南海大陆边缘盆地构造演化差异性及其与南海扩张耦合关系

Difference of tectonic evolution of continental marginal basins of South China Sea and relationship with SCS spreading

南海大陆边缘盆地由于边界条件的差异,不仅形成了不同类型的陆缘盆地,如离散型、走滑-伸展型和伸展-挠曲复合型,而且这些盆地构造演化存在明显的非同步性。这些陆缘破裂过程与南海扩张作用过程呈现明显不一致性。研究表明,南海扩张时期南海南、北大陆边缘均形成了一系列裂陷盆地,然而,南海南部、北部大陆边缘盆地裂陷作用结束时间不同,北部大陆边缘盆地裂陷作用结束于23Ma或21Ma,而南部大陆边缘盆地裂陷作用结束于15.5Ma,显然北部大陆边缘盆地裂陷结束时间明显早于南部大陆边缘盆地。南海扩张停止后,南海南、北部陆缘仍表现出明显差异,北部陆缘仍以伸展作用为主,晚中新世以来出现快速沉降幕,而南海南部陆缘则以挤压作用为主,且其挤压时间及强度呈现南早北晚的特点,即南部曾母盆地明显早于南薇西盆地和北康盆地。南海南、北大陆边缘盆地形成演化的差异性,特别是构造转型差异变化,为新生代南海扩张的迁移性提供了有力的佐证,可以推断南海不同期次海盆扩张可能存在向南的突然跃迁。因此,本次研究梳理出的南海不同陆缘盆地张裂-伸展的非同步性可为南海洋盆扩张演化过程解释提供新的证据。

Continental marginal basins in southern and northern South China Sea（SCS）show not only distinct tectonic regimes,including divergence,transform-extensional and flexural-extensional complex types,but also have different tectonic evolutionary stages,due to different boundary conditions.Discordance between the breakup of continental margins and South China Sea spreading processes are presented.The research results indicate that a series of rifting basins are developed in the southern and northern continental margins during the SCS spreading,which are resulted in the formation of main rifting stage.However,the time of breakup unconformity is quite different.The rifting terminates on the age of 23 Ma or 21 Ma in the northern margins of the SCS,while did not terminate until the age of 15.5Ma in the southern margins of the SCS.Those features indicate that the age of the break-up unconformity in the continental margins show a clear shift southward,which means that in the northern SCS is earlier than in the southern SCS.There are clear differences in tectonic regimes after the termination of the SCS spreading.Extension is dominated in the northern SCS and rapid subsidence episode is present since the Late Miocene,but compress regimes are present in the southern SCS,where the age and intension of compressive stress in the Zengmu basin are earlier and stronger than in the Beikang and Nanweixi basins.Transformation and distinct tectonic evolution in the southern and northern continental marginal basins indicate the shift of the SCS spreading,and are used to suggest the shift southward in the different ending times of the spreading processes for the three oceanic sub-basins of the SCS.Therefore,differences in formation and evolution of continental marginal basins in the northern and southern SCS provide a helpful evidence for the reasonable interpretation for the spreading processes of the SCS.