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Raman spectroscopy on carbonaceous material (RSCM) is a method widely used to infer the temperature of metamorphism in metasediments. Furthermore, anomalies in the Raman signature of carbonaceous particles contained in fault zones have been interpreted as reflecting short-lived heating events due to friction during earthquakes. These applications of RSCM rely all on the assumption that temperature is the main factor controlling the reorganization of carbonaceous material (CM) and its expression by Raman spectroscopy. Nonetheless, few examples of natural fault zones and shear zones recently raised questions about the possible role of strain in such reorganization.In this work, we studied the influence of deformation on the Raman spectra of CM. We carried out experiments of deformation on low-grade shales in the Paterson and Griggs-type rigs, corresponding to low and high pressure conditions, for variable conditions of temperature and strain rates, simulating deformation at the brittle-ductile transition. In parallel, we examined in these experiments the evolution of CM using the intensity ratio (IR), defined as the intensity ratio of the Defect band over the Graphite band of the Raman spectrum.Deformation proceeded as a combination of slip on discrete planes, corresponding to macroscopic stick-slip events, and of ductile shear in few hundreds of µm’s-thick zones. The effect of stick-slip events on RSCM signal is unclear, principally because the deformed layers are too thin to contain CM to be analyzed. In contrast, in zones of distributed strain we observed a systematic and significant increase in IR compared to undeformed domains, reflecting an enhancement in the structural organization of CM. On the basis of additional experiments of static heating we carried out in parallel, we show that the IR increase cannot be connected to local and transient increase in temperature during the experiment, and has therefore to be connected to strain itself.The zones of concentrated ductile strain are characterized by comminution of the clasts of quartz and feldspar initially present, accompanied by the development of a porosity at the submicron-scale. Chemical analysis revealed that along with intense grain-size reduction, the chemical composition of the feldspar was modified. On the basis of these microstructural and chemical evidence, it appears that the elementary processes behind ductile deformation include intense mechanical fracturing and mass transfer along the grain boundaries.As a conclusion, it appears that strain has an undisputable effect to increase IR of . When applied to nature, the relevant temperature and mechanical realm of these experiments is the brittle-ductile transition, for temperature of the order or below 300°C. These conditions are also the ones corresponding to seismic deformation. Therefore we show here that Raman spectra in fault zones reflect not only the temperature history (including coseismic flash-heating), but also, and to a large extent, the strain history of the rock.

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.

Fluid pressure is a key parameter in the brittle, upper portion of the lithosphere, as it controls the stress necessary for faults to slip. In turn, fluid pressure is affected by brittle deformation, as large permeability changes occur during the seismic cycle. The interplay between fluid pressure and deformation is nevertheless mostly a theoretical model, as few data are available regarding the amplitude of fluid pressure at depth and its variations with time. Quartz veins constitute a potential time record of variations in precipitation conditions, as suggested by microstructures such as crack-seal or growth features. We examine in this work different sets of veins from convergent settings, spanning the seismogenic zone (T

The 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.