detection of lower mantle scatterers northeast of the marianna subduction zone using short period array data short period university of washington northern california and southern california seismic array records of a sequence of deep mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following p we measure slowness arrival time and arrival azimuth of these phases relative to p using array techniques to find where they emanate in the mantle by combining slowness and back azimuth deviations and the move out with depth of the arrivals from different earthquakes we find their most probable origin is through s to p conversion at point scatterers in the mid to lower mantle at 25 7 n 148 2 e 1590 km and 25 5 n 151 0 e 1850 km respectively separated by 300 km these features indicate elastically distinct material in the middle of the lower mantle which if fragments of subducted slabs are not related to contemporary subduction they may represent upwelling convective material from the deep mantle or d but they are not purely thermal in nature as chemically distinct bodies they may represent potential reservoirs of the isotopic and trace element components recognized geochemically

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

in situ x ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions implications for density of subducted oceanic plate 1 phase relationships in klb 1 peridotite normal type mid oceanic ridge basalt nmorb and k rich basalt compositions were investigated experimentally from 28 to 143 gpa and 2050 to 3000 k a laser heated diamond anvil cell with the synchrotron x ray diffraction was developed to perform in situ observations of samples at simultaneous high pressures and temperatures we confirmed that the peridotite composition crystallized into an assemblage of mg perovskite ca perovskite magnesiowustite at pressures below 97 gpa and that nmorb and k rich basalt compositions both crystallized as mg perovskite ca perovskite stishovite an aluminous phase with a cafe2o4 type structure at pressures below 68 gpa the orthorhombic mg perovskite is stable in natural rock compositions at pressures less than 97 gpa corresponding to about 2200 km depth phase transition of silica phase from stishovite to cacl2 type phase was observed at 78 gpa in nmorb composition moreover we observed that nmorb crystallized in to an assemblage of cairo3 type mg fe sio3 ca perovskite pbo2 type silica an aluminous phase with a calcium cati2o4 type structure at 143 gpa corresponding to the core mantle boundary depth we measured the unit cell volumes of minerals in the nmorb composition at high p t and thereby estimated the density of subducted morb to a depth of 1800 km in the lower mantle in this depth regime morb is denser than model mantle densities e g prem and ak135 the estimated density of morb may contribute to an understanding of dynamics of the subducted slab in the deep lower mantle

mid mantle seismic anisotropy around subduction zones abstract there is increasing evidence for mid mantle seismic anisotropy around subduction zones whose interpretation remains elusive in this study i estimate the strain induced mid mantle fabric and associated seismic anisotropy developing in 3d petrological thermo mechanical subduction models where the slab is either stagnating over the 660 km discontinuity or penetrating into the lower mantle the modelling of synthetic lattice preferred orientation lpo development of wadsleyite and perovskite has been calibrated with results from deformational experiments and ab initio atomic scale models and the single crystal elastic tensor of the different mineral phases is scaled by local p t conditions the lower transition zone ringwoodite garnet is assumed to be isotropic mid mantle fabric develops in proximity of the subducting slab where deformation and stresses are high except at depths where upwelling or downwelling material undergoes phase transformations yielding to lpo reset the upper transition zone wadsleyite garnet is characterized by weak transverse isotropy 2 3 with symmetry axes oriented and fast s wave polarized dip normal a slightly stronger transverse isotropy develops in the lower mantle perovskite periclase where the symmetry axes the polarization of the fast s wave and the maximum vp and dvs are parallel to the slab dip and subduction direction for stagnating slab models this translates into negative and positive radial anisotropy in the upper transition zone and lower mantle back arc respectively minimum delay times for vertically travelling shear waves and large shear wave splitting for waves propagating horizontally in the lower mantle these results may help in reconciling the seismic anisotropy patterns observed in some subduction zones with subduction induced deformation such as those measured in the mid mantle between the australian plate and the new hebrides tonga kermadec trenches that i interpret as related to stagnating portions of the subducted pacific plate

the depth distribution of mantle anisotropy beneath the tonga subduction zone abstract shear wave splitting recorded by a temporary deployment of broadband seismometers located above the tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the tonga back arc region splitting parameters obtained for local s and teleseismic sks phases were modeled using a ray based method that predicts shear wave splitting on individual phase paths and determines the best fitting values of anisotropic strength orientation and depth extent splitting in teleseismic sks phases is identical to that in s phases from local earthquakes that occur at the base of the transition zone demonstrating that there is no significant splitting due to anisotropy in the lower mantle splitting times for local s phases do not vary significantly with depth and for models in which the strength of anisotropy is laterally uniform they rule out anisotropy in the transition zone finally the observed splitting indicates that anisotropy of more than 0 8 exists in the upper mantle with a fast symmetry axis at 60 w roughly parallel to the absolute plate motion vector of the subducting pacific lithosphere

upper and mid mantle anisotropy beneath the tonga slab measurements of source side splitting in s waves fromevents within the tonga slab reveal anisotropy in the upperand mid mantle beneath the slab we observed splitting forevents originating at both upper mantle and transition zonedepths anisotropic fast directions are trench parallel orsub parallel for both upper mantle and transition zoneevents delaytimes dt decreasewithdepthforuppermantleevents the source of anisotropy for the upper mantle eventsis likely in the sub slab mantle and is likely indicative oftrench parallel flow due to slab rollback the source ofanisotropy for the deeper earthquakes is more difficult toconstrain but the pattern of splitting measurements arguesfor an uppermost lower mantle anisotropic source and slabinduced deformation may be responsible for this anisotropyas well additional constraints from mineral physics studiesare necessary to interpret the mid mantle anisotropy signalin terms of geodynamical processes

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

tectonic evolution of the caribbean and northwestern south america the case for accretion of two late cretaceous oceanic plateaus it is widely accepted that the thickened oceanic crust of the caribbean plate its basaltic accreted margins and accreted mafic terranes in northwestern south america represent the remnants of a single ca 90 ma oceanic plateau we review geologic geochemical and paleomagnetic evidence that suggests that the caribbean colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus both dated as ca 90 ma the first of these plateaus the caribbean plateau formed ca 90 ma in the vicinity of the present day galapagos hotspot northeastward movement of the farallon plate meant that this plateau collided with the proto caribbean arc and northwestern south america 10 m y after the plateau s main phase of formation paleomagnetic evidence suggests that the second of these plateaus the gorgona plateau formed at 268 308s possibly at the site of the present day sala y gomez hotspot over the next 45 m y this plateau was carried progressively northeastward on the farallon plate and collided in the middle eocene with the proto andean subduction zone in northwestern south america the recognition of a second ca 90 ma pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events

new evidence for crustal accretion in the outer mariana fore arc cretaceous radiolarian cherts and mid ocean ridge basalt like lavas new age determinations on radiolarian cherts foraminifers and volcanic rocks document the presence of allochthonous fragments of cretaceous oceanic plate suggesting accreted terrane in the outer mariana fore arc more than 50 km from the trench three dredges from 3km 2 area along a steep scarp recovered a diverse assemblage of rocks representing an ophiolite suite chert mafic and intermediate lavas and intrusive rocks trace element patterns of the lavas suggest at least three tectonic associations island arc ocean island and oceanic plate the cherts contain two deep water assemblages of radiolaria of middle to late valan ginian 131 138 ma and albian 97 112 ma age foraminifers recovered with the chert are aptian to albian in age the lavas record a wide range of k ar ages 85 ma for a metabasalt with trace element signatures of mid ocean ridge basalt 71 ma for a highly metamorphosed alkalic basalt and 39 ma for a fresh glassy boninite these ages imply multiple volcanic events and at least two tectonic settings for magma genesis the cherts and metabasalts are too old to have formed in situ or to be part of trapped west philippine basin crust the mix of old oceanic plate with younger are rocks requires complex tectonic relations we suggest that one or more fragments of cretaceous oceanic plate chert mid ocean ridge basalt and alkalic lavas were accreted to the mariana fore arc and have been extensively faulted and probably intruded by arc lava island arc tholeiite and boninite

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

the emplacement of ophiolites by collision ophiolites recognized in most of the world s orogenic belts are generally interpreted to be oceanic crust and upper mantle lithosphere fragments that have been incorporated into continental margins at consuming plate boundaries we suggest that the mechanism for ophiolite emplacement is the same in both the alpine and andean type orogenes in both geological settings obduction of oceanic lithosphere onto the continental lithosphere is caused by the convergence of light buoyant bodies such as oceanic plateaus continental slivers island arcs or old hot spot traces for example the troodos ophiolite complex previously interpreted by some workers as resulting from continental collision may have been emplaced by the collision of cyprus with the eratosthenes plateau embedded in the oceanic eastern mediterranean crust on the other hand the upper jurassic coast range ophiolites of california previously interpreted as resulting from typical oceanic subduction may be the result of a continuous injection of thick nonsubductable packages of light continentally derived sedimentary rocks seamounts and plateaus into the subduction zones many other ophiolite complexes may be similarly related to accreted terranes

sound velocity of mgsio3 perovskite to mbar pressure abstract brillouin scattering measurements of the aggregate shear wave velocities in mgsio 3 perovskite were conducted at high pressure conditions relevant to the earth s lowermost mantle approaching 1 mbar infrared laser annealing of samples in a diamond anvil cell allowed us to obtain high quality brillouin spectra and to drastically extend the upper limit of pressure for brillouin measurements we found that the pressure derivative of the shear modulus d g d p g of mgsio 3 perovskite is 1 56 0 04 which is distinctly lower than that of previous lower pressure experiments below 9 gpa extrapolation of the high pressure shear velocities of perovskite to ambient pressure are in excellent agreement with earlier room pressure brillouin measurements the shear modulus shear velocity and longitudinal velocity at ambient pressure were determined to be g 0 172 9 15 gpa v s 6 49 3 and v p 10 85 3 km sec the mineralogical model that provides a best fit to global seismological 1 d velocity profiles is one that contains 85 90 vol perovskite in the lower mantle the results of this study are the first to demonstrate that the elastic wave velocities for a near adiabatic lower mantle with a bulk composition dominated by magnesium silicate perovskite are consistent with the average lower mantle seismic velocity structure the large pressure range over which acoustic measurements of mgsio 3 perovskite performed in this study has thus allowed us to put tighter constraints on compositional models of the earth s lower mantle

elasticity of subducted basaltic crust at the lower mantle pressures insights on the nature of deep mantle heterogeneity abstract the origin of the large low velocity structures observed in the lowermost mantle is still unclear to make new insights into deep mantle heterogeneity we performed ab initio modeling of the elasticity of representative petrology with the pyrolitic and basaltic compositions based on the elasticities of the following mineral phases magnesium silicate perovskite post perovskite ferropericlase calcium silicate perovskite stishovite cacl 2 and calcium ferrite titanate type mgal 2 o 4 calculations elucidated that the basalt composite is 2 denser than the pyrolitic one through all of the pressures in the lower mantle in addition the shear v s and bulk sound v velocities were found to be 2 slower and faster in the basalt composition in particular at 30 gpa and 120 gpa respectively while both the pyrolitic and basaltic composites have almost the same compressional wave speed v p thermochemically distinct piles which are thought to be related to accumulated hot and dense subducted crusts would therefore be expected to produce as much as a 5 to 6 low velocity anomaly in the s wave but only 1 in the p wave these suggest that the basaltic pile seems less compatible to the seismological signatures of the large low velocity structures if accompanied by thermal anomalies reaching 1000 to 1500 k

gravity signatures of terrane accretion abstract in modern collisional environments accreted terranes are bracketed by forearc gravity lows a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane the size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate along the central american trench the accretion of gorgona island caused a seaward trench jump of 48 to 66 km the relict trench axes show up as gravity lows behind the trench with minimum values of 78 mgal n of gorgona and 49 mgal s of gorgona respectively these forearc gravity lows have little or no topographic expression the active trench immediately seaward of these forearc gravity lows has minimum gravity values of 59 mgal n of gorgona and 58 mgal s of gorgona respectively in the north the active trench has a less pronounced gravity low than the sediment covered forearc in the mariana arc two cretaceous seamounts have been accreted to the eocene arc the northern seamount is most likely a large block the southern seamount may be a thrust slice these more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench however the minimum values of the mariana forearc gravity lows are modest only by comparison to the mariana trench 216 mgal their absolute values are more negative than at gorgona island 145 to 146 mgal we speculate that the forearc gravity lows and seaward trench jumps near gorgona island were produced by the accretion of a hotspot island from a strong plate the mariana gravity lows and seaward trench jumps or thrust slices were the result of breaking a relatively weak plate close to the seamount edifice these gravity lows resulting from accretion events should be preserved in older accreted terranes

la isla de gorgona colombia a petrological enigma a wide range of intrusive wehrlite dunite gabbro and olivine gabbro and extrusive komatiites picrites and basalts igneous rocks are found on the small pacific island of gorgona the island is best known for its 90 ma spinifex textured komatiites the only true phanerozoic komatiites yet discovered early work led to suggestions that the rocks of the island formed at a mid ocean ridge however more recent research supports an origin as part of a hot mantle plume derived oceanic plateau one of the main lines of evidence for this origin stems from the inferred high mantle source temperatures required to form the high mgo n15 wt komatiites and picrites another remarkable feature of the island considering its small size 8 2 5 km is the degree of chemical and radiogenic isotopic heterogeneity shown by the rocks this heterogeneity requires a mantle source region with at least three isotopically distinctive source regions two depleted and one enriched although these mantle source regions appear to be derived in significant part from recycled oceanic crust and lithosphere enrichments in 187os 186os and 3he in gorgona lavas and intrusive rocks suggest some degree of transfer of material from the outer core to the plume source region at dw modelling reveals that the komatiites probably formed by dynamic melting at an average pressure of 3 4 gpa leaving residual harzburgite trace element depletion in gorgona ultramafic rocks appears to be the result of earlier deeper melting which produced high mgo trace element enriched magmas the discovery of a trace element enriched picrite on the island has confirmed this model gorgona accreted onto the palaeocontinental margin of northwestern south america in the eocene and palaeomagnetic work reveals that it was formed at 26 8s it has been proposed that gorgona is a part of the caribbean colombian oceanic plateau ccop however the ccop accreted in the late cretaceous and was derived from a more equatorial palaeolatitude this evidence and differing geochemical signatures strongly suggests that gorgona and probably other coastal oceanic plateau sequences in colombia and ecuador belong to a completely different oceanic plateau to the ccop

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

spin transition of ferric iron in the nal phase implications for the seismic heterogeneities of subducted slabs in the lower mantle abstract al rich phases nal new hexagonal aluminous phase and cf calcium ferrite phase are believed to constitute 10 30 wt of subducted mid ocean ridge basalt morb in the earth s lower mantle in order to understand the effects of iron on compressibility and elastic properties of the nal phase we have studied two single crystal samples fe free na1 14mg1 83al4 74si1 23o12 and fe bearing na 0 71 mg 2 05 al 4 62 si 1 16 fe 2 0 09 fe 3 0 17 o 12 using synchrotron nuclear forward scattering nfs and x ray diffraction xrd combined with diamond anvil cells up to 86 gpa at room temperature a pressure induced high spin hs to low spin ls transition of the octahedral fe3 in the fe bearing nal is observed at approximately 30 gpa by nfs compared to the fe free nal the fe bearing nal undergoes a volume reduction of 1 0 1 2 a3 at 33 47 gpa as supported by xrd which is associated with the spin transition of the octahedral fe3 the fits of birch murnaghan equation of state b m eos to p v data yield unit cell volume at zero pressure v 0 183 1 1 a 3 and isothermal bulk modulus k t 0 233 6 gpa with a pressure derivative k t 0 3 7 2 for the fe free nal v 0 hs 184 76 6 a 3 and k t 0 hs 238 1 gpa with k t 0 hs 4 fixed for the fe bearing nal the bulk sound velocities v of the fe free and fe bearing nal phase are approximately 6 larger than those of al fe bearing bridgmanite and calcium silicate perovskite in the lower mantle except for the spin transition region where a notable softening of v with a maximum reduction of 9 4 occurs in the fe bearing nal at 41 gpa considering the high volume proportion of the nal phase in subducted morb the distinct elastic properties of the fe bearing nal phase across the spin transition reported here may provide an alternative plausible explanation for the observed seismic heterogeneities of subducted slabs in the lower mantle at depths below 1200 km

lower mantle seismic scatterers below the subducting tonga slab evidence for slab entrainment of transition zone materials abstract we show evidence that materials with significantly different elastic properties are juxtaposed in the lower mantle immediately below the subducting tonga slab depths 1000 km like the anomalies preferentially located beneath the lower mantle slabs at other pacific subduction zones kaneshima 2009 array analyses of wave form data of short period seismic networks at western united states and japan for deep earthquakes at the tonga slab reveal s to p scatterers with a size less than the wavelengths 10 km the scatterers are located mostly outside of the slab by several tens of kilometers assuming a locally planar interface for the geometry of the scatterers the amplitudes and polarities of the s to p waves are modeled to constrain the properties of the scatterers we find that the scatterers are steeply dipping the vs increases oceanward across the interface and the vs contrasts are at least comparable to that associated with the post spinel transformation 6 it is unclear at this stage what these subslab scatterers represent so we discuss about three mechanisms which seem plausible from mantle dynamics viewpoints 1 they may represent basaltic rocks which were emplaced by partial melting immediately beneath the former oceanic lithosphere asthenosphere boundary lab before the slab started subducting 2 alternatively the elastic anomalies of the scatterers may be caused by localized presence of dehydrated water or 3 the scatterers may correspond to a sharp boundary between fine grained isotropic rocks in the immediate vicinity of the slab and coarse grained anisotropic rocks more distant from the slab the presence of pronounced and localized elastic anomalies preferentially beneath the slabs in the shallow lower mantle whatever its mechanism is implies that a geophysically observable amount of transition zone material is entrained by the subducting slabs into the lower mantle

elasticity of single crystal periclase at high pressure and temperature the effect of iron on the elasticity and seismic parameters of ferropericlase in the lower mantle

subduction obstruction and the crack up of the pacific plate we consider two hypotheses for the origin of a linear trough extending 800 km eastward from the intersection of the louisville seamount chain with the tonga kermadec trench an extinct spreading center origin is suggested by the trough s eastward termination at a depth discontinuity that resembles a fracture zone the morphology of the trough however is inconsistent with the morphology of other extinct spreading centers and these two features do not appear to be connected to a larger paleo plate boundary on their eastern end we consider an alternative hypothesis in which the trough may be a recent rift in the pacific plate coupling between a large subducting seamount and the overriding australian plate may serve as a mechanism to focus stresses and nucleate a rift in the subducting pacific plate in this model the seamount would act as a rigid indenter producing tangential extension in the subducting plate about the region where it is coupled to the overriding plate impeded subduction is suggested by a prominent seismicity gap around the subducting seamount the graben morphology of the trough is also consistent with an extensional origin if hotspot volcanism can produce obstructions and local weaknesses conducive to plate coupling stress localization and nucleation of fractures it may provide a mechanism for subduction induced plate reorganizations

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

the temperature and salinity fine structure of the ocean under the ross ice shelf a series of temperature and salinity profiles was made in the ocean under the ross ice shelf at 82 22 5 s 168 37 5 w where the ice was 420 and the underlying seawater 240 m thick the water structure consisted of a fairly well mixed low salinity layer at the in situ freezing point of the ice water interface about 30 m thick a transition layer characterized by intrusions about 85 m thick a strongly stratified layer with increasing temperature and salinity about 50 m thick another transition layer characterized by intrusions about 45 m thick and an isohaline bottom layer with a nearly abiabatic temperature gradient about 30 m thick the temperature fluctuations in the two transition layers can be attributed mainly to intrusive activity even though internal wave activity was highest in these regions in the central stable layer internal waves probably contribute to the temperature fluctuations equally with intrusions the internal wave dropped displacement spectrum seems to be very similar to those found in the open ocean some profiles showed intrusions in the top transition layer colder than the top boundary layer and some showed intrusions in the bottom transition layer warmer than the bottom boundary layer indicating that horizontal shear must be present internal waves of near inertial frequency excited by the semidiurnal tides combined with shear induced instabilities seem to be the likely causes of the observed fine structure

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

distinct compositional thin layers at mid mantle depths beneath northeast china revealed by the usarray abstract we observe a clear seismic arrival at 35 45 s after the direct p wave in usarray recordings of two deep earthquakes occurring beneath northeast china velocity spectrum and beam forming analyses reveal that this arrival has a lower slowness value than the direct p wave and a back azimuth slightly different from the great circle direction the measured slowness and arrival time indicate that it is a transmitted s to p conversion from structures below the sources we employ the common conversion point ccp stacking and diffraction migration methods to determine the location and geometric features of the seismic structures the ccp stacking image indicates that the structure is a localized discontinuity at 1000 km with a dimension at 200 km by 50 km along the e w and n s directions respectively it is located at 150 km northeast to the two events the 2d migrated images on the other hand indicate that the sources structure are reflectors dipping northeastwards by 17 at a slightly shallower depths the reflectors have a length scale of 100 km ant their centers are 50 km away from the epicenters of the two earthquakes forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers the layers have a lower shear velocity and a higher density than that of the surrounding mantle which matches well with those predicted for mid ocean ridge basalt morb at mid mantle depths according to a recent ab initio study combined with the results from previous studies our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones

fine structure internal waves and intrusions in the marginal ice zone of the greenland sea two series of 40 temperature and salinity profiles were made in the marginal ice zone of the northeastern greenland sea fram strait region the first series showed much intrusive activity while the second was dominated by internal wave activity displacement dropped spectra showed spectral levels scaled by desaubies 1976 analytic representation of the garrett munk 1975 model to be comparable to those from lower latitudes though probably somewhat lower temperature dropped spectra showed severe contamination by intrusions it is concluded that temperature spectra cannot be used to determine internal wave activity of the upper ocean in the marginal ice zone

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

chronostratigraphy and tectonic deformation of the north ecuadorian south colombian offshore manglares forearc basin abstract forearc basins in convergent margins contain a nearly continuous record of sedimentation and deformation their chronostratigraphy reflects the manner in which the basin accommodates loads and vertical and horizontal motion caused by eustasy tectonics and sedimentation during plate convergence along the north ecuador and south colombia nesc margin 1 3 5 n thick cenozoic sedimentary sequences accumulated over blocks of oceanic basement that accreted between the late cretaceous and early cenozoic thus forming the onshore borbon and tumaco basins and the offshore pacific frontal basin based on morphology and seismic data analysis the nesc forearc basin named manglares basin consists of five acoustic units of sedimentary nature units a e separated by unconformities u1 u5 resting over the interpreted oceanic basement unit f by integrating these data with regionally compiled chronostratigraphic charts of the onshore borbon and tumaco basins and the geology of gorgona island we propose a model for the tectonic and stratigraphic evolution for the manglares basin and its oceanic basement this model shows that 1 unconformity u5 between units f and e marks a tectonic phase that may correlate with the accretion of the oceanic basement of the manglares basin to the northern andes possibly between the late cretaceous and paleocene 2 a marked change in seismic facies between units e and d associated with major tectonic unconformity u4 is compatible with the incaic compressive phase that affected the andes during the late eocene early oligocene 3 this tectonic phase likely peaked during the latest oligocene earliest miocene u3 and later during the middle miocene u2 4 the regional middle miocene erosion period u2 marked a change in the tectono sedimentary evolution of the forearc basin compressive deformation ceased in the southern margin segment which underwent long term basin subsidence while compression and uplift increased in the central and northern segments

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

spin transition of ferric iron in the calcium ferrite type aluminous phase we investigated fe free and fe bearing cf phases using nuclear forward scattering and x ray diffraction coupled with diamond anvil cells up to 80 gpa at room temperature octahedral fe3 ions in the fe bearing cf phase undergo a high spin to low spin transition at 25 35 gpa accompanied by a volume reduction of 2 0 and a softening of bulk sound velocity up to 17 6 based on the results of this study and our previous studies both the nal and cf phases which account for 10 30 vol of subducted morb in the lower mantle are predicted to undergo a spin transition of octahedral fe3 at lower mantle pressures spin transitions in these two aluminous phases result in an increase of density of 0 24 and a pronounced softening of bulk sound velocity up to 2 3 for subducted morb at 25 60 gpa and 300 k the anomalous elasticity region expands and moves to 30 75 gpa at 1200 k and the maximum of the v reduction decreases to 1 8 this anomalous elastic behavior of fe bearing aluminous phases across spin transition zones may be relevant in understanding the observed seismic signatures in the lower mantle

a statistical correlation between ridge crest offsets and spreading rate ridge crest offsets follow the statistical distribution n a exp b ln l lo where n is the cumulative number of ridge crest offsets with lengths greater than l lo is the cutoff length and a and b are constants we measured a and b for the slow spreading mid atlantic ridge the intermediate spreading juan de fuca ridge and the fast spreading east pacific rise the predicted mean distances between offsets greater than 4 km long are 30 32 and 74 km on the slow intermediate and fast spreading ridge crests these distances are in agreement with observations of other workers and calculations of modal ridge lengths

evidence of midmantle anisotropy from shear wave splitting and the influence of shear coupled p waves 1 until recently the midmantle region of the earth the transition zone and the uppermost lower mantle has been considered isotropic recent work by wookey et al 2002 presented evidence of anisotropy in the midmantle region near the tonga kermadec subduction zone inferred from shear wave splitting in teleseismic s phases at australian seismic stations this data set is revisited to examine the possibility of contamination by shear coupled p waves using reflectivity modeling we explore the effects of these phases on the measurement of shear wave splitting contamination of such measurements is demonstrated for short epicentral distances 30 wave field decomposition is a simple technique which can be used to separate these phases from the main s wave arrival the tonga australia data set is reprocessed after wave field decomposition shear wave splitting ranging between 0 7 and 6 2 s is observed a range comparable to that observed by wookey et al 2002 as before the polarization of the fast shear wave is observed to be predominantly horizontal however the magnitudes of splitting for several events at short epicentral distances are significantly reduced suggesting some influence of shear coupled p waves in the original analysis the results are compared with possible models of mantle anisotropy and the results can be best explained by significant anisotropy in the midmantle region the best candidate location for this anisotropy is in the uppermost lower mantle and scenarios involving the alignment of lower mantle or subducted materials by regional dynamic processes are suggested to account for this

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

shear wave anisotropy in textured phase d and constraints on deep water recycling in subduction zones abstract regions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation the interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases here we investigate the development of lattice preferred orientations lpo in phase d mgsi 2 h 2 o 6 10 18 wt h 2 o the ultimate water carrier in hydrous subducted peridotite the samples were deformed non hydrostatically up to 48 gpa in a diamond anvil cell and the texture and strength were obtained from analysis of the x ray diffraction patterns collected in radial diffraction geometry we find that at low strains the layered structure of phase d displays strong 0001 texture where the stacking fault axis c axis preferentially align parallel to the compression axis a subsidiary 1 0 1 0 texture develops at higher strains plasticity simulations in polycrystalline aggregates using a viscoplastic self consistent model suggest that these lpo patterns are consistent with shape preferred orientation mechanism during the first compaction steps and with dominant easy glide on basal planes and harder first order pyramidal slip respectively upon further compression we find that phase d displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone we further evaluate the effect of textured phase d on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in tonga subduction we show that 16 vol of phase d in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3 the extent of shear wave splitting 0 9 0 3 and the shear wave ray polarization geometry v sh v sv observed in a detached fragment of the tonga slab laying sub horizontally below the transition zone seismic observations may thus place constraints on the degree of hydration of the tonga slab beyond the transition zone leading to the requirement that a minimum of 1 2 wt h 2 o is retained in the slab by hydrous peridotite below 670 km depth

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

seismic anisotropy in the mantle transition zone induced by shear deformation of wadsleyite abstract mantle flow in the earth s mantle transition zone between 410 and 660 km depth plays a key role to understand the nature of mantle convection which can be mapped by observed seismic anisotropy combined with crystallographic preferred orientations of mantle minerals although wadsleyite is the most important mineral to cause seismic anisotropy observed in the mantle transition zone there have been limited experimental data on its crystallographic preferred orientation because of experimental limitations we experimentally evaluated the preferred orientation of wadsleyite developed by shear deformation at pressure temperature conditions of the mantle transition zone 17 6 gpa and 1800 1900 k using a deformation dia apparatus the deformation experiments reveal that the 0 0 1 axis and the 0 1 0 plane of wadsleyite tend to be subparallel to the shear direction and the shear plane during deformation respectively these results demonstrate that polarization seismic anisotropy velocity contrast between horizontally polarized and vertically polarized s waves vsh vsv observed in the mantle transition zone might be attributed to the preferred orientation of wadsleyite caused by horizontal mantle flow

equations of state of na k al host phases and implications for morb density in the lower mantle abstract we report equations of state eos of two important silicate phases known to store na k and al at pressures and temperatures prevailing in the earth s lower mantle using laser heated diamond anvil cell we produced the ca ferrite type cf and the new aluminum rich nal phases using different synthetic and natural starting materials at pressures ranging from 23 to 65 gpa unit cell volumes were investigated by means of angle dispersive x ray diffraction at the id30 beamline of the esrf synchrotron ring grenoble france cf phases were synthesized in naalsio4 mgal2o4 and naalsio4 mg fe 2sio4 systems and for a natural mid ocean ridge basalt morb the insertion of mgal2o4 in the naalsio4 based lattice has reduced effects on the cf phase eos whereas that of mg fe 2sio4 is mainly to increase the room pressure volume cf phases most relevant to the earth s mantle show compression behavior comparable to the pure naalsio4 cf phase with bulk moduli values close to k0 190 gpa and k0 4 we also synthesized nal phases in naalsio4 mgal2o4 system and using cana and k rich silicate glasses as starting materials all nal compounds investigated show similar compression behavior except for one anomalously naalsio4 rich composition for nal phase with the most natural conditions relevant composition we refine bulk moduli of k0 182 gpa and k0 6 we then model the density profiles expected for the cf and nal phases using our newly determined eos and the most accurate chemical compositions reported in the literature these two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 k however the density profiles calculated for mineral assemblages corresponding to typical morb compositions point out to denser morbs compared to the pyrolite this is due to the high fe content in morb yielding a densest fe rich perovskite that counterbalances the lower densities of the cf and nal phases the density gap is calculated between 0 4 and 4 0 depending on the starting morb composition and pressure

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

elastic properties of stishovite and the cacl2 type silica at the mantle temperature and pressure an ab initio investigation a b s t r a c t the elastic constant tensors of stishovite and the cacl2 type silica at the earth s mantle temperature and pressure were determined using first principles calculations with local density approximation the elastic properties of stishovite show not only strong pressure dependence but also temperature dependence by increasing temperature the shear instability of stishovite is shifted to an elevated pressure with a slope of 5 4 1 4m pa k the softening of the shear modulus and the positive clapeyron slope result in crossing of the sound velocities at different temperatures which leads to the unusual positive temperature dependence of the sound velocities around the phase boundary the transition from stishovite to the cacl2 type silica at the lower mantle s temperature occurs at a depth far deeper than 1200 km and is accompanied by a velocity jump of 0 98 0 08 km si n s wave velocity v s and 0 45 0 15 km s in p wave velocity v p this transition is likely related to the seismic discontinuity at the depth of 1670 km in the vicinity of mariana island the unusual positive temperature dependence of v s of stishovite and strong anisotropy of stishovite and the cacl2 type silica around the phase boundary

upper ocean internal waves in the marginal ice zone of the northeastern greenland sea oceanographic measurements obtained in the northeastern greenland sea fram strait region were studied in order to characterize internal wave activity in the marginal ice zone miz of the arctic ocean experiments were performed with an array of four horizontally separated current meters and a ctd instrument in 1984 as part of the marginal ice zone experiment spectra of horizontal and vertical motion show that within the frequency range from the inertial frequency to 3 cph the miz internal wave field is similar in spectral shape and energy content to the wave field found beneath arctic pack ice above 3 cph the miz spectra contain more energy than expected based upon both beneath ice measurements and the garrett munk model spectra elevated energy levels at high frequencies are shown to be caused by intermittent increases in internal wave activity occurring on time scales from 2 to 12 hours phase differences between pairs of time series computed during the passage of several packets of internal waves are used to estimate horizontal wave number vectors a comparison of the calculated wave number vectors to a dispersion relation derived from a simple normal mode model of internal waves suggests that the wave packets may be adequately described as being comprised of lowest mode horizontally propagating internal waves the interaction of ice bottom topography with a sharp elevated pycnocline is explored as a possible source of the intermittent increases in wave activity spectra of vertical displacement and velocity computed from time series of both temperature and density are presented in order to show the influence of intrusive activity in the miz differences in both spectral shape and energy content found at frequencies below 2 5 cph emphasize the need for careful interpretation of temperature based measurements of vertical motion obtained over long periods of time in the miz

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

sound velocity measurements of casio 3 perovskite to 133 gpa and implications for lowermost mantle seismic anomalies abstract we report the measurements of aggregate shear velocity v s of casio 3 perovskite capv at high pressure p between 32 and 133 gpa and room temperature t on the basis of brillouin spectroscopy the sample had a tetragonal perovskite structure throughout the experiments the measured p v s data show the shear modulus and its pressure derivative at ambient condition to be g 0 115 8 gpa and g 1 20 respectively the zero pressure shear velocity is determined to be v s0 5 23 km s in good agreement with the previous estimate inferred from the ultrasonic measurements on ca si ti o 3 perovskite at 1 bar our experimental results are broadly consistent with the earlier calculations on tetragonal capv but exhibit lower velocity at equivalent pressure such tetragonal capv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle while primitive mantle includes certain amount of capv a depleted peridotite former harzburgite layer in subducted oceanic lithosphere is deficient in capv and enriched in ferropericlase in the lower mantle such harzburgite exhibits 0 9 faster v s and 0 7 slower bulk sound velocity v at the lowermost mantle p t conditions if capv is present in the tetragonal form in the surrounding mantle the observed fast v s and slow v anomalies in the d layer underneath the circum pacific region might be attributed in large part in the presence of subducted harzburgitic materials

phase stability and elastic properties of the nal and cf phases in the namg2al5sio12 system from first principles new hexagonal aluminous nal phase and orthorhombic calcium ferrite cf type phase are considered to be major mantle components of the mid ocean ridge basalt morb at pressure and temperature conditions in the lower mantle which can potentially host alkali elements with large ionic radii the high pressure stability and elastic properties of both nal and cf phases are therefore of fundamental importance for understanding the fate of subducted morb here we report those properties of the namg2al5sio12 system studied by means of the first principles computation method nal was found to transform to the cf phase at 39 6 gpa accompanied by discontinuities in density 1 8 as well as compressional wave 0 2 shear wave 0 9 and bulk sound 1 0 velocities the property of subducted morb was evaluated using these results and the velocity contrast between pyrolite and morb of 5 was found to be quite comparable to the shear velocity anomaly observed for seismic scatterers at depths from 1100 to 1800 km however since the transformation of the nal to cf phase within morb produces insignificant increases in the seismic velocities it would be seismologically undetectable on the other hand the anisotropy change associated with the phase transition is significant and could be seismically detectable using observations such as shear wave splitting measurements since the cf phase is considerably more anisotropic compared to the nal phase

search for seismic discontinuities in the lower mantle summary indications of lower mantle discontinuities have been debated for decades but still little is known about their properties and their origins are enigmatic in our study broad band recordings of deep events are examined for the presence of signals from the lower mantle discontinuities with a novel technique we deconvolve vertical component of the p wave coda in the period range around 10 s by the s waveform and stack many deconvolved traces with moveout time corrections in synthetic seismograms for an earth model without lower mantle discontinuities the strongest signal thus detected in the time window of interest is often s 410 p phase generated as s and reflected as p from the 410 km discontinuity above the source in actual seismograms there are other phases that can be interpreted as converted from s to p at discontinuities in the lower mantle beneath the seismic source we summarize the results of processing the seismograms 1 of deep events in sunda arc at seismograph stations in east asia 2 deep kermadec fiji tonga events at the j array and freesia networks in japan and stations in east asia and 3 deep events in the northwest pacific region mariana izu bonin and the japan arc recorded at stations in north america in our data there are indications of discontinuities near 860 880 1010 1120 1170 1250 and 1670 1800 km depths the clearest signals are obtained from the discontinuity at a depth of 1200 km we argue that the 900 1200 and 1700 km discontinuities are global but laterally variable in both depth and strength seismic stratification of the lower mantle may have bearings on the patterns of subduction as revealed by tomographic models

internal waves and mixing in the marginal ice zone near the yermak plateau observations were made of oceanic currents hydrography and microstructure in the southern yermak plateau in summer 2007 the location is in the marginal ice zone at the arctic front northwest of svalbard where the west spitsbergen current wsc carries the warm atlantic water into the arctic ocean time series of approximately 1 day duration from five stations upper 520 m and of an 8 day duration from a mooring are analyzed to describe the characteristics of internal waves and turbulent mixing the spectral composition of the internal wave field over the southern yermak plateau is 0 1 0 3 times the midlatitude levels and compares with the most energetic levels in the central arctic dissipation rate and eddy diffusivity below the pycnocline increase from the noise level on the cold side of the front by one order of magnitude on the warm side where 100 m thick layers with average diffusivities of 5 3 10 25 m 2 s 21 lead to heat loss from the atlantic water of 2 4 w m 22 dissipation in the upper 150 m is well above the noise level at all stations but strong stratification at the cold side of the front prohibits mixing across the pycnocline close to the shelf at the core of the svalbard branch of the wsc diffusivity increases by another factor of 3 6 here nearbottom mixing removes 15 w m 22 of heat from the atlantic layer internal wave activity and mixing show variability related to topography and hydrography thus the path of the wsc will affect the cooling and freshening of the atlantic inflow when generalized to the arctic ocean diapycnal mixing away from abyssal plains can be significant for the heat budget around the yermak plateau it is of sufficient magnitude to influence heat anomaly pulses entering the arctic ocean however diapycnal mixing alone is unlikely to be significant for regional cooling of the wsc

elasticity of single crystal nal phase at high pressure a potential source of the seismic anisotropy in the lower mantle the new hexagonal aluminous phase named the nal phase is expected to be stable at depths of 1200 km in subducted slabs and believed to constitute 10 30 wt of subducted mid ocean ridge basalt together with the cafe2o4 type aluminous phase here elasticity of the single crystal nal phase is investigated using brillouin light scattering coupled with diamond anvil cells up to 20 gpa at room temperature analysis of the results shows that the substitution of iron lowers the shear modulus of the nal phase by 5 6 gpa but does not significantly affect the adiabatic bulk modulus the nal phase exhibits high velocity anisotropies with avp 14 7 and avs 15 12 for the fe bearing phase at ambient conditions the high avs of the nal phase mainly results from the high anisotropy of the faster vs1 13 9 15 8 while the slower vs2 appears almost isotropic 0 1 2 8 at ambient and high pressures the avp and avs of the nal phase decrease with increasing pressure but still have large values with avp 11 4 and avs 14 12 for the fe bearing sample at 20 4 gpa the extrapolated avp and avs of the fe free and fe bearing nal phases at 40 gpa are larger than those of bridgmanite at the same pressure together with its spin transition of iron and structural transition to the cf phase the presence of the nal phase with high velocity anisotropies may contribute to the observed seismic anisotropy around subducted slabs in the uppermost lower mantle

in situ observation of texture development in olivine ringwoodite magnesiowustite and silicate perovskite at high pressure magnesium silicates are the dominant minerals in the earth s mantle their preferred orientation is important for understanding the rheology and seismic anisotropy in the deep earth here we report results of radial synchrotron diffraction diamond anvil cell dac experiments on san carlos olivine axially compressed to 50 gpa experiments were performed at room temperature except for brief laser heating to induce phase transformations high stresses and development of preferred orientation were observed in diffraction images quantitative texture information was obtained by analyzing the images with the rietveld method with increasing pressure between 9 and 43 gpa olivine develops a texture with 001 axes perpendicular to the compression direction that is compatible with hk0 001 pencil glide ringwoodite between 20 and 50 gpa develops weak preferred orientation with 011 lattice planes perpendicular to the compression direction after the phase transformation perovskite and magnesiowqstite display transformation textures that are then modified by continuing deformation magnesiowqstite has a weak h111i maximum parallel to the compression direction that changes towards h001i with increasing deformation perovskite transforming from olivine has a pronounced 100 transformation texture and with increasing deformation a 012 maximum develops the transformation texture is probably produced by mechanical 110 twinning and nucleation in orientations that minimize elastic strain energy the deformation texture of perovskite is due to slip d 2004 elsevier b v all rights reserved

tectonically controlled origin of three unusual rock suites in the woodlark basin we propose alternative mechanisms for the origin of three unusual rock suites high mg andesites nati basalts and arclike rocks that have been dredged from the woodlark basin southwest pacific ocean we show that the high mg andesites and nati basalts are associated with an unusually cool ridge environment the cooling is due to increased hydrothermal circulation stimulated by an unusually high crustal permeability the high permeability is mainly due to cracking of the woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone although local processes such as faulting in fracture zones may also make some contribution this increased convective cooling affects magma dynamics and chemistry at the ridge crest the high mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite the nati basalts are also the indirect result of increased crustal cooling which causes anomalously low degrees of partial melting of their depleted mantle source the arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m y ago by a now inactive subduction zone to the north of the currently active one simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active as the old plate loses volatiles they rise into the mantle convection cell which feeds the woodlark basin ridge crest because ridge subduction increases the likelihood of ophiolite obduction these observations and explanations of woodlark basin tectonics are potentially important for ophiolites

elasticity of single crystal iron bearing pyrope up to 20 gpa and 750 k abstract elastic properties of the major constituent minerals in the earth s upper mantle at relevant high pressure temperature p t conditions are crucial for understanding the composition and seismic velocity structures of this region in this study for the first time we have measured the single crystal elasticity of natural fe bearing pyrope mg 2 04 fe 0 74 ca 0 16 mn 0 05 al 2 si 3 o 12 using in situ brillouin spectroscopy and x ray diffraction at simultaneous high p t conditions up to 20 gpa and 750 k in an externally heated diamond anvil cell the derived aggregate adiabatic bulk and shear moduli k s 0 g 0 at ambient conditions are 168 2 1 8 gpa the value in parentheses represents propagated uncertainties 1 and 92 1 1 1 gpa respectively consistent with literature results using the third order eulerian finite strain equation to model the high p t data the derived pressure derivatives of the bulk and shear moduli at constant temperature are k s p t 4 4 0 1 and g p t 1 2 0 1 respectively whereas the temperature derivatives of these moduli at constant pressure are k s t p 16 8 1 3 mpa k and g t p 5 1 1 1 mpa k respectively compared to literature values our results show that addition of 25 mol fe in pyrope increases the pressure derivative of the bulk modulus by 7 but has a negligible effect on other elastic parameters extrapolation of our results shows that fe bearing pyrope remains almost elastically isotropic at relevant p t conditions of the upper mantle indicating that it may not have a significant contribution to seismic v p and v s anisotropy in the upper mantle together with the elasticity of olivine and pyroxene minerals in the upper mantle we have constructed new velocity profiles for two representative compositional models pyrolite and piclogite along a representative geotherm of the earth s upper mantle these velocity models show v s profiles consistent with seismic observations although v p profiles are slightly lower than global seismic observations our analyses also show that approximately 30 garnet in a mineralogical model is needed to best match both seismic v p and v s profiles of the region although such high garnet content needs to be reconciled with our current geochemical understanding our results here provide new insights into seismic profiles and mineralogical models of the upper mantle region

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot