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

stress distribution within subducting slabs and their deformation in the transition zone abstract the mechanical behavior of slabs in the mantle transition zone is a poorly understood phenomenon to obtain a better understanding of the stress and deformation pattern inside subducting slabs we performed 2d numerical simulations of the subduction process in a purely viscous non linear rheological model including diffusion creep dislocation creep and a stress limiting mechanism the model calculations are used to investigate the effects of yield stress plate age and surface boundary condition free slip versus given plate velocity on the stress development in and around a subducting slab and on the deformation of the subducting plate in the transition zone the deformation and morphology of subducted slabs may differ between free slip and kinematic boundary condition models in some cases especially for young and relatively weak slabs even though kinematically driven subduction morphology may look realistic in most cases our results show that stress distributions can differ significantly if the discrepancy between free slip and kinematic subduction velocity is large and in some model cases even influence the ability of the plate to penetrate into the lower mantle stress distributions such as the ones presented in this study can be a valuable tool in understanding the complex and enigmatic interaction between subducting slabs and the transition zone

why cold slabs stagnate in the transition zone oceanic lithosphere sinks stagnates and is deflected sub horizontally beneath western pacific island arcs requiring buoyancy in the slab that is inconsistent with a thermal origin the transformation of pyroxene to majoritic garnet occurs by extremely slow diffusion and pyroxene is therefore unlikely to transform at equilibrium pressures and temperatures in the cold interior of slabs we present high resolution numerical simulations showing that when slow diffusion inhibits the dissolution of pyroxene into garnet the slab becomes buoyant relative to the ambient mantle and stagnates whereas when the phase transformations occur in equilibrium there is no effect on the slab we test the model by comparing slab temperature and geometry and find that sub horizontal slabs are more likely colder than average consistent with our numerical simulations

evidence of lower mantle slab penetration phases in plate motions it is well accepted that subduction of the cold lithosphere is a crucial component of the earth s plate tectonic style of mantle convection but whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate which has substantial implications for the chemical and thermal evolution of the mantle here we identify lower mantle slab penetration events by comparing cenozoic plate motions at the earth s main subduction zones with motions predicted by fully dynamic models of the upper mantle phase of subduction driven solely by downgoing plate density whereas subduction of older intrinsically denser lithosphere occurs at rates consistent with the model younger lithosphere of ages less than about 60 myr often subducts up to two times faster while trench motions are very low we conclude that the most likely explanation is that older lithosphere subducting under significant trench retreat tends to lie down flat above the transition to the high viscosity lower mantle whereas younger lithosphere which is less able to drive trench retreat and deforms more readily buckles and thickens slab thickening enhances buoyancy volume times density and thereby stokes sinking velocity thus facilitating fast lower mantle penetration such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle thus we identify a direct expression of time dependent flow between the upper and lower mantle

subduction mode selection during slab and mantle transition zone interaction numerical modeling global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660 km discontinuity or penetrate into the lower mantle the stagnating slabs also have various morphologies these are directly related to the interaction between the subducting slabs and the mantle transition zone mtz the dynamics of which are still debated using a 2 d thermo mechanical model we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles four basic subduction modes are obtained in the numerical experiments including one with slab penetrating through the 660 km discontinuity and three other modes with slab stagnating in the mtz i e folding lying and rolling back the numerical models indicate that the age of subducting oceanic plate the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and mtz interaction in general the young subducting slab favors the penetration or folding mode whereas the old subducting slab tends to result in lying or rolling back mode although other parameters can also affect our models also show a strong correlation between the subduction mode selection and dip angle of the slab tip when reaching the 660 km phase boundary

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

geostrophic circulation patterns in the northeastern adriatic sea and the effects of air sea coupling may september 2003 1 ctd data were collected weekly during 20 cruises from may to september 2003 over a 19 station grid in a coastal region of the northeastern adriatic sea as part of the adriatic sea integrated coastal areas and river basin management system pilot project adricosm relative geostrophic velocities indicated fine scale circulation patterns in the area consisting of two pronounced circulation cells of cyclonic and anticyclonic character and that were present in the area for most of the investigated period these motions were induced primarily by spatial variations in the temperature field and may be strengthened during episodes of very strong northeasterly bora wind when the investigated region is not heated cooled uniformly

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

non steady state subduction and trench parallel flow induced by overriding plate structure abstract the direction of plate tectonic motion and the direction of mantle flow as inferred from observations of seismic anisotropy measurements show a good global correlation far from subduction zones however this correlation is poor near subduction zones where below the slab seismic anisotropy is aligned parallel to the trench and above the slab has a complex pattern which has not been fully explained here we present time dependent three dimensional 3d fully dynamic simulations of subduction to study the effect of overriding plate structure on the evolution of slab geometry and induced mantle flow we find that along strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate the variation in slab geometry drives strong trench parallel flow beneath the slab and a complex flow pattern above the slab this new mechanism for driving trench parallel flow provides a good explanation for seismic anisotropy observations from the middle and south america subduction zones where both slab dip and overriding plate thermal state are strongly variable and correlated and thus may be an important mechanism in other subduction zones the location and strength of trench parallel flow vary with the time dependent evolution of the slab suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non steady state behavior of these systems

equilibration scales in silicic to intermediate magmas implications for experimental studies experimental phase equilibrium studies are increasingly being used for the determination of intensive variables p t fh2o fo2 in silicic to intermediate magmas in contrast silicic igneous bodies are now perceived as open periodically recharged systems involving only limited chemical equilibration thus the use of laboratory determined crystal liquid equilibrium data needs clarification here we review the field petrological and geochemical evidence concerning states and scales of chemical equilibrium in silicic magma bodies it is concluded that total chemical equilibrium is generally not the rule however a subsystem in local equilibrium the reactive magma can be identified equilibration scales in silicic magmas are rate limited either by diffusive flux in crystals dicl regime or by diffusive flux in the melt md regime the recognition that equilibrium in magmas is limited to a reactive subsystem requires phase equilibrium studies to be chemically scaled experiments either of total or partial equilibrium type should aim at a close reproduction of equilibrium states specific to natural systems the laboratory reconstruction of the natural equilibrium states guarantees a precise determination of the pre eruptive parameters and a reliable application of the experimental data to active volcanic systems

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

dynamics of free subduction from 3 d boundary element modeling 1 in order better to understand the physical mechanisms underlying free subduction we perform three dimensional boundary element numerical simulations of a dense fluid sheet with thickness h and viscosity 2 sinking in an ambient mantle with viscosity 1 the mantle layer is bounded above by a traction free surface and is either 1 infinitely deep or 2 underlain by a rigid boundary at a finite depth h d similar to the typical geometry used in laboratory experiments instantaneous solutions in configuration 1 show that the sheet s dimensionless stiffness s determines whether the slab s sinking speed is controlled by the viscosity of the ambient mantle s 10 time dependent solutions with tracers in configuration 2 demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides the extra edge drag exerted by the flow around the sides causes transverse deformation of the slab and makes the sinking speed of a 3 d slab up to 40 less than that of a 2 d slab a systematic investigation of the slab s interaction with the bottom boundary as a function of 2 1 and h h delineates a rich regime diagram of different subduction modes trench retreating slab folding trench advancing and reveals a new advancing folding mode in which slab folding is preceded by advancing trench motion the solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

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

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

subduction induced mantle flow finite strain and seismic anisotropy numerical modeling surface measurements of shear wave splitting patterns are widely used to infer the mantle circulation around subducting slabs however the relation between mantle flow and seismic anisotropy is still elusive finite strain is a direct measurement of time dependent deformation and has been proposed as a proxy for the crystal preferred orientation cpo of mantle minerals we have conducted a series of numerical models to systematically investigate the mantle flow finite strain olivine cpo and sks wave splitting in oceanic subduction zones with variable slab width they demonstrate that the preferred orientations of olivine a axes generally agree with the long extensional axes of the finite strain ellipsoid fse even in these very complex mantle flow fields however neither the a axis nor the fse axes necessarily aligns with the instantaneous mantle velocity vector we identify two domains with distinct deformation mechanisms in the central subplate mantle where simple shear induced by plate advance dominates at shallow depths and produces trench normal fast splitting while pure shear induced by slab rollback dominates the deeper mantle and results in trench parallel fast splitting the sks splitting patterns are thus dependent on the competing effects of these two mechanisms and also on the subduction partition ratio xp xt trench parallel when 1 in addition different mantle deformation mechanisms and sks splitting patterns are observed in the mantle wedge and around the slab edges which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones

dynamic models of downgoing plate buoyancy driven subduction subduction motions and energy dissipation abstract it is much debated whether the forces associated with the downgoing plate the overriding plate passive or active mantle flow are dominant in controlling the paths of plates into the mantle we investigate the dynamics and energetics for a free subduction system driven solely by downgoing plate buoyancy using a finite element model of a viscoelastic plate with a free surface sinking into a passive unbounded mantle represented by drag forces parameters are varied to study effects of an asthenosphere ridge push and a passive overriding plate for a range of subducting plate viscosities and densities such a single free plate achieves subduction mainly through trench retreat most of the energy dissipation occurs in driving the passive mantle response as a result the slab s sinking velocity is its stokes velocity determined by lithospheric buoyancy and mantle viscosity the total subduction velocity and dip adjust to minimize bending dissipation in the lithosphere and are affected by slab rheology as well as buoyancy a low viscosity asthenosphere and ridge push facilitate plate advance increasing plate dips and lowering subduction velocity while suction and buoyancy of a work free passive overriding plate decreases plate dips thus increasing subduction and rollback velocities however the geometrical relation between the different parameters is the same in all model cases because the slabs sink according to their stokes velocity the free subduction models thus provide a reference that can be used to distinguish the signature of downgoing plate buoyancy from that of other driving forces in global compilations of subduction parameters

effects of relative plate motion on the deep structure and penetration depth of slabs below the izu bonin and mariana island arcs an increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the earth s lower mantle below some island arcs but are deflected or rather laid down in the transition zone below others recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection with intermittent layering we present a multi disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth central in our discussion are the izu bonin and mariana subduction zones we argue that the differences in the tectonic evolution of these subduction zones in particular the amount and rate of trench migration can explain why the slab of subducted oceanic lithosphere seems to be at least temporarily stagnant in the earth s transition zone below the izu bonin arc but penetrates into the lower mantle below the mariana arc we briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones although further investigation is necessary our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate tectonic history of the subduction zones involved we therefore hope that the ideas outlined here will stimulate and direct new research initiatives

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

evolution of subduction zone curvature and its dependence on the trench velocity and the slab to upper mantle viscosity ratio 1 three dimensional laboratory models of upper mantle subduction are presented investigating the effect of the trench velocity vt and the slab to upper mantle viscosity ratio sp um on trench curvature and slab curvature one set of experiments varies sp um from 66 to 1375 another set of experiments modifies vt through applying different velocities at the trailing plate the results show that the radius of trench curvature rtc progressively decreases with continuous trench retreat due to quasi toroidal mantle return flow from the subslab region around the lateral slab edges and toward the mantle wedge region but starts to increase when trench retreat changes to trench advance due to quasi toroidal return flow in the opposite direction furthermore rtc increases with increasing sp um i e progressively stronger slabs are progressively less curved following a cubic root function the force required to curve the slab and trench varies with progressive subduction but is only 0 1 2 5 of the total negative buoyancy force of the slab while the viscous dissipation rate due to progressive slab curvature is only 0 01 0 60 of the potential energy release rate comparison of trench curvature in the models scaled slab width w 750 km with that of the scotia subduction zone w 800 km indicates that for the scotia subduction zone the effective sp um is of the order 1 2 102 finally the elastic sphere indentation model for arc curvature is revisited demonstrating that four main predictions are not met by observations implying that the model should be rejected

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

slab dynamics in the transition zone abstract seismic images of horizontal slab segments in across or below the transition zone invoke scenarios in which slabs are laid down in the mantle through progressive trench retreat however observations of subduction characteristics do not exhibit a clear correlation between trench retreat and slab dip in the transition zone instead analysis of a range of subduction characteristics demonstrates that while transition zone slabs include slabs of many ages and subduction velocities with both retreating and advancing trench motion they also have several enigmatic characteristics such as faster sinking rates and a larger range in slab dip than slabs that extend to either shallower or deeper depths comparison of subduction characteristics with several analytical and numerical models of subduction dynamics suggests that many of the possible mechanisms for trapping slabs in the transition zone e g positive buoyancy sources viscous resistance slab weakening are only viable if the slab is already shallow dipping two scenarios for the formation of stagnant slabs are proposed 1 trench retreat prior to slabs entering the transition zone or caused by the negative buoyancy forces associated with the wadsleyite and ringwoodite phase transitions and 2 slow lateral migration of slabs in stable subduction zones

a laboratory investigation of effects of trench migration on the descent of subducted slabs a laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours the slab dip in a uniform mantle is found to be steady or oscillatory depending on the rates of descent and trench migration in addition density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in earth s mantle if the slab is denser than the lower layer and its dip in the upper layer is steady it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface if the slab dip in the upper layer is unstable the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer the style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers estimates for mantle slabs indicate that they may range across the major regime transitions the experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere they also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present day tectonic processes at the surface may not be obvious tomographic images of western pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

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

bending mechanics and mode selection in free subduction a thin sheet analysis summary to elucidate the dynamics of free buoyancy driven subduction of oceanic lithosphere i study a model in which a 2 d sheet of viscous fluid with thickness h and viscosity 1 subducts in an infinitely deep ambient fluid with viscosity 1 numerical solutions for the sheet s evolution are obtained using the boundary element method bem starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally beneath an impermeable traction free surface interpretation of the solutions using thin viscous sheet theory shows that the fundamental length scale controlling the subduction is the bending length lb defined at each instant as the length of the portion of the sheet s midsurface where the rate of change of curvature is significant geophysically speaking lb is the sum of the lengths of the slab and of the region seaward of the trench where flexural bulging occurs the bending length in turn enters into the definition of the sheet s dimensionless stiffness s h lb 3 which controls whether the sinking speed of the slab is determined by the viscosity of the sheet itself s 1 or by that of the ambient fluid s 1 motivated by laboratory observations of different modes of subduction retreating versus advancing trench folding versus no folding etc in fluid layers with finite depth i calculate numerically the dip d of the slab s leading end as a function of and the normalized depth d h to which it has penetrated the contours of the function d d h strongly resemble the intermode boundaries in the laboratory based regime diagram of schellart supporting the hypothesis that the mode of subduction observed at long times in experiments is controlled by the dip of the slab s leading end when it reaches the bottom of the layer in particular the bem solutions explain why trenches advance in the laboratory only when lies in an intermediate range and why they retreat when is either smaller or larger than this application of the bem model to wu et al s compilation of the minimum curvature radii of subducted slabs suggests 140 510 for the earth this is too small to permit the laboratory type trench advancing mode in agreement with the lack of tomographic evidence for slabs that are bent over backwards

episodicity in back arc tectonic regimes abstract the evolution of back arc basins is tied to the development of the dynamics of the subduction system they are a part of we present a study of back arc basins and model their development by implementing 3d time dependant computer models of subduction including an overriding plate we define three types of episodicity pseudo quasi and hyper episodicity and find evidence of these in nature observations of back arc basin ages histories of spreading quiescence and compression in the overriding plate give us an understanding of the time development of these subduction zones and back arc basins across the globe today a number of trenches are advancing the izu bonin trench the mariana trench the japan trench the java sunda trench and the central portion of the peru chile trench the andes subduction zone the izu bonin mariana and japan all have established back arc basins while the others have documented episodes of spreading quiescence compression or a combination of these the combination of advancing and retreating trench motion places these subduction zones in the category of hyper episodicity quasi episodicity in which the back arc shifts between phases of rifting spreading and quiescence is the dominant form of episodic back arc development in the present we find this type of episodicity in models for which the system is dynamically consistent that we have allowed the subducting plate s velocity to be determined by the sinking slabs buoyancy quasi and hyper episodicity are only found in subduction zones with relatively high subducting plate velocities between 6 and 9 cm year finally those subduction zones for which the subducting plate is moving slowly such as in the mediterranean or the scotia sea experience only pseudo episodicity where the spreading moves linearly towards the trench but often does so in discrete ridge jump events

dynamical mechanisms controlling formation and avalanche of a stagnant slab abstract we performed a numerical study to understand the dynamical mechanism controlling the formation and avalanche of a stagnant slab using two dimensional dynamical models of the integrated plate mantle system with freely movable subducting and overriding plates we examined slab rheology as a mechanism for producing various styles of stagnating or penetrating slabs that interact with the 410 km and 660 km phase transitions the simulated results with the systematically changed rheological parameters are interpreted using a simple stability analysis that includes the forces acting on the stagnant slab slab plasticity that memorizes the shape produced by past deformation generates slab stagnation at various depths around the 660 km phase transition the slab stagnates even beneath the 660 km phase boundary with a gentle clapeyron slope feedbacks between trench backward migration and slab deformation promote each other during the slab stagnation stage slab viscosity also determines the final state of the subducted slab that is it continues stagnation or initiates penetration a low viscosity slab can finally penetrate into the lower mantle because the growth time of the rayleigh taylor instability is shorter after the avalanche the direction of the trench migration changes depending on the lower mantle slab viscosity

the effect of metastable pyroxene on the slab dynamics seismic studies show that some subducting slabs penetrate straight into the lower mantle whereas others seem to flatten near the base of the mantle transition zone slab stagnation is often attributed to an increase in viscosity and phase transformations in the olivine system however recent mineral physics studies showed that due to extremely low transformational diffusion rates low density metastable pyroxene may persist into the transition zone in cool slabs here we use a dynamically fully self consistent subduction model to investigate the influence of metastable pyroxene on the dynamics of subducting oceanic lithosphere our results show that metastable pyroxene affects slab buoyancy at least as much as olivine metastability however unlike metastable olivine which can inhibit slab penetration in the lower mantle only for cold old and fast slabs metastable pyroxene is likely to also affect sinking of relatively young and slow slabs

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

slab stiffness control of trench motion insights from numerical models 1 subduction zones are not static features but trenches retreat roll back or advance here we investigate the dominant dynamic controls on trench migration by means of two and three dimensional numerical modeling of subduction this investigation has been carried out by systematically varying the geometrical and rheological model parameters our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate a large viscosity ratio between plate and mantle and a small density contrast between plate and mantle or an intermediate width w 1300 km advancing slabs dissipate 45 to 50 of the energy in the system thin plates with relatively low viscosity or relatively high density or wide slabs w 2300 km on the other hand promote subduction in the retreating style i e slab roll back the energy dissipated by a retreating slab is 35 to 40 of the total dissipated energy most of the energy dissipation occurs in the mantle to accommodate the slab motion whereas the lithosphere dissipates the remaining part to bend and unbend with a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one plate stiffness stiffer slabs cause the trench to advance whereas more flexible slabs lead to trench retreat the reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface but stiff slabs have more difficulty unbending at depth when arriving at the 660 km discontinuity those bent slabs tend to cause the trench to advance in a similar way variation of the viscoplasticity parameters in the plate may change the style of subduction a low value of friction coefficient weakens the plate and results in a retreating style while higher values strengthen the plate and promote the advancing subduction style given the fact that also on earth the oldest and therefore probably stiffest plates have the fastest advancing trenches we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration

a regime diagram for subduction styles from 3 d numerical models of free subduction abstract previous models of subduction both analogue and numerical have observed a number of distinct styles of subduction each with particular subduction motions partitioned between slab rollback and forward plate advance and associated slab morphologies we use 3 d numerical models to investigate subduction dynamics by varying the strength of slabs as well as the buoyancy and propose a new classification based on these parameters the slab strength is specified both through the ratio of viscosities between the subducting plate and upper mantle plate um as well as the plate thickness h plate only a very restrictive range of plates strong plates with smaller buoyancy tend to favor modes of subduction which are exclusively advancing plates which have greater negative buoyancy will eventually transition into a retreating style we find that the flexural strength and the buoyancy determine the subduction style as distinguished by a characteristic slab morphology and control several subduction characteristics including the partitioning between slab rollback and plate advance the trench curvature and the slab s radius of curvature plates that are 80 100 km thick with plate um 100 300 are classified here as weak and are the only plates to exhibit slab geometry with several recumbent folds atop the more viscous lower mantle this regime of weak plates with their associated slab morphologies predominant folding is argued to be most similar to slabs on earth based on the presence of folded slab piles in earth s upper mantle as interpreted from seismic tomography

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

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

time dependent interaction between subduction dynamics and phase transition kinetics summary numerical subduction models are presented that account for the kinetics of the olivine spinel transformation and the equilibrium transformation of spinel into perovskite and magnesiowustite temperature depth and stress dependent rheology is used buoyancy due to temperature and the olivine spinel and magnesiowustite perovskite phases leads to free subduction into the transition zone and retarded penetration into the lower mantle a dynamic feed back mechanism is found between the different buoyancy forces the kinetics of the phase transition and the release of latent heat this mechanism modulates the parachute effect of the metastable olivine mo in a subducting slab and leads to characteristic temporal variations of the depth of mo and with a time delay variations of the subduction velocity time periods are of the order of 3 4 myr this mechanism allows to explain variations of the depths of the deepest earthquakes in otherwise similar subduction zones further conclusions are as follows the maximum depth of mo increases roughly linearly with lithospheric thickness and the maximum depth may reach as deep as 720 750 km different subduction zones may represent different stages of velocity depth phase loops of mo

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

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

dynamic buckling of subducting slabs reconciles geological and geophysical observations abstract ever since the early days of the development of plate tectonic theory subduction zones have been engrained in geological thinking as the place where steady linear slabs descend into the mantle at a constant uniform dip angle beneath volcanic arcs however growing evidence from geological and geophysical observations as well as analog and numerical modeling indicates that subducting slabs buckle in a time dependent manner in contrast to the steady state linear cartoons that dominate the literature to evaluate the implication of time dependent slab buckling of geological events we conduct a series of 2 d numerical dynamic kinematic subduction experiments by varying the viscosity increase across the 660 km discontinuity and the strength of the subducting slab our results show that slab buckling is a universal figure in all the experiments when rate of the trench migration vtrench is relatively slow vtrench the slab buckling behavior in our experiments explains a variety of puzzling geological and geophysical observations first the period of slab buckling is consistent with short periodic variations 25 myr in the motions of the oceanic plates that are anchored by subduction zones second the scattered distributions of slab dips from 20 to 90 in the upper mantle are snapshots of time dependent slab dip third the current compressional and extensional stress environments in back arcs are well correlated with shallowing and steeping slab dip resulting from slab buckling fourth the temporal evolution of stress environments in the andes is well correlated with alternate slab dip these correlations indicate that time dependent slab buckling is a major factor controlling subduction zone dynamics

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

upper plate controls on deep subduction trench migrations and deformations at convergent margins abstract thus far relatively simplistic models of free subduction in which the trench and plate motions are emergent features completely driven by the negative buoyancy of the slab have investigated the dynamics of a single isolated subducting plate here we extend such models to incorporate an overriding plate and present the results of how such an overriding plate feedbacks into the dynamics of free subduction in this study we address three fundamental aspects of these dynamics 1 how does the presence of an overriding plate change the force balance at the convergent margins 2 how are the forces from deep subduction propagated to the surface and 3 what controls the stress regime in a system of coupled upper and subducting plates and how is it expressed in the deformations and plate motions in general we find that the evolution of subduction zones is strongly controlled by both the interactions between the slab and the upper lower mantle discontinuity as well as the strength of the upper plate when either the subducting or upper plates are unable to move subduction motions are steady state and partitioned entirely into either slab rollback or plate advance respectively when conditions favour a quasi stationary trench the subducted lithosphere can form into a pile with multiple recumbent folds of slab material atop the lower mantle alternating between forwards and backwards draping slab the corresponding horizontal trench motions at the surface are frontward and rearward respectively resulting in either a compressive or extensional regime in the back arc time dependent forcing arising from the slab piling behaviour can have a feedback with upper plate and produce strongly non steady state intermittent phases of upper plate deformation as those commonly observed on earth two types of discontinuous back arc strain evolution are identified 1 periodic when recurrent phases of strain over finite durations are accommodated by viscous stretching thickening of the plate and 2 episodic when upper plate deformation localizes plastic strain and allows for punctuated episodes these phases can include extension quiescence and compression giving rise to a large variety of possible tectonic evolutions the models presented here provide insight into the dynamics behind the non steady state evolution of subduction which can help unravel seemingly erratic motions of major convergent margins and back arc deformations around the pacific and indian oceans during the cenozoic

interaction of subducted slabs with the mantle transition zone a regime diagram from 2 d thermo mechanical models with a mobile trench and an overriding plate transition zone slab deformation influences earth s thermal chemical and tectonic evolution however the mechanisms responsible for the wide range of imaged slab morphologies remain debated here we use 2 d thermo mechanical models with a mobile trench an overriding plate a temperature and stress dependent rheology and a 10 30 or 100 fold increase in lower mantle viscosity to investigate the effect of initial subducting and overriding plate ages on slab transition zone interaction four subduction styles emerge i a vertical folding mode with a quasi stationary trench near vertical subduction and buckling folding at depth vf ii slabs that induce mild trench retreat which are flattened horizontally deflected and stagnate at the upper lower mantle interface hd iii inclined slabs which result from rapid sinking and strong trench retreat isr iv a two stage mode displaying backward bent and subsequently inclined slabs with late trench retreat bir transitions from regime i to iii occur with increasing subducting plate age i e buoyancy and strength regime iv develops for old strong subducting and overriding plates we find that the interplay between trench motion and slab deformation at depth dictate the subduction style both being controlled by slab strength which is consistent with predictions from previous compositional subduction models however due to feedbacks between deformation sinking rate temperature and slab strength the subducting plate buoyancy overriding plate strength and upper lower mantle viscosity jump are also important controls in thermo mechanical subduction for intermediate upper lower mantle viscosity jumps 30 our regimes reproduce the diverse range of seismically imaged slab morphologies

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

effects of mantle and subduction interface rheologies on slab stagnation and trench rollback abstract trench rollback has been a widely discussed phenomenon in recent years and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the upper mantle here we concentrate on the effects of rheological description yield stress lower mantle viscosity viscosity of crust in controlling the rollback and associated stagnation of slabs in the transition zone 410 660 km depth we perform numerical simulations of slab evolution in a 2d cartesian model with strongly nonlinear rheology combining diffusion creep dislocation creep and a power law stress limiter we demonstrate that trench retreat develops in most models considered regardless of the subducting plate age or prescribed strength rollback then mostly produces slabs that are horizontally deflected at the 660 km phase boundary and remain subhorizontal at the bottom of the transition zone slab morphologies are in agreement with stagnant horizontally deflected structures reported in the transition zone by seismic tomography furthermore if the strength of the slab is limited to less than 0 5 gpa the slab experiences a significant amount of horizontal buckling the amplitude of the rollback velocity is sensitive to several model parameters as one might expect it increases with the age of the subducting plate thus reflecting its increasingly negative buoyancy on the other hand rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates high friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation also reducing the additional negative buoyancy imparted by the 410 km exothermic phase transition suppresses trench rollback interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models where for example buoyancy effects may be counteracted by associated yield stress weakening

dynamics of subduction and plate motion in laboratory experiments insights into the plate tectonics behavior of the earth 1 three dimensional laboratory experiments have been designed to investigate the way slab bearing plates move during subduction inside the mantle in our experiments a viscous plate of silicone lithosphere subducts under its negative buoyancy in a viscous layer of pure honey mantle varying thickness width viscosity and density of the plate and mantle three characteristic modes of subduction are observed a retreating trench mode mode i a retreating trench mode following a transient period of advancing trench mode ii and an advancing trench mode mode iii these modes are characterized by different partitioning of the amount of subduction into plate and trench motion our experiments show that the velocity of subduction can be modeled by the dynamic interaction between acting and resisting forces where lithospheric bending represents 75 95 of the total resisting forces however our experimental results also show the impossibility to predict a priori the plate velocity only from the velocity of subduction without considering trench migrations we find experimentally that the lithospheric radius of curvature which depends upon plate characteristics stiffness and thickness and the mantle thickness exerts a primary control on the trench behavior our results suggest that the complexity of the style of subduction could be controlled by geometrical rules of a plate bending inside a stratified mantle the earth system is in the crucial range for the interplay between the rigidity of the plate and the mantle stratification this setting may be the responsible for the complexity of the past and present tectonic styles

the influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms a possible mechanism suggested by several authors could be the weakening due to grain size reduction which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle we simulate a subducting slab in a two dimensional 2 d cartesian box in the numerical model with composite rheology including diffusion creep dislocation creep and a general stress limiting rheology approximating peierl s creep we concentrate on two rheologic effects the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology the effect of trench migration on slab deformation is also included in our study results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening however in case of a 4 cm yr trench retreat grain size weakening provides a viable mechanism to deflect the slab in the transition zone provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 pa s 2002 elsevier science b v all rights reserved

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102