The Alpine "orogenic lid" - upper plate of a metamorphic core complex?

The European Alps have a cold upper plate, often referred to as its "orogenic lid", but this lid appears not to be what previous authors have suggested. The term "orogenic lid" was proposed to describe low-grade metamorphic rocks that have undergone mostly brittle deformation during the Tertiary, while the underlying units have been ductilely deformed at higher metamorphic grade. The widely accepted explanation for this observation is that the orogenic lid was thrust on top of hotter rocks during Tertiary convergence of the African and European plates. Here we argue differently and show that in the late Eocene, whatever its previous history might have been, the orogenic lid in Eastern Switzerland has acted as the brittle upper plate of an extending metamorphic core complex. From ~33 Ma the rocks involved have not been deeper than 8 km, based on the results of K-feldspar argon geochronology. The lid is riddled with high-angle normal faults that terminate at a basal low-angle fault. Moreover this basal detachment is underlain by thick zones of intensely sheared mylonites. The last movement in rocks beneath the detachment occurred at ~30 Ma after a main phase at ~40-45 Ma, based on microstructurally-focussed white mica argon geochronology. These Barrovian facies metamorphic rocks have been exhumed from around 30 km depth and extracted from underneath the orogenic lid as the result of extensional tectonism. The geometry and thermal structure of the orogenic lid and underlying rocks have much in common with Cordilleran metamorphic core complexes. Hence we suggest that lithosphere scale extension is responsible for the primary tectonostratigraphic structure of the Alps, namely a fractured cold brittle orogenic lid on top of hotter ductile rocks.