| The establishment of river system and the spatiotemporal evolution of drainage pattern are the combined results of lithospheric tectonic movement,climate system evolution,and surface processes.Therefore,the fluvial sediments preserved in the sedimentary basin may record the evolution process of fluvial geomorphology under the interaction of tectonics and climate.Taking the eastern Tibet and the northern Tianshan as examples,this thesis discusses the roles of tectonics,climate and river self-regulation during river evolution at different time and space scales based on sedimentology and provenance results.Moreover,the topographic evolution history in eastern Tibet before and after the India-Eurasia collision is revealed,and the establishment timing of the present drainage pattern in eastern Tibet is further constrained.The specific contents of this thesis are as follows:(1)Combined the late Cretaceous early Paleogene stratigraphic and sedimentary characteristics of the southwestern Sichuan Basin,Xichang Basin,Huili basin and Chuxiong Basin at the eastern margin of the Tibetan Plateau with multi-technique provenance results of petro-stratigraphy,heavy-mineral analysis,and detrital zircon U-Pb dating,we indicate that there was a continental-scale southward drainage system in eastern Tibet from the late Cretaceous to early Paleogene,whose provenance was mainly dominated by the early Mesozoic sedimentary rocks in Songpan Ganzi,Yidun terranes,and Sichuan basin.Together with previous studies of paleoaltimetry,detrital zircon provenance and evaporite source,it is inferred that this large-scale fluvial system flowed through Simao and Khorat basins in the Indosinian terrane,and finally discharged into the Neo-Tethys ocean.These results challenge existing models of drainage networks that flowed toward the East Asian marginal seas and require revisions to inference of palaeo-topography during the Late Cretaceous.The presence of a continent-scale river may have provided a stable long-term base level which,in turn,facilitated the development of an extensive low-relief landscape that is preserved atop interfluves above the deeply incised canyons of eastern Tibet.The presence of a continent-scale river may have provided a stable long-term base level which,in turn,facilitated the development of an extensive low-relief landscape that is preserved atop interfluves above the deeply incised canyons of eastern Tibet.(2)Combining sedimentological results with a multi-technique sedimentary provenance dataset including detrital zircon U-Pb geochronology,heavy mineral,framework petrography,conglomerate component,and paleocurrent,we elucidate the evolution history the Eocene Ninglang basin at the eastern margin of the Tibetan Plateau.Our data indicate that the Ninglang basin was a small-scale,internally drained basin during the early Eocene(?56–45 Ma).The deposition period of first member of the Ninglang formation was characterized by a series of mountains-perpendicular river system with high-energy alluvial fan,which shifted to braided river and lake systems by the deposition period of the second member of the Ninglang formation.The basin changed from internally drained system to external one,and the drainage scale expanded rapidly during the deposition period of the third member of the Ninglang formation(?45 Ma).Specifically,in the northern basin,there is a nearly north-to-south axial river system,the upstream of which can be traced west to Gongjue area near the Tibet hinterland.It is similar to the present upper Jinsha River.At the same time,it is likely that another large-scale external river similar to the present Yalong River developed in the southern basin.During the deposition period of the fourth member of the Ninglang formation,the large river in the southern basin disappeared,whereas the northern basin was still dominated by external river system.Until ?35 Ma,the external drainage through Ninglang area died out.Combined with existing low-temperature geochronology and paleo-altimetric data,it is proposed that two periods of tectonic uplifts at ?45 Ma and ?35 Ma in eastern Tibet led to the transition from internal to external system,and the deposition termination in the Ninglang basin,respectively.In addition,due to the more humid climate since the middle–late Eocene,the enhanced rainfall would increase the river runoff,which likely promoted the occurrence of two regional drainage reorganization events to a certain extent.(3)We use an integration of new and published detrital zircon U-Pb ages from Pliocene–early Pleistocene Xigeda Formation and modern river sands to decipher the paleo-drainage evolution in eastern Tibet.Our new results confirm that the Jinsha River has flowed eastward at least before Pliocene time,but the Pliocene drainage patterns of its main tributaries(Dadu,Yalong,and Anning River)are different from the modern ones.Together with previously document fluvial incision in eastern Tibet,we interpret the excavation of the Paleo-Xigeda lake in the early Pleistocene initiated and accentuated fluvial incision of the Jinsha River and its main tributaries,which led to drainage reorganization and finally the establishment of the modern river pattern in eastern Tibet.(4)Here,only the accessible Kuitun River drainage basin in northern Tian Shan and incising late Miocene-Quaternary Dushanzi section were considered,and 1632 detrital zircon ages and 17 heavy mineral samples were comprehensively analyzed to investigate the signature of the driving forces for Miocene sedimentation in northern Tian Shan.This study first confirmed a previously recognized tectonic uplift at ca.7.0Ma,and further revealed that the basin sediments were mainly derived from the present glacier-covered ridge-crest regions during 3.3–2.5 Ma.It is suggested that Late Pliocene to Early Pleistocene sedimentation was likely a response to the onset of the northern hemispheric glaciation.Although complicated,this study highlights that the tectonic-climatic interaction during the Late Cenozoic orogenesis can be discriminated in the northern Chinese Tian Shan. |
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