a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

identification of density stratified waters in the late pleistocene north atlantic a faunal derivation abstract an expanded study of the radiolarian cycladophora davisiana in late pleistocene north atlantic marine sediments shows that over the last several hundred thousand years this species exhibits large variations in relative abundance the c davisiana curves in the north atlantic cores are quite similar with easily recognizable features common to all records minor deviations from the general pattern of this species abundance apparently reflect the response of c davisiana to specific oceanographic conditions characteristic of a particular area within the north atlantic c davisiana occurs today in high abundance 20 only in the sea of okhotsk extensive winter and early spring sea ice cover coupled with low surface water salinities during summer and fall is responsible for maintaining near freezing subsurface temperatures in this northwest pacific marginal sea as well as relatively stable temperatures and salinities at depths below a shallow subsurface temperature minimum during periods in the late pleistocene high c davisiana abundances 20 in the north atlantic were probably associated with oceanographic properties similar to those that exist in the sea of okhotsk today because of the relationship between relatively stable subsurface temperatures and salinities and high abundance levels of c davisiana analysis of this species abundance pattern at several locations throughout the high latitude north atlantic should assist in identifying source areas of deep water formation and determining the duration of deep convective processes at these sites

ridge spreading subduction and sea level fluctuations a numerical model of mantle convection shows that sea level fluctuations are not simply associated with temporal changes in ocean c plate spreading in the dynamic model sea level rises rapidly and then falls toward a steady value but one still higher than the initial following increased ridge spreading this time dependence results from profound changes in the deep thermal structure under ocean and continent the use of past variations in oceanic spreading to infer sea level fluctuations is called into question with more realistic models and better continental stratigraphy constraints may be placed on the viscosity structure of the mantle

investigating superswells and sea level changes caused by superplumes via normal mode relaxation theory 1 the superplume hypothesis particularly for one in the mid cretaceous has stimulated the search for evidence for or against such an event and the associated superswell an approach based on normal mode relaxation theory makes it possible to simulate the surface deformation caused by the rising plume using a spherical self gravitating and stratified viscoelastic earth model the results for the superswell are consistent with those of previous analyses using convection and experimental modeling the self consistent coupling of elastic and viscous properties based on maxwell rheology reveals that elasticity plays an important role contributing up to 30 to surface swelling when the plume head crosses the upper mantle we show that the deformation at great distances from the superswell is relatively small of the order of meters at most and opposite in sign with respect to the large uplift over the plume of the order of kilometers the global subsidence over a wide region of the planet surrounding the superswell counteracts its effects on sea level changes and triggers a eustatic signal of a few meters at most a negligible magnitude with respect to 250 m characteristic for the mid cretaceous previous arguments against the mid cretaceous superplume hypothesis which assume that the large amount of water displaced by the superswell causes a eustatic sea level rise of about 200 m thus competing with the displacement due to oceanic crust production are no longer tenable since such estimates did not account for the global behavior of the planet under internal loads

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

sea level fluctuations due to subduction the role of mantle rheology by means of a stratified viscoelastic earth model we study the effect of sinking slabs on the dynamic topography the non hydrostatic geoid and the long term sea level variations sea level fluctuations due to subduction are found to be sensitive to the nature of the 670 km seismic discontinuity and to the rheological layering of the mantle the response of our model to both a single subduction and a realistic distribution of slabs is studied by a numerical simulation based on a simplified approach consistent with previous results we find that an upper bound to relative sea level time variations associated with the initiation of a new subduction in the upper mantle is 0 1 mm yr relative sea level changes driven by the dynamic readjustment of internal mass heterogeneities may thus be comparable with those attributed to other changes in the tectonic regime on a large scale this confirms the relevance of subduction as an important contributor to long term sea level fluctuations copyright 1997 by the american geophysical union

effect of subductions and trends in seismically induced earth rotational variations in the past the influences of large earthquakes on earth rotation and on changes of the gravity field have been the subject of extensive investigations though it is now recognized that earthquakes cannot excite the chandler wobble and that they only drive a very modest secular drift of the earth s spin axis their rotational signatures present some intriguing peculiarities in fact they preferentially drive the rotation axis toward a well defined direction i e 0 140oe and systematically induce negative variations of the earth s oblateness in the first part of this work the static dislocation theory is employed to ascertain the origin of these two remarkable trends this task is accomplished by considering the effects of seismicity on earth rotation during the last two decades we find that the above tendencies are operating even on a yearly time scale and we explain their nature by means of simple geometrical and physical arguments we also find that the trend of the pole toward 140oe is essentially due to the seismicity which characterizes the western pacific subduction zones the seismic excitation function takes for shallow thrust earthquakes a direction roughly opposite to the excitation function associated with mass injection along active subduction zones this point highlights the potential effects of aseismic mass rearrangement at subduction zones on the secular drift of the pole our findings based on a simple model of steady state subduction show that in the last two decades this process substantially acts to counterbalance the polar drift due to global seismic activity we verify that this balance between earthquake and subduction induced rotational signatures also characterizes the time variations of the dynamic oblateness which is intrinsically independent of the inertia changes that drive polar motion from these results which may potentially have some impact on the interpretation of long term polar motion we conclude that the pole of rotation cannot be significantly perturbed by tectonic motions on a decade time scale

multiple processes causing sea level rise in the central mediterranean our analysis of a series of sea level records along the italian coast of the adriatic sea indicates a sea level rise not attributable to a global eustatic signal but rather to the combined effects of active tectonics and glacial isostatic adjustment henceforth gia the highest predicted sea level rise of the order of 0 9 1 0 mm yr occurs in proximity of the city of ravenna in the northern sector of the adriatic sea decreasing to 0 4 0 5 mm yr in the northernmost and southern parts of the adriatic sea while gia is the dominant mechanism of sea level rise in the southern sector of the adriatic sea active tectonics and gia contribute a comparable amount in the north our results are of importance for quantitative estimates of trends of sea level changes in a part of the mediterranean the northern adriatic sea where the historical cities of venice and ravenna are severely exposed to sea level rise

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

on the seasonal sea ice cover of the sea of okhotsk free of ice 4 months of the year the sea of okhotsk begins to freeze over in a few coastal locations in november attains its maximum ice coverage usually in mid march and by june retains ice in only a few coastal regions this sequence is well recorded by satellite imagery from the electrically scanning microwave radiometer esmr on board nimbus 5 four years of esmr data reveal significant interannual variations in the maximum ice cover and in the smoothness of the growth and retreat periods of the years 1973 1976 the heaviest ice cover occurred in 1973 and the growth period most interrupted by temporary ice edge retreats was in 1974 comparison of the general features of the cycle of sea ice distribution with basic oceanographic factors shows that ice forms first in cold shallow low salinity waters and then seems to drift in a direction approximating the okhotsk kuril current system two basic patterns of ice edge positioning which occur frequently in the esmr data are identified as a rectangular structure and a wedge structure each of these is strongly correlated with the bathymetry of the region and with the known current system suggesting that convective depth and ocean currents play an important role in determining ice patterns certain major interannual variations in the ice extents are accounted for by interannual contrasts in the atmospheric pressure fields

isostatic deformations and polar wander induced by redistribution of mass within the earth the effects at the surface of the earth of time dependent mantle mass anomalies are analyzed within the framework of a viscoelastic mantle with maxwell rheology the implications for the earth s rotation are developed using the linearized liouville equations valid for small polar displacements our approach is appropriate for a simplified modeling of subduction the displacement of the earth s axis of rotation called true polar wander is very sensitive to the viscosity profile of the mantle and to the nature of the 670 km seismic discontinuity phase change models generally yield a huge amount of polar wander except for large viscosity increases for chemically stratified models true polar wander is drastically reduced as a consequence of dynamic compensation of the mass anomalies at the upper lower mantle interface when the source is embedded in the upper mantle in the proximity of the chemical density jump the polar wander stops after a finite time controlled by the slowest isostatic mode

an ams study of magma transport and emplacement mechanisms in mafic dykes from the etendeka province namibia abstract the henties bay outjo dyke swarm hod in nw namibia is part of the early cretaceous parana etendeka large igneous province the dykes are dominantly doleritic compositionally equivalent to the erupted lava series and thus the hod provides a look at the feeder systems of a flood basalt province the subvertical dykes mostly strike ne sw and minor nw se parallel or perpendicular to the damara belt in which they intruded we present a magnetic fabric study using the anisotropies of low field magnetic susceptibility ams and of anhysteretic remanent magnetization aarm with the aim to derive magma flow directions and better constrain emplacement mechanisms within the dyke swarm magnetic susceptibility and its anisotropy in the dykes is mainly controlled by distribution anisotropy of titanomagnetite that mimics the flow oriented silicate fabric the anisotropy is low in most samples supporting a magmatic origin in 66 of 110 investigated samples the ams fabric is normal with the max axis inferred flow orientation within the dyke plane most samples yielded vertical to subvertical flow orientations regardless of location near or distant from the former rifted margin the anomalous magnetic fabrics where max is inclined to the dyke plane are attributed to two mechanisms one is the single domain effect of titanomagnetite which was found by unequal orientations of ams and aarm fabrics the other case anomalous fabric is rotation of the ams axes by shear within the magma producing symmetric imbrication of ams fabric on opposite dyke walls or more commonly asymmetric magnetic fabrics which we relate to tectonic shear at the dyke walls during emplacement field support for syn emplacement shear is given by dyke segmentation geometries including locally curved segment tips en echelon arrangements and left right stepping displacements regionally syn emplacement shear is consistent with the observed reactivation during gondwana breakup of transcrustal shear zones in the damara mobile belt

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

oceanographic conditions associated with high abundances of the radiolarian cycladophora davisiana abstract the cosmopolitan radiolarian cycladophora davisiana usually comprises less than 5 of the radiolarian fauna in holocene sediments in recent sediments from the sea of okhotsk however this species frequently represents more than 20 of the radiolarian assemblage at times during the late pleistocene abundances of this species in excess of 40 are recorded in marine sediments from high latitude oceans 40 of both hemispheres the sea of okhotsk apparently represents a modern analogue of climatic and oceanographic conditions that existed throughout large portions of high latitude oceans at times during the late pleistocene the near surface water structure of the sea of okhotsk is characterized by a low salinity surface layer with a strong temperature minimum near its base the low surface salinities are responsible for maintaining near freezing subsurface temperatures as well as establishing relatively stable temperatures and salinities at depths below the temperature minimum this water structure is produced at least in part by intense freezing of sea ice in winter with subsequent summer melting the physical characteristics of the upper water column affect the abundance and activity of shallow dwelling flora and fauna while providing a stable subsurface environment for deeper dwelling fauna

plate mantle coupling and continental flooding the flooding records of continents have been computed in dynamically self consistent models of plates and convection platforms flood following rapid translation of continental plates as may occur following supercontinent breakup in these models continental flooding is primarily controlled by deep mantle sources and continental hypsometry is strongly influenced by dynamic topography some models especially bottom heated ones predict more extensive flooding than has been observed during the phanerozoic however the computed flooding can be made consistent with the observations if the viscosity of the upper regions of the convecting system are significantly reduced compared to the deep regions the fluid is driven predominantly by internal heating or the convection is confined to a depth appreciably less than the width of the non subducting plate

topography of the calabria subduction zone southern italy clues for the origin of mt etna 1 calabria represents an ideal site to analyze the topography of a subduction zone as it is located on top of a narrow active wadati benioff zone and shows evidence of rapid uplift we analyzed a pattern of surface deformation using elevation data with different filters and showed the existence of a long wavelength 100 km relatively positive topographic signal at the slab edges the elevation of mis 5 5 stage marine terraces supports this pattern although the record is incomplete and partly masked by the variable denudation rate we performed structural analyses along the major active or recently reactivated normal faults showing that the extensional direction varies along the calabrian arc and laterally switches from arc normal within the active portion of the slab to arc oblique or even arc parallel along the northern and southern slab edges this surface deformation pattern was compared with a recent high resolution p wave tomographic model showing that the high seismic velocity anomaly is continuous only within the active wadati benioff zone whereas the northern and southwestern sides are marked by low velocity anomalies suggesting that large scale topographic bulges volcanism and uplift could have been produced by mantle upwelling we present numerical simulations to visualize the three dimensional mantle circulation around a narrow retreating slab ideally similar to the one presently subducting beneath calabria we emphasize that mantle upwelling and surface deformation are expected at the edges of the slab where return flows may eventually drive decompression melting and the mount etna volcanism

on postglacial sea level summary an exact method is presented for calculating the changes in sea level that occur when ice and water masses are rearranged on the surface of elastic and viscoelastic non rotating earth models the method is used to calculate the instantaneous elastic and delayed vi scoelastic sea level changes following the partial melting of late quaternary ice sheets we find that there can be large errors in the usual assumption that changes in sea level are uniform over the ocean basins if a quantity of ice equivalent to a uniform 100 m rise in sea level melts from the laurentide and fennoscandian ice sheets then in the south pacific the instantaneous rise in sea level can be as large as 120m in the north atlantic the instantaneous rise is always less than 100 m there is a zone in the north atlantic with almost no sea level change and near greenland and norway the sea level falls rather than rises by over 100 m one thousand years after the melting a forebulge migrating towards the ice loads causes water to flow from the south pacific into the north pacific suggesting that raised beaches should occur in the south pacific the gravitational attraction of an ice mass upon a nearby ocean tends to hold sea level high in the vicinity of the ice this extra load near the ice may have a significant influence on postglacial isostatic adjustment

on transient rheology and glacial isostasy theoretical predictions from analytical self gravitating layered earth models with maxwell burgers body and standard linear solid rheologies are employed to explore the extent to which transient creep may have influenced the inference of long term mantle viscosity from analysis of the various signatures derived from the process of glacial isostatic adjustment the compatibility of the various models is checked by their ability to fit simultaneously the relative sea level free air gravity and the rotational data we conclude that transient creep cannot operate throughout the entire mantle on account of the extreme sensitivity of the uplift and gravity data to any significant departures of the steady upper mantle viscosity from 1021 pa s transient rheology would most likely be unimportant in the upper mantle restricting transient rheology to the lower mantle we find that models with an adiabatic density jump at 670 km depth are not satisfactory because of their inability to produce sufficiently large gravity anomalies and polar wander speeds our calculations show that the preferred models with transient creep in the lower mantle are chemically stratified with a nonadiabatic density change of around 10 the long term viscosities in the lower mantle associated with compatible transient rheological models lie between 5 1022 and 1023 pa s these values are consistent with viscosity estimates derived from correlation of long wavelength geoid anomalies and the density perturbations in the deep mantle

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