The Mont Fort nappe is the upper tectonic subdivision of the former Grand St-Bernard (GSB) nappe (Escher 1988, Escher et al. 1997). Subsequent research has confirmed its tectonic and stratigraphic independence from the rest of the GSB nappe (e.g. Gouffon 1993). It consists of a Paleozoic basement overlain by a thin Mesozoic sedimentary cover, comprised of very thin, discontinuous Triassic quartzite and dolomite, a thin limestone layer, and for the greater part of breccias of variable thickness mostly made of dolomitic elements (“reconstituted Triassic”). These breccias are overlain by a more massive limestone topped by a thinner upper breccia rich in elements of quartzite. Called by Escher (1988) the Evolène series, this sequence presents remarkable similarities with the stratigraphic column of the Breccia nappe in the Prealps which typically belongs to the Prepiemontese paleogeographic domain (Lemoine 1961, Escher 1988). This similarity strongly suggests an earliest Jurassic age for the lower limestone, a late Early to Middle Jurassic age for the main mass of breccias (Lower Breccia), and a Late Jurassic to earliest Cretacous age for the upper limestone and the Upper Breccia. We will not discuss here the status of the Cretaceous calcschists (Série Rousse, Marthaler 1984) that overlie the Evolène series and are the subject of research in progress.
These ages are generally accepted. However, several authors proposed a different tectonic model where the Evolène series would be allochthonous over the Mont Fort basement (e.g. Sartori & Marthaler 1994, Marthaler et al. 2008). Their contact would be an Alpine thrust. The main argument would be that this contact surface often cuts with an angular discordance the stratigraphy of the Evolène series. Consequently, this series would not be the original sedimentary cover of the Mont Fort basement but would belong to a distinct tectonic unit that these authors identified with the Cima Bianca nappe (classically defined as a slice of Late Paleozoic to Mesozoic sediments separating the ophiolitic Zermatt-Saas and Tsaté nappes; Vannay & Allemann 1990, Steck et al. 2015). This proposition has been widely accepted (e.g. Tectonic map of Switzerland 2005).
Our observations don’t support this proposition. They rather confirm the tectonic reconstruction of Escher (1988) and suggest that the Evolène series is indeed the autochthonous cover of the Mont Fort basement. The main points are:
- At many places the contact is concordant and shows a good preservation of the basal levels of the Mesozoic sequence, without any hint of tectonic disturbance or anomalous rock deformation.
- The discordance observed at other places, which can put the Jurassic breccia in contact with all older formations, can be conveniently explained by synsedimentary normal paleofaults. By means of strain theory (e.g. Ramsay 1967) it is easy to demonstrate that passive deformation of such faults during Alpine compression can mimic thrusts.
- The sedimentary characteristics of the Evolène series, typicall of the Prepiemontese domain, are very different from those of the Cima Bianca unit, much poorer in breccias and whose relatively thick Triassic formations of quartzite and carbonates show a Briançonnais affinity.
Conclusion : The Evolène series is the sedimentary cover of the Mont Fort nappe. During Jurassic times it has been strongly affected by synsedimentary normal faulting, generating stratigraphic gaps, synsedimentary anomalous contacts and internal discordances. These structures provide a spectacular record of the extension of the European margin during the opening of the Alpine Tethys. Then Alpine compression passively deformed these faults in such a way that they mimic Alpine thrusts.
REFERENCES
Escher, A. 1988: Structure de la nappe du Grand Saint-Bernard entre le val de Bagnes et les Mischabel. Service hydrologique et géologique national, Berne.
Escher, A., Hunziker, J., Marthaler, M., Masson, H., Sartori, M., & Steck, A. 1997: Geologic framework and structural evolution of the western Swiss-Italian Alps. In O. A. Pfiffner et al. (Eds.): Deep structure of the Swiss Alps: results of NRP 20, 205–221. Birkhaüser, Basel.
Gouffon, Y. 1993: Géologie de la «nappe» du Grand St-Bernard entre la Doire Baltée et la frontière suisse (Vallée d’Aoste, Italie). Mém. Geol. (Lausanne) 12.
Marthaler, M., Sartori, M., Escher, A. & Meisser, N. 2008. Feuille 1307 Vissoie. Atlas géologique Suisse 1:25’000, Carte et Notice explicative 122.
Sartori, M. & Marthaler, M. 1994: Exemples de relations socle-couverture dans les nappes penniques du Val d’Hérens. C.-r. Excursion Soc. Géol. Suisse et Soc. Suisse Minér. Pétr., Schweiz. Miner. Petr. Mitt. 74, 503-509.