Ryukyu Arc: Difference between revisions

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Revision as of 10:44, 4 December 2022

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{{Short description|Volcanic island arc between Kyushu and Taiwan}}

[[File:The Ryukyu Arc Map.png|thumb|upright=1.35|Map of the Ryukyu Arc. Red triangles represent volcanoes based on Aster Volcano Archive (NASA-METI) data. Contours in 1 km intervals.]]

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 strikesruns parallel to the Ryukyu[[Okinawa TrenchTrough]], an active volcanic beltarc, and the [[OkinawaRyukyu Trough]]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.

== Geological setting ==

A number of studies defined the extent of the Ryukyu Arc geographically and morphologically into three parts: Northern Ryukyu, which includes the [[Ōsumi Islands]]; Central Ryukyu, which includes [[Amami Islands]] and [[Okinawa Islands]]; Southern Ryukyu, which includes [[Miyako Islands]] and [[Yaeyama Islands]].<ref name=":1" /><ref name=":5" /> The northernmost and southernmost parts of the Ryukyu Arc terminate in Kyushu and Taiwan, respectively.<ref name=":9">Foster, H. L. (1965). ''Geology of Ishigaki-shima, Ryukyu-retto''. US Geol. Surv., Prof. Paper. https://pubs.usgs.gov/pp/0399a/report.pdf</ref>

The geological and structural features of Southern Ryukyu are quite different from those of Northern and Central Ryukyu; Southern Ryukyu, and Northern and Central Ryukyu may have haddeveloped ain differentdisparate geological historysettings before the middle [[Miocene]].<ref name=":1" /> [[Accretionary wedge|Accretionary complexes]] in Northern and Central Ryukyu are considered as the extension of the Outer Zone of Southwest Japan,<ref name=":5" /><ref name=":25">Taira, A., Ohara, Y., Wallis, S. R., Ishiwatari, A., & Iryu, Y. (2016). Geological evolution of Japan: an overview. ''The geology of Japan'', 1–24.</ref><ref name=":26">Takami, M., Takemura, R., Nishimura, Y., & Kojima, T. (1999). Reconstruction of oceanic plate stratigraphies and unit division of Jurassic-Early Cretaceous accretionary complexes in the Okinawa Islands, central Ryukyu Island Arc. ''The Journal of the Geological Society of Japan, 105''(12), 866–880. https://doi.org/10.5575/geosoc.105.866</ref> whereas metamorphic rocks in Southern Ryukyu are associated with the Inner Zone of Southwest Japan.<ref name=":14" /> The arc might be united into the present configuration in the [[Pliocene]]-[[Pleistocene]] due to different rates of southeastward migration as tectonics proceeded.<ref name=":1" />

[[File:Location of the Ryukyu Islands.JPG|thumb|upright=1.5|Location of the Ryukyu Islands.]]

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</div>

=== Okinawa Trough ===

The Okinawa Trough is the [[back-arc basin]] of the Ryukyu Arc, and has been formed by lithospheric extension of the continental Eurasian Plate.<ref name=":2" /> En echelon, extensionalExtensional [[Graben|grabens]], withabout a10 widthkm ofwide and about 1050–100 km andlong, awith lengthen ofechelon about 50–100 kmalignment can be found in the middle to southern parts of Okinawa Trough.<ref name=":2" /><ref name=":4" /> Of the whole Okinawa Trough, the southern part is the most evolved and active, in that, its maximum depth, which is greater than other parts, is about 2200 m.<ref name=":3" /> The Okinawa Trough accommodates [[Terrigenous sediment|terrigenous sediments]] from the [[continental shelf]] of Asia and the Ryukyu Arc at a high rate.<ref name=":3" />

[[File:Volcanic Arc System SVG en.svg|thumb|upright=2|Schematic of island arc system.]]

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===Yaeyama Group===

The Yaeyama Group is a set of sedimentary rocks comprising sandstonesprimarily interbeddedsandstone with coalinterbeds seamsof conglomerate, mudstoneslimestone, conglomeratesmudstone, and limestonescoal seams, foundand cropping inout the Yaeyama IslandIslands, Southern Ryukyu,.<ref name=":1" /><ref name=":5" /> and theThe age of the group is Earlyearly Miocene, as suggested by [[Paleontology|paleontological]] data.<ref name=":15" /> Coal seams, cross laminae, and trace fossils reveal that the group is possibly derived from sediments in a continental shelf.<ref name=":1" /><ref name=":5" /> The group shows less deformation such as tilting and folding than the Eocene formations of Southern Ryukyu, suggesting that Southern Ryukyu has been stable since the early Miocene.<ref name=":1" />

===Shimajiri Group===

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=== Permian-Paleogene ===

Permian-Jurassic accretionary prisms accumulated along the eastern side of [[Pangaea|Pangea]], where the [[Panthalassa|ancient Pacific]] Plate subducted under the ancient Asian continental block.<ref name=":23">Taira, A. (2001). Tectonic evolution of the Japanese island arc system. ''Annual Review of Earth and Planetary Sciences, 29''(1), 109–134.</ref> Vast regions of the Japan Island Arc, including the Ryukyu Arc, developed from accretionaryaccretions processespertinent associated withto the subduction induring the Jurassic.<ref name=":23" /> Fossil and paleomagnetic evidence from the Shimanto Group suggests that subduction of a young oceanic plate took placeoccured in the Late Cretaceous (about 70 Ma) to the Paleogene.<ref name=":23" /> Subduction and accretion may have stopped in late Eocene prior to the deposition of the Neogene-Quaternary Shimajiri Group.<ref name=":21" />

=== Neogene-Quaternary ===

Deposition of continental shelf sediments (the Yaeyama Group) took place in Southern Ryukyu, which at the time was stable and had no crustal movement, during early Miocene.<ref name=":1" /><ref name=":6" /><ref name=":21">Ujiié, H. (1994). Early Pleistocene birth of the Okinawa Trough and Ryukyu Island Arc at the northwestern margin of the Pacific: evidence from Late Cenozoic planktonic foraminiferal zonation. ''Palaeogeography, Palaeoclimatology, Palaeoecology, 108''(3–4), 457–474. https://doi.org/10.1016/0031-0182(94)90246-1</ref> After a cessation of subduction around 10–6 Ma, the Philippine Sea Plate resumed subducting since the late Miocene (about 6 Ma), leading to back-arc spreading of the Okinawa Trough.<ref name=":22">Kamata, H., & Kodama, K. (1994). Tectonics of an arc-arc junction: an example from Kyushu Island at the junction of the Southwest Japan Arc and the Ryukyu Arc. ''Tectonophysics, 233''(1–2), 69–81. https://doi.org/10.1016/0040-1951(94)90220-8</ref> Initial rifting of the northern Okinawa Trough may have caused a counterclockwise rotation in Northern Ryukyu and southern Kyushu after 6 Ma.<ref name=":22" /> Meanwhile, paleomagnetic data suggestrecord thata Southernclockwise Ryukyurotation rotatedof clockwiseSouthern Ryukyu after 10 Ma.<ref name=":17" /> [[Reflection seismology|Seismic reflection]] surveys indicate thatinitial tectonicrifting processesof relatedthe tosouthern initialpart riftingof likethe crustalOkinawa doming,Trough erosionin the early Pleistocene, subsidencewhich andgave sedimentationrise dominatedto thedistinct Okinawatectonic Troughprocesses, duringi.e., the[[sedimentation]], earlycrustal Pleistocenedoming, [[erosion]], and [[Tectonic subsidence|subsidence]].<ref name=":3" /> The emergence of the Ryukyu Arc, together with the subsidence of the Okinawa Trough, may have occurred in the late Pleistocene (1.7–0.5 Ma) after the development of the Shimajiri Group and before that of the Ryukyu Group.<ref name=":21" /> The back-arc rifting and associated sedimentation in the southern Okinawa Trough have continued since 2 Ma.<ref name=":3" />

==See also==