西沙地块构造—地层特征及其记录的南海洋盆演化过程

Evolution of the South China Sea Recorded in the Tectono-Stratigraphy of the Xisha Block

西沙地块是南海岩石圈地壳拉伸减薄过程中发育于深水区的陆块,其保存了陆缘演化的重要信息。文章研究以西沙地块作为研究对象,基于研究区地质和地球物理资料,开展了地壳结构、盆地构造—地层分析和断层活动特征等研究。研究发现,西沙地块与其周缘的凹陷地壳结构具有显著的差异,西沙地块地壳厚度较大,发育了高角度断层控制的小型断陷盆地,基底断层活动一直可持续到T60地震界面发育时(~23 Ma);而西沙地块周缘发育的是规模较大的拆离断层及其控制的强烈减薄陆壳。结合区域动力学事件,研究认为渐新世早期拆离断层在南海西北次海盆的活动导致了西沙地块北部的岩石圈地壳的减薄,而中新世早期拆离断层在南海西南次海盆构造位置的活动使西沙地块与南沙地块分离。文章研究成果不仅深化了西沙地块裂解规律的认识,而且对该区的油气勘探具有启示意义。

The Xisha block,covered by deep water,is a continental fragment formed during the process of lithospheric crustal thinning on the South China Sea,which recorded important information about the evolution of the South China block.In this study,integrated with the geological and geophysical data available,we studied the crust structure,tectono-stratigraphic of basins and faults in the Xisha block.We observed that the crust in the Xisha block is much thicker,which is significantly different from the crust structure of the surrounding basins.We observed small faults with high dip cut the basement underlying the Neogene reef buildup.These basement faults continued to cut upward and ceased their activity at the seismic horizon T60(~23Ma).In contrast,to the north of the Xisha block the detachment fault system and extremely thinned the crust developed there.Integrated with the geological dynamic setting,we indicated that in the early Miocene,the activities of detachment faults in the Southwest Subbasin of the South China Sea facilitated the southward movement of the Nansha massif away from the Xisha block.The result from our study will not only improve our understanding of the break-up mechanism of the South China Sea,but also has significance implication for the petroleum exploration in this area.