NEW COALIFICATION PROFILES IN THE MOLASSE-BASIN OF WESTERN SWITZERLAND - IMPLICATIONS FOR THE THERMAL AND GEODYNAMIC EVOLUTION OF THE ALPINE-FORELAND

The coalification profiles of five oil wells (Chapelle, Eclepens, Esse rtines, Savigny and Treycovagnes) in the Western Swiss Molasse basin w ere determined using measurements of vitrinite reflectance (VR). The a nalysed sections cover essentially the Tertiary for the Chapelle and S avigny wells and the Mesozoic sediments for the other wells. VR values range from about 0.4 %Rr to 0.9 %Rr. In all wells an overall linear i ncrease in thermal maturity was observed. Coalification gradients rang e from 0.17 to 0.21 %Rr/km in the Plateau Molasse. In the Subalpine Mo lasse, a gradient of 0.07 %Rr/km was determined for the autochthonous part of the Savigny well. The isoreflectance lines in a NW-SE profile are discordant with respect to the stratigraphy. The thermal maturity of any given formation increases towards the Alpine front. The present -day VR values represent the ''frozen'' maturity level prior to Miocen e-Pleistocene uplift and erosion. Thermal modelling results suggest th at ''normal'' conditions prevailed in the area of the Plateau Molasse during the Tertiary. Average paleogeothermal gradients vary between 29 and 31 degrees C/km and the computed paleoheat flow values range from 60 to 70 mW/m(2). Lower values were reconstructed in the Subalpine Mo lasse (20 degrees C/km and 50 mW/m(2)). Modelling results demand erosi on of some 2000-2600 m in the Plateau Molasse and 4300 m in the Subalp ine Molasse. The level of heat flow (Tertiary and present-day) decreas es towards the German Molasse Basin. This trend corre lates with an in crease in flexural rigidity in the same direction. The thermal activit y and/or lithospheric thinning during the evolution of the Cenozoic ri ft system in Europe could explain to some extent these observations. H owever, the western Molasse Basin may have already inherited a weaker lithosphere due to greater Mesozoic extension and higher density of pr eexisting mechanical heterogenities (e.g. Permo-Carboniferous grabens) when compared to the German Molasse Basin.