Deformation and Metamorphism in the Lukmanier Region, Central Switzerland

Abstract

The Lukmanier region in the central-southern Swiss Alps contains parts of three important structural and stratigraphic units: the Gotthard Massif in the north, the parautochthonous Mesozoic cover of this massif, and the Lucomagno Massif, a frontal part of the Lower Pennine nappe complex to the south. Two successive phases of Alpine folding have been recognized in the Mesozoic cover, both of which occurred after the main northward horizontal transport of the Lower Pennine nappes. The earlier phase, called Phase B, produced three large-scale fold zones in the cover, with a penetrative axial-plane schistosity that strikes east-west and dips steeply north; fold hinge lines are markedly curved in the plane of the schistosity due to differential strain effects. Deformed fossils and ooids indicate the principal axes of Phase B strain and show that the steeply plunging, micaceous mineral lineation, which is found on most schistosity surfaces, is parallel to Phase B maximum strain extension. Regional Phase B folds were modified to various extents by Phase B movements on tectonic slides that lie parallel to the axial-plane schistosity and form the east-west contacts of both massifs with the Mesozoic rocks. The subsequent phase of deformation, called Phase V, produced no large-scale structures, but corrugated bedding and Phase B schistosity and mineral lineation mainly in the southern half of Lukmanier. Fold axial planes strike southeast and dip gently northeast, and fold axes plunge gently southeast. Complex systems of joints and rare, local strike-slip faults developed after Phase V, probably during postorogenic regional uplift. Effects of both phases of Alpine folding are found in the Gotthard and Lucomagno Massifs. Phase B produced the schistosity that strikes east-west and dips steeply north in both massifs, and the shapes of augen, together with deformed schlieren in the Hercynian granodiorite in the Gotthard Massif, indicate that the orientations of Phase B strain axes are similar to those in the Mesozoic cover, with the mineral lineation on the schistosity in both Massifs being parallel to Phase B maximum strain extension. Very intense Phase B deformation in the Gotthard Massif was restricted mainly to mylonitization in narrow east-west zones, and such movements in one zone of ancient paragneisses appear to have controlled the position of the Scopi synclinal zone, the most northerly large-scale Phase B fold zone in the Mesozoic cover. Phase V deformation had little effect in the Gotthard Massif, but Phase V folds are found everywhere in the Lucomagno Massif: their orientation is similar to Phase V folds in the Mesozoic cover. Complex systems of joints in both Massifs probably developed at about the same time as similar joints in the Mesozoic cover. Alpine metamorphism, generally of lower almandine-amphibolite facies, produced a diversity of porphyroblasts in the Mesozoic cover, and they show a variety of textural relations with phases of deformation. These relations change across the area and are due to migration of phases of deformation from south to north relative to stationary metamorphic conditions. Porphyroblast textures also show that shape and predeformational orientation may be important factors controlling amounts of porphyroblast rotation. Regional considerations of Alpine events in Lukmanier and adjacent areas lead to the conclusion that the main northward horizontal transport of the Lower Pennine nappes preceded Phase B deformation in Lukmanier, and in turn Phase B preceded and initiated, by uplift of the Gotthard Massif, the main northward horizontal transport of the Ultrahelvetic and Helvetic nappes.