Greater Himalayan Series Research Articles

Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya

Abstract The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ɛHf ratios decrease from between −10 and −15 down to values as low as −23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.

Protolith lithostratigraphy of the Greater Himalayan Series in Langtang, Nepal: implications for the architecture of the northern Indian margin

Abstract Reconstruction of the protolith lithostratigraphy of amphibolite-facies metasedimentary rocks of the Greater Himalayan Series (GHS) in Nepal documents a single, long-lived passive-margin succession that was deposited along the northern margin of the Indian Craton. In the Langtang area, Paleoproterozoic gneisses are unconformably overlain by a succession of upper Neoproterozoic–Ordovician fluvio-deltaic quartzite, basinal pelite and psammitic beds that grade upsection into micaceous semipelite and pelite. U–Pb zircon geochronology yields maximum depositional ages between c. 815 and 460 Ma for the GHS in Langtang. Regional variations in the composition and thickness of the GHS along the length of the Himalaya are attributed to siliciclastic depocentres centred on Zanskar in northern India, Langtang and Everest in central to western Nepal, which contrast with coeval marine carbonate shelf deposition in the Annapurna region. The protolith lithostratigraphy documented for Langtang provides a coherent framework for interpreting subsequent Cenozoic Himalayan deformation, specifically the homogeneously distributed layer-normal shortening (i.e. flattening) and layer-parallel stretching (i.e. transport-parallel stretching) that characterizes the GHS. Within the context of a single protracted northern Indian marginal sedimentary succession, the distinction between the Lesser, Greater and Tethyan Himalaya is structural rather than lithostratigraphic in origin.

The geology and tectonics of central Bhutan

Lithotectonic mapping, metamorphic observations and U–Pb zircon ages underpin a substantial revision of central Bhutan geology, notably a more extensive and continuous outcrop of the Tethyan Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene ( c . 17–20 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8 ± 0.2 and 17.9 ± 0.5 Ma. A 500 ± 4 Ma (U–Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic interpretations of the Bhutan Himalaya. Supplementary materials: Zircon U–Pb geochronological data, sample locations and descriptions, features of analysed zircons, sample processing method and detailed analytical conditions are available at http://www.geolsoc.org.uk/SUP18876 .

Deformation temperatures and flow vorticities near the base of the Greater Himalayan Series, Sutlej Valley and Shimla Klippe, NW India

We report new deformation temperature and flow vorticity data from the base of the Greater Himalayan Series (GHS) exposed in the Sutlej Valley and Shimla Klippe of NW India. We focus on three groups of transects across the hanging wall of the Main Central Thrust (MCT). In order of relative foreland – hinterland positions, they are the Shimla Klippe, Western and Eastern Sutlej transects. Deformation temperatures indicated by quartz c-axis fabric opening-angles increase both from foreland to hinterland at a given structural distance above the MCT and up structural section from the MCT within individual transects. Deformation temperatures in the immediate hanging wall to the MCT are estimated at ∼510–535, 535–550 and 610 °C on the Shimla, Western Sutlej and Eastern Sutlej transects, respectively. The steepest inferred field gradients in deformation temperatures are recorded adjacent to the MCT and progressively decrease up structural section following a power law relationship. Comparison with temperature estimates based on multi-mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions. Vorticity analyses indicate that shearing along the base of the GHS occurred under sub-simple shear conditions (Wm values of 0.9–1.0) with a minor component of pure shear.

Telescoping of isotherms beneath the South Tibetan Detachment System, Mount Everest Massif

Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Himalayan Series (GHS) schists and gneisses in the footwall to the South Tibetan Detachment System (STDS) have undergone extreme telescoping during penetrative flow associated with southward extrusion of the GHS. In the Rongbuk Valley, to the north of Mount Everest, we have made three vertical sampling traverses from the STDS down into the GHS and estimated temperatures associated with penetrative deformation using the opening angles of quartz c-axis fabrics measured on dynamically recrystallized grains. From north to south, the deformation temperature data indicate apparent thermal field gradients of 369, 385 and 420 °C per km for our three traverses, traced over a maximum vertical sampling distance of 0.5 km. Adopting a differential flow path model, simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment-parallel transport magnitudes of 25–170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the STDS, depending on assumed original geothermal gradient, dip of detachment, etc. These particle transport estimates are similar to those previously calculated from barometry data of GHS rocks in the Everest region and are compatible with channel flow models for extrusion and exhumation of the GHS.

Metamorphic history of the South Tibetan Detachment System, Mt. Everest region, revealed by RSCM thermometry and phase equilibria modelling

This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ∼340 °C. Temperatures increase down-section, reaching ∼650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ∼310 °C km−1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.

Progression from South-Directed Extrusion to Orogen-Parallel Extension in the Southern Margin of the Tibetan Plateau, Mount Everest Region, Tibet

In the South Tibetan Himalaya, major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the Ama Drime and Mount Everest massifs. The South Tibetan detachment system (STDS) accommodated exhumation of the Greater Himalayan Series (GHS) until the Middle Miocene. Field- and laboratory-based structural data presented here indicate that early melt-present deformation in the footwall of the STDS accommodated large-scale flow of a low-viscosity middle crust. Decompression-related anatexis and emplacement of leucogranites in the structurally highest positions of the GHS (∼16–18 Ma) mark the final stages of south-directed extrusion within a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions, from ultra-mylonitic leucogranite (∼400°–500°C; 51%–67% pure shear) to mylonitic marble (>300°C; 41%–55% pure shear) to moderately foliated marble ∼200°–300°C (46%–59% pure shear). Previous investigations demonstrated that the cessation of movement on the STDS at ∼16–13 Ma was followed by anatexis in the lower portion of the middle crust (∼12–13 Ma; Ama Drime Massif). The Ama Drime and Nyönno Ri detachments (∼8–12 Ma; Dinggyê graben) exhumed rocks from the deepest structural position in the central Himalaya during orogen-parallel extension. Brittle faulting dissected the upper crust (<4 Ma; Tingri–Kung Co graben) in a setting that was kinematically linked to extension in the interior of the Tibetan plateau.

Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite ( P = 14–18 kbar, T ≈ 800 °C) yield U–Pb ages of 24.0 ± 0.3 Ma and 18.8 ± 0.3 Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490 ± 3 Ma and 24.2 ± 0.4 Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0 ± 0.4 Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼ 24 Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17 Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼ 0.6 to ∼ 2.0 Ga with peak at 0.8–1.2 Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼ 600 Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼ 24 Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene.

Timing of Midcrustal Metamorphism, Melting, and Deformation in the Mount Everest Region of Southern Tibet Revealed by U(‐Th)‐Pb Geochronology

U(‐Th)‐Pb dating of zircon, monazite, and xenotime from metamorphic and igneous rocks at two outcrops along a north‐south transect in the Mount Everest region of southern Tibet provide new constraints on the timing and duration of thermal events associated with channel flow and the ductile extrusion of the Greater Himalayan Series (GHS). At the southernmost outcrop in the Kangshung Valley, Th‐Pb ages from monazite indicate that prograde metamorphism associated with crustal thickening following the India‐Asia collision occurred at least as early as $$38.9\pm 0.9$$ Ma. A subsequent sillimanite‐grade metamorphic event at $$28.0\pm 1.2$$ Ma was followed by two phases of leucogranite emplacement at $$20.8\pm 0.8$$ and $$16.7\pm 0.4$$ Ma. At Thongmön, ∼40 km to the northeast of the Kangshung Valley, prograde metamorphism was occurring at ∼25.4 Ma and lasted until $$16.1\pm 0.1$$ Ma, reaching ∼740°C and 5 kbar at $$22.4\pm 0.2$$ Ma. Immediately following metamorphism, two phases of leucogranite were emplaced at $$15.2\pm 0.2$$ and $$12.6\pm 0.2$$ Ma, with an intervening phase of ductile deformation. These data combined with ages from the Rongbuk glacier and Ama Drime range, north and east of Everest and the North Himalayan Mabja dome 100–140 km to the northeast, suggest that GHS metamorphism lasted ∼20 m.yr. and that migmatization and south‐directed channel flow peaked around ∼23–20 Ma and ended by ∼16 Ma. The youngest leucogranites crosscut all ductile fabrics related to the Miocene channel flow.

Structural insights into the early stages of exhumation along an orogen-scale detachment: The South Tibetan Detachment System, Dzakaa Chu section, Eastern Himalaya

Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20 Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision.

Mechanisms and timescales of felsic magma segregation, ascent and emplacement in the Himalaya

Abstract We combine field, petrological, geochemical and experimental observations to evaluate the timescales of compaction-driven and shear-assisted melt extraction and ascent in the Himalaya. The results show that melt migration via compaction and channelling is inescapable and operates on timescales of less than 1 million years and possibly as short as 0.1 million years. Field and petrological data show that such a fast and efficient melt transfer results from a combination of favourable factors, including: (1) low but constant melt viscosity (10 4.5 Pa s) during extraction and ascent; (2) grain size coarsening of the source rocks in response to prolonged heating prior to melting; and (3) high source fertility and thus high melt fraction, owing to elevated modal amounts of muscovite in leucogranite sources. All three factors dramatically increase source permeability. Calculations show that shear-assisted melt extraction had a time interval recurrence in the range 10 000–100 000 years (10–100 ka), leading to sill thicknesses of 1–30 m. Yet melts falling at the low end of the viscosity range when coupled to high shear velocities may lead to veins several hundred metres thick. The deepest structural levels (e.g. central Zanskar Range) show that in-situ melts formed where pure shear compaction was greatest and where simple shear was also operative. Magma extracted from migmatite leucosomes was injected along planes of weakness parallel to the ductile shear fabric, probably by some form of hydraulic fracturing crack propagation mechanism. Large High Himalayan leucogranite (HHL) bodies (e.g. c. 5 km thick sills at Manaslu, Makalu and northern Bhutan) may thus represent inflated laccoliths assembled via dykes that tapped a 100–300 m melt layer produced by compaction of the Greater Himalayan Series (GHS). Thermal simulations show that such melt layers may have incubation times of several million years. Although transport time for magmas associated with the HHL is short, the time for assembly may take several million years for the largest HHL, as geochronological data indicate (up to 5 million years for Manaslu, Shisha Pangma). Transport of leucogranite melt from mid-crustal levels towards the surface was concomitant with active low-angle normal faulting along the South Tibetan Detachment (STD) normal fault, a structure that effectively formed the lid to the extrusion of a partially molten layer of mid-crustal rocks (channel flow). Rapid cooling of the granites emplaced at the top of the GHS implies rapid extrusion and lateral flow of GHS rocks beneath the STD during the period c. 20–17 Ma. Weakening of the crust by partial melting is thus likely to be pulsatory in time, and future thermomechanical models should incorporate such aspects to model tectonic evolution of hot orogens.

Geologic control of Sr and major element chemistry in Himalayan Rivers, Nepal

Our study of the Seti River in far western Nepal shows that the solute chemistry of the river and its tributaries is strongly controlled by geology. The Seti flows through four distinct terranes, starting with the Tethyan sedimentary series (TSS) and Greater Himalayan series (GHS). TSS/GHS waters display 87Sr/ 86Sr ratios of <0.73 and high Sr and Ca, consistent with the composition of limestone and marble common in these terranes. The 87Sr/ 86Sr ratio and Mg increase markedly as the river passes into the Lesser Himalayan series (LHS), where tributaries have 87Sr/ 86Sr ratios from 0.75 to 1.02 and high Sr, Ca, and Mg. The high Mg in LHS waters correlate with high 87Sr/ 86Sr ratios, which we attribute to weathering of highly radiogenic (0.71–0.82) dolostones. Tributaries to the Seti River draining the largely carbonate-free Dadeldhura thrust sheet (DTS) have ratios near 0.74, but low Sr, Ca, and Mg and therefore have little impact on Seti mainstem chemistry. Mass balance calculations and CaMg-weathering indices show that carbonate weathering accounts for >70% of total dissolved solids to the Seti River. Sr/Ca ratios of river waters provide a minimum estimate of the %-carbonate weathering contribution to Sr, due to partitioning of Sr and Ca during incongruent dissolution and reprecipitation of calcite. Overall, we attribute high 87Sr/ 86Sr ratios in the Seti River and its tributaries to the weathering of metacarbonates (especially dolostones in the upper Nawakhot Group) which have exchanged Sr with silicates during metamorphism. Our modeling of Sr fluxes in the Seti River indicates that the TSS/GHS accounts for 36–39% of the Sr, the LHS for 40–53%, and 8–23% for the DTS. Prior to exposure of LHS rocks at ∼12 Ma, TSS and GHS carbonates with low 87Sr/ 86Sr ratios dominated Himalayan rivers. We attribute the elevated 87Sr/ 86Sr ratios of Himalayan paleorivers during the late Miocene and Pliocene to exposure and weathering of LHS metacarbonates.