Tektonisme langs die noordoostelike rand van die bosveldkompleks, Suid-Afrika

Abstract

Hierdie studie handel oar die struktuurgeologie en tektoniese geskiedenis van die noord-oostelike gedeel te van die Transvaalkom op die Kaapvaalkraton, Suid Afrika. Die strukturele grein van die studiegebied word deur 'n oos-noordoos-strekkende plooi- en stootverskuiwingsgordel, die Mhlapitsi-plooigordel, gedomineer, en vervorm gesteentes van die onderste gedeelte van die Opeenvolging Transvaal langs die kratonskaal ThabazimbiMurchisonlineament (TML). Die strukturele ontwikkeling van die plooi- en stootverskuiwingsgordel is geherinterpreteer deur gebruik te maak van onlangs-gepubliseerde idees omtrent tektoniese inversie in sedimentere komme. Gedetailleerde strukturele en sedimentologiese ondersoeke was nodig om die relatiewe ouderdom van die Mhlapitsi-gordel en die meganisme van plooiing bepaal. Nuwe insig omtrent die verwantskap tussen die ontwikkeling van die Mhlapitsi-gordel en Vroeg-Proteroso1ese tektoniek elders om en op die Kaapvaalkraton is verkry. Plooistrukture verwant aan die Mhlapi tsi-gordel is ge1nterpreteer as 'n reeks akkommodasiestrukture (opritantikliene) aan die dakkante van, tot dusver onge1dentifiseerde, suidhellende stootverskuiwings. stratigrafiese- en struktuurgetuienis dui aan dat hierdie fase van kompressietektoniek na afsetting van die Groep Chuniespoort ontstaan het. Dikteverskille van litologiee weerskante van sommige van hierdie verskuiwings impliseer egter dat hul voorheen onder 'n fase van ekstensietektoniek, as afskuiwings (groeiverskuiwings), tydens afsetting van die Groep Wolkberg en Formasie Swartrif aktief was. Die verandering in verplasingsrigting langs verskuiwings van af- na opskuiwings staan as positiewe tektoniese inversie bekend. Inversie is geakkommodeer langs elemente van die TML ('n kratonskaalse sone van vervorming wat periodies sedert Laat-Arge:i.ese tye aktief was). Verskeie modelle wat tektoniese inversie verklaar, word bespreek en daar word voorgestel dat 'n Alpynse tipe inversie, wat die gevolg is van 'n verandering in spanningstoestande langs die grense van die Kaapvaalkraton, data die beste pas. Vervolgens word daar voorgestel dat die TML positiewe inversie ondergaan het in reaksie op verandering in die regionale spanningspatroon langs die suidelike grens van die Kaapvaalkraton tussen 2.4 en 2.2 Ga. Die oorsaak van hierdie verandering is tans onbekend. Mikroskoopstudies het aangetoon dat die TML geheraktiveer is gedurende inplasing van die Bosveldkompleks teen ongeveer 2.06 Ga. Gedurende hierdie heraktivering was die beweging strekkingsglip en linkslateraal. Die meerderheid van die verskuiwings en plooistrukture wat die gesteentes van die Groep Pretoria vervorm, kan aan hierdie fase van beweging gekoppel word.

This thesis deals with the structural aspects and tectonic setting of the northeastern part of the Transvaal Basin, situated on the Kaapvaal Craton. The structural grain of the study area is dominated by the east-northeast trending Mhlapitsi Fold and Thrust Belt, which affects rocks of the lower part of the Transvaal Sequence, Murchison Lineament and also the craton-scale Thabazimbi- (TML). The deformation style of the Mhlapitsi Fold and Thrust Belt has been reinterpreted utilising recently published ideas on basin inversion tectonics. Detailed structural and sedimentological results have established the relative age of the Mhlapitsi Belt, and the mechanism of folding. Insight has also been gained into the relationship between the development of the Mhlapitsi Belt and Early Proterozoic tectonics elsewhere, on and around the Kaapvaal Craton. Folds defining the Mhlapitsi Belt are interpreted as a series of accommodation folds (ramp anticlines) in the hanging wall blocks of previously unidentified south-dipping thrust faults. Stratigraphy and structural evidence show that thrusting post-dates deposition of the Chuniespoort Group. In addition, however, variations in the thickness of lithological units across some of these faults indicate that they acted as extentional growth faults during deposition of the Wolkberg Group and Black Reef Formation. This change in displacement sense of the faults from normal to reverse movement, is termed positive structural inversion. This inversion was accommodated along elements of the TML (a craton-scale deformation zone that has been periodically active since the Late Archaean). Various models to account for the structural inversion are discussed, and it is suggested that an Alpine-type inversion due to a change in stress conditions along the margins of the Kaapvaal Craton best fits the available data. It is proposed, therefore, that the TML underwent positive structural inversion in response to changes in the regional stress pattern along the southern margin of the Kaapvaal Craton between 2.4 and 2.2 Ga. The cause of this change is at present unknown. Additional petrographic studies indicate that the TML was reactivated again during intrusion of the Bushveld Complex at about 2.06 Ga. During this reactivation, however, sense of movement was strike-slip and sinistral. The majority of the faults and folds that deform the Pretoria Group rocks can be related to this phase of reactivation.