Nappe du Mont Fort
Representation and status
- Index
- MF
- Color CMYK
- N/A
- Color RGB
- R: 241 G: 239 B: 237
- Rank
- nappe
- Validity
- Unit is in Use
- Status
- valid
Nomenclature
- Deutsch
- Mont-Fort-Decke
- Français
- Nappe du Mont Fort
- Italiano
- Falda del Mont Fort
- English
- Mont Fort Nappe
- Origin of the Name
- Historical Variants
-
nappe du Mont Fort (Escher in: Woodtli 1985, Allimann 1987, Escher 1988, Gouffon 1993, Escher et al. 1997, Sartori et al. 2006, Pantet et al. 2017), falda di Monfort [sic] (Dal Piaz et al. 1992a), Mont Fort nappe (Malusà et al. 2005), Mont Fort Nappe (TK500, Gouffon et al. 2024)
Links
- Synonyms
Description
- Description
-
Nappe du Pennique moyen constituée d'un socle monocyclique à reliques de métamorphisme à relativement haute pression, d'une couverture sédimentaire permo-triasique (série d'Evolène), ainsi que de brèches et de marbres crétacés. Elle repose sur le flanc normal de la nappe de Siviez-Mischabel par l'intermédiaire de niveaux de gypse et de cornieule de la zone du Barrhorn. Elle est issue, par inversion tectonique, d'un bassin permo-carbonifère.
Hierarchy and sequence
- Upper boundary
-
nappe du Tsaté
- Lower boundary
-
nappe de Siviez-Mischabel > zone du Barrhorn
Age
- Age at top
-
- Mesozoic
- Age at base
-
- Paleozoic
Geography
- Geographical extent
- Alpes valaisannes entre le Val d'Hérens et la Vallée d'Aoste. L'unité du Stockhorn, au SE de Zermatt, y est également rattachée.
- Type area
- Val de Bagnes (VS)
Palaenography and tectonic
- Paleogeography
- Prepiemont Domain
- Tectonic unit (resp. main category)
- Kind of protolith
-
- tectonic
- Metamorphism
- monocyclic
- Metamorphic facies
-
- greenschist facies (epizone)
- blueschist facies
- Note on metamorphism
-
Métamorphisme alpin uniquement
References
- Definition
- (1985) : Géologie de la nappe du Grand Saint-Bernard entre le Val de Bagnes et les Mischabel. Uromine, projet national de recherche N" 7, Fonds national suisse de la recherche scientifique.
- Definition
-
(2024) :
Tectonic Map of Switzerland 1:500000, Explanatory notes. Federal Office of Topography swisstopo, Wabern
p.66: The Mont Fort Nappe extends from the Valle d’Aosta in the SSW, south of the Col du Grand St-Bernard, to the Val d’Anniviers in the NNE. Its basement ends as an anticline in the Val d’Hérémence, just downstream of the Grande Dixence Dam, but its Permian – Mesozoic sedimentary cover continues towards the east and forms pinched recumbent frontal folds with the lower part of the Tsaté Nappe up to the Lac de Moiry. Southward, the Mont Fort Nappe is interrupted by the Col de Bard Fault (see p. 63 and § 10.4), but equivalent units are found further south in the Valle d’Aosta and in the Vanoise mountain range (Gouffon 1993). This nappe overthrusts the Siviez-Mischabel Nappe, while a tectonized synclinal zone could link these two nappes in the Verbier area (§ 6.2.2.3). Through the Val d’Hérémence and the Val d’Hérens, the contact between these two nappes is intersected by the basal thrust of the Tsaté Nappe which forms here a narrow recumbent syncline (Montset Syncline, Sartori & Epard 2011) stretching for more than 20 km.
The crystalline basement of the Mont Fort Nappe is similar to the upper, monocyclic part of that of the Siviez-Mischabel Nappe (Cambrian – Ordovician; Sartori et al. 2006). The sedimentary cover comprises Permian to Cretaceous rocks which are variously distributed (Escher 1998, Pantet et al. 2023). It is missing from most of the reverse limb, while a thick Permian – Triassic series surrounds the frontal part of the basement. On the normal limb of the nappe, a Permian – Jurassic series occurs intermittently, the top of which is characterized by a large accumulation of breccias; a deep marine Cretaceous series continuously covers the older sediments as well as the bedrock. Correlation with the stratigraphy of the Breccia Nappe in the Prealps (§ 6.1.2.3) allows the origin of the Mont Fort Nappe to be placed within the pre-Piemonte paleogeographic realm.
The discontinuity of the Permian – Jurassic cover against the basement is interpreted as the result of synsedimentary faulting creating tilted blocks during the phase of rifting. This also explains why the basement and the Permian – Jurassic cover are discontinuous below the Cretaceous sediments, which are considered to be the unconformity-bounded post-rift cover (Pantet et al. 2020, 2023). In another way, these discontinuities have been interpreted as overthrusts of the Permian – Jurassic series (“Sasseneire Nappe”) onto the Mont Fort Nappe (crystalline basement and Permian – Triassic sediments), and the Cretaceous series considered as the base of the Tsaté Nappe overthrusting the “Sasseneire Nappe” (Marthaler et al. 2020 and ref. therein). The same sedimentary series is found further east in the form of narrow and elongated bands or folds intercalated in the Tsaté Nappe. These bands are considered to be a continuation of the Mont Fort Nappe (Argand 1909, Escher 1988, Pantet 2022) or individually as Frilihorn Nappe and Cimes Blanches Nappe (Escher et al. 1993; see § 6.2.3.3).
- Important Publications
-
(1988) : Structure de la nappe du Grand St-Bernard entre le val de Bagnes et les Mischabel. Rapp. Serv. hydr. géol. Suissse 7