The Tambo nappe: insights into its internal deformation from geological mapping (Swiss-Italian Alps)

We present detailed 1:10’000-scale geological maps of the eastern flank of the Lepontine Dome, focusing on the Tambo nappe. These maps, produced for the Grono, Mesocco, and Hinterrhein sheets of the Geological Atlas of Switzerland 1:25’000 (GA25), reveal the nappe’s complex internal deformation history, influenced by inherited rheological anomalies, localized fluid percolation, and varying metamorphic conditions. The study of basement nappes internal deformation provides crucial insights into contrasting exhumation mechanisms, a topic central to understanding orogenic dynamics. While nappes exhumed through channel flow mechanisms typically exhibit incoherent internal deformation, those exhumed via Stokes flow or within an accretionary wedge are more structurally coherent.

The Tambo nappe is composed of heterogeneous paragneisses, micaschists, amphibolites, and metagranitoids. This 20 km-long body was emplaced northward as part of the Alpine nappe stack derived from the Briançonnais paleogeographic domain since the Paleocene. The dominant structural elements (foliation, fold axes, and lineation) are predominantly eastward-oriented.

In the southern part of the nappe, the Truzzo meta-granitoid, a folded Permian batholith, acted as a competent, elongated body embedded within micaschists and paragneisses. Below the batholith, pervasive greenschist facies mineral assemblage (chlorite and phengite bearing), associated with top-to-the-east shearing, overprinted earlier amphibolite-facies metamorphism. Deformation subsequently localized along the Forcola Fault. Quartz veins supplied the fluids responsible for the greenschist-facies metamorphism during orogen-parallel shearing that postdated nappe emplacement.

In the northern part, approximately 2 km from the nappe’s front, the lithologies steepen into a cusp-like geometry oblique to the upper nappe boundary. This sector is characterized by sporadic metacarbonate lenses within metasomatized gneisses and schists. Over several hundred meters, syn-foliation veins were folded and boudinated at local peak metamorphic (lower amphibolite) conditions. Locally, some veins cut across the foliation, indicating fluid percolation during deformation. Unlike the southern part, deformation and fluid activity here occurred at peak metamorphic conditions, obliterating earlier textures. We postulate that metacarbonates at the cusp’s core may represent remnants of squeezed Permo-Mesozoic grabens or half-grabens.

These findings suggest that the Tambo nappe behaved as an incoherent body incorporating Mesozoic fragments during nappe emplacement. Subsequent orogen-parallel shearing further thinned the sequence. Field evidence highlights the crucial role of fluids in both deformation stages, influencing rheology and metamorphism.