Vaalbara
Vaalbara is in Earth's history, a hypothetical continent that could have existed on the top Neoarchaikum to the lower Paleoproterozoic before about 2.7 to about 2.5 billion years ago. The existence of this geological continent has been confirmed by recent studies on. In contrast, the existence of the highly speculative hypothetical continent Ur by the relative position of the two constituent cratons, the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia is virtually refuted.
Naming and components
The name Vaalbara or geological continent was proposed and postulated by Erik S. Cheney, C. Roering and E. Stettler 1988. It consists of the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia. Here, the Pilbara craton was rotated by 60 ° clockwise northwest of the Kaapvaal Craton, based on the present north direction of the Transvaal Craton. The name was formed from part of their name - vaal of the Kaapvaal Craton and the name component of the Pilbara Craton - bara.
Pilbara region
Vaalbara as a geological continent
The oldest rocks in both cratons are predominantly mafic -ultramafic rocks of the Onverwacht Group, South Africa and the Warrawoona group in Western Australia. They are interpreted as fragments of an island arc complex. Both sequences contain mafic and ultramafic at the base of rocks, which are superimposed on the upper part of clastic siliceous shales. These cherts are now dated to 3.47 billion years and thus also result in a minimum age for the underlying rocks. From them were described with the oldest microfossils. The cherts are in turn overlain by pillow lava, which are selbt not superimposed by concordant felsic igneous complexes. Even before 3.45 billion both cratons were slightly deformed. After 3.3 billion years there was a strong deformation of both blocks. Between 2780 and 2.72 billion years intruded granites in the Kaapvaal craton, which as yet have no equivalents in the Pilbara craton. It was followed by a removal phase, and these consolidated sedimentary cores lie unconformably, only weakly metamorphic episodes, showing that the two cratons were consolidated at the latest before about 2.7 billion years ago. The sedimentary sequences of the Pilbara craton and Kaapvaal craton can be correlated ( Neoarchaikum to lowest Paleoproterozoic ) directly in the period 2717-2495 million years ago. The shelf - basin boundary running from northwest to southeast and is well documented on both blocks. On the basis of a sequence neoarchaischen impact location is turned on, which allows a further good correlation. This means that the two old continental nuclei hung up before at least about 2.5 billion years together. The Kaapvaal craton at the time was 52.5 ° north latitude. The further history of the Vaalbara - continent is uncertain since the sedimentary sequence aborts. The regionalmetamorphen events (around 2.045 billion years ) and the emergence of Vredevort - impact event (2025 million years) were associated with the intrusion of the Bushveld Complex, are not documented on the Pilbara craton. This means that the separation of the two longlifety occurred previously.