A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Palaecomagnetism and the origin of the Red Sea and Gulf of Aden

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Abstract
Tectonics, volcanism and petrology of the northern Afar (or Danakil) Depression have been investigated during two recent successive expeditions. The tectonics is characterized by numerous NNW trending normal faults. Two important active volcanic chains, namely Erta'Ale Range and Alayta Range, are disposed in en echelon structure. They both display differentiated lavas of subcrustal origin, ranging from abundant basalts to scarce soda-rhyolites with intermediate products, such as dark trachytes as well as oversaturated ones. $^{87}$Sr/$^{86}$Sr isotopic ratios are very low and approximately constant through the whole series. This fact, and other petrological considerations, suggest that this series has not been contaminated by any crustal sialic material. In addition to these two well-defined volcanic ranges, the Depression is covered by extensive basaltic fields and significant quantities of rhyolites. The variegated petrography of these silicic lavas and ignimbrites, as well as the $^{87}$Sr/$^{85}$Sr ratios, on the contrary strongly suggest that these rocks were formed by interaction between the subcrustal magma and the sialic crust. It can be tentatively concluded that the sialic crust is missing beneath the northern part of the Danakil Depression only below the two en echelon elongated volcanic ranges mentioned above.

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