Deformation at the base of the Bergell Pluton in the upper Valle dei Ratti (Italy)

The Bergell Pluton (BP) is a calc-alkaline intrusion located in the Italian Alps, north of the Insubrian Line. The intrusion continues into the Southern Steep Belt (SSB), an E-W-striking, north-block-up mylonite zone that affects the pluton and the underlying high-metamorphic nappe pile.

In the upper Valle dei Ratti, a large antiform (VRA) is exposed, which translates the base of the BP and the underlying gneisses into the SSB. We present a new geological map, structural field data and microstructural observations from this area. Petrological mapping reveals an alternating sequence of different gneisses and schists below the BP and metasedimentary and ultramafic rocks at its margins. Previous maps displayed the two limbs as derived from fundamentally different tectonic units below the pluton. Continuous metasedimentary layers, however, indicate a single basement unit. The VRA and associated small-scale folds show E-plunging fold axes and pronounced stretching parallel to fold-axes orientations. This stretching lineation is found in the entire area, also further north (Galli et al., 2013, doi:10.1007/s00015-013-0120-1), and is related to top-east shearing. In the study area, it continuously translates into the lineation associated with north-block-up, left-lateral shearing in the SSB. We propose that the shear zone of the SSB is folded together with the base of the BP and thus identical to right-lateral, south-block-up shearing at the northern border of the Gruf complex. This scheme would explain the metamorphic gap on both sides of the Gruf complex and why the SSB does not exist east of the BP.