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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 21 (1937)

Issue: 11. (November)

First Page: 1439

Last Page: 1463

Title: Orogeny of the Urals

Author(s): Anatole Safonov (2)


The present Urals are bordered on the west by a belt of Permian and Carboniferous sediments thrown into long, gentle north and south trending folds. East of this, there is a zone of overthrusts--the older beds pushed westward, over the younger, in an intricate pattern, the amount of dip increasing eastward. Next come the central ridges made up of metamorphic Cambrian and pre-Cambrian rocks mingled with a great variety of basic and acid intrusions. The whole eastern slope is but a partly deciphered maze of sediments, green schists, igenous rocks, et cetera. Among them, the basic massif of Nijni Tagil and north of there, together with the Djabyk granitic massif and its northern extension west of Alapaevsk, are prominent. The Djabyk is considered to be a batholith emplaced a a culmination of the main orogenic phase. A considerable part of the Uralian geology in the east is concealed by Tertiary and later sediments. The grain of the formations is longitudinal, with the regions of Kara-Tau and Ufaley as marked exceptions. The structure of the central metamorphic ridges is predominantly sharply anticlinal.

The Uralian geosyncline, apparently a rift zone between the Russian and Tobolsk shields, had at least one pre-Ordovician orogenic cycle. Existence of a comparatively shallow Ordovician trough of deposition, from Mugodjary to Novaya Zemlya, is well established. Maximum post-Ordovician submergence occurred in Middle Devonian; another submergence took place in Lower and Middle Carboniferous. Tectonic forces, probably connected with the Taconic, Caledonian, and Acadian revolutions, acted on the geosyncline, especially on its middle and southern parts. There were numerous minor disturbances intervening. The main orogenic phase took place in the Permian, persisting into the Triassic. The resulting post-Permian Urals were eroded to their batholithic core, during the later Triassic. The Juras ic and Cretaceous again saw a widespread sea transgression which reached its maximum in Lower Tertiary. The time of rise of the present Urals is as yet unknown. The latest known disturbances are post-Pleistocene.

The Permian disturbance was expressed in a wedge-like orogeny, with the main compressive stress coming from the east. This produced the belt of easterly dips in the zone of overthrusts. A belt of westerly dips runs along the line Alapaevsk-Chelyabinsk, then swings into the Ural River Valley, toward Orsk. One variation of the wedge theory is that the whole of the wedge is included between these two zones of inward dips, with the batholith offset in the direction of the stress (asymmetrical). Another variation is that the Djabyk occupies a central position, and there should be another belt of westerly dips, hidden under the Tertiary sediments of Western Siberia. Gravimetric measurements in that region reveal alternating longitudinal zones of positive and negative anomalies similar to th se farther west.

Because of the heterogeneous character of the Uralian geosyncline, no competent beds were deposited in wide areas, and the deforming stresses were transmitted through the basement complex. Thus a zone of underthrusts was formed in the River Belaya loop. Thrust faulting was followed by block faulting. Subsequent stresses were of non-compressive character. The mid-Ural strike-slip fault, with the amount of displacement up to 35 miles, is one of the latest adjustments to these stresses.

Oil was found in Permian strata, west of the zone of overthrusts.

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