Eugeosyncline

eugeosyncline [yü‚jē·ō′sin‚klīn]

(geology)

The internal volcanic belt of an orthogeosyncline.

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Eugeosyncline 

the inner, more mobile part of a geosynclinal system and the part permeated with products of magmatism, in contrast to the outer, less mobile miogeosynclinal zones that adjoin the platforms. Eugeosynclines are usually extended linear zones, stretching hundreds or thousands of kilometers. The lower parts of their stratigraphic cross sections are usually composed of ophiolites, and the upper parts, of detrital graywacke beds, flysch, and reef limestones, primarily associated with andesitic volcanic rocks. The layers are intricately folded, often forming imbricate structures and overthrust nappes; the latter may be shifted. The development of eugeosynclines ends with the formation of intricately constructed folded zones with a continental crust. High-temperature metamorphism develops parallel with deformation, in the course of which a granite-metamorphic layer gradually forms.

Examples of eugeosynclines are the Tagil’-Magnitogorsk Zone of the Urals and the Piedmont region of the Alps. Typical eugeosynclines first undergo an oceanic stage of development, with the formation of abyssal sediments and basic volcanic rocks. This is followed by the stage of island arcs and marginal seas. Thus, eugeosynclines emerge in place of former ocean basins (or marginal seas with an oceanic crust) and active transitional zones between the ocean and the continent, which were included in the folded belts of the continents during orogenic processes. An important group of minerals is found in eugeosynclines: ores of platinum, chromium, iron, and copper; complex ores and asbestos ores.

L. P. ZONENSHAIN