The Oligocene orogenic pulse in the southern Penninic arc (western Alps): structural, sedimentary and thermochronological constraints
Résumé
The Oligocene evolution of the southern branch of the western Alpine arc, more precisely the stack of metamorphic Briançonnais and Piedmont nappes composing the southern Penninic arc (SPA), are the focus of this study. We review published structural, sedimentological and thermochronological data in order to discuss exhumation of the SPA. At first, we compare bedrock zircon and apatite fission-track (FT) data from the SPA with detrital thermochronologic data (zircon FT, white mica 40Ar/39Ar) from Oligocene molasse deposits. Using improved stratigraphic ages for the Barrême basin, samples from the uppermost Rupelian "Conglomérat de Clumanc" and the Chattian "Molasse Rouge" provided zircon FT lag times of ~3.5 and 8 m.y., indicating source exhumation rates on the order of ~1.5 and ~0.75 km/m.y. respectively. These short lag times are consistent with lag times of 40Ar-39Ar ages of detrital white mica from the same formations in the same basin, and also from Oligocene molasse sediments in the Tertiary Piedmont basin. The sediment source for these grains as for the associated clasts of blueschist, is identified as the HP-LT metamorphic units of the SPA. The source cannot be the Ubaye-Embrunais nappes as classically considered, because these nappes do not bear the required metamorphic imprint. This interpretation is consistent with fast Oligocene cooling of the SPA, as attested by in situ zircon and apatite FT analyses. Such fast and relatively old cooling is a peculiarity of the southern branch of the western Alpine arc, when considering the entire arc. A second range of data concerns the structural building of the SPA. The initial stacking of metamorphic nappes in a poorly elevated accretionnary wedge was completed before the end of the Eocene. During the Early Oligocene collision, this wedge was severely refolded, acquiring its fan structure, as visible in cross section, and its curvature in map view. In such a context, we propose that fast exhumation and cooling of the SPA during the Oligocene resulted from active erosion of rapidly raised high topography. This is consistent with the sudden arrival of metamorphic Penninic clasts in the molasse basins along both flanks of the belt. Moreover, detrital and in situ thermochronological ages, suggest a strong slowing down of cooling and exhumation from the Miocene onwards, coinciding with brittle extension that dominates in the SPA during this long period. The brief Early Oligocene rise of a SPA cordillera, contrasts with the preceding and subsequent period of poor relief of the SPA. The mountainous character of the SPA today is not directly inherited from the Oligocene orogenic climax, as modern high relief and elevation are most likely related to rejuvenation under climatic control during the Quaternary.