Detrital zircon U-Pb ages and Hf isotopic constraints on the terrigenous flexural basins of the Western Alps and their paleogeographic implications

Abstract:

Detrital zircons from Cretaceous mica schist, Eocene sandstone and Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian, Caledonian (Ordovician) and Variscan (Carboniferous) events widespread in Western and Central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A small but distinguishable group of Triassic zircon, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the Oligocene "Schistes à Blocs" formation and the Brianconnais flysch. In contrast, this age group is absent in Eocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene flysch were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in Early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propoagation of allochthonous units. Based on previous studies and our new data, the Penninic Frontal Thrust zone was likely a paleorelief during the Eocene that accounts for the lack of detritus from the Internal Zone. Contemporary sediments were accumulated in the foredeep of the Adriatic Plate. From Oligocene time onwards, the blockage was cut through after a regional uplifting, and thus the Internal Zone started to provide detritus into the western flexural basins.