Quaternary River Response to Differential Uplift, Inferences from Steepness Index and Concavity

The thrust-bound Kashmir valley experiences differential uplift as a result of the presence of two thrust fault systems: the Zanskar Thrust and the Panjal Thrust in the Great Himalayan Range (GHR) and the Pir Panjal Range (PPR), respectively. These sub basins have been greatly affected by these thrust faults, with episodic uplift from the past five million years. The rivers from both the GHR and PPR respond to this differential uplift by adopting changes in their concavity and steepness. The influence of neotectonics on the topography of river longitudinal profiles was performed using semi-automated river profile analysis. Profiles of nine longitudinal rivers of the Jhelum basin, extracted from 30 m DEM, were modelled using area-slope analysis and stream power models in Arc GIS and Matlab software. Two sets of knick points, vertical-step and slope-break knick points, have been recognized, which were later compared with the geological settings and field observations to determine their genesis. We also used the stream-power incision model to calculate the channel steepness index ratio (ksn) and concavity (θ) of separated parts of the river system to compare differential uplift of sub basins. The present study shows that the transient states represented by the occurrence of slope break knick points and vertical-step knick points, related to inhomogeneous substrates and the presence of faults in these segments, are the findings of the present study. However, the relative differential uplift of the two mountain ranges was a key finding in the present work. Based on the steepness index ratio, the rivers from the PPR show higher uplift while the sub basins from the GHR show lower uplift, except for the Madhumati River and the Erin River, which show the higher uplift. However, the concavity index values clearly infer that the sub basins from the PPR show relatively higher uplift than the sub basins from the GHR. The higher uplift of Erin and Madhumati clearly indicates that, in addition to the regional geological structures, local geological structures also play an important role in the differential uplift of a particular area.