palaeostress configuration of pan-african orogeny: evidence from the igarra schist belt, sw nigeria

The stress configuration and tectonic analysis of the pan – african orogeny is drawn from the igarra schist belt, southwestern nigeria. the analysis of conjugate shear fractures of the schist belt indicates that two distinct compressive events (ne – sw and e – w) occurred in this pan – african mobile zone. the palaeostress systems reveal a clockwise rotation of compressional axis from d1 (ne – sw) to d2 (e – w) during the propagation of the schist belt. the dominance of e – w compression related structures and the identification of e – w compressive stress in other pan – african regions suggest that only the e – w compression probably had a strong regional effect during the pan – african orogeny, other episodes of deformation most likely had a partially regional or local effect. the occurrence of structures related to the e – w compression in the younger rocks (granite and syenite) and the absence of structures related to the ne – sw compression in these rocks indicates that the e – w σ1 is a younger episode of deformation (d2) than the ne – sw σ1 deformation (d1) and probably lasted for a longer period. the dihedral angles (2θ) of the conjugate shear fractures range from 35° to excess of 80° and this situation shows that the orogeny produced a combination of both brittle and ductile deformation.