NEW GEOLOGICAL AND HYDROGEOLOGICAL IMPLICATIONS OF THE RESISTIVITY DISTRIBUTION INFERRED FROM AUDIOMAGNETOTELLURICS OVER LA-FOURNAISE YOUNGSHIELD-VOLCANO (REUNION ISLAND)

An audiomagnetotelluric survey has been performed along an inactive fl ank of La Fournaise volcano massif in Reunion island, to study the sub surface resistivity structure. One-dimensional modelling of the AMT da ta at each site revealed an extensive low-resistivity (less than 10 Om ega m) zone at a few hundred meters below the surface. The significanc e of this unexpected conductive substratum is discussed in relation wi th the proposed impact of volcano-tectonic processes (caldera and land slides collapses), and a new interpretation of the geological structur e is proposed. Moreover, it is likely that these conductors are poorly permeable argilaceous materials; then they coincide with a limit in p ermeability and determine groundwater behavior. (C) 1997 Elsevier Scie nce B.V.