toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

a two phase model for compaction and damage 3 applications to shear localization and plate boundary formation a new two phase theory employing a nonequilibrium relation between interfacial surface energy pressure and viscous deformation bercovici et al this issue provides a model for damage void generation and microcracking and thus a continuum description of weakening failure and shear localization here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase the matrix representing for example silicate is much stronger more viscous than the other phase the fluid this calculation is motivated as a simple model of plate boundary formation in a shear zone even without shear the two phases eventually separate due to gradients in surface tension however the influence of shear on phase separation is manifest in several ways as shear velocity increases the separation rate of the phases increases demonstrating a basic feedback mechanism accumulation of the fluid phase causes focused weak zones on which shear concentrates causing more damage and void generation and thus greater accumulation of fluid beyond a critical shear velocity phase separation undergoes intense acceleration and focusing leading to a tear localization in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack at an even higher value of shear velocity phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer our two phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model applications of the theory to various fields such as granular dynamics metallurgy and tectonic plate boundary formation are numerous

forearc uplift and extension southern hawke s bay new zealand mid pleistocene to present the maraetotara plateau cape kidnappers area in southern hawke s bay new zealand lies in the forearc region of the obliquely convergent hikurangi margin where the pacific plate is presently subducting beneath the leading edge of the australian plate uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid pleistocene time principal strain directions and strain ratios determined from mesoscale fault arrays associated with major faults document the regional extension in the cape kidnappers area fault arrays record plane strain with northwest horizontal extension and vertical shortening in the maraetotara plateau area fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions either east west and north south or northwest and northeast and vertical shortening the morphology seismicity and nature and distribution of active faults of the southern hawke s bay region suggest that the maraetotara plateau cape kidnappers area is the on land expression of an outer arc high which is developed between the accretionary wedge on the east and the forearc basin on the west the cause of uplift and extension in the maraetotara plateau cape kidnappers area may be either upper plate deformation related to structural evolution of the outer arc high and underlying backstop underplating on the hikurangi subduction zone or a combination of these processes the uplift and horizontal extension occurring in the maraetotara plateau cape kidnappers area contrast sharply with active deformation in adjacent parts of the forearc reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

on the resolution of radial viscosity structure in modelling long wavelength postglacial rebound data summary studies of postglacial rebound pgr constrain the viscosity of the mantle in an absolute sense and have also been used to constrain the radial variation in mantle viscosity such radial variations probably have important effects on many aspects of solid earth dynamics including the existence and style of plate tectonics rotational dynamics thermal convection and mixing of chemical species by convection most studies of pgr as well as other studies constraining viscosity structure e g the geoid agree that the average viscosity of the upper mantle beneath the lithosphere is smaller than that of the lower mantle however there is marked disagreement regarding the amount of viscosity contrast and the thickness of an upper mantle low viscosity zone lvz with estimates for the viscosity contrast ranging from a mere factor of 2 to several orders of magnitude here we seek to both quantify and clearly illustrate the fundamental modelling trade off between viscosity contrast and layer thickness d and show that if only long wavelength constraints are available pgr models are largely indistinguishable for constant values of d 3 characterizing an lvz models employing both relative sea level rsl data from hudson bay and time variations in gravity from the grace satellite illustrate this principle models with lvz viscosity contrasts ranging from 3 orders of magnitude fit the data equally well depending on the thickness of the lvz other traditional data sets constraining pgr e g fennoscandian strand lines appear no more capable of independently resolving this ambiguity although some studies that incorporate both long wavelength rsl type constraints and shorter wavelength constraints such as tilting of continental margins appear to favour a relatively thin high viscosity contrast lvz new gps data from north america and scandinavia may be able to resolve this question even further however at the present time pgr studies do not preclude and perhaps favour a relatively thin lvz of viscosity contrast at least one and perhaps as much as 3 orders of magnitude beneath the lithosphere in some regions this conclusion has significant implications for understanding the phenomenon of plate tectonics and the evolution and dynamics of the mantle

plate generation and two phase damage theory in a model of mantle convection summary the formation of narrow rapidly deforming plate boundaries separating strong plate interiors are integral components of the generation of plate tectonics from mantle convection the development of narrow plate boundaries requires the interaction of a non linear rheology and convection one such non linear rheology is two phase damage theory which employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby forming a theoretical model for void generation two phase damage theory was recently extended to allow for deformational work to increase the fineness reduce the grain size of the matrix phase we present results testing two phase damage theory in a 2 d convectively driven system where we allow for 1 pure void generating damage 2 pure fineness generating damage and 3 combined void and fineness generating damage pure void generating damage is found to be unsuccessful at producing plate like features finenessgenerating damage is successful at inducing plate like behaviour in certain circumstances including increasing viscosity sensitivity to fineness and certain regimes of damage input and healing rate cases with combined void and fineness generating damage produce significantly more localization than the end members due to the apparent increase of deformational work input into fineness generation the interaction of microcracks and grain size reduction in two phase damage theory suggests a rheological model for shear localization necessary for the formation of plate tectonic boundaries

modelling plume related uplift gravity and melting on venus abstract three roughly circular regions on venus about 1000 km across are identified as potential hawaii scale plume sites on the basis of their gravity 70 90 mgal and topography 1 6 2 0 km anomalies and signs of melt generation and rifting axisymmetric isoviscous convection models are used to reproduce gravity profiles across these plumes and the line of sight acceleration of the magellan spacecraft as it passes over them the best fitting models have a conductive lid thickness of less than 150 km mantle viscosity of 8 9 1019 to 9 6 1020 pa s and a basal heat flux of 15 25 m w m 2 the lid thickness is constrained by requiring a modest amount of melt generation and a potential temperature of about 1300 c the high mantle viscosity relative to that of earth is probably a consequence of the absence of water in the mantle and may help to explain the current absence of plate tectonics on venus the low heat flux suggests that the thermal evolution of venus has differed from that of the earth

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

on the scale of mantle heterogeneity abstract a data set comprising 3313 residuals measured from digital records of 429 earthquakes is analyzed for the heterogeneity of the earth s mantle the smoothed ss residuals reveal a large pattern of heterogeneity and agree well with the major tectonic features on the earth s surface and the residual patterns predicted by upper mantle and whole mantle models the spectrum of ss residuals indicates that the heterogeneity in the earth s mantle which probably reflects the patterns of mantle convection is dominated by long wavelength features our analysis of the power of ss residuals as a function of harmonic degree shows a distinct difference in the rate of decrease of the power below and above degree 8 the rate is first slower than l 1 and then it exceeds l 2 this is also true for ss residuals predicted for model sh 425 2 and love wave phase velocities at 100 s measured by other researchers this indicates that there is a preference for features with dimensions larger than 2500 3500 km we find that the power spectrum of the velocity anomalies associated with the subducted slabs is two orders of magnitude lower for 1 l 8 than that of velocity anomalies obtained by inversion of waveform data for the ocean basins correlation of the ss residuals with the square root of age of the oceanic lithosphere is highly significant however the age correlation predicts only 10 of the variance this means that the thermal signature of the spreading center represents only a small fraction of the thermal anomalies introduced into the upper mantle by convection

a small unextractable melt fraction as the cause for the low velocity zone abstract throughout the oceanic asthenosphere there exists a zone of seismic velocities that are lower than would be predicted the cause of this low velocity zone lvz whether partial melt or solid state mechanisms has been debated for decades we investigate the lvz by considering seismic and magnetotelluric data from tectonically stable 70 myr old lithosphere in the central pacific ocean we utilise recent experimental advances on the influence of partial melt and seismic attenuation results show that the lvz is characterised by a small volume of interconnected melt and by low hydrogen contents beneath the lvz the asthenosphere does not contain partial melt but has high hydrogen contents and low attenuation values indicating large grain sizes and or low oxygen fugacities to explain these observations we propose that a small amount of unextractable melt is trapped in the asthenosphere after melting at mid ocean ridges

hysteresis in mantle convection plate tectonics systems 1 we use simulations of mantle convection with surface yielding to show that multiple tectonic regimes are possible for equivalent system parameter values models with the same lithospheric strength parameters and the same vigor of convection can display different modes of tectonics within the region of multiple solutions the evolutionary pathway of the system is the dominant factor that determines the tectonic mode e g whether mantle convection operates in a plate tectonic like mode the extent of the multiple regimes window is found to increase with the temperature dependent viscosity contrast across the system the implication for models that seek to predict the tectonic regimes of planets is that the temporal evolution of the planet needs to be taken into account a further implication is that modeling studies can lead to different conclusions regarding the tectonic state of a planet extra solar planets in particular despite the final model parameter values remaining equivalent

driving the earth machine the asthenosphere derived from the greek asthen s meaning weak is the uppermost part of earth s mantle right below the tectonic plates that make up the solid lithosphere first proposed by barrell 100 years ago 1 the asthenosphere has traditionally been viewed as a passive region that decouples the moving tectonic plates from the mantle and provides magmas to the global spreading ridge system recent studies suggest that the asthenosphere may play a more active role as the source of the heat and magma responsible for intraplate volcanoes furthermore it may have a major impact on plate tectonics and the pattern of mantle flow

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

a window for plate tectonics in terrestrial planet evolution abstract the tectonic regime of a planet depends critically on the contributions of basal and internal heating to the planetary mantle and how these evolve through time we use viscoplastic mantle convection simulations with evolving core mantle boundary temperatures and radiogenic heat decay to explore how these factors affect tectonic regime over the lifetime of a planet the simulations demonstrate i hot mantle conditions coming out of a magma ocean phase of evolution can produce a hot stagnant lid regime whilst a cooler post magma ocean mantle may begin in a plate tectonic regime ii planets may evolve from an initial hot stagnant lid condition through an episodic regime lasting 1 3 gyr into a plate tectonic regime and finally into a cold senescent stagnant lid regime after 10 gyr of evolution as heat production and basal temperatures wane and iii the thermal state of the post magma ocean mantle which effectively sets the initial conditions for the sub solidus mantle convection phase of planetary evolution is one of the most sensitive parameters affecting planetary evolution systems with exactly the same physical parameters may exhibit completely different tectonics depending on the initial state employed estimates of the early earth s temperatures suggest earth may have begun in a hot stagnant lid mode evolving into an episodic regime throughout most of the archaean before finally passing into a plate tectonic regime the implication of these results is that for many cases plate tectonics may be a phase in planetary evolution between hot and cold stagnant states rather than an end member

the geoid constraint in global geodynamics viscosity structure mantle heterogeneity models and boundary conditions summary the earth s non hydrostatic gravity field or geoid provides a first order constraint on mantle density structure and dynamics geodynamic models for the geoid have proliferated since the advent of seismic mapping of mantle heterogeneity structure tomography because the geoid offers perhaps the best measured independent constraint on mantle density heterogeneity however dynamic geoid models involve a number of questionable physical assumptions and uncertainties whose effects need to be evaluated before geodynamic inferences based upon the geoid can be considered sound troubling issues include the appropriate surface boundary conditions free slip no slip plates and parametrization of radial viscosity variations how many layers can be resolved in addition to lateral viscosity variations possible chemical layering of the mantle phase transitions etc there are also uncertainties in the density heterogeneity models used as input to dynamic geoid models most of which are derived from seismic tomography and require weakly constrained empirical conversion factors to go from seismic velocity variations to density variations here we address several of the most straightforward problems inherent in geoid modelling namely the issues of viscosity structure resolution uncertainties in appropriate boundary conditions and differences among mantle heterogeneity models a robust feature of all models is a lower mantle viscosity at least a factor of 30 greater than that of the upper mantle but there is little resolution with regard to finer details such as lithospheric or uppermost mantle low viscosity zone viscosity ironically free slip boundary conditions result in the best fits to the geoid in all cases but all boundary conditions exhibit predictable trade offs with the uppermost mantle viscosity models with a single viscosity layer representing the lower mantle yield similar dynamic topography estimates of the order of 700 1000 m in amplitude regardless of the finer details of upper mantle viscosity structure boundary conditions or input heterogeneity models comparing mantle heterogeneity models based on two independent seismological determinations harvard and berkeley models and on the history of subduction we find that these models are virtually indistinguishable regarding inferences of mantle viscosity structure and amplitude of dynamic topography and in terms of the effects of different boundary conditions uncertainties concerning which type of boundary condition is appropriate are much more important than which mantle heterogeneity model is chosen given other uncertainties in modelling the geoid particularly the strong effects due to lateral viscosity variations for intermediate 10 000 km wavelengths we conclude that the class of dynamic geoid models explored so far cannot reliably elucidate the details of upper mantle viscosity structure

fast asthenosphere motion in high resolution global mantle flow models a variety of geologic observations point to fast upper mantle flow that may exceed plate tectonic velocities by an order of magnitude at the same time there is mounting evidence from seismology for flow like structures in the upper 100 200 km of the mantle here we present a set of geodynamic simulations to link these observations in a synthetic setting we include asthenospheric channels of varying thickness with an extreme case of 100 km and a significant viscosity contrast of up to 4 orders of magnitude relative to the deeper mantle using our new global high resolution code terra neo we obtain an increase in velocity by a factor of 10 between a 1000 km thick and the very thin channel translating into velocities of 20 cm a within the narrow asthenosphere we further present and verify a simple poiseuille flow model predicting that the upper mantle velocity scales with the inverse of the asthenosphere thickness

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

degree 1 mantle convection and the crustal dichotomy on mars the surface of mars consists of an old heavily cratered and elevated southern hemisphere and younger resurfaced and depressed northern hemisphere a feature often termed the crustal dichotomy the global crustal structure zuber et al 2000 revealed by topography and gravity data from the mars global surveyor spacecraft and the possible late formation of the boundary zone between the hemispheres mcgill and dimitriou 1990 have been proposed to indicate an endogenic origin for the dichotomy however degree 1 mantle convection that is required for any endogenic process to be viable cannot be produced with conventional mantle convection models schubert et al 1990 we have studied the role of radially stratified viscosity on mantle deformation by using rayleigh taylor instability analyses in a spherical shell geometry our analyses reveal that when mantle viscosity is stratified with a weak asthenosphere deformation at long wavelengths is more efficient than that at short wavelengths the weaker the asthenosphere the longer the wavelength at which the deformation is the most efficient a thicker asthenosphere also favors the deformation at long wavelengths both the rayleigh taylor instability analyses and numerical modeling of mantle convection show that degree 1 convection can be produced within the martian mantle provided that the mantle had a weak asthenosphere v500 km thick and v10 2 times weaker than the underlying mantle early in planetary history the degree 1 convection causes preferential heating of one hemisphere that may explain the primary features associated with the dichotomy in crustal structure fl 2001 elsevier science b v all rights reserved

three regimes of mantle convection with non newtonian viscosity and stagnant lid convection on the terrestrial planets numerical simulations of convection with strongly temperature dependent viscosity suggest that non newtonian viscosity convection dislocation creep passes through three convective regimes similar to those observed for newtonian viscosity convection diffusion creep the small viscosity contrast regime the transitional regime and the stagnant lid regime for realistic viscosity contrasts mantle convection is in the stagnant lid regime characterized by formation of a very viscous slowly creeping lid on top of an actively convecting mantle this explains the tectonic style observed on the terrestrial planets and the moon on the other hand this eliminates the possibility that the plates on earth could be mobile due to non newtonian viscosity the nature of the mobility of lithospheric plates on earth has yet to be explained

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

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

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

long wavelength convection poiseuille couette flow in the low viscosity asthenosphere and the strength of plate margins summary mixed heated 3 d mantle convection simulations with a low viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere asthenosphere region is a poiseuille couette flow the poiseuille couette velocity magnitude ratio d u allows us to characterize solid state flow in the asthenosphere and to predict the regime transition the transition from dominantly pressure driven poiseuille flow at shorter wavelengths to dominantly shear driven couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion in the poiseuille regime significant resistance is provided by plate bending whereas in the couette regime most resistance is due to vertical shear in the bulk mantle the couette case corresponds to classical scaling ideas for mantle convection whereas the poiseuille case with asthenospheric velocities exceeding surface velocities is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the earth our simulations show that both modes can exist for the same level of convective vigour i e rayleigh number but at different convective wavelengths additional simulations with temperature and yield stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins our simulations establish a connection between the strength of plate margins solid state flow in the asthenosphere and the wavelength of mantle convection this connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned

low viscosity channels and the stability of long wavelength convection abstract mantle convection simulations with a low viscosity channel akin to the earth s asthenosphere are characterized by long wavelength flow structure boundary layer theory predicts that as the viscosity of the channel decreases the wavelength that maximizes heat transfer increases as a pattern selection criterion this analysis is not complete it provides no mechanism to relate the optimal heat transfer wavelength to the wavelength that is realized in nature we present numerical simulation suites for bottom and internally heated end members to demonstrate that the cell wavelengths that maximize heat transfer are also the most stable this does not rule out the possibility of multiple wavelengths being realizable but it does imply that wavelengths near the stability peak will be preferred and for the configurations we explore the stability peak corresponds to the energetically most efficient flow configuration

parameters controlling dynamically self consistent plate tectonics and single sided subduction in global models of mantle convection recent advances in numerical modeling allow global models of mantle convection to more realistically reproduce the behavior at convergent plate boundaries in particular the inclusion of a free surface at the outer boundary has been shown to facilitate self consistent development of single sided subduction this allows for a more extensive study of subduction in the context of global mantle convection as opposed to commonly used regional models our first study already indicated important differences between mantle convection with single sided subduction and mantle convection with double sided subduction here we further investigate the effect of various physical parameters and complexities on inducing earth like plate tectonics and its evolution in time results reinforce the previous finding that using a free surface instead of a free slip outer boundary dramatically changes subduction style with free surface cases displaying many episodes of single sided subduction which leads to more realistic slab dip stress state trench retreat rate and slab induced mantle flow longevity of single sided subduction is promoted by a layer of hydrated crust with a low yield strength to lubricate the subduction channel a low viscosity asthenosphere and a high strength of the slab determined by a combination of high diffusion creep viscosity and intermediate friction coefficient although its effective viscosity is in the observationally constrained range in the bending region the time evolution displays interesting events including subduction polarity reversals subduction shut off and slab break off

the effects of internal heating and large scale climate variations on tectonic bi stability in terrestrial planets abstract we use 3d mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet a proxy for thermal age planetary surface temperature and lithosphere strength at both high and low values of internal heating for moderate to high lithospheric yield strength hot and cold stagnant lid single plate planet states prevail for intermediate values of internal heating multiple stable tectonic states can exist in these regions of parameter space the specific evolutionary path of the system has a dominant role in determining its tectonic state for low to moderate lithospheric yield strength mobile lid behavior a plate tectonic like mode of convection is attainable for high degrees of internal heating i e early in a planet s thermal evolution however this state is sensitive to climate driven changes in surface temperatures relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile to a stagnant lid regime once a stagnant lid mode is initiated a return to mobile lid is not attainable by a reduction of surface temperatures alone for lower levels of internal heating the tectonic regime becomes less sensitive to surface temperature changes collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga year timescales within parameter space regions that allow for bi stable behavior any model based prediction as to the current mode of tectonics is inherently non unique in the absence of constraints on the geologic and climatic histories of a planet

degree 1 convection in the martian mantle and the origin of the hemispheric dichotomy 1 the surface of mars appears dramatically different between the northern and southern hemispheres any endogenic origin for this hemispheric dichotomy must involve a pattern of mantle convection that reflects the shape of the dichotomy primarily spherical harmonic degree 1 we investigated two mechanisms by which degree 1 convection may be initiated in the martian mantle 1 an endothermic phase change near the cmb and 2 viscosity layering in the mid mantle using two dimensional 2 d and 3 d spherical finite element convection models we explored the conditions under which each mechanism can produce degree 1 structures the phase transition is only effective at generating degree 1 structures when the mantle viscosity is constant or weakly temperature dependent activation energy 100 kj mol but the degree 1 pattern requires several billion years to develop increasing convective vigor in phase change models leads to reduced wavelengths for convective structures degree 1 convection can also develop in a layered viscosity mantle with temperature and depth dependent viscosity an overall sublithospheric radial viscosity variation of a factor of 100 including a factor of 8 25 jump in the midmantle can lead to formation of degree 1 structure in a timescale ranging from 100 my to several hundred my consistent with the timescale for the formation of the dichotomy neither convective vigor nor the internal heating rate greatly affects the formation of degree 1 structures we propose that degree 1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy

depth dependent viscosity and mantle stress amplification implications for the role of the asthenosphere in maintaining plate tectonics summary boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low viscosity asthenosphere the theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer the theory predicts that the confinement of horizontal mantle flow within a low viscosity sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel even if the absolute viscosity of the sublithosphere mantle does not change between the two cases thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow viscositychannelthickness if the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere then the plate normal stress is predicted to also increase with decreasing channel thickness we use 3 d spherical shell simulations of mantle convection with temperature depth and stress dependent rheology to test scaling trends the simulations and theoretical scalings demonstrate that a low viscosity layer asthenosphere can amplify convective stresses if the level of convective stress plays a role in maintaining and or reactivating plate boundaries this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics the numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active lid modeofconvectiontoastagnantlidstate collectively thesimulationsandtheoreticalscalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect associated with a low absolute asthenosphere viscosity but instead from a mantle flow channelization effect associated with a high viscosity contrast from the asthenosphere to the mantle below

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

an analytic model of convection in a system with layered viscosity and plates summary we present an analytic boundary layer model for thermal convection with a finite strength plate and depth dependent viscosity the model permits solutions in which convective flow rates in the mantle exceed the plate velocity the energy balance equations for the lithosphere and convective cell are distinct and the model yields the plate velocity plate thickness and heat flow as well as the laterally averaged horizontal flow profile for a convective cell we demonstrate by inspection of the lithospheric energy balance that the dominant plate driving force depends not only on the material properties of the mantle and lithosphere but also on the velocity of the plate multiple solutions are possible with three solution branches for the plate velocity representing three distinct modes of thermal convection the branch of solutions with the largest plate velocity corresponds to the classic boundary layer solution with a plate velocity approximately equal to the maximum mantle velocity this branch reproduces the classic convective scaling laws for an isoviscous fluid and its dynamics are controlled by the mantle material properties the branch of solutions with intermediate plate velocity represents a convective cell with a sluggish lid and a plate velocity that is less than the underlying mantle velocity the dynamics for this solution branch depend on the material properties of both the mantle and the lithosphere finally the lower branch of solutions for the plate velocity yields a convective solution in which the plate velocity is determined entirely by the local dynamics of a thick and strong relative to the mantle lithosphere the dynamics of this solution branch are independent of the material properties of the mantle and depend entirely on the properties of the lithosphere the introduction of an asthenosphere or low viscosity layer lvl beneath the plate can significantly alter the dynamics of the system by affecting plate mantle coupling lowering the lvl viscosity relative to the lower mantle can promote plate motion by providing a lubricating layer however a very low viscosity lvl beneath a strong plate can decouple plate and mantle and inhibit plate motion producing a solution with a slow moving plate and a channelized flow in the lvl thus a lvl can sometimes inhibit rather than promote plate tectonics