Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile). (December 2022)

• Record Type: Journal Article
• Authors: Masoch, Simone; Fondriest, Michele; Gomila, Rodrigo; Jensen, Erik; Mitchell, Thomas M.; Cembrano, José; Pennacchioni, Giorgio; Di Toro, Giulio
• Is Part Of: Journal of structural geology. Volume 165(2022)
• Publication Date: 2022-12
• Subjects: Seismogenic faults -- Fault structure -- Fault zone rocks -- Earthquakes -- Fluid-driven seismicity -- Atacama fault system; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2022.104745
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile). Highlights: We describe the structural variability of the Bolfin Fault Zone (BFZ). The BFZ has a wide fault core and an extensive hydrothermally altered damage zone. Ancient seismicity is attested by widespread occurrence of pseudotachylytes. Ancient seismicity was associated with intense fluid-rock interaction. Clusters of fault-vein networks represent ancient swarm-type sequences.