EXHUMATION OF COESITE-BEARING ROCKS IN THE DORA MAIRA MASSIF (WESTERN ALPS, ITALY)

Coesite-bearing, very high pressure rocks in the southern Dora Maira massif, western Alps, Italy, emerged from a depth of 100 km during Cretaceous time. These rocks are now covered only by a thin fraction of the synmetamorphic overburden. Tectonic contacts above the coesite-bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion: they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the Eoalpine baric and thermal structure. The result of this was the lateral dispersion of the Eoalpine orogenic wedge and the facilitation of the unroofing of the very high P rocks. Extensional unroofing is consistent with the absence of deeply cutting Cretaceous erosion, and with pressure-temperature paths that show that the very high P rocks cooled during decompression. The very high P rocks reached relatively shallow crustal levels by ca. 40 Ma, and the attenuated sequence overthrust a more external unit. These indicate that the exhumation of the very high P rocks took place within an overall plate convergence setting, prior to the final suturing of Europe and Adria.