The Namaqua-Natal Province

Abstract This paper provides an update of Chapter 16 in the Geology of South Africa (2006) and summarizes recent advances in understanding the Mesoproterozoic Namaqua-Natal Province (NNP). The NNP forms a major orogenic belt along the southern and southwestern margins of the Kaapvaal Craton, that developed between ~1.35 and 0.96 Ga during the assembly of Rodinia. It comprises two main parts: the Namaqua Sector in the west and the Natal Sector in the east, separated by Phanerozoic cover. In South Africa, the Namaqua Sector is a composite of five tectonostratigraphic entities separated by major thrusts and shear zones. The Richtersveld Subprovince in the west contains 1.9 to 1.86 Ga, arc-related, volcanic and plutonic rocks partly reworked during the Namaqua orogeny. The Bushmanland Subprovince in the south is dominated by high-grade supracrustal gneisses (1.21 to 1.13 Ga), granitic orthogneisses (~1.21 to 1.12 Ga), abundant late- to post-tectonic granites (~1.10 to 1.04 Ga) and minor mafic intrusions, accompanied by crustal heating, at ~1.03 Ga. The Kakamas Domain, also composed of granulite-facies paragneisses (~1.22 Ga), granitic orthogneisses (~1.23 to 1.15 Ga), and granites (~1.12 to 1.08 Ga), was thrust south-westward over the Richtersveld and Bushmanland Subprovinces along the Lower Fish River – Onseepkans Thrust Zone. East of the Kakamas Domain, the Areachap Terrane comprises 1.29 to 1.22 Ga island-arc volcanic and sedimentary rocks and juvenile granitoids generated after collision with the Kaapvaal-Rehoboth Craton at ~1.21 Ga. The easternmost Kaaien Domain represents a foreland thrust complex on the craton margin containing a possible back-arc volcanic sequence in the Wilgenhoutsdrif Group. Deformation in the Namaqua Sector involved several phases. Western Richtersveld Subprovince rocks preserve an older, Paleoproterozoic (~1.89 Ga), greenschist-grade folding event (D1). The Mesoproterozoic Namaqua Orogeny was polyphase, with an initial extended period of isoclinal folding and thrusting under high grade metamorphic conditions, accompanied by a penetrative gneissic foliation (D2 at ~1.20 to 1.12 Ga), followed by dome-and-basin type refolding (D3) and regional dextral shearing (D4 at ~1.00 to 0.96 Ga). The latter was associated with the emplacement of the Orange River Pegmatite Belt into D2 and D4 structures. Two contrasting geodynamic models have been proposed to explain the evolution of the Namaqua Sector. The traditional accretionary model interprets it as a collage of previously unrelated arc terranes successively juxtaposed with the Kaapvaal Craton, later affected by mantle delamination and localised thermal overprinting (metamorphism). The alternative continental back-arc model proposes long-lived (1.2 to 1.0 Ga) crustal extension and heating of a pre-existing crustal block in a continental back-arc setting, generating the widespread high-temperature/low-pressure metamorphism and voluminous granite magmatism that typify the Namaqua Sector. The Natal Sector comprises, from north to south, the Tugela, Mzumbe and Margate terranes. Over the past two decades, new geochronological datasets have refined the timing of key events but have not significantly changed the overall model. Juvenile island arcs formed south of the Kaapvaal Craton through southward subduction of the “Tugela Ocean” before 1 210 Ma. Subsequent northeast directed closure led to obduction of the Tugela oceanic arc terrane onto the craton’s southern margin and accretion of the Mzumbe and Margate arcs around 1 150 ± 20 Ma (D1), accompanied by high grade metamorphism, polyphase granite intrusion and minor mafic magmatism. Continued northeast-southwest convergence produced steep ductile sinistral shear belts in the southerly two terranes (D2) but not in the rigid, craton-underlain Tugela Terrane. The D2 phase was accompanied by extensive A-type granitoid magmatism (Oribi Gorge Suite). The Natal Sector is still regarded as an accretionary assemblage of juvenile Mesoproterozoic terranes.