The interplay between collapse structures,hydrothermal systems,and magma intrusions: the case of the central area of Piton de la Fournaise volcano |
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Authors: | Jean-François Lénat Patrick Bachèlery |
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Institution: | 1.Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans,Clermont-Ferrand,France;2.CNRS, UMR 6524, LMV,Clermont-Ferrand,France;3.IRD, R 163, LMV,Clermont-Ferrand,France;4.Laboratoire GéoSciences Réunion, Université de La Réunion, Institut de Physique du Globe de Paris, CNRS, UMR 7154-Géologie des Systèmes Volcaniques,Saint-Denis Cedex 9,France;5.Laboratoire Géologie des Systèmes Volcaniques, Institut de Physique du Globe de Paris, CNRS, UMR 7154,Paris,France |
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Abstract: | We explore the possible relationships between a structural heterogeneity, the hydrothermal system, and the intrusive activity
at Piton de la Fournaise volcano. Geological and geophysical data show that as the result of repeated collapses (the last
one in 2007), a cylinder of faulted, fractured, and crumbled rocks must exist between the surface and the top of a magma reservoir
at about sea level. This structure constitutes a major geological heterogeneity. An obvious spatial correlation exists between
this column of fractured and brecciated rock and the location of (1) most of the seismic activity, (2) a low-resistivity dome,
(3) a huge self-potential anomaly, (4) thermal evidence of hydrothermal activity, and (5) the root of magma intrusions. The
dominant factors that make this structural heterogeneity a trap for the activity are probably its higher permeability and
its weaker mechanical strength. Evidence exists for the presence of an active hydrothermal system confined in this permeable
zone. The long-term stability of the activated zone above sea level and the similarity of the pre-eruptive crises, in spite
of the inferred large perturbation of the magmatic system in 1998, suggest a common triggering mechanism for all the eruptions
since at least the first data recorded by the observatory in 1980. This mechanism can be purely magmatic, resulting from the
pressurization of a reservoir, but we also propose that the hydrothermal system may play a role in the development of volcanic
instabilities. A qualitative model is proposed to explain the triggering of magma intrusions by hydrothermal processes, and
its speculative aspects are discussed. This work represents a first attempt to integrate the structural and dynamic information
in a unified framework at Piton de la Fournaise. |
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