of the Labrador Sea and offshore southern West. Greenland has been given ...... Development of the Greenland-Scotland Ridge (Eds M. H. P.. Bott, S. Saxov, M.
Marine and Petroleum Geology 12 (1995) 205-217 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0264-8172/95/$10.00
r~UTTERWORTH ~E I N E M A N N
Labrador Sea: the extent of continental and oceanic crust and the timing of the onset of seafloor spreading James A. Chalmers* and Kirsten Holt Laursen Geological Survey of Greenland, ~ster Voldgade 10, DK- 1350 Copenhagen K, Denmark Received 4 March 1993; revised 31 December 1993; accepted 19 February 1994 Regional reflection seismic profiles across the Labrador Sea originally acquired in 1977 have been reprocessed and reinterpreted. Zones of different structural style have been identified. The seismic interpretations have been used to constrain magnetic modelling and oceanic crust has been confirmed from magnetic anomaly 27N and seaward. However, all attempts to model the area landward of magnetic anomaly 27N as a series of remanent magnetizations of alternating polarity have failed. Interpretations which fit the magnetic and seismic data consist of a zone of block-faulted and subsided continental crust on both the Greenland and Canadian sides, which is separated from oceanic crust by zones of continental crust intruded by and in places overlain by magnetized igneous material. It is concluded that seafloor spreading started in the Labrador Sea in the Palaeocene (Chron 27N) and that large areas under deep water formerly thought to be underlain by oceanic crust should now be considered to be continental. Keywords: Labrador Sea; plate tectonics; seismic interpretation; magnetic modelling
The understanding of the distribution of oceanic crust in the Labrador Sea that is current in published work (e.g. Ziegler, 1988) is based on the interpretation of oceanic magnetic anomalies originally made by Srivastava (1978). He identified the earliest anomaly as 31. Roest and Srivastava (1989a; 1989b) revised the interpretation to show anomaly 33 as the oldest, but their other identifications are essentially the same as those of Srivastava (1978). A consequence of these identifications is that the boundary between continental and oceanic crust is interpreted to lie under the steep bathymetric slopes at the edge of the continental shelves (Figure 1). The interpretations of Srivastava (1978) and Roest and Srivastava (1989a; 1989b) were made on magnetic data only, and were only loosely constrained by other geophysical data. In 1977, the Bundesanstalt fiir Geowissenschaften and Rohstoffe (BGR) acquired a regional seismic survey in the Labrador Sea (Hinz et al., 1979). Chalmers (1991) reported a test of the Roest and Srivastava (1989a; 1989b) interpretation along a reprocessed version of part of one of the B G R lines, BGR/77-12, which runs from the edge of the continental shelf off southern West Greenland for 275 km to the south-west (Figure 1). Chalmers (1991) used the 'basement' interpreted on the seismic line to constrain the magnetic modelling and confirmed Roest and Srivastava's (1989a; 1989b) interpretation of oceanic * Correspondence to Mr J. A. Chalmers
crust from magnetic anomaly 27N. However, Chalmers (1991) found that all attempts to model the area landward of anomaly 27N as a series of remanent magnetizations of alternating polarity failed. An alternative interpretation which fits the seismic and magnetic data better along seismic line BGR/77-12 was proposed by Chalmers (1991). In this, the Greenland margin of the Labrador Sea was divided into three zones. The zone nearest Greenland (Zone A) is interpreted as extended and block-faulted continental crust; the next zone to seaward (Zone B) is interpreted as extended and thinned continental crust, intruded and overlain by reversely magnetized igneous material; and the outermost zone (Zone C) is interpreted as oceanic crust with the oldest magnetic anomaly being 27N. Chalmers' (1991) reinterpretation was of only one seismic line. However, the implications of the reinterpretation of the plate tectonic and geological development of the area suggested that the interpretation by Roest and Srivastava (1989a; 1989b) should be tested on additional lines from the BGR/77 survey. Consequently, an additional 2670 km of the 1977 B G R survey were reprocessed and reinterpreted and magnetic modelling based on some of that interpretation carried out. The results confirm and extend those reported by Chalmers (1991) and have lead to a new map of the extent of continental and oceanic crust in the Labrador Sea (Figure 2). A review of the geology of the Labrador Sea and offshore southern West Greenland has been given by Chalmers et al. (1993).
Marine and Petroleum Geology 1995 V o l u m e 12 N u m b e r 2
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Seafloor spreading in Labrador Sea: J. A. Chalrners and K. H. Laursen mid-points are also common reflection points. I~ip move out corrections were used to compensate for this. Additional improvements in data quality have come from the suppression of multiple energy, especially from the sea-bed multiples. This was principally accomplished by means of an FK-demultiple process, combined with an inner trace mute. There is, however, a persistent pattern of noise on the data at two-way times greater than those of the first sea-bed multiple. No way of removing this noise has yet been found. Resolution of the data has been improved and interbed reverberations suppressed by means of a deconvolution before stack.
Magnetic data Magnetic data were recorded together with the seismic data by B G R in 1977. The data have been reduced to residuals by subtracting the IGRF. No magnetic data was available on the south-western part of line BGR/77-17 or on the north-eastern end of line BGR/77-21. There, the B G R data were supplemented by data read from the magnetics map in Srivastava (1986) and adjusted by a constant datum shift to fit the B G R data.
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Figure I Map of the Labrador Sea showing an interpretation typical of that in published papers. Seafloor spreading anomalies, the extinct spreading axis, fracture zones and the boundary between continental and oceanic crust are from Roest and Srivastava (1989b) south of about 63°N and from Tucholke and Fry (1985) further north. The positions of the Gjoa and Hecla Rises are from Tucholke and Fry (1985). Seismic lines shot by BGR in 1977 are shown and the reprocessed parts discussed in this paper are highlighted
Geophysical data Se&mic data The multichannel seismic survey recorded by B G R in 1977 was obtained using a 2400 m, 48-channel streamer. The source consisted of 23.451 (1430 in 3) airguns. The original processing sequence consisted of a two-fold vertical stack then a 12-fold weighted stack following velocity analysis. Post-stack operations were limited to deconvolution and time-variable frequency filtering. The reprocessing sequence was chosen during extensive trials made on the north-eastern part of line BGR/77-12 (Chalmers, 1991). This sequence has now been applied to an additional 2670 km of the survey and only parameters that change with the geology or water depth have been altered (stacking velocities, deconvolutions and time-variable filters). The reprocessed data are much more easily interpreted than those processed with the original sequence. This is primarily due to the reprocessed data having been migrated. However, the geological structure in the area is complex enough and some dips are steep enough to render invalid the assumption that common
206
Gravity data are also available along the BGR/1977 seismic lines, but it is suspected that many of the gravity anomalies originate from structures in the deep crust and upper mantle. This is not discussed in the present paper.
Seismic interpretation Figures 1 and 2 show the location of the BGR/77 seismic survey. Lines that cross the continent-ocean boundaries (COBs) and discussed here or by Chalmers (1991) are emphasized. Lines BGR/77-6, BGR/77-12 (Chalmers, 1991), BGR/77-21 and BGR/77-17 cross the margin on the Greenland side and BGR/77-17 crosses the margin on the Canadian side. Examples of the reprocessed data are shown in Foldouts 1-5. The seismic sections were interpreted both structurally and seismo-stratigraphically. Sequences (or more strictly rSUpersequences) of syn- and post-rift sediments were found and the structural style varied along each line. However, zones of similar structural style could be identified from line to line. Each line is discussed individually.
Line BGR/77-6 (Foldout 1) North-east of the shelf break at about shot point (SP) 9200 a sedimentary succession about 2 s two-way time (TWT) thick can be seen. This is interpreted to consist of post-rift sedimentary sequences. Some noise from incompletely removed multiples crosses the generally flat-lying reflections from the sediments. Below an unconformity at about 2 s TWT can be seen what appear to be remanent half-grabens containing northeastwards dipping reflections. No continuous reflections arrive from below these, which suggests that they lie on basement. From SP 8400 to SP 9200 the flat-lying sediments appear to lie directly on basement, which steps upwards
M a r i n e and P e t r o l e u m G e o l o g y 1995 Volume 12 Number 2
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Figure 5 Magnetic modelling along the north-east (Greenland) end of seismic line BGR/77-17 and comparison of the calculated and observed residual magnetic fields that result. (a) 'Oceanic crust' model in which the model profile was generated from the interpretations by Roest and Srivastava (1989b) for the locations of seafloor spreading anomalies as described in the text. (b) Alternative 'continental crust' (plus intrusions) model developed in this paper
seismic data from SP 600 to at least SP 1500 show clear evidence of fault blocks, similar to those known to involve basement elsewhere on the Labrador shelf (Balkwill et al., 1990). However, the large negative magnetic anomaly in this area can only be fitted by assuming a substantial thickness (about 6 - 7 km) of non-magnetized rock below the block-faulted surface. All of the above models were made assuming a constant rate and direction of spreading from Chron 33 to 26. Other possibilities than the seafloor spreading anomaly patterns shown in Figures 3a, 4a, 5a and 6a were tried, but not shown here. Attempts were made, for example, to vary the size and spacing of the normally and reversely magnetized blocks. This could happen if, for example, spreading rates between Chrons 33 and 27 varied, or if there had been 'jumps' in the location of the spreading axis. This latter suggestion would be implied by combining Roest and Srivastava's (1989a; 1989b) anomaly identifications over line BGR/77-6 with Tucholke and Fry's (1985) suggestion that the Hecla Rise may be a 'Late Cretaceous-Paleocene spreading ridge'.
210
Figure 6 Magnetic modelling along the south-west (Canadian) end of seismic line BGR/77-17 and comparison of the calculated and observed residual magnetic fields that result. The observed field shown in Figure 6a and 6b has had a regional gradient of 0.44 nT/km, increasing to the north-east, subtracted from it. (a) 'Oceanic crust' model in which the model profile was generated from the interpretations by Roest and Srivastava (1989b) for the locations of seafloor spreading anomalies as described in the text, (b) Alternative 'continental crust' (plus intrusions) model developed in this paper
On the north-east (Greenland) end of line BGR/77-17 it was possible to obtain a fit between the measured and calculated fields for a model containing bands of normally and reversely magnetized crust landwards of anomaly 27N. However, it was not possible to do so on any of the other lines. The conclusion must, therefore, be that the identification of seafloor spreading anomalies older than 27 made by Roest and Srivastava (1989a; 1989b) and earlier workers need to be reconsidered.
Alternative magnetic models Better fits to the observed magnetic data than those discussed above were obtained as shown in Figures 3b, 4b, 5b, 6b and 7. These all assume that the zones landward of anomaly 27N consist of continental crust, which has, in places, been intruded or overlain by igneous material.
Line BGR/77-12. Figure 3b is redrawn from Chalmers' (1991) Figure 5. Zone A is modelled as continental crust and a susceptibility of 0.04 (SI units) was found to give a best fit to the observed anomalies, except for a small wedge near SP 500, which has a susceptibility of 0.13 (SI units). Such a wedge could be
Marine and Petroleum Geology 1995 Volume 12 Number 2
Seafloor spreading in Labrador Sea: J. A. Chalmers and K. H. Laursen 800--~-- -
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due to granulite facies gneisses within an area of amphibolite facies such as those known to exist onshore (Thorning, 1986). Zone B is interpreted as continental crust [modelled by susceptibilities 0.02 and 0.025 (SI units)] intruded by reversely magnetized igneous material (modelled by magnetizations of 0.5 and 1.5 A/m) and overlain by reversely magnetized lavas (modelled by zero susceptibility and a remanent magnetization of 3 A/m). Oceanic crust with alternate bands of reverse and normal remanent magnetization occurs in Zone 3. The oldest seafloor spreading anomaly is 27N; however, the reversely magnetized rocks in Zone B may have been emplaced during Chron 27R.
Line BGR/77-21 (Figure 4b). Continental crust with a susceptibility of 0.1 (SI units) is interpreted from the north-east end of the line to approximately SP 2600 (Zone A). Between SPs 1650 and 2300 this is modelled to be overlain by a layer with remanent magnetization 2 A/m. Inspection of the seismic data (Foldout 3) suggests that the overlying layer may be due to sills intruded into sediments in a pre-Tertiary half-graben. A wedge of high susceptibility material (o = 0.25, SI units) is interpreted to dip seaward under the shelf break near SP 1000, similar to but of higher susceptibility than that interpreted on line BGR/77-12 (Chalmers, 1991 and Figure 3b). From about SP 2500 to about SP 4000 is Zone B, which is interpreted to consist of continental crust (o = 0.1, SI units) which has been thinned and intruded by igneous material. Two areas of different magnetization (2 and 1 A/m) were found to be necessary to fit the observed data. Oceanic crust may start at either SP 3900, in which case the first spreading anomaly is 27R, or at SP 4400, in which case the first spreading anomaly is 27N. In the latter case, the zone between SPs 3900 and 4400 can be interpreted as a reversely magnetized peridotite ridge similar to that known to exist in a similar location on the Galician continental margin (Boillot et al., 1987). The contrast in structure on the seismic data between this area and the oceanic crust to its south-west (Foldout 3)suggests
that the second of the two alternatives may be more likely. Zone C (oceanic crust) is therefore interpreted to start at SP 4400.
Line BGR/77-17 (north-easO (Figure 5b). Continental crust which was modelled with a susceptibility of 0.05 (SI units) exists from SP 15805 to approximately SP 15000 and 0.04 (SI units) from approximately SP 15000 to approximately SP 11400 is interpreted from the north-east end of line BGR/77-17 (Zone A). As on lines BGR/77-12 (Chalmers, 1991) (Figure 3b) and BGR/77-21 (Figure 4b), a wedge of high susceptibility material (o = 0.126, SI units) is interpreted to dip seawards under the shelf break. The magnetic anomalies in this area (Srivastava, 1986) are large and complex and, because the modelling procedure assumes infinite prisms at right angles to the profiles, no attempt has been made to fit them in detail. Zone B, where extended continental crust has been intruded and overlain by igneous material, appears to be between SPs 11000 and 14600 on seismic line BGR/77-17. Between SPs 13450 and 14600 the basement appears to be overlain by a layer with remanent magnetization 3 A/m. Inspection of the seismic data suggests that this layer could be due to sills intruded into sediments in a pre-Tertiary half-graben (as on line BGR/77-21). To fit the magnetic data, it was also necessary to replace the uppermost basement with a wedge of material with remanent magnetization 6 A/m. The lower wedge may also be due to intruded igneous rock, but if so it is perhaps some form of central complex or layered intrusion rather than sills. The latter interpretation can also be made for the other wedge of magnetized material (M = 7 A/m) between SPs 13200 and 13400. Between SPs 11000 and 12600 there is again a zone of remanent magnetization overlying thinned continental crust. Two wedges of different reversed magnetization are shown. The upper, M = 1 A/m, may represent lava flows or, in places, sills within sediments. The lower, M = 3 A/m, wedge probably consists partly of sills and lava flows too, but, in places, especially around SP 12200, may contain a more massive intrusion or this
Marine and Petroleum G e o l o g y 1 9 9 5 Volume 12 Number 2 211
Seafloor spreading in Labrador Sea: J. A. Chalmers and K. 14. Laursen structure could be a peridotite ridge similar to that interpreted on line BGR/77-21. Seafloor spreading anomalies appear to start at SP 11100, and the first anomaly is 26R. This area is termed Zone C in conformity with the interpretations made on the other lines. A seaward dipping reflection (Horizon N) can be seen between SPs 11450 and 11700 (Foldout 4), indicating that lava eruptions may have been subaerial here, and that the start of oceanic crust lies between SP 11450 and the first clear seafloor spreading anomaly at SP 11150.
Line BGR/77-17 (south-west) (Figure 6b). Zone A consists of stretched and thinned continental crust with a susceptibility (o) of 0.023 (SI units). It is still necessary to postulate a substantial thickness (about 6 - 7 km) of non-magnetized rock below the block-faulted surface from around SP 400 to approximately SP 2500. Zone B is an intermediate zone. From approximately SP 2500 to approximately SP 4000 it has been modelled as a wedge of material with a susceptibility of 0.023 and remanent magnetization 1 A/m. This can be interpreted as thinned continental crust intruded by normally magnetized igneous material. It is partially overlain by material which has been modelled to have remanent magnetization but zero susceptibility. Two areas with different normal remanent magnetizations of 5 and 4 A/m were modelled. North-east of this is Zone C, which consists of alternating stripes of normal and reverse magnetization, i.e. oceanic crust. Zone B can be interpreted as a volcanic continental margin formed during Chron 27N, when flood basalts overlie extended continental crust at its landward end. They grade seawards into oceanic crust which starts somewhere between SPs 3200 and 4900.
Line BGR/77-6 (Figure 7). Roest and Srivastava (1989b) show seafloor spreading anomalies extending across line BGR/77-6 between about SPs 3650 and 4500, the area called the Gjoa Rise by Tucholke and Fry (1985). The magnetic anomalies on this part of the line can be modelled as stripes of alternating remanent magnetic polarity, i.e. oceanic crust (Zone C) (Figure 7). Seaward dipping reflections can be seen on line BGR/77-6 dipping south-westwards between SPs 4700 and 5250 (Foldout 1) and north-eastwards between SPs 3300 and 3800. Magnetic modelling shows that there may be polarity changes conformable to the dipping surfaces in both areas. These observations suggest that the Gjoa Rise was formed, at least partly, by an episode of subaerial seafloor spreading. The change in magnetic anomaly pattern around SP 2500 occurs on the extension of where Roest and Srivastava (1989b) show the Hudson Fracture Zone. It is therefore probable that line BGR/77-6 crosses the Hudson Fracture Zone between SPs 2500 and 2850. The rocks under the buried rugged topography between SPs 2600 and 2850 appear to have zero remanent magnetization and a zero or small susceptibility. As discussed above, the part of line BGR/77-6 from SP 0 to about SP 2500 is difficult to understand (Zone D). Magnetic modelling suggests that the basalts may continue at least to SP 50, but that the reflection from
212
their top is difficult to follow on the seismic data partly because it lies below the sea-bed multiple and partly because it may be fractured by a large number of small faults. '"~ The magnetic modelling shown on Figure 7 suggests that Zone D could consist of a layer of reversely magnetized basalts partly underlain by normally magnetized basalts. However, other models can also be made to fit the data, including some where the lower layer is modelled with zero remanent magnetization and positive susceptibility, and it must be concluded that magnetic modelling is inconclusive in this area. One possibility is that the area could consist of oceanic crust produced during oblique seafloor spreading after the change of Euler Pole between Chrons 25 and 24. Roots and Srivastava (1984) showed that highly oblique seafloor spreading should lead to complex faulting and relative displacement of blocks of oceanic crust as they move away from the spreading axis. This displacement of blocks with the same remanent magnetization means that magnetic 'stripes' should be difficult or impossible to see in such areas. The presence of the fairly intense faulting visible on line BGR/77-6, SPs 0 to 1650, may mean that this mechanism operated here and that therefore this part of line BGR/77-6 could consist of oceanic crust generated later than the shift in Euler Poles between Chrons 25 and 24. Roest and Srivastava (1989a; 1989b) show no seafloor spreading anomalies north-east of the Gjoa Rise. However, Tucholke and Fry (1985) show oceanic crust extending to as far north-east as either SP 8100 or SP 9100 on line BGR/77-6 (Foldout 1 and Figure 7). The magnetic anomalies in the area of line BGR/77-6 north-east of about SP 5300 are complex and all have positive values. Figure 8, which is based on Geological Survey of Canada (1988), shows that the magnetic anomalies crossed by line BGR/77-6 are quite different north-east of SP 6000 from those south-west of that point. From SP 6000 to SP 9500 [which is landward of either of Tucholke and Fry's (1985) positions for the COB] line BGR/77-6 crosses the northern margin of an area of entirely positive magnetic anomalies up to over +750 nT. This area contrasts with that south-west of about SP 6000 and particularly south-west of about SP 5000, where the anomalies have been modelled by alternating stripes of normally and reversely magnetized crust (Figure 7, SPs 2900-4800). In contrast, the area of positive magnetic anomalies north-east of SP 6000 can be modelled only by assuming either normal remanent magnetization or finite (positive) susceptibilities in the crust. Attempts were made to try to model these anomalies with bands of alternating normal and reverse remanent magnetism, but all attempts to introduce reverse remanent magnetization into the model produced anomalies more negative than those observed. Negative anomalies just north of line BGR/77-6 between SPs 6000 and 9000 lie on the extension of the Hudson Fracture Zone shown by Roest and Srivastava (1989a; 1989b) and Tucholke and Fry (1985). It is the extension of the low values from this area onto line BGR/77-6 that causes the magnetic anomaly minimum between SPs 6900 and 7900. It is not possible to model such an off-line anomaly within the two-dimensional models used here, but a fit was obtained using the block
Marine and Petroleum Geology 1995 Volume 12 Number 2
Seafloor spreading in Labrador Sea: d. A. Chalmers and K. H. Laursen
Discussion
Magnetic signatures and oceanic crust
Figure 8 Map showing seismic lines BGR/77-6, BGRf77-12 and BGR/77-21 in relationship to the magnetic anomalies redrawn from the map Geological Survey of Canada (1988). Anomalies greater than +25 nT are shown as black, those less than -25 nT are shown as white and those between +25 nT and -25 nT are shown as grey. Magnetic anomaly values over the area interpreted to be oceanic crust or subsided continental crust intruded by igneous material (Zones B and C) are generally negative or around zero, whereas magnetic anomalies over areas interpreted to be continental crust without intrusions (Zone A) have positive (coded black) anomalies
of crust with susceptibility 0.02 (SI units) between SPs 700 and 7900. Variations in susceptibility are known from the high-grade metamorphic rocks exposed onshore southern Greenland (Thorning, 1986). There, zones of high susceptibility granulite facies gneisses which occur within or overlie much lower susceptibility amphibolite facies gneisses give rise to magnetic anomalies similar to those observed on line BGR/77-6. Tucholke and Fry's (1985) Hecla Rise extends from about SP 6000 to about SP 7200 on line BGR/77-6 and Figure 7 shows that it and the whole of line BGR/77-6 north-east of about SP 6300 can be modelled as continental crust without remanent magnetization. An alternative interpretation could be that the area consists of oceanic crust produced subaerially, so that the erupting basalts formed extensive lava flows rather than pillow lavas at the sea bed. However, such piles of lava flows are commonly visible as 'seaward dipping reflections' (Hinz, 1981; Eldholm et al., 1989) and there is no sign of such features on seismic line BGR/77-6 (Foldout 1) in this area. Under the trough between Tucholke and Fry's (1985) Gjoa and Hecla Rises (SPs 5250-6200), the basement can be modelled as continental crust (o = 0.025, SI units) intruded by reversely magnetized igneous material (M = 3 A/m), which supports the interpretation that sills intrude the sediment within the trough. This zone (Zone B) extends under the dipping reflections between SPs 4600 and 5250.
Oceanic crust is normally produced containing linear stripes of remanently magnetized material of alternating polarity. That such a mechanism has operated in the Labrador Sea has been shown by the agreement between the modelled and observed magnetic anomalies discussed above for the crust produced during Chrons 25 to 27. Magnetic 'stripes' produced later than Chron 24 are not apparent in northern areas of the central Labrador Sea, and Roots and Srivastava (1984) have suggested that this may be a consequence of oblique seafloor spreading. Further south, where the spreading direction was much less oblique, seafioor spreading anomalies younger than 24 can readily be discerned (Srivastava, 1986; Roest and Srivastava, 1989a; 1989b). It could be argued that Roots and Srivastava's (1984) mechanism might account for our inability to model the areas landwards of anomaly 26R or 27N as oceanic crust. If so, it would imply that the spreading before anomaly 27 was very oblique, and that therefore there had been a substantial change in Euler Pole at that time. Roest and Srivastava (1989a) make no such suggestion, which would also be inconsistent with their identifications of anomalies 31 and 33. Roest and Srivastava's (1989a; 1989b) interpretation of anomalies 31 and 33 partly involves a technique where they show that anomalies interpreted to be the same on conjugate sides of the Labrador Sea can be matched by 'closing' the ocean and 'refitting' Greenland and Labrador back to their positions relative to one another before seafloor spreading. When refitting their interpreted anomalies 31 and 33 to one another, they used a pole of rotation which implied that the direction of seafloor spreading was orthogonal to the direction of the resulting magnetic stripes. To argue that the Roots and Srivastava (1984) 'oblique-spreading' mechanism could account for the misfits in magnetic modelling in the area landward of anomaly 27 would invalidate the criterion used by Roest and Srivastava (1989a; 1989b) for acceptance of their identifications of anomalies 31 and 33, which was the 'goodness of fit' between the observed anomalies and those from the conjugate position restored to their pre-drift locations. It would also imply a substantial change in spreading direction during or just before Chron 27.
Extent of continental and oceanic crust Geological interpretations of the four B G R seismic lines across the Greenland margin are shown in Figure 9 and of line BGR/77-17 across the Canadian margin in Figure 10, which were originally published in Chalmers et al. (1993). The most readily obvious feature of the reprocessed seismic data is the pattern of tilted fault blocks and grabens that can be seen in the deep water parts of the sections off the Greenland shelf (Zone A) (Foldouts 1, 2 and 3, Figure 9 and Chalmers, 1991). This is a pattern normally associated with extended continental crust, although it can occur in oceanic crust. However, the failure of the magnetic modelling to reproduce a pattern of magnetic stripes of alternating polarity in this area suggests that the interpretation of Zone A as
Marine and Petroleum Geology 1995 Volume 12 Number 2 213
Seafloor spreading in Labrador Sea: J. A. Chalmers and K. H. Laursen a
BGR/77-6 SW
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Figure 9 Interpreted geological cross-sections across the transition from continental to oceanic crust off southern West Greenland,
based on seismic lines (a) BGR/77-6 (this paper), (b) BGR/77-12 (Chalmers, 1991), (c) BGR/77-21 (this paper) and (d) BGR/77-17 (this paper). See Figures 1 and 2 for locations. The sills within the sedimentary sections are visible on the seismic sections (see interpretations on Foldouts) and are confirmed by the magnetic modelling. The interpretation of intrusions within the basement is based on magnetic modelling only and other interpretations of the high remanent magnetizations may be possible. This figure was originally published in Chalmers et al. (1993)
extended continental crust is more likely to be correct. Block-faulted gneissic basement has been interpreted on seismic data under the southern West Greenland continental shelf (Chalmers, 1989; Chalmers, in press). That it consists of gneiss has been proved by the wells Nukik-1 and Kanghmiut-1 (Rolle, 1985). Seismic line BGR/77-12 (Chalmers, 1991) shows that the block-faulted terrain continues out under deep water without significant change in structural style to beyond where Roest and Srivastava (1989a; 1989b) interpret
214
seafloor spreading anomaly 33. Seismic line BGR/77-6 (Foldout 1 and Figures 7 and 8a) runs from close to the Nukik-1 well near its north-east end and again shows that fault blocks and grabens exist from there into the area of shallow basement which Tucholke and Fry (1985) named the Hecla Rise. They interpreted the Hecla Rise as a Late Cretaceous-Palaeocene spreading ridge, but the interpretations presented here suggest that a more likely explanation is that it is an area of continental
M a r i n e a n d P e t r o l e u m G e o l o g y 1995 V o l u m e 12 N u m b e r 2
S e a f l o o r spreading in Labrador Sea: J. A. Chalmers and K. H. Laursen BGRI77-17 SW
CONTINENTAL BASEMENT
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Figure 10 Interpreted geological cross-section across the transition from continental to oceanic crust off the coast of Labrador, based on seismic line BGR/77-17 (this paper). See Figures I and 2 for location. This figure was originally published in Chalmers eta/. (1993)
basement containing sediment-filled grabens which were probably formed during the same ?Cretaceous rifting episode identified by Chalmers (in press) on the southern West Greenland Shelf. However, the Hecla Rise area appears to have been uplifted and folded at a comparatively late date, possibly by some mechanism involving oblique compression along the Hudson Fracture Zone or its extension to the north-east which runs a short distance north of line BGR/77-6 (Chalmers et al., 1993, Figure 13). The part of Zone A landward of about SP 9000 on line BGR/77-6 certainly consists of continental crust, extended and block-faulted. The magnetic modelling has confirmed Roest and Srivastava's (1989a; 1989b) identifications of Palaeocene oceanic crust produced during Chrons 25, 26 and partly 27. However, all attempts to model the area landward of anomaly 27N as alternating stripes of normally and reversely magnetized material failed. Thus between the oceanic crust (Zone C) and the zone of block-faulting (Zone A) there is a zone (Zone B) whose nature is not immediately obvious. Chalmers (1991) suggested that Zone B on line BGR/77-12 could consist of highly extended continental crust overlain and intruded by igneous material. Zone B narrows to the north-west so that on seismic line BGR/77-6 (Foldout 1 and Figure 9a) the faulted terrain of Zone A can be traced into the graben south-west of the Hecla Rise (SPs 5300-6400). Here, sediments above basement are intruded by sills. These rocks disappear abruptly to the south-west below volcanic rocks containing seaward dipping reflectors and a landward facing volcanic escarpment similar to the situations known at the Faeroe-Shetland Escarpment (Smythe, 1983) and the Voring Plateau Escarpment (Eldholm et al., 1989). The Gjoa Rise to the south-west of the escarpment was interpreted by Tucholke and Fry (1985) as being due to 'off-axis volcanism'. However, Figure 7 shows that magnetic anomalies 25, 26 and 27 can be interpreted across the Gjoa Rise. A more probable explanation of it is that it was produced as a 'volcanic rifted margin' similar to those known from the northern North Atlantic (papers in Morton and Parson, 1988) and elsewhere (White and
McKenzie, 1989). Block-faulted continental crust can also be interpreted off the Greenland shelf on the landward parts of lines BGR/77-21 and BGR/77-17. In this more southerly area, however, the crust and overlying sediments have been intensely intruded by dykes and sills and larger igneous bodies (Figure 8c and 8d) and the location of the ocean-continent transition is more difficult to locate with confidence. However, a structure resembling the peridotite ridge on the Galician continental margin (Boillot et al., 1987) can be seen on line BGR/77-21 between SPs 3900 and 4400 (Foldout 3 and Figure 9c) and on line BGR/77-17 between SPs 12100 and 12400 (Foldout 4 and Figure 9d) may suggest that the COB lies just to its seaward. On Foldout 4 and Figure 9d, a seaward-dipping reflector similar to those known from the volcanic continental margins in the northern North Atlantic (papers in Morton and Parsons, 1989) is shown near SP 11500. Sediments of the Bjarni and Markland Formations can be readily followed seaward on seismic line BGR/77-17 (Foldout 5 and Figure 10) through an area of half-graben to where basalts onlap, cover and finally conceal them. Roest and Srivastava (1989a; 1989b) place the COB at SP 1700 on this line, but again the magnetic modelling failed to confirm stripes of alternating polarity landward of anomaly 26R, at SP 4900. Basalts overlying continental crust must merge into oceanic crust somewhere in the 85 km wide zone between SPs 3200 and 4900 (Foldout 4 and Figure 6b), and it is possible that this also is a volcanic continental margin. Although not relevant to the main discussion, the area of 'non-magnetic' rocks below the fault blocks between SPs 400 and 1700 on line BGR/77-17 is worthy of comment (Figures 6b and 9). What this consists of is unknown, but it is possible that it could indicate the presence of a deep pre-Cretaceous basin, possibly containing Ordovician to Carboniferous sediments (BalkwiU, 1987). Hinz et al. (1979, Figure 20) modelled such a basin from gravity data along a profile between seismic lines BGR/77-17 and BGR/77-21. The extent of continental (Zone A), oceanic (Zone
Marine and Petroleum Geology 1995 Volume 12 Number 2 2 1 5
Seafloor spreading in Labrador Sea: J. A. Chalrners and K. H. Laursen C) and 'intermediate' (Zone B) crust according to the interpretations outlined above is shown in Figure 2. The locations of the seafloor spreading anomalies 25 to 27 have been identified on the B G R seismic lines and they and the fracture zones have been interpolated between the seismic lines using the magnetic anomaly map in Srivastava (1986). The locations of seafloor spreading anomalies 21 and 24 have been taken from Roest and Srivastava (1989b).
Revised tectonic map of the Labrador Sea Figure 11 shows a revised tectonic map of the Labrador Sea based on Figure 2 and other sources. There are a number of important differences in addition to those discussed above between this map and earlier comparable maps [e.g. Tucholke and Fry (1985) Figure 4]. Some of the main features have already been discussed; that the Hecla Rise and substantial areas under deep water along both the Greenland and Labrador margins are underlain by continental crust, for example. Other important features of the map are that the entire area of the Davis Strait, including the Davis Strait High (Srivastava et al., 1982), is now also interpreted to be entirely composed of continental crust. Substantial early Tertiary volcanism undoubtedly
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occurred in this area, as indicated for example by the Palaeocene basalts penetrated by the Gjoa-1 well (Klose et al., 1982), but the implication of Figure 11 is that the volcanism was entirely intra-cratonic. The Ungava Fault complex (Menzies, 1982; Balkwill et al., 1990), which dissects this area, appears to be an intra-continental transform system which transfers the relative motion of Greenland and Canada in the Labrador Sea to Baffin Bay. The area in the central Labrador Sea west of the Hudson Fracture Zone is shown as oceanic crust on Figure 11. Although linear magnetic anomalies cannot be traced in this area, attempts to refit the continents before seafloor spreading (not discussed in this paper) suggest that there could be an area of oceanic crust here. The absence of visible magnetic stripes could be a consequence of highly oblique spreading in accordance with the mechanism suggested by Roots and Srivastava (1984). The faulting visible on the part of seismic line BGR/77-6 (Foldout 1 and Figure 9a) west of the Hudson Fracture Zone may support this conjecture. A discussion of the geological development of the Labrador Sea and offshore southern West Greenland area based on the interpretations presented here and elsewhere (Rolle, 1985; Chalmers, 1989; Chalmers, 1991; Ottesen, 1991a; 1991b; Chalmers and Pulvertaft, 1993; Chalmers, in press) has been presented by Chalmers et al. (1993).
48 ~ W 1168~ N
Conclusion
~-~
1.
Interpretation of multichannel seismic data from a regional survey in the Labrador Sea, which have been reprocessed and migrated, has been used to control magnetic modelling. This has confirmed earlier interpretations of Palaeocene oceanic crust in the Labrador Sea. However, all attempts to model the areas landwards of anomaly 27N as a series of stripes of alternating remanent magnetizations have failed. The seismic data show a zone of block-faulted crust, intruded by igneous material in places, extending from below the Greenland and Labrador continental shelves and out under deep water. This zone is interpreted to consist of continental crust, extended and faulted during one or more episodes of rifting. Between this zone and the oceanic crust is a zone whose nature is more difficult to determine, but it may consist of highly extended and thinned continental crust, intruded and overlain by igneous material. The magnetic modelling suggests that seafloor spreading started during the Palaeocene (Chron 27) and not during the Campanian (Chron 33) or Maastrichtian (Chron 31) as has previously been suggested. It still remains possible that seafloor spreading could have started earlier than Chron 27 in the south-eastern Labrador Sea, which was not interpreted in this study. . In some areas of the Labrador Sea, the continental margin appears to be a volcanic margin similar to those interpreted in the northern North Atlantic Ocean and elsewhere where large volumes of igneous material were erupted at the start of seafloor spreading. .
.
OCEANIC CRUST WiTH MAGNETIC ANOMALIES (NUMBERED) AREA S OF TERTIARY VOLCANIC$ ON CONTINENTAL CRUST C~;
MAJOR IGNEOUS INTRUSIONS
~
/~
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Figure 11 Tectonic map of the northern Labrador Sea. The map is based on Figure 2 plus material from Klose et al. (1982), Srivastava eta/. (1982), Tucholke and Fry (1985), Balkwill (1987),
Chalmers (1989), Ottesen (1991a), Ottesen (1991b) and Chalmers (in press). This figure was originally published in Chalmers et al. (1993)
216
Marine and Petroleum Geology 1995 Volume 12 Number 2
Seafloor spreading in Labrador Sea: J. A. Chalmers and K. H. Laursen Data availability Copies of the reprocessed BGR seismic lines at a scale of 5 cm per second TWT can be obtained from the
Geological Survey of Greenland for a nominal charge to cover the costs of reproduction, post, packing and overheads.
Acknowledgements Funding for the reprocessing of the seismic data was provided by The Danish Energy Research Programmes 1989 (EFP-89) project 1313/89-5 and 1990 (EFP-90) project no. 1313/90-0013. The seismic data were reprocessed by Prakla-Seismos Gmbh of Hanover, Germany and we especially wish to thank P. Briickner for all his work. We thank K. Hinz of BGR for allowing us access to the field tapes of the seismic data. We thank Trine Dahl-Jensen, Chris Pulvertaft and two anonymous referees for valuable comments which much improved the presentation of this paper, and Jette Halskov and Carsten Thuesen for much patience while doing the drafting work. We thank Dr J. R. Parker for permission to reproduce Figures 9, 10 and 11 and the Geological Survey of Canada for permission to reproduce Figure 8. The paper is published with permission of the Geological Survey of Greenland.
References Balkwill, H. R. (1987) Labrador Basin: structural and stratigraphic style. In: Sedimentary Basins and Basin-forming Mechanisms (Eds C. Beaumont and A. J. Tankard), Can. Soc. Petrol. Geol. Mem. 12, 17-43 Balkwill, H. R., McMillan, N. J., MacLean, B., Williams, G. L. and Srivastava, S. P. (1990) Geology of the Labrador Shelf, Baffin Bay, and Davis Strait. Part 1: Mesozoic-Cenozoic geology of the Labrador Shelf. In: Geology of North America, Vol. I-1, Geology of the Continental Margin of Eastern Canada (Eds M. J. Keen and G. L. Williams), Geological Survey of Canada, 295-324 Bell, J. S. (1989) East Coast Basin Atlas Series, Labrador Sea, Atlantic Geoscience Centre, Dartmouth, Nova Scotia, 111 pp Boillot, G., Winterer, E. L. eta/. (1987) Proc. /nit. Rep. (Pt. A), ODP 103, 663 pp Chalmers, J. A. (1989) A pilot seismo-stratigraphic study on the West Greenland continental shelf Rapp. GrCnlands geol. Unders. 142, 16 pp Chalmers, J. A. (1991) New evidence on the structure of the Labrador Sea/Greenland continental margin J. Geol. Soc. 148, 899-908 Chalmers, J. A. Project VEST SOKKEL Phase I. A seismic stratigraphic interpretation of the geology of the continental shelf of southern West Greenland between 64°15'N and 66°N Rapp. GrCnlands geoL Unders., in press Chalmers, J. A. and Pulvertaft, T. C. R. (1993) The southern West Greenland shelf - - was petroleum exploration abandoned prematurely? In: Arctic Geology and Petroleum Potential (Eds T. O. Vorren, E. Bergsager, 0. A. DahI-Stamnes, E. Holter, B. Johansen, E. Lie and T. B. Lund), Elsevier, Amsterdam, for Norwegian Petroleum Society, 55-66 Chalmers, J. A., Pulvertaft, T. C. R., Christiansen, F. G., Larsen, H. C., Laursen, K. H. and Ottesen, T. G. (1993) The southern West Greenland continental margin: rifting history, basin development, and petroleum potential. In: Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (Ed. J. R. Parker), Geological Society, London, 915-931 Eldholm, O., Thiede, J. and Taylor, E. (1989) The Norwegian continental margin: tectonic, volcanic, and paleoenvironmental framework. In: Proc. ODP Sci. Res. (Eds O. Eldholm, J. Thiede etaL), 104, 783 pp Geological Survey of Canada (1988) Magnetic anomaly map of the continental margin of Eastern Canada, GeologicalSurvey
of Canada Map 1709A, Scale 1:5 million Gibb, F. G. F. and Kanaris-Sotiriou, R. (1988) The geochemistry and origin of the Faeroe-Shetland sill complex In: Early Tertiary Volcanism and the Opening of the NE Atlantic (Eds A. C. Morton and L. M. Parson), Spec. Pub/. Geol. Soc. London No. 39, 241-252 Hinz, K. (1981) A hypothesis on terrestrial catastrophes. Wedges of very thick oceanward dipping layers beneath passive continental margins - - their origin and paleoenvironmental significance Geol. Jahr. E22, 3-28 Hinz, K., Schl0ter, H.-U., Grant, A. C., Srivastava, S. P., Umpleby, D. and Woodside, J. (1979) Geophysical transects of the Labrador Sea: Labrador to southwest Greenland Tectonophysics 59, 151-183 Klose, G. W., Malterre, E., McMillan, N. J. and Zinkan, C. G. (1982) Petroleum exploration offshore southern Baffin Island, northern Labrador Sea, Canada. In: Arctic Geology and Geophysics (Eds A. F. Embry and H. R. Balkwill), Can. Soc. Petrol GeoL 8, 233-244 Menzies, A. W. (1982) Crustal history and basin development of Baffin Bay. In: Nares Strait and the Drift of Greenland: a Conflict in Plate Tectonics (Eds P. R. Dawes and J. W. Kerr), Meddr. Gr~nland, Geosci. 8, 295-312 Morton, A. C. and Parson, L. M. (Eds) (1988) Early Tertiary Volcanism and the Opening of the NE Atlantic, Spec. Pub/. GeoL Soc. London No. 39 Ottesen, T. G. (1991a) A preliminary seismic stratigraphic study of the Paleocene-Eocene section offshore southern West Greenland between 66° and 68°N Gr~nlands geoL Unders. Open File Ser. 90/1, 46 pp + enc. Ottesen, T. G. (1991b) A preliminary seismic study of part of the pre-Paleocene section offshore southern West Greenland between 66°N and 68°N Gr~nlands geoL Unders. Open File Ser. 91/6, 28 pp + enc. Roest, W. R. and Srivastava, S. P. (1989a) Sea-floor spreading in the Labrador Sea: a new reconstruction Geology 17, 1000-1003 Roest, W. R. and Srivastava, S. P. (1989b) Seafloor spreading history 1. Labrador Sea. Magnetic anomalies along track. In: East Coast Basin Atlas Series, Labrador Sea (coordinator J. S. Bell), Atlantic Geoscience Centre, Dartmouth, Nova Scotia, 98 Rolle, F. (1985) Late Cretaceous-Tertiary sediments offshore central West Greenland: lithostratigraphy, sedimentary evolution, and petroleum potential Can. J. Earth Sci. 22, 1001-1019 Roots, W. D. and Srivastava, S. P. (1984) Origin of the marine magnetic quiet zones in the Labrador and Greenland Seas Mar. Geophys. Res. 6, 395-408 Smythe, D. K. (1983) Faeroe-Shetland escarpment and continental margin north of the Faeroes. In: Structure and Development of the Greenland-Scotland Ridge (Eds M. H. P. Bott, S. Saxov, M. Talwani and J. Thiede), Plenum Press, New York, 105-109 Srivastava, S. P. (1978) Evolution of the Labrador sea and its bearing on the early evolution of the North Atlantic Geophys. J. R. Astron. Soc. 52, 313-357 Srivastava, S. P. (1986) Geophysical maps and geological sections of the Labrador Sea Geol. Surv. Cab. Pap. 86-16, 11 pp + enc. Srivastava, S. P., MacLean, B., MacNab, R. F. and Jackson, H. R. (1982) Davis Strait: structure and evolution as obtained from a systematic geophysical survey. In: Arctic Geology and Geophysics (Eds A. F. Embry and H. R. Balkwill), Can Soc. Petrol Geol. Mere. 8, 267-278 Thorning, L. (1986) A decade of geophysical surveying in Greenland Gr~nlands geol. Unders. Rapp. 128, 123-133 Tucholke, B. E. and Fry, V. A. (1985) Basement structure and sediment distribution in northwest Atlantic Ocean Am. Assoc. Petrol. Geol. Bull. 68, 2077-2097 Vogt, P. R. and Tucholke, B. E. (Eds) (1986) The Geology o/North America, Vol. M, The Western North Atlantic Region, Geological Society of America, Boulder White, R. and McKenzie, D. (1989) Magmatism at rift zones: the generation of volcanic continental margins and flood basalts J. Geophys. Res. 94, (B6), 7685-7729 Ziegler, P. A. (1988) Evolution of the Arctic-North Atlantic and the Western Tethys. Am Assoc. Petrol. GeoL Mere. 43, 198 pp
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