Groupe de Géomagnétisme, Institut de Géologie, Neuchâtel, Switzerland

An Audio Magnetotelluric (AMT) profile has been carried out across the Plaine des Sables, a former caldera of the active Piton de la Fournaise volcano, Reunion Island. Located in the Western Indian Ocean, between the Mascarene and Madagascar basins, this basaltic shield volcano originates from the activity of a hot spot. Our aim was to determine the internal structure of the volcano, in particular the shallow electrical properties of an area extending between the old and the new caldera rims. Although several teams had already conducted AMT work in this region a few years ago, there was a need for more a detailed, in depth survey. Our final model displays a noticeable slope of the Plaine des Sables basement oriented toward the present Fournaise summit. This slope is interpreted as resulting from successive landslides toward the ocean. We conclude that this dipping, electrically good conducting layer, probably belongs to the flat layering of an older caldera.

The electromagnetic induction pattern in the Eastern Alps is characterised by a (continuous) large-scale zone on which the real parts of the induction arrows show anomalous directional behaviour. This zone extends from the Penninic Domain of Eastern Switzerland (Graubünden) probably into the Carpathian ranges. A coarse mesh of a Magnetotelluric (MT) and Geomagnetic Deep Sounding (GDS) station in the Alps of Graubünden and Valais (Western Switzerland) indicates that this electromagnetic anomaly is restricted to the Mesozoic sediments of the North Penninic Bündnerschiefer-facies that begins in Eastern Switzerland and extends towards the east beneath Austroalpine, South Penninic and Southalpine units. Striking similarities in position and arrangement between this zone and the magnetic signature in the Eastern Alps are found. The analysis of the GDS data with the method of the Hypothetical Event Analysis (HEA) shows that current channelling affects the electromagnetic fields in this zone and causes the anomalous direction of induction arrows. Based on the combined interpretation of GDS data from the Eastern Alps and West Hungary together with our recent data from Switzerland, the following geological implications are discussed: i) a spatial decoupling of induction processes from the upper to the lower crust; ii) a lower crustal conductive structure caused by the indentation of the Northern Adriatic promontory or terrane; iii) the eastward continuation of the Bündnerschiefer-facies at least to the tectonic window of Rechnitz.