Backarc basin

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Schematic representation of a subduction zone with back arc spreading.

Backarc basin ( English back-arc basin ) and forearc basin (English fore-arc basin or outer arc ) are basins or, if particularly well developed, even lake basins , which are subduced by one oceanic plate under another oceanic plate or continental plate, but are created in detail by completely different processes. The English prefixes back (hinter) and fore (vor) refer on the one hand to the position in relation to the volcanic arch of the island , which is located between these basins, and on the other hand to the position in relation to the subduction front, its position in roughly corresponds to the deep sea channel. The forearc basin lies on the side of the arch of the island facing the subduction front and the back arc basin on the side of the arch facing away from the subduction front.

Forearc basin

Forearc therefore means from the point of view of the subduction front located in front of the igneous area of ​​the island arc . Because there is usually no magmatism there, the forearc area is also called the magmatic gap . There, an accretion wedge forms due to the material delivered on the subducting plate . As a result of the tectonic tension at the outermost edge of the top plate, it can be raised and form a forearc ridge . This uplift can go so far that the forearc ridge reaches the sea surface, as in the case of Crete and Barbados , for example . The area between the forearc ridge and the volcanic arch that is not or less strongly raised is called the forearc basin . Forearc pools are always relatively shallow.

Backarc basin

The lake basin of the Sea of ​​Japan is interpreted as a backarc basin, which once began as a continental rift valley at the back of a continental volcanic arch on the eastern edge of the Eurasian plate . Since the corresponding oceanic spreading zone is inactive today, Japan is not regarded as a separate continental plate, but as part of the Eurasian plate. The subduction fronts are indicated by the blue zigzag lines in the figure

Backarc , on the other hand, means located behind the magmatic area of ​​the island arc from the point of view of the subduction front . There the earth's crust can stretch and, as a result, sink in. These sunken areas are called backarc basins . Backarc basins are primarily formed on arches of islands, in whose subduction zones relatively old, i.e. heavy, oceanic crust is subducted. The oceanic plate sinks there “by itself” and therefore at a relatively steep angle into the earth's mantle, rather than being actively driven over by the upper plate. As a result, the subduction front moves opposite to the drift direction of the subducted plate. This is called a slab roll-back . The slab roll-back generates horizontal tensile forces in the upper plate that act across the subduction front, an effect known as trench suction (literally: suction emanating from the deep sea channel ). These tensile forces cause an expansion in the back arc, which initially leads to the formation of a rift, but if the expansion continues, this can lead to the formation of an oceanic spreading zone and thus the opening of a deep ocean basin. Another model for the formation of backarc basins is based on the rupture of subducted oceanic crust ( slab break-off ) in the earth's mantle below the backarc area with subsequent upwelling of hot mantle material, i.e. the backarc - Stretching is caused by heating and thermal bulging of the crust.

Examples of relatively large backarc basins are the Japanese basin between mainland Asia and Japan , the South China Sea, and the Sulu Sea and the Celebes Sea .

Examples of active backarc spreading zones are the Mariana Trough in the backarc area of ​​the Mariana Arch in the eastern Philippine Sea and the Havre Trough (Lau Basin) between the Kermadec-Tonga Ridge and the Colville-Lau Ridge northeast of New Zealand .

literature

  • Wolfgang Frisch, Martin Meschede: Plate tectonics. 2nd Edition. Primus-Verlag, Darmstadt 2007, ISBN 3-89678-525-7 .

Individual evidence

  1. ↑ Introduction to Tectonics. Lecture notes, ETH Zurich
  2. ↑ Toshitsugu Yamazaki, Nobukazu Seama, Kazuya Kitada, Masato Joshima, Hirokuni Oda, Jiro Naka: Spreading process of the northern Mariana Trough: Rifting-spreading transition at 22 ° N. Geochemistry, Geophysics, Geosystems. Vol. 4, No. 9, 2003, 1075 doi : 10.1029 / 2002GC000492
  3. ^ E. Ruellan, J. Delteil, I. Wright, T. Matsumoto: From rifting to active spreading in the Lau Basin - Havre Trough backarc system (SW Pacific): Locking / unlocking induced by seamount chain subduction. Geochemistry, Geophysics, Geosystems. Vol. 4, No. 5, 2003, 8909 doi : 10.1029 / 2001GC000261

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