the variation of temperature with time and hydrocarbon maturation in sedimentary basins formed by extension abstract when a sedimentary basin is formed by extension both the subsidence history and the temperature gradient can be calculated hence the time temperature history of any stratum can be obtained three types of behaviour occur the temperature of material which never reaches depths greater than about 4 km increases at all times the time temperature history of material which finally sinks to greater depths reaches a maximum during the subsidence history the time at which this maximum occurs decreases as the depth increases at greater depths the maximum temperature occurs at the time the basin is formed the time temperature history is then used to calculate lopatin s maturation index for each stratum as a function of time and the index plotted as a function of depth and time for different amounts of extension the maximum temperature at a given depth and hence lopatin s index is almost independent of the amount of extension when the age of the basin is greater than about 150 ma similar calculations taking account of sediment compaction show the same behaviour though the maturity of the material at a given depth is increased because both lopatin s index and its calibration are empirical it is important to develop new maturation indices based on individual molecular reactions whose activation energies can be determined in the laboratory

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

tectonic plate generation and two phase damage void growth versus grain size reduction 1 the generation of plate tectonics from mantle convection requires shear localization in order to yield narrow weak plate boundaries separating broad strong plates a plausible shear localizing mechanism involves damage e g distributed microcracking one theoretical model of which involves two phase mechanics two phase damage theory employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a description of void generation and microcracking and hence weakening failure and shear localization here we examine the application of this theory to the problem of generating plate like behavior from convective type divergent convergent poloidal motion through a source sink formulation we extend the previous damage theory to consider two possible damage effects 1 growth and nucleation of voids associated with dilation of the host matrix and 2 increasing fineness i e reducing coarseness of the mixture by for example grain size reduction void generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate like flow fineness generating damage is found to be very efficient at generating plate like behavior if we assume that the matrix viscosity is a simple function of grain void size as is typical for diffusion creep the implied grain size reduction mechanism is different from that of dynamic recrystallization and appears to be more capable of generating the requisite shear localization for forming tectonic plates from mantle flow

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

geological consequences of super sized earths 1 the discovery of terrestrial scale extrasolar planets and their calculated abundance in the galaxy has prompted speculation on their surface conditions and thermal structure both are dependent on the tectonic regime of a planet which is itself a function of the balance between driving forces and the resistive strength of the lithosphere here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses and thus super sized earths are likely to be in an episodic or stagnant lid regime this effect is robust when associated increases in gravity are included as the more dominant effect is increased fault strength rather than greater buoyancy forces the thermo tectonic evolution of large terrestrial planets is more complex than often assumed and this has implications for the surface and conditions habitability of such worlds

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

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

mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

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

convection scaling and subduction on earth and super earths super earths are the smallest class of discovered extra solar planets owing to their relatively small mass some might resemble earth and perhaps be habitable because of the connection to habitability through thermal evolution we investigate the tectonic regime of massive terrestrial planets two independent studies o neill c lenardic a oct 2007 geological consequences of super sized earths grl 34 l19204 valencia d o connell r j sasselov d d nov 2007a inevitability of plate tectonics on super earths apjl 670 l45 l48 have reached opposing conclusions about the likelihood of plate tectonics here we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics deformation on faults negative buoyancy and energy dissipation during subduction we show that in general the ratio of driving force to plate resistance increases with planetary mass this is a consequence of increasing convective stresses thinning plates and similar plate structure we conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets the convective stress increases even more allowing deformation to take place also despite shorter timescales for plate cooling rocky super earths achieve negative buoyancy at subduction zones finally by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active lid tectonics in conclusion rocky super earths have more favorable conditions than earth for the subduction of plates and hence for sustaining plate tectonics

subduction initiation at a strike slip plate boundary the cenozoic pacific australian plate boundary south of new zealand 1 we first present a synthesis of the macquarie ridge complex mrc tectonic structures as well as paleo reconstruction models of the kinematic evolution of the pacific australia plate boundary south of new zealand since the eocene we then ascertain the geodynamical conditions that preceded subduction initiation and identify the nature and structures of the crust that first subducted at the puysegur subduction zone this synthesis is used to produce a subduction initiation model for the puysegur region concomitant to inception of the alpine fault ca 23 ma a 150 km wide transpressive relay zone developed along puysegur bank inherited structures enabling localization of compressive deformation right lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence subsequently the puysegur subduction zone initiated at the transpressive relay zone ca 20 ma upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the neogene the four individual segments of the mrc represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities the example of the mrc demonstrates that subduction can initiate from an oceanic spreading center through progressive changes in plate kinematics within a 10 15 myr time frame index terms 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8155 tectonophysics plate motions general 9604 information related to geologic time cenozoic keywords subduction initiation pacific australia plate motion cenezoic macquarie ridge

an estimate of the geosat altimeter wind speed algorithm at high wind speeds existing wind speed algorithms for satellite altimeters have been developed from data in the range 0 to 20 m s at higher wind speeds the available algorithms diverge alarmingly by comparing geosat altimeter values of the radar cross section with model predictions of the surface winds during satellite overpasses of tropical cyclones a wind speed algorithm valid for wind speeds between 20 and 40 m s is developed there is however considerable scatter in these data an error analysis shows that this observed scatter is consistent with the accuracy to which the tropical cyclone wind fields can be inferred an approximately linear dependence of the radar cross section on wind speed is however clearly evident within these data at the lower limit of its applicability u10 20 m s these data are consistent with the previously derived algorithms at its upper limit u10 40 m s mean squared slope values inferred from the radar cross section are consistent with the theoretical upper limit

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

a source sink model of the generation of plate tectonics from non newtonian mantle flow a model of mantle convection which generates plate tectonics requires strain rate or stress dependent rheology in order to produce strong platelike flows with weak margins as well as strike slip deformation and plate spin i e toroidal motion here we employ a simple model of source sink driven surface flow to determine the form of such a rheology that is appropriate for earth s present day plate motions in this model lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones respectively the source sink field is derived from the horizontal divergence of plate velocities and thus directly prescribes poloidal motion the toroidal flow field is solved through the non newtonian stokes equation for shallow layer tangential flow on the surface of a sphere two plate motion models are used to derive the source sink field the first is an idealized square plate which is used to explore the basic aspects of the model the second is an analytically continuous model of earth s present day plates bercovici and wessel 1994 as originally implied in the simpler cartesian version of this model bercovici 1993 the classical power law rheologies do not generate platelike flows as well as the hypothetical whitehead gans stick slip rheology which incorporates a simple self lubrication mechanism for the idealized plate geometry the power law rheologies yield much more diffuse strike slip shear i e radial vorticity than the stick slip rheology for the present day plate geometry the power law rheologies fail to reproduce the original proportion of left and right lateral strike slip shear whereas the stick slip rheology gives almost exactly the right proportion none of the fluid rheologies examined however produce more than approximately 60 of the original maximum shear for either plate model the viscosity fields produced by the power law rheologies are diffuse and the viscosity lows over strike slip shear zones or pseudo margins are not as small as over the prescribed convergent divergent margins in contrast the stick slip rheology generates very platelike viscosity fields with sharp gradients at the plate boundaries and margins with almost uniformly low viscosity quantitative comparisons with the toroidal poloidal kinetic energy partitioning and vorticity fields of the original plate model are also examined and the stick slip rheology is generally found to yield the most favorable comparisons power law rheologies with high viscosity contrasts however lead to almost equally favorable comparisons though these also yield the least platelike viscosity fields this implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related arid thus may present independent constraints on the determination of a self consistent plate mantle rheology the results of this study however predict that if such a rheology can indeed be uniquely determined it is likely to be in the class of stick slip self lubricating rheologies

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

the interaction of viscous heating with grain size dependent rheology in the formation of localized slip zones the formation of localized shear zones is important for understanding many local and global processes in geodynamics we have developed a self consistent thermal mechanical model together with a rheology which depends on temperature strain rate and grain size distribution the grain size distribution has contributions from both dynamic recrystallization and grain growth processes and is governed locally by a nonlinear ordinary differential equation a one dimensional model with 104 points is employed to resolve all of the scales involving grain size and temperature we found that grain growth inhibits the development of shear zones and that there is a delicate interplay between viscous heating and grain growth process in determining whether narrow fault zones are developed quickly for realistic parameters of rheology and grain boundary processes for wet olivine the magnitude of the rate of grain growth is crucial to determine whether shear zones are stable or unstable at temperature t 1000 k or shear stress 100 mpa

the driving mechanism of plate tectonics abstract plate velocities reflect the global mantle circulation which carries the heat from the deep part of the earth to the surface three other measurable parameters put strong constraints on the physical processes involved in mantle dynamics the surface topography the geoid and the lateral density heterogeneities mapped throughout the mantle by seismic tomography using these internal loads and the navier stokes equations a first generation of dynamic earth models have been developed in the last five years they have produced satisfactory predictions for the divergence of the surface velocities and for the geoid these predictions are quite sensitive to the assumed radial viscosity variations the best match with observations has therefore helped to discriminate between various possible viscosity structures a new set of models taking the existence of plates into account has been particularly successful in predicting the plate velocities including their vorticity the driving mechanism of plate tectonics is thus put on a firm physical basis and the computed solutions are quantitatively reasonable

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

generation of plate tectonics from lithosphere mantle flow and void volatile self lubrication abstract the formation of plate tectonics from mantle convection necessarily requires nonlinear rheological behavior recent studies suggest that self lubricating rheological mechanisms are most capable of generating plate like motion out of fluid flows the basic paradigm of self lubrication is nominally derived from the feedback between viscous heating and temperature dependent viscosity here we propose a new idealized self lubrication mechanism based on void e g pore and or microcrack generation and volatile e g water ingestion we test this void volatile self lubrication mechanism in a source sink flow model this leads to a basic nonlinear system which permits the excitation of strike slip toroidal motion a necessary ingredient of plate like motion out of purely divergent i e poloidal or characteristically convective flow with relatively inviscid void filling volatiles the void volatile mechanism yields a state of highly plate like motion i e with uniformly strong plate interiors weak margins and extremely focussed strike slip shear zones moreover the void volatile model obeys a chemical diffusion time scale that is typically much longer than the thermal convection time scale the model thus complies with the observation that plate boundaries are long lived and survive even while inactive the void volatile model of self lubrication therefore predicts self focussing shear zones plate generation and plate boundary longevity through what has long been suspected to be a key ingredient for the existence of plate tectonics i e water

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

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

a scanning electron microscope study of the effects of dynamic recrystallization on lattice preferred orientation in olivine effects of dynamic recrystallization on lattice preferred orientation lpo in olivine were investigated through the combination of two sem based techniques electron backscattered diffraction ebsd technique for crystallographic orientation measurement and backscattered electron imaging bei for dislocation observation samples are experimentally deformed olivine aggregates in simple shear geometry in the sample deformed at t 1473 k and high stresses 480 mpa only incipient dynamic recrystallization is observed along grain boundaries orientations of these small recrystallized grains are more random than that of relict grains suggesting an important role of grain boundary sliding at this stage of recrystallization in the sample deformed at t 1573 k and low stress 160 mpa dynamic recrystallization is nearly complete and the lpo is characterized by two 100 peaks one peak is located at the orientation subparallel to the shear direction and is dominated by grains with high schmid factor the other occurs at high angles to the shear direction and is due to the contribution from grains with low schmid factor grains with high schmid factor tend to have higher dislocation densities than those with low schmid factor based on these observations we identify two mechanisms by which dynamic recrystallization affects lpo 1 enhancement of grain boundary sliding due to grain size reduction leading to the modification of lpo caused by the relaxation of constraint for deformation 2 grain boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities based on the deformation mechanism maps and stress versus recrystallized grain size relation we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions

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

the conditions for plate tectonics on super earths inferences from convection models with damage abstract numerical simulations of mantle convection with a damage grainsize feedback are used to develop scaling laws for predicting conditions at which super earths would have plate tectonics in particular the numerical simulations are used to determine how large a viscosity ratio between pristine lithosphere and mantle l m can be offset by damage to allow mobile plate like convection regime diagrams of l m versus the damage number d show that the transition from stagnant lid to mobile convection occurs for higher l m as d increases a similar trend occurs for increasing rayleigh number we hypothesize a new criterion for the onset of plate tectonics on terrestrial planets that damage must reduce the viscosity of shear zones in the lithosphere to a critical value equivalent to the underlying mantle viscosity a scaling law based on this hypothesis reproduces the numerical results for the earth damage is efficient in the lithosphere and provides a viable mechanism for the operation of plate tectonics we scale our theory to super earths and map out the transition between plate like and stagnant lid convection with a planetary plate tectonic phase diagram in planet size surface temperature space both size and surface conditions are found to be important with plate tectonics being favored for larger cooler planets this gives a natural explanation for earth venus and mars and implies that plate tectonics on exoplanets should correlate with size incident solar radiation and atmospheric composition

modelling of the gravity effects of flexurally compensating thin layers under the afanasy nikitin seamount abstract modelling of the gravity data over the afanasy nikitin seamount is carried out to estimate the variation of the crustal and flexural parameters along the strike of the seamount it is inferred that the compensation of the seamount is due to both the bending of a very thin elastic plate and the occurence of low density material at depth the elastic plate thickness is inferred as 2 km and does not change significantly along the strike of the seamount furthermore the crustal thicknesses range within 12 22 km not showing any systematic variation along the strike of the seamount it is suggested that the afanasy nikitin seamount was emplaced on thin and weak lithosphere very near to a slowly migrating ridge which trapped a hot rising plume resulting in a higher crustal thickness

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

grain size evolution and the rheology of ductile shear zones from laboratory experiments to postseismic creep abstract geologic experimental and geodetic studies indicate that ductile shear zones do not deform in steady state we present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep in which grain size is allowed to evolve toward its equilibrium recrystallized value several grain size evolution laws are considered simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain weakening or strain strengthening episodes reminiscent of laboratory observations ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes non steady state ductile creep is difficult to differentiate from frictional sliding with velocity strengthening friction hence ductile creep may play a role in the transition from velocity weakening to velocity strengthening friction with increasing temperature

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s

variations in planetary convection via the effect of climate on damage abstract a new model for the generation of plate tectonics suggests an important interaction between a planet s climate and its lithospheric damage behavior and thus provides a simple explanation for the tectonic difference between earth and venus we propose that high surface temperatures will lead to higher healing rates e g grain growth in the lithosphere that will act to suppress localization plate boundary formation and subduction this leads to episodic or stagnant lid convection on venus because of its hotter climate in contrast earth s cooler climate promotes damage and plate boundary formation the damage rheology presented in this paper attempts to describe the evolution of grain size by allowing for grain reduction via deformational work input and grain growth via surface tension driven coarsening we explore the interaction of damage and healing in two dimensional numerical convection simulations we also develop a simple drip instability model to test the hypothesis that the competition between damage and healing controls convective and plate tectonic style by modulating episodicity at subduction zones at small values of damage f a or large values of healing k a the lithosphere remains strong enough to resist subduction on time scales of billions of years at intermediate values of f a and k a the lithosphere may become mobilized and allow for short bursts of tectonic behavior followed by periods of quiescence at large small values of f a k a the fineness is increased so that the viscosity of the plate boundary is reduced to allow for continuous unimpeded subduction of lithosphere and plate like deformation the results suggest the feasibility of our proposed hypothesis that the interplay of climate and damage control the mode of tectonics on a planet