Subduction

Subduction (from Latin sub " under " ducere " run " ) is called the concept of geodynamics in the framework of plate tectonics, the descent of the edge of a lithospheric plate with oceanic crust in the mantle sublithosphärischen, this plate boundary is approached simultaneously from the adjacent lithosphere. Therefore, a subduction zone is always located on a plate boundary. As two plates move towards each other, one also speaks of a convergent plate boundary. The subducting plate is called the lower plate and the overriding plate top plate. When it comes to the top plate up or thrust of fragments of the board, this is called a necropsy.

Subduction, as happens today, there is likely since Mesoarchaikum.

Causes and mechanism of subduction

Oceanic lithosphere consists of several layers. The upper layers have a much lower, the bottom slightly higher density than the stock under athenosphere. The total lift is at newly formed lithosphere sufficient to the laminate on the asthenosphere floating ie to adhere to the surface. By cooling and thus overall increasing density, as well as increase the already dense lower layer loses oceanic lithosphere in the course of millions of years so far to boost that they can sink into the asthenosphere and deeper. This happens in subduction zones by bends the edge of a lithospheric plate in a more or less steep downward angle. With seismological methods such hanging down into the mantle plate edges are (english slab called ) in many places of the earth detectable. In the depth they first undergo temperature-and pressure-dependent metamorphoses (see below), their density increases further. This also gravitational train grows ( engl. slab pull), the already sunken part of the plate exerts on the still on the earth's surface part. This Plattenzug is the main driving force of plate drift and therefore the overall plate tectonics. The depth and the manner in which the drop and eventually the existence of the slabs completed, is not yet fully understood. Thermal and seismic anomalies, which are interpreted as signatures of sinking slabs, have been determined to near the core - mantle boundary.

From the earth's surface vanishing material needs to be replaced - a geometric necessity, because the amount of the spherical surface of the earth is constant. Therefore the subduction zones is offset by a similar erdumspannendes extensive network of spreading zones ( see also → mid-ocean ridge ), in which continually aufdringendes Asthenosphärenmaterial formed into new oceanic lithosphere. At the same time rising from the core - mantle boundary called a mantle plume promote hot mantle material below the superficial lithosphere or even as a hotspot volcanism on the Earth's surface. Subduction plate drift, sea floor spreading and mantle plumes are sub-processes of the mantle of the earth.

Start, progress and end of the subduction

Subduction can either start spontaneously when an area of oceanic lithosphere has become thicker than its surface, or induced when mechanical stresses emanating from another, may far distant region of the lithosphere (eg, an extensive rift system ), cause an overthrust by another plate. If one located in subduction plate affixed by the absence of a spreading zone at the opposite edge, no new Lithosphärenmaterial or done the spread slower than the subduction, this leads to the closure of the corresponding ocean basin. This process normally ends with the collision of two continental blocks ( orogeny ), because the deep subduction of continental lithosphere is not possible, because this has a strong boost.

Structure of a subduction zone

From the ocean - continent subduction -, in which oceanic lithosphere subducted under a continental block because of its higher density, the ocean -ocean subduction, dives in the oceanic lithosphere beneath the oceanic lithosphere of another plate, distinguished. In ocean - continent convergence we also speak of an active continental margin.

In Abtauchbereich oceanic crust to deep-sea trenches form such as with up to 11,034 m deepest submarine trough the earth, the Mariana Trench. In addition, being built on the continental block on the subduction zone a volcano mountains such as the Andes. Sometimes it can also come in addition to raising the edge of the top plate, as in the example of the Central Andes. If only oceanic lithosphere involved in the subduction occurs on the subduction zone is an island arc.

Dipping angle and subduction of the plate affects the tectonic processes in the hinterland of the island arc or continental volcanic mountain range, called the backarc area. If the subduction velocity is low and the dipping angle steep ( > 50 ° ), found in the backarc often stretching of the lithosphere with the formation of a backarc basin instead of what at ocean - continent convergence may even lead to the creation of a small ocean basin with mittelozeanischem back ( backarc - spread). Particularly frequently, however, there backarc spreading on at ocean -ocean subduction. If the subduction velocity is high and the dipping angle flat ( <30 ° ), the backarc area is compressed and there arises a fold-and- thrust belt. This is fairly typical of ocean - continent subduction -.

Earthquake

Subduction zones are prone to earthquakes as a result of plate movements directed against each other. As you descend from catching the two plates and build considerable tension in the rock, whose sudden release at the Earth's surface earthquakes and undersea earthquakes can cause tsunamis (also called tsunamis ). Such a subduction zone earthquake occurred on 26 December 2004 in the Sunda Trench (see also Indian Ocean tsunami, 2004). Even the heavy Tōhoku earthquake of 11 March 2011, which was accompanied by a devastating tsunami, had its origins in the subduction. The zone in which these earthquakes occur, is called Wadati - Benioff zone.

Dehydration and metamorphism of the subducting plate

Oceanic lithosphere contains large amounts of water. This is either unbound ago, for example, in the column space of disorders or in the pore space of marine sediments that have accumulated on her, or bound in minerals. The water and other volatile (volatile ) compounds (such as CO2) when Subduktionsvorgang by the increase of pressure and temperature in several phases in the form of so-called fluids released ( devolatilization ). With the release of the bound in minerals water is a metamorphosis of the subducted rocks of the oceanic crust. Depending on the prevailing temperature conditions through MORB basalt, dolerite and gabbro, and incurred during the course of the ocean floor metamorphism rocks and amphibolite Spilit various so-called metamorphosis paths. At relatively "warm" subduction zones at a depth of about 50 kilometers, is a direct conversion to eclogite ( a high-pressure rocks, consisting of the Klinopyroxenmineral omphacite and garnet and jadeite ). At relatively " cold" subduction zones, first a blauschieferfazielle metamorphosis and the Eklogitisierung takes place only at depths greater than 100 kilometers. At subduction zones with a strong heat development due to the occurring shear forces occurs first grünschieferfazielle and with increasing depth of burial then amphibolite, followed by granulite and finally eklogitfazielle metamorphism at depths of less than 100 kilometers on the upper part of the lithosphere of the subducted plate. The olivine of the peridotite mantle lithosphere of the subducted plate is converted to depths of 350-670 km in spinel and from depths of more than 670 km, the conversion into perovskite and Magnesiumwüstit. With all of these transformations and rock and mineral respectively an increase in the density is associated. Only through the metamorphoses and the corresponding increase in density a really falling below the overthrust oceanic lithosphere into the asthenosphere and later in the lower mantle is possible.

Above all, released in greater depth in the Eklogitisierung of crustal rocks fluids that come from the breakdown of hornblende and clinozoisite of lawsonite and glaucophane and chlorite, are apparently also the cause of the volcanism at subduction zones.

Volcanism

By released during the metamorphism of the subducting slab fluids ( under these temperature and pressure conditions is referred to in terms of the physical state of water from a supercritical state, and therefore not of a liquid), the melting point of the surrounding rock is reduced and it comes to anatexis ( partial melting ) of the mantle wedge or the accretionary prism in very rare cases the subducting plate itself the resulting magma rises, but often remains within the coast stuck and crystallizes in large plutons.

The aufdringende to the surface magma forms characteristic chains of volcanoes. Where to plate boundaries oceanic lithosphere plunges beneath another oceanic lithosphere to form island arcs such as the Aleutians. If oceanic dive under continental lithosphere, created a chain of volcanoes such as the Andes. Most follow the earth's surface due to the phased drainage of the oceanic lithosphere with increasing depth (and thus zunehemdem pressure) at the subduction several parallel to the subduction volcanic chains together. The volcanic mountain and island arcs of the numerous subduction zones at the edges of the Pacific plate form the so-called Pacific Ring of Fire.

The typical subduction zones andesitic and thus relatively viscous fluxes lead to the formation of layer volcanoes that tend to explosive eruptions. Examples of particularly explosive eruptions are of Krakatoa in 1883, the Mount St. Helens in 1980 and Pinatubo in 1991.

Another possible consequence of subduction are Petit spots. 2006, this approximately 50 -meter-high volcanoes were observed in the Japan Trench at a depth of 5000 m on the subducting plate for the first time. It is believed that by the bending of the subducting plate there cracks and gaps arise and therefore can rise from the asthenosphere magma to the ocean floor.

Effects

Through this process, the entire ocean floor is constantly renewed, which is why there are oceanic lithosphere only in geological situations such as in the Mediterranean or in ophiolites, which is older than 200 million years.

Typical deposits

Typical primary ore deposits at subduction zones are porphyry copper deposits or so-called Iron -Oxide- Copper - Gold ( IOCG short ) deposits. However, there are also secondary sedimentary deposits, such as the Salare the Andean area, salt flats, in which over millions of years, lithium, which was washed out verwitterndem volcanic material has accumulated in mineable concentrations.