Geology of the Alps

Last updated

Satellite image of the Alps, March 2007 Alps 2007-03-13 10.10UTC 1px-250m.jpg
Satellite image of the Alps, March 2007
Folded rock layers exposed in the Swiss Alps Folded Rocks Layers - Tete a Pierre Grept and Arete Vierge - in Swiss Alps - aerial view.jpg
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.

Contents

The Alps arose as a result of the collision of the African and Eurasian tectonic plates, in which the Alpine Tethys, which was formerly in between these continents, disappeared. Enormous stress was exerted on sediments of the Alpine Tethys basin and its Mesozoic and early Cenozoic strata were pushed against the stable Eurasian landmass by the northward-moving African landmass. Most of this occurred during the Oligocene and Miocene epochs. The pressure formed great recumbent folds, or nappes , that rose out of what had been the Alpine Tethys and pushed northward, often breaking and sliding one over the other to form gigantic thrust faults. Crystalline basement rocks, which are exposed in the higher central regions, are the rocks forming Mont Blanc, the Matterhorn, and high peaks in the Pennine Alps and 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

Tectonic map of the Mediterranean, showing the position of the Alps within other structures of the Alpide belt Tectonic map Mediterranean EN.svg
Tectonic map of the Mediterranean, showing the position of the Alps within other structures of the Alpide belt

The Alps form a northward convex arc around their southeastern foreland basin, the Po river basin (to be precise the south is in fact their hinterland). Quaternary and Neogene sediments in this basin lie discordant over the southernmost thrust units. In the northeast, southward dipping and internally thrust Cenozoic foreland deposits (flysch and molasse) are found. This Bavarian and Swiss foreland basin is called the Molasse basin. The foreland basin deposits are overthrust from the south by the thrustfront of the Alpine nappes. In Switzerland the Molasse Basin is rimmed to the northwest by the Jura mountains, an external fold-and-thrust belt, which can be seen as part of the Alps geologically. The western part of the Molasse basin forms the plateau of the Mittelland between the Alps and Jura Mountains. The Jura Mountains' location is still a topic for debate. A possible tectonic factor is the north–south extensional Upper Rhine Graben to the north.

The Alps continue fairly smoothly into the following related Alpine mountain ranges: the Apennines to the southwest, the Dinarides to the southeast and the Carpathians to the northeast. In the east the Alps are bounded by 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 Eastern, Central and Western Alps, even though the boundaries between these subdivisions are arbitrary. The division between the Eastern and Central Alps is approximately the line between St. Margrethen, Chur and Sondrio; the division between the Central and Western Alps is unclear ( Pfiffner 2009 , p. 25). The main suture (big shear zone) in the Alps is called the Periadriatic Seam and runs through the Alps from east to west. This is the boundary between materials from the (former) European and Adriatic plate plates. South of this line are folded and thrust units of the Southern Alps.

North of the Periadriatic seam, rocks from three main palaeogeographic "domains" are found: the Helvetic or Dauphinois, the Penninic and the 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. Alps geology map en.jpg
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 Dent de Morcles S.jpg
Folded Helvetic nappe rock layers at Dent de Morcles, Switzerland

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 Eastern Alps, while in the west these nappes are, with the exception of a few places (the Dent Blanche and 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 antiforms called anticlinoria can be found, sometimes they are displayed in the outcrops as windows. At the level of one of these windows (the Hohe Tauern window) the Periadriatic seam curves to the north, which suggests that the Adriatic plate is more rigid in this particular spot, working as a so-called indentor. In the central part of Switzerland, uplift took place along a ductile north–south normal faultzone called the Rhône-Simplon line. The structure thus formed is called the Lepontin dome.

Intrusions

In older rocks from the lower crust intrusions are found that formed during or just after the Hercynian orogeny. These intrusions are older than the Alps and have nothing to do with their formation. Radiometric age determination yields ages around 320  Ma. Slightly younger felsic intrusions formed by Permian and Triassic extension can also be found.

Intrusions from the formation of the Alps themselves are relatively rare. The largest ones can be found along the Periadriatic seam, the largest one is the 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 metamorphism in the major Alpine phases in the Cenozoic. Any 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 blueschist or eclogite facies. These nappes show a Barrovian field gradient. This type of metamorphism can only occur when a rock is in pressuretemperature 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 (Cenozoic) contact- or Buchan metamorphism is rare in the Alps, because intrusions are rare.

Tectonic history

The Alps are a fold and thrust belt. Folding and thrusting is the expression of crustal shortening which is caused by the convergent movements of the European and Adriatic plates.

Breakup of Pangaea

At the end of the Carboniferous period (300  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 wind and water were able to chemically and mechanically erode destroy the Hercynic mountain ranges. In the Permian, the main deposits in Europe were sandstone and conglomerate, products of erosion in the Hercynic mountain range. At the same time, crustal extension took place because the mountain range was isostatically unstable (this is called orogenic collapse). Due to extension, basins formed along the axis of the mountain range and felsic volcanism occurred. This was the first phase of rifting between Europe and Africa. Due to the rising sealevel in the Triassic period, the eastern margin of Pangaea was flooded. Shallow shelf seas and epicontinental seas existed in which evaporites and limestones were deposited.

Jurassic

In the early Jurassic period (180  Ma), a narrow ocean began to form between the northern (North America and Eurasia) and southern (Africa and South America) parts of Pangaea. The oceanic crust that was formed in the process is known as the Piemont-Liguria Ocean. This ocean is generally regarded as a western extension of the Tethys Ocean. Although it was not really connected to it, a peninsular piece of continental crust of the African plate called the 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 microcontinent Iberia broke away from the European plate and the Valais Ocean was formed between the two plates. Both Piemont-Liguria and Valais Oceans were never large oceans such as today's Atlantic Ocean. What they might have been like is the opening below the Red Sea, continuing down through Africa, forming 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 divergent movement of the European and African plates was relatively short-lived. When the Atlantic Ocean formed between Africa and South America (about 100  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 anoxic clay took place in the deep-marine realms of the Piemont-Liguria and Valais Oceans. This clay would later become the Bündner slates from the Penninic nappes.

Paleocene and Eocene

When the Piemont-Liguria oceanic crust had completely subducted beneath the Adriatic plate in the Paleocene, the Briançonnais microcontinent, according to some a piece of the Iberian plate, arrived at the subduction zone. The Briançonnais microcontinent and Valais Ocean (with island arcs) subducted beneath the Adriatic plate. They stayed at around 70 km (45 mi) below the surface during the Eocene, reaching 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 slab broke off (known as slab breakoff, slab pull) and fell away, the subducted crust began moving up. This led to the uplift of the thickened continental crust which led, in the Miocene, to 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 Lake Neuchatel. The ice in this region reached some 1,000 m (0.6 mi) in depth and flowed out of the region behind Lake Geneva some 100 km (60 mi) to the South. Today large granite boulders are found scattered in the forests in the region. These were carried and pushed by the glaciers that filled this part of the western plain for some 80,000 years during the 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 Dauphiné Alps is limited by 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

Related Research Articles

<span class="mw-page-title-main">Central Eastern Alps</span> Portion of the Eastern Alps mountain range through Austria and parts of surrounding countries

The Central Eastern Alps, also referred to as Austrian Central Alps or just Central Alps, comprise the main chain of the Eastern Alps in Austria and the adjacent regions of Switzerland, Liechtenstein, Italy and Slovenia. South them is the Southern Limestone Alps.

<span class="mw-page-title-main">Helvetic (geology)</span>

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. The Helvetic zone correlates with the French Dauphinois zone, French geologists often prefer the French name but normally this is considered the same thing.

<span class="mw-page-title-main">Penninic</span> Geological formation in the Alps

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 Alps, 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.

<span class="mw-page-title-main">Austroalpine nappes</span> Geological formation in the European Alps

The Austroalpine nappes are a geological nappe stack in the European Alps. The Alps contain three such stacks, of which the Austroalpine nappes are structurally on top of the other two. The name Austroalpine means Southern Alpine, because these nappes crop out mainly in the Eastern Alps.

<span class="mw-page-title-main">Southern Alps (Europe)</span>

The Southern Alps are a geological subdivision of Alps that are found south of the Periadriatic Seam, a major geological faultzone across the Alps. The southern Alps contain almost the same area as the Southern Limestone Alps. The rocks of the southern Alps gradually go over in the Dinarides or Dinaric Alps to the south-east. In the south-west they disappear below recent sediments of the Po basin that are lying discordant on top of them.

<span class="mw-page-title-main">Sesia zone</span>

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 part of the Austroalpine nappes and correlated with the Dent Blanche nappe that crops out further to the northwest.

<span class="mw-page-title-main">Valais Ocean</span> Subducted ocean basin. Remnants found in the Alps in the North Penninic nappes.

The Valais Ocean is a subducted oceanic basin which was situated between the continent Europe and the microcontinent Iberia or so called Briançonnais microcontinent. Remnants of the Valais ocean are found in the western Alps and in tectonic windows of the eastern Alps and are mapped as the so-called "north Penninic" nappes.

<span class="mw-page-title-main">Piemont-Liguria Ocean</span> Former piece of oceanic crust that is seen as part of the Tethys Ocean

The Piemont-Liguria basin or the Piemont-Liguria Ocean was a former piece of oceanic crust that is seen as part of the Tethys Ocean. Together with some other oceanic basins that existed between the continents Europe and Africa, the Piemont-Liguria Ocean is called the Western or Alpine Tethys Ocean.

<span class="mw-page-title-main">Greywacke zone</span> Geological layer in Austrian Alps

The greywacke zone is a band of Paleozoic metamorphosed sedimentary rocks that forms an east-west band through the Austrian Alps.

<span class="mw-page-title-main">Lepontin dome</span>

The Lepontine dome or Lepontin dome is a region of tectonic uplift in the Swiss part of the Alps. It is located in the Lepontine Alps and Glarus Alps.

<span class="mw-page-title-main">Molasse basin</span> Foreland basin north of the Alps

The Molasse basin is a foreland basin north of the Alps which formed during the Oligocene and Miocene epochs. The basin formed as a result of the flexure of the European plate under the weight of the orogenic wedge of the Alps that was forming to the south.

<span class="mw-page-title-main">Western Carpathians</span> Mountain range along the border between Poland, Austria, the Czech Republic, Slovakia, and Hungary

The Western Carpathians are a mountain range and geomorphological province that forms the western part of the Carpathian Mountains.

The Helvetic nappes are a series of nappes in the Northern part of the Alps and part of the Helvetic zone. They consist of Mesozoic limestones, shales and marls that were originally deposited on the southern continental margin of the European continent. During the Alpine orogeny they were thrust north over a décollement and at the same time were internally deformed by folding and thrusting.

This is a list of articles related to plate tectonics and tectonic plates.

<span class="mw-page-title-main">Geology of the Western Carpathians</span>

The Western Carpathians are an arc-shaped mountain range, the northern branch of the Alpine-Himalayan fold and thrust system called the Alpide belt, which evolved during the Alpine orogeny. In particular, their pre-Cenozoic evolution is very similar to that of the Eastern Alps, and they constitute a transition between the Eastern Alps and the Eastern Carpathians.

<span class="mw-page-title-main">External massif</span>

An external massif is, in the geology of the Alps, a place where crystalline rocks of the European plate crop out. Such massifs are found north and west of the Penninic zone as tectonic windows in the Helvetic Zone. They differ from the crystalline nappes in that they were originally part of the European plate, while the Penninic nappes were part of the crust below various domains in the Tethys Ocean.

<span class="mw-page-title-main">Engadin window</span> Exposure of penninic rock lying below the austroalpine rocks

The Engadin window or 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 dimensions of 55 x 17 km.

<span class="mw-page-title-main">Geology of Germany</span> Overview of the geology of Germany

The geology of Germany is heavily influenced by several phases of orogeny in the Paleozoic and the Cenozoic, by sedimentation in shelf seas and epicontinental seas and on plains in the Permian and Mesozoic as well as by the Quaternary glaciations.

The geology of Austria consists of Precambrian rocks and minerals together with younger marine sedimentary rocks uplifted by the Alpine orogeny.

<span class="mw-page-title-main">Geology of Italy</span> Overview of the geology of Italy

The geology of Italy includes mountain ranges such as the Alps and the Apennines formed from the uplift of igneous and primarily marine sedimentary rocks all formed since the Paleozoic. Some active volcanoes are located in Insular Italy.

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. 1 2 3 Handy, Mark R.; Ustaszewski, Kamil; Kissling, Eduard (21 September 2014). "Reconstructing the Alps–Carpathians–Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion". International Journal of Earth Sciences. 104 (1): 1–26. Bibcode:2015IJEaS.104....1H. doi: 10.1007/s00531-014-1060-3 . S2CID   129726603.
  4. Champagnac, Jean-Daniel; Schlunegger, Fritz; Norton, Kevin; von Blanckenburg, Friedhelm; Abbühl, Luca M.; Schwab, Marco (September 2009). "Erosion-driven uplift of the modern Central Alps". Tectonophysics. 474 (1–2): 236–249. Bibcode:2009Tectp.474..236C. doi:10.1016/j.tecto.2009.02.024.
  5. Nocquet, J.-M.; Sue, C.; Walpersdorf, A.; Tran, T.; Lenôtre, N.; Vernant, P.; Cushing, M.; Jouanne, F.; Masson, F.; Baize, S.; Chéry, J.; van der Beek, P. A. (27 June 2016). "Present-day uplift of the western Alps". Scientific Reports. 6 (1): 28404. Bibcode:2016NatSR...628404N. doi:10.1038/srep28404. PMC   4921835 . PMID   27346228.
  6. Sternai, P.; Sue, C.; Husson, L.; Serpelloni, E.; Becker, T.; Willett, S.; Faccenna, C.; Di Giulio, A.; Spada, G.; Jolivet, L.; Valla, P.; Petit, C.; Nocquet, J.-M.; Walpersdorf, A.; Castelltort, S. (5 January 2019). "Present-day uplift of the western Alps: Evaluating mechanisms and models of their relative contributions". Earth-Science Reviews. 190: 589–604. Bibcode:2019ESRv..190..589S. doi:10.1016/j.earscirev.2019.01.005. hdl: 10281/229017 . S2CID   96447591.
  7. Evans, I.S. (2013). "Glacial landsforms, erosional features". In Elias, Scott A.; Mock, Cary J. (eds.). Encyclopedia of Quaternary Science (2nd ed.). Elsevier. p. 861. ISBN   978-0-444-53643-3.

Further reading