Eastern Nepal Himalaya Research Articles

Lithostratigraphy and structure of the Dharan–Mulghat area, Lesser Himalayan sequence, eastern Nepal Himalaya

The Dharan–Mulghat area of the eastern Nepal can be divided into three tectonic units: the Higher Himalayan Crystallines, the Lesser Himalayan Sequence and the Siwaliks from north to south separated by the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), respectively. The Lesser Himalayan Sequence is divided into two groups separated by Chimra Thrust: the Bhedetar Group and the Dada Bajar Group. The Bhedetar Group includes the Raguwa Formation, the Phalametar Quartzite, the Churibas Formation, the Sangure Quartzite, and the Karkichhap Formation from the bottom to top, respectively; overthrusted
 by the Dada Bajar Group consisting: the Ukhudanda Formation, the Mulghat Formation, the Okhre Formation, and the Patigau Formation, from lower to upper sections, respectively along the Chimra Thrust and the Bhorleni Formation as an individual formation overthrusted by Bhedetar Group along the Chhotimorang Thrust. The Main Central Thrust, the Main Boundary Thrust, the Chimra Thrust and the Chhotimorang Thrust are the major faults in Dharan–Mulghat area. The
 Leutiphedi Anticline and the Malbase Syncline are the major folds in the study area plunging towards east. The trend/plunge of anticline and syncline are 131o/24o and 096o/09o respectively. The microstructural study in the quartz grains reveals a sharp difference in the history across the MCT; dynamic in the rocks of the Lesser Himalayan Sequences and static in the rocks of the Higher Himalayan Crystallines.

Glacier area shrinkage in eastern Nepal Himalaya since 1992 using high-resolution inventories from aerial photographs and ALOS satellite images

ABSTRACTTo better understand the recent wide-scale changes in glacier coverage, we created and compared two glacier inventories covering eastern Nepal, based on aerial photographs (1992) and high-resolution Advanced Land Observing Satellite (ALOS) imagery (2006–10). The ALOS-derived inventory contained 1034 debris-free and 256 debris-covered glaciers with total and average areas of 440.2 ± 33.3 and 0.42 km2and 1074.4 ± 206.4 and 4.19 km2, respectively. We found that the debris-free glaciers have lost 11.2% (0.7 ± 0.1% a−1) of their area since 1992, whereas the number of glaciers increased by 5% because of fragmentation. The area change was significantly correlated by simple linear regression with minimum elevation (r= 0.30), maximum elevation (r= −0.18), altitudinal range (r= −0.50), glacier area (r= −0.62) and mean slope (r= 0.16), confirming that larger glaciers tended to lose a larger area (but a smaller percentage) than smaller glaciers. The intra-regional analysis of the glacier changes clearly showed higher shrinkage rates in the western massifs compared with the eastern massifs. In addition, 61 small glaciers covering an area of 2.4 km2have completely disappeared since 1992.

Dendroclimatic response of Abies spectabilis at treeline ecotone of Barun Valley, eastern Nepal Himalaya

A dendroclimatic study was conducted in the treeline ecotone of Barun Valley, eastern Nepal, to determine the tree-ring climate response and ring width trend of Abies spectabilis. A 160-year-old chronology, from 1850 to 2010, was developed from 38 tree-ring samples. No higher growth in recent decades was observed in tree-ring width in this area. The mean temperature of the current year in February and in the combined winter months of December, January, and February showed significant positive correlation with tree-ring width, although no significant correlation was found between tree-ring width and the precipitation pattern of the region. This tree-ring climate response result is different from that in other studies in Nepal, which could be attributed to location and elevation.

Site- and species-specific treeline responses to climatic variability in eastern Nepal Himalaya

Alpine treelines act as bio-indicators and bio-monitors of environmental change impacts in high elevation forests. This dendro-ecological study carried out in treeline ecotones in the Sagarmatha (Mt. Everest) National Park (SNP), eastern Nepal Himalaya, aimed to assess treeline dynamics and to understand the response of treeline forming Abies spectabilis (D. Don, Mirb) and Betula utilis (D. Don) to environmental change. At three treeline sites we placed two to four belt transects (size: 20m wide, variable length) which bisected the treeline as well as the tree species limit. The results revealed spatio-temporally heterogeneous regeneration with a higher regeneration of A. spectabilis compared to B. utilis. Warm temperatures during summer (JJA) growing seasons combined with sufficient moisture favored the growth of A. spectabilis while moisture stress during spring seasons (MAM) mainly limited the growth of B. utilis. The regeneration of A. spectabilis was favored by high temperatures throughout the year with sufficient moisture. The climatic response of the regeneration of B. utilis was spatiotemporally different and variable. Results predict a changing community structure in the treeline in response to future environmental change. During the past 200 years, A. spectabilis shifted upward by about 0.93m/yr and B. utilis by 0.42m/yr, with stabilization during the second half of the 20th century at the majority of the sites. The recent stability in treeline position of both species at most sites indicated that in addition to favorable climate, species-specific competitive abilities during the recruitment phase, recruitment suppression in the Krummholz and dwarf scrub belts, and grazing determine regeneration success and treeline position in the region.

An assessment of conditions before and after the 1998 Tam Pokhari outburst in the Nepal Himalaya and an evaluation of the future outburst hazard

On 3 September 1998, a glacial lake outburst flood (GLOF) that originated from Tam Pokhari occurred in the Hinku valley of the eastern Nepal Himalaya. This study analyses the lake's geomorphic and hydrologic conditions prior to the outburst, and evaluates the conditions that could contribute to a future flood through photogrammetric techniques. We processed high-resolution Corona KH-4A (2.7 m) and ALOS PRISM (2.5 m) stereo-images taken before and after the GLOF event, and produced detailed topographic maps (2-m contour interval) and DEMs (5 m × 5 m). We (re-) constructed lake water surfaces before (4410 ± 5 m) and after (4356 ± 5 m) the outburst, and reliably estimated the lake water surface lowering (54 ± 5 m) and the water volume released (19.5 ± 2.2 × 106 m3) from the lake, showing good agreement with the results obtained from ground-based measurements. The most relevant conditions that may have influenced the catastrophic drainage of Tam Pokhari in 1998 include the presence of: (i) a narrow (75 ± 6 m), steep (up to 50°) and high (120 ± 5 m) moraine dam; (ii) high lake level (8 ± 5 m of freeboard) and (iii) a steep overhanging glacier (>40°). The lake outburst substantially altered the immediate area, creating a low and wide (>500 m) outwash plain below the lake, a wide lake outlet channel (~50 m) and a gentle channel slope (~3–5°). Our new data suggest that the likelihood of a future lake outburst is low. Our results demonstrate that the datasets produced by photogrammetric techniques provide an excellent representation of micro-landform features on moraine dams, lake water surfaces and the changes in both over time, thereby allowing highly accurate pre- and post-GLOF (volumetric) change analysis of glacial lakes. Furthermore, it enables precise measurement of several predictive variables of GLOFs that can be useful for identifying potentially dangerous glacial lakes or prioritizing them for detailed field investigations. Copyright © 2015 John Wiley & Sons, Ltd.

Contemporary and historic population structure of Abies spectabilis at treeline in Barun valley, eastern Nepal Himalaya

Treeline ecotone dynamics of Abies spectabilis (D. Don) Mirb. in the Barun valley, Makalu Barun National Park, eastern Nepal Himalaya were studied by establishing seven plots (20 m × variable length) from the forestline to the tree species limit: three plots on the south- and north-facing slopes each (S1–S3, N1–N3), and one plot on the east-facing slope (E) in the relatively undisturbed forests. A dendroecological method was used to study treeline advance rate and recruitment pattern. In all the plots, most trees established in the early 20th century, and establishment in the second half of the 20th century was confined to the forestline area. Treeline position has not advanced substantially in the Barun valley, with only 22 m average elevational shift in the last 130 years, and with average current shifting rate of 14 cm/yr. Moreover, no significant relationship was found between tree age and elevation on the south-, north-, and east-facing slopes. The number of seedlings and saplings in near the treeline area was negligible compared to that near the forestline area. Therefore, A. spectabilis treeline response to the temperature change was slow, despite the increasing temperature trend in the region. Beside the temperature change, factors such as high inter-annual variability in temperature, dense shrub cover, and local topography also play an important role in treeline advance and controlling recruitment pattern above the treeline.

Agriculture and Environment in the Eastern Nepal Himalayas

Chapagain, Prem Sagar. 2009. Agriculture and Environment in the Eastern Nepal Himalayas, VDMVerlag Dr. Muller, Germany, pp 166 + viii, acknowledgements, tables, figures, pictures, diagrams,reference and annex, paperback, price not quoted.DOI: http://dx.doi.org/10.3126/ttp.v8i0.11519 The Third Pole: Journal of Geography Vol.8-10, pp. 73-74: 2010

Assessment of glacial lake development and prospects of outburst susceptibility: Chamlang South Glacier, eastern Nepal Himalaya

Chamlang South Tsho has been identified as one of the six high-priority glacial lakes in terms of glacial lake outburst flood (GLOF) danger in Nepal Himalaya, despite the fact that no detailed investigations of the lake had been hitherto undertaken. We conducted detailed mapping of the lake and its surroundings along with field surveys in October 2009 to determine the developmental history of Chamlang South Tsho and to assess its potential for GLOF. The lake expanded rapidly between 1964 (0.04 km2) and 2000 (0.86 km2) and has been stable ever since. Future lake expansion is improbable as its sides are confined by relatively stable landforms. The lake is 87-m deep with a water volume of approximately 34.9–35.6 × 106 m3. Hanging glaciers on the steep surrounding mountain slopes and prominent seepage water in the terminal moraine dam could be potential triggers for a future outburst flood. Additionally, the debris-covered dead-ice dam, which is higher than the lake water level, is narrow and low; therefore, it could be overtopped easily by surge waves. Furthermore, the pronounced difference in elevation between the lake and the base of the terminal moraine dam makes the lake susceptible for a large flood.

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

The type and kinetics of metamorphic CO2-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2 cycle. This paper focuses on CO2 production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al2O3–SiO2–CO2–H2O (CFAS–CO2–H2O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO2) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO2–H2O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle.

Ductile and brittle deformations in the Main Central Thrust zone at the frontal part of nappe in eastern Nepal Himalaya

Ductile and brittle deformations in the Main Central Thrust zone at the frontal part of nappe in eastern Nepal Himalaya

The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis, melt productivity, melt loss and the preservation of cordierite

AbstractCordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C−1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.

Modeling the Spatial Distribution of Snow Cover in the Dudhkoshi Region of the Nepal Himalayas

Abstract In this study, a distributed biosphere hydrological model with three-layer energy-balance snow physics [an improved version of the Water and Energy Budget–based Distributed Hydrological Model (WEB-DHM-S)] is applied to the Dudhkoshi region of the eastern Nepal Himalayas to estimate the spatial distribution of snow cover. Simulations are performed at hourly time steps and 1-km spatial resolution for the 2002/03 snow season during the Coordinated Enhanced Observing Period (CEOP) third Enhanced Observing Period (EOP-3). Point evaluations (snow depth and upward short- and longwave radiation) at Pyramid (a station of the CEOP Himalayan reference site) confirm the vertical-process representations of WEB-DHM-S in this region. The simulated spatial distribution of snow cover is evaluated with the Moderate Resolution Imaging Spectroradiometer (MODIS) 8-day maximum snow-cover extent (MOD10A2), demonstrating the model’s capability to accurately capture the spatiotemporal variations in snow cover across the study area. The qualitative pixel-to-pixel comparisons for the snow-free and snow-covered grids reveal that the simulations agree well with the MODIS data to an accuracy of 90%. Simulated nighttime land surface temperatures (LST) are comparable to the MODIS LST (MOD11A2) with mean absolute error of 2.42°C and mean relative error of 0.77°C during the study period. The effects of uncertainty in air temperature lapse rate, initial snow depth, and snow albedo on the snow-cover area (SCA) and LST simulations are determined through sensitivity runs. In addition, it is found that ignoring the spatial variability of remotely sensed cloud coverage greatly increases bias in the LST and SCA simulations. To the authors’ knowledge, this work is the first to adopt a distributed hydrological model with a physically based multilayer snow module to estimate the spatial distribution of snow cover in the Himalayan region.

Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya

In the Kanchenjunga area (far eastern Nepal), as well as in the whole Himalayan chain, the juxtaposition of the high-grade mid-crustal Higher Himalayan Crystallines (HHC) onto the low grade Lesser Himalayan Sequence (LHS) is structurally marked by the Main Central Thrust Zone (MCTZ). On the base of original field mapping and mesoand micro-structural data, the MCTZ has been here identified as a crustal-scale ductile to ductile-brittle shear zone roughly centred on the Inverted Metamorphic Sequence (IMS), in which different rocks are more pervasively deformed and sheared with respect to adjacent rocks. The boundaries of the MCTZ are not represented by single thrusts. The lower boundary is marked by phyllonites and mylonitic schists located at the uppermost portions of the LHS at the contact with strongly mylonitic augen gneisses of the IMS. The upper boundary of the MCTZ is roughly located at the base of Grt-Kfs-Ky-Sil anatectic gneisses in the lower portion of the HHC, being itself characterized by pervasive ductile shearing. Structural field data combined with petrologic results clearly indicate that the MCTZ is internally imbricated, resulting in the juxtaposition of rock packages characterized by different P-T evolutions and T/depth gradients, separated by “metamorphic discontinuities” which do not always correspond to evident structural breaks. This is the case of the metamorphic discontinuity identified at the upper structural levels of the IMS and juxtaposing the upper IMS rocks, locally anatectic, characterized by higher T/depth gradients on the lower IMS rocks, characterized by lower T/depth gradients. A similar metamorphic discontinuity was previously reported westward (Milke Danda transect) thus suggesting that it could be of regional importance in the tectonometmaoprhic architecture of eastern Nepal Himalaya. Citation: 2012. Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya . In: (Eds.) Michele Zucali, Maria Iole Spalla, and Guido Gosso, Journal of the Virtual Explorer, volume 41, paper 2, doi: 10.3809/jvirtex.2011.00294 Journal of the Virtual Explorer, 2012 Volume 41 Paper 2 http://virtualexplorer.com.au/

Rock type, precipitation, and the steepness of Himalayan threshold hillslopes

Abstract Studies on the dynamic coupling between tectonics, climate, and erosion at the margins of the Tibetan Plateau entail the notion of widely occurring threshold hillslopes along the southern Himalayan front. Here we show that differences in major lithological units are sufficient to explain trends in topographical relief and mean slope gradients across the eastern Nepal Himalaya. This lithological control is manifest in modal slope gradients that serve as proxies of peak rock-mass strength, and remain strikingly invariant per rock type on both sides of the Main Central Thrust (MCT) despite comparably high amounts of monsoon precipitation. We infer that lithology rather precipitation patterns plays an important though hitherto largely neglected role in modulating threshold hillslope steepness. A nonlinear relationship between mean slope gradients and relief further indicates that significant steepening of threshold hillslopes through relief increases is more readily facilitated in low-relief terrain. Most evidence of large (>10 8 m 3 ) bedrock landslides in the Himalaya clusters where topographical relief >2 km, and slope gradients are above average. This provides the mechanism necessary for rapidly lowering drainage divides and limiting hillslope heights, were the steepness of threshold hillslopes to decline via relief destruction.

Metamorphism, melting, and channel flow in the Greater Himalayan Sequence and Makalu leucogranite: Constraints from thermobarometry, metamorphic modeling, and U-Pb geochronology

[1] The Makalu leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation-parallel sheets along the top of the Greater Himalayan Sequence. It is part of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms ± Bt leucogranites but, unlike most other Himalayan granites, also locally contains coarse-grained cordierite. The cordierite-bearing leucogranite intrudes through and overlies lower sheets of “normal” tourmaline granites and represents the most recent phase of magmatism. Cross-cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting (∼700°C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800°C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in leucogranites record the last low-pressure decompression phase of melting. P-T determinations detail peak sillimanite grade metamorphism at 713°C/5.9 kbar, with a secondary cordierite overprint at 618°C/2.1 kbar; this P-T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different leucogranites. The main phase of leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite leucogranite and the migmatitic Barun gneisses at 15.6 ± 0.2 and 16.0 ± 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700°C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene.

Early Oligocene partial melting in the Main Central Thrust Zone (Arun valley, eastern Nepal Himalaya)

The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ. While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the P– T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal). Microstructural observations combined with P– T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820 °C, 13 kbar, the studied sample experienced decompression accompanied by cooling down to 805 °C, 10 kbar, which caused in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼ 31 Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.

Impact of tropical convective activity on monthly temperature variability during nonmonsoon season in the Nepal Himalayas

Surface air temperature, observed in the eastern Nepal Himalayas, showed large intraseasonal and year‐to‐year variability during the nonmonsoon season. A significant negative correlation was found in the monthly outgoing longwave radiation (OLR) between the area over the Himalayas and tropical areas between 80 and 150°E, which was confirmed by in situ observation data. The correlation was stronger in the months of December, March, and April with different correlative areas in the tropics. From comparison of data observed over 10 years with OLR and global precipitation data, the variability of monthly local surface air temperature in winter was primarily attributed to the indirect effect of convective activities in the tropics that caused a variation in the precipitation/snow cover condition at high elevations.

Equilibrium-line altitudes of the present and Last Glacial Maximum in the eastern Nepal Himalayas and their implications for SW monsoon climate

Equilibrium-line altitude (ELA) of a glacier can be an adequate indicator for glaciation and local climate. This study addresses the present ELA in eastern Nepal, in comparison with that of the Last Glacial Maximum (LGM). The present ELA of each glacier was identified by its surface topography using the latest aerial photograph interpretations. ELAs during the LGM were mostly estimated by the maximum elevation of lateral moraines (MELMs). The distinguished higher altitudes of the MELM were selected to estimate the ELAs during the LGM. Latitudinal profiles of the present equilibrium-line show southward gradients, interpreted as a product of the reduction in precipitation from monsoon humid wind blowing into the High Himalayas. Latitudinal ELA profiles of the LGM show the same southward inclination, suggesting that monsoon precipitation is likely to be a ruling resource on glacier nourishment. The eastern Nepal Himalayas, hence, was under summer monsoon environment during the LGM same as present, even if the SW monsoons were weaker.

Slow mass movement in the Kangchenjunga Area, Eastern Nepal Himalaya

The rates of the accumulated and continuous displacement of solifluction lobes in the Kangchenjunga area, eastern Nepal Himalaya, were determined using glass fiber tubes and a strain probe. Ground temperature, precipitation and soil moisture were monitored at two sites, whose altitude differed by approximately 100 m, to understand the solifluction process. The average movement rate of the glass fiber tubes on a 31° slope at altitudes of 5412–5414 m a.s.l. was approximately 11 mm/year, being almost threefold greater than that observed on a 22° slope at 5322–5325 ma.s.l. There was no significant difference in the depth of displacement at these sites. The continuous displacement measurement near the ground surface at 5414 m showed permanent downslope movement from early July. Such movement may be attributed to additional moisture supply during the monsoon season. The amplitude of the displacement cycle was highest at the ground surface, and decreased to virtually zero at and below 20 cm in depth. Probable factors leading to the relatively slow rates of downslope displacement at the surface and depth at the studied altitudes are the lack of concurrence of the freeze–thaw cycles and the high moisture condition in the soil, and the low moisture retention capacity of the soil because of steep slopes and superficial desiccation. The rate of displacement may be more pronounced at altitudes above 5600 m because of the freeze–thaw cycles during the summer season.

Active landslides on the lateral moraines in the Kanchanjunga Conservation Area, eastern Nepal Himalaya

This study identified the present development of landslide on the Holocene moraines (4,760-5,200 m) in the Kanchanjunga (Kangchenjunga) Conservation Area, eastern Nepal, which may result in disasters in future. A tension crack with ca. 100 m in length runs parallel to the moraine ridge on the debris flow deposits (5050 m) between the lateral moraine and mountain slope near Pangpema Base Camp. On the right bank of the Kanchanjunga Glacier, ground surface of the sliding blocks, moraines, mountain slope, and other deposits such as debris flow deposits are almost completely covered by Kobresia. On the other hand, the landslide scars show the fresh outcrops without vegetation. Such landslide scars ranging from 1 m to 70 m in relative height are developed in a few rows on the right bank around the Kanchanjunga Glacier, and continuously extend for about 7.2 km although at some places they are not clear. The interpretation of the air photographs taken in 1980 and field survey in 1989 clearly demonstrated that the landslides on the lateral moraines of the Kanchanjunga Glacier at the altitudes near the lower limit of the present permafrost (4,755 m: Ishikawa et al. 2001) have been created and enlarged between the two years. In the oblique air photographs taken in 1982, only one remarkable landslide scar can be seen on the lateral moraines near Lhonak and on the terminal moraine of the tributary glacier near Pangpema Base Camp. During the field survey in 1998, multiple landslide scars were observed at each site. This means that new landslides began to occur in this decade, especially in Pangpema and Lhonak. These new landslide scars are also identified in the 1992 air photographs. With the help of the studies on the glacial chronology study (e.g., Asahi and Watanabe 2000), the surface level of the former Kanchanjunga Glacier was estimated: the glacial ice of approximately 100 m thick melted in the past century. This estimate is also supported by an old photograph of the Jannu Glacier, some 9 km to the south, which is published in the book, “Round Kangchengjunga”, by Freshfield (1903). Therefore, unloading by rapid deglaciation might be a cause of the landslide although deglaciation alone cannot explain the development of the landslides. We applied electrical resistivity soundings to detect the existence of the permafrost. No clear existence, however, was confirmed with possible small bodies of the permafrost. The landslides may have been developed during the final stage of the permafrost melting. Many fresh and small cracks on the moraine surface indicate that the landslides are enlarging even today. This enlargement could cause hazards when the area is developed: the landslides occur on major trails and camping site. In the New Zealand Alps, mountain trails and recreation huts have been moved or demolished as a result of landslides on lateral moraines (Blair 1994). Because the similar features are observed in other Himalayan regions as well, monitoring landslides at higher altitudes should be a priority concern.