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

linking the bitter springs carbon isotope anomaly and early neoproterozoic oxygenation through i ca mg ratios abstract the ca 810 ma bitter springs carbon isotope anomaly is an abrupt and long lived 5 10 myr departure from the positive carbon isotope values 13 c carb that otherwise characterize early middle neoproterozoic carbonate rocks the onset of this isotope anomaly is manifested by a globally expressed 8 negative shift in 13 c values in shallow marine carbonate strata given increasing evidence that metazoan diversification began well before the ediacaran period the bitter springs anomaly is a logical interval to explore the potential relationship between biological innovation perturbations to the carbon cycle and oxygenation the iodine to calcium magnesium ratio i ca mg in marine carbonates is a sensitive proxy for seawater redox conditions that is increasingly being applied to reconstruct the oxygenation of the global surface ocean through earth s history we report i ca mg ratios along with carbon and oxygen isotope ratios and major and minor element concentrations from carbonate sections spanning the bitter springs anomaly in svalbard east greenland and the mackenzie mountains northwestern canada the results from svalbard and greenland collectively show two prominent features the early stage of the bitter springs anomaly is characterized by a negative 13 c shift coupled to low i ca mg ratios compared to carbonates pre and post dating the bitter springs anomaly the last stage of the bitter springs anomaly displays a positive excursion in i ca mg with the highest values at 7 mol mol yet documented in rocks older than 580 ma in contrast carbonates from the mackenzie mountains are uniformly low and display no variation in i ca mg across the bitter springs anomaly which we interpret to be the consequence of alteration of primary signatures during diagenesis the observed geochemical variations in our svalbard and greenland datasets are interpreted to be the result of a shift from pre bitter springs ocean conditions defined by well oxygenated surface waters and anoxic ferruginous bottom waters to syn bitter springs ocean conditions characterized by expanded euxinia

steady plumes in viscously stratified vigorously convecting three dimensional numerical mantle convection models with mobile plates 1 we present results from three dimensional 3 d cartesian geometry computations featuring vigorous high rayleigh number mantle convection in a system incorporating stiff tectonic plates with dynamically determined time dependent velocities we track plume related hot spot motion while calculating associated synthetic hot spot tracks allowing us to estimate plume migration rates relative to plate motion rates plate like surface motion is achieved explicitly by specifying both the plate geometry and rigidity however plate velocities evolve dynamically in response to the buoyancy distribution within the convecting system including buoyancy within the plate itself we find that convection is characterized by downwelling sheets and varying numbers of 3 d upwelling structures when the lower mantle to upper mantle viscosity ratio is varied from 9 to 90 as the lower mantle viscosity is increased relative to the upper mantle viscosity the upwellings in the calculations evolve into vigorous active plumes characterized by long narrow thermal conduits with broad disk like heads the total number shape and persistence of the plumes are affected by the specified viscosity stratification in the calculations our findings indicate that hot spots associated with mantle plumes drift by smaller distances in comparable amounts of time when the lower mantle viscosity is increased relative to the upper mantle viscosity in cases with a high lower mantle to upper mantle viscosity ratio the depth of the flow aligned with plate motion is diminished and the plate scale return flow is integrated into a deep unorganized sluggish layer as a result plume conduit morphology in the lower mantle of such calculations is less influenced by plate motion than in calculations with a small difference between upper mantle and lower mantle viscosity for lower mantle viscosities 30 times and 90 times greater than the upper mantle viscosity the lower mantle velocity field is dominated locally by active plumes anchored in the lower thermal boundary layer of the convecting system and plume migration rates are typically only about 10 of the overriding plate velocity in such cases we also find that plume related hot spots persist for periods that we estimate to be between 0 55 and 1 1 plume transit times

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

dynamic mantle density heterogeneities and global geodetic observables summary we investigate the influence of mantle dynamics on low degree deformations of the earth at geological timescale we first compute surface deformations and discuss the analytical form of the tangential surface displacement induced by internal loads in a reference frame related to the centre of mass of the planet we use the theoretical love numbers formalism since the earth has a viscoelastic behaviour at geological timescale then we quantify degree one and degree two deformations induced by upwelling domes and subducted plates sinking into the mantle we use a simple model in which the slabs are modelled as blobs diving vertically through the mantle and in which the domes are assumed to be stable over the last 120 ma their location is modelled from seismic tomography within the lower mantle the temporal evolutions of the j2 gravitational potential coefficient and of the geocentre motion are plotted since 120 ma we find that 1 the mantle density heterogeneities within the mantle can explain the present day non hydrostatic flattening of the earth however they vary at a too slow timescale to significantly perturb the coefficient 2 although there is a significant discrepancy of about a few hundred metres between the centre of figure and the centre of mass of the earth the secular variation of the geocentre motion is one order of magnitude smaller than the one induced by surface loads

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

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

mantle viscosity inferences from joint inversions of pleistocene deglaciatio induced changes in geopotential with a new slr analysis and polar wander joint inversions of post glacial rebound induced secular changes in low degree harmonics of the geopotential and true polar wander tpw systematically indicate that the upper mantle viscosity has a value that is considerably smaller than 10 21 pas the lower mantle viscosity is about one order of magnitude larger than the upper mantle viscosity discrepancies in the viscosity profiles inferred from even and odd zonals are likely an indication of recent unmodeled glacial forcings and or insufficient precision in the evaluation of the mass loss from pleistocene ice sheets the influence of systematic errors in slr data reduction due to thermal drag on the lageos satellites for the uneven zonals of the geopotential is avoided by making use of the starlette and stella satellites to recover the uneven zonals and by considering lumped coefficients for these uneven terms

the key role of global solid earth processes in preconditioning greenland s glaciation since the pliocene after 500 ma of absence major northern hemisphere glaciations appeared during the plio pleistocene with greenland leading other northern areas here we propose that three major solid earth processes underpinned build up of the greenland ice sheet first a mantle plume pulse responsible for the north atlantic igneous province at 60 ma regionally thinned the lithosphere younger plume pulses led to uplift which accelerated at 5 ma lifting the parts of the east greenland margin closest to iceland to elevations of more than 3 km above sea level second plate tectonic reconstruction shows a 6 northward component of greenland motion relative to the mantle since 60 ma third a concurrent northward rotation of the entire mantle and crust towards the pole dubbed true polar wander tpw contributed an additional 12 change in latitude these global geodynamic processes preconditioned greenland to sustain long term glaciation emphasizing the role of solid earth processes in driving long term global climatic transitions

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

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

low viscosity of the bottom of the earth s mantle inferred from the analysis of chandler wobble and tidal deformation abstract viscosity of the d layer of the earth s mantle the lowermost layer in the earth s mantle controls a number of geodynamic processes but a robust estimate of its viscosity has been hampered by the lack of relevant observations a commonly used analysis of geophysical signals in terms of heterogeneity in seismic wave velocities suffers from major uncertainties in the velocity to density conversion factor and the glacial rebound observations have little sensitivity to the d layer viscosity we show that the decay of chandler wobble and semi diurnal to 18 6 years tidal deformation combined with the constraints from the postglacial isostatic adjustment observations suggest that the effective viscosity in the bottom 300 km layer is 1019 1020 pa s and also the effective viscosity of the bottom part of the d layer 100 km thickness is less than 1018 pa s such a viscosity structure of the d layer would be a natural consequence of a steep temperature gradient in the d layer and will facilitate small scale convection and melt segregation in the d layer

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

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

palaeomagnetism of the loch doon granite complex southern uplands of scotland the late caledonian palaeomagnetic record and an early devonian episode of true polar wander abstract the southern uplands terrane is an ordovician silurian back arc foreland basin emplaced at the northern margin of the iapetus ocean and intruded by granite complexes including loch doon 408 3 1 5 ma during early devonian times protracted cooling of this 130 km 3 intrusion recorded magnetic remanence comprising a predominant a magnetisation linked to initial cooling with dual polarity and mean direction d i 237 64 95 4 palaeopole at 316 e 21 n subsidiary magnetisations include mesozoic remanence correlating with extensional tectonism in the adjoining irish sea basin b d i 234 59 and minority populations c d i 106 2 and d d i 199 1 recording emplacement of younger 395 ma granites in adjoining terranes and the variscan orogenic event the a directions have an arcuate distribution identifying anticlockwise rotation during cooling a comparable rotation is identified in the orthotectonic caledonides to the north and the paratectonic caledonides to the south following closure of iapetus continental motion from midsoutherly latitudes 40 s at 408 ma to equatorial palaeolatitudes by 395 ma is identified and implies minimum rates of continental movement between 430 and 390 ma of 30 70 cm year more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander silurian devonian palaeomagnetic data from the british scandinavian caledonides define a 430 385 ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from gondwana they reconcile pre 430 ma and post 380 ma apw from this supercontinent and show that laurentia baltica avalonia lay to the west of south america with a relict rheic ocean opening to the north which closed to produce variscan orogeny by a combination of pivotal closure and right lateral transpression

global tectonics under g having a minute westward tilt the convection seen to affect moon s mantle takes place under gravity having a component of asymmetry in the form of a minute westward slope or tilt constituting the external part of the field its effect is explored employing the equivalent potential denominated its nature is to bias convection of internal origin inducing east west polarization of a type prominent in global tectonics separately research at harvard has shown that by causing advection of non hydrostatic masses within the heterogeneous mantle deep seated convection inevitably displaces the axis of maximum moment compelling polar wander a reconstruction based on observed dissipation suggests that the combined effect of these factors is to contribute a not inconsiderable low latitude surface west component to the structure of the convection cumulative alteration in the direction west must eventually require global rearrangement of its cell structure offering explanation of the abrupt mesozoic tertiary transition and progressive net lithosphere rotation

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

reanalysis of ancient eclipse astronomic and geodetic data a possible route to resolving the enigma of global sea level rise predictions of the earth s response to the ice age appear to simultaneously reconcile a set of astronomical geodetic and ancient eclipse observations related to changes in rotation thus ruling out ice melting as a major contributor to 20th century sea level rise we demonstrate that the reconciliation disappears when an improved theory of rotational stability is applied furthermore our reanalysis of longer satellite records renders previous estimates of the secular change in rotation rate suspect the updated ice age predictions and observations permit an anomalous 20th century ice flux of 1 mm yr equivalent sea level rise thus the full suite of earth rotation observations are consistent with a connection between climatic warming and recent melting of ice reservoirs

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

quaternary glacial cycles earth s rotation and twentieth century global sea level rise perturbations of earth s rotation and tide gauge data with information for relative sea level during the past 140 200 years were analyzed to examine the rates and causes of twentieth century global sea level rise relative sea levels based on tide gauge data are sensitive to both the meltings of the polar ice caps and mountain glaciers and thermal expansion of seawater associated with the global warming changes in earth s rotation are however insensitive to the thermal expansion i compared the observations with predictions due to the meltings of both polar ice caps and examined the causes and rates of twentieth century global sea level rise by considering the sensitivities of these observables the present study suggests that the meltings of the greenland ice sheet and antarctic ice sheet around the weddell sea have equally contributed to the global sea level rise of 0 5mm yr the rates of the global sea level rise may be larger than 1 3mm yr it is however impossible to determine the components of the thermal expansion of seawater and melting of the mountain glaciers independently

effect of the viscoelastic lithosphere on polar wander speed caused by the late pleistocene glacial cycles previous studies of the wander of the rotation pole associated with the late pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity the thickness of the elastic lithosphere and the lower mantle viscosity in particular the m1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities in this study we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible maxwell viscoelastic earth models model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the m1 mode is similar to those associated with the density discontinuity at the core mantle boundary c0 mode and the lithosphere l0 mode we speculate that this is due to the interaction between the m1 mode and the transient mode associated with the viscoelastic lithosphere which reduces the magnitude of polar wander rates therefore the m1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity in this case predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an inverted parabola as predicted for the j 2 curve we caution however that these results are obtained for simplified earth models and the results for seismological earth models such as prem may be complicated by the interaction between the m1 mode and the large set of transient modes

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

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

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

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

reconstructing the cenozoic evolution of the mantle implications for mantle plume dynamics under the pacific and indian plates article history the lack of knowledge of the initial thermal state of the mantle in the geological past is an outstanding problem in mantle convection the resolution of this problem also requires the modelling of 3 d mantle evolution that yields maximum consistency with a wide suite of geophysical constraints quantifying the robustness of the reconstructed thermal evolution is another major concern to solve and estimate the robustness of the time reversed inverse problem of mantle convection we analyse two different numerical techniques the quasi reversible qrv and the backward advection bad methods our investigation extends over the 65 myr interval encompassing the cenozoic era using a pseudo spectral solution for compressible flow thermal convection in 3 d spherical geometry we find that the two dominant issues for solving the inverse problem of mantle convection are the choice of horizontally averaged temperature i e geotherm and mechanical surface boundary conditions we find in particular that the inclusion of thermal boundary layers that yield earth like heat flux at the top and bottom of the mantle has a critical impact on the reconstruction of mantle evolution we have developed a new regularisation scheme for the qrv method using a time dependent regularisation function this revised implementation of the qrv method delivers time dependent reconstructions of mantle heterogeneity that reveal 1 the stability of pacific and african large low shear velocity provinces llsvp over the last 65 myr 2 strong upward deflections of the cmb topography at 65 ma beneath the north atlantic the south central pacific the east pacific rise epr and the eastern antarctica 3 an anchored deep mantle plume ascending directly under the epr easter and pitcairn hotspots throughout the cenozoic era and 4 the appearance of the transient reunion plume head beneath the western edge of the deccan plateau at 65 ma our reconstructions of cenozoic mantle evolution thus suggest that mantle plumes play a key role in driving surface tectonic processes and large scale volcanism 2014 elsevier b v all rights reserved

palaeomagnetic study of the cairnsmoor of fleet granite and criffel dalbeattie granodiorite contact aureoles caledonian tectonics of the southern uplands of scotland and devonian palaeogeography the plutons of cairnsmoor of fleet 392 2 ma and criffel dalbeattie 397 2 ma both mineral isochron ages comprise two of four major post tectonic granitic complexes emplaced into the southern uplands an ordovician silurian back arc and foreland basin complex formed at the northern margin of the iapetus suture to expand the palaeomagnetic record of the southern uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks a uniform anisotropy of magnetic susceptibility ams fabric across the cairnsmoor of fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late acadian regional stress operative during or soon after emplacement of the pluton in middle devonian times magnetization during slow cooling recorded a dual polarity a remanence in granite and hornfels with mean direction d i 92 2 95 6 5 yielding a palaeopole q 6 at 2 n 265 e linked to cooling at c 392 ma subsidiary magnetizations are overprints imparted during variscan tectonism b d i 194 6 and jurassic rifting within the adjoining irish sea basin c c 160 140 ma d i 172 52 the criffel dalbeattie pluton has more complex ams fabrics recording both deformation and emplacement effects hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole which is otherwise dominated by paramagnetism the granodiorite is more strongly magnetized than the country rocks accounting for a positive aeromagnetic anomaly a fairly dispersed dual polarity remanence mean d i 115 55 95 18 in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of britain into southerly palaeolatitudes at c 410 ma and indicates an early devonian emplacement age the palaeofield at c 397 ma the currently accepted isotopic age is recorded by a minority overprinted remanence mean d i 272 2 95 12 similar to the record in the cairnsmoor of fleet pluton and granites from the adjoining lake district terrane granite complexes of the southern uplands block collectively record regional rotation and excursion of britain into southerly latitudes between c 410 and 390 ma comparable silurian devonian palaeomagnetic poles identify common apparent polar wander apw in paratectonic and orthotectonic terranes from the variscan front in the south to the laurentian foreland in the north following climactic acadian deformation apw between 430 and 390 ma embracing the post closure history of the caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander the 110 arc length is identical to polar shift identified between mid silurian and lower middle devonian poles from gondwana the two paths superimpose to show that the western margin of gondwana was in proximity to the se margin of laurentia during acadian deformation in early middle devonian times and remote from the caledonides the residual rheic ocean subsequently closed by a combination of pivotal and left lateral strike slip motions

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

astrometric and space geodetic observations of polar wander optical astrometric measurements of star positions taken during the past century have been recently re reduced using the final hipparcos star catalog in order to determine variations in the mean location of the earth s rotation pole with respect to the earth s crust this newly available polar motion series which is the longest homogeneous polar motion series currently available allows the drift in the pole path to be newly estimated during the 1900 0 to 1992 0 span of the smoothed hipparcos polar motion series the earth s rotation pole is observed to drift at a mean linear rate of 3 51 0 01 milliarcseconds year mas yr towards 79 2 0 2 w longitude this new estimate for the observed trend in the pole path which can be considered to be the present day expression of true polar wander is nearly the same as that estimated in previous studies using the homogeneous ils polar motion series

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

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

syntectonic carbonation during synmagmatic mantle exhumation at an ocean continent transition a jurassic extensional detachment associated with carbonated serpentinites and basalts is preserved at falotta platta nappe southeastern switzerland structural data indicate that fluid circulation occurred during late increments of extension along the detachment plane at the contact between serpentinites and basalts the homogeneity of the isotopic signatures 18o 16 and 13c centered on 0 1 can be best explained by a single sudden seawater derived carbonation event at temperatures of 100 c carbonation was focused in the high permeability zone along the detachment our model yields new insights for carbonation processes related to mantle exhumation

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

azimuthal seismic anisotropy constrains net rotation of the lithosphere 1 net rotation nr of the lithosphere is found in hot spot reference frames important for tectonics and plume models but difficult to constrain using mantle flow and crystallographic texture modeling i show that nrs lead to mantle shearing which is recorded in azimuthal seismic anisotropy the nr amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly smaller nrs such as predicted by flow models with stiff continental keels are consistent with seismology however

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

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

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 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

pattern of lobate scarps on mercury s surface reproduced by a model of mantle convection numerical simulations suggest that lobate scarps on the surface of mercury could have resulted from compressive stresses induced by convection within the planet s mantle in addition to those generated by global contraction mercury is the smallest and least tectonically active of the terrestrial planets1 2 although mercury s ancient cratered surface resembles the moon it has the largest ratio of metallic core to silicate mantle among the terrestrial planets3 as well as an internal magnetic field4 images from the mariner 10 spacecraft reveal lobate scarps so called because of their curved or scalloped edges which have been interpreted to be high angle thrust faults5 6 7 8 resulting from a period of global contraction a range of mechanisms has been invoked to explain the stresses leading to global contraction including cooling and core formation5 9 tidal effects due to gravitational interactions with the sun10 mantle convection11 and the impact that formed the caloris basin12 here i present numerical simulations of the three dimensional nature of convection within mercury s silicate mantle the model yields a regularly spaced pattern of convection in which upwelling regions of the mantle assume linear sheet like shapes at low latitudes and a nearly hexagonal pattern near the poles the distribution of resultant surface stresses is consistent with the observed pattern of lobate scarps suggesting that the compressive features record an ancient pattern of mantle convection11 in addition to global contraction the gravity and topographic data returned from the messenger 11 mission13 will help test this hypothesis

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

extraterrestrial chromite in middle ordovician marine limestone at kinnekulle southern sweden traces of a major asteroid breakup event the distribution of sediment dispersed extraterrestrial chromite grains and other cr rich spinels 63 mu m has been studied in middle ordovician orthoceratite limestone from two quarries at kinnekulle southern sweden in the thorsberg quarry all similar to 3 2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment dispersed extraterrestrial chromite grains typically 1 3 grains are found per kilogram of limestone in the nearby hallekis quarry the same beds show similarly high concentrations of extraterrestrial chromite grains but in samples representing the 9 m downward continuation of the section exposed at this site only 5 such grains were found in a total of 379 kg of limestone the extraterrestrial equilibrated ordinary chondritic chromite grains call be readily distinguished by a homogeneous and characteristic major element chemistry including 2 0 3 5 wt tio2 and stable v2o3 concentrations close to 0 7 wt terrestrial cr rich spinels have a wide compositional range and co exist with extraterrestrial chromite in some beds these grains may be derived for example from mafic dykes exposed and weathered at the sea floor considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied instead the difference may be primarily related to all increase in the ancient flux or extraterrestrial matter to earth in connection with the disruption of the l chondrite parent body in the asteroid belt at about this time the coexistence in some beds of high concentrations of chondritic chromite and terrestrial cr rich spinels however indicates that redistribution of heavy minerals on the sea floor related to changes in sea level and sea floor erosion and currents must also be considered less