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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|doi-access=free }}</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|doi-access=free }}</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 [[metamorphic rock]]s, 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">{{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|doi-access=free}}</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|doi-access=free }}</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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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>{{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|doi-access=free }}</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|doi-access=free }}</ref>
The Motobu Formation consists mainly of [[limestone]] interbedded with [[chert]] and [[phyllite]].<ref name=":11">{{cite report|last1=Flint|first1= D. E.|last2= Saplis|first2= R. A.|last3=Corwin|first3= G.|date=1959|title=Military geology of Okinawa-jima, Ryūkyū-Rettō|volume=5 (Geology)|publisher= US Army Pacific, Office of the Engineer|url=https://www.google.com/books/edition/Military_Geology_of_Okinawa_Jima_Ry%C5%ABky/65ocAQAAMAAJ|via=Google Books}}</ref> The limestone contains [[foraminifera]] fossils of Permian age.<ref name=":9" />
The Yonamine Formation, with strata of interbedded phyllite, [[slate]], [[sandstone]], limestone, greenstone, and chert,<ref name=":11" /> underlies the Motobu Formation<ref name=":12">{{cite journal|last1=Konishi|first1= K.|date=1963|title=Pre-Miocene basement complex of Okinawa, and the tectonic belts of the Ryukyu Islands|journal=Science Reports of Kanazawa University|volume=8|issue=2|pages= 569–602|url=http://scirep.w3.kanazawa-u.ac.jp/articles/08-02-017.pdf}}</ref> and contains Permian corals.<ref name=":13">{{cite journal |last1=Haikawa |first1=T. |last2=Ishibashi |first2=T. |title=Waagenophyllum (Waagenophyllum) okinawense, a new Permian coral from Okinawa-jima, Ryukyu Islands : Paleontological Study of the Ryukyu Islands－VII |journal=Memoirs of the Faculty of Science, Kyūsyū University. Series D, Geology |date=1981 |volume=24 |issue=3 |pages=179–188 |doi=10.5109/1546074|doi-access=free }}</ref>
===Tomuru Formation===
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===Shimajiri Group===
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|doi-access=free}}</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]]ing of the southern Okinawa Trough preceded the deposition of the group.<ref name=":3" />
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