琉球海沟的构造和运动特征

琉球海沟是欧亚板块与菲律宾海板块之间的边界，海沟西坡是大陆性质的琉球岛弧，东坡是大洋性质的菲律宾海地壳．海底地震反射探测和地震震源定位表明，菲律宾海板块沿海沟向琉球岛弧下俯冲，俯冲角度与深度沿海沟走向变化．有证据显示，由于俯冲板前缘的横向移动，海沟和岛弧正朝大洋方向后退，弧后盆地-冲绳海槽发生拉张变形．最近一次的海沟后退与冲绳海槽扩张可能是从上新世末(2Ma前)开始的，岛弧的后退移动和弧后拉张在南部与海沟走向垂直，在中部和北部与海沟走向斜交，总体上向南的运动分量占优势、与海沟后退相关的弧后拉张集中在冲绳海槽，没有证据表明对其西侧的中国东海陆架盆地产生影响、海沟后退的原因可能与俯冲板的动力不平衡以及它与周围地幔的相互作用有关．

Tectonics and motion of the Ryukyu trench

This review summarizes recent progress in studies on the tectonics and motion of Ryukyu trench. This trench, lying between Kyushu of Japan and Taiwan, is the boundary between the Eurasia plate and the Philippine Sea plate. Along this trench the Philippine Sea plate is subducting under the Ryukyu island arc, as evidenced by the seismic reflection surveys, earthquake source locating results, and fault plane solutions of earthquakes. There have been some indicators including GPS measurements showing that the Ryukyu island arc as well as the Ryukyu trench is retreating toward the ocean. This motion is associated with the backarc spreading in the Okinawa trough. Both the retreating motion and backarc spreading in the last phase very likely began since the late Pliocene time (2Ma ago). The extensional deformation of the backarc basin is perpendicular to the strike of the trough in its southern part and becomes more and more oblique when going northeastward. The southward components of motion and deformation are dominant, in particular in the south part of the island arc and the trough. Beyond the Okinawa trough going to the East Sea of China, the recent deformation seems little. The trench treating or subduction rollback, in relation to backarc basins , is still an enigmatic feature of converging plate boundaries, although many models have been proposed to explain the mechanism of it. It is popularly accepted that the dynamic nonbalance of the subducting slab and interaction between the slab and neighboring mantle are the important factors for the lateral migration of a subduction slab.