Journal of the Geological Society of India Vol: 64, No: 1, July 2004, pp: 21-31 Evolution of the Great Boundary Fault: A Re-Evaluation A. RAI CHOUDHURI and D. B. GUHA Abstract: The Great Boundary Fault (GBF) in southeastern Rajasthan originated as a thrust in a brittle-ductile/ductile regime. Since it has affected the youngest of Vindhyan formation, it is post-Vindhyan in age. Both the basement and the Vindhyan sediments occurring on the hanging wall side of the fault have been folded into large-scale low plunging folds trending parallel to the GBF. Increase in tightness and asymmetry of the folds near the fault suggest that these are fault related folds. Development of folds, occurrence of oriented fabric, recrystallisation in the vicinity of the GBF and presence of undisturbed mylonite as remnant bodies along the GBF suggest that the faulting took place in a brittle-ductile/ductile regime. Basin-ward vergence of the tight to isoclinal folds near the GBF and steep dip of the mylonite bands towards the basement indicate that the fault plane dips steeply towards the basement. Increasing tightness of the fault related folds towards the fault, moderate to steep dip of the fault towards the basement and juxtaposition of the basement rocks in the hanging wall with the younger sediments in the footwall prove that the fault originated as a thrust. Concomitant with the thrusting a conjugate set of brittle-ductile shear zone was also developed in the hanging wall side of the fault. The acute bisector of the conjugate set indicates a northwest-southeasterly directed compression, which is also perpendicular to the axial planes of the fault related folds. Subsequent to thrusting, several sets of brittle normal faults developed in the area. These faults transect the large scale fault related folds and even the GBF. One of these sets of normal faults strikes parallel to the GBF trace. Presence of such normal faults parallel to the GBF and fragments of mylonite as angular pieces within the breccia along the GBF imply that the GBF was subjected to a late brittle reactivation. Near the GBF, presence of close spaced GBF parallel joints also proves that the GBF was subjected to a late tensile stress.
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