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Non-tectonic deformations of Pleistocene sediments in the eastern Paris basin, France

¹ Institute of Radiological Protection and Nuclear Safety (IRSN), Seismic Hazard Division (BERSSIN), BP 17, 92262 Fontenay-aux-Roses, France. Tel : +33-1-58-35-93-50; fax : +33-1-58-35-81-30; e-mail : stephane.baize{at}irsn.fr
² UFR Sciences exactes et naturelles, Reims – Champagne-Ardennes University, BP 1039, 51687 Reims cedex 2, France
³ G2R– UMR CNRS 7566, Henri Poincaré (Nancy 1) University, BP 239, 54506 Vandoeuvre-lès-Nancy cedex, France
⁴ 11 rue Maryse Bastié, 10440 La Rivière-de-Corps, France

Deformations observed within Quaternary alluvium in the Champagne region (Paris Basin) comprise faults, folds and soft-sediment deformation structures. Their occurrence is linked to the subjacent weathered chalk. Previously interpreted as neotectonic features, the deformations are reinterpreted as karst subsidence features or/and soil displacements due to periglacial processes. Dissolution of chalk has produced superficial subsidence, explaining the geometry of some faults and their large offsets within surface deposits. The freezing-thawing cycles in the porous superficial layers have also favoured gravity instability and deformations, and this can explain local small-scale deformations but also mass movement (sliding). The seismotectonic hypothesis is rejected, because of the absence of regional faults able to generate such large co-seismic offsets. The fault directions and the apparent vertical offsets are not homogeneous at regional scale and they are often inconsistent with the Quaternary stress field. Moreover, the rooting of faults into the basement is not documented and therefore, the neotectonic origin is very doubtful.

Key Words: Deformation • Alluvial deposits • Karst • Periglacial • Neotectonics • Paris Basin