【摘要】 <正> 东海冲绳海槽,位于琉球岛弧-海沟系的陆侧,为半深海弧后边缘地.近年来,国内外学者作了一系列实际调查和专门研究,极大地促进了对海槽地壳演化的认识.但在探讨海槽深部构造时,均未论证异常地幔的存在;在剖析槽底地壳性质时,认为已出现新生洋壳或认为属过渡型地壳,分歧存疑之处颇多.在板块大地构造框架中,普遍认为边缘盆地是弧后扩张(形成新洋壳)的产物,但在冲绳海槽,地壳厚度明显大于正常洋壳,拖网取样采获大量中酸性岩石,从而对海槽的扩张成因产生了种种疑问.本文将在前人工作基础上,试就上述问题展开讨论,进而确定冲绳海槽在洋盆演化阶段中的特殊位置.更多还原

【Abstract】 The Okinawa Trough is a marginal basin at the Continental side of the Ryu -Kyu Are. The free-air gravity anomalies have been calculated in terms of refrac-ted seismic data measured from above the Moho at the observation stations. However, the calculated values are much greater than the observed ones. The difference in gravity anomaly values, which must be conditioned by the nature of the raantle under the Moho, suggests that there is an anomalous raantle of low density beneath the Okinawa Trough.According to the calculation of the residual mantle gravity anomaly, it seems that the thickness of the lithosphere underneath the Okinawa Trough is roughly less than 50km and probably 30km thinner than that in the adjacent regions. Moreover, the very high heat-flow value in these areas also suggests an upwelling of hightemperatured and low dense mantle material and a thinning of lithosphere there.The crustal thickness of the Trough calculated by some researchers in terms of gravity anomalies is 21 to 24km or even more, much greater than the values derived from seismic refraction. The reason why these values calculated from gravity appear so high is probably due to the existence of an anomalous mantle beneath the Trough had not been counted. Now we have an experimental relation-ship which can properly express the depth of the Moho (HM) under the Trough in terms of Bouguer anomalies(AgB):HM = 25. 1-0.06 gB From the relationship above, we can obtain a crustal thickness of about 13 to 20km.From a big quantity of intermediate-acidic volcanic rocks dragged from the bottom, the gradual thickening of the sediments toward the axial zone and the relatively thick crust, we can see that there is not a typical oceanic crust at the Okinawa Trough as consequently hoped there should be one in seafloor spreading. Instead, the Trough is rather constituted by a thinned Continental crust, although there may be a mixed oceanic-continental crust in the deep water area of the southern part of the Trough. In general, the Okinawa Trough can be regarded as a product in the first stage of back-arc extension, the stage of thinning and taphrogenesis of the Continental crust, and its southern part is seemingly to be confronted by a turning point from the taphrogenesis stage to the spreading stage. Thus, the Okinawa Trough is a typical place where the continental crust is being destroyed and changed into the oceanic cru.Gravity data demonstrate that the Trough is in isostatic imbalance. Although the compensation of the upwelling asthonosphere has apparently reduced the excessive mass, but a perfect compensation has not reached yet, so the Trough still exhibits certain positive isostatic anomalies. With the strong crustal extension and isostatic adjustment, the Trough subsides drastically. Its extension rate is about 0.4 to 1cm/yr, which can be rivaled by its subsidence rate. Compared with the East African Rift, the Trough is with a thicker crust, an higher heat-flow value, a more rapid spreading rate and deeper water. In comparison with Red Sea, the Trough with its relatively thicker crust, slower spreading rate, smaller dimension and shallower depth, has not presented a typical oceanic crust yet.In the process of oceanic basin evolution——Wilson Cycle, the Okinawa Troughis now situated in the midway of development from the embryonic stage(as in the East African Rift) to the young stage (as in Red Sea), i. e., in the so called born stage, and serves as a very rare example in the world. It can be concluded here that a further study of the nature and evolution of the Okinawa Trough has an important significance in understanding the origination of marginal basins and in the reconstruction of the crust.更多还原