Main Central Thrust Zone 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.

Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis

The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity.

Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.

Tectono-geomorphic and active deformation studies in the Ujh basin of Northwestern Himalaya

This study focuses on the morphotectonic analysis of the Ujh River basin, examining various aspects related to geometrical changes in the drainage basin caused by tectonic and erosional processes. The topographic development of the Ujh River basin is a response to enhanced tectonic deformation caused by Main Central Thrust (MCT) and Main Boundary Thrust (MBT). We assume that the basin asymmetry, tilting, incision, and migration of the river basin are regulated by the compressive force generated between MBT and MCT. The results were characterized by a high Asymmetry Factor (Af) indicating greater asymmetry suggesting a rightward tilting with low Valley Floor to Valley Width (Vf) values which indicate differential uplift and incision owing to ongoing tectonic processes. The high Stream Length Gradient (SL) Index values in the MCT zone, indicate sudden topographic breaks caused by the vertical tectonic uplift with active erosion processes. The Ujh River basin is in a disequilibrium state, with convex portions indicating uplift and concave portions indicating subsidence on the slope-area plot. The χ analysis, plotting χ values against elevation, highlights upliftment and erosion zones along the trunk stream, with major slope breaks and high χ values observed in the upstream region, particularly along the MCT. Overall, the Ujh River basin exhibits morphotectonic characteristics indicating ongoing tectonic activity and a state of disequilibrium.

Mineral resources of the Gandaki Province of Nepal: present status and prospect

Mineral resources serve as the foundation of prosperity for any country or community. However, the process of prospecting and exploring these resources is time-consuming and involves financial risks. It requires the combined expertise of both geoscientists and mining engineers, encompassing academic knowledge and professional skills. In this research, we provide an overview of the mineral resources within the Gandaki province of Nepal and evaluate their present status. The main objective is to examine the current status and potential of mineral resources in the province. To achieve this, we extensively utilize available reports, journal articles, and online sources. We gather both primary and secondary data on mineralization. Additionally, we systematically list the potential locations of all documented mineral resources within the Gandaki province, along with their GPS coordinates. Based on the current geological and mineralogical investigations, as well as our understanding of the province’s mineral resources, we identify ten minerals or mineral groups with potential economic value: iron, copper, uranium, tourmaline, syenite, thermal springs, limestone, dolomite, slate, and meta-basic rocks like amphibolite. We strive to provide geological evidence and context for supporting the presence of these minerals.
 In general, iron mineralization is observed as sedimentary deposits, exhibiting a syngenetic nature. Many copper ores are hydrothermally deposited originating from the underlying meta-basic rocks like amphibolite. The region is also known for its abundance of gemstones, including quartz crystals, garnet, kyanite, and tourmaline. These minerals are concentrated in the vicinity of the Main Central Thrust zone and its surrounding areas. Moreover, the province has significant potential for high-quality dimension stones for flooring, roofing, and pavements such as quartzite, slate, phyllite, schist, granite, and gneisses. Additionally, there are ample opportunities for the extraction of construction materials like sand and gravel from the river terraces and natural rock outcrops. Nevertheless, a systematic study focused on prospecting, exploration, and utilization of these potential and possible mineral resources is imperative for an accurate economic evaluation. To achieve this, the provincial government must develop plans and policies to facilitate the development of its own mineral resources. Furthermore, establishing strong collaboration between professionals and academia is crucial to make well-informed investments in the mineral sector. Considering the present status of minerals, the Gandaki province of Nepal holds promising prospects for harnessing its mineral resources for sustainable development.

Tectono-metamorphic transitions in the higher Himalayan sequence: A clue for Main Central Thrust (MCT) localization in Darjeeling-Sikkim Himalaya

This study recognizes a transition in the modes of ductile deformation across the Himalayan Crystalline Complex (HCC) from field and microstructural evidence. This transition leads to the Main Central Thrust (MCT) zone localization in the Darjeeling-Sikkim Himalaya. In the initial stage of crustal shortening (prior to ∼16 Ma), the HCC experienced temporally varying distributed ductile strains that produced regionally occurring planar fabrics and folds of three generations. Thereafter, the tectonic transition (from distributed to localized deformation) at ∼16 Ma occurred, forming the MCT zone in the HCC. This MCT zone (∼4–6 km thick) marks a diffuse metamorphic/rheological boundary between the hot HCC and the relatively colder Lesser Himalayan terrane to the south. The field data suggests that the base of compressed reverse paleo-isograds at the bottom of the MCT zone coincides with the transition of quartz creep mechanisms, from sub-grain rotation (SGR) to grain boundary migration (GBM). The present study documents the prograde metamorphic imprints to establish the P-T conditions of these multiple deformation episodes. Finally, this study presents a conceptual tectonic model, based on previous laboratory findings, to propose a two-stage tectono-thermal evolution of the Higher Himalaya.

Basement cross‐strike Bomdila Fault beneath Arunachal Himalaya: Deformation along curved thrust traces, seismicity, and implications in hydrocarbon prospect of the Gondwana sediments

Structural mapping and fieldwork in the Lesser and Higher Himalayan sequences in the western Arunachal Himalaya reveal crucial deformation fabrics in the Main Central Thrust (MCT) and Dirang Thrust (DT) zones. The top‐to‐SW ductile shear in the MCT and DT zones is correlated with the swing in the trend of MCT and DT from NE to NNW. The curved MCT and DT as traced by previous authors on the regional map of Arunachal Himalaya are studied. It is found that at places where these faults swing, shear senses developed at the meso‐scale. These shear senses are studied in meso‐ and micro‐scales. Seismicity in the western Arunachal Himalaya is influenced by basement cross‐strike crustal‐scale NW‐trending buried Bomdila Fault (BF). Landslides occur frequently along the Bhalukpong‐Bomdila–Sela traverse are also linked with the transverse BF. In its southern part, the BF coincides with the Dhansiri Lineament and is the basin margin fault in the upper Assam shelf. The Gondwana sediments extended further south below the Brahmaputra alluvium along the fault is to be explored for hydrocarbon exploration.

Assessment of Geothermal Renewable Energy with Reference to Tapoban Geothermal Fields, Garhwal Northwest Himalaya, India

Abstract The energy potential of geothermal resources at Tapoban Geothermal Field, situated in the orogenic region of the Chamoli district of Uttarakhand state in the Himalaya, is investigated based on physical parameters, reservoir temperature, borehole depth, and geochemical indicators. It is a prospective field of geothermal water exploitation for both electrical and non-electrical usages. The anomalous geothermal gradient due to the active Main Central Thrust (MCT) zone plays a significant role. To explore the feasibility of exploitation, the surface water temperature and pH from existing geothermal springs is calculated, which ranged from 45°C-93°C and 6.2-7.3, respectively. The study based on dissolved silica geothermometry shows an average reservoir temperature of ~125 ± 2.0°C. The springs emerge from the prominent joint/ weak zone in the country-rock with the discharge of hot waters up to 300 liter minute-1. The total geothermal heat energy of the reservoir is estimated as ~874.35 x 1011 KJ. The energy estimated for the binary power geothermal plant is ~1.02 MWe and 0.71 MWe for 20 years and 30 years, respectively. This energy can be increased manifolds by operating binary cycle power plant technique at multiple sites for an electric generation. The heat capacity from two dominant hot springs from Tapoban geothermal resource was found to be ~0.84 MWt, which can be harnessed by the local people for direct usage. The annual energy use from this reservoir is ~15.89 TJ per year with a capacity factor (CF) of ~ 0.60, indicating that the energy reserve of the Tapoban geothermal field can be used for low to moderate scale energy production to partially meet the energy gap and future requirements.

Tectonic Juxtaposition of Two Independent Paleoproterozoic Arcs by Cenozoic Duplexing in the Arun Tectonic Window of the Eastern Nepalese Himalaya

The tectonic evolution of the Himalayan orogenic belt before Cenozoic convergence is important to understand its modern structural framework. There is still controversy regarding the tectonic history of the lower Lesser Himalaya Sequence (LHS) in the frame of the Paleoproterozoic geological record. In this study, integrated analysis of field geology, zircon U-Pb age, Hf isotope, and whole-rock geochemistry was conducted across the LHS and the Main Central Thrust zone in the Arun Valley, eastern Nepal, to address their Precambrian tectonic evolution and the later convergence. Two groups of metasedimentary rocks with different age spectrums of detrital zircons and Hf isotope in the Paleoproterozoic can be distinguished in an imbricated duplexing system in the study area. One group with a maximum depositional age around ca. 1800 Ma has a unimodal detrital zircon pattern and negative zircon ƐHf (t) values ranging from −8.9 to 0.9. These data can be interpreted to reflect the deposition of zircons close to a Japanese-type arc that was isolated from the northern Indian Craton. The other group of calc-silicate rocks and quartzite with multiple peaks of detrital zircons in the Paleoproterozoic show a younger maximum depositional age around ca. 1,600 Ma and variable zircon ƐHf (t) values ranging from −6.7 to 8.8, indicating their deposition in a back-arc basin of an Andean-type arc established on the northern Indian Craton. The geochemistry of an orthogneiss sample dated at 1783 ± 11 Ma indicates high potassium, peraluminous granodiorite protolith emplaced in a volcanic arc or syn-collisional tectonic setting, supporting the existence of the Paleoproterozoic Andean-type arc. We hypothesize that possibly two arc systems developed, respectively, onto and in the proximity of northern Indian Craton in the Paleoproterozoic at the final stages of the Columbia supercontinent formation. These two arcs were juxtaposed either in the Paleoproterozoic or Cenozoic time, and were finally imbricated during the Cenozoic duplexing.

Normal reactivation of the Main Central Thrust Zone in Sikkim Himalaya

• The Main Central Thrust Zone, Sikkim , was reactivated as a normal shear zone. • The normal shear zone is 2.5 km wide with >5 km normal movement. • Normal movement followed thrusting marking the start of doming in footwall. • Doming caused steepening and destabilization of the MCTZ and normal slip. The Main Central Thrust Zone (MCTZ) controlled the Miocene development of the Himalaya. Its kinematic evolution is key to understanding the orogeny. The MCTZ accommodated shortening of the Himalayan orogen and thrusted hot rocks of the Greater Himalayan Sequence over cooler ones of the Lesser Himalayan Sequence, giving rise to an inverted metamorphic sequence. In Sikkim, NE India, the transport direction during thrusting describes a broad curved pattern indicating complex kinematics. In this paper we document further complexities in the motion of the MCTZ in the NW section of the Tista Dome in Sikkim, that reveals that SE-directed thrusting, at 45° to the orogenic front, was reactivated by top-to-NW normal movement. This produced a 2.5 km-wide ductile shear zone that accommodated >5 km of normal dip-slip movement, where the inverted metamorphic sequence is preserved with minor retrogression. This change from reverse to normal ductile movement likely occurred in the early stages of doming at ∼12 Ma, following peak metamorphism at 13.8 Ma. We argue that normal movement relates to the abandonment of the MCTZ and movement transfer to the Ramgarh Thrust and/or the Main Boundary Thrust, structurally deeper in the system. This new thrust gave rise to a duplex system responsible for the growth of the Tista Dome from ∼12 Ma onwards causing steepening and destabilization of the MCTZ leading to stress relaxation and normal movement reactivation. This must have occurred in the early stages of doming because the rocks were still sufficiently hot to respond in a ductile fashion.

Boron isotopes of tourmalines from the central Himalaya: Implications for fluid activity and anatexis in the Himalayan orogen

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values.

Subsurface Resistivity Image of Sikkim Himalaya as Derived from Topography Corrected Magnetotelluric Data

Abstract The Sikkim Himalaya is the eastern division of the Himalayan arc and is located between the Nepal and Bhutan Himalaya. Due to the complex geology, the abnormal trend of Main Central Thrust (MCT) and duplex structure in Higher Himalayan Crystalline (HHC) zone, the magnetotelluric (MT) data in Sikkim Himalayan region is modeled using topographic correction. In the present research, the three-dimensional (3-D) topographic correction using impedance tensor correction algorithm was applied to eight sites from the Sikkim Himalayan region and then inverted on the flat terrain. This paper presents the effectiveness of the 3-D topographic correction on two dimensional (2-D) modeling by comparing inversion models (a) model with distorted data on flat terrain, (b) distorted data with topography, (c) 3-D topography corrected data on flat terrain, to mitigate the effects of topography in MT data. The major outcome of the present study is in the form deciphering much bigger dimension in case of topography corrected data of the overlying resistive block (~9000 ohm.m) having depth up to 15 km between the sites Manglay and Lema, which correlate with the exposures of the Daling Formation in the northern Lesser Himalayan Sequence (LHS) and Lingste granite in the MCT zone. The topography corrected MT data brought out the topography of the resistive Indian crust more clearly.

Chemical mass transport during deformation and metamorphism: Insights from the Main Central Thrust and its footwall of Western Arunachal Himalaya, NE India

The Main Central Thrust (MCT) is one of the major tectonic discontinuities stretching about ~2500 km along the length of the Himalaya and it plays important role during the tectono-metamorphic evolution of Himalayan orogen. This contribution aims to find out the element mobility and loss or gain of chemical mass during mylonitization and migmatization of rocks and their relation with deformation and metamorphism within the Main Central Thrust Zone (MCTz) and its immediate footwall along the Bomdila-Dirang-Seppar section of the Western Arunachal Himalaya, India. Based on meso- and microstructural observations, we divided the Dirang Formation in the footwall of the MCT into 3 units: Lower Mylonite Zone (LMZ), Protomylonite Zone (PMZ) and Upper Mylonite Zone (UMZ), with increasing structural distance from Dirang thrust towards NW. Deformation of the Dirang Formation results in enrichment of Na, K and Si with significant gain in mass of ~55% in the UMZ relative to the PMZ. We inferred that shear strain localization during mylonitization in the UMZ induces textural transitions causing chemical alterations at peak metamorphism conditions of 0.62–0.66 GPa and 690°-710 °C. Migmatites of Lower Structural Position (LSP) show enrichment of Si, Na, K, Ca, Mn and P with depletion of Mg and Fe in the leucosome + mafic selvage (LM) relative to the paleosome with ~13% chemical mass gain. On the other hand, migmatites of Upper Structural Position (USP) are characterized by enrichment of Si, Mn and K and depletion of Ca, Mg, Na, P and Fe in the LM relative to the paleosome with ~17% chemical mass gain. Such chemical changes occur during near peak metamorphic conditions of 0.52–0.63 GPa and 680°-720 °C and it is accompanied by the appearance of sillimanite. This study improves our understanding of the chemical modification of rocks in the MCT and its footwall, which were subjected to deformation and metamorphism during its evolution in Himalayan orogen. The emplacement of the GHS along the MCT buried the Dirang Formation to a depth of ~23–25 km. Interaction of fluids, derived from dewatering of the underthrusted Dirang Formation, induced chemical alteration in the UMZ. We interpret that chemical mass transfer in migmatite zones directly influences the metamorphic reactions that occur during deformation and crustal anatexis in intracontinental shear zones, such as the MCT.

Seismic anisotropy beneath the Kumaun–Garhwal Himalaya using core-refracted shear wave phases

SUMMARY We investigate the nature of seismic anisotropy in the Kumaun–Garhwal Himalaya by analysing the core-refracted shear wave splitting phases (SK(K)S, PKS) recorded at 42 broad-band seismic stations. The mean fast anisotropy directions are mostly in ∼E–W and a few in the ∼NE–SW directions. The mean delay time decreases progressively from ∼1.0 s beneath the Sub Himalaya to ∼0.5 s at the Higher Himalaya. The observed seismic anisotropy suggests that the ongoing deformation beneath the Kumaun–Garhwal Himalaya has a large contribution by the relative rotation of the asthenospheric flow by the compression along the Indian Plate motion direction in this collision zone. Also, the southward transportation of the mid-crustal flow is probably responsible for the reduction in the delay times in the Main Central Thrust zone and further north than in the Sub-to-Lower Himalaya. The splitting parameters indicate a complex pattern of deformation beneath the Kumaun–Garhwal Himalaya with contribution from crust, lithospheric mantle and asthenospheric mantle.

Diverse Anatexis in the Main Central Thrust Zone, Eastern Nepal: Implications for Melt Evolution and Exhumation Process of the Himalaya

Abstract Sitting between the Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS), the Main Central Thrust zone (MCTZ) has experienced multiple episodes of anatexis, which presents an opportunity to explore the nature of partial melting and its response to Himalayan orogenic processes. A series of deformed rocks, including migmatites, gneisses, and leucosomes were collected across the MCT at Arun Valley, eastern Nepal. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, mineral chemistry, zircon geochronology and Hf isotopes, and conducted phase equilibria modeling. The protolith boundary between the GHS and LHS is recognized on the basis of Sr–Nd isotopes with εNd(0) of −16.7 to −8.0 for the GHS and −31.2 to −23.9 for the LHS. Samples from both the GHS and LHS have undergone partial melting, as revealed by in situ leucosomes at outcrops and melt inclusions at thin-section scale. Leucosomes separated from their host rocks are divided into four groups: those derived from hydration melting, muscovite dehydration melting, amphibole dehydration melting, and feldspar accumulation. Phase equilibria modeling results for the GHS migmatite show isothermal decompression from peak P–T conditions of 11 kbar and 795°C, accompanied by muscovite dehydration melting evolving into biotite dehydration melting. In contrast, rocks from the LHS are modeled to have undergone hydration melting at P–T conditions of 9 kbar and 685°C. Zircon U–Pb geochronology suggests that long-lived partial melting (35–13 Ma) occurred in the MCTZ. Moreover, anatectic zircon Hf isotopes show that the protoliths for partial melting changed from the GHS to the LHS with εHf(t) of −19.4 to −5.7 during the early Miocene, and lower values of −42.5 to −16.7 during the middle to late Miocene. These zircon geochemical results indicate that hydrous metasediments from the LHS were progressively accreted to the base of the GHS, resulting in hydration melting of both the GHS and LHS assisted by MCT. The timing of activity of the MCT is constrained to 25–13 Ma, coeval with movement of the South Tibetan detachment system. Integration of petrogenetic modeling, the chronology of partial melting, and metamorphic P–T paths allows us to propose that thickened Himalayan crust was heated from the middle to late Eocene, and widespread anatexis occurred during the Oligocene to middle Miocene, forming a large-scale melt channel. The hot GHS channel flow moved upward in association with the synchronous activity of the MCT system, triggered intense dehydration of LHS metasediments, resulting in fluid-present melting in both the GHS and LHS during middle to late Miocene, and the formation of leucogranite with mixture features of GHS and LHS. Furthermore, with the cooling of the melt channel, duplexing has gradually operated since the middle to late Miocene in the shallow crust.

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.

Complex tectonic setting and deep crustal seismicity of the Sikkim Himalaya: An electrical resistivity perspective

Tectonic setting of the Himalayan arc is dominated by thrust-type earthquakes mainly associated with rupture of the décollement surface, also called the Main Himalayan Thrust (MHT). However, the region encompassing eastern Nepal, Sikkim and western Bhutan departs from this general tectonic model of the Himalaya and experiences deep crustal seismicity of strike-slip nature with focal planes oblique to the convergence direction of the collision front. The 2011 Sikkim earthquake (Mw 6.9) is an example of such transverse tectonic deformation. Several geophysical studies have been carried out in Sikkim to understand the complex tectonics of this region. Among these, a crustal model derived from magnetotelluric (MT) data revealed contrasting crustal structure across the Main Central Thrust Zone (MCTZ) beside a small-scale MHT ramp. Here, we revisit the MT model by including long-period MT data (up to 10,000 s) at six sites along the profile, invert the distortion corrected impedance data, and analyse the results in conjunction with other published geophysical and structural information. Based on this study, we propose a tectonic model for the Sikkim Himalaya that reveals the complex nature of the lithospheric structure including two major contacts beneath the MCTZ and the Main Frontal Thrust (MFT), respectively. The contact beneath the MCTZ is a NW-SE trending lithospheric-scale seismogenic fault that separates two geologically and compositionally distinct blocks of the Indian plate underthrusting the Himalaya. The other contact beneath the MFT could be responsible for the necking of the Main Central Thrust (MCT) at this location.

2D Electrical resistivity imaging for geothermal groundwater characterization and rejuvenation of the Gaurikund hot spring in the Main Central Thrust (MCT) zone of the Garhwal Himalaya, Uttrakhand, India

Gaurikund is a famous geothermal spring near the Kedarnath Dham in the central Himalaya, Uttarakhand, India. During the Kedarnath Disaster in 2013, the thermal spring and most of the areas surrounding the Kedarnath temple lies ruined and covered by debris. In the present study, an effort has been made to image the debris covered source of the geothermal groundwater potential zones for rejuvenating the Gaurikund thermal spring and to understand the mechanism for enhancing the continuous recharge and flow of this hot spring. We have carried out hydrogeological measurements with these objectives at four locations (points 1–4). The obtained data indicates that spring water temperature varies from 35 to 58 °C and point-2 was observed to be the main source of the spring, situated adjacent to the Gauri Mata temple. Besides this, we performed 2D electrical resistivity imaging (ERI) profile with different configurations in the vicinity of the spring along the right bank of the Mandakini River. ERI reveals two distinct low resistivity horizons with resistivity variation from 20 to 50 Ωm within the depth of 5–15 m. The low resistivity zones have been interpreted to be the thermal spring's potential source, which occurs either along the fractured zones with channelized groundwater flow or along highly weathered zones. The study suggests that the source of the Gaurikund hot spring is located in the hanging-wall of the Vaikrita Thrust, a northerly dipping major structural unit and weak zone that can play a significant role in reviving the Gaurikund geothermal spring.

Tectonic discontinuity, partial melting and exhumation in the Garhwal Himalaya (Northwest India): Constrains from spatial and temporal pressure-temperature conditions along the Bhagirathi valley

The pattern and timing of tectonic discontinuity and partial melting in the Garhwal Himalaya, resulting from the India–Asia collision, are crucial factors for understanding the evolution of the Himalayan orogen, but these parameters remain poorly constrained. Middle- to high-grade metamorphic rocks from the Main Central Thrust (MCT) zone and the High Himalaya Crystalline Sequence (HHCS) along the Bhagirathi valley in northwest India have been investigated in this study. The primary mineral assemblage of the gneisses and migmatites in the HHCS is garnet + biotite + white mica + quartz + kyanite + rutile ± ilmenite ± sillimanite. The metamorphic grade at thermal peak rapidly increases upwards across the MCT from garnet schists (515–550 °C, 8–9 kbar) in the footwall to garnet-bearing migmatite (705–735 °C, 10–13 kbar) in the hanging wall. The P–T condition decreased upwards in kyanite gneiss of the lower HHCS (690 °C, 8.3 kbar), yielding a high field pressure gradient near the MCT. The prominent metamorphic discontinuity could have been caused by the southward extrusion of the HHCS along with the MCT. The base of the HHCS experienced high-grade metamorphism at 38–25 Ma, mainly at 31.9 ± 0.5 Ma. The new data, combined with previous studies, suggest that the prominent metamorphic discontinuity is characterized by the different periods of peak metamorphism and the overprint in the hanging wall (Eocene–Oligocene metamorphism and Early Miocene overprint) and footwall (Early Miocene metamorphism and Late Miocene to Pliocene overprint) of the MCT. The second metamorphic discontinuity occurred near the migmatite zone in the lower-middle HHCS, which experienced the anatexis (690–740 °C, 7–9 kbar) at 37–25 Ma, mainly at 33.9 ± 1.2 Ma. The Bhagirathi HHCS experienced a regional Barrovian-type Eohimalayan metamorphism associated with fluid-present melting of muscovite + plagioclase + quartz + H2O → melt ± kyanite at 34–32 Ma without a strong Miocene overprint. The period of partial melting is significantly older than the intrusion of ca. 22 Ma Early Miocene Gangotri leucogranites at the top of the HHCS. Protracted periods of crystallization (38–22 Ma) indicate a long duration of more than ca. 15 Myr of anatexis, from initiation of migmatization to pluton formation in the Bhagirathi HHCS. The protracted fluid-present melting and subsequent rapid exhumation in the Bhagirathi HHCS could be associated with heterogeneous extrusion forming the tectonic discontinuity in the lower-middle HHCS. The metamorphic grade decreased towards the upper HHCS (620 °C, 6.5 kbar) across the Jhala Normal Fault where occurs in the right-way-up sequence. On the other hand, in the central Himalaya, the thrusting occurred between upper HHCS and lower-middle HHCS in the inverted metamorphic sequence. The Early Miocene migmatites in the footwall of the significant tectonic discontinuity are dominant, and diachronous extrusion accompanied with the fluid-absent melting reactions is the common mechanism to exhume the HHCS, unlike the heterogeneous extrusion in the northwest Himalaya.

Middle Eocene‐Oligocene anatexis and exhumation of the Greater Himalayan Sequence in central Nepal

AbstractWe present structural analysis and zircon and monazite geochronology of the Greater Himalayan Sequence (GHS) in the Kali Gandaki valley, central Nepal. Although the foliation is moderately NE‐dipping, the upper GHS shows a top‐to‐WNW shear sense, whereas the lower GHS developed top‐to‐SW ductile shear zones. Quartz fabrics from mylonitic gneisses indicate deformation temperature from >650°C to ~450°C. Monazite from the GHS recorded anatexis at “near peak” conditions from ~43 to 28 Ma and exhumation near the sillimanite‐kyanite transition at 25–21 Ma. Ages of zircon rims constrain the onset of exhumation. From the upper GHS to the Main Central Thrust zone, pervasive deformation and anticlockwise rotation of the lineation from the orogen‐parallel to orogen‐perpendicular direction suggest exhumation of the GHS by general shear from ~31 to 21 Ma. A two‐stage exhumation model is proposed to explain the diffuse strain distribution and anatexis during early crustal thickening.