Tectonic geomorphology and morphometry of the frontal part of Kumaun Sub-Himalaya: Appraisal of tectonic activity

摘要

A section of the Main Boundary Thrust (MBT) in the Kumaun Himalaya suggests recent extensional tectonic activity. The surface expression of the tectonic activity is faulting of Quaternary fan deposit along the MBT zone. Faulting has caused ground rupturing producing fault scarp measuring about 37 m in height and 2.5 km in length that is exposed along the MBT. One sample from the fan has been dated using Optically Stimulated Luminescence (OSL) to about 17 ka. Two parallel fault traces are also observed in the western segment of this fault. In the Sukhidang-Shiala area normal faulting has produced a series of lakelets and paleolakes. Geomorphic evidence suggests the existence of more lakes, which have dried up due to headward erosion and landslides. Three pits were excavated in paleolakes to determine if any deformation event is preserved in the lake sediments. At Pit No.3 soft-sediment deformation structures are observed in strata comprising laminated silty sand and pebbly sand beds at a depth of 1 m. The OSL date of these deformed laminated silty sand and pebbly sand beds and the upper undeformed layer gives ages of 370 +/- 28 and 278 +/- 28 years, respectively. In the mountain front a fan deposit has been truncated by the Himalayan Frontal Thrust (HFT) exposing a fault scarp of about 5 km in length and 100 m in height. Geomorphic indices have been analyzed along the frontal part of the Kumaun Sub-Himalaya to assess tectonic activities in the area. Values corroborate the field evidence suggesting that this segment of the Himalayan mountain front is tectonically active.