Randell Stephenson

Late Precambrian to Triassic history of the East European Craton: dynamics of sedimentary basin evolution

Abstract During its Riphean to Palaeozoic evolution, the East European Craton was affected by rift phases during Early, Middle and Late Riphean, early Vendian, early Palaeozoic, Early Devonian and Middle-Late Devonian times and again at the transition from the Carboniferous to the Permian and the Permian to the Triassic. These main rifting cycles were separated by phases of intraplate compressional tectonics at the transition from the Early to the Middle Riphean, the Middle to the Late Riphean, the Late Riphean to the Vendian, during the mid-Early Cambrian, at the transition from the Cambrian to the Ordovician, the Silurian to the Early Devonian, the Early to the Middle Devonian, the Carboniferous to Permian and the Triassic to the Jurassic. Main rift cycles are dynamically related to the separation of continental terranes from the margins of the East European Craton and the opening of Atlantic-type palaeo-oceans and/or back-arc basins. Phases of intraplate compression, causing inversion of extensional basins, coincide with the development of collisional belts along the margins of the East European Craton. The origin and evolution of sedimentary basins on the East European Craton was governed by repeatedly changing regional stress fields. Periods of stress field changes coincide with changes in the drift direction, velocity and rotation of the East European plate and its interaction with adjacent plates. Intraplate magmatism was controlled by changes in stress fields and by mantle hot-spot activity. Geodynamically speaking, different types of magmatism occurred simultaneously.

On the origin of the southern Permian Basin, Central Europe.

Abstract A detailed study of the structural and stratigraphic evolution of the Southern Permian Basin during latest Carboniferous to Early Jurassic times, supported by quantitative subsidence analyses and forward basin modelling for 25 wells, leads us to modify the conventional model for the Rotliegend–Zechstein development of this basin. The Late Permian–Early Jurassic tectonic subsidence curves are typical for a Permian to Early Triassic extensional stage that is followed by thermal subsidence. However, a purely extensional model is extremely problematic because active faulting during this time is ‘minor’ and generally hard to document. Using inverse techniques to model the subsidence curves, we quantitatively show that a significant component of Late Permian and Triassic tectonic subsidence can be explained by thermal relaxation of Early Permian lithospheric thinning, and by delayed infilling of paleo-topographic depressions that developed during the Early Permian. In this interpretation, Stephanian–Autunian wrenching resulted in thermal destabilisation of the lithosphere, deep fracturing of the crust, disruption and erosion of its sedimentary cover and regional uplift of the area of the future Southern Permian Basin. Upon termination of wrench tectonics and associated volcanism, towards the end of the Autunian, the Southern Permian Basin began to subside in response to thermal contraction of the lithosphere. The evolving basin was isolated from the World oceans and had subsided possibly up to some 700 m below their level at the beginning of Upper Rotliegend sedimentation. After catastrophic flooding of this paleo-topographic depression at the beginning of the Zechstein, changing sea level, sedimentation and subsidence rates remained essentially in balance. Although the effects of Triassic rifting overprinted parts of the Southern Permian Basin, its overall subsidence pattern persisted well into the Jurassic. In contrast to the remainder of the Southern Permian Basin, Permian and Triassic crustal extension contributed significantly to the subsidence of the Polish Trough.

Late Vendian-Early Palaeozoic tectonic evolution of the Baltic Basin: regional tectonic implications from subsidence analysis.

Abstract Subsidence analysis was performed on 43 boreholes penetrating the Upper Vendian–Lower Palaeozoic sedimentary succession of the Baltic Basin. The results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development. Tectonic subsidence patterns are consistent throughout the basin for the time period studied. An extensional tectonic subsidence event, possibly of two phases, is indicated from the Late Vendian to the beginning of the Middle Cambrian. This event is seen in the southwestern part of the Baltic Basin (Peri-Tornquist zone) until the earliest Cambrian after which it is also observed in the SW–NE-trending Baltic Depression part of the basin. Basin development during this time is interpreted as recording the latest stages of break-up of the Precambrian super-continent Rodinia and ultimately the formation of the Tornquist Sea. The late Middle Cambrian to Middle Ordovician tectonic subsidence pattern of the Baltic Basin is characteristic of post-rift thermal subsidence of the newly formed passive continental margin of Baltica, developed along its southwestern edge. A gradual increase in subsidence rate is observed from the (Middle?) Late Ordovician and throughout the Silurian (particularly for Ludlow and Pridoli times) creating subsidence curves with convex shapes typical of foreland basin development. The rate of Late Silurian tectonic subsidence increases significantly towards the southwest margin of the Baltic basin, adjacent to the present location of the North German–Polish Caledonides. The Baltic Basin therefore appears to have developed primarily as a flexural foreland basin during Silurian oblique collision of Baltica and Eastern Avalonia. A foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the Late Silurian. Compressional deformation structures of Early Devonian (Lochkovian) age are seen in seismic sections in the central part of the Baltic Basin (Lithuania). These, together with a change in subsidence pattern, mark the end of the Caledonian stage of basin development of the Baltic Basin.

Tectonic evolution of the Mid-Polish Trough: modelling implications and significance for central European geology

Abstract The Polish Basin forms the easternmost part of the Permian-Mesozoic northwest European basin. The depocentral axis of the Polish Basin, the Mid-Polish Trough (MPT), is superimposed on the boundary between the west European Phanerozoic and east European Proterozoic crustal domains, within the Trans-European Suture Zone. The presence of this fundamental crustal boundary may be paramount in structurally controlling the position of the MPT, concentrating stresses during post-Variscan wrench and extensional tectonics in central Europe. Tectonic subsidence analysis of the preserved and reconstructed stratigraphic record of the Polish Basin indicates the occurrence of an initial Late Permian-Early Triassic (255−241 Ma) ‘rifting’ phase that was followed by subsequent episodes of increased tectonic subsidence during the Oxfordian-Kimmeridgian (∼ 157−152 Ma) and beginning in the Cenomanian (∼97 Ma). The Oxfordian-Kimmeridgian episode is interpreted as corresponding to a second extensional event, which correlated with intersified rifting and wrench activity within the Arctic-North Atlantic rift system and along the northern Tethyan margin, while the Cenomanian may be considered a precursor of compressional deformations in the basin which culminated in basin inversion in the latest Cretaceous and Paleocene. Forward modelling results, in view of existing geophysical interpretations which show the presence of a deep Moho and a very velocity lower crustal layer beneath theMPT, suggest that Permo-Mesozoic basic development may be related at least in part to the intrusion of mantle material into and densification of the lower crust rather than exclusively to crustal extension and thinning.

Some examples and mechanical aspects of continental lithospheric folding

Abstract Recent theoretical and observational advances in understanding the dynamics of lithosphere processes have yielded strong evidence in favour of the existence of large-scale lithosphere folds. Intraplate compressional stresses generated by plate tectonic forces can reach magnitudes that approach the plastic buckling strength of mechanical models of lithosphere with brittle-ductile rheological stratification. The models predict lower plastic buckling stresses for continental lithosphere than for oceanic lithosphere. Intraplate folding of relatively strong oceanic lithosphere, documented extensively in the northeastern Indian Ocean, occurs where stresses are concentrated by restricted geometric and dynamic conditions resulting in unusually high stress levels (several hundreds MPa). On continents, where geophysical data suggest that lithosphere folding may have occurred during the development of sedimentary basins in northern Canada and central Australia, buckling stresses can have magnitudes that are of a similar order to those thought typically to occur in the lithosphere (several tens MPa). Folding of continental lithosphere, although more difficult to document, is likely to be an important mechanism of large-scale continental deformation. In particular, thinned continental lithosphere, with a history of superposition of thick sedimentary successions, might be the preferential locus of such folding. Lithosphere folding could also be a controlling factor in the near-surface deformation of rifted basins as reflected by tectonic basin inversion.

Crustal-scale pop-up structure in cratonic lithosphere: DOBRE deep seismic reflection study of the Donbas fold belt, Ukraine

The DOBRE project investigated the interplay of geologic and geodynamic processes that controlled the evolution of the Donbas fold belt, Ukraine, as an example of an inverted intracratonic rift basin. A deep seismic reflection profile provides an excellent image of the structure of the Donbas fold belt, which is the uplifted and compressionally deformed part of the late Paleozoic Pripyat-Dniepr-Donets basin. Both the effects of rifting and those of later structural inversion are recognized in the seismic and geologic data. The interpretation of the reflection data shows that the inversion of the Donbas fold belt occurred at the crustal scale as a mega-pop-up, which involved a major detachment fault through the entire crust and an associated back thrust. The DOBREflection image provides a simple concept of intracratortic basin inversion, the crustal pop-up being uplifted and internally deformed. The association of such a structure with inverted intracratonic basins such as the Donbas fold belt implies brittle deformation of relatively cold crust.

The Mesozoic-Cenozoic tectonic evolution of the Greater Caucasus

Abstract The Greater Caucasus (GC) fold-and-thrust belt lies on the southern deformed edge of the Scythian Platform (SP) and results from the Cenozoic structural inversion of a deep marine Mesozoic basin in response to the northward displacement of the Transcaucasus (lying south of the GC) subsequent to the Arabia-Eurasia collision. A review of existing and newly acquired data has allowed a reconstruction of the GC history through the Mesozoic and Cenozoic eras. In Permo(?)-Triassic times, rifting developed along at least the northern part of the belt. Structural inversion of the basin occurred during the Late Triassic corresponding to the Eo-Cimmerian orogeny, documented SE of the GC and probably linked to the accretion of what are now Iranian terranes along the continental margin. Renewed development of extensional basin formation in the area of the present-day GC began in Sinemurian-Pliensbachian times with rift activity encompassing the Mid-Jurassic. Rifting led to extreme thinning of the underlying continental crust by the Aale-nian and concomitant extrusion of mid-ocean ridge basalt lavas. A Bathonian unconformity is observed on both sides of the basin and may either correspond to the end of active rifting and the onset of post-rift basin development or be the record of collision further south along the former Mesozoic active margin. The post-rift phase began with deposition of Late Jurassic platform-type sediments onto the margins and a flysch-like unit in its deeper part, which has transgressed the basin during the Cretaceous and Early Cenozoic. An initial phase of shortening occurred in the Late Eocene under a NE-SW compressional stress regime. A second shortening event that began in the Mid-Miocene (Sarmatian), accompanied by significant uplift of the belt, continues at present. It is related to the final collision of Arabia with Eurasia and led to the development of the present-day south-vergent GC fold-and-thrust belt. Some north-vergent retro-thrusts are present in the western GC and a few more in the eastern GC, where a fan-shaped belt can be observed. The mechanisms responsible for the large-scale structure of the belt remain a matter of debate because the deep crustal structure of the GC is not well known. Some (mainly Russian) geoscientists have argued that the GC is an inverted basin squeezed between deep (near)-vertical faults representing the boundaries between the GC and the SP to the north and the GC and the Transcaucasus to the south. Another model, supported in part by the distribution of earthquake hypocentres, proposes the existence of south-vergent thrusts flattening at depth, along which the Transcaucasus plunges beneath the GC and the SP. In this model, a thick-skinned mode of deformation prevailed in the central part of the GC whereas the western and eastern parts display the attributes of thin-skinned fold-and-thrust belts, although, in general, the two styles of deformation coexist along the belt. The present-day high elevation observed only in the central part of the belt would have resulted from the delamination of a lithospheric root.

The Donbas Foldbelt: its relationships with the uninverted Donets segment of the Dniepr–Donets Basin, Ukraine

Abstract The Donbas Foldbelt (DF) is a thrust-faulted and folded, inverted, segment of the Dniepr–Donets Basin (DDB). It lies between the Scythian and East European platforms north of the Black Sea and is conventionally thought to have formed in the Early Permian as part of the late Palaeozoic Hercynian–Uralian orogenic frame of cratonic Europe. The DDB itself formed as a result of Late Devonian intracratonic rifting and contains a thick, up to 22 km, late Palaeozoic and Mesozoic sedimentary succession. The deformed strata preserved at the erosional surface of the DF are mainly Carboniferous. The Donets segment of the DDB displays mild inversion structures that are transitional to those in the DF. Recently acquired regional seismic reflection data in the Donets part of the DDB allow these structures to be imaged in the subsurface for the first time. A widespread Early Permian unconformity occurring throughout the DDB is much more pronounced on its southern margin than on its northern one and appears to be related to profound uplift of the southern flank of the basin and the neighbouring Ukrainian Shield rather than to crustal shortening. Faults associated with the Early Permian unconformity are extensional in style. Gentle folds and other local structures in the southern flank and axis of the Donets segment are mainly located in areas associated with Early Permian salt movements. No significant development of compressional structures of Early Permian age can be seen in the seismic data where it is available. In contrast, there is ample evidence of latest Cretaceous–earliest Tertiary compressional deformation in the DDB, including the subsurface expression of structures along strike of the main folds of the DF. A review of geological evidence in the DF also indicates that the main period of shortening that can be definitely constrained in time is latest Cretaceous–earliest Tertiary. The eastern part of the DF records intense Late Triassic-aged (Cimmerian) deformation but none of clearly Permian age. It is tentatively concluded that the main phases of (trans)compressional tectonics forming the DF were Cimmerian and, especially, Alpine rather than Hercynian–Uralian.

Structural features and evolution of the Dniepr-Donets Basin, Ukraine, from regional seismic reflection profiles

Abstract A set of regional multichannel seismic reflection profiles crossing the Dniepr-Donets (DD) Basin in Ukraine, 100–200 km long and with a spacing of 10–15 km along strike illuminates the structure of the whole sedimentary cover. The rift, filled with pre- and syn-rift Devonian sequences and overlain with post-rift Carboniferous, Permian, Mesozoic and Palaeogene fill, is clearly seen on all sections. A ‘basement’ layer, previously identified as a Riphean rift succession by refraction seismic studies constitutes, in the main, strata of the Devonian and Early Carboniferous successions. The structure of the rift is characterised by large-scale rotational fault blocks and half-grabens, carried by basin-parallel to normal marginal faults or by major horst blocks. The total thickness of the Devonian and younger succession is up to 19 km in the study area. Post-rift Carboniferous and Mesozoic sediments cover the rift flanks and increase in thickness towards the rift axis and to the southeast. The cumulative thickness of the Carboniferous succession alone reaches 11 km, with the maximum depth of their base at about 15 km. Major post-rift tectonic events took place at the end of early Visean, in the middle of Serpukhovian, in Permian times, and between the Mesozoic and Cenozoic. Tectonic reactivations during Carboniferous and Early Permian times were most pronounced in the southeastern part of the DD Basin, while they were least pronounced in the northwestern part of the basin merging with the Pripyat Trough where they are barely observed. The most active development of salt structures and stocks corresponds with the periods of tectonic reactivation.

Subsidence analysis and modelling of the Roer Valley Graben (SE Netherlands)

Abstract The Roer Valley Graben (southeastern Netherlands) forms the most northern branch of the European Cenozoic Rift System. Sedimentary backstripping of exploration wells provides control on vertical tectonic motions during the complex post-Paleozoic geodynamic evolution of the area. The Roer Valley Graben evolved by repeated reactivation of Permo-Carboniferous fracture systems. At least three episodes of extension-related subsidence are recorded. These occurred during the Late Permian-Early Triassic (> 255-240 Ma), the Middle Jurassic (168-165 Ma) and the late Cenozoic (36-0 Ma). A Late Jurassic-Early Cretaceous stretching phase is weakly inferred from incomplete data. Crustal stretching values calculated from the tectonic subsidence curves can account for the reflection seismic controlled crustal configuration of the Roer Valley Graben area. Results demonstrate a close relationship between the Roer Valley Graben and the basins in the North Sea during the Mesozoic. In contrast, the Cenozoic evolution of the Roer Valley Graben is independent from that of the North Sea and is related to the development of the Rhine Graben.