Sinking of the Dharwar basin (south India): implications for Archaean tectonics

Abstract

Archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite-gneiss masses. The late Archaean Bababudan greenstone belt (Dharwar craton, South India) contains a low-grade volcanosedimentary basin sequence (the Dharwar Supergroup) which unconformably overlies a previously eroded basement made up mainly of granite and gneiss. Fabric trajectories, kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks, as well as strike-slip shearing. This is attributed to the progressive sagduction (gravity-sinking) of the greenstone belt into its basement along a décollement horizon and to late transcurrent shearing. We propose a deformation model for the Bababudan belt that may explain common features of many greenstone belts (such as décollements and superimposed upright fold patterns) which are attributed to the progressive sinking of the supracrustals into their basement. The results of our study and other field investigations in the crust of the Dharwar craton, as well as structural evidence from other cratons (Zimbabwe and Pilbara), raise the possibility that a unique continental tectonic regime existed during the Archaean, controlled by the emplacement of dense volcanic traps, mantle underplating and thermal blanketing of the continental crust. This tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity (i.e. Rayleigh-Taylor) instabilities between the volcanic traps and the rest of the continental crust. It allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating. The diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large-scale accretion events of the continental crust, perhaps in hot-spot environments.