Tectonostratigraphy of the northern Monviso Meta-ophiolite Complex (Western Alps)

Gianni Balestro (*), Bruno Lombardo (†), Gloria Vaggelli (**), Alessandro Borghi (***),Andrea Festa (*) & Marco Gattiglio (*)
(*) University of Torino, Earth Sciences Department, Via Valperga Caluso, 35 - 10125 Torino, Italy. E-mail: gianni.balestro@unito.it(**) CNR, Institute of Geosciences and Earth Resources, Via Valperga Caluso, 35 - 10125 Torino, Italy. (†) Passed away on 2014.

The Monviso Meta-Ophiolite Complex is a remnant of the Piedmont- Ligurian oceanic lithosphere stacked in the Western Alps, and consisting of dismembered HP meta-ophiolite sequences. In this work, focused on the northern sector of the Complex, we differen - tiate six tectonic units which structural, petrographic and stratigraphic characteristics are described in detail and discussed in the light of a comparison with the overall geology of the Monviso Meta-Ophiolite Complex. The structural evolution has been referred to i) an early syn-eclogitic deformation phase (D1), ii) a main deformation phase (D2) occurred in the blueschist- to greenschist-facies transition and characterized by the development of a regional foliation (S2) that is parallel to the tectonic contacts and to the axial plane of map-scale W-verging folds, and iii) a late-metamorphic deformation phase (D3) characterized by westward extensional tectonic. The northern Monviso Meta-Ophiolite Complex is characterized by a poor preservation of HP paragenesis and a widespread overprint of the blueschist- to greenschist-facies metamorphism, but the occurrence of garnet-, omphacite-, talc- and lawsoniteassemblage in a Fe-Ti metagabbro indicates P-T eclogitic conditions (2.5-2.7 GPa for 550-570 °C) very similar to those calculated in the southern sector of the Complex. The stratigraphic characteristics of the meta-ophiolite sequences point out that, differently from the southern sector of the Complex where basalt-poor and basalt-rich oceanic units have been distinguished, in the northern Monviso Meta-Ophiolite Complex the different types of metasediments may be the key to restore the oceanic tectonostratigraphy, marked by gabbro and mantle peridotite exposition on a puzzle-like ocean floor where basalt effusion and different sedimentation processes took place.

Monviso, HP meta-ophiolites, tectonic units, Western Alps.