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A preliminary lithospheric magnetic anomaly map of the Indian subcontinent (LAMI-1) was generated utilising seven years (April 2014–December 2020) of data recorded by the Swarm satellite constellation. To obtain a high-resolution lithospheric anomaly map, the fields originating from sources other than lithospheric sources are eliminated sequentially. The external field component is minimised by selecting the days of low geomagnetic activity (Kp < 2 and –20 < Dst < 20) within night-time hours. The main and the remaining external field sources are removed using data from CHAOS model. The model of residual data is achieved directly through the damped least square inversion technique by expanding the Legendre polynomial of order n = 6–50. The resulting satellite-derived lithospheric magnetic anomaly mostly reflects intermediate to long wavelength deep geological phenomena, with diverse tectonic provinces exhibiting discrete magnetic fingerprints/impressions with amplitudes ranging from high to low. Various tectonic blocks of the Indian subcontinent show distinct signature in the derived lithospheric anomaly map. The Himalayas and the Deccan Volcanic Province are associated with low magnetic signatures. The Central Indian Tectonic Zone and the Arakan Yoma Fold belt appear to have positive magnetisation. The division of Dhawar Craton into Western and Eastern Dharwar by Chitradurga Boundary shear is clearly evident in the anomaly map. A possible track of the Reunion hotspot is depicted as a north–south oriented high within the Marwar block. The comparison between the vertical (Z) component of LAMI-1 with the vertical component of MAGSAT data and MF7 lithospheric model from Champ satellite data indicates the LAMI-1 model shows far less noise and sharper anomalies with tectonic blocks better resolved compared to the other models. [ABSTRACT FROM AUTHOR]

The Nagaland-Manipur ophiolites (NMO), part of the Phanerozoic (538.8–0 Ma) Tethyan ophiolites, occur in the NNE-SSW trending Indo-Myanmar Orogenic Belt (IMOB), northeast India. The NMO hosts both high-Al (0.46 < Cr# < 0.53) and high-Cr chromitites (0.71 < Cr# < 0.79). These chromitite bodies are hosted in lherzolite, harzburgite, and dunite and show various textures, including massive, disseminated, nodular, and granular. The high-Al chromitite compositions in conjunction with the calculated Al2O3[melt] (15.66–16.39 wt.%), TiO2[melt] (0.65–0.94 wt.%), and FeO/MgO[melt] (0.65–0.83 wt.%) values indicate that they were derived from the tholeiitic melt that formed at the mid-ocean ridge centre through low-degree partial melting. In contrast, the high-Cr chromitites, coupled with the Al2O3[melt] (11.24–12.99 wt.%), TiO2[melt] (0.21–0.33 wt.%), and FeO/MgO[melt] (0.58–1.54) values show similar geochemical affinities to those derived from boninitic melts produced by partial melting of already depleted mantle due to the subduction of oceanic plate in a supra-subduction zone environment. The total platinum group element (PGE) contents (60–190 ppb) of high-Al chromitites are lower than the total PGE contents (118–2341 ppb) in high-Cr chromitites. Chondrite-normalized PGE patterns in high-Al chromitites are flat from Os to Rh and negatively sloping from Rh to Pd, whereas high-Cr chromitites show strongly fractionated chondrite-normalized PGE patterns. Total PGE contents and low Pd/Ir ratios (0.02–0.64) of chromitites are consistent with typical ophiolitic chromitites. Mineral chemistry and PGE systematics suggest that NMO chromitites were generated in two separate tectonic settings. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment. [ABSTRACT FROM AUTHOR]

The provenance and tectonic setting of the Stenian (<1200 to 1015 Ma) metasedimentary sequences of the South Delhi Supergroup, NW India, are least understood and thus debatable. The results of this study show that the protoliths of the metasedimentary rocks received detritus dominantly from felsic sources, particularly from Mesoproterozoic granitoids exposed in northern and eastern India. The weathering indices suggest low to moderate weathering in the source areas and that the sediment compositions reflect mixing of provenance components having different compositions. Overall, the REE patterns of the South Delhi rocks are similar with variable negative Eu anomalies and absolute ∑REE contents. The low REE abundances in the quartzites are the consequence of quartz dilution during hydraulic sorting. The tectonic multidimensional discrimination diagrams suggest a passive margin setting for the South Delhi metasedimentary rocks, which is in consonance with the detrital zircon age spectra and regional geology of the western margin of Aravalli orogen, NW India. [ABSTRACT FROM AUTHOR]

High‐resolution present‐day earth surface deformation maps from satellites provide important data constraints, which help us better understand tectonic processes and analyze seismic hazards. Here, we use Sentinel‐1 Radar images (2014–2020) and accurate positioning measurements (2009–2019) to get a high‐resolution three‐dimensional earth surface velocity map for the northeastern Tibetan Plateau, and we invert the slip rate and coupling ratio of major regional faults. We find ∼4 mm/yr uplift along an arc from the Qilianshan to Lajishan, relative to the neighboring low‐elevation area to the east, which indicates ongoing rapid orogeny. We find transient deformation along the Laohushan and 1920 M8.5 Haiyuan rupture segments of the Haiyuan fault, whereas the western Haiyuan, southern Liupanshan, central Lajishan and central‐western West Qinling faults are essentially locked above 15–20 km, suggesting a potentially high seismic hazard. Plain Language Summary: The northeast Tibetan Plateau, which is impeded by the tectonically stable Ordos block, continues to grow in response to the far‐field India‐Eurasia collision. This region has several large‐scale active faults that hosted some destructive earthquakes in the last 100 years. Although intensively studied, the fault kinematics of those seismogenic faults are still not clear, mainly because of poor observational constraints. Hence, high‐resolution present‐day deformation maps are essential to answering this question, either of scientific or societal concern. We provide satellite‐based ground deformation rate maps with mm‐accuracy for this region. They show shallow fault creep along some segments of the Haiyuan fault which is better constrained than previous results, active tectonic uplift in the Lajishan. Our work gives new constraints for the seismic hazard for the Northeastern Tibetan Plateau case. Key Points: We derive a new high‐resolution three‐dimensional present‐day deformation map for the Northeastern Tibetan PlateauGeodetic data reveals ongoing rapid uplift along an accurate orogenic belt from the eastern Qilianshan and LajishanNew constraints for the seismic hazard analysis for the Northeastern Tibetan Plateau [ABSTRACT FROM AUTHOR]

Mantle‐derived volatile degassing lacks quantitative evaluation in continental regions without active magmatism, such as the Tibetan Plateau. Ten new gas abundance and helium isotope data points combined with 286 hydrothermal volatile literature data points in India–Asia continental collision settings demonstrate widespread mantle‐derived volatiles across the thick (∼70 km) crust. The mantle‐derived 3He is best explained by direct mantle volatile input from subcontinental lithospheric mantle (1%–36.5%) or the asthenospheric wedge (1%–27.8%) instead of fossil residual magmatic fluids or Quaternary mantle‐derived melt intrusion into crustal depth. Mantle‐derived 3He is transported from the deep mantle to the surface at an upflow rate of 30–11,700 mm/year based on a newly developed steady‐state one‐dimensional flow model, corresponding to a mantle‐derived 3He flux of 17 to 1.5 × 107 atoms m−2 s−1 (81.7%–99.4% of the total 3He flux) and a mantle‐derived 4He flux from 2.0 × 106 to 1.8 × 1012 atoms m−2 s−1 (1.4%–36.5% of the total 4He flux). The mantle‐derived helium fluxes in the study area are comparable to those of other nonvolcanic hydrothermal systems in the tectonically active regions but lower than those of volcanic fields. The helium transit time ranges from 5.3 ka to 2.3 Ma, indicating that the spatial pattern of 3He/4He ratios in the India‐Asia continental collision settings can provide a snapshot of the state of the Indian mantle lithosphere between those revealed by potassic‐rich mafic rocks (25–8 Ma) and seismic methods (present). Key Points: 3He is derived directly from mantle depths instead of residual magmatic fluids or Quaternary mantle‐derived melt in the shallow crustThe vertical flow rate, flux and transit time of mantle‐derived 3He were systematically calculatedThe state of the Indian mantle lithosphere revealed by mantle‐derived 3He is between that revealed using petrologic and seismic methods [ABSTRACT FROM AUTHOR]

Northeast India encompasses numerous thrusts, faults, and lineaments with undulated surface topography and is one of the utmost tectonically active regions in the world. Owing to the results of the collision of the Indian Plate under the Tibetan Plate and Burmese Plate, respectively, this area has affected the highest seismic potential zone-V, triggering many earthquakes. The current study area is located in and around the Shillong plateau, Mikir Hills, Naga Hills, Arakan-Yoma fold belt, Bengal basin, and Mishmi hills of the Himalayan foothills and that fall under the northeast of India. The thrusts and faults information available in this area are very scanty due to limited availability of geoscientific data and revealing seismic survey. Henceforth, it is necessary to get enhanced geoscientific learning for a better understanding of thrusts, faults, and lineaments information, the most positive and most negative curvature attribute analyses have been carried out using ground gravity data in this area. The significant derived results from this study encourage supplementary findings of thrust, fault, and lineament information, which also correlate well with the previously found results of 3D Euler deconvolution and source edge detection. Although, gravity data interpretation has its own limitations, however, the current derived results using the latest curvature analysis approach utilizing gravity data show realistic invigorated solutions for a better understanding of the thrust, fault, and lineament locations in this area. [ABSTRACT FROM AUTHOR]

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC. [ABSTRACT FROM AUTHOR]

A comprehensive whole-rock geochemical and mineral chemistry study was carried out of granites from the Narnaul and surrounding isolated hills of North Delhi Mobile Belt, NW India to constrain their genesis, and tectonic environment. The distinctive features of granites are meta to peraluminous nature and decreasing trend of P2O5 with high SiO2 (>71%) ascribed to its metaigneous origin. In detail the chemical diversity of the studied samples is reflected by their affinity to Calc-alkaline to shoshonite series with high K2O/Na2O (mostly>1), low MgO (< 0.11%) and variation in Mg# (upto 0.5) and maficity with increasing SiO2. The positive Eu anomalies, high LILE, highly enriched LREE (La/Sm = 4.13@#@19.33) pattern and comparatively elevated concentrations of Cs, La, Ta, Dy, Zr, Pb and Yb suggest different episodes of magmatic differentiation. Our data indicate that melting of older subduction-related igneous rocks and the underwent fractional crystallization in hydrous conditions is the possible mechanism for the generation of I-type granites in collisional tectonic setting. [ABSTRACT FROM AUTHOR]

The Granulite Terrane of Southern India is a collage of Mesoarchean–Neoproterozoic crustal blocks that underwent high‐grade metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic–Cambrian. Here, we investigate the charnockites and associated sapphirine‐bearing semipelitic granulites from the eastern part of the Madurai Block (MB). We present new petrographic, mineral chemistry, and geochronological data to constrain the P–T–t evolution of the block and unravel the timescale and source of heat for the ultrahigh‐temperature metamorphism. Both the rock types contain coarse‐grained porphyroblastic garnet and orthopyroxene, yielding peak P–T conditions of 950 ± 30°C at 10.5 ± 0.8 kbar and 970 ± 40°C at 10 ± 0.5 kbar for semipelite and charnockite, respectively, using conventional thermobarometry. Peak ultrahigh temperatures are further supported by high Al content in the orthopyroxene (8.78 wt% Al2O3) coexisting with garnet (XMg: up to 0.57) and feldspar thermometry of the mesoperthites and antiperthites in the semipelite, yielding 950–980°C at 10 kbar. Subsequent decompression has led to the formation of coronal orthopyroxene3 + plagioclase3 in the charnockite and symplectic orthopyroxene3 + cordierite ± sapphirine ± plagioclase3 in the semipelite, yielding P–T range of 950–850°C and 9.5–6.8 kbar for semipelites and 950–820°C and 8–6.5 kbar for charnockite. Based on the obtained P–T estimates, preserved reaction textures, and phase equilibria modelling in the MnNCKFMASHTO system, a clockwise P–T evolution with isothermal decompression followed by cooling is inferred for both the rock types. Texturally constrained in situ monazite dating and rare earth element (REE) patterns show that the core of matrix monazite having low‐Th, Y, and extreme heavy rare earth element (HREE) depletion, yielding weighted mean ages of 582 ± 12 and 590 ± 22 Ma for semipelite and charnockite, respectively, dates the prograde evolution. The mantle of the matrix monazite in semipelite and comparable rim in charnockite, having relative Th‐enrichment compared to the core, yielding weighted mean ages of 552 ± 9 and 557 ± 13 Ma, respectively, dates extensive dissolution–reprecipitation from the melt at the peak stage. The relatively Th‐ and Y‐rich and moderately HREE‐depleted rim of matrix monazite in the semipelite, yielding weighted age of 516 ± 6 Ma, date initial garnet breakdown during post‐peak melt crystallization. By contrast, compositionally homogenous HREE + Y‐enriched monazite in the symplectite and retrograde monazites yielding weighted mean ages of 487 ± 47 Ma for semipelites and 508 ± 19 Ma for charnockites dates extensive garnet breakdown during final stages of melt crystallization and subsequent cooling. Our findings point to collision initiation at ~590 Ma, with the peak conditions attained at ~550 Ma followed by extensional collapse at ~510–490 Ma, resulting in rapid exhumation of lower crustal rocks to mid‐crustal levels under sustained ultrahigh‐temperature (UHT) conditions, followed by cooling to reach a stable geotherm. Our results suggest a long‐lived hot orogeny in the MB, where the UHT conditions were sustained for at least 40 MYr. The UHT conditions were most likely attained in the core of a long‐lived hot orogen by the combined effect of conductive heating through radioactive decay and mantle heat supply, with the former being the primary driver. [ABSTRACT FROM AUTHOR]

The mineral assemblages within ductile shear zones, which act as open systems, may be strongly influenced by the occurrence of mass transfer processes induced by the infiltrated composite H2O-rich (+ ±boron ±Al) fluid fluxes and mobilising major elements. Metasomatism is the re-equilibration of a rock involving a change in the chemical composition through the interaction of parent rock with an external fluid phase. In the Sakoli Fold Belt of central India, sillimanite occurrences are associated with two major ductile shear zones, viz., one near the eastern margin, trending N–S; and other, near the northwestern margin trending NE–SW formed contemporaneous with D1 and D3 deformations, respectively. The shear zone rocks include basement slices of granodiorite gneiss, quartzite, quartz vein and intrusive granite. Aluminosilicates of andalusite and sillimanite occur in two mutually exclusive domains. While andalusite is ubiquitous in the host metapelites of Sakoli Group, sillimanite is restricted within the shear zones, preferably in mylonitized granite protoliths. Other minerals associated with sillimanite include abundant fibrolite, tourmaline, dumortierite, rutile, sphene, pyrophyllite, quartz, rare biotite and garnet, together with and secondary muscovite (after feldspar) and kaolinite, etc. It is indicative of interaction between infiltrated high-temperature hydrothermal fluid derived possibly from intrusive granite and the host rocks in the shear zone. No evidence of polymorphic inversion between andalusite and sillimanite nor breakdown reaction from muscovite + quartz → sillimanite+ K-feldspar could be established. Textural evidences suggest the growth of sillimanite and associated tourmaline at two pulses, each coinciding with or outlasting the peak of shear zone deformations and metamorphism in Sakoli rocks. Ductile shear zones in the area may thus represent high alumina hydrothermal alteration zones in granitic rocks through the influx of high-temperature fluids from some deeper source. The infiltrating hydrothermal fluids rich in Al, B, and H2O were channelised along the ductile shear zones. It caused extensive chemical alteration of the host rock giving rise to sillimanite, tourmaline, kaolinite, mica, etc., as metasomatic products. The infiltrating fluid was sufficiently high in temperature (>550°C) to produce sillimanite in the later stages of prograde metamorphism of the Sakoli Group of rocks, restricted to ductile shear zones in two separate tectonic and deformation settings. The anomalous growth of sillimanite in the restricted domains was undoubtedly facilitated by high strain environment of shear zones. Highlights: Two sets of shear zones, N–S and NE–SW related to two generations of deformations, respectively, hosts sillimanite mineralisation. Textural characteristics indicate solid-state growth of sillimanite during the syn- to late-tectonic stages of mylonitization in the shear zones. Infiltrating hydrothermal fluids rich in Al, B, H2O along the ductile shear zones, caused extensive chemical alteration of host rocks. The temperature of the infiltrating fluid was high enough to form sillimanite instead of andalucite as the metastatic product. High strain within shear zones facilitated the growth. Minerals like sillimanite, tourmaline, kaolinite, mica, etc., are metasomatic products. [ABSTRACT FROM AUTHOR]

The present study attempts to ascertain the sediment provenance of around 6.5-m thick palaeochannel sediment exposed in the vicinity of the Ghaggar–Hakra river system of the alluvial plains of the Haryana state of India. The major element geochemistry indicates that the sediments are arkosic in nature, dominated by quartz, K-feldspar, micas, plagioclase, and clay minerals (chlorite, illite, and a very less amount of montmorillonite). The CIA (chemical index of alteration) values ranged between 63 and 77, indicating moderate degrees of chemical weathering. REE plots show that the Eu anomaly is negative for average Bhor Saidan samples similar to PAAS and UCC, suggesting that they could have been deposited in the foreland basin after being originally produced from differentiated silicic and/or recycled sedimentary sources; nonetheless, a little positive Eu anomaly in the chondrite normalised plot is the result of feldspar and arkosic nature of sediments indicating higher levels of physical over the chemical weathering. Mostly the samples are from Siwaliks. Optically stimulated luminescence (OSL) dating suggests that the sediments are ~11 ka old. Further, the palaeochannel is being incised ~3.3 ka and younger sediments are stratigraphically deposited at lower levels. The discriminant function plots (Roser and Korsch 1988) of sediment samples exhibited that they were deposited in a passive margin setting and came from the interior of cratons or a quartzose sedimentary orogenic terrain. The present work done on the geochemistry of the sediments of a palaeochannel present in the alluvial plains will work as a dot on the map to connect the history of the river drainage system of the area in future studies. [ABSTRACT FROM AUTHOR]

The Satpura Mountain Belt, or the Central Indian Tectonic Zone (CITZ), is a Paleoproterozoic mountain belt of Central India, with Mesoproterozoic and Neoproterozoic rejuvenation history. Structural studies carried out in the southern margin of this ancient orogenic belt have identified a two-phase evolution. The documentation of mapped fold interference of Type 2 'boomerang-shaped' pattern and the spread of early lineations over the hinge area of second-generation folds provide an estimate of ~40-43° angle between the regional axial directions of the folds of the two orogenic episodes. The map patterns indicate the presence of basement-involved recumbent fold nappes placed above the sedimentary cover sequence. The identified décollement structure on the surface provides information regarding the deformation of the upper crust in ductile manner. The patterns of the deep crustal reflectors mapped through previous deep seismic reflection study interpreted during the present investigation brought out the thick-skinned deformation in the region as well as identification of crustal low-velocity zones of lower strength and a domal geometry of the lower crust. The regional thrusts are found to deform deeper crustal layers, one thrust affecting the crust-mantle boundary. The behaviours of the upper and lower crust in terms of rheological layering have been used to interpret the regional structural style of the lithosphere. The structural features of the region do not support the argument regarding the status of the Central Indian Shear as a suture, but indicate it to be the outermost emergent thrust towards the foreland side. The patterns of dip of the thrusts and thrust transport directions indicate a northward dip of the South Indian Block during the subduction stage and the position of the collisional suture of the Satpura orogen to the north of the Sausar belt. A tectonic model has been proposed here to explain the formation of the Satpura Mountain Belt or CITZ involving subduction-collision at ~2250 Ma, followed by erosion, stretching and basin development between 2100 and 1500 Ma. A collisional event in the eastern and western craton margins of India, at ~1500 Ma, caused the second uplift of the Satpura Mountain Belt and its northern margin, developing a temporal unconformity of ~300 Ma gap between the Semri and Kaimur Groups (Vindhyan Supergroup). A third event in the region at ~950 Ma was similar to the second event. No continental amalgamation took place along the CITZ during the tectonothermal events at ~1500 Ma and ~950 Ma; these involved reactivation of old thrusts and basal accretion which were the effect of far-field stresses emanating from the orogenic events at the eastern margin of the Bastar Craton and western margin of the Aravalli Craton. [ABSTRACT FROM AUTHOR]

The India‐Eurasia collision is a key case study for understanding the influence of plate tectonic processes on Earth's crust, atmosphere, hydrosphere, and biosphere. However, the timing of the final India‐Eurasia continental collision is debated due to significant uncertainty in the age of the collision between the Kohistan‐Ladakh arc (KLA) and Eurasia along the Shyok suture zone. Here we present paleomagnetic results that constrain the Karakoram terrane in northwest India to a paleolatitude of 19.9 ± 8.9°N between 93 and 75 million years ago (Ma). Our results show that the Karakoram terrane was situated on the southern margin of Eurasia in the Late‐Cretaceous. Our results indicate that the KLA and Eurasian continent had a not converged until <61.6 Ma, placing a Paleocene older limit on the age of final closure of the Shyok suture zone. This suggests that the India‐Eurasia collision in northwestern India likely occurred after the closure of the oceanic basin between the KLA and Eurasia. The Paleocene collision event affecting India that has been widely interpreted to represent final India‐Eurasia collision instead records the arc‐continent collision between the KLA and the northern edge of India prior to final India‐Eurasia collision. Final India‐Eurasia collision in northwest India most likely occurred after the closure of the oceanic basin between the KLA and Eurasia. Plain Language Summary: We use the magnetism of rocks to determine the latitude of the southern edge of Eurasia before the India‐Eurasia continental collision. The results show that the southern edge of Eurasia was situated at 19.9 ± 8.9°N between 93 and 75 million years ago. This latitude means that a volcanic island chain called the Kohistan‐Ladakh arc (KLA) was situated significantly south of the Eurasian continent at the start of the India‐Eurasia collision. Our results suggest that the final stage of continental collision between India and Eurasia in northwest India occurred along the Shyok suture zone, a geological boundary where a major ocean basin once existed between the KLA and Eurasia. Key Points: The Karakoram terrane was on the Eurasian margin at 20°N between 93 and 75 MaThe final closure of the Shyok suture zone occurred after 61.6 million years agoThe Kohistan‐Ladakh arc accreted onto India prior to final India‐Eurasia continental collision [ABSTRACT FROM AUTHOR]

In the paleogeographic reconstructions of the Rodinia supercontinent, the circum-global 1.1–0.9 Ga collisional belt is speculated to skirt the SE coast of India, incorporating the Rodinian-age Eastern Ghats Province. But the Eastern Ghats Province may not have welded with the Indian landmass until 550–500 Ma. Instead, the ~1500-km-long, E-striking Central Indian Tectonic Zone provides an alternate option for linking the 1.1–0.9 Ga circum-global collisional belt through India. The highly tectonized Central Indian Tectonic Zone formed due to the early Neoproterozoic collision of the North India and the South India blocks. Based on a summary of the recent findings in the different crustal domains within the Central Indian Tectonic Zone, we demonstrate that the 1.03–0.93 Ga collision involved thrusting that resulted in the emplacement of low-grade metamorphosed allochthonous units above the high-grade basement rocks; the development of crustal-scale, steeply dipping, orogen-parallel transpressional shear zones; syn-collisional felsic magmatism; and the degeneration of orogenesis by extensional exhumation. The features are analogous to those reported in the broadly coeval Grenville and Sveconorwegian orogens. We suggest that the 1.1–0.9 Ga circum-global collisional belt in Rodinia swings westward from the Australo-Antarctic landmass and passes centrally through the Greater India landmass, which for the most part welded at 1.0–0.9 Ga. It follows that the paleogeographic positions of India obtained from paleomagnetic data older than 1.1–0.9 Ga are likely to correspond to the positions of the North and South India blocks, respectively, and not to the Greater India landmass in its entirety. [ABSTRACT FROM AUTHOR]

The NE-SW trending Jahazpur Belt in NW India has been considered correlative with the Aravalli Fold Belt, based on lithological similarities and greenschist facies metamorphic assemblages. The 2.54 Ga 'Jahazpur Granite' constitutes the basement for the overlying Hindoli and Jahazpur Group sediments in this belt. These meta sedimentary sequences presently occur as a greenschist facies ensemble of phyllite, marble, greywacke, and meta-conglomerate. In this study, the trace and Rare Earth Elements geochemistry of the Paleoproterozoic Hindoli Group metapelites and metagreywackes was evaluated for source characterization and tectonic setting. The transition metals and Th, La abundances, and Th/Sc, Zr/Sc and La/Sc ratios (1.19 and 1.5, 21.31 and 17.35 and 4.02 and 2.97, for metapelites and metagreywackes, respectively) of Hindoli Group suggest a felsic source for them and some degree of sediment maturity. Both Hindoli metapelites and metagreywackes are enriched in LREE and display significant negative Eu anomalies, further substantiating a predominant felsic component in the source region. The provenance modeling calculations indicate that the detritus was mainly sourced from Mangalwar Complex, along with some inputs from Jahazpur Granite, Bundelkhand TTG and Hindoli volcanics. Geochemical characteristics also discriminate a continental collision associated with an Active Continental Margin tectonic setting for the deposition of Hindoli sediments. [ABSTRACT FROM AUTHOR]

Miocene Tipam Sandstone Formation in parts of Cachar Fold Belt, Northeast India has been studied using petrographic and geochemical attributes to investigate the composition, paleoweathering, tectonic setting, and source rock characteristics. It comprises of coarse to medium grained false-bedded multistoried ferruginous sandstone and variegated sandy clay. Petrographically, these sandstones belong to sub-lithic and lithic arenite varieties possessing an average recalculated modal composition of Q67F13R20. Geochemically, Tipam sandstones have high SiO2, more Na2O than K2O, and relatively low Fe2O3, MgO, MnO content in accordance with the modal composition. Weathering indices like CIA, CIW, PIA and ICV suggest derivation of sediments from low to moderately weathered source terrain bearing plagioclase feldspar in the range of An10–An55 comparable to granodiorite and granite composition. Dominantly a passive margin setup with high degree of sediment recycling has been envisaged during the deposition of Tipam Sandstone Formation. [ABSTRACT FROM AUTHOR]

Cretaceous‐Miocene sedimentary rocks in the Nepalese Lesser Himalaya provide an opportunity to decipher the timing of India‐Asia collision and unroofing history of the Himalayan orogen, which are significant for understanding the growth processes of the Himalayan‐Tibetan orogen. Our new data indicate that detrital zircon ages and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks underwent two significant changes. First, from the Upper Cretaceous‐Palaeocene Amile Formation to the Eocene Bhainskati Formation, the proportion of late Proterozoic‐early Palaeozoic zircons (quantified by an index of 500–1200 Ma/1600–2800 Ma) increased from nearly 0 to 0.7–1.4, and the percentage of Mesozoic zircons decreased from ca. 14% to 5–12%. The whole‐rock 87Sr/86Sr and εNd(t = 0) values changed markedly from 0.732139 and −17.2 for the Amile Formation to 0.718106 and −11.4 for the Bhainskati Formation. Second, from the Bhainskati Formation to the lower‐middle Miocene Dumri Formation, the index of 500–1200 Ma/1600–2800 Ma increased to 2.2–3.7 and the percentage of Mesozoic zircons abruptly decreased to nearly 0. The whole‐rock 87Sr/86Sr and εNd(t = 0) values changed significantly to 0.750124 and −15.8 for the Dumri Formation. The εHf(t) values of Early Cretaceous zircons in the Taltung Formation and Amile Formation plot in the U‐Pb‐εHf(t) field of Indian derivation, whereas εHf(t) values of Triassic‐Palaeocene zircons in the Bhainskati Formation demonstrate the arrival of Asian‐derived detritus in the Himalayan foreland basin in the Eocene based on available datasets. Our data indicate that (1) the timing of terminal India‐Asia collision was no later than the early‐middle Eocene in the central Himalaya, and (2) the Greater Himalaya served as a source for the Himalayan foreland basin by the early Miocene. When coupled with previous Palaeocene‐early Eocene provenance records of the Tethyan Himalaya, our new data challenge dual‐stage India‐Asia collision models, such as the Greater India Basin hypothesis and its variants and the arc–continent collision model. [ABSTRACT FROM AUTHOR]

Brittle shallow crustal faults typically develop a complex fault zone architecture with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of such domains is typically controlled by the strength and composition of the protoliths, physical conditions of deformation, fluid ingress, and diachronous fault growth in response to stress accumulation and co-seismic slip. Herein, we studied the microstructuremineralogy- kinematics of fault rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating red and gray gouge layers, and a single black gouge layer. Our results from electron microscopy and X-ray diffractometry indicate that the protolith of the red gouge layers is argillaceous sandstone, whereas that of the gray and black gouge layers is sandstone. Microstructures suggest an initially distributed deformation (aseismic creep), followed by a protracted brittle deformation event, and a later aseismic creep stage. The brittle stage is marked by progressive localization of stress, fracture development, cataclasis, frictional sliding, and seismic slips. The black gouge layer acted as the principal slip zone and exhibited ultrafine bands of micrometerscale slip zones with vapor escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during seismic slips. The preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence on seismic slip in the Nahan Thrust. We also conclude that heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths. [ABSTRACT FROM AUTHOR]

In the Bhagirathi River Transect of the Garhwal Himalaya, India, the existence of the Jhala Normal Fault (JNF) and its movement sense are disputed. The JNF has been considered either as part of the South Tibetan Detachment System (STDS) or as a distinct, more southerly discontinuity within the Higher Himalayan Crystalline Sequence (HHCS). Field studies reveal that the JNF lies entirely within the HHCS, with both the JNF footwall and hanging-wall preserving thrust-related shear markers within amphibolite facies HHCS rocks. Rare extensional shear markers are, however, observed at the base of the JNF hanging-wall. New U–Pb zircon rim and monazite SHRIMP ages of 33.8 ± 0.8 Ma and 30.7 ± 0.5 Ma obtained in this study represent the timing of metamorphism in the JNF hanging-wall and footwall, respectively. Together, the field and geochronological evidence suggest that during Eocene–Oligocene channel flow in the HHCS, the slow-moving marginal part of the channel representing the JNF hanging-wall was trailing its more rapidly extruding footwall, resulting in apparent normal-sense movement across the JNF. The intrusion of 21.4 ± 2.3 Ma (monazite U–Pb age) tourmaline-bearing leucogranites within the JNF hanging-wall testifies to its ongoing uplift as part of the exhuming Miocene HHCS channel. The absence of any metamorphic break or distinct extensional shear zone at the JNF indicates that it originated as an intra-channel discontinuity rather than a major lithotectonic boundary. Highlights: The normal/reverse sense of movement at Jhala Normal Fault (JNF) is controversial Melt proportion and shear intensity sharply decrease in JNF hanging-wall No extensional shear zone or break in metamorphic grade observed at JNF Pulsed channel in footwall causes apparent normal movement along JNF Normal movements along JNF and STDS are coeval with ~21 Ma leucogranite intrusion [ABSTRACT FROM AUTHOR]

When and how the collision between India‐Asia occurred continue to be debated. We report new paleomagnetic data (Ds = 158.8°, Is = 7.8°, ks = 81.3, α95 = 5.1°, N = 11 sites) from limestone of the Zongpu Formation Member I in the Tingri, which indicate that the Tethyan Himalaya was situated at 3.9 ± 2.6°S during 62‐59 Ma. This implies that Greater India was ∼900 km and that Tethyan Himalaya did not break off from India, which left a ∼2,000 km Neo‐Tethys Ocean that challenges the prevailing hypothesis that the initial collision of India–Asia occurred during this period. The drift rate of the India plate, with an average rate of 160 ± 28 mm/a during 60‐50 Ma, implies that the northern margin (e.g., the Tingri, Gyangze, Gamba, and Zanskar areas) of Greater India almost simultaneously collided with Asia at ∼50 Ma, quasi‐synchronously, and closing the Neo‐Tethys Ocean. Plain Language Summary: The India‐Asia collision is one of the essential, important geologic events in the past 500 million years, which not only has affected the land and sea patterns and topography patterns but also affected the global climate and biodiversity. However, when and how this collision occurred remains a long‐standing debate. The size of Greater India first needs to be considered in any discussion of the timing and process of the India–Asia collision. Greater India is the part vanished continent that has been subducted and compressed since the India‐Asian collision. We provide new paleomagnetic evidence that defines a ∼900 km wide Greater India and confirms that the collision between India and Asia did not occur during 62‐59 Ma. The drift rate of the India plate, with an average rate of 160 ± 28 mm/a during 60‐50 Ma, implies that the terminal collision between India and Asia occurred at ∼50 Ma, followed by the quasi‐synchronously closing of the Neo‐Tethys Ocean. Key Points: The Tethyan Himalaya was situated at 3.9 ± 2.6°S during 62–59 Ma, and the Neo‐Tethys Ocean still opened ∼2,000 kmThe size of Greater India was only ∼900 km and Tethyan Himalaya did not break off from IndiaThe terminal collision between India and Asia occurred at ∼50 Ma, followed by the quasi‐synchronously closing of the Neo‐Tethys Ocean [ABSTRACT FROM AUTHOR]

Felsic volcanic rocks of rhyolite to dacite composition, hosting copper mineralization, are exposed near Mundiyawas-Khera area within the Alwar basin of Proterozoic North Delhi Fold Belt. Distinct variations in grain size, mineralogical composition, texture, sulfide mineral associations, structure, and outcrop patterns of the felsic volcanic rocks are observed in the field. In this study, litho-facies analysis of felsic volcanic rocks around Mundiyawas-Khera copper deposit has been attempted for the first time to understand their spatial distributions and bearing on copper mineralization. Signatures of five distinct facies variations in felsic volcanic rocks are recorded based on the field and laboratory studies. These facies variations recorded in the study area are (a) vitrophyric felsic volcanics; (b) fine grained felsitic fiamme bearing tuff; (c) fiamme-bearing welded ash flow tuff; (d) poorly welded ash flow tuff; and (e) volcanic ash. Presence of glass shards in the study area indicates phreatomagmatic eruptions in an aqueous to sub-aqueous environment. Occurrences of massive sulfide associated with the felsic volcanic unit can also correlate with well-known volcanic hosted massive sulfide (VHMS) deposit and is taken as an additional evidence of felsic volcanic rock occurrences at a submarine environment. [ABSTRACT FROM AUTHOR]

The paper presents data on the U–Pb chronology of detrital zircon grains and radiogenic isotopic composition (87Sr/86Sr, ɛNd) from the Proterozoic clastic successions of the Garhwal‐Kumaon Lesser Himalaya representing the extended northern Indian cratonic margin, NW India. The Proterozoic Lesser Himalayan Basin in Garhwal‐Kumaon Himalaya is divided into two sectors, namely, Inner Lesser Himalaya (ILH) and Outer Lesser Himalaya (OLH) by a tectonic boundary, namely the Tons Thrust (TT). Age distribution from inner and outer sectors of the Lesser Himalaya shows that the U–Pb chronology of most of these zircons provides Palaeoproterozoic (between 1.6 and 1.8 Ga) to Neoproterozoic (800 Ma) ages. The age data suggest sedimentation of the Rautgara Formation (Damtha Group) of ILH continued till the Neoproterozoic (~850 Ma), which was earlier regarded as ≤1.6 Ga. Tracking the detrital U–Pb zircon ages in the near adjacent cratonic parts point towards Aravalli Orogen as the major source region. Whole‐rock ƐNd(0) values for ILH rocks range from −37.6 to −14.6 and for OLH it ranges from −19.6 to −6.7. More negative ƐNd values along with dominance of Neoarchean‐Palaeoproterozoic ages in ILH indicates supply from more evolved protolith or recycled sources and less negative ƐNd values with major Neoproterozic zircon ages from OLH supports for less evolved source rock. The change from more negative ƐNd to less negative ƐNd values progressively upward the stratigraphy can be due to a shift in source with time. Both U–Pb zircon and ƐNd supports for a continuous sedimentation model, rule out the presence of ~500 Ma unconformity within the LH and argues for separate evolution of the Lesser Himalayan Basin on the trailing edge of the extended north Indian craton in "Columbia" configuration. [ABSTRACT FROM AUTHOR]

Subsurface source boundary identification is a major step in the interpretation of potential field anomalies in geophysical exploration. We investigated the behavior of wavelet space entropy over the boundaries of 2D potential field source edges. We tested the robustness of the method for complex source geometries with distinct source parameters of prismatic bodies. We further validated the behavior with two datasets by delineating the edges of (i) the magnetic anomalies due to the popular Bishop model and (ii) the gravity anomalies of the Delhi fold belt region, India. The results showed prominent signatures for the geological boundaries. Our findings indicate sharp changes in the wavelet space entropy values corresponding to the source edges. The effectiveness of wavelet space entropy was compared with the established edge detection techniques. The findings can help with a variety of geophysical source characterization problems. [ABSTRACT FROM AUTHOR]

Migmatites of Higher Himalayan Crystallines (HHC) along different river valleys are present above the Vaikrita Thrust in the upper package. They have experienced anatexis and protracted growth of in situ melt between ca. 46 Ma and ca. 20 Ma. Melt-filled conjugate set of extensional crenulation cleavage (ECC) of late-stage deformation indicating extension parallel to the preexisting foliation (Sm) and shortening normal to foliation plane. The field evidence indicates that ECC was formed during partial melting and later filled by the melt. During partial melting, the deformation got enhanced because of the accumulation of strain due to the prolonged presence of melt. That caused the increase in the melt pressure eventually leading to crossing the critical limit and forming extensional crenulation cleavage and making it the conduit for the emplacement of melt within the migmatite zone. The phenomenon of melt-filled ECC is due to melt-enhanced deformation occurring at the time of orogen parallel extension. The presence of episodic fluxing of melt as evident from U-Pb ages of zircon is very well be correlated with the building up of partial melt leading to an increase and drop of the melt pressure due to draining followed by rising of melt from the system along with the newly formed ECC until the next event. This episodic nature of fluxing appears to be a convenient mode to extract melt from partially melted terrains of migmatites leading to chaotic structures. [ABSTRACT FROM AUTHOR]

Scapolite occurrences are widely observed in the metasedimentary rocks exposed around the Khetri Copper Belt and adjoining Nim ka Thana copper mineralized area in western India. Amoeboidal to well-developed and rounded/elliptical-shaped marialitic scapolite (Na-rich end-member) rich zones with variable Cl contents ranging from 1.0 wt % to 2.9 wt % have been identified in proximity to the ore-bearing hydrothermal fluid activity zones. Although scapolite is formed as a product of regional metamorphism in many places, in this study, we propose a strong possibility that scapolite was formed by hydrothermal ore-bearing fluid interaction with metasediments. The evidence of hydrothermal activity and Cl sourcing is attributed to (i) the absence of evaporite beds in the area and no Na-rich plagioclase as inclusions within the scapolite suggesting the formation of marialitic scapolite from sodic plagioclase in the metasediments with the interacting hydrothermal fluid; (ii) an epithermal to mesothermal hydrothermal fluid with moderate salinity responsible for the Cu mineralization that is ascribed to be the source of Cl for the formation of marialitic scapolite; (iii) diffusion of SO2 in the scapolite in close association with the sulfide mineral phase (chalcopyrite) supporting the involvement of ore-bearing fluid in the development of scapolite; (iv) the absence of zoned scapolite, the spatial distribution of scapolite in a particular lithology, the occasional incorporation of sulfur into marialitic scapolite and the texture/geometry in the scapolite suggesting a broad hydrothermal linkage instead of a pure metamorphic origin. [ABSTRACT FROM AUTHOR]

Ambient noise tomography is a powerful tool that has found increasing application in reservoir analysis and imaging. The Cachar fold belt in lower Assam, northeast India encompasses several wells under active hydrocarbon production, along with several dry wells. To overcome the lack of active seismic data over the entire fold belt, a passive seismic study was carried out to image the concealed three‐dimensional sub‐surface structures. The data were recorded from February to November 2011 by a network of 65 wideband seismometers spanning an area of about 40 × 60 km2. The data are crosscorrelated in the 2–5 s band, followed by phase‐weighted stacking to estimate noise correlation functions with surface wave signatures. The traveltimes picked from the frequency‐time analysis are utilized in a tomographic inversion for Rayleigh wave group velocities. The group velocity anomalies have a lateral resolution of ~ 3.5 × 5.5 km2 and variations of up to ±20%$\pm 20\%$ for each period. The group velocities are in turn inverted for S‐wave velocity distribution as a function of depth. The three‐dimensional S‐wave velocity tomograms reveal the tight anticlines and broad synclines, with high‐ and low‐velocity zones corresponding to structural highs and lows, respectively. The structural interpretation is supported for the part of the region with producing wells and covered by active seismic data, wherein the post‐stack time migrated seismic section shows anticlinal and synclinal features similar to those obtained from ambient noise tomography. The structures revealed by ambient noise tomography can help identify zones of interest to be targeted by active seismic surveys in the Cachar fold belt. [ABSTRACT FROM AUTHOR]

South China, India, and their derivative blocks preserve many similar magmatic and sedimentary records related to the tectonic transition from Rodinia to Gondwana. They provide crucial insights into not only the paleogeographic correlation between them but also the geodynamic mechanism for such a transition. Our new results, combined with published data from these blocks, reveal that South China remained linked with India at least from ca. 830 Ma to ca. 510 Ma and formed the South China-India Duo, which is located at the western margin of Rodinia. The identical magmatism and sedimentation reflect that double late Neoproterozoic rift systems in the South China-India Duo developed owing to the rollback of subducting oceanic slab beneath them. For example, an intracontinental rift developed along the Jiangnan-Aravalli-Delhi fold belt, which separated the Yangtze-Marwar block from the Cathaysia-Bundelkhand block. Another intra-arc rift developed contemporaneously along the northern and western margins of the Yangtze block, through the Marwar terrane of western India, and then into the Seychelles and Madagascar terranes. Such an intra-arc rift is the most feasible explanation for the common development of coeval arclike and extension-related magmatic rocks and extensional sedimentary sequences on the western margin of the South China-India Duo, in Seychelles and Madagascar, and even at other subduction zones. South China was finally separated from Indian Gondwana at ca. 510 Ma due to the opening of the Proto-Tethys Ocean. [ABSTRACT FROM AUTHOR]

A comprehensive data of whole‐rock and stable isotopic geochemistry along with microfossil assemblages of the carbonate rocks of the ophiolite mélange zone of the Indo–Myanmar orogenic Belt (IMOB), north‐east India are discussed, to determine the influence of terrigenous contamination during the formation of these carbonate rocks and also to understand their depositional environment and ages. These carbonate rocks contain a diverse fauna with the dominance of foraminiferal assemblages of planktonic foraminifera (Globotruncana sp. and Heterohelix sp., etc.) which indicates they were formed during the Santonian to Maastrichtian age. Based on chemical compositions, these carbonate rocks have been identified as limestone (CaO/MgO > 50.1) to slightly dolomitic limestone (9.1 < CaO/MgO < 50.1). Total rare earth element (REE) contents in these carbonates are variable (22.39–146.05 ppm). The Post‐Archean Australian Shale (PAAS)‐normalized REE + Y patterns of these carbonates exhibit seawater‐like REE patterns with LREE depletion and relative HREE enrichment with negative Ce anomalies (Ce/Ce* = 0.32–0.79) and positive Y (4.42–27.81 ppm) and Eu anomalies (Eu/Eu* = 1.11–1.86), suggesting that they were deposited under an oxygenated environment with contamination by hydrothermal activity. They are also depleted in δ13C 0/00 (PDB) (1.02–1.570/00) and δ18O 0/00 (PDB) (−6.37 to −9.00%) values which characterize marine precipitates. Eu anomalies and spread in negative δ18O 0/00 (PDB) values to a lesser extent of δ13C 0/00 (PDB) values of these carbonates suggest their formation was altered by diagenesis in the shallow marine environment. Our new whole‐rock and stable isotope geochemical characteristics, in conjunction with microfacies, suggest that the investigated carbonate rocks might have been formed in low‐energy environments, and deposited in neritic to bathyal palaeoenvironments during the Santonian to Maastrichtian interval. [ABSTRACT FROM AUTHOR]

The along‐strike heterogeneity in the tectonics of Himalaya is well recognized; however, the eastern segment of Himalaya is less studied compared to the other segments. This study combines the comprehensive field data, as well as the U–Pb geochronology of sandstone samples from the Indo‐Burma Range (east India). The study shows that the Lower Disang Formation was deposited in an open marine environment with a steep gradient during the Late Cretaceous to Mid‐Eocene, accommodating the detritus solely from an Indian source. Furthermore, during the Early to Late Eocene interval, the sedimentary basin changed to a shallow marine shelf when the sediments from Himalayan as well as trans‐Himalaya were deposited. The Upper Disang Formation must have been deposited just before or during the collision process between the Indian and Asian plates. After the India‐Asian collision, there was a remnant ocean, in which the sediments of the Barail and Surma groups were deposited in the shallow marine to deltaic environments. The Tethys Ocean completely dried out in the Late Miocene resulting in a change from marine to fluvial environments, which facilitated the deposition of the sandstones of the Tipam Formation. Based on the U–Pb ages, the Palaeogene basin history in the Indo‐Burma Range was unlike the central Himalaya, that is, the detritus were mainly sourced from the Burmese Plate and adjoining magmatic rocks. [ABSTRACT FROM AUTHOR]

The study of brittle deformation of the collisional mountains can explain its shallow crustal tectonic evolution and the palaeostress regime. The Main Boundary Thrust (MBT) zone in the western Arunachal Himalaya displays imbrication in the Permian Gondwana sequence between the MBT‐1 (/Bome Thrust/MBT‐Upper) in the north and MBT‐2 (/MBT‐Lower) in the south with consistent northerly dip. The Lower Gondwana rocks occur in the footwall of the MBT‐1 with the Proterozoic Bomdila Group in the hangingwall. The upper Gondwana rocks constitute the hangingwall sequence for the MBT‐2 with Neogene Siwalik rocks in the footwall. This article analyses palaeostress using brittle fractures in the Gondwana rocks that crop out for ~120 km2 in the study area. The fault‐bounded imbricate zone depicts eight brittle shear indicators and four sets of joints (J1 and J2: inclined and J3 and J4: subvertical). The signatures of the inherited pre‐Himalayan extensional deformation are preserved in the Lower Gondwana Miri Formation. The Bichom and Bhareli rocks exhibit brittle deformation features of the Himalayan Orogeny under strong ~N‐S compression. The palaeostress analysis of all joint sets indicates three phases of brittle deformation in the Gondwana and Siwalik rocks of the area. The subvertical joint sets and normal faults in the Miri Formation indicate a north‐northwest (NNW)‐directed extensional phase of the pre‐Himalayan origin. The inclined joint sets of the Bichom and Bhareli formations of the Gondwana sequence depict Himalayan orogeny with ~N‐S compressional phases. The third phase of brittle deformation in the Siwalik sequence depicts an east‐west (~E‐W) extension. The arc‐parallel extension in the frontal fold belt of the Arunachal Himalaya may be due to oblique India‐Asia collisional tectonics. [ABSTRACT FROM AUTHOR]

The present study analyzes available aeromagnetic and recently acquired ground gravity data to understand geothermal gradients covering parts of northeastern India. The scaled spectral analysis of aeromagnetic data has been adopted for computing the bottom magnetic interface depths that help to estimate geothermal source beneath thrust and fold belts. The database indicates that high geothermal gradients observed in northwestern part of the study area encompass uplifted Shillong Plateau and thrust belts. The geothermal gradients are gradually decreasing towards eastern portion of Inner Fold Belts comprising northern fringe of Indo-Burmese Ranges. An elliptical-shaped northeast-trending conspicuous high aeromagnetic anomaly zone coincides with gravity signature of high density mafic/ultramafic rocks that may correspond to carbonatite complex, which are potential host for rare earth elements (REE) mineralization. An attempt has been made to comprehend the relation between crustal lateral heterogeneities of aeromagnetic and gravity interfaces and subsurface structures, in conjunction with the nature of deformation and seismic characteristics of northeast India. The estimated Euler depth solutions and modelling results of gravity and aeromagnetic data reveal that substantial crustal heterogeneities in the form of mafic intrusions or underplating beneath Shillong Plateau and Indo-Burmese arc display higher seismicity. The high anomalous geothermal gradients coincide with mafic intrusive or underplated rocks beneath Shillong Plateau and thrust belts that may act as potential geothermal reservoir. Research highlights: High geothermal gradients observed in northwestern part of the study area covering uplifted Shillong Plateau and thrust belts. The results of geothermal gradient beneath thrust and fold belts of NE India vary between 14 and 22°C/km and the heat flow values are varying from 34 to 56 mW/m2. The conspicuous high aeromagnetic anomaly upheld with gravity signature of high density mafic/ultramafic rocks may be associated with carbonatite complex. The present study established the relation between the crustal lateral heterogeneities of aeromagnetic–gravity interface depths and seismic characteristics of NE India. [ABSTRACT FROM AUTHOR]

Characterizations of hydrocarbon microseepage-induced surface sediments are essential to utilize remote sensing and geochemistry as effective petroleum exploration tools. However, only few researchers have attempted spectroscopic and geochemical characterizations of microseepage-affected sediments and that too are focussed on arid or semi-arid climatic regions. This article aims to delineate the spectral, mineralogical and geochemical characteristics of the microseepage-affected sediments in Assam–Arakan Fold Belt (AAFB), NE India, dominated by intense precipitation. The analytical approach followed consists of diffuse reflectance spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. Reflectance spectroscopy reveals that microseepage-induced sediments are enriched in illitic clay and deficient in goethite than the unaffected regions. XRD studies support the presence of illitic clays in the sediments. Major element analyses indicate higher Al2O3 and K2O and lower Na2O and SiO2 in the altered sediments. Trace element patterns point out enrichment of V, Cu, Zn, Ga, Zr, and Mo and depletion of Li, Cr, Co, Ni, Rb, Sr, Sc, and Y in the microseepage-affected sediments. Normalized rare earth element (REE) patterns are similar for both the altered and unaltered sediments, but the former are deficient in REE contents. Though based on limited sample analysis, the study indicates microseepage affected and unaffected sediments differ in spectral and geochemical characters. This may help in strengthening the microseepage model for hydrocarbons in high precipitation areas and thereby may help in reducing exploration risks in the basin. Research Highlightst: The study uses spectroscopy and geochemistry to characterize hydrocarbon microseepage-induced sediments. Spectroscopic study indicates abundance of clay minerals and deficiency of ferric iron in microseepage-affected sediments. Geochemical analysis shows minor compositional differences between microseepage-affected and unaffected sediments in terms of major and trace element distribution patterns. [ABSTRACT FROM AUTHOR]

In India, Atomic Minerals Directorate for Exploration and Research (AMD), the oldest unit under the Department of Atomic Energy (DAE) is mandated with the exploration and augmentation of atomic mineral(s) resources of the country to support the Nuclear Power Programme (NPP) of India. AMD, till date have established four (04) generations of uranium metallogeny in India, distributed in 46 deposits and several other occurrences, major share of which are contributed by the globally unique, stratabound carbonate hosted, syn-diagenetic Tummalapalle Group of deposit (∼59%), metamorphite type deposits along the Singhbhum Shear Zone (∼21%), unconformity related deposits in northern part of Cuddapah Basin (∼5%) and sandstone type deposits in Cretaceous Mahadek Basin (∼6%). Besides these, metasomatite type deposits along the North Delhi Fold Belt (4%) and granite related-structurally controlled, high grade deposits in Bhima Basin (2%) along with some small hydrothermal/metamorphite/migmatite type deposits in Aravalli, Kotri-Dongargarh, Higher Himalaya belts and Chhotanagpur Granite Gneiss Complex contribute to the national inventory which stands at 3,76,000 tonne uranium oxide. AMD have stockpiled substantial quantities of Nb-Ta, Be and Li minerals needed for the NPP of India from prospective pegmatite belts in the country. AMD have established immense potential for REE mineralization in carbonatite complexes of Ambadungar in Gujarat and per-alkaline granite-rhyolite of Siwana Ring Complex, Barmer district, Rajasthan. Exploration along the coastal and riverine placers of the country has established 130 heavy mineral placer deposits with substantial thorium and REE resources. The progressive advancements by AMD through adaptation of integrated multidisciplinary exploration techniques coupled with major thrust in airborne/ground geophysics and expansion of state-of-the-art analytical facilities have facilitated systematic planning for future augmentation of atomic mineral(s) resources. Exploration inputs are envisaged to be intensified in the brownfield target areas for resource augmentation while R&D and phase-wise exploration inputs will be focused for developing the identified greenfield areas following a planned roadmap to ensure self-sufficiency in atomic mineral resources for sustained growth of the NPP to reach the target of net zero carbon emissions by 2070. [ABSTRACT FROM AUTHOR]

The sandstones of the Bhuban Formation (Miocene) exposed in the Jampui hills of Tripura Fold Belt, India has been studied for provenance, tectonic setting and paleoclimate using the petrography, heavy mineral assemblage and clay mineral studies. The detrital mode of the sandstones indicate that the sediments consists predominantly of detrital quartz (avg. 92.19%), with subordinate proportions of feldspar (avg. 4.55%) and rock fragments (avg. 3.25%). The provenance discrimination diagrams depict the derivation of the detritus from both plutonic and metamorphic source rocks having affinity to continental block provenance. The plots of QFR and semi quantitative weathering index of the clastics support a moderate degree of paleoweathering in the source area under humid climatic condition. The presence of zircon, tourmaline, rutile, kyanite, epidote, sillimanite and garnet in the Bhuban sandstones is ascribed to the provenance of plutonic and metamorphic rocks. Dominance of zircon in the heavy mineral assemblage and the ZTR index values (75.13% avg.) indicate towards moderate degree of maturity with long distance of transport. The study of the shale samples by XRD and SEM analyses shows the dominance of illite over kaolinite and montmorillonite. The presence of minor amount of montmorillonite may suggest the addition of tuff and volcanic ash in the sediments under alkaline environment. [ABSTRACT FROM AUTHOR]

Northeast India is a seismically active region, with frequent major earthquakes. The impact of seismicity can be assessed by using the values fundamental resonant frequency (F0) and the site amplification factor (A0) to derive the seismic vulnerability index (Kg). In 2011, a network of 65 stations was installed in the Cachar fold belt, Assam to continuously record data for a continuous period of eleven months. In this study, the horizontal-to-vertical spectral ratio (H/V) method is employed to identify the resonant frequencies and site amplification factors using ambient noise data to assess the seismic hazard potential in the Cachar fold belt. The amplification factors ranged between 1.15 to 6, while the resonance frequencies varied between 0.25 to 5 Hz. The ambient noise interferometry is utilized to estimate surface waves between station pairs and invert the group velocity dispersion curves for the near-surface S-wave velocities. The S-wave velocity for the basement rocks varied between 600 to 1600 m/s. The S-wave velocities are in turn used to derive an empirical relationship between resonant frequency and alluvium thickness valid for the study region. It is found that stations located at regions with subsurface anticlines had a relatively high resonance frequency and low amplification factor, whereas receivers in synclinal locations and with a thick layer of alluvium sedimentary deposits have a substantially higher site amplification factor and lower resonant frequencies. Further, it is observed that regions with a substantial alluvium sediment cover and near the course of the Barak river are more seismically vulnerable and susceptible to natural hazards. [ABSTRACT FROM AUTHOR]

With the most prominent topography on Earth, the Tibetan Plateau has profound influences on the hydrologic cycle and climate dynamics in Asia. However, the regional uplift history of the Tibetan Plateau remains highly debated. Here, we use europium anomalies and LREE/HREE ratio in detrital zircon, combined with whole-rock La/Yb N ratio, to constrain the crustal thickness and topographic evolution of the Gangdese mountains in southern Tibet. Europium anomaly, controlled by pressure-sensitive minerals plagioclase and garnet, has been shown to correlate with crustal thickness. Our results reveal contrasting crustal thickening histories of the eastern (east of ∼88°E) and western (west of ∼88°E) parts of the Gangdese. Prior to the India-Eurasia collision (∼60–55 Ma), the crust of the eastern Gangdese thickened continuously from ∼40 km to nearly 60 km, while the western Gangdese maintained a mildly thickened crust of ∼50 km. Both parts underwent substantial thinning (to 40–45 km) immediately before the India-Eurasia collision, but the eastern Gangdese re-thickened rapidly afterwards. In the western Gangdese, post-collisional thickening was delayed until ∼40–20 Ma, which resulted in a mild elevation of ∼2000 m for most areas of southern Tibet until the late Oligocene. This diachronous uplift of the Gangdese after collision may have resulted from more mantle-derived magma additions in the eastern Gangdese before collision, which could rheologically weaken its lithosphere. We suggest that the diachronous uplift of the Gangdese after the India-Eurasia collision may have dominated the moisture transfer pattern across southern Tibet through the late Eocene and intensified the South Asian monsoon through the Oligocene-Miocene boundary (∼25–20 Ma). • Eastern and western Gangdese show different crustal thickening histories. • Uplift of western Gangdese was delayed by at least 20 Myr after collision. • Diachronous uplift of Gangdese might impact the regional and South Asian climate. [ABSTRACT FROM AUTHOR]

Major element compositional spectrum, formation in diverse magmatic and metamorphic rocks, and relative stability during sediment transport, burial, and diagenesis make garnet an important indicator of sedimentary provenances. However, a fundamental question yet to be answered is whether the low-grossular–high-pyrope detrital garnets reflect source characteristics or record sedimentary processes. To address this problem, we have analysed garnets from the beach sands of the East Coast of India and in the source lithologies of the catchment area within the Eastern Ghats Mobile Belt. Investigated garnets show a broad compositional spectrum; most garnets are dominated by almandine 49 – 86% with variable contents of pyrope 9 – 47% , minor grossular 0.8 – 15% , and low spessartine 0.5 – 4%. Using multiple proxies, including a state-of-the-art machine-learning-based garnet discrimination scheme, we found that 96 % of analysed beach sand garnets are of metamorphic origin and 4 % igneous. Amongst the metamorphic garnets, 85 % were derived from granulite facies, 5 % from eclogite/ultrahigh-pressure facies, 5 % from amphibolite facies, and 1 % from blueschist/greenschist facies rocks. Concerning host-rock bulk composition, 93 % of garnets belong to intermediate–felsic/meta-sedimentary class and 7 % belong to the mafic category. The chemistries of the beach sand detrital garnets of the East Coast of India are readily coupled with the compositions of the garnets from source rock lithologies of the Eastern Ghats Mobile Belt. Very low-grossular garnets are derived from meta-pelitic rocks (khondalites) and mafic granulites, whereas slightly grossular-rich garnets are derived from charnockites. The present study indicates that the low-grossular–high-pyrope detrital garnets are vital signposts of sedimentary provenances. We advocate that the high P-T granulite facies mobile belts are distinctive sources for the low-grossular–high-pyrope detrital garnets. • High P-T granulite facies mobile belts are distinctive sources for the low-grossular–high-pyrope detrital garnets. • Alm-Prp garnets are derived from meta-pelites, migmatites, and mafic granulites; Alm-Prp-Grs garnets are from charnockites. • The source for the Bengal Fan low-grossular–high-pyrope garnets possibly lies in the Eastern Ghats Belt. • Sedimentary basins with high proportions of low-grossular–high-pyrope garnets can be linked to (palaeo-) orogenic belts. [ABSTRACT FROM AUTHOR]

The fold belts, which border the cratons, are the building blocks for understanding the origin and modification of older continents. The Aravalli-Delhi Fold Belt (ADFB) in northwestern India provides evidence of crustal block accretion due to continental collision, whereas Malani Igneous Suite (MIS) modified with widespread magmatism. Imaging of upper mantle structure and lithospheric modifications, if any, of such regions has been intricate owing to reworking by subsequent superimposed tectono-magmatic processes. We applied a 2D modelling approach to model the lithospheric architecture along a 1000-km long WNW-SSE geotransect across northwestern India, which has been deformed in the past. Our modelling technique combines terrestrial gravity anomaly, heat flow data, satellite-based geoid, and topographic datasets using the basic premise of thermal steady-state and local isostasy. The overall 38 to 40 km thick crustal geometry underneath the Marwar Block had the maximum lithological heterogeneity. The region surrounding the MIS is characterised by 8–10 km thick high-density (2.78 g/cm3) sills deposited in the upper crust down to 9 km depth and another 10–15 km thick high-density (3.05 g/cm3) mafic mantle material near the Moho. About 42 km thick crust, including an 8 to 10-km thick high-density (3.05 g/cm3) underplated layer at its bottom, characterises the high-relief ADFB. The Vindhyan region of Bundelkhand craton is defined by a ∼ 1 km thick trap, having Moho extending at a depth of ∼40 km. The lithospheric thickness varies substantially from ∼143–168 km underneath the Marwar block, which thins to ∼135 km under the ADFB and thickens gradually to ∼150–165 km beneath the Vindhyan region. Substantial crustal density differences in distinct crustal domains, when integrated with the thin lithosphere, reinforce the concept that tectono-magmatic processes might have modified the lithosphere in NW India. [Display omitted] • A typical layered structure indicates no evidence of crustal modification underneath the Bundelkhand craton • Density inhomogeneity in Marwar Block and ADFB corresponds to crustal modification through collision and mafic intrusion. • Thermal perturbations and collisional tectonics are the twin causes of lithospheric modification in northwest India. [ABSTRACT FROM AUTHOR]

• Mid-Neoproterozoic magmatic and sedimentary rocks exposed in the Zhangbaling Uplift. • All the magmatic rocks were characterized by the A-type granites. • The variable geochemical signatures reveal the breakup of Rodinia supercontinent. • The Yangtze Craton should be located in the edge of Rodinia supercontinent. Rocks recording Neoproterozoic geological events related to the Precambrian supercontinent cycle are widely distributed in the Yangtze Craton, and can be used to investigate the evolution of the craton in the context of supercontinent assembly-breakup. However, the paucity of systematic research into Neoproterozoic magmatism and sedimentation in the northeastern Yangtze Craton has hindered our understanding of the evolution of Yangtze Craton. In this study, middle Neoproterozoic magmatic and sedimentary rocks from the Zhangbaling Uplift were analysed to investigate the tectonic development of the northeastern Yangtze Craton and its relationship to the evolution of the Rodinia supercontinent. The results of petrographic and geochronological analyses indicate that the samples record two main stages of silicic magmatism (807–787 Ma granitic gneisses and 757–740 Ma quartz-keratophyres), and that proximal sedimentary rocks deposited at 662 Ma. The granitic gneisses and quartz-keratophyres exhibit the geochemical characteristics of typical A-type granites, with high contents of total-alkalis and HFSEs, suggesting an extensional tectonic setting during their formation. The main residual minerals in the rocks are garnet (gneisses) and plagioclase (keratophyres), reflecting the different depths of partial melting of the source rocks. The gneisses and keratophyres have contrasting zircon Lu-Hf isotopic components (ε Hf (t) = −29.9 to −11.8; −5.8 to 2.5). These lines of evidence imply a crustal structure comprising ancient lower crust and juvenile middle-upper crust. The particular crustal structure is obviously different from adjacent Dabie-Sulu orogenic belt, hinting Zhangbaling Uplift is an in-situ uplift zone. The magmatic evolution recorded by the granitic gneisses and quartz-keratophyres reflects a progression through different stages of continental rifting, from initial extension to mature rifting during the breakup of Rodinia supercontinent. In addition, the prominent affinity between low δ18O magmatism and sedimentation in the Yangtze and India Craton implies that the Yangtze Craton was located on the margin of Rodinia supercontinent prior to breakup. [ABSTRACT FROM AUTHOR]

Knowing the pre-collisional extent of the northern Indian Plate margin ('Greater India') is vital to understanding the tectonic evolution of the India–Asia collision and the formation of the Himalayan–Tibetan orogen. However, suitable geological units for palaeomagnetic investigations along the Himalayan belt are limited, which makes it difficult to reconstruct Greater India during the pre-collisional period in Late Cretaceous to Palaeogene. Often the palaeomagnetic results from the Zongpu Formation at Gamba in southern Tibet (∼88.5°E) were used for estimates of Greater India, but their validity was recently questioned. As a contribution to closing the data gap, we performed a palaeomagnetic study of the Palaeocene/Lower Eocene Dibling limestone (DL) in the western Tethyan Himalaya of Zanskar (34.0°N/76.6°E). The results from 27 sites revealed a well grouping (k  = 71.7) syntectonic magnetization with best grouping at 52 per cent unfolding. The remagnetization of the DL was acquired shortly after ∼54 Ma, at the latest at ∼49 Ma, and is probably carried by fine-grained magnetite formed during the early orogenic phase. Assuming proportional tilting of the fold limbs, the corresponding palaeolatitude of 11.8 ± 2.4°N suggests a maximum Greater India extent of 810 ± 420 km and a first continental contact with the southern Eurasian margin at ∼12°N in the western part of the suture zone. The tectonostratigraphic equivalence of the DL with the Zongpu Formation at Gamba and a great similarity in their magnetic properties supports a secondary origin of the Gamba results. Through understanding the mechanism of remagnetization in the DL, an early orogenic remanence acquisition is also indicated for the Zongpu Formation, and thus the Gamba results deserve further credit for Greater India reconstructions. However, we note a large inconsistency of the available Late Cretaceous and Palaeogene palaeolatitude data from the Tethyan Himalaya by up to ∼20°, corresponding to differences of up to ∼2000 km in the size of Greater India. These discrepancies require further palaeomagnetic work in the Tethyan Himalaya, and in particular we recommend comparative studies at same locations and of same units. [ABSTRACT FROM AUTHOR]

Meta-granites of the South Delhi Fold Belt, northwestern India, contain spectacular reaction textures formed during the metamorphic replacement of primary minerals. Textural relationships imply that amphibole was replaced sequentially in two stages. Epidote + titanite + quartz symplectite formed syn-tectonically on amphibole grain boundaries/fractures, followed by post-deformational growth of euhedral garnet overprinting amphibole grains. Besides occurring as symplectite grown during deformation, titanite in this rock also developed as a post-tectonic corona around magnetite. Parent magnetite contains exsolutions of ilmenite and/or ultrafine lamellae of Ti-rich oxide (Ti-oxd). Textures involving coronal titanite suggest their formation through a magnetite + ilmenite(/Ti-oxd) + plagioclase → titanite reaction. Compositional attributes and the calculation of the gain versus loss of components during the reaction suggest that the Mn2+ for garnet (XSpss = 0.23–0.29) that grew replacing amphibole was supplied by ilmenite (Mn2+ is 0.118–0.128 apfu) as it disintegrated to form coronal titanite. The redistribution of components between the metamorphic reaction sites connects the texturally unrelated domains and suggests that these zones were in chemical equilibrium during metamorphism. We estimated the P–T conditions of metamorphism for these post-tectonic assemblages as ∼650–700 °C from pseudosection modelling and conventional thermometry. Zircon data from this study suggest that the granitic rock crystallized at 988.8 ± 8.8 Ma. We propose that the metamorphic phases replaced the primary minerals during the mid Neoproterozoic tectonic activity reported from this terrane. The syn-tectonic symplectitic assemblage formed as the temperature increased during prograde metamorphism, and the post-tectonic minerals developed at peak conditions following the cessation of deformation. [ABSTRACT FROM AUTHOR]

Under the shadow of the far-field effect of the India-Eurasia collision, the Tianshan orogenic belt underwent tectonic re-activation in the Cenozoic, accompanied by strong tectonic deformation and frequent large earthquakes. Bounded by two rigid cratonic blocks located in its north and south, a series of marginal foreland fold-and-thrust belts are developed within the Tianshan orogenic belt and continue to develop to the bilateral pull-apart basins. Meanwhile, the faults in the orogenic belt are reactivated. The deformation caused by thrust-related structure accounts for larger than 50% of the total convergence of the Tianshan Mountains, which results in the most active structure with large earthquakes in the Tianshan area. Therefore, it is of great significance to study the dynamic process of the newly generated and reactivated thrust-nappe structures in Tianshan orogen via numerical modeling. This paper selects a classical cross-section profile in the western segment of the Southwest Tianshan Mountains, which contains the Kalpin-Maidan-Nalati-Kemin fault system from the south to the north. We attempt to establish a two-dimensional plane strain, viscoelastic finite element model, by treating the regional faults as a whole fault system and considering the topography, fault geometry, and GPS data. The displacement and stress fields of the model are retrieved, the short-term cumulative deformation field of the overall fault system is analyzed, and the rate of Coulomb failure stress change of each fault is also considered. The results show that the deformation is concentrated in the middle and southern parts of the Southwest Tianshan Mountains. In contrast, the deformation of the Kemin fault in the north is relatively small. According to the Coulomb failure stress changes of these four faults and the historical earthquake catalog, the potential seismicity of each fault is qualitatively analyzed. Our preliminary results suggest that the possibility of large earthquake occurrence is higher in the Kalpin fault, Maidan fault, and Nalati fault but lower in the Kemin fault in the near future. [ABSTRACT FROM AUTHOR]

The India-Asia collision is the most spectacular, recent, and still active tectonic event of the Earth's history, leading to the uplift of the Himalayan-Tibetan orogen, which has been explained through several hypothetical models. Still, controversy remains, such as how and when it occurred. Here we report a paleomagnetic study of Cretaceous-Tertiary marine sediments from the Tethyan Himalaya (TH) in the Hazara area, north Pakistan, which aims to constrain timing for the onset of the India-Asia collision and to confirm the validity of already proposed models, particularly in western Himalaya's perspective. Our results suggest that the TH was located at a paleolatitude of 8.5°S ± 3.8° and 13.1°N ± 3.8° during the interval of ca. 84-79 Ma and 59-56 Ma, respectively. A comparison between paleopoles obtained from the current study and coeval ones of the India Plate indicates that the TH rifted from Greater India before the Late Cretaceous, generating the Tethys Himalaya Basin (THB). Our findings support a model for a multi-stage collision involving at least two major subduction systems. A collision of the TH with the Trans-Tethyan subduction system (TTSS) began first in Late Cretaceous-Early Paleocene times (ca. 65 Ma), followed by a later collision with Asia at 55-52 Ma. The onset of the collision between the TH (plus TTSS) and Asia could not have occurred earlier than 59-56 Ma in the western Himalaya. Subsequently, the India craton collided with the TH, resulting in the diachronous closure of the THB between ca. 50 and ca. 40 Ma from west to east. These findings are consistent with geological and geochemical evidence and have a broad implication for plate reconfigurations, global climate, and biodiversity of collisional processes. [ABSTRACT FROM AUTHOR]

Erinpura Granites form the basement for the Neoproterozoic Malani Igneous Suite in NW India. Based on this study, the Erinpura granites can be divided into Erinpura-East (gneissic fabric), a belt parallel to the southern sector of the South Delhi Fold Belt, and Erinpura-West (magmatic fabrics). EPMA dating on monazites gives a time frame of 890 to 860 Ma for crystallization of both types. The geochemically homogeneous peraluminous S-type granites with εNd values of −2.1 to −10.8 are interpreted as melting products of Archaean crust. REE pattern follows the pattern of the average continental crust, but Erinpura-East samples are more fractionated with steeper HREE depletion indicating melting in a deeper crustal level. A thermal pulse between 835 and 820 Ma constrains the timing of deformation in granite-gneisses during uplift along thrust planes, coeval with shear-bound exhumation of high-grade metamorphic rocks and initiated by delamination of the lower crust in this southern sector of the South Delhi Fold Belt. This is in contrast to the northern sector of the SDFB with arrested orogenic development and without considerable delamination or erosion of the lower crust. Latest movement related to the 200 Ma tectono-magmatic history overlaps with initiation of rifting during the Malani igneous event. A change from S-type to A-type granites and shift of isotopic signatures to εNd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process. [ABSTRACT FROM AUTHOR]

Digboi oil field, the oldest continuously producing oil field in India, is known to occur in an anticlinal trap (Digboi anticline) in the hanging wall of the frontal most thrust (Naga thrust) of Assam Arakan fold thrust belt. Since its discovery in 1890, about a thousand wells have been drilled in the Digboi anticline. The kinematic evolution of the Digboi anticline, however, is not well understood. In this article a geometrically valid balanced cross section and derive an internally consistent evolutionary model for the Naga thrust and related Digboi anticline are presented. It is shown that the Digboi anticline is a ramp anticline and can be best modelled as a fault-propagation fold with a high-angle breakthrough. There are two possible solutions (Model 1 and Model 2) to model the seismic events (sub-thrust high) in the footwall of Naga thrust. Model 1 considers the sub-thrust high to be real and conjectures a buried ramp anticline (Kusijan Anticline). Model 2 considers sub-thrust high to be a velocity pull up and the footwall rocks are undeformed. Both the models are geometrically and kinematically valid. Model 1 accommodates a total shortening of 32.9% while Model 2 accommodates a total shortening of 30.1%. From the present structural work and interpretation of seismic profiles, possible loci of branch line of Naga thrust along Digboi oil field is also traced. [ABSTRACT FROM AUTHOR]

Fragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia's break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings. [ABSTRACT FROM AUTHOR]

Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north‐east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt‐I and basalt‐II. Basalt‐I type shows the sub‐alkaline character with Nb/Y < 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB‐type mantle source. Investigated samples of basalt‐II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y > 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume‐related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings. [ABSTRACT FROM AUTHOR]

The article reviews the history and accomplishments of CSIR-NGRI over the past 60 years, related to elucidating the seismic structure of the crust and lithospheric mantle of the Indian shield. Extensive investigations have been carried out in diverse geological and tectonic provinces of India, employing seismic reflection, refraction/wide-angle reflection and passive seismology to decipher (a) the evolution of the Indian plate through geological time, (b) hazard and its mitigation and (c) accumulation and disposition of natural resources. These endeavours entailed the application and development of state-of-the-art methodologies. Synthesis of the results from active and passive seismology reveals that the thickness of the crust varies between 28 and 65 km in the Kachchh and Aravalli regions respectively, consistent with their evolutionary histories. The thickest crust is observed in the western Dharwar craton (WDC) and the shallowest lies in the west coast. The crust in the shield region is mostly thicker, while it is thin beneath the rift zones. Results from coincident reflection and wide-angle seismic reflection studies broadly suggest a three-layered crust with magmatic underplating. Interestingly, the seismic sections traversing the Aravalli fold belt, central Indian suture zone, Dharwar craton and Southern Granulite Terrain (SGT) depict paleo-collision and subduction environments. The diverse character of the Moho, crustal fabrics and structure in different geological provinces indicate that contrasting tectonic environments might have influenced their evolution and support the hypothesis that plate tectonic processes were operative since Neoarchean. The thickness of the lithosphere estimated from receiver functions varies from 80 to 140 km. An undulation in the Lithosphere Asthenosphere Boundary reveals evidence for a flexure on a regional scale, owing to the continental collision of the Indian and Asian plates. However, the lithospheric thickness derived from surface wave dispersion studies is somewhat larger, ranging from 100 to 250 km, with some body wave tomographic studies suggesting it to be ∼400 km, in consonance with the concept of Tectosphere. The thickness values derived from both the methods agree at a few locales such as the Eastern Dharwar Craton, SGT, Cambay, Singhbhum and western DVP. However, a broad disagreement prevails in WDC and northern part of the Indian shield where surface wave tomography reveals the thickness of lithosphere to be 140 to 200 km. [ABSTRACT FROM AUTHOR]

Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt <1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries. [ABSTRACT FROM AUTHOR]