Preprints
https://doi.org/10.22541/essoar.171995191.13613873/v1
https://doi.org/10.22541/essoar.171995191.13613873/v1
04 Jul 2024
 | 04 Jul 2024
Status: this preprint is open for discussion.

Earthquake swarms frozen in an exhumed hydrothermal system (Bolfin Fault Zone, Chile)

Simone Masoch, Giorgio Pennacchioni, Michele Fondriest, Rodrigo Gomila, Piero Poli, José Cembrano, and Giulio Di Toro

Abstract. Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatic systems and activate mesh-like fault networks. 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-filled veinlets. The epidote-rich fault-veins (i) include clasts of deformed magmatic quartz, with deformation lamellae and quartz-filled veinlets, and (ii) record cyclic events of extensional-to-hybrid veining and either aseismic or seismic shearing. Deformation of the wall-rock quartz is interpreted to record the large stress perturbations associated with the rupture propagation of small earthquakes. In contrast, 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.

Simone Masoch, Giorgio Pennacchioni, Michele Fondriest, Rodrigo Gomila, Piero Poli, José Cembrano, and Giulio Di Toro

Status: open (until 20 Aug 2024)

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Simone Masoch, Giorgio Pennacchioni, Michele Fondriest, Rodrigo Gomila, Piero Poli, José Cembrano, and Giulio Di Toro
Simone Masoch, Giorgio Pennacchioni, Michele Fondriest, Rodrigo Gomila, Piero Poli, José Cembrano, and Giulio Di Toro

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Short summary
We investigate an exhumed hydrothermal system in the Atacama Desert (Chile) to understand how earthquake swarms form. Wall-rocks near fault-veins experienced high-stress pulses, and fault-veins underwent cyclic crack opening and shearing. These findings suggest ancient earthquake swarm activity, from dynamic crack propagation to repeated crack opening and shearing. This system represents a unique geological record of earthquake swarms, providing insight into their initiation and evolution.