Characteristics of landslide in Koshi River Basin,Central Himalaya

Koshi River basin, which lies in the Central Himalayas with an area of 71,500 km2, is an important trans-boundary river basin shared by China, Nepal and India. Yet, landslide-prone areas are all located in China and Nepal, imposing alarming risks of widespread damages to property and loss of human life in both countries. Against this backdrop, this research, by utilizing remote sensing images and topographic maps, has identified a total number of 6877 landslides for the past 23 years and further examined their distribution, characteristics and causes. Analysis shows that the two-step topography in the Himalayan region has a considerable effect on the distribution of landslides in this area. Dense distribution of landslides falls into two regions: the Lesser Himalaya(mostly small and medium size landslides in east-west direction) and the TransitionBelt(mostly large and medium size landslides along the river in north-south direction). Landslides decrease against the elevation while the southern slopes of the Himalayas have more landslides than its northern side. Change analysis was carried out by comparing landslide distribution data of 1992, 2010 and 2015 in the Koshi River basin. The rainfallinduced landslides, usually small and shallow and occurring more frequently in regions with an elevation lower than 1000 m, are common in the south and south-east slopes due to heavy precipitation in the region, and are more prone to the slope gradient of 20°~30°. Most of them are distributed in Proterozoic stratum(Pt3ε, Pt3 and Pt2-3) and Quaternary stratum. While for earthquake-induced landslides, they are more prone to higher elevations(2000~3000 m) and steeper slopes(40°~50°).

Integrated approach for understanding spatio-temporal changes in forest resource distribution in the central Himalaya

Intense anthropogenic exploitation has altered distribution of forest resources. This change was analyzed using visual interpretation of satellite data of 1979, 1999 and 2009. Field and interactive social surveys were conducted to identify spatial trends in forest degradation and data were mapped on forest cover and land use maps. Perceptions of villagers were compiled in a pictorial representation to understand changes in forest resource distribution in central Himalaya from 1970 to 2010. For- ested areas were subject to degradation and isolation due to loss of con- necting forest stands. Species like Lantana camara and Eupatorium adenophorum invaded forest landscapes. Intensity of human pressure differed by forest type and elevation. An integrated approach is needed to monitor forest resource distribution and disturbance.

Assessing resource vulnerability quadrants under changing precipitation trends in Uttarakhand,Central Himalayan region

The Central Himalayan region is vulnerable to the adverse effects of climate change and characterized by regional climatic conditions.The livelihood of the mountain communities across the Himalaya is at risk owing to the consequences of variable precipitation patterns.There exists limited empirical research on precipitation variability due to inadequate hydro-meteorological stations at highaltitude regions.The study uses a novel methodology which integrates precipitation variability with resource sensitivity over the three verticals of Central Himalaya:Himadri,Himachal and Shivaliks and across four major river basins:Yamuna,Upper Ganga,Ghaghar and Ramganga.The magnitude of the significant precipitation trends was estimated through time series analysis at a 95%confidence interval.To assess the sensitivity of natural resources(forest,water and land)and human resources,fourteen mountain-specific indicators were identified which captured resource index using data standardization and principal component analysis.Sen’s slope and Resource index were plotted in a 2D Cartesian coordinate to draw precipitation-resource quadrants with their effective coverage area:High Precipitation and Scarce Resources(35.92%);Low Precipitation and Abundant Resources(30.10%);Low Precipitation and Scarce Resources(22.33%)and High Precipitation and Abundant Resources(11.65%).This helped in developing quadrant-specific adaptation strategies under regional variability of precipitation.The methodology and the research findings will certainly assist water experts,resource managers and policy makers to strengthen adaptive capacity and improve the resilience of vulnerable communities across Himalaya.

Mass balance and morphological evolution of the Dokriani Glacier,central Himalaya,India during 1999–2014

Glaciological mass balance(MB)is considered the most direct,undelayed and unfiltered response of the glaciers to climatic perturbations.However,it may inherit errors associated with stake underrepresentation,averaging over the entire glacier and human bias.Therefore,proper validation of glaciological MB with geodetic MB is highly recommended by the World Glacier Monitoring Service(WGMS).The present study focuses on the Dokriani Glacier,central Himalaya which is one of the bench-mark glaciers in the region and has glaciological MB records from 1993 to 2013 with intermittent gaps.In the present study,firstly the glaciological MB series is extended to 2014 i.e.,field-based MB for one more year is computed and,to compare with it,the geodetic MB is computed for the 1999–2014 period using high resolution Cartosat-1 digital elevation model(DEM)and SRTM DEM.Finally,the study assesses the regional representation of the Dokriani Glacier in terms of MB and evaluates the influence of the MB regime on its morphological evolution.Results show that the average glaciological MB(-0.34±0.2 m water equivalent(w.e.)y-1)is more negative than the geodetic MB(-0.23±0.1 m w.e.y-1)for the 1999–2014 period.This is likely because of the partial representation of glacier margins in the glaciological MB,where melting is strikingly low owing to thick debris cover(>30 cm).In contrast,geodetic MB considers all marginal pixels leading to a comparatively low MB.A comparative assessment shows that the MB of Dokriani Glacier is less negative(possibly due to its huge accumulation area)than most other glacier-specific and regional MBs,restricting it to be a representative glacier in the region.Moreover,continuous negative MB has brought a peculiar change in the epiglacial morphology in the lower tongue of the glacier as differential debris thickness-induced differential melting has turned the glacier surface into a concave one.This concavity has led to development of a large(10–20 m deep)supraglacial channel which is expanding incessantly.The supraglacial channel is also connected with the snout wall and accelerates terminus disintegration.Given the total thickness of about 30–50 m in the lower glacier tongue,downwasting at its current pace,deepening/widening of supraglacial channel coupled with rapid terminus retreat may lead to the complete vanishing of the lower one km glacier tongue.

Ethnobotanical Study towards Conservation of Medicinal and Aromatic Plants in Upper Catchments of Dhauli Ganga in the Central Himalaya

The present study broadly focused on medicinal plant species collected from wild by the villagers for different purposes in the upper catchment of Dhauli Ganga in Nanda Devi Biosphere Reserve(NDBR),in the central Himalaya.A schedule based survey was conducted during the years 2003-2005 in 15 villages of Chamoli district part of the NDBR.Information was collected from collectors, vaidya(medicine man)and those dealing with domestication and marketing of the medicinal plants. The aim of the study was to understand the prioritiesed medicinal plants,their mode of collection and document their ethnobotanical uses by the Bhotiya tribal communities,in this world heritage site. During the survey,50 medicinal plants belonging to 31 families and 44 genera were documented.Out of these,70% were harvested from the wild,22% were cultivated and 8% were cultivated as well as wild harvested.Of the cultivated species,8% were found growing in the kitchen gardens and 14%in the agricultural fields.However,42%of the plants had their roots and rhizomes used followed by leaves (26%),seeds(10%),seed and leaf(8%),bark and whole plant(6%)and flower(1%).Most plants were reported to be used for rheumatism(16),followed by stomach disorder(14),cold and cough(11),and jaundice(9).Thirty three plants species were reported to have more than one therapeutic uses,while 17 species were reported to be used against single ailment.The distance of villages from road head was one of the factors contributing to the decline in the medicinal plant population in their natural habitats. The availability of medicinal plants increased with increase in distance from road head and also the peoples'dependence on them.Documentation of the traditional knowledge will help in conservation of knowledge and also opportunity for using it for future training and use.The result of this study will help in promoting sustainable cultivation and implementation in conservation protocol of those species,which are in the verge of extinction in this region.

Hydrological Characteristics of the Rongbuk Glacier Catchment in Mt.Qomolangma Region in the Central Himalayas,China

From 8 April to 11 October in 2005,hydrological observation of the Rongbuk Glacier catchment was carried out in the Mt.Qomolangma (Everest) region in the central Himalayas,China.The results demonstrated that due to its large area with glacier lakes at the tongue of the Rongbuk Glacier,a large amount of stream flow was found at night,which indicates the strong storage characteristic of the Rongbuk Glacier catchment.There was a time lag ranging from 8 to 14 hours between daily discharge peaks and maximum melting (maximum temperature).As melting went on the time lag got shorter.A high correlation was found between the hydrological process and daily temperature during the ablation period.The runoff from April to October was about 80% of the total in the observation period.Compared with the discharge data in 1959,the runoff in 2005 was much more,and the runoff in June,July and August increased by 69%,35% and 14%,respectively.The rising of temperature is a major factor causing the increase in runoff.The discharges from precipitation and snow and ice melting are separated.The discharge induced by precipitation accounts for about 20% of the total runoff,while snow and ice melting for about 80%.

Deposition of atmospheric pollutant and their chemical characterization in snow pit profile at Dokriani Glacier,Central Himalaya

The uncertainty in assessing the numerous atmospheric pollutants transported via wind from arid and semi-arid regions is affecting the glacial ecosystem. In our study area due to the complexity of the system, a prominent seasonal difference noticed among major ions(Ca^(2+), Mg^(2+), SO_4^(2-), and NO_3^-). There is a need for understanding the ions cycling as a whole and the directionality of the feedback loops in the system. Therefore, we provide an appraisal of our current hypothesis for seasonal difference in major ion concentration from snow samples for two corresponding years(2013 and 2015) at Dokriani Glacier. A systematic study of chemical compositionsin the shallow snow pit from Dokriani Glacier was undertaken for the pre-monsoon season to understand the cycling of major ions from atmosphere to solute acquisition process. The intimating connections of ions cycling in snow and its temporal behavior was observed and analyzed through various statistical tests. Among major ions, the SO_4^(2-)has the highest concentration among anions on an average considered as 14.21% in 2013 and 29.46% in 2015. On the other side Ca^(2+) is the dominant cation contributing 28.22% in 2013 and 15.3% in 2015 on average. The average ratio of Na+/Cl-was higher in 2013 whereas lower in 2015. The backward trajectory analysis suggests the possible sources of the ions transported from Central Asia through the Western Disturbance(WD) as a prominent source of winter precipitation mainly in the Central Himalaya. Ionicconcentration of Ca^(2+) in cations was highly dominated while in anion SO_4^(2-)played the major role. Factor analysis and correlation matrix suggested that, the precipitation chemistry is mostly influenced by anthropogenic, crustal, and sea salt sources over the studied region. The elemental cycling through ocean, atmosphere and biosphere opens up new ways to understand the geochemical processes operating at the glacierized catchments of the Himalaya. Moreover, increasing the field-based studies in the coming decades would also have the certain advantage in overcoming the conceptual and computational geochemical modelling difficulties.

FRESH SNOW CHEMISTRY FROM HIGH MOUNTAIN REGIONS IN CENTRAL HIMALAYAS

During 1997 summer, fresh snow samples were collected from the high elevation region (5400－ 7000 m) of Dasuopu Galcier on the northern slope of Mt. Xixabangma (28° 33’N, 85° 44’E). Compared with other remote regions in the world, major ion concentrations in fresh snow are very low during summer in Mt. Xixabangma, suggesting that the atmosphere is very clean and may represent background value for the middle/upper troposphere in the middle/low latitude area. During summer at Mt. Xixabangma the fresh snow chemistry is minimally influenced by anthropogenic pollutants as revealed by the snow pH (mean value of 6.0). Conductivity of fresh snow are low and constant. A multi regression curve of pH vs conductivity shows a strong correlation; snow pH is negatively correlated with conductivity when pH < 6.0, and positively correlated when pH > 6.0. This suggests that the dominant chemical species of snow are interchanging between acid anions (e.g.) and crustal cations.

Assessing Anthropogenic Pressure and Its Impact on Hippophae salicifolia Pockets in Central Himalaya,Uttarakhand

Natural habitat of Hippophae salicifolia in Central Himalaya is continuously being degraded due to habitat destruction and harvesting.Although logging is prohibited,habitat destruction has increased because of regular road construction,repairing and broadening activities.In addition,Hippophae resources are continuously being harvested by lopping(both partial and complete) for fuelwood,fodder and fruits in higher Himalayan region.This paper presents a detailed analysis of relationship between density,demographic structure,and harvesting of H.salicifolia growing pockets in the five major valleys(Gangotri,Yamunotri,Niti,Mana and Bhyundhar) of Uttarakhand in Central Himalaya,India.A total of 120 quadrats were laid randomly to study population structure,regeneration,sex ratio and lopping using quadrats of 100 m 2(24 in each valley) in Hippophae growing patches.Our study shows that the density,size distribution,and regeneration of Hippophae vary considerably among the major valleys.Trees in the Yamunotri valley have the highest density of large trees but the lowest density of seedlings.In contrast,there are few large trees but many seedlings in the Mana valley.The number and size of lopped trees also varied among the valleys.Lopping was greatest in Bhyundhar(11.4%) and Yamunotri(19.7%) and least in Niti(3.9%).The size of lopped trees differed substantially as well.In Bhyundhar,the largest trees were taken while saplings were taken in Yamunotri.Our study revealed that unsustainable harvesting from plants for fuel,fencing and fruits along with road broadening activities in Central Himalaya are the main cause of habitat destruction.Our research highlights the urgent need for in-situ and ex-situ conservation of Hippophae salicifolia so that it's potential can be harnessed sustainably by rural hill societies for their socio-economic development.

Frequency and size change of ice–snow avalanches in the central Himalaya:A case from the Annapurna II glacier

Glaciers have retreated and shrunk in High Mountain Asia since the mid-20th century because of global warming,leading to glacier instability and hazardous iceesnow avalanches.However,the complex relationship between iceesnow avalanches and factors such as climate and potential triggers are difficult to understand because of the lack of observational data.Here,we addressed iceesnow avalanches on the Annapurna II glacier in Nepal,Central Himalaya.We constructed an iceesnow avalanche history using long-term multi-source remote sensing images(1988-2021)and mapped the velocity fields of glaciers using cross-correlation analysis on SAR and optical images.Then,we investigated the impact of climate change and earthquakes on the frequency and size of iceesnow avalanches.The results demonstrate that the frequency of iceesnow avalanches has increased from 10 in 1988 to 27 in 2020,but the average area of iceesnow avalanche deposits has decreased by approximately 70%,from 3.4×10^(5) m^(2) in 1988 to 1.2×10^(5) m^(2) in 2020.The evolutionary characteristic of ice avalanches is linked to the impact of glacier retreat(reduction in ice material supply)and increased activity under climate change.The glacier movement velocity controls the size of iceesnow avalanches and can be set as an indicator for iceesnow avalanche warnings.On the Annapurna Ⅱ glacier,an iceesnow avalanche occurred when the glacier velocities were greater than 1.5 m d^(-1).These results offer insights into iceesnow avalanche risk assessment and prediction in high-mountain areas,particularly in regions characterised by dense glacier distribution.

Vestiges of the Kerguelen Mantle Plume in Southern Tibet:Evidence from 123-117 Ma Magmatism in the Dingri Area of the Central Tethys Himalaya

The widely distributed Early Cretaceous magmatism in the Tethys Himalaya(TH)of southern Tibet is related to the Kerguelen mantle plume.Associated magmatic activity products are distributed in the eastern TH,where the active age is earlier than the peak ages of the Kerguelen mantle plume.This study investigated magmatic activity of the Dingri area in the central TH which was coeval with the Kerguelen mantle plume.The intrusion in the Dingri area contains diabases and monzonites.The zircon age of diabase is 123±1 Ma,and that of monzonite is 117±1 Ma.Geochemistry and Sr-Nd isotopic analyses show that the mafic-intermediate dikes were formed in an intraplate extensional environment.The diabase is derived from the enriched lithospheric mantle and monzonite is derived from partial melting of the lower crust,with both magmatic evolutions being contaminated by crustal materials.These characteristics are similar to those of the Rajmahal-Sylhet basalt,a typical Kerguelen mantle plume product.The discovery of the Dingri mafic-intermediate dikes of the central TH suggests that the TH and Rajmahal-Sylhet Traps formed a continuous mantle plume overflow magmatic belt which was a product of the continuous eruption of the Kerguelen mantle plume.

Glacier-Glacial Lake Interactions and Glacial Lake Development in the Central Himalaya,India(1994–2017)

Despite several regional glacier and glacier lake inventories, the relationship between receding glacier, glacial lake evolution(glacial-lake interactions) and their sensitivity to different forcing factors have not been properly understood yet. To better understand these processes, we used satellite images collected in 1994, 2015 and 2017 to monitor the spatially-explicit evolution of glacial lakes and glacier changes. The results show a total of 1 353 glacial lakes covering an area of 7.96 km;in the year 2015. Out of these, a total of 137 glacial lakes having an area of >0.01 km;and located within 2 km periphery of mother glacier have been selected for the monitoring of spatial development between 1994 and 2017. We found an increase in the total lake area from ~4.9 to ~7.73 km;between 1994 and 2017,corresponding to an overall expansion of ~57%. The total area covered by the glaciers associated with these lakes reduced from ~365 km;in 1994 to ~358 km^(2) in 2017, accounting for a glacier loss of ~7 km^(2) and corresponding to ~1.92% reduction. Our study results are in agreement with global glacier behavior, revealing a rapid glacier recession and accelerated glacial lake expansion under an unprecedented climate change scenario. In addition, the results suggest a significant reduction in the glacier area and a close relationship between the glacier melting and lake changes.

Strong link between large tropical volcanic eruptions and severe droughts prior to monsoon in the central Himalayas revealed by tree-ring records

Large tropical volcanic eruptions can cause short-term global cooling. However, little is known whether large tropical volcanic eruptions, like the one in Tambora/Indonesia in 1815, cause regional hydroclimatic anomalies. Using a tree-ring network of precisely dated Himalayan birch in the central Himalayas, we reconstructed variations in the regional pre-monsoon precipitation back to 1650 CE. A superposed epoch analysis indicates that the pre-monsoon regional droughts are associated with large tropical volcanic eruptions, appearing to have a strong influence on hydroclimatic conditions in the central Himalayas. In fact, the most severe drought since 1650 CE occurred after the Tambora eruption. These results suggest that dry conditions prior to monsoon in the central Himalayas were associated with explosive tropical volcanism. Prolonged La Ni?a events also correspond with persistent pre-monsoon droughts in the central Himalayas. Our results provide evidence that large tropical volcanic eruptions most likely induced severe droughts prior to monsoon in the central Himalayas.

Summer monsoon and dust signals recorded in the Dasuopu firn core, central Himalayas

In September 1997, a 15-m firn core was recovered from an elevation of 7 000 m a. s.l. from the Dasuopu Glacier in the central Himalayas. The analysis of δ18O values and major ion (Ca2+ , Mg2+ , NH4+ , SO2- and NO3-) concentrations shows that average annual accumulation is 0.75 m (water equivalent) in the Dasuopu firn core. The seasonal variations of δ18O values and major ion concentrations in the core indicate that present summer monsoon and

Numerical Modeling of Neotectonic Movements and State of Stresses in the Central Seismic Gap Region,Garhwal Himalaya

This paper presents finite element modeling(FEM) to simulate the present-day stress field and crustal deformation using NE-SW structural section in the central Seismic Gap region of the Garhwal Himalaya.Our study deals with the effect of geometrical characteristics and rock layer parameters on the upper crust.Modeling results show that two types of tectonic regimes developed in the central Seismic Gap region:the geotectonics of the northern part has been controlled by regional compression,whereas southern part is characterized by regional extension.Correspondingly,thrust faults are induced in the northern part and normal faults are extensively developed in the southern front.Those evidences noticeably indicate that the compressive tectonic environment of the Himalaya becomes change into the extensional tectonic regime in its front.The computed shear stress accumulation along the northern flat of Main Himalayan Thrust(MHT) implies that considerable amount of interseismic stress is building up along the MHT system in the Himalaya,which ultimately release through the possible future great Himalayan earthquake(M>8).The comparison between our modeled stress field,faulting pattern and horizontal shortening rate with the distribution of the microseismic events,focal mechanism solutions,active faulting and GPS data in the central Seismic Gap region shows good agreement.

PALEOCLIMATIC CHANGES DURING THE LAST 2.5Ma RECORDED IN THE KATHMANDU BASIN, CENTRAL NEPAL HIMALAYAS

The Kathmandu valley is an intermountain basin located at about 1300 and surounded by mountains around 2400m.Thick fluvio\|lacustrine sediments from late Pliocene to Quaternary are accumulated there under monsoon climate.We undertook palynological and sedimentological studies on 284\|m\|long drilled core that reaches the basement of the kathmandu basin,in order to clarify the paleoclimatic changes and their tectonic link with uplifting of the Himalayas..The core is lithologically divided into three members:sand predominant lower member (51m),slit and clay dominant middle member (182m) and organic silty clay predominant upper member (45m).We analyzed fossil pollens contained in muddy samples which were taken from the middle and upper member in every one meter interval.The pollen assemblage is characterized by predominance of \%Quercus,\%30%～80% of the all arboreal pollen.We inferred paleoclimatic changes on the basis of comparison between present vegetation and climate of Kathmandu valley and surrounded mountains and pollen diagram which we obtained.For the purpose of inference of paleoclimate,we used the following genera as climatic index:\%Pinus\% for cold climate, \%Quercus\% and \%Cyclobalanopsis\% for warm climate, Alnus for wet climate and Gramineae for dry climate.As the reference.we also examined amount of another eight genera,two family and all spores.The pollen diagram shows two pattern of paleoclimatic changes:oscillation pattern with seven times fluctuation of warm and cold climate in younger period (ca.115m in depth),and older less fluctuated pattern of warm climate.During the older period (209～215m in depth),one cold stage was recognized,and is characterized by predominance of Pinus and occurrence of Tsuga .

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS

The Siwalik Belt is a frontal fold\|thrust belt of the Himalayas and composed of thick sequence of foreland basin sediments derived from the Himalayas during the last 15 to 17 million years.From this Miocene belt in the central Nepal,we discovered exotic thrust packages of the Middle Proterozoic rocks,which has been regarded as the Siwalik Group or post\|collisional sediments correlatable with the Subathu or Murree Formation in India.The thrust belt,called the Bagmati Belt,is narrowly distributed in the Siwalik Belt,22km to the north of the Main Frontal Thrust (MFT or HFF) that is an active fault and considered to be the deformation front of the Himalayan orogen.The Main Boundary Thrust (MBT) which separates the Siwalik Belt from the Lesser Himalayan Belt runs 7km to the north of the thrust packages.Within the belt,tectonic slices of 400m to 1km in thickness are repeated three to five times due to thrusts,sandwiching a thin slice of the Siwalik beds.The thrust package consists of the pre\|Siwalik sedimetary rocks and sills of dolerite,and named as the Bagmati Group.The thickness is only about 800m due to tectonic repition by thrust,although the group has been considered to be a continuous sequence attaining 2200m in thickness.We divided the Bagmati Group into three formations,each of which shows an upward\|coarsening and thickening sequence of 200 to 350m in thickness.All sequence is composed of red\|brown orthoquartzite,pink quartzite,micaceous shale and thin sandstone interbed and rhythmite,mottled hematite and hematitic pisolite.We interpret that the Bagmati Group was deposited in shallow lacustrine and desert environments.

SLOPE INSTABILITY HAZARDS IN GRANITES OF THE LESSER HIMALAYAN EXAMPLE OF THE PALUNG GRANITE OF CENTRAL NEPAL

Granites are hard and sound rock at relatively fresh or unweathered condition. Steep rocky slopes are the characteristic features in the area occupied by granites of the Lesser Himalaya including Kathmandu nappe. Almost vertical to sometimes overhanging slopes in granites look stable in dry season, but the steeper slopes in the areas occupied by granitic rocks are metastable to unstable. The instabilities are related to: variation in texture and mineral composition of granite, nature and intensity of weathering (mechanical and chemical), altitude of the area, orientation of natural slope with reference to the predominant joint sets, quality of rock mass, stress release and activity of subsurface water during monsoon. The study is supposed to be an example for the study of the slope instabilities in the other part of the Himalaya occupied by granitic bodies.

Importance of Tourmaline Gneiss and Vein near Main Central Thrust in Sikkim Darjeeling Himalaya

Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and/or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.

METAMORPHISM IN THE LESSER HIMALAYAN CRYSTALLINES AND MAIN CENTRAL THRUST ZONE IN THE ARUN VALLEY AND AMA DRIME RANGE (EASTERN HIMALAYA)

The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone to the Tethyan Himalaya. At the northern end of the Arun tectonic window (ATW), the Ama Drime—Nyonno Ri range of south Tibet exposes a section of that portion of the Main Central Thrust (MCT) zone and Lesser Himalayan Crystallines (LHC) which elsewhere in Nepal is concealed below the overlying Higher Himalayan Crystalline (HHC) nappe (Fig. 1). As throughout the Himalaya at the structural level of the MCT, the ATW is characterized by an inverted metamorphic field gradient characterized by a progression from chlorite to sillimanite grade from low to high structural levels of the nappe pile. Metamorphic peak temperatures rise from circa 400℃ in the pelitic and psammitic Precambrian metasediments of the Lesser Himalayan Tumlingtar Unit, to 550～620℃ in the overlying LHC, to over 700℃ in the muscovite\|free Barun Gneiss, the lowermost HHC unit in the Arun valley.