seismic detection of folded subducted lithosphere at the core mantle boundary seismic tomography of the d region beneath the caribbean reveals a 100 km vertical step in an otherwise flat d shear velocity discontinuity and localized low velocities at the edge of the slab material which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the earth s lower mantle1 2 the fate of these slabs has remained unresolved but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle known as the d region relatively cold slab material may account for high seismic velocities imaged in d beneath areas of long lived plate subduction and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions3 4 the d discontinuity itself is probably the result of a phase change in relatively low temperature magnesium silicate perovskite5 6 here we present images of the d region beneath the cocos plate using kirchhoff migration of horizontally polarized shear waves and find a 100 km vertical step occurring over less than 100 km laterally in an otherwise flat d shear velocity discontinuity folding and piling of a cold slab that has reached the core mantle boundary as observed in numerical and experimental models can account for the step by a 100 km elevation of the post perovskite phase boundary due to a 700 c lateral temperature reduction in the folded slab we detect localized low velocities at the edge of the slab material which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer

oak leaves as biosensors of late neogene and early pleistocene paleoatmospheric co2 concentrations abstract complementary to the interpretation of 13 c values of biogenic carbonate and sedimentary organic carbon in marine sediments paleoatmospheric co 2 levels can be estimated by considering the inverse relationship between atmospheric co 2 concentration and stomatal parameters frequency size on leaves of land plants in woody plants the significance of this species specific physiological response to changing co 2 regimes is now repeatedly confirmed both experimentally and from historical sequences of leaves collected since the onset of industrialization a corollary of this relationship is that analysis of stomatal parameters on fossil leaves has the potential of determining changes in paleoatmospheric co 2 levels at different time scales well preserved cuticle remains of oak leaves from late miocene pliocene and early pleistocene sediments of the lower rhine embayment germany the netherlands give promise of extending the record of stomatal frequency response to the last 10 ma during intervals with warm temperate to subtropical climatic conditions oak leaves are characterized by a high stomatal resistance or low conductance to co 2 diffusion and low stomatal frequencies during cooler intervals we observe an opposite picture comparison with historical relations between co 2 concentration and stomatal properties suggests that paleoatmospheric co 2 concentrations were not significantly higher than during the last 200 years and fluctuated several times between 280 and 370 ppmv in covariation with contrasting regional climatic conditions on a global scale intervals with reduced co 2 levels match glacial pulses characterized by the occurrence of ice rafted detritus in high latitude oceanic sediments

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

differential contraction of subducted lithosphere layers generates deep earthquakes abstract the origin of forces for generating deep earthquakes remains elusive we propose a new mechanical model that involves constant release of shear strain accumulated between the crust and mantle lithosphere caused by differential volume changes during phase transformations as a slab sinks this generates increasing down dip compression inside the slab with depth consistent with the global distribution of deep earthquake focal mechanisms using experimentally calibrated slab rheology we show that the estimated distribution of slab internal stress agrees well with the depth frequency relationship of deep earthquakes in both cold and warm slabs with the peak in seismicity at 600 km depth corresponding to an induced stress maximum this mechanism of residual stress within a cold slab also provides a solution to deep earthquakes occurring beneath former subduction zones such as western tonga and spain we further suggest that this model may reconcile existing geodynamic paradoxes concerning the absolute strength of slabs discrepancies on mantle rock strength between laboratory and geophysical inversions as well as the competition between earthquake generating stresses and slab pull forces

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

paleobotanical evidence for near present day levels of atmospheric co2 during part of the tertiary understanding the link between the greenhouse gas carbon dioxide co 2 and earth9s temperature underpins much of paleoclimatology and our predictions of future global warming here we use the inverse relationship between leaf stomatal indices and the partial pressure of co 2 in modern ginkgo biloba and metasequoia glyptostroboides to develop a co 2 reconstruction based on fossil ginkgo and metasequoia cuticles for the middle paleocene to early eocene and middle miocene our reconstruction indicates that co 2 remained between 300 and 450 parts per million by volume for these intervals with the exception of a single high estimate near the paleocene eocene boundary these results suggest that factors in addition to co 2 are required to explain these past intervals of global warmth

competing influences of plate width and far field boundary conditions on trench migration and morphology of subducted slabs in the upper mantle abstract seismic tomography of the mantle provides a snapshot view of the present day structure of subducted tectonic plates slabs in the mantle however a correct interpretation of how such features developed requires knowledge of both the history of plate motions in particular the locations of subduction zones over time as well as an understanding of how slabs deform and interact with the lower mantle the understanding of seismic tomography requires a detailed knowledge of the motion of the trench with time as well as the time evolution of subduction zones and their associated slabs we use 3 d numerical models to investigate two effects which can strongly influence the kinematics of subduction in particular the migration of the trench the width of the plate and the far field boundary conditions of the subducting plate these models study the full range of plate widths 300 6000 km and are subjected to the full range of boundary conditions fixed free and pushed representative of the tectonic settings found on earth additionally our results indicate that trailing edge boundary conditions dominate over the effect of plate width each boundary condition is identified to produce a particular trench migration behavior with an associated upper mantle slab morphology 1 fixed edges result in retreating trenches and retrograde motion of the slab which ends up lying flat atop the lower mantle 2 free trailing edges tend to exhibit quasi stationary trenches with slabs developing into folded piles atop the lower mantle and 3 pushed trailing edges generally result in advancing trenches that overrun previously subducted material to form forward draping slabs lying prone atop the lower mantle this range of behavior can be observed in seismic tomography and an example of each is provided within each suite of models with similar boundary conditions plate width has a subsidiary role in the observed slab morphology but three distinct regimes of plate width can be distinguished in regards to trench migration thus plate width is an important effect on trench migration and does result in notable differences in slab morphologies between narrow and wide plate widths

buckling instabilities of subducted lithosphere beneath the transition zone a sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds recent tomographic images beneath subduction zones together with quantitative fluid mechanical scaling laws suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth beneath subduction zones such as central america and java the fast anomalies in elastic wavespeed are too broad 400 km to be explained by simple compressional thickening of subducted lithosphere that is initially 50 100 km thick by contrast a periodic buckling mechanism can account for both the characteristic wedge shapes of the anomalies and for their widths which agree closely with those predicted by the scaling laws in addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions interpretation of geoid anomalies and modeling of secular changes in earth s moment of inertia

relationship between slab dip and topography segmentation in an oblique subduction zone insights from numerical modeling slab dip controls the state of stress in an overriding plate and affects mountain building analog and numerical models have shown variations in tectonic regime induced by slab folding over the 660 km depth discontinuity zone in orthogonal convergence here using a three dimensional model of oblique subduction 30 and accounting for free top surfaces we show how slab folding generates an along strike slab dip segmentation inducing variations in topography of the overriding plate when the subducting plate begins to curve forward the elevation height rises inland and varies along the trench from 5 km to 2 km the andes are a suitable natural zone to compare our results with because of its linear margin and well constrained plates kinematics thus we provide a new explanation to the general decrease in elevation from the central to southern andes which still remains to be combined with other 3 d mechanisms to explain the actual andean topography

slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity subducting slabs can stagnate in the lower mantle high pressure laboratory experiments show that the viscosity of a dominant mantle phase increases dramatically at shallow lower mantle depths which could cause the slabs to halt their descent

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

multiple seismic reflectors in earth s lowermost mantle the modern view of earth s lowermost mantle considers a d region of enhanced seismologically inferred heterogeneity bounded by the core mantle boundary and an interface some 150 300 km above it with the latter often attributed to the postperovskite phase transition in mgsio3 seismic exploration of earth s deep interior suggests however that this view needs modification so called scs and skks waves which probe the lowermost mantle from above and below respectively reveal multiple reflectors beneath central america and east asia two areas known for subduction of oceanic plates deep into earth s mantle this observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the d region sensu stricto

mechanical coupling of the motion of the surface plate and the lower mantle slab effects of viscosity hill yield strength and depth dependent thermal expansivity abstract sinking rates of the subducted lithosphere in the earth s lower mantle inferred from seismic tomography images indicate much lower rates than that of the observed subducting surface plate motion this implies that the surface plate motion is independent of descending flow in the deep mantle seeking to understand the mechanical coupling of the slab penetrating into the deepest mantle with the surface plate motion we performed numerical modeling of the subducting lithosphere integrated into a whole mantle scale convection system viscosity layering slab strength and thermal expansivity were systematically varied using a two dimensional cartesian model with a free surface condition an increase in the lower mantle viscosity significantly diminished the surface plate motion by stress transmission of the slab when the slab strength was 300 mpa we observed slab buckling near the 660 km phase transition when the yield stress was 200 mpa the slab buckling partly absorbed the effects of the lowermost mantle viscosity on the surface plate pressure dependent thermal expansivity further enhanced slab buckling when the viscosity of the lowermost mantle was less than 1 1022 pa s even with a large viscosity hill in the lower mantle the slab sinking rate increased up to approximately 7 cm yr 1 when the slab reached the core mantle boundary region as a result the viscosity hill was not sufficient to reduce the sinking rate of the deepest slab slab deformation coupled with gradually increasing viscosity therefore plays a key role in the simultaneous occurrence of rapid motion in the surface lithosphere with sluggish slab motion in the deepest layer

subcontinental sinking slab remnants in a spherical geometry mantle model 1 comparisons of the paleogeographic record with seismic tomography sections from the mantle below asia indicate that detached slab remnants associated with a range of ages since subduction spanning roughly 100 myr appear to occupy mid mantle depths at different sites more than one explanation for the observed spectrum of ages at similar depths may be plausible to identify key parameters controlling the sinking rate of a slab we examine the effects of mantle viscosity rayleigh number and spherical geometry as well as the role of slab properties such as strike width depth extent at the time of slab detachment and slab rheology we model scenarios featuring detached mantle slabs in 3 d calculations and conclude that the initial depth extent of a subducted slab segment is a key parameter determining the rate at which the slab sinks we estimate that stiff slabs are able to occupy mid mantle depths for ages ranging from approximately 35 to 150 myr depending on the initial depth extent of the slab at the time of its detachment from the lithosphere this age range results from the time that different slab fragments require to cross the viscosity interface at the upper mantle lower mantle boundary in cases featuring mature slabs where the lower section of the slab has penetrated the lower mantle prior to detachment entrainment by the deep sinking segment helps the young upper mantle section of the slab cross into the lower mantle much more quickly than a slab segment initially confined to the upper mantle

coupled mantle dripping and lateral dragging controlling the lithosphere structure of the nw moroccan margin and the atlas mountains a numerical experiment abstract recent studies integrating gravity geoid surface heat flow elevation and seismic data indicate a prominent lithospheric mantle thickening beneath the nw moroccan margin lab 200 km depth followed by thinning beneath the atlas domain lab about 80 km depth such unusual configuration has been explained by the combination of mantle underthrusting due to oblique africa eurasia convergence together with viscous dripping fed by asymmetric lateral mantle dragging requiring a strong crust mantle decoupling in the present work we examine the physical conditions under which the proposed asymmetric mantle drip and drag mechanism can reproduce this lithospheric configuration we also analyse the influence of varying the kinematic boundary conditions as well as the mantle viscosity and the initial lithosphere geometry results indicate that the proposed drip drag mechanism is dynamically feasible and only requires a lateral variation of the lithospheric strength the further evolution of the gravitational instability can become either in convective removal of the lithospheric mantle mantle delamination or subduction initiation the model reproduces the main trends of the present day lithospheric geometry across the nw moroccan margin and the atlas mountains the characteristic time of the observed vertical movements the amplitude and rates of uplift in the atlas mountains and offers an explanation to the miocene to pliocene volcanism an abnormal constant tectonic subsidence rate in the margin is predicted

indian continental subduction and slab break off during tertiary collision high wavespeed seismic anomalies in the transition zone and uppermost lower mantle beneath the india asia collision zone imaged by body wave seismic tomography have been interpreted as subducted fragments of continental material in this study we focus on the prominent anomaly located beneath india between depths of about 450 and 900 km by combining the location of this anomaly with palaeogeographical positions of india we constrain the timing of the subduction event probably related to this anomaly we infer that a large portion of the north western margin of india initiated subduction at 35 5 ma along a 1500 km long wnw ese striking zone and ended with a progressive slab break off process this break off started most probably around 25 ma at the western end of the slab and propagated eastwards until complete break off around 15 ma this study helps to constrain better the amount of convergence between india and asia absorbed by continental subduction

coupled effects of phase transitions and rheology in 2 d dynamical models of subduction author s arredondo km billen mi abstract 2017 american geophysical union all rights reserved observations of seismicity and seismic tomography provide a present day constraint on the geometry of slabs within the mantle however it is challenging to directly relate the observed shape of slabs to their time evolution phase transitions modify the buoyancy forces within slabs affecting how slabs deform as they sink into the lower mantle and how forces are transmitted to the surface plates here we show that if the crustal shear zone is sufficiently weak 1020 pa s then the coupled effects of a compositionally dependent phase transition model and a stress dependent rheology lead to oscillatory plate speeds and trench motion in response to buckling and folding of the slab on average subducting plate velocity is 4 cm yr and trench motion is 0 7 cm yr however during folding subducting plate speed and trench motion increases by a factor of 3 and trench motion switches from advance to retreat in models with a higher viscosity shear zone 1021 pa s average plate speed is lower 2 cm yr and there is only trench advance stress dependent weakening due to increasing driving stresses can also cause unexpected changes in plate velocity i e decrease or no change in addition due to the added negative buoyancy from the phase transitions slab breakoff occurs in models with a yield stress 500 mpa because of the oscillatory motion of the trench long flat slabs do not form in the transition zone this suggests that other factors such as interaction of the slab with deep mantle flow may be required to create long flat slabs

folding of viscous plumes impinging on a density or viscosity interface summary the possibility that slabs of subducted oceanic lithosphere may buckle and fold under longitudinal compressive stresses when they encounter the major seismic discontinuity near a depth of 670 km in the mantle has led us to explore experimentally the behaviour of very viscous plumes falling onto a density interface or a viscosity step we begin with the case of axisymmetric and planar streams falling through air onto the free surface of a viscous fluid which has the same or smaller density and observe the conditions under which they coil or fold in order to remove surface tension and to introduce effects of a viscous environment we study plumes falling onto a density interface between two generally very viscous liquid layers stability to buckling is found to depend largely on a geometrical criterion based on the length to thickness ratio of plumes but also the tendency to buckle is greater for large density differences across the interface and larger viscosity contrasts between plume and upper layer when plumes are unstable coils or folds are laid down with a horizontal scale in a fixed proportion to the thickness of the plume and the frequency of folding is determined by the velocity and thickness of the slab as it enters the region of compressive stress entrainment of upper layer fluid can have a marked effect on the fate of the folded plume material which may spread at the interface or continue to descend into the lower layer similar behaviour is observed at a viscosity interface application of results to the mantle depends on the nature of the 670 km discontinuity and is therefore inconclusive however cool slabs are likely to be unstable if mantle circulation involves two layers separated by a density interface or if whole mantle circulation involves a large viscosity increase with depth near 700 km

understanding seismic heterogeneities in the lower mantle beneath the americas from seismic tomography and plate tectonic history we combine results from seismic tomography and plate motion history to investigate slabs of subducted lithosphere in the lower mantle beneath the americas using broadband waveform cross correlation we measured 37 000 differential p and s traveltimes 2000 pcp p and scs s times along a wide corridor from alaska to south america we invert the data simultaneously to obtain p and s wave velocity models we interpret slab structures and unravel subduction history by comparing our v s tomographic images with reconstructed plate motion from present day up to 120 myr convergence of the pacific with respect to the americas is computed using either 1 the pacific and indo atlantic hot spot reference frames or 2 the plate circuit passing through antarctica around 800 km depth four distinctive fast anomalies can be associated with subduction of the nazca cocos and juan de fuca plates beneath south central and north america respectively and of the pacific plate beneath the aleutian island arc the large fast anomalies in the lowermost mantle which are most pronounced in the s wave models can be associated with late cretaceous subduction of the farallon plate beneath the americas near 2000 km depth the images record the post 80 myr fragmentation of the proto farallon plate into the kula plate in the north and the farallon plate in the northeast near 1000 km depth we infer separate fast anomalies interpreted as the kula pacific juan de fuca and farallon slabs this interpretation is consistent with the volume and length of slabs estimated from the tomographic images and the plate history reconstruction

seismic observation of a high velocity slab 1200 1600 km in depth a slablike high velocity region 1200 1600 km beneath the east coast of north america apparently has a half width of 450 km and a lateral extent of 3000 km and a depth extent of more than 800 km this slab lies beneath the boundary where the farallon plate had been subducting for up to 120 ma the high velocity anomaly has been inferred previously from travel time studies this study uses body waveforms to verify and refine its structure the anomalous region has a peak shear wave velocity 1 5 greater than the surrounding region and compressional velocity 0 6 greater the width and smoothness of this anomaly is constrained by the complexity observed in sh waveforms from deep earthquakes beneath south america observed at gdsn stations in north america this slab is about 5 times broader and 5 times weaker in its velocity anomaly than most slabs observed in the upper mantle this observation implies that if this slab has passed through the 650 km discontinuity which is the most likely explanation for the observation it has been spread by more than simply thermal conductivity the possibility that there is two layer convection with thermal coupling is not ruled out however as thermal coupling could lead to a similar pattern

imaging the transition from flat to normal subduction variations in the structure of the nazca slab and upper mantle under southern peru and northwestern bolivia

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

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

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

on velocity anomalies beneath southeastern china an investigation combining mineral physics studies and seismic tomography observations abstract seismic tomography studies reveal distinct velocity and v p v s anomalies in the mantle transition zone mtz beneath the yangtze craton and cathaysia block in southeastern china the anomalies under the yangtze craton are characterized by high velocity both v p and v s and low v p v s ratio while those beneath the cathaysia block are characterized by low velocity especially v s and high v p v s ratio here we conduct analyses of phase relations and thermoelasticity to model the effects of thermal and chemical homogeneities in the mtz by taking advantage of recent simultaneous v p and v s seismic tomography results under southeastern china we attempt to quantify the seismic tomography results and examine the effects of temperature chemical composition and water or protonization on velocity anomalies in the deep mantle we find v p v s to be a powerful parameter in distinguishing the various effects of temperature chemical composition and protonization we conclude that an ancient stagnated oceanic slab is most likely the main cause of the observed fast velocity and low v p v s anomalies in the mtz under the yangtze craton this ancient slab material is most likely a product of paleo pacific subduction around 100 125 ma ago when the oceanic plate abruptly changed its direction of motion such an event has been shown to be closely related to the magmatic activities around eastern china the ultrahigh pressure metamorphism zone between the yangtze craton and the north china craton and the destruction of the lower crust of the north china craton the anomalies under the cathaysia block on the other hand are likely due to dehydration induced partial melting of subducted pacific slab materials here the large low v s anomaly in mtz coincides with the extensive mesozoic to cenozoic igneous features on the surface suggesting a state with lower viscosities in the upper mantle dehydration induced partial melting in mtz may have also promoted deformation of the south china fold belt our results suggest that these lithospheric processes are directly related to the tectonic interaction between the oceanic and continental plates in southeastern china and that a better understanding of past deep mantle dynamic processes may place important constraints on the evolution of the cratons in china

seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the caribbean summary broad band data from south american earthquakes recorded by californian seismic networks are analysed using a newly developed seismic wave migration method the slowness backazimuth weighted migration sbwm using the sbwm out of plane seismic p wave reflections have been observed the reflection locations extend throughout the earth s lower mantle down to the core mantle boundary cmb and coincide with the edges of tomographically mapped high seismic velocities modelling using synthetic seismograms suggests that a narrow 10 15 km low or high velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms but other explanations may exist considering the reflection locations and synthetic modelling the observed out of plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere we also detect weaker reflections corresponding to the tomographically mapped top of the slab which may arise from the boundary between the nazca plate and the overlying former basaltic oceanic crust the joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity linking processes and structure at the top and bottom of the earth s mantle supporting the idea of whole mantle convection

subducted slabs and lateral viscosity variations effects on the long wavelength geoid summary the characteristic broad local maxima exhibited by the long wavelength geoid over subduction zones are investigated with a numerical model of mantle flow in a spherical axisymmetric geometry a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle by the viscosity stratification and by lateral viscosity variations lvv in the lithosphere upper and lower mantle the presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle the viscosity of the lower mantle controls the long wavelength geoid to the first order ensuring a clear positive signal when it is at least 30 times greater than the upper mantle viscosity the presence of lvv in the lithosphere in the form of weak plate margins helps to increase the contribution of the surface topography causing a pronounced reduction of the geoid localized lvv associated with the cold slab play a secondary role if they are in the upper mantle on the other hand highly viscous slabs in the lower mantle exert a large influence on the geoid they cause its amplitude to increase dramatically way beyond the values typically observed over subduction zones long wavelength flow becomes less vigorous as the slab viscosity increases deformation in the upper mantle becomes more localized and power is transferred to short wavelengths causing the long wavelength surface topography to diminish and the total geoid to increase slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid it is shown that a phase change from perovskite to post perovskite above the core mantle boundary can cause the geoid to reduce significantly thereby helping to reconcile models and observations

quantifying seismic anisotropy induced by small scale chemical heterogeneities observations of seismic anisotropy are usually used as a proxy for lattice preferred orientation lpo of anisotropic minerals in the earth s mantle in this way seismic anisotropy observed in tomographic models provides important constraints on the geometry of mantle deformation associated with thermal convection and plate tectonics however in addition to lpo smallscale heterogeneities that cannot be resolved by long period seismic waves may also produce anisotropy the observed i e apparent anisotropy is then a combination of an intrinsic and an extrinsic component assuming the earth s mantle exhibits petrological inhomogeneities at all scales tomographic models built from long period seismic waves may thus display extrinsic anisotropy in this paper we investigate the relation between the amplitude of seismic heterogeneities and the level of induced s wave radial anisotropy as seen by long period seismic waves we generate some simple 1 d and 2 d isotropic models that exhibit a power spectrum of heterogeneities as what is expected for the earth s mantle that is varying as 1 k with k the wavenumber of these heterogeneities the 1 d toymodels correspond to simple layeredmedia in the 2 d case our models depict marble cake patterns in which an anomaly in shear wave velocity has been advected within convective cells the long wavelength equivalents of these models are computed using upscaling relations that link properties of a rapidly varying elastic medium to properties of the effective that is apparent medium as seen by long period waves the resulting homogenized media exhibit extrinsic anisotropy and represent what would be observed in tomography in the 1 d case we analytically show that the level of anisotropy increases with the square of the amplitude of heterogeneities this relation is numerically verified for both 1 d and 2 d media in addition we predict that 10 per cent of chemical heterogeneities in 2 d marble cake models can induce more than 3 9 per cent of extrinsic radial s wave anisotropy we thus predict that a non negligible part of the observed anisotropy in tomographic models may be the result of unmapped small scale heterogeneities in the mantle mainly in the form of fine layering and that caution should be taken when interpreting observed anisotropy in terms of lpo and mantle deformation this effect may be particularly strong in the lithosphere where chemical heterogeneities are assumed to be the strongest the authors 2017

changes in atmospheric co2 concentrations and climate from the late eocene to early miocene palaeobotanical reconstruction based on fossil floras from saxony germany abstract in the present study proxy data concerning changes in atmospheric co2 and climatic conditions from the late eocene to the early miocene were acquired by applying palaeobotanical methods fossil floras from 10 well documented locations in saxony germany were investigated with respect to 1 stomatal density index of fossil leaves from three different taxa eotrigonobalanus furcinervis laurophyllum pseudoprinceps and laurophyllum acutimontanum 2 the coexistence approach ca based on nearest living relatives nlr and 3 leaf margin analysis lma whereas the results of approach 1 indicate changes in atmospheric co2 concentration approaches 2 and 3 provide climate data the results of the analysis of stomatal parameters indicate that the atmospheric co2 concentration was higher during the late eocene than during the early oligocene and increased towards the late oligocene a lower atmospheric pco2 level after the late eocene is also suggested by an increase in marine palaeoproductivity at this time from the late oligocene onwards no changes in atmospheric co2 concentration can be detected with the present data for the considered sites the results of the coexistence approach and of the leaf margin analysis document a significant cooling event from the late eocene to the early oligocene the pco2 decrease from the late eocene to the early oligocene indicated by the stomatal data raised in this study was thus coupled to a temperature decrease which is reflected by the present datasets from the early oligocene onwards however no further fundamental climate change can be inferred for the considered locations the pco2 increase from the early oligocene to the late oligocene which is indicated by the present data is thus not accompanied by a climate change at the considered sites a warming event during the late oligocene is however recorded by marine climate archives according to the present data no change in pco2 occurred during the cooling event at the oligocene miocene boundary which is also indicated by marine data the quality and validity of stomatal parameters as sensors for atmospheric co2 concentration are discussed

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

age of the subducting pacific slab beneath east asia and its geodynamic implications abstract we study the age of the subducting pacific slab beneath east asia using a high resolution model of p wave tomography and paleo age data of ancient seafloor our results show that the lithosphere age of the subducting slab becomes younger from the japan trench 130 ma to the slab s western edge 90 ma beneath east china and the flat stagnant slab in the mantle transition zone mtz is the subducted pacific plate rather than the proposed izanagi plate which should have already collapsed into the lower mantle the flat pacific slab has been in the mtz for no more than 10 20 million years considerably less than the age of the big mantle wedge beneath east asia 110 million years hence the present flat pacific slab in the mtz has contributed to the cenozoic destruction of the east asian continental lithosphere with extensive intraplate volcanism and back arc spreading whereas the destruction of the north china craton during the early cretaceous 140 110 ma was caused by the subduction of the izanagi or the paleo pacific plate

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

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

long wavelength character of subducted slabs in the lower mantle abstract the high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle while the slab deformation in the transition zone is quite complex and differs among the individual subduction zones it could be generally concluded that most slabs seem to penetrate into the lower mantle the character of the fast seismic velocity anomalies however changes when the slabs cross the 670 km discontinuity while in the upper mantle tomography resolves relatively narrow plate like structures in the lower mantle it rather indicates thick blobs of seismically fast material such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle here we study the deformation of the slabs in the lower mantle in a viscous model with a non linear composite rheology including the diffusion creep the dislocation creep and a power law stress limiter we perform numerical simulations of the subduction process in a 2d cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle the thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress 0 1 gpa of the stress limiting mechanism and with a viscosity increase at the 670 km discontinuity further we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle

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

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

subducting slabs jellyfishes in the earth s mantle the constantly improving resolution of geophysical data seismic tomography and seismicity in particular shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle this observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle the distribution and character of seismicity reveal however that slabs undergo vertical extension in the upper mantle and compression near the transition zone in this paper we demonstrate that during the subduction process the shape of low viscosity slabs 1 to 100 times more viscous than the surrounding mantle evolves toward an inverted plume shape that we coin jellyfish results of a 3d numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head the head is linked to the body of the subducting slab by a thin tail a complete parametric study reveals that subducting slabs may achieve a variety of shapes in good agreement with the diversity of natural slab shapes evidenced by seismic tomography our work also suggests that the slab to mantle viscosity ratio in the earth is most likely to be lower than 100 however the sensitivity of slab shapes to upper and lower mantle viscosities and densities which remain poorly constrained by independent evidence precludes any systematic deciphering of the observations

development of topography in 3 d continental collision models understanding the formation and evolution of high mountain belts such as the himalayas and the adjacent tibetan plateau has been the focus of many tectonic and numerical models here we employ 3 d numerical simulations to investigate the role that subduction collision and indentation play on lithosphere dynamics at convergent margins and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper mantle scale model distinct dynamics are obtained for the oceanic subduction side trench retreat slab rollback and the continental collision side trench advance slab detachment topographic uplift lateral extrusion we show that slab pull alone is insufficient to generate high topography in the upper plate and that external forcing and the presence of strong blocks such as the tarim basin are necessary to create and shape anomalously high topographic fronts and plateaus moreover scaling is used to predict four different modes of surface expression in continental collision models i low amplitude homogeneous shortening ii high amplitude homogeneous shortening iii alpine type topography with topographic front and low plateau and iv tibet himalaya type topography with topographic front and high plateau results of semianalytical models suggest that the argand number governs the formation of high topographic fronts while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate applying these results to natural examples we show that the alps belong to regime iii the himalaya tibet to regime iv whereas the andes altiplano fall at the boundary between regimes iii and iv

paleoproductivity increase at the eocene oligocene climatic transition odp dsdp sites 763 and 592 abstract during the late eocene early oligocene period of rapid climate change across oxygen isotope shift oi 1 productivity showed major changes in the region around australia dsdp site 592 off new zealand odp site 763 off nw australia we estimated paleoproductivity at these sites using benthic foraminiferal accumulation rates as well as accumulation rates of siliceous and other calcareous microfossils and carbon isotope data in the eocene productivity was generally higher at site 763 where it reached 4 maxima but dropped at about 34 3 ma at both sites productivity increased strongly at oi 1 as recognized in the oxygen isotopic record at the sites and fluctuted during the early oligocene at higher levels than in the eocene we attribute the difference in eocene productivity to differences in oceanographic settings site 592 was probably under a n s flowing western boundary current of the pacific gyre whereas site 763 showed periodic high productivity as a result of local upwelling this upwelling occurred when a warm n s flowing current proto leeuwin current replaced the cold s n flowing eastern boundary current of the indian ocean gyre upwelling ended at the opening of the tasman sea the earliest oligocene increase in productivity occurred at many other locations in the southern oceans and is accompanied by a strong increase in carbon isotopic values indicating increased burial of organic matter on a global scale this productivity increase as well as the increased burial indicates that the oceanic carbon cycle may have been part of the climate change in the earliest oligocene

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

deformation modes of subducted lithosphere at the core mantle boundary an experimental investigation 1 ancient subducted lithosphere may be an important source of the seismically observed heterogeneity and anisotropy in the d layer proper interpretation of the seismic observations therefore requires an understanding of how subducted lithosphere deforms upon reaching the core mantle boundary cmb here we study this process experimentally using an analog model system in which a sheet of viscous sugar syrup representing the subducted lithosphere falls vertically onto an impermeable surface the cmb at the bottom of a reservoir of less viscous syrup the ambient mantle for sheet ambient viscosity contrasts less than a critical value c that depends on the ratio of the fall height to the sheet thickness the sheet impinges on the cmb as a stagnation flow with bilateral symmetry for c however the sheet undergoes a periodic folding instability we determine experimentally a universal scaling law for the folding amplitude as a function of the viscosities and densities of the two fluids and the thickness and vertical velocity of the sheet where it enters the folding region to apply this law to the earth we assume that the sheet s excess viscosity and density relative to the ambient mantle are both controlled by the same temperature anomaly t using estimated rheological parameters for diffusion creep in the lowermost mantle we find that the folding criterion c requires t 300 k the presence of folded lithosphere at the cmb beneath the cocos plate suggested recently on the basis of seismic data by hutko et al 2006 is unlikely to be the result of in situ folding because the temperature anomaly 400 700 k required to explain the observed variations in the depth of the postperovskite phase transition is probably too large to be preserved in a deeply penetrating slab however the seismic observations might be explained in an alternative way by a pile of folded lithosphere that formed at 660 km depth and subsequently sank to the cmb

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

an explanation for earth s long term rotational stability paleomagnetic data show less than approximately 1000 kilometers of motion between the paleomagnetic and hotspot reference frames that is true polar wander during the past 100 million years which implies that earth s rotation axis has been very stable this long term rotational stability can be explained by the slow rate of change in the large scale pattern of plate tectonic motions during cenozoic and late mesozoic time provided that subducted lithosphere is a major component of the mantle density heterogeneity generated by convection therefore it is unnecessary to invoke other mechanisms such as sluggish readjustment of the rotational bulge to explain the observed low rate of true polar wander

the opening of sirte basin result of slab avalanching abstract north africa s sirte basin opening is an enigmatic feature in the complex meso cenozoic rearrangement of mediterranean tectonics new borehole data inversion constrains its deformation history showing a stretching event starting 70 ma and terminating in a further abrupt increase at 50 ma rapidly fading afterwards the timing of this event hardly reconciles with the mesozoic major plates reorganization following the spreading of the atlantic and the neogene central mediterranean tectonics active at different times here we propose that sirte rifting could have been driven by the pull exerted by the hellenic subduction reconstructions of hellenic convergence and slab deep subduction as constrained by plate kinematics and tomography show that large slab mass accumulates in the upper mantle by late cretaceous paleogene eventually forcing further sinking in the lower mantle coeval to the growing strain evolution recorded in sirte furthermore the 20 m y phase of large pull here recorded the consequent rapid growth over 10 m y and following decay are compatible with the dynamics of slab avalanche as revealed by numerical models showing that the sirte basin opened in response to the large pull force developed during the mass flush and transferred from deep slab to surface

shear wave reflectivity imaging of the nazca south america subduction zone stagnant slab in the mantle transition zone 1 in this study we utilize over 5000 ss waveforms to investigate the high resolution mantle reflectivity structure down to 1200 km beneath the south american convergent margin our results indicate that the dynamics of the nazca subduction are more complex than previously suggested the 410 and 660 km seismic discontinuities beneath the pacific ocean and amazonian shield exhibit limited lateral depth variations but their depths vary substantially in the vicinity of the subducting nazca plate the reflection amplitude of the 410 km discontinuity is greatly diminished in a 1300 km wide region in the back arc of the subducting plate which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone the underlying 660 km discontinuity is strongly depressed showing localized depth and amplitude variations both within and to the east of the wadati benioff zone the width of this anomalous zone 1000 km far exceeds that of the high velocity slab structure and suggesting significant slab deformation within the transition zone the shape of the 660 km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending nazca slab impinges upon the base of the upper mantle citation contenti s y j gu a okeler and m d sacchi 2012 shear wave reflectivity imaging of the nazca south america subduction zone stagnant slab in the mantle transition zone geophys res lett 39 l02310 doi 10 1029 2011gl050064

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

seismic constraints on the morphology of deep slabs residual sphere images from deep earthquakes not only detect the presence of slab associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle we have compared travel times from deep events in the kuril and mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth we assume that the details of the deformation whether the slab buckles imbricates fragments etc are averaged out along the ray paths and hence our models constrain the scale not the mode of slab thickening the deep event travel times are best fit by undeformed models but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects although the mariana times are consistent with advective thickening factors of 5 or more factors larger than 3 are ruled out by the kuril data for all models examined the data require that slab material extends to depths of 900 1000 km global tomographic models and regional studies which delineate high velocity anomalies in the lower mantle beneath zones of cenozoic subduction are consistent with our results as is recent work on pulse distortion by slab gradients comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur it is largely aseismic

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

distribution of porphyry copper deposits along the western tethyan and andean subduction zones insights from a paleotectonic approach abstract along the western tethyan and andean subduction zones the distribution of cretaceous and cenozoic porphyry cu deposits is not random and shows that they were emplaced in distinct regional clusters to understand the appearance of these clusters within their geodynamical contexts and identify kinematic features which would favor the genesis of porphyry type ore bodies we use a paleotectonic approach two clusters in the aegean balkan carpathian area which were emplaced in upper cretaceous and oligo miocene and two others in the andes which were emplaced in late eocene and miocene are sufficiently well constrained to be studied in detail it appears that they are associated with a specific polyphased kinematic context related to the convergence of tectonic plates this context is characterized by 1 a relatively fast convergence rate shortly followed by 2 a drastic decrease of this rate from these observations and assuming that the major part of plate convergence is accommodated along subduction zones we propose a two phase geodynamic model favoring emplacement of porphyry cu deposits 1 a high melt production in the mantle wedge followed by 2 an extensional regime or at least relaxation of the compressional stress in the upper plate promoting ascension of fertile magmas to the upper crust melt production at depth and the following extensional regime which would be related to variations in convergence rate are thus associated with variations in plate and trench velocities themselves being controlled by both plate kinematics at the surface and slab dynamics in the upper mantle in particular along strike folding behavior of the subducting slab may strongly influence trench velocity changes and the location of porphyry cu deposits metallogenic data suggest that periods of slab retreat which would favor mineralization processes during 40 myrs would be separated by barren periods lasting 10 to 20 myrs corresponding to shorter episodes of trench advance as observed in laboratory experiments these results confirm the control of the geodynamic context and especially subduction dynamics on the genesis of porphyry cu deposits this study also shows that the paleotectonic approach is a promising tool that could help identify geodynamic and tectonic criteria favoring the genesis of various ore deposits

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

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

continental margin deformation along the andean subduction zone thermo mechanical models the chilean andes extend north south for about 3000 km over the subducting nazca plate and show evidence of local rheological controls on first order tectonic features here rheological parameters are tested with numerical models of a subduction zone incorporating slab pull and upper plate convergence which calculate the development of stress and strain over a typical period of 4 myr the models test the effects of subduction interface strength arc and fore arc crust rheology and arc temperature on the development of superficial near surface faulting as well as viscous shear zones in the mantle deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities upper plate shortening and trench advance are both correlated and favored to a first order by upper plate weakness and to a second order by interface strength in cases of a strong interface a weak forearc crust is dragged downward by tectonic erosion a scenario for which indications are found along the northern chilean margin in contrast for a resistant forearc the slab pull force transmits to the surface and produces topographic subsidence this process may explain present day subsidence of the salar de atacama basin and or the persistence of a central depression specific conditions for northern chile produce a shear zone that propagates from the subduction zone in the mantle through the altiplano lower crust into the subandean crust as proposed by previous studies models with a weak interface in turn allow buoyant subducted material to rise into the continental arc in case of cessation of the slab pull this buoyant material may rise enough to change the stress state in the continental crust and lead to back arc opening in a case of young and hydrated oceanic plate forced by the slab pull to subduct under a resistant continent this plate is deviated and indented by the continental mantle and stretches horizontally at 100 km depth this situation might explain the flat wadati benioff zone of central chile

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

a dipping thick segment of the farallon slab beneath central u s it has been hypothesized that much of the crustal deformation attributed to the laramide orogeny of the southwest north american cordillera was caused by dynamic effects induced by the flat subduction of a large oceanic plateau that was embedded within the farallon plate previous studies have identified within the north american mantle a seismic velocity anomaly that plausibly represents the remnants of the subducted plateau coupled plate kinematic and dynamic modeling of the anomaly as well as surface geological findings identify this anomaly as the subducted conjugate to the shatsky rise here we find clear evidence for a northeastward dipping 35 dip thick up to 400 km thick slab like seismic velocity anomaly within the top of the lower mantle below the central united states using a deep focus earthquake below spain we find that the observed seismic waveforms recorded with the dense usarray display multi pathing indicative of sharp surfaces plate tectonic reconstructions in which the anomaly is migrated back in time suggest strong coupling of the plateau thickened slab segment to the overriding north america plate as it was subducted in combination with the reconstructions we interpret the structure as arising from eastward dipping farallon subduction at the western margin of north america during the cretaceous in contrast with some recent interpretations it appears that the plateau area of the slab has been further thickened which might undergo a combination of pure shear bulk shortening during flat slab subduction and or by a folding instability during penetration into the lower mantle

compositional vs thermal buoyancy and the evolution of subducted lithosphere we formulate 2 d cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle subducted lithosphere is represented as a cold stiff layered composite of denser eclogite underlain by more buoyant harzburgite slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle approximately a factor of two thickening occurs via pure shear just above the discontinuity with additional enhancement due to folding by over a factor of two no separation of the individual slab components occurs at the discontinuity and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy these results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle

seismostratigraphy and thermal structure of earth s core mantle boundary region we used three dimensional inverse scattering of core reflected shear waves for large scale high resolution exploration of earth9s deep interior d and detected multiple piecewise continuous interfaces in the lowermost layer d beneath central and north america with thermodynamic properties of phase transitions in mantle silicates we interpret the images and estimate in situ temperatures a widespread wave speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite internal d stratification may be due to multiple phase boundary crossings and a deep wave speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick the core mantle boundary temperature is estimated at 3950 200 kelvin beneath central america a site of deep subduction the d is relatively cold t 700 100 kelvin accounting for a factor of two uncertainty in thermal conductivity core heat flux is 80 to 160 milliwatts per square meter mw m 2 into the coldest d region and 35 to 70 mw m 2 away from it combined with estimates from the central pacific this suggests a global average of 50 to 100 mw m 2 and a total heat loss of 7 5 to 15 terawatts

eocene oligocene transition in the southern ocean history of water mass circulation and biological productivity high resolution records of carbon and oxygen isotopes and benthic foraminiferal accumulation rates for the eocene oligocene section at ocean drilling program site 689 maud rise weddell sea paleodepth about 1500 m were used to infer variations in paleoproductivity in relation to changes in climate and ventilation of the deeper water column the benthic foraminiferal abundance and isotope records show short term fluctuations at periodicities of 100 and 400 ka implying orbitally driven climatic variations both records suggest that intermediate depth water chemistry and primary productivity changed in response to climate during the eocene productivity increased during cold periods and during cold to warm transitions possibly as a result of increased upwelling of nutrient rich waters in the oligocene in contrast productivity maxima occurred during intervals of low d18o values presumably warmer periods when a proto polar front moved to the south of the location of site 689 this profound transition in climate productivity patterns occurred around 37 ma coeval with rapid changes toward increasing variability of the oxygen and carbon isotope and benthic abundance records and toward larger amplitude d18o fluctuations therefore we infer that at this time temperature fluctuations increased and a proto polar front formed in conjunction with the first distinct pulsations in size of the antarctic ice sheet we speculate that this major change might have resulted from an initial opening of the drake passage at 37 ma at least for surface and intermediate water circulation

amount of asian lithospheric mantle subducted during the india asia collision abstract body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle focusing on the india asia collision zone we postulate the existence of several asian continental slabs based on seismic global tomography we observe a lower mantle positive anomaly between 1100 and 900 km depths that we interpret as the signature of a past subduction process of asian lithosphere based on the anomaly position relative to positive anomalies related to indian continental slab we propose that this anomaly provides evidence for south dipping subduction of north tibet lithospheric mantle occurring along 3000 km parallel to the southern asian margin and beginning soon after the 45 ma break off that detached the tethys oceanic slab from the indian continent we estimate the maximum length of the slab related to the anomaly to be 400 km adding 200 km of presently asian subducting slab beneath central tibet the amount of asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km using global seismic tomography to resolve the geometry of asian continent at the onset of collision we estimate that the convergence absorbed by asia during the indentation process is 1300 km we conclude that asian continental subduction could accommodate at most 45 of the asian convergence the rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction underthrusting processes continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like eurasia

initiation of the andean orogeny by lower mantle subduction abstract the cordillera of the andes is a double vergent orogenic belt built up by thickening of south american plate crust several models provide plausible explanations for the evolution of the andes but the reason why shortening started at 50 ma is still unclear we explore the evolution of the subduction zone through time by restoring the position of the nazca trench in an absolute reference frame comparing its position with seismic tomography models and balancing the evolution of the subducting slab reconstructions show that the slab enters into the lower mantle at 50 10 ma and then progressed moving horizontally at shallow lower mantle depth while thickening and folding in the transition zone we test this evolutionary scenario by numerical models which illustrate that compression in the upper plate intensifies once the slab is anchored in the lower mantle we conclude that onset of significant shortening and crustal thickening in the andes and its sustained action over tens of million years is related to the penetration of the slab into the lower mantle producing a slowdown of lateral slab migration and dragging the upper plate against the subduction zone by large scale return flow

thermal maturity of carboniferous strata ouachita mountains the ouachita mountains a relatively untested potential hydrocarbon province contain a thick paleozoic section of apparently favorable source beds reservoir beds and trap configurations to estimate the thermal maturity of these strata vitrinite reflectance was measured on 89 samples collected mostly from carboniferous rocks from throughout the ouachita outcrop area thermal maturity contours generally parallel structural grain in the western two thirds of the ouachitas reflectance values in oil range from 3 0 in the core area in oklahoma and from about 1 0 to 3 0 in analogous positions in arkansas older strata on upthrown sides of thrust faults are more thermally mature than younger strata on downthrown sides in the eastern one third of the outcrop belt thermal maturities are significantly higher 2 0 5 0 ro and thermal maturity contours cut obliquely across structural grain strata on either side of thrust faults display identical levels of maturity in the western two thirds of the ouachitas sedimentary and tectonic burial have been the major influences of thermal maturation although a mesozoic thermal overprint has increased levels of maturity in arkansas throughout this area strata are within the oil and wet gas windows the anomalous levels and patterns of thermal maturity in the eastern third of the ouachitas are probably the result of a thermal overprint related to mesozoic rifting and intrusive activity in the mississippi embayment this thermal overprint has elevated the maturity of strata in most of this area above the level of hydrocarbon preservation although maturities within the dry gas window are retained locally

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

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

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

laboratory experiments of slab break off and slab dip reversal insight into the alpine oligocene reorganization we present laboratory experiments to study the consequence of the rupture of a subducting slab on the deep geometry of the subduction zone in our experiments slab break off occurs after the entrance of buoyant material at trench causing a slowing down of the subduction and producing an increase of the shortening rate and a transient episode of subduction dip reversal we discuss the potential application of these processes to the recent collisional evolution of the alps proposing that the rupture of the slab produced a re organisation of the belt with a transition from one sided to a doubly vergent orogen

development of finite strain in the convecting lower mantle and its implications for seismic anisotropy 1 seismological observations have revealed patches of seismic anisotropy in regions related to mantle upwelling and paleosubduction within an otherwise isotropic lower mantle a combination of numerical modeling and mineral physics is used to constrain the source of anisotropy in these regions in an effort to better understand lower mantle dynamics and mineral physics specifically it is investigated whether lattice preferred orientation lpo can explain the anisotropy observed in regions of paleosubduction since lpo is caused by dislocation creep and is destroyed by diffusion creep we can develop deformation mechanism maps to determine which regions allow for the development of a mineral fabric strain is then calculated in these regions and is related to mineral physics experiments combined with high pressure elastic constants of lower mantle minerals in order to assess the predicted seismic anisotropy uncertainties in rheological parameters such as the transition stress between dislocation creep and diffusion creep necessitate a full evaluation of the parameter range the effect of variations in transition stress activations parameters and strength of slabs on fabric development is investigated it is shown that lpo is a likely candidate for the cause of lowermost mantle anisotropy in regions of paleosubduction

mapping the tonga slab we applied residual sphere analysis to 25 intermediate and deep focus earthquakes in the tonga subduction zone and mapped variations in slab strike and dip depth extent and deep lateral advection at all latitudes along the arc as in the northwest pacific travel time data from intermediate and deep focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity p t of 0 5 0 2 m s k i e peak velocity perturbations of about 3 7 in all regions where the seismicity extends to depths of 600 km or more the shortest thermal slab models that cannot be rejected at the 90 confidence level reach depths of 750 km northern tonga to 850 km southern tonga these models however require a value of p t 1 5 m s k 3 times the value suggested by laboratory studies if the magnitude of p t is constrained to be less than 1 0 m s k the minimum penetration depth varies from 800 km to 900 km the travel time data are equally consistent with slab structures that deform at the base of the upper mantle thickening by a factor of 3 and models that contain no advective thickening inferred slab structure varies considerably throughout the subduction zone and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near source heterogeneity travel times in southern tonga 21 5 s 27 s are consistent with and in some cases require slab models that bend to a steeper dip at depth this phenomenon is also observed in parts of the northwest pacific but is not required in central 19 s 21 5 s and northern 13 s 19 s tonga the residual spheres of both intermediate and deep focus events suggest a sharp bend in slab strike at 25 s this feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the louisville ridge northern tonga is the locus of considerable complexity in deep slab structure seismicity strain rate and travel time data are consistent with although they do not uniquely require a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle the contorted segment would have begun subducting roughly 10 m y ago during a major plate reorganization in northernmost tonga a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab compared to data from the northwest pacific where travel time observations also suggest slab penetration below the deepest earthquakes tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab these observations argue against convective models which are rigorously stratified at 650 km depth but they are consistent with a discontinuity in mantle properties such as an increase in viscosity or a small percent variation in chemistry if it merely resists and deforms the descending slabs without acting as a complete barrier to flow