GEOMETRY AND EVOLUTION OF SUPERPOSED FOLDING IN THE ZAWAR LEAD-ZINC MINERALIZED BELT, RAJASTHAN

The lead-zinc bearing Proterozoic rocks of Zawar, Rajasthan, show clas sic development of small-scale structures resulting from superposed fo lding 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 F -1 folds are deformed by a set of folds (F-2) having vertical or very steep axial planes. At many places a crenulation cleavage (S-2) has de veloped subparallel to the axial planes of F-2 folds, particularly in the psammopelitic rocks. The plunge and trend of F-2 folds vary widely over the area. Deformation of F-2 folds into hook-shaped geometry and development of another set of axial planar crenulation cleavage are t he main imprints of the third generation folds (F-3) in the region. In addition to these, there are at least two other sets of cleavage plan es with corresponding folds in small scales. More common among these i s a set of recumbent and reclined folds (F-4), developed on steeply di pping early-formed planes. Kink bands and associated sharp-hinged fold s represent the other set (F-5). Two major refolded folds are recogniz able in the map pattern of the Zawar mineralised belt. The larger of t he two, the Main Zawar Fold (MZF), shows a broad hook-shaped geometry. The other large-scale structure is the Zawarmala fold, lying south-we st of the MZF. Both the major structures show truncation of lithologic al units along their respective east 'limbs', and extreme variation in the width of formations. The MZF is primarily the result of superimpo sition of F-3 on F-2. F-1 folds are relatively smaller in scale and ar e recognizable in the quartzite unit which responded to deformation ma inly by buckle shortening. Large-scale pinching-and-swelling that appe ars in the outcrop pattern seems to be a pre-F-2 feature. The structur al evolutionary model worked out to explain the chronology of the defo rmational features and the large-scale out-crop pattern envisages extr eme east-west shortening following formation of F-1 structures, result ing in the formation of tight and isoclinal antiforms (F-2) with pinch ed-in synforms in between. These latter zones evolved into a number of ductile shear zones (DSZs). The east-west refolding of the large-scal e F-2 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 F-2 movement, the early lineations w hich were at high angles to the later ones (as is evident in the west of Zawarmala), became subparallel to the trend of F-2 folding over a l arge part of the area. Further, the virtually coaxial nature of F-2 an d F-3 folds and the refolding of F-3 folds by a new set of N-S folds i s an indication of continuous progressive deformation.