THE HIGH PRESSURE VOLTRI MASSIF (LIGURIAN ALPS, ITALY) AND 2D NUMERICAL MODELS: AN INSIGHT INTO EXHUMATION MECHANISM

High-pressure ophiolite massifs usually outcrop in the internal position of an orogenic chain. In the Ligurian Alps we studied a kilometric massif in which plurimetric to pluridecametric lenses of metagabbro and metabasites are surrounded by highly sheared serpentinite and metasediments. The massif is made of two tectono-metamorphic units (Palmaro - Caffarella and Voltri) which record distinct metamorphic histories with peak conditions ranging from the blueschist to the eclogite facies. The study of some metagabbro lenses outcropping in such massif revealed that a strong strain partitioning acted between the lenses and the host-rock (serpentinite or metasediment) and between the core and the rim of the lenses: in the cores the magmatic textures are preserved whereas tectonitic and mylonitic textures are often visible in the rims. In the Voltri and Palmaro - Caffarella units both the metamorphic peak (eclogite and blueschist facies, respectively) and the later greenschist facies retrogression are syn-kinematic.

Coupling the conventional petrographical analysis with P-T pseudosections (PERPLE_X; Connolly et al., 1990) we have been able to define the metamorphic history of each analyzed sample. The Palmaro-Caffarella metagabbros record peak metamorphic conditions of 10<P(kbar)<15 and 450<T(°C)<500 typical of the blueschist facies. The Voltri metagabbros reach peak metamorphic conditions in the lawsonite stability field, varying from about 21 kbar and 450-490 °C to 22-28 kbar and 460-500 °C. All the analyzed samples were subject to a clockwise P-T trajectory, which includes an almost isothermal exhumation path.

To study the mechanism for exhumation of the Voltri Massif we performed a series of 2D numerical simulations (Gerya & Yuen, 2003): the starting set up depicts an oceanic basin of prescribed amplitude surrounded by continental margins. To best reproduce the intra-oceanic subduction that started in Mesozoic time inside the Ligurian-Piedmontese branch of the Western Tethys, we defined an oceanic lithosphere with a non-layered structure typical of slow and ultra-slow spreading ridges; gabbros form discrete bodies inside the serpentinized lithospheric mantle and a discontinuous basaltic layer covers the latter.

We tested two different rheologies of serpentinites (Gerya et al., 2002; Hilairet et al., 2007) and in both cases slab dehydration causes formation of a viscous serpentinitic channel in the mantle wedge. Ductile deformation of serpentine (Hilairet et al., 2007) favors the mixing of sediments in the serpentinitic channel also during the initial stages of subduction. Furthermore the serpentinitic mélange includes slices of subducted oceanic lithosphere which are scraped from the slab; in this way serpentinite deriving from the mantle wedge hydration are therefore closely associated with slab-derived serpentinites. The serpentinitic mélange is finally exhumed thanks to the buoyant effect of the ultramafic rocks that decreases the bulk density of the high-pressure terrains below the mantle value (Hermann et al., 2000).

The P-T paths predicted by the model for the exhumed metagabbro and metasediments are comparable to the clockwise P-T path obtained for the Voltri Massif. The peak metamorphic conditions vary from P= 12,5 kbar and T=250°C to 20<P(kbar)<25 and 420<T(°C)<500 and the exhumation path is almost isothermal. In addition the size of exhumed units in the model grossly fits the size of the different rock bodies actually cropping out in the Voltri Massif.

Such evidence suggests that buoyancy could be considered an effective mechanism that contributed to the final exhumation stages of the high-pressure rocks of the Voltri Massif.

REFERENCES

Connolly J. (1990). Multivariable phase diagrams: an algorithm based on generalized thermodynamics. American Journal of Sciences, 290, 666-718.

Gerya T., Stöckhert B. (2002). Exhumation rates of high pressure metamorphic rocks in subduction channels: the effect of rheology. Geophysical research letters, vol. 29, no. 8.

Gerya T., Yuen D.A. (2003). Characteristics-based marker-in-cell method with conservative finite-differences schemes for modeling geological flows with strongly variable transport properties. Physics of the Earth and Planetary Interiors, 140, 293-318.

Hermann J., Müntener O., Scambelluri M. (2000). The importance of serpentinite mylonites for subduction and exhumation of oceanic crust. Tectonophysics 327, 225-238.

Hilairet N., Reynard B. (2007). High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. Science 318, 1910.