Paleo-erosion Rates In Qilian Mountain Since5Ma Derived From Cosmogenic¹⁰Be

The research on paleo-erosion rates of Cenozoic,as a key to understand the relationship of tectonic, surface process and climate, has long been a hotspot in geo-science. Spatial controls on erosion of different mountainous settings has been basically reached a consensus view that erosion rate is closely associated with topography, steepness of hillslopes, rainfall and lithology. However, Due to limitation of measurement accuracy and lack of better natural record,it is hard to get erosion record on long timescares. While some studies suggest that worldwide erosion rates increased at the Plio-Pleistocene climatic transition, the validity of this observation and its significance is a matter of debate due to potential biases of the sedimentary record,which indicates a long-term stability of global erosion rates.There are three main techniques of quantitative research in a long time-scale erosion rate:(Ⅰ) Mineral closure temperature thermochronology;(Ⅱ)Fission track thermochronology;(Ⅲ)cosmogenic isotope analysis.we can get an average erosion rate from a few to tens of million years by the first two methods. By the third method, however, we can get an average erosion rate from tens of thousands to hundreds of thousands of years. Importantly, the cosmogenic nuclide concentration measured in river sands has the remarkable property of averaging the concentration of all surficial rocks outcropping in the drainage basin. It is thus possible to determine the average erosion rate for a whole drainage basin by measuring the mean cosmogenic nuclide concentration in river sand. Fundamentally, for samples of old fluvial sediment, if we know the deposition age, we can do radioactive decay correction, then we should be able to learn the erosion rate at the time of deposition.Radionuclide accumulation in the ancient river sediments by accelerator measurement consists of three components:Accumulation in the exposure process, this part associated with the erosion rate; Accumulation during burial until overlying sediments to be thick enough to shadow cosmic rays, this part associated with the accumulation rate; Accumulation after recent exposure, this part associated with the recent exposure history of the profile. Therefore, if the latter two components can be deducted,we can get the erosion rate. Based on this theory, we analyzed samples for10Be to determine paleo-erosion rates since about5Ma from the Laojunmiao anticline profile which belong to the Jiuquan Basin and at northeast of Qilian Mountain.We focus on an intracontinental endorheic watershed draining the northeast Qinghai-Tibet Plateau in Central Asia.A good natural record was found at Laojunmiao anticline,which is part of Jiuquan Basin. In the present study, we estimate erosion rates over the last～5Ma using in situ produced cosmogenic10Be concentrations measured in magnetostratigraphically dated continental sediments. The Yumen Conglomerate is divided into three parts by two unconformities. the Lower Yumen Conglomerate spans the period of4.92～3.11Ma. The Middle Yumen Conglomerate spans the period of3.11～1.25Ma. The Upper Yumen Conglomerate spans the period of1.25～0.84Ma.While Miocene to Quaternary stratas of the Laojunmiao anticline profile exposed perfectly due to deeply cut by Shiyou river, stratas from3.11to1.25Ma lost due to unconformities. Samples were collected from The Yumen Conglomerate, and Jiuquan gravel layer,which has previously been dated by magnetostratigraphy.So accumulation during burial can be acquired after accounting for accumulation rate. We collected the samples from caves in the steep bank that Shaped by the river’ s lateral erosion.Therefore, accumulation after recent exposure can be ignored.While erosion rates between10and300m/Ma are derived from most of our record, they spans the values of180～3000m/Ma from0.8to1.2Ma. When we assume that the elevation has remained the same as present, which is about3600m in average, they spans the values of60-285m/Ma from0to2Ma and4-300m/Ma from3to5Ma,respectively. Meanwhile the rising elevation model, according the paleo-elevation estimation by the work of Fu et al,shows that they spans the values of55-230m/Ma from0to2Ma and30-112m/Ma from3to5Ma,respectively. Modern erosion rates derived from sands and gravels are85m/Ma and288m/Ma,respectively.They present cycle characteristics from0to2Ma,expecially between0.5and0.8Ma.Erosion rates by monitoring data from hydrological observation station are during20to200m/Ma and the mean value of QiLian Mountain is80m/Ma. Hetzel,et al, obtained a catchment-wide denudation rates derived from cosmogenic10Be in Yumu and Longshou mountain ranges,which both at the margin of NE Tibet,and their denudation rates are317±100m/Ma and155±25m/Ma,respectively.They also obtained mean denudation rates that is175±33m/Ma at the western and central Qilian Shan catchments. On the whole, we obtained a similar modern erosion rate in Qilian Mountain.The erosion rate keeps steady and is similar to the modern erosion rate on the whole. Obviously, there is a short-term low radionuclide concentration from0.8to1.2Ma,which shows a clear high erosion temporary increase.There may be three explanations for the spike of erosion rates:(ⅰ) It may be related to tectonic activity (Kun Huang movement), but it is difficult to explain with a single tectonic event of a substantial change of the erosion rate;(ⅱ) It may be correlated with The Middle Pleistocene Transition;(ⅲ) It may be correlated with the authenticity of the10Be concentration signal. Linking the formation missing from3.11to1.25Ma and the piggyback basins lcated in the south of the anticline, we can propose the following hypothesis:when the anticline rapid uplift at about3.11Ma, the river did not immediately cut through the anticline, and thus nearly a hundred years, erosion material from Qilian mountain temporarily accumulated in the piggyback basin. Until about1.25Ma, the anticline was re-cut through. Therefore, the peak samples may be mixed with re-handling material from the piggyback basin and cause the "dilution effect".In addition, these samples are in the growth wedge, so this transient signal may be also caused by the "dilution effect" owing to the erosion of the anticline itself when the anticline rapid uplift. In the rising elevation model, erosion rates from0to1.25Ma are slightly larger than from3to5Ma, This may be due to the samples mixed with older constituents cycle characteristics from0to1.25Ma may be the reaction of glacial-interglacial.