南海北部东沙运动的构造特征及动力学机制探讨

南海北部陆缘在南海扩张结束后发生了一次重要的构造运动—东沙运动.因前人对其研究较少,目前对其发生时间、影响范围和形成机制等都存在较大的争议.通过对东沙海区及其邻区新近纪地层二维、三维地震资料的详细解释,确定了东沙运动发生在晚中新世晚期,并在晚中新世末/早上新世初(5.5Ma)停止活动.东沙运动主要波及东沙隆起和潮汕坳陷地区,构造上主要表现为断块升降,其中隆起区沉积物遭受剥蚀,造成中新世及部分上新世地层的缺失.这次运动还形成了大量次级的NW-NWW向张性、张扭性断裂,构造运动整体上具有东强西弱的特点.东沙运动可能与新近纪以来菲律宾海板块持续向NWW方向运动导致的吕宋岛弧与欧亚大陆在9～6Ma期间开始发生的弧陆碰撞有关.同时,由于南海向马尼拉海沟下的俯冲及洋壳的冷却沉降作用,南海北部陆缘处于拉张环境,岩浆底侵到下地壳底部形成高速层,破坏了该区域的地壳均衡,从而造成上部地壳的隆升.

Structures and dynamic mechanism related to the Dongsha movement at the northern margin of South China Sea

After the end of the spreading of the South China Sea(SCS),an important structural movement-the Dongsha Movement took place at the Northern Margin of the SCS.However,timing,range and dynamic mechanism of the Dongsha Movement is still unclear.Based on the 2D/3D seismic data in the Dongsha area and its neighboring,the Dongsha Movement mainly occurred in the late period of the Late Miocene and ended at the Mio/ Pliocene boundary(5.5Ma) in this paper.The Dongsha Movement mainly affects the Dongsha Rise and the Chaoshan Depression.Besides,the eastern part of the Panyu Low-High and the southern and northeastern part of the Zhu Ⅰ Depression are also deformed.Its main deformation is characterized by faulting with the coeval sedimentation in the area of uplift eroded which causes the hiatus of part of Miocene and Pliocene stratums.It also caused clear angular unconformity which marked the time of the Dongsha Movement.Little igneous activity could be found in the Neogene stratum in the study area.However,magma intruded or erupted around the ocean and continent boundary,southeast of Dongsha Rise,probably due to the weakness of the crust.The movement causes abundant secondary NW-NWW tensional and transtensional faults,the length of which is mainly 1-10 km.The deformation is more intensive in the eastern area than in the western area.The Dongsha Movement might be related to a collision between 9-6 Ma of the Luzon Arc to the Eurasian Plate which was caused by the NWW movement of the Philippine Sea Plate in Neogene.At the mean time,the northern margin of SCS is in a setting of extension for a subduction of SCS towards the Manila Trench and a cooling of oceanic crust,which caused magma underplating at the bottom of lower crust in the study area.This mechanism resulted in the unbalanced isostasy of the area and the uplift of the upper crust.