Thesis defense of Jessica Uzel

Morphotectonic evolution of the Pyrenan range during the post-compressive period: weathering and dismantling of an uplifting planation surface

Abstract

The late to post-compressive evolution of the Pyrenees is characterized by the formation of a planation surface whose relics are observed today on top of the range. This surface is the subject of an abundant literature, where authors focused on the characterization of its geometry, extent and on the age and mechanism of its formation. The most recent studies showed that the planation surface probably formed at high elevation during the Oligocene - Upper Miocene interval. However, this elevation remains unconstrained.

Our work brings a new qualifying element of the relics of the pyrenean planation surface, which is the description of subtractive weathering profiles developed on these surfaces. Weathering in samples collected below surface remnants was dated by the 40Ar/39Ar applied to potassic manganese oxides. A Cretaceous age, consistent with the timing of the Pyrenean rifting during which the top of tilted blocs surfaced, and a Quaternary age were obtained. The lack of geochronological information between those two ages is interpreted as a result of the reactivation of weathering profiles due to uplift during the post-compressive period. These results show that weathering continued until Quaternary times in the Pyrenees.

The geomorphological analysis of the dismantling of the planation surface, based on miocene to present-day fluvial incisions analysis in the Western Pyrenees reveals the occurrence of an uplift of the range, which started during middle Miocene times. This uplift is confirmed for present-day by the analysis of a new InSAR dataset. The forcing mechanism of this uplift is discussed ar the African and Eurasian plates-scale. The middle Miocene to present-day uplift of the Pyrenees which provokes the dismantling of the planation surface may be explained by a propagation of the deformation from active plates boundaries toward intraplate areas caused by a reorganization of African - Eurasian plate motion.