Subduction

Manifestations of the subduction

The subduction is the process allowing disappearance of the old oceanic plate. Consequently, she plays a big role in the renewal of the oceanic funds.

Markers of the subduction

Volcanic activity and seisms

The seisms and the volcanos have an intense activity along the zones of Convergence so called active margins. One primarily finds them around the Pacifique (fire girdles), on the level of the the Antilles, but also in Italy and Indonesia. Moreover, volcanicity is of explosive type because its magma is charged with silica as well as gas.

Relief and deformation

A pit narrow and deep exists all along the edge where a plate starts to be inserted under the other. This pit is dissymmetrical and is responsible for a negative gravimetric anomaly. To approximately 100-150 km of the pit, on the plate overlapping, a chain of volcanos located is:

on the edge of a continent (cordillera);
like a succession of volcanic islands laid out in arc (insular arcs - Japan, the Antilles, etc).

On the side overlapping of the pit, the oceanic sediments continuously brought by the plate ride are " rabotés" , deformed considerably and piled up in superimposed scales. Fault S opposite, fold S faulted and zones of Chevauchement S testifies to an important shortening in a Prisme of accretion (like the Barbados S).

Depression of the oceanic lithosphere

Seismic hearths

The seismic hearths are increasingly deep under the plate overlapping since the pit towards the insular Arc and beyond.

They are registered on a tilted level (the Plan of Benioff), whose angle varies according to the type and the age of the plate subduite, as well as the speed of convergence.

Several phenomena are responsible for this seismicity

Thermic anomalies

One observes an unequal distribution of the thermal Flux. The Isogéotherme S inflect, which materializes a plunging of its isogeotherms. These thermic anomalies and deep seismic hearths materialize the diving of the lithosphere on the level of the confrontation of the two plates. These anomalies are types negative with the top of the pit and positive to the top of the volcanic arc.

Note: The techniques of thermic anomaly made it possible to highlight the fact that the slab is much more deeply prolonged in the Manteau (ground) that what one had been able to highlight with the Plan of Benioff (limited to approximately 670km).

Gravimetric anomalies

Measurements of the terrestrial field of gravity were undertaken by Pierre Bouguer at the XVIIIe century already. These measurements had made it possible to realize that the distribution of the rock masses is not uniform in-depth. From these measurements was born also the concept of isostasy, according to which masses lithospheric " flottent" on an asthenosphere according to the law of Archimedes. It is the Vening-Meinesz Dutchman who in 1923 discovered important gravimetric anomalies on the level of the pit off Indonesia whereas it carries out measurements with a gravimeter on board a submarine. He concludes a flexuration with large scales from it from the earth's crust at this place and that this crust thus had elastic properties.

Engine responsible for this depression

The difference in density between the oceanic lithosphere increasing with the age and that of an asthenosphere which grows less quickly (towards 50 million years the lithosphere becomes denser than the asthenosphere) is the main motor of the subduction. Indeed, more the lithosphere moves away from the dorsal (place of creation of the lithosphere) more its temperature decreases and more its density thus increases it becomes heavier and has tendency has to be inserted.

Confrontation of two different plates

Continental lithosphere against oceanic lithosphere

It is always the oceanic plate which plunges under the continental plate because it is denser (approximately 3,3 per 2,7). The angle of inclination of the plan of Wadati-Benioff is weak. This plan is materialized by the presence of many seismic hearths along the plate subduite. There is formation of a Assembly line volcanic, or of an arc of volcanic islands, on the continental plate (it is named cordillera). These volcanos are generally of andesitic type or rhyolitic (the lava is more acid than those which one finds on the level of the oceanic dorsals) which are explosive because the lava contains an high rate of SiO2 what makes them viscous (the gases have more difficulty of escaping).

Note:: The meeting of two continental plates involves a subduction and then a zone of collision such as for example for the the Alps preceded by the alpine Océan.

Oceanic lithosphere against oceanic Lithosphere

The oldest plate thus densest plunges under the younger plate. The plan of Wadati Benioff is very sloping. There is formation of a insular Arc and a Bassin back-arc.

Consequences related to this confrontation

(see also the Glossary of the minerals)

effusive Rocks: the magma (lava). There are no grains: presence of a paste in which some phenocrysts bathe. Microlites represent a fast cooling. Example:

the Basalt: phenocrysts of Olivine, Pyroxene, Plagioclases.

plutonic Rocks: with the naked eye, one observes differentiated grains. Under the microscope, one observes grains and minerals. Example:

gabbros: plagioclases, pyroxenes, olivine
peridotites: 80% of olivine, pyroxene

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In the zones of subduction, the magmatic rocks which one finds on the level of the subductée plate are Basalte S, Gabbro S, and Péridotite S. One finds there also Sédiment S.

Mineralogical transformations (metamorphism)

Methods of transformation of minerals

The various minerals are formed in conditions of precise temperature and pressure. The Lithosphere, while being inserted, will be confronted with new conditions: increase in the pressure, but little increase in the temperature.

The plate which deviates from the dorsal undergoes a cooling, a hydration, but little increase in the pressure.

Minerals will be in an unstable state because of the change of these conditions of temperature and pressure. That thus will involve a modification of these minerals. But, as these modifications will be done in a solid state, one speaks about Métamorphisme. About Gabbro, one speaks about Métagabbro.

Transformations related to the fall of temperature, hydration and low pressure:

gabbro: Plagioclase S (Anorthite) + H₂0 ==> plagioclases recent + Amphibole + Pyroxene (Augite) + Olivine ==> Hornblende: Facies green Schist

On the level of the subductée crust:

the crust is inserted. There is thus an increase in the pressure and little increase in the temperature. The pressure will drive out the water of minerals.

Actinolite + Chlorite + Plagioclase - H₂0 ==> Amphibole: Glaucophane: facies blue Schist.

While continuing to be inserted:

Amphibole - H₂0 ==> Amphibole + Jadeite (Pyroxene) + Garnet-red: facies eclogite S

Because of increase in the pressure, one witnesses a mineralogical evolution which transforms the contents of the rocks and their aspect. Gabbros (dorsal), transformed into métagabbros, that one finds on the level of the facies green schist, blue schist, eclogite.

Note: The facies eclogitic, testifying to a metamorphism to High pressure and High temperature, is in general the markers of a continental subudction, i.e., that the continental terrace, attached to the oceanic lithosphere subduite, it also underwent a subduction.

Consequences of the subduction and the associated metamorphism

First of all, the markers metamorphic S, are of very good indices of ground to know the history of a given site. By identifying rocks characteristic the such blue schists of the island of Groix, one can thus understand that the place observed was formerly a zone of subduction… returned on the surface later, thanks to erosion.

However, the phenomenon of subduction succeeds, in the long term, with the recycling of the oceanic crust, in the Manteau (ground). Thus for example, the oceanic old fogey still present on Earth dates from Jurassic (off Japan). What does not mean that there was no oceanic crust before the Jurassic one: the older oceanic crusts in fact were entirely recycled by subduction. This then poses many problems with the lithospheric geologists wanting to study the dynamics of the plates during old times, because the data contained by the crusts oceanic of the time, are at the same time lost by the hiding of its rocks, and by the metamorphism which affects them.

Formation of magma

Water is released in the peridotites of the coat located above the plan of subduction. These peridotites are thus hydrated.

The Diagram pressure/temperature shows that the melting point of a hydrated peridotite, is lower than that of a dry peridotite. The géotherme cut the curve of the solidus between -80 and -200 km. In this zone, one can thus have peridotites which begin to melt temperatures lower than 1000°C. There is thus formation of a magma. This lighter magma tends to go up in the wall-rocks, that is to say quickly: andesitic Volcanicity: formation of Andesite or Rhyolite; that is to say more slowly: a grained rock: Granodiorite. This is why the volcanos are laid out and aligned the margin parallel to. They are of eruptive type violate (one speaks about explosive volcanicity).

Zones of subduction

Most of the current zones of subduction are distributed around the Pacific Ocean, called the " Belt of fire " , which runs since the Patagonie until in Alaska, then Kamchatka with the Filipino . It implies the plates the Pacific, Nasca, Coco and Juan de Fuca. One also counts:

subduction of the plate of Nazca under the Andes cordillera;
subduction of the basin of the Southernmost China Sea under the island of Luzon (Filipino) on the level of the pit of Manila. This subduction is at the origin of volcanicity still present on the arc of Luzon today (eruption of the Mount Pinatubo in 1991)
the subduction of the margin indonésienne of the Indian Ocean, at the origin of the Séisme and the Tsunami of December 26th, 2004 (more than 250.000 dead).

See too

subduction|subduction

External bonds

Objective-Ground, the cordilleras
Geopedia, subduction and earthmovings

Simple: Subduction

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