Impact of active faulting on the Post LGM infill of Le Bourget Lake(western Alps, France)
Résumé
We have used high resolution seismic imaging to detect and characterize the recent deformation recorded by the Quaternary sediments of Le Bourget Lake. The last glacial episodes (MIS 6a and 2, i.e. Riss and Würm) scoured out an elongated over-deepened basin to more than 300 m below the present lake level and the basin accumulated 150 m of post-LGM to Holocene sediments. The well-stratified character of the infill is locally disturbed by tectonic deformations and gravity reworking. A northern fault zone, in continuation with the left-lateral strike-slip Culoz Fault, is imaged within the Holocene and Late Glacial accumulations. A southern fault zone is also detected, which can be related to the sublacustrine continuation of a much smaller fault affecting the Jura alpine foreland: the Col du Chat left lateral strike-slip fault. Different generations of fractures have been identified in the lake, allowing correlation and mapping. In pre-Quaternary substratum, the Culoz Fault has a N 160° orientation. Within the post-LGM sediments, fractures related to the Culoz Fault have an orientation between N135° and 95°. A Cloos model (1932) is thus proposed to explain the observed pattern of lacustrine deformations. The calculated horizontal slip rate for Culoz Fault during Holocene is about 1.3 mm.yr-1, and for the Col du Chat fault is around 0.6 mm.yr-1.