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Recent crustal movements along the Red Sea and Gulf of Aden coasts in Afar (Ethiopia and T.F.A.I.)
Tectonophysics, 29 (1975) 479-486 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
479
RECENT CRUSTAL MOVEMENTS ALONG THE RED SEA AND GULF OF ADEN COASTS IN AFAR (ETHIOPIA AND T.F.A.I.“)
HIJGWES FAURE Laboratoire de Ge’ologie du Quaternaire, Meudon-Belleuue (France)
Centre National
de la Recherche
Scientifique,
(Revised version accepted September 11,1975)
ABSTRACT Faure, II., 1975. Recent crustal movements along the Red Sea and Gulf of Aden coasts in Afar (Ethiopia and T.F.A.I.). In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 479-486. Mean velocity rates and gradients of vertical movements along the Red Sea and Gulf of Aden coasts have been calculated from deformed Pleistocene coral shorelines. Mean rates of vertical displacement have also been calculated along ancient shorelines of the Afar in Ethiopia and T.F.A.I. (French Territory of the Afars and the Issas). Systems of shorelines with ages of 200,000 and 120,000~80,000 years B.P. could be identified by uranium-thorium dating. The rates of uplift are calculated in different areas and are given in mm/year of mean vertical (positive or negative) relative movement since the deposition close to mean sea level. Results are of the same order of magnitude for the rates at different periods of time (0.1-1.0 mm/year) but are quite different from place to place. They confirm a tectonically active area in the Danakil rift and Gulf of Tadjoura and a more stabilized zone at the Bab el Mandeb Strait between the Danakil block and Arabia. These quantitative results will later be compared with the results of Ghoubbet-Asal Lake geodetic leveling.
INTRODUCTION
For more than fifty years, the Red Sea and the Gulf of Aden have been taken as a model for the initial stage in the fractumtion of a continent. Several evolution models have been suggested to reconstruct the bulge of the earth’s crust, its fracturation into grabens, followed by stretching which caused separation of the edges and the appearance of a new oceanic crust based on an axial rift (see bibliography in CNR-CNRS, 1973; Tarling and Runcorn, 1973; Pilger, 1974; Faure, 1973b etc.). The Magmatic and geochemical evolution which accompanies these phenomena has been described in de* T.F.A.I.:
Territoire Francais des Afars et des Issas.
480
tail (Varet, 1973; Treuil, 1973). An attempt was made to improve the classic model by quantifying it with the help of the results of the most recent investigations on the Afar region (Bannert et al., 1970; Mohr, 1970,1973; CNR-CNRS, 1973; Barberi et al., 1972a; Tazieff, 1971,1973; Varet, 1973; Faure, 1973a; Pilger, 1974 etc.). The quantitative model proposed here stresses the importance of vertical movements throughout the evolution of this sector in tension (Faure, 1973b). As with the oceanic ridges (Menard, 1969,1973; Sclater and Harrison, 1971), but with a different surface expression, a correlation appears to exist between vertical movements and horizontal displacement. An abnormal mantle of shallow depth (Lepine et al., 1972; Makris et al., 1972), a probably high thermal flux (Marinelli, 1971) and intense seismicity (Gouin, 1970; Fairhead and Girdler, 1970; Lepine and Ruegg, 1973) accompany the exceptional uplift of the margins in the region and the subsidence of its axial zone in the Afar. METHOD
The Quaternary marine formations outcropping along the present and ancient shorelines were investigated (CNR-CNRS, 1973; Lalou et al., 1970; Faure et al., 1973; Hoang et al., 1974). They exhibit the conditions necessary to obtain valid quantitative measurements (Faure, 1974) : (1) The possibility of locating the sea level in relation to observed marine deposits (morphology, sedimentology, ecology, etc.). (2) Altimetric and topographic measurements with respect to a known altitude reference (present sea level, horizontal surface of the Dalol salt plain). (3) Possibility of radiometric measurements (zs0Th/234 U) on corals. The marine deposits which are mainly coralline in these regions, correspond in their upper zones (reef flat) to a halt of the sea or to a sea level close to an interglacial high. The mean tidal levels, of which the tidal range is lower than 1.8 m, permit an accurate estimate of the reference paleolevel to within 0.5 m. Marine deposits occur along the present shorelines (above and below the present zero), and along the ancient shorelines of the Pleistocene Danakil Gulf, which occupied the north of the Afar rift depression. This ancient gulf extended 275 km into the hinterland, with a width ranging from 30 km to 65 km at the maximum, and a narrow gully connecting it to the present Gulf of Zula. This configuration stimulated rapid and thick evaporitic sedimentation beyond the high sea-level phases, and the bed of the depression is currently occupied by a thick formation of salts, bounded at the top by a surface located 118 m below sea level. The observation of marine outcrops can thus be effected in the open air at the periphery of the salt plain, and at the feet of the escarpments of the Ethiopian plateau and the Danakil horst. The ancient sea level in the gulf and along the shorelines intersects various geological fomrations and tectonic zones of the margins of the graben, or of the axial volcanic range of the rift. At the feet of the bordering escarpments,
481
the coralline formations rest on Miocene or Pliocene volcanic or sedimentary rocks (Barberi et al., 1972b), or on Pleistocene continental conglomerates and sands (CNR-CNRS, 1973). Their lower surface matches an ancient topography and fault escarpments which continued their morphological and tectonic evolution after the withdrawal of the sea. Along the axial volcanic chain, the corals, calcarenites and calcareous oolites rest on the basalt flows of the Tat’Ali, less than 7 km from the fault of the central rift. They are covered by more recent flows of the same volcanic block. The deposits in this region also constitute atolls constructed on hyaloclastite rings (Bonatti et al., 1971). The marine deposits were subjected to more than forty datings throughout the region in question. They belong to several lines of successive shorelines of the Middle Holocene (6000-3000 years B.P.), (Delibrias et al., in preparation) and the Upper and Middle Pleistocene (48-65,OOO years?), (73--80,000 years), (124-133,000 years) and (X30-200,000 years), (Lalou et al., 1970; Faure et al., 1973; Hoang et al., 1974). By referring to a single identifiable unit in the field, it is possible to define the maximum observable deformations to which it was subjected since its installation. In more favorable cases, it is possible to determine the deformation occurring between installation and the deposit of a more recent unit. These deformations relate to the action of faults, the tilting of plates, or epeirogenic movements of regional significance. RESULTS
In order to render the values of vertical movements obtained for periods of time of widely differing durations mutually comparable, the results are expressed in mm of mean movement per year (rate or velocity in mm/year). As recommended by Nikonov (1971), the velocity gradients between two points were also calculated when the position of the velocity isolines was identifiable in the field. Cenozoic
vertical movements
The uplift of the Ethiopian plateau (where the Jurassic marine limestones were progressively raised to an altitude of 2500 m in the Cenozoic) occurred at a mean velocity of about 0.1 mm/year. This was an epeirogenic movement acting on areas comprising hundreds of thousands of square kilometers, and the velocity gradients are very low or nil. The collapse of the central rift during the same period indicates a negative vertical movement of about 0.2 mm/year. Pleistocene
vertical movements
The marine deposits (Fig. 1) of several high levels occurring between 200,000 and 60,000 years (Lalou et al., 1970; Hoang et al., 1973) permitted
4 2”
Fig. 1. Pleistocene
marine
deposits
id0
in Afar (marine
sediments
in black).
the calculation of a certain number of rates of vertical movement along the ancient shorelines af the Afar (Faure, 1975). Some approximate values are summarized in Table I. It can be observed that the mean vertical movement velocities generally amount to a few tenths of mm per year since the Pleistocene. The maximum velocities measured reach 1 mm/year and perhaps 2 mm/ye~, according to certain field indications.
Holocene vertical movements Observations deal exclusively with the present shorelines of the Red Sea and the Gulf of Tadjoura, since the Danakil Gulf, isolated by a shelf, was not invaded by the sea during the final Postglacial rise. The lower coastal regions, edged with coastal plains, correspond to a Holocene filling. The strongest indications of marine deposits in place consist of shelly bars, the tops of which do not appear to rise more than 5 m above the present sea level (north of Sanda, west of Tadjoura). By comparison with present bars (caused by high tides or storms), this indicates a m~imum I-Iolocene sea level undoubtedly lower than +2 m.
483 TABLE
I
Mean rates of vertical
movements
in Afar
Place
Dates for datum lines (years B.P.)
Rates (mm/year)
Massawa Dalol Khaffala N. Aih Allah E. Affrera S. Assab Bab el Mandeb Bab el Mandeb Obock Hamboktou Tadjoura Goubbat* Djibouti
100 200 100 100 73 125 125 60 125
0.2 0.45 0.4 0.15 0.3 0.02
*Observations
000 ? 000 000 000 000 000 ? 000 000 000 ? 125000? 125 000 made by L. Stieltjes
(1973),
age modified.
Over long portions of rocky shorelines, several morphological clues and the absence of marine deposits appear to indicate that, at these points, the sea did not rise above the present level. In other sectors, such as the southeast of Ghoubbet al Kharab, marine shells dated at 5900 years B.P. (Delibrias et al., in preparation) are encountered up to +4.5 m in consolidated bars, and indicate a Holocene level as high or higher (+l m or +2 m) than the present level. Moreover, a notch in these formations proves a subsequent halt of the sea around the same level. These overall observations indicate that the amplitude of the deformations which affected the geoid over the past 5000 years is of the order of 5 m in absolute terms, with due consideration of negative movements in certain sectors. The mean vertical movement velocity hence approaches 1 mm/year, and is certainly lower than 2 mm/year. An exceptionally high velocity of 15 mm/year given by Tazieff (1972) pertained to the volcanic island of Ginni Koma. Consequently this is an isolated point subjected to very rapid vertical movement linked to a volcanic phenomenon. However, the oysters of 5800 years B.P., located at an altitude of 90 m, which were employed for this calculation, may have originated in an anthropic deposit. With the exception of this point, the maximum velocities of Holocene movements are generally comparable to those of the Pleistocene. Recent vertical movements The tidal gauge nearest to the region investigated, and one which has operated for almost a century, is that from Aden to Yemen. The annual means enable the calculation of a gradual rise in sea level of about 2 mm/year be-
484 mm
19io
1950
1960
1970
Fig. 2. Aden tide gauge. Evolution of the annual averages (1937-1967). manent Service for Mean Sea Level’, The Observatory, Bidston.)
(Data from ‘Per-
tween 1950 and 1960,2.4 mm/year for the period 1935-1960, and 2.7 mm/year for 1937-1967 (Fig. 2). These figures appear to indicate that the overall eustatic rise occurs in conjunction with a negative vertical movement of the continent, which is of the order of 1 mm/year at the Aden station. A geodetic and levelling network was installed in 1973 across the rift to permit the measurement of deformations covering periods of a few years (Chabbert, 1975). CONCLUSIONS
In the tectonically highly active region of the Afar and its surroundings it can be observed that the mean velocities of vertical movements remain of the same order of magnitude, regardless of the time scale or the period in question, from the Cenozoic to the present. On the other hand, the measured gradients vary widely, in time as well as in space. These results extend, to a zone in tension, the conclusions of Pavoni (1971) concerning vertical movements of the terrestrial crust. ACKNOWLEDGEMENTS This work was carried out at the Laboratoire de Geologic du Quaternaire thanks to a grant from the RCP 180 of the Centre National de la Recherche
485
Scientifique and the ATP Gkodynamique of the CNRS (Contract No. 3206). Fieldwork was made possible by the assistance given to us by the civil and military authorities of Ethiopia and the French Territory of the Afars and the Issas. REFERENCES Bannert, D., Brickmann, I., Kading, K.Ch. et al., 1970. Zur Geologie der Danakil-Senke (Nordliches Afar-Gebiet, N.E. Athiopien). Geol. Rundsch., 59: 409-443. Barberi, F., Borsi, S., Ferrara, G., Marinelli, G., Santacroce, R., Tazieff, H. and Varet, J., 1972a. Evolution of the Danakil depression (Afar, Ethiopia) in the light of radiometric age determinations. J. Geol., 80: 720-729. Barberi, F., Tazieff, H. and Varet, J., 1972b. Volcanism in the Afar depression. Its tectonic and magmatic significance. Tectonophysics, 15: 19-30. Bonatti, E., Emiliani, C., Ostlund, G. and Rydell, H., 1971. Final desiccation of the Afar Rift, Ethiopia. Science, 172: 468-469. Chabbert, C., 1975. Geodetic measurements across part of the Afar depression, East Africa. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 471-477. C.N.R.C.N.R.S. Afar Team, 1973. Geology of Northern Afar (Ethiopia). Rev. Geegraph. Phys. Geol. Dyn., Ser. 2,15: 443-490. Delibrias, G., Faure, H. and Hoang, C.T., in preparation. Mid-holocene shoreline in Gulf of Tadjoura and Red Sea (Afar and T.F.A.I.). Fairhead, J.D. and Girdler, R.W., 1970. The seismicity of the Red Sea, Gulf of Aden and Afar triangle. Philos. Trans. R. Sot., London, 267: 49-74. Faure, H., 1973a. Cadre morphotectonique megametrique de I’Afar. Rev. Geograph. Phys. Geol. Dyn., Ser. 2,15: 387-392. Faure, H., 1973b. Le modele cenozoi’que de 1’Afar applique a l’ouverture de 1’Atlantique au Mesozoi’que. Proc. 2nd Conf. on African Geology, Addis Abeba (in press). Faure, H., 1975. Neotectonics in the Afar (Ethiopia, T.F.A.I.). In: R.P. Suggate and M.M. Cresswell (Editors), Quaternary Studies. R. Sot. N.Z., pp. 121-126. Faure, H., in press. Coastal deformations and the Afar geodynamic model. In: A. Pilger (Editor), Afar Monograph. Proc. Symp. on the Afar region of Ethiopia and related rift problems. Schweizerbart, Stuttgart. Faure, H., Hoang, C.T. and Lalou, C., 1973. Structure et geochronologie (230Th/234U) des r&ifs coralliens souleves a 1’Ouest du Golfe d’Aden (T.F.A.I.). Rev. Gdograph. Phys. Geol. Dyn., Ser. 2,15: 393-403. Gouin, P., 1970. Seismic and gravity data from Afar in relation to surrounding area. Philos. Trans. R. Sot. London. 267: 339-358. Hoang, C.T., Faure, H. and Lalou, C., 1974. Les r&ifs souleves a 1’Ouest du Golfe d’Aden (T.F.A.I.) et les hauts niveaux de coraux de la depression de 1’Afar (Ethiopie). Geochronologie et paleoclimatologie interglaciaires. Coil. Int. C.N.R.S. no 219. Les mdthodes quantitatives d’etude des variations du climat au tours du Plhistoche, Gif, pp. 132-146. Lalou, C., Nguyen, H.V., Faure, H. and Moreira, L., 1970. Datation par la methode Uranium-Thorium des hauts niveaux de coraux de la depression de 1’Afar (Ethiopie). Rev. Geogr. Phys. Geol. Dyn., 12: 3-8. Lepine, J.C. and Ruegg, J.C., 1973. Premiers enseignements de la crise sismique survenue dans la region de Djibouti (T.F.A.I.) en mars-avril 1973. C.R. Acad. Sci., Paris, 277: 33-36. Lepine, J.C., Ruegg, J.C. and Steinmetz, L., 1972. Seismic profiles in the Djibouti area. In: R.W. Girdler (Editor), East African Rifts. Tectonophysics, 15: 59-64.
486 Makris, J., Menzel, H. and Zimmermann, J., 1972. A preliminary interpretation of the gravity field of Afar. Northeast Ethiopia. Tectonophysics, 15: 31--39. Marinelli, G., 1971. La province geothermique de la depression Dankali. Ann. Mines, May 1971. Menard, H.W., 6: 275-284.
1969.
Elevation
and subsidence
Menard, H.W., 1973. Depth anomalies Geophys. Res., 78: 5128-5137. Mohr, P.A., 1970. 7340-7352.
The Afar triple
and-the
junction
of oceanic bobbing
and sea-floor
crust.
motion
Earth
Planet,
of drifting
speading.
Sci. Lett.,
islands.
J. Geophys.
J.
Res.,
75:
Mohr, P.A., 1973. Crustal deformation rate and the evolution of the Ethiopian rift. In: D.H. Tarling and S.K. Runcorn (Editors), Implication of Continental Drift to the Earth Sciences. Academic Press, London, pp. 767-776. Nikonov, A.A., 1971. On recent vertical movements of the earth’s crust in seismically active areas of middle Asia. Tectonophysics, 12: 119-127. Pavoni, N., 1971. Recent and Late Cenozoic movements of the earth’s crust. R. Sot. N.Z., Bull., 9: 7-17. Pilger, A. (Editor), 1974. Afar Monograph. Proc. Symp. on the Afar region of Ethiopia and related rift problems. Schweizerbart, Stuttgart (sous presse). Sclater, J.G. and Harrison, C.G.A., 1971. Elevation of mid-ocean ridges and the evolution of the southwest Indian Ridge. Nature, 230: 175-177. Stieltjes, L., 1973. Evolution tectonique recente du rift d’Asa1 T.F.A.I. Rev. Geograph. Phys. Geol. Dyn., Ser. 2, 15: 425-436. Tarling, D.H. and Runcorn, S.K. (Editors), 1973. Implications of Continental drift to the Earth Sciences. Acad. Press, London, 2 vol. Tazieff, H., 1971. Sur la tectonique de 1’Afar Central. C.R. Acad. Sci., Paris, Ser. D, 272: 1055-1058. Tazieff, H., 1972. Tectonics of Central Afar. J. Earth Sci. Leeds, 8: 171-182. Tazieff, H., 1973. La signification tectonique de 1’Afar. Rev. Geograph. Phys. Geol. Dyn., Ser. 2,15: 241-346. Treuil, M., 1973. Criteres petrologiques, geochimiques et structuraux de la gbnese et de la differentiation des magmas basaltiques. Exemple de 1’Afar. These, Univ. of Orleans, 494 p. Varet, J., 1973. differentiation
Criteres petrologiques, geochimiques des magmas basaltiques. Exemple
et structuraux de la genese et de la de 1’Afar. These, Univ. of Orsay, 494
p.
479
RECENT CRUSTAL MOVEMENTS ALONG THE RED SEA AND GULF OF ADEN COASTS IN AFAR (ETHIOPIA AND T.F.A.I.“)
HIJGWES FAURE Laboratoire de Ge’ologie du Quaternaire, Meudon-Belleuue (France)
Centre National
de la Recherche
Scientifique,
(Revised version accepted September 11,1975)
ABSTRACT Faure, II., 1975. Recent crustal movements along the Red Sea and Gulf of Aden coasts in Afar (Ethiopia and T.F.A.I.). In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 479-486. Mean velocity rates and gradients of vertical movements along the Red Sea and Gulf of Aden coasts have been calculated from deformed Pleistocene coral shorelines. Mean rates of vertical displacement have also been calculated along ancient shorelines of the Afar in Ethiopia and T.F.A.I. (French Territory of the Afars and the Issas). Systems of shorelines with ages of 200,000 and 120,000~80,000 years B.P. could be identified by uranium-thorium dating. The rates of uplift are calculated in different areas and are given in mm/year of mean vertical (positive or negative) relative movement since the deposition close to mean sea level. Results are of the same order of magnitude for the rates at different periods of time (0.1-1.0 mm/year) but are quite different from place to place. They confirm a tectonically active area in the Danakil rift and Gulf of Tadjoura and a more stabilized zone at the Bab el Mandeb Strait between the Danakil block and Arabia. These quantitative results will later be compared with the results of Ghoubbet-Asal Lake geodetic leveling.
INTRODUCTION
For more than fifty years, the Red Sea and the Gulf of Aden have been taken as a model for the initial stage in the fractumtion of a continent. Several evolution models have been suggested to reconstruct the bulge of the earth’s crust, its fracturation into grabens, followed by stretching which caused separation of the edges and the appearance of a new oceanic crust based on an axial rift (see bibliography in CNR-CNRS, 1973; Tarling and Runcorn, 1973; Pilger, 1974; Faure, 1973b etc.). The Magmatic and geochemical evolution which accompanies these phenomena has been described in de* T.F.A.I.:
Territoire Francais des Afars et des Issas.
480
tail (Varet, 1973; Treuil, 1973). An attempt was made to improve the classic model by quantifying it with the help of the results of the most recent investigations on the Afar region (Bannert et al., 1970; Mohr, 1970,1973; CNR-CNRS, 1973; Barberi et al., 1972a; Tazieff, 1971,1973; Varet, 1973; Faure, 1973a; Pilger, 1974 etc.). The quantitative model proposed here stresses the importance of vertical movements throughout the evolution of this sector in tension (Faure, 1973b). As with the oceanic ridges (Menard, 1969,1973; Sclater and Harrison, 1971), but with a different surface expression, a correlation appears to exist between vertical movements and horizontal displacement. An abnormal mantle of shallow depth (Lepine et al., 1972; Makris et al., 1972), a probably high thermal flux (Marinelli, 1971) and intense seismicity (Gouin, 1970; Fairhead and Girdler, 1970; Lepine and Ruegg, 1973) accompany the exceptional uplift of the margins in the region and the subsidence of its axial zone in the Afar. METHOD
The Quaternary marine formations outcropping along the present and ancient shorelines were investigated (CNR-CNRS, 1973; Lalou et al., 1970; Faure et al., 1973; Hoang et al., 1974). They exhibit the conditions necessary to obtain valid quantitative measurements (Faure, 1974) : (1) The possibility of locating the sea level in relation to observed marine deposits (morphology, sedimentology, ecology, etc.). (2) Altimetric and topographic measurements with respect to a known altitude reference (present sea level, horizontal surface of the Dalol salt plain). (3) Possibility of radiometric measurements (zs0Th/234 U) on corals. The marine deposits which are mainly coralline in these regions, correspond in their upper zones (reef flat) to a halt of the sea or to a sea level close to an interglacial high. The mean tidal levels, of which the tidal range is lower than 1.8 m, permit an accurate estimate of the reference paleolevel to within 0.5 m. Marine deposits occur along the present shorelines (above and below the present zero), and along the ancient shorelines of the Pleistocene Danakil Gulf, which occupied the north of the Afar rift depression. This ancient gulf extended 275 km into the hinterland, with a width ranging from 30 km to 65 km at the maximum, and a narrow gully connecting it to the present Gulf of Zula. This configuration stimulated rapid and thick evaporitic sedimentation beyond the high sea-level phases, and the bed of the depression is currently occupied by a thick formation of salts, bounded at the top by a surface located 118 m below sea level. The observation of marine outcrops can thus be effected in the open air at the periphery of the salt plain, and at the feet of the escarpments of the Ethiopian plateau and the Danakil horst. The ancient sea level in the gulf and along the shorelines intersects various geological fomrations and tectonic zones of the margins of the graben, or of the axial volcanic range of the rift. At the feet of the bordering escarpments,
481
the coralline formations rest on Miocene or Pliocene volcanic or sedimentary rocks (Barberi et al., 1972b), or on Pleistocene continental conglomerates and sands (CNR-CNRS, 1973). Their lower surface matches an ancient topography and fault escarpments which continued their morphological and tectonic evolution after the withdrawal of the sea. Along the axial volcanic chain, the corals, calcarenites and calcareous oolites rest on the basalt flows of the Tat’Ali, less than 7 km from the fault of the central rift. They are covered by more recent flows of the same volcanic block. The deposits in this region also constitute atolls constructed on hyaloclastite rings (Bonatti et al., 1971). The marine deposits were subjected to more than forty datings throughout the region in question. They belong to several lines of successive shorelines of the Middle Holocene (6000-3000 years B.P.), (Delibrias et al., in preparation) and the Upper and Middle Pleistocene (48-65,OOO years?), (73--80,000 years), (124-133,000 years) and (X30-200,000 years), (Lalou et al., 1970; Faure et al., 1973; Hoang et al., 1974). By referring to a single identifiable unit in the field, it is possible to define the maximum observable deformations to which it was subjected since its installation. In more favorable cases, it is possible to determine the deformation occurring between installation and the deposit of a more recent unit. These deformations relate to the action of faults, the tilting of plates, or epeirogenic movements of regional significance. RESULTS
In order to render the values of vertical movements obtained for periods of time of widely differing durations mutually comparable, the results are expressed in mm of mean movement per year (rate or velocity in mm/year). As recommended by Nikonov (1971), the velocity gradients between two points were also calculated when the position of the velocity isolines was identifiable in the field. Cenozoic
vertical movements
The uplift of the Ethiopian plateau (where the Jurassic marine limestones were progressively raised to an altitude of 2500 m in the Cenozoic) occurred at a mean velocity of about 0.1 mm/year. This was an epeirogenic movement acting on areas comprising hundreds of thousands of square kilometers, and the velocity gradients are very low or nil. The collapse of the central rift during the same period indicates a negative vertical movement of about 0.2 mm/year. Pleistocene
vertical movements
The marine deposits (Fig. 1) of several high levels occurring between 200,000 and 60,000 years (Lalou et al., 1970; Hoang et al., 1973) permitted
4 2”
Fig. 1. Pleistocene
marine
deposits
id0
in Afar (marine
sediments
in black).
the calculation of a certain number of rates of vertical movement along the ancient shorelines af the Afar (Faure, 1975). Some approximate values are summarized in Table I. It can be observed that the mean vertical movement velocities generally amount to a few tenths of mm per year since the Pleistocene. The maximum velocities measured reach 1 mm/year and perhaps 2 mm/ye~, according to certain field indications.
Holocene vertical movements Observations deal exclusively with the present shorelines of the Red Sea and the Gulf of Tadjoura, since the Danakil Gulf, isolated by a shelf, was not invaded by the sea during the final Postglacial rise. The lower coastal regions, edged with coastal plains, correspond to a Holocene filling. The strongest indications of marine deposits in place consist of shelly bars, the tops of which do not appear to rise more than 5 m above the present sea level (north of Sanda, west of Tadjoura). By comparison with present bars (caused by high tides or storms), this indicates a m~imum I-Iolocene sea level undoubtedly lower than +2 m.
483 TABLE
I
Mean rates of vertical
movements
in Afar
Place
Dates for datum lines (years B.P.)
Rates (mm/year)
Massawa Dalol Khaffala N. Aih Allah E. Affrera S. Assab Bab el Mandeb Bab el Mandeb Obock Hamboktou Tadjoura Goubbat* Djibouti
100 200 100 100 73 125 125 60 125
0.2 0.45 0.4 0.15 0.3 0.02
*Observations
000 ? 000 000 000 000 000 ? 000 000 000 ? 125000? 125 000 made by L. Stieltjes
(1973),
age modified.
Over long portions of rocky shorelines, several morphological clues and the absence of marine deposits appear to indicate that, at these points, the sea did not rise above the present level. In other sectors, such as the southeast of Ghoubbet al Kharab, marine shells dated at 5900 years B.P. (Delibrias et al., in preparation) are encountered up to +4.5 m in consolidated bars, and indicate a Holocene level as high or higher (+l m or +2 m) than the present level. Moreover, a notch in these formations proves a subsequent halt of the sea around the same level. These overall observations indicate that the amplitude of the deformations which affected the geoid over the past 5000 years is of the order of 5 m in absolute terms, with due consideration of negative movements in certain sectors. The mean vertical movement velocity hence approaches 1 mm/year, and is certainly lower than 2 mm/year. An exceptionally high velocity of 15 mm/year given by Tazieff (1972) pertained to the volcanic island of Ginni Koma. Consequently this is an isolated point subjected to very rapid vertical movement linked to a volcanic phenomenon. However, the oysters of 5800 years B.P., located at an altitude of 90 m, which were employed for this calculation, may have originated in an anthropic deposit. With the exception of this point, the maximum velocities of Holocene movements are generally comparable to those of the Pleistocene. Recent vertical movements The tidal gauge nearest to the region investigated, and one which has operated for almost a century, is that from Aden to Yemen. The annual means enable the calculation of a gradual rise in sea level of about 2 mm/year be-
484 mm
19io
1950
1960
1970
Fig. 2. Aden tide gauge. Evolution of the annual averages (1937-1967). manent Service for Mean Sea Level’, The Observatory, Bidston.)
(Data from ‘Per-
tween 1950 and 1960,2.4 mm/year for the period 1935-1960, and 2.7 mm/year for 1937-1967 (Fig. 2). These figures appear to indicate that the overall eustatic rise occurs in conjunction with a negative vertical movement of the continent, which is of the order of 1 mm/year at the Aden station. A geodetic and levelling network was installed in 1973 across the rift to permit the measurement of deformations covering periods of a few years (Chabbert, 1975). CONCLUSIONS
In the tectonically highly active region of the Afar and its surroundings it can be observed that the mean velocities of vertical movements remain of the same order of magnitude, regardless of the time scale or the period in question, from the Cenozoic to the present. On the other hand, the measured gradients vary widely, in time as well as in space. These results extend, to a zone in tension, the conclusions of Pavoni (1971) concerning vertical movements of the terrestrial crust. ACKNOWLEDGEMENTS This work was carried out at the Laboratoire de Geologic du Quaternaire thanks to a grant from the RCP 180 of the Centre National de la Recherche
485
Scientifique and the ATP Gkodynamique of the CNRS (Contract No. 3206). Fieldwork was made possible by the assistance given to us by the civil and military authorities of Ethiopia and the French Territory of the Afars and the Issas. REFERENCES Bannert, D., Brickmann, I., Kading, K.Ch. et al., 1970. Zur Geologie der Danakil-Senke (Nordliches Afar-Gebiet, N.E. Athiopien). Geol. Rundsch., 59: 409-443. Barberi, F., Borsi, S., Ferrara, G., Marinelli, G., Santacroce, R., Tazieff, H. and Varet, J., 1972a. Evolution of the Danakil depression (Afar, Ethiopia) in the light of radiometric age determinations. J. Geol., 80: 720-729. Barberi, F., Tazieff, H. and Varet, J., 1972b. Volcanism in the Afar depression. Its tectonic and magmatic significance. Tectonophysics, 15: 19-30. Bonatti, E., Emiliani, C., Ostlund, G. and Rydell, H., 1971. Final desiccation of the Afar Rift, Ethiopia. Science, 172: 468-469. Chabbert, C., 1975. Geodetic measurements across part of the Afar depression, East Africa. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 471-477. C.N.R.C.N.R.S. Afar Team, 1973. Geology of Northern Afar (Ethiopia). Rev. Geegraph. Phys. Geol. Dyn., Ser. 2,15: 443-490. Delibrias, G., Faure, H. and Hoang, C.T., in preparation. Mid-holocene shoreline in Gulf of Tadjoura and Red Sea (Afar and T.F.A.I.). Fairhead, J.D. and Girdler, R.W., 1970. The seismicity of the Red Sea, Gulf of Aden and Afar triangle. Philos. Trans. R. Sot., London, 267: 49-74. Faure, H., 1973a. Cadre morphotectonique megametrique de I’Afar. Rev. Geograph. Phys. Geol. Dyn., Ser. 2,15: 387-392. Faure, H., 1973b. Le modele cenozoi’que de 1’Afar applique a l’ouverture de 1’Atlantique au Mesozoi’que. Proc. 2nd Conf. on African Geology, Addis Abeba (in press). Faure, H., 1975. Neotectonics in the Afar (Ethiopia, T.F.A.I.). In: R.P. Suggate and M.M. Cresswell (Editors), Quaternary Studies. R. Sot. N.Z., pp. 121-126. Faure, H., in press. Coastal deformations and the Afar geodynamic model. In: A. Pilger (Editor), Afar Monograph. Proc. Symp. on the Afar region of Ethiopia and related rift problems. Schweizerbart, Stuttgart. Faure, H., Hoang, C.T. and Lalou, C., 1973. Structure et geochronologie (230Th/234U) des r&ifs coralliens souleves a 1’Ouest du Golfe d’Aden (T.F.A.I.). Rev. Gdograph. Phys. Geol. Dyn., Ser. 2,15: 393-403. Gouin, P., 1970. Seismic and gravity data from Afar in relation to surrounding area. Philos. Trans. R. Sot. London. 267: 339-358. Hoang, C.T., Faure, H. and Lalou, C., 1974. Les r&ifs souleves a 1’Ouest du Golfe d’Aden (T.F.A.I.) et les hauts niveaux de coraux de la depression de 1’Afar (Ethiopie). Geochronologie et paleoclimatologie interglaciaires. Coil. Int. C.N.R.S. no 219. Les mdthodes quantitatives d’etude des variations du climat au tours du Plhistoche, Gif, pp. 132-146. Lalou, C., Nguyen, H.V., Faure, H. and Moreira, L., 1970. Datation par la methode Uranium-Thorium des hauts niveaux de coraux de la depression de 1’Afar (Ethiopie). Rev. Geogr. Phys. Geol. Dyn., 12: 3-8. Lepine, J.C. and Ruegg, J.C., 1973. Premiers enseignements de la crise sismique survenue dans la region de Djibouti (T.F.A.I.) en mars-avril 1973. C.R. Acad. Sci., Paris, 277: 33-36. Lepine, J.C., Ruegg, J.C. and Steinmetz, L., 1972. Seismic profiles in the Djibouti area. In: R.W. Girdler (Editor), East African Rifts. Tectonophysics, 15: 59-64.
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