Mountain building and rift-inheritance in the Pyrenees: insights from a new balanced cross-section and detrital thermochronometry

Providing accurate estimates of shortening, as well as the duration and vertical amplitudes of tectonic events in collisional belts is critical to better understanding the retroactions between the mechanics of crustal deformation and surface processes during mountain building. However, structural and bedrock geochronological constraints are usually lacking accuracy for the early stages of convergence that are generally overprinted by complex deformation patterns and synorogenic burial. As a consequence, new detrital low-temperature thermochronometry (detrital AFT dating, (U-Th)/He on zircons) and geochronology (U/Pb ages on zircons) provided on both flanks of the Pyrenean orogen are combined with in-situ thermochronometric constraints to fill the lack of accuracy for the early stages of orogenesis. Together with the exceptional constraints on the tectono-sedimentary evolution of the Pyrenean thrust fronts, these new data offer the unique opportunity to precisely determine the kinematics on both sides of the Pyrenean mountain belt from Late Cretaceous to Miocene. Based on these recent constrains and the re-appraisal of local structural cross-sections, we propose a new crustal cross-section of the central Pyrenees. Intermediate restorations are then produced for well-suited and key time intervals (Early Oligocene, Middle Eocene, Cretaceous-Paleogene transition, Late Campanian and Late Santonian) in order to discuss the kinematic evolution within the orogenic wedge and mass balance. This study shows that the initial stage of plate convergence accommodated very limited subduction. Mountain building in the central Pyrenees produced the inversion of North Pyrenean flysh basins on top of reactived of S-dipping crustal detachment that previously exposed mantle to the floor of the basin during the extension phase (Pyrenean Lherzolites) and inversion of the south-central Mesozoic Organyà basin. The amount of accreted material from the Iberian crust increased significantly after the Paleocene continuing until the late Oligocene to form an antiformal stack of basement units in the backbone of the chain. Preliminary mass balance estimates show a long-term unsteady pattern of orogenic evolution characterized by a remarkable increase of the outcoming erosional flux during the Late Eocene-Oligocene with no commensurate incoming flux of crustal material. This result appears consistent with a major renewal of the rapid exhumation in the hinterland for which both internal and external drivers are discussed.