Volcano core collapse triggered by regional faulting

It has been proposed recently that large-scale caldera-like structures may result from the viscous lateral spreading of a hydrothermal system located in the central part of volcanic edifices. For this to occur, an unbutressed boundary resulting in reduced lateral stresses must exist or be created not far from the hydrothermal system. Different ways to achieve such a weak lateral confinement may be proposed. In this paper, we show that regional tectonics may provide an unconfined boundary if a normal fault lowers part of the volcanic edifice. A natural example is the island of Nuku Hiva in the Marquesas Archipelago (French Polynesia) for which geological data strongly suggest that a regional fault oriented N75°E is responsible for the present-day shape of the island, i.e. an ellipse cut in the middle by a straight side along which two nested calderas are breached. Analogue modelling shows the mechanical consistency of the model and reveals that very low velocity faulting is needed for the formation of a caldera-like structure.