ALBERT

Notes: In 1984 an extensive geophysical investigation was conducted in the northernmost part of the Ryukyu Island Arc, south of Kyushu Island. Japan. The aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements (seismic profiling, gravity, magnetism, sea beam and heat flow).This paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines. One line, 190 km long, was taken behind the northernmost part of the Ryukyu Island Arc, parallel to the major tectonic units, i.e. the Okinawa Trough, Ryukyu Arc and Ryukyu Trench. The other, 295 km long, was perpendicular to the tectonic units. Along these profiles, we deployed ocean bottom seismograms spaced 10-20 km apart. As a controlled source, we used both explosives and an airgun array. The experiment was successful and provided quite important information on the tectonics of a trench-island arc-back arc system.The velocity structure obtained for the profile behind the Ryukyu Islands has continental properties. The sediment thickness is 3-4 km. although it shows lateral variations due to basement undulation. The velocities of the upper and lower crust are 5.8-6.2 and 6.6-6.8 km s-1, respectively. The total crustal thickness decreases southwestward, from 27-30 to 23-24 km. This is direct evidence for crustal thinning associated with the process of back-arc spreading.The velocity structure along the profile from the trench to the island arc clearly shows the subduction, accretion and deformation at this margin. The crust beneath the trench has oceanic properties. The sediment thickness is 1.3-3.0 km, beneath which the igneous basement shows severe undulation. The total crustal thickness is 7-8 km. The Pn velocity is slightly less than 8.0 km s-1. Our seismic data revealed a huge sedimentary wedge located 50-150 km landward of the trench. The maximum thickness of this wedge exceeds 12 km. The origin of this wedge may be oceanic because the P-wave velocity in its eastern half is almost comparable to that obtained for the trench area. The velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc.