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Opening of the Gulf of Aden and Afar by progressive tearing
Physics of the Earth and Planetary Interiors, 21(1980)343—350 © Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
343
OPENING OF THE GULF OF ADEN AND AFAR BY PROGRESSIVE TEARING VINCENT E. COURTILLOT Institut de Physique du Globe de Paris, Université P. et M. Curie, Paris (France) and Département des Sciences de la Terre, Université Paris VII, Place Jussieu, 75005 Paris (France)
(Received October, 1978; revised and accepted May, 1979)
Courtillot, V.E., 1980. Opening of the Gulf of Aden and Afar by progressive tearing. Phys. Earth Planet. Inter., 21: 343—350. A new detailed aeromagnetic survey of the Republic of Djibouti and surrounding area reveals a wealth of new information which can be correlated with other data, in particular geologic and tectonic maps. Oceanic magnetic anomalies are identified from the Gulf of Aden westward to the Ghoubbet—Asal rift in Afar. Identification of the anomalies, together with a reinterpretation of earlier magnetic profiles, indicates that rifting started earlier in the east and provides clear evidence for the westward propagation of a crack through the lithosphere at an approximate 1. The crack tip is now thought to lie somewhere close to lake Asal and should continue its movelocity of 3to cmthe y tion further northwest. Some first consequences of this non-rigid model of plate opening are discussed and independent support from a number of sources is obtained.
1. Introduction The model presented in this paper is based on data from a new high-precision aeromagnetic survey of the Republic of Djibouti and the western termination of the Gulf of Aden (Courtillot and Le Mouël, 1978), and from recently published results of a sea survey in the western Gulf of Aden by Girdler and Styles (1978). The technical aspects of the aeromagnetic survey are discussed in detail by Courtillot and Le Mou~l (1978). The survey was flown in two panels, one over land at an altitude of 2000 m, the other one over the sea at an altitude of 600 m. A total of 54 north— south oriented lines were flown with a 5 km spacing. A detailed discussion of the precision of localisation, magnetic measurements and data reduction leads one to the conclusion that the precision of the magnetic anomaly maps is better than 10 nT, except in areas of very intense gradients where it can be slightly larger.
2. The age of oceanic anomalies and the idea of crack propagation The sea part of the survey is shown in Fig. I a. It is clear that east—west trending oceanic magnetic lineations can be recognised which are almost parallel to each other throughout the western termination of the Gulf of Aden. In the three most eastern profiles of the survey near 44°E(Fig. ib), anomalies are easily recognised on both sides up to anomaly 3 and slightly beyond (about 5 My) and correspond to an average spreading rate of 0.65 cm y~1along the direction of the north—south profile (or 0.8 cm y1 along the NE transform). Anomaly 4 is not recognised and the magnetic style changes to a much more subdued one. In the Gulf of Tadjourah, anomalies can be recognised only up to anomaly 2’ (2.5 My) and in the Ghoubbet—Asal rift (Fig. 2) only the axial anomaly 1 is found (<1 My). Although only a small area is
344
covered by the survey, it can already be suggested that the opening of the western end of the Gulf of Aden has been progressive, due to the westward propagation of a crack in the Arabian and Somalian plates. The change in style of magnetic anomalies (for simplicity we will say that oceanic anomalies represent magnetic style A and will call the more subdued style B) would thus correspond to the passage of the crack tip and to the inception of spreading from an oceanic ridge in steady state,
430
Two profiles recently published by Girdler and Styles (1978) support this model. An earlier analysis of magnetic profiles in the Red Sea had led Girdler and Styles (1974) to believe that two distinct phases of spreading could be recognised there (41—34 My and 5—0 My in a first interpretation; 30—15 My and 5—0 My in a more recent one), separated by a long quiet interval. From a discussion of new profiles, obtained this time in the western Gulf of Aden, these authors believe that the two distinct phases of
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spreading can also be recognised there. It is our opinion that the profiles are at least as well accounted for by a more or less continuous spreading as originally proposed by Laughton et al. (1970), although with some modifications (Fig. ib). Our interpretation of the Girdler and Styles (1978) profiles is given in more detail in Courtillot et al. (in preparation); it supports the idea of time-transgressive rifting of the Gulf of Aden and is summarized in Fig. 3. It can be seen that the oceanic anomalies terminate fairly abruptly as one goes west against the
boundaries of the zone of B style anomalies. The crack split the Gulf open to initiate a stable plate boundary 12 My ago at the level of profile B (about 45.7°E),10 My ago at the level of profile F (about 45°E)~ 5 My ago at the level of our easternmost profile (about 44°E,Figs. lb and 3), less than 2.5 My ago in the Gulf of Tadjourah and less than 1 My ago in the Ghoubbet—Asal rift. That is where the crack tip would now lie. Indeed, Stieltjes et al. (1976) find on a geochemical basis that, despite many tectonic and petrographic features similar to those of oceanic
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lake Asal and to the south of lake Asal, over the Manda Inakir(Figs. volcanic chain, magnetic anomalies are of this style 2—4). (b) lower intensity, longerwavelength anomalies which are found over the Danakil “horst”, Mak’arrasou area and close to the Somalian border (quiet style B). Style A anomalies are offset by faults in a number of instances. Style B anomalies in the Mak’arrasou area are in agreement with the suggestion that this zone should be considered as a young transform fault in the making (Tapponnier and Varet, 1974; Courtillot et al., 1974).
45°
Fig. 3. Schematic interpretation of the western termination of the Gulf of Aden. The locations of the aeromagnetic survey (A) and of the Girdler and Styles’ (1978) profiles (B and F in Girdler and Styles, 1978) are shown. The zone of subdued (quiet) magnetic anomalies is outlined in gray. Oceanic anomalies (heavy lines) are seen to terminate against the quiet zone. This figure also incorporates data from Laughton et al. (1970).
ridges, the Asal rift has not yet reached steady state, The average rate of motion of the crack tip westwards is slightly less than 3.0 cm y”. A number of new radiometric dates from tholeitelike basalts on both sides of the Gulf of Tadjourah were recently obtained by Olivier Richard (pers. comm.). The ages become progressively younger as one goes west; 3.5 My at the longitude of Djibouti, ‘~2My at the longitude of Arta and indeed <1 My in the Ghoubbet—Asal rift. The total of 12 ages yields a regular westward velocity of 2.5 cm y’ for the propagation of the crack tip, in good agreement with our first estimate,
3. Magnetic anomalies on land Detailed analysis of the land portion of the survey is to be published elsewhere (Courtillot et al., in preparation). Only the relevant results of this study are summarized here (Figs. 2 and 3). Again, two distinct magnetic styles are clearly recognised: (a) lineated, high-intensity, short-wavelength anomalies which are typical of oceanic crust formed at spreading centers (style A). Over the Gulf of Tadjourah, Ghoubbet,
4. The crack model in the context of rigid plate tectonics
Attempts have been made to account for geological and geophysical data obtained in the Afar area in terms of rigid plate tectonics. Barberi and Varet (1977) try to identify the present plate boundaries within Afar. However, they mention the fact that deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates themselves. The more quantitative analysis of Le Pichon and Francheteau (1978) also reveals some misgivings about the rigid plate assumption. The crack model implies intense and diffuse deformation of the crust (and lithosphere) in the vicinity of the crack tip. Although it is not easy to determine the characteristic lateral dimensions of this deformation, we see that the crust is rifting along a number of linear, discontinuous tears (the axial ranges); eight presently active segments have been recognised (Barberi and Varet, 1977). The crust is also dissected by numerous strike-slip faults (Courtillot et al., 1974; Tapponnier and Varet, 1974). The tears jump and may eventually die while new segments are activated. The two magnetic styles we observe in Afar may well correspond to the two spreading phases which have been invoked by a number of authors. Style B would correspond to the early crustal stretching and thinfling phase with intense tectonism(such as in parts of the southern Danakil or Mak’arrasou). Isolated anomalies with magnetic style A could appear as new oceanic crust is emplaced, by non-rigid tearing of the earlier thinned crust and correlative injection of basalts along zones of weakness. However, style A
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349
anomalies would only really be established with the inception of truly oceanic spreading with a welllocalised rift (plate boundary) in steady-state (after the passage of the crack tip). This model is not in contradiction with plate tectonics, it only stresses the fact that plate models are large-scale, first-order models that may not apply to more local problems, in particular when steady-state has not been reached (e.g. in the case of the initiation or the death of a plate boundary). The general framework for describing the global evolution of Afar and the Gulf of Aden still is that of relative rigid motions of the Arabian and African plates. However, our crack model accounts for some field observations which rigid plate tectonics cannot account for. In particular it explains the earlier spreading and generation of oceanic crust in the Gulf of Aden than in the Afar (and the Red Sea), a point which has been made by Barberi and Varet (1977) and Le Pichon and Francheteau (1978). One often tends to think of the break-up of continents as the quasi-instantaneous formation of large stretches of plate boundaries, but fracture must be somehow initiated somewhere. A simple look at the geographical maps shows that the separation between Arabia and Africa is hindered in the Afar—southern Red Sea area. In all plate tectonic models, the Danakil “horst” is thought to have behaved as an individual microplate in the last 4 My, yet, it is still attached to the African plate near the Gulf of Zula and to the Arabian plate near the straights of Bab-el-Mandeb. Thus, the two major continental blocks have not yet ruptured and the far away distance of their relative rotation pole implies intense deformation of the lithosphere in Afar and in the southern Red Sea. It may be suggested that the crack tip, which presently lies near the western end of the Asal depression, will continue its motion northwestwards, and will possibly pass through some of the presently active axial ranges. The crack tip may eventually reach the Red Sea trough, and/or it may meet a corresponding crack tip heading south from there (such a crack might be responsible for the opening of the southern Red Sea). Clearly, the data pertaining to the Red Sea need to be reanalysed in this context; crack propagation associated with a very slow rate of spreading may be partly responsible for past difficulties in analysing Red Sea features (e.g., Girdler and Styles, 1974).
Although one must partly abandon the powerful quantitative tools provided by plate tectonics when working in areas such as the Afar (or the Himalayas and the Mediterranean), one is not left without quantitative or semi-quantitative models. One can for example think of applying slip-line field theory in order to account for some observed tectonic features (Molnar and Tapponnier, 1975; Tapponnier, 1977; Burke et al., 1978). This has been quite successfully applied by Molnar and Tapponnier (1975), in the case of converging continent-bearing plates. There is an interesting duality between their problem (death of a subduction zone) and ours (birth of an accreting plate boundary). Such semi-quantitative models are only suggested here and of course need to be developed further. They clearly point to a fruitful line of research.
Acknowledgements The aeromagnetic survey of the Republic of E~iboutiis dedicated to the memory of the late Professor E. Le Borgne, founder of the French aeromagnetic survey service. The authorities of Ethiopia and of the then French Territory of the Afar and Issa, and the French National Navy are to be thanked for making the survey practically feasible. I thank A. Galdeano and M. Armanet and the technical division of Institut National d’Astronomie et de Géophysique for considerable help with the data processing part of this work. Discussions with C.J. Allêgre, JL. Le Mouël, J.P. Poirier, 0. Richard and P. Tapponnier are gratefully acknowledged. Comments from F. Albarède, K. Burke, J. Ducruix and J.C. Ruegg were very helpful.
References Barberi F. and Varet J.~1977. Volcanism of Afar: small-scale plate tectonics implications. Geol. Soc. Am. Bull., 88: 12511266 Burke K., Fox P.J. and Sengör A.M.C., 1978. Buoyant ocean floor and the evolution of the Carribean. J. Geophys. Res., 83: 3949—3954. Courtillot V.~Tapponnier P. and Varet J., 1974. Surface features associated with transform faults: a comparison between observed examples and an experimental model. Tectonophysics, 24: 3 17—329.
350 Courtillot V. and Le Mouël J.L., 1978. Le Levé aéromagnétique de Ia République de Djibouti, IPGP Observations Magnétiques, Paris, 39 pp. Courtillot V., Galdeano A. and Le Mouël J.L. (in preparation). Propagation of an accreting plate boundary: a discussion of new aeromagnetic data in the Gulf of Tadjourah and southern Afar. Submitted to Earth Planet, Sci. Lett. Girdler R.W. and Styles P., 1974. Two stages Red Sea floor spreading. Nature (London), 247: 7—li. Girdler R.W. and Styles P., 1978. Seafloor spreading in the western Gulf of Aden. Nature (London), 271: 615—617. LaBrecque J.L., Kent D.V. and Cande S.C., 1977. Revised magnetic polarity time scale for late Cretaceous and Cenozoic time. Geology, 5: 330—335. Laughton A.S., Whitmarsh, R.B. and Jones M.T., 1970. The evolution of the Gulf of Aden. Philos. Trans. R. Soc. London, Ser. A, 267: 227—266.
Le Pichon X. and Francheteau 1., 1978. A plate-tectonic analysis of the Red Sea—Gulf of Aden area. Tectonophysics, 46: 369—406. Molnar P. and Tapponnier P., 1975. Cenozoic tectonics of Asia: effects of a continental collision. Science, 189: 419—426. Stieltjes L., Joron J.L., Treuil M. and Varet J., 1976. Le rift d’Asal, segment de dorsale émergé: discussion pétrologique et géochimique. Bull. Soc. Géol. Fr., 18: 851—862. Tapponnier P., 1977. Evolution tectonique du système alpin en Méditerranée: poinçonnement et écrasement rigideplastique. Bull. Soc. Géol. Fr., 19: 437—460. Tapponnier P. and Varet J., 1974. La zone de Mak’arrasou en Afar: un equivalent émergé des failles transformantes océaniques. CR. Acad. Sci. Sér. D, 274: 209—212.
343
OPENING OF THE GULF OF ADEN AND AFAR BY PROGRESSIVE TEARING VINCENT E. COURTILLOT Institut de Physique du Globe de Paris, Université P. et M. Curie, Paris (France) and Département des Sciences de la Terre, Université Paris VII, Place Jussieu, 75005 Paris (France)
(Received October, 1978; revised and accepted May, 1979)
Courtillot, V.E., 1980. Opening of the Gulf of Aden and Afar by progressive tearing. Phys. Earth Planet. Inter., 21: 343—350. A new detailed aeromagnetic survey of the Republic of Djibouti and surrounding area reveals a wealth of new information which can be correlated with other data, in particular geologic and tectonic maps. Oceanic magnetic anomalies are identified from the Gulf of Aden westward to the Ghoubbet—Asal rift in Afar. Identification of the anomalies, together with a reinterpretation of earlier magnetic profiles, indicates that rifting started earlier in the east and provides clear evidence for the westward propagation of a crack through the lithosphere at an approximate 1. The crack tip is now thought to lie somewhere close to lake Asal and should continue its movelocity of 3to cmthe y tion further northwest. Some first consequences of this non-rigid model of plate opening are discussed and independent support from a number of sources is obtained.
1. Introduction The model presented in this paper is based on data from a new high-precision aeromagnetic survey of the Republic of Djibouti and the western termination of the Gulf of Aden (Courtillot and Le Mouël, 1978), and from recently published results of a sea survey in the western Gulf of Aden by Girdler and Styles (1978). The technical aspects of the aeromagnetic survey are discussed in detail by Courtillot and Le Mou~l (1978). The survey was flown in two panels, one over land at an altitude of 2000 m, the other one over the sea at an altitude of 600 m. A total of 54 north— south oriented lines were flown with a 5 km spacing. A detailed discussion of the precision of localisation, magnetic measurements and data reduction leads one to the conclusion that the precision of the magnetic anomaly maps is better than 10 nT, except in areas of very intense gradients where it can be slightly larger.
2. The age of oceanic anomalies and the idea of crack propagation The sea part of the survey is shown in Fig. I a. It is clear that east—west trending oceanic magnetic lineations can be recognised which are almost parallel to each other throughout the western termination of the Gulf of Aden. In the three most eastern profiles of the survey near 44°E(Fig. ib), anomalies are easily recognised on both sides up to anomaly 3 and slightly beyond (about 5 My) and correspond to an average spreading rate of 0.65 cm y~1along the direction of the north—south profile (or 0.8 cm y1 along the NE transform). Anomaly 4 is not recognised and the magnetic style changes to a much more subdued one. In the Gulf of Tadjourah, anomalies can be recognised only up to anomaly 2’ (2.5 My) and in the Ghoubbet—Asal rift (Fig. 2) only the axial anomaly 1 is found (<1 My). Although only a small area is
344
covered by the survey, it can already be suggested that the opening of the western end of the Gulf of Aden has been progressive, due to the westward propagation of a crack in the Arabian and Somalian plates. The change in style of magnetic anomalies (for simplicity we will say that oceanic anomalies represent magnetic style A and will call the more subdued style B) would thus correspond to the passage of the crack tip and to the inception of spreading from an oceanic ridge in steady state,
430
Two profiles recently published by Girdler and Styles (1978) support this model. An earlier analysis of magnetic profiles in the Red Sea had led Girdler and Styles (1974) to believe that two distinct phases of spreading could be recognised there (41—34 My and 5—0 My in a first interpretation; 30—15 My and 5—0 My in a more recent one), separated by a long quiet interval. From a discussion of new profiles, obtained this time in the western Gulf of Aden, these authors believe that the two distinct phases of
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(b) Fig. 1(b). The three easternmost profiles of the sea survey (profile 50 is located along the meridian 44°E)together with a synthetic profile (using the reversal timescale of LaBrecque et al., 1977). As usual, magnetization in the central block is taken to be larger than in adjacent blocks. Beyond anomaly 3 magnetization is assumed to be negligible (B style).
spreading can also be recognised there. It is our opinion that the profiles are at least as well accounted for by a more or less continuous spreading as originally proposed by Laughton et al. (1970), although with some modifications (Fig. ib). Our interpretation of the Girdler and Styles (1978) profiles is given in more detail in Courtillot et al. (in preparation); it supports the idea of time-transgressive rifting of the Gulf of Aden and is summarized in Fig. 3. It can be seen that the oceanic anomalies terminate fairly abruptly as one goes west against the
boundaries of the zone of B style anomalies. The crack split the Gulf open to initiate a stable plate boundary 12 My ago at the level of profile B (about 45.7°E),10 My ago at the level of profile F (about 45°E)~ 5 My ago at the level of our easternmost profile (about 44°E,Figs. lb and 3), less than 2.5 My ago in the Gulf of Tadjourah and less than 1 My ago in the Ghoubbet—Asal rift. That is where the crack tip would now lie. Indeed, Stieltjes et al. (1976) find on a geochemical basis that, despite many tectonic and petrographic features similar to those of oceanic
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lake Asal and to the south of lake Asal, over the Manda Inakir(Figs. volcanic chain, magnetic anomalies are of this style 2—4). (b) lower intensity, longerwavelength anomalies which are found over the Danakil “horst”, Mak’arrasou area and close to the Somalian border (quiet style B). Style A anomalies are offset by faults in a number of instances. Style B anomalies in the Mak’arrasou area are in agreement with the suggestion that this zone should be considered as a young transform fault in the making (Tapponnier and Varet, 1974; Courtillot et al., 1974).
45°
Fig. 3. Schematic interpretation of the western termination of the Gulf of Aden. The locations of the aeromagnetic survey (A) and of the Girdler and Styles’ (1978) profiles (B and F in Girdler and Styles, 1978) are shown. The zone of subdued (quiet) magnetic anomalies is outlined in gray. Oceanic anomalies (heavy lines) are seen to terminate against the quiet zone. This figure also incorporates data from Laughton et al. (1970).
ridges, the Asal rift has not yet reached steady state, The average rate of motion of the crack tip westwards is slightly less than 3.0 cm y”. A number of new radiometric dates from tholeitelike basalts on both sides of the Gulf of Tadjourah were recently obtained by Olivier Richard (pers. comm.). The ages become progressively younger as one goes west; 3.5 My at the longitude of Djibouti, ‘~2My at the longitude of Arta and indeed <1 My in the Ghoubbet—Asal rift. The total of 12 ages yields a regular westward velocity of 2.5 cm y’ for the propagation of the crack tip, in good agreement with our first estimate,
3. Magnetic anomalies on land Detailed analysis of the land portion of the survey is to be published elsewhere (Courtillot et al., in preparation). Only the relevant results of this study are summarized here (Figs. 2 and 3). Again, two distinct magnetic styles are clearly recognised: (a) lineated, high-intensity, short-wavelength anomalies which are typical of oceanic crust formed at spreading centers (style A). Over the Gulf of Tadjourah, Ghoubbet,
4. The crack model in the context of rigid plate tectonics
Attempts have been made to account for geological and geophysical data obtained in the Afar area in terms of rigid plate tectonics. Barberi and Varet (1977) try to identify the present plate boundaries within Afar. However, they mention the fact that deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates themselves. The more quantitative analysis of Le Pichon and Francheteau (1978) also reveals some misgivings about the rigid plate assumption. The crack model implies intense and diffuse deformation of the crust (and lithosphere) in the vicinity of the crack tip. Although it is not easy to determine the characteristic lateral dimensions of this deformation, we see that the crust is rifting along a number of linear, discontinuous tears (the axial ranges); eight presently active segments have been recognised (Barberi and Varet, 1977). The crust is also dissected by numerous strike-slip faults (Courtillot et al., 1974; Tapponnier and Varet, 1974). The tears jump and may eventually die while new segments are activated. The two magnetic styles we observe in Afar may well correspond to the two spreading phases which have been invoked by a number of authors. Style B would correspond to the early crustal stretching and thinfling phase with intense tectonism(such as in parts of the southern Danakil or Mak’arrasou). Isolated anomalies with magnetic style A could appear as new oceanic crust is emplaced, by non-rigid tearing of the earlier thinned crust and correlative injection of basalts along zones of weakness. However, style A
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Fig. 4. Simplified geological map of part of Afar redrawn from the CNR-CNRS maps and Barberi and Varet (1977). Also shown are a number of geographic names used in the text and the boundaries of the aeromagnetic survey.
349
anomalies would only really be established with the inception of truly oceanic spreading with a welllocalised rift (plate boundary) in steady-state (after the passage of the crack tip). This model is not in contradiction with plate tectonics, it only stresses the fact that plate models are large-scale, first-order models that may not apply to more local problems, in particular when steady-state has not been reached (e.g. in the case of the initiation or the death of a plate boundary). The general framework for describing the global evolution of Afar and the Gulf of Aden still is that of relative rigid motions of the Arabian and African plates. However, our crack model accounts for some field observations which rigid plate tectonics cannot account for. In particular it explains the earlier spreading and generation of oceanic crust in the Gulf of Aden than in the Afar (and the Red Sea), a point which has been made by Barberi and Varet (1977) and Le Pichon and Francheteau (1978). One often tends to think of the break-up of continents as the quasi-instantaneous formation of large stretches of plate boundaries, but fracture must be somehow initiated somewhere. A simple look at the geographical maps shows that the separation between Arabia and Africa is hindered in the Afar—southern Red Sea area. In all plate tectonic models, the Danakil “horst” is thought to have behaved as an individual microplate in the last 4 My, yet, it is still attached to the African plate near the Gulf of Zula and to the Arabian plate near the straights of Bab-el-Mandeb. Thus, the two major continental blocks have not yet ruptured and the far away distance of their relative rotation pole implies intense deformation of the lithosphere in Afar and in the southern Red Sea. It may be suggested that the crack tip, which presently lies near the western end of the Asal depression, will continue its motion northwestwards, and will possibly pass through some of the presently active axial ranges. The crack tip may eventually reach the Red Sea trough, and/or it may meet a corresponding crack tip heading south from there (such a crack might be responsible for the opening of the southern Red Sea). Clearly, the data pertaining to the Red Sea need to be reanalysed in this context; crack propagation associated with a very slow rate of spreading may be partly responsible for past difficulties in analysing Red Sea features (e.g., Girdler and Styles, 1974).
Although one must partly abandon the powerful quantitative tools provided by plate tectonics when working in areas such as the Afar (or the Himalayas and the Mediterranean), one is not left without quantitative or semi-quantitative models. One can for example think of applying slip-line field theory in order to account for some observed tectonic features (Molnar and Tapponnier, 1975; Tapponnier, 1977; Burke et al., 1978). This has been quite successfully applied by Molnar and Tapponnier (1975), in the case of converging continent-bearing plates. There is an interesting duality between their problem (death of a subduction zone) and ours (birth of an accreting plate boundary). Such semi-quantitative models are only suggested here and of course need to be developed further. They clearly point to a fruitful line of research.
Acknowledgements The aeromagnetic survey of the Republic of E~iboutiis dedicated to the memory of the late Professor E. Le Borgne, founder of the French aeromagnetic survey service. The authorities of Ethiopia and of the then French Territory of the Afar and Issa, and the French National Navy are to be thanked for making the survey practically feasible. I thank A. Galdeano and M. Armanet and the technical division of Institut National d’Astronomie et de Géophysique for considerable help with the data processing part of this work. Discussions with C.J. Allêgre, JL. Le Mouël, J.P. Poirier, 0. Richard and P. Tapponnier are gratefully acknowledged. Comments from F. Albarède, K. Burke, J. Ducruix and J.C. Ruegg were very helpful.
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