Open Session - SWISS GEOSCIENCE MEETINGs

• Tectonic breccias, most likely associated with syn-sedimentary faulting, have been found in the Cretaceous limestones
along the Leschaux Fault.
• A steep fault, with a fault trace trending towards N150, parallel to the plane of the picture in Fig.1, separates the Leschaux
cliff from the Gérats cliff, the latter representing the downward-thrown block.
Relying on these new observations, the following interpretations can be proposed:
• The Dresse fault and the Leschaux fault represent one single ancient (probably upper Cretaceous) structure that has been
cut through and offset during a post middle-Oligocene phase of thrusting.
• Using the two offset segments of the Leschaux-Dresse paleo-fault as markers, the throw along the thrust plane is estimated
at about 750 m.
• The Gérats cliff has been down-faulted from the Leschaux cliff and is not directly related to the Bouchat cliff. Similar
structures can be observed on both flanks of the Borne Valley, suggesting the latter is partly of tectonic origin, and emphasizing
the orogen-parallel stretching in the Bornes massif.
Besides providing a new interpretation and a kinematic model for the Rochers de Leschaux, this study confirms the existence
of a late Cretaceous tectonic phase in the Subalpine domain and provides a prime example of the cutting trough of a
Mesozoic subvertical fault by a thrust ramp in a 150 m-thick massive limestone.
Fig. 1: Oblique aerial view of the Rochers de Leschaux showing its main structural elements.
REFERENCES
Bütler H. 1923: Le pli-faille des Rochers de Leschaux (vallée du Borne, Haute-Savoie). C. R. Séanc. Soc. Phys. Hist. Nat. Genève
40, 137-139.
Charollais, J., Pairis, J.-L. & Rosset, J. 1977: Compte rendu de l'excursion de la Société Géologique Suisse en Haute-Savoie
(France) du 10 au 12 octobre 1976. Eclogae geol. Helv. 70, 253-285.
Gidon, M. 1996: Vues nouvelles sur la structure des massifs des Bornes et des Bauges orientales. Géol. Alp. 72, 35-39.
Huggenberger, P. & Wildi, W. 1991: La tectonique du massif des Bornes (Chaînes Subalpines, Haute-Savoie, France). Eclogae
geol. Helv. 84, 125-149.
Maillard G. 1889: Note sur la géologie des environs d’Annecy, La Roche, Bonneville et de la région comprise entre le Buet et
Sallanches (Haute-Savoie). Bull. Serv. Carte géol. France 6, 1-64.
Symposium 1: Structural Geology, Tectonics and Geodynamics
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