Constraints On The India-Eurasia Collision And Early Tectonic Evolution Of The Himalayan Orogen On The Basis Of Provenance Change In Nepalese Upper Cretaceous-miocene Strata

The uplift of the Tibetan Plateau is a major event during the Cenozoic geological history,which records the processes of oceanic subduction and continental collision as well as the tectonic deformation law of lithosphere and crust.It also shapes Asian topography,controls the birth and development of large rivers in East Asia,and changes atmospheric circulation model,which have profound effects on Asian and global climate,marine geochemistry and animal migration and evolution.Therefore,the uplift of the Tibetan Plateau and its environmental effects have long been the focus of the international Geoscience.However,there are some different opinions on the uplift processes and mechanisms of the Tibetan Plateau,which hinders our understanding of the above major scientific questions.Knowledge of the timing and pattern of the IndiaEurasia collision and Cenozoic tectonic evolution of the Himalayan orogen are critical to studying the tectonic uplift processes and mechanisms of the Tibetan Plateau.Therefore,more systematic work in key regions is needed to obtain more evidence to test current opinions or propose a new point.Basin sediments are regarded as important records to study the evolution of orogenic belt,because they are the products of weathering and erosion of rocks in orogenic belt and thus document tectonic process of surrounding mountains in succession.The timing and pattern of the India-Eurasia collision as well as uplift and erosion history of the Himalayan orogen can be constrained by tracking and identifying sources of sedimentary rocks in adjacent basins of the Himalayan orogen.For this reason,we target the Upper Cretaceous-Miocene strata which are exposed in Nepalese Lesser Himalaya and analyze the paleocurrents,sedimentary environment and clastic components of sandstones in two representative parallel sections.We also measure detrital zircon U-Pb ages,in-situ Hf isotope of zircons and whole-rock Sr-Nd isotope of samples collected from different horizons.Combined our provenance analysis with previous research results,we discuss the timing and pattern of India-Eurasia collision as well as tectonic and uplift history of the Himalayan orogen during the early stage.The main insights are as follows:1.Upper Cretaceous-Paleocene Taltung and Amile formations were deposited in a fluvial environment in the Indian continental margin.Detrital zircon U-Pb ages show one main cluster at 1600-2000 Ma with two subsidiary clusters at 2300-2600 and 100-130 Ma.Cretaceous zircon U-Pb-Lu-Hf values are plotted in the field of an Indianaffinity source.Whole-rock Sr-Nd isotope indicates both the Lesser Himalaya and Cretaceous zircon provenance as sources for the two formations.Combined with northwestward paleocurrents,we suggest that the Lesser Himalaya,Indian continent,Rajmahal and Aulis volcanics are the sources for the Taltung and Amile formations.2.Lower-middle Eocene Bhainskati Formation was deposited in a shallow marine environment.Detrital zircon U-Pb ages are distinct from those in Taltung and Amile formations,and contain three main clusters at 700-1200,1600-2000 and 2300-2600 Ma and two subsidiary clusters at 100-230 and 500-600 Ma.One Paleocene(57 Ma)zircon is also founded.Detrital zircon U-Pb ages and whole-rock Sr-Nd isotope indicate both the Tethyan Himalaya and Mesozoic zircon provenance as sources for the Bhainskati Formations.U-Pb-Lu-Hf values of Triassic-Paleocene zircons indicate the Lhasa terrane as one of sources for the Bhainskati Formation.Combined with southward paleocurrents,we suggest that the Tethyan Himalaya,Lhasa terrane and ophiolite within the Indus-Yarlung suture zone are the sources for the Bhainskati Formation.3.Lower-middle Miocene Dumri Formation was deposited in a deltaic-fluvial environment.Sandstone components are dominated by quartz and lithic fragments.Detrital zircon U-Pb ages are similar to those in the Bhainskati Formation except that500-600 and 700-1200 Ma components increase and 1600-2000 and 2300-2600 Ma components decrease,especially the Mesozoic zircons nearly disappear.Combined with the whore-rock Sr-Nd isotope and southward paleocurrents,we suggest that both the Greater Himalaya and Tethyan Himalaya are sources for the Dumri Formation.4.The significant provenance transition from the Amile Formation to the Bhainskati Formation indicates that the timing of India-Eurasia collision was no later than the early-middle Eocene in the middle part of the Himalayan orogen,which not only supports the India-Eurasia continent-continent collision model,but also favors no obvious diachroneity from west to east for the India-Eurasia collision when combining previous studies on the India-Eurasia collision along the Himalayan orogen.5.Since the India-Eurasia collision,the Tethyan Himalaya,Greater Himalaya and Lesser Himalaya have successively uplifted in the Eocene,early Miocene and middlelate Miocene,and the uplift of the Himalayan orogen was dominated by in-sequence forelandward expansion.