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

Mr G:O Mosuro,Usman Kaigama,Dr Ta Laniyan,Dr Kamaldeen Olakunle Omosanya,Dr S.O Ariyo

doi:10.5539/jgg.v7n3p24

Abstract

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.

Highlights

The basement complex of Southwestern Nigeria is located in a triangular portion of the Nigerian basement, an extension of the Dahomeyide shield of the West African Craton (WAC; Figure 1)
Deformation of the Nigerian basement complex occurred in two phases, a ductile phase, which is responsible for the formation of planar structures and a brittle phase resulting in jointing and fractures, many of which have been filled with quartzo-feldspathic veins, dolerite dykes, pegmatite and aplitic veins and dykes
Since macroscopic structures remain relevant to the study of regional structures and events (Sander, 1950; Wilson, 1951; Price & Cosgrove, 1990), this research is aimed at investigating the degree to which the polycyclic nature of the basement complex in Nigeria is preserved at local or outcrop scale

Summary

Introduction

The basement complex of Southwestern Nigeria is located in a triangular portion of the Nigerian basement, an extension of the Dahomeyide shield of the West African Craton (WAC; Figure 1). The attitudes of tectonic structures in the Nigerian basement have been documented in terms of orientation and magma-induced veins and dykes such as quartz veins and pegmatites (Rahaman et al, 1983; Ajibade et al, 1987). The polycyclic nature of the basement complex is well-documented at regional scale. Since macroscopic structures remain relevant to the study of regional structures and events (Sander, 1950; Wilson, 1951; Price & Cosgrove, 1990), this research is aimed at investigating the degree to which the polycyclic nature of the basement complex in Nigeria is preserved at local or outcrop scale

Objectives

Methods

Results

Discussion

Conclusion