COALIFICATION, SHALE DIAGENESIS AND THERMAL MODELING IN THE ALPINE FORELAND BASIN - THE WESTERN MOLASSE BASIN (SWITZERLAND FRANCE)

The analysis of coalification and shale diagenesis in Oligocene and Lower Miocene sediments in the western part of the Molasse basin (Western Switzerland and Haute-Savoie, France) shows a close relationship between the very low-temperature metamorphism in the foreland and the tectonic history in the orogenic belt. The degree of thermal maturation generally increases towards the alpine front. Isomaturity lines, however, run slightly oblique to today's alpine front. This is probably due to the tectonic burying under the alpine nappes in Early Miocene time. Second order variations of the coalification pattern are explained as resulting from a regional discharge of warm fluids controlled by tectonic structures.