Polyphase Folding Deformation in the Hutti-Maski Schist Belt, Karnataka

Abstract Three phases of folding deformation (F1, F2 & F3) are recognised in the Hutti-Maski schist belt. The first two phases (F1 and F2) are quite pronounced producing regional folds whereas the last one (F3) is of mild intensity. F1 folding is accompanied by the development of schistosity (S1) in metabasalt and metasedimentaries, mylonite banding (Sd) in gold quartz veins and minor folds on bedding plane (So) and vein quartz. Two major F1 folds, viz., a syncline and an anticline could be traced in the area on the basis of orientation of pillow structure in metabasalt. Gold mineralisation in Hutti-Maski schist belt is mainly controlled by first phase of folding (F1) and associated regional metamorphism. F2 fold shows clear evidence of superposition on F1. F1 axial trace is folded by F2 folds. F2 folding is accompanied by the development of crenulation or slip cleavage (S2), minor folds on S1 and Sd and shear fractures and faults. F2 fold is represented by two major folds, viz., an antiform and a synform having axial traces along NNW-SSE to NW-SE direction and plunge around 45° to 50° towards Nw to NNW. A time gap between F1 and F2 is marked by the intrusion of a granodiorite body. F3 is developed on the limbs of F2 folds, and is accompanied by the development of S3 cleavage and kink band or chevron folds. Axial plane of F3 fold trends N75°E-S75°W (approx.) with vertical dip. Fold axis (F3) shows very steep westerly plunge.