Vaalbara

from Wikipedia, the free encyclopedia

Vaalbara is a hypothetical continent in the history of the earth , which could have existed in the upper Neo-Archean to the lower Paleoproterozoic about 2.7 to about 2.5 billion years ago. The existence of this geological continent has been further confirmed by recent studies. In contrast, the existence of the highly speculative hypothetical continent Ur is refuted by the relative position of the two constituent cratons, the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia.

Naming and components

The name Vaalbara or the geological continent was proposed or postulated by Erik S. Cheney, C. Roering and E. Stettler in 1988. It consists of the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia. The Pilbara craton was rotated 60 ° clockwise northwest of the Kaapvaal craton based on today's north direction of the Transvaal craton. The name was formed from the component -vaal of the Kaapvaal-Kraton and the component -bara of the Pilbara-Kraton.

  • Kaapvaal Kraton

  • Pilbara region

Vaalbara as a geological continent

The oldest rocks in both cratons are the predominantly mafic to ultramafic rocks of the Onverwacht Group in South Africa and the Warrawoona Group in Western Australia. They are interpreted as fragments of an island arch complex. Both sequences contain mafic and ultramafic rocks at the base, which are overlaid by clastic silica schists in the higher part . These silica schists are now dated to 3.47 billion years and thus also result in a minimum age for the rocks below. Some of the oldest microfossils have been described from them . The silica slates are in turn overlaid by pillow lavas , which themselves are not concordantly overlaid by volcanic-rock complexes.

Both cratons were slightly deformed before 3.45 billion. After 3.3 billion years, both blocks were severely deformed. Between 2,780 and 2,720 million years ago, granites intruded into the Kaapvaal craton that as yet have no equivalents in the Pilbara craton. This was followed by an erosion phase, and over these consolidated cores lie discordant sedimentary, only weakly metamorphic consequences, which show that the two cratons were consolidated at the latest about 2.7 billion years ago. The sedimentary sequences of the Pilbara Kraton and Kaapvaal Kraton can be directly correlated in the period between 2717 and 2495 million years ( Neo-Archean to the lowest Paleoproterozoic ). The shelf-basin boundary ran northwest to southeast and is well documented on both blocks. At the base of the neoarchaic sequence, an impact position is switched on, which allows a further, good correlation. This means that the two ancient continental cores were connected until at least 2.5 billion years ago. The Kaapvaal Kraton was then at 52.5 ° north latitude. The further history of the Vaalbara continent is uncertain because the sedimentary sequence breaks off. The regional metamorphic events associated with the intrusion of the Bushveld Complex (around 2045 million years) and the formation of the Vredevort Impact (2025 million years) have not been documented on the Pilbara craton. This means that the separation of the two cratons took place beforehand.

swell

literature

  • Andrew H. Knoll and Nicolas J. Beukes: Introduction: Initial investigations of a Neoarchean shelf margin-basin transition (Transvaal Supergroup, South Africa). Precambrian Research, 169: 1-14, Amsterdam 2009 ISSN  0301-9268 doi : 10.1016 / j.precamres.2008.10.009
  • Michiel Olivier de Kock, DAD Evans, JL Kirschvink, NJ Beukes, E. Rose, I. Hilburn: Paleomagnetism of a Neoarchean-Paleoproterozoic carbonate ramp and carbonate platform succession (Transvaal Supergroup) from surface outcrop and drill core, GriqualandWest region, South Africa . Precambrian Research, 169: 80-99, Amsterdam 2009 ISSN  0301-9268 doi : 10.1016 / j.precamres.2008.10.015
  • DR Nelson, AF Trendall and Wladyslaw Altermann: Chronological correlations between the Pilbara and Kaapvaal cratons. Precambrian Research, 97: 165-189, Amsterdam 1999 ISSN  0301-9268 doi : 10.1016 / S0301-9268 (99) 00031-5
  • Erik S. Cheney, C. Roering, and E. Stettler: Vaalbara (Ext. Abstr.). Geological Socitety of South Africa Geocongress, 88: 85-88, Johannesburg 1988

Individual evidence

  1. ^ JW Schopf and BM Packer: Early Archean (3.3-billion to 3.6-billion-year-old) microfossils from Warrawoona Group, Australia. Science, 237: 70-73, New York 1987