Slab

The English word slab , equivalent of the French words " plaque" or " tranche" , became usual in Plate tectonics to indicate the part of a oceanic Lithosphère committed in a Subduction.

A slab is defined and recognized by:

  • an important seismicity distributed in a plan called Plane of Wadati/Benioff (of the name of the two seismologists which discovered it).

  • a temperature lower than the Lithosphere and the Asthenosphere around, with identical depth (locatable thermic anomaly by the technique of the telesismic Tomography).

The most remarkable effects of the subduction of a slab on its environment are the following:

  • the hydration of the coat of the overlapping plate. Indeed, the plunging slab towards the lower coat is dehydrated gradually and the released fluids percolent the corner of overlying coat which they feed in elements coming from the crust subduite and of its sediments.

  • modification of isostatic balance

  • the appearance of a volcanicity which presents the geochemical double Signature of the oceanic Croûte subduite and of the corner of overlying coat. This volcanicity is of type Calco-alcalin, frequently explosive (Pelean eruptions or pliniennes). The lava of the zones of subduction (basalts, andesites, dacites) as a whole is supersaturated out of silica and systematically rich in hydroxylated minerals. The volcanos of the zones of subduction form chains parallel with the pit of subduction, at an average vertical distance of 110 kilometers of the roof of the slab subduit. This volcanicity is fed by the fusion of the corner of overlying coat to the slab, at a temperature of 1200 degrees approximately.

  • the opening or the closing of a Basin of back-arc. If the plate lithospheric subduite then forms an angle higher than 50° it is an extensive mode of this basin on the other hand if the plate forms an angle lower than 50° the mode is compressive. (Attention, the age of the plate has nothing to do with the mode! It is false to say that a young plate implies a mode in compression or conversely. That is checked only for some cases in the world; example: Mariannes

With approximately the 650 km depth, the slab penetrates in the lower coat. During this descent, the peridotite is transformed into denser rocks: garnet-rocks (wadsleyite, ringwoodite, majority) starting from 400 km; perovskite and magnésiowürstite to 650 km.

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