Fault reactivation and growth at rift-related calderas.

• 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]