Geomorphic assessment of the factors contributing to the evolution of landforms in Ukhaldhunga area, Kosi River valley, Kumaun Himalaya, Uttarakhand

This paper documents the various factors contributing to the evolution of landforms in the Kosi River valley, Ukhaldhunga area of the Kumaun Himalaya, Uttarakhand. The area falls in the major tectonic boundary between the Lesser Himalaya and the sub-Himalaya. Passing through the Parewa-Ukhaldhunga-Dabara area, this tectonic contact is known as the Main Boundary Thrust (MBT), which is characterized by presence of varied landforms. Study of the development of geomorphology, particularly those evolved in the MBT zone, suggests that the morphological features are intimately associated with the tectonic activities related to the movement of rocks along the MBT and the surface processes such as fluvial and mass movement. The various landforms observed in the MBT zone include strath terraces, fill terraces, paleochannels and V-shaped valley that pertain to fluvial genesis; gravity induced landforms such as landslide debris and colluvial fans; and tectonic landforms such as linear fault traces, swerving/swing of river course and linear depressions. Signature of recent tectonic activity along the MBT is observed between Dabara and Khakrakot. The activity is evident in the form of 8-km-long active fault trace trending NW-SE. Normal fault related landforms are observed in the hanging wall block of the MBT. Neotectonic strath terraces and fill terraces are well developed in the MBT zone. Multiple events of recurrence of landslides are evident from exposed section of landslide debris fans. The bedrocks are highly sheared, suggesting successive tectonic movements along the MBT. As a consequence, tectonic wedges have developed in the bedrocks. These wedges have facilitated for the recurrence of a number of landslides. The anthropogenic activities have also aggravated the slope stability. Truncation of older debris fan surfaces by an erosional/fault scarp is observed and the scarp separates the older fans from the youngest debris fan deposit. The scarp runs parallel to the MBT and Kosi River. Swerving of the Kosi River is as a result of the tectonic forcing along active lineaments/faults. The trend of lineaments of the maximum and high density is NE-SW, which is transverse to the trend of the NW-SE trending Himalayan fold-thrust belt.