Precambrian Eras of Folding
Precambrian Eras of Folding
eras of increased tectonomagmatic activity that were manifested during the Pre-cambrian history of the earth. They covered the time interval from 570 million to 3.5 billion years ago. The eras have been established on the basis of a number of geological data, such as changes in structural plan, manifestations of breaks and unconformities in the bedding of rock, and sharp changes in the degree of metamorphism. The absolute age of the Precambrian eras of folding and their interregional correlation are established by determining the time of the metamorphism and the age of the magmatic rock using radiological methods. The methods for determining the age of ancient rock allow the possibility of errors on the order of 50 million years for the late Precambrian and 100 million years for the early Pre-cambrian. Therefore, the dates of the Precambrian eras of folding can be established with considerably less certainty than those of the Phanerozoic eras of folding. The data of radiometric readings show the existence of a number of eras of tectonomagmatic activity in the Precambrian. These eras appeared approximately simultaneously throughout the world. The Precambrian eras of folding have received different names on different continents.
The oldest of them was the Kolian (Saamian; the Baltic Shield), or the Transvaalian (South Africa), which appeared about 3 billion years ago and was expressed by the formation of the most ancient archicontinents. The relics of these archicontinents are encountered on all ancient platforms (as yet with the exception of the Sino-Korean and Southern Chinese). The manifestations of the next era were even more widespread. On the Baltic Shield this era was named the White Sea, on the Canadian Shield it was known as the Kenoran, and in Africa as the Rhodesian. It developed 2.5 billion years ago, and the formation of the large shield cores of the ancient platforms was connected with it. Of great significance was the Early Karelian (Baltic Shield), or Eburnean (West Africa), era (about 2 billion years ago) which, along with the subsequent late Karelian era (the Hudsonian for the Canadian Shield and the Mayomban for Africa) which occurred some 1.7 billion years ago, played a decisive role in forming the basements of all the ancient platforms. The tectonomagmatic eras in the interval between 1.7 and 1.4 billion years ago have been established only on certain continents (for example, the Laxfordian in Scotland, some 1.55 billion years ago).
Occurring about 1.4 billion years ago, the Gothian (Baltic Shield), or Elsonian (Canadian Shield), era was of planetary significance. However, it was expressed not so much in the folding of geosynclinal formations as in the repeated metamorphism and granitization of individual zones within the basement of the ancient platforms. The next era, the Dalslandian (Baltic Shield), Grenvillian (Canadian Shield), or Satpurian (Hindustan), which occurred about 1 billion years ago, was the first major era of folding of the geosynclinal belts of the Neogaea. The concluding Precambrian era of folding was the Baikalian (Assyntian in Scotland, Cadomian in Normandy, and Katangan in Africa). It was manifested very widely on all the continents, including Antarctica, and led to the consolidation of significant areas within the geosynclinal belts of the Neogaea. The Baikalian movements began about 800 million years ago, their main pulsation occurred about 680 million years ago (before the depositing of the Vendian complex), and the concluding pulsation at the beginning or in the middle of the Cambrian.
Among the Baikalian folded systems in the USSR are the systems of the Timan, the Enisei Ridge, parts of the Vostochnyi Saian, and the Patom Plateau. Baikalian folded systems of this age are widely found in Africa (the Katangides, the Western Congolides, and the Atakora and Mauritano-Senegalese zones), South America (the Brasilides), Antarctica, Australia, and other continents. A common feature of the Precambrian eras of folding is the significant development of regional metamorphism and granitization which diminished in intensity from the ancient eras to the more recent. On the contrary, the scale of orogenesis and the folding itself were apparently weaker than those of the Phanerozoic. Granite-gneiss domes were characteristic structural forms, particularly for the Early Precambrian.
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Salop, L. I. “Dokembrii SSSR.” In Geologiia dokembriia. (Mezhdunarodnyi geologicheskii kongress: XXIII sessiia: Doklady sovetskikh geologov: Problema 4.) Leningrad, 1968.
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Shoubert, G. A., and A. For-Muret. “Legenda karty [Afriki].” Ibid., p. 83.
V. E. KHAIN