青藏高原可可西里盆地晚白垩世—早中新世地层年代学新进展及其对地层和古环境的指示意义

Progress of Late Cretaceous to Early Miocene stratigraphic chronology in the Hoh Xil Basin of the Xizang Plateau and its significance for stratigraphy and paleoenvironment

青藏高原构造隆升是新生代重大地质事件之一,是东亚构造—气候演化的重要边界条件之一,也是研究全球气候变化绕不开的一个重要因素。目前,关于青藏高原的隆升历史和机制等存在较大的争议,其中,一个最重要的原因是地层年代学问题,可靠的地层沉积年龄是后续研究的基础。青藏高原中北部可可西里盆地保存有晚白垩世至中新世较连续的沉积,是研究青藏高原构造演化不可多得的研究材料。笔者等基于可可西里盆地最新的晚始新世至中新世地层年龄研究结果,同时,结合其他已发表的、有绝对年龄控制点的地层年龄,认为风火山群和沱沱河组是两套独立的地层单元,即在可可西里盆地,地层划分从老到新可划分为:风火山群、沱沱河组、雅西错组和五道梁组。在可可西里盆地地层沉积年龄最新进展基础之上,综合盆地的古高度、古纬度、古温度、古地磁、沉积相变化、地层接触关系、全球温度、大气二氧化碳以及亚洲季风和高原隆升之间关系的模拟等证据,指出:①青藏高原面上类似于现在向东、东南逃逸的GPS速度运动场方向可能在始新世就已经形成,沱沱河盆地以东的物质向东南逃逸,以西没有这种趋势,沱沱河盆地可能是一个重要的边界;②类似于现在的东亚季风—内陆干旱化格局形成于晚渐新世—早中新世;③青藏高原中北部晚始新世以来发生了明显的两阶段隆升,>38.5~26(24)Ma的隆升主要由印度与欧亚板块的碰撞挤压缩短所致,16 Ma—现在的隆升由岩石圈地幔对流拆沉引起,26(24)~16 Ma是隆升相对平静期;④青藏高原中北部主夷平面可能发育于26(24)~16 Ma期间;⑤角度不整合接触不是构造事件发生的可靠判别标志;⑥石膏等盐类矿物的出现不是干旱化的可靠指标,但似乎表明在盐类矿物沉淀析出之前,区域应该存在至少一期湿润的气候,按此结果,可能暗示了至少在古新世我国中东部可能已经存在季风气候。

Objectives:The tectonic uplift of the Xizang(Tibet)Plateau is one of the major geological events of the Cenozoic,one of the important boundary conditions for the tectonic—climate evolution of the East Asia,and an important factor in the study of global climate change.The history and mechanism of the uplift of the Xizang Plateau are controversial,one of the most important reasons is the stratigraphic chronology.Reliable stratigraphy depositional ages are the basis for subsequent studies.The Hoh Xil Basin in the north—central part of the Xizang Plateau preserves continuous Late Cretaceous to Miocene sediments,which is an indispensable research material for the study of the tectonic evolution of the Xizang Plateau.In this paper,we will propose a new interpretation of the relevant traditional viewpoints,etc.,based on the new progress in stratigraphic chronology of the Hoh Xil Basin,while synthesizing the existing geological evidence.Methods:Using the latest stratigraphic chronological advances in the Hoh Xil Basin,while integrating the various geological evidence available in the surrounding area.Results:Based on the latest Late Eocene to Miocene stratigraphic age results in the Hoh Xil Basin,and combining with other published stratigraphic ages with absolute age control points,this paper argues that the Fenghuoshan Group and the Tuotuohe Formation are two distinct units.Following the new result,the stratigraphic divisions of the Hoh Xil Basin from the oldest to the newest are as follows:the Fenghuoshan Group,the Tuotuohe Formation,the Yaxicuo Formation Group,and the Wudaoliang Formation.On the basis of the latest advance in stratigraphic depositional age in the Hoh Xil Basin,the evidence of the basin’s palaeoelevation,palaeolatitude,palaeotemperature,palaeomagnetism,changes in sedimentary facies,stratigraphic contact relationships,global temperature,atmospheric carbon dioxide,and the simulation of the relationship between the Asian monsoon and the uplift of the plateau,we drew the conclusion that:①The direction of the GPS velocity kinematic field on the surface of the Xizang Plateau similar to the present eastward and southeastward escape may have been formed in the Eocene,with material east of the Tuotuohe Basin escaping to the southeast,and no such trend to the west,and the Tuotuohe Basin may have been an important boundary;②The East Asian monsoon—inland aridification pattern similar to the present was formed in the Late Oligocene—Early Miocene;③The Late Oligocene—Early Miocene uplift of the north central Xizang Plateau has occurred in two main phases since the Eocene.The uplift from>38.5 to 26(24)Ma was mainly caused by the collision and extrusion shortening of the Indian and Eurasian plates,while the uplift from 16 Ma to the present was caused by the convective removal of the lithospheric mantle,and the uplift from 26(24)to 16 Ma was a period of relative quiescence;④The main planation surface of the north—central Xizang Plateau may have been developed during the period 26(24)~16 Ma;⑤Angular unconformity contact is not a reliable discriminator of the occurrence of tectonic events;⑥The occurrence of salt minerals such as gypsum is not a reliable indicator of aridification,but seems to indicate that at least one period of wet climate should have existed in the region prior to the precipitation of salt minerals.According to this theory,it may imply that a monsoon climate already existed in the east—central part of our country at least in the Paleocene.Conclusions:New advances in the stratigraphic chronology of the Hoh Xil Basin and its palaeoenvironmental significance suggest that some indicators need to be interpreted with caution.