Geology of the Alps

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Satellite image of the Alps, March 2007
Folded rock layers exposed in the Swiss Alps

The Alps form part of a Cenozoic orogenic belt of mountain chains, called the Alpide belt, that stretches through southern Europe and Asia from the Atlantic all the way to the Himalayas. This belt of mountain chains was formed during the Alpine orogeny. A gap in these mountain chains in central Europe separates the Alps from the Carpathians to the east. Orogeny took place continuously and tectonic subsidence has produced the gaps in between.

The Alps arose as a result of the collision of the

Hohe Tauern (Stampfli & Borel 2004
).

Subsequently, the formation of the Mediterranean Sea covered terranes originating within the African plate south of the mountains.

Geologic boundaries

Mediterranean, showing the position of the Alps within other structures of the Alpide belt

The Alps form a northward

Upper Rhine Graben
to the north.

The Alps continue fairly smoothly into the following related Alpine mountain ranges: the

Viennese Basin and the Pannonian Basin
, where east–west stretching of the crust takes place.

Geologic structure

The Alps have a complex geology, but the general structure is the same as for other mountain ranges formed by continental collision.

Subdivisions

The Alps are often divided into

Southern Alps
.

North of the Periadriatic seam, rocks from three main

Austroalpine domains. This subdivision is made according to the paleogeographical origins of the rocks: the Helvetic Zone contains material from the European plate, the Austroalpine Zone material from the Adriatic plate, the Penninic Zone material from the domains that existed in between the two plates.[1]

Simplified geological map of the Alps, showing the tectonic subdivision and the largest geological structures. Some details are based on controversial assumptions.

Structural geology

Folded Helvetic nappe rock layers at Dent de Morcles, Switzerland[2]

Folds and thrusts north of the Periadriatic seam are generally directed to the north, the dominant vergence (direction of fold asymmetry) in these units is to the north. In the Southern Alps the thrusts are to the south so the vergence is dominantly southward.

The rocks of the Austroalpine nappes form most of the outcrops in the

Sesia units), eroded away. In the Western Alps the Helvetic nappes can be found to the north and west, sometimes still under klippes of the Penninic nappes, as in the Préalpes du Sud south of Lake Geneva
.

In many spots in the central zone north of the Periadriatic seam large

normal faultzone called the Rhône-Simplon line. The structure thus formed is called the Lepontin dome
.

Intrusions

In older rocks from the lower

extension
can also be found.

Adamello granite. In the Penninic nappes migmatites
and small melts can be found.

Metamorphism

The rocks of the Helvetic and Austroalpine nappes and the southern Alps did not experience high grade

metamorphic rocks
in these units will not have become metamorphic due to the formation of the Alps. Other possibilities are:

Cenozoic eclogites do occur in the Penninic nappes, which contain material that has been through

eclogite facies. These nappes show a Barrovian field gradient. This type of metamorphism can only occur when a rock is in pressure–temperature conditions that normally occur in the Earth's mantle. This means the Penninic nappes consist of material that was subducted into the mantle and was later obducted
onto the crust.

Alpine (

contact-
or Buchan metamorphism is rare in the Alps, because intrusions are rare.

Tectonic history

The Alps are a

crustal shortening which is caused by the convergent
movements of the European and Adriatic plates.

Breakup of Pangaea

At the end of the

Ma), the Hercynian or Variscan orogeny, in which the supercontinent Pangaea formed from Gondwana and Laurasia, was ended. East of the terranes that now form the Alps was the Paleo-Tethys Ocean
.

The effects of

epicontinental seas existed in which evaporites and limestones
were deposited.

Jurassic

In the early

Adriatic plate
lay in between the African and European plates and was involved in subdividing the Tethys and early Alps formation. Sometimes the names Alpine Tethys or Western Tethys Ocean are used to describe a number of small oceanic basins that formed southwest of the European plate, to distinguish them from the Neo-Tethys Ocean in the east. Because the Jurassic was a time with high sealevels, all these oceans were connected by shallow seas. On the continents, shallow sea deposits (limestones) were formed during the entire Mesozoic.

In the late Jurassic the

Great Rift Valley
. Eventually, a new ocean will cut through east Africa as the rift develops, dividing a large section of land from the main continent.

When at the end of the Jurassic the Adriatic plate began to move toward the European plate, oceanic trenches formed in the eastern Alps. In these, deep marine sediments were deposited, such as radiolarites and lutites.

Eo-Alpine phase in the Cretaceous

The

Ma
) Africa began moving northeast.

As a result of this process, the soft layers of

ocean sediment in the Alpine Tethys Oceans were compressed and folded as they were slowly thrust upwards. Caught in the middle of the merging continents, the area of the Tethys Sea between Africa and Eurasia began to shrink as oceanic crust subducted beneath the Adriatic plate. The tremendous forces at work in the lower continental foundation caused the European base to bend downward into the hot mantle and soften. The southern (African) landmass then continued its northward movement over some 1,000 km (600 mi). The slow folding and pleating of the sediments as they rose up from the depths is believed to have initially formed a series of long east–west volcanic island arcs. Volcanic rocks
produced in these island arcs are found among the ophiolites of the Penninic nappes.

In the late Cretaceous the first continental collision took place as the northern part of the Adriatic subplate collided with Europe. This is called the Eo-Alpine phase, and is sometimes regarded as the first phase of the formation of the Alps. The part of the Adriatic plate that was deformed in this phase is the material that would later form the Austroalpine nappes and the Southern Alps. In some fragments of the Piemont-Liguria Ocean now in the Penninic nappes an Eo-Alpine deformation phase can also be recognized.

Apart from the Eo-Alpine fold and thrust belt other regions were still in the marine domain during the Cretaceous. On the southern margins of the European continent shallow seas formed limestone deposits, that would later be (in the Alps) incorporated into the Helvetic nappes. At the same time sedimentation of

Bündner slates
from the Penninic nappes.

Paleocene and Eocene

When the Piemont-Liguria oceanic crust had completely subducted beneath the

eclogite facies and becoming intruded by migmatites
. This material would later become the Penninic nappes, but a large part of the Briançonnais terrane subducted further into the mantle and was lost. Meanwhile, at the surface the upper crust of the Adriatic plate (the later Austroalpine nappes) was thrust over the European crust. This was the main collisional phase in the formation of the Alps.

Oligocene and Miocene

When the subducting

extension. In the case of the Alps, the extension could only take place in a west–east direction because the Adriatic plate was still converging from the south. An enormous thrustzone evolved that would later become the Periadriatic Seam. The zone also accommodated dextral shear that resulted from the west–east extension. With the exception of the allochthon Austroalpine material, this thrust evolved at the boundary of the Adriatic and European plates. The central zones of the Alps rose and were subsequently eroded. Tectonic windows and domes as the Hohe Tauern window
were formed in this way.

Meanwhile, the thrust front of the Penninic and Austroalpine nappes moved on, pushing all material in its way northward. Due to this pressure a

decollement
developed over which thrusting took place. The thrust material would become the Helvetic nappes.

Adriatic plate started rotating counterclockwise.[3]

Quaternary

After subduction of oceanic crust of the European plate collision nearly completely stopped in the Western and Central Alps (See map Figure 2).,[3][4] These parts are still uplifted up to 2.5 mm/year in some areas.[5][6] It is thought it is mainly due to rebound after weight loss from melting ice caps after the last ice age, intensive erosion during glaciation and some processes in the lithosphere and mantle. Adriatic plate, pushed by the African plate, still rotates counterclockwise around the axis near Ivrea in northwestern Italy and is subducted in Eastern Alps and causes tectonic uplift (thrust) there.[3]

Geomorphology

The formation of the Alpine landscape seen today is a recent development – only some two million years old. Since then, five known ice ages have done much to remodel the region. The tremendous glaciers that flowed out of the mountain valleys repeatedly covered all of the Swiss plain and shoved the topsoil into the low rolling hills seen today. They scooped out the lakes and rounded off the limestone hills along the northern border.

The last great glacier advance in the Alps ended some 10,000 years ago, leaving the large lake now known as

last ice age. From their composition it has been possible to determine the precise area from which they began their journey. As the last ice age ended, it is believed that the climate
changed so rapidly that the glaciers retreated back into the mountains in only some 200 to 300 years time.

Besides leaving an Arctic-like wasteland of barren rock and gravel, the huge moraine of material that was dropped at the front of the glaciers blocked huge masses of melt water that poured onto the central plain during this period. A huge lake resulted, flooding the region to a depth of several hundred meters for many years. The old shoreline can be seen in some places along the low hills at the foot of the mountains – the hills actually being glacial side-moraines. As the Aare, which now drains western Switzerland into the Rhine, eventually opened the natural dam, the water levels in the plain fell to near the present levels .

In the last 150 years humans have changed the flow and levels of all the rivers and most of the extensive wetlands and small lakes have disappeared under the effects of farming and other development.

It has been proposed that the height of mountains in the

glacier erosion, an effect referred to as the glacial buzzsaw.[7]

Geologic research

The Alps were the first mountain system to be extensively studied by geologists, and many of the geologic terms associated with mountains and glaciers originated there. The term Alps has been applied to mountain systems around the world that exhibit similar traits.

Geophysics

In the 1980s and 1990s, a number of teams began mapping the structures in the lower crust by seismology. The result was a number of detailed geological cross-sections of the deep structures below the Alps. When seismic research is combined with insights from gravitational research and mantle tomography the subducting slab of the European plate can be mapped. Tomography also shows some older detached slabs deeper in the mantle.

See also

References

  1. ^ See for a detailed subdivision of the geologic units in the Alps for example (Schmid et al. 2004), (Compagnoni 2003), (Pfiffner 2009, pp. 25–27)
  2. ^ Schuster, Ralf; Stüwe, Kurt (2010). "Die Geologie der Alpen im Zeitraffer" (PDF). Mitteilungen des Naturwissenschaftlichen Vereines für Steiermark (in German). 140: 5–21.
  3. ^
    S2CID 129726603
    .
  4. doi:10.1016/j.tecto.2009.02.024
    .
  5. PMID 27346228
    .
  6. S2CID 96447591
    .
  7. ISBN 978-0-444-53643-3
    .

Further reading

External links

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