thermal response of the mantle following the formation of a super plate 1 evidence indicating that the mantle below pangea was characterized by elevated temperatures supports the widely held view that a supercontinent insulates the underlying mantle implementing a 3d model of mantle convection featuring distinct oceanic and continental plates we explore different effects of supercontinent formation on mantle evolution we find that a halt in subduction along the margins of the site of the continental collision is sufficient to enable the formation of mantle plumes below a composite super plate and that the addition of continental properties that contribute to insulation have little effect on sub continental temperature our findings show that the mean temperature below a supercontinent surpasses that below the oceanic plates when the former is a perfect insulator but that continental thermal insulation plays only a minor role in the growth of sub supercontinent mantle plumes we suggest that the growth of a super oceanic plate can equally encourage the appearance of underlying upwellings

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

gassmann s fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld someareundergoing enhanced oil recovery most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones but applying relations developed for sandstones to carbonates is problematic at best we measure in the laboratory nine carbonate samples from the same reservoir at seismic 3 3000 hz and ultrasonic 0 8 mhz frequencies samples are measured dry humidified and saturated with liquid butane and brine our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation however we explore the applicability of gassmann s theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion andgassmann stheoryassumptions forourcarbonatesetat high differential pressures and seismic frequencies the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by gassmann s relation we also explore in detail some of the assumptions of gassmann s relation especiallyrock framesensitivitytofluidsaturation our carbonate samples show rock shear modulus change from dry to brine saturation conditions and we investigate several rock fluid mechanisms responsible for this change to our knowledge these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange

true polar wander in mantle convection models with multiple mobile continents a b s t r a c t a r t i c l e i n f o the geologic record supports numerous instances during which continents apparently moved at speeds signi cantly faster than any of today s tectonic plates while the time dependence of convective driving forces likely explains some such observations rapid motions of large continents in particular are often attributed to true polar wander tpw in order to gauge the potential for connections between continents mantle temperature anomalies and polar motion we present the rst calculations of tpw derived from models that couple mantle convection with multiple mobile continents we nd that the aggregation and dispersal of supercontinents can lead to two types of tpw driven either by a well developed hot upwelling axis that creates a stable maximum moment of inertia or by the homogenization of mantle thermal structure following continent dispersal that leads to destabilization of the principal axis and possible large magnitude polar wander these supercontinent modulated thermal heterogeneities drive model tpw events as large as 90 at rates of up to 2 5 ma 1 such magnitudes and speeds are greater than those attained in similar models lacking continents but comparable to those for episodes inferred from paleomagnetic data for some large continents in the past published by elsevier b v on behalf of international association for gondwana research

mantle convection flow reversals due to continental collisions rigidly moving continental and oceanic plates with distinct thermal and mechanical properties and finite thickness are included in a two dimensional numerical model of mantle convection model plates typically span six vertical increments of a finite difference mesh we model two identical continents being carried towards each other by a pair of underlying counter rotating mantle convection cells upon meeting at the model mid plane the model continents form a motionless rigid conducting supercontinent along a portion of the upper boundary while the model oceanic plates continue to move and recycle through the mantle the resulting changes in the mechanical boundary conditions at the upper surface prove to be important factors in facilitating flow reversal below the supercontinent leading to a subsequent dispersal of the individual continental blocks we find the following factors to favour the development of sustained flow reversal below our model supercontinent wider continents lower thermal diffusivity of continental plates thicker plates and lower proportions of internal radiogenic heating within the mantle

mantle dynamics and the heat flow into the earth s continents reducedcontinental heat flow defined as the measured heat flow minus that due to radiogenic sources in the upper crust initially decreases with time after the last thermotectonic event in a region1 but then tends to stabilize at a roughly constant value which is higher than would be predicted by a conductive cooling model fig 1 here we argue that this behaviour results from the influence that continents have on mantle heat loss the finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale as is oceanic crust allows stable continental blocks to limit the local heat flux out of the mantle numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents causing lateral temperature variations at their base that parallel those in the mantle just below this tends to establish a constant flux of heat from the mantle into the continents which accounts for the trend in reduced heat flow

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

generation of plumes under a localized high viscosity lid in 3 d spherical shell convection we conduct numerical simulations of incompressible infinite prandtl number convection in a spherical shell with a single localized high viscosity lid hvl on the top surface to understand the possible effects of the continental lithosphere on plume generation the temperature under the hvl increases rapidly after the emplacement of the hvl on constant viscosity convection with internal and bottom heatings later upwellings at the bottom merge into a large scale flow and a large plume emerges under the hvl this time scale depends on the existence of phase changes in our model whose rayleigh number is 10 6 the large plume is formed on a time scale of 1 gyr a simple scaling suggests that this time scale may be reduced by a several times if the rayleigh number is around 10 despite the complicated 3 d thermal structure it is dominated by an i 1 pattern controlled by the position of the hvl

temperature beneath continents as a function of continental cover and convective wavelength geodynamic modeling studies have demonstrated that mantle global warming can occur in response to continental aggregation possibly leading to large scale melting and associated continental breakup such feedback calls for a recipe describing how continents help to regulate the thermal evolution of the mantle here we use spherical mantle convection models with continents to quantify variations in subcontinental temperature as a function of continent size and distribution and convective wavelength through comparison to a simple analytical boundary layer model we show that larger continents beget warming of the underlying mantle with heating sometimes compounded by the formation of broader convection cells associated with the biggest continents our results hold well for purely internally heated and partially core heated models with rayleigh numbers of 10 5 to 10 7 containing continents with sizes ranging from that of antarctica to pangea results from a time dependent model with three mobile continents of various sizes suggests that the tendency for temperatures to rise with continent size persists on average over timescales of billions of years

fluids and frequency dependent seismic velocity of rocks summary compressional and shear velocities of rocks are dependent on frequency and this dispersion may be significant even within the seismic band the amount and position of dispersion will be largely a function of fluid properties distribution and motion in the pores velocities are thus directly coupled to rock permeability and pore compliance propagation often will be in the high frequency regime even at a few tens of hertz static or low frequency models such as gasmann will fail under such conditions in addition biot theory will not correctly model wave propagation in many cases

the timescales of plume generation caused by continental aggregation abstract to understand the thermal evolution of the mantle following the aggregation of non subductable thick continental lithosphere we study a numerical model in which a supercontinent simulated by high viscosity raft hvr covers a part of the top surface of a convection layer we model infinite prandtl number convection either in a three dimensional 3d spherical shell 3d rectangular box aspect ratios 8 and 4 or two dimensional 2d rectangular box aspect ratio 8 and except for the hvr we specify a constant viscosity the hvr which has a viscosity higher than that of its surrounding is instantaneously placed on the top surface of a well developed convection layer and its position is fixed our results from 3d spherical shell cases with and without phase transitions show the emergence of a large plume characterized by a long wavelength thermal anomaly a degree one pattern for a pangea like geometry we analyze the volume averaged temperature under the hvr t c the remaining oceanic area t o and total area t m to determine the timescale of plume generation the difference between t c and t o t co and t m show the existence of two characteristic timescales t co exhibits an initial rapid increase and may become constant or continue to gradually increase meanwhile t m shows a similar behavior but with a longer timescale we find that these timescales associated with the increase of t co and t m can be attributed to the formation of large scale flow i e plume and response of the whole system to the emplacement of the hvr respectively for 3d spherical cases we find that the timescale of plume generation is 1 2 gyr if the rayleigh number is 10 6 to determine the effects of the viscosity of the hvr 2d versus 3d modeling and the effects of the internal heating we have also studied 2d and 3d rectangular box cases a factor of about two variation exists in the timescale of plume generation it appears that the timescale becomes greater for a smaller amount of internal heating this may be attributed to the time dependent flow caused by the internal heating for 2d cases we find that the timescale of the high rayleigh number 10 7 case is shortened by a factor of three to five when compared to the ra 10 6 case which is consistent with the simple boundary layer theory this may imply that well developed plumes may arise with the timescale of 0 2 to 0 4 gyr for ra 10 7

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

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

windblown desert sands in coeval shallow marine deposits a key for the recognition of coastal ergs in the mid cretaceous iberian basin spain introductionthe action of the wind in ancientsedimentary records is commonlybased on the direct observations ofdune and interdune deposits within ahierarchy of aeolian bounding surfa ces a direct observation of ancientaeolian systems is not always possibleas they are prone to erosion and to theremoval of sand into other uvial andmarine systems although the aeoliandeposits may no longer be present their former existence can be inferredon the basis of grain size characteris tics and aeolian features on grainsurfaces aeolian sediments indeedconstitute an important source formarine deposits kukal and saadal lah 1973 fryberger et al 1983 widespread ne grained very well sorted quartz sandstones characterizeshallow marine facies in the mid cre taceous of the eastern spain iberianbasin the good sorting and round ness of the grains hint at the presenceof a nearby erg system suggestive ofan arid climatic zone along the west ern tethyan margin during the mid cretaceous geological settingthe mesozoic iberian basin along thewestern tethyan margin has one ofthe more complete lower middle cre taceous sedimentary records in eur ope the iberian range is a nw sestriking intraplate fold belt generatedby the tectonic inversion of theiberian basin during the palaeogene fig 1a b salas and casas 1993 capote et al 2002 this basin wasformed by crustal thinning duringthe late permian triassic and latejurassic early cretaceous rift stages salas and casas 1993 arche andlo pez go mez 1996 during thesecond rift stage extensional sub basins were formed as part of thelarger maestrazgo basin i e the east ern sector of the iberian basin fig 1b soria 1997 sands of boththe last synrift unit of these sub basins the escucha formation and of the rst post rift unit of the iberian basin the utrillas formation were depos ited across the estuarine deltaic and uvial environments fig 1c quer ol et al 1992 the sands are domi nantly ne grained show excellentsorting and as they consist almostpurely of quartz they are mineralog ically very mature the escucha and the utrillasformationsthe upper aptian middle albian es cucha formation aguilar et al 1971 sole de porta et al 1994 fig 1c was formed during the last phases ofthe second rift stage that a ected theiberian basin salas and casas 1993 it is a tidally in uenced heterolithicunit dominated by very well sorted ne sand and clay with coal bearingdepositsinitslowerpart andlocallyinthe basal part coarse grained siliciclas tic levels it lies upon the limestonesand marls of the urgonian platform aptian and is covered by sandstonesof the utrillas formation fig 1c pardo 1979 querol 1990 the es cucha formation was interpreted as adeltaic estuarine succession querolet al 1992 with a barrier island sys tem in the northern part of the olietesub basin rodri guez lo pez et al 2005 it was deposited in extensionalsub basins bounded by palaeogeo graphic thresholds produced by listricand block rotation faults the utrillasformation overlies the escucha for mation and is late albian early cen omanian in age and represents uvialdeposits pardo 1979 however theabstract

supercontinent cycles disrupted by strong mantle plumes a theoretical basis for the regularity of supercontinent cycles is lacking here we show that periodic supercontinent cycles are unlikely if thermal instabilities originating at the core mantle boundary are of sufficient strength we couple multiple mobile continents with vigorous mantle convection in a spherical geometry regular supercontinent cycles lasting 400 50 m y occur in idealized models with three continents and a mantle heated purely from within by radioactive decay in a model incorporating six continents and strong mantle plumes this regularity is broken and supercontinents form only sporadically our results suggest that periodic supercontinent cycles are unlikely to occur in realistic earth models

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

pressure and fluid effect on frequency dependent elastic moduli in fully saturated tight sandstone we developed a system to explore the effects of pressure and fluid viscosity on the dispersion and attenuation of fully saturated tight sandstones especially at seismic frequencies calibration of the new system revealed that the system can operate reliably at frequencies of 2 200 106 hz tight sandstone with a crack pore microstructure was tested under nitrogen gas dry brine and glycerin saturation a frequency dependent effect was not found for the dry case however apparent dispersion and attenuation for the undrained unrelaxed transition was clearly observed for sample under brine or glycerin saturation the magnitude of which was largely suppressed by increasing effective pressure the measurement results illustrated that increasing the fluid viscosity or the effective pressure will shift the dispersion curve to the lower frequency range a simple squirt flow model with dual porosity scheme was used to compare with the measurement results although the estimated values deviated slightly from the data the trend fitted the saturated data relatively well especially at low effective pressures therefore considering the crack pore microstructure of the tight sandstone dispersion and attenuation are induced predominantly by the squirt flow stiffening effect from cracks to pores

heterogeneity and time dependence in 3d spherical mantle convection models with continental drift abstract feedback between continents and large scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and wilson cycles paleomagnetism provides evidence for extensive continental displacements 10 000 km on time scales of 100 200 million years comparable to an intrinsic overturn in whole mantle convection here we model continental motions in vigorous 3d spherical convection models focusing on the effects of continent size mantle heating mode and a strong increase in lower mantle viscosity continents covering 30 10 and 3 of earth s surface representative of the former supercontinent pangea present day asia and antarctica respectively are introduced into simple end member mantle convection models characterized by pure core or internal heating and uniform or layered mantle viscosity supercontinents promote temperature anomalies on the largest scales spherical harmonic degrees 1 and 2 primarily through the organization of the long wavelength convective planform inherent in models with a high viscosity lower mantle bottom heating can promote long wavelength heterogeneity by clustering plumes beneath the continent however in isoviscous models small scale structure persists away from the continent regardless of the heating mode supercontinents respond to long wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years smaller continents are unable to promote long wavelength structure and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years continental velocities are roughly a factor of 3 smaller than those in oceanic regions an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates

dynamic feedback between a continentlike raft and thermal convection seismic observations of the mantle which include long wavelength structure a k 1 dependence of heterogeneity on harmonic k and a heterogeneous upper boundary layer and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection we have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry the azimuthal interconnectivity of this geometry is vital to resolve the two way dynamics between rafts and convection computations show that 1 raft motion is periodic 2 long wavelength thermal structure is significant within both thermal boundary layers and the fluid interior and 3 the large scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion these three results which are observed for a range of rayleigh numbers internal heating rates and raft sizes are a direct consequence of the dynamic interaction between the raft and convection the physical processes for a model with a rayleigh number of 10 5 are representative when the raft is stationary due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer heat accumulates beneath the raft and results in long wavelength thermal anomalies the long wavelength thermal anomalies enhance raft motion accompanying the enhanced raft movement the long wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest since convection models without rafts generate less long wavelength heterogeneity compared to the models with rafts or continental plates we suspect that continental plates may play a crucial role in mantle dynamics interestingly raft motion with a period of about 10 transit times is usually significant 10 transit times is about 600 m y if scaled to the earth this is close to the observed 300 500 m y period of supercontinent aggregation and dispersal

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

mantle avalanches and the dynamic topography of continents abstract the dynamic topography of continents associated with mantle avalanche events are discussed our flow simulations involve two dimensional axisymmetric multi phase mantle models which incorporate compressibility and a realistic factor of 40 increase in viscosity with depth the surface kinematic boundary conditions involve a mix of free and rigid i e no slip constraints with the latter intended to mimic the influence of a spherical continental cap we find that avalanche events support regional vertical deflections of up to 2 5 km the horizontal length scale of the dynamic topography is 2500 km and non negligible deflections can persist on the order of 100 200 ma we also find that convergence rates in regions surrounding the continents are variable but can reach rates of the order of 3 cm a finally we speculate that an avalanche event may have been responsible for the development of the large scale intracontinental basin system in the western interior of north america from the early devonian to late carboniferous 1

the impact of rayleigh number on assessing the significance of supercontinent insulation several processes unfold during the supercontinent cycle more than one of which might result in an elevation in subcontinental mantle temperatures thus multiple interpretations of the concept of continental insulation exist although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones there are differing views on what role continental insulation plays in the production of elevated mantle temperatures here we investigate how the heating mode of the mantle can change the influence of the thermal blanket effect we present 2 d and 3 d cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates the evolution of mantle thermal structure is examined after continental accretion at subduction zones e g the formation of pangea for a variety of different mantle heating modes our results show that in low rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases when compared to models with higher convective vigor broad hot upper mantle features generated in low rayleigh number models due in part to the thermal blanket effect are absent at higher rayleigh numbers we find that subcontinental heating in a high rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin rather than the influence of continental insulation in our models featuring earth like convective vigor we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation

interaction between the supercontinent cycle and the evolution of intrinsically dense provinces in the deep mantle shear wave travel times in the earth s deep mantle reveal broad steep sided seismologically distinct provinces lying on the core mantle boundary cmb the longevity and permanence of the two large principal provinces located below the sites of present day africa and the pacific ocean have become a matter of great interest examination of the flood basalt record and kimberlite eruption dating suggests the presence of these provinces may disclose a deep mantle component with a compositionally distinct origin that plays a role in the generation of mantle plumes at preferred locations by extension the presence of these provinces may affect the supercontinent cycle implementing a mantle convection model featuring distinct continental lithosphere and a compositionally anomalous and intrinsically dense caid component we study the distribution and mobility of naturally forming compositionally distinct provinces and their impact on model supercontinent assembly we employ 2d cartesian geometry calculations of thermo chemical convection with extremely low compositional diffusion to model earth like convective vigor on a global scale and find that an intrinsically dense mantle component generally aggregates into one or two broad provinces the positions of the provinces are time dependent but in many of our calculations the province locations are characterized by periods of fixity that reach several hundred million years eras of province and associated plume fixity are punctuated by periods of relatively rapid migration a correlation between supercontinent position and the locations of caid provinces is not supported by our findings however we find the frequency of supercontinent assemblies increases when caid provinces are present

influence of supercontinents on deep mantle flow abstract the assembly of supercontinents should impact mantle flow fields significantly affecting the distribution of subduction upwelling plumes lower mantle chemical heterogeneities and thus plausibly contributing to voluminous volcanism that is often associated with their breakup alternative explanations for this volcanism include insulation by the continent and thus elevated subcontinental mantle temperatures here we model the thermal and dynamic impact of supercontinents on earth like mobile lid convecting systems we confirm that insulating supercontinents over 3000 km extent can impact mantle temperatures but show the scale of temperature anomaly is significantly less for systems with strongly temperature dependent viscosities and mobile continents additionally for continents over 8000 km mantle temperatures are modulated by the development of small scale convecting systems under the continent which arise due to inefficient lateral convection of heat at these scales we demonstrate a statistically robust association between rising plumes supercontinental interiors for a variety of continental configurations driven largely by the tendency of subducting slabs to lock onto continental margins the distribution of slabs also affects the spatial positioning of deep mantle thermochemical anomalies which demonstrate stable configurations in either the sub supercontinent or intraoceanic domains we find externally forced rifting scenarios unable to generate significant melt rates and thus the ultimate cause of supercontinent breakup related volcanism is probably related to dynamic continental rifting in response to mantle reconfiguration events

a laboratory study of low frequency wave dispersion and attenuation in water saturated sandstones abstract laboratory measurements of elastic and anelastic parameters of dry and water saturated sandstones with low 0 7 and 1 1 md and high 425 md permeability have been conducted at seismic frequencies 0 1 to 120 hz the experiments were performed with a laboratory apparatus that measures the stress strain relationship in the linear regime the extensional attenuation in water saturated low permeability sandstones exhibits prominent peaks in the seismic band accompanied by considerable dispersion variations in attenuation and dispersion in the high permeability sandstone are below the measurement error as is the moduli dispersion in all the sandstones in dry condition the experiments demonstrate that for low permeability rocks seismic frequencies do not necessarily correspond to the low frequency limit relaxed pore fluid pressures of acoustic wave dispersion

continental collisions in wide aspect ratio and high rayleigh number two dimensional mantle convection models distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two dimensional numerical model of mantle convection we investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models in addition we implement models of whole mantle flow by specifying high rayleigh number convection and thinner nondimensional plates we are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection for each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells upon collision the continents form a motionless rigid model supercontinent while oceanic plate material continues to recycle through the mantle following the continental collision our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress in contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent the integrated effect of which is capable of causing continental rifting the conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario

a new laboratory apparatus for the measurement of seismic dispersion under deviatoric stress conditions abstract a better understanding of seismic dispersion and attenuation of acoustic waves in rocks is important for quantitative interpretation of seismic data as well as for relating seismic data sonic log data and ultrasonic laboratory data in the present work a new laboratory setup is described allowing for combined measurements of quasistatic deformations of rocks under triaxial stress ultrasonic velocities and dynamic elastic stiffness young s modulus and poisson s ratio at seismic frequencies the setup has been used mainly for the study of shales for such rocks it is crucial that the saturation of the samples is preserved which requires fast sample mounting the design of our setup together with a technique that was developed for rapid mounting of strain gauges onto the sample and subsequent sealing of the sample allows for sample preservation which is of particular importance for shales the performance of the new experimental setup and sample mounting procedure is demonstrated with test materials aluminium and polyetheretherketone and two different shale types mancos shale and pierre shale furthermore experimental results are presented that demonstrate the capability of measuring the impact of saturation stress and stress path on seismic dispersion for the tests with mancos shale and pierre shale large dispersion up to 50 in young s modulus normal to bedding was observed increased water saturation of mancos shale results in strong softening of the rock at seismic frequencies whereas hardening is observed at ultrasonic frequencies due to an increase in dispersion counteracting the rock softening the poisson s ratio of mancos shale strongly increases with the level of saturation but appears to be nearly frequency independent we have found that the different types of shale exhibit different stress sensitivities during hydrostatic loading and that the stress sensitivity is different at seismic and ultrasonic frequencies

constraints on mantle viscosity structure from continental drift histories in spherical mantle convection models abstract earth s continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate mantle coupling thus the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics mantle structure and dynamics here we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on earth we employ global spherical models of mantle convection featuring self consistently generated plate tectonics which are used to compute time evolving continental configurations for different mantle and lithosphere structures our results highlight the importance of the wavelength of mantle flow for continental configuration evolution too strong short wavelength components complicate the aggregation of large continental clusters while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies earth like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long wavelength flow but also allows for shorter wavelength contributions such a criterion alone is a rather permissive constraint on internal structure but it can be improved by considering continental oceanic plate speed ratios and the toroidal poloidal partitioning of plate motions the best approximation of earth s recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are 103 more viscous than the characteristic upper mantle which itself is 100 200 less viscous than the lowermost mantle such a structure causes continents to move on average 2 2 1 0 slower than oceanic plates consistent with estimates from present day and from plate reconstructions this does not require a low viscosity asthenosphere globally extending below continental roots however this plate speed ratio may undergo strong fluctuations on timescales of several 100 myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions

frequency dependent seismic attenuation in shales experimental results and theoretical analysis claudio delle piane 1 joel sarout 1 claudio madonna 2 erik h saenger 2 david n dewhurst1 and mark raven3 1csiro earth science and resource engineering australian resources research centre 26 dick perry avenue kensington wa 6151 australia e mail claudio dellepiane csiro au 2geological department eth sonneggstrasse 5 ch 8092 zurich switzerland 3csiro land and water waite rd urrbrae sa 5064 australia

influence of subsurface injection on time lapse seismic laboratory studies at seismic and ultrasonic frequencies seismic monitoring of reservoir and overburden performance during subsurface co2 storage plays a key role in ensuring efficiency and safety proper interpretation of monitoring data requires knowledge about the rock physical phenomena occurring in the subsurface formations this work focuses on rock stiffness and elastic velocity changes of a shale overburden formation caused by both reservoir inflation induced stress changes and leakage of co2 into the overburden in laboratory experiments pierre shale i core plugs were loaded along the stress path representative for the in situ stress changes experienced by caprock during reservoir inflation tests were carried out in a triaxial compaction cell combining three measurement techniques and permitting for determination of i ultrasonic velocities ii quasi static rock deformations iii dynamic elastic stiffnesses at seismic frequencies within single test which allowed to quantify effects of seismic dispersion in addition fluid substitution effects connected with possible co2 leakage into the caprock formation were modelled by the modified anisotropic gassmann model results of this work indicate that i stress sensitivity of pierre shale i is frequency dependent ii reservoir inflation leads to the increase of the overburden young s modulus and poisson s ratio iii in situ stress changes mostly affects the p wave velocities iv small leakage of the co2 into the overburden may lead to the velocity changes which are comparable with one associated with geomechanical influence v non elastic effects increase stress sensitivity of an acoustic waves iv both geomechanical and fluid substitution effects would create significant time shifts which should be detectable by time lapse seismic this article is protected by copyright all rights reserved

temporal evolution of the stress state in a supercontinent during mantle reorganization summary previous numerical simulation models of isoviscous mantle convection in both 2 d and 3 d geometries have revealed that the presence of a supercontinental lid produces large scale horizontal mantle flow thereby reorganizing the thermal structure of the mantle interior large scale upwelling plumes arising from the core mantle boundary cmb beneath the supercontinent are observed in some models these upwelling plumes have been hypothesized to produce tensional stresses in the supercontinent and to be responsible for the subsequent continental lifting and breakup in this study numerical simulations using a 3 d spherical shell model are conducted in order to elucidate temporal changes in stress state in an idealized supercontinent during mantle reorganization the model supercontinent is assumed to be an undeformable highly viscous supercontinental lid hvsl with respect to the mantle the mantle is heated from the base and from within the hvsl is imposed abruptly on well developed convection systems and remains fixed during the simulations i first consider the scenario in which the hvsl is imposed on isoviscous or weakly temperature dependent viscosity convection exhibiting short wavelength thermal structures the simulation results reveal that a tensional stress regime rapidly predominates throughout the hvsl within 50 100 myr in model time in response to the horizontal mantle flow beneath the hvsl the subsequent large scale plumes upwelling from the cmb also induce tensional stresses in the supercontinent but the effects of these stresses on the overall stress state appear to be secondary for most of the models investigated herein the maximum deviatoric tensional stress generated in the hvsl once convection is re established is 30 90 mpa which is probably comparable to the minimum stress magnitude required to break up the supercontinent to investigate the effects of the hvsl on other convection regimes i next consider the scenario in which an hvsl having a viscosity contrast of 10 is imposed on much longer wavelength i e degree one or degree two dominant convection systems with moderately temperaturedependent viscosity the patterns of such low degree dominant convection are affected little by the imposition of the hvsl and notable mantle reorganization does not occur on the other hand when a reasonable yielding stress is imposed at the continental margins newly formed weak upwelling plumes develop due to enhanced downwelling at the margins rather than large scale mantle reorganization however the newly upwelling plumes do not seem to produce a visible tensional stress regime in the hvsl interior and the spatial distributions of the stress regime and stress magnitude in the hvsl depend on the spatial relationship between the hvsl and the convection pattern since the series l models of the present study do not include the yield stress in the oceanic part of the lithosphere plate like behaviour and subducting plates are not actualized the results for series s models in which the oceanic lithosphere freely sinks and the convection pattern is easily organized imply that the broadly tensional regime prevailing in the supercontinent may support the penetration of mantle plumes into the supercontinent the emergence of hotspots and subsequent continental uplift and break up

the influence of continental surface area on the assembly time for supercontinents 1 we employ a kinematic numerical model of circular continental blocks moving about randomly on a two dimensional periodic model of the earth s surface and monitor the time required for all of the blocks to assemble into a single model supercontinent we examine the sensitivity of the assembly times of our model supercontinents to the size of individual continental blocks the fraction of the model earth s surface covered by the continents and the degree of continental deformation that occurs upon collision assembly times are found to be most sensitive to the total continental surface area we show that in the absence of any dynamic effects due to deep mantle flow continents of present day scale moving randomly about the earth s surface would naturally aggregate into a single mass in approximately 0 4 gy

convection under a lid of finite conductivity heat flux scaling and application to continents 1 a scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed this scaling is constructed in order to quantify the heat transfer out of the earth s mantle taking into account the effect of the dichotomy between oceans and continents which imposes heterogeneous thermal boundary conditions at the surface of the mantle the effect of these heterogeneous boundary conditions is studied here using simple two dimensional models with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model we use free slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents independently of all mechanical effect a systematic study of the heat transfer as a function of the rayleigh number of the fluid of the width of the lid and of its thermal properties is carried out we show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss at least locally the heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the earth s history

biot gardner theory of extensional waves in porous rods many measurements have been made on fluid saturated porous rods executing extensional flexural and torsional motion measurements for extensional and flexural motion yield a loss parameter for young s modulus waves qy and the measurement for torsional motion yields qs for shear waves qp has then been calculated for compressional waves in bulk rock on the assumption that the fluid saturated rock is an isotropic solid i point out the fallacy of computing qp from these measurements and also urge workers to recognize the losses due to simple fluid viscosity in interpreting their data on extensional waves in rods by application of published theory i show that peaks in attenuation of extensional waves are to be expected at frequencies of several hertz to several kilohertz depending upon rod radius computed curves are compared with published measurements on navajo sandstone saturated with water ethanol and n decane in each case computed peak frequency agrees with published measurements shift of th

models of convection driven tectonic plates a comparison of methods and results summary recent numerical studies of convection in the earth s mantle have included various features of plate tectonics a number of different methods for modelling plate like behaviour have been used the differences in the methods of modelling plates may assume or predict significantly different plate deformation we describe three methods of modelling plates through material properties force balance and a thin power law sheet approximation we compare the results obtained using each method on a series of simple calculations from these results we are able to develop scaling relations between the different parametrizations while each method produces different degrees of deformation within the surface plate the surface heat flux and average plate velocity agree to within a few per cent the main results are not dependent upon the plate modelling method and therefore are representative of the physical system we set out to model

fluid mobility and frequency dependent seismic velocity direct measurements the influence of fluid mobility on seismic velocity dispersion is directly observed in laboratory measurements from seismic to ultrasonic frequencies a forceddeformation system is used in conjunction with pulse transmission to obtain elastic properties at seismic strain amplitude 10 7 from 5 hz to 800 khz varying fluid types and saturations document the influence of pore fluids the ratio of rock permeability to fluid viscosity defines mobility which largely controls pore fluid motion and pore pressure in a porous medium high fluid mobility permits pore pressure equilibrium either between pores or between heterogeneous regions resulting in a low frequency domain where gassmann s equations are valid in contrast low fluid mobility can produce strong dispersion even within the seismic band here the low frequency assumption fails since most rocks in the general sedimentary section have very low permeability and fluid mobility shales siltstones tight limestones etc most rocks are not in the lowfrequency domain even at seismic frequencies only those rocks with high permeability porous sands and carbonates will remain in the low frequency domain in the seismic or sonic band