Sedimentology and Basin analysis of the Thakkhola-Mustang Graben, Central Nepal

Der Thakkhola-Mustang Graben, der eine neogene Extensionstektonik im Tibetischen Plateau und Himalaya belegt, liegt nördlich der Dhaulagiri-Annapurna Kette und südlich der Yarlung-Tsangpo Suturzone. Detaillierte Geländeaufnahmen und sedimentologische Profilaufnahmen wurden durchgeführt sowie Geröllimbrikationen vermessen. Schwermineraluntersuchungen, Dünnschliffanalysen und stabile Isotopenuntersuchungen an Karbonatgesteinen, palynologische Untersuchungen, Komponentenbestand an Sedimenten und petrographische Untersuchungen an Sandsteinen zur Identifikation von Herkunftsgebieten wurden durchgeführt. Im Untersuchungsgebiet konnten 12 Lithofazies in 3 Lithofaziesassoziationen unterschieden werden. Die Assoziationen sind (1) matrix-reiche Konglomerate-kiesige Sandsteine, (2) matrix-reiche Konglomerate mit Sandsteinen und Tonsteinen, und (3) massive Siltsteine mit Tonsteinen und Karbonatlagen. Die Untersuchungen zeigen, dass die Sedimente in Alluvialfächern, in Seen, in Braided Rivers und in glazio-fluviatilen Environments abgelagert wurden. Die Paläoströmungsdaten von Geröllimbrikationen aller Formationen zeigen generell Transportrichtungen nach Süden. Niedrig- bis hochmetamorphe Liefergebiete zeigen die Schwermineralzusammensetzungen der neogenen Sedimente. Pellets, charophytische Algen und onkolithische Algenmikrite sind häufig in der Thakkhola-Formation. In der Tetang-Formation wurden mikritische Kalke mit Ostrakoden, mikritische Tonsteine mit Wurzelstrukturen und Onkolithe gefunden. Die δ18O Werte der Kalke des Thakkhola-Mustang Grabens sind sehr ähnlich jenen von publizierten rezentem meteorischen Wasser des Gebietes. Das belegt, dass der Thakkhola-Mustang Graben schon vor der Ost-West-Extensionsphase seine heutige Höhenlage erreicht hatte. Palynologische Untersuchungen zeigen ein trockenes Klima während der Ablagerung. Es wird auf ein signifikant wärmeres Paläoklima während der Sedimentation im Thakkhola-Mustang Graben im Vergleich zu heute geschlossen.The Thakkhola-Mustang Graben, which reflects Neogene extensional tectonics in the Tibetan Plateau and Himalaya, lies north of the Dhaulagiri-Annapurna ranges and south of the Yarlung-Tsangpo Suture Zone. Detailed field mapping was undertaken in conjunction with sedimentological profile logging and measuring imbricate pebble orientations. Heavy mineral analyses, stable isotope and thin-section analyses of carbonate rocks, palynological investigations, compositional analyses of sediments and petrographic studies of sandstones for provenance analysis were carried out. In the study area, twelve lithofacies were found in three lithofacies associations. The associations are (1) matrix-rich conglomerate-gravelly sandstone association, (2) matrix-rich conglomerate with sandstone and mudstone and (3) massive siltstone with mudstone, alternating with carbonate layers. The investigation revealed that the sediments were deposited in alluvial fan, lacustrine, braided river and glacio-fluvial environments. The paleocurrent data of imbricate pebbles from all formations of the basin show a generally southward flow direction. Minerals from low-to high-grade metamorphic sources are reflected in the heavy mineral assemblages of the Neogene deposits. Pelletal, charophytic algae and oncolitic algal micritic limestones are present in the Thakkhola Formation. In the Tetang Formation micritic limestone with ostracods, micritic mudstone with roots and oncolites have been found. The δ18O values of the limestones from the Thakkhola-Mustang Graben are very similar to those obtained from the recent meteoric water analyzed by previous researchers from the area, indicating that the Thakkhola-Mustang Graben attained its current elevation prior to east-west tectonic extension of the Himalaya. A palynological study indicates a dry climate during sediment deposition. Consequently, it is presumed that the paleoclimate during the evolution of the Thakkhola-Mustang Graben was significantly warmer than the present-day climate

Geomorphological and hydrological controls on sediment export in earthquake-affected catchments in the Nepal Himalaya

Large earthquakes can contribute to mountain growth by building topography but also contribute to mass removal from mountain ranges through widespread mass wasting. On annual to decadal or centennial timescales, large earthquakes also have the potential to significantly alter fluvial sediment dynamics if a significant volume of the sediment generated reaches the fluvial network. In this contribution, we focus on the Melamchi–Indrawati and Bhote Koshi rivers in central Nepal, which have both experienced widespread landsliding associated with the 2015 Gorkha (Nepal) earthquake. Using a time series of high-resolution satellite imagery, we have mapped exposed sediment along the rivers from 2012–2021 to identify zones of active channel deposition and document changes over time. Counter to expectations, we show negligible increases in coarse-sediment accumulation along both river corridors since the Gorkha earthquake. However, an extremely high-concentration flow event on 15 June 2021 caused an approximately 4-fold increase in exposed sediment along a 30 km reach of the channel with up to 12 m of channel aggradation in the Melamchi–Indrawati rivers; this event was localised and did not impact the neighbouring Bhote Koshi catchment. Based on published reports, new helicopter-based photography, and satellite data, we demonstrate that this event was sourced from a localised rainfall event between 4500 and 4800 m and that a significant fraction of the sediment was supplied from sources that were unrelated to the landslides generated by the Gorkha earthquake.</p

The impact of adsorption–desorption reactions on the chemistry of Himalayan rivers and the quantification of silicate weathering rates

A.K. was supported by a NERC DTP studentship (NE/S007164/1). This work was funded by the NERC grant NE/T007214/1.Common environmental adsorbents (clay minerals, metal-oxides, metal-oxyhydroxides and organic matter) can significantly impact the chemistry of aqueous fluids via adsorption–desorption reactions. The dissolved chemistry of rivers have routinely been used to quantify silicate mineral dissolution rates, which is a key process for removing carbon dioxide (CO2) from the atmosphere over geological timescales. The sensitivity of silicate weathering rates to climate is disproportionately weighted towards regions with high erosion rates. This study quantifies the impact of adsorption-desorption reactions on the chemistry of three large Himalayan rivers over a period of two years, utilising both the adsorbed and dissolved phases. The concentration of riverine adsorbed cations are found to vary principally as a function of the concentration and cation exchange capacity (CEC) of the suspended sediment. Over the study period, the adsorbed phase is responsible for transporting ∼70% of the mobile (adsorbed and dissolved) barium and ∼10% of the mobile calcium and strontium. The relative partitioning of cations between the adsorbed and dissolved phases follows a systematic order in both the monsoon and the dry-season (preferentially adsorbed: Ba > Sr & Ca > Mg & K > Na). Excess mobile sodium (Na*=Na-Cl) to silicon (Si) riverine ratios are found to vary systematically during an annual hydrological cycle due to the mixing of low temperature and geothermal waters. The desorption of sodium from uplifted marine sediments is one key process that may increase the Na*/Si ratios. Accounting for the desorption of sodium reduces silicate weathering rate estimates by up to 83% in the catchments. This study highlights that surficial weathering processes alone are unable to explain the chemistry of the rivers studied due to the influence of hydrothermal reactions, which may play an important role in limiting the efficiency of silicate weathering and hence modulating atmospheric CO2 concentrations over geological time.Peer reviewe

Phytochemical screening, antimicrobial activity and cytotoxicity of Nepalese medicinal plants Swertia chirayita and Dendrobium amoenum

Research on medicinal plants are important to Nepal because most of its rural population relies on it as mode of medicine. Medicinal plants namely Swertia chirayita and Dendrobium amoenum were collected from mid hills of Nepal. The present study was undertaken to find the antimicrobial activity, phytochemical presence and their cytotoxicity in different extraction medium. The percentage yield from the plants were highest in warm methanol extraction with 12.6%, followed by ethyl acetate and lowest was for cold methanol. Plant extract showed the presence of antioxidants like alkaloid, terpenoids, flavonoids, tannin, glycosides. The Brine Shrimp Bioassay of methanol and ethyl acetate extract showed cytotoxicity. Chiraito extract showed LC50 of 199 ppm for Dhunche sample, 128.82 ppm for Daman sample and 131.82 ppm of Illam sample. The antibacterial activity of methanol extract of Chiraito and Dendrobium amoenum showed significant bioactivity by inhibiting growth of microbial species selected for the test. The zone of inhibition shown by the extracts was comparable to the standard antibiotics. Similarly, methanol extract of Chiraito also showed significant antifungal activity with the zone of inhibition comparable to amphotericin.Nepal Journal of Biotechnology. Dec. 2015 Vol. 3, No. 1: 48-5

Geotechnical Field Reconnaissance: Gorkha (Nepal) Earthquake of April 25, 2015 and Related Shaking Sequence

The April 25, 2015 Gorkha (Nepal) Earthquake and its related aftershocks had a devastating impact on Nepal. The earthquake sequence resulted in nearly 9,000 deaths, tens of thousands of injuries, and has left hundreds of thousands of inhabitants homeless. With economic losses estimated at several billion US dollars, the financial impact to Nepal is severe and the rebuilding phase will likely span many years. The Geotechnical Extreme Events Reconnaissance (GEER) Association assembled a reconnaissance team under the leadership of D. Scott Kieffer, Binod Tiwari and Youssef M.A. Hashash to evaluate geotechnical impacts of the April 25, 2015 Gorkha Earthquake and its related aftershocks. The focus of the reconnaissance was on time-sensitive (perishable) data, and the GEER team included a large group of experts in the areas of Geology, Engineering Geology, Seismology, Tectonics, Geotechnical Engineering, Geotechnical Earthquake Engineering, and Civil and Environmental Engineering. The GEER team worked in close collaboration with local and international organizations to document earthquake damage and identify targets for detailed follow up investigations. The overall distribution of damage relative to the April 25, 2015 epicenter indicates significant ground motion directivity, with pronounced damage to the east and comparatively little damage to the west. In the Kathmandu Basin, characteristics of recorded strong ground motion data suggest that a combination of directivity and deep basin effects resulted in significant amplification at a period of approximately five seconds. Along the margins of Kathmandu Basin structural damage and ground failures are more pronounced than in the basin interior, indicating possible basin edge motion amplification. Although modern buildings constructed within the basin generally performed well, local occurrences of heavy damage and collapse of reinforced concrete structures were observed. Ground failures in the basin included cyclic failure of silty clay, lateral spreading and liquefaction. Significant landsliding was triggered over a broad area, with concentrated activity east of the April 25, 2015 epicenter and between Kathmandu and the Nepal-China border. The distribution of concentrated landsliding partially reflects directivity in the ground motion. Several landslides have dammed rivers and many of these features have already been breached. Hydropower is a primary source of electric power in Nepal, and several facilities were damaged due to earthquake-induced landsliding. Powerhouses and penstocks experienced significant damage, and an intake structure currently under construction experienced significant dynamic settlement during the earthquake. Damage to roadways, bridges and retaining structures was also primarily related to landsliding. The greater concentration of infrastructure damage along steep hillsides, ridges and mountain peaks offers a proxy for the occurrence of topographic amplification. The lack of available strong motion records has severely limited the GEER team’s ability to understand how strong motions were distributed and how they correlate to distributions of landsliding, ground failure and infrastructure damage. It is imperative that the engineering and scientific community continues to install strong motion stations so that such data is available for future earthquake events. Such information will benefit the people of Nepal through improved approaches to earthquake resilient design

Geotechnical Effects of the 2015 Magnitude 7.8 Gorkha, Nepal, Earthquake and Aftershocks

This article summarizes the geotechnical effects of the 25 April 2015 M 7.8 Gorkha, Nepal, earthquake and aftershocks, as documented by a reconnaissance team that undertook a broad engineering and scientific assessment of the damage and collected perishable data for future analysis. Brief descriptions are provided of ground shaking, surface fault rupture, landsliding, soil failure, and infrastructure performance. The goal of this reconnaissance effort, led by Geotechnical Extreme Events Reconnaissance, is to learn from earthquakes and mitigate hazards in future earthquakes