STRUCTURAL EVIDENCE FOR PAN-AFRICAN EVENT IN THE SW BASEMENT BLOCK OF NIGERIA: THE IGARRA EXAMPLE

Fractures and folds are common evidences for post-formational deformation in rocks. They result from the action of stress on the rocks and as such bear imprints of the nature and magnitude of the deformative force. The style and orientation of the recorded strain reflect the interaction between the applied force (stress) and the pre-existing rock body. Force is a vector quantity, having both magnitude and direction. The stress acting normal against a surface can be resolved into three mutually perpendicular principal axes of maximum stress σ1, intermediate stress σ2 and minimum stress σ3; while the resultant strain can also be resolved into three mutually perpendicular principal strain axes. There exist some relationship between the applied stress and the resultant strain (structures). This relationship shows correspondence between the principal stress axes and the principal strain axes. Structural analysis using the stress and strain relationship technique was carried out at Igarra. Fractures and folds characterized in the crystalline basement rocks at Igarra shows that the orientation of the strike of the fractures and the fold axes of the folds are dominantly in the N-S direction. Assuming pure shear, and that the strain on a large scale is essentially homogeneous statistically, reflecting both the orientation and size of the bulk finite strain; the orientation, style and intensity of the mapped structures suggest a dominantly E-W directional compressional stress. This generally agrees with the tectonic location and orientation, geologic and lithologic relationships of the Pan-African suites and indicates that the Pan-African event played a major role in the structural evolution of the Schist Belt.