<p>Brittle deformation along the fault zone plays a key role in modulating the physical and chemical properties of the rock in the vicinity by generating dense fracture system, and enhancing fluid circulation, which collectively enhance chemical weathering, reduce rock strength, and promote slope instabilities. In the tectonically active Himalaya, numerous faults compound the deformation processes, making it an ideal natural laboratory to investigate the fault-induced geomorphic and geochemical alteration. The geological mapping and structural analysis reveal the dextral slip behaviour of the Kaliasaur Fault, a prominent NE-SW trending strike-slip fault in the Garhwal Lesser Himalaya, which is accommodated by a family of synthetic and antithetic Riedel shear system. The fault system has significantly influenced the surrounding litho-units, especially the metabasic rocks, by increasing the fracture density and promoting the chemical weathering. Weathering indices indicate intermediate weathering with the formation of illite–smectite clays in the fault vicinity. Numerous small outcrop-scale wedge and planar failures observed along with two major landslides, namely the Kaliasaur and Narkota landslides, situated within the fault zone which exhibit chronic instability and large-scale mass failure in the region. The findings highlight the critical role of the Kaliasaur Fault in controlling the local geomorphic evolution, chemical weathering dynamics, and slope stability in the area, which has implications for infrastructure planning and hazard mitigation in the monsoon-dominated, tectonically active Garhwal Lesser Himalaya.</p>