Kaapvaal Craton

The Kaapvaal craton is incurred in the Archean crust stable block in southern Africa. In addition to the Pilbara craton in Western Australia, he is the only remaining unchanged tectonic crust of the earth, which has an archaic age from 3.6 to 2.5 billion years ago.

The structural development of the African continent is generally characterized by several mountain building processes, where within the respective period of processed Faltungstektonik addition, larger areas of metamorphic overprinted, as were crisscrossed by numerous deep - seated plutonic intrusions. In this way, areas corresponding to stable continental blocks or cratons, which, conditionally, fused together by plate tectonic collision and subsequent Orogenprozesse and eventually formed the so-called African plate developed. The full Kratonisierung of the African continent is given in time with the lower Paleozoic, have taken place at which time a total of five orogenic processes of regional extent.

Description

The Kaapvaal craton occupies approximately an area of 1.2 × 106 km2 and is the north by the Zimbabwe craton, bounded on the west and the south by the Proterozoic rock belt. In the East, among other Jurassic volcanic rocks are open that are set with the break-up of Gondwana in conjunction. In addition, the craton can be divided that have arisen in the gradual development of a small continent to the Kraton in its present size in the period 3.7 to 2.7 billion years ago in twelve different terrains. The formation age of the first continental crust of the Kaapvaal Craton is 3.7 to 3.3 billion years ago, with especially the eastern shield complex was kratonisiert or stabilized by extensive intrusions of granites intracontinental before about 3.0 billion years ago. This north-eastern section of the Kaapvaal Craton thus represents the oldest shield in southern Africa and is referred to as the eastern shield, or as Witwatersrand block. The central and western part of the Kaapvaal Craton, defined as the Kimberley block, however, has a significantly younger age of about 3.2 to 2.7 billion years. The old core of the Kaapvaal - shield is now represented by the ancient gneiss complex of the Ngwane gneiss - and - from the Barberton greenstone terrain.

Witwatersrand block

The eastern Kaapvaal shield built before about 3.2 billion years ago by tectonic convergence as a cohesive unit and includes, among other continental crust, which must have formed 3.7 to 3.3 billion years ago. The disintegrated Lochiel granite in the nature reserve Malolotja in Swaziland is interpreted as the marker for the Kratonisierung of the eastern shield and recent studies also make the assumption that has specially been the stabilization of the Witwatersrand block before about 3.08 billion years and a crust-mantle coupling has played in association with the formation of a stable Tektosphäre since that date. The Natal- granite - greenstone terrain with an age from 3.4 to 3.2 billion years ago, which is younger from north to south, sedimentary packages containing shallow-marine affinity, in which before 3.3 -3.2bn years Trondhjemit - tonalite magmas, or about 3.0 billion years ago intruded post- tectonic granites. Specifically, the post- tectonic granites point to the end of the accretion of the Natal- terrains with the Kaapvaal Shield back and allowed the stabilization of the crustal fragments newly formed. The extended above the Kaapvaal Shield crustal melting and the elapsed Granitoid plutonism - 3.1 to 3.08 billion years ago, consequently led to the eruption of the volcano - sedimentary Dominion Group, which in 3.09 to 3.07 billion years ago a continental rift system was deposited. The following marine sedimentation ages of the Witwatersrand Supergroup was defined using detrital zircons to 2.97 billion years ago, the subsequent eruption age of the Crown Lava Formation is given as 2.91 billion years. The further deposition time of the so-called Central Rand Group could be determined with detrital Zirkonaltern turn, therefore, began about 2.89 billion years and stood in addition to the beginning of the intrusion of the post- Pongola granitoids before 2.82 billion years back. The relatively undeformed volcano- sedimentary Pongola sequence unconformably overlaid also the units of the Ancient gneiss complex, and the Natal- greenstone terrain. The main Riftprozesse in the central and western part of the Kaapvaal Craton are characterized by the ultramafic to intermediate volcanic rocks of the Klipriviersburg and the Platberg group that are associated with the Venterdorp supergroup and is indicated with an age of 2.71 billion years ago.

Kimberley block

The Kimberley block occupying the western area of ​​the Kaapvaal Craton one, is separated from the eastern plate of the Witwatersrand block by the Colesberg lineament and is characterized by the absence of the Dominion Group and Witwatersrand Supergroup. It should be noted that the merger of the Witwatersrand and the Kimberley block is a relatively new process in the development of the Kaapvaal Craton, with a minimum age of 2.71 billion years for this coexistence is determined by the oldest Ventersdorp - sequence corresponding to both blocks covered, accepted. The crystalline basement of the Kimberley block is largely below the Phanerozoic cover, which is characterized by neoarchaische sediments and volcanic rocks. Outcrops of the crystalline basements are represented by the Schweizer- Reneke dome, the Kraaipan - Greenstone Belt, and neighboring granitic gneisses, which dated zircons from gneisses and schists of the respective environments, as well as crustal xenoliths from Kimberley an age between 3.25 exhibit and 2.79 billion years ago. The Kraaipan - greenstone belt consists among other things of rhyolitic tuffs together that have a dating age of 2.93 billion years, although this age refers to the time when one hand felsic volcanism, on the other hand, marine chemical and clastic sedimentation within the Kraaipan - Greenstone belt has taken place. In addition to these past processes occurred within this period to extensive lime - alkaline plutonic activity which has a north-south trend within the Kraaipan - Greenstone belt and may have contributed to the geophysical signature of the Colesberg - lineament. In this connection, it is pointed to the existence of a volcanic island arc and a backarc basin on the eastern edge of the Kimberley block prior to about 2.93 billion years. In the period around 2.88 billion years, the tectonic structure of the deformed Kraaipan belt of potassium-rich granitoids, which are represented by the open-minded Swiss - Reneke granite, pierced transversely, these post- tectonic granites in this context, the occupy time in which it has come to the welding of the Kimberley block to the Witwatersrand block. In summary it can be stated that the Kimberley block, unlike the Witwatersrand block a strong increase in magmatism shows (from 2.93 to about 2.88 billion years ), a ubiquitous deformation of rock packages and a high-grade metamorphism of the middle crust areas. Also there is a relevant difference in the degree of geology on the Colesberg anomaly, with mean crustal areas were exposed in the corresponding portions of the Kimberley block, and can be compared with the simultaneous subsidence and deposition on the eastern shield. This comparison is used to model that of the Witwatersrand block is defined as the lower plate is subducted, defined in a subduction zone beneath the Kimberley block as the upper plate.

Postarchaische development

The further development of the Kaapvaal Craton in the early Proterozoic was dominated by intra- continental extension processes. The Transvaal sedimentary basins developed at this time, associated sediments of the Transvaal Supergroup unconformably overlaid the rock units of the Ventersdorp Group. Prior to about 2.05 billion years ago, it also came in the ensuing widespread intrusion of the Bushveld Complex in the upper Transvaal sediments.