冲绳海槽轴线地质特征

GEOLOGIC FEATURES OF THE OKINAWA TROUGH AXIS

冲绳海槽位于东海大陆架与琉球岛弧之间 ,海槽中发育着一系列平行轴向延伸的地堑型正断层。穿越海槽的地震剖面证实了海槽扩张中心的存在 ,扩张轴沿海槽轴线方向延伸。由于地堑构造中心的扩张速率较大 ,由其引起的海槽中心下陷的速率大于沉积作用的补偿速率 ,使海槽中心海底出现明显的下陷。在轴线的大陆坡一侧 ,陆源沉积占绝对优势 ;轴线的岛坡一侧 ,以火山玻屑、浮岩。

The Okinawa Trough is located between the East China Sea continental shelf and the Ryukyu Island Arc. It has the following geologic features. (1) The formation and development of the Trough is controlled by tectonic spreading, which has been obviously revealed in many seismic sections. Now the grabens in the middle of the Trough are still an actively spreading axis, and in the either side of the graben axis are developing a series of symmetric imbricate tensional fractures, some of which have penetrated the seafloor. Since the graben tectonic center has a comparatively high spreading rate and it, in turn, causes the Trough center to depress at a higher rate than the compensation rate of sedimentation, the Trough central seafloor shows an evident subsidence. (2) The graben spreading has controlled the development of topographic and geomorphic features in the Okinawa Trough. And the subsidence at the Trough bottom gives a topographic form called as “trough in trough”. This well developed graben trough form makes the tectonically spreading center visually shown from the angle of topography and geomorphology. All the graben troughs lie in the central area of the Trough and extend along the axis for tens or hundreds of kilometers. The axis goes through the Trough from south to north and geologically becomes the central axis. (3) The graben trough axis has obvious sedimentary characteristics. The continental slope side is dominated by continental sediments, while the island arc side mainly consists of volcanic glass fragments, pumice and foraminifer ooze.