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.
