Higher Himalayan Crystalline Research Articles

300 MW Baspa II — India's largest private hydroelectric facility on top of a rock avalanche-dammed palaeo-lake (NW Himalaya): Regional geology, tectonic setting and seismicity

This study aims for the characterization of the geological setting of 300MW Baspa II, India's largest private hydroelectric facility which was built on top of a relict rock avalanche dammed palaeo-lake (Baspa Valley, NW Himalaya). Geologically, the hydroelectric installation is located in the Higher Himalayan Crystalline, just above the active Karcham Normal Fault, which is reactivating the Early Miocene Main Central Thrust, one of the principal Himalayan faults. The area is seismically active and mass-movements are common. At ca. 8200yr BP the Baspa River was blocked behind a 142×106m3 rock avalanche dam which created a ca. 260m deep palaeo-lake. The whole palaeo-lake was completely filled with sediments in about 3100years, making the Sangla palaeo-lake to a very rare example of a mass-movement dam with very long duration.The hydroelectric installation was built with its intake situated directly on top of the mass-movement dammed palaeo-lake of Sangla, utilizing the convex knick point in the river profile to increase the head for Baspa II for ca. 125m, compared to the reconstructed longitudinal profile of the river prior to the mass-movement, which amounts for about 18% of its design head.At least 5 levels of soft-sediment deformation have been recorded in the exposed part of the lacustrine sediments of Sangla palaeo-lake, including brecciated laminae, overturned laminae, folds, faults and deformation bands, separated by undeformed deposits. They are interpreted as seismites, indicating at least 5 earthquakes within 2495±297years strong enough to cause liquefaction. These observations extend the local seismicity record considerably into the past and indicate more and possibly stronger seismic events than might be expected from the instrumental measurements.

Structurally-controlled Melt Accumulation: Himalayan Migmatites and Related Deformation – Dhauli Ganga Valley, Garhwal Himalaya

Abstract In the uppermost parts of the Higher Himalayan Crystallines (HHC) of the Great Himalaya, widespread in situ partial melting of sillimanite+K-feldspar gneiss resulted in the formation of migmatite and resultant melt accumulation near the South Tibetan Detachment System (STDS) during various deformation events along the Dhauli Ganga valley in Garhwal. The oldest migmatite phase, designated as the Me1, parallels the main foliation Sm as the stromatite layers and concordant leucogranite bands. Younger melt phases Me2, Me3 and Me5 are recorded along small-scale ductile thrusts, extensional fabric and structureless patches, respectively. It is only the Me4 melting phase that is evidenced by large-scale melt migration along cross-cutting irregular veins. These were possible conduits for migration and accumulation of melt into larger leucogranite bodies like the Malari granite (19.0± 0.5 Ma).

Structural, metamorphic and geochronological relations between the Zanskar Shear Zone and the Miyar Shear Zone (NW Indian Himalaya): Evidence for two distinct tectonic structures and implications for the evolution of the High Himalayan Crystalline of Zanskar

The geologic structures and metamorphic zonation of the northwestern Indian Himalaya contrast significantly with those in the central and eastern parts of the range, where the high-grade metamorphic rocks of the High Himalayan Crystalline (HHC) thrust southward over the weakly metamorphosed sediments of the Lesser Himalaya along the Main Central Thrust (MCT). Indeed, the hanging wall of the MCT in the NW Himalaya mainly consists of the greenschist facies metasediments of the Chamba zone, whereas HHC high-grade rocks are exposed more internally in the range as a large-scale dome called the Gianbul dome. This Gianbul dome is bounded by two oppositely directed shear zones, the NE-dipping Zanskar Shear Zone (ZSZ) on the northern flank and the SW-dipping Miyar Shear Zone (MSZ) on the southern limb. Current models for the emplacement of the HHC in NW India as a dome structure differ mainly in terms of the roles played by both the ZSZ and the MSZ during the tectonothermal evolution of the HHC. In both the channel flow model and wedge extrusion model, the ZSZ acts as a backstop normal fault along which the high-grade metamorphic rocks of the HHC of Zanskar are exhumed. In contrast, the recently proposed tectonic wedging model argues that the ZSZ and the MSZ correspond to one single detachment system that operates as a subhorizontal backthrust off of the MCT. Thus, the kinematic evolution of the two shear zones, the ZSZ and the MSZ, and their structural, metamorphic and chronological relations appear to be diagnostic features for discriminating the different models. In this paper, structural, metamorphic and geochronological data demonstrate that the MSZ and the ZSZ experienced two distinct kinematic evolutions. As such, the data presented in this paper rule out the hypothesis that the MSZ and the ZSZ constitute one single detachment system, as postulated by the tectonic wedging model. Structural, metamorphic and geochronological data are used to present an alternative tectonic model for the large-scale doming in the NW Indian Himalaya involving early NE-directed tectonics, weakness in the upper crust, reduced erosion at the orogenic front and rapid exhumation along both the ZSZ and the MSZ.

Origin and Emplacement of the Higher Himalayan Leucogranite in the Eastern Himalaya: Constraints from Geochemistry and Mineral Chemistry

Abstract The Higher Himalayan Leucogranites (HHLG) intruded into the high grade rocks of the Higher Himalayan Crystallines (HHC) in western Arunachal Pradesh, eastern Himalaya, yield distinctive field data, petrography, geochemical and mineral chemistry data. The HHLG mostly occur as sill like bodies of limited thickness and lateral extent within the HHC. The Arunachal HHLG are characterized by the presence of two micas; normative corundum; high contents of SiO2 (67 - 78 wt.%), Al2O3 (13 - 18 wt.%), A/CNK (0.98 - 1.44) and Rb (154 - 412 ppm); low contents of CaO (0.33 - 1.91 wt.%) and Sr (19 - 171 ppm), and a high ratio of FeO(tot)/MgO in biotite (2.54 - 4.82). These distinctive features, along with their strong depletion in high field strength elements (HFSE), suggest their affinity to peraluminous S-type granite generated by the partial melting of crustal material. Since the HHLG associated with high grade rocks of HHC and lack of basaltic magmatism, strongly suggests that the high grade rocks of HHC might represent the melt source and HHLG are product of pure crustal melt without any contamination of mantle material. Geothermobarometric estimations and mineral assemblages of the HHC metapelites confirm that the HHLG were probably generated in the middle crust (∼20 km) and the produced melts intruded the HHC in the form of sills/dykes.

Variability of the southwest monsoon since the last 25 000 years and their possible causes: Role of Northern Hemisphere versus Tibetan Plateau

High-precision, clay sediment oxygen and hydrogen isotopes analyses of Pleistocene- Holocene deep-sea sediments from the Bay of Bengal (BOB) are presented for the first time. Our study shows that the major source of sediments in the study area, since the last ~25 000 years, is likely to be the Higher Himalayan crystalline rocks. Further, the study of these stable isotope data displays the variation of southwest monsoon (SWM) in the BOB region since the last ~25 000 years and the cause behind the variation has been interpreted. The δ 18 O values of the clay sediments are compared with δ 18 O values of the BOB seawater. This comparison shows that the clay sediment δ 18 O values of the studied sediment cores temporally vary along with the changes in strength of the SWM. Based on the changes in the clay sediment δ 18 O values of the studied sediment, we evaluate the variance in the SWM since the last 25 000 years in the BOB. Our results are consistent with previous work in the region based on other proxies. To evaluate the factors influencing the intensity of the SWM since the last gla- cial maxima, we conducted comparative analyses of the studied clay sediment δ 18 O values with δ 18 O values in the Greenland ice cores (GISP2) and Tibetan ice cores (Guliya). The results from this com- parative study show that large-scale changes in the intensity of the SWM since 25 000 years are affected by the climate oscillations of the Northern Hemisphere, but rapid and abrupt fluctuations in the SWM seem to be controlled by the amount of snow cover in the Tibetan Plateau.

Nd and Sr isotope characteristics of Quaternary Indo-Gangetic plain sediments: Source distinctiveness in different geographic regions and its geological significance

Quaternary sediments of the Himalayan foreland basin, known also as the Indo-Gangetic plains, have been studied for their Sr and Nd isotope geochemistry to constrain their sources and to infer their geological significance. The data show a large variation in the 87Sr/86Sr values and relatively a minor variation in the εNd(0) values, for compositionally homogeneous sediments. Sediments of the Ganga–Yamuna and Satluj systems have high 87Sr/86Sr (>0.75) and low εNd(0) (<−16) values, characteristics of High Himalayan Crystallines. Sediments of the Ghaggar river and the Thar desert in the western part of the Indo-Gangetic plains in India have distinctly lower 87Sr/86Sr (<0.75) and higher εNd(0) (>−16), all relative to the previous group, suggesting their derivation from the Sub-Himalayan sediments including Tertiary Subathu Formation. The Subathu was likely derived from the Indus-Tsangpo Suture Zone basaltic volcanics and Tibetan Sedimentary Series. The Thar desert/Ghaggar river sediments are isotopically similar to those of the Indus plain indicating a shared provenance, either immediate or ultimate. The exposed sediments of the incised Yamuna river at Kalpi in the Ganga–Yamuna plains were derived from Peninsular India. These southern Ganga–Yamuna plain sediments are isotopically similar to the Thar Desert/Ghaggar river sediments although the juvenile components of their provenance differ — the Vindhyans–Deccan traps and Indus-Tsangpo suture zone volcanics, respectively. In the southern Ganga plain the Peninsular wedge of sediments could be present at depths below about 12m, interbedded or mixed with Himalayan sediments. The isotope geochemistry of sediments also shows that the earliest influx of the Himalayan sediments to the Ganga–Yamuna interfluve occurred around 68–70ka. This indicates that the Yamuna river has been flowing in the region at least since the Late Pleistocene. The dynamic relationship between the Yamuna and Chambal rivers in the region negates the idea of Yamuna being a tributary of Ghaggar–Saraswati river system during the Harappan time (3500 to 1900 BC).

Soft sediment deformation structures and their implications for Late Quaternary seismicity on the South Tibetan Detachment System, Central Himalaya (Uttarakhand), India

The South Tibetan Detachment System (STDS) defines the lithological and tectonic boundary between the Higher Himalayan crystallines and the Tethyan sedimentaries. Earlier studies have suggested that the STDS has been dormant since its inception during the Miocene along with the Main Central Thrust (MCT). However, recent studies indicate that the STDS was active during the Pleistocene–Holocene period. We provide additional support for this more recent activity based on the occurrence of seismically induced Soft Sediment Deformation Structures (SSDS) preserved in relict lake sediments in the Dhauli Ganga, Gori Ganga and Kali Ganga river basins of the Central Himalaya.The relict lakes are located on the hanging wall of the STDS. An optical chronology of the lake sediments brackets the seismically induced SSDS between 20ka and 11ka with a major seismic event of magnitude >6.5 occurring between 17ka and 13.5ka. Since MCT and STDS are considered to be the coupled structures, our observation supports the hypothesis that the STDS is providing accommodation space to the strain gradient arising due to the north–south compression in the Himalaya during the late Quaternary.

Higher Himalaya in the Bhagirathi section (NW Himalaya, India): its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms

The Higher Himalayan Crystalline (HHC) in the Bhagirathi river section (India) on fieldwork reveals two extensional ductile top-to-N/NE shear sub-zones—the ‘South Tibetan Detachment System’ and the ‘Basal Detachment’—besides a preceding top-to-S/SW ductile shear. A top-to-N/NE brittle shear was identified as backthrusts from the HHC (except its northern portion) that occur repeatedly adjacent to numerous top-to-S/SW brittle shears as fore-thrusts. The northern portion of the HHC—the Gangotri Granite—exhibits infrequent total six extensional and compressional brittle shear senses. The backthrusts could be due to a low friction between the lower boundary of the HHC (i.e. the Main Central Thrust-Zone) and the partially molten hot rock materials of the HHC. Subduction of the Eurasian plate towards S/SW below the Indian plate more extensively in the Garhwal sector could be the second possible reason. Presence of two ductile extensional shear sub-zones may indicate channel flow (or several exhumation mechanisms) of the HHC in a shifting mode (similar to Mukherjee et al. in Int J Earth Sci 101:253–272, 2012). The top-to-S/SW extensional brittle shear exclusively within the upper (northern portion) of the HHC and a top-to-S/SW brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism. Thus, this work provides the field evidences of possibly both channel flow and critical taper conditions from a Higher Himalayan section, besides that by Larson et al. (Geol Soc Am Bull 122:1116–1134, 2010).

Microstructural and geochemical studies of Higher Himalayan Leucogranite: implications for geodynamic evolution of Tertiary Leucogranite in the Eastern Himalaya

The Higher Himalayan Leucogranites (HHLG) intruded into the high grade rocks of the Higher Himalayan Crystallines (HHC) in Arunachal Himalaya of the Eastern Himalaya, yield distinctive field data, petrography, microstructures, geochemical and mineral chemistry data. The Arunachal HHLG are characterized by the presence of two micas; normative corundum; high contents of SiO2 (67–78 wt.%), Al2O3 (13–18 wt.%), A/CNK (0.98–1.44) and Rb (154–412 ppm); low contents of CaO (0.33–1.91 wt.%) and Sr (19–171 ppm), and a high ratio of FeO(tot)/MgO in biotite (2.54–4.82). These distinctive features, along with their strong depletion in high field strength elements (HFSE), suggest their affinity to peraluminous S-type granite generated by the partial melting of crustal material. Geothermobarometric estimations and mineral assemblages of the HHC metapelites confirm that the HHLG were probably generated in the middle crust (~20 km) and the produced melts intruded the HHC in the form of sills/dykes. Microstructurally, the HHLG shows high temperature deformation features including chessboard extinction in quartz and cuspate/lobate grain boundaries between quartz and feldspars (plagioclase and K-feldspar). The deformation microstructures suggest that the HHLG was deformed under early high temperature ductile deformation conditions. These fabrics were subsequently superimposed by later brittle deformation features associated with decreasing temperatures during the exhumation of the HHLG towards shallow structural levels at the time of Himalayan orogeny. Copyright © 2013 John Wiley & Sons, Ltd.

Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Hima- layan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillima- nite. Peak metamorphic conditions were 8-9 kbar and *800 C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by micro- beam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of meta- morphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to meta- morphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales charac- terizes melting which occurred over a period of *15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (*0.6 Ma). An older *36 Ma high-grade event is recorded in an al- locthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic dis- continuity. The higher structural levels reached melting and peak conditions later (*26-23 Ma) than the lower structural levels (*31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.

Apatite fission track and (U–Th)/He ages from the Higher Himalayan Crystallines, Kaghan Valley, Pakistan: Implications for an Eocene Plateau and Oligocene to Pliocene exhumation

Apatite fission track and apatite and zircon (U–Th)/He ages were obtained from high- and ultra high-pressure rocks from the Kaghan Valley, Pakistan. Four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24.5±3.7 and 15.6±2.1Ma and apatite (U–Th)/He ages between 21.0±0.6 and 5.3±0.2Ma. These data record cooling of the formerly deeply-subducted high-grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated, normal-acting Main Mantle Thrust. Overlap at around 10Ma between fission track and (U–Th)/He ages is recognised at one location (Besal) showing that fast cooling occurred due to brittle reactivation of a former thrust fault. Widespread Miocene cooling is also evident in adjacent areas to the west (Deosai Plateau, Tso Morari), most likely related to uplift and unroofing linked to continued underplating of the Indian lower crust beneath Ladakh and Kohistan in the Late Eocene to Oligocene. In the southernmost part of the study area, near Naran, two significantly younger Late Miocene to Pliocene apatite fission track ages of 7.6±2.1 to 4.0±0.5Ma suggest a spatial and temporal separation of exhumation processes. These younger ages are best explained by enhanced Late Miocene uplift and erosion driven by thrusting along the Main Boundary Thrust.

Miocene andalusite leucogranite in central-east Himalaya (Everest–Masang Kang area): Low-pressure melting during heating

The studied Miocene andalusite-bearing leucogranites intrude the upper part of the High Himalayan Crystallines (HHC) and the north Himalayan domes and outcrop in an area stretching from Mt. Everest to the Kula Khangri massif (Bhutan) towards the east.The leucogranites constitute both dykes as well as sills and parts of larger andalusite-free leucogranite plutons (e.g., Makalu). They represent mainly of two-mica (muscovite+biotite±tourmaline±cordierite±andalusite±sillimanite±dumortierite) leucogranite, and tourmaline (muscovite+tourmaline±biotite±andalusite±sillimanite±garnet±kyanite±spinel±corundum) leucogranites. Microstructures reveal several generations of andalusite (from residual/peritectic early magmatic to cotectic late magmatic), even in the same sample. The occurrence of residual and/or peritectic andalusite, together with inclusions of sillimanite+biotite in cordierite, indicates that melts formed by dehydration melting of biotite at T=660–700°C during prograde heating at low-pressure conditions (P<about 400MPa).According to current models, leucogranites are produced by dehydration melting of muscovite and/or biotite during exhumation of the HHC. In this case, micas are consumed in the sillimanite stability field. As a consequence, these models cannot explain the occurrence of residual and/or peritectic magmatic andalusite. Conditions for anatexis in the andalusite field may have been achieved by heat transfer within the exhuming (extruding) HHC, from structurally lower and hotter rocks towards upper and colder fertile lithologies.

New geochronological constraints on the thermal and exhumation history of the Lesser and Higher Himalayan Crystalline Units in the Kullu–Kinnaur area of Himachal Pradesh (India)

New geochronological, petrological and structural data from the Beas–Sutlej area of Himachal Pradesh (India) are used to reconstruct the tectonothermal and exhumation history of this part of the Himalayan orogen.Sm–Nd garnet ages at 40.5±1.3Ma obtained on a pegmatoid from the inverse metamorphic High Himalayan Crystalline (HHC) in the Malana–Parbati area probably mark local melting during initial decompression. Ongoing exhumation in ductilely deformed leuco-gneiss is constrained by Sm–Nd garnet ages at 29±1Ma and white mica Rb–Sr ages around 24–20Ma, while Bt Rb–Sr ages indicate a drop of regional metamorphic temperatures below 300°C between 15 and 12Ma.The deep Sutlej gorge exposes medium-grade paragneisses and Proterozoic orthogneisses of the Lesser Himalayan Crystalline (LHC), overthrust by the HHC along the Main Central Thrust (MCT). Mica cooling ages in the HHC are in the range of 14–11Ma. Above the extruded wedge of the HHC, the Leo Pargil leucogranite and associated dykes intrude the Haimanta Unit (HU) below the weakly metamorphic Palaeo-Mesozoic sediments of the Tethyan Himalayas (TH). The Leo Pargil leucogranite yielded a mean Sm–Nd garnet age of 19±1Ma and Rb–Sr muscovite and biotite cooling ages between 16.4 and 11.6Ma.Marked young extrusion of LHC units resulted in differentiated exhumation/cooling of more frontal parts of the orogen. Very young ductile deformation of LHC gneisses near Wangtu is constrained by late-kinematic pegmatite intrusions crosscutting the main mylonitic foliation. Sm–Nd garnet and Rb–Sr muscovite ages of these pegmatites range between 7.9±0.9 and 5.5±0.1Ma. Published apatite FT ages down to 0.6Ma also document accelerated diachronous sub-recent exhumation of different parts of the orogen.Together with geochronological data from the literature, the new results demonstrate that the HHC and the HU were deformed by shortening and crustal thickening during the Eohimalayan phase (Late Eocene–Oligocene), followed by a strong thermal overprint and intrusions of granitoids during the Neohimalayan Phase (Early to Middle Miocene). The LHC experienced amphibolite facies metamorphic conditions in the Late Miocene prior to extrusion between the HHC and the very low-grade Lesser Himalayan sediments. In conjunction with climate changes, young tectonic activity in this central part of the Himalayan orogen may have strongly influenced fluvial incision and erosion, and therefore, contributed to the accelerated uplift, as indicated by extensive accumulation of Late Miocene to Early Pleistocene fluviatile–lacustrine sediments in the Zanda basin, the Transhimalayan headwaters of the Sutlej, in Western Tibet.

Partial Melting in the Higher Himalayan Crystallines of Eastern Nepal: the Effect of Decompression and Implications for the ‘Channel Flow’ Model

Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis.Tounderstand the influenceofdecreasingpressure onpartialmelting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet^K-feldspar^kyanite^sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC)ofeasternNepal.Themainmelt-producing reactions, the amount of melt produced during heating vs decompression, and the effects ofmelt loss on themineral assemblagesand compositions have been investigated along four idealP^T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have beeninterpreted intermsofmelt-producingvsmelt-consumingreactions (e.g. growth ofperitecticgarnetwith preserved ‘nanogranite’inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P^T pseudosection modelling predicts that at least 15^20 vol.% ofmeltwasproducedat peakP^Tconditions through dehydrationmelting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect ofdecompression.The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss ofmost of this melt.TheP^Tevolution inferred forsamplesfrom different, strategically located, structural levelsoftheBarunGneiss isconsistentwiththeexpectationsofa‘channel flow’model, including: (1) the clockwise shape of the P^T paths; (2) the estimatedPat peakT(newdata:10^8 kbarat 8008C;model:13^7 kbar at 8008C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel.

Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya

In the Kanchenjunga area (far eastern Nepal), as well as in the whole Himalayan chain, the juxtaposition of the high-grade mid-crustal Higher Himalayan Crystallines (HHC) onto the low grade Lesser Himalayan Sequence (LHS) is structurally marked by the Main Central Thrust Zone (MCTZ). On the base of original field mapping and mesoand micro-structural data, the MCTZ has been here identified as a crustal-scale ductile to ductile-brittle shear zone roughly centred on the Inverted Metamorphic Sequence (IMS), in which different rocks are more pervasively deformed and sheared with respect to adjacent rocks. The boundaries of the MCTZ are not represented by single thrusts. The lower boundary is marked by phyllonites and mylonitic schists located at the uppermost portions of the LHS at the contact with strongly mylonitic augen gneisses of the IMS. The upper boundary of the MCTZ is roughly located at the base of Grt-Kfs-Ky-Sil anatectic gneisses in the lower portion of the HHC, being itself characterized by pervasive ductile shearing. Structural field data combined with petrologic results clearly indicate that the MCTZ is internally imbricated, resulting in the juxtaposition of rock packages characterized by different P-T evolutions and T/depth gradients, separated by “metamorphic discontinuities” which do not always correspond to evident structural breaks. This is the case of the metamorphic discontinuity identified at the upper structural levels of the IMS and juxtaposing the upper IMS rocks, locally anatectic, characterized by higher T/depth gradients on the lower IMS rocks, characterized by lower T/depth gradients. A similar metamorphic discontinuity was previously reported westward (Milke Danda transect) thus suggesting that it could be of regional importance in the tectonometmaoprhic architecture of eastern Nepal Himalaya. Citation: 2012. Structural and metamorphic features of the Main Central Thrust Zone and its contiguous domains in the eastern Nepalese Himalaya . In: (Eds.) Michele Zucali, Maria Iole Spalla, and Guido Gosso, Journal of the Virtual Explorer, volume 41, paper 2, doi: 10.3809/jvirtex.2011.00294 Journal of the Virtual Explorer, 2012 Volume 41 Paper 2 http://virtualexplorer.com.au/

Two-stage partial melting and contrasting cooling history within the Higher Himalayan Crystalline Sequence in the far-eastern Nepal Himalaya

The timing of partial melting and the pressure–temperature (P–T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P–T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil+Grt+Bt+Kfs+Pl+Qtz and Ky+Sil+Grt+Bt+Ms+Pl+Qtz migmatites at different structural level of the HHCS display ages of c. 33–28Ma (Early Oligocene) and c. 21–18Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P=c. 7–10kbar and T=c. 730–780°C, and at P=c. 8–14kbar and T=c. 720–770°C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27–23Ma (Late Oligocene) and at c. 18–16Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P=c. 4–7kbar and T=c. 650–725°C. Early Miocene Ms–Bt leucogranites with two successively overgrown zircon rims at c. 18.3±0.3Ma and c. 16.3±0.2Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P–T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P–T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30–40°C/My) than the Early Oligocene (c. 15–25°C/My) migmatites, inferred from peak-T conditions and FT (c. 6Ma for both migmatites) and U–Pb zircon ages. The P–T–t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.

Geology along the Arniko Highway between Barabise and Kodari (China-Nepal Boarder) area, central Himalaya

The area between Barabise and Kodari in central Nepal along the Arniko Highway is geologically located into Higher Himalayan Crystallines (HHC) and Lesser Himalayan Sequence (LHS) that is separated by the Main Central Thrust (MCT). The HHC consists of amphibolite facies rocks (pelitic schist, psamitic schist, pelitic gneiss and quartzite), while LHS is comprised by green schist to amphibolite facies rocks (phyllite, calcareous phyllite, garnet-mica schist, black schist, quartzite and augen gneiss) in uppermost section and carbonate (dolomite and limestone) with phyllite, and metasandstone in lower section. The MCT in the area is oriented in E-W direction with about 30° dip due north and S-C structure preserved in augen gneiss of LHS characterizes the top-to-south sense of shearing, which could be related to the movement along the MCT. Mineral lineation marked by stretched mica indicates N to NNE direction in both HHC and LHS.&#x0D; Metamorphism of inverse grade from biotite at stratigraphically lower most section of Kuncha Formation to garnet at the uppermost section having schist and augen gneiss is obvious close to the MCT in the section. However, the Kuncha Formation contains tiny crystals of garnet in the rocks of greenschist facies. Kyanite and sillimanite isograds are developed in pelitic and psamitic schists, and pelitic schists appeared at the basal part of HHC above the MCT. The transformation of garnet to chlorite at the margin and fractures and formation of chlorite within bulk rocks of the MCT zone and HHC are the indicators of traces of retrograde metamorphism because of dropping in pressure-temperature probably related to post deformation event.

Equilibrium vs. disequilibrium melting relations in the Higher Himalayan Crystalline (HHC) pelitic migmatites, Sikkim Himalaya

Higher Himalayan Crystalline (HHC) complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites. In this study, reaction textures, mineral/bulk rare earth elements (REE), trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis. With the help of equilibrium REE and trace element partitioning model, it is shown that strong enrichment of Effective Bulk Composition (EBC) is responsible for the zoning in garnet in these rocks. The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.

Opposite vergent synclines on the flanks of a large-scale box fold in the Chamba–Lahaul region, northwest Himalaya, India

This article focuses on two regional-scale synclines of the Chamba Thrust Sheet, northwest Himalaya. These synclines were explained by two different tectonic events. Earlier studies ascribe the NE vergent Tandi syncline and the SW vergent Chamba syncline to the older NE-directed nappe stacking and the younger SW-directed Himalayan deformation events, respectively. The new field data and structural analysis reveal that these synclines define the flanks of a large-scale asymmetric box fold referred to here as the Hadsar–Chobia box fold and to a common phase of deformation. The box fold in the Chamba Thrust Sheet has developed over a ductile–brittle substrate referred to here as the Chamba Thrust. This thrust has translated a portion of the Tethys Himalaya over both the Higher Himalayan Crystallines and the Lesser Himalaya. Structural analysis, particularly the parallelism between the trends of hinge lines of the Hadsar–Chobia box fold and the strike of the regional thrust planes indicate that the folding and translation may have occurred simultaneously during the D1 deformation episode.

Microstructures in quartz and feldspars of the Bomdila Gneiss from western Arunachal Himalaya, Northeast India: Implications for the geotectonic evolution of the Bomdila mylonitic zone

Microstructures of the Bomdila Gneiss (BG) that intruded the metapelites of the Lesser Himalayan Crystallines (LHC) in the western Arunachal Himalaya were studied to understand the deformation mechanisms and temperatures during deformation. The LHC is delimited at the base by the Bomdila Thrust (BT) and the Main Central Thrust (MCT) at the top. The microstructural features of quartz and feldspar in the BG indicate that the rocks underwent an early stage of deep level ductile deformation superimposed by shallow level brittle deformation during a later stage of deformation. Grain boundary migration and subgrain rotation recrystallization are the main deformation mechanisms for recrystallization of quartz and feldspars. Pressure solution and solution transfer were also important at the base of the Bomdila mylonitic zone. Both the feldspars (plagioclase and k-feldspar) and quartz show dynamic recrystallization with growth of myrmekite at the upper level while the development of quartz subgrains and fractured in feldspars are dominate at the lower level. The deformation microstructures of BG suggest that the Bomdila mylonitic zone shows an inverted thermal profile towards the MCT, i.e. increase in deformation temperature from ∼400 °C at the lower zone to ∼500–600 °C at the upper zone of the gneiss. The microstructural variations and decreasing temperatures of deformation from west to east indicate that the LHC, initially buried below Higher Himalayan Crystallines (HHC) along the MCT, have been exhumed along the BT. Shear criteria indicates top-to-the-SE displacement, in agreement with the kinematics of the LHC along the BT. Producing of an inverted thermal profile in the study area may be related to imbrication and post metamorphic shearing.