Mid-Proterozoic tectonic evolution along the Orange River on the border between South Africa and Namibia

The border region between South Africa and Namibia straddles tectono-stratigraphic terranes that formed during different orogenic periods, these being 2.0 to 1.7 Ga (the Orange River Orogeny) and 1.4 to 1.1 Ga (the Namaqua Orogeny). Terranes of the western part of the Namaqua mobile belt situated in the Northern Cape Province were amalgamated during the latter orogeny and are easterly trending and lozenge shaped in plan (hundreds of km scale). These include the Grunau, Pofadder plus Steinkopf, Aggeneys and Okiep Terranes. They are bounded by major thrust zones that were subsequently re-activated during progressive ductile shear deformation. The Namaqua orogenic deformation has effectively overprinted the Orange River deformational features. The tectonic style and terrane evolution were dominated by horizontal movements during progressive ductile shear processes in a mid-crustal setting. Transport direction during the Namaqua Orogeny was from northeast to southwest and inter-terrane displacement exceeded 100 km. Nappes and southward verging folds are revealed by structural sections. Selected structures from the Pofadder and Aggeneys terranes are discussed as examples and are used to constrain a three-dimensional model for the entire region. It is concluded that the structural development of the region was dominated by northeasterly oriented compressional stress associated with plate convergence, resulting in the accretion of terranes onto the Kaapvaal Craton.