intermediate depth earthquakes facilitated by eclogitization related stresses eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10 15 one consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing pacific plate and reveal new evidence in favor of this mechanism tensional earthquakes lying 1 km above compressional earthquakes and earthquakes with highly similar waveforms lying on well defined planes with complementary rupture areas the tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate these observations provide seismic evidence for the role of volume change related stresses and possibly fluid related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere

meridional and downward propagation of atmospheric circulation anomalies part ii southern hemisphere cold season variability abstract as in the northern hemisphere there exists a simultaneous poleward propagation of temperature anomalies in the stratosphere and equatorward propagation in the troposphere in the southern hemisphere s cold season it takes about 110 days for anomalies of one polarity to propagate from the equator to the pole or half the period of the complete cycle nearly twice as long as in the northern hemisphere the earlier poleward propagation of temperature anomalies in upper levels compared with those in lower levels results in an apparent downward propagation in the stratosphere accompanying the poleward and downward propagating warm cold anomalies is a successive leveling steepening of isentropic surfaces reflecting a simultaneous reduction strengthening of the meridional temperature gradient and increase decrease of the vertical static stability following changes in the thermal fields are poleward and downward propagating zonal wind anomalies of the opposite sign the arrival of the pole

algebraic reconstruction of a two dimensional velocity inhomogeneity in the high hazles seam of thoresby colliery a high proportion of faces started in mechanized coal mines run into underground faults the faults take many forms from the splitting of a seam through a hidden stress pattern caused by subsidence or folding to a washout or a vertical throw all faults reduce face output a throw of only 1 5 m can lead to a face being abandoned faults of this order cannot be mapped reliably from the surface they may be mapped in an underground seismic survey roadways of a mine may give access to fast refracting horizons above and below a coal seam waveguiding in the plane of the seam if it occurs simplifies migration of wave trains reflected from discrete faults the reduction problem involved in a fault mapping is only two dimensional given guiding whether leaky or not it is possible to map distributed faults of low reflectivity by shooting in transmission across a panel of coal algebraic reconstruction techniques are used here to reduce first break times of flight through a 425 x 950 m rectangular block of coal into the profile of a velocity inhomogeneity input data are derived by static correction from hand picked arrival times the reduction itself is effected using an algorithm which accommodates underground site access restrictions in back projecting first break velocities a truncated cosine is used to weight the relative contributions of rays passing at different distances from any given mapping point the reconstructed velocity field suggests that the coal panel is bisected by a ridge of higher velocity the suspicion of a ridge is reinforced by results of an aberration test based on a standard huygens kirchhoff migration the ridge is found to follow the general line of a system of pillars left in place during the mining of a lower horizon it is concluded that channel waves may be used to map subsidence into old workings underground coal seams apparently share with other sedimentary rocks the property of a pressure sensitive seismic velocity

the roles of fluid transport and surface reaction in reaction induced fracturing with implications for the development of mesh textures in serpentinites the distinct element method was used to simulate chemical mechanical hydraulic processes that occur during serpentinization volume increasing hydration within the oceanic lithosphere the proposed model considers water transported in two ways advective flow along fractures and through matrices variations in fracture pattern and system evolution were examined using two nondimensional parameters the ratios of the rates of flow in fracture f and matrix m to the surface reaction rate in cases of fixed f and m with sufficiently low reaction rates the fracture pattern is not dependent on the surface reaction rate otherwise the fracture pattern varies systematically as a function of f and m at low f 1 and low m 1 the reaction proceeds from the boundaries inward and forms fine fractures layer by layer at high f 10 000 and low m 10 the reaction proceeds from the boundaries inward and forms polygonal fracture networks as m increases 100 the reaction tends to proceed homogeneously from the boundaries without fracturing a comparison of natural and simulated textures reveals that the following conditions are necessary to develop mesh textures during serpentinization in the oceanic lithosphere 1 the surface reaction rate must be similar to or higher than the fluid flow rate in the matrix or than the diffusive transport of water and much lower than the fluid flow rate along fractures 2 original olivine grain boundaries act as pathways for fluid transport these pathways may result from thermal or tectonic stress induced cracking prior to serpentinization

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

crosshole seismic tomography many tomographic interpretations of crosshole seismic traveltimes have approximated the raypaths with straight lines connecting the source and receiver this approximation is valid where the velocity does not vary greatly but in many regions of interest velocity variations of 10 20 percent or more are observed causing significant ray curvature other work has taken this nonlinear effect into account but there do not appear to be many cases of demonstrated success in its application to the crosshole seismic problem we present here an iterative inversion scheme based on two dimensional ray tracing and its successful application to field data the interpretation method iteratively ray traces and then updates the velocity model within each iteration the differences between the data and the current model traveltimes obtained by ray tracing are related to the unknown velocity perturbations through a system of linear equations a damped least squares method solves for the velocity perturbations which updat

softening trigerred by eclogitization the first step toward exhumation during continental subduction direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn exhumation deformation most ultrahigh pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces low viscosity in the subduction channel overburden removal by erosion and normal faulting a basic question is seldom considered why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface this event seminal to exhumation processes must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it our field observations in the bergen arc show that caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the caledonian subduction thus the sequence of deformation preserved in the bergen arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation this observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions i e underthrusting of the subducting slab in this context of simple shear metamorphic reactions assisted by fracturating fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab these tectonic units can then be incorporated into the channel circulation and start their upward travel

precursory signals of east asian winter cold anomalies in stratospheric planetary wave pattern the impacts of the stratospheric polar vortex changes on the tropospheric climate variability have been emphasized in previous studies the present study presents evidence for precursory signals of cold anomalies over east asia in the stratospheric wave pattern during november through march the leading two empirical orthogonal function eof modes of daily 50 hpa geopotential height anomalies along 65 n correspond to changes in the amplitude and phase of the stratospheric planetary wavenumber 1 respectively composite analysis is performed to unravel the temporal evolution of anomalies during 8 phases of 34 events identified based on the second eof mode with a complete life cycle for the period 1979 2016 the surface cold anomalies over eastern china emerge in phase 3 corresponding to negative and positive 50 hpa height anomalies on the north atlantic and russian far east sector respectively and peak in phase 5 corresponding to positive 50 hpa height anomalies over eurasia when the siberian high extends southeastward and the east asian trough deepens with anomalous surface northerly winds downward propagation of stratospheric signal is detected over western europe and western siberia the former is a source region of the tropospheric rossby wave trains propagating along the polar front jet and subtropical jet the downward propagation of stratospheric signal is related to the reflection of stratospheric planetary wavenumber 1 pattern and contributes to the development of the tropospheric rossby wave train that in turn leads to the strengthening of the siberian high and the deepening of the east asian trough

processes leading to eclogitization densification of subducted and tectonically buried crust 1 the transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling however a complex of partially eclogitized granulites on the island of holsnoy in the western part of the norwegian caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust the rocks on holsnoy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex metamorphism then occurred in a spatially heterogeneous manner with several distinct physical processes involving feedbacks between deformation fluid infiltration and chemical reactions operating simultaneously in different parts of the rock mass field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks a reservoir flux systems model is used to represent the various processes contributing to eclogitization of the granulites which operated at vastly different rates over disparate intervals of time the model suggests that the metamorphic process may have been brief 1 myr and ultimately self limiting arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite

backus averaging scattering and drift differences between traveltimes from sonic to seismic frequencies commonly known as drift can be attributed to a combination of multiple scattering and absorption the portion due to scattering can be estimated directly by calculating synthetic seismograms from sonic logs a simple alternative approach is suggested by the long wave equivalent averaging formulae for the effective elastic properties of a stack of thin layers which gives the same traveltime delays as the low frequency limit of the scattering dispersion we consider the application of these averaging formulae over a frequency dependent window with the hope of extending their use to frequencies higher than those allowed by the original validity conditions however comparison of the time delay due to window averaging with the scattering dispersion predicted by the o doherty anstey formula reveals that it is not possible to specify a form of window that will fit the dispersion across the spectrum for arbitrary log statistics a window with a width proportional to the wavelength squared matches the behaviour at the low frequency end of the dispersive range for most logs and allows an almost exact match of the drift across the entire spectrum for exponential correlation functions we examine a real log taken from a hole in nearly plane layered geology which displays strong quasi cyclical variations on one scale as well as more random smaller scale fluctuations the details of its drift behaviour are studied using simple models of the gross features the form of window which gave a good theoretical fit to the dispersion for an exponential log correlation function can only fit the computed drift at high or low frequencies confirming that there are at least two significant scale lengths of fluctuation a better overall fit is obtained for a window whose width is proportional to the wavelength the calculated scattering drift is significantly less than that observed from a vertical seismic profile but the difference cannot be wholly ascribed to absorption this is because the source frequency of the sonic tool is not appropriate for its resolution receiver spacing so that the scattering drift from sonic to seismic frequencies cannot be fully estimated from the layer model derived from the log

two types of antigorite serpentinite controlling heterogeneous slow slip behaviours of slab mantle interface abstract it is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip ets zones in order to examine the petrological controls on the large scale structure we compared recent geophysical observations in the shikoku area southwest japan with petrological models of the hanging wall mantle wedge as a result we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the shikoku area short term etss take place in the antigorite olivine stability field and silent long term slow slip events sses take place in the antigorite brucite stability field based on observations of natural antigorite serpentinites we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes the serpentinization reaction in the antigorite brucite stability field olivine h 2 o antigorite brucite proceeds at the expense of water this is consistent with moderately elevated pore pressures inferred for long term sses the existence of weak brucite enhances the development of shear zones oblique to the main foliation the resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes in contrast progress of the serpentinization reaction in the antigorite olivine stability field olivine h 2 o sio 2 antigorite results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short term sses our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3 d models for strain evolutions along plate boundaries

halogen contents of eclogite facies fluid inclusions and minerals caledonides western norway primary multiphase brine fluid inclusions in omphacite and garnet from low to medium temperature eclogites have been analysed for cl br i f li and so4 halogen contents and ratios provide information about trapped lower crustal fluids even though the major element na k ca contents of inclusion fluids have been modified by fluid mineral interactions and step daughter crystal formation after trapping halogens in the inclusion fluids were analysed with crush leach techniques cl br and cl i mass ratios of eclogite fluids are in the range 31 395 and 5000 33 000 respectively most fluids have a cl br ratio lower than modern seawater and a cl i ratio one order of magnitude lower than modern seawater fluids with the lowest cl br and highest cl i ratios come from an eclogite that formed by hydration of granulite facies rocks and may indicate that br and i are fractionated into hydrous minerals reconstructions indicate that the inclusion fluids originally contained 500 4000 ppm br 1 14 ppm i and 33 438 ppm li electron microprobe analyses of eclogite facies amphibole biotite phengite and apatite indicate that f and cl fractionate most strongly between phengite f cl mass ratio of 1469i1048 and fluid f cl mass ratio of 0 008 and the least between amphibole and fluid the chemical evolution of cl and br in pore fluids during hydration reactions is in many ways analogous to cl and br in seawater during evaporation the cl br ratio remains constant until the ah2o value is sufficiently lowered for cl to be removed from solution by incorporation into hydrous minerals

characterization of dispersion attenuation and anisotropy at the buena vista hills field california we create a log of intrinsic dispersion and attenuation for the antelope shale formation of the buena vista hills field san joaquin valley california high dispersion or low q values correlate with thin sand and carbonate beds within the antelope shale these beds are at least ten times as permeable as the host shale formation so this effect provides a possible avenue for seismic prediction of permeability the dispersion log is formed through comparison of crosswell seismic velocities measured at approximately 1 khz and sonic log velocities measured at approximately 10 khz in order to provide a proper basis for comparison the sonic log must first be adjusted for field anisotropy scaling effects and resolution of measurement we estimate a local shale anisotropy of about 20 based on correlations generated from published measurements of other shale fields we apply resolution enhancement to capture the thin sand and carbonate beds and windowed backus averaging to match the measurement scales a modeling study verifies the technique and shows that beds of thickness greater than 30 cm have a measurable signature the actual resolution is on the order of the crosswell fresnel length or about 7 m for the model study

geophysics hot fluids or rock in eclogite metamorphism reply arising from a camacho j k w lee b j hensen j braun nature 435 1191 1196 2005 camacho et al reply the mechanisms by which mafic rocks become converted to denser eclogite in the lower crust and mantle are fundamental to our understanding of subduction mountain building and the long term geochemical evolution of earth based on larger than expected gradients in argon isotopes camacho et al 1 propose a new explanation co seismic injection of hot 700 c aqueous fluids into much colder 400 c crust for the localized nature of eclogite metamorphism during caledonian crustal thickening as recorded in the rocks of holsnoy in the bergen arcs western norway we have studied these unusual rocks2 3 4 which were thoroughly dehydrated under granulite facies conditions during a neoproterozoic event about 945 million years 945 myr ago we also concluded that fracture hosted fluids were essential as catalysts and components in the conversion to eclogite about 425 myr ago5 however we are sceptical of the assertion by camacho et al that eclogite temperatures were reached only in the vicinity of fluid filled fractures determining whether these rocks were strong enough to fracture at depths of 50 km because they were cold or because they were very dry is crucial to understanding the mechanics of the lower crust in mountain belts including for example the causes of seismicity in the indian plate beneath the modern himalayas6

high resolution velocity and attenuation logs from long spaced sonic data a long spaced sonic survey may be thought of as a special case of ray theoretical tomographic imaging with such an approach estimates of borehole properties at a resolution of 6 inches 0 15 m have been obtained by inversion compared with a resolution of 2 ft 0 6 m from standard borehole compensated techniques bhc the inversion scheme employs the conjugate gradient technique which is fast and efficient unlike bhc the method compensates for variable refraction angles and provides estimates of errors in the measurements results from synthetic data show that these factors greatly improve the imaging of the properties of a finely layered medium though amplitude decay and coupling are less well defined than velocity and mud traveltime results from real data confirm the superior quality of logs from inversion furthermore they indicate that measured amplitudes can be dominated by errors that cause deterioration of bhc estimates of amplitude decay and coupling

intragranular replacement of chlorapatite by hydroxy fluor apatite during metasomatism abstract the pseudomorphic replacement of chlorapatite by hydroxy fluor apatite is associated with metasomatism of the odegarden metagabbro bamble south norway primary fluor chlorapatite in the protolith gabbro is non porous and homogenous in composition with f content of up to 3 5 wt and cl content up to 2 wt metasomatism transforms apatite during multi stage replacement reactions i magmatic apatite was transformed to chlorapatite with cl content of up to 6 8 wt during scapolitisation ii a secondary pseudomorphic replacement reaction which transforms chlorapatite to porous hydroxy fluor apatite with only minor cl is related to albitisation laser ablation inductively coupled mass spectrometry la icpms data from the primary fluor chlorapatite shows in general a high content of sr y ree and th chlor and hydroxy fluor apatite show depletion in co ni sr cd pb th and u and enrichment in v and ba relative to the primary apatite the replacement interface of chlorapatite to hydroxy fluor apatite is studied by energy filtered transmission electron microscopy tem the interface is sharp on a nano scale and the crystallographic orientation of the apatite is preserved these observations together with the porosity development in the hydroxy fluor apatite suggest that the replacement mechanism is by interface coupled dissolution reprecipitation

potential coordinate mislocations in crosshole tomography results from the grimsel test site switzerland tomographic techniques based on borehole to borehole and tunnel to borehole traveltime data are now being employed in a wide range of studies associated with the exploration and exploitation of hydrocarbons and metallic minerals the disposal of chemical and radioactive waste diverse civil engineering projects and archaeology a fundamental assumption of currently employed tomographic inversion strategies is that the coordinates of the boreholes and tunnels containing the seismic sources and receivers are accurately known by inverting both synthetic and observed traveltime data we demonstrate that relatively minor coordinate errors 1 2 in the deeper parts of long boreholes 100 m may produce artifacts in the tomographic images that are comparable in extent and amplitude to true velocity anomalies to address this problem we introduce the coupled inverse method commonly used in earthquake studies as a means to determine simultaneously borehole coordinate adjustments and an estimate of the tomog

meridional and vertical out of phase relationships of temperature anomalies associated with the northern annular mode variability 1 using the ncar ncep reanalysis we here present evidence suggesting that the out of phase relationships of temperature anomalies both between the low and high latitudes and between the stratosphere and troposphere are intimately related to the meridional and downward propagation of anomalies of both signs the temperature anomalies propagate poleward and downward above the tropopause and propagate equatorward below the tropopause the characteristic time scale for anomalies of one polarity to propagate from the equator to the pole or the half period of the complete cycle is about 55 days the relatively slow meridional propagation helps to explain the well known seesaw oscillatory pattern between low and high latitudes found in monthly data the equatorward propagation in the troposphere is synchronized with the poleward propagation of the stratospheric temperature anomalies of the opposite sign in both low and high latitudes responsible for the out of phase relation between the stratospheric and tropospheric temperature anomalies in the polar region since it takes about 55 days for anomalies of one polarity to propagate from the tropics to the pole such an intimate linkage between the anomalies in the deep tropics and high latitudes would imply a longer lead time for intra seasonal climate prediction in the extratropics

zircon and quartz inclusions in garnet used for complementary raman thermobarometry application to the holsnoy eclogite bergen arcs western norway mineral inclusions are common and have been widely used to investigate complex geological history when a rock undergoes cooling and decompression after the entrapment of an inclusion into a host mineral residual pressure may develop within the inclusion because of the differences in thermal expansivity and compressibility between the inclusion and host by combining laser raman spectroscopy and experimental data relating hydrostatic pressure and raman shift it is possible to estimate the entrapment pressure temperature p t conditions using an isotropic elastic model in this study we report raman spectroscopic data on both zircon and quartz inclusions in garnet host from the holsnoy eclogite bergen arcs norway averaged residual pressures based on different raman peaks for zircon and quartz inclusions are obtained to be ca 0 6 gpa and ca 0 65 gpa respectively using the equation of state for zircon and quartz the entrapment p t conditions are constrained to be 1 7 1 9 gpa 680 760 c consistent with previous estimates based on phase equilibria heating cooling experiments are performed on an entrapped zircon inclusion a clear trend is found between the residual zircon inclusion pressure and the externally controlled temperature we show that the residual zircon inclusion pressure sealed in garnet host is very sensitive to the entrapment temperature and can be used as a raman thermometer the effects of laser heating and the thermo elastic anisotropy of zircon inclusion are quantified and discussed

the high pressure stability of hydrous phases in orogenic belts an experimental approach on eclogite forming processes abstract metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases stability relations of hydrous phases depend on h 2 o activity availability during metamorphism however hydrous phases do occur in synthetic and natural systems also at water undersaturated conditions h 2 o undersaturated experiments in the cao al 2 o 3 sio 2 h 2 o model system were performed in order to predict the role of hydrous phases in h 2 o undersaturated compositions at constant pressure and temperature these experiments yielded different assemblages of hydrous anhydrous phases depending on the experimental bulk h 2 o content in natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole lawsonite zoisite mg chloritoid paragonite and talc complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases such continuous reactions can be used as powerful petrogenetic tools to derive p t conditions of natural eclogites derivation of h 2 o undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in h 2 o undersaturated bulk compositions at eclogite facies conditions comparison of experimentally determined phase relationships in mafic rocks with pressure temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward h 2 o saturation is necessarily maintained during prograde p t paths in orogenic belts as far as dehydration reactions occur any prograde p t path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized h 2 o preservation is achieved during metamorphism

a comparison of laboratory and field measurements of p wave anisotropy1 p wave velocity anisotropy determined from a cross hole survey at the imperial college borehole test site compares favorably with that measured in the laboratory on core from the holes the holes penetrate a layered sequence of sandstones shales and carbonates of the namurian upper limestone group the laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit p wave anisotropies of over 20 but that the sandstones and limestones are only slightly anisotropic these discrete measurements have been used in combination with wireline data to produce a log of p wave anisotropy including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross hole data set this implies that there is little frequency dependence of intrinsic p wave anisotropy inversion of the cross hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters the observed p wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks the intrinsic anisotropy is found to be the dominant factor

crosshole seismic tomography with cross firing geometry abstractwe have developed a case study of crosshole seismic tomography with a cross firing geometry in which seismic sources were placed in two vertical boreholes alternatingly and receiver arrays were placed in another vertical borehole there are two crosshole seismic data sets in a conventional sense these two data sets are used jointly in seismic tomography because the local sediment is dominated by periodic flat thin layers there is seismic anisotropy with different velocities in the vertical and horizontal directions the vertical transverse isotropy anisotropic effect is taken into account in inversion processing which consists of three stages in sequence first isotropic traveltime tomography is used for estimating the maximum horizontal velocity then anisotropic traveltime tomography is used to invert for the anisotropic parameter which is the normalized difference between the maximum horizontal velocity and the maximum vertical velocity finally anisotropic waveform tomography is impl

coexisting clinopyroxene spinel and amphibole spinel symplectites in metatroctolites from the buck creek ultramafic body north carolina blue ridge buck creek metatroctolites preserve evidence of three stages in the geologic history of the region a relict igneous stage a high pressure granulite facies metamorphic stage and a hydrous alteration stage during high pressure granulite facies metamorphism adjacent olivine and plagioclase reacted to produce complex coronitic textures relict olivine is surrounded by radial columnar orthopyroxene and plagioclase is rimmed by fine grained symplectite composed of separate grains of clinopyroxene spinel tschermakitic amphibole spinel symplectite and minor clinopyroxene sapphirine symplectite examination of textural relationships in back scattered electron images and x ray element maps of the symplectites demonstrates that amphibole spinel and clinopyroxene spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks clinopyroxene sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples a successful model for the main corona forming reaction in sample bc7 1 00 pl 1 72 ol 0 17 h 2 o 0 59 di 0 17 amp 0 68 spl 0 79 opx results only if amphibole is included the model predicts relative volumes of product clinopyroxene amphibole and spinel that are generally consistent with the proportions in imaged symplectites proportions of clinopyroxene and amphibole hosted symplectite appear to have been governed by local mass balance of na and al in plagioclase and or availability of h 2 o the most successful model for formation of the inner clinopyroxene sapphirine corona 1 00 pl 1 47 ol 0 17 h 2 o 0 59 di 0 17 amp 0 30 spr 10 ox 0 57 opx also includes amphibole which is present in association with clinopyroxene sapphirine symplectite sufficient h 2 o for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high p t conditions

geostatistical traveltime tomography in elliptically anisotropic media in geological materials anisotropy may arise due to different mechanisms and can be found at different scales neglecting anisotropy in traveltime tomographic reconstruction leads to artefacts that can obscure important subsurface features in this paper a geostatistical tomography algorithm to invert cross hole traveltime data in elliptically anisotropic media is presented the advantages of geostatistical tomography are that the solution is regularized by the covariance of the model parameters that known model parameters can be used as constraints and fitted exactly or within a prescribed variance and that stochastic simulations can be performed to appraise the variability of the solution space the benefits of the algorithm to image anisotropic media are illustrated by two examples using synthetic georadar data and real seismic data the first example confirms suspected electromagnetic anisotropy in the vadose zone caused by relatively rapid water content variations with respect to wavelength at georadar frequencies the second presents how sonic log data can be used to constrain the inversion of cross well seismic data and how geostatistical simulations can be used to infer parameter uncertainty results of both examples show that considering anisotropy yields a better fit to the data at high ray angles and reduces reconstruction artefacts

seismic detection limits of small deep man made reflectors a test at a geothermal site in northern germany a key question for the development of geothermal plants is the seismic detection and monitoring of fluid injections at several kilometers depth the detection and monitoring limits are controlled by several parameters for example the strength of seismic sources number of receivers vertical stacking and noise conditions for a known reference reflector at 2 66 km depth at a geothermal site in northern germany the results of a simple surface seismic experiment were therefore combined with numerical forward modeling for different injection scenarios at 3 8 km depth the underlying idea is that changes of reflectivity from the injection at 3 8 km must be larger than the variance of the measurements to be observable assuming that the injection at 3 8 km depth would produce a subhorizontal disklike target with a fracture porosity of 2 or 5 the critical porosity the water injection volume has to be at least 443 and 115 m 3 respectively to be detectable from the surface if the injection on the other hand does not create subhorizontal but subvertical pathways or only reduces the seismic velocities via the increased pore pressure in the immediate vicinity of the bore hole the injection is undetectable from the surface the most promising approach is therefore to move sources and or receivers closer to the target that is the use of borehole instrumentation

inhibited eclogite formation the key to the rapid growth of strong and buoyant archean continental crust subduction is the principal mechanism by which the hydrosphere and interior of earth interact today subduction involves the dehydration of ocean crust at depths of 60 120 km depending on the age of the slab release of the water leads to generation of arc magmas future continental crust and the slab is then transformed into denser eclogite that helps to pull more of the slab into the trench however it is unlikely that the first continental crust formed this way growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the archean when the planet was probably too hot for modern plate tectonics to operate a significant increase in the kinetics of eclogite forming reactions may have been the key to the transition from archean to modern tectonics under the higher geothermal gradients of the archean tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures because rapid eclogitization is dependent on water as a medium for advective ion transport the very shallow dehydration in the archean may have inhibited the formation of eclogite facies minerals the importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites in holsnoy western norway in which reaction progress was limited by the availability of water when water is scarce or absent metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle and it may have formed the substrate for the first continental lithosphere

full waveform processing and interpretation of kilohertz cross well seismic data high frequency cross well seismic data from the midale oil field of southeastern saskatchewan are analyzed for direct and reflected energy the goal of the analysis is to produce interpretable sections to assist in enhanced oil recovery activities co2 injection in this field direct arrivals are used for velocity information while reflected arrivals are processed into a reflection image raw field data show a complex assortment of wave types that includes direct compressional and shear waves and reflected shear waves a traveltime inversion technique layer stripping via ray tracing is used to obtain p and s wave interval velocities from the respective direct arrivals the velocities from the cross well inversion and the sonic log are in reasonable agreement the subsurface coverage of the cross well geometry is investigated it covers zones extending from the source well to the receiver well and includes regions above and below the source receiver depths upgoing and downgoing primary reflections

applications of algebraic reconstruction techniques to crosshole seismic data tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data the experiments were carried out at the retsoff salt mine in new york and at the underground radioactive waste study site at the stripa mine facility in sweden the traveltimes at restoff were high quality and were obtained over raypaths of up to 500 m in length the structure was quite complicated with velocity contrasts up to 50 percent the stripa site was in granitic rock with velocity contrasts of only a few percent the dimensions of the experiment were small with maximum ray lengths of just over 10 m the data at this site were collected with very high accuracy source and receiver locations were measured to better than 1 0 mm and traveltimes were read to 0 001 ms a number of algorithms similar to the algebraic reconstruction techniques art used in medical imaging have been applied to the data some modifications of the algorithms such as the application of weighting schemes damping parameters and curved raypaths were performed the resulting velocity fields were compared to the known fields and with each other to determine an optimal method the algorithms were found to be a rapid reliable means of reconstructing the slowness field of real data low velocity zones were recovered with accuracy in location and value it was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken otherwise poor reconstructions will result

reaction induced fracturing during replacement processes replacement processes are common transformation mechanisms in minerals and rocks at a variety of conditions and scales the underlying mechanisms are in general poorly understood but both mechanical and chemical processes are thought to be important replacement of leucite kalsi2o6 by analcime naalsi2o6 h2o is common in silica poor igneous rocks a 10 increase in volume is associated with the replacement process and this generates stresses that eventually cause fracturing of the reacting leucite experimentally reacted leucite samples display characteristic fracturing patterns that include both spalling of concentric onion skin like layers near the reacting interface and the formation of cross cutting often hierarchically arranged sets of fractures that divide the remaining leucite into progressively smaller domains these structures may explain the patchy alteration patterns observed in natural leucite samples and similar so called mesh textures associated with the serpentinization of olivine grains during hydration of mafic or ultramafic rocks they are also strikingly similar to larger scale patterns formed during spheroidal weathering processes a simple discrete element model illustrates the mechanics that control the formation of such systems and shows how these replacement processes may be accelerated due to the generation of new reactive surface area by hierarchical fracturing processes

reconciliation of crosshole seismic velocities with well information in a layered sedimentary environment in sedimentary environments horizontal fine layering can cause significant complications when analyzing and comparing seismic data with different frequencies and propagation directions at the whitchester test site three boreholes penetrate upper carboniferous cyclical sediments involving interbedded carbonates sandstones and mudstones with seismic velocities ranging from less than 3 0 km s in the mudstones to over 4 5 km s in the carbonates initial crosshole results from two boreholes approximately 200 m deep and 75 m apart showed a poor correlation with the borehole logs recorded in a third intermediate borehole furthermore the tomographic images were inconsistent with the expected geology the objective of this paper is to reconcile the crosshole seismic data with the borehole log information the borehole information must be upscaled to match the resolution of the crosshole experiment however upscaling techniques based on backus averaging lead to the prediction of significant anisotropy of the order of 20 in places with a vertical symmetry axis this anisotropy is a result of a combination of fine layering and the intrinsic mineral anisotropy measured on the core samples although it is the layer induced anisotropy that dominates at the crosshole frequencies of 400 hz this prediction was tested by including anisotropy parameters into the analysis of the crosshole data using anisotropic velocity tomography in the central part of the final anisotropic velocity tomograms where the ray coverage is adequate these crosshole results are consistent with the anisotropy predictions once the anisotropy was properly accounted for the tomographic images are consistent with the layered nature of the geology at the site and they show the location and throw of a known fault in the section we conclude that at this site and at crosshole frequencies seismic layer induced anisotropy plays a significant role and must be accounted for when processing and interpreting crosshole seismic data

on the footprint of anisotropy on isotropic full waveform inversion the valhall case study summary the validity of isotropic approximation to perform acoustic full waveform inversion fwi of real wide aperture anisotropic data can be questioned due to the intrinsic kinematic inconsis tency between short and large aperture components of the data this inconsistency is mainly related to the differences between the vertical and horizontal velocities in vertical transverse isotropic vti media the footprint of vti anisotropy on 2 d acoustic isotropic fwi is illus trated on a hydrophone data set of an ocean bottom cable that was collected over the valhall field in the north sea multiscale fwi is implemented in the frequency domain by hierar chical inversions of increasing frequencies and decreasing aperture angles the fwi models are appraised by local comparison with well information seismic modelling reverse time migration rtm and source wavelet estimation a smooth initial vti model parameterized by the vertical velocity v0 and the thomsen parameters andwere previously developed by anisotropic reflection traveltime tomography the normal moveout vnmo v0 1 2 and horizontal vh v0 1 2 velocity models were inferred from the anisotropic models to perform isotropic fwi the vnmo models allows for an accurate match of short spread reflection traveltimes whereas the vh model after updating by first arrival traveltime tomog raphy fatt allows for an accurate match of first arrival traveltimes ray tracing in the velocity models shows that the first 1 5 km of the medium are sampled by both diving waves and reflections whereas the deeper structure at the reservoir level is mainly controlled by short spread reflections starting from the initial anisotropic model and keeping fixed and models anisotropic fwi allows us to build a vertical velocity model that matches reasonably well the well log velocities isotropic fwi is performed using either the nmo model or the fatt model as initial model in both cases horizontal velocities are mainly reconstructed in the first 1 5 km of the medium this suggests that the wide aperture components of the data have a dominant control on the velocity estimation at these depths these high velocities in the upper structure lead to low values of velocity in the underlying gas layers either equal or lower than vertical velocities of the well log and or a vertical stretching of the structure at the reservoir level below the gas this bias in the gas velocities and the mispositioning in depth of the deep reflectors also shown in the rtm images are required to match the deep reflections in the isotropic approximation and highlight the footprint of anisotropy in the isotropic fwi of long offset data despite the significant differences between the anisotropic and isotropic fwi models each of these models produce a nearly equivalent match of the data which highlights the ill posedness of acoustic anisotropic fwi hence we conclude with the importance of considering anisotropy in fwi of wide aperture data to avoid bias in the velocity reconstruc tions and mispositioning in depth of reflectors designing a suitable parameterization of the vti acoustic fwi is a central issue to manage the ill posedness of the fwi

controls on rock weathering rates by reaction induced hierarchical fracturing abstract weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface the hydrosphere and the biosphere it is still poorly understood how this coupling affects weathering rates and patterns produced by weathering here we show that the observed spheroidal weathering patterns in basaltic intrusions dolerites in the karoo basin in south africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division the fracture processes generates new reactive surface area in a self accelerating manner thus providing a first order control on the total weathering rate

evolution of microstructure and elastic wave velocities in dehydrated gypsum samples 1 we report on changes in p and s wave velocities and rock microstructure induced by devolatilization reactions using gypsum as a reference analog material cylindrical samples of natural alabaster were dehydrated in air at ambient pressure and temperatures between 378 and 423 k dehydration did not proceed homogeneously but via a reaction front moving sample inwards separating an outer highly porous rim from the remaining gypsum which above approximately 393 5 k concurrently decomposed into hemihydrate overall porosity was observed to continuously increase with reaction progress from approximately 2 for fully hydrated samples to 30 for completely dehydrated ones concurrently p and s wave velocities linearly decreased with porosity from 5 2 and 2 7 km s to 1 0 and 0 7 km s respectively it is concluded that a linearized empirical raymer type model extended by a critical porosity term and based on the respective time dependent mineral and pore volumes reasonably replicates the p and s wave data in relation to reaction progress and porosity

propagation of the arctic oscillation from the stratosphere to the troposphere geopotential anomalies ranging from the earth s surface to the middle stratosphere in the northern hemisphere are dominated by a mode of variability known as the arctic oscillation ao the ao is represented herein by the leading mode the first empirical orthogonal function of low frequency variability of wintertime geopotential between 1000 and 10 hpa in the middle stratosphere the signature of the ao is a nearly zonally symmetric pattern representing a strong or weak polar vortex at 1000 hpa the ao is similar to the north atlantic oscillation but with more zonal symmetry especially at high latitudes in zonal mean zonal wind the ao is seen as a north south dipole centered on 40 45 n in zonal mean temperature it is seen as a deep warm or cold polar anomaly from the upper troposphere to 10 hpa the association of the ao pattern in the troposphere with modulation of the strength of the stratospheric polar vortex provides perhaps the best measure of coupling between the stratosphere and the troposphere by examining separately time series of ao signatures at tropospheric and stratospheric levels it is shown that ao anomalies typically appear first in the stratosphere and propagate downward the midwinter correlation between the 90 day low pass filtered 10 hpa anomaly and the 1000 hpa anomaly exceeds 0 65 when the surface anomaly time series is lagged by about three weeks the tropospheric signature of the ao anomaly is characterized by substantial changes to the storm tracks and strength of the midtropospheric flow especially over the north atlantic and europe the implications of large stratospheric anomalies as precursors to changes in tropospheric weather patterns are discussed

oxygen bulk diffusion measurements and tem characterization of a natural ultramylonite implications for fluid transport in mica bearing rocks oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample c 5 m grain size 15 20 biotite 20 quartz 60 65 feldspars and minor fe oxides from the gerrish island shear zone se maine usa the diffusion experiments were performed at 250 550 c and 100 mpa water pressure oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample the arrhenius parameters for transport parallel to the foliation are dbulk0 2 0 10 11 m2 s 1 and q 30 6 kj mol 1 the bulk diffusivity perpendicular to the foliation is about a factor of 3 5 less than that parallel to the foliation with the same activation energy the values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions the microstructure of the ultramylonite was characterized using transmission electron microscopy tem the nature and distribution of the interconnected fluid cannot be completely resolved from the tem analysis however the low percentage of three grain channels and open grain interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces the results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates strong lithological control and pervasive nature of fluid transport in mica bearing rocks

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

2 d ray based tomography for velocity layer shape and attenuation from gpr data tomographic inversion for near surface structure from multioffset ground penetrating radar gpr reflection data is implemented the model is parameterized as layers with constant velocity and constant attenuation the solution is divided into two parts first the velocity and shape of each layer are estimated from the observed reflection traveltimes then using this geometry and the corresponding ray paths the attenuation of each layer is estimated from the observed relative amplitudes solution of both tomographic problems is performed by singular value decomposition of the sensitivity matrix to compute information density resolution and covariance matrices as well as the solution tests on synthetic data for which the solution is known show that the algorithm converges efficiently to the neighborhood of the correct solution inversion of multichannel field data in a gpr line from a fluvial eolian environment produces a model that is consistent with structural images and velocity estimates obtai

reaction enhanced permeability during retrogressive metamorphism abstract retrogressive metamorphism is in most cases associated with influx of external fluids into igneous or metamorphic rocks with low to negligible initial permeabilities thus fluid migration is associated with some process that creates the necessary channels for migration one such mechanism is associated with the volume changes resulting from volatilization reactions petrographic studies of a partly serpentinized troctolite from the duluth complex in minnesota reveal how the increase in solid volume associated with hydration of olivine phenocrystals generates an extensive network of microfractures in the surrounding plagioclase dominated matrix this microfracture network allows hydrous fluids to penetrate further into the rock a 2 d spring network model illustrates how expanding polygons or non spherical grains in an elastically deforming matrix cause dilatant microfracturing and porosity generation even if the overall stresses are compressive the model emphasises how fluid infiltration may be promoted by stress heterogeneities set up by metamorphic reactions

understanding the variation of stratosphere troposphere coupling during stratospheric northern annular mode events from a mass circulation perspective we revisit the various stratosphere troposphere coupling relation from the perspective of the meridional mass circulation we constructed 10 hpa northern annular mode nam phase composites to show the typical spatiotemporal evolution of circulation anomalies during the nam s life cycle our results indicate that there is large case to case difference in the temporal evolution and vertical profile of polar temperature anomalies during nam events which shows no strong dependence on the intensity and duration of nam events but agrees well with the variations of the three branches of mass circulation at 60 n the stratospheric poleward warm air branch st the poleward warm air branch in the upper troposphere wb and the equatorward cold air branch in the lower troposphere cb such correspondence is due to the dynamic heating and cooling anomalies associated with the redistribution of air masses by the anomalous meridional mass circulation in different isentropic layers the various relationship among the three mass circulation branches is attributed to anomalous wave activities the amplitude and westward tilt of waves are always stronger weaker throughout the stratosphere before after the peak time of negative nam events leading to a stronger weaker st before after the peak time variations in wb and cb are mostly dependent on wave variabilities in the mid to lower troposphere leading to variations in the timing of in or out of phase coupling of the st with the wb and cb and thus various thermostructure during nam events at a later stage of the negative nam events when the polar temperature becomes colder and the polar jet recovers the weakened baroclinic instability in the lower stratosphere provides favorable conditions for the strengthening of the wb and cb if wave activities strengthen in the troposphere during that period

the significance of grain scale stresses in the kinetics of metamorphism during metamorphism phases grow and dissolve dominantly along grain boundaries bulk diffusion models for the development of metamorphic textures often assume that diffusion is homogeneous in regions of each phase rather than being localised along phase boundaries under isotropic stress chemical potentials of components involved in reactions are likely to be buffered along grain boundaries between phases thus inhibiting diffusion and growth motive forces must exist which overcome this and it is proposed here that local heterogeneous stresses will be important a mathematical theory for stress controlled growth is derived this theory predicts that the growth rates of two phases along their mutual interface will be linked in a quantifiable way it forms a basis for considering the important effects of stresses induced by local volumetric changes during metamorphism and it provides a link between these effects and those related to pressure solution in deforming rocks

regional scale development of opening mode calcite veins due to silica diagenesis the formation and distribution of natural fractures in cretaceous paleogene strata in jordan are strongly tied to diagenetic processes which in turn reflect the lithology of the host material observations collected from subsurface cores show that widespread fracturing began before compaction of the host sediment was complete based on ptygmatic folding of one set of mineral filled fractures veins non folded veins are preferentially developed within heavily cemented layers calcium carbonate is the greatest volumetric component of the host sediment and most fractures are at least partially filled by calcite dolomite and silica bearing fractures are present in dolomitized and silicified host beds respectively horizontal veins are filled by cone in cone calcite or rarely silica or dolomite the stratigraphic arrangement and degree of compaction around ptygmatically folded calcite veins and chert nodules suggest that silica diagenesis was an important driver of early fractures nevertheless those fractures were filled with carbonate cements as they opened based on crack seal texture of the vein fill the volume loss associated with silica diagenesis created fracture porosity which was filled coevally by carbonate cements the distribution of later veins reflects embrittlement of host layers by cementation and is consistent with crustal deformation as the primary fracture driver

reaction assisted hierarchical fracturing during serpentinization abstract hierarchical fracture patterns are the result of a fracturing process that successively divides rocks into smaller domains in a 2d outcrop section such fracture patterns are characterized by four sided domains and t junctions where new fractures stop growing at right angles to pre existing fractures here we describe fracturing of mm to dm thick serpentinized orthopyroxenite dykes in a dunite matrix from the leka ophiolite complex in nord trondelag norway the fracturing process is driven by volume changes associated with the serpentinization of the dunites during this process the orthopyroxenite dykes are extended by the expanding dunite and undergo considerable fracturing the resulting fracture patterns are hierarchical the cumulative distributions of fracture lengths show a scaling behavior that lies between a log normal and power law fractal distribution this is consistent with a simple fragmentation model in which domains are divided according to a top hat distribution of new fracture positions within unfractured domains reaction assisted hierarchical fracturing is also likely to be responsible for other structures commonly observed in serpentinized ultramafic rocks including the mesh textures observed in individual olivine grains and the high abundance of rectangular domains at a wide range of scales this mechanism may provide first order controls on fluid migration rates in a wide variety of geological settings

a field study of seismic attenuation in layered sedimentary rocks ii crosshole data summary a series of crosshole seismic experiments have been performed at the imperial college borehole test site in northumberland england using a detonator source and hydrophone receivers the holes have been drilled through a horizontally layered sequence of carboniferous limestones shales and sandstones estimates of intrinsic q were obtained for frequencies between approximately 500 hz and 1 5 khz a comparison with the results of part i suggests evidence of frequency dependent intrinsic q with attenuation at 1 khz being significantly greater than at 100 hz no change of intrinsic attenuation with ray direction from horizontal oo to 60 was observed however since no high frequency crosshole data for ray angles between 60 and vertical is available we cannot entirely discount the possibility that the difference in intrinsic q estimates from the vertical vsp data part i and the crosshole data is due at least in part to anisotropic intrinsic attenuation

reaction induced fracturing in a hot pressed calcite periclase aggregate abstract the chemo mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally dense calcite periclase aggregates with

impact of fluid pressure on failure mode in shear zones numerical simulation of en echelon tensile fracturing and transition to shear abstract en echelon veins are a set of tensile fractures that develop during the initiation of shear zones under certain conditions but factors that control fracture patterns remain unclear in this study we performed numerical simulations using the two dimensional distinct element method to understand the influence of effective normal stress on the development of fractures during shear deformation at low effective normal stress 10 mpa dilation takes place at an angle of 40 45 to the shear boundary and en echelon arrays of tensile fractures develop in contrast at high effective normal stress 150 mpa en echelon veins are not developed but a distinct shear plane forms a transient behavior characterizes intermediate effective normal stress 50 mpa early tensile cracks are generated and then propagate to form larger brittle faults in this case the larger strain energy released in the later stage is due to the greater shear strength under crustal conditions effective normal stress varies due to the change in fluid pressure our results suggest that en echelon veins are indicators of high fluid pressure close to lithostatic conditions i e effective normal stress close to zero and that a transition of the failure mode from tensile to shear could occur in response to a dynamic change of fluid pressure during fracturing

constraints on element mobility associated with the conversion of granulite to eclogite along fractures in an anorthositic complex on holsnoy norway on holsnoy an island off the coast of western norway an anorthositic complex metamorphosed to granulite facies was partially overprinted by a later eclogite facies metamorphism eclogite facies rocks containing omphacite garnet kyanite and hydrous phases such as mica and zoisite occur in shear zones of various scales and adjacent to veins previous studies of shear zones on holsnoy reported evidence for substantial element mobility jamtveit et al 1990 mattey et al 1994 in this work we compare chemical compositions of granulite and its undeformed eclogitized equivalent adjacent to veins in locations where a single band of granulite can be traced and sampled as it approaches the vein this tracing is crucial because the pre granulite rocks cover a substantial compositional range indicative of a petrologically variable protolith consisting of anorthosite gabbro and jotunite we analysed multiple core samples collected across nine separate granulite eclogite transition zones located at veins in anorthositic jotunitic and gabbroic protoliths for major and trace elements for each transition no compositional difference between the average granulite and average eclogite composition was found at the 90 confidence level except for loi loss on ignition which was consistently significantly higher in the eclogite samples although not significant at the 90 confidence level for any single traverse the average eclogite concentrations of sio2 na2o cs as and br exceed the average granulite concentrations for eight or all nine of the traverses for most traverses statistical analysis of the data limits any gain of sio2 in the eclogites to no more than a few relative per cent other than the introduction of volatile substances presumably an h2o rich fluid eclogitization associated with vein formation was essentially isochemical

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

interslab earthquakes and phase changes in subducting lithosphere at 0033 ut on 9 june 1994 a great deep earthquake occurred 635 km beneath the amazonian rain forest of northern bolivia the deep rupture of this record setting magnitude 8 3 shock lasted about a minute harvard university centroid moment tensor catalogue the seismic strain release of this single event as indicated by its seismic moment was greater than the total strain release for all deep earthquakes in the harvard cmt catalogue during the previous 17 years of its coverage of global seismicity incredibly ground motion was felt as far north as seattle washington and toronto canada evidently it occurred within the subducting nazca plate that is descending beneath south america this earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and in particular how such earthquakes may reflect the inner workings of slabs

seismic waveform inversion in the frequency domain part 2 fault delineation in sediments using crosshole data a crosshole experiment was carried out in a layered sedimentary environment in which a normal fault is known to cut through the section initial traveltime inversions produced stable but low resolution images from which the fault could be only vaguely inferred to image the fault wavefield inversion was used to produce a velocity model consistent with the detailed phase and amplitude of the data at a number of frequencies our wavefield inversion scheme uses a classical descent type algorithm for decreasing the data misfit by iteratively computing the gradient of this misfit by repeated forward and backward propagations our propagator is a full wave equation frequency domain acoustic finite difference method the use of the frequency space domain yields computational advantages for multisource data and allows an easy incorporation of viscous effects by running wavefield inversion on the field data a quantitative velocity image was produced that yielded a significantly improved image of the fault when compared with the original traveltime inversions because the original field data were noisy and contained a high degree of multiple scattering from the layering of the sediments the transmitted arrivals were selectively windowed to enhance the image the sediments at the site were strongly attenuating we therefore used a viscoacoustic model during the modeling and the inversion that correctly simulated the observed decrease in amplitude with increasing frequency and source receiver offset furthermore since the traveltime inversion indicated a high degree of anisotropy at the site a fixed homogeneous level of anisotropy was used during the inversion tests at varying levels of anisotropy confirmed the improvement in image quality and in data fit when anisotropy was incorporated the final image was verified by examining the distribution of the residuals in the frequency domain by comparing time domain modeled wavefields with the observed data and by direct comparison with borehole logs

high resolution cross well reflection imaging potential and technical difficulties the case study presented in this paper demonstrates that reflections can be found in high frequency cross well data these reflections can be enhanced by a processing sequence similar to the one applied to vsp data to produce very high resolution images vertical resolution of the order of 3 5 ft 1 1 5 m for well distances of a few hundred feet is potentially feasible although this study produced a good tie between the cross well reflection image and the sonic log our research also illustrated some of the technical difficulties associated with cross well reflection imaging specifically we found that the signal to noise ratio of the image decreases as the distance between the imaged reflection points and the wells increases and that aliased tube waves as well as shear and converted waves were the main components of the noise we also observed that interpretation of wide angle reflections might be misleading since these events are significantly stretched both horizontally and vertically during ima

combining wave equation imaging with traveltime tomography to form high resolution images from crosshole data traveltime tomography is an appropriate method for estimating seismic velocity structure from arrival times however tomography fails to resolve discontinuities in the velocities wave equation techniques provide images using the full wave field that complement the results of traveltime tomography we use the velocity estimates from tomography as a reference model for a numerical propagation of the time reversed data these backpropagated wave fields are used to provide images of the discontinuities in the velocity field the combined use of traveltime tomography and wave equation imaging is particularly suitable for forming high resolution geologic images from multiple source multiple receiver data acquired in borehole to borehole seismic surveying in the context of crosshole imaging an effective implementation of wave equation imaging is obtained by transforming the data and the algorithms into the frequency domain this transformation allows the use of efficient frequency domain numerical propagat

apatite and monazite an effective duo to unravel superimposed fluid flow and deformation events in reactivated shear zones abstract mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid driven metasomatic retrogression and reactivation however the relationships between fluid rock interaction retrogression deformation and mylonitisation and the timing thereof are often cryptically recorded this study focuses on the granulite facies boothby orthogneiss from the reynolds range central australia which contains a large scale mylonitic shear zone with an apparent record of structural inheritance fluid infiltration and reactivation the chosen site provides an ideal natural laboratory in which to investigate the timing of deformation associated fluid flow and mass transport u pb isotope analyses of monazite indicate an average pb recrystallization age of c 1560 ma demonstrating that the orthogneiss fabric developed during the mesoproterozoic chewings orogeny 1590 1550 ma structural mapping suggests that this shear zone represents a riedel branch of larger structures that were subsequently reactivated during the paleozoic alice springs orogeny 450 300 ma the timing of reactivation and fluid flow is constrained by u pb dating of apatite which is present as a stable u bearing mineral in both orthogneiss and mylonite modelling of apatite radiogenic pb retention ages accounting for a wide potential range in common pb compositions demonstrates at least some growth and or recrystallization at c 1500 ma and c 400 ma confirming apatite precipitation during alice springs shearing and the reactivation of chewings age structures in addition alice springs aged apatite is found along pre existing fabrics in the orthogneiss in the vicinity of the shear zone indicating pre kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected the combined datasets demonstrates that integrated apatite and monazite u pb geochronology is an effective method to unravel the record of superimposed fluid flow and deformation events this includes the detection of an inverse younging relationship where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone such effects are potentially common where deformation is driven by pre kinematic fluid rock interaction with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks

two dimensional velocity models from wide angle seismic data by wavefield inversion summary modern wide angle surveys are often multi fold and multi channel with densely sampled source and receiver spacings such closely spaced data are potentially amenable to multi channel techniques involving wavefield propagation methods such as those commonly used in reflection data processing however the wide angle configuration requires techniques capable of handling very general wave types including those not commonly used in reflection seismology this is a situation analogous to that faced in cross borehole seismics where similar wave types are also recorded in a real cross borehole example we compare pre stack migration traveltime tomography and wavefield inversion we find that wavefield inversion produces images that are quantitative in velocity as are the tomograms but are of significantly higher resolution the wavefield inversion results have a resolution comparable to that of the qualitative pre stack migration images we seek to extend this novel development to the larger scale problem of crustal imaging an essential element of the approach we adopt is its formulation entirely within the temporal frequency domain this has three principal advantages 1 we can choose to decimate the data by selecting only a limited number of frequency components to invert thus making inversion of data from large numbers of source positions feasible 2 we can mitigate the notorious non linearity of the seismic inverse problem by progressing from low frequency components in the data to high frequency components and 3 we can include in the model any arbitrary frequency dependence of inelastic attenuation factors q and indeed solve for the spatial distribution of q an initial synthetic test with an anomaly located within the middle crust yields a velocity image with the correct structural features of the anomaly and the correct magnitude of velocity anomaly this is related to the fact that the reconstruction is obtained from forward scattered waves under these conditions the method thus behaves much like tomography a second test with a deeper more extensive anomaly yields an image with the correct velocity polarity and the correct location but with a deficiency in low and high wavenumbers in this case this is because the reconstruction is obtained from backscattered waves under these conditions the method behaves not like tomography but like migration a more extensive test based on a large wide angle survey in south eastern california and western arizona demonstrates a real potential for high resolution imaging of crustal structures although our results are limited by the acoustic approximation and by the relatively low frequencies that we can model today the images are sufficiently encouraging to warrant future research the problem of local minima in the objective function is the most significant practical problem with our method but we propose that appropriate layer stripping methods can handle this problem

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

a comparison of cross hole electrical and seismic data in fractured rock cross hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available a strong correlation between electrical resistivity anisotropy and seismic compressional wave velocity anisotropy has been observed analysis of core samples from the site reveal that the shale rich rocks have fabric related average velocity anisotropy of between 10 and 30 the cross hole seismic data are consistent with these values indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures one region with velocity anisotropy greater than 30 has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross hole resistivity tomography data however in general the study highlights the uncertainties that can arise due to the relative influence of rock fabric and fluid filled fractures when using geophysical techniques for hydrological investigations

contrasting fracture patterns induced by volume increasing and decreasing reactions implications for the progress of metamorphic reactions abstract hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock and some of these reactions proceed with fracturing for this paper we have developed a model for coupled hydraulic chemical mechanical processes using the distinct element method in order to understand the relationships among chemical reactions fluid flow and fracturing during metamorphism the model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure with the help of the model volume decreasing dehydration reactions and volume increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions respectively both types of reaction proceed inwards from drained boundaries or fractures but they show contrasting fracture patterns a volume decreasing dehydration reaction produces tree type fractures with the new fractures generated as branches of a pre existing fracture a volume increasing hydration reaction produces a polygonal network of fractures where new fractures nucleate at sites far from the pre existing fracture and extend to form t junctions these contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient in the case of the tree type fractures the fractures are continuously generated at the reaction front and the reaction proceeds smoothly by positive feedbacks between reaction fracturing and fluid flow in contrast in a volume increasing hydration reaction fracturing initially occurs in response to the irregular boundary shape but as the reaction progresses a compressive stress field is generated which inhibits further fracture generation the compressive stress field also prevents fluid flow by closing the pre existing fractures which slows down the reaction these contrasting feedback systems between volume decreasing dehydration and volume increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved

characteristics of dewatering induced drawdown curve under blocking effect of retaining wall in aquifer summary for deep excavation pits that require the pumping of confined groundwater a combination of a retaining wall and dewatering with large diameter wells is usually adopted during excavation to improve safety since a retaining wall has a much lower hydraulic conductivity than the surrounding material in the aquifer blocking of seepage to prolong the seepage path of the groundwater outside of the pit is effective the retaining walls used during excavation dewatering cause hydraulic head drawdown inside the pit much faster than outside the pit thus difference in hydraulic head between inside and outside of the pit increases to investigate the mechanism of the blocking effect numerical simulation using the finite difference method fdm was conducted to analyze the effects of pumping in the pit the fdm results show that drawdown varies along the depth of the confined aquifer the influence factors of drawdown inside and outside the pit include insertion depth of retaining walls anisotropy of a confined aquifer and screen length of pumping wells in addition fdm results also show that the drawdown time curve can be divided into four stages in stage i drawdown inside the pit is very small and outside the pit it is almost zero in stage ii drawdown increases quickly with time in stage iii the drawdown curve is parallel to the cooper jacob curve on semi log axes and in stage iv the drawdown becomes constant these characteristics of the drawdown curve under the blocking effect of a retaining wall in an aquifer provide a way of estimating hydrogeological parameters according to pumping test results

transport of low ah2o dehydration products to melt sites via reaction zone networks milford sound new zealand the gabbroic dioritic pembroke hornblende granulite phg of milford sound displays a geometrically simple mesoscopic network of sub planar garnet reaction zones grz in which the meta igneous hornblende granulite has been depleted of na si and h2o and c 25 vol almandine rich garnet has formed some studies postulate the initial presence of melt along the centres of all grz explaining the frequent absence of feldspathic veins by selective melt loss a more parsimonious model is necessitated by structural evidence and together with chemical data suggests a relationship between mid range metasomatic transport and anatexis the pembroke outcrops show a process of incipient melting of gabbro diorite in an environment of relatively low ah2o in lithologies that have limited free quartz a non equilibrium steady state is proposed in which a sodic dehydration fluid moves some distance via the grz network towards areas of partial melting only in these areas are na and si reconstituted as albite with more garnet as byproduct having avoided the need for melt percolation the combined structural and chemical evidence directs a focus on mass transport in low ah2o gabbroic environments in subsequent events of shearing and complete transposition both sets of garnet the atypical grz residue and partial melt melanosomes were inherited by the milford gneiss facies of the phg

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

short lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids collision tectonics and the associated transformation of continental crust to high pressure rocks eclogites are generally well understood processes but important contradictions remain between tectonothermal models and petrological isotopic data obtained from such rocks here we use 40ar 39ar data coupled with a thermal model to constrain the time integrated duration of an orogenic cycle the burial and exhumation of a particular segment of the crust to be less than 13 myr we also determine the total duration of associated metamorphic events to be 20 kyr and of individual heat pulses experienced by the rocks to be as short as 10 years such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events earthquakes such events triggered transient heat advection by hot fluid along deformation shear zones which cut relatively cool and dry subducted crust in contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust our non steady state cold crust model satisfactorily explains several otherwise contradictory geological observations the duration of the geological processes that shape the earth generates vigorous debate mountain building where crust is consumed continuously by burial and recycled as buried remnants rise to the surface is one process where new dating methods can resolve the issue high precision dating of rock remnants left by continental collision in the caledonides of southern norway shows that the entire cycle can happen quickly in about 13 million years in addition the rapid transport of hot fluid zones through the cold crust may explain many puzzling geological observations

40 70 day meridional propagation of global circulation anomalies 1 this paper reports a diagnostic study of circulation anomalies in a semi lagrangian pvlat coordinate by following contours of the daily potential vorticity pv field on isentropic surfaces using the ncep ncar reanalysis ii data set from 1979 to 2003 the leading eof mode which explains about 69 of the total variance of daily northern hemisphere pv anomalies in the pvlat coordinate and is highly correlated with the stratosphere northern annular mode nam index is used to construct a composite life cycle of the nam variability composite circulation anomalies of both signs propagate poleward in the stratosphere and equatorward in the troposphere with an average time of about 44 days for warm anomalies and 72 days for cold anomalies to travel from the equator to the pole accompanying the meridional propagation there exist a simultaneous downward propagation of stratospheric circulation anomalies in both the tropics and extratropics a global mass circulation paradigm is proposed to explain the simultaneous meridional and vertical propagation of global circulation anomalies that appears responsible for the annular mode variability

a geostatistical framework for incorporating seismic tomography auxiliary data into hydraulic conductivity estimation abstract a geostatistical approach is presented for the inclusion of seismic tomography data and sonic log data into the estimation of hydraulic conductivity the procedure accounts for the errors in seismic tomography inversion and for the correlation of such errors the proposed methodology consists of two steps 1 the cross variogram inference is carried out using only the data at the wellbore both hydraulic and seismic 2 a co kriging procedure takes the cross well data into account to interpolate between boreholes no postulated a priori relationship is needed in order to illustrate the methodology a synthetic data set is generated on the basis of evidence from published case studies and simplified physical considerations the numerical experiments show that the choice of the excitation frequency is critical a trade off exists between the need for a high resolution survey asking for higher frequencies and the need for a good correlation between hydraulic conductivity and seismic properties asking for frequencies below the squirt frequency of the medium in the simulation using seismic data with the best excitation frequency 1 khz in this case the mean squared error of the hydraulic conductivity estimate is two thirds lower than using hydraulic data alone it is important to note also that only a part of the interwell region is adequately sampled by the tomographic experiment such a region can be readily identified by calculating the energy of the quasi null space through singular value decomposition of the tomographic matrix in planning this type of experiments it is necessary to carefully verify case by case whether the adopted range of high frequencies does not prevent the seismic energy from propagating effectively from sources to receivers

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

fracture initiation during volume increasing reactions in rocks and applications for co2 sequestration abstract volume increasing processes in rocks can induce various types of fractures such as hierarchical domain division of the unreacted material and spalling of the reacted material the mechanism responsible for the selection of a particular fracture type is generally not known we study such deformations and fracture mechanisms in a simplified system with a range of numerical techniques and find that sharpness of reaction fronts and magnitude of volume increase are the main parameters controlling which fracture type forms spalling dominates when there is sharp reaction fronts and large magnitude of volume change otherwise we get mainly domain dividing fractures increasing reaction front sharpness and magnitude of volume change also increases the fracture density and we argue that the change in fracture density can be important for the viability of in situ co 2 sequestration in ultramafic rock

predicting the downward and surface influence of the february 2018 and january 2019 sudden stratospheric warming events in subseasonal to seasonal s2s models using the real time predictions from 11 models this study analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 sudden stratospheric warmings ssws these two ssws differed both in their morphology types 2018 split 2019 displacement followed by split and magnitudes the former being stronger with a large sample size 2 200 of multimodel ensemble forecasts it is revealed that the strength of the ssw is more important than the vortex morphology in determining the magnitude of its downward impact with strong ssws more likely to propagate downward than weak ssws therefore based on the probabilistic forecasts the observed strong ssw in february 2018 was more likely to have a downward and surface impact than the january 2019 ssw the relationship between the 10 hpa dominant wave number and the 100 hpa polar cap height or the northern annular mode is weak implying that the dominant wave number might not be the primary factor determining the downward propagation of ssws in the two analyzed cases hence the high polar cap height or negative northern annular mode response in the lower stratosphere and troposphere following the february 2018 ssw is mainly attributed to its strong intensity rather than the split morphology further the 2 m temperature anomaly pattern following the january 2019 ssw is not forecasted due to its weak downward propagation whereas the 2 m temperature in north eurasia middle east south china and eastern united states could be forecasted for the downward propagating february 2018 ssw however regional rainfall anomalies are poorly forecasted both in a deterministic and probabilistic sense for both ssws

sub critical fracture and associated retrogression of garnet during mylonitic deformation garnets from the alpine fault mylonites are fractured most fractures have a retrogressive fracture fill a small number have garnet fill a detailed sem study has been carried out to elucidate the mechanisms of fracture and associated retrogression wide fracture zones contain free floating islands of garnet in a retrogressive matrix electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations where rotations are observed they are small and systematic some large fractures show clearly that fracture and retrogression occurs by a replacement process fracture surface morphologies are indicative of surface dissolution a model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet