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

Abstract : 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.
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Submitted on : Tuesday, March 13, 2012 - 11:50:28 AM
Last modification on : Wednesday, December 9, 2020 - 3:10:39 PM
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  • HAL Id : tel-00642171, version 2

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Pierre-Yves Filleaudeau. Croissance et dénudation des Pyrénées du Crétacé supérieur au Paléogène : apports de l'analyse de bassin et thermochronométrie détritique. Sciences de la Terre. Université Pierre et Marie Curie - Paris VI, 2011. Français. ⟨tel-00642171v2⟩

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