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

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

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

stresses in the vanuatu subducting slab a test of two hypotheses 1 stress inversions of intermediate depth focal mechanisms in vanuatu were used to test two hypotheses that the slab acts as a stress guide and whether intermediate focus earthquakes occur on preexisting faults although the strike of the vanuatu slab varies by over 70 the maximum compression 1 is everywhere slab normal and the least compression 3 is everywhere within the plane of the slab this provides strong confirmation of the stress guide hypothesis in the depth range 61 120 km the solution space for 3 forms a girdle pattern in the plane of the slab indicating bi axial tension this gradually changes to down dip tension at greater depth we find that preexisting faults may become reactivated by intermediate depth earthquakes but only when the slab stresses favor it otherwise new faults are formed this contradicts the idea that the mechanism of intermediate focus earthquakes requires serpentinite dehydration on preexisting faults

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

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

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

predicting plate velocities with mantle circulation models 1 we predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models plate driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction derived density fields observed and predicted plate velocities agree well for a variety of models leading to varied conclusions about the relative importance of forces the dominance of the subduction related density pattern in the mantle is confirmed it appears that p wave models do not satisfactorily image all of the slab associated anomalies in the upper mantle furthermore lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only we show that the average torques from the lower mantle scale with the radial flow through the 660 km phase transition the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km we also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit the main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench ridge system and plates move from ridges to trenches

episodic tectonic plate reorganizations driven by mantle convection abstract periods of relatively uniform plate motion were interrupted several times throughout the cenozoic and mesozoic by rapid plate reorganization events r hey geol soc am bull 88 1977 1404 1420 p a rona e s richardson earth planet sci lett 40 1978 1 11 d c engebretson a cox r g gordon geol soc am spec pap 206 1985 r g gordon d m jurdy j geophys res 91 1986 12389 12406 d a clague g b dalrymple us geol surv prof pap 1350 1987 5 54 j m stock p molnar nature 325 1987 495 499 c lithgow bertelloni m a richards geophys res lett 22 1995 1317 1320 m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 it has been proposed that changes in plate boundary forces are responsible for these events m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 we present an alternative hypothesis convection driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle this instability which has not been described before is responsible for episodic reorganizations of plate motion numerical mantle convection experiments demonstrate that high rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events changes in plate boundary forces or plate geometry are not required

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

pacific plate motion change at the time of the hawaiian emperor bend constrains the viscosity of earth s asthenosphere important constraints on asthenospheric viscosity come primarily from modeling the glacial rebound of the past 20 kyr but remain somewhat loose because of the intrinsic resolving power of these models we obtain narrower bounds by building on the notion that the asthenosphere also controls the ability to change plate motions over myr we focus on the pacific kinematic change at the time of the hawaiian emperor bend event which is linked to the coeval inception of subduction in the western pacific we sample plausible asthenospheric viscosity and thickness values by requiring the rate at which torque varied to generate the observed kinematics consistent with the nature of subduction initiation uncertainties on the bend event duration and the occurrence of pacific hot spots drift do not hamper our results that suggest that the asthenosphere viscous response to vertical shear over kyr is consistent with that to horizontal shear over myr

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

earthquake mechanics and deformation in the tonga kermadec subduction zone from fault plane orientations of intermediate and deep focus earthquakes 1 we make use of rupture directivity to analyze 82 deep earthquakes 100 km depth in the tonga kermadec subduction zone identifying the fault planes for 25 of them we are able to place new constraints on both the physical mechanism of intermediate and deep focus earthquakes and deformation within the subducting slab we find that half of deep earthquakes with mw 6 have detectable directivity we compare the obtained fault orientations with those expected for the reactivation of outer rise normal faults and with those expected for the creation of new faults in response to the ambient stress field earthquakes 300 km depth match the patterns expected for the creation of a new system of faults we observe both subhorizontal and subvertical fault planes consistent with a downdip compressional stress field slip along these faults causes the slab to thicken rupture propagation shows no systematic directional pattern in contrast at intermediate depths 100 300 km all ruptures propagate subhorizontally and all identified fault planes whether in the upper or lower region of the double seismic zone are subhorizontal rupture propagation tends to be directed away from the top surface of the slab after accounting for the angle of subduction the subhorizontal fault plane orientation is inconsistent with the orientation of outer rise normal faults allowing us to rule out mechanisms that require the reactivation of these large surface faults subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation if all deformation takes place on these subhorizontal faults it would cause the slab to thin assuming the slab is incompressible this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths

long term interactions between intermediate depth and shallow seismicity in north chile subduction zone we document interactions between intermediate depth and interplate seismicity in the north chile subduction zone over a 25 year period 1990 2015 we show that the 2005 mw 7 8 tarapaca slab pull earthquake was followed by 9 years of enhanced deep and shallow seismicity together with the decrease of eastward average gps velocities and associated interplate coupling eventually leading to the 2014 mw 8 1 iquique megathrust earthquake in contrast megathrust ruptures e g mw 8 0 antofagasta in 1995 or mw 8 1 iquique in 2014 initiate several years of silent background seismicity in the studied area both at shallow and intermediate depths the plunge of a rigid slab into a viscous asthenospheric mantle could explain the observed synchronization between deep and shallow seismicity and their long term interactions

global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones abstract the maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on earth with some subduction zone segments producing giant subduction zone thrust earthquakes e g chile alaska sumatra andaman japan and others producing relatively small earthquakes e g mariana scotia here we show how such variability might depend on various subduction zone parameters we present 24 physical parameters that characterize these subduction zones in terms of their geometry kinematics geology and dynamics we have investigated correlations between these parameters and the maximum recorded moment magnitude m w for subduction zone segments in the period 1900 june 2012 the investigations were done for one dataset using a geological subduction zone segmentation 44 segments and for two datasets rupture zone dataset and epicenter dataset using a 200 km segmentation 241 segments all linear correlations for the rupture zone dataset and the epicenter dataset r 0 00 0 30 and for the geological dataset r 0 02 0 51 are negligible low indicating that even for the highest correlation the best fit regression line can only explain 26 of the variance a comparative investigation of the observed ranges of the physical parameters for subduction segments with m w 8 5 and the observed ranges for all subduction segments gives more useful insight into the spatial distribution of giant subduction thrust earthquakes for segments with m w 8 5 distinct narrow ranges are observed for several parameters most notably the trench normal overriding plate deformation rate v opd i e the relative velocity between forearc and stable far field backarc trench normal absolute trench rollback velocity v t subduction partitioning ratio v sp v s the fraction of the subduction velocity that is accommodated by subducting plate motion subduction thrust dip angle st subduction thrust curvature c st and trench curvature angle t the results indicate that m w 8 5 subduction earthquakes occur for rapidly shortening to slowly extending overriding plates 3 0 v opd 2 3 cm yr slow trench velocities 2 9 v t 2 8 cm yr moderate to high subduction partitioning ratios v sp v s 0 3 1 4 low subduction thrust dip angles st 30 low subduction thrust curvature c st 2 0 10 13 m 2 and low trench curvature angles 6 3 t 9 8 epicenters of giant earthquakes with m w 8 5 only occur at trench segments bordering overriding plates that experience shortening or are neutral v opd 0 suggesting that such earthquakes initiate at mechanically highly coupled segments of the subduction zone interface that have a relatively high normal stress deviatoric compression on the interface i e a normal stress asperity notably for the three largest recorded earthquakes chile 1960 alaska 1964 sumatra andaman 2004 the earthquake rupture propagated from a zone of compressive deviatoric normal stress on the subduction zone interface to a region of lower normal stress neutral or deviatoric tension stress asperities should be seen separately from frictional asperities that result from a variation in friction coefficient along the subduction zone interface we have developed a global map in which individual subduction zone segments have been ranked in terms of their predicted capability of generating a giant subduction zone earthquake m w 8 5 using the six most indicative subduction zone parameters v opd v t v sp v s st c st and t we identify a number of subduction zones and segments that rank highly which implies a capability to generate m w 8 5 earthquakes these include sunda north sulawesi hikurangi nankai northern ryukyu kamchatka kuril japan aleutians alaska cascadia mexico central america south america lesser antilles western hellenic and makran several subduction segments have a low score most notably scotia new hebrides and mariana

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

continuity and episodicity in the early alpine tectonic evolution of the western carpathians how large scale processes are expressed by the orogenic architecture and rock record data

a geodynamical view on the steadiness of geodetically derived rigid plate motions over geological time 1 the ability to map ocean floor magnetization is key to infer past plate motions the advent of geodesy in the earth sciences offered an independent snapshot of contemporary plate kinematics averaged over decades early studies suggested plate motions steadiness through geological time because contemporary rates were similar to past 3 myr averages recent data however show that geodetic and paleo magnetic motions exhibit differences beyond confidence ranges which might suggest geodesy samples time scales shorter than a few centuries these differences pose important questions on the character of plate kinematic variability through time here i derive inferences on the steadiness of geodetically derived plate motions by combining geodynamical arguments with kinematic data sets i exploit data sets to build scenarios for the recent evolution of most of the major plates i compare the minimum rate at which torque needs to vary to generate these scenarios with the maximum rate at which geological processes can contribute torque this allows assessing the geodynamical plausibility of each scenario results indicate that plate motion changes recorded since 3 ma must have taken at least 1 myr to occur two exceptions are the pacific and australian plates whose motions changed by less than 5 and therefore required torque variations that might be built over periods as short as 0 1 myr it remains unresolved for how long geodetically derived plate motions kept steady in the geological past however results indicate it is indeed plausible that they did over the past 1 2 myr these inferences call for a shift in the way we regard the figure of geodetic plate motions

dynamical consequences on fast subducting slabs from a self regulating mechanism due to viscous heating in variable viscosity convection the authors have studied 2 d time dependent convection for a rheology which is both non newtonian and temperature dependent strong effects associated with viscous heating are found in the downwelling sheets which are heated on both sides with an intensity around o 10 sup 2 times the chondritic value the magnitude of viscous heating increases strongly with the subduction speed the slab interior is weakened by viscous heating and slab breakoff then takes place this process provides a self regulating mechanism for governing the speed of intact slabs able to reach the deep mantle timescales associated with viscous heating are quite short a few million years internal heating by radioactivity decreases the amount of shear heating 13 refs 5 figs

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

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

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

timing of deformational events in the rio san juan complex implications for the tectonic controls on the exhumation of high p rocks in the northern caribbean subduction accretionary prism abstract an integrated structural petrological and geochronological study was undertaken to constrain the tectonic history and controls on the exhumation of the high p rocks of the rio san juan complex in the northern caribbean subduction accretionary wedge in the main structural units of the complex microtextural analyses were performed to identify the fabrics formed at peak of metamorphism in eclogite facies conditions and during the main retrogressive event toward the low p amphibolite or blueschist greenschist facies conditions u pb shrimp dating on zircon rims 71 3 0 7 ma coupled with 40 ar 39 ar analyses on phengite 70 69 ma in felsic sills placed temporal constraints on the exhumation of the jagua clara serpentinite matrix melange during the blueschist facies stage at the early maastrichtian in the cuaba unit u pb tims zircon ages of 89 7 0 1 ma and 90 1 0 2 ma obtained for the crystallization of tonalitic trondhjemitic melts in the lower guaconejo and upper jobito subunits respectively are similar these ages coupled with a u pb shrimp zircon age of 87 1 8 ma obtained in a garnet amphibolite and a group of older 40 ar 39 ar cooling ages on calcic amphibole constrain the exhumation of the guaconejo subunit from the high p stage to the low p stage at the 90 83 ma time interval further the age data collectively supports a genetic relationship between the distributed extensional ductile shearing the related decompression and the local partial anatexis in the subunit at least from the turonian coniacian boundary to the early campanian a group of younger 40 ar 39 ar ages obtained in the mylonitized amphibolites of the basal jobito detachment zone indicates late ductile deformation and exhumation cooling in the late campanian to maastrichtian 75 70 ma therefore structural and age data established deformation partitioning and reworking of retrograde fabrics during 20 ma in the cuaba unit the different exhumation rates obtained for the jagua clara melange can be explained by uplift in two contrasting tectonic settings a first stage of slow exhumation 1 4 mm yr in the subduction channel largely lower than plate velocities and a second stage of relatively fast exhumation 7 6 mm yr up to the surface therefore the exhumation was temporally discontinuous and the velocity increase at 70 ma probably was triggered in response to the entrance of buoyant material in the subduction zone such as a caribeana continental ribbon or the distal part of yucatan bahamas continental margin in contrast the exhumation path of the guaconejo subunit is composed of a first segment from the baric peak to the low p amphibolite stage at 84 83 ma with an exhumation rate of 7 2 mm yr a second segment around the closure temperature of calcic amphibole at 82 70 ma with a rate of 0 4 mm yr and a third segment to the surface exposure at 60 ma with a rate of 1 8 mm yr these velocity differences can be correlated with the p t path proposed for the exhumation of the subunit with an initial isothermal decompression from the peak in the high p amphibolite to the eclogite facies produced by distributed extensional shearing followed by a relatively slow cooling at low p in the newly acquired structural position and the final tectonics mainly partitioned in the late jobito detachment zone the initial fast exhumation can be related to a major modification in the convergence conditions across the intra oceanic subduction zone as a jump of the basal subduction thrust beyond a previously accreted arc terrane

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

large trench parallel gravity variations predict seismogenic behavior in subduction zones we demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench parallel gravity anomaly tpga whereas regions with strongly positive tpga are relatively aseismic these observations suggest that over time scales up to at least 1 million years spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore arc the correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench parellel variations in fore arc topography gravity and seismogenic behavior

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

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

bivergent thrust wedges surrounding oceanic island arcs insight from observations and sandbox models of the northeastern caribbean plate at several localities around the world thrust belts have developed on both sides of oceanic island arcs e g java timor panama vanuatu and the northeastern caribbean in these localities the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt for example in the northeastern caribbean a north verging accretionary prism lies to the north of the eastern greater antilles arc hispaniola and puerto rico whereas a south verging thrust belt called the muertos thrust belt lies to the south researchers have attributed such bivergent geometry to several processes including reversal of subduction polarity subduction driven mantle fl ow stress transmission across the arc gravitational spreading of the arc and magmatic infl ation within the arc new observations of deformational features in the muertos thrust belt and of fault geometries produced in sandbox kinematic models along with examination of published studies of island arcs lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity without subarc mantle fl ow and without magmatic infl ation we suggest that the eastern greater antilles arc and comparable arcs are simply crustalscale bivergent or doubly vergent thrust wedges formed during unidirectional subduction sandbox kinematic modeling suggests in addition that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly in such cases the arc acts as a backstop that transmits compressive stress into the backarc region further modeling shows that when arcs behave as rigid blocks the strike slip component of oblique convergence is accommodated entirely within the prowedge and the arc the retrowedge hosts only dip slip faulting frontal thrusting the existence of large retrowedges and the distribution of faulting in an island arc may therefore be evidence that the arc is relatively rigid the rigidity of an island arc may arise from its mafi c composition and has implications for seismic hazard analysis

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

plate motion changes the hawaiian emperor bend and the apparent success and failure of geodynamic models abstract geodynamic models explain present day plate motions in terms of mantle buoyancy forces arising from subducted lithosphere and lithospheric thickening or from velocity anomalies mapped by seismic tomography however such models do not account for sudden plate tectonic reorganizations such as the dramatic change in pacific plate motion implied by the sharp bend in the hawaiian emperor seamount chain about 43 million years ago candidate mechanisms for the hawaiian emperor bend such as subduction initiation ridge subduction or the possibly time coincident collision between india and asia remain weakly formulated and largely untested we test the india asia collision hypothesis using a model for cenozoic plate motions driven mainly by buoyancy forces introduced at paleo subduction zones and we show that pacific plate motion is virtually unaffected by the development of compressional stresses along the india asia margin geologic evidence suggests subduction initiation at about 43 48 ma along a transform boundary on the western pacific plate margin but this mechanism is difficult to test we speculate that transform boundaries may guide plate motions for long periods of time and that rapid plate motion changes may result from the creation or destruction of major transform faults speculations aside our results show that the character of global plate motion changes is not adequately explained by current geodynamic models

pattern identification in the dynamic fingerprint of seismically active zones abstract the aim of this paper is to present a methodology designed to characterize seismically active areas from the point of view of their dynamic fingerprints which rely on space time aspects of seismicity patterns according to this methodology earthquake events are represented as points in phase space which leads to an events thread connecting the points in their temporal sequence a multi scale analysis procedure is applied to the events thread and scaling exponents are identified the proposed framework considers projections of the events thread corresponding to different spatial orientations for each orientation the events thread is characterized from the point of view of signal persistence the resulting persistence diagrams portray anisotropy aspects of the dynamic fingerprint the identification of anisotropic properties of the dynamic fingerprint provides a new perspective on seismically active zones complementing the information from geological and geophysical studies the paper describes the proposed methodological framework and presents applications focusing on two different regions the aleutian arc and hawaii

great earthquakes and slab pull interaction between seismic coupling and plate slab coupling great earthquakes the few largest earthquakes that account for most of the earth s seismic energy release have occurred at only a few subduction zones around the world strong locking or seismic coupling of the interface between plates at certain subduction zones is often invoked to explain these great earthquakes although past studies have correlated strong seismic coupling with a compressional stress environment that is characterized by back arc compression and caused by trenchward motion of the overriding plate the consequences of this compressional environment for the tectonic forces that drive global plate motions are not yet clear to examine these consequences we compared subduction zone earthquake magnitudes to tectonically constrained estimates of the degree to which each slab transmits its excess weight as a direct pull force on a subducting plate at seismically uncoupled subduction zones that generate only moderate sized earthquakes we find that slabs must transmit nearly their entire upper mantle weight as a pull force on the subducting plate at seismically coupled subduction zones that produce great earthquakes however we find that slabs must be nearly completely detached from their subducting plates this suggests that slabs subducting in a compressional environment experience stress induced weakening that prevents the effective transmission of the slab pull force convergent mantle flow above a descending slab that becomes decoupled from its surface plate may induce additional surface compression that further locks the subduction zone and leads to additional slab decoupling and detachment the resulting redistribution of plate driving forces may be responsible for rapid changes in plate motion c 2003 elsevier b v all rights reserved

how mantle slabs drive plate tectonics the gravitational pull of subducted slabs is thought to drive the motions of earth s tectonic plates but the coupling between slabs and plates is not well established if a slab is mechanically attached to a subducting plate it can exert a direct pull on the plate alternatively a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate from the geologic history of subduction we estimated the relative importance of pull versus suction for the present day plates observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

toroidal poloidal partitioning of plate motions since 120 ma changes in plate motions and plate configurations during the cenozoic and mesozoic have been investigated extensively but most geodynamical models have concentrated on present day plate motions we have investigated the recent evolution of plate tectonics by examining the history of toroidal poloidal partitioning of plate motions taking into account estimated errors our results suggest a significant increase in the ratio of toroidal to poloidal motions postdating the hawaiian emperor h e bend at 43 ma corresponding to an overall decrease in global plate motions these changes may reflect greater mantle plume activity in the mesozoic but a causal mechanism is not obvious in general observed cenozoic and mesozoic plate motions do not appear to be random which implies that they are correlated we also find perhaps three significant changes in net rotation of the lithosphere with respect to hotspots since 120 ma

effect of margin curvature on plate deformation in a 3 d numerical model of subduction zones summary the large amount of data acquired along most subduction zones underlines the importance of three dimensions in the subduction mechanism in this work we use a fully 3 d mechanical numerical model to analyse the impact of the plate boundary geometry on the deformation of the upper plate the model consists of two initially horizontal independent plates overlying an inviscid fluid and continuously pushed towards each other the initial geometry of the contact zone between the two plates controls the stress regime and the strain pattern in the upper plate the case with a linear plate boundary and orthogonal convergence is first considered and used as a reference model with this simple model the effects of some important parameters i e the interplate friction and the lithosphere asthenosphere density contrast are investigated finally cases with curved plate boundaries are considered and the results are used to shed light on the importance of the dip direction of the interplate plane for the upper plate strain pattern unnecessary oceanward convexity causes an accumulation of the subducted material beneath the upper plate and induces an important uplift of the convex area on the contrary the material escapes from an oceanward concavity and provokes subsidence in the forearc zone such a behaviour induces preferential zones of weakeness in the overriding plate and may allow for explaining some local stress regime variations along convergent margins

gravitational potential energy of the tibetan plateau and the forces driving the indian plate we present a study of the vertically integrated deviatoric stress field for the indian plate and the tibetan plateau associated with gravitational potential energy gpe differences although the driving forces for the indian plate have been attributed solely to the mid oceanic ridges that surround the entire southern boundary of the plate previous estimates of vertically integrated stress magnitudes of 6 7 x 1012 n m in tibet far exceed those of 3 x 1012 n m associated with gpe at mid oceanic ridges calling for an additional force to satisfy the stress magnitudes in tibet we use the crust 2 0 data set to infer gravitational potential energy differences in the lithosphere we then apply the thin sheet approach in order to obtain a global solution of vertically integrated deviatoric stresses associated only with gpe differences our results show large n s extensional deviatoric stresses in tibet that the ridge push force fails to cancel our results calibrate the magnitude of the basal tractions associated with density buoyancy driven mantle flow that are applied at the base of the lithosphere in order to drive india into tibet and cancel the n s extensional stresses within tibet moreover our deviatoric stress field solution indicates that both the ridge push influence 1 x 1012 n m and the vertically integrated deviatoric stresses associated with gpe differences around the tibetan plateau 3 x 1012 n m have previously been overestimated by a factor of two or more these overestimates have resulted from either simplified two dimensional approximations of the thin sheet equations or from an assumption about the mean stress that is unlikely to be correct

multiscale dynamics of the tonga kermadec subduction zone our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle while geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle the influence of short wavelength variations in viscosity is still poorly understood we present 2 d and 3 d finite element models of mantle flow including strong lateral viscosity variations and local sources of buoyancy owing to both thermal and compositional effects we first use generic 2 d models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure in particular the slab and lower mantle viscosity and the presence of a low viscosity region in the mantle wedge above the slab we find that 1 the strain rate provides a strong constraint on the absolute viscosity of the slab 10 23 pa s 2 stress orientation within the slab is sensitive to the relative viscosity of the slab lower mantle and the wedge and 3 the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy in particular the state of stress in the overriding plate changes from compression to extension with the addition of a low viscosity wedge we then use observations of strain rate stress orientation dynamic topography and the geoid for the tonga kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3 d regional dynamic model together these observations are used to develop a self consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics over a broad range of length scales the presence of a low viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc and down dip compression in the shallow portion of the slab these results suggest that a low viscosity wedge plays an important role in controlling the presence of backarc spreading however for a model with a low viscosity and low density region that provides a good fit to the observed topography we find that a reduction of the slab density by a factor of 1 3 relative to the reference density model is required to match the observed geoid these results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long wavelength modelling of the geoid

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

effects of plate and slab viscosities on the geoid abstract the effects of plate rheology strong plate interiors and weak plate margins and stiff subducted lithosphere slabs on the geoid and plate motions considered jointly are examined with three dimensional spherical models of mantle flow buoyancy forces are based on the internal distribution of subducted lithosphere estimated from the last 160 ma of subduction history while the ratio of the lower mantle upper mantle viscosity has a strong effect on the long wavelength geoid as has been shown before we find that plate rheology is also significant and that its inclusion yields a better geoid model while simultaneously reproducing basic features of observed plate motion slab viscosity can strongly affect the geoid depending on whether a slab is coupled to the surface in particular deep high viscosity slabs beneath the northern pacific that are disconnected from the surface as a result of subduction history produce significant long wavelength geoid highs that differ from the observation this suggests that slabs in the lower mantle may be not as stiff as predicted from a simple thermally activated rheology if the slab model is accurate

slab pull slab weakening and their relation to deep intra slab seismicity received 8 march 2005 revised 3 june 2005 accepted 21 june 2005 published 21 july 2005 1 subduction zone seismicity is highly variable great earthquakes occur at few subduction zones around the world with significant variation in size and frequency of deep events interactions between overriding and subducting plates and slab pull strength for individual plates provide a framework for understanding these variations previous work suggests an inverse correlation between great earthquake moment release and the degree to which the subducted slab is connected to the surface plate we find positive correlations between degree of plate slab attachment and moment release from intermediate and deep earthquakes this implies that shallow slab weakening that occurs at trenches where compressive stresses and great earthquakes dominate not only detaches slabs from plates but is maintained as the slab descends discouraging deep seismicity regions of low shallow moment release are consistent with extensional shallow stress regimes and undamaged slabs such slabs maintain mechanical strength during descent and deform seismogenically at depth citation bilek s l c p conrad and c lithgowbertelloni 2005 slab pull slab weakening and their relation to deep intra slab seismicity geophys res lett 32 l14305