GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 Earthquake swarms frozen in an exhumed hydrothermal system (Bolfn Fault Zone, Chile) S. Masoch 1 , G. Pennacchioni 1 , M. Fondriest 1 , R. Gomila 1 , P. Poli 1 , J. Cembrano 2,3 , G. Di Toro 1,4 1. Dipartmento di Geoscienze, Università degli Studi di Padova (Italy) 2. Departamento de Ingeniería Estructural y Geotécnica, Pontfcia Universidad Católica de Chile (Chile) 3. Andean Geothermal Center of Excellence (Chile) 4. Sezione di Tetonofsica e Sismologia, Isttuto Nazionale di Geofsica e Vulcanologia (Italy) Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatc systems and actvate mesh-like fault networks within zones of fault geometric complexity. How these networks develop through space and tme along seismic faults is poorly constrained in the geological record. Here, we describe a spatally dense array of small-displacement (< 1.5 m) epidote-rich fault-veins within granitoids, occurring at the intersectons of subsidiary faults with the exhumed seismogenic Bolfn Fault Zone (Atacama Fault System, Northern Chile). Epidote faultng and sealing occurred at 3-7 km depth and 200-300 °C ambient temperature. At distance ≤ 1 cm to fault-veins, the magmatc quartz of the wall-rock shows (i) thin (<10- µm-thick) interlaced deformaton lamellae, and (ii) cross cutng quartz-healed veinlets. The epidote-rich fault-veins (i) include clasts of deformed magmatc quartz, with deformaton lamellae and quartz-healed veinlets, and (ii) record cyclic events of extensional-to-hybrid veining and either aseismic or seismic shearing. Deformaton of the wall-rock quartz is interpreted to record the large stress perturbatons associated with the rupture propagaton of small earthquakes. In contrast, dilaton and shearing forming the epidote- rich fault-veins are interpreted to record the later development of a mature and hydraulically- connected fault-fracture system. In this later stage, the fault-fracture system cyclically ruptured due to fuid pressure fuctuatons, possibly correlated with swarm-like earthquake sequences. Corresponding author: Simone Masoch simone.masoch@phd.unipd.it