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Fuerteventura palaeomagnetism and the evolution of the continental margin off Morocco — Comments
236
Physics of the Earth and Planetary Interiors, 23(1980) 236—237 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
FUERTEVENTURA PALAEOMAGNETISM AND THE EVOLUTION OF THE CONTINENTAL MARGIN OFF MOROCCO COMMENTS -
CHRISTOPHER R. LLOYD 1 Jessops Close, Oxford 0X3 ONU (Great Britain)
(Received and accepted March 6, 1980)
Lloyd, C.R., 1980. Fuerteventura palaeomagnetism and the evolution ofthe continental margin off Morocco — comments. Phys. Earth Planet. Inter., 23: 236—237.
Storetvedt (1980) has recently discussed the sequence of tectonic and magmatic events in Fuerteventura (Canary Islands), and more generally on the northwest African continental margin, in the light of new palaeomagnetic evidence. The interpretation of this evidence is sound, but the final paragraph, in which these events are related to the broader framework of Atlantic evolution, should be criticised on two counts. 1. Timing of early South Atlantic opening Storetvedt cites biostratigraphic and palaeomagnetic evidence which implies that break-up between Africa and South America and initial opening of the South Atlantic took place during the late Cretaceous. There is, however, considerable evidence indicating that these events took place during the early Cretaceous. Biostratigraphic evidence The study of Reyment and Tait (1972) is referred to by Storetvedt. This concluded, on the basis of biostratigraphic evidence, that only a narrow, intermittent sea passage separated South America from Africa during the late Albian. However, there has been some controversy on this matter in more recent years. For example, Kennedy and Cooper (1975) argued that similarities between ammonite faunas of Texas, Mexico and Angola indicate a free South—Central Atlantic connection by the latest Albian. Also,
Förster (1978) concluded that ammonite faunas in southeastern Nigeriaindicate that an open seaway existed between the Central and South Atlantic by the late Albian. Palaeomagneticevidence Storetvedt cites the palaeomagnetic data of Gidskehaug et al. (1975) which indicates that the South Atlantic had not opened appreciably by 112 My ago. These results are based on palaeomagnetic pole determinations on the early Cretaceous Serra Geral and Kaoko basalts of the South American and African margins, respectively. They do show that total rotation about the South Atlantic early opening pole (in extreme West Africa Larson and Ladd, 1973) could only have been slight by Aptian times. However, with the opening pole in that position, while separation was slight in the north, it was significant further south by the Aptian, as shown by sedimentary evidence (Burk and Dewey, 1974; Pastouret and Goslin, 1974) and ocean-floor magnetic anomalies (Larson and Ladd, 1973). Therefore, contrary to the impression given by Storetvedt, South Atlantic opening was well-established by the mid-Cretaceous. —
2. Central and South Atlantic opening histories The late Cretaceous igneous basement complex of Fuerteventura is related by Storetvedt to the initiation of ocean-floor spreading at that latitude in the Atlantic. This suggestion is apparently based on the
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misconceived timing of events in the South Atlantic discussed above, and a lack of appreciation that the Central and South Atlantic Oceans had quite separate evolutionary histories, Separation ofWest Africa (Morocco to Senegal— Guinea) from North America probably began around 200 My ago (Heirtzler, 1973) with active ocean-floor spreading, of the Central Atlantic only, becoming established south of the Gibbs fracture zone by about 180 My ago (Pitman and Talwani, 1972). By the midCretaceous, the Central Atlantic had opened considerably (see, for example, the 100 My map of Smith and Briden, 1977), and consequently the axis of ridge-associated magmatic activity was at some distance from the continental margin by that time. It is therefore unlikely that the late Cretaceous magmatic activity in Fuerteventura was associated with continental separation and ocean-floor spreading in that part of the Atlantic, unless the dating of the basement complex is incorrect. It is more likely that Triassic basalts in western Morocco (e.g., Dewey et al., 1973) and the 200 My old Palisades sill of eastem North America (Heirtzler, 1973) are related to break-up between North America and Africa.
with motion on the probably complex plate boundary between Eurasia and Africa (see, for example, Dewey et a!., 1973), which in this region was probably associated with the South Atlas fault.
Conclusions
Geol. Soc. London, 131: 283—288. Larson, R.L. and Ladd, J.W., 1973. Evidence for the opening of the south Atlantic in the early Cretaceous. Nature (London), 246: 209—212. Pastouret, L. and Goslin, 1., 1974. Middle Cretaceous sediments from the eastern part of the Walvis Ridge. Nature (London), 248: 495 —496.
(a) Magmatic activity in Fuerteventura was too late even to be related to the early Cretaceous initial opening of the South Atlantic Ocean. Basaltic igneous activity on the South Atlantic margins, associated with continental break-up and separation, occurred much earlier than activity in Fuerteventura. (b) Early Cretaceous separation of South America and Africa, and the subsequent opening of the South Atlantic Ocean, almost certainly had no connection with events on the Central Atlantic continental margin off Morocco, where separation from North America took place in the late Triassic. The alternative suggestion is made here that late Cretaceous magmatic and tectonic activity in Fuerteventura was associated
References Burk, K. and Dewey, J.F., 1974. Two plates in Africa during the Cretaceous? Nature (London), 249: 313—316. Dewey, J.F., Pitman, W.C., III, Ryan, W.B.F. and Bonnin, J., 1973. Plate tectonics and the evolution of the Alpine systern. Geol. Soc. Am. Bull., 84: 3137—3180. Forster, R., 1978. Evidence for an open seaway between northern and southern proto-Atlantic in Albian times. Nature (London), 272: 158—159. Gidskehaug, A., Creer, K.M. and Mitchell, J.G., 1975. Palaeomagnetism and K—Ar ages of the South-West African basalts and their beaiing on the time of initial rifting of the south Atlantic Ocean. Geophys. J.R. Astron. Soc., 42: 1—20. Heirtzler, J.R., 1973. The evolution of the north Atlantic Ocean. In: D.H. Tailing and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences, Vol.1. Academic Press, London, pp. 191—196. Kennedy, W.J. and Cooper, M., 1975. Cretaceous ammonite distributions and the opening of the south Atlantic. J.
Pitman, and Talwani, Sea-floor in theW.C., northIIIAtlantic. Geol. M., Soc.1972. Am. Bull., 83: spreading 619— 646. Reyment, R.A. and Tait, E.A., 1972. Biostratigraphical dating of the early history of the south Atlantic Ocean. Phios. Trans. R. Soc. London, Set. B, 264: 55—94. Smith, A.G. and Briden, J.C., 1977. Mesozoic and Cenozoic paleocontinental maps. Cambridge University Press, Cambridge, London and New York, 63 pp. Storetvedt, KM., 1980. Fuerteventura palaeomagnetism and the evolution of the continental margin off Morocco. Phys. Earth Planet. Inter., 21: P1—P6.
Physics of the Earth and Planetary Interiors, 23(1980) 236—237 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
FUERTEVENTURA PALAEOMAGNETISM AND THE EVOLUTION OF THE CONTINENTAL MARGIN OFF MOROCCO COMMENTS -
CHRISTOPHER R. LLOYD 1 Jessops Close, Oxford 0X3 ONU (Great Britain)
(Received and accepted March 6, 1980)
Lloyd, C.R., 1980. Fuerteventura palaeomagnetism and the evolution ofthe continental margin off Morocco — comments. Phys. Earth Planet. Inter., 23: 236—237.
Storetvedt (1980) has recently discussed the sequence of tectonic and magmatic events in Fuerteventura (Canary Islands), and more generally on the northwest African continental margin, in the light of new palaeomagnetic evidence. The interpretation of this evidence is sound, but the final paragraph, in which these events are related to the broader framework of Atlantic evolution, should be criticised on two counts. 1. Timing of early South Atlantic opening Storetvedt cites biostratigraphic and palaeomagnetic evidence which implies that break-up between Africa and South America and initial opening of the South Atlantic took place during the late Cretaceous. There is, however, considerable evidence indicating that these events took place during the early Cretaceous. Biostratigraphic evidence The study of Reyment and Tait (1972) is referred to by Storetvedt. This concluded, on the basis of biostratigraphic evidence, that only a narrow, intermittent sea passage separated South America from Africa during the late Albian. However, there has been some controversy on this matter in more recent years. For example, Kennedy and Cooper (1975) argued that similarities between ammonite faunas of Texas, Mexico and Angola indicate a free South—Central Atlantic connection by the latest Albian. Also,
Förster (1978) concluded that ammonite faunas in southeastern Nigeriaindicate that an open seaway existed between the Central and South Atlantic by the late Albian. Palaeomagneticevidence Storetvedt cites the palaeomagnetic data of Gidskehaug et al. (1975) which indicates that the South Atlantic had not opened appreciably by 112 My ago. These results are based on palaeomagnetic pole determinations on the early Cretaceous Serra Geral and Kaoko basalts of the South American and African margins, respectively. They do show that total rotation about the South Atlantic early opening pole (in extreme West Africa Larson and Ladd, 1973) could only have been slight by Aptian times. However, with the opening pole in that position, while separation was slight in the north, it was significant further south by the Aptian, as shown by sedimentary evidence (Burk and Dewey, 1974; Pastouret and Goslin, 1974) and ocean-floor magnetic anomalies (Larson and Ladd, 1973). Therefore, contrary to the impression given by Storetvedt, South Atlantic opening was well-established by the mid-Cretaceous. —
2. Central and South Atlantic opening histories The late Cretaceous igneous basement complex of Fuerteventura is related by Storetvedt to the initiation of ocean-floor spreading at that latitude in the Atlantic. This suggestion is apparently based on the
003 1-9201/80/0000—0000/$02.25 © 1980 Elsevier Scientific Publishing Company
237
misconceived timing of events in the South Atlantic discussed above, and a lack of appreciation that the Central and South Atlantic Oceans had quite separate evolutionary histories, Separation ofWest Africa (Morocco to Senegal— Guinea) from North America probably began around 200 My ago (Heirtzler, 1973) with active ocean-floor spreading, of the Central Atlantic only, becoming established south of the Gibbs fracture zone by about 180 My ago (Pitman and Talwani, 1972). By the midCretaceous, the Central Atlantic had opened considerably (see, for example, the 100 My map of Smith and Briden, 1977), and consequently the axis of ridge-associated magmatic activity was at some distance from the continental margin by that time. It is therefore unlikely that the late Cretaceous magmatic activity in Fuerteventura was associated with continental separation and ocean-floor spreading in that part of the Atlantic, unless the dating of the basement complex is incorrect. It is more likely that Triassic basalts in western Morocco (e.g., Dewey et al., 1973) and the 200 My old Palisades sill of eastem North America (Heirtzler, 1973) are related to break-up between North America and Africa.
with motion on the probably complex plate boundary between Eurasia and Africa (see, for example, Dewey et a!., 1973), which in this region was probably associated with the South Atlas fault.
Conclusions
Geol. Soc. London, 131: 283—288. Larson, R.L. and Ladd, J.W., 1973. Evidence for the opening of the south Atlantic in the early Cretaceous. Nature (London), 246: 209—212. Pastouret, L. and Goslin, 1., 1974. Middle Cretaceous sediments from the eastern part of the Walvis Ridge. Nature (London), 248: 495 —496.
(a) Magmatic activity in Fuerteventura was too late even to be related to the early Cretaceous initial opening of the South Atlantic Ocean. Basaltic igneous activity on the South Atlantic margins, associated with continental break-up and separation, occurred much earlier than activity in Fuerteventura. (b) Early Cretaceous separation of South America and Africa, and the subsequent opening of the South Atlantic Ocean, almost certainly had no connection with events on the Central Atlantic continental margin off Morocco, where separation from North America took place in the late Triassic. The alternative suggestion is made here that late Cretaceous magmatic and tectonic activity in Fuerteventura was associated
References Burk, K. and Dewey, J.F., 1974. Two plates in Africa during the Cretaceous? Nature (London), 249: 313—316. Dewey, J.F., Pitman, W.C., III, Ryan, W.B.F. and Bonnin, J., 1973. Plate tectonics and the evolution of the Alpine systern. Geol. Soc. Am. Bull., 84: 3137—3180. Forster, R., 1978. Evidence for an open seaway between northern and southern proto-Atlantic in Albian times. Nature (London), 272: 158—159. Gidskehaug, A., Creer, K.M. and Mitchell, J.G., 1975. Palaeomagnetism and K—Ar ages of the South-West African basalts and their beaiing on the time of initial rifting of the south Atlantic Ocean. Geophys. J.R. Astron. Soc., 42: 1—20. Heirtzler, J.R., 1973. The evolution of the north Atlantic Ocean. In: D.H. Tailing and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences, Vol.1. Academic Press, London, pp. 191—196. Kennedy, W.J. and Cooper, M., 1975. Cretaceous ammonite distributions and the opening of the south Atlantic. J.
Pitman, and Talwani, Sea-floor in theW.C., northIIIAtlantic. Geol. M., Soc.1972. Am. Bull., 83: spreading 619— 646. Reyment, R.A. and Tait, E.A., 1972. Biostratigraphical dating of the early history of the south Atlantic Ocean. Phios. Trans. R. Soc. London, Set. B, 264: 55—94. Smith, A.G. and Briden, J.C., 1977. Mesozoic and Cenozoic paleocontinental maps. Cambridge University Press, Cambridge, London and New York, 63 pp. Storetvedt, KM., 1980. Fuerteventura palaeomagnetism and the evolution of the continental margin off Morocco. Phys. Earth Planet. Inter., 21: P1—P6.