The geological map of Valmalenco

The 'Carta Geologica della Valmalenco' (Montrasio et al., 2005) is the result of a thirty five-year cooperation between ETH and University of Zürich and the University and CNR-Italia at Milano. The map covers an area of about 350 km² with the mapping having been carried out by over fifty people, mostly as part of students' theses. The Valmalenco area contains one of the largest ultramafic masses of the Alps, which occurs together with a well preserved, Jurassic to Cretaceous ocean floor suite. These units are sandwiched in between nappes derived from the Adriatic microplate and the European plate. The ultramafic rocks are of subcontinental origin and preserve on their top a Permian, continental, crust-mantle transition. During Jurassic rifting these rocks were exposed at the Adriatic margin of the Piemontese-Ligurian ocean basin. Two orogenic cycles affected the Adriatic margin: the first is related to Late Cretaceous nappe stacking, whereas the second is due to the Tertiary continental collision. At the end of collision, the Malenco rocks were, in their southwestern part, intruded by the calc-alkaline, Oligocene, Bergell complex. The Bergell rocks present an exceptionally rich variety of magmatic phenomena and a well-defined contact aureole. The area affected by contact metamorphism is an outstanding natural laboratory to study metamorphic and metasomatic processes. Isograds of prograde metamorphism of serpentinite and of ophicarbonate rocks were mapped for the first time in the Valmalenco, and the results integrated into a consistent petrological model.