NEW GEOLOGICAL OBSERVATIONS BETWEEN THE JURA AND THE ALPS IN THE GENEVA AREA, AS DERIVED FROM REFLECTION SEISMIC DATA

A total of 350 km reflection seismic records have been interpreted in the Geneva Basin and Bornes Plateau. Using boreholes as calibration po ints, 10 seismic markers can be readily identified from the Cenozoic t o the Palaeozoic. This interpretation allows a correlation, across the study area, of tectonic and lithological observations derived from su rrounding Mesozoic and Cenozoic outcrops. In addition, seismic interpr etation of a Permo-Carboniferous sedimentary sequence permits the esta blishment of a schematic picture of top basement. Subsequently, the po tential relationship between the Palaeozoic and Mesozoic-Cenozoic stru ctural configuration of the area is analyzed. The tectonic framework o f the Geneva area is dominated by SW-NE and NW-SE trending Permo-Carbo niferous lineaments, which were rejuvenated at different stages up to the present-day: - SW-NE lineaments coincide with the front of the sub alpine massifs, prealpine units and Saleve thrust, and are marked by P ermo-Carboniferous half-grabens. Sedimentary and tectonic evidence ind icates a multi-phrase reactivation of these trends, culminating with t he late Alpine orogeny. The southernmost ridge of the Jura Mountains o verlies an inverted SW-NE trending Permo-Carboniferous graben. These o bservations suggest that the deformation of the Jura Mountains is more likely to have originated from basement shortening than from the larg e-scale translation across the foreland basin of the Mesozoic-Cenozoic sequence over the Triassic evaporites. - NW-SE lineaments linking the Jura Mountains and the Alpine front: interpreted for many years as wr ench fault zones in the Mesozoic outcrops around Geneva, their subsurf ace continuation can now be traced with certainty, particularly that o f the Vuache, Cruseilles and Le Coin wrench zones. Active from Permo-C arboniferous times up to now, they often form structural highs related to flower structures at Top Mesozoic level. Besides these basement-re lated lineaments, the Geneva Basin shows SW-NE trending, low-relief, a nticlinal and synclinal flexures in the Cenozoic and Mesozoic sequence . These are related to the fate Alpine orogeny. The anticlinal structu res form the characteristic molasse hills of the Geneva landscape. Fin ally, this paper illustrates the contribution of seismic stratigraphy to a better understanding of both local tectonics and sedimentary faci es distribution: for example, thickness variations in the Lower and Mi ddle Jurassic and onlaps of the molasse onto the Mesozoic demonstrate the reactivation of basement trends; the recognition on seismic sectio ns of sedimentary facies known in Jurassic outcrops of the Jura Mounta ins helps to refine palaeogeographical reconstructions.