Morphology and controls of the alluvial fan systems along the Zanskar Shear Zone, Northwest Himalaya, India

Abstract

Alluvial fans are prominent geomorphic landforms that develop at the junction of mountain slopes and valley floors. This study investigates the morphological characteristics of alluvial fans along the Zanskar Shear Zone to identify the key variables influencing their development and morphodynamics. Despite being among the world's most significant semi-arid terrestrial fans, their morphology and controlling factors remain largely unexplored. Using advanced remote sensing and GIS techniques, combined with field investigations, we conducted a detailed spatial analysis of 37 fans, located on both sides of the Doda River. This approach enabled precise mapping and characterization of these fans within the complex depositional settings of the Doda Valley, particularly where fans coalesce into bajadas. Our analysis revealed significant variability in fan attributes, including Flow Expansion Angle (FEA), Fan Entrenchment (FME), Base length (BF), Fan Area (FA), Fan Slope (SF), and Radius (R). A morphometric analysis was then conducted to evaluate the correlation between the fans and their corresponding basins. To deepen this understanding, power law regression was applied, revealing both positive and negative relationships between fans and basin characteristics. The findings underscore the key role of upstream basins in regulating sediment delivery to the fans. Larger basins contribute to the development of larger, less steep fans, driven by higher sediment supply and greater flows from basins with denser drainage networks. Low values of Smf Index, Vf ratio and Drainage Basin Shape suggest that upstream basins are significantly influenced by tectonic forces. These forces result in linear mountain fronts, V-shaped valleys and elongated basins, indicative of active tectonic deformation. Along the mountain fronts of the Doda Valley, fan morphology is controlled by a complex interplay of long-term tectonic activity, climate, upstream lithology, and basin characteristics. Tectonic forces, particularly those associated with the NW-SE-trending ZSZ, exert a primary influence on fan morphology by controlling sediment-flux and accommodation space. This is evident in tectonically modified landforms, such as wine-glass valleys with narrow outlets, triangular facets with broad bases and active mountain fronts, all indicative of recent uplift and ongoing tectonic activity. These findings strongly suggest that tectonic structures like the ZSZ, which dips approximately 20 degrees NE and continuously responds to tectonic collision, play a pivotal role in shaping fan morphodynamics. Additionally, climatic factors, such as increased glacial melt and freeze-thaw cycles, enhance erosion in upstream basins, particularly those fed by glaciers on the southwestern side of the valley. This process amplifies sediment supply to the fans, highlighting the dual influence of climate and tectonics in shaping regional fan systems. Overall, the findings demonstrate that ZSZ exert a dominant influence on the geometry and evolution of these fans, with climatic factors significantly contributing to their formation.