the influence of plate kinematics convection intensity and subduction geometry on the earth s upper mantle dynamics in the vicinity of a subduction zone summary this study reports the results of a 2 d numerical mantle convection model featuring plate kinematics and non vertical subduction the main question we address is the combined evects of plate velocities convection intensity and subduction dip angle on the earth s upper mantle dynamics in the vicinity of a subduction zone through a systematic investigation using rayleigh numbers in the range 1 2 104 6 105 a motionless overriding plate subduction dip angles of 70 4 5 and 30 and subduction velocities in the range 0 3 10 cm yr 1 we aim to study the interactions of the subducting and overriding plates with the underlying mantle we focus on the possible transitions of flow structures from a multicellular to a monocellular circulation and from an unsteady to a steady global structure unsteady states can be either periodical or nonperiodical the model predicts that unsteady structures are characterized by instabilities that avect both horizontal thermal boundary layers under the moving plate but only the upper thermal boundary layer under the motionless overriding plate the movement of the subducting plate may be suycient to drive the lower part of the circulation under the motionless overriding plate characterized by an elongation of cells this latter influence is favoured by a decreasing subduction dip angle thus diverent transition relations for the flow structure are obtained according to whether we consider the moving plate or the motionless overriding one and whether we consider a shallowor a steep dipping subduction by focusing first on the dynamics under the moving plate we are better able to quantify the relationship between the buoyancy and boundary forces associated with the subducting lithosphere for steep to intermediate subduction angles we find as in previous studies with no down going slab boundary that the greater the convection intensity the greater the plate velocity that is required to achieve the dynamical mantle plate coupling this is not however characteristic of a shallowdipping subduction since we show that the down going plate has a stabilizing influence suggesting the existence of a critical subduction dip angle for the mantle plate coupling thus the investigation of internal dynamics under the motionless overriding plate reveals a surprising single cell circulation under this plate for a suyciently high convection intensity this single cell circulation is obtained for smaller subducting plate velocities since the value of the subduction dip angle decreases the diverent lateral convection styles under this plate at moderate subduction velocities namely multicellular and monocellular for dip angles of 70 and 30 respectively are consistent with stress states in the overriding lithosphere indeed steep and shallow dipping subductions are likely to be associated with back arc extension and compression respectively for a dip angle of 70 under the overriding plate the model predicts an upwelling next to the trench the location of which is consistent with observed back arc basins

reply to comment on high and highly anisotropic electrical conductivity of the asthenosphere due to hydrogen diffusion in olivine by dai and karato earth planet sci lett 408 2014 79 86

anaerobic diagenesis of silica and carbon in continental margin sediments discrete zones of tco2 production pore water profiles of dissolved si ca2 so42 ch4 and tco2 dissolved inorganic carbon dic were determined from multicores and gravity cores collected at nine sites off southern california the west coast of mexico and within the gulf of california these sites were located within the eastern north pacific oxygen minimum zone at depths of 400 to 900 m and in settings where bottom water oxygen concentrations were 3 m and sediments were laminated pore water profiles were defined at a resolution of millimeters whole core squeezing centimeters sectioning and squeezing and meters gravity core sectioning and squeezing and diffusive fluxes were calculated for different zones within the sediment column the flux of dissolved silica across the sediment water interface swi ranged from 0 3 to 3 4 mmol si m 2d 1 and tco2 fluxes ranged from 0 8 to 4 6 mmol c m 2d 1 a positive correlation r 0 74 existed between these fluxes yet these two constituents exhibited significantly different diagenetic behavior downcore dissolved si generally reached a constant concentration between 450 and 900 m in the upper few cm whereas tco2 concentrations increased monotonically with depth methane was detected at micromolar levels in sediment intervals between 0 and 60 cm and at five sites increased to millimolar levels at depths of 80 to 170 cm at the horizon marking the appearance of millimolar levels of methane there was a distinct change in slope of the sulfate and tco2 gradients a flux budget for this horizon was determined by using linear fits to pore water profiles these budgets indicate that the upward tco2 flux away from this horizon is 40 to 50 greater than the downward sulfate flux to this horizon given that the tco2 flux to this horizon from below was quite small this imbalance suggests that anaerobic oxidation of methane by sulfate is not the only process producing tco2 within this horizon a budget for tco2 at this horizon is balanced when 40 to 80 of the sulfate flux is attributed to organic carbon remineralization of the dic that diffuses across the swi 20 to 40 is generated by reactions occurring within or below this deep reaction horizon

rates of formation of modern phosphorite off western mexico abstract phosphorite deposition represents a significant but poorly constrained sink in the marine p cycle this study establishes conclusively that phosphorite forms today throughout a widespread region along the upper continental slope off southern baja california mexico in this region o2 depleted 5 50 m bottomwater overlies the seafloor and a narrow range of suboxic to anoxic redox conditions prevail in the modern 0 18 000 yr old near surface 0 50 cm organic c rich 2 10 wt sediment interstitial gradients of total alkalinity ta and inferred effects of organic matter decay correlate directly with sedimentation rates calculated from 14c dating other porewater and solid phase data indicate that francolite currently precipitates authigenically at each of five study sites moderately high concentrations 20 100 m of dissolved inorganic phosphate dip produced in the upper 5 cm of sediment sustain francolite growth at slightly greater depths while the necessary f diffuses into the sediment from above neither organic matter decay nor fish bone dissolution seem capable of producing the steep near surface gradients of dip measured in this study at some sites dip production coincides spatially with the reversible production of dissolved fe2 but thorough evaluation of the possible link between these processes requires further study the mechanism responsible for the near surface enrichment of interstitial dip thus remains unidentified box cores from each site contain an unreworked sequence of up to six thin 1 3 cm discrete francolite rich horizons several of which qualify as true phosphorite 50 wt francolite estimated diffusional burial fluxes of inorganic p and f range respectively from 2 40 g cm 2 y 1 and 4 12 g cm 2 y 1 or the equivalent in francolite of 16 260 g cm 2 y 1 these present day accumulation rates lie within the range of those determined for several ancient phosphorite deposits and for the extensively studied region of modern phosphorite deposition off peru

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

formation and growth of deep mantle plumes summary if whole mantle convection occurs in the earth s mantle then the core mantle boundary constitutes the lower boundary layer for mantle convection this boundary layer appears to be unstable on a small scale and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere producing hot spots a finite amplitude numerical code is used to study the formation of such plumes and their growth through the mantle the plumes are restricted to being two dimensional sheets rather than cylinders the initial conditions consist of a steadily convecting mantle and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle two main results are obtained 1 a critical perturbation size has been found for a mantle with a rayleigh number of lo small perturbations produce plumes which fail to penetrate the mantle and instead are swept up by the pre existing convective pattern while large perturbations succeed in penetrating the mantle and reaching the lithosphere the critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation a perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200k or more 2 deep mantle plumes appear to require on the order of 50 100myr to penetrate the mantle episodic plumes on shorter time scales appear unlikely a similar time is required for plumes forming in an initially static uniform temperature mantle

on the aspect ratios of two layer mantle convection abstract some dynamic implications of separate convection systems in the upper and the lower mantle of the earth are investigated it is shown that the horizontal scale of convection cells in the lower mantle is likely to determine the scale of flow in the upper mantle this does not preclude the nearly independent realization of convection cells with horizontal dimensions comparable to the depth of the upper mantle

chaotic axisymmetrical spherical convection and large scale mantle circulation abstract this paper presents a study of high rayleigh number up to 200 times supercritical axisymmetrical convection in a spherical shell with an aspect ratio relevant for the earth s lower mantle both bottom heated and internal heated cases have been considered computations have been carried out for an infinite prandtl number isoviscous fluid with free slip isothermal boundary conditions the first part of the paper is devoted to the influence of the resolution on the accuracy of the numerical results it is shown that the resolution strongly influences the onset of time dependence recent methods of non linear physics have been used to prove that the time dependence and the chaotic behaviors of the solutions are real ones from these results we can confirm that convection is chaotic in this particular geometry even for rayleigh numbers 200 times critical aperiodic boundary layer instabilities are found to be incapable of breaking up the large scale flow owing to the shear of the global circulation spectral analysis of the power associated with the thermal anomalies shows that there is an upward cascade of energy due to small scale chaotic instabilities from l 2 to l 4 6 at the bottom boundary in agreement with new seismic observations at the core mantle boundary 1 3

convection experiments in high prandtl number silicones part 1 rheology equipment nomograms and dynamic scaling of stress and temperature dependent convection in a centrifuge it has been impossible to model mantle convection by other than numerical methods because dynamically scaled laboratory experiments of practical size in high prandtl number fluids would require a temperature gradient reaching far above the sublimation temperature of any known fluid however convection can be established in high prandtl number fluids without large temperature gradients and length scales if the thermal buoyancy forces are enhanced by increasing the gravitational acceleration this has been realized by placing the test tank in a high speed centrifuge up to 3000 rpm at the hans ramberg tectonic laboratory uppsala the centrifugal acceleration imposes an apparent gravity field several thousand times the earth s gravity by using this method supercritical rayleigh numbers up to 106 can be achieved in a transparent polymer fluid sgm36 of prandtl number 108 the carefully developed test tank and control unit and a rigorous derivation of the conditions for dynamic similarity of convection in fluids with temperature and stress dependent rheology facilitates detailed laboratory studies of high prandtl number convection the working philosophy section 2 equipment section 3 non newtonian and temperature dependent scaling procedure section 4 and practical nomograms section 5 are presented here to aid such laboratory studies the significance of this new laboratory method for studying convection is outlined in section 6 real convection experiments in high viscosity polydimethylsiloxanes now have the potential to contribute to the understanding of six major aspects of convection these are 1 1 progressive deformation and displacement in three dimensional convective flow fields 2 2 convective mixing 3 3 convection with high prandtl numbers 4 4 non newtonian flow 5 5 temperature dependent flow and 6 6 high rayleigh number convection detailed applications will be discussed in a companion paper part 2 of the present project

convective heat transfer as a function of wavelength implications for the cooling of the earth 1 attempting to reconstruct the thermal history of the earth from a geophysical point of view has for a long time been in disagreement with geochemical data the geophysical approach uses parameterized models of mantle cooling the rate of cooling of the earth at the beginning of its history obtained in these models is generally too rapid to allow a sufficient present day secular cooling rate geochemical estimates of radioactive element concentrations in the mantle then appear too low to explain the observed present mantle heat loss cooling models use scaling laws for the mean heat flux out of the mantle as a function of its rayleigh number of the form q ra b recent studies have introduced very low values of the exponent b which can help reduce the cooling rate of the mantle the present study instead focuses on the coefficient c in the relation q cr a b and in particular on its variation with the wavelength of convection the heat transfer strongly depends on the wavelength of convection the length scale of convection in earth s mantle is that of plate tectonics implying convective cells of wide aspect ratio taking into account the long wavelength of convection in earth s mantle can significantly reduce the efficiency of heat transfer the likely variations of this wavelength with the wilson cycle thus imply important variations of the heat flow out of the earth on a intermediate timescale of 100 ma which renders parameterized models of thermal evolution inaccurate for quantitative predictions

mantle convection flow reversals due to continental collisions rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness are included in a two dimensional numerical model of mantle convection model plates typically span six vertical increments of a finite difference mesh we model two identical continents being carried towards each other by a pair of underlying counter rotating mantle convection cells upon meeting at the model mid plane the model continents form a motionless rigid conducting supercontinent along a portion of the upper boundary while the model oceanic plates continue to move and recycle through the mantle the resulting changes in the mechanical boundary conditions at the upper surface prove to be important factors in facilitating flow reversal below the supercontinent leading to a subsequent dispersal of the individual continental blocks we find the following factors to favour the development of sustained flow reversal below our model supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and lower proportions of internal radiogenic heating within the mantle

time dependent convection in the earth s mantle abstract the range of rayleigh numbers accessible to 2 dimensional numerical models of mantle convection has been increased by almost two orders of magnitude through the use of high resolution numerical grids with up to 200 intervals in each of the horizontal and vertical directions the range of rayleigh numbers studied is believed to encompass conditions which would have existed in a hot archaean mantle although the models are highly idealized in that they neglect the compressibility and variations of viscosity in the earth s mantle they suggest that during the archaean and earlier boundary layer instabilities developed more frequently than at present and did so with a scale and separation comparable to the thickness of the thin thermal boundary layers rather than to the depth of the convecting layer itself the rising and sinking plumes which develop from these instabilities drive transient flows which nevertheless have a horizontal scale comparable to the depth of the convecting layer thus although surface motions during the archaean may have been more variable than at present in terms of speed and direction they might have occurred on a horizontal scale which is similar to that of present day plate motion the numerical models indicate that motions at the upper surface of time dependent convection cells are 1 predominantly controlled by transient sinking plumes and 2 converge directly towards the source region of the sinking plumes on the other hand rising plumes have a more diffuse influence on the upper surface and surface divergence does not always occur immediately above a rising plume if oceanic plates on earth constitute the upper surface of mantle wide convection cells the more pronounced geophysical signature of subduction zones compared to oceanic ridges may be simply due to the fact that buoyancy is most concentrated at the earth s surface in the form of sinking plumes the total surface heat flow is found to fluctuate significantly in the time dependent convection models since estimates of the present day heat loss from the earth s interior are at best instantaneous measurements it is possible that these estimates are not representative in a time averaged sense of the present thermal state of the planetary interior

the potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle abstract recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics we have considered the possible impact from radiative heat transfer which can be described by a nonlinear temperature dependent coefficient in the thermal conductivity in 2d numerical simulations results for rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior this nonlinearity is responsible for producing a strong attractor in the mantle convective system which greatly simplifies its time dependent dynamics the possible link of the simplified lower mantle time dependent dynamics with polar wander is discussed the main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long term rotational dynamics

some geodynamical effects of anisotropic viscosity summary rheological anisotropy in the mantle may arise from various causes the most important are the preferred orientation of single crystals and the orientation of streaks of eclogitic material in a marble cake mantle in both cases the orientation can be achieved by the flow kinematics and in steady state convection the planes of easy slip will eventually be orientated parallel to the stream lines the dynamic equations for a 2 d incompressible anisotropic viscous fluid are derived and some geodynamic consequences are studied a difference of the order 10 100 between shear and normal viscosity is considered post glacial rebound would be affected by the mechanical properties in deeper parts of the mantle the observed uplift history near the ice margin has been taken as argument against strong viscosity stratification in the mantle it is shown here that with strong viscous anisotropy at least in the upper mantle the uplift history is compatible with a significant increase in viscosity between upper and lower mantle the topographic and geoid signal can show a spatial offset from a generating mass anomaly in the mantle this may provide the opportunity for in situ observation of anisotropic flow properties in the mantle in numerical case studies of steady state convection streamline orientated anisotropy is considered at high rayleigh number for bottom heated convection the formation of velocity boundary layers is observed with no flow in the stagnant core of the convection cell with internal heating or strong pressure and temperature dependence of viscosity this is no longer found but in the latter case anisotropy enhances the channelling of flow into lowviscosity regions e g hot rising plumes a few experiments have been performed for time dependent convection the results suggest that the onset of time dependent boundary layer instabilities may be somewhat retarded but would not be strongly suppressed by anisotropy strong time dependence may destroy any large scale coherence in the orientation of anisotropy and thus render the mantle to appear viscously isotropic

whole mantle convection and the thermal evolution of the earth abstract three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the earth s thermal history the first and perhaps most important of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by v s safronov and others this work shows that the terrestrial planets accrete on a time scale of 10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce immediate differentiation of iron from silicates the second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the earth as a whole although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow there are good reasons to believe that this model is valid for the earth the third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations this paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness using the idea of parameterized convection the radial heat transfer by the mantle convective circulation is included in a thermal history model of the earth this model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3 8 ga have ever been found an observation that has heretofore proven enigmatic

the thickness of the continental lithosphere two independent sets of geophysical data are shown to be particularly sensitive to the thickness of the continental lithosphere both are connected with the response of the planet to the last deglaciation event of the current ice age the first set of data consists of relative sea level histories in the age range 0 10 kyr b p from sites along the eastern seaboard of north america such data appear to require a continental lithospheric thickness in excess of 200 km the second set of data consists of the ils record of the motion of the rotation pole over the time range 0 75 years b p which reveals a secular drift at the rate of 0 95 0 15 deg 106 years toward hudson bay when the viscosity profile of the mantle is fixed to that required to explain free air gravity and relative sea level data from within the ice margin and the observed nontidal acceleration of planetary rotation then these observations also suggest a continental lithospheric thickness of the same order

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

surface deflections due to transient subduction in a convecting mantle abstract transient surface deflections associated with non continuous subduction are studied through the use of a sequence of idealized numerical models of time dependent mantle convection the major concerns of this study are the magnitude and duration of dynamically supported topographic fluctuations resulting from either the initiation or termination of a subduction zone at the earth s surface the former is modelled by prescribing an initial temperature structure which mimics the presence of a cold slab of lithosphere immersed in a warm uniform temperature mantle while the latter is induced by thermally detaching this slab from the upper surface of the convection cell the model slabs are thus defined thermally rather than mechanically and their negative buoyancy induces convective flow in the neighbouring mantle the full hydrodynamic equations governing natural convection are solved numerically in order to follow the evolution of the temperature and velocity fields with time the resulting model flows are therefore dynamically self consistent and differ from previous kinematic models in that the flow velocities are determined at each instant from the evolving thermal field rather than prescribed as boundary conditions as the lithospheric slab sinks into the mantle the induced flow produces normal stresses which in turn result in a broad topographic depression subsequently as the slab surface distance increases the topography rebounds at a rate which is strongly dependent upon the imposed temperature contrast between the slab and the surrounding mantle material assuming that these depressions are filled with sediments if a subsequent episode of subduction were to begin before the initial depression was eliminated new sediments would be superimposed upon the old repeated episodes of subduction at a continental margin may therefore be an important factor contributing to repeated cycles of platform sedimentation

on the structure of the lowermost mantle beneath the southwest pacific southeast asia and australasia abstract the region of the lowermost mantle beneath the southwest pacific australasia and southeast asia 50 s 50 n and 80 190 e has been studied using a wide variety of seismic techniques we complement these studies with results obtained from long period global digital seismograph network gdsn data using a recently developed phase stripping technique that permits the isolation of d reflections from the stronger neighbouring s and scs signals we identify patches with d reflections and areas where we cannot confidently determine the presence or absence of a d reflection a synthesis of our results with other studies suggests that d varies dramatically through the region generally thickening from 100 150 km in a central zone to about 300 km at the eastern and western margins however there are irregularities in this overall pattern including areas where there seems to be little evidence for a d discontinuity inspection of waveform amplitudes shows considerable scatter in not only the d reflected phases but also core mantle boundary reflected phases experiments with synthetic seismograms for a variety of d models and the observed lateral variability through the region suggest that this is to be expected furthermore ray theory calculations for large scale 3d earth models predict significant variations 30 in s wave amplitudes of lowermost mantle turning rays finally we investigate possible correlations between lower mantle velocity flow and d thickness we find some correlation between d thickness and lower mantle velocities obtained from tomographic inversions with a thin d layer in high velocity regions and a thickening of the layer toward slower regions the relationship between predicted lower mantle flow and d thickness is less clear these results are qualitatively consistent with thermal boundary layer predictions for d but do not preclude the possibility of compositionally distinct material in the layer

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

mantle convection models of continental collision and breakup incorporating finite thickness plates abstract a two dimensional numerical model has been developed to study mantle convection during continental collision and breakup the model incorporates rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness we systematically investigate the influences of continental width diffusivity thickness and internal heating on continental collision and breakup in addition we consider the influence of different degrees of internal heating in the mantle for each case considered we model a pair of identical continents being carried towards a site of plate convergence by underlying counter rotating mantle convection cells the continents collide at the mid plane of the model to form a motionless rigid conducting supercontinent whereas oceanic plate material continues to recycle through the mantle we find that changes in the mechanical boundary conditions at the upper surface are important factors in initiating flow reversals below the supercontinent more generally our findings based on 144 models are that the following factors favour the initiation of flow reversals below the supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and continental crustal heating furthermore lower percentages of internal heating in the mantle are necessary to sustain and promote the subcontinental flow reversals in our models

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

on the existence of a second scale of convection in the upper mantle summary this paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea floor deviates from the prediction of boundary layer theory for ages in excess of about 100 ma the energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large scale convective circulation associated with plate creation and destruction here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability if the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022poise as suggested by post glacial rebound data then the existence of the second scale is extremely unlikely on the other hand if a sufficiently pronounced low viscosity zone does exist under old sea floor then the development of such a second scale cannot be ruled out completely some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case

effects of lithospheric rigidity on ocean floor bathymetry and heat flow the observed quasi rigid behaviour of surface plates in the course of their relative motion is a consequence of the high viscosity which obtains in the cold near surface region by assigning a particular constant velocity as the upper boundary condition in a numerical model of mantle convection we have investigated the effect of lithospheric rigidity on the variation of oceanic bathymetry and heat flow as a function of ocean floor age predicted variations of both bathymetry and heat flow at the surface of mantle wide convection cells which are partially heated from within exhibit the same qualitative behaviour as data compiled for the major ocean basins the bathymetry varies with distance x from the spreading centre initially as x sup 1 2 but subsequently flattens with respect to an x sup 1 2 reference curve whereas the heat flow closely follows an x sup 1 2 decay over most of the convection cell consequently the viability of the mechanism for sea floor flattening proposed by jarvis and peltier 1980 is increased when a constant surface velocity characteristic of rigid plates is incorporated in the model this model successfully predicts both that the bathymetry should flatten and that the heat flow should not

methane along the western mexican margin we investigated the processes controlling the water column and sediment methane distributions at 14 stations along the western mexican margin in and around the gulf of california stations were grouped into two categories coastal basins and open margins diffusive methane fluxes from the sediment at all sites as estimated from sediment methane gradients were 0 24 5 5 mmol m 22 d 21 with the highest fluxes observed on the pacific margin of baja california at both basin and open margin sites these high rates occur despite the lack of significant terrestrial input to these sediments reflecting the importance of upwelling induced productivity methane concentrations in the upper water column were supersaturated with respect to the present atmosphere at all sites with sea air fluxes of methane of 0 5 5 9 mmol m 22 d 21 four of the open margin sites had seafloor depths extending below the oxygen minimum zone 400 800 m and contained low methane concentrations below the subsurface methane maximum the remaining margin site was shallow 593 m with a seafloor that intersected the oxygen minimum zone and had elevated methane concentrations throughout the water column this indicates that such sediments may be a significant source of methane to the eastern tropical north pacific etnp the seawater within silled basin sites also had supersaturated methane concentrations reflecting the anoxic conditions within the basins however methane levels were low at the sill depth indicating that the silled basins were unlikely to be significant sources of methane to the etnp we observed an inverse relationship between methane concentration and d 13 c ch4 value in the basin waters consistent with biological aerobic oxidation of methane being released from the sediment an apparent kinetic isotopic fractionation factor of 1 0100 0 0038 was calculated for this process isotopically heavy methane resulting from similar oxidation of seafloor derived methane may be the source of large pools of heavy methane previously

surface deformation gravity and the geoid from a three dimensional convection model at low rayleigh numbers abstract the two dimensional surface deformation gravity field and geoid are calculated from the temperature fields of a number of numerical models of constant viscosity three dimensional convective flows heated from within and from below using the appropriate green s functions the admittance is positive with positive gravity anomalies above hot rising regions except for large aspect ratio circulations with undeformable lower boundaries the surface deformation and the geoid are insensitive to the short wavelength features of the temperature variation the gravity field is less smooth though still does not clearly indicate the narrowness of the upwelling and downwelling regions at large rayleigh numbers when the lower boundary of the convecting region is deformable the gravity field is dominated by lateral temperature variations within the upper thermal boundary layer even when their contribution to the overall circulation is small the variation of surface deformation with rayleigh number agrees well with that expected from simple boundary layer arguments when the circulation is driven by heating from below but less well when the heating is internal these results suggest that the convective upwelling beneath regions showing positive geoid and residual depth anomalies is more localized than the horizontal extent of these features would suggest

archean komatiites and geotherms solution to an apparent contradiction the high liquidus temperatures of archean komatiites and the low geothermal gradients in archean continents appear to give contradictory indications of temperatures in the archean mantle we resolve this contradiction in terms of an idealized model of convection in the archean mantle we suggest that temperatures in the archean mantle were no more than 100 c higher than at present that komatiites originate from the hot lower boundary layer of a non steady convection cell and that archean continents were formed near subduction zones where near surface temperature gradients were lower than elsewhere in the convecting mantle

low wavenumber signatures of time dependent mantle convection abstract spectral analysis is applied to idealized two dimensional numerical models of mantle convection examining the spectral signature of known model temperature fields represents the forward problem corresponding to the inverse problem of inferring the unknown temperature field in the mantle from the spectral components of lateral heterogeneity obtained by seismic tomography previous two dimensional fourier analyses of steady convection cells indicate that with current levels of resolution tomographic techniques have a much better chance of detecting horizontal boundary layers than vertical plumes from an examination of the shape of the spectral envelopes of the power spectrum of lateral heterogeneity at various depths in steady convection cells it has been suggested that evidence for thermal boundary layers in the mantle may be contained in the published spectral amplitudes of lateral heterogeneity this possibility is expanded on in the present study here we examine the effects on the spectral signature of lateral heterogeneity of averaging the model temperature field over vertical intervals of a few hundred kilometers of large amounts of internal heating of time dependent flow and of aspect ratios greater than unity the vertical averaging is an attempt to mimic the finite vertical resolution of tomographic inversions the various complications considered here are found to render the distinction between the spectra of lateral heterogeneity inside and outside of thermal boundary layers less clear than in the simpler models considered previously nevertheless the boundary layer spectra continue to contain significant information on the dominant scale of the model flow solutions

an extensional model of graben subsidence the first stage of basin evolution abstract the formation of grabens in continental crust is generally associated with tensile faulting crustal thinning anomalously warm mantle below the rift zone and an updoming of the outer flanks of the rift zone these features are also common to the initial stages of sedimentary basin formation and continental rupture creating new sea floor this similarity suggests that similar tectonic processes but of varying duration may be responsible for all three phenomena in this study subsidence within continental grabens which have been formed by a sudden local stretching event is examined within the zone of extension two competing effects act to determine the sense uplift or subsidence and magnitude of initial epeirogenic movements these are the thinning of the low density crust and the upwelling of hot mantle material on continental crust depending on which effect is dominant either rapid uplift or rapid subsidence may occur initially subsequently thermally controlled subsidence occurs as the anomalously warm mantle below the rift cools due to lateral heat flow subsidence occurs more rapidly in grabens than in basins and may be accompanied by a transient uplift of the graben flanks

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

dynamical and thermal structure of a subduction zone influence of slab geometry on the convective state of the earth s upper mantle preliminary results received 29 march 1996 accepted 3 july 1996 abstract the aim of this study is to investigate the influences of plate kinematics and of the downgoing slab s geometry on internal dynamics and thermal structure in the vicinity of a subduction zone a systematic study depending on these parameters has been carried out based on a two dimensional finite differences code in a convective domain representing the upper mantle plate displacements are prescribed by kinematic boundary conditions in particular the subducting plate moves and dips at a constant velocity under the slower overriding one typical subduction dips 30 45 and 70 have been studied for a realistic rayleigh number of about l05 and for plate velocities around a coupling velocity of 2 3 cm year 1 this limit value was inferred from the balance between surface forces and buoyancy forces for any subduction dip and plate velocities lower than this coupling velocity plate kinematics variations induce significant effects on thermal and mineralogical patterns at greater plate velocities these patterns stabilize for plate velocities of the order of the coupling velocity a subduction dip decrease of 40 involves a heating of the downgoing plate of 100degrees and a depth increase of 20 km of the olivine spinel phase transition in this plate in the particular case of a non moving overriding plate the balance between this plate and underlying internal dynamics requires a moving plate velocity lower than 2 3 cm year j for a lower dip moreover a shallow subduction enhances a lateral large scale convection under the upper plate even if this plate is motionless in contrast to the case of a steep subduction this is consistent with observed back arc extension at the surface which is often associated with steep subductions our model suggests a spreading zone of between 500 and 1000km in the upper plate for a subduction velocity lower than 2 3 cm year i and a rayleigh number of about l05 1 introduction the earth s mantle undergoes a rayleigh benard thermal convection coupled to oceanic lithospheric plate movements rigid and moving lithospheric corresponding author plates form the thermal and mechanical limit bound aries of the earth s mantle convection cells one of the concerns of numerical modelling is to account for a real dynamic convection model coupled to plate tectonics to force the movement of rigid plates one may either model a dynamic plate by prescribing a lower viscosity at plate margins or impose the horizontal surface velocity davies 1989 0031 9201 97 17 00 copyright 1997 elsevier science b v all rights reserved ph s0031 9201 96 03209 8

the initial gradient of sulfate in gulf of california sediments abstract results of pore waters sulfate analysis from 26 cores from the gulf of california were used to calculate initial gradients of sulfate the values range from 0 05 to 0 72 mm cm 1 the highest values are from the continental shelf the intermediate and lowest values are from deep basins regression analysis between sedimentation rates and sulfate gradients gives an equation of the form w b c x o with an empirical constant b 1 49 this constant differs from that obtained for marine sediments from other areas this difference may be due to the fact that gulf of california sediments apparently contain less reactive organic matter than do other marine sed ments with similar sedimentation rates

in search for a relationship between harmonic resolutions of the geoid convective stress patterns and tectonics in the lithosphere a possible explanation for the betic rif orocline abstract convective stress fields implied by the runcorn model have been integrated with geological structures in the overlying lithosphere this is dangerous since small scale convection patterns in the mantle might have remained stationary for at least 50 ma while particular parts of the lithosphere are likely to have passed over distinct convective stress regimes in such periods the mantle component of the stress history of lithospheric structures younger than 32 ma is potentially tractable if a mean plate velocity of 5 cm y 1 is assumed this is because the satellite gravity data used here allow the detection of convection cells with horizontal length scales of the order of 1600 km which the lithosphere may cross in 32 ma for example the sublithospheric stress field might have accounted for the uplift and subsequent subsidence of the alboran sea basin w mediterranean between 25 and 20 ma ago an immature diapiric rise of the mantle due to upwelling is likely to have occurred beneath this region between 25 and 20 ma ago the resulting lithospheric swell caused extensive tectonic unroofing by nappe spreading with or without some sliding the subsidence of the alboran sea after the rise of the dome seems an excellent example of the combined effects of lithospheric cooling connected with ceasing mantle diapirism and mechanical thinning of the lithosphere by nappe translations

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

submarine rifting at mid ocean ridges an analytical model for the formation of mid ocean ridges is presented in which the central rift zone is interpreted as a belt of localized extension of width 2w centred on the ridge axis upwelling of the asthenosphere occurs passively below the zone of extension at the rate required to conserve mass at distances greater than w from the ridge axis the lithospheric plates move at a uniform horizontal velocity u0 the model is thermally and mechanically consistent and predicts the surface heat flow and topography across the rift zone plus the vertical thermal structure and the thickness of the lithosphere below the zone of extension measurements of ridge heat flow and topography can be used to constrain estimates of the width of the zone of intrusion this width may be as large as 100 km comparable to the thickness of the lithosphere and that of mantle plumes as proposed in some models of mantle convection a comparison of the thermal structure at a ridge crest as produced passively by horizontal extension and actively by a mantle convection cell shows a qualitative similarity this similarity illustrates the difficulty in distinguishing cause and effect with regard to plate formation and motion

sedimentary carbon sulfur and iron relationships in modern and ancient diagenetic environments of the eel river basin u s a abstract depositional and diagenetic controls on the distributions of carbon sulfur and iron c s fe in modern sediments and upper pleistocene mudrocks of the eel river basin erb northern california continental margin were investigated using a combination of geochemical radioisotopic and sedimentological methods a mass balance based on down core profiles of porewater and solid phase constituents and diagenetic modeling suggests that only 12 30 of the pyrite s produced via so4 2 reduction during burial is retained in modern shelf and upper slope deposits of the erb bioturbational reoxidation of initially reduced s is inferred to be the major control on s preservation on the basis of an observed inverse relationship between pyrite s retention and biological mixing intensity estimated from profiles of excess 234th importantly these findings argue that massive depositional episodes on the shelf following floods of the eel river have a negligible long term impact on bioturbating macrofauna and the potential to affect geochemical properties of the sediments down core profiles of reactive fe3 and py fe ii for the modern deposits suggest that highly reactive fe phases are sulfidized well within 500 2000 years of burial thereby limiting later pyritization which could occur through sulfidation of less reactive phases this result explains the low 0 4 degree of pyritization dop values exhibited by both modern and ancient deposits of the erb and lends support to the notion that pyritization in aerobic continental margin sediments is largely associated with highly reactive detrital fe oxides comparable mean c s weight ratios for modern sediments 5 4 3 3 1 and mudrocks 6 9 4 5 of the erb suggest that the upper pleistocene strata reflect a geochemical environment analogous to that of the modern margin specifically the c s fe signatures shared by the modern and ancient deposits are a consequence of similar detrital fe mineralogies initial organic matter content corg 1 and composition c n 13 to 17 13corg 22 to 25o oo burial rate and importantly bioturbation intensity the findings of this study have important implications for the use of c s fe signatures as indicators of diagenetic processes in dynamic continental margin environments

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

mantle convection and core mantle boundary topography explanations and implications abstract recent seismic inferences of the topography on the core mantle boundary cmb constitute a new and potentially important constraint on the dynamics of the thermal convective circulation in the earth s mantle we employ viscous flow models for the mantle to predict the flow induced deflections of the cmb that are expected on the basis of seismic tomographic inferences of internal mantle density heterogeneity the good agreement between our theoretical predictions and the seismically inferred cmb deflections of morelli and dziewonski 1987 suggests that the latter may indeed be good approximations to the dynamically induced cmb topography the amplitude of the seismically inferred cmb topography may be construed to provide a direct constraint on the temperature derivative of seismic p wave velocity in the lower mantle and we determine a value for this parameter in the course of fitting the data that differs significantly from the value determined by laboratory measurements as appropriate for upper mantle phases the seismically observed topography on the cmb is also shown to provide evidence favouring the whole mantle convection hypothesis and in this connection we argue that deep penetration of subducted lithospheric slabs into the lower mantle is likely to be responsible for controlling many of the features of this topography

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

organic carbon burial rates across the arctic ocean from the 1994 arctic ocean section expedition abstract estimates of organic carbon burial rates were obtained from the canada us 1994 arctic ocean section expedition using a relationship between dissolved ammonium gradients in sediment pore water and organic carbon burial rates data from 9 locations more than 200 km west of the lomonosov ridge indicate that organic carbon burial rates average 0 01 g c m 2 year 1 at 4 stations within 100 km of the lomonosov ridge carbon burial rate increases 9 fold to an average of 0 09 g c m 2 year 1 enhanced carbon burial rates and organic carbon concentrations at the lomonosov ridge and in the eurasian basin appear to be related to higher terrestrial carbon supply to the eurasian basin and to the role that the lomonosov ridge plays in the circulation and mixing of the different water masses from the two major basins on either side of the lomonosov ridge

dynamo waves and palaeomagnetic secular variation summary a dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000 3000yr thermal convection in the lower mantle imposes a spatially variable heat flux at the core mantle boundary if the total heat loss from the core exceeds approximately 3 x 10 2w then both i a convective layer and ii large scale quasigeostrophic shear flows are likely to exist beneath the core mantle boundary the combined effects of convection and shear at the top of the outer core can generate three dimensionally propagating magnetic field disturbances called cuw dynamo waves an analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east west shear flow nonaxisymmetric disturbances exhibit the following characteristics i dynamo waves propagate in each hemisphere with a poleward or equatorward component depending on the sign of the a effect ii both eastward and westward propagation is allowed althcugh westward propagation is preferred the apparent velocity at the earth s surface is the sum of propagation and drift velocities in the core iii dynamo wave disturbances are transient with exponential growth and decay phases iv traces of inclination versus declination of individual fourier components obey runcorn s circularity rule a comparison is made with lake sediment secular variation records from central north america measured by lund banerjee the band passed time series and the major inclination declination loops can be modelled from 2600 to 10 000 yr bp by a single dynamo wave periodic in 2400 yr propagating under the site along an azimuth of 345 between 0 and 2600 yr bp the record can be modelled by addition of a wave with 1200yr period propagating along an azimuth of 30 this indicates that the palaeomagnetic secular variation record in north america to 10 000 yr may be dominated by poleward propagating wave like disturbances

continental lithospheric thickness and deglaciation induced true polar wander the secular drift of the earth s rotation pole which is evident in ils data is a memory of the planet of the last deglaciation event of the current ice age in order to fit the observed rate and direction of drift with an earth model whose viscosity profile is capable of reconciling the free air gravity and relative sea level data of postglacial rebound requires a lithospheric thickness which is substantially in excess of that which might be considered an appropriate average thickness for the entire planet this required thickness which is greater than 250 km is nevertheless in accord with independent estimates of the thickness of the continental lithosphere

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

deglacial land emergence and lateral upper mantle heterogeneity in the svalbard archipelago ii extended results for high resolution load models summary in paper i breuer wolf 1995 a preliminary interpretation of the postglacial land emergence observed at a restricted set of six locations in the svalbard archipelago was given the study was based on a simple model of the barents sea ice sheet and suggested increases in lithosphere thickness and asthenosphere viscosity with increasing distance from the continental margin in the present paper the newly developed high resolution load model barents 2 and land uplift observations from an extended set of 25 locations are used to study further the possibility of resolving lateral heterogeneity in the upper mantle below the northern barents sea a comparison of the calculated and observed uplift values shows that the lithosphere thickness is not well resolved by the observations although values above 110 km are most common for this parameter in contrast to this there are indications of a lateral variation of asthenosphere viscosity whereas values in the range 10 8 1020 pa s are inferred for locations close to the continental margin 1020 102 pa s are suggested further away from the margin a study of the sensitivity of the values found for lithosphere thickness and asthenosphere viscosity to modifications of load model barents 2 shows that such modifications can be largely accommodated by appropriate changes in lithosphere thickness whereas the suggested lateral variation of asthenosphere viscosity is essentially unaffected an estimate of the influence of the fennoscandian ice sheet leads to the conclusion that its neglect results in an underestimation of the thickness of the barents sea ice sheet by about 10 per cent

the long wavelength component of mantle convection abstract theoretical models of mantle convection predict that the major temperature fluctuations within the mantle are confined to narrow horizontal boundary layers and vertical plumes these fluctuations result in heterogeneities in seismic body wave velocities which could in principle be detected by seismic tomographic techniques however recent tomographic images of the spatial variations of temperature in the mantle are considerably out of focus in that only the longest wavelength components can be resolved to assess this partial recovery of the total tomographic image theoretical temperature fields have been generated with a numerical model of high rayleigh number mantle convection and then fourier analysed in two dimensions upon re synthesizing the model temperature fields the fourier series expansions were truncated at various levels of resolution the truncated expansions containing only the long wavelength components of the model temperature fields are compared to both the complete field and the tomographic images of the mantle at the current level of resolution it seems unlikely that seismic tomography could distinguish between layered and whole mantle convection estimates based on current tomographic data of long wavelength fluctuations of temperature and surface topography are predicted in the case of whole mantle convection to represent approximately 10 of the total temperature variation and approximately 50 of the total topographic relief thus topography at the core mantle boundary may be more accurately inferred from seismic tomography than may the characteristic lateral temperature fluctuation which drives the convective circulation

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

speculations on the thermal and tectonic history of the earth summary the connection between the earth s thermal history and convection in the mantle is exploited to elucidate the early evolution of the earth it appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the earth s history only in boundary layers at the surface and at a depth of 650 700 km is conduction likely to be important the resulting evolution appears to be consistent with geological observations on early precambrian rocks various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km the large scale flow is related to the motion of major plates and must be ordered over distances of more than 5000 km its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the earth s surface and to reconcile the geophysical observations with the results of numerical experiments though the existence of the small scale flow is at present speculative various tests of its presence are proposed

modelling planetary dynamics by using the temperature at the core mantle boundary as a control variable effects of rheological layering on mantle heat transport abstract in planetary convection there has been a great emphasis laid on the usage of the rayleigh number as a control parameter for describing the vigor of convection however realistic mantle rheology not only depends on temperature pressure strain rate and composition but also on the nature of the dominant creep mechanism which varies with pressure and also with temperature it is difficult to study the effects of varying influences from the convective strength without also changing the mantle flow law in the process we have adopted the approach of using as the sole control parameter the temperature at the core mantle boundary t cmb in modelling planetary dynamics with a composite non newtonian and newtonian rheology which is temperature dependent in the upper mantle and both temperature and pressure dependent in the lower mantle increasing the t cmb strengthens convective vigor and leads to a non linear increase of averaged temperature heat flow and root mean squared velocity the interior viscosity decreases strongly with t cmb and internal heating due to radioactivity a viscosity maximum is found in the horizontally averaged viscosity profile at a depth around 2000 km this viscosity hill moves downward with diminishing amplitude in the face of increasing dissipation number and internal heating the bottom third of the lower mantle appears to be superadiabatic as a consequence of the stiff lower mantle rheology the scaling relationship between the nusselt nu number and t cmb shows a relatively insensitive increase of nu with t cmb in terms of an effective rayleigh number of the whole system ra e the power law exponent of the nu ra e relationship is very low around 0 12 strong pressure dependence of lower mantle rheology and its large volume relative to the entire mantle would induce a much lower cooling rate of the planet than previous models based on parameterized convection with a temperature dependent viscosity

tests of crustal divergence models for aphrodite terra venus aphrodite terra the largest highland region of venus is a likely site of mantle upwelling active volcanism and extensional tectonics we examine three alternative kinematic models for the interaction of mantle convection and the surface one in which little horizontal surface displacement results from mantle flow vertical tectonics one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation plate divergence and one in which strain from large horizontal motions is broadly accommodated distributed deformation in support of a divergent plate model for aphrodite head and crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones small scale bilateral symmetry of topographic elements hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus and subsidence of topography proportional to the square root of distance consistent with that expected for a divergent thermal boundary layer we undertake quantitative tests of each of these assertions and we compare the results with those from similar tests of the mid atlantic ridge we find that apart from the long wavelength symmetry of broadly elevated regions there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet the fit of the topography of aphrodite to that of a thermal boundary layer is in general much poorer than for the earth and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography the broad saddle shaped region between thetis and atla regiones shows the best fit to the root distance relation yielding apparent spreading half rates of a few centimeters per year but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two plate model regardless of past pole motions a multiple plate geometry is therefore called for and one or more of the lineaments must act as a plate boundary along nearly its full length if the plate divergence hypothesis is correct such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming magellan radar images in the absence of such evidence as long as the existence and distribution of the lineaments are verified then broad disruption of a thin lithosphere is favored in such a model lineaments may be surface manifestations of mantle convective flow a model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by magellan

on the interaction between small and large scale convection and postglacial rebound flow in a power law mantle abstract some consequences arising from the superposition of flows of two different kinds or scales in a non newtonian mantle are discussed and applied to the cases mantle convection plus postglacial rebound flow as well as small plus large scale mantle convection if the two flow types have similar magnitude the apparent rheology of both flows becomes anisotropic and the apparent viscosity for one flow depends on the geometry of the other if one flow has a magnitude significantly larger than the other the apparent viscosity for the weak flow is linear but develops direction dependent variations about a factorn n being the power exponent of the rheology for the rebound flow lateral variations of the apparent viscosity about at least 3 are predicted and changes in the flow geometry and relaxation time are possible on the other hand rebound flow may weaken the apparent viscosity for convection secondary convection under moving plates may be influenced by the apparent anisotropic rheology other mechanisms leading to viscous anisotropy during shearing may increase this effect a linear stability analysis for the onset of convection with anisotropic linear rheology shows that the critical rayleigh number decreases and the aspect ratio of the movement cells increases for decreasing horizontal shear viscosity normal viscosity held constant applied to the mantle this model weakens the preference of convection rolls along the direction of plate motion under slowly moving plates rolls perpendicular to the plate motion seem to have a slight preference these results could be useful for resolving the question of newtonian versus non newtonian or isotropic versus anisotropic mantle rheology

mantle convection under simulated plates effects of heating modes and ridge and trench migration and implications for the core mantle boundary bathymetry the geoid and benioff zones summary numerical convection models are presented in which plates are simulated by imposing piecewise constant horizontal velocities on the upper boundary a 4 1 box of constant viscosity fluid and two dimensional 2 d flow is assumed four heating modes are compared the four combinations of internal or bottom heating and prescribed bottom temperature or heat flux the case with internal heating and an isothermal base is relevant to lower mantle or whole mantle convection and it yields a lower thermal boundary layer which is laterally variable and can be locally reversed corresponding to heat flowing back into the core locally when scaled to the whole mantle the surface deflections and gravity and geoid perturbations calculated from the models are comparable to those observed at the earth s surface for models with migrating ridges and trenches the flow structure lags well behind the changing surface plate configurations this may help to explain the poor correlation between the main geoid features and plate boundaries trench migration substantially affects the dip of the cool descending fluid because of induced horizontal shear in the vicinity of the trench such shear is small for whole mantle convection but is large for upper mantle convection and would probably result in the tonga benioff zone dipping to the se opposite to the observed dip for the case of upper mantle convection

small scale convection and divergent plate boundaries this paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small scale convection in the upper mantle this may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures this concept is quantitatively assessed by numerical modeling of the flow caused by divergent plate motions in a viscous temperature and pressure dependent nonlinear fluid significant time dependent small scale convection is generated at the lower model viscosities and or higher temperatures and or sharper rift geometries these models can provide the required thick igneous crust observed at volcanic margins however they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable an additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed this increases its viscosity by 2 to 3 orders of magnitude in the melting zone above 80 km the high viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness while at the same time allowing for vigorous small scale convection during the early history of the rift and the delivery of thick igneous crust against the margin this factor coupled to the models for flow described above allow the main features of volcanic margins to be simulated besides the thick igneous crust predicted at the margins these models suggest that time dependent small scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis the periodicity of this time dependent convection is primarily dictated by the model viscosities the models suggest that episodicities of tens of million years are reasonable these episodicities may be reflected in the record of vertical motions at rifted margins

high spatial resolution models of time dependent layered mantle convection summary the degree of geochemical mixing that occurred during the mantle s early evolution is dependent on the history of convective motions in the mantle we present time dependent tnodels of mantle convection that show for certain conditions in the newly formed earth s mantle a transient phase of layered convection results during which geochemical differences between the upper and lower mantle could have been preserved the basic model assumes that the initial temperature profile for subsolidus mantle convection is strongly superadiabatic the mantle is represented by a constant viscosity boussinesq fluid in a 2 d cartesian box heated from the bottom the models have considerably greater spatial resolution 90 x 90 uniformly spaced grid points than our previous work parameters that have been vaned include the rayleigh number ra lo5 lo8 the aspect ratio 1 4 9 the buoyancy of the initial temperature profde and the maximum temperatures allowed for subsolidus convection the models support our previous assertion that there is a critical value of ra above which superadiabatic initial conditions result in a burst of convection that inverts the temperature gradient in the bulk of the mantle sufficiently to temporarily prohibit a single layer of convection instead two layers result this critical value is about ra lo6 for a mantle with the maximum amount of buoyancy for less buoyant initial conditions this critical value is somewhat higher the layering lasts on the order of lo9 yr for ra lo6 10 considering that ra for the early mantle was certainly much larger than lo6 this suggests that a transient phase of multiple layering may have prevented vigorous convection from geochemically homogenizing the early mantle however if a less buoyant initial condition prevailed or one where supersolidus convection controls the maximum temperature of the upper mantle the transient layering could not have taken place

the influence of tectonic plates on mantle convection patterns temperature and heat flow summary the dynamic coupling between plate motion and mantle convection is investigated in a suite of cartesian models by systematically varying aspect ratios and plate geometries the aim of the study presented here is to determine to what extent plates affect mantle flow patterns temperature and surface heat flux to this end we compare numerical convection models with free slip boundary conditions to models that incorporate between one and six plates where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow we also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature heat flux and plate velocity of mantle convection models we have investigated the influence of plates for three whole mantle convection cases that differ in their heating modes internally heated and basally heated and rheologies isoviscous and depth dependent viscosity we present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time dependent systems using the insight gained from simpler models in models with aspect ratios from 0 5 to 12 we find that for the same heating mode variations in temperature can be as much as 40 per cent when comparing calculations with unit width plates to models incorporating plates with widths equal to five times the model depth mean surface heat flux may decrease by 60 per cent over the same range of plate widths we also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that we believe has not been described previously in mantle convection studies specifically in internally heated models plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa these flow reversals occur in internally heated convection and are caused by a build up of heat in the interiors of wide convection cells close to mantle downwellings we find that flow reversals occur rapidly and are present in both single plate and multiple plate models that include internal heating this behaviour offers a possible explanation for why the pacific plate suddenly changed its direction some 43 ma

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

the combined inversion of seismological and goce gravity data new insights into the current state of the pacific lithosphere and upper mantle abstract in this study we combine seismological and goce satellite gravity information by using a bayesian like technique with the aim of inferring the density structure of the pacific 90 n 90 s 121 e 60 w lithosphere and upper mantle we recover a 1 1 3 d density model down to 300 km depth which explains gravity observations with a variance reduction of 67 41 the model with an associated a posteriori standard deviation provides a significant contribution to understanding the evolution of the pacific lithosphere and answers to some debated geodynamic questions our methodology enables us to combine the recovery of density parameters with the optimum density v sv scalings the latter account for both seismological and gravity observations in order to identify the regions characterized by chemically induced density heterogeneities which add to the thermally induced anomalies chemically modified structures are found west of the east pacific rise epr and are of relevant amplitude both below the north western side of the pacific plate at the base of the lithosphere and up to 100 km depth beneath the hawaiian and super swell regions thus explaining the anomalous shallow regions without invoking the thermal buoyancy as the sole justification coherently with the chemically modified structures our results a support a lighter and more buoyant lithosphere than that predicted by the cooling models and b are in favor of the hypothesized crustal underplating beneath the hawaiian chain and beneath the volcanic units in the southern branch of the super swell region the comparison between calculated mantle gravity residuals and residual topography a suggests a lateral viscosity growth associated with the increasing thickness and density of the plate and b correlates well with sub lithospheric mantle flow from the epr towards west up to the kermadec and tonga trench in the south and the kuril kamchatka trench in the north

lateral heterogeneity in the convecting mantle thermally induced lateral heterogeneity in the earth s mantle has been studied in terms of idealized two dimensional constant viscosity numerical models of mantle convection steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity by examining the changes in spectral signature produced by varying the rayleigh number the degree of internal heating and the depth in the convection cell it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer the spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography the forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data

effect of a shallow low viscosity zone on small scale instabilities under the cooling oceanic plates we have studied the role of a low viscosity zone in the uppermost mantle on convective instabilities in the cooling oceanic plates the upper mantle is represented by three layers in a finite element model a conducting lid overlies a low viscosity zone which in turn overlies a layer of constant viscosity extending to the base of the upper mantle at 675 km in depth the viscosity contrast the rayleigh number the relative fluid layer thicknesses and the conducting lid thickness are varied if the rayleigh number of the low viscosity zone exceeds a critical value then convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the rayleigh number however the scale of the convection will eventually grow and extend throughout the upper mantle we compare our model to the small scale geoid and topography anomalies observed in the central pacific which are 50 80 cm in geoid and 250 m in topography 10 40 m y after plate formation and whose wavelength 150 500 km increases with age the magnitude early onset time and lifetime of the anomalies suggest a viscosity contrast of greater than 2 orders of magnitude the increase in wavelength with time also suggests a high rayleigh number 106 and the initial 150 250 km wavelength indicates a low viscosity zone thickness of 75 125 km small scale convection does not alter the slope of the depth age curve but does elevate the depth age curve by 250 m it is not until the onset of long wavelength convection that the depth age curve departs from age predicted by a cooling half space these results agree with the depth age behavior observed in the pacific where the depth age curve is elevated in the region of small scale anomalies and departs from age at 70 m y the models also predict heat flow values on old ocean floor which are higher than the plate model 50 55 mw m2 but which are in agreement with heat flow measurements in the atlantic and pacific

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system