Architettura di una faglia normale a basso angolo regionale: rilevamento geologico-strutturale ed analisi alla meso- e micro-scala di una porzione della “Simplon Fault Zone” Alpi Centrali (Svizzera)

In this thesis a structural-geology analysis, at different scales, was carried out. A portion of the Simplon Fault Zone outcropping along the Simplon Pass in Switzerland (Central Alps) was studied. The Simplon Fault Zone is an archetype of low angle normale fault (LANF) active in syn-collisional contest. An original structural-geological map of the area, at 1:5.000 scale, accompanied with a geological section and stereographic projections of the main structural elements were realized. Cartographic and meso-scale observations allowed the recognition of four main tectono- metamorphic unit and the occurrence of an important regional scale tectonic contact, with a top-to-the-SW normal kinematics. This one is characterized by a thick ductile shear zone surmounted by a narrow cataclastic zone, poorly developed in the study area. The ductile shear zone is characterized by a pervasive mylonitic foliation, i.e. the main foliation of the area, with a variable orientation, striking NW-SE and NE-SW, and dipping 25-30° towards the SW. On this foliation a mineralogical lineation trending SW-NE and plunging around 25° towards SW. It has also observed the occurrence of lineation-parallel folds showing their axes orientated parallel to the mineralogical lineation. These folds are related at a complex deformation history, occurred in a progressive deformation framework characterized with a sub-simple shear regime and constrictional strain. This main phase, related to the development of mylonites of the Simplon Fault Zone, was followed by a later deformation phase, linked to the development of folds with axes orientated about nearly N-S. Microtectonic observations, made with the aid of optic microscope, of four selected thin sections from oriented samples, allowed to recognize the main kinematic indicators at the micro-scale (mineral-fish, porphyroclasts with asymmetrical recrystallization tails, asymmetrical boudinage and obliqued foliation in quartz) of the shear zone and to qualitatively infer possible deformation temperature during the shearing.