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Natural non‐vesiculated pseudotachylyte of the Bolfín Fault Zone: evidence for solidification from melt and of seismic faulting in a fluid rich environment. (a) Injection vein (scan of thin section from sample B15‐19, parallel nicols) cutting through a chlorite‐rich cataclasite. (b) Albite microlites in the pseudotachylyte matrix (sample AT0717). (c) Albite microlites and quartz clasts in a chlorite‐rich matrix (presumably devitrified and recrystallized glass; sample B15‐19). (d) Calcite and quartz clasts in a microlite‐rich pseudotachylyte matrix. (e and f) cross‐cutting relations of the natural pseudotachylyte with calcite (early and late) veins and K‐feldspar vein. (b–f) BSE‐FESEM images. Ab, albite; Qz, quartz; Ep, epidote; Cal, calcite; Chl, chlorite; Kfs, K‐feldspar.
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Tectonic pseudotachylytes are thought to be unique to certain water–deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid–rich faults hosted in the continental crust. Pseudotachylytes were found...
Citations
... Thus far, porosity has been characterized to control fault strength in upper-crustal cataclasites and fault gouge (Kirilova et al., 2020), or to be related to melt degassing during coseismic slip in fluid-rich upper-crustal pseudotachylytes (Boullier et al., 2001;Gomila et al., 2021;Magloughlin, 2011;Rowe et al., 2005). In this contribution we report a detailed microstructural account of porosity in pristine and mylonitized pseudotachylytes from the lower-crustal anorthosites of Lofoten, Norway. ...
... The microstructures in Figures 6-8 are interpreted to be pores and not vesicles, because at lower-crustal conditions, high lithostatic pressures would render H 2 O-and CO 2 -bearing fluids (if present in any quantity) completely miscible in the frictional melt (Gomila et al., 2021). At these conditions, it is unlikely that there would be degassing of the melt during crystallization to form vesicles. ...
Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.
... This indicates that, as rapid sliding proceeds in a natural fault, the dominant melting mechanism at any specific location may change with time ( Fig. 4e, f). As reported from the observations of natural and experimental pseudotachylytes [37][38][39][40] , the water either initially present in fault slip zones or generated due to mineral decomposition during slip may be involved in melting reactions, possibly reducing the melting temperatures of grains 41,42 . In our experiments, water in room air and newly released water from dehydrated minerals probably worked in Stage 1, and Stages 2 and 3, respectively. ...
Rapid slip, at rates in the order of 1 m/s or more, may induce frictional melting in rocks during earthquakes. The short-lived melting has been thought to be a disequilibrium process, for decades. We conducted frictional melting experiments on acidic, basic, and ultrabasic silicate rocks at a slip rate of 1.3 m/s. The experiments and microstructural observations reveal that all minerals in the rocks are melted at temperatures below their known melting temperatures (Tm); e.g., quartz is melted at ~ 1000–1200 °C, not ~ 1720 °C, while olivine at ~ 1300 °C, rather than ~ 1700 °C. The low-temperature melting is incompatible with the conventional disequilibrium melting, and may be caused predominantly by grain size reduction and phase boundary reactions during the early and later stages of slip, respectively. The newly estimated Tm and the melting mechanisms should be considered for understanding the mechanics of earthquakes, landslides, and caldera collapses.
... The chemical composition of the glass-like matrix was similar to that of illite-smectite (Fig. 7e, f), indicating that the glass-like matrix resulted from the frictional melting of the clay minerals (e.g., Gu et al., 2021;Han et al., 2014;Wang et al., 2023). Gomila et al. (2021) reported that the breakdown of carbonate-bearing minerals during frictional heating can release CO 2 , which has a low solubility in the basaltic melt and can affect the formation of vesicles. In this study, calcite was observed as an accessary mineral and a fracture-filling mineral in the wall-rock fragment, and the clay-rich matrix of the mud ball samples also contained calcite clasts (Fig. 7a), which supplied CO 2 to the melt. ...
The 5.5 magnitude (Mw) earthquake in Pohang, South Korea in 2017 was one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. Faults that ruptured in Pohang were not identified by preliminary geological investigations or geophysical surveys, and the subsequent study of the fault rocks at the Pohang EGS site was limited to depths of 3,790–3,816 m. In this study, we present new observations of fault rocks from drill cuttings retrieved from the Pohang EGS. The drill cuttings obtained from 3,256–3,911 m contained “mud balls,” which showed a clay matrix with foliation and a cataclastic texture, indicating a typical fault gouge or breccia. Furthermore, the mud ball samples retrieved from depths of 3,256 m and 3,260 m contained black fragments. Scanning and transmission electron
microscopy revealed that the black fragments consisted of glass-like material, which is indicative of frictional melting during coseismic slip. The presence of these black fragments suggests that at least one seismic event had occurred at the Pohang EGS site prior to the hydraulic stimulation test.
... Vein and fault abundance was quantified (P 10 fracture density; Mauldon and Dershowitz, 2000) for the above defined units (i)-(iv) along scanlines (9-17 m long) traced perpendicular to the main trace of the BFZ. However, to quantify the P 10 values, we considered only veins and faults sealed and decorated by hydrothermal minerals (chlorite, epidote, quartz, prehnite) associated with Early Cretaceous fault activity Gomila et al., 2021;Herrera et al., 2005;Masoch et al., 2021;Olivares et al., 2010). In this way, we did not overestimate the fracture density by including structural elements (e.g., fractures, veins, faults) possibly associated with the post-Miocene extensional fault reactivation (see Masoch et al., 2021 for details) and passive exhumation. ...
... Pseudotachylytes cut chlorite + epidote ± calcite-bearing veins and are cut by chlorite-, K-feldspar-and calcite-bearing veins. Some pseudotachylytes include amygdalae or vesicles filled by quartz + calcite + epidote + chlorite ( Fig. 15f; see Gomila et al., 2021 for further details). ...
... Fig. 6. Structural map of the distribution of the fault zone structural units and structural data at Playa Escondida locality (northern segment, modified fromGomila et al., 2021). The fault core consists of two fault core strands, dipping towards SW, and exploited the magmatic foliation of the meta-diorites (Bolfin Complex). ...
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).
... Contrary to older tectonic models of faulting as a purely thermal process affecting petrologically inert rocks (e.g. England and Molnar 1993), faults act, albeit often episodically (Caine et al. 1996;Gomila et al. 2021), as a preferential conduit for aqueous fluids, with attending mineral recrystallization (Kerrich et al. 1980;Proyer 2003). On a fault plane the fluidinduced recrystallization is usually nearly complete and mineral chronometers mostly were reset (Tartese et al. 2012). ...
Dating deformation is difficult, as textures and petrogenesis of deformed rocks are complex. Moreover, geochronometer categories are pursued by communities that often do not communicate.
Hygrochronology dates the retrograde metasomatic/metamorphic reactions caused by aqueous fluid circulation events.
Thermochronology models time-temperature histories by assuming that mineral ages can be uniquely assigned to a "closure temperature T c ", the only process occurring in rocks being Fick's Law diffusion. Diffusion by definition produces a bell-shaped concentration profile. In contrast, patchy intra-grain isotope concentration profiles denounce aqueous retrogression, whose rate is orders of magnitude faster than diffusion.
Petrochronology is based on opposite assumptions, as the mobility of structure-forming major cations is higher than that of radiogenic Pb, Ar, and Sr. Whenever the formation of a mineral occurs at T < T c , its apparent age dates its formation.
Nanochronology analyzes samples at the nm-scale. These analyses illuminate atomic-scale processes, e.g. open-system transport of soluble ions along self-sealing networks of nanopores.
The key to dating deformation and producing correct, regional-sized (up to 100s of km) tectonic models is the realization that minerals consist of atoms, whose behavior is only firmly constrained by nm-scale analyses.
Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation
... The faults developed primarily in the late Cretaceous at 6-10 km depth, and have been passively exhumed from depth as strike-slip deformation migrated arcwards onto the West Fault system. It is not clear whether the faults experienced seismic or aseismic slip, although pseudotachylyte has recently been recognized on the nearby larger displacement Bolfín fault (Gomila et al., 2021). ...
Fault damage zones dominate the mechanical, hydraulic and seismological properties of faults yet the relative contributions from processes leading to their development and growth is obscure. In this study, we investigate the damage development related to slip on rough faults and passage of earthquake ruptures. We compare the cumulative damage with slip and damage distribution from numerical models against field data from exhumed faults with slip less than 3.5 m within the Atacama Fault Zone in northern Chile. Models are constrained by experimentally determined mechanical properties of the host rock. We perform simulations of damage accumulation during quasistatic slip on rough faults and during sequences of earthquakes on planar and rough faults governed by rate and state friction laws. Both sets of simulations include Drucker–Prager rheology of the bulk to identify off‐fault damage where the yield stress is exceeded. Our results indicate that the extent and distribution of damage depend on the characteristics of fault roughness, amount of slip, and, when present, the intensity and variability of dynamic ruptures. When typical values for fault roughness are used, the scaling of damage zone width versus slip during quasistatic slip is comparable to that observed in the field data. Earthquake rupture on smooth faults by itself does not explain the field data. Simulations of earthquake sequences on rough faults leads to significantly larger damage zone widths with slip than that observed in the field data, suggesting the development of damage for small displacement is dominated by quasistatic slip on rough faults.
... Contrary to older tectonic models of faulting as a purely thermal process affecting petrologically inert rocks (e.g. England and Molnar, 1993), faults act, albeit often episodically (Caine et al., 1996;Gomila et al., 2021), as a preferential conduit for aqueous fluids, with attending mineral recrystallization (Kerrich et al., 1980;Proyer, 2003). On a fault plane the fluid-induced recrystallization is usually nearly complete and mineral chronometers mostly were reset (Tartèse et al., 2012). ...
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 (i.e., hybrid extensional-shear veins) within granitoids, occurring at the intersections of subsidiary faults with the ex-humed seismogenic Bolfin Fault Zone (Atacama Fault System , northern Chile). Epidote hybrid extensional-shear vein-ing occurred at 3-7 km depth and 200-300°C ambient temperature. At a distance of ≤ 1 cm to fault veins, the magmatic quartz of the wall rock shows (i) thin (< 10 µm thick) in-terlaced deformation lamellae and (ii) systematically cross-cutting veinlets healed by quartz and feldspars, and it appears shattered at the vein contact. Clasts of deformed mag-matic quartz, with deformation lamellae and healed vein-lets, are included in the epidote-rich fault veins. Deformation of the wall-rock quartz is interpreted to record the transient large stress perturbation associated with the propagation of small earthquakes preceding conspicuous epidote mineral-ization. Conversely, the epidote-rich fault veins record cyclic events of extensional-to-hybrid veining and either aseismic or seismic shearing. The dilation and shearing behavior of 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.
In cratonic interiors, long-lived brittle shear zones host records of polyphase deformation, representing inherited structures that can host damaging earthquakes. Here, we explore the internal structure of the Kgomodikae Shear Zone (KSZ), signifying the western continuation of the ∼800-km long Precambrian Kgomodikae-Thabazimbi-Murchinson Fault System which extend along a region of widespread seismicity in southern Africa. At satellite-scale, the KSZ exhibits ENE-to-NE-striking subparallel zones of alternating high/low lineament clustering intensities, with peak-intensity zones that represent hydrologically-permeable principal brittle shear bands. In outcrops, we find pervasive occurrence of slip surfaces with dominant strike-slip paleo-slip vectors, and silica-cemented fault rocks hosting collocated quartz and pseudotachylyte vein clusters. Ground-based scanline fracture mapping reveals peak damage intensity in proximity of the satellite-mapped lineaments (localized high strain zones?), but with the pseudotachylytes occurring in both the peak- and flanking lower-intensity damage zones. The results suggest that the KSZ hosted paleoseismic ruptures that were not confined to its principal slip zones but may have nucleated on- or ruptured into off-fault splays; and that the NW-striking splays have greater reactivation tendency in contemporary stress field. In general, our findings highlight the nature of preexisting off-fault damage networks that accommodated earthquake rupture and propagation patterns in intraplate regions.
