a highly mobile, linearly elongated segment of the earth’s crust, sharply dissected into longitudinal troughs and uplifts, within which an oceanic crust is usually transformed into a continental crust as the result of extended development. (However, many geosynclinal systems have formed on a continental crust.) The geosynclinal system is characterized by the high speed and great scope and contrast of its vertical movement, by its intensive folding, by intensive and varied magmatic processes, and by phenomena of regional metamorphism and endogenic mineralization. The geosynclinal troughs and uplifts of a system are separated from one another and from neighboring structures of the earth’s crust by abyssal fractures.
The external parts of geosynclinal systems, which usually occur on the deeply and smoothly submerged edge of neighboring platforms, are called (according to H. Stille) miogeosynclines, and their internal parts, or the internal troughs that form on the sharply faulted and reworked bases, are called eugeosynclines.
In structure, the earth’s crust of a geosynclinal system is transitional between the oceanic and the continental crusts and is distinguished by great heterogeneity. Under geosynclinal troughs the crust is closer to oceanic (it has a reduced thickness with a very thin “granite” layer, which in places is completely absent); in uplifts the crust is closer to continental (its thickness is increased because of growth of the “granite” layer).
A number of stages can be singled out in the history of each geosynclinal system. In the initial stage of the geosynclinal phase, the system experiences general submersion accompanied by vulcanism and the accumulation of sediments and is occupied by a deep sea, which is particularly deep above geosynclinal troughs. Miogeosynclines are distinguished by the absence or weak manifestation of vulcanism and are filled primarily by argillo-arenaceous deposits of the so-called lower terrigenous (aspidic or graywacke) formation or carbonaceous rock. In this stage eugeosynclines typically exhibit intense initial vulcanism, with massive underwater flows of basic lavas. Therefore, eugeosynclines are filled primarily with volcanogenic and volcanogenic-sedimentary strata. Of the sedimentary rocks themselves, flinty shales and jaspers are typical in this stage. Along the fractures that bound the eugeosyncline, intrusions of basic and ultrabasic deep-seated magmatic rock are introduced. The composition of the latter, as well as the confinement of deep-focused earthquakes to geosynclinal systems, indicates that these fractures go deep into the earth’s mantle. In the next stage, the preorogenic stage or stage of maturity, the geosynclines that constitute the geosynclinal system are separated by secondary (newly formed) uplifts, or geoanticlines (according to A. D. Arkhangel’skii), or by intrageoanticlines (according to M. M. Tetiaev and V. V. Belousov) into narrow daughter troughs, or intrageosynclines (according to M. M. Tetiaev and V. V. Belousov), which fill with carbonaceous rocks and rhythmically stratified flysch formations, whereas in eugeosynclines they fill with the products of continuing volcanic activity, which is already primarily of andesitic composition. The development of this process is accompanied by intrusions and folded deformations.
The next stage is a turning point in the development of the geosynclinal system. It is expressed in a transition to a general rise in the system (a general inversion of the tectonic regime, according to V. V. Belousov). The geosynclinal system enters the orogenic, or mountain-forming, phase. It coincides with maximum fold and thrust formation, the appearance of granitoid masses (batholiths), regional meta-morphism of rock, and the most intensive endogenous ore formation. The geosynclinal systems are transformed into folded rock structures (folded-block and folded-mantle) that are made up of a system of complex folds—meganticlinoria and megasynclinoria. Intermontane troughs occur between them, and foredeeps occur on the borders of the folded system and the platform. Both are filled with the clastic products of the disintegration of the growing mountains. In the initial (early orogenic) phase of the orogenic stage the intermontane troughs and foredeeps are filled primarily with argillo-arenaceous material deposited under marine or lagoon conditions (formation of the lower molasse). In the late orogenic phase they are replaced by rough sand and conglomerates of continental origin (formation of the upper molasse). The growing mountain structures are cut by faults, upthrusts, and steep overthrusts, with the formation of internal graben-like depressions and surface outflows of lavas that are at first more acidic (rhyolites and dacites) and later increasingly basic (from andesites to basalts), which is called orogenic vulcanism or, according to Stille, subsequent and final vulcanism. With the conclusion of the final, orogenic phase, the geosynclinal system is transformed from a highly mobile segment of the earth’s crust into a tectonically stable folded system—the basis of a future platform. The phase that precedes final orogenesis has come to be called the primary geosynclinal phase.
Geosynclinal systems are distinguished according to their time of occurrence and, most importantly, the time of the conclusion of geosynclinal development and their transformation into folded systems.
The most common age generations of folded systems are Precambrian, early Paleozoic (Caledonian or caledonides), late Paleozoic (Hercynian or hercynides), middle Mesozoic (Cimmerian), and Cenozoic (Alpine). Some of the latter have not yet fully completed the geosynclinal cycle of development.
Adjacent and more or less simultaneously developing geosynclinal systems, together with median masses, are part of geosynclinal regions, which form vast geosynclinal belts.
The distribution of a number of the most important types of minerals is confined primarily to geosynclinal systems and the folded systems arising from them. Deposits of asbestos, chromite, magnetite, copper, and complex (pyrite) ores are likely to be found in their internal parts; the external parts contain deposits of copper, gold, tin, tungsten, molybdenum, lead, and zinc ores. Deposits of gold, silver, complex ores, sulfur, mercury, arsenic, and antimony are associated with orogenic vulcanism. Large deposits of petroleum, gas, mineral coal, common salt, and potassium salts are located in the foredeeps and intermontane troughs.
V. E. KHAIN and M. V. MURATOV