strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

the strength profile for bimineralic shear zones an insight from high temperature shearing experiments on calcite halite mixtures abstract shearing experiments on mixed halite calcite layers 0 7 mm thick have been performed to understand the behavior of bimineralic fault zones using a high temperature biaxial testing machine at a slip rate of 0 3 m s and shear strains to about 30 temperature was increased to 700 c in linear proportion to the normal stress with the experimental geotherm of about 22 c mpa simulating the natural geothermal gradient the experimental data clearly demonstrate that the effect of mineral composition on the ultimate frictional strength is distinctly different from that on the residual frictional strength tharp s framework model 1983 quantitatively accounts for the ultimate frictional strength whereas the residual frictional strength at large shear strains can be predicted by jordan s two block model 1988 thus the frictional behavior of a bimineralic shear zone changes from the framework model to the two block model with increasing displacement inclusion of halite the weaker member in quantities as small as 5 by volume reduces the friction almost to the level of pure halite and suppresses stick slip at large shear strains because halite grains are extremely sheared at the zone of strain concentration the strength profile and the slip mode including the lower limit of seismic behavior of bimineralic shear zones at large shear strains are controlled primarily by the weaker member the present results disprove strehlau s fault model 1986 the role of individual constituent mineral is not clarified in scholz s fault model 1988 and his model disagrees with existing experimental data

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

water films at grain grain contacts debye huckel osmotic model of stress salinity and mineralogy dependence abstract water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure solution during geological processes in the upper crust this mechanism assumes that matter is dissolved inside the contact between two grains the resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film the main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains in this paper we show that the electrically charged surface of a mineral can attract counter ions from the pore and by the related change of osmotic pressure keep water within the contact this is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore this lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film these notions are made more precise by combining the theory of the debye huckle double layer with equations of osmotic pressure the resulting d h o theory predicts the dependence of the water film thickness on stress across the contact composition of the pore fluid and the identity of the minerals involved

ringwoodite growth rates from olivine with 75ppmw h2o metastable olivine must be nearly anhydrous to exist in the mantle transition zone it has been previously demonstrated that as little as 300 ppmw h 2o increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis to further test this hypothesis we present new ringwoodite growth rate measurements from olivine with 75 ppmw h 2o at 18 gpa and 700 900 and 1100 c these growth rates are nearly identical to those from olivine with 300 ppmw h 2o and significantly higher than those from nominally anhydrous olivine we infer that transformation of olivine with 75 300 ppmw h 2o is primarily enhanced by hydrolytic weakening of reaction rims which reduces the elastic strain energy barrier to growth we present a new method for fitting non linear nominally anhydrous data to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine based on previous thermokinetic modeling these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone metastable persistence of olivine into the mantle transition zone would therefore require 75 ppmw h 2o

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

ahrensite γ fe 2 sio 4 a new shock metamorphic mineral from the tissint meteorite implications for the tissint shock event on mars ahrensite ima 2013 028 fe 2sio 4 is the natural fe analog of the silicate spinel ringwoodite mg 2sio 4 it occurs in the tissint martian meteorite where it forms through the transformation of the fayalite rich rims of olivine megacrysts or fe rich microphenocrysts in contact with shock melt pockets the typical sequence of phase assemblages traversing across a tissint melt pocket into olivine is quenched melt or fayalite pigeonite intergrowth bridgmanite wustite ahrensite and or ringwoodite highly deformed olivine nanocrystalline ringwoodite deformed olivine we report the first comprehensive set of crystallographic spectroscopic and quantitative chemical analysis of type ahrensite and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible we also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets the ahrensite and ringwoodite crystals show no preferred orientation consistent with random nucleation and incoherent growth within a highly strained matrix of olivine grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration 1 10 ms

kinetics of the olivine ringwoodite transformation and seismic attenuation in the earth s mantle transition zone abstract in regions of the mantle where multi phases coexist like at the olivine wadsleyite ringwoodite transitions the stress induced by the seismic waves may drive a mineralogical reaction between the low to high pressure phases a possible source of dissipation in such a situation the amount of attenuation critically depends on the timescale for the phase transformations to reach equilibrium relative to the period of the seismic wave here we report synchrotron based measurements of the kinetics of the olivine to ringwoodite transformation at pressure temperature conditions of the co stability loop for iron rich olivine compositions both microstructural and kinetic data suggest that the transformation rates are controlled by growth processes after the early saturation of nucleation sites along olivine grain boundaries transformation time data show an increase of reaction rates with temperature and iron content and have been fitted to a rate equation for interface controlled transformation g k 0 t exp n x fa exp h a p v r t 1 exp g r r t where x fa is the fayalite fraction the exponential factor n 9 7 ln k 0 9 1 m s 1 x fa 1 and h a 199 kj mol assuming v 0 cm 3 mol including these new kinetic results in a micro mechanical model of a two phase loop ricard et al 2009 we predict q k 1 and q 1 significantly higher than the prem values for both body waves and normal modes this attests that the olivine wadsleyite transition can significantly contribute to the attenuation of the earth s mantle transition zone

impact of phase change kinetics on the mariana slab within the framework of 2 d mantle convection abstract recent high pressure and high temperature experiments indicate that metastable olivine might persist in the cold core of a slab due to the low reaction rate of the olivine wadsleyite phase transformation recent seismological observations detected a metastable olivine wedge that survives to a depth of 630 km in the mariana slab to consider the problem of non equilibrium phase transformation we developed a two dimensional 2 d cartesian numerical code that incorporates the effects of kinetics into a thermal convection model we consider the kinetics of the 410 km olivine wadsleyite and the 660 km ringwoodite pv mw phase transformations including the effects of water content at the 410 km phase boundary the latent heat release of the 410 km non equilibrium phase transformations inside the slab is also considered the results show positive correlations between some of the controlling parameters and the length of the metastable olivine wedge the faster the subducting velocity and the lower the water content the deeper is the metastable olivine wedge with increasing depth of phase transformation the effect of latent heat release is enhanced heating of at most 100 c occurs if olivine transforms into wadsleyite at a depth of approximately 570 km in our model setting temperature increase due to the latent heat released stimulates further phase transformation resulting in further temperature increase acting as a positive feedback effect we also attempt to explain the seismological observations by calculating the temperature and phase structures in the mariana slab if we assume that the age of the mariana slab is 150 myr the subduction velocity is 9 5 cm yr phase transformation occurs from the grain boundary of the parental phase and the water content is 250 wt ppm for a grain size of 1 mm 300 wt ppm for one of 5 mm and 100 wt ppm for intracrystalline transformation then the metastable olivine wedge survives to a depth of 630 km which is in good agreement with the seismological observations this suggests that the deeper portion of the mariana slab is relatively dry assuming that the depression of the 660 km discontinuity by 20 30 km within the mariana slab as is indicated by seismological observation is explained by a combination of the depression caused by a negative clapeyron slope in the cold slab and that due to the kinetics of the 660 km phase transformation we obtain a gentle clapeyron slope of 0 9 mpa k for the phase transformation from ringwoodite to pv mw

evidence for a metastable olivine wedge inside the subducted mariana slab abstract an anomalous seismic velocity structure is revealed inside the subducted pacific plate at northern mariana near the deepest earthquake foci we observe anomalous later phases in the p wave coda for a small cluster of deep earthquakes and an anomalous differential p wave slowness between a few pairs of the earthquakes belonging to the cluster we interpret these observations with a 5 low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km the wedge is bounded by two steeply dipping 60 70 transition layers with the thickness of about 4 km the highly localized seismic anomaly inside the slab represents a wedge of metastable olivine embedded in dry ringwoodite deep earthquakes occur across only one of the boundaries of the metastable wedge with some of them outside the wedge the mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge

double seismic zone for deep earthquakes in the izu bonin subduction zone a double seismic zone for deep earthquakes was found in the izu bonin region an analysis of sp converted phases confirms that the deep seismic zone consists of two layers separated by 20 kilometers numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 to 550 c which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high pressure phase in the subducting slab

olivine wadsleyite pyroxene topotaxy evidence for coherent nucleation and diffusion controlled growth at the 410 km discontinuity we have synthesized a hydrous peridotite composition sample at 13 gpa and 1400 c with co existing coarse grains 100 lm of olivine wadsleyite clinoenstatite plus melt in a multi anvil press some of the olivine grains contain fine scale 0 5 2 lm wide lamellae of wadsleyite and clinoenstatite that likely resulted from transformation caused by small temperature fluctuations during the four hour experiment phase compositions were determined by electron probe microanalysis epma and secondary ion mass spectroscopy sims the olivine ranges from fo94 to fo90 in composition and contains about 4000 ppm wt h2o the wadsleyite is fo87 1 in composition and contains about 10 000 ppm wt h2o the clinoenstatite is en93 1 in composition and about 1400 ppm wt h2o transmission electron microscopy of the wadsleyite lamellae and host olivine shows that the two phases share their close packed oxygen planes so that the wadsleyite lamellae are nearly planar and perpendicular to the 1 0 0 of olivine the wadsleyite lamellae thus have their 1 0 1 and 0 2 1 planes parallel to the 1 0 0 plane of olivine additionally larger incoherent grains of wadsleyite in olivine are found dislocation microtexures in the olivine and iron concentration profiles across the lamella interface suggest heterogeneous nucleation and diffusion controlled growth of coherent wadsleyite lamellae on defects in the olivine followed by the nucleation of faster growing incoherent grains on the lamellae the results show that under hydrous conditions the olivine wadsleyite transformation occurs close to equilibrium at conditions of the 410km discontinuity furthermore inheritance of crystallographic preferred orientations and therefore seismic anisotropy across the 410 km discontinuity is unlikely to be significant in addition hydrogen distributions among the various phases indicate that dehydration by melt extraction at 410 km will be inefficient and that h contents greater than about 4000 ppm wt h2o are needed to initiate melting at 410 km 2012 published by elsevier b v

seismological and experimental constraints on metastable phase transformations and rheology of the mariana slab abstract a wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the mariana slab based on these seismic observations and numerical calculation of thermal structures of the slab we suggest that the phase transformations from olivine to ringwoodite and from ringwoodite to perovskite ferropericlase the post spinel transformation occur at the depth of 630 km and 550 c and at 690 km and 700 c respectively in the central coldest part of the mariana slab combining this information with recent experimental kinetic data we constrain the details of non equilibrium phase transformations and rheology of the mariana slab the observed depth of the metastable olivine inside the mariana slab can be explained by growth controlled olivine ringwoodite transformation under relatively dry condition such as the water content of about 150 wt ppm h2o on the other hand nucleation process controls the depth of the post spinel transformation in the mariana slab and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative clapeyron slope for the transformation when the overpressure for the nucleation is 0 5 gpa the observed depressions can be explained by the clapeyron slope of 0 7 mpa k grain size evolution and viscosity structures in the mariana slab are estimated based on these metastable phase transformations the slab weakening due to the grain size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post spinel transformation these viscosity structures are consistent with the behaviors of the mariana slab estimated from seismic tomography the slab vertically descends to the bottom of the transition zone and substantially deforms and thickens at the top of the lower mantle

using shear wave amplitude patterns to detect metastable olivine in subducted slabs 1 olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to phase this may produce a tapering low seismic velocity wedge within the slab in the transition zone transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic motivated by previous two dimensional numerical calculations for upgoing and downgoing seismic waves a three dimensional gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions five slab velocity structures are considered a thermal model with horizontal phase transitions at the same depths as in ambient mantle two models incorporating equilibrium transitions within the slab and two models with tongues of metastable olivine a depth and temperature dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to phase are included in the thermal modeling three dimensional dynamic ray tracing and gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models for sources near 400 km depth the presence of low velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models comparison with observed amplitudes for four events in the kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models this indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

low effective viscosity during high pressure metamorphism due to dissolution precipitation creep the record of hp lt metamorphic carbonates and siliciclastic rocks from crete the micro structural record of high pressure low temperature hp lt metamorphic rocks t 400 50 c p 10 2 kbar of the phyllite quartzite unit in western crete greece is interpreted in terms of deformation mechanisms and flow stress phyllites were deformed at low stress by dissolution precipitation creep governed by strongly enhanced dissolution along quartz mica 001 phase boundaries quartzites and quartz veins were deformed by dislocation creep at higher flow stress the contrasting effective viscosities caused stress concentration in the quartzites and quartz veins also microstructures from aragonite marbles indicate that dissolution precipitation creep was the dominant deformation mechanism and that dislocation creep has not been activated in these rocks pervasive ductile deformation was restricted to hp lt metamorphic conditions and the microstructural record of deformation at maximum depth has been preserved with all subsequent deformation localized and confined to the brittle field constraints on the timing of deformation allow an estimation of strain rates experimentally determined flow laws for dislocation creep are used to pose upper bounds on flow stress and bulk viscosities of rocks deformed by dissolution precipitation creep for the phyllites a conservative estimate is about 1019 pa s or below in contrast to 1020 pa s derived for the quartzites this compares well to the viscosity contrast of 1 2 orders of magnitude reflected by the mesoscopic structures since phyllites and carbonate rocks form large portions of the subducted sedimentary pile the low flow stress during rapid deformation of these rocks at hp lt metamorphic conditions and the lack of deformation along the burial and exhumation path imply very low strength of the plate boundary shear zones and negligible shear heating

kinetics of the olivine spinel transformation in subducting lithosphere experimental constraints and implications for deep slab processes abstract the persistence of metastable olivine to depths greater than 400 km in subducting slabs has implications for the generation of deep focus earthquakes the magnitude of buoyancy forces driving plate motion and the state of stress in the slab the depth to which metastable olivine can survive in a subduction zone and the depth interval over which transformation to or mg fe 2sio4 occurs have been evaluated from experimental kinetic data for the mg2geo4 and ni2sio4 transformations and the mg2sio4 transformation the data were extrapolated using a kinetic model for grain boundary nucleation and interface controlled growth under non isothermal and non isobaric conditions the results predict that metastable mg1 8fe0 2sio4 olivine survives to depths greater than 500 km in the cold interior of rapidly subducting slabs of old lithosphere the onset of transformation to mg fe 2sio4 spinel depends only on growth kinetics and coincides with the 550 50 c isotherm including the effects of latent heat production causes the transformation to occur by a runaway process over a very narrow depth interval at the onset of transformation high nucleation rates and low growth rates are consistent with the formation of very fine grained reaction products which are required for the transformational faulting mechanism of deep focus earthquakes when olivine crosses the equilibrium boundaries at higher temperatures e g higher than 700 c at 400 km depth transformation to or occurs much closer to equilibrium at a depth controlled by nucleation kinetics in this case the effect of latent heat production on the transformation kinetics is small and microstructural evolution is unlikely to result in transformational faulting below the depth of olivine breakdown cold slabs are likely to have a complex rheological structure owing to temperature dependent microstructural evolution during the phase transformation

hydration reduced lattice thermal conductivity of olivine in earth s upper mantle earth s water cycle enables the incorporation of water hydration in mantle minerals that can influence the physical properties of the mantle lattice thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs however the effect of hydration on lattice thermal conductivity remains poorly understood and has often been assumed to be negligible here we have precisely measured the lattice thermal conductivity of hydrous san carlos olivine mg0 9fe0 1 2sio4 fo90 up to 15 gigapascals using an ultrafast optical pump probe technique the thermal conductivity of hydrous fo90 with 7 000 wt ppm water is significantly suppressed at pressures above 5 gigapascals and is approximately 2 times smaller than the nominally anhydrous fo90 at mantle transition zone pressures demonstrating the critical influence of hydration on the lattice thermal conductivity of olivine in this region modeling the thermal structure of a subducting slab with our results shows that the hydration reduced thermal conductivity in hydrated oceanic crust further decreases the temperature at the cold dry center of the subducting slab therefore the olivine wadsleyite transformation rate in the slab with hydrated oceanic crust is much slower than that with dry oceanic crust after the slab sinks into the transition zone extending the metastable olivine to a greater depth the hydration reduced thermal conductivity could enable hydrous minerals to survive in deeper mantle and enhance water transportation to the transition zone

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

effects of slab mineralogy on subduction rates although velocities of subducting slabs should be controlled primarily by their negative thermal buoyan cies their mineralogy can also have signicant eects we explore this by using thermo kinetic modeling to predict mineralogy and compare the resultant buoyant driving force to the opposing viscous drag phase transitions of mg fe 2sio4 in subducting slabs depend on thermal struc ture in two ways first equilibrium phase boundaries should be deflected causing local buoyancy anomalies whose sign depends upon that of the clapeyron slope as slabs rst enter the transition zone negative anomalies should acceler ate them but positive anomalies that arise when they fully penetrate the transition zone should slow them such ef fects may induce geologically abrupt changes in plate mo tions second olivine that persists metastably in slabs will form regions of positive buoyancy which should reduce slab velocities the coldest and fastest slabs should be slowed more greatly thus narrowing the range of feasible subduc tion rates decreased descent rates however allow slabs to warm and metastable wedges to thermally erode such neg ative feedback mechanisms may serve to regulate subduction rates

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

metastable olivine wedge beneath the japan sea imaged by seismic interferometry the metastable olivine wedge mow within subducted slabs has long been hypothesized to host deep focus earthquakes 300 km its presence would also rule out hydrous slabs being subducted into the mantle transition zone however the existence and dimensions of mow remain debatable here we apply inter source interferometry which converts deep earthquakes into virtual seismometers to detect the seismic signature of mow without influence from shallow heterogeneities with data from the hi net we confirm the existence of mow beneath the japan sea and constrain its geometry to be 30 km thick at 410 km depth and gradually thinning to a depth of 580 km at least our result supports transformational faulting of metastable olivine as the initiation mechanism of deep earthquakes although large events m7 0 probably rupture beyond the wedge furthermore the slab core must be dehydrated at shallower depth and only transports negligible amount of water into the transition zone

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

seismic evidence for a metastable olivine wedge in the subducting pacific slab under japan sea abstract we apply a forward modeling approach to high quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting pacific slab beneath the japan sea our results show that a finger like anomaly exists within the subducting pacific slab below 400 km depth which has a p wave velocity 5 lower than the surrounding slab velocity or 3 lower than that of the normal mantle suggesting the existence of a metastable olivine wedge mow in the slab the mow top and bottom depths are 400 and 560 km respectively the mow is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary at 560 km depth the mow is located at about 25 km below the slab upper boundary most of the deep earthquakes are located in the mow our results favor transformational faulting as the mechanism for deep earthquakes

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

mid mantle anisotropy in subduction zones and deep water transport the earth s transition zone has until recently been assumed to be seismically isotropic increasingly however evidence suggests that ordering of material over seismic wavelengths occurs there but it is unclear what causes this we use the method of source side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe we find significant amounts of splitting 2 4 s confirming that the transition zone is anisotropic here however there is no decrease in the amount of splitting with depth as would be the case for a metastable tongue of olivine which thins with depth suggesting this is not the cause the amount of splitting does not appear to be consistent with processes in the ambient mantle such as lattice preferred orientation development in wadsleyite ringwoodite or mgsio3 perovskite we invert for the orientation of several mechanisms subject to uncertainties in mineralogy and deformation and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions we suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab implying significant water transport from the surface down to at least 660 km depth

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

a temperature profile of the mantle transition zone the temperature distribution across the transition zone has been determined on the high pressure phase equilibria in the system mg2sio4 fe2sio4 recently determined by katsura and ito 1988 and ito and takahashi 1988 the temperature at the base of the transition zone 655 km depth is estimated to be about 1600 c assuming that the spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves this estimation would be little modified for the chemically stratified mantle models since the lower mantle is characterized by the stability of perovskite the temperatures in the region from 350 km to 655 km depth are calculated for a peridotitic mantle model assuming an adiabatic temperature gradient the and transformations in mg fe 2sio4 make the temperature profile steep in the depth regions of 390 415 km and 500 530 km respectively the temperature would increase from about 1400 c to 1600 c over the depths interval 350 between 655 km

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

stress fields associated with metastable phase transitions in descending slabs and deep focus earthquakes inside subducting slabs the interaction of different metastable phase transformations olivine spinel spinel spinel spinel perovskite magnesiowustite associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates we have described the thermal field within the downgoing slab in a lagrangian framework by a self consistent high resolution two dimensional thermokinetic coupling model which also incorporates the effects of nucleation site saturation using an adaptive method the threshold for the nucleation site saturation is found to be less than 0 1 transformation degree in the cold slab interior this can significantly shift the metastable wedge of olivine deeper as suggested from previous thermokinetic slab models however this effect is compensated by the latent heat release during the olivine spinel and spinel spinel phase transitions latent heat release produce very sharp phase boundaries in the slab for fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior however the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field we have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up to date temperature field and physical properties determined from recent high p t experiments we find that the slab is denser up to 100 kg m 3 than the surrounding mantle remarkable denser portions up to 260 kg m 3 are located just below the uplifted and equilibrium phase transitions while lighter portions up to 210 kg m 3 can be found just above the depressed spinel to perovskite magnesiowustite transition and for the metastable wedge of olivine these density differences effectively act as a body force and produce a significant stress field which we calculated with a two dimensional finite element code the results show that the buoyancy induced forces produce maximum shear stress up to 23 mpa along the metastable wedge and deeper portion just above the depressed last phase transition involving spinel for the latter the dominant state of stress is down dip compression the calculated p t dependent state of stress is very similar to the depth distribution of deep focus

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

preparation and deformation of synthetic aggregates of quartz synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder impure silica gel and high purity silicic acid by hydrothermal isostatic pressing at 300 mpa the resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests the flow stress at 1200 1300 k was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity it is questionable that the natural quartz origin specimens were fully equilibrated in respect to the activity of the water but equilibration has probably been achieved in the amorphous silica origin specimens which crystallized under the experimental conditions in the deformation of the latter materials the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kj mol 1 for both however the impure gel origin material gave a stress exponent of about 2 3 whereas the high purity silicic acid origin material gave a stress exponent of about 4 in spite of the grain sizes being about 90 m and 20 m respectively it is concluded that while the impure specimens have higher intragranular strengths there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high purity specimens

fault zone weakening processes along the reactivated outer hebrides fault zone scotland the outer hebrides fault zone is a major reactivated structure cutting amphibolite grade lewisian basement gneisses in nw scotland during a regionally important phase of sinistral strike slip movements the influx of chemically active hydrous fluids along the fault zone was associated with the formation of a network of greenschist facies phyllonitic shear zones later ese directed extensional strain was preferentially focused into these pre existing zones of weakness the syn tectonic alteration of a relatively strong feldspar hornblende dominated load bearing framework microstructure to an interconnected weak layer microstructure of fine grained strongly aligned phyllosilicate aggregates leads to the long term weakening in the fault zone comparison with experimental data suggests that this produces a shallowing of the frictional viscous creep 9brittle ductile9 transition and a substantial reduction in total crustal strength similar processes may account for the apparent weakness of many long lived fault zones

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

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

elasticity and strength of hydrous ringwoodite at high pressure oh bearing hydrous ringwoodite compressed non hydrostatically in a diamond anvil cell supports a differential stress that increases from 2 9 to 4 5 gpa over the pressure range of 6 7 13 2 gpa at room temperature this result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart kaver and duffy geophys res lett 28 2001 2691 2694 the elastic anisotropy 2c44 c11 c12 of hydrous ringwoodite is measured to be 0 87 7 throughout this pressure range similar to measured values for anhydrous ringwoodite kaver and duffy geophys res lett 28 2001 2691 2694 this lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure these results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature resulting in stronger preferred lattice orientation this in turn may be seismically detectable which opens the possibility of using seismic anisotropy as a marker for local volatile containing areas within the upper mantle and transition zone

time dependent interaction between subduction dynamics and phase transition kinetics summary numerical subduction models are presented that account for the kinetics of the olivine spinel transformation and the equilibrium transformation of spinel into perovskite and magnesiowustite temperature depth and stress dependent rheology is used buoyancy due to temperature and the olivine spinel and magnesiowustite perovskite phases leads to free subduction into the transition zone and retarded penetration into the lower mantle a dynamic feed back mechanism is found between the different buoyancy forces the kinetics of the phase transition and the release of latent heat this mechanism modulates the parachute effect of the metastable olivine mo in a subducting slab and leads to characteristic temporal variations of the depth of mo and with a time delay variations of the subduction velocity time periods are of the order of 3 4 myr this mechanism allows to explain variations of the depths of the deepest earthquakes in otherwise similar subduction zones further conclusions are as follows the maximum depth of mo increases roughly linearly with lithospheric thickness and the maximum depth may reach as deep as 720 750 km different subduction zones may represent different stages of velocity depth phase loops of mo

pressure solution in siliciclastic hp lt metamorphic rocks constraints on the state of stress in deep levels of accretionary complexes abstract the magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured the only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high pressure low temperature metamorphic rocks recorded during their crustal evolution microfabrics in hp lt metamorphic t 400 50 c p 10 2 kbar phyllites and quartzites of the phyllite quartzite unit on the island of crete southern aegean reveal 1 that progressive deformation was by pressure solution dissolution precipitation creep 2 that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation 3 that the unilaterally rational 001 mica quartz phase boundaries were sites of strongly enhanced dissolution and 4 dislocation creep was restricted to a minor role in quartzites poor in mica consequently the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial to a depth of more than 30 km and subsequent exhumation currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 mpa at temperatures around 400 c at a depth of 30 km this implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep

experimental pressure solution deposition on quartz grains the crucial effect of the nature of the fluid abstract experimental deformation by pressure solution was performed on an aggregate of small grains subjected to deviatoric stress 50 mpa for a long time several weeks or months at relatively high temperature and pressure in contact with various fluids air water 0 1 to 1 n naoh for quartz water and 5 nh 4 cl for calcite the change in shape of the grains by solution deposition depended on the duration of the experiment with the same fluid and on the concentration of the solid in solution with the same duration but various fluids significant shape changes were obtained for quartz grains but only with both long duration and very good solvents 1 n naoh by comparison with previously obtained results on the change of shape of fluid inclusions where the kinetics of dissolution was the rate controlling process the limiting process of the deformation of the quartz grains was inferred to be the rate of diffusion along grain boundaries saturated with trapped fluid

detection of metastable olivine wedge in the western pacific slab and its geodynamic implications abstract seismic tomography and numerical simulations show that the western pacific slab bends horizontally when it reaches the boundary between the upper mantle and lower mantle beneath northeast asia it is expected that a metastable olivine wedge mow exists in the cold core of the slab because of a delayed phase transition from olivine to its high pressure polymorphs however it is still debated whether the mow actually exists or not and even if it exists its physical properties such as seismic velocity and density are still unclear in this work we use high quality arrival time data of 17 deep earthquakes occurring within the pacific slab under northeast asia to study the detailed structure of the slab the deep earthquakes are relocated precisely by applying a modified double difference location method to arrival time data recorded at both chinese and japanese stations based on the precise hypocentral locations a forward modeling method and differential travel time residuals data are used to estimate seismic velocity within the deep source zone which can decrease or remove the influence of ambient velocity heterogeneities our results show that the mow does exist within the pacific slab under northeast asia and the mow has a mean velocity anomaly of 7 9 lower than the iasp91 earth model the existence of mow in the slab has important geodynamic implications it can reduce the speed of slab subduction and affect the generation of deep earthquakes

shear wave splitting and mantle flow associated with the deflected pacific slab beneath northeast asia 1 a total of 361 sks and five local s wave splitting measurements obtained at global and regional seismic network stations in ne china and mongolia are used to infer the characteristics of mantle fabrics beneath northeast asia fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features the dominant fast directions at the eastern part beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone mtz are about 100 from north which are almost exactly the same as the motion direction of the eurasian plate relative to the pacific plate and are independent of the direction of local geological features the splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3 anisotropy the shear wave splitting observations complemented by the well established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the paleozoic era can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected pacific slab in the mtz or by back arc asthenospheric flow in the mantle wedge above the slab

interface kinetics grain scale deformation and polymorphism deviatoric stress generated within subducting slabs by the olivine spinel transformation has been modeled by assuming the phases to be simply connected rather than comprising a mixture in which one phase is embedded within another here we use a simplified model to explain how transformation strain is incorporated into a continuum model and we then use the simplified model to explain quantitatively the origin of the unreasonably large deviatoric stresses predicted by existing slab models we review experiments on the transformation of single crystal samples and argue that they are consistent with the occurrence at the grain scale of deviatoric stresses comparable with those predicted erroneously to exist at the slab scale by those slab models using a simple example we show that although large deviatoric stresses can exist at the grain scale their average over a sample containing many grains can be hydrostatic this leads us to the problem of modeling the microscale structure we outline the therm

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

on the mechanical properties of synthetic kaolinite quartz fault gouge summary the mechanical properties and microstructures of mixed kaolinite quartz fault gouge have been studied by means of triaxial tests wherein a 1 mm thickness of fault gouge was sheared between rigid sintered alumina sliders test conditions ranged up to 200 mpa confining pressure 175 mpa pore water pressure and temperatures to 600 c constant displacement rate tests were carried out at 5 10 4 mm s 1 and the stress relaxation technique was used to access a wide range of lower displacement rates down to 10 10 mm s 1 the effects of different ratios of clay quartz were studied and some experiments were cafried out using crushed tennessee sandstone gouge and pre faulted cylinders of tennessee sandstone deformation induced microstructures were studied by optical scanning and transmission electron microscopy and mineral chemistry changes were identified using eds analysis in the stem the most important factor determining the strength of these gouges is effective confining pressure and the effective stress law was obeyed at all test conditions increasing the proportion of quartz in the gouge slightly increases the strength as does increase of temperature under all test conditions the gouges deformed by strain hardening stable sliding at 2 mm displacement the friction coefficient is similar to that for rock and rock sliding 0 85 over wide ranges of strain rate and temperature the strength of these gouges is little affected if the effective pressure is high but strength drops rapidly with decreasing strain rate if effective pressure is low or if pore water pressure is high reloading of the gouge after stress relaxation at high temperatures demonstrated hardening arising from hydrothermal cementation during stress relaxation which led to sudden rupture of the cemented gouge this phenomenon may be responsible for cyclic rupture of natural faults new mineral phases were produced in these experiments only at 600 c at 400 c anticipated production of pyrophyllite from kaolinite plus quartz was not found but recrystallization of kaolinite is believed to have occurred and evidence of healing of cracks in quartz was seen microstructural studies revealed the ubiquitous development of p and r1 foliations and other features which are identical to those found in natural clay bearing fault zones microstructural evolution is associated with the strain hardening and apparent hardening is strongly associated with the stress path during loading

stresses along the metastable wedge of olivine in a subducting slab possible explanation for the tonga double seismic layer abstract a numerical calculation of the stresses associated with changes in volume during phase transitions of olivine in a descending slab results in a double layer of high shear stress along the metastable olivine wedge in the depth range 350 460 km stress in the upper layer is in plane tensional and stress in the lower layer is down dip compressional the modeled stress field agrees with observations of stress in the tonga double seismic zone high shear stress also exists in the slab at depths below the metastable wedge this stress distribution involves down dip compression and trench parallel tension which agrees with about half of the focal mechanisms in the tonga slab at depths of 460 690 km the model supports the idea that at least two possible stress release mechanisms for deep earthquakes may act in the tonga subducting slab one transformational faulting is restricted to the metastable wedge while the other one acts below the metastable wedge

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

deformation processes and weakening mechanisms within the frictional viscous transition zone of major crustal scale faults insights from the great glen fault zone scotland abstract the great glen fault zone ggfz scotland is a typical example of a crustal scale reactivated strike slip fault within the continental crust analysis of intensely strained fault rocks from the core of the ggfz near fort william provides a unique insight into the nature of deformation associated with the main phase of sinistral movements along the fault zone in this region an exhumed sequence of complex mid crustal deformation textures that developed in the region of the frictional viscous transition ca 8 15 km depth is preserved fault rock fabrics vary from mylonitic in quartzites to cataclastic in micaceous shear zones and feldspathic psammites protolith mineralogy exerted a strong control on the initial textural development and distribution of the fault rocks at lower strains crystal plastic deformation occurred in quartz dominated lithologies to produce mylonites simultaneously with widespread fracturing and cataclasis in feldspar and mica dominated rocks at higher strains shearing appears to increasingly localise into interconnected networks of cataclastic shear zones many of which are strongly foliated textures indicative of fluid assisted diffusive mass transfer mechanisms are widespread in such regions and suggest that a hydrous fluid assisted grainsize controlled switch in deformation behaviour followed the brittle comminution of grains the fault zone textural evolution implies that a strain induced fluid assisted shallowing and narrowing of the frictional viscous transition occurred with increasing strain it is proposed that this led to an overall weakening of the fault zone and that equivalent processes may occur along many other long lived crustal scale dislocations

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

nearshore waveheight during storms abstract this paper considers storm waves generated locally over deep water and travelling into shoaling water the offshore deepwater spectrum is assumed to be of the jonswap form which is approximated by the phillips spectrum with the same h mo this is carried ashore assuming that breaking operates to produce a constant spatial steepness the resulting formula for h mo as a function of depth is simple and is shown to agree well with values computed from the tma spectrum which is the finite depth equivalent of the jonswap spectrum the result can be used to predict 50 year maximum waveheight in finite depth or the actual nearshore waveheight in a storm using offshore predictions from a wave forecasting model the theory has been tested against data taken off holderness on the yorkshire coast in two depths the predictions of h mo are remarkably accurate and are apparently robust against the actual form of the spectrum not being quite as assumed the measured spectra in the storms approximate to the tma formula when they are under active local generation

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust