Denudation history and palaeogeography of the Pyrenees and their peripheral basins: an 84-million-year geomorphological perspective

International audienceThis review provides a synthesis of the evolution of the Pyrenees since ~84 Ma and is uniquely focused on analysing jointly and comparatively its peripheral pro-foreland, retro-foreland and Mediterranean basins. The reconstructions adopt a geomorphological perspective focused on the waxing and waning of palaeorelief, and is underpinned by (i) the denudation history of the mountain belt encoded in the sedimentary record of its basins, (ii) rock-cooling histories inferred from low-temperature thermochronology, and (iii) the age and spatial distribution of tectonic and erosional landforms. Existing geological reconstructions of the Pyrenees commonly terminate at the end of the syntectonic collision period (early Miocene). Here, the no-less eventful post-shortening period of the last 25-30 m.y. is also addressed. Accordingly, emphasis is given to the record provided by nonmarine clastic sequences, and to the often understated depositional biochronology documented by the continental fossils they contain. Sedimentological and provenance analysis of coarse clastic deposits further documents the fine-scale palaeogeography of sources and sinks, and is correlated with different generations of eustatic, tectonic, and volcanic features, as well as extant populations of land surfaces such as rock pediments, palaeovalleys, and other landforms indicative of palaeoelevation and palaeotopography. These interconnected and age-bracketed diagnostic features are correlated with independent evidence concerning the structural evolution of the orogenic belt at crustal and lithospheric scale. They show that the Ancestral (i.e., Paleogene) Pyrenees were in many aspects dissimilar to the successor mountain range we observe today. They also suggest that, despite its prima facie topographic continuity from the Mediterranean to the Atlantic, the modern mountain range, particularly in its eastern half, is in a transient topographic state. This would appear to have been driven by large-scale asthenospheric flows contributing to regional uplift and erosion of not just the mountain range but also its foreland basins during the last ~12 m.y. Despite these early hopes for a unified understanding of the mountain range, over 200 years later we find that the investigation of the Pyrenees has suffered from the usual disconnect between geological and geomorphological approaches, between orogen-scale desktop modelling and local-scale field observations, between a focus on Alpine tectonics and a neglect of Quaternary neotectonics, with many of its consequences listed above.Restricting research to the confines of the time or spatial bundles listed above has generated persistent blind spots with an entrenched risk of confirmation bias. This review attempts to transcend some of those conventional boundaries of inquiry by operating from two key angles: (i) we address the metabolism of the orogen as a mountain range rather than just as a crustal wedge or mosaic of geological structures, and thus focus on its erosional history, topographic evolution, and landform assemblages; (ii) we articulate the geodynamic history of the Pyrenees with the palaeogeography of its Ebro, Aquitaine and Mediterranean sedimentary basins simultaneously, spanning the earliest plate convergence episodes of the latest Mesozoic to glaciation in the Pleistocene. As a result, rather than a small, asymmetrically bi-vergent orogen captured over simplifyingly short intervals of geological time, it portrays instead a mountain range evolving continuously by lateral and longitudinal growth or destruction through a succession of three quite distinct avatars defined hereafter as the Proto-Pyrenees, the Ancestral Pyrenees, and the Modern Pyrenees - respectively and schematically during the late Cretaceous, Paleogene and Neogene-Quaternary. Each of these Pyrenean ranges has displayed transient periods of crustal and topographic symmetry and asymmetry, successions of range-parallel and range-transverse drainages, the construction and unequal preservation of range-front megafans, uneven intensities of crustal uplift and depths of rock denudation, unequal depths of fluvial incision and patterns of alluvial deposition, and unequal intensities of glacial imprint in more recent time. Many place names (massifs, peaks, towns, etc.) mentioned in the text are located in the figures, but for precision and completeness it is recommended to use an Earth navigation browser.