Showing papers in "Geological Society, London, Special Publications in 2000"

The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution

Abstract: Abstract The mid-European segment of the Variscides is a tectonic collage consisting of (from north to south): Avalonia, a Silurian-early Devonian magmatic arc, members of the Armorican Terrane Assemblage (ATA: Franconia, Saxo-Thuringia, Bohemia) and Moldanubia (another member of the ATA or part of N Gondwana?). The evolution on the northern flank of the Variscides is complex. Narrowing of the Rheic Ocean between Avalonia and the ATA occurred during the late Ordovician through early Emsian, and was accompanied by formation of an oceanic island arc. By the early Emsian, the passive margin of Avalonia, the island arc and some northern part of the ATA were closely juxtaposed, but there is no tectonometamorphic evidence of collision. Renewed extension in late Emsian time created the narrow Rheno-Hercynian Ocean whose trace is preserved in South Cornwall and at the southern margins of the Rhenish Massif and Harz Mts. Opening of this ‘successor ocean’ to the Rheic left Armorican fragments stranded on the northern shore. These were later carried at the base of thrust sheets over the Avalonian foreland. Closure of the Rheno-Hercynian Ocean in earliest Carboniferous time was followed by deformation of the foreland sequences during the late lower Carboniferous to Westphalian. Closure of narrow oceanic realms on both sides of Bohemia occurred during the mid- and late Devonian by bilateral subduction under the Bohemian microplate. In both these belts (Saxo-Thuringian, Moldanubian), continental lithosphere was subducted to asthenospheric depths, and later partially obducted. Collisional deformation and metamorphism were active from the late Devonian to the late lower Carboniferous in a regime of dextral transpression. The orthogonal component of intra-continental shortening produced an anti-parallel pair of lithospheric mantle slabs which probably joined under the zone of structural parting and became detached. This allowed the ascent of asthenospheric material, with important thermal and rheological consequences. The strike slip displacements were probably in the order of hundreds of kilometres, since they have excised significant palaeogeographic elements.

When did the western Anatolian grabens begin to develop

Abstract: Abstract To solve a long-lasting controversy on the timing and mechanism of generation of the western Anatolian graben system, new data have been collected from a mapping project in western Anatolia, which reveal that initially north-south trending graben basins were formed under an east-west extensional regime during Early Miocene times. The extensional openings associated with approximately north-south trending oblique slip faults provided access for calc-alkaline, hybrid magmas to reach the surface. A north-south extensional regime began during Late Miocene time. During this period a major breakaway fault was formed. Part of the lower plate was uplifted and cropped out later in the Bozdağ, Horst, and above the upper plate approximately north-south trending cross-grabens were developed. Along these fault systems, alkaline basalt lavas were extruded. The north-south extension was interrupted at the end of Late Miocene or Early Pliocene times, as evidenced by a regional horizontal erosional surface which developed across Neogene rocks, including Upper Miocene-Lower Pliocene strata. This erosion nearly obliterated the previously formed topographic irregularities, including the Bozdağ elevation. Later, the erosional surface was disrupted and the structures which controlled development of the Lower-Upper Miocene rocks were cut by approximately east-west trending normal faults formed by rejuvenated north-south extension. This has led to development of the present-day east-west trending grabens during Plio-Quaternary time.

The falling stage systems tract: recognition and importance in sequence stratigraphic analysis

Abstract: Until recently, sequence stratigraphic models have attributed systems tracts to periods of relative sea-level rise, highstand and lowstand. Recognition of a discrete phase of deposition during relative sea-level fall has been limited to a few studies, both in clastic and carbonate systems. Our work in siliciclastic ramp settings suggests that deposition during relative sea-level fall produces a distinctive falling stage systems tract (FSST), and that this is the logical counterpart to the transgressive systems tract. The FSST lies above and basinward of the highstand systems tract, and is overlain by the lowstand systems tract. The FSST is characterized by stratal offlap, although this is likely to be difficult or imposs- ible to recognize because of subsequent subaerial or transgressive ravinement erosion. The most practical diagnostic criteria of the FSST is the presence of erosive-based shoreface sandbodies in nearshore areas. The erosion results from wave scouring during relative sea- level fall, and the stratigraphically lowest surface defines the base of the FSST. Further off- shore, shoaling-upward successions may be abruptly capped by gutter casts filled with HCS sandstone, reflecting increased wave scour on the shelf during both FSST and LST time. The top of the FSST is defined by a subaerial surface of erosion which corresponds to the sequence boundary. This surface becomes a correlative submarine conformity seaward of the shoreline, where it forms the base of the lowstand systems tract. Differentiation of the FSST and LST may be difficult, but the LST is expected to contain gradationally-based shoreface successions because it was deposited when relative sea level was rising. Intern- ally, the FSST may be an undifferentiated body of sediment or it may be punctuated by internal regressive surfaces of marine erosion and ravinement surfaces which record higher- frequency sea-level falls and rises superimposed on a lower-frequency sea-level fall. The corresponding higher-order sequences are the building blocks of lower-order sequences. The addition of a falling stage systems tract results in a significant reduction in the pro- portion of strata within a sequence that are assigned to the classical highstand and lowstand systems tracts. Many outcrop and subsurface cross-sections use an overlying ravinement, or maximum flooding surface as datum. Those surfaces might be flat, but they are not horizontal. Both dip seaward at slopes that generally are steeper than the fluvial system responsible for cre- ating the sequence boundary. When a section is restored to such a datum, the falling stage systems tract will appear to record stratigraphic climb, whereas in fact it does not.

The tectonic evolution of the Norwegian Sea Continental Margin with emphasis on the Vøring and Møre Basins

Abstract: Abstract The Norwegian Sea continental margin is dominated by two major basins with a very thick Cretaceous basin fill: the Vøring and Møre Basins. The basins are flanked by the uplifted mainland and the Cretaceous Trøndelag Platform to the east and by the Møre and Vøring Marginal Highs capped by Eocene lavas to the west. The tectonic development of the area is controlled by two structural trends: NE-SW and NW-SE. The area has been tectonically active from Carboniferous to Late Pliocene time with the main tectonic phases in Late Palaeozoic, late Mid-Jurassic-Early Cretaceous and Late Cretaceous-Early Tertiary time. The general tectonic development comprised a long period of extension and rifting that ended in Early Eocene time by continental separation, major volcanism and subsequent sea-floor spreading in the Norwegian-Greenland Sea. In Carboniferous to Early Cretaceous time the extensional tectonics were related to within-plate continental rifting. The tectonics of the Late Cretaceous and the Tertiary periods were controlled by the relative movements along plate boundaries. The overall NE-SW structural grain is constituted by faults and basin axes that probably originated in Late Palaeozoic time and were active during all subsequent tectonic phases. The transverse NW-SE trend is expressed as major lineaments that probably reflect the old, Precambrian grain of the basement. These lineaments, two of which are the continuation into the continental crust of major oceanic fracture zones, controlled the tectonic activity throughout Cretaceous and Tertiary time and constitute the boundaries between the major structural provinces of the area. The differentiation into the Cretaceous basins and the bounding platforms and marginal highs started by the late Mid-Jurassic-Early Cretaceous extensional phase. The subsequent Cretaceous subsidence history, where the basin flanks formed by flexuring rather than faulting, resulted in an exceptionally thick basin fill. In the Vøring Basin the Cretaceous development comprised an early thermal subsidence phase and a post-Cenomanian phase of tectonically driven subsidence involving intermittent phases of normal faulting and compression and folding. The Vøring Basin was tectonically active also during Tertiary time with the main phases of strike-slip-compression coinciding with the Alpine orogenies in Late Eocene and Mid-Miocene time. Within the Vøring Basin there is evidence of the formation of a fossil opal A-opal-CT transition and extensive regional marine erosion in Mid-Miocene and Late Pliocene times. In contrast, the Møre Basin was generally tectonically quiet throughout the Cretaceous and Tertiary periods, experiencing mainly continuous subsidence.

Tectonics and Magmatism in Turkey and the Surrounding Area

Abstract: This volume contains 23 papers from a range of international contributors, describing recent research into the tectonics and magmatism of Turkey and its surroundings. This region is sited at the collision zone between Eurasia and Afro-Arabia and, as such, provides an extraordinarily complete and well-exposed record of the staged tectonic evolution of this sector of the Alpine-Himalayan orogen. The geological history of this area involves separation of continental fragments from the margin of Gondwana, their migration across the Tethyan oceans, the subsequent closure of these oceans and, finally, the development of the neotectonic regime, which continues to evolve to the present day. Such a comprehensive record is relevant to the understanding of collisional zones worldwide. The volume is divided into five sections: Tethyan evolution, Neotethyan ophiolites, post-Tethyan basin evolution, neotectonics and igneous activity. The first two sections deal with Tethyan oceans, whose growth and subsequent closure dominated the geodynamic framework in the Mesozoic and Cenozoic. The subsequent sections deal with more recent geological developments from the Balkan Peninsula in the west to the Transcaucasus in the east that followed consumption of the Tethyan oceans. There is a broad mix of papers throughout the volume: wide-ranging review papers on ocean development and extensional tectonics are followed by detailed descriptions of petrology and geochemistry and geographically focused studies on basin evolution, specific aspects of extensional and strike-slip tectonics and discussions of the relationship of magmatic activity to the tectonic development of the area. Tectonics and Magmatism in Turkey and the Surrounding Area presents up-to-date results and ideas from a large number of international contributors on a wide range of current research activity in this region. It is essential reading for all geoscientists with an interest in both academic and applied aspects of eastern Mediterranean geology.

Orogenic processes: quantification and modelling in the Variscan belt

Abstract: Research into the orogenic processes that shaped the continental crust of Europe has a long-standing tradition. Why the need to quantify and model? It is not just satisfactory to identify subduction zones, accretionary prisms, island arcs, extensional collapse and other standard items of the geodynamic menu. Such interpretations need to be quantified: extent and composition of subducted crust, angle and speed of subduction, amount and composition olmelts produced, heat sources for metamorphism. All such interpretations have to conform to first principles, and also to stand the test of quantitative balancing – a concept first developed for the conservation of length or volume in tectonic cross sections. Also in other fields, the correlation of causes and effects and the internal consistency of dynamic models requires a numerical approach. The present volume combines review articles with reports on recent progress in an attempt to address these aims. There is a foldout map of the region, which locates the main areas of outcrop and tectono-stratigraphic units, and a reassesment of the Palaeozoic time scale permits correlation of tectonic, metamorphic and magmatic events with the sedimentary record of the upper crust.

NE Atlantic continental rifting and volcanic margin formation

Abstract: The NE Atlantic rift system, which has been under extensional deformation since the Caledonian orogeny, experienced a series of rift episodes in Late Paleozoic to Early Cenozoic times (Fig. 1). Late Paleozoic rifting is poorly constrained, particularly with respect to timing. However, rifted basin geometries, inferred to be of this age, are observed at depth in seismic data from the flanks of the younger rift structures.

Mesozoic-Tertiary Tectonic-Sedimentary Evolution of a South Tethyan Oceanic Basin and its Margins in Southern Turkey

Abstract: This paper focuses on the Mesozoic-Tertiary tectonic evolution of southern Turkey and offshore areas of the easternmost Mediterranean. The area is discussed and interpreted utilizing three segments from west to east. In the far west, the Lycian Nappes represent emplaced remnants of mainly Mesozoic rift, passive margin and oceanic units that formed within a northerly strand of the Mesozoic (i.e. Neotethyan) ocean. Further east, the Hoyran-Beyşehir-Hadim Nappes, likewise encompass sedimentary and igneous units that formed within a northerly Neotethyan oceanic basin, although lithologies, structure and timing of emplacement differ from the Lycian Nappes. Further east (Adana region), ophiolites and ophiolitic mélange also formed in a northerly oceanic basin and were thrust southwards over the regionally extensive Tauride carbonate platform initially in latest Cretaceous time (e.g. Pozanti-Karsanti Ophiolite). By contrast, further south the regionally important Antalya Complex records northerly areas of a separate, contrasting southerly Neotethyan oceanic basin. This comprised a mosaic of carbonate platforms and interconnecting seaways, similar to the Caribbean region today. In particular, an ocean strand separated Tauride carbonate platforms to the west (Bey Dağlari) and east (e.g. Akseki Platform) within the Isparta Angle area. In the centre of southern coastal Turkey, the metamorphic Alanya Massif is interpreted as a Triassic rift basin bordered by two small platform units that was located along the northern margin of the southerly Neotethys which collapsed in latest Cretaceous and was finally emplaced in Early Tertiary time. Remnants of the southerly Neotethyan oceanic basin remain today in the non-emplaced continental margin of the Levant and North Africa, and neighbouring seafloor areas (e.g. Levant and Herodotus Basins). In southern Turkey, emplaced Neotethyan units are unconformably overlain by a complex of mainly Miocene basins. These largely reflect the effects of southward directed crustal loading as convergence of Africa and Eurasia continued, although the basins were also influenced by an inferred more southerly subduction zone (near Cyprus). Further east, in southeastern Turkey, ophiolites, ophiolitic mélange and continental margin units were emplaced southwards onto the Arabian Margin, a promontory of North Africa in latest Cretaceous time. The south Neotethyan basin’s north margin experienced northward subduction, accretion, arc volcanism and ophiolite emplacement in Late Cretaceous time. The intervening southerly Neotethyan oceanic basin remained partly open in the Early Tertiary, finally closing by diachronous collision in Eocene-Oligocene time, followed by further convergence and overthrusting in the Miocene. The Eocene later stages of convergence were marked by renewed arc volcanism and extensive subduction accretion (e.g. Maden Complex). In the west, subduction remained active in Late Oligocene-Early Miocene time giving rise to sedimentary mélanges (olistostromes) of the Misis-Andirin Mountains (Adana region) as an accretionary wedge. By the Miocene the subduction zone accommodating Africa-Eurasia convergence had been relocated to its present position south of Cyprus. Areas behind this subduction experienced crustal extension (e.g. Antalya and Adana-Cilicia Basins) from the Late Miocene onwards. After onset of westward ‘tectonic escape’ of the Turkish Plate in the Early Pliocene, southeastern Turkey was transected by the South Anatolian Transform Fault. Strike-slip was dissipated though the Kyrenia-Misis Lineament into Cyprus. Today, southeastern Turkey records a post-collisional setting, whereas areas to the west experience incipient collision of the African and Turkish Plates.

The evolutionary biology of the bivalvia

Abstract: Bivalves are key components of Recent marine and freshwater ecosystems and have been so for most of the Phanerozoic. Their rich and long fossil record, combined with their abundance and diversity in modern seas, has made bivalves the ideal subject of palaeobiological and evolutionary studies. Despite this, however, topics such as the early evolution of the class, relationships between various taxa and the life habits of some key extinct forms have remained remarkably unclear. In the last few years there has been enormous expansion in the range of techniques available to both palaeontologists and zoologists and key discoveries of new faunas which shed new light on the evolutionary biology of this important class. This volume integrates palaeontological and zoological approaches and sheds new light on the course of bivalve evolution. This series of 32 original papers tackles key issues including: up to date molecular phylogenies of major groups; new hard and soft tissue morphological cladistic analyses; reassessments of the early Palaeozoic radiation; important new observations on form and functional morphology; analyses of biogeography and biodiversity; novel (palaeo)ecological studies

Aspects of the stratal architecture of forced regressive deposits

Abstract: Abstract Forced regression refers to the process of seaward migration of a shoreline in direct response to relative sea-level fall. Recognition criteria for forced regressive deposits include: (1) presence of a significant zone of separation between successive shoreface deposits, (2) the presence of sharp-based shoreface/delta front deposits, (3) the presence of progressively shallower clinoforms going from proximal to distal, (4) the occurrence of long-distance regression, (5) the absence of fluvial and/or coastal plain/delta plain capping the proximal portion of regressive deposits, (6) the presence of a seaward-dipping upper bounding surface at the top of the regressive succession, (7) the presence of increased average sediment grain size in regressive deposits going from proximal to distal and (8) the presence of ‘foreshortened’ stratigraphic successions. The principal factors driving the stratal architecture of forced regressive deposits include: (1) the gradient of the sea floor progressively exposed by falling relative sea-level, (2) the ratio of the sediment flux to the rate of relative sea-level fall, (3) the ‘smoothness’ of relative sea-level fall, (4) the variability of sediment flux and (5) the changes of sedimentary process that occur as sea-level falls and progressively more of the shelf is subaerially exposed. Forced regressive deposits are grouped into attached v. detached, and smooth-topped v. stepped-topped. Attached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally in contact with each other. In contrast, detached deposits are defined as successive downstepped stratigraphic units whose shoreface/delta front deposits are generally not in contact with each other. Rather, in this instance a zone of sedimentary bypass exists. Stepped-top forced regressive deposits are characterized by a succession of horizontally topped though downstepping stratigraphic units. In contrast, smooth-topped forced regressive deposits are characterized by a seaward-dipping, albeit smooth, upper bounding surface. The bounding surfaces of forced regressive deposits commonly are expressed as a ravinement surface at the top and an unconformity to correlative conformity at the base.

From Cadomian subduction to Early Palaeozoic rifting: the evolution of Saxo-Thuringia at the margin of Gondwana in the light of single zircon geochronology and basin development (Central European Variscides, Germany)

Abstract: Abstract Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian-Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent-continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660–540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540–530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530–500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500–490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490–470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440–435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia.

Palaeomagnetism and Palaeozoic palaeogeography of Gondwana and European terranes

Abstract: Abstract Neoproterozoic to Late Palaeozoic times saw the break-up of the supercontinent Rodinia, and the subsequent construction of Pangaea. The intervening time period involved major redistribution of continents and continental fragments, and various palaeogeographical models have been proposed for this period. The principal differences between these models are with regard to the drift history of Gondwana, the timing of collision between northern Africa and Laurussia, and formation of Pangaea. Palaeomagnetic evidence provides basically two contrasting models for the Ordovician to Late Devonian apparent polar wander (APW) path for Gondwana involving either rapid north and southward movement of this continent, or gradual northward drift throughout Palaeozoic time. In contrast, palaeobiogeographical models suggest contact between Laurussia and Gondwana as early as mid-Devonian time with the continents basically remaining in this configuration until break-up of Pangaea in the Mesozoic era. This is in conflict, however, with most palaeomagnetic data, which demonstrate that in Late Devonian time, north Africa and the European margin of Laurussia were separated by an ocean of at least 3000 km width. This is also in agreement with the geological record of present-day southern Europe, which argues against any collision of northern Africa with Europe in Devonian time. With regard to formation of Laurussia, however, palaeobiogeographical and palaeomagnetic data are in excellent agreement that by mid-Devonian time the oceanic basins separating Baltica, Laurentia, Gondwana-derived Avalonia and the Armorican Terrane Assemblage (ATA) had all closed. Palaeomagnetic and geological data are also in agreement that the Palaeozoic basement rocks of the European Alpine realm formed an independent microplate, which was situated to the south of Laurussia. In Late Silurian times it was separated by an ocean of c. 1000 km, and by Late Devonian time was approaching the southern Laurussian margin. According to palaeomagnetic data, the northern margin of Gondwana was still further to the south in Late Devonian time, and according to the geological record in southern Europe, the main continent-continent collision of northern Africa with European Laurussia and closure of the intervening ocean occurred in Late Carboniferous times. Location of this suture is situated to the south of the Palaeozoic alpine units (e.g. the Greywacke zone, Carnic Alps, Sardinia and Sicily), but has been obscured by younger deformational events and cannot be precisely positioned. Assessing available evidence and as discussed in the text, it is proposed that the most likely scenario is that the northern margin of Gondwana drifted gradually northwards from Ordovician to Late Carboniferous times when it collided with Laurussia, resulting in formation of Pangaea.

Timing of Extension on the Büyük Menderes Graben, Western Turkey, and Its Tectonic Implications

Abstract: Abstract The Büyük Menderes Graben is one of the most prominent structures of western Anatolia (Turkey) and borders the Aegean. New structural and stratigraphic evidence demonstrates that the (?)Miocene fluvio-lacustrine, coal-bearing red clastic sediments exposed along the northern margin of the graben are northward back-tilted, locally folded and overlain unconformably by horizontal terraced Pliocene-Pleistocene sediments. Also, there is no evidence that these red clastics at the base of the Neogene sequence were deposited during neotectonic extension. It is suggested here that these sediments cannot be regarded as passive neotectonic graben-fill deposits. This new evidence further indicates that the age of the modern Büyük Menderes Graben is Pliocene, younger than previously considered (Early-Middle Miocene) and that initiation of north-south neotectonic extensional tectonics in the graben, and thus in western Anatolia, is unlikely to have resulted from orogenic collapse. The Pliocene estimate of the start of extension is in close agreement with the start of slip on the North Anatolian Fault Zone. The north-south extensional tectonics, and associated east-west faulting and basin formation, commenced during the Pliocene due to the effect of westward tectonic escape of the Anatolian block along the North and East Anatolian Faults. New mammal evidence also constrains the start of slip on the younger faults which bound the present-day graben floor to c. 1 Ma. The Büyük Menderes Graben has experienced a two-stage extension. An initial extension (latest Oligocene-Early Miocene) along initially moderately, steeply dipping normal faults was superseded by movement on steeper normal faults during the (?)Pliocene. The two phases of deformation appear to reflect significant changes in the tectonic setting of western Anatolia and are attributed to orogenic collapse followed by tectonic escape.

The myth of metabolic cold adaptation: oxygen consumption in stenothermal Antarctic bivalves

Abstract: Antarctic marine ectotherms are often described as only being capable of living in a restricted temperature range, i.e. they are stenothermal. However, few data exist demonstrating that for a given group this is the case. The Antarctic bivalve molluscs Laternula elliptica and Limopsis marionensis are similar to other Antarctic invertebrates and can only exist within a temperature window of 6–12°C. This is two to six times smaller than the range for temperate and tropical bivalves, thus demonstrating their stenothermal nature. The possibility of elevated metabolic rates of cold-water ectotherms has been a topic of debate over many years. Recently, the suggestion that metabolic rates must be elevated at low temperatures to overcome constraints has been supported by findings that mitochondrial contents of muscles in ectotherms are higher at low temperatures. Data, presented here for standard or routine metabolic rates of 41 species of bivalve mollusc from polar, temperate and tropical sites, indicate that oxygen consumption is not elevated at low temperatures. Indeed, analysis of Q10 coefficients between 0 and 25°C suggests that metabolic rates of polar species may be lower than would be expected by comparison with temperate bivalves

Micromorphological analyses of microfabrics and microstructures indicative of deformation processes in glacial sediments

Abstract: Abstract Various forms of sediment deformation can be detected within glacial sediments at the microscopic scale. Analyses of these forms leads to a preliminary classification of microfabrics and microstructures of brittle, ductile and polyphase modes of deformation in glacial sediments. With the development of a taxonomy of different microfabrics and microstructures these processes, once differentiated, permit insights into glacial sediments to a scale and level of detail hitherto unknown. Examples are presented that illustrate some of these different forms of deformation often within a single glacial sediment sample. This research suggests many of the past ideas with regard to details of glacial depositional processes, especially in terms of diamicton deposition and subsequent classifications, need to be re-evaluated.

The eastern termination of the Variscides: terrane correlation and kinematic evolution

Abstract: Abstract Analysis of tectonostratigraphic units in the West Sudetes reveals the same geological events as in the areas west of the Elbe Fault Zone: a late Proterozoic (Cadomian) orogenic event, Cambro-Ordovician to Devonian rift-drift, and late Devonian to early Carboniferous subduction-collision. There is no conclusive evidence of an Ordovician orogenic event. Tectonic units in the Sudetes are shown to be related to terranes defined in western parts of the Bohemian Massif. The Lausitz-Izera Block, the Orlica-Śnieżnik Unit and the Staré Město Belt represent easterly continuations of the Saxo-Thuringian Terrane. The Rudawy Janowickie Unit and the Sudetic Ophiolite contain fragments of the Saxo-Thuringian Ocean. The protoliths of the Görlitz-Kaczawa Unit, the South Karkonosze Unit, the Góry Sowie and the Kłodzko Units either belong to the Bohemian Terrane or else were welded onto it during mid-late Devonian metamorphism and deformation. Relicts of the Saxo-Thuringian Foreland Basin are marked by flysch with olistoliths in the Görlitz-Kaczawa Unit and in the Bardo Basin. The spatial array of terranes in and around the Bohemian Massif reveals a disrupted orocline, dissected by dextral transpression along the Moldanubian Thrust. This orocline was formed when central parts of the Variscan belt were accommodated in an embayment of the southern margin of the Old Red Continent.

Modelling western North Sea palaeogeographies and tidal changes during the Holocene

Abstract: Abstract Analysis of cores collected from Late Devensian (Weichselian) and Holocene sediments on the floor of the North Sea provides evidence of the transgression of freshwater environments during relative sea-level rise. Although many cores show truncated sequences, examples from the Dogger Bank, Well Bank and 5 km offshore of north Norfolk reveal transitional sequences and reliable indicators of past shoreline positions. Together with radiocarbon-dated sea-level index points collected from the Holocene sediments of the estuaries and coastal lowlands of eastern England these data enable the development and testing of models of the palaeogeographies of coastlines in the western North Sea and models of tidal range changes through the Holocene epoch. Geophysical models that incorporate ice-sheet reconstructions, earth rheology, eustasy, and glacio- and hydroisostasy provide predictions of sea-level relative to the present for the last 10 ka at 1-ka intervals. These predictions, added to a model of present-day bathymetry, produce palaeogeographic reconstructions for each time period. The palaeogeographic maps reveal the transgression of the North Sea continental shelf. Key stages include a western embayment off northeast England as early as 10 ka bp; the evolution of a large tidal embayment between eastern England and the Dogger Bank before 9 ka bp with connection to the English Channel prior to 8 ka bp; and Dogger Bank as an island at high tide by 7.5 ka bp and totally submerged by 6 ka bp. Analysis of core data shows that coastal and saltmarsh environments could adapt to rapid rates of sea-level rise and coastline retreat. After 6 ka bp the major changes in palaeogeography occurred inland of the present coast of eastern England. The palaeogeographic models provide the coastline positions and bathymetries for modelling tidal ranges at each 1-ka interval. A nested hierarchy of models, from the scale of the northeast Atlantic to the east coast of England, uses 26 tidal harmonics to reconstruct tidal regimes. Predictions consistently show tidal ranges smaller than present in the early Holocene, with only minor changes since 6 ka bp. Recalibration of previously available sea-level index points using the model results rather than present tidal-range parameters increases the difference between observations and predictions of relative sea-levels from the glacio-hydro-isostatic models and reinforces the need to search for better ice-sheet reconstructions.

Was the Late Triassic orogeny in Turkey caused by the collision of an oceanic plateau

Abstract: A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Niltifer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 x 105 km 3. The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Niliifer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Niltifer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Niltifer Plateau. An east-west trending belt of latest Triassic deformation and regional metamorphism extends for over 1100 km in northern Turkey. The Early Mesozoic deformation (but not the regional metamorphism) was known previously ($eng6r 1979; Bergougnan & Fourquin 1982) and was referred to as the Cimmeride deformation ($eng6r et al. 1984). The Cimmeride deformation was ascribed to the closure of the Palaeotethys ocean following the collision of a Cimmerian continental sliver with the southern margin of Laurasia ($eng6r 1979; Seng6r et al. 1984). Here, an alternative explanation, involving the collision and partial accretion of an oceanic plateau to the southern margin of Laurasia, is proposed for the origin of the latest Triassic deformation and metamorphism in northern Turkey. A tectonic map of Turkey and the surrounding region is shown in Fig. 1. During the Palaeozoic and Mesozoic, the various continental blocks that make up present-day Turkey were situated on the continental margins of the Tethys Ocean. The Pontides, which comprise the Strandja, istanbul and Sakarya Zones, show Laurasian stratigraphic affinities, while the Anatolide-Tauride Block and the Klr~ehir Massif are tectonically and stratigraphically related to Gondwana ($eng6r & Yllmaz 1981; Okay et aL 1996; Okay & Ttiystiz 2000). The istanbul Zone is a continental fragment, which was translated south from the Odessa Shelf with the Cretaceous opening of the oceanic West Black Sea Basin (Fig. 1; Okay et al. 1994). Its stratigraphy is similar to that of the Scythian and Moesian platforms, with a fully developed Palaeozoic sedimentary sequence unconformably overlain by Triassic and younger sedimentary rocks (Haas 1968; Dean et al. 1997; G6rtir et al. 1997). In the Istanbul Zone, a weak latest Triassic deformation is marked by an unconformity between the Norian siliciclastic turbidites and the overlying Upper Cretaceous carbonates. The Strandja Zone consists of a Late Hercynian metamorphic and granitic basement unconformably overlain by Lower Triassic-Middle Jurassic sedimentary rocks (Chatalov 1988; Okay et al. 1996). The Anatol ide-Tauride Block and the Klr~ehir Massif are also devoid of Triassic metamorphism, and of any significant Triassic deformation. Several well studied Lower Mesozoic stratigraphic sections in the Taurides, including those in the Bornova Flysch Zone (Erdo~an et al. 1990.). and in the central Taurides (Gutnic et al. 1979; Ozgti11997), show a continuous transition between Triassic and Jurassic with no evidence of an intervening deformation phase. The pre-Jurassic thrusting, described by Monod & Akay (1984) from a small locality in the central Taurides, is as yet of unknown significance. Late Triassic deformation and regional metamorphism in Turkey are predominantly found in the Sakarya Zone, which will form the main subject of this paper. From: BOZKURT, E., WINCHESTER, J. A. & PIPER, J. D. A. (eds) Tectonics and Magmatism in Turkey and the Surrounding Area. Geological Society, London, Special Publications, 173, 25-41.1-86239-064-9/00/$15.00 (C) The Geological Society of London 2000.

A Geotraverse Across Northwestern Turkey: Tectonic Units of the Central Sakarya Region and their Tectonic Evolution

Abstract: Abstract In the Central Sakarya area of Turkey there are two main Alpine continental units, separated by a south verging ophiolitic complex which represents the root zone of the İzmir-Ankara Suture Belt. The Central Sakarya Terrane in the north includes two ‘Variscan’ tectonic units in its basement. The Söğüt Metamorphic rocks represent a Variscan ensimatic arc complex and the Tepeköy Metamorphic rocks are characteristically a forearc-trench complex. The unconformably overlying Triassic Soğukkuyu Metamorphic rocks correspond to a part of the Karakaya Formation and are interpreted as a Triassic rift basin assemblage. These units are unconformably overlain by a transgressive sequence of Liassic-Late Cretaceous age that represents the northeastward deepening carbonate platform of the Sakarya Composite Terrane. The middle tectonic unit (the Central Sakarya Ophiolitic Complex) comprises blocks and slices of dismembered ophiolites, blueschists and basic volcanic rocks with uppermost Jurassic-Lower Cretaceous radiolarite-limestone interlayers. Geochemical data from basalt blocks suggest mid-ocean ridge basalt (MORB)- and suprasubduction-type tectonic settings within the Neotethyan İzmir-Ankara Ocean. The southern tectonic unit includes basal polyphase metamorphosed clastic rocks (Sömdiken Metamorphics), intruded by felsic and basic dykes and overlain by thick-bedded marbles. This assemblage is unconformably overlain by continental clastic rocks gradually giving way to thick-bedded recrystallized limestones, cherty limestones and pelagic limestones intercalated with radiolarites, and finally by a thick high pressure-low temperature (HP-LT) metamorphic synorogenic flysch sequence. This succession is identical to the passive continental margin sequences of the Tauride Platform. It is suggested that this passive margin was subducted during the Late Cretaceous in an intraoceanic subduction zone and affected by HP-LT metamorphism. The emplacement of the allochthonous oceanic assemblages and the collision with the Central Sakarya Terrane was complete by the end of the Cretaceous.

The fundamental Variscan problem: high-temperature metamorphism at different depths and high-pressure metamorphism at different temperatures

Abstract: Abstract The evolution of the crystalline internal zone of the European Variscides (i.e. Moldanubian and Saxo-Thuringian) is best understood within a framework of two distinct subduction stages. An early, pre-Late Devonian (older than 380 Ma), subduction stage is recorded in medium-temperature eclogites and blueschists derived from low-pressure basaltic and gabbroic protoliths now found as minor relics in amphibolite facies meta-ophiolite or gneiss-metabasite nappe complexes. A second subduction and exhumation event produced further nappe complexes containing different types of mantle peridotites, along with their enclosed pyroxenites and high-temperature eclogites, associated with large volumes of high-T-high-P (900–1000°C, 15–20 kbar) felsic granulites. Abundant geochronological evidence points to a Carboniferous age (c. 340 Ma) for the high-P-high-T metamorphism as well as an extremely rapid exhumation because the fault-bounded, granulite-peridotite-bearing tectonic units are also cut by late Variscan granitic plutons (315–325 Ma). The massive heat energy for the characteristic, and most widespread feature of the Variscan event, the low-P-high-T metamorphism (750–800°C, 4–6 kbar) and voluminous granitoid magmatism (325–305 Ma), comes from three sources. An internal heat component comes from imbrication of crust with upper-crustal radiogenic heat production potential in the region parallel to the subduction zone; an external mantle heat component is undoubtedly contributing to the transformation of crust taken to mantle depths (i.e. the granulites); and a heat component advected to the middle and lower crust seems inescapable if the hot granulite-peridotite complexes were exhumed and cooled as rapidly as petrological and geochronological evidence seems to suggest. Major mantle delamination and asthenospheric upwelling as a cause of heating in Early Carboniferous times is not supported by geochemical, geophysical or petrological-geochronological studies, although slab break-off probably did occur.

The Brunovistulian: Avalonian Precambrian sequence at the eastern end of the Central European Variscides?

Abstract: Abstract An outline is presented of the present state of research on the Precambrian evolution history of the Brunovistulian, a large (30 000 km2), mainly sediment covered Peri-Gondwana basement block at the eastern end of the Central European Variscides. On the basis of recent chemical, isotopic and geochronological data it is argued that the eastern half of the Brunovistulian (Slavkov Terrane) originated in an island-arc environment, documenting the rare case of Neoproterozoic crustal growth in central Europe. The western half of the Brunovistulian, the Thaya Terrane, includes more mature, recycled cratonic material and is considered to have been originally part of the Neoproterozoic Gondwana continent margin. A phase of regional metamorphism at c. 600 Ma, followed by extensive granitoid plutonism, probably marks the stage when the Slavkov Terrane was accreted to the Thaya Terrane by arc-continent collision. A belt of metabasites, which is intercalated between the two terranes, may represent relics of the incipient arc or a back-arc basin. A comparison of geochronological data shows that the timing of geological events recorded in the Brunovistulian does not correlate with the evolution history of the Cadomian crust in the Teplá-Barrandian zone and the Saxo-Thuringian belt. This supports the theory that the Brunovistulian is not part of Armorica but derived from a different sector of the Neoproterozoic Gondwana margin. A correlation with the Avalonian superterrane appears feasible.

Functional anatomy, chemosymbiosis and evolution of the Lucinidae

Abstract: Abstract All Lucinidae species studied so far possess sulphide-oxidizing, chemosymbiotic bacteria housed in bacteriocytes of gill filaments. The ecology, functional anatomy and evolution of the Lucinidae must be considered in relation to this symbiosis. The ctenidia have been extensively studied but other anatomical structures peculiar to lucinids have received much less attention. Reviewed are the morphological diversity of living lucinids, highlighting features of their anatomy including ctenidia, pallial apertures, anterior adductor mucsle, pallial blood vessel and mantle gills. The latter are much more complex than previously understood and are here redescribed. They comprise folded structures located near the anterior adductor muscle in Codakia, Phacoides and Lucina, and on the septum of Anodontia. These are interpreted as secondary respiratory surfaces, their location enabling the separation of the anterior inflow of oxygenated water from sulphide-containing water. The latter is released from the sediment by the probing activities of the highly extensible foot and is pumped over the gill through the pedal gape and perhaps also via the exhalant tube. The shell features of Ilionia from the Silurian Period suggests that the lucinid chemosymbiosis is an ancient association.

Suprasubduction Zone Origin of the Pozanti-Karsanti Ophiolite (Southern Turkey) Deduced from Whole-rock and Mineral Chemistry of the Gabbroic Cumulates

Abstract: Abstract The Pozanti-Karsanti Ophiolite Complex is situated in the eastern Tauride Belt and represents a remnant of the Mesozoic Neotethyan Ocean. It consists of three distinct nappes: (1) an ophiolitic mélange; (2) a metamorphic sole; and (3) ophiolitic rocks. The oceanic lithosphere section of the Pozanti-Karsanti Ophiolite comprises mantle tectonites, ultramafic-mafic cumulates, isotropic gabbros, sheeted dykes and basaltic volcanic rocks. These units are cut by isolated microgabbro-diabase dykes at all structural levels. New results are presented on the whole-rock and mineral chemistry of the gabbroic cumulates. Well-layered, low-Ti gabbroic cumulates, showing adcumulate to mesocumulate textures, are represented exclusively by gabbronorites. The mineral chemistry of gabbronorites from the Pozanti-Karsanti Ophiolite indicates that these cumulate rocks have been produced by the low-pressure crystal fractionation of basaltic liquid. Magnesium numbers (Mg-numbers) of clinopyroxene, orthopyroxene and amphibole range from 89 to 73, 80–66 and 80–72, respectively. Plagioclase compositions range from An94 to An84. The coexistence of calcic plagioclase, magnesian clinopyroxene and orthopyroxene indicates that the cumulate gabbronorites from the Pozanti-Karsanti Ophiolite were formed in an arc environment. The covariation of Al2O3 and Mg-numbers of both clinopyroxene and orthopyroxene show features typical of low-pressure igneous intrusions such as the Skaergaard and Tonsina Complexes, but differ from the high-pressure ultramafic cumulates found in the same arc. The cumulate gabbronorites probably represent shallower levels in the arc which were subsequently juxtaposed against deeper level ultramafic cumulates either during accretion or later faulting.

The Late Palaeozoic relations between Gondwana and Laurussia

Abstract: Abstract Reconstructions based on biogeography, palaeomagnetism and facies distributions indicate that, in later Palaeozoic time, there were no wide oceans separating the major continents. During the Silurian and Early Devonian time, many oceans became narrower so that only the less mobile animals and plants remained district. There were several continental collisions: the Tornquist Sea (between Baltica and Avalonia) closed in Late Ordovician time, the Iapetus Ocean (between Laurentia and the newly merged continents of Baltica and Avalonia) closed in Silurian time, and the Rheic Ocean (between Avalonia and Gondwana and the separate parts of the Armorican Terrane Assemblage) closed (at least partially) towards the end of Early Devonian time. Each of these closures was reflected by migrations of non-marine plants and animals as well as by contemporary deformation. New maps, based on palaeomagnetic and faunal data, indicate that Gondwana was close to Laurussia during the Devonian and Carboniferous periods, with fragments of Bohemia and other parts of the Armorican Terrane Assemblage interspersed between. It follows that, after Early Devonian time, the Variscan oceans of central Europe can never have been very wide. The tectonic evolution of Europe during Devonian and Carboniferous time was thus more comparable with the present-day Mediterranean Sea than with the Pacific Ocean.

Episodic Graben Formation and Extensional Neotectonic Regime in West Central Anatolia and the Isparta Angle: A Case Study in the Akşehir-Afyon Graben, Turkey

Abstract: Abstract Central and Western Anatolia form a continental back-arc region related to the Hellenic-Cyprus convergent plate boundary of the Anatolian and African Plates. The Akşehir-Afyon Graben (AAG), the easternmost extension of the west Anatolian horstgraben system, is located at the junction of Central Anatolia and eastern limb of the Isparta Angle. The AAG is 4–20 km wide and 90 km long. It trends west-northwest-east-southeast and is an actively growing rift containing two sedimentary infills of continental fluviolacustrine origin bounded on both sides by oblique-slip normal faults. The older infill is folded, thrust faulted and early Late Miocene in age. The younger infill, which is nearly horizontally bedded, is Plio-Quaternary in age and rests on the older infill with angular unconformity. The deformation of the older infill and the angular unconformity indicate a Late Miocene phase of compression, which separates two extensional periods. The second phase of extension has lasted since the Pliocene and is part of the current extensional neotectonic regime of both west Central Anatolia and the Isparta Angle, despite being previously reported as a compressional neotectonic regime. The graben-bounding Akşehir Fault Zone (AFZ) and the Karagöztepe Fault Zone display well-preserved fault surfaces and slickenlines. Although stereographic plots of the fault slip data show that the graben-bounding structures are oblique-slip normal faults, the AFZ has also been described as a single reverse fault. Both the field and seismic data, particularly the 1921 Argıthanı-Akşehir and 1946 Ilgın-Argıthanı earthquakes, indicate that the AAG is an active neotectonic structure. However, it can also be interpreted to lie in a seismic gap when its rate of seismicity is compared with that of the Gediz-Simav Graben forming its west-northwestern extension.

Crustal structure in the northern North Sea: an integrated geophysical study

Abstract: Abstract This study focuses on the deep structure of the Viking Graben and adjacent areas of the northern North Sea (60–62°N), and its implications for the amount, timing and nature of lithospheric extension. Two regional transects have been constructed based on an integrated analysis of deep seismic reflection and refraction data, gravity and magnetic data, and correlations between offshore and onshore geology. The shallow interpretation is based on high-quality conventional seismic reflection data calibrated against a large number of exploration wells. The new and partly reprocessed seismic data, combined with the other geophysical data, make possible a better documentation of the crustal configuration, such as the pre-Jurassic sediment distribution, basement and Moho relief, and deep fault geometries. A lower-crustal body characterized by an 8+ km s−1 velocity and an average bulk density of 2.95 g cm−3 is present beneath the Horda Platform. This body probably represents a deep crustal root of partially eclogitized rocks that formed during the Caledonian orogeny. Heterogeneities within this body give rise to the non-typical velocity-density relation. The crust-mantle boundary is located at the base of this body at a depth of 30–35 km and does not coincide with the seismically defined Moho. The geometry of crustal thinning reflects the cumulative effect of several post-Caledonian rift phases. Results show that Permian rifting affected a wide area, from the Øygarden Fault Complex to the Hutton Fault.

Kinematic indicators of subglacial shearing

Abstract: Abstract Criteria to distinguish between sediments that have been subglacially deformed and those that are undeformed, or deformed by other mechanisms, are sparse. In this paper we develop structural criteria to reconstruct the deformation history of glacial sediments that can be readily applied in the field as well as to analyses of thin sections of tills and related materials. Progressive simple shear is the simplest model to describe the deformation history of subglacially deformed sediments. It includes most of their characteristic structural aspects and provides tools for the kinematic analysis of subglacially deformed sediments. Progressive simple shear generates asymmetric structures, in which the principal direction of finite extension is subparallel to the direction of shearing. This is the simple shear fabric’s most distinctive characteristic, and that which most reliably defines the palaeo-ice flow direction. At a moderately strong intensity of deformation a typical shear zone in unlithified sediments may contain folded and strongly attenuated sediment layers, producing a transposed foliation which must not be mistaken for a sedimentary layering. Original sedimentary and deformation structures may completely disintegrate in the most intensely deformed sediments leading to its homogenization, although the typical shear zone fabric may still be identified in thin section.

Chronological constraints on the pre-Variscan evolution of the northeastern margin of the Bohemian Massif, Czech Republic

Abstract: Abstract New single zircon ages enable us to provide an evolutionary scenario for the Neoproterozoic to Cambro-Ordovician tectonic history of part of the easternmost Sudetes along the northeastern margin of the Bohemian Massif. The easternmost crustal segment (Brunia) yields Neoproterozoic ages from both autochthonous and allochthonous Variscan units; these ages document a Cadomian (Pan-African) history that may be linked with the northern margin of Gondwana. A Cambro-Ordovician magmatic-thermal event in Brunia is represented by granitic to pegmatitic dykes intruding Neoproterozoic crust and by localized partial anatexis. Farther west a narrow zone of Cambro-Ordovician rifting is identified (Staré Městro belt), marked by gabbroic magmatism, bimodal volcanism and medium-pressure granulite facies metamorphism. The westernmost crustal domain (Orlica-Sniezník dome) is represented by Neoproterozoic crust intruded by Cambro-Ordovician plutons consisting of calk-alkaline granitoid rocks and affected by widespread Cambro-Ordovician anatexis. The geodynamic setting of the Neoproterozoic and Cambro-Ordovician domains is similar to that of the Western Sudetes, where both Cambro-Ordovician rifting and calc-alkaline magmatism were identified. We discuss the rifting mechanics in terms of sequential crustal thinning along the northern margin of Gondwana. The calc-alkaline magmatism, in conjunction with crustal rifting, is related to a back-arc geometry in front of a retreating south-dipping subduction zone during progressive closure of the Tornquist Ocean southeast of Avalonia.

Structural Correlation of the Southern Transcaucasus (Georgia)-Eastern Pontides (Turkey)

Abstract: Abstract The eastern Pontides (northeastern Turkey) and Transcaucasus (Georgia) belong to the same geological belt representing an active margin of the Eurasian continent. According to palaeotectonic-palaeogeographic reconstructions, based on regional geological, palaeomagnetic, palaeobiogeographical and petrological data, the eastern Pontides and the major part of the Transcaucasus, situated to the north of the North Anatolian-Lesser Caucasian ophiolitic suture, comprise island arc, forearc, back and interarc basins. The eastern Pontide segment of the belt consists of three structural units which, from north to south, are the northern, central and southern units. The northern unit, the southeastern Black Sea coast-Adjara-Trialeti Unit, represents a juvenile back arc basin formed during the Late Cretaceous (pre-Maastrichtian). This unit separates the southern and northern Transcaucasus zones. The central Artvin-Bolnisi Unit is also known as the northern part of the southern Transcaucasus and is characterized by Hercynian basement, unconformably overlying the Upper Carboniferous-Lower Permian molasse and Upper Jurassic-Cretaceous arc association. The southern unit is the imbricated Bayburt-Karabakh Unit and is known as the southern part of the southern Transcaucasus. This unit has a similar basement to the Artvin-Bolnisi Unit and also includes a chaotic assemblage; it unconformably overlies the Upper Jurassic-Cretaceous forearc association. The eastern Pontide system is interpreted as the product of interference between a spreading ridge and subduction zone during Late Jurassic-Cretaceous times. The North Anatolian-Lesser Caucasus Suture, comprising ophiolites, mélanges and an ensimatic arc association, separates the overlying system from the Anatolian-Iranian Platform in the south. Maastrichtian-Lower Eocene cover rocks in the region unconformably overlie all the other units. Middle Eocene rifting resulted in the formation of new basins, some of which closed during an Oligocene-Early Miocene regression. Others, such as the Black Sea and Caspian Basins, have survived to the present day as relict basins.

Quaternary forced regression deposits in the Adriatic basin and the record of composite sea-level cycles

Abstract: High-resolution seismic and sedimentological data from the Central Adriatic basin reveals a Quaternary shelf-perched wedge that is comprised of four prograding units, the top surfaces of which are truncated by regional erosional surfaces. Internal reflector geometry indicates that each unit developed during highstand to falling sea-level conditions. Falling sea level resulted in successive downward and seaward shifts of the shoreline (forced regression). Because progradation occurs as a depositional continuum from highstand to lowstand conditions, sequence boundaries are difficult to recognize and correlate on a regional scale. In fact, continued sea-level fall produces several downshift surfaces of limited lateral extent. The regional erosional surfaces that truncate and hence bound the prograding units are composite in origin. The erosion surfaces formed during times of shelf subaerial exposure and were modified by shoreface and marine erosion during each subsequent rapid sea-level rise. All of these composite erosional surfaces become conformable seaward of the youngest depositional shoreline break formed at the end of each phase of progradation. These composite sequence-bounding erosional surfaces are draped by mud that is the distal equivalent of the overlying progradational unit deposited following rapid landward shifts of the shoreline. Facies analysis and stratigraphic data in the youngest progradational unit indicate that each of the four progradational units formed in response to fourth-order (100–120 ka) cyclicity. The four progradational units stack to form an aggradational-retrogradational sequence set that records a longer-term relative sea-level rise. Such a trend can reflect regional subsidence and/or a longer-term component of rise in the Quaternary eustatic signal. In the Adriatic basin, the key factors that favour the preservation of the forced regression deposits within the Quaternary shelf-perched wedge are: (1) the composite nature of relative sea-level cycles where a longer-term relative sea-level rise interacts with shorter-term (fourth to fifth-order) sea-level cycles; (2) the asymmetry of the eustatic signal that, reinforced by local subsidence, yields relative rises of large magnitude and short duration; (3) the high amplitude of the higher-frequency fifth-order signal that drives relative sea-level falls of short duration. This kind of composite cyclicity also controlled the formation and preservation of forced regression deposits on other Quaternary continental margins as well as in ancient sedimentary successions where these deposits may occur in backstepping or aggradational multistorey sequence sets.