Lesser Himalayan Sequence Research Articles

Lithostratigraphy of the Nawakot Complex (Lesser Himalayan Sequence) from Malekhu area (south-west) to Syabrubensi area (north-east) along the Trishuli River, central Nepal Himalaya

All formations (Kuncha Formation, Fagfog Quartzite, Dandagoan Phyllite, Nourpul Formation, Dhading Dolomite of the Lower Nawakot Group and Benighat Slate, Malekhu Limestone and Robang Formation of the Upper Nawakot Group) of the Nawakot Complex are well exposed along the Malekhu-Thopal Khola and Galchi - Kaljeri section s, west of the Kathmandu Valley. The Phalangu Khola and Syabrubensi sections, north of the Kathmandu Valley do not show the exposures of all formations (exposures of the Kuncha Formation, Fagfog Quartzite, and Benighat Slate only). The stratigraphic sequences from the Kuncha Formation to Dhading Dolomite (Lower Nawakot Group) along the Malekhu-Thopal Khola section are overturned, showing the younging strata to the deeper sections shown by the sedimentary structures (ripple marks, mudcracks, etc.) and stromatolites. The se overturned sequences could be related to the development of the post Main Central Thrust (MCT)/Mahabharat Thrust (MT) deformational structure (Gorkha - Pokhara anticlinorium, Likhu Khola anticline). The stratigraphic sequences are normal in the formations of the Upper Nawakot Group along the Malekhu-Thopal Khola section. This normal sequence can be also observed along the Galchi - Kaljeri, Phalangu Khola and Syabrubensi sections. In the northern part of the study area along the Likhu Khola, Tadi Khola, Phalangu Khola and Syabrubensi sections, the rocks of the Benighat Slate just below the MCT are highly sheared, deformed, metamorphosed with the development of the garnet and staurolite (medium amphibolite facies metamorphism) due to the movement of the MCT. However, these minerals are not developed in the Robang Formation (immediate below the MT) along the Malekhu-Thopal Khola and Galchi - Kaljeri sections. This formation could be caused by shallow depth metamorphism (green-schist to lower amphibolite facies) along the movement of the MT.

Metamorphic history of the central Himalaya, Annapurna region, Nepal, and implications for tectonic models

Pressure-temperature-time (P-T-t) conditions of metamorphism have been determined in the Annapurna region of central Nepal that place new constraints on the structural and tectonic evolution of the Himalayan orogenic wedge. Peak P-T conditions increase structurally upward: ∼525 °C and 8 kbar in the Lesser Himalayan sequence, 650 °C and 12 kbar at the base of the Greater Himalayan sequence across the Main Central thrust, 750 °C and 12 kbar in the middle of the Greater Himalayan sequence, and 775 °C and 13 kbar near the top of the Greater Himalayan sequence. Metamorphic monazite ages in the Greater Himalayan sequence also increase structurally upward: 16–21 Ma for subsolidus growth at the base of the Greater Himalayan sequence to ∼25 Ma for peak-T metamorphism and anatexis near the top of the Greater Himalayan sequence. These ages are several million years older than at equivalent structural levels at Langtang, ∼200 km to the east. The P-T-t data recommend reinterpretation of the Bhanuwa fault within the Greater Himalayan sequence as a thrust, and the presence of a different thrust structurally above the Bhanuwa thrust, here named the Sinuwa thrust. The new data are consistent with progressive stacking of tectonic slices, with calculated overthrust rates that are consistent with some (but not all) models that presume ∼2 cm/yr convergence across the Himalaya since 25 Ma. Despite differences in absolute ages, similarities among the chemical systematics of monazite, peak P-T conditions, and overthrust rates calculated for Annapurna when compared to Langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers, although the timing and duration of movement on discrete thrust surfaces may differ.

Structural and geochronological evidence for the leading edge of the Greater Himalayan Crystalline complex in the central Nepal Himalaya

The Himalaya is commonly described as a three layer-two fault stack. Namely, a high-grade crystalline core featuring an inverted metamorphic field gradient, the Greater Himalayan Crystalline complex (GHC), is separated from units above and below by shear zones. The Lesser Himalayan Sequence (LHS) underlies the GHC below the Main Central thrust, and the Tethyan Himalayan Sequence (THS) overlies it along the South Tibet detachment. However, the southern Main Central thrust hanging wall consists of a lower unit dominated by a right-way-up metamorphic sequence of biotite ± garnet schists (Bhimphedi Group) and an upper unit with only anchizone metamorphism (Pulchauki Group). The Bhimphedi Group is commonly equated to the GHC, while the Pulchauki Group is well correlated to the THS. However, no shear zone separates these units. We present new structural and geochronological data along the boundary between the GHC and the Bhimphedi Group in the Kathmandu region. These data reveal an ~ 200 m thick Early–Middle Miocene top-north-northeast shear zone that we term the Galchi shear zone. We correlate this shear zone to the South Tibet detachment on the basis of lithological, metamorphic, structural, and chronological criteria. The Galchi shear zone merges with the Main Central thrust to the south, bounding the leading edge of the GHC. This result, combined with recent work in the western Himalaya, suggests that the locally preserved leading edge of the GHC is sub-parallel to the arc of the orogen and the southern Main Central thrust hanging wall is dominated by THS rocks. This orogenic architecture rules out wedge extrusion and channel flow-focused denudation kinematic models for the Himalayan orogen, but is accommodated by tectonic wedging kinematic models, including channel tunneling models with modified timing.

Cenozoic anatexis and exhumation of Tethyan Sequence rocks in the Xiao Gurla Range, Southwest Tibet

In order to advance our understanding of the suturing process between continental landmasses, a geologic and geochronologic investigation was undertaken just south of the India–Asia suture in southwestern Tibet. The focus of this study, the Xiao Gurla Range, is located near the southeastern terminus of the active, right-lateral strike-slip Karakoram fault in southwestern Tibet. The range exposes metasandstone, phyllite, schist (locally of sillimanite facies), calc-gneiss and marble, paragneiss (± pyroxene), quartzite, metagranite, and variably deformed leucogranite. These metamorphic rocks are exposed in the footwall of a domal, top-to-the-west low-angle normal (detachment) fault, structurally beneath Neogene–Quaternary basin fill and serpentinized ultramafic rocks of the Kiogar-Jungbwa ophiolite. The detachment is interpreted to be the northeastern continuation of the Gurla Mandhata detachment fault system that bounds metamorphic rocks of the Gurla Mandhata Range ~ 60 km to the southwest. U–Pb geochronology on five detrital zircon samples of schist, phyllite, and quartzite yielded maximum depositional ages that range from 644–363 Ma and age probability distributions that are more similar to Tethyan sequence rocks than Lesser Himalayan sequence rocks. A felsic gneiss yielded a metamorphic zircon age of 35.3 ± 0.8 Ma with a significant population of early Paleozoic xenocrystic core ages. The gneiss is interpreted to be the metamorphosed equivalent of the Cambro-Ordovician gneiss that is exposed near the top of the Greater Himalayan sequence. Leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct U–Pb zircon ages of ~ 23 Ma and ~ 15 Ma. Locally, rocks exposed in the hanging wall of this fault and of the southward-dipping, northward-verging Great Counter thrust to the north consist of serpentinite-bearing mélange and conglomerate of inferred Paleogene age dominated by carbonate clasts. The mélange is intruded by a 44 Ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon U–Pb ages similar to Tethyan sequence rocks. We attribute the middle Eocene magmatism south of the suture to break-off of the Neo-Tethyan oceanic slab. In addition, our observations are consistent with the late Eocene shortening and crustal thickening within the Tethyan Himalayan sequence, early-middle Miocene leucogranite emplacement being related to underthrusting and melting of the Greater and possibly Lesser Himalayan sequences, and late Miocene arc-parallel extension in the hinterland of the southward propagating Himalayan thrust belt.

Isotopic and structural constraints on the late Miocene to Pliocene evolution of the Namche Barwa area, eastern Himalayan syntaxis, SE Tibet

Within the Namche Barwa area, SE Tibet, the Indus–Yarlung suture zone separates the Lhasa terrain in the north from the Himalayan unit including the Tethyan (sedimentary and volcanic rocks), Dongjiu (greenschist to lower amphibolite facies), Namche Barwa (granulite facies), Pei (amphibolite facies) and Laiguo (greenschist facies) sequences in the south. Two fault systems were distinguished in the Namche Barwa area. The former includes a top-down-to-the-north normal fault in the north and two top-to-the-south thrust zones in the south named as Upper and Lower Thrusts, respectively. The Namche Barwa and Pei sequences were exhumed southwards from beneath the Dongjiu sequence by these faults. Thus, the fault system is regarded as a southward extrusion structure. Subsequently, the exposed Dongjiu, Namche Barwa, Pei and Laiguo sequences were displaced northwards onto the Lhasa terrain by the top-to-the-north fault system, thus, marking it as northward indentation structure. Monazite TIMS U–Pb dating demonstrates that the normal fault and the Lower Thrust from the southward extrusion system were probably active at ~ 6 Ma and ~ 10 Ma, respectively. Zircon U–Pb SHRIMP and phlogopite K–Ar ages further suggest that the Upper Thrust was active between 6.2 ± 0.2 Ma and 5.5 ± 0.2 Ma. The northward indentation structures within the core portion of the eastern Himalayan syntaxis were perhaps active between 3.0 Ma and 1.5 Ma, as inferred by published zircon U–Pb SHRIMP and hornblende Ar–Ar ages. The monazite from upper portions of the Pei sequence dated by U–Pb TIMS indicates that the precursor sediments of this sequence were derived from Proterozoic source regions. Nd isotopic data further suggest that all the metamorphic rocks within eastern Himalaya (εNd = − 13 to − 19) correlate closely with those from the Greater Himalayan Sequences, whereas the western Himalayan syntaxis is mainly comprised of Lesser Himalayan Sequences. The two indented corners of the Himalaya are, thus, different.

Evidence for structural discordance in the inverted metamorphic sequence of Sikkim Himalaya: Towards resolving the Main Central Thrust controversy

Inverted metamorphism in the Himalayas is closely associated with the Main Central Thrust (MCT). In the western Himalayas, the Main Central Thrust conventionally separates high grade metamorphic rocks of the Higher Himalayan Crystalline Sequence (HHCS) from unmetamorphosed rocks of the Inner sedimentary Belt. In the eastern Himalayas, the Inner sedimentary Belt is absent, and the HHCS and meta-sedimentary Lesser Himalayan Sequence (LHS) apparently form a continuous Barrovian metamorphic sequence, leading to confusion about the precise location of the MCT. In this study, it is demonstrated that migmatitic gneisses of the sillimanite zone in the higher structural levels of the HHCS are multiply deformed, with two phases of penetrative fabric formation (S 1HHCS and S 2HHCS ) followed by third folding event associated with a spaced, NW-SE trending, north-east dipping foliation (S 3HHCS ). The underlying LHS schists (kyanite zone and lower) are also multiply deformed, with the bedding S 0 being isoclinally folded (F 1LHS ), and subsequently refolded (F 2LHS and F 3LHS ). The contact zone between the HHCS and LHS is characterized by ductile, top-to-the southwest shearing and stabilization of a pervasive foliation that is consistently oriented NW-SE and dips northeast. This foliation is parallel to the S 3HHCS foliation in the HHCS, and the S 2LHS in the LHS. Early lineations in the HHCS and LHS also show different dispersions across the contact shear zone, implying that pre-thrusting orientations of the two units were distinct. The contact shear zone is therefore interpreted to be a plane of structural discordance, shows a shear sense consistent with thrust movement and is associated with mineral growth during Barrovian metamorphism. It may well be considered to represent the MCT in this region.

Provenance of the Greater Himalayan sequence: Evidence from mafic granulites and amphibolites in NW Bhutan

Mafic granulites and amphibolites in the Masang Kang area of NW Bhutan Himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history. Bulk-rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics. U–Pb zircon SIMS data suggest an age of 1742 ± 39 Ma for mafic magmatism. The age-corrected ε Nd (1742) values of the rocks are highly variable, ranging from high positive (+ 8.4) to negative (− 3.3). The positive value suggests a primitive magma source, similar to that of rift-related tholeiites. We suggest that the rocks of the Masang Kang suite were produced during a major late Paleoproterozoic thermal event that caused the mobilization and enrichment of the sub-continental lithospheric mantle beneath the north Indian margin. The geochemical signature of these rift-related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub-lithospheric mantle may have been derived. Though their occurrence is rare, Paleoproterozoic igneous rocks within the Greater Himalayan Sequence (GHS), in addition to sources identified throughout the LHS, may have contributed to the detrital zircon population that form the 1.7–1.9 Ga peak in the age spectra of the Lesser Himalayan Sequence (LHS). In addition, the coeval Paleoproterozoic magmatism in both LHS and GHS suggests that the two lithotectonic units may have belonged to the same continental plate at that time period.

The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.

Sm-Nd isotopic systematics in whole rocks of the Kathmandu and Nawakot complexes, central Nepal Himalaya

In central Nepal, medium- to high-grade rocks of the Kathmandu Complex overlie lower-grade metamorphic rocks of the Nawakot Complex. These two units are separated by a clear high strain thrust belt marked by mature dynamic recrystallisation. These rock sequences have been folded to form the NW-SE trending, essentially upright Mahabharat Synclinorium.
 Whole rock Sm-Nd isotopic analysis was carried out in thirteen samples collected from the Nawakot Comp lex, Kathmandu Complex, and the thrust zone between these two rock sequences from different parts of central Nepal. This analysis demonstrates important distinctions between the two rock packages. Results show a clear linear array of the data points from the different units in a 143Nd/144Nd vs. 147Sm/144Nd isochron plot with lowest values in the Nawakot Complex phyllite and highest values in the Kathmandu Complex gneiss while Ꜫ(0) Nd values show a marked difference with Nawakot Complex rocks ranging at -20 or higher, while the Kathmandu Complex rocks range at about -13 in schist and at about-6 in gneiss giving rise to major differences in their model ages. The comparison of the obtained data from the previous one clearly shows the affinity of the Nawakot Complex rocks with the Lesser Himalayan Sequence and of the Kathmandu Complex with the Higher Himalayan Sequence.

A new kinematic evolutionary model for the growth of a duplex — an example from the Rangit duplex, Sikkim Himalaya, India

The Lesser Himalayan duplex (LHD) is a prominent structure through much of the Lesser Himalayan fold–thrust belt. In the Darjeeling - Sikkim Himalaya a component of the LHD is exposed in the Rangit window as the Rangit duplex (RD). The RD consists of ten horses of the upper Lesser Himalayan Sequence (Gondwana, Buxa, Upper Daling). The duplex varies from hinterland-dipping in the north, through an antiformal stack in the middle to foreland-dipping in the south. The Ramgarh thrust (RT) is the roof thrust and, based on a balanced cross-section, the Main Himalayan Sole thrust is the floor thrust at a depth of ~ 10 km and with a dip of ~ 3.5° N. Retrodeformation suggests that the RD initiated as a foreland-dipping duplex with the Early Ramgarh thrust as the roof thrust and the RT as the floor thrust. The RT became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent RT reactivation. This kinematic history best explains the large translation of the overlying MCT sheets. The restoration suggests that RD shortening is ~ 125 km, and the original Gondwana basin extended ~ 142 km northward of its present northernmost exposures within the window.

The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy

Our understanding of the geologic evolution of the Himalaya remains incomplete, particularly in regard to structural and geochronologic details of the Proterozoic-Paleozoic Lesser Himalayan Sequence. We conducted an integrated field mapping, geochronological study, and geochemical analysis of the Lesser Himalayan Sequence strata in the Kumaun and Garwhal Himalaya of NW India (78° 00′–80°30′E). Structural observations reveal a systematic change in deformation styles from lower to higher structural levels in the Main Central Thrust footwall. In the south, and at lower structural levels, the Main Central Thrust footwall is characterized by parallel folding and sparse development of axial cleavage. Quartz microstructures indicate that deformation occurred at temperatures below 350 °C. In contrast, in the north and at higher structural levels close to the Main Central Thrust, deformation is characterized by replacement of original bedding, the development of penetrative cleavage, and schistosity locally. Folds are tight, in places isoclinal and overturned. The corresponding quartz microstructures indicate that deformation occurred above 350 °C. Dating of detrital zircons from Lesser Himalayan Sequence metasedimentary units, and igneous zircons from schistose gneisses throughout the Lesser Himalayan Sequence allow refinements to the stratigraphic framework. First, unlike has been observed in western Nepal immediately west of our study area, U-Pb zircon dating suggests the absence of Paleoproterozoic strata in the Ramgarh Thrust hanging wall. This suggests that the thrust cuts upsection laterally along strike. Second, U-Pb detrital zircon dating suggests that metasedimentary strata in the Ramgarh Thrust hanging wall are correlative with basal units of the Outer Lesser Himalayan Sequence, removing the necessity that the Ramgarh Thrust hanging wall was allochthonous with respect to its footwall Lesser Himalayan Sequence units. Schistose gneisses with U-Pb ages of ca. 1850 Ma are older than Lesser Himalayan Sequence units in the area, suggesting that they are thrust-emplaced Lesser Himalayan Sequence basement. We conclude that the Ramgarh Thrust is an out-of-sequence thrust postdating a folding event in its footwall. The earlier proposal that Neoproterozoic-Cambrian Lesser Himalayan Sequence strata are the southern extension of the Tethyan Himalayan Sequence requires that the Tons Thrust, which separates the distal and proximal facies of the Lesser Himalayan Sequence, be a major, south-directed structure with a slip magnitude of >50–100 km.

The Kumaun and Garwhal Lesser Himalaya, India: Part 2. Thermal and deformation histories

An integrated petrologic, thermochronologic, and numerical modeling study constrains the thermal evolution of the Lesser Himalayan Sequence in the Kumaun and Garwhal regions of India. South of the Tons thrust, peak metamorphic temperatures do not exceed 330 °C. On the northern side, a transition toward higher temperatures adjacent to the Main Central thrust hanging wall is documented. In the immediate footwall of the Main Central thrust, peak temperatures average ~550 °C. An inverted thermal field gradient of ~30 °C/km beneath the Vaikrita thrust is calculated. An inverted thermal field gradient of ~20 °C/km is also documented beneath a klippe of the Main Central thrust hanging wall ~25 km south of the Vaikrita thrust, and a peak footwall temperature of 530 °C is recorded close to the shear zone at the base of the klippe. The mechanism for thermal metamorphism in the Lesser Himalayan Sequence is interpreted to be conduction of heat from the Main Central thrust fault and/or hanging wall to the footwall. A localized thermal field gradient is also associated with the Tons thrust. The ^(40)Ar/^(39)Ar thermochronology reveals that the exposed rock in the Kumaun and Garwhal Lesser Himalaya underwent cooling below the closure temperature (Tc) for white mica at different times in the late Tertiary. The proximal footwall of the Munsiari thrust experienced cooling below the white mica Tc at ca. 8.5 Ma. South of the Munsiari thrust, in the proximal footwalls of the Chaukori and Askot klippen, muscovites ages are ca. 11.5 Ma, ~3 Ma older than samples to the north. The age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the Main Central thrust hanging wall atop the Lesser Himalayan Sequence. Results of thermal modeling achieve good fits to the data when scenarios involve an early Miocene phase of overthrusting of a hot hanging wall over a downgoing footwall, followed by the initiation of a duplex within Lesser Himalayan Sequence rocks.

Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India

In this study, we reconstruct the inverted metamorphic sequence in the western Arunachal Himalaya using combined structural and metamorphic analyses of rocks of the Lesser and Greater Himalayan Sequences. Four thrust-bounded stratigraphic units, which from the lower to higher structural heights are (a) the Gondwana rocks and relatively weakly deformed metasediments of the Bomdila Group, (b) the tectonically interleaved sequence of Bomdila gneiss and Bomdila Group, (c) the Dirang Formation and (d) the Se La Group are exposed along the transect, Jira–Rupa–Bomdila–Dirang–Se La Pass. The Main Central thrust, which coincides with intense strain localization and the first appearance of kyanite-grade partial melt is placed at the base of the Se La Group. Five metamorphic zones from garnet through kyanite, kyanite migmatite, kyanite-sillimanite migmatite to K-feldspar-kyanite-sillimanite migmatites are sequentially developed in the metamorphosed low-alumina pelites of Dirang and Se La Group, with increasing structural heights. Three phases of deformation, D 1–D 2–D 3 and two groups of planar structures, S 1 and S 2 are recognized, and S 2 is the most pervasive one. Mineral growths in all these zones are dominantly late-to post-D 2, excepting in some garnet-zone rocks, where syn-D 1 garnet growths are documented. Metamorphic isograds, which are aligned parallel to S 2 were subsequently folded during D 3. The deformation produced plane-non-cylindrical fold along NW–SE axis. In the garnet-zone, peak metamorphism is marked by garnet growth through the reaction biotite + plagioclase → garnet + muscovite. An even earlier phase of syn-D 1 garnet growth occurred in the chlorite stability field with or without epidote. In the kyanite-zone metapelites, kyanite appeared via the pressure-sensitive reaction, garnet + muscovite → kyanite + biotite + quartz. Staurolite was produced in the same rock by retrograde replacement of kyanite following the reaction, garnet + kyanite + H 2O → staurolite + quartz. These reactions depart from the classical kyanite- and staurolite-isograd reactions in low-alumina pelites, encountered in other segments of eastern Himalaya. In the metapelites, just above the kyanite-zone, melting begins in the kyanite field, through water-saturated and water-undersaturated melting of paragonite component in white mica. Leucosomes formed through these reactions are characteristically free of K-feldspar, with sodic plagioclase and quartz as the dominant constituents. With increasing structural height, the melting shifts to water-undersaturated melting of muscovite component of white mica, producing an early K-feldspar + kyanite and later K-feldspar + sillimanite assemblages and granitic leucosomes. Applications of conventional geothermobarometry and average P– T method reveal near isobaric (at P ∼ 8 kbar) increase in peak metamorphic temperatures from 550 °C in the garnet-zone to >700 °C for K-feldspar-kyanite-sillimanite-zone rocks. The findings of near isobaric metamorphic field gradient and by the reconstruction of the reaction history, reveal that the described inverted metamorphic sequence in the western Arunachal Himalaya, deviates from the classical Barrovian-type metamorphism. The tectonic implication of such a metamorphic evolution is discussed.

Geology of the western Arunachal Himalaya in parts of Tawang and west Kameng districts, Arunachal Pradesh

The Western Arunachal Himalaya comprises migmatitic gneisses of the Sela Group intruded by Tertiary leucogranite (tourmaline, muscovite and K-feldspar bearing), metasedimentaries of Lumla and Dirang Formations and granite gneiss of the Bomdila Group. The litho units of Sela Group are completely migmatised with distinct leuco, melano and mesosomes (Neosome). The mineral assemblages indicate a sedimentary protolith (para gneiss). Concordant metabasic bodies are observed (rarely discordant) indicating older dykes/sills. The migmatites of Sela Group are of Significantly higher grade than the tectonically underlying Dirang Formation, characterized by kyanite ± biotite± staurolite ± sillimanite assemblages. The tourmaline bearing pegmatitic High Himalayan leucogranite crops out in the higher altitudes of the High Himalayan Sequence. The primary mineral assemblages recorded are tourmaline + muscovite + biotite + garnet apart from quartz and K-feldspar. This intrudes the migmatite of Sela Group parallel as well as oblique to earlier gneissosity as lens shaped bodies. The Sela's are thrust over the metasedimentary rocks of Dirang Formation. Earlier workers marked this thrust as the Main Central Thrust (MCT). During the present studies the whole Dirang Formation was found to be a high strained zone with many shear planes, almost all the lithounits are sheared with prolific graphite/carbon emplacement along the permeable shear planes. The Dirang Formation comprises quartzite, kyanite ± staurolite + garnet ± biotite schist, crystalline limestone and carbon phyllite. Basic and ultrabasic rocks intrude these lithounits. The Bomdila Gneiss, which forms a part of the Lesser Himalayan Sequence, is a large body of granite orthogneiss. The primary mineral assemblage includes quartz, K-feldspar, biotite and garnet. Though the gneisses have an intrusive relationship with the Dirang metasediments the contact is marked by a thrust. Metamorphic grade increases from the base of Dirang to the top of the Sela Group. The base of the Dirang Formation is characterized by biotite + garnet assemblages followed by kyanite + staurolite at the top the Dirang Formation. The base of the Sela Group is marked by kyanite ± garnet ± biotite followed by sillimanite ± biotite at the top of Sela Group. Two phases of deformations are recorded from Sela and Dirang rocks with type III interference pattern folds.

Petrology of an inverted Barrovian sequence of metapelites in Sikkim Himalaya, India: Constraints on the tectonics of inversion

The lesser Himalayan sequence in Sikkim exposes an unusually complete inverted Barrovian sequence of metapelitic rocks. A number of different bulk compositions are interlayered along the prograde sequence, providing an excellent natural laboratory for studying the controls of pressure, temperature and composition on stability of mineral assemblages. Using three different approaches for determining pressures and temperatures, and making use of the bulk chemical constraints, we show that all three methods yield consistent estimates of pressures and temperatures once the strengths and weaknesses inherent to each approach are critically evaluated. The metamorphic field gradient along two separate traverses are found to be positive (60 °C/kbar in east Sikkim and 70 °C/kbar in north Sikkim) - pressure as well as temperature increases continually up to the highest grade. The various bulk compositions allow different assemblages to be used to establish the coherence of the sequence. In combination these place tight constraints on the permissible tectonic mechanisms for the production of this inverted metamorphic sequence. Neither older hot iron type models, nor more recent models that invoke thrusting (post- or syn- metamorphic) as a mechanism of inversion can produce such a coherent package inverted in pressure as well as temperature. On the other hand, the observations are more consistent with the predictions of channel flow type tectonics. In particular, a recent geodynamic model of subduction-collision that takes into account high heat generation (as found in these Himalayan metapelites) and incorporates the effects of the resulting melting (reduction of viscosity and enhancement of buoyancy) on the tectonic evolution, predicts such inversion of coherent blocks as an unavoidable consequence. Such melting triggered inversion and exhumation as a coherent, fault bounded block explains many features of the inverted sequence in Sikkim and it is not necessary to invoke inversion by multiple thrusting events.

Nd isotopic compositions of the Tethyan Himalayan Sequence in southeastern Tibet

The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread metamorphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sampled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a ɛ Nd-T DM plot. The first is defined by the ɛ Nd(210 Ma) values of −3.45 to −7.34 and T DM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with ɛ Nd(130 Ma) values from −15.24 to −16.61 and T DM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has ɛ Nd(130 Ma) values of −10.89 to −16.32 and Nd model ages (T DM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.

Tectonic implications of Palaeoproterozoic anatexis and Late Miocene metamorphism in the Lesser Himalayan Sequence, Sutlej Valley, NW India

Unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units, and structural elements that bound them, are properly identified and characterized. In the Sutlej Valley (western Himalaya), the Munsiari and Vaikrita thrusts have both been correlated with the Main Central Thrust. The sequence of amphibolite-grade rocks (the Jutogh Group) bounded by these faults has been variously assigned to the Lesser Himalayan Sequence (based on provenance ages) and to the Greater Himalayan Sequence (from its metamorphic grade). Trace-element and geochronological data from leucogranites in the Jutogh Group (1) indicate crustal melting at c . 1810 Ma, before the deposition of the Greater Himalayan Sequence, thus correlating the Jutogh Group with the Lesser Himalayan Sequence, and (2) record Proterozoic metamorphism overprinted at 10.5 ± 1.1 Ma (established from U–Pb analysis of uraninite) during the Himalayan orogeny. Pressure–temperature–time data indicate that the Jutogh Group and Greater Himalayan Sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion. This precludes extrusion along a single, widening channel, and requires a southward shift of the locus of movement during the Late Miocene, coincident with present-day precipitation patterns.

P-T-t data from central Nepal support critical taper and repudiate large-scale channel flow of the Greater Himalayan Sequence

A new synthesis of pressure-temperature conditions and pressure-temperature-time (P-T-t) paths is presented for high-grade metamorphic thrust sheets associated with the Main Central Thrust, in the Langtang and Darondi regions, central Nepal. From structurally low to structurally high, major structures include the Lesser Himalayan Duplex, Munsiari Thrust, Main Central Thrust (thrust contact between the Greater and Lesser Himalayan Sequences), and Langtang Thrust. Key P-T-t results include the following: in a transect from Lesser Himalayan Duplex to Langtang Thrust rocks, peak metamorphic P-T conditions are uniformly ∼550 °C and 8 kbar in the Lesser Himalayan Duplex, and show a strong gradient to ∼725 °C and 10–12 kbar over a structural distance of less than 2 km associated with the Munsiari Thrust and Main Central Thrust; T9s then increase and P9s decrease gradually upsection, reaching ∼825 °C and 8 kbar in the Langtang Thrust. Juxtaposition of thrust sheets occurred at moderate pressure (8–12 kbar) on thrust surfaces roughly coincident with the modern Main Himalayan Thrust. Published monazite ages demonstrate synmetamorphic thrusting, with peak metamorphic ages decreasing progressively downward: 21 ± 2 Ma (Langtang Thrust), 16 ± 1 Ma (Main Central Thrust), 10.5 ± 0.5 Ma (Munsiari Thrust), and 3.5 ± 0.5 Ma (Lesser Himalayan Duplex). Together with published thermochronologic results, these data constrain initial cooling ages and rates: 15–20 Ma and ∼40 °C/m.y. for the Langtang Thrust and Main Central Thrust, and 3–10 Ma and ≥100 °C/m.y. for the Munsiari Thrust and Lesser Himalayan Duplex. Overall, metamorphic and chronologic patterns are matched well by expectations of critical taper models, including (1) uniformly high pressures of metamorphism (8–12 kbar) for all structural levels and thrust movement along the paleo–Main Himalayan Thrust, (2) isobaric cooling from the peak of metamorphism for Greater Himalayan rocks (deep juxtaposition of thrust sheets), (3) “hairpin” P-T paths for Lesser Himalayan rocks, and (4) relatively slow cooling rates for Greater Himalayan rocks. However, observations contrast significantly with published channel flow models, which predict (1) peak P-T conditions within the sillimanite stability field for Lesser and lower Greater Himalayan rocks (versus observations of P-T conditions in the kyanite stability field), (2) peak metamorphic pressures that decrease structurally downward—7–13, 6, 5, and 5 kbar for rocks achieving temperatures recorded by Langtang Thrust, Main Central Thrust, Munsiari Thrust, and Lesser Himalayan Duplex rocks (versus observations of 8, 10–12, 10, and 8 kbar), (3) retrograde isothermal exhumation P-T trajectories for Greater Himalayan rocks (versus isobaric cooling of the Main Central Thrust and Langtang Thrust), (4) cooling of migmatitic Greater Himalayan rocks after 10 Ma (versus observations of 15–20 Ma), and (5) isobaric heating of the Lesser Himalayan rocks (versus observations of simultaneous increases in P and T for some Lesser Himalayan rocks). Neither model matches “clockwise” P-T paths observed in structurally high Lesser Himalayan rocks, or the extraordinary cooling rate of the Lesser Himalayan Duplex, which points to complications in their evolution in the context of end-member models. Most generally, although channel flow may have initiated since ca. 10 Ma due to focused erosion above the Lesser Himalayan Duplex, it does not appear responsible for past transport and exhumation of the migmatitic core of the Himalaya.

Geodynamics of Chamba Nappe, western Himalaya

Chamba Nappe is a SW-directed thrust sheet, which is composed of the Tethyan Sequence and was exhumed by thrusting along the Chamba thrust that is equivalent to the Panjal Thrust or the Main Central Thrust (MCT) over the Lesser Himalaya Sequence. Internal deformation within Chamba nappe is responsible for a large amount of crustal thickening and shortening (Sharma and Bhola 2004). The Chamba nappe is folded by regional scale fold viz the Chamba-Bharmaur syncline, Tandi syncline and Tisa anticline (Frank 1995, Sharma et al. 2003, Thakur 1998) and is the result of southwestward propagation of cover rocks on their basement (HHC) due to topographic uplift. The southward propagating thrust system also led to the development of major folds DF 1 , DF 2 and minor DF 3 folds in a single progressive deformation. DF1 is the main compression phase, which corresponds to early SW-directed movements associated with the formation of the Chamba Nappe. Deformation associated with DF2 phase propagated gradually from the Indus Suture Zone towards the SW as the Indian plate was progressively subducted below Asia along the Indus Suture Zone. DF3 last phase is marked by kinkband formation as a result of brittle deformation in the slate/ phyllites sequence due to amplification of Chamba nappe. These deformations resulted with the southwestwarddirected exhumation of the Tethyan Himalayan Sequence over the Higher Himalayan Crystalline. The southwestward propagation of the Chamba Nappe also affected the basal series of the Tethys Himalaya, which will ultimately form the High Himalayan Crystalline Sequence. In initial stage, mainly Proterozoic, basal series were still welded to the Tethys Himalaya, but as deformation proceeded, and as the Tethys Himalaya formed an increasingly thick stack of rocks, the lower part of the Tethys Himalaya became ductile enough to accommodate the compressive deformation through ductile shearing (Robyr et al. 2002). From that time onwards, the compressive forces were essentially concentrated along a ductile shear zone at the base of the Chamba nappe. Movements along Panjal thrust/Chamba thrust (MCT-1) zone contributed to the further underthrusting and metamorphism of the basal proterozoic series, which leads to the ultimate metamorphic differentiation between the Tethys Himalaya and the High Himalayan Crystalline Sequence.

Isotopic study of the Himalayan metamorphic rocks in the far-eastern Nepal

Previous studies have documented that Sr-Nd and U-Pb zircon isotopic compositions are different between the Lesser Himalayan and the Higher Himalayan metamorphic rocks in Nepal, India, and Pakistan (Parrish and Hodges 1996, Ahmad et al. 2000). However, not much attention was paid to the metamorphic rocks of Main Central Thrust (MCT) zone. We report the result of Sr-Nd isotopic studies in the Higher Himalayan zone, the Lesser Himalayan zone and the MCT zone in far-eastern Nepal. Far-eastern Nepal (Kangchenjunga-Taplejung-Ilam region) comprises three distinct tectonic units: the Higher Himalaya, the Lesser Himalaya and the Sub-Himalaya (Shelling and Arita 1991). Each of these tectonic units is a fault-bound tectonic package. The Higher Himalayan metamorphic rocks consist of mediumto high-grade paragneisses and orthogneisses. The Lesser Himalayan metamorphic rocks comprise primarily metasandstones with intercalations of phyllites and meta-quarzites. The Ilam Nappe is composed of the Higher Himalayan metamorphic rocks. It is a geologically siginificant unit in fareastern Nepal. The Ilam Nappe with no overlying the Tethyan sedimentary rocks has been thrust over the Lesser Himalayan thrust package along the MCT zone up to near the Sub-Himalaya zone in the south. The MCT zone is a ductile-brittle zone with a thickness of less than 1 km to several km. The upper boundary of the MCT zone is known as the Upper MCT (UMCT) and the lower one as the Lower MCT (LMCT). The lithology of the MCT zone is characterized by mylonitic augen gneiss, biotite-muscovitechlorite phyllite with S-shaped garnet and graphitic phyllite. Compositions and zoning patterns of garnets can be discriminated between the MCT zone and the Higher Himalaya. Information on the tectonic disposition prior to the Himalayan orogeny of the geologic units juxtaposed by the MCT is important for understanding the crustal shortening and changes of thermal structure due to the MCT activity. In general, the Higher Himalayan sequence has been considered to be Indian basement in origin, and the Lesser Himalayan sequence has been deposited on the northern margin of the Indian continent in the Precambrian times. However, the Higher Himalayan sequence in the Langtang area, central Nepal yields zircon U-Pb ages of 0.8 to 1.0 Ga, implying a sedimentary provenance of the Late Proterozoic. On the other hand, the Lesser Himalayan sequence contains 1.8 to 2.6 Ga zircons. Therefore, it was proposed that the Higher Himalayan sequence is metasedimentary rocks that were originally deposited north of continental margin than the Lesser Himalayan sequence (Parrish and Hodges 1996). Furthermore, Nd isotope data are useful in distinguishing between Higher Himalayan and Lesser Himalayan zones (Ahmad et al. 2000). These data show that the Nd values for t=1000 Ma in the Higher Himalayan are -10 to -3, whereas these of the Lesser Himalayan are -17 to -7. We have recalculated Nd values at 1000 Ma as this time corresponds to a “Grenville” thermal event in the Himalayas. 17 Samples in the far-eastern Nepal were collected from the Lesser Himalayan, the Higher Himalayan and the MCT zones by the tectonostratigrapic units (Figure 1). The isotopic work presented in this study follows on the structural and petrological studies of Schelling and Arita (1991). In study area the Nd values for t=1000 Ma from rocks of the Higher Himalaya and the Lesser Himalaya are almost within the range of the previous data except for the Tamar Khola Granite. The Nd values for t=1000 Ma obtained are -10 to -2 in the Higher Himalaya and -17 to -15 in the Lesser Himalaya. The Tamar Khola Granite has the Nd value for t=1000 Ma of –11 and the high 147Sm/144Nd. Most samples from the MCT zone have the middle Nd values for t=1000 Ma (-15 to -11) of the samples from Higher Himalaya and Lesser Himalaya (Figure 2a). Similarly, the 87Sr/86Sr values obtained from the MCT zone show the values between those of Higher Himalayan and Lesser Himalayan zones although a few rocks units of Lesser Himalaya have very high 87Sr/86Sr values (Figure 2b). These data may serve to strengthen the opinion that the Higher Himalayan sequence is not Indian basement in origin. Further, these data may suggest that the rocks of the MCT zone fill a gap between the Higher and Lesser Himalayan sequence although Sm-Nd isotopic signiture from them indicate that they had different sediment sources area. Finally, both the Higher and the Lesser Himalayan sequence, including the rocks of the MCT zone between them, may represent thick clastic successions deposited on the north of the thinned continental margin of the Indian basement.