The eastern termination of the Southern Steep Belt (Central Alps)

The Southern Steep Belt (SSB), an E–W-striking, north-block-up mylonite zone, juxtaposes high-grade metamorphic Europe-derived basement nappes and the unmetamorphic Southern Alps. At its eastern end in the upper Valle dei Ratti, the SSB apparently terminates at the calc-alkaline Bergell pluton (BP). There, a large antiform at the base of the BP translates the intrusive rocks and underlying gneisses into the SSB. However, the relationship between the SSB and the units farther north (Gruf complex, Adula nappe) remains unclear.

We present a new geological map of this critical area. An alternating sequence of Bt-rich gneisses and schists adjacent to more leucocratic gneisses, as well as metasedimentary and meta-ultramafic rocks, clearly represents a single basement unit beneath the BP. This contradicts previous maps, which assigned different tectonic units to the two limbs of the antiform. Stretching lineations and fold axes consistently plunge eastward. Structural, microstructural, and EBSD data indicate upper-unit-to-the-east shearing, which transitions into north-block-up, left-lateral shearing within the SSB. We propose that the mylonitic shear zone of the SSB is folded together with the base of the BP and continues into the right-lateral, south-block-up shear zone at the northern border of the Gruf complex. Hence, top-to-the-east shearing beneath the BP would exhume the entire Lepontine, not just the Gruf complex. This scheme would explain the metamorphic gap on the northern and southern sides of the Gruf complex as well as the absence of the SSB east of the BP.