Basin–scale Denudation Rates In The Tibetan Plateau Revealed By Cosmogenic Nuclides

The feedbacks between tectonics,climate change and landform evolution is the key to understanding the relationship between landform formation and evolution,climate change and global carbon cycle,and is also the frontier core scientific issue of the relationship between surface Earth system dynamics and global change.The Tibetan Plateau is the highest and largest orogenic plateau in the world.It is also one of the regions where the Indian plate has been continuously subducted and collided with the Eurasian plate since the Cenozoic,resulting in the strongest tectonic uplift and the most significant geomorphological evolution.In the late 1980s,with the improvement of accelerator mass spectrometry analysis technology and the continuous improvement of theoretical models of cosmogenic nuclides,the application of in-situ cosmogenic nuclides in the quantitative study of surface denudation rates provided an important means for the development of geomorphology.The radionuclides ¹⁰Be and ²⁶Al have been widely used to solve scientific problems such as geomorphology and river changes since the Quaternary because of their half-lives on the scale of millions of years.Under steady-state denudation,the denudation rate of the basin can be obtained from the content of ¹⁰Be and ²⁶Al in the sediment,which provides key information for understanding the variation of denudation rate on different time scales.This paper focuses on the southern margin,southeastern margin and eastern margin of the Tibetan Plateau,and collects 82 sediment samples from these regions.Using high-energy accelerator mass spectrometry technology,the contents of cosmogenic nuclides ¹⁰Be and ²⁶Al in the sediments are analyzed.The basin-scale denudation rate was calculated under the assumptions of the steady-state denudation model.By comparing the differences of denudation rate under different tectonic,geomorphic and climatic backgrounds in the Tibetan Plateau,the main controlling factors of the surface denudation process were clarified.At the same time,¹⁰Be-²⁶Al dual nuclides were firstly used in the Tibetan Plateau to reasonably evaluate the unsteady-state denudation processes,and reconstructed the burial history and the paleo-denudation rate of the sediments before burial.In addition,the author summarizes and analyzes the denudation rates of more than a thousand existing ¹⁰Be basins in the Tibetan Plateau and its surrounding areas,and discusses the long-term geomorphological evolution model and trend of the Plateau based on geomorphological evidence.The main research conclusions are as follows:（1）The content of ¹⁰Be and ²⁶Al in river sediment samples in the Tibetan Plateau are（（0.97±0.15）～（365.50±11.00））×10⁴ atoms g^-1 and（（0.74±0.07）～（287.00±4.79））×10⁵ atoms g^-1,respectively.The denudation rates ranged from 11±1 to3490±612 m Myr^-1,with an average of 278 m Myr^-1.Spatially,the denudation rate in the Namche Barwa area is the highest(78±7～3490±612 m Myr^-1,with an average value of 682 m Myr^-1),and the highest value is located in the Layue Qu,a tributary of Parlung Tsangpo.The eastern region on the plateau is second(47±9～476±54 m Myr^-1,mean=198 m Myr^-1),and the basins are mostly distributed in the upper reaches of the Dadu River and the Minjiang River.The denudation rate in the Three River Region is relatively small(11±1～224±25 m Myr^-1,mean=69 m Myr^-1).The middle and upper reaches of the Yarlung Tsangpo had the lowest denudation rate(21±2～154±14 m Myr^-1,mean=63 m Myr^-1).（2）The variation of ²⁶Al/¹⁰Be ratio ranges from（1.37±0.24）to（8.27±0.36）.The correlation between ¹⁰Be and ²⁶Al/¹⁰Be shows that most basins are in a steady-state of denudation.While some basins are in an unsteady-state of denudation.Among them,ratios of 11 sediment were lower than the steady-state denudation island,indicating that the samples experienced a long period of burial.The comparison of the single nuclide¹⁰Be denudation rate and the paleo-denudation rate calculated by the simple denudation burial model shows that the former is at least 32%higher than the paleo-denudation rate,and this difference can be up to 520%.The sediments with the largest deviation are located in the Namche Barwa area.While in the TRR,it may be that precipitation has promoted the transportation of landslides or debris flow materials,resulting in the deviation of the basins from steady-state denudation.The ²⁶Al/¹⁰Be ratios of 9sediments are higher than those of the steady-state denudation island,indicating that these sediments may have been mixed with material from deeper sources.The ¹⁰Be denudation rate of sediments above the continuous exposure line should be regarded as the upper limit of denudation rate for this baisn.The combined use of ²⁶Al and ¹⁰Be is an effective indicator to identify the state of denudation,sediment recycling in the basin,or extreme denudation events such as landslides and debris flows.（3）The controlling factors of denudation rate in different regions of the Tibetan Plateau are obviously different.In the Yarlung Tsangpo River basin,with the Jiacha Knick zone as the boundary,geomorphological and climatic factors play an equally important role in controlling the surface denudation process in the upper basin.From the knick zone to the Nyang River basin,tectonic uplift is the main factor controlling the denudation rate.In the surrounding area of Namche Barwa,the unsteady-state denudation process is the main cause for the decoupling of denudation rate from tectonic,topographic and climatic factors.Under steady-state denudation conditions,the average basin slope and annual precipitation are the main factors controlling the denudation rate.In the TRR,however,only precipitation and denudation rate are significantly correlated.In the eastern part of the plateau,there is a strong correlation between glacier coverage and denudation rate.Combined with ¹⁰Be denudation rates and their structural,topographic and climatic parameters,the results show that the ¹⁰Be denudation rate on the Plateau has a significant correlation with the distance from the Main Boundary Thrust zone.The area closer to the fault zone has a larger denudation rate than that farther from the fault zone,indicating that the activity of the fault zone has played an important role in controlling the large-scale surface denudation process of the Tibetan Plateau.（4）Under steady-state denudation conditions,the shortest exposure age of ¹⁰Be denudation rate in this study is～170 yr.By comparing ¹⁰Be denudation rates with modern hydrological sediment flux records and million-year-scale U/Th-He denudation rates,it shows that the ¹⁰Be denudation rates is basically consistent with the modern hydrological sediment flux records,indicating that the denudation rate has been relatively stable for the past 1 Myr.The increase in the denudation rate since the Pliocene may be closely related to the global cooling event during this period.The comparison results of the ¹⁰Be denudation rates and the distribution characteristics of the channelχ’value show that the denudation rate in the tectonic active area around the plateau is～3–112 times that of the inner plateau area.The southern,eastern and western parts of the Plateau are affected by strong tectonic activities and climate,and denudation rates in these regions are higher than those in the interior and northern parts of the Plateau.The analysis of the distribution characteristics of theχ’value shows that the landforms in the interior area of the Plateau are relatively stable,and the landforms and slopes will not change significantly with time.The Plateau border areas（such as the southern slopes of the Himalayas）are suffering from intense headward erosion and incision by numerous rivers.