The stratigraphy and structure of the chitradurga region: an illustration of cover-basement interaction in the late Archaean evolution of the Karnataka craton, Southern India

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The succession in the late Archaean supracrustal belts in the Chitradurga region begins with basal quartz-pebble conglomerates and quartzites which were deposited on a basement of tonalitic—granitic gneisses, ca. 3000 Ma, containing tracts and enclaves of older supracrustal rocks. The basal beds are overlain by basic metalavas and amphibolites interbedded with cross-bedded quartzites and siliceous phyllites. This series passes up into siliceous phyllites, locally containing a large lenticular mass of polymict conglomerate (Talya Conglomerate), overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates. Greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks, the latter being more abundant at higher levels. With progressive deepening of the whole basin, fine-grained, graded greywackes and local polymict conglomerates (Aimangala Conglomerate) were deposited; the turbidite sequence forms the youngest sedimentary rocks exposed. Estimated total stratigraphic thickness is at least 10 km in deeper parts of the basin. Deformation led to a series of upright anticlines and synclines and related LS fabrics with extreme variations in plunge of the coaxial fold axes and L fabrics within axial surfaces which maintain a steep orientation. Low-grade metamorphism outlasted the deformation. Sedimentation and volcanism in the Chitradurga supracrustal belt is linked to variable subsidence of the basement foundation, probably as a result of rifting. The structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history. Variable displacement of the basement during these late Archaean events in the Karnataka craton took place on shear zones and related fractures that acted as channels for water and CO 2 as indicated by vein minerals and alteration products. Uplift of basement segments and their distension by granitic diapirs led to deformation of the supracrustal cover made ductile by its high water content.