Origin of the Monte Rosa nappe in the Pennine Alps—A new working hypothesis

Abstract

On the basis of lithological and structural evidence, a new solution is proposed for the classical problem of the restoration of folded continental and oceanic nappes in the Pennine Alps of Switzerland and Italy. According to this working hypothesis, the Monte Rosa nappe, a high-pressure metamorphic gneiss unit, represents basement of the northern European margin of Alpine Tethys. Its paleogeographic origin was formerly sought either in the Brianconnais microcontinent or in the southern Adriatic continental margin. The Monte Rosa nappe is enveloped by the lithologically heterogeneous and extremely deformed Furgg zone, interpreted here as a melange zone formed during subduction and collisional closure of the Valais ocean basin, the northern subbasin of Alpine Tethys. In a late stage of the Alpine orogeny (late Eocene), postdating closure of the Valais ocean basin, the European continental margin including the Monte Rosa basement was subducted southward under the Pennine nappe stack and reached eclogite facies depth. From there, the Monte Rosa nappe quickly ascended back toward the surface in an internal (southeastern) position, separated from the other Europe-derived units located farther northwest by the rootless, Brianconnais- derived Bernhard nappe system.