Stress Patterns in the Cape Fold Belt

Rocks outcropping north of the Worcester Fault near Worcester, South Africa, belong to the Malmesbury Group and the Peninsula Formation of the Table Mountain Group. These exposures were studied to get an idea of their structural evolution. Flattening strains produced the planar fabrics in the rocks, with flattening being the dominant deformation mechanism resulting from successive compressive events. This flattening was responsible for the preferred attitude of the rocks and the compressional crenulation cleavages. The compression direction varied from NE-SW to NW-SE during the formation of the composite foliations during the Saldanian Orogeny. These foliations were subsequently crenulated during the Cape Orogeny. In general, NW-SE directed compressional stresses produced the deformation in the Peninsula Formation, where σ1 is oriented NW-SE. Extension direction is perpendicular to this, i.e. NE-SW. For the Malmesbury Group of rocks, NW- directed compressive stresses produced the fabrics in these rocks, with the extension direction being approximately perpendicular to this compressive direction. In general, it would appear as if the regional stress field varied somewhat in its orientation, and the structures in the Malmesbury rocks developed as a relatively continuous sequence of events within a geologically short period of time. It is also very clear that the later Cape Orogeny had a profound effect on the Malmesbury basement rocks, with stress orientations of the former influencing those of the latter.