Structural Analysis of Archean Rocks in the 

Negaunee Area, Michigan Constraints on 

Archean Versus Early Proterozoic Deformation

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

Structural Analysis of Archean Rocks in the 

Negaunee Area, Michigan Constraints on 

Archean Versus Early Proterozoic Deformation

By S.A. NACHATILO and R.L. BAUER

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 

Robert M. Hirsch, Acting Director

Manuscript approved for publication December 11,1992 

Published in the Central Region, Denver, Colorado 

Photocomposition by Shelly A. Fields 

Graphics by Mari L. Kauffmann

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does not imply endorsement by the U. S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993

For sale by

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Library of Congress Cataloging-in-Publication Data

Nachatilo, S.A.

Structural analysis of Archean rocks in the Negaunee area, Michigan 

constraints on Archean versus early Proterozoic deformation / by S.A. Nachatilo 

and R.L. Bauer.

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

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

Includes bibliographical references.

Supt.ofDocs.no.: I 19.3:1904-0

1. Geology, Stratigraphic Archean. 2. Geology, Stratigraphic 

Proterozoic. 3. Geology, Structural Michigan Negaunee Region. I. Bauer, 

R.L. II. Title. III. Series. IV. Series: Contributions to Precambrian geology 

of Lake Superior region ; ch. O. 

QE75.B9 no. 1904-O 

[QE653.3]

557.3 s dc20 93-12779 

[551.7'12'09774] CIP
CONTENTS

Abstract Ol 

Introduction Ol

Regional geology and tectonic framework O3 

Archean O3 

Early Proterozoic O3 

Rock units and lithologies O3 

Archean Rocks O4

Kitchi Schist O4 

Mona Schist OS 

Undifferentiated greenstone OS 

Compeau Creek Gneiss O6 

Dead River pluton O6 

Reany Creek Formation O6 

Early Proterozoic rocks O6 

Metamorphism O6 

Structural features and their geometries O6

Structural correlation with adjacent areas O6 

Structural domains and analyzed features O6 

Regional 82 foliation and F2 folding O7 

Domain I O7 

Domain II O7 

Domain III O7

03 north-side-up, dip-slip shear zones O8

Distribution and timing of 03 shearing O8 

Carp River Falls shear zone (domain I) Oil 

Dead River shear zone (domain III) O12 

03 shear zones in domain II O12

04 structures O13

D4 crenulations O13

D4 kink bands O13

D4 strike-slip shearing O14 

Early Proterozoic folding, domain II O15 

Early Proterozoic reactivation of shear zones O16 

Finite strain analysis O16 

Introduction O16

Kitchi Schist and lower member of the Mona Schist O17 

Sheared rhyolite tuff member of the Mona Schist O18 

Early Proterozoic metasedimentary rocks O18 

Kinematic framework of the deformations O18 

Focus of discussion and results O18 

Archean deformations O19

D2 deformation O19

D3 deformation O19

D4 deformation O21

Relationship of D4 features to shear zones O21 

Stress orientations and kink band development 021 

Early Proterozoic deformation O22 

Effects of Early Proterozoic deformation on Archean basement rocks O23

Contents III
Interpretation of finite strain data O23 

Strains in domain I O23

D2 fabrics and finite strains O23 

Effects of D3 shearing O24 

Strains in domain III O24

Type-a rhyolite O25 

Type-b rhyolite O25

Implications for Archean tectonic history O26

Summary and conclusions O27

References cited O27

FIGURES

1. Regional geologic map of Upper Michigan O2

2. Geologic map of the Negaunee study area O4

3. Map outlining structural domains within the study area O8

4. Equal-area projections of D2 fabric elements O9

5. Photomicrograph of F2 crenulations and 82 cleavages O10

6. Photograph of F2 folds in basalt O10

7. Equal-area projections of 83 foliation poles Oil

8. Photograph of 03 en-echelon tension veins O12

9. Photomicrograph of asymmetrical pyrite porphyroclast deformed during 03 O12

10. Photomicrograph of sigma porphyroclasts formed during 03 O13

11. Equal-area projection of D4 crenulation hinge lines O14

12. Photograph of D4 kink bands O14

13. Schematic drawing of conjugate kink bands O15

14. Equal-area projections of D4 kink planes and kink hinges O15

15. Photograph of asymmetrical (D4) fold in a 03 shear zone O16

16. Equal-area projections of Early Proterozoic fabric elements and Early Proterozoic 

finite strain axes O17

17. Equal-area projection of finite strain axes and Flinn diagram of finite strains from 

domain I O19

18. Equal-area projections of finite strain axes and D4 stretching lineations, and Flinn 

diagram of finite strains from domain III O20

19. Equal-area projection of ai orientations estimated from D4 kink bands, and sketch 

of D4 kink band model O22

20. Schematic model of kinematic framework during D2, D3 , and D4 O24

TABLE

1. Deformation events affecting the Archean rocks of the Negaunee area O7

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Structural Analysis of Archean Rocks in the 

Negaunee Area, Michigan Constraints on 

Archean Versus Early Proterozoic Deformation

£yS.A. Nachatilo1 anc/R.L Bauer1

Abstract

A detailed analysis of deformation features and finite 

strain in the Archean rocks of the Ishpeming greenstone belt 

near Negaunee, Michigan, was undertaken to evaluate the 

deformation history of the area and to determine whether the 

Archean rocks sustained significant ductile deformation during 

the Early Proterozoic Penokean orogenic event. Our study 

indicates that the Penokean orogeny was primarily a thinskinned event in the Negaunee region and produced little ductile deformation in the Archean rocks.

Four periods of deformation, interpreted to be of Archean 

age, affected the Archean rocks. D} deformation is of local 

extent and is defined by a flat-lying foliation that is folded by 

F2 crenulations. D2 deformation significantly shortened the 

greenstone belt in a north to north-northeast direction and produced a nearly vertical west-northwest-striking S2 foliation that 

is axial-planar to upright F2 folds and crenulations. Finite strain 

analyses indicate constrictional D2 strains with steeply plunging X axes. D3 deformation produced brittle-ductile steeply 

dipping shear zones with north-side-up dip-slip movement that 

is consistent with a north-plunging maximum principal stress 

axis. D4 deformation produced prominent crenulations, local 

kink bands with monoclinic symmetry, and steeply dipping 

strike-slip shear zones during a period of northwest-directed 

shortening.

The S2 foliation in the Archean rocks and the orientation of the axial planes of Early Proterozoic folds are subparallel. However, D3 and D4 deformation features that deform 

the Archean rocks are not recognized in Early Proterozoic 

rocks and apparently developed during the Archean. Therefore, the D2 structures in Archean rocks, which are parallel 

to regional Early Proterozoic structures, are Archean in age. 

We conclude that the Early Proterozoic Penokean orogeny

'Department of Geological Sciences, University of Missouri, Columbia, Columbia, MO 65211.

caused little or no ductile deformation in the Archean basement in the Negaunee area.

INTRODUCTION

The Negaunee area of Michigan's Upper Peninsula 

(fig. 1) contains good local exposures of both Archean and 

Early Proterozoic rocks. The Archean rocks, which are the 

focus of this study, are part of the Ishpeming greenstone 

belt of the Archean northern complex (see Morgan and 

DeCristoforo, 1980) and are among the southernmost 

exposures of metavolcanic and metasedimentary rocks of 

the Superior province of the Canadian Shield (Card and 

Ciesielski, 1986). The Early Proterozoic metasedimentary 

rocks in this area occupy broad basins that overlie the 

Archean basement rocks, and they were deformed during 

the Early Proterozoic Penokean orogeny (approximately 

1,850 Ma) (Cannon, 1973).

Several workers (for example, Cannon and Klasner, 

1972; Klasner, 1972, 1978; Cannon, 1973; Klasner and others, 1988) have proposed thin-skinned deformation models 

to explain the strong regional deformation fabrics in the 

Early Proterozoic rocks and, in part, to explain their juxtaposition with Archean rocks. They suggested that a decollement exists between the Archean and Early Proterozoic 

rocks because of differences in the style of deformation 

between the Early Proterozoic rocks and the underlying 

Archean rocks. Recent work, however, has led to the recognition of two distinct and widely separated structural regimes 

in the Early Proterozoic rocks of northern Michigan (Klasner 

and others, 1988; Klasner and others, 1991; Klasner and 

Sims, 1992): (1) a foreland thrust belt in the north, in which 

Penokean deformation was largely thin skinned, and (2) an 

imbricate fold and thrust belt in the south, which is part of

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O1
90'

47° -

Middle Proterozoic 

Early Proterozoic 

Archean

0 10 20 30 40 50 KILOMETERS

I . I

Figure 1. Distribution of Precambrian rocks in the western Upper Peninsula of Michigan (modified from Puffett, 1974). Boxes outline the Negaunee SW (left) and Negaunee (right) quadrangles of this study. On Niagara fault zone, sawteeth on overthrust block.

a broad basement arch in which deformation involved both 

Archean basement and Early Proterozoic supracrustal rocks. 

Although thick-skinned Early Proterozoic deformation 

has been documented in Archean rocks of the southern complex (fig. 1), separating Archean from Early Proterozoic 

deformation within the Archean basement rocks in the northern complex is complicated. This is because the major deformation fabrics (Puffett, 1974; Clark and others, 1975) and 

the finite strain axes (Westjohn, 1978, 1986, 1987; Meyers, 

1983; Carter, 1989) in both Archean and Early Proterozoic 

rocks of the area are subparallel. Based only on these 

regional fabrics, one might conclude that Penokean orogenic 

activity was thick skinned, characterized by pervasive ductile deformation of Archean basement rocks. Even though 

regional deformation of the Early Proterozoic rocks in the 

Negaunee area is well documented (for example, Cannon, 

1973; Klasner, 1978, 1972; Klasner and others, 1988;

Klasner and others, 1991), detailed structural studies in the 

Archean rocks of the area have not been made. This report 

presents a detailed analysis of small-scale deformation structures, deformation fabrics, and finite strain in several of the 

Archean rock units. It was aided considerably by published 

geologic maps of the Archean rocks of the region (such as 

Gair and Thaden, 1968; Puffett, 1974; Clark and others, 

1975), which provided an excellent geologic base for our 

more detailed structural analysis.

The primary objectives of our structural studies were (1) 

to develop a structural model that is consistent with the 

kinematic framework indicated by ductile to brittle-ductile 

deformation features in the Archean rocks of the Negaunee 

area, and (2) to evaluate the relationship of this deformation 

to that in the adjacent Early Proterozoic rocks. To pursue 

these objectives, we analyzed deformation fabrics and finite 

strains within selected regions of the Archean rock

O2 Precambrian Geology of Lake Superior Region
exposures in the Negaunee area that are adjacent to 

deformed Early Proterozoic rocks of the Dead River basin 

and the Marquette trough (fig. 2). We also analyzed the 

geometry of an Early Proterozoic fold adjacent to the 

Archean rocks, which we used as a local standard for Early 

Proterozoic deformation.

The analysis presented in this report indicates that the 

Archean rocks of the Negaunee area contain deformation 

features associated with four Archean deformation episodes 

(Di, D2, 03, and 04). The most widespread and penetrative 

deformation (D2) produced a regional foliation (82) in the 

Ishpeming greenstone belt (Archean rocks north of the Marquette trough, fig. 2). Although the 82 foliation and D2 finite 

strain axes have orientations similar to those in adjacent 

Early Proterozoic rocks, post-D2 deformation features in the 

Archean rocks such as D3 dip-slip shears and 04 kink 

bands, strike-slip shears and crenulations indicate reorientation of the regional principal stress axes during the late 

stages of the orogenesis recorded in the Archean rocks. 

Because these structures are not recognized in Early Proterozoic rocks, they predate the Early Proterozoic deformation. We conclude that the Early Proterozoic Penokean 

orogeny imposed little or no ductile deformation on the 

previously deformed Archean rocks in the Negaunee area.

REGIONAL GEOLOGY AND 

TECTONIC FRAMEWORK

Archean

The Archean rocks within the region are part of the 

Ishpeming greenstone belt (Morgan and DeCristoforo, 1980) 

and consist mainly of metasedimentary rocks, felsic 

metavolcanic rocks, and metabasalts that are intruded by 

felsic plutons and local mafic and ultramafic bodies. These 

Archean rocks occur as nearly vertical easterly striking units 

(fig. 2) that are locally truncated by brittle to brittle-ductile 

faults of both Archean and Early Proterozoic age. Largescale recumbent folds that are refolded by upright, east-westtrending folds have been recognized in northern parts of the 

Ishpeming greenstone belt (Johnson and Bornhorst, 1991; 

Johnson, 1990); however, early recumbent structures have 

not been recognized in the Negaunee area.

Sims (1980) and Sims and others (1980) have suggested that the Ishpeming greenstone belt is an extension 

of the greenstone-granite terrane of Minnesota (Wawa 

subprovince) that lies north of the major Archean crustal 

boundary known as the Great Lakes tectonic zone 

(GLTZ). The GLTZ is a regional Archean suture zone 

extending from the central Dakotas eastward across central 

Minnesota, northern Wisconsin, and Upper Michigan. It is 

a zone of tectonism, first recognized in central Minnesota, 

that is associated with the suturing of a northern

greenstone-granite terrane and a southern gneissic terrane 

(Morey and Sims, 1976; Sims, 1991).

The Late Archean greenstone-granite terrane yields 

radiogenic ages of 2.6-2.8 Ga (Peterman, 1979), whereas 

the Middle Archean gneissic terrane yields ages as old as 

3.0-3.8 Ga (Goldich and others, 1970; Sims and Peterman, 

1976; Peterman, 1979; Sims and others, 1980). U-Pb isotopic ages from the metavolcanic and granitic rocks of the 

northern complex are about 2.75 Ga (Hammond and Van 

Schmus, 1978), and are consistent with those derived from 

other parts of the greenstone-granite terrane. The southern 

complex is interpreted to lie south of the GLTZ and primarily to be part of the gneiss terrane (see Sims, 1991), but no 

rocks in the southern complex have been found with isotopic 

ages as old as 3.0-3.8 Ga.

Early Proterozoic

Early Proterozoic metasedimentary rocks of the Marquette Range Supergroup were deformed during the 

Penokean orogeny (approximately 1.85 Ga). These rocks 

represent an extensive continental margin assemblage (Cambray, 1978) that is truncated on the south by the Niagara 

fault zone (Van Schmus, 1976; Klasner and others, 1988; 

Sims, 1990) (fig. 1), a broad zone of shearing and highly 

flattened rocks as much as 10 km wide (Klasner and others, 

1988) that separates the northern continental margin assemblage from a southern magmatic arc terrane (Van Schmus, 

1976; Sims and others, 1989). Sedlock and Larue (1985) 

interpreted the Niagara fault zone as a 1.85 Ga suture zone 

associated with the development of the Penokean fold belt 

in Upper Michigan and Wisconsin.

Deformation of the Early Proterozoic rocks in the 

Negaunee area is relatively simple compared to that imposed 

on rocks of this age near the Niagara fault zone. Most of the 

deformation fabrics are associated with upright folding. In 

the sparse outcrops exposing the contact between the Early 

Proterozoic rocks and the Archean rocks of the northern 

complex, the rocks are deformed in a brittle fashion and 

contain a cataclastic fabric (Gair and Thaden, 1968). Klasner 

and others (1988) suggested that these brittle structures were 

formed during the late stages of the Penokean orogenesis 

and are associated with vertical tectonic stresses resulting 

from block uplifts.

ROCK UNITS AND LITHOLOGIES

Precambrian rocks in the Negaunee area were first 

mapped and described by the U.S. Geological Survey in the 

late 19th century (Williams, 1890; Van Hise and Bayley, 

1895; Van Hise and others, 1897). During more recent mapping (Gair and Thaden, 1968; Puffett, 1974; Clark and 

others, 1975), the units were remapped and their 

stratigraphic nomenclature was revised. The stratigraphy in

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O3
87°35'

46°35' -

46°30' -

5 KILOMETERS

J

EXPLANATION

EARLY PROTEROZOIC 

Marquette Range Supergroup

ARCHEAN 

Dead River pluton 

Compeau Creek Gneiss 

Deer Lake Peridotite 

Reany Creek Formation

Lighthouse Point Member 

Sheared rhyolite tuff member 

Nealy Creek Member 

Lower member 

Undifferentiated greenstone

>:-Wks:-V- Kitchi Schist

 Contact Dashed where covered by water 

^^ ^^ ^^ Shear zone Dashed where covered by water 

 ^ ^ Axial trace of synform

A, locality referred to in text

Figure 2. Geology of the southern two-thirds of the Negaunee SW and Negaunee quadrangles. Letters A-E are outcrop localities 

described in the text. (Modified from Puffett, 1974, and Clark and others, 1975.)

this report is consistent with that of more recent studies, 

except for Johnson and Bornhorst (1991). Johnson and Bornhorst have proposed that the Lighthouse Point Member of 

the Mona Schist be redefined as the Lighthouse Point Basalt, 

of formational status. They continue to consider the rocks 

south of the Dead River shear zone (fig. 2) as part of the 

Mona Schist restricted. In this report, we use the 

nomenclature of Puffett (1974) and Clark and others (1975). 

The mapped distribution of these units is shown in figure 2.

Archean Rocks 

Kitchi Schist

The Kitchi Schist (unit Wks, fig. 2) comprises felsic 

metavolcanic rocks and amphibolite. The Carp River Falls 

shear zone truncates the southern boundary of the unit, adjacent to the Early Proterozoic metasedimentary rocks of the 

Marquette trough, and the Dead River basin defines part of

O4 Precambrian Geology of Lake Superior Region
its northern boundary (figs. 2, 3). The nature of the northeastern contact of the Kitchi Schist with the Mona Schist is 

unclear (see discussion following). Van Rise and Bayley 

(1895) named and first described the Kitchi Schist; Morgan 

and DeCristoforo (1980) subsequently subdivided it into 

amphibolite units and intermediate to felsic metavolcanic 

units. The relatively undeformed amphibolitic units (to the 

northwest) and the felsic metavolcanic units are separated 

by the northeast-trending ultramafic intrusive unit the 

Deer Lake Peridotite (fig. 2). The felsic metavolcanic rocks 

are dacitic to andesitic in composition and are locally intercalated with agglomerates. Puffett (1974) suggested that 

these rocks consist of subaerially deposited ash flows and 

that the agglomerates are more felsic pyroclasts.

Mona Schist

The Mona Schist, named by Van Hise and Bayley (1895), 

was subdivided by Gair and Thaden (1968) into a lower 

member consisting mostly of metabasalt interlayered with 

lesser amounts of metagabbro, metadiabase, and metapyroxenite, and upper members consisting of felsic pyroclastic 

rocks and rhyolite dikes. The south margin of the Mona 

Schist is bounded by the Carp River Falls shear zone (fig. 

2) (Puffett, 1974) and is in structural contact with Early 

Proterozoic metasedimentary rocks of the Marquette trough 

(fig. 2). The southwest margin is marked by a contact of 

uncertain origin with the Kitchi Schist. (See Puffett, 1974; 

Clark and others, 1975.)

Gair and Thaden (1968) and Puffett (1974) suggested 

that the lower member (unit Wl, fig. 2) is the oldest unit of 

the Mona Schist because the stratigraphic topping directions 

obtained from cusps of the pillowed metabasalts in the unit 

are consistently northward. However, the relative age relationship of the lower member of the Mona Schist and the 

adjacent Kitchi Schist is uncertain. Morgan and DeCristoforo (1980) suggested that the two units were deposited or 

emplaced during two different Archean volcanic cycles, 

with the lower member being the youngest because it unconformably overlies the Kitchi Schist. However, Puffett (1974) 

suggested that the relative ages of the lower member of the 

Mona Schist and the Kitchi Schist cannot be determined 

because of the poor exposure of their contact. Clark and 

others (1975) mapped this contact as a shear zone.

The upper part of the Mona Schist was subdivided by 

Puffett (1974) into the Nealy Creek Member (unit Wnc, fig. 

2), the sheared rhyolite tuff member (unit Wrt, fig. 2), and 

the Lighthouse Point Member (unit Wlp, fig. 2). The Nealy 

Creek Member, exposed both north and south of the Dead 

River Storage Basin (fig. 2), is a fine-grained, massive, 

sericitic schist and contains local compositional layering 

(S0). Gair and Thaden (1968) and Puffett (1974) suggested 

that the Nealy Creek Member is a subaerially deposited, 

felsic-pyroclastic unit because of its dacitic composition and

because it contains some pyroclasts. The unit also contains 

pelitic and psammitic metasedimentary rocks.

The sheared rhyolite tuff member of the Mona Schist 

(fig. 2) was mapped by Puffett (1974) as a broad zone of 

sheared rocks. In a detailed study of relative age and petrologic and geochemical relationships of rocks in the sheared 

rhyolite tuff member, MacClellan and Bornhorst (1989) concluded that rock units of different relative ages within this 

member are deformed differently. They described three different components of this unit: (1) a "rhyolite," designated 

here as type-a rhyolite, (2) an "undifferentiated foliated pillowed basalt," and (3) a "highly foliated rhyolite," designated here as type-b rhyolite. The type-a and type-b 

designations are arbitrarily chosen for this report to avoid 

possible confusion in later discussions of these rock units 

and because both of the rhyolites are locally foliated. The 

sheared rhyolite tuff member consists mainly of the type-b 

rhyolite. This unit, which locally is clearly a tuff, agglomerate, or volcanogenic metasediment, is a quartz-sericite 

schist containing quartz and plagioclase phenocrysts or 

clasts that are aligned in the foliation (Puffett, 1974; MacClellan and Bornhorst, 1989). The type-a rhyolite also has 

quartz and plagioclase phenocrysts, but it intrudes the typeb rhyolite. MacClellan and Bornhorst (1989) suggested that 

the type-a rhyolite correlates with the intrusive rhyolite of 

Fire Center mine located about 1 km north of the Dead River 

Storage Basin. (See Owens and Bornhorst, 1985; Johnson 

and others, 1986.) The rhyolite of the Fire Center mine is 

interpreted as an Archean intrusive rock because it crosscuts 

the volcanic rocks of the Lighthouse Point Member (discussed following) (fig. 2) but does not crosscut the Late 

Archean Reany Creek Formation (MacClellan and Bornhorst, 1989).

The Lighthouse Point Member of the Mona Schist consists of layered amphibolite and felsic pyroclastic rocks with 

subordinate intercalated chert and iron-formation (Morgan 

and DeCristoforo, 1980; Owens and Bornhorst, 1985; MacClellan and Bornhorst, 1989). The oldest and most voluminous rocks of this unit are amphibolites. Morgan and 

DeCristoforo (1980) suggested that these are pillowed basalt 

locally intruded by gabbro. Van Schmus (1974) obtained a 

Rb-Sr age of 2.75 Ga from a felsic tuff in this unit, but its 

age relative to other Archean rocks in the Ishpeming greenstone belt is not firmly established. Johnson and Bornhorst 

(1991) have described the nature and geometries of the 

deformational features in this unit.

Undifferentiated Greenstone

Puffett (1974) mapped a fault-bounded block of metaamphibolites, with lesser amounts of felsic metavolcanic rocks 

and gray sericitic shale, as undifferentiated greenstone (unit 

Wsg, fig. 2). This unit is strongly sheared and contains parts 

of the adjacent greenstone belt as well as Proterozoic 

metasedimentary rocks from the Marquette trough.

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O5
Compeau Creek Gneiss

The Compeau Creek Gneiss (unit Wg, fig. 2) is a variably 

foliated rock that ranges from tonalite to granite in composition (Puffett, 1974). This unit was first named and 

described by Gair and Thaden (1968), who observed that it 

intrudes parts of the Ishpeming greenstone belt. It is typically 

well foliated along its contact with wall rocks but is less 

foliated within interior parts of the unit.

Dead River Pluton

The Dead River pluton (unit Wp, fig. 2) consists of 

comagmatic syenite, diorite, and granodiorite (Puffett, 1974; 

MacClellan and Bornhorst, 1989) that are relatively unfoliated. Attempts to date this unit radiometrically have been 

unsuccessful (Van Schmus, 1974); however, it is interpreted 

to be of Late Archean age because of its nonconformable 

contact with adjacent Early Proterozoic metasedimentary 

rocks (Puffett, 1974).

Reany Creek Formation

Puffett (1974) described the Reany Creek Formation 

(unit Wrc, fig. 2) as consisting of a basal conglomerate, a 

middle chloride shale, and an upper unit of interbedded 

conglomerate, slaty graywacke, and horizons of quartzite 

and arkose. The clasts within the unit are derived from parts 

of the granite-greenstone belt, and consist mostly of lightcolored felsic intrusive rocks from the adjacent Dead River 

pluton. Puffett (1969) suggested that the clasts are dropstones, indicating that the deposit has a glacial origin during 

the Early Proterozoic. Sims (1991), however, believes that 

the unit was deposited in a fault-bounded transtensional 

basin (compare Norris and Carter, 1982) developed during 

Late Archean time.

Early Proterozoic Rocks

In parts of Upper Michigan, Early Proterozoic metasedimentary rocks of the Marquette Range Supergroup (Cannon 

and Gair, 1970) occupy broad, linear basins (fig. 1) that 

overlie Archean rocks. These Early Proterozoic rocks (unit 

Xs, fig. 2) are composed mainly of slate, metaquartzite, 

marble, and intercalated iron-formation that were deposited 

along a rifted continental margin prior to the Penokean orogeny (Van Schmus, 1976; Cambray, 1978; Sedlock and 

Larue, 1985; Klasner and others, 1988). The principal rock 

units exposed in the study area include the Ajibik Quartzite, 

Goodrich Quartzite, and Michigamme Formation (Clark and 

others, 1975; Puffett, 1974).

metamorphic conditions in the southern part of the belt (Puffett, 1974) and amphibolite-facies metamorphic conditions 

in the northern part of the belt (Johnson and Bornhorst, 

1991). All Archean rocks in our study area are within the 

greenschist-facies part of the belt.

The Early Proterozoic rocks of the study area lie in the 

chlorite zone (greenschist facies) of the Republic node of 

regional metamorphism of James (1955), which reached 

sillimanite grade (amphibolite facies) about 17 km to the 

west of the study area. Studies of the relationship between 

growth of metamorphic minerals and deformation have been 

done by Klasner (1972, 1978), primarily in pelitic rocks of 

the Marquette Range Supergroup (Cannon and Gair, 1970). 

Klasner showed that peak metamorphism postdated the formation of the regional west-northwest-trending structural 

fabric in the Early Proterozoic strata.

STRUCTURAL FEATURES AND 

THEIR GEOMETRIES

Structural Correlation with Adjacent Areas

Recent mapping and structural analysis of Archean 

metabasalt in the Ishpeming greenstone belt north of the 

Negaunee area show that these rocks were deformed during 

at least two phases of Archean folding (Johnson, 1990; 

Johnson and Bornhorst, 1991). The earliest deformation 

(Dj) is characterized by kilometer-scale recumbent folds 

(Fi) in tholeiitic basalt. These FI folds have west- to northwest-trending axial traces and shallow, easterly plunging 

hinge lines (Puffett, 1974; Johnson, 1990; Johnson and 

Bornhorst, 1991). Johnson (1990) described a second deformation (D2) north of the Negaunee area that folds a flatlying foliation (Si) that is axial-planar to FI recumbent folds. 

These F2 folds are widespread in the area; they are upright 

with nearly vertical, west-striking axial planes and contain 

a locally well developed $2 axial-plane foliation.

We have not recognized FI folds in the Negaunee area, 

although the Sj foliation is locally developed in the Lighthouse Point Member of the Mona Schist in the northern part 

of the area. The most widespread deformation fabric in the 

Negaunee area is the steeply dipping west-northwest-striking foliation that we correlate with the $2 foliation of 

Johnson and Bornhorst (1991). However, structural features 

and their geometries examined during our study (table 1) 

define two additional stages of deformation (D3 and 04). In 

the following section, we describe the deformation geometries and fabrics associated with each of these deformations 

in selected structural domains.

Metamorphism

Archean rocks in the Ishpeming greenstone belt contain 

mineral assemblages consistent with greenschist-facies

Structural Domains and Analyzed Features

Three individual domains (fig. 3) were chosen for 

detailed structural analysis and strain analyses of Archean

O6 Precambrian Geology of Lake Superior Region
Table 1 . Deformation events affecting the Archean rocks of the Negaunee area

Event Locati ons/Doma i ns Characteristics

Dl

D2

Domain III

Domains I, II, and III

FJ: recumbent folds; 

Sj: flat-lying foliation.

F2: upright folds and crenulations;

Carp River Falls shear zone

and 

Dead River shear zone, mostly.

Domains I, II, and III

S2: regional near-vertical west-northwest 

foliation.

East-west trending, dip-slip, northside-up, brittle/ductile shear zones. 

S3 (C) foliation.

Kink bands, crenulations; 

strike-slip shearing.

rocks; part of one of these domains (domain II) was also 

examined to evaluate the structural geometries in the adjacent Early Proterozoic rocks. Domain I, located in the 

southern part of the Negaunee SW quadrangle (fig. 1), north 

of the Marquette trough and south of the Dead River basin, 

contains a well-developed S2 foliation and a locally well 

developed 83 shear foliation associated with the Carp River 

Falls shear zone. Domain II includes exposures of adjacent 

Archean and Early Proterozoic rocks along the south margin 

of the Dead River basin. The Archean rocks of this domain 

contain local F2 folds and a well-developed S2 foliation that 

is locally cut by 03 shear zones and deformed by 04 crenulations and kink bands. The Early Proterozoic rocks of 

domain II have been folded by a west-northwest-plunging 

syncline (fig. 2) that is not evident in the adjacent Archean 

rocks. Domain III, in the northern part of the Negaunee 

quadrangle along the north margin of the Dead River basin, 

contains local F2 folds and crenulations and a well-developed S2 foliation that is deformed by 03 shearing associated 

with the Dead River shear zone. The deformation features 

in these domains are described in the following sections in 

their approximate age order of deformation.

Regional S2 Foliation and F2 Folding

Domain I

The Kitchi Schist in domain I contains both felsic 

metavolcanic and mafic amphibolite units. The felsic 

metavolcanic rocks are primarily tuffaceous, but include 

local agglomerates. The felsic units contain a weak to moderate S to L-S fabric that predates the Carp River Falls shear 

zone. However, the amphibolite units of the Kitchi Schist 

(northwest of the area of the Deer Lake Peridotite, fig. 2) 

show little or no deformation fabric and were clearly more 

competent than the felsic units during deformation (Morgan 

and DeCristoforo, 1980). Alignment of platy minerals in

the felsic rocks, mainly chlorite, defines a west-northweststriking nearly vertical S2 foliation (fig. 44).

The matrix of the felsic agglomerate rocks in domain I 

sustained more deformation than the matrix-supported felsic 

clasts. Felsic clasts are composed mostly of sericite with 

lesser amounts of albitic plagioclase. Strain shadows, 

defined by elongate clusters of quartz + calcite ± chlorite, 

occur extensively on the termini of the felsic clasts in a 

nearly vertical orientation. The orientation of the clast and 

their strain shadows indicate that tectonic elongation was 

nearly vertical during the development of S2 foliation.

Domain II

The metavolcanic and metasedimentary rocks of 

Archean Nealy Creek in domain II contain a well-developed, 

penetrative subvertical S2 foliation that generally strikes 

west-northwest (fig. 4B). The foliation is locally crenulated 

and kinked, but these structures were formed during latestage deformations.

Domain III

Features of deformation D2 in domain III include welldeveloped F2 folds, crenulations, and an associated axialplanar S2 foliation. The dominant S2 foliation within 

domain III is nearly vertical and strikes east (fig. 4Q. 

Sparse, shallowly plunging F2 crenulations of SQ and S\ 

occur in the Nealy Creek Member of the Mona Schist in this 

domain. The crenulations are typically asymmetrical microfolds with wavelengths from 1 to 10 mm. The axial planes 

of the crenulations are parallel to both the regional nearly 

vertical, west-northwest-striking S2 foliation and the axial 

planes of larger scale F2 folds. A well-developed S2 foliation 

that is axial-planar to the crenulations is present in many 

samples and occurs in two forms (see fig. 5). The first is a 

spaced foliation that ranges from slightly anastomosing to 

smooth (compare Powell, 1978) and contains thin layers of 

dark, insoluble residue indicative of a pressure solution

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O7
46°35' -

46°30'

R27W R26W

EXPLANATION

Early Proterozoic 

Archean

Figure 3. Distribution of structural domains I, II, and III of this study. Numbers are localities from which strain analyses were 

obtained. Contact dashed where approximate.

origin (right side of fig. 5). The second is a zonal to incipient 

zonal crenulation cleavage (compare Gray, 1977; Powell, 

1978) that is variably developed on the short limbs of asymmetrical crenulations (left side of fig. 5). $2 is also axialplanar to sparse, open to isoclinal upright folds (p2> of primary compositional layering (So) (fig. 4D). The axial planes 

of these shallowly to steeply plunging ¥2 folds are nearly 

vertical and strike east-northeast (fig. 4D).

Figure 6 shows an example of symmetrical ¥2 folds in 

the undifferentiated foliated pillow basalt within the 

sheared rhyolite tuff member of the Mona Schist (locality 

A, fig. 2). The metabasalt is intensely folded by 10-cmto 1-m-amplitude ¥2 folds that range from close to isoclinal. Type-a rhyolite dikes intrude the foliated pillow 

basalt but are not folded.

Most of the strong foliation within both the type-a and 

type-b rhyolites of the sheared rhyolite tuff member of the 

Mona Schist developed in response to localized shearing

(Puffett, 1974; discussed following) rather than regional D2 

shortening.

D3 North-Side-Up, Dip-Slip Shear Zones 

Distribution and Timing of D3 Shearing

Two major brittle-ductile shear zones within the study 

area, the Carp River Falls shear zone (CRFSZ) (domain I) 

and the Dead River shear zone (DRSZ) (domain III) (figs. 

2, 3, 7), are nearly vertical shear zones that may have been 

initiated during the early stages of deformation (Di or 02) 

in the Ishpeming greenstone belt. However, inasmuch as an 

83 shear foliation locally cuts both the earlier 82 foliation 

and upright F2 folds, a significant component of deformation 

along these (D3) shear zones clearly occurred after D2- Kinematic indicators within the shear zones, such as rotated

O8 Precambrian Geology of Lake Superior Region
Figure 4. Equal-area projections of D2 fabric elements in the Negaunee area. Great circles indicate the average S2 orientation, 

defined as the plane normal to the highest contoured concentration of plotted poles to S2 . A, Poles to S2 foliation (n=82) from the 

Kitchi Schist (felsic metavolcanic rocks) in domain I. B, Poles to S2 foliation (n=81) from domain II. C, Poles to S2 foliation (n=69) 

from domain III. D, Poles to F2 fold axial planes (solid squares, n=35) and F2 fold hinge lines (crosses, n=23) from domain III.

asymmetrical porphyroclasts and asymmetrical tension 

veins, are consistent with north-side-up relative 

displacements. These shear zones were locally reactivated 

during both Archean 04 deformation and Early Proterozoic

deformation (Puffett, 1974; Bornhorst, 1988). Bornhorst 

(1988) concluded that the CRFSZ is an Archean structure 

because it is crosscut by less deformed Archean metadiabase 

dikes (Baxter and Bornhorst, 1988).

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O9
Figure 5. Photomicrograph (plane light) of upright F2 crenulations of the ST foliation 

in the Nealy Creek Member of the Mona Schist in domain III. The section, cut normal 

to the nearly horizontal east-trending crenulation hinge line, contains an incipient 

zonal (nonpenetrative) S2 crenulation cleavage (left side of photo) and a smooth disjunctive S2 cleavage with dark lines of residue from pressure solution (right side of 

photo).

Figure 6. Close, upright, shallowly plunging ?2 folds in the undifferentiated foliated pillowed basalt from the sheared rhyolite tuff member of the Mona Schist (north shore of the 

Dead River Storage Basin, locality A in fig. 2). Hammer handle is 14 cm long. Arrow on 

photo indicates north.

O10 Precambrian Geology of Lake Superior Region
Carp River Falls Shear Zone (Domain I)

The east-trending Carp River Falls shear zone (CRFSZ) 

truncates the southern boundaries of both the Kitchi Schist 

and the lower member of the Mona Schist (fig. 2), and 

juxtaposes Archean rocks to the north and Early Proterozoic 

rocks to the south.

Shear fabric and kinematic data were gathered from primarily three areas in domain I (figs. 2, 3): (1) within the 

shear zone itself, (2) along the northeast shoreline of the 

southern segment of Deer Lake (locality B, fig. 2), and (3) 

northwest of Teal Lake (locality C, fig. 2). The orientation 

of poles to the 83 shear foliation (C planes of Lister and 

Snoke, 1984), shown in figure 1A, is subparallel to the 

regional 82 foliation in domain I. (See fig. 44.) These three 

shear zone areas, defined by well-developed phyllonitic fabrics, are most commonly confined to topographic depressions, and they are marked by deposition of carbonate 

minerals and quartz (Puffett, 1974) associated with hydrothermal alteration.

An excellent exposure of sheared rock within domain I 

occurs along the northeast shore of the southern part of Deer 

Lake (locality B, fig. 2). In this area, a conspicuous easttrending topographic low, approximately 30 m wide, defines 

the trace of a shear zone and contains phyllonites whose 

fabrics become stronger toward the center of the zone.

Rotated en-echelon tension veins in sheared rocks containing a strong planar fabric indicate north-side-up, dip-slip 

relative displacement (fig. 8). C-foliations (83) occur in the 

most strongly deformed parts of the shear zone. Agglomeratic clasts seem to be lacking in these strongly deformed 

areas, perhaps because they were destroyed by the large 

localized shear strains. The clasts in less deformed parts of 

the shear zone are elongated vertically.

Sheared Archean metavolcanic rocks within the mapped 

area of the CRFSZ are well exposed near the northwest shore 

of Teal Lake (locality C, fig. 2), and contain brittle-ductile 

kinematic indicators, such as asymmetrical agglomerate 

clasts, indicating north-side-up, dip-slip shear. This zone 

was also reactivated during the Penokean orogeny (Bornhorst, 1988), which produced brittle shears in the Archean 

rocks along the fault zone.

Shear zones of 03 within the Kitchi Schist northeast of 

Teal Lake also indicate north-side-up, dip-slip shearing 

(localities 1, 2, 4, and 6, fig. 3). Kinematic indicators in the 

shear zones include asymmetrical strain shadows on porphyroclasts (compare Passchier and Simpson, 1986), shear 

bands (compare Lister and Snoke, 1984), and rotated tension 

veins. Figure 9 is a photomicrograph of a sheared pyrite 

porphyroclast from this particular shear zone. Asymmetrical 

quartz-filled strain shadows that were precipitated in areas 

of relatively low pressure during the noncoaxial deformation

B

Figure 7. Equal-area projections of poles to 83 shear foliation 

(C planes) from A, domain I in Carp River Falls shear zone 

(n=33), and B, domain III in Dead River shear zone (n=48).

occur on the termini of the porphyroclast within the plane 

of 83 shear foliation. The sigmoidal shape of the quartz and 

chlorite grains is further confirmation of the noncoaxial 

strain imposed on these rocks.

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O11
Figure 8. D3 en-echelon tension veins; barbs demonstrate 

dextral shear in plane of photograph. Actual D3 shear within the shear plane is north-side-up within the Kitchi Schist 

(north lobe of southern part of Deer Lake, locality B, fig. 2). 

Arrow indicates north.

Dead River Shear Zone (Domain III)

The Dead River shear zone (DRSZ) (fig. 2) is a structure 

that we believe sustained most of its early deformation during DS and 04 events. Bornhorst (1988) has suggested that 

the DRSZ is a broad, regional shear zone that extends from 

the northwest margin of the Dead River basin to Marquette. 

MacClellan and Bornhorst (1989) and Johnson and Bornhorst (1991) have also suggested that initial displacements 

along the shear zone are Archean, and P.K. Sims (written 

commun., 1991) postulated that it accommodated some of 

the transtensional deformation associated with deposition of 

the Reany Creek Formation. Puffett (1974) has shown that 

the most recent movement along the shear zone occurred 

after deposition of the Early Proterozoic Michigamme Slate 

of the Marquette Range Supergroup and prior to intrusion 

of Middle Proterozoic diabase dikes.

The DRSZ contains deformation features indicating that 

shearing was brittle-ductile in nature. Fabrics range from 

strong S to L-S. The type-b rhyolite incorporates the

Figure 9. Photomicrograph (crossed nicols) of asymmetrical pyrite porphyroclast with asymmetric strain 

shadows of quartz that indicate sinistral (north-side-up) 

shear (see photomicrograph orientation markings). The 

shear sense barbs are parallel to the subvertical S3 , C foliation. The pyrite grain is approximately 1 cm long. 

Sample collected from S'/2 sec. 10, T. 48 N., R. 26 W., 

north of Teal Lake.

strongest phyllonitic shear fabric, although metabasalts and 

the type-a rhyolite dikes are also sheared. Kinematic indicators, such as asymmetrical porphyroclasts and en-echelon 

tension veins, give evidence for both transcurrent shearing 

that we attribute to D4 deformation (thus 84) and dip-slip 

shears that we attribute to 03 deformation.

D3 Shear Zones in Domain II

Some areas within domain II (for example, locality D, 

fig. 2) contain well-foliated felsic metavolcanic rocks (unit 

Wnc) that show clear evidence of brittle-ductile shearing. 

Sigma-type asymmetrical porphyroclasts (Passchier and 

Simpson, 1986; Simpson, 1986) with asymmetrical pressure 

shadows indicate north-side-up shearing (fig. 10). The 

strain-shadow mineral assemblage consists of quartz and 

chlorite, whereas the matrix consists of white mica, biotite, 

chlorite, and quartz.

O1 2 Precambrian Geology of Lake Superior Region
1 mm

Figure 10. Photomicrograph (crossed nicols) of sigma-type porphyroclasts formed 

during D3 shearing in the Nealy Creek Member of the Mona Schist along the contact 

of the Dead River pluton (domain III). Asymmetry of the porphyroclasts is consistent 

with dextral (north-side-up) displacement, indicated by barbs parallel to the subvertical S3/ C foliation. Sample collected from locality D (fig. 2).

D4 Structures

The youngest brittle-ductile deformation (04) affecting 

Archean rocks of the study area deformed rocks that contained a well-developed $2 foliation or 83 foliation. Folding 

associated with this deformation includes widely distributed 

crenulations and local kink bands. These structures developed independent of and subsequent to 03 as indicated by 

(1) kink bands and crenulations that deform 03 fabrics, (2) 

D4 shear features that indicate strike-slip relative displacements in contrast to 03 structures that display dip-slip relative displacements, and (3) maximum principal stress 

direction inferred from D4 kink bands being inconsistent 

with the moderately plunging, north-trending maximum 

principal stress axis associated with 03 deformation. (See 

discussion in subsequent sections.)

D4 Crenulations

D4 crenulations are evident in the field as steeply plunging (fig. 11) linear ridges on well-developed $2 or 83 foliation planes. They occur in Archean rocks of all sites studied 

but are particularly well developed in metasedimentary and 

metavolcanic rocks of domains II and III that contain abundant sheet silicates. D4 crenulations are always associated 

with D4 kink bands, but they are much more common than 

the kink bands. These D4 crenulations are most commonly 

asymmetrical with locally developed zonal crenulation 

cleavages. (See Gray, 1977, p. 768.) In some places, they

occur as well-developed microbuckles with an associated 

discrete crenulation cleavage.

The type-a rhyolites of the sheared rhyolite tuff member 

of the Mona Schist contain D4 crenulations that occur as 

closely spaced lineations on 82 or 83 foliation surfaces. The 

wavelength of these buckles is on the order of millimeters.

D4 Kink Bands

D4 kink bands are common in the sheared rhyolite tuff 

member of the Mona Schist (domain III), but they are less 

common in the Nealy Creek Member (domains II and III). 

Kink bands are associated with crenulations, but their morphology is distinct from the crenulations. The kink bands 

are defined by sharp angular displacements of 82 or the 83 

C-foliation surfaces (fig. 12). The crenulations, in contrast, 

have smoothly curved hinge areas, and have the geometry 

of similar-type microfolds.

Kink bands may form as a result of layer-parallel shortening or layer-parallel extension. However, the occurrence 

of the D4 kink bands locally as conjugate sets indicates that 

they are of contractional rather than extensional origin. 

(Compare Ramsay and Huber, 1987, p. 427.) The contractional origin of the kinks is also indicated by the equality of 

the angle between the kinked foliation and the kink plane 

(angle a) and the acute angle between conjugate kink planes 

(angle P) (fig. 13). Experiments conducted by Donath (1969) 

and Anderson (1974), in which highly foliated rocks were

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O13
EXPLANATION

0.9-10.0 percent 

10.0-20.0 percent 

20.0-24.6 percent

Figure 11 . Contoured equal-area projection of orientations of 

D4 crenulation hinge lines from all domains (n=112).

shortened parallel to the foliation, produced contractional 

kink bands with angle a approximately equal to angle p. 

Several field studies (for example, Kleist, 1972; Murphy, 

1988; Stubley, 1989) have verified the similarity of angles 

a and P associated with constrictional kink bands. Extensional kink bands, in contrast, are characterized by a geometry in which angle P is much greater than angle a (Ramsay 

and Huber, 1987, p. 430).

Most kink bands of this study, however, do not exhibit 

a conjugate relationship. Kink bands take on either an S or 

a Z symmetry (fig. 13) depending upon the local rotational 

sense imposed on the rock anisotropy. For example, figure 

12 shows monoclinic kink bands having only S symmetry. 

Throughout most of domains II and III only the steeply 

dipping, northwest-striking S-symmetry kink bands occur 

(fig. 144, B). Most of the sparse Z-symmetry kink bands dip 

steeply and have easterly strikes.

The hinge lines of the kink bands plunge vertically (fig. 

144, B) and are parallel to both the intersection of conjugate 

kink planes (S and Z symmetry) and the hinge lines of the 

04 crenulations (fig. 11). This parallelism and the fact that 

both the kink bands and the crenulations formed as a result

Figure 12. Well-developed S-symmetry kink bands in 

metasediment of the sheared rhyolite tuff member of the Mona 

Schist. The kinked foliation, S2, is parallel to compositional 

layering (S0) in the rock. Outcrop is on north shore of Dead 

River Storage Basin (locality A, fig. 2). Arrow indicates north.

of similarly oriented layer-parallel compression suggest 

their concomitant development.

04 Strike-Slip Shearing

Rotated asymmetrical tension veins, consistent with both 

dextral and sinistral strike-slip displacement, indicate latestage, brittle-ductile transcurrent shearing along both the 

Dead River and Carp River Falls shear zones.

A prominent shallowly to steeply plunging stretching 

lineation, which may have developed during the transcurrent 

shearing, occurs within both the type-b rhyolite and the typea rhyolite of the sheared rhyolite tuff member of the Mona 

Schist. Sparse agglomeratic clasts within the type-b rhyolite 

are elongate in the shear foliation (C plane) and plunge 

shallowly. These lineations are deformed by both 04 kink 

bands and 04 crenulations. Stretching lineations within the 

type-a rhyolite are defined by elongate quartz phenocrysts, 

and have a highly variable plunge within the steeply dipping, 

easterly striking plane of the shear foliation (compare fig. 

18Z?). Aggregates of white micas are aligned in the foliation 

and are locally deformed by well-developed 04 crenulations.

O14 Precambrian Geology of Lake Superior Region
CONTRACTIONAL KINK BANDS

Compression

<D

Figure 13. Schematic illustration of the geometry of conjugate kink bands formed by compression parallel to a preexisting planar rock fabric (modified from Ramsay, 1962). Note the 

angles a and p\ Barbs show relative direction of kink formation.

Steeply plunging folds in the most intensely deformed 

parts of the shear zones (fig. 15) have vertical hinge lines 

that are parallel to the hinge lines of 04 kink bands, but they 

are distinct from 04 kink bands because they have curved 

hinge zones. These vertically plunging folds could not have 

formed as a result of the nearly vertical shear displacement 

associated with 03 shearing. We suggest, however, that they 

may have formed in response to transcurrent shearing. The 

S-symmetry of the folds is consistent with folding of perturbed shear planes during the late stages of sinistral strikeslip shearing. (Compare Hudleston and others, 1988; Bauer 

and Bidwell, 1990.) The relationship of these folds to strikeslip shearing, however, cannot be unequivocally established.

Early Proterozoic Folding, Domain II

Recent workers (Klasner, 1972, 1978; Klasner and others, 1988) have suggested that the Early Proterozoic rocks 

of western Upper Michigan were subjected to at least three 

deformations. The first deformation formed gently plunging, 

upright folds with west-northwest-striking axial surfaces in 

response to north- to north-northeast-directed shortening 

(Klasner and others, 1988). (Note for example the westerly 

plunging synformal axial traces shown at locality E in fig. 

2.) The second deformation, associated with uplift of 

Archean rocks during the waning stages of the Penokean 

orogeny, is largely responsible for the development of structures such as the Marquette trough (Klasner and others, 

1988). The third deformation, also associated with the latestage uplift of Archean basement blocks, produced sparse 

conjugate kink bands in slaty rocks such as the Michigamme 

Formation (Klasner, 1978). The kink bands dip vertically 

and strike close to N. 20° E. (S symmetry) and N. 50° W. 

(Z symmetry). However, kinks are quite rare and most commonly occur in Early Proterozoic rocks that crop out near 

the west end of the northern complex (fig. 1).

The Early Proterozoic deformation fabrics that we examined in the Negaunee area are consistent with the regional

Figure 14. Equal-area projection of poles to (D4) kink 

planes of S symmetry (square) and Z symmetry (triangle), and 

the plunge of (D4) kink hinges (cross) from A, domain II; B, 

domain III.

orientation of Early Proterozoic fold axial planes associated 

with the first two Early Proterozoic deformations described 

by Klasner (1972, 1978) and Klasner and others (1988). We 

analyzed the geometry of folded Early Proterozoic Goodrich

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O15
Figure 15. Steeply plunging, asymmetrical fold in a D3 

shear zone along north shore of south lobe of Deer Lake 

(domain I). This shear zone was locally reactivated during 

D4 strike-slip shearing; the fold is interpreted to have 

formed as a result of sinistral strike-slip shearing during D4 

deformation. Arrow indicates north.

Early Proterozoic Reactivation of Shear Zones

Well-exposed segments of the Carp River Falls shear 

zone (domain I) north of Teal Lake (locality C, fig. 2) 

indicate Early Proterozoic reactivation of Archean basement 

rock. The Early Proterozoic Ajibik Quartzite contains a welldeveloped cataclastic fabric within the fault zone. Bedding 

of the quartzite in the fault zone is nearly vertical and strikes 

east, but bedding dips to the south at less than 60° just tens 

of meters south of the Carp River Falls shear zone. Vertical 

slickensides and vertical stretching lineations in the quartzite 

indicate that the last recognizable displacement on the fault 

was vertical. These observations and interpretations are consistent with those reported by Gair and Thaden (1968) in 

which well-exposed Archean rocks adjacent to Early Proterozoic rocks in the Marquette trough were displaced along 

a nearly vertical shear zone in a north-side-up relative sense.

Early Proterozoic brittle deformation in the Archean 

rocks in the fault zone is defined by veins that contain quartz 

and associated hydrothermal minerals. The vein assemblage 

in both the Late Archean Kitchi Schist and the Early Proterozoic Ajibik Quartzite across the fault zone includes quartz 

± carbonate minerals ± galena ± chalcopyrite ± pyrite. R.C. 

Johnson (written commun., 1989) suggested that this assemblage is a common base-metal mineralization producit precipitated from Early Proterozoic hydrothermal fluids.

In domain III (fig. 3), the structural juxtaposition of the 

Archean rocks to the north of the Early Proterozoic rocks 

also implies north-side-up, dip-slip relative displacement 

associated with Early Proterozoic reactivation of the Dead 

River shear zone. We did not, however, attempt to better 

define this younger brittle deformation, due to poor 

exposures.

Quartzite exposed in the west end of domain II (locality E, 

fig. 2), and used it as a local standard for Early Proterozoic 

deformation. Structural elements measured include bedding, 

cleavage that is axial-planar to folds in the bedding, and a 

lineation formed by the intersection of bedding and the 

cleavage. Poles to bedding (fig. 16A) define a well-constrained great circle with a best-fit Pi-axis plunging 23° to 

N. 69° W. The axial plane of the syncline dips steeply and 

strikes N. 69° W., subparallel to the spaced axial-planar 

cleavage that strikes approximately N. 70° W. (fig. 16A). 

The intersection lineation is well developed on foliation 

surfaces of the slaty layers and clusters about the best-fit Piaxis in figure 16A.

The eastern segment of the Goodrich Quartzite is adjacent to the Nealy Creek Member of the Mona Schist. However, the axial trace of the synform defined by bedding plane 

orientations in the Goodrich Quartzite does not extend into 

the adjacent Archean rocks.

FINITE STRAIN ANALYSIS 

Introduction

Analyses of finite strains in the rocks of the Negaunee 

area were completed recently by Carter and Palmquist 

(1986) and Carter (1989) in Archean rocks, and by Westjohn 

(1978, 1986, 1987), Meyers (1983), and Carter (1989) in 

Early Proterozoic rocks (fig. 16B). These studies were conducted to examine how the finite strains are related to the 

deformation fabrics developed in these rocks. The finite 

strain analyses completed for our study were obtained to (1) 

extend the work of Carter (1989), who attempted to distinguish Early Proterozoic and Archean finite strains in rocks 

of the southern Ishpeming greenstone belt, (2) assess the 

extent to which shear strains modified finite strain in the 

felsic Archean rocks in southern parts of the Ishpeming 

greenstone belt, and (3) determine the nature of finite strains 

imposed on type-a and type-b rhyolites in the Dead River 

shear zone. The results of our analysis indicate that (1) the

O16 Precambrian Geology of Lake Superior Region
XY principal plane of finite strain is subparallel to the 82 

foliation, and nearly vertical extension occurred during the 

development of 82 foliation; (2) finite strains determined for 

samples of the Kitchi Schist, which has obvious 03 shear 

indicators, are indistinguishable from finite strains determined for samples that do not have obvious 03 shear indicators; and (3) the orientations of the principal planes of the 

finite strain ellipsoid in the type-a and type-b rhyolites of 

the sheared rhyolite tuff member of the Mona Schist are 

significantly different from those in other parts of the Ishpeming greenstone belt. These different orientations reflect 

complex deformation within the Dead River shear zone.

Two-dimensional finite strain analyses were obtained 

using the Rf/0 technique, which has been widely used to 

determine strains from ellipsoidal particles with initially random orientations (Ramsay, 1967; Dunnett, 1969; Siddans 

and others, 1984). We obtained two-dimensional strain 

ellipse data from oriented hand samples, from photographs 

of flat outcrop surfaces, and from direct ellipticity measurements on flat outcrop faces containing deformed lapilli, 

agglomeratic clasts, or quartz phenocrysts. To maximize the 

accuracy of the analysis, the outcrop faces and slab cuts used 

in the analysis were near the orientation of the principal 

planes of the finite strain ellipsoid estimated from the rock 

fabrics. To evaluate the possibility of initial primary orientation of the clasts, we used the Isym technique of Lisle 

(1985). We concluded that the Rf/0 method was applicable 

to these rocks because 90 percent of the two-dimensional 

finite strain analyses met the Isym criteria for a 90-percent 

confidence interval (Nachatilo, 1991).

We applied Owens' (1984) technique to determine the 

best-fit finite strain ellipsoid from the two-dimensional finite 

strain ellipses obtained from three nonorthogonal planes. 

Because we used two-dimensional finite strain data for three 

independent planes in each analysis, the solution to the symmetrical strain tenor is overdetermined. Owens (1984) used 

this overdetermined condition to develop a criterion for 

goodness of fit to an ellipsoid. In all cases, our calculated 

ellipsoid shape fell within the acceptable criteria suggested 

by Owens (1984).

Kitchi Schist and Lower Member of the 

Mona Schist

Finite strain analyses were performed on agglomeratic 

clasts of the Kitchi Schist because they are similar in composition to their matrix, indicating a low competency contrast between the clasts and the surrounding matrix. A low 

clast-matrix competency contrast should produce relatively 

homogeneous clast-matrix deformation, so that the strain 

sustained by the clasts is equivalent to the bulk strain. However, petrographic observations showed that fabric development is stronger in the matrix, and that strain shadows 

developed parallel to the maximum principal axis of

Figure 16. Equal-area projection of Early Proterozoic fabrics 

and finite strain axes of the Negaunee area. A, Structures associated with the folded Goodrich Quartzite in domain II (fig. 2, 

locality E). Poles to bedding (open circle), poles to axial plane 

foliation (cross), bedding-foliation intersection lineations 

(small triangle), and K axis (large triangle) of the best-fit great 

circle (solid line) of poles to bedding. B, Orientation of principal axes (X, square; Y, open circle; Z, triangle) of finite strain in 

slate from the Early Proterozoic Kona Dolomite in the Marquette trough (data of Westjohn, 1978). The great circle shown 

is the best-fit XY principal plane of finite strain.

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O17
elongation (X) on the margins of the clasts. The strain analyses based on clast shape therefore record the minimum 

finite bulk strain.

We analyzed two types of agglomeratic samples one 

group containing obvious shear features associated with D$ 

north-side-up shear (type 1) and a second group lacking 

indication of shear (type 2). We believe that agglomerates 

subjected to the largest shear strains were tectonically disrupted and are not recognizable.

Figure 11 A shows the orientation of the principal axes of 

finite strain in both agglomerate of the Kitchi Schist (localities 1-8, fig. 3) and metabasalt of the lower member of the 

Mona Schist. The principal axes of the finite strain ellipsoid 

are not significantly different for type 1 and type 2 samples; 

the principal axes of finite elongation (X, Y, and Z) are 

nearly vertical, east trending, and north trending, respectively. The orientation of the Y principal plane of finite strain 

is subparallel to the 82 foliation (compare fig. 4A). The 

relative dimensions of the principal axes of the finite strain 

ellipsoid of the Kitchi Schist agglomerate, depicted on a 

logarithmic Flinn diagram, indicate moderately constrictional strains (fig. \1E). The relative dimensions of the principal axes of type 1 and type 2 samples are similar.

Finite strains from the metabasalt of the lower member 

of the Mona Schist (Carter, 1989) are similar to those determined from the Kitchi Schist agglomerate. (See fig. 17A, fi.) 

Carter (1989) used deformed varioles within pillow basalts 

as strain markers (Rf/0 technique) for his finite strain analyses. The orientations of the principal axes of finite strain 

are indistinguishable from the orientations of the principal 

axes of finite strain of the Kitchi Schist agglomerate, but the 

relative dimensions of the principal axes of finite strain 

(logarithmic Flinn diagram, fig. 17E) indicate that the magnitude of finite strain evident on the Kitchi Schist agglomerate is somewhat greater than and more constrictional than 

that evident in the lower member of the Mona Schist. A 

recent study of quartz strain shadows on hematite porphyroclasts in a chlorite schist within the lower member of the 

Mona Schist indicates almost 100 percent nearly vertical 

elongation (Palmquist, 1990), consistent with our estimates 

of finite strain.

Sheared Rhyolite Tuff Member of the 

Mona Schist

Two types of samples containing finite strain markers 

appropriate for the Rf/0 technique were chosen for finite 

strain analysis. The first type (type-a) contains deformed 

quartz phenocrysts from the type-a rhyolite (localities 10, 

12, and 13, fig. 3). The second type (type-b) contains matrixlike clasts from the agglomeratic parts of the type-b rhyolite 

(localities 9 and 11, fig. 3). The specimens were carefully 

chosen so that the phenocrysts were not kinked or 

crenulated.

Quartz phenocrysts were used as strain markers in the 

type-a rhyolite samples. These markers were significantly 

more competent than their matrix; accordingly, they record 

a relatively smaller proportion of the bulk finite strain sustained by the samples. The least principal axis of finite 

elongation (Z) trends northeast and plunges shallowly. The 

XY principal plane of finite strain is nearly vertical and 

strikes east-northeast (open symbols in fig. ISA). The orientations of the long (X) axes of finite elongation vary from 

shallowly to moderately plunging in the XY plane and are 

consistent with the variable orientation of stretching lineations in this unit (fig. 18fl). The relative dimensions of finite 

strain indicate both apparent constrictional and apparent flattening strains (fig. 18C).

Type-b rhyolite samples contain clasts that are similar in 

composition to the matrix and thus record a larger proportion 

of the finite bulk strain sustained than the type-a rhyolite. 

The orientation of the minimum axis of finite elongation (Z) 

plunges shallowly to the south-southwest and the XY plane 

is nearly vertical and strikes west-northwest.

Early Proterozoic Metasedimentary Rocks

Finite strain in Early Proterozoic rocks of the area was 

not measured during this study. However, previous studies 

of Carter (1989), Westjohn (1987, 1986, 1978), and Meyers 

(1983) provide representative finite-strain data for the Early 

Proterozoic rocks. The principal axes of finite elongation 

(Rf/0 technique) determined from reduction spots in slate in 

the Early Proterozoic Kona Dolomite from the Marquette 

trough (Westjohn, 1978) are shown in figure 16B. The maximum principal axis of finite strain (X) is nearly vertical, the 

intermediate principal axis of elongation is east-trending, 

and the minimum principal axis of elongation is north-trending. The XY plane of finite strain is consistent with the 

steeply dipping, east-striking rock cleavage we observed in 

the area of our reference Proterozoic fold (locality E, figure 

2; compare fig. 16A). The associated ellipsoid shape data, 

shown on the logarithmic Flinn diagram (fig. 18C), indicate 

apparent flattening.

KINEMATIC FRAMEWORK OF THE 

DEFORMATIONS

Focus of Discussion and Results

In our study we became convinced that the probable 

orientation of the regional stresses and related finite strain 

axes that developed during orogenesis are related to fabric 

elements in the study area. As an aid to interpreting the 

finite strain data and considering the implications for 

Archean orogenesis in the Ishpeming greenstone belt, we 

have outlined the kinematic framework of each deformation, 

including a mechanical interpretation of each separate

O18 Precambrian Geology of Lake Superior Region
deformation. This analysis indicates that the principal axis 

of stress (GI) was reoriented during Archean orogenesis. GI 

was subhorizontal and roughly north to north northeast 

trending during 02; moderately plunging and north trending 

during 03; and subhorizontal and east to northeast trending 

during 04. This change in the kinematic framework is 

lacking in the Early Proterozoic rocks of the region, which 

nevertheless contain fabric elements similar to those generated during the Archean D2 deformation. From this we 

conclude that deformation features and finite strains in the 

Proterozoic rocks postdate the 04 deformation in the 

Archean rocks and that the Archean rocks were subjected to 

little or no ductile Early Proterozoic deformation.

Archean Deformations

D2 Deformation

The relationships among fabric orientation, the orientation of the principal axes of finite strain (X, Y, and Z), and 

the instantaneous principal stress axes (GI, G2, and 03) during progressive deformation have been addressed by numerous workers (for example, Wood, 1974; Williams, 1978; 

Gray and Durney, 1979). These studies indicate that foliation normally develops parallel to the XY principal plane of 

the finite strain ellipsoid. Although crenulation cleavages 

may be initiated within the plane of maximum shear stress 

in rocks containing a strong preexisting deformation fabric 

(Tewksbury, 1986; Hobbs and others, 1976; Cosgrove, 

1976), this relationship is uncommon (Tewksbury, 1986), 

and clearly does not provide an appropriate model for the 

development of 82 foliation considered here.

The regional orientation of the 82 foliation in the study 

area is steeply dipping and west northwest striking (fig. 

4A-C). 82 foliation is nearly parallel to the XY plane of the 

finite strain ellipsoid defined by finite strains attributed to 

D2 (fig. 17A) and is consistent with north- to north-northeast-directed subhorizontal coaxial shortening (Nachatilo 

and Bauer, 1990). The strain analyses further indicate that 

the maximum principal axis of elongation (X) was nearly 

vertical during D2 deformation. The orientation of D2 deformation fabrics in the study area is consistent with those in 

other areas of the Ishpeming greenstone belt. (See Johnson 

and Bornhorst, 1991; Clark and others, 1975; Puffett, 1974.)

03 Deformation

Brittle-ductile 03 shearing is localized within the Dead 

River and the Carp River Falls shear zones as well as along 

the contact between the Dead River pluton and the Nealy 

Creek Member of the Mona Schist. 03 shear features, 

including en-echelon tension veins (fig. 8), S-C mylonitic 

fabrics, and asymmetrical strain shadows (fig. 9), indicate 

localized noncoaxial deformation leading to nearly vertical, 

north-side-up relative displacement on east-striking shear

B

0.5 -|

~ 0.3 -

0.1 -

0.0

0.0 0.1 0.3

Log(Y/Z)

0.5

Figure 17. Finite strain data (X, Y, and Z) from rocks of the 

southern Ishpeming greenstone belt (domain I). A, Equal-area 

projection of the orientation of principal axes of finite strain in 

felsic agglomerate of the Kitchi Schist (open symbols, this 

study) and metabasalt of the lower member of the Mona Schist 

(solid symbols, data of Carter, 1989). Data points from samples 

of the agglomerate that contain obvious shear fabrics (type 1) 

include a cross in the open symbol; open symbols without a 

cross are data points representing samples with no indication 

of shear (type 2). B, Log Flinn diagram of finite strain from agglomerate of the Kitchi Schist (triangles, this study) and metabasalt of the lower member of the Mona Schist (crosses, data of 

Carter, 1989). Solid triangle, type-a rhyolite with obvious shear 

fabrics; open triangle, type-b rhyolite lacking shear fabrics.

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O19
zones (fig. 7A, B). In contrast to the D2 deformation, the 

kinematic framework of 03 features developed in response 

to a steeply to moderately north plunging maximum principal stress.

The geometry and kinematic framework of shearing 

deduced by Johnson (1990) in other parts of the Ishpeming 

greenstone belt are consistent with the results from this 

study, but the interpretation of the origin of the shearing 

differs. Johnson (1990) reported a shear zone in the Lighthouse Point Member of the Mona Schist north of the DRSZ 

that dips steeply to the north and is consistent with northside-up relative displacement. The shear zone cuts across 

the flat-lying structural grain of bedding and Si foliation. 

Johnson interpreted the shear zone to be caused by north- to 

north-northeast-directed shortening that accompanied DI 

and D2 deformations.

Two possible scenarios could explain the orientation and 

relative displacement on the 03 shear zones. In the first 

model, consistent with Johnson's interpretation, the shear 

zones were initiated at relatively low angles while the greenstone belt was being horizontally shortened (D2), and the 

shear zones were progressively rotated to nearly vertical 

orientations. In the second model, the shear zones were 

initiated at their current nearly vertical orientations in a 

kinematic framework in which the maximum principal stress 

axis (GI) was more steeply dipping than that envisioned for 

the D2 deformation.

The first model requires the accumulation of large shear 

strains during progressive rotation of fault planes from typical low-angle reverse faults (approximately 20°-40°) to 

nearly vertical orientations. Our strain data from the Kitchi 

Schist, which are clearly distinct in orientation and magnitude from the D2 strains (figs. 17, 18), do not indicate that 

such large shear strains exist in 03 shear zones. We are not 

certain that our strain data from samples containing D3 shear 

indicators necessarily reflect the full magnitude of D2 strains 

that may have been concentrated within the shear zones. 

Nevertheless, we think it unlikely that the contact between 

the Dead River pluton and the metavolcanic rock of domain 

II, which displays locally well developed 03 shear (fig. 10),

Figure 18 (facing column). Finite strain data from rocks of the 

sheared rhyolite tuff member of the Mona Schist. A, Equal-area 

projection of the orientation of principal axes of finite strain (X, 

Y, and Z) from samples in domain III. Three of the samples are 

type-a rhyolite that contain deformed quartz phenocrysts (open 

symbols) and two samples are type-b rhyolite that contain matrix-like clasts (solid symbols). X, dots/circles; Y, squares; Z, triangles. B, Equal-area projection of the orientation of 04 

stretching lineations from the type-a rhyolite (domain III) defined by elongate quartz phenocrysts. C, Log Flinn diagram of 

finite strains from the sheared rhyolite tuff member (domain III) 

and slate of the Early Proterozoic Kona Dolomite (solid 

squares, data of Westjohn, 1978). The data from the present 

study include analyses from type-a rhyolite (open circles) and 

type-b rhyolite (solid triangles).

0.5 -i

0.3-

0.1 -

0.0

o

o

o

0.0 0.1 0.3 0.5 

Log(Y/Z)

O20 Precambrian Geology of Lake Superior Region
could have originated as a shallow southerly dipping shear 

zone that was rotated to its current vertical orientation.

We believe the mechanical constraints imposed by our 

observations adjacent to the Dead River pluton and our finite 

strain data to be more consistent with the second model for 

the development of 03 shear zones, in which the shear zones 

developed after D2 deformation during which steep fabrics 

in the rock sequences were developed. This model requires 

a steepening of the greatest principal stress in the vicinity of 

the shear zones in order to effect the north-side-up displacement. We cannot, however, eliminate a link between the D$ 

shearing and progressive D2 deformation. The shear zones 

may have become localized during the progressive steepening of the east-trending rock units during D2. This could 

explain the consistency of the principal axes of strain 

between rocks affected only by D2 and those containing both 

D2 and 03 features. Our data do not definitely resolve this 

relationship.

04 Deformation

04 deformation is characterized by the common development of 04 crenulations and by a more restricted development of crenulation cleavages, kink bands (fig. 12), and 

strike-slip shear features. D4 structures are best developed 

along 03 shear zones and within Archean rocks that contain 

a strong 82 foliation. Well-developed 04 crenulations are 

generally asymmetrical, and 04 contractional kink bands 

have monoclinic symmetry and do not commonly occur in 

conjugate sets. 04 strike-slip brittle-ductile shearing promoted the development of en-echelon tension veins.

Relationship of 04 Features to Shear Zones

We have discussed the small-scale buckles associated 

with 04, assuming that they are microbuckles developed 

through foliation-subparallel shortening. However, recent 

workers (for example, Platt and Vissers, 1980; Platt, 1983, 

1984; Dennis and Secor, 1987, 1990; Hudleston and others, 

1988; Bauer and Bidwell, 1990; Williams and Price, 1990) 

have shown that buckling phenomena may occur in response 

to continued noncoaxial strains in developing shear zones. 

In such cases, C foliations developed within the shear zone 

may undergo buckling in response to local perturbations in 

the finite strain field during continued shear. Figure 15 

shows a steeply plunging fold that may have developed by 

this mechanism during the 04 strike-slip shearing.

Williams and Price (1990) conducted shear deformation 

experiments on initially foliated rock and showed that kink 

bands with conjugate geometries can be developed in shear 

zones where the boundaries of the shear zone are inclined 

to the preexisting foliation. Although 04 kink bands are 

found within shear zones in the study area, we interpret the 

development of these structures as a result of layer-subparallel shortening, in contrast to rotational deformation during

progressive shear. Two observations lead to this conclusion. 

First, kink bands and crenulations with locally developed 

crenulation cleavages occur within rocks that do not appear 

to be sheared and that are in areas far from the Dead River 

and the Carp River Falls shear zones. For example, they are 

observed in domain II more than 5 km outside the region 

mapped as the Dead River shear zone. The 04 crenulation 

hinge lines (fig. 11), parallel to the vertical hinge zones of 

kink bands (fig. 14A, fl), are a well-developed fabric element 

in the study areas. Fabric development within the boundaries 

of a shear zone, in contrast, is much more localized (Ramsay, 

1980). Although 04 crenulations occur extensively in the 

shear zones, such mechanical behavior is predicted for rocks 

containing a strong preexisting fabric, that are shortened 

subparallel to the rock anisotropy (Paterson and Weiss, 

1966; Cobbold and others, 1971; Cosgrove, 1976).

Also, the morphology of the kink bands (fig. 12) is inconsistent with crenulations having developed during progressive shear (Dennis and Secor, 1987, 1990). The angular 

relationship of the kink planes with respect to the foliation 

planes of the kink bands is consistent with those produced 

in experimental compressional deformation of foliated rocks 

(Donath, 1969; Anderson, 1974); that is, angle a is approximately equal to angle (3 (fig. 13).

Although we interpret the development of 04 folds outside shear zones to be a result of buckling instabilities due 

to layer-subparallel shortening, some of the folding that 

occurs locally within the shear zones may have developed 

in response to progressive shear. For example, asymmetrical 

folds, like the fold shown in figure 15, occur in strongly 

sheared rock in domain I and may have resulted from sinistral, strike-slip shearing.

Stress Orientations and Kink Band Development

The orientation of kink band propagation bears a predictable relationship to the imposed stress field (Ramsay, 1962; 

Paterson and Weiss, 1966; Gay and Weiss, 1974; Weiss, 

1980). In triaxial compression experiments where a material 

with a strong preexisting foliation is loaded with GI parallel 

to the plane of the foliation, conjugate kink bands develop; 

GI bisects the obtuse angle between the kinks and G2 is 

parallel to the intersection of the kink planes. In experiments 

where strongly foliated rocks are loaded with G\ slightly 

oblique to the plane of foliation, one set of kink bands tends 

to develop preferentially over the other (Donath, 1969; Cobbold and others, 1971; Gay and Weiss, 1974). On the basis 

of these experiments, several field studies have used the 

orientation of conjugate kinks to constrain the orientation of 

the principal stress axes associated with kink band development. (See for example Kleist, 1972; Tobish and Fiske, 

1976; Tewksbury, 1986; Murphy, 1988; Babaie and Speed, 

1990).

The 04 kink bands observed in this study are more commonly of S symmetry than Z symmetry, indicating that the

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O21
axis of maximum principal stress (GI) was inclined to the 

rock anisotropy. By assuming that Oi bisects the obtuse 

angle between sets of conjugate kink bands and noting that 

the obtuse angle between these conjugate kink bands averages 115°, Oi can be computed from the kink band data set 

(fig. 14A, B). Figure 19A shows the trend of GI assuming 

that it was inclined 57.5° from the kink band within a horizontal plane. The results indicate that during D4 the trend 

of GI was west to northwest, G2 was nearly vertical, and 03 

was north to northeast trending.

Experimental deformation studies of phyllites under 

moderate to high confining pressure indicate that the principal stress cannot be inclined more than approximately 30° 

to the rock anisotropy and still produce kink bands (Paterson 

and Weiss, 1966; Donath, 1968). Gay and Weiss (1974) 

suggested that if the rock anisotropy is inclined more than 

5° to GI, a single set of kink bands (S or Z symmetry, see 

fig. 12) would develop in most cases. The results of these 

deformation experiments predict that S-symmetry kink 

bands would form more easily than the Z-symmetry kink 

bands in rocks of this study. If the orientation of the preexisting 82 foliation was nearly vertical and striking approximately N. 70° W. (its present orientation), a northwestdirected compression would impose a preferential counterclockwise (S-symmetry kink) rotation on the foliation in this 

stress regime (fig. 19fl). The calculated west- to northwestdirected compression is within the 30° limit (Paterson and 

Weiss, 1966; Donath, 1968) of GI inclination from the rock 

anisotropy.

Early Proterozoic Deformation

Klasner and others (1988) suggested that three periods 

of deformation affected Early Proterozoic metasedimentary 

rocks in Upper Michigan. The first two were coaxial (Klasner, 1972, 1978; Klasner and others, 1988; Klasner and 

others, 1991). The foliations associated with these deformations are nearly vertical and strike west-northwest (see 

Klasner, 1972, 1978), consistent with north- to north-northeast-directed shortening.

The second deformation was associated with late-stage 

block uplifts and reactivation of Archean faults. This deformation produced features such as the Marquette trough (fig. 

2). Klasner (1978) suggested that this stage of deformation 

occurred within a kinematic framework defined by a steeply 

plunging maximum principal stress axis. The relative displacement on dip-slip shear zones developed during this 

deformation was brittle and north-side-up.

The third deformation produced conjugate kink bands. 

However, the geometry of conjugate kink bands in Early 

Proterozoic rocks indicates compression at an inclination of 

less than 5° to preexisting foliation. Klasner (1978) 

reported that these rare, vertically dipping kink bands strike 

N. 20° E. (Z symmetry) and N. 50° W. (S symmetry). The 

bisector of the obtuse angle formed between these kink

Figure 19. Principal stress orientations inferred from kink 

bands in Archean rocks. A, Equal-area projection of the calculated orientation of the maximum principal stress axis (CTJ) from 

kink band orientation data collected from domains I (solid 

square), II (triangle), and III (cross). Not shown are 02- which 

is nearly vertical, and 03, which is north to northeast trending. 

B, Schematic representation of the development of kink bands 

within the inferred D4 stress regime (northwest-directed compression). Note that compression oblique to preexisting deformation fabric results in kink bands with primarily S symmetry. 

(See also fig. 13.)

planes, approximating the orientation of the maximum principal stress axis (GI) that kinked the foliation, trended N. 

75° E. The intermediate principal stress axis (02), parallel

O22 Precambrian Geology of Lake Superior Region
to the intersection of the conjugate kink bands, was nearly 

vertical. The minimum principal stress axis (03), bisecting 

the acute angle between conjugate kink bands, trended N. 

15° W. Klasner and others (1991) described a north-northwest-striking Proterozoic 83 foliation dipping moderately 

to the west that deforms a mylonitic foliation in Archean 

gneiss of the Plumbago Creek area along the west margin 

of the northern complex. This foliation may have formed 

under the same regional stress regime that produced the 03 

Early Proterozoic kink bands, but this foliation orientation 

is not observed in the Archean rocks of the Negaunee area.

Effects of Early Proterozoic Deformation on 

Archean Basement Rocks

The kinematic frameworks indicated by deformation 

features in Archean and Early Proterozoic rocks are similar; 

both north-side-up, dip-slip relative displacement and horizontally transmitted shortening subparallel to foliation are 

indicated by small-scale deformation features in Early Proterozoic and Archean rocks. However, certain notable differences exist between these Early Proterozoic and Archean 

deformation features. First, the orientation of GI inferred 

from the geometry of contractional kink bands in Early 

Proterozoic metasedimentary rocks indicates a late-stage, 

horizontal compression oriented N. 75° E. However, the 

geometry of kink bands in the Archean rocks indicates a 

horizontal compression that is west to northwest trending 

(fig. 19A). Second, the occurrence of kink bands in Early 

Proterozoic rocks in Upper Michigan is not extensive; they 

are only reported in areas near the west end of the northern 

complex (Klasner, 1978). Early Proterozoic slate outcrops 

are common, containing well-developed rock cleavage that 

is subparallel to Archean 82 foliation. That a late-stage, 

foliation-subparallel shortening during Early Proterozoic 

orogenesis could have produced D^ and 04 deformations 

features in the Archean rocks but not in the Early Proterozoic rocks is unlikely. For example, D4 fabric elements, 

such as crenulations, occur extensively in the Archean 

rocks of the study areas but are not well developed in the 

Early Proterozoic rocks. Third, the 03 shearing imposed 

on Archean rocks is brittle-ductile in nature, whereas shearing of Early Proterozoic rocks in the Marquette syncline is 

brittle. It is more likely that Early Proterozoic deformation 

is imposed on Archean rocks along reactivated fault zones 

in a brittle fashion. Finally, an Early Proterozoic syncline 

(locality E, fig. 2) that overlies Archean rocks in domain II 

contains locally developed rock cleavage that is axial-planar to folded phyllite and quartzite bedding (fig. 16A). 

These Early Proterozoic planar fabrics and the axial plane 

of the upright fold are subparallel to the 82 foliation in 

adjacent Archean rocks (see fig. 4fl). Despite this parallelism, the adjacent Archean rocks show no evidence of folding by this Early Proterozoic fold.

INTERPRETATION OF FINITE STRAIN DATA

Several mechanisms can reasonably be put forth to 

explain the finite strains in rocks of the southern (domain I) 

and central (domain HI) parts of the Ishpeming greenstone 

belt based on the geometry of fabric data. Our data herein 

provide support for the kinematic framework outlined in the 

previous section.

Rocks of the Ishpeming greenstone belt may have accumulated increments of finite strain through a variety of 

deformation mechanisms that are summarized schematically 

in figure 20. These mechanisms include (1) shortening 

associated with early FI recumbent folding (not observed in 

the study area), (2) shortening associated with upright F2 

folding, (3) dip-slip shearing during D3 deformation, (4) 

shortening subparallel to the 82 and 83 foliations that led to 

buckling of these preexisting deformation fabrics during D4 

deformation, (5) strike-slip shearing during D4 deformation, 

and (6) shortening associated with a thick-skinned Penokean 

orogeny.

From the available finite strain data, we conclude that 

(1) finite strain imposed on the felsic metavolcanic rocks of 

domain III and the metabasalt of the lower member of the 

Mona Schist resulted primarily from D2 deformation. 

Although some of the rocks from this domain show evidence 

of 03 shearing, the strains measured in obviously sheared 

samples are not significantly different than those measured 

in apparently unsheared samples. (2) Finite strains imposed 

on the type-a rhyolite of the sheared rhyolite tuff member 

of the Mona Schist are strongly affected by strike-slip shearing associated with D4 deformation.

Strains in Domain I

D2 Fabrics and Finite Strains

The XY principal plane of finite strain in the Kitchi 

Schist in domain I (figs. 3 and 17A) strikes west-northwest 

and is parallel to the local 82 foliation (fig. 4A). Furthermore, the X axis of principal elongation is parallel to steep 

linear elements in the schist. The orientation of principal 

axes of finite strain in the adjacent lower member of the 

Mona Schist (Carter, 1989) is indistinguishable from those 

of the Kitchi Schist (fig. ISA); however, the magnitude of 

strain and the degree of apparent constrictional strain are 

both higher in the Kitchi Schist. The differences in the 

relative ratios of the principal axes of finite strain between 

the agglomerate of the Kitchi Schist and the lower member 

of the Mona Schist could be explained by differential volume loss, problems in the application of the Rf/0 technique, 

or differential ductility of these rock units.

Carter (1989) suggested that volume loss did not affect 

the strain markers of the lower member of the Mona Schist 

metabasalt because the devitrified glass varioles are

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O23
A

H-

\

Figure 20. Schematic representation of the proposed 

Archean fabric geometries, kinematic framework of A, D2 ; B, 

D3 ; and C, 04 deformations, and orientations of the principal 

axes of stress (a-j, 02, and 03) (D^ was not recognized in the 

study area). The principal axes of both elongation (X, Y, and Z) 

and principal stress are only constrained for D2 deformation 

where the coincidence of stress and strain axes assumes progressive pure shear strain.

relatively immobile and therefore dissolution was negligible 

during greenschist-facies metamorphic conditions. Substantial volume loss within the clasts may also be ruled out for 

the Kitchi Schist because foliation resulting from pressure 

solution effects does not crosscut the clasts.

Problems with the finite strain analysis technique in all 

probability did not contribute significantly to errors in the 

analysis because the fabric data coincide with the principal 

axes of finite strain and the Isym test indicates no contribution of initial preferred orientation of the strain markers to 

the finite strain. Carter (1989) found that the Rf/0 technique 

was also effective in accurately defining the finite strain

ellipsoid. For example, the XY principal plane of finite strain 

determined from the varioles is subparallel to the foliation, 

and symmetrical strain shadows on plagioclase porphyroclasts indicate near-vertical tectonic elongation. Carter also 

found no problem with a preexisting fabric orientation based 

on the Isym test.

Differences in the finite strains between the Kitchi Schist 

and the Mona Schist may, however, be explained by differences in their relative ductility and competence contrast 

during deformation (Nachatilo and Bauer, 1990). Ramsay 

(1983, p. 118) suggested that under greenschist-facies temperature and pressure conditions, metabasalt is one of the 

most competent rock units found within the crust. The Mona 

Schist may have been less responsive to the imposed strain 

than the Kitchi Schist. Furthermore, Freeman and Lisle 

(1987) have shown that clasts that are more competent than 

their matrix (such as the clasts used for the strain determination in the Kitchi Schist) show a more prolate strain than 

the bulk strain of the rock.

Effects of 03 Shearing

Well-exposed parts of the Carp River Falls shear zone 

along the north shore of the southern lobe of Deer Lake 

(locality B, fig. 2) contain agglomeratic zones. The agglomerates that are spatially associated with well-developed shear 

zones contain clasts that show a well-defined nearly vertical 

preferred orientation. However, the finite strains of rocks 

that contain obvious shear fabrics (type 1) and those that 

lack shear fabrics (type 2) are indistinguishable (fig. 17A, 

B). Two possibilities can explain these similarities: (1) 03 

shearing did not significantly affect the finite strain accumulated by the clasts, and (2) 03 shearing affected all the rocks 

but did not result in the preservation of kinematic indicators 

in all the rocks. Our data do not resolve these possibilities.

Two logistical problems hindered evaluation of these 

hypotheses. First, the boundaries of the shear zones are often 

difficult to delineate because 82 and 83 foliations are subparallel (figs. 4A and 7A) and outcrops are poor and discontinuous. Second, small and large finite strains are not 

represented in the analysis; this occurs because agglomerate 

clasts within zones of the highest shear strains were either 

absent or destroyed, and samples that did not demonstrate 

obvious tectonic strains were not analyzed.

Strains in Domain III

Recent work by Johnson (1990) indicates that the finite 

strains in rocks of the northern part of the Ishpeming 

greenstone belt have an east-trending maximum principal 

axis of elongation, whereas rocks of the southern Ishpeming greenstone belt display finite strains with a nearly 

vertical maximum principal axis of finite elongation.

O24 Precambrian Geology of Lake Superior Region
Johnson (1990) suggested that shallowly plunging stretching lineations, defined by the alignment of amphiboles that 

occur in metabasalt units north of domain III, were caused 

by the superposition of ?2 upright folding on FI recumbent folds. This shallowly plunging elongation (X) is also 

indicated by shallowly plunging stretching lineations (fig. 

185) in both the quartz phenocrysts of the type-a rhyolite 

and agglomeratic clasts of the type-b rhyolite of the 

sheared rhyolite tuff member. However, these samples 

were collected in the vicinity of the Dead River shear zone 

and were subjected to substantial 03 and 04 deformation. 

In the following discussion of the type-a and type-b rhyolites, we compare the fabrics and finite strains recorded in 

these units with the regional fabric and strain orientations, 

and consider the probable 03 and D4 contribution to the 

finite strain in the rhyolites.

Type-a Rhyolite

The geometry of fabric and the finite strains developed 

in the type-a rhyolites differ substantially from those developed regionally in both Archean and Early Proterozoic 

rocks. For example, the planar fabrics and XY principal 

plane of finite strain in the type-a rhyolites are nearly vertical 

and strike east-northeast. This is in contrast to the nearly 

vertical west-northwest-striking plane of flattening, defined 

by 82 foliation and the XY principal plane of finite strain, 

in Archean rocks of the Negaunee area (fig. 4A-C, fig. 17/4). 

Furthermore, type-a rhyolite dikes crosscut the undifferentiated foliated pillowed basalt (MacClellan and Bornhorst, 

1989) that contains upright F2 folds (fig. 6), indicating that 

the dikes postdate most of the D2 deformation.

The foliation in these dikes is deformed by 04 crenulations and 04 kink bands; therefore, the dikes must have 

intruded between the late stages of D2 deformation and the 

early stages of D4 deformation. Furthermore, the dikes lack 

shear fabrics that indicate north-side-up, dip-slip relative 

displacements, implying that they are not deformed by D$. 

Johnson and Bornhorst (1991) suggested that these type-a 

rhyolite dikes, as well as those in other shear zones within 

the volcanic rocks of the Lighthouse Point Member, intrude 

areas of relatively high D2 strain and that the fabrics developed in these dikes are primarily a result of post-D2 shearing. 

03 deformation is highly localized, and although 03 shear 

zones do not commonly occur in type-a rhyolite dikes, 03 

deformation is evident in the type-b rhyolite.

04 is expressed by two deformation mechanisms: coaxial finite shortening associated with buckling phenomena 

such as crenulations and kink bands, and noncoaxial shearing associated with such features as asymmetrical tension 

veins. The finite strains that deformed the quartz phenocrysts were imposed either by 04 shear strains or by earlier 

deformation (late stages of 02). For example, the long axis

of finite elongation (X) of many quartz phenocrysts is at a 

relatively high angle to the axial plane of 04 crenulations. 

Experiments by Paterson and Weiss (1966) show that little 

bulk shortening (about 5 percent) is required to promote the 

development of kink bands, but the finite strains indicate 

large finite shortening (about 45 percent, fig. 18C). Furthermore, because the matrix is less competent than the 

strain markers, the strains we obtained using the Rf/0 technique indicate the minimum bulk finite strain experienced 

by the rock.

Localization of finite strains associated with strike-slip 

shearing could explain the shallowly plunging maximum 

principal axes of elongation (X, fig. ISA) and shallowly 

plunging stretching lineations (fig. 185). During progressive ductile shearing, the maximum principal axis of finite 

elongation within a shear zone is progressively rotated 

toward parallelism with the orientation of tectonic transport 

(Ramsay, 1980). This rotation would reorient finite strain 

markers toward the near-vertical and east-striking shear foliation (C planes) of the Dead River shear zone (fig. IB). 

Three possible mechanisms thus exist to explain the finite 

strain data in the type-a rhyolite dikes: (1) 04 strike-slip 

shearing, (2) a late-stage of D2 shortening that was north to 

north-northwest directed, or (3) some combination of D2 and 

04 strains.

Type-b Rhyolite

The finite strain indicated by the type-b rhyolite (fig. 

ISA, C) of the sheared rhyolite tuff member is more consistent with the fabrics developed in the northern part of the 

Ishpeming greenstone belt as reported by Johnson (1990) 

than with the type-a rhyolite dikes. However, these rocks are 

strongly sheared (Puffett, 1974), and that they share the same 

deformation history as the metabasalts north of the Dead 

River shear zone is unlikely. The XY principal plane of finite 

strain is nearly vertical and strikes west-northwest (fig. ISA), 

consistent with north- to north-northeast-directed shortening 

associated with D2 deformation. The maximum principal 

axis of elongation (X, fig. ISA) indicates approximately N. 

80° W.-trending elongation (fig. 185). Rocks in the southern 

part of the Ishpeming greenstone belt, in contrast, indicate 

nearly vertical elongation (fig. 17A).

We believe that a significant component of 04 deformation was partitioned into the type-b rhyolite relative to other 

rocks in the Ishpeming greenstone belt because these tuffaceous rocks were relatively less competent than the other 

Archean rocks. However, a detailed interpretation beyond 

this level of complexity is not feasible given the limited 

size of the data set, the complicated structural dynamics of 

the Dead River shear zone, and the difficulty in establishing 

the extent to which FI recumbent folding is developed in 

domain III.

Structural Analysis of Archean Rocks, Negaunee Area, Michigan O25
IMPLICATIONS FOR 

ARCHEAN TECTONIC HISTORY

The dynamics of present-day mountain building processes are not well understood because Theological models 

of rocks in multilayered sequences are not yet adequate to 

predict large-scale tectonic interactions associated with orogenesis (Ramsay, 1983). Nevertheless, the end results of the 

dynamic interactions involved in mountain building processes are readily observed in many exhumed mountain 

belts, and are defined by small-scale deformation features. 

Models for the mechanical development of the small-scale 

structures described in this study, such as foliation, shear 

fabrics, crenulations, and kink bands, are well documented 

in the literature (Ramsay, 1967; Hobbs and others, 1976; 

Ramsay andHuber, 1983,1987). Such models, coupled with 

the kinematic interpretation presented in this study, 

informed our attempt to set constraints on the tectonic evolution of the Ishpeming greenstone belt.

Dewey and Bird (1970) proposed several models of orogenesis that involve formation of mountain ranges at plate 

boundaries resulting from the convergence of oceanic crust, 

an island arc, or continental crust with another lithospheric 

plate. Their models are based on comparison with plate 

tectonic processes that are well established for the Phanerozoic. However, it is not certain that these plate tectonic 

processes are applicable to the Archean. Nevertheless, models that describe the mechanical behavior of an erogenic 

wedge impacted by a rigid indenter (Platt, 1986, among 

others) are applicable to any convergent orogenic setting. 

The mechanical development of the Ishpeming greenstone 

belt can be roughly extrapolated from current models of 

convergent tectonism regardless of the true nature of 

Archean plate interactions.

The reorientation of the principal stress axes during the 

Archean (fig. 20) seems to be well founded from the kinematic data. Prolonged Archean north- to north-northeastdirected shortening is supported by the geometry of folding 

and associated foliation and finite strains. However, latestage deformation features, such as shear zones, crenulations 

with locally developed crenulation cleavages, and kink 

bands, indicate that reorientation of the regional stress field 

occurred during the late stages of Archean orogenesis.

Tobish and Fiske (1976) suggested that late-stage reorientation of the regional stress field is a common occurrence 

in many Precambrian (Naha and Halyburton, 1974) as well 

as Phanerozoic (Stubley, 1989) orogenic settings. They suggested the following model to explain late-stage reorientation of the stress field without changing the orientation of 

tectonic convergence: During progressive shortening in an 

orogenic wedge, rocks extend in the XY principal plane of 

finite strain. Vertical extension is accommodated by crustal 

thickening and formation of a stretching lineation, but the 

horizontal principal axis of finite elongation (Y), which is

parallel to the margin of the fold belt, is constrained by space 

restrictions. Elastic strain is taken up in the Y direction, and 

as margin-normal compression wanes, "elastic recovery" 

occurs, inducing compression parallel to the preexisting 

deformation fabric.

Application of this genetic model, however, to the Ishpeming greenstone belt is problematic. Although finite strain 

and fabric data indicate near-vertical elongation, consistent 

with the Tobish and Fiske (1976) model, two arguments are 

in apparent opposition to the model. First, the Tobish and 

Fiske model predicts that "elastic recovery" will impose a 

compression that is nearly parallel to the preexisting foliation and produce kink bands with orthorhombic symmetry. 

Clearly the data indicate an asymmetric development of kink 

bands promoted by compression oblique to the preexisting 

foliation.

The second argument is that an intermittent deformation 

occurs between D2 and 04 with a different kinematic framework (fig. 20). 03 north-side-up shearing is associated with 

a steeply inclined maximum principal stress axis. The existence of nearly vertical shear zones, which strike parallel to 

the structural grain of the belt, implies that shearing could 

not have occurred until the waning stages of orogenesis. For 

instance, Platt (1986) showed that extensional faulting at 

near-vertical orientations can occur if the stress field associated with convergence relaxes and nearly vertical gravitationally driven stresses locally exceed horizontally 

transmitted tectonic stress. However, it is unlikely that the 

rocks would retain the elastic strains required by the Tobish 

and Fiske (1976) model to induce compression parallel to 

the margin of the Ishpeming greenstone belt.

Recent models of deformation in the Superior province 

have emphasized convergent margin plate tectonic processes 

as a mechanism for the construction of the province (Card, 

1990; Williams, 1990). These models call on northerly convergence resulting in early north-directed, south-side-down 

underthrusting along north-dipping shears followed by 

upright folding in a transpressive regime that culminated in 

widespread strike-slip shear (Hudleston and others, 1986; 

Hudleston and others, 1988; Williams, 1987; Percival and 

Williams, 1989; Bauer and Bidwell, 1990). General features 

of the deformation in the Negaunee area can be considered 

in the context of these models; however, the changes in 

kinematic framework during the progress of deformation in 

the Negaunee area are not entirely consistent with such 

models. It may be that the DRSZ and CRFSZ are early 

thrust-generated structures that have been repeatedly reactivated during the deformation sequence, but this idea is not 

supported by the finite strain data collected from the Carp 

River Falls shear zone (fig. 18). The early north-side-up dipslip displacement in these zones is consistent with early dipslip observed by Williams (1987) in volcanic sequences in 

the Beardmore-Geraldton area of Ontario; however, we 

interpret this displacement to postdate D2 folding and fabric 

development. 04 strike-slip shear could be consistent with

O26 Precambrian Geology of Lake Superior Region
late-stage strike-slip developed during the waning stages of 

regional transpression; however, kinematic indicators associated with strike-slip shearing indicate both dextral and 

sinistral relative displacements within the shear zones. As a 

result, we believe that further detailed investigations of the 

history of deformation in the shear zones of the region would 

be useful.

SUMMARY AND CONCLUSIONS

Four Archean deformations affected the rocks of the 

southern and central parts of the Ishpeming greenstone belt. 

DI deformation is confined to the northern part of the study 

area and is defined by a flat-lying foliation that is deformed 

by D2 crenulations. D2 deformation pervasively affects the 

entire Ishpeming greenstone belt, and is defined by a nearly 

vertical west-northwest-striking 82 foliation that is axialplanar to upright ?2 folds and ?2 crenulations (domain HI, 

fig. 3). 03 deformation is defined by north-side-up dip-slip 

in narrow shear zones that are subparallel to the structural 

grain of the belt. D4 deformation is defined by well-developed crenulations, kink bands with monoclinic symmetry, 

and strike-slip shear reactivation of D3 shear zones.

Early Proterozoic Penokean deformation in Early Proterozoic metasedimentary rocks of the area also produced a 

nearly vertical, west-northwest-striking foliation that is 

axial-planar to upright Early Proterozoic folds. Although the 

Archean 82 foliation and Early Proterozoic axial-plane orientations are subparallel, late-stage Archean deformations 

indicate a significant reorientation of the regional stress 

field. Inasmuch as these late-stage structures are absent in 

the Early Proterozoic rocks, we conclude that the Penokean 

orogeny did not significantly shorten the underlying 

Archean basement rocks in the Negaunee area.

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O28 Precambrian Geology of Lake Superior Region
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Structural Analysis of Archean Rocks, Negaunee Area, Michigan O29

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