Helvetic nappes
From Wikipedia, the free encyclopedia
| Geology of the Alps |
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 |
| Tectonic subdivision |
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Helvetic nappes |
| Penninic nappes |
| Austroalpine nappes |
| Southern Alps |
| Formations & rocks |
| Geological structures |
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Aarmassif | Dent Blanche klippe | Engadine window | Flysch zone | Giudicárie line | Greywacke zone | Hohe Tauern window | Molasse basin | Penninic thrustfront | Periadriatic Seam | Ivrea zone | Lepontin dome | Rechnitz window | Rhône-Simplon line | Sesia unit |
| Paleogeografic terminology |
| Briançonnais microcontinent |
| Piemont-Liguria Ocean |
| Apulian or Adriatic plate |
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This article is about the geology of the (European) Alps. For the main article see: geology of the Alps
The Helvetic nappes, Helvetic system or the Helveticum are one of three nappe stacks in the Alps. The Helvetic nappes crop out mainly in Switzerland, hence their name (derived from Helvetica: latin for Switzerland). Rocks in these nappes were originally deposited at the southern margin of the European plate. The French parts are often called Dauphinois nappes by French geologists, but normally this is considered the same thing.
[edit] Occurrence
In Switzerland the Helvetic nappes are found in outcrops on the northern side of the Alpine mountain ranges. The French Alps consist mainly of Helvetic material. In Austria and the German Alps the Helvetic nappes crop out as a narrow band.
[edit] Subdivision
Because they originate from the European continent the rocks in the Helvetic nappes are similar to their counterparts from other parts of Western Europe. Roughly speaking the main units found are:
- the European crystalline basement of Paleozoic metamorphic and igneous rocks. The same basement occurs in other tectonic highs of Western Europe like the Vosges, the Eifel, the Ardennes or the French Massif Central. The basement was deformed and metamorphosed during the Variscan orogeny. Seen from the north (or in France from the west) the first high mountain chains of the Alps consist of this basement. These chains are (from southwest to northeast): the Mercantour, the Massif des Écrins, the Belledonne, the Aiguilles Rouges and the Mont Blanc, the Aarmassif and the Gotthardmassif.
- Mesozoic to early Tertiary sedimentary rocks. These are mostly shallow marine rocks (limestones) and deeper marine shales and claystones, deposited at the continental margin of the European plate. Nowadays these rocks form the external Jura mountains and the Swiss calcareous Alps. These rocks were deposited on top of Permian evaporites that formed a tectonically weak zone over which the Mesozoic rocks could move (such a zone is sometimes called a decollement horizon).
- early Tertiary (Eocene and Oligocene) flysch, deposited in the foreland basin that evolved when the Alps started to rise in that period. The flysch was later, when the thrustfront reached the foreland basin, transported along with the older Mesozoic rocks. It now forms a band along the northern feet of the Alps, called the Flysch zone.
- middle Miocene to recent molasse was deposited in the northern foreland (for example in the Swiss Molasse basin). This is, however, mostly not considered part of the Helvetic nappes. In the French Alps the molasse is less thick, sometimes non-existent.
The Mesozoic and Tertiary rocks of the Helvetic nappes form the northern foreland of the Alps. They were pushed northward by the more metamorphic rocks of the Penninic nappes and form the northern thrustfront of the Alps. In the northwest the Jura mountains were formed by this thrustfront.
In the Oligocene and Miocene extension took place in the foreland, which formed a deep basin where the flysch and later molasse could be deposited. The topography on top of the Molasse basin is relatively flat, this region in Switzerland is called the mittelland.
