Refraction-seismic investigations of the northern Massif Central (France)

Abstract

New refraction-seismic data have been obtained in the northern French Massif Central. Seven shots were recorded simultaneously on two parallel, NNW-SSE (i.e., perpendicular to the surface structures) oriented profiles 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 crustal 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 Cézallier, however, velocity is reduced to about 7.7 km/s, corresponding to the area of crustal thickening. Therefore, the low velocity in this zone is explained by the remainders of cooling magma chambers and heating by ascending hot plume material. The Cenozoic tectonic evolution of the Massif Central has affected crust and upper mantle in different ways. Oligocene rifting corresponds well to local crustal thinning, but no correlation is detected to velocities of the upper mantle. In contrast, the volanic event occurring since Miocene times is related to upper mantle velocity reduction and crustal thickening due to underplating.