densification of polar snow measurements modeling and implications for altimetry density profiles in the upper 10 14 m of snow have been measured along a 500 km traverse across the greenland ice sheet using a neutron scattering technique repeat measurements over periods ranging from a few days to 5 years allow strain rates to be determined as a function of depth very large strain rates are observed in the surface layer of snow over summer periods in the underlying multiyear snow strain rate decreases with decreasing porosity however once this effect has been removed the effect of increasing overburden pressure is counteracted by increasing strength of the material there are fluctuations in strain rate associated with the annual layering which indicate that winter and summer snow have different strengths based on these observations we derive a new densification equation which includes the effect of snow density and snow type and the effect of temperature described by an arrhenius expression with activation energy of the order of 110 kj mol 1 and an exponential prefactor determined simply by the temperature history of the snow for multiyear snow and meteorological conditions that do not vary from year to year our equation reduces to a form similar to the herron and langway equation for first stage densification using the new equation we calculate the sensitivity of compaction rate to short term fluctuations in temperature and accumulation as 0 11 0 20 m a 1 k 1 and 0 33 0 95 m a 1 meters water equivalent 1 respectively and discuss the consequent uncertainty in satellite measurements of the long term elevation trend in this area of the greenland ice sheet

seismic imaging of melt in a displaced hawaiian plume the precise location of the mantle plume upwelling beneath hawaii is debated seismic data reveal a thick layer of melt in the mantle beneath western hawaii implying that the upwelling plume may be deflected around an ancient resistive root beneath the island

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

an s receiver function analysis of the lithospheric structure in south america 1 we make use of s receiver function to investigate the structure and thickness of the crust and mantle lithosphere in the south american continent and adjacent areas the moho discontinuity has been detected at all stations and goes from 18 km beneath the coast of the continent to 80 km in the andean region the depth of the lithosphere asthenosphere boundary can be clearly detected below those stations that are located on stable areas from 50 km to 160 km the identification of this phase becomes more difficult when the stations are located near subduction zones we also observed the base of the subducted oceanic lithosphere down to depths of 220 km

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

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

upper mantle seismic shear discontinuities of the pacific 1 shear wave impedance discontinuities are inventoried for eight paths connecting circum pacific earthquakes with seismic stations on hawaii and oahu in addition to the transition zone discontinuities we observe a consistent impedance decrease at a depth of 80 km that marks the transition from the fast seismic lid to the low velocity zone the interval over which this transition occurs is less than 30 km the requisite impedance decrease ascribed almost entirely to diminished velocities in the low velocity zone exceeds predictions for peridotite chemistries of appropriate lithospheric age in all cases the transition is better matched by lithosphere of roughly half of the true age four paths show clear evidence of the x discontinuity an impedance increase of 3 8 near 300 km depth the only viable explanations would require extensive eclogite or basalt depleted mantle in the upper mantle of the western pacific the impedance contrast of the 410 km discontinuity which depends on the modal fraction of mantle olivine does not signal the presence of either although a mixture of the two remains a possibility we find no evidence of a low velocity layer atop of the 410 km discontinuity if present it is either consistently thin 15 km highly variable in thickness topography in excess of 60 km or has little impedance contrast 2 3 with overlying mantle the apparent absence of melt atop the 410 km discontinuity an intermittent and weak 520 km discontinuity and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central pacific

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

cone penetration testing cpt in antarctic firn an introduction to interpretation commercial cone penetration testing cpt equipment was adapted to allow penetrative testing in hard polar firn to depths of 10 m the apparatus is hydraulically driven rate controllable and able to penetrate firn with a resistance of 10 mpa it can be mounted on many types of typical polar vehicles requiring connection to only hydraulics and 12 v electricity data recorded include both cone tip resistance and sleeve friction a parameter not previously examined through such testing this paper describes the development and calibration of the equipment and examines factors including snow density penetration rate and cone size and shape that are shown to affect cpt interpretation cpt can be used efficiently in polar environments to potentially provide estimates of physical parameters in hard firn to substantial depth

age progressive volcanism opposite nazca plate motion insights from seamounts on the northeastern margin of the galapagos platform highlights four of the seven seamounts northeast of the galapagos platform are drowned islands the ages of the seamounts range from 5 2 ma to 0 5 ma seamount morphology changes from conical to elongate at 1 5 ma the locus of volcanism appears to migrate eastward at the rate of nazca plate motion abstract we present new geochemical and 40ar 39ar analyses from seven seamounts located off the northeastern margin of the shallow galapagos platform initial volcanism at 5 2 ma created a small island pico over the current location of the hotspot with geochemically enriched lavas there is no further record of magmatism in the study area until 3 8 to 2 5 ma during which four roughly conical volcanoes sunray grande fitzroy and beagle formed through eruption of lavas derived from a depleted mantle source sunray fitzroy and grande were islands that existed for 3 m y ending with the submergence of fitzroy at 0 5 ma the youngest seamounts largo and iguana do not appear to have been subaerial and were active at 1 3 ma and 0 5 ma respectively with the style of edifice changing from the previous large cones to e w elongate composite structures the progression of magmatism suggests that pico erupted near 91 5 w near the location of the galapagos plume while the others formed well east of the plume center if the locations of initial volcanism are calculated using the eastward velocity of the nazca plate there appears to be a progression of younger volcanism toward the east opposite what would be expected from a fixed mantle plume source the rate that initial volcanism moves eastward is close to the plate velocity a combination of higher temperature and geochemical enrichment of the thickened lithosphere of the galapagos platform could have provided a viscosity gradient at the boundary between the thick lithosphere and the thinner oceanic lithosphere to the northeast as this boundary moved eastward with the nazca plate it progressively triggered shear driven mantle upwelling and volcanism

melting dehydration and the dynamics of off axis plume ridge interaction 1 we present results from a series of two dimensional numerical experiments in which a thermally buoyant off axis mantle plume interacts with a nearby ridge axis these experiments incorporate melting and a number of related dynamical feedbacks including energy loss to latent heating and fe depletion buoyancy as well as viscosity increases due to dehydration during melting results indicate that dehydration which acts to increase the viscosity of the residual mantle during melting profoundly impacts the morphology of flow between plume and ridge axis as upwelling plume material begins to melt it becomes significantly more viscous and readily accretes to the overlying lithosphere this creates a viscous plug that grows downward from the base of the lithosphere directly above the plume conduit plume material traveling to the ridge axis is then deflected horizontally at subsolidus depths by this plug allowing pristine unmelted plume material to reach the ridge axis experiments without dehydration show that the loss of thermal energy to latent heating during melting marginally decreases the flow of plume material along the base of the lithosphere to the ridge axis alternatively extraction of fe from the solid during melting slightly increases plume flow to the ridge these results also demonstrate that the flow of plume material to the ridge axis is sensitive to the slope of the base of the rheological lithosphere consistent with previous studies of off axis plume ridge interaction

anomalously thin transition zone beneath the galapagos hotspot differential arrival times of p to s conversions at 410 and 660 km depth measured from radial receiver functions are used to determine the thickness of the mantle transition zone beneath the galapagos hotspot for an area of approximately 700 km 2 surrounding the galapagos archipelago p660s3p410s times are not significantly different from those for the pacific basin in contrast a subset of the galapagos data yields differential times indicating thinning of the mantle transition zone by 18 3 8 km within an area approximately 100 km in radius centered about 40 km southwest of the center of the island of fernandina this anomaly is consistent with an excess temperature of 130 3 60 k within this volume of the transition zone comparable to that inferred beneath the iceland and society hotspots the most straightforward interpretation of this anomaly is that a mantle plume upwelling from depths greater than 410 km underlies the galapagos hotspot

the lithosphere asthenosphere boundary in the north west atlantic region abstract a detailed knowledge of the thickness of the lithosphere in the north atlantic is an important parameter for understanding plate tectonics in that region we achieve this goal with as yet unprecedented detail using the seismic technique of s receiver functions clear positive signals from the crust mantle boundary and negative signals from a mantle discontinuity beneath greenland iceland and jan mayen are observed according to seismological practice we call the negative phase the lithosphere asthenosphere boundary lab the seismic lithosphere under most of the iceland and large parts of central greenland is about 80 km thick this depth in iceland is in disagreement with estimates of the thickness of the elastic lithosphere 10 20 km found from postglacial rebound data in the region of flood basalts in eastern greenland which overlies the proposed iceland plume track the lithosphere is only 70 km thick about 10 km less than in iceland which is located directly above the proposed plume at the western greenland coast the lithosphere thickens to 100 120 km with no indication of the iceland plume track identified below jan mayen the lithospheric thickness varies between 40 and 60 km

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

three dimensional coordination number from two dimensional measurements a new method la valeur de n 3 moyenne des coordonnees du neve a trois dimensions peut etre estimee a partir de n 2 nombre moyen des mesures a deux dimensions et a partir d une fonction qui depend uniquement du rapport entre le rayon moyen de voisinage et de celui du grain de l echantillon

frequency dependence of q in earth s upper mantle inferred from continuous spectra of body waves q0 f f0 a and applied a grid search to determine the best fit a and q0 the inferred frequency dependence 0 2 a 0 4 in most regions is consistent with laboratory data on solid olivine but not on partially molten olivine rich rocks except those data for deep events close agreement between seismological and laboratory observations on frequency dependence of q suggests that physical mechanisms of seismic wave attenuation are common and therefore laboratory data can be used to infer the physical state of earth s upper mantle from seismic wave attenuation index terms 5144 physical properties of rocks wave attenuation 7218 seismology lithosphere and upper mantle 8162 tectonophysics rheology mantle citation shito a s karato and j park 2004 frequency dependence of q in earth s upper mantle inferred from continuous spectra of body waves geophys res lett 31 l12603

an upper mantle seismic discontinuity beneath the galapagos archipelago and its implications for studies of the lithosphere asthenosphere boundary an upper mantle seismic discontinuity the gutenberg or g discontinuity at which shear wave velocity decreases with depth has been mapped from s to p conversions in radial receiver functions recorded across the galapagos archipelago the mean depth of the discontinuity is 91 8 km beneath the southeastern archipelago and 72 5 km beneath surrounding regions the discontinuity appears deeper beneath the portion of the nazca plate that we infer passed over the galapagos mantle plume than elsewhere in the region we equate the depth of the g discontinuity to the maximum depth extent of anhydrous melting which forms an overlying layer of dehydrated and depleted mantle we attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the galapagos spreading center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 30 c the g discontinuity lies within a high seismic velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile bearing mantle material at the depth of the solidus for anhydrous mantle material removal of remaining water creates a sharp decrease in velocity with depth this discontinuity may also mark a site of melt accumulation results from seismic imaging the compositions of galapagos lavas and rare earth element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer we conclude that the g discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

seawater causes rapid trace metal mobilisation in coastal lowland acid sulfate soils implications of sea level rise for water quality abstract coastal floodplains are highly vulnerable to inundation with saline water and the likelihood of inundation will increase with sea level rise sediment samples from floodplains containing coastal lowland acid sulfate soils class in eastern australia were subjected to increasing seawater concentration to examine the probable effects of sea level rise on acidity and metal desorption ten soils were mixed with synthetic seawater concentrations varying from 0 to 100 at a solid solution ratio of 1 10 for 4 h there was a slight decrease in ph 0 5 units with increasing seawater concentration following treatment yet calculated acidity increased significantly in most soil treatments al was the dominant component of the calculated acidity pool al dominated the exchange complex in the class and correspondingly was the major metal ion desorbed in general concentrations of soluble and exchangeable al fe 2 ni mn and zn in all soil extracts increased with increasing salinity increasing trace metal concentrations with increasing seawater concentration is attributed to the combined effects of exchange processes and acidity the increasing ionic strength of the seawater treatments displaces trace metals and protons adsorbed on sediments causing an initial decrease in ph hydrolysis of desorbed acidic metal cations can further contribute to acidity and increase mobilisation of trace metals these findings imply that saline inundation of class environments even by relatively brackish water may cause rapid shorter term water quality changes and a pulse release of acidity due to desorption of acidic metal cations

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

scattered wave imaging of the lithosphere asthenosphere boundary several recent studies have imaged discontinuities interpreted as the lithosphere asthenosphere boundary using receiver function methods these are exciting results since this boundary is fundamental and yet relatively elusive its location is not well imaged nor are its properties well understood the receiver function method provides high depth resolution and the multitude of studies using this method have increased lateral resolution a better global understanding of the location and nature of the boundary is developing which ultimately will have implications for mantle dynamics and planetary evolution however these new results do not all form a clear and consistent picture and in some cases introduce new complexities and questions some apparent contradictions may represent unmodeled earth structure overall interpretation and synthesis requires careful consideration of the approaches and sensitivities of individual studies

p sh conversions in layered media with hexagonally symmetric anisotropy a cookbook reflectivity synthetic seismograms demonstrate that the type layering and orientation of 1 d anisotropy influences strongly the coda of teleseismic p waves at periods t 1 sec particularly p sh converted waves we assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis of arbitrary orientation the resulting phase velocities vary as cos 2 with respect to that axis using three families of simple crustal models we compare the effects of an anisotropic surface layer with reverberations caused by both thick and thin layers of anisotropy at depth if anisotropy in the surface layer is significant the polarization of direct p can be distorted to generate a transverse component followed by ps and a prominent shear reverberation converted from direct p at the free surface if the anisotropic layer is buried the first and often the most prominent arrival on the transverse component is the p to sh conversion at its upper surface if the anisotropic layer is sufficiently thin p to sh conversions from its boundaries interfere to form a derivative pulse shape on the transverse component which could be mistaken as the signature of shear wave splitting if is horizontal compressional p and shear s anisotropy both produce similar waveform perturbations with four lobed azimuthal patterns suggesting that a weighted stack of p coda from different back azimuths would improve signal to noise for tilted between the horizontal and vertical however the effects of p and s anisotropy differ greatly the influence of p anisotropy on p to s conversion is greatest for a symmetry axis tilted at 45 to the vertical where its azimuthal pattern has two lobes rather than four combinations of p and s anisotropy typically lead to a composite azimuthal dependence in the p coda reverberations

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

major element chemistry of galapagos rift zone magmas and their phenocrysts abstract basalts dredged from the galapagos rift zone between 85 w and 100 w were analyzed by electron microprobe to determine the chemistry of the glass exteriors and included phenocrysts microphenocrysts and quench minerals the basalts come from both normal mid ocean ridge segments and from ridge segments that cross the galapagos platform the basalts fall into two chemical and geographical groups group a basalts come from outside the central region of the galapagos platform i e outside 89 92 5 w and are chemically similar to basalts from normal ocean ridge segments group b basalts come from the center of the galapagos platform 89 92 5 w and are enriched in incompatible elements like plume influenced basalts from the mid atlantic ridge the spinel olivine plagioclase and clinopyroxene phenocrysts in both groups of basalts are low pressure equilibrium phases but the chemical difference among basalts from within each group indicates high pressure fractional crystallization is also responsible for the chemical evolution of some of these basalts presently no crystallization or partial melting model can relate the chemistry of the two groups of basalts and the compositional influence of a large ion lithophile elements and water rich mantle beneath the galapagos platform is a viable alternate hypothesis the eruption temperature of magmas from the normal ridge segments as determined by olivine liquid thermometry is 1217 10 c suggesting steady state conditions but on the galapagos platform the eruption temperatures are lower and more variable than on the normal ridge 1186 c 30 c suggesting a more complex plumbing system and the absence of a steady state magma chamber beneath the platform

a 1 5 ma record of plume ridge interaction at the western galapagos spreading center 91 40 92 00 w abstract shallow elevated portions of mid ocean ridges with enriched geochemical compositions near hotspots document the interaction of hot geochemically enriched plume mantle with shallow depleted upper mantle whereas the spatial variations in geochemical composition of ocean crust along the ridge axis in areas where plume ridge interaction is taking place have been studied globally only restricted information exists concerning temporal variations in geochemistry of ocean crust formed through plume ridge interaction here we present a detailed geochemical study of 0 1 5 ma ocean crust sampled from the western galapagos spreading center wgsc axis to 50 km north of the axis an area that is presently experiencing a high influx of mantle material from the galapagos hotspot the tholeiitic to basaltic andesitic fresh glass and few bulk rock samples have incompatible element abundances and sr nd pb isotopic compositions intermediate between depleted normal mid ocean ridge basalt n morb from 95 5 w along the wgsc and enriched lavas from the galapagos archipelago displaying enriched e morb type compositions only limited and no systematic geochemical variations are observed with distance from the ridge axis for

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

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

crustal structure beneath the galapagos archipelago from ambient noise tomography and its implications for plume lithosphere interactions 1 to constrain the seismic velocity structure of the crust beneath the galapagos archipelago we conducted a tomographic study using high frequency rayleigh waves obtained from cross correlations of ambient noise we analyzed waves with periods between 5 and 8 5 s sensitive to shear wave velocity vs structure between about 3 and 10 km depth after accounting for the effect of water depth crustal velocities are up to 25 lower than those of very young crust at the east pacific rise and are comparable to those of hawaii we attribute the lower than normal velocities to the combined effect of heating and the presence of melt in the crust above the galapagos plume as well as the construction of a highly porous volcanic platform emplaced atop preexisting oceanic crust on average vs between 3 and 10 km depth beneath the western archipelago is up to 15 lower than beneath the eastern archipelago we attribute the west to east velocity increase to a decrease in porosity of the volcanic platform and to cooling of the crust after its passage above the galapagos plume the results of this study in combination with previous work indicate that many of the unusual aspects of the galapagos archipelago are the result of variations in the thickness and internal structure of the chemical and thermal lithospheres our findings indicate that observed variations in the flexural response to loading observed in the galapagos cannot be explained by the current thermal state of the lithosphere instead the flexural response likely represents varying elastic strength at the time of loading we also propose that the northwest and northeast trending alignments of volcanic centers found throughout the archipelago the darwinian lineations may be associated with preexisting zones of weakness in the lithosphere formed during earlier episodes of ridge jumping and ridge propagation that were later reactivated by stresses generated by plume lithosphere interactions

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

seismic attenuation near the east pacific rise and the origin of the low velocity zone abstract low shear wave velocities beneath mid ocean ridges and in the low velocity zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water but several recent studies have challenged that interpretation the alternative is that the anelastic effects of increasing temperature may cause the observed drop in velocity along with a predicted increase in attenuation we report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor near the east pacific rise there is much less attenuation than is predicted by models in which the velocity is controlled solely by the direct elastic and anelastic effects of changing temperature suggesting that melt and water concentration do play an important role there also is somewhat less attenuation than is found in global studies we speculate that scattering from unresolved velocity heterogeneities contribute to the apparent attenuation in global studies

influence of sediment redox conditions on uranium mobilisation during saline intrusion abstract in the uk several coastal nuclear sites have been identified as vulnerable to future sea level rise legacy contamination at these sites has accumulated in sub surface sediments at risk of future seawater inundation and intrusion porewater salinization changes in ph and the influx of oxygen into sediments may impact the stability of sediment associated uranium u in this study saturated column experiments were performed to compare the mobilisation of u from oxic and reduced sediments into seawater under environmentally relevant flow conditions uranium release profiles were independent of the initial geochemistry of the sediments uranium release from the sediments was kinetically controlled showing relatively slow desorption kinetics with release initially limited by the impact of the sediments on the ph of the seawater significant u release only occurred when the ph was sufficiently high for the formation of u carbonate complexes phoxic 6 3 phreduced 7 5 uranium was more strongly bound to the reduced sediments and after 400 pore volumes of seawater flow release was more extensive from the initially oxic 46 compared with initially nitrate reducing 27 and iron reducing 18 sediments the products of iron cycling appeared to act as a buffer limiting u mobilisation but the on going dissolution of the fe phases suggests that they did not form a permanent protective layer

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

repeat ridge jumps associated with plume ridge interaction melt transport and ridge migration 1 repeated shifts or jumps of mid ocean ridge segments toward nearby hot spots can produce large long term changes to the geometry and location of the tectonic plate boundaries ridge jumps associated with hot spot ridge interaction are likely caused by several processes including shear on the base of the plate due to expanding plume material as well as reheating of lithosphere as magma passes through it to feed off axis volcanism to study how these processes influence ridge jumps we use numerical models to simulate 2 d in cross section viscous flow of the mantle viscoplastic deformation of the lithosphere and melt migration upward from the asthenospheric melting zone laterally along the base of the lithosphere and vertically through the lithosphere the locations and rates that magma penetrates and heats the lithosphere are controlled by the time varying accumulation of melt beneath the plate and the depth averaged lithospheric porosity we examine the effect of four key parameters magmatic heating rate of the lithosphere plate spreading rate age of the seafloor overlying the plume and the plume ridge migration rate results indicate that the minimum value of the magmatic heating rate needed to initiate a ridge jump increases with plate age and spreading rate the time required to complete a ridge jump decreases with larger values of magmatic heating rate younger plate age and faster spreading rate for cases with migrating ridges models predict a range of behaviors including repeating ridge jumps much like those exhibited on earth repeating ridge jumps occur at moderate magmatic heating rates and are the result of changes in the hot spot magma flux in response to magma migration along the base of an evolving lithosphere the tendency of slow spreading to promote ridge jumps could help explain the observed clustering of hot spots near the mid atlantic ridge model results also suggest that magmatic heating may significantly thin the lithosphere as has been suggested at hawaii and other hot spots

upper mantle structure beneath the galapagos archipelago from surface wave tomography 1 we present a rayleigh wave tomographic study of the upper mantle beneath the galapagos archipelago we analyze waves in 12 separate frequency bands 8 50 mhz sensitive to shear wave velocity vs structure in the upper 150 km average phase velocities are up to 2 and 8 lower than for 0 to 4 my old and 4 to 20 my old pacific seafloor respectively laterally averaged vs is 0 05 0 2 km s lower between 75 and 150 km depth than for normal pacific mantle of comparable age corresponding to an excess temperature of 30 to 150c and 0 5 melt a continuous low velocity volume that tilts in a northerly direction as it shoals extends from the bottom of our model to the base of a high velocity lid which is located at depths varying from 40 to 70 km we interpret this low velocity volume as an upwelling thermal plume that flattens against the base of the high velocity lid the high velocity lid is 30 km thicker than estimated lithospheric thickness beneath the southwestern archipelago above the main region of plume upwelling we attribute the thicker than normal high velocity lid to residuum from hot spot melting the thickness of the lid appears to control the final depth of melting and the variability of basalt composition in the archipelago at depths less than 100 120 km plume material spreads in directions both toward and against eastward plate motion indicating that plume buoyancy forces dominate over plate drag forces and suggesting a relatively high plume buoyancy flux b 2000 kg s

the lithosphere asthenosphere boundary beneath the western united states summary s receiver functions from 67 broad band seismic stations in the western united states clearly reveal the existence of a mantle discontinuity with velocity reduction downward which we interpret as the lithosphere asthenosphere boundary lab the average depth of the lab is 70 km the boundary is relatively sharp with an overall sharpness of less than 20 km the boundary is more prominent south of the mendocino triple junction where the farallon plate has completely subducted this may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window a double low velocity zone is observed at base of the lithosphere beneath southern sierra nevada implying a second melting zone at a depth of 100 km well correlated with previous studies of lithospheric delamination in the area

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

basal reflector under the philippine sea plate summary to place constraints on the observability and properties of the lithosphere asthenosphere boundary lab we sought sh precursors to ss from earthquakes beneath the philippine sea plate as a result a discontinuity with a velocity reduction downwards could be detected at depths of 50 70km across the philippine sea by using both an array method and seismic imaging technique in the frequency band of 0 01 0 3hz the lateral coherence and amplitude ratio relative to ss provide constraints on possible mechanisms by which the discontinuity arises the amplitude ratios though scattered suggest minimum shear wave contrasts of 5 8percent the change in depth of the discontinuity along the profile does not suggest a thermal origin of the signal the lateral variation of the discontinuity depth indicates that it is neither a solid solid phase transition nor a thermally mediated change in rheological fabric of the plate we find that the depths and discontinuity properties agree with previously reported properties of the lab in the philippine sea plate attributed to melt presence between 0 01 and 7 vol percent

receiver function imaging of lithospheric structure and the onset of melting beneath the galapagos archipelago abstract the galapagos archipelago represents an opportunity to investigate the properties of young oceanic lithosphere the effects of a hotspot anomaly on lithospheric thickness and melting dynamics in a hotspot ridge interaction here we use data recorded by the signet array and permanent station payg on the islands santa cruz and isabela respectively we used p to s ps and s to p sp receiver functions to constrain crust and mantle structure a simultaneous deconvolution method was used to constrain 1 d structure and also for the modeling of robust features a migrated extended multitaper method was used to investigate 3 d structural variations ps images a velocity increase with depth at 11 7 km probably the base of the pre plume crust or old moho sp imaging and modeling images a second deeper velocity increase at 37 7 km depth a velocity decrease with depth is imaged on average at 75 12 km likely associated with the lithosphere asthenosphere boundary this discontinuity is imaged deeper 82 km in the southwest and shallower 66 km in the northeast near the spreading ridge although the trend is consistent with lithospheric thickening with age the thickness is much larger than predicted by conductive cooling models of 0 10 my oceanic lithosphere we infer a compositional contribution to velocity variations finally a velocity increase with depth is imaged at 125 to 145 15 km depth that is likely associated with the onset of melting the discontinuity is imaged deeper in 3 sectors of the galapagos platform ridge region all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km one is located in the southwest in a hypothesized plume location the other two are to the northwest and northeast possibly illuminating multiple plume diversions related to complex plume ridge interactions

imaging the seismic lithosphere asthenosphere boundary of the oceanic plate 1 the seismic lithosphere asthenosphere boundary lab or g discontinuity a seismologically characterized abrupt drop in wave speed in the uppermost mantle is one of the key issues in current geodynamics although plate tectonics started as a theory for the ocean reports on lab for normal oceanic regions are scarce due to paucity of seismic data and whether or not the oceanic lab grows with age is the key issue to be resolved we conduct a systematic survey for the oceanic lab using s to p converted seismic waves along three margins of oceanic plates whose crustal age ranges from 10 myr to 130 myr and we observe laterally continuous oceanic lab images the thickness of the oceanic plate estimated from labs increases with the plate age though scattered suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic lab represents a boundary that grows with age

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

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

shear wave attenuation and dispersion in melt bearing olivine polycrystals 2 microstructural interpretation and seismological implications 1 the torsional forced oscillation tests of melt bearing olivine aggregates reported by jackson et al 2004 consistently show a peak in attenuation that is absent from melt free aggregates tested under similar conditions and grain sizes characterization by sem shows that the melt resides in triple junction tubules and larger pockets as previously described tem imaging and eds analysis reveals that olivine olivine grain boundaries are characterized by a region 1 nm wide which is structurally and chemically distinct from olivine grain interiors from the possible mechanisms that can produce an anelastic attenuation peak melt squirt can be eliminated for our samples and experimental conditions we attribute the observed attenuation peak to elastically accommodated grain boundary sliding requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt while the nanometer scale grain boundary structure in the melt bearing aggregates is essentially the same as for melt free aggregates studied previously elastically accommodated sliding in the latter is apparently inhibited by tight three grain edge intersections the exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle

ponded melt at the boundary between the lithosphere and asthenosphere the boundary between earth s rigid lithosphere and ductile asthenosphere is marked by a seismic discontinuity laboratory experiments on basaltic magmas show that melts should pond at pressures that correspond to the lithosphere asthenosphere boundary thus magma ponding could explain the observed seismic discontinuity

does partial melting explain geophysical anomalies abstract the existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity i review various experimental and theoretical studies to evaluate the plausibility of this explanation in order for a partial melt model to work not only the presence of melt but also the presence of appropriate amount of melt needs to be explained using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport i conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work one needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting however most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and or implausible therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid and some solid state mechanisms must be invoked the situation is different in the deep upper mantle where melt could completely wet grain boundaries and continuous production of melt is likely by dehydration melting at around 410 km in the ultralow velocity zone in the d layer where continuous production of melt is unlikely easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co existing solid minerals or if there is a strong convective current to support the topography of the ulvz region compositional variation such as fe enrichment is an alternative cause for the anomalies in the d layer

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

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

implications for melt transport and source heterogeneity in upwelling mantle from the magnitude of sp converted phases generated at the onset of melting recently detected converted phases in upwelling mantle have been attributed to seismic velocity gradients at the onset of melting in this study we investigate conditions required to generate a melting onset phase by combining melt migration models with synthetic receiver functions we find that increasing upwelling velocity increasing mantle viscosity and decreasing water content in the mantle source increases the predicted strength of a melting onset phase differences in these parameters between study regions may explain observed variations in converted phase magnitude for a wet mantle source the amplitude of synthetic receiver functions calculated using a standard relationship for dependence of seismic velocity on melt fraction is lower than observed amplitudes one possibility is that the observed receiver functions indicate a heterogeneous mantle source in which wet and dry components melt independently alternatively the dependence of seismic velocity on melt fraction at very low melt fractions may be much stronger than that used here as suggested in recent studies

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

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

seismic imaging of thickened lithosphere resulting from plume pulsing beneath iceland ocean plates conductively cool and subside with seafloor age plate thickening with age is also predicted and hot spots may cause thinning however both are debated and depend on the way the plate is defined determining the thickness of the plates along with the process that governs it has proven challenging we use s to p sp receiver functions to image a strong persistent lab beneath iceland where the mid atlantic ridge interacts with a plume with hypothesized pulsating thermal anomaly the plate is thickest up to 84 6 km beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 6 km beneath regions formed during colder intervals we performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere instead periods of increased plume temperatures likely increased the melting depth causing deeper depletion and dehydration and creating a thicker plate this suggests plate thickness is dictated by the conditions of plate formation

the gutenberg discontinuity melt at the lithosphere asthenosphere boundary the lithosphere asthenosphere boundary lab beneath ocean basins separates the upper thermal boundary layer of rigid conductively cooling plates from the underlying ductile convecting mantle the origin of a seismic discontinuity associated with this interface known as the gutenberg discontinuity g remains enigmatic high frequency ss precursors sampling below the pacific plate intermittently detect the g as a sharp negative velocity contrast at 40 to 75 kilometer depth these observations lie near the depth of the lab in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base i propose that the g reflectivity is regionally enhanced by dynamical processes that produce melt including hot mantle upwellings small scale convection and fluid release during subduction

laboratory experiments on shear failure of snow using a direct simple shear apparatus snow samples 115 mm in diameter 16 18 mm in height taken from a so called homogeneous layer small rounded particles density 290 kg m 3 were tested in a cold laboratory experiments were performed for strain rates between 7 10 6 s 1 and 5 10 3 s 1 at test temperatures of 5 c 10 c and 15 c the effects of strain rate and temperature on failure stress failure strain stiffness initial tangent modulus and toughness were studied the transition between the ductile and brittle sudden fracture state of failure was found to be at about 1 10 3 s 1 for the snow types tested independent of temperature stiffness proved to be the most temperature dependent property of alpine snow it strongly increases with decreasing temperature failure strain and toughness decrease with decreasing temperature failure stress was found to increase slightly with decreasing temperature the effect is not very distinct but close to statistically significant and might be partly hidden by the scatter in the stress data due to variations inherent in sampling and testing

upper mantle structure beneath the hawaiian swell constraints from the ocean seismic network pilot experiment 1 data from two broadband ocean bottom seismographic stations deployed 225 km southwest of oahu hawaii during the ocean seismic network pilot experiment provide constraints on upper mantle structure beneath the hawaiian swell receiver functions show that the mantle transition zone is thinned by 50 km relative to reference model pa5 which in the absence of compositional changes implies excess temperatures of 350 k in the transition zone the combination of the measurements reported here and the thickness variations reported by li et al 2000 imply that the transition zone is thinned by 30 15 km over an along swell dimension of at least 700 km at 80 km depth p to s converted phases are identified from the gutenberg discontinuity marking the lid of the oceanic low velocity zone and the base of the lithosphere shear wave splitting measurements imply that fast polarization azimuths are intermediate between the absolute plate motion vector and the fossil spreading direction multi event stacked values of o and t are 80 and 1 5 s respectively

triaxial tests on snow at low strain rate part i experimental device a deformation controlled triaxial apparatus has been developed in order too investigate the mechanical behaviour of alpine snow cylindrical specimens 58 mm in diameter 126 mm in height can be axially deformed by applying tensile or compressive strain rates between 2 4 10 7 s 1 and 2 4 10 2 s 1 compressed air is used to laterally load the snow specimens with pressures of up to 40 kpa the volumetric deformation is found by carefully encapsulating the snow samples in an airtight mylar foil and measuring the expelled pore air during sample deformation the multidimensional stress strain behaviour can therefore be determined and constitutive theories of snow advanced the device is located run the cold rooms of the weissfluhjoch research station of the swiss federal institute for snow anti avalanche research seine 180 tests have been performed over the last four winters the density of the snow specimens varied between 190 and 435 kg m 3 part i of this two part paper discusses the development of the apparatus and the applied testing methodology qualitative results are provided in part ii a detailed analysis of the results is presented and the viscoclastic properties of snow quantified