THE TIMING OF AND NATURE OF GREENSCHIST FACIES DEFORMATION AND METAMORPHISM IN THE UPPER PENNINE ALPS

The upper Pennine nappes in the western Alps are cut by structures tha t locally place European basement rocks over those of the Piemonte zon e, which represent the sutured remains of Alpine ocean crust. In the T aschalp area of Switzerland the Mischabel ''backfold'' and associated SE verging shearing carries gneisses of the Grand Saint Bernard nappe over eclogitic material of the Zermatt-Saas zone. Greenschist-facies a ssemblages are developed locally in the foot wall and hanging wall of the Mischabel structure as well as in the sheared rocks of the thrust zone. Rb-Sr, Ar-40/Ar-39, and U-Pb studies of white micas, amphiboles and sphenes show that greenschist assemblages developed at circa 44 Ma in the Zermatt-Saas zone of the footwall and at circa 40 Ma in the th rust zone. Rb-Sr data from the Literature document muscovite and bioti te ages in strongly foliated Grand Saint Bernard gneisses of the hangi ng wall of circa 36 and 32 Ma, respectively. These are thought to indi cate that deformation on the Mischabel structure continued for at leas t 5 m.y. Rapid cooling inferred for the Zermatt-Saas ophiolites in the mid-Eocene may document either the tectonic emplacement of the ophiol ite over the underlying Monte Rosa massif or their extensional woofing . It is not possible to determine with certainty whether the Mischabel structure is extensional or contractional on a crustal scale. The age of cooling and deformation reported here show that it is not possible that the Mischabel structure was responsible for the exhumation of th e high-pressure assemblages in the Zermatt-Saas and Monte Rosa rocks.