The structural framework of the southern margin of the Limpopo Belt, South Africa

Abstract

The Southern Marginal Zone is characterized by two prominent regional structural features: (i) massive crustal wedges (20 km × 50 km), containing large oval-shaped closed-fold structures, which are related to an early (D 1) phase of deformation, and which are bounded by, (ii) major ductile shear zones that strike in a general E-W direction. The Hout River Shear Zone forms the southern boundary of the Southern Marginal Zone where granulites are juxtaposed against lower grade granite-greenstones of the Kaapvaal Craton. This fault system also acted as the sole thrust for the other high-grade shear zones in the Southern Marginal Zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the Southern Marginal Zone. The Hout River Shear Zone and related faults within the Southern Marginal Zone clearly post-date D 1 and a period of peak metamorphism (M 1). They are composite structures showing dominantly reverse-sense displacements and are responsible for the emplacement of the granulite terrane over and onto the Kaapvaal Craton. In contrast to these older structures are the reactivated shear zones. These structures clearly cross-cut and displace the high-grade shear zones in the Southern Marginal Zone as well as the retrograde orthoamphibole isograd. The fact that the Hout River, and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism, i.e., ∼ 2670 Ma (Barton and Van Reenen, 1992).