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In the repeatedly reactivated Gavilgarh–Tan Shear Zone of central India, sheared and foliated pseudotachylyte (Pt–M) veins, with characteristic signatures of viscous deformation, occur as discrete layers within granitic mylonite. Deformation history and age constraints described by earlier workers suggest that semi-brittle reactivation of the shear zone possibly occurred near the frictional-viscous transition at an intermediate depth of the crust that formed these pseudotachylyte layers. Detailed microstructural study and electron back-scattered diffraction (EBSD) analyses of the mylonite and the Pt–M layers were carried out in the present study, which suggest that the mylonite deformed at a high temperature (>500°–600°C) with dominance of Regime 3 Grain Boundary Migration (GBM) recrystallization. Quartz grains in the mylonite deformed dominantly by prism and rhomb slip. In the Pt–M, significant grain size reduction occurred by initial cataclasis and later recrystallization processes. Deformation of quartz grains by a combination of basal and (subordinate) rhomb slip suggests relatively lower temperatures for Pt–M deformation. Grain boundary misorientation distribution (MAD) shows the dominance of low-angle subgrain boundaries in neighbour-pair grains, suggesting the dominance of subgrain rotation (SGR) recrystallization in the Pt–M matrix. Extensive development of chlorite, sericite, and epidote in Pt–M veins suggest hydration during shearing. Prism slip was possibly activated at a lower temperature (<400°C) due to the hydrolytic weakening of quartz. The Pt–M veins developed an interconnected weak layer of biotite, chlorite, and/or epidote during shearing and retrogression that helped in concentrating viscous deformation in these layers. [ABSTRACT FROM AUTHOR]

In this paper, we report an occurrence of ultramafic pseudotachylytes, providing fault-rock evidence of paleo-earthquakes, from the Luobusha ophiolite complex in the Yarlung Zangbo suture zone. The pseudotachylytes form hairline-thin aphanitic veinlets and vein networks bounded by micro-damage zones cutting through the host harzburgite, forming flow banding in some places. The pseudotachylyte veins are dominated by close-knit ultrafine-grained minerals consisting of olivine, orthopyroxene, serpentine, spinel, and magnetite, cemented by an extremely fine matrix. As the primary component of the pseudotachylyte veins, olivine occurs as microphenocryst showing zoning from core to rim and as irregularly shaped microlite immersed in the interstitial material. Zoned crystals of olivine developed with Mg-rich cores and more Fe-rich rims. Microlite diagnosis of crystallization in a quenched melt includes dendritic, skeletal, and poikilitic olivine crystals, which are typical of ultramafic pseudotachylyte. The olivine microlites contain higher amounts of Ca, Al, and Cr but a lower Ni content compared with the host harzburgite olivine. Irregularly shaped chrome-spinel crystals are chemically zoned as well, indicating an Fe-rich rim overgrowth. Ni sulfide droplets interspersing among the matrix imply melt occurrence. The presence of a micro-fibrous and micro-vesicular interstitial matrix also indicates a melting-related origin. Ultracataclastite veins associated with pseudotachylyte transecting serpentine are observed, which convey that heat was generated during rapid comminution and injection. The characteristic petrography, microtextures, and chemical inhomogeneities meet the criteria of ultramafic pseudotachylyte and reveal a mixed genesis via a combination of crushing and melting. The development of extremely tiny globular prograde serpentine inclusions (∼100 nm) in the olivine microlites is ascribed to the dehydration reaction of serpentine to olivine within the pseudotachylyte. The Luobusha metamorphogenic peridotite was subjected to serpentinization after having emplaced in the crust and subsequently to high-pressure metamorphism. The pseudotachylytes were generated in the crust after the high-pressure metamorphism and did not descend to a greater depth. Flash ultra-comminution associated with frictional heating may release fluids via localized heat-driven prograde reactions in the crust.

Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep. Plain Language Summary: Earthquakes create fractures and increase the porosity in crustal rocks. These fractures can help transport fluids to newly accessible regions in the crust, which in turn may kickstart metamorphic reactions, and potentially alter the rheology. However, very little is known about the mechanisms, the microstructural context, and the morphology of this increased porosity. We analyzed ancient earthquake‐generated frictional melts (pseudotachylytes) and their immediate damage zone in the host rock, as well as plasticly deformed pseudotachylytes, that have since been exhumed from depth and are now exposed at the surface in Lofoten, Norway. We analyzed these rocks to determine the processes that create porosity and how this porosity evolves with increasing plastic deformation. The pseudotachylyte hosts more porosity than the damage zone immediately flanking the vein, in particular there is a high concentration of porosity around garnets. We interpret this porosity to have formed as a result of the metamorphic growth of garnet. Much of the fracture‐related porosity created during the initial earthquake has been efficiently sealed. Porosity is greatly reduced in the sheared pseudotachylytes because of solution‐precipitation processes that operated during ductile deformation. Porosity reduction may reflect fluid consumption, leading to shear zone hardening and possibly new pseudotachylyte formation. Key Points: Fracture porosity is healed in the wall rock, while porosity from metamorphic recrystallization is preserved in the pseudotachylyte veinPorosity in the pseudotachylyte is poorly interconnected and is concentrated around garnets and in survivor clasts from the wall rockPorosity in the mylonite is greatly reduced because of precipitation of phases in transient pores during grain‐size sensitive creep [ABSTRACT FROM AUTHOR]

The development of multiple paleotemperature proxies over the last twenty years has led to an increasing number of coseismic temperature measurements collected across a variety of faults. Here we present the first compilation of coseismic temperature rise measurements and frictional energy estimates to investigate the contribution of frictional heating to the earthquake energy budget and how this varies over different fault and earthquake properties. This compilation demonstrates that there is no clear relationship between coseismic temperature and displacement or thickness of the principal slip zone. Coseismic temperature rise increases with the depth of faulting until ~5 km and below this depth temperature rise remains relatively constant. Frictional energy, similarly, increases with depth until ~5km. However, frictional energy is remarkably similar across all of the faults studied here, with most falling below 45 MJ/m2. Our results suggest that dynamic weakening mechanisms may limit frictional energy during coseismic slip. We also demonstrate a basic difference between small and large earthquakes by comparing frictional energy to other components of the earthquake energy budget. The energy budget for small earthquakes (1-10 m of displacement), frictional, radiated, and fracture energy contribute somewhat equally to the earthquake energy budget.

Earthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure. Plain Language Summary: Earthquake initiation within strong, dry rock types in the Earth's lower continental crust requires high driving stresses if fluids are absent. Several mechanisms have been proposed to generate these unusually high stresses deep in the lower crust, many of which imply very short‐term stress increases. High short‐term stresses are proposed by field‐based studies and numerical modeling, but the geological record for these stress increases occurring in the build‐up to earthquakes is not always obvious because the effects of the subsequent earthquake normally will overprint the evidence. However, in some cases a complete record of stress change before, during and after an earthquake may survive. In this study, detailed observations of the deformation of pyroxene (a silicate mineral) characterizes the short‐term changes in stress and temperature experienced before and during earthquakes recorded in the lower crust. Pyroxene crystals close to faults exhibiting ancient earthquake‐generated frictional melts (pseudotachylytes) are investigated from an exhumed fault zone in Lofoten, northern Norway. The microstructures imply high stress increases prior to the earthquake, followed by widespread fragmentation and grain growth linked to the passage of earthquake slip. These results support very high stresses localized within the lower crust and show that crystal deformation can capture such changes. Key Points: Microstructures capable of recording transient stress changes prior to and during earthquake rupture in the lower crustLocalized differential stresses exceeding 1 GPa during prerupture loading accommodated in dislocation glide of pyroxenesCoseismic deformation represented by pulverization style fragmentation and thermally activated grain growth within orthopyroxene [ABSTRACT FROM AUTHOR]

The mechanisms of earthquake rupture in lower continental crust, below the usual frictional‐viscous transition, remain uncertain. In addressing this problem, the study of pseudotachylyte (quenched frictional melt produced during seismic fault slip) and related structures from deeply exhumed rocks can provide direct observational constraints. A felsic granulite from the Musgrave Ranges (central Australia) exceptionally preserves pristine microstructures spatially related to a pseudotachylyte. This sample remained dry, without introduction of hydrous fluids, during pseudotachylyte development and subsequent exhumation. It was therefore unaffected by alteration and metamorphic re‐equilibration. Fractures in the damage zone developed asymmetrically to either side of the pseudotachylyte and are marked by new, randomly oriented quartz, feldspar, and garnet grains. Pulverization of garnet occurred locally between intersecting fractures with powder injected into dilatant fractures in a quartz inclusion within the garnet host. Injection of pulverized material can explain the growth of new, compositionally different minerals (quartz, feldspar, kyanite, ilmenite, magnetite, and rutile) along dilatant fractures developed in a short‐lived seismic event. The pseudotachylyte contains only clasts of quartz, suggesting an unusually high melting temperature. The sequentially developed microstructures provide a time‐lapse record of thermomechanical processes during a single earthquake event, including initial rupture propagation with associated off‐fault damage and local pulverization during very dynamic fluctuations in the local stress field; frictional heating and eventual melting during fault slip; flow and injection of melt; and rapid solidification (quenching) and crystallization of new minerals. This occurred under lower continental crustal conditions of ca. 650°C and 1.2 GPa about 550 Myrs ago. Plain Language Summary: Some special rocks (pseudotachylytes: quenched melts produced by frictional heating during seismic slip on a fault) exhumed from deep in the Earth's crust provide the opportunity to study old earthquakes that occurred at depths that are not directly observable. The pseudotachylyte we studied from the Musgrave Ranges in central Australia was formed during a single earthquake some 550 million years ago at depths of ca. 40 km and temperatures of ca. 650°C, but remained remarkably well preserved on its path to the Earth's surface. As a result, the sample preserves a complete time‐lapse record of the sequence of microstructures representative of different thermal and mechanical processes, which lasted only seconds to minutes during a seismic event, including initial fracture propagation, off‐fault damage with local pulverization, and development, flow, and final solidification of frictional melt. Key Points: Time sequence of microstructures related to pseudotachylyte formation in the lower continental crust (Musgrave Ranges, central Australia)Dry conditions with no infiltration of water‐rich fluid before, during, or after seismic rupturing and pseudotachylyte developmentAsymmetric fracturing, local garnet pulverization, and high‐temperature frictional melting represent different stages of a single earthquake rupture [ABSTRACT FROM AUTHOR]

Transient creep during an earthquake cycle is very important to understand the rheology of fault and deformation mechanisms in the brittle–plastic transition zone. In this paper, we analyzed the microstructures of samples for mylonites, pseudotachylyte, and cataclasite under optical microscope, SEM, and EBSD, which were collected from the Red River fault in southwest of China, where we uncovered a series of ductile to brittle deformed rocks which recorded transient creep related to earthquakes. The results show that mylonites formed at the inter-seismic creep were overprinted by pseudotachylyte and cataclasite which were produced during co-seismic rupture, and cracks in cataclasite were healed during the post-seismic relaxation. Based on the analysis of the microstructures and deformation mechanism of fault rocks, we propose the oscillation deformation model to explain transient creep of the brittle–plastic transition zone during the seismic cycle in the Red River fault.

Mantle peridotites exhumed in mountain belts provide important insights on the composition and evolution of the upper mantle, and additionally inform on metamorphic, geochemical, and tectonic processes -including seismic activity-at convergent margins. In this contribution, we present field, microstructural, and mineralogical data of fresh, pseudotachylyte-bearing mantle peridotites from the lawsonite eclogite-facies San Petrone unit, Alpine Corsica, France. The present case study represents the first example of subducted fresh peridotite associated with fresh lawsonite eclogite-facies assemblages. Two bodies of fresh peridotite are embedded in fully serpentinized ultramafic rocks forming the substratum of a subducted ocean-continent transition of the Piemonte-Liguria Basin. Clinopyroxene and spinel mineral chemistry indicates that the investigated peridotite samples were part of a refertilized mantle and, therefore, the San Petrone unit likely belonged to the more distal part of the ocean-continent transition. Because small bodies of fresh peridotite embedded in fully serpentinized rocks can hardly be identified by means of geophysical investigations, this finding suggests that small, yet disseminated bodies of fresh mantle peridotite can potentially be more abundant than previously supposed at ocean-continent transition and, potentially, at mid-ocean ridges and in subduction zones. The preservation of fresh mantle peridotite bodies in subducting slabs is also discussed with respect to its potential implications on intermediate depth seismicity and geochemical cycling -including production of natural energy sources-from rifting to subduction. [ABSTRACT FROM AUTHOR]

Abstract Megasplay fault branching from plate boundaries of subduction zones is thought to be important sources of earthquakes and tsunamis. In this study, we performed structural and geochemical analyses on a fossilized megasplay fault (the Nobeoka Thrust of the Shimanto accretionary complex) to understand fluid-rock interaction and how the splay fault plays a role in fluid flow in the seismogenic zone. As a result of structural observations, we report that the principal slip zone (PSZ) of the Nobeoka Thrust is composed of foliated cataclasite originating from a sandstone-shale mélange and includes a thin (~ 1.5 mm thick) pseudotachylyte layer. Major and trace element composition analysis and EPMA element mapping revealed that the pseudotachylyte is enriched in Li and Cs within the PSZ, as well as in the slip zone of a minor fault in the footwall. Li and Cs enrichment in pseudotachylyte is interpreted as a result of fluid-rock interaction in the postseismic stage, because such an anomaly only results from a large fluid/rock ratio (R > 512–24 at 250–350 °C) under the influence of Li- and Cs-enriched fluids. The amount of fluid that reacted with the pseudotachylyte is estimated to be 1.78 × 100 to 7.61 × 103 m3.

Devastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear‐heating event. The well‐preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti‐poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10−6 [SI] volume, with P′ ranging from 1.05 to 2.5) to reconstruct the co‐seismic deformation parameters, that is, fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included mostly normal mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite‐hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution. Key Points: Ultramafic pseudotachylytes inform on rupture kinematics in subducted slabsMiniature‐size anisotropy of magnetic susceptibility records seismic viscous flow in frictional meltsEarthquake focal mechanisms can be inferred from magnetic fabrics [ABSTRACT FROM AUTHOR]

Pseudotachylyte (Pt) veins associated with ancient fault zones are considered as reliable palaeoseismic indicators as they are produced by a combination of cataclasis and frictional melting of the host rock during rapid (seismic rate) fault slip. In spite of a large amount of work done on pseudotachylytes, several issues related to the frictional melting process are not completely resolved for example, the nature and origin of domain‐scale chemical heterogeneity of the Pt matrix, the contribution of different minerals of the host rocks in forming the melt product, temperature variation in different domains of the Pt veins, among others. To revisit these issues, pseudotachylyte veins hosted entirely within pelitic gneisses (Pt‐I) and those formed along the contact of gneisses with intrusive meta‐dolerite dykes (Pt‐II) within the Sarwar‐Junia Fault Zone in Rajasthan, India, are studied in detail using optical and scanning electron microscope, X‐ray diffraction (XRD), X‐ray fluorescence, and electron probe micro‐analyser, with an intention to better understand the evolution of the microstructures and the heterogeneous chemical character of the pseudotachylyte veins and their host rocks. Unequivocal melt‐origin microstructures (e.g., microlites, spherulites, melt flow structure) are observed in the pseudotachylyte matrix, and an indirect evidence of amorphous (glass?) phase is found through XRD. The Pt‐matrix is more mafic than the host rock or the bulk Pt‐vein, irrespective of the rock types involved, indicating preferential melting of mafic minerals of the host rock(s). Microdomain‐scale variation in mineralogical and chemical character of Pt‐matrix suggests co‐existence of melts of different composition which remained unmixed due to rapid quenching of the melt. Mineral chemistry of fine microlites and spherulites within the Pt‐matrix differs significantly from the corresponding parent mineral grains in the respective host rocks suggesting their formation by direct crystallization from a melt phase. Formation of microlites by devitrification of solidified glass is discounted due to the absence of fluid activity, and due to the spatial distribution of finer microlites (near the Pt‐vein boundary), and coarser spherulites (near the centre). It is surmised that frictional heating attains different temperatures within a single Pt‐vein, and in adjacent Pt‐veins, and the constituent minerals melt selectively as the temperature rises beyond the melting points of different constituent minerals of the host rock. Neither preferential crushing of mafic minerals followed by bulk melting, nor eutectic (equilibrium) melting of the host rock can explain the strongly heterogeneous character of pseudotachylyte melt observed in this study. [ABSTRACT FROM AUTHOR]

To constrain the conditions of the brittle‐ductile transition in peridotite, we studied the deformation sequence of a small‐displacement fault system developed in the Balmuccia peridotite body, northern Italy. The ambient pressure‐temperature conditions of the deformation were estimated using equilibrium mineral assemblages and mineral compositions. Fault‐related deformed rocks, including pseudotachylytes, cataclasites, and mylonites, were classified into three groups according to the metamorphic facies at the time of their formation: Group 1 consists of mylonites, cataclasites, and pseudotachylytes of the spinel‐lherzolite facies; Group 2 consists of cataclasite and pseudotachylyte, but no mylonite, of the plagioclase‐lherzolite facies; and Group 3 consists of cataclasite and pseudotachylyte of a lower‐grade hydrous facies. Cataclasite of the spinel‐lherzolite facies coeval with fault‐vein pseudotachylyte was identified for Group 1 and ambient temperature of the earthquake was determined by applying geothermometers to the mylonite and cataclasite. Among the deformed rocks of the spinel‐lherzolite facies (Group 1), mylonite is truncated by veins of pseudotachylyte. However, the mineral compositions of the mylonite and the cataclasite associated with the pseudotachylyte are essentially identical, yielding a temperature for the ductile‐to‐brittle transition in the peridotite of ~720 °C at a pressure of ~0.6–1.6 GPa. In the pressure range, low‐pressure side, which overlaps with conditions of surrounding crustal rocks, is more consistent with obtained fault structures and regional tectonics. This condition lies on a geotherm of an oceanic lithosphere of the age of ~10 Ma and is consistent with a predicted temperature and depth using published flow laws of olivine.Key Points: Fault rocks in a peridotite body record three distinct periods of pseudotachylyte formation during uplift and exhumationThe earliest pseudotachylytes were formed at a deformation condition close to the brittle‐ductile transitionThe deepest seismic activity in the mantle rock occurred at an ambient temperature of ~720 °C [ABSTRACT FROM AUTHOR]

Quartz/feldspar fragment (‘clast’) sizes were measured in thin sections of three types of fault zone rocks, e.g., melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and cataclasite (Ct), from two well-studied Precambrian shear/fault zones in the Indian craton (e.g., the Gavilgarh–Tan Shear zone in central India and the Sarwar–Junia Fault zone in western India). Logarithmic plots of clast area vs. cumulative frequency in the pseudotachylytes demonstrate a fractal clast-size distribution (c.s.d.) for the intermediate size range, whereas the finer and coarser clast size fractions clearly deviate from the fractal trend. Under-representation of the finer size clasts in the pseudotachylyte samples may be attributed to their preferential melting and removal from the clast population. The relative paucity of coarse clasts, on the other hand, is possibly due to a sampling bias against coarse clasts. The c.s.d of the cataclastic rock shows a multi-fractal character with two different slopes (i.e., lower D-value for finer clast sizes) and absence of the left-hand (finer size) fall off. This suggests less efficient crushing in the finer clast size fraction. The proportion of clasts, compared to the matrix, is very small in M-Pt, increases in C-Pt and is highest in Ct, suggesting that melting of rock/mineral fragments is a dominant process in forming M-Pt, whereas it is less significant in C-Pt, and is absent in Ct, which corroborates the microscopic observations. Research Highlights: Clast size and cumulative frequency were measured in pseudotachylyte and cataclasite samples from two shear zones. Size-frequency relationship follows a power law in melting- and crushing-dominated pseudotachylytes. The power law does not hold good for the finest and coarsest size ranges in both types of pseudotachylytes. For cataclasite, a multi-fractal power law relationship exists between the size and cumulative frequency of clasts. The non-uniform power law relationship is explained by selective melting of finer clasts in pseudotachylyte. Clast size distribution in a fault rock is likely controlled by the degree of melting involved in its genesis. [ABSTRACT FROM AUTHOR]

Understanding the ability of the lower crust to support transient changes in stresses and strain rates during the earthquake cycle requires a detailed investigation of the deformation mechanisms and rheology of deep crustal fault rocks. Here, we show that lower crustal pseudotachylyte‐bearing shear zones are able to accommodate short‐term episodes of high strain rate and high stress deformation by accelerated viscous creep, followed by a reduction in stresses to some ambient deformation condition. Quartz microstructure within pseudotachylyte‐bearing shear zones in otherwise undeformed granulites from Lofoten, Norway, indicates that dynamic recrystallization occurred during viscous creep under rapid strain rates and high stresses of ~10−9 s−1 and ~100 MPa, respectively. Lower stress microstructures (i.e., foam textures) are also recorded in the shear zones, indicating spatial and temporal variations of stress and strain rate during deformation cycles. Both the high and lower stress quartz recrystallization took place under granulite facies conditions of 650°C–750°C and 0.7–0.8 GPa and represented a record of highly localized viscous creep within the lower crust. This implies that lower crustal pseudotachylytes are potentially able to form extremely localized weak zones within strong lower crust, enabling a deep mechanical response to perturbations in stress and strain rate such as those experienced during the seismic cycle, for example, seismogenic loading followed by subsequent postseismic relaxation. Plain Language Summary: Detailed investigation of the strength and deformation style of fault rocks sourced from the Earth's lower crust is important to understand how the lower crust reacts to short‐term variations in stress and strain rate, which can occur, for example, between earthquakes. Here, we show that solidified pseudotachylytes (initially melts produced due to frictional heating along the fault plane during an earthquake) occurring at depths of 25–30 km in the lower crust can accommodate deformation at particularly high strain rates and high stresses via solid‐state creep. We look at pseudotachylytes formed in lower crustal shear zones that are now exhumed in Lofoten, Norway. Deformation microstructures in quartz within these pseudotachylytes have recorded rapid strain rates and high stresses. These microstructures are occasionally transformed into lower stress versions, indicating that during the deformation the stress and strain rate varied through both time and space. Both stages, however, record the same deformation temperatures and pressures, indicating that these are snapshots of ongoing deformation within the lower crust. We conclude that, when the lower crust is strong, pseudotachylytes will form important weak zones that accommodate deformation even during rapid variations in the deformation conditions, for example, as occurs during the postseismic period immediately after an earthquake. Key Points: Viscous creep at high differential stress and strain rate is observed in microstructures within lower crustal mylonitized pseudotachylytesThe high stress and strain rate deformation was transient and localized, as shown by partial development of lower stress microstructuresPseudotachylytes could support transients in stress and strain rate within strong lower crust, for example, as observed during postseismic relaxation [ABSTRACT FROM AUTHOR]

We report the first evidence of fault rocks (FRs) developed during high-pressure/low-temperature (HP/LT) subduction-related metamorphism, within quartz+epidote pods embedded in the glaucophane–lawsonite-bearing ophiolitic metabasalts of the Diamante–Terranova unit (Calabria, Italy). FRs occur as relic injections appearing as thin dark seams, locally showing an internal foliation characterized by tabular, curvilinear and meander-like shapes, and consist of very fine grains of glaucophane and titanite, locally including survivor clasts of epidote and lawsonite. Some boudinaged veins show glaucophane fibres in the boudin necks, marking a clear HP/LT syn-metamorphic origin at c. 30 km depth. The injected FRs can be alternatively interpreted either as pseudotachylytes or as fluidized ultracataclasites. Although subsequent recrystallization largely obliterated primary diagnostic features, the occurrence of (i) different-coloured flow streaks, characterized by alternating layers of glaucophane and titanite, (ii) well-developed flow folds and (iii) corroded epidote survivor crystals could indicate a viscous flow of molten material characterized by a non-uniform chemical composition. With this in mind, we support the hypothesis that these fine-grained veins were originally pseudotachylytes generated by the frictional melting of the glaucophane-rich layers of the Diamante–Terranova metabasalts, likely related to seismic events occurring during the Eocene along thrust faults within the subducting oceanic Ligurian lithosphere. The lack of evidence for pseudotachylyte relics in the metabasalt source rock argues for a selective preservation, largely dependent on the efficient mechanical shielding action of the stiffer quartz+epidote pods. [ABSTRACT FROM AUTHOR]

Vein networks of pseudotachylytes and ultracataclastic rocks are composed mainly of ultrafine-to fine-grained materials, including fault gouge and microbreccia, and are widely considered indicators of past seismic faulting events. I show that such vein networks of pseudotachylyte with both melt and crush origins, as well as ultracataclastic rocks, form by the rapid injection of ultrafine-to fine-grained material sourced from pulverized ultracataclastic rocks in seismogenic fault zones under thermal pressurization and fluidization during seismic events. The thermal expansion of seismic slip zones caused by frictional heating results in the rapid fluidization of ultrafine-to fine-grained materials along with expanded fluids and gases (e.g., water vapor and melt) that are injected under pressure into fracture void spaces within fault zones in a gas–solid–liquid system during large earthquakes. I propose that the thermal expansion of water vapor and ultrafine-to fine-grained materials caused by frictional heating in the fault slip zone is the main mechanism responsible for the dramatic increase in pore pressure that results in the dynamic coseismic weakening of faults. • Vein networks of pseudotachylyte (Pt) are indicators of seismic faulting. • Pt veins are sourced from by pulverized ultracataclastic rocks in seismic fault zones. • Thermal pressurization and fluidization occurred in fault zone during seismic faulting. • Thermal expansion of seismic slip zones caused by frictional heating in fault zones. • Thermal expansion is a main mechanism of dynamic coseismic weakening of faults. [ABSTRACT FROM AUTHOR]

We investigate the influence of unmelted clasts on the 40Ar 39Ar age record of a Miocene pseudotachylyte within the early Oligocene Riesenferner pluton (eastern Alps). This case study is ideal to investigate the effect of inherited Ar retained in survivor clasts because, although seismic faulting was only slightly younger (<20 Ma) than the igneous source rock, the pseudotachylyte includes layers with significant variations in K content, of either dominant plagioclase and biotite microlites or dominant submicrometric muscovite matrix. Numerical modeling, assuming pure diffusive loss and a short-lived thermal pulse due to frictional heating, indicates that unmelted plagioclase clasts represent the most critical issue in interpreting Ar data. Plagioclase clasts of only a few to tens micrometers in size are expected to retain most of radiogenic Ar accumulated before coseismic faulting. A comparable behavior is only predicted for much larger (ten to fifty times larger) K-feldspar clasts. In-situ 40Ar 39Ar laser analyses of low-K microlitic domains, free of discernable clasts, yield slightly, but significantly, older ages than those from the high-K matrix. Results suggest that older ages (up to ~4% on average) from low-K domains are due to contamination by plagioclase clasts and suggest an age of 17.12 ± 0.22 Ma for coseismic slip. This age dates the last stage of activity of the Giudicarie Fault during tectonic indentation of the Dolomites block of the southern Alps against the nappe stack of the eastern Alps. • Numerical modeling indicates that unmelted plagioclase clasts represent the most critical issue in interpreting Ar data. • Small (few μm) plagioclase clasts may induce detectable effect in the age record of pseudotachylytes. • Ar Ar data suggest an age of 17.12 ± 0.22 Ma for coseismic slip in the Rieserferner Pluton. • Ar age dates the last phase of the Giudicarie faulting and indentation of the Dolomites indenter against the Alpine belt. [ABSTRACT FROM AUTHOR]

Basalt is one of the main components of megathrust faults in subduction zones where the world's largest earthquakes are generated. Paradoxically, pseudotachylytes have never been reported from basaltic rocks deformed at conditions compatible with the thermally-defined seismogenic zone. We report the first discovery of pseudotachylyte from oceanic crustal basalt, identified by microstructural criteria. The fault is located within a Late Cretaceous subduction complex, the Mugi Mélange, and either formed along the plate interface during subduction or within the upper plate during underplating. After solidifying, the pseudotachylytes were fragmented by cataclasis and subsequently partially hydrated to produce phyllosilicates. Previous experiments using dry, felsic lithologies as starting material have shown that pseudotachylytes can be as strong as their host rock, deterring re-activation and favoring their preservation over geologic time. However, we show that where fluid is present, pseudotachylyte can be replaced by phyllosilicate-rich layers with a low frictional strength. The rate of dissolution is dependent on the bulk composition of the pseudotachylyte, with dissolution of mafic glasses occurring at least three orders of magnitude faster than felsic glasses. Under hydrothermal conditions, the replacement of pseudotachylytes by layers of frictionally weak phyllosilicates is predicted to occur over time intervals shorter than the megathrust earthquake cycle. This process likely reduces fault strength and promotes reactivation. • First reported occurrence of pseudotachylyte within a basalt-hosted fault. • Pseudotachylytes are altered along margins to minerals with low frictional strengths. • Pseudotachylytes are reworked indicating reactivation during subsequent slip. • Calculated alteration rates are faster than megathrust earthquake cycle. • Altered pseudotachylytes may weaken fault zones and promote reactivation. [ABSTRACT FROM AUTHOR]

Caldera fault zones, identified in many modern and ancient volcanoes, have been the subject of geological and geophysical observations and of analog and numerical modeling. However, the physicochemical processes in fault zones during a caldera collapse are still poorly understood. Here, we present field observations from a caldera fault zone in the Cretaceous Jangsan Caldera, SE Korea. The fault zone is ∼30 m wide and juxtaposes an intracaldera rhyolitic volcanic complex against older dacitic rocks; it consists of a minor fault, a main fault, and a series of fault-related intrusions (rhyolite and tuffisite). The main fault dips 90°–88°NW (vertically to steeply inward) and strikes N30°–40°E. A layer of pseudotachylyte (less than ∼10 cm thick) with some injection veins occurs along the main fault. The pseudotachylyte contains rounded to sub-rounded clasts, commonly displaying fuzzy or embayed boundaries, as well as euhedral feldspar microlites (<10 μm) and flow structures similar to those typically observed in tectonic pseudotachylytes. The tuffisite not only intrudes sharply into the pseudotachylyte and dacitic wall rocks but also displays a mingling structure with the pseudotachylyte. The contact between the rhyolitic intrusion and the tuffisite is nonplanar and complex. Based on the observed field relationships, we conclude that frictional melting and magmatic intrusion were near-coeval and that intrusion outlasted frictional melting by a short period. Although volcano–tectonic pseudotachylytes have rarely been identified in calderas (e.g., in the Glencoe Caldera, Scotland), our results suggest that frictional melting may be an important fault zone process during caldera collapse. • Pseudotachylyte, rhyolitic intrusions and tuffisite are identified in a caldera fault zone of the deeply eroded and ancient Jangsan Caldera, SE Korea. • The intrusion of pyroclastic and magmatic materials occurred shortly after frictional melting and outlasted by a short period. • Frictional melting may be an important fault zone process during caldera collapse. [ABSTRACT FROM AUTHOR]

Abstract The over 600 km long Woodroffe Thrust developed at lower to mid-crustal levels during the intracontinental Petermann Orogeny at ca. 560–520 Ma. Ductile deformation with a top-to-north shear sense was accommodated along a shallowly (≤30°) south-dipping surface. Metamorphic conditions during deformation are established along a 60 km N-S transect, providing an ideal framework for studying variation in microstructure and crystallographic preferred orientations with changing temperature (ca. 520–620 °C) and pressure/depth in dominantly dry felsic crust. In the Woodroffe Thrust mylonites, dynamic recrystallization of quartz was dominated by subgrain rotation, whereas feldspar underwent grain size reduction by neocrystallization. Differential stress, estimated from quartz grain size piezometry, decreases with increasing metamorphic grade (i.e., deeper structural levels), and indicates a long-term average strain rate of around 10−11–10−12 s−1. We propose a qualitative rheological model to explain the observed cyclic interplay between ductile shearing (mylonitization) and brittle fracturing (pseudotachylyte formation) in the relatively dry middle crust. The model involves the downward migration of earthquake ruptures from the overlying seismogenic zone, which transiently triggers seismic slip at mid-crustal levels. Highlights • The Woodroffe Thrust involves an interplay between brittle and ductile deformation. • Microstructures are analyzed along a ~60 km section in the direction of thrusting. • Quartz recrystallized mainly by subgrain rotation, feldspar by neocrystallization. • The long-term average strain rate of the Woodroffe Thrust was ca. 10–11–10-12 s-1. • Transient coseismic strain rate increase can trigger earthquakes at mid-crustal levels. [ABSTRACT FROM AUTHOR]

Pseudotachylytes, produced by frictional heating during seismic slip, convey information that is critical to understanding the physics of earthquakes, their geochemical features triggered by frictional melting shed light on the seismic processes and frictional strength as well. However, whether their geochemical properties changed during exhumed process is still uncertain. To provide insight to this topic, we conducted a case study of pseudotachylytes in the Longmen Shan thrust belt, eastern Tibet. Pseudotachylyte samples drilled from the WFSD boreholes at deep depth and collected at surface outcrops were analyzed by μXRF core scanner to estimate their geochemical variations. Macro- and micro-structural observations suggest that the pseudotachylytes are from melt-origin. Comparative analyses of the geochemical characteristics between surface and deep pseudotachylytes show that the former are K- and Ti-depleted in an oxidation environment, while the latter have low Si, high K, Fe, Ti and Mn in a reductive environment. Besides the selective melting process, another reasonable explanation for these differences is that the geochemical properties of the surface pseudotachylytes have changed due to the prolonged fluid influence during the long-term exhumation processes, which cannot exactly reflect the nature of the initial frictional melt. Therefore, we conclude that primary pseudotachylytes are characterized by enrichment of K, Fe, Ti, Mn and depletion of Si. The frictional melt was generated in high-temperature reductive environment, and fluids have influenced their chemical composition during the exhumation process. These results are of great significance for correctly understanding earthquake mechanics and fault behavior. [ABSTRACT FROM AUTHOR]

Although the Indus‐Tsangpo Suture (ITS) is the most spectacular thrust system of continent‐continent collision in the world, fundamental questions about its strength evolution and deformation behavior transition remain unanswered. Here we reported, for the first time, frictional melting‐induced pseudotachylytes in the intensively deformed felsic rocks along the ITS zone in southern Tibet. This study reveals that pseudotachylytes induced profound weakness of the boundary fault between Indian and Asian plates. The intrinsically low strength of the foliated microlites crystallized from frictional melt or glass (i.e., pseudotachylyte) at seismogenic depths compared with the surrounding coarse‐grained quartzofeldspathic rocks in the brittle and semi‐brittle regime is sufficient to explain the localization of shear strain, the development of ductile shear zones embedded in strong wall rocks, and the transition from the strong to weak fault behaviors without invoking the presence of high fluid pressure or low friction coefficient metasomatic materials (e.g., smectite or lizardite) within the faults. [ABSTRACT FROM AUTHOR]

The strike‐slip Davenport Shear Zone in Central Australia developed during the Petermann Orogeny (~550 Ma) in an intracontinental lower crustal setting under dry subeclogite facies conditions (~650 °C, 1.2 GPa). This approximately 5‐km‐wide mylonite zone encloses several large low‐strain domains, allowing a detailed study of the initiation of shear zones and their progressive development. Quartzo‐feldspathic gneisses and granitoids contain compositional layers, such as quartz‐rich pegmatites, mafic bands, and dykes, which should preferentially localize viscous deformation if favorably orientated. This is not observed, except for long, continuous, and fine‐grained dolerite dykes. Instead, many shear zones, typically a few millimeters to centimeters in width but extending for tens of meters, commonly exploited pseudotachylytes and are sometimes parallel to a network of little overprinted fractures. The recrystallized mineral assemblage in the sheared pseudotachylyte is similar to that in the host gneiss, without associated hydration due to fluid‐rock interaction. Lack of localization in quartz‐rich, coarser‐grained (typically >50 μm) rocks compared to mafic dykes, precursor fractures, and pseudotachylytes implies that localization in the dry lower crust preferentially occurs along elongate, planar fine‐grained layers. Transient high stress repeatedly initiated fractures, providing finer‐grained, weaker, planar precursors that localized subsequent ductile shear zones. This intimate interplay between brittle and ductile deformation suggests a local source for lower crustal earthquakes, rather than downward migration of earthquakes from the shallower, usually more seismogenic part of the crust. Key Points: Ductile shearing in the dry lower crust localizes on precursor pseudotachylytes and fracturesStress fluctuations range from 10 MPa to at least 1 GPaEarthquakes in the lower crust can be locally triggered by stress concentrations without the transfer of stresses from the upper crust [ABSTRACT FROM AUTHOR]

Abstract A model of Mesoarchaean spreading ridge subduction and related metamorphism, magmatism and deformation in an active continental margin is presented here. The eclogites exposed along northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield were formed as a result of Mesoarchaean–Neoarchaean subduction and collision. The Belomorian eclogite province contains two eclogite associations have been recognized within TTG gneisses: (1) the subduction-type Salma association and (2) the Gridino mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising oceanic layered gabbro, formed at ∼2.9 Ga in a slow-spreading ridge. The subduction began in a time span 2.9–2.87 Ga, and eclogite-facies events occurred between ca.2.87 and ca.2.78 Ga. Injection of a mafic magma into an active continental margin, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. The dykes represent typical fissure intrusions and underwent metamorphism to eclogite, granulite and amphibolite facies. The dykes divide into two compositions (Fe-Ti gabbro and Mg-Cr gabbronorite) which represent alternating episodes of arc and slab window magmatism. The Fe-Ti gabbros have high Ti and low abundances of fluid-mobile and other LIL elements compared to HFSE and plot in the MORB and within-plate oceanic basalt fields. The Fe-Ti gabbros formed from low-degree of partial melting from a depleted mantle source. This petrogenesis is consistent with slab window model in which magmas were derived by decompression melting of suboceanic mantle that upwelled into the opening beneath continental margin following spreading ridge subduction. The Mg-Cr gabbronorites have low Ti along with high abundances of fluid-mobile elements and other LILE compared to HFSE and plot in arc basalt fields that imply melting of subduction-modified hydrous subcontinental mantle. As the mid-ocean spreading ridge entered a trench, continental rocks above subduction zone underwent deformations with pseudotachylyte-bearing strike-slip faults development that is an evidence of a fossile earthquake. Paleoseismic event was limited to crustal conditions above the brittle-ductile transition, corresponding to temperatures below 450 °C and depths above 15 km. The slab window opening induced upwelling of anhydrous mantle and formation of the Fe-Ti gabbro dykes. The Mg-Cr gabbronorite was formed at slab window edges from metasomatized mantle. Summarizing evidences, such as ophiolites, eclogite, arc magmatism, we suggest that the modern style of subduction and plate tectonics began already in Mezoarchaean time 2.9-2.8 Ga ago. Special aspect of the paper is an argumentation of Meso-Neoarchaean age of eclogite facies metamorphism in the Belomorian eclogite province. [ABSTRACT FROM AUTHOR]

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Copyright of Acta Geoscientica Sinica is the property of Acta Geoscientica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Pseudotachylytes (quenched frictional melts produced on a fault by seismic slip) in dry rocks exhumed from the mid-lower crust are potential indicators of earthquakes that either nucleated at, or propagated to, depths below the main shallow brittle-ductile transition zone. Establishing whether these pseudotachylytes effectively record deep-crustal earthquakes, or shallow-level earthquakes overprinting the mid-lower-crustal rocks during the exhumation path, may represent a major challenge. This challenge is mainly related to the fact that the mineral assemblage of a pseudotachylyte develops out of equilibrium during the coseismic thermal transient leading to melting and melt quenching. Here we investigate pseudotachylytes within peraluminous, sillimanite-garnet-rich, migmatitic paragneiss of the Serre Massif in Calabria (Southern Italy). These exhumed lower-crustal rocks experienced granulite-facies metamorphism (∼700–800 °C; ∼600–800 MPa), partial melting and dehydration during the late Variscan Orogeny (ca. 320–280 Ma). The crosscutting pseudotachylytes contain hercynite and sillimanite microlites, globular-shaped poikilitic cordierite and plagioclase, and rare cauliflower- to subhedral-shaped garnet. The pseudotachylytes are pristine, not affected by ductile deformation, recrystallisation or extensive alteration by fluid after their formation. A Rb-Sr isochron age of 51.4 ± 5.1 Ma is obtained for the pervasively kinked biotite in the host rock immediately adjacent to the pseudotachylyte and associated with earthquake damage, while an age of 105.3 ± 4.1 Ma is obtained for the undeformed host-rock biotite. This indicates that the granulites were cooler than the closing Rb-Sr temperature of biotite (ca. 300–400 °C) in the Cretaceous and that the studied pseudotachylytes formed by shallow seismic faulting. Therefore, sillimanite, hercynite, garnet, plagioclase, and cordierite all formed during quenching of the frictional melt well above the ambient temperature. Modelling of cordierite growth during melt quenching indicates that cordierite should have started to crystallise at T > 900 °C to achieve the grain size (up to 10 μm in diameter) observed in the pseudotachylyte. Modelling and microstructural observations allow the crystallisation sequence of microlites during melt cooling to be established. These microlites include cauliflower garnet which, in this case, did not develop in a deep-seated faulting context as commonly reported. • Pristine pseudotachylytes (pst) hosted in lower-crustal granulites. • Pst contain microlites of hercynite, sillimanite and locally garnet. • Poikilitic plagioclase and cordierite crystallised later than microlites. • In-situ Rb-Sr dating of biotite constrains pst formation to a shallow environment. • Thermal modelling of pst melt quenching and growth of cordierite in the melt. [ABSTRACT FROM AUTHOR]

Abstract The redox state of pseudotachylyte provides valuable information on the physicochemical properties related to fluid activity during paleoseismicity. In this study, we evaluate the behavior of fluids in pseudotachylytes using fluid buffering minerals such as graphite and other secondary minerals. Two types of pseudotachylyte were identified in the Hidaka metamorphic belt, Hokkaido, Japan; Pst I and Pst II. Pst I is composed of pyrrhotite and K-feldspar generated by the breakdown of phyllosilicates, whereas Pst II includes recrystallized biotite microlites, K-feldspar, and plagioclase due to the breakdown of quartz and feldspars, as well as phyllosilicates. The selective frictional melting of phyllosilicates is indicated by significant changes in the concentrations of large ion lithophile elements (e.g., Ba, Sr, Rb, Cs, B, and K). In addition, the graphite associated with titanite and hydroxyapatite is often distributed within the Pst II matrix. The graphite exhibits different crystallinity, morphology, and higher carbon stable isotopic compositions (δ13C = −21.9 ± 2.3‰, n = 16), when compared with metamorphic graphite extracted from the protolith (δ13C = −25.1 ± 0.8‰, n = 15), cataclasite, ultracataclasite, and Pst I (δ13C = −25.2 ± 0.8‰, n = 15). The microstructure and geochemical signatures in both pseudotachylytes are controlled by dehydration–redox reaction of phyllosilicates coexisting with sulfide minerals and graphite. In particular, the formation of 13C enriched graphite in the Pst II can be deduced by co-precipitation of fluid-deposited graphite with hydrous minerals from H 2 O-CH 4 fluids and/or degassing of H 2 O-CH 4 fluids from residual graphite. Our data demonstrated that the carbonic fluids of a reducing nature could be produced, and then dissolved into the pseudotachylyte melt by frictional melting of metasedimentary rocks. The production of carbonic fluids by coseismic slip may be an overlooked process in the remobilization of refractory graphite locked in the crust. Highlights • Two types of pseudotachylyte were identified. • Significant enrichments of large ion lithophile elements by frictional melting of phyllosilicates • Changes in redox state of pseudotachylyte depending on the fluid buffering minerals • Generation of isotopically light carbonic fluids by the frictional melting of graphite [ABSTRACT FROM AUTHOR]

The efficiency of thermal pressurization as a dynamic weakening mechanism relies on the thermal and hydraulic properties of the rocks forming the fault core. Here we assess the effectiveness of thermal pressurization by comparing predictions of temperature rise to field estimates based on pseudotachylyte‐bearing rocks. We measure hydraulic and transport properties of a suite of fault rocks (a healed cataclasite, an unhealed breccia, and the intact parent rock) from the pseudotachylyte‐bearing Gole Larghe fault in the Adamello batholith (Italy) and use them as inputs in numerical simulations of thermal pressurization. We find that the melting temperature can be reached only if damaged, unhealed rock properties are used. A tenfold increase in permeability or a fourfold increase in pore compressibility of the intact rock is required to achieve melting. Our results emphasize the importance of damage processes that strongly modify fault rock properties and dynamic weakening processes during earthquake propagation. Plain Language Summary: During earthquakes, faults slide rapidly past each other, typically at several meters per second. Such fast sliding rates at great depth in the crust are expected to generate large amounts of heat and should melt the rocks at the sliding interface. However, such melted rocks are not systematically observed in faults. A number of mechanisms have been suggested to explain this apparent discrepancy. One convincing explanation is that the presence of water within the porosity of the fault rocks buffers the fault temperature by being rapidly pressurized and reducing the fault friction. This effect has been predicted from physical models, but these predictions depend strongly on poorly constrained rock properties and have not yet been tested in nature. Here we measured key physical properties in rocks adjacent to a fault that underwent frictional melting, and we test whether model predictions for the effects of pressurized water are consistent with the presence of melt. We find that it is the case only if rock properties are altered to account for the effect of microfractures that are likely created during earthquake rupture. Our results provide constrains to improve earthquake simulations and highlight the key role of microfracture damage on fault sliding processes. Key Points: We test if the presence of pseudotachylytes in the Gole Larghe fault is consistent with predictions from thermal pressurizationThermal pressurization is too efficient to explain melting, except if "damaged" rock properties are considered [ABSTRACT FROM AUTHOR]

The incursion of meteoric-hydrothermal fluids into the ductile crust can be evaluated with the stable isotope ratios of minerals in exhumed ductile shear zones, but the incursion mechanisms are yet to be well-demonstrated. Field structural and microstructural relations and regional oxygen isotope patterns from an exhumed, Late Cretaceous shear zone-fault system in the central Sierra Nevada, CA, are used to reconstruct the deformation history and develop insights into fluid incursion mechanisms. A ductile-to-brittle transition was partially accommodated by a strike-slip duplex involving flexural slip-accommodated folding along faults. Two main NNW-striking brittle fault strands overlap, the northern fault curves into the southern fault, which is sealed by a >4 m wide composite quartz vein. Based on structural relationships, we interpret the transition from ductile to brittle deformation to have occurred during cooling of the adjacent Late Cretaceous Tuolumne Intrusive Complex. Quartz-sealed fault veins in the pendant have δ 18 O values ranging from −3.2 to 14.5‰, and are interpreted to have precipitated partially or fully from meteoric-hydrothermal fluids, including quartz veins that have been recrystallized. The lowest values are only found within the region of the jog. The δ 18 O values for pseudotachylyte matrix in one sample from this fault system are δ 18 O = 3.3‰, δ 2 H = −137‰. Together, these data indicate meteoric-hydrothermal incursion into a deforming, seismogenic brittle-ductile transition. [ABSTRACT FROM AUTHOR]

Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults. [ABSTRACT FROM AUTHOR]

In the Bergen Arc, western Norway, rocks exhumed from the lower crust record earthquakes that formed during the Caledonian collision. These earthquakes occurred at about 30–50 km depth under granulite or amphibolite facies metamorphic conditions. Coseismic frictional heating produced pseudotachylytes in this area. We describe pseudotachylytes using field data to infer earthquake magnitude (M ≥ ~6.6), low dynamic friction during rupture propagation (μ d  < 0.1) and laboratory analyses to infer fast crystallization of microlites in the pseudotachylyte, within seconds of the earthquake arrest. High resolution 3D X-ray microtomography imaging reveals the microstructure of a pseudotachylyte sample, including numerous garnets and their corona of plagioclase that we infer have crystallized in the pseudotachylyte. These garnets 1) have dendritic shapes and are surrounded by plagioclase coronae almost fully depleted in iron, 2) have a log-normal volume distribution, 3) increase in volume with increasing distance away from the pseudotachylyte-host rock boundary, and 4) decrease in number with increasing distance away from the pseudotachylyte -host rock boundary. These characteristics indicate fast mineral growth, likely within seconds. We propose that these new quantitative criteria may assist in the unambiguous identification of pseudotachylytes in the field. [ABSTRACT FROM AUTHOR]

Abstract: In the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single Mw > 6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60°SW. The average width of the main slip surface is ~ 5 mm. A dense network of thin (20–200 μm) injection and ultramylonite‐like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing ≈ 1% of structurally bound H2O. In the ultramylonite‐like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2–2 μm) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250°C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40 km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient ≪ 0.1). Because of the low viscosity of slightly hydrated ultramafic liquids (≈ 1 Pa·s), we argue that lubrication was only transient, as the melt could rapidly flow into tensile fractures, which led to rapid cooling and promoted strength recovery and sliding arrest. Combined together, our observations suggest that this pseudotachylyte is the frozen record of a deep (>40 km) earthquake of 6 < Mw < 7. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction. [ABSTRACT FROM AUTHOR]

Abstract: Melt‐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. [ABSTRACT FROM AUTHOR]

The 5.5 magnitude (Mw) earthquake in Pohang, South Korea in 2017 was one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. Faults that ruptured in Pohang were not identified by preliminary geological investigations or geophysical surveys, and the subsequent study of the fault rocks at the Pohang EGS site was limited to depths of 3790–3816 m. In this study, we present new observations of fault rocks from drill cuttings retrieved from the Pohang EGS. The drill cuttings obtained from 3256 to 3911 m contained "mud balls," which showed a clay matrix with foliation and a cataclastic texture, indicating a typical fault gouge or breccia. Furthermore, the mud ball samples retrieved from depths of 3256 m and 3260 m contained black fragments. Scanning and transmission electron microscopy revealed that the black fragments consisted of glass-like material, which is indicative of frictional melting during coseismic slip. The presence of these black fragments suggests that at least one seismic event had occurred at the Pohang EGS site prior to the hydraulic stimulation test. • Drill cuttings from the Pohang enhanced geothermal system (EGS) site were studied. • Drill cuttings contained "mud balls" with a typical fault gouge structure. • Some mud balls contained black fragments, consisting of amorphous material. • Black fragments may be a result of frictional melting during the coseismic slip. [ABSTRACT FROM AUTHOR]

Abstract: We investigate how the rheological evolution of shear zone rocks from beneath the brittle‐ductile transition (BDT) is affected by coeval ductile shear and pseudotachylyte development associated with seismicity during the earthquake cycle. We focus our study on footwall rocks of the South Mountains core complex, and we use electron backscatter diffraction (EBSD) analyses to examine how strain is localized in granodiorite mylonites both prior to and during pseudotachylyte development beneath the BDT. In mylonites that are host to pseudotachylytes, deformation is partitioned into quartz, where quartz exhibits crystallographic‐preferred orientation patterns and microstructures indicative of dynamic recrystallization during dislocation creep. Grain size reduction during dynamic recrystallization led to the onset of grain boundary sliding (GBS) accommodated by fluid‐assisted grain size‐sensitive (GSS) creep, localizing strain in quartz‐rich layers prior to pseudotachylyte development. The foliation‐parallel zones of GBS in the host mylonites, and the presence of GBS traits in polycrystalline quartz survivor clasts indicate that GBS zones were the ductile precursors to in situ pseudotachylyte generation. During pseudotachylyte development, strain was partitioned into the melt phase, and GSS deformation in the survivor clasts continued until crystallization of melt impeded flow, inducing pseudotachylyte development in other GBS zones. We interpret the coeval pseudotachylytes with ductile precursors as evidence of seismic events near the BDT. Grain size piezometry yields high differential stresses in both host mylonites (~160 MPa) and pseudotachylyte survivor clasts (> ~200 MPa), consistent with high stresses during interseismic and coseismic phases of the earthquake cycle, respectively. [ABSTRACT FROM AUTHOR]

Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4-2,000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory-produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore, these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks. [ABSTRACT FROM AUTHOR]

The Musgrave Block in central Australia exposes numerous large-scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560-520 Ma. The most prominent structure is the crustal-scale, over 600 km long, E-W trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximately top-to-north sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation from predominantly midcrustal (circa 520-620°C and 0.8-1.1 GPa) to lower crustal (~650°C and 1.0-1.3 GPa) levels in the direction of thrusting, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south dipping thrust plane but is only small, implying that a ≥60 km long N-S segment of the Woodroffe Thrust was originally shallowly dipping at an average estimated angle of ≤6°. The reconstructed geometry suggests that basement-cored, thick-skinned, midcrustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g., large volumes of syntectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low-angle, near-planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers. [ABSTRACT FROM AUTHOR]

Coseismic frictional melting and the production of quenched glass called pseudotachylyte is a recurring process during earthquakes. To investigate how glassy materials affect the postseismic strength and stability of faults, obsidian gouges were sheared under dry and wet conditions from 200°C to 300°C at ~150 MPa effective normal stress. Dry glass exhibited a brittle rheology at all conditions tested, exhibiting friction values and microstructures consistent with siliciclastic materials. Likewise, wet glass at 200°C exhibited a brittle rheology. In contrast, wet gouges at 300°C transitioned from brittle sliding to linear-viscous (Newtonian) flow at strain rates <3 × 10−4 s−1, indicating melt-like behavior. The viscosity ranged from 2 × 1011 to 7.8 × 1011 Pa-s. Microstructures show that viscous gouges were fully welded with rod-shaped microlites rotated into the flow direction. Fourier transform infrared spectroscopy along with electron backscatter imaging demonstrate that hydration of the glass by diffusion of pore water was the dominant process reducing the viscosity and promoting viscous flow. As much as 5 wt % water diffused into the glass. These results may provide insight into postseismic-slip behaviors and challenge some interpretations of fault kinematics based on studies assuming that pseudotachylyte formation and flow is solely coseismic. [ABSTRACT FROM AUTHOR]

Pseudotachylyte, a fault rock formed through coseismic frictional melting, provides an important record of coseismic mechanics. In particular, injection veins formed at a high angle to the fault surface have been used to estimate rupture directivity, velocity, pulse length, stress drop, as well as slip weakening distance and wall rock stiffness. These studies have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt generation, transport, and solidification have little influence on the final vein geometry. Using a pressurized crack model, an analytical approximation of injection vein formation based on dike intrusion, we find that the timescales of quenching and flow propagation may be similar for a subset of injection veins compiled from the Asbestos Mountain Fault, USA, Gole Larghe Fault Zone, Italy, and the Fort Foster Brittle Zone, USA under minimum melt temperature conditions. 34% of the veins are found to be flow limited, with a final geometry that may reflect cooling of the vein before it reaches an elastic equilibrium with the wall rock. Formation of these veins is a dynamic process whose behavior is not fully captured by the analytical approach. To assess the applicability of simplifying assumptions of the pressurized crack we employ a time-dependent finite-element model of injection vein formation that couples elastic deformation of the wall rock with the fluid dynamics and heat transfer of the frictional melt. This finite element model reveals that two basic assumptions of the pressurized crack model, self-similar growth and a uniform pressure gradient, are false. The pressurized crack model thus underestimates flow propagation time by 2–3 orders of magnitude. Flow limiting may therefore occur under a wider range of conditions than previously thought. Flow-limited veins may be recognizable in the field where veins have tapered profiles or smaller aspect ratios than expected. The occurrence and shape of injection veins can be coupled with modeling to provide an independent estimate of minimum melt temperature. Finally, the large aspect ratio observed for all three populations of injection veins may be best explained by a large reduction in stiffness associated with coseismic damage, as injection vein growth is likely to far exceed the lifetime of dynamic stresses at any location along a fault. [ABSTRACT FROM AUTHOR]

In an ophiolitic nappe of Alpine Corsica, a major fault zone superimposes metagabbro over serpentinite and peridotite. Ductile and brittle deformation structures are observed in the fault damage zones. In the metagabbro damage zone, early deformation culminates in blueschist or eclogite facies conditions and consists of west-verging mylonitization alternating with pseudotachylyte-forming faulting with undetermined vergence. This early deformation is likely coeval with west-verging seismic (pseudotachylyte-forming) reverse faulting in the footwall peridotite or with aseismic distributed cataclastic deformation of footwall serpentinite. These early events (aseismic mylonitization or distributed cataclasis and seismic faulting) are interpreted as reverse faulting/shear in an east-dipping subducting oceanic lithosphere in Cretaceous to Eocene times. Late deformation events consist of ductile shear and seismic faulting having occurred under retrograde greenschist conditions. Kinematics of the ductile shear is top-to-the-east. These events are interpreted as the result of syn-to post-collision extension of Alpine Corsica in Eocene to Miocene times. The heterogeneous distribution of pseudotachylyte veins along the fault zone (abundant at peridotite-metagabbro interfaces, rare or absent at serpentinite-metagabbro interfaces) is interpreted as the consequence of contrasted frictional properties of the rocks in contact. High-friction peridotite-metagabbro contacts could correspond to asperities whereas low-friction serpentinite-metagabbro contacts could correspond to creeping zones. [ABSTRACT FROM AUTHOR]

In order to study seismo-mechanochemical processes, two kinds of shock compression experiments – the recovery and the in-situ VISAR (velocity interferometer system for any reflector) experiments – were performed, using a single crystal of forsteritic olivine and a single-stage powder propellant gun. The generated peak shock pressures were 31.1 GPa for both the recovery and the VISAR experiments. Many shear planes were generated in the olivine crystal. The VISAR experiment revealed detailed time profiles for the particle and shock wave velocities during the compression and yielded the Hugoniot elastic limit (HEL) of the olivine to be 7.44 GPa. We observed, in one recovered sample, a clear local melting occurring along a shear plane. The microstructural observations of the shear plane and its walls using a field-emission scanning electron microscopy (SEM) and an analytical transmission electron microscopy (ATEM) revealed that plastically-deformed olivine was pulverized into a few hundred-nanometer size particle assemblages within a few-micrometers narrow zone along the shear plane and, locally have partially melted. Moreover, injection veins filled with olivine melt were found in the wall of the same shear plane. It was shown from the estimate of the critical resolved shear stress (CRSS) along the shear planes and the inferred slip velocity using the Hugoniot elastic limit (HEL) data that the power generated by the friction was large enough for melting to occur in the olivine. The whole process occurred in a time scale <0.6 microsecond. Pulverization (comminution) before melting is considered to be a common phenomenon in the formation of pseudotachylytes and in dynamically expanding seismic faults upon earthquakes. [Display omitted] • Shock compression experiments were performed using a forsterite single crystal and a powder propellant gun. • Particle velocity profile on the olivine surface was obtained by laser in situ measurement. • Many shear bands were generated in the olivine, where a clear local melting was observed. • It was found out that plastic deformation and pulverization took place before melting. • Pulverization before melting may be a common phenomenon in co-seismic faults in earthquakes. [ABSTRACT FROM AUTHOR]

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. • Frictional power Q ˙ [W/m2] is shear stress times slip rate during faulting. • Q ˙ controls fault temperature increase and dynamic weakening during earthquakes. • Q ˙ cannot be estimated by seismological methods. • We use microstructural observations calibrated by experiments to estimate Q ˙. • Q ˙ ranges from 4 to 60 MW/m2 in an upper crustal fault patch. [ABSTRACT FROM AUTHOR]

At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustrés). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.

The occurrence of pseudotachylyte-ultracataclasite veins at the Main Central Thrust (central Nepal) is reported here for the first time. The veins are planar, mm-thin layers concordant to the main foliation and layering of the wallrock. They contain elongate domains with variable amount of glasses and µm to mm-sized angular to sub-angular fragments. The glasses indicate the presence of pseudotachylyte and its origin by frictional fusion and subsequent rapid cooling. Amoeboid and partly cuspate domain geometries with diffuse contacts result from simultaneous movement and mingling of pseudotachylyte and ultracataclasite materials during injection into opening foliation-parallel cracks. Frequency distributions of fragment sizes, distribution patterns of fragments, and lobate boundaries of mingled materials are quantified by fractal geometry methods. They follow various power laws on various scales and indicate repeated processes of fragmentation, injection of mobile material, flow, mingling, mixing, and solidification. The formation of composite pseudotachylyte-ultracataclasite veins correlates with thrusting along the Main Central Thrust, specifically with the reactivation of the thrust in the Late Miocene to Early Pliocene. [ABSTRACT FROM AUTHOR]

Pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting. Here we perform step-heating 40Ar/39Ar, clay mineral K-Ar, and zircon fission track (ZFT) analyses on the pseudotachylyte samples collected from the Yingxiu-Beichuan coseismic rupture of the Longmen Shan fault belt (LSFB) to provide time constraints for the tectonic evolution of the LSFB during the Indosinian orogeny. 40Ar/39Ar results from the matrix show that the frictional melting occurred 226-235 Ma ago. Combined with mylonite dating of the host rock, the age of the ancient earthquakes is constrained at 231-238 Ma, with a formation depth of 10-14 km. As a response to the earthquakes, a series of soft-sediment deformation structures are widely preserved in the Middle and Late Triassic strata along the Yingxiu-Beichuan fault (YBF), indicating that the LSFB was seismically active since the Late Anisian and persisted episodically until the end of the Indosinian orogeny. Both clay mineral K-Ar and ZFT analyses record a younger deformation or alteration with an age of approximately 195 Ma, corresponding to a postcollisional orogeny. These new data represent the first direct evidence of the regional thrusting (YBF) in the central LSFB during the Indosinian orogeny, concurrent with the initial ductile deformation of the western boundary fault. Tectonic inheritance then strongly influenced the evolution of the LSFB as most of the Mesozoic faults are reactivated by major Tertiary tectonic deformations. [ABSTRACT FROM AUTHOR]