Source-to-sink Responses To The Migration Of Depocenters During Cenozoic In The Qaidam Basin

The Qaidam Basin is the largest sedimentary basin in the Qinghai-Tibetan Plateau.It is adjacent to the Altyn Tagh Mountain to the west,south Qilian Mountain to the north and east Kunlun Mountain to the south.The tectonic uplift and movements of peripheral mountains since the Cenozoic controlled the subsidence and sedimentation process of th e Qaidam Basin.Based on the previous studies,the analysis of seismic and logging data and a large number of analysises and testing,this study summarizes the distribution and migration law of depocenters in the major periods of Cenozoic in the Qaidam Basin.Combined with apatite fission track dating data,sensitive heavy mineral indexes of sedimentary strata and calculation of deposition rate in piedmont zones,the source-to-sink response relationships between the peripheral mountains and the basin in different periods of the Cenozoic and its controls are clarified.Through the analysis of chloride ion and carbonate content,the changes of paleoclimate in the Cenozoic as a response to the migration of depocenter in the study area are discussed.The analysis of core,logging data shows that there are many types of sedimentary facies in the Qaidam Basin in the Cenozoic,including alluvial fan,meandering river,braided river,deltaic and lacustrine facies.By revising the Cenozoic sedimentary facies distribution map and residual thickness map of the Qaidam Basin,the migration process of depocenter during the Cenozoic in the study area was revealed.The depocenter was initially located at the southwest corner of the basin,then migrated to the Yiliping and Sanhu areas in the central basin successively,showing the migration law from southwest to northeast,and then to southeast and east.The statistical analysis of apatite fission track data shows that the uplift process of the three mountains around the Qaidam Basin is synchronous and different.The Altyn Tagh orogenic belt mainly experienced five periods of tectonic uplift events:65～60Ma,55～40Ma,38～30Ma,26～17Ma and 9～7Ma;Three periods of tectonic uplift events occurred in the east Kunlun orogenic belt:60～50Ma,30～16Ma,from 2.8 Ma to now;Three periods of tectonic uplift events occurred in the south Qilian orogenic belt:65～50Ma,25～15Ma and5～2.1Ma,65～60Ma is the common uplift period of the three mountains,in which the Altyn Tagh Mountain is during the stage of left strike-slip movement;55～40Ma:the Altyn Tagh Mountain experienced the second stage of tectonic uplift;38～30Ma:the Altyn Tagh Mountain is in the stage of right walking and sliding;30～15Ma is the common uplift period of the three mountains,in which the tectonic uplift of the east Kunlun Mountain and the Altyn Tagh Mountain is more significant;9～2.8Ma:affected by the overall rapid uplift of the Qinghai-Tibetan Plateau,the three mountains are in a strong tectonic uplift period.Through the analysis of sensitive heavy mineral indexes,the calculation of deposition rate in piedmont and the development of growth strata,it is shown that there are mainly three provenance supply areas in the Qaidam Basin during the Cenozoic,namely,the Altyn Tagh Mountain area,the east Kunlun Mountain area and the south Qilian Mountain area.The composition of heavy minerals in different provenance areas is different.Among them,the Altyn piedmont ZTR index shows a relatively low value in E₁₊₂,N₁,N₂¹ and N₂³ periods;The ZTR index of the east Kunlun and the south Qilian mountains is relatively low in E₁₊₂,N₁ and N₂³ periods;It shows that strong tectonic uplift occurred in the three mountains during this period.The Qaidam Basin is surrounded by three orogenic belts.The migration and sedimentary evolution of depocenter during the Cenozoic are controlled by the tectonic activities and interactions of the three surrounding mountains:（1）65～43.8Ma:with the collision between the Indian plate and the Eurasian plate,the Altyn piedmont in the west of the Qaidam Basin and the east Kunlun piedmont in the south of the basin subsided relatively rapidly,and the depression lake basin began to form in the southwest of the Qaidam Basin.At this time,the depocenter is mainly located in the Youshashan area in front of the Altyn Tagh mountain and the Yiliping area in the central basin;（2）25～14.9Ma:the east Kunlun Mountain have undergone significant uplift,and the Altyn Tagh uplift range and amplitude reach the maximum,and the depocenter begins to migrate northeastward;（3）14.9～2.8Ma:the east Kunlun Mountain and the south Qilian Mountain are in a relatively quiet period of structure,and the Altyn Tagh Mountain is in a stage of left strike-slip,strong compression and uplift,resulting in the continuous migration of the depocenter of the basin to the southeast;（4）From 2.8 Ma to now:the Qinghai-Tibetan Plateau has risen rapidly as a whole,and the depocenter has further moved eastward to the Sanhu area in the southeast of the basin.The analysis of the changes of chloride ion content and carbonate content shows that the Qaidam Basin was in a humid climate environment during the Paleogene and more arid during the Neogene.The Cenozoic paleoclimate in the study area presents the change law of moisture-drought-moisture-drought,and its climate change is mainly controlled by the uplift of mountains around the basin.At 8 Ma,the Qinghai-Tibetan Plateau rose rapidly as a whole,and the surrounding mountains were in the period of tectonic uplift.While the depocenter continued to migrate eastward,the lake basin shrank in a large area,Formation of Asian monsoon climate,the climate became extremely arid.From the brackish water lake basin of the Paleogene to the quaternary brackish water or salt water lake basin of the Neogene.