Abstract During Europe–Adria collision in Tertiary times, the Monte Rosa nappe was penetratively deformed in several stages after an eclogite-facies pressure peak: (1) top-to-the-NW thrust shearing (Mattmark phase, after 40 Ma); (2) orogen-parallel, top-to-the-SW extensional shearing and folding (Malfatta phase); (3) orogen-perpendicular, top-to-the-SE extensional shearing and folding (Mischabel phase, before 30 Ma); and (4) large-scale, upright, SE-vergent folding (Vanzone phase, c . 29–28 Ma). Structural analysis and neutron texture goniometry of quartz mylonites show that the Stellihorn shear zone in the Monte Rosa nappe accommodated a complex and multidirectional sequence of shearing movements during the Mattmark, Malfatta and Mischabel phases, and was folded in the Vanzone phase. In the tail-shaped eastward prolongation of the Monte Rosa nappe in the Southern Steep Belt of the Alps, both dextral and sinistral mylonites (Olino phase) were formed during and after the formation of the Vanzone fold, reflecting renewed orogen-parallel (SW–NE) extension contemporaneous with NW–SE shortening from c . 29 Ma onward. A similar sequence of deformation stages was identified in the Adula nappe at the eastern border of the Lepontine metamorphic dome. Important consequences arise for the Insubric fault at the southern border of the Lepontine dome: (1) the NW- to N-dipping orientation of the Insubric fault is not a primary feature but resulted from rotation of an originally SE-dipping shear zone after c . 30 Ma; and (2), the strong contrast in metamorphic grade across this fault (upper amphibolite facies to the north versus anchizone to the south) results from north-side-up faulting coupled with orogen-parallel extension of the northern block (Lepontine dome), while no such extension occurred in the southern block (Southern Alps). Extension in the northern block started in the Malfatta phase and continued in the Mischabel phase when the foliation in the area which later became the Southern Steep Belt still dipped towards south. During Vanzone/Olino deformation, further unroofing and uplift of the Lepontine dome relative to the South Alpine block took place while the Southern Steep Belt was progressively rotated into its present, overturned position, changing its character from a normal fault into a backthrust. Complex deformation paths in the Southern Steep Belt resulted from the combination of extension of the northern block with strike-slip motion along the Insubric fault.

Eclogites occur in the Monte Rosa and Gran Paradiso Penninic nappes of the Western Alps as small bodies within metasedimentary rocks and, in Monte Rosa, rarely, as boudinaged dikes within late-Variscan granitoids. Minor but characteristic constituents of these eclogites are glaucophane and paragonite. Bulk composition of the protoliths ranges from basaltic to Fe-Ti basaltic. High-pressure assemblages comprising chloritoid, phengite, garnet ± kyanite, talc, Mg-chlorite and glaucophane also occur in micaschists of both nappes. The Early Alpine high-pressure assemblages postdate the intrusion of late-Variscan granitoids and are overprinted by the greenschist, mid-Tertiary Lepontine metamorphism. Garnet-clinopyroxene geothermometry and phase compatibilities suggest high-pressure metamorphic temperatures ranging from 440°C to 530°C in the Monte Rosa nappe, with pressure in excess of 8-10 Kb but lower than approximately 14 Kb. These conditions are in the same range as established for the overlying Piedmont ophiolite nappe and for parts of the Austroalpine basement nappe, implying a similar tectonomet-amorphic evolution during Early Alpine time. Garnet-omphacite pairs from the amphibole eclogites of the Gran Paradiso nappe yield lower equilibration temperatures (T ≃ 400°C), possibly recording a later stage in the P-T-t path of this unit.