MASSIF-CENTRAL (FRANCE) - NEW CONSTRAINTS ON THE GEODYNAMICAL EVOLUTION FROM TELESEISMIC TOMOGRAPHY

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by dist inct volcanic episodes, the most recent dated at 20 Ma to 4000 years B P. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment. During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent s tations. Inversion of P-wave traveltime residuals of teleseismic event s recorded by this dense array yielded a detailed image of the 3-D vel ocity structure beneath the Massif Central down to 180 km depth. The u pper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channel s of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from He rcynian time and reactivated during Oligocene times. The teleseismic i nversion model yields no indication of a low-velocity zone in the mant le associated with the graben structures proper. The observation of sm aller velocity perturbations and a change in the shape of the velocity pattern in the 60-100 km depth range indicates a smooth transition fr om the lithosphere to the asthenosphere, thus giving an idea of the li thosphere thickness. A broad volume of low velocities having a diamete r of about 200 km from 100 km depth to the bottom of the model is pres ent beneath the Massif Central. This body is likely to be the source r esponsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.