Tectonic implication of drainage set-up in the Sub-Himalaya: A case study of Papumpare district, Arunachal Himalaya, India

The Sub-Himalaya, deformed between the Main Boundary Thrust and/or Bomdila Thrust towards north and the Himalayan Frontal Thrust (HFT) towards south, is characterized by the presence of fault-propagated folds and an imbricate thrust system. Along the HFT, the mountain front is offset for about 10 km sinistrally along a NW–SE trending transverse fault, the Banderdewa Fault, which runs through the Dikrang River valley that seems to have followed a pull-apart basin developed along the Banderdewa Fault. The western part of study area, associated with a major folding, faulting and the development of fault-bounded Quaternary basins with normal faulting along their northern margins, is interpreted to be tectonically more transported towards southeast than the eastern part where no Quaternary basins are developed. The channel forms and landforms have been studied in relation to the neotectonics of the area. The watersheds exhibit their structurally controlled nature, with drainages following tectonic alignments. Rivers/streams experienced compressed meandering, narrowing, widening and deflection of channels. Longitudinal profiles of river/stream channels exhibit knick points. Unpaired terraces are observed at higher levels. Factors like the unconsolidated Siwalik rocks, tectonic instability along faults and high rainfall with the advent of the Holocene, led to the development of the present-day drainage set-up. A tectonic model is proposed for development of tectonic geomorphology of the area influenced by ultra-tremor to low magnitude earthquakes in seismically very active northeastern India, as moderate to big earthquakes (M ≥ 6.0) are scarce in this region.