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

rayleigh wave tomography beneath intraplate volcanic ridges in the south pacific sources recorded over a 12 month period provides resolution of phase velocities at periods upto100s theaveragephasevelocitiesforthestudyarearevealapronouncedlow velocity zone reaching a minimum shear velocity of 3 95 km s the negative velocity gradient defining the base of the lithosphere observed at 40 15 km abruptly reverses at 70 km depth the underlying positive gradient changes slope at 125 km we attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in thepresenceofwaterat125km followedbyincreasedmeltproductionat70kmthatleadsto dehydration of the residual matrix and migration of melt to the surface spreading center rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the sojourn ridge and the hotu matua volcanic complex with relatively high velocities between these volcanic chains these observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking dynamic models invoking flow in the asthenosphere in the form of small scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies

effects of change in slab geometry on the mantle flow and slab fabric in southern peru the effects of complex slab geometries on the surrounding mantle flow field are still poorly understood here we combine shear wave velocity structure with rayleigh wave phase anisotropy to examine these effects in southern peru where the slab changes its geometry from steep to flat to the south where the slab subducts steeply we find trench parallel anisotropy beneath the active volcanic arc that we attribute to the mantle wedge and or upper portions of the subducting plate farther north beneath the easternmost corner of the flat slab we observe a pronounced low velocity anomaly this anomaly is caused either by the presence of volatiles and or flux melting that could result from southward directed volatile rich subslab mantle flow or by increased temperature and or decompression melting due to small scale vertical flow we also find evidence for mantle flow through the tear north of the subducting nazca ridge finally we observe anisotropy patterns associated with the fast velocity anomalies that reveal along strike variations in the slab s internal deformation the change in slab geometry from steep to flat contorts the subducting plate south of the nazca ridge causing an alteration of the slab petrofabric in contrast the torn slab to the north still preserves the primary fossilized petrofabric first established shortly after plate formation

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

buoyant mantle upwelling and crustal production at oceanic spreading centers on axis segmentation and off axis melting numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers buoyancy results from 1 the depletion of residual mantle in fe relative to mg mantle depletion buoyancy 2 the presence of low density melt melt retention buoyancy and 3 thermal expansion thermal buoyancy thermal buoyancy drives off axis convective rolls that develop closer to the spreading axis at slower spreading rates melt retention buoyancy drives focused upwelling into isolated melting centers with increasing spreading rate the amplitude of on axis crustal segmentation decreases the melting region being essentially two dimensional at full spreading rates greater than 100 km m y these numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations this discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow at fast spreading rates when the lithosphere is thinner melt retention buoyancy gives rise to isolated focused centers of off axis upwelling and melt production off axis melting centers begin to form at about 300 km from the axis with a characteristic along axis spacing of 200 km successive off axis melting centers are shifted along axis so that crustal thickness far from the axis is increased at a 100 km wavelength

eocene reorganization of the pacific farallon spreading center north of the mendocino fracture zone during a 1984 sea beam and magnetics survey in the north pacific we collected data in the area north of the mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducted detailed surveys of the bends in the surveyor and mendocino fracture zones associated with this change in plate motion we have analyzed these data combined with all available magnetics and bathymetric data from previous cruises to determine whether the ridge reorientation and reorganization occurred by way of rift propagation or rift rotation through differential asymmetric spreading the reorganization began just after anomaly 24 to the north of the surveyor fracture zone but did not begin between the mendocino and surveyor fracture zones until the time of anomaly 22 a 2 5 m y delay produced by the old mendocino transform trend constraining the farallon plate lithosphere between the mendocino and surveyor transforms to move in the old spreading direction until the fracturing through of a new mendocino transform fault in the new orientation most of the reorientation of spreading rift segments occurs within 2 3 m y the entire reorganization requires about 7 m y we find direct evidence for seven propagation episodes accounting for the primary reorientation of 75 of the length of the ridge system and nowhere is the data more consistent with rift rotation than with rift propagation as the principal means of reorientation we conclude that the primary mechanism by which the reorganization occurred was rift propagation assuming that rift propagation can account for all of the ridge reorganization we present forward models of magnetic anomaly profiles which succeed in fitting the data well we then present a model for the tectonic evolution of the reorganization which fits the data well using 11 episodes of both northward and southward propagation

upper mantle structure beneath the eastern pacific ocean ridges 1 we analyze vertical component body and surface waves for 10 mw 5 earthquakes recorded by ocean bottom seismometers at regional and teleseismic distances through waveform modeling we place new constraints on along axis variation in temperature and partial melt beneath the eastern pacific ridges the resulting best fit models show over 9 variation in average lithosphere shear velocities between different ridge segments we demonstrate that lid velocity correlates with the square root of plate age consistent with a conductive cooling process but we find a more rapid dependence on age close to the rise axis we map the average plate age into a mean lithospheric temperature for each of our profiles using a half space cooling model and the temperature derivatives dvs dt determined from least squares linear fits are 1 1 and 0 26 m s 1 deg 1 respectively for temperatures above and below 1000 c the former estimate is more negative than values determined by earlier reports 0 4 to 0 7 m s 1 deg 1 using global or regional data from a much wider range of seafloor age but with less resolution at young ages the high dvs dt value may suggest the presence of limited partial melt at shallow mantle depths even after accounting for the strong effect of anelasticity and anharmonicity resulting from temperature and grain size variations our data also show a strong north south difference in mantle structure the surface waves that traverse the southern east pacific rise epr experience shear velocities as low as 3 75 km s 1 more than 0 2 km s 1 slower than the average mantle structure at comparable depths beneath the northern epr and the galapagos spreading center this difference cannot be explained by the simple conductive cooling process or spreading rate variations between ridge segments the slow seismic speeds in the low velocity zone lvz appear to require partial melt the velocity difference might be solely caused by higher temperatures under the southern epr but it may also suggest more melt in the lvz beneath the southern ridge axis due to potential differences in melt production or extraction compared to the north in addition differences in width or symmetry of the partial melt zones can affect the observed path averaged velocities beneath these two segments finally we determine more accurate depths for ocean transform earthquakes by analyzing the amplitude ratios between body and surface waves the depths of these transform earthquakes are consistent with brittle deformation within the oceanic crust

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

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

warping and cracking of the pacific plate by thermal contraction 1 lineaments in the gravity field and associated chains of volcanic ridges are widespread on the pacific plate but are not yet explained by plate tectonics recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere we show that the large thermoelastic stress produced by top down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite derived gravity our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required index terms 1208 geodesy and gravity crustal movements intraplate 8110 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1219 geodesy and gravity local gravity anomalies and crustal structure 3045 marine geology and geophysics seafloor morphology and bottom photography 8159 tectonophysics rheology crust and lithosphere keywords thermal contraction pacific plate gravity anomalies

placing bounds on lithospheric deformation in the central pacific ocean abstract lineations in the gravity field in the central pacific ocean have been explained by models of both small scale convection and lithospheric boudinage the boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation we calculate separations between a pair of fracture zones on the pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the nazca plate where no lineations have been observed satellite and surface ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs we found that between anomalies 13 and 7 the difference in fracture zone spacing on the pacific plate compared to the nazca plate appears to increase as the age of oceanic lithosphere decreases the maximum difference calculated was 97 km about 4 of the distance between the fracture zones and we attribute this difference to the re orientation of the ancient pacific farallon spreading centre between anomalies 7 and 5 the main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results

the role of hydrous phases in the formation of trench parallel anisotropy evidence from rayleigh waves in cascadia 1 the cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research we provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well localized region of trench parallel fast directions in rayleigh wave phase velocities near the cascade arc at 45 to 66 s periods we constrain the location of the anisotropic material to within or directly above the oceanic plate using the depth sensitivity of rayleigh waves as a function of frequency and the accurate slab imaging available for cascadia from scattered wave studies we infer that the likely source of trench parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

seafloor age dependence of rayleigh wave phase velocities in the indian ocean variations in the phase velocity of fundamental mode rayleigh waves across the indian ocean are determined using two inversion approaches first variations in phase velocity as a function of seafloor age are estimated using a pure path age dependent inversion method second a two dimensional parameterization is used to solve for phase velocity within 1 25 1 25 grid cells rayleigh wave travel time delays have been measured between periods of 38 and 200 s the number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22 272 paths at a period of 40 s at periods 100 s the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half space cooling model for a mantle potential temperature of 1400 c the inferred thermal structure beneath the indian ocean is most similar to the structure of the pacific upper mantle where phase velocities can also be explained by a half space cooling model the thermal structure is not consistent with that of the atlantic upper mantle which is best fit by a plate cooling model and requires a thin plate removing age dependent phase velocity from the 2 d maps of the indian ocean highlights anomalously high velocities at the rodriguez triple junction and the australian antarctic discordance and anomalously low velocities immediately to the west of the central indian ridge

pacific plate gravity lineaments diffuse extension or thermal contraction 1 we detect strain very close to zero on the pacific plate relative to the nazca plate for anomalies c8 c13 our observations are in the region of the satellite gravity lineaments 10 40 s in the central pacific strain was measured by comparing fracture zone spacing on the pacific and nazca plates in an approach similar to that of goodwillie and parsons 1992 but with an improved data set which allowed us to place tighter tolerances on estimated strain we used a standard finite rotation technique that rotates points of a given isochron and corresponding active transform fault from the pacific plate to the nazca plate our strain measurements discount the diffuse extension hypothesis for gravity lineament formation which requires at least 10 strain sandwell et al 1995 the best known hypothesis that small scale convection rolls in the upper mantle aligned in the direction of plate motion produce the lineaments was rejected because of observations of seamount chains in the gravity lows and observations of lineaments in seafloor topography younger than 2 3 ma we propose a third hypothesis that the gravity lineaments are signatures of the bending stresses resulting from vertical variations in cooling rate we interpret the lineament lows as areas of brittle failure and the highs as convex upward flexure due to greater contraction of the base of the plate association of the lows with volcanoes could result either from both being caused by cracks or by the load of volcanoes causing brittle failure we explored the feasibility of this idea using the equations for thermal bending moment and vertical lithosphere displacement as presented by turcotte 1974 and the equation for thermal stress over elastic thickness presented by parmentier and haxby 1986 results of this simple model show vertical displacement or convex upward flexure ranging from 80 m to 140 m with associated gravity anomalies of 6 10 mgal these amplitudes are within a factor of 2 of topographic and free air gravity anomaly amplitudes observed in the region of the gravity lineaments

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

upper mantle seismic wave attenuation effects of realistic partial melt distribution frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated we consider the contribution of the melt squirt mechanism through which pressure differences between disk shaped inclusions are equalized by melt passing through connecting tubes the pressure differences arise as a result of shear strain compressing disk shaped pores differently on the basis of disk orientation with respect to the applied shear the frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores the pressure equalization process is modeled by a system of first order linear differential equations whose eigenvalues are the characteristic frequencies for melt squirt relaxation it is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity as well as distribution of melt inside pores and conduits use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation some conceivable melt distributions however would result in detectable attenuation in the seismic band in all cases investigated attenuation increases with frequency indicating that melt squirt is not responsible for global upper mantle q observations

analysis of gravity and topography in the glimpse study region isostatic compensation and uplift of the sojourn and hotu matua ridge systems 1 the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the formation of a series of non hot spot intraplate volcanic ridges in the south pacific and their relationship to cross grain gravity lineaments detected by satellite altimetry using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate we estimate effective elastic thicknesses ranging from 2 km beneath the sojourn ridge to a maximum of 10 km beneath the southern cross seamount these elastic thicknesses are lower than predicted for the 3 9 ma seafloor on which the volcanoes lie perhaps due to reheating and thinning of the plate during emplacement anomalously low apparent densities estimated for the matua and southern cross seamounts of 2050 and 2250 kg m 3 respectively probably are artifacts caused by the assumption of only surface loading ignoring the presence of subsurface loading in the form of underplated crust and or low density mantle using satellite free air gravity and shipboard bathymetry we calculate the age detrended residual mantle bouguer anomaly rmba the rmba corrects the free air anomaly for the direct effects of topography including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices there are broad negative rmba anomalies along the sojourn and brown ridges and the hotu matua seamount chain that extend nearly to the east pacific rise these negative rmba anomalies connect to negative free air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross grain gravity lineaments subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor this swell topography and the negative rmba anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges favoring models with a dynamic mantle component such as small scale convection or channelized asthenospheric return flow

a comprehensive survey of lignin geochemistry in the sedimentary organic matter along the kapuas river west kalimantan indonesia abstract in this first study of lignin geochemistry in the world s longest river on an island surface sediments were collected along the kapuas river three lakes in the upper river a tributary in the lower river and a separate river during june july 2007 and december 2007 january 2008 the samples were analyzed for lignin derived phenols and bulk elemental and stable carbon isotope compositions values the sum of eight lignin phenols expressed as mg 100 mg organic carbon oc ranged from 0 13 to 3 70 ratios of syringyl vanillyl s v and cinnamyl vanillyl c v ranged from 0 34 to 1 18 and 0 28 to 1 40 respectively indicating the presence of non woody angiosperm tissues the high vanillic acid to vanillin ad al v 0 71 2 01 and syringic acid to syringaldehyde ad al s 0 72 2 12 ratios indicate highly degraded lignin materials in the upper kapuas river highly degraded soil materials discharged from lands that were barren as a result of deforestation activities were detected in the locations directly in those vicinities the middle kapuas river showed rapid organic matter degradation probably due to the presence of fresh terrestrial and phytoplankton organic matter fueling the biogeochemical cycling the kapuas kecil river one of the two branches in the lower reach of the kapuas river showed higher levels and diagenesis of sedimentary organic matter due to input from anthropogenic sources and increased marine organic matter near the mouth this study shows that different stretches along the river exhibit different levels and composition of sedimentary organic matter as well as different carbon dynamics which is directly attributable to the varying landscapes and quality of organic matter

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

crustal structure beneath the gravity lineations in the gravity lineations intraplate melting petrologic and seismic expedition glimpse study area from seismic refraction data 1 the active source seismic component of the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the velocity structure and crustal thickness of the pacific plate along a 450 km line paralleling the southern east pacific rise where prominent intraplate ridges lie within lows of the lineated free air gravity pattern compressional wave traveltime data collected by 10 ocean bottom seismometers indicate that the median crustal thickness is 6 2 km and increases to 8 2 km beneath the sojourn ridge the conspicuous absence of crustal thinning beneath both the sojourn and hotu matua ridge systems is strong support against a boudinage style model for the formation of these volcanic complexes our results suggest the ridge loads are partially supported by both plate flexure and underplating with an additional mantle component inferred from the analysis of gravity data rayleigh wave tomography and teleseismic body wave delays small scale convection could produce the observed crustal thickness pattern but fails to explain the geochemical and formation age trends along the ridges we prefer a model where the channelized flow of low density material in the upper mantle possibly undergoing small scale convection is advected toward the ridge crest lows in the gravity field reflect the density contrast within the asthenosphere and decompression melting amid upwelling limbs of the convective cells sustains the development of the associated ridge systems

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

upper mantle structure of the kaapvaal craton from surface wave analysis a second look 1 surface waves from earthquakes recorded during the two year southern africa seismic experiment have been used to estimate the average velocity structure beneath the kaapvaal craton by cross correlating the data with synthetics derived using mode summation techniques i measure frequency dependent phase delays for love and rayleigh waves as well as higher modes for 12 events ranging in epicentral distance between 12 and 91 i simultaneously invert the traveltime delays for structure beneath the array and outside of the array in this way heterogeneity along the path lying outside of the array is not mapped erroneously into structure beneath the array the differential dispersion between the love and rayleigh waves require some anisotropy in the upper mantle however models with a range of anisotropic depths as shallow as 150 km and as deep as 220 km can equally well fit the data the sensitivity of the fundamental mode surface waves used in this study begins to decline significantly below 200 km depth limiting interpretation of deeper structure nonetheless this range of anisotropic depths is significantly different than the 100 km depth proposed in a recent study by freybourger et al 2001 using a similar technique and different data from the same array i attribute the difference in their dispersion measurements and modeling to the improper use of a phaseless i e frequency dependent filter on the data which systematically biased their measurements at lower frequencies in addition i find no evidence of a low velocity layer as proposed by qui et al 1996 and priestley 1996

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

sea ice diatom contributions to holocene nutrient utilization in east antarctica combined high resolution holocene 30sidiat and 13cdiat paleorecords are presented from theseasonal ice zone east antarctica both data sets re ect periods of increased nutrient utilization by diatomsduring the hypsithermal period circa 7800 to 3500 calendar years cal years b p coincident with a higherabundance of open water diatom species fragilariopsis kerguelensis increased biogenic silica productivity bsi and higher regional summer temperatures the neoglacial period after circa 3500 cal years b p isre ected by an increase in sea ice indicative species fragilariopsis curta and fragilariopsis cylindrus upto50 along with a decrease in bsi and 13cdiat 18 to 23 however over this period 30sidiatdata show an increasing trend to some of the highest values in the holocene record average of 0 43 competing hypotheses are discussed to account for the decoupling trend in utilization proxies including ironfertilization species dependent fractionation effects and diatom habitats based on mass balance calculations we highlight that diatom species derived from the semi enclosed sea ice environment may have a confoundingeffect upon 30sidowncorecompositions of the seasonal sea ice zone a diatom composition with approximately28 of biogenic silica derived from the sea ice environment diat si can account for the increased averagecompo sition of 30sidiatduring the neoglacial these data highlight the signi cant role sea ice diatoms can playwith relation to their export in sediment records which has implications on productivity reconstructions fromthe seasonal ice zone

rayleigh wave phase velocities in the pacific with implications for azimuthal anisotropy and lateral heterogeneities summary the lateral distribution of fundamental mode rayleigh wave phase velocities in the pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate the importance of azimuthal anisotropy and the presence of secondary lateral heterogeneities the data set used in this study consists of phase velocity measurements in the period range 20 125 s for 178 paths traversing the pacific both the pure path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime the first order velocity age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure path or spherical harmonic techniques azimuthal anisotropy is the dominant second order effect and is best developed in regions less than 80 myr in age in this age zone the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present day absolute plate motion in the western pacific there is a correlation with paleo relative motion fossil seafloor spreading for periods less than 50 s lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure path inversions and are correlated with regions associated with shallow residual depth anomalies this effect slow velocities with respect to the velocity age model monotonically decreases as a function of period and is indicative of a shallow origin age independent anomalies were also modelled by the application of the pure path and spherical harmonic methods in a sequential inversion

transition from seamount chain to intraplate volcanic ridge at the east pacific rise a number of large submarine intraplate volcanic ridges have been discovered throughout the south pacific basin but their origins are enigmatic recent shipboard geophysical surveys reveal that the sojourn ridge one of these large intraplate ridges becomes a chain of discrete seamount volcanoes that intersects the ridge axis this seamount chain exhibits several features that suggest that it is directly related to the sojourn ridge the sojourn seamount chain grows continuously in both volume and number of seamounts with distance from the spreading axis several loci of recent volcanic activity along the chain are evident in the side scan imagery and a mantle bouguer anomaly low underlies the entire length of the chain this evidence provides new constraints on the origin of intraplate volcanic ridges the continuation of the sojourn ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism such as flexural stresses induced by the mass of the volcanic pile as the probable mechanism for developing volcanic ridges and long seamount chains

ridge asymmetry and deep aqueous alteration at the trench observed from rayleigh wave tomography of the juan de fuca plate using rayleigh wave tomography of noise removed ocean bottom seismometer obs data from the cascadia initiative we illuminate the structure of the upper mantle beneath the juan de fuca plate beneath the juan de fuca ridge there is strong asymmetry with a pronounced low velocity zone in the 25 65 km depth range extending to the west from the spreading axis this anomaly has velocities low enough to indicate the presence of melt the asymmetry in velocity structure and the much greater abundance of seamounts on the west flank of the ridge suggest that dynamic buoyant upwelling is important perhaps triggered by thermal or compositional anomalies beneath axial seamount in contrast there is no evidence for asymmetry in the axial zone or lower than expected velocities beneath the gorda ridge on the eastern flank of the juan de fuca ridge the shear velocity in the 25 65 depth range is higher than expected the lithosphere appears to be colder and thicker than predicted by standard plate cooling models perhaps caused by the downwelling counterpart of the upwelling on the west side of the ridge close to the trench there is a sharp decrease in shear velocity we interpret this as aqueous alteration caused by hydrothermal circulation through deep normal faults associated with bending of the plate beneath the astoria and nitinat fans where abyssal plain sediment is thickest the velocity decrease is much smaller which is consistent with a thick sediment cap that prevents hydrothermal alteration of the plate

origin of the oceanic lithosphere in a global examination of mid ocean ridge basalt morb glass compositions we find that na8 fe8 depth variations do not support modeling of morbs as aggregates of melt compositions generated over a large range of temperature and pressure however the na 8 fe8 variations are consistent with the compositional systematics of solidus melts in the plagioclase spinel lherzolite transition in the cao mgo al2o3 sio2 na2o feo cmasnf system for natural compositions the p trange for melt extraction is estimated to be 1 2 1 5 gpa and 1250 12808c this p trange is a close match with the maximum p t conditions for explosive pressurerelease vaporization of carbonate bearing melts it is proposed that fracturing of the lithosphere induces explosive formation and escape of co2 vapor this provides the vehicle for extraction of morbs at a relatively uniform tand p the upper portion of the co2 bearing and slightly melted seismic low velocity zone flows toward the ridge rises at the ridge axis to lower lithosphere depths melts much more extensively during this rise and releases morb melts to the surface driven by explosively escaping co2 vapor the residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere the entire process of oceanic lithosphere creation involves only the upper 140 km when lithospheric stresses shift fracture formation to other localities escape of co2 ceases the vehicle for transporting melt to the surface disappears and ridges die inverse correlations of na8 vs fe8 for morb glasses are explained by mantle heterogeneity and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short ascending melting columns the uniformly low temperatures of morb extraction are not consistent with the existence of hot plumes on or close to ocean ridges in this modeling the southern atlantic mantle from bouvet to about 268n is relatively homogeneous whereas the atlantic mantle north of about 268 ns hows significant long range heterogeneity the mantle between the charlie gibbs and jan mayen fracture zones is strongly enriched in feo mgo perhaps by a trapped fragment of basaltic crust iceland is explained as the product of this enrichment not a hot plume the east pacific rise galapagos ridge gorda ridge and juan de fuca ridge sample mantle that is heterogeneous over short distances the mantle beneath the red sea is enriched in feo mgo relative to the mantle beneath the northern indian ocean

broadband ocean bottom seismology broadband ocean bottom seismographs bbobss were first developed in the 1980s and have since been used to explore the structure of earth s interior beneath oceanic regions for example mid oceanic ridges subduction zones hot spots and the oceanic lithosphere asthenosphere boundary the best approach for broadband seismic observations in oceanic regions is that of a borehole seismic observatory attached to the ocean floor cable which is realized in several near coast networks considering the high cost of such networks there is still a need to develop autonomous bbobss with a better signal to noise ratio for temporary observations of focused scientific targets far from the coast

asymmetric mantle dynamics in the melt region of the east pacific rise abstract the mantle electromagnetic and tomography melt experiment found a surprising degree of asymmetry in the mantle beneath the fast spreading southern east pacific rise melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 scheirer et al science 280 1998 1221 1224 evans et al science 286 1999 752 756 pressure release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick canales et al science 280 1998 1218 1221 seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 evans et al science 286 1999 752 756 at depths of several tens of kilometers melt is more abundant beneath the pacific plate to the west of the axis than beneath the nazca plate to the east melt investigators attributed the asymmetry in melt and geophysical properties to several possible factors asymmetric flow passively driven by coupling to the faster moving pacific plate interactions between the spreading center and hotspots of the south pacific an off axis center of dynamic upwelling and or anomalous melting of an embedded compositional heterogeneity melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 evans et al science 286 1999 752 756 here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies asthenospheric flow from hotspots in the pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies

a broadband ocean bottom seismometer system based on a 1 hz natural period geophone recent work with broadband sensors has demonstrated that useful data can be obtained from seismic sensors deployed on the deep seafloor for periods as long as 300 sec but ocean bottom seismometer obs systems using conventional broadband sensors have proven fragile and expensive to build and operate the intrinsic thermal brownian motion noise limit for large 1 hz geophone sensors such as the mark products l 4 is far below typical noise levels at seafloor sites in response to the need for large numbers of obs systems for experiments studying the oceanic upper mantle and crust we have developed a very low noise amplifier that can be used with short period sensors the new amplifiers push noise levels at the 100 sec period some 40 db below standard short period obs noise levels useful data can then be obtained from these short period sensors in the broad frequency range from 0 005 to 50 hz noise levels rise from 5 10 16 m sec 2 2 hz at a 20 sec period to 5 10 14 m sec 2 2 hz at a 200 sec period power requirements are less than 35 mw for three channels these amplifiers have been installed in the first 24 instruments of the new nsf sponsored ldeo obs instrument center we believe this system is an excellent compromise between cost reliability and noise level the new amplifiers may also prove useful for work on land extending the useful range of short period sensors to a much longer period

convection beneath young oceanic lithosphere implications for thermal structure and gravity small scale convection under the oceanic lithosphere which begins in the first 5 m y of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central pacific and southern indian oceans using seasat altimeter data the trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small scale convection models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed the persistence of short relatively uniform wavelength anomalies 200 km to crustal ages of 50 ma may indicate that they were produced when the lithosphere was very young and thin and were frozen in as cooling thickened the elastic lithosphere small scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age after convection has begun the subsidence due to thermal contraction within the lithosphere varies linearly with age in the absence of mantle heat sources although the rate of change of these quantities is affected by convection much of the variation of the geoid height across fracture zones can be fit by a model which includes small scale convection

distribution of recent volcanism and the morphology of seamounts and ridges in the glimpse study area implications for the lithospheric cracking hypothesis for the origin of intraplate non hot spot volcanic chains 1 lithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the pacific that clearly do not form above fixed hot spots as part of the gravity lineations intraplate melting petrology and seismic expedition glimpse project designed to investigate the origin of these features we have mapped two volcanic chains that are actively forming to the west of the east pacific rise using multibeam echo sounding and side scan sonar side scan sonar reveals the distribution of rough seafloor corresponding to recent unsedimented lava flows in the hotu matua volcanic complex recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide with an apparent irregular age progression from older flows in the west to younger in the east the 550 km long southern cross seamount sojourn ridge brown ridge chain appears to have been recently active only at its eastern end near the east pacific rise a third region of recent flows is found 120 km north of southern cross seamount in seafloor approximately 9 myr old no indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area thermoelastic cracking could be a factor in the formation of a few small very narrow volcanic ridges but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks the sojourn and brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges perhaps under the influence of membrane stresses from self loading we suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role

the bauer scarp ridge jump a complex tectonic sequence revealed in satellite altimetry abstract we investigate here the ridge jump that led to abandonment of the galapagos rise and formation of the bauer scarp during the initiation of the present day configuration of the east pacific rise since the lower miocene we use recently available high resolution satellite derived gravity data to investigate in detail the tectonic structure of the eastern pacific from the equator to 20 s with this data we identify fracture zones abandoned spreading ridges scarps and other seafloor features that provide evidence for discerning tectonic history based on our structural interpretation of the satellite derived gravity field we make the following conclusions 1 the galapagos rise spreading center appears to have originated by opening of the marquesas mendana transform complex as a result of the change in spreading direction following breakup of the farallon plate 2 the galapagos rise was not the sole locus of spreading following plate reorganization at 20 ma through to the initiation of the bauer scarp at 8 ma as had been previously hypothesized rather it and a second western spreading axis were likely active concurrently forming a counterclockwise rotating bauer microplate at a much earlier stage than thought previously 3 the bauer scarps are pseudofaults associated with northward rift propagation propagation proceeded in several stages a first propagator emanating from the garrett transform complex stalled at the future location of the wilkes transform creating an area of complex morphology near its northern tip a second propagator also emanating from the garrett complex followed in the first s wake and broke through the complex region at this point the propagation proceeded very rapidly to the northern end of the bauer microplate the gallego fracture zone later to become the yaquina transform fault ridge propagation continued north in two more stages creating the gofar and quebrada transforms at the terminus of each stage

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

mantle heterogeneity and off axis volcanism on young pacific lithosphere plate tectonics and mantle plumes explain most volcanism on earth but there are numerous actively forming linear volcanic chains in the middle of tectonic plates that are not explained by these theories using the multidisciplinary geophysical dataset of the melt and glimpse experiments we show that associated with 3 volcanic chains west of the east pacific rise there are low seismic velocities and densities in the asthenosphere that extend to the east pacific rise spreading center analogous to the hawaiian swell the low density anomalies produce swells beneath the volcanoes on young seafloor the associated gravity anomalies are part of a set of gravity lineaments that have been previously interpreted as being due to thermo elastic cracking of the lithosphere or small scale convection the correlation between the surface volcanism and subsurface density and velocity anomalies and their extension to the spreading center suggest that pre existing buoyant or fertile asthenospheric mantle heterogeneities are stretched in the direction of plate motion by shear between the plate and the underlying mantle these heterogeneities seed small scale convection producing upwelling and pressure release melting forming volcanic chains that extend nearly to the ridge axis

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

upper mantle low anisotropy channels below the pacific plate a new 3d anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals new insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure the model is best resolved in the pacific plate the largest and the most active tectonic plate superimposed on the large scale radial parameter and azimuthal anisotropy of vsv velocity within and below the lithosphere correlated with present or past pacific plate motions are smaller scale 1000 km lateral variations of anisotropy not predicted by plate tectonics channels of low anisotropy down to a depth of 200 km hereafter referred to as lac are observed and are the best resolved anomalies one east west channel between easter island and the tonga kermadec subduction zones observed on both radial and azimuthal anisotropies and a second one only observed on azimuthal anisotropy extending from the south west pacific up to south east hawaii and passing through the polynesia hotspot group for plate older than about 40 ma these features provide strong constraints on the decoupling between the plate and asthenosphere they are presumably related to cracking within the pacific plate and or to secondary convection below the rigid lithosphere predicted by numerical and analog experiments the existence and location of these lacs might be related to the current active volcanoes and hotspots possibly plumes in the central pacific lacs which are dividing the pacific plate into smaller units might indicate a future reorganization of plates with ridge migrations in the pacific ocean

high temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle summary mineralogical models suggest that low velocity zones of the upper mantle may be accounted for by elasticity of dry rocks along geotherms away from the mid oceanic ridges mor while closer to mor anelasticity plays a significant role in reducing shear wave velocities vs and generating attenuation we investigate the potential influence of elastic softening precursor of high temperature phase transition in orthopyroxenes on the seismic properties of the upper mantle in situ brillouin and raman spectroscopy were used to evidence pre transitional behaviour at high temperature in natural san carlos orthopyroxene pre transitional behaviour induces a large softening of the acoustic and low frequency modes resulting in a large anharmonic decrease of sound velocities similar to that observed in the mgsio3 end member orthoenstatite the high temperature high pressure phase diagram of enstatite is revised to account for the new phase transition and a simple model is developed to evaluate elastic softening effects on the upper mantle seismic properties whose results depend much on the as yet poorly constrained pressure and compositional dependence of the phase transition boundary within the tested range of parameters opx softening is likely to affect the seismic properties of mantle rocks at depths shallower than 80 km in hot regions thus elastic softening of pyroxene is unlikely to affect the lvz or continental lithospheric mantle unless the transition temperature is drastically reduced by incorporation of aluminium in orthopyroxene it will contribute to a decrease of vs near mor hot spots and evolved continental rifts where it can explain part of the non linear high temperature decrease of vs in the lithospheric oceanic mantle it will affect the magnitude of anelastic effects and the nature of the likely mechanisms of attenuation required to match seismological observations

evidence for shallow isostatic compensation of the southern rocky mountains from rayleigh wave tomography in the past decade seismologists have found that the topography in colorado correlates with seismic velocities in the upper mantle not with crustal thickness and postulated that isostatic compensation of the southern rocky mountains takes place largely in the mantle to test the validity of this hypothesis we use rayleigh wave phase and amplitude data recorded in the rocky mountain front passcal experiment to obtain crustal thickness and shear wave structure in the crust and upper mantle the thickest crust in the model is beneath one of the two most elevated regions the san juan volcanic field the other most elevated region in central colorado the sawatch range is underlain by crust of anomalously low velocity the seismically defined mantle lithosphere is thicker beneath the great plains than beneath the colorado plateau and is largely absent beneath the southern rocky mountains but the correlation of topography with mantle velocities is weaker than the correlation with crustal anomalies we construct a model that matches the observed bouguer gravity anomaly primarily with variations in crustal thickness and density by assuming that density anomalies are proportional to the observed velocity anomalies the anomalous mantle may also contribute to maintaining regional isostasy in colorado but its contribution may be much less important than the crust

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

p and s wave delays beneath intraplate volcanic ridges and gravity lineations near the east pacific rise the gravity lineations intraplate melting petrology and seismic expedition glimpse explored the origin of two intraplate non hot spot ridge systems associated with gravity lineations in the south central pacific using the 11 month ocean bottom seismometer obs deployment of the glimpse experiment we determined the average p and s station delays for the region across the array station delays varied by 1 22 s for p waves and 2 13 s for s waves the presence of ultraslow shear velocity

geosat derived geoid anomalies at medium wavelength geosat profiles of the exact repeat mission have been averaged over a 1 year period and high pass filtered using inverse method techniques the geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band shaped anomalies preferentially elongated in the east west direction these anomalies have an average amplitude of 30 cm and dominant wavelengths to 750km and 1100 km we have performed numerous tests to show that the lineations are not artefacts created by the filtering process moreover two dimensional 2 d filtering with the inverse method applied on a regional basis over the pacific gives essentially similar results indicating that the filtered geoid is not affected by directional bias seafloor topography in the pacific filtered by 2 d inverse method also shows east west trending depth anomalies positively correlated to medium wavelength geoid lineations along the east pacific rise there is a clear correlation between geoid lineations and regional variations in axial depth cross spectral analysis carried out on geoid and topography data over the pacific area gives coherence maxima at 750 km and 1100 km wavelengths and admittance values of 2 3 m km observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models suggesting that the lineations are related to regional variations in the plate cooling process convection models produce much higher admittances than observed unless a low viscosity layer is assumed so that dynamical support cannot be completely discarded in most instances however the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin in the south central pacific where the lineations appear parallel to absolute plate motion there may be a combination of both lithospheric and sublithospheric processes

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

geodynamic evidence for a chemically depleted continental tectosphere the tectosphere namely the portions of earth s mantle lying below cratons has a thermochemical structure that differs from average suboceanic mantle the tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle inversions of a large set of geodynamic data related to mantle convection using tomography based mantle flow models indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below earth s surface the approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time

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

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

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