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TheLate Pleistocene-HoloceneDilo-Dukana and Mega volcanic fields (Ririba rift, South Ethiopia) formedthrough monogenetic eruptions of limited volumes of alkaline-basalts, rich in mantle xenoliths. This activity postdated the emplacement of voluminous Pliocene subalkaline basaltic lavas related to the main rifting phase. Both volcanic fields display vent alignments that abruptly cut the rift-related features, suggestive of an emplacement after rift abandonment.We provide new petrological, geochemical and isotopic data on these still poorly studied products aimed at investigating the nature and temporal variation of the mantle source compared to the previous Pliocene activity, and the main differentiation processes.All data discriminate the younger alkaline lavas from the Pliocene products, indicating a variation of the mantle source during the two activities possibly related to the local rifting evolution. The Dilo-Dukana and Mega products overlap both in major, trace elements and radiogenic (Sr-Nd-Pb) isotopes. Well-defined trends, as displayed by incompatible trace elements and by major elements variations, indicate the prominent role of fractional crystallization driving magma differentiation, without evidence of shallow storage levels. No clear geochemical correlation is evident with respect to location, xenoliths content or eruptive style. Light-REEs roughly discriminate between the products erupted at Dilo-Dukana and Mega, pointing to possible variations in the degree of melting and/or crystallization conditions of different magma pulses in the two areas.These data indicate that the volcanic fields are fed by two different systems of deep structures, unrelated to rifting but associated to old inherited fabrics, directly transferring mantle melts to the surface.&nbsp, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

International audience; The Main Ethiopian Rift, in (East Africa), is a classic example where to analyse the interplay between tectonics and magmatic activity during rifting. This continental rift records indeed all different stages of rift evolution and is characterised by significant volcanic activity spanning from the Oligocene to present. However, despite recent improvements in the knowledge of single volcanic centres thanks to recent scientific projects (e.g., RiftVolc), volcanic activity in many rift sectors still remains poorly documented. In this work we provide new constraints on the timing, evolution and characteristics of the poorly documented volcanic activity at the southern termination of the Main Ethiopian Rift. We present new fieldvolcanological, petrographic, geochemical and geochronological data, together with and the results of a statistical analysis of the distribution of volcanic vents from remote sensing, together with petrographic, geochemical and geochronological analyses on the collected samples, for the Mega and the Dilo-Dukana volcanic fields, near the Kenya-Ethiopia border. In line with previous works, our data delineate the occurrence of two distinct groups of rocks: 1) a group of subalkaline basalts, observed only in the Dilo-Dukana volcanic field, forming a Pliocenic lava basement during Pliocene, faulted during an important rifting phase; 2) a group of alkaline basalts, characterising the Quaternary activity in both areas and sampled from Quaternary volcanic vents or from their associated lava flows, that are associated to several, mainly monogenetic volcanic centres. The products of this activity lay in discordance above the lava platform and seal the rift-related faults. Ar/Ar dating allowed to constrain the large basal lava plateau to 3.7 Ma, whereas the youngest volcanic activity characterizing the two areas dates back to 134 ka for Dilo-Dukana and Holocene age for Mega volcanic field. Interestingly, no direct relations are observed between the Pliocene, roughly N-S major boundary faults of the Ririba Rift and the NE-SW structures feeding the Quaternary volcanic activity, indicating a non-correlation between recent volcanism and rifting in the area. Conversely, our analysis suggests a major influence exerted by deep, inherited structures on the distribution of the recent volcanic vents under a complex, local stress field resulting from a major episode of reorganization of extensional structures in the region, recording the history of related to rift propagation and abandonment.

Continental rift systems commonly form by progressive growth of isolated rift segments that interact and link, eventually evolving into continuous zones of deformation. Propagation and interaction between adjacent rift segments are important features of rifting, influencing the distribution of volcanoes, seismicity and sedimentary basins. The process also controls the morphology of the rift at breakup with possible implications for the segmentation of mid-ocean ridges. However, how the punctuated, discontinuous zones of extension propagate and interact to form a continuous rift system and how the complex zones of interaction evolve remain poorly documented and understood. The Turkana depression in East Africa is an ideal place where to analyse these processes: within this NW-SE lowland, two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact giving rise a complex, >300km-wide region of faulting and volcanic activity. In this work, we present geological-structural, geochemical and geochronological data from the poorly documented Ririba rift (South Ethiopia), which is part of the complex, wide region of volcano-tectonic activity in the Turkana depression. This rift is of special interest because it is commonly accepted to represent the southernmost, youngest and less-developed sector of the Ethiopian Rift, having formed from recent southward propagation of this rift into the Turkana depression. Our data confirm the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary, when rift-related deformation ceased. Integration of seismicity data with new numerical models indicates that abandonment of laterally offset and overlapping tips of the Ethiopian and Kenyan rifts resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone which directly connects the rift sectors. These sectors link by near pure extension without forming the strike-slip transfer faults interpreted as precursors of oceanic transform faults