The structural anatomy of the Sandur schist belt-a greenstone belt in the Dharwar craton of south India

Abstract

Two belts of metasedimentary rocks occurring at the eastern and western flanks and a central terrane of metavolcanic rocks make up the Late Archaean Sandur schist belt in the Dharwar craton of South India. Contrary to the traditional interpretations, the regional structure of the belt is not a synclinorium. The earliest episode of deformation produced major and minor isoclinal folds in the banded iron formation (BIF); their axial planes are parallel to the regional attitude of bedding. D₂ folds have a consistent sinistral sense of vergence throughout the schist belt and they refold the D₁ folds. The D₂ axial planes make small angles with regional bedding. There was a strong component of layer-parallel simple shear during D₂ deformation. The styles of D₁ and D₂ folds show a general similarity. The fold axes have variable plunge, though in general it is moderate to steep. The variability of the geometry of minor folds is ascribed to progressive development of folds and the rotation of their axes and axial planes during non-coaxial deformation (simple shear). It is likely that the D₁ and D₂ structures were not products of entirely unrelated episodes of deformation and they might have formed at the early and late stages of a progressive deformation history. A noteworthy feature is that the deformation is mostly concentrated in the two metasedimentary belts and the internal strain is low in the central metavolcanic unit. This is probably due to concentration of layer-parallel simple shear in the metasedimentary rocks. Both flattening and simple shear played major roles in the development of the structural pattern and the deformation regime may be compared to one of transpression.