The structural development of the Aggeneys Hills, Namaqua Metamorphic Complex

Abstract

The Aggeneys terrane is composed of gneisses, supracrustal sequences and Broken Hill type base metal sulphide deposits that have been intensely deformed and metamorphosed to upper amphibolite grade. This terrane forms part of the Namaqua metamorphic complex that was amalgamated during the 1.1 to 1.2 Ma Namaqua Orogeny. Preserved stratigraphic sequences can be followed on strike (hundred km scale) in domains of relatively low strain. In high strain (more than 12γ) zones, the sequences are discontinuous. Inversion and duplication owing to thrusting and folding are more prevalent in the latter than in the former. The Aggeneys Hills is situated in a regional high strain zone and comprises a late synformal macro-structure, superimposed on five older structural domains. The exposed dimensions of the macro-structure is 20km by 5km and the strike length of the five recumbent folds is 6 to 17km. Zones of high strain that are localised in relatively ductile quartzite-schist sequences separate the recumbent folds. The geometry and stratigraphic relationships of the macro-folds indicate that the stratigraphic packages in the Aggeneys Hills were tectonically emplaced as fold nappes. Collectively these nappes represent a duplex structure. The five fold nappes have been named Soutkloof, Quarry, Klipbok, Dassie and Dabbiepoort. The Quarry fold nappe was chosen as an example to illustrate the details of the structural development of the Aggeneys Hills. Although the deformation is interpreted to have been a progressive ductile shear process, five successive structural events can be locally recognised (D1–5). The Quarry fold nappe is interpreted to be a large kilometre scale sheath fold. The sheath fold provides information about the nature and orientation of the regional strain ellipsoid; the XY-plane trends easterly and is subhorizontal with the X-direction subparallel to the southwesterly trending tectonic transport direction. The mega sheath fold has an allochtonous structure in its northern limb containing correlates of mineralised stratabound units (the Gams member of the Hotson Formation). Structural duplication of ore bearing units may well be present in the unexposed core of the Quarry structure. Similar structural duplication is to be expected in the rest of the high strain zone of the Aggeneys terrane.