REFRACTION-SEISMIC INVESTIGATIONS OF THE NORTHERN MASSIF-CENTRAL (FRANCE)

New refraction-seismic data have been obtained in the northern French Massif Central. Seven shots were recorded simultaneously on two parall el, NNW-SSE (i.e., perpendicular to the surface structures) oriented p rofiles allowing for inline and fan interpretations. The results of a 2D-interpretation reveal a large-scale updoming of the Moho of about 2 km, compared to average western Europe, corresponding to the Cenozoic uplift of the Massif Central, Local crustal thinning of up to 20% is restricted to the Cenozoic grabens as seen at the surface. Beneath the volcanic area of Cantal, one of the largest Cenozoic European shield volcanoes, which was active in Miocene to recent times, significant cr ustal thickening of 2-3 km is observed. Velocities in the upper mantle are generally in the range of 7.9-8.1 km/s beneath the Limagne graben and surrounding undisturbed areas. Underneath the volcanic area of Ce zallier, however, velocity is reduced to about 7.7 km/s, corresponding to the area of crustal thickening. Therefore, the low velocity in thi s zone is explained by the remainders of cooling magma chambers and he ating by ascending hot plume material. The Cenozoic tectonic evolution of the Massif Central has affected crust and upper mantle in differen t ways. Oligocene rifting corresponds well to local crustal thinning, but no correlation is detected to velocities of the upper mantle. In c ontrast, the volcanic event occurring since Miocene times is related t o upper mantle velocity reduction and crustal thickening due to underp lating.