Geology of the Alps

The the Alps belong to the alpine chains perished-téthysiennes, formed during the Mesozoic and the Cénozoïque, which extend from the the Maghreb (for example the High Moroccan Atlas) to the the Far East (the Himalayas). A part as of these assembly lines (perish-Mediterranean chains) is resulting from the opening, then of the closing of oceanic basins of the system téthysien. The existence of these orogene S is related on the convergence of the plates African and European and to the interposition of blocks or microplaques. The Alps themselves extend on a thousand from kilometers, between Genoa and Vienna, with a width ranging between 100 and 400 kilometers. One can subdivide them on geographical, geological and topological criteria, in three distinct parts: the Western Alps, which form an arc between the Mediterranean and the Valais, the central Alps, between Were worth and the Grisons (Eastern Suisse), and the Alps Eastern and southernmost, which are ensellent in the Plaine of Pannonia in the west of the Carpates.

The arc of the Western Alps is classically subdivided in two parts, separated by the crustal Chevauchement pennic: the external zone and internal zones. This major Chevauchement juxtaposes distinct paleogeographic units, having had stories Tectonique S and metamorphic different: broadly, the units of the external zone correspond to the proximales parts of the European margin, which were little shortened and little metamorphized at the time of the alpine history, whereas the internal units correspond to the more distal parts and the ocean floor, which underwent a metamorphism and a stronger shortening. The deformations in the Alps (observable thanks to the Seism S and with the Geodesy) are currently weak with very weak. However, of the rates of rising of about 1 mm/an is observable in Switzerland; comparable data are not available in France or Italy, but it is probable that vertical speeds are comparable. The origin of such a rising remains largely discussed today, the shortening in the Western Alps being weak with null.

Chain hercynienne with the alpine ocean (Téthys alpine)

The Hercynienne chain is formed Carbonifère with the Permien and disappears at the end from the Paléozoïque by a tectonic crustal thinning (characterized by large Faille (normal and taking down), from the sedimentary basins and the granitic intrusions . These structures are covered by the ic sediments Trias. The direction North-Be-South-West, currently expressed very well in the Alps by the metamorphic foliation and the Faille S of age hercynien (in the external crystalline solid masses of the Mont Blanc and Belledonne in particular) is one of the principal directions hercyniennes. These directions constitute a prédécoupage continent pangéen which made it possible to locate the deformations during the Jurassic extension and of alpine compression.

Thus, the Rift ing (Jurassic inferior) which led to the Jurassic average with the opening of the alpine Téthys (liguro-Piedmontese Ocean) re-uses these directions. Another oceanic opening, with the Crétacé takes again the Sédiment S post-rift and leads more to North with the formation of the Valaisan Océan. This narrow oceanic basin delimits the microphone-continent Briançonnais, also integrated into the alpine prism. These oceanic openings belong to a total context of extension which sees the bursting of Pangée and the formation of the Atlantic Ocean during the Jurassic average. They are expressed by a rifting creating basins in half-graben of decakilometric size separated by large normal faults crustal, then the creation of passive margins subsidentes. The sedimentary sequences of the continental terraces result from rocked blocks of base and sediments deposited on these blocks. The oceanic sedimentary sequences, as for them, result from the ocean floor (basic rocks and ultrabasic) and from the overlying sediments.

Continent with the ocean, one finds the paleogeographic fields following :

the field From the Dauphine (or Swiss ) consists of thick series marno-limestones Jurassic and is underlined by large bars of massive Calcaire of the Cretaceous, which surmounts the paleozoic base (external crystalline solid masses, MCE).

the fields Ultradauphinois (or Ultrahelvéthique ) and Valaisans primarily consist of flyschs Paléocène S with Eocene S produced by the emergence of the chain éoalpine.

the field Briançonnais is composed of paleozoic base composing the internal crystalline solid masses (MCI) covered with a thick sedimentary pile (Sorted with Cretaceous mainly).

the fields Liguro-Piedmontese correspond to the ocean floor covered with deep sediments (the futures glossed Schistes).

the austro-alpine units come from the base and the cover of the African margin. These units are particularly expressed in the Eastern Alps but constitute only the tablecloth of the White Dent and the Sésia zone in the Western Alps.

During the alpine Orogenesis, these various paleogeographic units will constitute the base of the metamorphic and tectonic zonations. With local scales, important distinctions are to be made, and of the juxtaposed units can have had different metamorphic stories.

Subduction then collision (compressive history)

With the Cretaceous higher, the opening of the Northern Atlantique causes the stop of the alpine opening of Thétys, the bringing together between Africa and Europe and the initiation of a Subduction with South-western vergency. The European margin subducte under the Adriatic microphone Plate. This microphone-plate, in relation to the African Plate, played a determining role in the alpine history. The subduction (cretaceous with Eocene) transports to great depth certain parts of the margin European (tablecloths Lépontines, or tablecloths Penniques Inférieures), of the microphone-continent Briançonnais (Mont Pink, Grand Paradise and Dora Maira) and the ocean floors Ligure (glossy Schists) and Valaisan (metamorphic Flysch S). The metamorphism éoalpin, of high pressure indicates depths of hiding from 50 to 100 km following the affected units. Certain parts of the ocean floor are not subductées, and one observes in the Alps a Suture, characterized by basic rocks, of affinity oceanic, obductées and integrated into the prism pennic (Ophiolites of the Chenaillet Mount).

The subduction is followed as of the Oligocène by a continental collision s.s. between the margins European and apulienne (“African”). This collision causes large haulages, folds, Chevauchement S, Schistosité S, and, generally, important a ductile deformation compressive with vergency pro (towards North and the West) then retro (towards the South and Is). This ductile deformation is complex and polyphase. The juxtaposition of the Austro-Alpine units (Outlier of Tooth-White), pennic (metamorphic internal zones) and external (not very metamorphic European margin) is directly resulting from the early stages of this collision. With the Miocène, the deformation is expressed mainly in more breakable field, with overlappings which are propagated internal fields towards the more external fields (towards North and the West) and which affect mainly the European margin. The tablecloths of cover (tablecloths from the Dauphine/Swiss) installation during the Oligocène are deformed by the rising of the external crystalline solid masses. The subalpine massive more external (From the Dauphine) continue to become deformed during the Miocène, in front of the external crystalline solid masses which are raised. Formation, with the higher Miocène of the chains of tectonic foreland (“folds and thrust belt”) of the the Jura and the Tablecloth Worthy sign last compressive episodes of alpine orogene.

During this compressive long story, an important basin of tectonic foreland (the Basin molassic) subsidy by Flexure, in front of the alpine chain of Oligocene with the average Miocène. The sediments which settle there are gradually deformed and integrated into the alpine structures. The transfer of the deformation in front of the basin molassic, with the face of the the Jura, causes the rising of this one and the stop of the sedimentation of Molasse. The alpine history is also marked by an important deformation taking down which affects all the tectonic units. The role of these taking down movements related to rotations is regarded as major by certain authors. The origin Géodynamique of these setbacks remains discussed, but they are for a great part related on the movements and anti-clockwise rotation of the microphone-plate apulienne.

Late extensive evolution

The most recent history of the internal Western Alps is expressed by an extensive deformation mainly and taking down, whereas the early evolution was characterized in compressive deformations mainly . Extensive deformations of the internal zones are described as of the Oligocène, in ductile condition . Breakable extensive deformations were observed in the whole of the internal zones, and reflect a dominating mechanism in the internal part of the Western alpine arc; it is necessarily placed during the Néogène, while the external zones are subjected to a compressive mode. The active deformation, characterized by the Sismotectonique, is mainly extensive in the internal zones, whereas the tectonic mode of the external zone is taking down and compressive. The comparison with other orogeneses (the Himalayas, the Andes, the Apennines, Basin and Arrange, Chaîne hercynienne…) show that the late evolution of an assembly line passes by extensive processes. The geodynamic origins of these extensive processes are numerous, and depend on balance between the internal forces (forces of volumes) and external with the chain (forces in extreme cases).

References

Random links: