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== 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" />
===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>
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, RyukyuRyūkyū-retto.Rettō|volume=5 (Geology)|publisher= US Army Pacific, Office Eng.,of 5,the 88Engineer|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 ''Sci.Reports Rep.of Kanazawa Univ., 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. (1981). |title=Waagenophyllum (Waagenophyllum) okinawense, a new Permian coral from Okinawa-jima, Ryukyu Islands : Paleontological Study of the Ryukyu Islands-VII.Islands－VII ''Mem.|journal=Memoirs Fac.of Sci.the KyushuFaculty Univ.of Science, serKyūsyū University. Series D, Geology, |date=1981 |volume=24''( |issue=3), |pages=179–188 |doi=10.5109/1546074}}</ref>
===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 [[Ultramafic rock|ultramafic rocks]] and high pressure/temperature (P/T) metamorphic rocks, i.e., metagabbro and [[mafic]], [[Siliceous rock|siliceous]], and pelitic [[Schist|schists]].<ref name=":14"></ref><ref name=":16">{{cite book|last1=Miyazaki,|first1= K.,|last2= Ozaki,|first2= M.,|last3= Saito,|first3= M.,|last4= Toshimitsu,|first4= S. (|date=2016). |chapter=The Kyushu-Ryukyu Arc. ''|title=The Geologygeology of Japan'' (pp.|pages=139–174)|doi=10.1144/GOJ.6|editor-first1=T.|editor-last1= GeologicalMoreno|editor-first2= Society,S. London|editor-last2=Wallis|editor-first3= T. |editor-last3=Kojima|editor-first4=W. |editor-last4=Gibbons|isbn=9781862397064|publisher=The Geological Society}}</ref>
===Fusaki Formation===
===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=KohtaroK. |title=Off-scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc |journal=Tectonics |date=April 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 [[Metasedimentary rock|metasedimentary rocks]] metamorphosed up to [[greenschist]] [[metamorphic facies|facies]], including [[flysch]]-type sandstone and slate with mafic greenstones.<ref name=":5" />
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. (2002). |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), 937-952.|pages=937–952 https://|doi.org/=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 [[Trace fossil|trace fossils]] have been found in the formation, it is thought to be Cretaceous to early Eocene in age by correlations with other formations in the island.<ref name=":27" /> The Kayo Formation features beds of turbidite containing mudstone, sandstone, and [[nummulite]] fossils that indicate middle Eocene age.<ref name=":21" /> ] It underlies the Nago Formation along a northwest-dipping thrust fault. <ref name=":27" /> Thrust-folding and metamorphism of the formation suggest a trench origin.<ref name=":21" /><ref name=":24" />
===Miyara and Nosoko Formations===
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, [[Echinoderm|echinoderms]], [[Bryozoa|bryozoans]], and [[Gastropoda|gastropods]]— have been preserved in the limestones, and [[Mollusca|molluscs]] were also found in the conglomerate.<ref name=":9" /> Foraminifera and calcareous algae in the limestones suggest a late Eocene age.<ref name=":9" />
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. (1995). Two‐phase|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. https://|doi.org/=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" />
===Yaeyama Group===
===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 Paper,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|Rifting]] of the southern Okinawa Trough preceded the deposition of the group.<ref name=":3" />
===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. http://|hdl.handle.net/=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"></ref> The group comprises predominantly the Ryukyu Limestone and Terrace Deposits of sand and gravel.<ref name=":19"></ref> The Ryukyu Limestone is generally 40–60m thick and is characterised by post-depositional faulting, which resulted in the formation of terraces and the associated sediments (Terrace Deposits).<ref name=":19"></ref>
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'Uruma Movement’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>
Southwest Japan (in Japanese with English abstract). ''Mem. Geol. Sco. Japan, 2'', 15–24</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 cm/year.<ref name=":7" /> The subduction angles become increasingly oblique to the arc toward south.<ref name=":7">{{cite journal |last1=Seno, |first1=T., |last2=Stein, |first2=S., & |last3=Gripp, |first3=A. E. (1993). |title=A model for the motion of the Philippine Sea platePlate consistent with NUVEL‐1NUVEL-1 and geological data. ''|journal=Journal of Geophysical Research: Solid Earth, |date=1993 |volume=98''( |issue=B10), |pages=17941–17948 |doi=10.1029/93JB00782}}</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">{{cite journal |last1=Sagiya |first1=T. |last2=Miyazaki |first2=S. |last3=Tada |first3=T. |title=Continuous GPS Array and Present-day Crustal Deformation of Japan: |journal=Pure and Applied Geophysics |date=2000 |volume=157 |issue=11 |pages=2303–2322 |doi=10.1007/PL00022507}}</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>
=== 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. (2001). |title=Tectonic evolutionEvolution of the Japanese islandIsland arcArc system.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 occured 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">{{cite journal |last1=Ujiié, |first1=H. (1994). |title=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. ''|journal=Palaeogeography, Palaeoclimatology, Palaeoecology, |date=1994 |volume=108''( |issue=3–4), |pages=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">{{cite journal |last1=Kamata, |first1=H., & |last2=Kodama, |first2=K. (1994). |title=Tectonics of an arc-arc junction: an example from Kyushu Island at the junction of the Southwest Japan Arc and the Ryukyu Arc. ''|journal=Tectonophysics, |date=1994 |volume=233''( |issue=1–2), |pages=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 record a clockwise rotation of Southern Ryukyu after 10 Ma.<ref name=":17" /> [[Reflection seismology|Seismic reflection]] surveys indicate initial rifting of the southern part of the Okinawa Trough in the early Pleistocene, which gave rise to distinct tectonic processes, i.e., [[sedimentation]], crustal doming, [[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==
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