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The present study area represents one of the active segments of the frontal part of the Arunachal Himalaya. The major structural boundaries traversing the area are the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) and other local faults are tectonically active, as evident from the landforms that evolved in the fault zones. Strath terraces and uplift fluvial terraces are observed in the hanging‐wall block of the HFT and MBT, and along the HFT straight to curvilinear mountain fronts are observed. Normal faults both transverse and parallel to the strike of HFT are observed in the hanging‐wall block of the HFT. West of Pasighat the sub‐Himalaya terrain resembles a duplex, where there is gradual rise in elevation from the Brahmaputra Plain to the low‐lying hills of the sub‐Himalayan and sudden rise in elevation across the MBT. Drainage aligning to the bedrock structures and stream migrations are observed in the fault zones. Along the MBT, the sub‐Himalayan rocks are thrusted over either by quartzite or black carbonaceous shales, in some sections the MBT is behaving as a back‐thrust that dips towards the south. The MBT has also been displaced by almost N–S‐trending transverse faults where highly sheared bedrocks are also observed. Tilting of Quaternary fluvial deposits are observed in the hanging‐wall blocks of intraformational thrusts at two locations. Signatures of paleoseismic activities in the form of various soft‐sediment deformational structures are observed in Quaternary deposits in the HFT zone and also in the hanging‐wall block of the MBT. To determine the deformation and strain rate accumulation, we have re‐analysed the available GNSS results of the area, which indicate a deformation rate of up to 16 mm/year in the vicinity of the Pasighat region. The GPS‐derived deformation rates are well corroborated with the existing results of peak ground acceleration (PGA) anomalies. The component–wise motion indicates higher fault perpendicular motion in this part, which reveals an accumulation of more strain along the major Himalayan faults. [ABSTRACT FROM AUTHOR]

In the northwest Himalayan region, the ongoing out-of-sequence deformation caused emergence of Back Thrust (BT) around 43ka, which attributed the growth of anticlines parallel to the strike of major thrusts. This spatial and temporal geomorphic deformation is accompanied by coexisting river incision in the axial zone of anticlines and base-level fall along with upstream migration of knickpoints in the longitudinal river course. We used the Stream Power Incision model (SPIM) which relates the normalized steepness index (k sn), and concavity index (θ ref) with the slope (S) of longitudinal river profile as a power-law function of upstream area (A). and estimated these parameters for Beas, Soan, and Sutlej rivers to decipher the spatial distribution of deformation pattern. Using SRTM DEM and based on strike parallel variations of the k sn we identified segmented nature of the NW Himalayan thrusts. Further, the dynamics of river basins and transience along the tributaries and main stem rivers has been analyzed by Chi analysis (χ), which is attributed to geometric disequilibrium of the rivers. The results of k sn and χ analysis together suggests that the crustal shortening of NW Himalaya is caused by segmental reactivation of the thrusts. Further, investigating the change in steepness along longitudinal course aids in understanding the differential uplifts patterns in actively deforming segments of major NW Himalayan thrusts. [ABSTRACT FROM AUTHOR]

We have analysed the geomorphic signatures in aggregation with the geomorphic indices with respect to hinterland and foreland neotectonic variability across the major Himalayan thrust system along the Kali River valley of eastern Kumaun Himalaya. The valley floor morphology in the vicinity of the major thrust gave rise to the accommodation space for the aggradation of the recent fluvial sediments. For a longitudinal distance of 212 km between the South Tibet Detachment (STD) and Himalayan Frontal Thrust (HFT), the valley has preserved significant aggradational landforms. These landforms have been physically examined to explain the spatial and temporal variability in phases of aggradation/incision in response to the tectonic activity during the late Quaternary. Fossil valleys and associated epigenetic gorges, cut‐and‐fill terraces with thick alluvial cover, debris‐flow terraces, bedrock strath terraces, and alluvial fan terraces are the significant aggradational landforms observed within the valley. They provide signatures of tectonic activity and past climatic records. We have analysed various geomorphic indices, namely, stream‐gradient index (SL), steepness index (Ks), hypsometric curve and index (HI), and asymmetric factor (AF) to map the spatial variability in tectonic processes across the major thrusts. The channel morphology (depth and width) of a river becomes adjusted in geological time because of long‐term tectonic and erosional processes. However, the channel geometry and deviations can be highly adjustable in a tectonically disturbed mountainous region. The deviations along the river course can be attributed to active tectonic movement along thrusts/faults or because of erosion/sedimentation processes. We applied a novel method called the river Gradient Length Anomaly (GLA) for 37 subbasins of the Kali River to deduce active deformation (uplift/subsidence) corresponding to the increased sedimentation and erosional rates, both from the upstream and downstream parts of the thrusted blocks, from where river flows through. The results of basin‐wise morphometric indices were corroborated with GLA anomaly and field observations, which suggests that the area has undergone active deformation between the Main Central Thrust (MCT) and Himalayan Frontal Thrust (HFT), which is attributed to the regional compression. The results of geomorphic and morphometric data are validated with the focal mechanisms of moderate earthquakes. The geomorphic analysis suggests that the hinterland part of eastern Kumaun is more active than the foreland region. [ABSTRACT FROM AUTHOR]

Himalaya is one of the youngest mountain systems with a fragile ecosystem due to continued collision tectonics. The endogenic and exogenic processes in the Himalaya have made landslide one of the critical surface processes continually sculpting the Himalaya. Constituting the hanging wall of the Main Boundary Thrust (MBT), the south-facing slope of the Mussoorie Syncline is prone to various slope movements. Several old and active landslides are taking place along the Dehradun-Bhitarli Chhoti-Mussoorie road section. The main causative factors for landslides are tectonics, slopes, and human interference. The highly sheared bedrocks and high numbers of joint sets in the bedrocks are the signatures of the successive phases of tectonic uplift along the MBT imprinted on the bedrocks. The intersecting joint sets developed in the bedrocks have formed wedges, and these wedges have facilitated the recurrence of several landslides. Wedge failure, sliding (bedrock and debris), and toppling are the primary types of failure mechanisms, of which wedge failure-induced landslides accounts for the maximum. The south-facing slopes are very steep and are also one of the contributing factors to the instability of the hill slope. Anthropogenic activities have further aggravated slope stability as the roads are constructed across steep slopes and highly sheared rocks. The onslaught of rains easily destabilizes the weak rocks during monsoon, where the region receives a good amount of rainfall. The landslides occur primarily during or right after monsoon when the slope-forming materials become saturated with water. Recent tectonic activity along the MBT is evident from tectonic landforms such are uplifted terraces, linear fault traces along the MBT, and triangular facet cones. [ABSTRACT FROM AUTHOR]

Torrential rains, landslides, and seismic activity are the common factors that are causing unprecedented damage to life, property and infrastructure in Mandakini and Bhilangana basins of Garhwal Himalaya. Owing to such conditions, we demonstrate the feedback this landscape in Garhwal Himalaya in response to episodic tectonic uplift and monsoon precipitation. We calculated conventional geomorphic parameters to access the tectonic deformation across the major thrusts and faults. We further analyzed the normalized steepness index (K sn), Chi (χ), and knickpoints along the longitudinal course of rivers. Additionally, the study attributed to active nature of Chail/Ramgarh thrust along the Balganga valley as envisaged by five levels of unpaired fluvial terraces, entrenched stream course, river ponding, active and stabilized landslide deposits, etc. Moreover, the flooding during the heavy rainfall events induced river toe cutting makes an effect on settlement over fluvial deposits. Therefore, we suggested that the highly dissected and tilted basins with deep V shaped valleys and ongoing seismicity also fabricates the region more vulnerable for hazards which threaten the human lives. [ABSTRACT FROM AUTHOR]

The morphotectonic elements of the Baijnath Klippe (BK) have been examined to understand the kinematics of the Inner Kumaun Lesser Himalaya from Tertiary to Late Quaternary. The BK is bounded by two thrusts: the Askot Thrust (AT), to the north and the Baijnath Thrust (BjT), to the south. The older (20 Ma) hanging wall rocks (mylonites and phyllites) are underlain and surrounded by quartzites (equivalent to Nagthat Formation of Outer Lesser Himalaya). The kinematics study and strain analysis along Dewal–Narayanbagarh transect provides the varied intensity of deformation (D1, D2, and D3) within the zone of BK. The D1 event is characterized by isoclinal tight folds in phyllite and mylonite; the D2 phase is characterized by open folding in mylonite. However, the youngest event D3 phase is observed as a regional synclinal folding of BK and associated rocks. The presence of 1–3-km-thick mylonite zone parallel to the thrust planes is related to the compressional event of Miocene age. The geomorphic pattern and chronologically constraints (OSL ages) with spatial variability in the incision/uplift rate suggests that the terrain is undergoing differential crustal deformation during Late Quaternary. Kinematic indicators such as streaky foliations, S–C foliation, mantled porphyroclasts, micro-shears with bookshelf gliding, and mica fish found in mylonites (macroscopic as well as microscopic) indicate ductile deformation in the area. We used Flinn plot (Rf/ϕ) and Fry (Tectonophysics 60:89–105, 1979) analysis to delineate the finite strain pattern in 21 oriented samples collected across the BK bound thrusts (BjT and AT). The strain analysis revealed an increasing trend of strained quartz grains, which presents triaxialoblate strain geometry (flattening i.e., 0 < k < 1) towards the structural discontinuity. The textural (chessboard, myrmekite, undulose extinction and fracturing in quartz and feldspar) and mineralogical properties (non-flaky) of quartz and feldspar indicate the temperature of deformation and mylonitization ranging 300–500 °C. [ABSTRACT FROM AUTHOR]