Study of tectonic mélanges from a fossil plate interface: probing geodynamic phenomena

Mélanges are abundant in both accretionary and collisional orogenic belts. Their chaotic, block-in-matrix structure can have different origins: sedimentary mélanges can be overprinted by later metamorphic and deformative events or, conversely, tectonic mélanges can form directly at the plate interface, at different tectonic levels and either in prograde (i.e. during underplating) or retrograde (i.e. during exhumation) conditions.

The HP-metaophiolitic Voltri Massif (W Alps, Italy), considered as an exhumed piece of the plate interface of the Alpine orogen, includes various, well-preserved examples of tectonic mélanges at different scales (from m- to km-scale). Here, we investigate a 100 meters-thick tectonic mélange, where blocks of various metamorphic lithologies (e.g. metagabbro, eclogite, serpentinite, calchschist and qtz-micaschist) and sizes (0,1-m- to 10-m scale) are dispersed within an intensely foliated, lithologically heterogeneous matrix made of a mixture among serpentinite-schist, chlorite-actinolite schist and graphitic schist, predominantly equilibrated at grenschist facies conditions.

Preliminary field investigations reveal a pronounced strain and metamorphic partitioning between the matrix and the blocks. These latter show internal metamorphic layering, shear zones and extensional veins discordant to the pervasive s-c-fabric and folding that characterize the enclosing matrix. Locally, eclogitic blocks show progressive internal fragmentation (e.g., fracturing/veining) up to pervasive brecciation. Petrographic/microanalytical investigations on the most preserved (Fe-Ti-bearing) metagabbro and metabasalt blocks indicate prograde peak metamorphism either in eclogite (grt + omp + rt ± Na-amp ± ph assemblage) or blueschist-facies (Na-amp + ttn + chl ± ep ± ph assemblage); some eclogites show either a retrograde syn-tectonic stage in blueschist facies or a static greenschist overprint. PT estimates on eclogitic blocks indicate a peak stage at P = 18,6 ± 1,0 Kbar (gnt-ph-cpx geobarometer) and T = 530 ± 10°C. The block-matrix transition is characterized by dm- to cm-thick metasomatic rinds rich in hydrous minerals, such as tremolitic amphiboles, biotite, chlorite and minor titanite, tourmaline, adularia and sulphides. Locally, tensile fractures filled by a polymineralic gouge material with the same mineral composition (±biotite) and syntectonic extensional veins with fibrous amphibole depart from the rinds and intrude the prisitne blocks. Abundant hydrothermal fluid circulation is suggested, among other, by peculiar microstructures, i.e. the growth of chlorite in vermicular form.

The block-in-matrix structures and microstructures (shear zones and extensional cracks repeatedly crosscuting eachother) point to the occurrence of a cyclic deformation characterised by episodic switch between brittle and ductile regimes and changes in the rehological properties of blocks and matrix. The occurrence of (i) abundant mélange matrix, (ii) metasomatic rinds digesting blocks with (iii) sets of veins/cracks irradiating inside the intact rocks suggest the key-role played by fluids in the evolution of the Piota River mélange.

The evidence recorded in the studied lithologies, such as episodic switch between deformation regimes assisted by transient exceed of the rock tensile strenght by pore fluids overpressure, would permit to better understand the mechanisms controlling slow earthquake generation at shallow plate interface. Morover, this study, combined with studies of other melange occurrences of the Voltri Massif, will help to better understand the complex geodynamic phenomena acting on collisional orogens.