Contrasting thermal evolution of thrust systems: An analytical and modeling approach in the front of the western Alps

Abstract

In foreland fold and thrust belts, the study of the maturity of the organic matter and kinematic and thermal modeling techniques is useful in unraveling the thermal history and maturation of source rocks. The case study presented here is the classical example, the western Alps. This work evidences that contrasting thermal evolutions can develop in relatively closely adjacent areas of the same thrust belt. Analytical results show that the Mesozoic rocks have undergone a wide range of temperatures up to 250C locally. The higher temperature values are associated with the tectonic emplacement of early, widespread allochthonous nappes that were emplaced obliquely in the northern part of the area, prior to the thrust tectonics of the Subalpine chains. Folding and offsets of the isomaturity surfaces are observed, which implies that in some parts of the thrust belt, the thermal peak was reached before fold and thrust deformation. Erosion rates vary greatly along the thrust front. Extensive erosion (probably more than 4 km [2.5 mi] of sediments from the Tertiary and the Prealpine nappes) occurred in the north of the area studied along the Bornes transect. Away from the area of influence of the Prealpine nappes, numerical modeling shows differences in 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 the syntectonic flexural series preceding thrust tectonics (Molasse Basin), or else, during thrust tectonics (most of the Chartreuse area).