Formation, burial and exhumation of basement nappes at crustal scale: a geometric model based on the Western Swiss-Italian Alps

Information from geological and reflection seismic data from the Western Swiss-Italian Alps, and from numerical models, is used to build a geometrical model that can explain some of the major tectonic-metamorphic features of Alpine-type basement nappes. The model gives a geometrical and mechanical explanation for the initiation, burial, and subsequent uplift and partial exhumation of basement nappes at a crustal scale. Three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures. The first two stages are single vergent (NW) and correspond to the subduction of continental margin crust, and the formation and uplift of high-pressure rocks. Simple-shear flow and superimposed wedge-shaped pure shear flow is proposed for the creation and intrusion of high-pressure nappes of the Monte Rosa type. The third stage is characterized by a doubly-vergent style with both pro- and retro-movements. The former created NW-vergent nappes, as seen in the external Alpine massifs and the latter caused backfolding and thrusting, typical of the internal Alps. This third stage corresponds to the Neoalpine movements in the Western Swiss-Italian Alps, and is accompanied by a generalized uplift, mountain building and molasse sedimentation.