Strain accumulation during basal accretion in continental collision — A case study from the Suretta nappe (eastern Swiss Alps)

Abstract

Stacked crystalline basement nappes are a typical feature in the core of many orogens. The Suretta nappe, exposed in Eastern Switzerland, consists of Briançon-derived crustal slices which were assembled in a south-dipping subduction zone during the Alpine orogenic cycle. The nappe contains post-Variscan rocks of the Rofna Porphyry Complex (RPC) and Permo-Mesozoic cover sequences, which are ideal to study the strain evolution during Alpine nappe formation. We present new structural cross-sections, finite strain analyses and a retrodeformation for the frontal part of the Suretta nappe. The overall geometry of the Suretta nappe is the result of two main deformation phases: (1) Eocene top-to-the-NNW directed thrusting and folding (Ferrera phase), which is overprinted by (2) backfolding and backshearing (Niemet–Beverin phase). We suspect that this backshearing was caused by the buoyant rise of light continental crust beneath the Suretta nappe and by frictional resistance at the top contact of the Suretta nappe, which had been accreted to the upper plate in the course of nappe stacking. In the lower and interior parts of the Suretta nappe, weakly to undeformed boudins are generally surrounded by L-tectonites indicating WSW–ENE stretching; foliated equivalents reveal a plane strain deformation state. The upper part of the Suretta nappe, which was strongly affected by backshearing, shows flattening strain. Strain data were used to reconstruct the palinspastic evolution of the Suretta nappe at two distinct time slots: Prior to Niemet-Beverin backfolding, the Suretta nappe is characterized by thrust faults and large-scale detachment folds. Before the onset of Ferrera phase nappe stacking, Jurassic normal faults and the RPC intrusion shape trigger the localization of subsequent deformation.Our structural study confirms a complex, polyphase evolution for the Suretta nappe, which might be characteristic for crystalline basement nappes in continental collision zones elsewhere.