| The formation of the Tibetan Plateau is one of the most important geological events in the Cenozoic,which not only changed the tectonic and geomorphic characteristics of the plateau and its surrounding areas,but also recorded the continental collision process,lithosphere and crustal deformation,and had a profound impact on the Asian climate and ecological environment as well as the global climate.However,in many researches on the formation of the Tibetan Plateau and its environmental effects,there are still any divergences in the understanding of the core issues of the uplifting time,process,amplitude and mechanism of the Tibetan Plateau.It`s in urgent need of using difference methods to study the key areas for testing those previous hypotheses or coming up with new ideas which solve the above mentioned problems.The Sanjiang region,in the southeastern margin of the Tibetan Plateau,had taken place large-scale strike-slip shear,compressional fold,thrusting nappe and uplift in the Cenozoic.Therefore,it is a key area to study the formation and evolution of the Tibetan Plateau and the mechanism with lateral extrusion of continental blocks along major strike-slip faults.This paper chooses typical paleogene formations in the Jianchuan Basin which is located in the Sanjiang region for research.Based on antecedently studies,we do some research on the sedimentary characteristics and detrital zircon U-Pb age spectrum of the Cenozoic continuous sedimentary profile,and combine the analysis of conglomerate and sandstone composition,heavy minerals and major and trace elements and Hf isotopes,in order to clarify the early Cenozoic provenance and the relationship between its changes and the paleo-drainage,regional tectonics and basin evolution of the Jianchuan Basin.This provides a basis for further study of the Cenozoic drainage and tectonic deformation and the formation and evolution of the Tibetan Plateau.The main insights are as follows:1.The Paleogene stratigraphic sequence of the Jianchuan Basin consist of the Baoxiangsi Formation,the Jiuziyan Formation,the Shuanghe Formation and the Jianchuan Formation from bottom to top.The detrital zircon U-Pb age spectrum of the Baoxiangsi Formation has multiple age peak ranges of 200-320,390-490,690-920920-1120 and 1700-2000 Ma(distinctly unlike the overlying strata),but the Shuanghe Formation is almost all concentrated at 35-45 and 200-280 Ma,and the Jianchuan Formation compared with the Shuanghe Formation has increased the age peak range of 720-900 Ma.Combined analysis of conglomerate and sandstone composition,heavy minerals,major and trace elements and Hf isotopes,these results indicate that significant provenance changes occurred between the Baoxiangsi Formation and the overlying strata(at ?41 Ma)in the Jianchuan Basin.Comprehensive provenance analyses illustrate that the Hoh-Xil,Songpan-Ganzi,North Qiangtang,Yidun and western Yangtze terranes served as major sources for the Baoxiangsi Formation,the adjacent Triassic and Eocene igneous rocks for the Shuanghe Formation,and the western Yangtze block,adjacent Triassic and Eocene igneous rocks for the Jianchuan Formation.2.Before 41 Ma of the middle Eocene(during the deposition of the Baoxiangsi Formation),there may have been a large-scale paleo-drainage connecting the Tibetan Plateau with various basins in the southeastern margin of the Tibetan Plateau.The re-organization of this paleo-drainage occurred at ?41 Ma as one of the responses to coeval topographic uplift in the southeastern Tibetan Plateau.3.The formation of the diluvial coarse clastic deposits at the bottom of the Baoxiangsi Formation is in angular unconformity with the underlying Mengyejing Formation,which represents the distal response of the India-Eurasia collision and the uplift of the Tibetan Plateau at the beginning of the Cenozoic(? 55 Ma).After that,the Baoxiangsi Formation changed into braided fluvial deposits,and lots of detrital materials from the northern plateau were delivered into the Jianchuan Basin.The tectonic activities of the Tibetan Plateau around 41 Ma caused the faulting of the Jianchuan Basin,and the provenance changed to the surrounding mountains,and sedimentary environment of the basin switched from the fluvial environment to the lacustrine carbonate deposit of the Jiuziyan Formation and the lacustrine and marsh coal measures deposit of the Shuanghe Formation,and extensive ultrapotassic magmatic intrusion and eruption.The tectonic activities of the Tibetan Plateau around36 Ma caused ?E-W compression and shortening of the Jianchuan Basin,which led to the strata folding and denudation,and formed the unconformity contact between the Jianchuan Formation and the underlying strata.During Oligocene-Miocene,the large-scale sinistral strike-slip along the Ailaoshan-Red River shear zone resulted in long-term uplift and denudation in the Jianchuan Basin. |
PDF Full Text Request