Response of basement and cover rocks to multiple deformations: a study from the precambrian of Rajasthan, Western India

Metasedimentary rocks of the Aravalli Group of early Proterozoic age (2000-2500 Ma) are underlain by migmatitic gneisses forming the Banded Gneissic Complex (BGC), with radiometric ages varying widely from 3500 Ma to much younger. Both the basement and the cover rocks are, however, involved in superposed folding of identical style and sequence. In the metasedimentary cover rocks, isoclinal folds of the first set (F1), with an axial planar cleavage, have been affected by coaxial folds (F14) in some places, and have been overprinted by a set of open, upright folds (F2) with N-S-striking axial planes. These structures are affected by small-scale conjugate folds and kink bands (F3) with subhorizontal axial planes caused by a vertical compression, and by upright kinks and conjugate folds (F4) developed by longitudinal shortening. The F1, F1a and F2 structures match in their entirety with those in the migmatites forming the basement gneisses, the migmatization being synkinematic with the first folding. This suggests that the BGC in its present state represents a largely mobilized basement