Cenozoic Sediment Flux And Geochemical Records In The Qaidam Basin,northern Tibetan Plateau,and Their Implications For Weathering And Denudation

The global climate,tectonics and physiognomy pattern had changed significantly during the Cenozoic,such as the uplift of the Tibetan Plateau,global cooling and aridification in the inner Asia.The continental weathering and denudation are the driving force to shape the landform evolution and surficial deposititon,which are closely related to tectonics and climates.Meanwhile,the continental weathering is the most important mechanism to balance the Earth's temperature,the Tibetan Plateau uplift could increase chemical weathering rate of silicate rocks and cause global and regional cooling.Hence,the relationship between Tibetan Plateau uplift,weathering and denudation,and climate change has become long-standing research topics.However,a shortage exists of stable and continuous weathering records from northern Tibetan Plateau to provide convincing evidence to improve our understanding of weathering and denudation and its relationship with tectonics and climate in the northern Tibetan Plateau.The Qaidam Basin is the largest inland closed basin in the northern Tibetan Plateau,and its thick sequences of basin sediments provide a unique opportunity to study the relationships between weathering and denudation,tectonic uplift,and climate change during the Cenozoic.In this study,we identified the Qaidam Basin has been an intermountain basin since the Mesozoic,thus the sediment flux in the Qaidam Basin could reflect the denudation of the basin bounding mountains.We taking full advantages of previous achievements about the Cenozoic high-resolution chronostratigraphic sequence,the revealed tectonic and environmental change events in the Qaidam Basin.Based on the restoration of seven NE-SW balanced cross-sections,compiled thickness data of outcrop sections and boreholes,sedimentary features?e.g.,sedimentary association,strata thickness and sedimentary rate?,and their space variation pattern during different periods of the Cenozoic era to reconstruct the basin boundaries,develop isopach maps,and calculate the sediment flux?represent denudation rates of the source area?in the Qaidam Basin.Then,by comparing the sediment flux with previously documented regional tectonic events and climate changes,we discuss the key factors that controlled the basin sediment flux and denudation rate around the basin.Meanwhile,we used the geochemistry of clays?<2?m?in Huaitoutala section?15.3-1.8 Ma?in the northeastern Qaidam Basin to reconstruct the history of silicate weathering intensity and the climate evolution in the catchment,and to study the relationship between denudation,chemical weathering intensity,and climate change in the northern Tibetan Plateau.Main conclusions and insights in this study are summarized as follows:?1?The results from the restoration of seven NE-SW seismic sections in the Qaidam Basin indicate that the Qaidam Basin has been undergoing continuous shortening since the beginning of the Cenozoic with two relatively faster shortening phases occurred during 43.8-35.5 Ma and15.3-0 Ma,and two relatively slower shortening phases occurred during 53.5-43.8 Ma and35.5-15.3 Ma.Moreover,the shortening rates and shortening ratioes gradually decreased from the west to the east of the Qaidam Basin,which indicate more intense deformation in the western edge of the basin during the Cenozoic.?2?Based on the shortening of seven NE-SW balanced cross-sections,strata thickness,boreholes data,and sedimentary features,we reconstructed the basin boundaries,developed isopach maps,and calculated the sediment flux?denudation rates?in the Qaidam Basin.Our results show that the sediment flux in the Qaidam Basin increased gradually between 53.5 and35.5 Ma,decreased to its lowest value from 35.5 to 22 Ma,increased again since 22 Ma,especially,increased dramatically after 2.5 Ma.?3?By comparing the changes in the sediment flux with regional tectonic events and regional and global climate changes,we suggest that the sediment flux from 53.5 to 43.8 Ma was controlled by both the tectonic uplift of the Tibetan Plateau and the relatively warm and humid climate,and the gradual increase in the sediment flux from 43.8 to 35.5 Ma was mainly controlled by the tectonic uplift of the northern Tibetan Plateau.The low sediment flux from 35.5 to 22 Ma was a response to the relatively cold and arid climate in a relatively stable tectonic setting.The relatively high sediment flux between 22 and 15.3 Ma was related to tectonic activity and the warm and humid climate.The Miocene-Pleistocene episodic and persistent uplift of the NE Tibetan Plateau caused the high sediment flux after 15.3 Ma?especially caused the dramatic increase after 2.5 Ma?.?4?The geochemical proxy records?e.g.,CIA,Rb/Sr and Na₂O/Al₂O₃ ratios?of clay particles?<2?m?in the Huaitoutala section in the northeastern Qaidam Basin demonstrate that the chemical weathering intensity was strong from ca.15.3 to 12.6 Ma,and the intensity of chemical weathering was continually decreased since ca.12.6 Ma.Combined with biological characteristics and previous researches,we conclude that the Qaidam Basin was relatively warm and humid from ca.15.3 to 12.6 Ma,and became relatively semi-humid and semi-arid from ca.12.6 to 6.6 Ma,and the extreme aridity was initiated after 6.6 Ma.?5?The chemical weathering intensity exhibits a pattern that similar to the long-term global climate change pattern,which demonstrates that chemical weathering intensity is mainly controlled by global climate change.