the rotational stability of an ice age earth summary predictions of glaciation induced changes in the earth s rotation vector exhibit sensitivities to earth structure that are unique within the suite of long wavelength observables associated with glacial isostatic adjustment henceforth gia and despite nearly a quarter of a century of research these sensitivities remain enigmatic previous predictions of present day true polar wander tpw speed driven by gia have indicated for example a strong sensitivity to variations in the thickness of the elastic lithosphere and the treatment phase or chemical of the density discontinuity at 670 km depth nakada recently presented results that suggest that the predictions are also sensitive to the adopted rheology of the lithosphere however his results have introduced an intriguing paradox in particular predictions generated using a model with an extremely high viscosity lithospheric lid do not converge to results for a purely elastic lithosphere of the same thickness mitrovica as cited by nakada has suggested that the paradox originates from an inaccuracy in the traditional rotation theory e g wu peltier associated with the treatment of the background equilibrium rotating form upon which any load and rotation induced perturbations are superimposed we revisit these issues using a new treatment of the linearized euler equations governing load induced rotation perturbations on viscoelastic earth models we demonstrate that our revised theory in which the background form of the planet combines a hydrostatic component and an observationally inferred excess ellipticity resolves the apparent paradox calculations using the revised theory indicate that earlier predictions based on earth models with purely elastic lithospheric lids are subject to large errors indeed previously noted sensitivities of tpw speed predictions to the thickness and rheology elastic versus viscous of the lithosphere largely disappear in the application of the new theory significant errors are also incurred by neglecting the stabilizing influence of the earth s excess ellipticity finally we demonstrate that the contribution from rotational feedback on predictions of present day rates of change of the geoid sea surface and crustal velocities are overestimated by the traditional rotation theory and this has implications for analyses of ongoing satellite e g grace missions and geodetic gps surveys

a complete formalism for the inversion of post glacial rebound data resolving power analysis summary a formal inverse theory for mantle viscosity based upon the data of glacial isostatic adjustment is here formulated and applied to synthetic data in this theory full account has been taken of the normal mode nature of the forward problem for realistic viscoelastic maxwell models of the planet since it is impossible to accurately infer the excitation and decay constants of the individual normal modes from the observations the formalism is cast in terms of the observed gross earth data in the time domain in this analysis expressions are required for the first order perturbations in both the modal amplitudes and relaxation times that are induced by an arbitrary radial perturbation to the starting viscosity profile a numerical technique is developed which enables us to accurately determine differential kernels for the modal amplitudes the analogous kernels for the modal decay times are derived analytically peltier 1976 and the complete set of kernels is shown to satisfy the physical constraint imposed by the uniqueness of the state of isostatic equilibrium for the viscously incompressible maxwell models that we employ when the problem is parametrized in terms of the logarithm of viscosity the kernels are capable of accurately predicting shifts in the normal mode characteristics for at least an order of magnitude variation in mantle viscosity using bayesian statistics a formal inversion is applied to a set of synthetically generated data these data chosen to reproduce the space time coverage of the actual observables include a subset related to the global gravity field and a large sequence of idealized relative sea level rsl curves it is found that even very weak a priori constraints can provide a stable and accurate inversion a resolving power analysis indicates a spatial resolution of approximately 1200km near the core mantle boundary cmb with a gradual improvement to better than 350km in the middle of the upper mantle subsets of the synthetic data are inverted in order to examine conditions on stability and accuracy and to determine their relative contributions to the spatial resolution data from progressively older beaches are shown to contribute most to the spatial resolution at all depths though the improvement in lower mantle resolution converges for data obtained from beaches formed within the last 5000 yr furthermore the rsl curves in the vicinity of the peripheral bulge of the ancient laurentide ice sheet are significant contributors to lower mantle resolution as demonstrated in previous analyses of the forward problem the inversion of this subset of the data also appears to be encouragingly stable

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

electromagnetic active source sounding near the east pacific rise an active source electrical conductivity sounding was carried out near the east pacific rise at 21 n the average electrical conductivity from 2 km beneath the seafloor into the upper mantle was found to be about 0 004 s m there is no evidence of fully molten magma at a distance of 7 km from the spreading axis the inferred conductivity with depth of the crustal rock is consistent with brace s model of connected seawater filled pores in rocks heated from beneath

geoid anomalies over cooling lithosphere source for a third kernel of upper mantle thermal parameters and thus an inversion summary the geoid anomaly over an isostatic mass redistribution on an infinite flat earth is simply 27 c g times the differential mass moment on a spheroidal earth the limiting value can never quite be reached because the external effect of a spherical shell of mass dipole is identically zero so distant portions of the spherical surface subtract from the effect of the region under the point of observation the width of the integrating kernel varies with the depth of the mass distribution but the observable fraction of the limiting geoid slope can be estimated at least for ocean basins of moderate extent where it should reach about 90 per cent for very small 3 cm yr 1 ocean basins the observable geoid slope is smaller a consideration of published geoid data together with gravity anomalies in the same area compared to the theoretical free geoid gravity anomalies gravity observed on the geoid leads with great caution to a crude estimate of the thermally produced slope of 0 14 0 02 m myr 1 given a value of the thermal geoid signal it is possible to conduct an inversion for the upper mantle thermal parameters if two the density and specific heat are assumed to be known the parameter kernels for the heat flow topographic decay and geoid give three equations to solve for expansion coefficient diffusivity and initial temperature ti the results are different from common modelling assumptions 3 1 10 5 c 1 17 per cent 0 0065 cm2 s l 45 per cent and 1660 c 29 per cent manipulation of the equations in the case of general temperature variable parameters leads to partial explanations appears multiplied by k0 7 k0 conductivity at 0 7 of t1 at surface temperature while k is weighted toward the lower values in the low to medium temperature range an estimate of t1 using the shape of the temperature variations of k and k in the most reliable available data for olivine remains at about 1660 c 30 per cent in spite of the large error band the higher initial temperature is plausible because decompression of upwelling material produces enough partial melting to generate the oceanic crust the latent heat of melting must be supplied from somewhere and the only other possible source is a large amount of mechanical work applied to the system in the divergent flow

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

pleistocene deglaciation and the earth s rotation implications for mantle viscosity summary recent results from the analysis of postglacial rebound data suggest that the viscosity of the earth s mantle increases through the transition region models which fit both relative sea level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle in this paper we analyse the effect of deglaciation upon the earth s rotation and thereby show that the observed secular trend polar wander evident in the ils ipms pole path and measurements of the non tidal acceleration of the length of day are both consistent with the viscosity profile deduced from postglacial rebound the two analyses are therefore mutually reinforcing

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

effects on post glacial rebound from the hard rheology in the transition zone summary we analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post glacial rebound ranging from the perturbations in the earth s rotation to the short wavelength features associated with the migration of the peripheral bulge a self gravitating model is adopted consisting of an elastic lithosphere a three layer viscoelastic mantle and an inviscid core the horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone the hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near field and in the far field from the ice sheets these findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle the stress field induced in the lithosphere by the pleistocenic ice sheet disintegration is a very sensitive function of mantle viscosity stratification the existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle with the viscosity of the transition zone acting as a controlling parameter a viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today

glaciation induced perturbations in the earth s rotation a new appraisal we revisit the theory and predictions of glaciation induced perturbations in both the rotation rate of the earth or j 2 and the wander of the rotation pole with respect to the surface geography the two most common theoretical formulations associated with polar wander driven by the surface loading of a viscoelastic earth e g sabadini et al 1982 wu and peltier 1984 appear to disagree by one on the number of rotational normal modes associated with a surface gravitational response having n normal modes we show using a generalized theory that the two formalisms yield essentially equivalent predictions we next consider a benchmark comparison of recent predictions of present day polar wander speed due to glacial isostatic adjustment henceforth gia these comparisons confirm the validity of certain previous analyses e g spada et al 1992a they also reveal that predictions of polar wander speed can be sensitive to an assumption of incompressibility previous studies have noted that predictions of j 2 rotation rate as a function of lower mantle viscosity differ significantly in form from analogous predictions of polar wander speed we investigate these differences by considering the contribution to the predicted anomalies from the individual normal modes of relaxation we find that the normal mode structure of the j 2 rotation rate predictions is fundamentally different from that which characterizes the polar wander response and this difference reflects the distinct physical mechanisms which govern these anomalies finally using a large suite of forward computations as well as the calculation of a set of frechet kernels we investigate the sensitivity of our predictions to variations in the viscoelastic earth model we find for example that a previously noted sensitivity of polar wander speed predictions to variations in lithospheric thickness is only valid for earth models with relatively weak lower mantle viscosities 5 x 10 21 pa s

contribution at satellite altitude of electromagnetically induced anomalies arising from a three dimensional heterogeneously conducting earth using sq as an inducing source field summary the induced magnetic signature at satellite altitude associated with crustal and mantle electrical heterogeneities is examined using five different earth models the three dimensional induction problem is solved in these models using the solar quiet daily variation as an inducing source field every model has an upper shell representing the land ocean electrical conductivity distribution the different models have either a homogeneous mantle or a heterogeneous mantle containing either subduction zones a ridge system or conductive blocks electromagnetic anomalies presented in the form of geographical maps are calculated for wavelengths related to spherical harmonics of degrees in the ranges 1 12 and 13 27 respectively magnetic anomalies are calculated at an altitude of 400 km and for three different local times 6 12 and 18 h

magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift although conductive intracrustal layers appear to be characteristic of continental rift zones they also have been detected in stable crustal environments the low resistivity layers are often explained by free water or crustal magma however alternative explanations are possible purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings certain minerals such as graphite and hydrated minerals e g serpentine may be important thermally activated processes associated with ductile flow mechanisms may also enhance conductivity two dimensional analysis of a portion of 25 new magnetotelluric stations in the central rio grande rift supports the presence of a conductive zone at 10 km depth or less however very significant differences are apparent in the crustal sections at two close localities mid crustal magma lenses well defined by seismic data cannot explain these differences in fact the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels only the possible effects of regional three dimensional current channeling could conceivably alter this conclusion to explain the unexpected result it is suggested that a conductive horizon occurs where an impermeable ductile cap traps pore fluids beneath the cap may be nearly an order of magnitude more conductive 100 s m than the dry brittle crust above the zone of trapped pore fluids much more conductive by more than an order of magnitude 10 m where active magma injection destroys the integrity of the ductile cap trapped fluids may escape and produce an overall decrease in conductivity the final electrical signature with this dynamic concept depends on the thermal gradient the relative impermeability of the cap the extent of pore fluids beneath and the amount and frequency of magma intrusion the temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection

polar wander caused by the quaternary glacial cycles and fluid love number abstract perturbations of the earth s rotation caused by the quaternary glacial cycles provide an important constraint on the viscosity of the deep mantle because they represent a long wavelength response of the earth to surface load redistribution the predicted present day polar wander speed pws is however sensitive to both the lower mantle viscosity lm the density jump at 670 km depth and the lithospheric thickness and viscosity e g sabadini and peltier geophys j r astron soc 66 1981 553 578 yuen et al j geophys res 87 1982 10745 10762 peltier and wu geophys res lett 10 1983 181 184 wu and peltier geophys j r astron soc 76 1984 753 791 peltier j geophys res 89 1984 11303 11316 vermeersen et al j geophys res 102 1997 27689 27702 mitrovica and milne j geophys res 103 1998 985 1005 johnston and lambeck geophys j int 136 1999 537 558 nakada geophys j int 143 2000 230 238 for earth models with lm 21 pa s and an elastic lithosphere the present day pws is very sensitive to the m1 mode buoyancy mode related to the density jump at 670 km depth mitrovica and milne j geophys res 103 1998 985 1005 the contribution of the m1 mode however is less significant for earth models with a viscoelastic lithosphere nakada geophys j int 143 2000 230 238 this is due to the fact that this contribution depends on the relative strength of the m1 mode k 2 t m1 k f t where k 2 t m1 is the magnitude of tidal love number k 2 t of the m1 mode and k f t is the value of k 2 t in the fluid limit fluid love number the magnitude of k f t for earth models with a viscoelastic lithosphere is larger than that for an elastic lithosphere and it is smaller for a thicker elastic lithosphere than for a thinner one thus for earth models with a viscoelastic lithosphere the pws is mainly sensitive to the lower mantle viscosity regardless of the behavior of the 670 km density discontinuity this relation also explains why the predicted pws increases with increasing thickness of an elastic lithosphere that is since the value of k 2 t m1 k f t with a thicker elastic lithosphere is larger than that with a thinner elastic lithosphere the m1 mode will have a higher contribution in the case of a thicker elastic lithosphere

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

polar wander of the earth associated with the quaternary glacial cycle on a convecting mantle summary observed true polar wander tpw at the present day with the speed of 1 myr 1 towards hudson bay has been generally examined based on the glacial isostatic adjustment gia for the quaternary glacial cycle the observation is however affected by the present day melting events of polar ice sheets and mountain glaciers and convective processes in the mantle in this study i examine tpw due to gia and convective processes by developing the liouville equation including these two processes referred to as generalized liouville equation here the generalized liouville equation makes it possible to evaluate the effects of convective related non forcing inertia elements i11 i22 i33 and i12 and forcing elements i13 and i23 on the polar wander for the quaternary ice age phase and clearly indicates that the excess flattening of the earth inferred from the non hydrostatic geoid stabilizes the polar motion as discussed by mitrovica et al 2005 also numerical experiments for examining the effects of time dependent convective processes on tpw indicate that reasonable convection scenarios with a time dependence in the inertia tensor of 1031 kg m2 myr 1 ricard et al 1993 insignificantly contribute to the observed present day tpw moreover the predicted tpw with magnitude of convective related forcing rates di13 dt and di23 dt larger than 5 1031 kg m2 myr 1 is significantly different from the observation these results may provide a framework for separating the contributions of gia and convection to the observed present day polar wander

mapping convection in the mantle long period 100 250 sec love and rayleigh waves are used to map heterogeneity and azimuthal anisotropy in the upper mantle spherical harmonic descriptions of anisotropy up to l m 3 and 2 are derived azimuthal anisotropy obtains values as high as 1 there is good correlation of fast rayleigh wave directions with upper mantle return flow directions derived from kinematic considerations this is consistent with the a axis of olivine being aligned in the flow direction the main differences between the flow models and the rayleigh wave azimuthal variation maps occur in the vicinity of hotspots hawaii for example appears to perturb the return flow there is strong correlation of the geoid and surface wave velocity at l 4 and 5 slow regions at this scale are associated with geoid highs and high heat flow consistent with upwelling convective flow or with isostatically compensated regions of low density the correlation of azimuthal anisotropy with upper mantle return flow directions rather than with plate directions suggests that part of the return flow is in the upper mantle and this in turn implies a low viscosity channel

water in the mantle results from electrical conductivity beneath the french alps 1 a deep magnetotelluric sounding in the french alps provided a vertical electrical conductivity profile between 200 1000 km two prominent features are observed first the conductivity in the depth range 400 800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm second the data do not require the conductivity to change throughout the transition zone 410 660 km in this part of the mantle a temperature of 350 450 c less than normal explains the magnetotelluric conductivity profile at 200 400 km our model favors a cold mantle with 1000 1500 ppm of water dissolved in olivine if correct this model suggests that the subducted slab is dehydrated before reaching the transition zone

significance of the fundamental mantle rotational relaxation mode in polar wander simulations summary in some studies on glacially induced true polar wander tpw the tidal effective relaxation of the fundamental mantle mode mo is lacking we show that this is caused by the deletion of the chandler wobble in an early stage of the theory development to facilitate the retrieval of the rotational relaxation modes we derive an analytical approximation formula for the mo rotational relaxation mode including the chandler wobble which can be of practical value for tpw simulations with realistically stratified earth models however we point out that the contribution of the mo rotational relaxation mode has to a high approximation the same effect on secular tpw as the contribution from an elastic term in models that do not have the mo rotational relaxation mode the two model approaches lead to the same polar wander results whenever the chandler wobble is filtered from models in which the mo rotational relaxation mode is retained

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

the rotational stability of a triaxial ice age earth 1 mitrovica et al 2005 following calculations by nakada 2002 demonstrated that the traditional approach for computing rotation perturbations driven by glacial isostatic adjustment significantly overestimates present day true polar wander tpw speeds by underestimating the background oblateness on which the ice age loading is superimposed the underestimation has two contributions the first originates from the treatment of the hydrostatic form and the second from the neglect of the earth s excess ellipticity supported by mantle convection in mitrovica et al 2005 the second of these two contributions was computed assuming a biaxial nonhydrostatic form i e the principal equatorial moments of inertia were assumed to be equal to their mean value in this article we outline an extended approach that accounts for a triaxial planetary form we show that differences in the tpw speed predicted using the mitrovica et al 2005 approach and our triaxial theory are relatively minor 0 1 myr and are limited to earth models with lower mantle viscosity less than 5 1021 pa s however for this same class of earth models the angle of tpw predicted for a triaxial earth is rotated westward toward the axis of maximum equatorial inertia by as much as 20 relative to the biaxial case we demonstrate that these effects are a consequence of the geometry of the ice age forcing which has a dominant equatorial direction that is intermediate to the axes defining the principal equatorial moments of inertia of the planet we complete the study by computing updated frechet kernels for the tpw speed datum which provide a measure of the detailed depth dependent sensitivity of the predictions to variations in mantle viscosity we show in contrast to earlier efforts to explore this sensitivity based on the traditional rotation theory that the datum does not generally have a sensitivity to viscosity that peaks near the base of the mantle

the lageos constraint on deep mantle viscosity results from a new normal mode method for the inversion of viscoelastic relaxation spectra the recently observed acceleration in the node of the orbit of the lageos satellite has previously been shown to provide a good constraint on the viscosity of the earth s mantle beneath the 670 km seismic discontinuity this paper explores the extent to which the previously inferred lower mantle viscosity may be traded off against variations of other properties of the radial viscoelastic structure we show explicitly that the nontidal acceleration of rotation inferred from the lageos data and predicted as a consequence of the earth s response to the last deglaciation event of the current ice age is relatively insensitive both to variations of lithospheric thickness and to the presence or absence of plausible chemical heterogeneity across the 670 km interface it therefore provides a high quality constraint on the viscosity of the deep mantle with the upper mantle viscosity fixed at the nominal value of 1021 pa s the bounds on lower mantle viscosity lm are such that 2 1021 pa s lm 6 1021 pa s even when allowance is made for the presence of a viscous boundary layer at the base of the upper mantle such as would exist if the mantle convective circulation were layered

geomagnetic variation sounding of the asthenosphere beneath the juan de fuca ridge two ocean bottom magnetometers have been deployed at sites on either side of the juan de fuca ridge 20 and 40 km from the crest near latitude 47 n twelve days of data were recorded simultaneously at the two sites and these data have been used to determine the electrical conductivity structure beneath the region the vertical field variations at both stations are of very low amplitude in the period range 0 25 to 8 hours with no detectable phase reversal across the ridge electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to victoria and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of oldenburg 1979 resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section the average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of shankland and waff 1976 for a basalt melt fraction within an olivine matrix for the low resistivity zone and depending on the water content of the melt 0 to 10 the estimated temperature range is 1400 c to 1250 c and the corresponding melt fraction 0 05 to 0 09 the data suggest that the melt fraction decreases below 80 km the conductivity profile from magnetotelluric data on 72 ma ocean floor in the north central pacific filloux 1977 has similar characteristics to the 1 ma old conductivity structure near the juan de fuca ridge but with the low resistivity layer at depths of 140 to 220 km in both cases the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere rather than with the base of the lithosphere the increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

upper mantle lateral heterogeneities and magnetotelluric daily variation data we use telluric and magnetic data of the diurnal variation recorded in europe australia and north america to study the magnetotelluric tensor in the 6h 24h period range we use associate directions and we eliminate the effects of deviation of telluric currents we thus obtain for each observatory reliable phases and apparent resistivity values representative of the neighbouring stratified substratum it appears that the values obtained in the four european observatories saint maur france ebro spain toledo spain nagycenk hungary give similar results and that these results are different from those obtained either in tucson usa or in watheroo australia

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

electrical conductivity of dry lower crustal rocks this study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings we present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 c to 1000 c we observe statistically significant differences between rock types mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group aphanitic fine grained rocks have higher conductivity than phaneritic coarse grained ones best fitting curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity because the laboratory rocks are dry their conductivities are lower at a given temperature than they would be if fluids or volatiles were present hence any temperatures inferred from

interior structure of terrestrial planets modeling mars mantle and its electromagnetic geodetic and seismic properties 1 we present a new procedure to describe the one dimensional thermodynamical state and mineralogy of any earth like planetary mantle with mars as an example the model parameters are directly related to expected results from a geophysical network mission in this case electromagnetic geodetic and seismological processed observations supplemented with laboratory measurements we describe the internal structure of the planet in terms of a one dimensional model depending on a set of eight parameters for the crust the thickness and the mean density for the mantle the bulk volume fraction of iron the olivine volume fraction the pressure gradient and the temperature profile and for the core its mass and radius currently available geophysical and geochemical knowledge constrains the range of the parameter values in the present paper we develop the forward problem and present the governing equations from which synthetic data are computed using a set of parameter values among all martian models fitting the currently available knowledge we select eight candidate models for which we compute synthetic network science data sets the synergy between the three geophysical experiments of electromagnetic sounding geodesy and seismology is emphasized the stochastic inversion of the synthetic data sets will be presented in a companion paper

magnetotelluric imaging of the society islands hotspot in april june 1989 seafloor magnetotelluric data across and along the leading edge of the tahiti hotspot were obtained the magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced the corrected response functions are consistent with a two dimensional structure inversion of the data shows a slightly higher conductivity relative to a reference site located away from the hotspot down to 130 km depth beneath the active area southeast of tahiti underlain by a more resistive structure there is a suggestion for a change in conductivity in the 400 450 km depth range which is consistent with elevated temperatures this result is consistent with a mantle plume of limited extent less than 150 km radius located near the leading edge of the tahiti hotspot the magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area as would be required by a superswell model

laboratory electromagnetic model results for the emslab region apparent resistivities and vertical to horizontal magnetic field ratios for the lincoln line traverse in the emslab project region are studied using a scaled laboratory analogue model the model includes a simulation of the juan de fuca plate subducting the canadian u s pacific northwest region in the model the subducting plate is taken to be horizontal offshore dipping at 10 beneath the coast then inland bending farther to dip at 45 in the washington oregon region and at 30 in the british columbia region the model results show that for inland sites the apparent resistivity for the telluric field component parallel to the coast and thus also parallel to the strike of the dipping conductive substructure depends strongly on period while the resistivity for the telluric field perpendicular to the coast is largely a galvanic effect and changes less with period the amplitude of the vertical to horizontal magnetic field ratio along the lincoln line shows a saddle shaped response with the coastal maximum followed by a second maximum inland due to the dipping substructure further rather than decreasing rapidly with distance inland as expected for a simple coast effect the field ratios retain significant values large distances inland for periods of 3 120 min model results for 120 min period are compared with one example of field results for the emslab project region

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

free carbon electrical conductivity in the earth s mantle laboratory measurements indicate that solid carbon either amorphous or as graphite enhances the electrical conductivity of rocks carbon in quantities of only a few ppm providing it forms an interconnected phase could give rise to electrical conductivities of the order seen in the high conductivity layers in the earth s mantle this possibility is consistent with carbon petrology in mantle rocks and puts the formation and migration of abiogenic carbon compounds into a broader context

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

changes in rotation induced by pleistocene ice masses with stratified analytical earth models the rotational response of the earth to pleistocene deglaciation is studied by means of a multilayered viscoelastic earth model based on the preliminary reference earth model prem incompressible viscoelastic deformation is evaluated from a self compressed initial state the novelty of our approach stands on the application of a fully analytical normal mode theory to the response of the earth to surface loads and variations in the centrifugal potential for large numbers of viscoelastic layers as requested by prem assuming that both present day true polar wander tpw and changes in the second degree component of the geopotential are solely due to pleistocene postglacial rebound we obtain for a two layer viscosity model that the upper mantle viscosity must be lowered to about 1 5 1020 pa s with respect to the classical value of 1021 pa s this upper mantle viscosity is accompanied by an increase of the lower mantle viscosity by a factor of 25 in agreement with some recent relative sealevel rsl data analyses and convectively supported long wavelength geoid anomalies when the viscosity contrast is located at 1470 km depth tpw and require a viscosity of 1021 pa s in the upper part of the mantle above 1470 km depth with a moderate viscosity increase in the lowermost portion of the mantle this result indicates that a viscosity of 1021 pa s is appropriate for a wider portion of the mantle than the upper mantle in agreement with haskell s 1935 estimate that was not limited to the seismically inferred 670 km boundary

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

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

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

magnetotelluric experiment over the rose area seafloor observations in natural electromagnetic fluctuations diffusing into the earth have been made in the vicinity of the pacific rise at 12 sup 0 n and 21 sup 0 n to gather information on the electrical conductivity structure of the oceanic basement near a spreading ridge at 21 sup 0 n the close proximity of land baja california introduces a noticeable distortion of the em fields and consequently the magnetotelluric sounding for this area must be interpreted with caution at both locations the conductance of the upper 200 km appears exceptionally high and the conductivity rises rapidly below reaching 1 s m sup 1 or more at 350 km the statistical significance of a prominent conductivity increase near a depth of 22 5 km detected in the inverted data from 12 sup 0 n and of a less developed one at 30 km in the case of 21 sup 0 n is discussed records of the magnetic variations of 21 sup 0 n taken above the spreading center compared to records from 40 and 120 km to the west reveal a conspicuous enhancement of the high frequency horizontal variations in a direction slightly west of magnetic north we interpret this distinctive feature as the signature of an electric current sheet concentrated in an extremely highmore conductance relatively shallow layer electrically bridging the two seafloor areas adjacent to the spreading center an approximate calculation suggests a conductance in excess of 3000 s per meter of ridge crest in the close vicinity of the accretion center equivalent to 1 to 8 km of molten basalt depending on temperature 1000 sup 0 1200 sup 0 c and depth 2 10 km this structure is consistent with seismic refraction information for the same area and with stratigraphic reconstruction of a fossil accretion center in samail ophiolite an interpretation in terms of a well developed magma chamber is irresistable although confirmation of the existence of this feature as well as improvement of its resolution by means of additional em data would be of major interest less

postglacial rebound and sea level contributions to changes in the geoid and the earth s rotation axis summary the rate of change of the spherical harmonic degree 2 component of the earth s gravitational potential coa 20 and polar wander velocity are two signals which are sensitive to late pleistocene deglaciation current changes in sea level and deep mantle viscosity diierent load and earth models have been used in earlier papers to predict the component of these geophysical signals caused by the collapse of the last great ice sheets and recent melting of polar ice caps in this paper we present a systematic analysis of the dependence of the predictions on parameters of the ice and earth model we show that the key parameters of the ice model which govern the predictions are the mass the location of the centre of mass and the midpoint of the deglaciation phase of secondary importance is the length of the deglaciation phase and the mean ice load prior to the last glacial maximum these conclusions enable us to make a more robust inference of mantle viscosity than has been made before allowing for the uncertainties in the model of late pleistocene and present deglaciation as previous authors have shown the lower mantle viscosity is the most important rheological parameter and therefore thecoa 20 observation complements sea level observations which are primarily sensitive to lithospheric thickness and the viscosity of the upper part of the mantle using realistic constraints on the sizes locations and timing of deglaciation of the late pleistocene ice sheets and current changes in polar ice caps the observation of coa 20 is used to infer lower mantle viscosity as a function of the present rate of sea level change if the present rate of non steric sea level change is 1 mm yr 1 and that change has been occurring for less than 1000 years then the lower mantle viscosity satis es log10 glm 21 82 0 15 which is consistent with inferences drawn from recent sea level analyses and con rms other analyses of thecoa 20 observation if the polar wander signal is produced entirely by postglacial rebound and current sea level change no more than 20 per cent of the present contribution to global sea level change comes from greenland the above conclusions also hold if the density discontinuities at 420 and 670 km are modelled as phase boundaries rather than material chemical boundaries

comparison of analogue model and field station em responses on southern vancouver island abstract measurements of geomagnetic induction responses for a laboratory analogue model that includes a simulation of the subducting juan de fuca plate are compared with those obtained at three sites on vancouver island good agreement between model and field responses at the central vancouver island site is observed over the period range 3 60 min while at the east and west coastal sites good agreement is achieved only for periods greater than 20 min at shorter periods departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model nevertheless the analogue model results are consistent with the premise of a juan de fuca plate underlain by the conductive asthenosphere subducting at a shallow depth beneath vancouver island