ON THE CYCLES OF TECTOGENESIS OF THE URALS

A. G. Bakirov

I. INTRODUCTION

The present work is a revised chapter, “On the Cycles of Tectogenesis of the Urals,” from the diploma project “Geological Structure of the Southwestern Part of the Kempirsai Hyperbasite Massif,” devoted to the geotectonic history of the Urals. The project was carried out in the 1938–1939 academic year at the Tomsk Industrial Institute named after S. M. Kirov under the supervision of Professor A. M. Kuzmin. In comparison with the above-mentioned chapter, additions have been made here concerning the structure of various regions of the Urals and Timan. The section dealing with the history of geosynclinal development has been considerably expanded.

In this work, on the basis of factual material on Ural geology, a somewhat different interpretation of its geotectonic history is given. The question is raised of the necessity of recognizing the manifestation of the Telbes cycle of tectogenesis for the Urals, alongside other cycles. Here, too, the structural features of individual regions of the Urals are noted, and certain considerations are expressed regarding the development of geosynclines.

In his work the author proceeds from recognition of the fundamental commonality in the history of formation of all regions of the Ural geotectonic province, which includes Mugodzhar, the Southern, Middle, Northern, and Polar Urals, Novaya Zemlya, and also, in part, Timan. Hence the principal conclusions, based on material from individual regions, are extended by the author to the entire Ural geotectonic province.

With each year the material on the geology of the Urals is augmented by new factual data. These pertain chiefly to the Middle and Lower Paleozoic, and also to the Precambrian. These data now make it impossible to doubt the presence in the Urals of Cambrian and Precambrian deposits, with consideration of which we shall begin the exposition of the history of the Urals.

II. THE PRECAMBRIAN OF THE URALS

The Precambrian deposits of the Urals are still insufficiently studied at the present day.

The schemes of their stratigraphic subdivision proposed at one time by N. N. Dinnel’shtedt (30), M. I. Garin (22), and other investigators are in most cases conventional in character, to say nothing of the history of volcanism and tectonics.

The author did not have at his disposal sufficiently complete factual material to draw corresponding conclusions regarding Precambrian geotect—

Note: This article is published for discussion purposes. (Ed.)

tectonic history. Therefore, he dwelt only in general terms on the Precambrian, and devoted his principal attention to the history of the Paleozoic and Meso-Cenozoic.

The Precambrian exposures are spatially confined to the axial part of the Ural Range, the western slope of the Southern Urals, and to the eastern belt of granite-gneiss massifs.

The rocks of the Precambrian are represented by gneisses, granite-gneisses, the most varied schists, marbles, ferruginous quartzites, conglomerates, metamorphosed sandstones, quartzites, marbled limestones and dolomites. Among these rocks there occur, in varying degrees of metamorphism, acidic and basic effusives, as well as gabbro-amphibolites, with which the Kusinsk and Kopansk deposits of titanomagnetites are spatially and genetically connected. Serpentinites also occur. In many cases they have been schistose and transformed into serpentine and talc-chlorite schists. With some qualification, the Berdyush rapakivi may be assigned to the Precambrian on the basis of their parallelization with similar rocks of Jotnian age in Finland, Karelia, and Ukraine.

The unconformities present in the Precambrian between individual series, and the diverse effusive and intrusive formations, testify to a complex and varied pre-Paleozoic geotectonic life of the Urals, still insufficiently studied at the present day.

III. LOWER PALEOZOIC

The first reliable data on a faunistically characterized Cambrian were obtained in 1931 in the Southern Urals by geologists of the group of N. I. Razumovsky, in connection with their discovery of archaeocyathid limestones.

Since then our knowledge of the Cambrian of the Urals has expanded considerably. At present we already possess reliable information on the presence of faunistically characterized Cambrian on Vaigach Island, Pai-Khoi, Novaya Zemlya, and in the Southern Urals—in the Orsk district and in the basins of the Tirlyan and Yurezani rivers. The fauna is represented chiefly by archaeocyathids and algae of the Lower and Middle Cambrian; in the phyllites of the metamorphic sequence of Novaya Zemlya, according to M. M. Ermolaev (32), Middle Cambrian trilobites have also been encountered.

Cambrian deposits are represented by archaeocyathid and algal marbled limestones and dolomites, phyllites, siliceous, siliceous-graphitic, and chloritic schists, as well as quartzites. Their thickness is approximately 4000–5000 meters.

It is now possible to raise the question of the Cambrian age of some part of the metamorphic schists of Timan on the basis of the discovery in them, in 1936 by Vollosevich, of algae from the stromatolite group, which, according to A. G. Vologdin (17), resemble the algae Cm₁ and Cm₂ of the basin of the Aldan River.

In the Urals, ancient granites are associated with the metamorphic sequence; in many cases these have been transformed into granite-gneisses. Indications of them are given for Mugodzhar by A. A. Petrenko (74) and G. I. Vodorezov (13), for Timan and the Polar Urals by S. N. Volkov and N. V. Yatsyuk (15—16), and for Novaya Zemlya by M. M. Ermolaev (33). E. A. Kuznetsov (48) also notes them for the Urals. In the Mugodzhar, as G. I. Vodorezov (13) indicates, these granites form stock-like and laccolith-like bodies cutting the metamorphic rocks. Granitic intrusions also cut the metamorphic sequence of Timan. Aplites are associated with the granites.

and aplite-like granites, as well as quartz–feldspar and quartz veins¹.

The presence of quartz and quartz–feldspar veins is a quite characteristic feature of the ancient metamorphic rocks of the Urals. This has been noted for the Southern Urals by F. N. Shakhov (90) and N. N. Dingelstedt (29), for Mugodzhar by A. A. Petrenko (74) and G. I. Vodorezov (13), and for Timan by S. N. Volkov and N. V. Yatsyuk (16).

On Novaya Zemlya, quartz veins, presumably of pre-Silurian age (A. B.), were recorded in phyllites of faunistically characterized Cambrian. They have also been found in considerable quantity in the Cambrian phyllites of the Ebeity River basin² in the Southern Urals. The Troitsk granite massif, as E. A. Kuznetsov (52) notes, metamorphoses Cambrian phyllites. On Timan, granites with their vein derivatives are the youngest intrusive formations in the metamorphic sequence.

At the same time, pebbles of vein quartz, granites, granite-gneisses, and aplite-gneisses occur in the sandy-conglomeratic strata lying at the base of the Lower Silurian; references to this are found in K. I. Dvortsova, G. I. Vodorezov (14), K. A. Lvov and A. I. Olli (58), N. N. Dingelstedt (28), and E. A. Kuznetsov (52). On Novaya Zemlya, ancient granites are directly overlain by a Silurian coarse-clastic sequence, which may be correlated with the Ashinskaya suite of the Lower Silurian of the Southern Urals.

This factual material compels one to assume the existence in the Urals, alongside Precambrian granites, of younger—Cambrian—granites as well.

The Cambrian and Precambrian deposits constitute a complex and dislocated metamorphic complex in which it is not easy to identify unconformities. A somewhat different picture is seen when comparing the Silurian with the underlying metamorphic rocks.

In many regions of the Urals, Lower Silurian deposits rest upon the underlying strata with a sharp angular unconformity. In particular, A. I. Ivanov (39) established on the western slope of the Southern Urals the occurrence of the Lower Silurian on various Precambrian suites. Transgressive occurrence of Lower Silurian deposits with erosion of the underlying strata was noted for the Southern Urals by N. N. Dingelstedt (27), G. I. Kirichenko (43), and B. I. Borsuk (8); for Mugodzhar by G. I. Vodorezov (14); and for the Polar Urals by S. N. Volkov and N. V. Yatsyuk (15), E. A. Kuznetsov, and K. I. Astashenko (51).

An angular unconformity between the Silurian and the underlying metamorphic rocks was recorded in the Northern Urals by A. A. Chernov (86). In the basin of the Ebeity River in the Southern Urals, the Silurian, with a sharp unconformity, rests upon Cambrian phyllites.

Coarse-clastic sandy-conglomeratic strata are developed in the Lower Silurian. The composition of the conglomerates is varied. They contain pebbles of phyllites, various schists, marbles, amphibolites, pebbles of vein quartz, granites, granite-gneisses, and gneisses—precisely those rocks that enter into the composition of the Ural metamorphic complex. These data are found in G. I. Kirichenko (43), N. N. Dingelstedt (28), K. A. Lvov and A. I. Olli (58), I. A. Ivanov (39), G. I. Vodorezov (14), and other investigators of the Urals.

An interesting fact is the absence of the Upper Cambrian, as indicated—

...are indicated for the Southern Urals by N. N. Dingelstedt (30) and V. N. Pavlinov (69), and for Novaya Zemlya by M. M. Ermolaev (33).

All that has been stated above testifies that in the Urals, after intensive tectogenesis in the Cambrian, a folded region was formed which, at the end of the Cambrian and at the beginning of the Lower Silurian, underwent intensive denudation, exposing the deep horizons of this region. The latter is attested by the pebbles of metamorphic rocks of the Urals in the conglomerates of the Lower Silurian, which transgressively overlap the rocks of the metamorphic sequence.

Thus, the Silurian deposits lie on the eroded surface of the dislocated Cambrian and Precambrian. Hence it is quite natural that there is also a differing degree of dislocation and metamorphism—though, to be sure, not everywhere—of the pre-Silurian metamorphic rocks and the Silurian-Devonian deposits. The latter was noted in its time by N. K. Vysotsky (21), A. A. Chernov (86), and N. N. Dingelstedt (27).

The absence of $Cm_3$ first allowed N. N. Dingelstedt (30) to raise the question of the manifestation in the Southern Urals of Salair folding. The possibility of its existence is admitted by E. A. Kuznetsov (52), V. N. Pavlinov (69), S. N. Volkov and N. V. Yatsuk (15), as well as D. G. Ozhiganov (65). E. A. Kuznetsov, in this connection, tentatively associates the intrusion of granites with the Salair folding.

However, despite the factual material and individual statements, the manifestation of Salair folding in the Urals is in most cases underestimated.

Thus, the transgressive occurrence of Lower Silurian deposits with angular unconformity upon an already metamorphosed Cambrian and Precambrian sequence; the deep erosion of the metamorphic basement, with the finding of pebbles of metamorphosed rocks in the deposits of the Lower Silurian; the absence from the stratigraphic section of the Upper Cambrian; the differing degree of metamorphism of the Cambrian and Precambrian in comparison with the Silurian-Devonian deposits that cover them; the association of granite intrusions, probably, with the end of the Cambrian and the beginning of the Lower Silurian, together with their abundant vein derivatives—such is the factual material that compels one to speak more definitely of the manifestation of Salair diastrophism in the Urals. As was clear from the foregoing, these data are not isolated; they have been established by many investigators for various regions of the Urals.

Given such factual material, it will hardly be correct to suppose only that there are merely traces of the manifestation of Salair folding. At the very least, the available data compel one to speak of the great role of the Salair cycle in the developmental history of the Urals. It is probably to it that the closure of the Cambrian geosyncline and the formation in its place of the Salair folded belt are due; this belt extended from the northern extremities of Novaya Zemlya to the Southern Mugodzhary; Timan was its northwestern branch. At the present day, deep horizons of this belt have been exposed by denudation, represented by a complex of metamorphic rocks¹. Salair folding masked the unconformities within the geosynclinal sequence. Hence the difficulty of subdividing it and distinguishing the Cambrian. It also caused the disappearance of Upper Cambrian deposits in various regions of the Urals. The possibility is not excluded that this phenomenon is connected with the establishment of a continental regime in the Upper Cambrian. On Timan the latter, apparently, persisted also during the Lower Silurian, which led to the omission of this division from the stratigraphic column. As a result of the Salair folding, meridional structures were formed—

structures of the Urals, established as early as the Precambrian. They were maintained subsequently during the manifestation of later cycles of tectogenesis.

IV. MIDDLE PALEOZOIC

At the present time, as noted above, the information on the Precambrian and Cambrian is insufficient and fragmentary. The situation with the Silurian is considerably better. Numerous factual data and the diverse fauna that has been preserved have made it possible to carry out its stratigraphic subdivision and to reconstruct an idea of the history of the Urals in this period.

Silurian deposits are considerably developed on both slopes of the Urals. Among them we encounter both sedimentary and effusive formations. In the Lower Silurian, coarse clastic conglomerate–sandstone deposits are widespread, with subordinate shales, limestones, and dolomites, making up a sequence from 700 to 1200 meters thick. This sequence on the western slope of the Southern Urals has been distinguished as the independent Asha suite. In the sedimentary formations of the Upper Silurian, clastic deposits play a subordinate role; limestones and siliceous and clayey shales predominate.

In the Upper Silurian, and partly also in the Lower Silurian, the Urals served as an arena of enormous outpourings of basic lavas. This is evidenced by thick, kilometer-scale effusive–tuffaceous sequences. They consist mainly of diabases, spilites, porphyrites, variolites, mandelsteins, and their tuffs. Basic varieties of effusives predominate; acid varieties occur more rarely. In most cases the effusives are represented by an extrusive facies. Intrusive bodies of gabbro and gabbro-diabase also occur.

The presence in the Silurian of intrusive bodies of the above-mentioned rocks permits the supposition that the cycle of basic effusions was completed at the end of the Silurian by intrusions of the gabbro series. The possibility is not excluded of a connection with the Silurian intrusive cycle of diorites and plagiogranites. The granodiorites of the Kargaly Mountains may be assigned to these.

The Silurian deposits of the western and eastern slopes of the Urals differ from one another. Whereas on the western slope the Silurian is composed in its main mass of sedimentary deposits with subordinate effusives, on the eastern slope the dominant role is played by effusives, and the subordinate role by sedimentary rocks. In addition, the thickness of the Silurian deposits of the eastern and western slopes is different. In the west it reaches 2000–3000 meters, while in the east it reaches 5000–6000 meters. Exceptions to this are the Orsk Urals and, in part, the northern Mugodzhary, where the geological structure of both slopes is close to one another.

These facts indicate that in the Silurian these slopes experienced a somewhat different history.

The presence of clastic deposits at the base of the Lower Silurian compels one to think that they were formed at the expense of the denudation of the Salair folded belt.

The thick sedimentary–effusive sequences of the Silurian indicate that in this period the Urals were drawn into a new geosynclinal cycle. At the same time, the western peripheral parts of the Salair folded belt, being the least consolidated, gradually subsided and favored the accumulation of enormous sequences of sedimentary formations with subordinate, rare effusives. The regions of the eastern slope of the Urals also underwent geosynclinal regeneration. But on this slope it occurred more intensively and was accompanied by powerful outpourings of Silurian effusives.

The presence of enormous sequences of effusive formations compels the supposition that the geosynclinal regeneration of the eastern slope was accompanied by fault fissures of the basement, with the formation of a system of graben—

... which in the Silurian and in subsequent periods served as the receptacle for effusive-sedimentary strata. The complex tectonics, the island-like character of the occurrence of the rocks, and the preserved separate areas of ancient metamorphic strata, established on the eastern slope, testify to the probable step-like type of these grabens.

An important event that occurred against the background of the geosynclinal downwarping of the Salair folded belt was Caledonian folding. The author does not examine the manifestation of all phases of Caledonian tectogenesis, including the Taconic phase, for which some data exist, but dwells mainly on the manifestation of the Erian phase. This is due to the fact that the question of this folding is one of the disputed questions in the geology of the Urals.

Let us examine the factual material concerning this folding. In 1901 E. S. Fedorov (83), while studying the Bogoslovsk mining district, established a break in the Upper Silurian at the boundary with the Lower Devonian. He also proposed a connection between the Lower Devonian transgression and the beginning of meridional folding, which led to the formation of the Ural Range. An angular unconformity was noted in the Southern Urals by A. V. Khabakov (84) and G. I. Vodorezov (14). The former, in the Kos-Istek district, identified the discordant occurrence of Lower Devonian conglomerates upon Upper Silurian shales. The conglomerates contained pebbles of shales with a graptolite fauna of the Upper Silurian. A. V. Khabakov there, near the village of Usrgan, established the presence of nappe effusives of $D_{1-2}$, occurring at two different horizons of the underlying strata (on spilites and siliceous shales of the Silurian–Devonian).

An angular unconformity between the Silurian and Devonian was described by V. N. Pavlinov (70) in the region of the Belaya River basin. In the Sakmara River basin, the Lower Devonian jasper-conglomerate sequence lies upon the eroded surface of Silurian deposits, covering the faunistically characterized Lower and Upper Silurian. Traces of erosion of $S_2$ at the boundary with $D_1$ have also been established in the Poltavo-Bredinsky district. In Timan, Devonian clays and sandstones lie transgressively upon Silurian limestones. Caledonian folding led to the establishment of a continental regime in certain regions of the Urals, in connection with which the Lower Devonian dropped out of the stratigraphic section in some parts of the western slope of the Southern Urals, in Pay-Khoy and on Novaya Zemlya. In the first of these, as K. A. Lvov and A. I. Olli (58) indicate, the Upper Silurian is also partly eroded. This continental regime also explains the appearance of a weathering crust at the boundary of $S_2$ and $D_1$ (the bauxite deposits of Krasnaya Shapochka and Bogoslovsk). D. V. Nalivkin (61) also admits Caledonian folding for the western slope of the Southern Urals, explaining by it the great dislocation of Silurian deposits in comparison with Devonian ones. The manifestation of Caledonian folding for Novaya Zemlya, recognized by M. M. Ermolaev (33) and other geologists, is beyond doubt.

Forty years have already passed since E. S. Fedorov (83) presented facts and spoke in favor of the manifestation of folding at the boundary of $S_2$ and $D_1$. By the present time, as was clear from the foregoing, considerable factual material has already accumulated confirming Caledonian folding. But for some reason the question of it still remains controversial. At best, the possibility of its manifestation is admitted for the western slope of the Southern Urals and for Novaya Zemlya only. At worst, the facts testifying to this folding are regarded as isolated, as having no decisive significance, and as indicating local uplifts. In his time N. K. Vysotsky (21) also denied Caledonian folding. It would not be entirely correct to regard these facts as little convincing, especially since angular unconformity in the presence of folding could have manifested itself not in all regions of the Urals.

Thus, the presence of an angular unconformity between the Upper Silurian and the Lower Devonian; the occurrence of the Lower Devonian at different levels of the underlying strata; the erosion of the Upper Silurian and the absence of the Lower Devonian in some regions of the Urals; the difference in the degree of dislocation of Silurian and Devonian rocks; and the confinement of intrusions of the gabbro series to the end of the Silurian—these are the facts that testify to the manifestation of Caledonian folding throughout the entire Ural geotectonic region.

After the Caledonian folding, in the Lower Devonian, some individual parts of the Urals experienced a continental regime, while the majority experienced a marine one. In the latter, sandy and calcareous sediments continued to accumulate. On the eastern slope and, in part, on the western slope, at this time there were powerful outpourings of effusives, the formation of jasper, limestones, and other sedimentary formations. The downwarp continued. Among the effusives, basic varieties of andesite-basalt magma predominated, with rarer dacitic and liparitic derivatives. Individual varieties of rocks were connected with one another by a whole series of gradual transitions, which is a characteristic feature of effusives not only of the Lower Devonian, but also of the Upper Silurian. The appearance of acidic varieties is confined mainly to the end of the Lower Devonian; they occur in the Middle and Upper Devonian, and also in the Silurian. Quantitatively, the acidic rocks are clearly subordinate to the basic ones.

During the Middle Devonian, limestones and, more rarely, sandstones accumulated on the western slope of the Urals. Individual parts of it experienced a continental regime. On the eastern slope, the Middle Devonian epoch was characterized by a downwarp with weakened effusive activity. Calcareous and siliceous sediments were deposited.

In some areas of the Urals the Middle Devonian has not been established. This fact has been noted for individual districts of Mugodzhar and of the western slope of the Southern Urals. The Eifelian stage is also absent in the Middle Devonian of Novaya Zemlya. In many places the Middle Devonian is partly eroded. Indications of this are given by E. Ya. Pern (76), E. Razumovskaya (77), and E. A. Kuznetsov (52).

Usually, at the base of the overlying Upper Devonian deposits lie basal sandy-conglomeratic formations, which have been noted for various districts of Mugodzhar, the Southern Urals, Timan, and Novaya Zemlya. In the Southern Urals, the clastic deposits of the Upper Devonian, with subordinate shales and limestones, were distinguished by L. S. Librovich (56) as an independent Zilair formation. V. N. Pavlinov (70), on the western slope of the Southern Urals, established the occurrence of Upper Devonian graywackes upon rocks of different ages. G. I. Vodorezov (12), in the Berchogur district of Mugodzhar, revealed an angular unconformity between Lower and Upper Devonian deposits.

In the clastic deposits of the Upper Devonian there occur pebbles of jaspers, diabases, and spilites \(D_{1-2}\); more rarely there are pebbles of mica schists, phyllites, siliceous schists, gneisses, gabbro-diabases, and other rocks. The composition of the pebbles indicates that the Upper Devonian sea eroded not only the deposits of \(D_{1-2}\), but also an ancient metamorphic sequence that had already been formed long before this. At the same time, F. N. Shakhov (90) notes that Lower Devonian rocks entered the conglomerate already partly metamorphosed. These facts compel one to suppose the possibility of tectonic movements in the Middle Devonian.

For the first time, M. Yanishevsky (91), in 1905, after visiting the Mugodzhar and Guberlinsky Mountains, put forward the suggestion that “mountain-forming movements began in Devonian time. This is evidenced by the Devonian conglomerates” (p. 16). There are indications of tectonic movements at the boundary between the Middle and Upper Devonian.

indications in F. N. Shakhov (90), N. N. Vysotskii (21), A. L. Yanshin, V. S. Koptev-Dvornikov (44), V. N. Pavlinov (70), A. D. Arkhangelskii (4), and other investigators.

It should be noted that the manifestation of these tectonic movements, established mainly in the Southern Urals, was not extended by the authors to the whole of the Urals; moreover, in many cases the character of these movements remained unclear and their existence hypothetical. The latter may be illustrated by the statement of L. S. Librovich (57), who, while recognizing tectonic movements in the Southern Urals at the boundary between the Middle and Upper Devonian, denied their orogenic character. He writes: “However, the variegation of facies in the Upper Devonian deposits and the presence in them of conglomerate beds and interlayers nevertheless show that, even if there were no folded orogenic processes here at that time, fairly significant tectonic movements nevertheless took place”¹) (p. 84). And what these tectonic movements represented remains unclear.

In the Urals, ultrabasic rocks are very widespread. Many of their massifs are complexly differentiated plutons, in whose structure dunites, peridotites, pyroxenites, olivine gabbro, gabbro²), and gabbro-diabases take part. Dioritic and granodioritic varieties are genetically associated with them (the Tagil massif, etc.).

To this day the view that the ultrabasic rocks are of Carboniferous age remains widespread. It was first formulated in the works of N. K. Vysotskii and was reflected in B. P. Krotov (45), A. N. Zavaritskii (35), and other investigators of the Urals. But in recent years other statements have appeared concerning the stratigraphic position of the Ural hyperbasites. A theoretical supposition concerning the intrusion of rocks of the gabbro-peridotite family in the Lower Devonian is found in B. M. Romanov (78). Indications of a possible pre-Carboniferous age of the ultrabasic rocks are present in V. S. Koptev-Dvornikov (44), E. Razumovskaia (77), and, for the Middle Devonian, in V. N. Pavlinov (70). S. N. Volkov and N. V. Yatsuk (15) consider part of the peridotites of the Polar Urals to be Upper Devonian. N. G. Kassin and G. I. Vodorezov assign the intrusion of the hyperbasites of Mugodzhar to the time of the Caledonian folding, to which they attach great significance in the history of the Southern Urals. There is an indication by E. A. Kuznetsov (52) of a probable intrusion of peridotite masses of the Southern Urals at the boundary between the Middle and Upper Devonian.

As is evident from the foregoing, at the present time the Carboniferous age of the Ural hyperbasites has been called into question, and there is no generally accepted point of view on this question.

There is also no doubt that the majority of the Ural hyperbasites cut through the complex of rocks of the Upper Silurian, Lower and partly Middle Devonian. Metamorphism of Lower Devonian limestones in contact with hyperbasites was noted by L. Duparc and G. Sigg (31). B. P. Krotov (45), studying the Miass dacha region, described epidotes formed as a result of the contact action of hyperbasites on faunistically characterized Lower Devonian limestones. In the Kos-Istek district of the Southern Urals, G. I. Vodorezov (14) observed a breakthrough of faunistically characterized Silurian by hyperbasites. They also cut through faunistically

¹) On the geological map of the Urals (24), one of whose editors is the same L. S. Librovich, to the south of the Kraka peridotite massifs angular unconformities between \(D_1\) and \(D_3\) are clearly expressed. It is unlikely that they could have resulted from non-orogenic movements.

²) Thus, for the Urals three gabbros of different ages are outlined: one Precambrian, the second Caledonian, and the third younger, genetically associated with the hyperbasites.

a characterized Lower Devonian, which rests unconformably upon the Upper Silurian. In the Northern Mugodzhary, K. I. Dvortsova (26) established the contact effect of peridotites upon limestones of the Lower Devonian containing fauna. In the Southern Urals, according to the observations of V. N. Pavlinov (70), ultrabasic rocks cut through the faunistically characterized Upper Silurian and the sedimentary siliceous-jasper sequence of the Lower and Middle Devonian. Thus the lower boundary of the intrusion of the hyperbasites is rather clearly indicated.

For the Urals, no cutting by hyperbasites of faunistically characterized Upper Devonian has been established. On the contrary, there are facts of finds of pebbles and sand grains of serpentinites in deposits of the Lower Carboniferous and Upper Devonian, as well as data on the latter overlying ultrabasic rocks.

In 1913 N. K. Vysotsky (66) found pebbles of serpentinites in conglomerates of the Lower Carboniferous. E. A. Kuznetsov (48) notes that “among the graywacke sandstones of the Upper Devonian of the Southern Urals there occur grains of gabbro rocks, plagiogranites, and also serpentinites. The same has been established in the conglomerates of the Lower Carboniferous of the greenstone belt of the Miass district” (pp. 17–18). According to D. G. Ozhiganov (65), in the clastic deposits of the Zilair Formation of the Upper Devonian of the basin of the Abshak River in the Southern Urals, fragments of serpentinites and chromites have been encountered. An observation by V. S. Koltev-Dvornikov (44) is of interest. He writes: “On the surface of serpentinites and effusive rocks, sometimes with conglomerates at the base, there rests unconformably a sequence of siliceous shales. Still higher are limestones of Visean age” (p. 113). For the southeastern part of the Miass district, G. A. Mirlin (60) describes the transgressive overlap of a shale sequence of the Upper Devonian—Lower Carboniferous, with conglomerates at the base, upon effusives and serpentinites.

The cutting by hyperbasites of the Lower and partly Middle Devonian, and the occurrence of their clastic material in deposits of the Upper Devonian and Lower Carboniferous, compel one to regard the viewpoint of a Middle Devonian age for a considerable part of the ultrabasic rocks of the Urals as the most probable. The very rare manifestations of contact effects of ultrabasic rocks upon the enclosing sequences, the uncertainty of the age of many sedimentary suites cut by the hyperbasites, and the complex tectonics, quite often observed in the contact zones of ultrabasic rocks with the enclosing rocks, introduce confusion into the determination of their stratigraphic position.

The disappearance of the Middle Devonian from the stratigraphic section in individual regions of the Urals, the transgressive occurrence of Upper Devonian deposits with angular unconformity upon rocks of different ages—reflected also on the geological map—and the considerable spread of clastic deposits at the base of the Upper Devonian indicate that during the Middle Devonian epoch quite definite tectonic movements took place throughout the Urals, accompanied by the intrusion of the ultrabasic magmatic complex. These Middle Devonian tectonic movements, noted for Western Siberia by Academician M. A. Usov (81), were distinguished by the latter as a special Telbes cycle of tectogenesis. The factual material presented here compels recognition of the manifestation of this cycle in the Urals as well. The association with it of hyperbasites, to which deposits of platinum, chromites, asbestos, nickel, and other useful minerals are confined, gives this cycle great significance.

As was already said above, clastic deposits play a significant role in the composition of the Upper Devonian deposits. In the Southern Urals, graywacke deposits of the Upper Devonian are widespread on both its slopes in areas directly adjoining the metamorphic rocks—

of the central part of the Urals. Among the Upper Devonian deposits of the western slope there occur limestones and bituminous deposits of the Domanik horizon, while on the eastern slope there are effusives, their tuffs, and limestones. The great thickness of the Upper Devonian deposits—about 1800 meters—testifies to the continuing downwarping of the western and eastern slopes of the Urals in the Upper Devonian epoch.

The total thickness of the Devonian deposits is approximately 6000 meters. We do not possess data that would testify to intensive folding processes associated with the Caledonian and Telbes cycles of tectogenesis, but nevertheless they were clearly manifested. This suggests that their role was reduced to the formation of relatively gentle folded structures in the effusive-sedimentary strata of the Silurian and Devonian, which became attached to the metamorphic complex of rocks of the Salair folded belt.

In these cycles the ancient Salair basement was subjected to block faulting, with corresponding uplifts and subsidences of its individual blocks. In connection with block faults, meridionally elongated deep tectonically weakened zones were formed, to which the emplacement of basic and ultrabasic intrusive complexes was confined. Along fractures, mass outpourings of basic effusive rocks took place.

Thus, we see that the geosynclinal trough of the Urals in the Silurian-Devonian epoch was accompanied by Caledonian and Telbes folding. They had a subordinate significance in the history of the formation of Ural structures, but played an important role by complicating the geosynclinal phase of the development of the Urals.

V. UPPER PALEOZOIC

The Lower and Middle Carboniferous were characterized, for both slopes of the present-day Urals, by downwarping and the accumulation of sedimentary calcareous and sandy-clayey deposits, coal-bearing in the Lower Carboniferous. Acidic and basic effusives occupied a subordinate position in the Lower and Middle Carboniferous.

The early phases of the Variscan cycle of tectogenesis marked the beginning of the folding processes of the Upper Paleozoic. They were responsible for the emergence of individual regions of the Urals from below sea level. By the end of the Carboniferous, folding of the eastern slope had been completed, accompanied by the emplacement of granite intrusions. At the same time, most regions of the central part of the Urals and of the eastern slope were finally raised above sea level, forming a high-mountain country. The regions of the western slope continued at this time to subside and to accumulate clastic sediments, which arrived from the eroded eastern slope.

The paleogeography of the Lower Permian is analogous to that of the Upper Carboniferous: continued mountain building and erosion of the young mountain country in the east, subsidence and accumulation of clastic deposits in the west. At the end of the Lower Permian and the beginning of the Upper Permian, folding of the geosynclinal sequence of the western slope was completed. At the same time the eastern and central parts of the Urals were subjected to faults, block movements with uplifts, and then also to denudation, which caused the accumulation of coarse-clastic deposits of the Buzuluk horizon of the Tatarian stage at the end of the Permian.

The thickness of the Carboniferous deposits is approximately 6000 meters, that of the Permian deposits—4000 meters.

The Variscan cycle of tectogenesis was of great importance in the history of the formation of the structures of the Urals. It was responsible for the closure of the Ural Paleozoic geosyncline, accompanied by intensive folding, the restructuring of the ancient Salair structures, and the complication of the Caledonian-

Telbess. Associated with it was the formation of block faults and thrust dislocations, which imparted a scaly and scaly-block structure to many regions of the Urals.

The completion of folding on the eastern slope of the Urals was accompanied by the intrusion of granite massifs with a distinctive formation of nepheline syenites.

In the effusive formations of the Carboniferous, the extreme members of andesite-basalt and liparite-dacite magmas are encountered. A predominance of acidic rocks and the absence of transitional varieties are characteristic. These features of effusive volcanism distinguish the Lower Carboniferous and partly the Upper Devonian from the Silurian and Lower Devonian.

The thick sedimentary sequences of Carboniferous and Permian deposits testify to the geosynclinal conditions that still existed in these epochs in various regions of the Urals. The early phases of Variscan folding marked the beginning of the cycle that closed the Paleozoic geosyncline. In this they differed qualitatively from the phases of the preceding two cycles and determined the above-noted peculiarity of Carboniferous effusive volcanism.

VI. MESO-CENOZOIC

During the Permian and partly the Triassic periods, the Urals were peneplanized. On the peneplain, under the conditions of the subtropical climate of the Triassic and possibly the Lower Jurassic, a weathering crust was formed. During the Jurassic and Cretaceous, the greater part of the present-day Urals experienced a continental regime. Separate episodes of marine transgression were observed, involving the southern and partly the northern regions of the Urals.

In the Mesozoic era, individual parts of it underwent differential uplifts and subsidences. As a result, ascending movements became evident for the central parts of the Urals and especially for the western slope. On the eastern slope, descending movements were partly manifested, which caused the gradual downwarping of some Paleozoic troughs. In them accumulated continental sedimentary sequences ranging from hundreds to a thousand meters in thickness, as, for example, in the Baimak basin, in the region of the Chelyabinsk coal basin, and elsewhere. In places, fault dislocations appeared with the outpouring of basic lavas. In the young Cimmerian phase of the Pacific cycle, Jurassic deposits were dislocated and Paleozoic structures were accentuated by block movements.

We have no data that would indicate that the Pacific cycle of tectogenesis caused the formation of scaly and overthrust structures. Moreover, this would hardly have been possible because of the strong consolidation of the folded belt in Variscan time.

The Alpine cycle of tectogenesis was an epoch of block uplifts of the western slope of the Urals and its central areas. Apparently, the manifestation of the young Mio-Pliocene and Riss-Würm (Baku) phases of this cycle was of greater significance here. Associated with them was the formation of the latest fault dislocations, in which Eocene and Quaternary deposits participated, and the orographic shaping of the western slope of the Urals as a modern mountainous country. Uplifts in the Alpine cycle occurred chiefly in the northern and southern regions and, to a lesser degree, in the Middle Urals. The possibility of hydrothermal activity in individual regions of the Urals is not excluded. It may explain the formation of epidotized cemented sandstones in the Eocene sands and quartz veins in the Jurassic clays of the Mugodzhar, as well as the appearance of the warm springs of Pym-va-shor in the region of the Chernyshev Ridge.

The covering of the continental weathering crust by sedimentary deposits of the Jurassic, Cretaceous, and Tertiary periods on the eastern slope

of the Urals and the insignificant role in them of deeply clastic rocks indicates that, by the beginning of the Jurassic and in subsequent periods, this slope represented a peneplain. This fact compels us to regard the area of the eastern slope of the Urals not as a Tertiary abrasion plateau, but as a pre-Jurassic peneplain buried beneath Meso-Cenozoic sediments¹.

We see that in the Cenozoic, as compared with the Paleozoic, the center of gravity of the intensive manifestation of tectonic movements in the Urals shifted from the eastern slope to the less consolidated western one. Hence the orographic asymmetry and heteromorphism of the modern Ural Range, most clearly expressed in the Southern Urals.

At the present time most regions of the Urals, especially its southern part, are undergoing uplift, as we judge from the rejuvenation of the erosional cycle of the river systems.

VII. STRUCTURAL FEATURES OF THE URALS

At the present day the Urals constitute a complex folded belt with intricate structural elements of different ages. Of these, the most ancient are the metamorphic series of the Cambrian and Precambrian in the central and western parts of the Urals, and separate areas of the belt of granite-gneiss massifs of the eastern slope. They may be regarded as preserved horst-like projections of the ancient Salairian basement, which at present in many places have tectonic contacts with the rocks adjoining them. On the western and, especially, the eastern slopes, where sedimentary-effusive sequences of the Middle and Upper Paleozoic are now situated, it underwent deep subsidence. Here, in many places, the post-Cambrian rocks are so strongly dislocated and metamorphosed that they are at times difficult to distinguish from the rocks of the metamorphic series of the Salairian basement, especially where they are metamorphosed by granitic intrusions. Hence also the difficulties in determining the age of the metamorphic rocks of the Urals. It may be thought that individual regions in which the Salairian basement now comes to the surface experienced, in post-Cambrian time, episodes of comparatively slight subsidence with the deposition of thin cover sediments. The latter have been preserved to the present only in isolated cases and are represented by weakly metamorphosed, almost horizontally lying sediments. Such may include the Silurian sandstones described by N. N. Dingelstedt (27) in the Tirlyan district of the Southern Urals. He states that the faunistically characterized sandstones of the Lower Silurian, “whose total thickness probably reaches 100–200 meters, are thrown into gentle wave-like folds and usually lie very flat (the dip sometimes does not exceed 10–20° to the E) upon steeply dipping (with predominance to the E) and strongly dislocated rocks, close in character to the rocks of the quartzite-schist series (the series of the Salairian basement—A. B.) adjoining the Tirlyan limestones from the east” (p. 120). Further he writes that “a characteristic feature is also the complete absence, in the Silurian quartz sandstones, of signs of metamorphism” (p. 122).

The western slope of the modern Urals is basically an area of Caledonian-Telbes and Variscan folded structures, complicated by thrusts which in many places have a plicative character; imbricate structures are widespread. Imbricate-block structures and faults are of subordinate importance.

As was said earlier, the region of the eastern slope of the Urals may be regarded as a system of grabens with preserved island-like areas of the Salair basement. Characteristic of this region is the presence of folded, scaly, and block structures, with developed fault dislocations and with an extremely varied and tangled tectonics.

The views of those who support the existence of nappe structures in the Urals find no confirmation.

The Middle Urals, and especially Timan, attract attention because they possess certain distinctive features that distinguish them from other regions of the Urals.

1. The Middle Urals. In discussing the Paleozoic cycles of tectogenesis, facts were not cited that would testify to their manifestation in the regions of the Middle Urals adjoining the Ufa horst. Usually geologists who deny the manifestation of pre-Variscan cycles refer to the Middle Urals, where there supposedly exists complete concordance in the occurrence of pre-Carboniferous deposits. There are also other peculiarities. Thus, for its eastern slopes the manifestation of intense tectonics and the presence of alkaline magmatic rocks are characteristic (nepheline syenites of the Vishnevye and Ilmen Mountains). In addition, the region of the Middle Urals underwent, during the Quaternary period, a far smaller uplift than the regions situated to the south and north of it.

At present, the complex tectonics of the eastern parts of the Middle Urals is explained by the influence of the Ufa subterranean block. In this connection, A. N. Zavaritskii (35) makes dependent upon this block also the appearance of alkaline rocks. The thought naturally arises of a possible connection between the difficulties in detecting angular unconformities in the Paleozoic deposits of the Middle Urals and the influence of the Ufa block.

By way of posing the question, one may advance the hypothesis that the Ufa Precambrian block exerted, in the middle parts of the Urals, a braking effect on the processes that took place in the Ural geosynclinal-orogenic zone and thereby caused a lesser mobility of this part of the zone. As a result of this action, after the corresponding foldings, the regions of the Middle Urals emerged from beneath sea level only partially, which in turn led to the absence of a noticeable angular unconformity. If such an unconformity did exist, then, in all probability, it was small in magnitude and could have escaped the notice of investigators. The latter was further aided by disjunctive tectonics, which complicated the normal relationships of the series. Such a case is highly probable for the Caledonian and Telbes foldings, which were not distinguished by particular intensity and played a subordinate role in the formation of the structures of the Urals. And since most geologists consider angular unconformity, clearly observed in all regions of a geotectonic area, to be the sole criterion of tectogenesis, the dispute being conducted concerning the manifestation in the Urals of the generally recognized Caledonian folding—let alone the Telbes and Salair foldings—is quite understandable.

The influence of the Ufa horst may explain the less intense dislocations of the Permian deposits of the western slope of the Middle Urals, as compared with the northern slope, and the smaller uplift of the Middle Ural regions, relative to others, during the Quaternary period.

Thus, the northern and southern parts of the Urals throughout its geological history were apparently the most mobile segments of the Ural geosynclinal-orogenic zone. Therefore, in them the epochs of tectogenesis manifested themselves more clearly and in the corresponding periods.

The curvature of the middle part of the Urals and the westward overturning of the folds of the sedimentary strata of the western slope are usually explained by the pressure of the Siber—

…of the platform and the presence of the Ufa massif, which served as a buttress that opposed the movement of masses from the east.

Apparently, when the Ural geosyncline was laid down, it did not have a consistent meridional extent; bends existed in its far northern part—the Polar Urals and Novaya Zemlya—and in the middle part, which were accentuated later during subsequent tectonic movements. The intense tectonics of the eastern slope of the Middle Urals and the westward overturning of the folds of the sedimentary sequences of the western slope can be explained not only by pressure on the Urals from Siberia alone. A similar effect could also have resulted from the combined pressure of the Siberian and East European platforms upon the Ural geosynclinal-orogenic zone. In this process, the Ufa massif apparently played not only the role of a buttress, but also that of a wedge. The overturned folding of the western slope could have formed as a result of the reflection of folds from the Ural Salair massif, with corresponding upthrow and thrust dislocations.

Tectonic pressure from the west may explain the intense tectonics on the western margins of Timan, where Paleozoic rocks, being in tectonic contact with a horst-like projection of a block of metamorphic rocks, are intensely dislocated. This effect would have been expected on its eastern margins if the movement of masses had manifested itself only from the east.

2. Timan. At present there is no generally accepted view regarding the formation of Timan. On this question there are two most widespread points of view. The first of them belongs to Academician A. D. Arkhangelsky (3–4), who regards Timan as one of the tectonic structures of the East European plate, of the type of the Don–Medveditsa anticlinal zone and the Oka–Tsna swell. According to the second, the Timan Ridge is a folded structure having nothing in common with the platform. In this connection M. M. Tetyaev (80) considers it a component element of the Ural Hercynides, while D. V. Nalivkin considers it a continuation of the Grampian geosyncline. According to the views of the latter author, the main role in the formation of the Timan structure was played by Caledonian folding.

On Timan there is a pre–Upper Silurian metamorphic sequence, with which ancient gabbro-diabases are spatially associated, as well as granites with abundant aplite and quartz veins. This metamorphic sequence is unconformably overlain by cover Upper Silurian and Devonian deposits. There are sedimentary formations of the Upper Paleozoic and Meso-Cenozoic.

The structural separateness of the Timan Ridge from the East European platform and the commonality of many features of its geological history with the Urals compel one to regard Timan as a component part of the Ural geotectonic region.

A different impression is obtained when other facts are considered. Whereas the Middle and Upper Paleozoic deposits in the Urals constitute a geosynclinal sequence many kilometers thick, on Timan they have a thickness measured in hundreds of meters. These cover Paleozoic deposits are characterized by brachyanticlinal tectonics and a complete absence of intrusive formations. These features bring Timan closer to the platform.

Thus, Timan combines within itself both elements of a platform and elements of a folded belt. Therefore, the formal-logical posing of the question with respect to it is hardly correct: “Either an element of the platform, or a folded belt.”

The metamorphic sequence of Timan can be parallelized with the deposits…

...of the Cambrian and parts of the Precambrian of the Urals, in connection with the discovery in it by Vollosovich of algae resembling Lower and Middle Cambrian forms. The latter compels one to think that the complex of metamorphic rocks of the Timan Ridge was formed in the Cambrian at the site of the NW branch of the Ural geosyncline, which closed in the Salair cycle. Its closure was accompanied, as in the Urals, by the intrusion of granite intrusions with aplite and quartz veins. After the Salair folding, Timan experienced an epoch of continental regime not only throughout \(Cm_3\), but also \(S_1\), in connection with which the Lower Silurian deposits also dropped out of the stratigraphic section.

At the end of the Cambrian, Timan, being a consolidated folded belt, came under the influence of the East European Platform and subsequently did not undergo geosynclinal regeneration. At the same time, however, it did not finally sever its connection with the Urals. This is evidenced by their many common features, both in the composition of the rocks of the Middle and Upper Paleozoic and in the manifestations of the cycles of tectogenesis that took place in the Urals. With the latter were associated on Timan effusive activity in the Devonian, the appearance of angular unconformities, and the formation of brachyanticlinal structures in the cover deposits.

The Timan Ridge today is a horst of the Salair folded belt, overlain by cover Paleozoic and Meso-Cenozoic sediments. In its historical development it constituted a distinctive node of intersection of Ural and platform structures. Timan may be regarded as a distinctive orogen-platform structure, an intermediate link between the East European Platform and the Ural folded belt.

VIII. THE URALS AS A SINGLE GEOTECTONIC WHOLE

Until now, among researchers of the Urals there is still no unanimous opinion regarding the manifestation of Paleozoic cycles of tectogenesis for the entire Ural geotectonic region. At the present time people still speak of the manifestation of the corresponding tectogenesis in one region of the Urals and its absence in another.

Many geologists believe that Caledonian folding is the domain of the Southern Urals and Novaya Zemlya, and that it did not exist at all in the Middle Urals, judging by the absence of angular unconformity. Prof. M. M. Tetyaev (80), for example, wishing to reconcile opponents and supporters of the manifestation of Caledonian folding, states that in their dispute “both sides are right, each in its own region” (p. 234). Further he writes that the existence of disagreements on the question of Caledonian folding is “a reflection of a contradiction existing in nature itself” (p. 234). Of course, there is a share of truth in this last statement. But it is important not only to note the existence of contradictions in one phenomenon or another, but also to show which of its contradictory aspects is the leading one, determining the unity of opposites. That is, in the present example it would have been necessary to give a clear answer to the question posed: “Did the Caledonian cycle manifest itself throughout the entire Ural geotectonic region, or not?” Prof. M. M. Tetyaev did not do this, but confined himself to an evasive answer—both yes and no—which brings no clarity to the problem of the Caledonian cycle for the whole of the Urals.

As regards the Salair folding, people speak of its probable manifestation somewhere to the west of the Ural folded belt, in the adjoining parts of the platform, in the regions of Pai-Khoi and the Tirlyan plant.

The Middle Devonian tectonic movements of the Telbes cycle are either ignored altogether or underestimated. At best they are considered...

are regarded as a manifestation of a phase of the Caledonian cycle, and only within the limits of the Southern Urals. There is no doubt about the Variscan cycle, which is recognized by all.

The tendency toward an artificial dismemberment of the Ural folded zone into separate districts, allegedly characterized by their own pre–Upper Paleozoic history, unlike the history of other districts, is revealed rather distinctly among some geologists, including Prof. M. M. Tetyaev (80, p. 234). This fact may be regarded as an attempt, in the process of analysis, to tear parts away from the whole, with a subsequent hypertrophy of their peculiarities and their opposition to one another. As a result of an underestimation of synthesis, the special features of the whole as a whole—in the present case, of the pre–Upper Paleozoic history of the Urals—have been blurred. Instead, we are sometimes presented with contradictory histories of separate districts, where each part of the Urals exists by itself and without connection with another.

The meridional persistence of the magmatic and, in particular, intrusive rocks of the Urals, as well as of the sedimentary rocks, beginning from the far north and ending with Mugodzhary, and the manifestation of the above-mentioned cycles in the various districts of the Urals, testify to the common character of the developmental history of all parts of the Ural geotectonic region. If there are certain peculiarities in one or another district, they may have arisen owing to the influence of the Ufa underground block, the varying degree of consolidation of individual parts of the Urals, and other causes, which require study in each separate case. One may speak only of a somewhat different form of manifestation of these cycles in individual districts of the Urals, but not of their absence.

On the basis of all the factual material set forth above, we are convinced that the cycles of tectogenesis first distinguished in Western Siberia—namely, the Salair cycle by A. M. Kuzmin (53), and the Telbes cycle by M. A. Usov (81)—are not peculiar only to that region.

Academician A. D. Arkhangelsky (4), who proposes that in tectonic studies one be guided by the hypothesis of the local significance of major tectonic cycles (pp. 288–289), is hardly right. This cannot be said with respect to the Caledonian, Variscan, and other generally recognized cycles, which manifested themselves in various parts of the earth’s surface. As for the Salair cycle, its manifestation has been established not only in regions of Western Siberia, the Tien Shan, and the Urals, but also beyond the borders of the USSR. Thus, for example, S. Bubnoff (9) notes the presence of Upper Cambrian tectonic movements in Spain. There are indications for North America in Schuchert, concerning the presence there of pre-Caledonian movements that occurred at the end of the Cambrian. The zone of Middle Devonian tectonic movements, distinguished as the Telbes cycle of tectogenesis, is not confined only to Western Siberia, but extends also into the Urals. The possibility is not excluded of identifying these cycles in other folded belts.

IX. CONCLUSIONS

Since the beginning of the Paleozoic, the Urals experienced two geosynclinal stages. They were completed in the Salair and Variscan cycles of tectogenesis, which thus were the leading ones in the history of the development of the Urals. These cycles corresponded to moments of powerful paroxysms of compression. They were characterized by intense folding of sedimentary formations, by the restructuring and complication of ancient structures, and by the intrusion of energetically strong granitic intrusions in the final phases of these cycles. As a result, their geosyncline was transformed into a consolidated folded belt. This condition of the folded belt as a whole for the entire Urals, with isolated exceptions—

...was experienced in the Upper Cambrian after the main phases of the Salair cycle, and also in the Mesozoic and Cenozoic after Variscan tectogenesis.

During the period from the Silurian to the Carboniferous, geosynclinal regeneration of a significant part of the Salair folded belt took place. Expansion and subsidence prevailed in the Ural geosynclinal zone. But the continuing struggle between compression and expansion, as an expression of the self-development of the earth’s matter, found its reflection here as well. Separate epochs of paroxysmal compression manifested themselves in the form of the Caledonian and Telbes cycles of tectogenesis. They played a subordinate role in the formation of the Ural structures, but were important epochs that complicated the geosynclinal trough of the Urals. Two sharply expressed paroxysms of extension in the Upper Silurian and Lower Devonian were characterized by an effusive cycle in its powerful extrusive manifestation on the eastern slope and by a weak, mainly dike-like manifestation on the western slope. The volcanic cycles of the Caledonian and Telbes epochs of folding were characterized by the intrusion of energetically weak basic and ultrabasic intrusions and by the outpouring of basic effusives. Acid rocks in these epochs played a subordinate role. These secondary cycles were characterized by relatively gentle folding of the Silurian and Devonian and by a definite type of magmatic rocks. Since both of these tectogeneses appeared against the general background of subsidence, after them the regions of the Urals that had taken part in folding in most cases retained geosynclinal conditions, in contrast to the leading cycles.

The Caledonian and Telbes cycles, while playing a subordinate role in the formation of the Ural structures, had great importance in the history of volcanism. This is evidenced by the predominance in the Urals of basic and ultrabasic magmatic rocks over acid ones. This fact is a characteristic feature of the Urals, distinguishing it from other folded belts of the Soviet Union. It is directly connected with the long period of geosynclinal subsidence of the Salair folded belt in the Paleozoic, which left its imprint also on the type of magma formation.

The Mesozoic was an epoch of tectonic preparation of the Variscan folded belt for the subsequent Alpine cycle. Differential uplifts and subsidences of individual parts of the Urals, the subsidence of certain Paleozoic troughs with the accumulation in them of Jurassic sedimentary strata, their subsequent dislocation, and block movements along old tectonic seams were connected with Pacific tectogenesis. As a result of it, the most mobile areas were revealed in the Ural Variscan folded belt. Among these were the regions of the central part of the Urals and, especially, of the western slope. The upward movements that were outlined for them in the Mesozoic, in the Tertiary and Quaternary periods, in connection with the Alpine cycle of tectogenesis, developed into genuine mountain-forming processes, which led to the formation of the Urals as a modern mountainous country.

The data set forth above compel us to recognize the subordinate role of the Pacific and Alpine cycles in the history of the development of the Urals. But they differed from the Caledonian and Telbes cycles. Whereas the latter were manifested mainly under conditions of a mobile geosynclinal zone and were accompanied by a powerful magmatic complex of basic and ultrabasic rocks, the Pacific and Alpine cycles occurred under conditions of a consolidated folded belt with an almost complete absence of magmatic manifestations. All this indicates that the cycles of tectogenesis manifested in the Urals differed from one another in their role in its geological history.

The different role, in this connection, of the Caledonian and Salair tectogenesis compels us to regard Salair diastrophism not as a phase of Caledonian fold...

as a local, but as an independent tectonic cycle. Academician M. A. Usov (82) also came to an analogous conclusion for Western Siberia.

Using the Urals as an example, one can see that individual folded zones in their history may pass through not necessarily one, but two and, perhaps, several geosynclinal stages, with the subsequent regeneration of already formed folded belts. In the development of geosynclines it is possible to distinguish two types of cycles—some leading ones, which close the geosyncline, and others secondary, taking place against the general background of the geosynclinal trough and accompanying it. In this case, the formation of geosynclines is accompanied by an energetically weak basic and ultrabasic magmatic complex, while its closure is accompanied by an energetically strong acid one.

Geosynclines, being mobile regions with a constant tendency toward subsidence, are not devoid of major revolutionary leaps before the moment of their closure; on the contrary, these leaps are present, and the possibility of their manifestation is inherent in the very nature of geosynclines and organically connected with it.

Geosynclines are the weakest and most mobile zones of the Earth’s crust. In them the principal factor of self-development of earthly matter—the struggle between compression and extension—is manifested most sharply. As long as extension predominates in a geosyncline, revolutionary leaps of compression are expressed in the form of secondary cycles that complicate the geosynclinal trough. As soon as predominance passes to compression, the geosyncline closes and is transformed into a consolidated folded belt.

It follows from the foregoing that the consolidation of a geosynclinal zone is prepared by the entire course of its historical development. Folding processes in a geosyncline, as elements of the new, begin to appear within the bowels of the old—the dominant trough of the geosynclinal zone—long before its closure. And the final phases of the leading cycles, accompanied by acid intrusions, merely complete what had been initiated by secondary cycles in the depths of the ages of the geosyncline’s history.

Since individual cycles of tectogenesis have different significance in the formation of folded zones, it would be not quite correct to depersonalize them. In each case it is necessary to ascertain the character of the manifestation of tectonic cycles under the concrete historical conditions of individual geosynclinal-orogenic zones, and to emphasize the distinctive features of each of them in the formation of structures, in volcanism, and in mountain building. This must be kept in mind, since leading and secondary cycles in different geotectonic regions may also have different forms of manifestation.

Not only is there a difference between cycles of different kinds, leading and secondary, but also between cycles of the same type. Such a difference is inevitable, since the subsequent cycle manifests itself under different conditions than the preceding one, and on a different foundation, complicated by the preceding course of development.

X. CONCLUSION

In the preceding chapters, on the basis of a study of the available literary material on Ural geology, the author has attempted to give a brief summary of the geological history of the Urals with a somewhat different interpretation of its geotectonic development. Far from all the propositions advanced in the present work are indisputable. Some of them require clarification, supplementation, and possibly correction, owing to the incompleteness of the factual material available and used by the author.

It cannot be said that the Urals have been well studied at the present time, as is evidenced by contradictory opinions and unclear and disputed points concerning

many questions of its geology. It is quite natural that the researchers of the Urals are faced with a great many tasks, especially in the study of the Precambrian, Lower and Middle Paleozoic. Among such most important problems requiring solution may be included the following.

1. The completion of the subdivision of the metamorphic rocks of the Salair structural complex, with the separation of the Cambrian, Proterozoic, and Archean.

2. The study of volcanism and metallogeny of the Precambrian and Cambrian.

3. The elucidation of regularities in the history of volcanism¹ and metallogeny of the Caledonian, Telybes, and Variscan cycles of tectogenesis.

4. The subdivision of the Ural plagiogranites. The available facts compel one to assume that this rather vague category includes groups of rocks genetically connected with intrusions of different ages.

5. The study of the structural features of the sedimentary-effusive series of the western and eastern slopes of the Urals and of the metamorphic complex of its central and eastern areas.

6. The identification of regularities in changes in the composition, facies, and thickness of coeval series over their extent throughout the entire Urals.

7. The revision of the identified tectonic contacts. It is possible that they will not prove to be such. A sharp change of facies and the presence of brecciated horizons is not yet proof of a tectonic contact. Both are possible under normal stratigraphic occurrence. Brecciated horizons may also arise as a result of folding during interformational movements.

8. The identification of local features in the history of development of individual regions of the Urals.

9. The study of the question of the possible continuation to the northwest from Timan, along its strike, of the ancient Salair folded zone.

Noteworthy is the absence of a generally accepted view concerning the criteria for distinguishing cycles and phases of tectogenesis and concerning many other questions of modern geotectonics. As a result, different interpretations are obtained of one and the same factual material, introducing confusion into the reproduction of the actual course of events.

At present, the study of volcanism and metallogeny of the Urals, as well as the compilation of a forecast map, is hindered by its insufficient tectono-stratigraphic study. The clarification of the above-mentioned problems will help to outline the paths toward a correct assessment and the most effective utilization of the natural riches concealed in the Ural subsurface.

In conclusion, I consider it my duty to express gratitude to Professor A. M. Kuzmin and to Professor-Doctor M. K. Korovin for a number of valuable comments and suggestions made by them while reviewing the manuscript.

LIST OF REFERENCES USED

Abbreviations:

Bull. MOIP—Bulletin of the Moscow Society of Naturalists.
Zap. VMO—Proceedings of the All-Russian Mineralogical Society.
Zap. Gor. Inst.—Proceedings of the Mining Institute.
Zap. Ur. Ob-va lyubit. estestv.—Proceedings of the Ural Society of Lovers of Natural Science.
Izv. AN—Proceedings of the Academy of Sciences.
Izv. VGRO—Proceedings of the All-Union Geological Prospecting Association.
Izv. GGRU—Proceedings of the Main Geological Prospecting Administration.
Izv. GGO—Proceedings of the State Geographical Society.
Izv. GK—Proceedings of the Geological Committee.
Izv. SOGK—Proceedings of the Siberian Branch of the Geological Committee.
PSG—Journal “Problems of Soviet Geology.”
Sov. geol.—Journal “Soviet Geology.”
Tr. BGT—Transactions of the Bashkir Geological Trust.
Tr. GK—Transactions of the Geological Committee.
Tr. MGRI—Transactions of the Moscow Geological Prospecting Institute named after Ordzhonikidze.
Tr. NIIMS—Transactions of the Scientific Research Institute of Mineral Raw Materials.
Tr. TsNIGRI—Transactions of the Central Scientific Research Geological Prospecting Institute.
Uch. zap. LGU—Scientific Notes of Leningrad State University.
FO KGU—Archive Division of the Kazakh Geological Administration.

1. Artaev S. A. and Aleshkov A. N.—A brief geological-petrographic sketch of the southeastern part of the Kempirsai hyperbasite massif. 1936. FO KGU.

2. Arkhangel’skii A. D.—On the question of the cover tectonics of the Urals. Bull. MOIP, geological section, vol. X (1), 1932.

3. The same.—Geological structure of the USSR. 1934.

4. Arkhangel’skii A. D. and others.—A short course on the geological structure and geological history of the USSR. 1937.

5. Bezrukov P. L. and Yanshin A. L.—Jurassic deposits and aluminum-ore deposits in the Pri-Mugodzhar steppes. Tr. NIIMS, vol. 1, issue 110, 1937.

6. Blokhin A. A.—New data on the geological structure of the Southern Urals. Bull. MOIP, geological section, vol. X (1), 1932.

7. Bogdanik N. S.—On the “theory” of cycles in modern geology. Sov. geol., No. 7, 1939.

8. Borsuk B. I.—Geological sketch of the Kumak district. 1936. FO KGU.

9. Bubnov S.—Geology of Europe.

10. Bubnov S.—Fundamental problems of geology. 1934.

11. Varsanof’eva V. A.—Geomorphological observations in the Northern Urals. Izv. GGO, vol. XIV, issue 2, 1932.

12. Vodoresov G. I.—Southern Mugodzhary. Geological description of the Kaidinsk and Berchogur districts. 1936. FO KGU.

13. The same.—Mugodzhary. Geological description of the Kara-Butak, Kairaktinsk, and Uzun-Taldyk districts. 1935. FO KGU.

14. The same.—Mugodzhary. Geological structure and mineral resources of the Kosistek district. 1937. FO KGU.

15. Volkov S. N. and Yatsyuk N. V.—Geological sketch of the Polar Urals. Explanatory note to the geological map of the northern part of the USSR. 1937.

16. Volkov S. N. and Yatsyuk N. V.—Geological sketch of Timan. Explanatory note to the geological map of the northern part of the USSR. 1937.

17. Vologdin A. G.—On the Cambrian of the Urals. Izv. AN, No. 4, 1937.

18. Vysotskii N. K.—Platinum deposits of the Isovsk and Nizhne-Tagil districts in the Urals. Tr. GK, issue 62, 1913.

19. The same.—Platinum and the districts of its extraction. 1923.

20. Vysotskii N. K.—Metamorphic schists and the more nearly indeterminate Paleozoic. Explanatory note to the geological map of the Urals. 1931.

21. The same.—A brief sketch of the development of orogenic and volcanic cycles in the Urals. Explanatory note to the geological map of the Urals. 1931.

22. Garan’ M. I.—Precambrian deposits of the western slope of the Southern Urals and the minerals associated with them. Transactions of the XVII Session of the International Geological Congress, vol. II, 1937.

23. Geological Map of the Urals. 1931.

24. Geological Map of the Urals at a scale of 1:500,000. 1939. Edited by I. I. Gorskii, L. S. Librovich, and V. M. Sergievskii.

25. Gorskii I. I.—Geological study of the Urals over the 20 years after October. PSG, No. 10, 1937.

26. Dvortsova K. I.—Geological sketch of the Northern Mugodzhary (upper reaches of the Medes, Terekta, and Kolymbai rivers) for 1934. FO KGU.

1. Dingelstedt N. N.—On a find of \(S_1\) fauna in the Southern Urals near the Tirlyan plant. GG RU, vol. 49, 1930.

2. The same—On the question of the geological structure of the Urals. Tr. VGRO, issue 243, 1932.

3. The same—Geological investigations in the Tirlyan district of the Southern Urals. Tr. VGRO, issue 277, 1939.

4. The same—On the stratigraphy and tectonics of the Southern Urals. PSG, No. 8, 1935.

5. Duparc L. and Sigg G.—Copper deposits in the Sysertskaya dacha in the Urals. Tr. GK, issue 101, 1914.

6. Ermolaev M. M.—Geology of Novaya Zemlya. Transactions of the Arctic Institute, vol. XI, 1936.

7. The same—Geological sketch of Novaya Zemlya. Explanatory note to the geological map of the northern part of the USSR. 1937.

8. Zavaritsky A. N.—On the tectonics of the Urals. Izv. GK, issue 42, 1923.

9. The same—Principal features in the development of the volcanic cycle in the Urals. Izv. GK, vol. 43, issue 3, 1924.

10. The same—Magmatic and metamorphic rocks of the Urals. Explanatory note to the geological map of the Urals, 1931.

11. The same—Peridotite massifs of the Polar Urals and the rocks surrounding them. Petrography of the Urals, collected volume.

12. The same—Petrography of the Berdyaush pluton. Tr. TsNIGRI, issue 96, 1937.

13. Ivanov A. I.—On the stratigraphy and ancient orogenesis of the western slope of the Southern Urals. Tr. BGT, issue 7, 1937.

14. Kassin N. G.—A brief geological sketch of northeastern Kazakhstan. Tr. VGRO, issue 165, 1931.

15. The same—Sketch of the tectonics of Kazakhstan. PSG, No. 6, 1934.

16. The same—Volcanism of Kazakhstan. PSG, No. 9, 1934.

17. Kirichenko G. I.—The metamorphic series “M” of the southern extremity of the Urals. Zap. VMO, part XV, No. 2.

18. Koltev-Dvornikov V. S.—The geological structure of the Urals along the South-Ural Railway (Chelyabinsk–Zlatoust). For the XVII International Geological Congress. Ural excursion.

19. Krotov B. P.—Petrographic investigation of the southern part of the Miass dacha. Tr. Society of Naturalists at Kazan University, vol. XVII, issue 1, 1915.

20. The same—Geological investigations in the Alapaevsk district in the Urals. Izv. GK, vol. XVIII, No. 2, 1929.

21. Kuznetsov E. A.—On the tectonics of the eastern slope of the Middle Urals. Bull. MOIP, vol. XII(2), 1933.

22. The same—General review of the geological structure of the Urals. For the XVII International Geological Congress. Ural excursion.

23. The same—The Precambrian of the Urals, 1937. Transactions of the XVII Session of the International Geological Congress.

24. The same—The development of views on the tectonics of the Urals from A. P. Karpinsky to our days. Izv. AN, No. 1, 1937.

25. Kuznetsov E. A. and Astashenko K. I.—The geological structure of the northwestern part of the Pai-Khoi Range. Izv. AN, No. 4, 1938.

26. Kuznetsov E. A. and Luchitsky V. I.—Petrographic provinces of the USSR. 1936.

27. Kuzmin A. M.—Materials on the stratigraphy and tectonics of the Kuznetsk Alatau, Salair, and Kuzbass. Izv. SOGK, VII—2, 1928.

28. Librovich L. S.—On a find of graptolite fauna \(S_2\) in the Southern Urals. Izv. GGRU, vol. 49, 1930.

29. The same—The Paleozoic of the eastern slope of the Urals. Explanatory note to the geological map of the Urals. 1931.

30. The same—On the geology of the southern part of the Bashkir Urals. Tr. VGRO, issue 144, 1932.

31. The same—Principal features of the geological history of the Kizil-Urtazym district in the Southern Urals. Zap. VMO, No. 1, 1933.

32. Lvov K. A. and Olli A. I.—On the relation of \(D_2\) to the Asha Formation. Zap. VMO, part 64, issue 2, 1935.

33. Mazarovich A. N.—Fundamentals of the geology of the USSR. 1938.

34. Mirlin G. A.—On the so-called “more or less indeterminate Paleozoic” of the eastern slope of the Urals. Izv. AN, No. 3, 1937.

35. Nalivkin D.—On the geological structure of the Southern Urals. Zap. Mining Institute, vol. VII, 1926.

36. The same—The Paleozoic of the western slope of the Southern and Middle Urals. Explanatory note to the geological map of the Urals. 1931.

37. The same—Geological regions of the USSR. PSG, vol. 1, No. 1, 1933.

38. The same—Scientific results of the Perm Conference. PSG, No. 7, 1937.

39. Ozhiganov D. G.—On the age of the Ural Range and the volcanic cycles of the Southern Urals. PSG, No. 3, 1937.

40. Report on the condition and activity of the Geological Committee in 1913. Izv. GK, vol. XXI, 1914.

1. Report on the condition and activity of the Geological Committee in 1914. Izv. GK, vol. XXXIV, No. 1, 1915.

2. Report on the condition and activity of the Geological Committee in 1915. Izv. GK, vol. XXXV, No. 1, 1916.

3. Pavlinov V. N.—On the stratigraphy and tectonics of the Sakmara-Karaganda district in the Southern Urals. Tr. MGRI, vol. 1, 1936.

4. He—Stratigraphy and tectonics of the southern part of the Southern Urals. Tr. MGRI, vol. IX, 1937.

5. Padalka G. L.—The western belt of rocks of the gabbro-peridotite formation of the Urals, 1937.

6. Panteleev P. G.—On the question of the geochemistry of titanium, vanadium, and chromium in the titanomagnetites of the Urals. Izv. AN, No. 3, 1938.

7. Petrenko A.—Geology of the northeastern part of the Orsk district. Tr. GTRU, issue 250, 1932.

8. He—Geological structure of the Primugodzhary district. Manuscript, 1932.

9. Postelov G. L.—Materials on the tectonics of intrusions. Dissertation. Tomsk, TII, 1938.

10. Perna E. Ya.—The Paleozoic of the eastern slope of the Urals between the town of Verkhneuralsk and the Magnitnaya station. Izv. GK, vol. 31, 1912.

11. Razumovskaya E.—Geology of the southern part of the Southern Urals (district of Orsk, Khalilovo, and Blyava). For the XVII International Geological Congress. Ural excursion.

12. Romanov V. M.—Experience in geological regionalization of the Urals. Zap. Ural. ob-va lyubiteley estestv., vol. XL, issue 1, 1926.

13. Solovyev S. T.—An attempt to study the distribution of igneous rocks within the USSR. 1936.

14. Tetyaev M. M.—Geotectonics of the USSR. 1938.

15. Usov M. A.—The Telbes iron-ore district. Izv. SOGK, VI—5, 1927.

16. He—Phases and cycles of tectogenesis of the West Siberian region. Tomsk, 1926.

17. Fedorov S. S.—Geological structure of the Bogoslovsk district, 1901.

18. Khabakov A. V.—Traces of Caledonian diastrophism in the southern part of the Southern Urals. Zap. VMO, part LXIV, No. 1.

19. He—Pre-Jurassic relief and ancient weathering crust in the southern part of the Southern Urals. Izv. GGO, vol. 67, issue 2, 1935.

20. Chernov A. A.—The Paleozoic of the western slope of the Northern Urals. Explanatory note to the geological map of the Urals. 1931.

21. Chernyshev F.—Description of the central part of the Urals and its western slope. Tr. GK, vol. III, 1885.

22. Chumakov A. A.—Materials on the petrography of the Mugodzhar Mountains, part 1. Uch. zap. LGU, No. 21, issue 5, 1939.

23. He—Materials on the petrography of the Mugodzhar Mountains, part 2. Uch. zap. LGU, No. 34, issue 7, 1939.

24. Shakhov F. N.—Materials on the geology of the Tanalyk-Baymak copper-ore district in the Southern Urals. Izv. Sib. tekhnolog. instituta, vol. 49, issue 1, 1928.

25. Yanishevsky M.—Report on a trip to the Mugodzhar and Guberlin Mountains. Izv. Tomskogo tekhnolog. instituta, book 3, 1905.

1. The works on this question by V. M. Sergievsky and G. A. Sokolov, as well as a number of studies by other geologists on the volcanism and tectonics of the Urals, remained inaccessible to the author. ↩↩↩↩

2. A courteous communication from the geologist V. L. Malyutin. ↩