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

Gianni Balestro (a), Andrea Festa (a), Stefano Ghignone (a), Costanza Morino (a), Emanuele Scaramuzzo (a) & Paola Tartarotti (b)
(a) Dipartimento di Scienze della Terra – Università degli Studi di Torino, Via Valperga Caluso, 35, 10125 – Torino. E-mail: andrea.festa@unito.it (b) Dipartimento di Scienze della Terra – Università degli Studi di Milano, Via Mangiagalli, 34, 20133 – Milano

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-in-matrix 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 Meta-ophiolite Complex) units, that experienced the overprint of different tectonic stages from subduction to collision. A Sedimentary Mélange characterizes the meta-sedimentary 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 tectono-metamorphic evolution of the northern sector of the Monviso Meta-ophiolite Complex and its pre-orogenic physiography.

Western Alps, Monviso Meta-ophiolite Complex, plate interface, mélange, broken formation