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Earthquake swarms frozen in an exhumed hydrothermal system (Bolfin Fault Zone, Chile)
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  • Simone Masoch,
  • Giorgio Pennacchioni,
  • Michele Fondriest,
  • Rodrigo Gomila,
  • Piero Poli,
  • Jose Cembrano,
  • Giulio Di Toro
Simone Masoch
Dipartimento di Geoscienze, Università degli Studi di Padova

Corresponding Author:[email protected]

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Giorgio Pennacchioni
University of Padova
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Michele Fondriest
Università degli Studi di Padova
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Rodrigo Gomila
University of Padua
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Piero Poli
Universita Di Padova
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Jose Cembrano
Pontificia Universidad Catolica de Chile
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Giulio Di Toro
University of Padua
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Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatic systems and activate mesh-like fault-fracture networks at zone of fault complexity. How these networks develop through space and time along seismic faults is poorly constrained in the geological record. Here, we describe a spatially dense array of small-displacement (< 1.5 m) epidote-rich fault-veins within granitoids, occurring at the intersections of subsidiary faults with the exhumed seismogenic Bolfin Fault Zone (Atacama Fault System, Northern Chile). Epidote faulting and veining occurred at 3-7 km depth and 200-300 °C ambient temperature. At distance ≤ 1 cm to fault-veins, the magmatic quartz of the wall-rock shows (i) thin (<10- µm-thick) interlaced deformation lamellae, and (ii) crosscutting quartz-healed veinlets. The epidote-rich fault-veins (i) include clasts of deformed magmatic quartz, with deformation lamellae and quartz-healed veinlets, and (ii) record cyclic events of extensional-to-hybrid veining and either aseismic and seismic shearing. Deformation of the wall-rock quartz is interpreted to record the large stress perturbations associated with the rupture propagation of small earthquakes. Instead, dilation 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 latter stage, the fault-fracture system cyclically ruptured due to fluid pressure fluctuations, possibly correlated with swarm-like earthquake sequences.
25 Oct 2023Submitted to ESS Open Archive
08 Nov 2023Published in ESS Open Archive