Penninic nappes
Encyclopedia
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 (in Austria
), where they crop out
as a narrow band. The name Penninic is derived from the Pennine Alps
, an area in which rocks from the Penninic nappes are abundant.
Of the three nappe stacks the Penninic nappes have the highest metamorphic grade
. They contain high grade metamorphic rock
s of different paleogeographic origins. They were deposited as sediment
s on the crust
that existed between the European and Apulian plate
s before the Alps were formed. They are characteristically ophiolite sequences and deep marine
sediments, metamorphosed to phyllite
s, schist
s and amphibolite
s.
The Piemont-Liguria Ocean and the Valais Ocean are, together with some other small oceanic basins, called Alpine Tethys Ocean or Western Tethys Ocean. The Tethys Ocean
itself is sometimes considered to have begun east of the Apulian and African plates, but normally the Alpine Tethys is regarded as part of it.
are found in the Hohe Tauern window
, the Kőszeg Mountains
and at the northern boundary of the Alps:
It is not clear which of these units can be correlated with the Penninic units of the Western Alps. Some of them are clearly Penninic, some clearly Helvetic
, and some are disputed. The oceanic trench deposits of the Penninic nappes are found through the Alps and called Bündner slate
s.
What is clear at least is that the Briançonnais terrane is not found in the Eastern Alps
. The conclusion that can be drawn is that the microcontinent wedged out in the east in the Alpine Tethys Ocean. Some authors suggest the ophiolites that occur at the Hohe Tauern window must be correlated with the Piemont-Liguria terrane of the western Alps, because trench deposits such as radiolarite
s occur in both.
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...
zones in which the Alps can be divided. In the western Alps the Penninic nappes are more obviously present than in the eastern Alps (in 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...
), where they 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...
as a narrow band. The name Penninic is derived from the Pennine Alps
Pennine Alps
The Pennine Alps are a mountain range in the western part of the Alps. They are located in Switzerland and Italy...
, an area in which rocks from the Penninic nappes are abundant.
Of the three nappe stacks the Penninic nappes have the highest metamorphic grade
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...
. They contain high grade metamorphic rock
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...
s of different paleogeographic origins. They were deposited as sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s on the 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...
that existed between the European and Apulian 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...
s before the Alps were formed. They are characteristically ophiolite sequences and deep marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
sediments, metamorphosed to phyllite
Phyllite
Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...
s, 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 amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s.
Subdivision in the Western Alps
Four paleogeographic domains can be recognized in the Penninic nappes of the Western Alps:- rocks from the former European continental marginContinental marginThe continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
that were subductedSubductionIn geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
and obductedObductionObduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode....
again. - rocks from the continental crustContinental crustThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
of the Valais OceanValais OceanThe Valais Ocean is a disappeared piece of oceanic crust which was situated between the continent Europe and the microcontinent Iberia or so called Briançonnais microcontinent...
, metamorphosed ophiolites and other sedimentary rockSedimentary rockSedimentary 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 from this disappeared oceanic basinBasin (geology)A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines...
. The occurrence of eclogiteEclogiteEclogite 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 shows these rocks were subducted to great depths in the Earth’s mantle. Valais sedimentary rocks include thin CretaceousCretaceousThe 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...
limestoneLimestoneLimestone 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 (now marbleMarbleMarble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...
s) and TertiaryTertiaryThe 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...
flyschFlyschFlysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...
which is now turned into (micaMicaThe mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
-) schistSchistThe 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. - rocks from the former BriançonnaisBriançonnais microcontinentThe Briançonnais zone or Briançonnais terrane is a piece of continental crust found in the Penninic nappes of the Alps.According to some paleogeographic reconstructions the rocks of the Briançonnais zone were in fact a part of the microcontinent Iberia, that encompassed not only the Iberian...
microcontinent. These are rocks from the lower continental crustContinental crustThe continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
deformed and intruded by VariscanVariscan orogenyThe 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:...
graniteGraniteGranite 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, but also metamorphosed sedimentary rocks: graphiteGraphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
-bearing CarboniferousCarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
rocks, red sandstoneSandstoneSandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s from the PermianPermianThe 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...
period, TriassicTriassicThe Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
evaporiteEvaporiteEvaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s and thin limestones of the JurassicJurassicThe Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
and lower Cretaceous. Examples of Briançonnais terraneTerraneA terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s are the Sankt Bernard and Monte Rosa nappes; the Monte RosaMonte RosaThe Monte Rosa Massif is a mountain massif located in the eastern part of the Pennine Alps. It is located between Switzerland and Italy...
and the MischabelhörnerMischabelhörnerThe Mischabelhörner, or just Mischabel, is a massif in the Pennine Alps of Valais, Switzerland, which forms the divide between the Saastal and Mattertal. Of all its summits, the highest is the Dom; at 4545 m the highest peak completely inside Switzerland. The other two chief summits are the...
are formed by hard Briançonnais gneissGneissGneiss 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. - rocks from the former Piemont-Liguria OceanPiemont-Liguria OceanThe Piemont-Liguria basin or the Piemont-Liguria Ocean was a former piece of oceanic crust that is seen as part of the Tethys Ocean...
, mainly ophiolites (fragments of the oceanic crustOceanic crustOceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
of this domain); limestone deposited in shallower parts of the Piemont-Liguria ocean and turned into marble and originally deep marine mudstoneMudstoneMudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s formed in the oceanic trenchOceanic trenchThe oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor....
that existed at the northern edge of the Apulian plateApulian PlateThe 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...
. From the Cretaceous onward the oceanic crust of the Piemont-Liguria ocean subducted at these trenches beneath the Apulian plate.
The Piemont-Liguria Ocean and the Valais Ocean are, together with some other small oceanic basins, called Alpine Tethys Ocean or Western Tethys Ocean. The Tethys Ocean
Tethys Ocean
The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...
itself is sometimes considered to have begun east of the Apulian and African plates, but normally the Alpine Tethys is regarded as part of it.
Subdivision in the Eastern Alps
The following Penninic lithologiesLithology
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...
are found in 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 Kőszeg Mountains
Koszeg Mountains
Kőszeg Mountains is a mountain range in the Alpokalja area, the easternmost region of the Alps. The territory of the range is shared between Austria and Hungary. Its highest point is the Írott-kő with a height of 883 metres....
and at the northern boundary of the Alps:
- old PrecambrianPrecambrianThe Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
and CambrianCambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
gneissGneissGneiss 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 of continental basementBasement (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...
. - various paleozoicPaleozoicThe Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
metamorphic rockMetamorphic rockMetamorphic 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...
s (ophiolites, schistSchistThe 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) - younger HercynicVariscan orogenyThe 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:...
(late CarboniferousCarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
) graniteGraniteGranite 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...
intrusionIntrusionAn intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s (turned into gneisses by deformation in the Alpine orogenyAlpine orogenyThe Alpine orogeny is an orogenic phase in the Late Mesozoic and Tertiary that formed the mountain ranges of the Alpide belt...
). - TriassicTriassicThe Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
and JurassicJurassicThe Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
sedimentary rocks and ophiolites, turned into calcareous phyllitePhyllitePhyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...
s and green schists by metamorphosis. StratigraphicallyStratigraphyStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
on top of these Cretaceous to early Tertiary flyschFlyschFlysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...
deposits are found sometimes.
It is not clear which of these units can be correlated with the Penninic units of the Western Alps. Some of them are clearly Penninic, some clearly Helvetic
Helvetic nappes
The Helvetic zone, Helvetic system or the Helveticum is a geologic subdivision of the Alps. The Helvetic zone crops out mainly in Switzerland, hence the name . Rocks in the Helvetic zone are sedimentary and were originally deposited at the southern margin of the European plate...
, and some are disputed. The oceanic trench deposits of the Penninic nappes are found through the Alps and called Bündner slate
Bündner slate
The Bündner schist or Bündner slate is a collective name for schistose rocks that form a number of geologic formations in the Penninic nappes of the Alps...
s.
What is clear at least is that the Briançonnais terrane is not found 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 conclusion that can be drawn is that the microcontinent wedged out in the east in the Alpine Tethys Ocean. Some authors suggest the ophiolites that occur at the Hohe Tauern window must be correlated with the Piemont-Liguria terrane of the western Alps, because trench deposits such as radiolarite
Radiolarite
Radiolarite is a 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 indurated radiolarian oozes and sometimes as a synonym of radiolarian earth...
s occur in both.