Basement-cover relations between a granite gneiss body and its metasedimentary envelope: a structural study from the Early Precambrian Dharwar tectonic province, southern India

Abstract

The Sirankatte granite gneiss, an oval-shaped body within the metasedimentary rocks of the Chitradurga Group in the Dharwar tectonic province of Early Precambrian age, occupies the core of a slightly overturned antiform with axial culmination. This antiform, with steep and moderate ENE dips of the western and eastern limbs, respectively, and with axes plunging gently NNW in the northern part and SSE to ESE in the southern part, represents a second generation fold. Isoclinal folds on stratification in the cover rocks as well as on gneissic banding in the granite gneiss have been involved in this folding coaxially. An associated axial planar cleavage dipping steeply towards ENE has been superimposed on both the granitic and metasedimentary rocks. Although conglomerates with pebbles, cobbles and boulders of quartz and granite gneiss at the contact of the granite at a number of places suggest the granite to be the basement, synkinematic migmatization and ductile deformation during the first folding show that the original granitic basement must have been extensively remobilized during the first deformation. Coaxial second folding due to compression in an almost horizontal WSW-ENE direction resulted in the development of an antiform with granite core resembling a mantled gneiss dome. Late folds on ENE-WSW axial planes accentuated the culminations and depressions of the folds of the first and second phases.