Munsiari Thrust Research Articles

Estimation of active surface deformation using PSInSAR technique of the Central Himalayan region

The Kumaun Himalaya is considered as the most active part of the Central Seismic gap in the Indian Sub-continent. In this paper, we presented active surface deformation rates of the Kumaun region from February 2017 to February 2021 using the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSI) technique. The cumulative displacement that occurred during the span of 4 years is ±55 mm, whereas the Line of Sight (LOS) deformation velocity rate ranges is ±7 mm/yr. Apart from PSI, we also estimated the b-value of the Kumaun region from 1803 to 2021 (399 events) and its value is 0.54 ± 0.03. A distinct NE-SW trend of b-value is observed where earthquakes with M > 6 occurred towards NE of this trend. The PSI-derived deformation reveals that the central part of the Inner Lesser Himalaya along with the Main Central Thrust (MCT) zone is dominated by uplift. The zone between the Munsiari Thrust (MT) and MCT in the central region shows the maximum uplift ranging 5–7 mm/yr which exactly lies above the mid-crustal ramp of the Main Himalayan Thrust (MHT). Our results are well corroborated with available observations of geodetic strain and peak ground acceleration values. However, the deformation patterns and high-velocity rates in the central part of the study area between MT and MCT indicate the accumulation of high stress.

Evaluating the distinct identity and the orogen-scale nature of the Munsiari thrust in the Himalaya

The Munsiari thrust (MT) was first recognized in northwest India within the Paleoproterozoic Lesser Himalayan rocks, but debate persists over its distinct identity and orogen-scale nature. Some researchers have included the MT within a ∼3–10 km-wide Main Central thrust (MCT) zone, whereas others proposed that, it is the northern and deeper extension of the Ramgarh thrust (RT). This study integrates field mapping, quantitative microstructural analysis, grain-size measurement, and piezometry to evaluate the distinct identity of the MT from North Sikkim, in eastern Himalaya. Our study reveals the presence of a ∼1700 m-thick prominent shear zone showing thrust-sense motion. The mylonites in the ∼300 m-thick core of the shear zone deformed by DRX (dynamic recrystallization)-assisted dislocation creep, and by climb-assisted dislocation creep in the ∼400–1000 m-thick flanking zones. We infer the mapped shear zone represents Sikkim-equivalent of the MT, because it transported Paleoproterozoic Lesser Himalayan crystalline rocks over Paleoproterozoic Lesser Himalayan quartzites and phyllites. The MCT and the RT lie ∼6 km structurally above-and-below the MT respectively. By comparing our findings with published results, we infer that, the MT is an orogen-scale structure, traceable from northwest Himalaya to far-eastern Himalaya, and it facilitated rapid exhumation of the Lesser Himalayan crystalline rocks between mid-Miocene and Pliocene. Therefore, we propose to reserve the appellation of the “Munsiari thrust” for a shear zone that transported Paleoproterozoic Lesser Himalayan crystalline rocks over Lesser Himalayan quartzites and phyllites of similar age, and suggest not to confuse it with the MCT or the RT.

Geochronology of Himalayan shear zones: unravelling the timing of thrusting from structurally complex fault rocks

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales ≪ 10 µm and are almost always altered to various degrees. First, electron probe microanalysis is necessary to assess both inventory and spatial distribution of minerals and their retrogression or alteration phases. We then used 40 Ar/ 39 Ar step-heating combining two independent indicators that allow the discrimination of coexisting mica generations from each other: (1) mica stoichiometry, which is proxied by 39 Ar concentration in combination with 37 Ar/ 39 Ar and 38 Ar/ 39 Ar (Ca/K and Cl/K) ratios; (2) furnace temperature at which the degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on average bond strength in the crystal structure, it is predicted and observed that the temperature of the differential Ar release peak is variable among different minerals. We observe that the Ca/Cl/K signatures of pure micas coincide with the Ar release peak. The Main Central Thrust zone in the Garhwal Himalaya records a protracted history. Foliation of the Vaikrita Thrust formed at 15–8 Ma, followed by static decompression at 7 Ma; foliation of the structurally lower Munsiari Thrust formed around 5 Ma. Our elaborate and time-consuming petrochronological procedure should become routine whenever analysing polydeformed metamorphic rocks. Supplementary material: Complete data for the studied samples are available at https://doi.org/10.6084/m9.figshare.c.5357212 Thematic collection: This article is part of the Isotopic dating of deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Himalayan cross faults affect thrust sheet geometry: An example from the Munsiari thrust sheet near the Gish Transverse fault zone, frontal Darjiling Himalaya, India

The Munsiari thrust (MT) sheet is a dominant thrust sheet in the Himalaya and forms the frontal ridge in the Gorubathan recess of the Darjiling Himalaya. The geometry of the MT sheet in the recess was poorly constrained thereby inhibiting the understanding of how the geometry and kinematics of the sheet varied from the Nepal to the Bhutan Himalaya. We present new structural data and infer the deformation mechanisms and the geometry of the MT sheet in the Gorubathan recess in the Darjiling Himalaya to fill this knowledge gap. The MT sheet was dominated by large-scale, overturned antiforms and synforms. The entire MT sheet was later folded to a north-easterly plunging synformal fold with NE-SW axial trend due to the effect of a Himalayan cross fault in the region named the Gish Transverse fault. A new brittle, out-of-sequence and neotectonically active, Phaparkheti thrust, was recognized in the MT sheet. Deformation microstructures indicate that the temperature of deformation in the MT fault zone and the sheet was ~500 °C to 700 °C and ~400 °C, respectively. Our approach of first mapping major and minor structures and then integrating these studies with deformation microstructure and tectonic geomorphology data resulted in an improved understanding of the structural characteristics of the MT sheet in the Darjiling Himalaya. Our study also suggests that there is considerable variation in the geometry and kinematics of frontal thrust sheets along the Himalayan arc that must be studied systematically in the context of Himalayan salients and recesses.

Three‐dimensional vorticity and time‐constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India)

AbstractVorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two‐dimensional data. We describe a 3D approach based on the use of X‐ray micro‐computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non‐coaxial shearing. 40Ar/39Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow.

Major shear zone within the Greater Himalayan Sequence and sequential evolution of the metamorphic core in Sikkim, India

The Greater Himalayan Sequence (GHS), bounded by the South Tibetan detachment system, and the Main Central thrust (MCT), constitutes majority of the Himalayan Metamorphic Core. Recent studies identified several shear zones within the GHS, which played significant role in its evolution. Here, we report the presence of a major shear zone, the Chungthang-Thambi thrust (CTT), which is situated within the GHS in Sikkim, eastern India. The CTT exhibits “zone-type” geometry, with mylonite core flanked by protomylonite zones, and its thickness varies from ∼1km to ∼5.6km along the transport direction. New 40Ar/39Ar mica ages, and apatite fission track dates from this study, when integrated with published UThPb monazite, and LuHf garnet ages indicate that the CTT, the MCT, and the Munsiari thrust (MT), at base of the crystalline Lesser Himalayan Sequence (LHCS), developed in-sequence between ∼27Ma and ∼12Ma, and controlled the sequential evolution of the metamorphic core (GHS+LHCS) in Sikkim. Our imbricate model suggests that the upper-middle GHS was initially underthrusted below the Tethyan unit, and underwent prograde metamorphism from ∼37Ma to ∼28Ma. Thrusting on the CTT exhumed the middle GHS, and underthrusted the lower GHS between ∼27Ma and ∼22.5Ma. The MCT exhumed the lower GHS between ∼22.5Ma and ∼12.1Ma, and underthrusted the LHCS, which was later exhumed by the MT between ∼12Ma and ∼10Ma. The thrust sheets exhumed to near-surface conditions between ∼2.6 and ∼0.5Ma, during growth of the underlying Lesser Himalayan Duplex. The MCT showed protracted activity till ∼7.2Ma, and leading edge of the CTT was later reactivated at ∼13.5Ma, and at ∼1.8Ma. The CTT is equivalent to the High Himalayan Discontinuity, and forms part of a regional structure that, spans ∼800km arc-length of the Himalaya from Sikkim to west-central Nepal.

U-Pb zircon ages and Sm-Nd isotopic characteristics of the Lesser and Great Himalayan sequences, Uttarakhand Himalaya, and their regional tectonic implications

Detrital zircons (DZ) and Nd isotopic characteristics constraint maximum depositional ages of two distinct Paleoproterozoic and Neoproterozoic terranes across the Main Central Thrust zone (Munsiari Group) in the Himalaya. New DZ ages and Nd isotopic characters are reported from the Inner Lesser Himalaya (iLH) sedimentary belt (Berinag Group quartzite) and the Munsiari Group through the Great Himalayan Sequence (GHS–Vaikrita Group) across the MCT to the lower parts of the Tethyan Himalayan Sequence (THS) along the Alaknanda–Dhauli Ganga valleys, Uttarakhand Himalaya. The iLH Berinag Group quartzite yielded nearly unimodal DZ U-Pb ages between 2.05 and 1.80 Ga with εNd(0) values of −17 and −23, while the overthrust Munsiari Group, bounded by the Munsiari Thrust at the base and the Vaikrita Thrust (MCT) at the top, represents the Proterozoic magmatic arc with ∼1.95 and 1.89 Ga U-Pb zircon age population with an average of −25 εNd(0) value; the arc developed during the Columbia Supercontinent assembly. In contrast, overthrust Great Himalayan Sequence (GHS–Vaikrita Group) above the MCT is characterized by entirely new Neoproterozoic 1.05–0.85 Ga zircon population, which appears for the first time in this sequence, and has higher εNd(0) values (average −16). Tectonically overlying the GHS, the Tethyan Himalayan Sequence (THS) has first appearance of the Early Paleozoic detrital zircons, with its εNd(0) values like the GHS. Broadly, these characters persist throughout the Himalayan belt from Himachal to NE Himalaya. The iLH sediments were possibly derived from northernly ∼1.9 Ga magmatic arc, and southern the Archean–Proterozoic Aravalli–Bundelkhand nuclei of the Indian craton. Potential sources for the GHS sediments may be a northerly ‘destroyed’ Neoproterozoic magmatic arc whose remnants exists within the Himalaya as the Neoproterozoic granitoids, and possibly be the iLH sedimentary belt, an ‘In-board’ Aravalli–Delhi Fold Belt (ADFB)–Central Indian Tectonic Zone (CITZ) in the south.

Earthquakes in the Garhwal Himalaya of the Central Seismic Gap: A Study of Historical and Present Seismicity and Their Implications to the Seismotectonics

The historical and the present seismicity catalogues of the Garhwal Himalaya have been studied for their spatio-temporal variations and their implications on the seismotectonics of the region. The Micro-Seismicity, Fractal dimensions (Dc) and Frequency Magnitude Distribution (b-value) coupled with the available literature on geology, geomorphology and geophysics have been used to derive the seismotectonics and stress level changes in the region. The seismic cross sections for the relocated micro-seismicity, focal mechanisms and the swath profiles (for the presence of Physiographic Transition 2 (PT2) at the foothill of the Higher Himalaya) indicate the constant presence of the Mid-Crustal Ramp (MCR) in the detachment plane and its active seismogenic nature. The comparison of this scenario suggests the constant presence of seismogenically active MCR structure throughout the Central Seismic Gap. The seismic cross sections further reveal that the sensu stricto Main Central Thrust (Munsiari Thrust) is also a site of generation of the micro-seismicity in few segments due to its reactivation by thrusting along the MCR. The high fractal dimension value (Dc = 1.47) suggests the heterogeneous nature of the region, owing to the presence of local faults and transverse tectonics. The high stress accumulation in the Garhwal Himalaya with low b value (b = 0.70) suggests the high probability of occurrence of a larger or greater earthquake in the near-future. Further, the study also reveals that the 2011 Chamoli earthquake of M 5.0, preceded by a quiescence period of nearly a year shows different stress levels before and after its occurrence, which is well constrained with the increased moderate earthquake activity around the Chamoli region. This increased seismic activity and stress conditions in the Chamoli region suggest the high possibility of the occurrence of major earthquakes, hence the study recommend for a detailed seismic hazard evaluation of the region.

Examining the tectono-stratigraphic architecture, structural geometry, and kinematic evolution of the Himalayan fold-thrust belt, Kumaun, northwest India

AbstractExisting structural models of the Himalayan fold-thrust belt in Kumaun, northwest India, are based on a tectono-stratigraphy that assigns different stratigraphy to the Ramgarh, Berinag, Askot, and Munsiari thrusts and treats the thrusts as separate structures. We reassess the tectono-stratigraphy of Kumaun, based on new and existing U-Pb zircon ages and whole-rock Nd isotopic values, and present a new structural model and deformation history through kinematic analysis using a balanced cross section. This study reveals that the rocks that currently crop out as the Ramgarh, Berinag, Askot, and Munsiari thrust sheets were part of the same, once laterally continuous stratigraphic unit, consisting of Lesser Himalayan Paleoproterozoic granitoids (ca. 1850 Ma) and metasedimentary rocks. These Paleoproterozoic rocks were shortened and duplexed into the Ramgarh-Munsiari thrust sheet and other Paleoproterozoic thrust sheets during Himalayan orogenesis. Our structural model contains a hinterland-dipping duplex that accommodates ∼541–575 km or 79%–80% of minimum shortening between the Main Frontal thrust and South Tibetan Detachment system. By adding in minimum shortening from the Tethyan Himalaya, we estimate a total minimum shortening of ∼674–751 km in the Himalayan fold-thrust belt. The Ramgarh-Munsiari thrust sheet and the Lesser Himalayan duplex are breached by erosion, separating the Paleoproterozoic Lesser Himalayan rocks of the Ramgarh-Munsiari thrust into the isolated, synclinal Almora, Askot, and Chiplakot klippen, where folding of the Ramgarh-Munsiari thrust sheet by the Lesser Himalayan duplex controls preservation of these klippen. The Ramgarh-Munsiari thrust carries the Paleoproterozoic Lesser Himalayan rocks ∼120 km southward from the footwall of the Main Central thrust and exposed them in the hanging wall of the Main Boundary thrust. Our kinematic model demonstrates that propagation of the thrust belt occurred from north to south with minor out-of-sequence thrusting and is consistent with a critical taper model for growth of the Himalayan thrust belt, following emplacement of midcrustal Greater Himalayan rocks. Our revised stratigraphy-based balanced cross section contains ∼120–200 km greater shortening than previously estimated through the Greater, Lesser, and Subhimalayan rocks.

Geomorphic characteristics of landscape development and formation of lakes in the zone of Munsiari Thrust, Garhwal Himalaya, Uttarakhand, India

The present investigation is focused on imprint of tectonic perturbation along a major tributary, known as the Lastar Gad (Gad = River), in southeast catchment of the Mandakini river in Garhwal Himalaya. Based on morphotectonic studies across the watershed, we recognized evidences of neotectonics along the Jutogh/Munsiari Thrust (MT/MCT-II). Various geomorphic features, e.g., ponding of streams and formation of lakes, fluvio-lacustrine deposits, waterfalls, stabilized and active landslide fans, dissected hills, paired and unpaired terraces, fault scarps and offsetting of drainage pattern etc., point to the active tectonics. The field investigations are supported by computation of morphometric indices, such as, low values of Valley Floor Width to Valley Height (Vf), Asymmetry Factor (Af), elongated and tilted river basin with narrow valley floor as well as higher Bifurcation Ratio (Rb) and sinuosity parameters, immature topography and landscape, elongation and tilting of drainage basin and accelerated erosion-all show that the basin is structurally controlled. The uplift along the valley is further manifested by two significant indexes, e.g., Stream Length (SL) and Steepness Index (Ks). Also, the mud deposits, lacustrine flats and swampy and soggy land within the valley suggest the existence of an ancient lake basin, named as Gethina palaeolake, downstream of present day Badanital lake. The 14C AMS radiocarbon chronology of the Badanital lake core as well as Gethina-Jagtoli palaeolake profiles indicate the formation of these lakes around 5–6 ka BP due to tectonic upheaval along the MT. We assume that a huge landslide activity blocked the course of a small stream (Badani Gad), forming an ancient lake at Gethina locality, followed by deposition of a landslide fan on which was created the Badanital. Subsequently, the palaeolake basin was filled with sediments due to accelerated erosion from almost vertical hills in the catchment. Owing to this, the Gethina (ancient) lake was disappeared and the modern Badanital lake shrank to its present state.

Fault activity, tectonic segmentation, and deformation pattern of the western Himalaya on Ma timescales inferred from landscape morphology

The location and magnitude of Himalayan tectonic activity has been debated for decades, and several aspects remain unknown. For instance, the spatial distribution of crustal shortening that ultimately sustains Himalayan topography and the activity of major fault zones remain unknown at Ma timescales. In this study, we address the spatial deformation pattern in the data-scarce western Himalaya. We calculated catchment averaged, normalized river-steepness indices of non-glaciated drainage basins with tributary catchment areas between 5 and 200 km(2) (n = 2138). We analyzed the spatial distribution of the relative change of river steepness both along and across strike to gain information about the regional distribution of differential uplift pattern and relate this to the activity of distinctive fault segments. For our study area, we observe a positive correlation of averaged k(sn) values with long-term exhumation rates derived from previously published thermochronologic datasets combined with thermal modeling as well as with millennial timescale denudation rates based on cosmogenic nuclide dating. Our results indicate three tectono-geomorphic segments with distinctive landscape morphology, structural architecture, and fault geometry along the western Himalaya: Garhwal-Sutlej, Chamba, and Kashmir Himalaya (from east to west). Moreover, our data recognize distinctive fault segments showing varying thrust activity along strike of the Main Frontal Thrust, the Main Boundary Thrust, and in the vicinity of the steep topographic transition between the Lesser and Greater Himalaya. In this region, we relate out-of-sequence deformation along major basement thrust ramps, such as the Munsiari Thrust with deformation along a mid-crustal ramp along the basal decollement. We suggest that during the Quaternary, all major fault zones in the Western Himalaya experienced out-of-sequence faulting and have accommodated some portion of crustal shortening.

Spatial interrelationship of landslides, litho‐tectonics, and climate regime, Satluj valley, Northwest Himalaya

Landslides have been considered widely as principal mass wasting agents in the valleys experiencing varied influence of tectonics and climate. However, pattern of landslides is rarely addressed in the literature that may act as a surface manifestation of interrelationship of tectonics, climate, and lithology. Here, we have attempted to understand such interrelationship in the context of landslide distribution pattern in the Satluj valley, Northwest Himalaya. Geomorphic indices such as steepness index, valley floor width to valley height ratio, and topographic swath profile were used for tectonic inference. Daily rainfall data of the year 2000–2016, using 5 rain‐gauge stations and swath profile of Normalized Difference Vegetation Index, were used to deduce spatial variability of climate. Influence of lithological variability and regional faults; Sangla Detachment, Main Central Thrust, and Munsiari Thrust on the landslide distribution are also inferred. A total of 55 landslides (20 rock avalanche, 19 debris slides, and 16 rockfalls) are found to exist in 6 clusters along 130 km stretch of the Satluj valley. These landslides, covering a total area (A) and volume (V) of 1.05 × 107 m2 and 4.4 × 107 m3, respectively, are also noted to follow a power law (R2 = 0.8) and result in a scaling relationship of V = (0.180)A1.208.

Pliocene episodic exhumation and the significance of the Munsiari thrust in the northwestern Himalaya

The Himalayan thrust belt comprises three in-sequence foreland-propagating orogen-scale faults, the Main Central thrust, the Main Boundary thrust, and the Main Frontal thrust. Recently, the Munsiari–Ramgarh–Shumar thrust system has been recognized as an additional, potentially orogen-scale shear zone in the proximal footwall of the Main Central thrust. The timing of the Munsiari, Ramgarh, and Shumar thrusts and their role in Himalayan tectonics are disputed. We present 31 new zircon (U–Th)/He ages from a profile across the central Himachal Himalaya in the Beas River area. Within a ∼40 km wide belt northeast of the Kullu–Larji–Rampur window, ages ranging from 2.4±0.4 Ma to 5.4±0.9 Ma constrain a distinct episode of rapid Pliocene to Present exhumation; north and south of this belt, zircon (U–Th)/He ages are older (7.0±0.7 Ma to 42.2±2.1 Ma). We attribute the Pliocene rapid exhumation episode to basal accretion to the Himalayan thrust belt and duplex formation in the Lesser Himalayan sequence including initiation of the Munsiari thrust. Pecube thermokinematic modelling suggests exhumation rates of ∼2–3 mm/yr from 4–7 to 0 Ma above the duplex contrasting with lower (<0.3 mm/yr) middle-late Miocene exhumation rates. The Munsiari thrust terminates laterally in central Himachal Pradesh. In the NW Indian Himalaya, the Main Central thrust zone comprises the sheared basal sections of the Greater Himalayan sequence and the mylonitic ‘Bajaura nappe’ of Lesser Himalayan affinity. We correlate the Bajaura unit with the Ramgarh thrust sheet in Nepal based on similar lithologies and the middle Miocene age of deformation. The Munsiari thrust in the central Himachal Himalaya is several Myr younger than deformation in the Bajaura and Ramgarh thrust sheets. Our results illustrate the complex and segmented nature of the Munsiari–Ramgarh–Shumar thrust system.

Out-of-sequence reactivation of the Munsiari thrust in the Relli River basin, Darjiling Himalaya, India: Insights from Shuttle Radar Topography Mission digital elevation model-based geomorphic indices

Quantitative tectonic geomorphology has emerged as a powerful discipline for studying evolution of topography, landscapes, and neotectonics using geomorphic indices computed from digital elevation data such as the Shuttle Radar Topography Mission (SRTM) data. We computed SRTM-based geomorphic indices to study neotectonics in the Relli River basin in the Darjiling Himalaya. We also used Real Time Kinematic Global Positioning System (RTKGPS) independent checkpoints to assess the quality of the SRTM data used to compute the geomorphic indices along with their uncertainties. Our analysis revealed that though the SRTM C-Band 90-m resolution (C90) digital elevation data has been used extensively for geomorphic studies, the 30-m resolution (C30) data were significantly more accurate. Moreover, geomorphic indices computed using SRTM C30 and C90 elevations in the Relli basin indicate that normalized, nondimensional indices such as the relief ratio (Rh), hypsometric integral (HI), basin elongation (Re), and valley floor width-to-height ratio (Vf) are statistically indistinguishable with uncertainty (1σ) at least an order of magnitude below the index value. The geomorphic indices in the Relli basin reveal neotectonic activity in the Munsiari thrust (MT) fault zone and intraformational faults in its footwall in the Lesser Himalayan rocks and also indicate that the basin is at an early mature stage close to equilibrium between tectonic and erosional process. However, analysis of the uncertainties associated with the indices suggest that the normalized or nondimensional geomorphic indices have the lowest uncertainties and that neotectonics in the Relli basin may only be confined to reactivation in the MT. The reactivation in the MT fault zone by out-of-sequence neotectonics implies the possibility of large earthquake events in the Darjiling Himalaya and significant seismic and landslide hazard for populations in large towns specifically located on the MT. Our new approach of looking at geomorphic indices and their uncertainties delivers a novel perspective for improved understanding of out-of-sequence neotectonics in river basins that may be applied more broadly across the Himalaya and elsewhere.

Seismotectonics and crustal stress field in the Kumaon–Garhwal Himalaya

We present fault plane solutions of 94 well located small-to-moderate sized (1.5≤ML≤5.4) earthquakes, which occurred in the Kumaon–Garhwal Himalaya during 2005–2008, using P-wave polarity and body wave amplitudes. These earthquakes show a mixture of thrust, normal and strike-slip type mechanism, with a majority of thrust type. Most of the thrust earthquakes occur at a depth of 8–22km in the Main Central Thrust (MCT) zone and the Lower Himalaya. The spatial distribution of these earthquakes suggest that the strain resulting from the ongoing collision of the Indian plate with the Eurasian plate is being consumed by thrust fault movement mainly on the north dipping Munsiari Thrust and south dipping Tons Thrust. The strike-slip earthquakes are mainly observed in the Lower Himalaya as well as around the Munsiari region in the MCT zone. The normal earthquakes are also observed in different parts of the Kumaon–Garhwal Himalaya and the Gangetic plain. Their occurrence is attributed to the local structure(s) as well as the flexure of the Indian plate. Stress tensor inversion of the calculated fault plane solutions indicates that the maximum compressive stress in the Gangetic plain is N–S directed and near vertical; whereas in the Kumaon–Garhwal Himalaya, it is near horizontal and NNE–SSW directed, and correlating with the prevailing stress condition due to northward movement of Indian plate.

A review on out-of-sequence deformation in the Himalaya

Abstract Out-of-sequence deformation in the Himalaya has been caused mainly by thrusting. Out-of-sequence thrusts, usually north- to NE-dipping foreshear planes, occur inside the Sub-Himalaya (SH), Lesser Himalaya (LH) and Greater Himalayan Crystalline (GHC) sequences. Where absolute dates are available, the youngest slip within the SH occurred near the Janauri Anticline (India) at c. AD 1400–1460. The Munsiari Thrust (India) activated within the LH at c. 1–2 Ma and the Main Central Thrust zone in the Marsyandi valley (Nepal) in the GHC was formed during the Holocene ( c. 0.3 ka). Except for the Riasi Thrust (Kashmir, India), the Paonta Thrust (Himachal Pradesh, India) in the Siwalik and the Tons Thrust (Garhwal region, India) within the Main Central Thrust zone, crustal shortening related to out-of-sequence thrusting in the Himalaya has been insignificant. The major litho-/stratigraphic contacts within the SH and the GHC at some places acted as out-of-sequence thrusts. Out-of-sequence thrusts in the SH have been detected mainly based on geomorphological observations. However, more quantitative geochronological studies have detected out-of-sequence thrusting from c. 22 Ma up to Holocene age in the GHC based on age jumps, especially within the Main Central Thrust zone. Crustal channel flow (specifically for the GHC) and/or the critical taper model with or without erosion can be used to explain the Himalayan out-of-sequence thrusts.

Redefining the tectonostratigraphic and structural architecture of the Almora klippe and the Ramgarh–Munsiari thrust sheet in NW India

Abstract We integrate U–Pb zircon geochronology and ε Nd (0) values with field mapping to determine which tectonostratigraphic units are represented to the north, south and within the Almora klippe in Kumaun, NW India. Rock in the Almora klippe and north of the Main Central thrust (MCT) have Neoproterozoic ( c. 900 Ma) detrital zircon ages, coupled with similar ε Nd (0) (−7.6 to −11.8) values, suggesting that these two units are the same tectonostratigraphic unit, and that the Almora klippe is the southern continuation of the MCT or another thrust in the Greater Himalayan thrust system. However, north of the Almora klippe, detrital zircon age populations establish the presence of Palaeoproterozoic rock instead of the previous interpretation of Neoproterozoic rocks. These Palaeoproterozoic Lesser Himalayan (LH) rocks are carried by the Ramgarh–Munsiari thrust (RMT) dipping south and folded underneath the klippe. South of the klippe, the RMT carries both the less metamorphosed Palaeoproterozoic and Neoproterozoic LH rocks, in disagreement with the idea that only Neoproterozoic–Cambrian LH rocks are present south of Almora klippe. These results suggest that previous cross-sections in Kumaun are incorrect and must be re-evaluated. Supplementary material: U–Pb zircon geochronological data table is available at http://www.geolsoc.org.uk/SUP18777 .

Structural, geochronological and geochemical evidence for two distinct thrust sheets in the ‘Main Central thrust zone’, the Main Central thrust and Ramgarh–Munsiari thrust: implications for upper crustal shortening in central Nepal

Abstract Two orogen-scale thrusts structurally underneath Greater Himalayan (GH) rocks characterize the structural architecture of Himalaya in central Nepal. The Main Central thrust (MCT) is at the base of the GH with the Lesser Himalayan (LH) Robang Formation in the footwall, which is the hanging wall of the Ramgarh–Munsiari thrust (RMT). At Kodari-Tatopani and Malekhu, U–Pb detrital zircon age populations from the RMT sheet yield a maximum depositional age of c. 1838 and c. 1871 Ma. U–Pb analyses of igneous zircons from the RMT sheet yield a crystallization age of c. 1750 Ma at both Galchhi and Kodari-Tatopani. The εNd(0) values of pelitic rocks from the RMT sheet at Kodari-Tatopani range from c. −23 to −25; whereas, GH rocks have values from c. −12 to −18. These data indicate that the RMT sheet carries the Palaeoproterozoic LH rock and the MCT carries the GH rock. At Kodari-Tatopani, the thrust previously mapped as the MCT is interpreted to be the RMT. Positively identifying the RMT sheet in all three locations yields a more accurate kinematic evolution and confirms its orogenic-scale presence in central Nepal. Supplementary material: U–Pb geochronological analyses are available at http://www.geolsoc.org.uk/SUP18775

An assessment of seismicity parameters in northwest Himalaya and adjoining regions

In this study, an assessment of seismicity parameters in the northwest Himalaya and adjoining regions using an earthquake catalog from India Meteorological Department covering a period from June 1, 1998 to June 30, 2011 has been carried out. The spatial distributions of seismicity parameters, namely magnitude of completeness, M C, a value, b value, and correlation fractal dimension, D C, are estimated for the studied region. The M C, a, and b values are found to be 2.5, 4.601, and 0.83, respectively. Despite significant gaps, the spatial distributions of a and b values are seen to follow similar trend and are found scattering in between Main Boundary Thrust (MBT) and South Tibet Detachment, adjoining areas of Mahendragarh-Dehradun Fault (MDF), Delhi-Haridwar Ridge (DHR) and Moradabad Fault (MF), and the southern flank of Karakoram Fault and Indus-Tsangpo Suture Zone. The estimated spatial distribution of b and a values is within 90 % of confidence level, thereby indicating non-uniform stress accumulation or higher rock fracturing density in the studied region caused by strong tectonization following several earthquakes. Negative correlation between low b value and high D C is observed predominantly in the region between the MBT and Munsiari Thrust or Main Central Thrust-I of Garhwal and Kumaon Himalaya, adjoining zones of MDF, DHR, and MF of Indo-Gangetic plain, and the eastern flank of the studied region, suggesting the presence of asperities in the zone. At the same time, active creeping process can be inferred in between the MBT and Main Central Thrust of Garhwal Himalaya and the surrounding areas of Shimla region of the Himalayan arc to the northwestern part of the studied region from the positive correlation between b value and D C. The results indicate that the structural heterogeneity caused by different stress accumulation and rock fracturing densities exists due to continuous tectonic adjustments between different geomorphic features of the studied region. An attempt has also been made to classify the studied region into smaller seismic zones by observing the spatial patterns of b value and D C that are fractal properties of the observed seismicity, along with the prevalent fault networks.

GIS-based morpho-tectonic studies of Alaknanda river basin: a precursor for hazard zonation

Alaknanda river basin is considered to be tectonically active where damaging earthquakes and landslides have occurred. The whole basin was divided into 8 sub-basins to carry out morphometric analyses, hypsometric integral (HI) analysis and valley floor width to valley height ratio (V f) factor. The sub-basins 2 and 3 show that they are highly active, because of the higher values of bifurcation ratio, stream frequency asymmetric factor, and lower values of form factor, elongation ratio and circulatory ratio. In these areas, HI values are very low indicating that the landscape is highly eroded, deeply dissected and tectonically active. The result obtained from V f was similar which classified both these basins as highly active. Morphometric analysis, HI and V f analyses along with structural map of study area are used to prepare morpho-tectonic map classifying the whole area into very high, high, moderate and low zones of tectonic activity. This map clearly indicates that the areas near MCT II (Munsiari Thrust), MCT III (Ramgarh Thrust) and North Almora Thrust are tectonically very active which fall in sub-basins 2, 3, 4 and parts of 5. Various locations such as Chamoli, Birahi, Pipalkoti, Rudraprayag, etc. are situated in these zones where many earthquakes and landslides occur every year. Moreover, the data plotted for earthquakes and landslides occurrences are consistent with morpho-tectonic map and can be used as a precursor for demarcation of natural hazard vulnerable zones.