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

double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

rotational bulge and one plume convection pattern influence on martian true polar wander abstract motion of the entire solid planet with respect to its spin axis have been proposed on mars this movement is known as true polar wander tpw according to the conservation of angular momentum with no external torque on geological time scales the axis of maximum inertia of a planet is aligned with the rotation axis then rearrangement of masses within the mantle disturbs the planet s inertia and induces tpw the convection pattern on mars is possibly controlled by a sequence of single plumes originating from the core mantle boundary using a homogeneous model of the martian mantle and modelling the plume as a sphere we calculate the inertial tensor perturbations caused by the plume mass anomaly we investigate the stabilizing influence of the remnant rotational bulge due to the lithosphere elasticity on these perturbations it appears that during early martian history the elastic lithosphere was thin enough to allow its fractures under the inertia perturbations induced by a hot plume consequently the lithosphere s behaviour became effectively viscoelastic and the plume could induce large tpw we conclude that one plume convection pattern should have greatly influenced the rotation pole behaviour during early mars history around 4 gyr ago mars already could have experienced two tpw events lasting possibly only a few million years each we then compare our scenario with others already published in the literature

origin of lode gold deposits of the juneau gold belt southeastern alaska mesothermal gold bearing quartz veins along the juneau gold belt formed in the early tertiary from h 2 o co 2 n 2 h 2 s ch 4 rich low salinity fluids at temperatures of 200 to 325 c and pressures in excess of 1 1 5 kbar isotopically heavy ore forming fluids with 18 o values between 8 and 12 and d values of 20 to 30 indicate a deep seated fluid source the data are compatible with a model of ore fluid generation via metamorphic devolatilization within material subducted below the southern alaska continental margin prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the coast range megalineament and formed the gold bearing veins within brittle units at higher retrograding crustal levels deposition of gold was accomplished by several mechanisms including boiling fluid wall rock reactions and decreases in pressure and temperature of the hydrothermal fluids

true polar wander in mantle convection models with multiple mobile continents a b s t r a c t a r t i c l e i n f o the geologic record supports numerous instances during which continents apparently moved at speeds signi cantly faster than any of today s tectonic plates while the time dependence of convective driving forces likely explains some such observations rapid motions of large continents in particular are often attributed to true polar wander tpw in order to gauge the potential for connections between continents mantle temperature anomalies and polar motion we present the rst calculations of tpw derived from models that couple mantle convection with multiple mobile continents we nd that the aggregation and dispersal of supercontinents can lead to two types of tpw driven either by a well developed hot upwelling axis that creates a stable maximum moment of inertia or by the homogenization of mantle thermal structure following continent dispersal that leads to destabilization of the principal axis and possible large magnitude polar wander these supercontinent modulated thermal heterogeneities drive model tpw events as large as 90 at rates of up to 2 5 ma 1 such magnitudes and speeds are greater than those attained in similar models lacking continents but comparable to those for episodes inferred from paleomagnetic data for some large continents in the past published by elsevier b v on behalf of international association for gondwana research

anomalous late jurassic motion of the pacific plate with implications for true polar wander abstract true polar wander or tpw is the rotation of the entire mantle crust system about an equatorial axis that results in a coherent velocity contribution for all lithospheric plates one of the most recent candidate tpw events consists of a 30 rotation during late jurassic time 160 145 ma however existing paleomagnetic documentation of this event derives exclusively from continents which compose less than 50 of the earth s surface area and may not reflect motion of the entire mantle crust system additional paleopositional information from the pacific basin would significantly enhance coverage of the earth s surface and allow more rigorous testing for the occurrence of tpw we perform paleomagnetic analyses on core samples from ocean drilling program odp site 801b which were taken from the oldest available pacific crust to determine its paleolatitude during the late jurassic and early cretaceous 167 133 ma we find that the pacific plate underwent a steady southward drift of 0 49 0 74 my 1 except for an interval between kimmeridgian and tithonian time 157 147 ma during which it underwent northward motion at 1 45 0 76 my 1 1 this trajectory indicates that the plates of the pacific basin participated in the same large amplitude 30 rotation as continental lithosphere in the 160 145 ma interval such coherent motion of a large majority of the earth s surface strongly supports the occurrence of tpw suggesting that a combination of subducting slabs and rising mantle plumes was sufficient to significantly perturb the earth s inertia tensor in the late jurassic

the magellan seamount trail implications for cretaceous hotspot volcanism and absolute pacific plate motion abstract the magellan seamount trail mst delineates a northwest trending chain of four cretaceous guyots in the west pacific seamount province wpsp seamount morphology 40ar 39ar geochronology and sr nd pb geochemistry of the mst provides evidence for a hotspot origin between the samoa rarotonga and society hotspots of the south pacific isotopic and thermal anomaly sopita the mst yields an excellent linear age progression of 47 6 1 6 mm yr r2 1 000 mswd 0 23 1 se including vlinder guyot 95 1 0 5 ma n 5 2 sd pako guyot 91 3 0 3 ma n 3 and ioah guyot 87 1 0 3 ma n 2 the mst also exhibits a small range in sr nd pb isotopic compositions indicating enriched mantle sources with an affinity of emi nevertheless three volcanic events are found out of sequence with linear mst hotspot volcanism 1 an independent volcanic pedestal was formed 4 7 myr before shield volcanism started at vlinder guyot 2 a post erosional volcanic cone was formed at least 20 30 myr after drowning of vlinder guyot and 3 ita mai tai guyot 118 1 0 5 ma n 3 was formed 34 36 myr before the mst hotspot arrived at the predicted location of this guyot by identifying and ruling out discordant volcanic events we can use the age progression in mst to test the fixity of its hotspot when presuming the fixed hotspot hypothesis the local age progressions of the mst 47 6 1 6 mm yr and the copolar musicians seamount trail 55 8 6 4 mm yr are not compatible with their 100 80 ma euler pole we investigate two options 1 acceptance of a forced euler pole obeying the hotspot hypothesis by using both the age progressions and the azimuths of the studied seamount trails or 2 acceptance of a best fit euler pole by using the azimuths of the studied seamount trail exclusively in the first option the angular speed of the pacific plate during the 100 80 ma stage pole is calculated at 0 502 0 017 myr in the second option the best fit euler pole is found approximately 35 different from the forced euler pole we argue that the observed age progressions can only be reconciled with the best fit pole when allowing for the relative movement of the mst and musicians mantle plumes with respect to one another the calculated maximum velocity component parallel to the line of age progression could then be as much as 23 mm yr for the mantle plumes when assuming one fixed hotspot in this alternate model

polar wandering in mantle convection models we calculate polar motion in models of 3 d spherical mantle convection at rayleigh numbers up to 108 which include internal heating radial viscosity variations and an endothermic phase change isoviscous models yield rapid polar motion of order 3 myr but a factor of 30 increase in viscosity with depth reduces the rate of polar motion to about 0 5 myr due to stabilization of the large scale pattern of convection avalanching due to an endothermic phase change causes pulsating inertial interchange polar excursions of order 80 110 and of duration 20 70 myr a layered viscosity model with an endothermic phase change yields only one inertial interchange event in 600 million years these models show that the slow observed rate of post paleozoic true polar wander is not incompatible with higher rates inferred for earlier times

mantle convection with longest wavelength thermal heterogeneity in a 3 d spherical model degree one or two 1 the formation of longest wavelength mantle convection in the sluggish lid regime is investigated using a three dimensional spherical model the bottom rayleigh number is fixed at 107 considering temperature dependent rheology degree one dominant thermal convection occurs for both purely basal heating and mixed i e basal and internal heating modes for the purely basal heating mode degree one convection occurs when the viscosity contrast due to temperature dependent rheology is 103 104 in both boussinesq and extended boussinesq fluids however with extended boussinesq fluid degree one convection may only occur in the basal heating mode in the mixed heating mode degree one convection shifts to one with high degree modes presumably because of enhanced viscous dissipation in the highly viscous lid over up downwelling plumes the geophysically relevant degree two convection with sheet like downwellings is not observed in this study the inclusion of visco plastic rheology in the top part of the mantle breaks down degree one convection

scaling rates of true polar wander in convecting planets and moons abstract mass redistribution in the convecting mantle of a planet causes perturbations in its moment of inertia tensor conservation of angular momentum dictates that these perturbations change the direction of the rotation vector of the planet a process known as true polar wander tpw although the existence of tpw on earth is firmly established its rate and magnitude over geologic time scales remain controversial here we present scaling analyses and numerical simulations of tpw due to mantle convection over a range of parameter space relevant to planetary interiors for simple rotating convection we identify a set of dimensionless parameters that fully characterize true polar wander we use these parameters to define timescales for the growth of moment of inertia perturbations due to convection and for their relaxation due to true polar wander these timescales as well as the relative sizes of convective anomalies control the rate and magnitude of tpw this analysis also clarifies the nature of so called inertial interchange tpw events and relates them to a broader class of events that enable large and often rapid tpw we expect these events to have been more frequent in earth s past

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

palaeointensity determinations palaeodirections and magnetic properties of basalts from the emperor seamounts summary we measured palaeodirections and palaeointensities by the thellier method on 93 samples from three of the emperor seamounts 20 from detroit seamount 81 ma 48 from nintoku seamount 56 ma and 25 from koko seamount 48 ma reliable palaeodirections obtained from three lava flows on nintoku seamount give an average palaeolatitude of 32 7 which is different from the present day latitude of hawaii and supports the hypothesis of a moving hotspot according to the selection criteria traditionally used in palaeointensity determination 17 samples give a reliable result the samples show a very wide variety in unblocking temperatures revealing an important variation in titanium content and the oxidation state of titanomagnetites in order to assess the reliability of the palaeofield recording in the accepted samples we carried out measurements of saturation isothermal remanent magnetization at low temperature and thermomagnetic curves we found curie temperatures varying from 250 to 580 c not only between seamounts but even within one lava flow thermomagnetic curves enabled us to identify titanomaghemite in several lava flows after rejecting the results from samples showing evidence of maghemitization only four samples all from nintoku seamount can be considered as truly reliable the palaeointensity values range between 34 2 and 36 9 t the low virtual axial dipole moment vadm values calculated from the palaeofield values are consistent with the most reliable vadm estimates in this time range and they are very close to the average vadm in the 0 3 300 ma time interval

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

timing of mineralization and source of fluids in a slate belt auriferous vein system hill end goldfield nsw australia evidence from 40ar39ar dating and o and h isotopes abstract 40 ar 39 ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the hill end goldfield n s w australia has distinguished four major geological events including the timing of gold mineralization the earliest hydrothermal event occurred during the middle devonian tabberabberan orogeny 370 380 ma and resulted in the formation of quartz veins barren of au a second and major episode of vein emplacement occurred in the early carboniferous during the principal phase of metamorphism and deformation at 359 363 ma this was followed by au accumulation in two stages 1 after the major phases of quartz deposition and 2 during and after the development of conspicuous internal vein laminations 357 ma and 343 ma respectively two sources of fluid are proposed for vein and ore formation the first is a local metamorphic fluid characterized by 18 o h 2 o values of 8 9 to 12 5 per mil and d h 2 o values of 87 to 90 per mil the second is a mixed ore fluid with 18 o and d values in the range of 18 o h 2 o 8 4 to 11 per mil and d h 2 o of 49 to 36 per mil progressive entry of this second fluid sourced from trough fill or deeper crustal rocks is linked closely to cycles of gold precipitation at hill end

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

fluid evolution in a slate belt gold deposit a fluid inclusion study of the hill end goldfield nsw australia auriferous quartz veins in the hill end goldfield nsw australia comprise bedding parallel vein sets and minor extension and fault controlled veins which are hosted by a multiply deformed late silurian slate metagreywacke turbidite sequence fluid inclusions in quartz either from bedding parallel veins or from narrow steeply n dipping veins leader veins indicate a similar range in homogenisation temperatures th from 350 c to 110 c within this range th data demonstrate five groupings in the temperature intervals 350 280 c 280 250 c 250 190 c 190 150 c and 150 110 c corresponding to a variety of primary and secondary inclusions developed during five periods of vein growth under a generally declining temperature regime inclusion fluids are characterised by a low salinity of around 0 1 to 3 6 wt nacl equivalent laser raman microprobe inclusion analysis indicates that gas phase compositions relate to the paragenetic stage of the host quartz h2o g and n2 dominate in the primary inclusions from barren stage i quartz ch4 and ch4 h2o g are important in inclusions related to the early gold forming events equivalent to stages ii and iii quartz but inclusions developed during the last episode of gold deposition are characterised by h2o g co2 rich and liquid co2 bearing fluids precipitation of gold was aided by sulphidation reactions or phase separation in response to periods of vein opening late in the paragenesis gold deposition may have been promoted by oxidation of the ore fluid

time dependent convection models of mantle thermal structure constrained by seismic tomography and geodynamics implications for mantle plume dynamics and cmb heat flux summary one of the outstanding problems in modern geodynamics is the development of thermal convection models that are consistent with the present day flow dynamics in the earth s mantle in accord with seismic tomographic images of 3 d earth structure and that are also capable of providing a time dependent evolution of the mantle thermal structure that is as realistic earth like as possible a successful realization of this objective would provide a realistic model of 3 d mantle convection that has optimal consistency with a wide suite of seismic geodynamic and mineral physical constraints on mantle structure and thermodynamic properties to address this challenge we have constructed a time dependent compressible convection model in 3 d spherical geometry that is consistent with tomography based instantaneous flow dynamics using an updated and revised pseudo spectral numerical method the novel feature of our numerical solutions is that the equations of conservation of mass and momentum are solved only once in terms of spectral green s functions we initially focus on the theory and numerical methods employed to solve the equation of thermal energy conservation using the green s function solutions for the equation of motion with special attention placed on the numerical accuracy and stability of the convection solutions a particular concern is the verification of the global energy balance in the dissipative compressible mantle formulation we adopt such validation is essential because we then present geodynamically constrained convection solutions over billion year timescales starting from present day seismically constrained thermal images of the mantle the use of geodynamically constrained spectral green s functions facilitates the modelling of the dynamic impact on the mantle evolution of 1 depth dependent thermal conductivity profiles 2 extreme variations of viscosity over depth and 3 different surface boundary conditions in this case mobile surface plates and a rigid surface the thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature i e the geotherm in the mantle one important goal of this study is to obtain a steady state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the earth s evolution we obtain surface heat flux in the range of earth like values 37 tw for a rigid surface and 44 tw for a surface with tectonic plates coupled to the mantle flow also our convection simulations deliver cmb heat flux that is on the high end of previously estimated values namely 13 tw and 20 tw for rigid and plate like surface boundary conditions respectively we finally employ these two end member surface boundary conditions to explore the very long time scale evolution of convection over billion year time windows these billion year scale simulations will allow us to determine the extent to which a memory of the starting tomography based thermal structure is preserved and hence to explore the longevity of the structures in the present day mantle the two surface boundary conditions along with the geodynamically inferred radial viscosity profiles yield steady state convective flows that are dominated by long wavelengths throughout the lower mantle the rigid surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4 and the plate like surface condition yields a pattern dominated by degree 1 our exploration of the time dependence of the spatial heterogeneity shows that for both types of surface boundary condition deep mantle hot upwellings resolved in the present day tomography model are durable and stable features these deeply rooted mantle plumes show remarkable longevity over very long geological time spans mainly owing to the geodynamically inferred high viscosity in the lower mantle

stability of the rotation axis in high resolution mantle circulation models weak polar wander despite strong core heating 1 growing evidence points to a substantial heat flow across the core mantle boundary cmb but the rotational stability of strongly bottom heated mantle flow with prominent upwelling plumes is poorly known here we calculate polar motion for the past 100 myr induced in a new class of isochemical high resolution mantle circulation models mcms with earth like convective vigor and up to 12 tw core heat flux our mcms include internal heating and a simple three layer viscosity profile associated with the lithosphere 1023 pa s and the upper 1021 pa s and the lower mantle 1023 pa s separated at 100 and 650 km depth respectively a published mantle mineralogy model in the pyrolite composition consistent with our assumption of whole mantle flow allows us to relate thermal to density variations in a thermodynamically self consistent way all models yield modest polar motion on the order of 0 5 myr 1 or less in accordance with paleomagnetic data and agreeing with a number of studies that demonstrate the stabilizing effect of the rotational bulge although a substantially reduced lower mantle viscosity would increase this rate the good agreement between mcm and seismic mantle heterogeneity lends independent support for our viscosity profile as otherwise slabs in the mcm would rapidly sink to depth levels where they are tomographically not observed in general there is good agreement between the long wavelength geoids predicted from our mcms and recent satellite derived models of earth s geoid correlation coefficient of around 0 4 but noticeable differences at intermediate wavelengths for example in the western pacific and in africa suggest the use of gravity data to distinguish between competing plate reconstruction models

long term true polar wander of the earth including the effects of convective processes in the mantle and continental drift summary long term true polar wander tpw of the earth was examined by taking into account the effects of simplified convective processes in the earth s mantle and continental drift the tpw for a given viscoelastic earth model is wholly determined by both the magnitude of non forcing elements of moment of inertia i 11 i 22 and i 33 and i 12 element of product of inertia and the rates for forcing elements di 13 dt and di 23 dt the forcing rates are largely related to time dependent convective processes in the mantle and also continental drift in this study i examined the tpw on a convecting mantle with oscillating moments of inertia inferred from a convective process of alternating degree one and degree two structure changes of mantle convection by zhongetal 2007 in the phase for a relatively hydrostatic earth corresponding to largely degree one planform the predicted tpw is sensitive to the viscosity structure of the mantle particularly to the lithospheric viscosity structure and its magnitude may be larger than 30 even for the forcing rates with 10 30 kg m 2 myr 1 related to continental drift dickman 1979 in the phase for a non hydrostatic earth characterized by largely degree two planform such as for the present day however the tpw is less sensitive to the viscosity structure and its magnitude may be 10 at most these results may provide quantitative constraints on examining relationship between rheological structure and convective processes in the mantle continental drift and tpw

on the fate of mantle plumes at density interfaces abstract the motion of plumes through compositionally layered systems is investigated by laboratory experiments in order to understand the ascent modes of deep mantle plumes experimental plumes entrain the ambient heavier fluid and the average density of the plume head p t increases with time the degree of entrainment depends on the viscosity ratio of the matrix fluid matrix to the buoyant fluid buoyant and the mass flux two types of plume penetration through the density interface are possible the first which can be characterized as a pass through mode ptm exhibits relatively little entrainment here the whole plume head passes directly through the interface because p t is smaller than the density of the upper layer u in contrast in the second type of interaction named the rebirth mode rbm a large entrainment effect takes place the plume is now greatly disturbed as it passes through the interface between the two fluid layers and entrained material remains at the density interface since p t u in addition a new diapir emerges from the plume conduit and rises through the upper layer the difference between these two types of interaction is reflected in the spatial and the temporal characteristics of geochemical data obtained from mantle plume products such as ontong java plateau ojp

shrimp s isotope evidence for fluid mixing during gold mineralization in a slate belt gold deposit hill end new south wales australia abstract sensitive high resolution ion microprobe shrimp sulfur isotope data show that pyrrhotite pyrite galena and chalcopyrite in syntectonic auriferous quartz veins at the hill end goldfield are characterized by two populations with 34sfluid values of 7 and 0 these two populations are accounted for by fluid mixing involving two sources of sulfur during evolution of the vein systems a local sulfur source and reduced fluids derived from trough fill siluro devonian sedimentary and volcano sedimentary sequences are responsible for the first two gold bearing events whereas a more oxidized fluid and a sulfur source from turbidites and volcanics in deeper parts of the crustal sequences appear responsible for gold deposition late in the second gold forming event and are the dominant influences in the final gold forming stage for fine grained mixtures where mineral separation is difficult the shrimp analyses are particularly useful for tracing detailed 34s variations of sulfides precipitated in different paragenetic stages

true polar wander abstract a re evaluation of the existence of true polar wander tpw since the late cretaceous and a comparison among the various approaches are made using updated paleomagnetic hotspot and relative motion datasets previous attempts to determine the existence of tpw had resulted in different conclusions comparison of hotspot locations and paleomagnetic poles required significant pole motion although lithospheric plate displacement analysis yielded insignificant motion however these earlier determinations cannot be directly compared to find the reason for the discrepancies because each used different datasets for this study the different approaches are applied to a single updated model with three alternative relative motions of east and west antarctica although the results are model dependent in general there was not significant motion of the pole relative to the lithosphere 1 5 since the early tertiary but a large motion 10 12 relative to the hotspot framework it is unlikely that errors in the determinations could account for this disagreement the a95 of the plate reconstruction is about 3 the uncertainty in antarctica motion is estimated to no larger than 3 and cumulative errors in the relative plate motions may also amount to 3 only if all these errors are present in the maximum estimated amount and in the same direction could they account for the 10 12 gap between the two approaches this conclusion of pole motion relative to the hotspots but not the lithosphere may indicate an independent shift of the mesosphere relative to the lithosphere or mantle roll of hargraves and duncan

the rotational stability of a convecting earth the earth s figure and tpw over the last 100 myr summary palaeomagnetic records spanning the last 100 myr indicate that the reorientation of the earth s rotation axis relative to the surface geography or true polar wander tpw has been confined to a range less than 6 from its present location this limited tpw is unexpected given that a canonical theory for the rotational stability of the earth generally predicts that mantle convection should drive larger displacements of the pole we argue following earlier work thatthemutedtpwisaconsequenceofthestable excessflatteningoftheearth sfiguredriven by plate subduction and deep mantle superplumes rising beneath africa and the pacific in particular we show that the tpw record is consistent with convection induced perturbations to the earth s inertia tensor of order 20 per cent or less of the excess flattening over the last 100 myr this upper bound will be higher if the earth s lithosphere retains any significant elastic strength over such long timescales this inferred stability of the earth s figure has important implications for our understanding of deep mantle structure and the long term global scale evolution of the earth

the case for metamorphic base metal mineralization pyrite chemical cu and s isotope data from the cu zn deposit at kupferberg in bavaria germany the stratiform cu zn sulfide deposit at kupferberg in germany represents bavaria s largest historic base metal producer the deposit is hosted by early paleozoic volcano sedimentary strata at the margin of a high grade allochthonous metamorphic complex the present paper reports on the first cu and s isotope data as well as trace element analyses of pyrite from this unusual deposit the new data point to syn orogenic mineralization that was driven by metamorphic fluids during nappe emplacement primary cu ore occurs as texturally late chalcopyrite within stratiform laminated pyrite in black shale in two different tectonostratigraphic units of very low and low metamorphic grade respectively that were juxtaposed during the variscan orogeny trace element contents of different pyrite types suggest the presence of at least one hydrothermal pyrite generation mean co ni 35 with the other pyrite types being syn sedimentary early diagenetic mean co ni 3 7 copper isotope analyses yielded a narrow 65cu range of 0 26 to 0 36 for all ore types suggesting a hypogene origin for the principal chalcopyrite mineralization the ore lenses in the two different tectonostratigraphic units differ with regard to their 34s values but little difference exists between poorly and strongly mineralized domains within a given locality a genetic model is proposed in which syn sedimentary early diagenetic pyrite with subordinate chalcopyrite and sphalerite formed in black shale beds in the two different stratigraphic units followed by late tectonic strata internal hydrothermal mobilization of fe cu and zn during syn orogenic thrusting which concentrated especially cu to ore grade in agreement with this model cu distribution in stream sediments in this region shows distinct enrichments bound to the margin of the allochthonous complex thus kupferberg can be considered a rare example of a syn orogenic cu deposit with the cu probably being derived from syn sedimentary early diagenetic pyrite contained in early paleozoic shale units

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

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

upwelling plumes superswells and true polar wander summary the geological evolution of the rotational axis of the earth is most likely controlled by internal mass redistribution within the mantle palaeomagnetic observations suggest that it is episodic in nature with periods of quasi standstill alternating with periods of faster wander here we investigate two models for the influence of mantle plumes that vary at different spatial wavelengths on the time variations of the rotational axis true polar wander tpw in the first model we represent an upwelling plume as a sphere whose radius varies as a function of the flux of material in the conduit and that traverses the mantle at the stokes velocity such a plume produces very little wander of the rotational axis we then study the effects of two superswells that mimic the ones observed with seismic tomography and conclude that a doming regime within the mantle involves significant polar wander some of the features of this tpw that are directly linked to the periodicity of doming are reminiscent of observed phases of slow and fast tpw with similar peak velocities

polar wander of mars evidence from giant impact basins abstract we investigate the polar wander of mars in the last 4 2 ga we identify two sets of basins from the 20 giant impact basins reported by frey frey h 2008 geophys res lett 35 l13203 which trace great circles on mars and propose that the great circles were the prevailing equators of mars at the impact times monte carlo tests are conducted to demonstrate that the two sets of basins are most likely not created by random impacts also fitting 63 771 planes to randomly selected sets of 5 6 or 7 basins indicated that the identified two sets are unique we propose three different positions for the rotation pole of mars besides the present one accordingly tharsis bulge was initially formed at 50 n and moved toward the equator while rotating counterclockwise due to the influence of the two newly forming volcanic constructs alba patera and elysium rise the formation of the giant impact basins subsequent mass concentrations mascons in argyre isidis and utopia basins and surface masses of volcanic mountains such as ascraeus pavonis arsia and olympus caused further polar wander which rotated tharsis bulge clockwise to arrive at its present location the extensive polar motion of mars during 4 2 3 9 ga implies a weak lithosphere on a global scale deduced from a total of 72 000 polar wander models driven by tharsis bulge alba patera and elysium rise as the major mass perturbations different compensation states 0 100 are examined for each of the surface loads and nine different thicknesses are considered for an elastic lithosphere the lithosphere must have been very weak with an elastic thickness of less than 5 km if the polar wander was driven by these mass perturbations

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

palaeopoles on an expanding earth a comparison between synthetic and real data sets abstract using an ad absurdum reasoning applied to synthetic plates and associated palaeopoles it is possible to prove that if the earth s radius during the early cambrian had been half its present size palaeopoles of the continents or of their particular sub sets should show typical patterns these effects due to the variable curvature of the earth s surface suggest possible tests on the real situation using the available palaeomagnetic data if such effects could be detected on the real earth they could constitute a new clue favouring the hypothesis of an expanding earth the results of applying palaeopoles and ocean floor magnetic anomaly boundary conditions to palaeogeographical reconstructions for a variable radius earth are presented taking into consideration the real continental shapes

the geomagnetic field in permo triassic time summary a least squares fit of the 500 fathom or 1000 metre contour of the principal continental masses has been constructed which may approximate to the arrangement of the continents in permo triassic time reliable permotriassic palaeomagnetic data have been plotted on this reassembly mean pole positions for the larger fragments have been calculated and grand mean poles for permian and triassic time have been estimated to a first approximation results are consistent with a geocentric dipole field but small systematic departures have been detected these may be attributed to superimposed multipole components coaxial with a dipole field although other explanations such as anomalies originating in the crust cannot be completely excluded tests for the existence of multipole components are suggested the effect of such components on the accuracy of continental reassemblies based on palaeomagnetism is briefly noted

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

dynamic topography and long term sea level variations there is no such thing as a stable continental platform abstract globally uniform i e eustatic sea level trends with time scales approaching 100 myr have been inferred from both seismic and backstripping stratigraphic analysis at a small set of geographic sites that are presumed to lie on stable continental platforms and passive continental margins characterized by simple thermal subsidence histories e g the new jersey margin the western african margin we demonstrate using mantle flow simulations based on high resolution seismic tomography that both the new jersey margin and the conjugate western african margin have been subject to orders of 100 m of dynamic i e flow induced topography change over the last 30 myr we also show that the changing pattern of downwelling mantle flow associated with plate subduction is a significant contributor to the background eustatic sea level trend which is also of order 100 m during the 30 myr time window therefore late cenozoic variations of dynamic topography on these passive margin sites are comparable to the eustatic sea level changes and can partially mask the latter furthermore even if the observed trend could be accurately corrected for local dynamic topography variation the residual eustatic signal does not merely reflect changes in mean spreading rates at mid ocean ridges we conclude that the observed long term sea level variations at so called stable sites cannot be interpreted as eustatic moreover previous analyses that have used long term sea level trends as a proxy for spreading rates and geochemical fluxes must be revisited

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

a new global paleocene eocene apparent polar wandering path loop by stacking magnetostratigraphies correlations with high latitude climatic data a new apparent polar wander path apwp from the beginning of the paleocene 65 ma to the middle of the mid eocene 42 ma is shown to be correlated with polar climatic data of the same time period rather than applying the classical method based on analysis of site based poles we stacked the apwps obtained from magnetostratigraphies magnetostratigraphies have the advantage of displaying an unbroken record of local apwps through time and for a magnetozone defined as the a combination of normal and reversed polarity intervals the instantaneous poles are synchronous seven magnetostratigraphies located on 4 different plates covered sufficient time to be used in the analysis an average apwp was then determined with respect to age at the magnetozone level for the african plate which was arbitrarily chosen as a reference frame virtual geomagnetic poles were transferred onto the african plate using ocean kinematic euler rotations the calculated apwp is characterized by a loop with two main changes of direction at magnetozones 26 25 61 5 56 5 ma and 24 22 56 5 48 6 ma distinct at a 95 level of probability and indistinct poles related to magnetozones 29 27 65 5 61 5 ma and 21 19 48 6 40 6 ma we also show that the implied rapid shift of the lithosphere with respect to the geographic pole possibly an episode of true polar wander was coeval with the time evolution of vertebrate occurrence on ellesmere island canadian arctic and with the tree ring growth rate in western antarctica

mantle dynamics geoid inertia and tpw since 120 myr abstract we investigate the effect of internal masses redistributions on the position of the earth rotational pole for the last 120 myr we use a geodynamic model based on plate reconstructions that estimates the location and rate of subducted slabs under the assumption that they sink vertically into the mantle ricard et al 1993b our model also takes into account the effect of large scale upwellings domes derived from an analysis of seismic tomography their location is assumed to remain stable with time we then compute the geoid associated with the time dependent mantle density heterogeneities in order to reconcile the computed and observed geoids we investigate the influence of the depth down to which the subducted pacific plates beneath the americas present a significant density contrast with respect to the surrounding mantle on the present day geoid and propose a plate model in which we obtain a variance reduction greater than 0 9 for the degree 2 we show that the vertical oscillation of domes within mantle only modulates the amplitude of the associated geoid the temporal variation of the mantle density heterogeneities is consequently essentially due to changes in the subduction history the temporal evolution of the principal inertia axis pia of the earth derived from our model the rotational axis is aligned to the maximum pia is then investigated and finally compared to estimations of tpw both the maximum and intermediate pias have moved in a plane perpendicular to africa along a circle corresponding to the low of geoid induced by subduction around the pacific the minimum pia seems to be relatively stable since 120 myr and close to the maximum degree 2 geoid high under africa this can account for observed tpw or african apw in the last 200 myr

the 125 150 ma high resolution apparent polar wander path for adria from magnetostratigraphic sections in umbria marche northern apennines italy timing and duration of the global jurassic cretaceous hairpin turn a new high resolution apparent polar wander path apwp segment has been obtained from the magnetostratigraphy of four kimmeridgian to lower aptian sections in the northern apennines italy the use of paleomagnetic data for determination of the adria apwp was hampered by the large local rotations linked to apennine tectonics characterized by folds and thrusts developed during the neogene to overcome this problem we have computed relative rotations between time overlapping sections and realigned them in a common declination reference frame namely the bosso section we synthesized a new high resolution 150 to 125 ma apwp for adria which has a similar shape to the time equivalent segment of the synthetic apwp of africa of besse and courtillot j besse v courtillot apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr j geophys res 107 b11 2002 doi 10 1029 200jb000050 a 26 clockwise rotation of our combined adria apwp places it in almost perfect overlap with african data of same age confirming that the adria promontory moved coherently with africa during this time span whereas the counterclockwise rotation of adria with respect to africa was introduced later most probably during apennines orogenesis finally we discuss in relation with worldwide plate evolution the peculiar shape of our apwp which displays a hairpin turn during berriasian time and dates the main late jurassic early cretaceous change in plate motion at around anomaly m16

on detecting changes in the earth s radius summary estimates of the earth s radius in the geological past can be made from paleomagnetic evidence a method appropriate to the spherical environment of the data for dealing with this problem is given which is applied to devonian permian and triassic data from europe and siberia yielding estimated radii for these periods of 1 12 0 94 and 0 99 times the present radius respectively these estimates are not considered to be significantly different from the present radius egyed 1960 has suggested that paleomagnetic data could be used to find probable ancient radii for the earth the idea is that the mean direction of magnetization of rocks of the same geological age from widely separated collecting sites should be consistent with a geocentric dipole field the problem is of interest since it has been suggested on geological and cosmological grounds that the earth s radius has changed greatly in the geological past for example egyed 1956 1957 requires a change of z per cent since the paleozoic and carey 1958 a change of 45 per cent in the same time cox and doell 1961 have applied egyed s method of calculation to permian data from europe and siberia and gave an estimated permian radius of 0 991 times the present radius in this article a generalization of egyed s method of calculation is given which is applied to devonian and triassic data as well as permian data from this region into which some recent values not available to cox and doell have been incorporated yielding estimates of the ancient radii for these three geological periods not significantly different from the present radius triassic and carboniferous data from the north american continent have also been studied but as yet there are not enough data from this continent to yield a meaningful result the method given here like that of egyed is based on the assumptions that during any change in the earth s radius the size of each continent remains constant and that during the particular geological age being studied the position of the pole has not changed markedly together with the usual assumptions involved in paleomagnetic studies that the earth s field at the time in question may be represented by a geocentric dipole and that it is possible by a study of rocks from a given rock unit to determine the approximate direction of the earth s field at that place at a known time the set of relevant observations from a rock unit consists of four quantities which are average values for the rock unit as a whole the mean longitude and latitude a of the rock unit and the declination di and inclination it of the estimated

an explanation for earth s long term rotational stability paleomagnetic data show less than approximately 1000 kilometers of motion between the paleomagnetic and hotspot reference frames that is true polar wander during the past 100 million years which implies that earth s rotation axis has been very stable this long term rotational stability can be explained by the slow rate of change in the large scale pattern of plate tectonic motions during cenozoic and late mesozoic time provided that subducted lithosphere is a major component of the mantle density heterogeneity generated by convection therefore it is unnecessary to invoke other mechanisms such as sluggish readjustment of the rotational bulge to explain the observed low rate of true polar wander

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

paleo movement of continents since 300 ma mantle dynamics and large wander of the rotational pole abstract apparent polar wander apw is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large scale upwellings in the mantle we investigate plate motions during the last 410 million years in a reference frame where africa is fixed indeed africa has remained a central plate from which most continents diverged since the break up of pangea the exact amount of subduction is unknown prior to 120 ma we propose an approach based on one hand on the study of the past subduction volcanism to locate ancient subduction activity and on the other hand microplate motion history in the tethyan area derived from geology and paleomagnetism the peri pacific subductions seem to be a quasi permanent feature of the earth s history at least since the paleozoic with however localized interruptions the tethyan subductions have a complex history with successive collisions of continental blocs hercynian indo sinian alpine and himalayan and episodical rebirth of e w subduction trending zones assuming that subducted slabs sink vertically into the mantle and taking into account large scale upwellings derived from present day tomography and intra plate volcanism in the past we compute the time variation of mantle density heterogeneities since 280 ma due to conservation of the angular momentum of the earth the temporal evolution of the rotational axis is computed in a mantle reference frame where the africa plate is fixed and compared to the apparent polar wander apw observed by paleomagnetism since 280 ma we find that a major trend of both paleomagnetic and computed apw are successive oscillatory clockwise or counter clockwise motions with tracks separated by abrupt cusps around 230 ma 190 ma and 140 110 ma we find that cusps result from earlier major geodynamic events the 230 ma cusp is related to the end of active subduction due to the closure of the rheic ocean basin after the hercynian continental collision 340 300 ma and to renewed subduction zone west of laurentia whereas the 190 ma cusp results from the indo sinian collision 270 230 ma and the subsequent end of the neo tethys ocean subduction

mantle stratification and long term polar wander there is accumulating evidence1 2 that the average location of the earth s magnetic pole has moved relative to the deep mantle over the past 200 myr this phenomenon of true polar wander may be caused by the advance and retreat of ice sheets3 or by mass redistribution in the earth s interior due to changes in the pattern of mantle convection4 5 new analyses1 2 6 of polar wander data show a significant shift of the pole in the late cretaceous but this period is thought to have been too warm for glaciation to have occurred thus most of the true polar wander must be due to mass movements in the mantle here we show by analysing the appropriate equations for polar wander that both viscosity and chemical stratification in the mantle are important in determining the rate of polar wander the dynamical effects of chemical stratification and high viscosity 1023 poise in the lower mantle are to decrease polar wander speed to a level consistent with the averaged velocities inferred from palaeomagnetic data whole mantle convection models without non adiabatic density jumps at 670 km depth and with viscosities less than 1023 p would produce wandering rates in excess of 5 per myr

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

the spin and inertia of venus one of the most remarkable rotational features of venus is the presence of a relatively large offset between spin and maximum inertia axis according to the most recent gravity field models this offset may amount to 0 5 considerably larger than the one which characterizes the earth s chandler wobble 0 3 at present it is not clear whether the offset of venus is the result of the limited resolution of the gravity models or alternatively it may be regarded as an evidence in support to the existence of a wobble purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time scales of mantle convection due to the peculiar rotational characteristics of venus we also show that the inertia perturbation associated to the rising of hot thermal plumes from the core mantle boundary to the surface is large enough to maintain offsets ranging between 0 1 and 1 of course this does not preclude that other processes such as venusquakes or periodic atmospheric variabilities may contribute significantly to the observed offset on shorter time scales

changes of the earth s rotation axis owing to advection of mantle density heterogeneities polar wander the secular motion of the earth s rotation axis relative to its surface has been studied for many years dynamical arguments1 3 show that polar wander can arise from the redistribution of mass in a plastic deformable earth the rate depending on both the rate of mass redistribution and the rate at which the earth s rotational bulge can readjust to the changing rotation axis here we use a viscosity structure obtained through geoid modelling4 a mantle flow field consistent with tomographic anomalies5 and time dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree two geoid during the cenozoic era we show that the rotation axis will follow closely any imposed changes of the axis of maximum non hydrostatic moment of inertia the resulting path of the rotation axis agrees well with palaeomagnetic results7 with the model predicting a current rate of polar motion that explains 40 of that observed geodetically8

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

development of diapiric structures in the upper mantle due to phase transitions solid state phase transitions in time dependent mantle convection can induce diapiric flows in the upper mantle when a deep mantle plume rises toward phase boundaries in the upper mantle the changes in the local thermal buoyancy local heat capacity and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir this mechanism may explain episodic hot spot volcanism the nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes allowing hot plumes to go through and retarding the tepid ones

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

rotational instabilities of the earth induced by mantle density anomalies from the seismic tomographic models the authors estimate the offset between the geographic pole and the main inertial axis this offset which is large for a static earth is drastically reduced when the dynamical behavior of the mantle is taken into account for a mantle stratification with a viscosity increase by a factor of about 50 this offset is comparable to what can be estimated on the basis of remanent post glacial depression the authors also discuss the rate of polar drift induced by both internal mass heterogeneities and pleistocenic deglaciation contributions from internal sources and ice sheets disintegration are consistent with the observed secular trend for high viscosity contrasts the results are quite insensitive to the phase transition of chemical nature of the upper lower mantle interface this study shows that there is no contradiction between the tomographic models of the earth and their implications for the rotational stability of our planet the findings also agree with recent analyses of paleomagnetic data

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