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Palaeomagnetic evidence relating to the cenozoic drift of Greenland
Tectonophysics, 29 (1975) 209-221 0 Elsevier Scientific Publishing Company,
PALAEOMAGNETIC OF GREENLAND*
P.V. SHARMA
EVIDENCE
RELATING
University of Copenhagen,
Palaeomagnetic Laboratory, version
- Printed
in The Netherlands
TO THE CENOZOIC
DRIFT
and R.N. ATHAVALE
Institute of Geophysics,
(Revised
209 Amsterdam
accepted
Copenhagen
(Denmark)
National Geophysical Research Institute, Hyderabad (India)
July 15, 1975)
ABSTRACT Sharma, P.V. and Athavale, R.N., 1975. Palaeomagnetic evidence relating to the Cenozoic drift of Greenland. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4) : 209-221. An analysis is made of the available palaeomagnetic data for the Early Tertiary basalts of Greenland. The Early Tertiary pole for Greenland, calculated from stable remanent magnetic directions of 81 lava flows on Disko Island, is located at 67.5°N,1650W, with dp = 5O and dm = 6’. The Disko pole is widely separated from the pole obtained for contemporaneous lava flows on Baffin Island, Canada, and this suggests a significantly large separation between Canada and Greenland as early as 65 m.y. ago. The Disko pole is also displaced from poles of similar age in northwestern Europe (British Isles and the Faeroe Islands). This is apparently inconsistent with the idea that the split of Greenland from Europe occurred concurrently with the widespread Early Tertiary igneous activity on the continental borderlands of the northeast Atlantic. The main results of the palaeomagnetic comparisons between eastern Canada, Greenland and North Europe are discussed in the light of available evidence for sea-floor spreading in the North Atlantic.
INTRODUCTION
Greenland is considered to be a link between the Eurasian and American plates and is believed to have separated first from North America and then from Europe as a consequence of the opening of the Baffin Bay-Labrador Sea and the northeastern part of the Atlantic Ocean. Palaeomagnetic data from Greenland are particularly interesting as Greenland holds a key position between North America and Europe in any attempted pre-drift configuration of these continents. Whereas a considerable amount of palaeoma~etic data has been obtained * Contribution
No. 5, Institute
of Geophysics,
Copenhagen
University.
210
from North America and Europe in the past two decades, such data from Greenland are as yet sparse. Mainly because of the difficulty of access few rocks from this area have been studied. There is a conspicuous lack of palaeomagnetic data for the Palaeozoic and Mesozoic rocks excepting the Triassic sediments of East Greenland for which some preliminary results have been reported (Bidgood and Harland, 1961; Athavale and Sharma, 1974; Reeve et al., 1974). To date a greater number of studies have been made on younger rocks, particularly on the Early Tertiary volcanics of East and West Greenland. The results of palaeomagnetic studies (Tarling, 1967; Deutsch and Krist-
Fig. 1. Known Early Tertiary igneous activity on the continental borderlands of the North Atlantic plotted on approx. 60 m.y. B.P. reconstruction modified from Fitch (1965). I = West Greenland basalt province; 2 = East Greenland basalts; 3 = Faeroes basalts; 4 and 5 = British Tertiary igneous province; 6 = Baffin Island basalts. Stippled area represents pre-60 m.y. ocean crust. Partly reproduced from Vogt and Avery (1974).
211
jansson, 1974; Athavale and Sharma, 1975) based on extensive sampling of these rocks are of great interest in relation to the proposed movement of Greenland between North America and Europe. The basaltic lavas of Early Tertiary age exposed on the western and eastern margins of Greenland (Fig. 1) are related in age to the other rocks of the North Atlantic volcanic province exposed on Baffin Island, the Faeroes and the British Isles. By their distribution, these areas can provide crucial palaeomagnetic sites for testing proposed reconstructions that involve separation of Greenland from eastern Canada and northwestern Europe. In particular, palaeomagnetic comparisons across the Baffin Bay and the northeast Atlantic are potentially useful for tracing the drift history of Greenland. We discuss here the results of palaeom~etic comparisons across the Baffin Bay and the northeast Atlantic together with their bearing on the Cenozoic drift of Greenland. The comp~isons are based mainly on the data from Early Tertiary volcanics of Greenland, Baffin Island, the Faeroes and the British Isles. The interpretations are hampered by lack of reliable radiome tric age data. SELECTION OF THE EARLY TERTIARY DATA
The palaeomagnetic data used for the comparative study of the Early Tertiary pole positions are listed in Table I. The list comprises published results of selected studies from West and East Greenland, Baffin Island, the Faeroes and the British Isles. The stability of magnetization for all selected results is reported to be established by alternating-field and/or thermal-cleaning tests. In most studies (excepting the one on Baffin Island), the sampling was extensive enough for the mean direction to be reasonably assumed to represent the geomagnetic field at that time, and in all cases the age of magnetization was determined within reasonable limits. Rock ages quoted in the list are from original references. In Fig. 2, the Early Tertiary pole positions for Greenland are shown along with poles for Baffin Island, the Faeroes and the British Isles. The East Greenland result is based on 28 Palaeocene lava flows (age 60-47 m.y.) from the Scoresby Sund area (Tarling, 1967). Palaeomagnetic results of a recent study (Hailwood et al., 1973) based on more extensive sampling of the Scoresby Sund basalts are yet not published; however, these authors report radiometric ages of 52-47 m.y. for the rocks studied. The most reliable West Greenland result is that obtained from 81 Disko basalt flows (of Late Danian age, i.e. -65-60 m.y.) by combining the results for northern and southern Disko (Athavale and Sharma, 1975). In contrast to this the Baffin Island pole based on five Cape Dyer flows (Deutsch et al., 19’71) dated 58 m.y. has much lower statistical reliability. From the northweste~ Europe results the most reliable Early Tertiary pole is that obtained from 253 Faeroese basalt flows (Tarling, 1970) for
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171-E
77’N
-
2’
II’
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11.5O
4.9’
8”
5.5”
.-
( 197 0 )
(1967)
1974).
__._____-_-
.._._
et al. ( 197 1)
Various studies (mean quoted in Tarling, 1970)
Tariing
Tarling
Deutsch
and Kristjansson,
2.5’
13”
12O
Mean of 1 and 2
5.6’
and Sharma
Kristjansson and Deutsch (1973)
Athavale { 1975)
Reference
9’
6”
Radii of 95% confidence oval
and Pulvertaft, 1969). has given a date of 70 * 4 m.y. (Deutsch
___._~__^_.
L61’E
77” N
.-_,.-_-
174’ E
63’N
__
55” w
83-N
58 m.y. (E. Farrar in Clarke and Upton, 1971) 60-55 m.y. (Beckinsale et al., 1970) 52-47 m.y. (~ailwood et al., 1973) 59-53 m.y. (Tarling and Gale, 1968) 64-50 m.y. (Various sources quoted in Duncan et al., 1972; Purdy et al., 1972)
165’W
67”3ON
154’3OW
72”N
Late Danian*
Long.
Lat.
Pole position ~_.___
II~-_-
province
169” w
_.._
volcanic
62”N
m.y.)
Atlantic
Late Danian* *
Late Danian* (i.e.- 65-60
North
* Age inferred from palaeontological evidence (Rosenkrantz ** A single K/Ar result obtained by a commercial laboratory
7.
6.
c5 .
4.
3.
Lava flows, Faeroe Islands Intrusives and extrusives, NW Scotland and N. Ireland
Lava flows N. Disko, W. Greenland Lava flows S. Disko, W. Greenland Disko Island, W. Greenland Lava flows Cape Dyer Baffin Island, Canada Lava flows Scoresby Sund, E. Greenland
1.
2.
Formation
Serial No. Age
_~_-.-~.
___.__..~
and locality
poles from the Early Tertiary
Palaeomagnetic
TABLE I
213
180
Fig. 2. Palaeomagnetic pole positions for Early Tertiary lavas from Baffin Island (BF), Disko Island (D), East Greenland (EG), Faeroe Islands (F) and British Isles (BI). The oval of confidence for the latter pole (BI) is not shown because the mean is computed from sixteen studies of variable reliability (Tarling, 1970). Lambert equal-area projection with North pole at the pivot.
which the K/Ar ages are from 59 to 53 m.y. It is significantly different from the Greenland (Disko) pole but lies close to an average Early Tertiary (-64-50 m.y.) pole for the British Isles based on sixteen different studies (mean pole quoted in Tarling, 1970). PALAEOMAGNETIC
COMPARISONS
ACROSS
BAFFIN
BAY
Palaeomagnetic comparisons across Baffin Bay are potentially useful for establishing whether the opening of Baffin Bay predates the time of major extrusions on West Greenland and on Baffin Island. If one could assume the Disko and Baffin Island poles (Fig. 2) to be representative of the two coast areas, then it is possible to interpret the difference in pole positions in terms of relative separation of the two landmasses. We have discussed this point in greater detail in a previous paper (Athavale and Sharma, 1975). Figure 3 summarizes the results of this comparison. The displaced position of the Disko pole, from D, to D2 , is after the Bullard et al. (1965) reconstruction model which involves a rotation of Greenland towards North America through 18” around a pole of rotation at 70.5”N,
Fig. 3. Comparison between Baffin Island (BF) and Disko poles. The displaced position of Disko pole (from D, to D2 ) is after rotation of Greenland towards Canada by 18 according to the Builard et al. (1965) reconstruction model. The pivot for rotation is shown by a diamond. Lambert equal-area projection.
94.4”W (Wells and Verhoogen, 1967). It is apparent from Fig. 3 that the amount of rotation suggested is not adequate to bring the Disko and Baffin Island poles close together. A larger rotation by about 50” is required to make the poles in agreement. It follows therefore that to accommodate such a large rotation Early Tertiary Greenland should be located further east. This implies a significantly large separation between Greenland and Canada as early as 65 m.y. ago. The above interpretation suffers from a major weakness. The Baffin Island pole is associated with a large statistical error; the computed pole is based on only five flows at Cape Dyer and may not be representative of the axial dipole field prevailing at that time. The proposed interpretation is inconsistent with the suggestion (Clarke and Upton, 1971) that the Baffin Island and West Greenland olivine basalt areas were once continuous and began rifting apart during the time of volcanism itself. HOWever, in the light of other geological and geophysical evidence (discussed later) a mid-&Iesozoie rifting between Greenland and Canada and a larger separation (than at present) between the two landmasses is not at all improbable.
215 PALAEOMAGNETIC
COMPARISONS ACROSS NORTHEAST
ATLANTIC
Palaeomagnetic comparisons of the Early Tertiary data on either side of the northeast Atlantic are meaningful in that they could establish if this part of the North Atlantic Ocean had already opened at that time or that the opening occurred concurrently with the Early Tertiary volcanism on the continental borderlands of the northeastern Atlantic (see Fig. 1). Such comparisons are statistically feasible now as reliable palaeomagnetic data are available from Greenland and northwestern Europe. For this comparison we have taken the mean pole positions for Disko and the Faeroes/ British Isles (see Table I) to be representative poles for Early Tertiary Greenland and northwestern Europe. From Fig. 4 it is apparent that there is a real difference between the Greenland and European results. The separation of the Disko pole from the Faeroese and British Isles poles is not reduced on rotation of Greenland towards Europe by 22” about a pivot (73”N,965”E) after the Bullard et al. (1965) rotation model for the fit of the northern continents.
Fig. 4. Comparison of Early Tertiary palaeomagnetic poles from Disko (Di , Ds), Faeroe Islands (F) and the British Isles (BI). The displaced position of Disko pole (from Di to Ds) is after rotation of Greenland towards Europe by 22’ according to the Bullard et al. (1965) reconstruction model. The pivot for rotation is shown by a diamond. Lambert equal-area projection.
216
There are two possible explanations for the observed difference between the Disko and the British Isles/Faeroes poles. It is likely that the noncoincidence of the poles may be due to age differences between the DiskoSvartenhuk volcanism and the British Isles/Faeroes volcanism. An alternative explanation is that the Norwegian-Greenland Sea (at least in part) had already opened at the time these rocks were laid down. The available radiometric age data for these rocks (see Table I) are insufficient to decide between these alternatives. GREENLAND
AND THE EVOLUTION
OF THE NORTH
ATLANTIC
The history of sea-floor spreading in the North Atlantic is very relevant to any discussion of Greenland’s separation from North America and Europe. Our present knowledge of the spreading history depends mainly on the interpretation of detailed magnetic surveys between southern Greenland and Labrador on one side and between southern Greenland and the British Isles on the other (Vogt et al., 1971; Le Pichon et al., 1971; Laughton, 1972; Pitman and Talwani, 1972; Vogt and Avery, 1974). The spreading history northeast of Iceland is complicated (Vogt et al., 1970). The anomaly picture in the northern Labrador Sea (north of 60”) is not clear; only a few magnetic lineations have been identified (Hood and Bower, 1973) and there is uncertainty as to their age. Major magnetic anomalies identified in the northeast Atlantic and Labrador Sea are shown in Fig. 5. The magnetic-lineation pattern south of Iceland is very clear, where anomalies going back to no. 24 (60 m.y. B.P.) and parallel to the NE--SW trend of the Reykjanes Ridge have been identified. In the Labrador Sea, southwest of Greenland, magnetic anomalies numbering from 24 to 19 (corresponding to age 60-47 m.y. B.P.) have been identified and these have a general trend in the NW-SE direction. These anomalies taken together with those of corresponding age, parallel to the Reykjanes Ridge, suggest that Greenland was pushed away both from North America and northwestern Europe during the Early Tertiary. The general picture that evolves from interpretation of anomalies in Fig. 5 leads to the following scheme of Greenland’s movement during the Cenozoic : (1) About 60 m.y. (anomaly 24): Greenland separated from North EUrope, breaking off from Rockall Plateau and opening the northeast branch of the Atlantic. (2) 60-47 m.y. (anomaly 24-19): Simultaneous spreading in the Labrador Sea and northeast Atlantic led to a resultant movement of Greenland in the northward direction. The northward movement of Greenland is also supported by the mean remanent direction of the Early Tertiary Disko lavas that gives a palaeolatitude of 56” (Athavale and Sharma, 1975). (3) About 47 m.y.: Spreading ceased in the Labrador Sea. Greenland virtually stopped moving relative to North America.
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218
(4) 47 m.y. to present: Continued spreading in the North Atlantic and growth of Reykjanes Ridge. Greenland began to move northwestward as part of the American plate to occupy its present position. TIMING
AND PATTERN
OF GREENLAND’S
DRIFT
The timing and pattern of Greenland’s drift from northwestern Europe are now better understood in the light of sea-floor spreading evidence as discussed above. This evidence suggests that the spreading between Greenland and North Europe started around 60 m.y. ago (anomaly 24). The initiation of this spreading is linked in some way with the widespread Early Tertiary igneous activity along certain parts of the continental borderlands of the northeast Atlantic (Fig. 1). Vogt (1971) considers that within the accuracy of the dating methods the splitting between Greenland and North Europe did occur concurrently with the Brito-Arctic igneous activity. The available palaeomagnetic data (Fig. 4), however, do not substantiate this and instead suggest that the Norwegian-Greenland Sea, in part, had already opened at that time. The possibility that rifting may have existed long before sea-floor spreading began should not be overlooked. Greater uncertainty exists in the timing of initial rift and subsequent drift between Greenland and eastern Canada. The timing of initial rift has been variously estimated from Late Palaeozoic to Cretaceous-Early Tertiary. In contrast to the southern Labrador, there is a lack of recognizable and dateable magnetic-anomaly pattern in the northern Labrador-Baffin Bay region (Fig. 5). Available geological--geophysical information in this area is in scattered pieces and their tying up is linked with speculation. At its northern extremity the Baffin Bay-Labrador Sea is connected with the Arctic basin (see Fig. 1) through the Nares Strait lineament. From a study of t.he North Greenland fold belt Dawes (1973) considers the Nares Strait-Baffin Bay lineament to represent a fracture of the crust as early as the Palaeozoic. Recent seismic studies in Baffin Bay show that it has an oceanic crust and the thickness of sediments deposited on this crust is up to 7 km (Keen and Barret, 1972). Sediments of comparable thickness are found on the shelf margins on either side of the Baffin Bay. Based on studies of marine sediments exposed on land and on the shelf of West Greenland, Henderson (1973) has suggested that the offshore sediments could be of Jurassic age or older. In the northern Labrador Sea thickness of sediments is reported to be at least 1.5 km (Johnson et al., 1973a) and on average about 2 km on the western extremity of the Labrador basin (Le Pichon et al., 1971). A north to northwest trending basement high in the northern Labrador Sea is considered by Johnson et al. (1973a) to represent the ancestral Labrador Sea spreading centre. They also report dredged samples of sedimentary rocks of Late Cretaceous age.
219
Various studies of the southern Labrador Sea have been reviewed by Vogt and Avery (1974) and they suggest that the larger deeper areas may be parts of an older mid-Atlantic rift that extended northwest into a protoLabrador basin. A similar conclusion has been suggested earlier by other investigators (Le Pichon et al., 1971; Laughton, 1971). According to Johnson et al. (197313) this rifting commenced probably as early as Late Jurassic. Watt (1969) considered that the Jurassic age of the coast-parallel dykes of southwestern Greenland suggests that initial rifting between Greenland and Labrador took place in mid-Mesozoic time. In contrast to this, Le Pichon et al. (1971) propose a two-stage opening of the Labrador Sea during 76-49 m.y. Clarke and Upton (1971) consider the similar stratigraphy, chemistry and Palaeocene age of the Baffin Island and West Greenland olivine basalts to be suggestive of a rifting apart during the time of volcanism itself. However, this is not supported by the available palaeomagnetic data from Disko and Baffin Island; the observed difference between the two poles (Fig. 3) is suggestive of a relatively large separation between Canada and Greenland as early as about 65 m.y. Whether or not a seaway existed between Greenland and Canada before the Cretaceous is a matter of speculation. Palaeomagnetic data from Greenland, especially on Jurassic dykes from southwest Greenland would be of great potential value. Also, it is important to obtain more palaeomagnetic data on Tertiary and Mesozoic rocks from northeastern Canada for resolving uncertainties in the timing and pattern of Greenland’s drift.
REFERENCES Athavale, R.N. and Sharma, P.V., 1974. Preliminary paleomagnetic results on some Triassic rocks from East Greenland. Phys. Earth Planet. Inter., 9: 51-56. Athavale, R.N. and Sharma, P.V., 1975. Paleomagnetic results on Early Tertiary lava flows from West Greenland and their bearing on the evolution history of the Baffin Bay-Labrador Sea region. Can. J. Earth Sci., 12: l-18. Beckinsale, R.D., Brooks, C.K. and Rex, D.C., 1970. K-Ar ages of the Tertiary of East Greenland. Bull. Geol. Sot. Denmark, 20: 27-37. Bidgood, D.E.T. and Harland, W.B., 1961. Paleomagnetism of some East Greenland sedimentary rocks. Nature, 189: 633-634. Bullard, E.C., Everett, J.E. and Smith, A.G., 1965. The fit of the continents around the Atlantic. In: P.M.S. Blackett, E.C. Bullard and S.K. Runcorn (Editors), A Symposium on Continental Drift. Philos. Trans. R. Sot. London, 258A: 41-51. Clarke, D.B. and Upton, B.G.J., 1971. Tertiary basalts of Baffin Island: Field relations and tectonic setting. Can. J. Earth Sci., 8: 248-258. Dawes, P.R., 1973. The North Greenland fold belt: A clue to the history of the Arctic Ocean basin and the Nares Strait lineament. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Science, 2. Academic Press, London, pp. 925-947. Deutsch, E.R. and Kristjansson, L.G., 1974. Palaeomagnetism of Late CretaceousTertiary volcanics from Disko Island, West Greenland. Geophys. J. R. Astron. Sot., 39: 3437-360.
220 Deutsch, E.R., Kristjansson, L.G. and May, B.T., 1971. Remanent magnetism of Lower Tertiary lavas on Baffin Island. Can. J. Earth Sci., 8: 1542-1552. Duncan, R.A., Petersen, N. and Hargraves, R.B., 1972. Mantle plumes, movements of the European plate and polar wanderings. Nature, 239: 82-86. Fitch, F.J., 1965. The structural unity of the reconstructed North Atlantic continent. A symposium on continental drift. Philos. Trans. R. Sot., London, 258A: 191-193. Hailwood, E.A., Tarling, D.H., Mitchell, J.G. and Lovlie, R., 1973. Preliminary observations on the palaeomagnetism and radiometric ages of the Tertiary basalt sequence of Scoresby Sund, East Greenland, Rapp. Gronlands Geol. Unders., 58: 43-47. Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pitman, W.C.111, and Le Pichon, X., 1968. Marine magnetic anomalies, geomagnetic field reversals, and motions of the ocean floors and continents. J. Geophys. Res., 73: 2119-2136. Henderson, G., 1973. The implications of continental drift for the petroleum prospects of West Greenland. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences, 1. Academic Press, London, pp. 599--607. Hood, P. and Bower, M.E., 1973. Low-level aeromagnetic surveys of the continental shelves bordering Baffin Bay and the Labrador Sea. In: P.J. Hood (Editor), Earth Science Symposium on Offshore Eastern Canada. Geol. Surv. Can., Pap., 71-23: 573-598. Johnson, G.L., Campsie, J., Rasmussen, M., Egloff, J., Dittmer, F. and Freitag, J., 1973a. A sedimentary basin in the northern Labrador Sea. Bull. Geol. Sot. Denmark, 22: 1-6. Johnson, G.L., Egloff, J., Rasmussen, M., Dittmer, F. and Freitag, J., 1973b. Sediment distribution and crustal structure of the southern Labrador Sea. Bull. Geol. Sot., Denmark, 22: 7-24. Keen, C.E. and Barret, D.L., 1972. Seismic refraction studies in Baffin Bay: An example of developing ocean basin. Geophys. .J. R. Astron. Sot., 30: 253-271. Kristjansson, L.G. and Deutsch, E.R., 1973. Magnetic properties of rock samples from the Baffin Bay coast. In: P.J. Hood (Editor), Earth Science Symposium on Offshore Canada. Geol. Surv. Can., Pap., 71-23: 573-598. Laughton, A.S., 1971. South Labrador Sea and the evolution of the North Atlantic. Nature, 232: 612-617. Laughton, A.S., 1972. The southern Labrador Sea - A key to the Mesozoic and Early Tertiary evolution of the North Atlantic. In: Initial Reports of the Deep Sea Drilling Project, 12. U.S. Govt. Printing Office, Washington, D.C., pp. 1115-l 179. Lc Pi&on, X., Hyndman, R.D. and Pautot, G., 1971. Geophysical study of the opening of the Labrador Sea. J. Geophys. Res., 76: 4724-4743. Pitman, W.C.111 and Talwani, M., 1972. Sea-floor spreading in the North Atlantic. Geol. Sot. Am. Bull., 83: 619--646. Purdy, J.W., Mussett, A.E., Charlton, S.R., Eckford, M.J. and English, H.N., 1972. The British Tertiary igneous province: Potassium-Argon ages of the Antim basalts. Ckophys. J. R. Astron. Sot., 27: 327--335. RWVC, S.C., Lcythaeuser, D., Helsley, C.E. and Bay, K.W., 1974. Paleomagnetic results from the Upper Triassic of East Greenland. J. Geophys. Res., 79: 3302-3307. Rosenkrantz, A. and Pulvertaft, T.C.R., 1969. Cretaceous-Tertiary stratigraphy and tectonics in northern West Greenland. In: M. Kay (Editor), North Atlantic -Geology and Continental Drift. Am. Assoc. Pet. Geol., Mem., 12: 883-898. Tarling, D.H., 1967. The paleomagnetic properties of some Tertiary lavas from East Greenland. Earth Planet. Sci. Lett., 3: 81-88. Tarling, D.H., 1970. Palacomagnetic results from the Faeroe Islands. In: S.K. Runcorn (Editor), Palaeogeophysics. Academic Press, London, pp. 193-208. Tarling, D.H. and Gale, N.H., 1968. Isotopic dating and paleomagnetic Polarity in the Faeroe Islands. Nature. 218: 1043--1044.
221 Vogt, P.R., 1971. Asthenosphere motion recorded by the ocean floor south of Iceland. Earth Planet. Sci. Lett., 13: 153-160. Vogt, P.R. and Avery, O.E., 1974. Detailed magnetic surveys in the North Atlantic and Labrador Sea. J. Geophys. Res., 79: 363-389. Vogt, P.R., Ostenso, N.A. and Johnson, G.L., 1970. Magnetic and bathymetric data bearing on sea-floor spreading north of Iceland. J. Geophys. Res., 75: 903-920. Vogt, P.R., Johnson, G.L., Holcombe, T.L., Gilg, J.G. and Avery, O.E., 1971. Episodes of sea-floor spreading recorded by the North Atlantic basement. Tectonophysics, 12: 211-234. Watt, W.S., 1969. The coast-parallel dike swarm of southwest Greenland in relation to the opening of the Labrador Sea. Can. J. Earth Sci., 6: 1320-1321. Wells, J.M. and Verhoogen, J., 1967. Late Paleozoic paleomagnetic poles and the opening of the Atlantic Ocean. J. Geophys. Res., 72: 1777-1782.
PALAEOMAGNETIC OF GREENLAND*
P.V. SHARMA
EVIDENCE
RELATING
University of Copenhagen,
Palaeomagnetic Laboratory, version
- Printed
in The Netherlands
TO THE CENOZOIC
DRIFT
and R.N. ATHAVALE
Institute of Geophysics,
(Revised
209 Amsterdam
accepted
Copenhagen
(Denmark)
National Geophysical Research Institute, Hyderabad (India)
July 15, 1975)
ABSTRACT Sharma, P.V. and Athavale, R.N., 1975. Palaeomagnetic evidence relating to the Cenozoic drift of Greenland. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4) : 209-221. An analysis is made of the available palaeomagnetic data for the Early Tertiary basalts of Greenland. The Early Tertiary pole for Greenland, calculated from stable remanent magnetic directions of 81 lava flows on Disko Island, is located at 67.5°N,1650W, with dp = 5O and dm = 6’. The Disko pole is widely separated from the pole obtained for contemporaneous lava flows on Baffin Island, Canada, and this suggests a significantly large separation between Canada and Greenland as early as 65 m.y. ago. The Disko pole is also displaced from poles of similar age in northwestern Europe (British Isles and the Faeroe Islands). This is apparently inconsistent with the idea that the split of Greenland from Europe occurred concurrently with the widespread Early Tertiary igneous activity on the continental borderlands of the northeast Atlantic. The main results of the palaeomagnetic comparisons between eastern Canada, Greenland and North Europe are discussed in the light of available evidence for sea-floor spreading in the North Atlantic.
INTRODUCTION
Greenland is considered to be a link between the Eurasian and American plates and is believed to have separated first from North America and then from Europe as a consequence of the opening of the Baffin Bay-Labrador Sea and the northeastern part of the Atlantic Ocean. Palaeomagnetic data from Greenland are particularly interesting as Greenland holds a key position between North America and Europe in any attempted pre-drift configuration of these continents. Whereas a considerable amount of palaeoma~etic data has been obtained * Contribution
No. 5, Institute
of Geophysics,
Copenhagen
University.
210
from North America and Europe in the past two decades, such data from Greenland are as yet sparse. Mainly because of the difficulty of access few rocks from this area have been studied. There is a conspicuous lack of palaeomagnetic data for the Palaeozoic and Mesozoic rocks excepting the Triassic sediments of East Greenland for which some preliminary results have been reported (Bidgood and Harland, 1961; Athavale and Sharma, 1974; Reeve et al., 1974). To date a greater number of studies have been made on younger rocks, particularly on the Early Tertiary volcanics of East and West Greenland. The results of palaeomagnetic studies (Tarling, 1967; Deutsch and Krist-
Fig. 1. Known Early Tertiary igneous activity on the continental borderlands of the North Atlantic plotted on approx. 60 m.y. B.P. reconstruction modified from Fitch (1965). I = West Greenland basalt province; 2 = East Greenland basalts; 3 = Faeroes basalts; 4 and 5 = British Tertiary igneous province; 6 = Baffin Island basalts. Stippled area represents pre-60 m.y. ocean crust. Partly reproduced from Vogt and Avery (1974).
211
jansson, 1974; Athavale and Sharma, 1975) based on extensive sampling of these rocks are of great interest in relation to the proposed movement of Greenland between North America and Europe. The basaltic lavas of Early Tertiary age exposed on the western and eastern margins of Greenland (Fig. 1) are related in age to the other rocks of the North Atlantic volcanic province exposed on Baffin Island, the Faeroes and the British Isles. By their distribution, these areas can provide crucial palaeomagnetic sites for testing proposed reconstructions that involve separation of Greenland from eastern Canada and northwestern Europe. In particular, palaeomagnetic comparisons across the Baffin Bay and the northeast Atlantic are potentially useful for tracing the drift history of Greenland. We discuss here the results of palaeom~etic comparisons across the Baffin Bay and the northeast Atlantic together with their bearing on the Cenozoic drift of Greenland. The comp~isons are based mainly on the data from Early Tertiary volcanics of Greenland, Baffin Island, the Faeroes and the British Isles. The interpretations are hampered by lack of reliable radiome tric age data. SELECTION OF THE EARLY TERTIARY DATA
The palaeomagnetic data used for the comparative study of the Early Tertiary pole positions are listed in Table I. The list comprises published results of selected studies from West and East Greenland, Baffin Island, the Faeroes and the British Isles. The stability of magnetization for all selected results is reported to be established by alternating-field and/or thermal-cleaning tests. In most studies (excepting the one on Baffin Island), the sampling was extensive enough for the mean direction to be reasonably assumed to represent the geomagnetic field at that time, and in all cases the age of magnetization was determined within reasonable limits. Rock ages quoted in the list are from original references. In Fig. 2, the Early Tertiary pole positions for Greenland are shown along with poles for Baffin Island, the Faeroes and the British Isles. The East Greenland result is based on 28 Palaeocene lava flows (age 60-47 m.y.) from the Scoresby Sund area (Tarling, 1967). Palaeomagnetic results of a recent study (Hailwood et al., 1973) based on more extensive sampling of the Scoresby Sund basalts are yet not published; however, these authors report radiometric ages of 52-47 m.y. for the rocks studied. The most reliable West Greenland result is that obtained from 81 Disko basalt flows (of Late Danian age, i.e. -65-60 m.y.) by combining the results for northern and southern Disko (Athavale and Sharma, 1975). In contrast to this the Baffin Island pole based on five Cape Dyer flows (Deutsch et al., 19’71) dated 58 m.y. has much lower statistical reliability. From the northweste~ Europe results the most reliable Early Tertiary pole is that obtained from 253 Faeroese basalt flows (Tarling, 1970) for
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~-
171-E
77’N
-
2’
II’
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11.5O
4.9’
8”
5.5”
.-
( 197 0 )
(1967)
1974).
__._____-_-
.._._
et al. ( 197 1)
Various studies (mean quoted in Tarling, 1970)
Tariing
Tarling
Deutsch
and Kristjansson,
2.5’
13”
12O
Mean of 1 and 2
5.6’
and Sharma
Kristjansson and Deutsch (1973)
Athavale { 1975)
Reference
9’
6”
Radii of 95% confidence oval
and Pulvertaft, 1969). has given a date of 70 * 4 m.y. (Deutsch
___._~__^_.
L61’E
77” N
.-_,.-_-
174’ E
63’N
__
55” w
83-N
58 m.y. (E. Farrar in Clarke and Upton, 1971) 60-55 m.y. (Beckinsale et al., 1970) 52-47 m.y. (~ailwood et al., 1973) 59-53 m.y. (Tarling and Gale, 1968) 64-50 m.y. (Various sources quoted in Duncan et al., 1972; Purdy et al., 1972)
165’W
67”3ON
154’3OW
72”N
Late Danian*
Long.
Lat.
Pole position ~_.___
II~-_-
province
169” w
_.._
volcanic
62”N
m.y.)
Atlantic
Late Danian* *
Late Danian* (i.e.- 65-60
North
* Age inferred from palaeontological evidence (Rosenkrantz ** A single K/Ar result obtained by a commercial laboratory
7.
6.
c5 .
4.
3.
Lava flows, Faeroe Islands Intrusives and extrusives, NW Scotland and N. Ireland
Lava flows N. Disko, W. Greenland Lava flows S. Disko, W. Greenland Disko Island, W. Greenland Lava flows Cape Dyer Baffin Island, Canada Lava flows Scoresby Sund, E. Greenland
1.
2.
Formation
Serial No. Age
_~_-.-~.
___.__..~
and locality
poles from the Early Tertiary
Palaeomagnetic
TABLE I
213
180
Fig. 2. Palaeomagnetic pole positions for Early Tertiary lavas from Baffin Island (BF), Disko Island (D), East Greenland (EG), Faeroe Islands (F) and British Isles (BI). The oval of confidence for the latter pole (BI) is not shown because the mean is computed from sixteen studies of variable reliability (Tarling, 1970). Lambert equal-area projection with North pole at the pivot.
which the K/Ar ages are from 59 to 53 m.y. It is significantly different from the Greenland (Disko) pole but lies close to an average Early Tertiary (-64-50 m.y.) pole for the British Isles based on sixteen different studies (mean pole quoted in Tarling, 1970). PALAEOMAGNETIC
COMPARISONS
ACROSS
BAFFIN
BAY
Palaeomagnetic comparisons across Baffin Bay are potentially useful for establishing whether the opening of Baffin Bay predates the time of major extrusions on West Greenland and on Baffin Island. If one could assume the Disko and Baffin Island poles (Fig. 2) to be representative of the two coast areas, then it is possible to interpret the difference in pole positions in terms of relative separation of the two landmasses. We have discussed this point in greater detail in a previous paper (Athavale and Sharma, 1975). Figure 3 summarizes the results of this comparison. The displaced position of the Disko pole, from D, to D2 , is after the Bullard et al. (1965) reconstruction model which involves a rotation of Greenland towards North America through 18” around a pole of rotation at 70.5”N,
Fig. 3. Comparison between Baffin Island (BF) and Disko poles. The displaced position of Disko pole (from D, to D2 ) is after rotation of Greenland towards Canada by 18 according to the Builard et al. (1965) reconstruction model. The pivot for rotation is shown by a diamond. Lambert equal-area projection.
94.4”W (Wells and Verhoogen, 1967). It is apparent from Fig. 3 that the amount of rotation suggested is not adequate to bring the Disko and Baffin Island poles close together. A larger rotation by about 50” is required to make the poles in agreement. It follows therefore that to accommodate such a large rotation Early Tertiary Greenland should be located further east. This implies a significantly large separation between Greenland and Canada as early as 65 m.y. ago. The above interpretation suffers from a major weakness. The Baffin Island pole is associated with a large statistical error; the computed pole is based on only five flows at Cape Dyer and may not be representative of the axial dipole field prevailing at that time. The proposed interpretation is inconsistent with the suggestion (Clarke and Upton, 1971) that the Baffin Island and West Greenland olivine basalt areas were once continuous and began rifting apart during the time of volcanism itself. HOWever, in the light of other geological and geophysical evidence (discussed later) a mid-&Iesozoie rifting between Greenland and Canada and a larger separation (than at present) between the two landmasses is not at all improbable.
215 PALAEOMAGNETIC
COMPARISONS ACROSS NORTHEAST
ATLANTIC
Palaeomagnetic comparisons of the Early Tertiary data on either side of the northeast Atlantic are meaningful in that they could establish if this part of the North Atlantic Ocean had already opened at that time or that the opening occurred concurrently with the Early Tertiary volcanism on the continental borderlands of the northeastern Atlantic (see Fig. 1). Such comparisons are statistically feasible now as reliable palaeomagnetic data are available from Greenland and northwestern Europe. For this comparison we have taken the mean pole positions for Disko and the Faeroes/ British Isles (see Table I) to be representative poles for Early Tertiary Greenland and northwestern Europe. From Fig. 4 it is apparent that there is a real difference between the Greenland and European results. The separation of the Disko pole from the Faeroese and British Isles poles is not reduced on rotation of Greenland towards Europe by 22” about a pivot (73”N,965”E) after the Bullard et al. (1965) rotation model for the fit of the northern continents.
Fig. 4. Comparison of Early Tertiary palaeomagnetic poles from Disko (Di , Ds), Faeroe Islands (F) and the British Isles (BI). The displaced position of Disko pole (from Di to Ds) is after rotation of Greenland towards Europe by 22’ according to the Bullard et al. (1965) reconstruction model. The pivot for rotation is shown by a diamond. Lambert equal-area projection.
216
There are two possible explanations for the observed difference between the Disko and the British Isles/Faeroes poles. It is likely that the noncoincidence of the poles may be due to age differences between the DiskoSvartenhuk volcanism and the British Isles/Faeroes volcanism. An alternative explanation is that the Norwegian-Greenland Sea (at least in part) had already opened at the time these rocks were laid down. The available radiometric age data for these rocks (see Table I) are insufficient to decide between these alternatives. GREENLAND
AND THE EVOLUTION
OF THE NORTH
ATLANTIC
The history of sea-floor spreading in the North Atlantic is very relevant to any discussion of Greenland’s separation from North America and Europe. Our present knowledge of the spreading history depends mainly on the interpretation of detailed magnetic surveys between southern Greenland and Labrador on one side and between southern Greenland and the British Isles on the other (Vogt et al., 1971; Le Pichon et al., 1971; Laughton, 1972; Pitman and Talwani, 1972; Vogt and Avery, 1974). The spreading history northeast of Iceland is complicated (Vogt et al., 1970). The anomaly picture in the northern Labrador Sea (north of 60”) is not clear; only a few magnetic lineations have been identified (Hood and Bower, 1973) and there is uncertainty as to their age. Major magnetic anomalies identified in the northeast Atlantic and Labrador Sea are shown in Fig. 5. The magnetic-lineation pattern south of Iceland is very clear, where anomalies going back to no. 24 (60 m.y. B.P.) and parallel to the NE--SW trend of the Reykjanes Ridge have been identified. In the Labrador Sea, southwest of Greenland, magnetic anomalies numbering from 24 to 19 (corresponding to age 60-47 m.y. B.P.) have been identified and these have a general trend in the NW-SE direction. These anomalies taken together with those of corresponding age, parallel to the Reykjanes Ridge, suggest that Greenland was pushed away both from North America and northwestern Europe during the Early Tertiary. The general picture that evolves from interpretation of anomalies in Fig. 5 leads to the following scheme of Greenland’s movement during the Cenozoic : (1) About 60 m.y. (anomaly 24): Greenland separated from North EUrope, breaking off from Rockall Plateau and opening the northeast branch of the Atlantic. (2) 60-47 m.y. (anomaly 24-19): Simultaneous spreading in the Labrador Sea and northeast Atlantic led to a resultant movement of Greenland in the northward direction. The northward movement of Greenland is also supported by the mean remanent direction of the Early Tertiary Disko lavas that gives a palaeolatitude of 56” (Athavale and Sharma, 1975). (3) About 47 m.y.: Spreading ceased in the Labrador Sea. Greenland virtually stopped moving relative to North America.
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218
(4) 47 m.y. to present: Continued spreading in the North Atlantic and growth of Reykjanes Ridge. Greenland began to move northwestward as part of the American plate to occupy its present position. TIMING
AND PATTERN
OF GREENLAND’S
DRIFT
The timing and pattern of Greenland’s drift from northwestern Europe are now better understood in the light of sea-floor spreading evidence as discussed above. This evidence suggests that the spreading between Greenland and North Europe started around 60 m.y. ago (anomaly 24). The initiation of this spreading is linked in some way with the widespread Early Tertiary igneous activity along certain parts of the continental borderlands of the northeast Atlantic (Fig. 1). Vogt (1971) considers that within the accuracy of the dating methods the splitting between Greenland and North Europe did occur concurrently with the Brito-Arctic igneous activity. The available palaeomagnetic data (Fig. 4), however, do not substantiate this and instead suggest that the Norwegian-Greenland Sea, in part, had already opened at that time. The possibility that rifting may have existed long before sea-floor spreading began should not be overlooked. Greater uncertainty exists in the timing of initial rift and subsequent drift between Greenland and eastern Canada. The timing of initial rift has been variously estimated from Late Palaeozoic to Cretaceous-Early Tertiary. In contrast to the southern Labrador, there is a lack of recognizable and dateable magnetic-anomaly pattern in the northern Labrador-Baffin Bay region (Fig. 5). Available geological--geophysical information in this area is in scattered pieces and their tying up is linked with speculation. At its northern extremity the Baffin Bay-Labrador Sea is connected with the Arctic basin (see Fig. 1) through the Nares Strait lineament. From a study of t.he North Greenland fold belt Dawes (1973) considers the Nares Strait-Baffin Bay lineament to represent a fracture of the crust as early as the Palaeozoic. Recent seismic studies in Baffin Bay show that it has an oceanic crust and the thickness of sediments deposited on this crust is up to 7 km (Keen and Barret, 1972). Sediments of comparable thickness are found on the shelf margins on either side of the Baffin Bay. Based on studies of marine sediments exposed on land and on the shelf of West Greenland, Henderson (1973) has suggested that the offshore sediments could be of Jurassic age or older. In the northern Labrador Sea thickness of sediments is reported to be at least 1.5 km (Johnson et al., 1973a) and on average about 2 km on the western extremity of the Labrador basin (Le Pichon et al., 1971). A north to northwest trending basement high in the northern Labrador Sea is considered by Johnson et al. (1973a) to represent the ancestral Labrador Sea spreading centre. They also report dredged samples of sedimentary rocks of Late Cretaceous age.
219
Various studies of the southern Labrador Sea have been reviewed by Vogt and Avery (1974) and they suggest that the larger deeper areas may be parts of an older mid-Atlantic rift that extended northwest into a protoLabrador basin. A similar conclusion has been suggested earlier by other investigators (Le Pichon et al., 1971; Laughton, 1971). According to Johnson et al. (197313) this rifting commenced probably as early as Late Jurassic. Watt (1969) considered that the Jurassic age of the coast-parallel dykes of southwestern Greenland suggests that initial rifting between Greenland and Labrador took place in mid-Mesozoic time. In contrast to this, Le Pichon et al. (1971) propose a two-stage opening of the Labrador Sea during 76-49 m.y. Clarke and Upton (1971) consider the similar stratigraphy, chemistry and Palaeocene age of the Baffin Island and West Greenland olivine basalts to be suggestive of a rifting apart during the time of volcanism itself. However, this is not supported by the available palaeomagnetic data from Disko and Baffin Island; the observed difference between the two poles (Fig. 3) is suggestive of a relatively large separation between Canada and Greenland as early as about 65 m.y. Whether or not a seaway existed between Greenland and Canada before the Cretaceous is a matter of speculation. Palaeomagnetic data from Greenland, especially on Jurassic dykes from southwest Greenland would be of great potential value. Also, it is important to obtain more palaeomagnetic data on Tertiary and Mesozoic rocks from northeastern Canada for resolving uncertainties in the timing and pattern of Greenland’s drift.
REFERENCES Athavale, R.N. and Sharma, P.V., 1974. Preliminary paleomagnetic results on some Triassic rocks from East Greenland. Phys. Earth Planet. Inter., 9: 51-56. Athavale, R.N. and Sharma, P.V., 1975. Paleomagnetic results on Early Tertiary lava flows from West Greenland and their bearing on the evolution history of the Baffin Bay-Labrador Sea region. Can. J. Earth Sci., 12: l-18. Beckinsale, R.D., Brooks, C.K. and Rex, D.C., 1970. K-Ar ages of the Tertiary of East Greenland. Bull. Geol. Sot. Denmark, 20: 27-37. Bidgood, D.E.T. and Harland, W.B., 1961. Paleomagnetism of some East Greenland sedimentary rocks. Nature, 189: 633-634. Bullard, E.C., Everett, J.E. and Smith, A.G., 1965. The fit of the continents around the Atlantic. In: P.M.S. Blackett, E.C. Bullard and S.K. Runcorn (Editors), A Symposium on Continental Drift. Philos. Trans. R. Sot. London, 258A: 41-51. Clarke, D.B. and Upton, B.G.J., 1971. Tertiary basalts of Baffin Island: Field relations and tectonic setting. Can. J. Earth Sci., 8: 248-258. Dawes, P.R., 1973. The North Greenland fold belt: A clue to the history of the Arctic Ocean basin and the Nares Strait lineament. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Science, 2. Academic Press, London, pp. 925-947. Deutsch, E.R. and Kristjansson, L.G., 1974. Palaeomagnetism of Late CretaceousTertiary volcanics from Disko Island, West Greenland. Geophys. J. R. Astron. Sot., 39: 3437-360.
220 Deutsch, E.R., Kristjansson, L.G. and May, B.T., 1971. Remanent magnetism of Lower Tertiary lavas on Baffin Island. Can. J. Earth Sci., 8: 1542-1552. Duncan, R.A., Petersen, N. and Hargraves, R.B., 1972. Mantle plumes, movements of the European plate and polar wanderings. Nature, 239: 82-86. Fitch, F.J., 1965. The structural unity of the reconstructed North Atlantic continent. A symposium on continental drift. Philos. Trans. R. Sot., London, 258A: 191-193. Hailwood, E.A., Tarling, D.H., Mitchell, J.G. and Lovlie, R., 1973. Preliminary observations on the palaeomagnetism and radiometric ages of the Tertiary basalt sequence of Scoresby Sund, East Greenland, Rapp. Gronlands Geol. Unders., 58: 43-47. Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pitman, W.C.111, and Le Pichon, X., 1968. Marine magnetic anomalies, geomagnetic field reversals, and motions of the ocean floors and continents. J. Geophys. Res., 73: 2119-2136. Henderson, G., 1973. The implications of continental drift for the petroleum prospects of West Greenland. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences, 1. Academic Press, London, pp. 599--607. Hood, P. and Bower, M.E., 1973. Low-level aeromagnetic surveys of the continental shelves bordering Baffin Bay and the Labrador Sea. In: P.J. Hood (Editor), Earth Science Symposium on Offshore Eastern Canada. Geol. Surv. Can., Pap., 71-23: 573-598. Johnson, G.L., Campsie, J., Rasmussen, M., Egloff, J., Dittmer, F. and Freitag, J., 1973a. A sedimentary basin in the northern Labrador Sea. Bull. Geol. Sot. Denmark, 22: 1-6. Johnson, G.L., Egloff, J., Rasmussen, M., Dittmer, F. and Freitag, J., 1973b. Sediment distribution and crustal structure of the southern Labrador Sea. Bull. Geol. Sot., Denmark, 22: 7-24. Keen, C.E. and Barret, D.L., 1972. Seismic refraction studies in Baffin Bay: An example of developing ocean basin. Geophys. .J. R. Astron. Sot., 30: 253-271. Kristjansson, L.G. and Deutsch, E.R., 1973. Magnetic properties of rock samples from the Baffin Bay coast. In: P.J. Hood (Editor), Earth Science Symposium on Offshore Canada. Geol. Surv. Can., Pap., 71-23: 573-598. Laughton, A.S., 1971. South Labrador Sea and the evolution of the North Atlantic. Nature, 232: 612-617. Laughton, A.S., 1972. The southern Labrador Sea - A key to the Mesozoic and Early Tertiary evolution of the North Atlantic. In: Initial Reports of the Deep Sea Drilling Project, 12. U.S. Govt. Printing Office, Washington, D.C., pp. 1115-l 179. Lc Pi&on, X., Hyndman, R.D. and Pautot, G., 1971. Geophysical study of the opening of the Labrador Sea. J. Geophys. Res., 76: 4724-4743. Pitman, W.C.111 and Talwani, M., 1972. Sea-floor spreading in the North Atlantic. Geol. Sot. Am. Bull., 83: 619--646. Purdy, J.W., Mussett, A.E., Charlton, S.R., Eckford, M.J. and English, H.N., 1972. The British Tertiary igneous province: Potassium-Argon ages of the Antim basalts. Ckophys. J. R. Astron. Sot., 27: 327--335. RWVC, S.C., Lcythaeuser, D., Helsley, C.E. and Bay, K.W., 1974. Paleomagnetic results from the Upper Triassic of East Greenland. J. Geophys. Res., 79: 3302-3307. Rosenkrantz, A. and Pulvertaft, T.C.R., 1969. Cretaceous-Tertiary stratigraphy and tectonics in northern West Greenland. In: M. Kay (Editor), North Atlantic -Geology and Continental Drift. Am. Assoc. Pet. Geol., Mem., 12: 883-898. Tarling, D.H., 1967. The paleomagnetic properties of some Tertiary lavas from East Greenland. Earth Planet. Sci. Lett., 3: 81-88. Tarling, D.H., 1970. Palacomagnetic results from the Faeroe Islands. In: S.K. Runcorn (Editor), Palaeogeophysics. Academic Press, London, pp. 193-208. Tarling, D.H. and Gale, N.H., 1968. Isotopic dating and paleomagnetic Polarity in the Faeroe Islands. Nature. 218: 1043--1044.
221 Vogt, P.R., 1971. Asthenosphere motion recorded by the ocean floor south of Iceland. Earth Planet. Sci. Lett., 13: 153-160. Vogt, P.R. and Avery, O.E., 1974. Detailed magnetic surveys in the North Atlantic and Labrador Sea. J. Geophys. Res., 79: 363-389. Vogt, P.R., Ostenso, N.A. and Johnson, G.L., 1970. Magnetic and bathymetric data bearing on sea-floor spreading north of Iceland. J. Geophys. Res., 75: 903-920. Vogt, P.R., Johnson, G.L., Holcombe, T.L., Gilg, J.G. and Avery, O.E., 1971. Episodes of sea-floor spreading recorded by the North Atlantic basement. Tectonophysics, 12: 211-234. Watt, W.S., 1969. The coast-parallel dike swarm of southwest Greenland in relation to the opening of the Labrador Sea. Can. J. Earth Sci., 6: 1320-1321. Wells, J.M. and Verhoogen, J., 1967. Late Paleozoic paleomagnetic poles and the opening of the Atlantic Ocean. J. Geophys. Res., 72: 1777-1782.