EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Rediscovery of a major alpine thrust : the Helvetic Basal Decollement

Antoine Mercier1, Philippe Hervé Leloup1, Gabriel Courrioux2, Séverine Caritg2, Simon Lopez2, and Amir Kalifi1
Antoine Mercier et al.
  • 1Laboratoire de Géologie de Lyon – Terre, planètes et environnement (LGL-TPE) – CNRS : UMR5276, Université de Lyon , Lyon, France
  • 2Bureau de Recherches Géologiques et Minières (BRGM), Orléans, France

Since two centuries the European Alps are a natural laboratory to study continental lithosphere deformation during mountain building. Since the early studies, a constant question has been to evaluate the importance of vertical versus horizontal displacements in the building of reliefs. Whilst the occurrence of large thrust sheets, as initially proposed from field observations, are now well explained in the frame of plate tectonics, controversies still arise on the precise geometry, amount, and timing of major thrusting during the orogeny.

We present a new detailed 3D structural study of the cover/basement relationships in the Chamonix synclinorium in between the Mont-Blanc (MB) and Aiguilles Rouges (AR) ranges. These massifs are two of the main external basement ranges of the western Alps.  The study allows deciphering the area structural history: the Mesozoic sedimentary cover has been thrust at least 10km NW above the Helvetic Basal Décollement (HBD) before to be offset by late steep thrusts during exhumation in the Miocene.

Such interpretation fundamentally diverges from the classical view of the sedimentary cover of the Chamonix synclinorium being expulsed from a former graben during a single deformation phase and implies that a major thrust phase lasting ~10 Ma has been overlooked. Our observations show that the HBD was a major thrust system active between ~30 and ~20 Ma, possibly until 15 Ma, with a shortening of more than 10km in the south to 20km in the north. It extends below most of the subalpine ranges and emerges in front of the Bauges and within the Chartreuse and Vercors massifs, and was rooted east of the External Cristalline Massifs (Mont-Blanc and Belledonne). During the Miocene, the HBD was cut by steep reverse faults and uplifted above the basement culmination of the External Cristalline Massifs obscuring its continuity and precluding its recognition as a major structure even if it was previously described at several localities.

How to cite: Mercier, A., Leloup, P. H., Courrioux, G., Caritg, S., Lopez, S., and Kalifi, A.: Rediscovery of a major alpine thrust : the Helvetic Basal Decollement, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8595,, 2021.

Corresponding presentation materials formerly uploaded have been withdrawn.