The Gotthard nappe (german: Gotthard-Decke, 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 Hel ...

of 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, macros ...

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.

Position and outcrops

The Gotthard nappe is found south of its larger neighbour, the

Aarmassif The Aarmassif or Aaremassif (German: ''Aarmassiv'') is a geologic massif in the Swiss Alps. It contains a number of large mountain chains and parts of mountain chains. Name The massif is named after the Aar, a river that has its source in the A ...

. 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 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 ...

cover rocks of the Tethys sea. In the south, the Simano and Adula 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 possibly Hungary (if one includes the Kőszeg Mountains). This article describes the de ...

. This outcrop begins west of the mountain ridge between the

Binntal The Binn Valley ( German: ''Binntal'') is a valley of the Alps, located on the north side of the Lepontine Alps in the Swiss canton of Valais. The valley is drained by the ''Binna'', a tributary of the Rhone, at Grengiols. The valley is named ...

and the Goms (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-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 latter part ...

. It stays south of the Goms and south of the

Furka Pass Furka Pass (french: Le Col de la Furka), with an elevation of , is a high mountain pass in the Swiss Alps connecting Gletsch, Valais with Realp, Uri. The Furka Oberalp Bahn line through the Furka Tunnel bypasses the pass. The base tunnel o ...

Andermatt Andermatt ( rm, ) is a mountain village and 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 cross of north-south and ...

, north of

Airolo Airolo (''Airöö'' in Lombard, in rm, italic=yes, ) 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 ...

in the Valle Leventina, south of the

Oberalp Pass Oberalp Pass ( rm, Alpsu or ''Cuolm d'Ursera''; german: Oberalppass) (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 closur ...

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

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 (German; English: ''Anterior Rhine''; Sursilvan: ; Sutsilvan: ''Ragn Anteriur''; Rumantsch Grischun, Vallader, and Puter: ''Rain Anteriur''; Surmiran: ''Ragn anteriour'') is one of the two sources of the Rhine. Its catchment area ...

Geology

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

orthogneiss Gneiss ( ) 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. Gneiss forms at higher temperatures an ...

, called the ''Streifengneis'' ("striped gneiss"); flanked on both sides by zones of mica schist 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 or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an a ...

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 (flak ...

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, ...

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 prop ...

(mostly thoroughly serpentinized) and

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

. The southern paragneiss zone contains characteristic

hornblende Hornblende is a complex 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 in igneous and metamorphic ro ...

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...

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. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...

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

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

. 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 ultramafi ...

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 generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

bodies intruded, especially

granite Granite () is a coarse-grained ( phaneritic) 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 cools and solidifies und ...

. During the

Hercynian orogeny 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'', comes ...

, 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 Rotondo is an Italian surname. Notable people with the surname include: * Anthony Rotondo (born 1957), American mobster * Giovanni Rotondo, composer *Nunzio Rotondo (1924–2009), Italian jazz trumpeter and bandleader *Paolo Rotondo Paolo Rotond ...

north of the

Val Bedretto Val may refer to: Val-a Film * ''Val'' (film), an American documentary about Val Kilmer, directed by Leo Scott and Ting Poo Military equipment * Aichi D3A, a Japanese World War II dive bomber codenamed "Val" by the Allies * AS Val, a So ...

, the Fibbia and Gamsboden granites close to the

Gotthard Pass german: Gotthardpass , photo = File:Gotthardpass 2008.jpg , photo_caption = The area of the Gotthard Pass from the west , elevation_m = 2106 , elevation_ref = , traversed = National Road 2 Old paved road ( Tremola) Gotthard Rail Tunnel G ...

and the

Cristallina granite The Cristallina is a mountain of the Lepontine Alps, located in the Swiss canton of Ticino. It is situated between the valleys of Leventina, Val Bavona and Valle di Peccia (the latter two belonging to the Valle Maggia. On the west side of the ...

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