The Great Lakes Tectonic Zone Revisited

U.S. GEOLOGICAL SURVEY BULLETIN 1904-S
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Chapter S

The Great Lakes Tectonic Zone Revisited

By P.K. SIMS and W.C. DAY

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION 

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

BRUCE BABBITT, 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: 1993

For sale by

USGS Map Distribution 

Box 25286, Building 810 

Denver Federal Center 

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

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

Creat Lakes tectonic zone revisited / by P.K. Sims and W.C. Day.

p. cm. (U.S. Geological Survey bulletin ; 1904-S) 

(Contributions to Precambrian geology of Lake Superior region ; ch. S)

Includes bibliographical references.

Supt. of Docs, no.: 119.3:19045

1. Geology, Stratigraphic Archaean. 2. Geology Middle West. 3. Faults 

(Geology) Great Lakes Region. I. Day, Warren C. II. Title. 

III. Series. IV. Series: Contributions to Precambrian geology of Lake Superior 

region; ch. S. 

QE75.B9 no. 1904-S 

[QE653.3] 

557.3 s dc20

[551.7'12'0977] 92-42437

CIP
CONTENTS

Abstract SI

Introduction SI

Great Lakes tectonic zone S3 

Minnesota segment S3 

Wisconsin segment S4 

Marenisco segment S6 

Marquette segment S6

Vergence S7

Evolution and significance S8

Conclusions S9

References cited S9

FIGURES

1. Simplified tectonic map of Lake Superior region showing Great Lakes tectonic zone 

and adjacent Archean terranes S2

2. Migrated composite section of COCORP north-south seismic-reflection profiles, 

Minnesota S4

3. Generalized geologic map of northwestern Michigan and Wisconsin, showing 

approximate trace of the Great Lakes tectonic zone S5

4. Diagrammatic sketch map showing approximate configuration of the continental margin in Late Archean time S9

Contents III

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

The Great Lakes Tectonic Zone Revisited

By P.K. Sims anofW.C. Day

Abstract

The Great Lakes tectonic zone (GLTZ) is a Late Archean 

crustal boundary (paleosuture) at least 1,200 kilometers long 

that juxtaposes a Late Archean greenstone-granite terrane 

(Wawa subprovince of Superior province) on the north and an 

Early to Late Archean gneiss terrane (Minnesota River Valley 

subprovince) on the south. Recent mapping of an exposed segment in the Marquette, Michigan, area provides new data on 

the vergence of the structure. These data necessitate reexamination of the COCORP seismic-reflection profiling in central 

Minnesota, which has been the principal basis for past views 

on the vergence of the GLTZ.

In the Marquette area, the GLTZ is a northwest-striking 

mylonite zone about 2.3 kilometers wide that is superposed on 

previously deformed rocks of both Archean terranes. Shear 

zone walls strike N. 55°-60° W., and foliation in mylonite 

within the GLTZ strikes (average) N. 70° W. and dips 75° SW. 

A stretching lineation plunges 42° in a S. 43° E. direction. 

Hinges of tight to open (sheath?) folds of both Z- and S-symmetries plunge parallel to the lineation. The attitude of the lineation (line of tectonic transport and X finite strain axis), together 

with asymmetric kinematic indicators, indicates that collision 

at this locality was oblique; the collision resulted in dextralthrust shear along the boundary, northwestward vergence, and 

overriding of the greenstone-granite terrane by the gneiss 

terrane.

In contrast, the seismic-reflection profiling in central 

Minnesota has been interpreted by several investigators to indicate that the GLTZ is a shallowly north dipping (=30°) structure, which implies southward vergence on a north-dipping 

subduction zone. We suggest, alternatively, that the shallowdipping reflectors in the seismic profiles indicate lithologic 

contacts related to recumbent and gently inclined folds (D^, 

perhaps enhanced by ductile deformation zones, and that the 

Morris fault is indeed the GLTZ. The Morris fault strikes about 

N. 70° E., dips steeply southeastward, is transparent in seismic 

profiles, appears to be narrow, and coincides with the inferred 

position of the GLTZ as shown on earlier maps.

The oblique collision along northwest-trending segments 

of the GLTZ would be expected to produce dextral transpression across a large region north of the GLTZ, and may have

produced an early nappe-forming event (D^ as well as 

younger upright folds (D2), and as a later, more brittle event, 

the numerous dextral faults and conjugate sinistral faults that 

are widespread in the Wawa and adjacent subprovinces.

INTRODUCTION

The Great Lakes tectonic zone (GLTZ), named in 1980 

(Sims and others, 1980), forms the boundary between an 

Archean gneiss terrane (Minnesota River Valley subprovince of Superior province) on the south and a Late 

Archean greenstone-granite terrane (Wawa subprovince) 

on the north. These terranes differ in age, rock assemblages, metamorphic grade, and structural style (Morey 

and Sims, 1976). The GLTZ has now been traced across 

the Lake Superior region (Sims, 1991; fig. 1), and is 

inferred to extend westward to the Trans-Hudson orogen 

in central South Dakota (Sims and others, 1991) and eastward through the Sudbury structure to the Grenville front 

(Sims and others, 1980). The postulated length of the 

GLTZ exceeds 1,200 km; thus, it is a feature of subcontinental length.

This boundary was first recognized in Minnesota from 

regional geologic relations. Because the boundary is covered by glacial deposits in Minnesota, its position was 

determined initially from aeromagnetic and gravity data 

(Morey and Sims, 1976). Later (Sims, 1980), the boundary 

was approximately located using regional geologic relations 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. More recently it was mapped in the Marquette, 

Michigan, area (Sims, 1991; 1993), where it is exposed 

along a strike length of about 10 km.

The purpose of this report is to (1) describe the general 

geology of each segment of the GLTZ within the Lake 

Superior region, (2) describe the kinematics determined 

from the one area where the GLTZ is exposed, (3)

The Great Lakes Tectonic Zone Revisited S1
ueuquieaajj A8o|oaQ jo ajjei jouadns uoiSan 
speculate on the trajectory of stress into the Archean 

greenstone-granite crust resulting from collision along the 

GLTZ, and (4) propose an alternative to the prevailing 

interpretation of vergence based on geophysical data from 

central Minnesota that is consistent with the kinematics 

determined in the exposed area.

GREAT LAKES TECTONIC ZONE

Geologic and geophysical data in the Lake Superior 

region indicate that the GLTZ is characterized by angular 

bends, so that it alternately trends northeast and westnorthwest (fig. 1). The Minnesota and Marenisco segments 

trend northeast; the Wisconsin and Marquette segments 

trend west-northwest. Brief descriptions follow for each 

segment of the GLTZ.

Minnesota Segment

The Minnesota segment of the GLTZ trends about N. 

70° E. (fig. 1). It is not exposed, but its position has been 

delineated rather accurately using aeromagnetic and gravity anomaly maps (Morey and Sims, 1976). It is a conspicuous linear feature in west-central Minnesota on the 

shaded-relief aeromagnetic maps of the State (Chandler, 

199la) and on the aeromagnetic anomaly map of Minnesota (Chandler, 199 Ib).

The Archean greenstone-granite terrane (Wawa subprovince), to the north of the GLTZ, as exposed in northern Minnesota and adjacent Ontario, Canada, comprises a 

metamorphosed succession of volcanic and volcanogenic 

sedimentary rocks of Late Archean age intruded by several episodes of Late Archean plutonic rocks (Sims, 1976; 

Day, 1990b). In the Shebandowan greenstone belt of 

adjacent Ontario, southwest of Thunder Bay (fig. 1), the 

volcanic succession has a U-Pb zircon age of about 2,730 

Ma (Corfu and Stott, 1986). A first deformation (Di) 

occurred during or before intrusion of a major (Shebandowan Lake) pluton at 2,696±2 Ma. A second deformation (D2) caused by regional transpression occurred after 

2,689+3-2 Ma and before 2,684+6-3 Ma. New mapping 

in the Wawa subprovince of northern Minnesota by Boerboom and Jirsa (1992) has identified two main periods of 

defonnation that are correlative with the two deformations 

in the Vermilion district (Hudleston and others, 1988), and 

almost certainly also with the two events in the Shebandowan belt. An older (D!) event associated with recumbent folding is followed by a younger (02) transpressive 

event. Boerboom and Jirsa reported that D2 ended 

between 2,685 Ma and 2,674 Ma, apparently slightly later 

than D2 defonnation in the Shebandowan belt. The D2 

event, clearly demonstrated by several investigators in the

Vermilion district, extends into the Quetico subprovince 

(Bauer, 1985; Bauer and Bidwell, 1990) and the adjacent 

Wabigoon subprovince (to the north) (Poulsen, 1986; Day, 

1990b). The D2 deformation throughout its known extent 

was caused by northwest-directed compression.

The Archean gneiss tenane exposed in the Minnesota 

River Valley south of the GLTZ consists of complexly 

folded rriigmatitic felsic gneisses of presumed volcanic 

affinity, local amphibolite units, and lesser amounts of 

metagabbro and metasedimentary rocks (Grant, 1972). The 

rocks have been metamorphosed to upper amphibolite and 

(or) granulite facies (Himmelberg and Phinney, 1967). 

Protoliths of the older gneisses date from =3,500 Ma; 

high-grade metamorphism occurred at =3,050 Ma and was 

followed by emplacement of the late-tectonic Sacred Heart 

Granite at =2,600 Ma (Goldich and others, 1980; Goldich 

and Wooden, 1980; Doe and Delevaux, 1980). Tonalitic 

gneiss recovered in a drill core from a covered block south 

of the Minnesota River Valley was recently dated at 

2,642±57 Ma (U-Pb zircon method; D.L. Southwick, written commun., 1992). This isotopic age indicates that at 

least some rocks of Late Archean age are present in the 

Minnesota River Valley subprovince in southwestern 

Minnesota.

The Minnesota segment of the GLTZ is the type area 

for the feature and has been extensively studied. In the 

initial report on the structure, Morey and Sims (1976, figs. 

1, 8) visualized the GLTZ as a narrow zone, at most a few 

kilometers wide, that separates a major greenstone-granite 

tenane to the north from a gneiss terrane of mini-continental dimensions to the south. Because the boundary 

along the Marenisco segment, Upper Michigan (fig. 1), 

coincides with a broad belt of distinctive tectonism in the 

overlying Early Proterozoic supracrustal rocks (Sims, 

1980), the use of the term GLTZ was later modified to 

include the Early Proterozoic (Penokean) deformation as 

well as the Archean (Sims and others, 1980). In central 

Minnesota, the zone within which deformation of Early 

Proterozoic age was superposed on the Late Archean 

structure was estimated to be =50 km wide (Morey and 

others, 1982). This broader usage was continued in a later 

report on seismic-reflection profiling across the GLTZ 

(Gibbs and others, 1984). Shallow drilling and potentialfield geophysics to aid in interpreting the seismic profiles 

(Southwick and Chandler, 1983; Southwick and others, 

1986) have indicated that the GLTZ in central Minnesota 

is indeed narrow; the rocks within the "wide" tectonic 

zone are typical of volcanic subprovinces within the 

Archean greenstone-granite tenane. Because of their low 

magnetization (Chandler, 1991b), the rocks in this zone 

have been called the "quiet zone" (Chandler and Southwick, 1990).

In addition to the seismic-reflection profiling acquired 

by COCORP (Consortium for Continental Reflection Profiling) (Gibbs and others, 1984), the GLTZ in central

The Great Lakes Tectonic Zone Revisited S3
NORTH GLTZ 

Greenstone-granite terrane Gneiss terrane

SOUTH

GRANTS

RANGE

BATHOLITH

Seismic stations 300 

|

400

|

500

|

800

I

4 

O 8' 

u

12 

16 1 Upper mantle(?)

20 KILOMETERS

Figure 2. Migrated composite section of COCORP north-south seismic-reflection profiles, Minnesota. Dashed lines, seismic surfaces; dotted line, Morris fault. Modified from Gibbs and others, 1984, fig. 6. Great Lakes tectonic zone (GLTZ) is reinterpreted 

as the Morris fault.

Minnesota has been investigated by a detailed aeromagnetic survey (Chandler, 1991b). These data have been 

interpreted using various computer-generated graphical 

enhancement techniques, computer-generated mapping of 

the second vertical derivative of the gravity field, and 

shallow test-drilling (Southwick and Chandler, 1983; 

Southwick and others, 1986).

The seismic-reflection profiling in central Minnesota 

(fig. 2) has defined abundant shallowly dipping seismic 

surfaces in the upper 30 km of greenstone crust. One particularly conspicuous zone of north-dipping reflectors 

projects to the surface near station 400 (fig. 2). The moderate dip of this zone of reflectors, their continuity over 

tens of kilometers, and their extension to midcrustal 

depths led Gibbs and others (1984) and other investigators 

to infer that the reflectors correspond to a zone of imbricate thrust faults (GLTZ) that displaced rocks of the 

greenstone-granite terrane (Wawa subprovince) southward 

over rocks of the gneiss terrane (Minnesota River Valley 

subprovince). This interpretation implies southward vergence for the GLTZ on a shallowly north dipping subduction zone.

Wisconsin Segment

The Wisconsin segment of the GLTZ strikes northwest, 

as indicated by trends of gravity and aeromagnetic anomalies and by available geologic data. Some aspects of the

S4 Precambrian Geology of Lake Superior Region

geologic relations in the area are not definitive, however, 

because the Archean rocks are poorly exposed. Also, the 

Archean rocks were tilted by movements on the Middle 

Precambrian (Keweenawan) Pelton Creek fault (fig. 3), a 

southward-directed, listric thrust fault (Cannon and others, 

in press). At the time of the initial geologic mapping, Sims 

and others (1985) recognized textures in the Archean 

rocks indicative of a nearly pervasive brittle-ductile deformation that formed at about 1,050 Ma, and which is superposed on Archean structures, but the cause of the 

deformation was not known. We now know from more 

recent mapping that all but the southernmost rock exposures in the area shown in figure 3 were rotated at least 

35° northward during the tilting along the Pelton Creek 

fault. Interestingly, evidence for Penokean deformation is 

lacking along the Wisconsin segment of the GLTZ (Sims 

and others, 1985).

Despite the sparse exposures and the tilting caused by 

the Pelton Creek fault, the position of the GLTZ in northwestern Wisconsin (fig. 2) is believed to be accurate 

within a few kilometers. The rocks to the north of the tectonic zone comprise a bimodal Late Archean volcanic succession (interlayered felsic gneiss and amphibolite) that 

correlates with the succession (greenstone-granite terrane) 

exposed between Marenisco and the Watersmeet dome to 

the northeast. These bimodal rocks are similar chemically 

to bimodal volcanic rocks in the Rainy Lake area (Day, 

1990a) of the Wabigoon subprovince, along the 

Minnesota-Ontario border. Small granite to tonalite bodies
10 30 40 50 KILOMETERS

 J Paleozoic rocks

Keweenawan Supergroup (Middle 

Proterozoic)

Rocks of Pembine-Wausau terrane 

of Wisconsin magmatic terranes 

(Early Proterozoic)

Marquette Range Supergroup (Early

Proterozoic) 

Greenstone-granite terrane (Late

Archean) 

Puritan Quartz Monzonite (Late

Archean)

EXPLANATION

Volcanic rocks-Interlayered biotite

gneiss and amphibolite 

Gneiss terrane (Late to Early

Archean) 

Gneiss, migmatite, and amphibolite

Contact 

High-angle faultThrust fault of Middle Proterozoic 

(Keweenawan) age-Sawteeth on upper plate; dashed where uncertain

-A -A. Thrust fault of Early Proterozoic 

(Penokean) age-Sawteeth on upper plate /-* I T7

Great Lakes tectonic zone80

55

Approximately located 

Primary layering-Showing dipFoliation-Showing dip

Figure 3. Generalized geologic map of northwestern Michigan and Wisconsin, showing approximate trace of the Great Lakes tectonic zone. Modified from Sims (1992).
cut the interlayered succession, and the Late Archean 

Puritan Quartz Monzonite batholith is exposed north of 

the Mineral Lake fault (fig. 3). The interlayered biotite 

gneiss and amphibolite unit has a U-Pb zircon upper intercept age of 2,735±16 Ma and a lower intercept age of 

1,052±70 Ma (Sims and others, 1985); the lower intercept 

age is nearly identical to Rb-Sr and K-Ar biotite ages from 

the gneisses and is interpreted as the result of 

Keweenawan disturbance. The 2,735±16 Ma age is virtually identical to that determined from the same unit in the 

Marenisco area. The rocks in the interlayered unit (unit 

Wv, fig. 3) strike northwest and dip nearly vertical. 

Before Middle Proterozoic rotation on the Pelton Creek 

fault, the general dip of rocks in the unit would have been 

steep to the south. This rotation did not appreciably 

change the strike of the rocks.

Sparse data from drilling (Sims and others, 1985, and 

references therein) and from outcrops indicate that the 

Archean gneiss terrane to the south of the GLTZ in this 

area consists mainly of quartzo-feldspathic gneisses. Aeromagnetic and gravity trends indicate a predominant eastwest strike for the gneisses. Gneisses from the linear 

northeast-trending Archean block south of the Flambeau 

Flowage thrust fault (fig. 3) have a Rb-Sr whole-rock age 

of =2,700 Ma (Sims and others, 1985, fig. 3), and gneiss 

from the small body exposed south of Park Falls has a UPb zircon upper intercept age of 2,984±51 Ma (W.R. Van 

Schmus, written commun., 1988).

Marenisco Segment

The Marenisco segment of the GLTZ (fig. 1) trends 

northeast and is covered by mafic volcanic rocks of the 

Early Proterozoic Blair Creek Formation of the Marquette 

Range Supergroup (not shown separately in fig. 3; Sims 

and others, 1984; Sims, 1990). In the area east of Marenisco the boundary lies between exposures of Archean 

metagraywacke (included in unit Wv, fig. 3) of the greenstone-granite terrane and Archean gneiss in the Watersmeet dome (Sims, 1990); the position of the boundary is 

constrained within a distance of 6 km.

The rocks immediately northwest of the GLTZ in the 

greenstone-granite terrane are the same bimodal volcanic 

rocks (unit Wv, fig. 3) that occur in northwest Wisconsin 

(fig. 3). These volcanic rocks are intruded by granitic rocks 

of the Puritan batholith (unit Wp, fig. 3; Sims and others, 

1977) and by associated granite pegmatite. One sample of 

biotite gneiss from the bimodal volcanic unit has a U-Pb 

zircon concordia-intercept age of about 2,750 Ma (Sims and 

others, 1984, fig. 12). The Puritan Quartz Monzonite has a 

Rb-Sr whole-rock isochron age of 2,650±140 Ma (Sims and 

others, 1977). The bimodal volcanic and graywacke units 

were deformed during a Late Archean event into upright, 

moderately tight north-northeast-trending folds. The axial

surfaces of these folds are commonly marked by lenses of 

quartz. A foliation that strikes northeast and dips steeply 

southeast (Sims and others, 1984, fig. 19) that formed during the Early Proterozoic Penokean orogeny is superposed 

on these folds. The exposed width of the zone of superposed 

Penokean foliation is about 6 km. This zone is terminated 

to the west by the Pelton Creek fault (Sims, 1990).

The rocks that comprise the Archean gneiss terrane 

exposed in the Watersmeet dome (fig. 3) consist of older, 

Early Archean tonalitic augen gneiss and biotite gneiss 

overlain by a supracrustal succession of interlayered 

gneiss and amphibolite of bimodal composition. The 

older gneisses have a U-Pb zircon age of 3.56±0.04 Ga, 

comparable in age to gneisses in the Minnesota River 

Valley; the supracrustal rocks have a U-Pb zircon age of 

about 2.64 Ga (Peterman and others, 1986). Small bodies 

of leucogranite (=2.59 Ga) intrude the layered rocks 

(Sims and others, 1984, fig. 9). The Watersmeet dome 

(fig. 3) is mantled by Early Proterozoic rocks. A strong, 

east-trending moderately northward dipping foliation was 

imposed on gneisses in the dome during the Penokean 

deformation, as indicated by the deformation of Early 

Proterozoic metagabbro dikes that cut the Archean rocks 

in the dome. Rb-Sr and K-Ar whole rock and mineral 

systems in the Archean rocks within the core were reset 

at the time of the doming, 1,800-1,750 m.y. ago (Sims 

and others, 1984). This event followed collision along the 

Niagara fault zone, dated at =1,852 Ma (Sims and others, 

1989), by 50 to 100 m.y.

Marquette Segment

The Marquette segment of the GLTZ is a 2.3-km-wide 

mylonite zone that trends northwest and dips steeply 

southwest (Sims, 1991). It is exposed along strike for a 

distance of =10 km and extends northwestward beneath 

Early Proterozoic rocks of the Marquette syncline. The 

mylonite zone is superposed on previously deformed rocks 

of the greenstone-granite terrane on the north and the 

gneiss terrane on the south.

Rocks of the greenstone-granite terrane exposed on the 

south side of the Marquette syncline (but north of the 

GLTZ) consist predominantly of foliated tonalite-granite 

but include small masses of biotite and hornblende schists, 

similar to those rocks exposed to the north of the syncline 

(Johnson and Bornhorst, 1991). Foliated tonalite from the 

northern complex (north side of the Marquette syncline) 

(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). The

S6 Precambrian Geology of Lake Superior Region
rocks south of the Marquette syncline in the greenstonegranite terrane have variable strikes because of folding, 

generally moderate to gentle dips, and shallow plunging 

fold axes and mineral lineations (Sims, 1993). In the Ishpeming greenstone belt on the north side of the Marquette 

syncline, the rocks in this terrane record early recumbent 

folds (Fj) and younger (D2> west-trending upright, 

upward- and downward-facing folds formed by transpression that are mainly Z-shaped in plan view (Johnson and 

Bornhorst, 1991). These rocks are cut by younger northwest- to west-trending faults (or shear zones), some of 

which have demonstrable dextral movement. Commonly, 

these faults in the northern complex, which belong to the 

same family of transcurrent faults deciphered in northern 

Minnesota (Hudleston and others, 1988), separate volcanic 

rock domains having opposite stratigraphic facings 

(Johnson and Bornhorst, 1991). The granitoid rocks that 

surround and intrude the greenstone bodies are mostly 

strongly foliated as a result of regional deformation D2.

Rocks that compose the gneiss terrane to the south of 

the GLTZ along the Marquette segment are compositionally layered, medium-grained gneiss, migmatite, and 

amphibolite cut by pink aplitic granite and granite pegmatite (Sims, 1993). A sample of gray gneiss collected in 

SEi/4 SWi/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 802176 Ma (recalculated by Zell E. Peterman). 

A post-tectonic intrusion about 4 km in areal extent (granite near Tilden of Hammond (1978)) has a U-Pb zircon 

age of 2,585±15 Ma (Sims and Peterman, 1992). The 

dominant structures in the Archean gneiss to the south of 

the GLTZ are gently inclined to recumbent folds oriented 

northwestward and plunging gently to moderately northward (Sims, 1991, fig. 4). A younger steep, northwestoriented foliation occurs locally. This younger foliation is 

subparallel to the northwest-trending antiformal structure 

of the southern complex of the Marquette district, and presumably is related to the deformation that produced the 

antiform. Both the recumbent folds and the younger foliation predate the GLTZ.

In the exposed part of the Marquette segment (Sims, 

1991; 1993), the shear zone walls of the GLTZ are subparallel and strike N. 60° W. Foliation in mylonite within 

the GLTZ strikes (average) N. 70° W. and dips 75° SW. 

A stretching lineation in the mylonite foliation plunges 

42° in a S. 43° E. direction. Hinges of tight to open 

sheath(?) folds of both Z- and S-symmetries plunge parallel to the lineation. The attitude of the lineation (line of 

tectonic transport and X finite strain axis), together with 

asymmetric kinematic indicators, indicates that collision 

along this segment of the GLTZ was oblique; the collision 

resulted in dextral-thrust shear along the boundary, northwestward vergence, and overriding of the greenstonegranite terrane by the gneiss terrane.

VERGENCE

Two interpretations have been proposed for the vergence on the GLTZ: (1) the interpretation based on geophysical data in Minnesota regards the GLTZ as 

characterized by southward vergence on a shallowly northward dipping subduction zone (Gibbs and others, 1984; 

Chandler and Morey, 1989), and (2) the interpretation 

based on geologic data using kinematics in outcrops in the 

Marquette, Michigan, area (Sims, 1991) characterizes the 

GLTZ as a northward-verging, high-angle, south-dipping 

structure on which the gneiss terrane was displaced northward over the greenstone-granite terrane. We have proposed from the kinematic data along the Marquette 

segment that vergence along each of the segments was 

northward, and have suggested an alternative interpretation 

of the north-dipping seismic reflectors in the Minnesota 

profiles (Sims and Day, 1992). The alternative interpretation is consistent with data from the exposed area.

Seismic reflection zones such as those in the greenstone (Wawa subprovince) crust in Minnesota can be 

attributed to (1) lithologic contacts of units of contrasting 

composition, which can provide velocity contrast, (2) 

igneous layering, and (3) fluid-rich zones, as well as (4) 

brittle-ductile deformation zones. That the Minnesota seismic planes could be lithologic contacts perhaps enhanced 

by shallow-dipping faults or shear zones is suggested by 

the informative seismic-reflection profiles in the southern 

Abitibi greenstone belt, Canada (Jackson and others, 

1990), which is along strike to the northeast from Minnesota. The seismic profiles in the Abitibi belt reveal abundant subhorizontal reflection zones of regional extent in 

the upper 15 km of crust. These reflectors resemble those 

in the greenstone crust of Minnesota in generally being 

discontinuous and having variable shallow dips. The shallow-dipping reflectors in the Abitibi belt are interpreted by 

Jackson and others (1990) as layering and (or) tectonic 

high-strain zones, possibly like the low-angle faults that 

produced the "out of sequence" stratigraphy demonstrated 

by U-Pb geochronology (Corfu and others, 1989). The 

shallow-dipping reflectors are truncated and in part offset 

by regional, steeply dipping, ductile-brittle shear zones 

such as the Dexter-Porcupine and Cadillac-Larder Lake 

fault zones, which are transparent and cannot be identified 

directly in the seismic profiles. Some of these faults penetrate to depths of 15-18 km.

We suggest that the GLTZ would not be identifiable 

on the COCORP profiles in Minnesota because of its 

steep dip; we also suggest that the steep southeast-dipping fault shown by Gibbs and others (1984) near station 

500 on line 3 (fig. 2) is the GLTZ. This structure has the 

local name Morris fault. At the time of the COCORP 

investigation, the Morris fault was considered as the 

southeast margin of a "wide" GLTZ (Gibbs and others,

The Great Lakes Tectonic Zone Revisited S7
1984, fig. 2). That the Morris fault could indeed be an 

expression of the GLTZ is suggested by drilling by the 

Minnesota Geological Survey (Southwick and others, 

1986). A hole (1985-11) located on the approximate 

trace of the Morris fault in Todd County and near station 

500 on COCORP line 3 (Gibbs and others, 1984) penetrated 11 ft of mylonite at depths of 333-344 ft (1 

ft=0.328 m). The hole bottomed in mylonite. Previously, 

Herrmann (1979) determined from surface-wave focal 

plane data of a recent earthquake near Morris that the 

fault probably trends northeast and has a steep (=70°) 

southeast dip. A steep southward dip of this structure is 

consistent with the observed southward dip of the GLTZ 

in the Marquette, Michigan, area, as well as with northward vergence on the structure. The apparent narrowness 

of the tectonic zone in Minnesota also is consistent with 

the observed »2-km-wide mylonite zone that comprises 

the GLTZ in the outcrop area in Michigan.

Structural data from northern Minnesota and Michigan 

support our suggestion that the shallowly dipping seismic 

reflectors are predominantly lithologic contacts rather than 

faults or ductile shear zones. Early (Di) recumbent or 

gently inclined folds, some of large areal extent, have 

been recognized over wide areas of the Wawa subprovince 

(Hudleston and others, 1988; Day and Sims, 1984; 

Johnson and Bornhorst, 1991). Such large structures could 

produce the strong seismic reflections shown in the 

COCORP profiles. Also, no evidence has been recognized 

in exposed areas of the Wawa subprovince in northern 

Minnesota for imbricate shallow-dipping ductile-brittle 

deformation zones, such as postulated for the entire upper 

crust from the COCORP profiles (Gibbs and others, 

1984). The faults and shear zones in the region north of 

the GLTZ, such as the Vermilion fault in northern Minnesota (Sims, 1976), are nearly vertical, and formed from 

regional transpression (Hudleston and others, 1988).

The COCORP seismic profiles in central Minnesota 

show shallow-dipping reflection zones in the gneiss crust 

south of the GLTZ that extend to depths of about 36 km. 

These reflectors accord with the generally shallow dipping 

gneissic foliation and open folds in gneiss exposed at the 

surface in the Minnesota River Valley, about 100 km to 

the south (Bauer, 1980). Gibbs and others (1984, p. 291) 

noted that because of differences in stacking velocity this 

crust differs in seismic character from the greenstone crust 

north of the GLTZ.

EVOLUTION AND SIGNIFICANCE

The Great Lakes tectonic zone was formed by the final 

collision in the assembly of the Superior province of the 

Canadian Shield. According to our model, the collision 

resulted from northwest-directed tectonic transport of the

gneiss terrane (micro-continent) against the previously 

assembled, predominantly island-arc terrenes that comprise most of the Superior province. Collision was oblique 

along the northwest-trending segments, resulting in dextral-thrust shear along the boundary and probable overriding of the island arc terrane by the gneiss terrane. 

Southward subduction may have resulted in emplacement 

of the large =2,600-Ma batholithic Sacred Heart Granite 

into the gneisses in southwestern Minnesota.

The deformation along the GLTZ took place under 

ductile conditions, but near the brittle-ductile transition. 

These conditions are indicated by the presence of dominant orthomylonite rather than ultramylonite and a slight 

retrogressive alteration in the zone. Under typical crustal 

conditions, and assuming a geotherm of 30 °C km"1 , the 

brittle-ductile transition occurs at a depth of 8-12 km, corresponding to vertical stresses of 2-3 kbar and temperatures of 240-360 °C (Kerrich and Feng, 1992).

The northwest-directed tectonic transport during suturing of the Archean terranes, determined from the 

Marquette, Michigan, area, provides a means for establishing the evolution of the GLTZ and the trajectory of stress 

into the Archean greenstone-granite crust. The systematic 

angular bends in the GLTZ that shape segments alternately trending northeastward and west-northwestward 

(fig. 1) may well reflect original irregularities in the margin of the greenstone-granite terrane, which was a continental margin before convergence and collision with the 

southern Archean gneiss terrane. This zigzag-shaped 

Archean continental margin is similar in broad outline to 

the Appalachian-Ouachita Paleozoic orogenic belt (Thomas, 1983). The Archean continental margin could have 

resulted from rifting, as proposed for the Paleozoic continental margin. If so, the Minnesota and Marenisco segments probably represent rifted margins, and the 

Wisconsin and Marquette segments represent former transform faults.

Collision along the Archean continental margin can be 

discussed with respect to figure 4, a diagrammatic sketch 

map showing the approximate configuration of the continental margin in Late Archean time. In the reconstruction, 

the Middle Proterozoic Midcontinent rift system has been 

removed, thus bringing the Minnesota segment in juxtaposition with the Wisconsin segment.

We presume that the oblique collision determined 

along the Marquette segment is indicative of convergence 

along this irregularly shaped continental margin. Thus, the 

tectonic transport direction of the converging plates was 

about N. 45° W. (fig. 4), with upthrusting toward the 

northwest. In this scenario, collision would have been 

diachronous along the Archean continental margin. The 

Wisconsin recess (a concave curve toward the continental 

margin; terminology of Thomas, 1983) would have acted 

as a buttress against which compressive stress was 

directed into the greenstone-granite crust to the north.

S8 Precambrian Geology of Lake Superior Region
Greenstone-granite terrane crust

Minnesota 

segment

Marquette 

segment

M

Gneiss

\

terrane crust

100 KILOMETERS

M

EXPLANATION

High-angle dextral fault Barbs show direction of 

relative movement; Qu, Quetico fault; RL-SR, 

Rainy Lake-Seine River fault; V, Vermilion fault

Direction of tectonic transport 

Marquette, Michigan

Figure 4. Diagrammatic sketch map showing approximate 

configuration of the continental margin in Late Archean time. 

The Middle Proterozoic Midcontinent rift system has been removed, thus juxtaposing the Minnesota and Wisconsin segments. Scale is approximate. Refer to figure 1 for geographic 

positions of segments of Great Lakes tectonic zone.

Stott, 1986) areas, Canada, indicate that these faults 

formed in the approximate interval 2,692-2,686 Ma. Inasmuch as these faults are attributed to collision along the 

GLTZ, these ages date the approximate time of collision 

along the GLTZ.

The northwest-directed compression was nearly perpendicular to the northeast-trending segments of the 

GLTZ, and accordingly the rocks in the foreland deformed 

zone adjacent to these segments were folded on northeasttrending axes during the collision. This structural fabric is 

particularly evident along the Marenisco segment (Sims 

and others, 1984; Sims, 1990, 1992).

CONCLUSIONS

We have suggested a reinterpretation of the COCORP 

seismic-reflection profiles in central Minnesota (Gibbs and 

others, 1984) that is consistent with kinematic data from 

the 10-km-long exposed segment of the GLTZ near Marquette, Michigan. We propose that the GLTZ in Minnesota is a north-verging steep southeast-dipping structure, 

locally called the Morris fault. We also propose that north 

vergence characterizes each segment of the tectonic zone. 

Such vergence on the structure can account for the early 

recumbent folds (Di), younger upright folds (02), and 

major transcurrent faults formed by regional dextral 

transpression in rocks in northern Minnesota (Hudleston 

and others, 1988) and adjacent areas in Ontario and Michigan. The relatively young age (=2,600 Ma; Doe and Delevaux, 1980) of the Sacred Heart Granite in the gneiss 

terrane in southwestern Minnesota and its volcanic-arc 

granite affinity (Sims, 1992) strongly support an origin for 

the GLTZ suture through southward subduction prior to 

continent-continent collision, at about 2,690 Ma.

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Transmittal of dextral-thrust shear into the crust to the 

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(1990b) from northern Minnesota, and Poulsen (1986) 

from adjacent Canada. In addition to early recumbent 

folds (Di) and younger upright east-west folds, the generally northwest to west trending dextral transcurrent faults 

and conjugate sinistral faults in the region developed as a 

later more brittle expression of the shear regime. These 

faults include the Vermilion, Rainy Lake-Seine River, and 

Quetico faults, which form the margins of or transect the 

major Archean subprovinces in northern Minnesota and 

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____1985, Correlation of early recumbent and younger upright 

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____1990b, Bedrock geologic map of the Rainy Lake area, 

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and Suda, R.U., 1980, Origin of the Morton Gneiss, 

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Hammond, R.D., 1978, Geochronology and origin of Archean 

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Transpression in an Archean greenstone belt, northern 

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Johnson, R.C., and Bornhorst, T.J., 1991, Archean geology of the 

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fluids at transpressive collisional boundaries and the origin 

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Morey, G.B., and Sims, P.K., 1976, Boundary between two 

Precambrian W terranes in Minnesota and its geologic 

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Morey, G.B., Sims, P.K., Cannon, W.F., Mudrey, M.G., Jr., and 

Southwick, D.L., 1982, Geologic map of the Lake Superior 

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Peterman, Z.E., Zartman, R.E., and Sims, P.K., 1986, A protracted 

Archean history in the Watersmeet gneiss dome: U.S. 

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Poulsen, K.H., 1986, Rainy Lake wrench zone An example of an 

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deWit, M.J., and Ashwal, L.D., eds., Tectonic evolution of 

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Texas, Technical Report 86-10, p. 177-179.

Sims, P.K., 1976, Early Precambrian tectonic-igneous evolution in 

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____1980, Boundary between Archean greenstone and gneiss 

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___1990, Geologic map of Precambrian rocks, Marenisco, 

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area, Michigan Implications for Archean tectonics

S10 Precambrian Geology of Lake Superior Region
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tectonic zone: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-2355, scale 1:24,000.

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1904-M, 19 p.

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Great Lakes tectonic zone in the Marquette area, 

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Shannon, 1991, Precambrian basement map of the TransHudson orogen and adjacent terranes, northern Great Plains, 

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northern Michigan: U.S. Geological Survey Journal of 

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Proterozoic rocks in the Marenisco-Watersmeet area, 

northern Michigan: U.S. Geological Survey Professional 

Paper 1292-A, 41 p.

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1985, Geology and geochronology of granitoid and 

metamorphic rocks of Late Archean age in northwestern 

Wisconsin: U.S. Geological Survey Professional Paper 

1292-C, 17 p.

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1989, Tectonostratigraphic evolution of the Early 

Proterozoic Wisconsin magmatic terranes of the Penokean 

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investigations of the Great Lakes tectonic zone, west-central 

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Programs, v. 15, p. 692.

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drilling in central Minnesota Summary of lithologic and 

geochemical results: Minnesota Geological Survey 

Information Circular 23,186 p.

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intracratonic structures: Geology, v. 11, p. 270-272.

Manuscript approved for publication November 2,1992 

Published in the Central Region, Denver, Colorado 

Photocomposition by Shelly A. Fields 

Graphics by Dennis L. Welp and Springfield & Springfield 

Edited by Lorna Carter

The Great Lakes Tectonic Zone Revisited S11

<rU.S. GOVERNMENT PRINTING OFFICE: 1993-774-049/66062

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