FISSION-TRACK RECORD OF EXHUMATION OF NORTHERN CORSICA (FRANCE)

The island of Corsica is characterized by two distinct geologic terranes. The western portion consists of European Hercynian crust intruded by Carboniferous-Permian granitoid rocks and covered by Permian acid volcanic rocks and scattered Mesozoic sediments. The eastern portion, Alpine Corsica, is a nappe stack, dominated by Jurassic oceanic crust and its sedimentary cover, mostly metamorphosed and emplaced during the Alpine orogeny. Major thrusting in the late Eocene is responsible for much of the now visible nappe stacking and metamorphism (Caron, 1994). Eclogitic facies of Late Cretaceous age (Lahondere and Guerrot, 1997) are widespread in different rocks of the Alpine units and were retrogressed in the blueschist facies during Eocene. Our fission-track study yields zircon ages between 165 and 115 Ma from samples collected in the Hercynian basement. These ages can be related to the opening of the Jurassic Tethys ocean. “Tethyan zircons” have been widespread detected by Vance (1999) all along the Western Alps. Most of the samples collected across the different units of the Alpine Corsica show ages between 21 and 19 Ma, which are interpreted as the record of the exhumation after Alpine metamorphism. Two anomalous ages of 74 and 69 Ma have been detected along the south-western border of the Tenda massif. The Tenda massif consists mostly of Hercynian granitoid rocks and Permian volcanics, recrystallized during westward Alpine thrusting under medium-pressure conditions. These zircon ages give a new constrain to the unravelling of the metamorphic conditions and indicate that part of the European basement was exhuming during the Late Cretaceous subduction. European basement and Alpine nappe stack were juxtaposed before 18 Ma, since apatite fission-track ages indicate an Early Miocene exhumation episode which was synchronous for all the different structural units of northern Corsica.

References: Caron, J.M., 1994, Schweiz. Miner. Petrogr. Mitt., 74, 105-114. Lahondere, J.C. and Guerrot, C., 1997, Geol. France, 3, 3-11. Vance, J.A., 1999, Mem. Sci. Geol., 51/2, 473-476.