Syn-rift Cretaceous deformation in the Agly Variscan Massif (Eastern Pyrenees, France)

Abstract

A petro-structural and geochronological study has been conducted in the Agly Variscan Massif located in the Northern Pyrenean Zone (France). The Lower Gneiss Unit (LGU) displays highly ductile deformation with a NNE-SSW oriented stretching lineation and opposite senses of shear in the directions top-to-the-NNE and top-to-the-SSW. Field observations and thin section analysis show that these two senses of shear are coeval with bulk coaxial vertical shortening and horizontal lengthening. Focusing on syntectonic minerals located within shear bands, Th-U/Pb monazite and 40Ar/39Ar mica dating yield a 94–127 Ma age bracket for the mylonitic deformation. The principal conclusion from these results is that the main ductile strain (i.e., stretching lineations and kinematic indicators) in the LGU should be ascribed to the Cretaceous rifting. A PTt path for the LGU is proposed showing the diachronism between the Cretaceous metamorphic evolution in the Agly Massif (peak temperature at 127 Ma) and the sedimentary basins (peak temperature at 95 Ma) bordering it to the north and south. Finally, a north-south crustal scale evolution of the whole area is put forward to explain this diachronism.