comparison between the hot spot and geomagnetic field reference frames hot spots are located on the earth s surface and are caused by hot plumes of upwelling mantle material they manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume it has been suggested1 3 that hot spots are relatively stationary with respect to each other and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the earth another frame of reference can be partially defined by the position of the mean magnetic dipole field of the earth as the dipolar field is believed to be closely related to the spin axis this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates but since the position of the ancient dipole field of the earth does not give any information about palaeolongitude and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest hot spot reference frames have been favoured by many people we now report a comparison of the two frames of reference and show that over the past 200 myr there has been relative movement between the two frames of reference of about 20 and that this motion has not been constant with time

polar wandering and paleomagnetism an important hypothesis of paleomagnetism is that apparent polar wander the motion of the paleomagnetic pole relative to a continent or plate is caused entirely by plate motion relative to the mesosphere which is the relatively stronger and slowly deforming mantle beneath the astheno sphere recent research has challenged this hypothesis and suggests that polar wandering i e true polar wandering affects apparent polar wander apw paths the debate about polar wandering has been revived by studies comparing paleomagnetic poles to plate motion relative to hotspots morgan 1 98 1 what makes the new work interesting is that the under lying hotspot model makes many predictions that are testable by available and obtainable data one prediction that the hawaiian hotspot has shifted southward over cenozoic time has been severely tested and confirmed kana et al 1 978 kana 1 980 gordon cape 1 98 1 gordon 1 982 recent tests have mainly but not entirely taken the form of a reexamination of existing data in the light of the new model future tests can and should be made however by obtaining high quality data from paleomagnetic study of critical time intervals already identified by analysis of existing data this paper discusses what polar wandering is and reviews how the available paleomagnetic data and plate reconstructions have been used to investigate past polar wandering implications of work to date possible causes of polar wandering and promising directions of future research are also discussed

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

true polar wander and plate driving forces a net torque on the global lithosphere can be exerted both by ridge push and trench pull forces a net ridge push torque is present if there is an asymmetry in the age distribution of seafloor about one or more ocean ridges a net torque due to trench forces is present if the forces per trench length acting on the subducting and overthrust plates are not equal in magnitude the combined net torque on the lithosphere contributed by ridge and trench forces must be balanced by an opposing torque most likely due to basal shear tractions associated with a global rotation of the lithosphere relative to the underlying mantle such a rotation should be identifiable as true polar wander the lack of significant rotation of the lithosphere with respect to the magnetic dipole axis during the cenozoic therefore places a strong constraint on the nature of torques acting on the lithosphere we suggest that the most likely explanation for negligible true polar wander in the cenozoic is that the net ridge and trench torques nearly cancel one another the net ridge and trench torque vectors are in nearly opposite directions and have been so through the cenozoic so the condition of near cancellation amounts to a restriction on the magnitude of the force imbalance along trench boundaries such a near cancellation of net torques and the resulting negligible true polar wander are likely to be consequences of present plate geometries and need not be representative of earlier eras

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

mantle flow in the rivera cocos subduction zone summary western mexico where the young and small rivera plate and the adjacent large cocos plate are subducting beneath the north american plate is a unique region on earth where tearing of subducting oceanic plates as well as fragmentation of the overriding continental plate is occurring today characterizing the mantle flow field that accompanies the subduction of the rivera and adjacent cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary here we report observations of seismic anisotropy as manifested by shear wave splitting derived from local s and teleseismic sks data collected by the mapping rivera subduction zone array that was deployed from 2006 january to 2007 june in southwestern mexico and from data collected by two of mexico s servicio sismologico nacional stations sks and local s wave splitting parameters indicate that the fast directions of the split sks waves for stations that lie on the central and southern jalisco block are approximately trench normal following the convergence direction between the rivera plate and jalisco block s wave splitting from slab events show a small averaged delay time of 0 2 s for the upper 60 km of the crust and mantle therefore the main source of anisotropy must reside in the entrained mantle below the young and thin rivera plate trench oblique fast sks split directions are observed in the western edge of the rivera plate and the western parts of the cocos slab the curved pattern of fast sks split directions in the western jalisco block and beneath the rivera cocos slab gap indicates 3 d toroidal mantle flow around the northwestern edge of the rivera slab and the rivera cocos gap which profoundly affect the finite strain field in the northwestern edge of the rivera slab and the mantle wedge both the tomographic images and shear wave splitting results support the idea that the rivera and western cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the colima rift is intimately related to the tearing between the rivera and cocos plates

hotspot spin axis motion or magnetic far sided effect palaeomagnetic data from central pacific sediments and from the hawaiian emperor volcanic chain indicate that during the tertiary either the derived magnetic pole was located alternately on the far and near side of the geographical pole or that the hotspot coordinate systems thought to be fixed relative to the earth s mantle oscillated relative to the earth s spin axis

late mesozoic and cenozoic palaeomagnetism of australia ii implications for geomagnetism and true polar wander summary new palaeomagnetic results from australia indicate that throughout the cenozoic era the continent lay further south than suggested by hot spot data moreover while hot spots give a uniform rate of drift during most of the cenozoic the drift rate obtained from apparent polar wander varies considerably the discrepancies between the palaeomagnetic and hot spot results are analysed by comparing the australian data with those of europe and the central pacific the analysis suggests that the discrepancies are due to 1 departures of the earth s magnetic field from the geocentric axial dipole model and 2 either true polar wander or a non axial inclined dipole component it is found that since the mid tertiary the dominant non dipole component has been a quadrupole and that during this period both the quadrupolar field and the true polar displacement non axial dipole component decreased progressively during the quaternary and also at the earliest tertiary the non dipole components appear to have been moderate or small the comparison of data sets demonstrates that considerable errors may be incurred when cenozoic and presumably earlier poles from one geographic region are used to derive those of another widely separated region the results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors and that hot spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes finally the new early tertiary pole for australia is used in conjunction with updated early tertiary poles from other lithospheric plates to reapply the mckenzie test for true polar wander the results indicate a small true polar displacement since the beginning of the tertiary the amount and direction of the displacement however differ from those generally obtained from hot spot data

plate motions and the geomagnetic field i quaternary and late tertiary summary we have tested the hypothesis that the apparent increase in the northward offset of the axial dipole i e the ratio g gy with age for the last 25 ma wilson mcelhinny is due to the failure to correct for plate motions spherical harmonic analyses were performed on two types of data palaeomagnetic poles from continents islands and seamounts and magnetic inclinations from deep sea cores after returning the sampling sites to their predrift locations in fixed hotspots coordinates the results show that the quadrupole term g maintained a value of about 0 05 gy throughout the last 35 ma and that the axial octupole had a small value of about 0 02 gy for the last 5 ma sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years independent data sets for the past 2 ma and for the period 2 6 ma gave nearly identical solutions showing that the persistence of the axial terms is not a phenomenon of the more densely sampled quaternary field alone nor is it an artifice of predominantly northward motion of the plates returning all 0 5 ma data to a 5 ma reconstruction failed to eliminate the quadrupole the non zonal coefficients for the 0 5 ma field are typically an order of magnitude less than the zonal terms with the exception of h and are probably insignificant suggesting that longitudinal drift is generally effective in averaging out these components the present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 ma but there is evidence that g may have changed little during the last 35 ma in addition there has been very little relative motion between the palaeomagnetic dipole axis as defined by the first degree terms and the axis of the hotspots reference frame

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

cenozoic latitudinal shift of the hawaiian hotspot and its implications for true polar wander abstract pacific plate equatorial sediment facies provide estimates of the northward motion of the pacific plate that are independent of paleomagnetic data and hotspot tracks analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the hawaiian emperor chain the discrepancy is largest 60 70 ma b p the 60 to 70 ma equatorial sediment facies data agree with recent paleomagnetic results from deep sea drilling on suiko seamount 1 and from a northern pacific piston core 2 equatorial sediment facies data and paleomagnetic data combined with k ar age dates along the emperor chain 3 indicate a position of the spin axis at 65 ma b p of 82 n 205 e in the reference frame in which the pacific ocean hotspots are fixed this pole agrees well with the position of the spin axis in the reference frame in which the atlantic ocean hotspots and the indian ocean hotspots are fixed 4 5 supporting the joint hypotheses that 1 the pacific ocean hotspots are fixed with respect to the hotspots in other oceans 2 the hotspots have shifted coherently with respect to the spin axis and 3 the time average of the earth s magnetic field 65 ma b p was an axial geocentric dipole global neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the neogene in the same direction as the shift between 65 ma b p and the present all data are consistent with a model in which the hotspots and by inference the mantle have shifted with respect to the spin axis about a fixed euler pole at a constant rate of rotation for the last 65 ma

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

late cretaceous apparent polar wander of the pacific plate evidence for a rapid shift of the pacific hotspots with respect to the spin axis paleomagnetic data from the pacific plate show that the average azimuth of the pacific plate decreased by 12 between 90 ma and 81 ma whereas trends of late cretaceous seamount chains suggest that the pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval this difference is used to infer a 13 4 shift of the spin axis relative to the hotspots in the pacific ocean basin this shift occurred within 5 m y of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34 this coincidence in timing suggests there may be a causal link between these three phenomena

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

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

ice cascade growth and decay a thermodynamic approach the ice volume evolution of a frozen waterfall or ice cascade was studied using a thermodynamic model the model was developed from meteorological data collected in the vicinity of the waterfall and validated from ice volume measurements estimated from terrestrial lidar images the ice cascade forms over a 45 m high rock wall located in northern gaspesie quebec canada two stages of formation were identified during the first stage the growth is mainly controlled by air convection around the flowing and free falling water the ice cascade growth rate increases with decreasing air temperature below 0 c and when the water flow reaches its lowest level during the second stage the ice cascade covers the entire rock wall surface water flow is isolated from the outside environment and ice volume increases asymptotically heat is evacuated from the water flow through the ice cover by conduction the growth is controlled mainly by the conductive heat loss through the ice cover but also by the longwave radiation emitted at the ice surface during the night in spring melting of the ice cascade is dependent on the air convection over the ice surface but also on the sensible heat carried by the increasing water flow and the solar radiation received during the day

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

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

mantle flow and plate motions summary new data on mantle viscosity variation and plate motions suggest that the author s previous theory for the forces driving and resisting plate tectonics needs revision the flow in the mantle associated with plate movements probably pervades the whole mantle instead of being almost entirely in a thin asthenosphere at the top of it the first task undertaken in this paper is a calculation of the viscous torque exerted on a hemispherical plate by a vertical point force in the mantle directly beneath the plate boundary this prototype problem is simple enough even with asthenosphere and mesosphere assigned different viscosities to allow the computing to be done with a large number of terms of the relevant spherical harmonic series it shows that forces in the lithosphere and asthenosphere are the major source of torques on plates especially if the asthenosphere viscosity is low and that truncating the series at a pre assigned degree can seriously underestimate the effect of a force in the asthenosphere these results are used as a guide to the approximations to adopt for more realistic boundary conditions where accurate computing by the same method as the prototype problem would be too time consuming to do the second task is estimating the torques actually driving and resisting the motion of the earth s plates it includes a recalculation of the ridge push driving mechanism which gives a zero resultant torque about the centre of the earth in agreement with archimedes principle but contrary to a recent conclusion of davis solomon the recalculation does not affect slab pull which still has a non zero resultant balanced mainly by a non zero mean angular velocity of the plate system relative to the mean mesosphere in which the hot spots are assumed in this work to lie this angular velocity agrees well with observational data and gives a test of the approximations on data not used in deriving them misfits between driving and resisting torques are found for each plate they are smaller for the present method than its best fitting competitor conclusions common to both are that the slab pull driving force must be appropriately weighted for age and speed and that the most important resisting

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

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

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