The Lesser Himalayan Sequence of the Taplejung Window in the far eastern Nepal Himalaya can be divided into Taplejung Formation, Mitlung Augen Gneiss and Linkhim Schist (from bottom to top respectively). The window is a large domal shaped anticline plunging to the east.
Two-mica granite bodies (Amarpur Granite, Kabeli Khola Granite and Tamor River Granite) have intruded the metasediments of Taplejung Formation. The granite bodies are discordant to subconcordant in relation to the country rocks. Quartz, alkali feldspar, plagioclase, muscovite, biotite and tourmaline are the main constituent minerals of the granite. Generally, the core of granite bodies is undeformed, whereas the marginal part is gneissfied with S-C mylonitic texture showing the top to south sense of shear. This sense of shear is related to the movement along the Main Central Thrust (MCT). All the samples from the granitic bodies fall under the granite field in the normative quartz-alkali feldspar-plagioclase (QAP) triangular diagram. The mineral composition shows that the granite is peraluminous in nature. The Kabeli Khola Granite has yielded a 40Ar/ 39Ar muscovite age older than 1.6 Ga indicating its magmatic age. The granites of the study area can also be correlated with the 1.8 Ga Ulleri type augen gneiss of central Nepal.
In the northern part of the Taplejung area, two tectonic units are recognised. These are the Higher Himalayan and Lesser Himalayan Sequences separated by the Main Central Thrust (MCT). The Lesser Himalayan Sequence is exposed in the Taplejung Window represented by Taplejung Formation, Mitlung Augen Gneiss, and Lingkhim Schist from the bottom to top. The main rock types in the Lesser Himalayan Sequence are phyllite, schist, metasandstone, quartzite and augen gneiss. Three Proterozoic granitic bodies are intruded into Taplejung Formation: Tamor River Granite, Amarpur Granite and Kabeli Khola Granite. The rocks of the Higher Himalaya on the hanging wall of the MCT are composed of kyanite-sillimanite bearing banded gneiss, orthogneiss, migmatite, quartzite.
Three major landslides in the area, viz., Hangdewa, Hireba, and Paire lie along the valley slope sides of the Tamor River. Stream erosion and incision, presence of highly weathered rocks, surface and subsurface drainage are responsible in activating the landslides. Due to the presence of mainly weathered phyllites, favourable structural orientations and wet cultivation practice in the area have contributed to the development of these landslides. The Hangdewa and Hireba landslides have endangered the villages in between them and ultimately the Suketar airport. The landslides are gradually destroying more and more areas of cultivated land and property and increased loss of human lives.
Excessive anthropogenic interference in the form of development activities like unplanned urbanization, deforestation and unfavorable agricultural practices have greatly aggravated the situation. The intensity of damages amplifies during the late part of monsoon each year. The study suggests that further detailed surface and subsurface investigation of the landslides are needed to prevent further loss of lives and property.
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