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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]

This paper documents the various factors contributing to the evolution of landforms in the Kosi River valley, Ukhaldhunga area of the Kumaun Himalaya, Uttarakhand. The area falls in the major tectonic boundary between the Lesser Himalaya and the sub-Himalaya. Passing through the Parewa-Ukhaldhunga-Dabara area, this tectonic contact is known as the Main Boundary Thrust (MBT), which is characterized by presence of varied landforms. Study of the development of geomorphology, particularly those evolved in the MBT zone, suggests that the morphological features are intimately associated with the tectonic activities related to the movement of rocks along the MBT and the surface processes such as fluvial and mass movement. The various landforms observed in the MBT zone include strath terraces, fill terraces, paleochannels and V-shaped valley that pertain to fluvial genesis; gravity induced landforms such as landslide debris and colluvial fans; and tectonic landforms such as linear fault traces, swerving/swing of river course and linear depressions. Signature of recent tectonic activity along the MBT is observed between Dabara and Khakrakot. The activity is evident in the form of 8-km-long active fault trace trending NW-SE. Normal fault related landforms are observed in the hanging wall block of the MBT. Neotectonic strath terraces and fill terraces are well developed in the MBT zone. Multiple events of recurrence of landslides are evident from exposed section of landslide debris fans. The bedrocks are highly sheared, suggesting successive tectonic movements along the MBT. As a consequence, tectonic wedges have developed in the bedrocks. These wedges have facilitated for the recurrence of a number of landslides. The anthropogenic activities have also aggravated the slope stability. Truncation of older debris fan surfaces by an erosional/fault scarp is observed and the scarp separates the older fans from the youngest debris fan deposit. The scarp runs parallel to the MBT and Kosi River. Swerving of the Kosi River is as a result of the tectonic forcing along active lineaments/faults. The trend of lineaments of the maximum and high density is NE-SW, which is transverse to the trend of the NW-SE trending Himalayan fold-thrust belt. [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]

Geomorphic evidences and geochronology suggest recent tectonic activity in the Belt of Schuppen of Dimapur and Peren districts of Nagaland. Multiple levels of strath terraces forming remarkable geomorphic expressions are observed along the Chathe and Jharnapani river valleys. They indicate episodic uplift of the hanging‐wall blocks of the thrust systems. The major lineaments prominently trend parallel to the NE–SW regional thrusts, some being perpendicular or oblique. Active normal faults displacing bedrock and overlying fill sediments point to recent tectonic activity. The parallel and trellis drainage patterns are lineament‐controlled. Uplift of the terrain is evident from successive palaeochannels of the Chathe River younging towards the southwest. Three major aggradational phases of the terrace sediments are inferred from optically stimulated luminescence (OSL) chronology, the oldest beginning around 21 ka, the second around 11–7 ka, and the youngest between 3.5 and 1 ka. A sand lens of a tilted strath terrace exposed between Kukidolong and Chumukedima is dated at 7 ka, indicating tectonic events after 7 ka. Five major phases of bedrock uplift/incision have been estimated during 20 ka (0.13 mm/year), 11 ka (0.92 mm/year), 9 ka (2.29 mm/year), 6 ka (0.67 mm/year), and 1.8 ka (2.6 mm/year). Transverse topographic symmetry and asymmetry factor data suggest tilting towards the NE and SW. Other morphometric indices, such as channel sinuosity, stream length gradient index, valley floor to valley height ratio, and mountain front sinuosity, suggest that the area is tectonically active, which corroborates field observations. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The strath terraces were once active stream channels that were uplifted due to tectonic activity. • Different phases of tectonic upheaval caused the development of various levels of strath terraces. • Tectonic activity continued in the Schuppen Belt of Mokokchung after the deposition of the Quaternary sediments. • The anticlines of the area are rising. Abstract A stretch of the schuppen belt, trending NE-SW in the western sector of the Indo-Myanmar Ranges, represents a tectonically active zone. Deformed Quaternary deposits and various geomorphic features lend evidence for the same. In a portion of the footwall block of the Sanis-Chongliyimsen Thrust, the Nazira Sandstone of the Tipam Group of Mio-Pliocene age is thrust over tilted Quaternary fluvial deposits near Tzudikong, with a displacement of more than 2 m along the plane of the thrust fault. Deformed Quaternary deposits such as tilted terraces and faulted Quaternary sediments are indicative of ongoing tectonic activity in the area. The sediments are tilted towards the southeast by varying degrees (21°–39°) similar to the trend of stream migrations observed at four locations in and around Tuli. In some sections, faulting has not only affected the underlying bedrock but also deformed the overlying fluvial sediments. An important morphotectonic feature representing tectonic movement along the Kongan Thrust is evident from drainage migration in the hanging wall block of the thrust. The stream has migrated towards the southeast as a result of NW verging along the Kongan Thrust, which has left behind a trail of palaeochannels. Numerous palaeochannels, mostly linear in nature, associated with the scarp of the terrace, are indicative of migration of the river. Stream migrations suggest that the two anticlines noted in the study area are possibly growing in the present tectonic regime. Multiple levels of strath terraces lend evidence to tectonic pulses in the study area, each strath representing a tectonic phase. Lineaments have been extensively mapped in the area, out of which, around 70% are parallel to the regional NE-SW trend and the other 30% are approximately perpendicular to this trend. Any movement along the NW-SE plane may be normal faults. The mountain front sinuosity index of the emerging mountain front is close to 1, which is indicative of a relatively active mountain front. Steam length gradient index and steepness index correspond with each other, with high values in the thrust zones, which is indicative of a neotectonically active region. [ABSTRACT FROM AUTHOR]

In the Kumaun Himalaya, a portion of the Kosi River valley of ~90 km in length is chosen to study the fluvial morphology that provides first‐order information about the dynamic response of bedrock channels to tectonic impulse. The Kosi River flows across/along major tectonic boundaries such as the South Almora Thrust, Ramgarh Thrust, Main Boundary Thrust, and the Himalayan Frontal Thrust, and local transverse and longitudinal faults. Varied fluvial landforms correspond to different tectonic settings, lithologies, bedrock channels, hillslopes, large landslides, terraces, and fans. The longitudinal valleys are also the sites for thick aggradational landforms. Some portion of these valleys fall in the areas of active extensional tectonics and is characterized by one of the widest valley floor sections in the Lesser Himalaya. In contrast, the transverse valley sections are incised by deep‐cut v‐shaped valleys and the narrowest valley section. Swerving of the Kosi River is observed in the Ramgarh Thrust and Amel Fault zones and also in the Main Boundary Thrust zone. Recent tectonic activity is evident from the presence of the faulted Quaternary deposits, linear fault scarps, abandoned channels, incised meandering, and multiple levels of terraces/strath terraces. Field observations and the computed ratio of valley floor width to valley height (Vf) corroborate each other. Valleys developed parallel to the strike of faults and bedrocks have relatively broader valleys with higher Vf values whereas in contrast, the valleys developed across the bedrock strike are narrow with smaller Vf values. The results of computed stream length gradient (SL) and steepness (Ks) indices show considerable correlations between the obtained SL and Ks data and the field evidences; high values of SL and Ks are characterized by the presence of knick points observed at the prominent thrusts and faults. [ABSTRACT FROM AUTHOR]

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]

The present study has been carried out with special emphasis on the aggradational landforms to explain the spatial and temporal variability in phases of aggradation/incision in response to tectonic activity during the late Quaternary in the Saryu River valley in central Kumaun Himalaya. The valley has preserved cut-and-fill terraces with thick alluvial cover, debris flow terraces, and bedrock strath terraces that provide signatures of tectonic activity and climate. Morphostratigraphy of the terraces reveals that the oldest landforms preserved south of the Main Central Thrust, the fluvial modified debris flow terraces, were developed between 30 and 45 ka. The major phase of valley fill is dated between 14 and 22 ka. The youngest phase of aggradation is dated at early and mid-Holocene (9–3 ka). Following this, several phases of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces. Seven major phases of bedrock incision/uplift have been estimated during 44 ka (3.34 mm/year), 35 ka (1.84 mm/year), 15 ka (0.91 mm/year), 14 ka (0.83 mm/year), 9 ka (1.75 mm/year), 7 ka (5.38 mm/year), and around 3 ka (4.4 mm/year) from the strath terraces near major thrusts. We postulate that between 9 and 3 ka the terrain witnessed relatively enhanced surface uplift (2–5 mm/year). [ABSTRACT FROM AUTHOR]

A portion of the Kosi River in the outer Kumaun Lesser Himalaya is characterized by wide river course situated south of the Ramgarh Thrust, where huge thickness (~200 m) of the landslide deposits and two to three levels of unpaired fan terraces are present. Brittle normal faults, suggesting extensional tectonics, are recognized in the Quaternary deposits and bedrocks as further supported by surface morphology. Trending E-W, these faults measure from 3 to 5 km in length and are traced as discontinuous linear mini-horst and fault scarps (sackungen) exposed due to cutting across by streams. Active normal faults have displaced the coarsely laminated debris fan deposits at two sites located 550 m apart. At one of the sites, the faults look like bookshelf faulting with the maximum displacement of ~2 m and rotation of the Quaternary boulders along the fault plane is observed. At another site, the maximum displacement measures about 0.60 cm. Thick mud units deposited due to blocking of the streams by landslides are observed within and above the fan deposit. Landslide debris fans and terrace landforms are widely developed; the highest level of fan is observed ~1240 m above mean sea level. At some places, the reworking of the debris fans by streams is characterized by thick laminated sand body. Along the South Almora Thrust and Ramgarh Thrust zones, the valleys are narrow and V-shaped where Quaternary deposits are sparse due to relatively rapid uplift across these thrusts. Along the South Almora Thrust zone, three to four levels of fluvial terraces are observed and have been incised by river exposing the bedrocks due to recent movement along the RT and SAT. Abandoned channel, tilted mud deposits, incised meandering, deep-cut V-shaped valleys and strath terraces indicate rapid uplift of the area. Thick mud sequences in the Quaternary columns indicate damming of streams. A ~10-km-long north-south trending transverse Garampani Fault has offset the Ramgarh Thrust producing tectonic landforms. [ABSTRACT FROM AUTHOR]

Conjugate sets of normal faults formed in the Quaternary fan sediments lying near the top of footwall rocks of the Ramgarh Thrust are analysed. These faults are recognized on left hillslope of Kosi River valley, Kumaun Lesser Himalaya. The Ramgarh Thrust marks the mountain front of the uplifted Central Crystallines, which have been under thrust along the Ramgarh Thrust by its footwall of the Nagthat Formation belonging to the Lesser Himalayan Sequence. The existence of a regional-scale footwall anticlinal structure along the Kosi River suggests that the compressional stress regime is active in the subsurface region related to the Himalayan thrust tectonics. Analysis of structural data reveals that the normal faults have been formed by pure shear due to gravity. The WNW-ESE trending normal faults are recognized within the Quaternary fan deposit and also at the top of the country rocks just below and adjacent to the fan deposit. Therefore, it is interpreted that the deformation related to N-S extensional tectonics has taken place at the uppermost crustal level due to gravity, where influence of the Himalayan subsurface compressional tectonics is no more significant. [ABSTRACT FROM AUTHOR]

We present the results of integrated studies of geomorphic indices of drainage networks and landforms developed across the mountain front along the Himalayan Frontal Thrust (HFT) between the Dabka and Baur rivers, Kumaun Himalaya. The HFT is a morphogenic structure in nature, creating a 100-m-high E–W trending escarpment that extends ~ 21 km. Geomorphological evidence indicates ~ 10.5 km westward migration of the Dabka River and ~ 5.2 km eastward migration of the Baur River. These migrations are a result of uplift of the hanging wall along the HFT. The HFT is offset by a transverse fault, which suggests that the latter postdates the reactivation of the HFT between 500 and 100 ka. Presence of different levels of strath terraces along the mountain front suggests the active nature of the HFT. To assess the relative tectonic activity, morphometric indices such as stream-gradient ( SL ) index, mountain front sinuosity ( S mf ) index, and ratio of valley floor width to valley height ( Vf ) have been analyzed. Results of the former two are consistent with the tectonic landforms developed in thrust zones. Paleochannels of the Dabka and Baur rivers are characterized by high Vf values while other valleys show low Vf values. Quaternary alluvial sediments have been deformed along the Pawalgarth Thrust, a splay of the HFT. Deformation has resulted in the formation of the Pawalgarh Anticline, a thrust-related asymmetric fold. [ABSTRACT FROM AUTHOR]

[Display omitted] • Geomorphic and drainage anomaly suggests dominant strike-slip movement in the Ringui area. • Our analysis suggests presence of wrench geometry of the Thoubal fault. • The chronological data suggests that the fluvial aggradation begins around 8 Ka. • The cross cutting relationship of lineaments suggests multiple phases of deformation. The present study is an attempt to gain insight of the geodynamic implications on the evolution of Quaternary landforms in the Yangui River basin in the Indo-Myanmar Range (IMR), taking a multi-disciplinary approach. Originating at an elevation of ~2568 m asl, the Yangui River flows almost parallel to the regional strike of the IMR and drains into the Imphal valley at Yaripok and exhibits varied valley floor morphology. The valley floors are narrowest in the transverse valley sections. The fluvial landforms are represented by fill terraces, strath terraces and paleochannels. The chronologically constrained ages from fluvial stratigraphy suggests that the oldest phase of aggradation in the area began much before 8 ka. A total of 1164 lineaments were mapped, most trending NE-SW, which is slightly oblique to the regional strike of the IMR. East of the study area, the most prominent lineaments are those trending NE-SW. Lineament analysis suggests that the NNE-SSW trending lineaments are the first generation; followed by the ENE-SWS, while the youngest are the NW-SE trending lineaments. Quaternary landforms related to tectonic activity are observed in the Ringui area, where the morphology suggests transpressional tectonics. Cross-cutting relationships amongst faults suggest multiple phases of tectonic activity; linear valleys are observed along the fault planes. The cross conjugate faults and NW-SW lineaments of the area confirm tectonic activity in the area, which corroborates well with the seismogenic potential and oblique geometry of the Churachandpur-Mao Fault (CMF). Morphometric data also suggest ongoing tectonic activity in the area. [ABSTRACT FROM AUTHOR]

Rapid melting of snow/ice and heavy rainfall have resulted in the formation and expansion of morainedammed lakes, creating a potential danger from glacial lake outburst floods (GLOFs). Around 9:30-10 p.m. on 3 September 2023, the South Lhonak Lake, a glacial lake in the upper reaches of Sikkim Himalaya, burst its banks following a cloudburst in the catchment area of Lachung River. The water, with all the debris it picked up along the way, rammed into the dam, causing parts of it to give way. This led to massive flooding downstream. This catastrophic event changed the landscape in many parts of the upper reaches of Sikkim, making the whole region more fragile and vulnerable. [ABSTRACT FROM AUTHOR]

In the present study, we are reporting an unnamed glacier located (30.28089N-80.69344E) in the upper Kali Ganga valley, Pithoragarh district, Uttarakhand, India, which has been affected by active fault and climate change. Recent tectonic activity and change in climatic conditions have changed the course and morphology of this glacier. An active fault has produced a fault scarp that measures about 250 m in height with a northerly dip, the fault trace is 6.2 km in length and trend towards NW-SE. The observation is based on the satellite image, toposheet and Google Earth images. We observed that the 5 km long unnamed glacier, which covered ∼4 km2 area in Kuthi Yankti valley (Tributary of Kali River) has abruptly changed its main course. The NE moving glacier has been abruptly truncated and forced to move towards SE and ultimately merges with the adjacent glacier named Sumzurkchanki as a result of tectonic forcing. It is one of the unique behaviors of the glacier, which is for the first time reported from the Himalayan glacier as no such observation have so far been reported on this type of glacier kinematics. The study will help to understand the glacial-tectonic interaction and provides key information for future studies. [ABSTRACT FROM AUTHOR]

Here a report of a lake of about one km in length that resulted from damming of a river by landslide near Kuwari village at the altitude of 1733 m asl and located between N 30° 5′ 16″–30° 5′ 16″ and E 79° 48′ 30″–79° 48′ 29″. On the basis of Google Earth images, it has been observed that a huge landslide took place from 16th to 17th June, 2013 and also in 2019 (as per the local sources) that blocked the Shambhu Gad (Baura Gad) in Bageshwar district and formed a huge lake, which covered ∼15666 m2 area. It was also observed that an old landslide was reactivated due to cloud burst or toe erosion and affected an area ∼862913 m2 between 2664 and 1761 m asl. The area affected by the landslide is ∼4 km, the maximum width is about 650 m. In the vicinity of Kuwari village the impact of this Kuwari landslide also observed in the Jhaliya village (district Chamoli) established over south facing slope in the right bank of Pinder river. Both the villages are separated by E-W trending ridge (3700 m asl). [ABSTRACT FROM AUTHOR]

Rana and Sharma (2017) dispute our tectonic interpretation mainly on the basis of what they believe (climate?). However, we welcome their comments, as this gives us a chance to highlight the ambiguity inherent in discriminating the climate-tectonic imprints in morphotectonic records that are prevalent in current research. We should note that the paper published by Kothyari et al. (2017) was reviewed by national/international reviewers. We would like to emphasize the fact that the paper does not rule out the role of climate. However, most importantly, it presents significant features and observations that collection/assemblage points toward the dominant role of tectonics in their shaping, and not solely climate, as postulated by Rana and Sharma (2017). The objective of this paper is to identify tectonic signatures (geomorphology) in a monsoon - dominated, tectonically active terrain like the North Almora Thrust (NAT). These faults are marked by previous workers based on field evidence such as folding and faulting of lithological units; presence of slickensides parallel to the fault; offset of NAT owing to a transverse fault; and offset of drainage, drainage basin analysis, strath terraces, fluviolacustrine terraces, development of scarp, narrow river course, and deeply incised valleys. However, we disagree with the comments raised by Rana and Sharma (2017), because they are highly skewed toward the climate school of thought, and did not perceive the setting as a collection of landforms. Instead, they attempted to view them in isolation. Because these comments are important, we will try to further our research incorporating issues related to isolation of climate and tectonics imprints in the immediate future. We would like to thank Rana and Sharma (2017) for raising some basic questions on our work as this gave us an excellent opportunity to summarize and present the dominance of various processes and related landforms as earlier reported by Kothyari et al. (2017). A point-by-point detailed rebuttal/explanation of their queries is provided below. [ABSTRACT FROM AUTHOR]

The study discusses detail valley formation and sedimentation processes in the monsoon dominated non-glaciated catchment of the Ramganga river in the Lesser Himalaya. The geomorphic and sedimentological studies in this basin indicates phases of massive aggradation that was controlled mainly by channel bound processes and debris flows/landslides. The luminescence chronology of the fill sequences suggests that the valley filling occurred mainly in response to the enhanced monsoon after the Last Glacial Maxima (LGM), during Medieval Warm Period (MWP) and Little Ice Age (LIA). This phase is common in both glaciated and the non-glaciated catchments of Himalaya. The Ramganga River that flows through various tectonic structures of the Lesser Himalaya shows development of wide valleys with thick fill deposits in the fault zones. Chaukhutiya Fault (CF) and Binau-Bhikiyasain-Naurar Fault (BBNF) are the two main transverse faults where the evolved geomorphology pertains to their tectonic activity. The computed morphometric variables such as Ratio of valley floor width to valley height (Vf) and Stream Gradient Index (SL) show higher values in the transverse fault zones. Basin asymmetry vectors along the South Almora Thrust and BBNF are characterized by preferred stream migration in NE and SW direction suggesting BBNF with dip slip movement. Thick clay deposits at different sites along the Ramganga River resulting from blocking of the river, particularly along the BBNF, also point towards tectonically induced landslide and channel blockage. Later phase of tectonic activity, bracketed between 27 and 24 ka, is evident from deformed fluvial deposits in the form of folds and faults. Evidences of tectonic activity in the form of soft sediment deformation structures (SSDS) generic to seismic activity in layers comprising alternation of clay and sand are observed in the Himalayan Frontal Thrust (HFT) zone. The diagnostic features such as dykes, faults and folds suggests that the shaking event took place between 38 ka and 30 ka. [ABSTRACT FROM AUTHOR]

The landform development and valley floor morphology of active regions is significantly controlled by tectonic processes. In the present study the upper catchments of Kosi and Gagas river valleys have been investigated with special emphasis on aggradational landforms to explain the spatial and temporal variability of aggradation/incision in response to tectonic activity during the late Quaternary and Holocene in Central Kumaun Himalaya. Major tectonic elements such as the North Almora thrust (NAT), Rasiyari fault (RF), and Gagas fault (GF) are considered to be controlling the development of landforms in the region. The valleys have preserved debris flow terraces, bedrock strath terraces, and fluviolacustrine terraces that provide signatures of tectonic activity. Morphostratigraphy of the terraces reveals that the oldest landforms are preserved in the hanging wall block of the NAT, RF, and along the GF. Reconstructions based on morphostratigraphy through the application of optical chronology suggests multiple phases of fluvial aggradation in the upper catchment of the Kosi and Gagas rivers that were initiated around 34 ka. The youngest phase of aggradation is dated to be around early to mid-Holocene (7–1 ka). Based on terrace morphology, four major phases of enhanced uplift have been estimated during 34, 15.8, 7, and 3 ka. The older uplift event dated to be around 34 ka is represented uplift along GF, which accommodated onset of valley-fill aggradation in Gagas valley. The 15.8 ka event represents uplift along RF, which led to the generation of debris flow from adjacent hillslopes. The 7 ka event represents uplift of a hanging wall block of the NAT and deposition of debris flow terraces. However, the youngest 3 ka event represents enhanced uplift, which is responsible for the incision of the older valley fill sediments and bedrock. Following this, a phase of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces between 7 and 1 ka. [ABSTRACT FROM AUTHOR]

The present study incorporates quantitative analysis of drainage of the Ramganga basin, Garhwal Himalaya, Uttarakhand. The drainage parameters (linear, areal and relief) of the Ramganga basin from Dudhatolidhar peak its point of origin to Kalagarh from where the river Ramganga enter into the plain, are covered. The impact of the morphological characters on the terrain is reflected by the drainage basin of the area. The calculated values of all the morphometric parameter of 38 V order intrabasins for different lithotectonic units were computed under four sectors viz., NE of NAT, between NAT and SAT, between SAT and MBT and downstream of MBT. In the lower reaches of Ramganga basin the various morphometric parameters shows existence of proportionality between stream length and catchment area while such proportionality is non-existence in the crystalline zone. Structural control on the drainage patterns of the Ramganga river and its tributaries are observed in the form of straight course, wide valleys with thick fluvial deposits and relatively gentle stream gradient along the transverse faults to incised valleys; less fluvial deposits in the banks and steeper stream gradient with knick points in the hanging wall of thrusts. [ABSTRACT FROM AUTHOR]

Main Boundary Thrust (MBT) zone is constituted of some of the landslide prone areas in southeastern part of Kumaun Sub-Himalaya. Role of landslides as natural hazard and hill slope modifying agent is well documented from various part of Himalayan region and southern hills of Kumaun particularly in the MBT zone, which are susceptible to various type of mass movement. The rocks making up the slopes has been put to a number of brittle deformation phases during the movement along the MBT, and are traversed by number of joint sets. In the open slope these intersecting joint sets forms wedges and are the most favorable site for initiation of rockfalls and other types of landslides. Landslides are taking place primarily due to high angle slopes, formation of structural wedges along the free steep slopes, sheared nature of the rocks due to proximity to the MBT and neotectonic activities along the MBT and other transverse faults. Wedge failure is a common type of landslides in rock slopes characterized by multiple joints and acts as sliding planes for the failed blocks. Field observations and wedge failure analysis indicates most of the landslides taking place in MBT zone of Kumaun Sub-Himalaya are joint controlled. Safety Factor analysis suggests MBT zone of Kumaun Sub-Himalayan region is prone to landslides and related mass movements. This zone is also neotectonically active as indicated by various geomorphic signatures such as structurally controlled drainage pattern, offsetting of fan by MBT and formation of number of small lakes. [ABSTRACT FROM AUTHOR]

The co-seismic deformation of the 28th April 2021 Assam earthquake with a magnitude of 6.4 has been investigated using the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR). The results obtained from the PSInSAR are validated with the results of the Global Positioning System (GPS). It is observed that the co-seismic deformation of the earthquake is not visible because of the atmospheric noise. Therefore, we used the Sentinel-1 time-series data of the area to minimize the atmospheric noise effect the data using the Atmospheric Phase Screening (APS) processing of Sentinel-1 time series data. The time-series analysis shows minor changes that occurred during the earthquake. Firstly, the detection of coherence-based changes was carried out to identify the most affected areas, caused by the earthquake. The results of time series analysis suggest that the region located to the north of the Brahmaputra River is subsiding at a much faster rate (−25 to -75 mm/y) than its southern counterpart, where the southern part is subsiding with an average velocity of −0.1 to −15 mm/y. The subsided locations identified using the PSInSAR are further confirmed by the presence of liquefaction features, ground cracks, and tilted buildings related to post-liquefaction ground settlement. To see the deformation along the Bomdila Fault (BF), re-analysis of the available GPS results has been carried out and the derived results reveal a significant amount of deformation (∼4 mm/y) associated with BF and Jorhat Fault (JF); whereas the deformation associated with Kopili Fault (KF) is 3.14 mm/y. Further, our results suggest the presence of compressional strain in the BF zone, while extensional strain in the KF zone. The earthquake focal mechanism analysis suggests that the event of the April 2021 was primarily associated with the BF rather than the KF. • We investigated co-seismic deformation caused by 28th April 2021 Assam earthquake. • Time-series analysis reveals significant amount of ground subsidence in the area. • We identified higher subsidence rates towards the north of Brahmaputra River • GPS result suggests that the Bomdila fault (BF) is deforming much faster than KF. • The identified co-seismic deformation of BF is ∼4 mm/y and 3.14 mm/y along the KF. [ABSTRACT FROM AUTHOR]

The aim of the present research is to provide the base line details of the NNW-SSE trending Raintoli fault (RF) which is running parallel to the North Almora Thrust (NAT) along the Saryu valley from Seraghat-Naichun to Seri in the central sector of the Uttarakhand Himalaya, India. The RF is characterized as dextral strike slip fault and behaves as a ductile shear zone within the zone of NAT. The dextral sense of shear movement of RF is delineated by the fabric of the shear zone rocks including microscopically observed indicators such as sigma and delta porphyroclasts, quartz c-axis, and the field structural data. Additionally, in the quaternary period the dextral strike slip fault is reactivated with oblique slip component as characterized by various geomorphic indicators, for example, triangular facets, abandoned river channels, unpaired fluvial terraces, and V-shaped valleys with recurrent seismicity. Further, the morphometric parameters including Valley Floor Width to Valley Height (Vf), asymmetry factor (AF), and gradient index (GI) further prove active nature of RF as suggested by low values of hypsometric integration, V-shaped valley, higher gradient index, and tilting of Saryu basin. [ABSTRACT FROM AUTHOR]