The Gotthard nappe is, in the geology of the Alps

Geology of the Alps

The Alps form part of a Tertiary 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 Alpine orogeny. A gap in these mountain chains in central...

 a nappe

Nappe

In geology, a nappe is a large sheetlike body of rock that has been moved more than or 5 km from its original position. Nappes form during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart. The resulting structure is a...

 in the Helvetic zone of Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

. It consists of crystalline rocks that were, before the formation of the Alps, part of the lower crust

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 of the southern margin of the European continent (the northern margin of the Tethys Ocean

Tethys Ocean

The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...

).

Position and outcrops

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

Aarmassif

The Aarmassif or Aaremassif is a geologic massif in the Swiss Alps. It contains a number of large mountain chains and parts of mountain chains.-Geography:...

. The two crystalline zones are separated by a thin zone of metamorphosed

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

 and strongly deformed

Deformation (mechanics)

Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...

 sedimentary

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 cover rocks of the Tethys Ocean. In the south, the Simano and Adula nappes lie on top of the Gotthard nappe.

The Gotthard nappe crops out

Outcrop

An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...

 in an elongated strip through the Central Alps. This outcrop begins west of the mountain ridge between the Binntal and the Goms (the valley of the Rhone

Rhône

Rhone can refer to:* Rhone, one of the major rivers of Europe, running through Switzerland and France* Rhône Glacier, the source of the Rhone River and one of the primary contributors to Lake Geneva in the far eastern end of the canton of Valais in Switzerland...

) east of Brig. It stays south of the Goms and south of the Furka Pass

Furka Pass

Furka Pass 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...

, Andermatt

Andermatt

Andermatt is a municipality in the canton of Uri in Switzerland.With Realp and Hospental, it is located in the Urseren valley, south of Altdorf.-Geography:...

, north of Airola

Airola

Airola is a comune in the Province of Benevento in the Italian region Campania, located about 35 km northeast of Naples and about 20 km southwest of Benevento in the Valle Caudina, facing the Monte Taburno. Nearby is the confluence of the Tesa and Faenza streams into the Isclero River...

 in the Valle Leventina, south of the Oberalp Pass

Oberalp Pass

Oberalp Pass is a high mountain pass in the Swiss Alps connecting the cantons of Graubünden and Uri between Disentis and Andermatt.-Winter closure:...

, Disentis and ends south of the eastern part of the Surselva (the valley of the Vorderrhein

Vorderrhein

The Vorderrhein is one of the two sources of the Rhine. Its catchment area of is located predominantly in the Canton of Graubünden . The Vorderrhein is about long, thus more than 5% longer than the Hinterrhein...

).

Geology

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

Lithology

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, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...

, such as calcareous

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:...

 schists, amphibolite

Amphibolite

Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...

, eclogite

Eclogite

Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...

, peridotite

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

 (mostly thoroughly serpentinized) and gabbro

Gabbro

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

. The southern paragneiss zone contains characteristic hornblende

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....

-garnet

Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...

 schists called Garbenschiefer, part of the Tremola series.

The crystalline rocks of the Gotthard nappe carry strong traces of at least two older orogenies

Orogeny

Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

 before the Alpine orogeny

Alpine orogeny

The Alpine orogeny is an orogenic phase in the Late Mesozoic and Tertiary that formed the mountain ranges of the Alpide belt...

. The oldest phase, sometimes (erroneously) called "Caledonian

Caledonian orogeny

The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...

", took place in the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

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

Accretionary wedge

An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...

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

Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

 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

Granitoid

A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...

 magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 bodies intruded, especially granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

. During the Hercynian orogeny, 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

Intrusion

An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

s. Four Hercynian intrusive bodies are found in the current outcrop of the Gotthard nappe: the Rotondo granite north of the Val Bedretto, the Fibbia and Gamsboden granites close to the Gotthard Pass and the Cristallina granite around the Lukmanier Pass

Lukmanier Pass

Lukmanier Pass is a pass in the Swiss Alps.The road from Disentis/Mustér in the canton of Graubünden leads through the Val Medel across the pass to the Blenio valley and Biasca in the canton of Ticino.The Pass is kept opened during the winter.-See also:* List of highest paved roads in Europe*...

. These intrusions have ages between 305 and 290 Ma.