U-Th/He and fission track dating in north-eastern Corsica (France): extensional versus erosional exhumation.

During the Late Oligocene-to-Miocene, the Alpine Units of north-eastern Corsica experienced a rapid exhumation related to the extensional collapse coeval with the anticlockwise rotation of Corsica and the formation of the Provençal-Ligurian basin (Jolivet et al., 1998). During the same period of time, shallow-water basins formed in eastern Corsica where alluvial sediments interfingered with marine deposits and recorded the time of exposure to erosion of the Alpine Units and the growth of relief (Orszag-Sperber and Pilot, 1976). New apatite (U-Th)/He and fission-track ages from Corsica record cooling rates as high as 30-40°C/Myr during Early-Middle Miocene when this region is affected by a relatively high geothermal gradient, and possibly by topographic thermal perturbations. Cooling rates are commonly less than 7.5°C/Myr from Middle Miocene to Present when the growth of relief is detected by fluvial adjustments as indicated by the variations in the source areas of alluvial deposits of Late Miocene age. To analyze the roles of extensional exhumation versus topographic thermal perturbations and erosion, a simple one-dimensional model based on age-elevation-distance profiles has been applied to two case studies: namely the St. Florent basin, and the Tenda massif. The Early-to-Middle Miocene St. Florent basin is bordered by ductile-to-brittle extensional shear zones dissecting the Alpine units. The thermochronometric data from the Alpine units along a transect across the base and the shoulder of the basin are analyzed through an age-elevation-distance profile. The model derived from this profile shows that under low-temperature conditions the fault bordering the St. Florent basin produced a vertical throw of 3-4 km, and therefore supports the major role of extensional exhumation versus that of topographic thermal perturbations and erosion. The Tenda massif is the most prominent relief of north-eastern Corsica with elevations up to 1535 m: to the east it is bordered by a low-angle extensional ductile shear zone, and to the west by a high-angle extensional fault. Rounded pebbles from the Tenda massif are included in the sediments of the St. Florent basin indicating that this massif has been exposed to erosion since Middle Miocene, at least. Apatite fission track and U-Th/He data from this massif show an inverse relation with relief, and this relation can be interpreted as either deriving from a thermal topographic perturbations followed by a decrease of relief due to erosion, or to extensional faulting. These two end-member hypothesis are investigated through an age-elevation-distance profile, and this suggests that the present relief of the Tenda massif could be the remnant of a much more elevated paleorelief, and also that extensional faulting under low-temperature conditions might have played a significant role in the surface uplift of this massif.