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Evidence for the mode of formation of Porcupine Seabight
EARTH AND PLANETARY SCIENCE LETTERS 11 (1971) 140-146. NORTH-tlOLLAND PUBLISHING COMPANY
EVIDENCE FOR THE MODE OF FORMATION
OF
PORCUPINE SEABIGHT R. A. SCRUTTON, A. P. STACEY and F. GRAY Department of Geodesy and Geophysics, University of Cambridge, U.K. Received 17 January 1971
Data collected in the southern part of Porcupine Seabight and on the Porcupine Abyssal Plain are presented. An interpretation of the magnetic data provides a date of opening for the North Atlantic at 50°N. When this is combined with seismic reflection, seismic refraction and gravity results, it can be shown that a rotation of Porcupine Bank, as a mechanism for the formation of Porcupine Seabight, may not have taken place.
1. Introduction Porcupine Seabight is a bathymetric feature of the continental margin south-west of Ireland (fig. 1). The depth of water in the Seabight increases from 200 fathoms (371 m) at its narrow north end to 2000 fathoms (3708 m) where it opens into the Porcupine Abyssal Plain. It separates Porcupine Bank from the continental shelf west of Ireland. The bathymetry within the Seabight is characterised by gradual slopes, in contrast with the steeper continental slopes north and west of Porcupine Bank and around Goban Spur. The geophysical data collected on and around Porcupine Bank indicate that the Bank is part of the continental shelf [ 1 - 4 ] . Stride et al. [1] have suggested that Porcupine Seabight has been formed by the clockwise rotation of Porcupine Bank away from Ireland. This suggestion is supported by the Bullard et al. [5] reconstruction of the North Atlantic continents in which Porcupine Bank overlaps the Newfoundland continental shelf. A clockwise rotation of Porcupine Bank during the opening of the North Atlantic would explain the overlap in the reconstruction. However, seismic reflection studies by Bailey et al. 12] and Le Pichon et al. [6] indicate that there is a vertical component of crustal movement associated with the formation of Porcupine Seabight. In order to obtain more evidence for the mode o f
formation of the Seabight the Department of Geodesy and Geophysics, Cambridge ran a geophysical cruise to the southern part of the Seabight in May, 1969. R.R.S. John Murray was engaged in acquiring seismic reflection, seismic refraction, magnetic and gravity data. The purpose of this paper is to present and interpret this data.
2. Seismic reflection and seismic refraction data Seismic reflection profiles A to D (fig. 1) were obtained in Porcupine Seabight during the trials of a newly acquired Flexotir-G6om6chanique hydrophone array. The prof'fling system provided a maximum penetration of 3 sec two-way travel time. (Travel times quoted in the remainder of the paper will refer to two-way time.) A sequence of flat lying reflectors is observed in the southern part of the Seabight. Each reflector overlaps onto the margins of the Seabight, where the acoustic basement is very irregular. The irregularity may be due to faulting. The short profile over Godan Spur, D (fig. 2), shows a very undulating acoustic basement. Above this basement the reflectors indicate a geological sequence interrupted by folding and erosion, which suggests that the Spur, probably a ridge of metamorphic basement rocks possessing an
141
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double reflector consistent over the area of the Seabight deeper than 1000 fathoms (1854 m). On profile C (fig. 2), an undulating reflector at 1.2 sec below the sea floor is sometimes seen. The depth and nature of this reflector correlates it with an unconformity beneath the Seabight observed by Stride et al. [ 1 ] and Le Pichon et al. [6]. On the basis of the known geology of adjacent parts of the British Isles and the
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the sediments. The sedimentary basin continues northwards as a fault bounded trough [2]. Unreversed data from two seismic refraction lines were obtained on the John Murray cruise. The results are presented in fig. 3. Line 1 runs parallel to reflection profile C (fig. 1) and indicates a refracting interface at about 1.5 km beneath the sea floor. The interface may correlate with the unconformity observed on profile C. The unreversed velocity of 4.4 km/sec probably represents consolidated sedimentary rocks; because of the horizontal nature of the bedding 4.4 krn/sec will not differ greatly from the true velocity. 5.1 km/sec probably indicates basement rocks at a depth of about 7 km beneath the sea floor. This depth must be considered very approximate in view of the unreversed data. The velocity of 7.7 kin/ sec at a depth of about 12 km may represent an intermediate layer of the Earth's crust, as suggested by Bunce et al. [7], rather than the upper mantle. Velocities of 7.2 to 7.7 km/sec are often found at the base of the crust beneath a subsiding continental margin [9]. The second unreversed seismic refraction line, 2 in fig. 1, is situated on the Porcupine Abyssal Plain. It reveals an oceanic crust overlying a low velocity upper mantle. 7.8 km/sec is very close to the 7.7 km/sec velocity assigned to crustal rocks in the structure for line 1. However, in the oceanic environment of the North East Atlantic 7.8 km/sec is typical o f upper mantle velocities [10].
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South-West Approaches, and on the basis of reflective characteristics, Stride et al. tentatively assign this horizon to an unconformity separating the Mesozoic from overlying Tertiary rocks. Since the formation of the reflector at 1.2 sec, sedimentation appears to have been continuous in this part of the Seabight. There is no evidence for diapiric disturbance within
In order to discover the trend and characteristics of the magnetic anomalies over the Porcupine Abyssal Plain the magnetic survey shown in fig. I was carried out by R.R.S. John Murray. The survey has been reduced to the International Geomagnetic Reference Field and shows anomalies up to 500 gamma in amplitude. A large negative anomaly and its adjacent positives are continuous across the survey on a strike o f 340 °. Similarly shaped anomalies have been observed to the south on Discovery'cru~¢ 23 data (C.A.Williams, personal communicatio.n). "the2~ anomalies form the easternmost magneti~ ]iheadons of the North Atlantic at this latitude. The two positives can be identified
R.A.Scrutton et aL, Mode or formation of Porcupine Seabight
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143
with some certainty as anomalies 31 and 32 on the reversal time scale of McKenzie and Sclater [ 11 ] (fig. 4). The wavelengths of the anomalies correspond to a mean half-rate of spreading of 1.4 cm/yr. To the east of anomaly 32, which is equivalent to 75 mybp, the magnetic profiles indicate that the first formed oceanic crust is approximately 80 my old. The oceanic magnetic lineations give way to a smoother magnetic field at the mouth of Porcupine Seabight. Upward continuation of some continental magnetic anomalies indicates that the anomalies observed in the Seabight could be caused by a continental crust at a depth of 3 to 5 km below the sea floor.
4. Interpretation The seismic reflection data collected on the cruise of R.R.S. John Murray, and on other cruises [1,2, 6], show that a sedimentary basin exists beneath Porcupine Seabight. This seismic reflection data, and seismic refraction and magnetic data obtained by the John Murray, suggest that the basin may be of the order of 5 km deep. Gray and Stacey [3] have outlined the problems of the origin of Porcupine Seabight. The results of their gravity survey, and the results of a 3 dimensional interpretation made on gravity data from the John Murray cruise (tracks I and II in fig. 1), suggest that crustal thinning may have occurred beneath the Seabight. The crustal thinning, which was probably associated with the formation of the Seabight, may have been the result of the process of rifting and sea-floor spreading along the axis of the Seabight, producing a rotation of Porcupine Bank away from the continental shelf west of Ireland. Alternatively, the crustal thinning may have bee,l caused by vertical movements of the Earth's crust induced by density changes at the crust-mantle boundary. The data presented in this paper can be used to place limitations on the possible crustal movements
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144
R.A .Scrutton et al., Mode or formation o f Porcupine Seabight
in the Porcupine Seabight region. The identification of anomaly 32 striking across the mouth of Porcupine Seabight indicates that at this latitude the North Atlantic began opening about 80 mybp. It is difficult to see how Porcupine Bank could have moved any great distance laterally before this time, when it was situated within a continental block. Consequently, the formation of Porcupine Seabight by the rotation of Porcupine Bank is most likely to have taken place at or since 80 mybp. It can be shown, however, that the seismic reflection data may be incompatible with this hypothesis. The seismic reflection records reveal an unconformity at 1.2 sec beneath the seabed in Porcupine Seabight. Beneath the unconformity our records show over 0.9 sec, and those of Le Pichon et al. [6] at least 1.4 sec, of undeformed sediments. These sediments are continuous across the full width of the Seabight. It seems reasonable to assume that formation of the Seabight by lateral extension must predate these sediments, otherwise severe faulting would be observed within them. Clearly, if it is possible to demonstrate that these sediments were accumulating before 80 mybp (the proposed time for the formation of Porcupine Seabight by rotation of Porcupine Bank), then the case for lateral extension is much weaker. Assuming the unconformity at 1.2 sec represents the top of the Mesozoic succession, by applying a velocity of 4.4 km/sec (measured on seismic refraction line 1) to the time interval of 1.4 sec observed by Le Pichon et al. below this unconformity, a thickness of at least 3.1 km of Mesozoic rocks is obtained. Before compaction and lithification the thickness could have been as much as three times greater [12]. The deposition of these sediments between the suggested time of rotation of Porcupine Bank, probably not greater than 80 mybp, and the end of the Mesozoic, 65 mybp, requires a mean sedimentation rate of 2 0 - 6 0 cm (depending on the degree of compaction) per 1000 years for the 15 my period. This is a very high rate of sedimentation and is considered unlikely to be correct. (For the North Sea the mean sedimentation rate during the Mesozoic was 3.4 cm per 1000 years [13]. The rate for the Tertiary in Porcupine Seabight is 2.3 cm per 1000 years assuming 1.5 km of Tertiary rocks as indicated by the seismic reflection
and seismic refraction results.) A more plausible solution is that the deeper sediments were deposited before 80 mybp. It must be stressed, however, that the use of sedimentation rates is not always a reliable criterion for age determination, and that the Tertiary-Mesozoic interface suggested by Stride et al. [ 1] has yet to be verified. Other geophysical results may also provide limitations on the nature of crustal movements necessary to the formation of Porcupine Seabight. If the Seabight has been formed by the rotation of Porcupine Bank relative to the European continent then fracture zones and perhaps trenches should be observed. It is therefore worth noting that neither the gravity profiles by Gray and Stacey [3] across the western margin of Porcupine Bank, nor a gravityseismic reflection traverse across the northern margin of Porcupine Bank (fig. 5), obtained on Discovery cruise 29 in October 1969, show any evidence of a trench feature where crust would have been consumed. The calculated Free-Air anomaly across the northern margin of the Bank, based on Airy's hypothesis of isostasy for a 2 dimensional model, provides a good fit with the observed Free-Air anomaly. The small negative residual at the foot of the continental slope can be explained by the sediments observed on the reflection profile in fig. 5. Similarly, no fracture zones have been identified adjacent to Porcupine Bank. A fracture zone might have been expected, for example, to the west of the mouth of the Seabight if Porcupine Bank had moved away from Europe. However, the magnetic lineations are clearly continuous in this region and can be traced northwards to latitude 51°N, the southernmost magnetic profile of the survey of Gray and Stacey [3]. The lack of evidence for fracture zones or trenches suggests Porcupine Bank may not have rotated. Finally, Sheridan and Drake [14] have observed a sedimentary basin on the Newfoundland continental shelf which, from the Atlantic fit of Bullard et al. [5], may have been the pre-continental drift continuation of the Porcupine Seabight basin. The Newfoundland basin is about 5 km deep and is thought to contain Carboniferous to Recent sedimentary rocks.
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146
R.A.Scrutton et aL, Mode or formation o f Porcupine Seabight
5. Conclusions Porcupine Seabight contains sediments which may may be of the order of 5 km thickness. The formation of the Seabight and the sedimentary basin may have been by rotation or translation of Porcupine Bank away from the continental shelf west of Ireland. However, there are now several lines of geophysical evidence to suggest that a major lateral movement of Porcupine Bank may not have taken place. From the magnetic data the initial opening of the North Atlantic at 50°N was approximately 80 mybp. It is probable that this is the earliest date for the rotation of Porcupine Bank proposed by Stride et al. ttowever, the rotation of Porcupine Bank as a mechanism for the formation of Porcupine Seabight appears to be incompatible with the seismic reflection and seismic refraction data. These data suggest that the sediments beneath the Seabight may have been accumulating since before 80 mybp. Furthermore the rotation of Porcupine Bank may be doubted from the lack of evidence for fracture zones or trenches around the Bank. Accepting that Porcupine Bank is unlikely to have rotated, the alternative mode of formation of Porcupine Seabight is by vertical movements in the Earth's crust. Such movements would be induced by density changes at the crust-mantle boundary as summarised by Sheridan [9], and would result in subsidence at the top of the crust.
Acknowledgements We thank Dr. D.H.Matthews for his encouragement, help and discussion. We are indebted to Dr. D.P. McKenzie and his assistants for their help in reducing the magnetic data, and to Miss J.M.Lort for her help in reducing the seismic refraction data. Thanks also to all those who helped us collect the data, in particular the Master and Officers of R.R.S. John Murray. This work was supported by the Natural Environment
Research Council, London under grant no. 1294 to the University of Cambridge.
References [ 1] A.H.Stride, J.R.Oarray, D.G.Moore and R.H.Belderson, Marine geology of the Atlantic continental margin of Europe, Phil. Trans. Roy. Soc. (London) A264 (1969) 31 [2] R.J.Bailey, R.tI.Clarke and D.Taylor-Smith, The continental margin west of Ireland, S.C.O.R. Cambridge Symposium "Geology of the East Atlantic Continental Margin", Report Inst. Geol. Sci., in press. [3] F.Gray and A.P.Stacey, Gravity and magnetic interpretation of Porcupine Bank and Porcupine Bight, Deep Sea Res. 17 (1970) 467. [4 ] T.D.AIIan, Magnetic measurements at sea, Ph.D. Thesis, Cambridge University (1960). [5] E.C.Bullar, J.E.Everett and A.G.Smith, The fit of the continents around the Atlantic, Phil. Trans. Roy. Soc. (London) A258 (1965) 41. [6] X.Le Pichon, A.Cressard, J.Mascle, G.Pautot and B. Sichler, Structures sous-marines des bassins Mdimentaires de Porcupine et de Rockall, Rdsultats scientifiques de la campagne de N.O. Jean Charcot en Atlantique Nord, aofit-septembre-octobre 1969, publication no. 2 (1970) 8 pp. [7] G.A. Day and C.A. Williams,Gravity compilation in the N.E.Atlantic and interpretation of gravity in the Celtic Sea, Earth Planet. Sci. Letters 8 (1970) 205. [8] E.T.Bunce, S.Crampin, J.B.Itersey and M.N.HilI, Seismic refraction observations on the continental boundary west of Britain, J. Geophys. Res. 69, 18 (1964) 3853. [9] R.E.Sheridan, Subsidence of continental margins, Tectonophysics 7 (1969) 219. [10] J.Ewing and M.Ewing, Seismic refraction measurements in the Atlantic Ocean basins, in the Mediterranean Sea, on the Mid-Atlantic Ridge, and in the Norwegian Sea, Geol. Soc. Am. Bull. 70 (1959) 291. I 11 ] D.P.McKenzie and J.G.Sclater, The Evolution of the Indian Ocean, in preparation. [ 12] H.Kuenen, Marine Geology (Wiley, New York, 1950) 551 pp. [ 13] J.H.l:.Umbgrove, Periodal events in the North Sea Basin, Geol. Mag. 82 (1945) 237. [ 14] R.E.Sheridan and C.L.Drake, Seaward extension of the Canadian Appalachians, Can. J. Earth Sci. 5 (1968) 337.
EVIDENCE FOR THE MODE OF FORMATION
OF
PORCUPINE SEABIGHT R. A. SCRUTTON, A. P. STACEY and F. GRAY Department of Geodesy and Geophysics, University of Cambridge, U.K. Received 17 January 1971
Data collected in the southern part of Porcupine Seabight and on the Porcupine Abyssal Plain are presented. An interpretation of the magnetic data provides a date of opening for the North Atlantic at 50°N. When this is combined with seismic reflection, seismic refraction and gravity results, it can be shown that a rotation of Porcupine Bank, as a mechanism for the formation of Porcupine Seabight, may not have taken place.
1. Introduction Porcupine Seabight is a bathymetric feature of the continental margin south-west of Ireland (fig. 1). The depth of water in the Seabight increases from 200 fathoms (371 m) at its narrow north end to 2000 fathoms (3708 m) where it opens into the Porcupine Abyssal Plain. It separates Porcupine Bank from the continental shelf west of Ireland. The bathymetry within the Seabight is characterised by gradual slopes, in contrast with the steeper continental slopes north and west of Porcupine Bank and around Goban Spur. The geophysical data collected on and around Porcupine Bank indicate that the Bank is part of the continental shelf [ 1 - 4 ] . Stride et al. [1] have suggested that Porcupine Seabight has been formed by the clockwise rotation of Porcupine Bank away from Ireland. This suggestion is supported by the Bullard et al. [5] reconstruction of the North Atlantic continents in which Porcupine Bank overlaps the Newfoundland continental shelf. A clockwise rotation of Porcupine Bank during the opening of the North Atlantic would explain the overlap in the reconstruction. However, seismic reflection studies by Bailey et al. 12] and Le Pichon et al. [6] indicate that there is a vertical component of crustal movement associated with the formation of Porcupine Seabight. In order to obtain more evidence for the mode o f
formation of the Seabight the Department of Geodesy and Geophysics, Cambridge ran a geophysical cruise to the southern part of the Seabight in May, 1969. R.R.S. John Murray was engaged in acquiring seismic reflection, seismic refraction, magnetic and gravity data. The purpose of this paper is to present and interpret this data.
2. Seismic reflection and seismic refraction data Seismic reflection profiles A to D (fig. 1) were obtained in Porcupine Seabight during the trials of a newly acquired Flexotir-G6om6chanique hydrophone array. The prof'fling system provided a maximum penetration of 3 sec two-way travel time. (Travel times quoted in the remainder of the paper will refer to two-way time.) A sequence of flat lying reflectors is observed in the southern part of the Seabight. Each reflector overlaps onto the margins of the Seabight, where the acoustic basement is very irregular. The irregularity may be due to faulting. The short profile over Godan Spur, D (fig. 2), shows a very undulating acoustic basement. Above this basement the reflectors indicate a geological sequence interrupted by folding and erosion, which suggests that the Spur, probably a ridge of metamorphic basement rocks possessing an
141
R.A.Scrutton et al., Mode o f formation o[ Porcupine Seabight
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Fig. 1. Bathymetry of the Porcupine Seabight region and locations of profiles. Hercynian trend [7], has experienced some vertical and horizontal displacements during its formation. The topmost rocks, lying above the unconformity, have imparted a smooth topography to the Spur. The reflection profiles show that beneath Porcupine Seabight a sedimentary basin exists. The deepest continuous reflectors, at 1.6 sec and 2.1 sec below the sea floor, constitute a characteristically fiat-lying
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R.A.Serutton et al., Mode o f formation o f Porcupine Seabight
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the sediments. The sedimentary basin continues northwards as a fault bounded trough [2]. Unreversed data from two seismic refraction lines were obtained on the John Murray cruise. The results are presented in fig. 3. Line 1 runs parallel to reflection profile C (fig. 1) and indicates a refracting interface at about 1.5 km beneath the sea floor. The interface may correlate with the unconformity observed on profile C. The unreversed velocity of 4.4 km/sec probably represents consolidated sedimentary rocks; because of the horizontal nature of the bedding 4.4 krn/sec will not differ greatly from the true velocity. 5.1 km/sec probably indicates basement rocks at a depth of about 7 km beneath the sea floor. This depth must be considered very approximate in view of the unreversed data. The velocity of 7.7 kin/ sec at a depth of about 12 km may represent an intermediate layer of the Earth's crust, as suggested by Bunce et al. [7], rather than the upper mantle. Velocities of 7.2 to 7.7 km/sec are often found at the base of the crust beneath a subsiding continental margin [9]. The second unreversed seismic refraction line, 2 in fig. 1, is situated on the Porcupine Abyssal Plain. It reveals an oceanic crust overlying a low velocity upper mantle. 7.8 km/sec is very close to the 7.7 km/sec velocity assigned to crustal rocks in the structure for line 1. However, in the oceanic environment of the North East Atlantic 7.8 km/sec is typical o f upper mantle velocities [10].
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South-West Approaches, and on the basis of reflective characteristics, Stride et al. tentatively assign this horizon to an unconformity separating the Mesozoic from overlying Tertiary rocks. Since the formation of the reflector at 1.2 sec, sedimentation appears to have been continuous in this part of the Seabight. There is no evidence for diapiric disturbance within
In order to discover the trend and characteristics of the magnetic anomalies over the Porcupine Abyssal Plain the magnetic survey shown in fig. I was carried out by R.R.S. John Murray. The survey has been reduced to the International Geomagnetic Reference Field and shows anomalies up to 500 gamma in amplitude. A large negative anomaly and its adjacent positives are continuous across the survey on a strike o f 340 °. Similarly shaped anomalies have been observed to the south on Discovery'cru~¢ 23 data (C.A.Williams, personal communicatio.n). "the2~ anomalies form the easternmost magneti~ ]iheadons of the North Atlantic at this latitude. The two positives can be identified
R.A.Scrutton et aL, Mode or formation of Porcupine Seabight
M
143
with some certainty as anomalies 31 and 32 on the reversal time scale of McKenzie and Sclater [ 11 ] (fig. 4). The wavelengths of the anomalies correspond to a mean half-rate of spreading of 1.4 cm/yr. To the east of anomaly 32, which is equivalent to 75 mybp, the magnetic profiles indicate that the first formed oceanic crust is approximately 80 my old. The oceanic magnetic lineations give way to a smoother magnetic field at the mouth of Porcupine Seabight. Upward continuation of some continental magnetic anomalies indicates that the anomalies observed in the Seabight could be caused by a continental crust at a depth of 3 to 5 km below the sea floor.
4. Interpretation The seismic reflection data collected on the cruise of R.R.S. John Murray, and on other cruises [1,2, 6], show that a sedimentary basin exists beneath Porcupine Seabight. This seismic reflection data, and seismic refraction and magnetic data obtained by the John Murray, suggest that the basin may be of the order of 5 km deep. Gray and Stacey [3] have outlined the problems of the origin of Porcupine Seabight. The results of their gravity survey, and the results of a 3 dimensional interpretation made on gravity data from the John Murray cruise (tracks I and II in fig. 1), suggest that crustal thinning may have occurred beneath the Seabight. The crustal thinning, which was probably associated with the formation of the Seabight, may have been the result of the process of rifting and sea-floor spreading along the axis of the Seabight, producing a rotation of Porcupine Bank away from the continental shelf west of Ireland. Alternatively, the crustal thinning may have bee,l caused by vertical movements of the Earth's crust induced by density changes at the crust-mantle boundary. The data presented in this paper can be used to place limitations on the possible crustal movements
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Fig. 4. From top to bottom: the magnetic profiles observed from north to south across the survey shown in fig. 1, the synthetic profile calculated from the reversal time scale of McKenzie and Sclater [ 11 ] with a half-rate of spreading of 1.4 cm/yr, and a model of the reversals. All profiles are projected normal to the strike of the anomalies.
144
R.A .Scrutton et al., Mode or formation o f Porcupine Seabight
in the Porcupine Seabight region. The identification of anomaly 32 striking across the mouth of Porcupine Seabight indicates that at this latitude the North Atlantic began opening about 80 mybp. It is difficult to see how Porcupine Bank could have moved any great distance laterally before this time, when it was situated within a continental block. Consequently, the formation of Porcupine Seabight by the rotation of Porcupine Bank is most likely to have taken place at or since 80 mybp. It can be shown, however, that the seismic reflection data may be incompatible with this hypothesis. The seismic reflection records reveal an unconformity at 1.2 sec beneath the seabed in Porcupine Seabight. Beneath the unconformity our records show over 0.9 sec, and those of Le Pichon et al. [6] at least 1.4 sec, of undeformed sediments. These sediments are continuous across the full width of the Seabight. It seems reasonable to assume that formation of the Seabight by lateral extension must predate these sediments, otherwise severe faulting would be observed within them. Clearly, if it is possible to demonstrate that these sediments were accumulating before 80 mybp (the proposed time for the formation of Porcupine Seabight by rotation of Porcupine Bank), then the case for lateral extension is much weaker. Assuming the unconformity at 1.2 sec represents the top of the Mesozoic succession, by applying a velocity of 4.4 km/sec (measured on seismic refraction line 1) to the time interval of 1.4 sec observed by Le Pichon et al. below this unconformity, a thickness of at least 3.1 km of Mesozoic rocks is obtained. Before compaction and lithification the thickness could have been as much as three times greater [12]. The deposition of these sediments between the suggested time of rotation of Porcupine Bank, probably not greater than 80 mybp, and the end of the Mesozoic, 65 mybp, requires a mean sedimentation rate of 2 0 - 6 0 cm (depending on the degree of compaction) per 1000 years for the 15 my period. This is a very high rate of sedimentation and is considered unlikely to be correct. (For the North Sea the mean sedimentation rate during the Mesozoic was 3.4 cm per 1000 years [13]. The rate for the Tertiary in Porcupine Seabight is 2.3 cm per 1000 years assuming 1.5 km of Tertiary rocks as indicated by the seismic reflection
and seismic refraction results.) A more plausible solution is that the deeper sediments were deposited before 80 mybp. It must be stressed, however, that the use of sedimentation rates is not always a reliable criterion for age determination, and that the Tertiary-Mesozoic interface suggested by Stride et al. [ 1] has yet to be verified. Other geophysical results may also provide limitations on the nature of crustal movements necessary to the formation of Porcupine Seabight. If the Seabight has been formed by the rotation of Porcupine Bank relative to the European continent then fracture zones and perhaps trenches should be observed. It is therefore worth noting that neither the gravity profiles by Gray and Stacey [3] across the western margin of Porcupine Bank, nor a gravityseismic reflection traverse across the northern margin of Porcupine Bank (fig. 5), obtained on Discovery cruise 29 in October 1969, show any evidence of a trench feature where crust would have been consumed. The calculated Free-Air anomaly across the northern margin of the Bank, based on Airy's hypothesis of isostasy for a 2 dimensional model, provides a good fit with the observed Free-Air anomaly. The small negative residual at the foot of the continental slope can be explained by the sediments observed on the reflection profile in fig. 5. Similarly, no fracture zones have been identified adjacent to Porcupine Bank. A fracture zone might have been expected, for example, to the west of the mouth of the Seabight if Porcupine Bank had moved away from Europe. However, the magnetic lineations are clearly continuous in this region and can be traced northwards to latitude 51°N, the southernmost magnetic profile of the survey of Gray and Stacey [3]. The lack of evidence for fracture zones or trenches suggests Porcupine Bank may not have rotated. Finally, Sheridan and Drake [14] have observed a sedimentary basin on the Newfoundland continental shelf which, from the Atlantic fit of Bullard et al. [5], may have been the pre-continental drift continuation of the Porcupine Seabight basin. The Newfoundland basin is about 5 km deep and is thought to contain Carboniferous to Recent sedimentary rocks.
R.A.Scrutton et al., Mode or formation of Porcupine Seabight
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146
R.A.Scrutton et aL, Mode or formation o f Porcupine Seabight
5. Conclusions Porcupine Seabight contains sediments which may may be of the order of 5 km thickness. The formation of the Seabight and the sedimentary basin may have been by rotation or translation of Porcupine Bank away from the continental shelf west of Ireland. However, there are now several lines of geophysical evidence to suggest that a major lateral movement of Porcupine Bank may not have taken place. From the magnetic data the initial opening of the North Atlantic at 50°N was approximately 80 mybp. It is probable that this is the earliest date for the rotation of Porcupine Bank proposed by Stride et al. ttowever, the rotation of Porcupine Bank as a mechanism for the formation of Porcupine Seabight appears to be incompatible with the seismic reflection and seismic refraction data. These data suggest that the sediments beneath the Seabight may have been accumulating since before 80 mybp. Furthermore the rotation of Porcupine Bank may be doubted from the lack of evidence for fracture zones or trenches around the Bank. Accepting that Porcupine Bank is unlikely to have rotated, the alternative mode of formation of Porcupine Seabight is by vertical movements in the Earth's crust. Such movements would be induced by density changes at the crust-mantle boundary as summarised by Sheridan [9], and would result in subsidence at the top of the crust.
Acknowledgements We thank Dr. D.H.Matthews for his encouragement, help and discussion. We are indebted to Dr. D.P. McKenzie and his assistants for their help in reducing the magnetic data, and to Miss J.M.Lort for her help in reducing the seismic refraction data. Thanks also to all those who helped us collect the data, in particular the Master and Officers of R.R.S. John Murray. This work was supported by the Natural Environment
Research Council, London under grant no. 1294 to the University of Cambridge.
References [ 1] A.H.Stride, J.R.Oarray, D.G.Moore and R.H.Belderson, Marine geology of the Atlantic continental margin of Europe, Phil. Trans. Roy. Soc. (London) A264 (1969) 31 [2] R.J.Bailey, R.tI.Clarke and D.Taylor-Smith, The continental margin west of Ireland, S.C.O.R. Cambridge Symposium "Geology of the East Atlantic Continental Margin", Report Inst. Geol. Sci., in press. [3] F.Gray and A.P.Stacey, Gravity and magnetic interpretation of Porcupine Bank and Porcupine Bight, Deep Sea Res. 17 (1970) 467. [4 ] T.D.AIIan, Magnetic measurements at sea, Ph.D. Thesis, Cambridge University (1960). [5] E.C.Bullar, J.E.Everett and A.G.Smith, The fit of the continents around the Atlantic, Phil. Trans. Roy. Soc. (London) A258 (1965) 41. [6] X.Le Pichon, A.Cressard, J.Mascle, G.Pautot and B. Sichler, Structures sous-marines des bassins Mdimentaires de Porcupine et de Rockall, Rdsultats scientifiques de la campagne de N.O. Jean Charcot en Atlantique Nord, aofit-septembre-octobre 1969, publication no. 2 (1970) 8 pp. [7] G.A. Day and C.A. Williams,Gravity compilation in the N.E.Atlantic and interpretation of gravity in the Celtic Sea, Earth Planet. Sci. Letters 8 (1970) 205. [8] E.T.Bunce, S.Crampin, J.B.Itersey and M.N.HilI, Seismic refraction observations on the continental boundary west of Britain, J. Geophys. Res. 69, 18 (1964) 3853. [9] R.E.Sheridan, Subsidence of continental margins, Tectonophysics 7 (1969) 219. [10] J.Ewing and M.Ewing, Seismic refraction measurements in the Atlantic Ocean basins, in the Mediterranean Sea, on the Mid-Atlantic Ridge, and in the Norwegian Sea, Geol. Soc. Am. Bull. 70 (1959) 291. I 11 ] D.P.McKenzie and J.G.Sclater, The Evolution of the Indian Ocean, in preparation. [ 12] H.Kuenen, Marine Geology (Wiley, New York, 1950) 551 pp. [ 13] J.H.l:.Umbgrove, Periodal events in the North Sea Basin, Geol. Mag. 82 (1945) 237. [ 14] R.E.Sheridan and C.L.Drake, Seaward extension of the Canadian Appalachians, Can. J. Earth Sci. 5 (1968) 337.