Three geochrons are distinguished: (1) Protogey (before 1,600 m.y.), (2) Mesogey (1,600-600 m.y.), and (3) Neogey (younger than 600-550 m.y.).

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1. The oldest nuclear volcanic regions and primary upheavals (e.g., Yengr) existed during the Protogey geochron within the limits of the Aldan and Sino-Korea shields. After the consolidation of the early Archean mobile regions in interblock protogeosyncline subsidences, the Stanovik fold system formed. Later, around 1,900-1,800 m.y., a gigantic collapse (Umbruch) took place. During the last 300 m.y. of the Protogey geochron large sags developed.

2. During the 1,000 m.y. which comprised the Mesogey geochron, a calm, catastable regime predominated. At the beginning and end of the Mesogey, outbursts of volcanic activity occurred.

3. During the Neogey the following stages are distinguished. (a) The early Paleozoic, during which broad geosynclines formed. The geosynclines were filled mainly with terrigenous and carbonate material. The structures of the late Baikalides formed with widespread granitoid magmatism. (b) During the middle Paleozoic thick eugeosynclinal and miogeosynclinal downwarping occurred. (c) The late Paleozoic, an extension of the middle Paleozoic, was completed by orogenesis in the Mongol-Okhotsk, Sikhote-Alin, and Nippon systems. (d) In the Mesozoic several types of mobile regions and systems existed. The Verkhoyansk and Chukotsk geosyncline systems developed on weakly crushed basement with terrigenous sedimentation and trap-type magmatism. The Mongol-Okhotsk system formed on the depressed mar ins of rigid massifs. The Selenge-Yablonovyy and Stanovoy regions, with rift basins (Jurassic-Cretaceous), are uplifted mountain arches and block-faulted regions that developed on stabilized structures. They are characterized by continental sedimentation and intrusive (granitoid commonly alkaline) and effusive magmatism. The Chugoku (Japan), eastern Sikhote-Alin, Okhotsk-Chukotsk, and Nunivak (Alaska) regions are marginal-continental volcanogenic belts. Other regions were slowly evolving orthogeosyncline belts on the boundary between the continent and the ocean. (e) The Cenozoic was a period of intense mountain building accompanied by evolution of intermontane basins on continents and the formation of systems of island arcs in transitional continental-oceanic regions (Kuril-Kamchatka and Aleutian arc systems). During this time the accelerated rate of tectonic processes is apparent.

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