THE EROSION OF THE ALPS - ND ISOTOPIC AND GEOCHEMICAL CONSTRAINTS ON THE SOURCES OF THE PERI-ALPINE MOLASSE SEDIMENTS

Sm-Nd data from molasse sedimentary deposits from eastern France and S witzerland are used to quantify the erosion of the Alps during the Oli gocene and Miocene. The average present day epsilon(Nd) value of the c ontinental sedimentary deposits increases from -11 for Cretaceous and Eocene substratum to -9 for the first Oligocene molasse sediments. Thi s increase requires the erosion of Mesozoic marine sediments to explai n the average epsilon(Nd) value (-9.1) of the Rupelian and Lower Chatt ian sediments. Then the average epsilon(Nd) value (-9.7) and the chemi cal compositions of the Upper Chattian and Aquitanian sediments are co nsistent with the erosion of granitic rocks of the Variscan crust. Thi s change in source allows us to define a second cycle in the peri-Alpi ne molasse which began at 24.5 Ma with the deposition of the ''Calcair es et Dolomies''. This basin-wide unit represents the sedimentary reco rd of a halt in the input of Alpine detritus in the western part of th e 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 epsilon(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 grani tes having high epsilon(Nd) values between -1.8 and -5.2. This study c oncludes that the erosion of the Alps increased the epsilon(Nd) Values of the sedimentary mass in two stages: (1) during the Rupelian and Lo wer Chattian, by recycling of marine chemical sediments having epsilon (Nd) values similar to that of Tethys seawater (epsilon(Nd) approximat e to -8), and which represent 90% of the eroded materials, and (2) dur ing the Burdigalian, by the erosion of a Variscan crust representing a t least 20% of the eroded material. Half of this Variscan material was composed of alkaline granites with high epsilon(Nd) values, suggestin g the addition of mantle-derived material to the crust during late Var iscan events.