Quaternary landform study in Kosi and Dabka river valleys in Kumaun sub‐Himalaya: Implication of reactivation of thrusts

Abstract

Landforms developed along the Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) zones in Ramnagar area of Kumaun sub‐Himalaya are studied. The geomorphic analyses suggest their spatial and temporal relationship with the reactivation of the boundary thrusts, faults, and associated later developed transverse faults. Mapping of geomorphic features and structural analysis of the MBT and HFT established an intimate relationship between thrusting of rocks along these major thrusts and evolution of landforms. The reactivations of the MBT and HFT have resulted in recent rejuvenation of landscapes formed in the thrust zones. In the MBT zone, the swerving of the channels, linear development of elongated depressions, sudden topographic breaks, and presence of relatively fresh‐looking escarpments, uplifted present ground surfaces, V‐shaped valleys, and occurrence of linear series of dormant and active landslides are observed. Along the Kosi River section, the vertical‐dipping Neoproterozoic Lesser Himalayan rocks have overridden the recent landslide debris along the subsidiary steep reverse faults associated with the MBT. The HFT is topographically represented by an escarpment trending ESE‐WNW along the Himalayan mountain front. In the HFT zone particularly at Ramnagar area, four levels of terraces are noted, the highest being a strath terrace. The surface trace of the HFT is offset by a transverse N‐S trending Ramnagar Fault, which coincides with the course along which Kosi Rver flows. Uplift of the hanging wall block near the HFT has resulted in subsidence of the present ground surface towards the northeast away from the HFT trace. During this process, the transverse streams have vigorously incised the uplifted surfaces. Rivers and streams realigning with the bedrock structure suggest tectonic control on morphology of drainage patterns. Neotectonic activity along the intra‐Siwalik thrust has offset a terrace surface into two surfaces that are separated by a fault scarp of about 50 m high. The bedrocks in the HFT region are gentle to steep dipping towards NW/NE/ESE. Both normal and reverse faults are recognized in the bedrocks. The general trend of the lineaments, which are concentrated in the thrust zones, is E‐W. Two transverse faults trending nearly N‐S cut and offset the HFT. Tectonic activity during the deposition of the Upper Siwalik rocks is evident from the occurrence of liquefaction features. Soft‐sediment deformation structures are observed in fine‐ to medium‐grained friable sandstones. These include up‐welled structures (with vertical to inclined bedding planes), sand dykes, faults (reverse and normal), folds, pseudo‐nodules, and deformed laminations.