Ellesmere-Greenland Fold Belt: Structural evidence for left-lateral shearing

215

Tectonophysics, 100 (1983) 215-225 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ELLESMERE-GREENLAND LEFT-LATERAL SHEARING

FOLD BELT: STRUCTURAL

EVIDENCE

FOR

H. HUGON Deparrment of Geology, University of Toronto, Toronto, Ont., MSS IA I (Canada)

(Accepted September 19, 1983)

ABSTRACT Hugon, II., 1983. Ellesmere-Greenland Fold Belt: structural evidence for left-lateral shearing. In: M. Friedman and M.N. Toksaz (Editors), Continental Tectonics: Structure, Kinematics and Dynamics. Tecfonophysics, 100: 215-225. Regardless of whether they were passive or active markers of the Eurekan deformation, fold-axial traces in the Elksmere-Greenland fold belt define a regional pattern typical of left-lateral ductile shear zones. It is therefore postulated that a left-lateral mega-shear zone spanned the entire east-west width of Ellesmere Island plus adjacent northern Greenland, and that the shear direction was subparallel to Nares Strait. A left-lateral motion along Nares Strait is required by the plate tectonic models of J.T. Wilson and E.C. Bullard. To explain the opening of the Labrador Sea and Baffin Bay by sea-floor spreading, these models involve a left-lateral displacement between Greenland and North America of 200~400 km. Because of the lack of lithological and structural offsets across Nares Strait, the models have been controversial. The mega-shear zone proposed herein reconciles all the conflicting evidence. Accordingly, the ductile behaviour of the sedimentary strata on Ellesmere Island permitted a large left-lateral displacement of Greenland relative to North America without a large offset of geological features across Nares Strait.

INTRODUCTION

According to plate-tectonics models by Wilson (1963, 1965) and Bullard et al. (1965), total tangential displacement between Greenland and Ellesmere Island is 350 km. Srivastava’s (1978) model predicts a left-lateral displacement of about 200 km along Nares Strait, and that of Peirce (1982) as much as 320 km. Geophysicists have suggested that this left-lateral displacement was accomplished by means of a transform fault in Nares Strait. Some geologists oppose this interpretation on the grounds that there is no significant strike separation of geological structures between northwestern Greenland and eastern Ellesmere Island (Dawes, 1966, 1973, 1982; Kerr, 1967, 1981; Chirstie et al., 1982), and favour models that lack a transform fault in Nares Strait (Beh, 1975; Le Pichon et al., 1977; Kerr, 1981). As shown by ~-1951/83/$03.~

0 1983 Elsevier Science Publishers B.V.

216

Srivastava and Falconer also McWhae, 1981). To reconcile suggested

(1982), all these models lead to severe misfits of plates (see

the geophysical

that the left-lateral

was distributed

and geological

displacement

interpretations,

between

across a wide fault zone containing

to a single transform

fault (Peirce,

several authors

Greenland

and Ellesmere

Nares Strait rather than confined

contribution

is based

on an analysis

Tertiary

of the tectonic

Ellesmere-Greenland fold belt (EGFB), and offers a new solution Strait problem. This solution reconciles the need for a large left-lateral between Greenland and Ellesmere Island geologic features across Nares Strait. TECTONIC

AND GEOLOGICAL

The Ellesmere-Greenland

1983, this

1980; Keen and Peirce. 1982; Miall.

volume). This idea is supported by sinistral strike-slip affecting easternmost Ellesmere Island (Mayr and DeVries. 1982). The following

have Island

strata

on

map of the to the Nares displacement

with the lack of strike separation

of linear

SETTING

fold belt (Fig. 1) of Thorsteinsson

and Tozer (1970) is

the result of two main tectonic pulses, i.e. (1) the Ellesmerian

Orogeny

which created

the Inuitian

Orogeny

which created

fold belt in the Devonian

the Eureka Sound Tertiary. According Orogeny

occurred

and (2) the Eurekan

fold belt (Fortier, 1963) in the Upper to Balkwill (1978), the main compression between

Eocene

and Lower Miocene.

Cretaceous to Lower phase of the Eurekan

The Inuitian

fold belt is

separated from the northern Canadian and Greenland shields by a narrow zone of unmetamorphosed Paleozoic sediments devoid of folds, known as the Arctic Platform (Fig. 1). This fold-free zone widens on southernmost Ellesmere Island (Okulitch, 1982). Little is known about the intensity most parts of Ellesmere Island, Ellesmerian

Fig. 1. Index map of the regions

referred

3 -Bathe

Peninsula.

Peninsula.

4 -Boothia

of the Ellesmerian deformation. structures are strongly affected

to in the text. 1 -Peary

Land,

2-Judge

As in by the

Daly Promontory,

217

Eurekan

Orogeny

the Eurekan related

(Kerr, 1982; Okulitch,

tectonism

overturned

is characterized

folding

(Mayr

1981). Geological by eastward

and

DeVries,

mapping

has shown that

to southeastward 1982;

Okulitch,

thrusting 1982;

and

Osadetz,

1982). The EGFB

has a sigmoidal

fault traces and fold axial-plane Okulitch

geometric

pattern

accentuated

(1982) showed that the folds are considerably

ern Ellesmere

Island

by subparallel

traces in Paleozoic-Tertiary

than in the southernmost

sediments

tighter in central-southwest-

or the central

parts of the Island.

estimated that, locally, the amount of shortening in central-southwestern Island is between 25 and 50%. Thus difference in horizontal shortening the effects of the Eurekan

compression,

thrust

(Fig. 2A).

which seems to diminish

He

Ellesmere is related to

southward

(Bal-

kwill and Bustin, 1975; Kerr, 1981). It would appear that the Eurekan style of deformation is one of thin-skinned tectonics (Osadetz, 1982; Okulitch and Osadetz, 1982; Van Berkel et al., 1983). Osadetz (1982) showed that structures on northerncentral Ellesmere Island developed at high angles to the Eurekan fold trends. He concluded that these normal list& faulting

structures were produced by epeirogenic to late erogenic subpe~endicular to the Eureka Sound fold belt. Numerous

normal faults have been mapped subperpendicular western Ellesmere Island (Kerr, 1968; Okulitch, suggested

that faults occurring

part of the island

are conjugate

to the erogenic trend on south1982). Mayr and DeVries (1982)

at high angle to the fold trend on the northeastern strike-slip

faults related

to a major

sinistral

strike

along the Judge Daly fault zone. This zone makes an angle of about .5”-10” with the Nares Strait direction. The few data available the folding

intensity

for northern

which

Greenland

is attributed

indicate

a northward

to the Ellesmerian

increase

of

tectono-metamorphic

event (Dawes and Soper, 1973; Dawes and Peel, 1981). Pedersen (1979) showed that southward thrusting and related overturned folding have occurred in Peary Land, which slightly modified land. Most of the major known,

or thought,

the general northward thrusting

or reverse

to be of Tertiary

vergence observed faulting

in northern

in northern

Green-

Greenland

is now

age (Dawes, 1982; Soper et al., 1982). An event

of retrograde metamorphism related to the thrusting in Peary Land indicates a Tertiary overprinting in northern Greenland too (Dawes and Sopper, 1973; Soper et al., 1982). Hence one can support the possibility of a Eurekan event responsible an accentuation of the variation of the folding intensity in northern Greenland. INTERPRETATION

OF THE ELLESMERE-GREENLAND

FOLD

for

BELT

In the present paper the “Ellesmere-Greenland fold belt” will include the Eureka Sound fold belt of Fortier (1963). The present geometry of the EGFB could have developed in three different ways. Firstly, it is possible that the Ellesmerian Orogeny produced a mechanical anisotropy which strongly controlled the trend of the

?.I8

Eurekan

structures.

Accordingly,

the Ellesmerian

structures

were amplified

in the

Eurekan Orogeny, and they induced additional structures in the post-Paleozoic sediments. Secondly, it is possible that the mechanical anisotropy produced by Ellesmerian Eurekan

tectonism

Orogeny.

the result

was rather

of Eurekan

tectonism.

maximum

Eurekan

extension.

is unrealistic.

geometry

of the EGFB

inherited

Ellesmerian

Eurekan

deformation,

present

geometry

pieces of structural

and

Finally,

Ellesmerian mechnical anisotropy enough to rotate the Ellesmerian first possibility

weak,

If this is true then the present

to be insignificant

geometry

in the is mainly

that, even if the

was marked, the Eurekan deformation was large structures into subparallelism with trajectories of

Owing

is mainly

to the tectonic

the result of Eurekan anisotropy

it will be assumed

of the EGFB

of the EGFB

there is the possibility

weakness

Each of the other alternatives

mechanical

evidence

proved

is mainly

further

of most rocks, the

implies

Although

the

must have had some effect upon

the

herein

deformation.

as a working

hypothesis.

the result of the Eurekan

support

that the present

that the

tectonism.

this choice: (1) the absence

Two

of patterns

of interference structures, such as domes and basins (cf. Ramsay, 1967, p. 531) and (2) the strong effect of the Eurekan deformation upon Ellesmerian structures throughout

central

Ellesmere

Island

(Kerr. 1981; Okulitch.

1982).

Fig. 2. A. Horizontal traces of fold-axial planes and thrusts on Ellesmere Island and northern Greenland. Data from Thorsteinsson (1974), Dawes (1982), Frisch and Dawes (1982) and Landsat photo reconstitution of Ellesmere island (EMG f762-NTS-29-39-49).

Heavy lines = thrust traces; thick dash lines = fold

axial-plane traces; thin dash lines = trends of marble units; dotted lines = major lineaments. B. Strain trajectories defined by the fold axial traces in a left-lateral ductile shear model.

219

Accordingly, it is assumed herein that fold axial-plane traces and thrust fault plane traces are approximately parallel to the trend lines of the X, Y principal surface of Eurekan deformation (following Ramsay, 1967, X is the principal direction of extension and Y is the intermediate principal direction of deformation). Hence the fold axial-plane and thrust traces define a pattern (Fig. 2) strikingly analogous to the schistosity pattern in ductile shear zones (cf. Ramsay and Graham, 1970). From this analogy, the author deduces that (1) the post-Cambrian sedimentary pile in Ellesmere Island could have been strained during the formation of a wide transcurrent shear zone, (2) the sigmoidal geometry is characteristic of a left-lateral transcurrent shear zone, (3) the wide shear zone is mainly distributed over Ellesmere Island and is divided into two deformation bands, the first lies in Nares Strait the second is slightly oblique to Nares Strait and located on central-southweste~ Ellesmere Island (Fig. 2B). If the fold axial-plane traces on Ellesmere Island are indeed reliable indicators of the Eurekan deformation then an attempt can be made to calculate the left-lateral displacement using a method suggested by Ramsay and Graham (1970). Such a calculation made for eastern Ellesmere Island (cf. rectangle Fig. 2B and Fig. 3). gives a minimum value of about 180-185 km of left-lateral displacement between Ellesmere Island and Greenland but being distributed over 150 km on Ellesmere Island. This amount was obtained by (1) assuming that the deformation is strictly continuous, (2) taking the direction of the Nares Strait lineament as the shear direction and (3) adopting a simple-shear model for the Nares Strait area. Strain ratios thus obtained are between 6 and 8 in Nares Strait. If one uses a more general deformation model, these strain ratios have lower values. Thrusting and reverse faulting indicate that a certain amount of vertical extension

3Q

2s

ELLESMERE

ISL.

GREENLAND . . -.

,

I

1

1

SO

between maximum

see Ramsay

and Graham,

Strait and central

value of 360-370

1

Island.

200

about

central

330

shear zone as computed

(i.e. Nares Strait lineament). boundaries

Eilesmere

The

(for calculation

km of displacement

was made for northern

the Nares Strait direction.

--__ -__ Km , l-

250

the two chosen

No calculation between

K_ 1

1970). The area under the curve gives 180-185

Elksmere

km of displacement

land is given by a curve symmetric

between

*\ l.

I

makes with the shear direction

the curve gives the total displacement

Nares

‘.

1967) across the western part of the transcurrent

from the angle that the strain trajectory procedure,

i I50

100

Fig. 3. Shear strain (y, Ramsay, area under

I

3-.,

Greenland.

Island and northern

A

Green-

220

must have occurred. Moreover if Nares Strait is a fault zone, then strain ratios within faults blocks and away from the fault zone can stay below the critical ratio of 2.5 where schistosity

is assumed

The latter assumptions

to start developing

(cf. Pfiffner

and Ramsay.

work of Mayr and DeVries (1982). Note that the Nares Strait Iineament taken

as the shear

1982).

seems to be much more realistic and is also supported direction

for the second

deformation

band.

by the

cannot

Indeed

be

such

a

boundary condition is impossible for two reasons; (1) because of enormous values of displacement and strain ratios and (2) because the shear direction intersects the extension

trajectories

(fold

traces)

more

than

once

of this high-strain

Although area cannot

the value obtained for the differential displacement be precise (cf. Ramsay and Graham, 1970; Ramsay

it gives, nonetheless predicted

zone will be presented

(cf. Fig. 3). A more

explanation

a minimum

by plate-tectonics

in the Nares Strait and Allison, 1979)

estimate which can be compared

models (cf. Srivastava,

viable

below.

with the magnitude

1978; and others}. This value of

differential displacement seems to indicate that either the Ellesmerian structures have been rotated during the Eurekan deformation or that they agree fortuitously with a large left-lateral

displacement

of Greenland

relative

to Ellesmere

Island.

BASEMENT AND COVER RELATION

Many problems are encountered if the overall deformation of the post-Cambrian sedimentary rocks on Ellesmere island is attributed to a clockwise rotation of Greenland with respect to North America. A major problem is the compatibility of deformation between the Canadian-Greenland shield and the post-Cambrian sedimentary cover. The folding in the sedimentary pile suggests that sediments deformed mainly in a ductile manner. The same cannot be invoked for the shield as it is overlain

by unmetamorphosed

thin-skinned

tectonics

sediments.

in Ellesmere

Island

(Okulitch

a decoupling

between

1982). This tectonism

requires

of decollement

in Ordovician

zones

A solution

Osadetz, 1982; Osadetz, 1982). Di?collement allows the basement

to this problem and Osadetz, basement

and Carboniferous to deform

is provided

by

1982; Osadetz,

and cover by means

evaporites

quasi-compatibly

(Okulitch with

and

the post-

Cambrian cover rocks, albeit in a discontinuous manner, by developing or using either a set of transcurrent faults (Fig. 4), several sets of conjugate shear fractures or both types of structures. Depending on the spacing between the transcurrent faults or the conjugate shear fractures, the strain can be described either as discontinuous at the outcrop scale or as ductile or ductile-like at the belt scale (Price, 1973: Schwerdtner, 1973). Mayr and DeVries (1982) demonstrated the existence of conjugate sets of fractures related to the Judge Daly fault zone in the Nares Strait area. The left-lateral Judge Daly fault zone makes an angle of 5”-10” with the Nares Strait lineament. Therefore, this fault zone fits the model of Riedel’s shear fractures

Fig. 4. Idealised model allowing for a ductile deformation of a sedimentary cover during severe transcurrent faulting in the basement gneiss (schematic sketch).

(Riedel, 1929) and is then a piece of indirect evidence for a larger left-lateral displacement parallel to the Nares Strait lineament. If we consider the mechanism suggested in Fig. 4, the extension direction X in the cover should be subparallel to the fold axes at any point in the fold belt, The normal

Fig. 5. First major motion of Greenland relative to North America between magnetic anomalies 32 and 25 (after Srivastava, 1978).

I Fig. 6. Second major motion

13 (after Srivastava, Strait lineament

of Greenland

relative to North America,

1978). The small circles of the total

(Peirce,

between magnetic

pole of this rotation

anomalies

are parallel

24 and

to the Nares

1982).

faulting and listric faulting subp~~~ndicular to the fold belt trend (cf. p. 217) if not due to lateral ramps, is in agreement with a wide Ieft-lateral shear zone in Ellesmere Island and represent a late discontinuous response to a strong extension subparallel to the fold trend. The deformation band on central southeastern Ellesmere Island (cf. Fig. 2B) cannot be explained by simple shearing with a shear direction parallel to the Nares Strait lineament (cf. p. 220). A possible explanation for this high-strain band

is that it was initiated

America implied

during

(cf. Fig. 5 and Srivastava, a shortening

the first motion 1978). According

of 25% for the Sverdrup

of Greenland

relative

to North

to Peirce (1982), this motion

Basin. This deformation

band can then

be explained by a squeezing of the sedimentary strata, in southwestern Ellesmere Island, between two rigid bodies, i.e. the Boothia uplift on the west side and the Canadian-Greenland shield on the east side. This could not have happened at the northern edge of the Sverdrup Basin because the Boothia uplift dies away northward (Kerr, 1981). Presumably, the amount of shortening was distributed within a wider area at the northernmost portion of the basin. During the second motion of Greenland (cf. Fig. 6), the deformation band central-southweste~ Ellesmere Island became

more pronounced.

CONCLUDING If it is mainly

DISCUSSION

accepted that the present geometry of the Ellesmere-Greenland fold belt is result of Eurekan tectonism, then one can hardly avoid the conclusion

the

223

that the sedimentary cover rocks on Ellesmere Island were subjected to severe left-lateral shearing in the Tertiary. This offers a solution to the Nares Strait problem and explains (1) the large tangential displacement between the centres of Greenland and the Queen Elizabeth Islands as predicted by plate tectonics models, (2) the continuity of the linear features in the sedimentary cover across Nares Strait and (3) the main compression phase of the Eurekan tectonism in Eocene-Miocene times (Balkwill, 1978), i.e. during the second main motion of Greenland with respect to North America (e.g., between magnetic anomalies 24 and 13, cf. Srivastava 1978; Srivastava and Falconer, 1982). While the sedimentary cover was capable of shearing in the ductile manner, the basement rocks must have been fairly brittle and developed a transcurrent system along Nares Strait. It follows that major geological markers should be offset across the southern portion of Nares Strait, where the ~anadi~-Greenland shield outcrops. The Bathe Peninsula Arche was originally believed to be a structural marker implying no strike-slip between Greenland and Ellesmere Island. However, according to Miall (1983, this volume), the so called Bathe Peninsula Arche can no longer be taken as a structural high predating the Tertiary deformation. Marble units (cf. Fig. 2) are then the only geological markers that show an apparent continuity across Nares Strait. According to Dawes (1982) these markers can also be interpreted as the two limbs of a large fold structure generated prior to the left-lateral motion of Greenland. Accordingly, the author proposes that the 250-350 km left-lateral displacement between Greenland and Ellesmere Island predicted by plate-tectonics models was distributed in the cover rocks across a 200-300 km wide zone of horizontal shearing. A test of this proposal would consist of a detail study of the Canadian-Greenland shield adjacent to the Arctic platform and along the southern portion of Nares strait. ACKNOWLEDGEMENT

This study was done while under a postdoctoral fellowship supported by the National Science and Engineering Research Council (Strategic Grant to A-D. Miall and Colleagues), The author wishes to thank A.V. Okulitch and KG. Osadetz for helpful comments and A.D. Miall for continuous encouragement. Special thanks are due to W.M. Schwerdtner for helpful discussion and critical reading of early drafts. REFERENCES Balkwill, H.R., 1978. Evolution of Sverdrup Basin, Arctic Canada. Am. Assoc. Pet. Geol. Bull., 62: 1004-1028. Balkwili. H.R. and Bustin, R.M., 1975. Stratigraphic and structural studies, central Ellesmere Island and eastern Axe1 Heiberg Island. Geol. SUIT. Can. Pap., 751-A: 513-517. Beh, R.L., 1975. Evolution and geology of western Baffin Bay and Davis Strait, Canada. In: C.J. Yorath,

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