“三江”北段沱沱河盆地古近纪—新近纪沉积格架与盆地演化分析

An analysis of Paleogene-Neogene sedimentary framework and basin evolution of Tuotuohe basin in the northern segment of the Sanjiang region

通过对青藏高原腹地沱沱河盆地古近纪—新近纪沉积序列、区域不整合面、岩性特点及分布特征等的分析研究,认为沱沱河盆地古近纪—新近纪沉积由下而上可分为沱沱河组、雅西措组、五道梁组和曲果组4个向上变浅序列,构成两个完整的陆相造山磨拉石建造序列。盆地分析表明,古近纪—新近纪沱沱河盆地经历了前陆盆地演化阶段（56.5~45.0 Ma）→走滑拉分盆地阶段（45.0~30.0 Ma）→整体抬升,山间残留盆地阶段（23.5~16.0 Ma）→前陆盆地-局限盆地-山间残留盆地阶段（16~2.6？Ma）等阶段。根据构造岩相古地理的演化史认为,在雅西措组沉积早期,大约在45 Ma左右,区域大地构造背景发生了大的转换,由区域挤压增厚阶段转变为以板块间的侧向走滑作用为主,由此进入陆内板块汇聚演化阶段。从沱沱河盆地古近纪—新近纪的沉积演化来看,印度板块与欧亚板块的碰撞是脉动性的,整个古近纪—新近纪的沉积中4个区域不整合面和2个磨拉石建造序列是脉动造山过程的沉积响应,初始碰撞可能发生在白垩纪与古近纪之交,时间在56.5 Ma之前。

Based on an analysis of sedimentary sequence,regional unconformity,lithologic character and distribution of Paleogene-Neogene sediments,the authors hold that the Paleogene-Neogene sediments can be divided into four upward-shallowing sequences of Tuotuohe Formation,Yaxicuo Formation,Wudaoliang Formation and Quguo Formation,which constitute two complete continental orogenic molasses formation sequences.Basin analysis shows that the Paleogene-Neogene Tuotuohe basin has experienced such evolutionary stages as foreland basin evolution stage （56.5～45.0 Ma）→strike-slip pull-apart basin stage（45.0～30.0 Ma）→overall uplift and intermountain residual basin stage（23.5～16.0 Ma）→foreland basin-confined basin-intermountain residual basin stage（16～2.6？ Ma）.According to tectonic,lithofacies and paleographic evolution,it is considered that at the early stage of the deposition of Yaxicuo Formation（45 Ma or so）,regional geotectonic background experienced remarkable transformation from the regional compression-thickening stage to the domination of inter-plate lateral strike-slip action,thus entering into the intracontinental plate convergence evolution stage.Viewed from Paleogene-Neogene sedimentary evolution of the Tuotuohe basin, the collision between the Indian plate and the Eurasian plate was of pulsation nature,the four regional surfaces of unconformity and two molasses formation sequences in the whole Paleogene-Neogene sedimentation seemed to be sedimentary response to pulsating orogenic process,and the initial collision probably took place at the time when Cretaceous changed into Paleogene before 56.5 Ma.