pseudotachylytes from corsica fossil earthquakes from a subduction complex subduction zones are the most seismically active tectonic environment on earth here we report fault rock evidence of palaeoearthquakes from a subduction complex pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites spherulites and acicular minerals characteristic of blueschist facies conditions the veins have been found in both mantle peridotite and gabbro our discovery from corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released

three dimensional variable viscosity convection of an infinite prandtl number boussinesq fluid in a spherical shell we investigate a three dimensional spherical shell model of mantle convection with strongly temperature dependent viscosity numerical calculations of convection in an infinite prandtl number boussinesq fluid heated from below at a rayleigh number of ra 10 5 are carried out for the isoviscous case and for a viscosity contrast across the shell of 1 000 in the isoviscous case convection is time dependent with quasi cylindrical upflow plumes and sheet like downflows when viscosity varies strongly across the shell convection is also time dependent but major quasi cylindrical downflows with spider like extensions occur at both poles and interconnected upflow plumes occur all around the equator the surface expression of mantle convection in the earth downwelling sheets at trenches upwelling plumes at hot spots and upwelling sheets at midocean ridges resembles structures seen in both the isoviscous and variable viscosity models the dominance of spherical harmonic degree l 2 in the variable viscosity model agrees with the l 2 dominance in the earth s geoid topography and seismic tomography the overall pattern of convection in the variable viscosity case is similar to the distribution of major highlands and volcanic rises on venus

the relationships between large scale variations in shear velocity density and compressional velocity in the earth s mantle a large data set of surface wave phase anomalies body wave travel times normal mode splitting functions and long period waveforms is used to investigate the scaling between shear velocity density and compressional velocity in the earth s mantle we introduce a methodology that allows construction of joint models with various levels of scaling complexity d ln d lnvs d lnvs d lnvp in order to detect seismological signatures of chemical heterogeneity we demonstrate that the data sets considered cannot be fit concurrently with a uniform or a positive and uniform throughout the mantle the variance reductions to p wave travel times and vp sensitive modes are up to 40 higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear velocity model s362ani m which was constructed assuming a uniform throughout the mantle several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets anticorrelation between bulk sound and shear velocities in the lowermost mantle as well as an increase in with depth in the lower mantle are largely independent of the regularization scheme when correlations between density and shear velocity variations are imposed in the lowermost mantle variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 recent measurements of the splitting of 0s2 in particular are largely incompatible with perfectly correlated shear velocity and density heterogeneity throughout the mantle a way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle our preferred joint model consists of denser than average anomalies 1 peak to peak at the base of the mantle roughly coincident with the low velocity superplumes the relative variation of shear velocity density and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle with implications for the long term stability and evolution of superplumes

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

flow fields within the tonga slab determined from the moment tensors of deep earthquakes the seismic strain rates and instantaneous velocity fields within the plane of the tonga slab are investigated using 450 harvard centroid moment tensor cmt solutions from the period 1977 1992 down dip seismic strain rates at the bottom of the slab are around 1 10 15 s 1 spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab solutions show a northward motion of the 200 km depth contour in the tonga slab at rates of 8 2 mm yr relative to the 670 km depth contour of the slab relative to the deep southern portion of the tonga slab down dip velocities increase from near zero in southern tonga to 10 2 5 mm yr in northern tonga consistent with the increased subduction rate from south to north described by the pacific australia pole of rotation moreover patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1 1025 dyn cm along strike extensional strain rates in the deep zone of northern tonga are approximately 0 5 10 15 s 1 this is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate suggesting that the seismic strain rates within tonga represent a large fraction of the total strain rate budget within the seismogenic zone an estimated maximum of 60 of the relative motion between the surface and 670 km is absorbed as strain within the slab indicating considerable slab thickening and penetration of the slab into the lower mantle

constraining odd degree earth structure with coupled free oscillations several pairs of normal mode multiplets sensitive to long wavelength mantle structure are potential coupling partners geometric selection rules render such coupling the only means by which normal modes sample odd degree mantle structure that overtone coupling can be strong and can significantly impact normal mode spectra is illustrated using synthetic seismograms for a realistic earth model these observations motivate the generalization of normal mode spectral fitting to permit simultaneous estimation of even and odd degree structural constraints when generalized spectral fitting is applied to a data set of approximately 150 high signal to noise vertical records dominated by data from the northern bolivian event 6 9 1994 it is found to reduce misfits appreciably for coupled multiplets 1 s 5 2 s 4 and 2 s 5 1 s 6 to yield odd degree constraints generally consistent with recent mantle models and to improve agreement between estimated and predicted even degree constraints

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

the influence of thermodynamic formulation on simulations of subduction zone geometry and history we introduce varying approximations to the thermodynamic properties of a convecting system similar to earth s mantle a realistic geometry rheology and faults are included to insure that the variations we see are robust in a dynamic self buffering environment we find two levels of response in the system first some approximations lead to very different patterns of flow in the simulations second other sets of approximations lead to similar patterns of flow but the time history of the flow varies significantly with the formulation used last we note that only minor changes to an existing code would be needed to integrate a complex thermodynamic model

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

thermal expansion measurements at very high pressure systematics and a case for a chemically homogeneous mantle new data on thermal expansivity of iron to over 400 kbar in laser and electrically heated diamond anvil cells and on mgo and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats boehler 1982 imply that at high pressure may be described by a constant value of 1n 1nv t 5 5 0 5 this behavior is supported by previous measurements at lower pressures on a large variety of materials densities of magnesio wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and at high pressures using the above systematics match those in prem through the entire lower mantle making a strong case for a uniform mantle

wide band coupling of earth s normal modes due to anisotropic inner core structure summary we investigate the importance of wide band coupling of normal modes due to inner core anisotropy we compare four different seismic models of inner core anisotropy which were obtained by others using the splitting of earth s normal modes these models have been developed using a self coupling sc approximation which assumes that coupling between nearby modes through anisotropic inner core structure is negligible we test the sc approximation by comparing the frequencies and quality factors of 90 inner core sensitive modes computed for these models using either the sc approximation or full coupling fc among large groups of modes we find significant shifts in the quality factors and frequencies for some modes groups of modes which significantly couple together are constructed for six target modes these groups are model dependent and in some cases contain large numbers of modes synthetic seismograms are calculated to show that the difference between sc and fc is observable on the scale of seismograms and of the same order of magnitude as the difference between synthetic and observed seismograms thus future models of inner core anisotropy should take cross coupling between large groups of modes into account

seismic slip and down dip strain rates in wadati benioff zones the rate of accumulation of seismic moment in wadati benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere between depths of 75 and 175 kilometers a typical down dip strain rate is about 10 15 per second which implies that slabs in this depth range typically accumulate strain of order 10 1 this result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes mantle seismicity repeated catastrophic shear failure is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs slabs are penetratively deformed and they have low flexural rigidity compared to oceanic plates at the earth9s surface

joint inversion of normal mode and body wave data for inner core anisotropy 1 laterally homogeneous anisotropy 1 we use mantle and inner core sensitive normal modes to invert for laterally varying mantle models and a transversely isotropic inner core to obtain an inner core model that is compatible with long period mode and short period body wave data we incorporate mantle corrected absolute travel time and differential travel time data in our inversions using this diverse data set we investigate the dependence of inner core models upon mantle parameterization and possible radial structure within the inner core inner core anisotropy obtained from a simultaneous inversion for mantle and inner core structure has a weak dependence upon the parameterization within the mantle that is the results for the inner core are insensitive to whether the mantle is inverted only for shear velocity structure or for shear and compressional velocity structure as well as density an inner core model with constant anisotropy is capable of fitting all types of data but mode and absolute travel time data are more compatible with one another than with differential travel time data allowing for radial variations within the inner core reduces this incompatibility and it is possible to obtain a radially dependent model that fits normal mode absolute travel time and differential travel time data with a variance reduction greater than 75

distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry examples from tethyan ophiolites in the western alps large amounts of fluid bound up in the hydrated upper layers of the ocean crust are consumed at convergent margins and released in subduction zones through devolatilization the liberated fluids may play an integral role in subduction zone processes including the generation of arc magmas however exhumed subduction zone rocks often record little evidence of large scale fluid flow especially at deeper levels within the subduction zone basaltic pillows from the high pressure corsican and zermatt saas ophiolites show a range of 18o values that overall reflect seafloor alteration prior to subduction however comparison between the 18o values of the cores and rims of the pillows suggests that the 18o values of the pillow rims at least have been modified during subduction and high pressure metamorphism pillows that have not undergone high pressure metamorphism generally have rims with higher 18o values than their cores whereas the converse is the case in pillows that have undergone high pressure metamorphism this reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid rock interaction occurring during subduction and high pressure metamorphism however the preservation of different 18o values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled resetting of the 18o values in the pillow rims was probably due to fluid hosted diffusion that occurred over relatively short time scales 1 myr

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

interpretation of the d zone at the base of the mantle dependence on assumed values of thermal conductivity a common assumption in modeling dynamic processes in the earth s lower mantle is that thermal conductivity is sensibly constant throughout the lower mantle and equal to approximately 4 w m 1 k 1 this value is the result of extrapolation using a linearized theory with estimated parameters for lattice contributions to thermal conductivity furthermore this value is highly dependent on the assumed geotherm excluding theoretical uncertainties in the extrapolation recent experimental data for the pressure dependence of conductivity coupled with a revised geotherm lead to substantially higher estimated values for the lower mantle the hypothesis that the seismically resolved anomalous zone at the base of the mantle d is a thermal boundary layer can be questioned in this case of a higher estimation for thermal conductivity

the state of stress in subducting slabs as revealed by earthquakes analysed by moment tensor inversion abstract we have used a data set of earthquakes whose mechanisms have been studied here or elsewhere by moment tensor inversion to confirm some earlier ideas concerning the state of stress in subducting slabs the moment tensor method offers the advantages of speed and objectivity over earlier methods usually based exclusively on first motion data deep earthquakes below 300 400 km tend to have compression axes aligned with the dip of the subducting slab the behavior of intermediate earthquakes from 70 to 300 km is less clear there is some evidence for down dip tension but the alignment is not as obvious as it is in the case of the compression axes of deep events tonga shows a tendency toward down dip compression at intermediate depths but other deep extending slabs as well as shallow extending slabs slabs with maximum depths shallower than 300 km seem to be closer to down dip tension

calculation of normal mode spectra in laterally heterogeneous earth models using an iterative direct solution method summary normal mode observations play an important role in studying broad scale lateral variations in the earth such studies require the calculation of accurate synthetic spectra in realistic earth models and this remains a computationally challenging problem here we describe a new implementation of the direct solution method for calculating normal mode spectra in laterally heterogeneous earth models in this iterative direct solution method the modecoupling equations are solved in the frequency domain using the preconditioned biconjugate gradientalgorithm andthetime domainsolutionisrecoveredusinganumericalinversefourier transform a number of example calculations are presented to demonstrate the accuracy and efficiency of the method for performing large full coupling calculations as compared to methods based on matrix diagonalization and the traditional direct solution method

structural and geochemical data on the rio magno unit evidence for a new apenninic ophiolitic unit in alpine corsica and its geodynamic implications the rio magno unit rmu tectonically overlies the schistes lustres units in south eastern alpine corsica it is represented by an ophiolitic sequence showing remarkable differences with respect to the commonly recognized corsican ophiolites this unit can be distinguished from the schistes lustres by the lack of hp lt metamorphism reflecting its different geodynamic setting although both were involved early in the same tectonic events similarly the rmu can be distinguished from the balagne unit by the presence of normal morb basalts and the scarcity of continent derived sedimentary input testifying to a different oceanic palaeogeographical setting moreover the petrochemical and stratigraphic features of the rmu ophiolitic sequence show close analogies with the internal ligurides of the northern apennines the rmu represents the first record of a nonmetamorphic apenninic type ophiolitic unit in alpine corsica supporting the hypothesis that the alpine corsica northern apennine system represents a double vergent accretionary wedge

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

bayesian inference of mantle viscosity from whole mantle density models the long wavelength geoid is sensitive to earth s mantle density structure as well as radial variations in mantle viscosity we present a suite of inversions for the radial viscosity profile using

interaction of weak faults and non newtonian rheology produces plate tectonics in a 3d model of mantle flow according to the theory of plate tectonics relatively rigid plates are bounded by large faults plate motion has negligible internal strain with significant toroidal component to the velocity by contrast models of mantle flow with viscous rheology in an intact medium predict little toroidal component and substantial internal strain in surface motion it has been suggested5 that the observed characteristics of plate motion are related to faulted plate margins which are observed to be weak here we confirm this suggestion using three dimensional models of mantle flow that incorporate faults and the forces exerted on plates by subducting slabs slab pull and mid ocean ridges ridge push our models show that plate like motion results from the interaction between weak faults and a strain weakening power law rheology weak transform faults tend to guide plate motion this guiding effect and the decoupling that occurs at thrust faults may result in oblique subduction convergent margins are associated with realistic trench and fore bulge topography by simultaneously predicting surface kinematics topography and gravity the models achieve a useful degree of tectonic realism

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

seismic strain rate and deep slab deformation in tonga seismic strain rates estimated from volume averaged moment tensor data are combined with the strike dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep tonga slab tonga strain rate orientation tensors constructed from the harvard centroid moment tensor catalog are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650 km discontinuity those which sample lithospheric volumes from 18 s to 25 s at 500 700 km depth indicate that the axis of maximum shortening lies within 15 of the slab s downdip azimuth and plunge the discrepancy between shortening and slab dip may be larger in northernmost tonga 17 s 18 s where slab morphology is less well constrained but overall the slab is compressing roughly parallel to its trajectory tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab the proportion of along arc strain rate decreases from northern to southern tonga the scalar rate of seismic strain peaks at certain latitudes but its values averaged over northern and central tonga and southern tonga are roughly equal strain rate data may be related nonuniquely to constraints on slab structure and finite strain inferred from seismicity and travel time data assuming that aseismic and seismic flow have the same geometry and that the total rate of strain is constant along the arc the degree of cross strike advective thickening increases to the south while along arc strain decreases if along arc strain rate exclusively produces southward shear of the deeper lithosphere and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the louisville ridge thickening of at least 50 is required in central tonga this lower bound improves the resolution of minimum slab width provided by travel time observations if 50 thickening occurs an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab larger thickening factors or flow in the outer regions of the slab require aseismic deformation assuming that the observed travel time residuals are the result of a single coherent slab a synthesis of seismicity strain rate and travel time data indicate a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes roughly to the north at shallower depths and in the lower mantle such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m y and the tectonic history of the region

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

the architecture of an incipient oceanic basin a tentative reconstruction of the jurassic liguria piemonte basin along the northern apennines alpine corsica transect in this paper a scenario for the early evolution of the jurassic oceanic liguria piemonte basin is sketched for this purpose four selected examples of ophiolite sequences from the northern apennines and corsica are described and analyzed in the external ligurian units northern apennines the ocean continent transition of the adria plate was characterized by a basement made up of subcontinental mantle and lower continental crust covered by extensional allochthons of upper crust both the basement rocks and the extensional allochthons are cut by basaltic dikes and covered by basalts and pelagic deposits the conjugate ocean continent transition of the corsica margin represented by the balagne nappe corsica was composed of mantle peridotites and gabbros covered by basaltic flows and minor breccias that in addition include continent derived clasts by contrast the innermost i e closest to the ocean preserved area observed in the internal ligurian northern apennines and inzecca corsica units consists of former morphological highs of mantle peridotites and gabbros bordered by small basins where the basement is covered by a volcano sedimentary complex characterized by ophiolitic breccias and cherts interlayered with basaltic flows the overall picture resulting from our reconstructions suggests an asymmetric architecture for the liguria piemonte basin with a central area bounded by two different transition zones toward the continental margins this architecture can be interpreted as the result of a rifting process whose development includes a final stage characterized by passive asymmetric extension of the lithosphere along an east dipping detachment fault system

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

tectonic evolution of the corsica apennines alps region studied by the method of successive approximations paleotectonic and paleogeographic maps are often very complex users find it difficult to evaluate the extent to which the features shown are based on reliable geologic data and it is not generally possible to estimate uncertainties in shapes and positions in the scientific tradition of designing simple models to represent complex reality this study uses systematically simplified palinspastic models to study the tectonic evolution of the central mediterranean the prime constraint is northwestward motion of the african continent toward europe at 7 5 km m y in the first approximation model only unidirectional plate motion is allowed and the model is unable to generate both the alpine and apennine orogenic belts at right angles to each other the second approximation allows rotations it is surprising how well this very simple model with its rigidly restrictive rules reproduces the known evolution of the central mediterranean this agreement allows some optimism that the second approximation is correctly modeling the primary features of the tectonic evolution of the central mediterranean revising the shapes of the tectonic entities in a third approximation would allow the final pattern to mimic even more closely the present tectonic situation in the mediterranean but here the advantages of rigid simplification would start to diminish the first conclusion of the paper is thus that strongly simplified models are useful in tectonic studies and are justified in light of the uncertainties simplified models are used to explore the recently proposed ancient apennine hypothesis in which apennine deformation begins with a reversal of subduction polarity at about 100 90 ma this view contrasts with the standard young apennine hypothesis in which subduction reversal and the onset of apennine orogensis do not occur until 35 30 ma study of the simplified tectonic models leads to the second conclusion of the paper that the ancient apennine hypothesis honors regional constraints as well as the standard view does the simplified modeling suggests specific tests in alpine corsica for choosing between the young apennine and ancient apennine hypotheses

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

thermal anomalies and magmatism due to lithospheric doubling and shifting abstract we present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere upper mantle system including an oceanic ridge lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle in both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a normal continental or oceanic geothermal situation the perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism we speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma the original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes we dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general lithospheric recycling is inherent to the mechanism of lithospheric doubling

effects of depth variations in creep laws on the formation of plates in mantle dynamics we have studied the influences of depth varying rheologies on the structure of mantle convection and the near surface flow fields upwelling flows crossing the upper lower mantle boundary become considerably thinner and faster streamlines are concentrated near the surface and other plate like characteristics are enhanced by this type of rheological stratification in which the averaged effective viscosity of the upper mantle is about two orders of magnitude lower than that of the lower mantle the presence of a lithosphere with a higher non linear dependence in the flow law further promotes plate like behavior plate like character of the top boundary layer is encouraged by a decreasing depth dependent viscosity in the non newtonian upper mantle and by greater convective vigor

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

the curvature of wadati benioff zones and the torsional rigidity of subducting plates it is an axiom of plate tectonics that lithospheric plates are to a good approximation torsionally rigid that is inextensible in the plane of the plate1 3 although few scientists would question this assumption when it is applied to oceanic lithosphere at the earth s surface there is no consensus as to whether or not oceanic lithosphere maintains its mechanical integrity during subduction com pare refs 4 7 with refs 8 11 i show here that the shapes of many wadati bemoff zones are inconsistent with the notion that they manifest subducted portions of oceanic plates subject only to very small 1 membrane strains

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

neogene to quaternary rifting and inversion in corsica retreat and collision in the western mediterranean 1 stratigraphic structural and fission track data have been integrated in order to unravel the neogene to quaternary tectonic evolution of northeastern corsica results indicate that during early to middle miocene the study area was affected by a rapid cooling at low temperatures accompanied by exhumation and rifting basin inversion occurred during the tortonian again followed by extension in the messinian to early pliocene compressional deformations are finally recorded by early pliocene to quaternary deposits alternating rifting and inversion events are recorded not only in northeastern corsica but also in many peri tyrrhenian regions this alternation can be explained in terms of intraplate propagation of the compressional stress related to the africa europe convergence and to a discontinuous slab retreat along the apennine subduction system a pause in the slab retreat implies a break of the back arc extension in the upper plate and it allows a temporary propagation of africa europe compression

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

fossil earthquakes recorded by pseudotachylytes in mantle peridotite from the alpine subduction complex of corsica paleo earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of alpine corsica pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the schistes lustres of cape corse ultramafic pseudotachylyte fault and injection veins are found within well preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures the newly formed minerals are zoned olivine fo93 89 clino and ortho pyroxene with compositions indicative of high crystallization temperatures 1300 1400 8c zoned cr spinel and a glassy to micro vesicular hydrous matrix showing that frictional melts contained up to 4 water frictional heating on co seismic faults raised the temperature from ambient blueschist facies conditions 450 8c and 1 1 5 gpa to more than 1700 8c which is required for 75 disequilibrium melting of spinel peridotite at 1 5 gpa the characteristic fault vein thicknesses observed are 1 to 3 cm but several fault veins are thicker than 10 cm co seismic displacement of 1 m a stress of 300 mpa and seismic efficiency of 5 may melt ca 60 kg peridotite pr m 2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte the ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the alpine subduction zone if the interpretations of typical displacements of approximately 1 m are correct the most common pseudotachylyte fault veins are related to magnitude ca 7 or larger subduction earthquakes d 2005 elsevier b v all rights reserved

deformation of a weak subducted slab and variation of seismicity with depth oceanic lithosphere is assumed to possess platelike properties whether it is lying at the earth s surface or descending deep into the mantle yet the geometries of subducting slabs indicate significant deformation as they descend through the mantle1 4 suggesting that lithospheric plates might be so weakened during subduction as to act not as a rigid solid but as a viscous fluid numerical and laboratory experiments have shown that fluid slabs can indeed take on realistic profiles5 7 however it has not been clear whether a fluid or weak model of slab dynamics can account for deep earthquakes which are usually ascribed to the deformation of a rigid or strong slab here we show using numerical simulations of slab evolution that a weak slab model is in fact consistent with seismic observations assuming that earthquakes occur at a rate proportional to deformation rate we reproduce the observed variation with depth of seismicity rate and focal mechanisms and the cessation of seismicity at 670 km depth provided that sinking material encounters resistance at depth here modelled as a viscosity jump at 670 km the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the ophiolite bearing schistes lustres nappe in alpine corsica a model for the emplacement of ophiolites that have suffered hp lt metamorphism

evolutionary model for alpine corsica mechanism for ophiolite emplacement and exhumation of high pressure rocks a tectonic model of alpine corsica is proposed based on geological studies its evolution starts after the jurassic with intraoceanic subduction followed by mid cretaceous subduction of the european continental margin under the oceanic segment of the adriatic plate after subduction of the continental crust to a depth of 150 km slices of crustal material are buoyantly uplifted together with high pressure oceanic rocks ophiolites and schistes lustres high pressure low temperature continental gneiss units overthrust the outer segments of the european crust while producing a normal sense motion along the upper surface of the rising crustal body during the eocene the closure of the remnant ligurian oceanic basin separating the proto corsican belt and adria resulted in a second orogenic phase with the emplacement of unmetamorphosed ophiolitic nappes which overthrust the previously exhumed and eroded hp belt this corsican model suggests an original evolutionary path for orogenic belts when continental collision is preceded by intraoceanic subduction

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

geological evidence for a global tectonic polarity inferences about plate motions over the past 40 ma based on structural and geodetic data suggest that there is a coherent movement direction on a global scale moreover a westward drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot spot reference frame independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative eastward or northeastward mantle flow also support this global tectonic polarity unfortunately most hot spot reference frames are based on assumptions which strongly limit their validity for example many combine magmatic sources originating from different depths in the mantle including hot spots located along oceanic ridges which are not fixed relative to one another using a hot spot reference frame filtered to exclude shallow hot spots and anomalous wet spots we would predict a much higher relative westward drift of the lithosphere plate motions are not random rather that they are controlled by the relative motion between lithosphere and asthenosphere and by gradients in lithospheric thickness and composition triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere e g the clockwise rotation of south america plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the earth s rotation poles plate rotations induced by triangular shape differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of euler poles this might have inhibited the recognition of a coherent global tectonic pattern

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

hp lt metamorphism in the volparone breccia northern corsica france evidence for involvement of the europe corsica continental margin in the alpine subdution zone the alpine belt in corsica france is characterized by the occurrence of stacked tectonic slices derived from the corsica europe continental margin which outcrop between two weakly or non metamorphic tectonic domains the autochthonous domain of the hercynian basement to the west and the balagne nappe ophiolitic unit belonging to the nappes superieures to the east these slices including basement rocks permian granitoids and their palaeozoic host rocks late carboniferous permian volcano sedimentary deposits coarse grained polymict breccias volparone breccia and middle eocene siliciclastic turbidite deposits were affected by a polyphase deformation history of alpine age associated with a well developed metamorphic recrystallization this study provides new quantitative data about the peak of metamorphism and the retrograde p t path in the alpine corsica the tectonic slices of volparone breccia from the balagne region previously regarded as unmetamorphosed were affected by peak metamorphism characterized by the phengite chlorite quartz albite assemblage using the chlorite phengite local equilibria method peak metamorphic p t conditions coherent with the low grade blueschist facies are estimated as 0 60 0 15 gpa and 325 20 c moreover the retrograde p t path characterized by a decrease of pressure and temperature is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions 0 35 0 06 gpa and 315 20 c the occurrence of metamorphic peak at high pressure low temperature hp lt conditions is evidence of the fact that these tectonic slices derived from the corsica europe continental margin were deformed and metamorphosed in the alpine subduction zone during their underplating at 20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non metamorphic oceanic units during a complex exhumation history

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

how valid are dynamic models of subduction and convection when plate motions are prescribed a detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection plate velocity is computed self consistently from fully dynamic plate models with temperature and stress dependent viscosity and preexisting mobile faults in fully dynamic models the flow is driven solely by internal buoyancy while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy only a temperature dependent viscosity close to the effective viscosity determined from the fully dynamic models is used in the kinematic models the two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical in kinematic plate models the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere in models with periodic lateral boundary conditions slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions

the influence of temperature and depth dependent viscosity on geoid and topography profiles from models of mantle convection abstract we consider the geoid and topography profiles from convection models with depth and temperature dependent viscosity models in a 2 d cartesian geometry we use a very low viscosity region in the top center of the geometry to generate plate like viscosity models we focus on subduction regions which have narrow 100 200 km trenches the narrow trench is only a minor feature in the regional geoid in depth dependent viscosity models we change the geometry of the weak zone around the ocean trench to study the impact of the weak zone on trench topography and the geoid we also vary the vertical viscosity structure of the models the weak zone geometry has very little effect on the surface velocity of the ocean plate or the large scale pattern of convection the width of the weak zone within a reasonable range has little effect on topography and geoid profiles there is a narrow trough in the geoid and topography profiles this trough becomes wider and deeper with a high viscosity upper mantle this may suggest that the high viscosity upper mantle is not suitable for our models in temperature dependent viscosity models we vary the viscosity values for different layers temperature dependent viscosity is also important to producing realistic subduction models however the activation energy must be weaker than laboratory estimates of olivine under mantle pressures and temperatures from both depth and temperature dependent viscosity models the viscosity in the lower mantle should be at least ten times greater than in the upper mantle consistent with previous studies of the geoid in regions of subduction the topography and geoid profiles can match the observations when the viscosity increases with depth

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

exhumation of deep crustal metamorphic rocks and crustal extension in arc and back arc regions we adress the problem of exhumation of metamorphic rocks in arc and back arc positions based on observations of the aegean and tyrrhenian back arc regions in both regions back arc extension has been active for 25 ma at least while convergence and crustal thickening was going on at the thrust front compressional structures are reworked in the back arc domain as extensional flat lying detachments which allow unroofing of the metamorphic pile metamorphic rocks suffer first a dynamic burial during nappe stacking and then fast exhumation when they reach the extensional region we describe a continuous process of burial and exhumation that is contemporaneous with the migration of the thrust front toward external zones the front of extension follows behind at approximately one hundred kilometers at the convergence front continuous subduction of a cold oceanic slab or continuous accretion of continental units keeps a cool p t gradient and hp lt metamorphic rocks are formed at depth the overall cool environment leads to a deep brittle ductile transition and thus to a resistant crust which can sustain its large thickness while the thrust front propagates forward parts of the accretionary wedge which were formed earlier enter the back arc domain which can be already under extension and which is warmer because of a greater distance to the cold slab and the presence of the volcanic arc the brittle ductile transition rises in the thick crust which then becomes unstable and subsequently collapses the exhumation history of metamorphic rocks below detachments is two fold 1 a constantely thickening crust is unroofed by flat lying detachments which rework early thrusts this first period is characterized by a good preservation of hp lt parageneses 2 the second period is when rocks are in the zone of crustal scale extension of the back arc region deep crustal metamorphics are exhumed below flat lying detachements which affect the whole crust

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

cenozoic plate driving forces past studies of plate driving forces have concluded that the forces due to subducted slabs in the upper mantle and those due to the thickening of the oceanic lithosphere are the principal driving forces we reexamine the balance of driving forces for the present day and extend our analysis through the cenozoic using an analytical torque balance method which accounts for interactions between plates via viscous coupling to the induced mantle flow we use an evolving mantle density heterogeneity field based on the last 200 myr of subduction to drive plate motions an approach which has proven successful in predicting the present day mantle heterogeneity field we find that for plausible upper mantle viscosities the forces due to subducted slabs in the cenozoic and mesozoic account for in excess of 90 of plate driving forces and those due to lithospheric thickening for less than 10

the metastable olivine wedge in fast subducting slabs constraints from thermo kinetic coupling abstract the effects of a strong non linear thermo kinetic coupling in the olivine spinel transition are investigated with a 2d model for fast subducting slabs the thermo kinetic coupling problem is solved in the lagrangian framework by the differential algebraic method and by using experimental input parameters narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity while the thermo kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist in contrast to recent kinetic models both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge both processes trigger the ongoing phase transformation in the cold slab interior a thermal instability over a short timescale 10 4 10 5 yr can occur due to a positive thermo kinetic feedback this effect is enhanced at greater depths the depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature slab width and the activation energy for diffusion variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge hence predictions about the exact position of the metastable olivine wedge are rather difficult we propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating while the thermal instability is enhanced by the latent heat release associated with the olivine spinel phase transition

burial and exhumation of corsica france in the light of fission track data 1 the mesozoic and cenozoic exhumation and cooling history of corsica is reconstructed by fission track ft data on basement and sedimentary rocks apatite ages are 105 16 ma zircon ages are 160 145 ma the jurassic and cretaceous ages show that parts of the variscan basement escaped alpine influence the basement was thermally affected by rifting prior to jurassic opening of the ligurian piedmont ocean then it cooled to near surface temperatures in paleocene eocene times subduction buried parts of the basement and overlying flysch to greater depth in the oligocene both collapse of the nappe stack and rifting prior to opening of the ligurian provencal basin affected the apatite ft system of the basement in different partly overlapping areas causing a complex age pattern the study shows that thorough analysis of ft data and thermal modeling allow to assign age populations to distinct cooling processes even when several thermotectonic events contributed to generate an intricate age pattern

an o isotope profile through the hp lt corsican ophiolite france and its implications for fluid flow during subduction abstract subduction zones are important sites for fluid generation as the downgoing slab undergoes progressive high pressure metamorphism these fluids are thought to play an important role in the generation of arc magmas but their affect on the subducted slab itself is less clear in this study the stable isotope geochemistry of the corsican ophiolite which is variably affected by subduction related metamorphism was investigated to assess the impact of subduction zone fluid flow on the downgoing slab a total of 170 stable isotope analyses oxygen and carbon showed the following features 1 overall the ophiolite preserves the same oxygen isotope profile with depth as unmetamorphosed ophiolites 2 the variation in oxygen isotope values for different areas does not correlate with metamorphic grade 3 oxygen and carbon isotope values for calcite in the ophiolite are not in equilibrium with the ophiolite but are similar to those of the overlying calc schists schistes lustres and 4 hydrogen isotopes indicate that serpentinitisation of ultramafics occurred during sub seafloor hydrothermal alteration and probably also during tectonic emplacement of the ophiolite these features indicate that large scale fluid flow did not affect the corsican ophiolite during subduction zone metamorphism calcite in the ophiolite was probably precipitated from co2 bearing fluids derived from decompression dehydration reactions affecting the overlying schistes lustres during exhumation of the ophiolite bearing schistes lustres nappes given that ocean crust must undergo at least some dehydration during prograde subduction zone metamorphism the evolved fluids must be either highly channelled or exit the slab relatively quickly so as to not interact with it the results of this study indicate that the important fluid pathways on corsica have yet to be identified but are probably the shear zones and fault systems that bound different slices of the high pressure nappe stack

the thermal evolution of corsica as recorded by zircon fission tracks abstract new zircon fission track zft ages from corsica record multiple thermal events that can be tied to the structural evolution of the western mediterranean region the corsican zircons have a wide scatter of zft grain ages 243 14 ma which together define several age domains western corsica consists largely of stable hercynian basement characterized by zft ages in the range 161 114 ma we interpret these ages late jurassic early cretaceous as the product of a long lived tethyan thermal event related to continental rifting and subsequent drifting during the separation of the european and african plates and the formation of the liguro piemontese ocean basin in contrast to hercynian corsica alpine corsica northeast corsica experienced widespread deformation and metamorphism in late cretaceous tertiary time dated samples from alpine corsica range in age from 112 to 19 ma and all are reset or partially reset by one or more alpine thermal events the youngest zft grain ages are from the northernmost alpine corsica and define an age population at 24 ma that indicates cooling after tertiary thermal events associated with the alpine metamorphism and the opening of the liguro provencal basin a less well defined zft age population at 72 ma is present in both alpine corsica and hercynian basement rocks the thermal history of these rocks is not clear one interpretation is that the zft population at 72 ma reflects resetting during a late cretaceous event broadly synchronous with the early alpine metamorphism another interpretation is that this peak is related to variable fission track annealing and partial resetting during the tertiary alpine metamorphic event across central to north eastern corsica this partial age resetting supports the presence of a fossil zft partial annealing zone and limits the peak temperature in this area below 300 c for both the affected pre alpine and alpine units

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

self lubricated mantle convection two dimensional models we present results from convection models allowing for self lubrication of downflows models impose a line source of chemically light low viscosity material at the top of a convecting layer of temperature dependent viscosity material low viscosity surface material serves as an analog to hydrated sediment crust and the high viscosity upper portion of the convecting layer as an analog to mantle lithosphere slow near surface motion in the convecting layer entrains low viscosity material into zones of downflow which has a lubricating effect once entrained lubricant is deeper than the cold high viscosity portion of the convecting layer rapid upper boundary layer overturn occurs and system properties change e g heat flux doubles this marks transition to a lubricated state before and after transition transport properties are dominantly determined by respectively the viscosity of mantle lithosphere and that of interior mantle lubricated and nonlubricated states appear as distinct regions in system output space suggesting that exchange between them is akin to a phase transition that such exchange depends on a near surface lubricant implies that the geodynamics of planets lacking such lubricants may fundamentally differ from that of planets possessing them

back arc extension and denudation of mediterranean eclogites eclogite facies rocks exposed in mediterranean back arcs are delimited from above by low angle normal faults and detachments nevertheless our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites extension in mediterranean back arcs began in the late oligocene or early miocene but the pressure temperature time p t t paths of eclogite facies rocks exposed in these areas indicate that a major part of the overburden several tens of kilometers has been removed from above these rocks prior to the oligo miocene we show that the time period bracketed between the peak of eclogite metamorphism eocene in the central aegean probably upper cretaceous in corsica and the betics and the onset of back arc extension in the oligo miocene was characterized by thrust faulting in the central aegean corsica and the betics eclogite bearing units were partly unroofed and then overthrusted lower pressure units we emphasize that with one exception tinos island greece the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower grade sequences thus mediterranean back arc extension lags behind a major part of the denudation process and is superposed on orogenic wedges that contain eclogite facies rocks at relatively shallow structural levels we emphasize that the mode of occurrence of eclogites in mediterranean back arc regions involves a continuum of in situ crustal accretion below the eclogites widespread p t paths that show cooling or isothermal decompression and lower grade rocks at the bottom of the structural pile thus instead of reflecting whole crust back arc extension the tectonic style associated with the denudation of mediterranean eclogites better fits an active accretionary wedge setting this is similar to the mode of occurrence of eclogite facies rocks in mountain belts such as the western alps where decompression was synorogenic and back arc extension played no role

normal mode splitting function measurements of anelasticity and attenuation in the earth s inner core we have used the iterative spectral fitting method to measure both the elastic and anelastic splitting functions of 20 inner core sensitive normal modes these modes show significant improvement in spectral fit when anelastic splitting function coefficients dst are introduced in addition to the elastic splitting function coefficients cst we employ two separate anelastic treatments i fully anelastic measurement in which a complete set of anelastic splitting function coefficients is measured in addition to the elastic coefficients and ii zonal anelastic measurement in which anelasticity is only allowed in zonal splitting function coefficients together these two approaches confirm that normal modes sensitive to the earth s inner core resolve zonally dominant elastic and anelastic structures the zonal dominance of anelasticity suggests that the inner core exhibit cylindrical attenuation anisotropy in addition to cylindrical velocity anisotropy in particular the zonally dominant anelasticity correlates with zonal elastic structure that is directions of higher velocity in the inner core also appear more attenuating

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

horizontal shear flow in the mantle beneath the tonga arc in the tonga subduction zone a systematic shear deformation in which deep material moves relatively to the south is superimposed on the mode of down dip shortening which is characteristic of the region we interpret this observation together with a tectonic reconstruction in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle

even degree lateral variations in the earth s mantle constrained by free oscillations and the free air gravity anomaly the recent occurrence of several large earthquakes in particular the 1994 june 9 bolivia event has motivated a re examination of the earth s large scale heterogeneity from a normal mode or free oscillation perspective compared to earlier studies the number of normal mode constraints on lateral variations in the mantle has increased five fold and toroidal and cross coupled modes complement the traditional spheroidal mode data set it is demonstrated that this large collection of mode data combined with the free air gravity anomaly can reliably constrain even degree lateral variations in wave velocities as well as density we present the first whole mantle density model constrained by seismology our shear and compressional velocity models are consistent with existing models based upon traveltimes and waveforms and are reasonably well correlated throughout the mantle shear and bulk sound velocity models exhibit a gradual decrease in correlation with depth and are anti correlated near the core mantle boundary we find that lateral variations in density are poorly correlated with wave velocities and are locally anti correlated with shear velocity in the lowermost mantle the correlations between wave velocities and density suggest both a thermal and a compositional origin to lateral heterogeneity in addition to traditional maps of lateral variations in wave velocity we also present maps of lateral variations in shear and bulk moduli the inversion puts weak constraints on even degree topographic variations on the core mantle boundary the 660 km discontinuity and dynamic free surface topography finally we determine both radially and laterally varying scaling relationships including poisson s ratio