• Anupam Chattopadhyay

      Articles written in Journal of Earth System Science

    • Propagation and coalescence of en-echelon cracks under a far-field tensile stress regime: An experimental study

      Nilanjana Ghosh Kalyanbrata Hatui Anupam Chattopadhyay

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      Experiments were carried out to observe the variation in propagation and linkage of parallel en-echelon cracks with varying orientation of the crack array and different relative position of the cracks within the array in an extensional regime. Two-layered analogue model, with a basal layer of pitch overlain by a layer of kaolin paste was used in the experiments. En-echelon cracks were pre-cut within the kaolin layer maintaining specific geometrical parameters of the cracks (e.g., length, centre spacing, separation) in such a manner that there was a weak (though not negligible) local tip-induced stress favouring curvature of adjacent crack tips towards one another. The results obtained were matched with natural pattern of linkage of veins, rift basins and spreading ridges, as described in the relevant literature. The experimental results showed that the final pattern of linkage between the cracks was a result of initial deflection of crack tip from its plane due to combined effect of local and far-field stress. When the deflection of tip from the crack plane was between 0$^{\circ}$ to 45$^{\circ}$, a ‘tip to wall’ linkage took place between adjacent cracks isolating a rhombohedral area in the interaction zone. The resultant structure could be geometrically comparable to a micro-plate-like structure isolated due to linkage of ridge segments initially forming an overlapping spreading centre (OSC). When the deflection of tip from the crack plane was greater than 45$^{\circ}$, a ‘tip to tip’ linkage between adjacent cracks took place resulting in a structure similar to a transform fault between spreading ridges and or rift basins. When effect of the remote stress opposed the tip induced stress, no linkage took place between the adjacent cracks, and finally the tips propagated straight along a plane perpendicular to the remote extension direction.

    • Roundness of survivor clasts as a discriminator for melting and crushing origin of fault rocks: A reappraisal

      Arindam Sarkar Anupam Chattopadhyay Tusharika Singh

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      Roundness of survivor clasts (mineral/rock fragments) in fault rocks (e.g., pseudotachylyte and cataclasite/gouge) has been used by some workers to distinguish melt-origin from crush-origin of such rocks. Keeping in view the large overlap in the published data on the roundness of fault rocks, the reliability of such a discriminator appears somewhat uncertain. The present study attempts to reappraise the aforesaid criterion through roundness analysis of quartz and feldspar clasts in melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and fault-related cataclasite (F-Ct) collected from Sarwar–Junia Fault Zone in Rajasthan and from Gavilgarh–Tan Shear Zone in central India. Our analysis shows that roundness of clasts cannot reliably distinguish between fault rocks of melt-origin and crush-origin (especially M-Pt and F-Ct) as the roundness values overlap and a distinct limit of roundness value for each rock type cannot be established. While the roundness of clasts in M-Pt and C-Pt may be enhanced due to melt-induced rounding off of the initially angular clasts, rounding of clasts can also occur by abrasion during rolling of crushed material in F-Ct. Furthermore, anomalous thermal expansion of clasts in melt-origin pseudotachylyte may cause disintegration of larger clasts into smaller angular fragments, thereby increasing the percentage of angular clasts in melt-origin fault rocks. Therefore, roundness of survivor clasts cannot be solely used as a discriminator between melt-origin and crush-origin fault rocks.

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