Using monazite and zircon petrochronology to constrain the P–T–t evolution of the middle crust in the Bhutan Himalaya

The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition (HREE, Y, Eu_N/Eu*_N) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time (P–T–t) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c. 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐T, medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.     

Luminescence chronology of the Sankosh river terraces in the Assam- Bhutan foothills of the Himalayas: Implications to climate and tectonics

Himalaya is an active fold and thrust belt formed due to continent-continent collision between the Eurasian and Indian plates. It comprises a 3000 km long chain of mountains that span ∼1000 km across, with major boundary thrusts viz., Main Central Thrust (MCT), Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). MFT is marked as mountain front and is the most active thrust; however, evidence of tectonic activity along MCT and MBT also exists.Tectonic activity along MFT created uplifted terraces which now serve as geomorphic archives of past tectonic events. The present study focussed on a glacial-fed river Sankosh that originates in northern Bhutan, and crosses MCT, MBT and MFT before joining the Brahmaputra River in Assam. Due to tectonic uplift, the river shows a deflection at MFT, incising and thus forming four levels of strath terraces. Luminescence chronology, geomorphic studies and analysis of satellite images suggest four levels of terraces T4 (highest level, 195 m asl), T3, T2 and T1 (lowest level, 120 m asl).The quartz was found insensitive for luminescence dating, and thus fading corrected Infra-Red Stimulated Luminescence (IRSL) ages on feldspar minerals were measured that provided ages of 143-77 ka (T4), 65-36 ka (T2) and 35-14 ka (T1), respectively. The T3 terrace was present only on the right bank of the river and could not be accessed. These ages accord with other studies at the Chalsa and Malbazar, North Bengal (west of the study area) and this regional disposition of similar ages suggest that these formed during glacial-interglacial periods. The strath terraces indicate a time-averaged tectonic uplift with a 0.5 mm/year rate over the past 150 ka.     

Pressure–temperature–structural distance relationships within Greater Himalayan rocks in eastern Bhutan: implications for emplacement models

In the Himalayan orogen, Greater Himalayan (GH) rocks were buried to mid‐ to lower‐crustal levels and are now exposed across the strike of the orogen. Within the eastern Himalaya, in the Kingdom of Bhutan, the GH is divided into structurally lower (lower‐GH) and upper (upper‐GH) levels by the Kakhtang thrust (KT). Pressure–temperature estimates from lower‐ and upper‐GH rocks collected on two transects across the KT yield similar P–T–structural distance trends across each transect. In the eastern transect, temperatures are similar (from 730 to 650 °C) over a structural thickness of ~11 km, but peak pressures decrease from ~10 to 6 kbar with increasing structural level. In comparison, peak temperatures in the central Bhutan transect are similar (from 730 to 600 °C), but pressures decrease from 10 to 6.5 kbar with increasing structural level over a structural thickness of ~6 km. The structurally highest sample reveals slightly higher pressures of 8.0 kbar in comparison to pressures of ~6.5 kbar for samples collected from within the KT zone, ~4 km below. Within each transect, there are increases in pressure ± temperature within the overall upright P–T gradient that may demarcate intra‐GH shear zone(s). These P–T results combined with evidence that the timing of initial melt crystallization becomes older with increasing structural level suggest that the intra‐GH shear zones emplaced deeper GH rocks via progressive ductile underplating. These shear zones, including the KT, likely aided in the initial emplacement and construction of the GH as a composite tectonic unit during the Late Oligocene to Early Miocene, from c. 27 to 16 Ma.     

Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan

U–Pb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LA–MC–ICP–MS U–Pb lower intercept cooling ages of 10.1 ± 0.4, 10.8 ± 0.1 and 10.0 ± 0.3 Ma, respectively. Numerical finite‐difference diffusion models constrained by previously published temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ～800 °C at 14 Ma in the granulite‐bearing unit and ～650 °C at 12 Ma in the amphibolite‐bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation‐related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile‐bearing samples. Instead, the titanite age suggests cooling from ～650 °C at an earlier time of 17–15 Ma, implying that the high‐grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high‐grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 °C Ma^?1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along‐strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.     

Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year^?1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.     

Abnormal disappearance of Duoqing Co lake between November 2015 and April 2016, due to far-field aseismic creeping of the southern Yadong-Gulu rift of Tibet,triggered by the 2015 Ms 8.1 Nepal earthquake

ABSTRACT

Duoqing Co is a 60 km² outflow lake in the N-trending Pagri graben, located at the southern end of the Yadong-Gulu rift in Tibet. The water in this lake suddenly disappeared between November 2015 and April 2016, closely following the Ms 8.1 (Mw 7.8) Nepal earthquake in April 2015. Both, geomorphological and remote sensing data indicate the existence of blind faults striking NNE along the east boundary of Duoqing Co lake. There were also several nearly NE-trending extensional cracks preserved in the dried lakebed, apparently formed in response to creeping deformation of the underlying rock. Based on field studies and analysis of meteorological and remote sensing data, it is suggested that this phenomenon cannot be explained by evaporation linked to climate change nor can it be related to human activity. Instead, it is considered that the lake water drained through the extensional cracks formed in the lakebed as it responded to the far-field effects of the 2015 Nepal earthquake. It is proposed that a shift in regional tectonics occurred as a result of the Nepal earthquake, causing a sharp increase in stress accumulation along the seismically locked Bhutan–Sikkim zone on the Main Himalayan Thrust (MHT) fault, which was accommodated by the extension of the Pagri graben in the southern Yadong-Gulu rift. It is believed that the crust may have reached a critical stress-state that resulted in strain hardening and brittle failure throughout the region along the Bhutan–Sikkim segment of the MHT. If so, considering the high potential for tectonic activity along the segment of the MHT, it may be worth paying attention to deformational changes and potential geomorphological precursors that might appear in the seismically locked Bhutan–Sikkim gap to predict future earthquakes.     

Metamorphic history of a syn‐convergent orogen‐parallel detachment: The South Tibetan detachment system,Bhutan Himalaya

The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.     

Prediction and assessment of multiple glacial lake outburst floods scenario in Pho Chu River basin,Bhutan

In this study, the characteristic of multiple glacial lake outburst floods (GLOFs) in the Pho Chu River basin in Bhutanese Himalayas is evaluated to help assess the potential impact. Thorthormi Cho (TC) and Lugge Cho (LC) in the east branch and two unnamed lakes labelled A and B in the west branch of Pho Chu are chosen for the study. Numerical models were employed to simulate different involved processes. The results show that the peak sediment discharge in the east branch of the Pho Chu River by the TC dam breach reached about 5000 m³/s (during the first GLOF) at 4 km whereas by the LC dam breach is about 600 m³/s (second GLOF) at 6 km. However, the highest peak hydrographs (sediment and water mixture) calculated during the first and second GLOF are about 10 000 m³/s at the 18‐km section and about 23 000 m³/s at the 10‐km section, respectively. In the west branch of Pho Chu, erosion and depositions are the frequent intermittent local processes during the first GLOF event from Lake A. Because the first event stabilized the irregular river bed profile, there is not much sediment discharge developed during the second GLOF from Lake B. At the 17‐km section of the west branch, the peak hydrograph reached about 9000 m³/s during the first event against the peak of about 800 m³/s during the second event. The results suggest that even if multiple dam breaches occur simultaneously, GLOF surges pass through the main river channel at different times with very different flood characteristics. The differences in travel time and flood characteristics mostly depend on the distributions of bed slope and potential erosion depth along the reach. Further, the amount of sediment accumulated in and transported by each surge is reliant on the temporal geomorphologic setting of the river and therefore on the impact of the previous GLOF on riverbed profile and potential erosion depth. The robustness in peak GLOF hydrographs is associated with sediment flow dynamics. As a consequence, serious inundation of Punakha, Lobeysa and major portion of Wangdue Phodrang is anticipated. Copyright © 2011 John Wiley & Sons, Ltd.     

GROSS NATIONAL HAPPINESS AND ENVIRONMENTAL STATUS IN BHUTAN*

ABSTRACT. The Himalayan Kingdom of Bhutan is in the midst of transformation as it moves from an isolated past to a modern nation‐state and participant in the global community. Its development path embraces the concept of “Gross National Happiness,” a philosophy and policy instrument that seeks to promote human development and manage environmental conservation within a sustainable strategy guided by Buddhist ethics. After examining Bhutan's approach to development and the governance and environmental policies stemming from it, this essay assesses its impacts on environmental conditions in the country.     

Carbon neutral policy in action: the case of Bhutan

Climate policy across the world is proceeding at a highly variable pace, with some places very committed to decarbonizing their economies and others just beginning. Emerging nations are generally just starting along this journey. However, among the few nation states that have pledged to achieve carbon neutrality, is Bhutan, a least developed country. Carbon neutrality is an ambitious climate policy that is increasingly being recognized as necessary in order to stabilize global temperature rise at 1.5°C. However, Bhutan is likely to face significant challenges in maintaining this status as the country balances its desire to grow in economic opportunities (GDP) and in human happiness (GNH). Little research has been conducted inside the policy processes to better understand how Bhutan will maintain carbon neutrality. Through open-ended, semi-structured interviews with key stakeholders, this study provides an inside view on the current situation and future challenges that Bhutan may face, along with the complexities associated with implementing and maintaining an ambitious carbon neutral policy. The paper highlights Bhutan's story and how it could be useful for policy learning and knowledge sharing, especially in the context of emerging nations’ climate governance.