Coupled Response Of The Cenozoic Tectonic Uplift And Climate Change On The Northern Tibetan Plateau

The most important collisional orogeny in the Cenozoic is the collision between the India and the Eurasia,leading to the formation of the "roof of the world" Tibetan Plateau.As the existed largest and highest orogeny,the Tibetan Plateau is particularly important for people to understand the mechanics of continental deformation and further the evolution of the earth.There has been much debate on the Cenozoic uplift history and mechanics of the Tibetan Plateau.The uplift of the Tibetan Plateau has been suggested to largely affected the Cenozoic global change,including the global cooling,the formation of polar ice sheets and the Asian monsoon,and the aridification of inland Asia.As the largest sand sea in Asia and one of the most important dust sources to the global aerosol system,the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic.However,the age of the Taklimakan remains controversial,with the view being from tens of kyr to 7 Ma.The thrusting of the rigid Tarim Block under the northern Tibetan Plateau supports the extremely high relief of the plateau.Meanwhile.large-scale foreland basin systems were formed driven by the load of boundary ranges.The large thicknesses of sedimentary rocks preserv ed in the foreland basin provide plenty of information on the history of tectonic deformation and aridification in the Northern Tibetan Plateau and inland Asia.In this study,we first focus on the volcaniclastic sequence and present detailed sedimentary,petrological,geochemical and chronological results;these provide abundant information on the source to sink processes of the sequense,as well as useful constraints for regional strata and tectonics.The sequence was divided into Lower and Upper Members.The Lower Member is dominated by dark gray,thick-bedded,massive,medium to coarse sand-sized clasts,intercalated with beds of well-rounded,pebble to cobble,polymictic conglomerates.The Upper Member is comprised of massive,poorly sorted,and matrix-supported debris flow breccia.Petrologic observations indicate that the volcaniclastic sequence comprises of trachyte and phonolite clasts.Geochemistry results indicate that the sequence shows affinities with shoshonitic and ultrapotassic-potassic series.Another obvious character of these rocks is their enrichment in large ion lithophile elements and light rare earth elements.Several lines of evidence indicate that the volcaniclastic sequence was likely derived from the Dunkeldik Volcanic Complex.The sediments of the Lower Member were transported in the form of dilute streamflows before deposition.The dilute streamflows most likely formed during or soon after the eruptions,when the source region provided abundant clasts.The Upper Member was formed by a fast moving volcanic debris flow,likely transformed from an initial slope failure at the volcanic center and which is inferred to be triggered by a volcanic eruption.E stimates of the vertical descent height and the run-out length of the volcanic debris flow indicate that it may have the longest run-out length and is one of the most mobile volcanic debris flows derived from slope failures.The uplift of the Pamir-West Kunlun generated a long valley and provided a critical condition for the long-run-out and high mobility volcanic debris flow.Constrained by the age of volcaniclastic rocks,we are able to reconstruct a magnetostratigraphy for the Cenozoic sequence in the West Kunlun foreland basin based on new measurements from Aertashi and published data from other sections,which provides age constraints for several critical geological events.From?35.5 to 33 Ma,significant tectonic activity occurred in the Pamir-West Kunlun,resulting in the major hiatus in lithostratigraphy,increase in grain size of Oligocene sequence of the southwestern Tarim Basin and the deposition in the centre Tarim Basin.At-27-26 Ma,another tectonic activity occurred,which leaded to the fast northward indentation of the Pamir and the birth of the Taklimakan Desert.We suggest that the Taklimakan was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts,both of which are closely linked to the tectonically driven surface uplift of the Tibetan-Pamir Plateau,which had reached a climatically sensitive threshold at this time.Since the Oligocene-Miocene boundary,the intensed tectonic uplift,the global cooling and the retreat of the Paratethys may together have played an important role in further aridification of inland Asia,represented by enlargement of the Taklimakan Desert and the wide occurrence and increase of dust accumulation in the Chinese Loess Plateau and the Pacific Ocean.The abrupt increases in grain size and the significant upward decreases in stratal dip angle of the widely distributed Xiyu Comglomerate at?15 Ma likely indicate the beginning of a phase of increased compressional deformation caused by the propagation of the Tibetan Plateau,marking the formation of landform structure similar to present.At?11 Ma,the relative motion between the Pamir and the Tarim Basin may have terminated,as the northward velocity of the Tarim increased to the same level with the North Pamir.