Stress in the Geneva Basin

In this paper we explore the stress orientation and magnitudes of the Greater Geneva Basin in the detached Alpine Foreland in Switzerland and France. The interpretation is based on shallow earthquake focal mechanisms from strike-slip faults. The area of investigation comprises the westernmost Molasse Basin, the Vuache mountain range formed by the Vuache Fault Zone and the meridional thrusts and folds of the Haute Chaine Jura (Internal Jura). All these domains are detached along a main décollement in the Triassic Keuper Group evaporites and transported by some 30 km to the NW. The whole foreland is presently in a state of critical stress, considering the ongoing seismic activity, with hydrostatic fluid pressure conditions and an Andersonian stress setting. It is thus possible to derive locally the full stress tensor and analyze the stress state of known individual faults. The Mohr circle analysis and the distance to criticality demonstrate that the hydrostatic fluid pressure conditions allow seismic activity on optimally oriented vertical faults. Less optimally oriented faults require an additional raise in fluid pressure conditions. Differential stress values are in the range of 100-150MPa, and max horizontal stress values range up to 200MPa. Similar values are derived from 2D numerical mechanical analysis using limit analysis theory of the basin applied to a very weak décollement and steep reverse faults. In this approach different friction states were explored. The fact that not all faults are hydraulically conductive, despite favorable orientation, suggest that a valve pressure system and repeated sealing of the faults is a likely process operation in the area. This has profound consequences on fluid circulation models in geothermal systems.