Active tectonic deformation of the Shillong plateau, India: Inferences from river profiles and stream-gradients

Abstract

The Shillong plateau in northeast India constitutes an actively deforming anticlinal basement uplift in the foreland of the Himalayas in the north and the Indo-Burman ranges in the east. The study aims to document zones of active crustal deformation in the Shillong plateau through the interpretation of the geometry and the quantitative analysis of stream profiles derived from 30-m SRTM DEM.Longitudinal profiles of the streams draining the plateau exhibit conspicuous convex upward reaches, mainly to the upstream of faults and in areas of deep river incision. Analysis of river profiles using Hack’s stream-gradient index (SL-index) reveals perturbations in profiles and steep channel-gradients in the middle reaches of the streams.Spatial distribution of the SL-index brings out extensive zones of anomalously high stream-gradients along the monoclinal flexure at the southern margin of the plateau adjacent to the Dauki fault and along the northern margin, where multiple en echelon faults define step-like topography. In the western part of the plateau, spatial association of zones of high stream-gradients with well-defined fault-scarps is suggestive of active crustal deformation by differential vertical movement along faults.The observed spatial distribution of anomalously high stream-gradients is uncorrelated with the lithological variations and hence, perturbations in river profiles and abrupt changes in stream-gradients are suggestive of differential tectonic uplift of the plateau. The study suggests that the rate of active tectonic deformation in the plateau decreases from the east to the west.