Post-orogenic topography and sediment flux of mountain-foreland basin systems: Miocene erosion and sediment yield from the Pyrenees

Abstract

Considerable research has been focused on the syn-orogenic evolution of mountain belts in order to explore interactions between tectonic, climate and surface processes. Comparatively, limited work has been carried on the post-orogenic stage of mountain ranges. Because of the relatively small-sized of the orogen, the good quality of exposure and large volume of dataset, the Pyrenees are a natural laboratory to study orogenic processes. Moreover, the Pyrenees are characterized as a post-orogenic system since at least 20 Ma and constitute a perfect target for this thesis. Our aim in not to solely resolve the post-orogenic history of the Pyrenees, but rather provide an understanding of fundamental behaviours and processes of the late syn- to post-orogenic systems. The purpose of this study is to better constrain the transition from crustal thickening to post-orogenic decay in mountain ranges in term of topographic and sediment flux toward foreland basins and surrounding continental margins. This is achieved through a multi-disciplinary approach combining analyses of low-temperature thermochronological data (apatite and zircon fission track, apatite helium and 4He/3He apatite data), topographic analyses from digital elevation model of the Pyrenees and landscape evolution models. Thermal histories inferred using a similar inverse modelling from pre-existing apatite and zircon fission track and apatite (U-Th)/He data document the syn-exhumation history and reveal the late syn- to post-orogenic transition in the Central Pyrenees. The main exhumation associated with the orogenic growth is diachronous in the Central Pyrenees and younging from north to south. In the Northern Pyrenean zone, the main exhumation occurred during Middle Miocene times (i.e. 40-45 Ma) whereas in the southern Axial Zone, main exhumation is recorded during Oligocene times (i.e. 30-25 Ma). Thermal histories also reveal an abrupt and diachronous cooling cessation across the Central Pyrenees. Cooling cessation, associated with the post-orogenic transition occurred from 36-37 Ma in the northern Pyrenees (Arize massifs) to 23 Ma in the southern Pyrenees (southern Axial Zone). Because of late laboratory difficulties, 4He/3He data were not available for the thesis publication. However, implications of these results for potential late post-orogenic exhumation signals are discussed. Extraction of topographic indices from digital elevation models of the Pyrenees allowed us to observe a systematic correlation between river channel steepness, elevation and rock strength associated to variation of lithologies. Rock strengths were determined using Schmidt hammer measurements. This correlation demonstrates that rock types have affected the topographic development of the Pyrenees since cessation of orogenesis. The landscape of the Pyrenees reveals this behaviour with the main drainage divide tracking the granitoid plutonic massifs which have the highest rock strength. We propose also that the abrupt deceleration of exhumation recorded through thermal histories could be associated with the exhumation of these high resistant plutonic massifs. Landscape numerical modelling confirms that exhumation of harder rocks results in catchment vulnerabilities during transient landscape response and that readjustment is principally made by divide migrations. Other mountains range as the Western European Alps show similar drainage divide patterns. Analysis of a coupled mountain range and foreland basin model reveal that early post-orogenic sediments of the foreland basin should drape and seal structures of the thrust wedge. This trend is explained by a continuation of highly eroded sediment flux from the range and a diminution of accommodation space in the foreland basin because of reduction and cessation of lithospheric flexure. The northern Aquitaine retro-foreland basin shows similar evidence with Early to Middle Miocene sediments draping the Sub-Pyrenean Zone and North Pyrenean Thrust Belt. Alluvial cover of the thrust-wedge during the post-orogenic stage in combination with low lithosphere elastic thickness and the presence of resistant lithologies have important implications for topographic survival. With this three conditions, landscape evolution models allow long post-orogenic persistence of topography and relief. Our model results suggest also that higher sediment flux should be recorded in surrounding continental margin depocenters during the post-orogenic stage. The Pyrenean system confirms again this trend with an increase of sediment accumulation in the Bay of Biscay. Overall, this multi-disciplinary study brings new constraints on how the landscape of an orogenic system evolves during the post-orogenic decay stage. We highlight multiple interactions and coupled dynamic from the mountain range toward surrounding foreland basin and continental margin.