摘要
冲绳海槽属西太平洋沟弧盆系,其成因各说不一.根据海槽地壳厚度13—20km,为减薄的大陆壳,具有正的空间异常值,中新世至第四纪沉积由两侧向轴部增厚,海底拖网取样见大量中酸性火山岩,活断层和轴部地堑发育,中浅源地震活跃,有理由相信海槽是在陆壳上发生发展而成,是张应力控制下的断陷带,属裂谷型边缘海盆地,现在仍然处于大陆裂谷的裂陷阶段,构造作用与岩浆活动十分强烈.没有明显的、大规模的扩张,新生洋壳尚未形成.
Lying between East China Sea and Ryukyu Arcs,, Okinawa Trough is in NE-SE strike and curved southeastwards. Tectonically, it is a tensional rift valley. The Okinawa Trough has experienced mantle upwelling, crustal uplift. fault and depression.
1.The Okinawa Trough is floored by thin Continental crust, indicating deep mechanism of mantle upwelling. Continental crust at 25 21'N and 123 54'E is determined by seismic refraction analysis to be l3.6km thick, and based on inversion of gravity, the thickniss of the crust throughout the Okinawa Trough ranges from 14 to 20km.
2.The Okinawa Trough is a tensional rift valley in which normal faults and grabens widely develop. It is bordered by Tokara Ridge on the east and by the East China Sea slope on the west respectively. 7-25km wide grabens, in an en echelon pattern, have developed in the axial part of the central and southern Okinawa Trough. It is formed by post-Miocsne subsidence.
3.In the Okinawa Trough, there are frequent volvanic activities, mainly made up of intermediate and acid eruptions. pumice rocks are most widely distributed in the Okinawa Trough. The dredged samples on the seamount at site 398 are rhyolite. There are dacite, andesite, and basalt distributing not far south of the site. Locally found basalts in the Okinawa Trough cannot be considered as the new oceanic crust formed by sea-floor spreading.
4.The Okinawa Trough belongs to an intermediate to shallow earthquake zone with high heatflow value. Since the recent few decades, 20 earthquakes of more than 5 magnitudes have been recorded in . the Okinawa Trough. 90% of the intermediate earthquakes in the Ryukyu Are area are concentrated on the trough. The Okinawa Trough is one of the Pacific basins with very high heat-flow values. lts highest value is up to 30 HFU.
5. The development of the Okinawa Trough is controlled by the subduction of the Philippine Sea plate and the slow seaward extension of the continent, as well as by vertically d'ifferential uplifts and subsidences. During late Miocene-early Pliocene, it underwent mantle upwelling and crustal tensionally thinned, so faults were formed. In late Pliocene, faults gave way to depression-Since Quaternary, Volcanism and earthquakes have been active, and still active now.
出处
《海洋地质与第四纪地质》
CAS
CSCD
北大核心
1989年第1期43-51,共9页
Marine Geology & Quaternary Geology
