Advances in Grassland Ecosystem Research in Northern Tibetan Plateau

With the aggravation of global change, the response and adaptation of the unique ecosystem in Qinghai-Tibet Plateau to global change have been increasingly concerned by scientific community day by day, which makes the sensitivity and fragility of this ecosystem in response to global change widely recognized by scholars. On the basis of introducing the present research process on the degenerate mechanisim, measures of and approaches to recovery, carbon cycle and primary productivity toward global change, we put forward several propositions on studying the alpine grassland ecosystem in Northern Tibetan Plateau.

Early Cenozoic Mega Thrusting in the Qiangtang Block of the Northern Tibetan Plateau

Recent mapping and seismic survey reveal that intensive compression during the Early Cenozoic in the Qiangtang block of the central Tibetan Plateau formed an extensive complex of thrust sheets that moved relatively southward along several generally north-dipping great thrust systems. Those at the borders of the ~450 km wide block show it overrides the Lhasa block to the south and is overridden by the Hohxil-Bayanhar block to the north. The systems are mostly thin-skinned imbricate thrusts with associated folding. The thrust sheets are chiefly floored by Jurassic limestone that apparently slid over Triassic sandstone and shale, which is locally included, and ramped upward and over Paleocene-Eocene red-beds. Some central thrusts scooped deeper and carried up Paleozoic metamorphic rock, Permian carbonate and granite to form a central uplift that divides the Qiangtang block into two parts. These systems and their associated structures are unconformably overlain by little deformed Late Eocene-Oligocene volcanic rock or capped by Miocene lake beds. A thrust system in the northern part of the block, as well as one in the northern part of the adjacent Lhasa block, dip to the south and appear to be due to secondary adjustments within the thrust sheets. The relative southward displacement across this Early Cenozoic mega thrust system is in excess of 150 km in the Qiangtang block, and the average southward slip-rate of the southern Qiangtang thrusts ranged from 5.6 mm to 7.4 mm/a during the Late Eocene-Oligocene. This Early Cenozoic thrusting ended before the Early Miocene and was followed by Late Cenozoic crustal extension and strike-slip faulting within the Qiangtang block. The revelation and understanding of these thrust systems are very important for the evaluation of the petroleum resources of the region.

Carbon and oxygen isotopic constraints on paleoclimate and paleoelevation of the southwestern Qaidam basin, northern Tibetan Plateau

We investigate the growth of the northern Tibetan Plateau and associated climate change by applying oxygen and carbon isotopic compositions in Cenozoic strata in the southwestern Qaidam basin. The X-ray diffraction and isotopic studies reveal that the carbonate minerals are mainly authigenic and they do not preserve any evidence for detrital carbonate and diagenesis. The isotope data show large fluctuations in the δ^(18)O and δ^(13)C values in the middle-late Eocene, indicating relatively warm and seasonal dry climate.The positive correlation of the δ^(18)O and δ^(13)C values in the Oligocene and the positive shift of the δ^(13)C values from the Eocene to Oligocene suggest that the climate changed to arid in the Oligocene. However,the δ^(18) values show negative shift, which is closely related to the global cooling event. During the Miocene, the δ^(13)C values vary between-2‰ and-4‰, whereas the δ^(18)O values show continuous negative shift. The mean δ^(18) values decrease from-8.5‰, in the early Miocene to-10.0‰, in the late Miocene. The stable isotope-based paleoaltimetry results suggest that the elevation of the southwestern Qaidam basin was approximately 1500 m in the middle-late Eocene and Oligocene. Subsequently, during Miocene the crustal uplift process started and the elevation reached approximately 2000 m in the early Miocene and 2500 m in the late Miocene, which suggests large-scale growth of the northern Tibet Plateau during the Miocene.

Alpine grassland fPAR change over the Northern Tibetan Plateau from 2002 to 2011

In this study, two different methods including Digital Camera and Reference Panel （DCRP） and traditional in situ fPAR observation for measuring the in situ point fPAR of very short alpine grass vegetation were compared, and the Moderate Resolution Imaging Spectroradiometer （MODIS） fPAR products were evaluated and validated by in situ point data on the alpine grassland over the Northern Tibetan Plateau, which is sensitive to climate change and vulnerable to anthropogenic activities. Results showed that the MODIS alpine grassland fPAR product, examined by using DCRP, and traditional in situ fPAR observation had a significant relationship at the spatial and temporal scales. The decadal MODIS fPAR trend analysis showed that, average growing season fPAR increased by 1.2 × 10^-4 per year and in total increased 0.86% from 2002 to 2011 in alpine grassland, when most of the fPAR increments occurred in southeast and center of the Northern Tibetan Plateau, the alpine grassland tended to recover from degradation slightly. However, climatic factors have influenced the various alpine grassland vegetation fPAR over a period of 10 years; precipitation significantly affected the alpine meadow fPAR in the eastern region, whereas temperature considerably influenced the alpine desert steppe fPAR in the west region. These findings suggest that the regional heterogeneity in alpine grassland fPAR results from various environmental factors, except for vegetation characteristics, such as canopy structure and leaf area.

An off-line simulation of land surface processes over the northern Tibetan Plateau

In order to further understand the land surface processes over the northern Tibetan Plateau, this study produced an off-line simulated examination at the Bujiao site on the northern Tibetan Plateau from June 2002 to April 2004, using the Noah Land Surface Model （Noah LSM） and observed data from the CAMP/Tibet experiment. The observed data were neces- sarily corrected and the number of soil layers in the Noah LSM was changed from 4 to 10 to enable this off-line simulation and analysis. The main conclusions are as follows： the Noah LSM performed well on the northern Tibetan Plateau. The simulated net radiation, upward longwave radiation, and upward shortwave radiation demonstrated the same remarkable annual and seasonal variation as the observed data, especially the upward longwave radiation. The simulated soil temperatures were acceptably close to the observed temperatures, especially in the shallow soil layers. The simulated freezing and melting processes were shown to start from the surface soil layer and spread down to the deep soil layers, but they took longer than the observed processes. However, Noah LSM did not adequately simulate the soil moisture. Therefore, additional high-quality, long-term observations of land surface-atmosphere processes over the Tibetan Plateau will be a key factor in proper adiustments of the model parameters in the future.

Air Temperature Estimation with MODIS Data over the Northern Tibetan Plateau

Time series of MODIS land surface temperature(Ts) and normalized difference vegetation index(NDVI) products,combined with digital elevation model(DEM) and meteorological data from 2001 to 2012,were used to map the spatial distribution of monthly mean air temperature over the Northern Tibetan Plateau(NTP). A time series analysis and a regression analysis of monthly mean land surface temperature(Ts) and air temperature(Ta) were conducted using ordinary linear regression(OLR) and geographical weighted regression(GWR). The analyses showed that GWR,which considers MODIS Ts,NDVI and elevation as independent variables,yielded much better results [RAdj2> 0.79; root-mean-square error(RMSE) =0.51℃–1.12℃] associated with estimating Tacompared to those from OLR(RAdj2= 0.40-0.78; RMSE = 1.60℃–4.38℃).In addition,some characteristics of the spatial distribution of monthly Taand the difference between the surface and air temperature(Td) are as follows. According to the analysis of the 0℃ and 10℃ isothermals,Tavalues over the NTP at elevations of 4000–5000 m were greater than 10℃ in the summer(from May to October),and Tavalues at an elevation of3200 m dropped below 0℃ in the winter(from November to April). Taexhibited an increasing trend from northwest to southeast. Except in the southeastern area of the NTP,T d values in other areas were all larger than 0℃ in the winter.

Lithospheric Electrical Structure across the Eastern Segment of the Altyn Tagh Fault on the Northern Margin of the Tibetan Plateau

Project INDEPTH （InterNational DEep Profiling of Tibet and the Himalaya） is an interdisciplinary program designed to develop a better understanding of deep structures and mechanics of the Tibetan Plateau. As a component of magnetoteUuric （MT） work in the 4th phase of the project, MT data were collected along a profile that crosses the eastern segment of the Altyn Tagh fault on the northern margin of the plateau. Time series data processing used robust algorithms to give high quality responses. Dimensionality analysis showed that 2D approach is only valid for the northern section of the profile. Consequently, 2D inversions were only conducted for the northern section, and 3D inversions were conducted on MT data from the whole profile. From the 2D inversion model, the eastern segment of the Altyn Tagh fault only appears as a crustal structure, which suggests accommodation of strike slip motion along the Altyn Tagh fault by thrusting within the Qilian block. A large-scale off-proffie conductor within the mid-lower crust of the Qilian block was revealed from the 3D inversion model, which is probably correlated with the North Qaidam thrust belt. Furthermore, the unconnected conductors from the 3D inversion model indicate that deformations in the study area are generally localized.

The Beila Ophiolite from the Bangong-Nujiang Suture Zone, Northern Tibetan Plateau

The Beila ophiolite is located in the middle part of the Bangong-Nujiang suture zone,northern Tibetan plateau.It is a complete ophiolite suite,and plays a key role in understanding the evolution of the Bangong-Nujiang suture zone,as well as the Meso-Tethys Ocean.The Beila ophiolite was composed of peridotite,serpentinite,gabbro,pillow basalt,and minor rodingite.Peridotites comprisemainlymedium–tocoarse–grained serpentinized harzburgites and minor plagioclase-bearing lherzolites and dunites.There are some felsic-ultramafic dykes within the peridotite and they are mainlypegmatoidal pyroxenites,coarse to fine-grained gabbros,and diabases.Gabbros included isotropic and cumulate gabbros,and they commonly contain minor pegmatoidal gabbros veins.Pillow basalts and basaltic andesites overlaid on the margin of the serpentinized peridotites.Rodingite occurs as lenses and/or dykes within the host serpentinized peridotites.Zircon SHRIMP U–Pb dating for two rodingite samples yielded the ages ranging from172 to 164 Ma.Whole-rock geochemical and zircon Hf isotopic data show that the Beila ophiolite shows SSZ-type ophiolite affinity.Finally,we suggest that the Beila ophiolite was generated in an initial subduction process at the middle Jurassic(164–172 Ma).

Study on potential evapotranspiration and wet-dry condition in the seasonal frozen soil region of northern Tibetan Plateau

This study was based on the CEOP/CAMP-Tibet observed data at AWS （Automatic Weather Station） of MS3478 in the seasonal frozen soil region of northern Tibetan Plateau from March 2007 to February 2008. The variation characteristics of PE （potential evapotransph＇ation） were analyzed based on the Penman-Monteith method recommended by FAO （the Food and Agriculture Organization of the United Na- lions）. The contributions of dynamic, thermal and water factors to PE were discussed, and the wet-dry condition of the plateau region was further studied. The results indicated that daily PE was between 0.52 mm and 6.46 mm for the whole year. Monthly PE was over 107 mm from May to September, but decreased to less than 41 mm from November to February. Annual PE was 1,037.8mm. In the summer, thermal PE was significantly more than dynamic PE, but conversely in the winter. Annual variation of thermal PE was of sine wave pattern. In addition, drought and semi-drought climate lasted for a long time while semi-humid climate was short. The effect of water and dynamic factors on PE varied considerably with the seasons. Annual variation of thermal PE was of sine wave pattern.

Deep structure of the Kumkol basin in the northern Tibetan Plateau and its resource environmental implications

The Kumkol basin is located in the northern Tibetan Plateau and is a closed plateau basin with an average altitude of>4000 m and an area of nearly 20000 km^(2). Its boundaries are limited by the Altyn Tagh fault, East Kunlun orogen and Qimantag orogen. Studying the deep structure of the Kumkol basin reveals 2 significant implications:(1) the basin has developed a large thickness of >7000 m Cenozoic continental sediments, recording the uplift history of the northern Tibetan Plateau, and(2)preliminary work indicates that the basin is likely to have oil and gas prospects. However, owing to the adverse natural conditions of the area and the strong tectonic activity in the Cenozoic, the latter of which was not conducive to hydrocarbon preservation,only regional geological mapping and petroleum exploration route surveys have been carried out, and there is no consensus on strata, structure and tectonic evolution. From 2021 to 2022, a deep seismic reflection profile implemented by the Second Tibetan Plateau Scientific Expedition and Research(STEP) project was the first high-resolution geophysical survey across the Kumkol basin. This study uses seismic reflection migration profiles, first-arrival wave tomographic imaging and previous research results to analyze the deep structure of the basin. The final merged model contains many features of tectonic and resource significance:(1) The Kumkol basin is ~90 km wide from north to south, with a basement depth of >9000 m. The main component is the Cenozoic continental deposits, which are divided into two major parts: the southern composite basin and the northern faulted basin. Owing to the later compression, the southern composite basin experienced significant deformation, but most parts still preserved their original sedimentary formations.(2) The structural deformation characteristics of the basin reveal a two-stage tectonic evolution process of the northern Tibetan Plateau in the Cenozoic: from the Oligocene to the Pliocene, the main mechanism was vertical differential uplift and subsidence, and after the Pliocene, it transformed to north-south compression and shortened deformation.(3) The strata, formation time, and source-reservoir-cap conditions of the Kumkol basin are similar to those of the Qaidam basin. If a breakthrough can be achieved, it is expected to expand the production capacity of the oil field in the Qaidam basin with a low-cost investment. Thus, further exploration is recommended.

Transformation and mechanisms of climate wet/dry change on the northern Tibetan Plateau under global warming:A perspective from paleoclimatology

The northern Tibetan Plateau is a climatically sensitive zone influenced by monsoon and westerly winds.In summer,water vapor transport can reach Qinghai Lake and the eastern section of the Qilian Mountains;in winter,westerly winds mainly control the climate.This article compares the wet/dry changes in the region during the mid-Holocene(MH)warm period,the medieval climate anomaly(MCA),the current warm period(CWP),and the future warm period from the perspective of paleoclimate.We found that the MH warm period was mainly affected by the orbit-controlled East Asian summer monsoon,and the region showed warm and humid climate characteristics.The MCA was mainly controlled by solar radiation,and there was a warm and dry phenomenon.The CWP and the future warm period are mainly controlled by the rise in temperature caused by the increase in greenhouse gases,and the climate is becoming more arid.The wet/dry patterns in the CWP and the future warm period in the next century on the northern Tibetan Plateau are similar to those in the MCA.Continued warming will lead to the expansion of the westerly belt and a gradually humid climate.The future wet/dry changes will be more similar to the MH warm period.

Sources of uncertainty in exploring rangeland phenology： A case study in an alpine meadow on the central Tibetan Plateau

Global climate change has been found to substantially influence the phenology of rangeland,especially on the Tibetan Plateau. However, there is considerable controversy about the trends and causes of rangeland phenology owing to different phenological exploration methods and lack of ground validation. Little is known about the uncertainty in the exploration accuracy of vegetation phenology.Therefore, in this study, we selected a typical alpine rangeland near Damxung national meteorological station as a case study on central Tibetan Plateau, and identified several important sources influencing phenology to better understand their effects on phenological exploration. We found man-made land use was not easily distinguished from natural rangelands, and therefore this may confound phenological response to climate change in the rangeland. Change trends of phenology explored by four methods were similar, but ratio threshold method(RTM) was more suitable for exploring vegetation phenology in terms of the beginning of growing season(BGS) and end of growing season(EGS). However, some adjustments are needed when RTM is used in extreme drought years. MODIS NDVI/EVI dataset was most suitable for exploring vegetation phenology of BGS and EGS. The discrimination capacities of vegetation phenology declined with decreasing resolution of remote sensing images from MODIS to GIMMS AVHRR datasets. Additionally, distinct trends of phenological change rates were indicated in different terrain conditions, with advance of growing season in high altitudes but delay of season in lower altitudes. Therefore, it was necessary to eliminate interference of complex terrain and man-made land use to ensure the representativeness of natural vegetation. Moreover, selecting the appropriate method to explore rangelands and fully considering the impact of topography are important to accurately analyze the effects of climate change on vegetation phenology.

EFFECT OF SOIL MOISTURE-ENERGY DISTRIBUTION AND MELTING-FREEZING PROCESSES ON SEASONAL SHIFT IN TIBETAN PLATEAU

As one of the major projects of GAME (GEWEX Asian Monsoon Experiment), the GAME\|Tibet aimed to mainly examine the energy and water cycle in Tibetan Plateau and its effects on Asian monsoon. In this paper, based on the in\|situ high\|resolution observation data of GAME\|Tibet, the soil energy\|moisture distribution and the melting\|freezing progresses and their effects on seasonal shift were preliminarily discussed.The soil energy\|water distribution and freezing\|melting processes varied at different sites in northern part of Tibetan Plateau. The temporal and spatial variation of the soil moisture content is more complex than that of temperature. The soil moisture content increased with depth in certain layers but decreased in other layers. The freezing and melting processes and the temperature distribution were largely influenced by the existence of higher soil moisture content layer. During summer monsoon, the soil moisture at 10cm at all sites is relatively high, but the spatial difference existed. Generally speaking, the shallow layers start to freeze in October and to melt from April at all sites, with about 6 months frozen period. However, the beginning time of freezing\|melting and frozen period varied at different sites.

Food-Chain Model of Grassland Degradation and Its Restoration Process in Northern Tibet Plateau:A Case Study in Nierong County

Based on the model of grassland climate ecological productivity, the process of grassland degradation and its restoration mechanism in northern Tibetan Plateau were discussed by the model of food-chain in which the environmental and human factors were corrected. The results of case study in Nierong County showed that： ① the climate trend of becoming warmer, more droughts and gales were conflicted with the restoration of grassland degradation, even under level of perfect management the climate ecological productivity was declined from 89. 3 kg/m^2 of 1983 to 71.8 kg/m^2 of 2003; ② from 1983 to 2003, the population increased fast, while the variation of livestock on hand was little, and the cost of its maintaining is rapid grassland degradation; ③ on the present condition of overgrazing, the livestock on hand can be maintained on the level of theoretical carrying capacity in 2033 by applying the mechanism of food-chain in grassland ecological system controlled with expected coefficients, so that to realize the policy of determining the quantity of livestock according to grass growth.

柴北缘尕海南山晚志留世—晚泥盆世火山岩浆组合对早古生代造山后伸展时限的约束

柴达木盆地北缘(柴北缘)构造带经历了早古生代的大洋俯冲到大陆俯冲,形成了广为人知的柴北缘超高压变质带。早古生代造山带何时开始垮塌一直存在争论,火山岩和侵入岩作为深地岩石探针能为约束地壳活动提供关键制约。应用锆石LA–ICP–MS U–Pb年代学和Lu–Hf同位素方法对柴北缘东段尕海南山地区出露的牦牛山组火山碎屑岩和侵入其中的花岗岩开展研究。锆石U–Pb年代学结果显示,牦牛山组火山碎屑岩的形成时代约为423 Ma,侵入其中的花岗岩的形成时代为370 Ma,表明火山岩喷发的年龄在晚志留世,后期侵入的花岗岩结晶年龄为晚泥盆世;锆石Lu–Hf同位素结果显示,晚志留世熔结凝灰岩ε_(Hf)(t)值集中在–11.5~–8.3,其两阶段Hf模式年龄集中在1945~2133 Ma,显示火山岩主要源于古老地壳物质熔融;而晚泥盆世侵入的花岗岩的ε_(Hf)(t)值分布在3.9~9.1,其两阶段的Hf模式年龄集中在792~1118 Ma,显示花岗岩主要源于中—新元古代地壳物质的部分熔融。结合对区域地质、岩石学等资料的综合分析认为,晚志留世—早泥盆世时期,大陆深俯冲导致的强烈造山作用造成柴北缘地壳发生明显加厚,加厚的欧龙布鲁克地壳基底发生部分熔融,形成了该时期的火山岩;晚泥盆世时期,加厚地壳的拆沉作用导致软流圈地幔上涌,引发区域地壳伸展,上涌的软流圈物质与地壳相互作用并发生部分熔融作用。因此区域牦牛山组形成时代跨度较大,不能笼统地用牦牛山组代表造山结束的时限,晚泥盆世岩浆岩的出现才预示着柴北缘地区进入显著的地壳伸展状态。

柴北缘骆驼泉剖面新生代地层磁组构特征及其构造意义

青藏高原东北部新生代构造演化对理解高原隆升和变形模式具有重要意义,而目前对于该地区挤压应力方向转变过程仍存在很大争议.本文对柴北缘逆冲带北西部骆驼泉剖面新生代地层开展系统磁组构(本文特指磁化率各项异性)研究,以揭示该地区挤压应力方向的转变特征.系统岩石磁学结果表明,骆驼泉剖面新生代样品中主要磁性矿物是顺磁性组分和赤铁矿以及少量磁铁矿.通过对磁组构特征分析及其与古水流方向对比表明,骆驼泉剖面新生代地层磁组构主要为初始变形磁组构,可用于指示沉积成岩时期的挤压应力方向.磁组构结果揭示,骆驼泉地区挤压应力方向在上干柴沟组下部沉积时期为NNE-SSW向,而上干柴沟组上部和油砂山组沉积时期转变为NE-SW向.结合柴北缘逆冲带已有磁组构结果指出,该地区早期N-S向或NNE-SSW向挤压应力可能与印度—欧亚板块早新生代以来近N-S向碰撞挤压过程有关,指示印度—欧亚碰撞的挤压应力自下干柴沟组下部沉积时期就已传播至高原东北部地区;而后期NE-SW向挤压应力方向与该地区现今GPS揭示的上地壳运动方向一致,可能与该时段高原东北部巨型走滑断裂构造体系(尤其是阿尔金断裂)有关.此外,柴北缘逆冲带新生代挤压应力方向转变在其北西部起始于上干柴沟组下部沉积时期,而南东部起始于上油砂山组下部沉积时期,与地震反射剖面揭示的断裂活动等地质证据共同揭示柴北缘逆冲带新生代的构造活动自靠近阿尔金断裂的北西部向南东部传播和扩展.综合分析青藏高原东北部地区挤压应力方向转变和其他地质证据发现,挤压应力方向转变显示出自柴北缘逆冲带北西部向东、西和南向扩展特征,与阿尔金断裂在上干柴沟组下部-上油砂山组下部沉积时期剪切应力集中于断裂本身,而上油砂山组下部沉积以来开始散布于高原东北部内部地区的两阶段走滑活动相关.

放牧牲畜粪尿返还对季节性冻融高寒草原N_(2)O排放的影响

放牧牲畜粪尿斑块是草地氧化亚氮(N_(2)O)排放的热点区域,季节性冻融会不同程度改变粪尿养分返还效率和土壤特性,使粪尿作用下的土壤N2O排放特性复杂且作用机理尚不明确。本文采用静态箱-气相色谱法开展为期1年的野外控制试验,探究牦牛和藏绵羊粪尿处理对季节性冻融藏北高寒草原土壤氮动态及N_(2)O排放通量的影响和可能的作用机制。结果表明:尿液施加对土壤N2O排放的短期促进作用明显,且均在处理第1天达到排放通量峰值;牦牛粪尿施加对土壤矿化氮含量的增加更为突出,且牛粪和牛尿处理N_(2)O年累积排放量显著高于羊粪和羊尿处理(P<0.05);冻融期各处理N_(2)O累积排放量占全年的比例为29.3%~42.4%,且消融期对非生长季的贡献最大。研究结论有助于为优化牲畜排泄物管理模式和促进季节性冻融高寒草地温室气体减排等提供理论参考。

藏北高原牧户家庭收支的主要影响因素

基于对藏北高原12个县137户牧户的家庭人员组成、草场及家畜组成和家庭收支等的调查数据,文章旨在了解藏北高原牧民收支的主要影响因素,进而探索增加牧民收入的有效途径。分析结果表明:牧户的主要收入来源于政府补贴和出售牛羊,奶制品和牦牛绒的收入占比很低;所调查牧户年平均收入7万元左右,而年平均总支出占总收入的90%左右,其中41%用于家畜的补饲,且31%用于非劳动力人群以及家庭贷款,导致牧户净收入普遍偏低。文章在此基础上分析了实现牧民增收过程中存在的问题,并提出通过产业结构调整、加强惠民政策管理及实施等措施来促进牧民增收的合理建议,将为促进藏北高原经济发展及草原可持续发展等相关政策的实施提供重要依据。

The northern boundary of the Asian summer monsoon and division of westerlies and monsoon regimes over the Tibetan Plateau in present-day

Precipitation patterns and their variations over the Tibetan Plateau(TP) are mainly dominated by the Asian summer monsoon, westerlies, and their interactions. The exact extent of the Asian summer monsoon’s influence, however, remains undetermined. Referencing the climatological northern boundary index of the East Asian summer monsoon, we demonstrate that the 300 mm precipitation isoline from May to September can be utilized as an indicator of the northern boundary of the Asian summer monsoon over the TP, allowing for an analysis of the spatial distribution characteristics of the climatological and interannual northern boundary. Our results indicate that the climatological northern boundary of the Asian summer monsoon over the TP lies along the eastern Qilian Mountains-Tanggula Mountains-Qiangtang Plateau-Gangdise Mountains-Western Himalayas during 2001–2020. This position corresponds well with the position of the convergence of westerly(westerlies) and southerly wind(monsoon) in the lower troposphere, representing the interface between dry and wet regions in the rainy season over the TP. There is a significant positive correlation between changes in the zonal/meridional water vapor budget and variations in precipitation to the north/south of the climatological northern boundary, respectively. Additionally, a close relationship exists between the interannual fluctuation range of the northern boundary and the distribution of vegetation across the TP. Compared to the northern boundary of the summer monsoon defined by meteorological criteria, which is established based on 5-day(pentad)mean precipitation(exceeding 4 mm day^(-1)), our climatological northern boundary offers a more objective portrayal of the region that experiences persistent influence from the summer monsoon. These indicate that climatological northern boundary has a clear significance for natural geographical distribution such as the westerlies-monsoon circulation, ecology, and climate. Based on the interannual fluctuation range of the northern boundary, we divided the TP into domains of westerlies, monsoon, and westerliesmonsoon transition. This study could serve as a foundation for further investigation into the interactions between westerlies and monsoon, variations in precipitation patterns and hydrological-ecological systems over the TP.

Teleseismic shear wave splitting and intracontinental collision deformation of the northern Tibetan Plateau and the eastern Tarim basin

Shear wave splitting measurement of teleseismic data has been used to determine the fast polarization directions and delay times for 38 temporary stations and 15 permanent stations from a NW linear seismic array across the eastern Tarim basin(ETB) and the northern Tibetan Plateau(NTP),and 10 permanent stations on both sides of the array.We present an image of upper mantle anisotropy in the ETB and NTP using the 63 new measurements.The results show that the fast directions and delay times have complex spatial distribution characteristics.The delay times within the interior of the Tarim basin are very small,with an average value of 0.6 s,which is not only smaller than that in the Altyn Tagh fault and Tianshan on the southern and northern margins of the basin,but also smaller than that in the NTP,reflecting that the delay time of stable blocks is smaller than that of active blocks.Along the array,from east to west,the fast directions contrarotate from NNW in the southern Songpan-Garze terrane to NW in the northern Songpan-Garze terrane,to near E-W or ENE in the north of the East Kunlun fault and southern margin of the Qaidam basin,then first abruptly rotate to NW in the Qiman Tagh fault on the northwestern margin of the Qaidam basin,second abruptly rotate to ENE in the Altyn Tagh fault and south of the ETB,and third abruptly rotate to NW in the north of the ETB,then finally rotate to WNW in the Tianshan.The comparative analysis between the fast wave directions measured by shear wave splitting and predicted from the surface deformation field shows that,with the exclusion of the five observations with larger misfits within the interior of the ETB(with an average misfit of 27°),the misfits in the NTP and northern and southern margins of the Tarim basin are relatively small(with an average misfit of 9°).In addition,the fast wave directions of the tectonic units such as the Altyn Tagh fault,East Kunlun fault,and Tianshan are parallel to the strikes of faults and mountains in the region,which indicates that the deep and shallow deformations of the NTP and northern and southern margins of the ETB are consistent,where the crust-mantle coupling extent of lithospheric deformation is higher,according with the vertical coherent deformation of the lithosphere.Conversely,the crust-mantle coupling extent within the interior of the Tarim Basin is weak,and it is characterized by weak anisotropy,stable rigidity,and thick lithosphere,which may remain the “fossil” anisotropy of ancient craton.