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Abstract The East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain. 

As continents break apart, the dominant mechanism of extension transitions from faulting and lithospheric stretching to magma intrusion and oceanic crust formation in a new ocean basin. A common feature of this evolution preserved at magmatic rifted margins worldwide are voluminous lava flows that erupted close to sea level during the final stages of development of the continent-ocean transition (COT). The mechanisms responsible for the generation of the melts that contribute to these voluminous flows, the so-called seaward dipping reflectors (SDR), and their significance in the context of COT development, are relatively poorly understood; they lie deep below post-rift strata along submarine rifted margins where they cannot be studied directly. Extensive coring of the Afar Stratoid Series - an areally-extensive sequence of Pliocene-aged basalts and intercalated sediments that lie atop the developing COT in the sub-aerial Afar Depression, northern Ethiopia - offers fresh scope to address this issue. We present a numerical model simulating the formation of enriched metasomes within the continental lithospheric mantle by the passage of magmas resembling modern axial basalts. Thermal destabilization of the metasome, caused by plate stretching, initiates melt formation within the metasome. These melts, when mixed with a depleted lithospheric mantle component, closely match the range of compositions of the Afar Stratoid Series lavas in this study. Metasomatic re-enrichment and subsequent melting of the lithospheric mantle during the COT may contribute to further plate thinning. These results demonstrate a novel mechanism by which large-volume flows may be erupted during the COT. 

Strain localization is central to the transition between continental rifting and seafloor spreading. In the East African Rift System (EARS), there is an emerging understanding of the link between extensional pulses and magmatic episodes. We investigate modern magmatism located within the Turkana Depression and its relationship with the distribution of extensional strain. We probe the source of magmatism at South Island volcano using bulk rock, melt inclusion and olivine geochemical data and find that the magmas are derived from sub-lithospheric sources equivalent to magmatism in the more mature sectors of the rift. The depth extent of the magmatic plumbing system of South Island is constrained using vapour saturation pressures derived from bubble-corrected H 2 O and CO 2 concentrations in melt inclusions and the results indicate a magmatic system resembling modern axial volcanic systems observed in other parts of the EARS. The zone of focused axial magmatism that South Island represents has evolved contemporaneously with a region of focused axial faulting that has accommodated the majority of regional Holocene extension and subsidence at this latitude. We conclude that at South Island there has been a migration of magmatic and tectonic strain towards the modern zone of focused intrusion along this portion of the EARS. Supplementary material: S1–S2 image files, data table files S3–S6 and caption file S7 are available at https://doi.org/10.6084/m9.figshare.c.6026627