Réorganisation structurale des matériaux carbonés lors de l’enfouissement et de la déformation : Expériences de déformation et exemples naturels

Particles of carbonaceous material (CM), present in most sediments, experience a progressive evolution during their burial from immature and amorphous structure to fully mature and crystalline graphite. This transformation, decomposed in two successive stages (carbonization then graphitization), can be quantitatively followed with Raman spectroscopy (RSCM), and is assumed to be controlled at the first order by temperature, while the potential role of other parameters is still controversial. In this work we explore the effect of strain on CM crystallinity with natural examples from accretionary complexes (Shimanto belt, Kodiak Accretionary Complex, Alps) and experiments encompassing both high strain rate (seismic) and low strain-rate (non-seismic) deformation. We focused on the low temperature range from 200 to 350°C, spanning the carbonization and the early graphitization stages. We used high-resolution profiles to correlate RSCM signal to microstructures. Intensity ratio (IR) appears to be the most relevant RSCM parameter to record deformation. Irrespective of the deformation mechanism (i.e. in low and high strain-rate examples), IR is increased in the zone where strain is localized, demonstrating the large influence of strain on Raman signal. In addition, using geological examples of contact metamorphism, combined with flash heating experiments, we demonstrated the potential of a RSCM feature (D3/Gsl Ratio) to detect short-lived and intense heating events. When applied to Black Fault Rocks considered as the result of quenching of a melt formed during fast seismic slip, these new tools (profiles of IR and D3/Gsl Ratio) can discriminate fault cores that melted from those where only mechanical comminution was active.Les particules de matériaux carbonés (CM), présentes dans la majorité des sédiments, enregistrent une évolution progressive lors de l’enfouissement, passant d’une structure amorphe à une structure parfaitement ordonnée (i.e. graphite). Cette évolution s’effectue via de deux phases successives, la carbonisation et la graphitisation, qui peuvent être quantitativement suivies grâce à la spectroscopie Raman (RSCM). Cependant, le contrôle unique supposé de la température, ignorant d’autres potentiels facteurs, est encore débattu. Dans cette étude, l’effet de la déformation sur la cristallinité de la CM a été exploré à travers l’analyse d’échantillons naturels, provenant de prismes d’accrétions (Shimanto Belt, Kodiak Accretionary Complex, Alpes), et d’expériences de déformation reproduisant des déformations à vitesses faibles (non-sismique) et rapides (sismique). L’analyse a été focalisée sur des températures allant de 200 à 350°C, couvrant les processus de carbonisation et de graphitisation précoce. L’utilisation de profils de haute-résolution ont permis de corréler les microstructures et le signal RSCM. Le ratio d’intensité (IR) s’est montré comme un paramètre RSCM approprié à l’étude de l’effet de la déformation. Indépendamment du mécanisme de déformation (i.e. sismique ou non-sismique), l’IR augmente lorsque la déformation est localisée, montrant l’effet de la déformation sur le signal Raman. De plus, basé sur des échantillons ayant connu un métamorphisme de contact, couplés à des expériences de flash-heating, le Raman a montré son efficacité (D3/Gsl Ratio) dans la détection des chauffages courts et intenses. Cette méthode a été appliquée à des Black Fault Rocks considérées comme le résultat d’une trempe d’une veine fondue formée lors de glissement sismique. Les profils d’IR et le D3/Gsl Ratio ont permis de discriminer les mécanismes de formations (i.e. fusion ou comminution seul) de ces objets

Réorganisation structurale des matériaux carbonés lors de l’enfouissement et de la déformation : Expériences de déformation et exemples naturels

Particles of carbonaceous material (CM), present in most sediments, experience a progressive evolution during their burial from immature and amorphous structure to fully mature and crystalline graphite. This transformation, decomposed in two successive stages (carbonization then graphitization), can be quantitatively followed with Raman spectroscopy (RSCM), and is assumed to be controlled at the first order by temperature, while the potential role of other parameters is still controversial. In this work we explore the effect of strain on CM crystallinity with natural examples from accretionary complexes (Shimanto belt, Kodiak Accretionary Complex, Alps) and experiments encompassing both high strain rate (seismic) and low strain-rate (non-seismic) deformation. We focused on the low temperature range from 200 to 350°C, spanning the carbonization and the early graphitization stages. We used high-resolution profiles to correlate RSCM signal to microstructures. Intensity ratio (IR) appears to be the most relevant RSCM parameter to record deformation. Irrespective of the deformation mechanism (i.e. in low and high strain-rate examples), IR is increased in the zone where strain is localized, demonstrating the large influence of strain on Raman signal. In addition, using geological examples of contact metamorphism, combined with flash heating experiments, we demonstrated the potential of a RSCM feature (D3/Gsl Ratio) to detect short-lived and intense heating events. When applied to Black Fault Rocks considered as the result of quenching of a melt formed during fast seismic slip, these new tools (profiles of IR and D3/Gsl Ratio) can discriminate fault cores that melted from those where only mechanical comminution was active.Les particules de matériaux carbonés (CM), présentes dans la majorité des sédiments, enregistrent une évolution progressive lors de l’enfouissement, passant d’une structure amorphe à une structure parfaitement ordonnée (i.e. graphite). Cette évolution s’effectue via de deux phases successives, la carbonisation et la graphitisation, qui peuvent être quantitativement suivies grâce à la spectroscopie Raman (RSCM). Cependant, le contrôle unique supposé de la température, ignorant d’autres potentiels facteurs, est encore débattu. Dans cette étude, l’effet de la déformation sur la cristallinité de la CM a été exploré à travers l’analyse d’échantillons naturels, provenant de prismes d’accrétions (Shimanto Belt, Kodiak Accretionary Complex, Alpes), et d’expériences de déformation reproduisant des déformations à vitesses faibles (non-sismique) et rapides (sismique). L’analyse a été focalisée sur des températures allant de 200 à 350°C, couvrant les processus de carbonisation et de graphitisation précoce. L’utilisation de profils de haute-résolution ont permis de corréler les microstructures et le signal RSCM. Le ratio d’intensité (IR) s’est montré comme un paramètre RSCM approprié à l’étude de l’effet de la déformation. Indépendamment du mécanisme de déformation (i.e. sismique ou non-sismique), l’IR augmente lorsque la déformation est localisée, montrant l’effet de la déformation sur le signal Raman. De plus, basé sur des échantillons ayant connu un métamorphisme de contact, couplés à des expériences de flash-heating, le Raman a montré son efficacité (D3/Gsl Ratio) dans la détection des chauffages courts et intenses. Cette méthode a été appliquée à des Black Fault Rocks considérées comme le résultat d’une trempe d’une veine fondue formée lors de glissement sismique. Les profils d’IR et le D3/Gsl Ratio ont permis de discriminer les mécanismes de formations (i.e. fusion ou comminution seul) de ces objets

Réorganisation structurale des matériaux carbonés lors de l’enfouissement et de la déformation : Expériences de déformation et exemples naturels

Particles of carbonaceous material (CM), present in most sediments, experience a progressive evolution during their burial from immature and amorphous structure to fully mature and crystalline graphite. This transformation, decomposed in two successive stages (carbonization then graphitization), can be quantitatively followed with Raman spectroscopy (RSCM), and is assumed to be controlled at the first order by temperature, while the potential role of other parameters is still controversial. In this work we explore the effect of strain on CM crystallinity with natural examples from accretionary complexes (Shimanto belt, Kodiak Accretionary Complex, Alps) and experiments encompassing both high strain rate (seismic) and low strain-rate (non-seismic) deformation. We focused on the low temperature range from 200 to 350°C, spanning the carbonization and the early graphitization stages. We used high-resolution profiles to correlate RSCM signal to microstructures. Intensity ratio (IR) appears to be the most relevant RSCM parameter to record deformation. Irrespective of the deformation mechanism (i.e. in low and high strain-rate examples), IR is increased in the zone where strain is localized, demonstrating the large influence of strain on Raman signal. In addition, using geological examples of contact metamorphism, combined with flash heating experiments, we demonstrated the potential of a RSCM feature (D3/Gsl Ratio) to detect short-lived and intense heating events. When applied to Black Fault Rocks considered as the result of quenching of a melt formed during fast seismic slip, these new tools (profiles of IR and D3/Gsl Ratio) can discriminate fault cores that melted from those where only mechanical comminution was active.Les particules de matériaux carbonés (CM), présentes dans la majorité des sédiments, enregistrent une évolution progressive lors de l’enfouissement, passant d’une structure amorphe à une structure parfaitement ordonnée (i.e. graphite). Cette évolution s’effectue via de deux phases successives, la carbonisation et la graphitisation, qui peuvent être quantitativement suivies grâce à la spectroscopie Raman (RSCM). Cependant, le contrôle unique supposé de la température, ignorant d’autres potentiels facteurs, est encore débattu. Dans cette étude, l’effet de la déformation sur la cristallinité de la CM a été exploré à travers l’analyse d’échantillons naturels, provenant de prismes d’accrétions (Shimanto Belt, Kodiak Accretionary Complex, Alpes), et d’expériences de déformation reproduisant des déformations à vitesses faibles (non-sismique) et rapides (sismique). L’analyse a été focalisée sur des températures allant de 200 à 350°C, couvrant les processus de carbonisation et de graphitisation précoce. L’utilisation de profils de haute-résolution ont permis de corréler les microstructures et le signal RSCM. Le ratio d’intensité (IR) s’est montré comme un paramètre RSCM approprié à l’étude de l’effet de la déformation. Indépendamment du mécanisme de déformation (i.e. sismique ou non-sismique), l’IR augmente lorsque la déformation est localisée, montrant l’effet de la déformation sur le signal Raman. De plus, basé sur des échantillons ayant connu un métamorphisme de contact, couplés à des expériences de flash-heating, le Raman a montré son efficacité (D3/Gsl Ratio) dans la détection des chauffages courts et intenses. Cette méthode a été appliquée à des Black Fault Rocks considérées comme le résultat d’une trempe d’une veine fondue formée lors de glissement sismique. Les profils d’IR et le D3/Gsl Ratio ont permis de discriminer les mécanismes de formations (i.e. fusion ou comminution seul) de ces objets

Réorganisation structurale des matériaux carbonés lors de l’enfouissement et de la déformation : Expériences de déformation et exemples naturels

Les particules de matériaux carbonés (CM), présentes dans la majorité des sédiments, enregistrent une évolution progressive lors de l’enfouissement, passant d’une structure amorphe à une structure parfaitement ordonnée (i.e. graphite). Cette évolution s’effectue via de deux phases successives, la carbonisation et la graphitisation, qui peuvent être quantitativement suivies grâce à la spectroscopie Raman (RSCM). Cependant, le contrôle unique supposé de la température, ignorant d’autres potentiels facteurs, est encore débattu. Dans cette étude, l’effet de la déformation sur la cristallinité de la CM a été exploré à travers l’analyse d’échantillons naturels, provenant de prismes d’accrétions (Shimanto Belt, Kodiak Accretionary Complex, Alpes), et d’expériences de déformation reproduisant des déformations à vitesses faibles (non-sismique) et rapides (sismique). L’analyse a été focalisée sur des températures allant de 200 à 350°C, couvrant les processus de carbonisation et de graphitisation précoce. L’utilisation de profils de haute-résolution ont permis de corréler les microstructures et le signal RSCM. Le ratio d’intensité (IR) s’est montré comme un paramètre RSCM approprié à l’étude de l’effet de la déformation. Indépendamment du mécanisme de déformation (i.e. sismique ou non-sismique), l’IR augmente lorsque la déformation est localisée, montrant l’effet de la déformation sur le signal Raman. De plus, basé sur des échantillons ayant connu un métamorphisme de contact, couplés à des expériences de flash-heating, le Raman a montré son efficacité (D3/Gsl Ratio) dans la détection des chauffages courts et intenses. Cette méthode a été appliquée à des Black Fault Rocks considérées comme le résultat d’une trempe d’une veine fondue formée lors de glissement sismique. Les profils d’IR et le D3/Gsl Ratio ont permis de discriminer les mécanismes de formations (i.e. fusion ou comminution seul) de ces objets.Particles of carbonaceous material (CM), present in most sediments, experience a progressive evolution during their burial from immature and amorphous structure to fully mature and crystalline graphite. This transformation, decomposed in two successive stages (carbonization then graphitization), can be quantitatively followed with Raman spectroscopy (RSCM), and is assumed to be controlled at the first order by temperature, while the potential role of other parameters is still controversial. In this work we explore the effect of strain on CM crystallinity with natural examples from accretionary complexes (Shimanto belt, Kodiak Accretionary Complex, Alps) and experiments encompassing both high strain rate (seismic) and low strain-rate (non-seismic) deformation. We focused on the low temperature range from 200 to 350°C, spanning the carbonization and the early graphitization stages. We used high-resolution profiles to correlate RSCM signal to microstructures. Intensity ratio (IR) appears to be the most relevant RSCM parameter to record deformation. Irrespective of the deformation mechanism (i.e. in low and high strain-rate examples), IR is increased in the zone where strain is localized, demonstrating the large influence of strain on Raman signal. In addition, using geological examples of contact metamorphism, combined with flash heating experiments, we demonstrated the potential of a RSCM feature (D3/Gsl Ratio) to detect short-lived and intense heating events. When applied to Black Fault Rocks considered as the result of quenching of a melt formed during fast seismic slip, these new tools (profiles of IR and D3/Gsl Ratio) can discriminate fault cores that melted from those where only mechanical comminution was active

Réorganisation structurale des matériaux carbonés lors de l’enfouissement et de la déformation : Expériences de déformation et exemples naturels

Les particules de matériaux carbonés (CM), présentes dans la majorité des sédiments, enregistrent une évolution progressive lors de l’enfouissement, passant d’une structure amorphe à une structure parfaitement ordonnée (i.e. graphite). Cette évolution s’effectue via de deux phases successives, la carbonisation et la graphitisation, qui peuvent être quantitativement suivies grâce à la spectroscopie Raman (RSCM). Cependant, le contrôle unique supposé de la température, ignorant d’autres potentiels facteurs, est encore débattu. Dans cette étude, l’effet de la déformation sur la cristallinité de la CM a été exploré à travers l’analyse d’échantillons naturels, provenant de prismes d’accrétions (Shimanto Belt, Kodiak Accretionary Complex, Alpes), et d’expériences de déformation reproduisant des déformations à vitesses faibles (non-sismique) et rapides (sismique). L’analyse a été focalisée sur des températures allant de 200 à 350°C, couvrant les processus de carbonisation et de graphitisation précoce. L’utilisation de profils de haute-résolution ont permis de corréler les microstructures et le signal RSCM. Le ratio d’intensité (IR) s’est montré comme un paramètre RSCM approprié à l’étude de l’effet de la déformation. Indépendamment du mécanisme de déformation (i.e. sismique ou non-sismique), l’IR augmente lorsque la déformation est localisée, montrant l’effet de la déformation sur le signal Raman. De plus, basé sur des échantillons ayant connu un métamorphisme de contact, couplés à des expériences de flash-heating, le Raman a montré son efficacité (D3/Gsl Ratio) dans la détection des chauffages courts et intenses. Cette méthode a été appliquée à des Black Fault Rocks considérées comme le résultat d’une trempe d’une veine fondue formée lors de glissement sismique. Les profils d’IR et le D3/Gsl Ratio ont permis de discriminer les mécanismes de formations (i.e. fusion ou comminution seul) de ces objets.Particles of carbonaceous material (CM), present in most sediments, experience a progressive evolution during their burial from immature and amorphous structure to fully mature and crystalline graphite. This transformation, decomposed in two successive stages (carbonization then graphitization), can be quantitatively followed with Raman spectroscopy (RSCM), and is assumed to be controlled at the first order by temperature, while the potential role of other parameters is still controversial. In this work we explore the effect of strain on CM crystallinity with natural examples from accretionary complexes (Shimanto belt, Kodiak Accretionary Complex, Alps) and experiments encompassing both high strain rate (seismic) and low strain-rate (non-seismic) deformation. We focused on the low temperature range from 200 to 350°C, spanning the carbonization and the early graphitization stages. We used high-resolution profiles to correlate RSCM signal to microstructures. Intensity ratio (IR) appears to be the most relevant RSCM parameter to record deformation. Irrespective of the deformation mechanism (i.e. in low and high strain-rate examples), IR is increased in the zone where strain is localized, demonstrating the large influence of strain on Raman signal. In addition, using geological examples of contact metamorphism, combined with flash heating experiments, we demonstrated the potential of a RSCM feature (D3/Gsl Ratio) to detect short-lived and intense heating events. When applied to Black Fault Rocks considered as the result of quenching of a melt formed during fast seismic slip, these new tools (profiles of IR and D3/Gsl Ratio) can discriminate fault cores that melted from those where only mechanical comminution was active

Raman Spectroscopy of Carbonaceous Material record in pseudotachylytes: heating or deformation?

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The impact of melt versus mechanical wear on the formation of pseudotachylyte veins in accretionary complexes

International audienceAbstract Whether seismic rupture propagates over large distances to generate mega-earthquakes or is rapidly aborted mainly depends on the slip processes within the fault core, including particularly frictional melting or intense grain-size reduction and amorphization. The record of seismic slip in exhumed fault zones consists in many instances in Black Faults Rocks, dark and glass-like-filled aphanitic veins that have been interpreted as resulting from the quenching of frictional melts, i.e. pseudotachylytes. Such interpretation has nevertheless been questioned as similar macro to nano-microstructures have been observed either on intensely comminuted natural fault rocks or on slow creep experiments conducted on crustal rocks, where melting is absent. Here, we report a new dataset of Raman Spectroscopy of Carbonaceous Material analyses, aimed at discriminating the slip weakening processes operating in the fault core during slip. Using high spatial resolution profiles on natural Black Fault Rocks from exhumed accretionary complexes and an experimentally calibrated modelling of Raman intensity ratio evolution with temperature, we assessed different scenarios of temperature evolution during fault slip. None of them is able to account for the distribution of Raman signal, so that in the three studied Black Fault Rocks interpreted so far as natural pseudotachylytes, Raman Spectroscopy of Carbonaceous Material rather reflects the effect of intense and localized strain during fault slip. Furthermore, the absence of thermal imprint on Raman signal puts upper bounds on the temperature reached within the fault zone. If one cannot rule out the occurrence of high and short-lived temperature increase due to friction, the latter was not high enough as to melt the large quartz fraction of the fault zone rocks

Frictional melting during seismic rupture? A new Raman Spectroscopy approach to detect short-lived heat pulses

International audienceWhether seismic rupture propagates over large distances to generate earthquakes or on the contrary slows down quickly, is heavily dependent on the slip processes operating within the fault core. One possible scenario is that during seismic slip, the frictional work induces a local and transient release of heat up to reach the melting of the rock. This melt-lubrication of the fault plane results in resistance drop and promotes further propagation of the fault. Nonetheless, assessing the occurrence of flash melting has turned problematic, especially in the metasediments that constitute a large fraction of seismically active collision or subduction zones.In this work, we explore the effects of short-lived intense heating on the crystallinity of the carbonaceous particles present in the fault core. For this purpose, we carried out flash-heating experiments on pellets of natural sediments. Using a pair of lasers, the sample temperature was raised to 1400°C for durations ranging from 0.5 to 60 seconds, resulting in partial to total melting. The carbonaceous particles were then analyzed by Raman Spectroscopy. The spectroscopic signal of particles intensely heated for a short period of time present an atypical shape, with a large D3 band centered around 1500cm-1. The D3/Gsl. ratio in Flash-heating experiments show an evolution from 0.2 for the starting material up to 0.7 after a couple of seconds of Flash-heating. Following this experimental work, we analyzed with Raman spectroscopy several independent examples of short-lived intense heating of carbon-bearing rocks: static heating, stick-slip, high-velocity-friction experiments, In all these cases, we observed the presence of a prominent D3 band and a D3/Gsl. ratio larger than reference material. Based on these observations, we established a new parameter, the D3/Gsl. ratio, as sensitive to short-lived intense heating.Finally, we applied this new Raman parameter in association with micro-structural observations to discriminate the formation process of five Black Fault Rocks (BFR) from the Shimanto and the Kodiak Accretionary Complex. Microstructures are in several cases ambiguous as to the occurrence of melting in the BFR. However, the D3/Gsl. ratio shows a large increase in the Kure and the Mugi BFR while the values are close to 0.2 in the host-rock. In contrast, Nobeoka, Okitsu and Kodiak BFR show similar values in comparing the BFR veins and the host-rocks. Accordingly, the Mugi and Kure BFR are associated with a molten origin when the three others BFR are the result of mechanical wear solely, without evidence for large temperature increase.In summary, the D3/Gsl. ratio is a parameter that can be easily retrieved in most fault rocks cutting across sediments and that efficiently tracks the occurrence of short-lived intense heating. The use of this parameter appears as a promising approach to decipher the dynamics of faulting and to discriminate faults with intense frictional work from faults where temperature increase was much more limited, either because of slow creep or inhibiting processes (e.g. fluid vaporization during slip)

Raman Spectroscopy of Carbonaceous Material record in pseudotachylytes: heating or deformation?

International audienceThe Raman Spectroscopy of Carbonaceous Material (RSCM) allows quantifying the degree of crystallinity of carbonaceous material, which increases upon geological heating. Evolution of crystallinity of CM is now routinely used as a reliable geothermometer. Recently, RSCM approach has been used to evidence frictional heating during seismic events. This new application assumes that CM spectra reflect only the thermal record irrespectively of the potential impact of geological strain on CM crystallinity. The aim of this study is to reconsider this postulate by using analyses of Raman spectra in order to understand the effects of seismic deformation on the CM structure. For this purpose, we analyzed six pseudotachylyte veins from the Shimanto Belt (Japan) and the Kodiak Accretionary Complexes (USA), through high-resolution cross-sections. Samples are composed of foliated tectonic mélanges cut by millimetric shear planes filled by black vitreous material accompanied by injection veins, and present most of features described in the melt-origin rocks. The Raman peak area ratio as well as the intensity ratio show a drastic increase within the pseudotachylyte compared to the host rock. In addition, these parameters show a very sharp evolution in few microns along cross-sections across the PST boundaries, which is at variance with thermal diffusivity models applicable for others intrusive bodies. In order to understand such an evolution of the Raman parameters, 1D thermal diffusion and kinetics of RSCM evolution based on static heating experiments modellings were applied. Diffusion models show that the temperature of the pseudotachylyte materials and the surrounding rocks drops down very quickly after few milliseconds and returns to the background temperature after few seconds. Additionally, the kinetics modelling shows that a very large temperature must be applied for at least 1.0 x 10² ms to increase the intensity ratio. Furthermore, the cooling down of a molten layer is not able to reproduce the sharp evolution in RSCM parameters across the PST boundaries. Therefore, these results are not consistent with observation made on natural samples. It suggests that deformation is the main factor controlling CM crystallinity in fault cores. These results therefore also question the maximum temperature reached in fault zones, possibly much lower than previously estimated

Pseudotachylyte veins in accretionary complexes: melt or mechanical wear?

International audienceWhether seismic rupture propagates over large distances to generate mega-earthquakes or on the contrary slows down quickly, is heavily dependent on the slip processes operating within the fault core, such as frictional melting or intense grain-size reduction and amorphization. The record, in fossil fault zones, of seismic slip, consists in many instances in Black Faults Rocks (BFR), that consists in a generally thin dark and aphanitic veins similar to volcanic glasses, which cross-cuts sharply a weakly foliated tectonic mélange, and have been interpreted as resulting from quenching of a melt (i.e. pseudotachylytes). Such interpretation has nevertheless been questioned because identical (micro- and nano-) textures have been observed on intensely comminuted natural fault rocks and on slow creep experiments on crustal rocks.In this study, we report a new dataset of high spatial-resolution Raman Spectroscopy of Carbonaceous Materials (RSCM) profiles across natural BFR from two accretionary complexes. RSCM is sensitive to both temperature and deformation. We have carried out analyses on Okitsu and Nobeoka BFR from the Shimanto Belt and Kodiak BFR from the Kodiak Accretionary Complex to discriminate the slip weakening process. The Raman Intensity Ratio (i.e. R1 in Beyssac et al., 2002) and the Area ratio (RA1 in Lahfid et al., 2010) show a drastic and discontinuous stepped increase along profiles across the BFR, revealing a higher crystallinity. Moreover, in spite of scattering, highest values have been measured on the rim between the BFR and the host-rock. Fluidization structures, interpreted as injection veins, show similar values to the ones in the host rock. Additionally, using an experimentally calibrated kinetics 1D modelling of Intensity ratio evolution with temperature, we compared the natural Raman spectroscopy profiles to different scenarios of temperature increase during seismic slip. In the three examples of BFR from accretionary complexes interpreted as natural pseudotachylytes, RSCM profiles are not consistent with a molten origin and must reflect mechanical wear during deformation.Consequently, these results bear major consequences on the dynamics of faulting in accretionary complexes, as the slip-weakening processes that occur during seismic slip rely on extreme grain-size reduction and fluidization rather than melting