Nappe de Siviez-Mischabel

Rappresentazione e statuto

Index

SM

Colore CMYK

N/A

Colore RGB

R: 241 G: 239 B: 237

Rango

falda

Uso

Unità in uso.

Status

valido

Discussione del statuto

Nomenclatura

Deutsch

Siviez-Mischabel-Decke

Français

Nappe de Siviez-Mischabel

Italiano

Falda Siviez-Mischabel

English

Siviez-Mischabel Nappe

Origine del nome

Siviez (VS)

Varianti storiche

Mischabel-Decke = nappe des Mischabels (Zimmermann 1955), zone du Mischabel [sic] (Bearth 1963), nappe des Mischabel (Burri 1983), nappe de Siviez-Mischabel (Escher 1988, Gouffon 1993, Sartori et al. 2006), Falda Siviez-Mischabel (Dal Piaz et al. 1992a), Siviez-Mischabel Nappe (TK500, Gouffon et al. 2024)

Descrizione

Descrizione

Nappe du Pennique moyen formant un vaste pli couché vers le NW, affecté par un rétroplissement ultérieur. Son socle anté-triasique est principalement constitué de gneiss clairs mylonitiques et de schistes verdâtres (métagrauwackes et métapélites). Sa couverture mésozoïque est encore localement présente sur son flanc normal (unités du Tounô et du Barrhorn), tandis qu'elle est détachée dans sa partie occidentale et forme actuellement la nappe des Préalpes Médianes.

Gerarchia e successione

Unità di rango inferiore

• zone de cisaillement de l'Ar du Tsan
• zone de cisaillement de la Brinta

Limite superiore

nappe d'Antrona

Paleogeografia e tettonica

Paleogeografia

terrane brianzonese :

microcontinent briançonnais

Termini generici

• Siviez-Mischabel

Tipo di origine

• tettonico

Metamorfismo

policiclico

Facies metamorfica

• facies a scisti verdi
• facies a scisti blu

Referenze

Revisione

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

p.65: The Siviez-Mischabel Nappe is made up of a crystalline basement and a typical Briançonnais sedimentary cover. The crystalline basement comprises rocks of sedimentary and magmatic origin of Proterozoic to Ordovician age. The lower, older part of this basement displays traces of polycyclic metamorphism, while the upper, junger part contains only mineral assemblages characteristic of the Alpine metamorphism. A Permian metagranite (Randa Orthogneiss) intruded the crystalline basement and the Permian cover. The sedimentary cover includes Permian to Triassic formations, completed by Jurassic to Eocene formations only in the normal limb of the nappe between Zermatt and the Val d’Anniviers (Sartori et al. 2006), especially well developed in the Barrhorn region. Further west, the sedimentary cover is detached at the level of the Middle Triassic evaporites and form the Préalpes Médianes Rigides (Sartori 1990; see § 6.1.2.2).
The Siviez-Mischabel Nappe occupies a large region of the Valais Alps south of the Rhône. In its central part, it overlies the Zone Houillère, but in many places the contact between these two units is marked by the extended and steep longitudinal faults related to the Rhône-Simplon Fault. From the Val de Bagnes southward, the Siviez-Mischabel Nappe overlies the Ruitor Nappe and decreases in thickness; it terminates in the Valle d’Aosta on the Col de Bard Fault (see § 6.2.2.2 and 10.4).
Between Visp and the Simplonpass, Permian – Triassic rocks form the St. Niklaus syncline which separates the overlying Siviez-Mischabel Nappe from the underlying Upper Stalden Nappe. The upper and lower contacts of the syncline are strongly sheared. Sartori et al. (2017) postulate that these rocks belong to the Siviez-Mischabel Nappe and that they thrust over the Upper Stalden Nappe, while Genier et al. (2008) consider that this syncline connects the two nappes.
In its western part, the Siviez-Mischabel Nappe is overlain by the Mont Fort Nappe. Their contact is tectonic, although a zone of Permian – Triassic rocks can be observed between these two units and interpreted as a tectonized syncline in the Verbier region. From the Val d’Hérens to the Zermatt Valley, the nappe is overlain by the Tsaté Nappe. In the latter valley and further east, the Siviez-Mischabel Nappe forms the large Mischabel Backfold, in whose steep overturned flank the nappe is in contact with, from west to east, the Zermatt - Saas Fee Nappe, the Portjengrat Nappe and the Antrona Nappe (see Pl. II, western cross-section). The contact with the latter terminates against the Simplon Fault, part of the Rhône-Simplon Fault. The Antrona Nappe seems to form here a synform truncated by this fault.

Pubblicazioni importanti

Gouffon Yves (1993) : Géologie de la "nappe" du Grand St-Bernard entre la Doire Baltée et la frontière suisse (Vallée d'Aoste - Italie). Mémoires de Géologie (Lausanne) 12, 147 pages

Escher Arthur, Masson Henri, Steck Henri (1987) : Coupes géologiques des Alpes occidentales suisses. Rapp. géol. Serv. hydrol. et géol. natl. 2, 12 pages