{{Geology of the Alps The Helvetic zone, Helvetic system or the Helveticum is a geologic subdivision of the Alps. The Helvetic zone crops out mainly in Switzerland, hence the name (derived from ''Helveticus'':

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

for Swiss). Rocks in the Helvetic zone are

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

and were originally deposited at the southern

margin Margin may refer to: Physical or graphical edges *Margin (typography), the white space that surrounds the content of a page *Continental margin, the zone of the ocean floor that separates the thin oceanic crust from thick continental crust *Leaf ...

of the European plate. The Helvetic zone correlates with the French Dauphinois zone, French geologists often prefer the French name but normally this is considered the same thing.

Occurrence

In Switzerland the Helvetic zone is found in outcrops on the northern side of the Alpine mountain ranges. The French Alps consist mainly of Helvetic (Dauphinois) material. In Germany and Austria the Helvetic nappes crop out as a narrow band.

Subdivision

The Helvetic zone consists of a number of

tectonically Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

very different units. The " Helvetic nappes" are a nappe stack that was thrust over the

molasse __NOTOC__ The term "molasse" () refers to sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flysc ...

of the Molasse basin in the Alpine foreland. They are composed of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

marine

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, marls and shales. The Helvetic nappes are completely

detached A stand-alone house (also called a single-detached dwelling, detached residence or detached house) is a free-standing residential building. It is sometimes referred to as a single-family home, as opposed to a multi-family residential dwelli ...

from their former basement. The Helvetic nappes are thrust over the " Infrahelvetic complex" in eastern Switzerland. The Infrahelvetic complex is composed of

autochthonous Autochthon, autochthons or autochthonous may refer to: Fiction * Autochthon (Atlantis), a character in Plato's myth of Atlantis * Autochthons, characters in the novel ''The Divine Invasion'' by Philip K. Dick * Autochthon, a Primordial in the ...

Mesozoic sediments on top of

Hercynian The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Variscan'', come ...

basement rock. The Mesozoic of this unit is contemporary with that of the Helvetic nappes, but deposited further north on the former

continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

and therefore shallower in

sedimentary facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

. The Infrahelvetic is internally deformed by thrusting and folding that continues into the Hercynian basement. Because basement and "cover" were not detached, geologists do not call the Infrahelvetic units "nappes". At places throughout the Alps the European basement was, after being detached of its cover rocks, tectonically uplifted in a late stage of the orogeny. Thus the "external massivs" were formed, places where the Hercynian basement rock crops out in large anticlinoria at the southern (or in France eastern) side of the Helvetic zone. Seen from the north (or in France from the west) the hard competent

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

rocks of these external massivs form the first of the higher ranges of the Alps. These chains are (from southwest to northeast): the

Mercantour Mercantour National Park (french: Parc national du Mercantour) a French national park located in the Alpes-de-Haute-Provence and Alpes-Maritimes departments. Since it was created in 1979, the park has proven popular, with 800,000 visitors ann ...

, the

Massif des Écrins The mountains of the Massif des Écrins (; Écrins Massif) form the core of Écrins National Park in Hautes-Alpes and Isère in the French Alps.Belledonne, the Aiguilles Rouges and the

Mont Blanc Massif The Mont Blanc massif (french: Massif du Mont-Blanc; it, Massiccio del Monte Bianco) is a mountain range in the Alps, located mostly in France and Italy, but also straddling Switzerland at its northeastern end. It contains eleven major indepen ...

, the Aarmassif and the Gotthardmassif. Sigriswiler Rothorn

References

Geology of the western and central Alps, website of S.M. Schmid
Geology of the Alps Structural geology Geology of Switzerland Geology of Austria de:Helvetisches System it:Elvetico