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{{Short description|Island arc between Kyushu and Taiwan}}
[[File:The Ryukyu Arc Map.png|thumb|upright=1.9|Map of the Ryukyu Arc. Red triangles represent volcanoes based on Aster Volcano Archive (NASA-METI) data. Contours in 1 km intervals. Figure made with GeoMapApp (www.geomapapp.org) / CC BY / CC BY (Ryan et al., 2009).<ref name="fig">{{Cite journal|last1=Ryan|first1=William B. F.|last2=Carbotte|first2=Suzanne M.|author-link2=Suzanne Carbotte|last3=Coplan|first3=Justin O.|last4=O'Hara|first4=Suzanne|last5=Melkonian|first5=Andrew|last6=Arko|first6=Robert|last7=Weissel|first7=Rose Anne|last8=Ferrini|first8=Vicki|last9=Goodwillie|first9=Andrew|last10=Nitsche|first10=Frank|last11=Bonczkowski|first11=Juliet|date=March 2009|title=Global Multi-Resolution Topography synthesis|journal=Geochemistry, Geophysics, Geosystems|volume=10|issue=3|pages=n/a|doi=10.1029/2008gc002332|bibcode=2009GGG....10.3014R|issn=1525-2027|doi-access=free}}</ref>]]
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">{{cite journal |last1=Kizaki |first1=K. |title=Tectonics of the Ryukyu Island Arc |journal=Journal of Physics of the Earth |date=1978 |volume=26 |issue=Supplement |pages=S301–S307 |doi=10.4294/jpe1952.26.Supplement_S301}}</ref><ref name=":2">{{cite journal |last1=Shinjo |first1=R. |last2=Kato |first2=Y. |title=Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin |journal=Lithos |date=2000 |volume=54 |issue=3–4 |pages=117–137 |doi=10.1016/S0024-4937(00)00034-7}}</ref><ref name=":3">{{cite journal |last1=Park |first1=J.-O. |last2=Tokuyama |first2=H. |last3=Shinohara |first3=M. |last4=Suyehiro |first4=K. |last5=Taira |first5=A. |title=Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system |journal=Tectonophysics |date=1998 |volume=294 |issue=1–2 |pages=21–42 |doi=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">{{cite journal |last1=Shinjo |first1=R. |last2=Chung |first2=S.-L. |last3=Kato |first3=Y. |last4=Kimura |first4=M. |title=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 |journal=Journal of Geophysical Research: Solid Earth |date=1999 |volume=104 |issue=B5 |pages=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">{{cite journal |last1=Nakae |first1=S. |title=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=Journal of Asian Earth Sciences |date=2013 |volume=73 |pages=21–30 |doi=10.1016/j.jseaes.2013.04.022}}</ref> It runs parallel to the [[Okinawa Trough]], an active volcanic arc, and the Ryukyu Trench.<ref name=":5">{{cite journal |last1=Kizaki |first1=K. |title=Geology and tectonics of the Ryukyu Islands |journal=Tectonophysics |date=1986 |volume=125 |issue=1–3 |pages=193–207 |doi=10.1016/0040-1951(86)90014-4}}</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 [[
== 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">{{cite report|last1=Foster|first1= H. L.|date=1965|title=Geology of Ishigaki-shima, Ryukyu-retto|series=US Geological Survey, Professional Papers|publisher=US Geological Survey|doi=10.3133/pp399A|volume=399-A}}</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 developed in disparate geological settings 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">{{cite book|last1=Taira|first1= A.|last2= Ohara|first2= Y.|last3= Wallis|first3= S. R.|last4=Ishiwatari|first4= A.|last5= Iryu|first5= Y. |date=2016|chapter=Geological evolution of Japan: an overview|title=The geology of Japan|pages=1–24|doi=10.1144/GOJ.1|editor-first1=T.|editor-last1= Moreno|editor-first2= S. |editor-last2=Wallis|editor-first3= T. |editor-last3=Kojima|editor-first4=W. |editor-last4=Gibbons|isbn=9781862397064|publisher=The Geological Society}}</ref><ref name=":26">{{cite journal |last1=Takami |first1=M. |last2=Takemura |first2=R. |last3=Nishimura |first3=Y. |last4=Kojima |first4=T. |title=Reconstruction of oceanic plate stratigraphies and unit division of Jurassic-Early Cretaceous accretionary complexes in the Okinawa Islands, central Ryukyu Island Arc. |journal=The Journal of the Geological Society of Japan |date=1999 |volume=105 |issue=12 |pages=866–880 |doi=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" />
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=== 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" /> Extensional [[
[[File:Volcanic Arc System SVG en.svg|thumb|upright=2|Schematic of island arc system.]]
=== Seismicity ===
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">{{cite journal |last1=Roecker |first1=S. W. |last2=Yeh |first2=Y. H. |last3=Tsai |first3=Y. B. |title=Three-dimensional P and S wave velocity structures beneath Taiwan: Deep structure beneath an arc-continent collision |journal=Journal of Geophysical Research: Solid Earth |date=1987 |volume=92 |issue=B10 |pages=10547–10570 |doi=10.1029/JB092iB10p10547}}</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"
[[File:M≥6.0 (1960-2022) Earthquakes in the Ryukyu Arc.png|thumb|upright=2.2|M≥6.0 (1960-2022) Earthquakes in the Ryukyu Arc. Earthquake data from USGS-ANSS Catalog. Contours in 1 km interval. Figure made with GeoMapApp (www.geomapapp.org) / CC BY / CC BY (Ryan et al., 2009).<ref name="fig" />]]
=== Volcanoes ===
An active volcanic front lies 100
== Geological units ==
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[[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>{{cite journal |last1=Miyagi |first1=N. |last2=Baba |first2=S. |last3=Shinjo |first3=R. |title=Whole-rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands |journal=The Journal of the Geological Society of Japan |date=2013 |volume=119 |issue=10 |pages=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">{{cite journal |last1=Ishibashi |first1=T. |title=Stratigraphy of the Triassic Formation in Okinawa-jima, Ryukyus |journal=Memoirs of the Faculty of Science, Kyūsyū University. Series D, Geology |date=1969 |volume=19 |issue=3 |pages=373–385 |doi=10.5109/1543682}}</ref>
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===Tomuru Formation===
The Tomuru Formation is distributed in the [[Iriomote Island|Iriomote]] and [[Ishigaki Island|Ishigaki]] Islands of the Yaeyama Islands, Southern Ryukyu.<ref name=":5" /><ref name=":14">{{cite journal |last1=Nishimura |first1=Y. |title=Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan |journal=Journal of Metamorphic Geology |date=1998 |volume=16 |issue=1 |pages=129–140 |doi=10.1111/j.1525-1314.1998.00059.x}}</ref><ref name=":15">{{cite journal|last1=Nakagawa|first1= H.|last2= Doi|first2= N.|last3= Shirao|first3= M.|last4= Araki|first4=Y.|date=1982|title= Geology of Ishigaki-jima and Iriomote-jima Yaeyama Gunto, Ryukyu Islands|journal= Contributions from the Institute of Geology and Paleontology, Tohoku University|volume= 84|pages=1–22}}</ref> The formation has an age of 220–190 [[megaannum|Ma]] (million years ago) (Late [[Triassic]]-Early [[Jurassic]]) and comprises [[
===Fusaki Formation===
The Fusaki Formation, located at the Ishigaki Island and [[Taketomi Island]] of the Yaeyama Islands in Southern Ryukyu,<ref name=":15" /> comprises weakly metamorphosed [[Olistostrome|olistromal rocks]]: [[allochthon|allochthonous blocks]] of chert, [[mudstone]], sandstone, and limestone are embedded in a muddy [[Matrix (geology)|matrix]].<ref name=":14" /> The metamorphic age of this formation ranges from
On Ishigaki Island, this formation is thrusted beneath the Tomuru Formation along the Sokobaru [[Thrust fault|thrust]] <ref name=":14" /> and is intruded by [[Oligocene]] granitic rocks, named the Omoto Pluton.
===Nakijin Formation===
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===Shimanto Group===
The Shimanto Group is a set of metamorphic rocks dated from Early Cretaceous to earliest Miocene which are associated with the Shimanto belt.<ref name=":5" /><ref name=":24">{{cite journal |last1=Ujiie |first1=K. |title=Off-scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc |journal=Tectonics |date=1997 |volume=16 |issue=2 |pages=305–322 |doi=10.1029/96TC03367|s2cid=129383358 }}</ref> The Shimanto belt is an accretionary complex in the Outer Zone of Southwest Japan which extends from [[Honshu]], [[Shikoku]], and Kyushu to Northern and Central Ryukyu.<ref name=":1" /> The group comprises sedimentary and [[
Deformations such as northwest-dipping, isoclinal, overturned [[Fold (geology)|folds]] and SE-verging thrust faults can be found in the Shimanto Group of Central and Northern Ryukyu.<ref name=":5" />
In Okinawa Island, Central Ryukyu, the Shimanto Group is separated into the Nago Formation and the Kayo Formation.<ref name=":27">{{cite journal |last1=Ujiie |first1=K. |title=Evolution and kinematics of an ancient décollement zone, mélange in the Shimanto accretionary complex of Okinawa Island, Ryukyu Arc |journal=Journal of Structural Geology |date=2002 |volume=24 |issue=5 |pages=937–952 |doi=10.1016/S0191-8141(01)00103-1}}</ref> The Nago Formation comprises pelitic and mafic schist, phyllite, and slate, with minor chert and limestone,<ref name=":27" /> and crops out most of northern Okinawa Island.<ref name=":11" /> Though only [[
===Miyara and Nosoko Formations===
The Eocene Miyara and Nosoko Formations crop out the Yaeyama Islands, Southern Ryukyu.<ref name=":5" />
The Miyara Formation is a south-southwest-dipping succession of [[Conglomerate (geology)|conglomerate]], sandstone, [[shale]], and limestone deposited along the coast of Ishigaki Island.<ref name=":9" /> A large variety of marine fossils—e.g., calcareous algae, foraminifera, corals, [[
The Nosoko Formation is a 300m thick sequence of [[tuff]], volcanic sandstone and [[breccia]], and lavas with [[Dike (geology)|dykes]], [[Sill (geology)|sills]], and other small [[Igneous intrusion|intrusions]].<ref name=":9" /> This formation is widely exposed at the Nosoko peninsula in northern Ishigaki Island.<ref name=":9" /> It also lies conformably above the Miyara Formation.<ref name=":21" /> [[Paleomagnetism|Paleomagnetic]] data of the Nosoko Formation indicate a mean deflection of the magnetic direction at about 30˚ clockwise away from the expected pole.<ref name=":17">{{cite journal |last1=Miki |first1=M. |title=Two-phase opening model for the Okinawa Trough inferred from paleomagnetic study of the Ryukyu arc |journal=Journal of Geophysical Research: Solid Earth |date=1995 |volume=100 |issue=B5 |pages=8169–8184 |doi=10.1029/95JB00034}}</ref> This data, coupled with [[Radiometric dating|radiometric]] ages, suggest that Southern Ryukyu might have rotated about 25˚ with respect to the Asian continent in the Miocene at 6–10 Ma.<ref name=":17" />
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The Shimajiri Group consists of an upper and a lower member. The upper member (Shinzato) of late Miocene or Pliocene age consists of tuff and shale; the lower member (Yonabaru) of Miocene age contains shale interbedded with siltstone and sandstone.<ref name=":18">{{cite report|last1=LeRoy|first1= L. W. |date=1964|title=Smaller foraminifera from the late Tertiary of southern Okinawa|doi=10.3133/pp454F|series=US Geological Survey Professional Papers|volume= 454|pages=1–58}}</ref>
The Shimajiri Group is the first geological unit to be found across Northern, Central, and Southern Ryukyu.<ref name=":1" /><ref name=":5" /> Northern, Central Ryukyu, and Southern Ryukyu may have had different basins and tectonic settings prior to the Late Miocene (the age of deposition of the group).<ref name=":1" /> Despite being widely distributed across the East China Sea, the Ryukyu Arc and its [[forearc]], the group does not occur in the southern Okinawa Trough.<ref name=":3" /> [[Rift
===Ryukyu Group===
The Ryukyu Group is Pleistocene deposits formed after the development of the Shimajiri Group but before [[Holocene]] sediments had been deposited.<ref name=":19">{{cite journal|last1=Furukawa|first1= H.|date=1979|title=Quaternary geologic history of the Ryukyu Islands|journal= Bulletin of the Science and Engineering Division, University of the Ryukyus (Mathematics and Natural Sciences)|volume= 27|pages= 99–161|hdl=20.500.12000/23768}}</ref> It is distributed in Central and Southern Ryukyu and is marked by a distinct [[unconformity]] above the Shimajiri Group.<ref name=":19"
Following the deposition of the Ryukyu Limestone, prevalent northwest–southeast faulting across the Ryukyu Arc resulted in an episode of dome-like [[tectonic uplift|uplifting]] of basement rocks in the Ryukyu Islands, named the 'Uruma Movement'.<ref name=":20">{{cite journal|author1=Quaternary Research Group of Southwest Japan|date=1968|title=西南日本の第四紀地殻変動|trans-title= Quaternary Crustal Movements in Southwest Japan|language=Japanese |volume= 2|pages= 15–24|journal=地質学論集|url=https://dl.ndl.go.jp/view/download/digidepo_10808937_po_ART0003485601.pdf?contentNo=1&alternativeNo=}}</ref>
== Tectonics ==
[[File:Evolution of the Ryukyu Arc.png|thumb|upright=1.8|right|Evolution of the Ryukyu Arc. Modified from Ujiie (1994).<ref name=":21" />]]
Along the Ryukyu Trench, the Philippine Sea Plate is subducting northwestward under the Eurasian Plate at an estimated velocity of 5–7
=== 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">{{cite journal |last1=Taira |first1=A. |title=Tectonic Evolution of the Japanese Island Arc System |journal=Annual Review of Earth and Planetary Sciences |date=2001 |volume=29 |issue=1 |pages=109–134 |doi=10.1146/annurev.earth.29.1.109}}</ref> Vast regions of the Japan Island Arc, including the Ryukyu Arc, developed from accretions pertinent to the subduction during the Jurassic.<ref name=":23" /> Fossil and paleomagnetic evidence from the Shimanto Group suggests that subduction of a young oceanic plate
=== Neogene-Quaternary ===
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==References==
{{reflist}}
* [https://www.gsj.jp/Map/EN/geology2-7.html Geological Map of Japan – Nansei Islands (Ryukyu Islands)]▼
[[Category:Natural history of the Ryukyu Islands]]
[[Category:Volcanic arcs]]
[[Category:Volcanism of Japan]]
▲== External link ==
▲* [https://www.gsj.jp/Map/EN/geology2-7.html Geological Map of Japan – Nansei Islands (Ryukyu Islands)]
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