seismic anisotropy in the crystalline upper crust observations and modelling from the outokumpu scientific borehole finland summary seismic velocity anisotropy measurements are made of a fractured metamorphic formation from the 2 5 km deep international continental scientific drilling programme icdp borehole in outokumpu finland three component walk away vertical seismic profile vsp measurements are made along two source line azimuths at three receiver depths 1000 1750 and 2500 m and incidence angle dependent qp and qs wave velocities are extracted with a p method the highest qp wave anisotropy 13 6 per cent vfast 6160 m s 1 vslow 5370 m s 1 is measured between 1000 and 1750 m depth with anisotropy of up to 9 4 per cent vfast 6090 m s 1 vslow 5540 m s 1 measured between 1750 and 2500 m depth the top 1300 m of the region is composed of a homogeneous strongly intrinsically anisotropic biotite rich schist and is sampled by the shallowest walk away profile anisotropy of up to 11 1 per cent vfast 5950 m s 1 vslow 5320 m s 1 is measured by the walk away vsp between 50 and 1000 m depth along with shear wave splitting averaging 5 per cent 180 m s 1 laboratory derived intrinsic anisotropy of the schist cannot by itself explain the degree or orientation of the anisotropy measured in the walk away vsps however a model which modifies the intrinsic stiffnesses by the inclusion of a single set of dipping aligned cracks allows the observed in situ velocities to be reproduced forward modelling of the qp wave walk away vsp measurements from 50 to 1000 m depth is undertaken using an effective medium model to develop a 3 d velocity model of this region an orthorhombic medium is used to represent the intrinsic anisotropy of the biotite rich schist and a single set of aligned cracks is added to give a bulk elastic stiffness the resulting model predicts the schist to have an overall anisotropy of 16 8 per cent with qp wave velocities of up to 6315 m s 1 the accuracy of the model is assessed through its fit to the walk away vsp measurements as well as a comparison to known geology of the region

multi stage exhumation of young uhp hp rocks timescales of melt crystallization in the d entrecasteaux islands southeastern papua new guinea abstract on the d entrecasteaux islands southeastern papua new guinea png a series of domes expose the youngest known ultrahigh pressure uhp to high pressure eclogites and associated gneisses abundant evidence of partial melting is preserved throughout the d entrecasteaux domes in the form of variably deformed leucosomes dikes and plutonic bodies in order to understand 1 the timing of melt emplacement relative to the uhp metamorphism and 2 the complete exhumation and deformational history of the eclogites and surrounding gneisses a series of leucosomes dikes and eclogites from the mailolo dome in western fergusson island were analyzed by u pb id tims zircon geochronology zircons extracted from the eclogite at the uhp locality yield 206pb 238u tims dates from 5 60 0 22 to 4 61 0 18 ma zircons from three foliation parallel leucosomes are interpreted to record melt crystallization from 3 49 0 01 ma to 2 98 0 02 ma in comparison weakly deformed granitic granodioritic dikes that cut the leucosomes yield zircon 206pb 238u dates of 2 48 0 01 ma 2 420 0 002 ma based on these new and previously published results the eclogites may have undergone uhp metamorphism for several million years from 7 1 0 7 ma to 4 61 0 18 ma based on tims dates alone a more restricted interval of 5 6 4 6 ma is proposed following uhp metamorphism the eclogite and gneisses were exhumed to lower crustal depths intruded by foliation parallel leucosomes by ca 3 5 3 0 ma and by ca 2 45 ma late discordant dikes multiple generations of leucosomes and dikes allow a timeline for the exhumation of the png uhp terrane to be constructed mantle 100 km depths to the base of the crust in 1 my storage in the lower crust for 1 my and final emplacement into the shallow crust 1 my later

crustal structure across the transition from rifting to spreading the woodlark rift system of papua new guinea summary the woodlark rift system is one of the few places where active ocean basin formation can be studied within this rift system continental extension rates are some of the fastest on the planet and extension progresses eastwards to full seafloor spreading we use results from a recent passive seismic experiment to address the role of magmatism prior to the onset of seafloor spreading we invert local earthquake p and s traveltimes for 3 d structure around and ahead of the active spreading tip from the local earthquake tomography we observe three main structures seismic velocities in the crust show a sharp contrast between regions that resemble continental crust ahead of the spreading tip and oceanic crust to the east in the continental portion p velocities increase from 5 6 km s 1 to 6 9 km s 1 between 10 and 25 km depth indicating a bulk felsic to intermediate composition similar to other continental regions beneath the seismic moho 23 28 km depth as defined by receiver functions a 10 15 kmthick gradient zone exist locally with velocities from 7 0 to 7 9 km s 1 which may reflect an underplated mafic granulite facies lowermost crust or perhaps magmatic intrusion into the lithospheric mantle in contrast farther east and closer to the active spreading tip velocities rapidly increase from 6 5 to 7 2 km s 1 between 8 and 18 km depth these fast crustal velocities appear in a narrow zone roughly 60 km wide and indicate a mafic crust similar to oceanic crust our velocity results suggest that the transition from diffuse continental rifting to localized seafloor spreading likely occurs across a narrow zone magmatism prior to the onset of seafloor spreading may not play a significant role in altering crust until the onset of seafloor spreading except through underplating at the base of the crust

late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

elastic anisotropy of tambo gneiss from promontogno switzerland a comparison of crystal orientation and microstructure based modelling and experimental measurements author s vasin rn kern h lokajicek t svitek t lehmann e mannes dc chaouche m wenk hr abstract the authors 2017 felsic and mafic gneisses constitute large proportions of the upper and lower continental crust gneisses often display high anisotropy of elastic properties associated with preferred orientations of sheet silicates here we study the elastic anisotropy of a sample of tambo gneiss from promontogno in the central alps we apply optical microscopy time of flight neutron diffraction neutron and x ray tomography to quantify mineral composition and microstructures and use them to construct self consistent models of elastic properties they are compared to results of ultrasonic measurements on a cube sample in a multi anvil apparatus and on a spherical sample in an apparatus that can measure velocities in multiple directions both methods provide similar results it is shown that models of microstructure derived elastic properties provide a good match with ultrasonic experiment results at pressures above 100 mpa at a pressure of 0 1 mpa the correspondence between the model and the experiment is worse this may be caused by an oversimplification of the model with respect to microfractures or uncertainties in the experimental determination of s wave velocities and elastic tensor inversion the study provides a basis to determine anisotropic elastic properties of rocks either by ultrasonic experiments or quantitative models based on microstructures this information can then be used for interpretation of seismic data of the crust

quantitative constraints to the complete state of stress from the combined borehole and focal mechanism inversions fox creek alberta abstract we develop a quantitative predictive model for the state of stress of a volume of crust encompassing the duvernay formation with a spatial extent of over 150 km 150 km centred near the municipality of fox creek an average direction of the maximum horizontal compression sh is of n43 e determined from analysis of 20 borehole image logs a model of the vertical stress sv corrected for topographic variations is provided from the integration of a 3d density volume constructed from 1125 density logs the minimum horizontal compressions sh and pore pressures pp are evaluated through analysis of 57 well tests carried out within the duvernay formation 3d models developed by kriging of the observed values stress inversion of the focal mechanism solutions for the earthquakes nearby validated the assumed andersonian stress regime and provided the shape ratio of the stress which further allowed estimation of maximum horizontal compression sh a supplementary program is provided allowing the user to calculate the full set of components necessary to describe the states of in situ stress for a location of interest near the duvernay formation within our study area

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

hf isotopic evidence for a cogenetic magma source for the bushveld complex and associated felsic magmas abstract here we test the hypothesis that the rhyolitic lavas of the rooiberg group and granophyres associated with the roof of the bushveld complex are differentiation products of bushveld age mafic liquids we present lu hf isotopic compositions in zircons from roof rocks that have been interpreted to represent thermally metamorphosed and remelted rooiberg group lavas and from granophyres interpreted to be differentiation products of the cumulate rocks that make up the bushveld complex all of these rocks were found to possess e hf 2 06 ga statistically indistinguishable from the intrusion wide average e hf 2 06 ga value of 8 6 1 2 of the bushveld complex our results combined with chronologic and field relations suggest that the felsic rocks were generated by fractional crystallization of bushveld mafic liquids including those that gave rise to the cumulate rocks of the bushveld complex

an approach for predicting stress induced anisotropy around a borehole abstractformation elastic properties near a borehole may be altered from their original state due to the stress concentration around the borehole this could result in a biased estimation of formation properties but could provide a means to estimate in situ stress from sonic logging data to properly account for the formation property alteration we propose an iterative numerical approach to calculate stress induced anisotropy around a borehole by combining a rock physics model and a finite element method we tested the validity and accuracy of our approach by comparing numerical results to laboratory measurements of the stress strain relation of a sample of berea sandstone which contains a borehole and is subjected to a uniaxial stress loading our iterative approach converges very fast and can be applied to calculate the spatially varying stiffness tensor of the formation around a borehole for any given stress state

multiple episodes of anatexis in a collisional orogen zircon evidence from migmatite in the dabie orogen abstract a combined study of mineral inclusions u pb ages and trace elements was carried out on zircon from migmatites in the dabie orogen the results provide insights into multistage anatexis of ultrahigh pressure metamorphic rocks in the continental collision orogen zircon grains in thin sections and mounts record four episodes of magmatic metamorphic and anatectic events 1 middle neoprotrozoic u pb ages for domains that contain qz ap pl inclusions and exhibit high th u ratios 0 1 steep hree patterns with marked negative eu anomalies dating protolith emplacement 2 late triassic u pb ages of 212 5 to 219 4 ma for domains that contain cpx grt inclusions and show low th u ratios

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

petrological and zircon evidence for anatexis of uhp quartzite during continental collision in the sulu orogen petrological evidence is provided for anatexis of ultrahigh pressure uhp metamorphic quartzite in the sulu orogen some feldspar grains exhibit elongated highly cuspate shapes or occur as interstitial cuspate phases constituting interconnected networks along grain boundaries elongated veinlets composed of plagioclase quartz k feldspar also occur in grain boundaries these features provide compelling evidence for anatexis of the uhp quartzite zircon grains from impure quartzite are all metamorphic growth with highly irregular shape they contain inclusions of coesite jadeite rutile and lower pressure minerals including multiphase solid inclusions that are composed of two or more phases of muscovite quartz k feldspar and plagioclase all zircon grains exhibit steep ree patterns similar u pb ages and hf isotope compositions with a weighted mean of 218 2 ma most grains have similar 18o values of 0 6 to 0 1 but a few fall in the range 5 2 to 4 3 thus these grains would have grown from anatectic melts at various pressures zircon o isotope differences indicate that anatectic melts were derived from different sources with contrasting o isotopes but similar hf isotopes that is one from the quartzite itself and the other probably from the country rock granitic gneiss zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths domains that contain eclogite facies minerals exhibit flat hree patterns no eu anomalies and concordant u pb ages of c 220 ma similar u pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep ree patterns and marked negative eu anomalies these observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures zircon enclosed by garnet gave consistent u pb ages of c 214 ma such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite the ree patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts therefore the anatexis of uhp metamorphic rocks is evident during continental collision in the triassic

modeling elastic anisotropy resulting from the application of triaxial stress abstractelastic wave velocities in rocks vary with stress due to the presence of discontinuities and microcracks within the rock we analytically derived a model to account for stress dependency of seismic velocities when a rock is subjected to triaxial stresses we first considered a linearly isotropic elastic medium permeated by a distribution of cracks with random orientations the geometry of cracks is not specified instead their behavior is defined by a ratio b of their normal to tangential excess compliances when this isotropic rock is subjected to triaxial stresses cracks tend to close depending on their orientation with respect to the applied stresses for small stresses the model predicts ellipsoidal anisotropy and expresses the ratios of thomsen s parameters in the three orthogonal planes of symmetry as a function of the compliance and poisson s ratios it also establishes relationships between anisotropy parameters and stress ratios deriving the model for larger stresses shows that su

precipitation of rutile and ilmenite needles in garnet implications for extreme metamorphic conditions in the acadian orogen u s a we report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the merrimack synclinorium connecticut and present a precipitation model for their origin the rocks were strongly metamorphosed and deformed during the devonian acadian orogeny the needles are primarily elongated parallel to in garnet rutile has anomalous extinction angles as great as 35 cf griffin et al 1971 rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long other oxide inclusions that may be present include submicrometer to micrometer scale twinned rutile bicrystals as well as srilankite and a crichtonite group mineral some garnet cores have unusual box shaped quartz inclusions which coexist with ti fe oxide needles and commonly contain micrometer scale rods of f oh cl apatite negative garnet crystal pores are also widespread ti fe oxide needles are restricted to garnet core regions rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite spinel cordierite and k feldspar consequently the garnet core regions represent an earlier distinct period of growth relative to the rims garnet cores contain 25 35 pyrope and a host of minor and trace constituents including tio2 0 07 0 6 wt cr2o3 0 01 0 10 wt na2o 0 01 0 03 wt p2o5 0 01 0 09 wt and zro2 up to 150 ppm na2o and zro2 correlate positively with tio2 titanium zoning is preserved in some garnets zoning profiles and two dimensional chemical mapping show that ti and to a lesser degree cr are depleted around ti fe oxide inclusions therefore we conclude that the needles are precipitates that formed from ti bearing garnet during exhumation and cooling garnet contained sufficient ti to form precipitates no ti source external to garnet was necessary titanium bearing garnets that contain oriented ti fe oxide needles are known primarily from ultrahigh pressure metamorphic rocks mantle peridotites and pyroxenites and high pressure granulites thus the presence of needle bearing garnets in connecticut strongly suggests that a previously unrecognized domain of extreme pressure and or temperature metamorphism exists in the acadian orogen

surface based passive microwave studies of multiyear sea ice multifrequency passive microwave observations are reported from surface based field experiments carried out over multiyear sea ice in the eastern arctic basin the beaufort sea the canadian archipelago and the northern greenland sea dual polarized brightness temperatures at 6 7 10 18 7 37 and 90 ghz were obtained at selected multiyear ice sites covering a wide range of surface properties considerable variation was found in the observed spectra over short distances on individual floes under winter conditions where values of ev 37 ghz ev 18 ghz ev 37 ghz ev 18 ghz ranged from nearly zero to approximately 0 08 concurrent surface characterization observations consisting of temperature density salinity and grain size profiles in the snow and the upper meter of the ice were carried out frequently in conjunction with cold regions research and engineering laboratory personnel the present analysis shows that the magnitude of the spectral gradient of emissivity is directly related to the existence and thickness of a decomposed surface ice layer with very high porosity spectra for melt ponds with a frozen surface layer closely resembled those of lake ice and show a positive spectral gradient as soon as the ponds were completely frozen their spectra became nearly the same as those for first year ice fine grained winter snow produced a mild decrease in the emissivities at 90 ghz with increasing snow thickness from 0 to 0 2 m at which point the snow appears to be optically thick during the melt season the ice emissivities e depend most strongly on whether the surface layers are experiencing temporary freezing or not under melting conditions e depends on the liquid water content only over a very small range e g 0 5 by mass at 90 ghz if the liquid water in the surface layers is freezing which can occur at intervals throughout the melt season the spectral gradient is negative with a slope which depends on the thickness of the refrozen layer the variance among emissivity spectra for multiyear ice appears to be influenced primarily by the distribution of melt ponds and by the presence of significant amounts of scattering inhomogeneities greater than 1 mm in diameter in the snow and in the upper 20 30 cm of the ice we expect the signatures of multiyear ice to be regionally dependent perhaps yielding environmental information about the arctic ice pack

modeling shear wave splitting due to stress induced anisotropy with an application to mount asama volcano japan we use numerical modeling to investigate the proposed stress based origin for changing anisotropy at mount asama volcano japan stress induced anisotropy occurs when deviatoric stress conditions are applied to rocks which are permeated by microcracks and compliant pore space leading to an anisotropic distribution of open crack features changes to the local stress field around volcanoes can thus affect the anisotropy of the region the 2004 eruption of mount asama volcano coincided with time varying shear wave splitting measurements revealing changes in anisotropy that were attributed to stress changes associated with the eruption to test this assertion we create a model that incorporates knowledge of the volcanic stress ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model anisotropy is calculated in two ways by considering a basic case of having uniform crack density and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress our results show that this approach is sensitive to crack density crack compliance and the regional stress field all of which are poorly constrained parameters in the case of dry crack closure results show that modeled stress conditions produce a much smaller degree of anisotropy than indicated by measurements we propose that the source of anisotropy changes at asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes such as the movement of pore fluid

azimuthal velocity variations caused by borehole stress concentrations in laboratory experiments borehole stress concentrations caused by far field uniaxial stress produce measurable variations in compressional wave velocities near the borehole uniaxial stress was applied perpendicular to the axis of a borehole drilled through a sandstone block inside the fluid filled borehole refracted compressional waves were propagated in the rock parallel to the borehole axis at discrete azimuths producing a scan of velocity versus azimuth as uniaxial stress increased significant variations of compressional wave velocity with azimuth developed of the order of 10 variation the azimuth of velocity minima provides an excellent indicator of the direction of maximum far field uniaxial stress accurate to 5 the detailed variation of velocity with azimuth while approximately sinusoidal has sufficient character to provide new information about the rock one interpretation yields the curvature of the velocity versus stress curve of the rock

the pre atlantic hf isotope evolution of the east laurentian continental margin insights from zircon in basement rocks and glacial tillites from northern new jersey and southeastern new york abstract this paper presents laser ablation u pb age and hf isotope data for zircons from basement rocks and glacial deposits in northern new jersey and southeastern new york the purpose is to understand the eastern laurentian continental margin s hf isotope record in relation to its geologic evolution prior to the opening of the atlantic ocean the basement samples encompass a meso to neoproterozoic continental margin arc an anatectic magmatic suite as well as a late ordovician alkaline igneous suite emplaced during post orogenic melting of the lithospheric mantle additional samples were collected from terminal moraines of two quaternary continental ice sheets across the entire dataset zircons with ages corresponding to the timing of continental margin arc magmatism 1 4 ga to 1 2 ga have positive e hf initial values that define the more radiogenic end of a crustal evolution array this array progresses towards more unradiogenic e hf initial values along a series of low 176 lu 177 hf 0 022 to 0 005 trajectories during subsequent anatectic magmatism 1 2 ga to 1 0 ga and later metamorphic and metasomatic re working 1 0 ga to 0 8 ga of the continental margin arc crust in contrast nearly chondritic e hf initial values from the late ordovician alkaline magmas indicate that the laurentian margin was underlain by a re fertilized mantle source such a source may have developed by subduction enrichment of the mantle wedge beneath the continental margin during the mesoproterozoic additionally preliminary data from a metasedimentary unit of unknown provenance hints at the possibility that some of the sediments occupying this portion of the laurentian margin prior to the ordovician were sourced from crust older than 1 9 ga

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

static and dynamic pressure sensitivity anisotropy of a calcareous shale optimizing the productivity of nonconventional low permeability shale reservoirs requires detailed knowledge of the mechanical properties of such materials these rocks elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional duvernay reservoir of alberta canada the complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 mpa wave speed anisotropies of up to 35 are observed at even the highest confining pressures furthermore the stress sensitivity of the wave speeds and hence moduli is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure whereas those along the bedding plane show unexpectedly nearly no pressure dependence these observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images these results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions for the concentration of stress within such formations and for estimation of static engineering moduli from sonic log wave speeds

cathodoluminescence guided zircon hf isotope depth profiling mobilization of the lu hf system during u hp rock exhumation in the woodlark rift papua new guinea abstract cathodoluminescence image guided hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the lu hf system s behavior during rapid exhumation of u hp rocks in the woodlark rift papua new guinea investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve 8 m thick domains in which ehf present differs by as little as 4 units in a metasedimentary gneiss 2 89 0 29 ma zircon overgrowths on cretaceous aged inherited cores have radiogenic ehf present indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization the garnet effect of zheng et al 2005 in a separate gneiss sample that originated as an exhumation related anatectic melt 3 66 0 13 ma zircons lacking inheritance contain sub domains that differ from each other by 15 ehf present some of these sub domains are radiogenic and can be explained by the garnet effect whereas others also contain highly elevated lu and yb in addition to their radiogenic hf compositions thus necessitating a medium derived from the complete breakdown of garnet zircons in this sample also contain non radiogenic sub domains that grew in the presence of hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the australian continental margin the results confirm that rapid exhumation of u hp rocks can result in the following 1 transmission of radiogenic hf and sometimes lu and the other hree from garnet bearing mafic lithologies into the quartzofeldspathic gneisses and 2 mobilization and transport of unradiogenic hf present within the quartzofeldspathic remnants of subducted continental crust

estimation of the anisotropy parameters of transversely isotropic shales with a tilted symmetry axis summary this paper reports a new approach for the estimation of thomsen anisotropy parameters and symmetry axis orientation from ultrasonic p wave traveltime measurements on transversely isotropic shale samples of arbitrary geometry this approach can be used for core samples cut in any direction with respect to the bedding plane because no a priori assumption regarding the symmetry axis orientation is made this orientation is rather part of the solution of the inverse problem together with the anisotropy parameters themselves very fast simulated reannealing is used to search for the best possible estimate of the model parameters the methodology is applied to spherical and cylindrical anisotropic shale samples

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

anisotropy effects on microseismic event location seismic anisotropy in sedimentary environments is significant microseismic waveforms often show strong shear wave splitting with differences reaching 40 between horizontally and vertically polarized shear wave velocities failure to account for this anisotropy is shown to result in large microseismic event location errors a method is presented here for determining the five elastic parameters of a homogeneous vertical transverse isotropic vti model from calibration shot data the method can also use data from mining induced seismic events which are then simultaneously located this simple model provides a good fit to arrival times from coal environment data and results in dramatic shifts in interpreted event locations

observed vertical redistribution of black carbon and other insoluble light absorbing particles in melting snow 1 light absorbing impurities in snow reduce snow albedo producing a positive radiative forcing warming the surface air and snowpack and accelerating snow melt as the snow melts black carbon bc and other insoluble light absorbing particulate impurities ilap are retained at the snow surface because their scavenging efficiency with meltwater is 100 so concentrations of ilap in surface snow increase with snow melt further reducing snow albedo the magnitude of this positive feedback depends on the scavenging efficiency of bc and other ilap with snow meltwater we present results from field measurements of the vertical distribution of bc and other ilap in snow near barrow alaska the dye 2 station in greenland and tromso norway during the melt season amplification factors due to melt are calculated for the concentrations in surface snow of bc and all ilap at barrow and dye 2 melt scavenging rates are estimated melt amplification appears generally to be confined to the top few centimeters of the snowpack where it increases concentrations of bc and other ilap by up to a factor of about five scavenging fractions of ilap due to percolation of meltwater are estimated at 10 30 with the rates for bc being comparable or a few percent lower the lack of distinction may result from the particles in snow being internal mixtures of both bc and other ilap so that scavenging efficiencies for these internally mixed particles are determined by the total particle size and hydrophobicity rather than being different for individual particle components

seismic anisotropy in sedimentary rocks part 1 a single plug laboratory method a single plug method for measuring seismic velocities and transverse isotropy in rocks has been rigorously validated and laboratory tested the method requires only one sample to measure the velocities needed to derive the five independent elastic constants for transversely isotropic materials in this method piezoelectric transducers are fitted to the top bottom and sides of the cylindrical sample laboratory velocity and anisotropy can be measured as functions of pressure temperature fluid saturation and fluid displacement because this method uses a horizontal core plug that has much higher permeability than a vertical core plug it is especially suitable for low permeability shale measurements it reduces the sample preparation and velocity measurement time by more than two thirds

dipole dispersion crossover and sonic logs in a limestone reservoir analyses of sonic logs in a horizontal well provide new information about mechanical properties of rocks made possible by recent developments in our understanding of acoustic wave propagation in prestressed formations most sections of this horizontal well exhibit azimuthal shear isotropy indicating isotropic stresses in the plane perpendicular to the well trajectory leading to stable wellbore conditions however two sections show dipole dispersion crossovers that confirm the presence of stress induced shear anisotropy caused by a difference between the maximum and minimum stresses in the plane perpendicular to the well trajectory the two dipole dispersions are obtained by processing the recorded waveforms by a modified matrix pencil algorithm the fast shear direction is estimated from alford rotation of the cross dipole waveforms one section of the well exhibits the fast shear direction parallel to the overburden stress as the maximum stress direction whereas the other section has the fast shear direction parallel to the horizontal stress that is larger than the overburden stress the cause of this change in the fast shear direction is believed to be the well s penetration into a 3 ft thick bed with lower porosity and permeability and significantly higher elastic stiffnesses than those in the other part of the homogeneous high permeability reservoir a stiff bed is likely to have greater stresses in its plane than perpendicular to it which would make the horizontal stresses greater than the vertical

the effect of confining pressure on elastic wave velocities and dynamic to static young s modulus ratio abstractwe carried out laboratory experiments under dry conditions on limestone core specimens of sarvak formation obtained from an oil well in the southwest of iran our objective was to study the effect of confining pressure on the compressional and shear wave velocities vp vs and on the dynamic to static young s modulus ratio ed es furthermore we made attempts to predict vp and vs at atmospheric pressure based on the same velocities at various confining pressures these analyses revealed that below a critical pressure with an increase in confinement vp and vs increased exponentially representing a poroelastic regime above a critical pressure however the trend was linear likewise we observed that with an increase in confinement ed es initially decreased exponentially followed by a linear decreasing trend above the critical pressure this indicated that es is more responsive than ed accordingly these observations infer that it is possible to predict es based on ed at different confinin

the anatectic effect on the zircon hf isotope composition of migmatites and associated granites zircon hf isotope composition is widely used to trace the growth and evolution of continental crust however it is controversial whether the hf isotope composition of magmatic zircons can faithfully reflect that of their sources especially for s type granites in order to provide an insight into this issue we have revisited the published lu hf isotope data of zircons from well studied migmatites and associated granites in the sulu orogen and the cathaysian terrane respectively the results show greatly elevated 176hf 177hf ratios by more than 10e units for newly grown zircon domains compared to the relict zircon domains this indicates considerable contributions from non zircon hf to anatectic melts during crustal anatexis and subsequent magmatism furthermore this more radiogenic hf isotope signature was not erased during magmatic processes such as crystal fractionation during melt ascent and emplacement the budget of hf isotopes in source rocks with respect to mineral lu hf ratios suggests the involvement of hf bearing major minerals in anatectic reactions by dissolving hf bearing major minerals into the anatectic melts the significant hf isotope variations in some anatectic and magmatic zircon domains from the migmatites and granites suggest not only the source heterogeneity but also the variable non zircon hf contributions as such the hf isotope compositions of anatectic and magmatic zircons are substantially dictated by the mass balance between the non zircon hf from anatectic reactions and the zircon hf from the dissolution of protolith zircons into the anatectic melts they are primarily controlled by p t conditions and mechanism of crustal anatexis and the magmatic processes during melt evolution the present study highlights the important contribution of non zircon hf to the anatectic and magmatic zircon domains in this regard the greatly elevated 176hf 177hf ratios for newly grown zircon domains in the migmatites and granites cannot reflect the hf isotope compositions of their source rocks such a deviation must be kept in mind when linking relatively depleted hf isotope compositions to the nature of source rocks in petrogenetic interpretation and to the growth of continental crust the contribution of non zircon hf can also better account for the variable hf isotope compositions often shown by zircons from s type granites

ultrasonic borehole velocity imaging we describe a new technique for making high resolution velocity images of rocks using refracted ultrasonic waves the use of refracted waves makes this technique potentially suitable for imaging borehole walls in the laboratory we use a single transmitter two receiver first arrival method for making velocity measurements with a spatial resolution on the order of 1 cm our acoustic pulses are centered near 200 khz scans of a borehole wall reveal dipping thin layers and fractures when external stress is applied perpendicular to the borehole stress concentrations appear on our images as axial bands of high and low velocities breakouts created by high stress also can be imaged on a planar sample a velocity image reveals shale laminations and carbonate stringers for field applications this technique offers the potential for imaging in both conductive and nonconductive muds and provides images based on a physical property velocity that currently is not used for fine scale borehole imaging

differences between static and dynamic elastic moduli of a typical seismogenic rock summary static and dynamic elastic moduli of calcare massiccio mudstone limestone the typical seismogenic rock in the italian apennines are measured using a standard uniaxial static compression test a dual cantilever forced oscillation test and ultrasonic measurement of elastic wave velocities these measurements cover nine decades in frequency including the seismic domain neither a significant frequency dependence nor a pronounced strain amplitude dependence was observed providing a young s modulus of 75 7 gpa and a poisson s ratio of 0 28 0 02 these values are characteristic of calcare massiccio in undamaged condition

albedo of dirty snow during conditions of melt the evolution of spectrally averaged albedo wavelengths between 0 28 m and 2 8 m of snow surfaces treated with known initial concentrations of particles of submicron sized soot and air fall volcanic ash was investigated during conditions of natural melt depending on the particle type and concentration the initial applications reduced the surface albedo to values ranging from 0 18 to 0 41 which were substantially lower than the albedo of the untreated natural snow about 0 61 many of the soot particles flushed through the snowpack with the meltwater and surface concentrations of soot greater than about 5 10 7 kg kg did not persist for more than a few days the migration of particles to depth caused the snow to brighten after the initial application thus limiting the amount of albedo reduction and the consequent effects on melting nevertheless the soot remaining near the surface had a substantial long term effect the residual concentration of 5 10 7 kg kg persisted for several weeks and compared to the untreated surface reduced the albedo by about 30 and increased melting by 50 particles of volcanic ash with diameters larger than about 5 m remained at or near the snow surface although many of the smaller particles flushed through the snow with the meltwater the surface albedo was not changed significantly by their removal the different behaviors of the ash and soot are probably related to the difference in their particle size distributions in relation to the thickness of water films that form the transport paths under conditions of partial saturation that are characteristic of melting snow

tectonics of the new guinea region the new guinea region evolved within the obliquely and rapidly converging australian and pacific plate boundary zone it is arguably one of the most tectonically complex regions of the world and its geodynamic evolution involved microplate formation and rotation lithospheric rupture to form ocean basins arc continent collision subduction polarity reversal collisional orogenesis ophiolite obduction and exhumation of ultra high pressure metamorphic rocks we describe the major onshore and offshore tectonic and geologic components including plate boundaries seismicity faults and magmatism and we integrate these with emerging ideas about mantle dynamics to evaluate the cenozoic tectonic evolution of new guinea future research opportunities to resolve the mantle structure beneath new guinea will enable mantle dynamics to be linked to lithospheric and surface processes virtually all plate tectonic and mantle processes have been active in the new guinea region throughout the cenozoic and as such

the effect of anisotropy on the young s moduli and poisson s ratios of shales young s modulus and poisson s ratio are required for geomechanics applications such as hydraulic fracture design analysis of wellbore stability and rock failure determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly anisotropy in shales results from a partial alignment of anisotropic clay particles kerogen inclusions microcracks low aspect ratio pores and layering for shales the young s modulus measured parallel to bedding e1 is usually greater than the young s modulus measured perpendicular to bedding e3 however the poisson s ratio 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than equal to or less than the poisson s ratio 12 for stress applied parallel to bedding and strain measured parallel to bedding for transverse isotropy the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients w200 and w400 which describe the impact of clay particle orientation on the anisotropy of shales disorder in the orientation of clay particles acts to reduce w400 faster than w200 since w400 is a higher order moment of the clay particle orientation distribution function than w200 this is confirmed by analysis of measured anisotropy parameters for shales a partial alignment of clay particles is consistent with the measured young s moduli for shales and with values of poisson s ratio 31 12 but not with values 31 12 these values can be explained if there exist kerogen inclusions microcracks or low aspect ratio pores aligned parallel to the bedding plane

nonlinear elasticity and stress induced anisotropy in rock the elastic nonlinear behavior of rocks as evidenced by deviations from hooke s law in stress strain measurements and attributable to the presence of mechanical defects compliant features such as cracks microfractures grain joints is a well established observation the purpose of this paper is to make the connection between the elastic nonlinearity and stress induced effects on waves in this case uniaxial stress induced transverse isotropy the linear and nonlinear elastic coefficients constitute the most condensed manner in which to characterize the elastic behavior of the rock we present both the second and the third order nonlinear elastic constants obtained from experimental data on rock samples assumed homogeneous and isotropic when unstressed as is normally the case the third order nonlinear constants are found to be much larger than the second order linear elastic constants contrary to results from intact homogeneous solids materials without mechanical defects rocks exhibit weak to strong nonlinearity and always in the same manner i e an increase of the moduli with pressure as a consequence the stress induced p wave anisotropy and s wave birefringence can be large the stress induced p wave anisotropy appears to be much larger than the s wave birefringence the fast direction is parallel to the stress direction and the anisotropy goes as sin2 being the angle between the propagation direction and the stress direction experiments on rocks indicate that at low applied stresses the proportionality of the stress and the induced s birefringence and p anisotropy predicted by theory is well corroborated

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight bakken reservoir abstractto improve the accuracy of microseismic event locations we developed a new inversion method with double difference constraints for determining the hypocenters and the anisotropic velocity model for unconventional reservoirs we applied this method to a microseismic data set monitoring a middle bakken completion in the beaver lodge area of north dakota geophone arrays in four observation wells improved the ray coverage for the velocity inversion using an accurate anisotropic velocity model is important to correctly assess the height growth of the hydraulically induced fractures in the middle bakken our results showed that 1 moderate to strong anisotropy exists in all studied sedimentary layers especially in the upper and lower bakken shale formations where the thomsen parameters and can be greater than 0 4 2 all the events selected for high signal to noise ratio and used for the joint velocity inversion are located in the bakken and overlying lodgepole formations i e no events a

geometry and kinematics of shear zones formed during continental extension in eastern papua new guinea abstract the d entrecasteaux islands eastern papua new guinea lie in an area of continental extension that has been active since the mid miocene during the last 4 ma the metamorphic basement rocks composing most of the islands have been uplifted and tectonically exhumed from depths of approximately 35 km tectonic exhumation appears to have been controlled by deformation in broad kilometre scale mylonitic shear zones a progressive evolution is documented in the shear zones characterized by increasing localized deformation a change from dominantly ductile to dominantly brittle processes and decreasing metamorphic grade i e retrograde metamorphism the evolution is believed to be the result of uplift and cooling of the basement accompanying shear zone movement the kinematic history of the shear zones is complex a number of sinistrally offset extensional detachment zones separate a multiply deformed high grade metamorphic basement from a cover of largely undeformed ultramafic and mafic cover rocks the detachment zones are connected by transverse shear zones these transverse zones appear to act as sinistral strike slip zones which also accommodate transfer motion between the detachment zones

noncontacting benchtop measurements of the elastic properties of shales we evaluated a laser based noncontacting method to measure the elastic anisotropy of horizontal shale cores whereas conventional transducer data contained an ambiguity between phase and group velocity measurements small laser source and receiver footprints on typical core samples ensured group velocity information in our laboratory measurements with a single dense acquisition of group velocity versus group angle on a horizontal core we estimated the elastic constants c11 c33 and c55 directly from ultrasonic waveforms and c13 from a least squares fit of modeled to measured group velocities the observed significant p wave velocity and attenuation anisotropy in these dry shales were almost surely exaggerated by delamination of clay platelets and microfracturing but provided an illustration of the new laboratory measurement technique although challenges lay ahead to measure preserved shales at in situ conditions in the lab we evaluated the fundamental advantages of the proposed method over conventional transducer measurements

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

elemental and organic carbon measurements with a two step heating gas chromatography system in snow samples from the tibetan plateau in this study an optimized two step heating gas chromatography system is used to measure elemental carbon ec and organic carbon oc content in snow and ice with the ability to quantify the elemental and organic carbon species in a snow or ice sample of 60 80 g in this system oc and ec are transformed into co2 in a stream of oxygen at 3408c and 6508c respectively the resulting co2 is accumulated in two molecular sieve traps and then put into a gas chromatograph equipped with a flame ionization detector by heating the traps to 2008c in a helium stream background contamination mainly caused by impurities in the oxygen stream and accuracy are dominated by the variability of the blank loads on the pre cleaned filters which are 0 50 0 04 1 mgc for oc and 0 38 0 04 1 mgc for ec the system is suitable for snow and ice sample measurements with the same precision as shown for the blank tests ec and oc concentrations have been measured in snow samples collected from different glaciers on the tibetan plateau the results allow quantification for the first time of the different carbonaceous particle contents on the tibetan plateau and other regions the concentrations of ec and oc particles in snow show a clearly decreasing trend from east to west and from north to south on the plateau excluding the pamirs region the highest mean ec content 79 2 ng g 1 was found in the northeast region and the lowest 4 3 ng g 1 was found in the western himalaya we note that even slight surface melting results in fresh snow getting dirtier especially in regions with higher pollution such as seen on a glacier in the qilian shan here the ec and oc concentrations in the fresh snow average 6 6 and 87 5 ng g 1 but after 2 days of surface melting they increased to 52 6 and 195 5 ng g 1 this suggests that surface snow melting can reduce snow albedo due to the accumulation of carbonaceous particles

high resolution velocity field imaging around a borehole excavation damaged zone characterization the excavation of a deep underground structure induces stress field redistribution that creates an excavation damaged zone edz the study of edz physical properties is of prime importance in the framework of nuclear waste geologic storage where edz constitutes a preferential pathway for storage materials toward the biosphere a probe for measuring the ultrasonic velocity field around a cylindrical excavation zone has been developed the probe uses pzt transducers operating at 150 khz to collect all required data with high accuracy and minimum effort the probe is fully monitored for automatic data acquisition also a numerical inversion method reconstructs a stress field using the measured velocity field this method was partially validated in the laboratory on a multilayer test bed following laboratory tests and probe development an in situ experiment was performed in the meuse haute marne underground research laboratory france to assess the safety of nuclear waste storage in the callovo oxfordian argillaceous layer the damaged zone extends up to 0 175 diameter of depth with an anisotropic damage pattern oriented according to the regional stress field the results show the potential of the method to forecast damage of similar larger structures

resorption growth solid state recrystallisation and annealing of granulite facies zircon a case study from the central erzgebirge bohemian massif zircon crystals have been investigated from a gneiss area in the central erzgebirge bohemian massif where small occurrences lenses of granulites and sometimes eclogites are located within amphibolite facies gneisses geological relationships indicate that leucocratic quartzofeldspathic rocks within the granulite boudins represent melts whereas garnet rich melanocratic rocks are considered restites derived through multiple extraction of the leucocratic melts the morphology of zircon crystals is quite different in these two types of granulites with rounding pointing to higher zircon resorption in garnet rich rocks due to multiple interaction with melts extensive new zircon growth can be demonstrated within granulitic boudins on the basis of zircon morphology internal zircon textures and age data the newly formed zircon grains and rims are non cathodoluminescent and yield an age of about 340 ma secondary ionisation mass spectrometry sims and zircon evaporation raman microprobe investigations revealed heterogeneous metamictisation within zircon grains and pronounced damage of the zircon structure in u rich non cathodoluminescent zircon rims the common reverse discordance of such non cathodoluminescent zircon rims may be explained as a result of preferential pb sputtering during sims analysis leading to apparent pb excess which seems to occur principally in the most damaged zircon domains it is proposed from field relationships that surrounding gneisses although equilibrated under amphibolite facies conditions experienced the same metamorphic pt history as the granulites these amphibolite facies rocks contain zircon distinct from the granulite zircon populations with morphologies and internal textures typical for their protolith rocks implying only slight zircon changes due to hp overprint new zircon growth is here mostly limited to the appearance of very rare new grains or more commonly to formation of thin overgrowths the internal texture of these zircon crystals shows clear signs of solid state recrystallisation leading in most cases to a partial resetting of their u pb ages mainly due to pb loss in addition u contents are slightly higher in cl dark rims that we interpret to have formed mainly due to solid state recrystallisation most evaporation and sims ages vary between protolith rock age and 340 ma so that the interpretation of the 340 ma age is not unambiguous it could be explained as the age of zircon crystallisation from a u rich and th depleted melt or alternatively as a resetting age due to solid state recrystallisation resulting from melt or fluid interaction with old zircon it is difficult to determine at which stage of metamorphic evolution new zircon growth occurred in the granulitic boudins and remains unclear that zircon growth occurred at peak metamorphic conditions

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

guidelines for reporting zircon hf isotopic data by la mc icpms and potential pitfalls in the interpretation of these data invited technical paper high quality analytical data are essential for the development of sound scientific interpretations to ensure the quality of the data published in chemical geology a new type of contribution has been introduced the invited technical paper when a particular technical issue needing discussion or clarification is identified specialists in the technique will be invited to share their expertise as is true for all articles published in the journal these contributions will be subject to peer review before publication the first topic to be treated concerns in situ hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry this is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world however if insufficient care is taken during analysis and data processing inaccurate results can be obtained notably because of the existence of very large isobaric interferences on the isotope of interest to address this issue we have asked christopher fisher jeffery vervoort and john hanchar to provide a set of guidelines that can be adopted to assure that reliable hf isotopic data are obtained by this technique abstract over the past decade the hf isotope composition of zircon as determined in situ by laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool there has been no uniformity however in the reporting of in situ hf isotopic data and quite often reviewers and readers of papers are not provided with sufficient information to assess data quality the goal of this invited contribution is to provide the non specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ hf isotopic data from zircon including details of how the challenging large isobaric interference corrections have been made in addition we discuss a number of potential pitfalls vis a vis the assignment of the incorrect age to the measured hf isotope composition non specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others

protolith control on fluid availability for zircon growth during continental subduction zone metamorphism in the dabie orogen abstract different episodes of zircon growth are recognized by a combined study of cl images mineral inclusions u pb ages trace elements and lu hf isotopes for ultrahigh pressure uhp eclogite facies metamorphic rocks in the dabie orogen the results provide insights into the effect of protolith property on fluid liberation during continental collision fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth zircon u pb dating for uhp metabasalt and metasediment eclogite and its host paragneiss yields two groups of ages at 244 3 and 225 2 ma respectively mineral inclusion trace element and lu hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation respectively in contrast zircon u pb dating for uhp metaintrusive rocks granitic orthogneiss and its hosted eclogite gave only one group of age at 222 2 ma mineral inclusion trace element and lu hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation the difference between the two groups of zircon u pb dates is attributed to the difference in their protolith origin volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water pore fluid so that considerable amounts of aqueous fluid can be released from them during subduction this episode of fluid action was recorded by the growth of anatectic zircon at 244 ma in contrast intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals so that little fluid can be released from them during subduction nevertheless large amounts of retrograde fluid were released from uhp metamorphic rocks regardless of their protolith origin during decompression exhumation this episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220 225 ma therefore the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction which has great bearing on partial melting element transport and mineral growth during continental subduction zone metamorphism

zirconological tracing of transition between aqueous fluid and hydrous melt in the crust constraints from pegmatite vein and host gneiss in the sulu orogen abstract zircon can grow from aqueous fluid and hydrous melt in crustal rocks when they undergo metamorphic dehydration and partial melting under high pressure hp to ultrahigh pressure uhp conditions a distinction between fluid and melt has important bearing on the development of crustal anatexis in collisional orogens a genetic transition between fluid and melt is recorded by zircons from pegmatite vein and host uhp gneiss in the sulu orogen this transition is elucidated by an integrated study of sims u pb and o isotope analyses with la icpms u pb isotope and trace element analyses in the zircons pegmatite veins yield zircon u pb ages of 147 153 ma for new growths and 700 800 ma for relict cores a one meter width pegmatite vein exhibits two episodes of zircon growth at 153 3 ma for inner domain and at 147 2 ma for outer domain the two types of domains have a series of differences in cl image inclusion type ree content and pattern trace element contents and ratios and ti in zircon temperature the inner domain grew from the hydrous melt at 730 840 c whereas the outer domain grew from the aqueous fluid at 520 650 c nevertheless they have similarly low 18o values of 1 0 2 3 suggesting their growth from o isotope homogeneous media despite the transition from melt to fluid the host gneiss of this pegmatite vein exhibits three generations of zircon growth with 180 205 ma at 700 770 c during amphibolite facies metamorphism 157 3 ma at 610 670 c for fluid assisted growth and 147 2 ma at 780 860 c for melt assisted growth the last two types of growth record the development of crustal anatexis from hydration melting to dehydration melting the other decimeter width pegmatite vein exhibits only one episode of zircon growth from melt at 149 2 ma and 660 860 c its host gneiss exhibits a residual melt texture but no significant growth of zircon indicating low degree of dehydration melting therefore the zircon domains in the host gneisses record their growth during a transition from aqueous fluid to hydrous melt along a temperature increasing path whereas the zircon domains in the pegmatite veins record their growth during a transition from hydrous melt to aqueous fluid along a temperature decreasing path despite the difference in the direction of fluid melt evolution the all zircon domains in these gneisses and pegmatite veins record the same event of crustal anatexis in the uhp gneisses the present study also provides insights into a genetic definition of anatectic melt and thus anatectic zircon

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

titanium solubility in coexisting garnet and clinopyroxene at very high pressure the significance of exsolved rutile in garnet exsolution microstructures including ilmenite garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the sulu ultrahigh pressure uhp terrane in order to understand the phase relations and ti solubility in both garnet and clinopyroxene in a natural tio2 bearing system several experiments at 5 15 gpa 1000 1400 c were carried out using the multianvil high pressure apparatus the hujianlin ilmenite rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material because it closely approaches a composition lying in the tio2 cao mgo feo al2o3 sio2 system except for minor melt in one experiment at 1400 c and 5 gpa other run products contain majoritic garnet clinopyroxene ilmenite or rutile and exhibit neoblastic texture with increasing pressure ti and ca mg and si contents of neoblastic garnet increase with decreasing al the principal coupled substitutions are ca2 ti4 2al3 and si4 mg2 2al3 responding to majorite component increase titanium solubility 0 8 4 5 wt as tio2 in garnet and grtti cpxti ratio have a pronounced positive correlation with pressure between 5 and 15 gpa on the other hand the coexisting clinopyroxene contains low ti 0 17 0 53 wt as tio2 and shows no significant pressure effect rutile exsolution in garnet is coupled to that of pyroxene exsolution both are exsolved from majoritic garnet on decompression therefore the amount of such exsolved lamellae is a potential indicator of high pressure metamorphism in exhumed rocks whereas the tio2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change

synexhumation anatexis of ultrahigh pressure metamorphic rocks petrological evidence from granitic gneiss in the sulu orogen abstract petrological evidence is provided for partial melting of ultrahigh pressure uhp metamorphic granitic gneiss in the sulu orogen petrographic observations show the occurrence of elongated highly cuspate feldspars in grain boundaries interstitial cuspate feldspars in triple junctions felsic veinlets mainly consisting of k feldspar quartz and feldspar crystal faces against quartz these features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss zircon domains grown from these melts were identified based on cl images mineral inclusions and ree patterns some large zircon grains 100 m contain small relict domains of magmatic origin suggesting nearly complete dissolution of the protolith zircon during the anatexis all newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions one group of domains contain no coesite inclusion and exhibit high u contents but low th u ratios pb ages of 217 2 to 224 2 ma the other group of domains contain coesite inclusions and exhibit low u contents and very low th u ratios pb ages of 221 5 to 226 3 ma the two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative eu anomalies that are absent for metamorphic zircons grown from aqueous fluids the zircon u pb ages of 217 2 to 226 3 ma are close to but slightly younger than known ages for major uhp metamorphism in the sulu orogen therefore the uhp gneiss experienced incipient melting during the initial exhumation subsequent to the peak uhp metamorphism and extensive anatexis later at lower pressures muscovite relicts coexist with cuspate feldspars suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the uhp granitic gneiss as the degree of partial melting increases with temperature significant fractions of the anatectic melts would be produced in the regional gneiss provided that the uhp rocks experienced hot exhumation such melts can be gathered together being eventually emplaced as synexhumation granitic intrusions

dynamic and static moduli static and dynamic bulk moduli ks and kd are measured as continuous functions of pressure from zero to 2 3 kilobars for two sandstones a tuff limestone granite and oil shale results for the sandstones and granite are in good agreement with previously reported data with ks kd varying from about 0 5 at atmospheric pressure to close to unity at pressures 2 kilobars and above for rocks behaving elastically under static loading the ks kd ratio is inversely related to the microcrack density for the limestone time dependent deformation associated with pore collapse results in ks kd ratios approaching 0 1 at high pressure upon unloading while initially high 1 0 at high pressures ks kd becomes lower than values obtained during loading at low pressures 1 kilobar due to opening of microcracks generated during pore collapse for the oil shale in which few microcracks exist ks kd remains relatively constant with pressure at a value of about 0 7

transversely isotropic stiffness parameters and their measurement in colorado shale drained stress path triaxial tests and confined torsion tests were conducted on colorado shale core samples to investigate transversely isotropic stiffness parameters at small strain deformation nonlinear plastic behaviour occurred along the primary loading path even at strains less than 0 01 nonlinear hysteric behaviour was only observed during the loading unloading path however the shale material displayed transverse isotropy in deformation and very small or nondetectable poisson s ratios in vertical and horizontal directions this special response alludes to the postulate that the colorado shale could be approximated by a transversely isotropic elasticity model at small strain levels with negligible yielding key words anisotropy elasticity stiffness triaxial test torsion test shale

focal mechanisms in anisotropic media summary focal mechanisms of seismic sources in anisotropic media are more complicated than in isotropic media planar shear faulting produces pure double couple dc mechanism in isotropy but generally non double couple non dc mechanism in anisotropy the non dc mechanism can comprise both the isotropic iso and compensated linear vector dipole clvd components the amount of the iso and clvd components depends on strength and symmetry of anisotropy and on the orientation of faulting shear faulting in anisotropy generates a pure dc mechanism provided that faulting is situated in symmetry planes of orthorhombic or of higher anisotropy symmetries the fault plane solution i e the orientation of the fault normal and the slip direction can be retrieved from the seismic moment tensor and from elastic parameters describing anisotropy at the source numerical modelling shows that shear faulting in anisotropic rocks present in the earth crust the earth mantle or in the subduction zones can produce mostly mechanisms with the clvd up to 30 per cent and with the iso up to 15 per cent the fault plane solutions calculated under the assumption of isotropy typically deviate from the true solutions by an angle of 5 to 10

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

exhumation of high pressure metamorphic rocks during crustal extension in the d entrecasteaux region papua new guinea the d entrecasteaux islands of eastern papua new guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea floor spreading in the west woodlark basin the complexes are defined by mountainous domes 2500 m high of fault bounded high grade metamorphic rocks including eclogite facies intruded by 2 4 ma granodiorite plutons garnet clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730 900 c the jadeite component of omphacite indicates minimum pressure of 21 kbar suggesting depths of 70 km the metamorphic rocks have undergone widespread retrogression to amphibolite facies retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes p t conditions in the early stages of shear zone activity determined using the garnet biotite exchange thermometer and the gasp and grips barometers were 570 730 c and 7 11 kbar a second phase of re equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons one re equilibrated gneiss indicated maximum temperature of 730 c at estimated pressures of approximately 4 kbar this late high temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes both closely coincide spatially with high temperature metamorphic rocks the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement these observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the d entrecasteaux islands metamorphic core complexes

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

mantle compensation of active metamorphic core complexes at woodlark rift in papua new guinea in many highly extended rifts on the earth tectonic removal of the upper crust exhumes mid crustal rocks producing metamorphic core complexes these structures allow the upper continental crust to accommodate tens of kilometres of extension but it is not clear how the lower crust and underlying mantle respond also despite removal of the upper crust such core complexes remain both topographically high and in isostatic equilibrium because many core complexes in the western united states are underlain by a flat moho discontinuity it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating these processes should decouple upper crust extension from that in the mantle in contrast here we present seismic observations of metamorphic core complexes of the western woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension these core complexes are actively being exhumed at a rate of 5 10 km myr 1 and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density as indicated by low seismic velocities we conclude that at least in this case the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon but must involve mantle extension

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

anisotropy of elastic wave velocities in deformed shales part 1 experimental results elastic wave velocity measurements in the laboratory are used to assess the evolution of the microstructure of shales under triaxial stresses which are representative of in situ conditions microstructural parameters such as crack aperture are of primary importance when permeability is a concern the purpose of these experiments is to understand the micromechanical behavior of the callovo oxfordian shale in response to external perturbations the available experimental setup allows for the continuous simultaneous measurement of five independent elastic wave velocities and two directions of strain axial and circumferential performed on the same cylindrical rock sample during deformation in an axisymmetric triaxial cell the main results are 1 identification of the complete tensor of elastic moduli of the transversely isotropic shales using elastic wave velocity measurements 2 assessment of the evolution of these moduli under triaxial loading and 3 assessment of the evolution of the elastic ani

focal time analysis a new method for stratigraphic depth control of microseismicity and induced seismic eventsfocal time microseismic depth control

lu hf garnet geochronology applied to plate boundary zones insights from the u hp terrane exhumed within the woodlark rift abstract high pressure and ultra high pressure u hp metamorphic rocks occur in many of the world s major orogenic belts suggesting that subduction of continental lithosphere is a geologically important process despite the widespread occurrence of these rocks relatively little is known about the timescales associated with u hp metamorphism this is because most u hp terranes are tectonically overprinted and juxtaposed against rocks with a different history an exception to this are the late miocene u hp metamorphic rocks found in active metamorphic core complexes mcc in the woodlark rift of southeastern papua new guinea this region provides a rare opportunity to study the garnet lu hf isotopic record of u hp metamorphism in a terrane that is not tectonically overprinted in order to constrain the timing of garnet growth relative to the history of u hp metamorphism and the evolution of the woodlark rift lu hf ages were determined in conjunction with measurements of lu and major element zoning for garnets from three metamorphic rocks garnets from the three samples yielded different ages that instead of recording the spatial and temporal evolution associated with a single metamorphic event provide information on the timing of three separate plate boundary events the youngest lu hf age determined was 7 1 0 7 ma for garnets in a late miocene coesite eclogite the age is interpreted to record the time when a garnet bearing partial melt of the mantle crystallized within subducted continental lithosphere at u hp conditions the young lu hf age from the coesite eclogite is in contrast to a 68 3 6 ma lu hf age obtained on large 1 2 cm garnet porphyroblasts from within the pleistocene amphibolite facies shear zone carapace bounding exposures of u hp rocks in the d entrecasteaux islands this older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of australia during late mesozoic early cenozoic time in order to relate the u hp metamorphic rocks to the larger scale tectonic evolution of the woodlark rift a third lu hf age of 11 2 2 1 ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift while the tectonic implications of the lu hf age from the southeastern rifted margin are not fully known this result suggests that the eastern and western parts of the woodlark rift might have a different metamorphic history the garnet lu hf results demonstrate that prior to rifting the woodlark rift underwent a complex history of metamorphism involving garnet growth and recrystallization the ability of the lu hf system to date late miocene garnet growth and at the same time preserve latest cretaceous garnets within pleistocene shear zones in the woodlark rift demonstrate that lu hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals

anisotropic approximations for mudrocks a seismic laboratory study an experimental technique for determining the in situ elastic properties of mudrocks with horizontal alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic ti media each subsequent approximation adds one more velocity parameter and includes the previous as a special case these approximations are convenient and robust because of their close relationship to standard geophysical measurements there exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks in this study the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers the mechanical characterization performed at constant pore pressure is accompanied by detailed microscopic observations and analysis the method was used to obtain accurate elastic constants for five well defi

experimental and theoretical study of the anisotropic properties of shale abstract the effect of bedding plane orientations on shear strength and failure mechanisms of shale is investigated with an emphasis on the shear parameters of the bedding planes this study is based on direct shear tests conducted on directional shale samples obtained from formations which exhibit well developed stratification the tested cylindrical specimens were prepared by coring at 30 intervals from the direction of the bedding planes the results indicate that the anisotropy of the shear strength of shale can be readily observed due to the anisotropic effects of bedding plane orientation and normal stress the shear parameters such as strength cohesion and internal friction angle reach their maximum and the minimum values at 60 and 0 respectively where is the angle between the bedding planes and the coring orientation which is also the direction of normal stress during the laboratory tests it is demonstrated that the bedding planes are weak not only with regards to cementing strength but also with regards to friction three different shear failure modes were built up depending on the shearing angle 1 sliding failure across the bedding planes 2 sliding failure along the bedding planes and 3 sliding failure across the bedding planes combined with tensile splitting along the bedding planes an expression of the shear stress concentration factor of the direct shear model is derived to evaluate the anisotropy of shear strength

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision