Miocene to Present differential exhumation in the Western Alps: Insights from fission track thermochronology

Abstract

Analysis of cooling age patterns yielded by low-temperature 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

Extensional neotectonics around the bend of the Western/Central Alps: an overview

“…The very complex exhumation processes in the Alps, the lithological variations and the various strain rates do not allow to give an approximate age for specific locations of the ductile-brittle transition in the belt. Nevertheless, according to Malusa et al (2005), such a transition should have occurred as early as in late Oligocene to early Miocene times in the internal Alps. To summarize, late Alpine faulting exhibits two directions of extension.…”

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

“…Different models have been proposed to explain this complex structural setting, such as westward extrusion (Coward and Dietrich 1989), dextral transcurrence at the scale of the chain induced by the counterclockwise rotation of the Adria plate since Oligocene (Vanossi et al 1994;Castellarin 2001;Collombet et al 2002;Agard et al 2003;Sue and Tricart 2003;Malusà et al 2005Malusà et al , 2009, and polyphasic models that associate south-westward extrusion (Sue et al 2007a;Champagnac et al 2004Champagnac et al , 2006 or dextral transcurrence Perello et al 2004b) with buoyancy forces.…”

The structural history of the Mont Blanc massif with regard to models for its recent exhumation

“…the Briançonnais front, e.g. Fügen-schuh et al 1999;Malusa et al 2005). In the current study area, the Penninic thrust can best be defined as the protolith boundary between the sediments of the Sion-Courmayeur zone and the Helvetic units of the ''root zone'', often marked by a thick Triassic evaporite layer.…”