Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution

Reconstruction de la dynamique précoce d'un orogène : mise en évidence de la transition Rifting-Collision dans le système Est-pyrénéen (France) par la géo-thermochronologie

Collisional orogens are classically described as the result of continental accretion of proximal margins. This accretion leads to the creation of relief and to the important export of erosion products in the directly adjacent foreland basins. In this geodynamic scheme, only the least deformed pre-orogenic domains are considered. However, a growing number of geological field evidences indicate the preservation or even the reuse of structures inherited from the rifting phase preceding convergence and collision within orogens. By conducting a geo-thermochronometric study of the easternmost, inverted hyperextended Aptian-Cenomanian rift system (Agly massif, North Pyrenean Zone) and the adjacent early retroforeland (Rennes-les-Bains syncline, Aquitaine Basin), this thesis aims to describe the evolution of a distal rifted margin during the first stages of convergence, to quantify the associated source-to-sink processes and to provide temporal and paleogeographic constraints regarding the creation of the very first Pyrenean reliefs resulting from inversion of the margin. Using the zircon and apatite (U-Th)/He multi-thermochronometers, I show that the Pyrenean retro-wedge records two clear phases of orogenic cooling, Late Campanian-Maastrichtian and Ypresian-Bartonian, which I relate to early inversion of the distal rifted margin and main collision, respectively. I have thus been able to propose a crustal-scale sequentially restored model for the tectonic and thermal transition from extension to peak orogenesis in the eastern Pyrenees, which suggests that both thrusting and underplating processes contributed to early inversion of the Aptian-Cenomanian rift system. The absence of Paleocene cooling record indicates little to no erosion of the Pyrenean retro-wedge, suggesting the existence of a more easterly source area supplying early retroforeland sediments at this time. To characterize this eastern edifice, which has since been destroyed by the Oligocene-Miocene opening of the Gulf of Lion, I used in situ (U-Th)/He - U/Pb double dating on detrital zircons and show rapid denudation rates during early convergence, characteristic of early topographic growth. This work shows for the first time clearly the progressive migration of deformation from east to west by inversion of inherited structures at the beginning of Pyrenean convergence. This suggests the existence of an open domain in the east at the end of the rifting phase preceding convergence. This study highlights the role of the architecture of hyper-thinned systems in the formation of collisional orogens and confirms the close links between an orogen and its foreland basins.Les orogènes collisionnels sont classiquement décrits comme le résultat de l'accrétion continentale de marges proximales. Cette accrétion conduit à la surrection des reliefs et à l'export important de produits d'érosion dans les bassins d'avant-pays qui les jalonnent. Dans ce schéma géodynamique sont alors uniquement considérés les domaines pré-orogéniques les moins déformés. Pourtant, un nombre croissant d'évidences géologiques de terrain indiquent la conservation voire la réutilisation de structures héritées de la phase extensive précédant la convergence et la collision au coeur des orogènes. À partir de l'étude géo-thermochronométrique de la bordure orientale du domaine hyper-étiré nord-pyrénéen (massif de l'Agly, Zone Nord Pyrénéenne) et de son avant-pays précoce (synclinal de Rennes-les-Bains, Bassin Aquitain), cette thèse a pour objectifs de décrire l'évolution d'une marge distale au cours des premiers stades de convergence, de quantifier les processus source-réceptacle associés et d'apporter des contraintes temporelles et paléogéographiques quant à la création des tout premiers reliefs pyrénéens issus de son inversion. Grâce à l'utilisation du multi-thermochronomètre (U-Th)/He sur zircon et apatite, deux épisodes de refroidissement sont mis en évidence dans le prisme nord-pyrénéen (Campano-Maastrichtien et Eocène), chacun d'eux synchrone d'une phase de subsidence dans le bassin d'avant-pays. J'ai ainsi pu proposer un modèle équilibré d'évolution d'une marge distale hyper-amincie par inversion de structures héritées, chevauchements de socle et sous-placage continental se matérialisant par une signature thermochronologique claire de refroidissement sans érosion au début de la convergence. L'absence d'enregistrement de refroidissement au Paléocène par l'arrêt prématuré de l'inversion précoce dans le prisme nord-pyrénéen indique l'absence significative d'érosion et la position bordière de ce prisme par rapport à un édifice déjà construit plus à l'est à cette époque. Pour caractériser cet édifice aujourd'hui disparu du fait de l'ouverture du Golfe du Lion, j'ai utilisé une approche détritique de double datation in situ (U-Th)/He - U/Pb sur zircon et mis en évidence une histoire de dénudation rapide pendant le Campano-Maastrichtien, caractéristique de la création d'une topographie précoce. Ce travail montre pour la première fois clairement la migration progressive de la déformation d'est en ouest par l'inversion de structures héritées au début de la convergence pyrénéenne, ce qui suggère l'existence d'un domaine ouvert à l'est à la fin de l'épisode extensif précédant la convergence. Cette étude met en avant le rôle de l'architecture des systèmes hyper-amincis dans la formation des orogènes collisionnels et confirme les liens étroits existant entre un orogène et ses bassins d'avant-pays

Decoding low-temperature thermochronology signals in mountain belts: modelling the role of rift thermal imprint into continental collision

International audienceResolving the timing of initiation and propagation of continental accretion associated with increasing topography and exhumation is a genuinely challenging task using low-temperature thermochronology. We present an integrated thermo-mechanical and low-temperature thermochronology modelling study of tectonically-inverted hyperextended rift systems. Model low-temperature thermochronology data sets for apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He and zircon fission-track systems, which are four widely used thermochronometric systems in orogenic settings, are generated from fourteen locations across a model collisional, doubly-vergent orogen. Our approach allows prediction of specific, distinct low-temperature thermochronology signatures for each domain (proximal, necking, hyperextended, exhumed mantle) of the two rifted margins that, in turn, enable deciphering which parts of the margins are involved in orogenic wedge development. Our results show that a combination of zircon (U-Th)/He and apatite fission-track data allows diagnostic investigation of model orogen tectonics and offers the most valuable source of thermochronological information for the reconstruction of the crustal architecture of the model inverted rifted margins. The two thermochronometric systems have actually very close and wide closure windows, allowing to study orogenic processes over a larger temperature range, and therefore over a longer period of time. Comparison of model data for inverted rifted margins with model data for non-inverted, purely thermally-relaxed rifted margins enables assessing the actual contribution of tectonic inversion with respect to thermal relaxation. We apply this approach to one of the best-documented natural examples of inverted rift systems, the Pyrenees. Similarities between our thermochronometric modelling results and published low-temperature thermochronology data from the Pyrenees provide new insights into the evolution of the range from rifting to collision. In particular, they suggest that the core of the Pyrenean orogen, the Axial Zone, consists of the inverted lower plate necking and hyperextended domains while the Pyrenean retrowedge fold-and-thrust belt, the North Pyrenean Zone, represents the inverted upper plate distal rifted margin (exhumed mantle, hyperextended and necking domains). This is in good agreement with previous, independent reconstructions from literature, showing the power that our integrated study offers in identifying processes involved in orogenesis, especially early inversion, as well as in predicting which domains of rifted margins are accreted during mountain building.Identifier au moyen de la thermochronologie basse température le moment à partir duquel l’accrétion continentale s’amorce et se propage est une tâche difficile. Dans cette étude, nous couplons modélisation thermomécanique de systèmes hyper-amincis inversés et thermochronologie basse température pour prédire la signature thermochronologique de quatre systèmes basse température communément utilisés en domaine orogénique ((U-Th)/He sur apatite, traces de fission sur apatite, (U-Th)/He sur zircon et traces de fission sur zircon). Notre approche de modélisation permet de prédire les signatures thermochronologiques basse température propres à chaque domaine de marge du système extensif (domaine proximal, zone de neck, domaine hyper-aminci et domaine de manteau exhumé). Ces signatures permettent en retour d’identifier les domaines de marge impliqués dans l’accrétion continentale qui suit l’extension. Nos résultats montrent que la combinaison des signatures thermochronologiques des systèmes (U-Th)/He sur zircon et traces de fission sur apatite renseigne sur les processus mis en jeu lors de la formation d’un orogène collisionnel à double vergence, ainsi que sur l’architecture des marges inversées durant l’orogénèse. Les deux systèmes thermochronométriques possèdent en effet des gammes de température de fermeture voisines et relativement larges, permettant d’étudier les processus orogéniques sur un intervalle de températures plus grand, et donc sur une période de temps plus longue. La comparaison des signatures thermochronologiques prédites dans un modèle thermomécanique d’inversion avec celles prédites dans un modèle de relaxation thermique, sans convergence postérieure à l’extension, permet d’apprécier la nécessaire contribution des processus tectoniques d’inversion par rapport au processus de relaxation thermique postrift. Nous appliquons notre approche à l’un des exemples naturels les mieux documentés de systèmes de rift inversé, les Pyrénées. Les similitudes qui existent entre nos prédictions thermochronométriques et les données de thermochronologie basse température publiées pour les Pyrénées fournissent de nouvelles informations sur la formation et l’évolution de l’orogène pyrénéen. En particulier, ces similitudes suggèrent que la zone interne de déformation pyrénéenne, la Zone Axial, est constituée des domaines de neck et hyper-aminci de la plaque inférieure, inversés durant l’accrétion, tandis que le prisme orogénique qui se développe sur la plaque européenne, la Zone Nord-pyrénéenne, représente la marge distale inversée de la plaque supérieure. Ce résultat est en accord avec les précédentes reconstructions publiées de la dynamique pyrénéenne, démontrant ainsi le potentiel de notre approche dans l’identification des processus impliqués dans l’orogenèse, notamment l’inversion précoce, ainsi que dans la prédiction du type de domaine de marge accrété lors de la formation des chaînes de montagnes

Resolving timing of early convergence from low-temperature thermochronology applied to Alpine-type systems

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Pre-, syn- and post-breakup evolution of northern South America from apatite fission track data and inverse thermal history modelling

International audiencePhanerozoic evolution of the Guiana Shield in northern South America is poorly known and remains an important question for source-to-sink studies. As part of the Source to Sink project TOTAL R&D-BRGM-INSU, this study presents a suite of 40 new apatite fission track analysis results from French Guiana, Surinam and north Brazil. Inverse thermal history modelling reveals a marked, albeit poorly constrained, period of relatively high temperatures (temperatures > 110 °C) during the Jurassic (200-150 Ma), i.e. prior to the generally accepted timing of the Equatorial Atlantic Ocean opening (150-110 Ma). Lower temperatures are recorded on the southerm flank of the Amazonas Basin and a little portion of the Surinam interior. All data reveal then a protracted phase of cooling starting from ~150 Ma and representing the main event during the Mesozoic and Cenozoic for French Guiana and Surinam basement. A second thermal event is nevertheless recorded after the deposition of Upper Jurassic and Lower Cretaceous detrital sedimentary rocks on the northern flank of the Amazonas Basin. Maximum temperatures for this event (80-100 °C) are reached during the middle Cretaceous (115-90 Ma). Implications of these new results for the Guiana Shied evolution and the Equatorial Atlantic opening are discussed

Pre-, syn- and post-breakup evolution of northern South America from apatite fission track data and inverse thermal history modelling

International audiencePhanerozoic evolution of the Guiana Shield in northern South America is poorly known and remains an important question for source-to-sink studies. As part of the Source to Sink project TOTAL R&D-BRGM-INSU, this study presents a suite of 40 new apatite fission track analysis results from French Guiana, Surinam and north Brazil. Inverse thermal history modelling reveals a marked, albeit poorly constrained, period of relatively high temperatures (temperatures > 110 °C) during the Jurassic (200-150 Ma), i.e. prior to the generally accepted timing of the Equatorial Atlantic Ocean opening (150-110 Ma). Lower temperatures are recorded on the southerm flank of the Amazonas Basin and a little portion of the Surinam interior. All data reveal then a protracted phase of cooling starting from ~150 Ma and representing the main event during the Mesozoic and Cenozoic for French Guiana and Surinam basement. A second thermal event is nevertheless recorded after the deposition of Upper Jurassic and Lower Cretaceous detrital sedimentary rocks on the northern flank of the Amazonas Basin. Maximum temperatures for this event (80-100 °C) are reached during the middle Cretaceous (115-90 Ma). Implications of these new results for the Guiana Shied evolution and the Equatorial Atlantic opening are discussed

Laser Ablation ( U‐Th‐Sm )/He Dating of Zircons: Analytical Age Bias and its Consequence for Studying Detrital Zircon Populations

International audienceThe in situ (U‐Th‐Sm)/He and U/Pb laser‐ablation double‐dating procedure is a valuable method that can provide a large dataset relatively efficiently in contrast with conventional bulk helium thermochronometry. In this study, we evaluate the potential age error associated with the double ablation procedure and report the in situ (U‐Th‐Sm)/He double‐ablation dating of 249 zircons from the Fish Canyon Tuff locality. With LA‐ICP‐MS pseudo‐depth profiling and 3D numerical modelling, we show that the concentric double‐ablation procedure in minerals with U‐Th‐Sm zoning can generate a significant (U‐Th‐Sm)/He age error (positive or negative), resulting in over‐scattering and/or an offset of the mean age. Pseudo‐depth profiling is insufficient to predict the individual age error, partly because of the superimposed ablations. To evaluate the consequence of this inherent bias, we confront a synthetic age distribution to the error expected for U‐Th‐Sm zoned zircons analysed with double‐ablation (U‐Th‐Sm)/He thermochronometry. As expected, a strong age bias causes the spreading of peak ages, downgrading the original signal. Yet, the throughput of the ablation‐based method can allow intra‐ and inter‐sample peak age identification and comparison, and the coupling of (U‐Th‐Sm)/He and U/Pb ages extends our ability to deconvolute a multimodal age spectrum

Investigating the Shallow to Mid-Depth (>100–300 °C) Continental Crust Evolution with (U-Th)/He Thermochronology: A Review

International audienceQuantifying geological processes has greatly benefited from the development and use of thermochronometric methods over the last fifty years. Among them is the (U-Th)/He dating method, which is based on the production and retention, within a crystal structure, of radiogenic 4He atoms associated with the alpha decay of U, Th and Sm nuclei. While apatite has been the main target of (U-Th)/He studies focusing on exhumation and burial processes in the upper levels of the continental crust (~50–120 °C), the development of (U-Th)/He methods for typical phases of igneous and metamorphic rocks (e.g., zircon and titanite) or mafic and ultramafic rocks (e.g., magnetite) over the last two decades has opened up a myriad of geological applications at higher temperatures (>100–300 °C). Thanks to the understanding of the role of radiation damage in He diffusion and retention for U-Th-poor and rich mineral phases, the application of (U-Th)/He thermochronometry to exhumation processes and continental evolution through deep time is now mainstream. This contribution reviews the (U-Th)/He thermochronometer principle and the influence of radiation damage in modifying the diffusion behavior. It presents applications of (U-Th)/He dating to problems in tectonic and surface processes at shallow to middle crustal depths (>100–300 °C). New and promising applications using a combination of methods will stimulate a research avenue in the future

Orogenic evolution of Western Europe controlled by lithosphere evolution

International audienceProcesses driving orogenic styles and long-term isostatic versus dynamic support of the topography have been largely debated in domains of plate convergence. Tectonics of orogens reflect the interactions between mantle flow driving plates and the inherited rheology and composition of moving plates, which are however still strikingly ill-defined. A recent review of the evolution of the weak European lithosphere, based on geological, geophysical, petrological data, has shed lights on the role played by lithospheric mantle chemo-magmatic history and structure, which inherits past subduction/collision (e.g. Variscan) and rifting events (Tethys/Atlantic), on crust-mantle coupling, plate-mantle coupling, defining Alpine-type orogens. While the details of the Cenozoic topographic history of peri-Mediterranean orogens are understood to be controlled by the rheology and architecture of rifted margins combined with changing large-scale kinematic boundary conditions (e.g. Atlas, Betics, Pyrenees, Alps), their post-10 Ma, quaternary to current surface (Insar) and tectonic (seismic) evolution appears to illustrate increasing control by magmatism and flow at the asthenosphere-lithosphere limit as well as local thermal re-equilibration. We argue that isostatic processes in western Europe linked in part to long lithosphere evolution can be first-order drivers of the post-collisional evolution of the peri-Mediterranean orogenic belts and their still active surface and tectonic evolution

New geo-thermochronometric constraints on the evolution of the Pyrenean-Provencal orogen

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