Rifted arch basins and post-breakup rim basins on passive continental margins

In its evolution by plate divergence to a passive continental margin, a continental arch marked by narrow rift valleys (intra-arch basins) and flanked by broad basins (inter- and extra-arch basins) is most likely to break up along a rift valley boundary fault. The resulting dismembered arch at the continental margin is a rim that constitutes the oceanward flank of a rim basin, and the rim basin succeeds one or other of the basins related to the previous arch. In offshore Western Australia, the juxtaposition of Mesozoic reservoir rock at a rift shoulder and source rock of the succeeding rim basin provide a mechanism for concentrating a large gas deposit.     

Seaward dipping reflectors and the continent-ocean boundary at passive continental margins

The definition of the continent-ocean boundary at passive continental margins has proved to be an elusive task. Even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally. Multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors, dipping seaward to form a wedge-shaped structure. Their general characteristics and velocity structure suggest that they may be largely volcanic in nature, but there is no agreed upon model for their origin. Nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean, and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins.     

Rheologic properties of the lithosphere and applications to passive continental margins

Thermal and petrologic models of the crust and upper mantle are used for calculating effective viscosities on the basis of constant creep rates. Viscosity—depth models together with pressure—depth models are calculated for continental and oceanic blocks facing each other at continental margins. It is found from these “static models” that the overburden pressure in the lower crust and uppermost mantle causes a stress which is directed from the ocean to the continent. The generally low viscosity of 10²⁰–10²³ poise in this region should permit a creep process which could finally lead to a “silent” subduction. In the upper crust static stresses act in the opposite direction, i.e. from the continent to the ocean, favouring tension which could produce normal faulting in the continent. Differences between observations and the results obtained from the static models are attributed to dynamical forces.     

Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood.We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite).Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties.The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.     

Evolution of the passive continental margins of India—a geophysical appraisal

The passive continental margins of India have evolved as India broke and drifted away from East Antarctica, Madagascar and Seychelles at various geological times. In this study, we have attempted to collate and re-examine gravity and topographic/bathymetry data over India and the adjoining oceans to understand the structure and tectonic evolution of these margins, including processes such as crustal/lithosphere extension, subsidence due to sedimentation, magmatic underplating and so on. The Eastern Continental Margin of India (ECMI) seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments, respectively, and bears similarities with its conjugate in East Antarctica. Crustal extension rates are uniform along the stretch of the ECMI in spite of the presence or absence of crustal underplated material, variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing. The Krishna-Godavari basin is underlain by a strong ( 30 km) elastic lithosphere, while the Cauvery basin is underlain by a thin elastic lithosphere ( 3 km). The coupling between the ocean and continent lithosphere along the rifted segment of the ECMI is across a stretched continental crust, while it is direct beneath the Cauvery basin. The Western Continental Margin of India (WCMI) seems to have developed in an oblique rift setting with a strike-slip component. Unlike the ECMI, the WCMI is in striking contrast with its conjugate in the eastern margin of Madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones. The break up between eastern India and East Antarctica seems to have been accommodated along a Proterozoic mobile belt, while that between western India and Madagascar is along a combination of both mobile belt and cratonic blocks.     

Volcanic passive margins

Compared to non-volcanic ones, volcanic passive margins mark continental break-up over a hotter mantle, probably subject to small-scale convection. They present distinctive genetic and structural features. High-rate extension of the lithosphere is associated with catastrophic mantle melting responsible for the accretion of a thick igneous crust. Distinctive structural features of volcanic margins are syn-magmatic and continentward-dipping crustal faults accommodating the seaward flexure of the igneous crust. Volcanic margins present along-axis a magmatic and tectonic segmentation with wavelength similar to adjacent slow-spreading ridges. Their 3D organisation suggests a connection between loci of mantle melting at depths and zones of strain concentration within the lithosphere. Break-up would start and propagate from localized thermally-softened lithospheric zones. These ‘soft points’ could be localized over small-scale convection cells found at the bottom of the lithosphere, where adiabatic mantle melting would specifically occur. The particular structure of the brittle crust at volcanic passive margins could be interpreted by active and sudden oceanward flow of both the unstable hot mantle and the ductile part of the lithosphere during the break-up stage. To cite this article: L. Geoffroy, C. R. Geoscience 337 (2005).     

Asymmetric Atlantic continental margins

We analyze the gross crustal structure of the Atlantic Ocean passive continental margins from north to the south, comparing eleven sections of the conjugate margins. As a general result, the western margins show a sharper continental-ocean transition with respect to the eastern margins that rather show a wider stretched and thinner margin. The Moho is in average about 5.7°±1° dipping toward the interior of the continent on the western side, whereas it is about 2.7°±1° in the eastern margins. Moreover, the stretched continental crust is on average 244 km wide on the western side, whereas it is up to about 439 km on the eastern side of the Atlantic. This systematic asymmetry reflects the early stages of the diachronous Mesozoic to Cenozoic continental rifting, which is inferred as the result of a polarized westward motion of both western and eastern plates, being Greenland, Northern and Southern Americas plates moving westward faster with respect to Scandinavia, Europe and Africa, relative to the underlying mantle.     

Elevated passive continental margins: Numerical modeling vs observations. A comment on Braun (2018)

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被动陆缘洋陆转换带和岩石圈伸展破裂过程分析及其对南海陆缘深水盆地研究的启示

作为伸展陆壳和正常洋壳之间重要的过渡和衔接,洋陆转换带(ocean-continent transition,简写为OCT)蕴含有丰富的地壳岩石圈伸展破裂过程的信息。文中通过系统的资料调研,在总结OCT研究历史、现状和发展趋势的基础上,阐明了OCT的现代概念、类型及其识别标志;详细介绍了以OCT为基础而建立的被动陆缘地壳岩石圈结构构造单元划分方案、表层沉积盆地构造地层格架及重要的构造变革界面特征;分析了大型拆离断层在地壳岩石圈薄化、地幔剥露过程中的控制作用;揭示了陆缘变形集中、迁移和叠合的规律,建立了被动陆缘岩石圈伸展、薄化、剥露和裂解模式。最后,论文对比了国际非岩浆型被动大陆边缘与我国南海OCT的研究,介绍了南海OCT和陆缘深水超深水盆地研究的新发现,提出深入研究南海OCT将为南海陆缘构造演化、洋盆扩张过程和深水超深水盆地的成因机制研究提供新的启示。     

The state of stress at continental margins

Three sources of stress at active (Andean) continental margins are considered: body forces on the plates which drive their motion, thermal stresses generated within the cooling lithosphereand bending stresses due to the flexure of the lithosphere at an ocean trench. It is argued that the bending stresses dominate. The evolution of passive (Atlantictype) continental margins is also considered. Models for the free and locked flexure of the continental and oceanic lithosphere are given. Based on observed gravity anomalies, it is argued that the continental margin fault system must remain active throughout much of the evolution of the margin. These displacements accommodate both the subsidence of the oceanic lithosphere due to its cooling and thickeningand the sedimentary loading. This loading may be responsible for the seismicity on the eastern continental margin of the United States e.g., the Charleston, South Carolina earthquake of 1884.     

大陆边缘增生造山作用

文章评述了增生造山作用的研究历史和进展,认为增生造山作用贯穿地球历史,是大陆增生的重要方式。用大陆边缘多岛弧盆系构造理解造山带的形成演化,提出巨型造山系的形成与长期发育的大洋岩石圈俯冲制约的两侧或一侧的多岛弧盆系密切相关。在多岛弧盆系演化过程中的弧 弧和弧 陆碰撞,弧前和弧后洋盆的消减冲杂岩的增生,洋底高原、洋岛/海山、外来地块(体)拼贴等一系列碰撞和增生造山作用形成大陆边缘增生造山系。大洋岩石圈最终消亡形成对接消减带,大洋岩石圈两侧的多岛弧盆系转化的造山系对接形成造山系的联合体。拼接完成后往往要继续发生大陆之间的陆 陆碰撞造山作用、陆内汇聚(伸展)作用,后者叠加在增生造山系上,使造山过程更加复杂。对接消减带是认识造山系形成演化的关键。大洋两侧多岛弧盆系经历的各种造山过程可以从广义上理解为一个增生造山过程。多岛弧盆系研究对于划分造山带细结构非常重要,是理解造山系物质组成、结构和构造的基础,并制约了造山后陆内构造演化。大陆碰撞前大洋两侧多岛弧盆系及陆缘系统更完整地记录了威尔逊旋回,记录的信息更加丰富。根据多岛弧盆系的思路对特提斯大洋演化提出新的模式,认为西藏冈底斯带自石炭纪以来受到特提斯大洋俯冲制约,三叠纪发生向洋增生造山作用,特提斯大洋于早白垩世末最终消亡。     

On the use of rayleigh wave group velocities for the analysis of continental margins

Rayleigh wave group velocities provide a low-cost means for a quick assessment of averaged local properties of the Earth's crust in continental margin regions of the Atlantic type. Sufficiently accurate measurements (with a standard error of 0.3 km/s or less) of group velocities in continental shelf areas at periods between 5 and 30 seconds provide important information about structural parameters. They may resolve the Moho depth to within 4 or 5 km, depending on crustal thickness and give useful estimates of the average velocities in the upper part of the crust. The group velocities of Rayleigh waves in this period range are influenced most by the shear velocity at all depths and the compressional velocity and the density near the surface. For continental rise regions, the dominating influence of the water layer limits the effectiveness of the method.     

Magmatism at continental passive margins inferred from Ambient‐Noise Phase‐velocity in the Gulf of Aden

Non‐volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced. We use ambient‐noise tomography to constrain Rayleigh‐wave phase‐velocity maps beneath the eastern Gulf of Aden (eastern Yemen and southern Oman). In the crust, we image low velocities beneath the Jiza‐Qamar (Yemen) and Ashawq‐Salalah (Oman) basins, likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin. Our results provide strong evidence that magma intrusion persists after breakup, modifying the composition and thermal structure of the continental margin. The coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning, as well as with transform faults, suggests that magmatism post‐breakup may be driven by small‐scale convection and enhanced by edge‐driven flow at the juxtaposition of lithosphere of varying thickness and thermal age.     

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.     

Ancient continental margins and comparative analysis of their oil-and-gas bearing

Analysis of peculiarities in the distribution of hydrocarbon accumulations within the basins of Phanerozoic continental margins, which had completed their evolution, and complicated peripheral regions of ancient Laurasian and Gondwanian platforms nowadays, has enabled us to reveal certain regularities related to two stages in the evolution of sedimentary basins. The first stage of evolution of sedimentary basins (period of existence of the continental margin proper) is related to large accumulations of fluid and gaseous hydrocarbons in the margins of continents belonging to the Laurasian megablock; for the margins of continents belonging to Gondwana, this period was reflected in the formation of large gas accumulation only (in the Permian). At the second stage of sedimentary basin evolution, large oil and gas accumulations were formed in areas associated with fore deeps, which were laid in the boundary of the Gondwanian platforms and fold belts. In comparison, in fore deeps that emerged in the marginal parts of Laurasian platforms, less significant accumulations of fluid and gaseous hydrocarbons were found (Table 1). The results of comparative analysis in oil-and-gas bearing basins located in the margins of the Laurasian and Gondwanian megablocks would help in purposeful exploratory works for oil and gas.     

Tectonotype of volcanic passive margins in the Norwegian-Greenland region

A tectonotype of volcanic passive margins exemplified in the conjugate Norwegian and East Greenland margins is considered, with discussion of the Paleogene igneous complexes and the regional rift structure before continental breakup. Fragments of asymmetrical rift have been retained on both sides of the ocean. Large Cretaceous pre-rift sedimentation basins marking the initial stage of the ocean opening are included into the passive margin as well. The continental breakup was accompanied by intense basaltic magmatism over a short time span. This magmatic episode was distinguished by (1) the formation of widespread plateau-basalt complexes on continents and in near-shore areas of the ocean; (2) the development of thick lava series that are recorded in seaward dipping reflector wedges; (3) thick high-velocity lower crust, resulting from magmatic underplating; (4) asymmetrical accretion of the crust and structure formation. The discussion is based on published seismic data and reference sections selected for each margin with consideration of the composition and thickness of the igneous rocks, their lateral variations, source composition, and eruption and crust formation conditions. The characteristic feature of both sections is the two-member structure of volcanic complexes with substantial geochemical differences between the rocks from the lower and upper parts of the section, which correspond to the pre-breakup and breakup phases. At the initial phase, small magma volumes were melted out from the lithosphere. The geochemical signatures of the upper parts of the sections testify to the melting of the asthenospheric mantle. Their spatiotemporal variations reflect the ascent and melting of the deep plume, which was active during and after continental breakup. In the Greenland area, near the central part of the plume, a N-MORB-type mantle magma source gave way to a depleted Iceland-type mantle, while apart from the central part of the plume, its effect is expressed only in the enormous volume of mantle-derived melt without migration of its source. A variety of evidence is provided for the plume’s activity: the great thickness of the volcanic complexes and the relatively stable composition of the melt; the elevated temperature in the mantle; the specific geochemistry of the breakup-related lavas and their lateral zoning; conclusions on the necessity of dynamic support of volcanic eruptions; and recent results of seismographic tomography. The continental breakup inherited a system of older sedimentary basins in the zone of prolonged extension of the lithosphere in the North Atlantic. The continuous dynamic support of extension was most likely provided by long-term ascent of the Iceland plume. The comparison of the considered tectonotype with other volcanic and non-volcanic margins opens the way to further elucidation of the geodynamic processes responsible for the ocean opening.     

Oil source rocks at the mesozoic and cenozoic continental margins: Communication 1. Oil source rocks at continental margins in the Triassic-Jurassic and Neocomian-Aptian

The presence of rocks capable of generating hydrocarbons (HC) in the section of sedimentaryrock basins is an essential criterion for their qualification as structures with oil and gas pools. Although organic matter (OM) is always present as dissemination in genetically different sediments, it is believed that rocks enriched with OM of the sapropel series (2 to 3% C_org) can generate a significant amount of liquid HC. However, rock sequences with the C_org ranging from 3–5 to 15–20% are considered oil source formations. The rock section of large petroliferous basins usually includes one or two source sequences, which generated liquid and gaseous HCs after submergence to high temperature and pressures zones. In the basin confined to the Arctic slope of Alaska, one of the main producers of liquid HC is represented by the Upper Triassic clays and limestones of the Shublik Formation. In the Barents Sea and North Sea basins, such rocks are represented by the Spekk Formation and the Kimmeridge Clay, respectively; in the West Siberian basin, by the Bazhenovo Formation; in the Persian Gulf, by the Fahlian, Sargelu, and Garau formations; in basins of the Caribbean region, by marls and clayey limestones of the La Luna Formation. In perioceanic basins of the South Atlantic, the major source sequences are represented by the Neocomian and Barremian clays and marls. The source rocks are identified as the Lagoa Feia Formation in the Campos and Santos basins. They are cognized as the Black Marlstone or Bukomazi Formation in the Lower Congo, Kwanzaa-Cameroon, and Angola basins.     

被动大陆边缘盐构造研究进展

内容提要:被动大陆边缘的含盐盆地多在重力作用下发育薄皮盐构造.这些构造不但记录了盆地的演化过程,而且往往富含大量的油气资源.因此,被动陆缘盐构造是学术界与工业界共同关注的热点.在传统的被动陆缘含盐盆地模型中,盆地主要在重力作用下形成上坡的拉张区,下坡的挤压区和两者之间的转移区.近年来,国际学术界围绕重力变形在盆地中的作...     

Triassic-Liassic basins and climate of the Atlantic passive margins

As the Laurasian Plate tracked north over the New England hotspots in the LateTriassic, the heated and stretched crust failed along re-activated basement structures including micro-plate sutures, and continental extensions of transforms. This created the rifted passive margins of the Atlantic and established the tectonic and climatic setting of wrench-generated coastal ranges and detrital basins bordering vast salt flats that were overlain with waters from the Tethys Sea.In tracking north from an equatorial position in the Late Triassic to a subtropical latitude in the Middle Jurassic, the plate transgressed first humid, then savanna and finally arid climatic zones, which were then bordered by a transgressing epeiric Tethyan Sea. Within these climatic zones, monsoonal circulation profoundly affected patterns of sedimentation as tropical air masses cooled and warmed adiabatically as they crossed the coastal ranges and broad salt flats.Where the basement had been pulled apart as in the Newark-Gettysburg Basin or the Argana Basin of Morocco, plutons intruded the axis of the basin in the form of dikes, lava flows and subaqueous fissure flows. Differential horizontal shear along strike-slip faults created assymetric basins with an upthrown leading plate and a subsiding trailing plate. Strata within the basins record a history of recurrent, but alternating, transtentional and transpressional episodes in an overall wrench-tectonic regime. While the borderfault facies is marked by complex unconformities, young basin sediment, volcanics, en-echelon folds, fanglomerates, turbidites and deep-water lacustrine deposits with organic-rich black shale, sediments on the trailing plate are marked by an older suite of gently inclined fluvialdeltaic sands that rest with profound unconformity on the Hercynian — Variscan basement.Where shallow marine waters of the Tethys Ocean transgressed sagged pull-apart basins (as in the Khemisset and Berrichid Basins of Morocco) or where the basement was faulted by straignt, non-branching transforms (as in Grand Banks), vast salt flats occurred forming thick, deposits of halite and potash salt. The extent of Tethyan transgression and concomitant subsidence of these basins is marked by salt diapirs in the Baltimore Canyon Trough and in the Aaiun Basin of Africa.

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Zusammenfassung Während sich die Laurasische Platte in der späten Trias nordwärts bewegte über die Hotspots Neuenglands hinweg, brach die erwärmte und gedehnte Kruste entlang reaktivierter Strukturen des Basements, sowie entlang von Mikroplatten-Rändern und entlang der Fortsetzungen von Querstörungen auf den Kontinenten. Dieser Vorgang schuf die abgesenkten passiven Ränder des Atlantik und etablierte die tektonische und klimatische Situation der Küstenketten und Sedimentationsbecken, die weite mit Tethys-Meerwasser bedeckte Salzpfannen säumten.Während der Drift der Platte von einer äquatorialen Lage zur späten Triaszeit hin in eine subtropische Breite zur mittleren Jurazeit durchlief sie zunächst humide, dann Savannen- und schließlich aride Klimazonen. Diese wurden gerahmt von dem transgredierenden epirischen Tethys-Meer. Innerhalb dieser Klimazonen wurde die Sedimentation nachhaltig durch Monsum-Zirkulation beeinflußt dadurch, daß tropische Luftmassen sich abkühlten und adiabatisch erwärmten beim Überqueren der Küstenketten und der breiten Salzebenen.Dort, wo das Basement aufriß, wie etwa im Newark-Gettysburg Becken oder im Argana Becken von Morocco, drangen Plutone in die Achse des Beckens ein in Form von Gängen, Lavaergüssen und subaquatischen Spaltenergüssen. Differentielle horizontale Schubspannungen entlang Blattverschiebungen sorgten für asymmetrische Becken mit aufgeschobener Leitplatte und abgesenkter Schlepp-Platte. Die Ablagerungen innerhalb der Becken bilden eine Geschichte periodischer aber alternierender durch Zug- und Druckspannungen beherrschte Episoden ab.Die Fazies des Randstörungssystems ist durch komplexe Diskordanzen markiert, durch junge Beckensedimente, vulkanische Gesteine, girlandenartige Faltenzüge, Fanglomerate, Turbidite und Tiefwasser-Seesedimente mit organogen-reichen Schwarzschiefern. Dagegen sind die Sedimente der Schlepp-Platten gekennzeichnet durch eine ältere Folge von schwach geneigten fluviatil-deltaischen Sanden, die mit markanter Diskordanz auf dem herzynisch-variskischen Basement ruhen.Dort, wo der flache Tethys-Ozean über die sich absenkenden Dehnungs-Becken (wie etwa die Becken von Khemisset und Berrichid von Morocco) transgredierte oder wo das Basement durch geradlinige, nicht verzweigte Querstörungen zerschnitten wurde (wie im Gebiet der Great Banks), breiteten sich weite Salzebenen aus, die dicke Halit- und Kalisalzlager bildeten. Die Ausdehnung der Tethys-Transgression und die einhergehende Absenkung dieser Becken wird durch Salz-Diapire im Baltimore Canyon Graben und im Becken von Aaiun in Afrika markiert.

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Résumé Tandis que la plaque laurasiatique se déplaçait à la fin du Trias vers le nord sur les points chauds de la Nouvelle Angleterre, il s'est produit dans la croûte échauffée et sous tension, des ruptures le long de structures réactivées du socle ainsi que le long de bordures de microplaques et des prolongements de dérangements transversaux sur les continents. Ce processus conduisit à l'affaissement des bords de l'Atlantique, et à fixer la situation tectonique et climatique des chaînes cotières et des bassins de sédimentation qui bordaient de vastes dépressions salées couvertes par les eaux de la Téthys.Pedant sa dérive, à partir d'une position équatoriale à la fin du Trias jusqu'à une latitude subtropicale au Jurassique moyen, la plaque traversa des zones climatiques d'abord humides, puis à savannes et finalement arides, qui se trouvaient en bordure des transgressions épiriques del a Thétys. Dans ces zones climatiques, la sédimentation fut fortement influencée par la mousson sous l'effet des masses d'air tropical qui se refroidissaint et se réchauffaient adiabatiquement à la traversée des chaînes côtières et des plaines salifères ouvertes.Là où le socle apparaissait, comme dans le bassin de Newark-Gettysburg ou dans le bassin d'Argan au Maroc, des plutons pénétraient dans l'axe des bassins sous la forme de dikes, de coulées de lav et de coulées fissurales subaquatiques. Des poussées différentielles horizontales suivant des failles conduisirent à des bassins asymétriques, la plaque motrice en voie de soulèvement entraînant la plaque en voie d'affaissement. Les dépôts dans les bassins représentent une histoire faite d'épisodes périodiques et alternants dominés par des tensions et compressions.Le facie dans le système en bordure des dérangements, est marqué par des discordances complexes, des sédiments de bassin jeunes, des roches volcaniques, des faisceaux de plis en guirlande, des fanglomérats, des turbidites, et des sédiments de mer profonde avec des schistes noirs riches en matières organiques. Par contre les sédiments des plaques entraînées sont caractérisés par une série plus ancienne de sables fluvio-deltaïques faiblement inclinés qui reposent avec une discordance bien marquée sur le socle hercynovarisque.Là où la Thétys, de faible profondeur, transgressait sur les bassins d'extension en voie d'affaissement (comme les bassins de Khemisset et de Berrichid au Maroc), ou là ou le socle était recoupé par des fractures transversales rectilignes sans bifurcation (comme dans les Great Banks), s'étendaient de vastes aires salées avec formation d'épaisses couches de halite et de sels potassiques. L'extension de la transgression thétysienne et la continuelle dépression de ces bassins est marquée par des diapirs salins dans le Graben de Baltymore et dans le bassin d'Aaiun en Afrique.

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