bacterial fossil record in whale falls petrographic evidence of microbial sulfate reduction abstract whale fossils from a variety of geologic ages and depositional environments were thin sectioned and analyzed for evidence of microbial activity comprising a combination of microborings authigenic pyrite botryoidal cements and micropeloids borings are dense in some samples with an average diameter of 8 02 m framboidal pyrite crystals with an average diameter of 61 84 m in diameter occur within the pore spaces of the bone in borings and at the bone sediment interface the pyrite crystals are densely packed at the edges of the bone and scattered more randomly throughout the pore spaces aragonitic botryoidal cements line pore spaces in samples from deep bathyal settings potential bacterial peloids occur within pore spaces with an average size of 77 45 m or diffuse to comprise clotted micrite since bacterial body fossils are extremely rare in the fossil record recognition of past microbial ecosystems must be based on multiple lines of supporting evidence of the independent signatures studied the bacterial peloids and botryoids provide the most convincing evidence of a sulfophilic stage these fossil biosignatures suggest that a sulfophilic stage is preserved at least as far back as the eocene

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

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

convection in a variable viscosity fluid newtonian versus power law rheology abstract a number of finite element calculations of convection in a variable viscosity fluid have been carried out to investigate the effects of non newtonian flow when rheology is also subject to a strong temperature and pressure influence a variety of cases has been studied in the range of effective rayleigh numbers between 104 and 106 including different modes of heating and a range of values for activation energy and activation volume power law creep with a stress exponent of 3 turns out to lead to considerably different flow pattern and heat transfer properties than newtonian rheology in general the effect is to reduce viscosity contrasts imposed by p t dependence which can lead in some circumstances to the mobilisation of otherwise stagnant regions within the cell the properties of non newtonian flow can be closely imitated by a newtonian fluid with a reduced value of the activation enthalpy bh with b 0 3 0 5 it appears possible that non newtonian rheology plays a key role in determining the convective style in a planetary mantle

uranium thorium and potassium in the venusian rocks at the landing sites of vega 1 and 2 concentrations of natural radioactive elements in venusian rocks at the landing sites of vega 1 and vega 2 were determined by gamma ray spectrometers on the landers as follows vega 1 k 0 45 0 22 u 0 64 0 47 10 4 th 1 5 1 2 10 4 vega 2 k 0 40 0 20 u 0 68 0 39 10 4 th 2 0 1 0 10 4 the abundances of natural radioactive elements in the investigated rocks are similar to that of basic rocks tholeiitic basalts and gabbros of the earth s crust the same conclusion about the type of the rocks at the landing site of vega 2 was made from the xrf experiment

thermal convection in a cylindrical annulus with a non newtonian outer surface this study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet to first order a geologically active terrestrial type planet may consist of a metallic core silicate mantle and lithosphere with the lithosphere being rheologically different from the mantle therefore we have developed a numerical model consisting of a thin non newtonian fluid hoop that is dynamically coupled to a thick newtonian fluid cylindrical annulus thus the rheological dichotomy between mantle and lithosphere is built into the model time dependent calculations show the existence of at least two regimes of behaviors in one regime the behavior of the hoop switches between periods characterized by low or high speeds in response to changes in convective vigor and planform this regime may apply to the planet venus where the available evidence indicates that prior to 500 myr ago the planet was resurfaced on a time scale of 100 myr since that time large scale tectonic activity on venus has been sharply curtailed in the other regime which is more like plate tectonics on earth the hoop speeds rise and fall on short time scales

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

numerical studies of non newtonian mantle convection abstract numerical model computations have been carried out to determine how the stress dependence of non newtonian viscosity affects the flow structure of thermal convection the viscosity laws have been chosen in accordance with present knowledge of upper mantle rheology based on the diffusion and dislocation creep laws of olivine the results show that there are important differences between the structures of newtonian and non newtonian convection while the newtonian models are insufficient in some respects the non newtonian solutions can explain the characteristics of the real mantle flow however this may require a faster plastic deformation than power law dislocation creep at least in the high stress regions of the mantle e g at the active plate margins

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

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

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

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

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

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

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

the effect of convection planform on the toroidal poloidal energy ratio abstract this study investigates the influence of thermal convection planform geometry on the pattern of surface deformation in a dynamic model of plate mantle coupling the lithosphere is viewed as a distinct rheological layer and is modeled as a thin non newtonian sheet that is dynamically coupled to a three dimensional convecting newtonian layer the results of numerical experiments show that planforms which possess a high degree of symmetry excite only small amounts of toroidal energy in the thin non newtonian sheet and thus have low toroidal poloidal energy ratios asymmetric planforms tend to possess toroidal poloidal energy ratios that are one to two orders of magnitude larger than symmetric cases the results of this study suggest the precipitous drop in the toroidal poloidal energy ratio for the earth s surface velocity field that occurred in the late cretaceous early cenozoic resulted from the late cretaceous superplume which organized mantle flow into a higher degree of symmetry

on the effects of the lithosphere on mantle convection and evolution abstract an idealized model is discussed for plate motion and mantle convection it consists of a numerical square box filled with a newtonian boussinesq fluid heated from below at the surface the fluid has a high viscosity i e it contains a stiff lid which may or may not be laterally decoupled by weakness zones lid stiffness and coupling were varied systematically only if the lid is effectively decoupled laterally does it move as a quasi rigid plate it is then driven by the lateral pressure differential from the rising and descending plumes in the fluid and it moves against viscous shear at its bottom gravitational sliding and sinking may also drive the plate if allowed for in the model a decoupled freely moving stiff plate effectively removes heat from the interior a strongly coupled stiff lid on the other hand or one with a very large aspect ratio inhibits heat transfer through the convecting layer if heating occurs by a given flux through the bottom or internal heat generation the internal cell temperatures become very high because the stagnant surface lid can only conduct the heat by assuming the required temperature gradient throughout its thickness since the earth is partly covered by conducting continental lids which in addition contain much of the heat sources the thermal history may be affected caution must be exercised in parameterized convection modelling in this respect the overall effect cannot easily be predicted because mantle flow and continental plates will interact the importance of mechanical decoupling for plate motion strongly calls for thorough studies of the decoupling mechanisms particularly at convergent plate boundaries bending stick slip processes and the role of volatiles

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

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

the relationship between plate velocity and trench viscosity in newtonian and power law subduction calculations convection with a newtonian temperature dependent rheology leads to little or no surface velocity unless zones of weakness are introduced plate like features are observed in calculations both with newtonian rheology employing imposed weak zones and with power law non newtonian rheology where high stresses at the trench reduce the effective viscosity since deformation at subduction zones involves faulting both of these parameterizations should be treated with some skepticism it is important to understand how the parameterizations affect the model results we study the relationship between trench viscosity and plate velocity using a newtonian rheology by varying the viscosity at the trench the plate velocity is a function of the trench viscosity for fixed rayleigh number and plate slab viscosity slab velocities for non newtonian rheology calculations are significantly different from slab velocities from newtonian rheology calculations at the same effective rayleigh number both models give reasonable strain rates for the slab when compared with estimates of seismic strain rate non newtonian rheology eliminates the need for imposed weak zones and provides a self consistent fluid dynamical mechanism for subduction in numerical convection models