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Pre-quaternary Geological Frame

2017; Springer International Publishing; Linguagem: Inglês

10.1007/978-3-319-60865-5_4

2364-6438

Jacques Varet,

Geological and Geophysical Studies Worldwide

Although the Afar depression is essentially floored by recent geological formations of quaternary age (dating less than 4My), units of older age outcrop along the Afar margins and in the plateaus escarpments. The study of these formations is of high interest as they allow to understand how pre-rift structures played a role in the rift process, as pre-existing lines of weakness appear to have determined the fracturation process of the continental crust. Besides this, in the period pre-dating the Red Sea-Gulf of Aden rifting through Afar, the study of the geological formations that developed between 24My (date of the initiation of the rifting process) and 4My (date of the initiation of the rifting process of oceanic type) show a succession of magmatic and tectonic events. Wide peralkaline granite intrusions – intruding older plateau basalts – found on both Nubian and Danakil plate margins as well as in central Afar mark the first event (20 to 24 My). It is followed by a trap basaltic unit itself followed by a major rhyolitic phase (Mabla formation 15 to 9 My) marked by important normal faulting and erosion, before a basaltic trap series (Dalha formation 8 to 4 My) covered this unit in discordance. These units are well exposed on the SW and SE Afar margins, in particular South of Assab and North of Tadjourah where these Dalha basalts lie unfaulted down to the Red Sea margin. The significance of these units is discussed in this chapter.