The Molasse fault zone in Lake Geneva, South-West Switzerland, From high-resolution seismic reflection data

The Oligocene-Miocene Molasse basin along the northern rim of the Swiss Alps is affected by a major fault zone that separates Plateau Molasse to the north from Subalpine Molasse to the south. As a result of alpine compression, thrusts along this zone brought up Subalpine Molasse rocks by an amount of up to 1000 m. On the northern shore of Lake Geneva, near the City of Lausanne, the fault zone is well defined (Fig. 1). However, its extension in the lake and on its southern shore is not well known. Geology on both sides of the lake, however, indicates that major changes in the distribution of Molasse units occur in this region. Locating the Molasse fault zone across Lake Geneva would help to better understand geology and would provide important constraints on tectonic models of the Alps. In order to study the extension of the Molasse fault zone across Lake Geneva, we conducted a 2-D high-resolution multi-channel seismic reflection survey. Molasse units in the lake are overlain by Quaternary sediments up to 300-400 m thick (Vernet et al. 1974). Our data imaged these sediments and detected several faults in the Molasse units.