High-pressure-low-temperature metamorphism and deformation in the Bundnerschiefer of the Engadine window: implications for the regional evolution of the eastern Central Alps

A study of the metamorphic and tectonic evolution of the Bundnerschiefer of the Engadine window shows that the individual nappes have been thinned by a large amount and that extension was active during and soon after nappe stacking. Based on contrasting P-T histories the Penninic Bundnerschiefer can be divided in two major units bounded by a horizontal contact. The lower (Mundin) unit shows typical high-P/low-T parageneses in metapelites (Mg-carpholite) and in metabasites (glaucophane); metamorphic conditions are estimated around 12 kbar, 375 degrees C. The upper (Arina) unit contains no specific high-P minerals; metamorphic conditions are estimated around 7 kbar, 325 degrees C. A minimum pressure gap of 5 kbar is thus observed. The contact between the two units is marked by a mappable normal shear zone with top-to-the-north-west sense of shear. Near the shear zone, fresh carpholite fibres trend parallel to the regional stretching lineation, implying that the detachment is an early structure active from the depth of stability of the carpholite and persisting during subsequent exhumation. The good preservation of carpholite and the absence of retrograde chloritoid below the shear zone show that exhumation occurred along a cooling path, whereas the deeper units are exhumed along an isothermal path. Exhumation probably occurred during convergence and further nappe stacking during the earlier Eocene. These results suggest that precollisional tectonic thinning of the Penninic oceanic units map be more widespread and significant than generally recognized.