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

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.


Results
Structures. The two exhumed accretionary complexes investigated here were both formed by stacking of tectonic units composed either of coherent turbidites or tectonic mélange units, both mostly composed of shales and bounded by thrust faults 36,37 . Some of these fault zones, contain Black Fault Rocks bearing characteristic features of pseudotachylytes 8,[12][13][14] . Black Fault Rocks are black and aphanitic thin veins, which cut sharply across, with straight boundaries, the host-rock that often contains itself brittle microstructures (Fig. 1). The two Black Fault Rocks from the Shimanto Belt are millimeter-thick or less but in Kodiak, the Black Fault Rock described by Rowe et al. 13 reaches up to 30 cm in thickness. In addition, in contact with meter-sized sandstone lenses, some Black Fault Rocks show structures interpreted as injection structures of a fluidized material perpendicular to the fault core 8,14 (Fig. 4a-c). The filling material is also black and has an aphanitic texture (Fig. 2).
Microstructures. The Black Fault Rock material is characterized by sub-rounded to rounded quartz clasts, feldspars (mostly albite), and sometimes calcite 8 , less than 100 µm in size, embedded in a ultra-fine black matrix. Clasts, sometimes, show embayment, corroded and fractured structures 12 . Clasts are wrapped in a matrix (Fig. 3a,b). Under SEM, the matrix seems composed of an ultra fine-grained material composed mostly of phyllosilicates, with the notable occurrence of pores 10,13 , micrometer scale scattered euhedral iron sulfides or titanium oxides 8,12,14 and scattered angular clasts (Fig. 3c). For example, the Kodiak Black Fault Rock is composed of 1 to 2 µm euhedral zoned plagioclases, in a matrix composed of acicular chlorite 14 (Fig. 3b). Iron sulfides and titanium oxides present euhedral to sub-euhedral shapes and sizes ranging from ca. 0.5 to 1 µm (Fig. 3b,c). In rare cases, framboïdal iron sulfides are observed in the BFR layer ( Supplementary Fig. 4). Titanium oxides are mostly observed in the homogeneous layers, whereas iron sulfides are more abundant in the coarse-grained layers. In the Nobeoka fault zone, Black Fault Rock observed in optical microscopy and SEM show flow textures, where the layering is defined by the distribution of sulfide grains 12 and the quantity and the shape of pores ( Figs. 1 and 3a), while the ultrafinegrained matrix and clasts remain homogeneous. This microstructural evolution is also observed in Okitsu and Kodiak Black Fault Rocks. However, on the extreme rim of the Nobeoka Black Fault Rocks, a smaller grain size of the clasts in the ultrafinegrained matrix is observed.
The host-rock of the Black Fault Rocks (Fig. 3d), is also composed of quartz and albite clasts embedded in a matrix composed of phyllosilicates with scattered iron sulfides and titanium oxides, the latter with a much lower abundance than in the Black Fault Rock. The grain size of quartz and albite clasts is larger than in the Black Fault Rock. Clasts embedded in the matrix show angular shapes and internal fractures. Corroded shapes on quartz and albite are often observed ( Supplementary Fig. 4). Titanium oxides have euhedral shapes, while the iron sulfides have either euhedral or framboïdal shapes. Iron sulfides show a clear metamorphic coronitic reaction including the production of iron oxides (Fig. 3d).
Polystaged vs single-staged. Moreover, two types of microstructures have been observed for these Black Fault Rocks. Samples from the Shimanto Belt are composed of a single homogeneous layer. In contrast, the thicker Black Fault Rock from Kodiak has a multi-layered structure, with "homogeneous" or "granular" layers, visible in both proportion and size of the clasts embedded in the vitreous matrix (Fig. 2). Homogeneous layers are composed principally of an ultrafinegrained matrix, bright in cathodoluminescence imaging, embedding scattered, micron-size, particles. Granular layers are composed of an ultrafinegrained matrix, dark in cathodoluminescence imaging, embedding a large proportion of clasts of various sizes from the homogeneous layer (Fig. 2c). These features support the hypothesis of a polystaged formation process for the Kodiak black Fault Rock.  Heat diffusion modelling is based on the heat equation applied to a 1D system, composed of an infinite matrix embedding a molten layer of thickness h, (= the current thickness of the Black Fault Rock). We considered three www.nature.com/scientificreports/ idealized end-member cases to model the evolution of the system before and after melting. Case (1) corresponds to a fault zone of thickness h, where heat is dissipated by mechanical work, up to melt the central portion of the fault. Heat production is described as Ė = τ . u h , with τ (shear stress) and u (slip velocity) inferred from geodetic and seismological models 40 . Cases (2) and (3) describe the temperature field in a molten layer of thickness h, without/with viscous shear heating, respectively. Heat production is modelled as Ė = µ u h 2 , where µ is the melt viscosity.
Slip parameters are subject to large uncertainties, even if mechanical and seismological data provide some constraints on the orders of magnitude. Different slip models, in terms of velocity u and total duration, were considered, in order to fit the intensity ratio measured in natural rocks.
The geometry and boundary conditions of the model were adjusted to the natural case study of the Nobeoka Black Fault Rock. The thickness of the molten layer of the modelling is fixed to 1 mm. The initial temperature and intensity ratio field are very uniform and equal to 200 °C and 0.50, respectively. The temperature of the molten layer is fixed at 1400 °C, based on the interpreted melting temperatures for Black Fault Rocks in accretionary complexes and experimental reproduction of molten origin BFR in the literature 11,14,20,41 . Complementary information are given in the Supplementary note 1 and 2.
Modelling results. The temperature evolution before melting (Case (1)) is extremely short, as the frictional melting is reached after only few hundredths of second. For this short heating time, the evolution of the intensity ratio of Raman spectra, around 0.003, appear not significant ( Supplementary Fig. 3).
After melting at 1400 °C, if viscous shear heating is negligible (Case (2)), inside the molten layer the temperature drops down very quickly (in few milliseconds) so that at the center of the layer, the temperature is reduced to 800 °C after less than 1 s. In the host-rock, heat diffusion leads to an increase of temperature, locally up to 800 °C, but for a very short time. In this scenario, the intensity ratio reaches 0.62 in the center of the molten layer and decreases rapidly laterally, down to unchanged values of 0.50 at the boundary with the host-rock.
The significant intensity ratio increase in the molten layer requires therefore incorporating the production of heat by viscous shear as the fault is slipping (Case (3)). We adjusted the slip velocity to 10 m/s and the duration to www.nature.com/scientificreports/ 1 s so that the modelled intensity ratios in the center of the molten layer are equal to the natural ones measured in Nobeoka Black Fault Rock (ca. 0.68; Fig. 6). In spite of significant heat production by viscous shear, the diffusion of heat away from the molten layer into the cold host-rock is extremely efficient, so that sharp temperature and intensity gradients develop exclusively within the molten layer (Fig. 6a). Therefore, in the outer domains of the molten layer, the intensity ratio remains the same as host-rock values throughout slip (Fig. 6b).

Discussion/conclusions
Shape of modelled vs. natural RSCM ratio profiles. Parametric models presented here show that frictional melting and viscous shear produce Gaussian shape profiles where symmetric gradients in intensity ratio are located within the molten layer. By contrast, natural profiles follow rectangular function shapes with major discontinuities (i.e. steps) coinciding with Black Fault rocks/host-rock physical boundaries with up to 40% variation of the intensity ratio confined to few micrometers. Furthermore, towards the outer boundary of Black Fault Rocks, the modelled peak values are much lower than those in natural cases, even considering extreme conditions of slip and heat production (Figs. 6b and 7). Therefore, the sharp increase in Raman spectra properties of carbonaceous material in the Black Fault Rocks appears inconsistent and therefore not to be controlled by the high and short-lived temperature increase during seismic slip. This conclusion is further supported in polystaged deformation faults such as the Black Fault Rock of Kodiak ( Fig. 5 and Supplementary Fig. 2), where the presence of multiple steps in Raman Intensity ratio is difficult to reconcile with thermal models of multiple melting stages.
Other features of Raman Spectroscopy of Carbonaceous Material profiles are at variance with the hypothesis of a temperature control. First, intensity ratios retrieved from injection structures are much lower than those in the fault core, again in contradiction with a common origin as a quenched melt (Fig. 4). Second, the highest intensity ratio values are mostly measured on the rim of the fault core layer, which is colder than the fault core www.nature.com/scientificreports/ in thermal models because of heat diffusion. Third, the microstructures do not show evolution from the center of the vein to the rim, except a thinner grain size on the extreme rim, whereas the temperature profile shows a Gaussian shape evolution.

Interpretation of microstructural observations in terms of formation process. The three Black
Fault Rocks described in this study show features very close to the ones described for the melt-origin pseudotachylytes. These samples are composed of an ultrafinegrained matrix with micro-scale sub-rounded clasts of quartz and plagioclase. Some of these clasts show fractures and corroded shapes interpreted as resulting from interactions with a melt 11,12 . However, the same corroded shape of the plagioclase has been observed in cataclastic rocks from the Kodiak Accretionary Complex, while gulf of corrosion in quartz are described in the Nobeoka BFR host-rock ( Supplementary Fig. 4). Moreover, the ultra-fine matrix material that composes the matrix of the Black Fault Rock cannot be considered as an undisputable evidence for the former presence of a melt. Indeed, microfault zones filled with ultra-fine matrix material have also been described in slow creep experiments where frictional temperature increase is not significant 16,17 . In addition, Ti-rich and Fe-rich particles, interpreted as droplets, have been described in the three samples 8,12-14 . Our observations confirm the presence of Ti-rich particles in the Black Fault Rock but their shape is systematically euhedral. We observed also in the Black Fault Rock the absence of Fe-rich particles, which are abundant in the host-rock, in the exception of the Nobeoka BFR where Fe-rich particles are still present. Incipient replacement of iron sulfides by Fe-rich oxides is already operative in the host-rocks, in the form of corona of Fe-oxides around iron-sulfide particles (Fig. 3d). The difference in the proportion of the Fe-rich and Ti-rich particles between host-rock and Black Fault Rock, induced by the destabilization of Fe-rich particles, can therefore be interpreted as a result of fluid-rock interactions under variable redox-conditions, associated with fluid circulation in fault zones 42 . Black Fault Rocks have also an increase porosity with respect to host-rock (Fig. 3). Increase in porosity might as well be the result of solid-state deformation, even at large depths, as demonstrated in viscously creeping crustal shear zones 43,44 . To finish, the presence of microlites, i.e. microcrystals crystallized from a melt, is not decribed in the three Black Fault Rocks considered here. The very fined grained phyllosilicates wrapping the clasts in the Black Fault Rocks have the same nature as the low-grade metamorphic mineral present in the host-rock 45 . In the Kodiak Black Fault Rock, micrometer-scale, euhedral and zoned plagioclase has been described 14 . The models to account for oscillatory zoning imply either coupling between mineral interface processes and elemental diffusion in the surrounding melt, or variations in temperature/melt composition related to convection in a magma chamber [46][47][48][49] . In both cases the formation of oscillatory zoning in plagioclase involves durations inconsistent with the temperature evolution of mm-thick molten layer ( Fig. 6 and Supplementary Fig. 3). However, these oscillatory zones can be www.nature.com/scientificreports/ interpreted as an local evolution of temperature, pressure or water 50,51 content and could be the result of intense fluid-rock reaction 21 . As a conclusion, in the three Black Fault Rock described here, none of the microstructures can be considered as irrefutable evidence to discriminate between melting 5 and strain-related ultra-comminution 6,16-18 as a formation process.
An alternative model to shear heating: Raman Spectroscopy of Carbonaceous Material (RSCM) ratio profiles record the distribution of strain. While microstructures are rather ambiguous as to the origin of the Black Fault Rocks, the RSCM signal, both in terms of values and spatial distribution, is not consistent with any scenario of frictional heating and melting [28][29][30][31] . Therefore, temperature is not the parameter that controlled the sharp spatial variations in the distribution of crystallinity of carbonaceous material observed in all the natural samples.
Alternately to temperature, strain might be the factor controlling RSCM signal variations. The RSCM intensity ratio used in this work has been shown in natural and experimental examples to be sensitive to deformation 30,32,52,53,35 .
The interpretation of the intensity ratio in terms of strain is nevertheless not straightforward for several reasons. (1) The parameters of deformation (total amount of strain, strain-rate), as well as the conditions of deformation (pressure, temperature, fluid abundance), are only partially known, especially in natural samples, Figure 6. Thermal and kinetic modellings obtained for a 1 mm-thick molten layer where heat is transferred by diffusion to the host rock and is produced in the molten layer by viscous shear heating. (a) Temperature and (b) Intensity ratio evolution at different times (curves). Also presented, intensity ratio of the Nobeoka Black Fault Rock. Slip parameters (slip rate and total slip) were adjusted so that the modelled intensity ratio reaches the natural values at the center of the molten layer. www.nature.com/scientificreports/ but also in experimental samples where macroscopic variables might be prescribed but local variables are only inferred.
(2) The evolution from disordered carbonaceous material to graphite is not a monotonic and simple process, but rather a combination of many elementary processes. This is reflected in the complex evolution of RSCM spectra with increasing metamorphic temperature 26,38,54 : in disordered carbonaceous material, from catagenesis to low-grade metamorphic conditions, increase in temperature results in the increase in Raman Spectroscopy of Carbonaceous Material intensity ratio. Subsequently, at higher-grade conditions, increase in temperature results in the decrease in intensity ratio, up to the graphite where defect bands, i.e., D bands are absent. Therefore, comparison should be preferentially be carried out on carbonaceous matter in the same range of crystallinity. Most existing studies deal with relatively well-ordered carbonaceous particles pertaining to the higher-grade metamorphic conditions 32,34,52,53,55 . In contrast, our work focuses on relatively disordered carbonaceous typically present in external domains of orogenic belts 38,45 .
In natural samples, relationships between strain and crystallinity of carbonaceous particles point rather to an increase in carbonaceous material ordering in viscous mylonites 52,53,55 . In these examples, the low strain-rate attested by the microstructures precludes any significant increase in temperature by shear heating. Beyond the total amount of strain, other parameters of deformation, such as strain-rate, are also likely to play a role: in high grades rocks from the Hidaka Belt, two opposite trends of evolution, i.e. towards higher and lower carbonaceous material ordering, were observed in two types of high strain zones 53 .
In addition, experiments can also shed some lights on the relationship between strain and carbonaceous material ordering. Low strain-rate experiments have correlated without ambiguity zones of high strain with higher carbonaceous material ordering 34 . High strain-rate deformation experiments, aimed at reproducing fault zone deformation, have also shown that zones of localized deformation have higher carbonaceous material ordering 28,30,32 , but without a clear quantitative connection between the parameters of slip and the RSCM signal evolution. The interpretation of these high velocity experiments in terms of strain-carbonaceous material ordering relationship is nevertheless difficult, as not only strain, but also temperature is increased in the domains where deformation localizes, so that the respective effect of strain and temperature are impossible to disentangle. In addition, the temperature increase estimation is difficult. In fact, for similar starting material, deformation apparatus and experimental conditions, the calculated temperature increases ranges from < 300 °C 32 to more than 1000 °C 28 .
The processes behind this increase in crystallinity as a result of shear are active at the nano-structural scale. Shear is responsible for the coalescence of pores and the parallelization of the basic structural units that conducts to a more regular organization of the aromatic carbon layers and to widen carbon sheets 33,34 . Therefore, shear will directly catalyze the carbonization reactions and generate higher crystallinity.
In the studied samples, the shape of RSCM profile provides the strongest argument in favor of the effect of strain on carbonaceous material ordering. The stepwise RSCM intensity ratio profiles across the natural Black Intensity ratio profile and the amount of strain or strain rate across the host-rock and the Black Fault Rock for the Nobeoka case (with median and standard deviation shown as black dotted lines). Intensity ratio derived from thermal models of a viscously sheared molten layer (case (3)) is superimposed as a grey dotted line. A hypothetical strain/strain rate profile is shown as a black bold line. The stepwise shape of natural profile matches strain/strain rate profile, while it stands at variance with thermal models. www.nature.com/scientificreports/ Fault Rocks coincides well with the microstructure profiles, without evolution from the rim to the center of the vein, which reflect strain or strain-rate distribution (Fig. 7). Conversely to temperature profiles, strain or strainrate profiles can be discontinuous, as a result of dynamic weakening processes. As a summary, from existing natural and experimental evidences, and from the characteristics of RSCM intensity ratio distribution, we conclude that carbonaceous material crystallinity therefore reflects at the first order mechanical processes instead of frictional temperature increase. The Black Fault Rocks are composed of extremely fine-grained material which may be in some cases down to amorphous state, as a result of comminution, i.e., a drastic mechanical work during fault slip. The increase of temperature during such deformation is not sufficient to influence the Raman Spectroscopy ratios within/outside the Black Fault Rocks, a feature that we use in the following to place upper bounds on the maximum temperature reached within the slipping zone.
Upper bounds on temperature increase during slip based on the RSCM data and thermal and kinetics modelling. High velocity friction experiments have attempted to reproduce the frictional processes that occur during fault seismic slip 28,56,57 . A severe limitation of these experiments is that the normal and shear stress are a few MPas at most [57][58][59][60][61] , far from the deviatoric stress that prevails at depth. As a consequence, the power dissipated on experimental fault planes is much smaller than during actual seismic slip 62 and large uncertainties remains as to the temperature reached in deep fault cores during earthquakes. First-order estimations have been made on the basis of albite and quartz microstructures 28,56 , however, as we reported above (Fig. 3) ambiguities remain as to the interpretation of such microstructures. In the following, we attempt to estimate the range of possible temperature increase based on the RSCM signal evolution and on kinetics modelling.
The main conclusion reached above, based on the shape of the IR ratio in the three BFR, is that such shape is the result of strain distribution, and that no thermal effect can be detected. In other words, if there had been any thermal imprint on IR signal, then it is undetectable, hence lower than the scattering of IR values within the BFR or the host rock, of the order of 0.06.
Considering a slip duration of 1 s (as Sibson 62 ), and a 1 mm-thick slip zone where the temperature is fixed during slip, then the maximum temperature is 1075 °C, above which the thermal imprint becomes larger than IR scattering along the step-wise profiles ( Supplementary Fig. 5).
High-velocity experiments at ambient pressure have shown that during flash heating of a polyphasic mixture, each phase behaves independently and according to its melting point as a pure phase 20,63 .Our estimate of upper bound on temperature, based on the RSCM, is higher than the melting point of albite, estimated from 900 to 1100 °C at 2 to 5kbar 51 but is largely lower than the melting point of the quartz (i.e. 1730 °C) 64,65 .
As a consequence, the distribution of IR in the three fault zones examined here is incompatible with temperature as high as to completely melt the rock; melting, if it ever occurred, was only partial and limited to the feldspar and phyllosilicate fraction of the rock. Therefore, the corrosion structures observed on albite could be the results of the melt of this mineral, on the contrary to quartz microstructures which are the result of hydrothermal or dissolution-precipitation processes. Alternatively, considering the ambiguity of all observed microstructures, the three Black Fault Rocks considered here might also be the product of mechanical wear, without any significant temperature increase nor partial melting.