The erosion of the Alps: Nd isotopic and geochemical constraints on the sources of the peri-Alpine molasse sediments

Abstract

SmNd data from molasse sedimentary deposits from eastern France and Switzerland are used to quantify the erosion of the Alps during the Oligocene and Miocene. The average present day ϵ Nd value of the continental sedimentary deposits increases from −11 for Cretaceous and Eocene substratum to −9 for the first Oligocene molasse sediments. This increase requires the erosion of Mesozoic marine sediments to explain the average ϵ Nd value (−9.1) of the Rupelian and Lower Chattian sediments. Then the average ϵ Nd value (−9.7) and the chemical compositions of the Upper Chattian and Aquitanian sediments are consistent with the erosion of granitic rocks of the Variscan crust. This change in source allows us to define a second cycle in the peri-Alpine molasse which began at 24.5 Ma with the deposition of the “Calcaires et Dolomies”. This basin-wide unit represents the sedimentary record of a halt in the input of Alpine detritus in the western part of the molasse basin, and we suggest that the limit between Lower and Upper Chattian sediments, at 24.5 Ma, corresponds to a major tectonic event in the Alps. A further increase of 1 ϵ Nd unit recorded by the Burdigalian marine sandstones (average of −8) defines a third molasse cycle which resulted from the erosion of late Variscan alkaline granites having high ϵ Nd values between −1.8 and −5.2. This study concludes that the erosion of the Alps increased the ϵ Nd values of the sedimentary mass in two stages: (1) during the Rupelian and Lower Chattian, by recycling of marine chemical sediments having ϵ Nd values similar to that of Tethys seawater ( ϵ Nd ≈ −8), and which represent 90% of the eroded materials, and (2) during the Burdigalian, by the erosion of a Variscan crust representing at least 20% of the eroded material. Half of this Variscan material was composed of alkaline granites with high ϵ Nd values, suggesting the addition of mantle-derived material to the crust during late Variscan events.