The role of pre-existing topography in the evolution of post-glacial fluvial landforms in the middle Satluj valley, north-western Himalaya, India

The present study is aimed at understanding the relationship between the valley and channel-fill sediments and the underlying bedrock topography in the evolution of fluvial landforms. Based on the detailed field mapping of the relict channel/fossil valley-fill sequences, geomorphometry and limited optical ages the study suggests that the bedrock topography is cumulative expression of multiple cycles of erosion while the post glacial valley-fill sediments (dated to 14 ka and 8 ka) represent the most recent aggradation phase. The discreet linear strath terraces and epigenetic gorges represent a combination of climate and tectonically induced mid-Holocene river adjustment particularly in the areas proximal to local thrusts. Further, the study suggests that the securely dated strath terraces may provide the average tectonically induced incision/uplift rates whereas the epigenetic gorges largely represent climatically controlled focused and local uplift/incision in the study area. A preliminary estimate based on the strath terraces indicate that the terrain is being incised/uplifted at ∼0.6-1.3 mm/yr whereas as expected the epigenetic gorges provide a relatively higher incision rate (3.8 mm/yr to 12.5 mm/yr). Finally, the study cautions against using the conventional age-depth relationship of valley-fills and the underlying incised bedrock to ascertain the incision/uplift rates. Instead it suggests that the morphology of the underlying bedrock must be taken in to account in order to avoid erroneous estimate of incision/uplift rates (crustal deformation) in a tectonically active orogen.