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LI Nai-sheng; JMNG Li-li; LI Chang-zhen(Institute of Oceanology The Chinese Academy of Sciences, Qingdao, 266071)  
Abstrsct The Okinawa Trough which lies at the southastem margin of the East China Sea,is an astive back-are rift and a part of the Ryukyu T-A-BA system. Many geological and geophysical studies have shown that the Okinawa Trough is associated with an extremely high heat flow,strong submarine hydrothermal activities,frequent earthquaks,strong volcanic eruptions, well-developed central grabens and active faults,and a thin crust and lithosphere. Up to now, the crustal nature of the Trough is still a key debating question.Based on the new geological and geophysical data collected since 1990,includ ing, single and multiple channel seismic reflections, ocean bottom seismometer (OBS) refractions, gravity and geomagnetic measurements,water depth sounding,submarine sampling and sea bottom dragging,the authors have undraken analyses of the crustal structure of the Okinawa Trough. The conclusions obtained in this study are listed below.1. Except the Andaman Sea with a complex oblique opening,the Okinawa Trough is the ouly example in the world of an active marginal back-arc rift opening along a continental boundary; it is formed by continental crustal extension,at a special high evoluting stage of a back-are rift,near the end of a rifting period and the beginning of a spread. Hence the Okinawa Trough is tectonicly a modern active rift valley.2. Analyses of igneous rock distribution patterns,sediment thickness and crustal structures,show that there is no oceanic crust in the central graben of the Trough and the Okinawa Trough has not begun to "spread" till now. The crust of the Trough is still continental in character because of the continental extension.3. Based on an seismic profile interpretation,the K-Ar dating for igneous rock sampes and heat flow calculations,as a submarine "Trough",the Okinawa Trough was formed 6 Ma B. P.; however,as a back-arc rift 4. The Okinawa Trough is a typical astive zone in the marginal area of the westem Pacific. The study on its crust nature and tectonic astivity is of great significance for many key questions about continental marginal tectonics such as back-arc rifting processes,influence from the subducted slab,mechanisms of the back-arc hydrothermal ctivity,mantle-crust interaction in the astive back-arc basin,relationships between the volcanic activity and back-arc rifting, differences in crustal fluid ectivities between mid-ocean ridges and back-arc basins,the tectonic evolution and geodynamics of active back are rifts. Therefore, it is required to carry out an ODP driling more accurate and more detailed geological and geophysical investigations for the Okinawa Trough.
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