南海北部陆架断裂系统特征及其对南海打开的启示意义

The Fault Assembly Characteristics on the Northern South China Sea Continental Margin and its Implications on the South China Sea Spreading

南海作为西太平洋最大的边缘海之一,处于太平洋、欧亚以及印‒澳三大板块的俯冲汇聚作用之下,具有复杂的形成机制和演化历史。白垩纪初期,受太平洋板块俯冲后撤的影响,南海北部和南部陆缘发育一系列NE-NEE向张性剪切岩石圈断层,渐新世以来断裂大多继承先存断裂继续活动。中新世晚期,菲律宾海板块向西持续俯冲,在南海北部陆缘出现大量的近EW-NEE向张性正断层和NWW-NW向剪性基底断裂。两期断裂在平面上相互交切,代表了不同时间板块间的相互作用,但这些断裂在垂向上经历了何种构造活动,如何控制南海北部盆地群的沉积分布,与周围板块的运动方式有何联系,这些问题仍未得到系统解答。最重要的是,这些断裂系统的形成过程对南海海盆的打开方式有何启示意义是一个值得深入探讨的问题。本文针对南海北部陆缘发育的NE-NEE向、NW向和E-W向断裂系统,选取NE-NEE向、NW向断裂系统最发育的珠江口沉积盆地,利用重磁异常、沉积地层和地震反射剖面资料,系统分析断裂结构特征及活动历史。结果表明,南海北部陆缘一系列NEE向的断裂带(如滨海断裂带),很可能为珠江口盆地内部控坳主控断层,且断裂活动经历了早期低角度滑脱,第二期高角度正断及第三期右旋走滑,三期活动分别对应南海陆缘地区裂陷阶段、裂后坳陷阶段和断块活动三大构造阶段,反映了南海周边板块俯冲汇聚速率和方向改变过程中该组断裂在南海打开的重要作用。

Being the largest marginal sea on the southeastern continental margin,the South China Sea is controlled by the subduction and convergence of the Pacific,Eurasian and Indo-Australian plates,and has a complex formation mechanism and evolutionary history.At the beginning of the Cretaceous,a series of NE-NEE-trending extensional shear lithospheric faults developed in the south and north of the South China Sea Basin.During the Oligocene,most of the faults inherited the old fault activities.Ever since the late Miocene,the Philippine Sea Plate continued to subduct westward,and a large number of near-EW-NEE-trending extensional normal faults and NWW-NW-trending shear basement faults occurred in the northern continental margin of the South China Sea.Intersecting of the two-phase faults represents the interaction between plates at different times.However,issues concerning the vertical tectonic activities,factors controlling the distribution of sediments,and their relationships with the movement of the surrounding plates remain elusive.Most importantly,the formation process of these fault systems and the opening mode of the South China Sea basin are issues worthy of deep digging.Aiming at the NE and EW trending fault systems in the continental margin of the northern South China Sea,this paper selects the Pearl River Mouth Basin where the NEE-and NW-trending fault systems are most developed,and uses the gravity and magnetic anomalies,sedimentary strata and seismic reflection profiles to systematically analyze the structural characteristics and activity history of the faults.The results show that a series of NEE-trending fault zones(such as the Littoral Fault Zone)in the northern continental margin of the South China Sea are likely to be the main faults controlling depressions in the Pearl River Mouth Basin,and the faults experienced the early low-angle detachment,and the second phase high angle normal fault and the right-lateral strike-slip since the third Oligocene period.They correspond to three tectonic stages:rifting stage,post-rifting depression stage and fault block activity,reflecting the subduction and convergence rates of the plates around the South China Sea Basin,which demonstrates that this group of faults played an important role in the opening of the South China Sea during the change of subduction direction.