Impact of active faulting on the post LGM infill of Le Bourget Lake (western Alps, France) Camille de La Taille a,b,c , François Jouanne a,b , Christian Crouzet a,b , Christian Beck a,b , Hervé Jomard c , Koen de Rycker d , Maarten Van Daele d a Université de Savoie Mont Blanc, ISTerre, 73376 Le Bourget-du-Lac, France b CNRS, ISTerre, 73376 Le Bourget du Lac, France c Institut de Radioprotection et Sureté Nucléaire, BERSSIN, 92260 Fontenay-aux-Roses, France d Renard Centre of Marine Geology, Ghent University, Krijgslaan 281, S8, B-9000 Gent, Belgium abstract article info Article history: Received 24 July 2014 Received in revised form 12 August 2015 Accepted 18 August 2015 Available online 29 August 2015 Keywords: Alps Jura Riedel model Active tectonic Lake Le Bourget Seismic imaging 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 sub-lacustrine 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 . © 2015 Elsevier B.V. All rights reserved. 1. Introduction High-resolution seismic reflection profiling has been used by differ- ent authors to image neotectonic faults. Doughty et al. (2013) employed this method to image faults in Lake Timiskaming (Canada). Adams et al. (1999) used it in the Lake Lahontan Basin (Nevada, California) to see isostatic rebound, active faulting and potential geomorphic effects. Van Daele et al (2011) saw Riedel faults in Gulf of Cariaco, along El Pilar Fault. In particular, we can consider the pattern of faults associated with the major strike–slip El Pilar Fault as possible analogue of the Cloos (1932) model. This model shows Riedel fractures forming in the clay on 2 metal plates acting in shear. The north-western Alps foreland and the Jura Mountains (Fig. 1) are currently subjected to moderate deformation induced by the anti- clockwise rotation of the Adria microplate relative to stable Eurasia (Biju-Duval et al., 1977; Delacou et al., 2008; Nocquet, 2012; Vigny et al, 2002). In addition to this kinematics, the fast disappearance of the Last Glacial Maximum (LGM) glacial cover and the mass transfer fol- lowing the LGM is considered to have significantly enhanced seismicity and gravity instabilities through the unloading effect (Beck et al., 1996; Vernant et al., 2013). Based on seismological, geological and geodetic surveys, detailed patterns of active faulting (including subsurface décollements, blind ramps and deeper crustal thrusts) have been proposed (e.g., Jouanne et al., 1995, 1998; Thouvenot et al., 1998), underlining the importance of NW–SE left-lateral strike–slip offsets as along the Vuache Fault (cf. the 1996 Epagny event; Thouvenot et al., 1998). In parallel with this tec- tonic evolution, the last glaciation/deglaciation cycles (isotopic stages 1 and 2) contributed to develop large and over-deepened lacustrine ba- sins represented either by thick lacustrine, marshy and fluvial deposits, or by still sub-aqueous sediments accumulations. A cross section of Lake Le Bourget (Fig. 2) illustrates this mixed heritage of alpine tectonics (Miocene growth strata in front of ramps), glacial erosion, and intergla- cial lacustrine accumulation (remnants of the last major cycles, MIS 6a and 2). In this paper, we propose to use the post-LGM infill of Lake Le Bourget as a recorder of the moderate deformation caused by the strike–slip faults that structured the southern Jura Mountains. To do this, we used high-resolution seismic profiles to highlight and map Quaternary fault activity in Lake Le Bourget. 2. Geological setting 2.1. The pre-Quaternary substratum The Jura fold and thrust belt represents the most external part of the Alpine Chain in its northwestern portion (Burkhard and Sommaruga, Tectonophysics 664 (2015) 31–49 http://dx.doi.org/10.1016/j.tecto.2015.08.024 0040-1951/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto