(folded geosynclinal belt, folded belt, geosyncline [second meaning]), a vast, linearly elongated, tectonically highly mobile belt of the earth’s crust. It is located either between ancient continental platforms or between the platforms and an ocean bed, including inland and marginal seas, island arcs, and deep-water trenches. Its length reaches several tens of thousands of kilometers and its width is hundreds and even thousands of kilometers. In the recent history of the earth (the Neogaea), that is, in the last 1.6 billion years, five main geosynclinal belts have developed. The Pacific belt encircles the Pacific Ocean and separates its bed from the platforms of North and South America, Asia, Australia, and Antarctica. The Mediterranean belt joins the Pacific belt in the area of the Malay Archipelago and stretches across the south of Eurasia and Northwest Africa as far as Gibraltar. The Ural-Mongolian (Ural-Mongolian-Okhotsk) belt skirts the Siberian platform on the west and south and divides it from the Eastern European and Chinese-Korean platforms. The Atlantic belt encompasses the coasts of the continents in the northern part of the Atlantic Ocean. The Arctic belt encircles the Arctic Ocean. The Pacific and Atlantic geosynclinal belts are subdivided by some authorities, respectively, into the eastern and western Pacific belts and the eastern and western Atlantic belts.
During the evolution of the geosynclinal belts numerous geosynclinal regions and systems were consecutively laid down and developed within the boundaries of these belts. At different times these geosynclinal regions and systems were affected by folding and by regional metamorphism and granitization and were transformed into folded mountain systems of different ages and later into young platforms. The oldest folded areas of the geosynclinal belts date from the late Proterozoic age (baikalides). They are usually located on the periphery of the belt, adjoining one or both of the ancient platforms bordering the belt. The younger folded regions— Paleozoic (caledonides, hercynides), Mesozoic, and Cenozoic—occupy a position which is correspondingly closer to the central part of the belt or to the fringe on the opposite side from the platform (in the case of the marginal continental geosynclinal belt).
In the present age most of the geosynclinal belts have taken on the character of folded mountain structures or young platforms. Thus, the Paleozoic structures over vast areas have been buried under a thick mantle of horizontally deposited sedimentary rock, forming the foundation of young platforms (for example, the Western Siberian platform). The youngest, Cenozoic, parts of the geosynclinal belts have not yet completed their geosynclinal development and to this day retain a high mobility, accompanied by increased seismic activity and active vulcanism. Examples of these are the regions of the Mediterranean Sea, the Malay Archipelago, and the island arcs that ring the eastern coast of Asia in the Pacific Ocean geosynclinal belt.
In addition to the main geosynclinal belts mentioned above, which include folded geosynclinal regions and systems of different ages, there are two belts that completed their geosynclinal development at the end of the Proterozoic age (in the age of the Baikal folding). One has been traced in Arabia and East Africa and the other in the eastern part of South America and West Africa. The contours of these belts have been defined differently by different investigators.
For bibliography seeGEOSYNCLINE.
V. E. KHAIN, M. V. MURATOV, and E. V. SHANTSER