Bhutan Himalaya Research Articles

State of Tectonic Stress in Northeast India and Adjoining South Asia Region: An Appraisal

An attempt is made to map the spatial variation of the tectonic stress pattern in northeast India and its adjoining south Asia region using stress tensor inversion of some 516 fault‐plane solutions. The Bhutan Himalaya and the Arunachal Himalaya are mapped with north–south to north‐northwest–south‐southeast compression. The eastern Himalaya syntaxis zone, on the other hand, shows a clockwise rotation; a north‐northeast compression is dominant. To the south, in the intraplate part of the region, the Shillong plateau, Assam valley, Bengal basin (Bangladesh), and Tripura fold belt exhibit north‐northwest to north‐northeast compression. Orthogonal horizontal extension is dominant in southern Tibet, Bhutan, and partly in the syntaxis zone, and the same is also observed in the Shillong plateau and Assam valley area of the intraplate region. The Indo–Burma ranges and the Sagaing fault in the Myanmar region show a northeast–southwest compression; an orthogonal horizontal northwest–southeast extension is also observed in the Sagaing fault zone. A depth variation of the tectonic stress is observed below the Indo–Burma ranges; it changes from north–south to northeast–southwest in the southern part, and from northeast–southwest to north‐northeast–south‐southwest in the northern part in the deeper seismogenic zone. The stress inversion results of clusters of events in individual zones, though mostly conformable with the average observations, indicate a variation in the Shillong plateau due to heterogeneity and tectonic complexity.

Tectonic landform around Gelephu in southern Bhutan Himalaya

Tectonic landform around Gelephu in southern Bhutan Himalaya

Seismic landscape from Sarpang re-entrant, Bhutan Himalaya foredeep, Assam, India: Constraints from geomorphology and geology

Geomorphic landscape and late Quaternary geological attributes from the Raidak–Manas interfluve in the Bhutan–Himalayan foothills, Kokrajhar District, Assam led towards documenting the east–west trending, south dipping, 30km long active Frontal Back Thrust (FBT), well within the foredeep south of the Main Frontal Thrust (MFT). Spectacular north facing 6–50m high tectonic-scarp generated by the north-propagating emerging thrust front along with a complementary subdued south-facing scarp defines the terrain as a pop-up structure. The entire belt is made up of 5 to 8km wide six distinct blocks, separated by antecedent rivers/streams. Scarp parallel east–west drainage along with linear lakes characterises the emerging thrust front. Field evidence for a major fault-propagation fold structure along with thrust faulting within the late-Quaternary fluvial sediments is ubiquitous. Clay beds deposited in lakes along the footwall of FBT have formed due to blockade of south flowing rivers by episodic upliftment of the hanging wall block; three such episodes of uplift since 16k years correspond to three morphogenic earthquakes of magnitude ~6.9 rupturing the FBT during late Pleistocene–Holocene. In light of geomorphological and geological studies, neotectonic activity has been modelled as an active south dipping backthrust that originates at shallow crustal depth from south vergent basal Himalayan Decollement in response to the advancing Himalayan wedge.

多時期のデジタル標高モデルから復元された1960 年代以降のブータンヒマラヤ・ルナナ地方の氷河の高度変化

多時期のデジタル標高モデルから復元された1960 年代以降のブータンヒマラヤ・ルナナ地方の氷河の高度変化

Recent melting status at the upstream part of the Gangjoe La Glacier, Bhutan Himalayas, determined by pollen analysis of an ice core

Recent melting status at the upstream part of the Gangjoe La Glacier, Bhutan Himalayas, determined by pollen analysis of an ice core

Comparative analysis of hydroclimatic changes in glacier-fed rivers in the Tibet- and Bhutan-Himalayas

Abstract Both glacial and climatic changes have increased the amount of water supplying Himalayan rivers. Kurichu River in the Bhutan-Himalayas (BH) and Karuxung River in the Tibetan Himalayas (TH) were selected as representative glacier-fed rivers with high glacier coverage (20.8% and 14.6%) in their respective watersheds and 20 years of hydrometeorological data. The Mann–Kendall trend test and correlation analysis were used to analyze data from the gauge stations (1600 m and 4550 m asl, respectively) and related meteorological stations (2600 m and 4450 m asl). The results indicate that there are close correlations between monthly air temperature and discharge in the studied watersheds. In contrast, annual runoff has a significant increasing trend with increasing air temperature and decreasing precipitation in TH, but annual runoff decreases in BH with increasing air temperature and decreasing precipitation. Monthly runoff was the most sensitive parameter to climatic warming, especially during autumn and winter. It was concluded that far more trends were observed than were expected to occur by chance. In the past 20 years, the mean annual air temperature has risen dramatically by 0.38 °C per decade in TH and 0.68 °C per decade in BH, and the mean annual air temperature in BH is 0.26 °C warmer than in TH. The increasing trend in runoff rate in TH varied with different times of the year, increasing by 44% for the winter flow (from October to February), 24% for the spring flow, and 27% for the summer flow, while the streamflow decreased in BH. The change in runoff in TH is mainly affected by climatic warming from April to June, while in BH, the runoff is affected by both air temperature and precipitation when the Indian Monsoon prevails, and summer rainfall has unpredictable influences on the runoff. During the non-monsoon period, the change in runoff is significantly influenced by air temperature and subsurface water, and the reduction in runoff in BH is balanced by the increasing runoff in TH.

Evaluation and parameterization of ATCOR3 topographic correction method for forest cover mapping in mountain areas

A topographic correction of optical remote sensing data is necessary to improve the quality of quantitative forest cover change analyses in mountainous terrain. The implementation of semi-empirical correction methods requires the calibration of model parameters that are empirically defined. This study develops a method to improve the performance of topographic corrections for forest cover change detection in mountainous terrain through an iterative tuning method of model parameters based on a systematic evaluation of the performance of the correction. The latter was based on: (i) the general matching of reflectances between sunlit and shaded slopes and (ii) the occurrence of abnormal reflectance values, qualified as statistical outliers, in very low illuminated areas. The method was tested on Landsat ETM+ data for rough (Ecuadorian Andes) and very rough mountainous terrain (Bhutan Himalayas). Compared to a reference level (no topographic correction), the ATCOR3 semi-empirical correction method resulted in a considerable reduction of dissimilarities between reflectance values of forested sites in different topographic orientations. Our results indicate that optimal parameter combinations are depending on the site, sun elevation and azimuth and spectral conditions. We demonstrate that the results of relatively simple topographic correction methods can be greatly improved through a feedback loop between parameter tuning and evaluation of the performance of the correction model.

The age and rate of displacement along the Main Central Thrust in the western Bhutan Himalaya

In western Bhutan, the Main Central Thrust (MCT) is broadly folded, creating multiple exposures of the fault surface over a ~70km across-strike distance. This unusual map pattern presents a unique opportunity to map the MCT and document both the magnitude and age of displacement. In situ Th-Pb (SIMS and LA-ICP-MS) geochronology of metamorphic monazite from the immediate hanging wall of the MCT indicates that prograde monazite growth in Greater Himalayan (GH) rocks continued until 20.8±1.1Ma, whereas crystallization of in situ melts, characterized by high Y monazite overgrowths, occurred during cooling from ca. 15–10Ma. Prograde monazite growth at 15Ma in Lesser Himalaya (LH) rocks in the immediate footwall requires that LH footwall strata began to be buried at this time, and the MCT had reached its southernmost, exposed extent. By combining prograde monazite ages in the immediate hanging wall and footwall, the duration of MCT displacement is bracketed between 20.8±1.1 and 15.0±2.4Ma. Immediately north of our study area, a published estimate of shearing along the outer-South Tibetan detachment (STD) argues for displacement between 20 and 15Ma, coeval with the age range for MCT displacement that we document in this study. However retrograde monazite grains as young as 10Ma suggest that GH rocks were cooling until ~10Ma, 5Myr later than motion on the outer-STD immediately to the north. This cooling was either the result of continued displacement on the MCT, or growth of a duplex that passively folded the MCT. Using a sequential reconstruction, we estimate a total displacement of ~230km, which is the sum of displacements on the MCT and the structurally-lower Paro Thrust, over a duration of 5.8±2.6Myr. This indicates a horizontal shortening rate of 4.0+3.2/−1.3cm/yr, which exceeds present rates estimated from geodetic measurements across the Himalaya, and MCT displacement rates (c. 2cm/yr) inferred from petrologic and thermal models in central Nepal but is indistinguishable from plate convergence rates calculated for eastern Bhutan between 23 and 20Ma (3.3±0.7cm/yr). Our study highlights that displacement on the MCT alone achieved plate velocity rates in western Bhutan, and that the age and rate of MCT displacement varied significantly across the Himalaya.

Extreme rainfall events over the Himalayas between 1871 and 2007

Currently there is much discussion regarding the impact of climate change and the vagaries of the weather, in particular extreme weather events. The Himalayas form the main natural water resource of the major river systems of the Indian region. We present a brief review of the available information and data for extreme rainfall events that were experienced in different sectors of the Himalayas during the last 137 years (1871–2007). Across the entire Himalayas, from east to west, there are now 822 rainfall stations. There was an increase in the rainfall station network from 1947 onwards, especially in the Nepal and Bhutan Himalayas. Extreme one-day rainfall has been picked out for each station irrespective of the period for which data are available. The decadal distribution of these extreme one-day rainfalls shows that there is a considerable increase in the frequencies during the decades 1951–1960 to 1991–2000, whereas there is a sudden decrease in the frequencies in the present decade during 2001–2007, indicating the need to understand the response of the systems to global change and the associated physical and climatological changes. This is essential in terms of preserving this natural resource and to encourage environmental management and sustainable development of mountain regions. Citation Nandargi, S. & Dhar, O. N. (2011) Extreme rainfall events over the Himalayas between 1871 and 2007. Hydrol. Sci. J. 56(6), 930–945.

Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling

[1] The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low-temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along-strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north-south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N-S transects in the high-relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low-temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along-strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal-scale ramp) strongly control the kinematics and exhumation history of the orogen.

Low seismicity in the Bhutan Himalaya and the stress shadow of the 1897 Shillong Plateau earthquake

The seismicity of the Bhutan Himalaya region is generally low as compared to its adjoining Himalayan segments. The topography in the region is slightly subdued and different from the neighbouring central Nepal region. Low seismicity in the region may possibly be due to the postulated low convergence rate in the Bhutan Himalaya, difference in subsurface structures, or the aseismic nature of the region. We suggest that the lower seismicity in the Bhutan Himalaya may also be due to stress change caused by the great 1897 Shillong Plateau earthquake as the low seismicity region of the Bhutan Himalaya coincides with the stress shadow of the earthquake.

Using isotopic and chronologic data to fingerprint strata: Challenges and benefits of variable sources to tectonic interpretations, the Paro Formation, Bhutan Himalaya

[1] We combine detrital zircons (DZ) and epsilon neodymium (Nd) signatures with field mapping in the Paro Formation in western Bhutan. DZ age spectra are strongly variable and display signatures that have been used to uniquely identify both Greater Himalayan (GH) and Lesser Himalayan (LH) strata. DZ age peaks from six quartzite samples require sources for ∼0.5, 0.8, 1.2, 1.4, 1.7, 1.8, and 2.5 Ga zircons in the Paro Formation. The youngest (∼0.5 Ga) zircons argue for a Cambrian maximum deposition age. Two samples have a youngest 1.8 Ga peak typically attributed to Paleoproterozoic LH rocks. A ∼450 Ma crystallization age from two granite samples constrains the minimum deposition age as Ordovician. New Nd signatures from six detrital samples from the Paro Formation show significant variation with lithology. Schists have Nd(0) values between −12.0 and −16.9, while quartzite values vary between −18.8 and −24.5. These data imply that the Paro Formation was derived from both young and old sources, with DZ and Nd values obtained from the same quartzite samples requiring old detritus while the Nd values obtained from interbedded schist require younger detritus. Using published isotopic and chronologic definitions of Himalayan strata, schist‐rich layers wouldbeconsideredGH,whiletheinterbeddedquartzite

The 2009 Bhutan and Assam felt earthquakes (Mw 6.3 and 5.1) at the Kopili fault in the northeast Himalaya region

Seismotectonics of the two recent earthquakes, one Mw 6.3 in the Bhutan Himalaya on 21 September 2009 and the other Mw 5.1 in the Assam valley on 19 August 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east–west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicentre of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone may be vulnerable to an impending larger earthquake (M > 7.0) in the region.

The integration of terrestrial laser scanning and numerical modelling in landslide investigations

Abstract Terrestrial laser scanning (TLS) can be used to either complement or replace traditional methods of characterizing both the geometry and structural geology of unstable slopes. TLS data collected from a failed bedrock slope threatening the main east–west highway in the Bhutan Himalaya are presented and interrogated for structural information. The structural data, along with TLS-derived slope geometry and cross-sectional profiles, are suitable for use within commercially available slope stability packages to derive solutions for the causes of instability, likely geometry of failure, and future activity under varied scenarios. The methods also allow the possibility of future model verification and calibration though TLS monitoring. The results of TLS-based numerical modelling utilizing a commercially available code are presented and the implications for slope surveying, numerical modelling, monitoring and management are discussed.

Placing limits on channel flow: Insights from the Bhutan Himalaya

Along the majority of the Himalayan orogenic belt, high-grade metamorphic rocks of the Greater Himalaya (GH) are separated from lower-grade metasedimentary rocks by two shear zones, the basal, top-to-the-south Main Central Thrust (MCT), and the upper, top-to-the-north South Tibetan Detachment (STD). Seemingly pervasive ductile deformation within GH rocks and a lack of exposed hanging wall or footwall cut-offs for the MCT and STD have permitted models that predict 100's of km of coeval displacement on both structures, extruding the GH section via gravitational loading from Tibet and focused erosion along the Himalayan topographic front. We present new mapping, stratigraphic columns, mineral assemblages, U/Pb zircon ages, and a suite of structural data from GH and Tethyan Himalayan (TH) rocks in eastern and central Bhutan. Our observations highlight significant, ∼ 250–300 °C along-strike changes in the deformation temperature conditions recorded by GH rocks. In eastern Bhutan, TH rocks are separated from GH rocks displaying ubiquitous partial melt textures by a top-to-the-north sense shear zone correlated with the STD. However, in central Bhutan, limited partial melt textures are only present near the base of the GH section, a biotite–muscovite–garnet mineral assemblage persists from 0.2 to 3 km above the MCT through TH rocks, and distinct lithologies interfinger at the GH–TH contact, which suggests that TH strata are in depositional contact above GH strata. The same strata exposed on both sides of the STD limits slip along this structure to ∼ 20 km. Thin-section scale top-to-the-south shear is focused in a 2–3 km-thick zone above the MCT while the overlying ∼ 11 km-thick section displays top-to-the-north shear, suggesting asymmetric channel behavior. Strained quartz grains allow us to quantify ∼ 23–34 km of top-to-the-north shear. Our data suggest that the GH–TH section in central Bhutan acted as a cool, viscous, low-displacement channel, and when compared to sections in eastern Bhutan highlights dramatic changes in temperature conditions, viscosity and possibly displacement along strike. However, we suggest that the erosion rates needed to remove low-viscosity material limit channel flow even in regions containing significant partial melt, and that the magnitude of channel flow is small compared to the total mass balance of the system.

Formation condition of debris-covered glaciers in the Bhutan Himalaya derived by satellite data

Formation condition of debris-covered glaciers in the Bhutan Himalaya derived by satellite data

Provenance of the Greater Himalayan sequence: Evidence from mafic granulites and amphibolites in NW Bhutan

Mafic granulites and amphibolites in the Masang Kang area of NW Bhutan Himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history. Bulk-rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics. U–Pb zircon SIMS data suggest an age of 1742 ± 39 Ma for mafic magmatism. The age-corrected ε Nd (1742) values of the rocks are highly variable, ranging from high positive (+ 8.4) to negative (− 3.3). The positive value suggests a primitive magma source, similar to that of rift-related tholeiites. We suggest that the rocks of the Masang Kang suite were produced during a major late Paleoproterozoic thermal event that caused the mobilization and enrichment of the sub-continental lithospheric mantle beneath the north Indian margin. The geochemical signature of these rift-related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub-lithospheric mantle may have been derived. Though their occurrence is rare, Paleoproterozoic igneous rocks within the Greater Himalayan Sequence (GHS), in addition to sources identified throughout the LHS, may have contributed to the detrital zircon population that form the 1.7–1.9 Ga peak in the age spectra of the Lesser Himalayan Sequence (LHS). In addition, the coeval Paleoproterozoic magmatism in both LHS and GHS suggests that the two lithotectonic units may have belonged to the same continental plate at that time period.

Regeneration and structure of mixed conifer forests under single-tree harvest management in the western Bhutan Himalayas

We examined the regeneration and structure of mixed conifer forests under single-tree harvest management in western Bhutan. Sixteen 900 m 2 (30 m × 30 m) plots were sampled at four Forest Management Units (FMUs; Chamgang, Gidakom, Paro-Zonglela, and Haa-East) representing the forest type, including half the plots in single-tree harvest stands and half in unlogged stands. In addition, we solicited information on traditional forest management practices from informants using survey questionnaires and collected tree species data from felling records from respective local forest offices. Rural timber demand is concentrated on the removal of straight and well-formed bluepine trees for beams, planks, and scaffolding. Single-tree harvest, however, has not significantly altered stand structures from unlogged stands. Similarly, tree regeneration is not different when comparing single-tree harvest and unlogged stands, except at Chamgang FMU, where seedling densities were generally higher in harvested stands than in unlogged stands. These results indicate that single-tree harvest is not detrimental to regeneration and utilization of mixed conifer forests in western Bhutan.

Regeneration of mixed conifer forests under group tree selection harvest management in western Bhutan Himalayas

Group selection tree harvest has been proposed as an ecologically sustainable silvicultural technique in mixed conifer forests of the western Bhutan Himalayas. To evaluate this silvicultural technique, we studied the ecological consequences of a group selection tree harvest in mixed conifer forests by assessing 127 circular plots (71 in logged and 56 in unlogged stands) in two forest management units (FMUs). Tree species composition and diversity were similar between logged and unlogged stands. Seedling density and height growth vary by species and were influenced by logging and microsites, with generally taller seedlings found in the logged versus unlogged stands. Early successional shade-intolerant species colonized logged stands. Seedlings growing on bare soil scarified by harvesting had medium vigour while seedlings growing on bryophyte mats showed good vigour in both logged and unlogged stands. Moist sites with a northerly aspect supported profuse conifer seedling regeneration, compared to sites with a dry southerly aspect. Damage to conifer seedlings from herbivore browsing was minimal. Conifer seedling density and height growth was negatively affected by competition from herbaceous vegetation, most notably Salvia officinalis. Group selection tree harvest in southern dry exposures in spruce-dominated stands is silviculturally unsuitable because it alters tree succession.

Structural and geomorphological features of landslides in the Bhutan Himalaya derived from Terrestrial Laser Scanning

This study presents data on landslides affecting the main west–east transport corridor in the Himalaya of eastern Bhutan close to the Main Central Thrust. The study focuses on the extraction of structural and geometrical information from Terrestrial Laser Scanning (TLS) data utilising a case study landslide, the Namling Landslide. Analyses of the TLS data allow extraction of slide geometry, discontinuity orientations and identification of the mode of failure. The TLS results are compared to a traditional engineering geological survey and prove to yield considerable added value and should form the basis of new approaches to geomorphological studies and site assessment. Macro-scale geomorphological reconnaissance findings are outlined and interpreted within a conceptual model of the evolution of landslide features, here termed hierarchical landsliding. It is concluded that the Nam Ling Landslide is indicative of deeper seated deformation and is not purely a function of road construction.