Structure and Thermal Evolution of the Tectonic Front of the Western Alps

Deville, Eric, Alain Mascle, William Sassi, Institut Francais du Petrole, Rueil-Malmaison, France

Several seismic lines shot in the foreland fold-&-thrust belt of the western Alps have evi­denced that the décollement of the thrust system is located in the Triassic or Liassic forma­tions and that changes occurred laterally in the depth of the décollement, the tkickness of the thrust sheets and the timing of the deformations. Geochemical analyses have evidenced that contrasting thermal evolutions developed laterally in relatively close areas of the same thrust belt. Analytical results show that the Mesozoic rocks have undergone a wide range of temperatures up to 250°C locally. The higher temperature values being associated to the tec­tonic setting of early widely allochthonous nappes that have covered obliquely the northern part of the area, prior to the thrust tectonics of the Subalpine chains. Folding and offsets of the isomaturity surfaces have been observed, which implies that, in some parts of the thrust belt, the thermal peak has been reached before the fold-and-thrust deformation. Erosion rates vary drastically along the thrust front. Extensive erosion (probably more than 4 km of sediments from the Tertiary series and the Prealps nappes) occurred along the Bornes tran­sect. Away from the area of influence of the Prealps nappes, numerical modeling has evi­denced differences concerning the timing of maturation of the source rocks. According to the area considered, maturation was attained either early, before thrust tectonics (most of the Vercors area), or during the deposition of syntectonic flexural series preceding thrust tectonics (Molasse Basin), or else during thrust tectonics (most of the Chartreuse area).