THE RELATIONSHIP BETWEEN ANTIFORMAL AND MONOCLINAL STEEP STRUCTURES IN THE OKIEP COPPER DISTRICT, NAMAQUALAND - A CASE-STUDY

Steep structures represent narrow zones of intense, subvertical, gneis sose foliations that are developed in subhorizontal, granulite-facies, Namaqua-age granite-gneisses of the Okiep Copper District. The steep structures have no reported analogue in the Literature and appear to b e a unique structural feature of the Okiep District. A prominent steep structure in the Hester Malan Nature Reserve illustrates the intimate structural relationships between contrasting steep structure geometri es, namely, cusp-like, antiformal structures and monoclinal warps of t he regional gneissosity. An antiformal and a monoclinal steep structur e are independently developed along the Hester Malan steep structure l ine. The transition between the two steep structure geometries is mark ed by the occurrence of anatectic migmatites which are locally referre d to as 'megabreccias'. Steep structure development in the Hester Mala n Nature Reserve can be described as a progression with both the antif ormal and monoclinal structures initiating as open, upright folds from the subhorizontal gneissosity. Folding occurs parallel to regional-sc ale D-3 folds, but is progressively obliterated by a subvertical, inte nsely developed, easterly-trending S-2/S-3 transposition fabric which reflects a subhorizontal, north-south-directed shortening strain. The S-2/S-3 fabric is axial planar to the regional-scale D-3 folds, suppor ting a D-3 Origin for the steep structures. In the antiformal steep st ructure, the predominant component of bulk shortening strain is accomm odated by lateral shear discontinuities, ductile flow (i.e. material e xtrusion), and volume loss in the core of the structure. Monoclinal st eepening of the S-2 gneissosity induced a foliation-parallel simple sh ear in the steepened monoclinal limb. This component of flexural slip resulted in the super imposition of a secondary antiformal steep struc ture onto the monocline. The megabreccias occur preferentially along t he structural transitions between the antiformal and monoclinal struct ures because of the localized strain incompatibility between the two g eometries. On a regional scale, steep structure development, that is, the progression from initial folding to the formation of induced struc tural anisotropies, is interpreted to reflect a large component of lay er-parallel shortening of the high-grade metamorphic granite-gneiss se quence during D-3 deformation.