Ryukyu Arc: Difference between revisions

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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 1000m interval.]]

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-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 strikes parallel to the Ryukyu Trench, an active volcanic belt, and the [[Okinawa Trough]].<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 had a different geological history 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=":24" />. 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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=== Okinawa Trough ===

The Okinawa Trough is the [[back-arc basin]] of the Ryukyu Arc and is formed by lithospheric extension of the continental Eurasian Plate.<ref name=":2" />. En echelon, extensional [[Graben|grabens]] with a width of about 10 km and a length of about 50-100 km 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.]]

=== 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">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|546x480px|M≥6.0 (1960-2022) Earthquakes in the Ryukyu Arc. Earthquake data from USGS-ANSS Catalog. Contours in 1000m interval.]]

=== Volcanoes ===

An active volcanic front lies 100km above the [[Wadati–Benioff zone|Wadati-Benioff zone]] (a planar zone of seismicity at the interface between the subducting and overriding plates) in Northern Ryukyu and gradually fades off in Central and Southern Ryukyu.<ref name=":4"></ref>.

== Geological units ==

===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>.

The Motobu Formation consists mainly of [[limestone]] interbedded with [[chert]] and [[phyllite]].<ref name=":11">Flint, D. E., Saplis, R. A., & Corwin, G. (1959). Military geology of Okinawa-jima, Ryukyu-retto. US Army Pacific Office Eng., 5, 88.</ref>. The limestone and chert of the formation contain fossils of [[foraminifera]] and [[Radiolaria|radiolarians]], respectively, suggesting a Permian age.<ref name=":9" /><ref name=":12">Konishi, K. (1963). Pre-Miocene basement complex of Okinawa, and the tectonic belts of the Ryukyu Islands. ''Sci. Rep. Kanazawa Univ., 8'', 569-602.</ref>.

The Yonamine Formation, with strata of interbedded phyllite, [[slate]], [[sandstone]], limestone, greenstone, and chert,<ref name=":11" />, underlies the Motobu Formation<ref name=":12" /> and contains Permian coral.<ref name=":13">Haikawa, T., & Ishibashi, T. (1981). Waagenophyllum (Waagenophyllum) okinawense, a new Permian coral from Okinawa-jima, Ryukyu Islands: Paleontological Study of the Ryukyu Islands-VII. ''Mem. Fac. Sci. Kyushu Univ., ser.D, Geology, 24''(3), 179-188</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">Nishimura, Y. (1998). Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan. ''Journal of metamorphic Geology, 16''(1), 129-140.</ref><ref name=":15">Nakagawa, H., Doi, N., Shirao, M., Araki,Y. (1982). Geology of Ishigaki-jima and Iriomote-jima Yaeyama Gunto, Ryukyu Islands. ''Contributions from the Institute of Geology and Paleontology, Tohoku University 84'', 1-22</ref>. The formation has an age of 220-190 Ma (Late [[Triassic]]-Early [[Jurassic]]) and comprises ultramafic rocks and high pressure/temperature (P/T) metamorphic rocks, i.e., metagabbro and mafic, siliceous, and pelitic schists.<ref name=":14"></ref><ref name=":16">Miyazaki, K., Ozaki, M., Saito, M., Toshimitsu, S. (2016). The Kyushu-Ryukyu Arc. ''The Geology of Japan'' (pp.139-174). Geological Society, London. </ref>.

===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.<ref name=":14" />. The metamorphic age of this formation ranges from 145-130 Ma (Early [[Cretaceous]]) and was determined by K-Ar phengitic micas dating.<ref name=":14" />. Radiolaria data show that the formation had accreted in the [[Toarcian]] in the Early Jurassic.<ref name=":6" />. This formation is part of an accretionary complex of a Middle Jurassic subduction zone.<ref name=":14" />.

On Ishigaki Island, this formation is thrusted beneath the Tomuru Formation along the Sokobaru thrust.<ref name=":14" />.

===Nakijin Formation===

The Nakijin Formation is a 450-500m thick deposit of limestone, basalt, and a minor amount of calcareous siltstone, tuffaceous and calcareous mudstone.<ref name=":10" />. Fossil records of [[Ammonoidea|ammonites]] and halobiids in this formation suggest a Late Triassic age.<ref name=":10" />.

The Nakijin Formation can be found in Central Ryukyu, i.e., [[Sesoko Island]] and the northwestern part of the Motobu peninsula of Okinawa Island.<ref name=":10" />. This formation overlies the Yonamine Formation along a reverse fault.<ref name=":10" />.

===Shimanto Group===

The Shimanto Group is a set of Early Cretaceous-earliest Miocene metamorphic rocks associated with the Shimanto belt.<ref name=":5" /><ref name=":24">Ujiie, K. (1997). Off‐scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc. ''Tectonics, 16''(2), 305-322. https://doi.org/10.1029/96TC03367</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 rocks metamorphosed up to greenschist facies, including flysch-type sandstone and slate with mafic greenstones.<ref name=":5" />.

Deformations such as NW-dipping, isoclinal, overturned folds and SE-verging thrust faults can be found in the Shimanto Supergroup of Central and Northern Ryukyu.<ref name=":5" />.

In Okinawa Island, Central Ryukyu, the Kayo Formation of the group features beds of turbidite containing mudstone, sandstone, and [[nummulite]] fossils that indicate middle Eocene age.<ref name=":21" />. Thrust-folding and metamorphism of the formation suggest a trench origin.<ref name=":21" /><ref name=":24" />.

===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 S-SW-dipping succession of conglomerate, sandstone, shale, and limestone that are 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 limestone, and [[Mollusca|molluscs]] were also found in the conglomerate.<ref name=":9" />. Foraminifera and calcareous algae 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 dykes, sills, and other small 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">Miki, M. (1995). Two‐phase opening model for the Okinawa Trough inferred from paleomagnetic study of the Ryukyu arc. ''Journal of Geophysical Research: Solid Earth, 100''(B5), 8169-8184. https://doi.org/10.1029/95JB00034</ref>. This data, coupled with 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===

The Yaeyama Group is a set of sedimentary rocks comprising sandstone interbedded with coal seams, mudstone, conglomerate, and limestone, found in the Yaeyama Island, Southern Ryukyu ,<ref name=":1" /><ref name=":5" />, and the age of the group is Early Miocene, as suggested by palaeontological data. 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===

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">LeRoy, L. W. (1964). Smaller foraminifera from the late Tertiary of southern Okinawa. ''US Geological Survey Professional Paper, 454'', 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 and 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 in the East China Sea, the Ryukyu Arc and its forearc, the group does not occur in the southern Okinawa Trough.<ref name=":3" />. 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">Furukawa, H. (1979). Quaternary geologic history of the Ryukyu Islands. Bulletin of the Science and Engineering Division, University of the Ryukyus (Mathematics and Natural Sciences), 27, 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 NW-SE faulting across the Ryukyu Arc resulted in an episode of dome-like uplifting of basement rocks in the Ryukyu Islands, named the ‘Uruma Movement’.<ref name=":20">Quaternary Reasech Group of Southwest Japan. (1968). Quaternary Crustal Movements in

Southwest Japan (in Japanese with English abstract). ''Mem. Geol. Sco. Japan, 2'', 15-24</ref>.

== Tectonics ==

[[File:Evolution of the Ryukyu Arc.png|thumb|513x480px|right|Evolution of the Ryukyu Arc.]]

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>.

=== 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 accretionary processes associated with the subduction in the Jurassic.<ref name=":23" />. Fossil and paleomagnetic evidence from the Shimanto Group suggests that subduction of a young oceanic plate took place 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 suggest that Southern Ryukyu rotated clockwise after 10 Ma.<ref name=":17" />. [[Reflection seismology|Seismic reflection]] surveys indicate that tectonic processes related to initial rifting like crustal doming, erosion, subsidence and sedimentation dominated the Okinawa Trough during the early Pleistocene<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==