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The Austroalpine nappes are a
geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...
nappe stack 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 European
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
. The Alps contain three such stacks, of which the Austroalpine nappes are structurally on top of the other two (meaning they were
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
over the other two). The name Austroalpine means ''Southern Alpine'', because these nappes crop out mainly in the
Eastern Alps Eastern Alps is the name given to the eastern half of the Alps, usually defined as the area east of a line from Lake Constance and the Alpine Rhine valley up to the Splügen Pass at the Alpine divide and down the Liro River to Lake Como in t ...
(the Alps east of the line
Lake Constance Lake Constance (german: Bodensee, ) refers to three bodies of water on the Rhine at the northern foot of the Alps: Upper Lake Constance (''Obersee''), Lower Lake Constance (''Untersee''), and a connecting stretch of the Rhine, called the Lak ...
-
Chur , neighboring_municipalities= Arosa, Churwalden, Tschiertschen-Praden, Domat/Ems, Felsberg, Malix, Trimmis, Untervaz, Pfäfers , twintowns = Bad Homburg (Germany), Cabourg (France), Mayrhofen (Austria), Mondorf-les-Bains (Luxe ...
Lake Como Lake Como ( it, Lago di Como , ; lmo, label=Western Lombard, Lagh de Còmm , ''Cómm'' or ''Cùmm'' ), also known as Lario (; after the la, Larius Lacus), is a lake of glacial origin in Lombardy, Italy. It has an area of , making it the thir ...
). Because the Austroalpine nappes consist of material from the former Apulian or Adriatic plate, that was thrust over the European plate, they are called
allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a nappe. If an erosional hole is created in the nappe that is called a window (geology)">window. A klippe is a solitary ou ...
nappes. In comparison with the other nappe stacks, they have experienced lower-grade
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
, which distinguishes them clearly from the
Penninic nappes The Penninic nappes or the Penninicum, commonly abbreviated as Penninic, 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 ...
on which they rest.


Lithologies

The Austroalpine nappes are fragments of the former
continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
and
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 margin ...
of the Apulian or Adriatic plate. These fragments contain rocks from the continental
basement A basement or cellar is one or more Storey, floors of a building that are completely or partly below the storey, ground floor. It generally is used as a utility space for a building, where such items as the Furnace (house heating), furnace, ...
as well as from
sedimentary rock 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 ...
s deposited in these environments. The basement rocks have experienced
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...
related to their original depth in the Earth's crust, but in the Austroalpine nappes
Alpine Alpine may refer to any mountainous region. It may also refer to: Places Europe * Alps, a European mountain range ** Alpine states, which overlap with the European range Australia * Alpine, New South Wales, a Northern Village * Alpine National P ...
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 Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite, ...
to
amphibolite facies 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 ...
, depending on their original depth. They are
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
s and (para-)
gneiss 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 a ...
es intruded by
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 un ...
s of
Variscan 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 f ...
and
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
age. On top of this basement rock,
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
and
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
sedimentary and
volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
s were deposited. Shallow marine
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
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 The Northern Limestone Alps (german: Nördliche Kalkalpen), also called the Northern Calcareous Alps, are the ranges of the Eastern Alps north of the Central Eastern Alps located in Austria and the adjacent Bavarian lands of southeastern Germany ...
''. Sometimes, the limestone has been turned into
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
, as in the
Austria Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...
n region
Salzkammergut The Salzkammergut (; ; bar, Soizkaumaguad, label=Central Austro-Bavarian) is a resort area in Austria, stretching from the city of Salzburg eastwards along the Alpine Foreland and the Northern Limestone Alps to the peaks of the Dachstein Mou ...
and the
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
region
Allgäu The Allgäu (Standard German: , also Allgovia) is a region in Swabia in southern Germany. It covers the south of Bavarian Swabia, southeastern Baden-Württemberg, and parts of Austria. The region stretches from the pre-alpine lands up to the ...
. A special unit is the
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 P ...
, a band of
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
metamorphosed
sedimentary rock 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 ...
s that forms an east-west band through the
Austria Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...
n
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
. 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 The Central Eastern Alps (german: Zentralalpen or Zentrale Ostalpen), also referred to as Austrian Central Alps (german: Österreichische Zentralalpen) or just Central Alps, comprise the main chain of the Eastern Alps in Austria and the adjacen ...
. Stratigraphically, the greywacke zone can be up to {{convert, 2, km thick. All of these
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 ...
were folded and thrust, so that the basement rock can be found on top of the sediments and vice versa.


Geographic position

In
Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
, the Austroalpine nappes have been
eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is disti ...
away except for a few isolated outcrops called the
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. It is widely seen as p ...
and the
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 nappe is na ...
(the
Matterhorn The (, ; it, Cervino, ; french: Cervin, ; rm, Matterhorn) is a mountain of the Alps, straddling the main watershed and border between Switzerland and Italy. It is a large, near-symmetric pyramidal peak in the extended Monte Rosa area of the ...
is the most outstanding example of an Austroalpine
klippe 350px, Schematic overview of a thrust system. The shaded material is called a window_(geology).html"_;"title="nappe._The_erosional_hole_is_called_a__window_(geology)">window_or_fenster._The_klippe_is_the_isolated_block_of_the_nappe_overlying_aut ...
). These remaining Swiss nappes have a different
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
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 Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for se ...
like the
Hohe Tauern window The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window (in German ''fenster'') in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out. The structu ...
and the
Engadin window The Engadin window or (Lower Engadin window) is a tectonic window that exposes penninic units lying below the austroalpine units in the alpine nappe stack. It has a roughly elliptical shape with the long axis striking northwest-southeast and di ...
.


Traces of the Eo-Alpine orogenic phase

Before the formation of the Alps in the lower and middle
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
period, the Austroalpine rocks experienced another deformation phase: the Eo-Alpine phase of mountain building that took place in the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
. The metamorphic grade increases 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. In Austria, however, Eo-Alpine
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, ...
lenses occur close to the
Hohe Tauern window The Hohe Tauern window is a geological structure in the Austrian Central Eastern Alps. It is a window (in German ''fenster'') in the Austroalpine nappes where high-grade metamorphic rocks of the underlying Penninic nappes crop out. The structu ...
. The Eo-Alpine phase is sometimes seen as the earliest phase of the Alpine orogeny. However, after the initial mountain-building, the
tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s moved away from each other. The next phase was more than 50 Ma later, so the events are often seen as unrelated.


References


Description of the geology of Austria, website of Christof Kuhn
Geology of the Alps Structural geology Geology of Austria Geology of Switzerland Geology of Italy