Oblique exhumation of HP metaophiolite at the southern termination of the Western Alps (Italy).

The exhumation of high-pressure (HP), metaophiolitic terrains is a long-studied process, and many different models have been proposed so far, since the problem of how dense HP metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep is crucial to understanding processes occurring at the plate interface and mantle wedge within subduction zones.

However, the exhumation process may obliterate peak-related structures and metamorphic associations at different degrees. In the Western Alps, one of the most studied orogen of the world, some HP terrains still preserve an almost complete stratigraphy of the oceanic lithosphere (e.g., the Monviso and Zermatt-Saas Massifs).

On the contrary, the Voltri Massif (VM), which crops out at the southern termination of the Alpine orogen (Ligurian Alps), is at places characterized by a high degree of disruption of the original stratigraphy.

The VM shows different features in the eastern and western sectors: in the eastern sector the high-pressure eclogitic-blueschist rocks are frequently embedded as bodies and lenses within serpentinite or metasediments, which act as a low strength “matrix” that accommodates most of the strain. This has led to the interpretation of the massif as a tectonic mélange, formed inside the subduction channel (Federico et al., 2007).

The western sector, on the contrary, contains relics of disrupted mélange associated to more coherent slices of metamorphic oceanic lithosphere.

Regarding the structural architecture, the VM eastern sector shows a steeply dipping foliation, steeply dipping blueschist to greenschist stretching lineation, high shear strain and prevalent structures typical of non-coaxial flow (Capponi and Crispini 2002). These structures are formed during the progressive exhumation from blueschist to greenschist facies conditions.

On the other side, the western part of the Massif is characterized by shallow-dipping fabrics and prevalent structures mostly dominated by strain flattening. Here structures related to the HP stage are better preserved and the greenschist-facies overprint is less pervasive and static at places.

Combination of new and reviewed structural data collected during several decades of fieldwork, geological mapping, PT-paths and geochronological data, points to a model of exhumation in which a non-coaxial transpressional zone played a fundamental role. Important rotation probably occurred at this stage, since the eastern high-strain zone is now perpendicular to the main orogen strike. This is likely due to the peculiar geodynamic position of the VM, at the tip of the alpine subduction zone and to the interference and lateral transition to the embryonic Apennine belt.

 

Capponi G., Crispini L. (2002) Structural and metamorphic signature of alpine tectonics in the Voltri massif (Ligurian Alps, North-Western Italy).

Federico L., Crispini L., Scambelluri M., Capponi G. (2007) Ophiolite mélange zone records exhumation in a fossil subduction channel Geology, 35/6; p. 499–502; doi: 10.1130/G23190A.1