Browsing by Author "Di Toro, Giulio"

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    Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)
    (2022) Masoch, Simone; Fondriest, Michele; Gomila, Rodrigo; Jensen, Erik; Mitchell, Thomas M.; Cembrano, Jose; Pennacchioni, Giorgio; Di Toro, Giulio
    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 degrees C in a fluid-rich environment. The BFZ in-cludes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudo-tachylytes, 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 Liquin similar to e-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).
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    Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)
    (2021) Masoch, Simone; Gomila, Rodrigo; Fondriest, Michele; Jensen, Erik; Mitchell, Thomas; Pennacchioni, Giorgio; Cembrano, Jose; Di Toro, Giulio
    How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and <= 300 degrees C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth.