Detrital Zircon U-Pb Geochronology Of The Xining Basin And Its Geological Implications In Cenozoic

The collision between India and Asia led to the formation of the Himalayan orogeic belt at the Early Cenozoic.The area of nearly 2,000 kilometers north of the Himalayan orogenic belt is uplifted and deformed.The uplift of the Tibetan Plateau during the Cenozoic had a great impact on the tectonic-geomorphica pattern and climate of Asian inland.Therefore,the study on the mechanism of the uplift and expansion of the Tibetan Plateau has become international research hotspots.With the continuous convergence between India and Asia,the tectonic deformation and geomorphology of the northeastern margin of the Tibetan Plateau are strongly evolved.It is a unique geomorphic tectonic pattern formed by a series of mountains and the Cenozoic basins sandwiched between them is an idea area for studying the Tibetan Plateau's uplift,mechanism and the distal effects of India-Asian collision.Basins and orogenic belts are the two basic tectonic units of continental geomorphology.They are inseparable coupling processes and tectonic framework under unified geodynamic background.In the basin,sediments eroded from the nearby mountains and transported by the rivers can record precise information on the unroofing history of the ranges.In this paper,the Xining Basin in the northeastern margin of the Tibetan Plateau is selected as the research object.Based on the previous studies,we will use the basin-mountain coupling relationship with the characteristics of the zircon U-Pb age distribution in different layers of the Cenozoic strata to analyze the provenance of the Xining Basin(including stratigraphic sedimentary facies analysis,paleocurrent analysis,gravel composition analysis).Mainly obtained the following understanding:1.There are five major changes in the zircon U-Pb age of the Cenozoic sediments in the Xining Basin: the detrital zircon U-Pb age populations before 41 Ma is 200-300 Ma as the main age peak,and 400-550 Ma is the secondary age peak;41->31Ma detrital zircon U-Pb age distribution with 400-550 Ma as the main age peak,200-300 Ma and 1000-2000 Ma as the secondary age peak;31->22Ma detrital zircon U-Pb age on the basis of the previous,the 700-1000 Ma group suddenly appeared and became the main age peak;the 22->12Ma detrital zircon U-Pb age was 200-300 Maand 400-550 Ma as the main age peak,and 1000-2000 Ma was the secondary age peak;12->3.6Ma detrital zircon U-Pb age with 1000-2000 Ma as the main age peak;since ~3.6Ma the detrital zircon U-Pb age with 400-550 Ma as the main age peak,200-300 Ma as the secondary age peak.2.The main source in the Early-Middle Eocene(52->41Ma)of Xining Basin was the East Kunlun Orogenic Belt,and the main source in the late Eocene-late Oligocene(41->22Ma)was transformed into the Qilian orogenic belt.In the early and middle Miocene(22->12Ma),the Central Qilian orogenic belt was the main source,and the Laji Shan began to uplift and became one of the basin sources.The main source of the Late Miocene-Pliocene(12->3.6Ma)came from the Precambrian of the Qilian orogenic belt in the northern margin of the basin.Since the early Pleistocene(~3.6Ma)the main source of sediments in the Huangshui terraces is the Paleozoic strata of the Qilian orogenic belt.3.After the collision between the Indian and Eurasian in the Cenozoic,the stress was transmitted to the northeastern margin of the Tibet Plateau,which led to the uplift and erosion of the Eastern Kunlun orogenic belt and the sedimentation of the Xining Basin depression.The late Eocene(~41Ma)Altun fault occurred the strong strike-slip movement caused the Central Qilian orogenic belt initially uplifted,and then accelerated uplifting at 35-31 Ma.In the early Miocene(~22Ma),a extensive tectonic movement occurred in the northeastern margin of the Tibet Plateau,and the Laji Shan began to gradually uplift and became the watershed of the Xining Basin and the Guide Basin;since the late Miocene(<12Ma),the strong tectonic activity of the northeastern margin of the Tibet Plateau has caused the mountains around Xining Basin to rise rapidly,~3.6Ma the basin was uplifted and denuded and the Huangshui River began to form.