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• Elongated calderas were often considered proxy for the regional extension. • We run circular caldera collapse analogue models afterward elongated by extension. • We prove that extension justifies only 13% of elongation for calderas in the MER. • Only specific sectors of caldera faults are capable of reactivation under extension. • Caldera structures influence the architecture of newly forming regional faults. Caldera collapses are paramount volcano-tectonic features because they form during hazardous explosive volcanic eruptions, they are ideal sites for geothermal development and mineral resources exploitation, and also because they preserve the evidence of the interaction between caldera magmatism and the regional tectonic processes. Despite this, many aspects of the caldera collapse process remain unclear, particularly concerning the interaction between caldera and tectonic related fault systems. We therefore used analogue models 1) to quantify the effect of regional strain on caldera elongation in extensional settings, such as the Main Ethiopian and the Kenya rifts, 2) to describe the effect of regional strain on caldera structures and, vice-versa 3) to document, for the first time, "the other side of the coin", that is how caldera structures affect the formation of newly forming regional extensional faults. Our models showed that tectonic extension only explains a small proportion (e.g. 13% for the Main Ethiopian Rift) of the elongation of most rift calderas. Furthermore, we showed how specific segments of caldera faults may accommodate regional extension by reactivating, therefore precluding caldera elongation. Finally, we showed how the presence of caldera structures may influence the geometry of newly forming regional normal faults, that display a marked curvature, "faking" caldera ring faults. We have suggested that these "fake" curved caldera ring faults may lead to incorrect estimations of caldera elongation in nature. In addition, such faults may also mislead geothermal fluid exploration, as they are likely disconnected from the caldera structures or the caldera plumbing system, and less likely the locus of hydrothermal fluid flow. [Display omitted] [ABSTRACT FROM AUTHOR]

The relationships between volcanic activity and tectonics at the southernmost termination of the Main Ethiopian Rift (MER), East Africa, still represent a debated problem in the MER evolution. New constraints on the timing, evolution and characteristics of the poorly documented volcanic activity of the Dilo and Mega volcanic fields (VF), near the Kenya-Ethiopia border are here presented and discussed. The new data delineate the occurrence of two distinct groups of volcanic rocks: 1) Pliocene subalkaline basalts, observed only in the Dilo VF, forming a lava basement faulted during a significant rifting phase; 2) Quaternary alkaline basalts, occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift-related faults. 40Ar/39Ar dating constrains the emplacement time of the large basal lava plateau to ~3.7 Ma, whereas the youngest volcanic activity characterising the two areas dates back to 134 ka (Dilo VF) to as recent as the Holocene (Mega VF). Volcanic activity developed along tectonic lineaments independent from those of the rift. No direct relations are observed between the Pliocene, roughly N-S-trending major boundary faults of the Ririba rift and the NE-SW-oriented structural trend characteristic of the Quaternary volcanic activity. We speculate that this change in structural trend may be the expression of (1) inherited crustal structures affecting the distribution of the recent volcanic vents, and (2) a local stress field controlled by differences in crustal thickness, following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment. • Multidisciplinary study of the Dilo and Mega volcanic fields in South Ethiopia • Two phases of volcanism: Pliocene tholeiitic basalts and Quaternary alkaline basalts • Quaternary volcanism is unrelated to rifting and rift-related structures. • Inherited fabrics and local stress field influence volcanism after rift abandonment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]