The Gotthard nappe (, in older literature called the Gotthard Massif) is, in the

geology of the Alps The Alps form part of a Cenozoic orogenic belt of mountain chains, called the Alpide belt, that stretches through southern Europe and Asia from the Atlantic all the way to the Himalayas. This belt of mountain chains was formed during the Alpin ...

nappe In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the ...

in the

Helvetic zone {{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 for Swiss). Rocks in the Helve ...

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 ...

. It consists of

crystalline rock 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, macrosco ...

s that were, before the formation of the Alps, part of the upper crust of the southern margin of the European continent (the northern margin of the Alpine Tethys Ocean). As it names suggests, the Gotthard nappe lies in close proximity to the

Gotthard Massif The Gotthard Massif or Saint-Gotthard Massif ( or ; ; ) is a mountain range in the Alps in Switzerland, located at the border of four cantons: Valais, Ticino, Uri and Graubünden. It is delimited by the Nufenen Pass on the west, by the Furk ...

Position and outcrops

The Gotthard nappe is found south of its larger neighbour, the Aarmassif. The two crystalline zones are separated by a thin zone of

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

and strongly deformed

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 ...

cover rocks of the Tethys sea. In the south, the Simano and

Adula Adula may refer to *The Adula Alps * Rheinwaldhorn, a mountain called in Italian "l'Adula" * Adula Formation, a geological formation in China {{disambig ...

nappes lie on top of the Gotthard nappe. The Gotthard nappe crops out in an elongated strip through the

Central Alps The Alps form a large mountain range dominating Central Europe, including parts of Italy, France, Switzerland, Liechtenstein, Austria, Slovenia, Germany and Hungary. This article describes the delimitation of the Alps as a whole and of subdivi ...

. This outcrop begins west of the mountain ridge between the Binntal and the

Goms GOMS is a specialized human information processor model for human-computer interaction observation that describes a user's cognitive structure on four components. In the book ''The Psychology of Human Computer Interaction'', written in 1983 by St ...

(the valley of the Rhone) east of

Brig A brig is a type of sailing vessel defined by its rig: two masts which are both square rig, square-rigged. Brigs originated in the second half of the 18th century and were a common type of smaller merchant vessel or warship from then until the l ...

. It stays south of the Goms and south of the

Furka Pass Furka Pass (; ) is a high mountain pass in the southern Swiss Alps connecting Gletsch, Valais with Realp, Uri via the seasonal Furkapassroute. The Furka Oberalp Bahn bypasses the pass through the high Furka Base Tunnel, which opened in 19 ...

Andermatt Andermatt () is a mountain village and municipalities of Switzerland, municipality in the canton of Uri in Switzerland. At an elevation of above sea level, Andermatt is located at the center of the Saint-Gotthard Massif and the historical center ...

, north of

Airolo Airolo (''Airöö'' in Lombard, in ) is a municipality in the district of Leventina in the canton of Ticino in Switzerland. Geography Airolo is located in Leventina valley and the Lepontine Alps, on the southern flank of the St. Gotthard ...

in the

Valle Leventina The Leventina District is one of the eight districts of the largely Italian language, Italian-speaking Cantons of Switzerland, canton of Ticino in Switzerland. The capital of the district is Faido but the largest town is Airolo on the southern flan ...

, south of the

Oberalp Pass Oberalp Pass ( or ''Cuolm d'Ursera''; ) (2044 meters above sea level) is a high mountain pass in the Swiss Alps connecting the cantons of Graubünden and Uri between Disentis/Mustér and Andermatt. Winter closure The public road that crosses t ...

Disentis Mustér (, Romansh) or Disentis (German), with its official name Disentis/Mustér is a village and a municipality in the Surselva Region in the Swiss canton of Grisons. The skiing and summer tourism resort high up in the Rhine valley is the si ...

and ends south of the eastern part of the

Surselva Surselva Region is one of the eleven administrative districts in the canton of Graubünden in Switzerland. It was created on 1 January 2017 as part of a reorganization of the canton.Vorderrhein The Vorderrhein (; ; ; ; ), or Anterior Rhine, is the left of the two initial tributaries of the Rhine (the other being the '' Hinterrhein''). It is longer than the ''Hinterrhein'', but has a lower discharge than the latter at their confluence, w ...

Geology

The outcrop can be divided into three parts or zones: a central core of Ordovician

orthogneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

, called the ''Streifengneis'' ("striped gneiss"); flanked on both sides by zones of

mica schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

or paragneiss. The northern paragneiss zone contains lenses of various

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lith ...

, such as

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...

schists,

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

(mostly thoroughly serpentinized) and

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

. The southern paragneiss zone contains characteristic

hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

schists called ''Garbenschiefer'', part of the Tremola series.Labhart (2005) The crystalline rocks of the Gotthard nappe carry strong traces of at least two older

orogenies Orogeny () is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involv ...

before the

Alpine orogeny The Alpine orogeny, sometimes referred to as the Alpide orogeny, is an orogenic phase in the Late Mesozoic and the current Cenozoic which has formed the mountain ranges of the Alpide belt. Cause The Alpine orogeny was caused by the African c ...

. The oldest phase, sometimes (erroneously) called " Caledonian", took place in the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

. The paragneiss of the Gotthard nappe was once oceanic sediment, which was deformed into an

accretionary wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the ...

during this orogeny. The gabbros and peridotites, now lenses in the paragneiss, probably represent pieces of

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

underneath these sediments, that were incorporated into the Ordovician accretionary wedge. The eclogite metamorphism has been dated at about 470 Ma (million years ago). In the late stages of the Ordovician orogenic phase,

granitoid A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

bodies intruded, especially

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

. During the

Hercynian orogeny The Variscan orogeny, 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 ...

, about 320 Ma, all of these units were strongly deformed. Today, the Ordovician intrusives form the ''Streifengneis'' of the central zone. The Hercynian orogeny ended with another phase of granitoid

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s. Four Hercynian intrusive bodies are found in the current outcrop of the Gotthard nappe: the Rotondo granite north of the

Val Bedretto Bedretto () is a municipality and a village in the Val Bedretto, the upper most part of the river Ticino. It belongs to the district of Leventina in the canton of Ticino in Switzerland. History In 1906, pre-Roman graves and a Roman era villa an ...

, the Fibbia and Gamsboden granites close to the

Gotthard Pass The Gotthard Pass or St. Gotthard Pass (; ) at is a mountain pass in the Alps traversing the Saint-Gotthard Massif and connecting northern Switzerland with southern Switzerland. The pass lies between Airolo in the Italian-speaking canton of Ti ...

and the Cristallina granite around the Lukmanier Pass. These intrusions have ages between 305 and 290 Ma.

References

*; 2005: ''Geologie der Schweiz'', h.e.p. verlag ag, Ott Verlag, 7. Auflage, Bern. {{in lang, de Geology of the Alps Geology of Switzerland

Position and outcrops

Geology

See also

References