Post-Nappe folding southeast of the Mischabelrückfalte (Pennine Alps) and some aspects of the associated metamorphism

Three important folding phases, F1, F2 and F3, have been found in the Pennine Nappes and the nappe-separating Palaeozoic and Mesozoic rocks between Saas Fee and Villadossola. The F1 folds were formed at the beginning of the temperature increase that marks the second Alpine metamorphic event, and they resulted from penetrative deformation of the nappes and the nappe-separating rocks during the final stage of nappe emplacement. This stage was preceded by a stage of non-penetrative deformation during which most of the nappe transport is believed to have taken place. This earlier stage was characterized by thrusting along the interfaces between undeformed slabs of Hercynian or older crystalline basement rocks, and in the zones of nappe-separating rocks. In this study, the structural and metamorphic history after the beginning of F1 will be chiefly considered, and a reconstruction of the structural geology before F2 will be made. Nearly all the F1 folds outside the Moncucco zone have the same asymmetry, which reflects penetrative directional movement during the last stage of nappe emplacement. The axial directions of the F1 folds, which are tight to rootless intrafolial folds of the layering, initially were not everywhere parallel. The folds occur on a microscopic and mesoscopic scale and have a pervasive axial-plane schistosity, S1, which is the regionally prevalent S-plane. F1 folding may have taken place at about 60-65 m.y. before present. The F2 folds postdate the nappe movements; they have refolded the nappe boundaries and the structures of F1 age. The relationships between plagioclase porphyroblastesis, F2 deformation and the time-temperature curve for the second Alpine metamorphic event show that F2 folding took place at about 36 m.y. before present. The similar-style F2 folds, that occur on every scale, are characteristically disharmonie and sometimes extremely so. A common type of disharmonic fold has limbs which are convex towards the axial plane, the outer layers having been more tightly folded than the core, and some major folds also have this geometry. Many of the F2 folds have an S2 axial-plane schistosity that in the majority of cases was formed by reorientation of pre-existing micas. It is found that the degree of reorientation depends on the lithology, on the fold geometry, and probably also on other factors. All the major folds except one have F2 age, and evidence for intensive F2 deformation can be found almost anywhere. Well-known structures like the ‘Synclinale rétrograde de Saas’ and the Antrona Syncline were formed at this time. It is believed that the Antrona Syncline continues through the Monte Rosa Nappe to Mattmark, where it meets another major F2 fold (the Trifthorn Antiform) and both folds terminate. The age and characteristics of F2 folding in the Moncucco zone differ from the remainder of the area. This and the fact that the lithology of the Moncucco zone and the style of F1 folding there also differ, suggests that the Moncucco zone may not be a part of the Pennine Nappes. The F3 folds postdate all mineral growth, and one major fold, the Brevettola Antiform, was formed during this phase. It is the hinge between the ‘root-zone’ of the nappes and the nappes themselves, and the overturning of the ‘root-zone’ is therefore the result of F3 folding. The Brevettola Antiform is believed to be the continuation in the Camughera-Moncucco Complex of the Vanzone Antiform. All other F3 folds occur on a microscopic to mesoscopic scale; they can be separated in two contemporaneous sets of folds with opposite asymmetry and opposed axial-plane dip directions.