Arunachal Himalaya Research Articles

Structure, Stratigraphy and Magnetic Susceptibility of Bomdila Gneiss, Western Arunachal Himalaya, India

Abstract: Bomdila Gneiss (BG) of Western Arunachal Himalaya (WAH), a suite of felsic magmatic rocks of different components of Palaeoproterozoic age is hosted within the Lesser Himalayan Sedimentary Sequence (LHSS) and placed between the two notable thrust systems-MCT at the top of the structural level to the north and MBT at the lower structural level to the south. Both MCT and MBT are highly sheared ductile zone of high strain and the felsic batholith around Bomdila signatures the protolithic history as porphyritic granite. The BG is highly sheared, strained and displays a number of shear sense indicators like σ, δ, mica fish, stretch ribbon, S-C fabric, dislocation fabric and effect of mylonitisation. Stratigraphic status, magnetic susceptibility and whole rock geochemistry of BG have been studied to evaluate the tectonostratigraphy and nature of granite as ilmanite series (reduced type) and the related data sets are correlated as supplements with the redox state (moderately reduced type).

Burial of thermally perturbed Lesser Himalayan mid-crust: Evidence from petrochemistry and P–T estimation of the western Arunachal Himalaya, India

Abstract In this work, we establish a dual prograde P–T path of the Lesser Himalayan Sequence (LHS) rocks from the western Arunachal Himalaya (WAH). The investigated metagranites, garnet- and kyanite-zone metapelites of the LHS are part of an inverted metamorphic sequence (IMS) that is exposed on the footwall side of the Main Central Thrust (MCT). Integrated petrographic, mineral chemistry, geothermobarometric (conventional and isopleth intersection methods) and P–T pseudosection modeling studies reveal a near isobaric (at P ~ 8–9 kbar) peak Barrovian metamorphism with increase in TMax from ~ 560 °C in the metagranite through ~ 590–600 °C in the lower and middle garnet-zone to ~ 600–630 °C in the upper garnet- and kyanite-zone rocks. The metamorphic sequence of the LHS additionally records a pre-Barrovian near isobaric thermal gradient in the mid crust (at ~ 6 kbar) from ~ 515 °C (in the middle garnet zone) to ~ 560–580 °C (in the upper garnet- and kyanite zone, adjoining the Main Central Thrust). Further burial (along steep dP/dT gradient) to a uniform depth corresponding to ~ 8–9 kbar and prograde heating of the differentially heated LHS rocks led to the formation of near isobaric metamorphic field gradient in the Barrovian metamorphic zones of the WAH. A combined critical taper and channel flow model is presented to explain the inverted metamorphic zonation of the rocks of the WAH.

Drainage migration and out of sequence thrusting in Bhalukpong, Western Arunachal Himalaya, India

In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5cm/yr in upper reaches and ∼13.5cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1ka) and differing incision rates of the surface of same terrace (T2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12mm/yr and 7mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan.

Anthropogenic Disturbances Led to Risk of Extinction ofTaxus wallichianaZuccarini, an Endangered Medicinal Tree in Arunachal Himalaya

ABSTRACT: Species extinction is the major biodiversity crisis in the twenty-first century. Today, anthropogenic activities are causing extinction of many species of ecological and economic importance. Arunachal Pradesh, home of high biodiversity and endemism, has many species facing tremendous stress which are on the verge of extinction. Taxus wallichiana, an endangered tree species distributed in temperate forests, has the anti-cancerous chemical Paclitaxel (Taxol®). It has other important uses in Ayurveda, Tibetan, and Unani medicine. Currently, Taxol is recognized as the most effective drug for a variety of cancers. Owing to its high economic demand, unsustainable extraction, and over-harvesting, the species ranked in the endangered category and is at risk of extinction in Arunachal Himalaya. Regeneration from seed is very poor due to a long dormancy period (nearly 18 months) and it is slow to reach maturity in nature. Moreover, consumption of fruits by birds, monkeys, rats, etc., is causing regenerati...

Rapid denudation of Higher Himalaya during late Pliestocence, evidence from OSL thermochronology

Optically Stimulated Luminescence (OSL) of quartz, with closure temperatures of 30–35°C in conjunction with Apatite Fission Track (AFT; closure temp. ∼120°C) and 40Ar-39Ar (biotite closure temperature ∼350°C), were used to obtain cooling ages from Higher Himalayan crystalline rocks of Western Arunachal Himalaya (WAH). Cooling age data based on OSL, AFT and Ar-Ar thermochronology provide inference on the exhumation — erosion history for three different time intervals over million to thousand year scale. Steady-state exhumation of ∼0.5 mm/yr was observed during Miocene (&gt;7.2 Ma) till Early Pleistocene (1.8 Ma). Onset of Pleistocene glacial/interglacial conditions from ∼1.8 Ma formed glaciated valleys and rapid erosion with rivers incising deep valleys along their course. Erosion enables midcrustal partial melts to move beneath the weak zone in the valley and causes an erosion-induced tectonic uplift. This resulted in a rapid increase in exhumation rate. The OSL thermochronology results suggest increased erosion over ∼21 ka period from Late Pleistocene (2.5 mm/yr) to Early Holocene (5.5 mm/yr) and these are to be contrasted with pre 1.8 Ma erosion rate of 0.5 mm/yr. Enhanced erosion in the later stage coincides with the periods of deglaciation during Marine Isotope Stages (MIS) 1 and 2. The results of the present study suggest that in the present setting OSL thermochronology informed on the short-term climatic effect on landscape evolution and techniques like the AFT and 40Ar-39Ar provided longer-term exhumation histories.

Geosciences of the Himalaya–Karakoram–Tibet orogen

The Himalaya–Karakoram–Tibet orogen, a product of India–Eurasia collision *55 Ma back, continues to attract global attention for its many geoscientific uniqueness. This orogen is the most exciting ‘natural laboratory’ to study continental dynamics and its evolution. During past few decades, phenomenal progresses have been made in uplift and extrusion models, dynamic metamorphism, magmatism, climate–tectonics interaction, etc. This thematic volume on the geosciences of the Himalaya–Karakoram– Tibet consists of eighteen papers and two Geosites. In the first paper, Kirby and Harkins correlated variation in slip rate along the Kunlun fault with topography of the Anyemaqen Shan Mountain at the eastern Tibetan plateau. Their work indicates that fault terminations might be associated with crustal thickening of the plateau and demonstrate that the upper crustal deformation of the Tibetan plateau to be pervasive and dispersed throughout the crustal blocks rather than localized along narrow fault zones. Robinson and Pearson compiled an orogen-wide correlation of footwall and hanging wall units of the Ramgarh and Munsiari thrust sheets. They considered the Ramgarh–Munsiari thrust sheet as a single ‘major orogenscale fault’ system. They hypothesized that the extensional shear within the South Tibetan Detachment was triggered possibly by ‘slip transfer’ from the Main Central Thrust into the Ramgarh–Munsiari Thrust during the Miocene. Thakur reviewed the tectonics of the Siwalik range of the Himalaya. He concluded in-sequence deformation and critical taper mechanism acted in this terrain. Presuming the crustal channel flow model to be correct, Mukherjee estimated a viscosity of 10–10 Pa s, and a Prandtl number of 10–10 for the Greater Himalayan Crystallines. Moharana et al., described the Munsiari Thrust from Kumaun Himalaya to consist of a core and a damage zone and described their structural geology in detail. Mukherjee described a ‘basal detachment’ of extensional ductile shear from the base of the Greater Himalayan Crystallines at Bhagirathi section of the Indian Himalaya and explained it in terms of a shifting crustal channel flow. Ubiquitous backthrusts within this terrain were another new finding and were explained by southward subduction of Eurasian plate below the Indian plate. Based on balanced crosssection studies, Khanal and Robinson estimated slip along major Himalayan faults from the Budhi-Gandhaki river section of central Nepal. Sen and Collins inferred dextral transpression and changing angle of convergence of the formation of the Ladakh magmatic arc. They also reported Late Eocene S-type granite magmatism in its central part. Mathew et al. discuss the tectonothermal evolution of the Arunachal Himalaya. They obtained isothermal decompression for the Greater Himalayan Sequence and isobaric cooling from the Main Central Thrust region. Jayangondaperumal et al. reevaluated co-seismic slip of the Himalayan Frontal Thrust. The estimated shortening around the Himalayan Frontal Thrust would lead a bigger earthquake in the western part of Indian Himalaya. Shah presented a geomorphologic study from Kashmir Basin in India, identified faults from remote sensing studies, and predicted a future earthquake of Mw 7.6. Srivastava et al. S. Mukherjee (&) Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India e-mail: soumyajitm@gmail.com

Arc parallel extension in Higher and Lesser Himalayas, evidence from western Arunachal Himalaya, India

The existence of E–W extensional features from northeast (NE) Himalaya is poorly documented. Our investigation in the western part of Arunachal Himalaya provides evidences of active Quaternary E–W arc-parallel extensional features in the Higher and Lesser Himalayas. They are represented by arc-perpendicular normal faults and arc-parallel sinistral strike-slip faults. We discuss the occurrences of these arc-parallel extensional features in terms of oblique convergence and radial expansion models. The partitioning of stress due to oblique convergence is argued based on evidences of left-lateral slip in NE-Himalaya, right-lateral slip in NW-Himalaya and absence of translation in the central part. The amount of arc-parallel extension in the hinterland regions is correlated to the amount of radial shortening in the foreland. The computation of arc-parallel extension in the NE Himalayan arc is carried out by defining a small-circle centered at 88°39′±0.7′E longitude and 33°40′±0.6′N latitude having a radius of 770.7 ± 15.1 km, for the segment between 92°01′ and 95°16′E longitudes. The amount of arc-parallel extension estimated is ~110 km for the NE Himalayan segment. Our result agrees closely with the 104 km extension determined based on geodetically computed extension rate and age of initiation of rifting in southern Tibet.

Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution

Determination of the peak thermal condition is vital in order to understand tectono-thermal evolution of the Himalayan belt. The Lesser Himalayan Sequence (LHS) in the Western Arunachal Pradesh, being rich in carbonaceous material (CM), facilitates the determination of peak metamorphic temperature based on Raman spectroscopy of carbonaceous material (RSCM). In this study, we have used RSCM method of Beyssac et al. (J Metamorph Geol 20:859–871, 2002a) and Rahl et al. (Earth Planet Sci Lett 240:339–354, 2005) to estimate the thermal history of LHS and Siwalik foreland from the western Arunachal Pradesh. The study indicates that the temperature of 700–800 °C in the Greater Himalayan Sequence (GHS) decreases to 650–700 °C in the main central thrust zone (MCTZ) and decreases further to 600 and 12 km) than the upper Siwalik sediments to its south at depths <8 km before they were exhumed. The 40Ar/39Ar ages suggest that the upper zones of Se La evolved ~13–15 Ma. The middle zone exhumed at ~11 Ma and lower zone close to ~8 Ma indicating erosional unroofing of the MCT sheet. The footwall of MCTZ cooled between 6 and 8 Ma. Analyses of P–T path imply that LHS between MCT and U.MBT zone falls within the kyanite stability field with near isobaric condition. At higher structural level, the temperatures increase gradually with P–T conditions in the sillimanite stability field. The near isothermal (700–800 °C) condition in the GHS, isobaric condition in the MCTZ together with T–t path evidence of GHS that experienced relatively longer duration of near peak temperatures and rapid cooling towards MCTZ, compares the evolution of GHS and inverted metamorphic gradient closely to channel flow predictions.

State of Tectonic Stress in Northeast India and Adjoining South Asia Region: An Appraisal

An attempt is made to map the spatial variation of the tectonic stress pattern in northeast India and its adjoining south Asia region using stress tensor inversion of some 516 fault‐plane solutions. The Bhutan Himalaya and the Arunachal Himalaya are mapped with north–south to north‐northwest–south‐southeast compression. The eastern Himalaya syntaxis zone, on the other hand, shows a clockwise rotation; a north‐northeast compression is dominant. To the south, in the intraplate part of the region, the Shillong plateau, Assam valley, Bengal basin (Bangladesh), and Tripura fold belt exhibit north‐northwest to north‐northeast compression. Orthogonal horizontal extension is dominant in southern Tibet, Bhutan, and partly in the syntaxis zone, and the same is also observed in the Shillong plateau and Assam valley area of the intraplate region. The Indo–Burma ranges and the Sagaing fault in the Myanmar region show a northeast–southwest compression; an orthogonal horizontal northwest–southeast extension is also observed in the Sagaing fault zone. A depth variation of the tectonic stress is observed below the Indo–Burma ranges; it changes from north–south to northeast–southwest in the southern part, and from northeast–southwest to north‐northeast–south‐southwest in the northern part in the deeper seismogenic zone. The stress inversion results of clusters of events in individual zones, though mostly conformable with the average observations, indicate a variation in the Shillong plateau due to heterogeneity and tectonic complexity.

Geochemistry of Proterozoic granitoids exposed between Dirang and Tawang, western Arunachal Himalaya, north-eastern India: petrogenetic and tectonic significance

Major and trace element geochemistry of Proterozoic granitoids from the Dirang and Galensiniak Formations, of Lesser and Higher Himalayas, respectively, emplaced in and around Dirang and Tawang regions of the western Arunachal Himalaya, is discussed. In general, these granitoids are massive as well as foliated in nature and are characterized by granitic mineralogical compositions. Porphyritic and hypidiomorphic textures are common in massive type, whereas others show porphyroblastic and foliated textures. Augen structure is also observed in a number of samples. Geochemical and normative compositions together with petrographic features classify them as peraluminous granitoids. Major and trace element geochemistry of most of these granitoids shows granitic nature, while few samples also show monzonitic characteristics. Observed geochemical characters, such as their peraluminous and alkali-calcic/calcic-alkalic nature, crudely defined geochemical patterns, different multi-element and rare-earth element patterns, together with low Mg# (Mg number) of these granitoids suggest their derivation from lower crustal material rather than a mantle source. Multi-element and rare-earth element patterns corroborate their genesis from different crustal melts. It is difficult to explain variations observed in granitoid rocks by partial melting alone; definitely different other processes like migration of melts, magma mixing, assimilation and fractional crystallization also played important role in the genesis of these granitoids. These melts were likely generated at low temperature (730–760 °C) and low pressure (2–5 GPa). The chemical compositions suggest that most of these Paleoproterozoic granitoids are emplaced within the syn-collisional tectonic setting, while few granitoid samples also indicate their volcanic-arc nature. Probably, later group of granitoids are slightly younger to the syn-collisional type.

Geochemistry and petrogenesis of metabasic rocks from the Lesser Himalayan Crystallines, Western Arunachal Himalaya, Northeast India

In the Lesser Himalayan Crystallines of the western Arunachal Himalaya, numerous metabasic rocks occur as disrupted dykes / sills and boudins mostly within gneiss and rarely interbedded with quartzite and phyllite. These metabasic rocks are tholeiitic in composition, resemble ortho-amphibolites, enriched in light rare earth elements and large ion lithophile elements, and depleted in high field strength elements (Nb, P, Ti). Amphiboles present in these rocks are typically calcic in nature and range from tschermakite to magnesiohornblende-actinolite, whereas plagioclase varies from oligoclase to labradorite (An29 to An52). Their mineral chemistry suggests that these rocks were metamorphosed at temperatures between 566 °C and 633 °C at a pressure >5 kb. Geochemical characteristics and petrogenetic modelling imply that the protolith of these metabasic rocks was probably derived from an enriched lithospheric mantle source by partial melting (5% to 20%) leaving a residue with 35% olivine, 50% orthopyroxene, 15% clinopyroxene; subsequently, the melt was modified by fractional crystallization of clinopyroxene and plagioclase. It is also suggested that the protoliths of these rocks were emplaced in a continental rift environment and were metamorphosed into amphibolites during the Himalayan orogeny.

Estimation of Source Parameters of Local Earthquakes Originated in Shillong–Mikir Plateau and its Adjoining Region of Northeastern India

Abstract We estimate the source parameters (seismic moment, source radius, stress drop, and source displacement) and scaling laws for local earthquakes that occurred in the Shillong–Mikir plateau, Assam Valley, and Arunachal Himalaya in northeast India during 2001–2008. The source parameters were determined using the spectral analysis of P waves from the vertical component seismograms, after correction for attenuation. Seismic moments are observed within the range from 9.51×10 12 to ; stress drop ranges from 4×10 5 to 9×10 7 Pa for the Brune model and 7×10 5 to 1×10 8 Pa for the Madariaga model. Seismic events in this study are prominent with an average stress drop of 0.1–10 MPa. The effect of site geology may be a contributing factor for such a variation in stress drop. Source dimensions are, however, found to be smaller within the major part of the plateau. It is suggested that local earthquakes in the region are associated with a brittle shear‐failure mechanism on fault segments and/or the presence of weakened zones, and earthquakes are triggered by low deviatoric stress. Empirical relations between M w – M L and M 0 – M L are developed leading to the future prediction of calibration coefficients for the local earthquakes in the Shillong–Mikir plateau and its adjoining region. Online Material: Tables of source parameters.

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.

Herbal medicines used in the treatment of diabetes mellitus in Arunachal Himalaya, northeast, India

Ethnopharmacological relevanceMedicinal plants have played an important role in treating and preventing a variety of diseases throughout the world. Khampti tribal people living in the far-flung Lohit district of the Eastern Arunachal Himalaya, India still depend on medicinal plants and most of them have a general knowledge of medicinal plants which are used for treating a variety of ailments. This survey was undertaken in Lohit district in order to inventory the medicinal plants used in folk medicine to treat diabetes mellitus. Materials and methodsField investigations were conducted in seventeen remote villages of Lohit district starting from April 2002 to May 2004 through interviews among 251 key informants who were selected randomly during our household survey. To elucidate community domains and determine differences in indigenous traditional knowledge of medicinal plants with anti-diabetic efficacy, we repeated our field survey starting from April 2008 to May 2010 with one hundred traditional healers locally called as “Chau ya” in Khampti of Lohit district. “Chau ya” traditional healers who know and use medicinal plants for treating diabetes mellitus were interviewed using a semi-structured questionnaire. ResultsThis study reports an ethnobotanical survey of medicinal plants in Lohit district of Arunachal Pradesh reputed for the treatment of diabetes mellitus. Forty-six plant species were identified in the study area to treat diabetes mellitus by the Khamptis “Chau ya” traditional healers. Comparative published literature survey analysis of this study with other ethnobotanical surveys of plants used traditionally in treating diabetes mellitus suggests that eleven plant species make claims of new reports on antidiabetic efficacy. These plant species are Begonia roxburghii, Calamus tenuis, Callicarpa arborea, Cuscuta reflexa, Dillenia indica, Diplazium esculentum, Lectuca gracilis, Millingtonia hortensis, Oxalis griffithii, Saccharum spontaneum, and Solanum viarum. Some of the plants reported in this study have an antidiabetic effect on rodent models but none have sufficient clinical evidence of effectiveness. ConclusionsThe wide variety of medicinal plants that are used to treat diabetes mellitus in this area supports the importance of plants in the primary healthcare system of the rural people of Lohit district of Arunachal Pradesh. The finding of new plant uses in the current study reveals the importance of the documentation of such ethnobotanical knowledge.

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.

Structural Analyses of the Crystalline Rocks between Dirang and Tawang, West Kameng District, Arunachal Himalaya

Abstract In Kameng Valley of Arunachal Pradesh, the crystalline rocks of Se La Group of Higher Himalaya are thrust over the Lesser Himalayan rocks of Dirang Formation, Bomdila Group along the Main Central Thrust and exhibit well preserved structures on macro- to microscopic scales. Detailed analysis of structures reveals that the rocks of the area have suffered four phases of deformation D1, D2, D3 and D4. These structures have been grouped into (i) early structures (ii) structures related to progressive ductile thrusting and (iii) late structures. The early structures which developed before thrusting formed during D1 and D2 phases of deformation, synchronous to F1 and F2 phases of folding respectively. The structures related to progressive ductile shearing developed during D3 phase of deformation, when the emplacement of the crystalline rocks took place over the rocks of Dirang Formation along the Main Central Thrust. Different asymmetric structures/kinematic indicators developed during this ductile/brittle-ductile regime suggest top- to -SSW sense of movement of the crystalline rocks of the area. D4 is attributed to brittle deformation. Based on satellite data two new thrusts, i.e. Tawang and Se La thrusts have been identified parallel to Main Central Thrust, which are suggestive of imbricate thrusting. Strain analysis from the quartz grains of the gneissic rocks reveals constriction type of strain ellipsoid where k value is higher near the MCT, gradually decreases towards the north. Further, the dynamic analysis carried out on the mesoscopic ductile and brittle-ductile shear zones suggest a NNE-SSW horizontal compression corresponding to the direction of northward movement of Indian Plate.

Understanding the tectonic behaviour of the Shillong Plateau, India using remote sensing data

Shillong Plateau in India is tectonically and geologically interesting entity in the subducted front of Indian Plate below Burmese Plate to the southeast and Tibetan Plate to the north and associated with thrusts and shears along the plate boundaries. Horse-tail geometry in the foothills of the Arunachal Himalaya, east of Jia Bhareli river, associated with south-convex foothill ranges in the eastern Himalaya and exactly similar structural geometry in the eastern part of Shillong Plateau in Meghalaya seems to develop due to resistance received by the plateau in its eastward journey. Wide separation of Karbi Anglong Plateau and Shillong Plateau to the southeast as compared to northwestern part defines the shape of Kopili graben. Low seismic activity in southeastern part of Shillong Plateau might be related to stress released field generated by its clockwise rotation. Satellite derived images and digital elevation data from Landsat ETM+ and SRTM data shows that the central part of Shillong Plateau possesses young topography with strong structural fabrics along with relatively high topography aligning NE-SW following Kolkota-Pabna-Mymansingh High and if extended passes through western part of Arunachal Pradesh in eastern Himalayas. This alignment has been observed in Precambrian gneissic complex west of the Proterozoic intracratonic Shillong Basin. The epicentral plot for the period 1918 to 2009 shows their high concentration within the Shillong Plateau aligning along this trend. The active geodynamics of Shillong Plateau is reflected in its seismic activity pattern in relation with the structural fabrics, northward migration of the Brahmaputra in the north front of the Plateau and by shrinking pattern of Chandubi Lake in the Kulsi river catchment, a north-flowing tributary of the Brahmaputra in the north-central part of the plateau.

Tectonic implication of drainage set-up in the Sub-Himalaya: A case study of Papumpare district, Arunachal Himalaya, India

The Sub-Himalaya, deformed between the Main Boundary Thrust and/or Bomdila Thrust towards north and the Himalayan Frontal Thrust (HFT) towards south, is characterized by the presence of fault-propagated folds and an imbricate thrust system. Along the HFT, the mountain front is offset for about 10 km sinistrally along a NW–SE trending transverse fault, the Banderdewa Fault, which runs through the Dikrang River valley that seems to have followed a pull-apart basin developed along the Banderdewa Fault. The western part of study area, associated with a major folding, faulting and the development of fault-bounded Quaternary basins with normal faulting along their northern margins, is interpreted to be tectonically more transported towards southeast than the eastern part where no Quaternary basins are developed. The channel forms and landforms have been studied in relation to the neotectonics of the area. The watersheds exhibit their structurally controlled nature, with drainages following tectonic alignments. Rivers/streams experienced compressed meandering, narrowing, widening and deflection of channels. Longitudinal profiles of river/stream channels exhibit knick points. Unpaired terraces are observed at higher levels. Factors like the unconsolidated Siwalik rocks, tectonic instability along faults and high rainfall with the advent of the Holocene, led to the development of the present-day drainage set-up. A tectonic model is proposed for development of tectonic geomorphology of the area influenced by ultra-tremor to low magnitude earthquakes in seismically very active northeastern India, as moderate to big earthquakes (M ≥ 6.0) are scarce in this region.

Mineralogy and geochemistry of biotites from Proterozoic granitoids of western Arunachal Himalaya: Evidence of bimodal granitogeny and tectonic affinity

Felsic magmatic rocks in Kameng corridor of western Arunachal Himalaya are represented by extensively exposed Palaeoproterozoic porphyritic muscovite-biotite granite (GGn) of the Bomdila Group and small stock-like Mesoproterozoic hornblende-biotite granite (HBG) of the Salari Group. Mineralogy and chemical composition of biotites from GGn and HBG have been utilized to understand the nature and tectonic environment of their parental felsic melts. Biotites in GGn (FeOt/MgO=3.1–4.6) are Fe-biotites and have shown affinity with primary biotites co-precipitating with muscovite in a peraluminous (S-type) felsic melt of syn-collisional tectonic environment. Biotites in HBG (FeOt/MgO=1.3–2.2) are transitional between Fe and Mg biotites evolved from Fayalite-Magnetite-Quartz (FMQ) to Nickel-Nickel Oxide (NNO) buffers and are related to primary biotites co-existing with amphibole and other ferromagnesian minerals in a calc-alkaline metaluminous (I-type) felsic melt mostly formed in a subduction setting. Both GGn and HBG biotites exhibit Mg⇌Fe substitution, which is more pronounced in HBG biotites. GGn biotites exhibit 2Al⇌3Fe2+ substitution as expected in peraluminous melt, whereas 3Mg⇌2Al substitution normally expected to operate in metaluminous melt is less pronounced in HBG biotites. GGn biotites are markedly enriched in siderophyllite, and depleted in phlogopite components as compared to HBG biotites, which point to diverse genetic conditions. HBG biotites indicate oxidizing environment of the felsic melt unlike the reducing nature of the porphyritic granite (GGn).

Geochemistry and petrogenesis of granitoids of Lesser Himalayan crystallines, Western Arunachal Himalaya

Low to medium grade crystalline rocks locally known as Bomdila Group extensively covers the Lesser Himalayan region in Western Arunachal Himalaya. This Group consists dominantly of mylonitic gneisses of granitic composition of Palaeoproterozoic age, named as Bomdila mylonitic gneiss (BMG) and a small body of hornblende bearing granite of Mesoproterozoic age known as Salari granite (SG). The BMG is affinity to peraluminous (A/CNK > 1.1) with high content of SiO2, K2O/Na2O ratio, normative corundum, high ratio of FeOt/MgO in biotite (3.21–5.11) that shows characteristics of S-type granite whereas SG has granodiorite composition with high Na2O, low K2O, presence of hornblende, normative diopside, low A/CNK ratio (<1.1) and low FeOt/MgO ratio in biotite (1.58–1.60) indicates metaluminous I-type granite affinity. The SG has more fractionated nature of REE [(Ce/Yb)N = 9.06–18.53] and minor negative Eu anomalies [EuN/Eu* = 0.69–0.94] as compared to BMG which has less fractionation of REE [(Ce/Yb)N = 5.95–9.16] and strong negative Eu anomalies [EuN/Eu* = 0.37–0.43]. Geochemical and petrological studies suggest that the SG and BMG are not genetically related; SG appears to have derived from igneous source whereas the BMG have been derived from sedimentary source, however these granitoids might have produced during the same thermal event.