Abstract The Istria ‘Depression’ or sub-basin of offshore Romania lies at the intersection of the trans-European Tornquist–Teisseyre ‘Zone’ and the Black Sea back-arc basin, just outboard of the East Carpathian orogenic welt. Its Late Mesozoic–Cenozoic succession records an extraordinary polyphase history of subsidence and sedimentation, interrupted by several quite spectacular second-/third-order erosional unconformities, reflecting the interplay between these tectonic domains. The unconformities divide the succession into a number of stratigraphic sequences. The sub-basin first developed as a transtensional rift in the Triassic–Early Jurassic, evolving into a narrow oceanized trough in the later Jurassic. This was tilted west during the Early Cretaceous, and the residual Late Jurassic topography was filled and buried by a west-facing clastic–evaporite wedge. Following Late Aptian–Albian(?) rifting, post-rift subsidence and spreading in the Western Black Sea imposed a strong easterly tilt, encouraging the partial evacuation of its Early Cretaceous sedimentary fill by gravity-driven mass wastage. The incised valley topography was subsequently infilled and buried during the later Cretaceous and Early Cenozoic. During the mid-Late Cenozoic, the Black Sea Basin experienced intermittent periods of partial to complete isolation from the world ocean and significant base-level drawdown. The first major sea-level fall occurred in the Eocene when the Istria ‘Depression’ was deeply incised, to be healed by Oligocene shales during the subsequent rise. Yet another period of drawdown and exposure occurred in the mid-Miocene, with extensive shelf-margin mass wastage and erosion, followed by re-flooding and deposition of a transgressive backstepping sequence in the middle-late Miocene. Messinian drawdown in the Mediterranean caused a further period of isolation and falling base level. The shelf margin was again exposed, and experienced widespread mass wastage and slumping. Rising sea level eroded the earlier slumped sequence and the margin was healed by a lowstand prograding wedge in the late Miocene–early Pliocene. This was followed by shelf sedimentation in the Plio-Pleistocene periodically interrupted by canyon-incision events, testifying to continued climatically or tectonically imposed base-level fluctuations. Several direct and indirect tectonic factors were responsible for valley/canyon incision within the Istria Depression and erosion of the Romanian Black Sea shelf margin. These include: (1) the local structural framework; (2) direct tectonic uplift and tilting; and (3) more indirect tectonically imposed isolation encouraging significant base-level falls.

ABSTRACT For the first time, modern seismic reflection data along with gravity and magnetic data were used to image the structure of a fold-and-thrust belt overlying the SW margin of the East European craton in SE Poland. These data demonstrate that the Variscan orogen extends eastward much farther than previously believed and terminates against the East European craton basement slope. The structural setting of this newly documented eastern extension of the Variscan fold-and-thrust belt in SE Poland is comparable to that of the Alleghanian orogen emplaced on the margin of the North American craton. Variscan deformation documented in SE Poland is more intense than anywhere else beneath the Permian–Mesozoic German-Polish Basin east of the Harz Mountains, probably because of buttressing by the relatively shallow basement of the East European craton. Our study focused on two regional tectonic units: (1) the Radom-Kraśnik block, a NW-SE–elongated structural high where early Paleozoic to Devonian strata subcrop beneath the Permian–Mesozoic cover, and (2) the Lublin Basin, a major Paleozoic sedimentary basin developed above the SW slope of the East European craton. The seismic data image the Radom-Kraśnik block as a thin-skinned fold-and-thrust belt with a 10–12-km-thick pile of Ediacaran (?) to Devonian sediments tectonically emplaced on the margin of the East European craton. These sediments are involved in a NNE-vergent stack of thrust units striking oblique to the East European craton margin slope. Individual thrusts branch off from a basal detachment that is located in the basal part of Ediacaran sediments unconformably overlying the East European craton crystalline basement. The frontal part of the Radom-Kraśnik fold-and-thrust belt is a triangle zone related to the jump of the basal detachment from the intra-Ediacaran position to the base of the Silurian shales. The base-Silurian detachment continues under the gently folded Lublin Basin and emerges along the Kock fault zone, which is a thin-skinned ramp placed over a basement step at depth. The Kock fault zone could be considered an analogue to the so-called mushwad structures described within the frontal part of the Appalachians.

The Bryansk-Kursk-Voronezh intracontinental collisional orogen was formed by the juvenile Middle Paleoproterozoic assemblages in combination with Early Paleoproterozoic complexes and areas of reworked Archean crust. The crucial events fall in the time interval 2.1–2.0 Ga. The orogen includes a sequence of second-order, near-meridional orogens, which, in turn, consist of tectonic belts. From the west eastward, these are the Krivoi Rog–Bryansk orogen, the orogen of the Kursk magnetic anomaly (KMA), and the East Voronezh orogen. The latter may be regarded as an axial (central) structural element, which marks a transient rupture of the continental crust. The north to south extent of the orogen along the strike of the second-order orogens reaches 900 km long and ca. 900 km in width. The Krivoi Rog–Bryansk orogen consists of two groups of tectonic belts. Its western region is composed of tectonic nappes pertaining to the Kulazhino gneiss and the Bryansk granulite-gneiss belts. The eastern region, including the Krupetsk-Znamensk belt and the Meshchevsk system of tectonic sheets, with significant role of intensely deformed banded iron formations (BIF) of the Kursk and Oskol groups, is also the western imbricated margin of the Archean Kursk craton. The volcanic-sedimentary BIF belts play the main role in the structure of the KMA orogen. The second important component is composed of subvolcanic and plutonic mafic-ultramafic, felsic, and alkaline rock complexes. Granite-gneiss domes are related to the final stage of the orogen evolution. The East Voronezh orogen consists of the Lipetsk-Losevka volcanic-plutonic belt and the Vorontsovka imbricated thrust belt, separated by the Losevka-Mamon suture. The evolution of the Bryansk-Kursk-Voronezh orogen as a whole includes: onset of rifting within the Archean Kursk craton (2.59–2.53 Ga); terrigenous and chemical sedimentation including BIF and volcanic activity (2.5–2.10 Ga), periodically accompanied by emplacement of intrusions (2.6–2.5 Ga and 2.1–2.05 Ga); metamorphism under granulite-facies conditions in the Kulazhino and Bryansk belts at 2.13 Ga and in the Vorontsovka belt at 2.10 Ga; intrusive magmatism in the Vorontsovka belt generally synchronous with high-temperature metamorphism; suprasubduction magmatism and formation of the Lipetsk-Losevka volcanic-plutonic belt 2.10–2.05 Ga ago; the collision-related reverse and thrust faulting and folding in the BIF belts accompanied by westward and south-westward overturning; rheomorphism and formation of granite-gneiss and granite-migmatite domes deforming the fold-nappe assemblages in the BIF belts of KMA; and the tectonothermal activity of postcollisional and anorogenic stages (2.07–1.9 Ga) expressed by emplacement of alkaline ultramafic and gabbro-syenite intrusions in the Lipetsk-Losevka belt and its hinterland. The most active phase of tectonic evolution spanned the time interval 2.1–2.0 Ga, which was no less than 100 Ma long and coincided with the active stage of extension, sedimentation, and high-temperature metamorphism in the northern part of the East European craton.

The earliest events in the geological history of the Early Precambrian crust of the East European craton (3.5–2.93 Ga) resulted in the emergence of spatially separate and genetically independent areas of continental crust (continental embryos), the dimensions of which rarely exceeded a few hundred kilometers across. The period between 3.05 Ga and ca. 2.75 Ga was a time of mainly plate-tectonic development: origin, evolution, and accretion of ancient island-arc systems, and collision of microcontinents. The Vedlozero-Segozero and Sumozero-Kenozero systems of greenstone belts, Tipasjärvi-Kuhmo-Suomussalmi and Central Belomorian greenstone belts in Karelia, and the Kolmozero-Voronya greenstone belt in the Kola Peninsula are interpreted as accretionary systems transformed in collisional orogens. The Belomorian eclogite province is structurally linked with the Central Belomorian greenstone (suture) belt. The Kovdozero granite-greenstone terrane is formed by granitoids and gneisses hosting metasediments and metavolcanics of several greenstone belts, which belonged to the Parandovo-Tiksheozero island arc. The amalgamation of the continental domains that made up the bulk of the Archean crust in the growing East European composite craton took from 2.82 to 2.66 Ga, but the main events had terminated by 2.75 Ga. During the period from 2.79–2.55 Ga, specific areas of intracontinental thermal and tectonic activity (hot regions) developed in the inner portions of the recently formed continent: Karelian-Belomorian and Kola areas in the eastern Fennoscandian Shield and extensive Volgo-Uralia granulite-gneiss area in the eastern part of the East European craton. These processes marked a principally new evolutionary episode in the Early Precambrian history of the East European craton. Widespread high-temperature magmatic and metamorphic processes and the development of synformal structures and linear sedimentary basins testify to an anorogenic extensional environment and a significant influx of heat to the crust, i.e., a significant event of mantle-plume type. During the Paleoproterozoic (2.53–1.87 Ga), a number of intracontinental collisional orogens were produced within the East European craton. The largest of them are the Lapland–Mid-Russia–South Baltia intracontinental orogen and the Svecofennian accretionary orogen. The Lapland–Mid-Russia–South Baltia orogen surrounds the Karelian craton as a wide arc, separating it from Volgo-Uralia and Sarmatia. The orogen extends for more than 3000 km; its width in the northern and central segments is 400–700 km and increases to 1000 km in the southwest. The Lapland sector of the orogen is characterized by spatial distribution of tectonic belts composed of low-grade metavolcanic-metasedimentary rocks and belts of high-grade metamorphic rocks, including granulite-gneiss complexes. The former are localized along the orogen boundaries; in turn, the axial zone of the orogen is mainly formed by alternation of low-angle tectonic sheets varying in thickness from a few to 20–25 km: sheets composed of Paleoproterozoic granulite-gneiss complexes with a predominance of metamorphosed juvenile intrusive and volcanic bodies and sedimentary rocks alternate with the sheets of Archean granite-greenstone and amphibolite-gneiss complexes. The Paleoproterozoic evolution of the Kola-Karelia continent and, accordingly, the Lapland–Mid-Russia–South Baltia orogen, is subdivided into four episodes: (1) ca. 2.53–2.3 Ga: failed rifting of the Archean continent; (2) 2.3–2.1 Ga: quiescent within-plate activity and diffuse rifting that can be regarded as “failed attempts” to break the supercontinent; (3) 2.1–1.95 Ga: rifting of the Kola-Karelia continent; and (4) 1.95–1.87 Ga: origin of the intercontinental collisional orogens. The Paleoproterozoic pulse of tectonic activity, which transformed the Neoarchean Kola-Karelia continent, continued for more than 600 m.y. Globally speaking, Early Paleoproterozoic magmatic and thermal activities were largely constrained within the ancient continent that then included North America and most of the eastern European continent, including the Fennoscandian Shield (Lauroscandia). Analysis of the extensive data leads us to distinguish a new type of tectonic structure: the intracontinental oval orogen formed in the inner portions of continents under the effect of large mantle plumes. It is an oval-shaped tectonic ensemble of regional rank with diameters from 600–1000 to 2500–3000 km, of which at least a part is characterized by concentric structure and metamorphic zonation or which contains bowl-shaped crustal structures. Intracontinental orogens contain granulite-gneiss complexes, derivatives of juvenile (though crust-contaminated) mafic magmas (gabbro-anorthosites and layered mafic-ultramafics), intrusions of “dry” high- temperature, within-plate–type granites, enderbites, and charnockites, and low-grade sedimentary-volcanic belts. The oval or oval-concentric structure excludes the possibility that intracontinental orogens originated as a result of processes at convergent plate boundaries. Their size and morphology and the evidence of a vast influx of mantle heat make intracontinental orogens comparable to oceanic plateaus and large igneous provinces on the continents. The fundamental changes in Earth's geological evolution that occurred at the Mesoarchean-Neoarchean boundary (ca. 2.75 Ga) can be related to the transition from Archean “microplate tectonics” to Paleoproterozoic “supercontinent tectonics” (or “microocean tectonics,” with regard to the limited size of the Red Sea–type oceans that were formed within the partly fractured supercontinent). The origin of Earth's first supercontinent, a landmass covering much of Earth's surface, by 2.80–2.76 Ga, should have played an extremely important role in restyling the system of convection cells in the underlying mantle. The style of tectonic processes and the geodynamic environment of plate tectonics in the Neoarchean–Paleoproterozoic differ from those in both the Archean and the Phanerozoic: the Archean tectonics of multiple “miniplates” was much more similar to Phanerozoic plate tectonics than to the Neoarchean–Paleoproterozoic “tectonics of supercontinents.”

Abstract We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies. Supplementary material: Sensitivity of backstripping results to flexural rigidity is available at http://www.geolsoc.org.uk/SUP18778 .