The geology of Valchiavenna in the frame of the Central Alps: recent insights from the CARG project 038 “Chiavenna”, 021 Passo dello Spluga, 022 Madesimo and 037 Bodengo

The study and comprehension of the geodynamic history and tectonic structure of the Central Alps has always represented a key to unravel the emplacement of the whole Alpine structure and to better understand a great range of pre-alpine magmatic and metamorphic events. The extreme geological complexity of this area still presents several unsolved problems, both at local and regional scale. For this reason, a great number of past works focused on this portion of the Alpine chain, trying to address a wide range of problematics and providing interesting insights for the review and understanding of the Alpine orogeny. New lymph on Central Alps geology has been recently provided by the beginning of new geological surveys in the area of Valchiavenna, lower Valtellina and Alto Lario, in the frame of the CARG Project (Carta Geologica d’Italia alla scala 1:50.000, geological sheets 038 “Chiavenna”, 021 “Passo dello Spluga”, 022 “Madesimo”, 037 “Bodengo”). Four geological sheets are underway in this province, that falls in the middle of the axial zone of the Central Alps, along the Italy-Switzerland national border. It comprehends a predominant mountain area of approximately 770 km2, characterized by a hostile topography with altitudes ranging from 200 to 3300 m a.s.l. Most of the territory sees the exposition of different poly-metamorphic basements belonging to the Penninic nappe stack (Adula, Tambo and Suretta Nappes) (Schmid et al., 2004), divided by slices of metasedimentary covers and/or tectonic mèlanges and intruded by some pre-Alpine granitoid bodies (e.g. Truzzo Granite and Roffna Porphyry Complex) (Marquer et al., 1998). The nappe stack is juxtaposed by some structural and paleogeographic uncertain units, as in the case of the Chiavenna Unit, a mafic-ultramafic complex, and the Gruf Complex, that represents one of the less studied units of the Central Alps. Due to the presence of unique high-grade rocks as sapphirine granulites, charnokites and migmatites (Galli et al., 2013), this unit has represented an enigma for its tectonic attribution and it could play a key role in the geological comprehension of the Central Alps. Two tertiary magmatic bodies, the Bergell Pluton and the Novate Granite, emplaced in the south-eastern part of the study area (Berger et al., 1996), surrounded by a multitude of tardive aplitic and pegmatitic swarms, the source of which is still to be definitely identified. Several lineaments cross the study area: the Forcola Line, westward, divides Adula Nappe from Tambo Nappe, while the Engadine Line, eastward, runs alongside the contact between Chiavenna Unit, Tambo Nappe and Gruf Complex. The Insubric Line, to the south, divides Penninic and Asutroalpine from Southalpine domain. This work aims to introduce the study area involved in the production of the four geological sheets of the CARG project Valchiavenna, and to present the advances on the field work, through several examples on the main geological features of the area, with the aid of traditional and innovative survey techniques, as digital mapping and drone photogrammetry. The research on a great number of previous bibliographical works, joined to field work and laboratory analyses, could be a chance to reconsider and solve the main issues of this portion of the Central Alps, providing a starting point for more in-depth studies that can finally shed light on the complex tectonic structure of the Alps.