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The Ethiopian Rift Valley (between 7° 00′ And 8° 40′ Lat. North)

G. M. D. Paola
Published 1972 · Geology

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The Ethiopian Rift Valley, which cuts the uplifted Ethio-Somalian Plateau, is one of the most important structures of East Africa, and nevertheless it is still largely unknown.A preliminary 1/500,000 geological map as well as volcanological and petrological descriptions of an important part of this structure are presented.This part of the Ethiopian Rift Valley is marked by a set of NNE-SSW normal faults. « En échelon » arrangements, rift-in-rift structures, asymmetry and open tensional fissures are its most important tectonic features. The region has been affected by volcanism since Eocene in the neighbouring Plateaux and since probably Pliocene within the Rift, with fissure eruptions and growth of individual volcanoes. Non-volcanic rocks consist exclusively of lacustrine sediments. Magmatic products on both Plateaux are represented mainly by huge piles of basaltic lava flows, whilst within the Rift most of the volcanics are widespread ignimbritic units.The presence of such large amount of ignimbrites can easily explain some important volcano-tectonic collapses which produced large regional calderas. Although still insufficient, the available petrological data suggest that the Plateau basalts have more alkalic character than those within the Rift, which show a transitional nature between alkali basalts and tholeiites. Intermediate rocks seem to be scarce and most of the silicic products are monotonously represented by peralkaline rhyolites (mostly pantellerites).The genesis of the peralkaline silicic rocks might be related to the presence, along the Rift axis, of a huge basic igneous body recently discovered by a geophysical investigation. This could explain the presence of large volume of these rocks within the Rift, although the apparent scarcity of intermediate rocks remains unexplained.RésuméLa Rift Valley d’Ethiopie qui recoupe le Plateau Ethio-Somalien est une des plus importantes structures de l’Afrique orientale, et néanmoins encore très peu connue.Ce texte présente une carte géologique préliminaire au 1/500.000 ainsi qu’une description volcanologique et pétrologique d’un secteur important de cette structure.La partie étudiée de la Rift Valley d’Ethiopie présente les caractéristiques suivantes: système le failles normales orientées NNE-SSW, style « en échelon », structures « rift in rift », asymétrie du graben et présence de nombreuses fissures de tension ouvertes. Toute la région a été affectée par le volcanisme depuis l’Eocène dans les Plateaux environnants et probablement depuis le Pliocène dans le Rift, avec éruptions fissurales et édification de volcans individuels. Les roches non volcaniques sont représentées uniquement par des dépôts lacustres. Les produits magmatiques des deux Plateaux sont, pour la plupart, représentés par des centaines de coulées de lave basaltique empilées, tandis que dans le Rift, la majeure partie des produits volcaniques est constituée par des ignimbrites. Les éruptions d’ignimbrites sont responsables de la formation de grandes caldéras régionales. Les données pétrologiques actuellement disponibles, bien qu’encore insuffisantes, suggèrent que les basaltes de Plateau ont un caractère plus alcalin que ceux du Rift qui montrent une nature transitionnelle entre les basaltes alcalins et les tholéiites. Les roches intermédiaires semblent être rares et la majorité des produits acides est représentée uniquement par des rhyolites peralcalines (pantellérites pour la plupart).Récentes données géophysiques indiquent la présence, le long de l’axe du Rift, d’une importante intrusion de magma basique qui pourrait ainsi expliquer la grande quantité de roches peralcalines que l’on trouve dans le Rift, bien que l’apparente rareté des roches intermédiaires reste inexpliquée.
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