Back-arc region

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A back-arc above a subduction zone

The back-arc region is the area behind a volcanic arc.[1] In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs.[1] In continental arcs, the back-arc region is part of the continental platform, either dry land (subaerial) or forming shallow marine basins.[1]

Formation

Back-arc deformation is a product of

upper mantle wedge caused by the downward movement of the subducted slab causes stress in the upper plate and the high heat flow that characterizes back-arcs.[3] The pulling effect of the slab as it goes down into the mantle causes a rollback motion of the trench, which also applies stress on the back-arc region of the upper plate.[2] However, this last process has less of an impact on deformation compared to upper plate motion.[2]

Back-arcs can form on either oceanic crust or continental crust. In the case of oceanic crust, most back-arc regions are subjected to tensional stresses and thus develop a spreading center where new oceanic crust is formed.[4] The composition of this new crust is similar to mid-ocean ridge basalt (MORB), although it contains higher amounts of water.[4]

Back-arc extension vs. compression

The back-arc deformation may be either extensional or compressional. The overriding plate will shorten when its motion is directed towards the trench, resulting in a compression of the back-arc region.[5] This type of deformation is associated with a shallow dipping subducted slab.[5] Inversely, an overriding plate moving away from the trench will result in extension, and a back-arc basin will form.[5] This extensional deformation is associated with a steeply dipping slab.[5]

The extreme cases of these two types of back-arc deformation can be found in Chile and at the Marianas arc. The shallow dipping slab subducting beneath Chile at an angle of about 10–15° causes a compressional stress on the back-arc region behind the Andes.[6] On the other extreme, the slab going down into the mantle at the Marianas subduction zone is so steep it is nearly vertical.[6] This is the perfect example of an oceanic back-arc basin experiencing extensional forces. The Oriente in Ecuador (the eastern part of the country covered by rainforest) is also a good example of an extensional back-arc basin, this time in a continental setting. The continental crust in this area east of the Andes has been stretched out and covered by layers of sediments.

See also

References

  1. ^
    ISBN 978-0-7167-2437-7
    .
  2. ^
    doi:10.1016/j.pepi.2004.08.022{{citation}}: CS1 maint: multiple names: authors list (link
    )
  3. doi:10.1016/0040-1951(82)90126-3
  4. ^
    doi:10.1016/S0012-821X(03)00167-5{{citation}}: CS1 maint: multiple names: authors list (link
    )
  5. ^
    doi:10.1016/j.epsl.2007.02.004
  6. ^
    doi:10.1029/JB084iB03p01049
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