comparison of dynamic flow models for the central aleutian and tonga kermadec subduction zones using three dimensional 3 d instantaneous dynamic models of the central aleutian subduction zone we show that a low viscosity low density deltarho 10 kg m3 region in the wedge is needed to match observations of topography the geoid and the stress state in the slab and overriding plate previous models of the tonga kermadec subduction zone also require a low viscosity low density deltarho 20 kg m3 region in the wedge a low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down dip tension to down dip compression if the low viscosity region is large as in the tonga kermadec models then the decoupling may also cause the overriding plate to go into extension these are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones

two dimensional modelling of subduction zone anisotropy with application to southwestern japan summary we present a series of 2 d numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate we use a kinematically defined slab geometry approximating the subduction of the philippine sea plate beneath eurasia through finite element modelling we explore the effects of different rheological and thermal constraints e g a low viscosity region in the wedge corner power law versus newtonian rheology the inclusion of thermal buoyancy forces and a temperature dependent viscosity law on the velocity and finite strain field in the mantle wedge from the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material we then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory and compare the predictions to 500 previously published shear wave splitting measurements from seventeen stations of the broad band f net array located in southwestern japan although the use of different model parameters can have a substantial effect on the character of the finite strain field the effect on the average predicted splitting parameters is small however the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge the splitting observed in southwestern japan agrees well with the predictions of trench perpendicular flow in the mantle wedge along with b type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench

gravity above subduction zones and forces controlling plate motions 1 short and intermediate wavelength gravity and geoid anomalies are used to provide constraints on the mechanical structure of subduction zones and on the forces involved this study is based on 2d cartesian dynamically self consistent models with newtonian or power law rheologies we show that both strong decoupling of the two convergent plates shear stresses of the order of 107 pa and weakened bending lithosphere are necessary to reproduce the observed geoid and gravity data good fits are found for relatively low failure stresses 30 50 mpa for all models providing reasonable predictions of gravity only a small fraction of the downgoing slab weight is transmitted to the surface plates about 10 of the energy is dissipated in the contact zone between the two plates 10 to 20 in the bending region and more than 70 in the sublithospheric mantle the basal tractions on the order of 1 4 1012 n m induce a net motion of the plates with the subducting lithosphere moving faster than predicted by the no net motion principle a marked positive geoid anomaly is predicted above subduction zones at intermediate wavelength 2000 4000 km in the case of pure whole mantle convection such large geoid highs are not observed introducing partial layering i e mass anomalies hampering mantle flow through the transition zone is necessary to reconcile model predictions and observations for these wavelengths

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

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

numerical comparison of different convergent plate contacts subduction channel and subduction fault summary at convergent plate boundaries the properties of the actual plate contact are important for the overall dynamics convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain we investigate two fundamental physical states of the subduction contact one based on a fault and the other based on a subduction channel using a finite element method we determine the specific signatures of both states of the subduction contact we pay particular attention to the overriding plate in a tectonic setting of converging plates where the subducting plate is freely moving the subduction channel reduces compression relative to the fault model in a land locked basin setting where the relative motion between the far field of the plates is zero the subduction channel model produces tensile stress regime in the overriding plate even though the amount of slab roll back is small the fault model shows a stronger development of slab roll back and a compressive stress regime in the upper plate based on a consistent comparison of fault and channel numerical models we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension we conclude that the type of plate contact plays a decisive role in controlling the backarc state of stress to obtain backarc extension roll back is required as an underling geodynamic process but it is not always a sufficient condition

asymmetric lithospheric instability facilitated by shear modulus contrast implications for shear zones summary viscoplasticity has been considered to be a dominant element in causing the nucleation of shear instability leading to lithospheric weakening here we propose that a simple contrast in shear moduli may be sufficient for explaining the fast timescale asymmetric shear instability in a bimaterial setting not much attention has been paid to heterogeneous elasticity in geodynamical modelling because it is dominant only for short timescales up to now no studies have been made on asymmetric shear instability induced by elastic modulus contrast thermal mechanical numerical simulations based on high resolution from 0 4 km 0 4 km to 0 2 km 0 4 km meshes finite element methods were performed to understand the effects of shear modulus contrast on inducing asymmetric instabilities strain rate and stress dependent rheology are used with a wide range of activation energy 0 850 kj mol 1 for all models numerical results with enough shear modulus contrast show asymmetric shear instability which is generated around the interface and then propagates across the interface two parts of the lithosphere with different shear moduli stiff for higher and soft for lower shear modulus lithospheres which are simply connected to each other without a pre defined weak zone were compressed at a constant rate of 2 cm yr 1 having different shear modulus is justified by chemical heterogeneity of geological minerals and their pressure temperature dependence to explore the dynamical effects generated by the contrast in the elastic modulus the shear modulus of the soft lithosphere is fixed at 32 gpa whereas that of stiff lithosphere is increased systematically from 32 up to 640 gpa we also examined the role of activation energy 0 850 kj mol 1 on the geometrical pattern and the initiation time of asymmetric shear localization the shear modulus contrast has to be close to two for triggering asymmetric shear instability and is found to be by far a more important controlling factor in causing shear instability than activation energy of the creep law the instability develops rapidly between 250 000 and 500 000 yr after deformation begins and thermal weakening in the shear zone is greater when a stronger shear modulus contrast is prescribed our work suggests that initiation of lithosphere scale asymmetric instability would be faster than previous considerations our finding stresses that naturally occurring shear modulus contrast has also important impact on many geological problems related to bimaterial instability

the dynamical control of subduction parameters on surface topography the long wavelength surface deflection of earth s outermost rocky shell is mainly controlled by large scale dynamic processes like isostasy or mantle flow the largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates having a mantle convection model with a free surface that is able to reproduce both realistic single sided subduction and long wavelength surface topography self consistently we are now able to better investigate this interaction we separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters results are diagnosed by splitting the topographic signal into isostatic and residual components and by considering various physical aspects like viscous dissipation during plate bending performing several systematic suites of experiments we are then able to quantify the topographic impact of the buoyancy rheology and geometry of the subduction zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws we identify slab dip and slightly less importantly slab buoyancy as the major agents controlling surface topography at subduction zones on earth only the island arc high and the back arc depression extent are mainly controlled by plate strength overall his modeling study sets the basis to better constrain deep seated mantle structures and their physical properties via the observed surface topography on present day earth and back through time

insights into the dynamics of an orogenic wedge from lubrication theory implications for the himalayan tectonics abstract using lubrication theory we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate the model suggests that the subducting plate motion produces a dynamic pressure in the wedge which supports the gravity load of the overriding plate lying above it stable condition a drop in the dynamic pressure results in a rotational collapse of the overriding plate unstable condition we present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges the lubrication model is also implemented in computational fluid dynamics cfd simulations to perform real scale numerical experiments considering mainly three variables 1 plate convergence angle 2 subduction rate us and 3 viscosity ratio r between the overriding plate and wedge the overriding plates attain mechanical stability when us exceeds a threshold value us otherwise they become unstable as us

an empirical relationship between seismic attenuation and velocity anomalies in the upper mantle we use recent p wave attenuation q 1 and velocity vp tomographic models of the tonga fiji region to obtain an empirical relation between attenuation and velocity for the upper mantle the attenuation and velocity anomalies show a strong inverse correlation allowing the determination of an empirical relationship the relationship is similar to a predicted relation derived from laboratory experiments on peridotite and dunite assuming the observed anomalies result from temperature variations the results here suggest that the observed anomalies are generally consistent with the effect of temperature and demonstrate that the combination of velocity and attenuation data provides powerful constraints on the physical state of upper mantle materials we find that the chosen geotherm and activation energy has a strong effect on the position of the solidus within the seismic data

separating rapid relocking afterslip and viscoelastic relaxation an application of the postseismic straightening method to the maule 2010 cgps the postseismic deformation captured with continuous global positioning system cgps monitoring following many recent mega thrust events has been shown to be a signal composed of two dominant processes afterslip on the plate interface and viscoelastic relaxation of the continental and oceanic mantles in response to the coseismic stress perturbation following the south central chile 2010 maule mw 8 8 earthquake the time series from the regional cgps network show a distinct curvature in the pathway of the horizontal motion that is not easily fit by a stationary decaying pattern of afterslip in combination with viscoelastic relaxation here we show that with realistic assumptions about the long term decay of the afterslip signal the postseismic signal can be decomposed into three first order contributing processes plate interface re locking plate interface afterslip and mantle viscoelastic relaxation from our analyses we conclude that the plate interface recovers its interseismic locking state rapidly model space ranges between an instant recovery and a period of 1 year a finding that supports laboratory experimental evidence as well as some recent studies of aftershocks and postseismic surface deformation furthermore re locking is the main cause of the curvature in the cgps signal and this study presents a plausible range of geodetic re locking rates following a megathrust earthquake

dynamics of slab rollback and induced back arc basin formation abstract we performed a numerical study to understand the dynamical mechanisms of back arc basin formation induced by slab rollback to this end we used two dimensional numerical models of an integrated plate mantle convection system with 410 and 660 km phase transitions retrograde slab migration occurs when the slab stagnates in the transition zone or when the deep section of the slab is vertical in both cases slab rollback occurs because the deep slab section obstructs the descending motion of the shallow slab section with an inclination buoyancy of the 660 km phase boundary acts as the obstructing force in the case of stagnant slab formation and an anchoring force against the horizontal motion works similarly in the case of vertical slabs to balance the horizontal component of the obstructing force a suction force at the plate boundary pulls the overriding plate toward the ocean back arc spreading is produced by means of slab rollback when the overriding plate with a weak area is fixed to the model boundary the back arc deformation becomes compressional when the overriding plate is freely movable despite trench retreat because the wedge mantle flow viscously drags the overriding plate toward the trench this implies that forces tending to actuate the overriding plate away from the trench are necessary to generate back arc extension even when trench retreat is generated by slab rollback

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

slab flattening and the rise of the eastern cordillera colombia abstract the topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening however deeper mantle processes may also influence the topography of the earth here we discuss the role of these processes in the eastern cordillera ec of colombia the ec is an active double vergent fold and thrust belt that formed during the cenozoic by the inversion of a mesozoic rift and topography there has risen up to 5 000 m cocuy sierra the belt is located 500 km away from the trench where two separate portions of the nazca plate subduct below the south american plate north of 5 n the ec rises above a flat slab subduction region volcanic arc migration implies slab shallowing by 10 ma and flattening up to the present day configuration at 6 ma the occurrence of a high vp vs anomaly and clustered seismicity below the belt at 160 km depth delineates the slab geometry and has been related to dehydration of the slab suggesting the presence of a hydrated mantle wedge we compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 ma using the age elevation relationship and the different closure temperatures of multiple thermochronologic systems results indicate that exhumation rates increased during the plio pleistocene at different wavelengths and amplitudes the small wavelength and large amplitude signals could be related to shallow crustal deformation whereas the source of the long wavelength and moderate amplitude signal has yet to be identified pulses of fast exhumation are found to be concomitant with the uplift that occurred from 7 ma to the present day previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment the highest residual topography is centered on the highest elevations of the ec whereas the lowest residual topography corresponds to the magdalena valley following the regional slab geometry we propose that the recent uplift and exhumation events were triggered by the transition from regular to flat slab subduction along with the hydration of the mantle wedge above the slab we test the dynamic feasibility of our hypothesis with a series of numerical models for the present day state predicting the correct trends in elevation requires a flat slab geometry and a weak and buoyant mantle wedge

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

stress dependent power law flow in the upper mantle following the 2002 denali alaska earthquake abstract far field continuous global positioning system gps time series data following the 2002 m7 9 denali alaska earthquake imply that mantle viscoelastic rheology is stress dependent a linear viscous mantle cannot explain fast early displacement rates at the surface that rapidly decay with time whereas a power law rheology where strain rate is proportional to stress raised to the power of 3 5 0 5 provides decay rates and spatial patterns in agreement with observations this is consistent with laboratory measurements for hot wet olivine implying a hydrated mantle and a relatively thin 60 km thick lithosphere beneath south central alaska these results suggest that the viscous strength of the lithosphere varies both spatially and temporally and that effective viscosities inferred from different loading events or observational time periods can differ by up to several orders of magnitude thus the very conditions that enable the inference of rheologic strength transient loading and unloading events significantly alter the effective viscosity

numerical modelling of lithosphere asthenosphere interaction in a subduction zone we developed a new 2 d numerical approach to study solid fluid coupling applied to subduction zones the lithosphere is characterised by an elastic or elastoplastic behaviour and the asthenosphere by a homogeneous isoviscous fluid the temperature effects are ignored and viscosity and density are constant in time the solid and the fluid problem are discretised by the finite elements method fem the same solid code used in hassani et al hassani r jongmans d chery j study of plate deformation and stress in subduction processes using two dimensional numerical models j geophys res 1997 102 17951 17965 has been used to compute the solution of the solid problem the stokes problem is solved by a direct solver with a stabilisation procedure we used a very simple staggered coupling method where the fluid domain is regularly re meshing we observed numerical instabilities when the time step is not sufficiently small especially when strong coupling between the solid and the fluid occurs we have tested different configurations where the lithosphere is elastic or elastoplastic and show how the slab geometry the topography and the stress regime in the plates are affected by the viscous resistance of the mantle we observed that the asthenosphere viscosity is a fundamental parameter in the subduction process for subduction with an extensional regime in the upper plate we observe a linear decrease of the extensional stress as a function of the asthenospheric viscosity

nature of the plate contact and subduction zones diversity abstract in recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision we investigated two fundamental states of the plate contact one based on a fault and the other based on a subduction channel using geodynamic modeling we determined the speci c signatures of both states of the subduction contact we established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate and is one of the controlling factors whether back arc extension develops or not in the present study we combine results of our previous numerical experiments with a re analysis of published observations overall our synthesis connects seismic moment release with back arc deformation and tectonic processes at the margin it leads us to identify four classes of subduction zones the rst two classes result directly from our numerical experiments in class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin in class 2 the plate contacts are largely channel type and have an erosive margin class 3 where the plate contact is entirely channel like consists of accretionary margins with a high sediment supply subduction zones of class 4 mostly characterized by an erosive convergent margin northern chile peru honshu and kuril are more complicated they can be explained by incorporating regional observations

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

plate motions and the viscosity structure of the mantle insights from numerical modelling abstract the viscosity structure of the earth s mantle is likely to play an important role with respect to the motion of the lithospheric plates a zone of low viscosity in the upper mantle as postulated by seismological studies and also by investigations of postglacial uplift has been proposed to facilitate plate motion another inferred feature of the viscosity profile in the mantle is a significant increase at greater depth we have employed a three dimensional mantle convection model to explore the relation between the appearance of plate motion and the viscosity structure of the mantle beneath the model allows for a complex fluid rheology strong temperature pressure and stress dependence of the viscosity and can so account for the self consistent formation of plates at the surface of the convecting mantle plate like motion appears only in a limited parameter range and we demonstrate that this appearance is closely linked to a specific viscosity structure the viscosity is characterized by a local minimum at shallow depth low viscosity zone and by a local maximum at greater depth self consistent plate motion is only observed if such a viscosity structure exists in order to determine the functional relationship between the viscosity structure and plate motion we also employ an approach in which a certain viscosity profile is prescribed these studies again reveal the importance of the local extrema in the viscosity profile taking into account an increased interior viscosity also obtained by the pressure dependence we find stable plate motion as a general result we observe that the balance of the temperature and stress dependence of the viscosity is necessary to obtain plate motion at the same time we find a viscosity profile showing a minimum at shallow depth and a maximum at greater depth

magnetotelluric study of the earth s crust along the deep seismic reflection profile dekorp 2 n in 1986 1987 53 magnetotelluric soundings were carried out along the deep seismic reflection line dekorp 2 n crossing the munsterland basin and the rhenish massif examination of the data suggests one dimensional interpretation to be appropriate for the munsterland sites and still reasonable as an approximation for the major part of the rhenish massif sites in some cases the data are disturbed by man made noise in particular at the northern border of the rhenish massif or affected by static shift distortion effects evidencing small scale near surface electrical conductivity inhomogeneities one dimensional modeling for undistorted stations reveals a good conductor at a depth of 6 8 km in the munsterland basin and at a depth of 14 16 km in the rhenish massif shallowing to the north thus far earlier mt results for these regions are largely corroborated comparison with reflection seismic results shows the conductor of the rhenish massif to coincide fairly well with a poorly reflective zone in the middle crust associated with strong reflectors at its upper and lower boundaries the discussion of possibly responsible electrical conduction mechanisms considers electronic conduction to be most favorable for explanation of the munsterland good conductor black shales with high organic content are regarded as being primarily capable of producing the observed high conductance because of adequate pre graphitization of organic matter which may form a conducting network at conditions of very low grade metamorphism in accordance with the crustal structure discernible from seismic reflections an extension of this black shale horizon to the south into the rhenish massif is discussed at the base of overthrusted rock masses it could have served as a gliding horizon during the variscan folding era mobilization of its organic content and redeposition in shear zones would explain the good conductor of the rhenish massif by means of electronic conduction therefore concurrence between the good conductor and strong seismic reflecting elements appears conceivable provided the latter represent shear zones as well electrolytic conduction often supposed to be causative to high conductivity layers at midcrustal depths would require considerable connected pore space if the latter interpretation is correct the good conductor observed will indicate a recent thermal or extensional process

viscous flow model of a subduction zone with a faulted lithosphere long and short wavelength topography gravity and geoid the essential features of observed topography free air gravity and the geoid over subduction zones have been reproduced with a dynamically self consistent viscous flow model of a subduction zone with a faulted lithosphere a fault between the overriding and subducting plates is crucial to understanding lithospheric deformation and gravity in a subduction zone the topography for models with a fault is characterized by a deep and narrow trench and a broad and shallow back arc depression quite different from viscous models without faults which only have a broad and large depression over subducting slabs for a model in which a slab is 600 km long and 80 myr old the resulting trench is approximately 3 km deep and 100 km wide and the back arc basin is about 1 5 km deep and 600 km wide the model is characterized by a long wavelength geoid high of about 30 m over the slab a local geoid low of 10 m over the trench and a 150 mgal gravity low over the trench the fact that trench depth and width are reproduced in models with a small resisting shear stress 50 bars suggests that convergent plate boundaries are probably weak

effects of oblique subduction on the 3 d pattern of small scale convection within the mantle wedge 1 we have developed a 3 d numerical model of obliquely subducting slab and mantle flow above using this model we explore the effects of oblique subduction on the small scale convection within the mantle wedge we find that the spatial pattern and time dependent characteristics of oblique subduction are similar to those of non oblique cases whose subduction speed is the same as the subduction velocity component normal to the strike of subducting slab this also implies that the axes of rolls of small scale convection are nearly normal to the plate boundary assuming that the plate boundary represents the strike of subducting plate only when the back arc lithosphere overlying the small scale convection moves along the strike of subduction a significant change of the orientation of roll axes is observed

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

overriding plate thickness control on subducting plate curvature subducting plate sp curvature exerts a key control on the amount of bending dissipation associated with subduction and the magnitude of the subduction resisting bending force however the factors controlling the development of sp curvature are not well understood we use numerical models to quantify the role of sp rheology on the minimum radius of curvature rmin we find that rmin depends strongly on the sp thickness when the rheology is viscous this dependence is substantially reduced when the sp behaves plastically in line with the lack of correlation between rmin and sp thickness on earth in contrast plasticity leads to a strong positive correlation between rmin and the overriding plate op thickness using an analysis of rmin versus op thickness we show that such a positive correlation exists on earth this suggests that op structure in conjunction with sp plasticity is crucial in generating slab curvature systematics on earth

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

a simple semi dynamic model of the subduction zone effects of a moving plate boundary on the small scale convection under the island arc summary a simple semi dynamic numerical model of the subduction zone which incorporates the plate velocity and a moving plate boundary as a priori constraints has been developed this model may be useful for studying specific subduction zones where a history of kinematic movements of plates is well constrained as an application of this model the effects of a moving plate boundary on the nature of the small scale convection in the mantle wedge were studied under the restriction of given relative velocity between two converging plates the absolute velocity of each plate was changed and i found that the characteristics of small scale convection are not significantly affected by the movement of the plate boundary at least within the parameter range used in this study i also found a small systematic change which may be related to the difference of the heat supply to the mantle wedge caused by the moving plate boundary thus the model with a fixed continental plate and moving oceanic plate with a given relative velocity may be sufficient for studying the present subject this model can be extended into a simple 3d subduction zone model

influence of lithosphere asthenosphere interaction on the overriding lithosphere in a subduction zone numerical modeling 1 several factors influence the topography of the overriding plate in a subduction zone mantle wedge flow caused by motion of the descending slab lithosphere elasticity and interaction between the solid lithosphere and the viscous asthenosphere this paper presents the results of new subduction modeling that incorporates both elastic elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere the effects of fluid solid coupling and interaction were considered using an arbitrary lagrangian eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate modeling results show that thicker lithosphere which is harder to deform contributes to the development of broadly shallower back arc basins as lithospheric strength decreases its ability to withstand outside force fluid load resulting from flow in the mantle wedge also declines leading to the development of deep and narrow basins large scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern which in turn increases the fluid load and thereby enhances topographic variation generally the effect of interaction is enhanced with lithospheric deformation the elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere similarly to previous studies back arc basins are deeper when mantle viscosity is higher for a 50 km thick elastic lithosphere the lithosphere asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 1019 pa s increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins

the bending mechanics in a dynamic subduction system constraints from numerical modelling and global compilation analysis abstract lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions to understand the mechanics of plate bending we derive scaling laws for the deflection at the margin i e radius and dip from numerical models of a subducting viscoelastic plate in such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip as well as the plate motions toward the trench this mechanical model successfully predicts the curvature of published three dimensional laboratory and numerical models for a thorough comparison with the observable we have also implemented forces additional to the slab pull such as the suction force and far field stresses by increasing or resisting the torque applied at the trench by the slab these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions although they do not alter the observed anticorrelation between radius and dip similar inverse correlation relationship and dip radius ranges are shown by most of the subduction zones analysed from a global compilation radii in the range of 100 350 km and dips of 30 70 for slabs that extends to the bottom of the upper mantle are compatible with the models and allow estimating an average lithospheric viscosity contrast of 200 in the bending with respect to the ambient mantle radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far field forces in all these subduction zones the correlation between dip radius and plate velocity is found to be compatible with that of the models showing how relevant bending is for the dynamics of earth

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

mantle convection with strong subduction zones because mantle viscosity is temperature dependent cold subducting lithosphere should be strong which implies that the rapid localized deformation associated with subduction should strongly resist plate motions due to computational constraints the deformation of a subducting plate cannot be accurately resolved in mantle scale convection models so its effect on convection is difficult to investigate we have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid by imposing velocity boundary conditions in the vicinity of the subduction zone we enforce a geometry for subduction producing a slab with a realistic thermal structure to make the model dynamically consistent we specify a rate for subduction that balances the energy budget for convection which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts this expression is determined here from a local model of bending for a strong viscous lithosphere by implementing subduction in this way we have demonstrated convection with plates and slabs that resemble those observed on earth but in which up to 40 per cent of the mantle s total convective resistance is associated with deformation occurring within the subduction zone this additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab for sufficiently strong lithosphere the bending deformation slows surface plates sufficiently that they no longer actively participate in global scale convection which occurs instead beneath a sluggish lid by introducing a low viscosity asthenosphere beneath the oceanic plate we demonstrate that small scale convection at the base of oceanic lithosphere may limit plate thickness and thus the resistance to bending and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle for a cooling earth the effective lithosphere viscosity should be nearly constant but the mantle viscosity should increase with time thus subduction resisted convection should produce nearly constant plate velocities and heat flow over time which has implications for the earth s thermal evolution we estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 10 23 pa s but only if some mechanism such as small scale convection prevents the bending resistance from stopping plates altogether such a mechanism could be fundamental to plate tectonics and earth s thermal history

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

dynamic interaction of cold anomalies with the mid ocean ridge flow field and its implications for the australian antarctic discordance abstract negative thermal anomalies beneath a mid ocean ridge are dynamically isolated from the ambient upwelling and diverging flow field in the asthenosphere whose viscosity is on the order of 5 10 19 pa s or less this study examines on what condition a near ridge cold anomaly ascends with the upwelling passive flow and spread off axis three dimensional numerical modeling demonstrates that for a given magnitude of the cold anomaly the viscosity of the asthenosphere the spreading rate and the interference from continental rifting are the dominant controlling factors to the ascent descent of the anomaly to overcome the weight of such an anomaly and couple it with the upwelling either the spreading rate or the asthenospheric viscosity has to be high in a low viscosity asthenosphere the cold anomaly also ascends during the early stage of continental rifting due to the enhanced upwelling induced by the thick continental lithosphere the dynamic interaction between the cold anomaly and the ambient flow renders a transient nature of the subsidence of the seafloor which may lead to exaggerated temperature variation estimated by using a conduction model alone the scenarios examined are employed to place a constraint on dynamic models recently proposed for the australian antarctic discordance in which the source of the negative anomaly is hypothesized to be deeply rooted in the upper mantle with the asthenospheric viscosity less than 10 20 pa s the upwelling of the cooler material from great depths which causes a significant topographic low at the discordance is made possible only by rifting of the australian continent off the gondwanaland

experimental evidence for the origin of lead enrichment in convergent margin magmas it has been proposed1 5 that the low ce pb ratio of subduction related basalts relative to their oceanic counterparts arises by the preferential transfer of lead to the mantle wedge overlying the subducting slab by non magmatic processes fluxing of the mantle wedge by low ce pb fluids generated by the dehydration of sub ducted oceanic crust is one mechanism favoured for this process see for example ref 5 here we report the results of a series of high pressure experiments which confirm that low ce pb fluids coexist with the dominant mineral phases garnet and clinopyroxene produced during high pressure dehydration of altered basalt our results show that the production of subduction zone magmas from mantle sources fluxed by basalt derived fluid is a mechanism by which relatively lead rich cerium poor mantle derived material is added to the continents the lead enrichment of the earth s continental crust is thus a continuing process occurring at conver gent margins

postseismic grace and gps observations indicate a rheology contrast above and below the sumatra slab more than 7 years of observations of postseismic relaxation after the 2004 sumatra andaman earthquake provide an improving view on the deformation in the wide vicinity of the 2004 rupture we include both gravity recovery and climate experiment grace gravity field data that show a large postseismic signal over the rupture area and gps observations in the back arc region with increasing time gps and grace show contrasting relaxation styles that were not easily discernible on shorter time series we investigate whether mantle creep can simultaneously explain the far field surface displacements and the long wavelength gravity changes we interpret contrasts in the temporal behavior of the gps grace observations in terms of lateral variations in rheological properties of the asthenosphere below and above the slab based on 1 d viscoelastic models our results support an almost order of magnitude contrast between oceanic lithosphere viscosity and continental viscosity which likely means that the low viscosities frequently found from postseismic deformation after subduction earthquakes are valid only for the mantle wedge next to mantle creep we also consider afterslip as an alternative mechanism for postseismic deformation we investigate how the combination of grace and gps data can better discriminate between different mechanisms of postseismic relaxation distributed deformation mantle creep versus localized deformation afterslip we conclude that the grace observed gravity changes rule out afterslip as the dominant mechanism explaining long wavelength deformation even over the first year after the event

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

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

origin of localized fast mantle flow velocity in numerical models of subduction 1 the origin of localized fast mantle flow near subduction zones found in regional jadamec and billen 2010 2012 and global stadler et al 2010 alisic et al 2010 numerical models using non newtonian rheology is explored using 3d models with simple geometry two suites of models using both a free slab and a slab attached to a lithospheric layer are presented to determine 1 the origin of high magnitudes of mantle flow 50 cm yr and 2 focusing of flow within the mantle wedge with mantle wedge velocities up to two times higher than slab sinking velocities the rheology in the models is either newtonian with a prescribed low viscosity wedge lvw or non newtonian wherein low viscosity regions form in response to high stress surrounding the sinking slab the results show that there are two aspects of the models that lead to localized fast velocities reduction in the mantle viscosity surrounding the sinking slab leads to faster overall flow rates induced by the negative buoyancy of the sinking slab and geometric effects that are caused by lateral variations in viscosity including retrograde pivoting motion of the slab

controls on segmentation and morphology along the back arc eastern lau spreading center and valu fa ridge back arc spreading centers increasingly depart from mid ocean ridge mor characteristics with proximity to the arc volcanic front the close association of these departures with slab derived materials in erupted lavas suggests that subduction related chemical effects are their primary cause the eastern lau spreading center elsc and valu fa ridge vfr are type examples of this process together they constitute a first order spreading center in the lau back arc basin that progressively converges on the tofua arc volcanic front from north to south here we use ship multibeam and deep towed side scan sonar data to examine variations in axial morphology and volcanism at the second and third order segment scale along these ridges and develop a model for the processes that control them closest to the arc vfr and the southern segment of the elsc shoal toward second order segment ends in contrast to mors northward and beyond 70 km from the arc the axis becomes abruptly deeper and flatter and no longer shoals toward second order segment ends at vfr along axis topographic highs correlate with the location of arc volcanoes along slab flow lines these correlations are weaker along the southernmost elsc segment and absent along elsc segments farther north the observations show a modulation of back arc segmentation with arc proximity that rapidly diminishes with distance they support a model of the mantle wedge with a strongly hydrous domain within 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc ridge interactions

sunda java trench kinematics slab window formation and overriding plate deformation since the cretaceous the kinematics and time dependence of back arc extension or compression is one of the most poorly understood aspects of plate tectonics and has nearly exclusively been studied from snapshots of present day observations here we combine absolute and relative plate motions with reconstructions of now subducted ocean floor to analyse subduction kinematics and upper plate strain from geological observations since 80 ma along the 3200 km long sunda java trench one of the largest subduction systems on earth combining plate motions and slab geometries enables us to reconstruct a time dependent slab window beneath sundaland formed through wharton spreading ridge subduction we find that upper plate advance and retreat is the main influence on upper plate strain but subduction of large bathymetric ridges and slab window effects also play a significant and at times dominant role compression in the sundaland back arc region can be linked to advance of the upper plate extension of the sundaland backarc region correlates with two patterns of upper plate motion a retreat of the upper plate and b advance of the upper plate combined with more rapid advance of the sundaland margin due to hinge rollback subduction of large bathymetric ridges causes compression in the upper plate especially wharton ridge subduction underneath sumatra over the period 15 0 ma our reconstructions unravel the evolving geometry of a slab window underlying the java south sumatra region and we propose that decreased mantle wedge viscosities associated with this slab window exacerbated palaeogene extension in the java sea region via active rifting and enabled sumatran continental extension to continue at 50 35 ma when upper plate advance would otherwise have led to compression

intraslab seismicity and thermal stress in the subducted cocos plate beneath central mexico we present a model of the subducting cocos slab beneath central mexico that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment based on the recently developed thermal models for the central mexico subduction zone the thermal stresses due to non uniform temperature contrast in the subducting slab are calculated using a finite element approach the slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep the calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 mpa on the other hand the top of the slab experiences high compressive thermal stresses up to 110 mpa depending on the elastic constants used and location along the flat part of the subducting plate these compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes and two different sources of stress are proposed the trenchward migration of the mexican volcanic arc for the last 7 ma indicates an increase of the slab dip through time this observation suggests that the gravity torque might exceed the suction torque considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0 5 10 16 1 5 10 16 n m the upper half would exhibit tensional stresses of 40 110 mpa that can actually balance the compressive thermally induced stresses an alternative stress source might come from the slab pull force caused by the slab positive density anomaly based on our density anomaly estimations 75 20 kg m 3 a 350 km slab length dipping at 20 into the asthenosphere induces a slab pull force of 1 7 10 12 4 6 10 12 n m this force produces a tensional stress of 41 114 mpa sufficient to balance the compressive thermal stresses at the top of the flat slab the linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60 180 mpa inside the flat slab core also our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40 80 mpa this study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath central mexico and also it shows that thermal stresses due to non uniform reheating of subducting slabs play a considerable role in the total stress field

variation in the distribution and properties of circumpolar deep water in the eastern amundsen sea on seasonal timescales using seal borne tags in the amundsen sea warm saline circumpolar deep water cdw crosses the continental shelf toward the vulnerable west antarctic ice shelves contributing to their basal melting due to lack of observations little is known about the spatial and temporal variability of cdw particularly seasonally a new dataset of 6704 seal tag temperature and salinity profiles in the easternmost trough between february and december 2014 reveals a cdw layer on average 49 db thicker in late winter august to october than in late summer february to april the reverse seasonality of that seen at moorings in the western trough this layer contains more heat in winter but on the 27 76 kg m3 density surface cdw is 0 32 c warmer in summer than winter across the northeastern amundsen sea which may indicate wintertime shoaling offshelf changes cdw properties onshelf in pine island bay these seasonal changes on density surfaces are reduced likely by gyre circulation

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

heterogeneity within the subducting pacific slab beneath the izu bonin mariana arc evidence from tomography using 3d ray tracing inversion techniques distinct zones of seismic heterogeneity along the izu bonin mariana arc have been investigated using complimenting regional bulk sound shear wave speed and p wave tomographic images the distribution of seismic anomalies and the inferred geometry of the subducting pacific plate have been modelled in unprecedented detail using joint tomography and new p wave model using a 3d inversion algorithm the use of 3d ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients therefore the models show much more detail about the structure and physical properties of the subduction zone the well defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the izu bonin horizontal slab and mariana vertical arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere changes in physical properties within the slab tear at the southern end of the izu bonin arc identified as a bgap or thinningq in the tomographic images could be the result of the distortion of the pacific plate as its shape transforms between near horizontal to near vertical a decrease in the rigidity and strength of the lithosphere the subduction of the marcus necker ridge change in subduction velocity or a combination of all these factors strong slow anomalies exist both in the mantle wedge above and below the slab beneath the izu bonin arc but are not present beneath the mariana arc at the junction between the izu bonin and mariana arcs it appears that the there are two separate pieces of the pacific plate the torn slab north of 268 n and a buckled near vertically dipping slab south of the tear the transition between the two distinct slab morphologies coincides with the location of the ogasawara plateau and the trench d 2005 elsevier b v all rights reserved

defect microdynamics in minerals and solid state mechanisms of seismic wave attenuation and velocity dispersion in the mantle the propagation of seismic waves in the earth s mantle can be significantly affected by relaxation processes causing attenuation and velocity dispersion reduction this paper reviews the solid state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals particularly in olivine for a given mechanism to have a significant effect on seismic wave propagation both the density and the mobility of the defects must be in an appropriate range the examination of the densities and geometry and mobilities of defects in olivine shows that dislocation and or grain boundary mechanisms can have a significant effect on seismic wave propagation although wide distributions of geometrical factors such as spacing of pinning points and of mobilities are required to explain all available data point defect mechanisms however are unlikely to be important because their densities are too small and or their mobilities are too large since the dislocation density and or grain size are determined in most cases by the long term tectonic stress seismic wave attenuation and velocity dispersion reduction involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature theoretical considerations suggest a wide range of dependence of seismic wave attentuation and velocity dispersion on the long term tectonic stress this is particularly the case for dislocation mechanisms and warrants careful experimental investigation dislocations and or grain boundaries cause anelastic behavior relaxation peaks when they are pinned or blocked at some points pinning or blocking becomes ineffective at high temperatures and or low frequencies causing a transition to viscoelastic behavior both laboratory and seismological observations of internal friction are dominated by the high temperature background where internal friction increases monotonically with temperature which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times however in most experimental studies to date the dislocation densities or the grain sizes were not well controlled making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to earth the need for better characterization of defect microstructures in experimental specimens is emphasized

abrupt upper plate tilting during slab transition zone collision abstract the sinking remnant of a surface plate crosses and interacts with multiple boundaries in earth s interior here we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper mantle transition zone and its corresponding surface elevation signal we unravel for the first time that the collision of the sinking slab with the transition zone induces a sudden dramatic downward tilt of the upper plate towards the subduction trench unraveling this crucial interaction was only possible thanks to state of the art numerical modelling and post processing the new model that is introduced here to study the dynamically self consistent temporal evolution of subduction features accurate subduction zone topography robust single sided plate sinking stronger plates close to laboratory values an upper mantle phase transition and simple continents at a free surface to distinguish the impact of the new physical model features three different setups are used the simplest model setup includes a basic high viscosity lower mantle the second adds a 660 km phase transition and the third includes additionally a continental upper plate common to all models is the clear topographic signal upon slab transition zone interaction the upper plate tilts abruptly towards the subduction trench by about 0 05 and over around 10 ma this dramatic increase in upper plate tilt can be related to the slab induced excitation of the high viscosity lower mantle which introduces a wider flow pattern a large change in horizontal extent of inundation of up to 900 km is observed as a direct consequence of the upper plate tilting such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large scale surface elevation and might explain continental tilting as observed in australia since the eocene and north america during the phanerozoic

deformation cycles of subduction earthquakes in a viscoelastic earth subduction zones produce the largest earthquakes over the past two decades space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes the short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle piecing together geodetic snapshots from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short term years and long term decades and centuries viscous behaviour of the mantle traditional views based on elastic models such as coseismic deformation being a mirror image of interseismic deformation are being thoroughly revised snapshots of subduction zones using space geodesy reveal that the viscous behaviour of the mantle controls crustal deformation requiring the revision of traditional elastic models for earthquake risk assessment space geodesy the use of satellites to monitor earth has revolutionized our view of crustal deformation between consecutive earthquakes however the brevity of such measurements means that studies must be done by comparing multiple subduction zones at different stages of the earthquake cycle by piecing together geodetic snapshots from the sumatra chile and cascadia subduction zones this review presents a unifying picture in which deformation is controlled by the viscous behaviour of the mantle in both the short term years and the long term decades to centuries

influence of the zone of weakness on dip angle and shear heating of subducted slabs we study the importance of the zones of weakness and the pattern of downgoing flow in steady state models of subducting lithosphere which interacts mechanically and thermally with the ambient mantle the non linear system of governing equations consists of i the momentum equation in stream function formulation and ii the steady state heat transfer equation including conduction and advection of heat and dissipation a finite element method has been applied to this system we consider the viscosity to be a non linear function of both the temperature and the stream function in steady state two dimensional 2d flow the stream function isolines follow material trajectories they are used to follow the top of the subducting slab which because of its possible increase in water content is assumed to have a lower viscosity the zone of weakness has been thus obtained in the self consistent fashion since the stream function as well as the temperature are the output from our modeling and no a priori assumptions about the shape of the bending lithosphere are taken into account it was shown that several orders decrease of viscosity in the zone of weakness is required to obtain the dip angle of about 45 if the decrease of viscosity is not sufficient enough the subducted slab either sinks almost vertically or does not exhibit a plate like behavior we have also demonstrated that shear heating can unrealistically increase at the zone of weakness for fast subductions if decrease of viscosity is underestimated 2004 elsevier b v all rights reserved

switching between alternative responses of the lithosphere to continental collision summary we study possible responses to arc continent or continent continent collision using numerical models our short term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones 1 mantle wedge 2 plate interface and 3 lower continental crust depending on which of these is the weakest zone in the system three different responses can be recognized 1 subduction polarity reversal 2 continuation of subduction and 3 delamination and back stepping subduction polarity reversal occurs if the mantle wedge is the weakest zone in the system this happens only if the viscosity of the mantle wedge is at least one order of magnitudelowerthantheaverageviscosityofthelithosphere incontinent continentcollision one additional condition needs to be satisfied for subduction polarity reversal to occur for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0 006 the time required for polarity reversal depends on several parameters the convergence rate the sinking velocity of the detached slab and the relative strength of the mantle wedge arc and backarc the response to collision is continued subduction if the plate interface is the weakest zone and is delamination and back stepping if the lower continental crust is the weakest area in the system our finding that a low viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed

constraining cretaceous subduction polarity in eastern pacific from seismic tomography and geodynamic modeling interpretation of recent mantle seismic images below the america ignited a debate on the cretaceous subduction polarity in the eastern pacific ocean the traditional view is that the massive vertical slab wall under eastern north america resulted from an eastward farallon subduction an alternative interpretation attributes this prominent seismic structure to a westward subduction of the north american plate against a stationary intraoceanic trench here i design quantitative subduction models to test these two scenarios using their implied plate kinematics as velocity boundary conditions modeling results suggest that the westward subduction scenario could not produce enough slab volume as seismic images reveal as is due to the overall slow subduction rate 2 5 cm yr the results favor the continuous eastward farallon subduction scenario which with an average convergence rate of 10 cm yr prior to the eocene can properly generate both the volume and the geometry of the imaged lower mantle slab pile the eastward subduction model is also consistent with most cretaceous geological records along the west coast of north america

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

gravity signature of crustal subduction inferred from numerical modelling summary gravitational signatures of subduction are a major feature of the earth s gravity pattern being visible as lineaments parallel to the arc trench systems the novelty of our study resides in the simulation of this subduction fingerprint via a newly developed dynamic subduction model based on the finite element approach which includes the compositional stratification of the lithosphere into a light upper crust and a dense lithospheric mantle crustal material sinks in the mantle in proximity to the top of the slab due to the viscous dragging forces and off scraping of the overriding plate we show that the sinking of light crustal material from both the subducting and overriding plates to depths of the order of 200 km may be responsible for a trough of about 100 mgal extending 100 200 km in a direction perpendicular to the trench this anomaly is superimposed on a global positive gravity anomaly due to thermal contraction of the mantle lithosphere system the negative gravity anomaly due to compositional stratification of the crust has comparable magnitude and horizontal extent of that due to the negative dynamic topography of the trench the gravity patterns from compositional stratification of the crust and from dynamic trench topography are evaluated for different tectonic subduction styles related to active convergence with and without roll back in general we found that case without roll back produces a broader and smoother dynamic topography gravity anomaly than with roll back while the gravity signature from compositional stratification is less sensitive to the style of subduction

flat slab subduction topography and mantle dynamics in southwestern mexico topography above subduction zones arises from the isostatic contribution of crustal and lithospheric buoyancy as well as the dynamic contribution from slab driven mantle flow we evaluate those effects in southwestern mexico where a segment of the cocos slab subducts horizontally the eastern part of the volcanic arc the trans mexican volcanic belt stands at an average elevation of 2 3 km nearly 1 3 km above the fore arc lateral changes in bulk crustal density are relatively small and seismic imaging shows that there is little variation in crustal thickness between these two regions thus the elevation difference between the arc and the fore arc should arise from differences in mantle properties we present finite element models of flat slab subduction that provide a simultaneous match to topography plate velocities and stress state in the overriding plate we find that the dynamic effects are primarily controlled by the amount of coupling at the subduction interface and in the mantle wedge the lack of slab anchoring into the lower mantle and the absence of continental mantle lithosphere with a mantle wedge and a subduction interface that are respectively 2 and 4 orders of magnitude weaker than the asthenosphere the flat slab exerts a downward pull that can explain most of the elevation difference between the fore arc and the arc we infer that lateral viscosity variations play a significant role in shaping dynamic topography in complex tectonic settings and that sublithospheric dynamics can influence the topography at wavelengths that are significantly shorter than previously recognized

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

mantle wedge asymmetries and geochemical signatures along w and e ne directed subduction zones article i nfo subduction zone kinematics predict that assuming a fixed lower plate the velocity of the subduction equals the velocity of the subduction hinge vs vh in all subduction zones the subduction hinge migrates toward the lower plate however two main types of subduction zones can be distinguished 1 those where the upper plate converges toward the lower plate slower than the subduction hinge mostly w directed and 2 those in which the upper plate converges faster than the subduction hinge generally e or ne directed along the first type there generally is an upward flow of the asthenosphere in the hanging wall of the slab whereas along the opposite second type the mantle is pushed down due to the thickening of the lithosphere the kinematics of w directed subduction zones predict a much thicker asthenospheric mantle wedge larger volumes and faster rates of subduction with respect to the opposite slabs moreover the larger volumes of lithospheric recycling the thicker column of fluids rich hotter mantle wedge all should favour greater volumes of magmatism per unit time the opposite e ne directed subduction zones show a thinner if any asthenospheric mantle wedge due to a thicker upper plate and shallower slab along these settings the mantle wedge where the percolation of slab delivered fluids generates melting mostly involves the cooler lithospheric mantle the subduction rate is smaller andesites are generally dominant and the lithosphere thickens there appears to be a greater contribution to the growth of the continental lithosphere another relevant asymmetry that can be inferred is the slab induced corner flow in the mantle along w directed subduction zones and an upward suction of the mantle along the opposite e or nne directed slabs the upward suction of the mantle inferred at depth along e ne directed subduction zones provides a mechanism for syn subduction alkaline magmatism in the upper plate with or without contemporaneous rifting in the backarc positive 11 ba nd high 143 nd 144 nd characterize w directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab however this observation disappears where large amounts of crustal rocks are subducted as along the w directed apennines sub duction zone

pull down basin in the central part of japan due to subduction induced mantle flow abstract the driving force for the basin subsiding against isostatic balance in and around lake biwa in the kinki district japan is discussed the lake region is characterized by strong negative bouguer anomalies especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake the large negative anomaly 50 mgal cannot be explained by low density sediments beneath it a down warping structure extending to the moho depth should be taken into account this conjecture has been strongly supported by a short period receiver function imaging which shows a clear offset of about 8 km for the moho discontinuity under the steep gravity gradient zone a question arises as to what is the driving force to create such a large down warping structure we consider that the subduction of the shallow dipping slab under the region philippine sea slab may cause crustal deformation by dragging the viscous mantle downward in order to verify this model we simulated the induced mantle flow due to the subduction of the philippine sea slab and the pressure distribution on the crust mantle boundary this numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction this offset of the slab is consistent with plate models deduced from hypocentral distributions and sp phases of the deep focus earthquakes

rapid mantle flow beneath the tonga volcanic arc abstract flow in the earth s asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates suggesting flow rates comparable to plate velocities however arc parallel shear wave splitting and geochemical observations imply along strike flow in the lau basin presenting a challenge to geodynamic models of plate driven subduction systems as plate driven flow models predict arc perpendicular fast direction with simple a type mineral texturing although b type mineral texturing would result in the fast directions developing 90 relative to a type and likely occurs in the forearc mantle such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail by tracking mineral texturing development within geodynamic flow models patterned after the tongan system in the lau basin that include along strike flow we find that the observations are best explained by models including rapid along arc flow within the subarc mantle the effect of melt on the anisotropy does not appear to be a primary control but needs to be investigated more fully the best fit to the observations is a model with along strike flow within a low viscosity channel beneath the arc at a rate of nearly 50 cm yr several times the plate motion velocities

subduction initiation with vertical lithospheric heterogeneities and new fault formation how subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied we investigate this with a geodynamic model for the puysegur incipient subduction zone pisz south of new zealand the model incorporates a true free surface elasto visco plastic rheology and phase changes our predictions fit the morphology of the puysegur trench and ridge and the deformation history on the overriding plate we show how a new thrust fault forms and evolves into a smooth subduction interface and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation consistent with two fault system at pisz the model suggests that the pisz may not yet be self sustaining we propose that the snares zone or snares trough is caused by plate coupling differences between shallower and deeper parts the tectonic sliver between two faults experiences strong rotation and low density material accumulates beneath the snares zone

numerical model of heat flow in back arc regions abstract we numerically modeled mantle wedge flow driven by subducting plates and its consequences to surface heat flow in back arc regions where an increase in surface heat flow has been observed in almost all subduction zones in order to calculate the steady state temperature and velocity field in a generic subduction zone model we utilized a new technique which does not require a priori assumptions about the position of the slab wedge boundary we demonstrated that calculated surface heat flow is very sensitive to the thermal boundary condition prescribed at the base of the model for a simple temperature dependent viscosity since an increase of surface heat flow is a common feature of many back arc regions it should be rather independent on various thermal conditions applied at the bottom we showed that such a robust behavior can be reproduced if moreover a strong pressure dependence of viscosity is taken into account the pattern of increased surface heat flow is then stable for a wide range of settings including various dip angles and age of the subducting slab the temperature and pressure dependent viscosity resulted in a nearly isothermal mantle wedge where temperatures exceeded 1200 c even in relatively shallow depths and consequently elevated surface heat flow is consistent with recent hot back arcs evidence circulation in the wedge was obtained since the employed viscosity law resulted in creation of a zone of substantially reduced viscosity in shallow depths below the continental crust

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

depth extent of the lau back arc spreading center and its relation to subduction processes seismic tomography and wave form inversion revealed that very slow velocity anomalies 5 to 7 percent beneath the active lau spreading center extend to 100 kilometer depth and are connected to moderately slow anomalies 2 to 4 percent in the mantle wedge to 400 kilometer depth these results indicate that geodynamic systems associated with back arc spreading are related to deep processes such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab the slow regions associated with the tonga arc and the lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers suggesting that slab components of back arc magmas occur through mixing at these depths

small scale convection under the back arc occurring in the low viscosity wedge abstract water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly thus we may expect a low viscosity wedge lvw above the subducting slabs the lvw coupled with a large scale flow induced by the subducting slabs may allow the existence of roll like small scale convection whose axis is normal to the strike of the plate boundary such a roll structure may explain the origin of along arc variations of mantle temperature proposed recently in northeast japan we study this possibility using both 2d and 3d models with without pressure and temperature dependent viscosity 2d models without pressure and temperature dependence of viscosity show that with a reasonable geometry of the lvw and subduction speed small scale convection is likely to occur when the viscosity of the lvw is less than 10 19 pa s corresponding 3d model studies reveal that the wavelength of rolls depends on the depth of the lvw the inclusion of temperature dependent viscosity requires the existence of further low viscosity in the lvw since temperature dependence suppresses the instability of the cold thermal boundary layer pressure i e depth dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities the model which shows a relatively moderate viscosity decrease in the lvw most of the lvw viscosity is 10 18 10 19 pa s and a wavelength of roll 80 km has a rather small activation energy and volume 130 kj mol and 4 cm 3 mol of the viscosity this small activation energy and volume may be possible if we regard them as an effective viscosity of non linear rheology

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

sea level and vertical motion of continents from dynamic earth models since the late cretaceous dynamic earth models are used to better understand the impact of mantle dynamics on the vertical motion of continents and regional and global sea level change since the late cretaceous a hybrid approach combines inverse and forward models of mantle convection and accounts for the principal contributors to long term sea level change the evolving distribution of ocean floor age dynamic topography in oceanic and continental regions and the geoid we infer the relative importance of dynamic versus other factors of sea level change determine time dependent patterns of dynamic subsidence and uplift of continents and derive a sea level curve we find that both dynamic factors and the evolving distribution of sea floor age are important in controlling sea level we track the movement of continents over large scale dynamic topography by consistently mapping between mantle and plate frames of reference and we find that this movement results in dynamic subsidence and uplift of continents the amplitude of dynamic topography in continental regions is larger than global sea level in several regions and periods so that it has controlled regional sea level in north and south america and australia since the late cretaceous northern africa and arabia since the late eocene and southeast asia in the oligocene miocene eastern and southern africa have experienced dynamic uplift over the last 20 to 30 m y whereas siberia and australia have experienced cenozoic tilting the dominant factor controlling global sea level is a changing oceanic lithosphere production that has resulted in a large amplitude sea level fall since the late cretaceous with dynamic topography offsetting this fall