GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Fluid-rock interacton in eclogite-facies meta- peridottes (Erro-Tobbio Unit, Ligurian Alps) S. Cacciari 1 , G. Pennacchioni 1 , M. Scambelluri 2 , E. Cannaò 3 , G. Tofol 1 1 Università degli Studi di Padova, Italy 2 Università degli Studi di Genova, Italy 3 Università degli Studi di Milano - La Statale, Italy The investgaton of exhumed blueschist/eclogite-facies ophiolitc serpentnites can provide informaton on dehydraton reactons, fuid actvity and, possibly, seismicity occurring at subducton zones intermediate depths. Fluids are progressively released during subducton by breakdown reactons occurring within serpentnites and meta-sediments, leading to periodic fuid pressure build up that may eventually result in britle failure (dehydraton embritlement mechanism). This mechanism is likely responsible for triggering deep Episodic Tremor and Slow Slip events (deep ETS), composed of correlated tectonic tremor (low-frequency seismic swarms) and aseismic slow-slip events (SSEs). These events are observed along the subducton interface at depth ranges of 25-60 km ( Behr and Bürgmann, 2021 ), a rheologically heterogeneous domain characterized by high Vp/Vs rato, indicatng the presence of pressurized fuids. Rheological heterogeneites (i.e. mantle peridottes afected by diferent degrees of serpentnizaton) allow for strain parttoning into low-strain domains, developing crack-seal veining and radiatng tremor, and high-strain domains accommodatng SSEs. These events are geophysically well constrained, however, only a few exhumed geological assemblages have been interpreted as geological records for deep ETS so far. The Erro-Tobbio meta-peridotte (Voltri Massif, Western Alps) records fuid-rock interactons and associated deformaton occurred mostly within the “deep ETS” depth range (Fig.1). The uneven serpentnizaton experienced by the lherzolite protoliths at the ocean foor and at the forearc region, led to parttoning of the eclogite-facies deformaton into high-strain domains of serpentnite mylonites (interpreted as horizons of SSEs), hostng overprintng britle and ductle structures and low strain domains of undeformed meta-peridottes (interpreted as the asperites reaching failure and triggering tremor), mainly afected by britle deformaton. These rocks, therefore, display associaton of tens-of- cm-thick mylonitc horizons with a pervasive network of metamorphic olivine ( Ol 2 ) and Ti-clinohumite ( Ti-chu ) veins and reacton bands related to breakdown of brucite ( Brc ) and antgorite ( Atg ) to Ol at 350-500 °C ( Hermann et al., 2000 ). At the mesoscale, the Ol, Ti-chu reacton bands can be grouped into two main sets: (i) Ol-fabric-1 (OlF1), steeply-dipping around 320°, oriented at high angle ( ~ 60°) to the mylonites, (ii) Ol-fabric-2 (OlF2) which trends parallel to the mylonitc domains and progressively increases in spatal density towards the mylonites. Locally, the structural arrangement is more complex and includes multple sets of olivine veins.