Late Pleistocene-Holocene record of north-western Alps seismic events in lacustrine sediments: combined relations with local active tectonic features and last deglaciation.
by
Christian Beck
Laboratoire de Géodynamique des Chaînes Alpines, U.M.R. C.N.R.S. 5025, Université de Savoie, Campus Savoie-Technolac, F-73 376 Le Bourget du Lac
Coauthors: Marc DE BATIST (Renard Centre of Marine Geology, University of Ghent), Pieter VAN RENSBERGEN (Renard Centre of Marine Geology, University of Ghent), Emmanuel CHAPRON (Institut des Sciences de la Terre d'Orléans), Vincent LIGNIER (Sciences de la Terre, Ecole Normale Supérieure de Lyon), Fabien ARNAUD (Sédimentologie et Géodynamique, Université de Lille I), Jean-François DECONINCK (Sédimentologie et Géodynamique, Université de Lille I), Marc DESMET (Laboratoire de Géodynamique des Chaînes Alpines, U.M.R. C.N.R.S. 5025, Université de Savoie, Campus Savoie-Technolac), Michel POURCHET (Laboratoire de Glaciologie et de Géophysique de l’Environnement, Université J. Fourier, Saint-Martin d’Hères)

Most of the largest natural lakes of the Northwestern Alps – as Léman, Le Bourget, or Annecy – developed within depressions mainly related to the alpine foreland structures. Based on many investigations (neotectonics, in situ stress measurements, historical seismicity, geodetic surveys) this – middle/upper Miocene - structuration is believed to have been prolongated during Pliocene and Quaternary, with possible different stress regime and lower rate of deformations and /or displacements. Beside the latters, a strong morphological imprint of the late Quaternary climatic fluctuations was superimposed to the morphostructures, by mean of glacial erosion and overdeepening of the oligocene-miocene siliciclastics (the so-called ”molasses”), leading to rather deep lacustrine basins and thick infills (ex.: for Lake Le Bouget: water depth: 147 m; late Quaternary fill: up to 180 m). Among active features, several major NW-SE (strikeslip) faults have an interesting situation as crossing (partly or completely) the substratum of some lakes. Thus, Lakes Le Bourget and Annecy were elected for different high resolution seismic and side scan surveys, and coring. Their post-LGM sedimentary fills underwent the effects of these faults, by mean of significant strain or guiding of channel migration (or both). Beside this continuous process, instantaneous – catastrophic – sediment disturbing and reworking appear at two different observation scales: i) thick and large debris-flows, slumps, and homogenites, imaged by high resolution seismic and side scan sonar, ii) turbidites, debris-flows, and homogenites (centimetric to decimetric thickness) observed and analyzed in cores. Associated seiche effects were detected, and we relate most of these sedimentary events to earthquakes. In high altitude smaller lakes, with terrigenous feeding, gravity reworkings of delta foreset sediments, were distinguished from runoff and flood events, and correlate with historical earthquake, depending on the magnitude of the latters and the distance to the epicentral area. The geographical and time distributions of these paleoseimicity point out to a higher frequency and stronger events during the Late Glacial period.

Date received: March 8, 2002