the interplay of dynamic topography and eustasy on continental flooding in the late paleozoic abstract global sea level change can be inferred from sequence stratigraphic and continental flooding data these methods reconstruct sea level from peri cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts and from spatio temporal correlations across basins however it has been understood that long wavelength typically hundreds of km and low amplitude

the influence of post perovskite strength on the earth s mantle thermal and chemical evolution abstract we have investigated the influence of post perovskite ppv viscosity on mantle convective dynamics and stirring efficiency using numerical modeling and simple analytical theory our results show that the strength of ppv has a dramatic influence on convective dynamics the presence of a weak ppv enhances heat transfer across the bottom thermal boundary layer resulting in higher temperatures lower mantle viscosities and considerably larger convective velocities this leads to a significant increase in stirring efficiencies with decreasing the ppv strength by at least one order of magnitude in addition using a simple parameterized convection evolution that includes the influence of ppv coupled to a mixing model we show that during the long term history of the earth s mantle the presence of ppv yields systematically hotter thermal evolution and more efficient convective stirring such a strong effect of ppv strength on mantle stirring efficiency suggests that the influence of ppv phase must be considered when interpreting both geochemical and geophysical observations

the gargano promontory an important italian seismogenic tsunamigenic area abstract gargano is a small mountainous promontory protruding into the southern adriatic and from a structural point of view it is part of the apulia foreland deformative structures both inland and offshore the promontory are quite common and can partially be associated with a substantial quaternary uplifting seismogenically this area is well known because several historical strong earthquakes have been reported to occur here and recent important seismic sequences have been recorded this paper is a contribution to evaluate the seismogenic as well as the tsunamigenic potential of this region particular attention has been given to study the major events some of which were able to produce tsunamis the epicenter location of some of these earthquakes computed solely on the basis of macroseismic data would point to focal regions on land but this is questionable if tsunami generation is taken into account as regards the tsunamigenic potential statistical analysis is used to evaluate the local geographical distribution of expected tsunamis and the mean return periods it is found that large events are expected on average every 228 years while a tsunami of any size is generated every about 84 years which is in good agreement with the experimental observations

the olivine spinel transformation and the rheology of subducting lithosphere during subduction relatively hot lithospheric slabs are expected to deform in a superplastic flow regime below a depth of 300 km if olivine reacts to fine grained spinel this process reduces the flow strength of such slabs below the phase boundary so that they become aseismic at about this depth in colder slabs temperatures are too low for superplastic deformation and a high flow strength and seismic activity are maintained to a depth of 600 700 km

stress distribution within subducting slabs and their deformation in the transition zone abstract the mechanical behavior of slabs in the mantle transition zone is a poorly understood phenomenon to obtain a better understanding of the stress and deformation pattern inside subducting slabs we performed 2d numerical simulations of the subduction process in a purely viscous non linear rheological model including diffusion creep dislocation creep and a stress limiting mechanism the model calculations are used to investigate the effects of yield stress plate age and surface boundary condition free slip versus given plate velocity on the stress development in and around a subducting slab and on the deformation of the subducting plate in the transition zone the deformation and morphology of subducted slabs may differ between free slip and kinematic boundary condition models in some cases especially for young and relatively weak slabs even though kinematically driven subduction morphology may look realistic in most cases our results show that stress distributions can differ significantly if the discrepancy between free slip and kinematic subduction velocity is large and in some model cases even influence the ability of the plate to penetrate into the lower mantle stress distributions such as the ones presented in this study can be a valuable tool in understanding the complex and enigmatic interaction between subducting slabs and the transition zone

thermochemical plumes and mantle phase transitions integral relations based on boundary layer theory are derived to study the motion of an isolated two dimensional thermal plume through a viscous mantle containing polymorphic phase changes analytical results are obtained which show that phase transitions alter average mantle convective velocities by less than 50 in particular we find that the olivine spinel transition approximated as univariant can enhance the circulation velocity of upper mantle convection by 30 40 while it can enhance the overall amplitude of whole mantle convection by a few percent only our calculations demonstrate that a possible endothermic phase change located at 650 km will not prevent deep mantle convection unless the clapeyron slope defining the transition exceeds 0 3 kbar k this large value is more than one order of magnitude greater than what has been proposed for the 650 km discontinuity we then extend the method to include compositional buoyancy and effects of the divariant nature of the olivine spinel transition analysis of the motion of a compositionally buoyant plume one having an anomalous mg fe ratio relative to the ambient mantle reveals that the chemical plume locally distorts the transition in a way which contributes buoyancy and enhances convective amplitudes by 10 or less finally we combine thermal and compositional buoyancy to investigate the interaction between a thermochemical plume and a compositionally induced density interface the results are used to determine the stability criterion for two layered mantle convection we find that a compositionally produced density increase of 4 at 650 km is needed to prevent mixing between the upper and lower mantle seismological observations indicate that density jump at the 650 km discontinuity is between 5 and 9 if all the density increase observed there is due to change in composition then two layered mantle convection is stable however if phase transitions account for part of the density increase so that the chemically produced component is less than 4 then two layered mantle convection may be unstable

finite amplitude convection through a phase boundary summary a finite amplitude numerical model of convective motions through a phase boundary is studied the phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered the motions are driven by horizontal temperature gradients on the boundaries or super critical vertical stratification in the case of rayleigh instability the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem when the parameters of the problem are chosen to simulate the olivine spinel transition in the upper mantle the following results are obtained 1 the vertical scale of motion is the entire depth of the fluid 2 the horizontal scale is not significantly changed from the case of a single phase fluid 3 the amplitude of the motion is not significantly changed due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat and 4 the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10 1 cm yr 1

polar wandering in mantle convection models we calculate polar motion in models of 3 d spherical mantle convection at rayleigh numbers up to 108 which include internal heating radial viscosity variations and an endothermic phase change isoviscous models yield rapid polar motion of order 3 myr but a factor of 30 increase in viscosity with depth reduces the rate of polar motion to about 0 5 myr due to stabilization of the large scale pattern of convection avalanching due to an endothermic phase change causes pulsating inertial interchange polar excursions of order 80 110 and of duration 20 70 myr a layered viscosity model with an endothermic phase change yields only one inertial interchange event in 600 million years these models show that the slow observed rate of post paleozoic true polar wander is not incompatible with higher rates inferred for earlier times

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

a possible local anomaly in the 670 km discontinuity beneath kerguelen abstract the mapping of the 670 km discontinuity in regions of ascending or descending flows is of major interest for a better understanding of the nature of this discontinuity in this paper an attempt is made to investigate the properties of deep mantle discontinuities beneath the kerguelen hot spot anomalous p to s converted waves are observed for some earthquakes recorded at the kerguelen long period geoscope station the characteristics of these phases are specified owing to a three dimensional polarization analysis of the signals a tilted discontinuity is able to explain the data this discontinuity dips toward the northeast with at 20 the depth at which the conversion takes place is 880 km assuming standard velocities in the upper mantle beneath kerguelen or 810 km if a hot upper mantle with a poisson s ratio of 0 30 0 31 is present however the constraints given by the data are not strong enough to specify unambiguously this depth a conversion at 600 km or 550 km assuming a hot upper mantle may alternatively be proposed a strong velocity increase at the discontinuity is in each case necessary to explain the amplitude of the converted waves geophysical arguments show that this tilted discontinuity must be a local feature possible other mechanisms such as wave focusing due to undulations of the discontinuity may also be proposed

on the penetration of the 660 km phase change by mantle downflows we present a simple analytic model of the inter action of cold convective downwelling currents with an dothermic phase change the model describes the ponding and lateral spreading of downflows along the phase transi tion interface a simple comparison of the vertical forces on the ponding material provides a necessary condition for a downflow to penetrate the phase boundary this con dition is fundamentally dependent on the geometry of the downflow for planar downwellings the model predicts a minimum ponding time before the structure can pen etrate the phase boundary for columnar axisymmetric downflows there is no minimum time of spreading required before penetration can proceed the model thus provides an explanation for the observation that in numerical mod els of three dimensional convection with an endothermic phase change cylindrical downflows penetrate the phase interface while planar ones do not since descending slabs in the earth s mantle display a wide spectrum of geome tries between planar and cylindrical given various trench curvatures as well as intersections of two or more subduc tion zones this phenomenon may explain in part why some slabs appear to extend into the lower mantle while others are deflected at the 660 km discontinuity

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

the diversity of tectonics from fluid dynamical modeling of the lithosphere mantle system abstract numerical modeling of the lithosphere has been carried out from many points of view most popular and prevalent is the use of the kinematic boundary conditions to move the lithosphere about in the dynamics we have approached the numerical modeling of the lithosphere mantle system by treating this as a fluid dynamical system in which crustal and mantle rheologies and thermal and compositional buoyant forces play dominant roles in delivering the dynamics in a self consistent manner we have investigated the development of extensional and compressional tectonic regimes from variations of the crustal density and compare their temporal evolutions in comparing with kinematically driven thickening of the lithosphere we find that the timescales are more self consistently determined in a full fluid dynamical treatment than in kinematic models we emphasize the delicate nature of balancing the chemical buoyancy of the crust with the thermal negative buoyancy of the mantle lithosphere as the modes and associated timescales of the delamination process are controlled critically by their relative importance the role of an actively participating buoyant crust cannot by ignored in the overall lithosphere mantle dynamics

mantle phase transitions and postglacial rebound the short term response of a univariant or divariant isochemical phase boundary to a pressure change is studied the pressure change can be caused by vertical displacement of material in response to surface loading the aim is to determine if the density discontinuity is nonadiabatic i e will affect fluid flow or adiabatic for the basic state both a quasi static mantle with a smooth thermal structure in the transition region and the case of a steady convection current through the boundary are considered in the latter case the peculiar temperature distribution near the boundary affects significantly the response to additional loading a univariant transition behaves fully nonadiabatic at low convective velocities below the verhoogen limit but increasingly adiabatic above a divariant transition behaves partly adiabatic already in a static mantle and the adiabatic character is strongly enhanced with increasing velocity of convection with realistic combinations of the relevant parameters for a divariant mantle phase change the response will be hardly more than 10 nonadiabatic and probably less than 1 this result affects especially the interpretation of the gravity anomaly above formerly glaciated areas in terms of mantle viscosity and layering of mantle convection

water circulation and global mantle dynamics insight from numerical modeling we investigate water circulation and its dynamical effects on global scale mantle dynamics in numerical thermochemical mantle convection simulations both dehydration hydration processes and dehydration melting are included we also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed due to effective slab dehydration at shallow depths water still affects significantly the global dynamics by weakening the near surface oceanic crust and lithosphere enhancing the activity of surface plate motion compared to dry mantle case as a result including hydrous minerals the more viscous mantle is expected with several orders of magnitude compared to the dry mantle the average water content in the whole mantle is regulated by the dehydration hydration process the large scale thermochemical anomalies as is observed in the deep mantle is found when a large density contrast between basaltic material and ambient mantle is assumed 4 5 comparable to mineral physics measurements through this study the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

mantle flow and deformation of subducting slab at a plate junction abstract at junctions between plate boundaries there are several general characteristic features these include trench normal fast polarization direction of s wave splitting in the mantle wedge along arc variation of the angle of subduction and the different behavior of slab stagnation along the strike of the trench in this paper we focus on the junction of the tohoku kurile arc we show the dynamical effects of the junction using a numerical model of mantle convection with a realistic curved shape of the trench we obtain 3d flow in the mantle wedge which is consistent with the observation of seismic anisotropy even with the oblique subduction we also find the along arc variation of subduction angle which agrees with the observations although the match becomes worse when we include the effect of dislocation creep that is non linear viscosity along arc variation of the angle of subduction partly contributes to the different behavior of slab stagnation in the tohoku kurile arc our results show that the shape of the trench is an important factor which considerably affects mantle flow and deformation of subducting slabs thus 3d modeling is necessary to constrain the dynamics of subduction zones near the junction

the non adiabatic nature of mantle convection as revealed by passive tracers abstract we have investigated the state of adiabaticity in mantle flows by using a lagrangian tracer technique to map out the thermal state of parcels in both downwelling and upwelling flows in mantle flush events the model baed on a core cooling boundary condition has two major phase transitions and a temperature dependent viscosity the temperature histories of the first tracers that enter into the lower mantle during a flushing event reveal a superadiabatic increase of up to 700 k due to conduction and viscous dissipation the lateral temperature anomalies in the deep lower mantle monitored by the tracers can exceed 1000 k the cold sinking anomalies follow a superadiabatic path and are heated up according to the time history of the flush event the time history of the rising parcels is strongly influenced non adiabatically by the two phase transitions in the upper mantle with only a small segment in the upper mantle below the surface where the parcels are found to be invariably slightly superadiabatic following a major flush event the horizontally averaged temperature profiles display strong subadiabaticity in the deep mantle and superadiabaticity in the transition zone during this time the mean temperature profiles do not follow an adiabat anywhere the concept of adiabaticity may have limited applications in strongly time dependent mantle convection with phase transitions where major overturns take place from time to time

phase transitions in the martian mantle implications for the planet s volcanic history the influences of phase transitions on martian mantle convection and melting in the martian mantle have been studied with an axisymmetric spherical shell model an extended boussinesq model in which viscous and adiabatic heatings are included has been used there are depth dependences in the thermal expansivity and gravity which taken together decrease by a factor of between 2 and 3 across the martian mantle the two destabilizing exothermic phase transitions olivine to spinel and and spinel transitions above the martian core mantle boundary cmb accelerate the mantle flow and result in an amplification and superheating of plumes in mars the additional consideration of the endothermic phase transition spinel to perovskite transition which was only likely present in the early evolution of the planet has little influence on martian mantle convection in this model strong localized viscous heating is generated near the cmb and underneath the lithosphere because here the flow bends over sharply a possible volcanic evolution of mars can be derived from a comparison of the temperature fields with the solidus of anhydrous peridotite if the cooling of the martian mantle is taken into account in the early evolution the mantle was molten to a high degree along the plume axis which possibly resulted in a strong differentiation of the mantle as the planet cooled down the region of melt generation receded to where the maximum of viscous heating occurred at the cmb and immediately underneath the lithosphere upon further cooling the deep mantle melt source region became subsolidus the most recent volcanism on mars was most likely generated at a shallow depth below the lithosphere

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

on some consequences and possible causes of layered mantle convection laboratory and numerical experiments are used to study convection confined to discrete layers the laboratory experiments which use the water content of a predominantly glycerine solution as a way of establishing chemical density variations show that convection will remain confined to superimposed layers when the chemical density contrast between layers is greater than the density change associated with the greatest temperature difference within the convecting system the numerical experiments show a double thermal boundary layer at the interface between layers a feature that would result in an average temperature increase of about 500 c in the mantle near 700 km depth if the lower mantle is isolated as suggested by recent geochemical models the possible causes of a layered mantle convection system include major element chemistry variations the combined effect of major element chemistry and a phase change or a phase change alone if it has a sufficiently negative clapeyron slope

is subducted lithosphere trapped at the 670 km discontinuity recent work1 2 on phase equilibria has shown that subducted oceanic lithosphere which is differentiated into a basaltic crust and a harzburgitic residue layer could be significantly denser than undifferentiated peridotite at depths between 650 and 700 km but would be less dense in the range 700 740 km it might thus be trapped in a gravitational stable layer between upper and lower mantle3 here using numerical experiments of isoviscous convec tion i explore the stability of such a layer assuming that it had previously been formed the results suggest that the survival of the intermediate layer is at best marginally feasible provided that new subducted lithosphere is efficiently trapped

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

sedimentary characteristics of late pleistocene bottom current deposits barents sea slope off northern norway abstract the barents sea slope off northern norway received large amounts of terrigenous sediments from an extensively glaciated norwegian continental margin during the late pleistocene a sedimentological study of seven gravity cores from the barents sea slope suggests that some terrigenous sediments were transported by both downslope currents and contour following bottom currents the bottom current deposits comprise four sedimentary facies 1 indistinctly laminated mud 2 layered mud 3 indistinctly layered mud and 4 bioturbated mud the indistinctly laminated mud on the mid to lower slope is characterized by a relatively low degree of bioturbation and consists of monotonous diffuse and discontinuous silt rich and silt depleted laminae 1 mm scale and or discontinuous trains of coarse grains these characteristics are suggestive of sedimentation from contour currents driven by water exchange with the north atlantic ocean these bottom current deposits can be distinguished from fine grained turbidites silt clay couplets and intensively bioturbated hemipelagites on the barents sea slope

the effects of adiabatic and viscous heatings on plumes we have assessed the influence of viscous and adiabatic heatings on the transient development of plumes by employing the time dependent convection equations in the extended boussinesq limit both temperature dependent and temperature and pressure dependent viscosities are employed for constant thermal expansion of coefficient we find the vigor of plume flows is diminished with larger magnitudes of the thermal expansivity this effects is caused by the local domination of adiabatic decompression over viscous heating inside the developing plume for both the cartesian and axisymmetric domains for deep mantle flows a thermal expansivity decreasing with depth is required for thin plume structures to exist values of the thermal expansivity contrast which can promote plume growth are in accord with estimates from recent studies in seismic equation of state copyright american geophysical union 1987

geoid anomalies from cenozoic subduction in semi dynamical flow models including a phase boundary in order to investigate the relationship between the geoid and plate subduction we develop a 3 d spherical shell model in which the circulation is driven by both buoyancy forces and an imposed surface velocity taken from plate reconstruction for the past 65 ma to avoid numerical resolution problems we use an enhanced value of thermal diffusivity which leads to an overly thick lithosphere the correct amount of buoyancy is re established by using a reduced value of thermal expansion coefficient first we calculate the present temperature field in the mantle due to the cenozoic plate motions for models with and without a phase transition at 660 km depth which is approximated by a locally modified effective thermal expansion coefficient in a second step the geoid anomalies are determined subject to a stress free upper boundary condition when the thermodynamic parameters of the boundary at 660 km allow slab penetration into the lower mantle the medium wavelength l 4 11 geoid agrees well with the observed geoid if there is a moderate increase of viscosity from the upper to the lower mantle when the clapeyron slope is sufficiently negative to prevent slab penetration the agreement is poor