南海南部陆缘礼乐盆地新生代的构造—沉积特征及伸展机制: 来自NH973-2多道地震测线的证据

针对“973”项目中“南海大陆边缘动力学与油气资源潜力”这一研究课题, 对在南海南部陆缘礼乐盆地采集的NH973-2测线进行了研究.对地震剖面的解释共划分出6个层序界面, 将地层划分为4个构造沉积单元.根据地震解释, 对不同时期断层的水平断距进行了测量及分析, 获取了与脆性拉张相关的伸展信息: 研究区的拉张作用可以分为2期, 主要的拉张作用发生在大陆裂谷阶段(古近纪), 形成了一系列的地堑—半地堑以及翘倾断块; 第2期拉张作用的时期为晚渐新世—早中新世, 断层活动强度明显变弱.在南海南部陆缘广泛发育了碳酸盐沉积, 其发育的时代和南海的海底扩张时期一致.对穿越礼乐滩区地震剖面伸展特征的分析表明, 根据断层水平断距获得脆性伸展因子与根据重力反演获得的全地壳伸展因子之间存在差异, 表明研究区的拉张在纵向上并非是均一的, 新生代的拉张经历了深度决定拉张模式. 

Seismic Stratigraphy,Tectonic Structure and Extension Model Across the Reed Bank Basin in the South Margin of South China Sea: Evidence from NH973-2 Multichannel Seismic Profile

One recently acquired regional multi-channel seismic profile (NH973-2) across the Reed Bank basin, the South China Sea (SCS), is interpreted.Based on the interpretations of these profiles, we have worked out its stratigraphic sequences, tectonic structures and extension factors.Four tectonic-stratigraphic units are determined, together with 6 sequence boundaries.Detailed analyses on the extension factors based on the measurement of fault heaves reveal two episodes of continental extension separated by a distinct unconformity.The main extension occurred during the continental rifting (Early Tertiary), and resulted in the formation of half-grabens and rotated blocks, controlled by deeply rooted detachment system.During the second extension phase (drifting period, Late Oligocene-Early Miocene) intensity reduced sharply and focused on the continental-oceanic transition area.A widespread carbonate platform developed across the Reed bank region concurrent with the drifting period of the SCS.Fault-related stretching factor (β_f) is found to be comparable and shows discrepancy with the whole crust stretching factors (β_c) in the study area.Thus we conclude that the continental crust of the Reed Bank basin experienced depth-dependent extension.