Lesser Himalayan Rocks Research Articles

Geological study along Sipaghat-Panchkhal-Sukute area, Kavreplanchowk and Sindhupalchowk districts, Lesser Himalaya, central Nepal

A detailed geological study has been carried out in the Lesser Himalayan rocks distributed in Panchkhal-Dolalghat- Sukute area in Kavre Palanchowk and Sindupalchowk districts. The study area belongs to Kuncha Formation of Lower Nawakot Group, Benighat Slate of Upper Nawakot Group, Nawakot Complex; Raduwa Formation, Bhaisedobhan Marble, Kalitar Formation, Chisapani Quartzite, Kulikhani Formation, and Markhu Formation of Bhimphedi Group and Tistung Formation of Phulchauki Group, Kathmandu Complex. These two complexes are separated by the Main Central Thrust (MCT). Four lithounits of the Lower Nawakot Group (i.e., Fagfog Quartzite, Dandagaon Phyllite, Nourpul Formation and Dhading Dolomite) and two lithological units (Malekhu Limestone, Robang Formation) of the Upper Nawakot Group are missing in the area. The area comprises intercalation of coarse-grained, thick bedded quartzite and gritty phyllite of Kuncha Formation, dark argillaceous slates with Jhiku Carbonate beds including sporadic amphibolite of the Benighat Slate. The Bhimphedi Group of the Kathmandu Complex comprises mainly micaceous schist, marble, metasandstone and augen gneiss whereas the Phulchauki Group comprises of metasandstone and granite. The lithological units strike NW-SE and dip towards south forming the eastern closure of the Mahabharat Synclinorium in the northern limb.

Exploring the tectonostratigraphy of the Pakistan Himalaya with new detrital zircon geochronology and neodymium isotope data from the Kaghan Valley

In the central Himalaya, the Greater and Lesser Himalayan tectonostratigraphic zones can be distinguished from each other by combining detrital zircon U-Pb ages and whole-rock Nd isotopic data. In the Hazara-Kashmir syntaxis of northern Pakistan, rock units in the Kaghan valley have been variously assigned to the Tethyan, Greater, and Lesser Himalayan zones. To determine which tectonostratigraphic zones are represented, we sampled across the Batal thrust in the northern Kaghan valley because this structure is identified in some research as the Main Central thrust. Samples from the footwall yield Paleo- to Mesoproterozoic maximum depositional ages (∼1.8–1.1 Ga) and an average εNd(0) value of −14.3. Samples from the hanging wall yield Neoproterozoic maximum depositional ages (∼1000–600 Ma) and εNd(0) values of −17.3. The contrasting detrital zircon ages show that the Batal thrust in the Kaghan valley is a major structural division. However, these data lack the characteristic detrital zircon age spectra and εNd(0) values of Greater and Lesser Himalayan rocks north and south of the Main Central thrust in the central Himalaya, respectively. Therefore, either the Batal thrust is not the Main Central thrust, or the Main Central thrust in the Kaghan valley cuts a different structural level and carries rock units not seen outside the Hazara-Kashmir syntaxis.

Magnetic fabric and geomorphic characteristic of Neotectonic activity along strike direction of North Almora Thrust, Kumaun Lesser Himalaya, India

Structurally, the North Almora Thrust (NAT), is placed in the northern proximity of the Almora Nappe (AN) in the Kumaun Lesser Himalaya. The NAT is characterized by the presence of a zone of mylonitic rocks of basal Saryu Formation (1800 ± 100 Ma) in the hanging wall and the metasedimentaries of Inner Lesser Himalayan rocks as the footwall block. The NAT has been dextrally offset by two estabilished major faults i.e. Saryu River Fault (SRF) in Saryu Valley and Dwarahat-Chaukhutia Fault (DCF) in Ramganga valley. In present study, we identified four new faults, based on field study, geomorphic landform and magnetic fabric analysis. These faults are N–S trending Pancheshwar Fault, NE–SW Rameshwer Fault, NNE-SSW trending Kosi Fault, and NNE-SSW trending Gagas Fault (GF). Our results show that the zone is bound by cross cutting relation with NAT and these fault zones are comparatively more active than other regions. Furthermore, we suggest that the steep and NW–SE orientation of magnetic foliation within the NAT zone is a result of NE–SW oriented progressive regional compression. The magnetic foliations represent the unseen internal foliations in the rocks developed due to preferred alignment of magnetic minerals and can be found through the Anisotropy of magnetic susceptibility (AMS) study. Variation in the alignment of the field and magnetic foliations are developed due to superimposed brittle deformation along the faults over the pre-existing field foliations. Magnetic foliations represent the impact of last stage deformation and significant to find brittle deformation and finite strain in the rocks of the study area. The lowering of anisotropy (Pj) away from the fault zone represents distribution of strain across the NAT zone. AMS fabric confirms the presence of faults developed across the NAT zone and also explains the deformation pattern along these faults. The geomorphic anomalies and steepness changes across the NAT zone are correlated with active deformation along the NAT and associated transverse faults.

Miocene development of the Main Boundary Thrust and Ramgarh Thrust, and exhumation of Lesser Himalayan rocks of the Kumaun-Garhwal region, NW-Himalaya (India): Insights from Fission Track Thermochronology

• New FT ages suggest that the MBT and Saleri Thrust were initiated during ∼14 Ma. • The Amritpur granite was emplaced over the Siwalik along the MBT during late Miocene . • The Ramgarh Thrust sheet was emplaced over the LHMS zone along the RT during ∼24 Ma. • Old ZFT ages suggest that the metamorphism of the outer LHMS zone occurred < 250 °C. • Reactivation of the MBT and its associated faults occurred since Plio-Plistocene time. Bounded between the northeast dipping Main Boundary Thrust (MBT) at the base and the Saleri Thrust (ST) at the top, the Amritpur granite in the Kumaun-Garhwal region, NW-Himalaya is emplaced over the Siwalik sediments. As it is devoid of microscopic/mesoscopic imprints of the Himalayan tectonics, there is a debate regarding its emplacement along the MBT. While some interpret it as a klippe, others describe it as an intrusive body. The new apatite (AFT) and the zircon (ZFT) fission-track thermochronological data from the Amritpur granite provide similar age ranges and trends from the MBT to the ST. AFT ages range between 11.3 ± 1.6 and 14.7 ± 1.0 Ma with a weighted mean of 13.4 ± 1.5 Ma and ZFT ages range between 12.4 ± 1.2 and 15.4 ± 1.7 Ma with a weighted mean of 13.9 ± 1.2 Ma. Such age ranges and trends could be explained by rapid upliftment and exhumation of the Amritpur granite as horse rooted from the basement along the MBT from a depth of ∼8–10 km during mid-Miocene (∼14 to 13 Ma). We interpret this age as the time of development of the MBT. The new ZFT ages from the Jaunsar Group of the outer Lesser Himalayan meta-sedimentary (LHMS) zone range between 49.4 ± 2.0 and 80.3 ± 4.1 Ma (with a weighted mean of 67.5 ± 4.01 Ma) while the AFT ages range between 1.7 ± 0.2 and 4.2 ± 1.0 Ma (with a weighted mean of 3.17 ± 0.6 Ma). The old ZFT ages indicate that the outer LHMS zone underwent low-grade pre-Himalayan metamorphism (∼250 °C) around ∼67–72 Ma and the ZFT age of outer LHMS was not reset during the Himalayan orogeny. Young AFT ages from the Jaunsar Group of the outer LHMS zone reflect accelerated exhumation, likely due to reactivation of the MBT and its associated faults, since Plio-Pleistocene time. The new ZFT ages from the Ramgarh Thrust (RT) sheet range between 23.2 ± 2.4 and 24.2 ± 2.7 Ma with a weighted mean of 23.7 ± 2.5 Ma. The mean ZFT age of the RT sheet is similar to the age range of the Main Central Thrust (MCT) movements. This indicates that the RT sheet has been thrusted along the RT during 24 Ma followed by the exhumation of the RT sheet.

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.

Impact of Late Pleistocene climate variability on paleo-erosion rates in the western Himalaya

It has been proposed that at short timescales of 102–105 yr, climatic variability can explain variations in sediment flux, but in orogens with pronounced climatic gradients rate changes caused by the oscillating efficiency in rainfall, runoff, and/or sediment transport and deposition are still not well-constrained. To explore landscape responses under variable climatic forcing, we evaluate time windows of prevailing sediment aggradation and related paleo-erosion rates from the southern flanks of the Dhauladhar Range in the western Himalaya. We compare past and present 10Be-derived erosion rates of well-dated Late Pleistocene fluvial landforms and modern river sediments and reconstruct the sediment aggradation and incision history based on new luminescence data. Our results document significant variations in erosion rates ranging from 0.1 to 3.4 mm/yr over the Late Pleistocene. We find that, during times of weak monsoon intensity, the moderately steep areas (hillslope angles of 27 ± 13°) erode at lower rates of 0.1–0.4 mm/yr compared to steeper (>40°) crestal regions of the Dhauladhar Range that erode at 0.8−1.3 mm/yr. In contrast, during several millennia of stronger monsoon intensity, both the moderately steep and high slope areas record higher erosion rates (>1-3.4 mm/yr). Lithological clast-count analysis shows that this increase of erosion is focused in the moderately steep areas, where Lesser Himalayan rocks are exposed. Our data thus highlight the highly non-linear response of climatic forcing on landscape evolution and suggest complex depositional processes and sedimentary signals in downstream areas.

Landslide characteristics and its impact on tourism for two roadside towns along the Kathmandu Kyirong Highway

Frequent landslide events affect the Kathmandu Kyirong Highway (KKH), one of the most strategic Sino-Nepal highways, with multiple social effects. Amongst them, the impacts on local tourism, although being substantial, have not been studied so far. The aim of this research is to analyze the characteristics of such landslides and their influence on road damages and/or blockages as well as on local tourism industry. We analyzed the co-seismic landslides triggered by the Gorkha Earthquake, 2015 (7.8 Mw), the post-seismic landslides that occurred during the monsoons following the earthquake, as well as landslides which occurred or reactivated in 2018, with relation to the damage that they caused to the highway. High resolution satellite images from 2015 to 2018, and field data were used for the analysis. The Langtang avalanche that locates off the highway was also mapped due to its high impacts on tourism. Between 2015 and 2018, the number of road damaging landslides in the Betrawati-Rasuwagadhi section of KKH (where Dhunche and Syafrubesi towns are located) was 101 in the main track (MT) and 103 in the new track (NT), with respective average density of 1.46/km and 3.63/km. The dominant observed landslide types were debris slides and rock falls. Landslides were mostly concentrated in the locations with the following characteristics: 1) having higher elevated area, 2) being located with the ‘main central thrust’ and other lineaments’ belts, 3) belonging to the Proterozoic lesser Himalayan rocks, 4) having a slope gradient of 25°–45°, 5) having northern, western and southern slope aspect, 6) being subjected to average annual rainfall of higher than 1,000 mm, and 7) having less than 4 km distance from the past earthquake epicenters. The results further indicated that 7 rain-induced and 4 co- and post-seismic landslides have great impact on tourist flows. An impact analysis was also assessed through a door to door questionnaire survey with local hotel operators from Dhunche and Syafrubesi towns (n = 29 + 31). The results reveal that out of six rigorously affected sectors by landslides leading to road blockage, tourism business is the most impacted livelihood sector in these towns. The reduction of visitors in different hotels ranged from 50%–100% in Dhunche and 70%–100% in Syafrubesi for the first year aftermath of the tremor. This is higher than the respective 5%–50% tourist reduction due to rain-induced landslides. Using as a reference the base year 2014, the income loss of hotels in both towns was found to be 50%–100% in 2015, 20%–100% in 2016, 5%–75% in 2017, and similar to 35% in 2018. These results provide insights on the synergic effect of contributing factors for cut slope as well as down slope instability along mountainous motorways and their impact on income sources for local communities.

Importance of Tourmaline Gneiss and Vein near Main Central Thrust in Sikkim Darjeeling Himalaya

Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and/or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.

Quaternary landform study in Kosi and Dabka river valleys in Kumaun sub‐Himalaya: Implication of reactivation of thrusts

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.

Geology and mineral resources of Khudi-Bahundanda area of west-central Nepal along Marshyangdi Valley

Geological study was carried out along the Khudi-Bahundanda area of the Marshyangdi Valley in the west central Nepal. The area lies partly in the Main Central Thrust (MCT) zone and partly in the Higher Himalayan Crystalline Zone. The aim of the study was to prepare a detail geological map and cross section in the scale of 1:25,000 to work out on stratigraphy, metamorphism and mineral resource potential of the area. The rocks of the Higher Himalaya have been mapped under a single unit as Formation I. This unit consists of kyanite-garnet para-gneiss. The lithological units of the MCT zone are mapped into three units as the Benighat Slate, the Malekhu Formation and the Robang Formation from the bottom to the top, respectively. The Benighat Slate consists of dark grey to black schist with some carbonate beds as members. The Malekhu Formation consists of creamy white siliceous dolomite marble with parting of schist. The Robang Formation comprises of light grey psammitic schist with garnet and white micaceous quartzite in various proportion.&#x0D; Many secondary structures are observed in the study area, but primary structures are missing due to extreme metamorphism. The large-scale structures are the MCT, which separates the Lesser Himalayan rocks to the south from the Higher Himalaya to the north, and the Bahundanda Thrust (BT). Numerous outcrop-scale structures like meso-scale folds, quartz veins, boudinage and ptygmatic folds are abundant. Folds in the MCT zone are mostly E-W trending, and rocks have experienced multiple metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalayan rocks resemble to the kyanite grade of metamorphism. As in the other sections of the Himalaya, the present section also clearly shows the inverted metamorphism in the MCT zone. The MCT zone is considered as the potential site for precious and semi-precious stones, of which the most potential ones are the garnet and kyanite.

Geology and micro-structure analysis of the MCT zone along Khudi- Bahundanda area of Lamjung District, west-central Nepal

Geological mapping was carried out along the Marsyangdi Valley in the Khudi-Bahundanda area of west-central Nepal covering the Main Central Thrust (MCT) zone. The main objectives of the study were to draw a clear picture of lithology, geological structures and micro-tectonics in the rocks. A detail survey on stratigraphy and correlation with central Nepal reveals geological rock units such as the Benighat Slate, the Malekhu Formation and the Robang Formation of the Lesser Himalaya and the Formation I of the Higher Himalaya. Both regional and small-scale geological structures have been studied. The MCT zone has been mapped as a major regional structure in the region. The Bahundanda Thrust (BT), which has brought the older Malekhu Formation over the younger Robang Formation, is an another significant structure mapped. The BT is marked on the basis of fault breccia, slickensides as well as large deposits of debris mass at the fault zone.&#x0D; The study area has undergone poly-metamorphism and dynamic crystallization of minerals. The Lesser Himalayan rocks resemble the garnet zone while the Higher Himalaya rocks resemble to the kyanite grade of metamorphism. The present section clearly shows the inverted metamorphism in the MCT zone as in the other sections of the Himalaya. Microscopic features like ribbon-quartz, polygonization of quartz crystals, grain boundary reduction, mica-fish and rotated garnet grains indicates the ductile shearing in the MCT zone suggesting the dynamic recrystallization during thrust propagation. Numerous outcrop-scale structures like meso-scalefolds, quartz veins, boudinage and ptygmatic folds are abundant folds in the MCT zone and these are mostly E-W trending.

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.

U-Pb monazite ages from the Pakistan Himalaya record pre-Himalayan Ordovician orogeny and Permian continental breakup

The Greater Himalayan Sequence in India and Nepal records crustal thickening processes that took place during and following the onset of India-Asia collision (ca. 54–50 Ma). These resulted in late Eocene–early Miocene kyanite- and sillimanite-grade regional metamorphism, and Oligocene–Miocene crustal anatexis, which formed migmatites and leucogranites. We present new U-Pb monazite data for kyanite- and sillimanite-grade gneisses of the Neoproterozoic to Cambrian Tanawal Formation in the Pakistan Himalaya, which have metamorphic ages of 482.4 ± 7.9 Ma and 464.5 ± 4.0 Ma, respectively. These ages, together with along-strike equivalent rocks in the Lesser Himalaya of India and Nepal, help to define an Ordovician orogeny (the Bhimpedian orogeny, also called the Kurgiakh orogeny) spanning at least from 490 to 460 Ma, which followed Neoproterozoic–Cambrian sedimentation and preceded Late Ordovician–Silurian postorogenic molasse deposition and development of a stable shelf margin at 460–440 Ma. These new ages for peak metamorphism of Neoproterozoic rocks in the Pakistan Himalaya overlap with those of widespread S-type granites that exist along the length of the Lesser Himalaya (e.g., Mansehra, Mandi, and Kathmandu granites), Greater Himalaya (e.g., Nanga Parbat, Kinnaur Kailas, and Ama Drime), and in the North Himalayan domes (e.g., the Tso Morari, Kangmar, and Kampa domes). Thus, based on stratigraphic, metamorphic, and U-Pb geochronological criteria, we equate rocks of the Tanawal Formation gneisses to the Lesser Himalayan rocks to the east and rocks of the Alpurai Group to the Higher Himalaya. Our data also confirm that the Lesser, Greater, and Tethyan Himalaya are all part of a single contiguous plate. Staurolite-grade schists from a higher structural level of the Tanawal Formation record U-Pb monazite ages of ca. 272–267 Ma, indicating the occurrence of Permian regional metamorphism in the area concomitant with major rifting and extrusion of the Panjal Trap volcanics.

The nature and evolution of the Main Central Thrust: Structural and geochronological constraints from the Sikkim Himalaya, NE India

The Main Central Thrust (MCT) is a prominent continental-scale fault within the Himalaya. Its definition has been the topic of some debate in the literature. After a brief consideration of the state of discussion to clarify the definition we use in this work, we report features from the field- to the microstructural- scale of a particularly well-exposed section in Sikkim, NE India. The nature of the protoliths as well as the overlying and underlying rocks is characterized in terms of ε-Nd. The dates of motion on the fault are constrained using U–Pb geochronology of zircon and monazite from pegmatitic dikes that cross-cut the deformation fabric. It is found that the mechanism of deformation recorded in the fault zone rocks is different compared to that found in the overlying Greater Himalayan (GH) or the underlying Lesser Himalayan (LH) rocks. The GH and LH have different protolith characteristics as well. Combined with existing data on P–T history, dates of metamorphism, and cooling- and exhumation-rates of the GH and the LH, our measurements show that major motion on this fault occurred before 20Ma at 450–700°C but after peak metamorphism of rocks (750–800°C) in this zone. Isolated events occurred in this zone as late as 11Ma, possibly in the brittle domain. This underscores the pulsed nature of movement over an extended period on such major faults, and the related difficulties in dating fault movement, determination of the rates of movement, and designating a fault plane as in- or out-of-sequence within a propagating deformation front.

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.

Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transport‐parallel stretching and inverted metamorphism

In order to understand mass and heat transfer processes that operated during Himalayan orogenesis, we collected temperature, finite and incremental strain, and kinematic vorticity data through a 5 km thickness of Lesser and Greater Himalayan rocks in southern Bhutan. This transect crosses two major shear zones, the Main Central thrust (MCT) and Shumar thrust (ST). Raman spectroscopy on carbonaceous material and garnet-biotite thermometry are integrated with deformation temperatures from quartz petrofabrics. These data define inverted field gradients that correspond in structural position with the MCT and ST, which are separated by sections in which temperatures remain essentially constant. Temperatures increase from ~400-500 °C to ~700-750 °C between 675 m below and 200 m above the MCT. This defines a 269 ± 44 °C/km inverted gradient, interpreted to have formed via high-magnitude (~100-250 km) shearing on a discrete MCT zone delineated by the limits of inverted metamorphism. Temperatures increase from ~300-400 °C to ~400-530 °C across the ST, which is attributed to differences in maximum burial depth of hanging wall and footwall rocks. Strain and vorticity data indicate that Lesser and Greater Himalayan rocks were deformed by layer-normal flattening. Transport-parallel lengthening and foliation-normal shortening increase from 38-71% and 36-49%, respectively, between 2.3-1.0 km below the MCT. The MCT acted as a ‘stretching fault’, with translation on the order of 100's of km accompanied by transport-parallel stretching of footwall and hanging wall rocks on the order of 10's of km. This demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer.

Back-thrusting in Lesser Himalaya: Evidences from magnetic fabric studies in parts of Almora crystalline zone, Kumaun Lesser Himalaya

The present study aims to understand evolution of the Lesser Himalaya, which consists of (meta) sedimentary and crystalline rocks. Field studies, microscopic and rock magnetic investigations have been carried out on the rocks near the South Almora Thrust (SAT) and the North Almora Thrust (NAT), which separates the Almora Crystalline Zone (ACZ) from the Lesser Himalayan sequences (LHS). The results show that along the South Almora Thrust, the deformation is persistent; however, near the NAT deformation pattern is complex and implies overprinting of original shear sense by a younger deformational event. We attribute this overprinting to late stage back-thrusting along NAT, active after the emplacement of ACZ. During this late stage back-thrusting, rocks of the ACZ and LHS were coupled. Back-thrusts originated below the Lesser Himalayan rocks, probably from the Main Boundary Thrust, and propagated across the sedimentary and crystalline rocks. This study provides new results from multiple investigations, and enhances our understanding of the evolution of the ACZ.

Detrital zircon U–Pb age and Hf isotopic composition from foreland sediments of the Assam Basin, NE India: Constraints on sediment provenance and tectonics of the Eastern Himalaya

Synorogenic Palaeogene–Neogene sediments of the Assam foreland basin, were derived by erosion of adjacent crustal and orogenic sources following the Greater India–Eurasia collision since ∼55Ma. To constrain source sediment influx, and its relation to Himalayan tectonics, from pre- to post-collision time, detrital zircon U–Pb geochronology and their Hf isotopic compositions were carried out. The varying detrital zircon spectral patterns analyzed from the Paleogene Jaintia, Barail and Neogene Surma and Tipam Groups, with sediment petrography, track source sediment derived from cratonic India, Gangdese and eastern Transhimalayan batholiths and the eastern Himalaya. These sources are tested against Cenozoic paleopositions proposed for the northeastward motion of the Indian plate.Precollisional cratonic detritus to Middle to Late Eocene Sylhet Formation shifted to Tethyan Himalaya and arc sources of the Gangdese and eastern Transhimalayan batholiths to Late Eocene Kopili and Barail Formations, consistent with the proposed paleoposition proximal to the Indus-Yarlung suture. This Sylhet–Kopili Formation transition, within the Jaintia Group, reflects one of the earliest Himalayan hinterland exhumation stages during the Late Eocene. Major shift in provenance to Higher Himalayan Crystalline and arc detritus is recorded from the Surma Group, constraining Mid Miocene Himalayan tectonic exhumation from the eastern Himalaya. Late Miocene Tipam Group preserves sediment of Higher Himalayan Crystalline detritus, ophiolite and likely Lesser Himalayan rocks.

Detrital zircon U–Pb geochronology of the Siwalik Group of the Nepal Himalaya: implications for provenance analysis

This paper deals with the provenance analysis of the Neogene foreland basin sediments of the Siwalik Group in the Nepal Himalaya. This study adopts the techniques of the optical petrography and detrital zircon U–Pb ages from two river sections: the Koshi Nadi in eastern Nepal and the Surai Khola in western Nepal Himalaya. The optical petrography data and resulting QFL plot show a “recycled orogeny” field for the studied sandstone samples, indicating northern lithotectonic units; Tethys Himalaya, Higher Himalaya and Lesser Himalaya as the source of the foreland basin sediments. The detrital zircon geochronological data set has clearly revealed that the cluster ages are younger than ~1000 Ma; however, the older grains (>1000 Ma) are significantly fewer. The obtained age spectrum is similar to the Tethys Himalaya and the upper Lesser Himalaya, but the lower Lesser Himalayan rocks were not distinct, which indicates that sediments in the Neogene foreland basin of the Nepal Himalaya were primarily sourced from the Tethys Himalaya and upper Lesser Himalaya. The minor subordinate scattered peaks that roughly correspond to the age of the Higher Himalaya and lower Lesser Himalaya may indicate that a lower proportion of the sediments might have a link with the Higher Himalaya and lower Lesser Himalaya. Therefore, the provenance of the Siwalik Group in the Nepal Himalaya might have witnessed a mixed type of provenance similar to the northwestern Himalaya.

Evidence for a far-traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 + 7/ − 10 Ma and ~570 Ma, with metamorphic zircon growth at 23–29 Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475 + 7/ − 3 and ~660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile “thrust sheets,” present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.