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The timing of and nature of greenschist facies deformation and metamorphism in the upper Pennine Alps
Author(s) -
Barnicoat A. C.,
Rex D. C.,
Guise P. G.,
Cliff R. A.
Publication year - 1995
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/94tc02017
Subject(s) - geology , greenschist , nappe , metamorphism , ophiolite , gneiss , massif , metamorphic facies , shear zone , basement , geochemistry , thrust fault , geomorphology , seismology , metamorphic rock , fault (geology) , facies , tectonics , structural basin , civil engineering , engineering
The upper Pennine nappes in the western Alps are cut by structures that locally place European basement rocks over those of the Piemonte zone, which represent the sutured remains of Alpine ocean crust. In the Täschalp 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 assemblages 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, 40 Ar/ 39 Ar, 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 thrust zone. Rb‐Sr data from the literature document muscovite and biotite ages in strongly foliated Grand Saint Bernard gneisses of the hanging wall of circa 36 and 32 Ma, respectively. These are thought to indicate that deformation on the Mischabel structure continued for at least 5 m.y. Rapid cooling inferred for the Zermatt‐Saas ophiolites in the mid‐Eocene may document either the tectonic emplacement of the ophiolite over the underlying Monte Rosa massif or their extensional unroofing. 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 the high‐pressure assemblages in the Zermatt‐Saas and Monte Rosa rocks.