Natural Pseudotachylytes Research Articles

Frictional power dissipation in a seismic ancient fault

The frictional power per unit area Q˙ (product of frictional traction τ and slip rate u˙ in MW m−2) dissipated during earthquakes triggers fault dynamic weakening mechanisms that control rupture nucleation, propagation and arrest. Although of great relevance in earthquake mechanics, Q˙ cannot, with rare exceptions, be determined by geophysical methods. Here we exploit theoretical, experimental and geological constraints to estimate Q˙ dissipated on a fault patch exhumed from 7-9 km depth. According to theoretical models, in polymineralic, silicate rocks the amplitude (< 1 mm) of the grain-scale roughness of the boundary between frictional melt (pseudotachylyte) and host rock decreases with increasing Q˙. The dependence of grain-scale roughness with Q˙ is due to differential melt front migration in the host rock minerals. This dependence is confirmed by friction experiments reproducing seismic slip where pseudotachylytes were produced by shearing tonalite at Q˙ ranging from 5 to 25 MW m−2. In natural pseudotachylytes across tonalites, the grain-scale roughness broadly decreases from extensional to compressional fault domains where lower and higher Q˙ are expected, respectively. Analysis of the natural dataset calibrated by experiments yields Q˙ values in the range of 4-60 MW m−2 (16 MW m−2 average value). These values, estimated in small fault patches, are at the lower end of broad estimates of Q˙ (3-300 MW m−2) obtained from frictional tractions (30-300 MPa) and fault slip rates (0.1-1 m/s) assumed as typical of upper crustal earthquakes.

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

Whether seismic rupture propagates over large distances to generate mega-earthquakes or is rapidly aborted mainly depends on the slip processes within the fault core, including particularly frictional melting or intense grain-size reduction and amorphization. The record of seismic slip in exhumed fault zones consists in many instances in Black Faults Rocks, dark and glass-like-filled aphanitic veins that have been interpreted as resulting from the quenching of frictional melts, i.e. pseudotachylytes. Such interpretation has nevertheless been questioned as similar macro to nano-microstructures have been observed either on intensely comminuted natural fault rocks or on slow creep experiments conducted on crustal rocks, where melting is absent. Here, we report a new dataset of Raman Spectroscopy of Carbonaceous Material analyses, aimed at discriminating the slip weakening processes operating in the fault core during slip. Using high spatial resolution profiles on natural Black Fault Rocks from exhumed accretionary complexes and an experimentally calibrated modelling of Raman intensity ratio evolution with temperature, we assessed different scenarios of temperature evolution during fault slip. None of them is able to account for the distribution of Raman signal, so that in the three studied Black Fault Rocks interpreted so far as natural pseudotachylytes, Raman Spectroscopy of Carbonaceous Material rather reflects the effect of intense and localized strain during fault slip. Furthermore, the absence of thermal imprint on Raman signal puts upper bounds on the temperature reached within the fault zone. If one cannot rule out the occurrence of high and short-lived temperature increase due to friction, the latter was not high enough as to melt the large quartz fraction of the fault zone rocks.

Microstructural evolution of pseudotachylyte-bearing rocks during increasing temperatures: Evidence from rock-heating experiments

Pseudotachylyte are solidified melts that form from frictional heating on a fault plane during earthquakes and play an important role in the mechanism of faulting. Pseudotachylytes interact with mineralizing fluids and their textures are altered over long periods of time; consequently, determining seismic parameters by characterizing pseudotachylytes in the field remains challenging. In this study, we performed heating experiments on faulted pseudotachylyte-bearing cataclasites recovered from the Wenchuan Earthquake Fault Scientific Drilling Project-2 (WFSD-2) to constrain the formation of temperatures and microstructural evolution of the pseudotachylytes. These experiments revealed that melting occurred at 1100 °C, while microlites formed at 1100 °C and disappeared at 1500 °C, and metallic iron spherulites formed at 1300 °C. Fractures and cracks occurred in the pseudotachylytes after rapid cooling, implying an increase in pore volume that allowed for fluid infiltration and subsequent alteration. Because natural pseudotachylyte of the WFSD-2 shows the lack of microlite and the presence of metallic iron spherulites, our results imply that its formation was at temperature of 1300–1500 °C under water-deficient conditions.

Selective clast survival in an experimentally-produced pseudotachylyte

On-fault processes during earthquakes contribute to seismic rupture propagation and slip. Here we investigate clast fragmentation in an experimental pseudotachylyte (solidified seismic melt) produced with a rotary shear machine. We slid for 0.44 m (corresponding to Mw ≥ 6 earthquakes), at slip rates > 1 m/s, pre-cut samples of quartz + phyllosilicates + plagioclase + sillimanite + garnet -bearing ultramylonite, that hosts pseudotachylytes in nature. The ultramylonite minerals extensively preserved as clasts in the experimental pseudotachylyte are quartz, plagioclase, and sillimanite. Garnet is scarcely preserved, despite having a melting temperature similar to plagioclase, probably due to having low thermal shock resistance. This selective clast survival is identical to the one found in the natural pseudotachylytes. Based on these experimental observations and assuming non-equilibrium melting, the preservation of a mineral, as a clast, in the melt appears to be controlled by its thermal shock properties as well as by its melting temperature. Since the mechanical effects of rupture propagation in these experiments were negligible, we conclude that, for Mw ≥ 6 earthquakes, (i) frictional slip and heating of the slipping zone plus (ii) thermomechanical properties of minerals, rather than fault rupture processes, control mineral comminution and clast survival in frictional melts.

Three‐dimensional texture of natural pseudotachylyte: Pseudotachylyte formation mechanism in hydrous accretionary complex

AbstractMelt‐origin pseudotachylyte is the most reliable seismogenic fault rock. It is commonly believed that pseudotachylyte generation is rare in the plate subduction zone where interstitial fluids are abundant and can trigger dynamic fault‐weakening mechanisms such as thermal pressurization. Some recent studies, however, have discovered pseudotachylyte‐bearing faults in exhumed ancient accretionary complexes, indicating that frictional melting also occurrs during earthquakes in subduction zones. To clarify the pseudotachylyte generation mechanism and the variation of slip behavior in the plate subduction zone, a pseudotachylyte found in the exhumed fossil accretionary complex (the Shimanto Belt, Nobeoka, Japan) was re‐focused and microscopic and three‐dimensional observations of the pseudotachylyte‐bearing fault were performed based on optical, electron, and X‐ray microscope images. Based on the patterns contained in the fragment, the pseudotachylyte is divided into four domains, although no clear domain boundaries or layering structures are not found. Three‐dimensional observation also suggests that the pseudotachylyte were fragmented or isolated by cataclasite or carbonate breccia. The pseudotachylyte was rather injected into the surrounding carbonate breccia, which is composed of angular fragments of the host rock and a matrix of tiny crystalline carbonate. The pseudotachylyte volume was extracted from the X‐ray microscope image and the heat abundance consumed by the pseudotachylyte generation was estimated at 2.18 MJ/m2, which can be supplied during a slip of approximately 0.5 m. These observations and calculations, together with the results of the previous investigations, suggest hydrofracturing and rapid carbonate precipitation that preceded or accompanied the frictional melting. Dynamic hydrofracturing during a slip can be caused by rapid fluid pressurization, and can induce abrupt decrease in fluid pressure while drastically enhancing the shear strength of the shear zone. Consequently, frictional heating would be reactivated and generate the pseudotachylyte. These deformation processes can explain pseudotachylyte generation in hydrous faults with the impermeable wall rock.

Pseudotachylyte increases the post-slip strength of faults

Abstract Solidified frictional melts, or pseudotachylytes, are observed in exhumed faults from across the seismogenic zone. These unique fault rocks, and many experimental studies, suggest that frictional melting can be an important process during earthquakes. However, it remains unknown how melting affects the post-slip strength of the fault and why many exhumed faults do not contain pseudotachylyte. Analyses of triaxial stick-slip events on Westerly Granite (Rhode Island, USA) sawcuts at confining pressures from 50 to 400 MPa show evidence for frictional heating, including some events energetic enough to generate surface melt. Total and partial stress drops were observed with slip as high as 6.5 mm. We find that in dry samples following melt-producing stick slip, the shear failure strength increased as much as 50 MPa, while wet samples had &amp;lt;10 MPa strengthening. Microstructural analysis indicates that the strengthening is caused by welding of the slip surface during melt quenching, suggesting that natural pseudotachylytes may also strengthen faults after earthquakes. These results predict that natural pseudotachylyte will inhibit slip reactivation and possibly generate stress heterogeneities along faults. Wet samples do not exhibit melt welding, possibly because of thermal pressurization of water reducing frictional heating during slip.

Three dimensional texture of natural pseudotachylyte and estimation of the slip behavior

Three dimensional texture of natural pseudotachylyte and estimation of the slip behavior

A friction to flow constitutive law and its application to a 2‐D modeling of earthquakes

AbstractEstablishment of a constitutive law from friction to high‐temperature plastic flow has long been a challenging task for solving problems such as modeling earthquakes and plate interactions. Here we propose an empirical constitutive law that describes this transitional behavior using only friction and flow parameters, with good agreements with experimental data on halite shear zones. The law predicts steady state and transient behaviors, including the dependence of the shear resistance of fault on slip rate, effective normal stress, and temperature. It also predicts a change in velocity weakening to velocity strengthening with increasing temperature, similar to the changes recognized for quartz and granite gouge under hydrothermal conditions. A slight deviation from the steady state friction law due to the involvement of plastic deformation can cause a large change in the velocity dependence. We solved seismic cycles of a fault across the lithosphere with the law using a 2‐D spectral boundary integral equation method, revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep including slow slip prior to earthquakes. Seismic slip followed by creep is consistent with natural pseudotachylytes overprinted with mylonitic deformation. Overall fault behaviors during earthquake cycles are insensitive to transient flow parameters. The friction‐to‐flow law merges “Christmas tree” strength profiles of the lithosphere and rate dependency fault models used for earthquake modeling on a unified basis. Strength profiles were drawn assuming a strain rate for the flow regime, but we emphasize that stress distribution evolves reflecting the fault behavior. A fault zone model was updated based on the earthquake modeling.

Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments

AbstractFault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine‐grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10−4 Am2/kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earth's field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.

Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks

Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite–pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9–11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.Mylonites have mineralogical, elemental and hydrogen isotope compositions (−80‰<δD<−78‰) similar to the host tonalite (−77‰<δD<−73‰), within the analytical error of ±5‰. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (−69‰<δD<−60‰) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (−81‰<δD<−59‰) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T>50°C and have low δD value (−93‰). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (−103‰<δD<−88‰), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD=−95‰). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (−76‰<δD<−74‰) or cataclasite (−68‰<δD<−62‰), with a slight decrease of the δD values in some samples (−85‰<δD<−81‰).The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes.

Frictional melting of peridotite and seismic slip

The evolution of the frictional strength along a fault at seismic slip rates (about 1 m/s) is a key factor controlling earthquake mechanics. At mantle depths, friction‐induced melting and melt lubrication may influence earthquake slip and seismological data. We report on laboratory experiments designed to investigate dynamic fault strength and frictional melting processes in mantle rocks. We performed 20 experiments with Balmuccia peridotite in a high‐velocity rotary shear apparatus and cylindrical samples (21.8 mm in diameter) over a wide range of normal stresses (5.4–16.1 MPa), slip rates (0.23–1.14 m/s), and displacements (1.5–71 m). During the experiments, shear stress evolved with cumulative displacement in five main stages (stages 1–5). In stage 1 (first strengthening), the coefficient of friction μ increased up to 0.4–0.7 (first peak in friction). In stage 2 (abrupt first weakening), μ decreased to about 0.25–0.40. In stage 3 (gradual second strengthening), shear stress increased toward a second peak in friction (μ = 0.30–0.40). In stage 4 (gradual second weakening), the shear stress decreased toward a steady state value (stage 5) with μ = 0.15. Stages 1 and 2 are of too short duration to be investigated in detail with the current experimental configuration. By interrupting the experiments during stages 3, 4, and 5, microstructural (Field Emission Scanning Electron Microscope) and geochemical (Electron Probe Micro‐Analyzer and Energy Dispersive X‐Ray Spectroscopy) analysis of the slipping zone suggest that second strengthening (stage 3) is associated with the production of a grain‐supported melt‐poor layer, while second weakening (stage 4) and steady state (stage 5) are associated with the formation of a continuous melt‐rich layer with an estimated temperature up to 1780°C. Microstructures formed during the experiments were very similar to those found in natural ultramafic pseudotachylytes. By performing experiments at different normal stresses and slip rates, (1) the “thermal” (as it includes the thermally activated first and second weakening) slip distance to achieve steady state from the first peak in strength decreased with increasing normal stress and slip rate and (2) the steady state shear stress slightly increased with increasing normal stress and, for a given normal stress, decreased with increasing slip rate. The ratio of shear stress versus normal stress was about 0.15, well below the typical friction coefficient of rocks (0.6–0.8). The dependence of steady state shear stress with normal stress was described by means of a constitutive equation for melt lubrication. The presence of microstructures similar to those found in natural pseudotachylytes and the determination of a constitutive equation that describes the experimental data allows extrapolation of the experimental observations to natural conditions and to the study of rupture dynamics in mantle rocks.

Experimental investigation of frictional melting of argillite at high slip rates: Implications for seismic slip in subduction‐accretion complexes

Discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite‐rich, argillite‐derived slip zones during subduction earthquakes. We conducted high‐velocity friction experiments on argillite at a slip rate of 1.13 m/s and normal stresses of 2.67–13.33 MPa. Experiments show slip weakening followed by slip strengthening. Slip weakening is associated with the formation and shearing of low‐viscosity melt patches. The subsequent slip strengthening occurred despite the reduction in shear strain rate due to the growth (thickening) of melt layer, suggesting that the viscosity of melt layer increased with slip. Microstructural and chemical analyses suggest that the viscosity increase during the slip strengthening is not due to an increase in the volume fraction of solid grains and bubbles in the melt layer but could be caused primarily by dehydration of the melt layer. Our experimental results suggest that viscous braking can be efficient at shallow depths of subduction‐accretion complexes if substantial melt dehydration occurs on a timescale of seismic slip. Melt lubrication can possibly occur at greater depths within subduction‐accretion complexes because the ratio of viscous shear to normal stress decreases with depth. Argillite‐derived natural pseudotachylytes formed at seismogenic depths in subduction‐accretion complexes are more hydrous than the experimentally generated pseudotachylytes and may be evidence of nearly complete stress drop.

Amorphous material formed by the mechanochemical effect in natural pseudotachylyte of crushing origin: A case study of the Iida-Matsukawa Fault, Nagano Prefecture, Central Japan

Pseudotachylytes from the Iida-Matsukawa Fault, Central Japan (Iida pseudotachylytes), are described down to the nanometer scale using high-resolution electron microscopy, scanning electron microscopy, analyses of chemical compositions, and mercury intrusion porosimetry. The pseudotachylyte matrix chiefly consists of nanoscale particles and elongated submicron fragments with biotite composition; amorphous material of several tens of nanometers in size is also observed, despite the absence of melt textures. Lattice fringe images reveal that the amorphous phase coexists with lattice distortion in deformed biotite fragments that are several hundreds of nanometers in size. The presence of these submicrostructures indicates that the amorphous material formed by mechanical stress during the comminution process; that is, via the “mechanochemical effect”. We conclude that amorphous material does not always provide evidence of the rapid cooling of melt in pseudotachylyte. The amorphous materials present in the Iida pseudotachylyte resulted from the mechanochemical effect due to both shear stress and normal stress during the comminution process that accompanied fault motion.

Numerical simulations of frictional melting: Small dependence of shear stress drop on viscosity parameters

Frictional melting during coseismic slipping strongly affects dynamic sliding. An idealized numerical simulation of shear stress drop due to lubrication of fault surfaces by frictional melting is performed to investigate dynamical adjustment of melt thickness and its dependence on material properties. We numerically solved one‐dimensional heat conduction equations coupled with hydrodynamic equations of motion of the melt layer. We adopt a simple Arrhenius‐type temperature dependence of melt viscosity with a parametric factor η0. After the initial transient stage, the evolution of the melt layer asymptotes to the late stage, where the melt thickness increases as and the generated shear stress drops as 1/. An analytic self‐similar solution of the temperature profile for the late stage is obtained. The balance between the viscous heat generation and conductive heat loss characterizes the late stage. For small η0, the melt thickness is proportional to η0, whereas it will saturate for a larger η0. This is because the strong temperature dependence of viscosity enables an automatic adjustment of viscosity, which drops through the temperature increase due to the viscous heating even if a large η0 is chosen. Numerical results and the thickness data of natural pseudotachylyte layers were compared. A maximum ratio X/ (X is thickness, D is sliding distance) exists above which no solution is found. The effective thermal conductivity of the melt layer should be large, probably due to the sliding surface roughness. Comparison with laboratory experimental results showed that the normal stress applied to the sample is an important parameter for stress drop.

Slip-Weakening Distance of Faults during Frictional Melting as Inferred from Experimental and Natural Pseudotachylytes

The slip-weakening distance, D c , over which a fault weakens during seismogenic fault motion, is one of most important parameters controlling fault instability. A paradox over the last decade has been the source of the large difference between laboratory-measured values of D c using conventional friction experiments (10 −5 –10 −3 m) and seismically determined D c values for natural earthquakes (10 −1 –10 0 m). D c is a difficult quantity to estimate for natural faults, but this study attempts to determine the order of D c values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes. Experiments at a constant slip velocity of 0.85 m/sec and a normal stress of 1.4 MPa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening. Slip weakening (a reduction in frictional strength with displacement) and the growth of the molten layer (an increase in the thickness of the molten layer with displacement) are both characterized by exponential changes with nearly the same characteristic distances. The comparison suggests that the order of D c can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement. Sibson’s (1975) data from the Outer Hebrides exhibit a similar pseudotachylyte-thickness-versus-fault-displacement curve with a characteristic distance of 0.38 m. This is the same order of magnitude as seismically inferred values of D c . The effect of frictional melting is a possible solution for the D c paradox, although it is not the only possible solution, since frictional melting along large faults is rather uncommon.

Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults

Tectonic pseudotachylytes might be used to constrain earthquake source parameters, such as dynamic shear stress resistance, average dynamic friction and slip-weakening distance. Estimation of dynamic shear stress resistance and dynamic friction from field studies is based on the assumption that the volume of melt produced during coseismic slip is proportional to the frictional work converted to heat on the fault surface. Conditions conducive to a realistic estimate of dynamic shear resistance are: (i) the presence of large outcrop exposures that allow for estimation of the volume of pseudotachylyte; (ii) the presence of structural markers offset by faults in order to relate the displacement accommodated by the fault with the volume of melt produced; (iii) data that provide an estimate of the initial melt temperature; and (iv) determination of host-rock temperature and pressure conditions that may have existed during seismic faulting. An independent indication that steady-state friction in the presence of melts might be achieved during coseismic slip arises from the dependence of the fractal dimension of the fault profile (intersection of the fault surface with the outcrop surface) with displacement. This relation could also indicate the slip-weakening distance ( Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). The above conditions are all satisfied in the case of the Gole Larghe Fault Zone, which consists of hundreds subparallel strike-slip faults that cut tonalites of the Adamello batholith (Italy). The thickness of pseudotachylyte-bearing faults increases with displacement. From displacement/thickness ratios and energy balance calculations, we determined the dynamic shear resistance for several pseudotachylyte-bearing faults. In the same faults, the fractal dimension of the fault profile increases from 1.0 to 1.16 with displacement. This was also observed in experiments where steady-state friction in the presence of melt was achieved ( Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). However, we will show that the estimate of the dynamic shear stress resistance, average dynamic friction and slip-weakening distance in the studied faults is limited by the uncertainties to attribute the measured displacement to a single seismic rupture. Since many pseudotachylytes in the upper seismogenic crust overprint preexisting cataclasites, it is suggested that future field and experimental work should be addressed to determine microstructural indicators (i.e. evolution of cataclastic fabric with displacement) within cataclasites, which might constrain the contribution of the cataclastic, pre-pseudotachylyte displacement to the total displacement accommodated by the fault.

Melt–clast interaction and power-law size distribution of clasts in pseudotachylytes

Before the onset of melting along frictional interfaces, the wall rocks of seismic faults are crushed to generate a power-law grain-size distribution pattern. Theoretical analysis with the help of numerical models shows that, under conditions of uniform rim melting of the grains, the pre-melting power-law pattern survives in a modified form in the relicts, which occur as clasts in the quenched product, i.e. pseudotachylyte. The size distribution of clasts in experimentally produced as well as natural pseudotachylytes, including those from the Sarwar–Junia fault zone in western India, shows a pattern similar to that predicted by theoretical analysis. A micron-scale mantle of glass±microlites around a large number of clasts, and minute beads of glass set within glassy matrix, are the observed evidence of rim melting. Post-melting clast size distribution in the numerical models follows the relationship N∝(1+ z/ z′) − D , where N is number of clasts of size ≧ z. Size z may be represented by mean diameter r, or sectional area a, or volume v, and D(= D r , D a or D v ) is the modified power-law exponent. z′ (= r′, a′ or v′) is a constant that depends on thickness ( ω) of uniform rim melting, and r′= ω, a′ =ω 2, and v′ =ω 3. The analysis suggests that the modified power-law clast-size distribution pattern is a characteristic feature of all fault-related pseudotachylytes.

Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes

High-velocity friction experiments on gabbro and monzodiorite, using a rotary-shear high-velocity friction apparatus, have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening, eventually reaching a nearly steady-state. The melting surface at the host rock/molten layer interface is initially very flat, but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the Gibbs–Thomson effect. This change in the melting-surface topography can be quantitatively expressed by the fractal dimension D, as determined by the divider method, from about 1.0 near the peak friction to around 1.1 near the steady-state friction. The ultimate fractal dimension at steady-state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting. A systematic correlation of D with mechanical behavior of the fault during frictional melting may provide a way of estimating slip-weakening distance and heat production rate at steady-state friction by measuring D for natural pseudotachylytes on slip surfaces with different displacements. The weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength. The experimental studies point to ways to estimate these difficult quantities for natural faults.

Size distribution of clasts in experimentally produced pseudotachylytes

High-velocity frictional experiments on monzodiorite, at sliding rates of 1.3 m/s and at normal stresses of 2 MPa, produced dark 100 μm thick pseudotachylyte layers on the sliding surfaces. The experimentally produced pseudotachylyte contains clasts of various sizes in an ultrafine matrix, similar to the texture observed for natural pseudotachylytes. The size distribution of clasts in the experimental pseudotachylyte obeys a power law of the form: N= N′(1+ r/ r′) − D , where N is the cumulative number of clasts with a size greater than r, r′ and D are material constants, and N′ is a constant that depends on the total number of measurements. The total area of ultrafine-grained clasts in a thin section, estimated from the size distribution is less than 0.5%, whereas the actual area of ultrafine matrix is measured as 60–70% of the total product. This suggests that the matrix cannot be regarded as a product of only comminution processes, consistent with natural pseudotachylytes, and that the ratio of actual area of the ultrafine-grained matrix to the hole product represents almost the ratio of the melt (glass). This estimate is consistent with the ratio of glass determined from an X-ray diffraction analysis.

Selective melting processes as inferred from experimentally generated pseudotachylytes

High-speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion. Experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1.0–1.5 MPa and at slip rates up to 2 m/s using a rotary-shear high-speed frictional testing machine. The experimentally-generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub-angular to rounded fragments. These locally occur as injected network veins in the fractured rock. X-ray diffraction analysis has revealed the presence of as much as 40–70 wt% glass. The mineral contents of clasts in the experimental pseudotachylyte in granite as determined by X-ray diffraction analysis, indicate that quartz is the most resistant to melting, biotite the least, and feldspar is intermediate. Thus, the SiO 2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting.