The Afar Stratoids
Continental extension thins the lithosphere ultimately leading to its breakup and the formation of a mid-ocean ridge where new crust is created. As plates diverge the underlying asthenosphere upwells and melts due to decompression. The buoyant magma migrates upward, intruding the plate and erupting to the surface, supplementing the mechanical extension that occurs by faulting and by ductile stretching and thinning [Bastow and Keir, 2011]. The early-stage development of a continental rift is thought to be controlled by growth of extensional fault systems while the rise of magma through the lithosphere marks the shift to highly localised intrusion at the incipient plate boundary hence playing a major role in continental rifting (Fig. 1) [Muirhead et al., 2016].
The divergence of the Nubian, Arabian and Somalian plates during the past 30 Myrs created Afar (Fig. 1a)[Barberi and Varet, 1977]. During the initial stage of rifting, normal faulting was concentrated along the border faults with formation of rift basins collecting synrift sediments. Ductile thinning of deeper parts of the lithosphere is thought to have occurred beneath the full width of the rift. Later (the last ~4 Ma) thinning of the crust and deeper lithosphere shifted to narrow axial magmatic segments, which mark the axis of the currently active plate boundary [Manighetti et al., 1998].
The transition from border fault to rift floor strain accommodation represents a critical process in the evolution of the Afar rift as it corresponds to the emplacement of a large igneous province, named the ‘stratoid’ series (4-0.5 Ma) [Lahitte et al., 2003; Acocella, 2010]. The stratoids consist mainly of basaltic effusive lavas with some localized rhyolites [Barberi et al., 1980]. The stratoids cover an area of over 50,000 km2, about two third of the Afar depression. Individual basalt flows are 1-6 m thick and the whole series is up to 1500 m thick. Acocella  estimated an eruption rate of approximately 5000 km3 per Ma, one of the highest rates for a plate divergent margin. The stratoids lie non-conformably on the Dalha Series (igneous rocks, 8-6 Ma) indicating a period of erosion and lowered magmatic activity between the two series [Varet, 1975].
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Barberi, F., L. Civetta, and J. Varet (1980), Sr Isotopic Composition of Afar Volcanics and Its Implication for Mantle Evolution, Earth Planet. Sc. Lett., 50(1), 247-259.
Barberi, F., G. Ferrara, R. Santacroce, M. Treuil, and J. Varet (1975), Transitional Basalt-Pantellerite Sequence of Fractional Crystallization, Boina Center (Afar Rift, Ethiopia), J Petrol, 16(1), 22-56.
Barberi, F., and J. Varet (1977), Volcanism of Afar - Small-Scale Plate Tectonics Implications, Geol. Soc. Am. Bull., 88(9), 1251-1266.
Hayward, N. J., and C. J. Ebinger (1996), Variations in the along-axis segmentation of the Afar Rift system, Tectonics, 15(2), 244-257.
Manighetti, I., P. Tapponnier, P. Y. Gillot, E. Jacques, V. Courtillot, R. Armijo, J. C. Ruegg, and G. King (1998), Propagation of rifting along the Arabia-Somalia plate boundary: Into Afar, J. Geophys. Res.,
Muirhead, J. D., S. A. Kattenhorn, H. Lee, S. Mana, B. D. Turrin, T. P. Fischer, G. Kianji, E. Dindi, and D. S. Stamps (2016), Evolution of upper crustal faulting assisted by magmatic volatile release during
early-stage continental rift development in the East African Rift, Geosphere, 12(6), 1670-1700.
Varet, J. (1975), Geological map of Central and Southern Afar, CNRS-CNR.
Geological map of the Afar region showing fault orientation and distribution of the magmatic segments. After Barberi & Varet, 1977; Hayward & Ebinger, 1996;