Nappe des Voirons

Rappresentazione e statuto

Colore CMYK

N/A

Colore RGB

R: 241 G: 239 B: 237

Rango

falda

Uso

Unità in uso.

Status

valido

Nomenclatura

Deutsch

Voirons-Decke

Français

Nappe des Voirons

Italiano

Falda dei Voirons

English

Voirons Nappe

Origine del nome

Les Voirons (France)

Varianti storiche

nappe ultrahelvétique (Vial 1975)

Descrizione

Descrizione

Equivalent de la nappe du Gurnigel dans les Préalpes du Chablais, avec développement plus important des conglomérats et absence de la série basale à calcaires blonds.

Geografia

Regione-tipo

Voirons (France)

Referenze

Revisione

Gouffon Yves (Editor) (2024) : Tectonic Map of Switzerland 1:500000, Explanatory notes. Federal Office of Topography swisstopo, Wabern

p.56: The Lower Penninic units in front of the Prealps comprise the Voirons Nappe in the Chablais Prealps and the Gurnigel Nappe in the Romandes Prealps; both are composed of flysch. Besides their structural position, both nappes show great similarity in terms of sedimentology. However, the Gurnigel flysch is dated Late Cretaceous to Eocene (van Stuijvenberg 1979), while the Voirons flysch is only of Paleogene age (van Stuijvenberg & Jan du Chêne 1980, Ragusa et al. 2018, 2021). The position of these two nappes below the front of the Préalpes Médianes Nappe implies that they are considered as Lower Penninic. The paleogeographic origin of these flysch formations is however still debated. Due to their close lithological and petrographical similarity with those of the Sarine Nappe (Upper Penninic), the flysches of the Gurnigel and Voirons nappes are thought to have originated in the Piemonte-Liguria Ocean (van Stuijvenberg 1979, Caron et al. 1980, 1989, Winkler 1983, Gasinski et al. 1997). A possible origin in the Valaisan Basin is also suggested (Schmid et al. 2005, Trümpy 2006, Pfiffner 2014). The first hypothesis implies that these flysches were transported over and beyond the Préalpes Médianes Nappe, and then, that the latter was emplaced above the Voirons and Gurnigel Nappes by an out-of-sequence thrust fault.
Both the Voirons Nappe and the Gurnigel Nappe are discontinuously bordered by the Bulle Mélange Zone (§ 4.2.8), which separates the former nappes from the underlying Subalpine Molasse. In the Romandes Prealps, the Gurnigel Nappe is overturned at the contact with the Préalpes Médianes Plastiques (§ 6.1.2.2). This is the result of an out-of-sequence thrust bringing the Préalpes Médianes Nappe over the internal part of the Gurnigel Nappe (de Kaenel et al. 1989). The contact between the two nappes is characterized by the presence of the Bulle Mélange Zone, which also marks an internal thrust fault within the Gurnigel Nappe in its western part.
At the eastern end of Le Léman, both nappes become extremely thin. No remnants of neither the Voirons Nappe nor the Gurnigel Nappe were encountered between the Bulle Mélange Zone and the Préalpes Médianes Nappe (Dupuy et al. 2014) in the deep well Chessel-1001 drilled about 5 km south of the Rhône’s mouth into the lake. It is therefore assumed that the Voirons Nappe is not connected to the Gurnigel Nappe beneath the alluvium of the Rhône Valley.
The Gurnigel and Voirons nappes can be correlated towards the east with the Schlieren and Wägital nappes, which consist of similar flysches and occur on top or in front of the Helvetic nappes (§ 6.3.1.1).