南海万安盆地构造—层序发育特征与构造—沉积充填演化

Characteristics of Structural⁃Sequence and Evolution of Tectonic and Sedimentary of Wan’an Basin in South China Sea

万安盆地是南海西南部重要的沉积盆地之一,深入分析其构造‒沉积充填特征对于认识南海南部主要构造事件及其沉积响应具有重要的科学意义.利用覆盖全盆地的二维地震资料,结合国内外的研究成果,对万安盆地构造‒层序特征及其构造‒沉积充填演化进行分析.研究表明,万安盆地内新生代以来可识别出8个主要的二级/三级层序界面.沉降模拟显示,盆地沉降整体表现出一个“快‒慢‒快”的过程,且整体呈现出东高西低,中高南低的特征.综合构造层序特征和沉降模拟结果,万安盆地新生代以来沉积演化可分为5个阶段:初始裂陷期、晚期裂陷期、断坳转换期、裂后热沉降期和裂后加速热沉降期.盆地自形成以来,沉降主要受东亚大陆边缘区域拉张所造成的深部断裂的影响,至上新世,万安断裂转而成为盆地沉降的主要影响因素,并由此造成了早期盆地沉降中心由中部向西迁移,然后再逐步向东迁移的特征.渐新世至早中新世为盆地裂陷阶段,以陆源碎屑岩沉积为主,断陷早期可能为湖相,晚期为浅海相;中中新世为盆地断坳转换阶段,晚中新世以来为盆地裂后热沉降阶段,二者均发育陆源碎屑岩和自生碳酸盐岩两种沉积类型,且裂后热沉降期碳酸盐岩沉积范围相对缩小,陆缘碎屑岩沉积范围相对扩大.

The Wan’an basin is one of the most important sedimentary basins in the southwestern South China Sea.Based on 2D seismic data covering the whole basin,combined with the latest research results,this study analyzes the characteristics of tectonicsequence and evolution of tectonics-sedimentary infillings of the Wan’an basin.In this study,8 major 2nd and 3rd order sequence boundaries have been identified in the Wan’an basin.Further subsidence simulation shows that the overall feature of subsidence rate of the basin shows a“high-low-high”process,with rough characteristics of higher in the east and lower in the west,higher in the north and lower in the south.Combined tectonic-sequences and simulation results of subsidence,evolution of tectonicsedimentary infillings of Wan’an basin since Cenozoic can be divided into five stages,which are early rifting stage,late rifting stage,rifting-post rifting transitional stage,thermal subsidence stage and accelerated thermal subsidence stage.Since the formation of the basin,the subsidence has mainly been affected by the deep faults caused by the extension of the East Asian continental margin.In Pliocene,the Wan’an fault became the main influencing factor of the basin subsidence,which resulted in the early migration of the basin subsidence center from the middle to the west,and then gradually to the east.From Oligocene to Early Miocene,the basin was in rifting stage,which was dominated by terrigenous clastic deposits.In Middle Miocene,the basin was in rifting-post rifting transitional stage and thermal subsidence has begun since Late Miocene.In these two stages,except for clastic deposits,carbonate deposits were also developed.In addition,compared to transitional stage,carbonate deposits in the thermal subsidence stage were relatively reduced,and the terrigenous clastic deposits were relatively expanded.