Influence of source basin lithology on alluvial fan morphology in the trans-Himalayan Cold Desert (Spiti Valley), India

Source basin lithology plays a vital role in shaping alluvial fan morphology, yet such studies are limited, often contradictory, and based on small datasets. This study quantitatively examines 173 fan–basins in the Spiti Valley cold desert (India), where similar climatic and tectonic conditions, but varied lithologies (six groups), provide an ideal setting for such a study. An integrated remote sensing and field approach, with proxies, such as ideal fan-area (Afi), slope of ideal fan (Sfi), selective basin-area (Abs), and lithology-specific angle of repose (to estimate sediment generation/storage), was employed. Results from morphometric analyses indicate that lithology strongly governs fan morphology, with clearer relationships emerging through lithological-group-wise analyses, despite weak correlations for the entire dataset. Erodible lithologies (shale-dominated) form small, steep fans under confined settings but create larger, flatter fans as basin-area and slope increase. Resistant lithologies (high-grade metamorphic and crystalline rocks) consistently produce small, steep fans due to limited sediment supply, while sandstone-dominated rocks generate larger, gentler fans. Limestone and mixed lithologies produce intermediate forms. Irrespective of basin-lithology, basins dominated by glacial processes produce the largest and gentlest fans through effective sediment transport. Basin-slope vis-à-vis angle of repose serves as a useful proxy for sediment dynamics but requires cautious application. Finally, confinement and depositional processes significantly modify fan-morphology, making Afi and Sfi more reliable indicators of lithology–fan relationships.