Paleomagnetism of selected neoarchean-paleoproterozoic cover sequences on the Kaapvaal Craton and implications for Vaalbara
- Authors: De Kock, Michiel Olivier
- Date: 2008-08-25T06:27:36Z
- Subjects: Paleomagnetism , Stratigraphic geology , Cratons , Paleogeography
- Type: Thesis
- Identifier: uj:3676 , http://hdl.handle.net/10210/905
- Description: 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
- Full Text:
- Authors: De Kock, Michiel Olivier
- Date: 2008-08-25T06:27:36Z
- Subjects: Paleomagnetism , Stratigraphic geology , Cratons , Paleogeography
- Type: Thesis
- Identifier: uj:3676 , http://hdl.handle.net/10210/905
- Description: 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
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Provenance ages and timing of sedimentation of selected Neoarchean and Paleoproterozoic successions on the Kaapvaal Craton
- Authors: Dorland, Herman Christiaan
- Date: 2009-01-27T07:16:59Z
- Subjects: Stratigraphic geology , Cratons , Geological time , Kaapvaal Craton (South Africa)
- Type: Thesis
- Identifier: uj:14820 , http://hdl.handle.net/10210/1945
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
- Authors: Dorland, Herman Christiaan
- Date: 2009-01-27T07:16:59Z
- Subjects: Stratigraphic geology , Cratons , Geological time , Kaapvaal Craton (South Africa)
- Type: Thesis
- Identifier: uj:14820 , http://hdl.handle.net/10210/1945
- Description: M.Sc. , Please refer to full text to view abstract
- Full Text:
Neoarchaean clastic rocks on the Kaapvaal Craton : provenance analyses and geotectonic implications
- Authors: Schneiderhan, Eva Anita
- Date: 2008-08-13T12:19:27Z
- Subjects: Geology South Africa , Stratigraphic geology , Cratons , North-West (South Africa)
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/377453 , uj:7672 , http://hdl.handle.net/10210/853
- Description: The provenance of the Neoarchaean Ventersdorp Supergroup and several age-related supracrustal successions was analysed to gain insight into the geotectonic evolution of the Kaapvaal Craton during the transition from the Archaean to Proterozoic Eras. The studied successions include, besides the siliciclastic formations of the Ventersdorp Supergroup, the upper Wolkberg and Buffelsfontein Groups, the Godwan Formation and the Schmidtsdrift Subgroup of the basal Transvaal Supergroup in Griqualand West. Petrographic, whole rock geochemical and Sm-Nd isotopic analyses were combined with SHRIMP U-Pb age dating of detrital zircons. Furthermore, Rb-Sr isotopic studies were carried out on carefully selected suites of samples from surface exposure or, wherever possible, on deep diamond drill core. The Ventersdorp Supergroup is an up to 5 km thick undeformed, only slightly metamorphosed volcano-sedimentary succession deposited on the Kaapvaal Craton between 2714 Ma and 2665 Ma. A lack of major time hiati to the underlying Mesoarchaean Witwatersrand Supergroup and covering Neoarchaean to Palaeoproterozoic Transvaal Supergroup render the Ventersdorp Supergroup very well suited for the investigation of the geotectonic evolution of the Kaapvaal Craton near the Archaean-Proterozoic boundary. This is supported by its excellent preservation, which also allowed detailed studies of sedimentological structures, such as seismites indicating Neoarchaean earthquakes. The provenance analyses carried out on the clastic formations of the Ventersdorp Supergroup point to a gradual change in tectonic evolution from typically Archaean to post-Archaean processes rather than a drastic, unique transition in the case of the Kaapvaal Craton. Texturally immature wackes of the Kameeldoorns Formation, representing the oldest clastic units of the Ventersdorp Supergroup, are derived mainly from Mesoarchaean source rocks, whereas the stratigraphically younger Bothaville Formation displays geochemical signatures comparable with Archaean trondhjemite-tonalite granodiorite-suites (TTGs), thus suggesting crustal addition in the so-called ‘Archaean-style’. The extension of provenance analyses to supracrustal successions that are tentatively correlated with the Bothaville Formation, revealed contributions from granitoid V sources that formed under post-Archaean and Archaean conditions. Furthermore, the geochemical data for all analysed formations support a passive margin setting. Arc settings, as indicated in some samples, are due to the input of less fractionated volcanic material that provides evidence of distal volcanism. Analyses of Nd-isotopic systematics and U-Pb ages of detrital zircons revealed a Mesoarchaean age for the source rocks of the formations. U-Pb age dating of detrital zircons of the Godwan Formation suggests that this formation is of Mesoarchaean age, and therefore not a correlative of the other Neoarchaean successions. Hence, the results suggest that the continental crust of the Kaapvaal Craton was thick enough since the Mesoarchaean (2.8 - 3.1 Ga) to allow long-term crustal recycling, and therefore modern plate tectonic processes could have operated earlier than on other well-studied cratonic blocks. During the Neoarchaean, however, crustal thickening of the Kaapvaal Craton took place by accretion of Archaean-style TTGs along the margins of the craton. Thus, Archaean and post-Archaean tectono-magmatic processes co-existed. Furthermore, the Neoarchaean supracrustal successions represent the first sedimentation events on an entirely stabilised and tectonically quiescent Kaapvaal Craton. Input from distal volcanic sources marks the last sign of volcanic activity prior to the craton-wide deposition of carbonate rocks of the Transvaal Supergroup. Geochronological data also imply a connection of the Neoarchaean Kaapvaal Craton to further cratonic blocks that may hold source rocks for the studied formations, as for some small age populations of older detrital zircons (ca. 3.1 - 3.4 Ga), no suitable source area could be identified on the Kaapvaal Craton itself. However, it seems unlikely that the Zimbabwe Craton was one of these cratonic blocks, because the Rb-Sr whole rock ages of all studied formations yield a model age of 2092 ± 55 Ma, which is thought to correspond to a craton-wide influence of the 2.05 Ga old Bushveld Igneous Complex on the Rb-Sr isotope systematics of all analysed clastic successions. This influence is apparently missing in the Southern and Central Marginal Zones of the Limpopo Belt, suggesting that the collision between the Kaapvaal and Zimbabwe Cratons only took place after the emplacement of the Bushveld Igneous Complex, i.e. after 2.05 Ga. , Dr. U. Zimmermann Prof. J. Gutzmer
- Full Text:
- Authors: Schneiderhan, Eva Anita
- Date: 2008-08-13T12:19:27Z
- Subjects: Geology South Africa , Stratigraphic geology , Cratons , North-West (South Africa)
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/377453 , uj:7672 , http://hdl.handle.net/10210/853
- Description: The provenance of the Neoarchaean Ventersdorp Supergroup and several age-related supracrustal successions was analysed to gain insight into the geotectonic evolution of the Kaapvaal Craton during the transition from the Archaean to Proterozoic Eras. The studied successions include, besides the siliciclastic formations of the Ventersdorp Supergroup, the upper Wolkberg and Buffelsfontein Groups, the Godwan Formation and the Schmidtsdrift Subgroup of the basal Transvaal Supergroup in Griqualand West. Petrographic, whole rock geochemical and Sm-Nd isotopic analyses were combined with SHRIMP U-Pb age dating of detrital zircons. Furthermore, Rb-Sr isotopic studies were carried out on carefully selected suites of samples from surface exposure or, wherever possible, on deep diamond drill core. The Ventersdorp Supergroup is an up to 5 km thick undeformed, only slightly metamorphosed volcano-sedimentary succession deposited on the Kaapvaal Craton between 2714 Ma and 2665 Ma. A lack of major time hiati to the underlying Mesoarchaean Witwatersrand Supergroup and covering Neoarchaean to Palaeoproterozoic Transvaal Supergroup render the Ventersdorp Supergroup very well suited for the investigation of the geotectonic evolution of the Kaapvaal Craton near the Archaean-Proterozoic boundary. This is supported by its excellent preservation, which also allowed detailed studies of sedimentological structures, such as seismites indicating Neoarchaean earthquakes. The provenance analyses carried out on the clastic formations of the Ventersdorp Supergroup point to a gradual change in tectonic evolution from typically Archaean to post-Archaean processes rather than a drastic, unique transition in the case of the Kaapvaal Craton. Texturally immature wackes of the Kameeldoorns Formation, representing the oldest clastic units of the Ventersdorp Supergroup, are derived mainly from Mesoarchaean source rocks, whereas the stratigraphically younger Bothaville Formation displays geochemical signatures comparable with Archaean trondhjemite-tonalite granodiorite-suites (TTGs), thus suggesting crustal addition in the so-called ‘Archaean-style’. The extension of provenance analyses to supracrustal successions that are tentatively correlated with the Bothaville Formation, revealed contributions from granitoid V sources that formed under post-Archaean and Archaean conditions. Furthermore, the geochemical data for all analysed formations support a passive margin setting. Arc settings, as indicated in some samples, are due to the input of less fractionated volcanic material that provides evidence of distal volcanism. Analyses of Nd-isotopic systematics and U-Pb ages of detrital zircons revealed a Mesoarchaean age for the source rocks of the formations. U-Pb age dating of detrital zircons of the Godwan Formation suggests that this formation is of Mesoarchaean age, and therefore not a correlative of the other Neoarchaean successions. Hence, the results suggest that the continental crust of the Kaapvaal Craton was thick enough since the Mesoarchaean (2.8 - 3.1 Ga) to allow long-term crustal recycling, and therefore modern plate tectonic processes could have operated earlier than on other well-studied cratonic blocks. During the Neoarchaean, however, crustal thickening of the Kaapvaal Craton took place by accretion of Archaean-style TTGs along the margins of the craton. Thus, Archaean and post-Archaean tectono-magmatic processes co-existed. Furthermore, the Neoarchaean supracrustal successions represent the first sedimentation events on an entirely stabilised and tectonically quiescent Kaapvaal Craton. Input from distal volcanic sources marks the last sign of volcanic activity prior to the craton-wide deposition of carbonate rocks of the Transvaal Supergroup. Geochronological data also imply a connection of the Neoarchaean Kaapvaal Craton to further cratonic blocks that may hold source rocks for the studied formations, as for some small age populations of older detrital zircons (ca. 3.1 - 3.4 Ga), no suitable source area could be identified on the Kaapvaal Craton itself. However, it seems unlikely that the Zimbabwe Craton was one of these cratonic blocks, because the Rb-Sr whole rock ages of all studied formations yield a model age of 2092 ± 55 Ma, which is thought to correspond to a craton-wide influence of the 2.05 Ga old Bushveld Igneous Complex on the Rb-Sr isotope systematics of all analysed clastic successions. This influence is apparently missing in the Southern and Central Marginal Zones of the Limpopo Belt, suggesting that the collision between the Kaapvaal and Zimbabwe Cratons only took place after the emplacement of the Bushveld Igneous Complex, i.e. after 2.05 Ga. , Dr. U. Zimmermann Prof. J. Gutzmer
- Full Text:
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