2,967 research outputs found
Orthogonal Tectonic and Magmatic Fabrics in a Layered Granite-Gneiss at Remal Dam Site, India: Implications for Fabric Generation and Superposition
No full textThe Remal granite-gneiss is situated close to the tectonic boundary between the Singhbhum Craton and the Rengali Province in the state of Odisha, eastern India. This granite-gneiss contains two prominent fabric elements - a sub-horizontal to gently dipping felsic fabric Sign, believed to be of igneous origin that predates a sub-vertical gneissosity S1 which is of tectonic origin. Sign layers have a non-uniform, arcuate geometry and grain-size within the layers show systematic variations. S1 is defined by metre-scale segregations of biotite-poor and biotite-rich domains whose orientations are constant. Sign layers are arranged rhythmically in cross-section and either curve into parallelism with or truncate against layers above and below; the entire assembly resembles cross beds developed in sediments. Some of the layers develop trough cross-bedding similar to those seen in mafic intrusions such as the Skaergaard Complex, indicative of slumping of a crystallizing mush along an inclined depositional plane at the time of crystallization. The Sign layers are composed of quartz, K-feldspar and plagioclase with abundant graphic intergrowths and myrmekite, and lack any evidence of compaction. Plagioclase grains are often zoned, and dihedral angles between mineral grains is significantly different from the equilibrium value of 120°, testifying to the preservation of the igneous nature of this fabric without significant solid state modification. In contrast, S1 is sub-parallel to localized mylonite zones within the granite-gneiss composed of chlorite and epidote, indicative of deformation under greenschist facies conditions. The mylonitized zones contain prominent dextral shear sense indicators and is believed to have originated due to the amalgamation of the Rengali Province with the Eastern Ghats Mobile Belt along the east-west trending, sub-vertical Brahmani Shear Zone further to the south. The S1 gneissosity appears to have developed as a result of this deformation event. EBSD analyses of quartz grains within the granite-gneiss reveal distinct variations in the distribution of axes in different domains. Close to the mylonite zones, deformation of quartz has been dominantly accommodated by basal slip with a dextral shearing overprint while away from these zones and S1, the axes are distributed in clusters without any systematic pattern. The persistence of an earlier igneous layering, despite the subsequent development of a gneissosity concomitant with localised mylonitisation, indicates that the later deformation event has not obliterated the earlier formed igneous fabric. The study also demonstrates that development of a gneissosity does not necessarily require deformation operating at moderate to high temperature, and can stabilize even under greenschist facies conditions
Direct Visualization of Grain Boundaries in Quartzites using Atomic Force Microscopy: Application to Study of Fluid Percolation in Continental Crust
No full textHydrous fluids play a vital role in the chemical and rheological evolution of ductile, quartz-bearing continental crust, where fluid percolation pathways are controlled by grain boundary domains. In this study, widths of grain boundary domains in seven quartzite samples metamorphosed under varying crustal conditions were investigated using Atomic Force Microscopy (AFM) which allows comparatively easy, high magnification imaging and precise width measurements. It is observed that dynamic recrystallization at higher metamorphic grades is much more efficient at reducing grain boundary widths than at lower temperature conditions. The concept of force-distance spectroscopy, applied to geological samples for the first time, allows qualitative estimation of variations in the strength of grain boundary domains. The strength of grain boundary domains is inferred to be higher in the high grade quartzites, which is supported by Kernel Average Misorientation (KAM) studies using Electron Backscatter Diffraction (EBSD). The results of the study show that quartzites deformed and metamorphosed at higher grades have narrower channels without pores and an abundance of periodically arranged bridges oriented at right angles to the length of the boundary. We conclude that grain boundary domains in quartz-rich rocks are more resistant to fluid percolation in the granulite rather than the greenschist facies
Origin of magmatic and tectonic fabrics in the Remal granite-gneiss, Singhbhum Craton, India
No full textIn the Singhbhum Craton of the Indian shield, the Remal granite-gneiss preserves felsic magmatic fabrics onto which a low-temperature segregation layering has been superposed. Planar, sub-horizontal to gently dipping layers (Sign1) comprise K-feldspar megacrysts, plagioclase and quartz, with the base of each layer defined by segregations of biotite. Sign2 consists of trough cross-bedded layers composed of K-feldspar phenocrysts, plagioclase and quartz with biotite schlieren defining the base of each layer. Microstructural features such as concentrically arranged mineral inclusions in K-feldspar phenocrysts and graphic intergrowth textures testify to the magmatic origin of these fabrics, with insignificant subsequent metamorphic reconstitution. The tectonic fabric S1 has developed sub-parallel to localized greenschist-facies mylonite bands, and is defined by weakly aligned flakes of biotite. Crystallographic preferred orientations away from the mylonitized domains show a strong alignment of K-feldspar, quartz and biotite parallel to the magmatic fabric due to efficient segregation during magmatic flow. Quartz crystallographic preferred orientations within the mylonitized domains show a strong preferred orientation and dextral asymmetry. Temperature constraints from synkinematic chlorites along with estimates of deformation temperature from quartz crystallographic preferred orientations indicate that mylonitization occurred at the lower limits of quartz crystal plasticity. The results of combined thermodynamic and multiphysics modelling studies show that felsic magmas can undergo significant convective motion for a wide range of crystallinities and water contents before solidification. Additionally, segregation layering resembling a gneissosity can develop at low temperatures owing to localized mylonitization and concomitant dissolution–precipitation of biotite
The Northern Margin of the Eastern Ghats Mobile Belt: Evidence for Strike-Slip Tectonics Along a Craton-Mobile Belt Boundary
No full textThe Eastern Ghats Mobile Belt (EGMB) lies to the east of the Archaean Bastar and Dharwar Cratons, and to the southeast of the Singbhum Craton in the Indian shield. Along its western boundary, the EGMB granulites have been thrust westward as a nappe over the Bastar Craton along a mylonitic contact zone. Earlier studies considered the northern boundary of the EGMB with the Singhbhum Craton to be a thrust, although this interface is geometrically parallel to the west-directed transport direction of the granulitic nappe. Detailed geological studies along this northern margin reveal that the c. 1.0 Ga granulites of the EGMB do not share a direct contact with the Archaean granite-greenstone terrane of Singhbhum, but are actually juxtaposed against a Late Archaean (2.8-2.5 Ga) high grade terrane referred to as the Rengali Province. Structural studies reveal that the EGMB-Rengali Province contact has a WNW-ESE strike with sub-vertical dip, with prominent asymmetric markers indicating dextral strike-slip shearing along a horizontal transport vector. Microstructural studies indicate that fabric formation during strike-slip deformation is controlled by plastic deformation of quartz, while other minerals remained passive or deformed in a brittle manner. Electron Back-Scatter Diffraction (EBSD) studies on selected samples from the contact zone indicate that quartz deformed mostly by prism < a >, rhomb < a > and basal < a > slip, with asymmetry indicating dextral simple shearing. The Rengali Province samples show that earlier shortening (pure shear) microstructures and quartz CPO patterns were also sheared dextrally by this later deformation. These results confirm that the northern boundary of the EGMB is not a thrust, but a strike-slip shear zone that operated well after granulite metamorphism, and at lower temperatures characteristic of the greenschist facies. Shortening structures in the Rengali Province and the Singhbhum Craton are related to an older deformation event unrelated to emplacement of the EGMB. Gravity studies across the contact confirm that the shear zone continues vertically to a depth of at least 25 km. Thus, integrated geological and geophysical studies confirm the strike-slip nature of the craton-mobile belt boundary
Nanoscale visualization of high-angle misorientations in quartz-rich rocks using SEM-EBSD and Atomic Force Microscopy
No full textHigh-angle misorientations can significantly influence material properties. In this study, optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM) have been used to investigate high-angle misorientations in quartz-bearing crustal rocks. Thin sections of high-grade quartzofeldspathic rocks were subjected to chemical mechanical polishing (CMP) with colloidal silica. In quartz, high-angle misorientations like random high angle grain boundaries (RHAGBs) and Dauphiné twin boundaries (DTBs) could be discriminated using EBSD techniques but not optical microscopy. In nanoscale AFM images, indented channels are observed along RHAGBs but not DTBs; these result from material removal during CMP, indicating lower compactness of RHAGBs compared to DTBs. Along any RHAGB, EBSD reveals different misorientations across segments between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° with parallel {101 ̅2} planes, and 81–84° with parallel {112 ̅2} planes, respectively. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments become low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Finally, these results suggest that apart from surface topography quantification and high-resolution nanoscale imaging, AFM in conjunction with SEM-EBSD can be used for precisely locating sites for TEM study
Data for Gupta et al., "Estimating the Meridional Extent of Adiabatic Mixing in the Stratosphere using Age-of-Air", JGR:Atmospheres,
No full textModel data and post-processed data supporting the creation of the manuscript "Estimating the Meridional Extent of Adiabatic Mixing in the Stratosphere using Age-of-Air" submitted to JGR:Atmospheres in August 2022.
1) The netCDF files created through post-processing of full model data in FORTRAN are shared in the /data/ directory. These file contains the zonal mean circulation statistics based on Gupta et al. (2020), age-of-air transport diagnostics based on Linz et al. (2021), and the novel \Gamma-\Theta circulation streamfunction introduced in this study. The /data/ directory also contains MATLAB .mat data files for the transport diagnostics obtained from WACCM. 150 days of actual GFDL-FV3 model data in the northern hemisphere, between 0.1 hPa-500 hPa pressure levels is also provided to support external computations and validation.
2) The Jupyter notebook used for final computation and figures production is provided in .ipynb, .html and .pdf formats in /code/. All the files referred to in the notebook are stored in the /data/ directory.
Corresponding author : Aman Gupta, [email protected], [email protected], [email protected]
Corrigendum: Capital Inflows and House Prices: Aggregate and Regional Evidence from China
No full textIn the paper ‘Capital Inflows and House Prices: Aggregate and Regional Evidence from China’ by H. An, et al., printed in the December 2016 issue, there was a missing acknowledgement section for funding resources.
On page 451, the acknowledgement section should appear after the corresponding information as:
“Correspondence: Rakesh Gupta, Department of Accounting, Finance and Economics, Griffith Business School, Griffith University, Nathan Campus QLD 4111. [email protected]
*This work was financially supported by the Humanities and Social Science Foundation of Ministry of Education of China (16YJA790001).”
The author apologises for this error and any confusion it may have caused.No Full Tex
First person – Akash Gupta
No full textFirst Person is a series of interviews with the first authors of a selection of papers published in Biology Open, helping early-career researchers promote themselves alongside their papers. Akash Gupta is first author on ‘A novel and cost-effective ex vivo orthotopic model for the study of human breast cancer in mouse mammary gland organ culture’, published in BiO. Akash conducted the research described in this article while a PhD Scholar in Rajendra Mehta's lab at IIT Research Institute, Chicago, USA. He is now an assistant research scientist in the lab of Syreeta L. Tilghman at the University of Arizona, Department of Medicine, Tucson, USA, investigating drug efficacy modeling using human organoids culture for the treatment of cancers
Engineering materials : research, applications and advances / author, K.M. Gupta.
No full text"A CRC title."Includes bibliographical references and index.596 p.
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