holm, 1986b). The strains inferred parallel extension (at least 228% and perhaps up overthrusting or obduction, erosion of this above for the early deformation (extension both to 293%; see Table 1) in the Thomson Forma- thickened crust will effect instantaneous pressure

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 MODEL FOR PENOKEAN OROGENY, MINNESOTA 1813

Emergent unit: variably REFERENCES CITED oriented folds superimposed Archibald, N. J., Bettenay, L. F., Binns, R. A., Groves, D. I., and Gunthorpe, R. J., 1978, The evolution of Archean greenstone terrains, eastern Gold- fields Province, western Australia: Precambrian Research, v. 6, p. 103-131. Bally, A. W., 1981, Thoughts on the tectonics of folded belts, in McClay, K. R., and Price, N. J., eds., Thrust and nappe tecotnics: Geological Society of London Special Publication No. 9, p. 13-32. Bridgewater, D., McGregor, V. R., and Myers, J. S., 1974, A horizontal tec- tonic regime in the Archean of Greenland and its implications for early crustal thickening: Precambrian Research, v. I, p. 179-197. Brocoum, S. J., and Dalziel, I.W.D., 1974, The Sudbury Basin, the southern province, the Grenville Front, and the Penokean orogeny: Geological Society of America Bulletin, v. 85, p. 1571-1580. Cambray, F. W., 1977, Field guide to the Marquette district, Michigan: Michi- gan Basin Geological Society Annual Meeting, 62 p. 1978, as a model for the environment of deposition and deformation of the early Proterozoic (Precambrian X) of northern Michigan: Geological Society of America Abstracts with Programs, v. 10, p. 376. Cannon, W. F., 1973, The Penokean orogeny in northern Michigan, in Young, G. M., ed., Huronian stratigraphy and sedimentation: Geological Asso- ciation of Canada Special Paper, v. 12, p. 211-249. Carmichael, D. M., 1978, Metamorphic bathozones and bathograds: A meas- ure of the depth of post-metamorphic uplift and erosion on the regional scale: American Journal of Science, v. 278, no. 6, p. 769-797. Chandler, V. W., Nordstrand, E., and Anderson, S„ 1984, Shaded relief aero- magnetic anomaly map of northeastern and east-central Minnesota: Figure 5. Schematic synopsis of the multiple deformation associated with the Penokean Minnesota Geological Survey Miscellaneous Map Series, Map M-53. Coward, M. P., and Kim, J. H., 1981, Strain within thrust sheets, in McClay, orogeny in east-central Minnesota. K. R., and Price, N. J., eds., Thrust and nappe tectonics: Geological Society of London Special Publication No. 9, p. 275-292. DeWit, M., 1982, Gliding and overthrust nappe tectonics in the Barberton Greenstone Belt: Journal of Structural Geology, v. 14, p, 117-136. Dixon, J. M., 1975, Finite strain and progressive deformation in models of diapiric structures: Tectonophysics, v. 28, p. 89-124. changes on a given rock, but at any depth below McGrath Gneiss and open to close, upright-to- Ellis, M. A., 1986a, Lithospheric strength in compression: Subduction initia- a few kilometers the temperature will increase overturned folds with axial-planar foliation in tion, migration of orogeny, and an origin of exotic terranes: Geological Society of America Abstracts with Programs, v. 18, p. 594. for some time. Although the temperature in- the Denham and Thomson Formations) super- 1986b, Structural morphology and associated strain in the central Cor- dillera (British Columbia and Washington): Evidence of oblique tecton- crease which occurs is dependent on a number imposed upon these early structures are related ics: Geology, v. 14, p, 647-650. of parameters (degree of thickening, crustal con- to imbrication of the footwall into the hanging 1988, Lithospheric strength in compression: Initiation of subduction, flake tectonics, foreland migration of thrusting, and an origin of dis- ductivity, and so on), heating will continue for a wall during uplift. Increasing temperature asso- placed terranes: Journal of Geology, v. 96, p. 91-100. Ellis, M. A., and Watkinson, A. J„ 1987, Orogen-parallel extension and considerable portion of the uplift. England and ciated with decreasing pressure (uplift) is ex- oblique tectonics: The relation between stretching lineations and relative Thompson (1984) showed that over a range of plained by conductive relaxation caused by plate motions: Geology, v. 15, p. 1022-1026. England, P. C., and Richardson, S. W., 1977, The influence of erosion upon the erosion rates, the rocks do not begin to cool until crustal thickening and erosion during over- mineral fades of rocks from different metamorphic environments: Geo- logical Society of London Journal, v. 134, p. 201-213. after they have experienced 20%~40% of the thrusting. England, P. C., and Thompson, A. B„ 1984, Pressure-temperature-time paths of total uplift. This would mean that the minimum regional metamorphism I: Heat transfer during the evolution of regions It is striking that the general structural fea- of thickened continental crust: Journal of Petrology, v. 25, p. 894-928. depth estimate obtained above (12.4 km) may tures of this Precambrian terrane in Minnesota Ferry, J. M., and Spear, F. S., 1978, Experimental calibration of the partition- ing of Fe and Mg between biotite and garnet: Contributions to Mineral- be 20%-40% lower than the true depth expe- (early recumbent folds overprinted by largely ogy and Petrology, v. 66, p. 113-117, rienced by the rocks (about 15-17.5 km). Flinn, D., 1979, The deformation matrix and the deformation ellipsoid: Journal upright folds) are characteristic of many Pre- of Structural Geology, v. 1, p. 299-307. Typical post-orogenic molasse-type deposits cambrian terranes throughout the world. Similar Fripp, R.E.P., van Nierop, D. A., Callow, M. J., Lilly, P. A., and du Plessis, L. U., 1980, Deformation in part of the Archean Kaapval Craton, South are not clearly delineated anywhere in the Lake features have been suggested to exist in southern Africa: Precambrian Research, v. 13, p. 241-251. Hoffman, P. F., 1987, Early Proterozoic foredeeps, foredeep magmatism, and Superior region. Young (1983), however, noted Africa (DeWit, 1982; Fripp and others, 1980), Superior-type iron-formations of the Canadian Shield, w Kroner, A., that the general absence of these late-stage de- western Australia (Archibald and others, 1978; ed., Proterozoic lithosphere evolution: American Geophysical Union Geodynamics Series, v. 17, p. 85-98. posits may be due to later uplift and erosion Piatt, 1980), west Greenland (Bridgewater and Holm, D. K., 1986a, A structural investigation and tectonic interpretation of the Penokean orogeny: East-central Minnesota [M.S. thesis]: Duluth, during the opening of the Keweenawan rift sys- others, 1974), and Canada (Poulsen and others, Minnesota, University of Minnesota Duluth, 114 p. tem in the Lake Superior region. 1980). This paper presents a tectonic model spe- 1986b, An account of multiphase deformation during the Penokean orogeny: Evidence from analysis of the Archean McGrath Gneiss and cifically for the Penokean orogeny in east- overlying early Proterozoic metasedimentary rocks in east-central Min- central Minnesota, but its implications for other nesota: Institute on Lake Superior Geology Abstracts, v. 32, p. 32-33. SUMMARY AND IMPLICATIONS Holm, D. K., and Ellis, M. A., 1986, Basement*cover deformation during the and more widespread areas may be worthy of Penokean orogeny: East-central Minnesota: Geological Society of America Abstracts with Programs, v. 18, p. 309. Figure 5 is a schematic synopsis accounting consideration. Hoist, T. B., 1982, Evidence for multiple deformation during the Penokean orogeny in the middle Precambrian Thomson Formation, Minnesota: for the multiple deformation associated with the Canadian Journal of Earth Sciences, v. 19, p. 2043-2047. Penokean orogeny in east-central Minnesota. 1984a, Penokean tectonics: Constraints from the structural geology in ACKNOWLEDGMENTS east-central Minnesota: Institute on Lake Superior Geology Abstracts, Early-formed structures (foliation in the Mc- v. 30, p. 19. 1984b, The early Proterozoic Penokean orogeny as a convergent plate Grath Gneiss and isoclinal recumbent folds with boundary: Geological Association of Canada Program with Abstracts, v. 9, p. 75. bedding-subparallel foliation in the Denham This work was supported by National Science 1984c, Evidence for nappe development during the early Proterozoic and Thomson Formations) and progressive Foundation Grant EAR8420089 (to Hoist) and Penokean orogeny, Minnesota: Geology, v. 12, p. 135-138. 1985a, Implications of a large flattening strain for origin of a bedding- metamorphism are interpreted to be associated by a Sigma Xi Grant-in-Aid of Research (to parallel foliation in the early Proterozoic Thomson Formation, Minne- sota: Journal of Structural Geology, v. 7, p. 375-383. with footwall deformation during oblique south- Holm). Very helpful reviews by R. L. Bauer, 1985b, Strain within early Proterozoic nappes in the Penokean orogenic dipping subduction beneath the gneissic craton F. W. Cambray, and P. K. Sims are gratefully belt, Minnesota: Geological Society of America Abstracts with Pro- grams, v. 17, p. 293. to the south. Later-formed structures (folding of acknowleged, as are helpful discussions with Keighin, C. W., Morey, G. B„ and Goldich, S. S„ 1972, East-central Minnesota, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A the foliation and crosscutting shear zones in the Jim Grant. centennial volume: Minnesota Geological Survey, p. 240-255.

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 1818 HOLM AND OTHERS

LaBerge, G. L., Schulz, K. J., and Myers, P. E., 1984, The plate tectonic history Ojakangas, R. W., 1983, Tidal deposits in the early Proterozoic basin in the Sims, P. K., Peterman, Z. E., and Schulz, K. J., 1985, The Dunbar Gneiss- of north-central Wisconsin: institute on Lake Superior Geology Ab- Lake Superior region—The Palms and Pokegama Formations: Evi- granitoid dome: Implications for early Proterozoic tectonic evolution of stracts, v. 30, p. 25-27. dence for subtidal-shelf deposition of Superior-type banded iron forma- northern Wisconsin: Geological Society of America Bulletin, v. 96, Larue, D. K., 1981, The Chocolay Group, Lake Superior region, USA: Sedi- tions: Geological Society of America Memoir 160, p. 49-66. p. 1101-1112. mentologic evidence for deposition in basinal and platform settings on Perchuk, L. L., and Lavrent'eva, I. V., 1983, Experimental investigation of Sims, P. K., Peterman, Z. E., Klasner, J. S., Cannon, W. F., and Schulz, K. J., an early Proterozoic craton: Geological Society of America Bulletin, exchange equilibria in the system cordierite-garnet-biotite, in Saxena, B., 1987, Nappe development and thrust faulting in the upper Michigan Part I, v. 92, p. 417-435. ed., Kinetics in equilibrium and mineral reactions: Berlin, Heidelberg, segment of the early Proterozoic Penokean orogen: Geological Society Larue, D. K., and Sloss, L. L., 1980, Early Proterozoic sedimentary basins of New York, Springer-Verlag, p. 199-239. of America Abstracts with Programs, v. 19, p. 246. the Lake Superior region—Summary: Geological Society of America Peterman, Z. E., 1966, Rb-Sr dating of middle Precambrian metasedimentary Smithson, S. B., Pierson, W. R., Wilson, S. R., and Johnson, R. A., 1985, Bulletin, Part I, v. 91, p. 450-452. rocks of Minnesota: Geological Society of America Abstracts with Pro- Seismic reflection results from Precambrian crust, in Tobi, A. C., and Maass, R. S., Medaris, L. G., Jr., and Van Schmus, W. R., 1980, Penokean grams, p. 686. Touret, J.L.R., eds., The deep Proterozoic crust in the North Atlantic deformation in central Wisconsin, in Morey, G. B., and Hanson, G. N., Piatt, J., 1980, Archean greenstone belts: A structural test of tectonic hypothe- provinces: London, D. Reidel, p. 21-37. Selected studies of Archean gneisses and lower Proterozoic rocks, ses: Tectonophyacs, v. 65, p. 127-150. Van Schmus, W. R., 1976, Early and middle Proterozoic history of the Great southern Canadian Shield: Geological Society of America Special Paper Poulsen, K. H., Borradaile, G. J., and Kehlenbeck, M. M., 1980, An inverted Lakes area, North America: Royal Society of London Philosophical 182, p. 147-157. Archean succession at Rainy Lake, Ontario: Canadian Journal of Earth Transactions, v. 280, p. 605-628. Malvern, L. E., 1969, Introduction to the Mechanics of a Continuous Medium: Sciences, v. 17, p. 1358-1369. 1980, Chronology of igneous rocks associated with the Penokean orog- Englewood Cliffs, New Jersey, Prentice-Hall, 713 p. Ramsay, J. G., 1980, Shear zone geometry: A review: Journal of Structural eny in Wisconsin: Geological Society of America Special Paper 182, Morey, G. B., 1973, Stratigraphic framework of middle Precambrian rocks in Geology, v. 2, p. 83-99. p. 159-168. Minnesota, in Young, G. M., fed., Huronian stratigraphy and sedimenta- Ramsay, J. G., Casey, M., and Kligfield, R., 1983, Role of shear in develop- 1981, Possible interpretations of the Penokean orogeny: International tion: Geological Association of Canada Special Paper 12, p. 211-249. ment of the Helvetic fold-thrust belt of Switzerland: Geology, v. 11, Proterozoic Symposium, University of Wisconsin, Madison, Abstracts 1978, Lower and Middle Precambrian stratigraphic nomenclature for p. 439-442. Volume, p. 44. east-central Minnesota: Minnesota Geological Survey Report of Investi- Richardson, S. W., 1968, Staurolite stability in a part of the system Fe-Al-Si-O- Wood, D. S., 1974, Current views of the development of slaty cleavage: Annual gations 21,52 p. H: Journal of Petrology, v. 9, p. 467-488. Reviews of Earth and Planetary Sciences, v. 2, p. 1-35. 1979, Field trip guidebook for the Precambrian geology of east-central Sanderson, D. J., 1982, Models of strain variation in nappes and thrust sheets: Wright, T. O., and Piatt, L. B., 1982, Pressure dissolution and cleavage in the Minnesota: Minnesota Geological Survey Guidebook Series No. 12, A review: Tectonophysics, v. 88, p. 201-223. Martinsburg Shale: American Journal of Science, v. 282, p. 122-135. 28 p. Schulz, K. J., LaBerge, G. L., Sims, P. K., Peterman, Z. E., and Klasner, J. S., Young, G. M., 1983, Tecxono-sedimentary history of early Proterozoic rocks of 1983, Lower Proterozoic stratified rocks and the Penokean orogeny in 1984, The volcanic-plutonic terrane of northern Wisconsin: Implica- the northern Great Lakes region: Geological Society of America Mem- east-central Minnesota: Geological Society of America Memoir 160, tions for early Proterozoic tectonism, Lake Superior region: Geological oir 160, p. 15-32. p. 97-112. Association of Canada Program with Abstracts, v. 9, p. 103. Morey, G. B„ and Ojakangas, R. W., 1970, Sedimentology of the middle Siddans, A.W.B., 1983, Finite strain patterns in some Alpine nappes: Journal of Precambrian Thomson Formation, east-central Minnesota: Minnesota Structural Geology, v. 5, p. 441-448. Geological Survey Report of Investigations 13,32 p. Simpson, C., and Schmid, S. M., 1983, An evaluation of criteria to deduce the Morey, G. B., and Sims, P. K., 1976, Boundary between two Precambrian W sense of movement in sheared rocks: Geological Society of America terrenes in Minnesota and its geologic significance: Geological Society Bulletin, v. 94, p. 1281-1288. of America Bulletin, v. 87, p. 141-152. Sims, P. K., 1976, Precambrian tectonics and mineral deposits, Lake Superior Morey, G. B., Sims, P. K., Cannon, W. F., Mudrey, M. G., Jr., and Southwick, region: Economic Geology, v. 71, p. 1092-1118. D. L., 1982, Geologic map of the Lake Superior region, Minnesota, Sims, P. K., and Peterman, Z. E., 1983, Evolution of the Penokean foldbelt, Wisconsin, and northern Michigan: Minnesota Geological Survey State Lake Superior region, and its tectonic environment: Geological Society Map Series S-13. of America Memoir 160, p. 3-14. Morey, G. B., and Southwick, D. K., 1984, Early Proterozoic geology of Sims, P. K., Card, K. D., Morey, G. B., and Peterman, Z. E., 1980, The Great MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 14,1987 east-central Minnesota—A review and reappraisal: Institute on Lake Lakes tectonic zone—A major crustal structure in central North Amer- REVISED MANUSCRIPT RECEIVED APRIL 19, 1988 Superior Geology Abstracts, v. 30, p. 35-36. ica: Geological Society of America Bulletin, Part 1, v. 91, p. 690-698. MANUSCRIPT ACCEPTED APRIL 20,1988

Printed in U.S.A.

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