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

Abstract

Abstract In tectonically active orogen like the Himalaya while computing the bedrock incision rate it is assumed that the underlying incised bedrock is genetically related to the overlying alluvium. This assumption may not be applicable universally, considering that the trans-Himalayan rivers were in existence before Miocene or earlier. Therefore, the sediments preserved at various locations represent the youngest phase of aggradation on the pre-existing valley topography – a cumulative expression of multiple cycles of erosion and aggradation. Considering that the fluvial terraces are extensively used to estimate the incision/uplift rates, it is therefore pertinent to ensure that the underlying bedrock topography is indeed genetically related to the overlying fluvial sediments and is not a relic of the pre-existing topography. 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.