• NILANJAN DASGUPTA

      Articles written in Journal of Earth System Science

    • Characteristics of pegmatoidal granite exposed near Bayalan, Ajmer district, Rajasthan

      Nilanjan Dasgupta Taritwan Pal Joydeep Sen Tamoghno Ghosh

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      The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D2 deformation of the Delhi orogeny along the axial planes of D2 folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses.

    • Tectonic imprints within a granite exposed near Srinagar, Rajasthan, India

      Trishit Ruj Nilanjan Dasgupta

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      Partial melting in the middle to lower crustal level produces melts of granitic composition during orogeny. Thrusts play a vital role in their exhumation after consolidation of these granitic melts. In this paper we focus on one such granite along the eastern margin of the Delhi Fold Belt (DFB) rocks near Srinagar, Rajasthan, India. This is the first report of granite within the area and holds a key stratigraphic position in the entire rock package. The said granite is found to be intrusive to the DFB metasediments as well as their basement popularly known as the Banded Gneissic Complex (BGC). We disentangle the deformation fabrics seen within the granite and associated DFB metasediments, suggesting that subsequent to emplacement and consolidation, the granite has co-folded along with the country rocks. Three deformational events could be identified within the DFB metasediments namely, D1D, D2D and D3D. The peak metamorphism was achieved in the D1D event. The granite magma is generated and emplaced late syn-kinematic to D1D and thereafter is deformed by D2D and D3D producing D1G and D2G structural fabrics. These compressive deformations resulted in the collapse of the basin; the combined package of DFB rocks and the granite was thrusted eastwards over the basement rocks. The tectonic transport direction during thrusting is suggested eastwards from our structural analysis. Transverse faults developed perpendicular to the length of the granite have led to partitioning of the strain thereby showing a heterogeneity in the development of fabric within it.

    • The role of pre-existing faults and fractures in shaping polygonal impact craters and its tectonic implications in the southern Margaritifer Terra region, Mars

      SUPRATIK BASU NILANJAN DASGUPTA ABHIK KUNDU SATADRU BHATTACHARYA DIPAYAN DASGUPTA

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      The presence of polygonal-shaped craters, i.e., craters with complete or incomplete polygonal rims along with circular or elliptical outlines, on the surface of celestial bodies has been known for nearly a century. However, many investigations on their distribution and formation have not been carried out, until recently. Scientists have proposed that the polygonal shapes of the crater rims owe their origin to the preexisting structurally weak planes like faults/fractures in the area adjacent to the impact. The present study area is the southern part of Margaritifer Terra, Mars; a mid-Noachian terrain, which has craters of different morphologies, including the polygonal impact craters (PICs). The study focuses on the reason for the development of PICs by identification and mapping of a population of 50 selected polygonal craters along with morphotectonic features namely grabens, wrinkle ridges, and lobate scarps. The analysis of orientations of the straight segments of polygonal crater rims shows marked resemblance with orientations of these morphotectonic features conforming to their control on the rim geometries.

      $\bf{Highlights}$

      ${\bullet}$ Structural mapping of the southern Margaritifer Terra region, Mars.

      ${\bullet}$ Generating rose diagrams and line graphs for the orientation of the straight edges of polygonal craters and the morphotectonic features within the southern Margaritifer Terra region.

      ${\bullet}$ Conducting statistical F-test to find out the bearing of weak planes on the formation of PICs.

      ${\bullet}$ Finding out the presence of buried weak planes by comparing the rose diagrams and spike graphs.

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