a frictional law for volcanic ash gouge abstract volcanic provinces are structurally active regions undergoing continual deformation along faults within such fault structures volcanic ash gouge containing both crystalline and glassy material may act as a potential fault plane lubricant here we investigate the frictional properties of volcanic ash gouges with varying glass fractions using a rotary shear apparatus at a range of slip rates 1 3 1300 mm s and axial stresses 0 5 2 5 mpa we show that the frictional behaviour of volcanic ash is in agreement with byerlee s friction law at low slip velocities irrespective of glass content the results reveal a common non linear reduction of the friction coefficient with slip velocity and yield a frictional law for fault zones containing volcanic ash gouge textural analysis reveals that strain localisation and the development of shear bands are more prominent at higher slip velocities 10 mm s the textures observed here are similar to those recorded in ash gouge at the surface of extrusive spines at mount st helens usa we use the rate weakening component of the frictional law to estimate shear stress resistance reductions associated with episodic seismogenic slip events that accompany magma ascent pulses we conclude that the internal structure of volcanic ash gouge may act as a kinematic marker of exogenic dome growth

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

deformation fabrics and strain localization mechanisms in graphitic carbon bearing rocks from the ailaoshan red river strike slip fault zone abstract graphitic carbon bearing rocks generally occur in low to high grade metamorphic units in many brittle faults graphitic carbon is often associated with gouge or low grade metamorphic rocks whereas in ductile faults graphitic carbon commonly occurs in marble schist or gneiss carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock in this contribution based on detailed field observations the variably deformed and metamorphosed graphitic gneisses to phyllites located within the footwall and hangingwall unit of the cenozoic ailaoshan red river strike slip shear zone are studied according to lithological features and temperatures determined by raman spectra of carbonaceous material these graphitic rocks and deformation fabrics are divided into three types type i is represented by medium grade metamorphism and strongly deformed rocks with an average temperature of 509 c and a maximum temperature of 604 c type ii is affected by low grade metamorphism and deformed rocks with an average temperature of 420 c type iii is affected by lower grade metamorphism and occurs in weakly deformed undeformed rocks with an average temperature of 350 c slip localized micro shear zones and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in types i and ii the electron back scattered diffraction ebsd lattice preferred orientation lpo patterns of graphitic carbon grains were firstly observed in comparison with lpo patterns of quartz and switch from basal rhomb to prism slip systems which indicate increasing deformation temperatures comparison of quartz and graphite lpos indicates that graphite lpos show higher temperature conditions according to the graphitic slip planes micro shear zones with grain size reduction and mylonitic foliation constituted by graphitic carbon minerals we also propose that the development of fine grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing

coseismic extension recorded within the damage zone of the vado di ferruccio thrust fault central apennines italy abstract recent high resolution hypocentral localisation along active fault systems in the central apennines illuminates the activation of seismogenic volumes dipping at low angle here we report field and microstructural evidence of seismogenic extensional faults localized within pre existing thrust fault zones the vado di ferruccio thrust fault vftf is a narrow fault zone

weak faults rotten cores earthquakes are triggered when crustal deformation produces displacements along fault zones the rock in these zones is found to be weaker than adjacent material but what are the mechanisms of this loss of strength in his perspective holdsworth 1 discusses how recent studies of exposed ancient fault zones combined with laboratory analyses are revealing that rock properties at the grain scale can determine the large scale behavior of crustal deformation 1 http www sciencemag org cgi content full 303 5655 181

frictional properties of natural gouges from longmenshan fault zone ruptured during the wenchuan mw7 9 earthquake abstract we investigated the properties of frictional sliding of natural gouges collected from longmenshan fault zone which ruptured during the wenchuan mw7 9 earthquake based on xrd analysis five gouge samples of different mineralogical assemblies and mineral contents are chosen for frictional sliding experiments with initial confining pressure of 50 mpa and at different temperatures from 25 c to 150 c to obtain the rate dependence of friction shearing rates are stepped up and down in the range of 0 0488 to 6 1 m s while the friction coefficient of most gouge samples decreases with the clay content following a general trend consistent with previous studies the organic matter bearing sample px 3 shows a much lower value than the prediction by the general trend indicating further strength reduction due to the 9 organic matter included we also found a significant effect of temperature on the sliding behavior of organic matter bearing and carbonate rich gouges when temperature increases to 150 c the px 3 sample with organic matter exhibits unstable oscillation that indicates velocity weakening frictional sliding behavior in contrast to the velocity strengthening behavior at lower temperatures the velocity dependence of carbonate rich samples px 1 px 2 with more than 50 wt carbonates is also sensitive to temperature and shearing rates and both samples have smaller velocity dependence of friction a b at higher temperatures the a b value of the px 2 sample with 80 carbonates even decreases to the velocity neutral point at 150 c and slow shearing rates based on the rate and state friction framework fast slip nucleation is possible for faults associated with the organic matter bearing gouge at 150 c whereas triggered seismic slips are possible in faults that include the carbonate rich samples which have a small a b value at 150 c the temperature dependent sliding behavior of carbonate dominant and organic matter bearing gouges probably provides a new clue to the problem of transition from shallow aseismic zone to seismic zone in subduction interfaces of the oceanic plate

on the role of phyllosilicates on fault lubrication insight from micro and nanostructural investigations on talc friction experiments 1 exposures of mature faults at the earth s surface show that in the shallow crust some of co seismic slip occurs within a narrow phyllosilicate rich gouge material mechanical behavior of phyllosilicates sheared at seismic slip velocities remains a complex issue as suggested by the experimental and theoretical studies reported so far talc represents a simple case for common phyllosilicates in natural faults and therefore we chose talc as an analogue to better understand the mechanical behavior of clay rich mature crustal faults we present a series of high velocity friction experiments conducted on talc at 1 31 m s 1 and normal stresses of 0 3 1 8 mpa for wet and dry conditions at 1 mpa normal stress both wet and dry experiments show a slip weakening behavior however with a higher decreasing rate in wet conditions dynamic shear stress evolves from a peak value of 0 24 0 52 mpa down to a residual state value of 0 08 0 18 mpa in wet conditions and from a peak value of 0 46 1 17 mpa down to a residual state value of 0 12 0 62 mpa in dry conditions based on a detailed microstructural analysis down to the nanoscale we propose thermal pressurization as a possible slip weakening mechanism for wet conditions on the contrary in dry conditions the long lasting weakening is interpreted to be due to a combination of i progressive disappearance of geometrical incompatibilities ii solid lubrication of talc lamellae and iii powder lubrication by nanometric aggregates we conclude that initial gouge humid conditions and inherited fabric from past sliding may have large influence on the slip weakening for subsequent slip

crystallographic controls on the frictional behavior of dry and water saturated sheet structure minerals 1 we compare the frictional strengths of 17 sheet structure mineral powders measured under dry and water saturated conditions to identify the factors that cause many of them to be relatively weak the dry coefficient of friction ranges upward from 0 2 for graphite leveling off at 0 8 for margarite clintonite gibbsite kaolinite and lizardite the values of dry correlate directly with calculated 001 interlayer bond strengths of the minerals this correlation occurs because shear becomes localized along boundary and riedel shears and the platy minerals in them rotate into alignment with the shear planes for those gouges with dry 0 8 shear occurs by breaking the interlayer bonds to form new cleavage surfaces where dry 0 8 consistent with byerlee s law the interlayer bonds are sufficiently strong that other frictional processes dominate the transition in dry friction mechanisms corresponds to calculated surface energies of 2 3 j m2 adding water causes to decrease for every mineral tested except graphite if the minerals are separated into groups with similar crystal structures wet increases with increasing interlayer bond strength within each group this relationship also holds for the swelling clay montmorillonite whose water saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure water in the saturated gouges forms thin structured films between the plate surfaces the polar water molecules are bonded to the plate surfaces in proportion to the mineral s surface energy and wet reflects the stresses required to shear through the water films

experimental frictional heating of coal gouge at seismic slip rates evidence for devolatilization and thermal pressurization of gouge fluids high velocity 1 m s friction experiments on bituminous coal gouge display several earthquake related phenomena including devolatilization by frictional heating gas pressurization and slip weakening stage i is characterized by sample shortening and reduction in the coefficient of friction from 1 to 0 6 stage ii is characterized by high frequency 5 hz oscillations in stress and strain records and by gas emissions stage iii is marked by rapid weakening 0 1 to 0 35 and sample shortening together with continued gas emissions stage iv produces stable stress records and continued weakness 0 2 but without gas emission stage i shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects stage ii behavior is interpreted as due to coal gasification and fluctuations in fluid pressure resulting in high frequency stick slip type behavior dramatic reduction in shear stress in stage iii is attributed to gas pressurization by pore collapse and corresponds to a frictional instability analogous to nucleation of an earthquake microstructural observations indicate the deformation was brittle during stages i and ii but ductile during stages iii and iv time dependent finite element frictional heat models indicate the center of the samples became hot 900 c during stage ii whereas the edge of samples remained relatively cold 300 c vitrinite reflectance of coal samples shows an increase in reflectance from 0 5 to 0 8 over the displacement interval 20 40 m 20 40 s indicating that the reflectance responds to frictional heating on a short time scale the energy expended per unit area in these low stress large displacement experiments is similar to that of higher stress 50 mpa short displacement 1 m earthquakes 107 j m2

textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone alpine fault new zealand graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization we examine processes of graphitic carbon emplacement and deformation in the active alpine fault zone new zealand by analysing samples obtained from deep fault drilling project dfdp boreholes optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain raman spectroscopy allowed interpretation of the degree of graphite crystallinity which reflects both thermal and mechanical processes in the amphibolite facies alpine schist highly crystalline graphite indicating peak metamorphic temperatures up to 640 c occurs mainly on grain boundaries within quartzo feldspathic domains the subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions 500 600 c resulting in clustered in protomylonites and foliation aligned graphite in mylonites in cataclasites derived from the mylonitized schists graphite is most abundant 50 as opposed to 10 elsewhere and has two different habits inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon tectonic hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences the increasing abundance of graphite towards the fault zone core may be a significant source of strain localization allowing fault weakening

extreme dynamic weakening of faults during dehydration by coseismic shear heating the dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release high velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0 6 to 0 15 as the slip velocity reaches 1 1 m s at normal stresses up to 24 5 mpa the extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding the rapid heating at asperities causes serpentine dehydration in impermeable fault zones in nature serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening this dynamic fault weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the san andreas

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

frictional strength and wear rate of carbonate faults during high velocity steady state sliding article history we ran an extensive series of shear experiments to test the effect of shear velocity and normal stress on wear rate and frictional strength the experiments were conducted on three types of carbonate samples with a rotary shear apparatus on solid ring shaped rock samples that slipped for displacements up to tens of meters at slip velocity of v 0 002 0 96 m s and normal stress n 0 25 6 9m pa the analysis reveals that during steady state stage the values of wear rate and frictional strength depend on both slip velocity and normal stress the wear rates at low slip velocity show linear relations to the normal stress archard s model however at high velocity v 0 5m s the wear rates are independent of the normal stress and may vanish at the highest velocity and normal stress of the present experiments the steady state friction coefficient correlates best with the experimental power density shear stress slip velocity we recognized three friction regimes high 0 8a t low power density low 0 3 at high power density and a transition regime of fast drop of friction coefficient as the power density increases from 0 03 to 0 3 mw m 2 runs of low power density high friction displayed fault surfaces covered with fine grained gouge whereas runs of high power density low friction displayed shiny smooth fault surfaces we interpret the observed intensity variations of wear rate and frictional strength as indicating a brittle to ductile transition associated with frictional heating

deep crustal roots beneath the northern apennines inferred from teleseismic receiver functions abstract teleseismic waveforms recorded by a regional array crossing the northern italian peninsula and northern corsica are analyzed using the receiver function technique to determine the first order crustal structure the receiver function approach is used to isolate receiver side ps conversions generated at the crust mantle boundary and any major velocity discontinuity beneath the stations we used the time delay between the direct p wave and the ps wave converted at the moho discontinuity to infer crustal thickness beneath the stations the crust mantle boundary is estimated at 25 1 km of depth in northern corsica 20 2 km beneath the elba island and 20 to 24 2 km beneath tuscany in the eastern portion of the array moho depth increases from 28 2 km beneath the adriatic coast to 49 3 km beneath the apennine chain in a distance of about 100 km a double ps conversion produced beneath the val tiberina graben to the west of the apennines corresponds to interfaces as deep as 20 2 and 52 2 km this observation supports the hypothesis of partial overlapping between the shallow tuscan moho and the deeper adriatic moho the westward deepening of the adriatic moho beneath the northern apennines can be explained by lithosphere delamination that has dragged downward the adriatic lower crust the deep crustal root estimated beneath the northern apennines indicates that this portion of the chain is isostatically overcompensated

thermal alteration of pyrite to pyrrhotite during earthquakes new evidence of seismic slip in the rock record seismic slip zones convey important information on earthquake energy dissipation and rupture processes however geological records of earthquakes along exhumed faults remain scarce they can be traced with a variety of methods that establish the frictional heating of seismic slip although each has certain assets and disadvantages here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep sea sediments cored from the japan trench one of the seismically most active regions around japan during the integrated ocean drilling program expedition 343 the japan trench fast drilling project fault zone sediments and adjacent host sediments were analyzed mineral magnetically supplemented by scanning electron microscope observations with associated energy dispersive x ray spectroscopy analyses the presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at 697 720 and 801 m below sea floor in the frontal prism sediments while it is absent in the adjacent host sediments elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite the presence of pyrrhotite in combination with pyrite to pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800 c the integrated mineral magnetic microscopic and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature

the origin and hydrothermal mobilization of carbonaceous matter associated with paleoproterozoic orogenic type gold deposits of west africa abstract the chemical and physical properties of carbonaceous matter cm were studied in paleoproterozoic metasediment hosted orogenic type gold deposits in burkina faso the inata deposit in mali the syama deposit and in ghana the obuashi and bogoso deposits two types of cm occur in all the studied deposits metamorphosed and hydrothermal metamorphosed cm prevails in all the deposits hydrothermal cm occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub parallel to c type cleavage within the shear zones the origin of hydrothermal cm which occurs in paragenesis with au bearing arsenopyrite or pyrite seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels the isotopic composition of carbon in bulk cm 33 1 to 26 2 vpdb indicates its biogenic origin the isotopic composition of carbon in hydrothermal carbonates ranges from 14 5 to 4 4 vpdb which suggest mixing of carbon derived from a deep seated source with carbon derived from an organic source the interaction of hydrothermal fluids with metamorphosed cm could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization the optical properties and raman spectra of the metamorphosed cm particles in the individual studied mineral deposits differ considerably the temperatures calculated on the basis of the raman spectra of metamorphosed cm vary between 280 and 440 c depending on the thermometer used and correspond to temperatures of metamorphism of upper sub grenschist and greenschist facies the temperatures calculated for hydrothermal cm at the individual deposits are only slightly lower compared to the metamorphosed cm at the same deposits which indicates approximately the same temperature of the metamorphic and hydrothermal processes

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

carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan abstract carbonaceous material in pelitic schists of the sanbagawa metamorphic belt in central shikoku japan was separated from the host rocks and its x ray diffraction and chemical composition were studied its crystal structure and chemistry change continously with increasing metamorphic grade and approach those of well ordered graphite near the biotite isograd as graphitization is a rate process the temperature of complete graphitization differs from one metamorphic terrain to another as a function of the duration of metamorphism in an individual metamorphic terraan however the degree of graphitization is a useful indicator of relative metamorphic temperature in lower grade rocks

microstructures in landslides in northwest china implications for creeping displacements abstract microstructures mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the gansu province china were examined with optical microscopy transmission electron microscopy tem x ray diffraction xrd and synchrotron x ray diffraction measurements common sheet silicates are chlorite illite muscovite kaolinite pyrophyllite and dickite other minerals are quartz calcite dolomite and albite in one sample graphite and amorphous carbon were detected by tem edx analyses and tem high angle annular dark field images the occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the suoertou and xieliupo landslides is particularly significant for reducing the frictional strength of the landslides it is proposed that the landslides underwent comparable deformation processes as fault zones the low friction coefficients provide strong evidence that slow moving landsliding is controlled by the presence of weak minerals in addition tem observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion for calcite and quartz grain size reduction was attributed to both pressure solution and cataclasis therefore besides landslide composition the occurrence of ultrafine grained slip zone material may also contribute to weakening processes of landslides tem images of slip zone samples show both locally aligned clay particles as well as kinked and folded sheet silicates which are widely disseminated in the whole matrix small newly formed clay particles have random orientations based on synchrotron x ray diffraction measurements the degree of preferred orientation of constituent sheet silicates in local shear zones of the suoertou and duang he ba landslide is strong this work is the first reported observation of well oriented clay fabrics in landslides

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

elemental and mineralogical analyses using geochemical logs from the cajon pass scientific drillhole california and their preliminary comparison with core analyses estimates of elemental and mineralogical abundances from geochemical logs are compared to preliminary chemical and modal analyses from cores in the cajon pass scientific drillhole accuracies of log computed weight percent oxide and mineralogy determinations range from 10 to 30

raman spectra of carbonaceous materials in a fault zone in the longmenshan thrust belt china comparisons with those of sedimentary and metamorphic rocks abstract we analyzed micro raman spectra of carbonaceous materials cm in natural and experimentally deformed fault rocks from longmenshan fault zone that caused the 2008 wenchuan earthquake to characterize degree of disordering of cm in a fault zone raman spectral parameters for 12 samples from a fault zone in shenxigou sichuan china all show low grade structures with no graphite low crystallinity and 13c values 24 to 25 suggest that cm in fault zone originated from host rocks late triassic xujiahe formation full width at half maximum values of main spectral bands d1 and d2 and relative intensities of two subbands d3 and d4 of cm were variable with sample locations however raman parameters of measured fault rocks fall on established trends of graphitization in sedimentary and metamorphic rocks an empirical geothermometer gives temperatures of 160 230 c for fault rocks in shenxigou and these temperatures were lower for highly sheared gouge than those for less deformed fault breccia at inner parts of the fault zone the lower temperature and less crystallinity of cm in gouge might have been caused by the mechanical destruction of cm by severe shearing deformation or may be due to mixing of host rocks on the footwall cm in gouge deformed in high velocity experiments exhibits slight changes towards graphitization characterized by reduction of d3 and d4 intensities thus low crystallinity of cm in natural gouge cannot be explained by our experimental results graphite formation during seismic fault motion is extremely local or did not occur in the study area and the cm crystallinity from shallow to deep fault zones may be predicted as a first approximation from the graphitization trend in sedimentary and metamorphic rocks if that case graphite may lower the friction of shear zones at temperatures above 300 c deeper than the lower part of seismogenic zone

geochemistry of shear zone hosted uranium mineralisation at the zadni chodov uranium deposit bohemian massif abstract samples from the zadni chodov uranium deposit of the bohemian massif were analysed using optical and chemical methods organic matter and brannerite were studied in detail to estimate conditions that prevailed in association with uranium enriched shear zones four major events are known in the high grade metasediments the pre ore stage is associated with the formation of shear zones during the uranium ore deposition stage temperatures ranged from 122 c to 258 c and coffinite and brannerite appeared two generations of post ore stage minerals are distinguishable a detailed study of a brannerite mineral grain revealed variations in its interior and surroundings but these are related to the composition of brannerite which is not a nominal mineral si and al are presented as chlorites but the main mineral phases are mixtures of anatase oxidized brannerite with non stoichiometric uranium oxide and carbonaceous matter carbonaceous matter in the samples studied appeared as a mixture of substances of different composition origin and age optical and structural characteristics showed variations in three coexisting types of metamorphosed carbonaceous matter including fine grained and larger lamella shaped particles that represent anthracite meta anthracite and semi graphite stages lamellas of crystalline graphite and pyrobitumen corresponding to impsonite the structure of organic particles was not affected by uranium minerals which is an interesting difference from changes that minerals undergo in for example coal biomarkers compounds used in assessing sources of organic matter were present in extracts but they were not a remnant of the original shear zone organic material they are likely products from a later impact probably of tertiary and younger sediments

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

carbon forming reactions under a reducing atmosphere during seismic fault slip graphite is a well known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults some natural fault zones contain carbonaceous material cm even where host rocks do not contain it and seismic fault motion can promote the graphitization of low grade cm thus the origin of cm in fault zones is an important issue in fault mechanics previous high velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion but most experiments have been conducted in an atmosphere under oxic conditions here we report experimental results on carrara marble free of cm conducted under n2 or h2 atmospheres at a slip rate of 1 3 m s and normal stresses of 2 0 3 1 mpa a small amount of blackish material formed in generated gouge only under reducing conditions with the h2 atmosphere and raman spectroscopic analysis revealed the presence of cm amorphous carbon in the material the cm is attributable to 1 the generation and pyrolitic dissociation of ch4 and or 2 a reduction reaction of emitted co2 due to calcite decomposition we confirmed the formation of ch4 using gas chromatography the cm produced in experiments resembles cm in the nojima fault japan gouge in terms of raman spectra the granitic host rock of this fault is free of cm and calcite is precipitated close to the cm therefore the cm probably formed through processes similar to those simulated in our experiments future research should investigate the amount and origin of cm in natural fault zones

generation of amorphous carbon and crystallographic texture during low temperature subseismic slip in calcite fault gouge identification of the nano scale to micro scale mechanochemical processes occurring during fault slip is of fundamental importance to understand earthquake nucleation and propagation here we explore the micromechanical processes occurring during fault nucleation and slip at subseismic rates 3 10 6 m s 1 in carbonate rocks we experimentally sheared calcite rich travertine blocks at simulated upper crustal conditions producing a nano grained fault gouge strain in the gouge is accommodated by cataclastic comminution of calcite grains and concurrent crystal plastic deformation through twinning and dislocation glide producing a crystallographic preferred orientation cpo continued wear of fine grained gouge particles results in the mechanical decomposition of calcite and production of amorphous carbon we show that cpo and the production of amorphous carbon previously attributed to frictional heating and weakening during seismic slip can be produced at low temperature during stable slip at subseismic rates without slip weakening

high velocity frictional behavior and microstructure evolution of fault gouge obtained from nojima fault southwest japan abstract high velocity experiments on fault gouge taken from the nojima fault that slipped during the 1995 kobe earthquake were conducted to investigate physical mechanism associated with the slip weakening behavior with increasing slip the friction values of the gouge sheared at 0 62 mpa normal stress and 1 03 m s slip velocity decrease exponentially from a peak value of more than 0 6 to a steady state value of 0 2 the textures of the gouge are characterized by grain comminution oblique and parallel shear planes and localized deformation zone with strongly preferred orientation in the friction weakening stage and folding and fluttering structures at the steady state friction stage numerical modeling based on the temperature measurements close to the gouge layer shows that the temperature inside the gauge layer did not exceed 400 c during the experiments in a slide hold slide test a full strength recovery of the fault gouge was observed only after 12 s slip pause and the slip weakening curves are the same between the two successive slips the steady state coefficient of friction decreased from 0 8 to about 0 2 when the slip velocity increased from 0 006 m s to 1 03 m s this high velocity weakening feature was observed in a synthetic quartz gouge as well as in the nojima gouge although it is unclear which mechanism causes the weakening among thermal pressurization silica gel lubrication flash heating moisture draining and so on the present experimental results suggest that the high velocity weakening is related to the high heat production rate finally the flow structures observed in the samples deformed up to the final steady state stages have never been reported in previous slow rate experiments and could be a key structure characteristic of high velocity frictional sliding

effects of sample preparation on the microstructural signatures of faulting in clay bearing fault gouge abstract clay rich fault gouge in the principal slip zones of faults stores abundant water within pores and between clay interlayers during the preparation of thin sections and rock chips for microstructural observations fault gouge samples are commonly air dried at room temperature or in an oven however during the drying process remnant liquid water between gouge grains produces an inter particle adhesion force liquid bridge force which rearranges the grain to grain structure likely resulting in a disturbance of the original fabric for this study we prepared gouge samples from the itozawa fault northeastern japan using a t butyl alcohol freeze drying method that mitigates drying induced fabric disturbance we then compared the microstructure of our samples with those prepared using the conventional air drying method the freeze dried samples preserve a smooth fault plane clearly defined nanoparticles and well developed shear sense indicators including slickenlines and riedel shear planes in contrast the air dried samples underwent shrinkage during drying which distorted the geometry of the fault plane these air dried samples lack nanoparticles and display only a weak shear fabric we conclude that microstructural observations on samples prepared using the t butyl alcohol freeze drying method compared with conventional air drying could preserve more evidence for the retrieval of fault information including the kinematics slip stability and dynamic weakening mechanism of a fault

energy partition for grain crushing in quartz gouge during subseismic to seismic fault motion an experimental study abstract brunauer emmett teller bet surface area of quartz gouge was measured before and after shearing deformation at subseismic to seismic slip rates to determine how much of frictional work and fracture energy was consumed during grain crushing measurement of bet surface area provides more direct data on surface area change than grain size measurement does twenty four experiments on quartz gouge composed of ottawa sands or crushed quartz were conducted dry with room humidity or wet with added water at slip rates of 0 006 1 3 m s and normal stresses of 0 76 3 37 mpa using a high velocity friction apparatus and a low to high velocity friction apparatus about 1 mm thick gouge was placed between cylindrical specimens of belfast dolerite of 25 mm in diameter results show that grain crushing absorbed only 0 02 0 22 of frictional work and only 0 05 1 06 of fracture energy under dry and wet conditions thus grain comminution is unlikely to be an important energy sink at least for mature faults with well developed slip zone surface area of gouge decreases with deformation at seismic slip rates this and sem observations suggest partial sintering due to frictional heating thus a simple scenario of grain crushing as an energy sink does not hold for estimating the energy budget during earthquakes

the role of graphite layers in gravitational deformation of pelitic schist abstract deep seated landslides in pelitic schists are common in many countries but are poorly investigated and understood in this study we present the first detailed examination and modelling of landslide mechanisms in these materials we found that pelitic schist commonly contains black graphite rich layers on a scale of millimeter to centimeter thickness that are typically weaker than neighboring layers by examining microscopic textures in borehole samples obtained from landslide masses of pelitic schist we find that ductile gravitational shearing commonly occurs within these weaker layers accompanied by brittle fracture in the surrounding layers to investigate these mechanisms we have performed high precision direct shear tests using a novel back pressured shearbox on artificial rock samples both with and without graphite layers placed between pre cut shear surfaces the tests used normal stresses up to 800 kpa equivalent to 32 m depth of burial we found that the coefficients of friction for samples with graphite layers embedded in the artificial rock samples 0 30 representing an angle of internal friction of 16 7 were much lower than those without graphite layers on the pre cut surface 0 85 the shear strength of the artificial rocks with embedded layers of graphite decreased abruptly with increasing areal extent of the graphite layer along the shear surface from which it can be inferred that the continuity of a graphite layer in natural pelitic schist has a considerable effect on shear resistance these results suggest that even comparatively low dip angles of schistosity in pelitic schist could initiate microscopic slip along the graphite rich layers

reconstruction of seismic faulting by high velocity friction experiments an example of the 1995 kobe earthquake 1 high velocity friction experiments on a fault gouge collected from the nojima fault activated during the 1995 kobe earthquake showed that the friction coefficient decreased from 0 63 to 0 18 over a slip weakening distance dc at high slip rates of 1 m s the dramatic drop in friction coefficient of more than 0 3 is consistent with that for the kobe earthquake estimated from seismological observations experimentally determined dc becomes 5 m at a higher normal stress of 1 85 mpa close to the order of magnitude of seismologically determined dc of 0 5 to 1 m the difference in dc is not significant because the fracture energy consumed during frictional slip is the same order of 106 n m for both cases here we show that frictional behavior of a fault during an earthquake can be predicted by conducting high velocity friction experiments

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

fault lubrication and earthquake propagation in thermally unstable rocks experiments performed on dolomite or mg calcite gouges at seismic slip rates v 1 m s and displacements d 1 m show that the frictional coefficient decays exponentially from peak values m p 0 8 in the byerlee9s range to extremely low steady state values ss 0 1 attained over a weakening distance d w microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products periclase and lime or portlandite were produced in the slip zone during the transient stage d w these observations integrated with co 2 emissions data recorded during the experiments suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes e g decarbonation reactions t 550 c during steady state d d w shear strength is very low and not dependent upon normal stresses suggesting that pressurized fluids co 2 may have been temporarily trapped within the slip zone at this stage a continuous layer of nanoparticles is developed in the slip zone for d d w a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone at this stage dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone experimentally derived seismic source parameter w b i e breakdown work the energy that controls the dynamics of a propagating fracture 1 matches w b values obtained from seismological data of the a d 1997 m6 colfiorito italy earthquakes which nucleated in the same type of rocks tested in this study and 2 suggests similar earthquake scaling relationships as inferred from existing seismological data sets we conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms which are activated or inhibited by physicochemical reactions in the slip zone

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

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records

frictional properties of simulated shale coal fault gouges implications for induced seismicity in source rocks below europe s largest gas field abstract we report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale coal mixtures our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the upper carboniferous source rocks underlying the groningen gas reservoir netherlands we used shale siltstone core recovered from beneath the groningen reservoir plus polish bituminous coal of similar age and origin to coals locally present in the groningen source rocks we performed friction experiments in velocity stepping constant velocity slide hold slide shs and slide unload slide sus modes under near in situ conditions of 100 c and 40 mpa effective normal stress employing sliding velocities of 0 1 100 m s and a variety of pore fluids samples with 0 50 vol coal showed friction coefficients 0 45 with minor slip weakening samples with 50 vol coal showed marked slip weakening from peak friction values of 0 47 to 0 3 regardless of experimental conditions presumably reflecting strain localization in weak coal rich shear bands possibly accompanied by changes in coal molecular structure however re sliding experiments sus showed that slip weakening is limited to small initial displacements 2 3 mm and does not occur during slip reactivation at near steady state almost all experiments performed at in situ stress pore water pressure 15 mpa and temperature conditions exhibited stable velocity strengthening behaviour regardless of coal content by contrast under dry and gas saturated ch4 argon conditions or using water at 1 atm 50 50 vol shale coal mixtures showed velocity weakening and even stick slip our results imply that faults in the groningen carboniferous shale siltstone sequence are not prone to induced earthquake nucleation at in situ conditions even when coal bearing or coal enriched by smearing however the mechanisms controlling coal friction remain unclear at the sliding velocities studied and the evolution of coal friction at seismic slip velocities remains unknown

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

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

mechanisms of fault mirror formation and fault healing in carbonate rocks abstract the development of smooth mirror like surfaces provides insight into the mechanical behaviour of crustal faults during the seismic cycle to determine the thermo chemical mechanisms of fault mirror formation we investigated carbonate fault systems in seismically active areas of central greece using multi scale electron microscopy combined with raman and electron energy loss spectroscopy we show that fault mirror surfaces do not always develop from nanogranular volumes the microstructural observations indicate that decarbonation is the transformation process that leads to the formation of smooth surface coatings in the faults studied here piercement structures on top of the fault surfaces indicate calcite decarbonation producing co2 and lime cao lime subsequently reacts to portlandite ca oh 2 under hydrous conditions nanoscale imaging and electron diffraction reveal a thin coating of a non crystalline material sporadically mixed with nano clay forming a complex composite material that smooths the slip surface spectroscopic analyses reveal that the thin coating is non crystalline carbon we suggest that ordering hybridisation of amorphous carbon led to the formation of partly hybridised amorphous carbon but did not reach full graphitisation calcite nanograins 100 nm and new nanograins formed by back reaction secondary nanograins