• A B Roy

      Articles written in Journal of Earth System Science

    • Influence of differential basement mobility on contrasting structural styles in the cover rocks: An example from Early Precambrian rocks east of Udaipur, Rajasthan

      A B Roy D K Nagori

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      Detailed structural and lithological mapping of the Aravalli rocks overlying the Mewar Gneiss in the area east of Udaipur, Rajasthan, suggests presence of blocks bounded by faults, showing a contrasting structural pattern. The contrast is reflected in the differential development and in the orientation of AF1, AF2 and AF4 folds in different blocks. In the central Umra block, the rocks constitute a virtually homoclinal sequence showing one dominant orientation of bedding and axial planar schistosity. Fold axes, lineations andβ orientations indicate presence of reclined folds of AF1 generation. AF2 folds are either absent or have developed only locally. The two other blocks which border the Umra block show development of large AF2 synforms and local minor antiforms having N-S or NNE-SSW trend. The folds interfere with AF4 folds producing irregular domes and basins in the western Kanpur-Kalarwas Block and minor plunge reversals in Bagdara-Dhamdhar Block. It is argued that the constituents of the different blocks which formed a collage of rift basins and horsts during sedimentation, responded differentially to deforming forces because of differential mobility of the underlying basement.

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

      A B Roy

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      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 (S1) axial planar to a phase of isoclinal folding (F1). The lineations which parallel the hinges ofF1 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 ofF2 folds, particularly in the psammopelitic rocks. The plunge and trend ofF2 folds vary widely over the area.

      Deformation ofF2 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 ofF3 onF2.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 ofF1 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-scaleF2 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 theF2 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 ofF2 folding over a large part of the area. Further, the virtually coaxial nature ofF2 andF3 folds and the refolding ofF3 folds by a new set of N-S folds is an indication of continuous progressive deformation.

    • Geochemistry of sericite deposits at the base of the paleoproterozoic aravalli supergroup, Rajasthan, India: Evidence for metamorphosed and metasomatised precambrian paleosol

      B Sreenivas A B Roy R Srinivasan

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      Fine grained sericite deposits occur at the interface between Archean Mewar Gneiss Complex and the Proterozoic Aravalli Supergroup independent of shearing. They show a gradational contact with the basement granites and gneisses and a sharp contact with the overlying quartz pebble conglomeratic quartzites. Rip-up clasts of these sericite schists are found in the overlying conglomerates. The sericite schists are rich in sericite towards the top and contain chlorite towards the base. The sericite in these schists was formed by metasomatic alteration of kyanite and not from the feldspars of the basement granitoids and gneisses. Uni-directional variations of SiO2 and Al2O3, high Al2O3 content (>30%), positive correlation between Al2O3 and TiO2, Ti/Al and Ti/Zr ratios, high pre-metasomatic chemical indices of alteration (> 90), and enrichment of heavy rare earth elements relative to the parent granites and gneisses—all these chemical characteristics combined with field evidence suggest that the sericite schists are formed from a paleosol protolith, which developed on Archean basement between 2.5 and ~2.1 Ga in the Precambrian of Rajasthan. The superimposed metasomatic alteration restricts the use of Fe2+/Ti and Fe3+/Ti ratios of these paleosols for interpretation of PO2 conditions in the atmosphere.

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