Active tectonic deformation along reactivated faults in Binta basin in Kumaun Himalaya of north India: Inferences from tectono-geomorphic evaluation

摘要

We discuss the tectono-geomorphic evaluation of the Binta basin (Lesser Himalaya, India) through quantitative and qualitative morphotectonic analyses at the watershed scale. The basin, drained by the Gagas River and its tributaries, is situated in the zone of the active North Almora Thrust (NAT). Developments of deeply incised V-shaped valleys indicate increased incision capability of streams in response to steepening of hillslope gradients due to neotectonic activity. Prominent features, e.g., off-setting of the NAT by later formed subsidiary faults, deflection of streams, vertical fault scarps, formation of palaeolakes, river ponding and development of fluvial terraces were likely responsible for reformation of the landscape and drainage system due to tectonic perturbations. The morphometric parameters using digital terrain data in the GIS technology and focusing on hydrography (stream length-gradient index, ratio of valley floor width to valley height, transverse topographic symmetry factor and topography (local relief and relief anomaly) verify the rejuvenation of landscape. Anomalously high and low values of stream length-gradient indices of main tributary streams associated with faults and multiple knickpoints along the channel profiles may have been associated with deformational events. Our observations on the tectonic instability of the faulted basin support the existing GPS based studies indicating the maximum crustal shortening and strain rate with a maximum aerial strain of 6-8 x 10(-7) strain/year and deformation of similar to 15 mm/year immediately north of the NAT, and proving that this tectonically active zone is vulnerable for the future earthquakes.