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