Quantitative finite strain and kinematic analyses of Dahomeyide orogenic belt (Benino-Nigerian shield) around Igarra, southwestern Nigeria

Abstract

The Igarra area (including Sebe-Ogbe and Otuo) in Igarra schist belt, southwestern Nigeria exposes a good section of the Dahomeyide orogenic belt (Benino-Nigerian shield). The area provides an opportunity to investigate quantitative finite strain, types of strain and strain ellipsoid, kinematic vorticity number (Wk) and proportions of pure and simple shear components from metaconglomerate outcrops. Measurements of the long, short and intermediate axes of deformed clasts in the metaconglomerates were carried out. The angular relationships between a constant reference line and the long axes of some of the clasts were determined. Combined strain in two dimensions (2-D) and three dimensions (3-D) analyses involving the Rf/ɸ (Phi), centre to centre (Fry) and Flinn methods were performed on the data with the aid of Ellipsefit software to generate Flinn, the tear-drop shaped diagrams and finite strain ellipsoids. The kinematic vorticity numbers were estimated in order to quantify the amount of shear. The results indicated that the rocks in the Dahomeyide orogenic belt around Igarra have ellipticity in initial state (Ri) and true strain (Rs) of 3.6 and 4.4, respectively. The Ri and Rs for Otuo are 2.43 and 3.35, respectively while the values for Sebe-Ogbe are 1.96 and 3.16, respectively. The results of centre to centre method showed strain ellipses with different orientations from near horizontal to vertical, while the Flinn diagram showed that the rocks in the three areas have both constrictional and flattening strains corresponding to prolate and oblate strain ellipsoids, respectively which are characteristic of L-S tectonites. The kinematic vorticity number (WK or Wm) for the locations are 0.97 (Igarra), 0.79 (Sebe-Ogbe) and 0.97 (Otuo). These probably suggest homogeneous subsimple shear transpressional deformation involving approximately 42% pure shear and 58% simple shear for Sebe-Ogbe and 17% pure shear and 83% simple shear for both Igarra and Otuo. The state of strain of the rocks at Igarra, Otuo and Sebe-Ogbe was relatively high and increased towards the south from Igarra (north) to Sebe-Ogbe (south) where there is highest strain probably due to clast support nature of the metaconglomerates where the clasts acted as single rigid body, and influence of contact metamorphism. The implication of this is that the rocks at Igarra have undergone less deformation than those at Otuo which in turn have undergone less deformation than those at Sebe-Ogbe.