• A Rai Choudhuri

      Articles written in Journal of Earth System Science

    • Superposed folding in the Honakere arm of the Chitradurga-Karighatta schist belt in the Dharwar tectonic province, southern India, and its bearing on the Sargur-Dharwar relation

      K Naha A Rai Choudhuri V Ranjan R Srinivasan

      More Details Abstract Fulltext PDF

      The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold (F1). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds 'er generation (F2),is proved by the varying attitude of both compositional bands (S0) and axial pranar schistosity (S1), which are effectively parallel in a major part of the area. A crenulation cleavage (S2) has developed parallel to the axial planes of theF2 folds at places. TheF2 folds range usually from open to rarely isoclinal style, with theF1 andF2 axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the Borikoppalu area to the north, coupled with younging directions from current bedding andS0-S1 inter-relation.

      Although statistically theF1 andF2 linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make as high an angle as 90°. Usually, in these instances, theF1 lineations are unreliable around theF2 axes, implying that theF2 folding was by flexural slip. In zones with very tight to almost isoclinalF2 folding, however, buckling attendant with flattening has caused a spread of theF1 lineations almost in a plane. Initial divergence in orientation of theF1 lineations due to extreme flattening duringF1 folding has also resulted in a variation in the angle between theF1 andF2lineations in some instances. Upright later folding (F3) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF1 andF2 axes, and increase in the angle between theF1 andF2 lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’ abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across the hinge of a large-scaleF2 fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group and the Dharwar Supergroup, untenable.

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