a comparison of mantle convection models featuring plates oceanic plates are an integral part of the earth s mantle and thus play an important role in its dynamics and evolution to allow plate behavior to arise naturally in numerical mantle convection models self consistent plate generation methods apply a fully rheological approach featuring a temperature pressure and stress dependent viscosity however due to the extreme local viscosity changes that the self generation of model plates entails their computational requirements are demanding alternative plate modeling methods specify the existence of plates explicitly but can also obtain dynamically determined velocities e g by employing a force balance method here we present modifications to a force balance model by utilizing a rheology dependent viscosity profile accordingly plate viscosity and plate thickness are no longer prescribed by the modeler but now follow as a dynamic consequence of the temperature and stress dependence of the viscosity and the model s evolution we describe the new method and present benchmark results for a rheologically self consistent mantle convection model and the modified force balance plate model our results show that both plate modeling methods lead to the same system behavior for a wide range of system parameters making the new method a powerful tool to also achieve plate like surface motion naturally

organic matter accumulation in the upper devonian duvernay formation western canada sedimentary basin from sequence stratigraphic analysis and geochemical proxies abstract we present a model for organic carbon accumulation in the upper devonian duvernay formation of the western canada sedimentary basin relating total organic carbon toc concentrations to a high resolution geochemical and organic petrologic database and comparing these to a core based sequence stratigraphic interpretation many previous source rock studies have suggested that organic matter is enriched in transgressive systems tracts or at maximum flooding surfaces an observation that is commonly attributed to an assumed relationship between water depth and oxygen levels our data set enables us to associate total organic matter content and organic assemblages with particular systems tracts and to examine how the organic assemblages change across the basin tested against proxies for redox conditions and bioproductivity to relate relative sea level to specific mechanisms for organic accumulation the duvernay formation comprises three third order depositional sequences superimposed on a second order late transgressive systems tract and early highstand systems tract sequences are composed almost entirely of transgressive systems tracts and highstand systems tracts with a lowstand systems tract only at the base of the upper cycle that marks the second order sea level turnaround depositional facies generally vary from bioturbated carbonate rich siltstones to siliceous mudstones from the platform margins to basin center organic petrologic analysis records predominantly amorphous organic matter and solid bitumen with much less abundant vitrinite and inertinite organic matter type indicated by rock eval analysis shows that in most cases high total organic carbon toc content is associated with better quality organic matter high hydrogen index and vice versa indicating more reducing conditions or higher bioproductivity were responsibility for organic enrichment however near carbonate reefs hydrogen index is uncorrelated with toc indicating that here dilution by carbonate minerals was the primary control on organic enrichment across most of the basin highest tocs are recorded just above the second order maximum flooding surface secondary peaks in toc are recorded above the maximum flooding surface in the upper third order cycle and in the transgressive systems tract of lower third order cycle low toc values are recorded in the lowstand systems tract and in the middle and upper third order transgressive systems tracts almost up to the maximum flooding surfaces throughout much of the basin proxies for relative sea level restriction of water masses mo toc redox conditions mo al and s fe and bioproductivity biogenic silica coincide these relationships demonstrate that organic carbon accumulation resulted from influxes of nutrient rich upwelled water during high sea level thus toc values are highest in the upper part of transgressive systems tracts and lower highstand systems tracts anoxia typically developed as a result of bioproductivity and enhanced organic matter accumulation but was not itself a trigger for organic sedimentation near carbonate reefs however varying sea level regulated carbonate sedimentation rate and organic matter dilution and bioproductivity had less impact on organic matter concentrations thus toc was high when carbonate deposition was low during transgressions which forced carbonate reefs to backstep limiting carbonate sedimentation in deeper water area

high accuracy mantle convection simulation through modern numerical methods ii realistic models and problems computations have helped elucidate the dynamics of earth s mantle for several decades already the numerical methods that underlie these simulations have greatly evolved within this time span and today include dynamically changing and adaptively refined meshes sophisticated and efficient solvers and parallelization to large clusters of computers at the same time many of these methods discussed in detail in a previous paper in this series were developed and tested primarily using model problems that lack many of the complexities that are common to the realistic models our community wants to solve today with several years of experience solving complex and realistic models we here revisit some of the algorithm designs of the earlier paper and discuss the incorporation of more complex physics in particular we re consider time stepping and mesh refinement algorithms evaluate approaches to incorporate compressibility and discuss dealing with strongly varying material coefficients latent heat and how to track chemical compositions and heterogeneities taken together and implemented in a high performance massively parallel code the techniques discussed in this paper then allow for high resolution 3d compressible global mantle convection simulations with phase transitions strongly temperature dependent viscosity and realistic material properties based on mineral physics data

optical characteristics of amorphous kerogens and the hydrocarbon generating potential of source rocks abstract four recurring types of amorphous kerogen are optically identified in petroleum source rock samples and are related to hydrocarbon generating potential defined here as geochemically interpreted oil prone or gas prone at this time a genetic classification is not possible the different amorphous kerogen types are distinguished on the basis of microscopic textural differences type a chunky compact masses with mottled network or weak polygonal textures type b very small dense elongate oval or rounded individual grains type c clumpswith granular fragmented or globular textures type d thin platy or rectangular individual grains geochemistry defined oil prone samples generally contain types a and or d separately or combined geochemically defined gas prone samples rarely contain type a and vary in amounts of types b c and or d the samples in this study contain dominantly one of the four types of amorphous kerogen so the geochemical analyses of the whole rock or bulk concentrated kerogen samples are assumed to be analyses of the dominant amorphous type the samples were chemically analyzed by several techniques to determine the oil or gas generating potential the establishment of relations between specific optical and chemical properties of kerogen types can make visual kerogen analyses more meaningful in predicting the hydrocarbon generation potential of source rocks

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

concepts and methods of high resolution event stratigraphy modern geological analyses involving sedimentary rocks require a high resolution system of dating integrating and correlating diverse strati graphic geochemical and paleontological data that also takes into account the possibility that short term phenomena 100 kyr or less exercise a strong control on sedimentation such phenomena may be extraterrestrial tectonic volcanic oceanographic climatic sedimentologic and or bio logic in origin some reflect unpredictable perturbations of local to global scale but most short term phenomena are predictable and may be either autocyclic locally regulated with limited stratigraphic continuity or allo cyclic in nature regionally to globally regulated with extensive strati graphic continuity beerbower 1964 a stratigraphic system based on short term phenomena would ideally be chronostratigraphic involving the identification and regional tracing of time lines isochronous surfaces or very thin event deposits and easily integrated with refined independently derived biostratigraphic and geochronologic systems an intensive search for such a new and revolutionary system of stratig raphy has been prompted in many parts of the phanerozoic by attain ment of plateaus in stratigraphic resolution utilizing the more standard tools of magnetostratigraphy biostratigraphy and geochronology most magnetic reversals have been identified biostratigraphic systems can be no more refined than the maximum evolutionary rates of component taxa and their major structures known for many favored groups and geo chronology is regulated by the analytical confidence limits of radiometric

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

sedimentation sea level change and tectonics on an early pleistocene continental shelf northern california the rio dell formation was deposited in a tectonically active area during a time of fluctuating eustatic sea level detailed interpretation of rio dell shelf sediments facilitated by comparison to the nearby analogous modern eel river shelf has proved a vital tool for decoding the record of these influences understanding of the nature and relative roles of processes on the modern and ancient shelf including those associated with storms and with river floods not only allows for detailed paleobathymetric reconstruction of the ancient sequence but also for recognition of relatively subtle effects of syndepositional tectonism shelf sediments of the rio dell formation comprise four depositional sequences bounded by thin submarine condensed sections the condensed sections formed during periods of rapid probably largely eustatic sea level rise the sequences each record progradation of the shelf predominantly during sea level highstands paleobathymetric reconstruction provides the basis for interpretation of the magnitude of local relative sea level changes within and between the sequences subsidence of the rio dell shelf apparently played a role in recorded local sea level fluctuations as did eustasy subsidence geohistory analysis of the sequence indicates that the shelf was subsiding throughout its existence due not only to sediment loading but also due to tectonism the analysis points to a period of rapid shelf subsidence accompanied by onland uplift between about 1 3 and 1 2 ma an increase in subsidence rates after about 1 3 ma apparently caused an increase in rio dell shelf gradient which changed the balance between flood and storm processes on the middle shelf increased shelf inclination evidently resulted in restriction of storm wave reworking to a narrower zone extending from the shoreline whereas distance from shoreline to the shelf area most influenced by river flood deposition remained unchanged the resultant increased segregation of flood and storm effects caused a fining of background grain size distributions at mid shelf sites

plate motions and the viscosity structure of the mantle insights from numerical modelling abstract the viscosity structure of the earth s mantle is likely to play an important role with respect to the motion of the lithospheric plates a zone of low viscosity in the upper mantle as postulated by seismological studies and also by investigations of postglacial uplift has been proposed to facilitate plate motion another inferred feature of the viscosity profile in the mantle is a significant increase at greater depth we have employed a three dimensional mantle convection model to explore the relation between the appearance of plate motion and the viscosity structure of the mantle beneath the model allows for a complex fluid rheology strong temperature pressure and stress dependence of the viscosity and can so account for the self consistent formation of plates at the surface of the convecting mantle plate like motion appears only in a limited parameter range and we demonstrate that this appearance is closely linked to a specific viscosity structure the viscosity is characterized by a local minimum at shallow depth low viscosity zone and by a local maximum at greater depth self consistent plate motion is only observed if such a viscosity structure exists in order to determine the functional relationship between the viscosity structure and plate motion we also employ an approach in which a certain viscosity profile is prescribed these studies again reveal the importance of the local extrema in the viscosity profile taking into account an increased interior viscosity also obtained by the pressure dependence we find stable plate motion as a general result we observe that the balance of the temperature and stress dependence of the viscosity is necessary to obtain plate motion at the same time we find a viscosity profile showing a minimum at shallow depth and a maximum at greater depth

global scale water circulation in the earth s mantle implications for the mantle water budget in the early earth abstract we investigate the influence of the mantle water content in the early earth on that in the present mantle using numerical convection simulations that include three processes for redistribution of water dehydration partitioning of water into partially molten mantle and regassing assuming an infinite water reservoir at the surface these models suggest that the water content of the present mantle is insensitive to that of the early earth the initial water stored during planetary formation is regulated up to 1 2 oms om ocean mass 1 4 10 21 kg which is reasonable for early earth however the mantle water content is sensitive to the rheological dependence on the water content and can range from 1 2 to 3 oms at the present day to explain the evolution of mantle water content we computed water fluxes due to subducting plates regassing degassing and dehydration for weakly water dependent viscosity the net water flux is almost balanced with those three fluxes but for strongly water dependent viscosity the regassing dominates the water cycle system because the surface plate activity is more vigorous the increased convection is due to enhanced lubrication of the plates caused by a weak hydrous crust for strongly water dependent viscosity the degassing history is insensitive to the initial water content of the early earth as well as rheological strength the degassing flux from earth s surface is calculated to be approximately o 10 13 kg yr consistent with a coupled model of climate evolution and mantle thermal evolution

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

subduction controls the distribution and fragmentation of earth s tectonic plates the theory of plate tectonics describes how the surface of the earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution 2 3 the reconstruction of global tectonics during the past 200 my 4 suggests that this layout is probably a long term feature of our planet but the forces governing it are unknown previous studies 3 5 6 primarily based on statistical properties of plate distributions were unable to resolve how the size of plates is determined by lithosphere properties and or underlying mantle convection here we demonstrate that the plate layout of the earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere using 3d spherical models of mantle convection with plate like behaviour that match the plate size frequency distribution observed for earth we show that subduction geometry drives the tectonic fragmentation that generates plates the spacing between slabs controls the layout of large plates and the stresses caused by the bending of trenches break plates into smaller fragments our results explain why the fast evolution in small back arc plates 7 8 reflects the dramatic changes in plate motions during times of major reorganizations our study opens the way to use convection simulations with plate like behaviour to unravel how global tectonics and mantle convection are dynamically connected

implementation and application of adaptive mesh refinement for thermochemical mantle convection studies numerical modeling of mantle convection is challenging owing to the multiscale nature of mantle dynamics high resolution is often required in localized regions with coarser resolution being sufficient elsewhere when investigating thermochemical mantle convection high resolution is required to resolve sharp and often discontinuous boundaries between distinct chemical components in this paper we present a 2 d finite element code with adaptive mesh refinement techniques for simulating compressible thermochemical mantle convection by comparing model predictions with a range of analytical and previously published benchmark solutions we demonstrate the accuracy of our code by refining and coarsening the mesh according to certain criteria and dynamically adjusting the number of particles in each element our code can simulate such problems efficiently dramatically reducing the computational requirements in terms of memory and cpu time when compared to a fixed uniform mesh simulation the resolving capabilities of the technique are further highlighted by examining plume induced entrainment in a thermochemical mantle convection simulation

sedimentary organic matter sources and depositional environment in the yegua formation brazos county texas abstract the complex depositional environment of the eocene yegua formation brazos county texas can be better understood by integrating organic matter om geochemistry with stratigraphy yegua sediments represent parasequences separated by exposure surfaces organic petrography and geochemistry biomarkers c n ratios and carbon isotopes indicate the presence of both terrestrial and marine om in transgressive sediments in contrast regressive sediments contain only terrestrial om these differences relate to contrasting om sources and depositional styles on the shelf om in the sediments is immature and the potential for generating hydrocarbons is poor the study suggests that organic geochemical data can help in distinguishing transgressive and regressive environments in sedimentary formations

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 comparison of numerical surface topography calculations in geodynamic modelling an evaluation of the sticky air method summary calculating surface topography in geodynamic models is a common numerical problem besides other approaches the so called sticky air approach has gained interest as a free surface proxy at the top boundary the often used free slip condition is thereby vertically extended by introducing a low density low viscosityfluid layer this allows the air crust interface to behave in a similar manner to a true free surface we present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface two cases are evaluated that characterize the evolution of topography on different timescales 1 isostatic relaxation of a cosine perturbation and 2 topography changes above a rising plume we quantitatively compare topographies calculated by six different numerical codes using finite difference and finite element techniques using three different topography calculation methods i direct calculation of topography from normal stress ii body fitting methods allowing for meshing the topography and iii lagrangian tracking of the topography on an eulerian grid it is found that the sticky air approach works well as long as the term st ch hst l 3 is sufficiently small where st and hst are the viscosity and thickness of the sticky air layer and ch and l are the characteristic viscosity and length scale of the model respectively spurious lateral fluctuations of topography as observed in some marker based sticky air approaches may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

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

convection in three dimensions with surface plates generation of toroidal flow this work presents numerical calculations of mantle convection that incorporate some of the basic observational constraints imposed by plate tectonics the model is three dimensional and includes surface plates it allows plate velocity to change dynamically according to the forces which result from convection we show that plates are an effective means of introducing a toroidal component into the flow field after initial transients the plate motion is nearly parallel to transform faults and in the direction that tends to minimizes the toroidal flow field the toroidal field decays with depth from its value at the surface the poloidal field is relatively constant throughout the layer but falls off slightly at the top and bottom boundaries layered viscosity increasing with depth causes the toroidal field to decay more rapidly effectively confining it to the upper low viscosity layer the effect of viscosity layering on the poloidal field is relatively small which we attribute to its generation by temperature variations distributed throughout the system the generation of toroidal flow by surface plates would seem to account for the observed nearly equal energy of toroidal and poloidal fields of plate motions on the earth a low viscosity region in the upper mantle will cause the toroidal flow to decay significantly before reaching the lower mantle the resulting concentration of toroidal flow in the upper mantle may result in more thorough mixing there and account for some of the geochemical and isotopic differences proposed to exist between the upper and lower mantles

a community benchmark for 2 d cartesian compressible convection in the earth s mantle benchmark comparisons are an essential tool to verify the accuracy and validity of computational approaches to mantle convection six 2 d cartesian compressible convection codes are compared for steady state constant and temperature dependent viscosity cases as well as time dependent constant viscosity cases in general we find good agreement between all codes when comparing average flow characteristics such as nusselt number and rms velocity at rayleigh numbers near 106 and dissipation numbers between 0 and 2 the results differ by approximately 1 per cent differences in discretization and use of finite volumes versus finite elements dominate the differences there is a small systematic difference between the use of the anelastic liquid approximation ala compared to that of the truncated ala in determining the onset of time dependence there was less agreement between the codes with a spread in the rayleigh number where the first bifurcation occurs ranging from 7 79 10 5 to 1 05 10 6

time domain parallelization for computational geodynamics 1 i present a time domain parallelization approach for geodynamic modeling this algorithm named parareal is based on the use of coarse sequential and fine parallel propagators to predict and to iteratively correct the solution of the governing equations over a given time interal although the method has been successfully used to solve differential equations in various scientific areas it has not been applied to model solid state convective motions relevant to the earth and other planetary mantles in that case the time dependence of the velocity is only implicit which requires modifications to the original algorithm the performances of this adapted version of the parareal algorithm were investigated using theoretical model predictions in good agreement with numerical experiments i show that under optimum conditions the parallel speedup increases linearly with the number of processors and speedups close to 10 were measured using only few tens of cpus this parareal approach can be used alone or combined with any spatial parallel algorithm allowing significant additional increase in speedup with increasing number of processors

relation of natural gas composition to thermal maturity and source rock type in san juan basin northwestern new mexico and southwestern colorado san juan basin is a roughly circular asymmetric structural depression located in northwestern new mexico and southwestern colorado ultimate recoverable reserves of predominantly nonassociated gas 23 tcf 0 65 1012 m3 are present in the structurally low part of the central basin the major producing intervals are low permeability sandstone reservoirs in the upper cretaceous dakota sandstone mesaverde group and pictured cliffs sandstone lesser amounts of oil and or gas are produced from pennsylvanian jurassic and cretaceous rocks along the southern and western flanks of the basin the gases display a trend of becoming isotopically heavier dgr13c1 values range from 48 7 to 31 4 pmil and chemically drier c1 c1 5 values range from 0 75 to 0 99 with increasing depth these changes are assumed to be the result of thermal cracking processes and the gases are interpreted to have been generated during the mature and post mature stages of hydrocarbon generation however there is considerable scatter in the data which is interpreted to result from a difference in source rock type gases generated from nonmarine humic source rocks are isotopically heavier and chemically drier than those generated from marine sapropelic source rocks at equivalent levels of maturity the gases also become isotopically heavier and chemically drier to the northeast following the trend of increasing maturity of all units in that direction the increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the san juan mountains area maximum burial and heat flow occurred during the oligocene which probably coincided with peak hydrocarbon generation lack of oil in the central basin is believed to be the result of two factors first gas in reservoirs such as the dakota sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature wet gas condensate and post mature dry gas stages of hydrocarbon generation second gas in reservoirs such as mesaverde group and pictured cliffs sandstone is nonassociated and probably was generated from nonmarine coaly organic matter during the mature and post mature stages minor amounts of condensate instead of oil may have been generated from nonmarine source rocks during the mature stage

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

limitations of use of organic petrographic techniques for identification of petroleum source rocks organic petrographic and organic geochemical studies have been conducted on 58 sediment samples of differing ages and depositional environments in this study interpretations of petroleum source rock potential based on data from transmitted light microscopy show a poor correlation with those derived from chemical data this lack of correlation arises from the failure to distinguish consistently between hydrogen poor and hydrogen rich amorphous organic matter and inability to detect hydrogen rich components exinite and resinite in coal fragments interpretations based on data from reflected light microscopy show a better correlation with those derived from chemical studies but difficulties still remain these difficulties arise from inability except on the basis of luorescence intensity to distinguish hydrogen rich from hydrogen poor organic matter in a finely dispersed state

characteristic trace fossil associations in oxygen poor sedimentary environments the sedimentologic record of oxygen poor depositional environments typically includes trace fossils produced by deposit feeding organisms we propose an oxygen controlled trace fossil model in which increasing oxygen concentration of interstitial water in sediment parallels a transition from fodinichnia dominated through pascichnia dominated to domichnia dominated trace fossil associations in certain places such as parts of the late paleozoic oquirrh basin in utah this model provides a reasonable alternative to the more traditional depth controlled model of trace fossil distribution

fluorescent microscopy of particulate organic matter sparta formation and wilcox group south central louisiana abstract particulate organic matter pom isolated from core samples of mudstones in the mid eocene sparta formation and the late paleocene early eocene wilcox group in louisiana was characterized using transmitted and fluorescent light microscopy and rock eval pyrolysis observations of maceral fluorescence combined with programmed pyrolysis results better define source potential than visual examination using transmitted white light alone transmitted light microscopic characterization of pom indicates that amorphous macerals of mixed terrestrial and marine origin dominate samples from both the lagoonal and shelf depositional environments microscopic examination under fluorescent light indicates that some of the amorphous matter including terrestrially derived material fluoresces to some degree hydrogen index values show a positive correlation with the total number of fluorescing particles in the isolate as well as with the relative fluorescence levels comparison of sparta and wilcox samples indicates that significant changes occur in the assemblages at levels of thermal maturation associated with the onset of petroleum generation thermally mature samples contain a higher percentage of moderately fluorescent particles and lesser percentages of highly and weakly fluorescent particles this phenomenon is readily apparent in specific particle types namely amorphous nonstructured protistoclasts miospores and dinoflagellate cysts

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

organic matter variations in transgressive and regressive shales abstract organic matter in the upper cretaceous mancos shale adjacent to the tocito sandstone in the san juan basin of new mexico was characterized using organic petrology and organic geochemistry differences in the organic matter found in these regressive and transgressive offshore marine sediments have been documented and assessed within a sequence stratigraphic framework the regressive lower mancos shale below the tocito sandstone contains abundant well preserved phytoclasts and correspondingly low hydrogen indices total organic carbon values for the regressive shale are low sediments from the transgressive systems tract tocito sandstone and overlying upper mancos shale contain less terrestrially derived organic matter more amorphous non structured protistoclasts higher hydrogen indices and more total organic carbon advanced stages of degradation are characteristic of the phytoclasts found in the transgressive shale amorphous material in the transgressive shale fluoresces strongly while that found in the regressive shale is typically non fluorescent data from pyrolysis gas chromatography confirm these observations these differences are apparently related to the contrasting depositional styles that were active on the shelf during regression and subsequent transgression it is suggested that data from organic petrology and organic geochemistry provide greater resolution in sedimentologic and stratigraphic interpretations particularly when working with basinward fine grained sediments petroleum source potential for the regressive lower mancos shale below the tocito sandstone is poor based on abundant fluorescent amorphous material high hydrogen indices and high total organic carbon the transgressive upper mancos shale above the tocito sandstone possesses excellent source potential this suggests that appreciable source potential can be found in offshore fine grained sediments of the transgressive systems tract below the condensed section and associated downlap surface organic petrology can be used to accurately predict petroleum source potential the addition of simple fluorescence microscopy greatly enhances this predictive ability because non generative amorphous material is generally non fluorescent organic petrology must also be used to properly evaluate the utility of t max from programmed pyrolysis as a thermal maturity indicator organic matter dominated by autochthonous amorphous protistoclasts exhibits lower t max values than that which is composed of mostly phytoclasts

dynamical consequences in the lower mantle with the post perovskite phase change and strongly depth dependent thermodynamic and transport properties article i nfo we have carried out numerical simulations of large aspect ratio 2 d mantle convection with the deep phase change from perovskite pv to post perovskite ppv using the extended boussinesq approximation for a fluid with temperature and pressure dependent viscosity we have investigated the effects of various ppv phase parameters on the convective planform heat transport and mean temperature and viscosity profiles since ppv is expected to have a relatively weak rheology with respect to pv and a large thermal conductivity we have assumed that the transition from pv to ppv is accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an increase in thermal conductivity by a factor of 2 furthermore we have analyzed the combined effects of a strongly decreasing thermal expansivity in pv and steeply increasing thermal conductivity according to recent evidence from high pressure experiments and first principle calculations as long as the thermal expansivity and conductivity are constant ppv exerts a small but noticeable effect on mantle convection it destabilizes the d layer causes focusing of the heat flux peaks and an increase of the average mantle temperature and of the temporal and spatial frequency of upwellings when the latest depth dependent thermal expansivity and conductivity models are introduced the effects of ppv are dramatic on the one hand without ppv we obtain a very sluggish convective regime characterized by a relatively cool mantle dominated by large downwellings that tend to stagnate beneath the transition zone with ppv on the other hand we observe an extremely significant increase of the average mantle temperature due to the formation of large sized and vigorous upwellings that in some cases tend to cluster thus forming superplumes if a very large thermal conductivity at the core mantle boundary is assumed k 20 wk 1 m 1 we obtain a quasi steady state regime characterized by large and stable plumes with long lifetimes the combination of strongly depth dependent expansivity and conductivity is a viable mechanism for the formation of long wavelength long lived thermal anomalies in the deep mantle even if a low viscosity ppv atop the core mantle boundary is included

the spiral grid a new approach to discretize the sphere and its application to mantle convection 1 this paper presents a new method to generate a three dimensional spherical grid using natural neighbor voronoi cells distributed by spiral functions a unique property of this grid is the complete removal of symmetries with arbitrary selectable lateral and radial resolution which are not restricted to discrete radial levels or geometrical constraints as compared to the commonly used grids based on projected triangulated platonic solids such as a cube a rhomboid or an icosahedron the spiral grid can be refined in certain areas of interest and makes it possible to have a very small inner radius to outer radius ratio cell volumes can be made almost constant throughout the computational domain analysis statistics and computation methods are described in detail as well as a possible domain decomposition suitable for parallel computing conductive temperature profiles were numerically calculated in the spherical shell and directly compared with the analytic solution as verification the grid is applied to numerical simulations of mantle convection using a finite volume scheme the model is validated by a comparison of steady state cubic and tetrahedral convection patterns with other published models

organic facies of middle cretaceous black shales in deep north atlantic the middle cretaceous black shales of the deep north atlantic consist of alternating layers of sediment rich in total organic carbon toc that contains abundant amorphous types of organic matter and sediment poor in toc that contains mostly woody and coaly types of organic matter the amorphous materials are derived mostly from marine organisms in the eastern north atlantic off africa in the caribbean and on the demerara rise off surinam and from higher land plants in the western north atlantic and off europe marine amorphous material is important off africa because this was an area of upwelling and highly productive surface water amorphous marine organic matter was deposited across the entire north atlantic basin in the cenomanian in response to an ocean wide upwell ng event connected with the opening of the equatorial atlantic the difference between the toc rich and background toc layers reflects deposition of the former under reducing conditions which allowed the preservation of labile amorphous materials of marine and terrestrial orgin and of the latter under oxidizing conditions when nearly all that was preserved was refractory organic material

trace fossil model for reconstruction of paleo oxygenation in bottom waters recognition of fluctuations in the degree of paleo oxygenation of bottom waters recorded in fine grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions general sedimentary fabric composition of trace fossil assemblages and burrow size and crosscutting relationships have been incorporated into a trace fossil tiering model that permits detailed reconstruction of changes in paleo oxygenation of bottom waters applications of this model to the miocene monterey formation california and the cretaceous niobrara formation colorado indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body fossil information

assessing the role of slab rheology in coupled plate mantle convection models reconstructing the 3d structure of the earth s mantle has been a challenge for geodynamicists for about 40 yr although numerical models and computational capabilities have substantially progressed parameterizations used for modeling convection forced by plate motions are far from being earth like among the set of parameters rheology is fundamental because it defines in a non linear way the dynamics of slabs and plumes and the organization of lithosphere deformation in this study we evaluate the role of the temperature dependence of viscosity variations up to 6 orders of magnitude and the importance of pseudo plasticity on reconstructing slab evolution in 3d spherical models of convection driven by plate history models pseudo plasticity which produces plate like behavior in convection models allows a consistent coupling between imposed plate motions and global convection which is not possible with temperature dependent viscosity alone using test case models we show that increasing temperature dependence of viscosity enhances vertical and lateral coherence of slabs but leads to unrealistic slab morphologies for large viscosity contrasts introducing pseudo plasticity partially solves this issue producing thin laterally and vertically more continuous slabs and flat subduction where trench retreat is fast we evaluate the differences between convection reconstructions employing different viscosity laws to be very large and similar to the differences between two models with the same rheology but using two different plate histories or initial conditions

a systematic 2 d investigation into the mantle wedge s transient flow regime and thermal structure complexities arising from a hydrated rheology and thermal buoyancy 1 arc volcanism at subduction zones is likely regulated by the mantle wedge s flow regime and thermal structure and hence numerous studies have attempted to quantify the principal controls on mantle wedge conditions in this paper we build on these previous studies by undertaking a systematic 2 d numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge s flow regime and associated thermal structure we quantify the role of a range of plausible i water contents 0 5000 h 106si ii subduction velocities 2 10 cm yr and iii upper plate ages 50 120 myr finding that small scale convection ssc resulting from rayleigh taylor instabilities or drips off the base of the overriding lithosphere is a typical occurrence the morphology of ssc varies with viscosity and subduction parameters with drips at their most prominent when subduction velocities and wedge viscosities are low our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner flow regime and strong upper plate erosion below the arc region by contrast we find that back arc upper plate erosion by ssc is largely controlled by wedge viscosity occurring when i viscosities are 5 1018 pa s and ii the length of the upper plate available for destabilization exceeds the characteristic wavelength of instabilities thus if hydrous weakening of wedge rheology extends at least 100 150 km from the trench our 2 d models predict an unstable flow regime resulting in temperature fluctuations of 50 100 k which are sufficient to influence melting and the stability of hydrous minerals

a community benchmark for viscoplastic thermal convection in a 2 d square box numerical simulations of thermal convection in the earth s mantle often employ a pseudoplastic rheology in order to mimic the plate like behavior of the lithosphere yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature here we present a suite of simple tests based on nonlinear rheologies featuring temperature pressure and strain rate dependent viscosity eleven different codes based on the finite volume finite element or spectral methods have been used to run five benchmark cases leading to stagnant lid mobile lid and periodic convection in a 2 d square box for two of these cases we also show resolution tests from all contributing codes in addition we present a bifurcation analysis describing the transition from a mobile lid regime to a periodic regime and from a periodic regime to a stagnant lid regime as a function of the yield stress at a resolution of around 100 cells or elements in both vertical and horizontal directions all codes reproduce the required diagnostic quantities with a discrepancy of at most 3 in the presence of both linear and nonlinear rheologies furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs as the most recent mantle convection codes can handle a number of different geometries within a single solution framework this benchmark will also prove useful when validating viscoplastic thermal convection simula tions in such geometries

late maastrichtian early danian sea level changes at gebel tawil shihaq in central jordan implications of chemostratigraphy and foraminiferal biostratigraphy abstract integrated foraminiferal and geochemical analyses of the upper maastrichtian lower danian succession of the gebel tawil shihaq section in the central jordan plateau has been conducted to interpret the nature of that succession within a sequence stratigraphic context the studied succession covers the interval of the cf4 p1c planktonic foraminiferal zones the incorporation of foraminiferal and geochemical proxies led to the recognition of four 3rd order depositional sequences three in the maastrichtian m sq1 m sq2 and m sq3 and one in the danian d sq1 these depositional sequences are bounded by four sequence boundaries detected based on abrupt faunal e g zonal interruption foraminiferal turnover and geochemical e g dolomitization sr ca and nearby ti al maxima mn and caco3 minima shifts based on the paleodepth variation revealed from the foraminiferal analyses and the consistent behavior of the different geochemical proxies a relative sea level curve was established and consequently the systems tracts of each depositional sequence were recognized the development of the inferred relative sea level curve reveals the twofold control of both global sea level changes and regional tectonics the latter resulted in amalgamation and elimination or modification of some sea level cycles relative to the previously recognized eustatic schemes at the cretaceous paleogene k pg boundary a highly agitated phosphatized event bed is intercalated and exposes distinctive unique lithological geochemical and biostratigraphical characteristics denoting a long lasting period of non deposition and erosion commonly interrelated to the tectonic impact of the syrian arc orogeny sao

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

recent advances in petroleum geochemistry applied to hydrocarbon exploration this paper presents a critical review of recent advances in the study of generation maturation and migration of petroleum the different source potentials for generating hydrocarbons depend on the type of kerogen in addition to the classic types i ii and iii a residual oxidized organic matter is also observed which has no potential for hydrocarbon generation coal is widely considered as a potential source for gas but some coals may generate sizeable amounts of oil as well the development of routine analytical tools particularly pyrolysis makes possible the preparation of geochemical logs that are the basis for screening samples and for interpreting other analyses among optical techniques the major subjects of discussion concern the nature of amorphous organ c matter which may encompass different chemical types of kerogen and the applicability of vitrinite reflectance techniques to type i and ii kerogens following important advances in identification of biological markers in sediments and crude oils these markers are used for oil source rock correlation and proposed for reconstruction of depositional environment and subsequent thermal evolution migration of hydrocarbons is now better understood and the importance of a hydrocarbon phase migration is widely recognized overpressuring of pore fluids is mainly responsible for expulsion of petroleum geologic models of basin evolution subsidence compaction thermal history and geochemical models hydrocarbon generation and migration become progressively available their integration int a comprehensive model should be one of the major developments in future petroleum exploration

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

fluidity a fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics we present a new computational modeling framework fluidity for application to a range of two and three dimensional geodynamic problems with the focus here on mantle convection the approach centers upon a finite element discretization on unstructured simplex meshes which represent complex geometries in a straightforward manner throughout a simulation the mesh is dynamically adapted to optimize the representation of evolving solution structures the adaptive algorithm makes use of anisotropic measures of solution complexity to vary resolution and allow long thin elements to align with features such as boundary layers the modeling framework presented differs from the majority of current mantle convection codes which are typically based upon fixed structured grids this necessitates a thorough and detailed validation which is a focus of this paper benchmark comparisons are undertaken with a range of two and three dimensional isoviscous and variable viscosity cases in addition model predictions are compared to experimental results such comparisons highlight not only the robustness and accuracy of fluidity but also the advantages of anisotropic adaptive unstructured meshes significantly reducing computational requirements when compared to a fixed mesh simulation

terraferma the transparent finite element rapid model assembler for multiphysics problems in earth sciences we introduce and describe a new software infrastructure terraferma the emph transparent finite element rapid model assembler for the rapid and reproducible description and solution of coupled multiphysics problems the design of terraferma is driven by two computational needs in earth sciences the first is the need for increased flexibility in both problem description and solution strategies for coupled problems where small changes in model assumptions can lead to dramatic changes in physical behavior the second is the need for software and models that are more transparent so that results can be verified reproduced and modified in a manner such that the best ideas in computation and earth science can be more easily shared and reused terraferma leverages three advanced open source libraries for scientific computation that provide high level problem description fenics composable solvers for coupled multiphysics problems petsc and an options handling system spud that allows the hierarchical management of all model options terraferma integrates these libraries into an interface that organizes the scientific and computational choices required in a model into a single options file from which a custom compiled application is generated and run because all models share the same infrastructure models become more reusable and reproducible while still permitting the individual researcher considerable latitude in model construction terraferma solves partial differential equations using the finite element method it is particularly well suited for nonlinear problems with complex coupling between components terraferma is open source and available at http terraferma github io which includes links to documentation and example input files this article is protected by copyright all rights reserved

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

interactions of scales of convection in the earth s mantle abstract the existence of undulations of the geoid gravity and bathymetry in ocean basins as well as anomalies in heat flow point to the existence of small scale convection beneath tectonic plates the instabilities that could develop at the base of the lithosphere are sufficiently small scale 60 ma they form networks that are shaped by the plate evolution slabs plumes and the geometry of continental boundaries plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere they create a wake in which new instabilities form downstream these wakes form channels that are about 1000 km wide and thus are possibly detectable by seismic tomography beneath fast plates cold sinking instabilities are tilted in the direction opposite to plate motion while they sink vertically for slow plates these instabilities are too small to be detected by usual seismic methods since they are about 200 km in lateral scale however this preferred orientation of instabilities below fast plates could produce a pattern of large scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models