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depressions on the earth’s surface enclosed on all or almost all sides. Their diameter usually varies between tens and hundreds (rarely, thousands) of kilometers. They are mainly tectonic in origin. Tectonic basins are regions of sustained sinking of the earth’s crust, filled with sedimentary and volcanic deposits. The youngest tectonic basins are visible in the earth’s surface relief. Their form is usually more or less rounded and isometric or oval in peaceful regions of the earth’s crust (platforms) and linear in mobile belts (geosynclinal and erogenous zones); in the latter, they are frequently bounded by faults. Tectonic basins include synclises and aulakogenes (intracratonal mobile belts) on platforms, separate geosynclinal depressions (intrageosynclines) in geosynclinal belts, and advanced (lateral, foothill) and intramontane depressions and graben-rifts in orogenous regions. The fundamental causes of the formation of tectonic basins are processes in the upper mantle of the earth causing compacting or diffusion of its material, as well as the stretching of the earth’s crust with increases or changes in the planet’s velocity of rotation and, consequently, of its shape. Basins are also situated on the ocean floor. A special type of basin is the volcano-tectonic, particularly calderas, which are formed as a result of the settling of the crust after the ejection of a great volume of magmatic material; their diameter is measured in tens of kilometers, rarely over 100. Occasionally, the word“basin” is used synonymously with the terms “hollow,” “depression,” and “tectonic trough.” In recent times an effort has been made to differentiate between the meanings of these terms.
V. E. KHAIN