tectono metamorphic evolution of a nappe stack a case study of the swiss alps fold and thrust belts are prominent structures that occur at the front of compressional orogens to unravel the tectonic and metamorphic evolution of such complexes kinematic investigations quantitative microstructural analysis and geothermometry calcite graphite calcite dolomite were performed on carbonate mylonites from thrust faults of the helvetic nappe stack in central switzerland paleo isotherms of peak temperature conditions and cooling stages fission track of the nappe pile were reconstructed in a vertical section and linked with the microstructural and kinematic evolution mylonitic microstructures suggest that under metamorphic conditions close to peak temperature strain was highly localized within thrust faults where deformation temperatures spatially continuously increased in both directions from n to s within each nappe and from top down in the nappe stack covering a temperature range of 180 380 c due to the higher metamorphic conditions thrusting of the lowermost nappe the doldenhorn nappe was accompanied by a much more pronounced nappe internal ductile deformation of carbonaceous rock types than was the case for the overlying wildhorn and gellihorn nappes ongoing thrusting brought the doldenhorn nappe closer to the surface the associated cooling resulted in a freezing in of the paleo isotherms of peak metamorphic conditions contemporaneous shearing localized in the basal thrust initially still in the ductile deformation regime and finally as brittle faulting and cataclasis inducing ultimately an inverse metamorphic zonation with ongoing exhumation and the formation of the helvetic antiformal nappe stack a bending of large scale tectonic structures thrusts folds peak temperature isotherms and cooling isotherms occurred while this local bending can directly be attributed to active deformation underneath the section investigated up to times of 2 3 ma a more homogeneous uplift of the entire region is suggested for the very late and still active exhumation stage

shear stress in the graphitization of carbonaceous matter during the low grade metamorphism from the northern parang mountains south carpathians implications to graphite geothermometry abstract low grade metamorphic rocks from the northern parang mountains south carpathians were examined to infer the temperature of metamorphism and to obtain new data to study the applicability of the geothermometers relying on the graphite crystallinity to rocks that underwent different shear stress conditions there were analyzed samples of graphitic carbonaceous matter bearing rocks that exposed dissimilar degrees of strain for which the temperature of metamorphism was calculated using the graphite geothermometers based on the lattice parameter c0 determined through x ray diffractometry and based on the peak area ratio of the first order raman spectrum the temperature was also inferred from the chemical composition of the chlorite and from the fe mg partitioning between chlorite and chloritoid the mineral chemistry being determined through electron microprobe analyses the results obtained have shown i differences of up to more than 150 c between vicinal strained and less strained zones when applying the geothermometers based on the graphite crystallinity and ii considerable discrepancies between the temperatures calculated based on the graphite crystallinity and those based on the chlorite and chloritoid chemical compositions these inconsistencies are most likely caused by the fact that the calibration of the graphite geothermometers considers temperature as being exclusively responsible for enhancing the degree of graphite crystallinity and therefore these graphite geothermometers should be adjusted accordingly by introducing a correction factor that quantifies the degree of strain of the samples analyzed in order to obtain more accurate results

calcite twin widths and intensities as metamorphic indicators in natural low temperature deformation of limestone abstract twins in naturally deformed coarse grained limestones of the northern subalpine chain france show a direct relationship of twin width and intensity number of twins per mm to the peak metamorphism under diagenetic and anchimetamorphic conditions the transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance ro of 1 60 and 2 60 or temperatures of between 150 and 200 c below the transition temperature increasing strain is apparently accommodated by adding new twins and above the transition temperature increasing strain is accommodated by twin widening similar transitions from thin to thick twinning with increasing metamorphism are also present in the helvetic alps the prealps and the central appalachian valley and ridge province these data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern subalpine chain occurred at or near peak metamorphism

deformation conditions in the northern subalpine chain france estimated from deformation modes in coarse grained limestone abstract in low temperature experiments the deformation mode and ductility in a single rock type are most directly controlled by the confining pressure and strain magnitude a graphical technique is developed from published experimental microstructural data that allows for the determination of deformation mode for coarsegrained limestone on the basis of microstructural point count data the deformation mode and strain are used to constrain the confining pressure during thin section scale deformation by comparison with experimental deformation results the technique is applied to coarse grained samples of cretaceous and tertiary limestone from the northern subalpine chain france and reveals that the majority of samples deformed mostly by twinning in the uniform flow mode at minimum depths estimated from deformation mode and strain of between 0 and 6 0 km and maximum depths interpreted from peak metamorphic temperatures and geothermal gradient of about 2 7 9 7 km in the remaining samples brittle failure probably occurred at relatively shallow depths

cooling induced strain localization in carbonate mylonites within a large scale shear zone glarus thrust switzerland abstract the glarus thrust switzerland offers a great field example of strain localization under retrograde conditions along the thrust mylonitic microfabrics are characterized by a temperature stress controlled balance of grain size reducing mechanisms and grain growth consequently mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact in an opposite manner calcite twin densities increase towards the contact cpos are strongest between 0 5 and 15 m away from the thrust but become generally weaker in the last centimeters where also cataclasites occur the cpo weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation the microfabric changes correlate with a decrease in 13c and 18o towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation based on the microfabric changes variations in stable isotopes and cross cut relationships a subdivision into a low and high temperature shear zone can be made the microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling promoting further strain localization of an existing high strain shear zone

evolution of the raman spectrum of carbonaceous material in low grade metasediments of the glarus alps switzerland terra nova 22 354 360 2010 abstract the raman spectrum of carbonaceous material cm from advanced diagenesis 200 c to low grade metamorphism 320 c is documented in the helvetic flysch of the glarus alps switzerland the spectrum is complex with several defect bands at 1200 d4 1350 d1 1500 d3 and 1620 d2 cm 1 we document the evolution of these bands relative to the graphite g band with increasing metamorphic grade and we show that this qualitative evolution may be used as a proxy for temperature in the glarus alps we develop a robust peak fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region further work in other geological contexts is needed to assess whether the spectral evolution observed in the glarus alps may be generalized and to critically assess the potential for calibrating a general empirical and quantitative thermometer based on the raman spectrum of cm in low grade rocks

biomarkers heat up during earthquakes new evidence of seismic slip in the rock record during earthquakes faults heat up due to frictional work however evidence of heating from paleoearthquakes along exhumed faults remains scarce here we describe a method using thermal maturation of organic molecules in sedimentary rock to determine whether a fault has experienced differential heating compared to surrounding rocks we demonstrate the utility of this method on an ancient pseudotachylyte hosting megathrust at pasagshak point alaska measurements of the ratio of thermally stable to thermally unstable compounds diamondoids n alkanes show that the melt bearing rocks have higher thermal maturity than surrounding rocks furthermore the mineralogy of the survivor grains and the presence of any organic molecules allow us to constrain the temperature rise during the ancient earthquakes to 840 1170 c above ambient temperatures of 260 c from this temperature rise we estimate that the frictional work of the earthquake was 105 228 mj m 2 using experimental friction measurements as a constraint we estimate that the minimum slip necessary for heating was 1 8 m this paper demonstrates that biomarkers will be a useful tool to identify seismic slip along faults without frictional melt

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

coal maturation by frictional heat during rapid fault slip 1 the detection of frictional heating effects along faults provides key insight into the dynamics of earthquakes and faulting thermal maturity of organic matter has been considered a possible fault thermometer that records the frictional heat signature of ancient earthquakes however whether or not organic matter can mature on the order of seconds typical earthquake rise time remains uncertain here we present the results of experiments aimed at revealing coal maturation by frictional heat generated at slip velocities representative of natural earthquakes of up to 1 3 m s our results show that coal can mature coseismically in 11 seconds at temperatures induced by frictional heat ranging from 26 to 266 c even with a temperature rise to only 28 7 c over 15 m displacement in 3 2 hours coal can slightly mature within a shear localized zone the commonly used kinetic model of vitrinite maturation cannot predict the experimental results a kinetic model involving the effect of flash temperature at grain contacts and mechanochemical effects on reaction kinetics is necessary to better estimate heat generation along a fault

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

grain size stabilisation by dispersed graphite in a high grade quartz mylonite an example from naxos greece high grade quartz mylonites from naxos greece consist of alternating thin layers of pure quartz and quartz layers with 0 3 3 vol finely dispersed graphite particles graphite free layers are coarse grained 100 300 mm show undulose extinction subgrains lobate grain boundaries and have a strongly developed crystallographic preferred orientation in these layers dislocation flow is interpreted to be the dominant deformation mechanism in contrast graphite rich layers are fine grained 30 70 mm have equant quartz grain shapes and have a crystallographic preferred orientation that becomes progressively weaker with decreasing quartz grain size given the high temperature of deformation and the need for a c axis fabric destroying mechanism grain boundary sliding is interpreted to be important in these layers analysis shows an inverse relationship between quartz grain size and the graphite dispersion suggesting stabilization of quartz grain size by graphite particles graphite particles larger than 5 mm are concentrated along quartz boundaries suggesting that stabilisation only operates above a certain critical graphite particle size this study shows that a dispersed second phase such as graphite in a naturally deforming rock can inhibit grain boundary migration stabilise the grain size and enhance grain boundary sliding at the expense of dislocation flow q 2002 elsevier science ltd all rights reserved

diagenetic thermal evolution of organic matter by raman spectroscopy abstract vitrinite reflectance is considered one of the most reliable indicators of thermal maturity in sedimentary basins and is widely used to calibrate numerical models that assess hydrocarbon generation expulsion from source rocks nevertheless ambiguities in vitrinite reflectance can occur from variations in composition and preservation state of organic facies temperature pressure conditions and the abundance of organic matter resulting in incorrect or uncertain assessments of petroleum systems this study presents an original application of raman spectroscopy to assess the diagenesis catagenesis of kerogen between the immature and mid mature stages of hydrocarbon generation kerogens were isolated from 33 intervals of drill cuttings from a 5 km thick oligocene miocene siliciclastic section of the malembo formation lower congo basin angola their raman spectra were obtained and derived parameters were compared to depth and to previously reported equivalent vitrinite reflectance values the best correlations between thermal maturity and raman parameters were found for d g band distance fwhm of the g band d g area ratio ra2 ratio calculated as s dl d dr gl g band ratio and the d g width ratio the raman parameters were not influenced by varying kerogen composition mixed type ii iii the technique offers the potential to reduce the risk of source rock thermal maturity assessment even when organic facies rich in amorphous organic matter are present

shear induced graphitization of carbonaceous materials during seismic fault motion experiments and possible implications for fault mechanics abstract carbonaceous materials often concentrate in fault zones developed in pelitic rocks among carbonaceous minerals graphite is known as a lubricant and possibly plays a key role in frictional properties of the fault graphite reported from slip localized zones suggests that graphitization can occur during seismogenic fault motion thus we performed friction experiments on amorphous carbon and graphite to investigate how graphite forms in association with fault motion and how these carbonaceous minerals affect frictional properties of faults experiments were done at normal stresses of 0 5 2 8 mpa and slip rates of 50 m s to 1 3 m s in atmospheres of air and n 2 gas using rotary shear apparatuses xrd and tem analyses revealed that graphitization can indeed occur during seismogenic fault motion perhaps due to large shear strain short lived flash heating and stress concentration at asperity contacts even at low temperatures and pressures under anoxic environments we found large differences in steady state friction coefficient ss between graphite ss 0 1 and amorphous carbon ss 0 54 at low slip rate but amorphous carbon exhibits marked velocity weakening at slip rate above 10 mm s and its steady state friction reduces to the same level as that of graphite at a slip rate of 1 3 m s faults with amorphous carbon are not weak at low slip rates but they can become dynamically weak to foster fault motion during the generation of large earthquakes enriched graphite in fault zones can lubricate at all slip rates even at great depths and should receive more attention

vitrinite reflectance and shear induced graphitization in orogenic belts a case study from the kandersteg area helvetic alps switzerland vitrinite reflectance vr is an important indicator of incipient metamorphism and it is generally assumed that temperature and to a lesser degree time are the two main variables the role of stress in vr is controversial and will be addressed in this study in the kandersteg area north of lake oeschinen four different helvetic tectonic units are present comprising mainly limestones marls and greywackes of mesozoic and lower tertiary age vr rr rmax rmin and illite crystallinity ic were determined from a very steep section at elevations between 1700 and 2900 m a s l in general rr rmax and ic increase from tectonically higher to lower units i e from upper diagenetic 2 3 to 4 5 rr to upper anchizonal conditions 3 4 to 5 6 rmax several samples contain in addition to coaly fragments abundant semigraphitized and graphitized particles that are believed to be of authigenic origin transitional matter showsrmax values ranging from 7 to about 11 and rmin of 1 3 3 4 optical graphite showsrmax of about 15 16 and rmin of 1 6 1 8 there is good textural evidence that tectonic deformation has played a major role in the development of these strongly coalified particles the first tiny needle like crystals of graphitic material occur within narrow shear zones transecting the rock matrix the beginning of graphite formation parallels the onset of plastic deformation and dynamic recrystallization of quartz elevated vitrinite reflectance rmax and bireflectance values are found near thrust planes where simple optical observation on quartz grain morphology indicates an increase in strain intensity it appears that frictional heating associated with thin shear zones may have been responsible for the formation of authigenic graphite in the kandersteg samples

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

area balancing as a test of models for the deep structure of mountain belts with specific reference to the alps abstract basic concepts of structural restoration are applied to crustal cross sections through mountain belts to explore large scale tectonic models and deep structure however restored sections should account for variations in pre orogenic crustal thicknesses crustal balancing approaches are reviewed and applied to two alpine sections coinciding with deep seismic experiments nrp 20 east central alps and ecors crop western alps existing studies assume large 300 km orogenic contraction and only moderately thinned pre orogenic crust the resulting restored sections contain more crust than is imaged beneath the present day alps the missing crust generally assumed to be subducted two kinematic modifications reduce the requirement for subduction thinning and buoyancy driven return flow of ultra high pressure metamorphic rocks during orogenesis and pre orogenic hyperextension using large stretching factors for the pre orogenic crust negates crustal subduction on both alpine transects if the lower crust was approximately rigid restorations of the central alps require strongly depth heterogeneous stretching of upper and lower crust during mesozoic rifting relaxing this requirement allows uniform lithospheric stretching a corollary consistent with published subsidence estimates restorations make implicit statements on the form of pre orogenic basins and the structure of continental margins incorporated into mountain belts that can in turn provide tests of tectonic models

graphitization in a high pressure low temperature metamorphic gradient a raman microspectroscopy and hrtem study the graphitization of carbonaceous material cm in a high pressure metamorphic gradient is characterized along a cross section in the schistes lustres formation western alps along this 25 km cross section both the cm precursor and the host rock lithology are homogeneous and the prograde evolution of the pressure temperature metamorphic conditions from the lower blueschist facies 13 kbar 330 c to the eclogite facies 20 kbar 500 c is tightly constrained by literature data raman microspectroscopy shows that at the micrometre scale this process is progressive and continuous with increasing metamorphic grade and that the structure of cm is very sensitive to temperature variations at the nanometre scale hrtem the cm is composed of a mixture of a microporous phase and an onion ring like phase both known as non graphitizing under the effect of temperature at ambient pressure the hp lt graphitization produces structurally and microtexturally heterogeneous cm with increasing metamorphic grade the graphitization of the two types of cm proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of cm the microporous material is progressively transformed into graphite through a macroporous transitional stage in this case graphitization mainly occurs on the pore walls as a result of pore growth in the case of concentric onion ring like material graphitization occurs in the regions with the largest radius of curvature i e on the outer part of the ring in comparison with 1 bar experiments pressure seems to induce microtextural changes which allows the subsequent structural modifications of the starting material

deformation of olivine at 5gpa and 350 900 c abstract knowledge of the rheology of the upper mantle is important for understanding both subduction related phenomena and mantle flow in general here we report the results of stress relaxation experiments on san carlos olivine at pressures between 5 3 and 1 3 gpa and from 350 to 900 c at 5 gpa and 500 c the deformation best follows a model governed by long range resistance to movement of dislocations the yield stress of olivine has been measured at low strain rates of 10 7 s 1 we conclude that the upper limit for the long term yield stress of olivine at 500 c is 68 180 44 mpa we observe significant weakening and infer an associated change in deformation mechanism in olivine between 500 and 730 c this change is coincident with the limit of the seismogenic layer and the greatest elastic thickness measurements in oceanic crust

the crucial role of temperature in high velocity weakening of faults experiments on gouge using host blocks with different thermal conductivities we study the important role of temperature rise in the dynamic weakening of fault gouge at seismic slip rates by using host blocks composed of brass stainless steel titanium alloy and gabbro with thermal conductivities h of 123 15 5 8 and 3 25 w m k respectively our experiments are performed mostly on fault gouge collected from the longmenshan fault sichuan china consisting primarily of illite and quartz high velocity weakening of gouge becomes more pronounced as h decreases because the temperature in the gouge increases microstructure observations reveal welded slip zone material and more compact slip surfaces for the gouge deformed with low h host blocks which is probably caused by a sintering process indicative of higher temperatures these conclusions are supported by temperature calculation performed using the finite element method the observed differences in frictional behaviors deformation microstructures and calculated temperature demonstrate that temperature rise driven by frictional heating is essential in causing dynamic weakening of gouge at seismic velocities we show that our data are in good agreement with the flash heating model though thermochemical pressurization may also be important some of our experiments where nanoparticles are present but show negligible weakening demonstrate that the presence of nanoparticles alone is not sufficient to cause dynamic weakening of faults

reduction of friction on geological faults by weak phase smearing most common crustal rock types display friction coefficients of 0 6 or higher but some faults must be frictionally weak as they slip when the stress state is unfavourably oriented i e the resolved shear stress is low for a given normal stress across the fault surface a role for low friction minerals and high pore fluid pressures either separately or in combination is frequently invoked to explain such slip but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening by means of mechanical tests on synthetic fault gouge and microstructural study of run products we show that the effective area of an embedded weak phase graphite on a slip plane can be substantially increased by mechanical smearing and that the enlarged area of the weak phase on the slip plane follows a linear mixing law this allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them

source rock maturation owing to igneous intrusion in rifted margin petroleum systems intrusive and extrusive volcanism is a common feature associated with many sedimentary basins formed along rifted margins as a result igneous intrusions are often found in potential hydrocarbon producing source rock horizons although direct thermal effects on source rocks have been studied in detail few studies have dealt with understanding the thermal effects on source rocks across a range of different thermal histories prior to intrusion the metamorphic aureoles of three intrusions have been characterized for source rock maturation using organic geochemistry and raman spectroscopy results show that a unique combination of organic geochemical and raman spectroscopic techniques adds further insight to intrusion and host rock interactions with an effective oil window and extended gas window located in the aureole of an immature and mature source rock respectively such varied and potentially beneficial geochemical responses further the need to re evaluate source rock intervals in rifted margin basins

the occurrence of graphite bearing fault rocks in the atotsugawa fault system japan origins and implications for fault creep abstract graphite in fault zones has received little attention even though it is a well known solid lubricant that could affect frictional properties of faults dramatically this paper reports the presence of abundant graphite in fault zones of the atotsugawa fault system central japan mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones one is a pressure solution process or diffusive mass transfer in general which removes water soluble minerals such as quartz and carbonates from rocks resulting in the enrichment of insoluble minerals including carbon the other process is precipitation of graphite from a high temperature carbon rich fluid forming graphite filling fractures within cataclasitic fault zones the two processes have led to the concentration up to 12 wt of graphite in the atotsugawa fault zones compared to 0 to 3 wt of carbonaceous materials in the host rocks this concentration is high enough for graphite to affect frictional properties at wide range of slip rates the presence of graphite may provide an explanation for the low resistivity the patterns of microearthquakes and fault creep along the western part of the atotsugawa fault system graphite should receive more attention as a weakening and stabilizing agent of faults

the role of veining and dissolution in the evolution of fine grained mylonites the mcconnell thrust alberta microstructural observations of the mcconnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the mcconnell thrust limestone mylonites the protolith to the mylonite is a micritic limestone the mylonite contains numerous discontinuous bedding parallel calcite veins that range from relatively undeformed except for twinning and undulose extinction to highly sheared the highly sheared veins show evidence for dynamic recrystallization and grain size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately 1 3 m the bulk of the calcite matrix within the mcconnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins the emplacement of relatively coarse grained veins promoted the activation of dislocation creep within the fault rocks of the thrust in the absence of the increase in grain size afforded by vein emplacement it is anticipated that the fine grained protolith would have deformed predominantly by solution transfer with grain boundary sliding and localized brittle shearing the layer parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite these clay seams may have contributed to the build up of high pore fluid pressures which if intermittently reaching lithostatic conditions during displacement would result in transient brittle failure and thrust surface parallel vein emplacement in an otherwise ductilely deforming rock

carbon ordering in an aseismic shear zone implications for raman geothermometry and strain tracking abstract determining the burial and strain history of sedimentary rocks is important for understanding crustal behaviour when rocks contain organic carbon increased temperatures at depth can alter the molecular structure of the carbon this change known as carbon ordering can be detected using raman spectroscopy as a result raman spectroscopy is increasingly used to estimate burial depths and associated maximum temperatures in carbon bearing rocks it is known from experiments and natural samples that other factors can affect raman derived maximum temperatures including frictional heating on fault planes and interaction with hot fluids for faulted samples a question remains as to whether it is purely frictional heating that causes carbon ordering or strain or a combination of the two in this study we use a mid crustal shear zone to show that strain related carbon ordering occurs in natural rock samples during aseismic shear strain a traverse across the shear zone whose relative strain we quantify with respect to the surrounding less deformed rock shows a marked decrease in raman d g peak intensity ratios indicating greater carbon ordering within the shear zone we interpret this as evidence for carbon ordering as a result of aseismic shear strain rather than inflated temperatures due to frictional heating commonly associated with seismic strain rates on faults our results have implications for the further development of raman spectroscopy as a geothermometer which may yield erroneous results in strained rock samples and for understanding strain localisation processes in the earth s crust and its associated rheological implications

gouge graphitization and dynamic fault weakening during the 2008 mw 7 9 wenchuan earthquake the longmenshan fault that ruptured during the 2008 mw 7 9 wenchuan china earthquake was drilled to a depth of 1200 m and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575 595 m we report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths results indicate that the longmenshan fault at this locality is extremely weak at seismic slip rates in situ synchrotron x ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products similar to the occurrence of graphite in the natural principal slip zone of the 2008 wenchuan rupture we surmise that graphitization occurred due to frictional heating of carbonaceous minerals because graphitization was associated with strong dynamic weakening in the experiments we further infer that the longmenshan fault was extremely weak at borehole depths during the 2008 wenchuan earthquake and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust

raman spectroscopy of selected carbonaceous samples abstract this paper presents the results of raman spectra measured on carbonaceous materials ranging from greenschist facies to granulite facies graphite anchimetamorphism and epimetamorphism zones raman spectroscopy has come to be regarded as a more appropriate tool than x ray diffraction for study of highly ordered carbon materials including chondritic matter soot polycyclic aromatic hydrocarbons and evolved coal samples this work demonstrates the usefulness of the raman spectroscopy analysis in determining internal crystallographic structure disordered lattice heterogeneity moreover this methodology permits the detection of differences within the meta anthracite rank semi graphite and graphite stages for the samples included in this study in the first order raman spectra the bands located near to c a 1350 cm 1 defects and disorder mode a 1g and 1580 cm 1 in plane e 2g zone centre mode contribute to the characterization and determination of the degree of structural evolution and graphitization of the carbonaceous samples the data from raman spectroscopy were compared with parameters obtained by means of structural chemical and optical microscopic analysis carried out on the same carbonaceous samples the results revealed some positive and significant relationships although the use of reflectance as a parameter for following the increase in structural order in natural graphitized samples was subject to limitations

determination of low b ca ratios in carbonates using icp qqq the very low b ca ratios characteristic of some natural biogenic carbonates are of interest for research in ocean acidification but represent an analytical challenge we describe a method using a novel instrument configuration icp qqq for which we are not aware of any previously published geological applications and for coccoliths a sample type unique in its low b content and organic phases detection limits as low as 0 41 mol mol were achieved isobaric interferences out of the reach even for sf icp ms can be solved using this instrument which permits the safe measurement of the lowest abundance ca isotope 46ca this allows maximizing the b concentration measured matrix concentration up to 800 ppm ca while maintaining both b and ca signals in counting mode more significantly for low b samples the icp qqq is also able to overcome the interference of the ubiquitous 12c tail on the 11b mass which otherwise leads to significant overestimates at very low b concentrations this could be a reason for the significantly lower b ca ratios observed for the low b content interlaboratory calibration standards carrara and oka while matching for the high b content standards was good finally results obtained in the analysis of coccoliths grown in laboratory culture seems to corroborate that sims analysis of the samples mounted in indium leads also to b ca overestimates due to porosity effects as previously observed using la icp ms this approach also permits the interference free measurement of p ca and s ca ratios which could be used as indicators of the complete removal of the organic matter from the samples

strain induced amorphization of graphite in fault zones of the hidaka metamorphic belt hokkaido japan abstract we report here on two distinct processes of deformation affecting graphite in fault zones of the hidaka metamorphic belt hokkaido japan one process involves the micrometer scale delamination mmd in the stacking of graphite and the other is the nanometer scale delamination nmd in the stacking plus pulverization p of carbon sheets to less than 10 nm graphite in mylonites mainly glide along the 002 planes and they exhibit layer separation kink banding and bending structures produced by mmd processes whereas nmd p processes induce a significant size reduction and amorphization of the graphitic structures in cataclasites ultracataclasites and pseudotachylytes the strain induced amorphization of graphite as inferred from xrd and micro raman spectroscopy corresponds well with nano to micro scale deformation structures observed under the hrtem suggesting a systematic increase of edge planes by interlayer delamination and pulverization of carbon sheets in addition the carbon isotopic compositions of the graphite in fault rocks have negative 13 c signatures indicating a sedimentary organic origin such graphite has a tendency to accumulate within the shear bands our data suggest that the graphitic structures record information on both the peak metamorphic temperatures and the deformation processes and they show the progressive amorphization with increasing brittle deformation

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

vitrinite reflectance and raman spectra of carbonaceous material as indicators of frictional heating on faults constraints from friction experiments abstract vitrinite reflectance r o and raman spectra of carbonaceous material rscm are both widely used as indicators of the maximum attained temperatures in sedimentary and metamorphic rocks however the potential of these methods to estimate temperature increases associated with fault slip has not been closely studied to examine this issue friction experiments were conducted on a mixture of powdered clay rich fault material and carbonaceous material cm at slip rates of 0 15 mm s and 1 3 m s in nitrogen n 2 gas with or without distilled water after the experiments we measured r o and rscm and compared to those in starting material the results indicate that when fault material suffers rapid heating at 500 c in 9 s at 1 3 m s r o and the intensity ratio of d1 and d2 raman bands of cm i d2 i d1 markedly increase comminution with very small temperature rise in 32 min at 0 15 mm s is responsible for very limited changes in r o and i d2 i d1 our results demonstrate that r o and rscm could be useful for the detection of frictional heating on faults when the power density is 0 52 mw m 2 however the conventionally used r o and rscm geothermometers are inadequate for the estimation of peak temperature during seismic fault slip the reaction kinetics incorporating the effects of rapid heating at high slip rates and studies of the original microtexture and composition of cm are required to establish a reliable thermometer for frictional heating on faults