Self‐potential and magnetic surveying of La Fournaise volcano (Réunion Island): Correlations with faulting, fluid circulation, and eruption

Abstract

Self‐potential (SP) and magnetic surveys have been performed on the basaltic La Fournaise volcano (Réunion Island, Indian Ocean) between 1989 and 1992. The 1992 SP mapping of the inner caldera (Enclos Fouqué) reveals high relative positive SP anomalies. They are interpreted as related to a large hydrothermal system. A large SP anomaly of 2 V is centered on the terminal cone. A horseshoe‐shaped positive SP anomaly intersects the cone. This anomaly outlines the main arcuate fracture zone (MFZ), oriented north‐south on the cone and extending to the northeast and southeast away from the summit. The zone controls the volcano dynamism. Most of the fissure eruptions occur along this volcanotectonic axis. A second mechanical weakness axis is evidenced by a positive SP anomaly lying along a 120°N trend. Both volcanoes of the island, Piton des Neiges and later La Fournaise, have been built up along it. A large structure with reverse magnetization is found at the southeastern tip of this axis. This 120°N axis is interpreted as an old regional structure associated with a lithospheric weakness zone located beneath Réunion Island. The SP anomalies are thought to be of electrokinetic origin. In the central part of the volcano, along the MFZ, and the 120°N axis, positive SP data suggest hydrothermal upwelling caused by thermal transfer from the magma chamber and the dikes of the last eruptions. Away from the summit, SP data suggest groundwater infiltrates the massif by gravity effect. Changes of the SP with time are related to volcanic activity. The SP anomaly associated with main fracture zone has been generated by the 23 eruptions occurring between 1981 and 1992. Short‐time local SP anomalies that disappear in a few months are associated with individual eruptions.