Superposed deformation and inherited structures in an ancient dilational step-over zone: Post-mortem of the Rengali Province, India

Abstract

In the eastern Indian shield, a dextral strike-slip system juxtaposed the Archaean Singhbhum Province against the Proterozoic Eastern Ghats Belt at ∼490–470 Ma. Two WNW–ESE trending strands of the strike-slip system enclose a multiply deformed (D1 to D3) intervening domain called the Rengali Province, with D3 representing dextral shearing. In a granulite lens within the province, an early fabric (Sgr) was deformed by an amphibolite facies D1–D2 deformation continuum in the late Archaean time, forming cylindrical folds. In the surrounding quartzofeldspathic gneisses, quartzites and mica schists of the province, superimposition of syn-D3 shortening on D1-D2 folds generated complex non-cylindrical geometries; the granulites escaped D3 strain. Microstructures in the province-bounding shear zones confirm that D3 deformation was associated with mylonitization, dynamic recrystallization and greenschist facies metamorphism. In the quartzites, syn-D3 folds can be correlated with rotation of D1–D2 structures through the shortening zone of bounding dextral shears. Since the province-bounding shears form a step-over zone, the structural complexity within the Rengali Province arises from superposition of syn-D3 shortening structures on initially asympathetically oriented inherited cylindrical D1-D2 folds. Hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied D3 as the step-over zone was dilational in nature.