SELF-POTENTIAL AND MAGNETIC SURVEYING OF LA FOURNAISE VOLCANO (REUNION ISLAND) - CORRELATIONS WITH FAULTING, FLUID CIRCULATION, AND ERUPTION

Self-potential (SP) and magnetic surveys have been performed on the ba saltic La Fournaise volcano (Reunion Island, Indian Ocean) between 198 9 and 1992. The 1992 SP mapping of the inner caldera (Enclos Fouque) r eveals high relative positive SP anomalies. They are interpreted as re lated to a large hydrothermal system. A large SP anomaly of 2 V is cen tered on the terminal cone. A horseshoe-shaped positive SP anomaly int ersects the cone. This anomaly outlines the main arcuate fracture zone (MFZ), oriented north-south on the cone and extending to the northeas t and southeast away from the summit. The zone controls the volcano dy namism. 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 degrees N trend. Both volcanoes of the isla nd, Piton des Neiges and later La Fournaise, have been built up along it. A large structure with reverse magnetization is found at the south eastern tip of this axis. This 120 degrees N axis is interpreted as an old regional structure associated with a lithospheric weakness zone l ocated beneath Reunion Island. The SP anomalies are thought to be of e lectrokinetic origin. In the central part of the volcano, along the MF Z, and the 120 degrees N axis, positive SP data suggest hydrothermal u pwelling caused by thermal transfer from the magma chamber and the dik es of the last eruptions. Away from the summit, SP data suggest ground water 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 b etween 1981 and 1992. Short-time local SP anomalies that disappear in a few months are associated with individual eruptions.