[J08-P] Poster

[J08-P-16] Inhomogeneous rifting structure in the northern Okinawa Trough, an active backarc basin southwest of the Japan Islands

^○Chiaki Okada¹, Azusa Nishizawa¹, Kentaro Kaneda¹, Mitsuhiro Oikawa¹, Daishi Horiuchi¹, Yukari Fujioka¹, Kosaku Arai² (1.Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo, Japan, 2.Research Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan)

The Ryukyu (Nansei-Shoto) island arc–trench system is located at a convergent plate margin where the Philippine Sea (PHS) plate is subducting under the Eurasian plate. The Okinawa Trough is a backarc basin of the system extending from north to south and the stage of its ongoing rifting process varies along the trough. The crustal extension in the northern trough is not progressing so much compared with the southern trough. The position of the trough axis is not determined clearly in the northern trough, though it corresponds to several en echelon narrow rifts in the southern trough. The seafloor topography in the northern trough shows different features between the eastern and the western parts. Because the volcanic front related to the PHS plate subduction exists in the eastern part, there are many seamounts and knolls in this part. In contrast the western part is rather flat seafloor.
We have conducted multi-channel seismic (MCS) reflection surveys to acquire detailed images of deformation in shallow structures reflecting variable tectonic backgrounds of the Okinawa Trough since 2008. We fired a trigun cluster with a total volume of 1,050 cubic inches at a 50 m shot interval and used 240-channel (3,000 m long) seismic streamer. The MCS data were processed by a standard procedure including band-pass filter, deconvolution, normal move out correction and CMP stacking.
The MCS results reveal a number of normal faults and intrusions beneath all the seismic lines inside the northern trough. In the western part of the trough with a flat seafloor, there are thick sedimentary layers with over 3,000 m thick at most. However we detected several intrusions below the seafloor and some of them penetrate into the uppermost sediments. On the contrary, the eastern part shows the rough seafloor and the existence of many volcanic features. The MCS profiles reveal many igneous intrusions reaching above the seafloor, which corresponds to the volcanic front in this region.