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African Rift Tectonics and Sedimentation, an Introduction H.G

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African Rift Tectonics and Sedimentation, an Introduction H.G Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 African Rift tectonics and sedimentation, an introduction H.G. Reading Rifts are one of the most spectacular features of Gulf of Elat rift lies along a lineament that global morphology. Within oceans they separate separates two lithospheric plates and, as Arabia plates as new oceanic crust is created. Within moves northwards relative to the Palestinian continents they may form deep valleys such as the (Mediterranean) block and faults curve or side- Rhine graben, within which runs one of the step each other, extensional pull-apart basins are world's busiest waterways. They are often floored formed, such as the Dead Sea whose surface lies by deep lakes, such as Lake Baikal, the deepest further below sea level than any other place in the lake in the world today whose floor lies 1700 m world. A feature of these strike-slip rift valleys is below the surface of the lake. Ancient rifts are the that the extensional valleys pass, along the rift, sites of petroleum accumulations beneath the into mountainous zones of compression and margins of the Atlantic, in the North Sea and in uplift. Rapid erosion from these mountains leads China, of coals and oil shales and of minerals, to substantial sediment supply for the basins from including phosphates, barite, Cu-Pb-Zn sul- within the fault system itself. Ancient strike-slip phides and uranium (Robbins 1983). Of all the rift basins are therefore characterized by very world's rifts none can match in scale and diversity thick, rapidly accumulated basin fills with areas the Great Rift System, which runs for over of contemporaneous compressional tectonics and 7000 km and includes the East African Rift unconformities not far away. System and its extension through the Red Sea Rift valley basins that are extensional features into the Dead Sea, to form a unique feature of on a regional scale tend to sink rather more global geology. Its study is important not only for slowly, have less sediment entering them, and what it can tell about the nature and origin of show no evidence of synchronous compressional present-day rifts, the thermal, magmatic and tectonics in their neighbourhood. Cross-sections tectonic processes which gave rise to them, clima- through them show normal faulting, probably tic changes, and sedimentary, particularly lacus- listric in form. trine and volcaniclastic, processes. It is also The main controversy with regard to extension- essential for the understanding of the processes al rift valleys is whether magmatic activity in rifts which formed passive continental margins, all of reflects an underlying heat source such as a which are underlain by rifted sedimentary basins, convection plume in the mantle or hot spot, the and failed rifts whether they formed within rifting then being the consequence of expansion continents or at the junction of continents and and uplift brought about by thermal activity, or oceans. whether the magmatic activity is merely the result Early workers on rifts were impressed by the of regional or local extensional stresses which wide valleys, 40-50 km across, filled by young have passively allowed magma to come to the alluvial and lacustrine sediments and bordered by surface. steep escarpments rising in some cases a few Some rifts that penetrate continents are clearly hundred metres, in others up to 3-4000 m above failed rifts or aulacogens that began as one arm of the valley floor. Many rifts are the sites of a triple junction. As ocean floor spreading deve- volcanic activity, sometimes of rather unusual loped along two arms, one arm was left as a 'failed composition. These unusual volcanic source rift' in the sense that new oceanic crust was not rocks may in turn be the cause of peculiar lake created. Perhaps the best known is the Benuc water chemistries, many rift lakes containing trough within which the Niger delta developed, unique suites of evaporitic minerals. However, the northern (Viking) graben of the Although at one time some people considered North Sea has been claimed to be one (Whiteman rifts to have been formed by compression (ramp- et al. 1975), and so has that part of the East valley hypothesis), discussion today revolves African Rift known as the Afar triangle where thc principally around the extent to which rift valleys Red Sea and the Gulf of Aden Rifts successfully are extensional features or are the result of drifted, whilst the Afar triangle 'failed'. oblique-slip movements between two laterally Molnar & Tapponnier (1975) have argued that moving blocks. other rifts are the result of continental collision, in There is no doubt now that some rift valleys are particular that the impact of India with Asia the result of strike-slip movement. The Dead Sea/ caused not only major lateral displacement along From FROSTICK, L.E. et al. (eds) 1986, Sedimentation in the African Rifts, Geological Society Special Publication No. 25, pp. 3-7. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 4 H.G. Reading faults such as the Red River and Altyn Tagh different from the West and Central African Rift strike-slip faults, but also E-W extension to System, and its southern and older portion is very produce the Baikal graben. The Rhine graben has different again from the northern Kenyan to also been interpreted as the result of stresses Ethiopian parts. The dominant feature of the generated in the Alps. East African Rift System is the wide zone of low However the rifts in Africa are not the result of density upper mantle which is expressed topo- continental collision, and there is little evidence graphically by the localized Ethiopian and for any substantial degree of strike-slip move- Kenyan domes where volcanic activity is most ment. However, strike-slip movement may occur active. In the northern portion the rifting appears locally and some African rifts lie along major to be relatively young (late Tertiary) though there shear zones or lineaments extending for thou- is evidence that rifting commenced during the sands of kilometres, which may have had a latest Mesozoic (Williamson et al., this volume). considerable amount of lateral movement at The amount of crustal extension is relatively some time in the past. small, about a quarter of that in the West and Although the East African Rift System is the Central African Rift System (I0 35 kin, Fair- best known of African rifts, other extensive rifts head, this volume), and there seems little doubt occur especially in West and Central Africa that the rift is the result of thermal activity in the (Fairhead, this volume). Some parts have been upper mantle and consequent thermal uplift. well studied, for example the Benue trough of Support for thermally-induced rifting comes Nigeria because of its petroleum resources, and from the migration of some rifts eastward with the Volcanic Cameroon Line is also well known. volcanism preceding doming, that in turn pre- Other parts, though, have little topographic cedes rifting. The reason it has not opened yet expression and therefore have only recently may simply be that regional stresses are not yet become known, due to hydrocarbon exploration. conducive to opening. Were they to be so it would In particular the nature of the geology across the probably open. Central African Shear Zone has not been under- Sedimentation in the northern portion is com- stood. monly away from the domes and main rift. For This complex pattern of shear zones, asso- example the Turkana basin is to the W of the ciated rifts and rift valleys, known as the West present active rift, though it probably started and Central African Rift System (Fairhead, this within a late Miocene-Pliocene rift before the rift volume), is apparently older than the East Afri- migrated eastward (Cerling & Powers 1977). can Rift System in that at the present day it is Sedimentation may also take place between rifts largely inactive except for some gentle subsi- in sag basins such as Lake Victoria. Sedimentary dence. The main rift phase was the Cretaceous thicknesses are not too great; volcanic accumu- when substantial and rapid stretching of the lations are considerable, and volcanic source lithosphere caused thinning of the crust and areas give rise to unusual and varied sedimentary passive upwelling of hot asthenosphere. It deve- compositions. loped as a result of large extensional stresses, The southern portion of the East African Rift continental disruption and high sea-floor spread- differs from the northern portion in that volcanic ing rates. Volcanism was minor and although activity is limited. Although there is still a wide, there was probably a large anomalously low low density zone on either side of the rifts, there is density body within the upper mantle during the no thermal doming. The initiation of rifting Cretaceous it did not produce domal uplift, appears to be earlier, and rifts follow Precam- except perhaps at the oceanward end of the Benue brian lineaments. Sediments in the rift basins are Rift. Instead deep rifts formed, open to the sea. thicker and up to 3-4 km of sediments may have This was due to the major crustal extension which accumulated. occurred at this time, perhaps some four times From the early days of the plate tectonics that of the East African Rift System. Towards the hypothesis, the East African Rift has been consi- end of the Cretaceous some lateral displacements dered to be a present-day example of the earliest took place along the Central African Shear Zone, stage of continental breakup, that is the rift stage and during the Tertiary wider and more extensive which precedes the drift stage represented by the sedimentary basins than the original rift basins Gulf of Aden and the Red Sea (e.g.
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