Great Lakes Tectonic Zone in Marquette 

Area, Michigan Implications for 

Archean Tectonics in North-Central 

United States

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Chapter E

Great Lakes Tectonic Zone in Marquette 

Area, Michigan Implications for 

Archean Tectonics in North-Central 

United States

By P.K. SIMS

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

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Library of Congress Cataloging-in-Publication Data

Sims, P.K. (Paul Kibler), 1918 

Great Lakes Tectonic Zone in Marquette area, Michigan implications for 

Archean tectonics in north-central United States/by P.K. Sims.

p. cm. (Contributions to Precambrian geology of Lake Superior region ; 

ch. E) (U.S. Geological Survey bulletin ; 1904-E)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1904-E

1. Geology, Stratigraphic Archaean. 2. Geology Middle West. 

3. Faults (Geology) Michigan Marquette Region. 4. Faults (Geology) 

Great Lakes Region. I. Title. II. Series. III. Series: U.S. Geological 

Survey bulletin ; 1904-E 

QE75.B9 no. 1904-E 

[QE653.3]

557.3 s dc20 90-13956 

[551.7'12] CIP
CONTENTS

Abstract El

Introduction El

Acknowledgments E2

Geologic setting E2

Archean greenstone-granite terrane E3

Granitoid rocks E3

Structure E3

Late-tectonic conglomerate E7 

Archean gneiss terrane E7

Gneiss and associated granitoid rocks E7

Structure E8 

Great Lakes tectonic zone E8

Mylonite E9

Interpretation E9

Kinematic analysis E13

Evolution E13 

Concluding remarks E14 

References cited E15

FIGURES

1. Geologic map of part of the Marquette I°x2° quadrangle E4

2. Quartz-alkali feldspar-plagioclase diagram for granitoid rocks of Archean 

greenstone-granite terrane E7

3. Quartz-alkali feldspar-plagioclase diagram for granitoid rocks of Archean gneiss 

terrane E8

4. Equal-area projection of poles to foliation, lineations, and fold hinges in gneisses 

of Archean gneiss terrane E9

5. Structure map of southwestern part of Sands 71/2-minute quadrangle showing 

Great Lakes tectonic zone E10

6. Isometric diagram illustrating geometry and deformational elements of 

Great Lakes tectonic zone Ell

7. Equal-area projection of poles to foliation, stretching lineation, and fold hinges in 

Great Lakes tectonic zone Ell

8. Simplified tectonic map of Lake Superior region showing Great Lakes tectonic 

zone and adjacent Archean terranes E12

TABLES

1. Approximate modes of granitoid rocks in Archean greenstone-granite terrane E6

2. Approximate modes of granitoid rocks in Archean gneiss terrane E8

Contents III

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Great Lakes Tectonic Zone in Marquette Area, 

Michigan Implications for Archean Tectonics in 

North-Central United States

By P.K. Sims

Abstract

The Great Lakes tectonic zone (GLTZ) is an Archean 

crustal boundary of subcontinental length that separates a 

greenstone-granite terrane (southern part of Superior province 

of Canadian Shield) on the north from a partly older gneiss 

terrane on the south. It is generally interpreted as a paleosuture 

resulting from continent-continent collision. The tectonic zone 

is covered at imost places in the Lake Superior region by 

Proterozoic rocks or Pleistocene glacial deposits, and its 

position and characteristics previously have been determined 

mainly by geophysical data. Geologic mapping in the Marquette, Michigan area provides for the first time direct observations of the structure.

In the Marquette area, the GLTZ is characterized by a 

zone of mylonite (orthomylonite) that has been superposed on 

previously deformed rocks of both the Archean greenstonegranite terrane and the Archean gneiss terrane. Foliation in the 

mylonite strikes about N. 60° W. and dips steeply southwest, 

presumably subparallel to the boundary between the 

greenstone-granite and gneiss terranes. A pronounced 

stretching lineation and tight fold hinges plunge about 45° S. 

45° E. The attitude of the stretching lineation (line of tectonic 

transport) together with asymmetric structures indicative of 

movement sense indicates that collision at this locality was 

oblique, resulting in dextral-thrust shear along the boundary, 

northwestward vergence, and probable overriding of the 

greenstone-granite terrane by the gneiss terrane. Transmittal of 

the dextral shear stress across a large area of the greenstonegranite crust (Superior province) to the north may have been 

responsible for the nearly east-west foliation, upright folds, and 

northwest- to east-west-trending dextral faults and shear zones 

at least as far north as the Quetico fault, in southern Ontario, 

a distance of about 250 kilometers.

Manuscript approved for publication June 26, 1990.

As a whole, the GLTZ is characterized by systematic 

angular bends that alternately trend west-northwestward, as in 

the Marquette area, and northeastward. This zigzag pattern 

probably reflects original irregularities in the continental 

margin (Superior province) composed of greenstone-granite 

crust. Late Archean convergence along this margin resulted in 

a variable trajectory of stress into the greenstone-granite crust 

and probably in along-strike diachroneity of orogeny. The 

major deformation resulted from oblique compression at 

promontories, which acted as buttresses against which 

compressive stress was directed into the crust. In addition to 

the dominant foliation, major brittle-ductile to brittle strike-slip 

faults, such as the Vermilion fault system in northern 

Minnesota and the Quetico and Rainy Lake-Seine River faults 

in southern Ontario, resulted from a more brittle continuum of 

the transcurrent shear caused by collision along the GLTZ.

INTRODUCTION

The Great Lakes tectonic zone (GLTZ) is an Archean 

crustal boundary more than 1,000 km long that separates a 

greenstone-granite terrane (southern part of Superior province) on the north from a gneiss terrane on the south (Sims 

and others, 1980; Sims and Peterman, 1981; Peterman, 

1979). It is covered throughout most of the Great Lakes 

region by younger Proterozoic rocks or Pleistocene glacial 

deposits, but recently it has been delineated and studied in 

outcrop in an area south of Marquette, Mich. (fig. 1).

The boundary was first recognized in Minnesota 

(Sims and Morey, 1973; Morey and Sims, 1976) from 

regional geologic relations, which indicated that the two 

basement terranes had different geologic histories and 

probably had evolved separately. Regional magnetic and

Great Lakes Tectonic Zone in Marquette Area, Michigan E1
gravity data were utilized to determine the position of the 

boundary. Later (Sims, 1980), the boundary was approximately delineated in the western part of Upper Michigan 

(Sims and others, 1984) and northwestern Wisconsin (Sims 

and others, 1985), east of the Middle Proterozoic Midcontinent rift system, and it was inferred on indirect evidence to 

extend eastward through the Sudbury structure, where it is 

truncated by the Middle Proterozoic Grenville tectonic zone 

(Sims and others, 1980).

Recent geologic mapping in the Sands 7 1/2-minute 

quadrangle, Michigan (fig. 1), previously mapped by Gair 

and Thaden (1968), has delineated this Archean boundary in 

outcrop for the first time. It is exposed on the south side of 

the Early Proterozoic Marquette synclinorium (or trough), 

and its northwestern projection into the trough coincides 

with a major Early Proterozoic fault, the Richmond fault. 

The GLTZ here is a mylonite zone about 2.4 km wide that 

mainly is overprinted on rocks of the greenstone-granite 

terrane but also affects an approximately 0.4-km-wide zone 

of the Archean gneiss. In this area, the GLTZ is interpreted 

as a continent-continent collision zone. The collision was 

oblique, resulting in dextral wrench shear on the N. 60° W.- 

trending boundary and northwestward vergence of the 

gneiss terrane against the greenstone-granite terrane.

The purpose of this report is to describe the exposed 

GLTZ in the context of the regional geology, to discuss 

genetic relationships between convergence along the boundary and structural features in the Archean rocks to the 

north, and to present a refined interpretation of the evolution 

of the GLTZ throughout the Lake Superior region.

ACKNOWLEDGMENTS

Critical reviews of an early draft by D.L. Southwick, 

G.B. Morey, and W.C. Day and of a later version by R.L. 

Bauer and W.C. Day significantly improved the manuscript. 

W.C. Day helped with the kinematic analysis.

GEOLOGIC SETTING

The Great Lakes tectonic zone is moderately well 

exposed in the Sands 7 1/2-minute quadrangle in Upper 

Michigan (see fig. 5). It separates the two distinctive 

Archean terranes in the area. The northern greenstonegranite terrane is composed largely of Late Archean granitoid rocks and approximately coeval metavolcanic and 

lesser metasedimentary rocks of greenstone affinity. The 

layered rocks and most of the granitoid rocks were metamorphosed (mainly to greenschist facies) and deformed 

during Late Archean orogeny. The southern Archean gneiss 

terrane is composed mainly of layered gneiss, migmatite, 

and amphibolite rocks that are distinctly different from 

those in the greenstone-granite terrane. Except for latetectonic to post-tectonic, generally small(?) granitoid 

bodies, the rocks are metamorphosed mainly to amphibolite 

facies. The rocks exposed within the two terranes in Upper 

Michigan are closely similar to those in Minnesota (Morey 

and Sims, 1976; Sims and others, 1980), thus establishing 

the identity of the GLTZ in the Marquette area.

The Archean rocks in Michigan are overlain in the 

Marquette synclinorium, the Republic trough, and the Dead 

River, Clark Creek, and Baraga basins by shelf deposits of 

the Early Proterozoic Marquette Range Supergroup (fig. 1; 

Cannon and Gair, 1970). For the Archean rocks north of the 

Marquette synclinorium, Van Hise and Bay ley (1897) 

introduced the name "northern complex"; the Archean rocks 

south of the synclinorium they named "southern complex."

A Late Archean age for the GLTZ is now established 

by regional geologic relationships in north-central United 

States. The Archean rocks in the greenstone-granite terrane 

in northern Minnesota (Hudleston and others, 1988) and 

northernmost Michigan (fig. 1) are characterized mainly by 

ductile and brittle structures formed in response to dextral 

shear, which accords with the deformation pattern observed 

in mylonite within the exposed GLTZ south of Marquette, 

Mich. These structures include a generally west trending 

steep foliation and upright folds, widespread Z-shaped 

folds, and northwest- to west-trending dextral faults, indicative of dextral shear. In contrast, Early Proterozoic 

Penokean deformation in the Marquette area had little effect 

on the Archean basement (Cambray, 1984). Cambray proposed that the Penokean deformation was produced by 

horizontal compression that was transmitted from the 

basement to the folded cover rocks by narrow ductile shears 

in the basement. Readjustment of rigid basement blocks 

along these shears using old weaknesses resulted in some 

shortening, but not folding of the basement rocks. For the 

Early Proterozoic Marquette trough, Cambray proposed a 

nearly north-south compressional axis, which initially 

produced reverse dip slip on faults bounding the trough and 

compressed the Early Proterozoic sedimentary rocks within 

the trough into west-trending folds. Subsequently, resistance to this movement resulted in a sinistral strike-slip 

motion and the development of F2 folds with northwesttrending axial surfaces and variable plunge.

Gair and Thaden (1968) applied the name "Compeau 

Creek Gneiss" to both the foliated granitoid rocks of the 

Archean greenstone-granite terrane and the layered gneisses 

and massive intrusions of the gneiss terrane, and subsequent 

investigators extended this terminology to the western parts 

of the southern complex (Cannon and Simmons, 1973). To 

apply this name to rocks in both Archean terranes, however, 

is inappropriate, because the two terranes consist of 

distinctive rock types of different origins. Accordingly, in 

this report informal lithologic names are used to describe 

the crystalline rocks in the two diverse terranes.

E2 Precambrian Geology of Lake Superior Region
ARCHEAN GREENSTONE-GRANITE 

TERRANE

The greenstone-granite terrane in the Marquette area 

(fig. 1) consists of several thousand meters of subaqueous 

mafic to felsic flows and pyroclastic rocks and volcanogenic 

sedimentary rocks in a succession that is intruded by small 

bodies of gabbro and ultramafic rocks and by large plutons 

of granitoid rocks (Bornhorst, 1988). The volcanic rocks 

have been named the Ishpeming greenstone belt (Morgan 

and DeCristoforo, 1980); they represent the southwestern 

extension of the Wawa subprovince of the Superior 

province (Card and Ciesielski, 1986). Felsic volcanic rocks 

adjacent to an ultramafic body north of Ishpeming host gold 

deposits of the Ropes mine (Brozdowski and others, 1986; 

Brozdowski, 1988, 1989), and the ultramafic body hosts 

additional mineral prospects (Bodwell, 1972). Foliated 

tonalite from the northern complex (Hammond, 1978) has a 

U-Pb zircon age of 2,703±16 Ma (recalculated by Zell E. 

Peterman), and associated rhyolite has a U-Pb zircon age of 

2,780+69 Ma (recalculated by Zell E. Peterman). These 

ages are consistent with more precise U-Pb ages in the 

Wawa (Shebandowan) subprovince in adjacent Canada 

(Corfu and Stott, 1986).

Granitoid Rocks

The Late Archean granitoid rocks in the greenstonegranite terrane (fig. 1) are dominantly pink to grayish-pink, 

generally medium grained, porphyritic, foliated, homogeneous tonalite or granodiorite (Gair and Thaden, 1968, 

p. 18-23). They contain xenoliths and schlieren of biotite 

schist and amphibolite. In the Sands quadrangle (fig. 1), 

weakly foliated granite also is a common rock type (table 1; 

fig. 2), but its age relation to the tonalite-granodiorite was 

not determined. The bimodal composition of the granitoid 

rocks (fig. 2) suggests, however, that the granite represents 

a discrete magmatic event.

The granitoid rocks of the greenstone-granite terrane 

in the Marquette area exposed on both sides of the Marquette trough are strongly altered (table 1). Plagioclase 

(oligoclase) is saussuritized and albite twinning is largely 

obscured, and biotite is largely changed to chlorite and 

opaque oxides (locally rutile). The rocks are highly fractured, and fracture surfaces are coated by chlorite and other 

propylitic minerals. Along the south margin of the Marquette trough, the granitoid rocks are exceptionally rich in 

quartz (table 1; fig. 2). The granitoid rocks in the Sands 

quadrangle also are mylonitic. The protomylonite is characterized by recrystallization of quartz to fine grain sizes and 

localized shear-induced recrystallization of plagioclase and 

potassium feldspar to fine-grained polycrystalline aggregates (type IP, IIP, 1M, and 11M structures of Hanmer,

1982). The protomylonite grades into orthomylonite at 

a distance of about 2 km north of the GLTZ. (See 

fig- 5.)

The relative homogeneity of the granitoid rocks, the 

occurrence of sharp-walled xenoliths, and the general 

absence of a lithologic layering that could represent original 

sedimentary or volcanic layering suggest that the granitoid 

rocks of the greenstone-granite terrane are of magmatic 

origin. Their pervasive foliation is attributed to deformation 

subsequent to primary crystallization, as discussed 

following.

Structure

The rocks in the greenstone-granite terrane, on the 

north side of the GLTZ (fig. 1), record early recumbent 

folding (F,) of metavolcanic rocks of the Ishpeming 

greenstone belt. Superposed deformation (D2) produced 

northwest- to west-trending upright, upward- and downward-facing folds (F2) that are Z-shaped in plan view 

(Rodney Johnson, written commun., 1989). An axial plane 

foliation was developed during F2. The Z-symmetry of the 

F2 folds is consistent with their development in a 

deformation regime with a dextral shear component. 

Associated granitoid rocks also were deformed by D2 . 

Younger, northwest- to west-trending faults, some of which 

have demonstrable dextral movement, transect and offset 

the folded rocks. Commonly, these faults separate volcanic 

rock domains having opposite stratigraphic facing 

directions, as shown in figure 1.

Foliation and lithologic layering in the vicinity of the 

Ropes mine (R, fig. 1) are puzzling with respect to the 

dominant regional structure. Here, foliation and lithologic 

layering strike about N. 70° E. and are nearly vertical 

(Brozdowski, 1988, p. A-44). The published geologic map 

that includes the Ropes mine area (Negaunee SW 

quadrangle; Clark and others, 1975) also shows a steep 

(stretch?) lineation that plunges southeastward in unit Wkf 

of the Kitchi Schist. Possibly, the northeast foliation in the 

Ropes mine area represents the east-northeast-trending limb 

of a large-scale D2 Z-fold.

The steep, northwest- to west-trending shear zones 

and faults in the greenstone-granite terrane are tens of 

meters to a few hundred meters wide. They are highly 

schistose zones characterized by an intense, close-spaced 

foliation, a steep stretching(?) lineation, and strong 

retrograde alteration. The faults are of both ductile and 

brittle-ductile types. Major structures include the Dead 

River shear zone (fig. 1, DRSZ) (Puffett, 1974), which 

forms the northern boundary of the Dead River basin; the 

Carp River shear zone (CRSZ), northeast of the Ropes 

mine, which separates two different blocks of Archean 

volcanic rocks; and the Carp River Falls shear zone 

(CRFSZ), which forms the north margin of the Marquette 

trough in this area. The Carp River Falls shear zone is

Great Lakes Tectonic Zone in Marquette Area, Michigan E3
87°30"

32

on 

I

o 

33

I(/)

I5'93. 

§

46°45

46°30

i %^\ii'-£<i^*Jli^^^i&«-

'&&&%£&&$l!3fi*X?- " &.*®ffi^W&&&*'< ci^% «; -,*. Jr»S vi^i.^.V* i * :*..- m
CORRELATION OF MAP UNITS

J3 

re re6

Middle 

Proterozoic

Unconformity

Penokea

®§?s

L 

I

n orogeny deformat

Xms

nconform

^^VxmuN

>\*rs3iT

nconform

/.'Xrtjc'.'

^Xbq

ty

Xmn

mmity

Xch

on and metamorphi

] 1 I Baraga 

| Group

J Menominee Group

} Chocolay Group

sm

Marquette 

Range 

Supergroup

Early 

Proterozoic

Continent-continent collision (=2,690 Ma)

Nor 

Greenston

L

'\^f,Y

th of GLTZ 

e-granite terrane

^>Wrc

Jnconformity

",Wv* Wu

4 t >

South c Gneiss

gWgtg

>f GLTZ 

terrane

Agn I Late Archean

Yj:

1

Xms

:>'Xmc>>

^ Xbq <

gXmu^

Xmn

Archean

DESCRIPTION OF MAP UNITS

Middle Proterozoic 

Jacobsville Sandstone

Early Proterozoic

Alkali granite (1,733 ±25 Ma) 

Marquette Range Supergroup

Michigamme Format ion-Dominantly slaty rocks in lower part

Clarksburg Volcanics Member of Michigamme Formation

Goodrich QuartzHe

Menominee Group, undivided, in Republic trough

Negaunee Iron-formationSiamo Slate and Ajibik Quartzite, undivided

Chocolay Group, undivided

^Wgt

Agn

75

CRSZ 

CRFSZ 

DRSZ 

GLTZ 

GGT

Archean

Greenstone-granite terrane 

Reany Creek Formation

Granitoid rocks-Dominantly granodiorite and tonalite but

including granite; generally foliated 

Mafic to felsic flows and pyroclastic rocks and

volcanogenic sedimentary rocks 

UI tram a fie rocks 

Gneiss terrane 

Granite near Tilden (Hammond, 1978)

Gneiss, migmatite, and amphibolite Includes foliated 

and massive granite

Contact

High-angle fault-Bar and ball on downthrown side 

1 Transcurrent fault Showing relative horizontal movement 

1 Thrust fault-Sawteeth on upthrown side

Strike and dip of foliation 

Inclined

Vertical 

Bearing and plunge of lineation May be combined with

foliation symbol 

Direction of stratigraphic tops

Bearing and plunge of minor fold 

Mylonite- Dominantly orthomylonitc Carp River shear zone 

Carp River Falls shear zone Dead River shear zone 

Great Lakes tectonic zone Greenstone-granite terrane Ropes mine

90° 89" 88°

0 40 KILOMETERS

KS

0 40 MILES

INDEX MAP

Figure 1. Geologic map of part of the Marquette 1°x2° quadrangle. Compiled by P.K. Sims, 1989, from various sources.
n

8

I 

S,fitIT S? "8

f

Table 1. Approximate modes of granitoid rocks, in volume percent, in Archean greenstone-granite terrane 

[Tr., trace; blank, absent]

Sample No. .............

Plagioclase .............Quartz.....................Potassium feldspar. 

Biotite...... ...............

Chlorite ..................Epidote ...................

Sphene..... ...............

Opaque oxides .......Accessory 

minerals... ......... ..

Muscovite...............

136R

373030 

Tr.

3

Tr.Tr.

177 A

3532531 

Tr.

1 3

Tr.

0.2

177B

6024

10 

Tr.

6

Tr.

Tr.

179A

6621.5

5.5 

Tr.

7

Tr.Tr.

179B

54528.5

2.5 

10

Tr.

4

Tr.

0.5

182A

4845

0 

Tr.

6

Tr.Tr.

182B

52 536

0.5 

Tr.

6.5

Tr.

0.5

4

183A

383424.5 

2 5

Tr.Tr.Tr.

1

186A

54524

14.5 

7

Tr.

Tr.

Tr.

199

3627.5

31 

5

Tr.

Tr.

0.5

Tr.

200

56.5

37 5

3 

Tr.

3

Tr.Tr.

202

60

28

1 

10

Tr.Tr.

1

208

393030 

0

Tr.

1

211A

41

2724 

Tr.

8

Tr.

Tr.

136-1

27.5

3932 

Tr.

1.5

Tr.Tr.

203

3328

37.5 

Tr.

0.5

Tr.

1

1

61.226

3.8 

0.2

3.6

0.600.2

Tr.

2.5

2

58.522.2

12.8 

Tr.

2.5

Tr.

0

Tr.Tr.

1.3

3

50.6

T75

1.4 11 8

Tr.Tr.Tr

7

	SAMPLE DESCRIPTIONS

136R. Pink, fine- to medium-grained, slightly porphyritic altered granite; contains wispy inclusions of biotite schist. Sands quadrangle, 1,800 ft W., 675 ft N., SE. cor. sec. 3, T. 47 N., R. 25 W.177A. Pink, medium-grained, porphyritic massive altered granite; cut by chlorite-coated fractures. Sands quadrangle, 775 ft S., 2,150 ft E., NW. cor. sec. 16, T. 47 N., R. 25 W.

177B. Pink, fine-grained, porphyritic massive altered granite. Sands quadrangle. Same locality as 177A.

179A. Pinkish-gray, medium-grained, porphyritic altered tonalite; contains oriented layers of biotite schist. Palmer quadrangle, 650 ft S., 250 ft W., NE. cor. sec. 26, T. 47 N., R. 26 W.

179B. Gray, medium-grained, foliated altered tonalite. Biotite is fresh. Palmer quadrangle. Same locality as 179A.182A. Pinkish-gray, medium-grained altered tonalite gneiss. Sands quadrangle, 1,350 ft E., 100 ft N., SW. cor. sec. 31, T. 47 N., R. 25 W.

182B. Pinkish-gray to medium-gray altered tonalite gneiss. Sands quadrangle. Same location as 182A.

183A. Pinkish-gray, fine- to medium-grained granite gneiss. Sands quadrangle. 675 ft N., 300 ft E., SW. cor. sec. 31, T. 47 N., R. 25 W.186A. Pinkish-gray, medium-grained altered granodiorite gneiss. Palmer quadrangle, 2,500 ft N., 2,200 ft E., SW. cor. sec. 36, T. 47 N., R. 26 W.199. Pinkish-gray, medium-grained granite gneiss. Sands quadrangle, 1,000 ft S., 2,000 ft W., NE. cor. sec. 16, T. 46 N., R. 25 W.

200. Pinkish-gray, fine- to medium-grained altered tonalite gneiss. Sands quadrangle, 775 ft N., 2,075 ft E., SW. cor. sec. 9, T. 46 N., R. 25 W.202. Gray, fine-grained biotite tonalite gneiss cut by chlorite-coated fractures. Sands quadrangle, 850 ft N., 2,350 E., SW. cor. sec. 5, T. 46 N., R. 25 W.208. Pink, medium-grained leucocratic granite gneiss. Sands quadrangle, 2,150 ft N., 2,000 ft W., SE. cor. sec. 8, T. 46 N., R. 25 W.

211 A. Pink, medium-grained altered granite gneiss. Sands quadrangle, 2,550 ft S., 150 ft W., SE. cor. sec. 7, T. 46 N., R. 25 W.

136-1. Pink, medium- to coarse-grained foliated granite gneiss. Fractures coated by chlorite. Sands quadrangle. Same location as 136R.

203. Pink, medium-grained leucocratic granite gneiss. Sands quadrangle, 200 ft N., 2,300 ft W., SE. cor. sec. 5, T. 46 N., R. 25 W.

1. Average of 14 samples of foliated tonalite. Marquette quadrangle, north of Marquette synclinorium (Gair and Thaden, 1968, table 6, no. 1).

2. Average of five samples of foliated granodiorite. Marquette quadrangle, north of Marquette synclinorium (Gair and Thaden, 1968, table 6, no. 9).

3. Average of 11 samples of quartz-rich foliated tonalite. Sands quadrangle, south of Marquette synclinorium (Gair and Thaden, 1968, table 6, no. 6).
90 80 70 60 50 40 30 20 10

Figure 2. Quartz-alkali feldspar-plagioclase diagram for 

granitoid rocks of Archean greenstone-granite terrane, Marquette area. Rock classification modified from Streckeisen 

(1976). Modes determined by 600 to 1,000 point counts on 

standard thin sections, and given in table 1. Open circles 1, 2, 

and 3, average compositions reported by Gair and Thaden 

(1968).

reported to be cut by relatively undeformed mafic dikes of 

presumed Late Arehean age, indicating a probable Archean 

age for the shear zone (Baxter and Bornhorst, 1988).

In the area north of the Palmer fault (fig. 1), foliated 

and fractured Late Archean granitoid rocks form partly fault 

bounded domes surrounded by Early Proterozoic rocks of 

the Marquette Range Supergroup. The large Archean granitoid body in the Sugarloaf Mountain area, north of Marquette, also is a dome; foliation in adjacent metavolcanic 

rocks (Puffett, 1974) dips gently to moderately away from 

the granite contact.

Late-Tectonic Conglomerate

The youngest Archean unit in the greenstone-granite 

terrane is the Reany Creek Formation (Puffett, 1969; 1974). 

On the basis of the reinterpretation of age and structural 

relationships, the Reany Creek Formation is no longer 

included as part of the Chocolay Group or the Marquette 

Range Supergroup. It is a heterogeneous body of conglomerate, arkose, chloritic slate, graywacke, and boulderbearing slate. It transects structures in the older volcanic 

rocks, is less deformed than the volcanic rocks, and is 

bounded along its south margin by the northwest-trending 

Dead River shear zone (fig. 1). Its age has been uncertain 

(see Puffett, 1974), but its penetrative foliation, asymmetry 

of basin fill, and relationship to the Dead River shear zone 

strongly suggest that it developed concurrently with dextral 

shear along the Dead River shear zone. It is similar to 

"Timiskaming-type" sequences, such as the Seine Group, in 

northern Ontario, now commonly interpreted as forming in 

Archean analogs to modern pull-apart basins (Poulsen, 

1986; Thurston and Chivers, 1990).

ARCHEAN GNEISS TERRANE

The Archean gneiss terrane in the Marquette area 

(fig. 1) constitutes the greater part of the southern complex, 

as defined by Van Hise and Bayley (1897). It consists of 

gneiss, migmatite, and amphibolite, substantial amounts of 

deformed and undeformed granite pegmatite, and massive 

to weakly foliated granite plutons. Cannon and Simmons 

(1973) have described the general rock types in much of the 

southern complex. One rock has been dated as Late 

Archean. A sample of gray gneiss (called Compeau Creek 

Gneiss by Gair, 1975) collected in SE1/4SW1/4 sec. 36, 

T. 47 N., R. 27 W. (Hammond, 1978) has a U-Pb zircon age 

of 2,779+21 Ma and a lower intercept age of 802±76 Ma 

(recalculated by Zell E. Peterman).

Gneiss and Associated Granitoid Rocks

Compositionally layered, medium-grained gneiss and 

migmatite are the dominant rock types in the gneiss terrane 

in the Marquette area. Layered felsic gneisses ranging in 

composition from tonalite to granite predominate. Smaller 

amounts of massive to layered amphibolite are intercalated 

with the felsic gneiss, but amphibolite constitutes layers 

several tens of meters thick at places, as can be seen on the 

geologic map of the Palmer 71/2-minute quadrangle (Gair, 

1975, pi. 1), which is immediately west of the Sands 

quadrangle. The felsic gneisses are gray to pinkish gray; 

typically, compositional layering is expressed by different 

proportions of the major silicate minerals, as for example, 

(1) plagioclase-quartz-biotite-microperthite, (2) microperthite-quartz-plagioclase-biotite, and (3) biotite-quartzplagioclase-microperthite. Textural differences at places 

emphasize the compositional layering. Pink aplitic granite 

and granite pegmatite commonly transect the gneiss and 

amphibolite and locally form migmatite. Metasedimentary 

rocks such as iron-formation, which forms layers in the 

felsic gneisses in the vicinity of the Republic trough 

(Cannon and Simmons, 1973), were not observed in the 

Marquette area.

Pinkish-gray to pink, medium-grained, massive to 

weakly foliated, homogeneous granite (table 2; fig. 3) 

intrudes the gneisses at places. Hammond (1978) delineated 

a body of massive granite about 4 km2 in areal extent south 

of Ishpeming, Mich. (unit Wgt, fig. 1), which he informally 

called the "Tilden granite." It is a gray to pink, mediumgrained, locally porphyritic massive granite that locally 

contains oriented xenoliths of mafic gneiss. It is cut by pink 

pegmatite and is highly fractured. The fractures have 

slickensided surfaces and a thin coating of chlorite and other 

propylitic alteration minerals. Isotopic data on samples of 

the granite near Tilden of Hammond (1978) indicate that 

both the Rb-Sr and U-Th-Pb systems are highly disturbed. 

Four zircon fractions from one sample of a red phase of the

Great Lakes Tectonic Zone in Marquette Area, Michigan E7
60 A A A A A Al A A A 40

90 80 70 60 50 40 30 20 10

Figure 3. Quartz-alkali feldspar-plagioclase diagram for 

granitoid rocks of Archean gneiss terrane, Marquette area. 

Modes given in table 2.

Table 2. Approximate modes of granitoid rocks, in volume 

percent, in Archean gneiss terrane

[Tr., trace; blank, absent]

Sample No....................

Plagioclase...................

Quartz........ ............ ......

Potassium feldspar ......

Biotite ..........................

Chlorite........................

Muscovite ....................

Epidote ........................

Sphene .........................

Opaque oxides.............

Accessory minerals .....

153A

34.7

28

28

9

Tr.

Tr.

0.3

146-88

33.5

23.8

34.2

8.0

Tr.

Tr.

Tr.

0.5

1

25

25

40

3

Tr.

2

Tr.

Tr.

Tr.

226A

26.3

38.2

33.0

2.2

Tr.

0.3

226B

39.3

32.7

25.0

3

Tr.

Tr.

Tr.

SAMPLE DESCRIPTIONS

granite yield an age of 2,545±71 Ma, but when combined 

with two zircon fractions from another red phase yield an 

age of 2,633±100 Ma (Hammond, 1978, p. 38). Six zircon 

fractions from two samples of the gray phase, however, 

yield an age of 2,345+20 Ma, which is comparable to a 

2,330 Ma Rb-Sr whole-rock age on the same rock body. 

Hammond (1978) concluded that the granite near Tilden 

probably dates at about 2,350 Ma. I suggest, however, that 

the granite is Late Archean in age and that the =2,350 Ma 

age records highly disturbed Rb-Sr and U-Pb systems.

A nearly circular body of alkali granite (unit Xga, 

fig. 1) about 2 km in diameter occurs about 3 km south of 

Humboldt, Mich. (Schulz and others, 1988). The granite is 

light red to brick red, generally massive, fine to medium 

grained, and equigranular to hypidiomorphic granular. The 

granite is similar compositionally to Sn-W mineralized 

alkali feldspar granites of the Arabian Shield (Jackson and 

Ramsay, 1986) and the Nigerian younger granite province 

(Kinnaird and others, 1985). The granite has a Rb-Sr 

whole-rock age of 1,733±25 Ma, which is interpreted as a 

crystallization age (Zell E. Peterman, written commun., 

1988); it is a post-tectonic intrusion.

Structure

Archean gneisses in the southern complex form a 

northwest-trending antiformal structure that closes to the 

west and is overlapped by Paleozoic rocks to the east (Sims, 

1991). An infolded belt of Early Proterozoic (Marquette 

Range Supergroup) rocks indents the Archean fold nose in 

the Republic trough (fig. 1). In the area west of the Republic 

trough, Taylor (1967) determined two principal phases of 

deformation: (1) early, probably flat lying folds with axial 

planes trending northeastward, and (2) younger upright 

folds with steep northwest-trending axial surfaces. The 

latter phase mainly controls the distribution of the rock 

units.

153A. Pinkish-gray, medium-grained, foliated granite, SEViNEVi 

sec. 7, T. 46 N., R. 26 W. Biotite is weakly altered.

146-88. "Tilden granite" of Hammond (1978). Quartz and biotite are 

recrystallized in shears; biotite slightly altered to chlorite.

1. "Tilden granite" of Hammond (1978, p. 63). Potassium 

feldspar is microperthite. Plagioclase has concentric 

zoning. Highly fractured.

226A. Light-gray, medium-grained, foliated granite. Cut by 

fractures. Biotite highly altered to chlorite. Quarry, 

SE'ASW'A sec. 21, T. 46 N., R. 26 W.

226B. Pale-reddish-brown, medium-grained foliated granite. Cut by 

shears, some with mylonite. Biotite highly altered to 

chlorite and calcite. SEV&SWV& sec. 21, T. 46 N., R. 26 

W. Same locality as 226A.

In the Marquette area, early gently inclined to 

recumbent folds that trend northwestward and plunge gently 

northward (fig. 4) are the dominant structure in the gneisses. 

These folds deform an older foliation (Sj). Boudinage 

accompanied the folding; the boudins plunge subparallel to 

gently inclined fold hinges. The flat foliation is overprinted 

at a distance of about 0.4 km from the GLTZ by the 

mylonite foliation in the GLTZ. Discernible remnants of the 

older, gently dipping foliation remain, however, in the 

mylonite zone.

A narrow ductile shear zone trending N. 45° W. and 

dipping 75° NE. was delineated in the eastern part of 

sec. 31, T. 47 N., R. 26 W., 1.5 km south of Palmer (fig. 1). 

A mineral lineation and mullions in the shear zone plunge 

70° N. 10° E. The rocks within this shear zone are fine 

grained (mylonite) and extensively serialized. The shearing 

obliterates the older, gently dipping foliation.

GREAT LAKES TECTONIC ZONE

The GLTZ is characterized in the Marquette area 

(figs. 1, 5, and 6) by a mylonite zone about 2.4 km wide that

E8 Precambrian Geology of Lake Superior Region
Lower hemisphere

Figure 4. Equal-area projection of poles to foliation (cross, 

n=26), lineations (dot, n=9), and fold hinges (open circle, n=4) 

in gneisses of Archean gneiss terrane.

has been superposed on dominantly massive granitoid rocks 

and schists of the Archean greenstone-granite terrane and 

previously deformed rocks of the Archean gneiss terrane. 

As mapped (fig. 5), the mylonite zone is about 2 km wide in 

the greenstone-granite terrane and 0.4 km wide in the gneiss 

terrane. The great width of this shear zone distinguishes it 

from the other, much narrower shear zones and faults in the 

region. The mylonite grades northward into protomylonite 

and highly fractured and altered rocks (fig. 5).

Foliation in the mylonite zone strikes west-northwest 

and dips steeply southwest (fig. 7), presumably subparallel 

to the N. 60°-65° W.-trending boundary between the 

greenstone-granite and gneiss terranes. A pronounced 

rodding (stretching) lineation in the mylonite, expressed 

mainly by comminuted and recrystallized quartz and quartzfeldspar aggregates, plunges about 45° S. 45° E. Hinges of 

tight folds are subparallel to the plunge of the stretching 

lineation (fig. 7). Locally, slickensides and tectonic grooves 

are also parallel to the lineation.

Mylonite

The mylonite is mainly orthomylonite, as defined by 

Wise and others (1984), inasmuch as surviving megacrysts 

compose 10-20 percent of the rock. In the terminology of 

Hanmer (1987), the mylonites are mainly "heteroclastic" 

because the porphyroclasts have variable size ranges, but

include ''homoclastic" mylonite having more uniform 

textural characteristics. Ultramylonite is absent except on a 

scale of a few centimeters.

Mylonite on the north side of the boundary between 

the greenstone-granite terrane and the gneiss terrane shows 

progressive sequential textural development from protomylonite to orthomylonite. In the zone of protomylonite, 

quartz in granitoid rocks is strained, sutured, and recrystallized into lensoid aggregates. In the northern part of the 

mylonite zone, as defined in figure 5, quartz typically is 

recrystallized into "ribbon quartz," yielding a pronounced 

rodding (stretch) lineation. Amphibolite xenoliths in the 

granitoid rocks in this part of the zone are virtually 

undeformed but are somewhat retrograded, as indicated by 

the presence of some actinolite and chlorite, indicating 

deformation under upper greenschist metamorphic conditions. Inward, the relatively stiff minerals, plagioclase and 

potassium feldspar, are progressively recrystallized to finer 

grain sizes, with the development of core-mantle structures 

(White, 1976) or type IP and 1M structures (Hanmer, 

1982); these structures yielded oriented aggregates of quartz 

and of quartz-and-feldspar that produce a prominent 

stretching lineation. Accompanying biotite is mainly recrystallized in planar or irregular shears.

Interpretation

The Great Lakes tectonic zone has been interpreted 

(Gibbs and others, 1984; Southwick and Sims, in press) as 

a paleosuture resulting from continent-continent collision 

that juxtaposed the Archean gneiss and greenstone-granite 

terranes. In the Marquette area, the stretching lineation, 

which represents the line of tectonic transport (Schackleton 

and Ries, 1984), indicates that collision was oblique, 

resulting in dextral shear along the N. 60°-65° W.-trending 

boundary (paleosuture). The parallelism of the fold axes 

with the extension direction (X finite strain axis), which is 

common in many ductile shear zones (Bryant and Reed, 

1969), can be accounted for by rotation of fold axes as a 

result of shear strain perpendicular to the displacement 

direction (Ridley, 1986; Ridley and Casey, 1989). The 

deformation resulted from a combination of thrust shear and 

wrench shear.

The oblique collision would be expected to produce 

dextral shear across a large region north of the GLTZ 

(fig. 8). The extent of the area affected by this dextral 

transcurrent shear is not definitely known, however, 

because dextral shear was the dominant mechanism of 

deformation throughout most of the Superior province 

(Card, 1990). Card has proposed that the Superior craton 

was constructed from the oblique subduction of a succession of arcs, seamounts, and microcontinents or cratonized islands (modern analogs such as Borneo or New 

Guinea) that progressed temporally from north (oldest) to 

south (youngest). This hypothesis is supported by existing

Great Lakes Tectonic Zone in Marquette Area, Michigan E9
87°27'30"

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Figure 5 (above and facing page). Structure map of southwestern part of Sands 7V2-minute quadrangle showing Great Lakes 

tectonic zone (modified from Gair and Thaden, 1 968, with 1 989 additions by Sims). Surficial materials above Archean not shown; 

outlines of outcrops are from plate 2 of Gair and Thaden (1968).

E10 Precambrian Geology of Lake Superior Region
25

EXPLANATION

Archcan

Greenstone-granite terrane

Biotite schist and granitoid rocks

Mylonite Protolith dominantly granitoid rocks but includes

biotite schist and amphibolite 

Gneiss terrane

Gneiss, migmatite, and amphibolite-lncludes foliated and

massive granite 

Mylonite Dominantly mylonitic quartzofeldspathic gneiss

Silicified rocks

Boundary between rocks of greenstone-granite terrane 

and gneiss terrane within the Great Lakes tectonic 

zone(GLTZ)

- Approximate outer limit of orthomylonile in Great Lakes 

tectonic zone

Strike and dip of foliation 

Inclined

Vertical 

Bearing and plunge of minor fold

Bearing and plunge of lineation May be combined with 

foliation symbols

geochronological data, particularly the southward-younging 

ages of the terminal granitoid plutons in the various subprovinces of the Superior province (Hoffman, 1989).

The absence of any known suture in the Wawa 

subprovince (fig. 8) suggests that it is a single structural 

terrane. Structures in the greenstone-granite rocks of northern Minnesota (Wawa subprovince) are remarkably similar 

to those in northern Michigan. In northern Minnesota, 

deformed and metamorphosed volcanic and sedimentary 

rocks of the Vermilion district (Sims, 1976; Sims and 

Southwick, 1985) compose an east-trending belt between 

higher grade rocks of the Late Archean Vermilion Granitic 

Complex (Quetico subprovince, fig. 8) (Southwick, 1972) 

and the Giants Range batholith to the south. The measured 

strain, a cleavage, upright folds, and a mineral lineation in 

this belt have been attributed to the "main" phase of 

deformation (D2) that followed an early nappe-forming 

event (Dt) (Bauer, 1985). The nappes show little evidence 

of a penetrative fabric (Hudleston, 1976). Hudleston and 

others (1988) attributed the (D2) deformation to regional 

dextral transpression, as the strain pattern requires a 

northeast-southwest component of shortening in addition to 

shear. They further proposed that major dextral faults, such 

as the Vermilion fault (fig. 8), are later more brittle 

expressions of this shear regime. They concluded that the 

D2 transpressive deformation resulted from oblique compression between the two more rigid crustal blocks to the 

north (Quetico subprovince) and south (Giants Range 

batholith). A similar tectonic regime has been recognized in 

the Rainy Lake area (Poulsen and others, 1980; Day and 

Sims, 1984; Wood, 1980), where early recumbent folding 

was followed by upright folding and dextral strike-slip 

faulting.

Figure 6. Isometric diagram (not to scale) illustrating 

geometry and deformational elements of Great Lakes 

tectonic zone. A, mylonitic foliation; B, quartz and quartzfeldspar aggregates that define a moderately plunging stretch 

lineation; C, dominantly granite-tonalite rocks of Archean 

greenstone-granite terrane; D, gneiss of Archean gneiss 

terrane.

Lower hemisphere

Figure 7. Equal-area projection of poles to foliation (cross, 

n=21), stretching lineation (dot, n=19), and fold hinges (open 

circle, n=3) in mylonite of Great Lakes tectonic zone, Sands 

and Palmer ZVz-minute quadrangles, Marquette area, 

Michigan.

Recent precise isotopic analyses of zircon, titanite, 

and rutile from the Rainy Lake area, Canada (Davis and 

others, 1989), which lies between the Quetico and Wabigoon subprovinces, have provided time constraints on these

Great Lakes Tectonic Zone in Marquette Area, Michigan E11
97° 95° 94° 93° 92° 90°

a55'n8

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46° 

Middle Proterozoic

Volcanic, sedimentary, and plutonic rocks of

Midcontinent rift system (=1,100 Ma) 

Early Proterozoic

Turbidites overlying shelf deposits

Plutonic and volcanic rocks

Complex continental-margin sequences

EXPLANATION 

Archean

Rocks of greenstone-granite terrane 

(Superior province)

Rocks of gneiss terrane 

Contact

High-angle fault

Transcurrent fault Showing relative horizontal movement

 * * Thrust fault Sawteeth on overthrust block ---- Trace of Great Lakes tectonic zone (GLT2)

Trend of magnetic anomaly

Figure 8. Simplified tectonic map of Lake Superior region showing Great Lakes tectonic zone and adjacent Archean terranes. Geology modified from Morey and others (1982), Southwick and Morey (in press), Sims (1991), and W.C. Day (written commun., 1990). Terminology of Early Proterozoic rocks modified from Southwick and Morey (in press).
structural events. The major deformation, including nappe 

emplacement, thrusting, and local doming, took place 

between 2,696 Ma and 2,692 Ma; this deformation was 

followed shortly by wrench faulting and simultaneous 

deposition of conglomerate/arenite (Seine Group), which 

occurred in the interval 2,692-2,686 Ma. Late (Algoman) 

granitic plutons were emplaced about 2,686 Ma, although 

some are older. In the Wawa subprovince, west of Thunder 

Bay, Ont. (fig. 8), Corfu and Stott (1986) found that the Dl 

deformation occurred during or before the intrusion of the 

Shebandowan Lake pluton at 2,696±2 Ma. Deformation D2 

in this area occurred between 2,689+3/-2 Ma and 

2,684+6/-3 Ma, similar to the age suggested by Davis and 

others (1989) for D2 in the Rainy Lake wrench zone. These 

ages are compatible with the less precise isotopic ages on 

rocks in northern Minnesota and Michigan (Peterman, 

1979), and it seems probable that the rocks and structures 

throughout the Wawa and Quetico subprovinces are 

approximately coeval (Percival, 1989). Although convergence along the GLTZ undoubtedly was diachronous, 

collision probably occurred in the approximate interval 

2,692-2,686 Ma (Davis and others, 1989).

Kinematic Analysis

The attitude of the stretching lineation (line of 

tectonic transport) in the mylonite exposed south of Marquette together with asymmetric meso- and micro-structures 

revealing sense of movement indicates that the oblique 

collision resulted in dextral-thrust shear along the GLTZ 

and northwestward vergence and probable overriding of the 

Archean greenstone-granite terrane by the Archean gneiss 

terrane. Kinematic indicators rotated mica grains within 

narrow compositional layers, asymmetric porphyroclasts 

with tails (a type; Simpson, 1986), and asymmetric microfolds in mylonitic layering indicate northwestward vergence. This information implies southward subduction of 

the Archean greenstone-granite terrane (Wawa subprovince) beneath the Archean gneiss terrane.

Evolution

The northwest direction of tectonic transport during 

suturing of the Archean terranes ascertained from the 

Marquette area provides a means for determining the 

evolution of the GLTZ and the variable trajectory of stress 

into the Superior province crust.

The GLTZ in the Lake Superior region is characterized by systematic angular bends that alternately trend 

northeastward and west-northwestward (fig. 8). Presumably 

this zigzag pattern reflects original irregularities in the 

margin of the Archean greenstone-granite terrane (or 

Superior province) crust, which was a continental margin 

before convergence and collision with the southern Archean 

gneiss terrane.

The northeast-trending and west-northwest-trending 

segments of the GLTZ have different structural styles. As 

discussed earlier, deformation along the northwest-trending 

segments of the GLTZ, as particularly shown by data from 

the Marquette segment, was principally caused by dextral 

transpression resulting from oblique collision. Transmittal 

of this transcurrent shear into rocks north of the GLTZ 

yielded a widespread, pervasive west-northwest- to weststriking foliation, subparallel upright folds, and northwestto west-trending dextral faults and shear zones in the 

Archean greenstone-granite terrane.

The similarly oriented northwest-trending segment of 

the GLTZ in northwestern Wisconsin has many structural 

features in common with the Marquette segment. Foliation 

and upright folds in low amphibolite-facies rocks of the 

Archean greenstone-granite terrane (unit Wga, fig. 4, Sims 

and others, 1985) strike west-northwest, and mineral 

lineations and fold hinges mainly plunge gently southeast. 

The boundary between the two terranes is not exposed 

because of a glacial cover, but is presumed to lie along the 

south edge of unit Wga. Numerous northwest-trending 

dextral faults, some of which reactivated in Early Proterozoic time, have been mapped in the area (Sims and others, 

1985; fig. 1).

Collision along the northeast-trending segments of 

the GLTZ, on the other hand, produced northeast-trending 

structures of apparently more restricted areal extent. In the 

northeast-trending Marenisco segment (fig. 8; Sims and 

others, 1984), the boundary is covered by Early Proterozoic 

sedimentary and volcanic rocks, but lithologic layering and 

foliation in rocks of the adjacent Archean greenstonegranite terrane near the boundary trend northeastward and 

are deformed into upright, moderately tight northeasttrending folds that plunge 45°-50° SW. These structures are 

presumably subparallel to the covered Archean boundary. 

Archean metamorphism has been overprinted by Early 

Proterozoic Penokean nodal metamorphism centered on the 

Watersmeet dome (Sims and others, 1985; Sims, 1990); the 

presence of relict garnet at a few places in the Archean 

rocks of the greenstone-granite terrane near the boundary 

suggests that these rocks were metamorphosed to at least 

upper greenschist facies in Archean time. In the same way, 

north-verging Penokean deformation in the boundary zone 

overprinted Archean structures (Sims and others, 1984). An 

axial plane S2 (Penokean) penetrative cleavage that strikes 

northeast and dips 45°-70° SE. was superposed on the 

previously folded rocks. Apparently the Archean rocks were 

not refolded, however, as a result of the Penokean 

deformation.

In the northeast-trending Minnesota segment of the 

GLTZ, neither the terrane boundary nor the Archean rocks 

on either side are exposed. They are covered in west-central 

Minnesota by thick Quaternary glacial deposits and in 

central Minnesota by Early Proterozoic sedimentary and 

volcanic rocks of the Animikie basin (Southwick and

Great Lakes Tectonic Zone in Marquette Area, Michigan E13
others, 1988). The GLTZ has been investigated, however, 

by a detailed aeromagnetic survey, by computer-generated 

mapping of the second vertical derivative of the gravity 

field, and by shallow test-drilling (see Southwick and Sims, 

in press); the boundary has been located rather accurately on 

the basis of these data. The drilling has shown that the rocks 

on the northwest side are volcanogenic sedimentary and 

mafic to intermediate volcanic rocks, metamorphosed to 

upper greenschist fades, which are intruded by Afchean 

tonalite (Southwick and Chandler, 1983). These rocks are 

typical of the Archean greenstone-granite terrane in exposed 

parts of the Lake Superior region. A seismic reflection 

profile in central Minnesota acquired by COCORP 

(Consortium for Continental Reflection Profiling) has been 

interpreted to indicate that the GLTZ in this area is a 

shallow (=30°) north-dipping tectonic feature (Gibbs and 

others, 1984). In east-central Minnesota, the GLTZ is 

covered by Proterozoic rocks of the Animikie basin. The 

structural style in the Proterozoic cover indicates northverging tectonism (Southwick and others, 1988), and as in 

the Marenisco segment, the Archean crustal boundary had a 

role in defining Penokean deformation.

Deformation along both of the northeast-trending 

segments of the GLTZ resulted mainly from northwestsoutheast shortening, probably dominantly by flattening 

strain. The direction of tectonic transport during convergence was virtually perpendicular to the juncture of the two 

terranes at these localities.

The origin of the zigzag pattern of the south edge of 

the Superior province, now marked by the GLTZ, is 

uncertain. The prevailing thought is that the Wawa subprovince is one of a sequence of stacked island arcs that formed 

progressively from north to south above north-dipping 

subduction zones as the continental mass to the south of 

the GLTZ (that is, the Archean gneiss terrane) migrated to 

the north (Card, 1990). With this interpretation, possible modern analogs of the Superior province are the 

convergent-plate boundaries of the western Pacific, as for 

example those of the Indonesian region (Hamilton, 1979).

The physical resemblance of the south margin of the 

Superior province to the Appalachian-Ouachita Paleozoic 

erogenic belt (Thomas, 1977), however, suggests a possible 

alternative interpretation for the origin of the Superior 

margin. In this interpretation, the Superior margin was a 

rifted continental margin. Two interpretations have been 

made for the origin of the Paleozoic continental margin: 

(1) rift segments offset by transform faults, as suggested by 

Thomas (1977, 1983), or (2) intersections between active 

rift arms at triple junctions (Rankin, 1976). Of these two 

suggestions, the rift-transform mechanism seems the more 

likely, with the Minnesota and Marenisco segments being 

the rifted segments (fig. 8) and the northwestern Wisconsin 

and Marquette segments being highly modified transform

faults. Regardless of the mechanism by which the zigzag 

Archean continental margin originated, the subsequent trace 

of the erogenic belt probably was inherited from the shape 

of the earlier margin.

CONCLUDING REMARKS

Convergence along the irregularly shaped margin of 

the Archean greenstone-granite terrane (GLTZ) resulted in a 

variable trajectory of stress into the continental crust and 

probably in along-strike diachroneity of orogeny. Structural 

data from the Marquette area, in particular, as well as 

elsewhere along the GLTZ, suggest that the major direction 

of tectonic transport was northwestward. Accordingly, 

promontories such as those along the concave part of the 

Marquette and Wisconsin segments of the GLTZ (fig. 8) 

must have projected as buttresses against which compressive stress was directed into the continental crust. Oblique 

compression at these points produced dextral shear across 

the region north of the suture, probably at least as far 

northward as the Quetico fault, a distance of about 250 km. 

This shear imposed a roughly east west, steep structural 

fabric on the rocks and, as a late, more brittle expression 

of the shear regime (Hudleston and others, 1988), the 

northwest- to west-trending dextral transcurrent faults.

Along the Marenisco and Minnesota segments of the 

GLTZ, where convergence was more nearly perpendicular 

to the ancient continental margin, a northeast-trending 

structural fabric was imposed on the rocks immediately 

cratonward from the suture.

I suggest that the main structural fabric (D2) in rocks 

of the Archean greenstone-granite terrane in the northcentral United States (Wawa and Quetico subprovinces; 

fig. 8) resulted from the collision along the GLTZ. The 

predominance of orthomylonite rather than ultramylonite 

and the nearly pervasive retrogressive alteration (greenschist facies) in rocks of the greenstone-granite terrane 

suggest that the exposed collision zone was developed at a 

moderately shallow crustal level. As discussed in a previous 

report (Sims and others, 1980), the Archean structures in 

this regime played a strong role also in subsequent tectonism, especially in the Early Proterozoic north-verging 

deformation.

I further suggest that the late-tectonic granite bodies 

in the Archean gneiss terrane are possibly related to the 

collision along the GLTZ and presumed southward subduction. The available age data on these granites are compatible 

with a presumed 2.69 Ga age for the collision. The "Tilden 

granite" of Hammond (1978) in Michigan has a probable 

Late Archean age, although both the U-Pb and Rb-Sr 

systems are disturbed. In the Minnesota River valley, in 

southwestern Minnesota, a large pluton of late-tectonic 

granite (Sacred Heart Granite) has a Pb-Pb age of about 

2,605 Ma (Doe and Delevaux, 1980) and a Rb-Sr age of 

about 2.7 Ga (Goldich and others, 1970). Doe and Delevaux

E14 Precambrian Geology of Lake Superior Region
(1980) have shown that 207Pb-204Pb values in the Sacred 

Heart Granite are characteristic of ensialic environments, as 

contrasted with the ensimatic (arc) granitoid bodies in the 

Superior province (greenstone-granite terrane). The ensialic 

environment indicates that the Archean gneiss terrane had 

been cratonized prior to emplacement of the Sacred Heart 

Granite. Precise ages are required to test the hypothesis that 

the Late Archean granites south of the GLTZ were indeed 

formed during continent-continent collision.

Cumulative data on the Archean Superior province 

(see Hoffman, 1989, for review) indicate that it consists of 

generally east trending belts of island arc and related rocks 

that were assembled progressively from north to south 

(Card, 1990), before finally colliding with the Archean 

gneiss terrane (continent) on the south at about 2,690 Ma. 

This pattern of accretion as well as the tectonic style is not 

unlike that in modern plate-tectonic regimes, indicating that 

plate-tectonic mechanisms existed in the Archean as well as 

in the Proterozoic and Phanerozoic.

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E16 Precambrian Geology of Lake Superior Region
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*U.S. GOVERNMENT PRINTING OFFICE: 1991-573-049/46001 Great Lakes Tectonic Zone in Marquette Area, Michigan E17



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