The Tonale Nappe follows the E–W striking part of the Tonale Fault (Insubric Fault) and is is characterized by a very distinct and lithologically variable association of predominantly sillimanite-bearing paragneisses, marbles, quartzites and amphibolites metamorphosed during the Variscan cycle (Tonale Gneiss Complex).

In the west, between Giubasco and Sondrio, the Tonale Nappe forms a steeply N-dipping narrow strip of mylonitic series that are part of the Southern Steep Belt (or “root zone” north of the Tonale Fault), bounded by the tonalites of the Bregaglia Intrusion to the north and Alpine mylonites and cataclasites of the Tonale Fault in the south. Between Sondrio and Tirano, it then represents the hangingwall of the Late Cretaceous Mortirolo Normal Fault. Further east, a second Late Cretaceous normal fault, the SE-dipping Pejo Normal Fault, accommodates top-E extension combined with a sinistral strike-slip component in the present-day map view. Finally, east of the Passo del Tonale, the Tonale Nappe displays a rock association that is lithologically distinct from the Tonale Gneiss Complex and that is referred to as the Ulten (or Ultimo) Unit. It overlies the Tonale Gneiss Complex along a pre-Alpine tectonic contact.

Paragneiss biotitici a granato e paragneiss e micascisti a granato, staurolite e sillimanite intercalati di anfiboliti, talora eclogitiche, di marmi e fels Ca-silicati, di ultrabasiti e di filoni pegmatitici.