Higher Himalayan Crystalline Research Articles

RAMAN Analysis of Carbonaceous Material and Deduced Peak Metamorphic Temperatures of Metasediments From Western Himalaya, NW Pakistan

ABSTRACTRaman spectroscopy of carbonaceous materials (RSCM) was applied to 10 carbonaceous material (CM) ‐rich low‐ to medium‐grade metasedimentary rock samples of Western Himalaya, Pakistan to assess their optimum thermal evolution. The RSCM thermometry is based on the degree of graphitisation of CM as a function of peak metamorphic temperature. Petrographic observations of the studied samples revealed two dominant CM morphologies. Type‐I CM, represented by fine‐grained discontinuous dust‐like and scattered to thin sporadic layers, was more prevalent in low‐grade metamorphic samples. Type‐II CM, corresponding to continuous and extended thick fibres and elongated grain constellations, was dominant in medium‐ to high‐grade metamorphic samples. The degree of crystallinity of CM and subsequent metamorphic temperatures were quantified by considering the intensity‐based R1 and the area‐based R2 ratio parameters. Low‐grade samples from the Lesser Himalayan Sequence (LHS), having Type‐I CM, resulted in RSCM peak temperatures between 306°C and 403°C. Medium‐ to high‐grade samples from Higher Himalayan Crystalline (HHC), containing Type‐II CM, yielded peak temperatures in the 482°C–560°C range. The extremely deformed sample from a fault zone in the LHS produced relatively higher temperature of 403°C, thereby suggesting heat associated with shearing due to fault activities resulting in CM recrystallisation and elevated temperature values. The study revealed that the peak metamorphic temperatures increase from south towards north following a progressive path of metamorphism in the region.

Origin and composition of dissolved and particulate matter in mountain rivers of the Nepal Himalayas: Insights from Sr and Nd isotopes and elemental ratios

The Ganga River exhibits elevated concentrations of dissolved strontium (Sr) and a higher radiogenic Sr (87Sr/86Sr) among global rivers, which impacts oceanic Sr composition. This has implications for understanding weathering and sediment flux into open oceans. The Nepal Himalayan section is a significant part of the upstream Ganga River catchment. We analyzed Sr and Nd isotopes and major element concentrations in water and sediments to trace compositional and seasonal variability in dissolved and particulate matter. The Sr and Nd isotopes, in particular, are crucial in this study as they provide unique signatures that can be used to trace the origin and composition of the dissolved and particulate matter. Representative rivers draining monolithological terrains were selected to investigate the isotopic and elemental provenance and weathering intensity. The elemental ratios (Ca/Na, Mg/Na) indicate the watersheds are carbonate-rich terrains. The Lesser Himalaya (LH) rivers transport radiogenic Sr with high values compared to rivers draining the Tethyan Sedimentary Series (TSS), Higher Himalayan Crystalline (HHC), Mahabharat, and Siwalik Hills. The dry season records a higher 87Sr/86Sr ratio than the monsoon. Due to the faster dissolution of carbonates compared to silicates, monsoon waters transport less radiogenic Sr. The significant correlation of 87Sr/86Sr in dissolved and particulate phases signifies that short-term interactions between sediment and water may alter the 87Sr/86Sr composition. Notably, sediments originating from continental rocks exhibit an inverse correlation between 87Sr/86Sr and εNd. In conclusion, the Sr and Nd isotopic records in this study categorize the fluvial catchments into geological clusters aligned with the TSS, LH, and Siwaliks, which advance our understanding regarding the provenance besides providing crucial insights into geological processes, weathering, landscape evolution, and sediment flux.

Presence and implications of petrogenic organic carbon in High Himalayan Crystalline lake sediment

AbstractTwelve lacustrine sediment samples from a relict lake in the Kalla Glacier valley were co-dated using AMS radiocarbon (14C) and infrared stimulated luminescence (IRSL) dating methods. In general, the radiocarbon ages of bulk organic matter were older by a minimum of 1500 years compared to (age depth) modeled luminescence ages after fading corrections. This is observed for the first time in the lake sediments of High Himalayan Crystalline zone. A combination of lipid n-alkane data, Raman spectra and geochemical proxies suggested that this was due to ancient organic carbon (OCancient) that is a mixture of pre-aged (OCpre-aged) and petrogenic (OCpetro) organic carbon within older glacial moraine debris that served as sediment source to the lake. Raman spectra suggest the presence of moderate to highly graphitized OCpetro in all the profile samples. The OCpetro contributed 0.064 ± 0.032% to the sediment and the lake stored 2.5 ± 0.7 Gg OCpetro at variable rates during the last 16 kyr, with the mean burial flux 160 kg OCpetro yr−1. This study implies (1) employing another independent dating method in addition to radiocarbon method using bulk sediment organic matter, if the carbon content is low, to observe any discrepancy, and (2) a need to investigate on the fate of OCpetro as many such small lakes become relict in this region.

Geology of Kerabari - Rajarani Area with Special Emphasis on Geological Structures and Strain Analysis of the Main Central Thrust, the Lesser Himalayan Sequence, and Higher Himalayan Crystalline, Eastern Nepal

The geological study was carried out in the Kerabari - Rajarani area that covers south-eastern part of Dhankuta district and northern part of Morang district of eastern Nepal. The study was mainly focused on the lithostratigraphy, structural analysis, and strain analysis of the Main Central Thrust (MCT) and its hanging and foot walls by integrating field study and laboratory investigation. Geologically, the study area can be divided into the Sub-Himalaya, the Lesser Himalayan Sequence, and the Higher Himalayan Crystalline, respectively. The Lesser Himalayan Sequence is divided into the Bhedetar Group and the Dada Bajar Group, separated by the Chimra Thrust. The Bhedetar Group consists of the Chiuribas Formation, while the Dada Bajar Group comprises of the Ukhudanda Formation, the Mulghat Formation, the Okhre Formation, and the Patigau Formation, respectively. The key geological structures include the Main Central Thrust, the Chimra Thrust, the Main Boundary Thrust, and the Main Frontal Thrust, from north to south, respectively. The microstructural analysis of the quartz grains from the Lesser Himalayan Sequence and the Higher Himalayan Crystalline on either side of the MCT provide evidence of multiple phases of deformation. Likewise, the strain analysis of quartz grains from the MCT characterizes the MCT as the stretching fault thrust.

Evidence for a pre-Himalayan metamorphism in the High Himalayan Crystalline of the Miyar Valley (NW India)

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle.

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.

Chromite chemistry as petrogenetic indicator of altered ultramafic rocks from Higher Himalayan Crystalline, Kishtwar, India

Chromite chemistry as petrogenetic indicator of altered ultramafic rocks from Higher Himalayan Crystalline, Kishtwar, India

Mio‐Pliocene exhumation of the hinterland and sediments provenance of the Neogene strata from Kumaun Himalaya of northwest India: Insights from detrital fission‐track thermochronology

Fission‐track (FT) thermochronological analysis of detrital apatite and zircon derived from the lower and middle Siwalik Group of the sub‐Himalaya allows the reconstruction of the long‐term exhumation history of the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Sequence (LHS), and also the provenance of the Siwalik sediments. Here, we report 14 detrital apatite and zircon fission‐track (AFT/ZFT) ages of lower and middle Siwalik subgroups of the Kumaun region, northwest Himalaya. Detrital FT ages suggest that all sandstone samples were never buried below the temperature range of 90° to 100°C and remained as unreset to retain a signal of source‐area exhumation. A detrital AFT age from sandstone sample at close proximity to the Main Boundary Thrust (MBT) is 4.4 ± 0.5 Ma (Pliocene time) which is completely reset and younger than the depositional age, while the detrital ZFT ages of the same sample are unrest. The AFT age represents the time of reactivation of the MBT during which ZFT ages were not affected. This study revealed that the provenances of Siwalik sediments are located majorly in the MBT hinterland areas and the nearest sources are Amritpur granite, the outer Lesser Himalayan meta‐sedimentary (LHMS) zone, Almora klippe, Ramgarh thrust sheet, and the overlying Tethyan Himalayan Sequence. These source areas can be attributed to the development of local drainage networks in the Ramganga‐Kosi drainage basin during late Miocene to Pliocene (i.e., ~9 to 4 Ma). Based on detrital FT datasets, it has been envisaged that different tectonic activities in the MBT hinterland area between Miocene and Pliocene Periods cause the tectonic upliftment and exhumation of source areas and supply the sediment to the Siwalik foreland basin.

Geochemical characterization of tourmaline from granitic pegmatites, Higher Himalayan Crystalline (HHC), Paddar area, India

Geochemical characterization of tourmaline from granitic pegmatites, Higher Himalayan Crystalline (HHC), Paddar area, India

Geochemistry of the Kali River terrace sediments in Central Himalaya, India: Implication on nature of provenance in the known tectonic environment

In the Central Himalaya, the Kali River flow through the rocks of Tethys sedimentary (TS) sequences and Higher Himalayan crystallines (HHC) towards the upstream and through the Lesser Himalayan metasedimentary (LHS) the Siwaliks rocks in the downstream. The terrace sediments of the Kali River are geochemically investigated to infer nature of source region i.e. provenance and degree of chemical changes in the known tectonic setting during their journey. The low to high chemical index of alteration (CIA) of sediments (CIA = 22.24–87) indicate a dominant role of chemical and subordinate physical processes during sediment-forming mechanism. The longitudinal course emanating from TS in upper reaches to the Siwaliks in the lower reaches, the sediments along the main Kali River bear higher content Na2O, K2O, CaO and silica fractionation. The high and low ratios of La/Sc, Th/Sc and Co/Th advocate the dominance of felsic component in the source rocks. The content of Al2O3, TiO2, Fe2O3, La, Th, Sc, and Co/Th, La/Sc and Sc/Th ratios indicate the derivation of sediments dominantly from the leucogranite, granite to granodiorite and its gneissic sources, which commonly prevail in the rocks of Higher Himalayan crystallines sequences (HHCS). However, recycling and contribution of some aluminous Tethyan metasediments in the Kali River cannot be ruled out. Geochemical plots involving mobile and immobile elements show that erosion (sediment sorting) and weathering played a dominant role in the gradual change of sediment chemistry from source regions to catchments bed-load in the basin.

Eocene to Miocene metamorphic evolution and tectonic implication of the Ilam Nappe in Nepal Himalaya: Constraints from P–T conditions and monazite petrochronology

The Ilam Nappe, consisting of the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal, was largely emplaced southward along the Main Central Thrust (MCT). Here we present the P–T–t path of the Ilam Nappe using conventional geothermobarometers, pseudosection modelling, and monazite petrochronology. Amphibole-bearing migmatite just above the MCT experienced prolonged fluid-present melting from prograde metamorphism at 39–31 Ma to peak upper amphibolite-facies metamorphism (694 °C and 10.3 kbar) during 24–19 Ma, followed by cooling. Kyanite-sillimanite migmatites record prograde metamorphism at 35–28 Ma and peak lower granulite-facies metamorphism (738–765 °C and 8.3–9.7 kbar) under kyanite stability via the muscovite dehydration reaction at 25–20 Ma, followed by isothermal decompression under sillimanite stability. The clockwise P–T path and Early-Middle Miocene peak metamorphism of the Ilam Nappe can be correlated to those in the lower-middle HHCS hinterland. The Eocene-Oligocene prograde metamorphism with a low geothermal gradient of 15–25 °C/km in the Ilam Nappe and the entire HHCS hinterland could represent the earlier crustal thickening accompanied by large-scale overthrusting before the normal faulting of the South Thrust Detachment System (STDS), followed by diachronous extrusions of the upper HHCS along the High Himal Thrust, and of the lower-middle HHCS along the MCT with nappe emplacement. The lower granulite-facies Ilam Nappe was exhumed during the Early-Middle Miocene from the deeper or lower structural level of the HHCS, compared to the earlier Oligocene exhumation of the upper amphibolite-facies Karnali and Kathmandu klippes from the shallower or uppermost structural level of the HHCS.

Miocene Crustal Anatexis of Paleozoic Orthogneiss in the Zhada Area, Western Himalaya

AbstractGranitic gneiss (orthogneiss) and Himalayan leucogranite are widely distributed in the Himalayan orogen, but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear. In this study, we present the petrological, geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area, Western Himalayas. Zhada orthogneiss is composed mainly of quartz, plagioclase, K‐feldspar, biotite and muscovite, with accessory zircon and apatite. Orthogneiss zircon cathodoluminescence (CL) images show that most grains contain a core with oscillatory zoning, which indicates an igneous origin. Sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb dating of the zircon cores in the orthogneiss shows a weighted 206Pb/238U age of 515 ± 4 Ma (early Paleozoic), with spongelike zircon rims of 17.9 ± 0.5 Ma (Miocene). Zhada leucogranite shows 206Pb/238U ages ranging from 19.0 ± 0.4 Ma to 12.4 ± 0.2 Ma, the weighted average age being 16.2 ± 0.4 Ma. The leucogranites have a low Ca content (<1 wt%), FeOt content (<1 wt%), Rb content (67.0–402 ppm), Sr content (<56.6 ppm), Ba content (3.35–238 ppm) and Rb/Sr ratio (0.5–14.7), which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites. The highly variable Na2O + K2O (4.33 wt%–9.13 wt%), Al2O3 (8.44 wt%–13.51 wt%), ∑REE (40.2–191.0 ppm), Rb (67.0–402 ppm) and Nb (8.23–26.4 ppm) contents, 87Sr/86Sr(t) ratios (0.7445–0.8605) and εNd(t) values (–3.6 to –8.2) indicate that the leucogranite is derived from a heterogenetic source. The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from –8.2 to –3.6 and from –8.7 to –4.1, respectively. Therefore, the general mixing equation was used to perform the Sr and Nd isotope mixing calculations. The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence (THS)/Higher Himalayan Crystalline (HHC) metasediments and Zhada orthogneiss. The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration. The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment (muscovite dehydration melting), but also the trace elements and Sr‐Nd isotopic characteristics of the Zhada orthogneiss. These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9 ± 0.5 Ma (Miocene) and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid, which may have caused the orthogneiss solidus lines to decline, triggering a partial melting of the Zhada orthogneiss. It is therefore proposed that Himalayan leucogranite is a crust‐derived granite rather than a S‐type granite, as previously hypothesized.

Denudation Process of Crystalline Nappes in a Continental Collision Zone Constrained by Inversion of Fission‐Track Data and Thermokinematic Forward Modeling: An Example From Eastern Nepalese Himalaya

AbstractThermochronological methods were applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan Sequences (LHS) to elucidate the denudation process for the middle‐ and upper‐crust of eastern Nepal over millions of years. Thermochronological inverse modeling was undertaken for new results of fission‐track (FT) age and FT length data of zircon and apatite in order to reconstruct the time‐temperature (t‐T) paths in the temperature range of 60–350°C. Eight t‐T paths calculated along the across‐strike section show that the cooling process of the HHC nappe in this study area is characterized by the following three aspects: (a) gradual cooling followed by rapid cooling and subsequent gradual cooling, (b) northward‐younging of the timing of the rapid cooling, and (c) gradual cooling followed by <2 Myr rapid cooling in the frontmost part of the HHC nappe. The observed FT ages and t‐T paths were then compared with those predicted by forwarding thermokinematic modeling. The results of the thermokinematic modeling for the “Flat‐Ramp‐Flat MHT model”, in which the HHC and the underlying LHS are denudated in direct proportion to the uplift of rocks transported along the Main Himalayan Thrust (MHT), reproduced the observed t‐T paths and FT ages in eastern Nepal. This indicates that the observed FT ages and t‐T paths reflect a denudation process driven by the movement of the MHT with a flat‐ramp‐flat geometry and that the denudation rate and its spatial distribution have roughly been constant in eastern Nepal since ca. 9 Ma.

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.

Geochemical and Boron Isotopic Evidence that Tourmaline Records Country Rock Assimilation of Leucogranites in the Himalayan Orogen

AbstractTourmaline geochemical and boron (B) isotopic compositions in two‐mica granites (TMG), tourmaline‐bearing leucogranites (Tou‐LG), tourmalites and metapelites from the Gyirong–Malashan areas of the Himalayan orogen provide evidence for country rock assimilation during the intrusion of Himalayan leucogranite. The schorls in Gyirong leucogranitic plutons show low contents of MgO (0.238%–1.160%) and δ11B values (–12.1‰––11.2‰), while dravites gathered in the contact zone between the leucogranitic veins and metapelites show high contents of MgO (4.815%–6.755%) and δ11B values (–10.7‰––9.3‰). This geochemical and isotopic variation of tourmalines can also be identified in the Malashan gneiss dome. As a result, three types of tourmaline were identified in the Himalayan orogen: (1) Tou‐I in the TMG and Tou‐LG, which is the most common tourmaline type of schorl; (2) Tou‐II (dravite and high‐Mg schorl) in the Tou‐LG and tourmalite at the margins of the leucogranite; and (3) Tou‐III (mainly dravite, with minor high‐Mg schorl) in metapelites of the High Himalayan Crystalline Sequence. The lenses and veins of Tou‐LG may have experienced metasomatism and assimilation as a result of interaction with the High Himalayan Crystalline Sequence metasedimentary country rocks, which can be traced by the geochemical and isotopic characteristics of the tourmaline therein.

Crustal Melting Evidence in Migmatites of Higher Himalayan Crystallines (HHC) along Bhagirathi, Dhauliganga Valleys, and Sikkim Himalaya, India

Abstract An attempt has been made to evaluate crustal melting evidence through textural studies within a narrow zone of migmatite present in NW Himalaya along Bhagirathi and Dhauliganga valleys and two zones close to MCT and throughout within in the Sikkim Himalayas, NE Himalaya. It appears that partial melting was initiated by muscovite dehydration melting with a positive volume change driving melt segregation and discontinuous crystallization of peritectic biotite in the leucosome. Further, during retrogression due to cooling, a certain amount of melt was consumed. The occurrence of isolated pseudomorphosed melt pockets and lack of euhedral magmatic flow textured feldspar further indicates that the melt fraction was low for the alignment of crystals. The processes of melt segregation and migration could have been limited. The migmatite leucosomes and a small volume of in situ tourmaline-bearing leucogranite along extensional crenulation cleavages and melt pods indicate water-saturated melting of pelitic metasedimentary rocks. The formation of migmatites happened at around 46 Ma, corresponding to a peak metamorphic event due to collisional tectonics of the Himalayan orogeny. The presence of feeder dikes for main tourmaline-bearing leucogranite indicates that the source for the main body could be migmatite which is also supported by the similarity in REE patterns of the main body and in situ tourmaline-bearing leucogranite.

Direct zirconU–Pbevidence forpre‐Himalayan HTmetamorphism in the Higher Himalayan Crystallines, eastern Garhwal Himalaya, India

The high‐grade Higher Himalayan Crystallines (HHC), located between the South Tibetan Detachment System and Main Central Thrust in the collision zone between the Indo‐Australian and Eurasian plates, have been subject to at least four significant phases of deformation and metamorphism. The earliest of those significantly predates the Cenozoic continent‐continent collision, but has been difficult to date isotopically because of later overprinting. Migmatitic paragneiss from the Badrinath Formation in the Dhauliganga Valley, northern Uttarakhand, some of the highest‐grade rocks in the HHC, preserves direct evidence of mid‐Ordovician metamorphism in the form of 465.8 ± 6.4 Ma zircon overgrowths with extremely low Th/U (0.0038–0.0074). The overgrowths have formed on ca. 2.63–0.71 Ga detrital zircon cores and are themselves overgrown by two generations of Miocene metamorphic zircon with mean Pb/U ages of ca. 21.5 and 18.2 Ma. Monazite from the same sample has a mean Pb/Th age of 19.4 ± 0.2 Ma. The oxygen isotopic compositions of the monazite (δ18O: 7.69 ± 0.08‰) and youngest zircon overgrowths (δ18O: 7.95 ± 0.12, 8.24 ± 0.09‰) are consistent with mineral growth in a metasediment, but either of the two minerals did not grow in isotopic equilibrium with each other, or the original composition of the monazite has not been preserved. If the quartz (δ18O: 13.29 ± 0.11‰) equilibrated with the youngest zircon and its composition has been preserved, then the last episode of zircon growth took place at low temperature, ca. 420°C, after the migmatization. The protolith of the Badrinath migmatite was a Neoproterozoic or Early Palaeozoic metasediment partially melted (and probably migmatized) in the Middle Ordovician. The strong planar foliation currently present in the migmatite is probably the result of mid‐crustal extrusional channel flow and HT decompressional partial melting in the Miocene.

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.

Orogenic Collapse and Stress Adjustments Revealed by an Intense Seismic Swarm Following the 2015 Gorkha Earthquake in Nepal

The April 25, 2015 Mw 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO2-rich fluid migration, more likely post-seismic slip and seasonal enhancements.

Reconstruction of the exhumation history of the eastern Nepal Himalaya based on provenance changes

The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan denudation history. In this study, we present sandstone petrography, heavy mineral assemblages, Nd isotopes and chemical composition of detrital garnet from the Siwalik succession along the Muksar Khola section. This study reveals the Tibetan Tethys Himalaya, Higher Himalayan Crystalline and Lesser Himalaya Sequence as the source of sediments and records a provenance change from the middle Miocene to Pliocene/Pleistocene. This was observed as a shift from shallow to deeper parts of the Higher Himalayan Crystalline as a source area around 10.6 Ma. The Lesser Himalaya Sequence supplied a significant amount of sediments after 7.5 Ma, but the deeper part was only exposed after the late Pliocene to Pleistocene. Around 4.0 Ma an increase in the sediments from the Higher Himalayan Crystalline was observed. This provenance study reveals two stages of the exhumation of the eastern Nepal Himalaya. The first occurred between 11.0 and 5.5 Ma and the second after 4.0 Ma, due to the activation of an out-of-sequence thrust known as the Sun Koshi Thrust, and formation of the Lesser Himalayan duplex respectively.