Neoproterozoic to Early Cambrian Crustal Evolution of the Pan-African Saldania Belt, South Africa

Abstract

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation. Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard. Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime. A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean. Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.