Discerning Variscan from Alpine deformation in the Pyrenean Axial Zone; insights from geochronologic and structural data

Abstract. The Axial Pyrenean Zone constitutes the axis of the Pyrenees, an Alpine orogen. The precambrian and paleozoic rocks outcropping in this zone underwent two orogenies (the Variscan and Alpine orogenies). Within this zone, distinguishing Variscan from Alpine structures represents a very complicated task due to the scarcity both of geochronological data constraining the age of deformation and units younger than Permian which would be only affected by Alpine deformation. In this work we have dated a rhyolitic sill interspersed within the regional Variscan foliation (S1) by zircon geochronology with U-Pb ion probe (SHRIMP). This sill is located in the central part of the Axial Zone, between the Garone Dome and the Aran Valley Syncline, and it is folded by open and upright folds that also deform Variscan structures. The results indicate that the sill have an Early-Middle Permian age of 274±1.5 Ma. This age postdates the Variscan deformation that occurred between 380 and 290 Ma and indicates that the folds deforming the sill must be of Alpine age. Moreover, these folds are associated with a penetrative crenulation lineation recognized in the Garonne Dome that points to a possible Alpine origin for the macrostructural configuration of this dome. This work highlights the necessity of incorporating geochronological data to correctly interpret deformation in complex areas as the Axial Pyrenean Zone affected by two orogenies.