Ryukyu Arc: Difference between revisions

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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">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 [[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">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 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 fault|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 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=Kohtaro |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}}</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]] 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">Ujiie, K. (2002). Evolution and kinematics of an ancient décollement zone, mélange in the Shimanto accretionary complex of Okinawa Island, Ryukyu Arc. ''Journal of Structural Geology, 24''(5), 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 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 rocks 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 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 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===

The Yaeyama Group is a set of sedimentary rocks comprising sandstones interbedded with coal seams, mudstones, conglomerates, and limestones, 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[[Paleontology|paleontological]] 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, 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">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–SEnorthwest–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">Quaternary Research Group of Southwest Japan. (1968). Quaternary Crustal Movements in

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