Strain estimation of folds, orbiculites and quartz phenocrysts in the Ambaji basin of the south Delhi terrane, Aravalli-Delhi Mobile belt, Nw India and its tectonic implication

Abstract

Ambaji basin in the south Delhi terrane of Aravalli-Delhi Mobile Belt consists of greenschist facies rocks which are intruded by 850–750 Ma Ambaji granites and several phases of quartz and pegmatite veins. The basin is tectonically juxtaposed against the Balaram-Kui-Surpagla-Kengora Granulite Complex. In this paper the deformation structures of the Ambaji basin has been studied and strain has been measured using different markers like folds, orbiculites and quartz phenocrysts and its tectonic implication has been discussed. Strain due to folding has been expressed in terms of shortening that has been estimated using wavelength / arc length ratio. Since the folds belong to Class 1C geometry, the arc length of the parent Class 1B fold has been calculated after removing the homogeneous strain component from Class 1C fold. The above technique was verified by drawing a Class 1B fold and modifying to Class 1C by homogeneous strain in Macromedia FreeHand MX software. Further, this was cross checked by measuring strain in two strain ellipses that represent respectively shortening in Class 1B and Class 1C folds. The shortening was estimated to be ca. 65% and 55% during F1 and F2 folding respectively. In order to know the finite strain in the folded rocks the deformed orbiculites in the metabasalt and quartz phenocrysts in the metarhyolite and granites have been analysed by Normalized Fry techniques. An apparent flattening strain with about 10 to 23 % volume loss has been estimated; correlation between inclinations of the longer axis of strain ellipse with strain suggests layer parallel shear to be the primary cause of folding. It has been suggested that buckling associated with moderate shortening and low volume loss are responsible for the deformation in the low grade rocks of Ambaji basin during evolution of South Delhi Terrane.