Climate‐driven alluvial fan aggradation and incision in an unglaciated Himalayan basin, Northwestern India

Summary Remnants of thick alluvial fills in Himalayan valleys and intermontane basins record past disruptions in sediment routing. However, the climatic drivers of these aggradational phases remain debated. Do they reflect enhanced hillslope sediment supply during intensified monsoon phases, or reduced fluvial transport capacity under drier conditions? To address this question, we analyze an 55‐m‐thick late Pleistocene alluvial fan succession deposited in the structurally confined Pinjore Basin of the Northwestern Himalaya, sourced from an 350‐km 2 unglaciated catchment. Using optically stimulated luminescence dating and in situ 10 Be measurements from buried sediments, we reconstruct the timing of fan deposition and catchment‐scale paleo‐erosion rates to assess links between monsoon variability, sediment supply and fluvial transport capacity. Fan aggradation persisted for 39 kyr between 52 and 13 ka, coinciding with the prolonged weakening of the Indian Summer Monsoon. Low and relatively stable erosion rates throughout most of this interval suggest that aggradation was primarily driven by reduced runoff and limited fluvial transport capacity, rather than increased sediment supply. After 26 ka, declining sedimentation rates, sediment coarsening and lower inherited 10 Be concentrations may reflect reduced slope stability due to vegetation changes associated with glacial cooling. Renewed fan incision after 13 ka coincides with monsoon strengthening, indicating a shift toward increased transport capacity. These results highlight a climate‐sensitive, threshold‐controlled sediment‐routing system in which changes in runoff and vegetation drive aggradation–incision cycles. The Pinjore Basin record underscores the potential for nonlinear fluvial responses to hydroclimatic variability in tectonically active mountain landscapes.