system appendpdf cover-forpdf - university of toronto t-space · 78 from the west end of lake...

Draft

Magnetization Age from Paleomagnetism of the Copper

Harbor Red Beds, Northern Michigan, USA, and its Keweenawan Geologic Consequences.

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2017-0094.R1

Manuscript Type: Article

Date Submitted by the Author: 26-Sep-2017

Complete List of Authors: Symons, Dave; Dept of Earth Sciences

Kawasaki, Kazuo; University of Toyama, Earth Sciences Diehl, J.; Dept Geol. and Mining Eng.and Sci.

Is the invited manuscript for consideration in a Special

Issue? : N/A

Keyword: Paleomagnetism, Remagnetization Ages, Red beds, Keweenawan Supergroup, North America

https://mc06.manuscriptcentral.com/cjes-pubs

Canadian Journal of Earth Sciences

DraftDraft

Magnetization Age from Paleomagnetism of the Copper

Harbor Red Beds, Northern Michigan, USA, and its Keweenawan Geologic Consequences.

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2017-0094.R1

Manuscript Type: Article

Date Submitted by the Author: 26-Sep-2017

Complete List of Authors: Symons, Dave; Dept of Earth Sciences

Kawasaki, Kazuo; University of Toyama, Earth Sciences Diehl, J.; Dept Geol. and Mining Eng.and Sci.

Is the invited manuscript for consideration in a Special

Issue? : N/A

Keyword: Paleomagnetism, Remagnetization Ages, Red beds, Keweenawan Supergroup, North America


Canadian Journal of Earth SciencesPage 1 of 80



DraftDraft

1

Magnetization Age from Paleomagnetism of the Copper Harbor Red Beds, Northern 1

Michigan, USA, and its Keweenawan Geologic Consequences. 2

3

D.T.A. Symons1*, K. Kawasaki2, and J.F. Diehl3 4

5

1University of Windsor, Windsor, ON, N9B3P4, Canada 6

2University of Toyama, Toyama-shi, Toyama, 930-8555, Japan 7

3Michigan Technological University, Houghton, MI, 49931, USA 8

9

*Corresponding author. D.T.A. Symons. Department of Earth and Environmental 10

Sciences, University of Windsor, Windsor, ON, N9B3P4, Canada (Tel: 11

+1-519-2353000 ext. 2493; Fax: +1-519-9737081; e-mail: [email protected]) 12

13

14

Page 1 of 79


Canadian Journal of Earth Sciences Page 2 of 80



DraftDraft

2

Abstract 15

The Copper Harbor Formation on Lake Superior’s Keweenaw Peninsula 16

records the transition from volcanic to sedimentary infilling of North America’s 1.1 Ga 17

Keweenawan rift. Radiometric dating shows that the formation’s primary mafic 18

sediments and interbedded “Lake Shore” flows were deposited between ~1092 and 19

~1082 Ma. Our regional paleomagnetic results for the Copper Harbor’s red beds yield a 20

dominantly-prefolding normal-polarity secondary chemical characteristic remanent 21

magnetization (ChRM) in hematite at 18 of 21 sites with a mean direction of D = 22

274.9°, I = +10.9° (k = 69.5, α95 = 4.2°) and a paleopole at 7.4°N, 181.7°E (A95 = 3.3°). 23

Using paleopoles from Keweenawan volcanic rocks with U/Pb zircon age dates, an 24

Apparent Polar Wander Path (APWP) is constructed from 1106±2 Ma to 1087±2 Ma. 25

Extrapolation of this path dates oxidation of the Copper Harbor’s primary gray beds to 26

red beds at 1060±5 Ma. The path implies an apparent polar wander rate of ~18 cm/yr 27

from ~1108 Ma to 1096 Ma and of 6.8 cm/yr from 1096 Ma to 1087 Ma, along with a 28

consistent clockwise rotation of 0.30±0.05°/Myr for the Laurentian Shield from ~1108 29

to ~1160 Ma. Further, most Keweenawan volcanic rocks around the Lake Superior 30

region carry an endemic ~1060 Ma normal-polarity hematite remanence overprint, 31

acquired during the initial stages of Grenvillian tectonic uplift, that has caused 32

Page 2 of 79





DraftDraft

3

asymmetry in a unit’s normal and reverse paleopoles. Also, the Copper Harbor 33

paleopole dates emplacement of the White Pine stratiform sedimentary copper 34

mineralization more precisely at 1060±5 Ma. 35

36

Key words: Paleomagnetism; Remagnetization Ages; Red beds; Keweenawan 37

Supergroup; North America 38

39

Page 3 of 79





DraftDraft

4

1 Introduction 40

The Copper Harbor Formation (CHF) is a rift-related sedimentary and 41

volcanic sequence that crops out mostly along the shoreline of the Keweenaw Peninsula 42

in Lake Superior as part of the ~1.1 Ga Midcontinent Rift (MCR) system of North 43

America (Fig. 1, Table 1). The CHF is unique in the MCR’s geologic history because it 44

records both the last significant stage of rift magmatism and the first significant stage of 45

clastic sedimentation into the rift. The Lake Shore Traps (LST) are short sequences of 46

andesitic basalt flows within the red sandstone and conglomerate sequences of the CHF. 47

One LST flow in the CHF has yielded a U/Pb zircon date of 1087.2±1.6 Ma to tightly 48

constrain the CHF’s primary depositional age (Davis and Paces 1990) and 49

paleomagnetic studies have shown that the LST flows retain a primary characteristic 50

remanent magnetization (ChRM) and paleopole (Diehl and Haig 1993; Kulakov et al. 51

2013). In contrast, paleomagnetic studies of the CHF red beds have been limited to 52

minor reconnaissance tests (DuBois 1962), a major paleomagnetic conglomerate test 53

(Palmer et al. 1981), and an extensive bedding inclination error test conducted on 54

cross-bedded strata at one site (Elmore and Van der Voo 1982). This study was 55

undertaken to provide a regional evaluation of the CHF’s ChRM and thereby to assess 56

the red beds’ mean oxidation/magnetization acquisition age. This assessment is 57

Page 4 of 79





DraftDraft

5

important because the world-class native copper lodes in the MCR’s main stage Portage 58

Lake volcanic rocks, which immediately underlie the CHF, are intimately associated 59

with secondary alteration minerals that include substantial hematite. It is important also 60

because the basal beds of the Nonesuch Formation, which rest conformably on the CHF, 61

host most of the chalcocite (copper-oxide) mineralization of the huge White Pine 62

stratiform sedimentary copper (SSC) deposit (Bornhorst and Barron 2011). 63

In this paper, we use the estimated rates of volcanic extrusion, of clastic gray 64

beds deposition and of Keweenawan apparent polar wander (APW) to obtain the 65

magnetization age of 1060±5 Ma for the CHF’s red beds. From this age, we then 66

propose an upper Keweenawan chronology for the geologic evolution of the Lake 67

Superior region of the MCR from ~1100 Ma to ~1040 Ma. The chronology posits that 68

the White Pine SSC mineralization was also emplaced at ~1060±5 Ma or ~20 Ma later 69

than previously proposed (Brown 2005, 2014). This result further delays the timing of 70

the onset of regional Grenvillian compression in the MCR more tightly from ~1080 Ma 71

to ~1060 Ma. 72

73

2 Geology 74

Page 5 of 79





DraftDraft

6

The ~3000 km-long MCR fractured the pre-existing Laurentian Shield of North 75

America at ~1.1 Ga (Cannon 1994; Stein et al. 2016) (Fig. 1A). Gravity and seismic 76

evidence shows that the rift extends in the subsurface through Kansas to mid Oklahoma 77

from the west end of Lake Superior and through Windsor and along the Grenville Front 78

to mid Alabama from the east end of Lake Superior. However most exposures of the 79

Keweenawan Supergroup’s rift-related rocks occur in the Lake Superior region (Fig. 80

1B), and they are especially well exposed on the Keweenaw Peninsula (Bornhorst and 81

Lankton 2006) (Fig. 2). 82

The Portage Lake Formation of the Bergland Group rests conformably on 83

volcanic strata of the Powder Mill Group or unconformably on older 84

pre-Mesoproterozoic basement rocks of the Keweenaw Peninsula except where 85

truncated by the Keweenaw fault (Table 1). The Portage Lake Formation is composed 86

of >200 basaltic lava flows with minor interbedded rhyolitic conglomerates 87

(Swanson-Hysell et al. 2014), and the flows form the uppermost unit of main-stage 88

MCR magmatism (Stein et al. 2016). U/Pb zircon dates from the Portage Lake 89

Formation have given 1096.2±1.8 Ma and 1094±1.5 Ma (Davis and Paces 1990). In the 90

White Pine area, the Portage Lake Formation is overlain by the Porcupine Formation. It 91

is a local shield volcano of ~50 km in diameter and up to ~3 km thick that forms a dome 92

Page 6 of 79





DraftDraft

7

and has given a U/Pb zircon age of 1093±1.4 Ma (Zartman et al. 1997; Woodruff et al. 93

2013). 94

The Oronto Group rests conformably on the Portage Lake and Porcupine 95

Formations. The group is composed, from bottom to top, of the Copper Harbor (CHF), 96

Nonesuch and Freda Formations (Table 1). The CHF is a basinward-thickening 97

upward-fining wedge of red volcanogenic conglomerate and sandstone that ranges in 98

thickness from ~200 m to ~2000 m (White 1968; Elmore 1984). U/Pb analyses of 29 99

detrital zircon grains from the middle of the CHF yielded an average age of 1104±2 Ma, 100

indicating that the primary gray CHF sediments were derived almost entirely from older 101

Keweenawan volcanic rocks (Davis and Paces 1990). Within the middle to upper 102

portions of the formation are 31 known andesitic basalt flows, informally known as the 103

Lake Shore Traps (LST), that are interbedded with the red beds. Three of the four short 104

series of 4-40 m thick flows are exposed onshore and the fourth is deep offshore under 105

Lake Superior. A 1087.2±1.6 U/Pb zircon date comes from a flow in the middle of the 106

CHF (Davis and Paces 1990). The CHF sediments are interpreted to have been 107

deposited as proximal to distal braided-stream and sheet-flood facies sediments in 108

coalesced alluvial fans and sand flats (Elmore 1984). Oxidation of these sedimentary 109

rocks to red beds is attributed by Brown (2005, 2014) to the subsequent subsurface fluid 110

Page 7 of 79





DraftDraft

8

flow of meteoric water that was a precursor to emplacement of the White Pine SSC 111

mineralization in the basal beds of the Nonesuch Formation immediately above the 112

CHF. 113

The Nonesuch Formation consists of 40-250 m of greenish gray-to-black 114

siltstones, shales and carbonate laminates with low-to-moderate contents of total carbon 115

and pyrite that are interpreted to have been deposited in a lake bottom setting (Elmore et 116

al. 1989; Suszek 1997). Anoxic conditions preserved carbon and pyrite preferentially in 117

the lower 15 to 30 m of the unit to form well-laminated dark silty shales that host most 118

of the economic SSC mineralization of the White Pine deposit (Brown 1971; White 119

1971). The Freda Formation, the uppermost unit of the Oronto Group, conformably 120

overlies the Nonesuch Formation and contains up to 4-6 km of fine-to-medium grained 121

red sandstone with red siltstone and shale layers (Henry et al. 1977; Brown 2014). 122

The tectonic evolution of the Keweenawa study area began at ~1109 Ma with 123

the onset of MCR extension (Heaman and Machado 1992). From ~1109 Ma to ~1093 124

Ma, the rift opened to a width of ~80 km and a thickness of ~10 km of early 125

Keweenawan mafic flows were extruded into the rift basin (Cannon 1992; Zartman et 126

al. 1997). As extension waned from ~1092 to ~1083, gray clastic sediment was eroded 127

from the surrounding rift highlands and deposited into the basin along with sporadic 128

Page 8 of 79





DraftDraft

9

flows to form the CHF with the LST (Davis and Paces 1990). The chronology of late 129

Keweenawan tectonics after ~1082 Ma becomes uncertain because of the lack of 130

stratigraphic age dates. Based on regional geology and cross-rift seismic profiles, 131

Cannon (1992; 1994; see also Cannon et al. 1989, 1993) estimated that ~30 km of 132

shortening occurred in the rift basin between ~1080 Ma and ~1040 Ma as the Grenville 133

Province collided with the Laurentian Shield along the Grenville Front (Fig. 1). The 134

Grenvillian collision caused shortening in which the basinal strata were uplifted, folded, 135

thrust-faulted and inverted. The shortening formed a regional syncline along the 136

northwestern side of the Keweenaw peninsula with its curving synclinal axis beneath 137

Lake Superior (Fig. 2). Consequently, the CHF dips northwestward at a gentle to 138

moderate angle into the syncline (Table 2). Paleomagnetic studies of the Portage Lake 139

and LST rocks have shown that the curving axis is a primary feature and not due to 140

oroclinal folding of the MCR (Hnat et al. 2006; Diehl et al. 2009; Kulakov et al. 2013). 141

The collisional event also created the major northwest-dipping Keweenaw thrust fault 142

along the middle of the peninsula (Fig. 2) and extensive co-eval block faulting 143

throughout the Keweenawan strata (Bornhorst 1997). Cannon et al. (1993) reported 16 144

Rb/Sr biotite dates for metamorphosed Archean to Middle Proterozoic rocks that were 145

uplifted by the Keweenaw and subsidiary thrust faults. Eight of the determinations gave 146

Page 9 of 79





DraftDraft

10

reset metamorphic ages of <1100 Ma with a mean age of 1055±24 Ma, leading Cannon 147

et al. (1993) to propose a date of 1060±20 Ma as the culmination of the thrust faulting 148

event. Following the compressional deformation event on the Keweenaw peninsula, 149

there appears to have been only epeirogenic events that include the deposition of the 150

clastic Jacobsville Formation over the Oronto Group and Recent glaciation (Bornhorst 151

and Rose 1994). 152

The Keweenaw peninsula hosts two world-class copper mining districts. The 153

native copper district, mostly north of Houghton (Fig. 2), is the world’s largest such 154

district. It produced ~6.5 Mt of copper from ~30 mines in the Portage Lake Formation 155

between ~1840 and 1968 (Weege and Pollock 1972; Bornhorst and Barron 2011). The 156

copper infills open spaces in vesicular and brecciated basaltic flow tops (~58% of 157

copper production), in interflow pebble-to-boulder conglomerate layers (~40%), and in 158

fracture-filling veins (~2%) that cut across the flows (Bornhorst 1997). The veins infill 159

fractures related to the Keweenaw thrust-faulting event that provided access for 160

ascending hydrothermal fluids to mineralize the stratabound flow top and sedimentary 161

layers. The age of the copper mineralization is poorly constrained. Alteration minerals 162

associated with the copper in the vesicles have given Rb-Sr isochron dates of 1060±20 163

Ma and 1047±33 Ma (Bornhorst et al. 1988). The presence of native copper in the 164

Page 10 of 79





DraftDraft

11

Keweenaw fractures suggests that copper emplacement postdates most or all thrust 165

faulting (Broderick et al. 1946). Similarly, the presence of native copper at the bottom 166

of a 1100 m drill hole in the Jacobsville Formation suggests that copper emplacement 167

postdates primary deposition of the Freda gray beds (White 1968; Bornhorst 1997) but 168

not necessarily their conversion to red beds. 169

The White Pine SSC deposit produced 2.6 Mt of copper between 1953 and 170

1996 (Bornhorst and Barron 2011). The mineralization occurs in the bottom 5-10 m of 171

the Nonesuch Formation where oxidation front mineralization has replaced primary 172

carbon- and pyrite-rich laminated dark gray-to-black shale (Brown 1971; White 1971). 173

The oxidation front extends upwards from the CHF contact as primary pyrite is 174

progressively replaced in the sequence pyrite � chalcopyrite � bornite � chalcocite 175

(Brown 2005). The polarity of the zone indicates it was formed by ascending 176

hydrothermal fluids. Within the front there are two main “ore” zones, each ~3 m thick, 177

with transgressive contacts that imply an epigenetic origin after gentle tilting of the 178

strata prior to mineralization (Brown 1971). Chalcocite is the main copper-bearing 179

mineral. Native copper veins are present in the uppermost CHF red beds, immediately 180

below the Nonesuch oxidation front. Mineral stability limits, calcite homogenization 181

temperatures, biomarkers, Rock-Eval pyrolysis and illite-smectite estimates indicate 182

Page 11 of 79





DraftDraft

12

that the ambient maximum temperature (Tmax) for the White Pine ore deposit never 183

exceeded ~105±15°C (Symons et al. 2013). The Tmax value is consistent with ore 184

emplacement having occurred near the Keweenawan paleoequator after burial by the 185

overlying Nonesuch and Freda formations when they were over a rift with a reduced 186

heat flow of ~70 mW/m2 (Pujol et al. 1985). Ohr (1993) reported a Pb207/Pb204 calcite 187

age of 1081±9 Ma from a limestone bed in the oxidation front. He interpreted this age to 188

be an estimate for both the deposition-diagenesis and the main and late- copper stages of 189

mineralization at the White Pine ore deposit. Ohr (1993) also reported an age of ≤1050 190

Ma from Sr87/Sr86 data from vein calcite in the White Pine deposit. Symons et al. 191

(2013), based on a paleomagnetic projection using only two paleopoles, estimated an 192

age of 1063±8 Ma for White Pine’s SSC mineralization. They interpreted the ≤1050 Ma 193

age of Ohr (1993) to approximately date the copper mineralization event. 194

Overall, the minimal time and tectonic constraints on the upper Keweenawan 195

geologic events have meant that models for the genesis of the Keweenaw peninsula’s 196

copper deposits are diverse and poorly constrained in time. 197

198

3 Methods and Results 199

Page 12 of 79





DraftDraft

13

3.1. Natural Remanent Magnetization (NRM) 200

Three block samples of red sandstone were collected at most of the 20 sites (Fig. 2) and 201

oriented using a magnetic compass. Drilling core samples was not feasible because, 202

with rare exceptions, useful CHF exposures are found only in the shoaling outcrops of 203

Lake Superior either in front of private residences or in state parks. Using the known 204

locations of the upper and lower contacts of the CHF and of the three exposed 205

sequences of the LST, the stratigraphic levels of the 20 sites of red beds can be a 206

crudely estimated as follows: lower third of CHF – sites 9, 10, 19 and 20; lower middle 207

– sites 11 to 18; upper middle – sites 7 and 8; and, upper third – sites 1 to 6. 208

Three to five paleomagnetic specimens were drilled from most block samples, 209

typically yielding ~10-15 specimens per site (Table 2). After measuring each 210

specimen’s magnetic susceptibility on an AGICO Systems KLY-CS3 Kappa Bridge, the 211

specimens were stored in a magnetically shielded room to allow their unstable viscous 212

remanent magnetization (VRM) components to substantially decay. The natural 213

remanent magnetization (NRM) of each specimen was then measured using a 214

vertical-axis 2G Enterprises three-axis DC SQUID magnetometer with a specimen 215

sensitivity of ~2x10-4 A/m. Except for sites 4 and 19, which gave distinctly higher NRM 216

Page 13 of 79





DraftDraft

14

intensities, the remaining 18 sites have mean NRM intensities in the 8.33 x10-3 to 217

56.2x10-3 A/m range (Table 2). 218

219

3.2 Progressive Thermal Demagnetization 220

One typical pilot specimen per site was thermally demagnetized using a 221

Magnetic Measurement's MMTD-80 oven in 15 to 22 temperature steps up to 680°C. 222

Based on the responses of the pilot specimens, the remaining specimens were thermally 223

demagnetized in 14 steps with most steps in the 500°C to 680°C range. Most specimens 224

show the initial removal of residual VRM up to ~300°C, revealed by a very slow 225

decrease of the NRM intensity and a 10°±10° shallowing of the remanence inclination. 226

Above ~300°C, most specimens exhibit one of two behaviours: a) a distributed intensity 227

decrease to ~640°C followed by a discrete relatively-low intensity decrease to ~680°C 228

(Fig. 3A); or, b) a minimal intensity decrease to ~640°C followed by a discrete large 229

intensity decrease to ~680°C (Fig. 3BC). Both types of laboratory unblocking 230

temperature spectra are typical of hematite. About 20% of the specimens exhibit a small 231

but distinct “plateau and drop” in intensity in the 500°C to 600°C range superimposed 232

on one of the above two types of temperature spectra, indicating the retention in the 233

Page 14 of 79





DraftDraft

15

remanence of trace magnetite along with the dominant hematite (Fig. 3DE). For each of 234

the above decay patterns, the stable endpoint or characteristic remanent magnetization 235

(ChRM) direction is directed westward with an inclination of about 10°±15°. 236

In contrast to the above behaviour, the specimens from two sites (4 & 19) 237

display a markedly different behavior. These specimens yield a large intensity drop in 238

the 500°C to 600°C unblocking temperature range with a smaller intensity drop 239

thereafter to 680°C. Their ChRM direction has an inclination of nearly +40° and their 240

NRM intensity is much greater than that found at adjacent sites (Fig. 3F; Table 2). 241

These properties indicate that the specimens at sites 4 and 19 have magnetite as a 242

substantial or dominant carrier of their ChRM. 243

244

3.3 Saturation Isothermal Remanent Magnetization (SIRM) 245

Isothermal remanent magnetization acquisition to saturation (SIRM) was carried 246

out on specimens from 17 sites to further examine the magnetic mineralogy of the CHF 247

red beds. The specimens were pulse magnetized in nine direct field steps up to 900 mT 248

using a Sapphire Instruments SI-6 pulse magnetizer and then AF demagnetized in six 249

steps up to 120 mT. Data from most specimens fall in a “data envelope” defined by the 250

Page 15 of 79





DraftDraft

16

lower 12 unnumbered SIRM acquisition curves (Fig. 4 A) and by the upper 15 decay 251

curves (Fig. 4 B). These curves indicate the presence of hematite remanence with a trace 252

of magnetite that gives the slight upward bulge in the envelope between 0 and 300 mT 253

on the acquisition curves (Fig. 4 A) and suggests that magnetite contributes about 2% to 254

20% of the remanence signal. Recognizing that the magnetite signal is at least one 255

hundred times more intense per unit volume than the hematite signal (Dunlop and 256

Özdemir 1997), the curves imply that the volume of magnetite remaining in these 257

specimens is only ~0.02% to ~0.2% of the volume of hematite, indicating that the 258

oxidation of magnetite to hematite has been very thorough. Conversely, the curves for 259

anomalous site 19 indicate the preservation of significant unoxidized primary magnetite 260

with an insignificant contribution of hematite to the remanence (Fig. 4). The remaining 261

numbered specimens shown in Figure 6 record an increased contribution of magnetite to 262

the remanence signal from the increased initial slope of the acquisition curve from zero 263

to 200 mT. 264

265

3.4 Characteristic Remanent Magnetization (ChRM) 266

Page 16 of 79





DraftDraft

17

Using orthogonal vector plots (Zijderveld 1967) and least-squares fitting 267

analysis (Kirschvink 1980), ChRM directions were determined for each specimen in the 268

lower “magnetite+hematite” 500°C to 585°C laboratory unblocking temperature (Tub) 269

range and in the higher “hematite” 600°C to 675°C Tub range. Following Fisher (1953) 270

the mean directions were calculated for each site for both the lower and higher Tub, and 271

the 20 site mean directions were averaged in turn for both temperature ranges to obtain 272

the two overall mean directions before tectonic tilt correction. The 500°C-585°C mean 273

ChRM direction is declination (D) = 268.1°, inclination (I) = +16.4° (number of sites 274

(N) = 20, radius of the 95% confidence cone (α95) = 5.5°, precision parameter (k) = 275

36.5) and the 600°C-675°C mean ChRM direction is D = 266.6°, I = +17.2° (N = 20, 276

α95 = 5.9°, k = 32.0). Given that the lower and higher temperature unit mean directions 277

differ by only 1.7° of arc and each is well within the other’s ~6° cone of 95% 278

confidence, they are not significantly different directions at >99% confidence, which 279

indicates that hematite is by far the dominant carrier of the ChRM. Therefore, the vector 280

average of the two mean ChRM directions is used to represent the mean direction for 281

each site (Table 2). 282

Of the 20 CHF site mean directions (Table 2), sites 4 and 19 as noted above, 283

have anomalously high NRM intensities, substantial magnetite contents on thermal 284

Page 17 of 79





DraftDraft

18

demagnetization and anomalously steeper ChRM inclinations. Both sites 4 and 19 are 285

deemed to be anomalous with respect to the remaining 18 site mean ChRM directions. 286

A fold test in paleomagnetism is used to see if a ChRM is prefolding, synfolding or 287

postfolding in origin (Graham 1949). Using the test of Enkin (2003), the optimum 288

correction for the 18 site mean directions of the CHF red beds occurs at 83 ± 5 % 289

unfolding, suggesting that their ChRM is synfolding and acquired very early in the 290

deformation process (Fig. 5B). In contrast, the fold test of McFadden (1998) suggests 291

that the ChRM is statistically consistent with acquisition at either 83% and 100% 292

unfolding. The location of the 83% synfolding and 100% prefolding poles differ 293

nonsignificantly by only 1.1°. Therefore, we use the 100% tilt corrected pole in our 294

discussion below because it provides a more likely estimate of the pole position at the 295

time most of the red beds were flat-lying and their ChRM was acquired. These 296

remaining 18 sites give a tilt-corrected unit mean ChRM direction for the CHF red beds 297

of D = 274.9°, I = +10.9° (N = 18, k = 69.5, α95 = 4.2°), indicating an equatorial 298

paleolatitude of 5.5°N (Fig. 5A). The paleomagnetic pole for the CHF is located at 299

7.4°N, 181.7°E (N = 18; A95 = 3.3°) (Table 3; Fig. 6) based on a mean site location of 300

47.35°N, 88.47°W. As a test for bias, the block mean ChRM directions were used to 301

calculate the site mean directions. The block means give a virtually identical 302

Page 18 of 79





DraftDraft

19

tilt-corrected unit mean direction of D = 275.0°, I = +10.8° (N = 18, k = 77.2, α95 = 303

4.0°), indicating no bias. The mean ChRM direction for the two anomalous sites (sites 4 304

& 19) is D = 283°, I = +36°. This direction is close to the primary ChRM direction from 305

the LST of D = 283.3°, I = +34.7° (α95 = 5.0°) (Kulakov et al. 2013), further suggesting 306

the presence of residual primary magnetite. 307

308

4 Discussion 309

4.1 Kamb Analysis 310

Henry et al. (1977) reported an intermediate Tub (350-550°C) remanence 311

component (Cint) in 34 of 127 specimens from the Nonesuch and Freda formations 312

collected near White Pine (Fig. 2). They suggested that Cint was likely a chemical 313

remagnetization overprint with a direction of D = 280.6°, I = +9.5° that was acquired 314

during the late Hadrynian or Cambrian (~700-500 Ma) when the White Pine copper 315

mineralization was emplaced. Later, Henry et al. (1979) isolated Cint in 11 specimens 316

from one slab of Freda sandstone, obtaining a direction of D = 257.4°, I = -10.1° (N = 317

11, k = 86.3, α95 = 4.9°) with Tub of 275°-600°C, residing in hematite pigment. Some 318

specimens that we measured also isolated the Cint direction in the CHF on thermal 319

Page 19 of 79





DraftDraft

20

demagnetization (e.g., Fig. 3B). Our main concern was to ensure that we measured the 320

ChRM resulting from the initial hematization of the CHF and not from some later 321

chemical or thermal event. Therefore we analyzed our entire progressive thermal 322

demagnetization data base using Kamb (1959) analysis to see if Cint was causing a bias 323

in the ChRM. Kamb (1959) plots show the areal density of vector directions on a 324

spherical surface by contour intervals based on Gaussian normal statistics (Fig. 7). The 325

contour pattern is difficult to see on a conventional stereonet for the CHF vector 326

population because it straddles the paleoequator, which causes the positive down and 327

negative up vector directions plot on top of each other. Therefore, the stereonet has been 328

rotated to be viewed from above D = 270°, I = 0° to show the tilt-corrected specimen 329

vectors as a central population that forms a nearly-circular anomaly after thermal 330

demagnetization at temperatures above 550°C (Fig. 7C, Table 4). The most stable VRM 331

components in the specimens are assumed to be acquired over the past ~105 years in a 332

time-averaged geocentric axial dipole direction of D = 0°, I = 65.3° for the study area 333

(Merrill and McElhinny 1983) or of D = 358.8°, I = 34.2° after tilt correction. The 334

population of NRM directions before thermal demagnetization shows an anomaly with a 335

well developed lobe that extends towards the corrected VRM direction and a peak that 336

is displaced ~9° towards the VRM direction (Table 4, Fig. 7A), indicating the presence 337

Page 20 of 79





DraftDraft

21

of a VRM component in the red bed specimens before thermal demagnetization. Further, 338

the Kamb plot shows evidence of Cint being present in the CHF also. The oscillation 339

between 200 and 500°C on progressive demagnetization in the peak anomaly direction 340

(Table 4, Fig. 8) suggests that Cint has a shallow negatively-inclined southwest ChRM 341

direction as Henry (1979) observed in the Freda Formation. Sandstones of the 342

Jacobsville Formation overlie the Oronto Group on the Keweenaw peninsula and they 343

have yielded a similar collinear dual-polarity ChRM direction of D = 262.2°, I = -13.1° 344

(N = 18, k = 33.5, α95 = 6.1°) (Roy and Robertson 1978; sites 1, 2, 4, 6-13, 17-22). On 345

reaching a laboratory Tub of ~500°C, the ChRM directions of the CHF have coalesced 346

into a single population up to 640°C. Peak anomaly directions from 200 to 400°C are 347

not significantly different at 95% confidence from the 450 to 640°C directions using the 348

collection α95 value of 4.2°. Thus we conclude that our mean ChRM direction for the 349

CHF, isolated at a Tub above 500°C, has not been biased by an unrecognized Cint 350

component. 351

352

4.2 Primary Deposition of the CHF 353

Page 21 of 79





DraftDraft

22

The onset of deposition of primary, gray volcaniclastic sediments of the CHF 354

can be reasonably estimated from stratigraphic and radiometric evidence from the 355

underlying Portage Lake Formation, which is well known from the extensive drilling for 356

native copper deposits (White et al. 1953; Bornhorst and Rose 1994). There are >200 357

andesitic basalt flows between the 1096.2±1.8 Ma Copper City flow and the 1094.0±1.5 358

Ma Greenstone flow that is ~2.3 km stratigraphically above (Davis and Paces 1990), 359

which extrapolates to an extrusion rate of ~1.0 km/Ma. An additional ~900 m of Portage 360

Lake flows lie above the Greenstone Flow. These data suggest that primary deposition 361

of the CHF gray beds likely began by ~1092 Ma. This estimate is supported also by a 362

1093±1.4 Ma date from the Porcupine volcanic rocks that overlie the Portage Lake 363

Formation in the White Pine area, indicating that CHF deposition began no earlier than 364

~1092 Ma (Fig. 9). 365

The duration of deposition of the primary gray beds is less certain. However, 366

the 1087.2±1.6 Ma U/Pb zircon age from the LST came from ~1200 m above the base 367

of the CHF and ~800 m below the top of the unit (Davis and Paces 1990). Again 368

assuming constant rates of sedimentation and volcanic extrusion, the age date suggests 369

CHF deposition began at ~1092 Ma and ended by ~1082 Ma. The Michipicoten Island 370

Formation is the other notable young magmatic event in the MCR. This sequence of 371

Page 22 of 79





DraftDraft

23

basaltic flows is more than 1300 m thick and unconformably overlies a quartz feldspar 372

porphyry that has yielded a U/Pb zircon age of 1086.5±1.3/-3.0 Ma (Palmer and Davis 373

1987). This relationship also suggests that intermittent rift magmatism continued during 374

gray bed deposition until ~1082 Ma (Fig. 9). 375

Early reconnaissance paleomagnetic work on the LST by DuBois (1962) and 376

Vincenz and Yaskawa (1968) yielded divergent ChRM directions of D = 290°, I = 37° 377

(N = 13 samples, α95 = 11°) and D = 282°, I = 12° (N = 6 sites, α95 = 13°), respectively. 378

Halls and Palmer (1981, see also Halls and Pesonen, 1982) reported paleomagnetic data 379

from three locations in the northern Keweenawan Peninsula that yielded different mean 380

ChRM direction D = 299.0°, I = +37.9° (N = 4, α95 = 3.9°) with a positive contact test 381

(Palmer et al. 1981), D = 287.4°, I = +19.5° (N = 4, α95 = 6.4°) and D = 284.7°, I = 382

+9.3° (N = 3, α95 = 2.6°). Noting these discordant directions, Diehl and Haig (1993) 383

measured 30 site mean directions from three stratigraphic sequences that also yielded 384

distinctly different directions. They suggested that each sequence had cooled too rapidly 385

to average out secular variation of the Earth’s magnetic field, and that their mean ChRM 386

direction after tilt correction of D = 286.1°, I = +27.5° (N = 30, α95 = 4.5°) was an 387

inaccurate measure of the field. Recently, Kulakov et al. (2013) collected 21 additional 388

sites. After using serial correlation techniques to avoid duplicate measures of the same 389

Page 23 of 79





DraftDraft

24

flow, they obtained a mean direction for the 31 known available LST flows of D = 390

283.3°, I = +34.6° (N = 31, α95 = 5.0°), which is significantly different from the 391

intervening CHF red beds’ direction of D = 274.9°, I = +10.9°, (N = 18, α95 = 4.2°). 392

The end of CHF gray bed deposition at ~1082 Ma, as discussed above, agrees 393

closely with the 1081±9 Ma Pb207/Pb204 calcite age of Ohr (1993) for limestone 394

deposition in the basal Nonesuch Formation. The inferred highly-reducing environment 395

recorded in the basal Nonesuch strata (Elmore et al. 1989) is consistent with any 396

significant oxidation of gray beds to red beds beginning after 1081 Ma (Fig. 9). One 397

way to date the oxidation event is to determine the Keweenawan apparent polar wander 398

rate from 1108 Ma to 1087 Ma in dated volcanic rocks and extrapolating that rate to 399

younger times to estimate the oxidation/magnetization age of the Oronto Group’s red 400

beds. 401

402

4.3 Keweenawan Apparent Polar Wander Path (APWP) 403

4.3.1 Paleopole Asymmetry 404

Numerous paleopoles have been determined from Keweenawan igneous rocks 405

around Lake Superior (Halls and Pesonen 1982; Symons et al. 1994; Swanson-Hysell et 406

Page 24 of 79





DraftDraft

25

al. 2014). These poles track along the southward-younging western arm of the 407

Laurentian Shield’s Logan Loop as first recognized by DuBois (1962). The poles listed 408

in Table 3 are mostly abstracted from the compilation of Swanson-Hysell et al. (2009, 409

as amended in 2010). They include only poles from igneous rocks with reliable U/Pb 410

zircon-baddelyite dates with some updates, plus additional poles from the overlying 411

Keweenawan sedimentary detrital formations. For the Keweenawan paleomagnetic data, 412

a major consideration is the reported asymmetry between the normal and reverse 413

polarity paleopoles (Palmer 1970). 414

Four basic hypotheses have been proffered to explain why the apparently coeval 415

normal and reverse paleopoles from the Keweenawan volcanics are not antipodal. The 416

four are: 1) geologic field relations, such as unrecognized fault repetition (e.g., Palmer 417

1970; Palmer et al. 1981); 2) rapid polar wander with age progression (e.g., Beck 1970; 418

Robertson and Fahrig 1971; Halls and Pesonen 1982; Pesonen and Halls 1983; 419

Lewchuk and Symons 1990; Diehl and Haig 1993; Symons et al. 1994; Swanson-Hysell 420

et al. 2009); 3) overprinting of antiparallel normal and reverse primary magnetite 421

ChRMs by secondary hematite with a shallower ChRM inclination, thereby shallowing 422

the normal ChRM inclination and steepening the reverse ChRM inclination (e.g., 423

Palmer 1970; Tauxe and Kodama 2009); and, 4) long-term non-dipole behavior of the 424

Page 25 of 79





DraftDraft

26

Earth’s magnetic field such as a co-axial two-dipole field (eg. Palmer et al. 1981; 425

Pesonen & Nevanlinna 1981). Recently Tauxe and Kodama (2009) studied the 426

paleomagnetism of the 1098±2 Ma North Shore Volcanics on the northwest shore of 427

Lake Superior (Fig. 1B). They reported evidence for pervasive overprinting of the 428

primary magnetite remanence in the volcanic rocks by secondary hematite with a 429

shallower west-directed remanence inclination (explanation 3 above). Alternatively, 430

Swanson-Hysell et al. (2009) concluded from their study of the Mamainse Point 431

volcanic sequence on the eastern shore of Lake Superior that there was polar wander but 432

no paleopole asymmetry (explanation 2 above). 433

Table 3 includes 8 reverse and 5 normal polarity Keweenawan paleopoles with 434

ages between 1108 Ma and 1100 Ma. The two paleopole populations give comparable 435

mean radiometric ages of 1106±3 Ma and 1105±3 Ma but divergent paleopoles at 436

44.1°N, 206.4°E (antipode, N = 8, A95 = 6.6°, k = 72) and 38.4°N, 186.6°E (N = 5, A95 437

= 11.2°, k = 47), respectively, that are significantly different at >>95% confidence. 438

Similarly, both the normal and reverse mean ChRM directions for the Coldwell 439

Complex, Duluth Complex and Osler Volcanics have cones of 95% confidence with 440

radii of <10°, but all three collections yield significantly different normal and reverse 441

paleopoles at 95% confidence. These tests for antiparallelism support a conclusion that 442

Page 26 of 79





DraftDraft

27

the primary ChRMs of these rock units have been overprinted by a later secondary 443

hematite remanence component with a shallow west-directed inclination. 444

Palmer et al. (1981) ran a conglomerate test on 150 volcanic pebble clasts from 445

three sites in the CHF red beds near our sites 7, 13 and 20 (Fig. 2). The remanence of 446

the clasts yield great circle tracks on AF step demagnetization to 100 or 110 mT, where 447

they yielded a specimen mean direction of D = 280.6°, I = +12.3° (N = 150, α95 = 12°, k 448

= 1.9) after tilt correction. The k value of <2 indicates a nearly random population for 449

the demagnetized magnetite ChRM directions in the clasts. However, because of the 450

large number of clasts, there is a minor undemagnetized hematite mean ChRM direction 451

that is aligned to within ~2° of the CHF red beds’ direction. These results indicate that 452

the volcanic clasts were oxidized along with the CHF gray beds. Also Hnat et al. (2006) 453

conducted a paleomagnetic conglomerate test on rhyolitic clasts from the Portage Lake 454

Formation. They showed that the clasts’ magnetite A components, carried by magnetite, 455

gave random primary ChRM directions whereas their B components, carried by 456

hematite, gave directionally biased secondary directions. We note that 7 directions from 457

the 11 tested clasts give a poorly clustered mean direction at D = 273°, I = +7° (α95 = 458

25°, k = 7) that is consistent with the measured remanence directions of the Oronto 459

Group red beds (Table 5). Further Tauxe and Kodama (2009) used bootstrap statistical 460

Page 27 of 79





DraftDraft

28

methods to analyse their large paleomagnetic data base from the North Shore Volcanics. 461

They found that the magnetite-dominated sites and mixed magnetite-hematite sites 462

yielded statistically indistinguishable ChRM directions whereas the hematite-dominated 463

sites gave a significantly different direction. Thus they concluded that the normal and 464

reverse Keweenawan directions incorporate a shallow west-directed hematite overprint. 465

The fact that a hematite component could be present in the lava flows from either 466

primary exsolution on cooling or weathering immediately after cooling 467

(Swanson-Hysell et al. 2009; Kulakov et al. 2013, 2014) does not negate the possibility 468

that a secondary hematite overprint may be also acquired much later by the flows. 469

470

4.3.2 APWP Construction 471

A shallow normal-polarity overprint acquired about 25±5 Ma after extrusion 472

will have an about equal and opposite effect on the primary ChRM directions of the 473

1108 to 1100 Ma normal and reverse polarity Keweenawan flows. Specifically, the 474

overprint will cause both the ChRM declination and inclination to be decreased in the 475

normal flows and to be increased about equally in the opposite polarity directions of the 476

reverse flows. We posit that the best estimate for the Earth’s true primary paleopole is 477

Page 28 of 79





DraftDraft

29

midway between the two mean poles for the five normal and eight reverse populations 478

of 1108-1100 Ma paleopoles (poles 1 and 2, Table 5, Fig. 6) at 41.3°N, 196.5°E (pole 479

3). The A95 value of 6.6° for pole 3 is obtained conservatively from averaging all 13 480

paleopoles (Table 5). Poles 1 and 2 are not antipodal (Fig. 6) despite having similar 481

mean ages of 1106±3 and 1105±3 Ma, respectively, potentially because of overprinting 482

by later hematization (Table 5). 483

Two features of the early Keweenawan paleopoles become apparent when they 484

are corrected for asymmetry using the average remanence corrections required to move 485

normal pole 1 and the antipode of reverse pole 2 to the mean pole 3 (Fig. 6). First, the 486

correction decreases the divergence between the paired normal and reverse paleopoles 487

for the Dulth Gabbro (paleopoles DCr, DCn), Osler Volcanics (OVr, OVn) and 488

Mamance Point Volcanics (MPLn, MPLr) from 28±9° to 15±8°, presumably because 489

these rocks were exposed to oxidation by post-extrusion hydrothermal fluids in the 490

MCR basin. In contrast, the correction degrades the divergence between the paired 491

paleopoles for the intrusive Coldwell Complex (CCr, CCn) and Nemogosenda 492

Carbonatite from 8±2° to 18±3°, presumably because they were intruded into basement 493

rocks of the Laurentian Shield and not exposed to the rift’s hydrothermal fluids. Second, 494

using corrected poles for the rift’s rocks and uncorrected poles for the Coldwell and 495

Page 29 of 79





DraftDraft

30

Nemogosenda complexes, the early Keweenawan APWP becomes significantly (2σ) 496

better defined with its mean longitude and width decreasing from 199.2±14.4°E to 497

191.3±5.2°E on tracking from ~60°N latitude at 1108 Ma to ~28°N latitude at 1100 Ma. 498

This improvement in the definition of the early Keweenawan’s APWP is strong 499

evidence for the presence of overprinting in the ChRM of the strata in the MCR. 500

Paleomagnetic data are available from four dated Keweenawan volcanic 501

sequences younger than 1100 Ma. All four have normal polarity and they provide two 502

pairs of paleopoles (Table 5), one at 1096±2 Ma (pole 4) and the other at 1087±2 Ma 503

(pole 5). The sequence of poles from 3 to 4 to 5 show two distinct features. First, they 504

record a change in the APWP trend from a present-day southwesterly (3-4) to a more 505

southerly (4-5) direction that projects on to the paleopoles from the Oronto Group 506

sedimentary rocks (poles 6 and 7) (Fig. 5). Geologically, this change in trend at pole 4 507

corresponds to the end of main-stage volcanism in the MCR and to the onset of 508

deposition of the Oronto Group gray beds. Pole 4 also marks a sharp decrease in the 509

translation velocity of the Laurentian Shield, which hosts the MCR, from a rapid 18.3 510

cm/yr down to 6.8 cm/yr (Table 6). Such a sharp decrease in velocity of a large 511

continental plate typically coincides with a major tectonic collision, such as the collision 512

Page 30 of 79





DraftDraft

31

of the Indian and Eurasian plates at ~50 Ma that resulted in the Indian plate’s velocity 513

dropping from ~18±2 cm/yr to ~4±1 cm/yr (Dupont-Nivet et al. 2010). 514

515

4.3.3 APWP Extrapolation 516

Latitudinal tectonic translation of a terrane in a continental setting is recorded 517

paleomagnetically by a temporal change in ChRM inclination, providing a minimum 518

translation rate. The corresponding temporal change in ChRM declination may measure 519

either terrane translation and/or rotation. Further, both translation measures may include 520

secular variation errors, especially in volcanic sequences that were rapidly extruded, and 521

dipole offset inclination errors of the Earth’s magnetic field in both volcanic and 522

sedimentary rocks (Merrill and McElhinny 1983; Butler 1992; Dunlop and Özdemir 523

1997; Tauxe 2010; Bilardello et al. 2011; Pavón-Carrasco et al. 2016). 524

Extrapolating the 6.8 cm/yr translation velocity for the 4-5 paleopole segment 525

across the 5-6 segment to the CHF red beds paleopole (pole 6) (Fig. 6) gives a 526

post-1087 Ma time span of 27 Ma, indicating that the CHF gray beds were oxidized on 527

average to red beds about 27±5 Myr later than ~1087 Ma at ~1060±5 Ma. Similarly, 528

extrapolating the 6.8 cm/yr rate from pole 5 to pole 7 indicates that the overlying 529

Page 31 of 79





DraftDraft

32

Nonesuch and Freda beds acquired their hematite-borne remanence at ~1057±6 Ma. 530

Extrapolating the 6.8 cm/yr rate beyond pole 7 to the Jacobsville paleopole at 183°E, 531

10°S suggests, speculatively at best, that the Jacobsville Formation acquired its 532

remanence at ~1040 Ma. This ~1040 Ma date suggests in turn that the CHF could have 533

acquired its Cint component at ~1040 Ma also as a second added overprint, perhaps in 534

response to burial or tectonic pressure or to fluid flow associated with faulting. 535

It is prudent to check that the calculated translation rates have not been biased 536

by changes in Laurentia’s vertical-axis rotation rate by calculating the rate of change in 537

ChRM declination along each of the paleopole segments. As shown in Table 6, the 538

rotation rate for Laurentia is both consistent in sense and small in magnitude, ranging 539

between 0.25 and 0.36°/Myr counterclockwise to indicate a negligible distortion in the 540

calculated drift rates by this potential error source between ca. 1106 Ma and 1060 Ma. 541

The paleopoles for the four post-1100 Ma Keweenawan volcanic sequences 542

may also contain a normal polarity overprint because progressive thermal step 543

demagnetization studies have shown that all four contain a minor to major hematite 544

component in their ChRM that is about collinear with their magnetite component 545

(Palmer and Davis 1987; Diehl and Haig 1993; Hnat et al. 2006; Tauxe and Kodama 546

2009; Kulakov et al. 2013). Fortunately, because these four paleopoles generate a linear 547

Page 32 of 79





DraftDraft

33

1096 Ma to 1087 Ma APWP segment that projects to both the older 1105 Ma normal 548

paleopole (Fig. 6, pole 1) and to the cluster of younger normal Oronto Group red beds’ 549

paleopoles (Fig. 6, poles 6&7), a normal polarity overprint would offset all of the 550

paleopoles similarly but it will not result in a significant error in the estimated ages of 551

ChRM acquisition by the red beds. 552

553

4.3.4 Translation Velocities 554

Our 18.3 cm/yr rate for Keweenawan APW from 1106 to 1096 Ma is 555

significantly less than the five 21.5 to 33.6 cm/yr (mean 27.4 cm/yr) rates from 1107 to 556

1097 Ma estimated by Swanson-Hysell et al. (2009). Their rates were obtained using the 557

angular distances from five older reverse-polarity paleopoles to the same one normal 558

polarity paleopole, thus adding ~8° to each of the angular differences from the 559

asymmetric overprint and increasing the estimated rate by ~9 cm/yr on average. In 560

contrast, their calculated 7.0 cm/yr rate from 1095 Ma to 1087 Ma comes from two 561

normal polarity paleopoles and agrees closely with our 6.8 cm/yr estimate from 1096 to 562

1087 Ma that is derived from four normal paleopoles. 563

564

Page 33 of 79





DraftDraft

34

4.3.5 Inclination-shallowing 565

Inclination shallowing can significantly bias a red bed’s ChRM direction. If 566

oxidation precedes sediment deposition in a fluvial or lacustrine setting, the detrital 567

plate-like specular hematite crystals are deposited with a horizontal bias to form a 568

detrital remanent magnetization with a significantly-shallowed ChRM inclination (Im). 569

Im is related to the true Earth’s magnetic field inclination (IF) by a flattening factor (f), 570

i.e., tan Im = f tan IF (King 1955). Determining f in order to correct directional data from 571

red beds has proven difficult because each unit tested varies in magnetic mineralogy, 572

depositional environment and subsequent burial and tectonic history (Bilardello 2016). 573

Using f factors obtained from anisotropy measurements or from the 574

elongation-inclination (E/I) distribution of ChRM measurements (Jackson et al. 1991; 575

Kodama 1997; Tan and Kodama 2002; Tauxe and Kent 2004; Kent and Tauxe 2005), 576

Bilardello and Kodama (2010) found that 15 red bed units give f factors ranging from 577

0.40 to 0.83 with a mean of 0.59±0.12. However, we suggest four reasons to support the 578

contention that correcting for inclination shallowing will not significantly change the 579

age estimate for the ChRM of the CHF red beds. First, the CHF red beds carry a 580

secondary chemical remanent magnetization in hematite pigment acquired during 581

oxidation that postdates burial by the overlying Oronto Group strata. Only the primary 582

Page 34 of 79





DraftDraft

35

detrital remanent magnetization carried by magnetite in the primary gray beds could 583

have recorded inclination shallowing (Elmore and Van der Voo 1982). Second, there is 584

no evidence that the CHF was ever buried by more than ~5 km by the overlying Oronto 585

Group beds. Conversely, most of the determined f factors from red beds have come 586

from the ~14 km-thick Newark Group that could have acquired substantial additional 587

inclination-shallowing from post-depositional compaction (Bilardello and Kodama 588

2010). Third, the ChRM directions from the CHF give a nearly circular anomaly on a 589

Kamb plot with an E/I ratio near 1.0 (Fig. 8c) or f factor near 1.0, indicating minimal 590

inclination shallowing. Fourth, volcanic rocks from the past 400 years give f factors of 591

0.71±0.10 for mid-latitudes that are similar to those of the 1095 and 1087 Ma 592

Keweenawan volcanic rocks used here for the time extrapolation (Pavón-Carrasco et al. 593

2016). Correcting paleopoles 4, 5 and 6 for inclination shallowing moves all three in the 594

same direction by similar amounts, resulting in little change in the extrapolated age 595

estimate. 596

597

4.4 Tectonics 598

Page 35 of 79





DraftDraft

36

The magnetite-borne ChRM of the LST is 100% prefolding and the 599

hematite-borne ChRM of the CHF red beds is 83 to 100% prefolding (Kulakov et al. 600

2014; this study). However, these tests do not preclude hematization and ChRM 601

acquisition by the red beds at 1060±5 Ma during the initial uplift phase of Grenvillian 602

continental collision. Further, the presence of normal-polarity hematite overprints that 603

are about collinear with the red beds’ ChRM directions in many Keweenawan volcanic 604

rocks and in volcanic clasts of the Portage Lake Formation and CHF affirm that the 605

causative tectonic event extended throughout the Lake Superior region. 606

Cannon (1994) concluded that shortening during continental collision along the 607

Grenville Front (Fig. 1) from about 1080 to 1040 Ma reduced the MCR’s width from 608

about 80 to 50 km. When the tilt-corrected mean ChRM directions for all Oronto Group 609

red beds’ sites are combined, they yield a well-defined direction of D = 276.2°, I = 610

+4.7° (N = 67, α95 = 2.8°, k = 40) with a paleopole at 5.9°N, 177.8°E (A95 = 2.1°). This 611

result implies that all three Oronto formations were oxidized in one major uplift event 612

between about 1063 and 1055 Ma as the Grenville Province collided with the 613

Laurentian Shield, and that the minor regional tilts of ≤3° were sufficient to cause major 614

regional fluid migrations that substantially converted the gray beds into red beds. As the 615

Grenville – Laurentian collision progressed, the MCR’s original margin-parallel normal 616

Page 36 of 79





DraftDraft

37

faults were inverted to thrust and reverse faults, including the major Keweenaw fault 617

(Fig. 2), with concomitant folding and faulting between about 1055 Ma and 1040 Ma. 618

Supporting evidence for structural deformation at this time comes from: 1) a K/Ar 619

biotite minimum date of 1063±34 Ma for the Bear Lake rhyolite intrusion in the Freda 620

Formation (White 1968); 2) Rb/Sr isochron dates of 1060±20 Ma and 1047±33 Ma for 621

alteration minerals associated with the native copper lodes in the Portage Lake 622

Formation (Bornhorst et al. 1988); 3) Rb/Sr isochron dates of 1047±35 Ma and 1020±18 623

Ma on calcite and chlorite, respectively, from a post-ore vein at White Pine (Ruiz et al. 624

1984; Ohr 1993); 4) nine Rb/Sr biotite dates of <1085 Ma from Archean rocks abutting 625

the Keweenaw fault near White Pine (Fig. 2) that are interpreted to indicate 626

metamorphism at ~1055±24 Ma (Cannon et al. 1993); and, 5) a Sm/Nd date of 1043±40 627

Ma for mineralized vein calcites at White Pine (Ohr 1993). 628

629

4.5 White Pine Copper Mineralization 630

Brown (2014) concluded that White Pine’s main-stage SSC copper 631

mineralization occurred rapidly in the basal beds of the 1081±9 Ma Nonesuch 632

Formation by about 1076 Ma. In an evolving progression of detailed geologic and 633

Page 37 of 79





DraftDraft

38

geochemical papers he proposed that: 1) meteoric water infiltrated highlands on the 634

northern side of the MCR and oxygenated brines in the underlying CHF; 2) as the brines 635

were driven southward under the rift basin, they converted the CHF gray beds to red 636

beds and acquired the released copper ions; 3) on the rift’s southern side, brines were 637

driven upward into the Nonesuch shale aquitard by either gravity-driven fluid flow or 638

residual heat from the underlying Porcupine volcanics’ caldera; and, 4) copper was 639

precipitated from the brines in an oxidation front reaction as disseminated sulphides in 640

the basal Nonesuch Formation (Brown 1971, 1997, 2005, 2006, 2009, 2014). 641

Clearly Brown’s (2014) mean age estimate of about 1078±2 Ma for main-stage 642

SSC copper mineralization conflicts with our estimated 1060±5 Ma age. There are 643

several reasons why we deem Brown’s (2014) 1078±2 Ma age to be unreasonable. If his 644

1078±2 Ma age is correct, then the required translation rate for Laurentian Shield drift 645

would be about 68 cm/yr. This rate is about four times faster than our estimated 18.3 646

cm/yr rate from 1106 to 1096 Ma that about matches the fastest known rate for a large 647

continent, i.e., the Indian continent from about 70 to 50 Ma (Dupont-Nivet et al. 2010). 648

When hematite nucleates, it forms magnetic domains with a ChRM direction 649

that is extremely stable and resistant to change by burial loading. If the 1078±2 Ma age 650

estimate for main-stage SSC ore genesis is true (Brown 2014), a pre-1078Ma hematite 651

Page 38 of 79





DraftDraft

39

remanence had to have been acquired in the CHF that was subsequently remagnetized at 652

1060±5 Ma. However, the White Pine mineralized strata have never been exposed to 653

temperatures above about 110°C, based on mineral stability limits, fluid inclusion 654

homogenization temperatures, biomarkers, Rock-Eval pyrolysis and illite-smectite 655

expandability (Brown 1971; Nishioka 1983; Mauk 1993; Price et al. 1996). Nor has the 656

mineralized strata seen pressures greater than that produced by burial by the overlying 657

4-6 km of the Nonesuch and Freda Formations. Thus there is no apparent mechanism or 658

evidence that a pre-1078 Ma hematite was dissolved and then reconstituted in the CHF 659

at 1060±5 Ma. 660

The hematite-borne ChRM of the Nonesuch sulphide-mineralized beds has a 661

mean paleomagnetic age of 1060±5 Ma, indicating statistically that <<1% of the 662

hematite in the beds was formed before about 1075 Ma. We hypothesize that the CHF 663

brines were not sufficiently oxygenated by 1079 Ma to acquire significant copper from 664

the CHF gray beds and to transport the dissolved cations to the White Pine region. 665

Further, it is unrealistic to estimate the conversion rate of gray beds to red beds using 666

modern analogies such as the “several million year” estimate for the Gulf of California 667

rift or the Ethiopian segment of the East African rift (Brown 2005, 2014). This is 668

because the percentage of oxygen in the Earth’s atmosphere during Keweenawan time is 669

Page 39 of 79





DraftDraft

40

estimated to be only about one-tenth of its present content (Canfield 2005; Kump 2008; 670

Lyons et al. 2014), which would correspondingly decrease the conversion rate of CHF 671

gray beds to red beds. In summary, we support the genetic model for White Pine’s 672

main-stage SSC mineralization of Symons et al. (2013) except that the mean age is 673

decreased by ~3 Myr from 1063±8 Ma to 1060 ±5 Ma (Fig. 9). 674

675

5. Conclusions 676

The red beds of the Copper Harbor Formation (CHF) carry a geologically 677

ancient and coherent ChRM that resides in pigmentary hematite and yields a direction 678

of D = 274.9°, I = +10.9° (N = 18, k = 69.5, α95 = 4.2°) and a paleopole at 7.4°N, 679

181.7°E (A95 = 3.3°). This paleopole differs by ~16° from the paleopole for the 680

1087.2±1.6 Ma Lake Shore Traps (LST) at >>95% confidence that provides an 681

estimated depositonal age for the primary CHF gray beds. By extrapolating the polar 682

wander rate across the 1096±2 to 1087±2 Ma segment of a constructed Apparent Polar 683

Wander Path (APWP) for the Keweenawan Supergroup, the mean age for oxidation and 684

magnetization of the CHF red beds is estimated to be 1060±5 Ma, which explains the 685

divergence between the CHF and LST paleopoles (Fig. 9). The 1060±5 Ma age for the 686

Page 40 of 79





DraftDraft

41

red beds’ ChRM also enables a temporal framework to be established for the 687

magnetization of the overlying Nonesuch and Freda formations, for the 1060±5 Ma 688

emplacement of White Pine’s SSC copper mineralization, and for the onset of the 689

Grenville Province – Laurentian Shield collision and tectonic deformation event that 690

folded and faulted the Keweenawan Group. Construction of an APWP for Keweenawan 691

time leads to the three further conclusions. First, the Earth’s magnetic field was a 692

symmetrical dipole through Keweenawan time and the much-discussed apparent 693

asymmetry in the normal and reverse ChRM directions was caused by a 1060±5 Ma 694

overprint remanence carried by secondary hematite. The definition of the early 695

Keweenawan APWP is greatly improved by correction of the paleopoles for the 696

1109-1100 Ma volcanic and intrusive rocks in the rift basin that acquired the 697

normal-polarity 1060±5 Ma hematite remagnetization component. Second, the rate of 698

Keweenawan polar wander slowed from ~18 cm/yr from 1106-1096 Ma to ~6.8 cm/yr 699

from 1096-1087 Ma to mark the cessation of active rifting at ~1096 Ma in the Lake 700

Superior region. Three, the Laurentian Shield experienced a very slow and steady 701

clockwise rotation of ~0.30°/Myr during Keweenawan translation and initial uplift of 702

the Midcontinental Rift region. 703

704

Page 41 of 79





DraftDraft

42

Acknowledgments 705

The authors wish to thank most gratefully: R. J. Barron, T. J. Bornhorst and 706

A. Smirnov of Michigan Technical University for very helpful collecting advice; A. 707

Grossi of the University of Windsor for preparing the specimens and making the 708

magnetic measurements; R. Van der Voo, R.D. Elmore and J.W. Geissman for helpful 709

comments on a 2013 draft of this manuscript; Reviewers J.W. Geissman and D.M. 710

Jurdy, and Associate Editor R.J. Enkin for careful and constructive comments on this 711

paper; and the Natural Sciences and Engineering Research Council of Canada for 712

funding this research through Discovery Grant 7834-2008 to D.T.A.S. 713

714

715

716

Page 42 of 79





DraftDraft

43

References

Beck, M.E. 1970. Paleomagnetism of Keweenawan intrusive rocks, Minnesota.

Journal of Geophysical Research 75: 4985-4996.

Bilardello, D. 2016. The do's and don'ts of inclination shallowing corrections. Institute

for Rock Magnetism Quarterly 26 (3): 1, 9-11.

Bilardello, D., and Kodama, K.P. 2010. A new inclination shallowing correction of the

Mauch Chunk Formation of Pennsylvania, based on high-field AIR results:

implications for the Carboniferous North American APW path and Pangea

reconstructions. Earth and Planetary Science Letters 299: 218-227.

Bilardello, D., Jezek, J., and Kodama, K.P. 2011. Propagating and incorporating the

error in anisotropy‐based inclination corrections. Geophysical Journal International

187: 75-84.

Books, K.G. 1968. Magnetization of the lowermost Keweenawan lava flows in the

Lake Superior area. U.S. Geological Survey Professional Paper 600-D: pp

D248-D254.

Books, K.G. 1972. Paleomagnetism of some Lake Superior Keweenawan rocks. U.S.

Geological Survey Professional Paper 760: 42 pp.

Page 43 of 79





DraftDraft

44

Bornhorst, T.J. 1997. Tectonic context of native copper deposits of the North

American Midcontinent Rift System. Geological Society of America Special paper

312: 127-136.

Bornhorst, T.J., and Barron, R.J. 2011. Copper deposits of the western Upper

Peninsula of Michigan. In Geological Society of America Field Guide 24. Edited by

J.D. Miller, G.J. Hudak, C. Wittkop, and P.I. McLaughlin. pp. 83-99.

Bornhorst, T.J., and Lankton, L.D. 2006. Keweenaw copper: Geology and history.

Ontonagon, MI. Great Lakes Geoscience LLC., Ontonagon, Michigan: 30 pp.

Bornhorst, T.J., and Rose, W.I. 1994. Self-guided geological field trip to the

Keweenaw Peninsula, Michigan. Proceedings of the Institute on Lake Superior

Geology 40: 183 pp.

Bornhorst, T.J., Paces, J.B., Grant, N.K., Obradovich, J.D., and Huber, N.K. 1988.

Age of native copper mineralization, Keweenaw Peninsula, Michigan. Economic

Geology 83: 619-625.

Broderick, T.M., Hohl, C., and Eidemiller, H. 1946. Recent contributions to the

geology of the Michigan copper district. Economic Geology 41: 675-725.

Brown, A.C. 1971. Zoning in the White Pine copper deposit, Ontonagon County,

Michigan. Economic Geology 66: 543-573.

Page 44 of 79





DraftDraft

45

Brown, A.C. 1997. World‐class sediment‐hosted stratiform copper deposits:

Characteristics, genetic concepts and metallotects. Australian Journal of Earth

Sciences 44: 317-328.

Brown, A.C. 2005. Refinements for footwall red-bed diagenesis in the

sediment-hosted stratiform copper deposits model. Economic Geology 100: 765-771.

Brown, A.C. 2006. Genesis of native copper lodes in the Keweenaw district, northern

Michigan: a hybrid evolved meteoric and metamophogenic model. Economic Geology

101: 1437-1444.

Brown, A.C. 2009. A process-based approach to estimating the copper derived from

red beds in the sediment-hosted stratiform copper deposit model. Economic Geology

104: 857-868.

Brown, A.C. 2014. Latent thermal effects from Porcupine Volcanics calderas

underlying the White Pine-Presque Isle stratiform copper mineralization, northern

Michigan. Economic Geology 109: 2035-2050.

Butler, R.F. 1992. Paleomagnetism: Magnetic Domains to Geologic Terranes.

Blackwell Scientific Publications, Boston.: 319 pp.

Canfield, D.E. 2005. The early history of atmospheric oxygen: homage to Robert M.

Garrels. Annual Reviews of Earth and Planetary Science 33: 1-36.

Page 45 of 79





DraftDraft

46

Cannon, W.F. 1992. The Midcontinent rift in the Lake Superior region with emphasis

on its geodynamic evolution. Tectonophysics 213: 41-48.

Cannon, W.F. 1994. Closing of the Midcontinent rift — A far-field effect of

Grenvillian compression. Geology 22: 155-158.

Cannon, W.F., Peterman, Z.E., and Sims, P.K. 1993. Crustal-scale thrusting and origin

of the Montreal River monocline — A 35-km-thick cross section of the Midcontinent

rift in northern Michigan and Wisconsin. Tectonics 12: 728-744.

Costanzo‐Alvarez, V., Dunlop, D.J., and Pesonen, L.J. 1993. Paleomagnetism of

alkaline complexes and remagnetization in the Kapuskasing Structural Zone, Ontario,

Canada. Journal of Geophysical Research: Solid Earth 98: 4063-4079.

Davis, D., and Green, J. 1997. Geochronology of the North American Midcontinent

rift in western Lake Superior and implications for its geodynamic evolution. Canadian

Journal of Earth Sciences 34: 476-488.

Davis, D.W., and Paces, J.B. 1990. Time resolution of geologic events on the

Keweenaw Peninsula and implications for development of the Midcontinent Rift

system. Earth and Planetary Science Letters 97: 54-64.

Davis, D.W., and Sutcliffe, R.H. 1985. U-Pb ages from the Nipigon plate and northern

Lake Superior. Geological Society of America Bulletin 96: 1572-1579.

Page 46 of 79





DraftDraft

47

Diehl, J.F., and Haig, T.D. 1993. A paleomagnetic study of the lava flows within the

Copper Harbor Conglomerate, Michigan: new results and implications. Canadian

Journal of Earth Sciences 31: 369-380.

Diehl, J.F., Durant, A.J., and Schepke, C. 2009. Paleomagnetism of Silver Island,

Keweenaw Peninsula, Michigan: Additional Support (?) for the Primary Curvature of

the MCR. American Geophysical Union, Spring Meeting 2009 abstract #GP11E-01.

DuBois, P.M. 1962. Palaeomagnetism and correlation of Keweenawan rocks.

Geological Survey of Canada Bulletin 71: 75 pp.

Dunlop, D.J., and Özdemir, Ö. 1997. Rock Magnetism: Fundamentals and frontiers.

Cambridge University Press, Cambridge: 571 pp.

Dupont-Nivet, G., Lippert, P.C., Van Hinsbergen, D.J., Meijers, M.J., and Kapp, P.

2010. Palaeolatitude and age of the Indo–Asia collision: palaeomagnetic constraints.

Geophysical Journal International 182: 1189-1198.

Elmore, R.D. 1984. The Copper Harbor Conglomerate: A late Precambrian

fining-upward alluvial fan sequence in northern Michigan. Geological Society of

America Bulletin 95: 610-617.

Page 47 of 79





DraftDraft

48

Elmore, R.D., and Van der Voo, R. 1982. Origin of hematite and its associated

remanence in the Copper Harbor Conglomerate (Keweenawan), Upper Michigan.


Elmore, R.D., Milavec, G.J., Imbus, S.W., and Engel, M.H. 1989. The Precambrian

Nonesuch Formation of the North American Mid-continent rift, sedimentology and

organic geochemical aspects of lacustrine deposition. Precambrian Research 43:

191-213.

Enkin, R.J. 2003. The direction–correction tilt test: an all-purpose tilt/fold test for

paleomagnetic studies. Earth and Planetary Science Letters 212: 151-166.

Fisher, R.A. 1953. Dispersion on a sphere. Proceedings of the Royal Society of

London A217: 295-305.

Graham, J.W. 1949. The stability and significance of magnetism in sedimentary rocks.


Halls, H. 1974. A paleomagnetic reversal in the Osler Volcanic Group, northern Lake

Superior. Canadian Journal of Earth Sciences 11: 1200-1207.

Halls, H.C., and Palmer, H.C. 1981. Remagnetization in Keweenawan rocks. Part II:

Lava flows within the Copper Harbor Conglomerate, Michigan. Canadian Journal of

Earth Sciences 18: 1395-1408.

Page 48 of 79





DraftDraft

49

Halls, H.C., and Pesonen, L.J. 1982. Paleomagnetism of Keweenawan rocks. In

Geology and tectonics of the Lake Superior Basin. Edited by R.J. Wold and W.J.

Hinze. Geological Society of America, Memoir 156. pp. 173-201.

Heaman, L.M., and Machado, N. 1992. Timing and origin of midcontinent rift alkaline

magmatism, North America: evidence from the Coldwell Complex. Contributions to

Mineralogy and Petrology 110: 289-303.

Heaman, L.M., Easton, R.M., Hart, T.R., Hollings, P., MacDonald, C.A., and Smyk,

M. 2007. Further refinement to the timing of Mesoproterozoic magmatism, Lake

Nipigon region, Ontario. Canadian Journal of Earth Sciences 44: 1055-1086.

Henry, S.G. 1979. Chemical demagnetization: methods, procedures, and applications

through vector analysis. Canadian Journal of Earth Sciences 16: 1832-1841.

Henry, S.G., Mauk, F.J., and Van der Voo, R. 1977. Paleomagnetism of the upper

Keweenawan sediments: the Nonesuch Shale and Freda Sandstone. Canadian Journal

of Earth Sciences 14: 1128-1138.

Hnat, J.S., Van der Pluijm, B.A., and Van der Voo, R. 2006. Primary curvature in the

Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern

Michigan, USA). Tectonophysics 425: 71-82.

Page 49 of 79





DraftDraft

50

Hubbard, H. 1971. Keweenawan geology of the Porcupine mountains, western Upper

Peninsula, Michigan. In Proceedings and Abstracts-Institute on Lake Superior

Geology, Annual Meeting.

Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., and Gruber, W. 1991. Detrital

remanence, inclination errors, and anhysteretic remanence anisotropy: quantitative

model and experimental results. Geophysical Journal International 104: 95-103.

Kamb, W.B. 1959. Ice petrofabric observations from Blue Glacier, Washington, in

relation to theory and experiment. Journal of Geophysical Research 64: 1891-1909.

Kent, D.V., and Tauxe, L. 2005. Corrected Late Triassic latitudes for continents

adjacent to the North Atlantic. Science 307: 240-244.

King, R. 1955. The remanent magnetism of artificially deposited sediments.

Geophysical Journal International 7: 115-134.

Kirschvink, J.L. 1980. The least-squares line and plane and the analysis of

palaeomagnetic data. Geophysical Journal of the Royal Astronomical Society 62:

699-718.

Kodama, K. 1997. A successful rock magnetic technique for correctng paleomagnetic

inclination shallowing: Case study of the Nacimiento Formation, New Mexico. Journal

of Geophysical Research: Solid Earth 102: 5193-5205.

Page 50 of 79





DraftDraft

51

Kulakov, E.V., Smirnov, A.V., Diehl, J.F., and Spence, G. 2013. Paleomagnetism of ∼

1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan): new results and

implications. Canadian Journal of Earth Sciences 50: 1085-1096.

Kulakov, E.V., Smirnov, A.V., and Diehl, J.F. 2014. Paleomagnetism of the ~1.1 Ga

Coldwell Complex (Ontario, Canada): Implications for Proterozoic geomagnetic field

morphology and plate velocities. Journal of Geophysical Research: Solid Earth 119:

8633-8654.

Kump, L.R. 2008. The rise of atmospheric oxygen. Nature 451: 277-278.

Lewchuk, M.T., and Symons, D.T.A. 1990. Paleomagnetism of the Late Precambrian

Coldwell Complex, Ontario, Canada. Tectonophysics 184: 73-86.

Lyons, T.W., Reinhard, C.T., and Planavsky, N.J. 2014. The rise of oxygen in Earth's

early ocean and atmosphere. Nature 506: 307-315.

Mauk, J.L. 1993. Geologic and geochemical investigations of the White Pine

sediment-hosted stratiform copper deposit, Ontonagon County, Michigan. Ph.D.

thesis, The University of Michigan, Ann Arbor, MI.

McFadden, P.L. 1990. A new fold test for palaeomagnetic studies. Geophysical

Journal International 103: 163-169.

Page 51 of 79





DraftDraft

52

McFadden, P.L. 1998. The fold test as an analytical tool. Geophysical Journal

International 135: 329-338.

Merrill, R.T., and McElhinny, M.W. 1983. The Earth's magnetic field: Its history,

origin and planetary perspective. Academic Press, London: 401 pp.

Nishioka, G.K. 1983. Origin of late veins in the White Pine copper deposit, northern

Michigan. M.S. thesis, The University of Michigan, Ann Arbor, MI.

Ohr, M. 1993. Geochronology of diagenesis and low-grade metamorphism in pelites.

Ph.D. thesis, The University of Michigan, Ann Arbor, MI.

Paces, J.B., and Miller, J.D. 1993. Precise U‐Pb ages of Duluth complex and related

mafic intrusions, northeastern Minnesota: Geochronological insights to physical,

petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1

Ga midcontinent rift system. Journal of Geophysical Research: Solid Earth 98:

13997-14013.

Palmer, H.C. 1970. Paleomagnetism and correlation of some Middle Keweenawan

rocks, Lake Superior. Canadian Journal of Earth Sciences 7: 1410-1436.

Palmer, H.C., and Davis, D.W. 1987. Paleomagnetism and U-Pb geochronology of

volcanic rocks from Michipicoten Island, Lake Superior, Canada: precise calibration

of the Keweenawan polar wander track. Precambrian Research 37: 157-171.

Page 52 of 79





DraftDraft

53

Palmer, H.C., and Halls, H.C. 1986. Paleomagnetism of the Powder Mill group,

Michigan and Wisconsin: A reassessment of the Logan loop. Journal of Geophysical

Research: Solid Earth 91: 11571-11580.

Palmer, H.C., Halls, H.C., and Pesonen, L.J. 1981. Remagnetization in Keweenawan

rocks. Part I: conglomerates. Canadian Journal of Earth Sciences 18: 599-618.

Pavón-Carrasco, F., Tema, E., Osete, M., and Lanza, R. 2016. Statistical analysis of

palaeomagnetic data from the last four centuries: Evidence of systematic inclination

shallowing in lava flow records. Pure and Applied Geophysics 173: 839-848.

Pesonen, L.J., and Halls, H.C. 1983. Geomagnetic field intensity and reversal

asymmetry in Late Precambrian Keweenawan rocks. Geophysical Journal of the Royal

Astronomical Society 73: 241-270.

Pesonen, L.J., and Nevanlinna, H. 1981. Late Precambrian Keweenawan asymmetric

reversals. Nature (London) 194: 436-439.

Price, K.L., Huntoon, J.E., and McDowell, D.S. 1996. Thermal history of the 1.1 Ga

Nonesuch Formation, North American Mid-continent Rift, White Pine, Michigan.

American Association of Petroleum Geologists Bulletin 80: 1-15.

Page 53 of 79





DraftDraft

54

Pujol, J., Fountain, D.M., and Anderson, D.A. 1985. Statistical analysis of the mean

heat flow/reduced heat flow relationship for continents and its tectonothermal

implications. Journal of Geophysical Research: Solid Earth 90: 11335-11344.

Robertson, W.A., and Fahrig, W.F. 1971. The Great Logan Paleomagnetic Loop - The

polar wandering path from Canadian shield rocks during the Neohelikian Era.

Canadian Journal of Earth Sciences 8: 1355-1372.

Roy, J.L., and Robertson, W.A. 1978. Paleomagnetism of the Jacobsville Formation

and the Apparent Polar Path for the Interval −1100 to −670 m.y. for North America.


Ruiz, J., Jones, L.M., and Kelly, W.C. 1984. Rubidium-strontium dating of ore

deposits hosted by Rb-rich rocks, using calcite and other common Sr-bearing minerals.

Geology 12: 259-262.

Stein, S., Stein, C., Kley, J., Keller, R., Merino, M., Wolin, E., Jurdy, D., Wiens, D.,

Wysession, M., Al-Equabi, G., Shen, W., Frederiksen, A., Darbyshire, F., Waite, G.,

Rose, W., Vye, E., Rooney, T., Moucha, R., and Brown, E. 2016. New insights into

North America's Midcontinent Rift. EOS, 97, August 4, 2016. doi:

10.1029/2016EO056659.

Page 54 of 79





DraftDraft

55

Suszek, T. 1997. Petrography and sedimentation of the Middle Proterozoic

(Keweenawan) Nonesuch Formation, western Lake Superior region: Midcontinent Rift

System. In Middle Proterozoic to Cambrian rifting, central North America. Edited by

R.W. Ojakangas, A.B. Dickas, and J.C. Green. Geological Society of America, Special

Paper 312. pp. 195-210.

Swanson-Hysell, N.L., Maloof, A.C., Weiss, B.P., and Evans, D.A.D. 2009. No

asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan

basalts. Nature Geoscience 2: 713-717.

Swanson-Hysell, N.L., Burgess, S.D., Maloof, A.C., and Bowring, S.A. 2014.

Magmatic activity and plate motion during the latent stage of Midcontinent Rift

development. Geology 42: 475-478.

Symons, D., and Garber, J. 1974. Pole position for the Neohelikian Nemegosenda

carbonatite complex, Ontario. Canadian Journal of Earth Sciences 11: 341-345.

Symons, D.T.A., Kawasaki, K., and Diehl, J.F. 2013. Age and genesis of the White

Pine stratiform copper mineralization, northern Michigan, USA, from

paleomagnetism. Geofluids 13: 112-116. doi: 10.1111/gfl.12024.

Symons, D.T.A., Lewchuk, M.T., Dunlop, D.J., Costanzo-Alvarez, V., Halls, H.C.,

Bates, M.P., Palmer, H.C., and Vandall, T.A. 1994. Synopsis of paleomagnetic studies

Page 55 of 79





DraftDraft

56

in the Kapuskasing structural zone. Canadian Journal of Earth Sciences 31:

1206-1217.

Tan, X., and Kodama, K. 2002. Magnetic anisotropy and paleomagnetic inclination

shallowing in red beds: Evidence from the Mississippian Mauch Chunk Formation,

Pennsylvania. Journal of Geophysical Research: Solid Earth 107 (B11): 2311, doi:

2310.1029/2001JB001636.

Tauxe, L. 2010. Essentials of Paleomagnetism. University of California Press,

Berkeley, California: 489 pp.

Tauxe, L., and Kent, D.V. 2004. A simplified statistical model for the geomagnetic

field and the detection of shallow bias in paleomagnetic inclinations: Was the ancient

magnetic field dipolar? In Timescales of the Paleomagnetic Field. Edited by J.

Channell, D.V. Kent, W. Lowrie, and J. Meert. American Geophysical Union,

Washington, D.C. pp. 101-115.

Tauxe, L., and Kodama, K.P. 2009. Paleosecular variation models for ancient times:

Clues from Keweenawan lava flows. Physics of the Earth and Planetary Interiors 177:

31-45.

Tauxe, L., and Watson, G.S. 1994. The fold test: an eigen analysis approach. Earth and

Planetary Science Letters 122: 331-341.

Page 56 of 79





DraftDraft

57

Vincenz, S., and Yaskawa, K. 1968. Phenomenon of partial self-reversal in

Keweenawan rocks: II. Magnetization of upper Keweenawan lavas and sediments and

of lower Keweenawan dikes. Journal of Geophysical Research 73: 2753-2767.

Weege, R.J., and Pollock, J.P. 1972. The geology of two new mines in the Native

Copper District of Michigan. Economic Geology 67: 622-633.

White, W.S. 1968. The native-copper deposits of northern Michigan. In Ore deposits

of the United States. Edited by J.D. Ridge. American Institute of Mining,

Metallurgical, and Petroleum Engineers. pp. 303-325.

White, W.S. 1971. A paleohydrologic model for mineralization of the White Pine

copper deposit, northern Michigan. Economic Geology 66: 1-13.

White, W.S., Cornwall, H.R., and Swanson, R.W. 1953. Bedrock geology of the

Ahmeek quadrangle, Michigan. United States Geological Survey, Geological

Quadrangle Map GQ 27.

Woodruff, L.G., Cannon, W.F., Nicholson, S.W., Schul, K.J., and Wild, R. 2013.

Geology of Keweenawan Supergroup. Porcupine Mountains, Ontonagon and Gogebic

Counties, Michigan. The Institute on Lake Superior Geology, Proceedings, v. 59, pt. 2,

Field Trip Guide Book, p. 69-96.

Page 57 of 79





DraftDraft

58

Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G. 1997. U-Th-Pb zircon

ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior.

Canadian Journal of Earth Sciences 34: 549-561.

Zijderveld, J.D.A. 1967. A.C. demagnetization of rocks: analysis of results. In

Methods in Paleomagnetism. Edited by D.W. Collinson, K.M. Creer, and S.K.

Runcorn. Elsevier, Amsterdam, The Netherlands. pp. 254-286.

Page 58 of 79





DraftDraft

59

Captions

Figure 1. Location of the: A) Midcontinent Rift of central North America; and, B)

Keweenawan outcrops around Lake Superior.

Figure 2. Generalized geologic map of the Keweenaw Peninsula with paleomagnetic

sampling site locations.

Figure 3. Examples of progressive thermal demagnetization of specimens from: A) site

10, J0 = 2.5x10-2 A/m; B) site 1, J0 = 2.95x10-2 A/m; C) site 12, J0 = 1.33x10-2 A/m; D)

site 8, J0 = 2.31x10-2 A/m; E) site 17, J0 = 1.14x10-2 A/m; and, F) site 19, J0 = 2.31x100

A/m. Solid circles denote projections onto the horizontal plane (N, north; E, east; S,

south; W, west) and open circles denote projections onto the vertical plane (U, up; E,

east; D, down; W, west). The axial intensities are expressed as a ratio of the NRM

intensity (J/J0). Labeled demagnetizing temperatures are in degrees Celsius.

Figure 4. Saturation isothermal remanent magnetization (SIRM) curves for 17

representative specimens of the Copper Harbor red beds showing: A) acquisition of

SIRM intensity (J) in direct magnetic fields (Hdc) up to 900 mT (J900); and then, B)

alternating field demagnetization up to 120 mT (Haf) of J900.

Page 59 of 79





DraftDraft

60

Figure 5. A) Equal-area projection showing mean remanence directions (circles) for

the Copper Harbor red beds after tilt correction with their mean (triangle) and cone of

95% confidence. Solid (open) symbols are in the lower (upper) hemisphere. The

square shows the location of the Lake Shore Traps’ tilt-corrected mean ChRM

direction with its cone of 95% confidence. B) Results of fold test of Enkin (2003) for

the 18 site mean directions where k is the precision parameter of Fisher (1953).

Figure 6. Part of the “Logan Loop” of the North American apparent polar wander path

with critical mean Keweenawan paleopoles for this study (Table 5).

Figure 7. Kamb (1959) contour plot of the areal density of all CHF redbed vector

directions from the 18 accepted sites for the : A) natural remanent magnetization

(NRM), 202 vectors; B) 450°C and 500°C thermal demagnetization steps, 383 vectors;

and, C) 550°C and 560°C steps, 199 vectors. The left hand panel shows the vector

directions with the density contours and the right hand panel shows only the contours.

The stereonet plot is centered on Decl. = 270°, Incl. = 0° so that “westward directed”

+ve down lower-hemisphere and -ve up upper hemisphere vectors of one population

straddling the equatorial 0° inclination circumference are show as one population. The

initial contour is set at 3σA where σA is the one standard deviation density for an

Page 60 of 79





DraftDraft

61

anomaly within a random population and the contour interval is 5σA. See Table 3 for

additional information.

Figure 8. Segment of an equal-area stereonet showing the progressive thermal

demagnetization track of the Kamb-plot anomaly peak direction for the CHF red beds

(Table 4).

Figure 9. Time sequence schematic for the geologic history of the Keweenaw

Peninsula from ~1100 Ma to ~1040 Ma. The solid (lined) bars indicate the duration

(approximate duration) of the labelled volcanic extrusion, sedimentary deposition or

oxidation, or copper mineralization event.

Figure 10. Paleopoles for the 1109-1100 Ma early Keweenawan rift event: A)

uncorrected for a 1060±5 Ma normal-polarity remagnetization component; and, B)

corrected for the remagnetization component except for the Coldwell and

Nemogosenda complexes’ poles that intruded the Laurentian Shield. Note the greatly

reduced scatter in the paleopoles as a result of the correction, and the resulting

improved definition of the apparent polar wander path. The paleopole acronyms are

listed in Table 3.

Page 61 of 79





DraftDraft

Table 1. Stratigraphy of the Keweenaw Peninsula, modified from Bornhorst and Barron (2011)

- glacial deposits

Cambrian to Silurian - sedimentary rocks

Bayfield Group

Jacobsville <3000 m of mature fluvial red-brown

Formation sandstones and conglomerates

Freda ~3700 m of immature fluvial red-brown

Formation sandstones, siltstones and mudstones

Nonesuch ~180 m of lacustrine gray siltstones, shales

Formation and minor sandstones on top of ~30 m of

black-gray pyritiferous shales with

White Pine stratiform copper ore at base

Copper Harbor ~2000 m of immature fluvial red sandstones

Formation and conglomerates with the interbedded

Lake Shore Traps (mafic flows) and

minor native copper in veins at top.

Porcupine - 0-3000 m of intermediate volcanic rocks with a

Formation caldera shape beneath the White Pine area

Portage Lake ~5000 m+ of mafic volcanics with minor

Formation conglomerate interbeds and world-class

native copper lodes

- volcanic rocks

- basement rocks

Oro

nto

Gro

up

Pre-Mesoproterozoic

Quaternary

Powder Mill Group

Kew

eenaw

an S

uperg

roup

Berg

land G

roup

Page 62 of 79





DraftDraft

Table 2. Site attitudes, intensities, and remanence data for the Copper Harbor red beds after 100% tilt correction.

Mean NRM

Strike Dip Intensity Dec. α95 k

° ° mA ° °1 283 18 2.75E-02 13, 13 278.9 9.4 4.1 102.62 283 17 2.76E-02 15, 15 266.4 12.0 3.8 103.83 272 16 2.57E-02 12, 12 268.7 5.2 5.9 54.94 * 295 17 7.59E-02 12, 12 291.0 37.1 2.0 432.75 293 17 2.17E-02 12, 12 275.0 7.0 5.1 74.06 293 17 2.76E-02 16, 16 280.4 1.0 3.2 134.87 236 36 2.86E-02 11, 8 275.0 -4.5 3.1 311.78 244 31 2.92E-02 7, 7 271.6 -3.9 8.9 47.49 248 33 1.30E-02 12, 11 281.4 17.2 5.7 65.2

10 243 32 2.32E-02 12, 12 273.4 15.0 4.8 81.411 271 33 5.62E-02 7, 7 272.7 10.9 8.5 51.612 267 37 1.06E-02 11, 11 274.8 27.1 5.1 81.613 268 37 8.33E-03 10, 10 274.5 16.6 10.4 22.514 264 38 8.38E-03 12, 10 264.0 21.5 3.1 241.515 271 30 1.33E-02 9, 9 281.1 15.5 7.3 50.616 254 43 9.89E-03 12, 12 273.3 7.5 5.8 56.417 269 32 1.02E-02 7, 4 271.6 14.8 7.0 171.918 275 34 1.20E-02 10, 10 285.8 10.5 7.4 44.619 * 262 28 1.26E+00 11, 11 274.3 39.7 4.8 90.620 265 41 2.20E-02 9, 8 279.7 11.4 4.8 135.8

Unit mean direction N = 18 274.9 10.9 4.2 69.5

* site omitted from mean of sites. When the specimen directions are averaged to blockdirections and then to site directions, the unit mean direction is D = 275.0°, I = 10.8°,

α95 = 4.0°, and k = 77.2.

°

Incl.

Used

Site

No.

Attitude Tilt Corrected ChRM DirectionSpecimens

Measured,

Page 63 of 79





DraftDraft

Table 3. Dated Keweenawan Paleopoles from the ~1.1 Ga Midcontinental Rift.

References

U/Ns λp φp A95 Nd Ma

LSr Logan Sills R 5/62 -49.1 39.6 18.6 1 1108±1 1, 2

CCr* Coldwell Complex R 3/62 -48.7 24.4 4.3 5 1108±1 3, 4, 5, 6

CCn* Coldwell Complex N 3/23 47.1 194.5 5.1 NA NA NA

NSr North Shore Volcanics R 3/38 -49.8 18.0 10.9 2 1108±2 2, 7, 8, 9

DCr Duluth Complex R 1/13 -33.3 34.3 8.8 2 1107±1 2, 10, 11

DCn Duluth Complex N 7/97 27.6 189.5 3.8 NA NA 12

PMr Powder Mill Volcanics R 1/33 -37.9 36.9 8.9 1 1107±2 2, 13

OVr Osler Volcanics R 2/37 -45.9 18.1 8.1 2 1106±3 2, 14, 15, 16

OVn Osler Volcanics N 1/5 34.0 177.6 7.9 NA NA 15

NCr Nemegosenda Complex R 1/4 -48.6 13.8 25.2 1 1105±3 17, 18, 19

NCn Nemegosenda Complex N 1/10 52.2 184.1 15.5 NA NA NA

MPIn Mamainse Point Volcanics N 1/10 30.7 187.8 7.7 1 1100±1 7, 20, 21

MPIr Mamainse Point Volcanics R 2/7 -36.8 23.2 17.0 NA NA NA

NSn North Shore Volcanics N 3/98 32.4 183.6 4.8 2 1097±2 2, 7, 8, 9

PLL* Portage Lake Volcanics N 1/28 26.8 180.6 2.1 2 1095±2 8, 22, 23

LST* Lake Shore Traps N 1/31 23.1 186.4 4.0 1 1087±2 23, 28

MIn* Michipicoten Island Volcanics N1/13 25.2 175.0 7.0 1 <1087±2 24

CHFs* Copper Harbor Formation N 1/18 7.4 181.7 3.3 NA NA 25

NOs* Nonesuch Formation N 2/29 7.3 174.7 3.0 NA NA 26, 27

FRs* Freda Formation N 1/20 3.0 174.2 4.0 NA NA 26

Notes: The paleopoles listed for magmatic rocks in this table are abstracted mainly from Table S3 in

Swanson-Hysell et al. (2009, as amended April 4, 2010). An asterisk (*) after the paleopole acronym

in the first column indicates a change resulting from new information and from the addition of rift

sedimentary data. N or R indicate currently normal or reversed polarity. U/Ns is the number of studies/

sites. λP, φp, A95 are the latitude, longitude and radius of cone of 95% confidence in degrees of the

paleopole. Nd, Ma are the number of U/Pb determinations and age in millions of years. The references

are: 1. Halls and Pesonen (1982), 2. Davis and Green (1997), 3. Robertson (1970; in Lewchuk and

Symons 1990), 4. Lewchuk and Symons (1990), 5. Kulakov et al. (2014), 6. Heaman and Machado

(1992), 7. Palmer (1970), 8. Books (1968), 9. Hubbard (1971), 10. Beck (1970), 11. Paces and Miller

(1993), 12. Books (1972), 13. Palmer and Halls (1987), 14. Palmer (1970), 15. Halls (1974), 16. Davis

and Sufcliffe (1985), 17. Symons and Garber (1974), 18. Constanzo-Alvarez et al. (1993), 19. Heaman

et al. (2007), 20. Robertson (1973), 21. Swanson-Hysell et al. (2009), 22. Hnat et al. (2006), 23. Davis

and Paces (1990), 24. Palmer and Davis (1987), 25. this paper, 26. Henry et al. (1977), 27. Symons

et al. (2013); 28. Kulakov et al. (2013).

U/Pb AgePaleopoleSite Information

Page 64 of 79





DraftDraft

Notes: The paleopoles listed for magmatic rocks in this table are abstracted mainly from Table S3 in

Swanson-Hysell et al. (2009, as amended April 4, 2010). An asterisk (*) after the paleopole acronym

in the first column indicates a change resulting from new information and from the addition of rift

sedimentary data. N or R indicate currently normal or reversed polarity. U/Ns is the number of studies/

are the latitude, longitude and radius of cone of 95% confidence in degrees of the

paleopole. Nd, Ma are the number of U/Pb determinations and age in millions of years. The references

are: 1. Halls and Pesonen (1982), 2. Davis and Green (1997), 3. Robertson (1970; in Lewchuk and

Symons 1990), 4. Lewchuk and Symons (1990), 5. Kulakov et al. (2014), 6. Heaman and Machado

(1992), 7. Palmer (1970), 8. Books (1968), 9. Hubbard (1971), 10. Beck (1970), 11. Paces and Miller

(1993), 12. Books (1972), 13. Palmer and Halls (1987), 14. Palmer (1970), 15. Halls (1974), 16. Davis

and Sufcliffe (1985), 17. Symons and Garber (1974), 18. Constanzo-Alvarez et al. (1993), 19. Heaman

et al. (2007), 20. Robertson (1973), 21. Swanson-Hysell et al. (2009), 22. Hnat et al. (2006), 23. Davis

and Paces (1990), 24. Palmer and Davis (1987), 25. this paper, 26. Henry et al. (1977), 27. Symons

Page 65 of 79





DraftDraft

Table 4. Kamb plot statistics

Demagnetization Number

Temperatures of Dec. Inc. Sig.

(°C) Vectors (°C) (°C) σA

20 202 279 15 49

200+275 235 275 8 60

300+400 279 274 7 66

450+500 383 277 8 70

520+530+540 308 278 6 56

550+560 199 277 5 51

620+640 394 278 4 56

Anomaly Peak

Page 66 of 79





DraftDraft

��

��

��

��

! "" #��" #�$% #��" # !!&�'( )�*�� +*�+ !&�� ),�, (-�+ !-*�* !!�)

�

) ./�#�""�#��/�#��"�# !!&*'( )�!�) *(�- ��& +�� ++�! )&*�+ *�*

��#�$%�#��"�#��

( ��!�0�)�� 1 !!&*'( )�+�+2��!

2 ��& (��, +!�( !�*�� *�*

+ �/ #�.. !&�*') )�&�- (,�� &3 )!�( )��* !-)�! (�,3

� ./� #�� !&-,') )--�* )��) *�-3 !��* )+�( !-&�, ��,3

* "45� 6 ),��+ !&�� +�) �� ,�+ !-!�, (�(

, �$�#�57� 6 ),��- )�) ��&3 !�! ��) !,+�� (�*3

��8��9�� :��(��#��#��9�� 1�� #��

��1��9�� 1�� 2�� 1��:�� !�� 1�)�� 1�

��9��;�� !(��3��9�� <��9��

�� 7��

Page 67 of 79





DraftDraft

�� 1

)�� 1��

��;��

Page 68 of 79





DraftDraft

Table 6. Projection of Remanence Data

Path Time

Length Differ-

ence Change Rate

Myr ° °/Myr

3 4 16.5 10 1.65 18.3 -3.6 -0.36 4 1096±2

4 5 5.5 9 0.61 6.8 -2.2 -0.25 5 1087±2

5 6 16.3 27* 0.61* 6.8* -7.2 -0.27 6 1060±5*

5 7 18.0 30* 0.61* 6.8* -12.8 -0.30 7 1057±6*

Notes: Group paleopole numbers are from Table 5. * is from the projected rate of apparent

polar wander.

Mean Age of

Oxidation

From To ° °/Myr cm/yr Pole Ma

RemanenceTranslationPaleopole

Group Velocity Declination

Page 69 of 79





DraftDraft

KANSAS

Tecto

nic

Zone

km0

N KeweenawanMidcontinent Riftexposed

Michigan

Wisconsin

Ontario

LakeSuperior

KeweenawPeninsula

kilometers0 200

Ontario

LakeNipigon

ColdwellComplex

SynclineFigure 2

Fig. 1

BA

exposed

underwater

Phanerozoic cover

MidcontinentRift System

CANADA

CANADA

USA

USA

400

50oN

35oN

75 oW100o

W

WindsorMamainsePoint

Powder MillVolcanics

Logan Sills

OslerVolcanics

MichipicotenIsland Volcanics

Minnesota

DuluthComplex

NemegosendaComplex

NorthShore

Volcanics

GRENVILLE Front

studyarea

Page 70 of 79





DraftDraft

0 3km

Copper Harbor

JacobsvilleFormations

FredaNonesuchCopper HarborPortage Lake

Basement

Houghton

LAKE SUPERIOR

N

LAKE SUPERIOR

KEWEENAW

FAULT

White Pine

47oN

90oW 89oW

88oW

CopperHarbor

7 810

11

1-6

A

km0 30

LAKE SUPERIOR SYMBOLSBRoadTown

Site

FaultAnticline

Syncline

Beddingabout flatshallow dip

medium dip

Fig. 2

12, 13

14

15

16

17

1819

20

9

Page 71 of 79





DraftDraft

E

N, U

S, D

W

N, U

S, D

EW

NRM500

200600

E

N, U

S, D

W

NRM500

200

600

E

N, U

S, D

W

NRM 500

200 600

A

JJ0

1.0

0.5

00 600(oC)

E

N, U

S, D

W

NRM500

600

200

B

JJ0

1.0

0.5

00 600(oC)

D

F

JJ0

1.0

0.5

00 600(oC)

JJ0

1.0

0.5

00 600(oC)

E

N, U

S, D

W

C

E

JJ0

1.0

0.5

00 600(oC)

JJ0

1.0

0.5

00 600(oC)

NRM

200

500

600

NRM

200

500 640

Fig. 3

Page 72 of 79





DraftDraft

1.0

0.5

0

JJ900

0 300 600 900mTHdc

198

7

15

5

0 40 80 120mTHaf

1.0

0.5

0

JJ900

16

19

FIg. 4

A B

Page 73 of 79





DraftDraft

k

83 5

-50 0 50 100 150% Untilting

20

40

60

80

180o

270o

0o

Fig. 5

A B

Page 74 of 79





DraftDraft

Fig. 6

180°E160 200

90°N

60°N

30°N

0°

1

2

3

4

5

67

CopperHarborredbeds

1087 Ma

1096 Ma

1106 Ma

Nonesuch& Fredastrata

Page 75 of 79





DraftDraft

0o 0o

+90o +90o

-90o

Fig. 7

-90o

360o

360o

-30o -30o

30o 30o

0o 0o

360o

-30o -30o

30o 30o

60o 60o

0o 0o

270o 270o

270o 270o

-30o -30o

30o 30o

-60o -60oA

B

C

NRM NRM

450+500°C 450+500°C

550+560°C550+560°C

Page 76 of 79





DraftDraftFig. 8

NRM

200, 275300, 400

450,500

550, 560

620,640

520, 530, 540

270°

280°

0° 10° 20°

Inclination

Dec

linat

ion

Page 77 of 79





DraftDraft

Fig. 9

1100 1090 1080 1070 1060 1050 1040

PLV CHG NSG FRG CHR, WSM WVM

NSV PCV LST, MIV NSO, FRR

rift extensionfast slow slow, uplift fast, deformation

collision compression

CHG Copper Harbor gray bedsCHR Copper Harbor red bedsFRG Freda gray bedsFRR Freda red bedsLSG Lake Shore TrapsMIV Michipicoten Island volcanicsNSG Nonesuch gray bedsNSO Nonesuch, oxidized

NSV North Shore volcanicsPCV Porcupine volcanicsPLV Portage Lake volcanicsWSM White Pine SSC mineralizationWVM White Pine vein mineralization

radiometric age date controlpaleomagnetic control

Ma

Page 78 of 79





DraftDraft

180°E

90°N

60°N

30°N

LSrCCr

CCn

NSr

DCr

DCn

PMrOVrNCr

NCn

MPIn

MPIr

OVn

0°

200160

A

180°E

90°N

60°N

30°N

LSrNSr

DCrDCnPMr

OVrMPIn

MPIr

OVn

0°

200160

B

Fig. 10

NCr

CCn

CCrNCn

Page 79 of 79





system appendpdf cover-forpdf - university of toronto t-space · 78 from the west end of lake...

Documents