latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

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

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

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

overriding plate shortening and extension above subduction zones a parametric study to explain formation of the andes mountains mountain building above subduction zones such as observed in the andes is enigmatic and the key parameter controlling the underlying dynamics remains a matter of considerable debate a global survey of subduction zones is presented here illustrating the correlation between overriding plate deformation rate and twelve physical parameters overriding plate velocity subducting plate velocity trench velocity convergence velocity subduction velocity subduction zone accretion rate subducting plate age subduction polarity shallow slab dip deep slab dip lateral slab edge proximity and subducting ridge proximity all correlation coefficients are low r 0 39 irrespective of the global reference frame relative plate motion model or overriding plate deformation model except for the trench velocity 0 33 0 68 exact value depends on adopted global reference frame and subduction velocity which shows an anticorrelation 0 55 0 57 this implies that no individual parameter can explain overriding plate deformation except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined orogenesis occurs in overriding plates bordering central regions of wide subduction zones 4000 km when the overriding plate is moving trenchward at 0 2 cm yr e g andes japan because the center of a wide slab offers large resistance to lateral migration the overriding plate effectively collides with the subduction hinge forcing the slab to attain a shallow dip angle e g nazca and japan slabs overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone but in the latter scenario maximum extension velocities are much lower than in the former scenario for subduction settings close to lateral slab edges overriding plate motion plays no significant role in overriding plate deformation thus for rapid overriding plate extension the key ingredient is rapid trench retreat which only occurs close to lateral slab edges while for overriding plate shortening the key ingredients are 1 the resistance to rapid trench and hinge retreat which occurs far from lateral slab edges and 2 trenchward overriding plate motion

trench migration and subduction zone geometry seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries we numerically investigate the effect of trench migration on the evolution of a slab with temperature dependent viscosity encountering a viscosity interface we find that subduction zone geometry is sensitive to even small rates 1 cm yr of retrograde motion increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities subduction zone geometry appears to be strongly time dependent we conclude that individual tectonic setting and time dependent slab behaviour can account for many different types of observable subduction zone anomalies our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography

viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the eastern great basin the deformed shorelines of lake bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle viscosity we describe and apply a new model to a recently augmented data set new data better constrain both the complex spatio temporal pattern of the lake load and the crustal deformation response to that load the history of lake level fluctuations has been significantly refined and somewhat modified this is due to both more radiocarbon dates from within the bonneville basin and to an improved calibration of the radiocarbon timescale itself the data which constrain the crustal deformation pattern consist of ages and shoreline elevations from several hundred points which sample three major levels of lake bonneville and corresponding elevations from the high stands of three smaller lakes situated to the west of lake bonneville the data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the edge of the big lake the geometry of the earth model incorporates an arbitrary number of layers overlying a half space and the rheology of each level can accommodate an arbitrary number of maxwell viscoelastic elements in parallel the inverse modeling comprises three complementary approaches for the simplest configurations we performed a direct search of the parameter space and delineated the irregular boundary of the subspace of acceptable models for more complex configurations we constrained the elastic parameters to their seismically determined values and then solved for viscosity versus depth profiles by either expressing the log viscosity versus log depth profile as a series of specially constructed orthogonal polynomials or by allowing each of 8 10 layers plus the half space to have an independently determined viscosity we found that the data do not strongly support nor can they conclusively exclude a more complex rheology than simple maxwell viscoelasticity the orthogonal polynomial solution exhibits an essentially monotonic decrease in viscosity with depth the most rapid change occurs at shallow depths decreasing from 1023 pa s at 3 km to 102o pa s at 30 km the decrease is much more gradual below with only another factor of 5 decrease between 30 and 300 km depth the unconstrained solution exhibits a rapid decrease in viscosity with depth from 2x 10 24 pa s in the top 10 km to 4x 1017 pa s at a depth of 40 km a nearly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity 2x 102o pa s extending from 150 to 300 km depth and by a uniform viscosity 1019 pa s half space below

observations of the transition from linear polarization to complex polarization in short period compressional waves we measure the polarization of compressional waves from seismograms of chemical explosion of the ontario new york new england refraction experiment recorded by the seven element eco array in the new york adirondack mountains after careful instrument calibration a precision of about 5 is achieved in measuring the azimuth of the compressional wave polarization direction the azimuth of the polarization of the onset of the p wave differs from the geometrical source receiver azimuth by as much as 20 possible due to deflection of the first arriving ray by lateral variation in crustal structure shortly after the onset of p the polarization changes from the linear polarization expected of a compressional wave to become very complex the time of this transition increases with source receiver distance from about 0 4 to 0 5 sec at 50 km distance and 0 7 to 0 8 sec at 150 km distance the complex polarization may be due to the arrival of strongly scattered waves that have propagated mainly in the shallow crust which would imply that the upper 1 to 2 km of the crust is particularly heterogeneous

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

slab mantle flow interaction influence on subduction dynamics and duration we investigate the influence of mantle flow relative to the lithosphere on subduction dynamics we use 2d thermo mechanical models assuming incompressible non newtonian fluid rheology different mantle flow velocities consistent with absolute plate motion models are tested as well as both directions of flow either sustaining or opposing slab dip the effects of different inflow outflow velocity profiles slab strengths and upper lower mantle viscosity contrasts are also evaluated slab dip deviations between models with opposite mantle flow directions range from 37 for relatively strong slabs max 1025 pa s to 50 for weaker slabs max 1024 pa s accounting for a significant amount of natural slab dip variability for imposed mantle flow supporting the slab the initial stage of slab steepening is followed by a stage of continuous slab dip decrease this slab shallowing eventually leads to mantle wedge closure subduction cessation and slab break off possibly driving subduction flips

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

mantle convection models of continental collision and breakup incorporating finite thickness plates abstract a two dimensional numerical model has been developed to study mantle convection during continental collision and breakup the model incorporates rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness we systematically investigate the influences of continental width diffusivity thickness and internal heating on continental collision and breakup in addition we consider the influence of different degrees of internal heating in the mantle for each case considered we model a pair of identical continents being carried towards a site of plate convergence by underlying counter rotating mantle convection cells the continents collide at the mid plane of the model to form a motionless rigid conducting supercontinent whereas oceanic plate material continues to recycle through the mantle we find that changes in the mechanical boundary conditions at the upper surface are important factors in initiating flow reversals below the supercontinent more generally our findings based on 144 models are that the following factors favour the initiation of flow reversals below the supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and continental crustal heating furthermore lower percentages of internal heating in the mantle are necessary to sustain and promote the subcontinental flow reversals in our models

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

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 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

differential rotation between lithosphere and mantle a consequence of lateral mantle viscosity variations the description of plate motions in the so called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle this rotation called toroidal field of degree 1 is roughly westward it reaches an amplitude of about 2 cm yr and has been consistently found in the different generation of plate tectonic models various authors have tried to relate this observation to the deceleration of the earth s rotation to polar wander or to tidal drag however these different physical mechanisms cannot explain the requested amplitude in this paper we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame in a model earth with lateral viscosity variations a differential rotation is predicted the observed net lithospheric rotation is consistent with the dynamics of a model earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents this relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping 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

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

the dynamics of subduction and trench migration for viscosity stratification summary although subduction plays a dominant role in plate tectonics related processes such as trench migration are not well understood we investigate the influence of viscosity stratification and different flow boundary conditions on the trench migration dynamics of a subducting lithosphere in a 2 d model using a finite difference code subduction and mantle flow are driven by a compositionally prescribed density contrast between the high viscosity slab and the ambient mantle no overriding plate is assumed the model rheology is viscoplastic at shallow and viscous at greater depth the mantle is stratified with a higher viscosity in the lower part of the computational domain the plastic rheology allows the plate to decouple from the top of the box and is described by a simplified friction law to mimic the brittle deformation of the shallow lithosphere the flow patterns and slab morphologies differ as a function of the velocity boundary conditions bcs for reflective bcs we model either a plate which is fixed on the box side or a plate that can freely move a slab with periodic bcs is always laterally free we find that when the slabs interact with the highly viscous lower mantle the slab with reflective bcs is more folded than the periodic one due to the confinement of mantle flow around the slab in general the trench is observed to roll back toward the oceanic plate for free slabs with reflective bcs the trench retreat velocity deceases after the interaction with the lower mantle in contrast for the free slab with periodic bcs it is nearly constant or even increases with time if described in a reference frame fixed to the lower mantle the free slab with periodic bcs exerts a force on the lower mantle which causes a net differential flow between the upper and lower mantle we also carried out additional experiments with different mechanical slab properties and thicknesses stiffer free slabs subduct more steeply and are able to penetrate straight into the lower mantle after penetration into the lower mantle the trench moves toward the continent thinner slabs deform more easily and are folded for all boundary conditions we can generalize the rollback systematics for free slabs by normalizing trench velocities by the plate speed free slabs being more representative of the west pacific subduction zones show large temporal variations of normalized trench velocities ranging between a retreat value of 1 0 to an advancing value of 0 3

structure and evolution of lithospheric slab beneath the sunda arc indonesia tomographic imaging reveals seismic anomalies beneath the sunda island arc indonesia that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers the sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the mesozoic tethys ocean in accord with previous studies the lithospheric slab was imaged as a continuous feature from the surface to the lower mantle below java with a local deflection where the slab continues into the lower mantle the deep slab seems to be detached from the upper mantle slab beneath sumatra this complex slab structure is related to the tertiary evolution of southeastern asia and the indian ocean region

state of stress in slabs as a function of large scale plate kinematics the state of stress of slabs subducting worldwide as revealed by seismicity is extremely variable both with depth and between different subduction zones although in principle slab pull should enhance downdip extension in the slab the reconstructed stress fields for intermediate depths between 100 and 300 km range from downdip compression to downdip extension using 2 d viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry dip of the slab and kinematics velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle of subduction zones we conclude that although the state of stress in slabs is also controlled by other processes downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab whereas downdip extension is favored by mantle flow in the same direction of the slab dip i e sustaining it these predictions are in agreement with available geophysical observations although exceptions to this simple pattern are observed worldwide in addition if the slab is decoupled from the upper plate convergence between upper and lower plates induces a downdip compressional component of stress within the slab decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction however these are second order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent although low values of viscosity of the lithospheric mantle render this prediction less stable

effects of depth dependent properties on the thermal anomalies produced in flush instabilities from phase transitions abstract the effects of depth dependent thermal expansivity viscosity and thermal conductivity on mantle convection with phase transitions have been examined with two dimensional finite element simulations in an aspect ratio four the model includes a core mantle thermal coupling boundary condition which takes into play the secular cooling of the core by the overlying mantle flow initial surface rayleigh numbers between 2 10 7 and 10 8 have been considered with time the surface rayleigh number decreases to a value upon which a transition takes place from layered to single cell convection this tumultuous period is marked by large scale coherent breakthrough of cold material trapped in the transition zone all the way to the base of the mantle and a violent reaction of hot plume in the upper mantle both hot and cold anomalies have large magnitudes cold anomalies with temperatures exceeding 1000 k are found at the base of the mantle the magnitude of the cold anomalies is largest with all three depth dependent properties the timescales of this catastrophic event are between 20 and 50 myr with the longest being produced by the model with depth dependent expansivity and viscosity results from these simple two dimensional cartesian models represent a lower bound to the larger cold thermal anomalies potentially capable of being generated in spherical shell convection models

plate kinematics slab shape and back arc stress a comparison between laboratory models and current subduction zones a combination of statistical studies on present day subduction zones and three dimensional 3d laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones in 3d laboratory models the analogue of a two layer linearly viscous lithosphere upper mantle system is achieved by means of silicon putty glucose syrup tank experiment the subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior 1 trenches migrate at a rate close to the overriding plate motion but always move slower than the overriding plates the mechanism at work is a direct consequence of slab anchoring opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability 2 an overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench 3 shallow and steep dips are found if the overriding plate moves toward and away from the trench respectively

laboratory experiments on the structure of subducted lithosphere abstract mantle seismic tomography images indicate a variety of shapes for slabs of oceanic lithosphere subducted into the transition zone in the vicinity of the 660 km discontinuity both horizontally deflected slabs and nearly vertical slabs have been inferred at subduction zones on the pacific and indian plates here we determine experimentally the behavior of a cold negatively buoyant and highly viscous slab incident on a fluid interface with a viscosity and a density increase the experiments are scaled to mantle conditions with realistic subduction peclet number ratio of trench migration to subduction velocities and physical properties of the interface a wide range of slab deformation styles is observed when slight changes in the parameters are imposed sinking slab stagnant slab spreading slab sinking pile and stagnant pile represent the five different modes identified we show that these deformation modes are governed by two velocity ratios which characterize the horizontal and the vertical components of the slab velocity near the interface slab penetration into the lower layer takes the form of folded slab piles for large values of the velocity ratios and stagnant behavior is promoted by rapid retrograde trench motion the sensitivity of slab behavior to transition zone density and viscosity changes and to trench migration can account for the coexistence of different modes of deformation in subducted lithosphere

impact of regional mantle flow on subducting plate geometry and interplate stress insights from physical modelling summary physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates the initial mantle flow direction beneath the overriding plate can be horizontal or vertical depending on its location with respect to the asthenospheric flow field imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern horizontal or vertical respectively overpressure promotes shallow dip subduction while underpressure tends to steepen the slab horizontal mantle flow with rates of 1 10 cm yr 1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of 60 while the same lithospheric slab sinks vertically when no flow is imposed vertical drag force due to downward mantle flow exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere the strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry when the slab dip is steeper than the interplate zone the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate when the slab dip is shallower than the interplate zone an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress however the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low

regional scale tectonic evolution and the seismic velocity structure of the lithosphere and upper mantle the mediterranean region we present a numerical modeling approach designed for the verification of tectonic reconstructions and for a quantitative analysis of the seismic velocity structure of the lower lithosphere and upper mantle in this forward modeling procedure we use published reconstructions of the tectonic evolution of a region to predict the thermal structure from which we calculate physical quantities like p wave velocity basin subsidence and basement heat flow we show an application of this approach to two reconstructions of the late mesozoic and cenozoic evolution of the mediterranean region the resulting synthetic velocity models are tested against seismological models of the mantle structure obtained independently by delay time tomography subsidence and heat flow predictions are compared with actual observations to further constrain the geological assumptions underlying the forward models the mantle models predicted from reconstructions of the evolution of the mediterranean region correlate with major features in the tomographic images from our model results we conclude that the present distribution of seismic velocities in the mediterranean region is largely determined by the tectonic evolution during at least the last 40 m y this means we can verify processes represented in different tectonic scenarios with the seismologically determined velocity structure seen from a different perspective the model results contribute to understanding the causes of the velocity structure imaged by delay time tomography by showing how the tectonic evolution results in this structure

modeling phase separation and phase change for magma ocean solidification dynamics just after accretion the earth s mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core the mineralogical observations and thermodynamics models of solid liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle solids were probably formed first at the bottom of the lower mantle or at mid mantle leaving two possible magma oceans a shallow one and an abyssal one near the bottom of the mantle the liquid phase might become denser than solids due to iron enrichment in the shallow magma ocean the crystallizing solid phase was denser and sank through the magma to settle and compact at depth to understand these complex dynamics we develop a two phase numerical code that can handle simultaneously convection in each phase and in the slurry and the compaction or decompaction of the two phases although our code can only run in a parameter range rayleigh number viscosity contrast between phases prandlt number far from what would be realistic we think it already provides a rich dynamics that illustrates what could have happened we show situations in which the crystallization front is gravitationally stable and situations were the newly formed solids are gravitationally unstable and can snow across the magma our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification

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

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones the thickness of earth s mechanical lithosphere is poorly defined to investigate whether rheology controls the thickness of the overriding plate s mechanical lithosphere in subduction zones the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity it was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner leading to unrealistically high heat flow since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere we suggest that it is instead controlled by buoyancy the source of buoyancy we assume is compositional e g buoyant crust two end member models with crustal thickness of 10 and 70 km respectively were then undertaken these had lower heat flow this work supports the assumption of some earlier workers e g plank and langmuir 1988 who equated the mechanical lithosphere with the crust of the overriding plate

role of lateral mantle flow in the evolution of subduction systems insights from laboratory experiments summary we present 3 d laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions in particular we present a set of experiments characterized by different conditions imposed at the trailing edge of the subducting plate that is plate fixed in the far field plate detached in the far field imposed plate motion experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle for each set we show differences between models that do or do not include the possibility of out of plane lateral flow in the mantle by varying the lateral boundary conditions our results illustrate how a subducting slab vertically confined over a 660 km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways on the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

mantle flow influence on subduction evolution abstract the impact of remotely forced mantle flow on regional subduction evolution is largely unexplored here we investigate this by means of 3d thermo mechanical numerical modeling using a regional modeling domain we start with simplified models consisting of a 600 km or 1400 km wide subducting plate surrounded by other plates mantle inflow of 3 cm yr is prescribed during 25 myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench perpendicular mantle inflow from either the back or front of the slab leading to 10 50 km changes in slab morphology and trench position while no strong slab dip changes were observed as compared to a reference model with no imposed mantle inflow in experiments with trench oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100 200 km lastly we investigate how subduction in the western mediterranean region is influenced by remotely excited mantle flow that is computed by back advection of a temperature and density model scaled from a global seismic tomography model after 35 myr of subduction evolution we find 10 50 km changes in slab position and slab morphology and a slight change in overall slab tilt our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion still these secondary effects occur on scales 10 50 km typical for the large scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes