Ryukyu Arc
[2][5][7] The geological units of the arc include igneous, sedimentary, and metamorphic rocks, ranging from the Paleozoic to Cenozoic in age.
[2] The Okinawa Trough is the back-arc basin of the Ryukyu Arc, and has been formed by lithospheric extension of the continental Eurasian Plate.
[12] Low seismic velocity zones, which are possibly associated with the upwelling of magma, have been discovered beneath active volcanoes and the Okinawa Trough.
[11] The metamorphic age of this formation ranges from 145 to 130 Ma (Early Cretaceous) and was determined by K-Ar phengitic micas dating.
[7][20] 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.
[2] The group comprises sedimentary and metasedimentary rocks metamorphosed up to greenschist facies, including flysch-type sandstone and slate with mafic greenstones.
[7] Deformations such as northwest-dipping, isoclinal, overturned folds and SE-verging thrust faults can be found in the Shimanto Group of Central and Northern Ryukyu.
[21] The Nago Formation comprises pelitic and mafic schist, phyllite, and slate, with minor chert and limestone,[21] and crops out most of northern Okinawa Island.
[8] A large variety of marine fossils—e.g., calcareous algae, foraminifera, corals, echinoderms, bryozoans, and gastropods— have been preserved in the limestones, and molluscs were also found in the conglomerate.
[8] The Nosoko Formation is a 300m thick sequence of tuff, volcanic sandstone and breccia, and lavas with dykes, sills, and other small intrusions.
[23] 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.
[18] Coal seams, cross laminae, and trace fossils reveal that the group is possibly derived from sediments in a continental shelf.
[2] 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.
[29] Fossil and paleomagnetic evidence from the Shimanto Group suggests that subduction of a young oceanic plate occurred in the Late Cretaceous (about 70 Ma) to the Paleogene.
[22] 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.
[23] 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 subsidence.
