Remodeling of σ3 field in the evolution of a polycyclic basement complex

Abstract

Dele SI, Kolawole FO, Ukane DP, Tope GC

The polycyclic nature of the Migmatized gneiss basement complex of South-western Nigeria is depicted by several tectonic imprints oriented in different manner; such setting is characterized by temporal adjustment of the minimum stress field. Attitude (strike and dip), length, width, average perpendicular distance and aspect ratios were taken for foliations, joints, veins, intrusions and boudinages. These parameters were analyzed on histograms, rose diagrams and stereo nets; outcrop maps were used to elucidate the heterogeneity of the stress field. Joints are the youngest tectonic structures in the study area; they include extensional and shear types, systematic, non-systematic and orthogonal sets with dihedral angle of <90°. The boudinages occur in gneissic rocks; the veins and intrusions are metasome in the host rock and are made of quartz, feldspar and pegmatite minerals. The dominant orientation of structures are NNE-SSW for foliations, E-W, NE-SW, and NNW-SSE for joints, N-S, NNW-SSE and NNESSW for veins, and N-S for intrusions with dip direction of North-west and South-east for foliations and west for intrusions. Initial orientation of σ3 was east-west during the emplacement of the veins and pegmatite intrusions and then ENE-WSW for the quartz-rich intrusions. Joints oriented NE-SW commonly intersected the other structures suggesting they are juvenile and created by NW-SE oriented σ3. Subsequently, shearing of the rocks and rotation of the least stress field produced the boudinages, conjugate joints, and tension gashes. Evidence for horizontally situated σ1 is shown by few slightly folded foliations seen in specific locations. In the study area, the joints were the product of multidirectional minimum stress which was dominantly oriented NW-SE prior to the Pan African orogeny.