Geometry and evolution of superposed folding in the Zawar lead-zinc mineralised belt, Rajasthan

Abstract

The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show classic development of small-scale structures resulting from superposed folding and ductile shearing. The most penetrative deformation structure noted in the rocks is a schistosity (S ₁) axial planar to a phase of isoclinal folding (F ₁). The lineations which parallel the hinges of F₁ folds are deformed by a set of folds (F₂) having vertical or very steep axial planes. At many places a crenulation cleavage (S₂) has developed subparallel to the axial planes of F₂ folds, particularly in the psammopelitic rocks. The plunge and trend of F₂ folds vary widely over the area. Deformation of F₂ folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are the main imprints of the third generation folds (F₃) in the region. In addition to these, there are at least two other sets of cleavage planes with corresponding folds in small scales. More common among these is a set of recumbent and reclined folds (F₄), developed on steeply dipping early-formed planes. Kink bands and associated sharp-hinged folds represent the other set (F₅). Two major refolded folds are recognizable in the map pattern of the Zawar mineralised belt. The larger of the two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-west of the MZF. Both the major structures show truncation of lithological units along their respective east 'limbs', and extreme variation in the width of formations. The MZF is primarily the result of superimposition of F₃ on F₂.F₁ folds are relatively smaller in scale and are recognizable in the quartzite unit which responded to deformation mainly by buckle shortening. Large-scale pinching-and-swelling that appears in the outcrop pattern seems to be a pre-F₂ feature. The structural evolutionary model worked out to explain the chronology of the deformational features and the large-scale out-crop pattern envisages extreme east-west shortening following formation of F₁ structures, resulting in the formation of tight and isoclinal antiforms (F₂) with pinched-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scale F₂ isoclinal antiforms seems to be the consequence of a continuous deformation and resultant migration of folds along the DSZs. The main shear zone which wraps the Zawar folds followed a curved path. Because of the penetrative nature of the F₂ movement, the early lineations which were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend of F₂ folding over a large part of the area. Further, the virtually coaxial nature of F₂ and F₃ folds and the refolding of F₃ folds by a new set of N-S folds is an indication of continuous progressive deformation.