Significance of contrast in the early stages of the structural history of the Delhi and the pre-Delhi rock groups in the Proterozoic of Rajasthan, western India

Abstract

Structures of four generations are decipherable both in the pre-Delhi rocks of central Rajasthan, and in the Delhi rocks of Khetri in northeastern Rajasthan and around Todgarh in central Rajasthan. There is a remarkable identity in the later phases of the deformational history of the two groups, with gravity-induced structures followed by conjugate folds due to longitudinal shortening (N-S in northeastern Rajasthan and NE-SW in central Rajasthan). The earlier stages of the structural history of the two groups are, however, significantly different. The E-W-trending reclined folds of the first generation in the pre-Delhi rocks are absent in the Delhi rocks throughout Rajasthan. The NNE- to NE-trending folds of the second generation in the pre-Delhi groups are upright, whereas these structures in the Delhi rocks are of two phases-recumbent folds, followed by coaxial upright folds. The folds of the first and the second phases in the Delhi rocks plunge gently NE or SW where they are not affected by subsequent deformations. But the NE-trending folds in the pre-Delhi rocks show an extreme variation in axial plunge from horizontal to vertical, even where they are unaffected by later movements. Evidence has been adduced to suggest that these differences in the earlier phases of the structural evolution of the two groups are due to an angular unconformity between the Delhi and the pre-Delhi rocks.