(folded geosynclinal region), a large, relatively separate sector of a geosynclinal belt distinguished from adjacent regions by the age of the folding and the characteristics of its history of development. The geosynclinal region consists of folded systems of the same or approximately the same age (for example, Caledonian and Hercynian). Folded systems extend within the geosynclinal region in two or more parallel rows, continuous or replacing one another along the course in an echelon-like manner. These folded systems are divided by deep faults and relatively immobile median masses, which are the remnants of the reworked ancient base on which the geosynclinal systems occurred. Thus, the Carpathian and Dinaric geosynclinal systems are separated by the Pannonian median mass. Examples of geosynclinal regions are the Tien-Shan, the Central Kazakhstan, the Altai-Saian (in the Ural-Mongolian, or Ural-Mongolian-Okhotsk, geosynclinal belt), and the Antilles-Caribbean (within the eastern Pacific Ocean belt) regions.
The term geosynclinal region was introduced by A. D. Arkhangel’skii and N. S. Shatskii in 1933, and its initial meaning was close to that of the present-day concept of geosynclinal belt or geosynclinal system.