The Schist Enclaves of Oban Massif, Southeastern Nigeria: Consistency of Dihedral Angle and Other Natural Physical and Mechanical Properties

The schist enclaves of Oban massif, southeastern Nigeria were deformed predominantly by brittle deformation and contain a plethora of fractures such as joints and conjugate shear fractures. Analysis of the conjugate shear fractures using stereographic projection technique and statistical methods reveal a consistency in dihedral angle (2θ) in the different schist enclaves studied, irrespective of location. The most frequently occurring (2θ) values range from 55°-60° in this basement terrain and this indicates that the schists were deformed mainly by brittle deformation under similar depth of burial, confining pressure and metamorphic level. The natural angle of internal friction (ɸ) range from 20°-40° while the coefficient of internal friction (µ) range from 0.3 -0.8. These values lend credence to the dihedral angle (2θ) by being consistent in all the schist enclaves. The natural earth pressure coefficient (K) mostly ranges from 2.0 to 4.0 and this implies that the schists of Oban massif even though somewhat variable in mineralogy, have a moderate natural competence. From all indications, natural rock deformation in a quarzo-feldspathic rock mass at the level that schistose grade is produced, may generate dihedral angles mainly in the range of 55°-60°.