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

Tertiary convergence, high-pressure - low-temperature (HP-LT) metamorphism and subsequent exhumation in the internal Western Alps were concurrent with detrital deposition in foreland basins at the external periphery of the orogen. In the present work we have probed the geochronology and chemistry of white mica grains and metamorphic pebbles from the foreland basins of SE France in order to gain additional perspective on exhumation rates and mountain building processes in the internal domain of the Tertiary orogen. Our data indicate that throughout the Tertiary these basins were mainly fed from a relatively low pressure source, dominated by Si-poor micas yielding pre-Alpine ⁴⁰Ar/³⁹Ar ages. Small amounts of detritus composed of high-pressure minerals and pebbles are first detected in early Oligocene (∼30-32 Ma) strata of the Barrême basin. The ⁴⁰Ar/³⁹Ar geochronology of phengites from garnet-blueschist pebbles from these strata yielded 34 ± 3 Ma ages indicating rapid ascent and exposure of HP-LT rocks in the internal part of the orogen shortly after metamorphism. To explain the preponderance of pre-Alpine micas alongside the rare presence of Alpine HP-LT detritus we suggest that the general architecture of the Western Alps, whereby inner HP-LT metamorphic units are cut off from the SE France basins by a low-grade lid of Brianconnais back thrusts, was rapidly built in the early Oligocene and did not change significantly since then.