mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

assimilating lithosphere and slab history in 4 d earth models we develop methods to incorporate paleogeographical constraints into numerical models of mantle convection through the solution of the convection equations the models honor geophysical and geological data near the surface while predicting mantle flow and structure at depth and associated surface deformation the methods consist of four constraints determined a priori from a plate history model 1 plate velocities 2 thermal structure of the lithosphere 3 thermal structure of slabs in the upper mantle and 4 velocity of slabs in the upper mantle these constraints are implemented as temporally and spatially dependent conditions that are blended with the solution of the convection equations at each time step we construct earth like regional models with oceanic and continental lithosphere trench migration oblique subduction and asymmetric subduction to test the robustness of the methods by computing the temperature velocity and buoyancy flux of the lithosphere and slab full sphere convection models demonstrate how the methods can determine the flow associated with specific tectonic environments e g back arc basins intraoceanic subduction zones to address geological questions and compare with independent data both at present day and in the geological past e g seismology residual topography stratigraphy using global models with paleogeographical constraints we demonstrate 1 subduction initiation at the izu bonin mariana convergent margin and flat slab subduction beneath north america 2 enhanced correlation of model slabs and fast anomalies in seismic tomography beneath north and south america and 3 comparable amplitude of dynamic and residual topography in addition to improved spatial correlation of dynamic and residual topography lows

the accretionary history of southern south america from the latest proterozoic to the late palaeozoic some palaeomagnetic constraints

the case for dynamic subsidence of the u s east coast since the eocene the dynamic subsidence of the united states east coast is addressed using the discrepancy between regional and global estimates of sea level elevation of paleoshorelines and adjoint models of mantle convection that assimilate plate motions and seismic tomography the positions of eocene and miocene paleoshorelines are lower than predicted by global sea levels suggesting at least 50 m and possibly as much as 200 m of subsidence since the end of the eocene dynamic models predict subsidence of the east coast since the end of eocene although the exact magnitude is uncertain this subsidence has been occurring during an overall global sea level fall with the eustatic change being larger than the dynamic subsidence this results in a regional sea level fall in the absence of land subsidence dynamic subsidence is consistent with the difference between eustasy and regional sea level at the new jersey coastal plain

simultaneous inversion of mantle properties and initial conditions using an adjoint of mantle convection through the assimilation of present day mantle seismic structure adjoint methods can be used to constrain the structure of the mantle at earlier times i e mantle initial conditions however the application to geophysical problems is restricted through both the high computational expense from repeated iteration between forward and adjoint models and the need to know mantle properties such as viscosity and the absolute magnitude of temperature or density a priori we propose that an optimal first guess to the initial condition can be obtained through a simple backward integration sbi of the governing equations thus lessening the computational expense given a model with known mantle properties we show that a solution based on an sbi generated first guess has smaller residuals than arbitrary guesses mantle viscosity and the effective rayleigh number are crucial for mantle convection models neither of which is exactly known we place additional constraints on these basic mantle properties when the convection induced dynamic topography on earth s surface is considered within an adjoint inverse method besides assimilating present day seismic structure as a constraint we use dynamic topography and its rate of change in an inverse method that allows simultaneous inversion of the absolute upper and lower mantle viscosities scaling between seismic velocity and thermal anomalies and initial condition the theory is derived from the governing equations of mantle convection and validated by synthetic experiments for both one layer viscosity and two layer viscosity regionally bounded spherical shells for the one layer model at any instant of time the magnitude of dynamic topography is controlled by the temperature scaling while the rate of change of topography is controlled by the absolute value of viscosity for the two layer case the rate of change of topography constrains upper mantle viscosity while the magnitude of dynamic topography determines the temperature scaling lower mantle viscosity when upper mantle lower mantle density anomaly dominates the flow field this two stage scheme minimizes the tradeoff between temperature and lower mantle viscosity for both cases we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition for the two layer model this error is manifest as a tradeoff between the temperature scaling and lower mantle viscosity

the ups and downs of north america evaluating the role of mantle dynamic topography since the mesozoic the driving force for transient vertical motions of earth s surface remains an outstanding question a main difficulty lies in the uncertain role of the underlying mantle especially during the geological past here i review previous studies on both observational constraints and physical mechanisms of north american topographic evolution since the mesozoic i first summarize the north american vertical motion history using proxies from structural geology geochronology sedimentary stratigraphy and geomorphology based on which i then discuss the published physical models overall there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction at the continental scale a largely west to east migrating deformation pattern suggests an eastward translation of mantle dynamic effects consistent with models involving an eastward subduction and sinking of former farallon slabs since the cretaceous among the existing models the inverse model based on an adjoint algorithm and time dependent data constraints provides the most extensive explanations for the temporal changes of north american topography since the mesozoic at regional scales debates still exist on the predicted surface subsidence and uplift within both the western and eastern united states where discrepancies are likely due to differences in model setup e g mantle dynamic properties and boundary conditions and the amount of time dependent observational constraints toward the development of the next generation predictive geodynamic models new research directions may include 1 development of enhanced data assimilation capabilities 2 exploration of multiscale and multiphysics processes and 3 cross disciplinary code coupling

towards adjoint based inversion of time dependent mantle convection with nonlinear viscosity we develop and study an adjoint based inversion method for the simultaneous recovery of initial temperature conditions and viscosity parameters in time dependent mantle convection from the current mantle temperature and historic plate motion based on a realistic rheological model with temperature dependent and strain rate dependent viscosity we formulate the inversion as a pde constrained optimization problem the objective functional includes the misfit of surface velocity plate motion history the misfit of the current mantle temperature and a regularization for the uncertain initial condition the gradient of this functional with respect to the initial temperature and the uncertain viscosity parameters is computed by solving the adjoint of the mantle convection equations this gradient is used in a pre conditioned quasi newton minimization algorithm we study the prospects and limitations of the inversion as well as the computational performance of the method using two synthetic problems a sinking cylinder and a realistic subduction model the subduction model is characterized by the migration of a ridge toward a trench whereby both plate motions and subduction evolve the results demonstrate 1 for known viscosity parameters the initial temperature can be well recovered as in previous initial condition only inversions where the effective viscosity was given 2 for known initial temperature viscosity parameters can be recovered accurately despite the existence of trade offs due to ill conditioning 3 for the joint inversion of initial condition and viscosity parameters initial condition and effective viscosity can be reasonably recovered but the high dimension of the parameter space and the resulting ill posedness may limit recovery of viscosity parameters

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

mantle circulation models with sequential data assimilation inferring present day mantle structure from plate motion histories data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow such an approach is essential in mantle circulation models where we seek to constrain an unknown initial condition some time in the past and thus cannot hope to use first principles convection calculations to infer the flow history of the mantle one of the most important observables for mantle flow history comes from models of mesozoic and cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle buoyancy forces due to subducted oceanic slabs here we present five mantle circulation models with an assimilated plate motion history spanning the past 120 myr a time period for which reliable plate motion reconstructions are available all models agree well with upper and mid mantle heterogeneity imaged by seismic tomography a simple standard model of whole mantle convection including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation reveals downwellings related to farallon and tethys subduction adding 35 bottom heating from the core has the predictable effect of producing prominent high temperature anomalies and a strong thermal boundary layer at the base of the mantle significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep factor 100 viscosity rise from the upper to the lower mantle results in substantial transition zone heterogeneity enhanced by the effects of trench migration implicit in the assimilated plate motion history an expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km which is influenced by jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the cretaceous this result implies that sequential assimilation of past plate motion models is ineffective in studying the temporal evolution of core mantle boundary heterogeneity and that a method for extrapolating present day information backwards in time is required for short time periods of the order of perhaps a few tens of myr such a method exists in the form of crude backward convection calculations for longer time periods of the order of a mantle overturn a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

deep recycling of oceanic asthenosphere material during subduction uncertainties in the origin and composition of oceanic asthenosphere lead to different views on its temporal evolution upon subduction we investigate the evolution of asthenosphere material during subduction using high resolution geodynamic models in contrast to some earlier models suggesting that limited amount of asthenosphere material can be entrained during subduction we find that much of the subslab mantle 100 km thick could recycle into the deep mantle following the slab even if this mantle layer remains buoyant and less viscous during entrainment our results support the hypothesis that observed trench parallel subslab seismic anisotropy is a downward continuation of the anisotropic asthenosphere the entrainment of sometimes buoyant asthenosphere material provides a new mechanism for reducing slab dip angle which is consistent the shallower pacific slab underneath japan than that farther south episodic release of entrained buoyant materials during subduction can also explain enigmatic intraplate volcanism such as the changbaishan volcano in northeast china

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

miocene drainage reversal of the amazon river driven by plate mantle interaction northern south america experienced significant changes in drainage patterns during the opening of the south atlantic ocean disappearance of a mega wetland in the western amazonian basins was followed by the formation of the eastward draining amazon river which has been attributed to andean uplift however south america s westward motion over cold dense subducted slabs implies that regional subsidence and uplift east of the andes may have been driven by mantle convection here we use a coupled model of mantle convection and plate kinematics to show that dynamic subsidence of up to 40 m myr 1 initially formed the amazonian mega wetland in our model the sustained westward motion of continental south america over subducted slabs resulted in rebound of the amazonian mega wetland region at rates of up to 40 m myr 1 after 30 million years ago paired with continued subsidence of the eastern amazonian sedimentary basins at 10 20 m myr 1 the resulting progressive tilt of northern south america to the east enabled the establishment of the amazon river suggesting that mantle convection can profoundly affect the evolution of continental drainage systems

inverse problem of thermal convection numerical approach and application to mantle plume restoration abstract modern seismic tomographic images of the earth s interior facilitate the inference of the complex trajectories of present day convective flow in the upper mantle quantitative reconstruction of both the observed mantle structure and temperature field backwards in time requires a numerical tool for solving the inverse problem of thermal convection at infinite prandtl number in this paper we present a variational approach to three dimensional numerical restoration of thermoconvective mantle flow with temperature dependent viscosity this approach is based on a search for the mantle temperature and flow in the geological past by minimizing differences between present day mantle temperature derived from seismic velocities or their anomalies and that predicted by forward models of mantle flow for an initial temperature guess the past mantle temperatures so obtained can be employed as constraints on forward models of mantle dynamics to demonstrate the applicability of this technique we restore numerically a fluid dynamic model of the evolution of upper mantle plumes and show that the initial shape of the plumes can be accurately reconstructed we then model the evolution of the plumes forward in time plume upbuilding starting from the restored state to the state they were before the restoration and demonstrate the high accuracy of the model predictions we also show that the neglect of thermal diffusion in the backward modeling of thermal plumes in order to simplify the numerical procedure results in erroneous restorations of the plumes

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

the role of a mantle plume in the formation of yellowstone volcanism the origin of the yellowstone volcanic province remains debated proposed hypotheses involve either a mantle plume or not recent tomographic images allow a quantitative evaluation of the plume hypothesis and its interaction with the farallon slabs using 4 d geodynamic models with data assimilation we find that the slab is always in the way of the initially rising plume and that the plume could reach the surface only through the broken slab hinge at 15 ma for most of the time the sinking slabs dominate the mantle flow and prohibit upwelling we find that a plume that satisfies the present mantle image beneath yellowstone fails to predict both voluminous hot materials at shallow depths beneath the western u s and the age migration of the hot spot tracks we suggest that a plume is likely to have much less influence on the yellowstone volcanism than previously thought

eroding dynamic topography 1 geological observations of mantle flow driven dynamic topography are numerous especially in the stratigraphy of sedimentary basins on the contrary when it leads to subaerial exposure of rocks dynamic topography must be substantially eroded to leave a noticeable trace in the geological record here we demonstrate that despite its low amplitude and long wavelength and thus very low slopes dynamic topography is efficiently eroded by fluvial erosion providing that drainage is strongly perturbed by the mantle flow driven surface uplift using simple scaling arguments as well as a very efficient surface processes model we show that dynamic topography erodes in direct proportion to its wavelength we demonstrate that the recent deep erosion experienced in the colorado plateau and in central patagonia is likely to be related to the passage of a wave of dynamic topography generated by mantle upwelling

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

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

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

improving teleseismic event locations using a three dimensional earth model abstract a comparison is made between seismic event locations derived from standard spherically symmetric earth models jb prem iasp91 and a recent earth model sp and a published data set of 82 well located earthquakes with a more even global distribution iasp91 and prem are shown to offer similar errors in event location and origin time estimates with respect to the jb model the three dimensional 3d model sp for the earthquakes the improvements are smaller corrections for crustal thickness beneath source and receiver are found to be of similar magnitude to the mantle corrections but use of station corrections together with the three dimensional mantle model provide the best locations

quasi reversibility method for data assimilation in models of mantle dynamics summary rapid progress in imaging deep earth structures using seismic tomography and in studies of physical and chemical properties of mantle rocks facilitates research in assimilation of data related to mantle dynamics in this paper we present a new numerical approach for data assimilation which allows for incorporating observations at present and unknown initial conditions in the past for mantle temperature and flow into a 3 d dynamic model in order to determine the initial conditions the dynamic model is described by the backward heat motion and continuity equations the use of the quasi reversibility qrv method implies the introduction into the backward heat equation of the additional term involving the product of a small regularization parameter and a higher order temperature derivative the data assimilation in this case is based on a search of the best fit between the forecast model state and the observations by minimizing the regularization parameter we apply the qrv data assimilation method to restore the evolution of i mantle plumes a synthetic case study and ii the lithospheric slab imaged by teleseismic body wave tomography in the southeastern carpathians for both models the present temperature and mantle flow are assimilated to the geological past and the prominent features of mantle structures are recovered we then model the evolution of the mantle structures forward in time starting from the restored state to the present state and estimate the accuracy of the model predictions the results of the qrv data assimilation are compared to that obtained by the variational var and backward advection data assimilation although the accuracy of the qrv data assimilation is lower than that of the var data assimilation the qrv method does not require any additional smoothing of the input data or filtering of temperature noise as the var method does based on the results and the comparison of the methods we consider the qrv method to be a highly promising approach to assimilation of data related to mantle dynamics

retrodictions of mid paleogene mantle flow and dynamic topography in the atlantic region from compressible high resolution adjoint mantle convection models sensitivity to deep mantle viscosity and tomographic input model abstract although mantle convection at earth like vigor is a chaotic process it has been shown by conceptual studies that one can constrain its flow history back in time for periods comparable to a mantle overturn 100 million years if knowledge of the tangential surface velocity field and estimates on the present day heterogeneity state are available such retrodictions enabled through computationally demanding adjoint methods are a promising tool to improve our understanding of deep earth processes and to link uncertain geodynamic modeling parameters to geologic observables here we present the first mantle flow retrodictions for geodynamically plausible compressible high resolution earth models with 670 million finite elements going back in time to the mid paleogene our retrodictions involve the dynamic effects from a low viscosity zone lvz in the upper mantle assimilate a past plate motion model for the tangential surface velocity field and probe the influence from uncertain modeling parameters by using two different estimates for the present day heterogeneity state of the mantle as imaged by two recent seismic tomographic studies and two different values for deep mantle viscosity focusing on the african hemisphere we find that our retrodictions produce a spatially and temporally highly variable asthenosphere flow with faster than plate velocities and a history of dynamic topography characterized by local doming events in agreement with considerations on plate driving forces and regional scale uplifts reported in the geologic literature our results suggest that improved constraints on non isostatic vertical motion of earth s surface provided for instance by basin analysis seismic stratigraphy landform studies or the sedimentation record will play an important role in our understanding of the recent mantle flow history

seismic tomography surface uplift and the breakup of gondwanaland integrating mantle convection backwards in time mantle density heterogeneities imaged using seismic tomography contain information about time dependent mantle flow and mantle structures that existed in the past we model the history of mantle flow using a tomographic image of the mantle beneath southern africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition if the resulting backwards integrated model for structures is used as a starting point for a forwards convection model today s mantle can be adequately reconstructed if we do not integrate backwards more than than about 50 75 ma flow can also be reliably reversed through the mesozoic but only if instability of the lower boundary layer can be suppressed our model predicts that the large seismically slow and presumably hot structure beneath southern africa produced 500 700 m of dynamic topography throughout the cenozoic since 30 ma uplift has moved from eastern to southern africa where uplift rates are 10 m myr consistent with observations during the mesozoic the modeled topographic high is situated near gondwanaland rifting raising the possibility that this buoyant structure may have been involved with this breakup

subduction zone evolution and low viscosity wedges and channels dehydration of subducting lithosphere likely transports fluid into the mantle wedge where the viscosity is decreased such a decrease in viscosity could form a low viscosity wedge lvw or a low viscosity channel lvc on top of the subducting slab using numerical models we investigate the influence of low viscosity wedges and channels on subduction zone structure slab dip changes substantially with the viscosity reduction within the lvws and lvcs for models with or without trench rollback overthickening of slabs is greatly reduced bylvws or lvcs two divergent evolutionary pathways have been found depending on the maximum depth extentofthelvwandwedgeviscosity assumingaviscositycontrastof0 1withbackgroundasthenosphere modelswithalvwthat extendsdown to 400km depthshow asteeply dipping slab while models with anlvw that extends to muchshallower depth such as 200 km can produce slabs that are flat lying beneath the overriding plate there is a narrow range of mantle viscosities that produces flatslabs 5to10 10 19 pas andthe slab flatteningprocessisenhanced bytrenchrollback slabcanbedecoupled fromthe overriding platewithalvcifthe thicknessisatleastafew10s ofkm the viscosityreduction isatleastafactor oftwoandthedepthextentofthe lvc is several hundred km these models have important implications for the geochemical and spatial evolution of volcanic arcs and the state of stress within the overriding plate the models explain the poor correlation between traditional geodynamic controls subductingplateageandconvergencerates onslabdip wepredictthatwhenvolcanicarcschangetheirdistancefromthetrench they could be preceded by changes in arc chemistry we predict that there could be a larger volatile input into the wedge when arcs migrate toward the trench and visa versa the transition of a subduction zone into the flat lying regime could be preceded by changes in the volatile budget such that the dehydration front moves to shallower depths our flat slab models shed some light on puzzling flat subductionsystems likeincentralmexico wherethereisnodeformationwithintheoverridingplateabovetheflatsegment thelack

viscosity jump in earth s mid mantle the viscosity structure of earth s deep mantle affects the thermal evolution of earth the ascent of mantle plumes settling of subducted oceanic lithosphere and the mixing of compositional heterogeneities in the mantle based on a reanalysis of the long wavelength nonhydrostatic geoid we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth we detect an increase in viscosity at 800 to 1200 kilometers depth far greater than the depth of the mineral phase transformations that define the mantle transition zone the viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation plume deflection and changes in large scale structure and offers a simple explanation of these phenomena

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

a hybrid approach to data assimilation for reconstructing the evolution of mantle dynamics quantifying past mantle dynamic processes represents a major challenge in understanding the temporal evolution of the solid earth mantle convection modeling with data assimilation is one of the most powerful tools to investigate the dynamics of plate subduction and mantle convection although various data assimilation methods both forward and inverse have been created these methods all have limitations in their capabilities to represent the real earth pure forward models tend to miss important mantle structures due to the incorrect initial condition and thus may lead to incorrect mantle evolution in contrast pure tomography based models cannot effectively resolve the fine slab structure and would fail to predict important subduction zone dynamic processes here we propose a hybrid data assimilation approach that combines the unique power of the sequential and adjoint algorithms which can properly capture the detailed evolution of the downgoing slab and the tomographically constrained mantle structures respectively we apply this new method to reconstructing mantle dynamics below the western u s while considering large lateral viscosity variations by comparing this result with those from several existing data assimilation methods we demonstrate that the hybrid modeling approach recovers the realistic 4 d mantle dynamics the best

a rapid burst in hotspot motion through the interaction of tectonics and deep mantle flow volcanic hotspot tracks featuring linear progressions in the age of volcanism are typical surface expressions of plate tectonic movement on top of narrow plumes of hot material within earth s mantle seismic imaging reveals that these plumes can be of deep origin probably rooted on thermochemical structures in the lower mantle although palaeomagnetic and radiometric age data suggest that mantle flow can advect plume conduits laterally the flow dynamics underlying the formation of the sharp bend occurring only in the hawaiian emperor hotspot track in the pacific ocean remains enigmatic here we present palaeogeographically constrained numerical models of thermochemical convection and demonstrate that flow in the deep lower mantle under the north pacific was anomalously vigorous between 100 million years ago and 50 million years ago as a consequence of long lasting subduction systems unlike those in the south pacific these models show a sharp bend in the hawaiian emperor hotspot track arising from the interplay of plume tilt and the lateral advection of plume sources the different trajectories of the hawaiian and louisville hotspot tracks arise from asymmetric deformation of thermochemical structures under the pacific between 100 million years ago and 50 million years ago this asymmetric deformation waned just before the hawaiian emperor bend developed owing to flow in the deepest lower mantle associated with slab descent in the north and south pacific

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

tectonic geomorphology of the southern sierra nevada mountains california evidence for uplift and basin formation hypotheses regarding uplift of the sierra nevada mountains sierra california usa vary from a single slowly tilting block to rapid pliocene and or quaternary uplift in the south exclusively to test these hypotheses we examined geomorphic indices of the western sierra we interpret longitudinal profiles of the larger westerly flowing rivers mountain front sinuosity valley floor to width to height ratio and relief ratio to show that relative tectonic activity is greater in the southern sierra near the kern river gorge fault the sierra range crest profile indicates that the increased tectonism is related to more recent uplift in the southern sierra only westward migration of basin and range extension is consistent with the locus of uplift in the southern sierra we hypothesize that the sierra topography is the result of pliocene delamination related uplift in the central sierra and post pliocene interaction of the san andreas garlock and sierra nevada frontal fault zones

rejuvenation of appalachian topography caused by subsidence induced differential erosion topographic relief continued to develop in the appalachian mountains eastern usa long after the tectonic forces that created the range had become inactive numerical modelling and reconstructions of sediment deposition in the gulf of mexico suggest that the topographic relief was rejuvenated by subsidence induced differential erosion caused by sinking of the subducted farallon slab in the underlying mantle in ancient orogens such as the appalachian mountains in the eastern united states the difference between the high and low points topographic relief can continue to increase long after the tectonic forces that created the range have become inactive climatic forcing1 and mantle induced dynamic uplift2 3 could drive formation of relief but clear evidence is lacking in the appalachian mountains here i use a numerical simulation of dynamic topography in north america combined with reconstructions of the sedimentation history from the gulf of mexico4 to show that rejuvenation of topographic relief in the appalachian mountains since the palaeogene period could have been caused by mantle induced dynamic subsidence associated with sinking of the subducted farallon slab specifically i show that patterns of continental erosion and the eastward migration of sediment deposition centres in the gulf of mexico closely follow the locus of predicted dynamic subsidence furthermore pulses of rapid sediment deposition in the gulf of mexico4 and western atlantic5 correlate with enhanced erosion in the appalachian mountains during the miocene epoch caused by dynamic tilting of the continent the model predicts that such subsidence induced differential erosion caused flexural isostatic adjustments of appalachian topography that led to the development of 400 m of relief and more than 200 m of elevation i propose that dynamically induced continental tilting may provide a mechanism for topographic rejuvenation in ancient orogens

subduction controlled mantle flow and seismic anisotropy in south america abstract seismic anisotropy records both the past and present deformation inside the solid earth in the mantle seismic anisotropy is mainly attributed to the lattice preferred orientation lpo of mineral fabrics caused by the shear deformation due to mantle flow however contributions from different tectonic processes remain debated and a single geodynamic model that simultaneously explains the observed mantle structures and various seismic anisotropy measurements is still lacking here we present a model for the cenozoic subduction history in south america using a geodynamic simulation constrained by both past plate reconstructions and present mantle seismic structures with a recently developed software package drexs we further predict azimuthal seismic anisotropy at different depths and generate synthetic shear wave splitting sws measurements using the resulting mantle flow our results provide a good match to both depth dependent surface wave anisotropy and various land based sws records we find that the dominant control on seismic anisotropy in south america comes from subduction induced mantle flow where anisotropy below the subducting nazca plate aligns with plate motion induced couette flow and that below the overriding south american plate follows slab induced poiseuille flow this large scale mantle flow can be diverted by secondary slabs such as that below the antilles subduction zone in contrast the contribution to sws from fossil continental anisotropy and from the effects due to mantle flow modulation by lithosphere thickness variation are minor upper mantle fast seismic anomalies beneath the southern atlantic margin should have close to neutral buoyancy in order to satisfy the observed seismic anisotropy

thermal structure beneath the snake river plain implications for the yellowstone hotspot basaltic magmatism associated with the yellowstone hotspot has been widely attributed to upwelling of a mantle plume yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism fundamental questions concern the influence of continental cratonic lithosphere in producing the differences and the extent to which upper plate processes contribute to magma production to better understand scenarios of melt generation p t conditions are estimated for segregation of primitive snake river plain srp basalts from the mantle combined with analysis of trace element and seismic constraints we conclude from this that 1 melt production was concentrated at depths between roughly 70 100 km 2 mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 c and 3 the mantle source was relatively fertile mg 90 these results suggest that the seismically imaged plume below yellowstone is significantly cooler than upwellings beneath hawaii iceland and many other oceanic hotspots our findings in combination with other geochemical and geodynamic considerations are permissive of magma generation within the ancient lithospheric mantle keel associated with the wyoming craton plume contributions while not excluded involve physical and geochemical implications that suggest they are subordinate