Mylonitic shear-zones in the Millas granite (Pyrenees, France). 40Ar/39Ar cretaceous ages and tectonic interpretation

Within the Axial Zone of the eastern Pyrenees, the granite of Millas is affected by mylonitic shear zones of controversial age. The kinematic development of these E-W to NW-SE trending shear zones results from heterogeneous, non-coaxial deformation along steep south-dipping reverse faults generated under greenschist facies conditions. A 40Ar/39Ar geochronological study of multi-grain populations and single grain by laser microprobe yields ages in the range 90-100 Ma which are believed to indicate that the mylonitic shear zones were active in mid-Cretaceous times. Therefore, this mylonitisation is coeval with the high-temperature metamorphism of the North-Pyrenean Mesozoic basins and more generally with the episode of crustal thinning which accompanied the sinistral strike-slip motion of Iberia. This extensional setting of the Axial Zone would have been modified by subsequent deformations related to Eocene Pyrenean compression that partly overprinted the Cretaceous mylonites. (authors). 74 refs., 11 figs., 1 tab