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Cinématique et thermicité du prisme orogénique des Pyrénées : nouvelles données géo-thermochronologiques de la Zone Axiale

Abstract : The Pyrenean range is an antiformal stack with in-sequence thrust propagation. However, during Paleo-Mesozoic times the basement underwent: Variscan deformation, fluid flow, metamorphism. Newly-formed phylosilicates induce a brittle-ductile regime in upper crust and allows significant distributed shortening through the Axial Zone during Alpine orogeny. Thus, Variscan and Mesozoic inheritance implies a weaker upper crust and distributed deformation. The K/Ar in situ protocol aims at constrain the age of distributed deformation and also benefits from a textural control. Results from the Bielsa massif suggest a Variscan sericitization and the formation of reverse shear zones during early stages of Alpine collision. Published and newly-acquired thermochronological data (AFT, ZFT, ZHe) constrain the sequential restoration of Pyrenean prism during Meso-Cenozoic compression. They also provides estimates of a strong inherited thermal gradient (up to ~ 50 ± 20 °C/km) and a syn-rift denudation in northern AZ. A relatively low orogenic cooling rate distributed on whole Axial Zone, and coeval with the activation of several shear zones, is recorded at Paleocene times (~ 65 Ma; 5-10 °C/Ma; ~200 m/Ma). From ~35 to 32 Ma, the deformation is localized on Rialp thrust and exhumation rate increase on the Maladeta massif (~ 800 m/Myrs and ca. 30 °C/Myrs). The Axial Zone underwent a distributed shortening (~20 km) during ca. 30 Myrs (from ~ 70 to 40 Ma). This phase is probably facilitated by both inherited geothermal gradient and significant petrological and structural inheritance. This shortening mode is notably active during the so-called Paleocene “Quiescence” phase in the Axial Zone.
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Maxime Waldner. Cinématique et thermicité du prisme orogénique des Pyrénées : nouvelles données géo-thermochronologiques de la Zone Axiale. Sciences de la Terre. Sorbonne Université, 2019. Français. ⟨NNT : 2019SORUS413⟩. ⟨tel-03004798⟩

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