Frictional Melts Research Articles

Pseudotachylytes in felsic lower-crustal rocks of the Calabrian Serre massif: A record of deep- or shallow-crustal earthquakes?

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.

Mid-crustal reactivation processes linked to frictional melting and deep void development during seismogenic slip: examples from the Lewisian Complex, NW Scotland

Exhumed examples of ancient fault voids formed during seismic slip at depths >10 km are well preserved in the Assynt Terrane of the Lewisian Complex, NW Scotland. They are interpreted to have formed during regional Mesoproterozoic ( c. 1.55 Ga; ‘Assyntian’) strike-slip faulting. Deformation is characterized by sinistral reactivation of pre-existing NW–SE-trending features including intrusive contacts of ( c. 2.4 Ga) mafic dykes and Paleoproterozoic ductile shear zone fabrics ( c. 1.75 Ga). Reactivation occurred at palaeodepths of 10–15 km, where frictional–viscous deformation synchronous with co-seismic frictional melting led to cycles of millimetre- to decimetre-scale cavity dilation and collapse. Although individual melt-generating slip surfaces may have become rapidly welded, faulting was able to repeatedly localize along adjacent pre-existing planar anisotropies favourably oriented for slip, leading to the creation of a mesh of foliation-parallel melt generation surfaces linked by foliation-perpendicular dilational voids. The latter features are filled by chaotic clast-supported wall rock collapse breccias, localized injected frictional melts and hydrothermal mineralization. The fills act as natural props, holding cavities open and preserving them as long-term, pipe-like fluid flow conduits. These exhumed features are likely to be typical of multi-rupture seismogenic fault systems formed by direct reactivation of pre-existing basement structures.

Shear zone evolution and the path of earthquake rupture

Abstract. Crustal shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high-resolution geologic map of the shear zone core mylonite zone. The core consists of ∼ 1–200 cm, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric layers of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes differs from interfaces lacking pseudotachylyte at wavelengths of ≳ 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path of earthquake ruptures. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.

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

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

The pseudotachylytes at the base of the Silvretta Nappe: A newly discovered recent generation and the tectonomometamophic evolution of the Nappe

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.

Kinematics of frictional melts at the base of the world's largest terrestrial landslide: Markagunt gravity slide, southwest Utah, United States

The pseudotachylyte of the Markagunt slide in Utah is the best example of gravity slide frictional melt. A key exposure near Panguitch provides opportunities to investigate the kinematics of frictional melts. Here we analyze the anisotropy of magnetic susceptibility (AMS) in eight oriented samples and show that magnetite dominates the magnetic susceptibility (97.7% in generation veins and 87.6% in injection veins) and most likely also the AMS. These rocks have magnetic susceptibility one order of magnitude greater (∼46,106 • 10−6 [SI]) than their host rock (∼1850 • 10−6 [SI]), which shows that magnetite must have formed during the gravity slide. The AMS of these rocks records the laminar flow kinematics of the frictional melt, with a North-South direction and a top-to-South sense of displacement consistent with regional observations. The dominantly planar symmetry of the AMS points to a flow regime dominated by pure shear, towards the end of the gravity slide movement. Our results, obtained on a landslide pseudotachylyte where the emplacement kinematics was already known, fundamentally validate the use and interpretation of AMS in other pseudotachylytes including those formed along seismogenic faults.

Lower crustal earthquake associated with highly pressurized frictional melts

Earthquakes at lower crustal depths are common during continental collision. However, the coseismic weakening mechanisms required to propagate an earthquake at high pressures are poorly understood. Transient high-pressure fluids or melts have been proposed as a viable mechanism, but verifying this requires direct in situ measurement of fluid or melt overpressure along fault planes that have hosted dynamic ruptures. Here, we report direct measurement of highly overpressurized frictional melts along a seismic fault surface. Using Raman spectroscopy, we identified high-pressure quartz inclusions sealed in dendritic garnets that grew from frictional melts formed by lower crustal earthquakes in the Bergen Arcs, Western Norway. Melt pressure was estimated to be 1.8–2.3 GPa on the basis of an elastic model for the quartz-in-garnet system. This is ~0.5 GPa higher than the pressure recorded by the surrounding pseudotachylyte matrix and wall rocks. The recorded melt pressure could not arise solely from the volume expansion of melting, and we propose that it was generated when melt pressure approached the maximum principal stress in a system subject to high differential stress. The associated palaeostress field demonstrates that a strong lower crust accommodated up to 1 GPa differential stress during the compressive stage of the Caledonian orogeny.

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

AbstractDevastating 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.

Selective clast survival in an experimentally-produced pseudotachylyte

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

Pseudotachylyte Alteration and the Rapid Fade of Earthquake Scars From the Geological Record

AbstractTectonic pseudotachylytes are solidified frictional melts produced on faults during earthquakes and are robust markers of seismic slip events. Nonetheless, pseudotachylytes are apparently uncommon fault rocks, because they are either rarely produced or are easily lost from the geological record. To solve this conundrum, long‐lasting (18–35 days) hydrothermal alteration tests were performed on fresh pseudotachylytes produced by sliding solid rock samples at seismic slip rates in the laboratory. After all tests, the pseudotachylytes were heavily altered with dissolution of the matrix and neo‐formation of clay aggregates. Post‐alteration products closely resemble natural altered pseudotachylytes and associated ultracataclasites (i.e., fault rocks affected by fracturing in the absence of melting), demonstrating that the preservation potential of original pseudotachylyte microstructures is very short, days to months, in the presence of hydrothermal fluids. As a consequence, pseudotachylytes might be significantly underrepresented in the geological record, and on‐fault frictional melting during earthquakes is likely to occur more commonly than generally believed.

Shear localisation, strain partitioning and frictional melting in a debris avalanche generated by volcanic flank collapse

The Arequipa volcanic landslide deposit to the east of Arequipa (Peru) originated from the Pichu Pichu volcanic complex, covering an area ~200 km2. The debris avalanche deposit exhibits internal flow structures and basal pseudotachylytes. We present field, microstructural and chemical observations from slip surfaces below and within the deposit which show varying degrees of strain localisation. At one locality the basal shear zone is localised to a 1–2 cm thick, extremely sheared layer of mixed ultracataclasite and pseudotachylyte containing fragments of earlier frictional melts. Rheological modelling indicates brittle fragmentation of the melt may have occurred due to high strain rates, at velocities of >31 m s−1 and that frictional melting is unlikely to provide a mechanism for basal lubrication. Elsewhere, we observe a ~40 cm thick basal shear zone, overprinted by sub-parallel faults that truncate topological asperities to localise strain. We also observe shear zones within the avalanche deposit, suggesting that strain was partitioned. In conclusion, we find that deformation mechanisms fluctuated between cataclasis and frictional melting during emplacement of the volcanic debris avalanche; exhibiting strain partitioning and variable shear localisation, which, along with underlying topography, changed the resistance to flow and impacted runout distance.

Pseudotachylyte as field evidence for lower-crustal earthquakes during the intracontinental Petermann Orogeny (Musgrave Block, Central Australia)

Abstract. Geophysical evidence for lower continental crustal earthquakes in almost all collisional orogens is in conflict with the widely accepted notion that rocks, under high grade conditions, should flow rather than fracture. Pseudotachylytes are remnants of frictional melts generated during seismic slip and can therefore be used as an indicator of former seismogenic fault zones. The Fregon Subdomain in Central Australia was deformed under dry sub-eclogitic conditions of 600–700 °C and 1.0–1.2 GPa during the intracontinental Petermann Orogeny (ca. 550 Ma) and contains abundant pseudotachylyte. These pseudotachylytes are commonly foliated, recrystallized, and cross-cut by other pseudotachylytes, reflecting repeated generation during ongoing ductile deformation. This interplay is interpreted as evidence for repeated seismic brittle failure and post- to inter-seismic creep under dry lower-crustal conditions. Thermodynamic modelling of the pseudotachylyte bulk composition gives the same PT conditions of shearing as in surrounding mylonites. We conclude that pseudotachylytes in the Fregon Subdomain are a direct analogue of current seismicity in dry lower continental crust.

Pseudotachylyte increases the post-slip strength of faults

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

Fault welding by pseudotachylyte formation

Abstract During earthquakes, melt produced by frictional heating can accumulate on slip surfaces and dramatically weaken faults by melt lubrication. Once seismic slip slows and arrests, the melt cools and solidifies to form pseudotachylytes, the presence of which is commonly used by geologists to infer earthquake slip on exhumed ancient faults. Field evidence suggests that solidified melts may weld seismic faults, resulting in subsequent seismic ruptures propagating on neighboring pseudotachylyte-free faults or joints and thus leading to long-term fault slip delocalization for successive ruptures. We performed triaxial deformation experiments on natural pseudotachylyte-bearing rocks, and show that cooled frictional melt effectively welds fault surfaces together and gives faults cohesive strength comparable to that of an intact rock. Consistent with the field-based speculations, further shear is not favored on the same slip surface, but subsequent failure is accommodated on a new subparallel fault forming on an off-fault preexisting heterogeneity. A simple model of the temperature distribution in and around a pseudotachylyte following slip cessation indicates that frictional melts cool to below their solidus in tens of seconds, implying strength recovery over a similar time scale.

Deciphering viscous flow of frictional melts with the mini-AMS method

The anisotropy of magnetic susceptibility (AMS) is widely used to analyze magmatic flow in intrusive igneous bodies including plutons, sills and dikes. This method, owing its success to the rapid nature of measurements, provides a proxy for the orientation of markers with shape anisotropy that flow and align in a viscous medium. AMS specimens typically are 25 mm diameter right cylinders or 20 mm on-a-side cubes, representing a volume deemed statistically representative. Here, we present new AMS results, based on significantly smaller cubic specimens, which are 3.5 mm on a side, hence∼250 times volumetrically smaller than conventional specimens. We show that, in the case of frictional melts, which inherently have an extremely small grain size, this small volume is in most cases sufficient to characterize the pseudotachylyte fabric, particularly when magnetite is present. Further, we demonstrate that the mini-AMS method provides new opportunities to investigate the details of frictional melt flow in these coseismic miniature melt bodies. This new method offers significant potential to investigate frictional melt flow in pseudotachylyte veins including contributions to the lubrication of faults at shallow to moderate depths.

Ultrafine spherical quartz formation during seismic fault slip: Natural and experimental evidence and its implications

In recent works on the determination of pseudotachylyte within the principal slip zone (PSZ) of the Chelungpu fault (Taiwan), we demonstrated that frictional melting occurred at shallow depths during the 1999 Mw 7.6 Chi-Chi earthquake. Thus, the characteristics of melts are of paramount importance to investigate processes controlling dynamic fault mechanics during seismic slips. We conducted rock friction experiments on siltstone recovered from the Taiwan Chelungpu fault Drilling Project (TCDP) at a slip rate of 1.3m/s and a normal stress of 1MPa. Here we not only target to characterize experimental pseudotachylyte and evaluate the associated frictional behavior, but also compare it with natural frictional melts of TCDP. Our results show that (1) initial shear stress drop was related to the generation of low viscosity melt patches, (2) the evolution of shear stress in the postmelting regime was congruent with frictional melt rheology, and (3) the slip strengthening presumably resulted from the extremely low content of water of the frictional melt. In particular, the state-of-art of in situ synchrotron analyses (X-ray diffraction and Transmission X-ray Microscope) determine the presence of ultrafine spherical quartz (USQ) grains (~10nm to 50nm) in the glassy matrices presumably produced at high temperature. Our observations confirm that the USQ grains formed in rock friction experiments do occur in natural faults. We surmise the USQ is the result of frictional melting on siltstone and represents the latest slip zones of the Chelungpu fault, and further infer that the viscous melts may terminate seismic slips at shallow crustal conditions.

Planar microstructures in zircon from paleo-seismic zones

Pseudotachylytes resulted from frictional melts associated with ultramylonites in high-grade metapelitic rocks from the Ivrea-Verbano zone in the Southern Alps (Northern Italy) were studied with focus on the deformation microstructures in zircon. The aims were to investigate the characteristics of zircon deformation in seismic zones, and to recognize specific microstructures generated in zircon during earthquakes, which could be useful for mineral dating of paleo-seismic events; helps to understand how seismic energy is released at depth and interacts with metamorphic processes. The interior of polished zircon grains ranging from 30 to 150 μm in length were investigated with optical microscope and scanning electron microscope (SEM) techniques, including secondary electron (SE), backscattered electron (BSE), forward scattered electron (FSE), cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction analysis (EBSD). Grains were studied in situ and as separated fractions embedded in epoxy disks. Among different cataclastic and crystal-plastic deformation microstructures in zircon we identified characteristic planar deformation bands (PDBs), planar fractures (PFs), and curviplanar fractures (CFs). Planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain, which varies from 0.4° to 2.7°. PDBs are usually parallel to {100} crystallographic planes, have width from 0.3 to 1 μm and average spacing of 5 μm in 2D sections. Planar deformation bands appear as contrast lamellae in orientation contrast images and in EBSD maps, and in rare cases can be observed with the optical microscope. PDBs form in specifically oriented grains due to high differential stresses, high temperatures, and high strain rates generated in seismically active environment and/or due to shearing in the vicinity of frictional melts. Discovered structures represent a result of crystal-plastic deformation of zircon grains with operating dislocations having {010} glide system and misorientation axis, therefore, they can be classified as a new type 4 lattice distortion pattern, according to the existing classification for zircon (Piazolo et al. 2012; Kovaleva et al. 2014). We have demonstrated that formation of planar fractures in zircon takes place not only during impacts, but also in seismically active zones. We observe at least two cases of formation of PFs with {100} orientation: (1) as a result of evolution of PDBs and (2) as micro-cleavage. This study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events, but also as a result of seismic events at depth due to unusually high differential stress, strain rate, and temperature. According to the new findings, PDBs in zircon from the deep-crust are supposed to represent newly recognized evidence of seismicity.

Fault rheology beyond frictional melting

During earthquakes, comminution and frictional heating both contribute to the dissipation of stored energy. With sufficient dissipative heating, melting processes can ensue, yielding the production of frictional melts or "pseudotachylytes." It is commonly assumed that the Newtonian viscosities of such melts control subsequent fault slip resistance. Rock melts, however, are viscoelastic bodies, and, at high strain rates, they exhibit evidence of a glass transition. Here, we present the results of high-velocity friction experiments on a well-characterized melt that demonstrate how slip in melt-bearing faults can be governed by brittle fragmentation phenomena encountered at the glass transition. Slip analysis using models that incorporate viscoelastic responses indicates that even in the presence of melt, slip persists in the solid state until sufficient heat is generated to reduce the viscosity and allow remobilization in the liquid state. Where a rock is present next to the melt, we note that wear of the crystalline wall rock by liquid fragmentation and agglutination also contributes to the brittle component of these experimentally generated pseudotachylytes. We conclude that in the case of pseudotachylyte generation during an earthquake, slip even beyond the onset of frictional melting is not controlled merely by viscosity but rather by an interplay of viscoelastic forces around the glass transition, which involves a response in the brittle/solid regime of these rock melts. We warn of the inadequacy of simple Newtonian viscous analyses and call for the application of more realistic rheological interpretation of pseudotachylyte-bearing fault systems in the evaluation and prediction of their slip dynamics.

Garnet growth in frictional melts of the Ivrea Zone (Italy)

Pseudotachylytes, coseismic frictional melts, have formed in metagabbro and within a kinzigitic shear zone of the Ivrea Zone (Italy). In both rock types, amoeboid garnet has crystallized in the pseudotachylyte matrix, locally as a rim overgrown on pre-existing garnet clasts. This new garnet has been previously used to constrain the frictional melt formation in such rocks by applying geothermo-barometric calculations. Here we present a complete microstructural, textural and compositional characterization, by high-resolution scanning electron microscopy, focused ion beam tomography, electron backscatter diffraction and electron microprobe analysis. The garnet in pseudotachylyte exhibits subtle major element compositional and microstructural differences between the core and the overgrown rim. This suggests that the new garnet formed by rapid epitactic overgrowth on clasts of relic host rock garnet, during frictional melt cooling and solidification.

Fluid and deformation induced metamorphic processes around Moho beneath continent collision zones: Examples from the exposed root zone of the Caledonian mountain belt, W-Norway

Abstract Exposed High Pressure (HP) and Ultra High Pressure (UHP) metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near Moho depth in continental root zones. The investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle. This applies to a) formation of granulite facies areas, b) transformation of granulites to eclogites, c) retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d) the spinel lherzolite to garnet lherzolite transition. Dry rocks both feldspar bearing and ultramafic remain with their Pre-HP and UHP structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones. A mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle. Pseudotachylytes (frictional melts or ultracomminuted material) are observed in both ultramafic and feldspar bearing lithologies spatially associated with HP and UHP rocks suggesting that rock properties at Moho depth allow earthquakes. Seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx. Ductile eclogite facies shear zones nucleate along the brittle structures. These observations point to Moho as a rock processing zone with the following facets: 1. A metastable dry and strong lower crust and upper mantle. 2. Earthquakes and tremors result in fluid flow and HP metamorphism. 3. A pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics (LPO) in the transformed rocks. Deep tremor and earthquakes at Moho depth may record ongoing metamorphic transitions.