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.

    • ModiBcation of pre-existing folds in a shear zone: A case study from Kumbhalgarh–Ranakpur area, South Delhi Fold Belt, Rajasthan, India


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      Pre-existing structures (e.g., folds, foliations, lineations) are usually rotated and modified within a shear zone, depending on their attitude with respect to the kinematic framework (i.e., orientation of instantaneous stretching axes (ISA), kinematical vorticity number of the flow ($W_{k}$)) of the shear zone. In addition, new folds may also form within a shear zone, and it is not always easy to distinguish between the pre- and syn-shearing structures in the field, especially if they form on the same rock type. The present contribution describes the reorientation and shape modification (tightening) of pre-existing and synshearing folds in metamorphosed calcareous rocks of the Kumbhalgarh Group (part of Delhi Supergroup) due to shearing along the Ranakpur Shear Zone (RSZ), in Kumbhalgarh–Sayra–Ranakpur area of South Delhi Fold Belt (SDFB), also called South Delhi Terrane (SDT). From field-based study and measurements, it is shown that the shallow-plunging, upright second generation ($\rm{DF}_{2}$) folds of SDFB/SDT have been rotated to subvertical, tight folds within the RSZ. Fold shape analysis using layer thickness and limb dip of folds (i.e., Ramsay’s classification) and by Fourier transform shape analyses of fold profile sections corroborates and roughly quantifies the tightening and shape modification of pre-existing folds within the RSZ. In contrast, syn-shearing folds have formed on the foliations in calc-silicate rocks which show strongly non-cylindrical geometry with apical defection in an oblique direction. From the available shear sense indicators like rotated porphyroclast tails and vergence of asymmetric folds, the shear sense of the RSZ is interpreted as oblique reverse (east-side-up) with subordinate sinistral (east-towards-north) shear component, which is similar to the shear sense interpreted by some earlier workers.

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