Control of mechanical heterogeneities on nappe detachment, transport and stacking in the Helvetic Nappe System

Abstract : Nappe systems are typical of many orogenic belts. A well-known example is the Helvetic nappe system in the Swiss Alps, which consist of sedimentary units that have been sheared and thrusted over the crystalline basement of the European passive margin during the Alpine orogeny. These sedimentary units form, for example, the parautochtonous Morcles nappe of the infrahelvetic complex, which is situated below the Helvetic Wildhorn super-nappe. Although nappes were recognized a century ago and have been studied since then, the mechanisms responsible for nappe generation and nappe stacking are still debated. We present 2D high-resolution thermo-mechanical numerical simulations of the shearing of basement-cover system with half-graben representing the upper crust of the European passive margin. The scope of the numerical simulations is to evaluate the impact of the (1) geometry of the basement-cover interface, (2) presence of mechanical layering resembling the alternation of shale-rich and carbonate-rich sedimentary units. We are identifying the controlling mechanisms of the formation of the basal detachment zone. The final aim of the simulations is to reconstruct the overthrusting of the Helvetic nappes over the nappes of the Infra-Helvetic complex