Articles written in Journal of Earth System Science

    • Lithofacies analysis of volcanics and volcaniclastics of an ancient volcanic terrain with signatures of subaerial plinian volcanism: An example from Neoarchaean-Palaeoproterozoic Nandgaon Group, Bastar Craton, central India


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      The N–S trending Neoarchaean to Palaeoproterozoic Dongargarh–Kotri belt in the Bastar Craton in central India exposes bimodal volcanics and volcaniclastics of the Nandgaon Group. In this contribution, lithofacies analysis of the Nandgaon Group has been attempted mainly based on distribution of rhyolites, basalts and pyroclastics in this ancient volcanic terrain, their field characteristics and petrography. The study area in the central part of the belt around Dongargaon–Gotatola area can be two distinct domains based on distribution and composition of volcanics and volcaniclastics. Domain-I comprises coherent and autoclastic rhyolite, and Domain-II with basic lava and pyroclastics. The rhyolites of Domain-I can be further divided into feldspar quartz phyric, quartz phyric, aphyric, and autoclastic rhyolites. Basic lava of Domain-II includes basalt and andesitic basalt. The associated volcaniclastics are mainly coarse lapilli tuff, fine lapilli tuff and ash beds. Field characteristics of coherent lavas and volcaniclastics attest to phreato-magmatically controlled plinian to sub-plinian style of eruptions. Effusive and explosive eruptive processes through multiple vents gave rise to the observed volcanic sequences. This study indicates that subaerial volcanism initially led to formation of felsic ignimbrites and ash deposits followed by effusive rhyolite lava flows. Subsequently, mafic volcanism occurred giving rise to pyroclastic flows and surges, followed by effusive mafic lava flows.

    • Clast size analysis of pseudotachylite co-existing with mylonite: Constraints on evolution of Mahanadi Shear Zone, Eastern Ghats Mobile Belt, India


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      The Mahanadi Shear Zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB) separates the southern segment of EGMB with dominant NE–SW structural trend from its northern segment with dominant E–W trend. The MSZ is a high-temperature ductile shear zone in which mylonite–ultramylonite occur sintimately associated with pseudotachylite. The association of two components of contrasting origins (ductile and brittle deformations) is possibly attributed to contemporaneous evolution or multiple stages of evolution. Therefore, field and petrographic studies including clast-size analysis of three samples from different localities across the MSZ have been carried out. Field observations suggest that pseudotachylite veins often cut across the mylonitic fabric. In addition, there is variation in orientation of gneissic and mylonitic fabrics in the clasts within the pseudotachylite zones. The pseudotachylites occasionally exhibitcrude flow structures and rims around clasts. These features clearly indicate that the mylonitic fabric in the MSZ predates brittle deformation, and the pseudotachylites have not suffered any subsequent deformation after their formation. Clast-size analysis in all three cases follows the Power Law and demonstrates linear distribution. Lower value for fractal dimension (D = 0.51–0.69) and involvement of large range of size distribution of clasts in the studied pseudotachylite samples are attributed to brittle deformation at high strain rate, independent of the ductile deformation event. The present study suggests evolution of the MSZ in two stages, i.e., early development of mylonite–ultramylonite by ductile deformation at deeper level followed by pseudotachylite development at later stage by brittle deformation at a shallower level, thereby overprinting of brittle deformation over the ductile deformation.


      $\bullet$ The Manuscript describes the coexistence of pseudotachylite, mylonite and ultramylonites in the MSZ.

      $\bullet$ Clast size analysis of pseudotachylite suggests a single brittle deformation event.

      $\bullet$ We have highlighted the deformation evolution of the MSZ in the EGMB.

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