Calcite twin analysis in syntectonic calcite, Cape Fold Belt, South Africa: Implications for fold and cleavage formation within a shallow thrust front

Lithologies of two formations in the frontal ranges of the Cape Fold Belt, South Africa, host synorogenic calcite precipitated at low metamorphic grade during the end-Permian deformation of this Gondwana-wide orogen. The calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins. Twinning strain analysis reveals a complex, rotational synfolding history in the upper, thin-layered Prince Albert Formation. Twinned calcite within the fold preserves two unique strain events. The primary shortening strain fabric is layer-parallel and transport-parallel (∼north–south). The shortening strain overprint (−5.82%) is layer-normal and plunges steeply to the northeast. These results are inconsistent with a flexural-slip folding mechanism. By contrast, a homogeneous syn-cleavage stress–strain field is recorded in the underlying, massive thick-bedded Dwyka Tillite. Analyses of calcite twins in clast-hosted, syn-cleavage fibrous calcite and rare limestone clasts define a sub-horizontal N–S shortening, and sub-horizontal, E–W extension. The intermediate axis (ε2) is vertical and preserves shortening (−5.3%). The extension axis (ε3) is horizontal and parallel to the clast ‘tension gash’ (fracture) plane. These deformed syn-cleavage calcite materials provide an independent constraint to the debate about contemporaneous stress–strain fields associated with folding and formation of an axial planar cleavage.