Abstract
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 NNE-SSW 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. Key words: Tectonic, stress, σ3, joints, veins, boudinages, Pan African.
Highlights
Tectonic stress applied to a rock is one or a combination of extension, compression or shearing (Ramsay, 1967)
The joints were the product of multidirectional minimum stress which was dominantly oriented NW-SE prior to the Pan African orogeny
Initial extension of the NW-SE veins was associated with σ3 NE-SW, which is older than the NESW oriented joints produced by NW-SE σ3
Summary
Tectonic stress applied to a rock is one or a combination of extension, compression or shearing (Ramsay, 1967). Extensional structures grow perpendicular to a minimum principal stress σ3 (Griggs and Handin, 1960; Kehle, 1964); σ1 is oriented perpendicular to compressional structures, and for strike-slip faults and other tectonic structures produced by shearing, the intermediate stress is vertical (Anderson, 1936). Extensional structures include normal faults, joints, veins (including unsheared tension gashes), and intrusions, both pegmatite and quartz-feldspar rich. Compressional structures are limited to ductile rock types producing folds and thrust faults in their brittle equivalents. Evidence for shearing most often includes the presence of LS tectonites characterized by S-C fabrics or Riedel shears along shear zones (Twiss and Moore, 2007). Joints as palaeostress markers that provide the record
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