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{{Short description|Volcanic island arc between Kyushu and Taiwan}}
[[File:The Ryukyu Arc Map.png|thumb|upright=1.
The '''Ryukyu Arc''' is an [[island arc]] which extends from the south of [[Kyushu]] along the [[Ryukyu Islands]] to the northeast of [[Taiwan]], spanning about {{convert|1200|km|mi}}.<ref name=":1">Kiszaki, K. (1978). Tectonics of the Ryukyu Island Arc. ''Journal of Physics of the Earth, 26''(Supplement), S301–S307. https://doi.org/10.4294/jpe1952.26.Supplement_S301</ref><ref name=":2">Shinjo, R., & Kato, Y. (2000). Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. ''Lithos, 54''(3–4), 117–137. https://doi.org/10.1016/S0024-4937(00)00034-7</ref><ref name=":3">Park, J. O., Tokuyama, H., Shinohara, M., Suyehiro, K., & Taira, A. (1998). Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system. ''Tectonophysics, 294''(1–2), 21–42. https://doi.org/10.1016/S0040-1951(98)00150-4</ref> It is located along a section of the [[Convergent boundary|convergent plate boundary]] where the [[Philippine Sea Plate]] is subducting northwestward beneath the [[Eurasian Plate]] along the [[Ryukyu Trench]].<ref name=":2" /><ref name=":3" /><ref name=":4">Shinjo, R., Chung, S.-L., Kato, Y., and Kimura, M. (1999), Geochemical and Sr-Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin, ''J. Geophys. Res., 104''( B5), 10591– 10608, doi:10.1029/1999JB900040.</ref> The arc has an overall northeast to southwest trend and is located northwest of the [[Pacific Ocean]] and southeast of the [[East China Sea]].<ref name=":6">Nakae, S. (2013). The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima, southern Ryukyu Arc, Japan. ''Journal of Asian Earth Sciences, 73'', 21–30.</ref> It runs parallel to the [[Okinawa Trough]], an active volcanic arc, and the Ryukyu Trench.<ref name=":5">Kizaki, K. (1986). Geology and tectonics of the Ryukyu Islands. ''Tectonophysics, 125''(1–3), 193–207.</ref> The Ryukyu Arc, based on its geomorphology, can be segmented from north to south into Northern Ryukyu, Central Ryukyu, and Southern Ryukyu; the Tokara Strait separates Northern Ryukyu and Central Ryukyu at about 130˚E while the [[Miyako Strait|Kerama Gap]] separates Central Ryukyu and Southern Ryukyu at about 127 ˚E.<ref name=":1" /><ref name=":4" /><ref name=":5" /> The geological units of the arc include [[Igneous rock|igneous]], [[Sedimentary rock|sedimentary]], and [[Metamorphic rock|metamorphic rocks]], ranging from the [[Paleozoic]] to [[Cenozoic]] in age.
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The Ryukyu Arc is a site of active [[seismicity]] characterised by shallow earthquakes, given the ongoing convergence between the Philippine Sea Plate and Eurasian Plate.<ref name=":9" /> Seismic data of earthquakes have been used to detect seismic structures below the Ryukyu Arc.<ref name="a">Roecker, S. W., Yeh, Y. H., & Tsai, Y. B. (1987). Three‐dimensional P and S wave velocity structures beneath Taiwan: Deep structure beneath an arc‐continent collision. ''Journal of Geophysical Research: Solid Earth, 92''(B10), 10547–10570.</ref> [[Low-velocity zone|Low seismic velocity zones]], which are possibly associated with the upwelling of magma, have been discovered beneath active volcanoes and the Okinawa Trough.<ref name="a"></ref>
[[File:M≥6.0 (1960-2022) Earthquakes in the Ryukyu Arc.png|thumb|
=== Volcanoes ===
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== Geological units ==
Major, but not all, geological units of the Ryukyu Arc are herein summarised.
[[File:Simplified Geological Map of Ishigaki Island.png|thumb|upright=1.5|Simplified Geological Map of Ishigaki Island, after Nakae (2013)<ref name=":6" /> and Nishimura (1998)<ref name=":14" />]]
[[File:Simplified Geological Map of northern Okinawa Island.png|thumb|upright=1.5|Simplified Geological Map of northern Okinawa Island, after Miyagi et al., (2013).<ref>Miyagi, N., Baba, S., & Shinjo, R. (2013). Whole-rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands. ''Journal of the Geological Society of Japan, 119''(10), 665-678. doi: 10.5575/geosoc.2013.0045</ref>]]
===Motobu and Yonamine Formations===
The Motobu and Yonamine Formations are [[Permian]] rocks that constitute the basement of the Motobu peninsula of [[Okinawa Island]], Central Ryukyu.<ref name=":10">Ishibashi, T. (1969). Stratigraphy of the Triassic formation in Okinawa-jima, Ryukyus. ''Mem. Fac. Sci. Kyushu Univ., ser.D, Geology, 19''(3),373–385</ref>
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== Tectonics ==
[[File:Evolution of the Ryukyu Arc.png|thumb|
Along the Ryukyu Trench, the Philippine Sea Plate is subducting northwestward under the Eurasian Plate at an estimated velocity of 5–7 cm/year.<ref name=":7" /> The subduction angles become increasingly oblique to the arc toward south.<ref name=":7">Seno, T., Stein, S., & Gripp, A. E. (1993). A model for the motion of the Philippine Sea plate consistent with NUVEL‐1 and geological data. ''Journal of Geophysical Research: Solid Earth, 98''(B10), 17941–17948.</ref>
Global Positioning System data show that southern Kyushu and the Ryukyu Arc migrate southeastward (toward the Ryukyu Trench) relative to Eurasia, as compared to the westward-northwestward migration of other arcs of Japan.<ref name=":8">Sagiya, T., Miyazaki, S. I., & Tada, T. (2000). Continuous GPS array and present-day crustal deformation of Japan. ''Pure and applied Geophysics, 157''(11), 2303–2322.</ref>
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