Open to closed basin: tectonic and climatic feedback in the evolution of the largest Himalayan lake system

The geomorphological evolution of landscapes is primarily governed by the coupled influence of tectonics and climate, with their relative dominance varying through time and space. In the Ladakh Himalaya, where active deformation intersects pronounced Quaternary climatic fluctuations, this coupling produces a distinct geomorphic signature. The Muglib Valley, formerly the outlet of Pangong Tso, demonstrates a system in which tectonic forcing initiated hydrological reorganisation, subsequently amplified by climatic variability. The region lies along the Karakoram Fault system, where oblique right-lateral slip with a vertical component (3–5 mm yr⁻¹) has modified basin geometry, offset valley alignments, and generated localised blockages that facilitated lake formation. Detailed geomorphic and sedimentological analyses across seven field sections reveal marked spatial variability: stacked gravels and conglomerates represent sustained fluvial aggradation, while thick fan and lacustrine deposits reflect progressive sediment overloading and hydrological stagnation, respectively. Optically Stimulated Luminescence (OSL) ages indicate a steady flow of water from Pangong Tso between ~54 ± 4.3 ka and 21 ± 3 ka. Thereafter, the channel was cut and eventually abandoned at ~9 ± 1 ka. The latter coincides with intensified monsoonal precipitation during the Holocene Climate Optimum, which enhanced sediment flux and triggered fan progradation, ultimately blocking the Muglib outlet. This geomorphic transformation converted Pangong Tso from an open to a closed basin, isolating an upstream catchment of ~2000 km² and terminating water and sediment supply to the Tangste River. The findings demonstrate that slow but persistent tectonic activity along the Karakoram Fault primarily governed drainage reorganisation, while monsoon intensification acted as an additive trigger that accelerated fan aggradation and hydrological isolation. The Muglib system thus provides an example of coupled tectonic–climatic feedback that has reshaped the fluvial architecture and sediment connectivity in the Trans-Himalayan landscape.