effect of quartz inclusions on plastic flow in marble to investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks we produced synthetic marbles with a grainsize of 5 25 m containing quartz of similar size with volume fractions of 0 5 1 and 20 4 vol and between 3 to 9 porosity these rocks were deformed in triaxial compression at a strain rate of 3 x 10 5 s 1 confining pressure of 200 mpa and 600 c marbles containing 20 4 vol quartz were almost 5 times stronger than those containing no quartz the work hardening rate at given strain increased with quartz content marbles containing 20 4 vol quartz work hardened up to 10 strain whereas in single phase samples work hardening was negligible beyond 3 strain the dislocation density in the calcite increased with increasing quartz content the data are consistent with models for the strength of ceramic matrix composites or with the hypothesis that strength is increased by decreasing porosity

constraints on mantle evolution from 187os 188os isotopic compositions of archean ultramafic rocks from southern west greenland 3 8 ga and western australia 3 46 ga initial 187os 188os isotopic compositions for geochronologically and geologically well constrained 3 8 ga spinel peridotites from the itsaq gneiss complex of southern west greenland and chromite separates from 3 46 ga komatiites from the pilbara region of western australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle the measured compositions of 187os 188os 0 0 10262 2 from an olivine separate and 0 10329 3 for a spinel separate from 3 8 ga peridotite g93 42 are the lowest yet reported from any terrestrial sample the corrections for in situ decay over 3 8 ga for these low re os phases are minimal and change the isotopic compositions by only 0 5 and 2 2 for the spinel and the olivine respectively resulting in 187os 188os 3 8 ga 0 1021 0 0002 and 0 1009 0 0002 respectively these data extend direct measurement of os isotopic compositions to much earlier periods of earth history than previously documented and provide the best constraints on the os isotopic composition of the early archean terrestrial mantle analyses of pilbara chromites yield 3 46 ga mantle compositions of 0 1042 0 0002 and 0 1051 0 0002 these new data combined with published initial os isotopic compositions from late archean and early proterozoic samples are compatible with the mantle or at least portions of it evolving from a solar system initially defined by meteorites to a modern composition of 187os 188os 0 0 1296 0 0008 as previously suggested from peridotite xenolith data meisel et al 2001 the associated 187re 188os 0 0 435 0 005 thus chondritic 187os 188os compositions were a feature of the upper mantle for at least 3 8 billion years requiring chondritic re os ratios to have been a characteristic of the very early terrestrial mantle in contrast nonchondritic initial compositions of some archean komatiites demonstrate that os isotopic heterogeneity is an ancient feature of plume materials reflecting the development of variable re os mantle sources early in earth history the lower average 187os 188os 0 1247 for abyssal peridotites snow and reisberg 1995 indicate that not all regions of the modern mantle have evolved with the same re os ratio the relative sizes of the various reservoirs are unknown although mass balance considerations can provide some general constraints for example if the unradiogenic 187os 188os modern abyssal peridotite compositions reflect the prevalent upper mantle composition then the complementary high re os basaltic reservoir must represent 20 to 40 by mass of the upper mantle taken here as 50 of the entire mantle depending on the mean storage age the difficulties associated with efficient long term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly re depleted rather abyssal peridotites sample anomalous mantle regions the existence of 3 8 ga mantle peridotites with chondritic 187os 188os compositions and with os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer these observations require that any highly siderophile element rich component must have been added to the earth and transported into and grossly homogenized within the mantle by 3 8 ga either large scale mixing of impact materials occurred on very short 0 100 myr timescales or the interpretation preferred here the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at 3 8 to 3 9 ga

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

mechanics of kink bands during torsion deformation of muscovite aggregate abstract the growth geometry and bulk mechanical behavior of grain scale kink bands were investigated during layer parallel non coaxial deformation torsion of synthetic polycrystalline aggregates of muscovite at constant displacement rate 3 10 4 s 1 and various confining pressures cp 50 300 mpa temperatures t 300 800 c and finite shear strains 0 2 5 0 the kink instabilities were observed during elastic to plastic transition yield at finite shear strain of ca 0 2 the shear stress required to form the kinks is a function of pressure and temperature and range from ca 20 to 90 mpa within the experimental conditions the kink bands formed in non coaxial torsion tests are asymmetric involve strain partitioning show complex microstructures with increased deformation and have a limited lifetime as the structure is destroyed at ca 5 these results suggest a domain of pressure and temperature conditions where kink band structures should be common results obtained from torsion experiments are compared to those of layer parallel axial compression conducted with the same displacement rate at 600 c and 300 mpa for 5 10 and 17 bulk shortening combining microstructures viewed on surfaces parallel and perpendicular to the torsion plane a 3d geometrical model is constructed to discuss the kink mechanism the microstructural evolution and the mechanical behavior

an analytical benchmark with combined pressure and shear loading for elastoplastic numerical models we discuss the benchmark strategy to check the accuracy of elastoplastic numerical solutions based on a fully two dimensional analytical solution associated rate independent non hardening plasticity with von mises or tresca criteria is assumed throughout algorithms for integration of the rate equations strategies for stress updating over a time step and tangent operators are discussed the accuracy of a simple incremental algorithm as a function of the time step is discussed

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

argon 40 argon 39 age spectra of apollo 17 highlands breccia samples by laser step heating and the age of the serenitatis basin we have obtained high resolution 21 63 steps 40ar 39ar age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from apollo 17 samples collected from the pre imbrian highlands in the easternmost part of the serenitatis basin the samples include poikilitic melt rocks inferred to have been formed in the serenitatis basin forming impact aphanitic melt rock whose compositions vary and whose provenance is uncertain and granulite gabbro and melt clasts three of the poikilitic melts have similar age spectrum plateau ages 72395 96 3893 16 ma 2 72535 7 3887 16 ma 76315 150 3900 16 ma with a weighted mean age of 3893 9 ma which we interpret as the best age for the serenitatis basin forming impact published 40ar 39ar age spectrum ages of apollo 17 poikilitic melts are consistent with our new age but are much less precise two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of 3869 ma 72558 7 and 3743 ma 77135 178 plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 16 ma to 3937 16 ma and have probably been affected by the ubiquitous older clasts and by post formation heating impact events plateau ages from two of the aphanitic melt blobs and two granulites in sample 72255 fall in the narrow range of 3850 16 ma to 3869 16 ma with a weighted mean of 3862 8 ma two of the aphanitic melt blobs from 72255 have ages of 3883 16 ma and 3894 ma whereas a poikilitic melt clast of different composition from the serenitatis melts has an age of 3835 16 ma which is the upper limit for the accretion of 72255 these data suggest that either the aphanitic melts vary in age as is also suggested by their varying chemical compositions or they formed in the 72255 accretionary event about 3 84 3 85 ga and older relict material is responsible for the dispersion of ages in any case the aphanitic melts do not appear to be serenitatis products our age for the serenitatis impact shows on the basis of the isotopic age evidence alone that serenitatis is 20 25 ma and probably 55 60 ma older than imbrium 3870 ma and probably 3836 ma dalrymple and ryder 1993 noritic granulite sample 78527 has a plateau age of 4146 17 ma representing a minimum age for cooling of this sample in the early lunar crust so far there is no convincing evidence in the lunar melt rock record for basin forming impacts significantly older than 3 9 ga

video based nearshore bathymetry estimation in macro tidal environments abstract video based depth inversion through the linear dispersion relation for free surface waves using the cross spectral correlation analysis cbathy holman et al 2013 is applied for the first time in a highly energetic macro tidal environment in the south west of england at porthtowan this application of cbathy reveals two main issues 1 inaccurate depth estimations on inter camera boundaries when multiple cameras are used and 2 significantly less accurate depth estimates over the whole domain during spring tide compared to neap tide inaccuracies of around 35 of the local depths are found during spring tide these two issues are not only important in macro tidal environments the camera boundary issue has been reported in numerous video camera sites and the deviation in accuracy during tidal levels is a function of the tidal range in combination of the vertical camera position to overcome the two issues a camera boundary solution and a floating pixel solution meaning moving pixels in a horizontal plane as function of the tidal elevation are proposed here with the modifications cbathy is capable of estimating depths in the sub tidal zone with an accuracy in the order of 10 of the local depth irrespective to the local tidal regime however for the very upper part of the beach face less accurate results are found due to the reduced validity of the linear dispersion relation in that region due to the non linear behaviour of breaking waves and wave current interactions the improvements persist across all bathymetry survey campaigns at porthtowan and when compared to other well known argus video system sites the importance of the floating pixels is apparent

role of weak flaws in nucleation of shear zones an experimental and theoretical study this paper investigates the role of inherent weak flaws in the formation of plastic zones in deforming solids to understand the development of geological shear zones physical experiments were carried out on polymethylmethaacrylate pmma models containing single and multiple circular cylindrical flaws under compression maintaining plane strain condition models with single flaws show development of shear zones against a flaw in the form of conjugate sets with an average dihedral angle of 84 and oriented at an angle of 42 to the bulk compression direction the shear zones are generally tapered with increasing width away from the flaw in models with multiple flaws shear zones nucleated against individual flaws which propagated and coalesced with one another forming through going band like shear zones with inclination varying from 35 to 53 with the bulk compression direction with an increase in flaw concentration the through going shear zones defined persistent conjugate sets we applied the plane theory of elasticity for numerical simulations of ductile shear zones under the influence of a single circular weak flaw the pattern of shear zones yielded from numerical runs grossly matches with those observed in physical model experiments theoretical analysis demonstrates that the presence of a flaw promotes nucleation of shear zones at a bulk stress below the yield strength of matrix this critical stress is a non linear function of the flaw matrix competence contrast and decreases asymptotically with increasing competence contrast

strain localization by shear heating and the development of lithospheric shear zones abstract analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization such domains commonly occur in the lithosphere through variations in composition or mineral fabric here we use a 2d plane stress non newtonian viscous model to explore how strain localization develops from an initial isolated weak inclusion we use a temperature dependent rheological law for which the material weakens as a result of work done by shear converted to heat the progress of strain localization follows a power law growth that is strongly non linear and may be regarded as an instability although this localization mechanism is ultimately limited by thermal diffusion this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere shear zones in the lower crust are typically depicted as the downward continuation of faults we argue that the depth extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2d numerical experiments when applied to the rheological laws for common lithospheric minerals the combination of temperature and stress dependence provides a direct means of predicting the depth below which the localization instability does not occur for an olivine based rheology the maximum depth at which rapid localization is expected is in the range of 20 to 60 km depending on heat flow strain rate and water fugacity we apply our calculations to two major continental strike slip zones the san andreas fault and north anatolian fault and compare our predicted maximum localization depths with published seismological images strain localization in the lower crust requires a dry rheology comparable to plagioclase observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine

precise re os mineral isochron and pb nd os isotope systematics of a mafic ultramafic sill in the 2 0 ga onega plateau baltic shield abstract we present re os sm nd and pb pb isotope and trace element data for the konchozero sill a layered mafic ultramafic intrusion in the early proterozoic onega plateau one of the oldest continental flood basalt provinces on earth the sm nd and pb pb combined mineral and whole rock isochron ages of 1988 34 and 1985 57 ma for the sill coincide with the age of ferropicrites from pechenga the kola peninsula the lithostratigraphic chemical and isotope evidence suggest the derivation of pechenga lavas and the onega plateau volcanics from a single mantle plume peridotite and gabbro whole rock samples and primary ulvospinel and ilmenite mineral separates from the sill yield a re os isochron with a slope corresponding to an age of 1969 18 ma os t 0 61 5 9 this age is consistent with the other isotope data and indicates the closed system behavior of re and os in the rocks the peridotites and ulvospinel have high os concentrations 2 5 14 ppb and low 187 re 188 os ratios 0 35 1 1 thus allowing a more accurate determination of the weighted average initial 187 os 188 os of 0 1144 0 0019 2 pop os t 0 77 1 7 this value is lower than that determined by walker et al geochim cosmochim acta 61 1997 3145 3160 for the pechenga lavas os t 6 1 0 7 and implies a substantial os isotope heterogeneity in this ancient plume compared to the onega plateau primary basalt magma pechenga ferropicrites are relatively enriched in iron and ni have lower nb th n ratios 2 1 vs 1 1 and less radiogenic nd isotope compositions e nd t 3 1 and 1 4 respectively but share similar low radiogenic pb isotope characteristics 1 8 57 and 8 60 incorporation of small amounts 1 5 of outer core material into the hotter central part of the plume and subsequent contamination of the pechenga ferropicritic magmas with the 2 9 ga belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had

the early archaean itsaq gneiss complex of southern west greenland the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution geochemical and isotopic studies of small volumes of variably preserved 3600 ma rocks in gneiss complexes are crucial for documenting early earth history in the itsaq gneiss complex of the nuuk region west greenland there is dispute whether the granitic sensu lato orthogneisses dominating it are mainly products of a single ca 3650 ma crust formation super event or whether they formed in several unrelated events between ca 3850 and 3560 ma which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record pb pb sm nd rb sr reveals about continental crust formation and early terrestrial differentiation there is also debate whether some west greenland metasedimentary rocks with 12c 13c data interpreted as evidence for life are 3850 ma or only 3650 ma old establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life controversies have arisen because of different approaches taken by different workers specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data in this paper field observations and sampling from low strain zones where the origin and geological context of the rocks are best preserved and understood are closely integrated with u pb zircon dates and cathodoluminescence cl imagery of the zircons this approach shows that most single phase well preserved meta granitoid samples have simple zircon populations dominated by oscillatory zoned prismatic grains formed when their host magmas crystallized on the other hand migmatites and some strongly deformed banded gneisses have much more complex zircon populations the combined field evidence and zircon geochronology on the itsaq gneiss complex demonstrate that 1 some areas contain exposed orthogneisses formed during multiple magmatic thermal events between ca 3850 and 3560 ma and are not as suggested by kamber and moorbath 1998 dominated by ca 3650 ma granitoids containing abundant 3650 ma zircons inherited from cryptic unexposed older rocks 2 abundant 3750 ma granitoids are present which are locally well preserved 3 some water lain sediments reported as showing c isotope evidence for life were deposited as early as 3850 ma 4 the whole rock sm nd isochron approach fails to distinguish with any confidence 3650 ma from 3800 ma rocks 5 however it reinforces previous indications for markedly depleted 2 5 nd domains in the pre 3750 ma mantle

progressive mixing of meteoritic veneer into the early earth s deep mantle komatiites are ancient volcanic rocks mostly over 2 7 billion years old from the archaean era that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1 in particular the platinum group element pge contents of komatiites provide a unique source of information on core formation mantle differentiation and possibly core mantle interaction2 3 4 5 6 7 8 most of the available pge data on komatiites are from late archaean 2 7 2 9 gyr old or early proterozoic 2 0 2 5 gyr old samples here we show that most early archaean 3 5 3 2 gyr old komatiites from the barberton greenstone belt of south africa and the pilbara craton of western australia are depleted in pge relative to late archaean and younger komatiites early archaean komatiites record a signal of pge depletion in the lower mantle resulting from core formation this signal diminishes with time owing to progressive mixing in to the deep mantle of pge enriched cosmic material that the earth accreted as the late veneer during the early archaean 4 5 3 8 gyr ago meteorite bombardment

the tungsten isotopic composition of the earth s mantle before the terminal bombardment it has long been speculated that a late heavy bombardment of earth by meteoritic material replenished the mantle s budget of siderophile iron loving elements such as tungsten that were largely lost to the core during its segregation however evidence for this late veneer remains indirect and its influence has been much debated matthias willbold and colleagues present high precision tungsten isotope analyses of ancient greenland rocks and show that they have significantly higher 182w 184w ratios than modern terrestrial samples this finding is in good agreement with the expected influence of a meteoritic late heavy bombardment they speculate that both the tungsten isotope data and the observed decrease in 142nd 144nd neodymium ratios can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection many precious iron loving metals such as gold are surprisingly abundant in the accessible parts of the earth given the efficiency with which core formation should have removed them to the planet s deep interior1 one explanation of their over abundance is a late veneer a flux of meteorites added to the earth after core formation as a terminal bombardment that culminated in the cratering of the moon2 some 3 8 billion year old rocks from isua greenland are derived from sources that retain an isotopic memory of events pre dating this cataclysmic meteorite shower3 4 these isua samples thus provide a window on the composition of the earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet using high precision less than 6 parts per million 2 standard deviations tungsten isotope analyses of these rocks here we show that they have a isotopic tungsten ratio 182w 184w that is significantly higher about 13 parts per million than modern terrestrial samples this finding is in good agreement with the expected influence of a late veneer we also show that alternative interpretations such as partial remixing of a deep mantle reservoir formed in the hadean eon5 6 more than four billion years ago or core mantle interaction7 do not explain the w isotope data well the decrease in mantle 182w 184w occurs during the archean eon about four to three billion years ago potentially on the same timescale as a notable decrease in 142nd 144nd refs 3 and 6 we speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection

strain localization during high temperature creep of marble the effect of inclusions abstract the deformation of rocks in the earth s middle and lower crust is often localized in ductile shear zones to better understand the initiation and propagation of high temperature shear zones induced by the presence of structural and material heterogeneities we performed deformation experiments in the dislocation creep regime on carrara marble samples containing weak limestone or strong novaculite second phase inclusions the samples were mostly deformed in torsion at a bulk shear strain rate of 1 9 10 4 s 1 to bulk shear strains between 0 02 and 2 9 using a paterson type gas deformation apparatus at 900 c temperature and 400 mpa confining pressure at low strain twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at 0 1 0 2 peak shear stress at the imposed conditions is about 20 mpa which is 8 lower than the strength of intact samples strain progressively localized within the matrix with increasing bulk strain but decayed rapidly with increasing distance from the inclusion tip microstructural analysis shows twinning and recrystallization within this process zone with a strong crystallographic preferred orientation dominated by r and c slip in recrystallization induced weakening starts at local shear strain of about 1 in the process zone corresponding to a bulk shear strain of about 0 1 in contrast torsion of a sample containing strong inclusions deformed at similar stress as inclusion free samples but do not show localization the experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips recrystallization induced grain size reduction may only locally promote grain boundary diffusion creep accordingly the bulk strength of the twisted aggregate is close to or slightly below the lower isostress strength bound determined from the flow strength and volume fraction of matrix and inclusions

application of the linear dispersion relation with respect to depth inversion and remotely sensed imagery remote sensing methods have been developed to estimate bathymetry through the use of a theoretical relationship between wave speed and water depth known as the linear finite depth dispersion equation for surface gravity waves the authors describe a validation effort encompassing several hundred observations of wavenumber magnitude for sea swell frequencies obtained over a wide variety of conditions to investigate possible error sources resulting from the practical application of this relationship these wavenumber estimates were computed from pressure gauge signals using signal processing algorithms that can be equivalently applied to measurements of wave phase as imaged through remote sensors the major goal was to determine the accuracy of the dispersion relation while attempting to minimize errors associated with sensor positioning tidal variations and doppler shifts due to mean currents for water depths outside the surf zone the linear dispersion relation is highly accurate with average depth estimation errors on the order of 3 9 of the observed depth in shallower regions nominally less than 4 m for this field site where wave breaking is evident and nonlinear shoaling effects are more pronounced normalized depth errors of over 50 were commonly observed with most predictions being deeper than observations strong correlation between these bias errors and measured wave heights emphasizes the importance of accounting for wave amplitude in the calculation of shallow water phase speeds for depth estimation a simple depth correction is provided to allow for bathymetry estimation within the surf zone

photoelastic and numerical investigation of stress distributions around fault models under biaxial compressive loading conditions abstract this paper presents the results of a photoelastic and numerical study of stress distributions contours and trajectories around fault models of various geometries submitted to a biaxial compressive load it aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well known uniaxial compressive load case stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field and they can be interpreted by particular shallow tension compression situations at depth fractures can be reactivated or can dilate under conditions of triaxial compression and behaviour is essentially controlled by a high stress ratio high 3 1 ratio in spite of the potential importance of such stress states on fracture permeability and fluid flow and although they are frequently found at depth in a reservoir context such stress conditions are poorly investigated particularly in terms of stress perturbations the presented analogue experiments consisted of compression tests done on polymethylmethacrylate pmma plates this material has mechanical properties comparable to those of brittle rocks in the upper crust and presents birefringence the samples contained open defects acting as faults and the stress trajectories around these faults were investigated using a photoelastic device comparable numerical experiments were realised with a finite element code franc 2d using frictionless fault models first the effect of an increasing biaxial compressive load ratio 3 1 on stress trajectories around an isolated open defect was explored it was shown that the stress trajectories were drastically modified when 3 1 increased from 0 2 to 0 4 this result being consistent with previous studies in particular when 3 1 was superior or equal to 0 4 external isotropic points around where trajectories diverged called repulsive isotropic points appeared near the tips but away from the defects they tended to move away from the defects towards the main load direction when 3 1 increased the described isotropic points were points of stress trajectory divergence i e points where stress decreased implying that zones around them were strongly unfavourable to shear reactivation second stress trajectories around fault models of various geometries oblique isolated defect dilational jog compressive jog and complex patterns were studied the applied biaxial compressive load ratio being 0 7 these biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes it enabled new stress trajectory geometries to be described and interpreted in particular numerous external repulsive isotropic points were observed and defect tips were shown to be zones of high convergence of stress trajectories furthermore in contrast to the uniaxial compressive load case stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load for both jog types the centre of the overlapping zone and the areas along the wall of a jog defining fault and facing the overlapping tip of the other fault proved to be zones of low mean stress implying that fluids may migrate towards them from the tips of the faults in response to mean stress gradients furthermore the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation which may facilitate fluids transfer and storage it was also observed that for both the uniaxial and biaxial compressive loads isotropic zones were localised at the acute angle between branching defects

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

are isolated stable rigid clasts in shear zones equivalent to voids abstract particles in shear enclose important information about a rock s past and can potentially be used to decipher the kinematic history and mechanical behavior of a certain outcrop or region isolated rigid clasts in shear zones often exhibit systematic inclinations with respect to the shear plane at small angles tending towards the instantaneous stretching direction of the shear zone this shape preferred orientation cannot be easily explained by any of the analytical theories used in geology it was recently recognized that a weak mantle surrounding the clast or a slipping clast matrix interface might be responsible for the development of the observed inclinations physical considerations lead us to conjecture that such mantled rigid clasts can be effectively treated as voids that are not allowed to change their shape the resulting equivalent void conjecture agrees well with numerical and field data and has the following important geological implications i clasts in shear zones can have stable positions in simple shear without the requirement of an additional pure shear component ii the stable orientation can be approached either syn or antithetically hence the clast can rotate against the applied shear sense iii the strain needed to develop a strong shape preferred orientation is small 1 and therefore evaluations based on other theories may overestimate strain by orders of magnitude iv the reconstruction of far field shear flow conditions and kinematic vorticity analysis must be modified to incorporate these new findings

role of pressure and temperature in inclusion induced shear localization an analogue experimental approach abstract using analogue model experiments this study investigates the effects of lithostatic pressure and temperature in controlling the pattern of shear localization around rigid inclusions compression experiments were conducted on polymethylmethaacrylate pmma by varying confining pressure p 30 70 mpa homologous temperature t o 0 67 0 80 corresponding to room temperature 80 c and axial strain yield 15 in a triaxial setup mechanical data showed temperature has a greater influence on the bulk yield strength and steady state flow of the analogue material than confining pressure increasing confining pressures multiply the shear bands in number and their overall pattern becomes progressively more complex leading to composite band structures on the other hand under ambient pressure increasing temperature results in a transition from incipient high strain zones to shear bands with sharp boundaries further increase in temperature switches the mode of shear localization sharp to diffuse type we finally show the inclusion induced shear localization as a two stage process and provide a micro mechanical explanation for the p t dependent shear band patterns attributed to three mechanically distinct domains of the inclusion matrix interface compression extensional and shear localization

os isotope systematics of komatiitic basalts from the vetreny belt baltic shield evidence for a chondritic source of the 2 45 ga plume the vetreny belt in the southeast baltic shield is a large volcano sedimentary basin containing a 4 to 8 km thick sequence of basaltic to komatiitic lavas which were erupted 2 45 ga ago in a continental rift setting during the interaction of a mantle plume and the archean continental crust of the karelian granite greenstone terrane re os isotope data for olivine cumulate samples and chromite separates from victoria s lava lake and golets flow 3 define isochrons with ages of 2387 57 and 2432 34 ma respectively these ages are in good agreement with the previously reported and new nd pb and u pb zircon ages these data coupled with the evidence for immobile behavior of the re and os indicate that the re os system remained closed since the lava eruption the weighted average initial 187os values range between 0 43 0 10 for golets flow 3 and 0 07 0 13 for the lava lake a single chromite separate from golets flow 1 has a 187os t of 0 06 0 15 nd pb isotope and lithophile trace element data for the vetreny belt komatiitic basalts were used to monitor the effect of crustal contamination on the os isotope system and to estimate the initial os isotope composition of the vetreny plume source to be approximately chondritic with a 187os t of 0 9 this implies that the mantle source evolved with a long term nearly chondritic re os ratio the data provide further evidence that by the end of the late archean the earth s upper mantle was well homogenized with respect to the highly siderophile elements added during the accretion of a late veneer on a time scale of 1 ga

flow mixing object matrix coherence mantle growth and the development of porphyroclast tails abstract the kinematic stability fields of common porphyroclast types have been defined using a computer program that simulates low reynolds number newtonian shearing flow in the vicinity of a rigid spherical object the principal variables governing the geometry of porphyroclast tails at a given finite shear strain are 1 porphyroclast matrix coherence and 2 mantle recrystallization history both of which affect the extent to which mantle material becomes mixed with the matrix as deformation progresses

chromite in the early archean akilia association ca 3 800 m y ivisartoq region inner godthabsfjord southern west greenland

finite element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting the development of shear zones initiating on random weaker initial perturbations is modelled numerically for low deborah number viscoelastic materials considering the influence of effective viscosity contrast power law rheology strain softening and different imposed bulk deformation geometries ranging from pure to simple shear conjugate shear zones initiate at 90 to one another and rotate with increasing bulk deformation the basic pattern not being markedly influenced by the vorticity of imposed deformation the rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power law rheology however the most marked effect is observed for strain softening behaviour where rapid propagation produces straighter and narrower shear zones the localisation of strain is reflected in a correspondingly heterogeneous stress distribution in particular mean stress or pressure is higher in the extending near planar weaker zones of localised shear melting of gneissic or pelitic compositions is pressure dependent with free water present increased pressure promotes melting whereas the opposite is true for water absent melting for water present conditions a positive feedback could develop between localised shearing increased pressure and partial melting this is potentially more effective in concentrating melt in shear zones than shear heating where melt related softening has a negative feedback effect

the study of molybdenites through the187re 187os chronometer we determined the re and os concentrations in 11 molybdenites of various ages 30 2700 m y and from various areas no common osmium has been detected and os was found to be purely radiogenic therefore by comparing the concentrations we were able to determine 187 re 187 os ages for most samples these ages are in good agreement with those measured in surrounding rocks and probably correspond to the age of mineralization however two or three re os ages seem unreliable being probably related to some rhenium leaching through later metamorphic and or hydrothermal events this shows that the petrographic characteristics of ores and the type of crystallization of mos 2 are important in choosing samples suitable for dating re and os determinations combined with careful petrographic investigations may give important clues on the genesis and evolution of molybdenum ores

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

chemical composition of archaean komatiites implications for early history of the earth and mantle evolution abstract estimates of the chemical composition of the archaean mantle derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data support a model of a multistage heterogeneous accretion history of the earth and synchronous core formation 4 6 ga ago most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for v ti and ca al depletion of vanadium is likely due to its partial incorporation into the iron core whereas fractionation of ca al observed in archaean al undepleted komatiites 1 20 times chondrites and in some modern fertile spinel lherzolite xenoliths 1 15 times chondrites could be due to small amounts of garnet rich in al but poor in ca segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the earth however this process may have had only a small effect on the overall chemical composition of the upper mantle simultaneous occurrence of early archaean al undepleted al ti chondrites and al depleted al ti 0 5 chondrites and depletion of sc and heavy ree peridotitic komatiites in the barberton area s africa and late archaean newton township canada argue against derivation of peridotitic komatiites from a circum global magma ocean garnet separation from a mantle diapir which intersects the solidus at great depth 200 km in a hotter early archaean mantle could explain the chemical characteristics of al depleted komatiites alternatively these two types of komatiites could have been derived from different layers in a fractionated mantle a limited amount of hf isotope data for archaean komatiites seems to suggest that both mechanisms are important this chemically and minerallogically layered mantle if it existed was homogenized by mantle convection after early archaean times constant p 2 o 5 tio 2 ni mg co mg fe mg ratios siderophile lithophile and pge abundances estimated for the mantle sources of komatiites from archaean to modern times strongly argue against continuous growth of the earth s core since the early archaean extensive crustal contamination might have been involved in the generation of archaean early proterozoic siliceous high magnesian basalts with boninite affinity however involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

power law viscous materials for analogue experiments new data on the rheology of highly filled silicone polymers abstract the selection of appropriate analogue materials is a central consideration in the design of realistic physical models we investigate the rheology of highly filled silicone polymers in order to find materials with a power law strain rate softening rheology suitable for modelling rock deformation by dislocation creep and report the rheological properties of the materials as functions of the filler content the mixtures exhibit strain rate softening behaviour but with increasing amounts of filler become strain dependent for the strain independent viscous materials flow laws are presented while for strain dependent materials the relative importance of strain and strain rate softening hardening is reported if the stress or strain rate is above a threshold value some highly filled silicone polymers may be considered linear visco elastic strain independent and power law strain rate softening the power law exponent can be raised from 1 to 3 by using mixtures of high viscosity silicone and plasticine however the need for high shear strain rates to obtain the power law rheology imposes some restrictions on the usage of such materials for geodynamic modelling two simple shear experiments are presented that use newtonian and power law strain rate softening materials the results demonstrate how materials with power law rheology result in better strain localization in analogue experiments

melt bearing shear zones analogue experiments and comparison with examples from southern madagascar abstract analogue model experiments were conducted to investigate the influence of irregularly distributed weak sites in localising strain as an aid to understanding shear zone development in partially molten rocks the very weak inclusions consisted of vaseline in a homogeneous matrix of paraffin wax which has a power law viscous rheology boundary conditions were those of pure shear at constant natural strain rate and confining stress 3 the inclusions were initially perfect cylinders with axes parallel to the intermediate bulk strain axis y conjugate shear zones nucleate on the inclusions and link up to form an anastomosing pattern of high strain zones of concentrated shear surrounding much more weakly deformed pods of near coaxial strain the zones initiate at angles near 45 to the bulk shortening axis z but stretch and rotate towards the x axis with increasing bulk strain all inclusions nucleate shear zones so that with increasing development of the anastomosing pattern weak material occurs only within the high strain zones the restriction of migmatite leucosomes to shear zones in natural examples could also reflect a corresponding control of melt on the sites of shear zone nucleation rather than implying accumulation from the surrounding wall rock the model geometry is very similar to that observed in small scale shear zones in migmatites of southern madagascar elongate zones rich in weak inclusions originally either perpendicular or at 45 to the z axis were also modelled for direct comparison with the regional scale geometry of the pan african high grade shear zones on madagascar

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

age significance of u th pb zircon data from early archaean rocks of west greenland a reassessment based on combined ion microprobe and imaging studies comment

anatomy of an early archean gneiss complex 3900 to 3600 ma crustal evolution in southern west greenland the early archean complex of southern west greenland consists of polyphase tonalitic trondhjemitic granodioritic ttg and granitic amitsoq gneisses with inclusions of volcanic and sedimentary rocks gabbros and ultramafic rocks in this complex rocks of similar appearance and composition were found to be of different ages by u pb zircon dating the amitsoq gneisses comprise 3870 3820 3810 3760 3730 3700 and 3625 ma ttg and 3660 3650 and 3625 ma granites and their inclusions belong to several supracrustal sequences with a similar spread of ages these results show that the complex grew by episodic addition of new ttg and welding together of rocks of different ages a possible plate tectonics scenario is as follows melting of subducted mafic oceanic crust formed 3700 ma microcontinents consisting of ttg suites with predominantly mafic inclusions at 3650 ma collision between microcontinents caused crustal thickening high grade metamorphism and emplacement of leucogranites at 3625 ma subduction at the edge of the 3625 ma continental mass created a new crustal addition comprising both ttg and granite while granites were emplaced into the 3625 ma continental mass

probing the mantle past until recently geologists generally believed that all evidence of the first few hundred million years of earth s 4 56 billion year history had been erased by mantle convection and plate tectonics although planetary analogs with mars and the moon suggest that earth likely experienced large scale melting soon after accretion vigorous convection in the early mantle due to higher heat production from radioactive decay core formation and large impacts followed by 4 billion years of crustal recycling at plate boundaries was thought to have efficiently mixed and homogenized the mantle obliterating all signs of earth s youthful exuberance that all changed with the discovery of zircon grains more than 4 0 billion years old from the jack hills in western australia in 1983 1 followed by the demonstration in 2003 that geochemical signals linked to decay of short lived radioactive isotopes are preserved in some ancient crustal terrains the recognition of 142nd isotopic variations produced by decay of 146sm with a half life of 103 million years in 3 8 billion year old terrestrial rocks 2 proved that traces of earth s earliest history are still legible in the rock record on page 1065 of this issue touboul et al 3 report highly precise measurements of the isotopic composition of tungsten w in 2 8 billion year old komatiites a type of basaltic rock known to be derived from source regions deep within the mantle the results help to produce a clearer picture of the dynamic processes involved in the formation of earth s early mantle