Intracontinental deformation in southern Africa during the late Cretaceous

Intracontinental deformation accommodated along major lithospheric scale shear zone systems and within associated extensional basins has been well documented within West, Central and East Africa during the Late Cretaceous. The nature of this deformation has been established by studies of the tectonic architecture of sedimentary basins preserved in this part of Africa. In southern Africa, where the post break-up history has been dominated by major erosion, little evidence for post-break-up tectonics has been preserved in the onshore geology. Here we present the results of 38 new apatite fission track analyses from the Damara region of northern Namibia and integrate these new data with our previous results that were focused on specific regions or sections only to comprehensively document the thermo-tectonic history of this region since continental break-up in the Early Cretaceous. The apatite fission track ages range from 449 ± 20 Ma to 59 ± 3 Ma, with mean confined track lengths between 14.61 ± 0.1 μm (SD 0.95 μm) to 10.83 ± 0.33 μm (SD 2.84 μm). The youngest ages (c. 80-60 Ma) yield the shortest mean track lengths, and combined with their spatial distribution, indicate major cooling during the latest Cretaceous. A simple numerical thermal model is used to demonstrate that this cooling is consistent with the combined effects of heating caused by magmatic underplating, related to the Etendeka continental flood volcanism associated with rifting and the opening of the South Atlantic, and enhanced erosion caused by major reactivation of major lithospheric structures within southern Africa during a key period of plate kinematic change that occurred in the South Atlantic and SW Indian ocean basins between 87-56 Ma. This phase of intraplate tectonism in northern Namibia, focused in discrete structurally defined zones, is coeval with similar phases elsewhere in Africa and suggests some form of trans-continental linkage between these lithospheric zones

The chronology and tectonic style of landscape evolution along the elevated Atlantic continental margin of South Africa resolved by joint apatite fission track and (U-Th-Sm)/He thermochronology

Atlantic-type continental margins have long been considered “passive” tectonic settings throughout the entire postrift phase. Recent studies question the long-term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U-Th-Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 µm, and mean (U-Th-Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130 Ma and 110–90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1–3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle-driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km

Natural age dispersion arising from the analysis of broken crystals, part II. Practical application to apatite (U-Th)/He thermochronometry

We describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite (U-Th)/He ages. The approach explicitly exploits the information about the shape of the 4He diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals. Additional dispersion arising from differences in grain size and in U and Th concentration of grains, and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage, is explicitly included. In this approach we calculate the ingrowth and loss, due to both thermal diffusion and the effects of α-ejection, of helium over time using a finite cylinder geometry. Broken grains are treated explicitly as fragments of an initially larger crystal. The initial grain lengths, L0, can be treated as unknown parameters to be estimated, although this is computationally demanding. A practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long L0 value to each fragment. We found that a good value for L0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments. Currently whole crystals and fragments with one termination are taken into account. A set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories, and including realistic amounts of random noise (5-15%), are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion. These include experiments where dispersion arises only from fragmentation of a single grain (length 400μm and radius 75μm, c. 6-50% dispersion), including the effects of grain size variation (for spherical equivalent grain radii between 74-122 μm, c. 10-70% dispersion) and the combined effects of fragmentation, grain size and radiation damage (for eU between 5-150 ppm, c.10-107% dispersion). Additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases. The viability of the new technique is demonstrated for a real data set of 25 single grain (U-Th)/He apatite ages obtained for a gabbro sample from the BK-1 (Bierkraal) borehole drilled through the Bushveld Complex in South Africa. The inversion produces a well constrained thermal history consistent with both the (U-Th)/He data and available fission track analysis data. The advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes, such as the effects of temporally variable diffusivity, zonation of U and Th and arbitrary grain size variations, and it works equally effectively for whole or broken crystals, and for the most common situation where a mixture of both are analysed. For the routine application of the apatite (U-Th)/He thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15-20 single grain analyses are typically required to characterise the age dispersion pattern of a sample. The experiments also suggest that picking very short crystal fragments as well as long fragments, or even deliberately breaking long crystals to maximise the age dispersion in some cases, would ensure the best constraints on the thermal history models. The inversion strategy described in this paper is likely also directly applicable to other thermochronometers, such as the apatite, rutile and titanite U-Pb systems, where the diffusion domain is approximated by the physical grain size

Evolution Néogène de l'arc~alpin~sud-occidental: Approches sismotectonique et thermochronologique

This thesis focuses on the Neogene evolution of the south-Western Alps. An integrated approach coupling in-field analysis, seismotectonics and low-temperature thermochronology was adopted and highlights a complex tectonic pattern as an original tecto-morphic evolution within the Alpine belt. The faulting analysis shows a strong original pattern of NW-SE dextral fault drawing orogen-parallel valleys and accompanying the Alpine curvature. These faults coexist with a second order network of steep normal faults accommodating orogen-parallel extension. Paleostress inversions reveal a complex history of stress/strain in the South-Western Alps, involving an extensional and a transcurrent regime. Chronological discrepancies suggest that these regimes have coexisted. I propose that they are end members of a fluctuating stress state that results from interference between a regional extensional field and a more general transpressional field. In our view, the more local field would be induced by gravity while the general field is likely induced by indentation of Apulia. The seismotectonic analysis shows that the south-Western Alps are undergoing extensional to transcurrent deformation in the Briançonnais zone and almost pure extension in the more internal Piemont zone. Extension is oriented E-W, as it is farther north in the arc, in apparent independence with the surface structural curvature. Discussing extension in terms of orogen-perpendicular vs. orogen-parallel is consequently no longer pertinent in the western Alpine arc. The area undergoing extension overspreads onto the inner fringe of the external zone, and is no longer strictly confined to the internal zones as it is further north in the rest of the arc. I propose that the area affected by extension correlates with high crustal thickness. This would promote buoyancy forces as the main factor governing extension. The arc of the Western Alps mainly reflects the upper-crustal structure and contrasts with more linear structures at depth. This implies a vertical uncoupling probably acquired during the Oligocene climax of the indentation-collision that could explain why the major thrust at the front of the internal arc was not reactivated as an extensional detachment like further to the north. The synthesis of new and previously published Apatite Fission Tracks data confirms that the south-Western Alps have undergone an original tectono-morphic evolution during Neogene times. Thermochronology reveals a rapid exhumation in the Internal zones just after the Oligocene collision climax and confirms a strong jump in age (around 20 Ma) with respect to the External Zone. The latter testifies for reactivation of the front of the internal arc as an extensional detachment soon after the collision climax. I relate the rapid exhumation of the south-Western Alps to the strong erosion of an elevated cordillera taking place in the internal zones. Quantification of relief evolution is tested by using the Apatite Fission track thermochronological system along two transects sampled in the Pelvoux and Dora-Maira massifs. Age-elevation relationships at several topographic wavelengths allow estimating a mean exhumation rate of 0.4km/Myr for the last 7Myr in the Pelvoux massif and of 0.1Km/Myr for the last 20Myr in the Dora-Maira massif. No clear evolution of relief through time is evidenced. We underline a general lake of resolution of the AFT system to quantify relief evolution. Synthesis of results allow proposing that the Neogene deformation results from a weak equilibrium between inner-to-the belt processes, related to a gravitational spreading of an overthickenned alpine crust, and outer processes related to the complex indentation/rotation of Apulia.Cette thèse cible l'évolution Néogène de la branche sud de l'arc alpin occidental. Une approche multi-méthodes associant études de terrain, sismotectonique et thermochronologie basse température y met en évidence un régime tectonique complexe ainsi qu'une évolution morphologique et structurale originale au sein de l'arc. La fracturation s'organise autour d'un faisceau d'accidents NW-SE, à jeu principalement dextre, accompagnant la courbure de l'arc et commandant structure et morphologie. Par ailleurs, à l'échelle des massifs, une structuration en failles normales conjuguées NE-SW traduit une extension parallèle à la chaîne. D'autres failles normales traduisent une extension perpendiculaire à la chaîne. L'inversion des données microtectoniques a permis de caractériser des régimes de paléocontraintes décrochant et extensif. L'absence d'une chronologie cohérente à l'échelle régionale suggère que ces régimes ont coexisté. Je propose que ces régimes constituent deux extrêmes d'un régime de contrainte unique variable, résultant de l'interférence entre un champ de contraintes extensif, à tendance multidirectionnelle et propre à la partie interne, surépaissie de la chaîne, et un champ de contraintes décrochant à transpressif commandé aux limites de la chaîne. L'analyse sismologique montre que l'arc interne subit actuellement un régime transtensif en zone Briançonnaise et un régime purement extensif en Zone Piémontaise, plus interne. La direction d'extension reste proche d'E-W, comme plus au nord à la latitude du Pelvoux, et indépendante de la courbure de l'arc. La caractérisation du régime extensif actuel en termes de directions tangentes ou radiales à l'arc est donc ici inadaptée. La Zone Externe présente un régime globalement transpressif à nuancer puisqu'elle est également affectée par de l'extension dans sa partie la plus interne. Je propose une corrélation entre la zone subissant l'extension et la zone d'épaisseur crustale maximum. Ce modèle s'inspire et étend celui proposé par [Delacou et al., 2004]. Il favorise un processus d'extension lié à un effondrement gravitaire de la chaîne. La structure arquée concerne surtout la croûte supérieure, les structures plus profondes étant plus linéaires. Ce découplage vertical des structures est probablement hérité du poinçonnement syncollision oligocène, responsable de la formation de l'arc interne. Il peut expliquer qu'au Néogène le découplage tectonique entre arc interne en extension et zone externe en transpression ne réactive pas, près de la surface, l'ancien front chevauchant de l'arc interne, comme c'est le cas plus au nord au sein de l'arc. Quelques nouvelles données thermochronologiques (traces de fission dans l'apatite), replacées dans une synthèse des données existantes, confirment que les Alpes sud-occidentales ont connu une évolution structurale et morphologique originale au Néogène. Les zones internes présentent un refroidissement final homogène et rapide dès la fin de l'Oligocène. Le saut d'âge entre zone externe et zones internes, récemment mis en évidence à la latitude du Pelvoux, est confirmé dans l'ensemble de la branche sud de l'arc. Il témoigne de l'inversion du front chevauchant des zones internes peu de temps après le paroxysme de la collision. Je relie l'exhumation rapide des zones internes de la branche sud à la forte érosion d'une cordillère alpine axée sur les zones internes au front du poinçon apulien. La quantification de l'évolution du relief à partir du système traces de fission sur Apatite a été testée sur la base de deux transects échantillonnés dans les massifs du Pelvoux et Dora-Maira. Les relations âges-altitudes à différentes longueurs d'onde permettent d'estimer un taux d'exhumation moyen de 0.4km/Ma pour le Pelvoux au cours des 7 derniers Ma, et de 0.1Km/Ma pour Dora-Maira au cours des 20 derniers Ma. Pour ces deux massifs, aucune évolution probante du relief n'est mise en évidence. On souligne dès lors le manque de résolution du système apatite pour quantifier l'évolution du relief. Une synthèse des résultats permet de proposer que la déformation soit le résultat d'un équilibre instable entre l'étalement de la chaîne sous l'effet des forces de volume et la cinématique complexe de l'Apulie qui intègre à la fois une composante en translation et une composante rotationnelle

Morphotectonic Evolution of Passive Margins Undergoing Active Surface Processes: Large-Scale Experiments Using Numerical Models

Extension of the continental lithosphere can lead to the formation of rifted margins with contrasting tectonic and geomorphologic characteristics. Many of these characteristics depend on the manner extension spatially distributed. Here we investigate the feedback between tectonics and the transfer of material at the surface resulting from erosion, transport, and sedimentation and discuss how they influence the rifting process. We use large-scale (1,200×600 km), high-resolution (1 km) numerical experiments coupling a 2-D upper-mantle-scale thermo-mechanical model with a planform 2-D surface processes model. We test the sensitivity of the coupled models to varying crust-lithospheric rheology and erosional efficiency. We confirm that the development and long-term support of topography is dependent on the strength of the coupling between the crust and the mantle lithosphere. Strong coupling promotes high topography as the integrated strength of the lithosphere is sufficient to support the additional stress. Weak coupling results in the stress being relaxed via viscous flow in the middle/lower crust and leads to more subdued topography. Erosion and transport of sediment modulates this behavior but has only minor effect on the overall structure of the rift. High erosion efficiency counters the development of high topography and creates complex landscape morphologies while low erosion efficiency allows for longer standing high topography and results in more simple landscape morphologies. The transfer of mass between the continent and the basin alters the stress field at the onshore-offshore transition and facilitates the development of faults, increasing their offsets and keeping them active over a longer period

Vasodilatation dépendante du flux et épaisseur intima-média carotidienne (utilisation en routine pour la mesure du risque vasculaire en prévention primaire)

L'altération de fonction endothéliale et l'épaississement de la paroi vasculaire surviennent de manière précoce dans le processus d'athérogénèse. Deux techniques échographiques d'évaluation, l'une structurelle, mesure de l'épaisseur intima-média de la carotide commune (IMT), et l'autre fonctionnelle, calcul de la vasodilatation dépendante du flux de l'artère brachiale (FMD), semblent prometteuses pour évaluer le risque vasculaire en prévention primaire. Le but de cette étude a été d'évaluer ces deux techniques d'imagerie vasculaire, en routine chez des sujets ne présentant pas de signes cliniques d'athérosclérose, mais ayant des facteurs de risque cardio-vasculaire. La première partie, concernant 126 sujets, met en évidence, une association entre l'altération de la fonction endothéliale d'une part, et l'épaisseur intima-média d'autre part, et certains facteurs de risque cardio-vasculaire. Cependant aucune corrélation n'est retrouvée entre ces deux techniques. La deuxième partie, concernant 46 sujets, ne montre pas d'évolution significative de la FMD et de l'IMT, à moyen terme, lorsque l'on contrôle les facteurs de risque traditionnels. Alors que l'évaluation de l'IMT est semi automatique, la mesure de la FMD se fait manuellement et reste donc très dépendant de l'opérateur. Elle est pour l'instant réservée à la recherche et des études de validation utilisant des techniques semi-automatiques sont nécessaires avant d'essayer son extension en routine.BORDEAUX2-BU Santé (330632101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Morphotectonic Evolution of Passive Margins undergoing Active Surface Processes: Model outputs

Extension of the continental lithosphere can lead to the formation of rifted margins with contrasting tectonic and geomorphologic characteristics. Many of these characteristics depend on the manner extension is spatially distributed. Here we investigate the feedback between tectonics and the transfer of material at the surface resulting from erosion, transport, and sedimentation and discuss how they influence the rifting process. We use large-scale (1200 x 600 km), high-resolution (1km) numerical experiments coupling a 2D upper-mantle-scale thermo-mechanical model with a plan-form 2D surface processes model. We test the sensitivity of the coupled models to varying crust-lithospheric rheology and erosional efficiency. We confirm that the development and long-term support of topography is dependent on the strength of the coupling between the crust and the mantle lithosphere. Strong coupling promotes high topography as the integrated strength of the lithosphere is sufficient to support the additional stress. Weak coupling results in the stress being relaxed via viscous flow in the middle/lower crust and leads to more subdued topography. Erosion and transport of sediment modulates this behaviour but has only minor effect on the overall structure of the rift. High erosion efficiency counters the development of high topography and creates complex landscape morphologies while low erosion efficiency allows for longer standing high topography and results in more simple landscape morphologies. The transfer of mass between the continent and the basin alter the stress field at the onshore-offshore transition and facilitates the development of faults, increasing their offsets and keeping them active over a longer period

Orogen-parallel brittle extension as a major tectonic imprint in the Neogene evolution of the south-western Alpine arc

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underworldcode/quagmire: Quagmire v0.5.0 Beta

Release Notes v0.5.0b This is the first formal "public" release of the code. High level summary of changes Introducing quagmire.function which is a collection of lazy-evaluation objects similar to underworld functions Introducing MeshVariables which wrap PETSc data vectors and provide interoperability with quagmire functions Providing context manager support for changes to topography that automatically update matrices appropriately Making all mesh variable data arrays view only except for assignment from a suitably sized numpy array (this is to ensure correct synchronisation of information in parallel). various @property definitions to handle cases where changes require rebuilding of data structures making many mesh methods private and exposing them via functions upstream integration is a function on the mesh upstream / downstream smoothing is via a mesh function rbf smoothing builds a manager that provides a function interfac