| The Okinawa Trough which lies at the southeastern margin of the East China Sea, is an active back-arc 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 earthquakes, 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, including, 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 undertaken analyses of the crustal structure of the Okinawa Trough. The conclusions obtained in this study are listed below.
l. Except the Andaman Sea with a complex oblique opening, the Okinawa Trough is the only 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 evolution stage of a back-arc rift, near the end of a rifting period and the beginning of a spread. Hence the Okinawa Trough is tectonically 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 samples and heat flow calculations, as a submarine “Trough", the Okinawa Trough was formed 6 Ma B. P.; however, as a back-arc rift valley, It has become strongly active since 2 Ma B. P. The central rifting graben of the Trough has been f0I‘lTl€d since 2 Ma B. P. and is well developed. The age of igneous rock obtained from the central graben is younger than l Ma. Therefore, the Okinawa Trough is a young and active back-arc rift.
4. The Okinawa Trough is a typical active zone in the marginal area of the western Pacific. The study on its crust nature and tectonic activity 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 activity, mantle-crust interaction in the active back-arc basin, relationships between the volcanic activity and back-arc rifting, differences in crustal fluid activities between mid-ocean ridges and back-arc basins, the tectonic evolution and geodynamics of active back-arc rifts. Therefore, it is required to carry out an ODP drilling more accurate and more detailed geological and geophysical investigations for the Okinawa Trough. |
