the q structure of the upper mantle constraints from rayleigh wave amplitudes 1 the three dimensional shear attenuation or q structure of the upper mantle is still poorly understood despite the increasing accuracy and sophistication of velocity models of the same part of the earth here we present a set of three dimensional q models of the upper mantle based on amplitude measurement of minor and major arc rayleigh waves in the period range 70 170 s the models show that areas of low attenuation underlie continents to a depth of around 300 km and areas of high attenuation are associated with oceans and plate boundaries particularly in the top 200 km below 350 km the sensitivity of the data to attenuation structure decreases rapidly it is well known that the amplitudes of surface waves can be strongly influenced by factors other than attenuation in particular focusing due to elastic structure and errors in source mechanisms we consider the effects of focusing by discussing the implications of the results of selby and woodhouse 2000 comparisons with focusing predictions using phase velocity maps allow us to identify features due to focusing in maps of attenuation distribution the influence of the source on the observed attenuation distribution is investigated by including a source term in the model inversion we find that successfully combining the data at each frequency into one three dimensional model requires that this source term must vary with frequency suggesting that a simple correction to the scalar moment of each event cannot explain the observations although the effects of focusing and the source can be significant we find that the observed attenuation pattern is robust up to degree 8 however shorter wavelength structure can be strongly influenced by other controls finally we use one of the models to predict the degree 2 attenuation pattern for 250 s rayleigh waves the observed pattern suggests that existing normal mode observations may be explainable in terms of structure above 350 km depth

variations in shear wave splitting in young pacific seafloor 1 analysis of sks phases recorded by ocean bottom seismometers in the glimpse experiment has expanded the region of in situ measurements of shear wave splitting in young seafloor near the southern east pacific rise average splitting times at individual glimpse sites range from 1 1 to 2 2 s with fast direction azimuths trending approximately in the direction of absolute plate motion and fossil spreading for the pacific plate the glimpse study area at 12 to 14 s is adjacent to the melt experiment the new observations demonstrate that splitting delays are not uniform on the pacific plate and that the contrast between the nazca and pacific plates is not a fundamental consequence of the asymmetric plate motion the variations in degree of splitting could be caused by small scale convection variations in water content or rate of deformation in the asthenospheric return flow to the east pacific rise

the stress dependence of olivine creep rate implications for extrapolation of lab data and interpretation of recrystallized grain size abstract based on measured values for the stress exponent n 3 5 combined with the empirically determined relationship between dislocation density and stress 1 37 and an analysis of diffusion kinetics in olivine we conclude that silicon pipe diffusion limits strain rate in the dislocation creep regime furthermore assuming that steady state recrystallized grain size is set by a dynamic balance between strain energy density associated with dislocations and surface energy density associated with grain boundaries the resulting dependence of recrystallized grain size on stress accurately describes experimental observations when the empirical dislocation density versus stress relationship is accounted for d 1 1 37 the improved physical understanding of the stress dependence of creep rate provides justification for incorporation of experimentally derived flow laws into models of geodynamical process and grain size evolution these lab constraints combined with independent analyses of the stress dependence of mantle viscosity based on geophysical data provide bounds on rheological properties such as the yield stress of the lithosphere

the pacific lithosphere asthenosphere boundary seismic imaging and anisotropic constraints from ss waveforms 1 the lithosphere asthenosphere boundary lab separating the rigid lid from the underlying weaker convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics however the exact depth and defining mechanism of the lab interface remain poorly understood the ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70 of earth s surface area and have a relatively simple geological history seismically imaging the oceanic lab at high resolution has proved challenging yet several studies have recently increased resolution with provocative results we summarize recent imaging of discontinuity structure beneath much of the pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean continent margins ss precursors and waveform modeling of multiple phases including multiple bounce s waves scs reverberations and surface waves overall there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the lab some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths yet when the results are considered together no single age depth relationship is illuminated there are also puzzling discrepancies in where the discontinuity is detected which again suggests greater complexity here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure we stack ss waveforms with bounce points in the central pacific into azimuthal bins we use two methods one that inverts for discontinuity structure based on subtle variations in the character of the ss waveform and another that considers ss at higher frequency we find azimuthal variation in the amplitude of the waveform including a polarity reversal we suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate and must be carefully considered to constrain the age depth dependence and defining mechanism of the oceanic lithosphere

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

insights on upper mantle melting rheology and anelastic behavior from seismic shear wave tomography in seismic tomography we map the wave speed structure inside the earth but we ultimately seek to interpret those images in terms of physical parameters this is challenging because many parameters can trade off with each other to produce a given wave speed the problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1 d reference model rather than absolute wave speeds using a full waveform tomography model of europe as a case study we quantify the extent to which thermochemical and dynamic properties can be constrained using only s wave speed expressed in absolute values the wave speed distributions of this tomography model are compared with 4 million thermochemical models whose seismic properties are computed via thermodynamic modeling these models sample the full range of realistic mantle compositions including variable water and melt contents and mineral intrinsic anelasticity is taken into account intrinsic anelasticity causes waves to travel more slowly at higher temperatures leading to seismic attenuation but the sensitivity of the wave speed reduction to temperature is in turn controlled by the wave frequency global studies of surface waves indicate an anticorrelation between s wave speed and attenuation we therefore only retain thermochemical models satisfying this anticorrelation our study indicates that the frequency dependence of anelasticity depends on temperature or rheology with 0 1 being most appropriate in cold or lithospheric mantle and 0 3 in warmer regions i e the asthenosphere additionally the slowest regions require specific compositions and or a velocity weakening mechanism such as partial melting elastically accommodated grain boundary sliding or water

comparison of azimuthal seismic anisotropy from surface waves and finite strain from global mantle circulation models summary we present global models of strain accumulation in mantle flow to compare the predicted finite strain ellipsoid fse orientations with observed seismic anisotropy the geographic focus is on oceanic and young continental regions where we expect our models to agree best with azimuthal anisotropy from surface waves finite strain derived models and alignment with the largest fse axes lead to better model fits than the hypothesis of alignment of fast propagation orientation with absolute plate motions our modelling approach is simplified in that we are using a linear viscosity for flow and assume a simple relationship between strain and anisotropy however results are encouraging and suggest that similar models can be used to assess the validity of assumptions inherent in the modelling of mantle convection and lithospheric deformation our results substantiate the hypothesis that seismic anisotropy can be used as an indicator for mantle flow circulation derived models can contribute to the establishment of a quantitative functional relationship between the two

imaging pacific lithosphere seismic discontinuities insights from ss precursor modeling oceanic lithosphere provides an ideal location to decipher the nature of the lithosphere asthenosphere system which is vital to our understanding of plate tectonics it is well established that oceanic lithosphere cools thickens and subsides as it ages according to the conductive cooling models yet this simple realization fails to explain various observations for example old oceanic lithosphere does not subside as predicted further precise imaging of the lower boundary of the oceanic lithosphere has proven challenging here we use ss precursors to image the discontinuity structure across the pacific ocean using 24 years of teleseismic data we image a sharp pervasive velocity discontinuity 3 15 drop over 36 my there is no age depth dependence and we image the discontinuity at an average depth of 60 1 5 km the amplitude and sharpness of the boundary suggests a compositional variation and or layered carbonatitic melt may be required to explain our observations rather than temperature alone the strength and pervasiveness of the boundary suggests that it is likely related to the lithosphere asthenosphere boundary an additional deeper discontinuity at 80 120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth

evolution of the oceanic lithosphere in the equatorial atlantic from rayleigh wave tomography evidence for small scale convection from the pi lab experiment

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

the unique dynamics of the pacific hemisphere mantle and its signature on seismic anisotropy abstract the portion of earth s mantle lying below the hemispherical region occupied by the pacific ocean basin is characterized by a unique style of convection the unique dynamics below the pacific hemisphere are revealed by analyzing the mantle flow predicted on the basis of high resolution seismic images of mantle structure in the pacific hemisphere we find a dome like upwelling originating in the lower mantle which actively transports material into the upper mantle a unique aspect of this flow found only in the pacific hemisphere is a demi tour or u turn of the circulation which occurs in the shallow mantle west of the east pacific ridge this demi tour connects the westward movement of the pacific plate to the dome like upwelling below the central pacific ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis in a global analysis of the correlation between mantle flow and seismic anisotropy we find the strongest agreement occurs in the pacific hemisphere mantle

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

grain growth in partially molten olivine aggregates grain growth rates in partially molten olivine aggregates were determined experimentally at a pressure of 1 gpa the aim of the experiments with fine grained starting material solution gelation derived fo90 olivine with a grain size of 1 m with 2 and 4 wt added basaltic glass was to determine growth rates during steady state grain growth a series of experiments was conducted at a fixed temperature of 1 350 c for durations from 2 to 700 h and a second series at fixed duration 168 h at temperatures from 1 250 to 1 450 c the resulting activation energy e 390 kj mol and growth exponent n 4 3 are comparable to melt free results nichols and mackwell 1991 the observed grain size distributions are broader than those predicted for normal grain growth or ostwald ripening and most similar to the log normal distribution the observed melt distribution is influenced by the continuous rearrangement of neighboring grains during growth the relatively large growth exponent observed in this study and by nichols and mackwell 1991 where secondary phases are present suggests that grain growth in the upper mantle is slower than predicted by karato 1989

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

grain size distribution and rheology of the upper mantle abstract the physical processes that govern the grain size of rocks in the upper mantle are examined the analysis is based on the experimental data on creep recrystallization and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere the grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate above this depth the microstructures record the thermomechanical history of rocks rather than the in situ stress in the case of the oceanic lithosphere where the thermomechanical history is best known the following features of grain size distribution are found at the uppermost mantle where the amount of grain growth is limited the grain size is determined by the initial value and the growth rate and where the effect of grain growth dominates it increases with depth when the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform the grain size is then determined by the applied stress this grain size is however frozen when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization thus at the intermediate depth region the grain size records the fossil frozen stress at which the microstructures of rock have been frozen since the frozen stress increases with age the grain size in this depth interval decreases with depth finally the grain size below this level reflects the in situ stress and increases with depth its extent being dependent on the nature of return flow in the deep mantle thus the grain size versus depth relation may show a sigmoid curve the qualitative features of this curve may be similar also in the case of the continental lithosphere if a similar thermal event i e the intrusion of hot material and subsequent cooling occurs the results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules ave lallemant et al 1980 the present model suggests that the depth of minimum grain size 65 and 150 km at the continental rift zone and the shield region respectively corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels 1 mpa and in the geological time scale this result leads to a new definition of the thickness of lithosphere in terms of its rheological properties this thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness the model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

upper mantle seismic anisotropy at a strike slip boundary south island new zealand new shear wave splitting measurements made from stations onshore and offshore the south island of new zealand show a zone of anisotropy 100 200 km wide measurements in central south island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi shear wave nearly parallel to relative plate motion with increased obliquity to this orientation observed farther from shore on the eastern side of the island fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast uniform delay times between the fast and slow quasi shear waves of nearly 2 0 s onshore continue to stations approximately 100 km off the west coast after which they decrease to 1 s at 200 km stations more than 300 km from the west coast show little to no splitting east coast stations have delay times around 1 s simple strain fields calculated from a thin viscous sheet model representing distributed lithospheric deformation with strain rates decreasing exponentially to both the northwest and southeast with e folding dimensions of 25 35 km approximately 75 of the deformation within a zone 100 140 km wide match orientations and amounts of observed splitting a model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if at depths of 100 130 km below the lithosphere typical grain sizes are 6 7 mm

seismic attenuation beneath northeastern japan constraints on mantle dynamics and arc magmatism we apply a three step approach to estimate three dimensional 3 d p wave attenuation qp 1 structure beneath northeastern japan first corner frequencies of earthquakes are determined using the spectral ratio method for s coda waves then whole path attenuation terms t and site amplification factors are simultaneously estimated by a joint inversion the set of t is finally inverted for 3 d attenuation structure the results show that the mantle wedge has low attenuation in the fore arc and high attenuation in the back arc a depth profile of qp 1 in the back arc mantle is explained by attenuation expected for a two dimensional 2 d thermal model with qp qs 2 and grain sizes of 1 and 3 cm however an inclined high attenuation zone observed in the back arc mantle wedge which is interpreted as an upwelling flow shows higher attenuation than that calculated from the 2 d thermal model the higher seismic attenuation is probably caused by the concentration of partial melt in the upwelling flow a combined interpretation of seismic attenuation and velocity structures further suggests that the degree of partial melt in the upwelling flow varies along the arc and that volcanoes are clustered transverse to the arc below which the upwelling flow contains a higher degree of melt these observations indicate that magmatism is controlled by a mantle wedge process that depends strongly on spatial variations in the degree of partial melt in the upwelling flow our results further imply the breakdown of hydrous minerals in a hydrous layer above the pacific plate at a depth of 120 km

phase distribution of small scale ocean surface roughness remote sensing is becoming an important tool for oceanographic research as the resolution of remote sensors is becoming comparable to the wavelength of dominant ocean surface waves the usual assumption of homogeneous surface roughness is no longer adequate in this paper laboratory data of surface wave displacement and small scale surface roughness wavelength 0 004 0 10 m measured by a scanning slope sensor are analyzed the results show that higher intensity of surface roughness occurs at the upwind quadrant of the wave crests of background waves over a wide wind speed range the maximum surface roughness is located close to the maximum of surface velocity divergence

imaging continental breakup using teleseismic body waves the woodlark rift papua new guinea this study images the upper mantle beneath the d entrecasteax islands papua new guinea providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers differential travel times from p and s wave teleseisms recorded during the 2010 2011 cdpapua passive seismic experiment are used to invert for separate vp and vs velocity models of the continental rift a low velocity structure marks the e w axis of the rift correlating with the thinnest crust high heat flow and a linear trend of volcanoes this slow region extends 250 km along strike from the oceanic spreading centers demonstrating significant mantle extension ahead of seafloor breakup the rift remains narrow to depth indicating localization of extension perhaps as a result of mantle hydration a high vp structure at depths of 90 120 km beneath the north of the array is more than 6 5 faster than the rift axis and contains well located intermediate depth earthquakes these independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction although the polarity of subduction cannot be resolved this geometry is gravitationally unstable downwelling or small scale convection could have facilitated rifting and rapid lithospheric removal although this may require a wet mantle to be realistic on the required time scales the high v structure agrees with the maximum p t conditions recorded by young ultra high pressure rocks exposed on the rift axis and may be implicated in their genesis

shear wave splitting and implications for mantle flow beneath the melt region of the east pacific rise shear wave splitting across the fast spreading east pacific rise has been measured from records of sks and skks phases on the ocean bottom seismometers of the mantle electromagnetic and tomography melt experiment the direction of fast shear wave polarization is aligned parallel to the spreading direction delay times between fast and slow shear waves are asymmetric across the rise and off axis values on the pacific plate are twice those on the nazca plate splitting on the pacific plate may reflect anisotropy associated with spreading induced flow above a depth of about 100 km as well as a deeper contribution from warm asthenospheric return flow from the pacific superswell region

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of obs array records at 60 ma seafloor we analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 ma seafloor in the southeastward of tahiti island the south pacific by the tomographic investigation by seafloor array experiment for the society hotspot project we first obtained one dimensional shear wave velocity model beneath the array from average phase velocities of rayleigh waves at a broadband period range of 5 200 s the obtained model shows a large velocity reduction at depths between 40 and 80 km where the lithosphere asthenosphere boundary might exist we then estimated shear wave azimuthal anisotropy at depths of 20 100 km by measuring azimuthal dependence of phase velocities of rayleigh waves the obtained model shows peak to peak intensity of the azimuthal anisotropy of 2 4 with the fastest azimuth of nw se direction both in the lithosphere and asthenosphere this result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid ocean ridge but is roughly parallel to the ancient plate motion at depths of 20 60 km the fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60 100 km additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190 210 km

mantle upwelling and melting beneath slow spreading centers effects of variable rheology and melt productivity abstract we examine the effects of non uniform rheology and melting models on the 3d structure of mantle upwelling beneath spreading centers our numerical models identify a range of conditions for which 3d upwelling can be considered as a possible mechanism for magmatic segmentation of slow spreading ridges in a highly viscous shallow region due to dehydration of the solid by melt extraction flow results essentially from the plate spreading this contrasts with a deeper buoyant low viscosity region where melt is present but upwelling solid is not yet dehydrated and where both solid and melt flows acquire their 3d nature the thickness of this buoyant region depending mainly on the temperature interval between the wet and dry solidi results in a specific characteristic length scale for the upwelling smaller segmentation wavelengths occur for smaller values of this thickness in contrast to isoviscous models where minimum segmentation wavelengths were large 150 km minimum wavelengths obtained in models with non uniform rheology and melt production 40 70 km are comparable to smallest observed segment lengths a single wavelength may be preferred for a given set of parameters but such a preferred wavelength is difficult to achieve because a range of initially prescribed wavelengths can persist for long times the persistence of 3d solutions for tens of millions of years of model time indicate that segment lengths observed at slow mid ocean ridges may be inherited from the initial stages of spreading

fluid migration in the mantle wedge influence of mineral grain size and mantle compaction mineral grain size in the mantle affects fluid migration by controlling mantle permeability the smaller the grain size the less permeable the mantle is mantle shear viscosity also affects fluid migration by controlling compaction pressure high mantle shear viscosity can act as a barrier to fluid flow here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2 d numerical fluid migration model our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged down dip by the mantle flow due to small grain size 1 mm and high mantle shear viscosity that develop along the base of the mantle wedge increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at sub arc depths fluid introduced into the mantle wedge at post arc depths migrate upwards due to relatively large grain size in the deep mantle wedge forming secondary fluid pathways behind the arc fluids that reach the shallow part of the mantle wedge spread trench wards due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55 65 km depths these results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc

focusing of upward fluid migration beneath volcanic arcs effect of mineral grain size variation in the mantle wedge we use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc previous coupled thermal grain size evolution t gse models predict small grain size 1 mm in the corner flow of the mantle wedge a down dip grain size increase by two orders of magnitude along the base of the mantle wedge and finer grain size in the mantle wedge for colder slab subduction zones we integrate these t gse modeling results with a fluid migration model in which permeability depends on grain size and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down dip by corner flow due to the small grain size and low permeability immediately above the slab as grain size increases with depth permeability increases resulting in upward fluid migration fluids released beneath the arc and the backarc are also initially dragged down dip but typically are not transported as far laterally before they begin to travel upward as the fluids rise through the backarc mantle wedge they become deflected towards the trench due to the effect of mantle inflow the combination of down dip migration in the forearc and trench ward migration in the backarc results in pathways that focus fluids to the arc this article is protected by copyright all rights reserved

aqueous volatiles in hydrothermal fluids from the main endeavour field northern juan de fuca ridge temporal variability following earthquake activity the main endeavour field northern juan de fuca ridge experienced intense seismic activity in june 1999 hydrothermal vent fluids were collected from sulfide structures in september 1999 and july 2000 and analyzed for the abundance of h2 h2s ch4 co2 nh3 mg and cl to document temporal and spatial changes following the earthquakes dissolved concentrations of co2 h2 and h2s increased dramatically in the september 1999 samples relative to pre earthquake abundances and subsequently decreased during the following year in contrast dissolved nh3 and ch4 concentrations in 1999 and 2000 were similar to or less than pre earthquake values aqueous cl abundances showed large decreases immediately following the earthquakes followed by increases to near pre earthquake values the abundances of volatile species at the main endeavour field were characterized by strong inverse correlations with chlorinity phase separation can account for 20 50 enrichments of co2 ch4 and nh3 in low chlorinity fluids while temperature and pressure dependent fluid mineral equilibria at near critical conditions are responsible for order of magnitude greater enrichments in dissolved h2s and h2 the systematic variation of dissolved gas concentrations with chlorinity likely reflects mixing of a low chlorinity volatile enriched vapor generated by supercritical phase separation with a cooler gas poor hydrothermal fluid of seawater chlorinity decreased abundances of sediment derived nh3 and ch4 in 1999 indicate an earthquake induced change in subsurface hydrology elevated co2 abundances in vent fluids collected in september 1999 provide evidence that supports a magmatic origin for the earthquakes temperature salinity relationships are consistent with intrusion of a shallow dike and suggest that the earthquakes were associated with movement of magma beneath the ridge crest these data demonstrate the large and rapid response of chemical fluxes at mid ocean ridges to magmatic activity and associated changes in subsurface temperature and pressure

the anisotropic and rheological structure of the oceanic upper mantle from a simple model of plate shear summary we have developed a channel flow model that dynamically couples plate motion and mantle stress with a composite rheology diffusion creep and dislocation creep to study the rheological and anisotropic structures of the oceanic upper mantle a semi analytic approach is used to solve for mantle stress and viscosity allowing fast calculations and exploration of a wide range of rheological parameters mantle stress in our model is due to shearing by a moving plate by comparing mantle stress with a transition stress for dislocation creep we identify regions where either diffusion creep or dislocation creep is active deformation by dislocation creep results in a mineral fabric that may be responsible for observed seismic anisotropy our study suggests that there is an important relation between plate motion seismic anisotropy mantle viscosity and transition stress using laboratory results for rheological parameters we find that dislocation creep exists only in a layer at certain depths in the upper mantle for a plate velocity of 10 cm yr 1 an asthenospheric viscosity of 10 19 pa s and an asthenospheric transition stress of 0 1 mpa our model predicts a 200 km thick dislocation creep layer which is broadly consistent with the observations of seismic anisotropy for a plate velocity of 10 cm yr 1 and an asthenospheric transition stress of 0 1 mpa the asthenospheric viscosity needs to be greater than 5 10 18 pa s to produce any dislocation creep deformation and the

a global tomographic model of shear attenuation in the upper mantle we present a global three dimensional model of shear attenuation in the upper mantle based on the measurement of amplitudes of low frequency 100 300s rayleigh waves observed at stations of the geoscope and iris networks attenuation coefficients are measured on r1 and r2 paths using a method which minimizes the effects of focussing due to propagation in a three dimensional elastic earth through a series of tests which in particular involve the computation of synthetic models of attenuation and focussing we demonstrate that long wavelength lateral variations in attenuation in the first 400 500 km of the mantle can indeed be resolved the model is obtained in a two step procedure the first step consists in the computation of maps of rayleigh wave attenuation at different periods using an inversion method without a priori parametrisation which involves the introduction of a correlation length chosen here at 3000 km to optimize the trade off between resolution and variance in the model in the second step after corrections for shallow structure an inversion with depth is performed assuming lateral heterogeneity is confined to depths between 80 and 650 km the resulting model presents lateral variations in qp that are correlated with tectonic features in particular ridges and shields in the first 250 km of the upper mantle below that depth the pattern shifts and becomes correlated with the hotspot distribution particularly so if the buoyancy strength of hotspots is taken into account two major low velocity zones appear to be located in the central pacific and beneath northern africa in the depth range 300 500 km this pattern seems to continue at greater depth but resolution becomes insufficient below 500 km to draw definitive conclusions the smooth lateral variations retrieved are on the order of 50 down to 400 km we propose an interpretation in terms of plume lithosphere ridge interaction in the upper mantle arguing for deflection of the bulk of hot upwelling material from plumes towards ridges which may be occurring between 200 and 300 km depth

using short term postseismic displacements to infer the ambient deformation conditions of the upper mantle 1 to interpret short term postseismic surface displacements in the context of key ambient conditions e g temperature pressure background strain rate water content creep mechanism we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress the flow law is then used to investigate mantle deformation beneath the eastern california shear zone following the 1999 m7 1 hector mine earthquake the flow law parameters are determined using finite element models of relaxation processes constrained by surface displacement time series recorded by 55 continuous gps stations for 7 years following the earthquake results suggest that postseismic flow following the hector mine earthquake occurs below a depth of 50 km and is controlled by dislocation creep of wet olivine diffusion creep models can also explain the data but require a grain size 3 5 mm that is smaller than the inferred grain size 10 20 mm based on the mantle conditions at these depths in addition laboratory flow laws predict dislocation creep would dominate at the stress grain size conditions that provide the best fit to diffusion creep models model results suggest a transient creep phase that lasts 1 year and has a viscosity 10 times lower than subsequent steady state flow in general agreement with laboratory observations the postseismic response is best explained as occurring within a relatively hot upper mantle e g 1200 1300 c at 50 km depth with a long term background mantle strain rate of 0 1 0 2 mstrain yr consistent with the observed surface strain rate long term background shear stresses at the top of the mantle are 4 mpa then decrease with depth to a minimum of 0 1 0 2 mpa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity these conditions correspond to a background viscosity of 10 21 pa s within a thin mantle lid that decreases to 5 10 19 pa s within the underlying asthenosphere this study shows the utility of using short term postseismic observations to infer long term mantle conditions that are not readily observable by other means

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

structures of the oceanic lithosphere asthenosphere boundary mineral physics modeling and seismological signatures 1 we explore possible models for the seismological signature of the oceanic lithosphere asthenosphere boundary lab using the latest mineral physics observations the key features that need to be explained by any viable model include 1 a sharp 20 km width and a large 5 10 velocity drop 2 lab depth at 70 km in the old oceanic upper mantle and 3 an age dependent lab depth in the young oceanic upper mantle we examine the plausibility of both partial melt and sub solidus models because many of the lab observations in the old oceanic regions are located in areas where temperature is 1000 1200 k significant partial melting is difficult we examine a layered model and a melt accumulation model at the lab and show that both models are difficult to reconcile with seismological observations a sub solidus model assuming absorption band ab physical dispersion is inconsistent with the large velocity drop at the lab we explore a new sub solidus model originally proposed by karato 2012 that depends on grain boundary sliding in contrast to the previous model where only the ab behavior was assumed the new model predicts an age dependent lab structure including the age dependent lab depth and its sharpness strategies to test these models are presented

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

effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries 1 hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate we have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge melting therefore does not occur directly above the region where hydrous fluids are released from the slab a melting front develops where hydrous fluids first encounter mantle material hot enough to melt wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material the region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region the extent of melting f and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front respectively the position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences f and melt production rates applying the understanding of wet melting from this study to geochemical studies of the aleutians may help elucidate the processes influencing fluid migration and melt production in that region estimates of the timescale of fluid migration seismic velocity variation and attenuation are also investigated

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

seismic wave attenuation energy dissipation in viscoelastic crystalline solids seismic imaging as a tool to understand the structure and perhaps the dynamics of the planet at depth involves the spatially resolved combined study of wave velocities and velocity dispersion wave birefringence and wave attenuation mechanical absorption when integrated with insights from petrology plus or minus input from magneto tellurics specific hypotheses concerning structure at depth can be formulated cf karato 1993 karato and karki 2001 the recent advances in tomography have allowed significant improvements in spatial resolution which also have allowed ever more exacting hypotheses of structure to be articulated nevertheless as is nicely illustrated by the relatively recent seismic analyses of an accreting plate margin the east pacific rise e g toomey et al 1998 webb and forsyth 1998 the structures inferred at depth can vary significantly clearly the interpretation of seismic data is limited specifically by a lack of understanding of the physical processes by which low frequency wave absorption occurs particularly in the cases a where melt is present and or b where the material is being actively plastically deformed this chapter emphasizes the mineral physics materials science of mechanical absorption specifically in dense materials at elevated temperature the interest then is in employing the physical study of mechanical absorption to understand natural phenomena such as those mentioned above beyond questions of the basic mineral physics of absorption the guiding interest the community of scholars pursuing the ideas is in isolating actual absorption mechanisms operative in the geological setting thereby allowing for a greater discrimination in the interpretation of seismic data at the outset i note that this contribution does not seek to be a comprehensive review of the ideas and work involved in the physics of mechanical absorption there are marvelous reviews in the literature that are both authoritative and current i refer to these frequently in contemplating my own

crustal and uppermost mantle shear velocity structure adjacent to the juan de fuca ridge from ambient seismic noise 1 based on 6 months of obs data from the cascadia initiative experiment near the juan de fuca ridge we obtain rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs we confirm the hypothesis that the dispersion data can be fit by a simple age dependent formula which we invert using a bayesian monte carlo formalism for an age dependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0 5 and 3 5 ma igneous crustal structure is age invariant with a thickness of 7 km water depth varies in a prescribed way and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age the mantle model possesses a shallow low shear velocity zone lvz with a velocity minimum at about 20 km depth at 0 5 ma with lithosphere thickening monotonically with age minimum mantle shear velocities at young ages are lower than predicted from a half space conductively cooling model hscm and the lithosphere thickens with age faster than the hscm providing evidence for nonconductive cooling in the young lithosphere the shallow lvz is consistent with expectations for a largely dehydrated depleted harzburgite mantle with a small retained near ridge partial melt fraction probably less than 1 with melt extending to a lithospheric age of approximately 1 ma i e 30 km from the ridge

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

shear wave velocity and radial anisotropy structures beneath the central pacific from surface wave analysis of obs records abstract the central pacific plate is an ideal area to study the dynamics of the lithosphere asthenosphere system as this region is far away from other dynamics systems such as subduction zones and volcanic hotspots of mantle plumes and has experienced simple oceanic evolution in this study we collect three component continuous seismic data and differential pressure gauge data recorded by 15 broadband ocean bottom seismometers obss from the nomelt experiment we process continuous seismic noise data to obtain phase velocities of the fundamental 6 30 s and first high 5 11 s mode rayleigh waves and fundamental mode love waves 5 12 s we apply a two plane wave method to teleseismic rayleigh waves to obtain the fundamental mode phase velocities at 20 150 s periods and apply high resolution linear radon transform to teleseismic love waves to obtain phase velocities for the fundamental mode love waves at 28 48 s periods then we jointly invert the average phase velocities of rayleigh and love waves to construct 1 d isotropic shear wave velocity and radial anisotropy from the seafloor to 150 km depth beneath the nomelt region our 1 d model shows a low velocity zone lvz beneath a high velocity lid by comparing our results with the predictions of partial melt models we suggest the lvz is attributed to the presence of a small amount of melt in the asthenosphere strong radial anisotropy 5 6 is observed in the crust but the mantle lithosphere is characterized by weak radial anisotropy

development of phyllonite from granodiorite mechanisms of grain size reduction in the santa rosa mylonite zone california abstract field and laboratory investigations indicate that phyllonite within the santa rosa mylonite zone developed from granodiorite without hydrous alteration of the original mineral assemblage initial grain size reduction within zones of phyllonite was accomplished through whole rock cataclasis subsequent deformation occurred through ductile flow the original cataclastic failure is recorded by sharp boundaries with the surrounding rock which are retained through the ductile overprint biotite in phyllonite has a mean grain size of 3 m and exhibits straight boundaries and a strong crystallographic preferred orientation trace analyses of hvem images indicate that the largest biotite grains are most strongly oriented and the grains are elongate parallel to lineation tem observations indicate that typical grains have irregularly spaced stacking faults and twins evidence for intracrystalline folding and cataclasis present in protomylonite and mylonite is absent bending or kinking of grains is rare these observations suggest that biotite in phyllonite accommodated deformation through dynamic recrystallization in concert with intracrystalline slip and mechanical rotation grain size reduction to form phyllonite is accompanied by macroscopically and microscopically visible changes in the character of foliations in the rock we suggest that these changes are linked to the deformation mechanisms operating within the fine grained phyllonite

hot upper mantle beneath the tristan da cunha hotspot from probabilistic rayleigh wave inversion and petrological modeling understanding the enigmatic intraplate volcanism in the tristan da cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it we measured phasevelocity curves of rayleigh waves using cross correlation of teleseismic seismograms from an array of ocean bottom seismometers around tristan constrained a region average shear velocity structure and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot the ocean bottom data set presented some challenges which required data processing and measurement approaches different from those tuned for land based arrays of stations having derived a robust phase velocity curve for the tristan area we inverted it for a shear wave velocity profile using a probabilistic markov chain monte carlo approach the model shows a pronounced low velocity anomaly from 70 to at least 120 km depth vs in the low velocity zone is 4 1 4 2 km s not as low as reported for hawaii 4 0 km s which probably indicates a less pronounced thermal anomaly and possibly less partial melting petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle mantle potential temperature of 1 410 1 4308c an excess of about 50 1208c compared to the global average and a melt fraction smaller than 1 both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65 70 km consistent with recent estimates from receiver functions the presence of warmer than average asthenosphere beneath tristan is consistent with a hot upwelling plume from the deep mantle however the excess temperature we determine is smaller than that reported for some other major hotspots in particular hawaii

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

seismic properties of anita bay dunite an exploratory study of the influence of water as a pilot study of the role of water in the attenuation of seismic waves in the earth s upper mantle we have performed a series of seismic frequency torsional forced oscillation experiments on a natural anita bay dunite containing accessory hydrous phases at high temperatures to 13008c and confining pressure pc o f 200 mpa within a gas medium high pressure apparatus both oven dried and pre fired specimens wrapped in ni fe foil within the poorly vented assembly were recovered essentially dry after 50 100 h of annealing at 13008c followed by slow staged cooling the results for those specimens indicate broadly similar absorptionband viscoelastic behaviour but with systematic differences in the frequency dependence of strain energy dissipation q 1 attributed to differences in the small volume fraction of silicate melt and its spatial distribution in contrast it has been demonstrated that a new assembly involving a welded pt capsule retains aqueous fluid during prolonged exposure to high temperaturescallowing the first high temperature torsional forced oscillation measurements under high aqueous fluid pore pressure pf at temperatures 410008c a marked reduction in shear modulus without concomitant increase in q 1 is attributed to the widespread wetting of grain boundaries resulting from grain scale hydrofracturing and the maintenance of conditions

lithospheric cooling trends and deviations in oceanic pp p and ss s differential traveltimes 1 the thermal and compositional structure of oceanic lithosphere which exerts an important control on plate behavior is still debated our set of 60 000 pp p and ss s traveltime differences with oceanic pp and ss bounce points provides a good constraint on both compressional and shear wave velocity by calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach we test whether lithospheric cooling can explain pp p and ss s traveltime variations with plate age the pp p and ss s traveltimes have substantial scatter but on average decrease by 0 2 and 0 7 s myr respectively when the pp and ss waves reflect off progressively older oceanic crust both a half space and a plate cooling model with a mid ocean ridge basalt source mantle potential temperature 1315 50 c explain the average values of the pp p and ss s anomalies and their decrease with plate age residual pp p and ss s anomalies relative to a cooling model reveal large scale patterns along a few paths e g tonga fiji to western north america seismic heterogeneity in the deep mantle is responsible for a significant fraction of the pp p and ss s traveltime variation most anomalies probably correspond to broad temperature variations in the upper mantle such as a very slow central northern pacific which may require a 100 c excess temperature and high and low velocity anomalies along the ridges that correlate with deep and shallow bathymetry respectively

radial viscous fingering of hot asthenosphere within the icelandic plume beneath the north atlantic ocean abstract the icelandic mantle plume has had a significant influence on the geologic and oceanographic evolution of the north atlantic ocean during cenozoic times full waveform tomographic imaging of this region shows that the planform of this plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plates at a depth of 100 200 km the distribution of these anomalies suggests that about five horizontal fingers extend radially beneath the fringing continental margins the best imaged fingers lie beneath the british isles and beneath western norway where significant departures from crustal isostatic equilibrium have been measured here we propose that these radial fingers are generated by a phenomenon known as the saffman taylor instability experimental and theoretical analyses show that fingering occurs when a less viscous fluid is injected into a more viscous fluid in radial miscible fingering the wavelength and number of fingers are controlled by the mobility ratio i e the ratio of viscosities by the peclet number i e the ratio of advective and diffusive transport rates and by the thickness of the horizontal layer into which fluid is injected we combine shear wave velocity estimates with residual depth measurements around the atlantic margins to estimate the planform distribution of temperature and viscosity within a horizontal asthenospheric layer beneath the lithospheric plate our estimates suggest that the mobility ratio is at least 20 50 that the peclet number is o 10 4 and that the asthenospheric channel is 100 20 km thick the existence and planform of fingering is consistent with experimental observations and with theoretical arguments a useful rule of thumb is that the wavelength of fingering is 5 1 times the thickness of the horizontal layer our proposal has been further tested by examining plumes of different vigor and planform e g hawaii cape verde yellowstone our results support the notion that dynamic topography of the earth s surface can be influenced by fast irregular horizontal flow within thin but rapidly evolving asthenospheric fingers

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

predictions and observations for the oceanic lithosphere from s to p receiver functions and ss precursors the ocean lithosphere is classically described by the thermal half space cooling hsc or the plate models both characterized by a gradual transition to the asthenosphere beneath scattered waves find sharp seismic discontinuities beneath the oceans possibly from the base of the plate active source studies suggest sharp discontinuities from a melt channel we calculate synthetic s to p receiver functions and ss precursors for the hsc and plate models and also for channels we find that the hsc and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate subtle phases are predicted to follow a similar trend as observations flattening at older ages therefore the seismic discontinuities are probably caused by a thermally controlled process that can also explain their amplitude such as melting melt may coalesce in channels although channels 10 km thick should be resolvable by scattered wave imaging

protracted fabric evolution in olivine implications for the relationship among strain crystallographic fabric and seismic anisotropy abstract crystallographic fabrics in olivine rich rocks provide critical information on conditions and mechanisms of deformation as well as seismic properties of earth s upper mantle previous interpretations of fabrics produced in laboratory experiments were complicated by uncertainty as to whether the steady state fabric was attained to examine the systematics of the evolution of olivine crystallographic fabrics at high strain we conducted torsion experiments on olivine aggregates to shear strains of up to 20 our results demonstrate that a steady state fabric is not reached until a shear strain 10 a much higher value than previously thought necessary fabrics characterized by girdles of 010 and 001 axes or by clusters of 010 and 001 axes are both observed until now these fabrics were associated with either two different deformation mechanisms or two different sets of deformation conditions here we establish that both fabrics are in fact part of the same evolutionary process an eigenvalue analysis allows the fabric shape to be quantitatively correlated with the magnitude of shear strain misorientation analysis suggests that the observed fabric evolution results from the competition of the two easiest slip systems in olivine 010 100 and 001 100 our results open up the possibility of using olivine crystallographic fabrics or seismic anisotropy to quantitatively evaluate strain histories in both field studies and geophysical investigations of upper mantle rocks

trans pacific upper mantle shear velocity structure 1 we use 50 tonga fiji events recorded at the broadband trinet array southern california to develop a pure path upper mantle shear velocity model across the pacific at the epicentral distances of 70 95 multibounce s waves up to s5 are observed including their triplicated branches which become particularly clear after stacking since these s wave multiples turn at various depths simultaneously modeling their differential traveltimes and waveforms provides strong constraints on the radial velocity structure we parameterize the velocity model according to a priori information from the previous oceanic models so that we can take a grid search approach to fully investigate possible interdependencies among the model parameters we construct synthetics with a reflectivity code and study both the sh and sv components by modeling the whole recordings from events at different depths with different mechanisms we are able to separate shallow low velocity zone lvz features from deeper structure our preferred model pac06 contains a fast lid vsh 4 78 km s 1 vsv 4 58 km s 1 with a thickness of 60 km the underlying lvz is prominent with the lowest velocities vsh 4 34 km s 1 and vsv 4 22 km s 1 occurring at a depth of 160 km these velocities are below the estimates of solid state lvz suggesting the presence of partial melt the anisotropy vsv vsh of pac06 extends to 300 km depth which is constrained by s5 turning at this depth besides the 406 km and 651 km discontinuities pac06 also has a small 1 velocity jump at 516 km we consider these main features of pac06 to be well determined since pac06 explains a large data set from various events therefore it is ideally suited for comparing with mineralogical models

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

constraints on the anisotropic contributions to velocity discontinuities at 60 km depth beneath the pacific strong sharp negative seismic discontinuities velocity decreases with depth are observed beneath the pacific seafloor at 60 km depth it has been suggested that these are caused by an increase in radial anisotropy with depth which occurs in global surface wave models here we test this hypothesis in two ways we evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests we do this by fitting an example surface wave data set near the east pacific rise we also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in s to p and p to s receiver functions and ss precursors using synthetic seismograms we test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering the result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0 10 ma seafloor which would also have a radial anisotropy signature an increase in radial anisotropy with depth at 60 km depth is not well resolved in surface wave models and could be artificially observed shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops 6 in ss precursor imaging but not receiver functions the effect is diminished if strong anisotropy also exists at 0 60 km depth as suggested by surface waves overall an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations

grain size evolution and the rheology of ductile shear zones from laboratory experiments to postseismic creep abstract geologic experimental and geodetic studies indicate that ductile shear zones do not deform in steady state we present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep in which grain size is allowed to evolve toward its equilibrium recrystallized value several grain size evolution laws are considered simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain weakening or strain strengthening episodes reminiscent of laboratory observations ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes non steady state ductile creep is difficult to differentiate from frictional sliding with velocity strengthening friction hence ductile creep may play a role in the transition from velocity weakening to velocity strengthening friction with increasing temperature

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

formation of lithospheric shear zones effect of temperature on two phase grain damage abstract shear localization in the lithosphere is a characteristic feature of plate tectonic boundaries and is evident in the presence of small grain mylonites localization and mylonitization in the ductile portion of the lithosphere can arise when its polymineralic material deforms by a grain size sensitive rheology in combination with zener pinning which can impede or possibly even reverse grain growth and thus promotes a self softening feedback mechanism however the efficacy of this mechanism is not ubiquitous and depends on lithospheric conditions such as temperature and stress therefore we explore the conditions under which self weakening takes place and in particular the effect of temperature and deformation state stress or strain rate on these conditions in our model the lithosphere like polymineralic material is deformed in a two dimensional simple shear driven by constant stress or strain rate the mineral grains evolve to a stable size which is obtained when the rate of coarsening by normal grain growth and the rate of grain size reduction by damage are in balance damage involves processes by which some of the deformational energy gets transferred into surface energy this can happen by i dynamic recrystallization grain damage and ii stretching deforming and stirring the material interface interface damage the influence of temperature enters through rheological laws which govern the rate of work and damage grain growth kinetics and the damage partitioning fraction which is the fraction of deformational work that goes into creating new surface energy we demonstrate that a two phase damage model in which the partitioning fraction depends on both temperature and roughness of the interface between the phases can successfully match the field data including the reported correlation of grain size and temperature the increasing dominance of dislocation creep at higher temperatures and a large range of grain sizes observed across the depth of a single shear zone

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data