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Évolution sédimentaire, structurale et thermique d'un rift hyper-aminci : de l'héritage post-hercynien à l'inversion alpine : exemple du bassin de Mauléon (Pyrénées)

Abstract : The western Pyrenean Mauléon basin allows to discuss the tectono-sedimentary and thermal evolution of a hyperextended rift through an entire Wilson cycle. During Permian time, the western Pyrenees record the shift between N-S convergence recorded in the Pyrenean Axial Zone (310 to 290 Ma) and dominant E-W extension (290 to 275 Ma). This latter stage is responsible for a complex thermal, structural and compositional inheritance of the Pyrenean lithosphere. The preservation of the original Permian-Triassic paleogeography and structure in the “Basque Massif” indicates that there was no major east-west Mesozoic strike-slip motion between the Iberian and European plates in this part of the Pyrenees, questioning the mechanisms responsible for the opening of the Early Cretaceous North-Pyrenean rift system. The Cretaceous Mauléon hyperextended rift basin was inverted during the Alpine orogeny. Reactivation of the former rift structures leads to the formation of a lithospheric scale pop-up whose edges are characterized by differing tectonic reactivation style, localized (Iberia) vs distributed (Europe). The previously exhumed mantle acts as a buttress inhibiting the complete closure of the basin. In 3D, the N120° thrusts systems edging the pop-up are composed of different thrust segments branching into the inherited Permian N20° transfer zones, highlighting the non-cylindrical geometry of the Pyrenean structures. Permian and Cretaceous structural inheritance both control the synrift and post-collisional thermal evolution of the Mauléon basin. Synrift paleogeothermal gradients gradually increased from the margins (~ 34°C/km) to the basin (~ 60°C/km). Maximum peak temperatures are both controlled by sedimentary burial and mantle heat flow (100 mW.m-2). The different post-collisional thermal responses of the Mauléon basin pop-up edges is linked to their tectonic reactivation style. On the European margin, the postrift isotherms were passively transported onto the proximal margin (« thin-skinned »). On the Iberian margin, peak temperatures were acquired after thrusting (« thick-skinned »). The review of the hyperextended Iberian-Eurasian plate boundary basins shows that they strongly differ from classical Atlantic-type passive margins. Three main stages of continental crust thinning can be inferred to describe the evolution of this smooth-slope type extensional basin : (1) a dominant ductile thinning of the lower crust, without significant brittle deformation of the upper crust, resulting in the formation of a symmetric sag basin; (2) basinward gliding of the prerift cover along the Late Triassic evaporites; (3) hyper-extension with brittle deformation on the proximal margins and dominant ductile thinning on the hyperextended rift domain (HT/LP metamorphism).
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Nicolas Saspiturry. Évolution sédimentaire, structurale et thermique d'un rift hyper-aminci : de l'héritage post-hercynien à l'inversion alpine : exemple du bassin de Mauléon (Pyrénées). Sciences de la Terre. Université Michel de Montaigne - Bordeaux III, 2019. Français. ⟨NNT : 2019BOR30040⟩. ⟨tel-02860515⟩

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