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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. 

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 

Abstract The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centered on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising the flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly-alkaline small-volume Akobo Basalts (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric-mantle metasomes, (2) the primitive and spatially restricted Amaro Basalts (45.2–39.58 Ma) representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly, and (3) the spatially extensive Gamo-Makonnen magmatic unit (38-28 Ma) representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere resulting in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 m.y. preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia associated with the East African Rift. Supplementary material at https://doi.org/10.6084/m9.figshare.c.5557626