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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]

In the Garhwal Himalaya, the steeply inclined Main Boundary Thrust (MBT) is characterized by landforms that indicate towards tectonic activity in the recent past. Linear active fault trace, sub‐recent fault scarps, strath terrace, uplifted terraces, deflected drainage, and triangular facet cones are some of the morphotectonic evidences of recent tectonic activity along the MBT. The MBT is defined by active fault trace in the form of linear depression and shows a prominent slope break across it. Thrusting of the Lesser Himalayan bedrocks over the colluvial deposits and back tilting of the colluvial deposits are also observed in the MBT zone. The formation of the sag pond and the deposition of the Quaternary lacustrine sediments followed by the deformation of the lacustrine deposits indicate phases of tectonic activities along the MBT. The later phase of deformation is represented by the folded lacustrine and colluvial deposits, where the amplitude of the fold measures about 3 m‐high and the deformed section measures about 23 m in length. The ongoing deformation patterns of the MBT zone have also been monitored with the help of Synthetic Aperture Radar‐based time series analysis. We used Sentinel‐1C dataset in ascending direction with polarization of the VV+ VH acquired between December 2, 2017 to September 7, 2021 to identify phase changes caused by ongoing active deformation. The Persistent Scatterer Interferometry (PSI) result based on Sentinel‐1A suggests that the Dehradun–Mussoorie and adjoining region has undergone an average deformation of 5.5 to −11 mm/year and the ground displacement of 1.5 to −3.8 cm. The time series analysis reveals that the Mussoorie hills in the MBT zone have higher rate of deformation. [ABSTRACT FROM AUTHOR]

The paper discusses the magnetotelluric derived electrical resistivity cross‐section of deep crust along the Lohit Valley in the Eastern Himalayan Syntaxial bend region. A low‐angle, north‐east dipping, intra‐crustal low resistive layer (IC‐LRL) is a dominant feature in the resistivity cross‐section. We interpret IC‐LRL as a signature of Main Himalayan Thrust. In the geoelectrical model, all the major thrusts separating different tectonic zones across the profile coincide with the dipping conducting features and these thrusts merged to IC‐LRL. The Lesser Himalaya and the Lohit Plutonic Complex are imaged as resistive features. Conductive zone between the Lesser Himalaya and Lohit Plutonic Complex reflect the Tidding Suture Zone. Conductivity of the IC‐LRL is lower in comparison with IC‐LRL imaged in another sector of the Himalaya. We suggest this difference is due to the significant difference in the intensity of the tectonic processes in this part of the Himalaya as compared to elsewhere. [ABSTRACT FROM AUTHOR]

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]

Landforms developed along the Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) zones in Ramnagar area of Kumaun sub‐Himalaya are studied. The geomorphic analyses suggest their spatial and temporal relationship with the reactivation of the boundary thrusts, faults, and associated later developed transverse faults. Mapping of geomorphic features and structural analysis of the MBT and HFT established an intimate relationship between thrusting of rocks along these major thrusts and evolution of landforms. The reactivations of the MBT and HFT have resulted in recent rejuvenation of landscapes formed in the thrust zones. In the MBT zone, the swerving of the channels, linear development of elongated depressions, sudden topographic breaks, and presence of relatively fresh‐looking escarpments, uplifted present ground surfaces, V‐shaped valleys, and occurrence of linear series of dormant and active landslides are observed. Along the Kosi River section, the vertical‐dipping Neoproterozoic Lesser Himalayan rocks have overridden the recent landslide debris along the subsidiary steep reverse faults associated with the MBT. The HFT is topographically represented by an escarpment trending ESE‐WNW along the Himalayan mountain front. In the HFT zone particularly at Ramnagar area, four levels of terraces are noted, the highest being a strath terrace. The surface trace of the HFT is offset by a transverse N‐S trending Ramnagar Fault, which coincides with the course along which Kosi Rver flows. Uplift of the hanging wall block near the HFT has resulted in subsidence of the present ground surface towards the northeast away from the HFT trace. During this process, the transverse streams have vigorously incised the uplifted surfaces. Rivers and streams realigning with the bedrock structure suggest tectonic control on morphology of drainage patterns. Neotectonic activity along the intra‐Siwalik thrust has offset a terrace surface into two surfaces that are separated by a fault scarp of about 50 m high. The bedrocks in the HFT region are gentle to steep dipping towards NW/NE/ESE. Both normal and reverse faults are recognized in the bedrocks. The general trend of the lineaments, which are concentrated in the thrust zones, is E‐W. Two transverse faults trending nearly N‐S cut and offset the HFT. Tectonic activity during the deposition of the Upper Siwalik rocks is evident from the occurrence of liquefaction features. Soft‐sediment deformation structures are observed in fine‐ to medium‐grained friable sandstones. These include up‐welled structures (with vertical to inclined bedding planes), sand dykes, faults (reverse and normal), folds, pseudo‐nodules, and deformed laminations. [ABSTRACT FROM AUTHOR]