the differing effects of compositional and thermal buoyancies on the evolution of mantle diapirs diapirs rising through the earth s mantle and lithosphere are hotter and generally also of a different chemical composition than their surroundings experiments demonstrate that diapirs driven by a temperature difference alone entrain material from their surroundings and that their velocity of rise decreases as they enlarge these diapirs are referred to as thermals however when the driving buoyancy is due only to a difference in chemical composition there is no entrainment and diapirs rise at a constant velocity assuming the surroundings have a constant viscosity the different flow is a result of radically different diffusion coefficients for heat and composition between these two extremes is a range of behaviours characterised by the density ratio r i t where i is the contribution to the total density difference resulting from the composition or intrinsic difference and t is the contribution from the temperature difference for r of order one buoyancy given to surrounding material by heat loss from the diapir must be taken into account and entrainment persists during a diapir s rise through a scale height for cooling beyond this height entrainment ceases if the two buoyancy forces reinforce r 0 or gravitational instability causes breakup of the diapir if the buoyancy forces are opposed and thermal effects are initially dominant r 0 furthermore only diapirs driven by an intrinsic density difference but having temperatures high enough to produce a low viscosity lubricating boundary layer are able to exceed the stokes terminal velocity

venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

sediment accumulation rates in the southern barcelona continental margin nw mediterranean sea derived from 210pb and 137cs chronology sediment cores were collected from the southern barcelona continental margin including the foix canyon in order to study sediment accumulation in this area for this purpose 210pb 137cs and to a lesser extent 239 240pu and heavy metals were determined in sediment sections using state of the art techniques 210pb concentration profiles and 137cs 239 240pu and heavy metals profile features were used to establish the sediment chronologies further information was also obtained from granulometric analyses the magnitudes studied and correlated in this work included surface activities mixing rates accumulation rates and inventories sediments on the continental shelf showed a varied degree of mixing except possibly cn 23 sedimentation rates were relatively small and the sedimentary material probably had a common origin cores from the slope showed less mixing and in cores cn 36 and eb 2 located in the foix canyon axis the 137cs signal vs depth was clear and well preserved accumulation rates and total 210pb inventories were maximum in core cn 36 at mid slope of the canyon which was considered to be an active depocenter core cn 38 collected on the open slope showed a much smaller accumulation rate therefore it was concluded that shelf material is being transported alongshore by the prevailing currents and becomes focused in the canyon which is acting as a natural trap within the canyon this material is preferentially deposited at a mid slope depocenter little extra material reaches the lower canyon area in this area core eb 2 appeared to show signs of the presence of resuspended material possibly derived from the canyon walls finally observed changes in sedimentation rates during the 1930s could not be explained on the basis of current knowledge and this is suggested to be an important subject for further research

tidal oscillations in the aegean and ionian seas abstract harmonic constants are estimated by analysing tidal observations from 20 ports in the aegean and the ionian seas areas where no similar data were available so far the analysis is based on tidal records 7 years long the resulting values for the m 2 constituent are compared with the results of a hydrodynamic numerical model of the m 2 tide for the mediterranean sea the qualitative agreement between the observations and the model indicates that in spite of the complex topography of the aegean sea the major features of the tidal propagation could be retrieved through the model if better tuned

delineation of fluvial sediment architecture of subalpine riverine systems using noninvasive hydrogeophysical methods river management and restoration measures are of increasing importance for integrated water resources management iwrm as well as for ecosystem services however often river management mainly considers engineering and construction aspects only and the hydrogeological settings as the properties and functions of ancient fluvial systems are neglected which often do not lead to the desired outcome knowledge of the distribution of sediment units could contribute to a more efficient restoration in this study we present two noninvasive approaches for delineation of fluvial sediment architecture that can form a basis for the restoration particularly in areas where site disturbance is not permitted we investigate the floodplain of a heavily modified low mountain river in switzerland using different hydrogeophysical methods in the first approach we use data from electromagnetic induction emi with four different integral depths 0 75 6 m and gamma spectrometry as well as the elevation data as input for a k means cluster algorithm the generated cluster map of the surface combines the main characteristics from multilayered input data and delineates areas of varying soil properties the resulting map provides an indication of areas with different sedimentary units in the second approach we develop a new iterative method for the generation of a geological structure model gsm by means of various emi forward models we vary the geological input parameters based on the measured data until the predicted emi maps match the measured emi values subsequently we use the best matched input data for the gsm generation the derived gsm provides a 3d delineation of possible ancient stream courses a comparison with an independent ground penetrating radar gpr profile confirmed the delineations on the cluster map as well as the vertical changes of the gsm qualitatively thus each of the approaches had the capacity for detecting sedimentary units with distinct hydraulic properties as an indication of former stream courses the developed methodology presents a promising tool for the characterization of test sites with no additional subsurface information

venus gravity a harmonic analysis an improved model of venusian global gravity has been obtained by fitting an eighteenth degree and eighteenth order spherical harmonic series to 78 orbital arcs of high altitude 950 1350 km at periapsis tracking data and 351 orbital arcs of lower altitude 150 200 km at periapsis data from the pioneer venus orbiter pvo compared to a recently published tenth degree model mottinger et al 1985 which is based on the 78 high altitude arcs only the current model provides a significant improvement in resolution and fidelity as a measure of this improvement we note that for the low altitude arcs alone the variance of the residuals for the present model is reduced to 19 of the data variance compared to 51 for the tenth degree model venus differs significantly from the earth in that it exhibits a significant correlation between long wavelength topography and gravity the gravity topography spectral admittances are inconsistent with either airy or pratt isostasy but they are consistent with dynamic support by mantle convection

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

instabilities of fluid conduits in a flowing earth are plates lubricated by the asthenosphere summary a laboratory and theoretical study of the stability of conduits of buoyant fluid in a viscous shear flow has been conducted the object of the study is to explain the formation of discrete islands in island chains such as the hawaiian emperor seamount chain and to investigate a new method by which the variation of shear with depth in the mantle may be determined the conduits were made by injecting oil into a more viscous oil of greater density initially a growing chamber of lower viscosity oil formed near the injector but when the chamber got sufficiently large it rose as a buoyant spheroid behind this trailed a vertical cylindrical conduit through which fluid could continue to rise to the surface as long as the source continued if the more viscous fluid was sheared laterally the conduit was gradually rotated to a more horizontal position the diameter of the conduit increased with time due to a decreasing component of gravitational force along the axis of the conduit when the conduit was tilted to more than 60 with the vertical it began to go unstable by developing bumps which ultimately initiated a new chamber which rose to a new spot in addition if the reynolds number of the conduit was greater than approximately ten an axisymmetric wavy instability appeared in the walls of the conduit and the conduit had to be tilted less before a new chamber was initiated if shear under the pacific plate has to tilt buoyant mantle plumes to as much as 60 to form the relatively regular island chains associated with hot spots most of the shear would be found in a zone with a vertical extent of less than 200 km

the dependence of convection planform on mode of heating a fundamental property of a convecting fluid is its planform the distribution in the horizontal plane of hot rising regions and cold sinking regions for the earth s mantle the planform might he visualized as a map of subduction zones hotspots and possibly ocean ridges here i report numerical experiments of convection at high rayleigh number which show a strong dependence of planform on heating mode when heat generation is distributed uniformly through the box the preferred planform consists of an ensemble of time dependent cold axial sinkers distributed in a hot diffuse upward flow when half of the heat is generated within the box and the other half is input through the base the preferred planform consists of an array of hot axial plumes and elongated cold sheets in the former case the mean horizontal wavelength is about equal to the layer depth for the latter it is about twice the layer depth

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion abstract to facilitate geologic interpretation of satellite elevation potential field data analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation a powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion linear transformations of the equivalent source field lead to corresponding geoidal anomalies pseudo anomalies vector anomaly components spatial derivatives continuations and differential magnetic pole reductions a number of examples using 1 averaged surface free air gravity anomalies and pogo satellite magnetometer data for the united states mexico and central america illustrate the capabilities of the method

horizontal motions due to post glacial rebound north american horizontal post glacial rebound velocities are computed for three five layered incompressible spherical maxwell earth models using a simple glacial loading cycle centered on hudson bay the reference earth model has a 120 km thick elastic lithosphere and upper and lower mantle viscosities of 1021 pa s and 2 1022 pa s respectively the second model has a lower viscosity upper mantle 5 1020 pa s and a higher viscosity lower mantle 1023 pa s and the third has a 250 km thick lithosphere the reference model and the low viscosity upper mantle model calculations are in good agreement with uplift and gravity data although they may somewhat overestimate the present day central uplift rate the thick lithosphere model fares less well because it fails to exhibit the well pronounced peripheral bulge sinking that is required by eastern north america tide gauge data the reference model generally has the largest horizontal rebound velocities reaching a maximum of 4 mm yr towards hudson bay at 2400 km from the load center the effect of a thick lithosphere or of a low viscosity upper mantle is to reduce present day horizontal velocities especially in the formerly glaciated region the sensitivity of horizontal motions to lithospheric thickness and upper mantle viscosity may assist in discriminating between different models of the earth s rheology once sufficient high quality geodetic data exist

stratigraphic condensed deposition and diagenetic evolution of green clay minerals in deep water sediments on the ivory coast ghana ridge abstract green grains concentrations were studied through the 22 m pleistocene sedimentary column recovered from ocean drilling project site 959 in 2100 m water depth located on a small plateau on the northern flank of the cote d ivoire ghana marginal ridge the green clay material usually fills the chambers of pelagic foraminifers taking into account the variance in color and mineralogic composition of the bulk green grains fraction observed in each level a methodological approach was especially performed mineralogical and chemical analyses were done successively on the bulk green material then on hand picked white pale green medium green and dark green grains and lastly surface and spot analysis of nanno structures were obtained using an energy dispersive microprobe coupled with a scanning electron microscope green grains concentrations are evidenced showing both rising up of green white ratio and increasing abundance of cracked dark green grains these irregular concentrations are not consistent directly with a general climatic forcing as evidenced by 18 o isotopic curve but are the result of recurrent bottom current activity thus winnowing process is the cause of the long lasting ionic exchange and mineralogical evolution at the water sediment interface clay infilling composed of smectite 35 40 kaolinite 45 50 traces of mica calcite and quartz was a precursor material along with dissolution of kaolinite crystallochemical modifications of smectites were observed both to the sequential concentration scale bulk samples and to the individual grain scale from white to dark green increment of fe content associated decrease of al and increase of layer charge and k content in the layers with the highest green grain concentrations and in agreement with crystallochemical formulae fe 3 rich montmorillonite displays octahedral charge tetrahedral charge appears only when fe content increased 1 2 fe per formulae with the maturation process the growing of the closed layers is observed mixed layer smectite glauconite 80 20 nearly similar evolution patterns were recorded at various levels 0 01 0 85 2 2 ma of the pliocene pleistocene accumulation however the nontronite way is evidenced in the lowest green grains concentrations and remind the previously described shelf processes independently of the morphologically induced winnowing in these marine tropical deposits the glauconitization process was especially favored by abundant iron supply and was not affected by low temperature 3 c it is suggested that in deep water environment bottom current controlled iron deposition and disponibility strongly influencing the mineralogic evolution a too lengthy dispersal of iron microparticles played an ultimate role in the development of the montmorillonite way rather than the nontronite way this would be the most commonly found in shallow water environment

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

deflection of plumes by mantle shear flow experimental results and a simple theory summary linear hotspot tracks indicate relative motion between some hotspot sources and the overlying lithosphere in order to better understand the effects of plate motions and mantle convection on mantle plumes and associated hotspots we consider the path of a continuous plume of buoyant material in a shear flow laboratory experiments show that steady plumes bent over by a linear velocity profile follow parabolic trajectories plume trajectories are also shown to evolve quadratically in time toward a new steady shape after a change in the ambient flow a remarkably simple description of plume dynamics provides good agreement with the experimental data the plume trajectory is determined by the combination of horizontal advection by mantle flow and vertical rise according to a modified stokes law these results are applied to some idealised examples which demonstrate effects of depth dependent mantle viscosity and return flow at least some mantle plumes are likely to be sufficiently robust that they remain close to vertical and therefore stable

recent sediment accumulation rates for the western margin of the barents sea abstract we investigate patterns processes and rates of sediment accumulation within the marginal ice zone of the western barents sea the barents sea is among the most productive of the arctic marginal seas with approximately half of the sediment burial flux derived from marine rather than terrestrial sources sediment accumulation rates were quantified by 210 pb geochronology at 14 stations ranging in water depths from 173 to 503 m along a south north latitudinal gradient of 6 75 81 n the average sediment accumulation rate for all stations is 0 7 0 4 mm yr in general lower sediment accumulation rates are associated with coarser sediment fractions at shallower water depths 200 m where currents remove fine grained sediments and transport material to the deeper regions higher sediment accumulation rates are detected in deeper water stations and are largely associated with specific sedimentation events and or features of the barents seafloor although the c n ratio of surface sediments 0 2 cm from all stations indicates a predominance of sediments of marine origin c n 9 0 1 0 there is no distinct depositional pattern indicating preferential burial in areas more heavily influenced by the marginal ice zone we conclude that a combination of mixed sediment sources large shifts in the location of the marginal ice zone over time and the benthic boundary layer processes obscure the productivity signal preserved in these seafloor sediment deposits of the western barents sea

blob tectonics a prediction for western aphrodite terra venus it is proposed that the western aphrodite terra region of venus was formed by four diapirs of heated mantle material or blobs rising to the surface in a region of convective upwelling the four blobs are in different stages of evolution producing different topography gravity anomalies and morphology we show that this model is consistent with available data and make specific predictions that can be tested with magellan radar data

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

influence of heating mode on three dimensional mantle convection numerical models of three dimensional thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets as the proportion of internal heating increases the number of upwelling plumes increases and downwelling sheets become more vigorous and time dependent with any amount of basal heating the entire convective pattern during its evolution is anchored to the upwelling plumes as the proportion of internal heating increases the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow downwelling sheets carry only a minor fraction approximately 30 of the basal heat flow even when the shell is entirely heated from below but they advect almost all of the internally generated heat the relatively large number of plumes in the earth s mantle inferred from hotspots the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from within copyright american geophysical union 1989

sinker tectonics an approach to the surface of miranda images of the uranian moon miranda show three areas of complex terrain trapezoidal to ovoid in shape with no previously known counterpart these features called coronae are characterized by an inner core of intersecting ridges and troughs surrounded by a series of subparallel and concentric bands two of the proposed explanations for these tectonic features involve mantle convection driven by density anomalies the present study determines the surface stresses induced by such density anomalies and their expected surface expression miranda is modeled as having a rigid core surrounded by a mantle capable of viscous flow which is overlain by a thin elastic lithosphere the density anomaly is modeled as a point mass representing the sinker or riser the model is developed in a spherical coordinate system having axial symmetry about a 0 axis passing through the sinker the flow field set up by the sinking or rising mass is expressed in terms of a stream function using legendre sums streamlines are functions of the position of the mass anomaly and the radii of the core mantle and mantle lithosphere boundaries only magnitudes of the stream function depend upon the size of the mass anomaly its local gravitational field and the mantle lithosphere radius the stresses on the base of the lithosphere are then determined from the equations of motion the viscosity of the mantle material has no effect on the stresses produced at any given time but does control the velocity of the anomaly expressions for displacements and stresses in the elastic shell are developed as legendre sums and coupled to the viscous mantle solution with appropriate boundary conditions the stresses derived yield a pattern of expected tectonic expression this tectonic style varies with the radius of the body for a small body surface stresses are dominant in the area above the mass anomaly while farther away stresses at the base of the lithosphere control the tectonics directly over a sinker and extending outward 15 is a zone of both latitudinal and longitudinal surface compression which produces folding and thrust faults with no preferred orientation beyond 15 longitudinal stresses at the base of the lithosphere are extensional while latitudinal stresses are compressional producing a zone of folding and thrusts concentric about the sinker stresses induced by a riser are opposite in sign and predict extensional features with the same orientation as the compressional features predicted for a sinker stress magnitudes generally decrease with distance from the sinker and also decrease with both increasing sinker depth and lithosphere thickness the predictions of the sinker model are in good agreement with the tectonic styles of the banded and ridged terrains of miranda as determined from the topography as imaged by voyager 2 this agreement supports the idea that the coronae on miranda were produced by dense masses originally located near the surface that sank into miranda s interior for larger bodies the stresses at the surface and base are nearly equal longitudinal stresses are greater than latitudinal stresses resulting in compressional features with a preferred orientation radial to the sinker location the changeover in tectonic styles occurs at a planetary radius of approximately 1500 km

sedimentary impacts of recent moderate earthquakes from the shelves to the basin floor in the western gulf of corinth abstract in seismically active areas long term records of large earthquakes are indispensable to constrain reccurence patterns of large earthquakes in the western corinth rift one of the most active areas in europe in terms of seismicity data about ancient earthquakes are still insufficient despite historical records covering the last two millenia and several studies in onshore paleoseimology in this paper we test the use of offshore sediments from the gulf of corinth to identify sediment failures and tsunamis that have been triggered by historical earthquakes two shelves 40 100 m deep one sub basin 180 m and the basin floor 330 m have been sampled by short gravity cores the cores were analyzed in order to identify and characterize event deposits the age control has been provided by 137cs and 210pb activity measurements showing that the cores represent 2 to 4 centuries of sedimentation in each site sandy event deposits are interbeded in the muddy hemipelagic sedimentation the age of event deposits has been compared to the record of historical earthquakes using new and published macroseismic data this comparison shows temporal coincidence of some event deposits and documented earthquakes with a macroseismic intensity vii in the area e g in 1861 ce 1888 ce and 1909 ce in nearshore shallow water settings the record of event deposits does not exactly fit with the historical record of large earthquakes because too few event deposits are present this may be due to the absence of sediment failures or to a lower preservation of the deposits in such settings in the deepest site in the basin floor the correspondence is better a sandy turbidite probably corresponds to each large earthquake since 1850 ce except one aseismic sediment density flow that occurred at the end of the 20th century surprisingly the ms 6 2 june 15 1995 aigion earthquake is only possibly recorded in one nearshore site on the aigion shelf in the form of a tsunami back wash flow deposit this study showed that moderate earthquakes m 5 8 6 5 can significantly impact marine sediments regarding the evaluation of seismic hazard in the area the basin floor is proposed as a promising site for long term paleoseismology in the gulf of corinth while shallower settings need to be considered more carefully

continental rifting and the origin of beta regio venus topographic maps based on pioneer venus altimetry suggest that beta regio an elevated feature centered at 27 n 282 e is analogous to domes associated with continental rift systems on earth this interpretation is consistent with the commonly quoted analogy between the east african rift system and the topography of the region from beta regio southward to phoebe regio if beta regio is a dome major structural uplift of the crust of venus is implied suggesting a more dynamic upper mantle than would be the case if beta regio were simply a large volcanic construct

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

a spatial domain stokes flow model for the gravity and topography of the middle latitudes of venus a new modeling strategy is developed and applied to the long wavelength gravity topography and internal density structure of venus geodynamic modeling of planetary scale phenomena is usually carried out in the wavenumber transform domain spherical harmonics or fourier transforms however because of limitations associated with the gravity and topography data available for venus a spatial domain approach is preferable smoothed representations of the gravity and topography of venus are developed in terms of quadratic b splines on 10 centers in latitude and longitude over the latitude interval from 45 s to 45 n these models reproduce 83 6 and 98 3 of their respective data variances a similarly constructed model is then presented for the dynamic support of gravity and topography on venus internal mass anomalies are specified at two discrete depths 50 and 500 km with lateral variations determined by two constraints 1 that the slow viscous flow induced by the anomalies will distort the free surface into a form matching the observed topography and 2 that the sum of the gravitational effects of the internal anomalies plus that due to the surface distortion they cause will match the observed gravity there are four major features evident in the gravity and topography data sets atla beta ovda and thetis regiones all four have compensating mass patterns consistent with a mantle plume origin the surficial flow pattern associated with this model does not resemble terrestrial plate tectonic patterns

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

210pb sedimentation rates from the northwestern mediterranean margin abstract a total of 83 cores were collected in the gulf of lions continental margins and analysed for 210 pb xs excess 210 pb in order to understand sedimentation patterns apparent sedimentation rates asr range from 0 65 cm year 1 in the vicinity of the rhone river mouth to 0 01 cm year 1 in the deep basin except for the prodelta area rates decrease with depth linearly with the water depth on the slope asr do not differ between canyons and open slope except for the western area where the rates are slightly higher in the lacaze duthiers canyon compared to its adjacent open slope in the canyon and open slope areas mass accumulation rates determined from 210 pb xs profiles 0 10 and 0 08 g cm 2 year 1 respectively are in good agreement with particulate fluxes calculated from 5 years of near bottom sediment trap data even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects however differences in sediment trap data between west and east portion of the slope are not observed in the sedimentation rates calculated with 210 pb xs the outer shelf area may have an important role in trapping sediment but it is not sufficiently documented sediment surface mixed layer depths decrease with water depth with a mean value for the whole margin of 8 6 cm 210 pb xs inventories in the sediment are systematically higher than the net 210 pb export flux estimated from the above water column over the margin the ratio between accumulated 210 pb and available 210 pb is about 3 suggesting boundary scavenging effects and advective transport

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

determination of 137cs activities in surface sediments and derived sediment accumulation rates in thessaloniki gulf greece surficial and core samples collected from the eastern thessaloniki gulf located in the nw aegean sea were analyzed for their sedimentation rate and inventories of 137cs and 210pb the study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels the sedimentation rates were calculated from the vertical distribution of 137cs and excess 210pb in the sediment cores the spatial distribution of 137cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content the activity concentrations were measured by means of gamma ray spectrometry using hpge detector for 137cs and gross alpha counting using alpha counter for 210pb the average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0 18 0 02 to 0 22 0 03 cm year 1 the 137cs inventories varied from 205 15 to 602 39 bq m 2 while the 210pb inventories of the studied cores exhibited average value of 140 9 bq m 2 elevated 137cs activities were observed compared to certain mediterranean marine areas however they were lower than the reported values in the black sea

geochemistry and mineralogy of the nw aegean sea surface sediments implications for river runoff and anthropogenic impact abstract surface sediments of the nw aegean sea were analyzed for clay mineral composition 94 samples organic c and carbonate content and major and minor elements 112 samples illite is the most abundant clay mineral followed by smectite chlorite and kaolinite clay minerals are preferentially deposited within the continental shelf and slope environment due to the prevailing cyclonic circulation and the formation of flocs in the rivers pro delta areas the spatial distribution of major lithogenic elements si al ti fe is substantially biased by the presence of relict sands located in the central eastern part of the thermaikos gulf continental shelf biogenic elements ca sr are noticeably pronounced on the northern flank of the north sporades islands organic c content and minor elements cu zn as and pb are enriched markedly over the thessaloniki bay and gulf suggesting substantial human impact deriving largely from untreated or partly treated domestic and industrial effluents the terrigenous component of the elements is preferentially contributed from the axios river zn and pb the aliakmon river cr co ni cu and as and to a lesser extent from the pinios river v co ni and cu a considerable part of the metals originates in ultra mafic and volcanic formations which are abundant in the rivers catchment basins carbonate content exhibits great variability with the highest values observed in biogenic sediments of the sporades basin in the latter area mn and fe oxides and hydroxides were found as coatings on calcite the coatings were enriched in cr co ni zn as mo and pb due to adsorption or ion exchange during early diagenesis the combined influence of the river systems the thessaloniki metropolis and the diverse sedimentological background generates great patchiness in the distribution of major and minor elements

regional linear cross strike discontinuities in western aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the highlands the detailed characteristics of these features are similar to terrestrial oceanic fracture zones and strike slip fault zones either interpretation implies the presence of large scale horizontal tectonic movement on venus

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

comparison of observed and predicted gravity profiles over aphrodite terra venus the authors compare observed pioneer venus orbiter pvo gravity profiles over aphrodite terra to profiles predicted from models of thermal isostasy mantle convection and airy compensation similar approaches are used in order to investigate how well the models can be distinguished with the pvo data topography profiles across aphrodite are compared to model spreading ridge profiles in order to further assess this model airy compensation depths and convection layer thicknesses are greater under eastern aphrodite than western aphrodite compensation depths in the east are greater than most estimates of lithospheric thickness suggesting that this part of the ridge is dynamically supported in parts of western aphrodite the spreading ridge model gravity provides a better fit to the data than either airy compensation or mantle convection best fit spreading rates are between 0 3 and 1 6 cm yr airy compensation and mantle convection cannot be distinguished in most places using only pvo data

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

terrestrial spreading centers under venus conditions evaluation of a crustal spreading model for western aphrodite terra abstract assessment of terrestrial spreading centers under venus environmental conditions indicates that the major difference between earth and venus would be the influence of the enhanced surface temperature on upper mantle temperatures on venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography crust on venus produced at average spreading centers would be about 15 km thick in contrast to the average crustal thickness on earth of about 5 km we have used this model and the pioneer venus altimetry and gravity data to test the hypothesis of head and crumpler 1987 6 that western aphrodite terra represents the location of crustal spreading on venus the topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis the topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on earth at 1000 km on either side of the axis steep topographic slopes exist this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain by a ridge jump or by mechanisms unrelated to crustal spreading for processing line of sight los acceleration data we first show that static and dynamic calculations give the same results for track numbers 427 449 this allows us to determine a synthetic orbit track parallel to the cross strike dis continuities csd s and to compare it with observed los accelerations gravity data for this domain were then processed in a manner similar to that for topography models give a reasonable fit to the data for half spreading rates between 0 3 and 0 5 cm yr we conclude that a spreading center model for ovda regio could account for the observed topography and los gravity anomalies if it had the following characteristics a central domain with a 30 km thick crust that has been spreading at about 0 5 cm yr for the last 200 myr over a mantle whose temperature is about 1600 c at about 200 myr ago a change in upper mantle temperature of about 100 c is required prior to this time the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center this thinner crust would now lie on the flanks of ovda regio at distances greater than 1000 km from the spreading center and would be in excess of 200 myr old on the basis of our analysis we conclude that the hypothesis that ovda regio in western aphrodite terra is the site of crustal spreading on venus is consistent with the topography and gravity data

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle