Ryukyu Arc: Difference between revisions

Revision as of 18:52, 2 December 2022

Revision as of 09:15, 3 December 2022

Line 64:

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 rocks metamorphosed up to greenschist facies, including [[flysch]]-type sandstone and slate with mafic greenstones.<ref name=":5" />

Deformations such as NWnorthwest-dipping, isoclinal, overturned 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 Groupaccretionary 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 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 S–SWsouth-southwest-dipping succession of 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 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 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" />