VARISCAN EXTENSION IN THE PYRENEES

Evidence for Variscan extension in the Pyrenees is summarized a.id used with other geological and geochronological data to reconstruct the Variscan extensional geometry and kinematics at a crustal scale. A regionally consistent pattern of ESE-WNW trending synmetamorphic stretching directions indicates Variscan extension roughly parallel to the present alpine trend of the belt. The spatial distribution of Variscan low pressure - high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping, crustal-scale shear zones allowing heterogeneous ductile extension of the Variscan middle and lower crust. Extension occurred synchronous with uplift and emergence of large parts of the crust and deposition, during the Upper Carboniferous and Permian, of up to 2500 m of continental sediments and intercalated basaltic-andesitic volcanics in fault-bounded extensional halfgrabens. The extensional structures and related metamorphism postdate a Namurian to early Westphalian stage of crustal thickening, characterized by thrusting and steep folding. It is suggested that Variscan extension in the Pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere, enhanced by detachment of a cold and gravitationally unstable lithospheric root, which caused progressive uplift, tectonic denudation, and a highly anomalous thermal structure of the Variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the Early Permian.