Bulk Strain Estimation on Gneisses in Central Nigeria: A Preliminary Assessment

Abstract

The challenges of regional metamorphism and the need to investigate the petro-structural strain and tectonic evolution of the Nigerian basement using central Nigeria contact zone is useful in mineral exploration. Meso-structural data were plotted using GEOrient to obtain general trend as well as the axial folds of the Pre-Pan African deformation. Strain estimation was done on photomicrographs of the gneisses cut normal to the planes of foliation across the lithologies in order to establish the homogeneity and amount of strain. The centres of the quartz grains were digitized using the Fry method from which the axial ratios of the strain ellipsoids were used to estimate and compare the amount and orientation of strain across the different rock types in the study area. Petro-structural bulk strain estimation on the gneisses around Kaduna, central Nigeria show a generally moderate but inhomogeneous strain with axial ratios of the finite ellipsoids ranging between 1.6 and 3.8 with strain geometries that are consistent with the NW- SE metamorphic foliation. The amount of strain across the rock types correspond to grain size variation signifying probably that quartz responds differently to strain with decrease in grain size. In response to the generally NW-SE tectono-metamorphic ductile deformation that produced the steeply dipping gneisses of the area, there is heterogeneity in the bulk strain. The orientation is essentially concordant to the metamorphic deformation and probably unconnected with the latter but brittle Pan-African and predominantly NE-SW fracture systems.