Precambrian Research, 42 (1988) 77-96 77 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands RESPONSE OF BASEMENT AND COVER ROCKS TO MULTIPLE DEFORMATIONS: A STUDY FROM THE PRECAMBRIAN OF RAJASTHAN, WESTERN INDIA K. NAHA Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, West Bengal (India) S. MOHANTY Department of Applied Geology, Indian School of Mines, Dhanbad 826004, Bihar (India) (Received July 21, 1987; revision accepted April 14, 1988) Abstract Naha, K. and Mohanty, S., 1988. Response of basement and cover rocks to multiple deformations: a study from the Precambrian of Rajasthan, western India. Precambrian Res., 42: 77-96. Metasedimentary rocks of the Aravalli Group of early Proterozoic age (2500-2000 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 (Fla ) 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, Fla 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. Introduction Basement-cover relationships in Precam- brian orogenic belts are of special interest be- cause of the light they shed on the question of evolution of the Precambrian crust. Structural complexities due to superposed deformations often blur the basement-cover interfaces, heightening the problem of identifying these contacts. In the Aravalli Mountain Belt of western India (Fig. 1), the crustal development possibly started as early as 3500 Ma (Sm-Nd dating by Macdougall et al., 1983). The meta- sedimentary rocks of the older Aravalli and the younger Delhi Groups were successively welded to the earlier crust, represented now by mig- matites forming the Banded Gneissic Complex (BGC). Crustal development during the Ara- valli cycle started with the outpouring of mafic and possibly ultramafic lavas. The existence of primitive life during this time is recorded in the stromatolitic phosphorite/dolomite. The Ara- 0301-9268/88/$03.50  1988 Elsevier Science Publishers B.V.