The impact of syn- and post-extension prograding sedimentation on the development of salt-related rift basins and their inversion: Clues from analogue modelling

Various studies have demonstrated the intrinsic interrelationship between tectonics and sedimentation in salt-related rift basins during extension as well as during their inversion by compression. Here, we present seven brittle-ductile analogue models to show that the longitudinal or transverse progradation of sediment filling an elongate extensional basin has a substantial impact on the growth of diapirs and their lateral geometrical variations. We use five extensional models to reveal how these prograding systems triggered diapir growth variations, from proximal to distal areas, relative to the sedimentary source. In the models, continuous passive diapir walls developed, after a short period of reactive-active diapiric activity, during syn-extensional homogeneous deposition. In contrast, non-rectilinear diapir walls grew during longitudinal prograding sedimentation. Both longitudinal and transverse post extensional progradation triggered well-developed passive diapirs in the proximal domains, whereas incipient reactive-active diapirs, incipient roller-like diapirs, or poorly developed diapirs were generated in the distal domains, depending on the modelled sedimentary pattern. Two models included final phases of 6% and 10% shortening associated with basin inversion by compression, respectively, to discriminate compressional from purely extensional geometries. With the applied shortening, the outward flanks of existing diapir walls steepened their dips from 8 degrees-17 degrees to 30 degrees-50 degrees. Likewise, 6% of shortening narrowed the diapir walls by 32%-72%, with their fully closing (salt welds) with 10% of shortening. We compare our results with the distribution of salt walls and minibasins of the Central High Atlas diapiric basin in Morocco, which was infilled with a longitudinally prograding mixed siliciclastic and carbonatic depositional sequence during the Early-Middle Jurassic with a minimum thicknesses of 2.5-4.0 km. (C) 2017 Elsevier Ltd. All rights reserved

Paleoenvironmental and diagenetic evolution of the Aptian Pre-Salt succession in Namibe Basin (Onshore Angola)

The Aptian Pre-Salt sedimentary succession cropping out in Cangulo palaeovalley onshore Namibe Basin (Angola) was studied by a combination of field and analytical techniques to constrain the sedimentary and diagenetic evolution of the uppermost sag sequence of the South Atlantic passive margin. Field observations allows definition of four transgressive-regressive cycles characterised by fluvial to tidal-influenced mixed clastic-carbonate and carbonate-dominated deposits, that locally show evidence of evaporite dissolution; highlighting that evaporite deposition started earlier than deposition of the regional South Atlantic Loeme-Bambata evaporite formations. Two separate pre-salt carbonate units have been differentiated within the Cangulo Fm; i) a lower transitional to marginal marine, and ii) a younger upper non-marine freshwater travertine system, that is documented for the first time in the west African margin. Transgressive-regressive cycles control the early diagenesis of the tidal carbonates that include dolomitization due to mixing fluids during transgressions, and karstification due to evaporite dissolution by meteoric water circulation during regressive events. Clastic supply appears to have been completely shut down during carbonate deposition, suggesting major climatic change associated with carbonate deposition. During the lowstand between the two carbonate units, fluid flow through Cangulo palaeovalley was re-established resulting in extensive karstification and formation of a large-scale erosional unconformity that is interpreted to be time equivalent to an intra Chela-Cuvo Fm. event. The top of the studied succession corresponds to the transgressive deposits of the Bambata evaporites that are not preserved in the Cangulo palaeovalley due to its erosion but are regionally developed. The results of this study can be directly linked to along strike age equivalent Pre-Salt successions cropping out in the Namibe, Benguela and Kwanza basins, and directly offshore Angola and Brazil using well and seismic data. These new data shed important new light and constraints on the depositional and diagenetic evolution of the complex Pre-Salt reservoir systems of the South Atlantic, and the depositional and bathymetric setting at the time of onset of the main south Atlantic evaporite deposition.publishedVersio

Jurassic rifting to post-rift subsidence analysis in the Central High Atlas and its relation to salt diapirism

The subsidence evolution of the Tethyan Moroccan Atlas Basin, presently inverted as the Central High Atlas, is characterized by an Early Jurassic rifting episode, synchronous with salt diapirism of the Triassic evaporite-bearing rocks. Two contrasting regions of the rift basin - with and without salt diapirism - are examined to assess the effect of salt tectonics in the evolution of subsidence patterns and stratigraphy. The Djebel Bou Dahar platform to basin system, located in the southern margin of the Atlas Basin, shows a Lower Jurassic record of normal faulting and lacks any evidence of salt diapirism. In contrast, the Tazoult ridge and adjacent Amezra¿ı basin, located in the centre of the Atlas Basin, reveals spectacular Early Jurassic diapirism. In addition, we analyse alternative Central High Atlas post-Middle Jurassic geohistories based on new thermal and burial models (GENEX 4.0.3 software), constrained by new vitrinite reflectance data from the Amezra¿ı basin. The comparison of the new subsidence curves from the studied areas with published subsidence curves from the Moroccan Atlas, the Saharan Atlas (Algeria) and Tunisian Atlas show that fast subsidence peaks were diachronous along the strike, being younger towards the east from Early-Middle Jurassic to Late Cretaceous. This analysis also evidences a close relationship between these high subsidence rate episodes and salt diapirism

Signatures géomorphologiques de l'activitée tectonique plio-quaternaire dans le sud des Andes centrales, Argentine

The mechanisms and the timing of the building of the Neuquén Andes in the southern central Andes of Argentina, are poorly documented during the Plio-Quaternary. In this work, we studied morphotectonics evidence to characterize the orogenic front propagation showing a compressional state of stress during the Plio-Quaternary. The southern Neuquén Andes correspond mechanically to an orogenic tectonic wedge while its northern part implies initially isolated crustal blocks in the foreland. In addition, a broad uplifting area in the current foreland shows that the lateral growth of the Neuquén Andes is registered far from the commonly recognized orogenic front. Moreover, we interpret contemporaneous extensional structures as large scale gravity glidings which are usually observed along passive margins. Crustal uplifts and ductile layers in the sedimentary cover would be the primarily control parameters of the gliding. Numerical models and an analytical solution show that the incision and sedimentation could modify the mechanical parameters and thus favour or inhibit the process.Les mécanismes qui mènent à la morphologie actuelle des Andes de Neuquén, dans le sud des Andes centrales (Argentine), sont mal compris. Dans ce travail, nous avons étudié les marqueurs morphotectoniques de la déformation au front de l'orogène qui montrent que le régime tectonique régional est compressif au Plio-Quaternaire. La partie méridionale des Andes de Neuquén constitue un prisme orogénique critique alors que la partie septentrionale implique plutôt la réactivation de blocs crustaux isolés dans l'avant-pays. Par ailleurs, le soulèvement dans l'avant-pays d'une large province crustale loin du front orogénique habituellement considéré à ces latitudes, traduit l'élargissement de l'orogène. Certaines structures extensives contemporaines de la compression s'expliquent par des glissements gravitaires similaires à ceux observés sur les marges passives. Les soulèvements crustaux et les couches sédimentaires ductiles contrôlent au premier ordre le processus. Des modèles numériques et une solution analytique montrent que l'incision fluviatile et la sédimentation favorisent ou inhibent l'initiation du processus

Dynamics of the undulation mode in swollen lamellar phases

We investigate the dynamics of the undulation mode (displacement of wave vector q parallel to the layers) in swollen lamellar phases. We calculate the dispersion equation of this mode for all wave vectors, from small q to high q (compared with the inverse of the layer spacing). We then calculate the static structure factor for scattering wave vectors Q parallel to the layers over the whole Q-range and also the dynamic structure factor at high Q.Nous étudions la dynamique du mode d'ondulation (vecteur d'onde de déplacement q parallèle aux couches) de phases lamellaires diluées. Nous calculons l'équation de dispersion de ce mode pour tous les vecteurs d'onde, i.e. des petits aux grands q (par rapport à l'inverse de la distance entre couches). Nous calculons ensuite le facteur de structure statique pour des vecteurs de diffusion Q parallèles aux couches sur toute la gamme de Q, ainsi que le facteur de structure dynamique aux grands Q

Diapiric architecture controlled by syn- and post-extension prograding sedimentary wedges

International audienc

The impact of syn- and post-extension prograding sedimentation on the development of salt-related rift basins and their inversion: Clues from analogue modelling

International audienceVarious studies have demonstrated the intrinsic interrelationship between tectonics and sedimentation in salt-related rift basins during extension as well as during their inversion by compression. Here, we present seven brittle–ductile analogue models to show that the longitudinal or transverse progradation of sediment filling an elongate extensional basin has a substantial impact on the growth of diapirs and their lateral geometrical variations. We use five extensional models to reveal how these prograding systems triggered diapir growth variations, from proximal to distal areas, relative to the sedimentary source. In the models, continuous passive diapir walls developed, after a short period of reactive–active diapiric activity, during syn-extensional homogeneous deposition. In contrast, non-rectilinear diapir walls grew during longitudinal prograding sedimentation. Both longitudinal and transverse post-extensional progradation triggered well-developed passive diapirs in the proximal domains, whereas incipient reactive–active diapirs, incipient roller-like diapirs, or poorly developed diapirs were generated in the distal domains, depending on the modelled sedimentary pattern. Two models included final phases of 6% and 10% shortening associated with basin inversion by compression, respectively, to discriminate compressional from purely extensional geometries. With the applied shortening, the outward flanks of existing diapir walls steepened their dips from 8°–17° to 30°–50°. Likewise, 6% of shortening narrowed the diapir walls by 32%–72%, with their fully closing (salt welds) with 10% of shortening. We compare our results with the distribution of salt walls and minibasins of the Central High Atlas diapiric basin in Morocco, which was infilled with a longitudinally prograding mixed siliciclastic and carbonatic depositional sequence during the Early–Middle Jurassic with a minimum thicknesses of 2.5–4.0 km

U–Pb dating of carbonate veins constraining timing of beef growth and oil generation within Vaca Muerta Formation and compression history in the Neuquén Basin along the Andean fold and thrust belt

We combine structural analysis of fractures with 22 U–Pb dates measured in fracture-filling carbonate cements from bed-parallel fibrous calcite veins (beef), conjugated veins and faults within the Vaca Muerta Formation along the Andean fold and thrust belt in the Neuquén Basin. The measured ages constrain accurately the relationships between overpressures caused by hydrocarbon generation and Andean compression as mechanisms for natural fracturing and vein formation. Two generations of fibres have been identified in beef. The first one, consists of dark fibres from the inner zones, which are perpendicular to bedding and contain abundant cone-in-cone structures and hydrocarbon inclusions. U–Pb dating of these fibres yielded Early to Late Cretaceous ages from 116.7 ± 17.7 to 78.8 ± 10.2 Ma. The second generation of fibres corresponds to the outer zones and consists of white fibres oblique to bedding, indicating growth during layer-parallel shortening. Bed-perpendicular veins cutting beef yielded Late Cretaceous-late Palaeocene dates from 72.8 ± 22.4 to 60.9 ± 10.4 Ma. Eocene ages from 52.0 ± 2.9 to 42.2 ± 18.9 Ma were measured in bed-parallel slip surfaces and reverse and strike-slip faults, whereas Miocene dates from 13.9 ± 2.6 to 6.2 ± 1.1 Ma were measured in E-W calcite veins. U–Pb dating of veins, structural analysis of fractures and subsidence curves, indicate that beef inner zones formed in the oil window during burial of the Neuquén basin, and that tectonic stresses could enhance their formation. Beef outer zones and bed-perpendicular veins formed during E-W Late Cretaceous-late Palaeocene layer-parallel shortening. Contrarily, late Palaeocene-late Eocene bed-parallel slip surfaces and faults and Miocene E-W veins formed during NE-SW and E-W syn-to post-folding deformation, respectively. In both cases, syn-to post-folding compression occurred synchronously with forelandward migration of magmatic activity attributed to flat subduction of the Pacific slab beneath the Andes

UPb dating of carbonate veins constraining timing of beef growth and oil generation within Vaca Muerta Formation and compression history in the Neuquén Basin along the Andean fold and thrust belt

We combine structural analysis of fractures with 22 U–Pb dates measured in fracture-filling carbonate cements from bed-parallel fibrous calcite veins (beef), conjugated veins and faults within the Vaca Muerta Formation along the Andean fold and thrust belt in the Neuquén Basin. The measured ages constrain accurately the relationships between overpressures caused by hydrocarbon generation and Andean compression as mechanisms for natural fracturing and vein formation. Two generations of fibres have been identified in beef. The first one, consists of dark fibres from the inner zones, which are perpendicular to bedding and contain abundant cone-in-cone structures and hydrocarbon inclusions. U–Pb dating of these fibres yielded Early to Late Cretaceous ages from 116.7 ± 17.7 to 78.8 ± 10.2 Ma. The second generation of fibres corresponds to the outer zones and consists of white fibres oblique to bedding, indicating growth during layer-parallel shortening. Bed-perpendicular veins cutting beef yielded Late Cretaceous-late Palaeocene dates from 72.8 ± 22.4 to 60.9 ± 10.4 Ma. Eocene ages from 52.0 ± 2.9 to 42.2 ± 18.9 Ma were measured in bed-parallel slip surfaces and reverse and strike-slip faults, whereas Miocene dates from 13.9 ± 2.6 to 6.2 ± 1.1 Ma were measured in E-W calcite veins. U–Pb dating of veins, structural analysis of fractures and subsidence curves, indicate that beef inner zones formed in the oil window during burial of the Neuquén basin, and that tectonic stresses could enhance their formation. Beef outer zones and bed-perpendicular veins formed during E-W Late Cretaceous-late Palaeocene layer-parallel shortening. Contrarily, late Palaeocene-late Eocene bed-parallel slip surfaces and faults and Miocene E-W veins formed during NE-SW and E-W syn-to post-folding deformation, respectively. In both cases, syn-to post-folding compression occurred synchronously with forelandward migration of magmatic activity attributed to flat subduction of the Pacific slab beneath the Andes