Comment on “Alpine thermal and structural evolution of the highest external crystalline massif : The Mont Blanc” by P. H. Leloup, N. Arnaud, E. R. Sobel, and R. Lacassin

Abstract : In this comment we discuss the approach used and the significance of Ar-Ar dating of synkinematic phengite within low-grade Alpine shear zones, and we comment the geodynamic models that can be derived from this method. The paper by Leloup et al. [2005] is a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data. Leloup et al. [2005] have proposed a polyphase Alpine history for the Mont Blanc Massif (west Alps) based on a multidisciplinary approach: Ar-Ar on biotite for the higher pressure-temperature events of the Mont Blanc, and Ar-Ar on K-feldspar, fission tracks (FT) on zircon and apatite for its later exhumation stages. However, at this point of our knowledge of Alpine deformation in the Mont Blanc Range, the polyphased tectonic evolution, in particular the timing of thrust and back thrust events are not in agreement with recently obtained Ar-Ar data.