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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 Himalayan Frontal Thrust (HFT) forms the southernmost active tectonic mountain front of the Himalaya. To understand the ongoing tectonics further, paleoseismological study has been carried out in the vicinity of the HFT system along the Himalayan Front near Kala Amb, India. The trench excavation survey conducted across an explicit surface exposure of the HFT exhibits two distinct faults considered to be associated with the reactivation of the HFT where the Middle Siwalik rocks (Late Miocene) have repeatedly thrust over the Late Pleistocene and Holocene sediments. Presence of large-sized coseismically induced sand-injection feature and its disposition recognized in the trench also suggest occurrence of large magnitude earthquakes in this region. An uplifted and upwarped strath terrace, 3 to 5m thick alluvium, resting over the 15m high Middle Siwaliks, abruptly truncated by the HFT indicates its latest activity. Optically Stimulated Luminescence dating techniques were employed to constrain the chronology of events. The long term slip rate of the abandoned terraces due to the activity of the HFT is estimated to be 3.4mm/yr or greater since Late Holocene. The paleoseismological investigations have provided unambiguous evidences of at least two large magnitude earthquakes occurred in this region where an earthquake with 12m or larger surface displacement and magnitude 7.5 or greater hit this region in the period between 29.3ka and 17ka in the Late Pleistocene and another great earthquake recurred with 20–22m or more surface displacement and magnitude of 7.7 or greater between 5.8ka and 2ka in the Holocene. The present study is the first time report of multiple large magnitude paleoearthquakes in the northwestern part of the Frontal Himalaya during Late Pleistocene and Holocene. The repeated reactivation of HFT substantiates high seismic potential of the Frontal Himalaya and calls for more extensive study of paleoearthquakes of this vastly populous mountainous region. [Copyright &y& Elsevier]

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]

Structural style, supplemented by magnetic and petrofabric strain studies, suggests that the Kangra recess is the result of combined frontal and oblique thrust ramps, which were initiated as a listric normal fault during the pre-Himalayan tensional phase and were later reactivated as a listric thrust fault during the Himalayan orogeny. The displacement out of the tectonic transport plane has resulted in the formation of a pull-apart basin parallel to the oblique ramp. The shear strain associated with the faulting was accommodated within a narrow zone characterized by the presence of salt. The structures in the hanging wall were developed as a result of a combination of simple shear and pure shear. The orientations of the folds were controlled by interference patterns between simultaneously developing folds, and thrust ramp structures.There is no marker horizon across the Main Boundary Thrust (MBT) which could be used to measure the amount of displacement, but a method based on the geometry of the pull-apart basin is proposed to estimate the strike-slip displacement along the oblique ramp. The method indicates a horizontal displacement of ∼33.5 km along the MBT, which corresponds well to other geological data. A sequence of stages in the structural evolution of the area is proposed. [Copyright &y& Elsevier]

Inhomogeneous strain at late stages of deformation of modelling clay models with frontal and oblique thrust ramp geometries results in formation of extension faults on the hanging wall. The extension faults trend nearly parallel to the axis of maximum compression and orthogonal to the trend of the frontal ramp. Faults with similar geometries occur in the Upper Satluj River Basin of the Tethys Himachal Himalaya. The experimental results help in understanding the present day normal faulting and related active tectonics of the area. [Copyright &y& Elsevier]

Overriding the underthrusting Indian plate along the low angle detachment surface of the Main Himalayan Thrust (MHT), the entire Himalayan wedge, consisting of several allochthonous and autochthonous sheets is actively deforming due to ongoing India-Asia convergence. However, the information on active tectonics of autochthonous sheets exposed in tectonic windows has often been eluded due to lack of directed investigations. The present study pertains to the active tectonics of the eastern part of an autochthonous sheet exposed in Kullu-Larji-Rampur window of the Lesser Himalaya. Geomorphic features and statistically analyzed geomorphic indices of Drainage Basin Shape (Bs), Hypsometric Integral (HI), Stream Length-Gradient Index (SL), Transverse Topography Symmetry Factor (T), and Basin Asymmetry Factor (AF) reveal active crustal deformation in the window caused by tectonic activities of spatially varying magnitudes. While the spatial distribution of Bs, HI, and SL suggest differential uplift of the anticlinally folded autochthonous sheet along the MHT, the spatial distributions of AF and T suggest an overall southeastward preferential shifting of the trunk streams of 3rd-order drainage basins of the terrane; the hinge zone and northwestern part of the sheet are experiencing relatively faster uplift. The faster uplift in the northwest has caused an overall southeastward down-tiling of the terrane, and thus preferential stream migration in that direction. The faster uplift of the hinge zone is considered to indicate active tightening of the fold. [ABSTRACT FROM AUTHOR]

Abstract: The present article is the first time reporting of a paleoearthquake that occurred during Late Pleistocene time along the Nalagarh Thrust (NT) in the Pinjaur Dun in northwestern sub-Himalaya. Using CORONA satellite photographs, multi-spectral IRS satellite data, and aerial photographs, a prominent active fault has been identified at Nalagarh in Pinjaur Dun. This fault in the alluvial fan is located very close to the NT which borders the topographic front of the Tertiary rocks against Quaternary deposits. A trench excavation survey was carried out at Nalagarh for detailed paleoseismic studies across this thrust fault. Displacing all the lithologic units of the fan sequence, the fault plane has an average dip of 30° due ENE and a vertical displacement of 1.6m and slip of ∼2.5m along the fault. The lithological units, consisting of alternating sand and gravel, show back tilting and asymmetrical tight folding. Based on Optically Stimulated Luminescence (OSL) ages, the oldest litho-unit in the trench is 85.83±7.2ka and the youngest is 67.05±8.4ka. The OSL age of the sample collected from the easterly exposure of the fault shows an age of 20ka. The faulting and associated induced deformation features suggest occurrence of a Late Pleistocene large magnitude earthquake along NT in the Nalagarh region of the Pinjaur Dun following the deposition of the Quaternary sedimentary units. The Late Pleistocene fault substantiates the seismic potential of Pinjaur Dun and calls for more exhaustive study of paleoearthquakes in this fast developing industrial belt and highly populous mountainous region. [Copyright &y& Elsevier]