Austroalpine nappes
Encyclopedia
The Austroalpine nappes are a geological
nappe stack
in the European Alps
. The Alps contain three such stacks, of which the Austroalpine nappes are structurally
on top of the other two (meaning they were thrust
ed over the other two). The name Austroalpine means Eastern Alpine, because these nappes crop out
mainly in the Eastern Alps
(the Alps east of the line Lake Constance
- Chur
– Lake Como
).
Because the Austroalpine nappes consist of material from the former Apulian or Adriatic plate
, that was thrusted over the European plate, they are called allochthon
nappes. In comparison with the other nappe stacks, they have experienced lower-grade metamorphism
, which distinguishes them clearly from the Penninic nappes
on which they rest.
and continental slope of the Apulian or Adriatic plate
. These fragments contain rocks from the continental basement
as well as from sedimentary rock
s deposited in these environments.
The basement rocks have experienced metamorphism
related to their original depth in the Earth's crust
, but in the Austroalpine nappes Alpine
metamorphism (i.e. metamorphism related to the formation of the Alps) is fairly low grade to non-existent. The basement rocks can be greenschist facies to amphibolite facies, depending on their original depth. They are Paleozoic
schist
s and (para-)gneiss
es intruded by granite
s of Variscan
and Tertiary
age.
On top of this basement rock, Permian
and Mesozoic
sedimentary and volcanic rock
s were deposited. Shallow marine limestone
s are abundant, these limestones now form the mountain chains of the northern part of the Eastern Alps, which are therefore together called the Northern Calcareous Alps. Sometimes, the limestone has been turned into dolomite
, as in the Austria
n region Salzkammergut
and the German
region Allgäu
.
A special unit is the greywacke zone
, a band of Paleozoic
metamorphosed sedimentary rock
s that forms an east-west band through the Austria
n Alps
. The greywacke zone crops out between the Mesozoic rocks of the Northern Calcareous Alps and the Austroalpine and Penninic basement rocks of the Central Eastern Alps
. Stratigraphically
, the greywacke zone can be up to 2 kilometres (1.2 mi) thick.
All of these lithologies
were fold
ed and thrusted, so that the basement rock can be found on top of the sediments and vice versa.
, the Austroalpine nappes have been eroded
away except for a few isolated outcrops called the Sesia unit
and the Dent Blanche klippe
(the Matterhorn
is the most outstanding example of an Austroalpine klippe
). These remaining Swiss nappes have a different tectonic and metamorphic history than their counterparts in Austria, which is why they are not always considered a part of the Austroalpine nappes.
On the other hand, in Austria, the Austroalpine nappes cover the largest part of that country, except for a few windows
like the Hohe Tauern window
and the Engadin window.
period, the Austroalpine rocks experienced another deformation phase: the Eo-Alpine phase of mountain building that took place in the Cretaceous
. The metamorphic field gradient is to the east-south-east, so in a west-north-western direction, the traces become less severe. In the west of Switzerland, the event cannot be recognized anymore. In Austria, however, Eo-Alpine eclogite
lenses occur close to the Hohe Tauern window
.
The Eo-Alpine phase is sometimes seen as the earliest phase of the Alpine orogeny. However, after the initial mountain-building, the tectonic plates moved away from each other. The next phase was more than 50 Ma later, so the events are often seen as unrelated.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
nappe stack
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 European Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
. The Alps contain three such stacks, of which the Austroalpine nappes are structurally
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
on top of the other two (meaning they were thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
ed over the other two). The name Austroalpine means Eastern Alpine, because these nappes crop 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...
mainly in the Eastern Alps
Eastern Alps
Eastern Alps is the name given to the eastern half of the Alps, usually defined as the area east of the Splügen Pass in eastern Switzerland. North of the Splügen Pass, the Posterior Rhine forms the border, and south of the pass, the Liro river and Lake Como form the boundary line.-Geography:The...
(the Alps east of the line Lake Constance
Lake Constance
Lake Constance is a lake on the Rhine at the northern foot of the Alps, and consists of three bodies of water: the Obersee , the Untersee , and a connecting stretch of the Rhine, called the Seerhein.The lake is situated in Germany, Switzerland and Austria near the Alps...
- Chur
Chur
Chur or Coire is the capital of the Swiss canton of Graubünden and lies in the northern part of the canton.-History:The name "chur" derives perhaps from the Celtic kora or koria, meaning "tribe", or from the Latin curia....
– Lake Como
Lake Como
Lake Como is a lake of glacial origin in Lombardy, Italy. It has an area of 146 km², making it the third largest lake in Italy, after Lake Garda and Lake Maggiore...
).
Because the Austroalpine nappes consist of material from the former Apulian or Adriatic plate
Apulian Plate
The Adriatic or Apulian Plate is a small tectonic plate carrying primarily continental crust that broke away from the African plate along a large transform fault in the Cretaceous period. The name Adriatic Plate is usually used when referring to the northern part of the plate...
, that was thrusted over the European plate, they are called allochthon
Allochthon
thumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...
nappes. In comparison with the other nappe stacks, they have experienced lower-grade metamorphism
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
, which distinguishes them clearly from the Penninic nappes
Penninic nappes
The Penninic nappes or the Penninicum are one of three nappe stacks and geological zones in which the Alps can be divided. In the western Alps the Penninic nappes are more obviously present than in the eastern Alps , where they crop out as a narrow band...
on which they rest.
Lithologies
The Austroalpine nappes are fragments of the former continental shelfContinental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
and continental slope of the Apulian or Adriatic plate
Apulian Plate
The Adriatic or Apulian Plate is a small tectonic plate carrying primarily continental crust that broke away from the African plate along a large transform fault in the Cretaceous period. The name Adriatic Plate is usually used when referring to the northern part of the plate...
. These fragments contain rocks from the continental basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
as well as from sedimentary rock
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....
s deposited in these environments.
The basement rocks have experienced metamorphism
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...
related to their original depth in the Earth's 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...
, but in the Austroalpine nappes Alpine
Alpine orogeny
The Alpine orogeny is an orogenic phase in the Late Mesozoic and Tertiary that formed the mountain ranges of the Alpide belt...
metamorphism (i.e. metamorphism related to the formation of the Alps) is fairly low grade to non-existent. The basement rocks can be greenschist facies to amphibolite facies, depending on their original depth. They are Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s and (para-)gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es intruded by 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...
s of Variscan
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
and Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
age.
On top of this basement rock, Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
sedimentary and volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
s were deposited. Shallow marine limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s are abundant, these limestones now form the mountain chains of the northern part of the Eastern Alps, which are therefore together called the Northern Calcareous Alps. Sometimes, the limestone has been turned into dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
, as in the Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
n region Salzkammergut
Salzkammergut
The Salzkammergut is a resort area located in Austria. It stretches from City of Salzburg to the Dachstein mountain range, spanning the federal states of Upper Austria , Salzburg , and Styria . The main river of the region is the Traun, a tributary of the Danube...
and the German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
region Allgäu
Allgäu
The Allgäu is a southern German region in Swabia. It covers the south of Bavarian Swabia and southeastern Baden-Württemberg. The region stretches from the prealpine lands up to the Alps...
.
A special unit is the greywacke zone
Greywacke zone
The greywacke zone is a band of Paleozoic metamorphosed sedimentary rocks that forms an east-west band through the Austrian Alps. The greywacke zone crops out between the Mesozoic rocks of the Northern Calcareous Alps and the Austroalpine and Penninic basement rocks of the Central Eastern Alps....
, a band of Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
metamorphosed sedimentary rock
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....
s that forms an east-west band through the Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
n Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
. The greywacke zone crops out between the Mesozoic rocks of the Northern Calcareous Alps and the Austroalpine and Penninic basement rocks of the Central Eastern Alps
Central Eastern Alps
The Central Eastern Alps comprise the main chain of the Eastern Alps with its highest peaks, located between the Northern Limestone Alps and the Southern Limestone Alps, from which they differ in geological composition....
. Stratigraphically
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, the greywacke zone can be up to 2 kilometres (1.2 mi) thick.
All of these 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...
were fold
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
ed and thrusted, so that the basement rock can be found on top of the sediments and vice versa.
Geographic position
In SwitzerlandSwitzerland
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....
, the Austroalpine nappes have been eroded
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
away except for a few isolated outcrops called the Sesia unit
Sesia unit
The Sesia unit or Sesia nappe, also called the Sesia-Dent Blanche unit is a tectonic unit or terrane in the Swiss and Italian Alps. The zone crops out in the Pennine Alps and in the southeastern part of the Aosta Valley...
and the Dent Blanche klippe
Dent Blanche klippe
The Dent Blanche nappe or Dent Blanche klippe is a geologic nappe and klippe that crops out in the Pennine Alps. The nappe is tectonostratigraphically on top of the Penninic nappes and by most researchers seen as Austroalpine....
(the Matterhorn
Matterhorn
The Matterhorn , Monte Cervino or Mont Cervin , is a mountain in the Pennine Alps on the border between Switzerland and Italy. Its summit is 4,478 metres high, making it one of the highest peaks in the Alps. The four steep faces, rising above the surrounding glaciers, face the four compass points...
is the most outstanding example of an Austroalpine klippe
Klippe
thumb|right|350px|Schematic overview of a thrust system. The shaded material is called a [[nappe]]. The erosional hole is called a [[window |window or fenster]]. The klippe is the isolated block of the nappe overlying autochthonous material....
). These remaining Swiss nappes have a different tectonic and metamorphic history than their counterparts in Austria, which is why they are not always considered a part of the Austroalpine nappes.
On the other hand, in Austria, the Austroalpine nappes cover the largest part of that country, except for a few windows
Window (geology)
thumb|right|350px|Schematic overview of a thrust system. The [[fault |hanging wall block]] is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a window. A [[klippe]] is a solitary outcrop of the nappe in the middle of autochthonous material.A tectonic window...
like the Hohe Tauern window
Hohe Tauern window
The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out...
and the Engadin window.
Traces of the Eo-Alpine orogenic phase
Before the formation of the Alps in the lower and middle TertiaryTertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
period, the Austroalpine rocks experienced another deformation phase: the Eo-Alpine phase of mountain building that took place in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
. The metamorphic field gradient is to the east-south-east, so in a west-north-western direction, the traces become less severe. In the west of Switzerland, the event cannot be recognized anymore. In Austria, however, Eo-Alpine 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...
lenses occur close to the Hohe Tauern window
Hohe Tauern window
The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out...
.
The Eo-Alpine phase is sometimes seen as the earliest phase of the Alpine orogeny. However, after the initial mountain-building, the tectonic plates moved away from each other. The next phase was more than 50 Ma later, so the events are often seen as unrelated.