The Mountain Shear Zone, 

Northeastern Wisconsin A Discrete 

Ductile Deformation Zone Within the 

Early Proterozoic Penokean Orogen

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

The Mountain Shear Zone, 

Northeastern Wisconsin A Discrete 

Ductile Deformation Zone Within the 

Early Proterozoic Penokean Orogen

By P. K. SIMS, J. S. KLASNER, and Z. E. PETERMAN

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

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

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L Peck, Director

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Library of Congress Cataloging-in-Publication Data

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

The Mountain shear zone, northeastern Wisconsin a discrete ductile 

deformation zone within the Early Proterozoic Penokean Orogen / by 

P.K. Sims, J.S. Klasner, and Z.E. Peterman.

p. cm. (U.S. Geological Survey bulletin ; 1904-A. Contributions to

Precambrian geology of Lake Superior region)

Includes bibliographical references.

Supt. of Docs, no.: 19.3:1904-A

1. Rock deformations Wisconsin Mountain Region. 2. Geology, 

Structural Wisconsin Mountain Region. 3. Geology, Stratigraphic 

Proterozoic. I. Klasner, J.S. II. Peterman, Zell E. III. Title. IV. Series: 

Contributions to Precambrian geology of Lake Superior region. 

QE75.B9 no. 1904-A 

[QE604] 

557.3 S-dc20

[551.8'09775'37] 89-600277

CIP
CONTENTS

Abstract Al 

Introduction Al 

Acknowledgments A2 

Geologic setting A2 

Mountain area A2 

Geology A2 

Regional structure A2 

Mountain shear zone A2

Principal structural features A5 

Younger structures A7 

Kinematic analysis A7 

Strain analysis A8 

Method A9 

Results All 

Conclusions A13 

Regional significance A14 

References cited A15

FIGURES

1. Generalized geologic map of eastern part of the Lake Superior region 

showing major features of Early Proterozoic Penokean orogen A3

2. Geologic map of Mountain shear zone A4

3. Concordia plot of zircon data for Hines Quartz Diorite AS

4. Contoured, equal-area projection of poles to foliation outside of shear 

zone AS

5. Isometric sketch illustrating the general geometry and deformational elements 

of the shear zone and adjacent country rock A7

6. Generalized geologic map showing equal-area stereoplots of structural data 

from selected outcrops A8

7. Field sketch of multiple phases of deformation in granitic gneiss A9

8. Lower hemisphere equal-area stereographic plots All

9. Photograph and sketch of narrow ductile shear zone A12

10. Photograph of S-C fabric in granitic gneiss A13

11. Block diagram showing relationship of strained clasts to simple shear 

envelope A13

12. Equal-area stereoplots of orientation of principal strain axes A14

TABLES

1. Sequence of events, structural elements, and nature of events in the Mountain 

shear zone A6

2. Data on rock type and strain analyses A10

Contents III

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

The Mountain Shear Zone, Northeastern Wisconsin 

A Discrete Ductile Deformation Zone Within the 

Early Proterozoic Penokean Orogen

ByP.K. Sims, J.S. Klasner, 1 andZ.E. Peterman

Abstract

The Mountain shear, a discrete ductile deformation 

zone containing mylonitic rocks, largely obliterates older 

fabrics (So/S,) in metavolcanic and granitoid rocks of Early 

Proterozoic («1,860 Ma) age. A relatively undeformed quartz 

diorite body (1,812.7±3.6 Ma; U-Pb zircon) that was intruded 

late in the deformational history of the shear zone provides a 

firm minimum age on the time of shearing.

The shear zone trends N. 55°-60° E., is from 1 to 2.5 km 

wide, and has a prevailing steep mylonitic foliation (S2) and 

stretching lineation (L^). The southeastern margin transects 

north-trending country rocks and is gradational across a 

width of 100 m or more. The shear zone is truncated on the 

southwest, northwest, and northeast by undeformed anorogenic intrusive rocks of the 1,470 Ma Wolf River batholith, 

which had no perceptible structural effect on the country 

rocks or the shear zone.

S-C mylonite fabrics and stretching lineations indicate 

that the southeast block of the Mountain shear zone is 

upthrown relative to the northwest block. Field relationships 

and kinematic data suggest that the sense of movement in 

the horizontal plane was dextral, as indicated by the apparent 

clockwise rotation of SQ/S, toward parallelism with the 

southeast margin of the shear zone and the small right lateral 

offset of a granodiorite body deformed in the shear zone.

Strain analyses indicate that the shear zone represents 

heterogeneous simple shear with accompanying heterogeneous volume change because (1) domains of relatively 

low strain are preserved within the shear zone, (2) the nature, 

magnitude, and orientation of displacement vary both across 

and along the zone, (3) the L-S tectonites were formed 

primarily by plane strain but locally were subjected to

Manuscript approved for publication July 7, 1989.

1U.S. Geological Survey and Western Illinois University,

Macomb, IL 61455.

apparent constriction, and (4) the rocks were differentially 

subjected to both volume loss and volume gain along the 

length of the shear zone. The magnitude of both flattening 

and stretching increased from the southeast margin toward 

the center of the zone.

The shear zone was formed 35-45 million years after 

the collisional event (« 1,850 Ma) that sutured the Wisconsin 

magmatic (arc) terranes to the Early Proterozoic continental 

margin along the Niagara fault zone. Shearing could have 

resulted from an unrecognized continent-continent or 

continent-arc collision to the south or southeast of the study 

area.

INTRODUCTION

The existence of an extensive belt of well-exposed 

mylonitic rocks near Mountain, in Oconto County, 

northeastern Wisconsin (fig. 1), was revealed during 

reconnaissance geologic mapping in 1980. A detailed 

geologic map (scale 1:24,000) was prepared during 1985 

and 1986 (Sims, 1989), and detailed structural studies 

were carried out later to determine the nature of the 

deformation and the sense of displacement. The purpose 

of this report is to (1) describe the structural features 

within the shear zone, (2) outline and document the 

sequence of deformational events, (3) discuss the kinematics, including an analysis of strain, and (4) relate 

structures within the shear zone to the evolution of the 

Penokean orogen as a whole.

The Mountain shear fits the definition of Ramsay 

(1980) of a discrete zone of ductile deformation. The 

term mylonite, as used herein, follows the terminology of 

Wise and others (1984, p. 393): "Mylonite is a general 

term for coherent rocks with at least microscopic 

foliation, with or without porphyroclasts, characterized 

by intense syntectonic crystal-plastic grain-size reduction

The Mountain Shear Zone, Northeastern Wisconsin A1
of the country rock to an average diameter less than 50 

microns (0.5 mm) and invariably showing at least minor 

syntectonic recovery/recrystallization."

ACKNOWLEDGMENTS

M.G. Mudrey, Jr. of the Wisconsin Geological and 

Natural History Survey assisted Sims in reconnaissance 

mapping of the Mountain area in 1980, and Bruce 

Brasaemle, G.L. LaBerge, and K.J. Schulz assisted in the 

geologic mapping during 1985-1986. In this report, Sims 

is responsible for the general geologic relationship at 

both local and regional scales, Klasner is responsible for 

the detailed field structural analysis and the strain 

analysis, and Peterman is responsible for the geochronology. The manuscript benefitted from critical 

reviews by R.L. Bauer and E.H. DeWitt.

QEOLOGIC SETTING

The Mountain shear zone is within the Early 

Proterozoic Penokean orogen (fig. 1), which lies along 

the southeastern margin of the Archean Superior 

province of the Canadian Shield. The Penokean orogen 

consists of two assemblages, a northern continentalmargin assemblage of metasedimentary rocks in 

northern Michigan (Marquette Range Supergroup and 

its equivalents), which overlies Archean basement, and a 

southern assemblage of island arc volcanic-granitic rocks, 

termed the Wisconsin magmatic terranes (Sims, 1987). 

The two are juxtaposed along the Niagara fault zone, 

believed to be a paleosuture (Sims and others, in press). 

The northern (Pembine-Wausau) terrane of the 

Wisconsin magmatic terranes was accreted to the 

continental margin about 1,850-1,860 Ma (Peterman 

and others, 1985; Sims and others, 1985); during the 

collision, rocks within both terranes were regionally 

deformed and metamorphosed.

The Mountain shear zone is one of several discrete 

shear zones in the Pembine-Wausau terrane (LaBerge 

and Myers, 1983; Klasner and LaBerge, 1985) that 

formed after the culmination of regional deformation 

and metamorphism.

MOUNTAIN AREA

Geology

Major rock units within and adjacent to the 

Mountain shear zone are Early Proterozoic metamorphosed volcanic and volcanogenic sedimentary rocks,

assigned to the Waupee Volcanics, and an unnamed 

granodiorite body (unit Xg) and its deformed and metamorphosed equivalent in the shear zone (unit Xgn, fig. 

2). The metavolcanic rocks and the granitic gneiss in the 

shear zone are overlain by a local meta-conglomerate, 

the Baldwin Conglomerate. The metavolcanic rocks are 

intruded by the Hines Quartz Diorite, a relatively undeformed body confined to the shear zone. Early Proterozoic rocks within and southeast of the shear zone are 

truncated on all sides by anorogenic granitic rocks of the 

Middle Proterozoic (1,470 Ma; Van Schmus and others, 

1975) Wolf River batholith.

The volcanic rocks and granodiorite were subjected 

to regional deformation and upper greenschist facies 

metamorphism prior to the shearing. These rocks were 

converted in the shear zone to mylonitic schists and 

gneisses predominantly of lower amphibolite facies (Sims 

and others, 1986). The Baldwin Conglomerate was 

deformed during the shear deformation, but less 

intensely than the volcanic and granitoid rocks. The 

Hines Quartz Diorite, which has a U-Pb zircon intercept 

age of 1,812.7±3.6 Ma (fig. 3), was emplaced after or 

near the end of the ductile deformation, and this places a 

firm minimum age on the time of shearing.

Regional Structure

Metavolcanic rocks southeast of the shear zone 

have a penetrative schistosity (Sj) expressed mainly by 

aligned biotite and hornblende that are subparallel to 

depositional layering (S0 ). The schistosity is axial planar 

to tight F! folds, and is expressed by pancake-shaped 

clasts that would fall within the region of apparent 

flattening on a Flinn (1962) diagram. Sj in the granodiorite (unit Xg, fig. 2) is expressed mainly by flattened 

inclusions of metavolcanic rocks, principally amphibolite. 

The intensity of Sj foliation southeast of the shear zone is 

variable, indicating that the deformation was not 

homogeneous. For example, Sj foliation in felsic volcanic 

rocks (unit Xwf, fig. 2) is more intense than in the 

granodiorite (unit Xg) and in mafic lava flows (unit 

Xwm). A summary plot of measured foliation orientations outside the shear zone is given in figure 4; the 

calculated mean (Sj foliation is N. 8° E., 90°.

As noted in table 1, Sj fabric was observed at 

isolated outcrops within the Mountain shear zone; 

accordingly, the F2 deformation of the shear zone was 

superposed on rocks that had been deformed previously 

to varying degrees during the regional ¥l event.

Mountain Shear Zone

The Mountain shear zone, which sharply truncates 

and is distinctly younger than the regional foliation

A2 Precambrian Geology of Lake Superior Region
11111111

EXPLANATION

Sedimentary rocks of Paleozoic age

Middle Proterozoic (Keweenawan) mafic igneous 

and sedimentary rocks of Midcontinent rift system 

(1,000-1,200 Ma)

Anorogenic igneous rocks (1,470-1,510 Ma)

WISCONSIN MAGMATIC TERRANES 

Metavolcanic and granitoid rocks in Pembine-Wausau

terrane (1,760-1,880 Ma) 

Metavolcanic and granitoid rocks in Marshfield terrane

(1,835-1,890 Ma)

Gneiss in Marshfield terrane (=2,800 Ma)

A 

E 

J 

M

CONTINENTAL-MARGIN ASSEMBLAGE 

Marquette Range Supergroup (1,820-2,100 Ma) and 

gneiss, granite, and greenstone (2,600-3,550 Ma)

Contact Dashed where concealed 

High-angle fault

Thrust fault Sawteeth on upthrust block 

Fault rocks in shear zone

Athens 

Eau Pleine 

Jump River 

Mountain

Figure 1 . Generalized geologic map of eastern part of the Lake Superior region, northern Wisconsin and Michigan, 

showing major features of Early Proterozoic Penokean orogen. Modified from Sims (1987).

The Mountain Shear Zone, Northeastern Wisconsin A3
25'

45° 

12' 

30"

Ywh

45° 

10'

Ywb

1 KILOMETER 

I

EXPLANATION

w

o

N 

K<

W

':' #'<':

Xb

Xwq

\i% Xwa .;£:;

MIDDLE PROTEROZOIC (1,600-900 Ma) 

Belongia Granite of Wolf River batholith 

M.480 Ma)

Hager Formation

EARLY PROTEROZOIC 

Hines Quartz Diorite

Baldwin Conglomerate

Granite gneiss Deformed and 

metamorphosed granodiorite

Biotite schist and amphibole schist 

Interlayered

Quartz diorite schist

Amphibole schist with thin layers of 

felsic schist

H cc1 

D <

Granodiorite Equivalent to unit Xgn 

in shear zone

WAUPEE VOLCANICS 

Dominantly felsic volcanic rocks

Mafic lava flows

Contact

1 Transcurrent fault 

Trace of Sj foliation 

Trace of S? foliation 

Southeastern boundary of shear zone

Figure 2. Geologic map of Mountain shear zone and vicinity, northeastern Wisconsin.

(So/Si), strikes N. 55°-60° E. It has a steep dip, and an 

exposed width of 1-2.5 km. Two northwest-trending, 

sinistral (left lateral) faults transect the shear zone near 

its midpoint (fig. 2), and the major one offsets the 

southeastern edge of the zone about 500 m. These 

structures are probably also shear zones, as judged from 

the presence of several small-scale, northwest-trending 

ductile zones in the Mountain shear zone. The southeastern margin of the Mountain shear zone the 

transition from mylonitic to nonmylonitic rocks is 

relatively abrupt, although narrow zones of high strain 

subparallel to the main zone occur sporadically for a 

distance of 100 m or more outward from the margin. 

Elsewhere, the shear zone is truncated by younger 

granitic rocks of the 1,470 Ma Wolf River batholith, 

which had little or no apparent tectonic or metamorphic

A4 Precambrian Geology of Lake Superior Region
0.335

0.325

0.315

g 0.305

0.295

0.285

Intercepts at

1812.7±3.6 and 872±39 Ma - 

r 147-85 (MSWD=1.3) 

(-200+250)

4.0 4.2 4.4 4.6 4.8 5.0 5.2

207pb/235u

Figure 3. Concordia plot of zircon data for sample 

147-85 and 150-85 of Hines Quartz Diorite. Intercepts 

defined by four fractions of zircon are 1,812.7±3.6 and 

872±39 Ma. Numbers in parentheses are mesh sizes.

N

]N8° E, 90°

Figure 4. Contoured, equal-area projection of 68 poles to 

foliation outside of shear zone. Contours are 1, 3, 5, and 7 

percent. The great circle (N. 8° E., 90°) is the calculated mean.

effect on the rocks in the shear zone. Accordingly, neither 

the full width nor the length of the shear zone is known. 

The shear zone is characterized by a prominent 

foliation (S2 ), an accompanying stretching lineation (L2 ), 

minor younger S-structures, and by partial tectonic 

transposition of SQ/SJ and formation of folds. In the shear 

zone, biotite and quartz are predominantly recrystallized, 

whereas plagioclase and microcline, which are less 

ductile than the matrix, are only partly recrystallized. The 

deformed rocks are mainly protomylonite or orthomylonite (Wise and others, 1984). Biotite is present in

irregular, anastomosing trains, commonly associated 

with recrystallized plagioclase and, locally, microcline. 

The biotite and associated feldspar are very fine grained, 

commonly ^0.2 mm in diameter. Much of the plagioclase, however, remains as porphyroclasts as much as 

5 mm in diameter, which commonly have mantles (type 

IP structures of Hanmer, 1982) of fine-grained 

(^0.1-0.2 mm) plagioclase. Quartz tends to be recrystallized into lenses or rods which are comparable in size 

to the plagioclase porphyroclasts. These lenses actually 

are coarse-grained aggregates of individual sutured 

grains less than 0.5 mm in diameter. Commonly, the 

quartz lenses and rods contain inclusions of plagioclase 

and biotite and, locally, sphene and amphibole.

Principal Structural Features

The geometry and complexity of deformation of 

rocks within the Mountain shear zone are shown 

schematically in an isometric diagram of part of the zone 

(fig. 5) and by stereoplots of structural data from 

selected, critical outcrops within it (fig. 6).

Within the shear zone, the rocks are penetratively 

deformed by structures assigned to F2 and younger 

events (table 1), but domains of relatively nonmylonitized 

rocks as much as tens of meters across remain. The F2 

structures consist mainly of an S-surface (S2 ; C, fig. 5) 

resulting from tectonic layering and a mylonitic foliation; 

a stretching lineation (L2 ) expressed by elongate clasts 

(E, fig. 5), rodding, and biotite streaks; and folds of the 

SQ/SJ surface (D, fig. 5). The resulting composite 

structures are L-S tectonites. Younger ductile fold 

structures (F3 , F4 (F and G on fig. 5)) and kink bands 

related to brittle deformation (H on fig. 5) are superposed locally on the older, dominant structures, but do 

not appreciably modify the distribution of the rocks.

Narrow zones of high strain are present outside the 

main shear zone, as at locality lib (fig. 6), where northstriking S1 foliation is overprinted by a local zone of 

crenulation folds that strike N. 70° E. and dip steeply 

southeast, parallel to a 1-m-wide shear zone that is 

subparallel to the trend of the main shear zone.

Deformation was inhomogeneous within the shear 

zone. In the granitic rocks (unit Xgn), which lacked 

appreciable initial anisotropy, S2 consists dominantly of 

mesoscopic and microscopic, thin anastomosing, biotiterich zones and oriented, flattened aggregates of quartz. 

S-C mylonites (as defined by Berth6 and others, 1979) in 

the biotite-rich zones resemble type I of Lister and Snoke 

(1984). Throughout the shear zone, the C foliation is 

most prominent (see fig. 9). A related lineation is 

expressed by biotite aggregates and, especially, quartz 

rods. In strongly anisotropic rocks such as bedded tuff, S2 

is expressed at places by brecciated and boudinaged,

The Mountain Shear Zone, Northeastern Wisconsin A5
Table 1. Sequence of events, structural elements, and nature of events in the Mountain shear zone

Event Structural element Symbol Explanation Nature of event

Deposition Bedding, pillow structures, 

intrusive contacts.

Regional foliation; flattened 

clasts.

Foliation

Fold axes

Long axes of strained clasts.

Stretched lineations. 

Foliation

Fold axes 

Foliation

Fold axes

Lineations

Shears

Kink bands

L2b

Primarily is layering 

between felsic and mafic 

rocks.

Best observed south of shear 

zone, but present in isolated 

outcrops within 

shear zone.

Tectonic layering, mylonitic 

foliation in granitic gneiss 

(unit Xgn).

Folds of S2 plane.

Stretched clasts that lie in S2 

plane.

Geometric element. Not generally observed.

Seen as fold axes on S2 

surface.

Fabric is poorly developed. 

Appears to fold F3 fabric.

Variously oriented kink 

bands and fold axes associated with kink bands.

Original depositional and 

intrusive features.

Flattened clasts suggest 

regional compression.

Foliation associated with 

simple shear due to relative 

upthrow from southeast. 

Laa formed by fold of Sj 

and S0. L2b caused by 

simple shear associated 

with upthrow.

Late folding that created a 

west-northwest-trending 

fabric.

Rarely observed. Yielded a 

north-northwest-trending 

fabric.

Shears are parallel to cross 

faults.

Late-stage brittle deformation. Not systematically 

studied. Number of events 

not known.

brittle (siliceous) layers, representing So/Sj foliation that 

has been technically transposed into parallelism with S2 . 

However, at many other places segments of SQ/SJ 

surfaces have not been completely transposed, and folds 

are present that contain axial planar S2 foliation (see D, 

fig. 5). In interlayered mafic and felsic volcanic rocks, 

especially in the area northeast of the cross faults 

(locality V, fig. 6), the felsic layers commonly have a 

conspicuous S2 foliation expressed by recrystallized biotite that obliterates SQ/SJ, whereas some of the mafic 

layers, especially those at a large angle to S2 , have a relict 

SQ/SJ and only a weak S2 foliation expressed by aligned 

hornblende or chlorite.

Stereoplots from localities studied in detail within 

the shear zone (black areas in fig. 6) show variable, 

generally northeast trending S2 foliation having a steep 

dip to the northwest or southeast. At locality III, a 

layered schist (unit Xwi) has a well-developed N. 58° E. 

foliation that dips steeply southeast; the long axes of 

deformed clasts (E, fig. 5) plunge steeply east (L2b on 

stereonet III, fig. 6). At locality IV, S2 foliation strikes 

N. 36° E. and dips steeply southeast; elongate quartz 

aggregates (L2b ) plunge steeply east. At locality V, Sa

foliation is preserved in the fold crest within a mafic layer 

of a folded layered felsic-mafic volcanic rock, and axial 

planar S2 foliation is most intensely developed in the 

felsic layer. At locality VI an orthomylonitic granitic 

gneiss (unit Xgn) S2 foliation strikes N. 53° E. and dips 

steeply southeast. At locality VII, on the northern margin 

of the shear zone, undeformed Hager Feldspar Porphyry 

of the Hager Formation cuts deformed Baldwin Conglomerate. Clasts within the conglomerate are variably 

deformed: quartzite and granitoid clasts are least 

strained, whereas mafic volcanic clasts are highly 

strained. Flattened volcanic clasts have an S2 foliation 

that strikes N. 56° E. and dips steeply northwest. At 

locality VIII, S2 foliation in mafic schist strikes N. 48° E. 

and dips steeply northwest. Three sets of kink bands that 

plunge southwest (L on stereonet VIII) are superposed 

on the S2 foliation. At locality IX, intermediate to mafic 

tuff has a penetrative S2 foliation that strikes N. 68° E. 

and is vertical; the long axes of volcanic clasts also plunge 

vertically. At locality X, interlayered felsic and mafic 

volcanic rocks have a conspicuous S2 foliation, N. 72° E., 

68° SE., which dips somewhat less steeply than elsewhere 

in the shear zone.

A6 Precambrian Geology of Lake Superior Region
PartD

Main 

shear zone

Southeastern boundary^ 

of shear zone ""

Parti

Cut-away block 

from above isometric 

diagram (not to scale)

Figure 5 Isometric sketch (diagrammatic, not to scale) 

illustrating the general geometry and deformational 

elements of the shear zone and adjacent country rock. 

See table 1 for discussion of symbols and sequence 

of events. Deformational elements are as follows: 

A Primary bedding (S0); B, F, generation foliation (SJ; 

C F2 generation foliation (S2); B,, granodionte (unit Xg) 

body deformed by F, event; C,, same granodionte body 

as B« but deformed into gneiss (unit Xgn) by both F, and 

Fv C2 small F2 deformational domain outside of main 

shear zone; D, domain of weak F2 deformation where 

pre-F2 fabric elements (S0 ,..dotted lines, and SJ are 

preserved; E, strained claststhat define steeply plunging 

stretch direction (L2b); F, F3 fold axes (L3) and axial 

planes (S3); G, F4 fold axes (L4) and axial planes (S4); 

H late-stage kink folds or bands; I, late syntectonic (F2) 

to' mostly posttectonic Mines Quartz Diorite intrusion 

within main shear zone; J, anorogenic granite pluton of 

1 470 Ma Wolf River batholith and accompanying undefo'rmed felsic dikes (J,) in main shear zone.

Younger Structures

Three minor, more restricted deformational events 

followed the main (F2) deformation in the shear zone 

(table 1). An (F3 ) episode of deformation is represented 

by local folds whose axial traces trend northwestward 

(F fig. 5). The nature of these folds can be seen in figure 

7 which shows multiple folded gneissic layering in the 

granitic gneiss (unit Xgn, fig. 2). Here, the S, surface is 

folded by F2 , and both the S1 surface and the axial traces

of the F2 fold axis have been folded by F3 deformation. 

Axial traces of the F3 fold trend northwest.

Additional data on the F3 event and evidence for an 

R deformation are given in figure 8. Stereoplot A 

(fig 84) from locality XII (locality shown in fig. 6), 

clearly shows F3 deformation: S2 foliation is folded about 

east- or east-southeast-plunging fold axes (L3a). Plot B, 

from locality X, shows that S2 foliation strikes N. 72 E. 

and dips moderately southeast, but the spread in 

distribution of S2 poles indicates that S2 has been folded 

by later events, as suggested by girdles 3 and 4 and 

corresponding beta axes, B3 and B4 . Stereoplot C 

(fig. 8C), also from locality X, provides additional data on 

F To facilitate interpretation, we have plotted girdle 4 

and beta axis B4 from fig. 85; we have also plotted the 

orientation of the S2 foliation plane from a selected 

outcrop within the area of locality X (fig. 6). Evidence for 

F folding was observed in these outcrops. Open folds 

having wavelengths of about 1 m and amplitudes of about 

30 cm have fold axes that plunge steeply southeast (L4a , 

Stereoplot C, fig. 8C). Intersection of measured S4 

foliation in the outcrop and S2 foliation forms a prominent rodding that also plunges southeast (L4b , Stereoplot 

C) The F4 deformation (G, fig. 5) is expressed locally by 

ductile shear zones interpreted as being related to the 

northwest-striking, sinistral faults that transect the 

Mountain shear zone (see fig. 2). A 4-cm-wide ductile 

shear zone of this set transects strongly foliated granitic 

gneiss (fig. 9) at locality XIII (fig. 6), just southwest of 

the major sinistral fault. Boundaries of the shear zone 

and the foliation within the zone are parallel to the S4 

axial planar fabric of fig. 8C and to the transcurrent

faults. .,.11. jThe last episode of deformation is kink banding 

(H fig 5) which represents late-stage brittle deformation in the shear zone. These structures were not 

systematically studied.

Kinematic Analysis

The sense of movement during the F2 shear deformation has been determined from S-C fabrics (Berthe 

and others, 1979; Lister and Snoke, 1984) in mylomtic 

granitic gneiss, by orientation of stretch fabrics (long axes 

of strain ellipsoids), and from other field data such as 

described by Simpson and Schmid (1983) and Simpson 

(1986) S-C mylonite fabrics (fig. 10) in granitic gneiss 

from several localities show that the southeast block of 

the Mountain shear zone is upthrown relative to the 

northwest block. Stretch fabrics, as indicated by the 

orientation of the long (X) axes of strain ellipses, also 

suggest the same sense of movement. Stereoplots at 

localities III, IV, and IX (fig. 6) show that stretch axes

(Lab) PlunSe steeP!y SOUth l° CaSt ThiS SteCply P ^ 

stretch fabric is possibly caused by a simple shear couple

The Mountain Shear Zone, Northeastern Wisconsin A7
45° 

12' 

30"

45° 

10'

88"27'30' 

I

25' 88"22'30' 

I

1 MILE 

I

1 KILOMETER 

J________

Figure 6. Generalized geologic map of study area showing lower hemisphere, equal-area stereoplots of 

structural data from selected outcrops. Contour intervals are variable. See text for discussion of outcrops I through 

XIV. Note that outcrop areas XI, XII, XIII, and XIV do not have accompanying stereoplots.

having a shallower dip than the dip of the stretch axis, as 

shown in figure 11. Stereoplots at localities III and IV 

(fig. 6) indicate a steep easterly L2b stretch fabric, 

whereas that at locality IX indicates a steep southerly 

stretch fabric. This difference in orientation of stretch 

fabrics suggests a varying sense of movement of the shear 

couple along the length of the shear zone.

Field data suggest that the sense of movement in 

the horizontal plane was generally dextral (right lateral). 

At the map scale (figs. 2 and 6), Sj foliation appears to 

have been rotated clockwise toward parallelism with the 

shear zone, which is consistent with dextral movement. 

Also, the granodiorite body (unit Xg) is displaced right 

laterally in the shear zone as granitic gneiss (unit Xgn). In 

addition, rare S-C fabrics observed at localities XIV 

(fig. 6) and VIII (fig. 9) suggest dextral movement. Such 

movement is supported also by measured axes of 

extension (L2b , fig. 6) as well as by computed ellipsoid 

axes; these axes plunge toward the east-southeast 

quadrant of stereoplots, indicating that the upthrust 

southeast block had a dextral component of movement in

the horizontal plane, as shown in figure 11. On the other 

hand, the variation in orientation of l^^ shown in figure 

6 suggests a different horizontal shear sense from place 

to place along the shear zone. This variation is supported 

by a well-exposed S-C fabric in metavolcanic rocks at 

field locality III, which unequivocally indicates a sinistral 

sense of horizontal movement.

Strain Analysis

In general, rocks within the Mountain shear zone 

lack structures suitable for strain analysis, and only a few 

outcrops exist where strain could be estimated. We 

selected samples from three areas for strain analysis, as 

shown in table 2. Samples from areas IV and X (fig. 6) 

were selected because preliminary analysis of thin 

sections indicated that these rocks had domains ( > V2 cm 

in smallest dimension) that consisted primarily of 

contiguous deformed grains of quartz or feldspar which 

could be analyzed by the Fry (1979) technique. A sample

A8 Precambrian Geology of Lake Superior Region
N

r

F2 Fold Axis ONE METER

Figure 7. Field sketch of multiple phases (F2 , F3) of deformation in granitic gneiss. $! foliation is parallel 

and has been folded during F2 deformation as shown by the F2 fold axial traces (L^). S, and Lj, have been 

folded by the third (F3) phase of deformation. Lg fold axes trend northwestward.

from area IX contained well-exposed flattened and 

stretched volcanic clasts whose dimensions could be 

measured in the field. Lithologic descriptions of these 

rocks are given in table 2.

Both nature of rock and extent of our experiments 

influenced the results of the strain analyses: (1) Clasts 

and quartz grains were not spherical prior to the onset of 

F2 deformation; their shape at the time of F2 deformation 

reflects varying degrees of F: deformation. (2) The strain 

estimates apply only to the immediate vicinity of the 

analysis; we did not construct strain estimates for the 

more intensely mylonitized rocks. (3) Strain estimates 

were constructed from rocks of variable composition, as 

shown in table 2; it is possible that the variable amount of 

strain among the different rock types mainly reflects 

differences in physical compositions.

Method

The strain in samples from localities IV and X 

(fig. 6) was determined by cutting three mutually 

perpendicular rock slabs from an oriented field sample. 

Images of oriented oversized (3 cmx61/2 cm) thin 

sections prepared from these slabs were projected onto a 

screen. Outlines of quartz grains were traced from

quartz-rich domains in these slides. Strain ellipses for 

each thin section were then determined using the 

graphical techniques of Fry (1979). The strain for locality 

IX (fig. 6) was determined by measuring strain ellipse 

parameters in the field from strained volcanic clasts on 

three roughly perpendicular faces. Ratios of the principal 

X> Y>Z axes of the strain ellipsoids were measured in 

the field and orientation of the long axis (stretch 

direction) of the strain ellipse was determined.

Finite strain was calculated using an unpublished 

computer program written by A.W.B. Siddans. This 

program calculates the ratios of the major axes of the 

strain ellipsoid, their orientation relative to the coordinate system used, and the Flinn (1962) K values for the 

ellipsoid. Siddans' program also provides a measure of 

the "internal consistency" of the ellipsoid parameters as 

determined by calculating the strain parameters from 

different ellipse pairs. The orientation and scatter of the 

principal strain axes for samples from localities X and IV 

are shown in figure 12/4 and B. These orientations are 

relative to an arbitrary set of coordinates used to input 

data into Siddans' computer program. We have rotated 

the axes on a stereonet to their proper position in space, 

as shown by the shaded areas in figure 12.

The Mountain Shear Zone, Northeastern Wisconsin A9
3

o Table 2. Data on rock type and strain analyses

Outcrop Rock description

sr* 

(D 

CO

 o

<D 

O 

33

Strain analysis 

approach

Percent Percent Flinn K Logarith

flattening stretching value K valu

IV Protomylonitic granite and granitic 

gneiss. Felsic domains as much as 1 

cm in dimension are separated by 

anastomosing fine-grained mylonitic bands. Felsic domains contain 

abundant contiguous quartz and 

some saussuritized feldspar.

IX Mylonitized metavolcanic rock with 

numerous highly deformed biotiterich, hornblende-bearing mafic 

clasts. Matrix consists of layers of 

clinopyroxene, plagioclase, and 

minor quartz. Mafic clasts are 

highly deformed.

X Granoblastic rock containing plagioclase, biotite, hornblende, and 

minor quartz and chlorite; strongly 

foliated but not banded. Lithic 

clasts have same composition as 

matrix.

Fry analysis of quartz-rich domains on 24.12 

three perpendicular faces. Quartzrich domains lie between mylonitic 

bands.

Field measurement of major axes of 65.62 

strained clasts on three perpendicular faces.

Fry analysis on three perpendicular 

faces. Used plagioclase grains.

44.18 2.824 2.621 

186.28 0.930 0.956 

21.04 29.54 0.922 0.914 
A

Locality X

Figure 8 (facing column). Lower hemisphere equal-area 

stereographic plots. A is from outcrop XII (fig. 6), Band Care 

from outcrop X. A, Girdle 3 contours at 2, 6, and 12 percent 

are constructed for 50 poles to S2 foliation; contours for Lga at 

2, 10, 20, 30, and 40 percent are constructed for 50 

measurements of F3 fold axes. B, Contours at 1,5,10, and 20 

percent are on 72 poles to S2 foliation; orientation of best S2 

foliation plane shown and beta axes B3 and B4 correspond to 

girdles 3 and 4 respectively. C, Girdle 4 and B4 are 

transferred for comparison purposes from B. Field-measured S4 spaced cleavage plane shown as well as S2 foliation at 

small outcrop within area X. L4a represents axes of F4 folds. 

L4b represents lineations measured in the field that formed by 

intersection of S2 and S4 foliation planes.

Results

Strain analysis of the sample from locality X 

(fig. 12/4) shows good internal consistency, and the 

results agree well with field measurements. Samples 

from localities IV (fig. 125) and IX, on the other hand, 

have poor internal consistency relative to the sample 

from locality X, inasmuch as the ellipse parameters 

determined from different ellipse pairs do not agree 

within 24 percent (table 2). We attribute these poor 

internal consistency values to inhomogeneous deformation in the sample. Strain ellipses for the sample from 

locality IV, for example, were determined from grains 

within quartz-rich domains in the mylonitic granitic 

gneiss. Although the ellipsoid axes determined from 

different pairs disagree by as much as 24 percent, and 

show considerable spread on stereographic plots 

(fig. 12), the general orientation of the computed axes is 

close to stretch axes measured in the field (fig. 12C).

Some general conclusions can be drawn from the 

strain analyses. The sample from locality X, which is near 

the southern margin of the shear zone, has a Flinn (1962) 

K value of 0.922, which lies very near the plane strain 

line, barely in the field of apparent flattening. Localities 

IV and IX are near the center of the shear zone. The 

sample from IX has a Flinn value of 0.930, also near the 

line of plane strain, whereas the sample from IV has a 

Flinn value of 2.824, clearly in the field of apparent 

constriction. These results suggest that the rocks in the 

southwestern part of the shear zone were slightly 

flattened, whereas rocks toward the northeast were 

stretched. Alternatively, the results could reflect original 

differences in shape and orientation caused by the earlier 

F! deformation.

Although the data on the percentage of F2 

flattening and stretching are not firm because of the 

relatively high internal consistency level, the unknown 

nature of ¥l deformation, and differences in rock types, 

they do suggest that strain was heterogeneous within the 

shear zone. The 21 percent flattening and 29 percent

The Mountain Shear Zone, Northeastern Wisconsin A11
Figure 9. Photograph and sketch of narrow ductile shear zone caused by F4 

deformation at outcrop XIII (location, fig. 6). The shear zone transects strongly foliated 

granitic gneisses and is parallel to and related to the major sinistral fault that crosscuts the 

main Mountain shear zone. Inset sketch shows S-C fabric in Mountain shear zone with 

dextral sense of shear.

stretching values for the sample from locality X (table 2) 

have good internal consistency, and probably accurately 

represent the bulk strain in these rocks. Flattening and 

stretching percentages of 66 and 186 respectively (table 

2) from the sample at locality IX reflect much higher

strain than in samples from IV and X. This high strain is 

likely a function of the mafic composition of the rock. 

The 24 percent flattening and 44 percent stretching 

values for sample IV probably are minimum values 

because they were derived from relatively undeformed

A12 Precambrian Geology of Lake Superior Region
NW

Stretched 

clast

Figure 10. Photograph of granitic gneiss slab from outcrop XIV (fig. 6) showing S-C fabric viewed from 

the northeast parallel to the shear zone. The slab is cut so that it is vertical and strikes perpendicular to the 

strike of S2 foliation. Direction of relative movement is shown diagrammatically adjacent to the photograph. 

The long (X) axis of the strained clast lies within the S plane of shear. Scale in centimeters.

Shear zone

Figure 11. Block diagram showing relationship of 

strained clasts (strain ellipsoids) to simple shear envelope. 

Note that the stretch (long) axes of the clasts (l_2b) dip 

more steeply than the shear zone.

quartz-rich domains between mylonitized domains in the 

rock. Despite the uncertainties of the strain analyses, the 

results presented in table 2 suggest that rocks in the 

center of the shear zone are more highly strained than 

those along the southeast margin.

Conclusions

Rocks within the Mountain shear zone are L-S 

tectonites that accord with the classic definition of a shear 

zone (Ramsay, 1980). The shear zone is a local (discrete) 

linear zone of relatively high ductile deformation. 

Neither the width nor the length is known, but we 

presume that the shear zone had parallel sides prior to 

emplacement of the Wolf River batholith. In this area, 

the batholith was emplaced passively.

Ramsay (1980) classified shear zones according to 

their displacement fields. The Mountain shear zone 

appears to best fit Ramsay's (1980) type (iii) classification; that is, it is a heterogeneous simple shear with 

accompanying heterogeneous volume change. Ramsay 

and Graham (1970) noted that most deformation from 

natural erogenic processes leads to states of heterogeneous strain, and Ramsay (1980, p. 89) noted that "the 

basic component of practically all shear zones is that of 

heterogeneous simple shear* **made up of a number of 

elements showing homogeneous simple shear." The 

Mountain shear zone has the following features that fit 

these criteria.

(1) Deformation within the shear zone was not 

uniform; domains of relatively low strain, where pre-F2 

features are preserved, as well as domains of high strain, 

where the country rocks have been mylonitized, are 

present.

(2) The nature, magnitude, and orientation of 

displacement vary both across and along the shear zone, 

as suggested by the variation in (a) shape of the strain 

ellipsoids at localities IV, IX, and X, (b) percentage of

The Mountain Shear Zone, Northeastern Wisconsin A13
-Y

-Y

-Y

Figure 12 (facing column). Lower hemisphere equal-area 

stereoplots of orientation of principal strain axes of the strain 

ellipsoid generated by A.W.B. Siddans' program. A is from 

outcrop X (fig. 6); B is from outcrop IV (fig. 6). 1, axis of 

maximum elongation; 2, intermediate axis of elongation; 

3, axis of minimum elongation. Orientations are relative to an 

arbitrary set of reference coordinates and must be rotated to 

show their true orientation in space. Shaded areas represent 

the correct position of the ellipsoid axes in space. C, Points 

represent orientation of long (stretch) axes of strained clasts 

measured in the field from localities III and IV. Small 

triangular area represents orientation of axis of maximum 

extension as determined from strain analysis of outcrop IV.

flattening and stretching among the three outcrops where 

strain analyses were carried out, and (c) movement 

pattern along the shear zone; the orientation of the 

maximum principal shortening direction apparently 

rotated from nearly west in the northern part of the 

exposed shear zone (localities III and IV, fig. 6) to 

north-northwest at its southwestern exposed segment 

(locality IX, fig. 6), as indicated by the orientation of 

deformed clasts (L2b ).

(3) L-S tectonites of the shear zone appear to have 

formed primarily by plane strain, as indicated by Flinn K 

values at localities IX and X that are very close to 1, the 

value for plane strain. Rocks at locality IV, however, have 

a K value of 2.4, indicating that they lie in the field of 

apparent constriction.

(4) As shown in table 2, the logarithmic or natural 

strain K values (Ramsay, 1967) suggest that rocks at 

outcrops IX and X underwent only minor volume loss, 

whereas those at outcrop IV underwent appreciable 

volume gain.

(5) The magnitude of both flattening and stretching 

increases from the southeast margin of the shear zone 

toward the center.

(6) Finally, Roy (1977) has pointed out that mylonites within shear zones develop in stages and that this 

development is accompanied by chemical mobility. 

Chemical mobility is suggested in the Mountain shear 

zone by an apparent substantial gain in modal quartz of 

the granodiorite gneiss (Xgn unit) within it, as compared 

to undeformed granodiorite (Xg unit), but this apparent 

gain has not been confirmed by chemical studies.

REGIONAL SIGNIFICANCE

Prior to this study, major northeast-trending shear 

zones in the Wisconsin magmatic terranes were known to 

have formed subsequent to the main regional deformation (Sims and Peterman, 1983, p. 8), but data on the 

precise timing were lacking. The fortunate existence 

of a datable intrusive rock (Hines Quartz Diorite, 

1,812.7±3.6 Ma) within the Mountain shear zone, which

A14 Precambrian Geology of Lake Superior Region
apparently was emplaced near the end of the ductile 

deformation and thus establishes a firm younger limit for 

the shearing, provides a benchmark for the time of 

shearing on a regional basis. Thus, it can now be 

presumed that the steep Jump River and Athens shear 

zones (fig. 2) as well as others of N. 50°-60° E. trend have 

comparable ages (Sims and others, in press). A deformed 

intrusive rock within the Athens shear zone, which has a 

crystallization age of 1,832±9 Ma (Sims and others, in 

press) is consistent with this interpretation that the 

shearing postdates 1,840 Ma. The recognition of a major 

ductile deformation event 25 to 35 m.y. after the main 

collisional event («1,850 Ma; Sims and others, 1985) 

indicates the complexity and long duration of tectonic 

events in the Penokean orogen and the need to identify 

specific events, perhaps by assigning names to them, such 

as has been done in the Wopmay orogen in the 

Northwest Territories (Hoffman and Bowring, 1984). It 

seems improbable, however, that the ^1,815 Ma ductile 

deformation could be a late tectonic episode related to 

the collision that sutured the Wisconsin magmatic (arc) 

terranes to the continental margin at about 1,850 Ma. 

Instead, we suggest that the «1,815 Ma ductile deformation possibly resulted from a continent-continent or 

continent-arc collision far removed from northern 

Wisconsin, probably to the south or southeast.

REFERENCES CITED

Berthe, D., Choukroune, P., and Jegouzo, P., 1979, Orthogneiss, mylonite, and noncoaxial deformation of 

granites The example of the South Armorican shear 

zone: Journal of Structural Geology, v. 1, p. 31-42.

Flinn, D., 1962, On folding during three-dimensional 

deformation: Geological Society of London Quarterly 

Journal, v. 118, p. 385-433.

Fry, N., 1979, Random point distributions and strain 

measurements in rocks: Tectonophysics, v. 60, p. 89-105.

Hanmer, S.K., 1982, Microstructure and geochemistry of 

plagioclase and microcline in naturally deformed granite: 

Journal of Structural Geology, v. 4, p. 197-213.

Hoffman, P.P., and Bowring, S.A., 1984, Short-lived 1.9 Ga 

continental margin and its destruction: Geology, v. 12, 

p. 68-72.

Klasner, J.S., and LaBerge, G.L., 1985, Structural geology and 

kinematics of Early Proterozoic shear zones in central 

Wisconsin [abs.]: 31st Annual Institute on Lake Superior 

Geology, Kenora, Ontario, p. 46.

LaBerge, G.L., and Myers, P.E., 1983, Precambrian geology of 

Marathon County, Wisconsin: Wisconsin Geological and 

Natural History Survey, Information Circular No. 45, 

88 p., 2 plates, scale 1:100,000.

Lister, G.S., and Snoke, A.W., 1984, S-C mylonites: Journal of 

Structural Geology, v. 6, p. 617-638.

Peterman, Z.E., Sims, P.K., Zartman, R.E., and Schulz, K.J., 

1985, Middle Proterozoic uplift events recorded in the 

Dunbar dome of northeastern Wisconsin, U.S.A.: 

Contributions to Mineralogy and Petrology, p. 138-150.

Ramsay, J.G., 1967, Folding and fracturing of rocks: New York, 

McGraw-Hill, 568 p.

____ 1980, Shear zone geometry; a review: Journal of 

Structural Geology, v. 2, p. 83-99.

Ramsay, J.G., and Graham, R.H., 1970, Strain variations in 

shear belts: Canadian Journal of Earth Sciences, v. 7, 

p. 786-813.

Roy, S., 1977, Mylonite microstructures and their bearing on 

development of mylonites An example from deformed 

trondhjemites of the Bergen arc region, SW Norway: 

Geological Magazine, v. 114, p. 445-458.

Simpson, Carol, 1986, Determination of movement sense in 

mylonites: Journal of Geologic Education, v. 34, 

p. 246-261.

Simpson, Carol, and Schmid, S.M., 1983, An evaluation of 

criteria to deduce the sense of movement in sheared 

rocks: Geological Society of America Bulletin, v. 94, 

p. 1281-1288.

Sims, P.K., 1987, Metallogeny of Archean and Proterozoic 

terranes in the Great Lakes region A brief overview: 

U.S. Geological Survey Bulletin 1694, p. 56-74.

____ 1989, Geologic map of Proterozoic rocks near 

Mountain, Oconto County, Wisconsin: U.S. Geological 

Survey Miscellaneous Investigations Series Map 1-1903, 

scale 1:24,000.

Sims, P.K., Day, W.C., and Klasner, J.S., 1986, The Early 

Proterozoic Mountain shear zone, northeastern Wisconsin: Geological Society of America Abstracts with 

Programs, v. 18, p. 752.

Sims, P.K., and Peterman, Z.E., 1983, Evolution of Penokean 

foldbelt, Lake Superior region, and its tectonic environment, in Medaris, L.G., Jr., ed., Early Proterozoic 

geology of the Great Lakes region: Geological Society of 

America Memoir 160, p. 3-14.

Sims, P.K., Peterman, Z.E., and Schulz, K.J., 1985, The Dunbar 

Gneiss- granitoid dome Implications for Early Proterozoic tectonic evolution of northern Wisconsin: 

Geological Society of America Bulletin, v. 96, 

p. 1101-1112.

Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, 

Z.E., in press, Tectonostratigraphic evolution of the 

Early Proterozoic Wisconsin magmatic terranes of the 

Penokean orogen: Canadian Journal of Earth Sciences.

Van Schmus, W.R., Medaris, L.G., Jr., and Banks, P.O., 1975, 

Geology and age of the Wolf River batholith: Geological 

Society of America Bulletin, v. 86, p. 907-914.

Wise, D.U., and others, 1984, Fault-related rocks Suggestions 

for terminology: Geology, v. 12, p. 391-394.

The Mountain Shear Zone, Northeastern Wisconsin A15

 A- U.S. GOVERNMENT PRINTING OFFICE: 1989 773-047/06,034 REGION NO. 8

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National Center, Reston, VA 22092.

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announcements, and publications may be incorrect Therefore, The 

prices charged may differ from the prices in catalogs, announcements, 

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