Structural Pattern in the Dharwar Craton

Abstract

High-grade and low-grade supracrustal belts (Sargur and Dharwar) set in a matrix of gneisses dotted with granitic plutons of different ages make up the Dharwar craton. The overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from NW-SE in the northern part to N-S in the south. Both the high-grade and low-grade belts show three principal episodes of folding. However, the correlation between the deformation episodes in the Sargur and the Dharwar rocks is controversial, and the structural relations between them are not unequivocally established: parallelism of the first phase structures in the Sargur and the Dharwar rocks can be interpreted either as synchronous development or rotation of the earlier Sargur structures into parallelism with the younger Dharwar structures during later deformation. The extension of the supracrustal belts is parallel to the axial traces of the second deformation. Granite diapirism and basement uplift further modified the structural pattern in the Dharwar belts. In the Sargur Group the peak of metamorphism (upper amphibolite to lower granulite facies) was attained during the first deformation, which may be older than 3000-3100 Ma. There is no consensus regarding the age of the first deformation of the Dharwars; it is either coeval with the Sargur first deformation or younger. The closing stage of the second deformation in the Dharwars is no younger than 2500-2600 Ma. The climax of Dharwar metamorphism (greenschist or lower amphibolite facies) is post-tectonic with respect to the first deformation. The relation of the oldest component (3400 Ma) of the polyphase Peninsular Gneiss to the Sargurs is not established. Another major component was emplaced at 3000-3100 Ma, syn- to post-tectonic with respect to the first phase Sargur deformation. A third component was emplaced at 2500-2600 Ma at the late- to post-tectonic stage of the second phase Dharwar deformation. Ductile deformation style and polyphase migmatization, interspersed with folding episodes, characterize the Peninsular Gneiss. The gneisses exhibit a complicated deformation history: no comprehensive regional analysis is currently possible. The high-grade granulite terrain in southern Karnataka evolved at about 2500-2700 Ma under essentially static conditions. Much charnockite formation was definitely later than the formation of the gneissic foliation in the Peninsular Gneiss, and the structures in the granulites and in the low- to medium-grade terrains are essentially the same. It remains to be seen whether this is a mimetic preservation. It is possible that the high-grade terrain contains even older granulites, perhaps resulting from a metamorphism at 3000-3100 Ma ago. The youngest deformation features are the E-W shear systems in southern Karnataka and northern Tamil Nadu, which are probably Proterozoic in age and correlate with the N-S thrust system in the Eastern Ghats. These have caused retrogression of the granulites and development of mylonites and augen gneisses.