Silvretta-Seckau-Deckensystem

The Masuccio Nappe, consisting exclusively of amphibolite-facies Variscan basement, is the lowermost unit of the Campo Nappe Complex. It overthrusts the Bernina Nappe and is located in the footwall of the Late Cretaceous Mortirolo Normal Fault, a transtensional top-E directed extensional fault that became folded around the Pass d’Ur Antiform during Cenozoic folding. In the south, the Masuccio Nappe is in a subvertical orientation and part of the Southern Steep Belt (or root zone) located north of the Tonale Fault, where it is juxtaposed with the structurally much higher Tonale Nappe, located further south and directly adjacent to the Tonale Fault.

The Campo Nappe is the largest unit of the Upper Austroalpine Campo Nappe Complex and predominantly consists of Variscan basement, locally intruded by Permian magmatites (e.g., Sondalo Gabbro, Martell Granite). This nappe tectonically overlies the Masuccio Nappe in the southwest and the Languard Nappe in the northeast and extends all along the base of the tectonically higher Ortler Nappe. Its former Permian–Mesozoic cover has been almost completely detached very early during WNW-directed Late Cretaceous thrusting and is not present nowadays due to erosion, except as small occurrences more or less attached to the basement of the Campo Nappe.

The Grosina Nappe forms a series of tectonic klippes on the southwestern parts of the Campo Nappe along a distinct mylonitic zone associated with top-NW to top-N senses of shear of Cretaceous age that possibly reworked a Jurassic décollement zone. The amphibolite-facies Variscan basement of the Grosina Nappe is in parts similar to that of the underlying Campo Nappe. However, the basement of the Grosina Nappe differs in that orthogneisses are much more frequent, and, additionally, in that there is often a pervasive greenschist-facies metamorphic overprint of the older high-grade fabrics. The root zone of the Grosina Nappe most probably occurs at the southern margin of the Campo Nappe Complex and in the immediate footwall of the tectonically higher Tonale Nappe.

The Laas Nappe exposes a series of often subvertically northward inclined garnet and staurolite bearing paragneisses, marbles and amphibolites metamorphosed under amphibolite-facies conditions during the Variscan orogeny. Cretaceous metamorphic overprint varies from greenschist facies in the west to lower amphibolite facies in the east. This nappe occupies a much-debated and still unclear position in the Upper Austroalpine nappe pile.

The Vinschgau Shear Zone represents the structurally highest unit of the Campo Nappe Complex and constitutes a greenschist-facies Alpine mylonitic belt with a thickness of up to 2–3 km that accommodates W-directed thrusting of the Ötztal Nappe Complex in the area north of the Vinschgau Valley floor. South of the valley floor and in the lowermost Val Müstair, the same mylonitic belt accommodates WNW-directed thrusting of the easternmost part of the Umbrail-Terza Slice Complex over the S-charl-Sesvenna Nappe.

The SW–NE striking Ducan Syncline comprises the Triassic cover of the Silvretta Nappe at its western edge. It formed during top-NW thrusting in the Cretaceous, and then was overprinted by normal faulting still in the latest Cretaceous. This was followed by top-N thrusting of the entire Silvretta Nappe over the Penninic units during Cenozoic times.

The SW–NE striking Landwasser Syncline comprises the Triassic cover of the Silvretta Nappe at its western edge. It formed during top-NW thrusting in the Cretaceous, and then was overprinted by normal faulting still in the latest Cretaceous. This was followed by top-N thrusting of the entire Silvretta Nappe over the Penninic units during Cenozoic times.

The “Phyllitgneiszone” is a narrow belt of low-grade basement representing the northern, strongly retrogressed and phyllonitic part of the Silvretta Nappe.

Das Silvretta-Kristallin besteht ungefähr zu gleichen Teilen aus Paragneisen, Amphiboliten und Orthogneisen vorwiegend granitischer Zusammensetzung. Mischgneise (Migmatite, Anatexite, Injektions- und Imbibitionsgneise - mit und ohne Stoffzufuhr) treten häufig auf, bilden aber nur selten grössere Komplexe. Richtige Granite, wie wir sie aus den Zentralmassiven und aus dem Seengebirge kennen, fehlen (Streckeisen 1986). Die interne Struktur ist durch langgestreckte Züge und grossräumige

Falten mit steilstehende Achsen (Schlingentektonik) charakterisiert.

Within the S-charl-Sesvenna Nappe, the Tavrü Slice (formerly known as “S-charl-Oberbau”) consists entirely of a Norian to Cretaceous succession, volumetrically dominated by an over 2 km thick Hauptdolomit. It is completely detached from the Carnian Raibl Group. This slice, separated from the tectonically higher Ötztal Nappe by the Schlinig Thrust, suffered substantial internal extension in a domino-style.

Quarzreicher Phyllit und Glimmerschiefer der Languard-Decke.