Steep structure formation in the Okiep Copper District, South Africa: bulk inhomogeneous shortening of a high-grade metamorphic granite-gneiss sequence

Subhorizontal gneissosities and lithologies of the 1200–1000 Ma granulite-facies granite-gneiss terrane of the Okiep Copper District, South Africa, are cut by narrow, discontinuous, E-trending cusp-like and/or monoclinal structures in which the regional gneissosity has been rotated to subvertical attitudes. Closely associated migmatization and charnockitization indicates that these ‘steep structures’ formed during high-grade metamorphic conditions. In this paper we present a model which relates the deformational style, strain and orientation of the steep structures to bulk inhomogeneous shortening. The development of steep structures can be described as a progression from initial folding, via explosive fold amplification to the progressive obliteration of folds by a subvertical, E-trending, intensely developed transposition fabric. This fabric is parallel to the axial planes of regional-scale, open folds. Shortening in the structures is mainly accommodated by a vertical material extrusion, resulting in the commonly upward-pointing steep structure geometries and volume loss. The strain compatibility with surrounding rocks is maintained by bounding shear zones. The progressive development of steep structures illustrates the close relationship between buckle folding and the formation of internal, induced anisotropies during deformation of the high-grade metamorphic granitic gneisses. The apparent discrepancy between large-scale open folding and simultaneously developed steep structures recording large finite shortening strains is interpreted to be the result of a large component of internal layer-parallel shortening and strain partitioning.