Stress-strain relationships at elongated calderas in extensional settings: what analogue models say

Collapsed calderas are circular to elongated large depressions originating from the subsidence induced by depletion and/or migration of magma from a shallow or deep reservoir during eruptions. Despite being distributed in all tectonic settings, they are particularly important in extensional settings where are often associated with rifting processes, e.g., the East African Rift System. Therefore, their structural architecture can be strongly perturbed by extensional faults associated with regional extension or related to earlier stages of caldera formation. Calderas often bear an elongated shape in plain view, and have been considered valuable proxies for the regional stress (e.g., Nakamura, 1977) and regional strain (e.g. Casey et al., 2006). Moreover, other authors have related the elongated calderas to the influence of preexisting structures reactivated during extension (Acocella et al., 2003). We therefore aim to investigate the mechanical interactions between collapsed calderas and regional extension leading to elongated edifices. Analogue models of caldera collapse were performed by placing a circular magma chamber (simulated with poly-glycerine) placed below a sand-mixture package. We induced the collapse by draining out the analogue magma from the base, reproducing the classical fault architecture observed at many collapsed calderas (i.e., early inner outward-dipping reverse faults and late outer inward-dipping normal fault). Once completed, the collapsed depression was stretched such that normal faulting produced caldera elongation and segmentation. Finally, we compared the elongation and the structural pattern deriving from the interacting caldera-related and rift-related structures with natural examples from the East African Rift System. Our results suggest that different interacting factors may contribute to the development of elongated calderas, thereby questioning whether elongated calderas can be considered as a fully reliable proxy for the regional strain.

Acocella, V., Korme, T., Salvini, F., and Funiciello, R. (2003). Elliptic calderas in the Ethiopian Rift: control of pre-existing structures. J. Volcanol. Geotherm. Res., 119, 189–203.

Casey, M., Ebinger, C., Keir, D., Gloaguen, R., and Mohamed, F. (2006). Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments. Geol. Soc. Lond. Spec. Publ., 259(1), 143–163.

Nakamura, K., (1977). Volcanoes as possible indicators of tectonic stress orientation— principle and proposal. Journal of Volcanology and Geothermal Research 2, 1–16