Aulacogens, the Donets Basin (eastern Ukraine, southwestern Russia), and the new classification of rifts: towards a proper terminology

Some intra-cratonic basins are traditionally called “aulacogens”. This term has persisted in the geoscience literature since its invention by Soviet geologists in the mid-20th century before the triumph of the plate tectonics, but its meaning has evolved. Attempts to change its meaning from descriptive to genetic have led to a broad spectrum of opinions on the definition of aulacogens. Some specialists related them to continental rifts, while others have restricted aulacogens to the only particular rift systems or peculiar stages in the evolution of young cratons. The Donets Basin is a typical aulacogen stretching across the southern margin of the East European Craton. A brief review of present knowledge of this basin shows that its nature is rather incompatible with the present understanding of aulacogens. Instead, the new classification of rifts offers a more precise terminology for its exact characteristics. It is suggested that the term “aulacogen” should only be restricted to those basins for which it has been applied historically.


Introduction
The term "aulacogen" was coined by the famous Russian geologist N.S.SHATSKIJ and his followers in the midst of the 20 th century (KOSYGIN & PARFJONOV 1970;PAFFENGOL'TS 1978).It has since been used by researchers outside the ex-USSR and Russia (e.g.,

What does the term "aulacogen" mean?
The term "aulacogen" was introduced by the Soviet geologist N.S.SHATSKIJ in 1964(KOSYGIN & PAR-FJONOV 1970;PAFFENGOL'TS 1978).It originally meant nothing more than "a trench-like complex structure between similar zones of the platform" (KOSYGIN & PARFJONOV 1970, p. 148).Soviet geologists also emphasized thick, often folded, sedimentary cover (measured by thousands of meters) of aulacogens and controls of major faults on these basins (KOSYGIN & PARFJONOV 1970;PAFFENGOL'TS 1978).Several types of aulacogens were distinguished (e.g., KOSYGIN 1969).KOSYGIN (1969) pointed out that the original meaning of the term had already changed a few years after it was coined, and presented a broad spectrum aulacogen definitions (see also KOSYGIN & PARFJO-NOV 1970;PAFFENGOL'TS 1978).Interestingly, formation of aulacogens has been often attributed to a particular stage in the evolution of cratons, when young platforms experienced destructive deformations (KO-SYGIN 1969;LAZ'KO 1975;POTAPOV, 1996).The East European Craton (= Russian Platform), which has been identified by Soviet geologists as an ideal object for cratonic studies, exhibited the formation of several aulacogens during the so-called Riphean (Meso-and Neoproterozoic -see RUBAN 2009 for more details), when this craton began to evolve into a "stable" tectonic block (LEJTES et al. 1970;BELOUSOV 1978;VALEEV 1978;POTAPOV 1996).This interpretation appeared so obvious that even elementary textbooks in general geology tended to relate the majority of aulacogens to the late Proterozoic evolution of young cratons (e.g., KORONOVSKIJ & JAKUSHOVA 1991).
It is important to note that ideas about aulacogens appeared before the wide acceptance of the plate tectonics as a universal tectonic theory (this is especially true for the Soviet geoscience community of 1960-70s).Aulacogens were treated in terms of fixism (or, more properly, the geosyncline concept) during the 1960s and the 1970s, when crucial information about them was accumulated (KOSYGIN & PARFJONOV 1970;PAFFENGOL'TS 1978).When the theory of plate tectonics became accepted and the attention of Soviet geologists turned to extensional structures (MILANOV-SKIJ 1976), the term "aulacogen" started to become related to "continental rift" and "graben".However, some specialists expressed caution about a mix of these terms (BELOUSOV 1978).Moreover, it appears that the original definition of aulacogens (see above) does not require the formation of these structures within continental rifts (sometimes, compressed and folded after the main deposition phase), but also allows also their formation via large-scale epeirogenic deformation of cratons (often characterized in terms of dynamic topography).Nevertheless, continental rifting seems to be the most plausible explanation for the majority of aulacogens.Decades after the first definition of the term, the aulacogen stage in the evolution of young platforms was described in terms of continental rift development and the onset of extension (e.g., NIKISHIN et al. 1996).Thus, although the discussed term was originally only descriptive, it has "gained" a genetic sense as the tectonic knowledge of the geoscience community advanced.
The body of Soviet/Russian literature on aulacogens is huge, but what about the international publications?Below, I give some examples from books published recently.BOGGS (2006) provides several opinions on the nature of aulacogens.He notes, for instance, that these might have been failed rifts, later reactivated under a compressional regime.Following SENGÖR (1995), he also mentions the possible role of strike-slip displacements and tectonic block rotations in the origin of these structures.BOGGS (2006) also emphasizes the thick sedimentary cover of aulacogens and their occurrence at high angles relative to the continental margin.Does this mean that aulacogens can occur only in the peripheral parts of cratons?!This author also lists (as examples) aulacogens of a very different age (including late Paleozoic and Cretaceous structures).Reviewing knowledge of the intra-continental sedimentary basins, BAYER et al. (2008) note , that aulacogens are old inverted stuck rifts.FRISCH et al. (2011) reject the validity of the term "aulacogen", which, in their opinion, is a graben structure with thick sedimentary cover.It should be noted that these authors refer to a rather broad understanding of grabens.In his recent monumental review, INGERSOLL (2012) treats aulacogens as fossil rifts evolved as a third arm in three-rift systems; according to this author, aulacogens are continental rifts that did not become oceans and were later compressed.Looking at recent research papers published in international journals (e.g., AITKEN & BETTS 2009;DICKINSON et al. 2010;TEIXEIRA et al. 2010;DUAN & DUAN 2011;JIN et al. 2011), it is easy to realize that the term "aulacogen" refers to basins related to continental extension and/or supercontinental break-up, and many of these basins are late Precambrian in age.It also appears (but this is a mere impression) that the term "aulacogen" is used historically for particular tectonic structures in some (if not many) cases.
The authors of the non-Soviet/Russian publications considered above tend to employ the term "aulacogen" with certain differences, and they always do so within the context of plate tectonics.This is far from the original understanding of aulacogens in the Soviet geoscience literature before the 1980s (see above), when this term was used within the geosyncline conceptual frame.Interestingly, none of the books or book chapters mentioned above (BOGGS 2006;BAYER et al. 2088;FRISCH et al. 2011;INGERSOLL 2012) refer to aulacogen formation as a particular stage (often, late Precambrian) in the evolution of cratons, which has been a "classic" concept in the Soviet/Russian geoscience community.
According to the most recent synthesis of the available knowledge (SACHSENHOFER et al. 2012), the opening of the Donets Basin occurred in the Late Devonian when the pre-existing Sarmatian Craton was divided into two parts, which are known today as the Ukrainian and Voronezh massives (STEPHENSON et al. 1996;RUBAN & YOSHIOKA 2005).We can not exclude the possibility that emplacement of a mantle plume could trigger, or at least contribute to, the appearance of this basin (WILSON & LYASHKEVICH 1996;RACKI 1998;BRINK 2009;SACHSENHOFER et al. 2012).The Donets basin, however, might have inherited some older structures (e.g., POTAPOV 1996) 2005)).These authors followed earlier ideas expressed by ARTHAUD & MAT-TE (1977).According to these studies, the Donets Basin was formed as the result of strike-slip displacements in the Variscan and adjacent structures.It is possible that dextral displacements along the southern margin of the East European Craton detached from the Ukrainian block and opened the elongated basin between this new terrane and the rest of the craton in the late Paleozoic (Fig. 2).Changes in the direction of displacements along the major shear zone located along the southern margin of the East European Craton in the early Mesozoic resulted in compression of the thick sedimentary complexes that were accumulated in the above-mentioned basin.This scenario requires some refinement, but it relates the nature of the Donets Basin to forces that are much larger in scale than those responsible only for the evolution of the East European Craton.The noted major shear zone was an element of the global system of shear zones, which stretched across Gondwana and the northern Palaeo-Tethyan margin (RUBAN 2007) (Fig. 2).
If the Donets Basin is an aulacogen, how does its nature, characterized above, fit the various definitions of aulacogen formation?If we take the only descriptive meaning of the term "aulacogen" from the Soviet literature of the mid-20 th century (see above), there is no difficulty in applying this term to the Donets Basin.However, it is impossible to relate the term "aulacogen" to the Riphean stage in the evolution of the young East European Craton.If this basin inherited any Precambrian lineaments (e.g., POTAPOV 1996), then it was formed in the mid-Paleozoic, when the craton was already "old".Moreover, as said above, the forces responsible for the formation of the Donets Basin were different from those responsible for the evolution of the craton.From various definitions of aulacogens proposed in international publications, that of INGERSOLL (2012) differs especially from what occurred in the Donets Basin.In particular, there is no any clear evidence that the Donets Basin evolved as the third arm of a three-rift system.
Recently, a new classification of rift structures has been proposed; plume-related, subduction-related, mountain-related, and transform-related rifts are distinguished on the basis of the tectonic environments that were present at their formation (MERLE 2011).Is it possible to apply this classification to the Donets Basin?Features of two types of rifts can be found in the Donets Basin.First, we already suggested that the emplacement of a mantle plume could facilitate or even provoke the Donets rift formation in the Late Devonian (WILSON & LYASHKEVICH 1996;RACKI 1998;BRINK 2009;SACHSENHOFER et al. 2012), and the activity of mantle plumes might have contributed to the evolution of this rift at the later stages (ALEXAN-DRE et al. 2004).If so, this plume-related rift (sensu MERLE 2011) is to be compared with the East African continental rifts (CORTI 2009(CORTI , 2012)).Second, the Donets Basin developed in the strike-slip environment (NATAL' IN & SENGÖR 2005;RUBAN & YOSHIOKA 2005;RUBAN 2007).In this case, it bears features typical of transform-related rifts described by MERLE (2011).It is important to note that judging the Donets Basin as aulacogen does not clarify its nature.In contrast, the application of the new classification of rift structures , (MERLE 2011) permits us to indicate the mechanism of its formation exactly.

Discussion and conclusion
Undoubtedly, the geologic recognition of aulacogens, and the intense study of these formations by Soviet/Russian specialists, played a great role in deciphering the geologic history of cratons.Because of this, I do not tend to judge the results of these studies too critically, although when doing so it is important to also consider the alternative understandings of the term "aulacogen" (stressed already by KOSYGIN 1969), and the fact that aulacogen development is not necessarily associated with cratonic evolution (see about the nature of the Donets Basin).A greater problem is the "diffuse" meaning of the term "aulacogen" in the modern international geoscience literature.This meaning differs somewhat from the original definition, because it attempts to explain aulacogens genetically in terms of the plate tectonics.Moreover, the genetic treatment of aulacogens implies formational explanations that are not relevant for all possible aulacogens, including such typical aulacogens as the Donets Basin.Instead, the new tectonic nomenclature provides better causative descriptions of basins than "simply" judging them to be aulacogens.For example, the classification of rift structures proposed by MERLE (2011) provides a proper tectonic terminology from which we can infer the nature of the Donets Basin formation (combined plume-and transformrelated).
Do the considerations presented above imply that the term "aulacogen" is improper or failed, as has been suggested by FRISCH et al. (2011).In my opinion, it is equally wrong to preserve one term that does not fit the present needs as it is to abandon it, especially if it remains relatively frequently used (Fig. 1).I propose the following solution to this dilemma: the term "aulacogen" may still be used, but for only those tectonic structures and sedimentary basins that were already judged aulacogens, e.g., the Donets Basin, the Pachelma Trough, and the Vyatka Aulacogen of the East European Craton (KOSYGIN 1969;BOGGS 2006).For these, "aulacogen" is the historically correct term.Moreover, the original Soviet meaning of the "aulacogen" is merely descriptive, which simplifies the preservation of the regional use of this term.Similarly, such terms "flysch" and "molasse" are used for particular sedimentary packages in the sedimentological, stratigraphic, and tectonic literature.As for other or future tectonic investigations, the term "aulacogen" should be avoided.

Fig. 1 .
Fig. 1.Changes in the number of papers available for the international readership (based on a search of titles, abstracts, and key words in the bibliographical database scopus.com;accessed on April 9, 2012).The number of papers is indicated by columns; 16 papers were published in 2011.
, NATAL'IN & SENGÖR 2005; RUBAN & YO-SHIOKA (2005), RUBAN (2007), MEIJERS et al. (2010), . Strong post-rift subsidence occurred in the late Paleozoic, and was followed by an inversion and uplift (SACH-SENHOFER et al. 2012).The age of the compressional event(s) is still debated, but it ranges from the Permian to the Cretaceous (SAINTOT et al., 2003b; NATAL'IN & ÞENGÖR 2005; RUBAN & YOSHIOKA 2005; see also brief review in SACHSENHOFER et al. 2012).RUBAN & YOSHIOKA (2005) and RUBAN (2007) discussed the evolution of the Donets Basin in a broader context (similar views were also expressed independently by NATAL'IN & SENGÖR ( The new classifications, such as those proposed by MERLE (2011), provide us with a proper tectonic terminology.