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Structural and morphotectonic signatures in conjunction with the geomorphic indices are synthesised to trace the role of transverse tectonic features in shaping the landforms developed along the frontal part of the eastern Arunachal sub-Himalaya. Mountain front sinuosity ( S mf ) index values close to one are indicative of the active nature of the mountain front all along the eastern Arunachal Himalaya, which can be directly attributed to the regional uplift along the Himalayan Frontal Thrust (HFT). However, the mountain front is significantly sinusoidal around junctions between HFT/MBT (Main Boundary Thrust) and active transverse faults. The high values of stream length gradient ( SL ) and stream steepness ( K s ) indices together with field evidence of fault scarps, offset of terraces, and deflection of streams are markers of neotectonic uplift along the thrusts and transverse faults. This reactivation of transverse faults has given rise to extensional basins leading to widening of the river courses, providing favourable sites for deposition of recent sediments. Tectonic interactions of these transverse faults with the Himalayan longitudinal thrusts (MBT/HFT) have segmented the mountain front marked with varying sinuosity. The net result is that a variety of tectonic landforms recognized along the mountain front can be tracked to the complex interactions among the transverse and longitudinal tectonic elements. Some distinctive examples are: in the eastern extremity of NE Himalaya across the Dibang River valley, the NW-SE trending mountain front is attenuated by the active Mishmi Thrust that has thrust the Mishmi crystalline complex directly over the alluvium of the Brahmaputra plains. The junction of the folded HFT and Mishmi Thrust shows a zone of brecciated and pulverized rocks along which transverse axial planar fracture cleavages exhibit neotectonic activities in a transverse fault zone coinciding with the Dibang River course. Similarly, the transverse faults cut the mountain front along the Sesseri, Siluk, Siku, Siang, Mingo, Sileng, Dikari, and Simen rivers. At some such junctions, landforms associated with the active right-lateral strike-slip faults are superposed over the earlier landforms formed by transverse normal faults. In addition to linear transverse features, we see evidence that the fold-thrust belt of the frontal part of the Arunachal Himalaya has also been affected by the neotectonically active NW-SE trending major fold known as the Siang antiform that again is aligned transverse to the mountain front. The folding of the HFT and MBT along this antiform has reshaped the landscape developed between its two western and eastern limbs running N-S and NW-SE, respectively. The transverse faults are parallel to the already reported deep-seated transverse seismogenic strike-slip fault. Therefore, a single take home message is that any true manifestation of the neotectonics and seismic hazard assessment in the Himalayan region must take into account the role of transverse tectonics. [ABSTRACT FROM AUTHOR]

Abstract: 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 10km 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. [Copyright &y& Elsevier]