Rapport quadriennal 1995-1998

Seismic approaches reveal the lithospheric structure and geodynamic contexts of Piton de la Fournaise and Etna. A set of tomographic experiments constrains their internal structures with a resolution providing insight into edifice evolution and eruptive behavior. The hotspot context of La Réunion island thus appears to differ from the Hawaiian one by a reduced plate flexure, large proportion of low velocity-density in the edifice, crustal underplating under the presently active part, which may be seen as the level of transposition from primary melts into the more tholeitic products erupted. Structural heterogeneities of the preexisting plate are documented which may control magmatism in addition to slow plate/hotspot motion. Two completely different tomographies establish a high-velocity central core under the active volcano summit area which contrasts with the surroundings interpreted as eruptives or transported material and rises from the top of the oceanic plate. Another distinct high-velocity body to the East, which top has been drilled, is imaged to be rooted in the plate. So-called rift-zones of the active volcano are not marked by velocity anomalies, hence do not appear to be magmatic. High resolution location of earthquakes, heterogeneity in Vp/Vs and magnitude-number evolution can be correlated for the preeruptive magma migration to the most recent eruption. Mt Etna is not in a simple geodynamic context typical for volcanism. Marine reflection seismics reveal extension of crustal scale, which has succeeded to the compression related to the subduction at the Calabro-Sicilian arc. It is interpreted as marking a change in slab coupling, with which the development of Etna appears coeval... (D'après résumé d'auteur