Ryukyu Arc: Difference between revisions

Revision as of 07:47, 18 November 2022

Revision as of 09:24, 18 November 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 1000m interval. Figure made with GeoMapApp (www.geomapapp.org) / CC BY / CC BY (Ryan et al., 2009).]]

The '''Ryukyu Arc''' is an [[island arc]] which extends from the south of [[Kyushu]] along the [[Ryukyu Islands|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|the 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|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|the Osumi 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" />.

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=== The 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. From user: MagentaGreen]]

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===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 [[toarcian|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" />.

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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===

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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 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 in ~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==