Structural evolution of Iwaraja shear zone, southwestern Nigeria

The Iwaraja shear zone is an easterly splay to the Ifewara shear zone in the eastern part of Ilesha schist belt, southwestern Nigeria. Results of field and microstructural investigations reveal that the Iwaraja shear zone was developed during the second deformation phase that affected the region under retrograde metamorphic conditions. Deformation within the shear zone involved the mylonitisation of the granitic gneiss generating rocks ranging from weakly deformed granitic gneiss to ultramylonite under amphibolite to greenschist facies, respectively. The shear zone is characterized by NNE-SSW trending, sub-vertical mylonitic foliation (ca. 022°/79°) with shallowly dipping stretching lineations (ca. 021°/10°). Minor folds in the shear zone include early and late folds of pegmatite dykes in the granitic gneiss mylonite. The early folds are typically open to close folds but the late folds show typical geometry of synthetic folds characterized by thinned short limbs. Kinematic indicators in the granitic gneiss mylonite include sigma-type (σ-type) porphyroclasts and displaced fractured feldspar porphyroclasts indicate dextral shear sense. These features show that dextral, transcurrent displacement was important in the final amalgamation of different crustal blocks during the closing stages of the Pan-African Orogeny.