[H-DS08] Landslides and related phenomena

[HDS08-P11] Geochemical and mineralogical study of low-grade metamorphic rocks and their influence on rock weathering and shallow landslide occurrence in Central Nepal

*Bishow Raj Silwal¹, Babu Ram Gyawali², Kohki Yoshida³ (1.Department of Science and Technology, Graduate School of Medicine, Science and Technology, Shinshu University, 2.Lumbini Engineering Management and Science College ,Pokhara University, Butwal, Nepal, 3.Department of Geology, Faculty of Science, Shinshu University)

Keywords:Weathering, Landslide, Clay Mineralogy, Metamorphic Rocks, Nepal Himalaya

The Nepal Himalaya region is characterized by complex topography, fragile geology, intense rainfall, and seismic activity, and thus, this region experience several types of natural disasters, including landslides and mass movement. This work aimed to explore the geochemical and mineralogical characteristics of low-grade metamorphic rocks of the Kuncha Formation in central Nepal and their influence on rock weathering and shallow landslide occurrence. The rocks of the Kuncha Formation consist of phyllites, metasandstones, and gritty phyllites and are characterized by the formation of shallow landslides. Field characterization of the rock mass and weathering grade within the landslide body, along with petrographic observations, clay mineral analysis and major bulk geochemistry, was adopted to establish a relationship between rock weathering and landslide occurrence mechanisms. The rocks within the landslide area have undergone physical and chemical weathering. Physical weathering was shown by well-developed microcracks and intergranular microfractures, whereas the occurrence of neo-formed clay minerals and the conversion of biotite-muscovite to vermiculite, kaolinite, and mixed-layer clays indicated an effect of chemical weathering. The weathering of rock and development of clay minerals were strongly correlated with the landslide typology in the study area. The CIA ranges between 71 and 80 for rock samples and between 72 and 84 for soil samples, which indicates moderate to extreme weathering effects. The higher values of PIA and CIW reveal K-feldspar and plagioclase alteration to clay minerals by weathering and alteration. CIA-LOI plots reveal significant relationships corresponding to weathering effects. The transition from a fresh to a moderately weathered state and the development of clay minerals played crucial roles in shallow landslide occurrence. The weathering of the rock mass has weakened the physical properties coupled with the fractures and joint conditions, leading to instability of the hillslopes in the study area.