Facies distributions and structure of a Dharwar volcanosedimentary basin: evidence for late Archaean transpression in Southern India?

Abstract

The Bababudan-Nallur basin (c. 60 km × 125 km) is part of a series of late Archaean basins accommodating volcanic and sedimentary rocks of the Dharwar Supergroup within the low-grade terrain of the province of Karnataka. Initial development of the Bababudan-Nallur basin was characterized by rapid subsidence with eruption of mainly basaltic lavas. Breaks in eruption were marked in some instances by thin but persistent deposits of shallow marine quartz sands and muds. Subsidence in the south continued with accumulation of banded ferruginous cherts and muds, whilst uplift of the volcanic pile took place in the north. This uplift promoted erosion of the volcanic cover and its basement gneisses and granodiorites with deposition of alluvial and shallow marine fans and debris flows adjacent to the uplifted blocks. The fans passed laterally into intertidal and offshore quartz sands, greywackes and muds with interbedded dolomitic limestones. Accumulation of rhyolites, banded ferruginous cherts and greywackes took place further north. The structure of the basin is dominated by mainly upright folds and faults, with the trend of the folds broadly parallel to the boundaries of exposed blocks of basement. The basement deformed cataclastically on myriad fractures in contrast with the ductile behaviour of the cover. Many cover-basement contacts are faults with mylonites, although unconformities are preserved, especially in the south. The facies distributions and structure of Bababudan-Nallur and other Dharwar belts have characteristics in common with mixed-mode basins developed as a consequence of strike- or oblique-slip on long-lived fault zones. These relations suggest that transcurrent plate interactions comparable with those of the Phanerozoic were significant in the late Archaean evolution of southern India.