temperature dependence of shear wave attenuation in partially molten gabbronorite at seismic frequencies summary torsion oscillatory deformation experiments have been performed at high temperatures 600 1170 c and over a wide range of low frequencies 20 2 10 3 hz on fine grained gabbronorite samples from the oman ophiolite in order to determine the shear wave attenuation as a function of temperature and melt fraction the specimens have a small and uniform grain size 0 25 0 3 mm and do not contain secondary hydrated minerals measurements of internal friction q 1 w ere performed using a forced oscillatory torsion apparatus at small strains 10 7 and with increasing small temperature steps to reduce thermal microcracking the general dependence of q 1 to frequency is q 1 w here is the angular velocity of forced oscillations and is an empirical exponent below the melting temperature 1050 c has average values of 0 15 at low frequency 0 5 hz and 0 06 at higher frequency above the melting temperature has average values of 0 22 at low frequency and 0 02 at higher frequency this frequency dependence of q 1 is attributed to a viscoelastic behaviour due to the diffusion controlled grain boundary sliding and partially to the squirt flow of the meltphase wetting grain boundaries the onset of melting is associated with a markedly higher q 1 and a stronger dependence of q 1 on temperature the melt related mechanical dissipation process could be a melt squirt flow the characteristic frequency for the melt squirt flow is m 0 15 300 hz when the melt pocket aspect ratio is 10 3 10 2 around the melting temperature the internal friction can be approximated by an experimental power law q 1 a 1 d 1 exp ea rt with 0 08 a 34 72 s m and ea 873 kj mol 1

thermal models of oceanic crustal accretion linking geophysical geological and petrological observations 1 models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges we have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables in particular the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large scale compositional stratification within the crust such as that observed in the oman ophiolite controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested including the many sills and gabbro glacier models both gabbro glacier and many sills models can broadly match the geophysical observations but the preferred model is a hybrid where the uppermost 25 or more of the lower crust forms in a gabbro glacier and the lowermost 25 or more of the lower crust forms by crystallization in situ the success of all models is dependent on the strength and distribution of hydrothermal cooling

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

the influence of microstructure on seismic wave speed anisotropy in the crust computational analysis of quartz muscovite rocks summary we study the influence of microstructural variables on seismic wave speed anisotropy in crustal rocks the bulk elastic properties and corresponding wave velocities are calculated for synthetic rock samples with varying amounts of muscovite and quartz different muscovite and quartzgrainorientationsandvaryingspatialdistributionsofthemuscovitegrainstoinvestigate the sensitivity of seismic wave speed anisotropy on these characteristics the asymptotic expansionhomogenizationmethodcombinedwithfiniteelementmodellingisusedtocalculate bulk stiffness tensors for representative rock volumes and the wave velocities are obtained from these tensors using the christoffel equation the aim of this paper is to 1 demonstrate how wave speeds computed from the rigorous asymptotic expansion homogenization method compare with those generated using stiffness tensors derived from commonly applied analytic estimates and 2 explore how different microstructural variables influence seismic wave speeds our results show that the muscovite grain orientations have a significant influence on the wave speeds increasing the modal fraction and alignment of muscovite grains leads to greater seismic anisotropy of the rock the p wave speed at an incidence angle of 45 between the

pore fluid effects on seismic velocity in anisotropic rocks a simple new technique predicts the high and low frequency saturated velocities in anisotropic rocks entirely in terms of measurable dry rock properties without the need for idealized crack geometries measurements of dry velocity versus pressure and porosity versus pressure contain all of the necessary information for predicting the frequency dependent effects of fluid saturation furthermore these measurements automatically incorporate all pore interaction so there is no limitation to low crack density the velocities are found to depend on five key interrelated variables frequency the distribution of compliant crack like porosity the intrinsic or noncrack anisotropy fluid viscosity and compressibility and effective pressure the sensitivity of velocities to saturation is generally greater at high frequencies than low frequencies the magnitude of the differences from dry to saturated and from low frequency to high frequency is determined by the compliant or crack like porosity predictions of s

olivine antigorite orientation relationships microstructures phase boundary misorientations and the effect of cracks in the seismic properties of serpentinites antigorite bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones here we present a detailed study of the microstructures and seismic properties in a sample of antigorite olivine schist previously studied by kern et al 1997 2015 we have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections microstructures indicate that deformation is localized in the bands with high antigorite fractions resulting in strong crystallographic preferred orientations cpos with point maxima of poles to 100 parallel to lineation and poles to 001 to the foliation normal olivine cpo suggests deformation under high temperature and low stress with a 100 fiber texture the cpo strength varies with grain size but is strong even in fine grained antigorite and larger grains tend to display higher internal misorientation orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis 100 ol 001 atg being more frequent than 001 ol 010 atg two new orientation relationships between olivine and antigorite have been documented seismic velocities decrease while anisotropy increases with increasing antigorite modal content antigorite grain shape has a weak effect on seismic velocities but it is important on anisotropy comparison between cpo derived seismic velocities using voigt reuss hill averages and geometric mean only showed good agreement in 1 3 of experimental velocities if the crack porosity of 1 63 measured experimentally at 600 mpa was used in the self consistent model with two crack orientations with planes normal to z and y good match with all experimental velocities was achieved the self consistent model implies important crack porosity in the foliation plane at 600 mpa that reduces vp normal to the foliation by 0 3 km s

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

full waveform tomography for radially anisotropic structure new insights into present and past states of the australasian upper mantle article i nfo we combine spectral element simulations and adjoint techniques in a non linear full seismic waveform tomography for the radially anisotropic structure of the australasian upper mantle our method allows us to explain 30 s waveforms in detail and it yields tomographic images with locally 2 lateral resolution in the course of 19 conjugate gradient iterations the total number of exploitable waveforms increased from 2200 to nearly 3000 the final model amsan 19 thus explains data that were not initially included in the inversion this is strong evidence for the effectiveness of the inversion scheme and the physical consistency of the tomographic model amsan 19 confirms long wavelength heterogeneities found in previous studies and it allows us to draw the following inferences concerning the past and present states of the australian upper mantle and the formation of seismic anisotropy 1 small scale neutral to low velocity patches beneath central australia are likely to be related to localised paleozoic intraplate deformation 2 increasing seismic velocities between the moho and 150 km depth are found beneath parts of proterozoic australia suggesting thermochemical variations related to the formation and fragmentation of a centralian superbasin 3 radial anisotropy above 150 km depth reveals a clear ocean continent dichotomy we find strong vshnvsv beneath the coral and tasman seas the anisotropy is strongest at the top of the inferred asthenospheric flow channel where strain is expected to be largest radial anisotropy beneath the continent is weaker but more variable localised patches with vshbvsv appear in accord with small scale intraplate deformation 4 the ocean continent dichotomy disappears gradually between 150 and 250 km depth where the continental lithospheric mantle and the oceanic asthenosphere pass into the underlying convecting mantle 5 significant anisotropy exists below 250 km depth its character can be explained by sublithospheric small scale convection and a change in the dominant glide system of olivine

deformation and melt transport in a highly depleted peridotite massif from the canadian cordillera implications to seismic anisotropy above subduction zones abstract seismic anisotropy in subduction zones results from a combination of various processes although it depends primarily on the orientation of olivine in response to flow the presence of water and melt in the wedge may modify the deformation of olivine the melt distribution also influences anisotropy direct observations of the deformation and melt rock interactions in a strongly depleted spinel harzburgite massif from the cache creek terrane in the canadian cordillera allow evaluating the relative contribution of each process structural mapping shows that this massif has recorded high temperature low stress deformation high degrees of partial melting and synkinematic melt rock interaction at shallow depths

microstructures and crystallographic preferred orientation of anorthosites from oman ophiolite and the dynamics of melt lenses 1 microstructures and crystallographic preferred orientation cpo of anorthosite samples interlayered in the upper and lower gabbro sections in the oman ophiolite were analyzed in this paper in the anorthosites registering the dynamics of the melt lenses foliation is flat lying and starts to develop a few meters below the root zone of the sheeted dike complex rzsdc microstructures and cpo of these rocks were developed in response to four different mechanisms 1 density controlled settling of plagioclase on the lens floor 2 deposition of anorthosites related to convection currents 3 melt compaction and 4 uncompacted melt accumulation in these anorthosites the poles to 010 of plagioclase are parallel to the flow plane of convection whereas the 100 axes and poles to 001 express the convection flow direction and the axis of convection rolls respectively the effect of subsidence of melt lens floor is recorded immediately below the rzsdc and is characterized by the rapid but progressive development of dipping foliation and lineation reflecting the increase of deformation downsection the degree of foliation and cpo development in the anorthosites is directly related to the distance of the center of the melt lenses before the subsidence starts despite the uncertain origin of the anorthosites from the lower gabbro section all the samples lost the magmatic microstructural characteristics and presently are reequilibrated aggregates however they still preserve plagioclase cpo where some of these patterns present similarities with the anorthosites from the upper gabbro section but no evidence of intracrystalline deformation under high temperatures

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

reinterpretation of seismic reflection data over the east pacific rise multichannel seismic reflection data over the axial region of the east pacific rise are depth migrated using detailed velocity information from laboratory measurements of ophiolite samples the data are from the lamont doherty geological observatory s line 17 shot across the rise at 9 sup 0 n the migrated data are interpreted to show the structure of the seafloor and what we believe is the magma chamber roof the polarity and apparent root mean squared velocity of the magma chamber roof are asymmetric with respect to the topographic axis of the ridge the asymmetry is probably real and not an artifact of data collection modeling of the magnetic data gathered along the line shows that spreading has not been proceeding normally a reasonable explanation for the asymmetry is the possibility that the line intersects a transform fault or an abandoned ridge segment near the ridge axis the shape of the roof reflection is convex upward with an approximate slope of 10 sup 0 and a width of 4 km if extrapolated symmetrically to the other side of the ridge axis the magma chamber roof event is consistent with the funnel shaped chamber proposed by pallister and hopson 1981 for the samail ophiolite if the chambermore roof steepens rapidly beyond the extent of the reflection it would be consistent with the mush filled model of sleep 1975 1978 and dewey and kidd 1977 less

microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers abstract flow laws for high temperature creep of olivine plagioclase and diabase are used to place constraints on the rheology of partially molten lower oceanic crust this analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene extrapolation of experimental flow laws indicates that at temperatures above 1100 c and stresses less than 10 mpa olivine may be the weakest phase in rocks with gabbroic composition by accounting for variations in the melt fraction 0 10 and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep calculated effective viscosities range from 10 15 to 10 19 pa s based on observations of the geometry of the partially molten zone beneath the east pacific rise and the microstructural and experimental constraints we favor a value of 10 18 pa s this value approaches estimates for the viscosity of the upper mantle beneath ridge axes but is significantly higher than previously suggested for the partially molten lower crust such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution however the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2d sheet like fashion

climate connections between the hemisphere revealed by deep sea sediment core ice core correlations abstract correlation of southern ocean deep sea sediment core records with ice core records of polar climate delineates with unprecedented detail the relationship between high latitude climate and the ocean s thermohaline circulation over the last 80 000 years our observations suggest that while north atlantic deep water variability manifests itself clearly in southern ocean nutrient proxy records over periods as short as 500 yr this deep water variability did not promote a direct link between climate variability in the high latitudes of the two hemispheres on millennial timescales in particular the proxy records indicate that on average northern hemisphere climate fluctuations lagged those of the southern hemisphere by 1500 yr

subduction of oceanic asthenosphere evidence from sub slab seismic anisotropy 1 the oceanic asthenosphere is characterized as a low viscosity channel down to 200 300 km depth separating the cold lithosphere from above and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins while subduction of tectonic plates in convergent margins is well recognized the fate of oceanic asthenosphere remains enigmatic we demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub slab shear wave splitting patterns where the fast splitting direction changes from predominantly trench parallel or sub parallel under relatively steep subduction zones to frequently trench normal under shallow subduction zones to explain the observed splitting time the thickness of the subducted asthenosphere is estimated to be 100 50 km

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

elastic properties of anisotropic synthetic calcite muscovite aggregates 1 a set of synthetic aggregates containing mixtures of calcite and muscovite were compacted with varying uniaxial loads ranging from 20 mpa up to 400 mpa their elastic properties have been measured using compressional and shear waves in confining hydrostatic pressures up to 475 mpa measured seismic velocities are shown to depend on 1 the ratio of calcite to muscovite 2 the uniaxial load used during sample manufacturing and 3 the porosity amount prior to velocity measurements the matrix framework may also affect the seismic velocities but this effect is not easily quantifiable in general measured seismic velocities decrease with increasing muscovite content and porosity elastic properties have been calculated based on texture measurements which were obtained by neutron diffraction the calculated velocities are based on the calcite and muscovite single crystal elastic tensors their orientation distribution functions and their modal fractions a large discrepancy is apparent between calculated and measured velocities where the former always overestimate the actual velocities shear waves display less of a difference between calculated and measured values than do compressional waves indicating that waves propagating by shearing are less affected by sample porosity and matrix framework a better agreement between measured and calculated seismic velocities arises when the initial porosity is taken into account using a differential effective medium model seismic anisotropy is evident from both measured and calculated velocities and is low for samples containing pure calcite but becomes prominent as the muscovite concentration increases the intensity of anisotropy further depends on the uniaxial load used during sample compaction and the initial porosity

seismic propagation across the east pacific rise finite difference experiments and implications for seismic tomography we use a full waveform acousto elastic finite difference technique to investigate seismic propagation across the east pacific rise at 9 30 n for a two dimensional velocity model based on that proposed by vera and others 1990 the primary feature of the model is an upper crustal low velocity region corresponding to the axial magma chamber which includes a small magma body located 1 6 km beneath the seafloor at the rise axis the high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber a qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9 30 n shows generally good agreement for paths that cross the rise axis the first arrival in the finite difference solutions diffracts above the magma chamber this phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment in such cases the first arrival with significant energy is a diffraction from below the magma chamber a high amplitude moho turning pmp phase which results from the large velocity change across the moho beneath the rise axis is apparent in both the finite difference solutions and the observations ray theoretical calculations of the paths of the diffracted arrivals are very unstable and for the diffractions above the magma chamber no solution can be found with a single precision algorithm synthetic delay time inversions using an approximate ray tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body synthetic inversions of spectral estimates of t show that q 1 models are compromised not only if the ray paths are incorrect but also if t estimates include significant contributions from more than one phase deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

seismic anisotropy and shear wave splitting associated with mantle plume plate interaction geodynamic simulations of the development of lattice preferred orientation in the flowing mantle are used to characterize the seismic anisotropy and shear wave splitting sws patterns expected for the interaction of mantle plumes and lithospheric plates models predict that in the deeper part of the plume layer ponding beneath the plate olivine a axes tend to align perpendicular to the radially directed plume flow forming a circular pattern reflecting circumferential stretching in the shallower part of the plume layer plate shear is more important and the a axes tend toward the direction of plate motion predicted sws over intraplate plumes reflects the asymmetric influence of plate shear with fast s wave polarization directions forming a pattern of nested u shapes that open in the direction opposing both plate motion and the parabolic shape often used to describe the flow lines of the plume predictions explain sws observations around the eifel hot spot with an eastward not westward moving eurasian plate consistent with global studies that require relatively slow net westward rotation of all of the plates sws at the hawaiian hot spot can be explained by the effects of plume plate interaction combined with fossil anisotropy in the pacific lithosphere in ridge centered plume models the fast polarization directions angle diagonally toward the ridge axis when the plume is simulated as having low viscosity beneath the thermal lithosphere such a model better explains sws observations in northeast iceland than a model that incorporates a high viscosity layer due to dehydration of the shallow most upper mantle

structural contribution from the oman ophiolite to processes of crustal accretion at the east pacific rise a comprehensive model for the activity of the elementary accretion segment at fast spreading ridges relies on integration of structural data from the oman ophiolite and geophysical results from the east pacific rise epr around 9 n which are of comparable size and spreading rates the axial melt lens at shallow crustal level provides a link between deval segmentation at the seafloor and a lower melt sill at moho level imaged at the epr as a crustal melt zone cmz and mapped in oman as the moho transition zone mtz both are attached to a mantle upwelling at the epr and to a frozen diapir in oman the physical link between diapiric mantle uprising at the moho and devals segmentation at the seafloor is the melt being injected from the mantle into the lower mtz ponding there and then being released by powerful injections into the upper melt lens the magma chamber covers the diapir at a distance of 5 km from the ridge axis

seismic velocity anisotropy in mica rich rocks an inclusion model summary we calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method dem spheroidal biotite crystals with their c axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent all crystals are aligned with their c axes parallel to the symmetry axis of the effective homogeneous medium which shows transverse isotropy the effect of crystal shape on anisotropy was studied by changing the aspect ratio the ratio between the minor axis and the major axis of the spheroid from 0 01 flat spheroid to 1 sphere the s velocity anisotropy becomes large as the crystal shape becomes flat whereas the p velocity anisotropy shows only small changes with changes of the crystal shape in particular biotite generates large s wave anisotropy and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller when the volume ratio of the mica mineral is large the p wave phase velocity surface shows considerable deviation from the ellipse and the sv wave phase velocity surface forms a large bulge and crosses the sh wave phase velocity surface singularity in the plane including the symmetry axis these results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy crack or pore shape affects the p velocity more than the s velocity we also calculated thomsen s anisotropic parameters e and as functions of the crystal aspect ratio and the mica volume ratio

modeling the impact of melt on seismic properties during mountain building initiation of partial melting in the mid lower crust causes a decrease in p wave and s wave velocities recent studies imply that the relationship between these velocities and melt is not simple we have developed a modelling approach to assess the combined impact of various melt and solid phase properties on seismic velocities and anisotropy the modelling is based on crystallographic preferred orientation cpo data measured from migmatite samples allowing quantification of the variation of seismic velocities with varying melt volumes shapes orientations and matrix anisotropy the results show non linear behaviour of seismic properties as a result of the interaction of all of these physical properties which in turn depend on lithology stress regime strain rate pre existing rock fabrics and pressure temperature conditions this non linear behaviour is evident when applied to a suite of samples from a traverse across a migmatitic shear zone in the seiland igneous province northern norway critically changes in solid phase composition and cpo and melt shape and orientation with respect to the wave propagation direction can result in huge variations in the same seismic property even if the melt fraction remains the same a comparison with surface wave interpretations from tectonically active regions highlights the issues in current models used to predict melt percentages or partially molten regions interpretation of seismic data to infer melt percentages or extent of melting should therefore always be underpinned by robust modelling of the underlying geological parameters combined with examination of multiple seismic properties in order to reduce uncertainty of the interpretation this article is protected by copyright all rights reserved

deformation in a partially molten mantle constraints from plagioclase lherzolites from lanzo western alps abstract we studied the deformation in a partially molten mantle through detailed petrostructural analysis of plagioclase lherzolites from the northwestern part of the central lanzo massif these peridotites display a pervasive anastomosed network of millimetre to centimetre scale plagioclase rich bands which grades locally into a planar layering marked by an intercalation of plagioclase rich and olivine rich layers up to tens of centimetres wide both the anastomosed and the planar layering are subparallel to the deformation fabric characterized by a foliation defined by the shape preferred orientation of olivine and by crystal preferred orientation cpo of olivine orthopyroxene and clinopyroxene this parallelism the coexistence of microstructural evidence for melt rock reactions and for deformation by dislocation creep and the predominance of axial 010 olivine cpo patterns characterized by 010 axes clustered normal to the layering indicate that deformation and the reactive melt percolation that formed the layering were coeval strong heterogeneity in mineral composition in the planar layering domains with fe enrichment in olivine and spinel and highly variable ti content in spinel in cm scale plagioclase rich bands implies that the latter interacted with higher melt volumes high ti contents in pyroxenes and spinel in the anastomosed layering domains point to changes in melt composition hinting to less effective melt transport we propose therefore that the planar layering records melt segregation in layers parallel to the shear plane whereas the anastomosed layering results from melt alignment along grain boundaries subparallel to the shear plane without segregation finite strain in the different layers cannot be quantified but comparison of the present observations with data from experiments and from other mantle outcrops displaying shear controlled melt organization suggests that the transition from the anastomosed to the planar layering might record an increase in finite strain that is strain localization associated with variations in the instantaneous melt fraction

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

the structure of mid ocean ridges recent research results on the structure of midocean ridges are reviewed the new view of ridge axis crustal structure obtained from high resolution seismology is reviewed emphasizing the variation of that structure with spreading rate and along axis at a given spreading rate recent results on upper mantle structure beneath ridges are examined including variations with seafloor age indications from anisotropy for directions of mantle flow and long wavelength along axis variations in structure and their implications for lateral heterogeneity in mantle temperature and composition

melt topology and seismic anisotropy in mantle peridotites of the oman ophiolite this paper presents shape measurements of plagioclase and clinopyroxene inclusions assumed to reflect melt topology in peridotites of the uppermost mantle section of the oman ophiolite the plagioclase and clinopyroxene grains are devoid of any intracrystalline deformation in all samples in contrast the olivine in these rocks has recorded a high temperature plastic deformation with different strengths of the crystallographic preferred orientation cpo of the olivine grains individual melt pockets are first described by ellipses in two dimensions they are more elongated when they have a larger area and they are preferentially oriented parallel to the lineation the x structural axis of the sample with a better defined preferred orientation for samples that have a stronger cpo in a second step an average melt phase shape is defined in three dimensions for each sample using the image autocorrelation technique the average shape is nearly spherical for the samples with weak cpos and it is ellipsoidal with a long axis parallel to x and the short axis parallel to z normal to the foliation for samples with strong cpos the long axis of the ellipsoid is 3 times as long as the short axis for the sample with the strongest cpo we use an anisotropic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites the melt pockets were modelled as basalt filled inclusions with the average shape and orientation given by the image analysis the cpo of the olivine crystals was used to calculate the elastic properties of the anisotropic background medium the calculated p wave seismic anisotropies ranged from 5 to 15 with the anisotropy increasing with the cpo strength and melt fraction the maximum p wave velocities are found along x with velocities above 8 km s at 0 melt and an average 0 5 km s reduction for 10 of melt the minimum p wave velocities are found along z with velocities generally below 7 5 km s at 0 melt and an average reduction of 0 8 km s for 10 of melt 1998 elsevier science b v all rights reserved

seismic imaging of santorini subsurface constraints on caldera collapse and present day magma recharge abstract volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir the mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge here we use a dense active source seismic tomography study to reveal the sub surface physical properties of the santorini caldera in order to understand caldera formation we find a 3 km wide cylindrical low velocity anomaly in the upper 3 km beneath the north central portion of the caldera that lies directly above the pressure source of the 2011 2012 inflation we interpret this anomaly as a low density volume caused by excess porosities of between 4 and 28 with pore spaces filled with hot seawater vents that were formed during the first three phases of the 3 6 ka late bronze age lba eruption are located close to the edge of the imaged structure the correlation between older volcanic vents and the low velocity anomaly suggests that this feature may be long lived we infer that collapse of a limited area of the caldera floor resulted in a high porosity low density cylindrical volume which formed by either chaotic collapse along reverse faults wholesale subsidence and infilling with tuffs and ignimbrites phreatomagmatic fracturing or a combination of these processes phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down drop of the wider topographic caldera followed the more limited collapse in the northern caldera this progressive collapse sequence is consistent with models for multi stage formation of nested calderas along conjugate reverse and normal faults the upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011 2012 edifice inflation this result supports the hypothesis that sub surface density anomalies may influence present day magma recharge events we postulate that past collapses and the resulting topographical and density variations at santorini influence magma focusing between eruptive cycles a feedback process that may be important in other volcanoes

seismic evidence for a crustal magma reservoir beneath the upper east rift zone of kilauea volcano hawaii an anomalous body with low vp compressional wave velocity low vs shear wave velocity and high vp vs anomalies is observed at 8 11 km depth beneath the upper east rift zone of kilauea volcano in hawaii by simultaneous inversion of seismic velocity structure and earthquake locations we interpret this body to be a crustal magma reservoir beneath the volcanic pile similar to those widely recognized beneath mid ocean ridge volcanoes combined seismic velocity and petrophysical models suggest the presence of 10 melt in a cumulate magma mush this reservoir could have supplied the magma that intruded into the deep section of the east rift zone and caused its rapid expansion following the 1975 m7 2 kalapana earthquake

consistent modeling of the electrical and elastic properties of sedimentary rocks an effective medium theory is formulated which reproduces many of the observed electrical and elastic characteristics of sedimentary rocks within a unified framework the effective medium consists of fluid solid and cement components with the component material properties related to those of the composite through a differential effective medium dem scheme the resulting microstructure of the composite both conducts electricity through the fluid phase and supports shear besides deriving archie s law the theory gives an excellent account of the shear wave and compressional wave velocity correlations and predicts corrections to the wyllie equation that are consistent with observed deviations the three component dem also accommodates clay conductivity and finite frequency dielectric behavior

compaction and seepage properties of crushed limestone particle mixture an experimental investigation for ordovician karst collapse pillar groundwater inrush as mining activity rapidly develops to deeper and deeper ground there have been an increasing number of groundwater inrush events occurring in china the ordovician limestone karst collapse pillar kcp which contains a lot of crushed rocks usually functions as a channel for groundwater inrush the factors such as the geological conditions groundwater flow and extraction percentage of coal seam in underground mines are the main causes of recompaction and permeability change of crushed rock particles in kcp the high permeability of rock will cause large volumes of groundwater flow thus posing a great threat of loss of lives in the mines it is important to take into account the compaction and seepage behaviors of the crushed rocks in ordovician limestone kcp the mts815 02 system and a self designed water flow apparatus were used to investigate the effect of particle size distribution on compaction and seepage behavior of crushed limestone particle mixture the reynolds number calculation of particle mixture shows that the seepage has been influenced by non darcy flow testing results indicate that the effective porosity of crushed limestone sample is strongly influenced by compaction and particle size distribution the effective porosity decreases with the increase in compaction and decrease in larger particle size respectively particle crushing during compaction is a main cause of size 0 2 5 mm materials whereas some fine particles are washed away due to the effect of water seepage which is a main cause of weight loss non darcy seepage properties of the crushed limestone are strongly influenced by compaction and particle size distribution in general during the compaction the permeability decreases while the non darcy coefficient increases with the decreasing of effective porosity the effective porosity particle crushing and seepage properties of crushed limestone are not only related to compaction levels and mixture sizes but also to style of arrangement and initial pore structure

ordovician limestone karst development law in feicheng coal field being the basement of coal measure strata in north china coal mine the ordovician karst aquifer is the baleful source of water for exploiting the carboniferous coal bed the coal mines that the exploitation transfers into the deep stratum will be threatened by deep high or super high pressure confined water from ordovician mastering the vertical development law is one of the basal and prerequisite works for emancipating deep coal resources and ensuring safety for exploiting the deep coal bed based on the statistics of exploratory boring data a slide test and an x ray diffraction experiment the results of ordovician vertical development were obtained the results are as follows limestone karsts 0 30 m below the top are undeveloped those 30 50 m away from the top are the most developed and those over 50 m are also undeveloped similar to the first ones the property of the aquifer of the ordovician limestone karsts from the working face floor numbered as 101002 in the caozhuang coal mine is explored using a 3d high density electrical technique for the well and drill hole data this confirms the vertical development law and provides evidence for floor prevention and control of water disasters

upper crustal velocity structure beneath the central lucky strike segment from seismic refraction measurements we present a three dimensional velocity model of the upper crust around the central volcano of the lucky strike segment mid atlantic ridge the model constructed from a 3 d array of air gun shots 37 5 m spacing along line and 100 m between lines to ocean bottom seismometers fired during a 3 d seismic reflection survey shows an off axis velocity increase 1 km s a low velocity region within the median valley and a low velocity anomaly underneath the lucky strike volcano our observations indicate a porosity decrease of 1 9 corresponding to a velocity increase of 0 5 1 km s over a distance of 8 km from the ridge axis 0 7 ma and a porosity decrease of 4 11 corresponding to a velocity increase of 2 km s between a depth of 0 5 and 1 75 km below seafloor a sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis we favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion lava sheeted dike boundary and active extension faults fractures and fissures the porosity variation with depth probably depends on pore space collapse hydrothermal alteration and a change of stress accommodation the off axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

representing anisotropic subduction zones with isotropic velocity models a characterization of the problem and some steps on a possible path forward despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy most p wave tomography efforts still rely on the assumption of isotropy in this study we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach we conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback the simulation results provide a self consistent distribution of isotropic thermal anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions we find that anisotropy induced artifacts are abundant and significant for teleseismic local and mixed event distributions the occurrence of artifacts is not reduced and indeed can be exacerbated by increasing richness in ray path azimuths and incidence angles the artifacts that we observe are in all cases important enough to significantly impact the interpretation of the images we test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results using approximate anisotropy fields can result in reduced rms misfit to the travel time delays and reduced abundance and severity of imaging artifacts we propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model

seismic analysis of the axial magma chamber reflector along the southern east pacific rise from conventional reflection profiling the thickness and internal properties of the magma sill located at the top of the axial magma chamber amc along the southern east pacific rise epr have been investigated through a combination of waveform modeling the near vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source receiver offset or slowness our results show that the amc reflector observed along the southern epr is best modeled by a thin 100 m thick sill of partial melt vs 0 km s sandwiched between higher velocity material and that the thickest sills are generally associated with the lowest p and s wave velocities the comparatively high p wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content this may have important implications for eruption mechanisms and along axis mixing of magma at the epr there is no simple relationship between morphologic indicators of magma supply e g axial depth or volume and sill thickness depth or velocity magma sill properties may be closely tied to the eruption and replenishment cycle of the amc and thus may vary on a much shorter spatial and temporal scale than axial morphology which reflects longer term variations in magma supply to the ridge

formation of large summit troughs along the east pacific rise as collapse calderas an evolutionary model summit troughs wider than 500 m and 30 110 m deep are present along 15 20 of the fast spreading east pacific rise epr they occur only along ridge segments with large cross sectional areas indicative of a time averaged robust magma supply where available seismic data confirm that these troughs are underlain by an axial magma chamber amc 1 2 1 6 km below the seafloor furthermore detailed investigation of the large summit troughs which notch the epr between 17 56 and 18 35 s indicates that the vertical relief of the troughs tends to be maximum where the amc is shallowest both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension rather we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated amcs wanes or nearly ceases because seismic studies constrain the melt lens along the epr to be only 30 80 m thick the proposed existence of collapsed calderas 40 110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply in particular we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases the resulting caldera will further widen with each subsequent dike intrusion when an abundant melt supply finally resumes the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera

shear wave anisotropy in northwestern south america and its link to the caribbean and nazca subduction geodynamics to investigate the subduction dynamics in northwestern south america we measured sks and slab related local s splitting at 38 seismic stations comparison between the delay times of both phases shows that most of the sks splitting is due to entrained mantle flow beneath the subducting nazca and caribbean slabs on the other hand the fast polarizations of local s waves are consistently aligned with regional faults which implies the existence of a lithosphere confined anisotropy in the overriding plate and that the mantle wedge is not contributing significantly to the splitting also we identified a clear change in sks fast directions at the trace of the caldas tear 5 n which represents a variation in the subduction style to the north of 5 n fast directions are consistently parallel to the flat subduction of the caribbean plate panama arc beneath south america while to the south fast polarizations are subparallel to the nazca south america subduction direction a new change in the sks splitting pattern is detected at 2 8 n which is related to another variation in the subduction geometry marked by the presence of a lithosphere scale tearing structure named here as malpelo tear in this region ne sw oriented sks fast directions are consistent with the general dip direction of the underthrusting of the carnegie ridge beneath south america further inland this ne sw trending mantle flow continues beneath the eastern cordillera of colombia and merida andes of venezuela finally our results suggest that the subslab mantle flow in northwestern south america is strongly controlled by the presence of lithospheric tearing structures

defect microdynamics in minerals and solid state mechanisms of seismic wave attenuation and velocity dispersion in the mantle the propagation of seismic waves in the earth s mantle can be significantly affected by relaxation processes causing attenuation and velocity dispersion reduction this paper reviews the solid state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals particularly in olivine for a given mechanism to have a significant effect on seismic wave propagation both the density and the mobility of the defects must be in an appropriate range the examination of the densities and geometry and mobilities of defects in olivine shows that dislocation and or grain boundary mechanisms can have a significant effect on seismic wave propagation although wide distributions of geometrical factors such as spacing of pinning points and of mobilities are required to explain all available data point defect mechanisms however are unlikely to be important because their densities are too small and or their mobilities are too large since the dislocation density and or grain size are determined in most cases by the long term tectonic stress seismic wave attenuation and velocity dispersion reduction involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature theoretical considerations suggest a wide range of dependence of seismic wave attentuation and velocity dispersion on the long term tectonic stress this is particularly the case for dislocation mechanisms and warrants careful experimental investigation dislocations and or grain boundaries cause anelastic behavior relaxation peaks when they are pinned or blocked at some points pinning or blocking becomes ineffective at high temperatures and or low frequencies causing a transition to viscoelastic behavior both laboratory and seismological observations of internal friction are dominated by the high temperature background where internal friction increases monotonically with temperature which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times however in most experimental studies to date the dislocation densities or the grain sizes were not well controlled making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to earth the need for better characterization of defect microstructures in experimental specimens is emphasized

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

modeling heat transfer from a convecting crystallizing replenished silicic magma chamber at an oceanic spreading center 1 most hydrothermal systems at oceanic spreading centers are underlain by basaltic magma bodies however some are underlain by higher silica magmas such as andesite or dacite the different viscosity of the latter magmas which results from their higher sio2 content lower liquidus and solidus temperatures and higher water contents affects the rate of heat transport from these magmas and the behavior of the overlying hydrothermal system we construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high rayleigh number crystallizing replenished axial magma chamber amc beneath a hydrothermal circulation system simulations comparing the time dependent heat flux from dry basalt 0 1 wt h2o andesite 3 wt h2o andesite and 4 wt h2o dacite indicate that higher viscosity magmas convect less vigorously resulting in lower heat flux possibly lower vent temperatures and a slower decay rate of the heat flux hydrothermal systems driven by unreplenished high silica melts may have slightly longer lifetimes than those driven by basalt however vent temperature and heat output decay on decadal time scales magma replenishment at a rate of 10 8 10 7 m s across the base of the amc can maintain relatively stable heat output between 107 109 watts and typical hydrothermal vent temperatures such replenishment rates are not likely to result from buoyancy driven melt transport by porous flow through the lower crust especially for high viscosity magmas such as andesite and dacite

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

resistivity imaging survey to delineate subsurface seepage of hydrocarbon contaminated water at karbala governorate iraq this research is an attempt to accomplish a 3 d resistivity imaging survey which was carried out near a water well contaminated with hydrocarbon materials in karbala governorate two dimensional resistivity imaging measurements were collected along four parallel profiles using a wenner array with electrode spacing of 1 m the res3dinv program was used to invert the apparent resistivity data the results displayed a resistivity distribution of the subsurface in a three dimensional volume thus both the horizontal and vertical extents of the contaminated zone were displayed this technique revealed a low resistivity zone at depth ranges from 3 to 6 m in the investigation area but the seepage starts at depth ranges between 2 and 3 m and continues down depth may be to the groundwater level this low resistivity zone is the most likely location for a subsurface seepage of contaminated water it is clear that the sufficient measurement points along 2 d lines in a small area can increase the 3 d imaging resolution and nearly real 3 d imaging can be achieved when the size of subsurface anomaly compared with the electrode spacing a of the wenner array is taken into consideration

the influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock abstract to better understand the microstructural evolution of a serpentinized mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge we have conducted the detailed microstructural analyses of a sample of tremolite chlorite antigorite schist collected from the moses rock dike central part of the colorado plateau we performed differential effective media dem modelling to study the effect of three hydrous phases forming two phase aggregates with olivine considering the crystallographic preferred orientation cpo of each phase and the shape ratio of the hydrous phases we have demonstrated that in a partially serpentinized peridotite the olivine cpo characteristic of 100 010 dislocation glide is still preserved and the high temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist the transformation of olivine into antigorite occurs predominantly 75 by the relationship 100 ol 001 atg with 001 ol 010 atg with the 010 ol 001 atg and 001 ol 010 atg relationship observed in areas of weak antigorite cpo chlorite results from the phase transformation of olivine in a relatively static environment as shown by the correlation between the olivine chlorite cpos with 100 ol 100 ch 010 ol 001 ch and 001 ol 010 ch the fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans lithospheric shear zones the presence of chlorite induces the most important drop on the p wave velocities and may help to explain some local low velocities in the fore arc mantle wedges but is unlikely to be of global importance due to its very high vp vs ratio 1 9 on the other hand antigorite is the only phase that causes important modification on the propagation directions of p and s waves and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

absorption band q model for the earth body wave surface wave and normal mode data are used to place constraints on the frequency dependence of q in the mantle with a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range the quality factor qs is proportional to in the band and to and 1 at higher and lower frequencies respectively as appropriate for a relaxation mechanism with a spectrum of relaxation times the parameters of the band are q min 80 0 15 and width 5 decades the center of the band varies from 101 seconds in the upper mantle to 1 6 103 seconds in the lower mantle the shift of the band with depth is consistent with the expected effects of temperature pressure and stress high qs regions of the mantle are attributed to a shift of the absorption band to longer periods to satisfy the gravest fundamental spheroidal modes and the scs data the absorption band must shift back into the short period seismic band at the base of the mantle this may be due to a high temperature gradient or high shear stresses a preliminary attempt is also made to specify bulk dissipation in the mantle and core specific features of the absorption band model are low q in the body wave band at both the top and the base of the mantle low q for long period body waves in the outer core an inner core qs that increases with period and low qp qs at short periods in the middle mantle the short period qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km the deformational q of the earth at a period of 14 months is predicted to be 463

rapid mantle flow beneath the tonga volcanic arc abstract flow in the earth s asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates suggesting flow rates comparable to plate velocities however arc parallel shear wave splitting and geochemical observations imply along strike flow in the lau basin presenting a challenge to geodynamic models of plate driven subduction systems as plate driven flow models predict arc perpendicular fast direction with simple a type mineral texturing although b type mineral texturing would result in the fast directions developing 90 relative to a type and likely occurs in the forearc mantle such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail by tracking mineral texturing development within geodynamic flow models patterned after the tongan system in the lau basin that include along strike flow we find that the observations are best explained by models including rapid along arc flow within the subarc mantle the effect of melt on the anisotropy does not appear to be a primary control but needs to be investigated more fully the best fit to the observations is a model with along strike flow within a low viscosity channel beneath the arc at a rate of nearly 50 cm yr several times the plate motion velocities

seismic wave velocity and anisotropy of serpentinized peridotite in the oman ophiolite shallow seismic measurements in harzburgite from the oman ophiolite performed in a zone where the maximum horizontal anisotropy is expected vertical foliation and horizontal lineation point to a dominant dependence of seismic properties on fracturing optical microscopy studies show that microcracks are guided by the serpentine lizardite penetrative network oriented subparallel to the harzburgite foliation and subperpendicular to the mineral lineation and that serpentine lizardite vein filling has a maximum concentration of 001 planes parallel to the veins walls the calculated elastic properties of the oriented alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are compared with seismic velocities measured on hand specimens laboratory ultrasonic data indicate that open microcracks are closed at 100 mpa pressure e g j geophys res 65 1960 1083 and proc odp sci results leg 118 1990 227 above this pressure laboratory measurements and modeling show that p compressional and s shear wave velocities are mainly controlled by the mineral preferred orientation veins scaled with serpentine are effective in slightly lowering p and s velocities and increasing anisotropy the penetrative lizardite network does not affect directly the geometry of seismic anisotropy but contributes indirectly in the fact that this network controls the microcrack orientations comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that p and s waves anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere c 2003 published by elsevier b v

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

variations in axial magma lens properties along the east pacific rise 9 30 n 10 00 n from swath 3 d seismic imaging and 1 d waveform inversion we use three dimensional multistreamer seismic reflection data to investigate variations in axial magma lens aml physical properties along the east pacific rise between 9 30 n and 10 00 n using partial offset stacks of p and s converted waves reflecting off the top of the aml we image four 2 4 km long melt rich sections spaced 5 10 km from each other one dimensional waveform inversion indicates that the aml in a melt rich section is best modeled with a low vp 2 95 3 23 km s and vs 0 3 1 5 km s indicating 70 melt fraction in contrast the aml in a melt poor section requires higher vp 4 52 4 82 km s and vs 2 0 3 0 km s which indicates 40 melt fraction the thicknesses of the aml are constrained to be 8 32 m and 8 120 m at the melt rich and poor sites respectively based on the aml melt mush segmentation imaged in the area around the 2005 2006 eruption we infer that the main source of this eruption was a 5 km long section of the aml between 9 48 n and 51 n the eruption drained most of the melt in this section of the aml leaving behind a large fraction of connected crystals we estimate that during the 2005 2006 eruption a total magma volume of 9 83 106 m3 was extracted from the aml with a maximum of 71 106 m3 left unerupted in the crust as dikes from this we conclude that an eruption of similar dimensions to the 2005 2006 one would be needed with a frequency of years to decades in order to sustain the long term average seafloor spreading rate at this location

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

effect of crystal preferred orientations on the thermal diffusivity of quartz polycrystalline aggregates at high temperature article i nfo thermal diffusivity of synthetic quartz single crystals and natural polycrystalline aggregates were measured using a modified angstrom method at ambient pressure and up to 800 c in order to investigate the effect of crystal preferred orientation on thermal diffusivityof quartz rich rocks the thermal diffusivitytensorof quartz was determined by measuring the thermal diffusivity of an oriented synthetic quartz single crystal along the 100 and 001 directions a naturally deformed fine grained quartzite mylonite was used to compare the single crystalproperties with aggregate properties using thecrystal preferred orientations cpo measuredby the electron back scattered diffraction ebsd method and the measured thermal diffusivity tensor of quartz the three dimensional aggregate thermal diffusivity was modelled using volume averaging models in order to test the validity of these models that are only based on the properties of the constituent minerals the thermal diffusivity was then measured on oriented cores as a function of temperature along three orthogonal directions parallel to the x lineation y z pole to foliation structural frame both modelled and direct measurement of thermal diffusivity show an anisotropy of thermal diffusivity heat transport being more effective parallel to y direction and less effective parallel to the lineation x direction thermal diffusivities measured in a given direction on the natural rock are lower by less than 10 than the modelled thermal diffusivity in the same direction showing that rock imperfections have a weak effect on heat transport in addition the modelled thermal diffusivity anisotropy based on the characterization of crystal orientations and thermal diffusivity tensor shows a good agreement with measurements in both and stability field this validation of the petrophysical simulation allows for evaluating the 3d thermal diffusivity of quartz rich rocks in different crustal conditions the 3d thermal diffusivity of five naturally deformed quartzites displaying quartz fabrics expected in different crustal pressure and temperature ranges were simulated the resulting thermal diffusivity anisotropies of quartzites range between 40 and 10 depending on temperature crystallographic fabric patterns and fabric strength under crustal conditions quartzites deformed in the granulite facies display a high thermal diffusivity in the xz foliation plane whereas quartzites deformed in greenschist facies have highest thermal diffusivity in the z direction normal to the foliation plane

influence of melt viscosity of basaltic and andesitic composition on seismic attenuation in partially molten gabbronorite the characteristic frequencies at which two different melt related attenuation mechanisms occur in partially molten gabbronorite are computed as a function of aspect ratios of the melt filled cracks and the melt viscosity the computations were done for basaltic and andesitic melts the melt viscosity is constrained by i laboratory measurements performed in the range 10 8 10 14 pa s with a creep apparatus and in the range 10 1 to 105 pa s with a rotational couette viscometer and ii modeling of viscosity at high temperatures the results of the characteristic frequency calculations suggest that melt squirt flow is a viable attenuation mechanism at seismic frequencies for aspect ratio of melt inclusions in the range 10 3 to 10 2 for the andesite and 10 4 to 10 3 for the two basalts 2008 elsevier b v all rights reserved

modelling the seismic properties of fast spreading ridge crustal low velocity zones insights from oman gabbro textures although considerable progress has been made in the study of fast spreading mid ocean ridge magma chambers over the past fifteen years the fraction of melt present in the chamber remains poorly constrained and controversial we present new constraints obtained by modelling the seismic properties of partially molten gabbros at the ridge axis p wave velocities at low frequencies are calculated in the foliation lineation reference frame using a differential effective medium technique the model takes into account the lattice preferred orientation of the crystalline phase and the average shape of the melt phase the structural parameters are obtained from the oman ophiolite the structural reference frame is given by the general trend of the gabbro foliation and the melt fraction and shape are estimated using the textures of nine upper gabbro samples the estimated melt fraction and shape depend on the assumptions regarding which part of the observed textures represent the melt in the gabbroic mush of the magma chamber however we can put limits on the reasonable values for the melt fraction and shape our results are consistent with a melt fraction of the order of 10 to 20 in the low velocity zone i e the magma chamber which is anisotropically distributed with the melt pockets preferentially aligned parallel to the foliation and approximated by oblate ellipsoids with approximate dimensions of 4 4 1 these results are also consistent with the seismic structure of the east pacific rise at 9o30 0 the anisotropic melt distribution can at least partially explain the vertical velocity gradient described in the lvz 1999 elsevier science b v all rights reserved

hypersolidus deformation in the lower crust of the josephine ophiolite evidence for kinematic decoupling between the upper and lower oceanic crust new mapping and structural observations in the lower crust of the josephine ophiolite provide insights into the geometry and kinematics of hypersolidus flow beneath an oceanic spreading center the lower crust defined here as the sequence of rocks overlying mantle peridotite and beneath exposure of 100 sheeted dikes can be divided into lower wehrlite dunite and upper gabbroic sections the contact between the two is mutually intrusive where exposed hypersolidus fabrics are the dominant structures observed no pervasive crystal plastic deformation is observed restoration of the hypersolidus foliations and igneous modal layers to their on axis orientation indicates that they strike approximately perpendicular to the strike of the inferred paleo spreading center as defined by the orientation of sheeted dikes and onaxis oceanic faults hypersolidus lineations define a dispersed 3 d flow pattern in the lower crust whereas extension directions in the upper crust i e poles to sheeted dikes oceanic normal faults are unidirectional and perpendicular to the paleo ridge axis collectively these observations are consistent with partitioning of deformation between the upper and lower crust and local ridge parallel extension in the partially molten lower crust due to possible subsidence of the thickened axial upper crust however some component of kinematic coupling between the lower crust and mantle peridotite driven by asthenospheric flow cannot be ruled out q 2003 elsevier ltd all rights reserved

petrographic interpretation of a complex seismic crust mantle transition in the central eastern alps abstract a recent reinterpretation of the existing dss profiles scarascia and cassinis tectonophysics 271 1997 157 188 led to a reconstruction of the moho boundary and of the lower crust in the central eastern alpine sector where the european and the adriatic domains are deeply indented in that work a complex crust mantle transition composed of alternating low and high velocity layers was observed in the deepest bottom of the european crust in the same work it was confirmed that the european lower crust beneath the alpine range is characterised by a very low seismic velocity while the velocity of the adriatic lower crust reaches the average values found in collisional belts the interpretation of these features has been performed employing a data base of ultrasonic seismic measurements the ivrea verbano zone has been assumed as an analogue of crust mantle transition a statistical approach was used to associate the bodies of the interpreted seismic models to the lithotypes a petrological model is attempted in order to suggest an explanation of the observed phenomena assuming physical and compositional constraints appropriate to the alpine collisional belt cumulitic peridotites and pyroxenites lenses in a gabbroic host rock fit the observed seismic structure of the crust mantle transition the upper mantle lies below a transition zone and is characterised by a velocity of 8 2 km s detected by p n phases in our study it is interpreted as a spinel to garnet peridotite the anomalous low velocity of the european lower crust is discussed but still remains an open subject

an experimental and numerical study of the microstructural parameters contributing to the seismic anisotropy of rocks 1 the elastic properties of rocks are influenced by several microstructural variables including the lattice preferred orientation and grain shape fabric of the mineral phases variations in the spatial distribution of the mineral phases the properties of the grain boundaries and the presence of porosity fractures consequently in principle directional variations in these variables can be inferred from seismic velocity anisotropy observations experimental seismic velocity measurements made on rocks of well characterized microstructure may be used to improve the quality of such interferences however since most rocks are microstructurally complex in order to interpret the measurements fully theoretical analyses which can accommodate all the relevant microstructural variables are required theoretical analyses of the requisite sophistication have only recently been developed we have tested one of these due to ponte castaneda and willis by using it to calculate the elastic properties of an upper mantle harzburgite and by then comparing the results with experimental velocity measurements to determine if the values of those microstructural variables which are difficult to quantify grain boundary properties fracture shape are physically realistic we find that they are and conclude that the ponte castaneda and willis analysis provides a powerful method for a more detailed assessment of the causes of elastic property anistropy in rocks than has previously been possible

amorphization in natural omphacite and its implications abstract high resolution transmission electron microscopic data have revealed the existence of nanoscale intragranular amorphous phase in omphacite of eclogite from the dabieshan uhp metamorphic belt eastern china the results of profiles in the omphacite across the amorphous phase and micro fractured regions indicate variations in compositions and oh band intensity respectively this implies that the occurrence of amorphization in the omphacite appears to be pressure induced and controlled by kinetics oh may diffuse outward and further react with the atom at m2 site during more distorted plastic deformation which occurs as the omphacite crystalline structure collapses to a denser amorphous phase the reported propensity to amorphization and the strong influence on the outcome of the initial structure and of the pressurization protocol indicate the presence of metastability and large kinetic barriers

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

seismic structure across the rift valley of the mid atlantic ridge at 23 20 mark area implications for crustal accretion processes at slow spreading ridges the results from a 53 km long wide angle seismic profile across the rift valley of the mid atlantic ridge south of the kane transform near 23 20 n mark area provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid ocean ridges anomalously low middle and lower crustal p wave velocities beneath the neovolcanic snake pit ridge are consistent with elevated axial temperatures and with the presence of 4 1 partial melt evenly distributed within the lower crust in preferentially oriented elongated thin films if the melt inclusions have larger aspect ratios melt fractions can be up to 17 3 this and other geological observations suggest that the study area is presently in a magmatically active period the igneous crust is anomalously thin beneath both flanks of the median valley 2 3 2 5 km thus the mantle rocks observed along the western rift valley wall at pink hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting the crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous with igneous crustal thickness variations of 2 2 km over horizontal distances of 5 km this heterogeneity indicates that the magma supply in the area has fluctuated during the last 2 m y thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data