Rapid exhumation and mountain building in the Western Alps: Petrology and ~(40)Ar/~(39)Ar geochronology of detritus from Tertiary basins of southeastern France

摘要

Tertiary foreland basins of SE France contain abundant detrital white micas yielding Permian-Jurassic ~(40)Ar/~(39)Ar ages. Recycling of Cretaceous-early Eocene flysch units of the Western Alps was likely the source of much of these micas. First appearance of inner Alpine HP-LT detritus in the SSC basins is recorded by rare Si-rich phengite grains in the lower Oligocene Gres de Ville formation of the Barreme basin, whose depositional age is 33-32 Ma [Evans and Elliott, 1999]. These were soon followed by the occurrence of garnet-epidote blueschist pebbles of Alpine age in the overlaying Clumanc conglomerate unit. The 34 ± 3 Ma ~(40)Ar/~(39)Ar age defined for white micas separated from a garnet-bearing blueschist pebble of the Clumanc conglomerate unit, is within error of its depositional age. This indicates a rapid (even instantaneous!) exhumation rate for Western Alps blueschist-facies rocks in the early Oligocene. Rapid cooling/exhumation rates were evaluated for the internal eclogite facies and UHP units of the Western Alps in previous studies [e.g., Gebauer et al., 1997; Rubatto and Hermann, 2001]. Yet, our work indicates that garnet-bearing blueschists were not only exhumed to shallower crustal levels, but were fully unroofed and being eroded by 32-30 Ma. We noted that although HP-LT rocks of the inner Alpine domain were exposed by early Oligocene times, they were a very minor source of sediment for the Tertiary SSC basins. Currently exposed (U)HP-LT units are almost completely cut off from the western foreland of the Western Alps orogen by a thick lid of Brianconnais carbonates. To explain why Tertiary foreland basins of SE France were nearly devoid of high-pressure detritus, we suggest that a similar architecture (whereby the Brianconnais seals the HP-LT structural core of the Western Alps) prevailed as early as early Oligocene.