New topographic and geophysical maps in the southwesternmost Ryukyu area give the key to understanding the role of the Ryukyu trench-arc-backarc system towards the Taiwan orogeny

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 against Taiwan from the northeastern side. We first compiled all the available multibeam bathymetric survey data in this area including Ryukyu trench, island-arc zone and Okinawa Trough. The data were collected since 1990’s up to now mainly by JAMSTEC fleets. The new topographic data obtained in 2009 by EM-122 swath bathymetric echo sounder on board R/V Marcus G. Langseth during the TAIGER Leg2 cruise were also added. 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. 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 feature 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 also surveyed by multibeam echo sounder and ROV reconnaissance in 2005 and 2008. The survey revealed that the fault consists of five segments bounded by kinking of fault strike. The main scarp of the southern and central segments is associated with a trough. However, topographic relief is very poor in the northern segments. The result suggests an episodic northward 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.