STRUCTURAL-GEOLOGICAL SURVEY OF AN ECLOGITIZED CHAOTIC COMPLEX: THE RIFFELBERG -GARTEN UNIT IN THE BREUIL DELL (ZERMATT-SAAS ZONE, ITALIAN WESTERN ALPS)

The Riffelberg-Garten Unit (RGU) is a tectono-stratigraphic unit, belonging to the Zermatt-Saas Zone (ZSZ), characterized by a chaotic mixture of metasediments, metabasites and serpentinites (Bearth, 1967; Dal Piaz & Ernst, 1978; Campari et al., 2004). In particular the RGU is composed by meter to cm-sized clasts of eclogites, glaucophanites, marbles, and serpentinites within a heterogenous matrix of micaschists, marbles and calcschists. This unit mainly occurs in the Valais and in upper Aosta Valley, and its originally complex lithostratigraphy is further complicated by polyphase deformation generated during Alpine subduction, collision, and the subsequent exhumation. Such a complexity makes the origin of this mélange still debated. The multiscale structural analysis was performed over an area of 12km2 in the upper Valtournanche valley (Breuil) and revealed the occurrence of four superposed groups of ductile structures. The new structural map at 1:5000 scale has been integrated by form surface maps performed at more detailed scales (1:100, 1:50, 1:200). Microstructural and petrologic investigations show that the first two groups of structures, D1 and D2, developed under eclogite facies conditions, while D3 and D4 developed under decreasing P and T during the progressive RGU exhumation. New quantitative PT estimates suggest that the thermal regime associated with the development of D1 and D2 structures is compatible with a cold subduction, as the one already inferred for ZSZ. On the contrary the T/P ratio characterizing D3 and D4 results to be higher, indicating that the exhumation occurred in a mature collisional setting