Fast exhumation rate during late orogenic extension: The new timing of the Pilat detachment fault (French Massif Central, Variscan belt)

Abstract A leucogranite sill in the footwall of the extensional Pilat shear zone (PSZ) Eastern French Massif Central was emplaced and sheared in a short time interval, between 301.8 ± 3.1 and 303.0 ± 1.6 Ma (zircon U–Pb and mica 40Ar/39Ar ages, respectively). Extension ended at 298.5 ± 1.5 Ma (40Ar/39Ar on mica) as shown by a non deformed dike cross cutting the sheared zone. On the hanging wall of the fault, the opening of the St. Etienne Basin filled by coarse clastic sedimentation confirms the existence of a relief south of the PSZ. U-Pb dating of a lithic-rich ignimbrite at the bottom of the basin and of a volcanic layer at the top delivered ages of 310.5 ± 3.4 Ma and 299.3 ± 1.3 Ma, respectively. Close U-Pb and 40Ar/39Ar ages from syn- to post-tectonic granites indicate very fast cooling (∼150 °C/Myr) and exhumation rate (6 mm/yr) corresponding to the emplacement of granites in the middle crust immediately followed by their fast tectonic exhumation and cooling into the upper crust. The development of the Pilat fault in the thermally softened crust is responsible for the rapid exhumation of the Velay dome, a Metamorphic Core Complex localized in the footwall of the sheared zone. Here we propose that its rapid exhumation was due to localized sub crustal lithospheric mantle dripping. These new data show that there was only one extensional phase in the PSZ coeval with the opening of the St. Etienne coal Basin. Finally, the new chronology obtained in this study is challenging previous ages suggesting 20 Ma activity for the Pilat shear zone.