Back-arc basins are typically very long and relatively narrow, often thousands of kilometers long while only being a few hundred kilometers wide at most. For back-arc extension to form, a subduction zone is required, but not all subduction zones have a back-arc extension feature. Back-arc basins are found in areas where the subducting plate of oceanic crust is very old. The restricted width of back-arc basins is due to magmatic activity being reliant on water and induced mantle convection, limiting their formation to along subduction zones. Spreading rates vary from only a few centimeters per year (as in the Mariana Trough), to 15 cm/year in the Lau Basin. Spreading ridges within the basins erupt basalts that are similar to those erupted from the mid-ocean ridges; the main difference being 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 sources 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.
This has prompted some to characterize the spreading in back-arc basins to be more diffused and less uniform than at mid-ocean ridges. The idea that back-arc basin spreading is inherently different from mid-ocean ridge spreading is controversial and has been debated through the years. Another argument put forward is that the process of seafloor spreading is the same in both cases, but the movement of seafloor spreading centers in the basin causes the asymmetry in the magnetic anomalies. This process can be seen in the Lau back-arc basin. Though the magnetic anomalies are more complex to decipher, the rocks sampled from back-arc basin spreading centers do not differ very much from those at mid-ocean ridges. In contrast, the volcanic rocks of the nearby island arc differ significantly from those in the basin.
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, 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 entirely asymmetric there. This situation is mirrored to the north where a large spreading asymmetry is also developed.
Other back-arc basins such as the Lau Basin have undergone large rift jumps and propagation events (sudden changes in relative rift motion) that have transferred spreading centers from arc-distal to more arc-proximal positions. Conversely, study of recent spreading rates appear to be relatively symmetric with perhaps small rift jumps. 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.
The extension of the crust behind volcanic arcs is believed to be caused by processes in association with subduction. As the subducting plate descends into the asthenosphere it sheds water, causing mantle melting, volcanism, and the formation of island arcs. Another result of this is a convection cell is formed. The rising magma and heat along with the outwards tension in the crust in contact with the convection cell cause a region of melt to form, resulting in a rift. This process drives the island arc toward the subduction zone and the rest of the plate away from the subduction zone. The backward motion of the subduction zone relative to the motion of the plate which is being subducted is called trench rollback (also known as hinge rollback or hinge retreat). As the subduction zone and its associated trench pull backward, the overriding plate is stretched, thinning the crust and forming a back-arc basin. In some cases, extension is triggered by the entrance of a buoyant feature in the subduction zone, which locally slows down subduction and induces the subducting plate to rotate adjacent to it. This rotation is associated with trench retreat and overriding plate extension.
The age of the subducting crust needed to establish back-arc spreading has been found to be 55 million years old or older. This is why back-arc spreading centers appear concentrated in the western Pacific. The dip angle of the subducting slab may also be significant, as is shown to be greater than 30° in areas of back-arc spreading; this is most likely because as oceanic crust gets older it becomes denser, resulting in a steeper angle of descent.
The thinning of the overriding plate from back-arc rifting can lead to the formation of new oceanic crust (i.e., back-arc spreading). As the lithosphere stretches, the asthenosphere below rises to shallow depths and partially melts as a result of adiabatic decompression melting. As this melt nears the surface, spreading begins.
There are a number of extinct or fossil back-arc basins, such as the Parece Vela-Shikoku Basin, Sea of Japan, and Kurile Basin. Compressional back-arc basins are found, for example, in the Pyrenees and the Swiss Alps.
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Sdrolias, M; Muller, R.D. (2006). "Controls on back-arc basin formations". Geochemistry, Geophysics, Geosystems. 7 (4): Q04016. Bibcode:2006GGG.....7.4016S. doi:10.1029/2005GC001090. S2CID 129068818. https://doi.org/10.1029%2F2005GC001090
Sdrolias, M; Muller, R.D. (2006). "Controls on back-arc basin formations". Geochemistry, Geophysics, Geosystems. 7 (4): Q04016. Bibcode:2006GGG.....7.4016S. doi:10.1029/2005GC001090. S2CID 129068818. https://doi.org/10.1029%2F2005GC001090
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Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
Klein, G.D. (1985). "The Control of Depositional Depth, Tectonic Uplift, and Volcanism on Sedimentation Processes in the Back-Arc Basins of the Western Pacific Ocean". Journal of Geology. 93 (1): 1–25. Bibcode:1985JG.....93....1D. doi:10.1086/628916. S2CID 129527339. /wiki/Bibcode_(identifier)
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Karig, Daniel (1970). "Ridges and basins of the Tonga-Kermadec island arc system". Journal of Geophysical Research. 75 (2): 239–254. Bibcode:1970JGR....75..239K. doi:10.1029/JB075i002p00239. /wiki/Bibcode_(identifier)