Back-arc basin

Back-arc basins (or retro-arc basins) are geologic features, submarine basins associated with island arcs and subduction zones.They are found at some convergent plate boundaries, presently concentrated in the Western Pacific ocean. Most of them result from tensional forces caused by oceanic trench rollback and the collapse of the edge of the continent. Back-arc basins were not predicted by plate tectonics theory, but they are consistent with the dominant model for how the Earth loses heat.

Characteristics

Back-arc basins are typically very long (several hundreds to thousands of kilometers) and relatively narrow (a few hundred kilometers). The restricted width of back-arc basins is probably due to the fact that magmatic activity depends on water and induced mantle convection and these are both concentrated near the subduction zone. Spreading rates vary from very slow spreading (Mariana Trough), a few centimeters per year, to very fast (Lau Basin), 15 cm/year. These ridges erupt basalt that is similar to those erupted from the mid-ocean ridges; the main difference is that back-arc basin basalts are often very rich in magmatic water (typically 1-1.5 weight % H2O), whereas mid-ocean ridge basalt magmas are very dry (typically <0.3 weight % H2O). The high water contents of back-arc basin basalt magmas is derived from water carried down the subduction zone and released into the overlying mantle wedge. Additional source of water could be the eclogitization of amphiboles and micas in the subducting slab. Similar to mid-ocean ridges, back-arc basins have hydrothermal vents and associated chemosynthetic communities.

Asymmetry

Back-arc basins are different from normal mid-ocean ridges because they are characterized by asymmetric seafloor spreading, but this is quite variable even within single basins. For example in the central Mariana Trough current spreading rates are 2-3 times greater on the western flank (Deschamps and Fujiwara, 2003) whereas at the southern end of the Mariana Trough the position of the spreading center adjacent to the volcanic front suggests that overall crustal accretion has been nearly 100% asymmetric there (Martinez "et al.", 2000). This situation is mirrored to the north where a large spreading asymmetry is also developed (Yamazaki "et al.", 2003). Other back-arc basins such as the Lau Basin have undergone large rift jumps and propagation events that have transferred spreading centers from arc-distal to more arc-proximal positions (Parson "et al.", 1990) although recent spreading rates appear to be relatively symmetric with perhaps small rift jumps (Zellmer and Taylor, 2001). The cause of asymmetric spreading in back-arc basins remains poorly understood. General ideas invoke asymmetries relative to the spreading axis in arc melt generation processes and heat flow, hydration gradients with distance from the slab, mantle wedge effects, and evolution from rifting to spreading (Barker, 1980; Martinez et al, 1995; Molnar and Atwater, 1978).

Formation and sedimentation

Back-arc basins are hypothesized to form as a result of a process termed trench rollback (also, hinge rollback). This term describes the backward motion of the subduction zone relative to the motion of the plate which is being subducted. As the subduction zone and its associated trench pull backward, the overriding plate is stretched, thinning the crust which is manifest in the back-arc basin. Sedimentation is strongly assymmetric, with most of the sediment supplied from the active magmatic arc which regresses in step with the rollback of the trench.

Location

Active back-arc basins are found in the Marianas, Tonga-Kermadec, S. Scotia, Manus, N. Fiji, and Tyrrhenian Sea regions, but most are found in the Western Pacific. Not all subduction zones have back-arc basins, some like the central Andes are associated with rear-arc compression. In addition, there are a number of extinct or fossil back-arc basins, such as the Parece Vela-Shikoku Basin, Sea of Japan, and Kurile Basin.

History of thought

With the development of plate tectonic theory, geologists thought that convergent plate margins were zones of compression, thus zones of strong extension above subduction zones - back-arc basins - were not expected. The hypothesis that some convergent plate margins were actively spreading was developed by Dan Karig while a graduate student at the Scripps Institution of Oceanography in the early 1970s. This resulted from several marine geologic expeditions to the Western Pacific.

References

*P.F. Barker, I.A. Hill, 1980, Asymmetric spreading in back-arc basins, Nature 285. 652-654.
*Deschamps, A., and T. Fujiwara (2003), Asymmetric accretion along the slow-spreading Mariana Ridge, Geochem., Geophys., Geosyst., 4, doi:10.1029/2003GC000537.
*F. Martinez, P. Fryer, N.A. Baker, T. Yamazaki, (1995) Evolution of backarc rifting: Mariana Trough, 20-24N, J. Geophys. Res., 100, 3807-3827.
*F. Martinez, P. Fryer, N. Becker, (2000), Geophysical Characteristics of the Southern Mariana Trough, 11N-13N, J. Geophys. Res., 105, 16591-16607.
*P. Molnar, T. Atwater (1978), Interarc spreading and Cordilleran tectonics as alternates related to the age of subducted oceanic lithosphere, Earth Planet. Sci. Lett. 41, 330-340.
*L.M. Parson, J.A. Pearce, B.J. Murton, R.A. Hodkinson, RRS Charles Darwin Scientific Party, Role of ridge jumps and ridge propagation in the tectonic evolution of the Lau back-arc basin, southwest Pacific, Geology 18(1990) 470-473.
*T. Yamazaki, N. Seama, K. Okino, K. Kitada, M. Joshima, H. Oda, J. Naka, (2003), Spreading process of the northern Mariana Trough: Rifting-spreading transition at 22 N, Geochem., Geophys., Geosyst. 4, DOI 10.1029/2002GC000492.
*K.E. Zellmer, B. Taylor, (2001) A three-plate kinematic model for Lau Basin opening, Geochem., Geophys., Geosyst. 2, Paper number 2000GC000106.