Tectono-thermal history of an exhumed thrust-sheet-top basin : an example from the south Pyrenean thrust belt

This paper presents a new balanced structural cross-section of the Jaca thrust-sheet-top basin of the southern Pyrenees combined with paleo-thermometry and apatite fission track (AFT) thermochronology data. The cross-section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south-directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax>~120°C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene-lower Oligocene molasse (Tmax<~60°C). AFT results indicate a late Oligocene-early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleo-maximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240°C at the base of the turbidite succession. Inverse modelling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite-derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution

Croissance et dénudation des Pyrénées du Crétacé supérieur au Paléogène : apports de l'analyse de bassin et thermochronométrie détritique

Orogenic area result directly from the interplay between surface and deep processes. Our ability in discriminating the various forcing parameters controlling the growth of mountain ranges is thus highly dependent on our knowledge of deformation, exhumation and sedimentation timings, combined with precise constraints on the orogen geometry that permit us to quantify accretion and erosion flux. In this regard, the Pyrenees are a very interesting study area. The Pyrenees are an intraplate collision range of the Alpine-Himalayan system. They are built by the shortening of continental thinned margins during the convergence between Iberia and Eurasia lasting from Late Cretaceous to Early Miocene. The Pyrenees consist in a doubly vergent asymmetric orogenic wedge. From Priabonian to Miocene times, the Ebro foreland basin underwent endorheic conditions due to the closure of all oceanic connexions by the the continuing convergence between Iberia and Eurasia. The result is the exceptional preservation of synorogenic strata. Because of the duration of orogenic growth and its sedimentary evolution that both happen during the Cenozoic climatic cooling, the Pyrenees are the perfect target to investigate coupling and retroactions between climate and tectonics. The first part of this work is focused on the infilling of peripheral basins based on well-log data. We discuss ante-convergence evolution, structural inheritance and synorogenic infill. We first calculate subsidence from Late Triassic to Miocene and then we derive sedimentary fluxes from isopach maps. The sedimentary flux increases at the beginning of the collision (Late Cretaceous) and can be correlated to the convergence rates between Iberia and Eurasia. After the Middle Eocene, sedimentary flux increases rapidly that cause the complete filling of the basin whereas subsidence has stopped migrating, convergence is slowing and the orogenic wedge is not widening. A detailed biostratigraphic and sedimentologic analysis of Lutetian-Bartonian boundary limestones show that the disappearance of the platforms is due to an environmental stress caused by the combination of regional tectonics and climatic cooling. In the second part of this work we bring new thermochronological data on the early stages of orogenic growth. This study uses multiple dating methods on single detrital grains (U/Pb and (U-Th)/He on detrital zircons) and Apatite Fission Tracks. The combination of these methods is a powerful tool to identify source-rocks and estimate their cooling histories. The past source-rocks in the retro-wedge to the North are characterized by dominant Cadomian granitoids whereas Variscan granitoids are highly prevailing in the pro-wedge to the South, that is in agreement with current distribution of sources. We identify two Mesozoic exhumation phase before the synorogenic cooling in our samples. The older, with Triassic age, is to relate to the post-Variscan denudation. The second phase is Albian, related to the ante-orogenic extension. Because the samples are non-reset, this demonstrates that Mesozoic burial of sedimentary strata was lower than 2-3 km. We calculate exhumation rates from 0.15 to 0.35 km after the onset of orogenesis based on the different populations recognized in the samples. Finally we propose a reconstruction of the Central Pyrenees based on a crustal-scale balanced and restored cross-section that takes into account the constraints on exhumation and the extreme thinning of continental margins during extension. The restoration gives a minimum shortening of 103 km from an initial stage with the lithospheric mantle exhumed along a south-verging crustal detachment. The comparison of shortening and kinematic models of Iberia's displacement precise the width of the thin- ned domain to about forty kilometers. The restored sections also provide the incoming and outcoming fluxes across the orogenic wedge. We highlight an important phase of wedge contraction, combined with important erosional fluxes that completely fill the Ebro foreland basin during Oligocene. This is coeval with Cenozoic global cooling so we suggest that climate is the major forcing control the the Pyrenean wedge at this time. With this study we demonstrate that with the appropriate coupled methods and data from well-constrained orogens like the Pyrenees we can discuss forcing and growth mechanisms on margin inversion.Les zones orogéniques sont le résultats d'interactions entre processus profonds et de surface. Notre capacité à distinguer les différents forçages sur la formation des chaînes de montagnes est fortement dépendante d'une connaissance précise du calendrier de la déformation, de l'exhumation et de la sédi- mentation, combinées à des contraintes précises sur la géométrie pour quantifier les flux de matières. Les Pyrénées sont en ce sens exemplaires. Les Pyrénées sont une chaîne de collision intraplaque du système Alpes-Himalaya formée par le rac- courcissement de marges continentales préalablement amincies, lors de la convergence entre la plaque Ibérique et la plaque Eurasiatique du Crétacé supérieur au Miocène. Il s'agit d'un prisme orogénique à double vergence déversé vers le Sud. L'évolution géodynamique de la plaque Ibérique a entraîné la ferme- ture de toutes les connexions océaniques du Bassin de l'Ebre du Priabonien au Miocène, ce qui a permis la préservation exceptionnelle des séries syn-orogéniques. Du fait de son histoire tectono-sédimentaire et sa longue durée de construction, contemporaine des changements climatiques du Cénozoïque, cette chaîne est donc la cible idéale pour l'étude des couplages et rétroactions entre climat et tectonique. La première partie de ce travail présente une étude des données de puits disponibles dans les bassins d'avant-pays. Ces puits sont utilisés pour discuter l'évolution pré-convergence des bassins flexuraux et le remplissage syn-orogénique. La subsidence du bassin du Trias au Miocène est calculée à partir de ces données, qui servent également à estimer le flux sédimentaire sortant du système orogénique à travers des cartes d'isopaques. Le flux de sédiments augmente à la fin du Crétacé, soit au début de la colli- sion et montre une évolution qui est corrélable aux vitesses de convergence de l'Ibérie vers l'Eurasie au début de la croissance de la chaîne. A partir de l'Eocène supérieur, le flux sédimentaire augmente rapidement alors que la convergence diminue et que la chaîne ne s'élargit pas, ce qui cause le com- blement total du Bassin de l'Ebre. Une étude sédimentologique et biostratigraphique a été menée à la limite Lutétien-Bartonien montre que la disparition des plateformes carbonatées est le résultat d'un cocktail fatal refroidissement/flux lors du passage à un régime climatique instable couplé à la tectonique pyrénéenne. Dans un deuxième temps, des nouvelles contraintes thermochronologiques sont apportées sur les premiers stades de croissance de la chaîne à partir de la double datation (U/Pb et (U-Th)/He) sur zircons détritiques, couplée au comptage de traces de fission sur apatites détritiques a permis de caractériser à la fois le type de source à l'affleurement ainsi que leur refroidissement. Une différence est identifiée entre le versant Nord où les granitoïdes Cadomiens sont dominants et le versant Sud où les granites Varisques sont fortement majoritaires, ce qui est en accord avec les roches qui constituent les bassins versants actuels. En plus de l'exhumation syn-orogénique, des phases plus anciennes sont reconnues au Trias et à l'Albien. Le fait que ces phases soient préservées dans les sédiments anciens montre que l'enfouissement pré-orogénique des séries a été limité à 2-3 km au Mésozoïque et permet de comparer les âges obtenus avec ceux disponibles dans la littérature. L'exhumation au Trias est interprétée comme la dénudation post-Varisque et l'exhumation albienne correspond au stade post-rift dans les Pyrénées. Des taux d'exhumation de 0,15 à 0,35 km/Ma sont calculés au début de l'orogenèse à partir des différentes populations reconnues dans les échantillons. La dernière partie de ce travail est une reconstitution de la géométrie des Pyrénées centrales à partir d'une nouvelle coupe crustale équilibrée et restaurée à quatre étapes clés de la croissance orogénique qui tiennent compte des contraintes sur l'exhumation ainsi que de l'amincissement des marges Ibérique et Européenne lors de leur amincissement extrême. La restauration montre un raccourcissement minimal de 103 km à partir d'un état initial où le manteau lithosphérique est exhumé le long d'un détachement crustal. La comparaison du raccourcissement et de la cinématique obtenus avec l'histoire de la convergence de l'Ibérie permet de préciser la largeur du domaine aminci à une quarantaine de kilomètres. Les coupes restaurées sont également utilisées pour estimer des flux de matière au travers du prisme orogénique. Celles-ci mettent en évidence une phase importante d'érosion et de rétrécissement du prisme à l'Oligocène, contemporaine du refroidissement global illustré par de l'établissement des premières calottes polaires, et liée au début du remplissage rapide du bassin de l'Ebre. Cette étude suggère qu'il est possible via des méthodes couplées, adaptées aux orogènes très bien contraints comme les Pyrénées, de discuter des mécanismes de croissance et de forçage contemporains de l'inversion de marges continentales

Thermo-tectonic evolution of the south-central Pyrenees from rifting to orogeny: insights from detrital zircon U/Pb and (U-Th)/He thermochronometry

International audienceConstraining the thermal and denudational evolution of continental margins from extensional episodes to early orogenic stages is critical in the objective to better understand the sediment routing during the growth of orogenic topography. Here, we report 160 detrital zircon U/Pb ages and 73 (U-Th)/He ages from Albian, Upper Cretaceous and Eocene sandstones from the south-central Pyrenees. All samples show dominant zircon U/Pb age peaks at 310-320 Ma, indicating a primary contribution from Variscan granites of the central Pyrenean Axial Zone. A secondary population at 450-600 Ma documents zircon grains sourced from the eastern Pyrenees. Zircon (U-Th)/He ages recovered from older samples document, a Triassic age peak at ca. 241 Ma, corresponding to denu- dation coeval with the initiation of Atlantic rifting. An Early Cretaceous cooling event at ca. 133 Ma appears consistent with rift-related exhumation and thermal overprint on the Iberian margin. The (U-Th)/He age peaks from ca. 80 Ma to ca. 68 Ma with decreasing depositional ages are inter- preted to reflect the southward-migrating thrust-related exhumation on the pro-wedge side of the Pyrenean orogen. The increase in lag times, from ca. 15 Ma in the Tremp Formation (ca. 65 Ma) to 28 Ma in the Escanilla Formation (ca. 40 Ma), suggests decreasing exhumation rates from 0.4 km Myr-1 to 0.2 km Myr-1. The apparent inconsistency with convergence rates is used to infer that rocks cooled at 68 Ma may have resided in the crust before final exhumation to the surface. Finally, the cooling event observed at 68 Ma provides support to the inferred acceleration of conver- gence, shortening and exhumation during Late Cretaceous times

Paleogene propagation of the southern Pyrenean thrust wedge revealed by finite strain analysis in frontal thrust sheets: Implications for mountain building

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Integrated stratigraphic and petroleum system modeling study of the southern upper Rhine Graben

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Large scale facies change in the middle Eocene South-Pyrenean foreland basin: The role of tectonics and prelude to Cenozoic ice-ages

International audienceThe present study reports a sedimentological analysis of the Guara Limestone Formation deposited during the Lutetian in the Sierras Exteriores, in the South-Pyrenean foreland basin. We provide a detailed facies analysis of the carbonates to precise the paleoenvironmental context during their deposition. We show that those limestones are mainly composed of shallow-water foraminifers and were deposited in relative shallow-water environments (< 120m) during the whole Lutetian (SBZ 13 to SBZ 16). The Guara Limestone Formation represents the last occurrence of carbonate platform in the South-Pyrenean foreland basin and disappeared definitely at the Lutetian to Bartonian transition. The demise of carbonate producers at the end of the Lutetian could be related to an increase of continental erosion, due to tectonic and/or climatic forcing. We illustrate that in the Jaca basin, this event correlates with a marked increase in subsidence rate. However, this deformation event is local and the carbonate systems in the Pyrenean foreland resisted to many deformation events during the whole basin history before. Paleobathymetric reconstructions in the Jaca basin, where shallow marine sections outcrop, suggest an increase of the amplitude of high-frequency sea-level cycles. This increase is contemporaneous with several climatic evidences, which suggest the appearance of early ice-sheets near the Lutetian-Bartonian boundary. The demise of carbonate producers seems, therefore, to be the result of a major environmental shift in the basin accompanying increased subsidence rates, switching from low nutrient oligotrophic conditions - favourable for shallow water benthic foraminifers - to eutrophic conditions due to the increase of erosion and terrigenous nutrient input associated with higher-frequency sea-level changes and river destabilization. © 2012 Elsevier B.V

Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence

International audienceEstimating shortening in collision belts is critical to reconstruct past plate motions. Balanced cross-section techniques are efficient in external domains but lack resolution in the hinterland. The role and the original extent of the continental margins during the earliest stages of continental convergence are debated. Here we combine existing and new sequentially restored cross sections in the central Pyrenees, with Iberia/Europe (IB/EU) plate kinematic reconstructions and new apatite fission track, zircon (U-Th)/He, and U/Pb ages to discuss higher and lower bounds of crustal shortening and determine the amount of distal margin sutured during collision. We show that after extension in the Albian (~110 Ma), a 50 km wide extremely thinned crustal domain underwent subduction at 83 Ma. Low-temperature data and thermal modeling show that synorogenic cooling started at 75–70 Ma. This date marks the transition from suturing of the highly extended margin to collision of the more proximal margin and orogenic growth. We infer a relatively low crustal shortening of 90 km (30%) that reflects the dominant thick-skinned tectonic style of shortening in the Pyrenees, as expected for young (Mesozoic) and weak lithospheres. Our proposed reconstruction agrees with IB/EU kinematic models that consider initially rapid convergence of Iberia, reducing from circa 70 Ma onward. This study suggests that plate reconstructions are consistent with balanced cross sections if shortening predicted by age-dependent properties of the continental lithosphere is taken into account