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      Volume 128, Issue 8

      December 2019

    • Delineating groundwater prospect zones in a region with extreme climatic conditions using GIS and remote sensing techniques: A case study from central India


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      Sustainable groundwater management of an extreme climatic region is very important from both social and economic point of view. This study attempts to delineate the groundwater potential zones of Sonepur district, Odisha, which falls under an extreme climatic region, using remote sensing, geographical information system and Saaty’s analytical hierarchy process (AHP). Different ancillary data, multiple data sets obtained from LANDSAT 8 OLI and ASTER Level-1T were used in conjunction with Cartosat-1 imagery to study the detailed physical attributes of the study area and to prepare the groundwater prospect map using a weighted overlay method in ArcGIS 10.4 software. The AHP technique was used for determining the weights of each layer. From the groundwater prospect map, it was found that 52% of the area belongs to the moderate groundwater prospect zone followed by good to very good, very good to excellent (32%) and poor (16%) groundwater prospect zones. Statistical analysis of the number of existing wells in each of these water potential zones and their water level was used to verify the accuracy of the water potentiality estimated in this study. The groundwater potentiality map prepared as a part of this study would serve as an important tool for identifying suitable zones for rainwater harvesting and also for managing groundwater abstraction for a safe and sustainable water supply.

    • Strong motion generation area modelling of the 2008 Iwate earthquake, Japan using modified semi-empirical technique


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      The Iwate–Miyagi earthquake (Mw 6.9) of 14 June 2008 is one of the largest intraplate earthquakes that struck north-east Japan. This earthquake has produced the largest peak ground acceleration (PGA) ever recorded. The acceleration values 4022 and 1036 gal were observed at the surface and borehole accelerometers of IWTH25. To understand the cause of this extremely large acceleration, it is highly essential to obtain the detailed rupture process of Iwate–Miyagi earthquake. The present paper estimates the rupture model for this earthquake using the modified semi-empirical technique (MSET). The detailed analysis proposes one strong motion generation area (SMGA) in the rupture plane and nucleation point in the extreme western corner of the SMGA. Using this estimated source model, a satisfactory match is observed between the simulated and actual records. The quantitative analysis of these waveforms provides an almost 1:1 match for PGA values. Furthermore, the variation of these PGA values with epicentral distance shows similar attenuation rate. These results confirm the reliability of MSET and the estimated source model of this earthquake. To the best of our knowledge, this study is the first to model SMGAs in the rupture model using MSET and provides sufficiently reliable information which will be useful for seismic hazard prevention management.

    • Analytical solution for solute transport from a pulse point source along a medium having concave/convex spatial dispersivity within fractal and Euclidean framework


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      In the present study, analytical solutions of the advection dispersion equation (ADE) with spatially dependent concave and convex dispersivity are obtained within the fractal and the Euclidean frameworks by using the extended Fourier series method. The dispersion coefficient is considered to be proportional to the nth power of a non-homogeneous quadratic spatial function, where the index n is considered to vary between 0 and 1.5 so that the spatial dependence of dispersivity remains within the limit to describe the heterogeneity in the fractal framework. Real values like n ¼ 0.5 and 1.5 are considered to delineate heterogeneity of the aquifer in the fractal framework, whereas integral values like n = 1 represent thesame in the Euclidean sense. A concave or convex variation is free from demanding a limiting value as in the case of linear variation, hence it is more appropriate in the ambience of many disciplines in which ADE is used. In this study, concentration at the source site remains uniform until the source is present and becomes zero once it is annihilated forever. The analytical solutions, validated through the respective numerical solutions, are obtained in the form of an extended Fourier series with only first five terms. They are convergent to the desired concentration pattern and are stable with the Peclet number. It has been possible because of the formulation of a new Sturm–Liouville problem with advective information. The analytical solutions obtained in this paper are novel.

    • A new occurrence of two-pyroxene granulites at Chicholi from Betul supracrustal belt in Central Indian Tectonic Zone (CITZ), MP, India


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      We report the new occurrence of two-pyroxene granulites from Chicholi, the Betul Group of the Central Indian Tectonic Zone (CITZ). The common mineral assemblage observed within different thin sections is orthopyroxene–clinopyroxene–hornblende–plagioclase–biotite–quartz. The textural relationship of these mineral phases shows the reaction: hornblende + quartz = orthopyroxene + clinopyroxene + plagioclase. The estimated $P–T$ condition of metamorphism of the two-pyroxene granulites is $901 \pm 30^{\circ}\rm{C}$ and $8.68 \pm 1.4$ kbar.

    • Quality of local scale surface weather analogs over the north-west Himalaya (NWH), India


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      An analog ensemble system was developed for the realisation of local-scale surface meteorological variables for independent test data (test data) at six stations over the north-west Himalaya (NWH), India. Extreme values (the maximum value and the minimum value) and the mean value in 10 analog days (the analog mean) and the climatological mean of each surface meteorological variable were compared with its corresponding observed values on the same day ($d0$, lead time 0 hour (h)), $d1 (d0 + 1$, lead time 24 h), $d2 (d0 + 2$, lead time 48 h) and $d3 (d0 + 3$, lead time 72 h) of test data. Pearson correlation coefficients(CCs), Mean Absolute Differences (MADs) and Root Mean Square Differences (RMSDs) of the extreme values in analog days, and the analog mean and climatological mean of each meteorological variable on $d0$ with its corresponding observed values on $d0, d1, d2$ and $d3$ of test data were computed at six stations over the NWH. CCs of extreme values in analog days and the analog mean of each meteorological variable on d0 with its observed values on $d0, d1, d2$ and $d3$ were found to be higher than the CCs of the climatological mean of each meteorological variable on d0 with its observed values on $d0, d1, d2$ and $d3$. MADs (RMSDs) of extreme values in analog days and the analog mean of each meteorological variable on $d0$ with its observed values on $d0, d1, d2$ and $d3$ were found to be lesser than the MADs (RMSDs) of the climatological mean of each meteorological variable on d0 with its observed values on $d0, d1, d2$ and $d3$. However, the MADs (RMSDs) of the extreme values of each meteorological variable in analog days were found to be higher than the MADs (RMSDs) of its analog mean. Results show that the analog mean of each meteorological variable holds better predictive skill than the extreme values in analog days and its climatological mean. MADs (RMSDs) of different surface meteorological variables in surface weather analogs comparable to Mean Absolute Errors (MAEs) and RootMean Square Errors (RMSEs) for their prediction with the help of different types of weather forecast models show that the surface weather analogs hold good promise for the local-scale prediction of surface meteorological variables over the NWH.

    • Stable chlorine isotopes in saline springs from the Nangqen basin, Qinghai–Tibet Plateau: Brine genesis and evolution


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      Chlorine isotopes can be used to study the evolution of different fluids, sources and the causes of various related deposits. In this study, Cl concentrations and chlorine isotope ($\delta^{37}\rm{Cl}$, IAEA ISL-354 NaCl standard) values were determined for brine samples from Nangqen basin, located on the southern boundary of the Qinghai–Tibet plateau to study the source and the processes of these saline springs. The results demonstrated that the saline springs are distributed around a fault or fault zone, with a high average salinity of 228.30 g/l and flow rates ranging from 1.7 to 0.01 l/s. $\rm{Na}^{+}$ and $\rm{Cl}^{-}$ are the predominant cations and anions, respectively, accounting for more than 90% of the total. The $\delta^{37}\rm{Cl}$ values range from -1.55 per thousand to +0.97 per thousand, and the Cl/Br ratios are from 1739 to 175,260. Coupled with the previous H, O and B isotope compositions ($\delta\rm{D}$, VSMOW2 standard, ranges from -100.91 per thousand to -132.98 per thousand, $\delta^{18}O$, VSMOW2 standard, from -12.88 per thousand to -16.05 per thousand and $\delta^{11}B$, NIST 951 standard, from +3.55 per thousand to 29.59 per thousand), it can be interpreted that the saline springs are mainly the result of the dissolution of halite hosted in mudstone and volcanic country rocks.

    • Spatio-temporal variability of binary weather patterns and precipitation amounts of short time intervals during winter period over the north-west Himalaya (NWH)


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      Spatio-temporal variability of binary weather patterns (precipitation event/no-precipitation event) and precipitation amounts of short time intervals of 15, 24, 48 and 72 hours (h) are examined by analysing data on the observed precipitation amount of 3377 common days of different winters (winter 1993–winter 2015) at 12 stations in the north-west Himalaya (NWH). Surface meteorological variables over the NWH are collected daily at 0300 and 1200 UTC and data on the precipitation amount collected daily at 0300 UTC are taken to conduct this study. Data on the precipitation amount collected at 0300 UTC daily represent the cumulative precipitation amount of a short time interval of the previous 15 h (1200–0300 UTC) hence the precipitation amount of the 15 h time interval is considered in addition to the precipitation amounts of 24, 48 and 72 h time intervals to examine the Spatio-temporal variability of the precipitation amounts at 12 stations over the NWH. The spatio-temporal variability in the binary weather patterns of short time intervals is examined by computing the normalised percentage differences in the observed precipitation events of short time intervals at 11 stations from corresponding observed precipitation events at a reference station and Spatio-temporal variability in the precipitation amount of short time interval at 12 stations is examined by computing Mean Absolute Differences (MADs) and Root Mean Square Differences (RMSDs) of observed precipitation amounts of short time intervals at each station from corresponding observed precipitation amounts at a reference station. Normalised percentage difference in precipitation events and MAD (RMSD) of the precipitation amount of 24 h time intervals at 11 stations from a reference station fall in the range $(-) 50.0%–(+) 20.7%$ and 4.2–7.2 mm (12.2–18.5 mm), respectively. The maximum difference in binary weather patterns is found for 24 h time interval and simultaneous precipitation events are not found up to 72 h time interval at 12 stations over the NWH. The spatial variability of binary weather patterns is found to decrease and the spatial variability of the precipitation amount is found to increase with the increasing length of short time intervals, i.e., 15–72 h. These findings show that binary weather patterns and precipitation amounts of short time intervals exhibit large Spatio-temporal variability over the NWH. Results of this study can be useful for various applications directly (or indirectly) influenced by weather and/or precipitation amounts of short time intervals over the NWH during the winter period.

    • Study on the north-east monsoon onset features using a ground-based microwave radiometer over SHAR


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      Convection has a significant role in maintaining the atmospheric dynamics and thermodynamics; particularly in the tropical regions, it can often lead to the formation of clouds and precipitation, releaseof latent heat, etc. Microwave radiometers (MWRs) have evolved as a powerful tool for monitoring the genesis and evolution of the convection over a site. Ground-based MWRs are installed at the Satish Dhawan Space Centre SHAR, (SDSC SHAR) Sriharikota ($13.72^{\circ}\rm{N}; 80.18^{\circ}\rm{E}$) located over south coastal Andhra Pradesh, the south-east coast of India in 2014. MWR provides high temporal resolution vertical profiles of temperature, vapour density and liquid column measurements. The mean profiles of temperature and vapour density of upper-air ascents of GPS radiosonde and microwave radiometer (MWR) are compared for the same location. After the middle of October, the rainfall pattern over south peninsular India and in particular Tamil Nadu and south coastal Andhra Pradesh is due to the arrival of the northeasterly/easterly winds. The aim of the present study is to use MWR products for the north-east monsoon (NEM) onset study. In addition to the European Centre for Medium-Range Weather Forecasts (ECMWF) and operational WRF models, reanalysis data products were also used for the study. The association between the temporal evaluation of thermodynamic profiles and NEM onset has been tested for 3 years. The present results suggest that during the period of NEM onset, 3–5 days early signatures can be observed by MWR.

    • Velocity anisotropy analysis for shale lithology of the complex geological section in Jaisalmer sub-basin, India


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      Measurement of velocity anisotropy is an essential parameter for capturing the heterogeneity of sub-surface geology to characterise the hydrocarbon-bearing reservoir. The incorporation of velocity anisotropy parameters during the preparation of the 3D velocity model represents a robust result in a challenging geological set-up during interpretation. Generally, we can observe that the shale formation is more sensitive to velocity anisotropy response in comparison with other formations such as sandstone, siltstone for clastic reservoir or limestone and dolomite for carbonate reservoir. This study was performed mostly in the high amount shale section mixed with limestone and claystone of the Jaisalmer sub-basin area which lies in the western part of India. The preparation of the velocity model for frequent changes of lithology in the clastic and carbonate reservoir is challenging due to several changes of velocity which show a limitation in the result of the gridded velocity model. The objective of this study is to capture thechanges of compressional and shear wave velocity in mixed lithology of the significant shale formation. The idea was due to the inclusion of the anisotropy incorporated changed velocity during the preparation of the gridded velocity model for correctly identified lithology. The shale formation which is the zone ofinvestigation of the current study is situated over a carbonate sequence, and an estimated velocity anisotropy factor of this shale formation will contribute significantly during the cumulative study of velocity modelling of all formation. The current study shows that shale formation shows the character of orthorhombic anisotropy; however, this study was performed based on significant changes of well log data and related effects of vertically transverse isotropic parameters of the shale formation. The fundamental Thomsen anisotropy parameters were estimated by capturing the deviation of five independent stiffness coefficients. Significant changes in evaluated shale velocity were observed after the incorporation of the estimated Thomsen parameter in velocity values.

    • Stable water isotope signatures of dual monsoon precipitation: A case study of Greater Cochin region, south-west coast of India


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      Precipitation samples of various spatio-temporal scales were collected from coastal, midland and urban regions of Greater Cochin, Ernakulam district, Kerala for a period of 1 yr (2015–2016). The collected samples were analysed for stable water isotopes (SWI) ($\delta\rm{D}$ and $\delta^{18}\rm{O}$), to understand these variations in the precipitation source and the factors governing its isotopic characteristics during precipitation. The $\delta^{18}O$ in rainwater varies from -8.73 per thousand to 0.29 per thousand in urban, -12.21 per thousand to 2.59 per thousand in midland and -9.99 per thousand to 0.97 per thousand in lowland regions. Spatio-temporal variations in SWI were observed in various regions, suggesting altitude and continental effect followed by the establishment of a regional overall local meteoric water line (LMWL) $\delta\rm{D}$ = $8.06 (\pm0.15)\delta^{18}O + 12.5 (\pm0.68)$. Among the coastal, midland and urban regions, the highest slope ($\sim8.3$) and intercept($\sim13.0$) were observed in the urban region, which designates the variations in temperature along spatial and different layers of the atmosphere in the urban region, resulting in the deviation of isotopic characteristics.The overall deuterium excess ($d$-excess) value is $\sim10$ per thousand during the south-west monsoon (June–September), suggesting a moisture source of marine origin. A $d$-excess of $\sim13$ per thousand is observed during the north-east monsoon, indicating a moisture source from the continental contribution (October–December). The results of the moisture source obtained from the $d$-excess value are also supported by back-trajectory analysis. Thus, the present study on isotopic characterisation of precipitation and its controlling factor may enhance our understanding of the Indian monsoon and its dynamics in the west coast region of India.

    • Erosion–deposition and land use/land cover of the Brahmaputra river in Assam, India


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      The Brahmaputra is a unique dynamic river in the world with intense braiding and critical bank erosion. Both erosion and deposition are continuous processes in the river in an attempt to reach a new equilibrium in channel geometry and morphology by the ever dynamic nature of flow. Erosion and deposition of the river have link to land use and land cover (LULC) as the land cover is under constant change in a dynamic landscape constantly shaped by continuous erosion and deposition. The objective of the present work is to evaluate the extent of erosion and deposition along the Brahmaputra river and change in the LULC of the Brahmaputra river in Assam, India. Remote sensing and geographic information system (GIS) techniques were utilised to extract information from Landsat images. Total area of erosion and deposition during 1973–2014 was 1557 and 204 $\rm{km}^{2}$, respectively. Increase in area (28%) of the Brahmaputra during 1973–2014 is not solely due to bank erosion, but also for the bifurcation of streams without the loss of land. LULC study has revealed that 29% area was occupied by active channels and 71% was occupied by bars in 2014. Maximum reaches experienced reduction of the submerged part in 2014 compared to 1994 in the post-monsoon months with an overall decrease from 37% to 29%. A reduction in natural grassland and forest has been observed with a corresponding increase in agricultural practices in different bars and islands of the Brahmaputra in Assam during 1994–2014.

    • Uranium enrichment at North Almora Thrust Zone, Kumaun Lesser Himalaya, Uttarakhand, India


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      Intensely deformed and mylonitised rocks of the Almora Crystalline Zone (ACZ) have shown the potential of $^{238}U$ and $^{232}Th$ enrichment. $^{232}Th$, $^{238}U$ and $^{40}K$ are investigated by gamma-ray spectrometry in the rock samples collected along the three selected traverses across the North Almora Thrust Zone (NATZ). Obtained average values of U and Th are much higher, 7.3 and 23.9 ppm, respectively, than the average upper crustal abundances (2.8 and 10.7 ppm). The Th/Ra(eU) varies between 2.1 and 23, indicating a higher concentration of Th. The enrichment is due to the remobilisation of the radioisotopes due to the intense mylonitisation and metamorphism from the granitic protolith. The zones of enrichment, although concentrated near NATZ, show a heterogeneous distribution.

    • Grain-scale anatomy of the Bundelkhand granite: Implications for the interplay of magmatic to sub-magmatic deformation mechanisms


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      Grain-scale structures of the granitoid rocks from the north-western part of the Bundelkhand craton, central India are analysed with the aid of an optical microscope and electron probe micro analyser. Although field-based studies and quick microscopic observations suggest an overall porphyritic texture ofthe Bundelkhand granitoid, detailed microstructural observations reveal a significant deviation from the first-order igneous porphyritic texture. Here, we show that the Bundelkhand granitoid has three distinct grain-scale structures: (i) original pristine igneous structures, (ii) ductile deformation-related structures, and (iii) brittle fracturing-related structures. Based on microstructural evidences, we argue that the deformation-induced structures (both brittle and ductile) are not restricted to solid state, rather thesestructures initiated in the sub-magmatic stage and nucleated in partially crystallised magma during the magmatic to sub-magmatic event of the crystallisation history.

    • Bed boundary identification from well log data using Walsh transform technique: A case study from NGHP Expedition-02 in the Krishna–Godavari basin, India


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      Identification of thin stratigraphic beds is often dealt with using the scale of core data analysis. The downhole wireline log data also provide accurate information on the characteristics of lithological boundaries. In the present study, we have used a Walsh low pass filter and bed boundary detection algorithm to develop an automated bed boundary identification approach. Initially, we ran a Walsh low pass filter against the wireline log responses. Afterwards, a bed boundary detection algorithm has been applied to the Walsh low pass filtered version of the wireline log data. The Walsh domain filter creates a stepped version of the well log data having a constant step width over the entire low pass version of the log signal. As the Walsh function basically represents the binary waveforms, the Walsh low pass filtering can appropriately identify the thin lithological units within a complex succession of sedimentary strata. Further, the proposed approach is an efficient way of resolving and understanding subsurface inhomogeneity from wireline log data. The feasibility of the technique is successfully tested on the downhole wireline log data acquired from the Krishna–Godavari basin during the Expedition 02 of Indian National Gas Hydrate Program (MGHP).

    • Structural framework of the Wagad uplift and adjoining regions, Kutch rift basin, India, from aeromagnetic data


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      The Kutch sedimentary basin formed during the Late Triassic breakup of Gondwanaland is characterised by horst and graben structures consisting of several east–west trending uplifts surrounded by low-lying plains. The eastern part of the basin has a diverse landscape comprising the Wagad uplift, Banni plain, Island Belt uplift and the Rann of Kutch. This area is bounded by major faults like the South Wagad Fault (SWF), Gedi fault and the Island Belt Fault. The lineaments/faults present in the region at different depth levels and the propagation of these features through the different sedimentary layers are studied using the semi-detailed aeromagnetic data collected over the basin. The aeromagnetic anomaly map depicts several major E–W, NE–SW and NW–SE oriented lineaments/faults, which probably represent structural trends associated with different stages of evolution of this rift basin. Power spectral analysis of the differential reduced to pole magnetic data indicates the presence of four magnetic interfaces. The slopes identified from the 1D power spectra were used for designing matched bandpass filters for isolating and enhancing the magnetic signatures present within those interfaces. Different edge detection techniques were used to delineate the magnetic contacts/faults/lineaments present in those interfaces. In addition, we have computed the radially averaged power spectrum of 121 subset grids each with a dimension of $\rm{20 km \times 20 km}$ from which three magnetic interfaces were delineated and compared with the stratigraphic sequence of the Wagad uplift and adjoining regions. A major NE–SW fault is delineated from this analysis and suggests that this fault has depth persistence as it dislocates the different magnetic interfaces. Integration with stratigraphic data suggests that this fault was formed prior to the deposition of Miocene Kharinadi formation. We have interpreted that this fault, forming the eastern limit of the Banni basin, might have formed during the passage of the Indian plate over the Reunion hotspot. Based on the results of the aeromagnetic data analysis and other published data, we propose a generalised evolutionary model for the study region.

    • Geochemical characterisation of the Neoarchaean newer dolerite dykes of the Bahalda region, Singhbhum craton, Odisha, India: Implication for petrogenesis


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      The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE–SSW to NE–SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5–25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of $\sim 2.75–2.8$ Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India.

    • Significance of the tuffaceous beds associated with the Bijaigarh Shale of the Kaimur Group, Vindhyan Supergroup, Central India and their correlation with tuffs in other contemporaneous Proterozoic basins


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      Bijaigarh Formation is lithologically heterogeneous with grain size varying from fine to medium sand to clay. About $\sim 1$ m thick tuffaceous beds of rhyolitic to rhyodacitic nature, sandwiched between the Bijaigarh siliciclastics have been reported. We have attempted to distinguish the tuffaceous beds from the adjacent terrigenous sediments of Bijaigarh Shale on the basis of their field disposition, petrographic and geochemical characteristics. The tuffs are characterised by a relatively high content of LILE, HFSE and LREE as compared to Bijaigarh siliciclastics. They reflect their affinity to the felsic magmatic source. The geochemical signatures of the Bijaigarh tuff exhibit quite comparable characters to the contemporaneous ($\sim 1000$ Ma) tuffs from other Proterozoic basins, viz., Chhattisgarh and Indravati. The extrabasinal sources for the deposition of the Bijaigarh tuff at ($\sim 1000$ Ma) can be inferred which can possibly be correlated with tectonothermal events of the Grenville orogeny. The integration of elemental compositionswith mineralogical and textural observations makes possible the establishment of the tuffaceous beds as a stratigraphic marker in the Upper Vindhyan stratigraphy. This will have wider implications for the precise and reliable correlations of the Proterozoic basins of India.

    • Kinematics of ductile shear zones with deformable or mobile walls


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      Shear zones are important phenomena in the Earth’s middle and lower crust and are of great interest to structural geologists. Models involving rigid boundaries moving parallel to themselves are extended here to include the case where (i) walls are deformable and (ii) mobile rigid walls approach each other. These models are combined with Couette and Poiseuille flow to define a broad range of kinematic possibilities. Deformable wall models lead to smooth transitions from deformed to undeformed materials as well as with the zone transitions to gentler and more spread out deformation. Mobile walls, on the other hand, lead to shear zones where shear sense can change along a shear zone boundary.

    • Reappraisal of the ‘early proterozoic gabbro-anorthosite suite’ rocks from the eastern Singhbhum craton, India: Insights from field features, petrography–mineralogy and geochemistry


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      The present study focuses on six mafic–ultramafic units from eastern Indian Singhbhum craton: (i) gabbroic rocks of Galudih, (ii) dolerite from North of Bisoi, (iii) Bangriposi wehrlite (BW), (iv) dolerite from North of Kuliana, (v) Kuliana-layered gabbro (KLG) and (vi) dolerite of Jashipur.These rocks had been grouped earlier as a co-genetic unit titled ‘early proterozoic gabbro-anorthosite suite’. Dolerites of Jashipur, North of Bisoi and North of Kuliana are found as undeformed intrusives into the Mesoarchaean Mayurbhanj granite (MBG) unit and show hydrothermal alteration features and variable LREE enrichment. Multiply-deformed gabbroic rocks of Galudih also intrude the MBGs with similar hydrothermal alteration features and comparatively lower REE abundances. KLG and BW are found as dismembered rocks, emplaced within a multiply-deformed metasedimentary assemblage of Mid-Proterozoic North Singhbhum mobile belt. Through a critical assessment of their field features, petrography, mineralogy and geochemistry, the earlier grouping was found to be invalid. Instead, these rock units should be categorised into three groups: Group I (Galudih), Group II (Jashipur, North of Bisoi, North of Kuliana) and Group III (KLG and BW). Group I rocks are the oldest among them, but their stratigraphic affinity remains unclear. Group II mafic rocks possibly belong to the Proterozoic newer dolerite dykes, whereas Group III represents fragments of oceanic crust and mantle.

    • Fluoride behaviour analysis in groundwater with reference to hydrogeochemical parameters in basaltic aquifers using remote sensing and GIS technique in parts of Burhner watershed, MP, India


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      High fluoride concentration in groundwater leads to health threat to millions of people around the world; therefore, a systematic study is required to understand the behaviour of fluoride in water in terms of the local hydrogeological setting and other hydrogeochemical parameters.The present study is an attempt to assess the hydrogeology of groundwater in the study area to understand the fluoride behaviour in groundwater and to deduce the chemical parameters responsible for the dissolution activity of fluoride. $\rm{F}^{-}$ value varies from 0.04 to 14.6 mg/l (mean: 3.15 mg/l) in the stuldy area. It is geologically occupied by basalt rocks where groundwater occurs in the weathered and fractured portions of the rocks and under semi-confined to confined conditions in fractured rocks. High $\rm{F}^{-}$ concentration is observed in deeper aquifers compared to shallow aquifers. Physicochemical conditions like decomposition, dissociation and subsequent dissolution along with long residence time are responsible for leaching of $\rm{F}^{-}$ into the groundwater. $\rm{F}^{-}$ has +ve or -ve correlation with other parameters of water samples as per their nature. Simple to compound pahoehoe basaltic lava flows are responsible for the fluoride contamination in the confined aquifers (bore wells) of the study area. Overall confined aquifer water quality on the basis of fluoride concentration was found to be unsatisfactory for drinking purposes. About 57.13% of confined aquifers showed higher fluoride than the permissible limit but 100% unconfined aquifers (dug wells) have a low level of fluoride concentration, i.e. below the permissible limit.

    • Satellite-based observation of lightning climatology over Nepal


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      The lightning climatology over Nepal is analysed in detail for the first time. For the analysis, we utilised the satellite-based lightning imaging sensor data for the period from 1998 to 2013. A comparison of these climatological results is also performed with two ground-based lightning detection networks, namely, the World Wide Lightning Location Network and the Global Lightning Network for 3 yr from 2011 to 2013. On analysing the data obtained from the three sources, we conclude that the months of April and May are extremely vulnerable in the perspective of lightning hazards in Nepal, in contrast to the results reported previously which indicated that the maximum lightning activity occurred in the month of June. The central and eastern regions of the country receive the majority of lightning strikes during the months of April and May. The present finding is supported by the thunderstorm frequency data obtained from the disaster Information Management System, Nepal and also from thunder-day data from NOAA.

    • Revisiting the boundary between the Lower and Upper Vindhyan, Son valley, India


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      The placement of the boundary between the Lower and the Upper Vindhyan in the Son valley, an unconformity, has long been at the centre of a raging debate. At the Bundelkhand sector, it is placedbetween the Rohtas Limestone and the Sasaram Sandstone (Lower Quartzite). On the other hand, in the Son valley sector, it is placed between the Bhagwar Shale and the Kaimur Formation. The recent study reveals the existence of ca. 12 m thick sandstone between the Bhagwar Shale and Rohtas Limestone, traced over 150 km in the Son valley sector. Based on in-depth facies constituents and facies tracts, this sandstone is an exact equivalent of the Sasaram Sandstone in the Bundelkhand sector. Its base is strongly erosional and limestone and chert clasts derived from the underlying Rohtas Limestone are abundantly present at the basal part of the sandstone and the unconformity between the Upper and Lower Vindhyan are likely to be present in between.

    • Geochemistry of surface sediments in parts of Bandipora–Ganderbal areas, Kashmir valley, western Himalaya: Implications for provenance and weathering


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      Sediment geochemistry is an important tool to understand the sediment provenance and weathering. The present study describes the geochemical distribution, the provenance and the degree of weathering in surface sediments in parts of Bandipora–Ganderbal areas, Kashmir valley, western Himalaya. Following the standard operating procedure of the Geological Survey of India, high-density sediment sampling over an area of $\rm{800 km}^{2}$ of toposheet Nos. 43J/11 and 43J/12 (part) was carried during the Field Season Programme of 2014–2015. A total of 200 stream and slope wash sediment samples collected on a $2 \times 2$ grid pattern, covering an area of $\rm{800 km}^{2}$ and 10 numbers of duplicate samples were used in this study. An analysis of major and trace element concentrations of the sediment samples revealed significant changes corresponding to different geological formations of the area. The correlation matrix of $\rm{SiO_{2}, Al_{2}O_{3}, TiO_{2}, Fe_{2}O_{3}, MnO, K_{2}O, Th}$ and $\rm{Y}$ showed a positive correlation with each other and negative correlation with $\rm{CaO, MgO}$ and $\rm{Sr}$, thereby indicating two dominant geological provenances of the sediments. Two dominant geochemical patterns revealed provenances from basaltic and carbonate rock formations in the area. Furthermore, an analysis of geochemical weathering indices (chemical index of alteration and chemical index of weathering) revealed a poor (27) to moderate (78) degree of weathering in this area. This observation is further substantiated by the immaturity of the soil sediments in the area.

    • Geochemical constraints on the origin and tectonic setting of Chargar intrusions in the Alborz orogenic belt, NW Iran


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      Chargar area is located in the southern border of Tarom subzone within the Alborz magmatic belt of NW Iran. Two types of intrusions, mainly present in the southern part of the area, have been identified. These bodies intruded into the Karaj Formation volcanic and volcaniclastic rocks. Type-I intrusions occur in the south and include two magmatic bodies: (a) Gabbro-pyroxene quartz monzodiorite-quartz monzodiorite and (b) quartz syenite. Type-II crops out in the west and has a gabbro – gabbro-diorite composition. Geochemically, Chargar intrusive rocks belong to the high-K calc-alkaline to shoshonitic series and are classified as I-type and metaluminous granitoids. These intrusive rocks show insignificant different distribution patterns of trace elements and REEs, but generally they are characterized by highly enriched large ion lithophile elements (e.g., K and Ba) and depleted Nb, similar to that of continental arc magmatism. Type-I intrusions and Type-II mafic intrusion were originated from mantle source with amphibole-bearing spinel peridotite composition. These intrusions formed in a continental arc to post-collisional tectonic setting.

    • Seasonal contrast in the vertical profiles of aerosol number concentrations and size distributions over India: Implications from RAWEX aircraft campaign


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      Aircraft measurements of the vertical profiles of aerosol total number concentrations and size distributions (in the size range of 0.5–20 lm) were made over seven geographically diverse locations of theIndian mainland during two contrasting seasons, winter (December 2012) and spring (April–May 2013), as a part of the regional aerosol warming experiment (RAWEX). Our observations revealed an increase in the vertical extent of aerosol loading during spring having a significant enhancement in coarse mode aerosols in the lower free-troposphere (FT) over western and central parts of India and the Indo-Gangetic plains (IGP). The particulate depolarisation ratio (PDR) derived from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) over the same region showed the presence of dust (including polluted dust) at higher altitudes in spring. Concurrent and collocated measurements ofaerosol scattering and absorption properties aboard the aircraft revealed that the FT enhancement in coarse mode aerosol loading during spring is associated with a decrease in single scattering albedo and an increase in columnar absorption aerosol optical depth. This confirms that the elevated layers of coarse mode aerosols seen during spring are absorbing in nature, especially over the IGP. The presence of such coarse-mode absorbing aerosols plays a crucial role in governing the radiation balance over the IGP in spring through the diabatic heating of the upper atmosphere.

    • Evaluation of the impact of high-resolution winds on the coastal waves


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      This study discusses the impact of high-resolution winds on the coastal waves and analyses the effectiveness of the high-resolution winds in recreating the fine-scale features along the coastal regions during the pre-monsoon season (March–May). The influence of the diurnal variation of winds on waves is studied for the Tamil Nadu coastal region using wind fields from weather research and forecast (WRF) (3 km) and European Centre for Medium-Range Weather Forecasts (ECMWF) (27.5 km). The improvement in the coastal forecast is then quantified with wave rider buoy observations. The high-resolution wind fields simulated fine-scale features like land–sea breeze events and showed good agreement with observation results. The error in the wave height and period is reduced by 8% and 46%, respectively, with the use of high-resolution forcing winds WRF over ECMWF, although the overestimation of wave energy on high frequencies due to overestimated WRF winds remains as a challenge in forecasting. The analysis also shows the importance of accurate wave forecast during a short-duration sudden wind ($\sim$12 m/s) occurrence in southern Tamil Nadu near Rameswaram during the pre-monsoon period. Low pressure forms over Tamil Nadu due to the land surface heating, resulting in a sudden increase of winds. High winds and steep waves which cause damage to the property of the coastal community near Rameswaram also were well simulated in the high-resolution forecast system with WRF winds.

    • Geochemistry and petrogenesis of acidic volcanics from Betul–Chhindwara Belt, Central Indian Tectonic Zone (CITZ), central India


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      Betul–Chhindwara belt is part of Central Indian Tectonic Zone (CITZ) that includes Proterozoic basalt, rhyolite, quartzite, mafic–ultramafic rocks, volcano sediments and banded iron formation (BIF). Studied rhyolites and leuco-micro granites are deformed due to shearing and includes quartz, K-feldspar (microcline), muscovite, biotite and epidote. In some samples, feldspar has been sericitized due to interaction with hydrothermal fluids. The major element geochemistry of volcanic rocks clearly indicates acidic nature and falls in the rhyolite field. Rhyolites show difference in the enrichment of REEs and major element composition which help us divide them into two groups and also indicate heterogenous source. The rhyolites show very strong negative Eu anomaly, which indicates fractionation of feldspar. Positive anomalies of U–Th–Zr for the rhyolites indicate crustal involvement. The $\varepsilon\rm{Ndt (t=1500)}$ for the Group I rhyolites vary from –1.42 to –0.19 and for the Group II rhyolites vary from –5.81 to +0.14 and DM model ages for Group I rhyolites vary from 2284 to 2464 Ma and for Group II vary from 2174 to 2863 Ma. It is suggested that contemporary mafic magma of the Betul–Chhindwara belt while ascending from mantle sources interacted with the continental crust at different levels, supplying heat and fluids which reduced the melting points of the crustal source rocks, producing felsic melt of varying compositions. Tectonic discriminant diagrams and geochemical data indicate subduction zone tectonic environment for the genesis of the Betul–Chhindwara acidic volcanism. The acidic volcanics of Betul–Chindwara, Sakoli and the Bijli rhyolites from the adjoining areas display similarity in terms of the total alkali vs. silica diagram and many of the major and trace elements, including rare earth element characteristics. Compared to Betul Rhyolite, Sakoli Rhyolites are derived from less enriched source with less involvement of crust and/or the latter represents high degree of partial melting of similar source. They are considered contemporaneous to Betul Rhyolite based on geochronological data. Contrastingly, Bijli Rhyolite show highly fractionated patterns with high LREE enrichment indicating considerable crustal involvement which is very obvious for within plate magmatism, assigned for the Bijli rhyolites.

    • Rock magnetic and palaeomagnetic studies on the alkaline complexes of western Rajasthan, India


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      Forty-five oriented block samples were collected from 12 sites of Mundwara and Sarnu-Dandali alkaline complexes of western Rajasthan for palaeomagnetic and rock magnetic investigations to constrain the timing of the emplacement of alkaline suites in relation to the time-frame of main Deccan volcanism. The rock magnetic studies indicated (titano) magnetite as the main magnetic carrier of stable remanence. The Lowrie–Fuller (L–F) test revealed SD and PSD type magnetic carrier in samples. AF and thermal demagnetizations were piloted on the samples to isolate Characteristic Remanent Magnetisation (ChRM) directions. The samples exhibited stable remanence between 5 and 35 mT during AFD. Thermaldemagnetizations unblocked remanence between $350^{\circ}$ and $500^{\circ}C$. The mean ChRM directions with $\rm{Decl_{m} = 342, Incl_{m} = –35 (\alpha_{95} = 4.39, K = 121, N = 9)}$ corresponds to paleopole position at $42^{\circ}\rm{N}$ and $274^{\circ}\rm{E}$ with a palaeo-latitude of $24.5^{\circ}\rm{S}$ is coincident with the Deccan Super pole position. The comparison of ChRM in alkaline complexes with those documented in DVP implies coeval emplacement of alkaline suites with Deccan eruption. Further, the ChRM marked largely by normal polarity suggests that alkaline intrusions were accomplished within the magnetic Chron C30N, during the onset of Deccan volcanism. The results also indicate that Deccan volcanism extended far beyond the present day boundaries of the traps, especially in the north.

    • A box-model approach for reservoir operation during extreme rainfall events: A case study


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      Extreme rainfall events in an urban area pose various challenges to the water resource managers in terms of flood mitigation, inundation, water conservation and harvesting for drinking water supply. The objective of this study is to apply the box-model approach to evaluate reservoir operation during extreme rainfall events. A large water supply reservoir in Chennai was chosen to carry out this study. A box model, based on input–output parameters, is proposed to simulate the reservoir operation and hydraulic behaviour. Hydrologic Engineering Centre-Hydrologic Modelling System (HEC-HMS) has been used to simulate the reservoir inflow hydrograph and to understand the run-off characteristics of the basin. Three extreme rainfall events occurred in past have been selected for the analysis. Three different scenarios have been framed to assess the reservoir performance. Reducing the initial storage to 50% and releasing water at the beginning of the event gives a possible solution for flood mitigation in reducing the outflow volume by 9–37% and delaying the time to peak by 1–6 h. Though the reduced outflow volume from this reservoir is less, it can help to mitigate the flood inundation to a significant extent. Thus the box-model approach presented here can be utilised as a simple tool to generate the various combinations of outflow hydrographs for any reservoir.

    • Kharif crop characterization using combination of SAR and MSI Optical Sentinel Satellite datasets


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      In the present study, the differences in the kharif crop reflectance at varied wavelength regions and temporal SAR backscatter (at VV and VH polarizations) during different crop stages were analyzed to classify crop types in parts of Ranchi district, East India using random forest classifier. The spectral signature of crops was generated during various growth stages using temporal Sentinel-2MSI (optical) satellite images. The temporal backscatter profile that depends on the geometric and di-electric properties of crops were studied using Sentinel-1 SAR data. The spectral profile exhibited distinctive reflectance at the NIR (0.842 $\mu$m) and SWIR (1.610 $\mu$m) wavelength regions for paddy (Oryza sativa; $\sim 0.25$ at NIR, $\sim 0.27$ at SWIR), maize (Zea mays; $\sim 0.24$ at NIR, $\sim 0.29$ at SWIR) and finger millet (Eleusine coracana, $\sim 0.26$NIR, $\sim 0.31$ at SWIR) during pre-sowing season (mid-June). Similar variations in crop’s reflectance at their different growth stages (vegetative to harvesting) were observed at various wavelength ranges. Further, the variations in the backscatter coefficient of different crops were observed at various growth stages depending upon the differences in sowing–harvesting periods, field conditions, geometry, and water presence in the crop field, etc. The Sentinel-1 SAR based study indicated difference in the backscatter of crops (i.e., $\sim -18.5$ dB (VH) and $\sim -10$ dB (VV) for paddy, $\sim -14$ dB (VH) and $\sim -7.5$ dB(VV) for maize, $\sim -14.5$ dB and $\sim -8$ dB (VV) for finger millet) during late-July (transplantation for paddy; early vegetative for maize and finger millet). These variations in the reflectance and backscatter values during various stages were used to deduce the best combination of the optical and SAR layers in order to classify each crop precisely. The GLCM texture analysis was performed on SAR for better classification of crop fields with higher accuracies.The SAR-MSI based kharif crop assessment (2017) indicated that the total cropped area under paddy, maize and finger millet was 24,544.55, 1468.28 and 632.48 ha, respectively. The result was validated with ground observations, which indicated an overall accuracy of 83.87% and kappa coefficient of 0.78. The high temporal, spatial spectral agility of Sentinel satellite are highly suitable for kharif crop monitoring. The study signifies the role of combined SAR–MSI technology for accurate mapping and monitoring of kharif crops.

    • CPO and kinematic analysis of the Bitou S-tectonites (Central Cameroon shear zone): AMS and EBSD investigations


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      The development of foliation is not always associated with mineral stretching lineation in deformed rocks. Sometimes, S-tectonites only display foliation with no mineral stretching lineation, and it becomes a real challenge to perform kinematic analysis, i.e., to define the $XZ$ section of the ellipsoid. In this study, we present in a portion of the Central Cameroon Shear Zone (CCSZ) (the Bitou biotite gneiss mylonite), the usefulness of anisotropy of magnetic susceptibility (AMS) studies to identify the three principal axes of the AMS ellipsoid ($K_{1} \leq K_{2} \leq K_{3}$), equivalents respectively to the principal axes of the strain ellipsoid ($X \leq Y \leq Z$). The $K_{1}K_{3}$ plane of the AMS ellipsoid is equivalent to the $XZ$ section of the strain ellipsoid. The fabrics developed in the studied mylonitised biotite gneiss strike ENE–WSW to E–W with steep dips for the mylonitic and magnetic foliations and moderate plunges for the magnetic lineation. The rock is paramagnetic. The AMS ellipsoids are mostly of oblate shape, while the quartz $c$-axis pattern is typical of non-coaxial flow. This implies that deformation partitioning took place during mylonitisation. Quartz crystallographic preferred orientation (CPO) measured using electron backscatter diffraction reveals the activation of prism $\langle a \rangle$ slip, implying that the mylonitisation occurs under moderate temperature conditions ($450^{\circ}C$ < $T$ < $550^{\circ}C$). Microstructures observed in the $K_{1}K_{3}$ section of the AMS ellipsoid and CPO of the quartz $c$-axis indicate sinistral top-to-SW sense of shear. These results support the shear senses of movement that earlier studies in the CCSZ have emphasised and are assumed to be related to the early $\rm{syn-D_{2}}$ and $\rm{D_{3}}$ events of the Pan-African tectonic dated at ca. 613–585 Ma.

    • Paleoproterozoic ($\sim 1.88–1.89 \rm{Ga}$) ultramafic–mafic sills, Cuddapah basin, India—revisited: Implications for interaction between mantle plume and metasomatized subcontinental lithospheric mantle


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      A number of mafic–ultramafic sills, which are supposed to be part of the widespread $\sim 1.88–1.89 \rm{Ga}$ Hampi–Bastanar Large Igneous Province of the Indian shield, are reported to intrude into sedimentary/ meta-sedimentary rocks of the Proterozoic Cuddapah basin. Mineral chemistry of chrome-spinel, olivine and pyroxene from the cumulate ultramafic rocks of the Pulivendla sills, emplaced at the base of Tadpatri Formation of the Cuddpaha basin, is presented for better understanding on their nature and genesis. The intermediate Cr#, low Mg#, high $\rm{Fe^{3+}}$# and Ti (apfu) coupled with relatively low Al (apfu) content of studied spinel indicate their Alaskan-type nature. Moreover, the Ca-rich clinopyroxene together with classic variations of $\rm{Al_{2}O_{3}}$, $\rm{TiO_{2}}$ and $\rm{SiO_{2}}$ follow the trend of arc type of intrusions. The calculated $\rm{Al_{2}O_{3}}$ content of parental melt of the studied rocks also suggests arc-type characteristics. Whole-rock geochemistry show fractionated chondrite-normalized REE patterns, higher Ba/Nb and low Nb/La that suggest contribution from an enriched mantle source, whereas higher Th/Yb and negative Ta–Nb–Ti anomaly on the primitive mantle-normalized multi-element spidergrams emphasize the involvement of a subducted component in the lithospheric mantle source. Although, the mineral chemistry and geochemistry is akin to Alaskan-type intrusion, lack of concentric zoning of lithologies (olivine-rich ultramafic rocks in centre surrounded by mafic rocks), rarity of primary hornblende and abundance of orthopyroxene contradict it to be an Alaskan-type intrusion. It is suggested that the mantle source region of the Pulivendla ultramafic–mafic rocks was modified by a fluid-induced metasomatism during an ancient subduction event. Further, Al-in-olivine thermometry suggests crystallization temperature between $1410^{\circ}$ and $1484^{\circ}\rm{C}$, which is $260^{\circ}\rm{C}$ higher than the average temperature of MORB. Similarly, the estimated mantle potential temperature is also significantly higher (> $1600^{\circ}\rm{C}$) than the different secular cooling models of the earth at $\sim 1.88–1.89 \rm{Ga}$ and consistent with the thermal regime of a mantle plume. The existence of mantle plume during 1.88–1.90 Ga, which has played an important role in the genesis of mafic–ultramafic sills of the Cuddapah basin, is also well supported by a radiating mafic dyke swarm, domal uplift and magmatic underplating.

    • Trace-element systematics of pyrite and its implications for refractory goldmineralisationwithin the carbonaceous metasedimentary units of Palaeoproterozoic South Purulia shear zone, eastern India


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      Globally, refractory gold occurs in significant proportions in many types of gold deposits. The present work reports the occurrence of sulphide-hosted refractory gold within the carbonaceous phyllites of the South Purulia shear zone in the Singhbhum crustal province, eastern India. Detailed textural characteristics, paragenesis and trace-element concentrations of different generations of pyrites were studied to understand their evolutionary stages and the mechanism of gold incorporation in pyrites. Four types of pyrites, which are closely associated with gold mineralisation were identified in the host rock, i.e., carbonaceous phyllite. Py I is of diagenetic origin, whereas Py II, Py III and Py IV are of hydrothermal origin. Laser ablation inductively coupled plasma mass spectrometry studies confirm the presence of invisible/refractory gold concentration up to 110.55 ppm within these pyrites. The positive correlation between Au and As in pyrite indicates the significant role of As in the incorporation of gold in pyrite. Fourier transform infrared spectroscopy confirms the presence of organic matter that provided suitable redox conditions for the precipitation of auriferous pyrites. The refractory gold mineralisation is attributed to widespread sulphidation during both sedimentation and hydrothermal ore-forming processes. Transformation of diagenetic pyrite to pyrrhotite during prograde metamorphism of carbonaceous rocks promoted the liberation of sulphur and Au from the lattice of abundant diagenetic pyrites to the hydrothermal fluid which later precipitated sulphides in the quartz $\pm$ carbonate veins.

    • EPMA monazite geochronology of the granulites from Daltonganj, eastern India and its correlation with the Rodinia supercontinent


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      We report the monazite dates of the granulites from Daltonganj (Palamau), Chhotanagpur granite–gneiss complex (CGGC) which covers the significant part of the granulite blocks in central India by using an electron micro probe analyser dating. The monazite grain varies between 70 and 80 $\mu\rm{m}$ and shows the distribution of U, Th and Pb in all monazite grains of both samples. Two different dates were obtained from different monazite grains; the first age suggests that the granulite from CGGC preserves the first remnant of the protolith of the Mesoproterozoic era at $\sim 1424$ Ma and second one at $\sim 972$ Ma which provides evidence of metamorphism of the protolith. The CGGC rocks preserve four regional metamorphic events, namely $\rm{M_{1}, M_{2}, M_{3} and M_{4}}$. But in this work, two different ages from the Daltonganj granulites were obtained which are similar to the $\rm{M_{2}}$ (<1500 Ma, i.e., the age of protolith of the granulitic gneiss) and $\rm{M_{3}}$ (1200–930 Ma) metamorphic events as reported in the CGGC. The $\rm{M_{3}}$ metamorphism attained its average $P–T$ condition at $\sim 7.35 \rm{kbar/792^{\circ}C}$, and it represents the prograde metamorphic event. The $\rm{M_{3}}$ metamorphic event supported the Grenville-orogeny, and it was responsible for the metamorphism of the magmatic protolith of granulitic gneiss from the CGGC at the time of amalgamation of the Rodinia supercontinent. The Rodinia assembly had occurred through the global Grenville-orogenic events between 1100 and 900 Ma, with continental blocks which exist at that time.

    • Geochemical characteristics of the organic matter in UPPMRs and the implication for fluid–rock exchange due to the retrograde metamorphism in the Dabie–Sulu orogenic belt, North China


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      The samples were collected from ultrahigh-pressure para-metamorphic rocks (UPPMRs) around 500–2000 m deep by Chinese Continental Scientific Drilling. The combined method of ultrasonic disintegrator and Soxhlet was strictly conducted to extract the indigenous hydrocarbons from the UPPMRs, to obtain non-contaminated and entire organic matter to reveal the geochemical characteristics and origin of hydrocarbons in the UPPMRs. Through gas chromatography–mass spectrometry analysis, the ratios of Pr/Ph in the soluble hydrocarbon were from 0.04 to 0.87. It was inferred that the precursor of extracts would be deposited in the anoxic setting. Based on the relative content among $\rm{C_{27}, C_{28}}$ and $\rm{C_{29}}$ steranes, it was found that the main source of organic matter was marine algae. But the isomer ratios of $\rm{C_{31}}$ hopane 22S/(22S + 22R) were 0.39–0.69, and the distribution range of $\rm{C_{29}}$ sterane 20S/(20S + 20R) was 0.41–0.63, both of which reflected the equivalent maturity of organic matter was no more than 1% $(R_{o}%)$. Therefore, the immature organic matter derived from algae obviously conflicted with its host rock, which experienced ultrahigh-pressure metamorphism. Therefore, this sort of immature organic matter probably is of secondary origin. Furthermore, the chromatography of $n$-alkane almost assumes bimodal distribution, and the data of $T_{max}$ ranges from $394$ to $565^{\circ}\rm{C}$, as $\rm{S_{1}}$/total organic carbon (TOC) ratios systematically increase with the corresponding $T_{max}$. Therefore, it could be further proved that the organic matter in the UPPMRs probably is of mixed origin and is mainly derived from immigrant hydrocarbons during the stages of subduction and retrograde metamorphism. The emergence of Zr-in-rutile also indicates that the geological temperature was lower than that during the peak of metamorphism. Therefore, itcould be inferred that the kerogen associated with rutile might go through diagenesis regularly at the stage of retrograde metamorphism. It was also shown that the $\delta^{18}\rm{O}$ value suddenly reduced up to -10 per thousand corresponding to the highest $\rm{TOC (1820 \mu g/g)}$ around the subduction fault. The relationship between the TOC and $\delta^{18}\rm{O}$ value indicated that the fluid–rock exchange reaction was the main reason for the immature organic matter present in the para-metamorphic rock. The returned subduction and retrograde metamorphism resulted in the activity of the formation fluid, which could prominently impact the geochemical characteristics of the para-metamorphic rock and should also be considered in the geodynamics research of the metamorphic orogenic belt.

    • Seismic modelling of $\rm{CO_{2}}$ fluid substitution in a sandstone reservoir: A case study from Alberta, Canada


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      The prime objective of this study is to find the suitable petrophysical parameters which depict the maximum change in seismic amplitude due to fluid substitution. Therefore, in the present study thepetrophysical parameters are analysed to detect the most sensitive parameters due to fluid substitution. The analysis is performed in three steps: In the first step, the Gassmann fluid substitution is performedand a considerable change in velocity, density, impedance, lambda–mu–rho parameters and Shuey’s parameters is examined. The study shows that the most sensitive parameters are $A$ (intercept), whichshows the maximum drop of 22% with respect to $\rm{CO_{2}}$ injection, and $B$ (gradient), which shows the maximum increase of 10% with $\rm{CO_{2}}$ injection in the formation. Thereafter, in the second step, the seismic forward modelling is performed to examine the changes in seismic amplitude by the fluid substitution in the formation. The analysis depicts that the seismic amplitude increases steadily with increasing $\rm{CO_{2}}$ saturation. The amplitude increases by 4% at 20% $\rm{CO_{2}}$ injection, by 8% at 50% $\rm{CO_{2}}$ injection and the seismic amplitude increases by 12% at 100% $\rm{CO_{2}}$ injection in the target zone. Finally, in the third step, the numerical modelling is performed to assess the ability of seismic methods to detect the $\rm{CO_{2}}$ plume accurately by injecting $\rm{CO_{2}}$ plume of cylindrical shape. The analysis shows that the $\rm{CO_{2}}$ plume can be detected more prominently by analysing the impedance volume rather than the seismic amplitude section. This study is helpful in deciding which parameters should be monitored carefully in fluid replacement modelling projects.

    • Characterisation of particulate matter at a high-altitude site in southwest India: Impact of dust episodes


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      Observations on a particulate matter $\rm{(PM_{10} and PM_{2.5})}$ were carried out during March 2015 to February 2017 over a high-altitude location Mahabaleshwar in the Western Ghats region in southwest India. Apartfrom temporal variation of PM and the ratio of $\rm{PM_{2.5}/PM_{10}}$, impacts of local meteorological parameters on the concentration of PM are examined. $\rm{PM_{10}}$ showed a maximum concentration during pre-monsoon, whereas $\rm{PM_{2.5}}$ showed it in winter. The monsoon season showed the lowest concentrations for both $\rm{PM_{10} and PM_{2.5}}$. Concentrations were significantly reduced in 2016 due to the washout effect from enhanced rainfall during that year. Diurnal variations of PM were related to the variation in a planetary boundary layer, mountain valley winds as well as changes in different local sources. Dominance of primary particles was observed from the $\rm{PM_{2.5}/PM_{10}}$ ratio. The Central Pollution Control Board (CPCB) threshold limit for PM was exceeded on several days mainly during pre-monsoon due to transported dust from the Arabian Peninsula and Thar Desert apart from an increase in the tourist activity. A typical case for transported dust event during March 2016 is studied. Organics and sulphate particles showed a significant enhancement during dust event. Overall, the study indicated emissions from mixed sources for PM from local as well as distant source regions over Mahabaleshwar.

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