Structure and Dynamics of Plumbing Systems of Oceanic Shield Volcanoes: An Example from Réunion Island

This study focuses on the volcano-tectonic evolution of the two main volcanoes that constitute Reunion Island. This island, located in the south-western part of the Indian Ocean, hosts two main shield volcanoes: the dormant Piton des Neiges, and the active Piton de la Fournaise. Due to the tropical climate of the island, combined with the presence of volcano-tectonic structures (caldera, landslides scarps), it is deeply eroded. This study is divided in two parts. The first part deals with the intrusive system of the eroded Piton des Neiges, the second with the relationship between the active plumbing system of Piton de la Fournaise and the deformation of the edifice.Piton des Neiges has three main rift zones, trending N30°, N120° and N160°, located in the western half of the edifice. The field data show that the rift zones evolved under different stress fields. Magma-generated stress field played a major role close to the central conduit, but its influence decreases with distance. In the N120° and N160° rift zones, the maximum compressive principal stress is vertical and controls the injection of magma. Here it is proposed that this stress field results in the repeated injection of magma along a N120° direction, that of crustal structures. By contrast, in the N30° rift zone, magma injection has been partly influenced by surface stresses leading to the injection of sills and shallow-dipping sheets. The field studies suggest that the many sill clusters in the N30° rift zone and in the Salazie cirque, are related to tough plastic breccia that acted as a mechanical barriers to dyke propagation. Despite these differences, blade-like dyke injections were common in all the three rift zones, with the denser magmas being injected at greater depths and the lighter magmas at shallower depths. The field data also indicate that the N120° rift zone generated mechanical instabilities resulting in landslides, in agreement with the common, debris avalanche deposits observed in the Cilaos and Salazie cirques.Similar dyke-injection mechanisms were previously proposed for Piton de la Fournaise. Intrusive events in this volcano, however, are commonly accompanied by an asymmetrical pattern of ground deformation. This pattern was previously interpreted as reflecting the dip of the intrusions. However, the field results presented here suggest that the rift zones are close in style to the N120° and N160° rift zones of Piton des Neiges. The results also suggest that the deformation asymmetry is related both to a curved magma path and to the presence of stiff heterogeneities in the edifice, inherited from the past eruptive centre. It is also shown that the steep flanks of the summit cone are related to (1) the past eruptive history and (2) the concentration of deformation on the eastern side of the summit cone. This preferential deformation of the eastern side of the cone also explains the presence of tectonic fractures observed.