STRUCTURAL CONFIGURATION OF THE WESTERN SWISS MOLASSE BASIN AS DEFINED BY REFLECTION SEISMIC DATA

In the areas of the cantons Geneva and Vaud, reflection seismic data c alibrated by wells and outcrops give a good structural definition of t he Molasse Basin between the Jura Mountains and the Alpine front, exce pt for the Lake Geneva where seismic resolution is too poor. Geologica l cross-sections based on representative depth-converted dip and strik e lines illustrate the following threefold structural subdivision of t he Molasse Basin from the northwest to the southeast: The Plateau Mola sse consists essentially of subhorizontal Lower Freshwater Molasse (US M), which onlaps the Top Mesozoic unconformity and thins towards the J ura Mountains. It is cross-cut by wrench fault zones linking the Alps with the Jura, which often can be identified as Palaeozoic lineaments reactivated in the Early and Late Tertiary. The transition zone betwee n the Plateau Molasse and Thrusted Molasse (or Subalpine Molasse) coin cides with major SW-NE oriented hinge lines probably inherited from Pa laeozoic lineaments. This zone corresponds in the Geneva area to the M ount Saleve trend which overlies a basement high; east of Lausanne (Va ud), it is marked by an important strike-slip feature reaching the sur face. These hinge zones seem to have influenced the facies development s during the Early and Middle Jurassic, the northern depositional limi t of the Lower Marine Molasse (UMM) and the location of the deformatio n front of the Subalpine Molasse. The Thrusted (Subalpine) Molasse con sists of a folded and thrusted complex of UMM and USM. It thickens tow ards the Alpine front where it overlies a deep-seated Permo-Carbonifer ous half-graben. Seismic data from Western Switzerland and adjacent Fr ance demonstrate the imprint of Palaeozoic lineaments on the structura l configuration of the Molasse Basin. The often postulated thin-skinne d northwestward translation of the Mesozoic-Cenozoic sediments of the Molasse Basin above a regional Triassic decollement horizon during the Alpine orogeny and the folding of the Jura Mountains is not supported by the studied reflection seismic data: firstly, such a large-scale t ranslation is not compatible with the observed fault geometries and th e spatial coincidence of Palaeozoic trends and younger overlying depos itional and structural features. Secondly, there are no indications of thrust faults or major halokinetic deformations at the level of the p ostulated detachment horizon. The structural configuration of the Mola sse Basin of Western Switzerland/France is very similar to the structu ral style of its Central and Eastern parts as illustrated by two publi shed cross-sections which are also based on reflection seismic data.