Different types of HP mélange and broken formation in the northern Monviso Meta-ophiolite Complex (Inner Western Alps, NW Italy)

In the Western Alps the superposition of tectonic and/or tectonometamorphic events, and mineral transformations, strongly concur in obscuring the original prevailing processes of formation of block-inmatrix structures. We document in this study the occurrence and nature of different types of block-in-matrix structures (i.e. sedimentary and tectonic mélanges, and broken formations), formed at the plate interface between continental (i.e. the Dora Maira Unit) and oceanic (i.e. the Monviso Metaophiolite Complex) units, that experienced the overprint of different tectonic stages from subduction to collision. A Sedimentary Mélange characterizes the metasedimentary cover of the Dora Maira Unit and consists of exotic blocks (i.e., olistholits and blocks of marble) gravitationally collapsed from the margins of the Triassic-to Jurassic carbonate platform of the European continental margin. The Broken Formation is obtained thanks to the superposition of two deformation phases that, during exhumation stages, dismembered the original coherent stratigraphic succession. The Tectonic Mélange started to be formed during the subduction via offscraping of exotic blocks from the heterogeneous oceanic seafloor. The superposition of exhumation-related deformation contributed to form its final "structurally ordered" block-inmatrix fabric. Our findings may provide better constraints for the tectonometamorphic evolution of the northern sector of the Monviso Metaophiolite Complex and its pre-orogenic physiography. © Società Geologica Italiana, Roma 2013.