Neotectonic Significance of Longitudinal River Profiles: An Example from the Banas Drainage Basin, Rajasthan

Longitudinal river profiles can be quantified by normalizing the elevation and the distance along rivers. Several parameters such as profile shape (concavity), gradient fluctuations (knickpoints), river grade and valley incision have been derived from longitudinal river profile. These quantified parameters and their interrelations are useful in comparing different drainage basins. These also help draw inferences on neotectonism. These parameters, either individually or in combination, substitute some hydrological factors such as river capacity and competence, and provide clues to the morphotectonic evolution of drainage basins.

Ten tributaries (5^th, 4^th and 3^rd order) whose cumulative catchment area is ca. 82% of the 6^th order Banas drainage basin, the largest in Rajasthan, have been studied. Their longitudinal profile parameters have been interpreted, especially in respect of neotectonic activities. It is concluded that the Aravalli hill range and the surrounding areas in the SW segment of the basin are being currently uplifted, while the NE block is generally subsiding. Within these major neotectonic blocks, there are several sub-Blocks that have differentially moved nearly vertically to give rise to horst-Graben structures bounded by active faults, generally trending N-S and NE-SW. Some of these faults represent reactivated old dislocation zones. These conclusions have been substantiated by neotectonically controlled geomorphic features. Strike-Slip faulting in the NE neotectonic block and differential vertical movement of the neotectonic sub-Blocks have played an important role in forming the structural depressions and grabens. One such structural depression is the Sambhar Lake that is likely to have formed as a pull apart basin where neotectonic uplift of the sub-Blocks caused stream capture and ponding. Several faults in the area are seismically and thermally active.