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

Abstract

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 (S₁) 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 the F₂ folds at places. The F₂ folds range usually from open to rarely isoclinal style, with the F1 and F₂ 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 and S₀ -S₁ inter-relation. Although statistically the F₁ and F₂ 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, the F₁ lineations are unreliable around the F₂ axes, implying that the F₁ folding was by flexural slip. In zones with very tight to almost isoclinal F₂ folding, however, buckling attendant with flattening has caused a spread of the F₁ lineations almost in a plane. Initial divergence in orientation of the F₁ lineations due to extreme flattening during F₁ folding has also resulted in a variation in the angle between the F₁ and F₂ lineations in some instances. Upright later folding (F₃) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of the F₁ and F₂ axes, and increase in the angle between the F₁ and F₂ 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-scale F₂ 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.