Craton(redirected from Cratons)
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craton(krā`tŏn): see continentcontinent,
largest unit of landmasses on the earth. The continents include Eurasia (conventionally regarded as two continents, Europe and Asia), Africa, North America, South America, Australia, and Antarctica.
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consolidated parts of the earth's crust that cannot be transformed by alpine-type folding. The German geologist H. Stille subdivided them into uplifted cratons, masses of primarily sialic (Si, Al) composition (the ancient Precambrian platforms or shields of Soviet geologists, which are characterized by a continental crustal structure with a developed “graniticmetamorphic” layer), and submerged cratons, masses with simatic (Si, Fe, and Mg) bases (primarily regions of the ocean floor without the “granitic-metamorphic” crustal layer). The term “craton” was proposed by the Austrian geologist L. Kober and is used extensively in foreign writing.
(central continental craton; in Russian, platforma), one of the main structural elements of the earth’s crust. Cratons are large and relatively stable blocks of the crust, having uniform thickness and measuring several thousand km across. They are characterized by very low seismicity, specific volcanic activity, and a slight topographic relief of the earth’s surface.
The concept of cratons first appeared at the turn of the 20th century in the works of A. P. Karpinskii, E. Suess and G. E. Haug. In 1932, A. D. Arkhangel’skii used the Russian term platforma for the first time in its modern sense to denote a central continental craton. The study of cratons was advanced primarily through the work of the Russian and Soviet scientists Karpinskii, A. P. Pavlov, Arkhangel’skii, N. S. Shatskii, A. L. Ianshin, and A. A. Bogdanov.
Cratons formed by the continental-type crust with a well-developed “granitic” layer some 35 to 45 km thick have an angular and isometric outline and are separated by marginal seams from the adjacent geosynclinal belts or ocean troughs. They gradually develop on the site of previously existing geosynclinal systems as a section of the earth’s crust with high mobility becomes a technically stable crust. A craton’s most characteristic structural feature is its two structural stages. The basement is the lower and more ancient stage and is composed of intensely dislocated metamorphosed and granitized rocks; it represents a formation of the precratonic, or geosynclinal, developmental stage of the earth’s crust. The upper, and younger, structural stage—the cratonic mantle—consists of unmetamorphosed, largely sedimentary rocks that overlie the basement usually horizontally and unconformably. Individual parts of the lithosphere passed from the geosynclinal stage to the cratonic stage at various times in the history of the earth. The formation time of the folded basement determines the craton’s geological age.
Cratons are classified as ancient and young. Ancient cratons were formed during the Precambrian, mostly by the start of the late Proterozoic. Ancient cratons include the Eastern European (Russian), Siberian, North American, Sino-Korean, South China, Indian, African, Australian, and Antarctic platforms. These platforms constitute the cores of the modern continents. Young cratons have a folded base of Paleozoic and, partly, late Precambrian origin. The geosynclinal stage of development of young cratons lasted until the beginning, middle, or end of the Paleozoic era or the start of the Mesozoic era, when the cratonic mantle began forming. Depending on the age of the final deformations of the basement, young cratons are classified as epibaikalian, epicaledonian, and epihercynian; epibaikalian cratons are sometimes grouped with ancient cratons.
Ancient cratons are characterized by a crystalline basement composed predominantly of granites, gneisses, and schists. The basements of young cratons contain moderately dislocated and slightly metamorphosed sedimentary and igneous rocks, with granitic intrusions—if they are present at all—of secondary significance. Such a basement is termed a folded cratonic bases. Young cratons include the plains of Western Siberia, Northern Kazakhstan, the Turan Lowlands, Ciscaucasia, and Western Europe.
The largest structural elements of a craton are shields and platforms. As a result of prolonged uplifting and water erosion, shields are almost completely without sedimentary mantles, and the basement of the craton appears on the surface. Platforms, however, have a thick sedimentary mantle some 3 to 5 km thick and the two-stage structure typical of cratons. After shields and platforms, the most important category of cratonic structures comprises anteclises and synclises. These structures are uplifts and depressions of the basement and sedimentary mantle with very gentle slopes. In addition to these structures there are graben-like depressions, or aulacogens. Arches are smaller, elongated structures measuring 200 to 300 km in length; they consist of chains of local uplifts, or placanticlines, and usually develop over faults of the basement.
The development of continental cratons is determined by the movement of the basement, which causes a general uplifting of the craton. This uplifting is complicated by the ruptures that accompany the formation of aulacogens and by the movements originating in adjacent, actively developing geosynclinal belts. The adjacent geosynclinal belts cause the margins of the craton to be periodically drawn into subsidences; the sediments that accumulate are initially continental detrital materials, followed by coal- or salt-bearing lagoon and shallow-pelagic and sandy-argillaceous carbonaceous formations, and then once more by lagoonal and continental deposits. Periodic activation of tectonic movements, associated primarily with orogenic epochs in geosynclinal belts, lead to partial transformation of cratons into epicratonic orogenic zones; this occurs chiefly on the peripheries of cratons. This is accompanied by an uplifting of the craton and the emergence of a secondary mountainous topography with great height fluctuations. Also related to the epochs of tectonic activation is the revival of magmatic activity on the craton, which, is expressed in the development of specific magmatic formations, including trap (plateau basalts, dikes and sills of dolerites), alkali-basaltic, alkali-ultrabasaltic (ring intrusions), and kimberlite.
The development of cratons is a process that lasts many hundreds of millions of years. The following major stages are distinguished: the genesis, or cratonization, stage, with general uplifting; the aulacogen stage, with the formation of graben-like downwarps; the platform stage, with subsidence, accumulation of a sedimentary mantle, and the formation of synclises and platforms; and the general uplifting stage, with partial washout of the mantle.
In the 1960’s, extensive study of the ocean floor lead to great advances in ideas concerning the global tectonics of the earth. Geologists identified structures within the oceans that are analogous to, but very different from, continental cratons. This led to a new distinction between continental cratons—the subject of all previous research on cratons—and oceanic cratons, or thalassocratons.
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V. E. KHAIN