Rotated and displaced clay balls as kinematic indicators along shear zones in sandstones of the Siwalik Group of Surghar-Shinghar Range, Trans-Indus Ranges, Pakistan
Abstract
Neo-tectonic events in the Siwalik Group of Surghar-Shinghar Range are characterized by compressional, extensional and conjugate shear fractures. The compressional fractures are in the form of reverse faults mainly along clay ball scoured surfaces. The sense of shear in these fractures is interpreted from rotated and displaced clay ball geometry. Shearing along these weak planes provide kinematics of clay ball geometrical behavior and their mechanical/strength characteristics under surface P/T conditions. Most of the displaced clay balls are moderately hard due to the presence of carbonates, while others constitute some fine carbonaceous material. Thus, the displacement in the clay balls is controlled by their hardness. Some displaced clay balls show nice geometrical arrangement of Riedel fracture sets accommodating strain during shearing. Despite near-surface conditions (i.e., incohesive-brittle) the clay balls, due to the inherited plastic character of clays, form structures resembling those in plastic regimes of deformation. These structures include asymmetric rotated clay balls with tail edges just like in porphyroclasts, while others are equantly rotated forming oxidized boundaries within the sandstone matrix with no tail edges. The kinematics of both rotated and displaced clay balls show reverse sense of shear all along the compressional faults.
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