interpreting gravity geoid and topography for convection with temperature dependent viscosity application to surface features on venus while venus and earth are similar in size venus does not appear to have developed the same system of mobile surface plates which dominates convective heat transfer from the interior of earth numerical models with temperature dependent viscosity do not produce plate boundaries and are more closely reminiscent of venus than earth we demonstrate that many puzzling aspects of surface features on venus are also found in the analysis of the surface observables in these models despite having no strong lithosphere our calculations exhibit very high geoid topography ratios and uncompensated short wavelength topography surface heat flux values are not required to be very different from earth scaled values and the mantle remains below the likely solidus

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

constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

subducted slabs and the geoid 1 numerical experiments with temperature dependent viscosity one of the most powerful constraints on mantle viscosity comes from the correlation of the long wavelength degree 4 9 geoid with that predicted by a density model for subducted slabs the effect of lateral variations of viscosity should be most pronounced at subduction zones due to the strong effect of temperature on viscosity an idealized slab model with temperature dependent viscosity is considered with various lateral and vertical viscosity structures using a two dimensional finite element formulation the viscosity parameterization affects the amplitude of the long wavelength geoid anomaly but not the sign of the correlation between the geoid and density anomalies depth dependent viscosity models with a high viscosity lithospheric layer do not completely match the temperature dependent laterally varying results suggesting that the rheology of the slab does have an effect on the long wavelength surface topography and geoid however this affect is minor suggesting that the radial models of mantle viscosity inferred from surface observables are correct to first order in contrast the short wavelength features are dramatically affected by the rheology of the region surrounding the downwelling further study of the shorter wavelength geoid over subduction zones may provide better insight into subduction zone processes

em responses of an elongated conductor near an ocean analogue model studies abstract magnetic induction responses of an elongated conductor at sites near an ocean are studied with the aid of laboratory analogue model measurements the dependence on the period the distance from the ocean and the depth extent of the conductor is examined for the case of the elongated conductor parallel to the ocean coastline the parkinson induction arrows at sites near the conductor reverse directions from pointing towards the conductor at short periods to pointing towards the ocean at longer periods and the period at which the reversals occur differs for the in phase and the quadrature arrows it is also shown that except for sites right at the coastline and at the conductor the responses of the conductor alone and the ocean alone can be added to yield the responses of the model that includes both the ocean and the conductor these results have application to the subtraction of ocean induction effects from geomagnetic measurements in coastal regions to delineate the responses of any nearby elongated anomalous conductors

magnetovariational soundings across the south island of new zealand difference induction arrows and the southern alps conductor abstract magnetovariational mv data from 14 sites on a transect of the south island of new zealand are presented the induction arrows clearly show the effects of induction in the surrounding oceans comparison with the responses of an analogue model of the effect of the oceans on induction in the new zealand region indicates that the induction arrows also contain information on anomalous on land conductivity structure the use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two and three dimensional numerical models pertinent to the new zealand situation difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal in both real and quadrature arrows across the south island two dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the southern alps of an anomalously conducting region previously identified by magnetotelluric mt sounding use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation

the coast effect response in geomagnetic field measurements abstract two dimensional 2d numerical models of electromagnetic em induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean conductor separation distances that would permit to within an acceptable approximation a simple vector subtraction of the coast effect response the anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth the coast effect response range yr is arbitrarily taken to be the distance from the ocean at which the b z b y n coast effect response is reduced to a value of 0 2 the response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths island widths and ocean depths the response ranges are tested in 2d numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host earth the induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor as an application difference arrows obtained by subtracting three dimensional laboratory analogue model coast effect responses from field site responses are employed in the interpretation of induction arrow responses available for a number of sites in the bohai bay region of china and the kii peninsula region of japan

geomagnetic induction in a heterogeneous sphere fully three dimensional test computations and the response of a realistic distribution of oceans and continents summary long period geomagnetic data can resolve large scale 3 d mantle electrical conductivity heterogeneities which are indicators of physiochemical variations found in the earth s dynamic mantle a prerequisite for mapping such heterogeneity is the ability to model accurately electromagnetic induction in a heterogeneous sphere a previously developed finite element method solution to the geomagnetic induction problem is validated against an analytic solution for a fully 3 d geometry an ov axis spherical inclusion embedded in a uniform sphere geomagnetic induction is then modelled in a uniform spherical mantle overlain by a realistic distribution of oceanic and continental conductances our results indicate that the contrast in electrical conductivity between oceans and continents is not primarily responsible for the observed geographic variability of long period geomagnetic data in the absence of persistent high wavenumber magnetospheric disturbances this argues strongly for the existence of large scale high contrast electrical conductivity heterogeneities in the mid mantle lastly for several periods the geomagnetic anomaly associated with a mid mantle spherical inclusion is calculated a high contrast inclusion can be readily detected beneath the outer shell of oceans and continents a comparison between observed and computed c responses suggests that the mid mantle contains more than one order of magnitude of lateral variability in electrical conductivity while the upper mantle contains at least two orders of magnitude of lateral variability in electrical conductivity

electromagnetic constraints on strike slip fault geometry the fraser river fault system magnetotelluric data from four profiles crossing the eocene strike slip fraser river fault in southwestern british columbia suggest that it penetrates the entire crust this conclusion is supported by seismic reflection observations of a 2 3 km step in the moho slightly to the east of the surface expression of the fault but is at variance with an interpretation of the seismic data in which the fault soles into mid crustal reflectors that seem to be continuous across the fault trace a crustal penetrating geometry supports the proposal that the fraser river fault forms part of a 2500 km long intracontinental transform fault system in northwestern north america modeling studies resolve a thin highly conducting mid crustal zone that is connected electrically to the conducting lower crust beneath the coast belt low 13 c values close to the fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon

mars north polar deposits stratigraphy age and geodynamical response the shallow radar sharad on the mars reconnaissance orbiter has imaged the internal stratigraphy of the north polar layered deposits of mars radar reflections within the deposits reveal a laterally continuous deposition of layers which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice the packet interpacket structure can be explained by approximately million year periodicities in mars obliquity or orbital eccentricity the observed 100 meter maximum deflection of the underlying substrate in response to the ice load implies that the present day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers alternatively the response to the load may be in a transient state controlled by mantle viscosity both scenarios probably require that mars has a subchondritic abundance of heat producing elements

electromagnetic induction in the new zealand south island abstract laboratory magnetic field measurements for a scaled analogue model of new zealand and the surrounding oceans are used to provide induction arrows for simulated geomagnetic variations of 5 120 min periods for a series of traverses over the south island as the island is relatively narrow the field responses for all on land locations show significant effects of the surrounding oceans the model simulates a uniformly resistive island the oceans and a conductive asthenosphere at depth and thus discrepancies between the model and field results for sites on the island are attributed to anomalous geological structures not simulated in the model removing the coast effects before interpretation by subtracting the model induction arrows from the field site arrows at eyrewell permits an obvious reinterpretation that attributes a significant component of the field responses to the nearby alpine and marlborough faults rather than to the channelled current in cook strait

implications of large elastic thicknesses for the composition and current thermal state of mars abstract the martian elastic lithosphere thickness t e has recently been constrained by modeling the geodynamical response to loading at the martian polar caps and t e was found to exceed 300 km at the north pole today geological evidence suggests that mars has been volcanically active in the recent past and we have reinvestigated the martian thermal evolution identifying models which are consistent with t e 300 km and the observed recent magmatic activity we find that although models satisfying both constraints can be constructed special assumptions regarding the concentration and distribution of radioactive elements the style of mantle convection and or the mantle s volatile content need to be made if a dry mantle rheology is assumed strong plumes caused by e g a strongly pressure dependent mantle viscosity or endothermic phase transitions near the core mantle boundary are required to allow for decompression melting in the heads of mantle plumes for a wet mantle large mantle water contents of the order of 1000 ppm are required to allow for partial mantle melting also for a moderate crustal enrichment of heat producing elements the planet s bulk composition needs to be 25 and 50 sub chondritic for dry and wet mantle rheologies respectively even then models resulting in a globally averaged elastic thicknesses of t e 300 km are difficult to reconcile with most elastic thickness estimates available for the hesperian and amazonian periods it therefore seems likely that large elastic thicknesses in excess of 300 km are not representative for the bulk of the planet and that t e possibly shows a large degree of spatial heterogeneity

numerical estimation of the sea effect on the distribution of induction arrows in the japanese island arc abstract anomalous distribution of induction arrows in the japanese island arc has been supposed to be due at least partly to induced electric currents flowing in the surrounding seas none the less no reliable estimation has been made of this regional problem here we attempt to solve the problem using the algorithm of bimodal electromagnetic induction in a thin sheet conductor the non uniform conductance of the thin sheet corresponds to seawater depths in the surrounding seas it is found out that induction arrows are controlled strongly by the open seas such as the pacific ocean the effect of the semi closed japan sea is somewhat weaker and is dependent on period a detailed comparison between the calculated and the observed induction arrows over the japanese island arc indicates however that the sea effects alone cannot fully account for the observations the differences between the observed and the calculated responses delineate lateral inhomogeneities in the subsurface conductivity structure which are not included in the present model

mantle convection controls the observed lateral variations in lithospheric thickness on present day mars 1 sounding radar observations of the north polar layered deposits on mars suggest an elastic lithosphere thickness of at least 300 km in that region significantly larger than any prior estimate of lithospheric thickness on mars previous studies interpreted this result as requiring either that mars is significantly sub chondritic in its abundance of radioactive elements or alternatively that there are large lateral variations in lithospheric thickness we find that the required depletion of radioactivity is geochemically unlikely on the other hand stagnant lid convection naturally produces factor of two or more lateral variations in both mantle heat flux and lithospheric thickness and is the preferred explanation for the polar elastic thickness measurement our mantle plume models which have previously been shown to explain observations of present day magma production on mars can also explain the new elastic thickness observations

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

plume induced topography and geoid anomalies and their implications for the tharsis rise on mars the topography and geoid anomalies produced from an upwelling plume are sensitive to the depth of plume buoyancy d p and elastic lithospheric thickness t e both of which control the degree of compensation we formulated spherically axisymmetric convection models of mantle plumes with temperature and pressure dependent viscosity to dynamically determine d p and t e and the resulting topography and geoid anomalies from the plume from 10 cases with different rayleigh number activation energy and activation volume we determined te ranging from 100 km to 180 km and d p that is approximately twice t e the ratio of geoid to topography r g t from these plume models is either negative or slightly positive 0 03 at wavelengths that are most relevant to the tharsis rise low harmonics from degrees 2 to 4 the modeled r g t is significantly smaller than that observed for the tharsis rise suggesting that tharsis is unlikely to be dynamically supported

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

geophysical implications of the long wavelength topography of rhea 1 we use limb profiles to investigate the long wavelength topography and topographic variance spectrum of rhea one dimensional variance spectra show a break in slope at a wavelength of 300 km a similar effect is seen on the moon and may be a signature of an elastic lithosphere having a thickness te 10 km the implied heat flux is 15 mw m 2 much higher than can be explained by radiogenic heating we use the 1 d spectral behavior to constrain our solution for the long wavelength global topography of rhea the degree 3 topography is large enough if uncompensated to contaminate estimates of the degree 2 gravity using existing flyby data current models of rhea internal structures which rely on these degree 2 estimates may thus be inaccurate illustrating the need to acquire further rhea gravity data

global variations in the geoid topography admittance of venus global representations of geoid height and topography are used to map variations in the geoid topography ratio admittance of venus the admittance values are permissive of two mutually exclusive models for convection driven topography in the first compressive highland plateaus are expressions of present mantle downwelling broad volcanic rises are expressions of mantle upwelling and lowlands overlie regions with no substantial vertical motion in the mantle in the second compressive highland plateaus are remnants of an earlier regime of high crustal strain and most other long wavelength topographic variations arise from normal convective tractions at the base of the lithosphere

on the support of the tharsis rise on mars abstract the most significant long wavelength geoid and topographic anomalies on mars are associated with the tharsis rise smith et al science 286 1999 94 97 smith et al science 284 1999 1495 1503 however the origin of the elevated geoid and topography in the tharsis region remains unresolved between two competing models in the first model the tharsis rise is dynamically supported by a thermal plume in the mantle kiefer and hager lpi tech rep 89 04 1989 48 50 harder and christensen nature 380 1996 507 509 breuer et al j geophys res 101 1996 7531 7542 while the second model attributes the tharsis anomalies to volcanic construction and its associated lithospheric flexural effects turcotte et al j geophys res 86 1981 3951 3959 phillips et al science 291 2001 2587 2591 here we resolve this ambiguity by modeling the ratio of gravity and topography at long wavelengths with a new loading formulation that simultaneously considers plume buoyancy and surface loads we demonstrate that a thermal plume contributes no more than 15 to the geoid and 25 to the topography for the tharsis rise for the range of lithospheric thicknesses that are constrained by observations this indicates that the tharsis rise is best explained by volcanic construction on lithosphere

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

mantle convection and the dynamic geoid of mars this study compares different 3d spherical models of thermal convection in the martian mantle the best fit to the tharsis bulge is obtained if the endothermic phase transition to perovskite occurs closely above the core mantle boundary in this case a large scale flow develops and the associated geoid anomaly explains approximately half of the observed long wavelength geoid the shallower exothermic phase transitions have little influence on the planform of the flow and on the shape of the geoid previous studies that included exothermic transitions were biased by effects of a soft lithosphere which introduces a large scale mantle flow even without a deep endothermic phase transition however such models predict a continous resurfacing and a dominant geoid low which is in conflict with observations

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

an analogue model study of em induction in the north china korea coastal region abstract the behaviour of the magnetic field variations over the north china korea coastal region is studied with the aid of a scaled laboratory analogue model the model source frequencies simulate naturally occurring geomagnetic variations of 3 60 min periods in phase and quadrature magnetic bx by and bz field measurements for the modelled region are presented for e and b polarizations large anomalous in phase and quadrature model magnetic fields are observed over the korea japan strait for e polarization and over the bohai strait for b polarization due to current channelling through the straits large responses of the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period in general the effects of peninsulas straits bays and irregularities in the coastlines play an important role in the magnetic field responses both on shore and off shore for this complex north china korea coastal region model and field site induction arrows are compared for three sites west of bohai bay

does mantle convection currently exist on mercury abstract the full equations of convective motion over the age of the solar system can be solved in a few hours 2d to several weeks 3d with numerical simulations in cartesian or spherical geometry using the 2d cartesian geometry code conman we model the thermal evolution of mercury our preferred calculation has an initial rayleigh number of 8 10 4 no mantle internal heating and a constant core heat flux of 9 75 mw m 2 in a non newtonian rheology these values all fall within the range of previously estimated parameters from 1d calculations and allow for weak mantle convection to persist until present supporting the outer core dynamo explanation of mercury s intrinsic magnetic field further the peak dynamic topography and geoid anomalies from our model are about 179 m and 37 m respectively values that are not unrealistic and should be well within the range of observation by the messenger spacecraft future spacecraft observations yielding new and more accurate data will help us better understand the thermal evolution of mercury and the origin of its magnetic field

a numerical study of a mantle plume beneath the tharsis rise reconciling dynamic uplift and lithospheric support models 1 the tharsis rise is an area of extensive volcanism containing the most significant long wavelength topographic and areoid anomalies on mars the mechanism for supporting this large topographic expression and correlating areoid remains controversial the two main competing ideas are dynamic support by a deep mantle plume and a volcanically constructed lithosphere followed by lithospheric flexure while both of these models separately can account for specific features of tharsis neither explain its entirety most of the support of the tharsis topographic anomaly is best explained by a volcanic structure modifying the lithosphere however this raises the important question of the origin of the heat source for volcanic construction leading to possible small scale convection this demonstrates that the remaining topography and areoid not accounted for in lower end members of volcanically constructed surface loads is consistent with a deeper mantle plume source our results show we can produce about 15 and 18 of the total tharsis long wavelength areoid and topography anomaly with a plume forming in a strong temperature dependent rheology additionally our plume model has a 165 km thick rheological lithosphere within the range for present day estimates on mars and generates about 1 partial melting more than sufficient to provide a heat source for recent volcanism this implies both a model with dynamic support from a plume and a volcanically constructed lithosphere not only can coexist but offer an explanation for both the surface anomalies and heat needed for the minor amount of recent volcanic activity at tharsis rise

long wavelength topographic relaxation for self gravitating planets and implications for the time dependent compensation of surface topography the support of planetary surface topography is controlled by the thickness of the elastic lithosphere and thus has implications for the thermomechanical structure of shallow planetary interiors previous analyses of the support of long wavelength topography have generally utilized elastic formulations which preclude consideration of the time evolution of lithospheric stresses and thermal state here we formulate a viscoelastic model for the support of topography on a spherical planet our stress relaxation model employs a multilayer linear viscoelastic rheology and includes the effect of self gravitation in these models we approximate internal rheology with an olivine flow law and we estimate the internal thermal structure assuming simple conductive cooling we analyze how relief at the surface and internal density interfaces evolves with time in response to a surface or internal load our analysis demonstrates that crustal compensation is strongly dependent on planetary radius with a tendency for a large terrestrial planet like earth to reach complete airy isostasy at long wavelengths within a time scale controlled by the mantle viscosity the correlation of degree of compensation with planetary radius is consistent with previous work turcotte et al 1981 which showed that membrane stress can support significant long wavelength gravity anomalies applied to the moon the degree of compensation associated with loading of 100 ma lithosphere is less than 0 3 and the load love numbers are greater than 0 6 for all harmonics including degree 2 even after 109 years of relaxation this conclusion does not change significantly even with a moderately low viscosity for the crust the tendency for a small body like the moon to support long wavelength stresses may be relevant in understanding the anomalously large degree 2 terms in the lunar shape the results also suggest that major impact basins would not be compensated throughout lunar geological history 4 b y even if the surfaces on which the basins were initially formed were relatively young i e 100 300 ma the fact that the south pole aitken basin is largely compensated suggests that the lateral variations in thermal and or mechanical structure induced by impact processes play significant roles in the compensation of this structure

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

topography and geoid induced by a convecting mantle beneath an elastic lithosphere summary in the absence of seismological measurements observations of the topography and gravity fields of solid planets are the primary constraints on their internal structure to compute the synthetic geoid and topography induced by the dynamics of planetary interiors we introduce a 3 d numerical tool describing mantle convection beneath an elastic lithosphere although the energy conservation is treated in the whole spherical domain the deformation aspect is solved using a hybrid technique finite volume method for the viscous flow spectral method for elastic deformation the mechanical coupling is achieved via the imposition of the traction at the surface of the viscous flow as a basal boundary condition for the elastic deformation we present both response functions and full thermal convection cases computed with our new method for planetary bodies of varying dimensions the filtering effect of the lithosphere on the dynamic topography and geoid is specific for each planetary body justifying the importance of such a tool furthermore since our approach specifically focuses on the mechanical coupling at the base of the lithosphere it will permit future more elaborate rheological treatments it also enables to discriminate between the radial and tangential components of the viscous traction the latter is found to have a significant influence on the elastic deformation the effect on geoid is prominent more specifically while a thin elastic lithosphere is usually considered to play little role on the dynamic topography and geoid of venus a 35 per cent reduction is obtained for geoid height in the numerical example we propose on a planet with thicker elastic lithosphere such as mars the consequence of this filtering effect is to rule out the possibility of a dynamical support for the tharsis rise even for the lowest admissible values of elastic thickness in this region

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

an analogue model study of em induction in the japan korea china region abstract electromagnetic induction in the coastal region of japan korea china is studied using a scaled laboratory analogue model the model includes a simulation of the known ocean bathymetry the complex coastlines and the deep conductive pacific trench model measurements of the electromagnetic field components are carried out for simulated periods of 15 180 min for two mutually perpendicular polarizations of a uniform horizontal inducing source field in phase and quadrature bx by and bz model measurements are examined in detail for a series of traverses over japan the korean peninsula and the coastal region of china and the u s s r large anomalous in phase model magnetic fields are observed over the korea japan strait for e polarization and over the bohai tsugaru and la perouse straits for b polarization owing to current channelling the significant responses observed at short periods over the peninsulas in the shallow coastal areas decrease with increasing period large gradients in the in phase b2 field are observed over japan for e polarization for both short and long periods owing to the effects of induced currents in the surrounding oceans thus induction arrow responses over all regions of japan are expected to show the dominant effects of the ocean a comparison of analogue model and field site induction arrows over the kii peninsula of japan indicates that the ocean effects alone cannot fully account for the observed responses the differences in the model and field results can be attributed to lateral inhomogeneities in the conductivity structure under japan

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

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

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

intraseasonal atmospheric teleconnection patterns during the northern hemisphere summer abstract at times the 30 60 day filtered outgoing longwave radiation olr perturbations exhibited a systematic eastward propagation across the equatorial indian ocean western pacific during the five summers 1 may 30 september of 1979 83 such occasions are defined as e phase while periods of irregular movement are designated as ne phase global scale behavior of the 30 60 day filtered streamfunction and velocity potential fields differs significantly from e to ne phase during e phase at 200 mb a series of time clustered space overlapping disturbances develop over the northern as well as the southern subtropics although individual disturbances are nearly stationary a wave packet clearly propagates eastward with an approximate phase speed of 8 longitude per day and a space scale of wavenumber 1 sandwiched between the southern and northern hemisphere wave packets are relatively weak equatorial zonal wind perturbations which also move eastward at 850 mb e phase behavior is characterized by st