Pénéplanation et dynamique de la lithosphère dans les Pyrénées

Abstract : High-elevation, low-relief surfaces in mountain belts is classically interpreted as remnants of post-tectonic peneplains. Their elevation is often used to quantify the uplift of the Earth’s surface. A recent alternative interpretation propose that these surfaces are the remnants of a peneplain that develops at high elevation, due to the progressive rise of the efficient base level of the drainage system in mountain belts, and to the resulting inhibition of erosion during the mountain belts building. The two interpretations involve radically different lithosphere dynamics. The present study is concerned with the emblematic case of the Pyrenees, where the two hypotheses concerning the development of the mid-Miocene Pyrenean peneplain are opposed. First, a new mapping methodology has been developed to restore the morphology of the chain during the mid-Miocene from the present-day high-elevation, low-relief surfaces. Second, the crust and lithosphere thicknesses have been determined using a simple approach that takes into account the densities and the thermal parameters within the Pyrenean lithosphere, and assuming local isostasy and thermal equilibrium. The comparison of the restored peneplain with the crustal and lithosphere thicknesses below the Pyrenees shows that the crustal root is preserved beneath the peneplain in the whole Pyrenees, and that the crust thickness progressively decreases to the east, up to its complete disappearance of the in the Mediterranean, and resulting in the proportional lowering of the peneplain elevation. The comparison of the crust and lithosphere thicknesses, using normal densities, with the crust and lithosphere thicknesses deduced from seismic data shows that the presented elevation of the chain is isostatically compensated at lithosphere level. These results refute the previous hypothesis of an overcompensation of the crustal root beneath the Pyrenees, as well as the previous hypothesis of a 2000 m post-tectonic, upper Miocene to Plio-Quaternary of the chain. They favor in return the interpretation of the development of the mid-Miocene Pyrenean peneplain at high elevation.
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Gemma De Vicente I Bosch. Pénéplanation et dynamique de la lithosphère dans les Pyrénées. Sciences de la Terre. Université Rennes 1, 2016. Français. ⟨NNT : 2016REN1S131⟩. ⟨tel-01578109⟩

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