SELF-POTENTIAL ANOMALIES AND CONVECTIVE S YSTEMS ON LA-FOURNAISE VOLCANO (REUNION ISLAND, FRANCE)

Piton de la Fournaise, 2,630 m high, is a basaltic volcano located in the Indian Ocean. The Self-Potential map of Enclos Fouque was determin ed in 1992 and 1993 with about 70 km of profiles. The cone is the seat of a huge positive anomaly whose amplitude is more than 1,850 mV. Dur ing the last 10 years an SP anomaly has developed on a N 130-degrees d irection indicating a new fracture zone. A large positive horseshoe-sh aped anomaly is superimposed on the huge anomaly associated with the c one. The shape and the amplitude of the SP anomalies suggest that the source, the magmatic complex, is at least at 2 km depth beneath the su mmit. Inside the cone hydrothermal ascendant circulations are trapped by the fractures due to dyke migration and are progressively channelle d along radial and concentric faults to the summit. The horseshoe-shap ed positive anomaly is the electric signature of reheated fluid circul ations escaping from Enclos Fouque to the Indian Ocean through the mai n fracture zone. The profile around Bory and Dolomieu craters has been made three times embracing one summital eruption. A local 1,250 mV, 1 50 m wide, anomaly has been observed in the axis of dry fissures and d isappeared 2 months later.