Tectono-magmatic evolution of Central Afar since 5 Ma: late syn-rift and break-up processes

Central Afar (Ethiopia) is an active example of the final stages of continental rifting. The Stratoid magmatic series (ages between 5 and 1 Ma) were emplaced in a large fissural volcanic province, following an episode of thinning by normal faulting and detachment at 5-6 Ma (Stab et al., 2016). The Gulf Basalt series (0.9-0.4 Ma) later emplaced in more restricted areas attesting for the localisation of the deformation. Current active magmatic axes are even more localized and the most recent lava geochemistry attests for very little crustal contamination (Ayalew et al., 2018) along with recent dyking episodes. This suggests that Central Afar is currently in a late syn-rift stage, possibly close to continental break-up with divergence accommodated by magmatic accretion. The detailed study of the tectono-magmatic evolution of the region will allow us to better constrain the break-up processes active during volcanic margin formation.

Our new mapping of Central Afar has consisted in defining Stratoid sub-series to better follow the interplay between magmatism and deformation during continent-ocean transition. This map is supported by field data, new mapping using satellite multispectral images, and new Ar/Ar dating. We defined three new units: the old Stratoid (5-3 Ma), the intermediate (3-2 Ma) and the young Stratoid (2-1 Ma). This mapping shows that the localisation processes started during the old Stratoid emplacement, which we interpret as an equivalent of Seaward Dipping Reflectors described in magma-rich margins. The detailed mapping of the normal faults in Central Afar is used to quantify the amount of deformation through space and time and discuss the mechanism of divergence accommodation (dyke vs normal faults) in order to track the timing and controlling parameters of the eventual switch from rifting to break-up processes. In the next future, we will study the chemical signature of each series to determine the evolution of magma sources and conditions of melting during the Stratoid phases we defined. Moreover, new dates will provide much needed data on this volcanic series's continuous vs discrete (with pulses) nature.