**2. Geologic background**

Volcanic activity commenced around 45 Ma in southern Ethiopia [7], resulting in volumetrically significant basaltic flows and associated rhyolites. Nevertheless, the peak of magmatism had occurred c. ~30 Ma ago, resulting from the impinging Afar mantle plume at the base of the Ethiopian lithosphere and leading to flooding basalt eruptions in Ethiopia and Yemen [8]. The Yemen plateau basalts were united to their Ethiopian counterparts prior to the opening of the Red Sea basin. At ~25 Ma continental rifting commenced in the southern Red Sea [9]. In Southern Ethiopia, extension began ~18 Ma ago [10] and was accompanied by basaltic magmatism, active for about seven to eight million years [7]. The southern Ethiopian rift propagated northward, reaching the present central MER ~14 Ma ago and ultimately joining the southern Red Sea rift ~11 Ma ago [11]. Contemporaneously to the connection between the main Ethiopian and Red Sea rifts, a flood basalt event occurred in this area. Beginning in the late Miocene and continuing throughout the Pliocene, silicic volcanic centres emerged from the rift floor [6]. Progressive weakening of the lithosphere in Afar, associated with heating and the thermomechanical erosion of the lower crust generated by the Afar mantle plume, resulted in the onset of oceanic rifting at 5.3 Ma [9]. Oceanic rifting is still active in Afar, whereas it has not commenced in the MER yet [11].

The Pliocene-Pleistocene boundary was marked by a change in the stress field, giving rise to oblique rifting [12]. Since then, an extension has been localized in narrow (50 km long, maximum 20 km wide) en-echelon arranged segments on the rift floor [6], with a system of bounding faults that are referred to as the Wonji Fault Belt [12]. Moreover, these segments have been the locus of volcanic activity throughout the Quaternary and are thus referred to as magmatic segments by [6]. Volcanic activity associated with the magmatic segment was initially characterized by large volumes of felsic lavas. When these faults reached the upper mantle in recent times, basaltic volcanism commenced [12]. At present, volcanic activity within the magmatic segment is dominated by fissural basalt eruptions [1, 6].
