• ABHINABA ROY

      Articles written in Journal of Earth System Science

    • Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

      H M Ramachandra Abhinaba Roy

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      The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).

    • Interpretation of stratigraphy and structure of the Neoarchaean Dharwar Supergroup of rocks in Chitradurga area, Dharwar Craton

      ABHINABA ROY H M RAMACHANDRA SILADITYA SENGUPTA

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      The Neoarchaean Dharwar Supergroup of rocks in the Chitradurga area unconformably overlie the Mesoarchaean Peninsular Gneissic Complex in the west and are tectonically juxtaposed with Javagondanahalli Schist Belt in the east. The rocks of the supergroup have been divided into older Bababudan and younger Chitradurga Groups. We support the recent division of the Bababudan rocks into a lower conglomerate–sandstone facies association and an upper sandstone–mudstone facies association indicating tidal flat depositional environment. The Talya Conglomerate sequence at the base of the Chitradurga Group is inferred to represent a fault-controlled debris flow deposit. The basin opens out to the east where sedimentation and volcanism took place on an uneven basement surface. The Vanivilas and Ingaldhal Formations likely represent contemporaneous and overlapping sequences indicative of facies variation in space. The KM Kere Conglomerate at the base of the Hiriyur Formation represents a facies series comprising a sequence of volcanic–pyroclastic–volcaniclastic–epiclast association. We propose a four-fold stratigraphic classification with introduction of a new ‘Kantaramanahalli Formation’, placed above the Vanivilas and Ingaldhal Formations and below the Hiriyur Formation. The interpretation of multiple deformed nature of Dharwar Supergroup of rocks and the dominance of the second deformation ($D_{2}$) is supported. The initial irregularities on basement surface and the $F_{1}$ folds have significant role in fold superposition and outcrop patterns. The intra- and interformational ductile shear zones have dominant sinistral transcurrent component. Structural studies are conclusive of simple shear ($D_{2b}$ ) superposed on intense pure shear ($D_{2a}$) indicative of an overall transpressional type of horizontal tectonics.

    • Study of small-scale structures and their significance in unravelling the accretionary character of Singhbhum shear zone, Jharkhand, India

      ABHINABA ROY ABDUL MATIN

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      Localized strain within tabular ductile shear zones is developed from micro- to meso- to even large scales to form complex structures. They grow in width and length through linkage of segments with progressive accumulation of strain and displacement, and finally produce shear zone networks characterized by anastomosing patterns. Singhbhum shear zone (SSZ) represents a large composite zone characterized by a collage of different dismembered lithotectonic segments, with heterogeneous structural features, within a matrix typical of a shear zone. Structural features indicate that the material properties of protoliths have a great role in controlling the mechanics of deformation. Meso- and micro-scale structural studies of the east-central part of the SSZ reveal ‘tectonic complex like’ (? deeper level equivalent of mélange type complex) assemblage of dismembered lithoteconic units. Shear-induced foliations, S, C and C$^{\prime}$, were developed while the main mylonitic foliation is represented by C-plane. Apart from that, shear lenses are exceptionally well developed in both meso- and micro-scale in most of the units, particularly in schistose rocks. They were formed from different processes during progressive simple shear, which includes (1) anastomosing C-planes, (2) intersection between C- and C$^{\prime}$-planes, (3) disruption of stretched out longer limbs of asymmetric folds, and (4) cleavage duplex. Fabrics recorded in rocks indicate that there was a progressive change in the development of predominantly flattening fabric (coaxial pure shear) in the northern part (outside the SSZ), to simple-shear non-coaxial type deformation producing shear fabric, dominating over the flattening fabric, in the southern part (within the SSZ) that is in close proximity with the Singhbhum Craton. Although an overall plane strain simple shear model is apparent, occasional presence of extensional features along two directions of the mylonitic foliation, demonstrative of three-dimensional deformation (simple shear and flattening: X > Y > 1 > Z), may indicate the stretching nature of the SSZ. From the orientation of oblique grain shape fabric [ISA$_{max}$ (${\theta}$ < 45$^{\circ}$)], there is slight deviation from simple shear, i.e., a sub-simple nature of plane strain shear could be inferred. However, in conformity with simple shear model the ubiquitously developed stretching lineation shows consistency in orientation being parallel to the movement direction. There is no evidence of transpression. Shear sense indicators invariably indicate up-dip ductile thrust movement with vergence top-to-the south. Microstructural deformational characteristics indicate that peak temperature attained during the deformation in shear zone was ${\sim}$600$^{\circ}$C. Prolonged period of metasomatism, induced by fluid influx, played an important role in strain softening during the development of SSZ.

      $\bf{Highlights}$

      $\bullet$ Meso- to micro-scale structures in Singhbhum Shear Zone (SSZ) show imprints of shear induced foliations, S, C and C$^{\prime}$ and formation of shear lenses attesting protracted progressive deformation.

      $\bullet$ SSZ characterizes accretionary character with dismembered heterogeneous lithotectonic units similar to tectonic complex zone.

      $\bullet$ Shear sense criteria consistently show top-to-south movement of the hanging wall of SSZ.

      $\bullet$ A gradual transition from predominantly flattening deformation in the northern hanging wall to dominantly simple shear with subordinate flattening in the SSZ. Boundary conditions suggest stretching nature of SSZ.

      $\bullet$ Fluid influx played immense role in strain softening during deformation.

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