Simone Masoch (Dipartimento di Geoscienze, Università degli Studi di Padova, Padua, Italy)

Structural evolution and architecture of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Simone Masoch

The architecture and spatio-temporal evolution of brittle fault zones play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures. Nevertheless, the along-strike architectural variability and structural evolution of crustal-scale seismogenic sources are still poorly investigated. Here, we describe the architectural variability and structural evolution of the >40-km-long, exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Atacama Desert, Northern Chile). The BFZ has a sinuous crustal-scale geometry cutting through plutonic rocks of the Mesozoic Coastal Cordillera. Ancient (125-118 Ma) seismic faulting occurred at 5-7 km depth and ≤ 300 °C in a fluid-rich environment, as attested by widespread occurrence of pseudotachylytes (i.e., solidified frictional melts), and extensive chlorite-epidote veining and propylitic alteration. Moreover, the multiple cataclastic fault core strands are embedded within a strongly altered damage zone, which includes high-dilatant hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. We discuss how the geodynamic setting of the magmatic arc played a pivotal role in controlling the architecture and spatio-temporal evolution of the BFZ with reference to (i) spatial arrangement of precursory magmatic anisotropies (magmatic foliations of plutons and dyke swarm) exploited as loci of fault nucleation and (ii) large availability of fluids and heat during late cooling stages of the batholiths, triggering fluid-driven small-in-magnitude seismicity distributed within the BFZ damage zone.