The Cenozoic magmatism of East Africa: Part IV – The terminal stages of rifting preserved in the Northern East African Rift System

Abstract

The Afar Depression is a broad region of subdued topography wherein the Red Sea, Gulf of Aden, and East African Rift System converge at a triple junction. This mature sector of the East African Rift System is characterized by attenuated continental lithosphere, with the implication that lavas erupted here can provide insights into the composition of sub-lithospheric magma reservoirs. Magmas in Afar and Yemen largely have geochemical characteristics consistent with a mixture of melts derived from the Afar plume, and from a hybrid endmember comprised of the depleted upper mantle and Pan-African lithosphere. Lavas erupted in the Erta ‘Ale range do not comport with this model and instead appear to have been derived from melting of a lithospheric mantle metasome that is likely related to prior Afar plume-lithosphere interaction. The extant data show no significant change in the reservoirs contributing to magmatism during the development of the rift in this region, however the surface distribution of magmatism reveals a relationship between rift evolution and magmatism. In Yemen, magmatic activity is limited to the regional Mid Miocene Resurgence event at ca. 10 Ma, and small-volume alkaline volcanism along the evolving Gulf of Aden margin from 6 Ma to present. In contrast, a much more extensive magmatic record is preserved in Afar. Coincident with the development of the rift margin, initial magmatism in Afar during this period (20–10 Ma) was dominantly in the form of large silicic eruptions, with lesser volumes of crustally contaminated Mablas basalts. The development of rift-marginal basins at ca. 7 Ma was coincident with the widespread eruption of fissural basalt of the Dalha Series. The Dalhoid Series represents a pulse of magmatism from ca. 5.6 to 3.9 Ma, which correlates with the initiation of sedimentation within the Sagantole Formation. The initiation of the lower member of the Afar Stratoid Series at ca. 3.9 Ma is broadly contemporaneous with sedimentation within the Hadar Formation, while the upper member (2.6 Ma) correlates with the Busidima Formation. The Gulf Basalt Series (2.8–0.3 Ma) temporally overlaps the upper member of the Afar Stratoid Series, but represents the first manifestations of the coincidence of strain localization and magmatism within rift axial grabens. The subsequent Axial Series (ca. 0.7 Ma to present), manifests as basaltic cones and central silicic volcanoes within three zones aligned with the Main Ethiopian Rift, Red Sea Rift, and Gulf of Aden. The evolution in Afar from initial large-scale silicic volcanic events to basin-wide fissural basaltic flows, to modern zones of focused magmatism, broadly parallels the localization of strain during the same period. This intimate relationship between the mechanism of strain accommodation and surface manifestation of magmatism in Afar has the implication that the modern zones of focused magmatism and strain are the pre-cursors of oceanic spreading centers. While Afar is clearly at an advanced stage in the rifting process, whether these modern zones are the direct precursors to oceanic spreading centers remains unresolved.