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Abstract: On a strand of the Norumbega fault system, a Paleozoic, subvertical, seismogenic fault system in northeastern New England, USA, we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite/phyllonite layers. We consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone. The Norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of ~10–15km, some of which preserve evidence for mutually-overprinting pseudotachylyte and mylonite. Along one of these, the Sandhill Corner shear zone, all stages of the pseudotachylyte to ultramylonite/phyllonite transformation are preserved, from (1) primary pseudotachylyte structures to (2) initial mineral crystallization, (3) grain coarsening and reactions, and (4) viscous deformation. Our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin. Using these features, we estimate that ~5–50% of the rock volume in the Sandhill Corner shear zone (mean ~30%, locally >50%) is deformed pseudotachylyte, suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone. The Sandhill Corner shear zone localized along the contact between two rheologically-contrasting units. Mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core, indicating that seismic rupture also localized there. A decrease in grain size promoting grain-size-sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers. The formation of multiple generations of weak, deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core. [Copyright &y& Elsevier]

Fault surface roughness is a principal factor influencing earthquake mechanics, and particularly rupture initiation, propagation, and arrest. However, little data currently exist on fault surfaces at seismogenic depths. Here, we investigate the roughness of slip surfaces from the seismogenic strike-slip Gole Larghe Fault Zone, exhumed from ca. 10 km depth. The fault zone exploited pre-existing joints and is hosted in granitoid rocks of the Adamello batholith (Italian Alps). Individual seismogenic slip surfaces generally show a first phase of cataclasite production, and a second phase with beautifully preserved pseudotachylytes of variable thickness. We determined the geometry of fault traces over almost five orders of magnitude using terrestrial laser-scanning (LIDAR, ca. 500 to <1 m scale), and 3D mosaics of high-resolution rectified digital photographs (10 m to ca. 1 mm scale). LIDAR scans and photomosaics were georeferenced in 3D using a Differential Global Positioning System, allowing detailed multiscale reconstruction of fault traces in Gocad. The combination of LIDAR and high-resolution photos has the advantage, compared with classical LIDAR-only surveys, that the spatial resolution of rectified photographs can be very high (up to 0.2 mm/pixel in this study), allowing for detailed outcrop characterization. Fourier power spectrum analysis of the fault traces revealed a self-affine behaviour over 3-5 orders of magnitude, with Hurst exponents ranging between 0.6 and 0.8. Parameters from Fourier analysis have been used to reconstruct synthetic 3D fault surfaces with an equivalent roughness by means of 2D Fourier synthesis. Roughness of pre-existing joints is in a typical range for this kind of structure. Roughness of faults at small scale (1 m to 1 mm) shows a clear genetic relationship with the roughness of precursor joints, and some anisotropy in the self-affine Hurst exponent. Roughness of faults at scales larger than net slip (>1-10 m) is not anisotropic and less evolved than at smaller scales. These observations are consistent with an evolution of roughness, due to fault surface processes, that takes place only at scales smaller or comparable to the observed net slip. Differences in roughness evolution between shallow and deeper faults, the latter showing evidences of seismic activity, are interpreted as the result of different weakening versus induration processes, which also result in localization versus delocalization of deformation in the fault zone. From a methodological point of view, the technique used here is advantageous over direct measurements of exposed fault surfaces in that it preserves, in cross-section, all of the structures which contribute to fault roughness, and removes any subjectivity introduced by the need to distinguish roughness of original slip surfaces from roughness induced by secondary weathering processes. Moreover, offsets can be measured by means of suitable markers and fault rocks are preserved, hence their thickness, composition and structural features can be characterised, providing an integrated dataset which sheds new light on mechanisms of roughness evolution with slip and concomitant fault rock production. [ABSTRACT FROM AUTHOR]

Abstract: The integration of structural analyses with 40Ar/39Ar dating of fault-related pseudotachylytes provides time constraints for the reconstruction of the Alpine evolution of the central portion of the South Alpine orogenic wedge. In the northern sector of the belt a Variscan basement is stacked southward on the Permian to Mesozoic cover along regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age, experienced a complex evolution through the ductile and brittle deformation regime, showing greenschist facies mylonites overprinted by a penetrative cataclastic deformation. Generation of fault-related pseudotachylyte veins marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples give two separated age clusters: Late Cretaceous (80–68Ma) and latest Palaeocene to Middle Eocene (55–43Ma). These new data provide evidence that the pre-Adamello evolution of the central Southern Alps was characterised by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle–ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the central Southern Alps was already active during the Late Cretaceous, concurrently with the development of a synorogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. Our reconstruction provides direct evidence for an important Cretaceous “EoAlpine” orogenic event which was nearly coeval to the HP metamorphism affecting the Austroalpine units of the eastern Alps. In our view, the Upper Cretaceous Southern Alps can be interpreted as the south-verging retrobelt, which formed during subduction phenomena active to the north in the Austroalpine realm. [Copyright &y& Elsevier]

Abstract: Thin (0.5–2 mm thick) pseudotachylyte veins occur within muscovite-bearing (∼10% volume), amphibolite-facies quartzites of the Schneeberg Normal Fault Zone (Austroalpine, Southern Tyrol, Italy). Pseudotachylytes are associated with precursor localized plastic microshear zones (50–150 μm thick) developed sub-parallel to the host-rock foliation and with conjugate sets oriented at a high angle to the foliation. Such microshear zones are characterized by recrystallization to ultrafine-grained (1–2 μm grain size) mosaic aggregates of quartz showing a transition from a host-controlled to a random crystallographic preferred orientation towards the shear zone interior. Subsequent coseismic slip mainly exploited these microshear zones. Microstructural analysis provides evidence of extensive friction-induced melting of the muscovite-bearing quartzite, producing a bimodal melt composition. First, the host-rock muscovite was completely melted and subsequently crystallized, mainly as K-feldspar. Then, about 60% volume of the ultrafine-grained quartz underwent melting and crystallized as spherulitic rims (mostly consisting of quartz ± Ti ± Fe) around melt-corroded quartz clasts. The two melts show immiscibility structures in the major injection veins exploiting microshear zones at high angles to the quartzite foliation. In contrast, they were mechanically mixed during flow along the main fault veins. [ABSTRACT FROM AUTHOR]

Abstract: Pseudotachylytes and two distinct types of cataclasite in the Otago Schist at Tucker Hill, South Island, New Zealand, provide evidence for both seismic slip and aseismic creep on a normal fault zone during regional crustal extension in late Cretaceous time. Regional geologic evidence suggests that the fault had its present low-angle dip (ca. 10°) at the time it was active. ‘Type A’ cataclasites, presumably aseismic, can be recognized by bi-fractal grain size distributions, monomict composition, angular clasts of uniform textural maturity, and a crude fabric defined by oriented grains and transgranular fractures. ‘Type B’ cataclasites, possibly cosesimic, have characteristics consistent with fluidized grain flow, including heterogeneous clast shapes and types, a bimodal grain size distribution, intrusive relationships with other rocks, and the absence of any fabric or transecting fractures. Pseudotachylyte, which occurs as fault veins, injection veins and more complex types of intrusive structures, consistently cuts across and invades Type A cataclasites but is both intrusive into and included as clasts in Type B cataclasites. These relationships are consistent with a fault evolution model in which the development of a damage zone through aseismic cataclasis (Type A) facilitates the formation of pseudotachylyte in a subsequent seismic event by providing a permeable matrix through which fluids can drain in the early stages of slip, thereby maintaining frictional contact between rock surfaces. The formation of pseudotachylyte, in turn, may seal the fault zone and lead to thermal pressurization in a later seismic cycle, forming fluidized (Type B) cataclasites. Seismic slip on the low-angle normal fault zone at Tucker Hill may have occurred by two distinct modes of dynamic weakening — melt lubrication and thermal pressurization — in successive seismic events. Although there is a perception among geologists that pseudotachylyte is most likely to form in intact, crystalline rocks, geophysical models of fault zones clearly demonstrate that pseudotachylyte formation is actually suppressed in low-permeability rock because any fluids present would be unable to escape the fault zone and thermal pressurization would rapidly reduce frictional resistance. The paradigmatic occurrences of pseudotachylyte in otherwise pristine crystalline rocks probably represent somewhat exceptional circumstances (single rupture events at very high effective stress in dry rock). Coseismic frictional melts may actually be more common in hydrated rocks like the schist at Tucker Hill, but harder to recognize and also vulnerable to overprinting as a fault zone matures. In such rocks, pseudotachylyte may represent an intermediate stage in the evolution of a fault zone, the period between the formation of a high-permeability damage zone and the development of a low-permeability fault core. [Copyright &y& Elsevier]

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3× enrichment of K2O, Ba, and Rb, and similar depletion of Na2O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.9±1.8 Ma as measured by total fusion 40Ar/39Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40Ar/39Ar plateau age of 96.0±0.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist. [ABSTRACT FROM AUTHOR]

Abstract: The factors controlling the development of different types of fault rock assemblages and, more specifically, the formation of friction melts are still not fully understood. In this study we compared two exhumed strike–slip faults in the Adamello batholith (Southern Alps): the Gole Larghe and the Passo Cercen fault zones, active at 9–11km depth and temperatures of 250–300°C. Each fault zone consists of hundreds of sub-parallel strands exploiting pre-existing joints. The Gole Larghe fault strikes N105±5° and is dextral; the fault rocks are cataclasites and widespread, centimetre-thick pseudotachylytes. The Passo Cercen fault strikes on average N130° and is formed by multiple fault horizons: fault segments striking N105°–N130° are mainly dextral, whereas faults striking N135°–N140° are mainly sinistral. Microstructural, mineralogical and geochemical investigations show that the fault rocks are cataclasites associated with thick epidote+K-feldspar+quartz veins and rare, millimetre-thick pseudotachylytes. Field evidence suggests that in both fault zones, the direction of the maximum horizontal stress σ 1 was N135°. The Gole Larghe fault strikes at about 30° to σ 1 and is favourably oriented for reactivation. By contrast, the Passo Cercen fault strikes at low angles to σ 1 and is unfavourably oriented for reactivation, therefore requiring the development of high pore pressures, as suggested by the occurrence of extensive epidote veining and hydraulic breccias. It is proposed that frictional melting in the Passo Cercen fault zone was inhibited by the development of high pore pressures and low effective normal stresses. [Copyright &y& Elsevier]

Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested. [ABSTRACT FROM AUTHOR]

Abstract: The seismic slip behavior during the 1999 Chi-Chi, Taiwan, earthquake (M w 7.6) was contrastive between the northern and southern segments of the activated Chelungpu fault; large, fast and smooth slips with large stress drop in the north, while smaller, slower and irregular slips with smaller stress drop in the south. We analyzed the pseudotachylyte samples recovered from 1194 m, 1243 m and 1314 m depths of Hole-B of Taiwan Chelungpu fault Drilling Project (TCDP) to reveal the spatial difference in friction mechanism. All pseudotachylyte layers are thin (0.7–2.8 cm), the volume fraction of protoliths is very large (more than 63%), and the estimated temperature distribution is very heterogeneous from ca. 750–1750 °C. These observations suggest that these pseudotachylyte melts were in the partial melting regime of Montgomery [Montgomery, R.S., 1976. Friction and wear at high sliding speeds. Wear 36, 275–298] where friction coefficient is abnormally large. Similar pseudotachylyte was found already in the core sample from 175 m depth of the Nanto borehole penetrating the southern fault. Since both pseudotachylyte samples from the two boreholes are older than the 1999 Chi-Chi event and have been uplifted from depths farther down-dip of their current locations, it is likely that recent seismic ruptures also would have encountered these mechanical barriers of viscous melt patches at deeper parts in the north than in the south. Elastohydrodynamic lubrication of clayey gouge worked effectively at the shallower parts of the northern segment, however there is no evidence that it played an important role in the south. These differences are the plausible causes of the contrastive local slip behaviors during the Chi-Chi earthquake. [Copyright &y& Elsevier]

Abstract: Zircons from pseudotachylyte and cataclasite veins in the Svarthumlevatnet metagabbro, SW Norway, are faulted, brecciated and display multiple sets of planar deformation features (PDFs) that we assign to coseismic deformation in the middle crust during Caledonian continent collision. The zircons give a U–Pb upper intercept age of 1507±4 Ma, interpreted as a minimum age of the gabbro, and a lower intercept age of 968±31 Ma, likely indicating the time of granulite-facies metamorphism, but they do not record the Caledonian coseismic faulting even when the fault planes are healed by thin veins of low-U zircon. The work indicates that PDFs are not exclusive to meteorite impacts, but may also develop during seismic faulting. [Copyright &y& Elsevier]

The base of the Silvretta Nappe (Austroalpine Unit, Eastern Alps) has localized extensive deformation, including the occurrence of multiple generations of pseudotachylytes, alternating with (ultra)mylonites, in the host amphibolite and gneiss. Previous works attributed the formation of pseudotachylytes and associated ultramylonites to the Eoalpine deformation phase (mid. Cretaceous). In this work, we report the presence of a younger generation of pseudotachylytes, which overprints the previously characterized pseudotachylyte-ultramylonite association. A detailed petrographic study of selected samples from the Jamtal, Tyrol (Austria), provides further constraints on the possible tectonic evolution of the Silvretta Nappe. Contrary to the Eoalpine pseudotachylytes, the younger pseudotachylytes are not completely recrystallized, foliated, and epidotized, but rather preserve the typical features of frictional melts (e.g., microlites, glassy groundmass, evidence of melt immiscibility, etc.). This suggests formation in relatively shallow conditions, rather than under greenschist facies, as assumed for the Eoalpine pseudotachylytes. The "young" pseudotachylytes, commonly discordant to the foliation, thus likely formed during the final subduction of the Penninic Unit in the Paleogene. This occurrence lends further support to the localization of deformation at the base of the upper plate, in this case represented by the Silvretta Nappe, as observed in other portions of the Alps. • Deformation localized at the base of the Silvretta Nappe, including pseudotachylyte formation • "Old" pseudotachylytes were deformed in viscous-plastic regime and epidotized. • "Young" pseudotachylytes, not reported before, did not record further deformation. • "Young" pseudotachylytes likely formed in the Paleogene final Alpine phase • Evidence of pseudotachylyte/earthquake occurrence at the base of the upper plate [ABSTRACT FROM AUTHOR]

Abstract: We document the occurrence of pseudotachylyte (solidified melt produced during seismic slip) along strike-slip faults in the Lake Edison granodiorite of the Mt. Abbot quadrangle, Sierra Nevada, California and provide constraints on ambient conditions during seismic faulting. The pseudotachylytes are less than 0.3mm thick and are found in faults typically up to 1cm in thickness. Total measured left-lateral offset along sampled faults is approximately 20cm. Field and microstructural evidence indicate that the faults exploited pre-existing mineralized joints and show the following overprinting structures (with inferred ambient temperatures): mylonites are more or less coeval with quartz veins (>400°C), cataclasites and pseudotachylytes (∼250°C) more or less coeval with epidote veins, and zeolite veins (<200°C). Based on observations of the microstructural textures of faults combined with theoretical heat transfer and energy budget calculations, we suggest that only a fraction (<30%) of the total offset was associated with seismic slip (i.e. pseudotachylyte). The presence of pseudotachylyte in sub-millimeter thick zones lends support for the concept of extreme shear localization during seismic slip. The elusive nature of these pseudotachylytes demonstrates that observations in outcrop and optical microscope are not sufficient to rule out frictional melting as a consequence of seismic slip in similar fault rocks. [Copyright &y& Elsevier]

We report a new type of ultramafic pseudotachylyte that forms a fault- and injection-vein network hosted in the mantle-derived Balmuccia peridotite (Italy). In the fault vein the pseudotachylyte is now deformed and recrystallized into a spinel-lherzolite facies ultramylonite, made of a fine (<2 μm) aggregate of olivine, orthopyroxene, clinopyroxene, and spinel, with small amounts of amphibole and dolomite. Electron backscattered diffraction study of the ultramylonite shows a clear crystallographic preferred orientation (CPO) of olivine. The fault vein pseudotachylyte overprints a spinel-lherzolite facies amphibole-bearing mylonite, indicating that shear localization accompanying chemical reaction had taken place in the peridotite before seismic slip produced frictional melting. The occurrence of amphibole in the host mylonite and that of dolomite as well as amphibole in the matrices of ultramylonite and pseudotachylyte may indicate that fluid was present and had evolved in its composition from H2O-rich to CO2-rich during ductile deformation with metamorphic reactions, which may account for the observed rheological transition from ductile to brittle behavior. The spinel-lherzolite facies assemblage in mylonites, P-T estimations from pyroxene geothermometry and carbonate reactions, and the type of olivine CPO in deformed pseudotachylyte indicate that both the preseismic and the postseismic ductile deformations occurred at ∼800 °C and 0.7-1.1 GPa. [ABSTRACT FROM AUTHOR]

Abstract: The determination of the earthquake energy budget remains a challenging issue for Earth scientists, as understanding the partitioning of energy is a key towards the understanding the physics of earthquakes. Here we estimate the partition of the mechanical work density into heat and surface energy (energy required to create new fracture surface) during seismic slip on a location along a fault. Earthquake energy partitioning is determined from field and microstructural analyses of a fault segment decorated by pseudotachylyte (solidified friction-induced melt produced during seismic slip) exhumed from a depth of ~10 km—typical for earthquake hypocenters in the continental crust. Frictional heat per unit fault area estimated from the thickness of pseudotachylytes is ~27 MJ m−2. Surface energy, estimated from microcrack density inside clast (i.e., cracked grains) entrapped in the pseudotachylyte and in the fault wall rock, ranges between 0.10 and 0.85 MJ m−2. Our estimates for the studied fault segment suggest that ~97–99% of the energy was dissipated as heat during seismic slip. We conclude that at 10 km depth, less than 3% of the total mechanical work density is adsorbed as surface energy on the fault plane during earthquake rupture. [Copyright &y& Elsevier]

Abstract: Fourier transform infrared (FTIR) microanalysis of pseudotachylytes (i.e. friction-induced melts produced by seismic slip) from the Nojima fault (Japan) reveals that earthquakes almost instantaneously expel 99 wt.% of the wall rock CO2 content. Carbon is exsolved because it is supersaturated in the friction melts. By extrapolation to a crustal-scale fault rupture, large events such as the M7.2 Kobe earthquake (1995) may yield a total production of 1.8 to 3.4×103 tons CO2 within a few seconds. This extraordinary release of CO2 can cause a flash fluid pressure increase in the fault plane, and therefore enhance earthquake slip or trigger aftershocks; it may also explain the anomalous discharge of carbon monitored in nearby fault springs after large earthquakes. Because carbon saturation in silicate melts is pressure-dependent, FTIR can be used as a new tool to constrain the maximum depth of pseudotachylyte formation in exhumed faults. [Copyright &y& Elsevier]

Abstract: The Dora Maira Massif (DM), Western Alps is a type example of an ultra-high pressure (UHP) metamorphic terrain. The occurrence of pseudotachylytes within the Val Gilba area of DM led to speculation regarding the role of seismic slip in the exhumation of the massif. In this study, new field, microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented. The formation of pseudotachylyte veins occurred late in the exhumation history at 20.1 ± 0.5Ma and at shallow depths (<3km), long after the early stages and rapid exhumation that is characteristic of UHP massifs. Preseismic, coseismic and postseismic deformation features suggest that seismic slip occurred at a period where the geologic strain rate was particularly low. This observation is explained by a stick-slip model along the mylonitic foliation plane in which slip was preferentially along phengite-chlorite rich layers. Frictional sliding instability is promoted by a slow slip rate, by a phenomenon similar to “brake shuttering” in automobiles. An alternative explanation could be provided by climatically driven, accelerated exhumation ca. 20Ma, as suggested by the 20Ma-old white micas reported in the Tertiary Piedmont Basin. [Copyright &y& Elsevier]

Laser-ablation 40Ar/39Ar step-heating analyses of 20 pseudotachylyte veins from a single location along the exhumed central portion of the active Alpine fault of New Zealand yield total gas age values between 1 and 19 Ma. Evidence shows that they are genetically related and were formed during coeval episodes of seismogenic melting at shallow crustal depth, contrasting with a spread in formation ages. The total gas ages show an exponential decrease with increasing proportion of melt matrix and K content, reflecting incomplete degassing and mixtures of radiogenic Ar sources. Calculation of intercepts for all-melted matrix and all-clast end-member components indicate ca. 570 ka (Quaternary) friction-melting ages of ca. 332 Ma (Lower Carboniferous) source rock. Assuming an average exhumation rate of 6–9 mm/yr for uplift and erosion, these results imply that friction melts were generated during major slip episodes at ∼3.5–5 km crustal depth. We conclude that reliable dating of young pseudotachylyte can be accomplished by combining chronologic study with clast-matrix quantification of genetically related vein assemblages. [ABSTRACT FROM AUTHOR]

Abstract: Pseudotachylyte-bearing fault zones, found in the Shimanto accretionary complex, southwest Japan, developed during subduction or underplating at seismogenic depths. The pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a mélange. The microstructures of pseudotachylytes are represented by a fragment-laden, glass-supported texture resulting from the rapid cooling of the frictional melt. Transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed. The compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite-rich slip zone with a minimum melting temperature of 1100°C. The viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains. The viscosities at 1100°C range from 85 to 290Pas, and the corresponding shear resistance along a 1-mm-thick melt layer at a slip rate of 1m/s was 0.1–0.3MPa. The formation of a melt layer in an illite-rich slip zone can possibly induce large stress drops, increase the slip rate and enhance rupture propagation, which together could affect the earthquake magnitude in a subduction zone. [Copyright &y& Elsevier]

Proterozoic tectonites within the Homestake shear zone (HSZ) in the northern Sitwatch Range, Colorado, record two major cycles of progressive deformation under contrasting P-T conditions. Field relations and microstructures in these tectonites can be used to constrain the geometry, kinematics, and rheology of rocks in the HSZ during these events. The first deformation cycle occurred -1.7 Ga at temperatures near the granite solidus and produced structures indicative of pervasive viscous flow at low differential stress levels. The second deformation cycle at -1.4 Ga resulted in localized plastic and brittle failure concentrated in relatively narrow shear zones and seismogenic faults. This second deformation occurred at temperatures of 350-450°C and produced mylonite, ultramylonite, and pseudotachylyte. Overprinting relationships between brittle-frictional faults and plastic shear zones suggest that both modes of deformation were active at about the same time. Thus, the latter deformation cycle probably occurred near the brittle-plastic transition. Simple calculations using standard rheologic relationships show that tectonic loads of 300-400 MPa--sufficient to drive brittle failure--could have been transmitted by ductile ultramylonite zones deforming at relatively high strain rates (on the order of 10-9-10-11). Thus, plastic shear zones and brittle-frictional faults probably acted together as a dynamically coupled deformation system [ABSTRACT FROM AUTHOR]

Abstract: Fission-track dating was carried out on zircon grains from pseudotachylyte associated with a Late Pleistocene landslide (Tsergo Ri landslide) in Langtang Himal, Nepal. Zircon grains were separated from an injection pipe of glassy and vesicular pseudotachylyte about 10cm in diameter. The resulting age of 51±13Ka lies between the two subpeaks (70 and 20Ka) of the Würm glacial period, and is concordant with the previous age estimate from geomorphological evidence (i.e. older than 25–30Ka). This is the first reliable isotopic age for a landslide-generated pseudotachylyte excluding the possibility of contamination by an inherited age. [Copyright &y& Elsevier]

Abstract: Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are ∼ 67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73±7 (2σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53±9 (2σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology. [Copyright &y& Elsevier]

Field evidence preserved in ancient subducted oceanic crust documents that eclogite facies frictional failure with melting (pseudotachylyte formation during intermediate-depth earthquakes) was accompanied and followed, not preceded, by infiltration of external fluids and progressive vein formation in the eclogites. Eclogitization began during seismic failure and fluid passage through the shear zone. Subsequent fluid flow produced hydraulic fracturing and continuous vein formation during ongoing burial. We suggest that this kind of shear zone may allow channelized fluid flow within and out of slabs. [ABSTRACT FROM AUTHOR]

Abstract: Out-of-sequence thrusts (OST) play an important role in the thickening of accretionary prisms, formation of forearc basins, and tsunami generation in subduction zones. By detailed investigations of geological and paleogeothermal structures, an ancient seismogenic OST is identified in the Shimanto accretionary complex, southwest Japan. This OST consists of a mappable en echelon fault system developed at a late stage of the accretionary process. The estimated accumulated displacement of 2.5–8.5km and formation depth of 2.5–5.5km, based on a thermal analysis, indicates much higher fault activities sense of slip with a sinistral slip, and the large displacement was distributed among multiple small faults each with a small offset. Most of the faults in the en echelon fault system recorded repeated brittle failure, and one fault preserves pseudotachylyte, a fault rock indicative of seismic faulting. This is the first report of pseudotachylyte along an OST in an accretionary prism. Repeated fracturing in the same narrow shear zones (each only a few millimeters thick) suggests the OSTs perform as major shear localized zone in the accretionary prism. Shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone. These findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region. This OST showed high activity and repeated faulting in the shallow portion of the accretionary prism, comparable to that of the submarine tsunamigenic OST in the present Nankai trough. [Copyright &y& Elsevier]

Abstract: Thermal annealing behavior of fission tracks (FT) has not yet been documented under short-term (<4 min) heating condition. We performed a new series of 17 laboratory heating experiments on spontaneous FTs in zircon at 550–910 °C for ∼4, 10 and 100 s under strict controls of time and temperature using a graphite furnace. FTs were almost completely annealed at 912 °C for 3.9 s, at 858 °C for 10.4 s and at 805 °C for 100.6 s. These results suggest that FTs in zircon are totally annealed at time–temperature conditions of ordinary pseudotachylyte formation (i.e. at 1000 °C for 5 s). The measured FT lengths in ∼4 and 10 s heating runs are shorter than that predicted by previous annealing kinetic models, suggesting a more rapid annealing therein. The component of short tracks in these samples increases suddenly as the annealing proceeds, which has not been observed for longer heating runs including the results of previous studies. Such rapid shortening for ∼4 and 10 s heating is caused by earlier emergence of the annealing process of segmentation. [Copyright &y& Elsevier]

Abstract: Paleo-earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of Alpine Corsica. Pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the Schistes Lustrés of Cape Corse. Ultramafic pseudotachylyte fault- and injection veins are found within well-preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses. Numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified. Veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures. The newly formed minerals are zoned olivine (Fo93–89), clino- and ortho-pyroxene with compositions indicative of high crystallization temperatures (1300–1400 °C), zoned Cr-spinel, and a glassy to micro-vesicular hydrous matrix showing that frictional melts contained up to 4% water. Frictional heating on co-seismic faults raised the temperature from ambient blueschist facies conditions (450 °C and 1–1.5 GPa) to more than 1700 °C, which is required for ∼75% disequilibrium melting of spinel peridotite at 1.5 GPa. The characteristic fault-vein thicknesses observed are 1 to 3 cm, but several fault-veins are thicker than 10 cm. Co-seismic displacement of 1 m, a stress of 300 MPa, and seismic efficiency of 5%, may melt ca 60 kg peridotite pr. m2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte. The ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the Alpine subduction zone. If the interpretations of typical displacements of approximately 1 m are correct, the most common pseudotachylyte fault-veins are related to magnitude ca. 7 or larger subduction earthquakes. [Copyright &y& Elsevier]

In the Kodiak accretionary complex, Kodiak Island, Alaska, pseudotachylyte occurs in black, locally vitreous ultrafine-grained fault rock. Microscopic observations show that the pseudotachylytes are composed of glass, with vesicles, amygdules, microlites, and flow structures, indicating a frictional melt. The pseudotachylyte is gradational to cataclasite and shows outcrop-scale injection and ductile deformation structures. The cataclasite was ductily mobile (i.e., fluidized) simultaneous with the formation and emplacement of pseudotachylyte melt. The pseudotachylytic rocks postdate the stratal disruption fabric of associated shear-zone mélanges and show similar direction of thrust transport, and have undergone limited subsequent deformation. We interpret the stratal disruption as resulting from underthrusting of the subducting plate and pseudotachylyte development as the final activity of this thrust surface. The gradational contacts between pseudotachylyte and cataclasite demonstrate that the cataclasite also formed as a seismic product and may represent paleoseismic rupture zones, possibly of very great earthquakes, with or without accompanying pseudotachylytes. The pseudotachylytes are voluminous, and many are spatially disconnected from generation surfaces. This style is distinct from pseudotachylytes described in other environments, and this may explain the rarity of documented examples of subduction-thrust pseudotachylyte. [ABSTRACT FROM AUTHOR]

Abstract: For the common rock-forming minerals, the Joule–Thompson effect produces cooling on compression and heating on decompression. Experimental rock friction studies and seismological evidence suggest that the strength of faults is controlled by high-stress asperities that represent a small fraction of the total fault area. During seismic rupture, the area immediately beneath asperities is inferred to undergo adiabatic compressions and decompressions on the order of 1–2.8GPa. Joule–Thompson coefficients are in the range 261–372°C/GPa, resulting in temperature changes of ±336–880°C. Quartz asperities with a compressive strength of 2.8GPa can produce a heating or cooling of 880°C immediately beneath an asperity. By comparison, frictional heating ‘flash temperatures’ are less than 1000°C for asperity contact areas &#60;300μm2. The Joule–Thompson effect is independent of asperity size and may augment or counteract frictional heating. The paucity of pseudotachylytes in the geologic record and the absence of localized heat flow anomalies on active faults might be explained by the Joule–Thompson cooling effect. [Copyright &y& Elsevier]

The Mugi Mélange located in western Shikoku of the Shimanto Belt shows systematic Y-P deformation fabrics formed by microshear and pressure solution that penetrate throughout the mélange pile. Magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility (AMS) are highly oblate. Maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the mélange and the mean pole of P surfaces, respectively. This agreement suggests that the Mugi Mélange was formed as a result of underthrusting of trench filling sediment. Vitrinite reflectance ranges from 2.52% to 3.08%, which corresponds to a maximum paleotemperature of ∼180-200°C. Pseudotachylyte, evidence of a seismogenic slip, was found in the upper boundary roof fault of the Mugi Mélange. However, there is not a thermal gap between the mélange and the overlying coherent piles, and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the mélange beyond the boundary fault, which suggests that paleoisotherms parallel the boundary fault orientation. The isotherms in the seismogenic zone are estimated as subparallel to the plate boundary décollement. Therefore the setting of the cataclastic boundary fault, which includes pseudotachylyte, appears to be a major plate boundary thrust or a subhorizontal splay fault. A probable geologic setting that accounts for the Mugi Mélange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time: interseismic slip in the mélange and seismic slip along the roof fault. [ABSTRACT FROM AUTHOR]

Abstract: Near Granite Falls, Minnesota sub-parallel pseudotachylyte, mafic dikes, and calcite veins crosscut Archean granulite facies rocks in the Minnesota River valley adjacent to the north-dipping Yellow Medicine Shear Zone (YMSZ; N80°E) that separates the Montevideo and Morton tectonic terranes. The docking of these two Archean terranes occurred prior to intrusion of the 2.067 Ga Kenora-Kabetogama dike swarm as demonstrated by aeromagnetic anomalies (correlated with field exposures) that cross the YMSZ without offset. Tectonic adjustments along the YMSZ associated with the Penokean Orogeny (∼1.8 Ga) are likely responsible for pseudotachylyte formation. Pseudotachylyte is exposed in 22 sub-parallel veins (∼N80°E, 90°) each less than 2 cm wide across an outcrop width of 45 m. The pseudotachylyte matrix is commonly banded, and contains crystal fragments (quartz, plagioclase, amphibole, rutile, apatite, ilmenite, ulvöspinel), magnetite microlites, flow banding swirls, amygdules (filled with calcite, ankerite and siderite), collapsed vesicles, and abundant lithic clasts. Pseudotachylyte formed in a number of phases. Kinematic reconstruction is complex, utilizing winged porphyroclasts, S-C structures in the country rock, and fault drag indicators along the pseudotachylyte zones. Dextral motion along the YMSZ is the most common observation. Mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike. Calcite veins are apparently contemporaneous with the pseudotachylyte; one set preserves twinning strains identical to the calcite amygdule strains, and the second set contains a horizontal, vein-parallel (N70°E) shortening strain. The pseudotachylyte contains a flow fabric, as determined by AMS techniques, that is a proxy for vertical flow (K max is vertical). The Kenora-Kabetogama dikes, identified geochemically, are locally parallel to the pseudotachylyte and the adjacent YMSZ tectonic suture and preserve a vertical-to-horizontal, dike-parallel AMS fabric from east (Franklin) to west (Granite Falls). Hornblende andesite dikes (055°, 1.8 Ga) are not found south of the suture, are not associated with pseudotachylyte and have a different paleopole and AMS fabric. [Copyright &y& Elsevier]

Abstract: In situ UV-laser ablation 40Ar/39Ar geochronological and geochemical data, together with rock and mineral compositional data, have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the UHP whiteschists of the Dora Maira Massif, Italy. Several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation. Whole rock XRF data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith. Minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss. The pseudotachylyte contains newly crystallized grains of biotite and K-feldspar in a matrix of glass with partially fused grains of quartz, zircon, apatite, and titanite. Electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer-scale changes in mineralogy. UV-laser ablation ICP-MS traverses across the mylonitic gneiss–pseudotachylyte contact are consistent with cataclastic communition of REE carriers such as epidote, monazite, allanite, zircon, and apatite before melting as an efficient mechanism of REE homogenization in the pseudotachylyte. The 40Ar/39Ar data from one band of pseudotachylyte indicate formation at 20.1±0.5 Ma, when the mylonitic gneisses were already in a near surface position. The variable effects of top-to-the-west shear deformation within outcrops of the coesite-bearing unit are reflected in localized zones of protomylonite, cataclasite, ultracataclasite, and pseudotachylyte. Preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the UHP rocks. It is speculated that deeper crustal seismic events potentially played a role in the unroofing of the UHP rocks at earlier stages in their exhumation history. [Copyright &y& Elsevier]

Abstract: Since a pseudotachylyte is generated through a combination of comminution and frictional melting, it may record an ambient Earth''s magnetic field as a thermal remanence during the cooling from friction melt. To test the reliability of determining the paleointensity, the Coe-modified Thellier method was carried out on a small suite of pseudotachylytes in large-displacement fault zones from the North-range Sudbury impact structure. Pseudotachylytes studied here are classified into two types in their protolith lithology: gneiss-origin and granitoid-origin pseudotachylyte. Petrologic observations confirm the presence of abundant fine-grained magnetite inclusions and coarse-grained titanomagnetites with exsolved ilmenite lamella in gneiss-origin pseudotachylyte. In granitoid-origin pseudotachylyte, we found a scatter distribution of multidomain magnetite. Although the granitoid-origin pseudotachylyte fails to estimate the paleointensity, the gneiss-origin pseudotachylytes yield weak paleointensity values, which are consistent with previous paleointensity data from the norite in Sudbury Igneous Complex. These results suggest that the protolith petrology controls a magnetic mineralogy in pseudotachylytes and the reliability as recorder of ancient Earth''s magnetic field. [Copyright &y& Elsevier]

Abstract: Compositional variations are documented in friction melts along the Hari Hari section of the Alpine Fault, New Zealand, with multiple stages of melt injection into quartzo-feldspathic schists. Intermediate to felsic melts were heterogeneous in composition, but all fractions show a common trend, with a tendency for the younger melt layers and glasses to be more alkali−(Na+K) and Si-enriched, while being depleted in mafic (Fe+Mg+Mn) components. These changes are attributed primarily to crystal fractionation of the melt during transport. Farther traveled molten layers were on the whole less viscous, mostly due to a higher melt-to-clast ratio; however, compositional change, together with a decrease in volatile content, produced a progressively more viscous liquid melt with time. The glass phase is interpreted as a remnant of this high viscosity felsic residual melt that was preserved during final quenching. Following initial failure, the formation of largely phyllosilicate-derived, volatile-rich, lower viscosity melt corresponds with a phase of fault weakening. Subsequent rapid crystal fractionation during melt transport, the loss of volatiles and freezing of residual melt contributed to the strengthening of the fault during seismic slip. [Copyright &y& Elsevier]

Abstract: In the North Cascade Mountains, Washington, rocks that underwent friction melting commonly contained sulfide minerals, mostly pyrrhotite and pyrite. During pseudotachylyte melt formation, sulfides melted to form immiscible sulfide droplets present in five distinct textural settings. The largest droplets formed through melting of lithic clasts, whereas micron-scale sulfide droplets are common in many of the pseudotachylytes veins. Microprobe analysis indicates that nearly all droplets are pyrrhotite. The disappearance of pyrite indicates that melt temperatures must have exceeded 750 °C, but other indications suggest that the melt temperature must have been much higher. The extremely common presence of pyrrhotite droplets suggests that pyrrhotite from the protolith melted, requiring a minimum melt temperature of ∼1200 °C. In some samples, evidence for fluid-rich bubbles, and possible silicate spherules indicates three coexisting immiscible phases within the silicate melt. The presence of sulfide droplets appears to be common, especially in relatively low oxygen-fugacity melts that formed at shallow crustal levels. This can provide a good textural marker of melting and therefore of pseudotachylyte formation, especially where other indications of melting (i.e., high temperature microlites, vesicles, etc.) are lacking, and illustrates the extreme temperatures possible along frictionally sliding surfaces during seismic events. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates ∼1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults 1–2–3 ). Fault displacement is in the range of few centimeters (faults 3 ) to a maximum of a few tens of meters in major faults 1 . Faults 1–2 nucleated on pre-existing joints, whereas faults 3 are newly generated fractures produced during slip along faults 1–2 . Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe3+ compared to host rock. We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios. [Copyright &y& Elsevier]

Abstract: The Woodroffe thrust, central Australia, is a >1.5-km-wide mylonitized shear zone marked by large volumes of mm- to cm-scale pseudotachylyte veins. The pseudotachylytes display typical melt-origin features, including rounded and embayed clasts, spherulitic and dentritic microlites, and flow structures within a fine-grained matrix. Three types of pseudotachylyte are identified on the basis of deformation texture, vein morphology, and host-rock lithology: cataclasite-related (C-Pt), mylonite-related (M-Pt), and ultramylonite-related (Um-Pt). The M-Pt and Um-Pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization. These pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine-grained vein matrix, and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite. These observations constrain the timing and environment of M-Pt and Um-Pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust. The M-Pt and Um-Pt veins, as well as the host mylonite, are overprinted by cataclasis and multiple generations of late-stage C-Pt veins that were generated in the brittle-dominated regime of the upper crust during uplift and exhumation of the shear zone. The coexistence of multiple generations of voluminous C-Pt, M-Pt, and Um-Pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the Woodroffe thrust. The timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle-dominated seismogenic zone and propagated down through the brittle–ductile transition into the ductile-dominated regime of the crust. [Copyright &y& Elsevier]

Abstract: The internal sectors of the Orobic Alps (Northern Italy) are characterised by Alpine age regional shortening showing a transition, through time, from plastic to brittle deformation. Thrust faults cut Alpine ductile folds and are marked by cataclasites and, locally, by pseudotachylytes, suggesting that motion was accommodated by seismic frictional slip. In the Eastern Orobic Alps the thrusting initiated at depths deeper than 10 km (the emplacement depth of the Adamello pluton) and possibly continued at shallower depths. This demonstrates that thrust motion occurred between 10 km depth and the brittle-ductile transition, i.e., at mid-crustal depths. The Orobic Alps exhumed paleoseismic zone shows different geometries along strike. In the central sectors of the Orobic Alps, thrust faults, associated with pseudotachylytes, have average dips around 40° and show no pervasive veining. Much steeper thrusts (dips up to about 85°) occur in the eastern Orobic Alps. In this area, faults are not associated with pervasive veining, i.e., fluid circulation was relatively scarce. This suggests that faulting did not occur with supralithostatic fluid pressure conditions. These reverse faults are severely misoriented (far too steep) for fault reactivation in a sublithostatic fluid pressure regime. We suggest that thrust motion likely started when the faults were less steep and that the faults were progressively rotated up to the present day dips. Domino tilting is probably responsible for this subsequent fault steepening, as suggested by a decrease of the steepness of thrust faults from north to south and by systematic rotations of previous structures consistently with tilting of thrust blocks. When the faults became inclined beyond the fault lock-up angle, no further thrusting was accommodated along them. At later stages regional shortening was accommodated by newly formed lower angle shear planes (dipping around 30–40°), consistently with predictions from fault mechanics. [Copyright &y& Elsevier]

Abstract: In Central Otago, New Zealand, a series of subparallel, NNE–SSW trending, gently dipping (10–30° E) brittle faults are hosted in quartzofeldspathic schist. These faults contain numerous (n>100) pseudotachylyte veins. Pseudotachylyte fault veins are traceable along strike for distances of up to 200 m, and are between &#60;1 mm and 5 cm thick. Fault separations inferred from offset quartz veins, dilational jogs and a piercing point analysis suggest that pseudotachylyte was generated by slip increments of &#60;1 cm to 20 cm, comparable to those believed to occur during small to moderate-sized earthquakes [Sibson, R.H., 1989. Earthquake faulting as a structural process. Journal of Structural Geology, 11 (1–2) 1–14.]. Dilational jogs, fault drag and quartz vein separations imply a top-to-the-north (normal) sense of shear. Evidence for melting is provided by chilled margins, microlites and the formation of mineral phases not present in the host rock. Melting temperatures of 900–1100 °C are inferred from petrological observations. Heat-work calculations, utilising displacements on pseudotachylyte-bearing faults, and inferred melting temperatures suggest that the faulting occurred at depths of 6–12 km, with average frictional shear resistance exceeding 100 MPa on pseudotachylyte-generating faults. These inferred depths and relatively high levels of frictional shear resistance on pseudotachylyte-generating faults are similar to those inferred by some previous studies of pseudotachylyte (e.g. [Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. The Geophysical Journal of the Royal Astronomical Society, 43 (3) 775–794; Killick, A.M. and Roering, C., 1998. An estimate of the physical conditions of pseudotachylite formation in the West Rand Goldfield, Witwatersrand Basin, South Africa. Tectonophysics, 284 (3–4) 247–259.]). [Copyright &y& Elsevier]

The seismic slip that occurred during the 1999 Chi-Chi earthquake in Taiwan showed contrastive behaviors in different regions along the Chelungpu Fault: A large and smooth slip occurred in the north, while a relatively small slip associated with high-frequency seismic wave radiation occurred in the south. The core samples from shallow boreholes at northern (Fengyuan) and southern (Nantou) sites penetrating the seismic Chelungpu Fault were analyzed. The fault zones at the northern site are characterized by soft clayey material associated with clayey injection veins. This suggests that the fault zones were pressurized during ancient seismic slip events, and hence the elastohydrodynamic lubrication occurred effectively. In contrast, the fault rock from the southern site is old pseudotachylyte that has been shattered by repeated ancient seismic slip events. Statistical analysis of many pseudotachylyte fragments reveals that the degree of frictional melting tended to be low. In this case, the seismic slip is restrained by the mechanical barrier of a highly viscous melt layer. These contrastive fault rocks were produced by repeated ancient seismic slip events, but the two corresponding mechanisms of friction are likely to have also occurred during the 1999 Chi-Chi earthquake, thus causing the contrastive slip behaviors in the north and south. [ABSTRACT FROM AUTHOR]

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(=Dr, Da or Dv) 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′=ω2, and v′=ω3. The analysis suggests that the modified power-law clast-size distribution pattern is a characteristic feature of all fault-related pseudotachylytes. [Copyright &y& Elsevier]

Pseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25–0.3 GPa and 250–300 °C. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly <2 cm) according to the relation: x=(0.29±0.12)a. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (Tmelt1450 °C). Melt temperatures estimated by the clast/matrix ratio () are considerably lower (316–577 °C). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes. [Copyright &y& Elsevier]

Fault-generated pseudotachylytes are the product of frictional melting during high-velocity slip associated with coseismic faulting. 40Ar–39Ar dating of pseudotachylytes represents a powerful tool to directly determine the age of brittle faulting which is otherwise dated only indirectly. However, the pronounced spatial heterogeneity of most endogenically generated pseudotachylytes, due to the intimate coexistence of unmelted mineral clasts from the source rock, frictional glass and neogenic igneous minerals, often precludes a straightforward interpretation of argon data. This requires careful evaluation of the role of clasts in plaguing the age record of the pseudotachylyte matrix. This study exploits the full potential of the 40Ar–39Ar method, using laser step-heating and laser in situ techniques in conjunction with textural and chemical characterisation at the microscale to disentangle the complexity of the first record of pseudotachylytes generated in the right-lateral fault systems which dissect the western shoulder of the West Antarctic Rift System. Pseudotachylytes occur as fault and injection veins. They exhibit glassy matrices with a potassium feldspar-like composition, and invariably contain, at a millimeter to microscopic scale, unmelted quartz and subordinate feldspars from the source rock. Injection veins characteristically contain much fewer mineral clasts. In situ ultraviolet (UV) laser analyses indicate that quartz from the host rock and from clasts within the fault veins contains significant amounts (∼10–50 ppb) of parentless 40Ar, most probably hosted in microscopic to submicroscopic fluid inclusions, associated with high Cl/K ratios. Infrared laser step-heating experiments on fault vein pseudotachylyte matrices yield strongly discordant age spectra with an overall saddle shape characterised by a minimum at ∼34 Ma, unrealistically old ages at high temperatures associated with high Cl/K ratios and total gas ages of 41.5–44.3 Ma. Petrographic data and the pronounced compositional similarity between in situ UV laser data on quartz and the high-temperature end member of step-heating analyses suggest that quartz is the main contaminant of the pseudotachylyte age record. In situ laserprobe analyses on pseudotachylyte matrices give concordant ages at ∼34 Ma for an injection vein and systematically older ages, clustering at ∼40–48 Ma, for fault veins. Although the age cluster fits into the Cenozoic tectonic framework of the region, it is an artefact due to the low spatial resolution of the argon laserprobe compared to the size and spatial distribution of quartz clasts within the pseudotachylyte. The isochron age of 34.11±0.96 Ma, derived from in situ data from the injection vein, overlaps with the emplacement age of syn-tectonic dykes from nearby areas and is interpreted to date a single episode of coseismic faulting. Regionally, the age of the studied pseudotachylytes represents the first direct onshore evidence of right-lateral strike–slip fault system activity in Victoria Land during the Cenozoic. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

The metamorphic complex of the Western Gneiss Region (WGR), Norway, constitutes the root of the Caledonian mountain belt and experienced temperatures of 700–800 °C and pressures in excess of 20 kbar during peak metamorphism. Mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures. The Kra˚keneset gabbro, located on the island of Va˚gsøy in the mixed HP/UHP zone of the western WGR, display evidence for extensive metastability through the entire prograde and retrograde P, T histories. Eclogite constitutes less than a few percent of the total volume of the body and high-pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration. The gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones, <50 cm wide, transecting the gabbro body. The pseudotachylytes contain μm- to mm-scale amoeboid and dendrite-like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene, omphacite, amphibole, and dolomite, suggesting rapid disequilibrium growth of minerals during high-pressure conditions. Textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid-state alteration of a microcrystalline pseudotachylyte matrix. The pseudotachylyte-lined fault zones are in close spatial association with numerous amphibole±carbonate-filled hydrofractures with conspicuous fracture-parallel alteration zones defined by hydrous eclogite facies assemblages. These eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high-grade metamorphic conditions. Geothermobarometric estimates (ca. T=650–700 °C, P=∼20 kbar) and petrological data imply that hydrofracturing, pseudotachylyte crystallization, and the subsequent pseudotachylyte alteration process must have occurred during high-pressure metamorphism. Our observations are suggestive of a deep-crustal earthquake scenario where a high-pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro. Metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition. The formation of weak deep-crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the Kra˚keneset gabbro during its residence deep in the former Caledonian collision zone. [Copyright &y& Elsevier]

Our understanding of frictional melting that occurs during large earthquake slip events has been hampered by the extremely fine-grain size of frictionally fused rock, and the lack of detailed high-resolution microscopy studies that describe features at the crystal lattice scale. In such a study, we here report the complex nature of melting and crystallization in a symmetrically layered, pseudotachylyte vein from the Alpine Fault, New Zealand. Two melt pulses are recognized, attributed to successive, but rapid, injections of frictionally generated material. The initial injection, preserved at the vein margins, was proximally derived and contains a high concentration of clasts and a Si-rich glass. This was quickly followed by a second generation of a more distally derived, melt-dominated fraction, which was injected into the weak vein center. Whereas fragments of wall-rock biotite are preserved in the marginal zones, neocrystallized microlitic biotite characterizes both margins and center. The vein biotite is different in composition, microstructure and polytypism from the metamorphic biotite of the wall rock. In all melt layers, newly formed biotite shows notable signs of syn-flow crystallization, strain features and erosion at crystal–glass contacts, with breakdown of neocrystallized microlites along both crystal edges and faces. These characteristics imply cyclic pulses of heating, melting and crystallization occurred during a single, large earthquake episode, and probably reflects the stick-slip propagation properties of coseismic faulting. 40Ar/39Ar total gas ages from the vein center give a 1.11±0.04 Ma date for cyclic melting which, based on current exhumation rates, occurred at a crustal of depth ∼11 km. [Copyright &y& Elsevier]

The Cenozoic intracontinental Teletsk basin in the Central Asian Altai Mountains is composed of a complexly structured northern and a more simple southern sub-basin. These sub-basins formed in two distinct kinematic stages when first the NNW-striking Teletsk- and then the NE-striking West-Sayan shear zones became reactivated in the Cenozoic under dominant NS-oriented horizontal compression. Although the entire Teletsk basin strikes roughly NS, the southern sub-basin is parallel to the NNW-trending, amphibolite facies Teletsk ductile shear zone, while the northern sub-basin is NS-striking and flanked by differently structured, greenschist facies basement. Basement reactivation closely controlled the southern sub-basin formation, but this is less clear for the northern sub-basin. Contrasts between northern and southern basement and the exclusive occurrence of pseudotachylytes along the margins of the southern basin are explored for their contribution to the formation of the Teletsk basin with two distinct sub-basins.In the ductile shear fabric of the basement flanking the southern sub-basin, concordantly interleaved pseudotachylytes and isolated breccia lenses reflect local brittle deformation along the ductile fabric. The genetic link between breccia lenses and pseudotachylyte occurrences was defined by microstructural investigation. It allows to explore their possible development in a dextral strike–slip zone. These rocks occur in a large fault-bounded segment of the basement. The geometry of the structures in the segment is comparable with a dextral strike–slip sidewall-ripout structure along the Teletsk shear zone. Seismic slip related to pseudotachylytes is attributed to the sudden stress release on the NNW-striking Teletsk shear zone, when the latter became unconstrained by reactivation of the NE-trending West-Sayan fault zone at its northern boundary. The boundary of the sidewall-ripout structure was reactivated as a large listric fault in a later stage. The northern sub-basins roughly develop along an NS strike and are assumed to reflect reactivation of the ductile shear zone underneath the variably structured greenschist facies basement outcropping along the flanks of the sub-basin. [Copyright &y& Elsevier]

Abstract Cataclastic rocks found in the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drill core and outcrops along the Nojima Fault zone on Awaji Island, southwest Japan, were examined at mesoscopic and microscopic scales. The damaged zone of this fault in granitic rocks, observed on the southeast side of the fault, is 50–60 m wide and is composed of fractured host rocks and cataclastic rocks including cataclasite, fault breccia, and fault gouge. The fault breccia and gouge of small scales are scattered in the damaged zone. Fault core (zone of extremely concentrated shearing deformation along a fault) consists of fault gouge measuring several tens to approximately 150 mm in width, as recognized both in the drill core and at outcrops of the Nojima Fault along which surface ruptures formed during the 1995 Kobe earthquake. Fault breccia, measuring a few meters wide, has developed pervasively in the damaged zone, just next to the fault core. Pseudotachylyte has been found interlayered with fault gouge within the fault core only at outcrops at Hirabarashi, not in the DPRI 500 m core. Petrological studies and powder X-ray diffraction analysis show that the pseudotachylyte and fault gouge are composed mainly of fine-grained angular clasts of the host granitic rocks, suggesting the pseudotachylyte is of ‘crush origin’. Foliated cataclasite is characterized by the preferred orientation of elongated biotite clasts and granular aggregates of quartz and feldspar clasts, and by the development of cataclastic shear bands. Unlike cataclastically deformed quartz and feldspar in the cataclasite, biotite in the foliated cataclasite shows combinations of brittle and plastic deformation, such as biotite ‘fish’, cleavage steps, bending and kinking. These textures suggest that the foliated cataclasite formed at a deeper level than the cataclasite, fault breccia and gouge, possibly before the Quaternary period during which... [ABSTRACT FROM AUTHOR]