| 摘要: |
| 根据反射地震、折射地震和磁力等资料,结合周边地质,探讨冲绳海槽南段基底组成。在海槽周边的东海陆架盆地、台湾褶皱带和琉球岛弧褶皱带,发育不同程度变质的晚古生代、中生代和早第三纪地层。多道反射地震表明,海槽南段沉积盖层由上第三系和第四系组成,声学基底由下第三系及更老地层构成。邻近海槽南段的折射地震揭示,除第四系中新统速度层之外,还存在纵波速度分别为4.7—5.3km/s和6.3km/s的下第三系和中生界速度层。磁力异常分析和正反演拟合计算结果表明,海槽磁性基底主要由变质岩系构成,次为燕山期中酸性岩浆岩和喜山期中基性岩浆岩,磁性基底大部分相当声学基底。综合分析表明,海槽南段基底主要由不同程度变质的下第三系、中生界和上古生界构成;在海槽某些构造部位,已有喜山期基性岩浆岩形成。 |
| 关键词: 冲绳海槽 基底组成 磁性基底 |
| DOI： |
| 分类号: |
| 基金项目:国家重点基础研究发展规划项目资助,G2000046702号 |
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| ANALYSES ON THE BASEMENT CONSTITUTION OF THE SOUTHERN OKINAWA TROUGH |
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LIU Jian-Hua,LIN Chang-Song,GAO Jin-Yao,FANG Yin-Xia,WU Shui-Gen
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Second Institute of Oceanography, Key Laboratory of Submarine Geosciences, State Oceanic Administration, Hangzhou,310012
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| Abstract: |
| The paper deals with the issues of basement constitution of the southern part of the Okinawa Trough, which has close links with scientific issues regarding the origin and evolution of the trough.
The analyses are based on the data of geology collected in adjacent areas and seismic reflect ion, seismic refraction and magnetism obtained in the trough. The seismic survey uses LRS-16A digital seismic system made by Western Geophysics Company as recorder, LRS-16 Kiloseis digital streamer as receiver, and compressor and Stagaray air gun as energy source. Marine magnetometer Model G801 produced by EG & G Company of USA is used for the magnetism survey. The main instrument for bathymetric survey is echo sounder Model 3200 made by ODOM Company of USA.
The research area is located in the southern Okinawa Trough to the southwest of Miyako depression, between 24°–27°N and 122°–127°E, also the deepest water in the trough.
The southern Okinawa Trough is bordered on the shelf basin of the East China Sea (ECS), the Taiwan folded belt and the Ryukyu Islands folded belt to the northwest, west and southeast respectively. The oldest stratum found in the adjacent areas of the southern trough so far is the Proterozoic metamorphic rocks encountered at the Lingfeng No. 1 well of the southern part of the shelf basin of ECS. The Upper Paleozoic strata outcrop in the eastern flank of the Central Mountains of Taiwan folded belt and the Ryukyu Islands. The seismic reflection sequence supposed the Upper Paleozoic exists in the eastern depression of the shelf basin of ECS. The Mesozoic strata are extensively developed belt in the shelf basin of ECS, western part of Taiwan folded belt and the Ryukyu Islands.
Interpretation of multichannel seismic reflection sequences and interval velocities indicate that four seismic reflection sequences can be separated. The sequence A, B and C are belonging to the overlying strata and are composed of Neogene to Quaternary deposits. Sequence D is an acoustic basement and consists of Eogene and even older strata. Seismic refraction profiles show that three Eogene andMesozoic velocity layers with longitudinal velocities of 4.7–5.3 km/s and 6.3 km/s respectively beneath the Quaternary-Miocene Layers existed.
Qualitative analysis and forward and reversed calculation of magnetic data show that the southern trough is short of typical magnetic lineations resulted from basaltic basement of oceanic crust. The magnetic basement is mainly composed of metamorphic rocks. The Yanshanian (a tectonic phase between Jurassic to Cretaceous in Chinese literature) intermediate-acid magmatic rocks and the Himalayan intermediate-basic magmatic rocks are the next components. The former is concentrated in the northwestern and northeastern margins, and the latter is scattered in the trough, especially in the western and southeastern margins. Both magnetic basement and acoustic basement are mostly well matched in the buried depth.
Comprehensive analyses demonstrated following conclusions. (1) The basement of the southern Okinawa Trough is mainly composed of Eocene, Mesozoic and Upper Paleozoic groups metamorphosed in varied degrees. (2) In geological history, magmatism was very strong in the trough. The intermediate-acid magmatic rocks were mainly formed in Yanshanian phase, and the intermediate-basic magmatic rocks are in Himalayan (a tectonic phase between the Latest Cretaceous to Miocene in Chinese literature) epoch. (3) The southern trough is short of typical magnetic lineations resulted from the basaltic basement of the oceanic crust, but the basic magmatic rocks of Himalayan epoch have formed in some parts of the trough. |
| Key words: Okinawa Trough , Basement institution, Magnetic basement |
