Constraining the provenance and evolution of the Western Alps Molasse Basin by detrital zircon U–Pb geochronology
Résumé
Molasse basin is one of the best-preserved pieces of evidence of the Alpine orogeny. Molasse and flysch sequences deposited during the convergence between the Adriatic and the European continents recorded various geological processes. However, detailed provenance analysis of the foreland basin in the Western Alps is still in need of precise data for molasse strata. This paper provides new detrital zircon U–Pb geochronology results from five sandstone samples to constrain the provenance of the Molasse Basin in the Western Alps. The main populations in zircon age spectra correspond to the four tectonothermal events defined in the Alpine domain: the late Neoproterozoic to early Cambrian magmatic event, the pre-Variscan rifting event, the Variscan orogeny, and the Permian extensional event, respectively. Two magmatic zircons (100 ± 2 Ma and 130 ± 5 Ma) and one metamorphic zircon grain (116 ± 3 Ma) yield Cretaceous age. The metamorphic one was probably originated in the Internal zone. A contribution of the Valaisan unit as part of the source terrane is possible to account for the Cretaceous magmatic zircons. Comparing our results with published detrital zircon age data using multidimensional scaling, we infer that the Austroalpine unit was an essential provenance of the Western Alps Molasse Basin during the late Oligocene and the middle Miocene. During the middle Miocene, the deposition of the Molasse Basin was strongly influenced by the exhumation of the External Massifs and the propagation of the subalpine fold-and-thrust belt. Meanwhile, the French Massif Central was also possible to provide a limited proportion of the material for the basin.