新特提斯洋打开于晚三叠世?——印度大陆北缘沉积响应

Opening of the Neo-Tethys Ocean in the Late Triassic?Sedimentary responses on the northern Indian continental margin

新特提斯洋是中生代位于北方欧亚大陆和南方冈瓦纳大陆之间的古大洋,它在青藏高原南部地区于新生代早期因印度-欧亚大陆碰撞而消亡,其遗迹为现今的印度河-雅鲁藏布缝合带。新特提斯洋打开以拉萨地块从冈瓦纳大陆的裂离为标志。准确约束新特提斯洋的开启时间是重建冈瓦纳大陆裂解过程和特提斯洋演化历史的关键,但目前学术界对于新特提斯洋的开启时间还存在很大争议,不同学科方法的认识从早二叠世到晚三叠世不等。本文对新特提斯洋南侧印度被动大陆边缘二叠纪—三叠纪沉积地层进行了系统的梳理,研究发现在早二叠世冰期结束之后,印度大陆北缘长期表现为稳定的沉积环境,显著的沉积环境变化仅发生在晚三叠世。晚三叠世的沉积环境变化伴随着沉积和沉降速率增加、沉积物源变化、双峰式火山活动以及古地理格局的重大改变。研究认为,晚三叠世印度大陆北缘沉积作用变化所记录的区域伸展作用很可能代表了新特提斯洋的开启。

The Neo-Tethys recorded by the Indus-Yarlung-Zangbo suture zone in southern Tibet was a vast ocean between Eurasian and Gondwanan continents during the Mesozoic that closed in southern Tibet as a result of Indo-Eurasian collision in the Early Cenozoic.It is a common view that the Neo-Tethys Ocean was formed as a result of rifting of the Lhasa block from the Eastern Gondwana.However,the timing of the rift remains controversial.Proposed timings for the opening of the Neo-Tethys vary from Early Permian to Late Triassic.In this study,we carried out an integrated stratigraphic and sedimentological study on the Permian-Triassic strata deposited on the north Indian continental margin.Our data indicated that the depositional environment remained stable since cessation of the glacial period in the Early Permian.A notable change in sedimentary environment occurred only in the Late Triassic,from a carbonate platform to a terrigenous clastic shelf.Facies change was accompanied by increasing depositional rate,accelerating tectonic subsidence,provenance change,bimodal volcanism,and paleogeographic reconstruction.These changes clearly revealed a regional extensional event,which is considered as a response to the opening of Neo-Tethys.