Paleomagnetism of selected neoarchean-paleoproterozoic cover sequences on the Kaapvaal Craton and implications for Vaalbara

• Main Author: De Kock, Michiel Olivier
• Published: 2008
• Subjects: Paleomagnetism; Stratigraphic geology; Cratons; Paleogeography
• Online Access: http://hdl.handle.net/10210/905

The Kaapvaal craton of southern Africa and the Pilbara craton of Western Australia, two of the best-preserved Archean cratons in the world, are covered by remarkably similar early Precambrian cover sequences. This has led to the proposal of the so-called Vaalbara hypothesis, which promotes the existence of the two cratons as a single crustal entity, and possibly, Earth’s oldest assembled continent in Neoarchean-early Paleoproterozoic times. Previous studies have failed to prove the existence of Vaalbara conclusively, principally due to a lack of reliable ages or because of uncertainty and gaps in the paleomagnetic record from the Kaapvaal craton. During the present study paleomagnetic samples were collected from selected Neoarchean- Paleoproterozoic cover sequences of the Kaapvaal craton for the establishment of well-defined paleomagnetic poles. In addition, the Hartswater Group of the Ventersdorp Supergroup was sampled for zircon SHRIMP analyses in order to constrain the ages of poles defined from that succession. The paleopoles established here, together with existing paleopoles from the Kaapvaal craton, are used to evaluate the apparent polar wander path of the craton during the Neoarchean-Paleoproterozoic and are compared with poles of similar age from the Pilbara craton as a test of the Vaalbara hypothesis. Regarding the age of the Hartswater Group, zircon SHRIMP ages of 2735 ± 3 Ma and 2724 ± 6 Ma cast doubt on younger ages from the Klipriviersberg Formation, which comprise the base of the Ventersdorp Supergroup. Traditional (younger) age constraints from the Ventersdorp Supergroup do not support the original Vaalbara correlation. A new correlation is suggested here, taking the new ages into account, showing that the Ventersdorp Supergroup overlaps in time with the Fortescue Group of the Pilbara craton. Most importantly, the new ages also provide constraints on the magnetization within the Platberg Group and the Allanridge Formation. Six new paleopoles, of various quality, are added to the existing database from that craton. These poles from the ~2.73 Ga Platberg Group and ~2.7Ga Allanridge Formation of the Ventersdorp Supergroup, the ~2.5Ga lower Transvaal Supergroup, the lower two unconformitybounded sequences of the Waterberg Group (2.05 Ga and ~1.99 Ga) and the upper Soutpansberg Group (~1.76 Ga) have, together with existing poles from the Kaapvaal craton, led to the definition of an APWP for that craton for a period ~2.78 to ~1.76 Ga. Particularly the poles from the Waterberg and Soutpansberg Groups provided the information to identify complexities (looping) in the APWP that have gone unrecognized in the past. The paleomagnetic data gathered and the newly defined APWP could be used in conjunction with geological evidence from the Kaapvaal and Pilbara cratons to evaluate, and validate, the Vaalbara hypothesis. A good match between the APWP’s of the two cratons for the period ~2.78 to ~2.70 Ga and the geological features (lithology and structure) of the two cratons provide the best evidence that Vaalbara existed as a cratonic unit in the late Archean. Paleomagnetic data constrain the position of the Pilbara craton in immediate proximity to the northwest of the Kaapvaal craton (in a Kaapvaal reference frame). The position of the Zimbabwe craton relative to the Pilbara and Kaapvaal cratons is still unresolved, but indications are that it was most likely in a proximal position to the Kaapvaal craton at 2.7 Ga in a configuration not much different from its present day configuration. This would imply that Vaalbara was most probably the Earth’s oldest assembled continent as proposed by earlier workers. The new paleomagnetic data further suggest that Vaalbara did not exist anymore at ~2.0Ga. When evaluated in conjunction with geological evidence a strong argument can be made for the existence of the Vaalbaran continent up until ~2.22 Ga and that the Pilbara and Kaapvaal cratons became separate entities from about ~2.05 Ga. === Prof. NJ Beukes Prof. DAD Evans