Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the “Nappe des Gypses” and the Internal Briançonnais at the metamorphic peak.
Deciphering the impact of short-term or long-term forcing on fluvial incision, as well as understanding the influence of local (channel lithology and morphology) and global (tectonic motions) parameters in the spatial variation of incision efficiency, are ongoing geomorphological research fields. To shed new light on these issues, we chose to study the “Hautes Gorges du Verdon” (High Verdon Gorges or HVG), located in the foreland of the Southwestern Alps. We collected 24 samples along three polished surfaces for Cosmic Ray Exposure (CRE) 36 Cl dating, which allowed us to constrain short-term incision rates ranging from 0.06 to 0.2 mm/yr between 60 and 15 ka. Compared to known regional uplift and denudation rates, incision rates obtained in the HVG suggest tectonic or isostatic uplift as the main driver of Verdon River incision in the Late Quaternary. This comparison allows us to propose that the downcutting of the Verdon Gorges started approximatively 1.5 to 2 Ma ago, even if the drainage network of the Verdon catchment area could have been shaped earlier, during the Messinian salinity crisis. Déchiffrer l’impact des forçages à court ou long terme sur l’incision fluviale, et comprendre l’influence des facteurs locaux (lithologie et morphologie du lit de la rivière) et globaux (mouvements tectoniques) sur la variation spatiale de l’efficacité de l’incision, sont des domaines de recherche en cours. Pour apporter un regard neuf sur ces questions, nous avons choisi d’étudier les « Hautes Gorges du Verdon » (HGV), localisées dans l’avant-pays des Alpes du Sud-Ouest. Nous avons collecté 24 échantillons le long de trois surfaces polies pour la méthode de datation Cosmic Ray Exposure (CRE) 36 Cl, qui nous a permis de contraindre les taux d’incision à court terme allant de 0,06 à 0,2 mm/an entre 60 et 15 ka. Comparés aux taux de soulèvement et de dénudation régionaux connus, les taux d’incision obtenus dans les HGV suggèrent que le soulèvement tectonique ou isostatique est le principal moteur de l’incision de la rivière DU Verdon au cours du Quaternaire récent. Cette comparaison nous permet de proposer que l’incision des Gorges du Verdon a commencé il y a approximativement 2 Ma, même si le réseau de drainage du bassin versant du Verdon a pu être formé plus tôt, pendant la crise de salinité messinienne.
Long-term study of fault system activity is crucial for understanding the dynamics of orogeny structuring and the formation of peripheral basins, the impact of tectonic inheritance, seismic hazard assessment, and the estimating the coupling of deformation and erosion. At the junction of several orogenic domains, the foreland basin of the Western Alps exhibits a complex structural pattern inherited from the superposition of tectonic events since the late Paleozoic. Despite this knowledge, the absolute age of fault formation and reactivation remains poorly understood, primarily due to the difficulty of dating uranium-poor minerals typically found in sedimentary environments. This study proposes an integrated approach of structural analysis of deformations in the field combined to the recently developed U-Pb in-situ dating method on syn-tectonic calcite to fill this gap. By focusing on the subalpine massifs (from the Vaucluse massif to the Bornes massif), this work aims to constrain the dynamics of the Alpine foreland structuring over a wide temporal and spatial scale. Additionally, this study area presents diverse geodynamic characteristics, making it an ideal site to test the applicability of recent U-Pb in-situ dating methods.
Abstract Metallogenic models of polyphase mountain belts critically rely on robust geochronology. We combine petrology with Rb–Sr and U–Th–Pb in situ geochronology, paired at thin-section scale, to date mineralization in deformed hydrothermal Pb–Zn–Ag deposits along an east-west transect in the Western Alps, France. The Pb–Zn–Ag veins occur in shear zones with kinematic structures consistent with the mylonitized host rocks. The ore consists mainly of galena in a quartz-phengite gangue. The paragenesis can be related to hydrothermal crystallization during periods of variable strain. Both isotope systems yield only Cenozoic ages (ca. 35 Ma and 15–20 Ma) without any pre-Alpine inheritance, clearly indicating orogenic mineralization. The metallogenic model proposed here includes significant fluid circulation along major tectonic contacts between basement and sedimentary cover during Alpine convergence.
