• Nibir Mandal

      Articles written in Journal of Earth System Science

    • Mode of development of sigmoidal en echelon fractures

      Nibir Mandal

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      The solution of stress distribution for a multicrack system and model experiments confirm that en echelon cracks mutually interact with each other during their growth. Such a mechanical interaction deviates the crack-tip stress axes orientations from that of the bulk stress field and leads to a continuous change in propagation direction of tension cracks, initially at a right angle to the bulk tension direction. The sigmoidal shape of en echelon fractures evolve through rotation and crack length increments with changing orientations. The theoretical analysis shows that the instantaneous fracture-tip stress orientation is a function of initial crack spacing, orientation of crack array with respect to the principal axes of far-field stress.

    • Is the outcrop topology of dolerite dikes of the Precambrian Singhbhum Craton fractal?

      Nibir Mandal Atin Kumar Mitra Santanu Misra Chandan Chakraborty

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      In the Precambrian Singhbhum Craton of eastern India, newer dolerite dikes occur profusely with varying outcrop lengths. We have analysed the nature of their length-size and orientation distributions in relation to the theory of fractals. Two orientational sets of dikes (NW-SE and NE-SW) are present. Both the sets show strongly non-power-law size distributions, as reflected in non-linear variations in logarithmic space. We analyzed thousands of data, revealing that polynomial functions with a degree of 3 to 4 are the best representatives of the non-linear variations. Orientation analysis shows that the degree of dispersions from the mean trend tends to decrease with increasing dike length. The length-size distributions were studied by simulating fractures in physical models. Experimental fractures also show a non-power-law distribution, which grossly conforms to those of the dolerite dikes. This type of complex size distributions results from the combined effects of nucleation, propagation and coalescence of fractures.

    • Varying frontal thrust spacing in mono-vergent wedges: An insight from analogue models

      Puspendu Saha Santanu Bose Nibir Mandal

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      Sandbox experiments are used to study frontal thrust fault spacing, which is a function of physical properties within the thrust wedge. We consider three styles of thrust progression in mono-vergent wedges: Style I, II and III. In Style I, frontal thrusts progress forelandward, maintaining a constant spacing, whereas Style II and Style III progression show increasing and decreasing spacing, respectively. The three styles are shown as a function of the following factors: basal friction (𝜇b), initial surface slope (𝛼) and basal slopes (𝛽), and surface erosion. For high 𝜇b (∼0.46), thrust progression occurs in Style II when 𝛼 > 2° and 𝛽 > 0.5°, and in Style III when 𝛼 and 𝛽 are high (𝛼 < 2° and 𝛽 < 0.5°). Style II transforms to Style I when the wedge undergoes syn-thrusting surface erosion. In contrast, low-basal friction (𝜇b = 0.36) gives rise to either Style I or III, depending on the magnitudes of 𝛼 and 𝛽. Conditions with 𝛼 = 𝛽 = 0 developed Style I, whereas Style III in conditions with any non-zero values of 𝛼 and 𝛽. In this case, surface erosion caused the process of thrust progression unsteady, and prompted outof-sequence thrusting in the wedge. This study finally presents an analysis of the three styles, taking into account the following two parameters: (1) instantaneous increase of hinterland thickness (𝛥 H2/He) and (2) forelandward gradient of wedge thickness (𝛿 H/𝛿x). Experimental data suggest that thrust sequences develop in Style II for low 𝛿 H/𝛿x and large 𝛿 He/He values and, in Style III as either 𝛿 H/𝛿x increases or 𝛥 He/He drops.

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