exhumation and subduction erosion in orogenic wedges insights from numerical models 1 at oceanic margins syn convergent exhumation subduction erosion and inter plate coupling are intimately related but ample questions remain concerning their interaction and individual mechanisms to analyze these interactions for a thick skinned visco elastic wedge we focus on properly modeling stresses energies and topographies at the inter plate and wedge bounding interfaces using a coulomb frictional contact algorithm in this innovative plane strain free surface lagrangian finite element model fault dynamics is modulated by retreating subduction subduction is dynamically driven by slab pull due to a slab sinking in a semi analytic computationally favorable approximation of three dimensional induced mantle flow nodal trajectories show that continuous underthrusting of a slab induces a steady state corner flow through forced underplating and subsequent trenchward extrusion due to gravitational spreading this flow pattern confirms early proposed models of syn orogenic deep seated rock exhumation propelled by coexisting extension and continuous shortening at depth a distinct reduction in upward flowing material and accompanying decrease of exhumation velocities to millimeters per year as observed in nature is induced by a diversion of orogenic wedge material toward the mantle once a subduction channel is formed the key parameter affecting model evolution and spontaneous formation of a subduction channel is basal friction which modulates the amount of erosion however formation of a subduction channel entrance needs to be ensured through the deformability of the overriding plate which is influenced by applied pressure at the overriding plate tip and material properties the down dragging of the overriding plate is sufficient above a threshold inter plate shear stress of about 2 7 mpa

deformation of subducted oceanic lithosphere summary the deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in benioff zones and by the increasingly well resolved images of the upper mantle obtained by seismic p wave tomography by using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation the simplified model assumes that the lithosphere has uniform physical properties it is denser and much more viscous than the upper mantle and its penetration below 670 km depth is resisted by a step like density increase at that level the primary determinant of the style of deformation is the buoyancy number f a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate u l defined in terms of subduction rate u and slab thickness l for a small buoyancy number deformation of the slab is dominated by viscous flexure for large f down dip extension of the slab appears to dominate and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level the transition value of f at which this change in behaviour is observed is about 0 05 for constant viscosity and about 0 2 for stressdependent viscosity with a stress versus strain rate exponent of n 3 with n 3 a boudinage type instability of the slab is observed for large f penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle resisted only by the density difference the slab may initially penetrate several hundred kilometres below the 670 km level for plausible density parameters before a buckling instability causes this part of the slab to rotate and ascend comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the tonga subduction zone suggests that the effective buoyancy number f for this slab is approximately equal to the transition value that we determined experimentally we use this constraint to estimate average rheological parameters for the tonga slab which when compared with published creep deformation laws for olivine are consistent with the average temperature of the slab being about 0 4 to 0 45 times the melting temperature based on constitutive laws for olivine at shallow depths 100 km these temperatures correspond to around 590 c for wet olivine or around 700 c for dry olivine

back arc strain in subduction zones statistical observations versus numerical modeling 1 recent statistical analysis by lallemand et al 2008 of subduction zone parameters revealed that the back arc deformation mode depends on the combination between the subducting vsub and upper vup plate velocities no significant strain is recorded in the arc area if plate kinematics verifies vup 0 5 vsub 2 3 cm a in the hs3 reference frame arc spreading shortening occurs if vup is greater lower than the preceding relationship we test this statistical law with numerical models of subduction by applying constant plate velocities far away from the subduction zone the subducting lithosphere is free to deform at all depths we quantify the force applied on the two converging plates to sustain constant surface velocities the simulated rheology combined viscous non newtonian and brittle behaviors and depends on water content the influence of subduction rate vs is first studied for a fixed upper plate after 950 km of convergence steady state slab pull the transition from extensional to compressive stresses in the upper plate occurs for vs 1 4 cm a the effect of upper plate velocity is then tested at constant subduction rate upper plate retreat advance with respect to the trench increases extension compression in the arc lithosphere and increases decreases the subducting plate dip our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere even if the modeled law is shifted toward higher rates of upper plate retreat using our set of physical parameters e g 100 km thick subducting oceanic plate and short term simulations our results make valid the choice of the hs3 reference frame for assessing plate velocity influence on arc tectonic regime the subduction model suggests that friction along the interplate contact and the mantle stokes reaction could be the two main forces competing against slab pull for upper mantle subductions besides our simulations show that the arc deformation mode is strongly time dependent

lithospheric stress states predicted from long term tectonic models influence of rheology and possible application to taiwan abstract the long term stress state of the lithosphere results from a combination of boundary forces surface loads and tractions from mantle flow here we have employed a numerical technique which allows for the self consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free surface we have used this to study lithospheric dynamics in order to gain insights on the present day conditions of stress and deformation of the lithosphere our numerical approach takes the observed topography and far field plate velocities as input and computes the resultant deformation as a function of the rheological stratification our model application is inspired by taiwan tectonics stress data and earthquake focal mechanisms indicate that the upper crust in taiwan may be predominantly under compression the recent pingtung earthquake in 12 26 2006 however suggests that the sub moho mantle lithosphere under southern taiwan may be under extension if this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region for addressing this issue we have performed a number of numerical simulations in which we studied the effects of rheological stratification boundary conditions and geometry on the stress state and lithospheric dynamics we have employed both layered viscosity models and models in which we use laboratory based non newtonian and temperature dependent creep laws results show that the rheological stratification plays an important role in lithospheric deformation the relatively hot lithospheric thermal state in taiwan makes it likely that part of the lower crust is relatively weak which results in decoupling from surface near deformation with deformation at depth bending of a subducting slab underneath taiwan causes an extensional stress state in the sub moho mantle lithosphere whereas the collision of the philippine sea plate with the eurasian margin is responsible for a compressional state of stress in the crust it is demonstrated that models with a weak lower crust are also in reasonable agreement with available gps data as well as with constraints on uplift rates from long term geodynamic modelling studies

subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

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

weak zone formation for initiating subduction from thermo mechanical feedback of low temperature plasticity abstract we have addressed the problem of subduction initiation with a solid mechanical and fluid dynamical approach using a finite element method the setup has been extended by a rate sensitive coupling at the bottom of a semi brittle lithosphere and a fully coupled thermo mechanical model the central element of our model is a broad asymmetric sedimentary loading function at the passive continental margin which grows with time to 15 km two fundamentally different modes of shear zone formation have been found depending on the rheology of the creep layer mode 1 for cases of low or absent yield stress in the creep layer only the semi brittle top develops a weak zone while the rate sensitive layer acts as a decoupling shear zone mode 2 for cases with a yield strength in the creep layer strain rates above 10 15 s 1 after yielding the entire mechanical lithosphere fails on a major shear zone mode 1 fails to model subduction initiation while mode 2 creates a weak major shear zone that severs through the entire lithosphere

the effect of rheological approximations in 3 d numerical simulations of subduction and collision abstract subduction and collision zones evolve differently from one another due to different rheological properties different amounts of regional isostatic compensation and the different mechanisms by which forces are applied to the convergent plates the rheology of mantle and lithosphere is known to have the largest influence on the dynamics of subduction and continental collision however previous 3 d geodynamic models of subduction collision processes have used various rheological approximations making their results difficult to compare since there is no clear understanding on the extent of these approximations on the dynamics here we test the effect of rheological approximations in a geometrically simplified 3 d model in which the dynamics of subduction and collision is entirely driven by slab pull i e free subduction the results exhibit a wide range of behaviors depending on the rheological law employed from linear viscous lv to temperature dependent visco elasto plastic vep rheology that takes into account both diffusion and dislocation creep rheological approximations exert a strong effect on slab morphology trench migration rates and topography in the upper plate for example we find that slab dynamics varies drastically between end member models in lv approximations slabs do not break off but instead buckle at the transition zone the dynamics is dominated by low strain rates and diffuse stresses and topography is controlled by lateral extrusion of material which homogenizes amplitudes in the upper plate in simulations with non linear vep rheology slab break off occurs at the corners of the indenting continent opening of a back arc basin is fast and topography has a laterally heterogenous distribution of amplitudes opening of a back arc basin is observed only in simulations with non linear vep rheology when the rheology is approximated with multiple discrete layers the dynamics also resembles more the non linear vep case

three dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the philippine sea plate southwest japan we investigated temperature and mantle flow distributions associated with subduction of the philippine sea phs plate beneath southwest japan by constructing a three dimensional parallelepiped model incorporating a past clockwise rotation the bathymetry of the philippine sea plate and distribution of the subducting velocity within its slab the geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3 d simulation using the model we estimated a realistic and high resolution temperature field on the subduction plate interface which was constrained by a large number of heat flow data and attempted to clarify its relationship with occurrences of megathrust earthquakes long term slow slip events l sses and nonvolcanic low frequency earthquakes lfes results showed that the oblique subduction coupled with the 3 d geometry of subducting phs plate was a key factor affecting the interplate and intraplate temperature distributions leading to a cold anomaly in the plate interface beneath western shikoku the bungo channel and the kii peninsula temperatures in the slab core in these regions at a depth near the continental moho were nearly 200 c lower than that in eastern shikoku indicating a high thermal lateral heterogeneity within the subducting plate the geothermal control of the lfes beneath western shikoku was estimated to be within a range from 400 to 700 c and the interplate temperature for the l sses with a slip larger than 15 cm beneath the bungo channel was estimated to be approximately 350 500 c a large horizontal temperature gradient of 2 5 c km was present where the lfes occurred repeatedly the steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid ocean ridge basalt of the subducting phs plate

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

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

the role of elasticity in slab bending previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates here we perform a systematic investigation of the role of elasticity in slab bending using fully dynamic 2 d models where an elastic viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle we derive a scaling relationship between the bending radius of viscoelastic slabs and the deborah number de which is the ratio of maxwell time over deformation time we show that de controls the ratio of elastically stored energy over viscously dissipated energy and find that at de 10 2 substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity elastically stored energy at higher de favors retreating modes of subduction via unbending while trench advance only occurs for some cases with de 1 where most zones have low de 0 1 slabs with de 10 2 either have very low viscosities or they may be yielding in which case our de estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in 60 of the subduction zones in support of such a role of elasticity in subduction we find that increasing de correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues

fast ductile failure of passive margins from sediment loading sediment loading at continental margins has been interpreted as a very inefficient mechanism for initiating subduction we have re investigated this problem using a time dependent non linear ductile failure analysis in which the effects of power law plastic strain hardening are assessed the results follow classical flexural rigidity solutions until wholesale lithosphere failure occurs sediment loading causes two shear zones to develop from the bottom of the lithosphere upwards for strong strain hardening shear zones nucleate earlier but the plate may never break for small hardening the instability grows much faster and the plate can break under the sediment load with a limiting yield stress lower than 0 4 gpa when the ductile faults growing from the top and the bottom of the plate merge in the center the flexurally supported gravitational energy is released the lithosphere becomes unstable and shears off super exponentially within a few million years

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

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

overriding plate velocity control on surface topography in 2 d models of subduction zones we study the dynamically induced flexural topography in subduction numerical mechanical models we focus on the topographic changes at the overriding plate op surface induced by variations in op kinematics particularly when the subducting plate sp has a stationary motion after having reached the rigid base of the upper mantle our models consist of two viscoelastic plates with free surfaces and an isoviscous mantle friction is imposed along the planar subduction interface we first characterize the main topographic features at a constant op velocity using spatial definitions based on geometrical estimations of the volcanic arc position the models exhibit the formation of a bulge in the forearc area followed landwards by a depression and a smaller second bulge both bracketing the arc region the steady state distance to the trench of these features increases with op velocity their amplitude is affected by the far field op tectonic regime that depends on kinematics and plates and subduction interface strength we next test the effect of sudden changes in op velocity an op acceleration yields a transient topographic tilt during which the outer forearc quickly subsides whereas the arc region uplifts and that is followed by reverse slower motions an op slowdown induces opposite motions the rates of elevation change during the tilt are approximately proportional to velocity variations and mainly sensitive to the sp strength the rates are higher than 0 1 mm yr for velocity changes higher than 1 cm yr we suggest that topographic accommodations of op velocity changes should be considered when quantifying nonisostatic topography

evolution of subducting mantle lithosphere at a continental plate boundary although it is apparent that at continental convergence mantle lithosphere can be accommodated by a subduction like style of deformation it is not clear how this process evolves and how it is different from classical plate subduction using numerical models of the crust mantle system these issues are considered for the case of a simplified continent continent plate collision the results suggest that subducting mantle lithosphere is prone to detach resulting in a reversal of subduction polarity alternatively it may undergo symmetric plate consumption with steepening and possible roll over polarity reversal the style of post collisional subduction is largely controlled by the strength e g dependent on temperature assumed rheology of the mantle lithosphere

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

trench parallel shortening in the forearc caused by subduction along a seaward concave plate boundary insights from analogue modelling experiments abstract three dimensional thermo mechanical analogue experiments are employed to test the hypothesis that oceanic subduction along a seaward concave plate boundary can generate trench parallel shortening in the forearc near the axis of curvature the model deformation is analyzed with a particle imaging velocimetry piv system that allows for comparison of forearc deformation along the oblique limbs of the curved plate boundary and near the axis of curvature moreover piv allows for separation of the trench parallel and trench perpendicular components of strain regardless of trench orientation the resulting deformation maps show a remarkable symmetry and indicate drag of the forearc above the interplate coupling area towards the axis of curvature trench perpendicular profiles show that along the oblique limbs of the plate boundary the forearc is submitted to trench normal shortening and trench parallel shearing but not trench parallel shortening or extension this contrasts with the situation near the axis of symmetry where the forearc is submitted to trench parallel and trench perpendicular normal shortening but is not sheared the experimental results confirm that trench normal thrusts observed in the fore arc of the central andes can be a mechanical consequence of subduction along a seaward concave plate boundary if the degree of interplate coupling is large

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

slab pull effects from a flexural analysis of the tonga and kermadec trenches pacific plate summary thin plate flexure models have been frequently used to explain the mechanical behaviour of the lithosphere at oceanic trenches but little attention has been paid to using them as a way to check the relative importance of diverent plate driving mechanisms a 2 d numerical algorithm accounting for the flexural deflection of the lithosphere controlled by multilayered elastic plastic rheology brittle elastic ductile has been applied to the seaward side of the tonga and kermadec trenches this approach gives a better fit to the bathymetry on both trenches than assuming classical homogeneous plate models and allows the interplate coupling forces and the lithospheric strength profile to be constrained our results show that in order to fit the observed deflection of the lithosphere a regional tensile horizontal force must act in both regions this tensile force and its flexural evects are discussed in terms of slab pull as a main plate driving mechanism the predicted stress and yielding distributions partially match the outer rise earthquake hypocentres within the subducting plate and thus do not invalidate the model

slab breakoff insights from 3d thermo mechanical analogue modelling experiments abstract the detachment or breakoff of subducted lithosphere is investigated using scaled three dimensional thermo mechanical analogue experiments in which forces are measured and deformation is monitored using high speed particle imaging velocimetry piv the experiments demonstrate that the convergence rate in a subduction zone determine if and when slab detachment occurs slow subduction experiments with scaled convergence rates 1 cm yr 1 have lower peclet numbers and are characterized by lower tensile strength subducted lithosphere causing detachment to occur when the downward pull force exerted by a relatively short subducted slab is relatively low therefore when continental collision is preceded by slow oceanic subduction the hot and weak subducted lithosphere need not be very long or extremely negatively buoyant to cause detachment under such conditions detachment may occur sooner after the onset of continental subduction than previously predicted in contrast if collision is preceded by rapid subduction 10 cm yr 1 breakoff will be delayed and occur only when the convergence rate has slowed sufficiently to thermally weaken the slab and cause its eventual failure the analogue experiments further confirm that slab detachment occurs diachronously as it propagates along the plate boundary stereoscopic piv reveals a characteristic strain pattern that accompanies the detachment horizontal contraction and subsidence with scaled values up to 1200 m in the trench and forearc area precedes the passage of the detachment and is followed by horizontal extension and uplift up to 900 m high frequency monitoring captures rapid propagation of the detachment along the plate boundary at scaled rates of up to 100 cm yr 1 however this rate is not constant and interaction between the slab and lower mantle or opening of a backarc basin in the upper plate can reduce or stop slab breakoff propagation altogether

thermomechanical evolution of a thinned continental lithosphere under compression implications for the pyrenees in order to demonstrate the interactions between plate boundary forces and preexisting zones of weakness and how those control the localization of continental deformation inside the lithosphere the thermomechanical evolution of an initially thinned and thermally perturbed continental lithosphere in compression was analyzed within the framework of the pyrenees while a concensus exists as to the shallow structures the extent and geometry of deformation at depth have been the subject of great controversy the thermomechanical implications of recent geological hypotheses on the initial state prior to the compression recently derived from balanced cross section reconstructions are here quantified and compared to the recent ecors deep seismic reflection profile across the pyrenees a thermomechanical model based on finite elastoplastic viscoplastic deformations has therefore been formulated using phenomenological constitutive equations derived from experimental studies the thermally coupled problem is solved by an appropriate vertical two dimensional plane strain finite element approximation in those models most of the shortening is accommodated inside a local zone of intense plastic shearing that creates a sharp wedge shaped root progressively sinking deep into the mantle the main thickening takes place inside the lower crust where it can reach a factor 2 this indentation and crustal imbrication process is controlled by a buckling instability occuring inside the uppermost part of the mantle the evolution of the topography can be related to these phenomena lateral conductive heat transfer overtakes the advective transfer in the vicinity of the inflow of colder crustal material inside a warmer mantle the deformation pattern can be explained by a three layer mechanical lithosphere with a soft ductile internal layer of potential decoupling between two more competent layers characterized by a significant elastic stored energy the overall response of this composite plate is not much influenced by smaller scale variations of crustal rheology but is very much dependent on the contrast in rates of viscous dissipation between the lower crust and the upper mantle and on the possibility of singling out a layer of significant elastic stored energy by the balance between rate of loading and rate of dissipation inside the uppermost mantle predicted deformation pattern is found to agree with the seismic reflection and gravity data even though extrapolation to other rates of shortening must be verified by appropriate numerical modelling these physical mechanisms may be regarded as a paradigm for other crustal imbrication phenomena found in the himalayas tibet or the alps

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

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

dynamic models of subduction geophysical and geological evidence in the tyrrhenian sea summary predictions based on a 2 d finite element model for subduction underneath the calabrian arc in southern italy are compared with a variety of geophysical and geological data such as the present day stress pattern within the slab uplift from the elevation of marine terraces in calabria and subsidence in the tyrrhenian marsili basin from odp leg 107 we model the behaviour of the slab driven by slab pull in agreement with the present tectonic style in this part of the mediterranean as suggested by several investigators the model accounts for the crustal lithospheric and mantle structures in a vertical cross section perpendicular to the calabrian subduction zone the shape of the slab is constrained on the basis of new tomographic images in the southern tyrrhenian sea which were obtained from the regional seismic stations of the istituto nazionale di geofisica while the rheological properties of the mantle are taken from global dynamic models density contrasts between the subducted slab and the surrounding mantle based on petrological models drive the flow in our viscoelastic model stress values displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached our estimates are appropriate for a time window of 100 kyr the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day two families of models with unlocked and locked subduction faults are considered the unlocked models allow for roll back of the trench of about 20 mm yr in agreement with some geological estimates the same family of models predicts uplift of the calabrian arc of about 1 mm yr l and subsidence in the marsili basin of 1 2 mm yr in agreement with geological surveys the deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern tyrrhenian sea with minor concentration within the lithospheric wedge locked models fail to reproduce these geophysical and geological observations predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern tyrrhenian sea deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate dynamic topography and horizontal motions are reduced when compared with the continuous plate with deviatoric stresses concentrated within the relict slab our results indicate that subduction is a major tectonic process in the southern tyrrhenian sea

3 d simulation of tectonic evolution of the mariana arc back arc system with a coupled model of plate subduction and back arc spreading abstract we developed a mechanical model for the nonlinear coupled system of plate subduction and back arc spreading on the basis of mathematical formulation for internal deformation due to a moment tensor in an elastic viscoelastic layered half space under gravity in our modeling the plate subduction and the back arc spreading are rationally represented by the increase of tangential displacement discontinuity at the plate interface and normal displacement discontinuity at the spreading center respectively through 3 d numerical simulation with this model we obtained a possible scenario for the tectonic evolution of back arc basins as follows at the first stage steady plate subduction gradually forms tensile stress fields in the back arc region when the accumulated tensile stress reaches a critical level back arc spreading starts at a structurally weak portion of the overriding plate the back arc spreading pushes out the frontal part of the overriding plate toward the plate boundary and leads to the increase of slip rates at the plate interface the local increase of slip rates at the plate interface produces additional tensile stress in the back arc region the incremental tensile stress is canceled out by further back arc spreading such a feedback mechanism is necessary to maintain steady back arc spreading the long duration of slip rate excess due to back arc spreading leads to the gradual protrusion of the plate boundary toward the descending plate trench retreat as the plate boundary moves away from the back arc spreading center the accumulation rate of tensile stress at the spreading center gradually decreases and the slip rate excess at the plate interface due to back arc spreading also decreases then the original back arc spreading center becomes less effective in releasing the tectonic tensile stress and new active back arc spreading will start somewhere closer to the plate boundary such a qualitative scenario accords with the evolution history of back arc basins in the mariana region we obtained the slip rate excess of 7 mm year at the plate interface and the 20 km trench retreat for the last 4 5 myr these simulation results are significantly smaller than observations in the mariana region the discrepancy between simulation results and observations can be ascribed to the effect of the negative buoyancy acting on cold descending slab which advances spontaneous plate subduction and directly increases the slip rate excess and the trench retreat in our model

the connectivity of two dimensional networks of spatially correlated fractures 1 continuum percolation thresholds for fracture systems are based on models with random fracture locations resulting in uniformly clustered systems in which all connections are fracture intersections fractures in natural systems can be either clustered or anticlustered and a significant portion of the connectivity is achieved through fracture abutments or splays analytical and numerical methods are combined to define critical properties of isotropic networks of constant length lines as a function of fracture clustering and of the ratio of abutments to intersections a second suite of numerical results indicates that the influence of these parameters varies as a function of the fracture length distribution a preliminary predictor of critical density of anisotropic nonrandom systems with lognormal length distributions is derived and applied to a suite of natural fracture systems predicted connectivities are closer to the observed connectivities when the predictor is used than when connectivity is estimated from the random threshold

present day vertical isostatic readjustment of the western alps revealed by numerical modelling and geodetic and seismotectonic data the active tectonics of the western alps reveals contrasting regimes ongoing extension at the heart of the chain and transpression compression at its external sectors the active processes currently affecting this region are still a matter of debate the classical models proposed in the literature invoke eurasia adria plate collision counterclockwise motion of the adria micro plate slab retreat of the subducted continental lithosphere and slab detachment more recently several authors prefer the hypothesis of tectonics driven by isostasy buoyancy forces to better understand the influence of these processes on the velocity strain and stress fields at the surface and in the crust we developed 2d viscoelastic numerical models along a vertical cross section perpendicular to the western alps we run our models with different driving forces in order to investigate one by one the geodynamic processes proposed in the literature results are compared with available geodetic geological and seismotectonic data in order to bring into coincidence model predictions and observations an important vertical isostatic readjustment must be included in the modelling together with a slight horizontal compression 0 5 mm year 1 probably due to africa eurasia convergence we show that the subduction process in this alpine region is likely to be dead and that buoyancy forces may be dominating the present day tectonics the geological society of london 2010

dynamics of plate bending at the trench and slab plate coupling 1 the bending strength of subducting lithosphere plays a critical role in the earth s plate tectonics and mantle convection modulating the amount of slab pull transmitted to the surface and setting the boundary conditions under which plates move and deform however it is the subject of a lively debate how much of the potential energy of the downgoing plate is consumed in bending the plate and how the lithospheric strength is defined during this process we model the subduction of a viscoelastic lithosphere driven solely by the downgoing plate s buoyancy freely sinking in a passive mantle represented by drag forces to investigate the dynamics of bending 1 we vary the density and the viscosity profile within the plate from isoviscous where strength is distributed to strongly layered where strength is concentrated in a thin core and 2 we map the stress strain and dissipation along the downgoing plate the effective plate strength during bending is not a simple function of average plate viscosity but is affected by rheological layering and plate thinning earth like layered plates allow for the transmission of large fractions of slab pull 75 80 through the bend and yield a net slab pull of fspnet 1 to 6 1012 n m 1 which varies with the buoyancy of plates in all models only a minor portion of the energy is dissipated in the bending surprisingly bending dissipation hardly varies with lithospheric viscosity because in our dynamic system the plates minimize overall dissipation rate by adjusting their bending curvature as a result bending dissipation b is mainly controlled by the bending moment work rate exerted by slab pull we propose a new analytical formulation that includes a viscosity dependent bending radius which allows for assessment of the relative bending dissipation in the earth s subduction zones using parameters from a recent global compilation this yields estimates of b tot 20 these results suggest that plates on earth weakly resist bending yet are able to propagate a large amount of slab pull

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

influence of overriding plate geometry and rheology on subduction 1 subduction dynamics is strongly dependent on the geometry and rheology of the subducting slab and adjacent plates as well as on the induced mantle flow driven by the evolution of tectonic configurations along subduction zones however these processes and the associated plate tectonic driving forces are difficult to study using time dependent 3 dimensional computer simulations due to limitations in computing resources we investigate these phenomena with a novel numerical approach using bem earth a stokes flow solver based on the boundary element method bem with a fast multipole fm implementation the initial bem earth model configurations self consistently determine the evolution of the entire lithosphere mantle system without imposing additional constraints in a whole earth spherical setting we find that models without an overriding plate overestimate trench retreat by 65 in a 20 m y model run also higher viscosity overriding plates are associated with higher velocity subducting slabs analogue to faster oceanic plates subducting beneath more rigid continental lithosphere in our models poloidal flows dominate the coupling between the down going and overriding plates with trench orthogonal length variations in overriding plates inducing flows at least 2 stronger than trench parallel width variations however deformation in the overriding plate is related to its length and width with narrower and longer plates extending more than wider and shorter plates

free surface formulation of mantle convection i basic theory and application to plumes in mantle convection models the top surface is traditionally approximated as a free slip boundary and the dynamic topography is obtained by assuming that the normal stress on the free slip boundary is compensated instantaneously through surface deformation it has already been shown that this approximation is valid for long wavelength topography based on both viscous and viscoelastic models with a free surface we have found that the characteristic time for topographic growth is comparable to the timescales of mantle convection 10 6 year for short and intermediate wavelengths 10 3 km or less and or a high effective lithospheric viscosity 10 24 pa s this suggests that the topography is history dependent under these conditions and that a free surface formulation is required to study the topography at these wavelengths we have developed a new eulerian finite element technique to model a free surface since the technique is based upon an undeformable eulerian grid this enables us to study long term free surface dynamics in the presence of evolving buoyancy we have compared numerical with analytic solutions of viscous relaxation for fixed buoyancy problems as long as the magnitude of topography is much smaller than the wavelength we find that the finite element method is very accurate with relative errors of less than 1 per cent this numerical technique can be applied to a variety of geophysical problems with free surfaces in applying this technique to dynamic models of mantle plumes we find that surface relaxation retards the topography at intermediate and short wavelengths and produces a smoother topography compared with topography from free slip calculations this reduced topography has a significant influence on the geoid at the corresponding wavelengths moreover free surface models by allowing vertical motion on the free surface yield a hotter lithosphere over ascending plumes than models with free slip boundaries

influence of surrounding plates on 3d subduction dynamics 1 our 3d modelling study shows that the presence of lithospheric plates around a subducting plate has a significant influence on subduction dynamics in particular on trench retreat rate slab dip and lateral shortening of the subducting plate neighbouring plates prevent unrealistic plate behaviour with no need for complex rheologies because at the earth s surface plates form a continuous shell they should not be neglected

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

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

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

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

stress and deformation mechanisms at a subduction zone insights from 2 d thermomechanical numerical modelling numerous processes such as metamorphic reactions fluid and melt transfer and earthquakes occur at a subducting zone but are still incompletely understood these processes are affected or even controlled by the magnitude and distribution of stress and deformation mechanism to eventually understand subduction zone processes we quantify here stresses and deformation mechanisms in and around a subducting lithosphere surrounded by asthenosphere and overlain by an overriding plate we use 2 d thermomechanical numerical simulations based on the finite difference and marker in cell method and consider a 3200 km wide and 660 km deep numerical domain with a resolution of 1 km by 1 km we apply a combined visco elasto plastic deformation behaviour using a linear combination of diffusion creep dislocation creep and peierls creep for the viscous deformation we consider two end member subduction scenarios forced and free subduction in the forced scenario horizontal velocities are applied to the lateral boundaries of the plates during the entire simulation in the free scenario we set the horizontal boundary velocities to zero once the subducted slab is long enough to generate a slab pull force large enough to maintain subduction without horizontal boundary velocities a slab pull of at least 1 8 tn m 1 is required to continue subduction in the free scenario we also quantify along profile variations of gravitational potential energy gpe we evaluate the contributions of topography and density variations to gpe variations across a subduction system the gpe variations indicate large scale horizontal compressive forces around the trench region and extension forces on both sides of the trench region corresponding vertically averaged differential stresses are between 120 and 170 mpa furthermore we calculate the distribution of the dominant deformation mechanisms elastoplastic deformation is the dominant mechanism in the upper region of the lithosphere and subducting slab from ca 5 to 60 km depth from the top of the slab viscous deformation dominates in the lower region of the lithosphere and in the asthenosphere considering elasticity in the calculations has an important impact on the magnitude and distribution of deviatoric stress hence simulations with increased shear modulus in order to reduce elasticity exhibit considerably different stress fields limiting absolute stress magnitudes by decreasing the internal friction angle causes slab detachment so that slab pull cannot be transmitted anymore to the horizontal lithosphere applying different boundary conditions shows that forced subduction simulations are stronger affected by the applied boundary conditions than free subduction simulations we also compare our modelled topography and gravity anomaly with natural data of seafloor bathymetry and free air gravity anomalies across the mariana trench elasticity and deviatoric stress magnitudes of several hundreds of mpa are required to best fit the natural data this agreement suggests that the modelled flexural behaviour and density field are compatible with natural data moreover we discuss potential applications of our results to the depth of faulting in a subducting plate and to the generation of petit spot volcanoes

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

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

studying the influence of a solid shell on lava dome growth and evolution using the level set method a finite element formulation of the level set method a technique to trace flow fronts and interfaces without element distortion is presented to model the evolution of the free surface of a spreading flow for a highly viscous medium on a horizontal surface as an example for this class of problem we consider the evolution of an axisymmetric lava dome equilibrium configurations of lava domes have been modelled analytically as brittle shells enclosing pressurized magma the existence of the brittle shell may be viewed as a direct consequence of the strong temperature dependence of the viscosity the temperature dependence leads to the formation of a thin predominantly elastic plastic boundary layer along the free surface and acts as a constraint for the shape and flow of the lava dome in our model we adopt iverson s assumption that the thin boundary layer behaves like an ideal plastic membrane shell enclosing the ductile interior of the lava dome the effect of the membrane shell is then formally identical to a surface tension like boundary condition for the normal stress at the free surface the interior of the dome is modelled as a newtonian fluid and the axisymmetry equations of motion are formulated in a eulerian framework we show that the level set is an effective tool to trace and model deforming interfaces for the example of the free surface of a lava dome we demonstrate that iverson s equilibrium dome shapes are indeed steady states of a transient model we also show how interface conditions in the form of surface tension involving higher order spatial derivative curvature can be considered within a standard finite element framework

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

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

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

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

change in tectonic force inferred from basin subsidence implications for the dynamical aspects of back arc rifting in the western mediterranean abstract a method has been developed that allows temporal changes in tectonic force during rift basin formation to be inferred from observed tectonic subsidence curves and has been applied to the gulf of lions the provencal basin and the valencia trough in order to gain some understanding of the dynamical aspects of back arc basin rifting in the western mediterranean sea two distinct tectonic force regimes active at different times during the evolution of each of these back arc basins are identified the first which can be seen in both basins is characterized by tensional forces that gradually abate with time to vanish some 20 my after the onset of rifting the magnitude of tectonic force required to initiate the rifting process is significantly greater in the valencia trough than in the provencal basin subsequently the dynamic development of these back arc basins differs in the provencal basin there is a renewal of force with extensional deformation concentrated in the central part of the rift whereas in the valencia trough the second tectonic force regime is inferred to be one that causes compression that subsequently relaxes such temporal patterns of tectonic force are interpreted to be related to the causative driving processes allowing constraints to be placed on the transient interaction between the overriding and subducting plates in a back arc setting the models also allow inferences to be made about the rheological structure of the lithosphere a significant variation of initial crustal thickness is inferred for the provencal basin but not for the valencia trough in both basins a wet rheology is required in order to initiate rifting given currently accepted bounds on tectonic force magnitudes adoption of a dry rheology leads to insufficiently high strain rates for significant lithosphere extension in both cases

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

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

spherical dynamic models of top down tectonics 1 we use the multipole boundary element method mp bem to simulate regional and global geodynamics in a spherical 3 d setting we first simulate an isolated subducting rectangular plate with length llitho and width wlitho varying between 0 5 and 2 times the radius of the earth rearth and with viscosity litho varying between 100 and 500 times the upper mantle um sinking in a layered mantle characterized by lower upper mantle viscosity ratio lm um varying between 1 and 80 in a mantle with small upper lower viscosity contrast 1 trench and plate motions are weakly dependent on wlitho plate motion is controlled by slab pull if llitho rearth while for longer plates plate speed strongly decreases because of the plate basal friction and flow reorganization an increasing viscosity ratio gradually breaks this pattern and for 10 combined with wlitho rearth and greater trench advance and retreat are simultaneously observed these results offer a first order explanation of the origin of the size llitho wlitho rearth of the largest plates observed over the past 150 myr finally two global plate tectonic simulations are performed from reconstructed plates and slabs at 25 ma before present and before 100 ma respectively it is shown that mp bem predicts present plate kinematics if plate mantle decoupling is adopted for the longest plates llitho rearth models for 100 ma show that the slab slab interaction between india and izanagi plates at 100 ma can explain the propagation of the plate reorganization from the indian to the pacific plate

free surface formulation of mantle convection ii implication for subduction zone observables summary viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self consistently and simultaneously predict long wavelength geoid highs over slabs short wavelength gravity lows over trenches trench forebulge morphology and explain the high apparent strength of oceanic lithosphere in trench environments the models use two different free surface formulations of buoyancydriven flows see for example part i lagrangian viscoelastic and pseudo free surface viscous formulations the lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault decoupling the overriding from underthrusting plates the lower oceanic lithosphere must have a viscosity of less than pa s in order to be consistent with the flexural wavelength of forebulges forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle and give rise to apparently stiffer oceanic lithosphere at trenches with purely viscous models using a pseudo free surface formulation we find that viscous relaxation of oceanic lithosphere in the presence of rapid trench rollback leads to wider and shallower back arc basins when compared to cases without viscous relaxation moreover in agreement with earlier studies the stresses necessary to generate forebulges are small 100 bars compared to the unrealistically high stresses needed in classic thin elastic plate models

fast mechanisms for the formation of new plate boundaries abstract we demonstrate the important role played by non linear feedback in promoting the fast breakup of the continental lithosphere and the formation of new plate boundaries we compare the difference in fluid dynamic and elasto visco plastic approaches and emphasize the advection of stresses in finite element models with large strain lagrangian formulation two types of instabilities were found at multiple time length scales ranging from 100 years and km scale to 0 1 myr and 100 km scale one mechanism relies on the thermo mechanical elasto plastic energy conversion and the other stems from the void volatile interaction elasticity takes on a guiding role in the propagation of shear zones closely followed by a thermal wave which is again followed by a wave of volatiles this last mechanism consolidates the fault zones and establishes the plate boundary for a geological time scale using a fully coupled thermal mechanical approach we obtain for the first time a two dimensional solution which shows that the constant velocity boundary conditions can lead to episodic avalanche type tectonic phases within

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

subduction of the lithosphere and back arc dynamics insights from physical modeling physical modeling of the subduction of an elastico plastic lithosphere underlained by a low viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by 1 a where a and 1 are the asthenosphere and lithosphere densities at 0 a steady state process develops which requires a horizontal lithosphere compression of h 0 3 0 4 s 108 pa where s is the lithosphere yield limit the central part of the leading wedge of the overriding plate is nonisostatically elevated in this case especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200 400 mgal a decrease in 0 compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction at some point compression changes into tension tensional nonhydrostatic stresses grow during the process to a value near h 0 1 0 2 s 0 4 108 pa both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the benioif dip angle accompany this process the gravity anomaly maximum is displaced into the volcanic arc in this subduction regime where it is normally less than 150 mgal if some weaknesses are present in the overriding plate tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment the arc plate starts to move counter to the subduction plate the arc plate slides along the underthrusting plate s surface without loss of contact and causes back arc opening hydrostatic suction prevents the plates from being separated this effect is caused by the peculiarity of a mechanical system whereby a low viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate if the overriding lithosphere is not weakened enough to fail under the existing tension then the hanging slab breaks down under its own weight when it reaches a specific length both interacting plates undergo isostatic recovery in this case and the whole cycle is repeated again the hydrostatic suction effect also provides back arc tension under other conditions such as when asthenospheric flow forces the subducted plate to move away from the island arc

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

the role of boundary conditions in numerical models of subduction zone dynamics abstract numerical studies of subduction zone dynamics on a regional scale usually use a limited spatial extent for their models and therefore need to define boundary conditions on all model edges these boundary conditions imply a choice for the mechanical and thermal state of the surrounding regions which may influence the evolution of the model system we investigate the role of the surface and lateral boundary conditions for simple mechanical subduction models using a standard finite element method we find that subduction is only possible if the slab can decouple from the surface this decoupling can be achieved by a true free surface a free slip surface with a weak crust for the subducting plate or a free slip surface with a layer of low viscosity low density material sticky air between the model surface and the crust models of slab dynamics that employ a free slip surface reproduce trench migration slab sinking velocities and slab geometry of models with a free surface as long as they use either a weak crust which can be viscous viscoelastic and or brittle or a sticky air layer the vertical topography will however not be reproduced for free slip models without a sticky air layer for ocean ocean convergent models we find that the application of inflow boundary conditions at the edges of the subducting or overriding lithosphere controls trench motion and the geometry of the subducting slab inflow on the overriding side causes trench retreat and a slab resting on the lower mantle whereas inflow restricted to the subducting side can cause trench advance and a slab which folds on the lower mantle

retracted signatures of downgoing plate buoyancy driven subduction in motions and seismic coupling at major subduction zones this article has been retracted at the request of the editor in chief and authors please see elsevier policy on article withdrawal http www elsevier com locate withdrawalpolicy reason after publication errors were discovered in the plate motion database that it was based on this dataset was an updated version of the dataset presented in sdrolias and muller 2006 provided to us by the first author the errors in this version were in the away from trench towards trench assignment for subduction zones with back arcs and also due to the fact that the next generation plate model had only partially been completed these errors affect the conclusions about seismic coupling they also change some of the points in most of the other plots and although this does not invalidate the other conclusions the discussion to reach them would be altered

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

effects of elasticity on the rayleigh taylor instability implications for large scale geodynamics summary although parts of the lithosphere may be expected to behave elastically over certain timescales this effect is commonly ignored in models of large scale mantle dynamics recently it has been demonstrated that elasticity and in particular viscoelasticity may have a significant effect on the buckling instability and on the creation of lithospheric scale shearzones it is however less clear whether elasticity also has an effect on mantle convection and density driven lithospheric instabilities the focus of this work is therefore to study the effects of elasticity on the twolayer rayleigh taylor rt instability consisting of a maxwell viscoelastic layer overlying a viscous layer of lower density we analyse this problem by performing systematic numerical simulations that are compared with newly derived analytical solutions it is demonstrated that elasticity can be important for certain parameter combinations it leads to a speedup of the rt instability the cause for this speedup is that the rt instability is only sensitive to the viscous fraction of deformation in the viscoelastic layer elasticity reduces the viscous fraction of deformation at timescales shorter than the maxwell relaxation time t m t m g where is the viscosity and g the elastic shear module for plate tectonics on earth the parameters are such that the effect of elasticity on instability growth is negligible for most boundary conditions whereas elasticity does not or only slightly change the timescales for lithospheric detachment of the upper mantle it does significantly alter the response and stress build up in the overlying crust numerical simulations illustrate this effect for lithospheric detachment and show that peak stresses in a viscoelastic crust are smaller than stresses that develop in a viscous crust moreover if the timescale for delamination of the mantle lithosphere is equal or smaller than the maxwell relaxation time of the crust the topography of the crust is increased compared to viscous models

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

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

the zone of influence of the subducting slab in the asthenospheric mantle due to the multi disciplinary nature of combined geodynamics and shear wave splitting studies there is still much to be understood in terms of isolating the contributions from mantle dynamics to the shear wave splitting signal even in a two dimensional 2d mantle flow framework this paper investigates the viscous flow development of lattice preferred orientation lpo and predicted shear wave splitting for a suite of buoyancy driven subduction models to shed light on the nature of the slab driven asthenospheric flow field the models show that the spatial dimensions of the slab driven zone of influence in the mantle lpo fabric and resulting synthetic splitting are sensitive to the slab dip and slab strength with the maximum lpo fabric development occurring for models with a moderate initial slab dip and weaker slab the models show that olivine lpo fabric in the asthenosphere generally increases in alignment strength with increased proximity to the slab but can be transient and spatially variable on small length scales the results suggest lpo formed during initial subduction may persist into the steady state subduction regime the vertical flow fields in the asthenosphere can produce shear wave splitting variations with back azimuth that deviate from the predictions of uniform trench normal anisotropy a result that bears on the interpretation of complexity in shear wave splitting observed in real subduction zones furthermore the models demonstrate that the corner flow paradigm should not be equated with a 2d subduction framework

global paleo lithospheric models for geodynamical analysis of plate reconstructions we present a general framework to generate time dependent global subduction history models from kinematic plate reconstructions and explore their associated coupled plate mantle dynamic behaviour slabs are constructed by advecting material into the mantle by prescribing its radial velocity and following the absolute tangential motion of the subducting plate a simple geodynamic scenario where plates and slabs define isopycnic and isoviscous regions in an homogeneous or layered mantle was explored using the boundary element method based software bemearth the resulting dynamic behaviour was used to predict the absolute plate motion directions for the present day and a particular mid cretaceous 125 ma kinematic model we show how the methodology can be used to compare and revise kinematic reconstructions based on their effect on the balance of plate driving forces and the resulting euler poles of subducting plates as an example we compare the farallon plate dynamics at 125 ma in a global model with two reconstructions in the context of the evolution of the western north american cordillera our results suggest a method to identify episodes of absolute plate motions that are inconsistent with the expected plate dynamics

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

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

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

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

relationship of d structure with the velocity variations near the inner core boundary 1 variations in regional differential times between pkikp i and pkikp i have been attributed to hemispheric p velocity variations of about 1 in the upper 100 km of the inner core referred to as hic the top of the inner core appears relatively fast beneath asia where d is also fast an alternative interpretation could be the lateral variation in p velocity at the lowermost outer core hoc producing the same differential times to resolve this issue we introduce the diffracted pkp phase near the b caustic bdiff in the range of 139 145 epicenter distances and the corresponding differential times between bdiff and pkikp and pkikp as observed on broadband arrays due to the long wavelength nature of bdiff we scaled the s wave tomography model with k values k dlnvs dlnvp to obtain large scale p wave velocity structure in the lower mantle as proposed by earlier studies waveform synthetics of bdiff constructed with small k s predict complex waveforms not commonly observed confirming the validity of large scaling factor k with p velocity in lower mantle constrained at large scale the extra travel time constraint imposed by bdiff helps to resolve the hoc hic issue our preliminary results suggest k 2 for the lowermost mantle and support hic hypothesis an important implication is that there appears to be a relationship of d velocity structures with the structures near the inner core boundary via core dynamics

controls on subduction reorganization in the hellenic margin eastern mediterranean in subduction modeling interactions between the down going slab and neighboring plates are often neglected here we have modeled the interactions between a subducting and an upper plate and show that in such a system subducting plate advance towards the stationary margin accommodates convergence when interactions with the lower mantle are included in this system deeper penetration of the slab allows transient increases in subduction rates and weakening in the upper plate however this does not last long enough to mobilize the trench away from the upper plate the constraints imposed by the convergence of major plates are more effective in reorganizing the tectonic style by hampering or even locking the subducting plate motions resulting in sustained rollback and back arc opening over large time periods a comparison with the tectonics of the hellenic margin suggests that such boundary conditions might be the key to the reorganization of plate motions in this area

rapid conversion of elastic energy into plastic shear heating during incipient necking of the lithosphere an important and novel mechanism for ductile failure of the lithosphere is identified here which is intrinsic to the thermal mechanical feedback in a temperature dependent plastic body with coupled elastic fields both a temperature dependent power law visco elasto plastic rheology and a temperature dependent elasto plastic rheology are employed to study in a self consistent fashion the deformation of the lithosphere subject to extension by means of a two dimensional finite element code a structural perturbation initially localizes elasto plastic deformation only in its immediate vicinity however after 800 000 years have elapsed the localized zone of deformation takes off in a crack like fashion and travels to the bottom of the lithosphere in about 50 000 years time when the plate is severed thermal runaway is caused by mechanical heating triggered by the rapid energy transfer of the globally stored elastic energy into localized plastic dissipation in the ductile fault

mantle flow mechanisms for the large scale subsidence of continental interiors evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large scale subsidence we propose that mantle flow associated with the descent of cold plumes and slabs and their interaction with the endothermic phase change at 660 km depth may provide a plausible mechanism for these epeirogenic events simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth and temperature dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary we find that the plume slab flow scenarios are capable of supporting topographic deflections of amplitudes of 1 km and horizontal wavelengths of 1000 km that persist over time scales of 100 150 m y

the relationship between plate velocity and trench viscosity in newtonian and power law subduction calculations convection with a newtonian temperature dependent rheology leads to little or no surface velocity unless zones of weakness are introduced plate like features are observed in calculations both with newtonian rheology employing imposed weak zones and with power law non newtonian rheology where high stresses at the trench reduce the effective viscosity since deformation at subduction zones involves faulting both of these parameterizations should be treated with some skepticism it is important to understand how the parameterizations affect the model results we study the relationship between trench viscosity and plate velocity using a newtonian rheology by varying the viscosity at the trench the plate velocity is a function of the trench viscosity for fixed rayleigh number and plate slab viscosity slab velocities for non newtonian rheology calculations are significantly different from slab velocities from newtonian rheology calculations at the same effective rayleigh number both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate non newtonian rheology eliminates the need for imposed weak zones and provides a self consistent fluid dynamical mechanism for subduction in numerical convection models

a modelling study of vertical surface displacements at convergent plate margins summary during the evolution of a subduction zone system variations are likely to occur in for example surface plate velocities and buoyancy of the subducting lithosphere we quantify vertical surface displacements at convergent plate margins resulting from such imposed variations for this purpose we use a 2 d numerical model in which the lithospheric plates have an effective elastic thickness we first define a model in which the subducting plate is driven by its negative buoyancy and a velocity at its surface side boundary its equilibrium topography after around 2 myr is the reference level for examination of surface displacements resulting from variations in buoyancy velocity of the surface plates friction along the interplate contact and subduction zone roll back we find that a decrease increase in buoyancy of the subducting material leads to a deepening uplift of the plate margins an increase in friction along the subduction fault deepens the overriding plate margin subduction zone roll back due to sinking of the negatively buoyant subducting plate induces subsidence of the overriding plate margin this subsidence is reduced when roll back takes place in a land locked basin setting trench retreat forced by the motion of the overriding plate is characterized by higher topography of the overriding plate margin than the case of retreat due to the sinking of the negatively buoyant slab in the first case in plane stress in the back arc region is compressive while it is tensional for roll back due to the sinking of the slab we conclude that vertical surface displacements during ongoing subduction may reach a magnitude of a few kilometres on the overriding and subducting plate margins