Structural Geology of Parautochthonous and 

Allochthonous Terranes of the Penokean 

Orogeny in Upper Michigan Comparisons 

with Northern Appalachian Tectonics

U.S. GEOLOGICAL SURVEY BULLETIN 1904-Q
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with the last offerings, are given in the current-year issues of 

the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior 

to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological 

Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available.

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which 

is for sale in microfiche from the U.S. Geological Survey, Book and Open-File Report Sales, Box 25286, Building 810, Denver Federal Center, 

Denver, CO 80225

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL

Books

Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications 

of general interest (such as leaflets, pamphlets, booklets), single 

copies of periodicals (Earthquakes & Volcanoes, Preliminary Determination of Epicenters), and some miscellaneous reports, including some of the foregoing series that have gone out of print at the 

Superintendent of Documents, are obtainable by mail from

U.S. Geological Survey, 

Book and Open-File Report Sales

Box 25286, 

Denver, CO 80225

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY 

from

Superintendent of Documents

U.S. Government Printing Office

Washington, DC 20402

(Check or money order must be payable to Superintendent of 

Documents.)

Maps

For maps, address mail order to

USGS Map Distribution

Box 25286, Building 810

Denver Federal Center

Denver, CO 80225

Residents of Alaska may order maps from

U.S. Geological Survey, Map Sales

101 Twelfth Ave., Box 12

Fairbanks, AK 99701

OVER THE COUNTER

Books

Books of the U.S. Geological Survey are available over the 

counter at the following U.S. Geological Survey offices, all of 

which are authorized agents of the Superintendent of Documents.

 ANCHORAGE, Alaska-Rm. 101,4230 University Dr.

 LAKEWOOD, Colorado-Federal Center, Bldg. 810

 MENLO PARK, California-Bldg. 3, Rm. 3128, 

345 Middlefield Rd.

 RESTON, Virginia-USGS National Center, Rm. 1C402, 

12201 Sunrise Valley Dr.

 SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105, 

125 South State St.

 SPOKANE, Washington-U.S. Courthouse, Rm. 678, 

West 920 Riverside Ave.

 WASHINGTON, D.C.-U.S. Department of the Interior Bldg., 

Rm. 2650,1849 C St., NW.

Maps

Maps may be purchased over the counter at the U.S. Geological Survey offices where books are sold (all addresses in above list) 

and at the following Geological Survey offices:

 ROLLA, Missouri-1400 Independence Rd.

 FAIRBANKS, Alaska-New Federal Building, 

101 Twelfth Ave.

 STENNIS SPACE CENTER, Mississippi-Bldg. 3101
Chapter Q

Structural Geology of Parautochthonous and 

Allochthonous Terranes of the Penokean 

Orogeny in Upper Michigan Comparisons 

with Northern Appalachian Tectonics

By WILLIAM J. GREGG

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

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

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

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

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993

For sale by

USGS Map Distribution 

Box 25286, Building 810 

Denver Federal Center 

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Cregg, William J.

Structural geology of parautochthonous and allochthonous terranes of the 

Penokean orogeny in upper Michigan comparisons with northern Appalachian 

tectonics / by William j. Cregg.

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

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

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:B1904-Q

1. Geology, Stratigraphic Precambrian. 2. Geology, Structural Michigan. 

I. Title. II. Series. III. Series: Contributions to Precambrian geology of Lake 

Superior region; ch. Q. 

QE75.B9 no. 1904-Q 

[QE653]

557.3 s dc20 92-27843 

[551.7'1'09774] CIP
CONTENTS

Abstract Ql

Introduction Ql

Acknowledgments Q3

Structural setting in northern Baraga County Q4

Structures in the Huron River parautochthon Q5

Folds Q5

Foliations Q5

Strain associated with BI deformation Q5

Faults Q6 

Structures in the Falls River slice Q6

Folds Q6

Foliations Q12

Strain associated with BI deformation Q14

Faults Q15 

Implications for Penokean tectonics in Michigan and Minnesota Q15

Nappe structures Q15

Correlation of thrust boundaries Q17

Correlation of mesoscopic structures Q17 

Comparisons with northern Appalachian tectonics Q20

Similarity in terrane assemblages Q20

Origin of gneiss domes Q21

Basement detachments in the Penokean orogen Q21

Origin of metamorphic nodes Q25

Rotation of gneiss domes Q25 

References cited Q26

FIGURES

1. Map showing tectonic setting of the Early Proterozoic of Upper Michigan Q2

2. Map showing macroscopic structural setting of the Baraga belt Q4

3. Sketches of typical BI fold profiles in Huron River parautochthon and Falls River 

allochthon Q7

4. Equal-area hemispherical plots showing mesoscopic structures from Keweenaw Bay 

to the Falls River and Little Mountain Q8

5. Photomicrographs of chlorite-white mica aggregates in slate of the Michigamme 

Formation of the Huron River parautochthon Q9

6. Photomicrograph showing microstructure of large opaque minerals in slate of the 

Michigamme Formation of the Huron River parautochthon Q10

7. Modified Flinn diagram of reduction spot measurements from the slate of the Michigamme Formation of the Baraga belt Qll

8. Sketch of telescoped iron-rich cherty bed in slate of the Michigamme 

Formation Q12

9. Equal-area hemispherical plots of faults and related features in the Baraga 

belt Q13

10. Photomicrographs of microstructures in slate of the Michigamme Formation Q14

Contents III
11. Graph of strain distribution shown by reduction spots in slate of the Michigamme 

Formation Q15

12. Photographs showing mylonites from Penokean thrust faults in Upper 

Michigan Q16

13. Equal-area hemispherical plots showing rotation of BI structural elements by the 

Covington section fault Q18

14. Photomicrographs showing microscopic structures in slates, Carl ton County, 

Minnesota Q19

15. Sketch section comparing tectonic terranes in the Northern Appalachians and Upper 

Michigan Q22

16. Map and structural model for the development of late Penokean faults in northern 

Michigan Q24

TABLES

1. Correlation chart of Early Proterozoic rocks in northern Michigan Q3

2. Reduction spot data for slate of the Michigamme Formation Q12

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Structural Geology of Parautochthonous and Allochthonous 

Terranes of the Penokean Orogeny in Upper Michigan 

Comparisons with Northern Appalachian Tectonics

By William J. Gregg1

Abstract

The Precambrian terrane of Michigan's Upper Peninsula 

consists largely of Archean massifs and mantled gneiss domes 

surrounded by weakly to highly strained belts of Proterozoic 

metasedimentary rocks deformed during the Penokean orogeny, about 1 .85 Ca. The rocks in these Early Proterozoic belts 

consist of a thin lower sequence of shelf and shallow-water 

sediments, overlain by a thick upper sequence of slate, siltstone, and discontinuous graywacke units, known as the upper 

slate member of the Michigamme Formation. The Baraga belt, 

in northern Baraga County, typifies the structural setting of the 

upper slate member. The belt can be divided into a weakly 

deformed terrane, the Huron River parautochthon, to the 

north, and a polyphase-deformed terrane, the allochthonous 

Falls River slice, to the south. The change in structural style and 

finite strain between the two terranes occurs abruptly along the 

east-west-trending Falls River thrust, which appears to correlate with a similar, previously recognized boundary in Carlton 

County, Minnesota.

In the parautochthonous terrane the rocks contain only 

one style group of folds and associated cleavage. Thrust faults 

typically deform early first generation structures such as S1 

cleavage, but do not occur in association with B2 structures. In 

the allochthonous terrane, these same faults are overprinted by 

B2 folds and S2 crenulation cleavage, suggesting a late Bj timing for the thrusting in both terranes.

The overall structural setting in northern Michigan, as 

well as in east-central Minnesota, appears to comprise a series 

of Early Proterozoic (Penokean) northward-verging folds that 

have been transported northward along imbricate thrusts. No 

large-scale overturned fold limbs have been observed in either 

area; therefore a "thrust nappe" setting appears more appropriate than a "fold nappe."

'Department of Geological Engineering, Geology and Geophysics, 

Michigan Technological University, Houghton, MI 49931.

In northern (Upper) Michigan this structural setting is part 

of a regional terrane assemblage that is strikingly similar to that 

of the northern Appalachians. Both regions contain a parautochthonous and allochthonous foreland, followed laterally by 

a detached basement massif, a tightly appressed metasedimentary synform, and a compressional mantled gneiss domesynform complex. The two regions differ in that the deformation of both basement and cover rocks is of higher intensity in 

the Appalachians.

In northern Michigan, curved metamorphic isograds may 

be related to relative structural position in the gneiss domesynform terrane, with the highest metamorphic grade being in 

the deep synformal hinges. The gneiss domes may have been 

rotated from an initial east-west trend by late Penokean faults 

produced by north-northeast-directed tectonic indentation by 

an irregular projection on the north edge of the Wisconsin 

magmatic arc terranes.

The tectonic interpretations presented in this report suggest that at the beginning of the Proterozoic, the southern complex and possibly part of the northern complex of the 

Marquette district existed as a surface of low relief, forming a 

single large sedimentary basin extending over all the structural 

terranes, rather than forming a series of small basins in tectonic 

troughs. This single large basin received uniform sedimentation in a subsiding continental-shelf-shallow-deltaic environment facing open waters to the south. The relatively weak 

imprint of the collisional event in Upper Michigan might be a 

result of the smaller size of the colliding magmatic arc terranes 

compared to that in the Appalachians.

INTRODUCTION

The Precambrian terrane of Upper Michigan consists 

largely of Archean massifs and mantled gneiss domes (fig. 

1) surrounded by weakly to highly strained belts of 

Proterozoic metasediments deformed during the Penokean

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q1
87°30'

46°50' --

46°40

46°30' H46°20' TT;

20 30 KILOMETERS 

_j_______I

EXPLANATION

Midcontinent rift system (Middle Proterozoic)

Metasedimentary rocks of Marquette Range Supergroup

(Early Proterozoic) 

Crystalline rocks (Archean)

Contact

Thrust fault Dashed where inferred; teeth on upper plate

 GLTZ Approximate trace of Great Lakes tectonic zone

- -garnet- - Metamorphic isograd

CSF Covington section fault

LM Lake Michigamme

TM Taylor mine

Figure 1 . Tectonic setting of the Early Proterozoic of Upper Michigan. Modified from Klasner (1978) and Cannon (1977, 1 986).

orogeny, about 1.85 Ga (Cannon and Klasner, 1975; Foose, 

1980; Klasner and others, 1982; Cannon, 1986; Sims and 

others, 1989). The rocks in the Proterozoic belts consist of 

a thin lower sequence of shelf and shallow-water sedimentary rocks near the Archean contacts, overlain by a thick 

upper sequence of slate, siltstone, and discontinuous 

graywacke units (table 1). In the western part of the area, 

the lower sequence consists of the Goodrich Quartzite and

the lower slate member and Bijiki Iron-formation Member 

of the Michigamme Formation, whereas in the eastern part 

additional units of the Menominee Group are present below 

the Goodrich (Cannon and Gair, 1970; Cannon, 1974, 

1977, 1986). In both areas the thick sequence of slate, siltstone, and graywacke above these lower units is known as 

the upper slate member of the Michigamme Formation. The 

volume of the upper slate member far exceeds that of all

Q2 Precambrian Geology of Lake Superior Region
Table 1 . Correlation chart of Early Proterozoic rocks in northern Michigan 

[Modified from Klasner (1978). Wavy line, unconformity; diagonal line pattern, hiatus]

Huron River parautochthon Western Marquette syncline

upper slate member

Bijiki Iron-formation Member

lower slate member

Goodrich Quartzite

03 

TO

TOm

Archean gneisses, granites, and metabasites

upper slate member

Bijiki Iron-formation Member

lower slate member

Clarksburg Volcanics Member

Greenwood Iron-formation Member

Goodrich Quartzite

Negaunee Iron-formation

Siamo Slate

Ajibik Quartzite

the lower units combined (Cannon, 1986); however, the 

discontinuous nature of the psammitic units within it makes 

stratigraphic mapping difficult.

Work by the U.S. Geological Survey over the past 50 

years has outlined the structural style and orogenic 

sequence of the Penokean in this region (Cannon, 1973). 

Few attempts have been made at detailed structural analysis, especially in the voluminous upper slate member, but 

that which has been done shows a widespread variation in 

the complexity of overprinting relationships, amount of 

finite strain, and degree of metamorphism.

The greatest amount of deformation (Klasner, 1978) 

occurs in the western Marquette syncline near the Twin 

Lake dome of the southern complex (fig. 1), where the 

rocks consist of staurolite-bearing phyliites and schists 

(Cannon and Klasner, 1972; 1976). Klasner (1978) 

showed that the mesoscopic structures in this area consist 

of at least four generations of foliation and three tectonic 

lineations. West of this area, in Baraga County, the 

degree of deformation decreases and the rocks are garnetbearing slate, phyllite, and siltstone. Farther to the north, 

in the Baraga belt, the grade of metamorphism in the 

upper slate member is below the biotite isograd (James, 

1955) and the rocks are much less deformed (Cannon, 

1986). In some locations the rocks contain only an incipient slaty cleavage and very low amplitude first-generation 

folds (Van Roosendaal, 1985). In the more highly 

deformed areas, particularly near the western Marquette

syncline, the scarcity of detailed information on the 

geometry of Penokean deformation in these rocks has 

placed severe constraints on tectonic modeling, and has 

rendered the recognition of Penokean structures in 

Archean rocks all but impossible.

The work outlined in this report is an extension of 

Klasner and Cannon's pioneering efforts, with an 

increased emphasis on microscopic and mesoscopic foliation studies, statistical geometric analysis, and finite strain 

measurements. Letter designations for style group elements follow Turner and Weiss (1963). The mapping, typically at scales of 1:720 or greater, was initiated in the less 

deformed rocks of northern Baraga County. This allowed 

a systematic identification of structural subfabrics and 

mesoscopic style groups that has subsequently been 

extended into other, more highly deformed areas to the 

south and east.

ACKNOWLEDGMENTS

I thank my former graduate students, Dan Van 

Roosendaal, Kevin Sikkila, Gary Dyke, and Dave Saja, for 

the highly detailed and rigorous structural analyses they 

conducted along several of the river sections in Baraga 

County. Thoughtful reviews by David B. Rowley, David 

L. Southwick, and Paul F. Williams were very helpful and 

greatly appreciated. I especially thank Paul Sims and

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q3
46°50' -

46°45' -

c-/ 

. Jacobsville - ' .- ./£/ '

. - .' ' .'/ Sandstone

Kovine 'X

HURON ^ RIVER PARAUTOCHTHON 

iver

Silver River 

THRUST

eweenaw Bay 

FALLS

FALLS RIVER \ SLICE

EXPLANATION

Jacobsville Sandstone (Middle Proterozoic)

Metasedimentary rocks (Early Proterozoic Marquette 

Range Supergroup)

Crystalline rocks (Archean) 

Contact

10 KILOMETERS 

J

 y y Thrust fault Dashed where inferred: teeth on upper plate

 ^ y Overturned fold showing direction of plunge Arrows indicate dip of limbs

Figure 2. Macroscopic structural setting of the Baraga belt. The Falls River thrust separates the belt into structurally dissimilar 

parautochthonous and allochthonous terranes. Boxes outline named areas discussed in text; capital letters refer to locations of sections in figure 3. Archean contacts modified from Cannon (1977).

Lorna Carter for their patience and thoroughness while 

editing this report.

STRUCTURAL SETTING IN 

NORTHERN BARAGA COUNTY

The main area of study in northern Baraga County 

consists of the east-northeast-trending belt of Early

Proterozoic rocks (fig. 2) previously referred to as the 

Baraga basin. The area is not a basin in the usual sense, 

because neither bedding nor cleavage dips inward from 

the northern and southern boundaries of the belt, and no 

thickening of sedimentary units toward its geographic center has been recognized. The northwest boundary of the 

belt is an angular unconformity; the Early Proterozoic 

rocks are covered by the Keweenawan (Middle Proterozoic) Jacobsville Sandstone. To the northeast and south,

Q4 Precambrian Geology of Lake Superior Region
the belt is in tectonic contact with the Archean Huron 

Mountains and northern complex rocks, respectively.

This report documents the foreland fold-thrust model 

proposed by Klasner, Ojakangas, and others (1988), 

Klasner, Sims, and others (1988), and Barovich and others 

(1989) in Upper Michigan, providing evidence for the 

division of the Baraga belt into the Huron River parautochthon to the north, and the allochthonous Falls River 

slice to the south, which are separated by the east-westtrending Falls River thrust. Evidence for this thrust comes 

from abrupt changes in structural complexity and finite 

strain between the adjacent terranes. The thrust can be 

identified along two north-trending sections in the belt, 

one along the Silver River and the other along the Falls 

River and Keweenaw Bay (fig. 2).

STRUCTURES IN THE HURON RIVER 

PARAUTOCHTHON

Folds

Rocks of the upper slate member of the Michigamme 

Formation throughout the Huron River parautochthon 

belong to only one structural style group. This style group 

is generally characterized by Bj folds with Si cleavage, 

accompanied by small synkinematic low-angle thrust 

faults. The folds have shallow axial plunges to the northwest or southeast, and axial planes that dip 45° to 60° SW. 

(Van Roosendaal, 1985; Dyke, 1988; Gregg, 1989a).

Throughout the Huron River parautochthon, structures 

associated with Bj deformation show an increase in intensity toward the south. In the northeast, along the Huron 

River (fig. 3), folds are gentle, with amplitudes on the 

order of 2 m, and occur only in close association with 

widely spaced synkinematic thrust faults. Bj fold amplitudes develop progressively to the southwest (fig. 3A-C), 

with tight, overturned folds occurring along the 

Keweenaw Bay section (fig. 4). Throughout the parautochthon, B! folds are upward facing and display northwardtrending Z-vergence. Only along the northern Silver River 

(fig. 2), within the hinge of the Silver Falls anticline, are 

the folds symmetrical over a large area. S-vergence folds 

are not generally present within the parautochthon.

Foliations

An incipient to moderately developed Sj slaty cleavage 

is symmetrically disposed about the axial surfaces of Bj 

folds, forming an LI bedding-cleavage intersection lineation parallel to the fold axes. In most rock types Si is a 

domainal slaty cleavage defined by anastomosing, continuous mica films of neomineralized white mica. Quartzofeldspathic microlithons between the Si mica films display

bedding-parallel fine detrital layer silicates and opaque 

minerals. In rare examples, where bedding in the slate is 

defined by fine, competent laminations, Si is a weakly 

developed discrete crenulation cleavage. In the northern 

part of the parautochthon, Sj is visible as a weak micafilm foliation in thin section but is not sufficiently well 

developed to produce a parting surface in outcrop. To the 

southwest, Si increases in intensity, and is readily visible 

on a mesoscopic scale.

Large chlorite-white mica aggregates (fig. 5) occur 

commonly within the microlithons in both slate and siltstone. These aggregates, which have (001) traces aligned 

along bedding in weakly deformed samples, have been 

shown to originate as prekinematic, possibly authigenic, 

chlorite porphyroblasts, that become interlayered with 

white mica when strained (Gregg, 1986). Both the aggregates (fig. 5C) and accompanying irregularly shaped 

opaque mineral grains (fig. 6) have strain shadows parallel 

to Si, which increase in length with increasing strain, a 

feature that distinguishes Si from all other foliations in 

both parautochthonous and allochthonous rocks throughout 

the entire region. In addition to these strain shadows, long 

seams of opaque minerals occur along some Si mica 

films, where late displacements parallel to cleavage have 

taken place. The microstructure of Si is compatible with 

Beutner's (1980) studies, which indicate that evidence is 

lacking for a soft-sediment, gravity-sliding origin 

(Klasner, 1978) for any of the early structural features.

Strain Associated with B 1 Deformation

Reduction spots ranging in size from 0.1 mm to 1.0 cm 

occur throughout the Baraga belt in red silty shale containing fine white silty laminae. Along the East Branch Huron 

River, where mesoscopic cleavage is poorly developed, 

the spots are nearly circular in the plane of foliation, with 

Ryz ranging from 1.33 to 1.35 (fig. 7 and table 2). Along 

the West Branch Huron River the spots are also circular in 

the xy plane, but Ryz ranges from 1.57 to 1.65 (table 2). 

All these values are below the lower limit of the deformation field for slaty cleavage (fig. 7; Ramsay and Wood, 

1973), and occur in rocks with lower fold limb rotation 

angles (Beutner, 1978) and lower finite strains (Cloos, 

1947; Wood, 1973; Ramsay and Wood, 1973) than typically observed in slate belts.

A direct estimate of the finite strain in these weakly 

deformed rocks was made using fine iron-rich chert laminae in a few rare samples, which have been deformed by 

telescoping and rotation during cleavage development (fig. 

8). Elongation strain was obtained by summing up the 

lengths of telescoped segments in the xz plane and comparing this initial length to the final length of the telescoped layer. A simple Mohr Circle for Finite Strain 

construction was employed on data from two beds

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q5
oriented normal to and at 40° to the xy plane of the strain 

ellipse to obtain elongation strain values of 63=-0.28 and 

£i=+0.04 with Rxz of 1.46. These figures compare well 

with the results from reduction spot samples and suggest a 

shortening of 25-30 percent for rocks in the northeast end 

of the parautochthon. Strain is minimal in both the x and 

the y directions, an observation compatible with the reduction spot results from other samples and the fact that the 

beds were not telescoped in the xy plane. It seems likely, 

therefore, that the deformation was dominated at these low 

strains by volume-loss mechanisms.

Samples from Keweenaw Bay on the southwest end of 

the parautochthon (fig. 2) show obvious ellipticity in the 

xy plane, and have Ryz ranging from 1.62 to 1.79. These 

samples lie across the lower limit curve (fig. 7) and display well-developed parting surfaces in outcrop. The 

results are indicative of the higher strains associated with 

the increase in Bj-fold amplitudes and tightening of fold 

profiles to the west. A minor rotation of the long axis of 

the finite strain ellipse occurs from east to west in the 

parautochthon, and is associated with a decrease in the 

angle of dip of Bj-fold axial planes (fig. 3).

Faults

Numerous small thrust faults occur throughout the 

Huron River parautochthon (Van Roosendaal, 1985). 

Along the East Branch Huron River, where Bj folding is 

weak, faults occur parallel to both bedding and Sj cleavage. Some of the larger faults form a series of macroscopic-scale, postkinematic structural domains, marked by 

changes in the trend of Lj and in the dip of Bj-fold axial 

surfaces and slaty cleavage (fig. 9). The magnitude of displacement along these thrusts is unknown, but the rotations of LI within the thrust slices are pronounced. Minor 

thrust faults within the domains are generally parallel to 

bedding or cleavage and in a few places are accompanied 

by flattening of Bj axial surfaces to about 25°.

Pitchblende and secondary uranium minerals 

(metatyuyamunite and volborthite) occur in the thrusts as 

small discontinuous stringers and pods surrounded by 

brecciated quartz and calcite (Johnson, 1977). Along the 

West Branch Huron River, several old silver prospect pits 

are aligned along these faults.

In the central part of the autochthon, along the Silver 

River, faults are exclusively parallel or subparallel to bedding and have relatively little displacement (Dyke, 1988). 

The faults generally strike west-northwest as in other 

areas, and display white quartz veins with slickenlines 

plunging to the south-southwest. In rare cases, black slate 

bodies adjacent to these faults show a weak crenulation 

cleavage that overprints Sf, however, no mesoscopic B2 

folds are present.

Along Keweenaw Bay (fig. 4), at the southwest end of 

the parautochthon, where Bj folds are very tight, faults are 

relatively uncommon. Large-scale rotations of Lj, such as 

those associated with faulting along the Huron River, have 

not been noted, and the displacements along most faults 

appear to be small.

The faults present in the Huron River parautochthon 

typically deform early first generation structures such as 

Sj cleavage, but do not occur in association with widespread B2 structures. In the allochthonous Falls River 

slice, however, faults identical in mineralogy and morphology to those in the parautochthon are clearly 

overprinted by numerous B2 macrokinks and S2 crenulation cleavage. These observations suggest a post-Bj timing 

for the faults in the parautochthon as well as the 

allochthon.

STRUCTURES IN THE FALLS RIVER SLICE

The deformational history of the slate and metagraywacke in the Michigamme Formation south of the 

Falls River thrust is remarkably different from that in the 

parautochthonous rocks to the north. Differences in the 

southern terrane include rotation of early first deformation 

structures to nearly recumbent positions, an increase in the 

finite strain associated with Sj, and the overprinting of 

first generation structures by second generation folds on 

all scales.

At the northern front of the allochthon, the Falls River 

thrust has been placed at the boundary between the contrasting structural terranes (Gregg, 1989a). In the western 

part of the Baraga belt, this boundary occurs between the 

Keweenaw Bay section and the Falls River (fig. 4) as well 

as along the Silver River (fig. 2), at a silver prospect about 

100 m north of Dynamite Hill Road.

Folds

Bj folds in the Falls River slice are tight to isoclinal, 

strongly asymmetric folds having a northward vergence 

(figs. 3D and 4; Sikkila, 1987; Sikkila and Gregg, 1987). 

The folds are generally overturned and commonly recumbent; axial surfaces and Sj cleavage typically dip gently 

southward except where affected by second generation

Figure 3 (facing page). Typical B1 fold profiles in Huron River parautochthon and allochthonous Falls River slice. Folds 

show increasing tightness toward Falls River thrust (A to C) and 

abrupt change in structural styles in allochthon (D). Locations 

of sections shown in figures 2 and 4. D, composite section of 

several localities on map in figure 4. Straight dashed line, 

foliation.

Q6 Precambrian Geology of Lake Superior Region
South North

northern Slate River

central Silver River

Keweenaw Bay

Little Mountain Falls River

B, macrokink

D

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q7
°27'

BEDDING

(1,5, 10 pet)

130

reduction spot 

samples

/IKE 

J KB-

(1,5, 10 pet)

absent

(KB-30

/

Keweenaw Bay-^

FR

140

N DOMAIN 

402

DOMAIN II 

(B2 kinks)

reduction spot ,-.-> 

absent samPle \ ''

/Little Mountain

2 KILOMETERS

Figure 4. Equal-area hemispherical plots showing mesoscopic structures from Keweenaw Bay to the Falls River and Little 

Mountain. In stereonet plots: H, hinge region; L, long limb of fold; O, overturned limb; number, number of measurements. Both 

Falls River (FR) and Little Mountain (LM) are in the Falls River slice.

folding. First order Bj folds are quite large, with long 

limb heights (Fleuty, 1964) ranging up to 150 m. In a few 

places short limb heights exceed 45 m, with axial-plane 

separations on the order of 15 m. Second order EI folds 

occur as parasitic folds on larger Bj structures, and show 

roughly the same limb proportions on a hand specimen 

scale. The amplitudes of first order Bj folds in the allochthon are commonly an order of magnitude greater than 

those in the parautochthonous rocks to the north.

B! folds have been overprinted on all scales by macroscopic B2 kink folds (Sikkila, 1987), producing a variation 

in the orientation of Bj structural elements within the kink

band boundaries. Outside these boundaries, B2 deformation is barely perceptible, and Bj folds have axial planes 

that dip gently south, with horizontal to gently plunging 

axes trending west-northwest. Within a typical macroscopic kink zone (fig. 3D), Bj folds are rotated so that 

axial planes and accompanying Sj cleavage (fig. 4) are 

commonly recumbent or even gently north dipping. El 

fold axes and associated Lj bedding-cleavage intersection 

lineations may plunge as much as 20° within B2 kink 

zones, and in rare cases they are doubly plunging. As in 

the parautochthonous rocks to the north, Bj deformation is 

characterized as a system of constant-vergence asymmetric

Q8 Precambrian Geology of Lake Superior Region
J §JT

a> 

O5C oa>8.oo

I

I5'

Figure 5. Photomicrographs of chlorite-white mica aggregates in slate of the Michigamme Formation of the Huron River parautochthon. A, B, Grains from the Silver 

River, showing rotation of (001 ) out of original bedding-parallel orientation (Gregg, 1 986). C, Grain from the Huron River, snowing development of strain shadows (dark 

areas) parallel to S^ cleavage.
Figure 6. Photomicrograph showing microstructure of large opaque minerals in slate of the Michigamme Formation of the Huron River parautochthon. Opaques typically have long axes parallel to bedding and show strain shadows parallel to Sj. Specimen is from the Silver River (fig. 2).

Q10 Precambrian Geology of Lake Superior Region
0.6

0.5

0.4

tr 0.3

0.2

0.1

0.0

Parautochthon/allochthon-X^ 

boundary v

^KB-20

Lower limit to deformation field of - 

slaty cleavage (Ramsay and Wood, 1973)

X

,KB-30

\

KB-13

\

\

-Falls River 

O

o

FR-3

FrV1

-Keweenaw Bay-

\

-Huron Riuer-

,HV-102

HS-69/ 

.HS-8 O .

HV-105 , ,HS-10

0.0 0.1 0.2 0.3 0.4 

In (Ryz)

0.5 0.6 0.7 0.8

Figure 7. Modified Flinn diagram of reduction spot measurements from the upper slate member of the Michigamme Formation 

of the Baraga belt. Location of samples shown in figure 4. Samples from the parautochthon include: KB, Keweenaw Bay; HV, East 

Branch Huron River; HS, West Branch Huron River. Allochthonous rocks are represented by the Falls River samples (FR). RXy and 

Ryz are the ratios of the axes in the xy and yz planes of the finite strain ellipsoid (table 2; Flinn, 1962; Ramsay, 1967, p. 138).

folds and associated thrusts, all of which were upward facing prior to B2 deformation. There is no indication of the 

lower, downward-facing regional fold limb that is commonly associated with fold nappes.

B2 folds are typically asymmetric flexures that deform 

Sj cleavage, Sj-parallel thrust faults, and bedding. They 

are accompanied by an S2 axial-planar crenulation cleavage (fig. 4) and an L2 wrinkle lineation, both of which 

are readily distinguished from first generation structures. 

Along the Falls River, the intensity of B2 folding 

increases systematically from open, gentle flexures in the 

north, to tight, short wavelength structures in the south. 

B2 axial surfaces, which commonly appear as large-scale 

kink-band boundaries, are vertical and strike west-northwest in the north, whereas in the south they dip as low

as 28° S., and strike about N. 39° W. This rotation of 

B2 axial planes and S2 cleavage is accompanied by a rare 

S3 cleavage, which strikes N. 85° E. and dips 70° S. 

Mesoscopic 63 folds have not been observed.

The size of macroscopic B2 kink domains is on the 

order of 30-50 m. Within these domains, which are 

essentially defined by large-scale kink-band boundaries, 

smaller kink domains less than 1 m wide are present. 

On both scales it is typical for the L2 wrinkle lineation 

to increase in amplitude as the boundary of the kink 

domain is approached from either direction; thus, the lineation and S2 cleavage may actually diminish somewhat 

inside the rotated kink zone. Outside the kink zone, 

well away from the boundary, these structures are generally absent.

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q11
Table 2. Reduction spot data for slate of the Michigamme Formation

[(rt), number of samples; *, not available. Sample designations: FR, Falls River; KB, Keweenaw Bay; HV, East Branch Huron River 

(Van Roosendaal, 1985); HS, West Branch Huron River (Saja, 1991)]

Sample No. R (n) yz S0 x Si

FR-1

FR-3

KB-13

KB-20

KB-30

HV-102

HV-105

HS-8

HS-lOb

HS-69

1.38±0.07 (82)

1.44±0.05 (33)

1.39±0.06 (38)

1.46±0.04(11)

1.42±0.06 (14)

1.02±0.03 (23)

1.00 * *

1.01±0.04 (73)

1.0210.04 (178)

1.03±0.04 (67)

2.10±0.11(117)

2.11±0.09(45)

1.62±0.07 (29)

1.7910.09 (13)

1.65±0.05 (10)

1.35±0.09 (52)

1.33±0.08 (22)

1.61±0.12(70)

1.57±0.10(100)

1.65±0.14(33)

17

21

32

14

68

37

*

44

44

72

l(d)=7.6 mm

S^in slate)

Figure 8. Telescoped iron-rich cherty bed in slate of Michigamme Formation. Finite strain in the xz plane was estimated by comparing length of summed segments (£l(up to length of telescoped span (l(d)) as shown. £3 is elongation strain value. Specimen is 

from the East Branch Huron River (fig. 2).

Foliations

8j varies in mesoscopic appearance from a well-developed penetrative slaty cleavage in pelite to an irregular 

domainal rough cleavage in graywacke (Sikkila, 1987). In 

the limbs of isoclinal Bj folds, it is typically impossible to 

distinguish Sj from S0 without a thin section. On a microscopic scale, Si is defined by continuous, anastomosing

mica films of oriented chlorite and white mica separated 

by quartzo-feldspathic domains. The width and degree of 

irregularity of the quartzo-feldspathic domains generally 

increase with sedimentary grain size. In thin sections of 

finer grained rocks, bedding can be readily distinguished 

within quartzo-feldspathic domains.

8j in the Falls River slice can be conclusively correlated with Sj in the Huron River parautochthon not only

Q12 Precambrian Geology of Lake Superior Region
w

FALLS RIVER 

N

FAULT ORIENTATIONS

SILVER RIVER

N

HURON RIVER 

N

n = 35 n = 7

HURON RIVER-FAULT-BOUNDED DOMAINS 

N

B

Figure 9. Equal-area hemispherical plots of faults and related features in the Baraga belt. A, Poles to fault planes in allochthonous rocks (Falls River) and parautochthonous rocks (Silver River and Huron River) in the Michigamme Formation. B, Rotation of L! bedding-cleavage intersection lineations across fault boundaries (diagrammatically shown in cross section) on the 

East Branch Huron River (fig. 2). Numbers, number of measurements; contours are 1 point, 10 percent, and 20 percent of 1 

percent area for all plots.

because of its morphologic similarity, but also because of 

its relationship with the ubiquitous predeformational chlorite-white mica aggregates. In the allochthon, these 

aggregates display obvious prekinematic textures relative 

to Sj, including deformation of the aggregates and development of long strain shadows parallel to slaty cleavage 

(fig. 10A). As in the parautochthonous rocks, the strain 

shadows show an increase in length parallel to Sj with 

increasing strain. Sj-parallel strain shadows are also 

present around large opaque mineral grains (fig. 10A, 5), 

as in some of the parautochthonous rocks. In both cases 

the strain shadows are deformed by S2 crenulation cleavage (fig. 105).

S2 typically consists of a zonal to discrete crenulation 

cleavage that overprints both Sj and bedding (fig. 10). 

Where macroscopic B2 folds are gentle, this cleavage is 

well developed only within the hinge regions of the folds. 

Where B2 folds are tight and numerous, S2 is pervasive 

and predominates in mesoscopic and microscopic appearance over other foliations.

S2 morphology also varies with rock type. In pelitic 

rocks near gentle B2 flexures, S2 may be accompanied by 

thin, widely spaced mica films in the short limbs of B2 

microcrenulations. These films may contain thin seams of 

opaque minerals in rocks that initially contained abundant 

disseminated opaques. Where B2 deformation is intense,

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q13
Figure 10. Photomicrographs of microstructures in slate of the Michigamme Formation. A, Typical chlorite-white mica aggregates and large opaque porphyroblasts in allochthonous rocks along the Falls River. Strain shadows occur parallel to ST slaty cleavage as in the parautochthonous rocks to the north. B, Deformation of strain shadows and porphyroblasts by B2 crenulations along 

the Falls River.

S2 is more tightly spaced, the angle of the short limb is 

higher, and the width of the domain is much smaller. In 

the pelites, this is commonly accompanied by a change in 

morphology to a more anastomosing appearance. In 

siltstones, S2 is more strongly defined, since most of the 

chlorite in the rock lies along the microfold limbs, which 

have been subjected to solution-transfer removal of quartz. 

In graywackes, S2 is poorly defined or absent.

S3 foliation is present only in the south area of the Falls 

River section (fig. 4), near a large Keweenawan diabase 

dike, where B2 folds and S2 cleavage have been subsequently deformed. In this area, originally east west striking, nearly vertical S2 crenulation cleavage has been rotated 

more than 70° about an east-northeast-trending horizontal 

axis. In the field, a steeply dipping 83 cleavage occurs as a 

set of closely spaced planes, 2 to 4 cm apart, which overprints all earlier foliations. On a microscopic scale, 83 is 

characterized by a series of simple, discontinuous fractures

filled with dusty opaque material. No reorientation or 

recrystallization of layer silicates is associated with this 

foliation. This foliation has not been observed in any other 

area, and mesoscopic B3 folds have not been found.

Strain Associated with B 1 Deformation

Reduction spots are relatively rare in the Falls River 

slice because of the scarcity of the characteristic redbanded pelites and psammites that contain them. Those 

spots that were measured are from samples that contained 

no S2 cleavage and were not located within B2 macrokink 

boundaries; however, some overprint of strain associated 

with B2 deformation cannot be ruled out. As shown by 

Hoist (1985), the superimposed B2 strain is in a direction 

that would tend to decrease the ellipticity of the reduction

Q14 Precambrian Geology of Lake Superior Region
spots, since the long axis of the ellipsoid lies in the plane 

of the nearly recumbent slaty cleavage and plunges southwest. The data can thus be viewed as a minimum strain 

associated with Bj deformation in the Falls River slice. 

The measurements obtained from typical red silty slate 

(table 2) show Rxy ranging from 1.38 to 1.44 and Ryz of 

about 2.10 (fig. 7).

The wide distribution of reduction spots allows a comparison of finite strain throughout the parautochthon and 

across the Falls River thrust as well (fig. 11). Along the 

Keweenaw Bay and Falls River sections (fig. 4), the 

change in finite strain does not seem particularly abrupt, 

being roughly comparable to those observed on local 

faults in the Cambrian slate belt of Wales (Wood and 

Oertel, 1980). However, the change in finite strain is 

accompanied by a large rotation of the ellipse axis due to 

overprinting by B2 structure. The strain ellipse along 

Keweenaw Bay has a relatively constant orientation, 

plunging about 60° SW., whereas the strain ellipse in the 

Falls River slice plunges from horizontal to gently northward in B2 kink hinges to gently southward outside kink 

zones. This rotation amounts to a large post-Bj strain that 

cannot be evaluated at the mesoscopic scale.

The reduction spot measurements show an increase in 

ellipticity from northeast to southwest in the parautochthon, toward the Falls River thrust. It is tempting to draw 

a smooth curve through the data (fig. 11), suggesting a 

relatively uniform strain distribution in the parautochthon. 

More detailed studies in progress, however, suggest that 

this may not be the case (Saja, 1991).

Faults

A system of Sj-parallel faults from a few centimeters 

to 1 m thick is present in the allochthon, typically appearing near large El fold hinges. Most of the faults contain 

white quartz and slate fragments, but some contain abundant cataclastic slate fragments surrounded by carbonate. 

The vein material, which is also present in joints located 

near the faults, ranges in thickness from a few centimeters 

to 1 m. The mineralogy of minor faults is consistent with 

that of the mylonites from the Falls River thrust (fig. 12).

The rotation of cleavage in the slate fragments within 

these smaller thrusts shows them to be post-Bj structures, 

as is the case with similarly oriented and mineralized 

faults in the parautochthonous rocks to the north. This 

suggests an association between the small thrusts and the 

Falls River thrust, which shows a much larger rotation of 

$! cleavage and is therefore also a post-Bj structure. This 

timing relationship suggests that the Falls River thrust 

may have more or less the same orientation as the smaller 

structures, that is, it strikes roughly east-west and dips 

about 30°-40° S., flattening to the south.

The timing of the thrusting relative to the B2 fold event 

is apparent from both field and macroscopic observations.

3.5

3.0

2.5

2.0

1.5

1.0

KB

E

HS

HV -

-2 2468 

DISTANCE, IN KILOMETERS

10 12

Figure 11 . Strain distribution shown by reduction spots in slate 

of the Michigamme Formation. Sample localities shown in figure 

4, except Huron River, general location shown in figure 2. Samples from the Huron River parautochthon and Falls River slice 

show an increase in finite strain toward the Falls River thrust. FR, 

Falls River; KB, Keweenaw Bay; HS, West Branch Huron River; 

HV, East Branch Huron River. Data calculated from values in 

table 2.

It is obvious that B2 folding did not occur before the 

thrusting, because the steeply inclined B2-fold axial surfaces are in no place cut by faults. On the other hand, in 

areas where B2 macrokinks occur, it is the faults which 

are overturned and have severely convoluted and crenulated surfaces. The most reasonable suggestion is that the 

faulting is more or less synchronous with B2 macrokink 

development, and that the folds represent the plastic deformation associated with ramping during the overthrusting 

event. This fits the macroscopic picture well, because only 

the transported rocks south of the Falls River thrust contain B2 structures. If B2 folding had occurred after the 

overthrusting, one would expect the B2 macrokinks to 

overprint the thrust boundary and to be found in the parautochthonous rocks as well.

IMPLICATIONS FOR PENOKEAN 

TECTONICS IN MICHIGAN AND 

MINNESOTA

Nappe Structures

Detailed structural analysis (Sikkila, 1987; Dyke, 1988; 

Gregg, 1989a) has shown that the rocks of northern 

Baraga County can be divided into a northern weakly 

deformed parautochthonous structural belt and a southern 

polyphase-deformed allochthonous belt, separated by the 

east-west-striking, southward-dipping Falls River thrust

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q15
Figure 12. Photographs showing mylonites from Penokean thrust faults in Upper Michigan. A, Banded quartz mylonite with 

graphitic slate layers from the Falls River thrust at the Silver River locality (fig. 2). B, Banded and crossbanded quartz mylonite 

from one of several minor thrusts along the West Branch Huron River (fig. 2). C, Contorted quartz mylonite with slate bands from 

a minor thrust along the Covington section (fig. 1). D, Quartz mylonite with cherty iron-formation fragments, cutting bedding in 

cherty iron-formation. Sample is from the Taylor mine area (fig. 1) and occurs along a large thrust, shown as a saddle reef in 

Klasner (1978, fig. 3).

(fig. 2). The structural setting is characterized by a series 

of northward-verging folds that have been transported 

northward along a set of imbricate thrusts, possibly of 

"sledrunner" morphology.

The general term "nappe" is widely used to describe 

tectonic settings such as those observed in the Michigan 

and Minnesota terranes, and is synonymous with 

"overthrust" (Rodgers, 1990) or "thrust sheet" (Ramsay

Q16 Precambrian Geology of Lake Superior Region
and Huber, 1987, p. 521). The term "fold nappe" 

(Ramsay, 1967) refers specifically to the classic nappe 

model, consisting of a regional-scale recumbent fold with 

a downward-facing hinge, an inverted lower limb, and an 

upward-facing "root" zone (Tobisch and Glover, 1971; 

Ager, 1980, fig. 14.5). The term "thrust nappe" (Ramsay, 

1967) is used to describe a large recumbent fold nappe 

partially or completely cut by a lower thrust (Ramsay and 

Huber, 1983, fig. 11.10; Park, 1983, fig. 12.3); a series of 

imbricate thrusts in nearly undisturbed rocks (imbricate 

fans or schuppen-structure); or even a series of folds with 

relatively constant fold vergence (Hobbs and others, 1976, 

p. 411).

Thrust nappes appear to be typical of many foreland 

regions, where fold vergence is uniformly towards the stable craton, and where no evidence for a lower recumbent 

fold limb exists. Fold nappes, on the other hand, seem to 

be associated with highly strained (Ramsay and Huber, 

1983) rocks within or adjacent to metamorphic core zones 

or tectonic root zones. The foreland terranes in Michigan 

and Minnesota are best described as thrust nappes because 

there appears to be no direct evidence for a large-scale 

lower limb in either area.

Although the location of the Falls River thrust is relatively well defined, the location of the southern boundary 

of the Falls River slice is as yet unknown. This boundary 

may occur against lower parautochthonous or allochthonous rocks, in which case the slice would be a klippe 

(Zen, 1972, p. 71), against an overlying allochthonous 

slice, or against the Great Lakes tectonic zone (Sims and 

others, 1980). The possibility of a parautochthonous terrane to the south of the Falls River slice seems unlikely, 

but its existence could be demonstrated in the field by the 

absence of pervasive B2 structures, as is the case in the 

Huron Bay parautochthon. Klasner (1978) reported B2 

structures at the Taylor mine area and in the Lake Michigamme area along the western Marquette syncline (fig. 1). 

Hoist (1989a), on the other hand, reported a boundary 

along the Baraga-Iron County line, 15 km south of Covington (fig. 1), which separates a southern terrane 

deformed by B2 folds from a northern terrane lacking second generation structures. He suggested that the boundary, 

referred to as a "nappe front," correlated with a similar 

boundary that he delineated in Carlton County, Minn. 

(Hoist, 1984b).

Correlation of Thrust Boundaries

Numerous reports, including this one, have shown the 

existence of B2 structures in rocks north of Hoist's boundary. Furthermore, recent mapping by me along the Covington transect shows evidence for large-scale post-Bj 

rotations of early folds along a fault located approximately

7 km south of Covington (figs. 1, 13), and well within 

Hoist's northern terrane. At this location an abrupt 

change in the orientation of Sj cleavage and the Lj intersection lineation occurs, with BI fold axes showing a 

more northerly plunge on the south side of the fault. This 

location corresponds closely with the boundary delineated 

by Barovich and others (1989) on the basis of Nd isotope 

studies. The relative scarcity of S2 along this section, as 

reported by Hoist (1989a), may be caused by the dominance of psammitic rocks, which do not readily form 

crenulation cleavages, rather than the absence of a B2 

deformational event.

Because of the strong similarity in structural setting, I 

suggest that Hoist's boundary in Carlton County, Minn., 

correlates with the Falls River thrust. The terrane south of 

the boundary in both areas contains first generation folds 

that are in places isoclinal and recumbent, and which contain an axial-planar slaty cleavagt. These folds are overprinted in both areas by open, upright second generation 

folds and a steeply dipping crenulation cleavage, neither 

of which occur in the northern terranes.

In addition to the strong structural similarity between 

the Baraga and Carlton County areas, evidence exists for a 

similar sedimentological contrast across the boundaries in 

both areas. In Minnesota the amount of graphitic sedimentary rocks increases sharply to the south across the fault 

(Hoist, 1984b). In Baraga County, the rocks in the northern area are weakly graphitic, whereas those south of the 

boundary are very graphitic, and economic graphite deposits are known to exist (Johnson, 1977).

Correlation of Mesoscopic Structures

In Minnesota, Hoist (1981, 1982, 1989b) correlated the 

second generation folds that occur in the southern Minnesota terrane with the first generation folds to the north on 

the basis of similar cleavage orientation. He theorized that 

the northern terrane contained no folds or cleavage at the 

time the isoclinal Fj folds and slaty cleavage developed in 

the southern terrane. He then postulated that FI folds were 

rotated and overprinted by open F2 folds and crenulation 

cleavage during nappe emplacement in the southern terrane, with synchronous development of tight folds and 

slaty cleavage in the footwall rocks of the northern terrane. Using this model, Hoist assumed that the northern 

rocks recorded only the strains due to F2 deformation. He 

then used strain measurements made in the northern area 

in a subtractive manner, to calculate supposed finite strain 

due to Fj deformation south of the boundary. This calculation led him to propose total strains that are uncommonly 

high for a typical fold and thrust belt setting (Ramsay and 

Wood, 1973), and which demand a peculiar subhorizontal 

orientation of the principal plane of shortening

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q17
BEDDING B1 STRUCTURAL ELEMENTS

NORTH OF CSF

SOUTH OF CSF

Figure 13. Equal-area hemispherical plots showing rotation of B^ structural elements across the fault within the Covington 

section, Michigan. S^ slaty cleavage; Lj, bedding-cleavage intersection lineation; CSF, Covington section fault (fig. 1). Contours are 1 point, 5 percent and 10 percent per 1 percent area for all plots. TC, pole axis SQ. Numbers, number of measurements.

Q18 Precambrian Geology of Lake Superior Region
Figure 14. Photomicrographs showing microscopic structures in slate, Carlton County, Minn. A, Large chlorite-white mica 

aggregates from the Thompson dam, showing original bedding-parallel orientation of (001), and development of strain shadows 

parallel to Sj slaty cleavage. B, Chlorite-white mica aggregate deformed by S2 crenulation cleavage (from Gillogly Road location 

in Hoist's southern terrane). Strain shadows parallel to S^ remain visible despite S2 overprint. C, Large opaque porphyroblasts 

with strain shadows parallel to Sv subsequently deformed by S2 (from same location as B). Compare with identical textures in 

figure 10.

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q19
during the first deformation (Hoist, 1985; Holm and others, 1988).

My recent studies in Carlton County, Minn., suggest an 

alternative interpretation of the relative timing of the foliations and associated folds in the northern and southern terranes. The slaty cleavage associated with first generation 

folds in the southern area immediately adjacent to the 

boundary appears to be identical in structural petrography 

to the cleavage associated with first generation folds to the 

north. Like the rocks of the Baraga, Mich., area, the slaty 

cleavage can be readily correlated on both sides of Hoist's 

boundary by its relationship with diagenetic chlorite porphyroblasts (fig. 144, 5). Although these grains are 

deformed by first and second generation folding south of 

the boundary, they display strain shadows parallel only to 

Si slaty cleavage. These strain shadows are later crenulated by S2 (fig. 145), thus allowing an easy distinction 

between the foliations when the chlorite porphyroblasts 

are present.

The structural correlation presented above is based on 

style group criteria, that is, morphological similarity of 

foliations, lineations, and folds, and overprinting relationships. Hoist's correlation is based on similarity in orientation between structural features, an approach which has 

been shown to be problematic (Weiss and Mclntyre, 1957; 

Turner and Weiss, 1963; Hobbs, 1965; Hobbs, Means, and 

Williams, 1976). Furthermore, no compelling reason presents itself to make the unusual correlation (Turner and 

Weiss, 1963, p. 464) of a regional slaty cleavage with a 

late crenulation cleavage.

The field evidence in Carlton County strongly supports the progressive deformation model proposed by 

Connolly (1981), whereby slaty cleavage developed 

synchronously in first generation folds in rocks from both 

the north and south terranes, with deformation increasing 

to the south. The evidence further suggests that the 

southern area was subsequently overthrust to the north, 

with the development of B2 folds in the southern 

transported hanging-wall terrane, rather than in the northern footwall terrane (Hoist, 1985).

COMPARISONS WITH NORTHERN 

APPALACHIAN TECTONICS

Similarity in Terrane Assemblages

Numerous authors have suggested a collisional plate 

tectonic model for the Early Proterozoic Penokean orogeny in Upper Michigan and Minnesota (Cambray, 1977,

1978a, 1978b; Larue, 1983; Schulz, 1984; Hoist, 1984a; 

Sims and others, 1987). The work presented here extends 

this concept, providing field documentation for the existence of a typical foreland fold and thrust belt in Upper 

Michigan (fig. 1). The terrane assemblage in this area, 

from the parautochthonous rocks at the northern tectonic 

front, to the metamorphic core zone, and finally to the 

well-documented Niagara fault zone in the south (Larue, 

1983; Sims and others, 1984; LaBerge and others, 1984) is 

remarkably similar to that of the Northern Appalachians.

Figure 15 is a structural section across the Northern 

Appalachians, also showing the size and sequence of similar terranes in northern Michigan. The section runs from 

Glens Falls, N.Y., to Claremont, N.H., and is situated 

close to lines 1, 3, 4, and 5 of the COCORP New England 

seismic survey. Seismic "picks" from Brown and others 

(1983, fig. 2) and Ando and others (1984, fig. 3) were 

incorporated in the structure section. The geology of the 

parautochthonous and allochthonous foreland terranes is 

based largely upon the work of Rowley and Kidd (1981), 

Bosworth and Rowley (1984), and Stanley and Ratcliffe 

(1985). Interpretations in the Green Mountains, Ludlow 

Valley, and Chester dome areas are based on sections by 

Ando and others (1984), and on field work by Thompson 

(1950), Nisbet (1976), and Gregg (1975, 1989b).

The foreland in the Northern Appalachians consists of 

a sequence of parautochthonous Cambrian and Ordovician 

sediments, overlain by the highly deformed allochthonous 

Taconic slices (Zen, 1967; 1972). To the east of the foreland lie three distinct terranes that make up the metamorphic core zone. The westernmost of these, the Green 

Mountains, is a large Precambrian massif that is detached 

from the basement along a shallow decollement (fig. 15 

and Brown and others, 1983). On the east side of this 

massif is the complexly deformed Ludlow Valley 

sequence of the Vermont Valley slices. This zone, 

formerly referred to as the Townshend-Brownington syncline, consists of a tectonic stacking sequence of faultbounded, polyphase-deformed metasedimentary rocks 

(Gregg, 1975). Finite strains within this pseudo-stratigraphic sequence are extremely high; as many as three 

secondary foliations are present, and transposition is widespread, commonly resulting in tectonic pseudo-conglomerates (Gregg, 1990). Tectonic slices of ultramafic rocks 

occur along many of the faulted lithologic contacts in this 

terrane. These ultramafics, marking an earlier overthrust, 

have been redistributed by a major doming event that 

marks the next terrane.

To the east of the Ludlow Valley sequence lies a series 

of mantled gneiss domes (Thompson, 1950), typified by 

the Chester dome in figure 15. Strong seismic reflectors 

occur beneath the domes (Brown and others, 1983; Ando

Q20 Precambrian Geology of Lake Superior Region
and others, 1984) and appear to truncate the root zones of 

the early thrusts present in the eastern fold and thrust terrane (Stanley and Ratcliffe, 1985).

Origin of Gneiss Domes

Since their initial description by Eskola (1949), mantled gneiss domes have been considered to be either 

wholly or partly diapiric in origin by many workers 

(Thompson, 1950; Thompson and others, 1968; Ramberg, 

1963; Stanley, 1975; Miller, 1983; Sims and others, 1985; 

Dietsch, 1989). A few workers have pointed out the strong 

possibility that the domes are compressional structures displaying large-scale fold interference patterns (Ramsay, 

1967). In the Northern Appalachians, Nisbet (1976) documented the compressional nature of the Chester dome, 

showing its large-scale box-fold form and its close association with a macroscopically penetrative synkinematic 

axial-planar crenulation cleavage. He showed that the 

dome event overprinted two earlier style groups of folds 

and associated thrusts, forming the Butternut Hill fold 

nappe on the west flank of the Chester dome (Nisbet, 

1976, p. 152), and folding the main ultramafic suture zone 

in central Vermont.

The broad antiformal gneiss domes in the Northern 

Appalachians, and in other areas such as Upper Michigan, 

are typically paired with tightly appressed synforms developed in the overlying metasedimentary rocks. The characteristic form of the gneiss dome-synform pair fits 

Ramsay's (1967, p. 383) mechanical model, in which a 

high viscosity contrast along a boundary between basement and cover causes the crystalline basement rocks to 

develop broad, rounded folds of large amplitude, while the 

overlying low-viscosity sedimentary rock is deformed into 

small-wavelength, "pinched" synforms. I suggest that 

gneiss dome-and-pinched synform structure originated 

during the buckling of crystalline thrust sheets (Hatcher 

and Williams, 1986) after detachment from the basement. 

This buckling may explain the box fold or angular fold 

profile observed in gneiss domes, because folds of this 

style commonly form above decollements in materials 

characterized by closely packed competent layering, or 

around single buckling layers (Ramsay, 1967, fig. 7-81). 

The varieties of crystalline thrust sheets described by 

Hatcher and Williams (1986) are all capable of producing 

thin basement slices that could approximate the singlelayer case. In the Chester dome, for example, the buckling 

is superimposed upon a composite-type crystalline thrust 

sheet, whereas domes in other areas, such as northern 

Michigan, may be developed in thin-skinned sheets.

Basement Detachments in the 

Penokean Orogen

The terrane assemblage outlined above is quite similar 

to the sequence present along a north-south line through 

the Precambrian of Upper Michigan (figs. 1, 15). The 

foreland in Upper Michigan is represented by the Huron 

River parautochthon. The Falls River thrust marks the 

southern limit of this terrane, separating it from allochthonous rocks to the south. This fault also appears to define 

the boundary between the Huron River parautochthon and 

the northern complex. I suggest, therefore, that the northern complex may be a detached massif similar to the 

Green Mountains in Vermont, transported northward with 

the polyphase-deformed metasediments of the Falls River 

slice along a shallow decollement. The very low finite 

strains present in the Huron River parautochthon, and the 

absence of allochthonous terranes in front of the northern 

complex, suggest that the amount of transport was significantly less than that which occurred in the Northern 

Appalachians.

Several small bodies of Archean rocks, completely surrounded by Baraga Group (Early Proterozoic) sedimentary 

rocks, occur along the northern boundary of the northern 

complex. The largest of these, at Arvon Hill (fig. 2), is a 

thin slice of Archean gneisses approximately 550 m wide 

and 3,500 m long. Cannon (1986) has suggested that this 

body is bounded by steep, east-west-trending faults. An 

alternative explanation might be that the body, along with 

an intact "cap" of Goodrich Quartzite, is an exposed ramp 

formed by a south-dipping thrust sheet. Field mapping has 

been inconclusive in regard to this problem, which might 

be better investigated with geophysical methods.

Because of the lack of modern structural mapping in 

the region northwest of Marquette (fig. 1), complete integration of the eastern Clark Creek and Dead River slate 

belts into the tectonic picture is not yet possible. Both 

belts lie south of the proposed extension of the Falls River 

thrust, so their current status as undisturbed sedimentary 

basins is questionable. If they are allochthonous terranes, 

they represent either slices that dip beneath part of the 

northern complex, or klippen resting on shallow decollements above the Archean rocks.

In northern Michigan, the second terrane of the metamorphic core is represented by the Marquette syncline, 

which lies between the basement massif and the southern 

complex gneiss domes. Both the Marquette syncline and 

the analogous Ludlow Valley sequence fit the gneiss 

dome-synform model presented in this report; however, the 

rocks in the Marquette syncline show far less deformation 

(Cambray, 1984) than the Ludlow Valley rocks (Gregg, 

1975) and contain no ultramafic zones of Penokean age. It

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q21
UPPER MICHIGAN

oE* N KFWFFNAWAN TOV

IXC VVtCIMAAVVrAliI \*\JV

1

1 

1

1

'ER HURON RIVER 

iPARAUTOCHTHON

i

NORTHERN COMPLEX

LU 

^~ '"

in _

O 2

1"

1'

(0

* 

o 

<

Q

z 

o 

DC

Q

PARAUTOCHTHONOUS

FORELAND UNITS

(C-0)

NORTHERN APPALACHIANS

ALLOCHTHONOUSTACONIC SLICES ( -O) GREEN MOUNTAIN 

MASSIF

VERMONT 

VALLEY

SLICES

( -0)

CT QBT

MIDDLE PROTEROZOIC

10

I

20

I

30 KILOMETERS

100

NEW YORK LINE 1

200 300 400 500 100

VERMONT LINE 3

200 300 40
f

O 

3

c o

9

 o 

aj1'

3

o

n

3

 o

 g

EXPLANATION

Rocks of Silurian and Devonian age

Slope-rise sedimentary rocks of Cambrian and Ordovician age

Parautochthonous shelf sedimentary rocks of Cambrian and Ordovician age

Highly tectonized phyllites and schists of Cambrian and Ordovician age

Cambrian Dalton and Hoosac Formations

Basement rocks of Middle Proterozoic age

Contact Dashed where reconstructed above ground surface

Fault Dashed where reconstructed above ground surface 

1 Ultramafic rocks occupying fault zone 

Structural trend in basement

74° 

43°

42°

NEW YORK

UlA?SAC

j CONNE

Figure 15. Comparison of tectonic terranes in the Northern Appalachians and Upper Michigan. The terranes occur in a similar o

The seismic section for the Northern Appalachians is based on combined interpretations of COCORP data by Ando and others (198

stations along seismic lines. CT, Champlain thrust; GBT, Giddings Brook thrust; HT, Hinesburg thrust. No vertical exaggeration. In

(1984).

s
>.

a^^-!1 ^ V' "~~ *-' * -*

E*^ "7,- * SOUTHERN"/"'

N<^% «<=COMPLEX5r*/s

/ ?** * = * -. ** ii =

- <^S ^

^ n^^rr^'i^'Av

------- MAGMATIC^ NV x ' ' N

\NES^\>j,'0/VO/X X

i'-'Wx. 

^/V/^C

\ ^ \ X- / '

^>^r^ v - ,WISCONSIN MAGMATIC^\V 

0/"^ \,^\ \TERRANES x^\"x // 

'^O^XJ-ix-v'/VO^!',. --;'-.V;i^!^<>/ x ^-;

v ^ ..\^I N X/

/v /-v ->\-

' Nx /X v/X \ X > ' ^

^//^.</^/;> x ^\

B

'x'

fefe.V^^V-v^

40 KILOMETERS

Figure 16. Structural model for development of late Penokean faults in northern Michigan. A, Structural features in WisconsinMichigan border region (modified from Cannon, 1986). Heavy lines, faults; barbs show direction of relative movement where 

known. Large curved hachured arrow shows clockwise sense of rotation near southern complex, possibly causing reorientation 

of gneiss domes. Large, wide arrow, direction of tectonic transport. B, Slip-line model showing fault pattern similar to A developed 

at the front of an irregular indentor (large, wide arrow) during continental collision (modified from Tapponier and Molnar, 1976).

Q24 Precambrian Geology of Lake Superior Region
seems likely, therefore, that major pre-doming detachments 

did not extend to this zone from the deeper Niagara suture 

(fault zone) to the south, and that faulting such as that along 

the Covington section (fig. 1) was limited to minor detachments associated with later stage crustal shortening. Larger 

movements may have occurred along the proposed detachment beneath the northern complex, or along higher detachments that may have ramped over the gneiss domesynform structures in the southern complex.

The gneiss domes of Upper Michigan, like the synforms, appear to be less deformed counterparts of the 

Grenville domes in Vermont, having developed on the surface of a large, relatively undisturbed basement block, 

rather than on a complexly deformed basement slice. They 

nevertheless show the usual box-fold or angular fold styles 

when viewed in the profile plane, and are bounded by a 

number of tightly appressed synforms. The gneiss domesynform model suggests a compressional origin for the 

domes, as well as the possibility of their northward transport along a shallow decollement during the Penokean 

orogeny. The model supports the suggestion of Ueng, 

Larue, and Sedlock (1984) that the Amasa dome represents "crossfolding" rather than a "gneiss dome," if one 

assumes that their reference to gneiss domes implies the 

classic diapiric model. It also supports the work of 

Klasner (1984) and Attoh and Klasner (1989) who showed 

intense involvement of basement during Penokean thrusting in the Wisconsin magmatic arc terranes (Schulz, 

1984), with northward transport of the Dunbar gneiss 

dome toward the continental foreland along southwarddipping overthrusts.

Origin of Metamorphic Nodes

James (1955) reported a series of elliptical metamorphic nodes (fig. 1) associated with the gneiss domes in 

Upper Michigan and proposed that they developed around 

a series of separate heat sources. Attoh and Vander 

Meulen (1984), however, have shown that a magmatic 

heat source alone is insufficient, and suggested that crustal 

thickening or doming might have contributed the additional heat required for regional metamorphism. I suggest 

that the isograd patterns may simply correlate with relative 

structural position in the gneiss dome-synform model, 

because mantling sediments in the deep synformal hinges 

would be expected to show the highest metamorphic 

grade. Subsequent isostatic uplift, tilting, and erosion of 

the gneiss dome-synform complex would eventually 

expose these hinges. The Twin Lake and Grant Lake 

domes may therefore represent a tilted cross section of the 

basement-cover relationship, with metamorphic grade

decreasing "upwards" out of the Republic syncline to the 

northwest.

Rotation of Gneiss Domes

Cannon (1973) pointed out that the axial traces of both 

the southern complex gneiss domes and the major folds in 

the Proterozoic rocks have very divergent trends, and presented the problems that this situation posed for a horizontal compression model. This redistribution of the gneiss 

dome-synform structure appears to have been synchronous with the development of a broad pattern of faults that 

radiate northward from the Wisconsin magmatic arc terranes (Cannon, 1978, 1986; Sims, 1990). I suggest that the 

dome rotation and the faults were the result of a local, 

north-northeast-directed tectonic indentation (Dewey and 

Burke, 1973; Sengor, 1976) by an irregular projection on 

the north edge of the Wisconsin magmatic terranes. This 

late transport direction, which differs from the northwest 

compression direction of the main Penokean event (Sims 

and Peterman, 1983, fig. 5), may be entirely local.

Figure 16 shows a comparison of the Penokean fault 

system with the indentor slip-line model of Tapponnier 

and Molnar (1976, fig. 6). I have modified the model by a 

20 percent pure-shear deformation in the thrusting 

direction. This amount seems reasonable, as it is lower 

than the strains measured even in remote parautochthonous rocks, and it results in a pattern which closely resembles that shown by the Penokean faults. Agreement is 

good between the sense of offset shown by the faults and 

the predicted shear directions in the slip-line model (fig. 

16). Near the Amasa dome, the shear sense on the faults 

shows an overall clockwise rotation of the area, which 

could account for the rotation of the dome axes out of an 

original east-west orientation (arrows in fig. 16).

The tectonic interpretations presented in this report 

suggest that at the beginning of the Proterozoic the southern complex and possibly part of the northern complex 

existed as a surface of low relief, forming a single large 

sedimentary basin extending over all the structural terranes, rather than a series of small basins in tectonic 

troughs (Larue and Sloss, 1980). This basin received uniform sedimentation in a subsiding continental shelfshallow deltaic environment facing open waters to the 

south. The Penokean collisional event outlined in Attoh 

and Klasner (1989) resulted in an assemblage of structural 

terranes very similar in style to, but less intensely developed than, those found in well-studied Phanerozoic collisional mountain belts. The relatively weak imprint of the 

collisional event in Upper Michigan might be due to a 

smaller size of the colliding volcanic-magmatic arc terranes (Schulz, 1984) compared to the Appalachians.

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q25
REFERENCES CITED

Ager, D.V., 1980, The geology of Europe: New York, John Wiley, 

535 p.

Ando, C.J., Czuchra, B.L., Klemperer, S.L., Brown, L.D., Cheadle, 

M.J., Cook, F.A., Oliver, I.E., Kaufman, S., Walsh, T., 

Thompson, J.B., Lyons, J.B., and Rosenfeld, J.L., 1984, 

Crustal profile of mountain belt COCORP deep seismic 

reflection profiling in New England Appalachians and 

implications for architecture of convergent mountain chains: 

American Association of Petroleum Geologists Bulletin, v. 

68, p. 819-837.

Attoh, K., and Klasner, J.S., 1989, Tectonic implications of 

metamorphism and gravity field in the Penokean orogen of 

northern Michigan: Tectonics, v. 8, p. 911-933.

Attoh, K., and Vander Meulen, M.J., 1984, Metamorphic 

temperatures in the Michigamme Formation compared with 

the thermal effect of an intrusion, northern Michigan: Journal 

of Geology, v. 91, p. 417-432.

Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 

1989, Nd isotopes and the origin of 1.9-1.7 Ga Penokean 

continental crust of the Lake Superior region: Geological 

Society of America Bulletin, v. 101, p. 333-338.

Beutner, E.G., 1978, Slaty cleavage and related strain in 

Martinsburg slate, Delaware Water Gap, New Jersey: 

American Journal of Science, v. 278, p. 1-25.

____1980, Slaty cleavage unrelated to tectonic dewatering The 

Siamo and Michigamme slates revisited: Geological Society 

of America Bulletin, v. 91, p. 171-178.

Bosworth, William, and Rowley, D.B., 1984, Early obductionrelated deformation features of the Taconic 

allochthon Analogy with structures observed in modern 

trench environments: Geological Society of America 

Bulletin, v. 95, p. 559-567.

Brown, L.D., Ando, C., Klemperer, S., Oliver, J.E., Kaufman, 

Sidney, Czuchra, B., Walsh, T., and Isachsen, Y.W., 1983, 

Adirondack-Appalachian crustal structure The COCORP 

Northeast traverse: Geological Society of America Bulletin, 

v.94,p. 1173-1184.

Cambray, F.W., 1977, The geology of the Marquette District: 

Michigan Basin Geological Society, Field Guide, 62 p.

____1978a, Plate tectonics as a model for the environment of 

deposition and deformation of Early Proterozoic 

(Precambrian X) of northern Michigan: Geological Society 

of America Abstracts with Programs, v. 10, p. 376.

____1978b, Plate tectonics as a model for the environment of 

sedimentation of the Marquette Supergroup and the 

subsequent deformation and metamorphism associated with 

the Penokean orogeny [abs.]: 24th Institute on Lake Superior 

Geology, Milwaukee, Wisconsin, p. 6. 

.1984, Proterozoic geology, Lake Superior, south shore:

_1978, A middle and late Precambrian fault system in 

northern Wisconsin and northern Michigan [abs.]: 24th 

Institute on Lake Superior Geology, Milwaukee, Wisconsin, 

p. 8.

_1986, Bedrock geologic map of the Iron River 1x2 degree

Geological Association of Canada Annual Meeting Field

Trip Guidebook, 55 p. 

Cannon, W.F., 1973, The Penokean orogeny in Northern Michigan:

Geological Association of Canada Special Paper 12,

p. 251-271. 

____1974, Bedrock geologic map of the Greenwood quadrangle,

Marquette County, Michigan: U.S. Geological Survey

Geologic Quadrangle Map GQ-1168, scale 1:62,500. 

____1977, Precambrian geology in parts of the Baraga, Dead

River, and Clark Creek Basins, Marquette and Baraga

Counties, Michigan: U.S. Geological Survey Open-File

Report 77-467, scale 1:62,500.

quadrangle, Michigan and Wisconsin: U.S. Geological

Survey Miscellaneous Investigations Series Map I-1360-B,

scale 1:250,000. 

Cannon, W.F., and Gair, J.E., 1970, A revision of stratigraphic

nomenclature of middle Precambrian rocks in northern

Michigan: Geological Society of America Bulletin, v. 81,

p. 2843-2846. 

Cannon, W.F., and Klasner, J.S., 1972, Guide to Penokean

deformational style and regional metamorphism of the

western Marquette Range, Michigan: 18th Annual Institute

on Lake Superior Geology, Field Trip Guidebook,

Houghton, Michigan, p. B1-B38. 

____1975, Stratigraphic relationships within the Baraga Group

of Precambrian age, central Upper Peninsula, Michigan: U.S.

Geological Survey Journal of Research, v. 3, p. 47-51.

.1976, Geologic map and geophysical interpretation of the

Witch Lake quadrangle, Marquette, Iron, and Baraga

Counties, Michigan: U.S. Geological Survey Miscellaneous

Investigations Map 1-1987, scale 1:62,500. 

Cloos, E., 1947, Oolite deformation in the South Mountain fold,

Maryland: Geological Society of America Bulletin, v. 58,

p. 843-918. 

Connolly, M.R., 1981, The geology of the Middle Precambrian

Thomson Formation in southern Carlton County, east-central

Minnesota: Duluth, Minn., University of Minnesota-Duluth

M.S. thesis, 133 p. 

Dewey, J.F., and Burke, K.C.A., 1973, Tibetan, Variscan, and

Precambrian basement reactivation Products of continental

collision: Journal of Geology, v. 81, p. 683-692. 

Dietsch, C., 1989, The Waterbury Dome, west-central

Connecticut A triple window exposing deeply deformed,

multiple tectonic units: American Journal of Science, v. 289,

p. 1070-1097. 

Dyke, G.A., 1988, Structure and stratigraphy of the Silver River

area, Baraga County, Michigan: Houghton, Mien., Michigan

Technological University M.S. thesis, 87 p. 

Eskola, P.E., 1949, The problem of mantled gneiss domes:

Geological Society of London Quarterly Journal, v. 104,

p. 461-476. 

Fleuty, M.J., 1964, The description of folds: Geological

Association Proceedings, v. 75, pt. 4, p. 461-489. 

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

deformation: Geological Society of London Quarterly

Journal, v. 118, p. 383-483. 

Foose, M.P., 1980, Geologic map of the Ned Lake quadrangle, Iron

and Baraga Counties, Michigan: U.S. Geological Survey

Miscellaneous Investigations Series Map 1-1284. 

Gregg, W.J., 1975, Structural studies in the Moretown and Cram

Hill units near Ludlow, Vt.: Albany, N.Y., State University

of New York at Albany M.S. thesis, 118 p. 

____1986, Deformation of chlorite-mica aggregates in cleaved

psammitic and pelitic rocks from Islesbore, Maine, U.S.A.:

Journal of Structural Geology, v. 7, p. 59-68. 

___1989a, Effects of large scale overthrusting on fold geometry

and finite strain in the Baraga thrust/fold belt, Upper

Michigan [abs.]: Lansing, Mich., 1st Geology of Michigan

Symposium, p. 14. 

____1989b, Structural geology of the Ludlow mining district,

Vermont, U.S.A. [abs.]: 28th International Geological

Congress, Proceedings, Washington, D.C., p. 588.

Q26 Precambrian Geology of Lake Superior Region
____1990, Transposition structures in deformed rock, with 

examples from the Appalachians and Upper Michigan [abs.]: 

36th Annual Institute on Lake Superior Geology, Thunder 

Bay, Ontario, p. 27-29.

Hatcher, R.D., and Williams, R.T., 1986, Mechanical model for 

single thrust sheets, Part I Taxonomy of crystalline thrust 

sheets and their relationships to the mechanical behavior of 

orogenic belts: Geological Society of America Bulletin, v. 

97, p. 975-985.

Hobbs, B.E., 1965, Structural analysis of the rocks between the 

Wyangala batholith and the Copperhannia Thrust, New 

South Wales: Journal of the Geological Society of Australia, 

v. 12, p. 1-24.

Hobbs, B.E., Means, W.D., and Williams, P.P., 1976, An outline of 

structural geology: New York, John Wiley, 571 p.

Holm, O.K., Hoist, T.B., and Ellis, M., 1988, Oblique subduction, 

footwall deformation, and imbrication A model for the 

Penokean orogeny in east-central Minnesota: Geological 

Society of America Bulletin, v. 100, p. 1811-1818.

Hoist, T.B., 1981, Evidence for multiple deformation in the Middle 

Precambrian Thomson Formation [abs.]: 27th Annual 

Institute on Lake Superior Geology, East Lansing, Michigan, 

p. 22.

____1982, Evidence for multiple deformation during the 

Penokean orogeny in the Middle Precambrian Thomson 

Formation, Minnesota: Canadian Journal of Earth Science, 

v. 19, p. 2043-2047.

____1984a, Penokean tectonics Constraints from structural 

geology in east-central Minnesota [abs.]: 30th Annual 

Institute on Lake Superior Geology, Wausau, Wisconsin, 

p. 19.

____1984b, Evidence for nappe development during the early 

Proterozoic Penokean orogeny, Minnesota: Geology, v. 12, 

p. 135-138.

____1985, Implications of a large flattening strain for the origin 

of a bedding-parallel foliation in the Early Proterozoic 

Thomson Formation, Minnesota: Journal of Structural 

Geology, v. 7, p. 375-383.

____1989a, The Penokean orogeny in Minnesota and Upper 

Michigan A comparison of the structural geology of the 

Michigamme and Thomson Formations [abs.]: 35th Annual 

Institute on Lake Superior Geology, Duluth, Minnesota, 

p. 27-28.

____1989b, Penokean structural terranes in east-central

Minnesota: 35th Annual Institute on Lake Superior 

Geology, Field Trip Guide, p. B1-B17.

James, H.L., 1955, Zones of regional metamorphism in the 

Precambrian of northern Michigan: Geological Society of 

America Bulletin, v. 66, p. 1455-1488.

Johnson, C.A., 1977, Uranium and thorium occurrences in 

Precambrian rocks, Upper Peninsula of Michigan: 

Houghton, Mich., Michigan Technological University M.S. 

thesis, 217 p.

Klasner, J.S., 1978, Penokean deformation and associated 

metamorphism in the western Marquette Range, northern 

Michigan: Geological Society of America Bulletin, v. 89, 

p. 711-722.

___1984, Gravity models of gneiss domes and a granite pluton 

in northeastern Wisconsin [abs.]: 30th Annual Institute on 

Lake Superior Geology, Wausau, Wisconsin, p. 24.

Klasner, J.S., Cannon, W.F., and Van Schmus, W.R., 1982, The 

pre-Keweenawan tectonic history of the southern Canadian 

shield and its influence on the formation of the mid-continent 

rift, in Wold, R.J., and Hinze, W.J., eds., Geology and

tectonics of the Lake Superior Basin: Geological Society of 

America Memoir 156, p. 27-46.

Klasner, J.S., Ojakangas, R.W., Schulz, K.J., and LaBerge, G.L., 

1988, Evidence for development of an early Proterozoic 

overthrust-nappe system in the Penokean orogen of northern 

Michigan [abs.]: 34th Annual Institute on Lake Superior 

Geology, Marquette, Michigan, p. 56-57.

Klasner, J.S., Sims, P.K., Gregg, W.J., and Gallup, Christina, 1988, 

A structural traverse across a part of the Penokean orogen 

illustrating early Proterozoic overthrusting in northern 

Michigan: 34th Annual Institute on Lake Superior Geology, 

Field Guide, v. 34, part 2, p. C1-C36.

LaBerge, G.L., Schulz, K.J., and Myers, P.E., 1984, The platetectonic history of north-central Wisconsin [abs.]: 30th 

Annual Institute on Lake Superior Geology, Wausau, 

Wisconsin, p. 25.

Larue, D.K., 1983, Early Proterozoic tectonics of the Lake Superior 

region Tectonostratigraphic terranes near the purported 

collision zone: Geological Society of America Memoir 160, 

p. 33-47.

Larue, D.K., and Sloss, L.L., 1980, Early Proterozoic sedimentary 

basins of the Lake Superior region: Geological Society of 

America Bulletin, v. 91, p. 450-452.

Miller, D.M., 1983, Strain on a gneiss dome in the Albion 

Mountains metamorphic core complex, Idaho: American 

Journal of Science, v. 283, p. 605-632.

Nisbet, B.W., 1976, Structural studies in the Northern Chester 

Dome of East Central Vermont: Albany, N.Y., State 

University of New York at Albany Ph. D. dissertation, 167 p.

Park, R.G., 1983, Foundations of structural geology: New York, 

Chapman and Hall, 135 p.

Ramberg, Hans, 1963, Experimental study of gravity tectonics by 

means of centrifuged models: Bulletin of the Geological 

Institutions of the University of Uppsala, v. 42,97 p.

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

McGraw Hill, 568 p.

Ramsay, J.G., and Huber, M.I., 1983, The techniques of modern 

structural geology, volume 1 Strain analysis: New York, 

Academic Press, 307 p.

____1987, The techniques of modern structural geology, volume 

2 Folds and fractures: New York, Academic Press, 391 p.

Ramsay, J.G., and Wood, D.S., 1973, The geometric effects of 

volume change during deformation processes: 

Tectonophysics, v. 16, p. 263-277.

Rodgers, John, 1990, Fold-and-thrust belts in sedimentary rocks, 

Part 1 Typical examples: American Journal of Science, v. 

290, p. 321-359.

Rowley, D.B., and Kidd, W.S.F., 1981, Stratigraphic relationships 

and detrital composition of the medial Ordovician flysch of 

western New England Implications for the tectonic 

evolution of the Taconic orogeny: Journal of Geology, v. 89, 

p. 199-217.

Saja, D.B., 1991, Structure and strain in the Proterozoic metapelites 

of the north-central Huron River parautochthon, northern 

Michigan: Houghton, Mich., Michigan Technological 

University M.S. thesis, 109 p.

Schulz, K.J., 1984, Early Proterozoic Penokean igneous rocks of 

the Lake Superior region Geochemistry and tectonic 

implications [abs.]: 30th Annual Institute on Lake Superior 

Geology, Wausau, Wisconsin, p. 65-66.

Sengor, A.M.C., 1976, Collision of irregular continental 

margins Implications for foreland deformation of Alpinetype orogens: Geology, v. 4, p. 779-782.

Sikkila, K.M., 1987, A structural analysis of Proterozoic 

metasediments, Northern Falls River, Baraga County,

Penokean Orogeny, Upper Michigan, and Comparisons with Northern Appalachians Q27
Michigan: Houghton, Mich., Michigan Technological 

University M.S. thesis, 103 p.

Sikkila, K.M., and Gregg, W.J., 1987, A structural analysis of 

Proterozoic metasediments, northern Falls River, Baraga 

County, Michigan [abs.]: 33rd Institute on Lake Superior 

Geology, Wawa, Ontario, v. 33, p. 65-66.

Sims, P.K., 1990, Geologic map of Precambrian rocks, Marenisco, 

Thayer, and Watersmeet 15-minute quadrangles, Gogebic 

and Ontonagon Counties, Michigan, and Vilas County, 

Wisconsin: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-2093, scale 1:62,500.

Sims, P.K., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, 

The Great Lakes tectonic zone A major crustal structure in 

central North America: Geological Society of America 

Bulletin, v. 91, p. 690-698.

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. 3-13.

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

Schulz, K.J., 1987, Nappe development and thrust faulting in 

the Upper Michigan segment of the early Proterozoic 

Penokean orogeny: Geological Society of America 

Abstracts with Programs, v. 19, p. 246.

Sims, P.K., Peterman, Z.E., and Schulz, K.J., 1984, A partisan 

review of the Early Proterozoic geology of Wisconsin and 

adjacent Michigan [abs.]: 30th Annual Institute on Lake 

Superior Geology, Wausau, Wisconsin, p. 73.

____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., 

1989, Tectono-stratigraphic evolution of the Early 

Proterozoic Wisconsin magmatic terranes of the Penokean 

orogen: Canadian Journal of Earth Sciences, v. 26, 

p. 2145-2158.

Stanley, R.S., 1975, Time and space relationships of structures 

associated with the domes of southwestern Massachusetts 

and western Connecticut: U.S. Geological Survey 

Professional Paper 888, p. 69-96.

Stanley, R.S., and Ratcliffe, N.M., 1985, Tectonic synthesis of the

Taconian orogeny in western New England: Geological

Society of America Bulletin, v. 96, p. 1227-1250. 

Tapponnier, P., and Molnar, P., 1976, Slip-line field theory and

large scale continental tectonics: Nature, v. 264, p. 319-324. 

Thompson, J.B., Jr., 1950, A gneiss dome in southeastern Vermont:

Boston, Mass., Massachusetts Institute of Technology Ph. D.

dissertation, 149 p. 

Thompson, J.B., Jr., Robinson, P., Clifford, T.N., and Trask, N.J.,

Jr., 1968, Nappes and gneiss domes in west-central New

England, in Studies of Appalachian geology: New York,

JohnWiley,p.203-218. 

Tobisch, O.T., and Glover, L.G., 1971, Nappe formation in part of

the southern Appalachian Piedmont: Geological Society of

America Bulletin, v. 82, p. 2209-2230. 

Turner, F.J., and Weiss, L.E., 1963, Structural analysis of

metamorphic tectonites: New York, McGraw Hill, 545 p. 

Ueng, W.L., Larue, O.K., and Sedlock, R.L., 1984, Geologic

history and palinspastic reconstruction of the early

Proterozoic Penokean collisional zone [abs.]: 30th Annual

Institute on Lake Superior Geology, Wausau, Wisconsin,

p. 78. 

Van Roosendaal, Dan, 1985, An analysis of rock structures and

strain in cleaved pelitic rocks, East Branch of the Huron

River, Baraga County, Michigan: Houghton, Mich.,

Michigan Technological University M.S. thesis, 82 p. 

Weiss, L.E., and Mclntyre, D.B., 1957, Structural geometry of

Dalradian rocks at Loch Leven, Scottish Highlands: Journal

of Geology, v. 65, p. 575-602. 

Wood, D.S., 1973, Patterns and magnitudes of natural strain in

rocks: Philosophical Transactions of the Royal Society of

London Academy, v. 274, p. 373-382. 

Wood, D.S., and Oertel, Gerhard, 1980, Deformation in the

Cambrian slate belt of Wales: Journal of Geology, v. 88,

p. 309-326. 

Zen, E-an, 1967, Time and space relationships of the Taconic

allochthon and autochthon: Geological Society of America

Special Paper 97,107 p. 

____1972, The Taconide Zone and the Taconic orogeny in the

western part of the northern Appalachian orogen: Geological

Society of America Special Paper 135,72 p.

Manuscript approved for publication July 6,1992 

Published in the Central Region, Denver, Colorado 

Type composed by Shelly A. Fields 

Graphics by Norma J. Macs 

Edited by Lorna Carter

Q28 Precambrian Geology of Lake Superior Region

. GOVERNMENT PRINTING OFFICE: 1993-774-049/66057
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports 

of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrology, availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that maybe cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering 

geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric bases at various scales, they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which 

"effect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

lopographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photogeologic maps on planimetric 

'lases which show geology as interpreted from aerial photographs. The 

rv?ries also includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, 

stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology in relation to specific mining or mineral-deposit problems; post-1971 maps 

are primarily black-and-white maps on various subjects such as environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; the principal 

scale is 1:24,000, and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive 

listings of U.S. Geological Survey publications are available under the 

conditions indicated below from USGS Map Distribution, Box 25286, 

Building 810, Denver Federal Center, Denver, CO 80225. (See latest 

Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a set 

microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a set 

of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982, 1983, 1984, 1985, 1986, and for subsequent years since the last permanent catalog may be purchased by mail 

and over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and Water-Supply Reports and Maps For (State)," may be purchased by mail and 

over the counter in paperback booklet form only.

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet 

form only.

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" is available free of charge by mail or may be 

obtained over the counter in paperback booklet form only. Those wishing 

a free subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the prices 

charged may differ from the prices in catalogs, announcements, and publications.
U.S. Geologica