Abstract
Graphitization, or the progressive maturation of carbonaceous material, is considered an irreversible process. Thus, the 13 degree of graphite crystallinity has been calibrated as an indicator of the peak metamorphic temperatures experienced by the 14 host rocks. However, discrepancies between temperatures indicated by graphite crystallinity versus other thermometers have 15 been documented in deformed rocks. To examine the possibility of mechanical modifications of graphite structure and the 16 potential impacts on graphite 'thermometry' we performed laboratory deformation experiments. We sheared highly 17 crystalline graphite powder at normal stresses of 5 and 25 MPa and aseismic velocities of 1 µm/s, 10 µm/s and 100 µm/s. 18
Abstract
The degree of graphite crystallinity both in the starting and resulting materials was analyzed by Raman microspectroscopy. 19
Abstract
Our results demonstrate consistent decrease of graphite crystallinity with increasing shear strain. Microstructural 20 observations show that brittle processes caused the documented structural disorder of graphite. We conclude that the 21 calibrated graphite 'thermometer' is ambiguous in active tectonic settings and we suggest that a calibration that accounts for 22 shear strain is needed. 23 2
Introduction 24
Organic matter, preserved in sedimentary rocks, can be transformed into crystalline graphite due to structural and 25 compositional changes during diagenesis and metamorphism, a process known as graphitization (Bonijoly et al., 1982; 26 Wopenka and Pasteris, 1993; Beyssac et al., 2002a; Buseck and Beyssac, 2014; etc.) . Graphitization is thought to be an 27 irreversible process and graphite is known to remain stable to the highest temperatures of granulite facies and the highest 28 pressures of coesite-eclogite facies (Buseck and Beyssac, 2014) . It is generally accepted that the degree of graphite 29 crystallinity, or its structural order, is determined mainly by the maximum temperature conditions experienced by the host 30 rocks, whereas lithostatic pressure and shear strain are considered to have only minor influence on graphitization (Bonijoly 31 et al., 1982 ; Wopenka and Pasteris, 1993; Bustin et al, 1995) . Therefore, graphite crystallinity has been calibrated as an 32 indicator of the peak temperatures reached during progressive metamorphism (Beyssac et al., 2002a ; Reitmeijer and 33 McKinnon, 1985) . However, in strained rocks discrepancies between temperatures indicated by the crystallinity of graphite 34 vs. other thermometers have been reported (Barzoi, Fault zone, New Zealand (Kirilova, et al., in press), the Hidaka metamorphic belt, Hokkaido, Japan (Nakamura et al., 2015) , 40 the Atotsugawa fault system, Japan (Oohashi, et al., 2012) , the Tanakura Tectonic Line, Japan (Oohashi et al., 2011) , the Err 41 nappe detachment fault, Switzerland (Manatschal, 1999) , and the KTB borehole, Germany (Zulauf et al., 1990) . In these 42 intensely deformed rocks its presence is of particular interest because its low friction coefficient of µ ~ 0.1 (Morrow et al., 43 2000) allows graphite to act as a natural solid lubricant (Savage, 1948) . The mechanical behavior of graphite has been 44 broadly investigated in both natural and experimental specimens, where it manifests with the lowest µ among sheet structure 45 minerals (Moore and Lockner, 2004; Oohashi et al., 2011 Oohashi et al., , 2013 Rutter, et al., 2013; Kuo et al., 2014, etc.) 
confirming it 46
could have a significant impact on fault mechanics. It has been experimentally proven that even a small fraction of graphite 47 can have a disproportionally large effect on frictional strength where graphite is concentrated by smearing into interlinked 48 layers (Rutter, et al., 2013) . 49 However, structural changes in crystalline graphite caused by tectonic deformation have not yet been systematically 50 explored. To examine this aspect and to investigate the potential impacts of structural disordering of graphite on the graphite 51 'thermometer', we have carried out laboratory deformation experiments on highly crystalline graphite powder. 52
Experimental methods 53

Sample description 54
As a starting material in the current study we used synthetic (commercially synthesized) graphitic carbon to avoid 55 complexities arising from variable degree of crystallinity in natural carbon materials. Initially, the material was crushed to 56 maximum grain size of 160 µm in a RockLabs Swing (TEMA) mill. The resulting fine graphitic powder was annealed at 57 700˚C for two hours in a Lindberg Blue M Muffle Furnace to achieve full graphitization, which is known to occur at this 58 temperature in the absence of other variations in physical conditions (Buseck and Beyssac, 2014) . This was used as the 59 starting material for the deformation experiments. 60
Experimental procedure 61
In total, 10 deformation experiments were performed at room temperature and room humidity in the Brittle Rock 62 deformAtion Versatile Apparatus BRAVA (Collettini et al., 2014) , at INGV, Rome. For each experiment two 3-mm thick 63 layers of synthetic graphite gouges were placed in between three grooved forcing blocks in a double-direct shear 64 configuration (e.g. Dieterich, 1972). The two side blocks are held stationary, and the central forcing block is driven 65 downward causing shear to occur within the graphite gouge layers. Normal stress is applied by the horizontal piston in load-66 feedback control mode and shear displacement accomplished by the vertical piston in displacement-feedback control mode. 67 velocities of 1 µm/s, 10 µm/s and 100 µm/s. The experiments were carried out to total displacements of 20 mm. In addition, 70 some experiments were stopped at 5 mm and 10 mm and the specimens were then recovered to reveal graphite structural 71 changes that took place during different amounts of total deformation. The coefficient of friction (µ) was calculated as the 72 ratio of measured shear load to measured normal load (µ = τ / σn, where τ is shear stress and σn is effective normal stress). 73
Experimental procedure 61
The average shear strain within the layer was calculated by dividing shear displacement increments by the measured layer 74 thickness and summing. The displacement values of the vertical and horizontal load points were corrected for the elastic 75 stretch of each load frame, taking into account that the machine stiffness is 1283 kN/mm on the horizontal axis and 928.5 76 kN/mm on the vertical axis. In addition, we calculated total frictional work for each experiment as a function of shear stress 77 integrated over the total displacement (Beeler, 2007) . 
Raman microspectroscopy 79
The degree of graphite crystallinity was measured by an Alpha 300R+ confocal Raman microscope (WITec, Ulm, Germany) 80 with a 532 nm laser (Coherent, Santa Clara, California), located at the Department of Chemistry, University of Otago, New4 Zealand. The laser (3.0 mW) was focused on the samples with a 50× Zeiss objective. The scattered light was dispersed with 82 a 1200 g/mm grating. The combination of the 50× objective and 532 nm laser wavelength produced a laser spot size of 83 approximately 412 nm in diameter. The integration time of each spectrum was 2 seconds with 50 co-additions (100 seconds 84 in total). The spectra were calibrated using the Raman band from a silicon wafer prior to each set of measurements. 85
Raman microspectroscopy 79
The collected spectra were pre-processed in GRAMS AI 9.1 (Thermo Fisher Scientific Inc.), where cosmic spikes were 86 removed and a multi-point linear baseline offset was performed. This was followed by peak fitting three Lorentzian-87
Raman microspectroscopy 79
Gaussian functions to each spectrum with a linear baseline over 1000 -1700 cm -1 . For each spectrum, the area ratio was 88 calculated (R2 = AD1 / (AG + AD1 +AD2), where Ai is the area of the ith peak, G band is the main high frequency band of 89 graphite, D1 and D2 bands are defect bands observed in the first order Raman spectrum of graphite) (Wopenka and Pasteris, 90 1993; Beyssac et al.., 2002a) . 91
Scanning electron microscopy 92
Microstructural analyses of the graphite gouge recovered from the biaxial apparatus were carried out using a scanning 93 electron microscope (SEM). Some SEM images were acquired from the shiny surfaces of the graphite layers that had been Germany, where high-resolution secondary electron images were collected at 20 kV and a WD of 10 mm. 100
Transmission electron microscopy 101
Transmission electron microscopy (TEM) was used for detailed microstructural characterization of the shiny surfaces. High-102 resolution TEM images were collected by using a JEM-2010 electron microscope, located at the University of Hokkaido, 103
Transmission electron microscopy 101
Sapporo, Japan. The instrument was operated at 200 kV with LaB6 filament. TEM foils (with size of 12 x 5 µm and 104 thickness of 1 µm) milled by FIB perpendicular to the shiny surface (X-Z section) were placed on a carbon coated film, and 105 examined by using dual-axis tilting holder. 106
Results 107

Mechanical data 108
Our experiments allowed us to investigate graphite mechanical behavior and structural modifications under various sliding 109 velocities, normal stresses and shear strain. These conditions are summarized in Table 1 . 110
Friction variations 111
Over several mm of displacement, the friction coefficient shows a similar evolution trend in all experiments. On a plot of 112 friction coefficient vs. displacement (Fig. 1a) , the friction coefficient (µ) delineates a curve characterized by a rapid increase 113 to an initial peak friction coefficient (µpeak), followed by a subsequent exponential decay towards a steady-state friction 114 coefficient (µss) over a slip weakening distance. The shapes of the friction-displacement curves vary with the normal stress 115 applied and are steeper for the experiments conducted at 25 MPa than the ones at 5 MPa (Fig. 1a) i.e. the displacement 116 required to achieve steady-state decreases at higher normal stress. In addition, the values of both µpeak and µss ( Fig. 1a ; Table  117 1) are significantly lower in the experiments at 25 MPa (µpeak = ~0.4; µss = ~ 0.1) than in the experiments at 5 MPa (µpeak = 118 ~0.5; µss = ~ 0.2) (where µss values were read at the end of each experiment). Plots of µ at all sliding velocities (Fig. 1a ) 119
Friction variations 111
show subtle variations in µpeak and µss with change of the applied sliding velocities ( Fig. 1a ; Table 1 ). 120
Shear strain variations 121
Plots of friction coefficient vs. shear strain ( Table 1 ). In addition, the 125 experiments at 25 MPa demonstrate a dramatic increase in shear strain with increasing slip velocity ( Fig. 1b ; Table 1),  126 whereas at low normal stress we do not observe any systematic variations associated with changes in sliding velocities ( Fig.  127 1b, c and d). Fig. 1c and d show the experiments at low shear strain used to characterize graphite structural changes in the 128 early stages of deformation (Table 1) . 129
Graphite crystallinity 130
All the experiments resulted in the development of shiny smooth surfaces with gentle slickenlines (macroscopic fine 131 grooves, parallel to the slip direction as defined by Toy et al., in press). Raman spectra obtained on the top of these surfaces, 132
Graphite crystallinity 130
that had accommodated most of the induced deformation, are compared to Raman spectra from the starting material to 133 identify the effects of mechanical deformation on graphite crystallinity.
Graphite crystallinity 130
Raman data from 20 spectra per sample are presented in Supplementary material 1 (S1). Representative spectra for each 135 sample are illustrated in Fig. 2 , which shows spectra displaying the least (left column) and the most (right column) 136 disordered graphite within a sample (i.e. lowest and highest R2 values respectively). Spectra that were typical of the average 137 for each sample are also presented (middle column). Experiments 3 and 7 were stopped at only 5 mm displacement and 138 resulted in extremely fragile deformed surfaces, which were unable to be extracted without them breaking into pieces too 139 small to obtain spectra from. Thus, graphite crystallinity was not measured in these experiments. 140
Graphite crystallinity 130
All the acquired spectra show typical G, D1 and D2 bands, respectively at ~1580 cm -1 , ~1350 cm -1 and ~1620 cm -1 (S1). The 141 degree of graphite crystallinity in each sample could thus be calculated by using the area ratio R2 ( Fig. 2; S1 ). Raman 142 spectra collected from the starting material show R2 values ranging from 0 to 0.327 (Fig. 2) , corresponding respectively to 143 fully crystalline and highly organized graphite. Spectra acquired from the deformed surfaces show higher R2 values ( Fig. 2 ; 144 S1). The most crystalline graphite with R2=0.330 was collected in Exp. 2 (Fig. 2) while the most disordered graphite with 145 R2=0.661 resulted from Exp. 10 (Fig. 2) . 146
Graphite crystallinity 130
As graphite crystallinity varies within a sample ( Fig. 2; S1 ), we examine average R2 values for each one and compare them 147 with applied normal stress, sliding velocity, shear strain, and total frictional work ( Table 2 ). The starting material has 148 average R2pre-shear graphite = 0.173, whereas all deformed samples have higher average R2 values (Table 2) . Analyzing the 149 average R2 values for deformed samples reveals that graphite is more disordered in the high normal stress experiments 150 (Table 2 ) than in the experiments at 5 MPa. Furthermore, in the experiments at 25 MPa the average graphite crystallinity 151 decreases with increasing sliding velocities ( Table 2 ). In contrast, at low normal stress, we do not observe any dependence of 152 the degree of graphite crystallinity on the applied sliding velocities (Table 2) . Overall graphite appears as most disordered in 153 the experiments where the highest shear strain was achieved ( Table 2) . The relationship between average R2 and shear strain 154 is illustrated in Fig. 3a by fitting a power function with a correlation coefficient R 2 = 0.95. Fitting a power function to 155 average R2 and total frictional force showed a consistent correlation (Fig. 3b) . The experiments 2 and 6 at low normal stress, 156 which were stopped at 10 mm displacement and accommodated the least amount of shear strain, contain the least disordered 157 graphite ( Fig. 3; Table 2 ). 158
Microstructural characteristics 159

Scanning electron microscopy (SEM) 160
Similar microstructural features were observed in all the deformed samples. SEM images obtained from the sample 161 deformed during experiment 8 are presented to demonstrate our observations (Fig. 4) . 162
Scanning electron microscopy (SEM) 160
These high-resolution images in Y-Z sections reveal that the shiny surfaces are decorated by closely spaced (from < 5 to 10 163 micrometers) slickenlines (Fig. 4a) , on top of a smooth continuous layer. In places, the continuity of this layer is interrupted 164 by fine (~1 to 2 micrometers in width) fractures (Fig. 4a) , with random orientation compared to the slip direction. 165
Scanning electron microscopy (SEM) 160
Occasionally, the deformed surface appears as completely disrupted, and is decorated with smaller graphite grains from 10 to 166 50 micrometers in size, oriented nearly parallel to the shear direction (Fig. 4b) . In X-Z sections this highly deformed surface 167 is observed as a thin slip-localized zone, composed of well-compacted layer of aligned graphite grains (Fig. 4c) . This 168 localized shear surface is underlain by a zone of randomly oriented, inequigranular, irregular graphite grains (Fig. 4d) . In 169 places, most of the graphite grains are aligned with their basal (001) planes parallel to the slip direction, and form compacted 170 layers, defining a weakly-developed fabric (Fig. 4e) . There has been some dilation along these cleavage planes, and the 171 spaces thus created are filled with smaller graphite grains with their (001) planes sub-perpendicular to the shear direction 172 (Fig. 4e) . Locally, intensely fractured grains are also observed (Fig. 4f) . 173
Transmission electron microscopy (TEM) 174
TEM was used to examine the microstructure of the material that makes up the shiny surfaces (Fig. 4c) . TEM analyses were 175 performed on foils cut perpendicular to this surface. Fig. 5 shows characteristic TEM images obtained from the sample 176 recovered from experiment 8. 177
Transmission electron microscopy (TEM) 174
Graphite grains in this well-compacted layer have basal planes predominantly aligned with the shear plane, as were observed 178 in SEM images. However, adjacent grains show slightly different orientations (Fig. 5a ). In addition, kink folded graphite 179 grains are observed in multiple locations in the foils (Fig. 5b, c) , which yields a 'wavy layering' at a small angle to the shear 180 direction (Fig. 5b) . In isolated areas, there are also some smaller grain fragments with random orientation (Fig. 5d) . 181
Discussion 182

Mechanical behavior 183
Graphite in our experiments shows mechanical behavior consistent with other mechanical studies of pure graphite gouges. carried out at seismic rates. We also acknowledge that the imposed velocities in the experiments by Oohashi et al. (2011) 196 were substantially different to ours, and shearing at those seismic rates may cause frictional heating. Therefore, graphite 197 frictional strength in their experiments may be related to thermally-activated weakening mechanisms (Di Toro et al., 2011) 198 that are only significant at these high velocities. 199
Mechanical behavior 183
We also observed shear strain variations in the various samples (Fig. 1b, c and d velocities also play a role in the accommodated total shear strain, and shear strain increases with increase in the applied 204 sliding velocities but only in the high normal stress experiments (Fig. 1b) . As we previously suggested, higher velocities may 205 result in more efficient reorganization of graphite grains, and thus further progressive thinning of the graphite gouges 206 occurred. However, we cannot explain the absence of similar trend at the 5 MPa experiments by our results. There are too 207 few of these relationships to fully characterize the effect of sliding velocity on shear strain accumulation in graphite gouges, 208 and more mechanical data of this sort need to be collected in future. 209
Structural disorder of graphite 210
Our experimental study clearly demonstrates transformation of fully/highly crystalline graphite (with R2 ratios ranging from 211 0 to 0.327; Fig. 2 ; S1) into comparatively poorly organized graphitic carbon (with R2 ratios up to 0.661; Fig. 2 ; S1), which 212 indicates significant graphite disorder with increasing strain and total frictional work at the tested aseismic sliding velocities 213 (Fig. 3) . The estimated bulk shear strains (Table 1 ) are likely to be significantly lower than the shear strains accommodated 214 within the thin shear surfaces. However, we expect the strain variations within these surfaces to be directly related to the 215 measured bulk shear strains. Nevertheless, we refer to the above relationship as a rough approximation. We also 216 acknowledge that the slickenlined surfaces that were produced experimentally contain some graphite that yield spectra 217 comparable to those acquired from the starting material i.e. there is highly crystalline graphite that appears as unaffected by 218 the deformation. However, at least some of these spectra are derived from undeformed graphite powder that underlies the 219 shear surfaces and could not be entirely removed during sample preparation due to the fragile nature of the samples. It is also 220 possible that some non-deformed graphite powder was accidently measured through the fractures that are cross-cutting theaccumulated shear surfaces (Fig. 4a) . But even if some graphite did not undergo mechanical modification during the 222 experiments, the results overall validate that structural disorder of graphite can result from shear deformation subsequent to 223 the graphitization process. 224
Structural disorder of graphite 210
We evaluate the documented disorder of the crystal structure of graphite by analyzing variations in R2 ratios, which depend 225 on the increase of defect bands (D1 and D2) in the Raman spectrum of graphite. This relationship is a well-known 226 crystallinity index of graphite that shows the degree of maturity of the carbonaceous material (Wopenka and Pasteris, 1993; 227 Beyssac et al., 2002a; etc.). Alternatively, it may reflect increase in the grain boundary density (Tunistra and Koening, 1970; 228 Pimenta et al., 2007) . However, we aimed to avoid grain boundaries during spectra acquisition, which was possible due to 229 the laser spot size of 412 nm, which is much smaller than the graphite grains in our samples (>10 microns, Fig. 4b ). We 230 acknowledge that some of the spectra may have been accidentally obtained in close proximity to grain boundaries, however 231 occasional measurements of this sort are unlikely to affect the average R2 per sample. Thus, we attribute the detected 232 increase in D bands in our experimental data to disorder of the internal structure of graphite rather than grain size reduction. We have experimentally proven that shear strain can not only affect the final structural order of graphite but also manifests 241 as a controlling parameter in the transformation process ( Fig. 3a; Table 2 ). Previous authors have suspected that shear strain 242 may play an important role for graphite modifications, and evidence for this has been found in graphite crystallinity 243 variations in natural samples from active fault zones (Kirilova et al, in press; Nakamura et al., 2015), and strained rocks in 244 metamorphic terrains (Barzoi, 2015; Large et al., 1994) . Thus, we conclude that the previously proposed model of 245 progressive graphitization due to increase of temperature (Bonijoly et al., 1982) does not completely reflect the graphite 246 formation mechanisms. 247
Structural disorder of graphite 210
Furthermore, graphite can form or be transported at various depths by tectonic processes, and therefore, it can be exposed to 248 different lithostatic pressures, and hence different normal stresses. We demonstrated that during shearing higher normal 249 stress results in an increase of shear strain (Fig. 1b) , and thus causes a higher degree of graphite disorder ( Fig. 3a; Table 2) . 250
Structural disorder of graphite 210
This outlines the significant effect of lithostatic pressure on graphite crystallinity that has been undervalued until now 251 (Bonijoly et al., 1982 ; Wopenka and Pasteris, 1993; Bustin et al, 1995; Beyssac et al. 2002b ). Previous experimental studieshave identified initiation and enhancement of graphitization under pressure (i. e. increase in graphite crystallinity) but only at 253 nanometer scale (Bonijoly et al., 1982; Beyssac et al., 2003) . Nevertheless, we speculate pressure should be also considered 254 as a factor that can determine the degree of graphite crystallinity during both graphitization and graphite structural 255
modifications. 256
We have investigated the effects of shear strain and pressure on graphite crystallinity during shear deformation with aseismic 257 velocities, using a starting material with uniform properties (i.e. highly crystalline graphite powder). In contrast, Kuo et al. 258 (2014) and Oohashi et al. (2011) simulated fault motions in synthetic and natural carbonaceous material with variable degree 259 of maturity at the start of the experiments (ranging from amorphous carbonaceous material to crystalline graphite). Both 260 studies reported graphitization of carbonaceous material due to localized frictional heating rather than structural disordering. 261
modifications. 256
These experiments reveal the impact of seismic velocities on graphite structural order and the fact their findings differ so 262 markedly from ours highlights the complexity of graphite transformations in fault zones. 263
modifications. 256
Our microstructural observations provide some indications of the deformation processes that affected graphite structural 264 order. The shiny slickenlined surfaces are composed of very fine-grained material visible as slip-localized zone on SEM 265 images (Fig. 4d) . Nanoscale observations reveal graphite grains within it occasionally form stacked kink-band structures, 266 (Fig. 5b, c) . This zone, which we assume accommodated most of the induced deformation, is underlined by a less deformed 267 zone composed of larger graphite grains in a finer matrix that in places has developed as an anastomosing fabric, typical of 268 creeping gouges (Fig. 4d) . In rare places at SEM scale brittely fractured grains also occur ( Fig. 4f and 5d) . The interpreted 269 structures suggest that brittle processes operated during shearing, and we conclude that these processes resulted in the 270 structural disorder of graphite, manifested as changes in the Raman spectra. This interpretation is in agreement with the 271 conditions of our experiments (i.e. shearing with aseismic velocities took place at room temperature conditions), that 272 typically would not induce temperatures high enough for crystal plastic processes. Furthermore, the microstructures and the 273 inferred processes are exactly the same as those observed by Nakamura et al. (2015) in the Hidaka metamorphic belt, Japan. 274
modifications. 256
However, crustal fault zones do not only accommodate brittle deformation. At higher temperatures and confining pressures, 275 localised shearing can operate by plastic mechanisms (White et al., 1980). We hypothesize that graphite crystallinity could 276 also be influenced by plastic deformation, as was also suggested in previous studies by Large et al. (1994) , Bustin et al. 277 (1995), Barzoi et al. (2015) . Investigating this hypothesis and identifying the exact effects of strain on graphite crystallinity 278 during ductile deformation remain goals for future research. 279
Implications for graphite thermometry 280
The crystallographic structure of graphite measured by Raman spectroscopy has been applied as a thermometer that relies on 281 progressive maturation of originally-organic carbonaceous material during diagenesis and metamorphism. Previous studieshave focused on calibrating this thermometer. The current best calibration is described by the following equation T (ºC) = -283 445 * R2 + 641 ± 50 (Beyssac et al. 2002) by inferring a linear correlation between R2 ratio and peak metamorphic 284 temperatures. However, this thermometer disregards the effects of mechanical modifications of the graphite structure, which 285 this study has identified as having a substantial influence on graphite crystallinity in deformed rocks at sub-seismic 286
velocities. 287
Our experiments demonstrate a shear strain-dependent increase of the R2 ratio of initially highly crystalline graphite powder 288 due to brittle deformation ( Fig. 3a; Table 2 ). In natural analogues, the pre-shear graphite would yield temperatures up to 641 289 ± 50 ºC (S1), which is the upper limit of the calibrated thermometer (Beyssac et al. 2002) . Whereas, the sheared samples 290 would indicate peak metamorphic temperatures as low as 347 ± 50 ºC (estimated from the most strained samples; S1). Thus, 291
velocities. 287
we experimentally prove that in active tectonic settings graphite thermometers may underestimate the peak metamorphic 292 temperatures by < 300 ºC. In cataclasites from the Alpine Fault zone, New Zealand (Kirilova et al., in press) and fault zones 293 of the Hidaka metamorphic belt, Japan (Nakamura, et al., 2015) , the graphite thermometer yields temperature discrepancies 294 of more than 100 ºC compared to temperature estimates derived both from the surrounding high-grade amphibolite facies 295 mylonites and the lower grade equilibrium cataclastic phases (marked by chlorite alteration). Barzoi (2015) also described 296 differences of ~ 150 ºC in graphite temperatures between strained and less strained low grade metamorphic rocks from 297 Parang Mountains, South Carpathians. 298
velocities. 287
We conclude that shear strain calibration of the current graphite thermometer is needed, and we propose an appropriate 299 adjustment based on our dataset. Fig. 3a illustrates good correlation between the average R2 and the bulk shear strain 300 measured within a sample, which can be described by the following equation ( However, a calibration of the existing graphite thermometer could be still insufficient to permit reliable temperature 304 estimates in active tectonic settings because both aseismic and seismic sliding velocities are likely to be encountered in fault 305 zones, resulting in structural disorder of graphite or graphitization (Oohashi et al., 2013) respectively. Furthermore, it can be 306 challenging to estimate shear strain in natural samples, so a strain-calibrated graphite thermometer may be impossible to use 307 in deformed rocks. 308
Conclusions 309
composed of powdered highly-organized graphite. Our results clearly demonstrate significant decrease in graphite structural 312 order, which is a function of the total shear strain attained during the various experiments. Microstructural data presented 313 here reveal that this is a result of brittle processes. We also observed a trend of increasing shear strain within a sample with 314 increase in the applied normal stresses and sliding velocities. This reveals the complexity of graphite structural modifications 315 and highlights the significance of the various parameters that can affect the graphitization process. Our findings compromise 316 the validity of the calibrated graphite thermometer by showing they may underestimate the peak metamorphic temperatures 317 in active tectonic settings. We further suggest a simple shear strain calibration of this thermometer. 318
Acknowledgments 319
The research was funded by the Department of Geology, University of Otago, New Zealand, and Rutherford Discovery 320
Acknowledgments 319
Fellowship RDF-UOO0612 awarded to Virginia Toy. We also acknowledge the 'Tectonics and Structure of Zealandia' 321 subcontract to the University of Otago by GNS Science (under contract C05X1702 to the New Zealand Ministry of Business, 322
Acknowledgments 319
Innovation and Employment). We thank our colleagues Gemma Kerr and Brent Pooley for assistance in sample preparation, 323
Acknowledgments 319
and Hamish Bowman for helping with data visualization. We also wish to express our gratitude to Laura Halliday for 324 generously offering to perform grain size analysis on our samples at the Department of Geography, University 

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