Ryukyu Arc
The Ryukyu Arc
The Ryukyu Arc is an island arc, an arcuate chain of islands, which extends from the south of Kyushu along the Ryukyu Islands to the northeast of Taiwan, spanning ~1200 km[1][2][3]. It is located along a section of the convergent plate boundary where the Philippine Sea Plate is subducting northwestward beneath the Eurasian Plate along the Ryukyu Trench[2][3][4]. The arc – parallel to the Ryukyu Trench, an active volcanic belt, and the Okinawa Trough – has an overall NE-SW trend and is located northwest of the Pacific Ocean and southeast of the East China Sea [5][6]. The Ryukyu Arc, based on its geomorphology, can be divided into three parts: Northern Ryukyu, Central Ryukyu, and Southern Ryukyu; the Tokara Strait separates Northern Ryukyu and Central Ryukyu at ~130˚E while the Kerama Gap separates Central Ryukyu and Southern Ryukyu at ~127 ˚E[1][4][5]. The geological units of the arc include igneous, sedimentary, and metamorphic rocks, and their age ranges from the Paleozoic to Pleistocene.
Tectonics
The Ryukyu Arc is developed by the northwestward subduction of the Philippine Sea Plate under the Eurasian Plate at a velocity of 5-7 cm/year[7]. GPS data show that southern Kyushu and the Ryukyu Arc migrate southeastward (toward the Ryukyu Trench) unlike other arcs at the Pacific side of Japan[8].
Geologic Setting
The northernmost and southernmost parts of the Ryukyu Arc terminate in Kyushu and Taiwan, respectively[9]. A number of studies defined the extent of the Ryukyu Arc geographically and morphologically into three parts: Northern Ryukyu, which includes the Osumi Islands; Central Ryukyu, which includes Amami Islands and Okinawa Islands; Southern Ryukyu, which includes Miyako Islands and Yaeyama Islands[1][5]. It is possible that the geological history of Southern Ryukyu might differ from those of Northern and Central Ryukyu before Middle Miocene[1]. The arc might be united into the present configuration in the Pliocene-Pleistocene due to different rates of southeastward shifts[1].
| Arc Segment | Geographical Location |
|---|---|
| Northern Ryukyu | Osumi Islands |
| Tokara Channel | |
| Central Ryukyu | Anami Islands, Okinawa Islands |
| Kerama Gap | |
| Southern Ryukyu | Miyako Island, Yaeyama Islands |
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. The Motobu Formation consists mainly of limestone interbedded with chert and phyllite. The limestone and chert of the formation contain foraminifera and radiolarian, respectively, suggesting a Permian age. The Yonamine Formation, with strata of interbedded phyllite, slate, sandstone, limestone, greenstone, and chert, underlies the Motobu Formation and contains Permian coral.
Tomuru Formation
The Tomuru Formation is distributed in the Yaeyama Islands, a group of islands in Southern Ryukyu. 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.
Fusaki Formation
The Fusaki Formation, located at the Yaeyama Islands in Southern Ryukyu, comprises weakly metamorphosed olistromal rocks: allochthonous blocks of chert, mudstone, sandstone, and limestone are embedded in a muddy matrix. The metamorphic age of this formation ranges from 145-130 Ma (Early Cretaceous) and was determined by K-Ar phengitic micas dating. Radiolaria data show that the formation had accreted in the Early Jurassic. This formation might be part of an accretionary complex of a Middle Jurassic subduction zone. On Ishigaki Island, one of the Yaeyama Islands, this formation is thrusted beneath the Tomuru Formation along the Sokobaru thrust.
Nakajin Formation
The Nakijin Formation is a 450-500m thick deposit of limestone, basalt, and a minor amount of calcareous siltstone, tuffaceous and calcareous mudstone. Fossil records of ammonoids and halobiids in this formation suggest a Late Triassic age. The Nakijin Formation can be found in Central Ryukyu, i.e., Sesoko Island and the northwestern part of the Motobu peninsula of Okinawa Island. This formation contacts and overlies the Yonamine Formation at a reverse fault.
Shimanto Supegroup
The Shimanto Supergroup is a set of Upper Cretaceous-Eocene metamorphic rocks associated with the Shimanto belt. 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. The supergroup comprises sedimentary and metasedimentary rocks metamorphosed up to greenschist facies, including flysch-type sandstone and slate with mafic greenstones. 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.
Miyara and Nosoko Formations
The Eocene Miyara and Nosoko Formations crop out the Yaeyama Islands, Southern Ryukyu. The Miyara Formation is a S-SW-dipping succession of conglomerate, sandstone, shale, and limestone that are deposited along the coast of Ishigaki Island. A large variety of marine fossils, e.g., calcareous algae, foraminifera, corals, echinoderms, bryozoans, and gastropods, have been preserved in the limestone, and molluscs were also found in the conglomerate. Foraminifera and calcareous algae suggest a late Eocene age. The Nosoko Formation is a 300m thick sequence of tuff, volcanic sandstone and breccia, and lavas with dykes, sills, and other small intrusions. This formation is widely exposed at the Nosoko peninsula in northern Ishigaki Island. Paleomagnetic data indicate that this formation might have rotated clockwise 40˚ after its deposition roughly in the Oligocene.
Yaeyama Group
The Yaeyama Group, situated in the Yaeyama Islands of Southern Ryukyu, is a set of sedimentary rocks which contain sandstone intercalated with coal seams, mudstone, conglomerate, and limestone. Coal seams, cross laminae, and trace fossils reveal that the group is possibly derived from littoral (intertidal) sediments. The age of the group is Early Miocene, as indicated by palaeontological data. Despite being structurally slightly tilted and faulted, the group is less deformed than the Eocene formations of Southern Ryukyu. This means that the environment of Southern Ryukyu has been stable since the Early Miocene.
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. The Shimajiri Group is the first geological unit to be deposited across Northern, Central, and Southern Ryukyu. This implies that Northern and Central Ryukyu, and Southern Ryukyu had different basins and tectonic settings prior to the Late Miocene (the age of deposition of the group).
Ryukyu Group
The Ryukyu Group is defined as Pleistocene deposits formed after the deposition of the Shimajiri Group but before that of Holocene sediments. It is distributed in Central and Southern Ryukyu and is marked by a distinct unconformity above the Shimajiri Group. The group comprises predominantly the Ryukyu Limestone and Terrace Deposits of sand and gravel. 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). 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’.
References
- ^ a b c d e 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
- ^ a b 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.
- ^ a b 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.
- ^ a b 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.
- ^ a b c Kizaki, K. (1986). Geology and tectonics of the Ryukyu Islands. Tectonophysics, 125(1-3), 193-207.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ Foster, H. L. (1965). Geology of Ishigaki-shima, Ryukyu-retto. US Geol. Surv., Prof. Paper. https://pubs.usgs.gov/pp/0399a/report.pdf