DEVELOPMENT OF TECTONIC MéLANGES DURING SUBDUCTION AND EXHUMATION PROCESSES: CASE-STUDY FROM THE LIGURIAN ALPS (ITALY)

Mélanges are mappable chaotic bodies of mixed rocks, with different ages and origin, embedded in an argillitic, sandy, or ophiolitic matrix. Their development in the context of subduction zones have been recognized in different convergent settings all over the world (e.g. Wilson et al., 1989; Froitzheim, 2001; Federico et al., 2007), and many different mechanisms may be involved in mélange formation in a subduction zone, e.g. tectonic erosion, diapirism, underplating, etc.

Recent numerical models (e.g. Gerya et al., 2002; Stöckhert and Gerya, 2005) actually predict the development of a mixing zone between the subducting and the overriding plates in a convergent margin. The growth of this "subduction channel" (that can be interpreted as a “mega-mélange”) is controlled by the progressive hydration (and consequent serpentinization) of the mantle wedge; widening of the subduction channel results in the onset of forced return flow. A tectonic intermingling of fragments of the previously subducted crust, hydrated upper mantle slices and lower crustal rocks of the upper plate is therefore envisaged. The size of these fragments is a function of the viscosity contrast between the coherent slices and the serpentinite matrix; the model predictions include an array of diverse P-T paths for subduction-related metamorphic complexes.

In our study we focus on the High-Pressure (HP) metaophiolites of the Ligurian-Piemontese units of the Ligurian Alps (Western Alps). Such units are known in literature as Voltri Massif, that encompasses blueschist and eclogite facies rocks: we discuss the interpretation of the Voltri Massif as a kilometre-scale tectonic mélange developed in a subduction channel.

In the Voltri Massif two decametre-scale tectonic mélanges occur and have been studied (Vissers et al., 2001; Federico et al., 2007). The first mélange is hosted in country serpentinites and encloses blocks of various types of metabasites and metasediments, not occurring in the surrounding Voltri Massif. The mélange matrix is a chlorite-actinolite schist, produced by blackwall reactions between serpentinites and enclosed blocks. The blocks equilibrated under different peak metamorphic conditions and record different segments of a typical subduction P-T path, with peak conditions ranging from eclogite- to garnet-blueschist-facies and prograde lawsonite-blueschist-facies assemblages. The matrix is widely retrogressed in greenschist facies, but it contains rare relics of Na-amphibole. Both structural evidences and P-T paths of the different blocks suggest coupling between blocks and matrix in the blueschist facies. By ³⁹Ar/⁴⁰Ar dating on phengite (Federico et al., 2007), one eclogitic metasediment records peak equilibration at 43.2 ± 0.5 Ma; two metabasites record blueschist equilibration at 39.95 ± 0.37 Ma and at 43.4 ± 0.5 Ma, respectively. These results are interpreted as diachronic metamorphic trajectories resulting from independent tectonic evolution of the different slices inside a subduction channel.

The second mélange zone have similar lithologic and structural features (Vissers et al., 2001) and can be interpreted in the same way.

As the eastern sector of the Voltri Massif is made of blueschist and eclogite facies metabasites and metasediments dismembered and enclosed in serpentinites, our question is: is it possible to extend this interpretation to a larger scale, i.e. to the whole eastern Voltri Massif?

This option is currently being tested through structural and petrologic investigations combined with a numerical modeling approach (Malatesta et al., 2010 and this congress). Such hypothesis predicts that the Voltri Massif originated in the subduction channel developed during Alpine subduction and was subsequently exhumed when subduction was still active or later on during continental collision and/or transpressional tectonics. To check this option we discuss geochronological data presently available for the Voltri Massif, together with petrographic and structural constraints.

Federico L., Capponi G., Crispini L., Scambelluri M., Villa I. M. (2005) EPSL 240, 668-680

Froitzheim, N., (2001) Geological Society of America Bulletin, v. 113, no. 5, p. 604–614.

Gerya T.V., Stockhert B., Perchuk A.L. (2002) Tectonics, 21(6) 1056, doi: 1029/2002TC001406.

Malatesta C., Gerya T., Federico L., Scambelluri M., Crispini L., Capponi G. (2010) Geophysical Research Abstracts Vol. 12, EGU2010-10785, 2010.

Stockert B., Gerya T.V. (2005) Terra Nova 17 (2), 102-110.

Vissers R. L. M., Hoogerduijn Strating E.H., Heimans M. & Krabbendam M. (2001) Ofioliti, 26 (1): 33-46

Wilson, T.J., Hanson, R.E., and Grunow, A.M. (1989) Geology, v. 17, p. 11–14.