chemical alteration of seawater by lava from kilauea volcano hawaii abstract to examine the chemical alteration of seawater by submarine lava seawater was sampled at a depth of 21 m east of kupapau point hawaii after contact with molten lava from kilauea volcano the warm mildly acid ph 6 5 at 27 c hydrothermal solutions are highly enriched in dissolved si 40 and mn 400 relative to ambient seawater chloride shows a slight but discernable enrichment in the most altered samples probably due to boiling and partial phase segregation steam loss during heating assuming cl behaves conservatively ca and mg are on the order of 1 depleted in the altered solutions which we attribute to anhydrite precipitation and secondary silicate formation during the water rock exchange dissolved iron is not enriched in the solutions although examination of filtered particles suggests that some fe is mobilized from the basalt these distinctive chemical changes are consistent with a brief moderate temperature

mantle flow drives the subsidence of oceanic plates the subsidence of the sea floor is generally considered a consequence of its passive cooling and densifying since its formation at the ridge and is therefore regarded as a function of lithospheric age only however the lithosphere is defined as the thermal boundary layer of mantle convection which should thus determine its structure we examined the evolution of the lithosphere structure and depth along trajectories representative of the underlying mantle flow we show that along these flow lines the sea floor depth varies as the square root of the distance from the ridge as given by the boundary layer equation along the entire plate without any flattening contrary to previous models no additional heat supply is required at the base of the lithosphere

convective interactions between oceanic lithosphere and asthenosphere influence of a transform fault abstract current estimates of the asthenospheric viscosity favor the occurrence of small scale convection ssc beneath oceanic lithosphere of sufficient age since transform faults tfs juxtapose lithospheres of distinct ages and thicknesses they are likely to influence their dynamics in this paper we describe 3d numerical models of oceanic spreading for two ridge segments offset by a tf in domains that are sufficiently large for ssc to fully develop a framework based on thermal convection with temperature dependent viscosity and both internal and basal heating is adopted and parameters such as surface velocity age offset at the tf activation energy mantle viscosity and internal heating rate are explored in our numerical experiments the onset time for ssc is identical on each side of the fracture zone fz it is not modified by the presence of a tf moreover scaling relationships for the onset time obtained in 2d experiments remain valid earlier studies focused on the influence of a tf based on 2d experiments corresponding to slices parallel to the ridge axis they noted the rapid development of a sinking thermal instability located at the step in the lithospheric relief our results indicate that the wake of the tf is characterized by the development of a lateral edge driven buoyant flow from the younger thinner side to the older thicker side of the fz however due to the global shearing context this instability does not sink before the onset of ssc characterized by small scale instabilities on each side of the fz as observed in earlier studies the essentially 3d velocity field combining this lateral flow and the prescribed surface spreading velocity results in an angle formed by the tf trail with the spreading direction we describe the parameters controlling this angle in our experiments and propose a scaling relationship in oceanic settings the specific geometry of the ridge axis segment length orientation of the tfs may affect this angle in addition a large scale mantle flow of other geodynamical origin could interact with the spreading flow and also modify the tf trail angle this departure at depth of the boundary between the thick and thin lithospheres from the surface fz may explain the presence of oceanic volcanoes on the older side of the fz usually considered as thicker

intra oceanic subduction related hydrothermal venting kermadec volcanic arc new zealand intra oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front more than half seven of 13 of the volcanoes surveyed along 260 km of the southern kermadec arc ne of new zealand are hydrothermally active our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow 2 km water depth vent fields expelling fluids of a unique and heterogeneous composition into the oceans

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

new evidence for dislocation creep from 3 d geodynamic modeling of the pacific upper mantle structure laboratory studies on deformation of olivine in response to applied stress suggest two distinct deformation mechanisms in the earth s upper mantle diffusion creep through diffusion of atoms along grain boundaries and dislocation creep by slipping along crystallographic glide planes each mechanism has very different and important consequences on the dynamical evolution of the mantle and the development of mantle fabric due to the lack of in situ observations it is unclear which deformation mechanism dominates in the upper mantle although observed seismic anisotropy in the upper mantle suggests the presence of dislocation creep we examined the thermo mechanical erosion of the lithosphere by thermal boundary layer instabilities in 3 d dynamical models this study demonstrates that the seismically derived thermal structure of the pacific lithosphere and upper mantle imposes an important constraint on the upper mantle deformation mechanism the predominant deformation mechanism in the upper mantle is dislocation creep consistent with observed seismic anisotropy the acceptable activation energy range of 360 540 kj mol is consistent with although at the lower end of those determined from laboratory studies

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

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

why intracontinental basins subside longer 3 d feedback effects of lithospheric cooling and sedimentation on the flexural strength of the lithosphere

gas rich submarine exhalations during the 1989 eruption of macdonald seamount in january 1989 we observed submarine eruptions on the summit of macdonald volcano during a french german diving programme with the ifremer submersible cyana gas streaming of large amounts of ch4 co2 and so2 from summit vents inferred from water column anomalies and observed by submersible was accompanied on the sea surface by steam bursts turbulence red glowing gases and black bubbles comprising volcanic ash sulphur and sulphides chloride depletion of water sampled on the floor of an actively degassing summit crater suggests either boiling and phase separation or additions of magmatic water vapour submersible observations in situ sampling and shipboard geophysical and hydrographic measurements show that the hydrothermal system of this hotspot volcano is distinguished by the influence of magmatic gases released from its shallow summit

the role of the heating mode of the mantle in intermittent reorganization of the plate velocity field summary the geological record indicates that stages of relatively steady plate motion have been punctuated by comparatively brief periods in which plate velocities have reorganized the distribution of buoyancy sources in the mantle has generally been regarded as evolving too slowly to explain these rapid transitions in plate velocity we investigate the feedback between mantle convection and plate velocity using 2 d and 3 d mantle convection models that incorporate mobile dynamic plates we focus on the influence of internal heating in the mantle and consider the effect of mantle viscosity stratification and different plate geometries on the plate velocity time dependence as either the rayleigh number or the internal heating rate is increased to magnitudes approaching mantle values the record of the plate motion from our calculations becomes characterized by intermittent changes in direction this behaviour is a result of the influence of plates on heat loss from the inherently unsteady internally heated convecting system plate motion instills a pattern of organization on the underlying convection that reflects the plate geometry and results in the formation of sheet like downwelling structures at convergent plate boundaries in both 2 d and 3 d calculations in contrast upwellings in 3 d models are not sheet like the role of the sheet like downwellings is critical in the observed episodic reorganization of the plate velocities warm material below the plates is entrained by plate motion into regions enveloping the downwelling sheets during periods of fairly steady plate motion buoyancy associated with the build up of heat around the downwelling sheets leads to the creation of an unstable convection pattern this build up of heat is dramatic in calculations with mantle like internal heating rates and resists continued long term plate motion towards mature downwellings when there are limitations on the degree of freedom of the direction of plate movement such as in 2 d models these effects become even more pronounced accordingly the effect of plates on mean global thermal quantities is more dramatic in 2 d calculations than it is in 3 d calculations nevertheless 3 d calculations incorporating plates of different sizes do exhibit rapid reorganizations in their convection patterns as the pull of young slablike features supersedes the pull of mature downwelling sheets we compare the timing and frequency of the reorganization events in our calculations with the general characteristics of plate motions determined from plate reconstruction studies

episodic tectonic plate reorganizations driven by mantle convection abstract periods of relatively uniform plate motion were interrupted several times throughout the cenozoic and mesozoic by rapid plate reorganization events r hey geol soc am bull 88 1977 1404 1420 p a rona e s richardson earth planet sci lett 40 1978 1 11 d c engebretson a cox r g gordon geol soc am spec pap 206 1985 r g gordon d m jurdy j geophys res 91 1986 12389 12406 d a clague g b dalrymple us geol surv prof pap 1350 1987 5 54 j m stock p molnar nature 325 1987 495 499 c lithgow bertelloni m a richards geophys res lett 22 1995 1317 1320 m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 it has been proposed that changes in plate boundary forces are responsible for these events m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 we present an alternative hypothesis convection driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle this instability which has not been described before is responsible for episodic reorganizations of plate motion numerical mantle convection experiments demonstrate that high rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events changes in plate boundary forces or plate geometry are not required

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

hydrography and geochemistry of sea surface hydrothermal plumes resulting from hawaiian coastal volcanism the entry of lava into the ocean at kilauea volcano hawaii has produced extensive sea surface hydrothermal plumes studies at this site over 1989 1991 showed that particle rich chemically altered thermal plumes of 1 3 m thickness can be mapped for distances up to 2 km from the entries using an array of physical and chemical tracers particle concentrations in the plumes were as high as 65 mg l 1 and produced light attenuation anomalies exceeding 11 m 1 the maximum observed seawater temperature was 69 c the following extremes in plume chemical markers were measured h2 2 1 m dissolved mn 320 nm dissolved si 55 m co2 1 6 mm alkalinity 1 5 meq l 1 and ph 7 1 dissolved h2 linearly increased and co2 ph and total alkalinity decreased with increasing seawater temperature dissolved mn and si also increased with increasing temperature but were better correlated with each other indicating that heat is not conserved in the surface plumes heat and mass inputs to the ocean are comparable to those from mid ocean ridge crest vent fields

plume capture by divergent plate motions implications for the distribution of hotspots geochemistry of mid ocean ridge basalts and estimates of the heat flux at the core mantle boundary abstract the coexistence of stationary mantle plumes with plate scale flow is problematic in geodynamics we present results from laboratory experiments aimed at understanding the effects of an imposed large scale circulation on thermal convection at high rayleigh number 106 ra 109 in a fluid with a temperature dependent viscosity in a large tank a layer of corn syrup is heated from below while being stirred by large scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank three regimes are observed depending on the ratio v of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary and on the ratio of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary when v 1 and 1 large scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes when v 10 and 100 plume formation is suppressed entirely and the heat flux is carried by a sheet like upwelling located in the center of the tank at intermediate v and depending on established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling heat transfer across the layer occurs through a combination of ascending plumes and large scale flow scaling analyses show that the bottom boundary layer thickness and in turn the basal heat flux q depend on the peclet number pe and when 10 q pe1 2 and when 1 q pe 1 3 consistent with classical scalings when applied to the earth our results suggest that plate driven mantle flow focuses ascending plumes towards upwellings in the central pacific and africa as well as into mid ocean ridges furthermore plumes may be captured by strong upwelling flow beneath fast spreading ridges this behavior may explain why hotspots are more abundant near slow spreading ridges than fast spreading ridges and may also explain some observed variations of mid ocean ridge basalt morb geochemistry with spreading rate moreover our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots

lithospheric stress and uppermantle dynamics in mainland china due to mantle flow based on combination of global and regional scale seismic tomography abstract in order to explore the importance of mantle flow to lithospheric stress field in mainland china seismic tomography based mantle flow models are used to predict the most compressive principal horizontal stress directions mcphsds considered that regional scale seismic tomography models have higher horizontal resolution to map the mantle structure while global scale models can present the information out of the imaging domains of regional scale models although this information has relatively poor horizontal resolution the combined global and regional scale seismic tomography based mantle flow models hereafter called combined models are mainly used in this paper after the comparison of the observed and our predicted mcphsds it is suggested that 1 a combined model compared with a only global scale seismic tomography based model could improve greatly the predictions in some regions of mainland china such as sichuan yunnan south china and north china blocks 2 the mantle flow model driven by both plate motions and mantle density heterogeneity hereafter called plate density driven model compared with the flow model driven only by mantle density heterogeneity hereafter called density driven model has much better predictions in the eastern china 3 the presence of density variations above 250 km could better dramatically the predictions in the eastern china and 4 sublithospheric mantle flow causes the lithosphere under compression in mainland china and plays an important role in forming the lithospheric stress in alashan qaidam western tibetan and eastern tarim blocks as well as the east of the eastern china

observations and sampling of an ongoing subsurface eruption of kavachi volcano solomon islands may 2000 a serendipitous encounter with an erupting shallow submarine volcano in the solomon islands provided a rare opportunity to map and sample the dispersal of volcanogenic emissions into the surrounding water column kavachi episodically active since at least 1939 is a forearc volcano located on the pacific plate only 30 km northeast of its convergent boundary with the downgoing indo australian plate during 14 may 2000 we observed explosive phreatomagmatic eruptions at several minute intervals creating a complex distribution of plumes of volcanic glass shards throughout the water column at a distance of 1 5 km from the summit at distances of 4 5 km shallow water 3 he fe and mn one sample only but not in co 2 we infer that the volcano flanks were essentially impermeable to fluid emissions and that the observed particle halo was created by magma shattering and resuspension most magmatic and hydrothermal fluids were thus discharged directly from the summit into the atmosphere

lithospheric heating by mantle plumes summary a broad topographic swell is associated with the hawaiian hotspot a kinematic model is developed assuming that downstream of the hotspot this swell is produced by lithospheric heating by a mantle plume material from the model plume flows radially in the asthenosphere and eventually heats the lower lithosphere upstream from the hotspot the model swell owes its existence mainly to hot material at asthenospheric depths the model predicts extensive lithospheric heating downstream from the hotspot interpreted data to tell if the lower lithosphere behaves in this way are lacking

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

mantle convection model with a dynamic plate topography heat flow and gravity anomalies summary surface observables have been calculated for a numerical convection model that includes a dynamic plate that is a stiff but mobile section of the upper boundary layer that is driven entirely by internal buoyancy forces the results for heat flux and topography are virtually identical to those obtained from a partially kinematic model in which plate mobility results from an imposed horizontal velocity on the top surface these results confirm the validity of the partially kinematic approach which has been used previously the partially kinematic approach is more efficient computationally

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

arrhenius rheology versus frank kamenetskii rheology implications for mantle dynamics 1 the viscosity of planetary mantle material is strongly temperature dependent this dependence is described by an arrhenius law but for the realistic viscosity contrast that appears over the depth of the mantle strong gradients in the upper thermal boundary layer occur these strong gradients are not realizable in numerical models therefore the frank kamenetskii approximation leading to a linearized exponential viscosity function is commonly used much research on the plate mantle system has been done applying the frank kamenetskii rheology the question though still arises if these results can be reproduced when using the arrhenius law here it has to be kept in mind that in numerical models the realistic viscosity contrast appearing over the mantle can neither be coped with by the arrhenius nor the frank kamenetskii approach thus the computational aspects to date only allow for viscosity contrasts being moderate as compared to realistic values and extrapolations to planet like values have to be made for both rheologies comparing results obtained by the commonly used forms of the arrhenius and the frank kamenetskii approach we observe the same change in flow behavior from mobile lid to stagnant lid convection the differences are only of quantitative nature in the stagnant lid regime some arrhenius formulations lead to a thinner top boundary layer which results in values of lithospheric thicknesses being more realistic here it has to be noted that different forms of the arrhenius law have been used which differ among themselves when properly scaled the differences between the frank kamenetskii and arrhenius rheology can however be strongly diminished to understand general features in the plate tectonics mantle convection system an additional stress dependence of the viscosity has to be considered the discrepancies between the various viscosity formulations are then of even less importance because for an earth like convection regime the top viscosity is more strongly influenced by the stress dependence rather than by the temperature dependence

tephra dispersal from myojinsho japan during its shallow submarine eruption of 1952 1953 a new and detailed bathymetric map of the myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952 1953 eruption especially how the silicic pyroclasts both primary and reworked enlarged the volcano and were dispersed into the surrounding marine environment myojinsho 420 km south of tokyo along the izu ogasawara arc was the site of approximately 1000 phreatomagmatic explosions during the 12 5 month eruption these explosions shattered growing dacite domes producing dense clasts that immediately sank into the sea minor amounts of pumice floated on the sea surface after some of these events the myojinsho cone has slopes of almost precisely 21 in the depth range 300 700 m we interpret this to be the result of angle of repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions many of these gravity flows resulted from explosions and associated dome collapse but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1 2 km of the volcano s summit entering water as deep as 400 m because the fall velocity of single particles decreased by a factor of 30 in passing from air into the sea we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra gravitational instabilities within these tephra choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained upon reaching the sea floor many of these currents probably continued to move downslope along myojinsho s submarine slopes fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption ocean currents carried this tephra to distal areas where it presumably forms a pyroclastic component of deep sea sediment

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

edge modulated stagnant lid convection and volcanic passive margins the initial oceanic crust along volcanic passive margins is a factor of 3 greater than that of typical oceanic crust 20 versus 6 7 km convection driven by the edge of the continental lithosphere may cause mantle material to circulate through the shallow zone of significant melting beneath the nascent ocean basin and cause the volume of melted mantle to exceed that required to replace the diverging lithospheric plates this hypothesis is a well known alternative to mantle plumes i obtain dimensional scaling relations and develop numerical models which indicate that the melted volume is unlikely to be enhanced by a factor of 3 this conclusion agrees with the results of numerical calculations by nielsen and hopper 2002 2004 dimensional scaling provides the relationship which shows that the vigor of convection in terms of laterally averaged heat flow or flow velocity depends on the square of local thickness of the rheological boundary layer for linear viscosity the derivation does not involve undulations in lithospheric thickness and thus provides the inference that the vigor of stagnant lid convection depends only weakly on those features numerical models support this inference lithospheric edges do nucleate instabilities but these instabilities are similar in magnitude to those in models started with a tiny perturbation from laterally homogeneous temperature subsequent convection is also similar for models started with tiny perturbations and with large lateral variations in lithospheric thickness in terms of the scaling relationship the asthenosphere beneath the thin lithosphere within rifts is an unfavorable site for circulation and extensive melting the rheological boundary layer is in general thin below thin lithosphere which causes stagnant lid convection there to be weak care needs to be taken in numerical modeling of convection beneath passive margins starting conditions that drive large instabilities of features that could not have persisted in the earth need to be avoided models should start rifting only after mature stagnant lid convection is established

the geoid constraint in global geodynamics viscosity structure mantle heterogeneity models and boundary conditions summary the earth s non hydrostatic gravity field or geoid provides a first order constraint on mantle density structure and dynamics geodynamic models for the geoid have proliferated since the advent of seismic mapping of mantle heterogeneity structure tomography because the geoid offers perhaps the best measured independent constraint on mantle density heterogeneity however dynamic geoid models involve a number of questionable physical assumptions and uncertainties whose effects need to be evaluated before geodynamic inferences based upon the geoid can be considered sound troubling issues include the appropriate surface boundary conditions free slip no slip plates and parametrization of radial viscosity variations how many layers can be resolved in addition to lateral viscosity variations possible chemical layering of the mantle phase transitions etc there are also uncertainties in the density heterogeneity models used as input to dynamic geoid models most of which are derived from seismic tomography and require weakly constrained empirical conversion factors to go from seismic velocity variations to density variations here we address several of the most straightforward problems inherent in geoid modelling namely the issues of viscosity structure resolution uncertainties in appropriate boundary conditions and differences among mantle heterogeneity models a robust feature of all models is a lower mantle viscosity at least a factor of 30 greater than that of the upper mantle but there is little resolution with regard to finer details such as lithospheric or uppermost mantle low viscosity zone viscosity ironically free slip boundary conditions result in the best fits to the geoid in all cases but all boundary conditions exhibit predictable trade offs with the uppermost mantle viscosity models with a single viscosity layer representing the lower mantle yield similar dynamic topography estimates of the order of 700 1000 m in amplitude regardless of the finer details of upper mantle viscosity structure boundary conditions or input heterogeneity models comparing mantle heterogeneity models based on two independent seismological determinations harvard and berkeley models and on the history of subduction we find that these models are virtually indistinguishable regarding inferences of mantle viscosity structure and amplitude of dynamic topography and in terms of the effects of different boundary conditions uncertainties concerning which type of boundary condition is appropriate are much more important than which mantle heterogeneity model is chosen given other uncertainties in modelling the geoid particularly the strong effects due to lateral viscosity variations for intermediate 10 000 km wavelengths we conclude that the class of dynamic geoid models explored so far cannot reliably elucidate the details of upper mantle viscosity structure

more constraints on internal heating rate of the earth s mantle from plume observations 1 our previous numerical studies of plume dynamics without secular cooling effects demonstrated that the internal heating rate of the earth s mantle is constrained to be 70 by inferred plume heat flux and plume excess temperature in the upper mantle in this paper by integrating the secular cooling effects into the numerical studies we demonstrate that the dependence of plume related observations on internal heating rate is not affected by secular cooling and that the plume related observations are only dependent on the internal heating rate and insensitive to its partitioning between radiogenic heating and secular cooling thus confirming the conclusions of 70 internal heating rate from our previous numerical studies furthermore we present a new analysis to constrain the internal heating rate with the plume related observations based on the energy balance of mantle convection the analysis shows that the internal heating rate of the mantle has a lower bound of 60 to satisfy the energy balance of the mantle with current plume observations which is consistent with the conclusions from our numerical studies

subsidence of normal oceanic lithosphere apparent thermal expansivity and seafloor flattening abstract seafloor topography has been a key observational constraint on the thermal evolution of oceanic lithosphere which is the top boundary layer of convection in earth s mantle at least for the first 70 myr the age progression of seafloor depth is known to follow the prediction of half space cooling and the subsidence rate is commonly believed to be 350 m ma 1 2 here we show that based on a new statistical analysis of global bathymetry the average subsidence rate of normal oceanic lithosphere is likely to be 320 m ma 1 2 i e 10 lower than the conventional value we define the normal seafloor as regions uncorrelated with anomalous crust such as hotspots and oceanic plateaus but the lower subsidence rate appears to be a stable estimate not depending on how exactly we define the normal seafloor this low subsidence rate can still be explained by half space cooling with realistic mantle properties if the effective thermal expansivity of a viscoelastic mantle is taken into account in light of a revised model of half space cooling the normal seafloor unperturbed by the emplacement of anomalous crust exists for all ages and the so called seafloor flattening seems to be mostly caused by hotspots and oceanic plateaus

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

effects of compositional and rheological stratifications on small scale convection under the oceans implications for the thickness of oceanic lithosphere and seafloor flattening 1 pressure release melting at mid ocean ridges generate compositional and rheological layering in the oceanic mantle that may control the evolution of the oceanic lithosphere we use dynamic models coupled with melting and petrological models to explore 1 the influence of this layering on the development of small scale convection under the oceans 2 its role in determining the thickness of oceanic lithosphere and 3 its feasibility as responsible for the deviations of seafloor and surface heat flow from predictions by conductive models in mature oceanic lithosphere here we show that the existence of small scale convection is entirely compatible with experimental creep parameters and flow laws and that the viscosity stratification due to melt extraction i e h2o removal is the main factor controlling the plate s thermal evolution its asymptotic thickness and the flattening of seafloor and surface heat flow at ages 70 ma the effects of al rich phase transitions and compositional layering are minor

influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization 1 small scale convection ssc rolls below the oceanic lithosphere have the tendency to align with the large scale shearing direction and thus with the plate motion direction relative to the deep mantle understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the pacific such as gravity lineations small scale seismic velocity variations and intraplate volcanism that cannot be explained by hot spots in this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed as suggested by the kink in the hawaii emperor seamount chain using three dimensional numerical flow models we investigate the realignment of ssc rolls after a change in plate motion direction from the nature of the ssc it is expected that rheological parameters dominate the characteristics of this realignment our results show that this is indeed the case we find that 1 using constraints from onset timing of ssc realignment of rolls can occur as fast as within 20 ma but might also take much longer dependent on the rheology 2 the realignment period is strongly correlated to the sum of large scale shear stress induced by plate motion and small scale shear stress from the ssc itself 3 in a mantle deforming by dislocation creep realignment occurs faster than by diffusion creep because dislocation creep ssc is more vigorous and 4 activation energy has little influence on the realignment time possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy

basin infill architecture and evolution from magnetostratigraphic cross basin correlations in the southeastern pyrenean foreland basin magnetostratigraphy numerical dating and facies mapping have been combined to provide the first cross basin correlation of the eocene southeastern pyrenean foreland basin in northeastern spain this has enabled 1 depositional systems on the northern and southern margins of the basin to be dated and correlated across the basin and 2 changes in the rates of sediment accumulation in time and space to be evaluated by sampling correlative intervals at several locations in a transect across the basin magnetozones were detected that otherwise may have been misinterpreted due to recent magnetic overprinting or missed by wide sampling intervals in previous magnetostratigraphic surveys results of this study indicate that marginal marine strata of the southeastern part of the foreland near the vic area of spain are 5 m y older than previously thought the base of the interval 46 ma being middle lutetian in age by proposing an age of at least 35 68 ma for the overlying cardona evaporite unit the age span of the marine interval in the studied part of the foreland is 10 m y contrary to earlier interpretations no major southward migration of the southeastern pyrenean foreland basin depocenter is recorded by the basin stratigraphy during the initial stages of middle eocene marine sedimentation in the southern margin of the basin however marked basin asymmetry is observed later in its evolution as the top of the marine deposits records northward increase in the thickness of chron c17n 1n 37 47 36 61 ma this asymmetry is also proved by the presence of a larger number of magnetozones within marine strata at the top of the marine interval in the northern sector these results imply that previous age calibrations and the estimated rates of tectonic processes and sedimentation within the foreland need to be reassessed the implications for basin infill architecture evolution through time and space subsidence analysis and depocenter evolution through time are addressed

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

intraplate volcanism with complex age distance patterns a case for small scale sublithospheric convection 1 many volcano chains in the pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions one possible explanation for non hot spot intraplate volcanism is small scale sublithospheric convection ssc and we explore this concept using 3 d numerical models that simulate melting with rheology laws that account for the effects of dehydration ssc spontaneously self organizes beneath relatively mature oceanic lithosphere whenever this lithosphere is sufficiently young and thin ssc replaces the shallow layer of harzburgite which was formed by partial melting at the mid ocean ridge with fresh peridotite this mechanism enables magma generation without any preexisting thermochemical anomalies however the additional effect of melting induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous ssc overturn of the compositional stratification and related magmatism the intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of ssc into very shallow and cooler levels on the one hand such a restriction precludes high degrees of melting but on the other hand it slows asthenospheric cooling and thus prolongs the duration of melting to 25 ma volcanism over such an elongated melting anomaly continues for at least 10 20 ma and occurs on seafloor ages of 20 to 60 ma these seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid ocean ridge melting the long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains ssc thus gives an explanation for previously enigmatic volcano ages along the line islands and the gilbert and pukapuka ridges as well as along the individual subchains of the wakes marshalls and cook australs

convective instability of a boundary layer with temperature and strain rate dependent viscosity in terms of available buoyancy summary cold mantle lithosphere is gravitationally unstable with respect to the hotter buoyant asthenosphere beneath it leading to the possibility that the lower part of the mantle lithosphere could sink into the mantle in convective downwelling such instabilities are driven by the negative thermal buoyancy of the cold lithosphere and retarded largely by viscous stress in the lithosphere because of the temperature dependence of viscosity the coldest and therefore densest parts of the lithosphere are unavailable for driving the instability because of their strength by comparing theory and the results of a finite element representation of a cooling lithosphere we show that for a newtonian fluid the rate of exponential growth of an instability should be approximately proportional to the integral over the depth of the lithosphere of the ratio of thermal buoyancy to viscosity both of which are functions of temperature and thus depth we term this quantity available buoyancy because it quantifies the buoyancy of material sufficiently weak to flow and therefore available for driving convective downwelling for non newtonian viscosity with power law exponent n and temperature dependent pre exponential factor b the instabilities grow superexponentially as described by houseman molnar 1997 and the appropriate timescale is given by the integral of the nth power of the ratio of the thermal buoyancy to b the scaling by the available buoyancy thus offers a method of determining the timescale for the growth of perturbations to an arbitrary temperature profile and a given dependence of viscosity on both temperature and strain rate this timescale can be compared to the one relevant for the smoothing of temperature perturbations by the diffusion of heat allowing us to define a parameter similar to a rayleigh number that describes a given temperature profile s tendency toward convective instability like the rayleigh number this parameter depends on the cube of the thickness of a potentially unstable layer therefore mechanical thickening of a layer should substantially increase its degree of convective instability and could cause stable lithosphere to become convectively unstable on short cales we estimate that convective erosion will in 10 myr reduce a layer thickened by a factor of two to a thickness only 20 to 50 per cent greater than its pre thickened value thickening followed by convective instability may lead to a net thinningof a layer if thickening also enhances the amplitude of perturbations to the layer s lateral temperature structure for the mantle lithosphere the resulting influx of hot asthenosphere could result in rapid surface uplift and volcanism

flattening of the oceanic topography and geoid thermal versus dynamic origin summary the origin of the flattening of the seafloor bathymetry and geoid at old ages is still a matter of debate is this departure from the square root of age law induced by dynamic uplift of the lithosphere linked to the flow in the mantle or by cooling of the lithosphere different from that predicted for an infinite half space the amplitude of the dynamic uplift should strongly decrease at short wavelengths and should not be correlated with the seafloor age on a local scale therefore in order to eliminate the dynamic signal the cooling of the lithosphere is studied here by means of depth age and geoid age relationships computed from high pass filtered bathymetric geoid and age fields or from local dtopography d and dgeoid dage slopes the geoid age and depth age relationships obtained here are in good agreement with those previously deduced from global non filtered data at all observation scales a significant departure from the half space solution is found however the data are not sufficiently accurate to discriminate between the plate and chablis constant heat flow at the base of the lithospheric isotherm models our results support the existence of small scale convection supplying heat at the base of the old oceanic lithosphere

ocean bathymetry and mantle convection 2 small scale flow numerical models demonstrate that a boundary layer instability of the lower lithosphere does not stop the lithosphere from subsiding and so it cannot explain the so called flattening of old seafloor the slower subsidence of the old seafloor is in any case better seen as a perturbation probably due to plumes furthermore if a boundary layer instability or any mode of upper mantle convection transported most of the heat emerging from old seafloor there would be pervasive large amplitude local bathymetrie gravity and geoid anomalies such anomalies have never been convincingly differentiated from the effects of the cooling lithosphere and hotspot tracks except for some low amplitude linear geoid anomalies near spreading centers the only modes of mantle convection with horizontal scales less than those of plates for which there is reasonably clear evidence are hotspot plumes and some possible near ridge boundary layer instabilities hotspots transport less than about 10 of the earth s heat flux and other small scale modes are probably less important

heat flow maintained in ocean basins of great age investigations in the north equatorial west pacific summary multipenetration heat flow measurements have been made at four sites in deep basins of the west central pacific ocean the west mariana basin central mariana basin nauru basin and central pacific basin the final heat flows are respectively 46 6 f 0 5 49 4 f 0 2 44 2 f 0 9 and 49 5 f 1 1 mw m each site was surveyed by single channel seismic reflection profiling and provided a gravity core the instrument measured thermal conductivity in situ over the entire depth intervals used for determination of the gradients and the reduction scheme iterated conductivity and heat capacity changes into the fitting procedure both of entry frictional decays and of conductivity heat pulse decays the absolute accuracy of the instrument should approach 2 per cent and the first site would make a good intercalibration standard for heat flow measurement the heat flow variation between the sites is real and there is also a significant variation in the isostatically reduced depths of the sites there is no age progression of either depth or heat flow and when five other good multidata points are included the relationship between depth and heat flow conforms to that expected from simple cooling models only in an average sense for the whole group the most plausible explanation for the variations is that heat flow and thermal elevation are dependent on different levels of deep lithosphere reheating at different times between 70 and 120myr ago it is suggested that additional topographic variation is caused by the different accumulations of sediment and lava flows at each site and to errors in the isostatically reduced depths due to incomplete knowledge of the stratigraphy down to the crust mantle interface these explanations of the topographic variation could be tested by seismic refraction measurements

receiver functions in the western united states with implications for upper mantle structure and dynamics 1 investigations into mechanisms driving surface tectonics commonly search for mantle sources but few observations constrain flow in the upper mantle and transition zone here variations in the upper mantle discontinuities at 410 km and 660 km below the western united states are revealed through mapping depths of compressional to shear wave conversions recorded by broadband seismometers the resulting image exhibits 20 and 30 km of topography on the 410 and 660 km discontinuities similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone the pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the rocky mountains colorado plateau or basin and range province providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region at least at scale lengths of a few hundred kilometers furthermore we find no clear correlation between the depths of the 410 and 660 km discontinuities undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km if the topography were due only to lateral temperature differences such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone alternatively the topography may reflect lateral variations in composition variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth

oceanic residual topography agrees with mantle flow predictions at long wavelengths dynamic topography the surface deflection induced by sublithosheric mantle flow is an important prediction made by geodynamic models but there is an apparent disparity between geodynamic model predictions and estimates of residual topography total topography minus lithospheric and crustal contributions we generate synthetic global topography fields with different power spectral slopes and spatial patterns to investigate how well the long wavelength spherical degrees 1 to 3 components can be recovered from a discrete set of samples where residual topography has been recently estimated an analysis of synthetic topography along with observed geoid and gravity anomalies demonstrates the reliability of signal recovery appropriate damping factors which depend on the maximum degree in the spherical harmonic expansion that is used to fit the samples must be applied to recover the long wavelength topography correctly large damping factors smooth the model excessively and suppress residual topography amplitude and power spectra unrealistically recovered long wavelength residual topographies based on recent oceanic point wise estimates with different spherical expansion degrees agree with each other and with the predicted dynamic topography from mantle flow models the peak amplitude of the long wavelength residual topography from oceanic observations is about 1 km suggesting an important influence of large scale deep mantle flow

numerical simulations of the cooling of an oceanic lithosphere above a convective mantle abstract numerical simulations of two dimensional rayleigh benard convection are designed to study lithospheric cooling above a convective mantle a strongly temperature and pressure dependent viscosity fluid is heated from below or from within an imposed velocity at the surface of the box mimicks the plate motion between the ridge on one side and the subduction zone on the other side as the lithosphere cools its upper part remains rigid and therefore conductive while its bottom part is convectively unstable dripping instabilities are not observed close to the ridge nevertheless the material flows along the slope defined by the lower part of the lithosphere and feeds the first descending drip afterwards cold downgoing instabilities develop continuously and randomly at the base of the lithosphere and are replaced by hot material from the convecting core of the box the lithosphere continues to thicken even after the onset of the first instability surface heat flow subsidence and lithospheric temperature structure obtained by the convective simulations are compared to the predictions of three conductive models the plate chablis and modified chablis models these models differ by their applied bottom boundary condition which represents the lithosphere asthenosphere convective coupling i e by the presence or absence of instabilities developing at the base of the lithosphere the conductive model which best explains the lithospheric cooling obtained by convective simulations is the modified chablis model in this model a variable heat flow depending upon the viscosity at the base of the lithosphere is applied along an isotherm located in the lower unstable part of the lithosphere

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

high resolution rock magnetic cyclostratigraphy in an eocene flysch spanish pyrenees 1 a rock magnetic cyclostratigraphy based on anhysteretic remanent magnetization arm intensity variations was developed for the eocene arguis formation in the spanish pyrenees the arguis formation was sampled for arm cyclostratigraphy rock magnetic and paleomagnetic analyses rock magnetic measurements indicate that the dominant magnetic mineral controlling the arm cyclostratigraphy is depositional magnetite using thermal demagnetization a detailed magnetostratigraphy was developed for the arguis to provide an absolute time framework for the arm cyclostratigraphy the magnetostratigraphy is carried by a combination of depositional magnetite and secondary iron sulfides spectral analysis of the magnetostratigraphically scaled arm time series reveals the presence of significant milankovitch frequencies including eccentricity obliquity and precession the arm time series was tuned to the orbital eccentricity model for the eocene coherency analysis indicates that the eccentricity tuned arm is in phase with october november insolation for this site s paleolatitude varying amounts of terrigenous input delivered by a fluvial source at orbital time scales during the rainy season and diluted by a relatively constant input of marine carbonate is the most likely explanation for the cyclicity of the arm record the absolute age resolution of the arm chronostratigraphy is 1 or less based on an age offset with the reference chronology that was used for the eocene the offset in age between the two chronologies may be due in part to the uncertainty in the chron ages for this part of the eocene the relative age resolution within the arm chronostratigraphy is much better

a sporadic low velocity layer atop the 410 km discontinuity beneath the pacific ocean waveforms of ss precursors recorded by global stations are analyzed to investigate lateral heterogeneities of upper mantle discontinuities on a global scale a sporadic low velocity layer immediately above the 410 km discontinuity lvl 410 is observed worldwide including east asia western north america eastern south america the pacific ocean and possibly the indian ocean our best data coverage is for the pacific ocean where the lvl 410 covers 33 50 of the resolved region lateral variations of our lvl 410 observations show no geographical correlation with 410 km discontinuity topography or tomographic models of seismic velocity suggesting that the lvl 410 is not caused by regional thermal anomalies we interpret the lvl 410 as partial melting due to dehydration of ascending mantle across the 410 km discontinuity which is predicted by the transition zone water filter hypothesis given the low vertical resolution of ss precursors it is possible that the regions without a clear lvl 410 detection also have a thin layer therefore the strong lateral heterogeneity of the lvl 410 in our observations suggests partial melting with varying intensities across the pacific and further provides indirect evidence of a hydrous mantle transition zone with laterally varying water content

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

discussion basin infill architecture and evolution from magnetostratigraphic cross basin correlations in the southeastern pyrenean foreland basin the chronostratigraphy of the middle eocene upper eocene successions of the southeastern pyrenean foreland basin has been revised by taberner et al 1999 1 this contribution was based on the correlation of the magnetic polarity stratigraphy mps for several partial stratigraphic sections

heat flow and the thermal origin of hot spot swells the hawaiian swell revisited we present 150 new heat flow measurements obtained at eight sites along a 1230 km long profile across the hawaiian swell about 700 km ese of midway island most of the measurements include in situ thermal conductivity determinations which helped to reduce the statistical uncertainties 95 confidence at all sites to 59 mw m 2 about 20 higher than the predicted heat flow for 100 ma seafloor based on simple lithospheric cooling models thus even though there is a small increase in heat flow with age along the swell when compared with the off swell heat flux the anomalous heat flow associated with the hawaiian swell is probably of the order of 5 10 mw m 2 and arguably may not exist at all a previous investigation von herzen et al 1982 may have overestimated the magnitude of the heat flow anomaly associated with the hawaiian swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models the heat flow anomalies associated with the bermuda rise and the cape verde rise may also be smaller than previously estimated given the uncertainties in the heat flux on the normal seafloor surrounding these swells the lack of a significant heat flow anomaly associated with the bathymetric expression of the hawaiian swell is inconsistent with simple models of lithospheric reheating dynamic support accompanied by modest temperature increases 80 ma oceanic lithosphere

models of convection driven tectonic plates a comparison of methods and results summary recent numerical studies of convection in the earth s mantle have included various features of plate tectonics a number of different methods for modelling plate like behaviour have been used the differences in the methods of modelling plates may assume or predict significantly different plate deformation we describe three methods of modelling plates through material properties force balance and a thin power law sheet approximation we compare the results obtained using each method on a series of simple calculations from these results we are able to develop scaling relations between the different parametrizations while each method produces different degrees of deformation within the surface plate the surface heat flux and average plate velocity agree to within a few per cent the main results are not dependent upon the plate modelling method and therefore are representative of the physical system we set out to model

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

the relationship between plate velocity and trench viscosity in newtonian and power law subduction calculations convection with a newtonian temperature dependent rheology leads to little or no surface velocity unless zones of weakness are introduced plate like features are observed in calculations both with newtonian rheology employing imposed weak zones and with power law non newtonian rheology where high stresses at the trench reduce the effective viscosity since deformation at subduction zones involves faulting both of these parameterizations should be treated with some skepticism it is important to understand how the parameterizations affect the model results we study the relationship between trench viscosity and plate velocity using a newtonian rheology by varying the viscosity at the trench the plate velocity is a function of the trench viscosity for fixed rayleigh number and plate slab viscosity slab velocities for non newtonian rheology calculations are significantly different from slab velocities from newtonian rheology calculations at the same effective rayleigh number both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate non newtonian rheology eliminates the need for imposed weak zones and provides a self consistent fluid dynamical mechanism for subduction in numerical convection models

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow