YO (1998-2000): TRACK

• Reseau sismologique et géodésique français (RESIF), France

Seismic velocity tomography, geodynamic modelling and geochemical characterictics of the tertiary and quaternary volcanic activity provide an evidence for the ascent of a mantle plume beneath the MC during Tertiary. To characterize the pervasive upper mantle structures and the upper mantle flow beneath the French Massif Central, we deployed 25 three component stations (short period, intermediate and broad-band) during the period 1998-1999 in the southern Massif central, from the Clermont Ferrand volcanic area to the Mediterranean sea. 3-D tomographic models have been reconstructed from the volcanic areas to the Mediterranean sea with main objectives of detecting the presence of the mantle plume but also of mapping the lateral variations of the lithosphere thickness and of characterizing the deep structures of the mantle beneath the southern Massif Central. Teleseismic shear waves (SKS, SKKS and PKS) were also used to quantify the upper mantle anisotropy by measuring the splitting parameters: the fast polarization directions and the delay times. Delay times generally smaller than 1 s are observed at most of the sites. The azimuths of the fast split shear waves trend homogeneously NW-SE in the southern part of the Massif central. This anisotropy pattern clearly differs from the Pyrenean anisotropy further south which trends homogeneously N100°E and which displays statistically higher delay times. This regional anisotropy variation does not favor the present-day motion of the Eurasian plate as the primary source of mantle deformation. Instead, this homogeneous trend of mantle flow could be correlated to either frozen Hercynian structures or more likely to the tertiary extension direction. Several arguments suggest that the rotation of the Corsica block, the roll-back to the SE of the Calabrian subduction and the Tertiary extension of the western Mediterranean during the convergence of Africa and Europe may have affected the pervasive upper mantle structures in the southern Massif Central.

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