Open Session - SWISS GEOSCIENCE MEETINGs

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Symposium 1: Structural Geology, Tectonics and Geodynamics
Figure 1. Subduction at an active margin - formation of a backarc basin: hydration above the descending oceanic plate promotes flux mel-
ting. The melts ascend, build a magmatic arc atop the continental plate and weaken the material below the arc so that necking of the
continental lithosphere is possible and trench retreat and decompression melting take place. The result is a backarc basin including a
new spreading centre.
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The Rochers de Leschaux (Bornes Massif, Subalpine domain, Haute-
Savoie, France): a complex record of Cretaceous, Paleogene, and
Neogene tectonics
Martin Marc*1, Kindler Pascal*, Sartori Mario*, Charollais Jean*
*Section of Earth Sciences, University of Geneva, Maraîchers 13, CH-1205 Geneva, Switzerland (pascal.kindler@terre.unige.ch)
1present address: IHS Energy, chemin de la Mairie 24, CH-1258 Perly, Switzerland
Based on new stratigraphic and structural data, the complex structure of the Rochers de Leschaux has been reinterpreted,
providing additional information on the type, the age, and the number of tectonic phases that have affected the Subalpine
domain, as well as on the amount of shortening this domain experienced during the Alpine orogeny.
The Rochers de Leschaux (Fig. 1) is a complex faulted fold located in the Borne Valley (NE part of the Bornes Massif, Subalpine
fold-and-thrust belt, Haute-Savoie, France). Particularly impressive is the tectonic superposition of two cliffs made of
Urgonian Limestones (Early Cretaceous): the Bouchat-Gérats cliff, at the base, and the Leschaux cliff, at the top (Fig. 1). The
presence of a thrust fault in this complex structure has long been recognized, but the overall geometry of this accident is
still controversial (Maillard, 1889; Butler, 1923; Charollais et al., 1977; Huggenberger and Wildi, 1991; Gidon, 1996). Estimates
of the amount of displacement along the thrust plane vary between