STRUCTURAL OBSERVATIONS AT THE EASTERN CONTACT OF THE BERGELL PLUTON

Macro-, meso- and microstructures indicative of syn-intrusive deformat ion are very prominent at the eastern contact of the Bergell pluton (b etween Val Forno and Valle Sissone). This deformation is documented, f or example, by boudinaged intrusive sills and folded dikes. On a large r scale, the syn-intrusive deformation produced foliations that are co ncordant with the general trend of the contact between pluton and coun try rocks. Mafic, microgranitoid enclaves in both tonalite and granodi orite reveal a strain concentration towards the contact of the pluton. In all cases these enclaves point to a flattening type of strain. Def ormation related to the emplacement of the Bergell pluton is further i ndicated by microstructures exhibited by contact metamorphic minerals. In some cases, the textural relationships suggest that the contact me tamorphism can be subdivided into two stages, an older static event fo llowed by a younger kinematic stage. In order to link these observatio ns with the western part of the Bergell, we constructed a NE-SW profil e through the entire pluton. This profile confirms that, due to region al tilting of the Central Alps and block rotation along the Engadine l ine, a significantly higher crustal level is exposed in the east relat ive to that exposed in the west. Furthermore, the construction suggest s that the outcrops in Val Forno represent upper levels of the side ra ther than the roof of the pluton.