a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

emplacement of mantle peridotite in the lower continental crust ivrea verbano zone northwest italy the ivrea verbano zone has been cited repeatedly as an exposed pre alpine crust mantle transition on the basis in part of the presence of lenses of mantle peridotite and a possible petrologic moho a 10 km thick mafic complex has been described as an intrusion at the contact between the mantle and a thick crustal sequence of metamorphosed wacke and pelitic rocks the significance of these features is now challenged by new mapping that demonstrates that mantle peridotites in the southern ivrea verbano zone were lenses tectonically interfingered with metasedimentary rocks prior to intrusion of the gabbroic complex although it remains possible that the ivrea verbano section exposes part of a complex crust mantle interface within which crustal and mantle rocks were interleaved the association of peridotite and metasedimentary rocks is also consistent with assembly in an accretionary wedge in either case present exposures are from an unknown distance above the pre alpine contiguous mantle and reference to the section as a complete crust mantle transition could be misleading

stability and localization of rapid shear in fluid saturated fault gouge 2 localized zone width and strength evolution field and laboratory observations indicate that at seismic slip rates most shearing is confined to a very narrow zone just a few tens to hundreds of microns wide and sometimes as small as a few microns rice et al 2014 analyzed the stability of uniform shear in a fluid saturated gouge material they considered two distinct mechanisms to limit localization to a finite thickness zone rate strengthening friction and dilatancy in this paper we use numerical simulations to extend beyond the linearized perturbation context in rice et al 2014 and study the behavior after the loss of stability neglecting dilatancy we find that straining localizes to a width that is almost independent of the gouge layer width suggesting that the localized zone width is set by the physical properties of the gouge material choosing parameters thought to be representative of a crustal depth of 7 km this predicts that deformation should be confined to a zone between 4 and 44 m wide next considering dilatancy alone we again find a localized zone thickness that is independent of gouge layer thickness for dilatancy alone we predict localized zone thicknesses between 1 and 2 m wide for a depth of 7 km finally we study the impact of localization on the shear strength and temperature evolution of the gouge material strain rate localization focuses frictional heating into a narrower zone leading to a much faster temperature rise than that predicted when localization is not accounted for since the dynamic weakening mechanism considered here is thermally driven this leads to accelerated dynamic weakening

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

dynamic rupture processes inferred from laboratory microearthquakes we report macroscopic stick slip events in saw cut westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory experiments were conducted under triaxial loading 1 2 3 at confining pressures 3 ranging from 10 to 100 mpa a high frequency acoustic monitoring array recorded particle acceleration during macroscopic stick slip events allowing us to estimate rupture speed in addition we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane is almost total in the breakdown zone for normal stress 75 mpa while the friction f recovers to values of f 0 4 within only a few hundred microseconds enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our post mortem microstructural analysis relationships between initial state of stress rupture velocities stress drop and energy budget suggest that at high normal stress leading to supershear rupture velocities the rupture processes are more dissipative our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes a power law scaling of the fracture energy with final slip is observed over eight orders of magnitude in slip from a few microns to tens of meters

stress release in exhumed intermediate and deep earthquakes determined from ultramafic pseudotachylyte stresses released by coseismic faults during subduction toward lawsonite eclogite facies conditions in the alpine subduction complex of corsica can be estimated based on the energy required to form pseudotachylyte fault veins where shear strain can be measured congruent peridotite melting at ambient conditions of 1 5 gpa and 470 c requires a temperature increase of 1280 c to 1750 c we assume that more than 95 of the work is converted to heat during faulting hence that the stress drop is nearly proportional to the amount of melting and inversely proportional to shear strain minimum estimates of released stress are typically greater than 220 mpa and as high as 580 mpa the abundance of pseudotachylyte on small faults in the studied peridotite suggests that melting is very common on intermediate and deep earthquakes and that shear heating is important for seismic faulting at depth

large subduction earthquakes along the fossil moho in alpine corsica subduction earthquakes release vast amounts of energy to crust and mantle lithosphere the products of such drastic events are rarely observed in the field because they are mostly lost by subduction we present new observations of deformation products formed by a few very large and numerous small intermediate depth alpine subduction earthquakes that are preserved along the exhumed gabbro mantle peridotite contact of the piemont liguria oceanic basin in corsica the abrupt release of energy resulted in shear heating events that completely melted both gabbro and peridotite the large volumes of melt that were generated can be studied in the fault and injection vein breccia complex along the fault zone the energy required for wholesale melting of a large volume of peridotite along the fault combined with previous estimates of stress drops show that very large earthquakes took place along the moho of the subducting plate because these fault rocks formed by intraplate seismicity we suggest by analogy with present day subduction that they represent a proxy for the lower seismogenic zone

the tectonic and rheological evolution of an attenuated cross section of the continental crust ivrea crustal section southern alps northwestern italy and southern switzerland the tectonic and rheological evolution of the southern alpine continental crust is reconstructed from structural petrological and radiometric studies in the ivrea and strona ceneri basement units the deep crust of the southern alps acquired its present compositional and metamorphic zonation during paleozoic magmatism and amphibolite to granulite facies regional metamorphism inferred strength contrasts between lower crustal and upper mantle rocks in the ivrea zone are low at the high temperatures of regional metamorphism late paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early mesozoic time extensional uplift and cooling of the basement section is coupled with crustal scale trends of increasing rheological stratification grain size reduction and strain localization mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the ivrea zone are inferred to have controlled the high strain rheology of the attenuating southern alpine crust the deep crust in the thinned ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust final uplift and exposure of the deep southern alpine crust occurred during late cretaceous to tertiary alpine thrusting

initiating intermediate depth earthquakes insights from a hp lt ophiolite from corsica abstract the hypocentres of intermediate depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals this relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field based research we evaluated the relationship of the coincidence of intermediate depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite the eocene schistes lustres complex in corsica these units comprise incompletely metamorphosed metagabbro and peridotite the wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals tremolite mg hornblende glaucophane in the metagabbro and serpentine tremolite and chlorite in the peridotite back scatter electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration vesicular and h 2 o rich melt veins are observed cross cutting partially molten pseudotachylyte fault veins and show evidence of h 2 o exsolution during melt solidification the crystallisation products of these melts indicate formation under high temperature high pressure conditions 1400 1700 c 1 5 gpa the peridotite hosted pseudotachylytes crystallised olivine orthopyroxene and diopside which are surrounded by interstitial al and h 2 o rich glass the metagabbro pseudotachylyte is dominated by al rich omphacite ilmenite and high fe anorthite xrf bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix entrained survivor clasts and the crystallisation products show that near total disequilibrium melting took place the peridotite hosted pseudotachylyte composition is skewed strongly towards chlorite however the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered indicating that preferential fusion of chlorite took place the metagabbro hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of mg hornblende and tremolite not all the pseudotachylytes are hydrous as the h 2 o content of the melts is highly variable the metagabbro pseudotachylyte ranges from 0 to 4 wt and the peridotite pseudotachylyte ranges from 0 to 14 wt the range in h 2 o content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate depth seismicity however we have observed that the pseudotachylytes with the most chaotic vein networks thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals possibly owing to repeated fluid ingress in between pseudotachylyte generating events this implies that free fluids enhance pseudotachylyte generation and possibly seismicity but are not a first order requirement microtextural and geochemical results from this study suggest that the presence of abundant h 2 o rich minerals in the slab exerts a strong rheological control during high strain rate deformation facilitating thermally triggered localising shear instabilities these field based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation and offer fresh insight into the debate of how intermediate depth earthquakes take place

polyphase seismic faulting in the ivrea zone italian alps revealed by 40ar 39ar dating of pseudotachylytes terra nova 23 162 170 2011 abstract most fault related pseudotachylytes in the val sesia area ivrea zone nw italian alps occur in two areas near the canavese line in the kinzigite formation paragneisses and in an elongate belt in the mafic complex gabbros away from the canavese line 40ar 39ar dating indicates that pseudotachylytes from the kinzigite formation were formed in the late eocene likely during the early stages of the collision between the nw corner of the adriatic indenter and the european margin pseudotachylytes from the mafic complex were formed under a transpressive strain regime as suggested by kinematic indicators and yield early cretaceous albian 40ar 39ar ages their formation is possibly related to shortening during the inception of the tectonic inversion of the adriatic passive margin

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

slip weakening distance of faults during frictional melting as inferred from experimental and natural pseudotachylytes the slip weakening distance d c over which a fault weakens during seismogenic fault motion is one of most important parameters controlling fault instability a paradox over the last decade has been the source of the large difference between laboratory measured values of d c using conventional friction experiments 10 5 10 3 m and seismically determined d c values for natural earthquakes 10 1 10 0 m d c is a difficult quantity to estimate for natural faults but this study attempts to determine the order of d c values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes experiments at a constant slip velocity of 0 85 m sec and a normal stress of 1 4 mpa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening slip weakening a reduction in frictional strength with displacement and the growth of the molten layer an increase in the thickness of the molten layer with displacement are both characterized by exponential changes with nearly the same characteristic distances the comparison suggests that the order of d c can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement sibson s 1975 data from the outer hebrides exhibit a similar pseudotachylyte thickness versus fault displacement curve with a characteristic distance of 0 38 m this is the same order of magnitude as seismically inferred values of d c the effect of frictional melting is a possible solution for the d c paradox although it is not the only possible solution since frictional melting along large faults is rather uncommon

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

pseudotachylyte increases the post slip strength of faults 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 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

stress drop and frictional heating during the 1994 deep bolivia earthquake we use the slip model of the bolivian earthquake obtained by ihmle 1998 to investigate the spatial distribution of stress drop and calculate the frictional heat generated locally on the fault we find that the stress drop associated with the earthquake averages between 5 and 10 m pa over the fault the total frictional energy is estimated at between 3 1017 and 2 1018j if the fault width is as thin as the one produced by transformational faulting in laboratory experiments where it is about two orders of magnitude smaller than the local shear displacement melting likely occurred on a significant portion of the fault in this event as proposed by kanamori et al 1998 once rupture is triggered shear melting can quickly promote extensive slip

friction and roughness of a melting rock surface we acknowledge takehiro hirose for providing sample hvr 687 stefan nielsen was granted by the miur project fumo and giulio di toro and stefan nielsen were granted by the european research council starting grant project 205175 usems and ca ri pa ro we thank leonardo tauro for thin section preparation luca peruzzo steven smith and piergiorgio scarlato for sem facilities and toshi shimamoto for his constant support finally we are grateful to nick beeler and chris marone for their constructive reviews

transient discontinuities revisited pseudotachylyte plastic instability and the influence of low pore fluid pressure on deformation processes in the mid crust abstract occurrences of pseudotachylyte cyclically introduced as melts into the outer hebrides thrust are demonstrably synkinematic with crystal plastic mylonites classical interpretations of these rock types as respectively products of pressure dependent frictional melting and thermally activated intracrystalline deformation create a paradox in that these processes are to a large extent mutually exclusive detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy the ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 c and stresses of 140 210 mpa on which were imposed much higher transient stresses in response to heterogeneous non uniform flow the latter conditions are sufficient for the development of plastic instabilities in feldspar rich crust if pore fluid pressures are sufficiently low the latter is consistent with the absence of hydration during exhumation observed in the rocks under study low pore fluid pressure during thrust exhumation of deep crust enables activation of high strength ductile processes dominated by dislocation glide which may be a prerequisite to instability whereas other studies have demonstrated the possible occurrence of such melt generating instabilities it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure temperature conditions grain size reduction to produce ultramylonites dominated by grainsize sensitive flow is achieved by both deformation induced dynamic recrystallization and crystallization of instability generated melts

energy balance from a mantle pseudotachylyte balmuccia italy 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 m w 6 earthquake the subvertical fault underwent a metric dextral coseismic displacement raking 60 degrees sw the average width of the main slip surface is similar to 5mm a dense network of thin 20 200m injection and ultramylonite like veins decorates the fault walls in the injection veins raman microspectrometry mapping reveals pockets of still preserved amorphous silicate containing approximate to 1 of structurally bound h2o in the ultramylonite like veins electron backscattered diffraction mapping reveals that ultrafine 0 2 2m olivine grains exhibit a strong fabric with 010 planes parallel to shearing consistent with temperatures above 1250 degrees 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 40km the energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding i e dynamic friction coefficient 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 40km earthquake of 6 m w 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 plain language summary in active mountain ranges rocky massifs gradually rise to the surface under the combined effect of convergence and erosion it is then possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep tens of millions of years ago thus it can be seen that contrary to what we experience on the surface the rock melts when it tears a magma forms and facilitates relative ground displacement the traces discovered by the geologists are the solidified remains of this magma here by field observation and detailed analysis of samples in the laboratory we describe the traces of an earthquake that occurred more than 40 km deep in the earth s mantle its magnitude was greater than 6 the sliding on the fault greater than 1 m and yet the trace is thinner than 5 mm and against all odds the most talkative witnesses are extremely small micrometric crystals which formed from the magma upon the rupture record crucial information unveiling its focal mechanism we still do not entirely know what an earthquake exactly is and nobody will ever be able to see what happens on the fault at depth during an earthquake hence these fossils are key

the role of rift inherited hyper extension in alpine type orogens alpine type orogens are interpreted to result from the collision of former rifted margins as many present day rifted margins consist of hyper extended domains floored by thinned continental crust 10 km and or exhumed mantle this study explores the influence of rift inheritance on the architecture and final evolution of alpine type orogens we propose that rift related necking zones separating weakly thinned 25 to 30 km thick crust from hyper extended domains may act as buttresses during the transition from subduction to collision as a result former necking zones may now be found at the boundary between a highly deformed and overthickened nappe stack made of relics of hyper extended domains and an external weakly deformed fold and thrust belt which largely escaped significant rift related crustal thinning and orogeny related thickening therefore the role of rift inheritance is of critical importance and is largely underestimated in controlling the architecture and evolution of alpine type orogens

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

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

disappearing ink how pseudotachylytes are lost from the rock record abstract melt origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record however reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed in this paper we document the processes whereby pseudotachylytes are overprinted destroyed and otherwise removed from the rock record we present examples of recrystallized altered and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults based on these observations we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes our results imply that pseudotachylytes are vastly under reported due to their vulnerability to destruction and the resulting difficulty in identification as a consequence the significance of frictional melting is underestimated the criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record

amphibolite facies pseudotachylytes in premosello metagabbro and felsic mylonites ivrea zone italy abstract pseudotachylytes solidified friction induced melts produced during seismic slip that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle ductile elasto frictional viscous plastic transition in the crust thin sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite facies premosello metagabbro and inside country felsic mylonites of the ivrea zone italian southern alps petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies 500 650 c metamorphic conditions the occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites the association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients related to downward propagation of seismic ruptures from the upper brittle crust followed by post seismic stress relaxation by ductile flow

fault plane processes and mesoscopic structure of a strong type seismogenic fault in tonalites adamello batholith southern alps abstract the gole larghe fault is an exhumed paleoseismic fault crosscutting the adamello tonalites italian southern alps ambient conditions of faulting were 9 11 km in depth and 250 300 c in the study area the fault accommodates 1100 m of dextral strike slip over a fault thickness of 550 m displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons faults1 2 3 fault displacement is in the range of few centimeters faults3 to a maximum of a few tens of meters in major faults1 faults1 2 nucleated on pre existing joints whereas faults3 are newly generated fractures produced during slip along faults1 2 each fault within the gole larghe fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production pseudotachylytes are usually generated at the host rock cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins pseudotachylytes and cataclasites have a similar chemical composition which is enriched in loss on ignition k rb ba u and fe3 compared to host rock we envision two models for the evolution of the gole larghe fault in both models synkinematic fluid rock interaction along a fault causes fault hardening by precipitation of abundant k feldspar epidote and minor chlorite in the cataclasite matrix conducive to final production of pseudotachylyte in the first model induration occurs progressively by differential precipitation related to fabric evolution in cataclasites in the second model induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir whatever the model the gole larghe fault represents a strong fault where hardening processes resulted in a low displacement fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement fault thickness ratios

chemical evolution of a large mafic intrusion in the lower crust ivrea verbano zone northern italy the ivrea verbano and adjacent strona ceneri zones have been described collectively as a section through the continental crust while resident in the lower crust amphibolite to granulite facies paragneiss of the ivrea verbano zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the mafic complex growth of the mafic complex involved hypersolidus deformation in an extensional environment isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle derived melts that were extensively contaminated by crustal material previous investigations determined that the contaminant was fingerprinted by 87sr 86sr 0 71 18o 10 12 5 and a positive eu anomaly in the present study the contaminant is also shown to have been enriched in ba with respect to rb and k charnockites associated with paragneiss septa in the lower part of the mafic complex have the appropriate chemistry to be samples of the contaminating material these chemical features can be explained by melting of granulite facies paragneiss which had previously been depleted in k and rb by an earlier melting event the ba enrichment in the core of the mafic complex can be modeled by a replenishment tapping fractional crystallization rtf process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by ba rich charnockite very high ba k in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa in contrast to the mafic complex the chemistry of coeval granites in the adjacent strona ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event significantly the incompatible trace element abundances in the mafic complex and strona ceneri granites are similar to model compositions for the lower and upper crust respectively

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

cenozoic paleoceanography 1986 an introduction new developments in cenozoic paleoceanography include the application of climate models and atmospheric general circulation models to questions of climate reconstruction the refinement of conceptual models for interpretation of the carbon isotope record in terms of carbon mass balance paleocirculation paleoproductivity and the regional mapping of paleoceanographic events by acoustic stratigraphy sea level change emerges as a master variable to which changes in the ocean environment must be traced in many cases and tests of the onlap offlap paradigm therefore are of crucial importance

fast slip with inhibited temperature rise due to mineral dehydration evidence from experiments on gypsum anomalously low heat flow around active faults has been a recurrent subject of debate over past decades we present a series of high velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large scale endothermic dehydration reaction the tests were performed at 1 mpa normal stress and a velocity of 1 3 m s 1 while measuring the temperature close to the sliding surface and the relative humidity around the sample the temperature close to the sliding surface is remarkably stable at 100 c during the dehydration reaction of gypsum microstructural and x ray diffraction investigations show that dehydration occurs at the very beginning of the test and progresses into the bulk as slip increases in the hottest parts of the sample anhydrite crystal growth is observed the half thickness of the dehydrated layer ranges from 160 m at 2 m slip to 5 mm at 68 m slip thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10 to 50 of the total mechanical work input the temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam we suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake this is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

fossil earthquakes recorded by pseudotachylytes in mantle peridotite from the alpine subduction complex of corsica paleo earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of alpine corsica pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the schistes lustres of cape corse ultramafic pseudotachylyte fault and injection veins are found within well preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures the newly formed minerals are zoned olivine fo93 89 clino and ortho pyroxene with compositions indicative of high crystallization temperatures 1300 1400 8c zoned cr spinel and a glassy to micro vesicular hydrous matrix showing that frictional melts contained up to 4 water frictional heating on co seismic faults raised the temperature from ambient blueschist facies conditions 450 8c and 1 1 5 gpa to more than 1700 8c which is required for 75 disequilibrium melting of spinel peridotite at 1 5 gpa the characteristic fault vein thicknesses observed are 1 to 3 cm but several fault veins are thicker than 10 cm co seismic displacement of 1 m a stress of 300 mpa and seismic efficiency of 5 may melt ca 60 kg peridotite pr m 2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte the ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the alpine subduction zone if the interpretations of typical displacements of approximately 1 m are correct the most common pseudotachylyte fault veins are related to magnitude ca 7 or larger subduction earthquakes d 2005 elsevier b v all rights reserved

laboratory earthquakes triggered during eclogitization of lawsonite bearing blueschist abstract the origin of intermediate depth seismicity has been debated for decades a substantial fraction of these events occurs within the upper plane of wadati benioff double seismic zones believed to represent subducting oceanic crust we deformed natural lawsonite rich blueschist samples under eclogite facies conditions 1 5 p 3 5 gpa 583 k t 1121 k using a d dia apparatus installed at a synchrotron beamline continuously monitoring stress strain phase changes and acoustic emissions aes two distinct paths were investigated i heating during deformation at pressures 2 5 gpa to maximum temperatures ranging from 762 to 1073 k during which lawsonite and glaucophane became gradually unstable while entering the stability field of lawsonite eclogite and the breakdown reaction of lawsonite was only crossed in case of the highest final temperature ii heating while deforming at a pressure

pseudotachylytes in the balmuccia peridotite ivrea zone as markers of the exhumation of the southern alpine continental crust terra nova 22 70 77 2010 abstract two types of pseudotachylytes are observed in the balmuccia peridotite of the ivrea zone southern alps italy a type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 c and pressures comprising between 0 6 and 1 2 gpa these conditions were met in the ivrea crust between 350 and 270 ma suggesting that a type pseudotachylytes were formed during variscan tectonics or permian transtensional tectonics b type pseudotachylytes post date a type pseudotachylytes textural characteristics of b type veins suggest a formation in the upper continental crust at depths of about 5 10 km or less petrological constraints indicate that the exhumation of the ivrea crust at such shallow depths was achieved later than c 70 ma thus providing a maximum age of 70 ma for b type veins pseudotachylytes appear as markers of the poly orogenic evolution of the alpine belt

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

a weakening mechanism for intermediate depth seismicity detailed petrographic and microtextural observations from blueschist facies pseudotachylytes cape corse corsica abstract gabbro and peridotite hosted pseudotachylytes from the alpine schistes lustres unit in corsica previously determined to have formed at blueschist to lawsonite eclogite facies conditions have been causally linked to the generation of intermediate depth earthquakes which occur at depths of 50 300 km detailed petrographic and microtextural analyses of these pseudotachylytes suggest that their initiation may be controlled by a thermally activated shear runaway process that is controlled by rheology rather than mineralogy this is documented by sheared out prolate kinked and twinned wallrock clasts that have been peeled off and entrained into the pseudotachylyte vein as sigmoid survivor clasts the presence of metastable high temperature crystallisation products in the pseudotachylyte such as hoppers and dendrites of olivine enstatite and diopside peridotite and al rich omphacite and fe rich anorthite in metagabbro are suggestive of a short lived high temperature event resulting from thermal instability these high temperature mineral assemblages are overprinted by ones indicating a return to ambient conditions of lower temperatures but still high pressures glaucophane albite and epidote in metagabbro and clinochlore and fine grained granoblastic olivine enstatite and diopside in peridotite the observations from this detailed study of natural samples suggest that intermediate depth seismicity may be generated by a thermal runaway process

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

ductile brittle transition in pre alpine amphibolite facies mylonites during evolution from water present to water deficient conditions mont mary nappe italian western alps in the austroalpine mont mary nappe italian western alps discrete zones of mylonites ultramylonites developed from coarse grained upper amphibolite facies metapelites of pre alpine age the syn mylonitic mineral assemblage is quartz biotite muscovite plagioclase garnet sillimanite ilmenite graphite and formed via the model hydration reaction grt 1 kfs h 2 o bt 2 ilm 2 qtz ms sil grain size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite and by compositional changes of garnet and biotite deformation took place at temperatures of 510 580 c under low pressure conditions 0 25 0 45 gpa and corresponds to the latest stages of pre alpine metamorphic evolution the pre alpine mylonitization conditions were close to the brittle ductile transition as indicated by syn mylonitic generation of pseudotachylytes and high diverential stress inferred from quartz grain size piezometry the brittle ductile behaviour at a relatively high temperature and the absence of annealing textures in quartz aggregates are suggestive of water deficient conditions during mylonitization these were accomplished through progressive consump tion of water by syn kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization

seismic evidence for thermal runaway during intermediate depth earthquake rupture 1 intermediate depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected there are two leading candidates for the physical mechanism behind these earthquakes dehydration embrittlement and self localizing thermal shear runaway a complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate depth earthquake rupture the combination of high stress drop and low radiation efficiency that we observe for mw 4 5 earthquakes in the bucaramanga nest implies a temperature increase of 600 1000 c for a centimeter scale layer during earthquake failure this suggests that substantial shear heating and possibly partial melting occurs during intermediate depth earthquake failure our observations support thermal shear runaway as the mechanism for intermediate depth earthquakes which would help explain differences in their behavior compared to shallow earthquakes

deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte bearing brittle faults a matter of a few seconds abstract tectonic pseudotachylytes i e quenched friction induced silicate melts record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites in this study we document the occurrence of recrystallization of quartz to ultrafine grained grain size 1 2 m aggregates along microshear zones 50 150 m thick in the host rock adjacent to pseudotachylytes from two different faults within quartzite schneeberg normal fault zone eastern alps and tonalite adamello fault southern alps in the brittle crust the transition from the host quartz to microshear zone interior includes i formation of high dislocation densities ii fine 0 3 0 5 m polygonization to subgrains defined by disordered to well ordered dislocation walls iii development of a mosaic aggregate of dislocation free new grains the crystallographic preferred orientation cpo of quartz towards the microshear zone shows a progressive misorientation from the host grain by subgrain rotation recrystallization to a nearly random cpo possibly related to grain boundary sliding these ultrafine aggregates appear to be typically associated with pseudotachylytes in nature we refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip thermal models indicate that the process of recrystallization including recovery processes occurred in a time lapse of a few tens of seconds

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

generation kinetics based method for correcting effects of migrated oil on rock eval data an example from the eocene qianjiang formation jianghan basin china abstract migrated hydrocarbons occur commonly in liquid rich source rock reservoirs and can distort analytical results of programed pyrolysis disregarding the effect may result in unreliable data interpretation and inappropriate hydrocarbon generation kinetics this paper proposes a method based on kerogen kinetics for characterizing the impacts estimating the severity and subsequently minimizing the effects by removal of the migrated components from the oil stained samples through a numerical approach the restored hydrocarbon pyrograms are then used for recalculation of relevant rock eval parameters and proper construction of generation kinetic models the proposed methods and workflow were applied to the source rock samples from the qianjiang formation a confined source rock system imbedded in salt layers where contamination from migrated hydrocarbons is common an additional rock eval dataset consisting of whole rock samples and post solvent extracted replicates was used to validate the methods the validation results show that the proposed numerical approach is a cost effective alternative to the traditional laboratory solution

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

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust intermediate depth earthquakes in cold subduction zones are observed within the subducting oceanic crust as well as the mantle in contrast intermediate depth earthquakes in hot subduction zones predominantly occur just below the mohorovicic discontinuity these observations have stimulated interest in relationships between blueschist facies metamorphism and seismicity particularly through dehydration reactions involving the mineral lawsonite here we conducted deformation experiments on lawsonite while monitoring acoustic emissions in a griggs type deformation apparatus the temperature was increased above the thermal stability of lawsonite while the sample was deforming to test whether the lawsonite dehydration reaction induces unstable fault slip in contrast to similar tests on antigorite unstable fault slip that is stick slip occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed microstructural observations indicate that strain is highly localized along the fault r1 and b shears and that the fault surface develops slickensides very smooth fault surfaces polished by frictional sliding the unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate a thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones indicating the potential for unstable frictional sliding within natural lawsonite layers

thermochronological constraints on the multiphase exhumation history of the ivrea verbano zone of the southern alps abstract the ivrea verbano zone of the western southern alps nw italy exposes a well preserved tilted section across the lower continental crust making it a key region for studying deep crustal and exhumation processes this paper refines the cooling and exhumation history of the ivrea verbano zone using k ar dating of mica and illite rich fault gouges as well as zircon fission track and u th he thermochronology the adjacent strona ceneri zone sesia lanzo zone and lower penninic nappes are included in the study to derive a broader picture of the low temperature history of the area in the strona profile of the ivrea verbano unit the biotite k ar zircon fission track and u th he geochronometers show well preserved but unusually wide partial retention zones the youngest ages representing the formerly deepest position are situated along the insubric line the main foliation of the ivrea verbano zone is vertical the exhumation of the ivrea verbano zone which section has a horizontal position on the surface now took place in three steps during jurassic time the ivrea verbano zone was exhumed to a shallow to mid crustal position by continental scale extension in this displacement the pogallo line probably played a dominant role the studied section occupied an oblique position with a calculated angle of ca 15 to 23 in the jurassic later the ivrea verbano zone experienced a minor cooling event in the late eocene 38 ma zircon fission track ages that was probably related to thrusting and erosion the final exhumation towards the surface took place in the mid miocene as documented by the ca 14 ma zircon u th he ages and a 12 8 ma k ar fault gouge age the magnitude and the high rate of final exhumation suggest orogen parallel extension as a driving force which is widespread in the alps in the lower to middle miocene and is most probably connected to orogenic collapse

kick and cook experiments on peridotite simulating coseismic deformation and post seismic creep terra nova 24 62 69 2012 abstract experiments comprising deformation at 600 c and annealing at 700 1000 c were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle the found olivine microstructures indicate that deformation occurs by low temperature plasticity at low and high annealing temperatures zones of high strain are replaced by subgrains and recrystallized grains respectively the microstructures after annealing at 1000 c resemble core and mantle structures from shear zone peridotites often interpreted as imprint of steady state dislocation creep our study shows that such structures can form by a sequence of low temperature plasticity at high stress and subsequent recrystallization at low stress as corresponding to coseismic deformation and post seismic creep in this case palaeopiezometers are not applicable and a local cpo in aggregates of new grains does not necessarily indicate the activated glide system

origin of pseudotachylites in slow creep experiments we conducted a series of experiments on granitoid cataclasites under mid crustal conditions pc 500 mpa t 300 c and slow displacement rates 10 8 ms 1 d 10 7 ms 1 i e within an order of magnitude of plate tectonic velocities the samples reach high peak strength 1250 1375 mpa then weaken slightly by 40 mpa and continue to deform without any abrupt stress drops up to a of 4 microstructural observations show that at peak strength nanocrystalline mean grain size 34 nm smallest crystalline particles 8 nm to partly amorphous material develops weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions during higher strain deformation deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop this material shares many similarities with naturally occurring pseudotachylites however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes 1 ms 1 we conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

rough faults distributed weakening and off fault deformation 1 we report systematic spatial variations in fault rocks along nonplanar strike slip faults cross cutting the lake edison granodiorite sierra nevada california sierran wavy fault and lobbia outcrops of the adamello batholith in the italian alps lobbia wavy fault in the case of the sierran fault pseudotachylyte formed at contractional fault bends where it is found as thin 1 2 mm fault parallel veins epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends we argue that the presence of fluids as illustrated by this example does not necessarily preclude the development of frictional melt in the case of the lobbia fault pseudotachylyte thickness varies along the length of the fault but the pseudotachylyte veins thicken and pool in extensional bends we conduct a quantitative analysis of fault roughness microcrack distribution stress and friction along the lobbia fault numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness resulting in an overall weaker fault than suggested by the corresponding static friction coefficient the models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip rather than being confined to the region close to the dynamically propagating crack tip

earthquake rupture dynamics frozen in exhumed ancient faults just by looking at the traces of solidified friction induced melt produced by ancient earthquakes it is possible to learn about fractures that lasted a few seconds during seismic events that took place millions of years ago in the italian alps large sections of an ancient fault have been uprooted by tectonic movements brought to the surface almost intact and polished by the abrasion of a glacier secondary fractures created during the earthquakes tend to lie to one side of the fault in a particular direction suggesting that rupture propagated in the same direction during repeated earthquakes most of our knowledge about co seismic rupture propagation is derived from inversion and interpretation of strong ground motion seismograms1 2 3 laboratory experiments on rock4 5 and rock analogue material6 or inferred from theoretical and numerical elastodynamic models7 8 9 however additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the earth s surface10 pseudotachylytes solidified friction induced melts11 12 are the most certain fault rock indicator of seismicity on ancient faults13 here we show how the asymmetry in distribution and the orientation of pseudotachylyte filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity rupture velocity and fracture energy by comparison with the theoretical dynamic stress field computed around propagating fractures in particular the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the rayleigh wave velocity

crystallographic preferred orientation produced by diffusional creep of harzburgite effects of chemical interactions among phases during plastic flow 1 experimental deformation experiments have been conducted on fine grained two phase aggregates of olivine and orthopyroxene to investigate the role of grain and phase boundary sliding on rheology and fabric development a suite of large strain 1 general shear experiments conducted at t 1200 c and p 1 6 gpa on aggregates ranging from 65 to 0 vol orthopyroxene to characterize the evolution of fabric were complemented by small strain axial compression experiments at t 1200 c and p 0 3 gpa to better constrain the rheology microstructural and rheological data suggest that deformation of these two phase aggregates in the diffusion creep regime occurs via interface reaction controlled diffusion creep that is accompanied by extensive migration of olivine orthopyroxene phase boundaries the resulting rheologies suggest that olivine orthopyroxene composites are weaker than the olivine end member at the conditions tested physically this behavior arises because long range i e grain scale diffusion of si4 is unnecessary in these pseudobinary two phase aggregates we further demonstrate that interface controlled diffusion creep leads to strong crystallographic preferred orientations cpo of the component minerals which develops in the near absence of dislocation activity the cpo formed in these anhydrous low stress experiments has the olivine a axis aligned perpendicular to the flow direction type b fabric argued by some to be the unique result of deformation under conditions of high differential stress and high water fugacity phase boundary dynamics thus are argued as a significant factor in the accumulation of strain in polyphase aggregates

instantaneous healing of micro fractures during coseismic slip evidence from microstructure and ti in quartz geochemistry within an exhumed pseudotachylyte bearing fault in tonalite abstract exhumed faults within the tonalitic adamello pluton southern alps were seismic at depth as indicated by the presence of pseudotachylytes solidified friction induced melts during cooling of tonalite early formed joints were first exploited by localized ductile shear zones associated with deposition of quartz veins at 500 c and later by pseudotachylyte bearing cataclastic faults at 250 300 c ambient temperature adjacent to pseudotachylytes quartz of the host tonalite shows pervasive thin 1 10 m wide healed micro fractures and ultra fine 1 2 m grain size recrystallized aggregates along micro shear zones under cathodoluminescence cl the healed micro fractures have a darker gray shade than the host magmatic quartz that reflects a change in ti concentrations ti as indicated by nanosims measurements ti vary from 35 55 ppm in the cl lighter host quartz to 10 13 ppm along the cl darker healed micro fractures these ti were inherited by the ultra fine recrystallized aggregates that overprinted both the magmatic quartz and the healed micro fractures during the high temperature transient related to frictional seismic slip based on ti in quartz thermometry we infer that micro fracture healing occurred at higher temperatures than the ambient temperatures of faulting 250 300 c at 0 2 gpa for which ti

frictional melting of peridotite and seismic slip 1 the evolution of the frictional strength along a fault at seismic slip rates about 1 m s is a key factor controlling earthquake mechanics at mantle depths friction induced melting and melt lubrication may influence earthquake slip and seismological data we report on laboratory experiments designed to investigate dynamic fault strength and frictional melting processes in mantle rocks we performed 20 experiments with balmuccia peridotite in a high velocity rotary shear apparatus and cylindrical samples 21 8 mm in diameter over a wide range of normal stresses 5 4 16 1 mpa slip rates 0 23 1 14 m s and displacements 1 5 71 m during the experiments shear stress evolved with cumulative displacement in five main stages stages 1 5 in stage 1 first strengthening the coefficient of friction increased up to 0 4 0 7 first peak in friction in stage 2 abrupt first weakening decreased to about 0 25 0 40 in stage 3 gradual second strengthening shear stress increased toward a second peak in friction 0 30 0 40 in stage 4 gradual second weakening the shear stress decreased toward a steady state value stage 5 with 0 15 stages 1 and 2 are of too short duration to be investigated in detail with the current experimental configuration by interrupting the experiments during stages 3 4 and 5 microstructural field emission scanning electron microscope and geochemical electron probe micro analyzer and energy dispersive x ray spectroscopy analysis of the slipping zone suggest that second strengthening stage 3 is associated with the production of a grain supported melt poor layer while second weakening stage 4 and steady state stage 5 are associated with the formation of a continuous melt rich layer with an estimated temperature up to 1780 c microstructures formed during the experiments were very similar to those found in natural ultramafic pseudotachylytes by performing experiments at different normal stresses and slip rates 1 the thermal as it includes the thermally activated first and second weakening slip distance to achieve steady state from the first peak in strength decreased with increasing normal stress and slip rate and 2 the steady state shear stress slightly increased with increasing normal stress and for a given normal stress decreased with increasing slip rate the ratio of shear stress versus normal stress was about 0 15 well below the typical friction coefficient of rocks 0 6 0 8 the dependence of steady state shear stress with normal stress was described by means of a constitutive equation for melt lubrication the presence of microstructures similar to those found in natural pseudotachylytes and the determination of a constitutive equation that describes the experimental data allows extrapolation of the experimental observations to natural conditions and to the study of rupture dynamics in mantle rocks

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths