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a term introduced in 1921 by the Austrian geologist L. Kober for the second stage of development of geosynclines or tectonically mobile belts of the earth’s crust. This stage is characterized by the predominance of ascending movements and the emergence of mountains. By analogy with the tectonic structure of the Alps, Kober felt that every orogen has a symmetrical structure and is located between rigid masses called kratogens, or cratons, under whose pressure the orogen arises.
Kober divided the orogen into a series of zones. The central part is occupied by rigid granite masses called internides, on both sides of which are the centralides, zones composed of sedimentary and volcanic rocks in the form of enormous overthrust nappes in the direction of the kratogens. The metamorphides, composed of metamorphosed sedimentary rocks and magmatic rocks (ophiolites), adjoin the centralides. The marginal zones of the orogen, called the externides, are composed of flysch.
Kober’s scheme fails to take into account the full diversity of the structure of the mobile belts; the overthrust overlapping of some zones by others, for example, is not obligatory. Moreover, Kober overestimates the role of symmetry in the structure of the orogen. In present-day terminology the term “orogen” is often replaced by such synonyms as “folded mountain structure” or, less precisely, “geosyncline” (in the broad sense of the word) or “geosynclinal system.”
V. E. KHAIN