• N Ghosh

      Articles written in Journal of Earth System Science

    • Polyphase deformation and garnet growth in politic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast–matrix relationship

      A Chattopadhyay N Ghosh

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      Polyphase deformation and metamorphism of pelitic schists of Chorbaoli Formation of Sausar Group in and around Ramtek area,Nagpur district,Maharashtra,India has led to the development of garnet and staurolite porphyroblasts in a predominantly quartz –mica matrix.Microstructural study of oriented thin sections of these rocks shows that garnet and staurolite have different growth histories and these porphyroblasts share a complex relationship with the matrix.Garnet shows at least two phases of growth –first intertectonic between D1 and D2 (pre-D2 phase)and then syn-tectonic to post-tectonic with respect to D2 deformation.Growth of later phase of garnet on the earlier (pre-D2 garnet grains has led to the discordance of quartz inclusion trails between core and rim portion of the same garnet grain.Staurolite develops only syn-D2 and shows close association with garnet of the later phase.The peak metamorphic temperature thus coincided with D2 deformation,which developed the dominant crenulation schistosity ($S_2$ ,regionally persistent in the terrain.The metamorphic grade reached up to middle amphibolite facies in the study area, which is higher than the adjoining southern parts of Sausar Fold Belt.

    • The initiation and linkage of surface fractures above a buried strike-slip fault: An experimental approach

      N Ghosh A Chattopadhyay

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      Surface fractures in the overburdened sedimentary rocks, formed above a deep-seated basement fault, often provide important information about the kinematics of the underlying master fault. It has already been established that these surface fractures dynamically evolve and link one another with progressive displacement on the master fault below. In the present study, two different series of riedel-type experiments were carried out with clay analogue models under different boundary conditions viz., (i) heterogeneous simple shear of the cover rocks above a buried strike slip fault (wrench system) and (ii) heterogeneous simple shear with a component of shear-normal compression of the overburden package above a basement fault (transpressional system), to observe the initiation and linkage of surface fractures with varying $T^\prime$ (where $T^\prime$ = thickness of the overburden normalized with respect to the width of the master fault). In the wrench system, Riedel (R) shears were linked by principal displacement (Y) shears at early stages (shear strain of 0.8 to 1) in thin (2 > $T^\prime$ > 18) models and finally (at a minimum shear strain of 1.4) gave rise to a through-going fault parallel to the basement fault without development of any other fracture. Conjugate Riedel (R′) shears develop only within the thicker ($T^\prime$ < 18) clay models at a minimum shear strain of 0.7. With increasing deformation (at a minimum shear strain of 1.2) two R′ shears were joined by an R shear and finally opened up to make a sigmoidal vein with an asymmetry antithetic to the major faultmovement sense. Under transpression, the results were similar to that of heterogeneous simple shear for layers 2 > $T^\prime$ > 15. In layers of intermediate thickness (15 > $T^\prime$ > 25) early formed high angle R shears were cross cut by low angle R shears (at a minimum shear strain of 0.5 and shortening of 0.028) and “Riedel-within-Riedel” shears were formed within thick ($T^\prime$ < 25) models (at minimum shear strain of 0.7 and shortening of 0.1), with marked angularity of secondary fault zone with the master fault at depth.

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