Himalayan Orogen Research Articles

Influence of reactivated basement structures on evolving orogens: Along-strike diachronous Himalayan metamorphism in far west Nepal

Determining the geometry and evolution of a basal detachment and its influence on orogenesis is a challenging, but important, aspect to understanding orogenic evolution. The basal detachment of the Himalayan orogen in far west Nepal is presently segmented by a documented tear fault. New pressure-temperature-time-deformation paths from the Himalayan metamorphic core along the Seti Khola river transect were integrated to compare the tectonometamorphic evolution on either side of the basal detachment tear fault to outline its history. Peak metamorphic conditions of 645−745 °C and 0.85−1.1 GPa were reached in the Seti Khola Himalayan metamorphic core rocks during the Oligocene to earliest Miocene, 10−14 m.y. prior to equivalent along-strike rocks in the adjacent Karnali valley, which indicates segmentation of the Himalayan metamorphic core across the tear fault. We interpret the segmentation of the orogen to have been caused by the development of the tear fault in the basal detachment of the Himalayan orogen and differing ramp-flat geometries on either side. The segmentation and change in basal detachment geometry is consistent with the reactivation of an underthrusted Indian plate inherited basement structure, the Great Boundary Fault, during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal highlights the basal detachment geometry through time and the need to consider the pre-orogenic structural features of the plates involved in orogenesis. This study reinforces the importance of combining tectonometamorphic studies with geophysical and geomorphological data to fully understand the causes of along-strike segmentation of orogenic systems through time.

Pre‐Vegetation Mixed (Wave‐Tide) Energy Trangressive Nearshore Sedimentation: Evidence From the Proterozoic Passive Margin Sequence of NW Himalaya, India

ABSTRACTThe coupled evolution of the Earth's atmosphere–biosphere system through time has caused irreversible changes in the geodynamics as well as surface processes and sedimentation patterns. One such significant change took place in sedimentation in the Palaeozoic (i.e., Silurian) by the appearance of vascular vegetation. While the impact of evolving vegetation on the terrestrial fluvial environment has been relatively well documented, vegetation‐induced effects down the system in marginal or nearshore marine settings have undergone little study. The Meso‐ to Neoproterozoic Rautgara Formation exposed in the Himalayan Orogenic Belt of NW India, offers a chance to study a well‐preserved fluvial–marine transition to nearshore sedimentation before the appearance of vascular vegetation. A detailed sedimentological analysis identifies six genetically linked facies associations (FA) probably deposited in barrier, back‐barrier, and subtidal deltaic environments. Contrary to the other transgressive barrier models (where beach‐barrier overlie the back barrier environments), in the present case, wave‐dominated barrier deposits mostly occupy the basal part of the stratigraphy. In the middle stratigraphic level, back‐barrier deposits lack thick mud flats and show a dominance of sandstone over mudstone. Stacked subtidal sand bar facies association represents the top part of the sequence. Two sequence stratigraphic surfaces, that is, subaerial unconformity and maximum flooding surface, have been identified and the whole succession is interpreted in terms of HST and TST. Barrier and back‐barrier sediments are deposited during HST and TST, respectively. Subtidal deltaic system developed in late TST. The lack of frequent interbedding between the barrier and back‐barrier facies indicates negligible landward migration of the barrier and demonstrates system stability. The barrier system might have resulted from vertical aggradations akin to modern vegetated systems. The study portrays that sandy barrier systems are common in the Proterozoic. Vegetation and thick mud flats are not always essential for the stability of a barrier‐beach system.

Northward drift of the Burma Terrane with India during the Cenozoic and implications for the India–Asia collision

The past location of the Burma Terrane during the convergence of the Indian and Asian tectonic plates is key to unravelling the regional geodynamic, palaeoenvironmental and palaeobiogeographical history of the eastern edge of the Himalayan orogen. Palaeomagnetic data provide the ability to constrain the location of the Burma Terrane, but it has been very difficult to find rocks with palaeomagnetic records of primary characteristic remanent magnetizations. We present here new palaeomagnetic results spanning the Paleocene to late middle Eocene within the Burma Terrane, complementing palaeolatitudes previously established from Late Cretaceous intrusive rocks and late middle Eocene sedimentary rocks. Our palaeomagnetic data indicate that the Burma Terrane remained at equatorial latitudes during the Paleocene and early Eocene, at a considerable distance from the South Asian margin. In addition, palaeomagnetic results from mid- to late Eocene sedimentary rocks yield a predominantly north–south orientation of the Burma Terrane over the past 45 Myr, showing that it was not part of the NW–SE-oriented Sundaland margin before its collision with India. Our results support collision models involving a Trans-Tethyan subduction system during the Late Cretaceous and early Paleocene. We propose that this system incorporated the Burmese volcanic arc and continental fragments of Argoland before drifting north with India towards Asia. The new palaeogeographical model considers a reduced amount of oblique subduction of the Indian plate below Burma during the Cenozoic. A possible source of sediments filling the thick Myanmar basins from the Gangdese belt during the Eocene supports the hypothesis of an India–Asia collision around ∼50 Ma. The new palaeogeography supporting the formation of the Myanmar Cretaceous amber on an isolated Trans-Tethyan Arc is also a key element in discussions of the palaeobiogeographical evolution of the numerous faunas it contains.

The making of Mount Everest: Channel Flow, and Low-angle normal faults in the compressional Himalayan orogen

Mount Everest (8849 m) spans the Greater Himalayan Series metamorphic rocks and the base of the unmetamorphosed Tethyan sedimentary rocks in the Nepal-South Tibet Himalaya. Two north-dipping, low-angle normal faults cut the massif, the upper Qomolangma Detachment placing Ordovician sedimentary rocks above Everest Series greenschist – amphibolite facies rocks, and the lower Lhotse Detachment placing Everest Series schists above sillimanite gneisses, migmatites and leucogranites. The two faults merge northwards into one large ductile shear zone (South Tibetan Detachment). Pressure-temperature constraints and structural restoration shows that the fault acted as a passive roof fault during extrusion of the footwall. At least 120 km southward flow of footwall rocks occurred during the Miocene resulting in exhumation of rocks that were buried to 5.5 kbar (∼18-22 km depth) below the detachment juxtaposing them against hangingwall rocks that are essentially unmetamorphosed. The low-angle normal faults were operative during north-south convergence and reflect exhumation of a locked passive roof fault, not to any crustal extensional processes. U-(Th)-Pb dating of peraluminous leucogranites exposed on Everest (21–20 Ma), Nuptse (∼19–18 Ma) and along the Rongbuk valley (15.6–15.4 Ma) show that ductile extrusion occurred during the Early Miocene, with brittle faulting at <15.4 Ma, during exhumation.

Abrupt topographic descent at the eastern end of the Himalayan orogen: Insights from geodetic analyses

The Indian-Eurasian convergence has formed the Himalayas, one of the most youthful and dynamic orogeny on Earth, which is characterized by a unique “perfect arc” observed by seismicity, crustal deformation, and topographic relief. However, the presence of a significant topographic descent at the eastern end of the Himalayas, near the Eastern Himalayan Syntaxis (EHS) challenges the existing paradigm. The reason behind such significant topographic difference compared to other regions along the Himalayan mountains is still unclear. Based on the GPS velocity field, we determined the clockwise rotation of the North Indian Block (NIB) relative to the stable India plate with a Euler pole estimation of (89.566 ± 0.06°E, 26.131 ± 0.05°N, 1.34 ± 0.11°/Myr), implying that the NIB has broken away from the stable India plate. By reconstructing the position of the Northeast Indian Block (NIB) based on the Euler pole, we found that the collisional boundary between India and Eurasia is moving southward. Subsequently, a coupled fault model that accounted for continuous motion of fault can effectively match the topographic descent. Our result underscored the significant impact of the NIB rotation on regional geological evolution, an aspect that has received less attention in previous studies.

Geochemical and Radiogenic Sr‐Nd Isotope Characterization of Widespread Sandy Surface Sediments in the Great Indian Desert, Thar: Implications for Provenance Studies

AbstractUnderstanding large desert formation/evolution contributing to regional‐to‐global dust cycles remains a challenge. This study presents the geochemical and Sr‐Nd isotope compositions of 51 surface sediment samples collected from the widespread hyper‐arid Thar Desert in northwestern India. The major objective is to determine sediment provenance for a better understanding of the formation/evolution mechanism of this Great Indian Desert as well as downwind dust contributions toward the Himalayas. The compositionally immature sandy Thar sediments (CIA ∼50 ± 4, WIP ∼49 ± 12, and EuN/Eu* ∼0.80 ± 0.13) are recycled materials derived from the Himalayan orogen and later modified by quartz addition and heavy mineral depletion/sorting processes. The 87Sr/86Sr (0.7259 ± 0.0012 and εNd (−12.5 ± 2.7) in the bulk of these Thar sediments are different from the earlier published compositions of the eolian sand deposits in northwestern India. The subcategories of Thar materials collected from different dune types exposed over different lithologies (Quaternary alluvium vs. Tertiary and Mesozoic sedimentary formations) are geochemically and isotopically indistinguishable, which indicates their cogenetic sources and/or sediment reworking. Thar sediments collected in this study have a predominant Indus origin along with significant contributions from the upwind Ghaggar‐Hakra paleochannels. The Indus sediments are most likely wind‐eroded from the shelf region exposed during the low sea stand of LGM and afterward deglaciation. Considering the new and published data sets, the Sr‐Nd isotope budget of dust deposited in the Himalayan frontal glaciers indicates that atmospheric mineral dust contribution from the upwind Indo‐Gangetic Plain proximal to the Himalayas is at par with dust parcels from distant natural deserts.

Revisiting the fluid regime during anatexis of metasedimentary rocks: New constraints from an integrated approach on the Cenozoic Himalayan granites

Water plays a critical role in the formation of granitic magmas and continental crust, but distinguishing water-present and water-absent anatectic scenarios using the geochemistry of granites is controversial. In this study we use an integrated approach that combines whole-rock major and trace element geochemistry, Ti-in-zircon thermometry, phase equilibrium modeling and trace element modeling to study the water regime that produced the Miocene granites from the Malashan-Gyirong area in central Himalaya to solve this controversy. The Gyirong granites have relatively low CaO, Sr and Ba and high Rb and (87Sr/86Sr)i and the Malashan granites have relatively high CaO, Sr and Ba and low Rb and (87Sr/86Sr)i, and were classified as group A and group B granites, respectively. Following previous interpretation, group A and group B granites are consistent with the products of water-absent and water-present melting of metasedimentary rocks, respectively. Ti-in-zircon thermometry yielded maximum values of 761–796 °C for the Gyirong granites and 730–764 °C for the Malashan granites. Distinct variation trends in zircon trace element compositions indicate that these two groups of granites were not linked by crystallization differentiation. Using average compositions of Proterozoic pelite as starting materials, phase equilibrium modeling was carried out at a variety of P–T–H2O conditions typical of the Himalayan orogen, P = 5, 10 and 15 kbar, T = 600–800 °C, H2O = 0–10 wt%. Pelite can produce melts with coupled CaO–Na2O contents for both groups at 10 kbar. Specifically, constraints from compositions and temperatures require that the bulk H2O content is ca. 1–2 wt% for group A granites and > ca. 4 wt% for group B granites. Compared with the maximum structural water content of the pelite at 10 kbar (1.77 wt%), this study testifies that group A granites formed under water-absent conditions and group B granites under water-present melting conditions. Modeling shows that water-present melting can produce melts with high Sr–Ba and low Rb contents resembling group B granites, while water-absent melting can produce melts with low Sr–Ba and high Rb contents resembling group A granites. This study highlights that water can indeed cause differences in granite geochemistry but a comprehensive investigation is required to better determine the role of water during crustal anatexis.

Role and origin of water-fluxed melting in the generation of High Himalayan leucogranites

Knowledge of the role water has played in the evolution of Earth’s continents is crucial for improving our understanding of how its layers (e.g. crust and mantle) interact with each other and co-evolved. Orogenic belts are the preferred pathway for material circulation, heat transfer, and water cycling. The Himalayan orogen preserves a large number of syn- and post-collisional rocks, of which the Himalayan leucogranites are key to studying the role of water in crustal reworking. Although some geochemical proxies support water-fluxed melting in the source of Himalayan leucogranites, the timing of water addition, the scale of water-fluxed melting, and how water affects or controls the chemical and physical properties of the partial melts are not well constrained. Here, we filtered the published data on High Himalayan leucogranites to ensure that they represent the geochemical characteristics of primary melts. Our results show that the primary leucogranites with high Th/U and low Rb/Sr ratios were produced by water-fluxed melting as a result of the dissolution of monazite, melting of plagioclase, and formation of peritectic K-feldspar. Furthermore, thermodynamic modeling demonstrates that water-fluxed melting generates a high water content and a large volume of felsic water-unsaturated melt with chemical compositions similar to those of the High Himalayan leucogranites. We suggest that the external water is released by the breakdown of hydrous crustal minerals during dehydration melting and prograde metamorphism.

Metamorphism of Dolomitic and Magnesitic Rocks in Collisional Orogens and Implications for Orogenic CO2 Degassing

Abstract Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important constituents of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e. impure limestones and dolostones, marls sensu stricto and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate–silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on (i) the main decarbonation reactions, (ii) the P–T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15–40% carbonate is significant (>5.5 ± 1.0 CO2 wt% and up to 10.5 ± 1.5 CO2 wt%), whereas for carbonate contents above 60–70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.

Magnetotelluric Imaging of the Central Himalayan Crust Away From Rift Zones

AbstractImaging the crustal structure and state of the Himalayan Orogenic Belt (HOB) is of great significance for understanding crustal deformation and its tectonic response in the collision front of the Indian‐Eurasian plates. To avoid the influence from the anomalies underneath the north‐south trending rifts, a three‐dimensional electrical resistivity model across the central HOB was obtained between the Dingjie‐Shenzha and Yadong‐Gulu rifts. No lower‐crustal conductor was found in the southern Lhasa Terrane, and the lower crust with a moderate resistivity value of approximately 100 Ωm is interpreted to be the residue of previous partial melting beneath the Gangdese porphyry copper deposits. The middle crust between the Main Himalayan Thrust and Southern Tibet Detachment System is electrically resistive to the north of the north Himalayan gneiss domes (NHGD) belt and conductive to the south. The middle crust in the northern Himalayas may have undergone partial melting and crystallization during southward migration and duplexing. There is a strong deep‐shallow coupling relationship in the central HOB, where an increase in the underthrusting angle of the Indian crust has resulted in a southward turn of the NHGD belt.

Constraining the exhumation history of the Greater Himalayan Sequence, Kali Gandaki, Central Nepal

To understand the evolution of the Himalayan orogen, we first need to understand how the South Tibetan Detachment System (STDS) evolved through time and space. We present new (and previously published) thermochronological results from a transect in the footwall and ductile shear zone of the basal structure of the STDS in the Kali Gandaki region: the Annapurna detachment. The exhumation history is interpreted from observations using 1D thermal–kinematic models that invert to give the exhumation rate of samples. Recently published data have suggested that high-temperature slip on the detachment persisted until at least c. 12 Ma, more recently than is commonly assumed for STDS deformation. Our new data and modelling support these findings and suggest that the cessation of slip coincided with a dramatic (>50%) decrease in the exhumation rate of the shear zone and its footwall at c. 12–10 Ma. Exhumation rates remained low until c. 3 Ma, after which they increased to levels comparable with those that characterized STDS activity. Plausible causes of this late pulse of exhumation include an intensification of the Asian winter monsoon and the establishment of today's Indian summer monsoon, glaciation and/or an internal structural reorganization of the Himalayan orogenic wedge driving localized rock uplift in the hinterland. Supplementary Material, including method descriptions, full datasets and supplemental figures, is available at https://doi.org/10.6084/m9.figshare.c.6949467

Petrography, geochemistry and detrital zircon U–Pb dating of the Pliocene‐Pleistocene Dupi Tila Formation from the Lalmai Anticline, Bengal Basin: Regional tectonic implications

The Bengal Basin is a well‐known foreland basin that archives the tectonic evolution of the Himalayan and Indo‐Burman Ranges (IBR). The Dupi Tila Formation is the youngest among the stratigraphic successions of the Bengal Basin, which witnessed the last phase of the Himalayan uplift. However, there is still controversy regarding sediment provenances since the Himalayan and IBR are in the late Neogene exhumation stage. In this study, the petrographical and geochemical compositions of the Dupi Tila sandstones from the Lalmai Anticline of the Bengal Basin are examined to infer the source rocks, tectonic settings, palaeoclimate and weathering intensity in provenance areas. The detrital modes and geochemical discriminations, as well as Chondrite‐normalized rare earth elements patterns with enrichment in light rare earth elements and nearly flat heavy rare earth elements and negative Eu anomalies (average Eu/Eu* = 0.77) indicate dominantly felsic source rocks. The geochemical data also suggest that these source rocks in provenance areas were influenced by weak to moderate chemical weathering (chemical index of alteration: 65.64–75.62) and multiple recycling under warm and humid to semi‐humid climatic conditions. The detrital zircon ages show three dominant peaks at ca. 100, 500 and 1000 Ma with minor 1700 Ma. The amount of young zircon grains (<120 Ma) is noticeably higher (~18%) in this Lalmai outcrop compared to the Dupi Tila samples from the Sylhet Trough and Chittagong‐Tripura Fold Belt. Results from integrated zircon geochronology and geochemistry indicate that the detritus are likely derived from a recycled orogen provenance related mainly to active continental margin setting with subordinate influence from passive setting. Therefore, the regionally extensive Himalayan Orogen appears to be the primary sediment source, with additional mixing from the IBR and recycled sediments originating from the Gangdese batholith of the Trans‐Himalaya.

Exhumation of the Cuonadong Sn–W–Be polymetallic deposit, Tethyan Himalaya: Implications for exploration

The Cuonadong Sn–W–Be polymetallic ore deposit is the first rare-metal deposit related to leucogranites with giant mineralization potential to be discovered in the Himalayan Orogen. However, the post-mineralization exhumation and preservation processes of this deposit have not yet been studied in detail, although these factors are key to the exploration and evaluation of rare-metal deposits in the Himalaya. Geophysical data have revealed that a hidden fault (i.e., the Jisong secondary fault) cuts the southern part of the Cuonadong ore deposit. In order to determine the effects of the Jisong secondary fault on the exhumation of the Cuonadong deposit, we undertook zircon and apatite (U–Th)/He dating, apatite fission-track dating, and forward and inverse thermal history modeling of the footwall and hanging wall of the Jisong secondary fault. Our results show that the Cuonadong deposit experienced two stages of rapid, post-mineralization exhumation at ca. 10–9 and ca. 5–4 Ma, separated by a period of differential exhumation at ca. 9–5 Ma. These exhumation events were likely caused successively by the arc-shaped Nading Fault, normal faulting on the Jisong secondary fault, and thrusting on the Main Himalayan Thrust, although climate-related erosion may have also had a role in the exhumation. Owing to normal displacement on the Jisong secondary fault, the Cuonadong ore deposit underwent differential exhumation of 300–700 m. This implies that the eastern part of the deposit is more favorable for exploration than its western part if the deposit has the same mineralization depth. This study demonstrates that low-temperature thermochronological methods can be useful in the exploration for rare-metal deposits in the Himalayan Orogen.

Diffusion of hydrogen in garnets reveals the rapid formation of spodumene pegmatite from Cuore in the Himalayan orogen

Diffusion of hydrogen in garnets reveals the rapid formation of spodumene pegmatite from Cuore in the Himalayan orogen

Formation Mechanism of the Kuday Leucogranite from the Sakya Dome and Constraints on the Evolution of the Himalayan Orogen

Formation Mechanism of the Kuday Leucogranite from the Sakya Dome and Constraints on the Evolution of the Himalayan Orogen

Intraoceanic Subduction System Within the Neo‐Tethys: Evidence From Late Cretaceous Arc Magmatic Rocks of the Eastern Himalaya

AbstractThe tectonic evolution of the Neo‐Tethys Ocean remains highly controversial, with several models existing in the community that conflict with each other. Here, we present new geochronologic and geochemical data for orthogneisses and amphibolites from the Greater Himalayan Sequence, eastern Himalayan orogen, which indicate that these rocks have Cenozoic metamorphic ages (∼52–3 Ma), but were derived from Late Cretaceous (∼89 Ma) magmas with arc‐like and depleted mantle geochemical signatures. Considering that northern India was a passive continental margin during the Mesozoic, and the previously reported Late Cretaceous magmatic rocks in the eastern Himalaya formed in a continental rifting setting, we suggest that the studied Late Cretaceous arc‐type magmatic rocks formed in an intraoceanic arc setting within the Neo‐Tethys, and accreted onto the passive margin of the Indian continent prior to the terminal continental collision. When combined with the existence of Late Mesozoic and intraoceanic arc‐type magmatic rocks in the western Himalaya, we suggest that a huge Late Cretaceous subduction system operated within the eastern Neo‐Tethys Ocean. This study supports two subduction zones having been responsible for the consumption and closure of the Neo‐Tethys basin, and a two‐stage collision history between India, Asia, and the intermediate island arc system. Our data therefore provide important constraints on the evolution of the Neo‐Tethys Ocean and India‐Asia collisional orogeny in southern Tibet.

Cosmogenic Nuclide Tracking of Sediment Recycling From a Frontal Siwalik Range in the Northwestern Himalaya

AbstractThe Himalayan orogen exports millions of tons of sediment annually to the Indo‐Gangetic foreland basin, derived from both hinterland and foreland fold‐thrust belts (FTB). Although erosion rates in the hinterland are well‐constrained, erosion rates in the foreland FTB and, by extension, the sediment flux have remained poorly constrained. Here, we quantified erosion rates and sediment flux from the Mohand Range in the northwestern Himalaya by modeling and measuring the cosmogenic radionuclide (CRN) 10Be and 26Al concentrations in modern fluvial sediments. Our model uses local geological and geophysical constraints and accounts for CRN inheritance and sediment recycling, which enables us to determine the relative contributions of the hinterland and foreland FTB sources to the CRN budget of the proximal foreland deposits. Our model predictions closely match measured concentrations for a crustal shortening rate across the Mohand Range of 8.0 ± 0.5 mm yr−1 (i.e., approximately 50% of the total shortening across the Himalaya at this longitude) since Ma. This shortening implies a spatial gradient in erosion rates ranging from 0.42 ± 0.03 to 4.92 ± 0.34 mm yr−1, controlled by the geometry of the underlying structure. This erosion pattern corresponds to an annual sediment recycling of ∼2.0 megatons from the Mohand Range to the downstream Yamuna foreland. Converted to sediment fluxes per unit width along the Himalaya, the foreland FTB accounts for ∼21% ± 5% of the total flux entering the foreland. Because these sediments have lower 10Be concentrations than hinterland‐derived sediment, they would lead to ∼14% overestimation of 10Be‐derived erosion rates, based on Yamuna sediments in the proximal foreland.

Geochemical variations of anatectic melts in response to changes of P–T–H2O conditions: Implication for the relationship between dehydration and hydration melting in the Himalayan orogen

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios, but low Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios, but low SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn-collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T–H2O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens.

Late Quaternary evolution of the Belan River Basin, Central India

The Marginal Gangetic Plain is the southernmost segment of the Ganga Plain, formed by Himalayan Orogen thrust fold stress flexing the Indian lithosphere. Rivers originating in the Himalayas have received much attention (i.e., Ganga, Yamuna, Kosi, Ghagra, etc.). In contrast, the tributaries draining from the peripheral bulge of the foreland basin have not been studied in detail, although it bears signatures of seismic activity in the basement of the craton. The Belan River, a tributary of the Tons River and sub-tributary of the Ganga River, drains through the Marginal Gangetic Plain. Various tectonic-induced geomorphic signatures have been identified using Landsat Images, such as fluvial incision, river shifting, gorges channel formation, etc., in the Belan River Basin. Geomorphic indices of active tectonics (asymmetry factor, SL index, basin elongation ratio, and long profile), drainage pattern, and lineament study have been performed for the Belan River Basin. These parameters indicate slope adjustment due to the uplift and subsidence of the parts of the basin area. The most recent and extensive valley fills started accumulating at ∼51 ± 7 ka and persisted until around ∼22 ± 2 ka. The aggradation was accompanied by a weakening summer monsoon, followed by incision after 22 ± 2 ka, indicating a return to warmer and wetter conditions and a stronger summer monsoon after the Last Glacial Maximum (LGM). Tectonic exhumation in the southern and south-eastern parts of the Belan River Basin revealed by geomorphic indices. These findings indicate that the drainage networks of the Belan River Basin were built over a tectonically controlled valley and that denudational processes have been transforming the basin for a considerable period.

From source to emplacement: The origin of leucogranites from the Sikkim-Darjeeling Himalayas, India

Himalayan leucogranites are important for understanding the tectonic evolution of collision zones in general and the causes of crustal melting in the Himalayan orogen in particular. This paper aims to understand the melt source and emplacement age of the leucogranites from Sikkim in order to decipher the deep geodynamic processes of the eastern Himalayas. Zircon U-Pb analysis of the Higher Himalayan Sequence (HHS) metamorphic core reveals a prolonged period of crustal melting between > 33 Ma and ca. 14 Ma. Major and trace element abundances are presented for 27 leucogranites from North Sikkim that are classified into two-mica and tourmaline leucogranite types. They are peraluminous in composition, characterized by high SiO2 (70.91–74.9 wt.%), Al2O3 (13.69–15.82 wt.%), and low MgO (0.13–0.74 wt.%). Elemental abundances suggest that Sikkim Himalayan leucogranites are derived from crustal melts. The two-mica leucogranites are derived from a metagreywacke source, whereas the tourmaline leucogranites are sourced from metapelitic sources, with inherited zircons indicating an HHS origin for both types. U-Pb zircon geochronology of the two mica leucogranites indicates ages of ca. 19–15 Ma, consistent with crustal melting recorded in HHS gneisses from Darjeeling. Monazites from both the two-mica and tourmaline leucogranites yield a crystallization age of ca. 15–14 Ma, coeval with movement on the Main Central Thrust and South Tibetan Detachment System which further provides constraints on the timing and mechanism of petrogenesis of leucogranites in the Sikkim Himalayas.