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Mesozoic evolution of the Newfoundland Basin
Earth and Planetary Science Letters, 37 (1977) 307-320 ©Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
307
[41
MESOZOIC EVOLUTION OF THE NEWFOUNDLAND BASIN C.E. KEEN Atlantic Geoscience Centre, Bedford Institute o f Oceanography, Dartmouth, N.S. (Canadaj B.R. HALL * Department o f Oceanography, Dalhousie University, Halifax, N.S. (Canada) and K.D. SULLIVAN Department of Geology, Dalhousie University, Halifax, N.S. (Canadaj
Received March 8, 1977 Revised version received August 2, 1977
We present here an interpretation of geological and geophysical data collected in the Newfoundland Basin. Geophysical measurements indicate that the area was formed by sea-floor spreading during the Cretaceous. Rocks collected from the Newfoundland Seamount chain support this interpretation and their chemistry suggests affinities with volcanic rocks found on oceanic islands but not with those sampled on mid-ocean ridges. A schematic model for early plate motions between Iberia and North America beginning about 115 m.y.B.P., consistent with the data, is discussed.
1. Introduction An extensive body of literature dealing with the predrift position o f the continents and relative motions of the circum-Atlantic plates has accumulated in the past 10 years. With the present widespread acceptance of a basic model for the evolution of the North Atlantic [ 1 - 3 ] has come greater interest in resolution of specific paleogeographic and kinematic problems. Two such problems are the initial position o f Iberia with respect to North America and the geometry and chronology of early (pre-73 m.y.B.P.) relative motions between these two plates. A number of investigators have collected data bearing on these questions from areas in the eastern North Atlantic and the Bay o f * Present address: Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964, U.S.A.
Biscay [4,5]. Most recently, Le Pichon et al. [8] have utilized this work and additional data in fitting the continents around the North Atlantic Ocean. These workers state that the largest uncertainty in their re- " assembly is in the Iberia/North America portion, citing kate Paleozoic crustal fragmentation as a possible cause of the complexity of the later development o f the area. Furthermore, no comprehensive account of the structure and evolution of the deep-water area west o f Portugal has yet been published. Publication of detailed results from DSDP kegs 47 and 48 [6,7] should do much to remedy this latter situation. We present here the first results to emerge from a number of Bedford Institute-Dalhousie University cruises to the Newfoundland Basin. The sea floor in this area, which is defined for the present purpose as the deep-sea region extending from the Northwest Atlantic Mid-Ocean Canyon to the Grand Banks and from Flemish Cap to the Southeast Newfoundland
308
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mary of our knowledge of the structure, age, and geochemistry of the Newfoundland Seamounts (Fig. 1).
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2. 2. Seismic studies Fig. 1. Location map. Open circles represent seamounts from the Newfoundland Seamount chain which were charted but not sampled, × 's represent seamounts from which samples discussed in text were dredged. Northern and southern geophysical survey areas are labelled N and S respectively. A and D are seismic reflection lines, and B and C are composite magnetic model lines; "7" is the crustal refraction seismic line. Solid circles 29, 32, and 33 are sonobuoy locations.
A reversed crustal seismic refraction line (line 7, Fig. 1), using retrievable sonobuoys, and several expendable sonobuoy lines were obtained in the Newfoundland Basin and are reported upon in detail by Jackson et al. [9]. The results from the crustal refraction line (Table 1 and Fig. 2) show that normal oceanic crust, with typical velocities for layers 2 and 3 [10], underlain by mantle at a depth of about 12 km, occupies at least part of the area. This refraction line and unreversed expendable sonobuoy results (sonobuoys 29, 32) show that between 2 and 3 km of sediment overlies basement. Nearer the eastern margin of the Grand Banks the sediment thickness increases to about 4 km (sonobuoy 33).
Ridge, should contain the western half of the record of Mesozoic spreading between Iberia and North America. The data are presented in two sections: (1) a regional synthesis of the geophysical data pertaining to the structural evolution of the Basin; and (2) a sumTABLE 1
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Fig. 2. Time-distance plot, line 7. The solid lines with superimposed circles represent velocities measured by means of explosives; other solid lines are velocities interpreted from expendable sonobuoy wide-angle reflections and refractions, the dashed lines represent assumed velocities. The column to the right of the time-distance plot contains the reversed velocities from the time-distance plot, drawn so that depth and layer thickness can be read directly in kilometres. A typical seismic reflection record obtained within the basin (Fig. 3) shows a prominent oceanic basement reflector. The basement topography is rough, with a relief of about 0.7 s (two-way travel time). There are two prominent reflectors within the overlying sedimentary column. The first of these, R1, seen at about 1.0 s beneath the sea floor, is often overlain by a wavy unit roughly 0.5 s thick. R 1 is present in most of the area south of the seamounts. The disturbed acoustic facies, which is developed on and adjacent to the flank of the Southeast Newfoundland Ridge, is likely due to bottom-current-controlled sedimentation. A second prominent reflector, R2, is found just above oceanic basement (about 1.7 s subbottom) in the western half of the southern part of the basin. Except for the local evidence of draping over basement highs (especially R2) and the wavy facies, the reflectors on all profiles seaward of the continental rise are flat-lying. It is tempting to correlate reflectors R1 and R2 with Horizons A and 13which are found on most profiles in the western Atlantic [I 1], but such a correlation is not definitive. It should be noted in this connection that the preferred magnetic anomaly identification discussed below gives a sea-floor age for this part of the basin which is perhaps 10 m.y. younger than the age of Horizon/3 given by Ewing and Hollister [12], however, there is recent evidence that this horizon is diachronous [ 13]. The velocities obtained from the expendable sono-
buoys in the Newfoundland Basin fall into two groups [9]. Generally, velocities above R I are between 1.6 and 2.1 km s -1, while the sedimentary interval below R 1 exhibits velocities in the range 2.6 3.5 km s -1. The crustal velocities measured for layers 2 and 3 are 5.2-5.8 km s -1 and 6.6 km s -1 respectively, very similar to those observed elsewhere for these layers. The mantle velocity measured was 7.8 km s -1. Seismic reflection lines across the eastern margin of the Grand Banks, one of which is shown in Fig. 4, indicate acoustic basement underlying a well-developed angular unconformity on the shelf. This is probably the Upper Cretaceous unconformity found in exploratory wells drilled on the Grand Banks and correlated with seismic reflection records by Jansa and Wade [14]. Basement appears to be truncated by a fault near the shelf edge (at 50 km, Fig. 4). Sedimentary layers higher in the column may continue across the shelf break onto the slope, although identification of the reflectors is hampered by hyperbolae in this part of the record. Across the slope the record shows no evidence of diapir complexes such as are common along the Scotian margin [15], nor does the basement appear to be block-faulted in the manner observed along the transform-fault margin of the southern Grand Banks [9]. Basement interpreted to be oceanic crust from the seismic refraction results can be followed westward from the basin proper to within 75 km of the shelf break.
2. 3. Magnetic surveys In the two areas labelled N and S in Fig. 1, geophysical surveys incorporating gravimetric, magnetic, bathymetric and seismic reflection measurements were undertaken. The prime purpose of these surveys was to delineate any prominent direction of magnetic lineations which might exist, to determine, from correlation between magnetic anomalies and layer 2 topography [ 16], the presence or absence of magnetic reversals and to attempt to date these reversals. Full details of the data collection, reduction, and interpretation are given by Hall [17]. The eastern boundary of the region of interest is taken as the negative anomaly between anomalies 31 and 32 (31/32). Work by Cande and Kristoffersen [ 18] indicates that, due to the high degree of skewness, anomalies 31 and 32 may have been misidentified and are actually anomalies 33 and
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311 34. However, this problem of 31/32 versus 33/34 does not affect the present discussion in any significant way. We follow the anomaly identifications of Srivastava [19], which are based on extensive data in the Labrador Sea and the western Atlantic north of the Azores. The data in the northern survey area (area N, Fig. 1) consist primarily of measurements made by early cruises that passed through the area. Very large cross-over errors in the magnetic anomalies (109 gammas r.m.s.) are attributed to either inadequate navigational control or to the presence of large diurnal variations during at least some period of data collection. An unsuccessful attempt was made to improve the cross-over values by shifting the tracks and by applying diurnal corrections using variations measured at St. John's, Newfoundland, during the survey periods. The results in this northern area are inconclusive, but are presented here for the sake of completeness. Fig. 5a shows the pattern obtained by contouring the magnetic anomalies. Some of the anomalies are well defined while others are defined by only one or two tracks. The contour map shows a predominantly eastwest pattern of anomalies. However, if individual profiles are stacked from south to north, as shown in Fig. 5b, there appears to be a reasonably good correlation in anomaly shapes in a north-south direction. Hence, we cannot at present adequately define the direction of lineations in this area. Analysis of magnetic data in the southern survey area (area S in Fig. 1) is more straightforward. The navigation was more accurate than in the northern area and a magnetometer housed in a moored buoy was used to measure diurnal variations in the area during the survey. Fig. 6a shows the anomalies after correction for diurnal variation and removal of the International Geomagnetic Reference Field [20]. The r.m.s. cross-over error for the corrected survey data is 22 gammas. The track spacing is sufficient to define magnetic lineations trending in a north to northeast direction. A break in the anomaly pattern, approximately perpendicular to the lineations, can be seen near the centre of the survey area. This may reflect the position of a fracture zone, but we have insufficient data north of this break to determine the sense and magnitude of anomaly offset. Fig. 6b shows the magnetic anomaly profiles along ship's track. The wavelengths and amplitudes of the anomalies are typical of those
found elsewhere in areas formed by sea-floor spreading [21].
2. 4. Magnetic anomaly modelling The magnetic anomalies have been modelled along four seismic reflection tracks in the southern survey area (lines A-A', B-B', C-C', D-D', Fig. 6a), in order to discriminate between the effects due to basement topography and those due to magnetic reversals. Because the spacing of the seismic lines does not allow basement topography to be contoured, the assumption of two-dimensionality was adopted. The source layer used has a planar base at 8.5 km depth and incorporates along its upper surface the basement topography obtained from the reflection profiles. Models using source layers of constant thickness did not give significantly different results. All models used a remanent magnetization of 0.0025 emu cm -3. Declination and inclination to the pole were obtained from the Cretaceous pole position for North America of Couillard and Irving [22]. The models, an example of which is shown in Fig. 7 (D-D') demonstrate that, within the limits of the two-dimensionality assumption, the anomalies are not caused by layer 2 topography alone (although the effect of basement topography is considerable). No model assuming normally magnetized crust with a uniform magnetization value could be fitted to the observed curve, although the observed magnetic profile might be fitted using crustal blocks of uniform magnetic polarity (positive or negative) with alternately high and low magnetization values. In the absence of any evidence that lineated marine magnetic anomalies are formed by the latter mechanism, we prefer to satisfy the measurements by including zones of reverse. ly magnetized material in our model. On first inspection two of the reversed zones (25 and 145 km) are more obviously required to fit the data. However, closer inspection of the figure will show that a significantly better fit to the observations is obtained by incorporating reversed zones at 90 km and 190 km. All four of the lines modelled show four major reversed zones, indicating that the assumption of twodimensionality is valid here. However, the extent of the reversal sequence within the detailed survey area alone is too short to permit correlation with known
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reversal sequences elsewhere. A composite anomaly profile for the southern portion of the basin was therefore constructed. This extends from anomaly 31/32 westward to the continental rise (lines B and C, Fig. 1). The model for this composite profile incorporated basement topography except in the section extending from the southern survey area east to anomaly 31/32. No seismic data were available along this latter section of track; instead, a layer 2 km thick (top at 6.5 km) was assumed. We justify the exclusion of topography along section C on the basis of the fact that the reversed zones within the survey area are consistently associated with the prominent magnetic anomaly minima regardless of the configuration of basement topography; we assume that this is also true for section C of the composite profile. Finally, a remanent magnetization of 0.05 emu cnr -3 (rather than 0.0025 emu cm -a) was used for the westernmost 200 km of the profile in order to improve the fit to the high-amplitude anomalies in that section. The model, with the observed and calculated profiles, is shown in Fig. 8. A single composite magnetic model in the southern Newfoundland Basin is scant evidence from which to
deduce the age of the basin. However, the magnetic anomaly character (Fig. 6b) and basement topography observed on the composite line are similar to that seen on other profiles south of the seamounts, which supports the magnetic model presented here. Therefore, age limits for the basin can be established from these data. First, the Newfoundland Basin is bounded on the east by anomaly 31/32 and is therefore older than 73 m.y.B.P. To the west of this anomaly one would expect to find crust formed during the Cretaceous quiet interval (110 to 81 m.y.B.P. [23]),and, if the basin is old enough, the Keathley disturbed interval (older than 108 m.y.B.P. [21]). Second, the large anomaly at the western end of the composite model is similar in amplitude and character to the J-anomaly observed to the south of the Newfoundland Basin [24]. A comparison of the character of this large anomaly to the J-anomaly in the area of DSDP drilling on the Janomaly Ridge (Spur Ridge, Fig. 1) during Leg 43 is shown in Fig. 9. The similarity in shape and amplitude is striking. Furthermore, the anomalies in each case are associated with a basement high [24]. Other crossings of this anomaly between the Southeast Newfoundland Ridge and the seamounts show that it is a northerly trending feature within the basin and that it is always associated with a basement high. Thus, we consider this anomaly ill the Newfoundland Basin to be an extension of the J-anomaly mapped to the south.
2. 5. Age of magnetic anomalies and rates of spreading There is some question as to the age of the J-anomaly. The results of Leg 43 drilling [24] suggest that it is associated with crust of Aptian age (about 110 m.y. B.P.) which implies that the J-anomaly represents the youngest part of the Keathley sequence, roughly M0 to M4. The correspondence of the J-anomaly to an exact position within the Keathley sequence has been difficult to deternune because its shape cannot be accounted for by the commonly used plane layer magnetic block model. Workers using this method in the eastern North Atlantic have correlated the peak and trough which characterize the J-anomaly with anomalies M2 to M4 [25,26] and the peak with anomaly M15 [27], and in the western North Atlantic to the south of the Newfoundland Basin, Barrett and Keen [28] correlated the trough of the J-anomaly with anom-
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alies M 16 to M 17. Finally, the peak of the J-anomaly on both sides of the central North Atlantic has been correlated by Rabinowitz et al. [29] with anomalies M1 to M0. In the Newfoundland Basin correlation of the trough of the J-anomaly with anomaly M3 gives an age range for the basin of about 115 to 73 m.y.B.P. and an average spreading half-rate of 1.36 cm yr-1; while correlation of the low with anomalies M16 to 250 I
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M17 gives a range of around 137 to 73 m.y.B.P, and a half-rate of 0.89 cm yr - ] . The alternative correlations discussed above require different interpretations for the anomalies mapped within the Newfoundland Basin. If the J-anomaly represents the younger part of the Keathley sequence, then the anomalies to the east of it must have formed during the Cretaceous quiet interval, presenting a problem concerning their origin similar to that discussed by Vogt and Johnson [30] in connection with the "Lynch sequence". Because we have shown that basement topography alone cannot cause the observed anomalies, one must postulate that during the Cretaceous quiet interval the intensity of the earth's field fluctuated dramatically and systematically or that true reversals occurred. The latter suggestion is substantiated by studies of DSDP sediments of Albian age (i.e., within the Cretaceous quiet interval) which show that at least four reversals are present within that time period [31]. Recent land studies [32] do not, however, find evidence for these reversals. On the other hand, the association of the J-anomaly with the middle Keathley anomalies requires that the younger portion of the Keathley sequence lies within the basin, most probably represented by the lineated anomalies mapped in the southern survey area. This correlation leads to a plausible average half-rate (0.89 cm yr -1) but to an unrealistically low rate during the Cretaceous quiet interval (less than 0.4 cm yr-1).
150
(kilometres)
2. 6. The N e w f o u n d l a n d seamounts Fig. 9. Comparison of the (negative) J-anomaly over the Spur Ridge (lower profile) and the assumed J-anomaly in the western Newfoundland Basin (upper profile). Basement features in each of the two areas are similar.
In the approximate centre of the Newfoundland Basin lies the east-west-trending Newfoundland Seamount chain, which remained unsampled until 1974
316 (Fig. 1). The origin of this lineament is likely to be very important in relation to the evolution of the basin. It is worth noting in passing that all the seamounts plotted in Fig. 1, which to our knowledge includes all the seamounts in the chain, are of normal magnetic polarity [33]. A study of the volcanic rocks dredged from two seamounts near the centre of the group (crosses, Fig. 1) has provided geochemical and geochronological data bearing on their origin [34]. The geochemical evidence, in particular the rare earth patterns (Fig. 10) clearly demonstrates the alkaline nature of the seamount basalts. Alkaline volcanism of this type characterizes oceanic islands [35], hot-spot traces [36], and major transverse lineaments or fractures [37,38], and is postulated to be characteristic of "leaky" transfbrm faults. 4°Ar/39Ar dating of plagioclases separated from two of the trachytic rocks in one of the dredge hauls established the first solid chronological datum in the seamount volcanism [34]. The isochron ages obtained for the samples were 97.6 -+ 1.4 m.y. and 96.4 + 2.1 m.y., giving a combined isochron age of 97.7 +- 5 m.y. Additional chronological information has come from micropaleontological studies of a slab of clastic limestone dredged from Shredder Seamount, at the eastern end of the chain. Preliminary identification of the fauna in this sample indicates deposition in a shallow-water environment in Middle Jurassic to Early Cretaceous time, though tentative identifications of several poorly preserved foraminiferal fragments suggest that an Early Cretaceous age is more likely (F. Gradstein, personal communication). The present depth below sea level of the Shredder Seamount limestone indicates that the Newfoundland Basin has subsided a maximum of 1960 m since Early to Middle Cretaceous time. To the south of the Southeast Newfoundland Ridge on the Spur Ridge, or Janomaly Ridge, DSDP Leg 43 cored a similar, approximately coeval shallow-water limestone 3060 m below sea level [24]. This implies a differential subsidence of 1100 m across the Southeast Newfoundland Ridge, with the greater subsidence to the south. We suggest that the appellation "Newfoundland fracture zone" be applied with caution to the feature extending southeast of the Tail of the Bank since it may owe its existence to predominantly vertical motions of the type implied above. Further support for
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this postulate is found in the small offset of the Janomaly across the ridge (50-100 km) which indicates that the amount of transcurrent faulting has been small.
3. Discussion
We have attempted to formulate a number of plate tectonic models to explain the data presented in this paper. The schematic model presented below begins with the onset of sea-floor spreading between Iberia and the Grand Banks and ends with a reconstruction of the Iberian and North American plates at the time of anomaly 31/32 (Fig. 12). The time of this anomaly, according to Williams [5] marks the end of spreading in the Bay of Biscay. The sequence of events presented below is based on a correlation of the trough of the Janomaly with the interval M3;correlation of this feature with the M16 to M17 interval would change the age for initiation of spreading, but not the basic model. (1) At about 115 m.y.B.P. (M3 time) active seafloor spreading perpendicular to a trend of about 020 ° commenced between Iberia and the Grand Banks. Well data from the Grand Banks and initial results from DSDP Leg 48 [7] suggest that the initial rifting and faulting process began well before active spreading, possibly as early as latest Upper Triassic on the Grand
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Fig. 11. Paleogeographic reconstruction of the North Atlantic at anomaly 3 t / 3 2 time, showing location of the J-anomaly as identified in the Newfoundland Basin and as k n o w n south of the Southeast Newfoundland Ridge. Dotted line, indicating anomaly 31/32, separates the Biscay S e a m o u n t s (solid circles to the east) from the Newfoundland S e a m o u n t s (solid circles to the west).
i Fig. 12. Schematic three-stage model for the evolution o f the Newfoundland Basin and related portions of the North Atlantic, modified after Williams [ 5]. Solid line represents the spreading axis, zig-zag lines indicate position of seamounts.
318 Banks. Galicia Bank, which originally was situated immediately to the south of Flemish Cap, was separated from tile Grand Banks during the initial rifting, producing a complex type of transform motion between Flemish Cap and Galicia Bank. This perhaps explains the confused nature of the northern survey area. Since the direction of separation was cast-west, we would expect to find northerly trending lineations to the south of this confused zone. (2) At about I00 m.y.B.P., following the suggestion of Williams [5], the spreading ridge extended into the Bay of Biscay and the rotation of lberia away from Europe began. Both the volcanic base and the limestone cap of Shredder Seamount existed by this time and volcanism was still occurring on some of the central Newfoundland Seamounts; the age of initiation of the volcanic activity is not known. We suggest that the seamounts reflect reactivation of an older structural discontinuity. This type of model could involve either synchronous volcanism or propagation of volcanism along this older feature. A hot-spot seamount model has also been considered. The time at which formation of the trace within the Newfoundland Basin began is not known, but the paleontologic date from Shredder proves that the hotspot had reached or passed this point by 100 m.y. Identification of the hot-spot east of this point is problematic. Fig. 11 shows that the Biscay Seamounts lie approximately in line with the Newfoundland Seamounts at anomaly 31/32.time. It could be postulated that the hot-spot also produced these seamounts, but this remains speculative in the absence of chemical and chronological data from the Biscay Seamounts. Extension of the hot-spot trace into the Biscay group also requires that the hot-spot crosses the ridge axis. The further eastward extent of the trace and the present location of the hot-spot remain unresolved problems. (3) At 73 m.y.B.P. (anomaly 31/32) the formation of the Newfoundland Basin and the Bay of Biscay would have been completed. Our reconstruction for this time is almost identical to that proposed by Laughton [39]. The geometrical alignment of the seamounts in the Newfoundland Basin and Bay of Biscay may or may not be coincidental, depending on the validity of the two alternative seamount models presented above.
4. Conclusion From magnetic data in the southern Newfoundland Basin, a magnetic anomaly reversal sequence has been delineated. This sequence incorporates the identification of the J-anomaly at its western end, the identification of anomaly 31/32 at its eastern end, and the existence of reversely magnetized zones corresponding to the magnetic anomalies mapped in tile southern survey area. Tile preferred correlation of the J-anomaly with the young end of the Keathley sequence requires that the reversals within the basin occurred during the Cretaceous quiet interval. The continuation of the anomaly pattern to the north through the region of the Newfoundland Seamounts is uncertain. The geochemical data from the seamounts are consistent with volcanism along an older structural discontinuity or along a hot-spot trace. Available geochronologic data are insufficient to determine whether there is an age progression along the chain. Given that the seamounts separate the basin into two regions of markedly different magnetic character and that the magnetic polarity of all seamounts thus far located is normal, we favour the hypothesis of structurally controlled, roughly synchronous volcanism. A schematic model has been developed for plate motions in the basin between 115 and 73 m.y.B.P. which succeeds moderately well in incorporating available data from the Newfoundland Basin and makes a number of testable predictions. Continuing studies in the Newfoundland Basin and the eastern North Atlantic will permit further refinement of the model.
Acknowledgements The authors are grateful to the officers and crew of CSS "Hudson", CNAV "Sackville", and MV "Martin Karlsen" for their willing assistance in gathering the data. Many people assisted us in the collection and reduction of the data. M. Keen, D. Piper, R. Jackson, and D. Barrett were particularly helpful during all aspects of this study. M. Keen provided invaluable support at sea. We thank S. Barr for her assistance during the initial stages of this project. S. Srivastava, M. Keen, and R. Haworth critically read the manuscript and provided many thoughtful and helpful comments. The contents of this paper form part of the Ph.D. thesis of K.D. Sullivan.
319
References 1 I';.C. Bullard, J.E. Everett and A.G. Smith, The fit of the continents around the Atlantic, in: A Symposium on Continental Drift, P.M.S. Blackett, E.C. Bullard and S.K. Runcorn, eds., Philos. Trans. R. Soc. kond., Set. A, 1088 (1965) 41 --51. 2 W.C. Pitmann III and M. Talwani, Sea-floor spreading in the North Atlantic, Geol. Soc. Ana. Bull. 83 (1972) 619 646. 3 X. kc Pichon and P.J. Fox, Marginal offsets, fracture zones, and the early opening of the North Atlantic, J. Geophys. Rcs. 76 (1971) 6294-6308. 4 D. McKenzie and C.A. Williams, The evolution of the North-East Atlantic, Nature 232 ( 1971 ) 168-173. 5 C.A. Williams, Sea-floor spreading in the Bay of Biscay and its relationship to the North Atlantic, Earth Planet. Sci. Lett. 24 (1975) 440-456. 6 W.B.t:. Ryan, J.-C. Sibuet and Scientific Party, Passive continental margin, Geotimes 21 (1976) 21 23. 7 L. Montadert, D.G. Roberts and Scientific Party, Glomar Challenger sails on Leg 48, Geotimes 21 (1976) 19-23. 8 X. Le Pichon, J.-C. Sibuet and J. Francheteau, The fit of the continents around the North Atlantic Ocean, Tectonophysics 38 (1977) 169-209. 9 R. Jackson, C.E. Keen and M.J. Keen, Seismic structure of the continental margins and ocean basins of southeastern Canada, Geol. Surv. Can. Paper 74-51 (1974) 13 pp. 10 R.W. Raitt, The Crustal Rocks, in: The Sea, 3, M.N. ttill, ed. (Wiley, New York, N.Y., 1963) 85-102. 11 J. Ewing and M. Ewing, Seismic Reflection, in: The Sea, 4, A.E. Maxwell, ed. (lnterscience, New York, N.Y., 1970) 1. 12 1.1. Ewing and C.D. Hollistcr, Regional aspects of deep sea drilling in the western North Atlantic, in: C.D. ltollister et al., Initial Reports of the Deep Sea Drilling Project 11 (U.S. Govermnent Printing Office, Washington, D.C., 1972) 951-973. 13 G. Mountain and B.E. Tucholke, Horizon ~: acoustic character and distribution in the western North Atlantic, LOS Trans. Am. Geophys. Union 58 (1977) 406. 14 L.F. Jansa and J.A. Wade, Geology of the continental margin off Nova Scotia and Newfoundland, in: Offshore Geology of Eastern Canada, 2, Geol. Surv. Can. Paper 74-30 (1975) 51 106. 15 C.E. Keen, M.J. Keen, D.L. Barrett and D.E. Heftier, Some aspects of the ocean-continent transition at the continental margin of eastern North America, in: Offshore Geology of Eastern Canada, 2, Geol. Surv. Can. Paper 74-30 (1975) 189 198. 16 D.L. Barrett and C.E. Keen, Lineations in the magnetic quiet zone of the Northwest Atlantic, Nature 253 (1975) 423. 17 B.R. Hall, Collection, reduction and interpretation of magnetic data from the Newfoundland Basin, M. Sc. Thesis, Dalhousie Univ., Halifax, N.S. (1977) unpublished. 18 S.C. Cande and Y. Kristoffersen, Late Cretaceous magnetic anomalies in the North Atlantic, Earth Planet. Sci. Lett. 35 (1977) 215-224.
19 S.P. Srivastava, Evolution of the Labrador Sea and its bearing on the early evolution of the North Atlantic, Geophys. J. (in press). 20 IAGA Commission 2, Working Group 4, J. Geophys. Res. 74 (1969) 4407 4408. 21 R.L. Larson and T.W.C. Hilde, A revised time scale of magnetic reversals for the early Cretaceous and late Jurassic, J. Geophys. Res. 80 (1975) 2586-2594. 22 R. Couillard and E. Irving, Paleolatitude and reversals: evidence from the Cretaceous period, Geol. Assoc. Can., Spec. Paper 23 (1975) 21-29. 23 E. Irving and G. Pullaiall, Reversals of the geomagnetic field, magnetostratigraphy, and relative magnitude of paleosecular variation in tile Phanerozoic, Earth-Sci. Rev. 12 (1975) 35-64. 24 B.E. Tucholkc, P.R. Vogt and Scientific Party, Glomar Challenger drills in the North Atlantic, Geotimes 20 (1975) 18-21. 25 A.S. Laughton and B.R. Whitmarsh, The Azores-Gibraltar plate boundary, in: Geodynamics of Iceland and the North Atlantic Area, L. Kristjansson, ed., (D. Reidel, Dordrecht, 1974) 63-81. 26 D.E. Hayes and P.D. Rabinowitz, Mesozoic magnetic lineations and the magnetic quiet zone off Northwest Africa, Earth Planet. Sci. Lett. 28 (1975) 105-114. 27 E. Uchupi, K.O. Emery, C.O. Bowin and J.D. Phillips, Continental margin off western Africa: Senegal to Portugal, Bull. Am. Assoc. Pet. Geol. 60 (1976) 809-878. 28 D.L. Barrett and C.E. Keen, Mesozoic magnetic lineations, the magnetic quiet zone, and sea-floor spreading in the northwest Atlantic, J. Geophys. Res. 81 (1976) 4 8 7 5 4884. 29 P.D. Rabinowitz, S.C. Cande and D.E. Hayes, The J-anomaly in the central North Atlantic Ocean, in: B. Tucholke and P.R. Vogt, lnitial Reports of the Deep-Sea Drilling Project 43 (in press). 30 P.R. Vogt and G.L. Johnson, Cretaceous sea-floor spreading in the western North Atlantic, Nature 234 (1971) 22-25. 31 B. Keating and C.E. Helsley, Magnetostratigraphy of the Cretaceous, Geol. Soc. Am. Abstr. Progr. 8 (1976) 948 (abstract). 32 W. Alvarez and W. Lowrie, Upper Cretaceous-Paleocene magnetic stratigraphy at Gubbio, Italy, IIl. Upper Cretaceous magnetic stratigraphy, Bull. Geol. Soc. Am. 88 (1977) 374-377. 33 K.D. Sullivan, Structure and evolution of the Newfoundland Basin, offshore eastern Canada, Ph.D. Thesis, Dalhousie Univ., Halifax, N.S. (in preparation). 34 K.D. Sullivan and C.E. Keen, Newfoundland seamounts: petrology and geochemistry, in: Volcanic Regimes in Canada, Geol. Assoc. Can. Spec. Paper 16 (1976) 4 6 1 476. 35 G.D. Borley, Oceanic islands, in: The Alkaline Rocks, H. S~brensen, ed. (J. Wiley and Sons, New York, N.Y., 1974) 311-330. 36 J.G. Schilling, Iceland mantle plume: geochemical study of the Reykjanes Ridge crest, Nature 242 (1973) 565-571.
320 37 F. Barberi, E. Bonatti, G. Marinelli and J. Varet, Transverse tectonics during the split of a continent ; data from the Afar Rift, Tectonophysics 23 (1974) 199-209. 38 G. Thompson and W.G. Melson, Petrology of oceanic crust across fracture zones in the Atlantic Ocean: evidence of a new kind o f sea-floor spreading, J. Geol. 80 (1972) 526 - 5 3 8 .
39 A.S. Laughton, The southern Labrador Sea - a key to Mesozoic and early Tertiary evolution of the North Atlantic, in: Initial Reports of the Deep-Sea Drilling Project 12 (U.S. Government Printing Office, Washington, D.C., 1972) 1155-1179.
307
[41
MESOZOIC EVOLUTION OF THE NEWFOUNDLAND BASIN C.E. KEEN Atlantic Geoscience Centre, Bedford Institute o f Oceanography, Dartmouth, N.S. (Canadaj B.R. HALL * Department o f Oceanography, Dalhousie University, Halifax, N.S. (Canada) and K.D. SULLIVAN Department of Geology, Dalhousie University, Halifax, N.S. (Canadaj
Received March 8, 1977 Revised version received August 2, 1977
We present here an interpretation of geological and geophysical data collected in the Newfoundland Basin. Geophysical measurements indicate that the area was formed by sea-floor spreading during the Cretaceous. Rocks collected from the Newfoundland Seamount chain support this interpretation and their chemistry suggests affinities with volcanic rocks found on oceanic islands but not with those sampled on mid-ocean ridges. A schematic model for early plate motions between Iberia and North America beginning about 115 m.y.B.P., consistent with the data, is discussed.
1. Introduction An extensive body of literature dealing with the predrift position o f the continents and relative motions of the circum-Atlantic plates has accumulated in the past 10 years. With the present widespread acceptance of a basic model for the evolution of the North Atlantic [ 1 - 3 ] has come greater interest in resolution of specific paleogeographic and kinematic problems. Two such problems are the initial position o f Iberia with respect to North America and the geometry and chronology of early (pre-73 m.y.B.P.) relative motions between these two plates. A number of investigators have collected data bearing on these questions from areas in the eastern North Atlantic and the Bay o f * Present address: Department of Geological Sciences and Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964, U.S.A.
Biscay [4,5]. Most recently, Le Pichon et al. [8] have utilized this work and additional data in fitting the continents around the North Atlantic Ocean. These workers state that the largest uncertainty in their re- " assembly is in the Iberia/North America portion, citing kate Paleozoic crustal fragmentation as a possible cause of the complexity of the later development o f the area. Furthermore, no comprehensive account of the structure and evolution of the deep-water area west o f Portugal has yet been published. Publication of detailed results from DSDP kegs 47 and 48 [6,7] should do much to remedy this latter situation. We present here the first results to emerge from a number of Bedford Institute-Dalhousie University cruises to the Newfoundland Basin. The sea floor in this area, which is defined for the present purpose as the deep-sea region extending from the Northwest Atlantic Mid-Ocean Canyon to the Grand Banks and from Flemish Cap to the Southeast Newfoundland
308
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mary of our knowledge of the structure, age, and geochemistry of the Newfoundland Seamounts (Fig. 1).
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2. 2. Seismic studies Fig. 1. Location map. Open circles represent seamounts from the Newfoundland Seamount chain which were charted but not sampled, × 's represent seamounts from which samples discussed in text were dredged. Northern and southern geophysical survey areas are labelled N and S respectively. A and D are seismic reflection lines, and B and C are composite magnetic model lines; "7" is the crustal refraction seismic line. Solid circles 29, 32, and 33 are sonobuoy locations.
A reversed crustal seismic refraction line (line 7, Fig. 1), using retrievable sonobuoys, and several expendable sonobuoy lines were obtained in the Newfoundland Basin and are reported upon in detail by Jackson et al. [9]. The results from the crustal refraction line (Table 1 and Fig. 2) show that normal oceanic crust, with typical velocities for layers 2 and 3 [10], underlain by mantle at a depth of about 12 km, occupies at least part of the area. This refraction line and unreversed expendable sonobuoy results (sonobuoys 29, 32) show that between 2 and 3 km of sediment overlies basement. Nearer the eastern margin of the Grand Banks the sediment thickness increases to about 4 km (sonobuoy 33).
Ridge, should contain the western half of the record of Mesozoic spreading between Iberia and North America. The data are presented in two sections: (1) a regional synthesis of the geophysical data pertaining to the structural evolution of the Basin; and (2) a sumTABLE 1
Layer velocities and thickness from the seismic refraction measurements Line/sonobuoy
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Fig. 2. Time-distance plot, line 7. The solid lines with superimposed circles represent velocities measured by means of explosives; other solid lines are velocities interpreted from expendable sonobuoy wide-angle reflections and refractions, the dashed lines represent assumed velocities. The column to the right of the time-distance plot contains the reversed velocities from the time-distance plot, drawn so that depth and layer thickness can be read directly in kilometres. A typical seismic reflection record obtained within the basin (Fig. 3) shows a prominent oceanic basement reflector. The basement topography is rough, with a relief of about 0.7 s (two-way travel time). There are two prominent reflectors within the overlying sedimentary column. The first of these, R1, seen at about 1.0 s beneath the sea floor, is often overlain by a wavy unit roughly 0.5 s thick. R 1 is present in most of the area south of the seamounts. The disturbed acoustic facies, which is developed on and adjacent to the flank of the Southeast Newfoundland Ridge, is likely due to bottom-current-controlled sedimentation. A second prominent reflector, R2, is found just above oceanic basement (about 1.7 s subbottom) in the western half of the southern part of the basin. Except for the local evidence of draping over basement highs (especially R2) and the wavy facies, the reflectors on all profiles seaward of the continental rise are flat-lying. It is tempting to correlate reflectors R1 and R2 with Horizons A and 13which are found on most profiles in the western Atlantic [I 1], but such a correlation is not definitive. It should be noted in this connection that the preferred magnetic anomaly identification discussed below gives a sea-floor age for this part of the basin which is perhaps 10 m.y. younger than the age of Horizon/3 given by Ewing and Hollister [12], however, there is recent evidence that this horizon is diachronous [ 13]. The velocities obtained from the expendable sono-
buoys in the Newfoundland Basin fall into two groups [9]. Generally, velocities above R I are between 1.6 and 2.1 km s -1, while the sedimentary interval below R 1 exhibits velocities in the range 2.6 3.5 km s -1. The crustal velocities measured for layers 2 and 3 are 5.2-5.8 km s -1 and 6.6 km s -1 respectively, very similar to those observed elsewhere for these layers. The mantle velocity measured was 7.8 km s -1. Seismic reflection lines across the eastern margin of the Grand Banks, one of which is shown in Fig. 4, indicate acoustic basement underlying a well-developed angular unconformity on the shelf. This is probably the Upper Cretaceous unconformity found in exploratory wells drilled on the Grand Banks and correlated with seismic reflection records by Jansa and Wade [14]. Basement appears to be truncated by a fault near the shelf edge (at 50 km, Fig. 4). Sedimentary layers higher in the column may continue across the shelf break onto the slope, although identification of the reflectors is hampered by hyperbolae in this part of the record. Across the slope the record shows no evidence of diapir complexes such as are common along the Scotian margin [15], nor does the basement appear to be block-faulted in the manner observed along the transform-fault margin of the southern Grand Banks [9]. Basement interpreted to be oceanic crust from the seismic refraction results can be followed westward from the basin proper to within 75 km of the shelf break.
2. 3. Magnetic surveys In the two areas labelled N and S in Fig. 1, geophysical surveys incorporating gravimetric, magnetic, bathymetric and seismic reflection measurements were undertaken. The prime purpose of these surveys was to delineate any prominent direction of magnetic lineations which might exist, to determine, from correlation between magnetic anomalies and layer 2 topography [ 16], the presence or absence of magnetic reversals and to attempt to date these reversals. Full details of the data collection, reduction, and interpretation are given by Hall [17]. The eastern boundary of the region of interest is taken as the negative anomaly between anomalies 31 and 32 (31/32). Work by Cande and Kristoffersen [ 18] indicates that, due to the high degree of skewness, anomalies 31 and 32 may have been misidentified and are actually anomalies 33 and
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311 34. However, this problem of 31/32 versus 33/34 does not affect the present discussion in any significant way. We follow the anomaly identifications of Srivastava [19], which are based on extensive data in the Labrador Sea and the western Atlantic north of the Azores. The data in the northern survey area (area N, Fig. 1) consist primarily of measurements made by early cruises that passed through the area. Very large cross-over errors in the magnetic anomalies (109 gammas r.m.s.) are attributed to either inadequate navigational control or to the presence of large diurnal variations during at least some period of data collection. An unsuccessful attempt was made to improve the cross-over values by shifting the tracks and by applying diurnal corrections using variations measured at St. John's, Newfoundland, during the survey periods. The results in this northern area are inconclusive, but are presented here for the sake of completeness. Fig. 5a shows the pattern obtained by contouring the magnetic anomalies. Some of the anomalies are well defined while others are defined by only one or two tracks. The contour map shows a predominantly eastwest pattern of anomalies. However, if individual profiles are stacked from south to north, as shown in Fig. 5b, there appears to be a reasonably good correlation in anomaly shapes in a north-south direction. Hence, we cannot at present adequately define the direction of lineations in this area. Analysis of magnetic data in the southern survey area (area S in Fig. 1) is more straightforward. The navigation was more accurate than in the northern area and a magnetometer housed in a moored buoy was used to measure diurnal variations in the area during the survey. Fig. 6a shows the anomalies after correction for diurnal variation and removal of the International Geomagnetic Reference Field [20]. The r.m.s. cross-over error for the corrected survey data is 22 gammas. The track spacing is sufficient to define magnetic lineations trending in a north to northeast direction. A break in the anomaly pattern, approximately perpendicular to the lineations, can be seen near the centre of the survey area. This may reflect the position of a fracture zone, but we have insufficient data north of this break to determine the sense and magnitude of anomaly offset. Fig. 6b shows the magnetic anomaly profiles along ship's track. The wavelengths and amplitudes of the anomalies are typical of those
found elsewhere in areas formed by sea-floor spreading [21].
2. 4. Magnetic anomaly modelling The magnetic anomalies have been modelled along four seismic reflection tracks in the southern survey area (lines A-A', B-B', C-C', D-D', Fig. 6a), in order to discriminate between the effects due to basement topography and those due to magnetic reversals. Because the spacing of the seismic lines does not allow basement topography to be contoured, the assumption of two-dimensionality was adopted. The source layer used has a planar base at 8.5 km depth and incorporates along its upper surface the basement topography obtained from the reflection profiles. Models using source layers of constant thickness did not give significantly different results. All models used a remanent magnetization of 0.0025 emu cm -3. Declination and inclination to the pole were obtained from the Cretaceous pole position for North America of Couillard and Irving [22]. The models, an example of which is shown in Fig. 7 (D-D') demonstrate that, within the limits of the two-dimensionality assumption, the anomalies are not caused by layer 2 topography alone (although the effect of basement topography is considerable). No model assuming normally magnetized crust with a uniform magnetization value could be fitted to the observed curve, although the observed magnetic profile might be fitted using crustal blocks of uniform magnetic polarity (positive or negative) with alternately high and low magnetization values. In the absence of any evidence that lineated marine magnetic anomalies are formed by the latter mechanism, we prefer to satisfy the measurements by including zones of reverse. ly magnetized material in our model. On first inspection two of the reversed zones (25 and 145 km) are more obviously required to fit the data. However, closer inspection of the figure will show that a significantly better fit to the observations is obtained by incorporating reversed zones at 90 km and 190 km. All four of the lines modelled show four major reversed zones, indicating that the assumption of twodimensionality is valid here. However, the extent of the reversal sequence within the detailed survey area alone is too short to permit correlation with known
312
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Fig. 7. Magnetic anomaly models incorporating basement topography. Calculated curves in (a) assume all normally magnetized bodies with differing magnetization values. Fig. 7b shows a curve calculated assuming normal and reversed polarity bodies as indicated in the sketch of basement topography. Normally magnetized bodies are stippled.
reversal sequences elsewhere. A composite anomaly profile for the southern portion of the basin was therefore constructed. This extends from anomaly 31/32 westward to the continental rise (lines B and C, Fig. 1). The model for this composite profile incorporated basement topography except in the section extending from the southern survey area east to anomaly 31/32. No seismic data were available along this latter section of track; instead, a layer 2 km thick (top at 6.5 km) was assumed. We justify the exclusion of topography along section C on the basis of the fact that the reversed zones within the survey area are consistently associated with the prominent magnetic anomaly minima regardless of the configuration of basement topography; we assume that this is also true for section C of the composite profile. Finally, a remanent magnetization of 0.05 emu cnr -3 (rather than 0.0025 emu cm -a) was used for the westernmost 200 km of the profile in order to improve the fit to the high-amplitude anomalies in that section. The model, with the observed and calculated profiles, is shown in Fig. 8. A single composite magnetic model in the southern Newfoundland Basin is scant evidence from which to
deduce the age of the basin. However, the magnetic anomaly character (Fig. 6b) and basement topography observed on the composite line are similar to that seen on other profiles south of the seamounts, which supports the magnetic model presented here. Therefore, age limits for the basin can be established from these data. First, the Newfoundland Basin is bounded on the east by anomaly 31/32 and is therefore older than 73 m.y.B.P. To the west of this anomaly one would expect to find crust formed during the Cretaceous quiet interval (110 to 81 m.y.B.P. [23]),and, if the basin is old enough, the Keathley disturbed interval (older than 108 m.y.B.P. [21]). Second, the large anomaly at the western end of the composite model is similar in amplitude and character to the J-anomaly observed to the south of the Newfoundland Basin [24]. A comparison of the character of this large anomaly to the J-anomaly in the area of DSDP drilling on the Janomaly Ridge (Spur Ridge, Fig. 1) during Leg 43 is shown in Fig. 9. The similarity in shape and amplitude is striking. Furthermore, the anomalies in each case are associated with a basement high [24]. Other crossings of this anomaly between the Southeast Newfoundland Ridge and the seamounts show that it is a northerly trending feature within the basin and that it is always associated with a basement high. Thus, we consider this anomaly ill the Newfoundland Basin to be an extension of the J-anomaly mapped to the south.
2. 5. Age of magnetic anomalies and rates of spreading There is some question as to the age of the J-anomaly. The results of Leg 43 drilling [24] suggest that it is associated with crust of Aptian age (about 110 m.y. B.P.) which implies that the J-anomaly represents the youngest part of the Keathley sequence, roughly M0 to M4. The correspondence of the J-anomaly to an exact position within the Keathley sequence has been difficult to deternune because its shape cannot be accounted for by the commonly used plane layer magnetic block model. Workers using this method in the eastern North Atlantic have correlated the peak and trough which characterize the J-anomaly with anomalies M2 to M4 [25,26] and the peak with anomaly M15 [27], and in the western North Atlantic to the south of the Newfoundland Basin, Barrett and Keen [28] correlated the trough of the J-anomaly with anom-
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alies M 16 to M 17. Finally, the peak of the J-anomaly on both sides of the central North Atlantic has been correlated by Rabinowitz et al. [29] with anomalies M1 to M0. In the Newfoundland Basin correlation of the trough of the J-anomaly with anomaly M3 gives an age range for the basin of about 115 to 73 m.y.B.P. and an average spreading half-rate of 1.36 cm yr-1; while correlation of the low with anomalies M16 to 250 I
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2. 6. The N e w f o u n d l a n d seamounts Fig. 9. Comparison of the (negative) J-anomaly over the Spur Ridge (lower profile) and the assumed J-anomaly in the western Newfoundland Basin (upper profile). Basement features in each of the two areas are similar.
In the approximate centre of the Newfoundland Basin lies the east-west-trending Newfoundland Seamount chain, which remained unsampled until 1974
316 (Fig. 1). The origin of this lineament is likely to be very important in relation to the evolution of the basin. It is worth noting in passing that all the seamounts plotted in Fig. 1, which to our knowledge includes all the seamounts in the chain, are of normal magnetic polarity [33]. A study of the volcanic rocks dredged from two seamounts near the centre of the group (crosses, Fig. 1) has provided geochemical and geochronological data bearing on their origin [34]. The geochemical evidence, in particular the rare earth patterns (Fig. 10) clearly demonstrates the alkaline nature of the seamount basalts. Alkaline volcanism of this type characterizes oceanic islands [35], hot-spot traces [36], and major transverse lineaments or fractures [37,38], and is postulated to be characteristic of "leaky" transfbrm faults. 4°Ar/39Ar dating of plagioclases separated from two of the trachytic rocks in one of the dredge hauls established the first solid chronological datum in the seamount volcanism [34]. The isochron ages obtained for the samples were 97.6 -+ 1.4 m.y. and 96.4 + 2.1 m.y., giving a combined isochron age of 97.7 +- 5 m.y. Additional chronological information has come from micropaleontological studies of a slab of clastic limestone dredged from Shredder Seamount, at the eastern end of the chain. Preliminary identification of the fauna in this sample indicates deposition in a shallow-water environment in Middle Jurassic to Early Cretaceous time, though tentative identifications of several poorly preserved foraminiferal fragments suggest that an Early Cretaceous age is more likely (F. Gradstein, personal communication). The present depth below sea level of the Shredder Seamount limestone indicates that the Newfoundland Basin has subsided a maximum of 1960 m since Early to Middle Cretaceous time. To the south of the Southeast Newfoundland Ridge on the Spur Ridge, or Janomaly Ridge, DSDP Leg 43 cored a similar, approximately coeval shallow-water limestone 3060 m below sea level [24]. This implies a differential subsidence of 1100 m across the Southeast Newfoundland Ridge, with the greater subsidence to the south. We suggest that the appellation "Newfoundland fracture zone" be applied with caution to the feature extending southeast of the Tail of the Bank since it may owe its existence to predominantly vertical motions of the type implied above. Further support for
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this postulate is found in the small offset of the Janomaly across the ridge (50-100 km) which indicates that the amount of transcurrent faulting has been small.
3. Discussion
We have attempted to formulate a number of plate tectonic models to explain the data presented in this paper. The schematic model presented below begins with the onset of sea-floor spreading between Iberia and the Grand Banks and ends with a reconstruction of the Iberian and North American plates at the time of anomaly 31/32 (Fig. 12). The time of this anomaly, according to Williams [5] marks the end of spreading in the Bay of Biscay. The sequence of events presented below is based on a correlation of the trough of the Janomaly with the interval M3;correlation of this feature with the M16 to M17 interval would change the age for initiation of spreading, but not the basic model. (1) At about 115 m.y.B.P. (M3 time) active seafloor spreading perpendicular to a trend of about 020 ° commenced between Iberia and the Grand Banks. Well data from the Grand Banks and initial results from DSDP Leg 48 [7] suggest that the initial rifting and faulting process began well before active spreading, possibly as early as latest Upper Triassic on the Grand
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i Fig. 12. Schematic three-stage model for the evolution o f the Newfoundland Basin and related portions of the North Atlantic, modified after Williams [ 5]. Solid line represents the spreading axis, zig-zag lines indicate position of seamounts.
318 Banks. Galicia Bank, which originally was situated immediately to the south of Flemish Cap, was separated from tile Grand Banks during the initial rifting, producing a complex type of transform motion between Flemish Cap and Galicia Bank. This perhaps explains the confused nature of the northern survey area. Since the direction of separation was cast-west, we would expect to find northerly trending lineations to the south of this confused zone. (2) At about I00 m.y.B.P., following the suggestion of Williams [5], the spreading ridge extended into the Bay of Biscay and the rotation of lberia away from Europe began. Both the volcanic base and the limestone cap of Shredder Seamount existed by this time and volcanism was still occurring on some of the central Newfoundland Seamounts; the age of initiation of the volcanic activity is not known. We suggest that the seamounts reflect reactivation of an older structural discontinuity. This type of model could involve either synchronous volcanism or propagation of volcanism along this older feature. A hot-spot seamount model has also been considered. The time at which formation of the trace within the Newfoundland Basin began is not known, but the paleontologic date from Shredder proves that the hotspot had reached or passed this point by 100 m.y. Identification of the hot-spot east of this point is problematic. Fig. 11 shows that the Biscay Seamounts lie approximately in line with the Newfoundland Seamounts at anomaly 31/32.time. It could be postulated that the hot-spot also produced these seamounts, but this remains speculative in the absence of chemical and chronological data from the Biscay Seamounts. Extension of the hot-spot trace into the Biscay group also requires that the hot-spot crosses the ridge axis. The further eastward extent of the trace and the present location of the hot-spot remain unresolved problems. (3) At 73 m.y.B.P. (anomaly 31/32) the formation of the Newfoundland Basin and the Bay of Biscay would have been completed. Our reconstruction for this time is almost identical to that proposed by Laughton [39]. The geometrical alignment of the seamounts in the Newfoundland Basin and Bay of Biscay may or may not be coincidental, depending on the validity of the two alternative seamount models presented above.
4. Conclusion From magnetic data in the southern Newfoundland Basin, a magnetic anomaly reversal sequence has been delineated. This sequence incorporates the identification of the J-anomaly at its western end, the identification of anomaly 31/32 at its eastern end, and the existence of reversely magnetized zones corresponding to the magnetic anomalies mapped in tile southern survey area. Tile preferred correlation of the J-anomaly with the young end of the Keathley sequence requires that the reversals within the basin occurred during the Cretaceous quiet interval. The continuation of the anomaly pattern to the north through the region of the Newfoundland Seamounts is uncertain. The geochemical data from the seamounts are consistent with volcanism along an older structural discontinuity or along a hot-spot trace. Available geochronologic data are insufficient to determine whether there is an age progression along the chain. Given that the seamounts separate the basin into two regions of markedly different magnetic character and that the magnetic polarity of all seamounts thus far located is normal, we favour the hypothesis of structurally controlled, roughly synchronous volcanism. A schematic model has been developed for plate motions in the basin between 115 and 73 m.y.B.P. which succeeds moderately well in incorporating available data from the Newfoundland Basin and makes a number of testable predictions. Continuing studies in the Newfoundland Basin and the eastern North Atlantic will permit further refinement of the model.
Acknowledgements The authors are grateful to the officers and crew of CSS "Hudson", CNAV "Sackville", and MV "Martin Karlsen" for their willing assistance in gathering the data. Many people assisted us in the collection and reduction of the data. M. Keen, D. Piper, R. Jackson, and D. Barrett were particularly helpful during all aspects of this study. M. Keen provided invaluable support at sea. We thank S. Barr for her assistance during the initial stages of this project. S. Srivastava, M. Keen, and R. Haworth critically read the manuscript and provided many thoughtful and helpful comments. The contents of this paper form part of the Ph.D. thesis of K.D. Sullivan.
319
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