{{Geology of the Alps The Helvetic zone, Helvetic system or the Helveticum is a

geologic Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...

subdivision of the

Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...

. The Helvetic zone crops out mainly in

Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

, hence the name (derived from ''Helveticus'':

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for Swiss). Rocks in the Helvetic zone are

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

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 ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

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

molasse __NOTOC__ In geology, "molasse" () are 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 flys ...

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

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

marine

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

marl Marl is an earthy material rich in carbonate minerals, Clay minerals, clays, and silt. When Lithification, hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. M ...

s and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s. The Helvetic nappes are completely

detached A single-family detached home, also called a single-detached dwelling, single-family residence (SFR) or separate house is a free-standing residential building. It is defined in opposition to a multi-family residential dwelling. Definitions ...

from their former

basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...

. The Helvetic nappes are thrust over the " Infrahelvetic complex" in eastern Switzerland. The Infrahelvetic complex is composed of autochthonous Mesozoic sediments on top of Hercynian 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 margi ...

and therefore shallower in sedimentary facies. 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 massives" 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, macrosc ...

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

Massif des Écrins Massif des Écrins (; Écrins Massif) are a massif in the French Alps. They form the core of Écrins National Park in Hautes-Alpes and Isère.Belledonne, the

Aiguilles Rouges The Aiguilles Rouges (, "Red Needles") are a crystalline mountainous massif of the French Prealps, opposite the Mont Blanc Massif. The colour of the iron rich gneiss (metamorphique) mountains gives the range its name. The highest summit is the Ai ...

and the Mont Blanc Massif, 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