Quantification of the concrete freeze–thaw environment across the Qinghai–Tibet Plateau based on machine learning algorithms

The reasonable quantification of the concrete freezing environment on the Qinghai–Tibet Plateau(QTP) is the primary issue in frost resistant concrete design, which is one of the challenges that the QTP engineering managers should take into account. In this paper, we propose a more realistic method to calculate the number of concrete freeze–thaw cycles(NFTCs) on the QTP. The calculated results show that the NFTCs increase as the altitude of the meteorological station increases with the average NFTCs being 208.7. Four machine learning methods, i.e., the random forest(RF) model, generalized boosting method(GBM), generalized linear model(GLM), and generalized additive model(GAM), are used to fit the NFTCs. The root mean square error(RMSE) values of the RF, GBM, GLM, and GAM are 32.3, 4.3, 247.9, and 161.3, respectively. The R^(2) values of the RF, GBM, GLM, and GAM are 0.93, 0.99, 0.48, and 0.66, respectively. The GBM method performs the best compared to the other three methods, which was shown by the results of RMSE and R^(2) values. The quantitative results from the GBM method indicate that the lowest, medium, and highest NFTC values are distributed in the northern, central, and southern parts of the QTP, respectively. The annual NFTCs in the QTP region are mainly concentrated at 160 and above, and the average NFTCs is 200 across the QTP. Our results can provide scientific guidance and a theoretical basis for the freezing resistance design of concrete in various projects on the QTP.

Spatial-temporal heterogeneity of ecological quality changes across the Qinghai-Tibet Plateau under the influence of climate factors and human activities

Over the last few decades,the ecological quality of the Qinghai–Tibet Plateau(QTP)has significantly changed due to climate warming,humidification,and increasing human activities.Thus,evaluating this region's ecological quality and dominant factors is crucial for sustainable development.In this study,the changes in the ecological quality on the QTP from 2000 to 2020 were evaluated based on aggregated indices and Sen–MK trend analyses,and the dominant factors affecting the ecological quality of the QTP were quantitatively analyzed using decision tree classification.The results revealed that(1)the ecological quality of the QTP exhibited an overall high trend in the east and a low pattern in the west;(2)the ecological quality of the QTP significantly increased from 2000 to 2020,and human activities were the dominant factors causing this change;and(3)the changes in the ecological quality and dominant factors exhibited obvious spatiotemporal heterogeneity.The area with an improved ecological quality occurred mainly in the northern QTP region.It was governed by human activities and precipitation.In contrast,the area with a deteriorated ecological quality occurred largely in the southern QTP region and was dominated by human activities and temperature.The 2000–2010 period was the most significant period of heterogeneity regarding of ecological quality and its driving factors.(4)The change in the ecological quality was mainly affected by the synergistic relationship between human activities and climate change in this region,which encompassed multiple dominant factors.This study provides important information on the spatiotemporal heterogeneity of ecological quality change and its dominant factors on the QTP and offers systematic guidance for the planning and implementation of ecological protection projects.

Contribution of external forcing to summer precipitation trends over the Qinghai-Tibet Plateau and Southwest China

在过去的60年中,全球气候经历了快速变暖和短暂的变暖停滞,而中国的区域降水也经历了多样而复杂的变化.本文分析了1961年至2014年外强迫因子对青藏高原和中国西南地区夏季降水趋势的影响.观测数据显示,青藏高原的夏季降水呈增加趋势,而中国西南地区的夏季降水呈减少趋势,这两个相邻地区的夏季降水变化趋势相反.利用CMIP6数据,本文研究了不同外强迫因子对两个区域夏季降水趋势的影响.结果表明,温室气体对青藏高原夏季降水的增加具有显著影响,而气溶胶在中国西南地区夏季降水减少中起主要作用。

Spatial and temporal change patterns of net primary productivity and its response to climate change in the Qinghai–Tibet Plateau of China from 2000 to 2015

The vegetation ecosystem of the Qinghai–Tibet Plateau in China,considered to be the′′natural laboratory′′of climate change in the world,has undergone profound changes under the stress of global change.Herein,we analyzed and discussed the spatial-temporal change patterns and the driving mechanisms of net primary productivity(NPP)in the Qinghai–Tibet Plateau from 2000 to 2015 based on the gravity center and correlation coefficient models.Subsequently,we quantitatively distinguished the relative effects of climate change(such as precipitation,temperature and evapotranspiration)and human activities(such as grazing and ecological construction)on the NPP changes using scenario analysis and Miami model based on the MOD17A3 and meteorological data.The average annual NPP in the Qinghai–Tibet Plateau showed a decreasing trend from the southeast to the northwest during 2000–2015.With respect to the inter-annual changes,the average annual NPP exhibited a fluctuating upward trend from 2000 to 2015,with a steep increase observed in 2005 and a high fluctuation observed from 2005 to 2015.In the Qinghai–Tibet Plateau,the regions with the increase in NPP(change rate higher than 10%)were mainly concentrated in the Three-River Source Region,the northern Hengduan Mountains,the middle and lower reaches of the Yarlung Zangbo River,and the eastern parts of the North Tibet Plateau,whereas the regions with the decrease in NPP(change rate lower than–10%)were mainly concentrated in the upper reaches of the Yarlung Zangbo River and the Ali Plateau.The gravity center of NPP in the Qinghai–Tibet Plateau has moved southwestward during 2000–2015,indicating that the increment and growth rate of NPP in the southwestern part is greater than those of NPP in the northeastern part.Further,a significant correlation was observed between NPP and climate factors in the Qinghai–Tibet Plateau.The regions exhibiting a significant correlation between NPP and precipitation were mainly located in the central and eastern Qinghai–Tibet Plateau,and the regions exhibiting a significant correlation between NPP and temperature were mainly located in the southern and eastern Qinghai–Tibet Plateau.Furthermore,the relative effects of climate change and human activities on the NPP changes in the Qinghai–Tibet Plateau exhibited significant spatial differences in three types of zones,i.e.,the climate change-dominant zone,the human activity-dominant zone,and the climate change and human activity interaction zone.These research results can provide theoretical and methodological supports to reveal the driving mechanisms of the regional ecosystems to the global change in the Qinghai–Tibet Plateau.

Effects of Freeze–thaw Cycles on Soil Mechanical and Physical Properties in the Qinghai–Tibet Plateau

Extreme freeze-thaw action occurs on the Qinghai-Tibet Plateau due to its unique climate resulting from high elevation and cold temperature.This action causes damage to the surface soil structure, as soil erosion in the Qinghai-Tibet Plateau is dominated by freeze-thaw erosion.In this research,freezing–thawing process of the soil samples collected from the Qinghai–Tibet Plateau was carried out by laboratory experiments to determinate the volume variation of soil as well as physical and mechanical properties, such as porosity, granularity and uniaxial compressive strength, after the soil experiences various freeze–thaw cycles.Results show that cohesion and uniaxial compressive strength decreased as the volume and porosity of the soil increased after experiencing various freeze–thaw cycles, especially in the first six freeze–thaw cycles.Consequently, the physical and mechanical properties of the soil were altered.However, granularity and internal friction angle did not vary significantly with an increase in the freeze–thaw cycle.The structural damage among soil particles due to frozen water expansion was the major cause of changes in soil mechanical behavior in the Qinghai–Tibet Plateau.

Evidence of the Pan-Lake Stage in the Period of 40-28 ka B.P. on the Qinghai-Tibet Plateau

The Qinghai-Tibet plateau is one of major saline lake regions in China, where saline lakes are widespread and constitute an important object of researches on the palaeoclimatic change in the region. On the basis of comprehensive investigations of the evolution of the lake's surface and sediments on the plateau, the authors have further demonstrated the existence of a pan-lake stage (river and lake flooding stage) on the Qinghai-Tibet plateau during the period of about 40+-28 ka B.P. and analyzed the palaeoclimatic characteristics of the pan-lake period and relationships between the ancient monsoons and the uplift of the plateau since the beginning of the Quaternary.

Temporal-Spatial Structure of Intraplate Uplift in the Qinghai-Tibet Plateau

The intraplate uplift of the Qinghai-Tibet Plateau took place on the basis of breakup and assembly of the Precambrian supercontinent, and southward ocean-continent transition of the Proto-, Paleo-, Meso- and Neo-Tethys during the Caledonian, Indosinian, Yanshanian and Early Himalayan movements. The intraplate tectonic evolution of the Qinghai-Tibet Plateau underwent the early stage of intraplate orogeny characterized by migrational tectonic uplift, horizontal movement and geological processes during 180-7 Ma, and the late stage of isostatic mountain building characterized by pulsative rapid uplift, vertical movement and geographical processes since 3.6 Ma. The spatial-temporal evolution of the intraplate orogeny within the Qinghai-Tibet Plateau shows a regular transition from the northern part through the central part to the southern part during 180-120 Ma, 65-35 Ma, and 25-7 Ma respectively, with extensive intraplate faulting, folding, block movement, magmatism and metallogenesis. Simultaneous intraplate orogeny and basin formation resulted from crustal rheological stratification and basin-orogen coupling that was induced by lateral viscous flow in the lower crust. This continental dynamic process was controlled by lateral flow of hot and soft materials within the lower crust because of slab dehydration and melted mantle upwelling above the subducted plates during the southward Tethyan ocean-continent transition processes or asthenosphere diapirism. Intraplate orogeny and basin formation were irrelevant to plate collision. The Qinghai-Tibet Plateau as a whole was actually formed by the isostatic mountain building processes since 3.6 Ma that were characterized by crust-scale vertical movement, and integral rapid uplift of the plateau, accompanied by isostatic subsidence of peripheral basins and depressions, and great changes in topography and environment. A series of pulsative mountain building events, associated with gravity equilibrium and isostatic adjustment of crustal materials, at 3.6 Ma, 2.5 Ma, 1.8-1.2 Ma, 0.9-0.8 Ma and 0.15-0.12 Ma led to the formation of a composite orogenic belt by unifying the originally relatively independent Himalayas, Gangdise, Tanghla, Longmenshan, Kunlun, Altyn Tagh, and Qilian mountains, and the formation of the complete Qinghai-Tibet Plateau with a unified mountain root after Miocene uplift of the plateau as a whole.

Monitoring the impact of climate change and human activities on grassland vegetation dynamics in the northeastern Qinghai-Tibet Plateau of China during 2000–2015

Climate change and human activities can influence vegetation net primary productivity(NPP), a key component of natural ecosystems. The Qinghai-Tibet Plateau of China, in spite of its significant natural and cultural values, is one of the most susceptible regions to climate change and human disturbances in the world. To assess the impact of climate change and human activities on vegetation dynamics in the grassland ecosystems of the northeastern Qinghai-Tibet Plateau, we applied a time-series trend analysis to normalized difference vegetation index(NDVI) datasets from 2000 to 2015 and compared these spatiotemporal variations with trends in climatic variables over the same time period. The constrained ordination approach(redundancy analysis) was used to determine which climatic variables or human-related factors mostly influenced the variation of NDVI. Furthermore, in order to determine whether current conservation measures and programs are effective in ecological protection and reconstruction, we divided the northeastern Qinghai-Tibet Plateau into two parts: the Three-River Headwater conservation area(TRH zone) in the south and the non-conservation area(NTRH zone) in the north. The results indicated an overall(73.32%) increasing trend of vegetation NPP in grasslands throughout the study area. During the period 2000–2015, NDVI in the TRH and NTRH zones increased at the rates of 0.0015/a and 0.0020/a, respectively. Specifically, precipitation accounted for 9.2% of the total variation in NDVI, while temperature accounted for 13.4%. In addition, variation in vegetation NPP of grasslands responded not only to long-and short-term changes in climate, as conceptualized in non-equilibrium theory, but also to the impact of human activities and their associated perturbations. The redundancy analysis successfully separated the relative contributions of climate change and human activities, of which village population and agricultural gross domestic product were the two most important contributors to the NDVI changes, explaining 17.8% and 17.1% of the total variation of NDVI(with the total contribution >30.0%), respectively. The total contribution percentages of climate change and human activities to the NDVI variation were 27.5% and 34.9%, respectively, in the northeastern Qinghai-Tibet Plateau. Finally, our study shows that the grassland restoration in the study area was enhanced by protection measures and programs in the TRH zone, which explained 7.6% of the total variation in NDVI.

Thermal dynamics of the permafrost active layer under increased precipitation at the Qinghai-Tibet Plateau

Precipitation has a significant influence on the hydro-thermal state of the active layer in permafrost regions, which disturbs the surface energy balance, carbon flux, ecosystem, hydrological cycles and landscape processes. To better understand the hydro-thermal dynamics of active layer and the interactions between rainfall and permafrost, we applied the coupled heat and mass transfer model for soil-plant-atmosphere system into high-altitude permafrost regions in this study. Meteorological data, soil temperature, heat flux and moisture content from different depths within the active layer were used to calibrate and validate this model. Thereafter, the precipitation was increased to explore the effect of recent climatic wetting on the thermal state of the active layer. The primary results demonstrate that the variation of active layer thickness under the effect of short-term increased precipitation is not obvious, while soil surface heat flux can show the changing trends of thermal state in active layer, which should not be negligible. An increment in year-round precipitation leads to a cooling effect on active layers in the frozen season, i.e. verifying the insulating effect of "snow cover". However, in the thawed season, the increased precipitation created a heating effect on active layers, i.e. facilitating the degradation of permafrost. The soil thermal dynamic in single precipitation event reveals that the precipitation event seems to cool the active layer, while compared with the results under increased precipitation, climatic wetting trend has a different influence on the permafrost evolution.

Early Permian–Late Triassic Magmatism in the Tuotuohe Region of the Qinghai–Tibet Plateau: Constraints on the Tectonic evolution of the Western Segment of the Jinshajiang Suture

In this paper we present new zircon U-Pb ages,whole-rock major and trace element analyses,and zircon Hf isotopic data for magmatic rocks in the Tuotuohe region of the western segment of the Jinshajiang suture.Our aim is to constrain the Early Permian-Late Triassic tectonic evolution of the region.Zircons from the magmatic rocks of the Tuotuohe region are euhedralsubhedral in shape and display fine-scale oscillatory zoning as well as high Th/U ratios （0.4-4.6）,indicating a magmatic origin.The zircon U-Pb ages obtained using LA-ICP-MS are 281 ± 1 Ma,258 ± 1 Ma,244 ± 1 Ma,and 216 ± 1 Ma,which indicate magmatism in the Early Permian-Late Triassic.A diorite from Bashihubei （BSHN） has SiO2 =57.18-59.97 wt％,Al2O3=15.70-16.53 wt％,and total alkalis （Na2O ＋ K2O） =4.46-6.34 wt％,typical of calc-alkaline and metaluminous series.A gabbro from Bashibadaoban （BSBDB） belongs to the alkaline series,and is poor in SiO2 （45.46-54.03 wt％） but rich in Al2O3 （16.19-17.39 wt％） and total alkalis （Na2O ＋ K2O =5.48-6.26 wt％）.The BSHN diorite and the BSBDB gabbro both display an enrichment of LREEs and LILEs and depletion of HFSEs,and they have no obvious Eu anomaly; they have relatively low MgO contents （2.54-4.93 wt％）,Mg# values of 43 to 52,and low Cr and Ni contents （8.07-33.6 ppm and 4.41-14.2 ppm,respectively）,indicating they differentiated from primitive mantle magmas.They have low Nb/U,Ta/ U,and Ce/Pb ratios （1.3-9.6,0.2-0.8,and 0.1-18.1,respectively）,and their initial Hf isotopic ratios range from ＋9.6 to ＋16.9 （BSHN diorite） and ＋6.5 to ＋12.6 （BSBDB gabbro）,suggesting their primary magmas were derived mainly from the partial melting of a mantle wedge that had been metasomatized by subduction fluids.Taking all the new data together,we conclude that the western and eastern segment of the Jinshajiang suture regions underwent identical processes of evolution in the Early Permian-Late Triassic：oceanic crust subduction before the Early Permian,continental collision during the Early-Middle Triassic,and post-collisional extension from the Late Triassic.

Collision Event during 177-135 Ma on the Eastern Marginof the Qinghai-Tibet Plateau: Evidence from 40Ar/ 39Ar Dating for Basaltson the Western Margin of the Yangtze Platform

Geochronology of continental flood basalts sampled from the Emei large igneous province (LIP) on the western margin of the Yangtze platform was investigated by the laser microprobe 40Ar/39Ar dating technique. These basalts yield a fairly wide range of 40Ar/39Ar ages, varying from 259 to 135 Ma. One basalt sample, at least altered, recorded the oldest 40Ar/39Ar age of about 259 Ma, corresponding to a peak eruption age of the Emei LIP continental flood basalts. Most of the samples yield much younger ages from 135 to 177 Ma, which are consistent with the K-Ar ages for the same samples (122.8-172.1 Ma). The dating data suggest that these Permian basalts had been widely affected by the regional tectonothermal event at 177-135 Ma. The event was probably caused by the convergence and collision among the Laurasia, Yangtze and Qiangtang-Qamdo continental blocks on the eastern margin of the Qinghai-Tibet plateau after the late Triassic. The age of the event reflects the timing of the peak collisional orogeny.

Lithospheric Evolution and Geodynamic Process of the Qinghai-Tibet Plateau: An Inspiration from the Yadong-Golmud-Ejin Geoscience Transect

Abstract The Tibet Geoscience Transect (Yadong-Golmud-Ejin) has revealed the basic structures, tectonic evolution and geodynamic process of the lithosphere of the Qinghai-Tibet plateau. The evidence of northward thrusting of the Indian plate beneath the Himalayans on the southern margin and to southward compression of the Alxa block on the northern margin has been found. They were the driving forces causing the plateau uplift. The plateau is a continent resulting from amalgamation of eight terranes. These terranes are separated by sutures or large-scale faults, and different terranes have different lateral inhomogeneities and multi-layered lithospheric structures. At depths of about 20–30 km of the crust in the interior of the plateau there commonly exists a low-velocity layer. It is an uncoupled layer of the tectonic stress; above the layer, the upper crustal slices were thrust and overlapped each other and the rocks underwent brittle deformation, thus leading to shortening and thickening of the upper crust. Below the layer, the lateral change of the structure of the lower crust varies most greatly and ductile deformation occurs. The lower crust velocity of southern Tibet shows the reversed feature; whereas the lower crust velocity of northern Tibet increases and displays strong gradient variation and the character of the double Moho. On the whole, the Moho of the plateau is greatly undulatory. Although the crust of the Qinghai-Tibet Plateau has a great thickness, the lithosphere does not thicken markedly. The plateau is in a state of bi-directional compression. The unstable change of the Moho, the interaction between the crust and mantle and between the lithosphere and asthenosphere caused by the sinking of the lithospheric mantle and the strike slip and extension of the crust are the major dynamic factors for maintaining the present height and scope of the Qinghai-Tibet Plateau.

Late Cenozoic Chemical Weathering and Environmental Changes Recorded in the Co Ngoin Sediments,Central Qinghai-Tibet Plateau

A series of faulted inland basins were developed in the central Qinghai-Tibet Plateau, among which the Co Ngoin Basin containing thick lacustrine sediments is located in the peripheral area of the Indian monsoon. In this paper, we present the weathering history and paleoclimatic changes in the last 2.8 Ma based on studies of high-resolution temporal distributions of Sr, Rb and Zr concentrations, Rb/Sr and Zr/Rb ratios and δ 13C and TOC for the Co Ngoin sediments, in combination with the sediment properties, grain size distribution and clay mineralogy. The sedimentary records indicate three environmental stages in the last 2.8 Ma. At the core depth of 197?170 m (about 2.8?2.5 Ma), low-intensity chemical weathering in the Co Ngoin catchment was experienced under warm-dry to cool-wet climate conditions with relatively low Sr concentration and high Rb/Sr and Zr/Rb ratios. The sudden occurrence of both subalpine coniferous forest and coarse sand and gravel sediments in the Co Ngoin core reflects a strong tectonic uplift. The high Sr concentrations and low Rb/Sr and Zr/Rb ratios reflect a relatively strong chemical weathering between 2.5 Ma and 0.8 Ma (at the core depth of 170?38.5 m) under a temperate/cool and wet climate, characterized by mud and silt with fine sand, probably indicating a stable process of denudation and planation of the plateau. Above the depth of 38.5 m (about 0.8?0 Ma), the coarsening of sediments indicates a strong tectonic uplift and a relatively low intensity of chemical weathering as supported by the record of sediments having relatively low Sr concentrations and high Rb/Sr and Zr/Rb ratios. Since then, the plateau has taken the shape of the modern topographic pattern above 4000 m a.s.l.

Effect of increasing rainfall on the thermal–moisture dynamics of permafrost active layer in the central Qinghai–Tibet Plateau

In the past several decades,the trend of rainfall have been significantly increasing in the Qinghai–Tibet Plateau,which inevitably leads to a change in the surface energy balance processes and thermal-moisture status of the permafrost active layers.However,the influence of mechanisms and associated effects of increasing rainfall on active layers are still poorly understood.Therefore,in this study,a validated coupled numerical water–vapor–heat model was applied for simulating the surface energy components,liquid and vapor water migration,and energy transfer within the permafrost active layer under the action of increasing rainfallin the case of an especially wet year.The obtained results demonstrate that the surface heat flux decreaseswith the increase in rainfall,and the dominant form of energy exchange between the ground and atmospherebecomes the latent heatflux,which is beneficial for the preservation of permafrost.The increasing rainfall will also cause the migration of liquid and vapor water,and the migration of liquid will be more significant.The liquid and vapor water migrationcaused by the increasing rainfallis also accompanied by energy transfer.With the increase in rainfall,the decrease in total soil heat flux directly leads to a cooling effect on the soil,and then the upper limit of the frozen soil rises,which alleviates the degradation of permafrost.These results provide further insights into engineering structures,regional ecological climate change,hydrology,and environmental issues in permafrost regions.

Sedimentary Evolution of the Qinghai-Tibet Plateau in Cenozoic and its Response to the Uplift of the Plateau

We have studied the evolution of the tectonic lithofacies paleogeography of Paleocene- Eocene, Oligocene, Miocene, and Pliocene of the Qinghai-Tibet Plateau by compiling data regarding the type, tectonic setting, and iithostratigraphic sequence of 98 remnant basins in the plateau area. Our results can be summarized as follows. （1） The Paleocene to Eocene is characterized by uplift and erosion in the Songpan-Garze and Gangdise belts, depression （lakes and pluvial plains） in eastern Tarim, Qaidam, Qiangtang, and Hoh Xil, and the Neo-Tethys Sea in the western and southern Qinghai-Tibet Plateau. （2） The Oligocene is characterized by uplift in the Gangdise--Himalaya and Karakorum regions （marked by the absence of sedimentation）, fluvial transport （originating eastward and flowing westward） in the Brahmaputra region （marked by the deposition of Dazhuka conglomerate）, uplift and erosion in western Kunlun and Songpan-Garze, and depression （lakes） in the Tarim, Qaidam, Qiangtang, and Hoh Xil. The Oligocene is further characterized by depressional littoral and neritic basins in southwestern Tarim, with marine facies deposition ceasing at the end of the Oligocene. （3） For the Miocene, a widespread regional unconformity （ca. 23 Ma） in and adjacent to the plateau indicates comprehensive uplift of the plateau. This period is characterized by depressions （lakes） in the Tarim, Qaidam, Xining-Nanzhou, Qiangtang, and Hoh Xil. Lacustrine facies deposition expanded to peak in and adjacent to the plateau ca. 18-13 Ma, and north-south fault basins formed in southern Tibet ca. 13-10 Ma. All of these features indicate that the plateau uplifted to its peak and began to collapse. （4） Uplift and erosion occurred during the Pliocene in most parts of the plateau, except in the Hoh Xil-Qiangtang, Tarim, and Qaidam. The continuous uplift and intensive taphrogeny in the plateau divided the original large basin into small basins, deposition of lacustrine facies decreased considerably, and boulderstone accumulated, indicating a response to the overall uplift of the plateau. Here, we discuss the evolution of tectonic lithofacies paleogeography in Cenozoic and its response to the tectonic uplift of the Qinghai-Tibet Plateau in relation to the above characteristics. We have recognized five major uplift events, which occurred during 58-53 Ma, 45-30 Ma, 25-20 Ma, 13-7 Ma, and since 5 Ma. The results presented here indicate that the paleogeomorphic configurations of the Qinghai-Tibet Plateau turned over during the late Miocene, with high elevations in the east during the pre-Miocene switching to high contours in the west at the end of Miocene.

Characteristics of daily extreme wind gusts on the Qinghai-Tibet Plateau, China

Severe wind is a major natural hazard and a main driver of deserdficadon on the Qinghai-Tibet Plateau. Generally, studies of Qinghai-Tibet Plateau＇s wind climatology focus on mean wind speeds and its gust speeds have been seldom investigated. Here, we used observed daily maximum gust speeds from a 95- station network over a 5-year period （2008-2012） to analyze the characteristics of extreme wind speeds and directions by fitting Weibull and Gumbel distributions. The results indicated the spatial distribution of extreme wind speeds and their direction on the Qinghai-Tibet Plateau is highly variable, with its western portion prone to greater mean speeds of extreme wind gusts than its eastern portion. Maximum extreme wind speeds of 30.9, 33.0, and 32.2 m/s were recorded at three stations along the Qinghai Tibet Railway. Severe winds occurred mostly from November to April, caused primarily by the westerly jet stream. Terrain greatly enhances the wind speeds. Our spatial analysis of wind speed data showed that the wind speeds increased exponentially with an increasing altitude. We also assessed the local wind hazard by calculating the return periods of maximum wind gusts from the observational data based on the statistical extreme value distributions of these wind speeds. Further attention should be given to those stations where the yearly maximum daily extreme wind speed increased at a rate greater than that of mean value of daily extreme wind speeds. Severe extreme wind events in these regions of the plateau are likely to become more frequent. Consequently, building structural designers working in these areas should use updated extreme wind data rather than relying on past data alone.

Cluster analysis on summer precipitation field over Qinghai-Tibet Plateau from 1961 to 2004

The summer day-by-day precipitation data of 97 meteorological stations on the Qinghai-Tibet Plateau from 1961 to 2004 were selected to analyze the temporal-spatial distribution through accumulated variance,correlation analysis,regression analysis,empirical orthogonal function,power spectrum function and spatial analysis tools of GIS.The result showed that summer precipitation occupied a relatively high proportion in the area with less annual precipitation on the Plateau and the correlation between summer precipitation and annual precipitation was strong.The altitude of these stations and summer precipitation tendency presented stronger positive correlation below 2000 m,with correlation value up to 0.604（α=0.01）.The subtracting tendency values between 1961-1983 and 1984-2004 at five altitude ranges（2000-2500 m,2500-3000 m,3500-4000 m,4000-4500 m and above 4500 m）were above zero and accounted for 71.4%of the total.Using empirical orthogonal function, summer precipitation could be roughly divided into three precipitation pattern fields：the Southeast Plateau Pattern Field,the Northeast Plateau Pattern field and the Three Rivers＇ Headstream Regions Pattern Field.The former two ones had a reverse value from the north to the south and opposite line was along 35°N.The potential cycles of the three pattern fields were 5.33a,21.33a and 2.17a respectively,tested by the confidence probability of 90%.The station altitudes and summer precipitation potential cycles presented strong negative correlation in the stations above 4500 m,with correlation value of-0.626（α=0.01）.In Three Rivers Headstream Regions summer precipitation cycle decreased as the altitude rose in the stations above 3500 m and increased as the altitude rose in those below 3500 m.The empirical orthogonal function analysis in June precipitation,July precipitation and August precipitation showed that the June precipitation pattern field was similar to the July＇s,in which southern Plateau was positive and northern Plateau negative.But positive value area in July precipitation pattern field was obviously less than June＇s.The August pattern field was totally opposite to June＇s and July＇s.The positive area in August pattern field jumped from the southern Plateau to the northern Plateau.

Glacier extent changes and possible causes in the Hala Lake Basin of Qinghai-Tibet Plateau

Glacier is a common sensitivity indicator of environmental and global climate change.Examining the relationship between glacier area and climate change will help reveal glacier change mechanisms and future trends.Glacier changes are also of great significance to the regulation of regional water resources.This study selected the Hala Lake Basin in the northeastern Qinhai-Tibet Plateau as a study area,and examined the relationships between the temporal and spatial change of glaciers in the northeastern Qinghai-Tibet Plateau and climate change based on remote sensing imagery,climatological data,and topographic data during the past 30 years.Results showed that glacier area in the Hala Lake basin fluctuated and decreased from106.24 km2 in 1986 to 78.84 km2 in 2015,with a decreasing rate of 0.94 km2·yr-1.The number of glacier patches,mean patch area,and largest patch index all decreased from 1986 to 2015,while the splitting index increased from 1986 to 2015,indicating that the landscape fragmentation of glacier in the Hala Lake Basin was increasing significantly during the study period.Glacier area change was mainly concentrated in the slopes>25°with an altitude of 4500-5000 m,and the retreating rate of glacier of sunny slope was obviously higher than that of shady slope.Geometric center of glacier in the basin moved from southwest to northeast towards high altitude.Results of the response of glacier extent to climate change showed that temperature was the dominant factor affecting glacier area dynamic change in the Hala Lake Basin.It is predicted that in future several years,the glacier area will decrease and fragment continually as a result of global warming on the Tibetan Plateau.

Late Cenozoic Stratigraphy and Paleomagnetic Chronology of the Zanda Basin,Tibet, and Records of the Uplift of the Qinghai-Tibet Plateau

The characteristics of Late Cenozoic tectonic uplift of the southern margin of the Qinghai- Tibet Plateau may be inferred from fluvio-lacustrine strata in the Zanda basin, Ngari, Tibet. Magnetostratigraphic study shows that the very thick fluvio-lacustrine strata in the basin are 5.89- 0.78 Ma old and that their deposition persisted for 5.11 Ma, i.e. starting at the end of the Miocene and ending at the end of the early Pleistocene, with the Quaternary glacial stage starting in the area no later than 1.58 Ma. Analysis of the sedimentary environment indicates that the Zanda basin on the southern Qinghai-Tibet Plateau began uplift at -5.89 Ma, later than the northern Qinghai-Tibet Plateau. Presence of gravel beds in the Guge and Qangze Formations reflects that strong uplift took place at -5.15 and -2.71 Ma, with the uplift peaking at -2.71 Ma.

Episodes of Cenozoic Gold Mineralization on the Eastern Margin of the Qinghai-Tibet Plateau:40Ar/39Ar Dating and Implication for Geodynamic Events

A lot of new gold deposits have been found on the eastern margin of the Qinghai-Tibet Plateau during the past two decades. Among them, three main types of gold deposits have been recognized, including quartz-vein-type, shear- zone-type and porphyry-type. The former two types of gold deposits are mainly hosted within metamorphic rocks, while the latter is related to Cenozoic magmatism. Although all of these gold deposits are believed to have been formed during the uplift process of the Qinghai-Tibet Plateau in the Cenozoic era (Wang et al., 2002b), precise isotopic age constraints have still been lacking until quite recently. This paper presents new 40Ar/39Ar data of some gold deposits on the eastern margin of the Qinghai-Tibet Plateau, which indicate that gold mineralization in the region occurred in response to the episodic stages of the orogenies. Recently obtained 40Ar/39Ar data on quartz and feldspars from several gold deposits, such as the Sandiao deposit, the Baijintaizi deposit, the Pusagang deposits, provide new constraints on gold mineralization on the eastern margin of the Qinghai-Tibet Plateau. Geochronological studies of gold deposits along the Daduhe River indicate that there are three stages of gold mineralization. The early two stages occurred as early as 65.1 Ma in the Shuibaiyang deposit and 58.95 Ma in the Ruoji deposit, while the latter stage occurred as late as 25.35 Ma in Baijintaizi and 24.70 Ma in Sandiao. Isotopic dating of three plagioclases from the Beiya deposit, Zhifanggou deposit and Luobodi deposit and a K-feldspar from the Jinchangqing deposit in Yunnan Province indicates that these deposits were formed at two stages. The Zhifanggou and Jinchangqing deposits have early stage records as old as 58.82 Ma in Zhifanggou and 55.49 Ma in Jinchangqing, but all of the above four deposits in Yunnan have late stage records of 23.18 Ma in Jinchangqing, 24.54 Ma in Zhifanggou, 24.60 Ma in Luobodi and 24.56 Ma in Hongnitang. The above results suggest that the gold deposits on the eastern margin of the Qinghai-Tibet Plateau were formed concentratedly at two main episodes, i.e. the end of the Paleocene (about 58 Ma) and the boundary between the Paleogene and the Neogene (about 25 Ma). The later episode appears to be looks like more important and was coupled with the Sichuan movement, which was extensively activated at that period. The beginning of the Cenozoic Era (about 65 Ma) might be another episode of gold mineralization, but only one deposit (Shuibaiyang) in this study has been proved to have been be formed at this stage and might be earlier than the initial collision between the Indian Plate and the Eurasia Plate. In view of geology, the above three episodes of gold mineralization are associated with three events of tectonic- magmatism and/or fluid events. Even though the gold deposits (for example, the Shuibaiyang deposit, Ruoji deposit and Pusagang deposit) were formed at different episodes, all of them are genetically related to tectonic movements in large- scale shear zones. It looks like theat tectonic events (including large-scale strike-slip) between Paleogene and Neogene had a wide influence upon gold mineralization, with new deposits formed and old deposits enriched or superimposed to be a higher grade by new stage of mineralization. The above data suggest that gold deposits were not only concentrated in some areas, but also formed mainly at different boundaries of geological times, indicating that there existed some peak stages of gold mineralization (metallogenic episodes), and that the gold deposits were formed mainly by episodic mineralization.