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The Austroalpine nappes are a
geological Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
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 ...
system in the European
Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...
. They
structurally A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
on top of the
Penninic 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 ...
(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 them). The prefix Austro in this term refers to Austria, rather than south/southern. This is because the bulk of the Austroalpine nappes (which constitute the Eastern Alps except for some tectonic
windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
) is in Austria, although they also reach into Eastern
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 ...
. The western boundary of the Eastern Alps is the
Lake Constance Lake Constance (, ) 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 Seerhein (). These ...
-
Chur '' Chur (locally) or ; ; ; ; ; ; or ; , and . is the capital and largest List of towns in Switzerland, town of the Switzerland, Swiss Cantons of Switzerland, canton of the Grisons and lies in the Alpine Rhine, Grisonian Rhine Valley, where ...
Lake Como Lake Como ( , ) also known as Lario, is a lake of glacial origin in Lombardy, Italy. It has an area of , making it the third-largest lake in Italy, after Lake Garda and Lake Maggiore. At over deep, it is one of the deepest lakes in Europe. ...
line.


Geographic position and nappes in the Western Alps

The Austroalpine nappes constitute the Eastern Alps, except for some tectonic
windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
such as the
Tauern window The 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 structure is ...
, the Rechnitz window (both in Austria) 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 orogeny, alpine nappe stack. It has a roughly elliptical shape with the long axis striking northwest-so ...
(Switzerland). They cover eastern Switzerland and the largest part Austria. In the Western Alps the
Sesia The Sesia (Latin ''Sesites'' or ''Sessites'') is a river in Piedmont, north-western Italy, tributary to the Po. Geography Its sources are the glaciers of Monte Rosa at the border with Switzerland. It flows through the Alpine valley Valsesia an ...
(
Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...
) and the
Dent Blanche The Dent Blanche is a mountain in the Pennine Alps, lying in the canton of Valais in Switzerland. At -high, it is one of the highest peaks in the Alps. Naming The original name was probably ''Dent d'Hérens'', the current name of the nearby De ...
(Switzerland)
units Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...
form a
klippe 350px, Schematic overview of a thrust system. The shaded material is called a window or fenster. The klippe is the isolated block of the nappe overlying autochthonous material. A klippe ( German for cliff">German language">German for cliff or c ...
over the
Penninic 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 ...
nappes. They have been labelled as Austroalpine because this term has also been generally used to refer to units derived from the
Adriatic plate The Adriatic or Apulian plate is a small list of tectonic plates, 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 usu ...
. However, they are distinct from and unrelated to the Austroalpine nappes of the Eastern Alps. They have a different palaeogeographic origin and tectonic history. Thus, they are not always considered a part of the Austroalpine nappes. Neukirchen (2022) The
Matterhorn The , ; ; ; or ; ; . is a mountain of the Alps, straddling the Main chain of the Alps, main watershed and border between Italy and Switzerland. It is a large, near-symmetric pyramidal peak in the extended Monte Rosa area of the Pennine Alps, ...
is the most outstanding example of this klippe.


Eoalpine formation of the Austroalpine nappes and the Eastern Alps

The formation of the Alps occurred in two
orogenic 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 ...
phases. The Austroalpine nappes of the Eastern Alps were formed in the first, early, Eo-Alpine orogeny in the
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
, 140-84 Ma (millions years ago). The Central and Western Alps were formed in the second orogenic phase which occurred much later, from the end of
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
, 35 Ma. The eo-Alpine orogeny resulted from the
continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...
between the north-western corner of the
Adriatic plate The Adriatic or Apulian plate is a small list of tectonic plates, 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 usu ...
(overriding plate) and a
continental fragment Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...
, Alcapa, (subducting plate). This followed the
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
and closure of a western embayment of the Meliata ocean which was between them. This ocean was to the east of the former and south of the latter. It was a northern branch of the
Neotethys Ocean The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasian ...
which had opened in the
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
.handy et al. (2010) In this orogeny the Austroalpine nappes and the
Western Carpathians The Western Carpathians () are a mountain range and geomorphological province that forms the western part of the Carpathian Mountains. The mountain belt stretches from the Low Beskids range of the Eastern Carpathians along the border of Poland w ...
ware formed on Alcapa, while the South Alpine nappes were formed on the Adriatic plate. Parts of Alcapa were accreted to the base and leading edge of the Adriatic plate. The movement of the thrust front was from southwest to northwest. At the time of the eo-Alpine orogeny the Adriatic plate was further southeast than its present location. It was separated from the European continental margin of the time by the Piemont part of the
Piemont-Liguria Ocean The Piemont-Liguria basin or the Piemont-Liguria Ocean (sometimes only one of the two names is used, for example: Piemonte Ocean) was a former piece of oceanic crust that is seen as part of the Tethys Ocean. Together with some other oceanic basi ...
. Later this ocean started to subduct. The Adriatic plate moved north-westward, converged with Europe and reached its current position. These two plates eventually collided, leading to the second orogenic phase.


Structure

The Austroalpine nappes (which originally were between the
Piemont-Liguria Ocean The Piemont-Liguria basin or the Piemont-Liguria Ocean (sometimes only one of the two names is used, for example: Piemonte Ocean) was a former piece of oceanic crust that is seen as part of the Tethys Ocean. Together with some other oceanic basi ...
, at its southern end, and the Meliata Ocean, at its northern end) are divided into the lower and upper nappes. Their structure is as follows: • Upper Austroalpine nappes. These were the nappes which were displaced the most during the eo-Alpine orogeny. They build up the
Northern Calcareous Alps The Northern Limestone Alps (), 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. The distinction from the l ...
. Their lower nappes tend to be in their north, while the higher ones are in their south. These nappes are divided into three juxtaposed nappes systems. From upper to lower position they are: A) ''Juvavic nappes''. These are a series of
klippe 350px, Schematic overview of a thrust system. The shaded material is called a window or fenster. The klippe is the isolated block of the nappe overlying autochthonous material. A klippe ( German for cliff">German language">German for cliff or c ...
n overlying the Tirolian nappes. It was the
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 ...
which was accreted to the Northern Calcareous Alps. It became completely
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 ...
. Its remnants are only preserved in rock blocks that were
overthrust A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
on top of Middle to Late
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
basin fills of basins that were in front or on top of the propagating thrust belt and were later overthrust. A formation in the Lower Juvavic nappes at the Eastern Alps eastern margin with deep-water
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
s,
radiolarite Radiolarite is a Siliceous ooze, siliceous, comparatively hard, fine-grained, chert-like, and homogeneous sedimentary rock that is composed predominantly of the microscopic remains of radiolarians. This term is also used for Friability, indurat ...
s
greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...
s and
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...
is seen as the remains of the
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 ...
of the subduction zone. The Upper Juvavic nappes originate from the distal (outer, next to the deep-waters of an ocean) area of the
passive continental margin 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 margi ...
of the
Adriatic plate The Adriatic or Apulian plate is a small list of tectonic plates, 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 usu ...
along the northern end of the Meliata ocean.Schmid at al. (2004) B) ''Tirolian nappes''. These nappes are the
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
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 ...
directly above the sediments of the greywacke zone. These two
units Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...
originally were in the same position in the passive margin along the northern end of the Meliata Ocean. The Tirolian nappes have internal thrusting and
faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...
but only minor
folding Fold, folding or foldable may refer to: Arts, entertainment, and media * ''Fold'' (album), the debut release by Australian rock band Epicure * Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Abov ...
because they are dominantly dolomite. Mandl (2000) C) ''Bavarian nappes''. These are lowermost nappes. They are at the northern rim of the Eastern Alps and directly overly
Penninic 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 ...
units derived from the Piemont-Liguria Ocean. Unlike the Juvavic and Tirolian nappes, they originate from the distal (outer) area of the passive continental margin at the southern end of the Piemont-Liguria Ocean and in a relatively more external position with respect to the earlier formed passive margin at the northern end of the Meliata Ocean which was further south. There is also the ''Greywacke zone'', which is a narrow strip of
Palaeozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma at the start of ...
sediments (
greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...
s,
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
s and
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
s and others). It is the former
substratum Substrata, plural of substratum, may refer to: *Earth's substrata, the geologic layering of the Earth *''Hypokeimenon'', sometimes translated as ''substratum'', a concept in metaphysics *Substrata (album), a 1997 ambient music album by Biosphere * ...
of the Tirolian nappes which were deposited directly above them. This zone was in the
passive continental margin 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 margi ...
at the northern end of the Meliata Ocean. It is thought to have been originally the Palaeozoic basement of the Tirolian nappes
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
rocks. They remained several km behind in the south during the nappe movement. The Northern Calcareous nappes and the greywacke zone form a large thin-skinned
fold and thrust belt A fold and thrust belt is a series of mountainous foothills adjacent to an orogenic belt, which forms due to contractional tectonics. Fold and thrust belts commonly form in the forelands adjacent to major orogens as deformation propagates outwards ...
. • Lower Austroalpine nappes. These were the (
Variscan 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 ...
)
basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...
s of the nappes of the eo-Alpine orogeny over which the other (
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 ...
) Austroalpine nappes were thrust. They are widespread in Eastern Switzerland, particularly along the SW margin of the Austroalpine nappes. They are also found along the NE margin and, occasionally, the northern margin of the
Tauern window The 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 structure is ...
. They were derived from a very external part of the Adriatic plate that faced the Piemont-Liguria Ocean. These nappes were not moved during the Eo-Alpine orogeny and were only slightly
overprinted An overprint is an additional layer of text or graphics added to the face of a postage or revenue stamp, postal stationery, banknote or ticket after it has been printed. Post offices most often use overprints for internal administrative purpos ...
during this orogeny. They were
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 and comprise only a few nappes. The border between the Eastern and Western Alps (eastern Switzerland and easternmost Austria) are the western end of the eo-Alpine orogeny.


The question of the suture zone

Suture zones mark the areas where two landmasses have amalgamated after the closure of an oceanic basin. The two elements which aid their identification is the presence of traces of
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...
s which have been obducted onto
continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...
during
continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...
and/or eclogitic
units Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...
which indicate
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
of rocks to great depths, reaching high or ultrahigh pressure in subduction metamorphism. In the Austroalpine nappes identifying sutures is particularly difficult because these have been heavily eroded. Findings related to a suture have led to question marks regarding the dynamics of the eo-Alpine orogeny and its timeframe. The
Koralpe The Koralpe (, , or ), also referred to as ''Koralm'' (), is a mountain range in southern Austria which separates eastern Carinthia from southern Styria. The southern parts of the range extend into Slovenia. Running from north to south, it drain ...
-Wölz nappe system is a narrow high-pressure metamorphism belt. It is a candidate for the Eo-Alpine suture. However, there are disagreements regarding its nature and timing. This nappe system comprises a series of basement units that have a significant eo-Alpine metamorphic
overprint An overprint is an additional layer of text or graphics added to the face of a Postage stamp, postage or revenue stamp, postal stationery, banknote or Ticket (admission), ticket after it has been Printing, printed. Post offices most often use ...
. Its largest occurrences are east of the Tauern window. They become gradually more spread out eastwards. There are eclogitic units which indicate subduction to great depths. These are underlain by
basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...
units that indicate lower pressures and temperatures
metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...
. Schmid et al. (2004) propose that they were exhumed northward in an extrusion wedge. The eo-Alpine high pressure metamorphic overprint and the subsequent extrusion were caused by the collision between the northern and southern margins of the Meliata ocean (eo-Alpine orogeny). However, the metamorphism in this high pressure belt occurred in the Late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, whereas the nappe
thrusting 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 syst ...
of the eo-Alpine orogeny was completed in the Early Cretaceous. Moreover, there are no traces 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 ...
. The eclogites are seen as being from intra-continental
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 ...
s, indicating an intracontinental subduction. Kurtz and Fritz (2003) propose that the eo-Alpine orogeny might have been a two-stage cycle. The first one involved subduction and closure of the Meliata ocean (Late
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
) and the formation of the Northern Calcareous Alps (Early Cretaceous). The second one involved southward
magmatic underplating Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity or within the Crust (geology), crust. Entrapment (or 'stalling out') of magmas within the crust occurs due to the dif ...
of the continental margin of the
Adriatic plate The Adriatic or Apulian plate is a small list of tectonic plates, 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 usu ...
and imbrication of the Austroalpine basements which underwent
metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...
. This increased from greenschist-facies in the northern parts to
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 ...
and
eclogite facies 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, zoisite, dolo ...
in its southernmost parts. The origin of the second phase was probably
rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...
,
crustal extension Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...
and crustal thinning in the
Permian The Permian ( ) is a geologic period and System (stratigraphy), 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 251.902 Mya. It is the s ...
accompanied by magmatic underplating in the internal parts of the orogen. Heating of the crust by the latter created high temperature metamorphism in some rocks. Kurtz and Fritz (2003) With later cooling, the crust thickened again. It became gravitationally unstable causing the onset of
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
inside the continent. The high-pressure metamorphic rocks were exhumed in the Late Cretaceous through a
detachment fault A detachment fault is a gently dipping normal fault associated with large-scale extensional tectonics. Detachment faults often have very large displacements (tens of km) and juxtapose unmetamorphosed hanging walls against medium to high-grade me ...
in the lower crust. On the basis of an analysis of
detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephen ...
chrome
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of ''spine,'' in reference to its pointed crystals. Prop ...
grains A grain is a small, hard, dry fruit ( caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and le ...
found southwest of
Salzburg Salzburg is the List of cities and towns in Austria, fourth-largest city in Austria. In 2020 its population was 156,852. The city lies on the Salzach, Salzach River, near the border with Germany and at the foot of the Austrian Alps, Alps moun ...
(in the Hallstatt Mélange area of the Saalach Zone, western central
Northern Calcareous Alps The Northern Limestone Alps (), 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. The distinction from the l ...
), Gawlick et al. (2015) propose that the eo-Alpine orogeny stated earlier than the usually indicated timeframe (Early Cretaceous). These authors argue that their study indicates that in the southern Northern Calcareous Alps
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...
obduction Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, ...
occurred earlier, in the Middle to early Late
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
. This obducted ophiolite stack erosion started in the
Kimmeridgian In the geologic timescale, the Kimmeridgian is an age in the Late Jurassic Epoch and a stage in the Upper Jurassic Series. It spans the time between 154.8 ±0.8 Ma and 149.2 ±0.7 Ma (million years ago). The Kimmeridgian follows the Oxfordian ...
(Late Jurassic) and lasted until the
Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...
(late Early Cretaceous). Erosion was slowed down by a Kimmeridgian to earliest Cretaceous
carbonate platform A carbonate platform is a Sedimentary rock, sedimentary body which possesses topographic relief, and is composed of Autochthon (geology), autochthonic calcareous deposits. Platform growth is mediated by Sessility (zoology), sessile organisms whose ...
that evolved on top of the nappe stack with the obducted ophiolites. It protected the ophiolites against erosion. Afterwards, the ophiolites were quickly eroded.


Lateral escape

Lateral escape or extrusion refers to the horizontal (sideways) movement of a landmass. It can be sinistral (left-lateral) or dextral (right –lateral). The sideways movement is caused by a
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 ...
. In the case of the Austroalpine nappes and the Eastern Alps thrusting was caused by indenter tectonics. In a
continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at Convergent boundary, convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroy ...
one of the two colliding
tectonic plates Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
is rigid (with strong
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
rocks) and the other is “soft” (with weaker lithosphere rocks). The former acts as an indenter which thrusts into the rocks of the latter, which become deformed, shortened and
extended Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (proof theory) * Extension (predicate logic), the set of tuples of values that ...
(stretched) due to the indentation, causing lateral escape. During the collision between the Adriatic plate and the European continental imaging, thrusting by the former exerted a compressive regime and N-S crustal shortening. It also acted as an indenter which is divided into two parts, the Insubric and the Dolomites indenters. The latter is in the Southern Alps which are on this plate, south of the eastern part of the Northern Calcareous Alps. Since it is a protrusion, the largest amount of shortening is at its tip. Due to indentation, instead of being thrust further northward towards the
foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
, the Eastern Alps underwent a lateral pull-apart. They were strongly stretched, narrowed and elongated eastward (through lateral escape) to almost twice their original length. The largest amount of shortening is at the tip of the Dolomites Indenter. Near this there has been the formation of the
Tauern window The 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 structure is ...
, a pull-apart structure which underwent substantial stretching and elongation. The
Ötztal The Ötztal () is an alpine valley located in Tyrol, Austria. The Ötztaler Ache river flows through the valley in a northern direction. The Ötztal separates the Stubai Alps in the east from the Ötztal Alps in the west. The valley is long. The ...
and Gurktal blocks, which are just east and west of the window, were moved 160 km apart.Frisch et al. (2000) Rosenberg et al. (2018) estimate that shortening in the western part of the Eastern Alps amounted to 75 km, 55 km of which were accommodated in the Tauern Window. These figures in Handy et al. (2011) are 125 km and 70 km of accommodation in the Tauern window. Frisch et al. (2000) estimate that at the northern tip of the Dolomite indenter it was 113 km. Shortening decreased eastward (there the Eastern Alps broaden) because convergence between the two tectonic plates decreased. Frisch et al. (2000) estimate an amount of 40 km the eastern margin; Rosenberg et al. (2018) estimate 30 km. Some estimates for the total amount of extension in the Eastern Alps are 170 km and 130 km (ca. 85 km of which is in the North-Calcareous Alps and ca. 45 km is immediately north of the indenter).Rosenberg et al. (2018) Frisch et al. (2000) propose that the eastward movement of the Eastern Alps was made possible because there was an oceanic basin in the area of the future
Pannonian Basin The Pannonian Basin, with the term Carpathian Basin being sometimes preferred in Hungarian literature, is a large sedimentary basin situated in southeastern Central Europe. After the Treaty of Trianon following World War I, the geomorpholog ...
where there was
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
with
slab rollback Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tr ...
. This provided the space to accommodate this movement. Extension also occurred in Central Alps. The total stretching in the Central and Eastern Alps is estimated by Frisch et al. (2000) to have been more than 300 km. These authors also propose that these two parts of the Alps are part of an extensional province that also includes the
Pannonian basin The Pannonian Basin, with the term Carpathian Basin being sometimes preferred in Hungarian literature, is a large sedimentary basin situated in southeastern Central Europe. After the Treaty of Trianon following World War I, the geomorpholog ...
.


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 islan ...
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 margi ...
of the Apulian or Adriatic plate. These fragments contain rocks from the continental
basement A basement is any Storey, floor of a building that is not above the grade plane. Especially in residential buildings, it often is used as a utility space for a building, where such items as the Furnace (house heating), furnace, water heating, ...
as well as from
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s deposited in these environments. The basement rocks have experienced
metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...
related to their original depth in the Earth's crust, but in the Austroalpine nappes Alpine metamorphism 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 (fl ...
, depending on their original depth. They are
Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
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 ...
s and (para-)
gneiss 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 ...
es intruded by
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 ...
s of
Variscan 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 ...
and
Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
age.
Permian The Permian ( ) is a geologic period and System (stratigraphy), 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 251.902 Mya. It is the s ...
and
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
sedimentary and
volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...
s were deposited on top of these basement rocks. Shallow marine
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
s are abundant, these limestones now form the''
Northern Calcareous Alps The Northern Limestone Alps (), 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. The distinction from the l ...
',' the mountain chains of the northern part of the Eastern Alps. Sometimes, the limestone has been turned into dolomite, as in the
Austria Austria, formally the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine Federal states of Austria, states, of which the capital Vienna is the List of largest cities in Aust ...
n region
Salzkammergut The Salzkammergut (, ; ) 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 Mountains. The main river of the region is the Traun (r ...
and the
German German(s) may refer to: * Germany, the country of the Germans and German things **Germania (Roman era) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizenship in Germany, see also Ge ...
region
Allgäu The Allgäu (Standard ) 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 Alps. The main rivers flo ...
. 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 first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
metamorphosed
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s that forms an east-west band through the
Austria Austria, formally the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine Federal states of Austria, states, of which the capital Vienna is the List of largest cities in Aust ...
n
Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...
. It crops out between the Mesozoic rocks of the Tirolian nappes of the upper Austroalpine nappes.
Stratigraphically Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...
, the greywacke zone can be up to 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 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 ...
, so that the basement rocks can be found on top of the
sedimentary rocks 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. Sedim ...
.


Notes


References

{{Reflist


Bibliography

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*Gawlick, H.-J., Frisch, W. (2003): The Middle to Late Jurassic carbonate clastic radiolaritic flysch sediments in the Northern Calcareous Alps: sedimentology, basin evolution and tectonics - an overview. – Neues Jahrbuch Geologie Paläontologie, Abhandlungen, Vol. 230 pp. 163–21

*Gawlick H-J., Schlagintweit F.,Missoni S. (2015) Ophiolitic detritus in Kimmeridgian resedimented limestones and its provenance from an eroded ophiolitic nappe stack south of the Northern Calcareous Alps (Austria) Geologica Carpathica, Vol 66 (6) pp. 473-4

*Handy M.R., Schmid S.M., Bousquet R., Kissling E., Bernoulli D.(2010) Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps, Earth Science Reviews, Vol. 102, pp. 121-1

*Handy M. R, Rosenberg C.L., Ustaszewski K., Spakman W. (2011) Fragmentation of the Adriatic Indenter and its bearing on changing slab configurations beneath the Alps-Carpathian-Dinaride chain, AGU Fall Meeting, 5-9 Decemb

*Kurtz W., Fritz H. (2003) Thermometamophic evolution in the Austroalpine nappe complex in the eastern Alps –Consequences for the eo-Alpine subduction zone, International Geological Review, Vol. 45, (12) pp. 587-6

*Mandl G.W., (2000) The Alpine sector of the Tethyan shelf - Examples of Triassic to Jurassic sedimentation and deformation from the Northern Calcareous Alps, Mitteilungen Osterreichishen Geologischen Gesellschaft, Vol. 92, pp. 61-

*Neukirchen F. (2022) The formation of Mountains, Springer, ISBN 978-3031113840 *Ratschbacher L., Frisch, W., Neubauer F., Schmid S.M., Neugebauer J. (1989) Extension in compressional orogenic belts: The Eastern Alps. Geology Vol. 17, pp. 404–407. *Rosenberg C.L., Schneider S., Scharf, Bertrand A., Hammerschmidt K., Rabaute A., Brun J-P., Relating collisional kinematics to exhumation processes in the Eastern Alps, Earth-Science Reviews, Vol. 176, 2018, pp.311-34

*Schmid S.M., Fügenschuh B., Kissling E., Schuster R. (2004) Tectonic map and overall architecture of the Alpine orogen, Eclogae geologicae Helvetiae, Vol. 97, pp. 93–11

*Stüwe K., Schuster R. (2010) Initiation of a subduction in the Alps: continent or ocean? Geology, Vol. 38, pp. 175-1

Geology of the Alps Structural geology Geology of Austria Geology of Switzerland Geology of Italy