An outcrop evidence for polycyclic orogenies in the basement complex of Southwestern Nigeria

In this work, the tectonic history of a classic outcrop of the Migmatized-Gneiss Complex of Southwestern Nigeria was systematically investigated. The aim of this research is to investigate how the polycyclic nature of the basement complex in Southwestern Nigeria is preserved at a local or outcrop scale. Detailed geological mapping of the outcrop was done using ‘bit-a-bit’ technique in four pre-defined quadrants. Structural measurements include a detailed inventory of fold geometry, attitudes of veins, joints, and boudins. The outcrop is characterized by alternating leucosomes and melanosomes, fractures, S-C fabric and microfolds. Three metamorphic events were interpreted in the outcrop M1, M2 and M3, which are linked to deformation D1 to D3. These events suggest that the outcrop evolved through a complex interplay of compression, extension, and late stage sinistral movement. Phase I extension is coincident with the emplacement of veins, which were later compressed into ptygmatic folds. These veins were intersected by joints associated with Phase II deformation. During the third episode of extension, quartzo-feldspathic veins were emplaced normal to the orientation of earlier joint sets. The N-S, NE-SW and NW-SE trends of the structures are consistent with the polycyclic fabric of the Nigerian basement complex. This work has shown that macroscopic structures in the study area present unique evidence for the regional polycyclic history of the bedrock in Southwestern Nigeria.