Consequences of foreland basin development on thinned continental lithosphere: Application to the Aquitaine basin (SW France)

We present an analysis of the consequences of foreland basin development on thinned continental lithosphere, inherited from pre-orogenic phases of extension. Bathymetry at the transition from pre-orogenic extensional basin to foreland basin and compaction of pre-orogenic sediments contribute to the accommodation space for foreland basin sediments and thrust loads. In addition, the extension-induced transient thermal state of the lithosphere, results in ongoing thermal subsidence, and a flexural rigidity which changes through time. Quantitative modelling of the phase of extension and the foreland basin stage of the Aquitaine basin (southern France) shows that the inherited transient thermal state of the lithosphere contributes significantly to (1) the total foreland basin depth and width, (2) the post-compressional subsidence history, and (3) the cratonward onlap pattern. Accounting for the thermo-mechanical effects of pre-orogenic extension significantly reduces the estimates of both the flexural rigidity (30–43% for the Aquitaine basin) and the required topographic or thrust load (40% for the Aquitaine basin) at foreland basins. Emplacement of thrust loads below sea level, as expected in a pre-orogenic extensional basin setting, further reduces the required topographic load. This sheds light on the wide range of flexural rigidity values reported for continental lithosphere from foreland basin modelling studies, and explains, in many instances, the inferred ‘hidden load’ or subsurface load in flexural modelling studies at foreland basins. The present study has shown that pre-orogenic extension phases significantly affect the record of vertical motion and the stratigraphy of the Aquitaine basin and is probably important for foreland basin evolution in general.     

Some consequences of compressional tectonics for extensional models of basin subsidence

Changes in intraplate stress levels have important consequences for the stratigraphy of rifted basins and provide a tectonic explanation for the generation of sequence boundaries. Late-stage compressional phases during the post-rift evolution of basins produce unconformities, with offlap phases that increase in magnitude with the age of the rifted basin. Late-stage compression has not only a strong bearing on the generation of unconformities, but also induces significant downbending of the centre of a basin. Ignorance of the vertical motions of the lithosphere induced by late-stage compression during post-rift evolution can, therefore, give rise to substantial errors in the estimates of crustal extension derived from analysis of basement subsidence using stretching models. Consequently, late-stage compression can be of great significance in estimates of depth and timing of the hydrocarbon-window inferred from extensional models of basin subsidence. Quantification of the subsidence induced by post-rift compression has important implications for integrated models of basin subsidence and hydrocarbon generation.

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Zusammenfassung Unterschiede in platteninternen Streß-Niveaus haben entscheidende Auswirkungen für die Stratigraphie von Rift-Becken und liefern eine tektonische Erklärung für die Erzeugung von Sequenzgrenzen. Späte kompressive Phasen während der post-Rift-Entwicklung von Becken erzeugen Umkonformitäten mit offlap-Phasen, deren Ausmaß mit dem Alter des Rift-Beckens steigt. Entscheidende Fehlerquellen in der Abschätzung der Krustendehnung können darauf basieren, daß die durch späte Kompression während der post-Rift-Entwicklung ausgelösten Vertikalbewegungen der Lithosphäre ignoriert werden. Das Ausmaß der Krustendehnung wird durch Analysen der Basement-Subsidenz mit Hilfe von Dehnungsmodellen entwickelt. Die Quantifizierung der Subsidenz, die von der post-Rift-Kompression gesteuert wird, hat also wichtige Bedeutungen für Extensionsmodelle von Beckensubsidenz.

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Résumé Les changements dans la distribution des contraintes à l'intérieur des plaques ont des répercussions importantes sur la stratigraphie dans les bassins de rift et fournissent une explication tectonique de la limitation latérale des séries. Des épisodes compressifs tardifs, qui marquent l'évolution postrift des bassins, sont à l'origine de lacunes dans lesquelles l'importance des phases régressives augmente avec l'áge du bassin. Les mouvements verticaux de la lithosphère induits par ces épisodes tardifs de compression n'étant pas connus, il peut en résulter des erreurs non négligeables dans l'estimation de l'allongement crustal, telle qu'elle est déduite de la valeur de la subsidence du socle dans des modèles d'extension. Dans ces conditions, l'évaluation quantitative de la subsidence produite par les compressions post-rift doit être prise en considération dans l'élaboration des modèles qui rendent compte de la subsidence des bassins par les processus d'extension.

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Crustal structure of the eastern Mediterranean inferred from Rayleigh wave dispersion

Rayleigh wave group velocity data from paths crossing the Levantine Sea are presented. We have derived a suite of models for the crustal structure of the Levantine Sea for extreme values of data errors and of the data corrections which were applied in order to account for lateral heterogeneity.We conclude that models with a crustal thickness less than 30 km are not consistent with the data. Our preferred models are characterized by a crustal thickness of 35–40 km. These results and the presence of an extremely thick sedimentary sequence point to a passive continental margin type of structure underlying the Levantine Sea. Additional data from the path Sicily-Jerusalem suggest that this type of structure is representative of the whole of the eastern Mediterranean (Levantine Sea and Ionian Sea).     

Gravitational potential stresses and stress field of passive continental margins: Insights from the south-Norway shelf

It is commonly assumed that the stress state at passive margins is mainly dominated by ridge push and that other stress sources have only a limited temporal and/or spatial influence. We show, by means of numerical modelling, that observed variations in lithosphere structure and elevation from a margin towards continental interiors may also produce significant gravitational potential stresses competing with those induced by ridge push forces. We test this hypothesis on an actual case where abundant geological and geophysical datasets are available, the shelf of southern Norway and adjacent southern Norwegian mountains (or Southern Scandes). The modelling results are consistent with the main features of three key-observables: (1) undulations of the truncated geoid (reflecting variations in gravitational potential energy in the lithosphere), (2) significant stress rotations both offshore and onshore and (3) the seismicity pattern of southern Norway. The contribution of the Southern Scandes to the regional stress pattern appears to be far more significant than previously anticipated. In addition, the modelling provides a physical explanation for the enigmatic seismicity of southern Norway. Gravitational potential stresses arising from variations in the lithospheric structure between a passive margin and its continental borderlands, can exert a significant control on the dynamic evolution of the margin in concert with ridge push.     

Erratum to: Sedimentary Basins of the East Siberian Sea and the Chukchi Sea and the Adjacent Area of the Amerasia Basin: Seismic Stratigraphy and Stages of Geological History

Geotectonics - erratum     

Non‐uniform splitting of a single mantle plume by double cratonic roots: Insight into the origin of the central and southern East African Rift System

Using numerical thermo‐mechanical experiments we analyse the role of an active mantle plume and pre‐existing lithospheric thickness differences in the structural development of the central and southern East African Rift system. The plume‐lithosphere interaction model setup captures the essential features of the studied area: two cratonic bodies embedded into surrounding lithosphere of normal thickness. The results of the numerical experiments suggest that localization of rift branches in the crust is mainly defined by the initial position of the mantle plume relative to the cratons. We demonstrate that development of the Eastern branch, the Western branch and the Malawi rift can be the result of non‐uniform splitting of the Kenyan plume, which has been rising underneath the southern part of the Tanzanian craton. Major features associated with Cenozoic rifting can thus be reproduced in a relatively simple model of the interaction between a single mantle plume and pre‐stressed continental lithosphere with double cratonic roots.     

A comparison of the Iberian and Ebro Basins during the Permian and Triassic, eastern Spain: A quantitative subsidence modelling approach

The Permian–Triassic sediments of the Iberian Plate are a well studied case of classical Buntsandstein–Muschelkalk–Keuper facies, with good sedimentological interpretations and precise datings based on pollen and spore assemblages, ammonoids and foraminifera. Synrift–postrift cycles are recorded in these facies, but there are only a few studies of quantitative subsidence analysis (backstripping method) and only a previous one using forward modelling for the quantification of synrift–postrift phases of this period.Here we present the results obtained by the quantitative analysis of fourteen field sections and oil-well electric log records in the Iberian and Ebro Basins, Spain. Backstripping analysis showed five synrift phases of 1 to 3 million years duration followed by postrift periods for the Permian–Triassic interval. The duration, however, shows lateral variations and some of them are absent in the Ebro Basin. The forward modelling analysis, assuming local isostatic compensation, has been applied to each observation point using one-layer and two-layer lithospheric configurations. The second one shows a better fit between observation and model prediction in a systematic way, so a two layer configuration is assumed for the sedimentary basin filling analysis. Lithospheric stretching factors β and δ obtained in the forward modelling analysis are never higher than 1.2, but sometimes β < 1 and simultaneously δ > 1 in the same section. If surficial extension is compensated by deep compression either at the roots of the rift basins or in far-away zones is not yet clear, but this anomaly can be explained using a simple shear extensional model for the Iberian and Ebro basins.     

On the initiation of subduction zones

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin.Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas.Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.