Role of the Ryukyu Arc towards the Taiwan orogeny

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The southwestern Ryukyu area is the key to understanding the role of the arcuate-shaped trench-arc-backarc system towards the orogeny of Taiwan ; with the viewpoint of the northwestern termination of subduction of the Philippine Sea Plate against the Eurasian Plate and the consequent collision of the Luzon Arc against Taiwan from the northeastern side. We joined the TAIGER programme and collected the new topographic data in this area by EM-122 swath bathymetric echo sounder on board R/V Marcus G.Langseth during the TAIGER-2009 Leg2 cruise. We also compiled all the available multibeam bathymetric survey data in this area from JAMSTEC fleet cruises and NOAA/NGDC multibeam database.The Ryukyu Trench area south of Miyako and Yaeyama Islands is classified into zonal topographic domains. The northernmost zone is characterised by the distribution of well-developed submarine canyons along the southern coasts. The next is by amphitheatres, slump sediment blocks and their large-scale subsidence. … More The southern zone is by quite complicated topography, such as several escarpments, ridges and troughs and an E-W trending half graben. The southernmost zone is the trench axis area with depth of about 6500. 6600 m, which is characterised by a flat plane. The width of the axial plain is about 40 km of Yaeyama Islands which shows maximum and decreases towards both east and west. However, the topographic features of the trench is unclear on the western side of the Gagua Ridge collision(123°E).One of the across-arc normal faults located off the eastern coast of Ishigaki Island was already studied by multibeam sounding and ROV reconnaissance. Precise analyses of the data revealed an episodic northwestward propagation of the fault, probably due to the arc-parallel extension caused by the rifting of the Okinawa Trough.The observed topographic features suggest that the roll back of the subducting Phippine Sea Plate and the southward retreat of the trench axis is taking place on the eastern side of 123°E due to the oblique subduction, which corresponds to the extension of the western Okinawa Trough. However, on the western side of 123°E, it is unclear whether subduction of Huatung Basin is taking place. It may be possible that Huatung Basin couples with the fore-arc and back-arc areas and that these three areas altogether collides against the Taiwan block. Clear Benioff zone is observed below the fore-arc area on the north of Huatung Basin. However, this might be derived from the West Philippine Basin subducting northwestward obliquely against the Eurasia Plate. Less