南海南部礼乐滩西洋陆过渡带的伸展减薄特征与机制研究

The crustal structure of the continent-ocean transition (COT) in the southern South China Sea (SCS) rifting margin preserved a wealth of information on the processes of the southwestern sub-basin (SWB) extension and breakup. It is a significant region for studying the evolution of the SWB, the tectonic of rifted basins and petroleum exploration. Comparing with the northern SCS margin, the tectonic attribute, the stretch mechanism and the rifting type of margin in the southern SCS are poorly known. This study utilizes a wide-angle seismic line OBS2017-S1, synchronized multi-channel reflection profile, and regional gravity anomaly data to acquire high resolution crustal structure. Based on the regional geological and geophysical data, we will focus on (1) the structural differences from onshore to offshore; (2) the structural feature of the large-scale detachment faults and work on the Moho, as well we their controls on the lithospheric thinning; (3) the spatial variation of crustal structure, which will provide some scientific basis for early-stage extensional mechanism of continental crust and hydrocarbon resources exploitation and utilization.

南海南部张裂陆缘洋陆过渡带记录了海盆张裂、破裂到海底扩张过程的重要信息，是揭开西南次海盆形成演化机制、开展陆缘裂陷盆地构造和油气成藏研究的关键区域。相对于北部陆缘，南部陆缘的地壳结构特征和伸展减薄机制等问题尚不清楚。本项目拟利用穿越南海南部陆缘礼乐滩西洋陆转换带的广角折射地震和多道反射地震探测剖面OBS2017-S1，辅以高分辨率区域重力异常数据，获取南海南部礼乐滩西侧从陆向海的高分辨率二维地壳速度结构。综合利用该区域地质与地球物理资料，重点解决（1）南部陆缘礼乐滩西侧洋陆过渡带地壳结构特征；（2）大型拆离断层的构造形态、对莫霍面的拆离作用以及在地壳伸展减薄中的控制作用；（3）南海南部陆缘地壳结构空间变化特征。为海盆早期的张破裂机制和油气资源的开发利用提供科学依据。

南海南部张裂陆缘洋陆过渡带记录了海盆张裂、破裂到海底扩张过程的重要信息，是揭开西南次海盆形成演化机制、开展陆缘裂陷盆地构造和油气成藏研究的关键区域。相对于北部陆缘，南部陆缘的地壳结构特征和伸展减薄机制等问题尚不清楚。本本项目利用广角反射地震和多道反射地震数据相结合的方法对南海南部礼乐滩西洋陆过度带地壳结构和伸展构造进行了详细研究，研究结果得出：（1）较厚的同张裂期地层和明显的莫霍面抬升表明礼乐西海槽南部发生了较大规模的伸展和减薄；（2）在陆坡与海盆交界处，地壳减薄受一组高角度正断层控制，未发现拆离断层，不具有明显超伸展构造特征，我们认为与岩石圈快速破裂有关；（3）洋陆过渡区地壳内发育一系列高角度正断层，造成壳内低速异常，并作为流体通道，使得海水通过断裂进入地幔，地幔发生蛇纹石化现象。