Tectono-magmatic evolution of the Mesoproterozoic Singhora basin, central India: Evidence for compressional tectonics from structural data, AMS study and geochemistry of basic rocks

摘要

Studies in Proterozoic sedimentary basins are generally fraught with continuous and conformable hypothesis. Unlike many such undeformed and unmetamorphosed Proterozoic sedimentary basins around the Globe, the Mesoproterozoic Singhora basin, a so called " proto-basin" for the Chhattisgarh basin of central India, registers signatures of pervasive deformation documented in both field and anisotropic magnetic susceptibility (AMS) based study. While three constituent Formations of the Singhora Group viz. Saraipali, Bhalukona and Chuipali record outcrop-scale deformation in presence of folds having plane non-cylindrical to non-plane cylindrical geometry, the signatures of deformation within the coarse granular sandstones and conglomerates of the basal Rehtikhol Formation are shown principally from angular relation between the magnetic foliation and bedding plane under AMS study. Systematic formation-wise structural analysis reveals that the basin, as a whole, is deformed in the form of a regional-scale non-plane non-cylindrical fold; whereas non-planarity can be visualized in the regional scale, non-cylindricity is more prominent in the outcrop-scale. Penetrative fabric is only observed in the south-eastern corner of the basin. Compression in two directions is inferred, one being broadly N-S and the other E-W, with resultant overall compressional direction of NW-SE. Additionally, a narrow high strain zone is developed at the southern boundary of the basin in contact with gneissic basement, evident from the sheared metabasite band.Basic rocks (sheared and massive), metamorphosed at greenschist facies, present at the basin margin, and unaltered basaltic intrusives within the basin allowed us to constrain the broad time frame of deformation. Despite variation in mineralogy and major element geochemistry between the metamorphosed and unaltered variety, the intrusives are identified as cogenetic from the similarity in REE pattern and trace element character. Considering the ～1420. Ma emplacement age of the intrusive into the Saraipali Formation, the deformation age is constrained as <1420. Ma.Signatures of compressional deformation traced within all formations of the Singhora Group suggest that the Group is pervasively deformed and thereby indicates an unconformable relationship with its overlying undeformed Chandarpur Group of sediments. The present study warrants necessity of further work in the area to establish causal relationship, if any, between the operative tectonics within the frontal thrust belt in the immediate eastern proximity of the basin and the compressional deformation event/s recorded within the basin.