Proterozoic transcurrent movements along the Kapuskasing lineament (Superior Province, Canada) and their relationship to surrounding structures

147

Earth-Science Reviews, 32 (1992) 147-185 Elsevier Science Publishers B.V., A m s t e r d a m

Proterozoic transcurrent movements along the Kapuskasing lineament(Superior Province, Canada) and their relationship to surrounding structures Craig R. Goodings i and M.E. Brookfield 2 Department of Earth Sciences, Unit'ersity of Waterloo, Waterloo, Ont. N2L 3G1, Canada (Received July 27, 1988; accepted after revision December 22, 1990)

ABSTRACT Goodings, C.R. and Brookfield, M.E., 1992. Proterozoic transcurrent movements along the Kapuskasing lineament (Superior Province, Canada) and their relationship to surrounding structures. Earth-Sci. Rec., 32: 147-185. Analysis of large-scale geological and geophysical lineaments provide evidence for a new model to explain the Kapuskasing Structural Zone (KSZ) and James Bay. James Bay is interpreted to have formed due to rifting related to lateral strike-slip along the KSZ. The Sutton Arc is interpreted to be an extension from Hudson Bay Arc, which has been displaced by rifting of James Bay. Magnetic anomalies which terminate at the KSZ on the west have correlated magnetic anomalies on the east which are displaced left-laterally. It is proposed that these displacements and the rifting of James Bay and the strike-slip along the KSZ were caused as a result of 160 km of left-lateral motion between East and West Superior Terranes with an accompanying 5 ° of anticlockwise rotation of the West Superior Terrane. Additional support for this model comes from structural evidence drawn from previous reports from the last twenty-six years.

INTRODUCTION

This study is concerned with the history of the Kapuskasing Structural Zone (KSZ) and its possible relation to James Bay. The KSZ is an intracratonic belt of uplifted granulite and upper amphibolite facies that transects the Superior Province. It has been interpreted as a zone of strike-slip and a zone of dip-slip. However, these previous interpretations were not based on a regional analysis of the belt. Rather, small detailed areas of it were studied from which interpretations have been made for the whole structure. This lack of regional study is particularly surprising since the KSZ is located within an intracra-

t P r e s e n t address: 138 River road, Bracebridge, Ont. P0B IC0, Canada. 2 Land R e s o u r c e Science, Univ. of Guelph, Ont. N1G 2W1, Canada.

tonic setting which may have once been continuous across the belt. Such a setting is ideally suited for studies which actively search out lineaments that would provide important and necessary information about the movements along this zone. The aim of this study is to investigate the KSZ and James Bay in terms of relative movements between the terranes which enclose them. An overall regional model is presented which is supported by displaced geological and geophysical lineaments as well as structural evidence drawn from previous reports over the past twenty-six years. This model not only assimulates a wide range of previously known geological data but also allows for predictions by which it can be tested. In addition, the model is reviewed in terms of possible wider implications for the Proterozoic belts which border the Superior Province.

0012-8252/92/$15.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

148

The KSZ is located in the Superior Province (Fig, 1). It is a north-easttrending belt of high grade gneisses that transects the east-westtrending grain of the Abitibi-Wawa and Quetico-Opatica sub-provinces. These sub-provinces are interpreted as having been once continuous across the KSZ and therefore the KSZ is likely an intracratonic lineament (Percival and Card, 1983, 1985). It trends from James Bay in a fairly straight south-westerly line but before it reaches the southern boundary of the Superior Province it bends in a southwest arc to the northeast shore of Lake Superior. The rocks within the KSZ, where exposed, reflect the rock types of the sub-provinces through which it trends though of a lower crustal equivalent (Percival and Card, 1983; Percival and McGrath, 1986). Therefore, in the north, where the KSZ passes through the metasedimentary belts of the Opatica and Quetico Belts, the rocks exposed in the KSZ are metasedimentary granulites, and in the south where the belt passes through the Wawa-Abitibi metavolcanic belts, metavolcanic upper amphibolites to granulites are exposed. This marked increase in metamorphic grade contrasts to the lower rnetamor-

Graig Goodings completed his studies in geology at the University of Waterloo, Ontario, Canada. He has studied Paleozoic sedimenting sequences in the Himalayas and Proterozoic tectonic orogehies in the Canadian shield. Currently, Craig Goodings is working with the Ministry of Northern Development and Mines on mineral occurrences in Ontario.

C.R. (}OODINGS AND M.E. BROOKF1ELD

phic grades of the rocks on either side of the KSZ (Bennet et al., 1967; Thurston et ai., 1977). In addition to the change in metamorphic grades, the KSZ is also in fault contact with the surrounding terranes (Bennet et al., 1967; MacLaren et al., 1968; Thurston et al., 1977; Percival and Card, 1985; Percival and McGrath, 1986). In the north, faults mark both sides of the KSZ, but southward along its length the west-bounding fault dies out and only the east boundary fault can be traced to Lake Superior (Corfu, 1987). The KSZ also exhibits magnetic and positive gravity anomalies. The magnetic anomaly changes in character from northeast to southwest along strike. In the north, the magnetic anomaly is intense and defines a narrow belt 10-20 km in width. Southward, the anomaly widens to about 60 km and becomes less intense until at Chapleau the anomaly dies out. The source of this anomaly is small disseminated magnetite grains associated with the granulities (MacLaren et al., 1968). The positive gravity anomaly extends from within James Bay southward to Chapleau where it, like the magnetic anomaly, dies out. Ductile deformation has totally destroyed

M.E. Brookfield is a professor of geology at the University of Guelph, Ontario Canada. He has undertaken studies in carbonates and eolian deposits. Currently, he is studying the tectonic evolution of the Himalayas.

149

PROTEROZOIC TRANSCURRENT MOVEMEN rs

LEGEND ~='~ Proterozoic.Phanerezoicrocks Subprovince boundary ARCHEAN SUBPROVINCETYPE F ~ Plutonic ~

olcano-plutonic

~

Metasedimentary

~

High-grade gneiss I

300km

r~l I

HUDSON BAY

CHURCHILL PROVINCE .~..t- J " ~ *

GRANOE

~"

R

PIKWITONE]

,o~.:-ff.v.' i

- ~ '

" "

i..,~ ~,,S,ASSIN, " ~- % ' '~" t

HOMOCLINE

MOOSE RIVER

'.--: "-:-,..:.:,'.

O P A T I CIAC A

" . ",

.

" r . , p

BASIN

r.

AB

T

B

",,,.'k,

Kapusl~$ing ..

~ - ~ .

.

, . . ; . :~ " ~

. . :,

~

~1

...... E~ R-'~: .~:B:~O.'O~

• .2.~.~COeALr ~,-

EMRAYMEN T

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Fig. 1. Subdivisionsof the Superior Province and the location of the Kapuskasing Structural Zone (after Card and Ciesielski, 1986).

the previous character of the rocks in the zone and has been replaced with a dominant northeasterly structural grain (Bennet et al., 1967; Thurston et al., 1977; Percival and Card, 1985; Percival and McGrath, 1986). However, like the geophysical anomalies, this ductile deformation does not extend south past Chapleau, instead the rocks in the KSZ at Lake Superior exhibit only a slight change in trend from the surrounding rocks (Corfu, 1987). The KSZ is also marked by post-movement carbonatite-alkalic complexes which have given K - A r ages ranging from 1750 Ma to 1100 Ma (Gittens et al., 1967). The age of the KSZ is unknown but it was active at least since the complexes were emplaced in the

Proterozoic Era and recent earthquake activity along it (Forsyth et al., 1983: reported in Percival and Card, 1985) indicates that it is a long-lived zone of crustal weakness.

Previous interpretations of the Kapuskasing Structural Zone (Table 1) The history of the Kapuskasing Structural Zone (KSZ) has been the subject of much debate since it was first detected in a gravity traverse by Garland (1950). He interpreted the positive gravity anomaly as reflecting a deep seated disturbance caused by thinning of the granitic layer bringing the denser lower crust to the surface during a mountain building episode.

150

C.R. G O O D I N G S

AND M.E. BROOKFIELD

TABLE 1 Previous interpretation of the Kapuskasing Structural Zone Proposed movement

Extent

Evidence

Source

gravity

Garland, 1950

gravity, controlled Paleozoic deposition in James Bay

Innes, 1960 Innes et al., 1968

Upwarp in the Conrad discontinuity

gravity

Wilson and Brisbin, 1962

Horst-sinistral

uplifted granulites and fault offset which later controlled southern extent of deposition in Moose River Basin

Bennet et al., 1967

Uplift by faulting or folding

exposed granulites

Skinner (MacLaren et a1.,1968)

Uplift due to mountain building Rift

Lake Superior through James Bay to Hudson Bay

Thrust fault with sinistral motion

extended north through James Bay into Hudson Bay

exposed granulites rotation of lineations in zone, north-south faults at mouth of James Bay

MacLaren (MacLaren et al., 1968)

Sinistral movement or uplift

extended north to Cape Smith fold belt

offset of Churchill/ Superior boundary

Ayres, 1971 (report in Thurston et al., 1977)

parallel to Thompson Belt

Bell, 1971

strikes at high angle to Belcher Basin and the mid-continental gravity high

Burke and Dewey, 1973

Rifting or transform movement into a westerly dipping horst

foliations, exposed granulites

Thurston et al., 1977

Main movement over 100 km of sinistral motion

deflection of granulites strips, ductile foliations

Watson, 1980

Thrust due to e a s t west or southwestnortheast compression with minor sinistral motion

north-west dipping eastern margin, ductile foliations

Perciwd and Card, 1983, 1985; Percival and McGrath, 1986.

Extension, thrusting, and sinistral motion

related to collision of the Superior/Churchill Provinces

Gibb, 1983

Sinistral movement Rift-related to Keweenaw rift and Belcher Basin

Lake Superior through James Bay to Hudson Bay

PROTEROZOIC TRANSCURRENT MOVEMENTS

The positive gravity anomaly detected by Garland (1950) was shown by Innes (1960) to extend north-east for 600 miles from Chapleau to the southern end of James Bay. Innes (1960) and Innes et al. (1968) interpreted the Kapuskasing Structure as a major tensional feature of the crust caused by rifting. They proposed that the rifting may be related to the mid-continental gravity high in the Great Lakes area and extended the zone of rifting north-east through James Bay to west of the O Hawa Islands. Wilson and Brisbin (1962) considered the KSZ to be an upwarp of the Conrad discontinuity overlain by a thin granitic crust. Based on the trend of the gravity to the west of the exposed high grade rocks in the KSZ, they suggested that the crust may be thicker to the west than to the east. Further exploration of the KSZ was sparked when magnetic maps of the area, released by the Geological Survey of Canada in 1964, delineated an anomalous magnetic belt paralleling the Kapuskasing gravity high on its eastern flank and extending from James Bay to Chapleau. Bennet et al. (1967) proposed that the Kapuskasing zone represents a complex structure which was faulted into place with an east-trending horizontal axis of rotation which uplifted the west block in relation to the east block with the greatest amount of uplift taking place in the zone itself. They also speculated that sinistral movement had occurred along the zone displacing the east-west faults which controlled the southern extent of the Paleozoic sediments in the Moose River Basin. Skinner (MacLaren et al., 1968) proposed that the KSZ is a major structural feature in which a deeper part of the earth's crust is exposed either by faulting, sharp-folding, or both. MacLaren (MacLaren et al., 1968) postulated the presence of a major fault passing through the granulites of the KSZ and north through James Bay to Hudson Bay. He proposed that this fault was a rotational fault which brought the west side up with respect to the east side. In addition, he proposed

151

that the fault had a sinistral transcurrent component which brought the east side north with respect to the west side. MacLaren also considered that faulting along the KSZ was the result of a series of thrust faults inclined toward the centre of the gravity high which uplifted the granulites in a solid state. Ayres (pers. c o m m u n . : r e p o r t e d in Thurston et al., 1977) suggested that KSZ could represent a fracture along which large-scale left-lateral movement and vertical movement had taken place. This predicted lateral m o v e m e n t shifted the Superior Province-Churchill Province boundary from its position in the Sutton Inlier, west of the KSZ, northward to the Cape Smith Belt, east of the KSZ. He alternatively proposed that the KSZ could be a relatively recent feature that uplifted metasediments of Huronian age. Bell (1971) considered the KSZ and the Thompson Belt as related, first magnitude north-east faults both with a large sinistral component of movement. Burke and Dewey (1973) proposed that the Kapuskasing Zone had a two-stage history. In its first stage, the KSZ extended up through James Bay to Hudson Bay and formed a triple-point junction with the gravity anomaly in the Belcher Basin. The KSZ was later reactivated and formed a second arm with the mid-continental gravity high in the south. Thurston et al. (1977) proposed that the KSZ was the site of Early Precambrian sedimentation and intrusion of mafic igneous rocks, later intruded by the Shawmere Anorthsite complex. The zone was then upfaulted by either simple rifting or by transform movement into a west-dipping horst. Watson (1980) postulated that the KSZ originated as a deep transcurrent shear zone which had a three-stage history. During the first stage of development, upfaulting and sinistral displacement occurred which uplifted the granulites and associated rocks before the intrusion of the Matachewan dyke swarm. In the second stage, sinistral transcurrent movement continued during the em-

152

C.R. GOODINGS

placement of the Matachewan swarm. In the third stage of development, displacements of various kinds took place during a number of later events spaced out over a period of more than 2000 Ma. Gibb (1983) suggested several different periods of movement along the KSZ related to a collision model of the Superior Province and Churchill Province. In this model the KSZ existed before collision. Dur-

NUD%ON

BAY

UDSON

I

I

BAY ARC

EAST SUPERIOR TERRANE

A N D M.E. B R O O K F I E L D

ing the initial stages of collision, dextral slicing of the Western Superior Craton developed causing local tension along the KSZ. With further collision, dextral slicing continued and resulted in thrusting along the K S Z and, in the final stages of collision, the KSZ received a sinistral component of movement. The most recent interpretation of the KSZ was made by Percival and Card (1983, 1985) and Percival and McGrath (1986) based on metamorphic pressure estimates and from gravity and magnetic anomalies. They proposed a three-part history for the Kapuskasing Structure. In its early history, the zone suffered ductile shearing possibly due to sinistral motion followed by a compressional event which thrust the part of the Superior Province west of the Kapuskasing Zone onto the eastern part of the Superior Province. Following this uplift, major normal faults and associated volcanism developed in response to crustal thickening. Support for their model came from Cook (1985) who detected a 35 ° 38 ° northwesterly dipping structure along the eastern edge of the KSZ in seismic reflection survey. Cook interpreted this structure to be the thrust fault predicted by Percival and Card (1983). THE MODEL

0 I

300 .,.I km

Fig. 2. Model for sinistral motion between the East and West Superior terranes showing the proposed tectonic elements formed as a result of this motion. The model interprets the James Bay as marking the site of leftlateral rifting, the Northern KSZ as resulting from convergent strike-slip and the Southern KSZ as marking the site of a restraining bend along which the West Superior terrane was thrust over the East Superior terrane. The thick arrows show the proposed direction of terrane movement.

This paper proposes that there has been left-lateral movement between the Western Superior Terrane and the Eastern Superior Terrane and that this movement took place along the boundary now marked by James Bay and the KSZ. As a consequence of movement along this boundary, three distinct tectonic features are proposed to have resulted and are shown in Fig. 2. These are that: (1) the James Bay formed as a result of rifting; (2) the Northern KSZ formed as a result of convergent strike-slip; and, (3) the Southern K S Z formed due to a restraining bend along which the West Superior Terrane was thrust over the East Superior Terrane.

PROTEROZOIC T R A N S C U R R E N T MOVEMENTS

| 53

Fig. 3. This map shows that the Sutton Arc (the Winisk and Sutton Inliers) has a similar radius as the circle which traces out the Hudson Bay Arc. Notice that the arrowed lines drawn between the Sutton Arc's present position and the transposed position are parallel to the Kapuskasing Structural Zone and the southeast end of James Bay. KSZ Kapuskasing Structural Zone; RG - Richmond Gulf; SI - Sutton Inlier, WI - Winisk Inlier. To test this model, linear features must be identified which indicate the direction and amount of motion. As well, the individual tectonic elements must exhibit the expected structural elements. This means that linear features displaced by the rifting of James Bay must show left-lateral divergent offset and linear features along the KSZ must show left-lateral displacement parallel to the KSZ. James Bay must also display structures consistent with rifting such as extensional faulting along the margins and the KSZ must display uplift and deformation consistent with strike-slip.

paleogeographical features which have been displaced along the zone of offset (Reading, 1980). The features used in this study which are interpreted to have once been continuous are: (1) the Sutton Arc, which includes the Sutton Inlier and the Winisk Inlier, and the Hudson Bay Arc now displaced by James Bay; and, (2) m a g n e t i c a n o m a l i e s which o n c e crossed the KSZ but now are displaced by the proposed strike-slip movement along the KSZ.

Evidence for left-lateral motion

Offset of the Sutton Arc and the Hudson Bay Arc

Lateral motion is best recognized directly by the matching of particular rock types or

The correlation of the Sutton Arc to the Hudson Bay Arc is based on geomorphic

154 evidence. The following discussion therefore deals with the structure of these features and shows that, in cross section and plan view, they are unique to the area yet similar to each other and once possibly formed a large continuous ridged arc surrounding eastern Hudson Bay (Fig. 3).

Geomorphology of the Hudson Bay Arc For the most part, the Hudson Bay shoreline does not differ markedly from other large bodies of water. The shoreline is irregular marked by extensive shallow mud flats along the south and west coast and a rock coastline of small relief along the east coast. The prominent exception to this "normal" coastline is the vast circular arc bounding the Belcher Basin and extending over 667 km from latitude 54 ° 48' to latitude 58 ° 40' (Fig. 3). Beals (1968) called this the Hudson Bay Arc. He constructed a best fit circle to the shoreline of the arc with a radius of 232 km and a center located at latitude 56 °45 longitude 80 ° 09'. According to Beals, the largest deviation of this circle from the shoreline is less than 7 km and he stated that "when consideration is given to the general improbability of any land form due to whatever cause conforming precisely to any formal geometric shape, it must be concluded that the correspondence to a circle is extremely close". In addition to the arc's circular shape, it is also bounded by a parallel inland ridge. Recent reports on the area do not make reference to this ridge, but when the area was first being explored, geologists noted it presence. Woodcock (1960) described the ridge as a "coastal mountain range, which extends from Cape Jones to Portland Promontory" and "lies along the east rim of the Belcher Basin". Stevenson (1968) examined the northern half of the arch and made this comment concerning the ridge: "The row of hills parallel with the coast is apparently the remains of an ancient mountain range that becomes gradually more subdued and nar-

C.R. G O O D I N G S

A N D M.l:5. B R O O K F I E L D

row to the north, until only a single row of hills with an elevation of about 500 feet remains in the vicinity of Port Harrison. North of Portland Promontory the coastline becomes more irregular and the topography more subdued". Beals (1968) described this topographic feature from air photographs and concluded "that there appears to be no doubt that there does exist a coherent circular raised ridge or rim, paralleling the arcuate shoreline for a distance of the order of 400 miles (667 km)". Since these early reports, topographic maps have been made for the area and cross-sections drawn from these maps are shown on Fig. 4. These cross-sections reveal that the rim is in the form of an east facing cuesta. The rim reaches its broadest limits and its most complicated structure at Richmond Gulf Graben (Woodcock, 1960, see Fig. 4: section F - F ' ) . This relationship is an important indication that the formation of the graben and the rim may have been related events. From the graben north and south the rim returns to the shoreline and continues with decreasing height. At its most southern and northern ends only isolated hills remain to mark its location. North and south of the arc, the shoreline returns to "normal" (cross-sections A - A ' and J - J ' , Fig. 4), and the coastline is irregular with topography rather low at the shoreline limited to a few tens of metres. Therefore, it would seem that the rim and the arch are related features and unique to this part of the shoreline.

Geology of the rim North and south of Richmond Gulf, the rim is formed of Archean rocks (Beals, 1968; Stevenson, 1968). Approaching the Richmond Gulf graben from the north and south, Proterozoic rocks of the Nastapoka Group lie in undisturbed attitudes upon the lower slopes of the Archean rim (Beals, 1968). In the gulf however the Archean rocks are not exposed and instead it appears that the Richmond Gulf Group sediments form the

155

PROTERf)ZO1C TRANSCURRENT MOVEMENTS

120 ' ~ 90, 60, 30. 0

9°L

60

30

0

A

A

270 240 210 180. 150 120 90. 60. 30, 0

B

,2o[ 9O 6O 30

O

C

480 420 360 3OO 24O 180 120 60 0

E 5 4 0 ~ 480 420 360 3O0 24C 180 120 60 0 F I 2704 2401 / ~ 2101 180, 150, 120, 90~ 60~ 30, 0 160 140 120 1O0 80 60 40 20 0

D

D'

0

10

I

krn

18o G~

150 120 90 60 30 0

H

H'

4°b 20 0

F"

'

J

~

~""""'"~

~ j'

Fig. 4. Topographic sections of the east coast of Hudson Bay. Sections B-B' to I-I' show the elevated rim around the Hudson Bay Arc. The rim is composed of an elevated Archean surface parallel to the Hudson Bay Arc. At Richmond Gulf, the rim reaches its greatest width inland (see F - F ' ) and Proterozoic rocks of the Richmond Gulf Group form the rim at the shoreline with the Archean ridge rising further Inland. Sections A - A ' and J-J' show that the topography north and south of the arc is not elevated. All vertical scales are in metres. Location of sections shown in Fig. 3 (topography from Department of Energy, Mines, and Resources, 1983).

156

C.R. GOODINGS AND M.E. BROOKFIELD

rim onto which undisturbed Nastapoka sediments unconformably lie (Woodcock, 1960; C h a n d l e r and Schwartz, 1980). T h e Nastapoka strata consist roughly in ascending order of carbonate, clastics, iron formation, and basalt and the Richmond Gulf Group consists of mainly fluvial deposits and terrestrial basalt (Woodcock, 1960; Chandler and Schwartz, 1980).

Nastapoka Group because these sediments lie in undisturbed attitudes upon the rim (Beals, 1968). Further evidence comes from the distribution of the Nastapoka Group. Nowhere does the group extend inland past the rim; the implication of this is that the rim acted as a barrier to the seas from which the group was deposited. In addition, the Nastapoka sediments dip radially away from the rim toward the centre of the basin suggesting further control by the rim and the Archean paleoslope. The time of formation of the graben may also be the time of formation of the rim.

Timing of rim formation The time of formation of the Archean rim is uncertain. However, it must have been formed before the deposition of the L 4150 l

~

M

M

+150 SF'A LEVEl. -150

•'•PPA• 0 I

".'300

Itm

16 I

VIMIS~IE~pRO INL T

J

ARCHF.AM

A~E

EROZOIC ID OZOIC

- 450

~4~50

| K'

I

ARCHEAM

•

PROTEROZO|C K

Fig. 5. Seismic sections of James Bay Lowlands. Section L - L ' and K - K ' show that the Archean surface is raised into a ridge or rim across the Sutton Arc similar to the Archean surface around the Hudson Bay Arc. Section M - M ' shows that the ridge does not extend west of the Arc's trace (location of sections shown in Fig. 3) (modified from Hobson, 1968).

157

PROTEROZOIC TRANSCURRENT MOVEMENTS

According to Chandler and Schwartz (1980), the graben formed after the deposition of the Richmond Gulf Group, but before deposition of the Nastapoka Group. As already mentioned, the graben is located where the rim reaches its broadest limits inland and the Richmond Gulf Group appears to be elevated along the mouth of the graben to form a continuation of the rim. To the north and south, the Richmond Gulf Group is downfaulted and the rim is replaced by Archean rocks (Chandler, 1982). These relationships suggest that the same event which formed the graben also formed the rim. The origin of the arc is beyond the scope of this paper. It is certainly a ring structure of some sort; whether internally caused (? by magmatism) or externally caused (? by meterorite impact). I have not been able to find any conclusive evidence for either hypothesis.

Continuation of the Arc Beals (1968) considered the arc to be all that remained from a once continuous circular structure. Noting that the height of the rim decreased south and north, Beals suggested that the whole structure may be been titled causing its western half to be buried under Hudson Bay. However, it seems more likely that erosion has been the cause of the decrease in the rim's height. It is most developed where it is partially covered by Proterozoic sediments which may have provided protection from erosion. In addition, the trace of the arc continues under the waters of the bay at the north and south ends. This is shown by the bathymetric chart of the bay. This bathymetric trace terminates at the gravity anomaly which trends n o r t h - s o u t h along the west coast of Hudson Bay (see Fig. 9). This relationship implies that the arc has been terminated or offset by the tectonic events which produced the gravity anomaly. This was first suggested by Wilson (1968). He stated that "the other half [of the arc] could have been split off and carried away during some past cycle of recurrent continental

drift". Evidence that the arc was once more extensive and part of it was "carried away" comes from the Sutton Arc which will be discussed next.

Geomorphology of the Sutton Arc The Sutton Arc, which consists of the Sutton Inlier and the Winisk Inlier, is located on the west side of James Bay. The arc lies in the geographic area called the Hudson Bay Lowlands. The lowlands, as described by Bostock (1976), is a low, swampy, marshy plain with subdued glacial features and elevations which rarely exceed 133 m. The most conspicuous feature in the area is the Sutton Ridge, an inlier of Precambrian strata that rises to 200 m in elevation, about 167 m above the surrounding surface. The Sutton Ridge is that part of the Sutton Arc which emerges above the Paleozoic strata of the Moose River Basin. As shown in Fig. 3, the outline of the Sutton Arc, as mapped by Bostock (1971) fits remarkably well into the curve of the circle extended from the Hudson Bay Arc. This match is too close to be a coincidence. In addition to the similarity in plan view, the Sutton Arc also marks the location of an elevated rim. This is shown by Hobson's (1968) seismic sections across the arc in two locations, one across the Winisk Inlier and the other across the Sutton Inlier (Fig. 5: section L - L ' and K - K ' ) . These seismic sections reveal an elevated rim comparable in shape and height to the rim found along the Hudson Bay Arc. Section M - M ' shows that the rim does not extend west of the Sutton Arc, suggesting the rim and the arc are related features but unique to the area.

Geology of the Sutton Arc Bostock (1971) considered the ridge revealed in the seismic sections of Hobson's to be most likely due to Archean crystalline rocks. He described the Sutton Arc as being underlain by a ridge of massive granitic

]58

C.R. GOODINGS

Archean rocks overlain by undeformed Proterozoic sediments consisting in ascending order of carbonates, clastics, iron formation, and basalt which have been correlated to the Nastapoka sediments found on the Hudson Bay Arc (Bostock, 1971; Chandler, 1984). Further similarities with the Hudson Bay Arc come from the distribution and attitudes

A N D M.E. B R O O K F I E L D

of the Proterozoic sediments and its relationship to the Archean rim. As in the Hudson Bay Arc, the Proterozoic sediments of the Sutton Arc are contained to the Belcher Basin side of the arc and dip away from the rim toward the centre of the Belcher Basin (Bostock, 1971; Riley, 1979). This implies that the elevated Archean surface acted as a

HUDSON BAY

SUTTON ARC WlNISK FA ~SYSTEM

TERRANE

i

Fig. 6. Magnetic map of Superior Province as it appears today (Gupta, 1991).

"1OO K

159

PROTEROZOIC TRANSCURRENT MOVEMENTS

barrier to the seas as did the rim around the Hudson Bay Arc. In summary, the interpretation of the Sutton Arc as an extension of the Hudson Bay Arc is based on the following similarities: (1) both fit into the trace of the same circle;

(2) both are bordered by an elevated Archean rim; (3) both are unique to the otherwise "normal" geomorphology of the area; (4) the rim appears to have acted as a barrier to the seas which deposited the overlying undisturbed Proterozoic sediments; and,

HUDSON BAY ARC

TERRANE

;ze -~-

o

lOO KM

Fig. 7. Magnetic map of Superior Province before movement along the Kapuskasing Structural Zone.

160

(5) the overlying sediments dip toward the centre of the Belcher Basin. These similarities imply that Sutton Arc and the Hudson Bay Arc once formed a continuous curved rim over 833 km in length around the Belcher Basin before they were displaced.

Faults related to the displacement of the Sutton Arc from the Hudson Bay Arc The simplest movement required to replace the Sutton Arc back onto the circle of the Hudson Bay Arc would be to move it toward the circle trace in a straight line. This direction of movement is shown by the arrowed lines in Fig. 3. Strike-slip faults which could have produced this movement would therefore be parallel to these lines. No known faults in the immediate area satisfy this condition nor does the trend of the Winisk River Fault complex provide the necessary displacement, therefore the search for compatible faults was extended to a larger area. Such a search revealed that the KSZ is parallel to the required movement needed (see Fig. 3). Thus, the KSZ was investigated, to see if any displaced lineaments along the KSZ would support the strike-slip movement required. The regional geology of the area surrounding the KSZ is not well mapped, therefore aeromagnetic maps were used to identify any lineations which would have been cut by the proposed strike-slip movement along the KSZ.

Displacement of aeromagnetic anomalies across the Kapuskasing Zone Further evidence for lateral motion between the West and East Superior terranes comes from aeromagnetic anomalies which terminate against the KSZ. Figure 6 shows the KSZ and Sutton Arc as they appear today. The reconstruction in Fig. 7 was made by realigning the Sutton Arc back onto the trace of the circle extended from the Hudson Bay Arc. It shows that four separate anomalies appear to match up in the reconstruction. These are anomalies X and Y and the

C.R. GOOD1NGS

A N D M.E. B R O O K F I E L D

north-trending Matachewan dykes (M) and north to northeasterly-trending Hearst dykes (H). Poor exposures and lack of field investigations have resulted in a lack of ground control for interpretation of the magnetic features X and Y. Therefore, correlation of these anomalies across the KSZ is based on a similarity of magnetic intensity and trend.

Anomaly X Anomaly X is an east-west-trending magnetic anomaly with a magnetic intensity of about 225 gammas that is interpreted to extend from the west side of the KSZ to the east side where it branches into two magnetic anomalies as shown in Figs. 6 and 7. On the west side of the KSZ, this magnetic anomaly follows the Phanerozoic/Archean boundary along the southern edge of the Moose River Basin and is mostly overlain by Phanerozoic rocks. It is exposed for a short section to the immediate west of the KSZ and here Skinner (MacLaren et al., 1968) noted that it was underlain by Archean granite gneiss. The anomaly resembles other east-west-trending magnetic anomalies in the Superior Province and therefore is likely an Archean structure. Card (1983) referred to the southern edge of this anomaly as marking a major easterly trending fault. West of the Phanerozoic cover, he observed cataclastic foliations in the Archean gneisses and concluded that this fault had probably been active both in the Precambrian and Phanerozoic. Card and Ciesielski (1986) interpreted this fault as a strike-slip fault extending from its termination at the KSZ west to the Sydney Lake Fault. The contact of this east-west-trending anomaly with the KSZ appears, from the magnetic map, to be a north-trending fault east of which are granulities of the KSZ belt.

Anomaly Y The correlation of this anomaly across the KSZ is based on a similarity in trend and magnetic intensity. Nothing is known about its geological significance. Percival and Mc-

PROTEROZOIC

TRANSCURRENT

161

MOVEMENTS

LEGEND

..,..b.,"Proterozoic, Phanerozolc rocks J Suborovince boundary ARCHEAN SUBPROVINCETYPE ~ ]

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Fig. 8. Proposed reconstruction of Superior Province before movement of the East and West Superior terranes (modified from Card and Ciesielski, 1986).

Grath (1986) noted only that west of the KSZ the Lepage Fault has cut this anomaly. The Lepage Fault is interpreted as a normal fault related to the movement of the Kapuskasing Zone. As shown in the reconstruction in Fig. 7, this anomaly with a magnetic intensity around - 2 5 to 25 gammas extends in a broad arc west of the KSZ across to the east of the KSZ. This anomaly appears similar in trend and intensity to other Archean anomalies in the Superior Province and is therefore likely Archean in age.

Dyke displacement Two sets of dyke swarms are interpreted in this report to have been offset by the Kapuskasing Zone. These are the n o r t h south-trending Matachewan dyke swarm and the north to northwest curved Hearst dyke

swarm (Ernst and Halls, 1984)• Ernst and Halls (1984) examined the Hearst dyke swarm to determine if they were correlatable with the Matachewan dyke swarm. They concluded that the Hearst dykes may be part of the same swarm or they may have had a slightly different time of emplacement. As shown in Fig. 7, the north-trending Matachewan dykes continue north on the west side of the KSZ and the north west-trending Hearst dykes continue west of the KSZ in a broad arc. Watson (1980) proposed that the dykes were emplaced after and during sinistral motion. However, Fig. 7 shows that they were displaced by the KSZ and therefore the KSZ is younger than the dykes. The age of the dykes is still uncertain• The Matachewan dyke swarm have in imprecise R b - S r age of 2633

162

+_ 186 Ma (Gates and Hurley, 1973) but Hanes et al. (1988) considered these dykes to be too extensively altered to yield a precise age of intrusion. The Hearst dyke swarm gave a 4°Ar/39Ar age of 2367 _+ 28 Ma which Hanes et al. (1988) interpreted to be the minimum age of emplacement.

Offset of the Superior Province sub-provinces An important check for this model comes from the boundaries of the Superior sub-provinces. In the reconstruction shown in Fig. 8 with the Sutton Arc realigned back into the Hudson Bay Arc, the boundaries of the Q u e t i c o / W a w a and Opatica/Abitibi match up slightly better than they do in their present positions. While Fig. 8 does not validate the model, it does not invalidate it either. Rather, it suggests that the mapping of these sub-province boundaries in this poorly exposed area are is not yet complete.

Amount and timing of displacement The distance measured parallel to the trend of the Northern KSZ between the Sutton Arc's present position and its proposed original position and between the displaced magnetic anomalies shows a left-lateral displacement of 160 km. In addition, the angle between the Sutton Arc's present position and its proposed original position indicate that, during this lateral motion, the Sutton Arc was rotated counterclockwise 5 °. Therefore, in a regional context, this implies that there has been 160 km of left-lateral motion parallel to the KSZ with an accompanying 5 ° of anticlockwise rotation between the terrane which enclose the Sutton Arc and the magnetic anomalies west of the KSZ, here called the West Superior Terrane and the terrane which enclosed the Hudson Bay Arc and the magnetic anomalies east of the KSZ here called the East Superior Terrane. Whether this amount of displacement represents the net amount of movement which has occurred since movement began depends

C.R, G O O D I N G S AND M.E. B R O O K F I E L D

on whether the displaced features identified in this study were formed before the onset of movement. The earliest age of movement at the boundary between the East and West Superior Terranes is not known with certainty. However, James Bay, which is here proposed to mark the boundary in the north, appears to have rifted during the Proterozoic (see section on tectonic elements). The age of the displaced features ranges from Archean, the X and Y anomalies, to Proterozoic, the dykes and circular ridge. Since the Archean features have been displaced the same amount as the Proterozoic features, the 160 km of left-lateral movement is probably the net amount of movement which has taken place along this boundary. No information is available from these displacements to determine whether the movement was constant or took place in discrete steps or even whether the movement took place in one direction. It is possible, for example, that there has been both left-lateral and right-lateral movement. To determine the history of the movement would require the identification of features successively younger than the onset of movement. These features will display less and less offset through time. In addition, features must be identified which cross the KSZ or the James Bay rift which are not displaced. The oldest of these will give the minimum age after which no movement took place.

Type of displacement Although the regional picture can be characterized as left-lateral movement between the East and West Superior terranes, the kinematics at the boundaries along which this movement took place can be divided into two settings: strike-slip and extension or rifting. Along the boundary marked by the KSZ, the magnetic lineaments which intersect with the KSZ have been displaced parallel to the KSZ. In this region, the boundary can be defined as a zone of strike-slip and therefore

163

PROTEROZOIC TRANSCURRENT MOVEMENTS

the KSZ can be classed as a strike-slip fault. The Sutton Arc has also beell displaced parallel to the KSZ, however it does not intersect with the KSZ since the KSZ ends at the southwest end of James Bay. Rather, the Sutton Arc is separated from the Hudson Bay Arc by James Bay. It is proposed, therefore, that James Bay marks the boundary between the East and West Superior terranes. Geometrically, the only way that the Sutton Arc can be displaced from the Hudson Bay Arc in a direction parallel to the KSZ is by left-lateral extension in the region now marked by James Bay. Therefore, James Bay is interpreted to

56"

Fig. 9. Bouger gravity anomalies for Hudson Bay and James Bay. (dot pattern > - 3 0 mgal.) Gravity data extracted from Earth Physics Branch, 1982.

mark a site of rifting or extension, formed as a result of the left-lateral movement between the East and West Superior terranes. This argument is based on geometry. However, there is structural evidence which supports the model of James Bay as forming due to rifting. TECTONIC ELEMENTS

The direction of movement as defined by the displaced features (see previous section) along the KSZ trend require that three tectonic elements should demonstrate distinct structural features; these are: James Bay and the Northern and Southern KSZ. James Bay is oriented at an angle to the direction of movement such that extension must have occurred in this zone. The Northern KSZ is oriented parallel to the direction of movement and therefore strike-slip is expected. The Southern KSZ is oriented at an angle to movement such that compression would have accompanied strike-slip and therefore is interpreted to be a restraining bend. The next section presents structural evidence which, in addition to the displaced features already discussed, lends further support to the regional model.

Previous interpretations of James Bay Innes et al. (1968) produced the first complete gravity survey of the Hudson Bay. One of the distinct features revealed from that survey was the belt of positive gravity anomalies that trend south from west of the Ottawa Island through the Belcher Basin to the northwest shore of James Bay where it extends inland 100 km up to the Sutton Inlier (Fig. 9). Noting that the axis of this gravity anomaly is associated with volcanics in the Belcher Basin and that it roughly follows the Paleozoic/Precambrian boundary which extends to the south end of James Bay, they proposed that the gravity anomaly and the Paleozoic/Precambrian boundary marked an ancient rift zone extending from west of the

164

C.R. GOODINGS AND M.E. BROOKFILLD

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Fig. 10. Proposed reconstruction showing the formation of James Bay as a result of rifting due to sinistral movement between the East and West Superior terranes. (a) James Bay before rifting with the Sutton Arc falling on the trace of the circle extending from the Hudson Bay Arc. The solid line centered in James Bay marks the proposed location of the rifting centre. (b) James Bay after rifting with the offset of the Sutton Arc. As predicted by the theoretical rifting model (see solid lines in James Bay), the southeast end of James Bay is parallel to the KSZ. The Paleozoic/Precambrian contact marks the east side of James Bay and Akimiski Island (and the pop-up structure, see crosssections in Figs. 10 and 11) the west side of James Bay. These indicate faulting along the margins of James Bay interpreted as resulting from extensional faulting during rifting.

Ottawa Islands south through the Belcher Basin to the south end of James Bay where it joined up with the KSZ gravity anomaly. Burke and Dewey (1973) also interpreted the James Bay as rift zone, citing as evidence that the bay and the KSZ strike at a high angle to the Belcher Basin gravity anomaly, the resulting geometry resembling a triplepoint junction. Structural evidence that James Bay has been tectonically active comes from the north-south-trending faults which cut through the sediments on Bear Island at the mouth of James Bay (reported in Bostock, 1971). These sediments have been correlated by Bostock (1971) with the Nastapoka Group. MacLaren (MacLaren et al., 1968)

proposed that these faults related to a northward extension of the KSZ along which strike-slip motion had occurred.

150I~

AKIMI~I IS

"°°I "450 "600 J N

km Nl

Fig. 11. Seismic section across James Bay Lowlands to Akimiski Island showing rise in the Archean surface under the island. The Archean high is interpreted to be the result of rifting along the margins of James Bay (modified from Hobson, 1968). Location of section N - N ' shown in Figs. 10 and 14.

PROTEROZOIC

TRANSCURRENT

165

MOVEMENTS

The James Bay Rift The model for its formation is shown in Fig. 10. Figure 10a and b were constructed using the Sutton Arc as a guide for the amount and direction of extension expected. The assumptions made in this construction are (1) that the proto-James Bay was parallel to the present James Bay, and (2) that rifting and separation started in the centre of the Bay (see solid line in centre of James Bay in Fig. 10a). The Precambrian geology underlying James Bay is poorly known due to cover by Paleozoic rocks and the waters of James Bay. As a result, geophysical surveys must be relied upon for information about the Precambrian structures. As a guide for the expected features underlying James Bay, other

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Fig. 12. a) Gravity profile across the west shore of James Bay. The pop-up structure is interpreted to have been caused by strike-slip movement and compression which occurred along the west coast of James Bay during rifting (location of section shown in Figs. 10 and 11) (see text for more details, from Percival and McGrath, 1986). b) A similar pop-up structure found along the right lateral strike-slip Riconada fault in the San Andreas fault system (from Dibble, 1976).

rifts formed in relation to strike-slip faults such as the Gulf of California/San Andreas Fault, are used here. According to these rifts, the east and west margins of the proposed James Bay Rift should be uplifted relative to centre of the rift zone which should form a rift valley. In addition, the rift margins can be elevated above the regions flanking the rift (Buck, 1986). That there is a change in elevation in the Precambrian surface along the east side of James Bay comes from the location of the Precambrian/Paleozoic boundary as mapped by Grant (1968) from seismic surveys. This boundary marks the eastern extent of the Paleozoic and as shown in Fig. 10b is in close proximity and runs parallel to the eastern margin of the theoretical rift especially in the southern end of James Bay (see solid line on east side of James Bay in Fig. 10b). It is proposed that the Paleozoic/Precambrian boundary indicates that the Precambrian surface to the east is elevated above the area to the west. Such a change in elevation is predicted by the rift model and therefore is supporting evidence for this model. A seismic section across the western boundary (see section N - N ' , Fig. 11) showed that the Paleozoic thins over Akimiski Island. Hobson (1968) suggested that the island marks the site of a Precambrian high. Such an uplift could mark the uplifted rift shoulder and therefore supports the model. Further to the south (section 0-0', Figs. 10 and 12a) along the western margin, Percival and McGarth (1986) modeled an uplifted pop-up structure over an intense gravity anomaly (see Fig. 9) that extends from the KSZ. They proposed that this was formed due to compression related to their model of compression for the KSZ. This could also be interpreted as resulting from uplift of the western margin during the proposed rifting similar to the uplift further north at Akimiski Island. However, the location of this pop-up structure close to the intersection of the

166

C.R. GOODINGS

r/, •

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K S Z / J a m e s Bay and the trend of gravity anomaly from the KSZ, suggest the possibility that it is not formed by rifting alone but also due to strike-slip motion related to the KSZ which squeezed and uplifted the zone between strike-slip fault (Fig. 12b). As shown in the structural map of the Gulf of California in Fig. 13a, in addition to normal faults, strike-slip faults extend up the east side of the Gulf of California. In the case of James Bay, the strike-slip faults may extend up the west coast. Therefore, it is possible that the gravity anomaly which overlies this pop-up structure and splays off from

the main gravity anomaly of the KSZ and the associated magnetic anomaly (Figs. 6 and 7) which also tends off from the main magnetic anomaly of the KSZ and follows this gravity splay may mark the site of a horsetail splay (as shown in Fig. 14). A horsetail splay is defined as "a set of curved faults-splays near the end of a strike-slip fault that merge with that fault: The set forms an array that crudely resembles a horse's tail" (Biddle and Christie-Blick, 1985). Supporting the proposal that the pop-up structure may be related to squeezing strike-slip fault activity in this location is shown in Fig. 12b. This is a cross-

PRO'FEROZOIC

TRANSCURRENT

167

MOVEMENTS

% \

~-,,

//

\

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WEST SUPERIOR TERRANE

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/~"

SOUTHERN KSZ

LAKE SbRERIOR

i

km

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Fig. 14. Idealized structural interpretation of the KSZ based on magnetic anomalies intended to show the general structure of the Northern and Southern KSZ and the strike-slip basin. Also shown is the 25 regal.gravity contour (fine dashed line). Note that the gravity anomaly crosses to the east of the KSZ in the region of the basin. Arrows show the direction of proposed motion. W - Wawa; C - Chapleau; K Kapuskasing (based on a compilation of magnetic anomaly interpretations by Bennet et al., 1967; MacLaren et al., 1968; Percival and Card, 1983; Percival and McGrath, 1986).

section of uplift found along the right lateral Riconada fault in the San Andreas Fault system. This pop-up structure was formed due to convergent strike-slip and is similar to the pop-up structure modelled by Percival and McGrath (1986). Further support for the rift model comes from the trend of the southeast end of James Bay. As shown in Fig. 10b, the trend of the southeast end is parallel to the trend of the theoretical rift's southeast end which follows

the trace of the KSZ. Crowell (1974) referred to the end of a rift as a half fault (see Fig. 13b) because only one previously existing wall or side of the displaced terrane is preserved. Rifts typically have a spreading ridge with associated volcanics that mark the rift valley floor and usually produce a positive gravity anomaly. Basalts have been identified overlying the Proterozoic sediments on the Sutton Arc (Bostock, 1971) and are correlated with the gravity anomaly which extends from the arc into the northern end of James Bay and north through the Belcher Islands (Innes et al., 1968; Gibb, 1983) (see Fig, 9). The volcanics and the gravity anomaly over the Sutton Arc have been interpreted as rift-related by Innes et al. (1968) and Barager and Scoates (1981) and as collision-related by Gibb (1983) and Ricketts and Donaldson (1981). The volcanics at the Sutton Arc correlate with the theoleiitic Flahrety Formation which overlies sediments on the Belcher Islands and with basalts which overlie the Nastapoka sediments around Hudson Bay Arc (Bostock, 1971; Ricketts and Donaldson, 1981). Although still under debate as to the origin of the Flahrety Formation, the basalts overlying the Nastapoka sediments have flood basalt affinities (Ricketts et al., 1982). In terms of the model proposed here for the rifting of James Bay, support is given to the rift-related origin of the volcanics at the Sutton Arc. This has implications as to the origin of the Flahrety Formation and will be discussed in the section on the Belcher Basin in a later section. The lack of a positive gravity anomaly in the southern part of James Bay is problematic. One possible explanation for this may be that crustal extension here was accommodated without volcanic flows and intrusions at depth. As Crowell (1974) noted, the crust is easily folded, faulted, stretched, and therefore some basins probably form in a broad transform system without volcanicity. Such extension is consistent with present rifting models and is termed passive rifting (Buck, 1986). Passive rifting is driven by stress trans-

168

mitted by the mechanically strong lithosphere which produces extension first which may or may not be followed by volcanism. For example, the Red S e a / G u l f of Aden rift system has recently been interpreted as a passive rift formed without extensive volcanism or doming driven by tensile stresses associated with the northwestward-propagating central Indian Ridge spreading system (Hempton, 1987). The source of the stress needed to rift James Bay could have been from the tectonic activity which produced the gravity anomaly and volcanics at the northern end of James Bay. Evidence that there may have been some volcanic activity during rifting at the southern end of James Bay comes from two diamond drill holes on the southwest side of James Bay. In one drill hole at 51045 ' N latitude, 80 ° 40' W longitude about 2.5 m of gabbro-diabase and brown sandstone underlie the Paleozoic (Hogg et al., 1953). The total thickness of this sequence is not known because drilling was stopped before the Archean was reached. In another at 50 ° 20' N latitude, 81050 ' W longitude 45 m of amygdaloidal basalt and red argillite occur between the Paleozoic strata and Archean basement gneisses (Satterly, 1953). Interpretation of these rock types as being rift-related is highly speculative because their ages are unknown. Nonetheless, the basalts could be rift volcanics and the sandstone and red argillite could be rift-related sediments. If volcanics are present at the southern end of James Bay in great abundance, then the expected associated positive gravity anomaly may be depressed by low-density sediments which may have accumulated in the newly formed rift valley. Thick accumulations of sediments are a common feature in rifts formed in continental settings (Burke and Dewey, 1973; Crowell, 1974). For example, during the opening of the head of the Gulf of California in the Salton Trough, sediments reached a vertical thickness of 6000 m (Crowell, 1974). In summary, the evidence for a rift origin

C.l~. G O O D 1 N G S

A N D M.E. B R O O K F I E L D

of James Bay, in addition to the displacement of the Sutton Arc are as follows: (1) The east coast of James Bay appears to be elevated relative to the area west of it as indicated by the eastern discontinuance of Paleozoic strata; (2) The west coast of James Bay appears to be elevated as indicated by the thinning of the Paleozoic strata over Akimiski Island and the pop-up structure at the southwest end of James Bay; (3) The southeast end of James Bay is parallel to the trend of the KSZ as predicted by the rift model; (4) Volcanics and a positive gravity anomaly mark the north end of James Bay and may represent rift-related volcanics; and, (5) Two drill holes at the southwest shore of James Bay penetrated volcanics and sediments underlying the Paleozoic strata that may be related to rifting.

Timing of rifting If the volcanics and the gravity anomaly at the north end of James Bay were produced during the rifting of James Bay, then the age of the volcanics would give an age of rifting. Unfortunately, no accurate radiometric ages exist in this part of the shield. However, the volcanics overlie the Proterozoic sediments on the Sutton Arc and therefore a relative age of rifting would be post-sedimentation or Proterozoic. This is supported by northsouth faults which cut correlated Proterozoic sediments on two small islands at the northern end of James Bay (Bostock, 1971). If these faults were formed due to rifting, then the faults also indicate a post-sedimentation age of rifting.

The Kapuskasing Structural Zone Two features distinguish the KSZ from the surrounding Superior Province. These are: (1) the dominant northeast structural grain in the Northern KSZ and northern part of the Southern KSZ which cuts across the

PROTEROZOIC TRANSCURRENT MOVEMENTS

regional east-west trend of the Superior Province; and, (2) uplifted granulites to upper amphibolite facies rocks. In this section the structural grain is discussed and then the types of uplift are discussed and placed in context with the model proposed in this paper. The north-east structural grain Important supporting evidence that part of the movement of the KSZ was sinistral strike-slip comes from the northeast structural grain imposed on the rocks within the zone. The northeast structural grain, as defined by the orientation of gneiss bands and lithological contacts, was formed by ductile shear at depth which warped the structures into northeast trends (Watson, 1980; Percival and Coe, 1981). The direction of this shear movement is given by the deflection of the aeromagnetic striping over the granulites (MacLaren et al., 1968; Watson, 1980) which indicate sinistral motion. These features also indicate that transcurrent movement along the Northern and northern part of the Southern KSZ was not pure strike-slip but that the motion between the east and west terranes was slightly convergent. This is not surprising as pure strike-slip systems are rare and almost all involve some degree of perpendicular motion (Reading, 1980). Strikeslip motion which is slightly convergent is termed transpression (Harland, 1971) or convergent strike-slip. Vertical movements related to the KSZ In addition to the prominent northeast structural grain, the KSZ lineament is marked by granulite and upper amphibolite facies rocks uplifted by reverse and thrusts faults (Percival and Card, 1983; Percival and McGrath, 1986). According to Percival and McGrath (1986), the uplift was caused by east-west or northwest-southwest compression. However, the information presented here indicates that there has been a considerable amount of sinistral strike-slip move-

169

ment along the KSZ and further that this sinistral strike-slip motion was transpressive. Transpressive regimes are marked by folding, thrust faulting, reverse faulting and uplift and subsidence (Reading, 1980). Therefore, it is necessary to review the vertical movements of the KSZ and compare them to vertical movements found in transpressive (convergent strike-slip) regimes. That vertical movement accompanies transcurrent movement was shown by the clay models of Wilcox et al. (1973). In simple strike-slip motion, where the clay blocks were moved parallel, compressional and tensional stresses were generated in the zone of movement. When the blocks were made to converge (transpression) or diverge (transtension) slightly, the compressional or tensional stresses were enhanced and in strong transpression in these models revealed a complex thrusting of clay wedges squeezed up and out of the strike-slip zone. These wedges were bounded by vertical or high-angle reverse faults that resembled upthrust blocks. In addition to vertical movements caused by transpression and transtension, uplifts and thrusting will develop at restraining bends or fault junctions and subsidence will develop at releasing bends, fault oversteps or fault junctions. Both of these features can develop d u r i n g t r a n s p r e s s i o n or t r a n s t e n s i o n (Crowell, 1974; Reading, 1980; Mann et al., 1983). The identification of vertical movements along Phanerozoic lineaments is relatively easy because the resulting areas of topographic uplift and subsidence are still present. In ancient lineaments such as the KSZ, erosion has removed these features and other methods must be used to identify areas which once had differential relief. Percival and Card (1983, 1985) and Percival and McGrath (1986) were able to determine the vertical displacements along parts of the KSZ from geobarometry. This method makes use of mineral assemblages, now at the surface, which indicate metamorphic pressures. The pressure can be related to depth of metamor-

170

C.R. GOODINGS

phism and therefore into the amount of erosion which has occurred. The resulting values can then be used to construct models of vertical movement along the zone of interest. In the following discussion, the uplifts along the KSZ are discussed and then compared to the type of uplift expected in the model proposed in this paper. As shown in Fig. 14 the Northern KSZ is parallel to the slip vector• In these areas, uplift is predicted by the model to be a result of transpression. Further south where the fault trace swings toward Lake Superior placing it at an opposing angle to movement, crust must be consumed to accommodate motion (see Fig. 7) and therefore, in this region, uplift should be the result of thrusting caused by strike-slip.

zone is interpreted as being bounded by faults on the east and west which show cataclastic deformation (Bennet et al., 1967; MacLaren et al., 1968; Thurston et al., 1977; Percival and Card, 1985; Percival and McGrath, 1986) with fault slices also occurring within the zone (Bennet et al., 1967; MacLaren et al., 1968). Rocks in the zone have a granulite metamorphic grade and structural trends are variable from northeast to northwest and dips are generally steep (Percival and McGrath, 1986). Based on geobarometry, Percival and McGrath (1986) concluded that the zone was at least 10 km above the area to the west and more than 10 km above the area to the east in terms of depth of erosion within the zone. Neither the east or west boundary faults have been modeled from gravity nor magnetics to determine the amount or direction of dip. Percival and McGrath (1986) proposed that the east fault is a moderately dipping thrust fault and the west fault is a steeply normal fault• The interpretation of the east fault is based on an assumed similarity with the east boundary fault in the Chapleau area further

Vertical movements along the Northern K S Z The Northern KSZ is approximately 10-20 km wide and is marked by a distinct aeromagnetic anomaly which trends from James Bay to latitude 49 ° 45'. This is the area which Percival and McGrath (1986) referred to as the Frazerdale-Moosonee Block. Based on the magnetic anomaly and field surveys, the

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Fig. 15. a) Sketch of proposed uplift of the Northern KSZ. Uplift here is interpreted to be the result of convergent strike-slip between the East and West Superior terranes which caused the KSZ to be uplifted out of the shear zone. See Fig. 14 for location of Section P - P ' (modified from Percival and McGrath, 1986). b) Conceptual model of uplift due to convergent strike-slip (from Sylvester and Smith, 1976).

171

PROTEROZOIC TRANSCURRENT MOVEMENTS

south which has been modeled by seismic to be a moderately northwesterly-dipping thrust. However, in the Chapleau area, the structures within the zone also dip moderately to the northwest whereas in the Northern KSZ, the structures are generally steep. According to Daly (1986), the structures in a shear zone will parallel the shear planes which in the KSZ, may be represented by the faults along which uplift occurred, (i.e., the bounding faults and internal faults). This relationship holds true in the Chapleau area, therefore the implication for the Northern KSZ is that the east fault is not a moderately dipping structure but a steeply dipping structure similar to the west bounding fault. For this reason, the Northern KSZ is shown in Fig. 15a as a uplifted block with steeply dipping bounding and internal faults. In the model proposed by Percival and McGrath (1986), this zone was first thrust up along the east fault above the terrane to the east and then, due to gravitational collapse, the terrane to the west was downfaulted along the west fault which left the Northern KSZ as a perched thrust tip. A simpler model of uplift would be that the zone was uplifted as a central block simultaneously along the east and west faults. If this is the case, then the uplift of the Northern KSZ resembles the type of basement uplift found in convergent strike-slip zones, a conceptual model of which is shown in Fig. 15b. According to Sylvester and Smith (1976), in convergent strike-slip shear zones, the basement will be sheared and compressed between the two laterally and convergently slipping blocks, so that it will be uplifted out of the shear zone. In the convergent strike-slip model, the Northern KSZ is interpreted as marking a sheared and compressed zone with northeast structural fabric which was uplifted as the East and West Superior Terranes slipped past one another. Another example where strike-slip has possibly uplifted the basement is Great Slave Lake Shear Zone. This shear zone has recently been interpreted as a

transform fault formed as a result of oblique collision between the Slave and Churchill Province (Hoffman, 1987). The Great Slave Lake Shear Zone similar to the Northern KSZ, is a 25 km wide ductile shear zone of granulites to greenschist facies rocks with vertically dipping structures (Hammer and Lucas, 1985). Other examples where transpression has been inferred include the Great Glen Fault of Scotland which reveals uplifted basement slivers and the Frazer Fault of Western Australia which is marked by linear outcrops of granulites (Watson, 1980).

Southern KSZ The Southern KSZ is defined here as that part of the KSZ that swings in a broad arc toward Lake Superior (see Fig. 14 for location). It includes the Groundhog River Block and the Chapleau Block of Percival and McGrath (1986) and extends to Lake Superior (Card, 1979; Corfu, 1987). As shown in Fig. 7, the crust here must have been consumed to accommodate the sinistral motion between the West and East Superior Terranes. Two models of uplift are shown in this region; one across the KSZ in the Chapleau area and another further south at Lake Superior (see Fig. 14: sections Q - Q ' and R R'). The geology and proposed uplifts will be discussed separately but the interpretation of how they fit into the model will be discussed together since they both are interpreted as marking a restraining bend in the boundary between the East and West Superior Terranes.

Uplift in Chapleau area (Fig. 16) The geology of the KSZ in the Chapleau area is dominantly orthogneiss including the Shawmere anorthosite complex and minor paragneiss and mafic gneiss (Bennet et al., 1967; MacLaren et al., 1968; Thurston et al., 1977; Percival and Card, 1983, 1985). Metamorphism grade is mostly upper amphibolite with four small areas exhibiting granulite facies (Percival and Card, 1985). Structural trends strike northeast and dip 20 ° and 40 °

172

to the northwest (Bennet et al., 1967; MacLaren et al., 1968; Thurston et al., 1977). The area is marked by a broad magnetic anomaly of comparatively moderate intensity and by a positive gravity anomaly. The boundary with the Abitibi Belt on the east is marked by a cataclastic zone (the Ivanhoe Lake Cataclastic Zone) and an abrupt transition from the high grade metamorphic rocks west of the fault to predominately low grade greenschist facies east of it (Percival and Card, 1985). In contrast, the boundary with the Wawa Belt on the west is gradational except in the north where it may be in fault contact (Percival and Card, 1985). The gradational boundary is identified where the complex structures of the Wawa Belt are warped into the northeast-striking gently northwest-dipping structural trends characteristic of the KSZ (Percival and Coe, 1981). Percival and Card (1983) mapped the changes in metamorphic grade over a 120 km wide west-east traverse. They found a difference in erosion level of at least 20 km from the low-grade Michipicoten Greenstone Belt in the west to the high grade granulites in the Southern KSZ in the east. They concluded that this difference in erosion level is the result of thrusting along the northwest dipping fault marked in the Ivanhoe Lake Cataclastic zone. Further confirmation of this interpretation came from Cook (1985) who ran a seismic reflection profile across the Ivanhoe Lake Cataclastic Zone. This profile revealed a 3 5 ° - 3 8 ° west-northwest-dipping thrust fault predicted by Percival and Card (1983).

C.R. GOODINGS

Wakuslml River Fault

AND

M.P2. B R O O K F I E L D

Ivanhoe Lake Cataclastlc Zone

-~"-;ccc',;'.-.~.'c

. k ' . " "3" ' L 5 ~-~

Q

Q'

Fig. 16. Idealized cross-section showing uplift of the KSZ north of Chapleau. The Wawa Belt is interpreted to have been thrust over the Abitibi Belt due to a restraining bend in the KSZ. It is proposed that uplift here was the result of thrusting and sinistral strike-slip. See Fig. 14 for location of section line Q - Q ' . (Modified from Percival and Card, 1983; Percival and McGrath, 1986; Halls, 1988).

Zone divides distinctively different terranes (Card, 1979; Grunsky, 1981; Corfu, 1987). The Montreal River Fault (MRF) marks a major change in the topography of the area. North of the fault the terrane is extremely rugged with relief commonly 100-150 m with many vertical cliffs faces, while south of the fault the area tends to be hummocky to swampy (Grunsky, 1981). The fault also separates terranes of different metamorphic grade. North of the fault the rocks are metamorphosed to upper amphibolite facies (Grunsky, 1981; Corfu, 1987). This terrane is known as the Agawa Migmatitic Terrane (Card, 1979; Corfu, 1987). South of the fault Montreal River Fault J

Rr

Uplift of the KSZ at Lake Superior (Fig. 17) The southern extension of the KSZ is not well defined because the high grade gneisses and distinct geophysical anomalies end at Chapleau. However, the east bounding fault is interpreted to extend to Lake Superior where it is called the Montreal River Fault. This fault like the Ivanhoe Lake Cataclastic

R

Fig. 17. Hypothetical cross-section showing uplift along the Southern KSZ at Lake Superior. The Southern KSZ is interpreted as marking a restraining bend in the KSZ. According to the model, at this location 80 km of crust must have been consumed due to sinistral motion between the East and West Superior Terranes. To accommodate this motion, it is proposed that Wawa Belt was thrust over the Abitibi Belt by a low angle thrust fault, the Montreal River fault. See Fig. 14 for location of section line R - R ' .

173

PROTEROZOIC TRANSCURRENT MOVEMENTS

lies the Batchawana Greenstone Belt (Coffu and Grunsky, 1987). It is composed essentially of a supracrustal sequence of granodiorite and granite metamorphosed to upper greenschist facies with some areas of lower amphibolite facies. Foliation measurements indicate a slight change of structural patterns from south-east trending south of the fault to east trending north of it (Grunsky, 1981; reported in Corfu, 1987). Evidence of deformation along the M R F was only observed by G-runsky (1981) in one area where fault gouge, breccia and quartz veining occurred. Based on the increase in metamorphic grade north of the M R F and detailed geochronological studies, Coffu (1987) proposed that the area north of the MRF, the Agawa Migmatic Terrane represented a lower level of the Crust than the area south of the MRF, the Batchawana Greenstone Belt and therefore represents a similar structural relationship to the terranes east and west of the Ivanhoe Lake cataclastic zone further north of Chapleau (Fig. 15). However, the level of crust exposed west of the Ivanhoe Lake Cataclastic Zone correlates with a deep crustal level which Corfu called the lower mega-layer whereas the level of crust represented by the Agawa Migmatic Terrane north of M R F correlates with a crustal level above this or as Corfu proposed represents a crustal level between the lower mega-layer and the intermediate ~ega-layer. Therefore, the amount of uplift north of the M R F is less than the amount of uplift west of the Ivanhoe Lake cataclastic zone. One way to account for this would be if the Montreal River Fault has a gentler dip than the Ivanhoe Lake cataclastic zone. It is proposed therefore that the Agawa Migmatic Terrane which represents the West Superior Terrane has been thrust over the Bachawana Greenstone Belt which represents the East Superior Terrane by a low angle thrust marked by the Montreal River Fault (Fig. 17). This is supported by Grunsky's (1981) observation that the amount of dip-slip on the M R F is not great. It is also supported by general

decrease in dip of the east boundary fault as it trends south (see Figs. 15 and 16).

Cause of uplift in the Southern KSZ The uplift of the Southern KSZ shown in Fig. 16 section Q - Q ' was proposed by Percival and McGrath (1986) and Percival and Card (1983, 1985), to be the result of thrusting caused by east-west compression. An alternative interpretation presented here is that the area was uplifted due to left-lateral strike-slip motion along a restraining bend. A restraining bend is a bend in a strike-slip fault associated with overall crustal shortening and uplift in the vicinity of the bend (Biddle and Christie-Blick, 1985). Based on the model and shown in Fig. 7, the amount of crust consumed to accommodate the proposed motion varies from a few kilometres in the Chapleau Area to 80 km at Lake Superior. The Southern KSZ therefore is interpreted as a zone which underwent strike-slip but with extreme compression, (i.e., was an area of extreme convergent strike-slip). This differs from the Northern KSZ which is interpreted to have been an area of moderate convergent strike-slip where little or no crust was consumed. In extremely convergent strike-slip faults, the structural style of uplift becomes similar to that of a thrust (ChristieBlick and Biddle, 1985). Therefore, the type of uplift shown in Figs. 16 and 17 is consistent with that expected at a restraining bend. The difference between the two in terms of uplift and deformation can be accounted for by the amount of crustal shortening required which is a direct result of the angle between the fault trace and the direction of movement. In Fig. 16 the amount of crust which must be consumed to accommodate the proposed motion is 25 km measured from Fig. 7. Here, the fault trace is at a slightly greater angle to the direction of movement than in the Northern KSZ but less than that at the location of Fig. 17. As a result, the East and West Superior Terranes must overlap, thereby forming a thrust. In addition, the

174

angle allows the terranes to slide past one another. Therefore, it is expected that a strike-slip shear zone will form which is evidenced by the northeast structural grain and uplift will also be a result of shearing and compression which would cause the zone to be uplifted out of the shear zone, similar to the Northern KSZ. The model for uplift shown in Fig. 16 as determined by Percival and Card (1983, 1985) is consistent with that expected. The only change made to their model by this study is the addition of the labels toward and away to indicate movement perpendicular to the page. At the location of Fig. 17 (see section R - R ' ) the amount of crustal shortening predicted by the model is 80 km. This is a result of the fault trace being at a greater opposing angle to the direction of movement. Uplift here would therefore have been more due to dip-slip than strike-slip. This is consistent with the hypothetical uplift shown in Fig. 17. In such a situation, the development of a strike-slip shear zone is not expected and is not observed. T H E STRIKE-SLIP BASIN

In the area between the Northern and Southern KSZ there is a gap in the aeromagnetic anomaly (see Figs. 6 and 14). In this gap, granulites are absent and the area is underlain by amphibolites with sub-horizontal foliations. Percival and McGrath (1986) referred to this area as the Val Rita Block. This gap may indicate that the uplifted zones to the north and south are separate uplifts, however the gravity anomaly does extend through this zone indicating a continuum between the north and south zones. Percival and McGrath (1986) proposed that this zone was uplifted at the same time as the north and south zones but, due to gravitational collapse, was later downfaulted by normal faults. Another possible interpretation would be that this zone was never uplifted but was downfaulted as the areas north and south were uplifted. Simultaneous

C.R. G O O D I N G S

A N D M.E, B R O O K F I E L D

extension and compression is difficult to model in a pure compression regime but is a common feature along strike-slip faults due to deviations in the fault trace (Reading, 1980). This paper proposes that the granulite gap was formed during active strike-slip due to the presence in this location of a releasing fault junction, releasing bend or overstep in the fault trace. A releasing fault junction, releasing bend or overstep is a diversion along a strike-slip fault in which overall crustal extension will occur in the vicinity of this diversion resulting in subsidence and the development of a basin (Biddle and ChristieBlick, 1985). Diversions in a fault trace can result from many factors including (1) incompatible slip at a fault junction; (2) rotations within one or more adjacent blocks; or, (3) intersection of a strike-slip fault with a zone of greater extension or convergent strain (Christie-Blick and Biddle, 1985). Of these, number (3) seems most applicable here as the zone lacking uplift is found between the Northern KSZ which is interpreted as a zone of strike-slip and the Southern KSZ, which is interpreted as a zone of greater convergent strain. Supporting the interpretation of this zone as a strike-slip basin are the magnetic lineations which mark the boundaries of this zone shown in Fig. 14. These have been interpreted as faults and resemble the classic rhomb outline formed by boundary faults around other strike-slip basins. The arrangement of normal and reverse faults as proposed by Percival and McGrath (1986) around the basin is also consistent with other strike-slip basins in which uplift and subsidence has occurred on opposite ends and sides of the basin (Christie-Blick and Biddle, 1985). Also shown in Fig. 14 are magnetic lineations which trend north and intersect with the basin at the southern margin. It is proposed that they may be extensional fractures or faults formed in conjunction with the extension which formed the strike-slip basin. An additional feature of this area besides the

PROTEROZOIC TRANSCURRENT MOVEMENTS

lack of uplift is the spatial pattern of the positive gravity anomaly. This gravity anomaly for the most part is only present west of the KSZ, however, in this zone, it crosses over to the east. The significance of this deviation is not clear, but the spatial coincidence of this deviation with the proposed strike-slip basin suggests a possible genetic relationship. It must be remembered that strike-slip basins are complex structures and the model presented here must be considered as a tentative approximation to the real case. Nonetheless, it is a viable working model and should help to direct future investigation of this poorly exposed area. Of further mention, the area to the immediate southwest of the strike-slip basin, the Groundhog River Block of Percival and McGrath (1986) is included in the Southern KSZ but it is also similar to the Northern KSZ. This zone, like the area to the north, is underlain by dense granulites with associated intense magnetic anomaly and contrasts with the area to the south which has four granulite outcrops and a moderate magnetic anomaly (Percival and Card, 1983, 1985). However, unlike the area to the north and immediately to the south, a gravity anomaly is absent implying that it is not a deep structure. Percival and McGrath (1986) proposed that this area is a perched thrust tip. Given its position between the proposed strike-slip basin and the thrusting to the south it is suggested that this uplift marks the location of a complicated transition zone which caused the granulites to be uplifted and thrust onto the eastern terrane.

Ages of mouement for the KSZ A large proportion of the ages published for the KSZ have come from rocks which have been uplifted during movement of the KSZ. These dates do not provide a reliable age for uplift because they are most likely rejuvenated ages or ages of primary crystallization before uplift. The only meaningful ages so far obtained for the KSZ come from

175

carbonatite-alkalic rock complexes which are associated with the KSZ. The oldest age for a complex intruded in the KSZ comes from the Borden Lake Complex which gave a P b / P b age of 1872 + 13 Ma (Bell et al., 1987). This complex intrudes the KSZ about 21 km north of Chapleau and displays a roughly circular aeromagnetic anomaly. According to Sage (1979), the complex was probably emplaced within, or along, a fault and post emplacement faulting is noted on drill logs. The Cargill Complex is the oldest alkaliccarbonatite complex in the area but is emplaced outside of the KSZ about 25 km to the west of the proposed strike-slip basin. It gave a K - A r date of 1740 Ma (Gittens et al., 1967) and preliminary U-Pb-zircon dates of 1860 Ma-1907 Ma (reported in Percival et al., 1988). This complex has a distinct dumbbell shaped aeromagnetic signature with two major positive anomalies aligned roughly northeast-southwest, called the north and south complexes (Sandvik and Erdosh, 1977). The intrusion consists of carbonatite and ultramafic rocks hosted by Archean rocks. Sandvik and Erdosh (1977) suggested that the two sub-complexes were formed from displacement of a single intrusion due to dextral motion of the northeast-trending fault along which it was emplaced. Percival and McGrath (1986) named this fault the Lepage Fault and interpreted it as a normal fault formed after uplift of the KSZ. However, it is associated with granulites (Percival and McGrath, 1986) and so it may also have been active during the strike-slip movement of the KSZ. Percival et al. (1988) interpreted the ages of the Borden Lake Complex and the Cargill Complex as representing post-uplift ages. This does not necessarily mean that they are post-movement ages however as alkaline basalts and related intrusive are associated with divergent strike-slip motion (Woodcock, 1986). Therefore, the complexes could represent an age when sinistral movement was still occurring along the KSZ but instead of con-

176

C.R. G O O D I N G S A N D M.E. B R O O K F I E L D

CS8 1900? 185.07 L8

1890-1830

\

TB , / EAST SUPER/OR TERRANE WEST SUPERIOR TERF~NE

GP KSZ

0

300

I

i

KM LSA 1890-1830

Fig. 18. The Superior Province and bordering Proterozoic belts showing possible relationships with this paper's model and the deformation which has occurred in these belts. T B - Thompson Belt; C S B - Cape Smith Belt; L B Labrador Belt; G P - Grenville Province; H S - Huronian Supergroup; L S A - Lake Superior Association; K S Z Kapuskasing Structural Zone; FRB - Fox River Belt. verging the two terranes were separating. According to Reading (1980), transcurrent faults may change from transpressive regimes to transtensile regimes over a period of millions of years. They may also reflect local areas where extension was occurring simultaneously with compression elsewhere. In view of the evidence which suggests that there has been post-emplacement deformation in these complexes, it cannot be assumed that the ages 1907 Ma and 1872 Ma are ages representing the minimum age of movement along the KSZ. SUMMARY The spatial arrangement and type of vertical movements observed along the KSZ can

be accounted for by the angle between the direction of motion and the fault trace. In the north, the direction of motion is parallel to the fault trace. As the terranes moved past one another, the compression was intense yet confined to a relatively small zone. This resulted in the squeezing and vertical uplift of granulites from deep in the crust along steeply dipping faults out of the zone of stress. Further south, the fault trace swings toward Lake Superior. H e r e crust must be consumed as the angle between the direction of motion and the fault zone becomes increasingly oblique. In the Chapleau Area, uplift resulted from a combination of thrusting and transpressive stress with the west terrane thrusting over the east terrane as the two terranes moved past one another. In the

PROTEROZOIC T R A N S C U R R E N T MOVEMENTS

south at Lake Superior, 80 km of crust was consumed by a low angle thrust which brought the west terrane over the east terrane. Between these zones, a granulite gap is present which may mark the site of subsidence and a strike-slip basin.

Wider implications One of the significant results of this model is that it provides a tectonic link from the Belcher Basin to the Southern Province and indicates that the east and west terranes moved independently of one another. It is also clear that the proposed movement of these terranes would have had an effect on the events at the margins of the Superior Province. This section therefore examines the margins of the Superior Province to investigate what implications the model may have on previous interpretations for these areas. Also by examining the ages of movement along the borders, additional information can be obtained to better constrain the time of movements of the KSZ and James Bay. Figure 18 shows the ages of deformation so far obtained for the bordering Proterozoic belts and the possible relationship of this papers model with the type of deformation which has occurred in these belts.

Southern Prot~ince The Southern Province extends across the southern boundary of the West Superior Terrane and the southern boundary of the East Superior Terrane. Therefore, the Southern Province is in a position to record any Proterozoic aged differential movement between the East and West Superior Terranes. The discussion which follows therefore reviews the history of deformation south of the West Superior Terrane marked by the rocks south of Lake Superior, the Great Lakes Association, and the history of deformation south of East Superior Terrane marked by the Huronian Supergroup.

177

History of deformation along the southern margin of the West Superior Terrane Proterozoic sediments of the Lake Superior Association form a thin veneer on the southern edge of the West Superior Terrane and thicken to the south. The first deformation event recorded along this margin produced high grade metamorphism and deformation of the Archean basement with local intrusions of granite in the Felch trough area of Upper Michigan. The granite gave a R b Sr age of about 2000 Ma and a U - P b analysis on a zircon of about 2100 Ma (Van Schmus, 1976). The next phase of deformation affected the sediments south of Lake Superior and as well as the edge of the West Superior Terrane (Sims et al., 1980). This deformation event produced folds overturned toward the Superior Province as well as right-lateral strike-slip faults (Sims and Peterman, 1983). The dates for this orogeny, known as the Penokean Orogeny, came from igneous and metamorphic rocks of the Northern Wisconsin Migmatic Terrane formed during this tectonic event. These rocks have yielded zircon ages of 1930-1890 Ma (Van Schmus, 1980). A later event unrelated to the Penokean Orogeny produced a suite of rhyolitic volcanic rocks and associated high silica epizonal granite in southern Wisconsin dated at 1760 Ma (Van Schmus, 1978).

History of deformation along the southern margin of the East Superior Terrane Proterozoic sediments of the Huronian Group overlap onto the southern edge of the East Superior Terrane and thicken southward. The first deformation event recorded in these sediments produced major folds in the Espanola Sudbury area (Card et al., 1972). This event is bracketed by granite intrusions inplaced before and during folding and by Nippissing Diabase Dykes emplaced after the folding. This places the age of de-

178

formation based on R b - S r whole rock isochrons between 2165 M a - 2 1 1 3 Ma (Stockwell, 1982). The age of the next deformation which affected the Huronian Group is controversial. Based on the dates available, Stockwell (1982) attributed it to the Hudsonian Orogeny which culminated in the R b - S r time scale at 1785 Ma. Anderson and Burke (1983) based this deformation event on U - P b and R b - S r dates from numerous granitic intrusions in the Huronian Group south of the Murray Fault. They concluded that this second deformation event occurred between 1750-1550 Ma. Young (1983) and Sims et al. (1980) placed this second phase of deformation at the same time as the Penokean Orogeny (1890-1830 Ma). However, they included in this event the Sudbury Irruptive and various dates on metamorphic materials which Stockwell (1982) regarded as rejuvenated. If the Sudbury Irruptive is a meteorite impact site as the evidence indicates, then there seems to be no isotopic evidence for a orogeny between 1890-1830 Ma in the Huronian Group. Considering the times of orogeny presented above, the Penokean Orogeny appears to stand out as an anomalous event. Anderson and Burke (1983) considered this event as being important only to sediments of the Lake Superior Association, and Stockwell (1982) considered it an early phase of the Hudsonian Orogeny.

Possible cause of deformation Sims and Peterman (1983) proposed that the Penokean Orogeny could have resulted from forces transmitted from a remote distance to the southeast in an area undergoing rifting and subsequent collision. This would require a continuous basement of Archean or older Proterozoic rocks extending form the Southern Province to this continental margin. The problem with their proposal is: (1) there is no evidence for such a continuous terrane which they note; and,

C.R. G O O D I N G S

A N D M.E. B R O O K F I E L D

(2) it doesn't explain why only the rocks south of Lake Superior were affected and not, as it appears from isotopic dates, the area north of Lake Huron. A simpler explanation for the deformation and one which is predicted by the movements of the model presented here is that the West Superior Terrane moved south-east and collided with the rocks of the Southern Province. The West Superior Terrane is a known area of continuous terrane and its movement southwest, without the same movement of the East Superior Terrane as predicted in the model would only deform the rocks south of Lake Superior thereby satisfying the observed conditions. Further support of this comes from the right-lateral strike-slip faults formed during the Penokean deformation (see Fig. 18). They may be indent-linked strike-slip faults (Woodcock, 1986). Such strike-slip faults form during collision in zones of uplift and crustal shortening. If this deformation is the result of sinistral movement along the KSZ then it places the age of movement from 1890 to 1830 Ma. This great time span of movement, 60 million years, is also consistent with strike-slip movement which can occur over tens of millions of years (Woodcock, 1986). The problem with this age is that it is not in agreement with the ages given by the Cargill and Borden Lake complexes.

Belcher Basin The first tectonic event recorded in the Belcher Basin was the formation of the Richmond Gulf Graben (Chandler and Schwartz, 1980; Chandler, 1984). Chandler (1984) proposed that the graben represents an aulacogen formed during the rifting of the Belcher Basin. However, postgraben fluvial sediments predicted to form in failed arms by the capture of major rivers are missing. Chandler (1984) suggested that if originally present they may have been removed during later erosion. An additional complicating factor, with the aulacogen model is the presence of

PROTEROZOIC TRANSCU R RENT MOVEMENTS

the Archean rim, which appears to have been formed simultaneously with the graben. The presence of this circular ridge extending for more then 833 km is not predicted in rifting models and therefore poses a serious obstacle to the rifting model of the graben. Another possible cause of this ridge and graben is the meteorite impact theory proposed by Beals (1968). No evidence has yet been produced which specifically discounts Beals' proposal and therefore this idea must be seriously reconsidered. If the graben was formed due to rifting, it was not formed during the rifting of James Bay because the volcanism and faults which correlate to the rifting of James Bay have a relative age younger than the graben. The second major tectonic event which occurred in the Belcher Basin is marked by the eruption of the theoleiitic Flaherty Formation. The significance of this event is currently under debate. Ricketts and Donaldson (1981) and Gibb (1983) suggested that the volcanics were erupted during the closure of an ocean basin whereas Innes et al. (1968) and Baragar and Scoates (1981) suggested that the eruptions represent a rifting stage. The Flaherty volcanics have been correlated with the massive volcanic flows that overlie the Nastapoka Group sediments around Hudson Bay Arc and the Nastapoka equivalent sediments around the Sutton Arc (Ricketts et al., 1982). These volcanic flows have characteristics of flood or rift type basalts and it is proposed here that the volcanics which overlie the Sutton Arc were erupted during the rifting of James Bay. Therefore, this places the time of rifting of James Bay during the eruption of the Flaherty Formation and supports Innes et al. (1968) and Barager and Scoates (1981) suggestion that the Flaherty Formation is related to rifting in the Belcher Basin. Additional support for this comes from the Precambrian/Paleozoic boundary which extends from James Bay up through the Belcher Basin where it follows the trend of the positive gravity anomaly. If this bound-

179

ary, as proposed, marks the rift margin in James Bay, then its extension through the Belcher Basin suggests that the rift may also extend up through the Belcher Basin as suggested previously by Innes et al. (1968). In addition, James Bay strikes at a high angle to the Belcher Basin and as suggested by Burke and Dewey (1973), James Bay may form one arm or aulacogen of a triple point junction centered in the Belcher Basin with the other arms extending north and west (Fig. 18).

Thompson Belt The Thompson Belt has been interpreted as a major transform fault formed during the Trans-Hudson orogeny (Lewry, 1981). It trends roughly parallel to the KSZ which lead Gibb (1983), Sutton and Watson (1974), Bell (1971) to proposed that the KSZ and the Thompson Belt may be related strike-slip features. Such a proposal is supported by this paper's model, and the Thompson Belt is shown in Fig. 17 to have a dextral sense relative to the KSZ although relative to the Tran-Hudson orogeny it is interpreted to be a sinistral strike-slip fault (Lewry, 1981; Green et al., 1985). The age of movement along the Thompson Belt is uncertain but recent U-Pb ages on zircons from the Trans-Hudson orogeny with which it must be also related yielded ages of 1890-1835 Ma (Van Schmus et al., 1987). Therefore, if the Thompson Belt and the KSZ were active at the same time and under a related regional stress field, then the KSZ was active between 1890-1835 Ma. This supports the age of movement of the KSZ given by the Penokean orogeny at 1890-1830 Ma and again contradicts the minimum age of movement of the KSZ given by the Cargill and Borden Lake Complexes at 1907 Ma and 1872 Ma respectively (Percival et al., 1988). Fox Riuer Belt The Fox River Belt is composed of mafic and ultramafic rocks with interbedded sediments. The sediments and volcanics have been correlated with sediments in the

180

C.R. GOOD1NGS AND M.E. B R O O K F I E t . D

Thompson Belt (Weber and Scoates, 1978) except that whereas the Thompson Belt is intensely deformed the Fox River Belt is not. This lack of deformation is a problem with current models of the Trans-Hudson orogeny which interprets the Superior Province as moving northwest and colliding with the Churchill Province along both the Thompson Belt and Fox River Belt (Gibb, 1983; Green et al., 1985; Lewry, 1981). Green et al. (1985) suggested that the lack of deformation may have been the result of the Fox River Belt being thrust out of the collision zone. However, the lack of deformation along this east-west boundary zone is not confined just to the Fox River Belt. The Sutton Arc also falls on this east-west zone and it is not deformed either. Therefore, an alternate interpretation shown in Fig. 18 is that this was not a collision zone but rather may have been a zone of moderate rifting during the deformation of the Thompson Belt and movement along KSZ.

Cape Smith Belt and the Labrador Belt As shown in Fig. 18, if the East Superior Terrane moved to the northeast as is possible based on the model, then it would have collided obliquely along the Cape Smith Belt and the Labrador Belt, possibly causing the observed deformation. The ages of deformation for these belts are uncertain but dating on the Cape Smith Belt yielded an age around 1900 Ma (reported in St. Onge et al., 1987) and the Labrador Belt around 1850 Ma (reported in Anderson and Burke, 1983). Therefore, these ages are also consistent with the ages for the Thompson Belt and Penokean orogeny, and suggest a possible of movement along the KSZ and James Bay of 1900-1850 Ma.

Suggestions for further studies One of the remaining unanswered questions regarding the KSZ is when and over what length of time did the uplift and movement occur? In Phanerozoic strike-slip zones,

this question is usually addressed by carefully mapping displaced geological features. The youngest feature which has been offset the total amount provides a maximum age for the beginning of movement. Features formed during movement will be displaced less and less coming up through time. These provide an idea of the movement history, (i.e., whether it was constant or moved intermittently). The oldest feature which is not displaced at all will provide a minimum at which movement ended. This study has identified displaced geological features which have probably been offset by the net amount of movement. However, better ages are required on the rim, and the Matachewan and Hearst dykes to provide a maximum age for the onset of movement. An age of the dykes can possibly be obtained by direct dating but the rim cannot be dated directly. If as proposed, the rim and the Richmond Gulf Graben were formed together, than an age on the sills and dykes which Chandler (1984) interpreted as being related to the graben formation may provide an age on the rim. Finding other displaced features which show less and less displacement is doubtful, because of the deep erosion which has occurred. Therefore, the use of displaced features to better date the KSZ will probably not be a successful technique here. Dating the KSZ directly from uplifted rocks and associated volcanism will provide more information on the KSZ, but it will remain uncertain as to the significance of these dates. Probably the most important conclusion from this study is that James Bay may have rifted simultaneously with the movement and uplift of the KSZ. Rifts are somewhat less complicated structurally than strike-slip zones and therefore it may be easier to arrive at a history for their formation. To determine a time of rifting of James Bay, ages are required on the volcanics, which overlie the Sutton Arc and Hudson Bay Arc sediments, and the correlated Flaherty Formation. If ages can be obtained from these volcanics then it will provide an age of rifting

PROTEROZOIC TRANSCURRENT MOVEMENTS

for James Bay and an age of strike-slip movement along the KSZ. Equally important in determining an age of movement along the James B a y / K S Z boundary are the ages of events which were occurring at the Superior Province's other boundaries and which may be related to this movement. Hempton (1987) used this method to obtain a better history of movement on the Red S e a / D e a d Sea fault system and thereby proposed a more complete model for movement of the Arabian Plate. From the timing and nature of convergence along the Northern Arabian Plate boundary and the timing and nature of extension along the western boundary of the Arabian Plate, Hempton was able to produce a model for movement which showed that the Red S e a / D e a d Fault system had an episodic history of movement. The preliminary analysis of the boundary events of the Superior Province indicate that the KSZ and James Bay were active between about 1900-1830 Ma. This conflicts with the carbonatite dates of the KSZ and therefore more work is needed to explain this conflict. In addition, the boundary events must be better dated particularly the events related to the Belcher Basin, Cape Smith Belt, and the Labrador Belt, and Huronian Supergroup. Further work needed to evaluate this paper's model are: (1) More evidence to support the predicted steeply dipping eastern boundary fault of the Northern KSZ. Seismic surveys are probably required to accomplish this; (2) More evidence to support the predicted low dipping thrust plane of the Montreal River Fault. Again, seismic surveys would probably be required; (3) More detailed mapping in the proposed strike-slip basin to find geological evidence for the bordering faults and their orientation; (4) More evidence for the rifting model of James Bay. This would require drilling and seismic surveys because the area is covered by Paleozoic strata; and

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(5) Geological evidence is needed to support the correlation of the displaced anomalies X and Y which are now only correlated on the basis of a similarity of trend and magnetic intensity. Finally, the extension of the curved rim around part of the Belcher Basin to include the Sutton Arc brings to light again the question of what was the cause of this feature. The most direct way to resolve this problem would be to place a drill hole through the Belcher Islands into the Archean. If the basin is the result of a meteorite impact, then here in the centre of the basin should be the needed evidence. If a sheared and cataclastic zone and fall back breccia are not encountered in this drill hole, then it will lay to rest the meteorite theory. CONCLUSIONS

By taking a regional approach to the KSZ, this study has proposed a new model for its development. The model views the KSZ and James Bay as boundary features formed as a result of 160 km of left-lateral motion between the East and West Superior Terranes with an accompanying 5 ° of anticlockwise rotation of the West Superior Terrane relative to the East Superior Terrane. As a result of this motion, James Bay formed as an extensional feature possibly due to active or passive rifting and the KSZ was the site of strike-slip motion. Evidence for this model comes from displaced geological and geophysical features. The Sutton Arc is interpreted to have been offset from the Hudson Bay Arc as a result of the rifting of James Bay implying that the Belcher Basin was bounded by an elevated circular rim with a radius of 232 km and 833 km in length. In contrast, displacement of various magnetic lineaments indicates that left-lateral strikeslip motion has occurred along the KSZ. Additional support for the model comes from structural evidence, that is in agreement with that predicted in the model. James Bay has features expected in a rift, with

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elevated east and west margins and a southeast end which is parallel to the trend of the KSZ. The KSZ is divided into three structural units, the Northern KSZ and the Southern KSZ separated by a strike-slip basin. These divisions and related structural features are interpreted to have formed as a result of the angle between the motion vector and fault trace. In the Northern KSZ, the area underwent convergent strike-slip which produced the ductile northeast structural grain and vertical uplift of granulites along steeply dipping boundary faults. The Southern KSZ trends at greater angle to the direction of movement forming a restraining bend and crust must be consumed to accommodate this motion. As a result, the zone is marked by thrusting with the West Superior Terrane overlying the East Superior Terrane. In the southern part of the Southern KSZ at Lake Superior, dip-slip seems to have been more important than strike-slip and accounts for the lack of a shear zone and the proposed low angle thrust fault. In the northern part of the Southern KSZ, the dip-slip and strike-slip operated together, producing the ductile northeast structural grain and thrusting. Separating the Northern and Southern KSZ is a zone which lacks uplift. It is proposed that this is a strike-slip basin formed as a result of extension during the uplift of the Northern and Southern KSZ. The model also can be extended to include some of the deformation which has occurred at the Superior Province's borders. Left lateral motion between the East and West Superior Terranes provides a simpler and more elegant model than the compressional model. The compressional model has difficulty in assimilating the wide range of structural features which have occurred along the KSZ without invoking several separate phases of movement. This is overcome if strike-slip is proposed because most of the features can be explained as occurring under the same stress field. This model is also important because it shows that Proterozoic terrane movements

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behaved in a similar fashion as Phanerozoic movements. The K S Z / J a m e s Bay can be described with a direct analogy to the R e d S e a / D e a d Sea Fault or the Gulf of Californ i a / S a n Andreas fault system. All three involve a rift zone connected to a strike-slip zone which formed together as a result of relative terrane movements. In addition, the K S Z / J a m e s Bay system will be important to Phanerozoic geologists. Usually, the d e e p e r crustal levels of Phanerozoic strike-slip belts are covered by higher crustal levels. The KSZ therefore provides a view of how basement rocks are involved and effected by strike-slip thereby providing a better understanding of the complex processes which occur deep in a strike-slip belt. Finally, this paper illustrates the importance of obtaining a regional overview of ancient lineaments and orogenic belts before conclusions can be made about the area as a whole. This is especially important in strikeslip belts because they can show simultaneous extension and compression along strike or movement in one area in contact with an area where no movement was occurring. Therefore using a detailed study of a small area to propose a history for the whole structure will likely give an erroneous interpretation. ACKNOWLEDGEMENTS This work was supported by a N.S.E.R.C. operating grant to M. Brookfield. I acknowledge discussion and help from P.B. R o b e r t son (Geol. Surv. Can.). G.S.C. allowed examination and sampling of their core in the Nastapoka arc on Nielsen Island. REFERENCES Anderson, S.L. and Burke, K., 1983. A Wilson cycle approach to some Proterozoic problems. In: J.L.G. Medaris, C.W. Byers, P.M. Mickelson, and W.C. Shanks (Editors), Proterozoic Geology: Selected Papers from an International Proterozoic Symposium. Geol. Soc. Am. Mem., 161: 75-95.

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