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- Volume 33, Issue 2, 2021
Basin Research - Volume 33, Issue 2, 2021
Volume 33, Issue 2, 2021
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Sedimentation and viscosity controls on forearc high growth
More Less[Sedimentation and viscosity controls on temperature, vertical motions and basin architecture in subduction wedges. Figure shows the crustal wedge at the end of model run for simulations with a maximum sedimentation rate increasing from upper left to lower rigth, and crustal viscosity increasing from upper right to lower left. The thick white line bounds basin isochrons (above) from basement mesh (below).
Crustal rheology and surface processes strongly influence strain distribution and shape of orogenic wedges at their front but how they influence the wedge rear is still unclear. Here, we analyse the coupled control of viscosity and sedimentation on forearc high growth during advanced stages of subduction accretion. We use 2D thermo‐mechanical finite element models constrained with data of the south Anatolian margin. Our simulations show that forearc highs grow as a thermally‐activated viscosity drop in the lower crust induces ductile deformation and viscous flow. Initial viscosity and the amount of sediments in the forearc basin control non‐linearly the occurrence and timing of the thermally‐activated viscosity drop, and thus of the growth of the forearc high. High sedimentation rates result in thicker forearc basins that stabilise the subduction wedge and delay the onset of uplift in the forearc high. Low viscosities promote earlier onset of forearc high uplift and lead to larger morphological variability along the subduction margin. Increasing either the sedimentation rate or viscosity may prevent forearc high formation entirely. The thermo‐viscous forearc highs grow at an age set by wedge thermal state as a function of accretionary flux, wedge viscosity, and synorogenic sedimentation. Our models explain vertical motions in south Anatolia and potentially in other accretionary margins, like the Lesser Antilles or Cascadia, during the formation of their broad forearc highs.
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Sediment dispersal pathways in the Po coastal plain since the Last Glacial Maximum: Provenance signals of autogenic and eustatic forcing
Authors Daniel Tentori, Alessandro Amorosi, Salvatore Milli and Kathleen M. Marsaglia[AbstractWe examined downstream trends in sediment composition within the modern Po River system and defined provenance changes within a selected sector of the Upper Pleistocene to Holocene coastal plain in response to paleogeographic reorganisation during the last glacial‐interglacial cycle. Sediment composition of the Upper Pleistocene to Holocene alluvial and coastal plain succession overlaps with the modern petrofacies, suggesting that ancient fluvial sand was sourced from a dynamic river system attributable to a distal paleo‐Po with episodic sediment input from South Alpine and Apennine rivers. In the wave‐dominated estuarine depositional system developed under transgressive conditions, Po‐derived detritus mixed in the coastal system with sediment fed by Central/Eastern Alpine rivers, which was redistributed southwards by alongshore currents. A similar, mixed composition of late Holocene highstand depositional systems reflects the progradation of Po delta lobes (similar in composition to Po River sands) and adjacent strandplains (typified by sediment mixing due to alongshore transport). Compositional variability across the main stratigraphic surfaces suggests that autogenic processes (e.g. marine reworking, sedimentary mixing, delta switching/abandonment) in turn influenced by glacioeustasy, controlled the development of the transgressive and highstand Po Plain succession. This study clarifies the effect of fluvial‐marine interaction during the Holocene evolution of the Po coastal system and can be used as a model to interpret the evolution of many mixed‐delta types and Quaternary successions that shows alternating river‐flood and wave‐dominated deposits.
,a) The Upper Pleistocene fluvial sand was sourced from a dynamic river system attributable to a distal paleo‐Po with episodic sediment input from South Alpine and Apennine rivers. b) In the Holocene wave‐dominated estuarine depositional system developed under transgressive conditions, Po‐derived detritus mixed in the coastal system with sediment fed by Central/Eastern Alpine Rivers, which was redistributed southwards by alongshore currents. c) and d) Late Holocene highstand depositional systems reflects progradation of Po delta lobes and adjacent strandplain resulting in mixed composition.
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The influence of mantle flow on intracontinental basins: Three examples from Australia
Authors Alexander Young, Nicolas Flament, Lisa Hall and Andrew Merdith[Mechanisms by which dynamic topography can influence basin subsidence.
During the Paleozoic, sedimentary basins developed within Gondwana without evolving to diverging plate boundaries. Such intracontinental basins present long subsidence histories with multiple phases of accelerated subsidence that are not always easily explained by far‐field tectonic forces, and may be driven by processes other than rifting and thermal subsidence. Here we investigate the subsidence of Paleozoic Australian intracontinental basins by comparing one‐dimensional backstripped tectonic subsidence histories from the western Australian Canning and Southern Carnarvon Basins and the central Australian Cooper Basin to forward subsidence models for pure shear lithospheric thinning. We make the hypothesis that differences between observed and model subsidence may be explained by mantle‐flow driven topography, in addition to tectonic forces. To test this hypothesis, we compute dynamic topography from the first geodynamic models of mantle flow spanning the entire Phanerozoic Eon, and we analyse the relationship between dynamic topography and anomalous basin subsidence to dynamic topography and mantle flow. Although reconstructions of mantle flow in deep geological times are uncertain, our results suggest that long‐wavelength dynamic topography could explain aspects of the complex tectonic histories intracontinental basins. In the presented reconstruction of mantle flow, topographic rebound following the sinking of a Cambrian aged slab resulted in a minor phase of dynamic uplift in the Cooper Basin in middle Permian times. Throughout Carboniferous‐Triassic times Australia was positioned above a mantle upwelling driven by a hot structure at the base of the mantle. Structural uplift in the Canning and Southern Carnarvon basins during the Triassic‐Jurassic interval was augmented by dynamic uplift produced by that large‐scale upwelling, and possibly augmented by a focused active mantle plume during the Permo‐Triassic. In Late Jurassic‐Cretaceous times, Australia drifted east away from the mantle upwelling, resulting in a period of subsidence in the Canning and Southern Carnarvon basins. During the Cretaceous the Cooper Basin moved over a downwelling produced by long‐lived subduction along the east Australian margin, resulting in a period of accelerated subsidence.
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Sedimentology and provenance of newly identified Upper Cretaceous trench basin strata, Dênggar, southern Tibet: Implications for development of the Eurasian margin prior to India–Asia collision
Authors Devon A. Orme, Andrew K. Laskowski, Misia F. Zilinsky, Wang Chao, Xudong Guo, Fulong Cai and Ding Lin[Provenance of Upper Cretaceous trench basin strata in southern Tibet are consistent with derivation from a margin‐parallel sediment dispersal system sourced near Lhasa City. Accumulation of trench strata along the southern margin of Asia during Late Cretaceous time suggests a period of accretion and increased sediment delivery directly to the trench possibly driven by a change in plate convergence and subduction of an oceanic ridge, respectively.
Trench basins preserved along the southern margin of the Lhasa Terrane, Tibet, are sedimentologic records of convergent margin processes preceding Cenozoic India–Asia collision. We present new sedimentologic, petrographic and geochronologic data from the Rongmawa Formation and surrounding strata near Dênggar, Tibet, to determine depositional environment, provenance and age. Depositional ages range from ca. 92 to 87 Ma and lithofacies are consistent with deposition by low‐ and high‐density turbidity currents and suspension settling of pelagic detritus in a deep‐marine, trench basin setting. Sandstone modal analyses and U–Pb geochronology indicate that trench basin detritus in this region was derived from the Lhasa Terrane. We interpret that the Cretaceous subduction trench received detritus from an axial sediment dispersal system that transported sediment from headwaters in the central‐southern Lhasa terrane near Lhasa City directly to the trench and then flowed westwards parallel to the trench. The preservation of trench basin strata deposited during Late Cretaceous time compared with the lack of trench deposits prior to ca. 92 Ma and after ca. 80 Ma suggests the margin experienced a period of significant accretion during this interval. In addition, deposition of trench basin strata occurred during Late Cretaceous adakitic magmatism and high‐temperature metamorphism, which are hypothesized to be explained by subduction of an oceanic ridge or subduction zone retreat and related upper plate extension along the southern margin of the Lhasa terrane. Subduction of an oceanic ridge may provide a mechanism to potentially erode forearc basin strata and promote increased sediment delivery directly to the trench.
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Strain migration during multiphase extension, Stord Basin, northern North Sea rift
[AbstractIn regions experiencing multiple phases of extension, rift‐related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre‐existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time‐structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β‐factor). Our results show that the locations of basin‐bounding fault systems were controlled by pre‐existing crustal‐scale shear zones. Within the basin, Permo‐Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E‐W extension direction. Rift faults control the locus of syn‐RP1 deposition, whilst during the inter‐rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β‐factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic‐Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.
,Permo‐Triassic and Middle Jurassic‐Early Cretaceous crustal extension in northern North Sea rift.
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Interplay of tectonics and magmatism during post‐rift inversion on the central West Iberian Margin (Estremadura Spur)
Authors Ricardo Pereira, Filipe Rosas, João Mata, Patrícia Represas, Cláudia Escada and Beatriz Silva[The combined effects of multiphase margin inversion and Late Cretaceous magmatism on the Central West Iberian Margin reveals that deformation is decoupled between Cretaceous and Tertiary strata, with a laccolith accommodating significant shortening and acting as a rigid body.
The combined effects of post‐rift magma emplacement and tectonic inversion on the hyper‐extended West Iberian Margin are unravelled in detail using multichannel 2D/3D seismic data. The Estremadura Spur, acting as an uplifted crustal block bounded by two first‐order transfer zones, shows evidence of four post‐rift tectonic events each with a distinctive seismic‐stratigraphic response that can be used to demonstrate the tectono‐magmatic interplay, namely: (a) the Campanian onset of magmatism (including the Fontanelas Volcano, the widespread evidence of multiple sill complexes and the detailed description of a >20 km long laccolith, the Estremadura Spur Intrusion; (b) the Campanian‐Maastrichtian NE‐SW event pervasively affecting the area, resulting in regional uplift, reverse faulting and folding; (c) the Paleocene‐mid Eocene inversion that resulted in widespread erosion and; (d) the Oligocene‐mid Miocene evidence of rejuvenated NW‐SE inversion marked by crestal faulting and forced‐fault folding establishing the final geometry of the area. The distinct deformation styles within each tectonic phase document a case of decoupled deformation between Late Cretaceous and Tertiary units, in response to the predominant stress field evolution, revealing that the magnitude of Late Cretaceous inversion is far more significant than the one affecting the latter units. A detailed analysis of the laccolith and its overburden demonstrate the distinct deformation patterns associated both with magma ascent (including extensional faulting, forced‐folding and concentric reverse faulting) and its interference as a rigid intrusive body during subsequent transpressive inversion. This reinforces the role that the combined tectono‐magmatic events played on the margin. Also analysed is the wider impact of post‐rift magmatism and the associate emplacement of sub‐lithospheric magma on the rheology of a thinned continental crust. This takes into account the simultaneous tectonic inversion of the margin, the implied alternative views on characteristic heat flow, and on how these can be incorporated in source rock organic maturity modelling.
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Cambrian syn‐rift tectonic pulses at unconformity‐bounded carbonates in the Avalon Zone of Newfoundland, Canada
More Less[AbstractIn the Avalon Zone of SE Newfoundland, several Terreneuvian to Cambrian Series 2 carbonate bodies occur associated with paraconformities and onlapping geometries. Their stratigraphic discontinuities and related gaps represent syn‐rift episodes characterized by sharply tilted and downfaulted blocks and deposition of chaotic megabreccia beds and conglomerates on neighbouring footwall areas. Microbial and shelly carbonate production nucleated on tectonically unstable palaeohorsts, where stromatolitic crusts and mats commonly reflect the onset of hiatal stratigraphic diastems, and mud‐mounds the episodic establishment of calm conditions. Short‐term uplift episodes of rift shoulders led to unroofing of the Neoproterozoic basement (yielding granitoid clasts and input of reworked exotic clasts and allochthonous fossils from underlying Cambrian strata), localized subaerial exposure and karst features, and the development of synsedimentary fracture networks that provided favourable conduits and pathways for hydrothermal fluids. These carbonate beds host vein networks of carbonate‐hosted polymetallic sulphide‐sulphate‐oxide mineral associations (Pb–Cu–Fe–barite stockworks). The mixture of parautochthonous and allochthonous bioclastic assemblages points to the record of event‐concentration strata, which may clarify the long stratigraphic ranges of some involved microfossils. The Cambrian Avalonia rift and the offshore to basinal rift sector of the neighbouring Atlas‒Ossa‐Morena rift, preserved in the Moroccan northern High Atlas and Coastal Meseta, share common temperate‐water carbonate facies and build‐ups, lacking ‘subtropical’ indicators such as archaeocyathan‐microbial reefs, ooidal shoals or evaporitic pseudomorphs. These diagnostic facies and minerals occur only in relatively stable, shallow‐water rift branches, such as those preserved in the Anti‐Atlas and the Ossa‐Morena Zone (Iberian Massif).
,Unconformity‐bounded carbonates in the Cambrian of West Avalonia reflect syn‐rift tectonic pulses associated with hydrothermal activity. These carbonate beds host vein networks of carbonate‐hosted polymetallic sulfide‐sulfate‐oxide mineral associations (Pb–Cu–Fe–barite stockworks).
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Regional correlation and seismic stratigraphy of Triassic Strata in the Greater Barents Sea: Implications for sediment transport in Arctic basins
AbstractThe Greater Barents Sea Basin (GBSB) in Arctic Russia and Norway is an intracratonic basin that accommodated an enormous amount of sediment during the Triassic. These deposits are up to 4.5 km thick over an area 2,500,000 km2, and consist of marine mudstones and mudstone‐rich fluvio‐deltaic topsets with sandstone‐dominated fluvial channels. The basin is well‐studied and data‐rich, but regional correlation between different parts is lacking. Provenance data from adjacent Arctic basins have been interpreted to imply sediment transport from the Ural orogen across the GBSB, but these are disputed because of great transport distances, poorly constrained sediment‐transport directions and unknown timing of bypass. We integrated data from 3,238 seismic lines, 257 wells and palynostratigraphy, as well as published outcrop data, to create the first unified stratigraphic framework for the Triassic deposits across the entire GBSB. Results show that (1) sediment was transported northwest by one linked sedimentary system stretching across the entire basin; (2) sediment was derived from a source in the east comprising the Urals and West Siberia; (3) the main stratigraphic boundaries are major flooding surfaces which can be traced throughout the basin; and (4) significant amounts of sediment overspilled from the Barents Sea into adjacent sedimentary basins, starting with the Lomonosov Ridge from the Early Triassic, and into basins to the northwest (e.g. Sverdrup, Chukotka) during the late Carnian. These results provide a better understanding of geodynamics and provenance data in the Arctic, to improve the prediction of reservoirs in the area, and indicate a protracted uplift‐history of the northernmost Urals that started in the Carnian ~237 Ma. Furthermore, it shows how large intracratonic basins interact with uplands and subside over tens of millions of years.
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From sink to source: Using offshore thermochronometric data to extract onshore erosion signals in Namibia
Authors Mark Wildman, Kerry Gallagher, David Chew and Andrew Carter[Pre‐ and post‐depositional rock thermal histories are obtained by inverting apatite fission track data collected from a borehole offshore Namibia. The thermal histories are interpreted as reflecting multiple episodes of syn‐ and post‐rift erosion onshore and burial and exhumation offshore.
Products of onshore passive continental margin erosion are best preserved in offshore sedimentary basins. Therefore, these basins potentially hold a recoverable record of the onshore erosion history. Here, we present apatite fission track (AFT) data for 13 samples from a borehole in the southern Walvis basin, offshore Namibia. All samples show AFT central ages older or similar to their respective stratigraphic ages, while many single grain ages are older, implying none of the samples has been totally annealed post‐deposition. Furthermore, large dispersion in single grain ages in some samples suggests multiple age components related to separate source regions. Using Bayesian mixture modelling we classify single grain ages from a given sample to particular age components to create ‘subsamples’ and then jointly invert the entire dataset to obtain a thermal history. For each sample, the post‐depositional thermal history is required to be the same for all age components, but each component (‘subsample’) has an independent pre‐depositional thermal history. With this approach we can resolve pre‐ and post‐depositional thermal events and identify changes in sediment provenance in response to the syn‐ and post‐rift tectonic evolution of Namibia and southern Africa. Apatite U‐Pb and compositional data obtained during the acquisition of LA‐ICP‐MS FT data are also presented to help track changes in provenance with time. We constrain multiple thermal events linked to the exhumation and burial history of the continental and offshore sectors of the margin over a longer timescale than has been possible using only onshore AFT thermochronological data.
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The structural evolution of pull‐apart basins in response to changes in plate motion
[The evolution of a pull‐apart basin can be considerably influenced by a change of plate motion that introduces transtension in the basin. Identifying structures of this type of pull‐apart basin evolution are asymmetric triangular basins and wide principal displacement zones with oblique‐normal faults. We identify a number of these basins across the world and model their evolution through physical analogue models.
Pull‐apart basins are structural features linked to the interactions between strike‐slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull‐apart basin's structure, using analogue modelling experiments with a two‐layer ductile‐brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike‐slip system to drive pull‐apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map‐view sigmoidal oblique slip faults that become normal‐slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull‐apart basin.
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Review of Iberia–Eurasia plate‐boundary basins: Role of sedimentary burial and salt tectonics during rifting and continental breakup
[AbstractWe document the role of sedimentary burial and salt tectonics in controlling the deformation style of continental crust during hyperextension. The Iberian‐European boundary records a complex history of Cretaceous continental extension, which has led to the development of so‐called smooth‐slope type basins. Based on the review of the available geological constraints (crustal‐balanced cross‐sections, sedimentary profile evolution, RSCM thermometer, low‐temperature thermochronology) and geophysical data (Bouguer anomaly, Moho depth, seismic reflection profiles and Vp/Vs velocity models) on the Tartas, Arzacq, Cameros, Parentis, Columbrets, Mauléon, Basque‐Cantabrian and Internal Metamorphic Zone basins, we shed light on the main characteristics of this type of basin. This synthesis indicates that crustal thinning was influenced by two decoupling horizons: the middle crust and Triassic prerift salt, initially located between the basement and prerift sedimentary cover. These two horizons remained active throughout basin formation and were responsible for depth‐dependent thinning of the crust and syn‐rift salt tectonics. We therefore identify several successive deformation phases involving (a) pure shear dominated thinning, (b) simple shear dominated thinning and (c) continental breakup. In the first phase, distributed deformation resulted in the development of a symmetric basin. Field observations indicate that the middle and lower crust were under dominantly ductile conditions at this stage. In the second phase, deformation was localised along a crustal detachment rooted between the crust and the mantle and connecting upwards with Triassic prerift salt. During continental breakup, basin shoulders recorded the occurrence of brittle deformation, whereas the hyperextended domain remained under predominantly ductile thinning. The formation of smooth‐slope‐type extensional basins was intrinsically linked to the combined deposition of thick syn‐rift and breakup sequences, and regional salt tectonics. They induced significant burial and allowed the continental crust and the prerift sequence to deform under high temperature conditions from the rifting to continental breakup stages.
,At the beginning of the syn‐rift stage, depth‐dependent crustal thinning is dominantly controlled by distributed pure shear thinning within the lower/middle crust due to the presence of two decoupling levels: (1) the middle crust, which allows the lower crust to be extracted laterally without disturbing significantly the upper crust, and (2) within pre‐rift Triassic salt beds, which act as a décollement between the upper crust and the overlying sedimentary cover. Then simple shear becomes localized along a crustal detachment connecting upward with the Late Triassic décollement layer, inducing shearing in the pre‐rift salt. When continental breakup occurs, the basin flanks are affected by brittle deformation while the hyperextended domain undergoes dominantly ductile thinning. The rise of the 300°C to 500°C isotherms in the hyperextended domain, from the syn‐rift to the continental breakup stage, implies that the originally crystalline upper continental crust and the overlying pre‐rift and syn‐rift sedimentary pile are affected by depth‐dependent ductile thinning.
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From rifting to hyperextension: Upper Jurassic–Lower Cretaceous tectono‐stratigraphy of the Porcupine Basin, Irish Atlantic Margin
Authors Lewis Whiting, Peter D. W. Haughton and Patrick M. Shannon[Conceptual model for the development of syn‐rift and transitional sequences during hyperextended rifting. During hyperextension, strain progressively localizes towards the rift axis, leading to the development of transitional depocentres above the tectonically inactive parts of the necking domain, later followed by younger transitional sequences above the hyperextended domain.
Hyperextended basins are increasingly recognized along the outboard parts of continental margins as aborted basins created during continental break‐up. Many of the concepts for understanding and modelling basin evolution and fill were developed for regions that have undergone modest crustal stretching (β < 2) and may not be valid in basins where the crust and upper mantle are heavily modified by extreme stretching. The present study uses extensive 2D and 3D seismic and well data to analyse the Late Jurassic–Cretaceous tectono‐stratigraphic evolution of the Porcupine Basin, bracketing the timing of hyperextension. It is an instructive basin, offshore west of Ireland, preserving low‐magnitude strain in the north, with increasing degrees of hyperextension in the south. Detailed mapping of strain domains (proximal, necking and hyperextended) across the Porcupine Basin reveals five main rift segments, each with a distinctive geometry and strain history. During early low‐strain rifting, inherited crustal structures strongly influenced the rift architecture by controlling the location and geometry of fault‐controlled marine depocentres. The transition from hyperextension to post‐rift subsidence was marked by locally developed, unconformity‐bounded, marine sequences that draped the underlying rift topography. Whilst these ‘transition sequences’ are dated as Tithonian above the necking domain, similar but younger Early Cretaceous transition packages developed in the hyperextended domain, suggesting extension migrated towards the rift axis during hyperextension. Early post‐rift sequences were broadly distributed across the rift centre and basin flanks before strong, thermally‐controlled subsidence of the hyperextended crust, along with hinging of the necking domain, locally to the point of slope failure, gave rise to axially‐focused marine deposition. Hyperextension may have left the basin susceptible to intra‐plate stress changes accounting for several unconformities within the post‐rift fill. This study provides an improved basin‐wide understanding of the tectono‐stratigraphic evolution of hyperextended basins.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)