Tectonically driven exhumation in the Western Alps: insights from fission track thermochronology

Analysis of cooling age patterns yielded by lowtemperature thermochronometers provides key information about the role played by tectonic discontinuities during the late stages of exhumation of metamorphic belts. In the Western Alps, fission track data published so far are heterogeneously scattered and concentrated in few structural domains, preventing analyses at the scale of the whole belt. The new apatite fission track data reported in this work, obtained with the external detector method as well as the population method in very low U content samples, fill this gap. They constrain the postmetamorphic evolution of the Western Alps along two transects from the foreland to the retroforeland, unraveling the effective role played by some major faults during the exhumation of the belt at shallow crustal levels. A clear regional pattern, characterized by decreasing ages moving from the axial sector to the European external sector of the belt and by an along-strike gradient with increasing ages from north to south, has been unraveled. Evident breaks in this age pattern have been detected in correspondence of faults that are near-parallel to the trend of the belt, pointing to the occurrence of active tectonics during and after exhumation. The most apparent breaks have been observed in the axial sector of the belt, where the postmetamorphic deformation would have been negligible according to classic tectonic models. Faults located in the axial sector split the belt into two major blocks (eastern and western). Since the Miocene, the western block experienced higher exhumation rates than the eastern one. Such differential exhumation was accommodated in the northern portion of the belt thanks to reverse motion along the Internal Houiller Fault, which occurred in a convergent transcurrent framework. To the south, it was accommodated instead by normal reactivation of the Brianc¸onnais Front and by activity of the Longitudinal Fault System, which occurred in a divergent transcurrent framework. The tectonic activity affecting the axial sector of the belt, in a context of regional dextral strike slip, is coeval with the forward propagation of the external thrusts, and of similar magnitude. We suggest that the contrasting kinematic regimes (i.e., convergent versus divergent transcurrence) observed in the Western Alps moving along strike were responsible of the increasing exhumation rates toward the north, revealed in both blocks by the along-strike age gradient. The higher exhumation rates recognized northward would be related to an increasing importance of crustal shortening that promoted erosion during the late stages of exhumation of the belt.