lunar global shape and polar topography derived from kaguya lalt laser altimetry a global lunar topographic map with a spatial resolution of finer than 0 5 degree has been derived using data from the laser altimeter lalt on board the japanese lunar explorer selenological and engineering explorer selene or kaguya in comparison with the previous unified lunar control network ulcn 2005 model the new map reveals unbiased lunar topography for scales finer than a few hundred kilometers spherical harmonic analysis of global topographic data for the moon earth mars and venus suggests that isostatic compensation is the prevailing lithospheric support mechanism at large scales however simple rigid support is suggested to dominate for the moon venus and mars for smaller scales which may indicate a drier lithosphere than on earth especially for the moon and venus

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

interpreting gravity geoid and topography for convection with temperature dependent viscosity application to surface features on venus while venus and earth are similar in size venus does not appear to have developed the same system of mobile surface plates which dominates convective heat transfer from the interior of earth numerical models with temperature dependent viscosity do not produce plate boundaries and are more closely reminiscent of venus than earth we demonstrate that many puzzling aspects of surface features on venus are also found in the analysis of the surface observables in these models despite having no strong lithosphere our calculations exhibit very high geoid topography ratios and uncompensated short wavelength topography surface heat flux values are not required to be very different from earth scaled values and the mantle remains below the likely solidus

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on venus models of coupled crustal deformation and mantle convection on venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine a previous model which assumed a relatively weak basalt flow law more appropriate for hydrated crust is compared to a similar model that assumes a strong basalt flow law more appropriate for dry crust a principal difference is the timescale for significant crustal thickening of the order of 108 years for the weak basalt model and 109 years for the strong basalt model these results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near surface weakness of the type associated with plate margins on earth the lithospheric overturn time which sets the timescale required for convective thickening of crust scales dominantiy with the rayleigh number defined by the average viscosity of the lithosphere theoretical scalings as well as numerical models suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103 such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present day earth the large values inferred for the thickness of the mechanical lithosphere on venus if correct would thus tend to favor timescales of the order of 109 years if one assumes relatively low convective mantle heat loss over an equivalent time this in turn would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows if instead one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust

stress strain rate and anisotropy in kyushu japan abstract seismic anisotropy the directional dependence of wave speeds may be caused by stress oriented cracks or by strain oriented minerals yet few studies have quantitatively compared anisotropy to stress and strain over large regions here we compare crustal stress and strain rates on the island of kyushu japan as measured from inversions of focal mechanisms gps and shear wave splitting over 85 000 shear wave splitting measurements from local and regional earthquakes are obtained from the nied network between 2004 and 2012 and on aso sakurajima kirishima and unzen volcano networks strain rate measurements are made from the japanese geonet stations jma determined s arrival times processed with the mfast shear wave splitting code measure fast polarisations related to the orientation of the anisotropic medium and time delays dt related to the path length and the percent anisotropy we apply the tessa 2 d delay time tomography and spatial averaging code to the highest quality events which have nearly vertical incidence angles separating the 3455 shallow depth 40 km earthquakes using square grids with 30 km sides for all the inversions the best correlations are observed between splitting from shallow earthquakes and stress axes of maximum horizontal stress shmax and correlate with a coefficient c of 0 56 significant at the 99 confidence level their mean difference is 31 9 axes of maximum compressional strain rate and shmax are also well aligned with an average difference of 28 but they do not correlate with each other meaning that where they differ the difference is not systematic anisotropy strength is negatively correlated with the stress ratio parameter determined from focal mechanism inversion c 0 64 significant at the 99 confidence level the anisotropy and stress results are consistent with stress aligned microcracks in the crust in a dominantly strike slip regime eigenvalues of maximum horizontal strain rate correlate positively with stress ratio c 0 43 significant at 99 confidence all three orientations are e w in central kyushu where the compressional strain rate is highest both splitting and stress suggest plate boundary parallel maximum principal stress just off the coast of kyushu where strain rate measurements are sparse south western kyushu has the largest difference between directions of strain rate and stress from shallow and deep earthquakes are not well aligned suggesting that the deep earthquake waveforms are not simply split in the crust causes for the anisotropy may be olivine crystals aligned by drag of the subducting philippine sea plate in the mantle and stress aligned microcracks in the crust

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

seismic ray path variations in a 3d global velocity model abstract a three dimensional 3d ray tracing technique is used to investigate ray path variations of p pcp pp and pp phases in a global tomographic model with p wave velocity changing in three dimensions and with lateral depth variations of the moho 410 and 660 km discontinuities the results show that ray paths in the 3d velocity model deviate considerably from those in the average 1d model for a pcp wave in western pacific to east asia where the high velocity 1 2 pacific slab is subducting beneath the eurasian continent the ray path change amounts to 27 km for a pcp ray in south pacific where very slow 2 velocity anomalies the pacific superplume exist in the whole mantle the maximum ray path deviation amounts to 77 km ray paths of other phases p pp pp are also displaced by tens of kilometers changes in travel time are as large as 3 9 s these results suggest that although the maximal velocity anomalies of the global tomographic model are only 1 2 rays passing through regions with strong lateral heterogeneity in velocity and or discontinuity topography can have significant deviations from those in a 1d model because rays have very long trajectories in the global case if the blocks or grid nodes adopted for inversion are relatively large 3 5 and only a low resolution 3d model is estimated 1d ray tracing may be feasible but if fine blocks or grid nodes are used to determine a high resolution model 3d ray tracing becomes necessary and important for the global tomography

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods the idea that the propagation of elastic waves can be anisotropic i e that the velocity may depend on the direction is about 175 years old the first steps are connected with the top scientists of that time people such as cauchy fresnel green and kelvin for most of the 19th century anisotropic wave propagation was studied mainly by mathematical physicists and the only applications were in crystal optics and crystal elasticity during these years important steps in the formal description of the subject were made at the turn of the 20th century rudzki stressed the significance of seismic anisotropy he studied many of its aspects but his ideas were not applied research in seismic anisotropy became stagnant after his death in 1916 beginning about 1950 the significance of seismic anisotropy for exploration seismics was studied mainly in connection with thinly layered media and the resulting transverse isotropy very soon it became clear that the effect of layer induced anisotropy on data ac

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

simulating the thermochemical magmatic and tectonic evolution of venus s mantle and lithosphere two dimensional models 1 numerical convection models of the thermochemical evolution of venus are compared to present day topography and geoid and recent resurfacing history the models include melting magmatism decaying heat producing elements core cooling realistic temperature dependent viscosity and either stagnant lid or episodic lithospheric overturn in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe which requires massive magmatism and produces very thick crust inconsistent with observations partitioning of heat producing elements into the crust helps but does not help enough episodic lid overturn interspersed by periods of quiescence effectively loses venus s heat while giving lower rates of volcanism and a thinner crust calculations predict 5 8 overturn events over venus s history each lasting 150 myr initiating in one place and then spreading globally during quiescent periods convection keeps the lithosphere thin magmatism keeps the mantle temperature constant over venus s history crustal recycling occurs by entrainment in stagnant lid convection and by lid overturn in episodic mode venus like amplitudes of topography and geoid can be produced in either stagnant or episodic modes with a viscosity profile that is earth like but shifted to higher values the basalt density inversion below the olivine perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn the classical stagnant lid mode with interior temperature rheological temperature scale lower than tcmb is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start core heat flow decreases with time possibly shutting off the dynamo particularly in episodic cases

mapping seismic anisotropy of the lithospheric mantle beneath the northern and eastern bohemian massif central europe abstract we evaluate 3d body wave anisotropic parameters shear wave splitting fast s polarizations and delay times of the split slow shear waves and directional terms of relative p wave residuals from data of passive seismic experiment bohema ii 2004 2005 data from 34 temporary stations increased substantial density of observations in the bohemain massif bm and allowed us to scan in detail the upper mantle structure of its northern and eastern parts for the first time we delimit domains of the mantle lithosphere in this part of the bm and infer 3d self consistent models of their fabrics by a joint inversion of the body wave anisotropic parameters fabrics of the northern and north eastern parts of the bm are approximated best by peridotite aggregates with the a c foliations dipping approximately to the n and ne respectively whereas a model with the westerly dipping a lineation fits best the fabric of the south eastern domain the saxothuringian fabric nw of the eger rift extends to the east across the elbe fault zone and continues along this zone to the southeast beneath the cretaceous basin the south eastward continuation of the elbe fault zone seems to be related to the boundary between two different fabrics of the northern and southern brunovistulian domains below the moravo silesian zone this study shows an underthrusting of the brunovistulian micro plate beneath the eastern rim of the bm and indicates that its northern and southern fragments might have originally belonged to baltica and to gondwana respectively according to a zone of distinctly decreased anisotropic signals the brunovistulian micro plate extends at least about 100 km westward beneath the moldanubian with these new findings we update the domain like mantle structure of the bm and compare the results with inferences from the upper mantle velocity tomography and depth changes of the lithosphere asthenosphere boundary retrieved in a detailed model from relative residuals and from the receiver functions inferences on anisotropic structure beneath the permanent observatories included in different experiments are stable and are validated by the bohema ii experiment

the low degree shape of mercury the shape of mercury particularly when combined with its geoid provides clues to the planet s internal structure thermal evolution and rotational history elevation measurements of the northern hemisphere acquired by the mercury laser altimeter on the mercury surface space environment geochemistry and ranging spacecraft combined with 378 occultations of radio signals from the spacecraft in the planet s southern hemisphere reveal the low degree shape of mercury mercury s mean radius is 2439 36 plusmn 0 02 km and there is a 0 14 km offset between the planet s centers of mass and figure mercury is oblate with a polar radius 1 65 km less than the mean equatorial radius the difference between the semimajor and semiminor equatorial axes is 1 25 km with the long axis oriented 15 g west of mercury s dynamically defined principal axis mercury s geoid is also oblate and elongated but it deviates from a sphere by a factor of 10 less than mercury s shape implying compensation of elevation variations on a global scale

the isostatic stagnant lid approximation and global variations in the venusian lithospheric thickness we use the long wavelength venusian topography and geoid to constrain the global lithospheric thickness by assuming that the stagnant lid overlying the convective mantle is in isostatic equilibrium the isostatic stagnant lid isl approximation two dimensional and three dimensional numerical simulations of convection show that the isl approximation is reasonably accurate in the tested parameter range of temperature dependant viscosity convection with bottom heating and internal heating the errors in predicting the thickness of the stagnant lid i e the lithosphere are somewhat larger for internally heated convection we attribute this to a flatter lid and as a result to a larger relative contribution to topography and geoid anomalies from the deeper parts of the layer application of the isl approximation to venus suggests that the average lithospheric thickness might be as large as 600 km which is on the high end of previously proposed estimates

three dimensional structure of p wave anisotropy beneath the tohoku district northeast japan 1 we retrieve three dimensional structures of isotropic and anisotropic velocities of p waves of the tohoku district from first p arrival time data assuming azimuthal anisotropy to be caused by hexagonal symmetry axes distributed horizontally in the earth the results show that the high velocity pacific slab is clearly imaged in the isotropic velocity structure even though the azimuthal anisotropy is taken into account in addition small scale low velocity regions and prominent low velocity anomalies are found just below the active volcanoes and in the mantle wedge above the high velocity pacific slab respectively the fast propagation axis of p waves is in mostly e w direction in the upper crust nearly n s and e w directions in the lower crust e w direction in the mantle wedge and n s direction in the descending pacific slab these features of the p wave anisotropy structure are consistent with those of lateral variations of the fast polarization directions measured previously by shear wave splitting observations the plausible factor that causes the crust anisotropy is interpreted as being alignment or preferred orientation of microcracks and crust minerals the mantle wedge anisotropy is attributed to lattice preferred orientation of the mantle minerals arising from present day mantle process such as the mantle wedge convection and the plate motion however the fast propagation axis of p waves in the slab is almost perpendicular to the magnetic lineation of the oceanic plate under the northwest pacific and thus the slab preserves the original anisotropic property that the pacific plate gained when it formed

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

constraints on mantle plumes on venus implications for volatile history abstract the analysis of venus gravity field and topography suggests the presence of a small number of deep mantle plumes 9 this study predicts the number of plumes formed at the core mantle boundary their characteristics and the production of partial melt from adiabatic decompression numerical simulations are performed using a 3d spherical code that includes large viscosity variations and internal heating this study investigates the effect of several parameters including the core mantle boundary temperature the amount of internal heating and the mantle viscosity the smallest number of plumes is achieved when no internal heating is present however scaling earth s radiogenic heating to venus suggests a value of 16 tw cases with internal heating produce more realistic lid thickness and partial melting but produce either too many plumes or no plumes if a high mantle temperature precludes the formation of a hot thermal boundary layer mantle viscosity must be reduced to at least 10 20 pa s in order to include significant internal heating and still produce hot plumes in all cases that predict melting melting occurs throughout the upper mantle only cases with high core temperature 1700 k produce dry melting over time the upper mantle may have lost significant volatiles depending on the water content of the lower mantle deep plumes may contribute to present day atmospheric water via volcanic outgassing assuming 50 ppm water in mantle 10 plumes with a buoyancy flux of 500 kg s continuously erupting for 4 myr will outgas an amount of water on the order of that in the lower atmosphere a higher level of internal heating than achieved to date as well as relatively low mantle viscosity may be required to achieve simulations with 10 plumes and a thinner lid alternatively if the mantle is heating up due to the stagnant lid the effect is equivalent to having lower rates of internal heating a temperature increase of 110 k byr is equivalent to 13 tw this value along with the internal heating of 3 tw used in this study may represent the approximate heat budget of venus mantle

resurfacing events on venus implications on plume dynamics and surface topography 1 global resurfacing events explain the relatively young and uniformly aged surface of venus in numerical models featuring these events a variety of plume classes occur ranging from strong stable to smaller wind driven upwellings we investigate this plume dynamics in a 3d cartesian geometry paying special attention to the surface topography while the stable upwellings of the stagnant lid phase correlate with an elevation of the surface the wind driven upwellings of the resurfacing phase do not necessarily correlate with a positive topography signal we rather observe that a thick plate can dominate the topography above upwellings

mapping p wave azimuthal anisotropy in the crust and upper mantle beneath the united states abstract much progress has been made on revealing seismic structure and mantle dynamics beneath the united states us with the earthscope usarray project seismic anisotropy revealed by shear wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images but the shear wave splitting observations have poor vertical resolution and so their interpretations are often not unique in this work we used a large number of arrival time data from local and distant earthquakes recorded by the usarray to determine the first p wave azimuthal anisotropy tomography of the crust and upper mantle beneath the us our results show that fast velocity directions fvds in the lithosphere under the tectonically active areas correlate well with the surface tectonic features suggesting that the p wave anisotropy mainly reflects the present deformation a circular pattern of the fvds centered in the great basin is revealed which is well consistent with the specific circular shear wave splitting observations there suggesting that the anisotropy occurs in the crust and uppermost mantle in contrast beneath the stable cratonic region the fvds revealed by this study differ from the shear wave splitting observations but consistent with the features of gravity and magnetic anomalies indicating that the p wave fvds mainly reflect the fossil anisotropy in the lithosphere whereas the s wave splitting observations mainly reflect the significant anisotropy in the asthenosphere the present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the us

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

iodine content of the carbonates from the doushantuo formation and shallow ocean redox change on the ediacaran yangtze platform south china abstract the ediacaran period 635 541 ma is characterized by remarkable carbon isotope 13c excursions significant ocean redox changes and rapid diversification of eukaryotes including early animals these biological and geochemical events are commonly thought to be linked but their causal relationships remain controversial a particular point relates to the shallow ocean redox change across the negative 13c excursions most existing geochemical proxy data focused on tracking the redox conditions of deep or bottom water columns and provided limited information about the surface ocean ratios of iodine to calcium magnesium i ca mg from carbonates represent one of the few redox proxies that track the oxidation state of the shallow oceans here we report paired carbonate carbon isotopes 13ccarb and i ca mg ratios of the doushantuo formation from a drillcore section at daotuo paleogeographically located at the slope of the ediacaran yangtze platform south china in addition we compare the i ca mg ratios across the shuram upper doushantuo 13ccarb excursion in four sections of a shelf to slope transect in the slope section at daotuo the 13ccarb of the doushantuo formation shows three negative anomalies that are correlatable with those documented from the yangtze gorges area shelf and the siduping section slope the i ca mg ratios of the doushantuo formation from the drillcore are dominated by low but non zero

mantle flow tectonics and the origin of ishtar terra venus it is proposed that western ishtar terra formed due to compression and crustal thickening above a cylindrical mantle downwelling a model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western ishtar although axisymmetric downwellings occur in numerical models of constant viscosity mantle convection there is no evidence for their existence in earth s mantle where downwellings are sheet like either modes of downflow in venus and earth are fundamentally different or differences in near surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes

how robust is isotropic delay time tomography for anisotropic mantle using isotropic inversion of teleseismic p travel times computed for laterally varying anisotropic structures we investigate how anisotropy may show up in classical tomographic studies based on isotropic 3 d inversions anisotropic bodies of several tens of kilometers in the upper mantle may significantly bias the estimation of the isotropic part of a p wave velocity image the largest artifacts are introduced by anisotropic bodies with a dipping olivine a axis which could be associated with fossil subduction zones however anisotropy induced artifacts are negligible for anisotropy caused by pure shear deformation of the lithosphere due to horizontal extension or compression the negative p wave velocity anomaly arising from isotropic inversion beneath a rift zone may be enhanced if the olivine crystal s a axes are preferentially oriented parallel to the asthenospheric flow along the rift axis examination of recently published tomographic models shows that the image of the rhine graben may contain significant anisotropic artifacts

shear wave anisotropy in the crust mantle wedge and subducting pacific slab under northeast japan to study the anisotropic structure beneath northeast ne japan we made 4366 shear wave splitting measurements using high quality seismograms of many earthquakes occurring in the crust and the subducting pacific slab our results provide important new information on the s wave anisotropy in the upper crust lower crust mantle wedge and subducting pacific slab in the upper crust the anisotropy is mainly caused by the stress aligned fluid saturated microcracks the measured delay times dts increase to 0 10 s at 10 11 km depth the fast velocity directions fvds are parallel to either the tectonic stress or the strike of active faults the maximum dts for the low frequency earthquakes near the moho are 0 15 0 17 s suggesting strong anisotropy at the base of the crust or in the uppermost mantle the measurements for the intermediate depth earthquakes in the pacific slab show dominant e w trench normal fvds in the back arc area and n s trench parallel fvds in the fore arc area the trench normal fvds in the back arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the pacific plate the maximum dts for the slab earthquakes reach 0 30 0 32 s at 100 km depth but only half of the total dts are produced in the mantle wedge the small dts in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge in the fore arc the dominant trench parallel fvds for the slab earthquakes are consistent with those for the upper crust earthquakes and 80 of the total dts can be accounted for by the anisotropy in the crust in the subducting pacific slab the trench parallel fvds may reflect either the original fossil anisotropy in the pacific plate when the plate was produced in the mid ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab

influence of observed mantle anisotropy on isotropic tomographic models there is a potential for isotropic tomographic models to be biased by ignored anisotropy anisotropy could be mapped as artificial isotropic velocity perturbations we investigate the distribution and strength of this potential bias for three tomographic s velocity models based on regional s and rayleigh waveforms we use observed sks delay times and fast axis orientations to compute equivalent s velocity perturbations for each wave path used for the tomographic models these synthetic perturbations are then combined into a three dimensional isotropic model which reflects the potential bias in the actual tomographic model we quantify anisotropic bias and find that it indeed exists in the isotropic tomographic models investigated here this bias gets weaker with increasing depth of the anisotropic material and compared to the isotropic velocity anomalies typically interpreted by tomographers the bias from anisotropy is small

magellan and comparative planetology magellan data have been widely used to provide explanations for the tectonics and volcanism on venus however it is also important to compare our understanding of these processes on venus with the earth moon and mars one essential question concerning the behavior of these bodies is how does the heat get out internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism on the earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells the subduction of these cold plates at ocean trenches contributes about 75 of the mantle heat flux it is now quite clear that active plate tectonics is not occurring on venus the surface of venus is a single rigid plate as is the case for the moon mars and mercury prior to magellan the general consensus was that venus was in a near uniformatarian state with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet however crater studies from magellan images provide direct evidence for a catastrophic or episodic evolution of venus one explanation is that venus died tectonically 500 m y ago plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased an alternative explanation is that venus experiences episodic global subduction events two major questions concerning the earth remain largely unanswered particularly when compared with venus 1 why does the earth have continents 2 why does the earth have plate tectonics a major difference between the earth and venus is the presence of liquid water oceans during much of the evolution of the earth it has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the earth and the absence of well defined continental masses on venus supports this hypothesis it has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates plate evolution in time results in geometrical incompatibilities that require overlaps or gaps on the earth these incompatibilities lead to broad zones of continental deformation i e the western united states and china the absence of plate tectonics on venus suggests that rheologically soft continents are necessary for plate tectonics a reasonable conclusion is that the earth is the remarkable planet in terms of tectonics and volcanism not venus

seismic tomography and anisotropy in the source area of the 2008 iwate miyagi earthquake m 7 2 in order to understand the generation mechanism of the iwate miyagi earthquake m 7 2 that occurred on 14 june 2008 in northeast japan we determined high resolution 3d seismic velocity vp vs and poisson s ratio structures as well as p wave azimuthal anisotropy in and around the source area using 246 623 p wave and 189 153 s wave high quality arrival times from 1941 aftershocks of the 2008 iwate miyagi earthquake as well as 4878 shallow and intermediate depth earthquakes in this area recorded by the dense high sensitivity seismic network hi net and the seismic stations of tohoku university our results show that hypocenters of the mainshock and three large aftershocks are located in a boundary zone where both seismic velocity and poisson s ratio change drastically in a short distance a zone with pronounced low velocity and high poisson s ratio is revealed in the lower crust and uppermost mantle under the source area which may reflect the arc magma and fluids ascending from the upper mantle wedge the p wave azimuthal anisotropy is complex under the study area which may also indicate the complex crustal structure there these results indicate that the generation of the 2008 iwate miyagi earthquake was influenced by the ascending arc magma and fluids associated with the dehydration reactions of the subducting pacific slab under northeast japan

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

evidence for active hotspots on venus from analysis of magellan gravity data abstract the 500 myr average crater retention age for venus has raised questions about the present day level of tectonic activity in this study we examine the relationship between the gravity and topography of four large volcanic swells beta atla bell and western eistla regiones for clues about their stage of evolution the magellan line of sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation local compensation a flexural model with local and regional compensation of surface and subsurface loads and a hotspot model of compensation that includes top loading by volcanoes and subsurface loading due to a deep low density mass anomaly the coherence is also calculated in each region but yields an elastic thickness estimate only at bell regio in all models the long wavelengths are compensated locally the long wavelength estimates of the deep compensation depth for bell atla western eistla and beta regiones are 125 175 200 and 225 km respectively with an error of approximately 35 km the flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at bell regio and from other studies that model the topographic expression of flexure at bell regio and in other tectonic settings globally assuming a crustal thickness of 30 km the fit to the hotspot model at atla regio gives an effective elastic thickness of 30 5 km atla regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature at bell regio effective elastic thickness is 30 5 km at short wavelengths and 50 5 km at long wavelengths using a crustal thickness of 10 km the 30 km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced the 50 km value is believed to reflect the present day effective elastic thickness bell regio has a relatively shallow compensation depth 125 km and a weak bottom to top loading ratio determined from the coherence spectra these results may indicate a relatively old possibly inactive plume the data at beta and western eistla regiones are of relatively poor quality and do not permit detailed interpretations these areas are interpreted as active hotspots because of their large compensation depths greater than typical of the rest of the planet

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

on the trade off between seismic anisotropy and heterogeneity numerical simulations and application to northeast japan p wave tomography has been recently used to study 3 d azimuthal and radial anisotropy of subduction zones and continental regions however the fundamental issue about the trade off between the isotropic and anisotropic structures is still unclear in this study we investigate this issue systematically with comprehensive synthetic tests our results indicate that good ray coverage in the azimuth for azimuthal anisotropy and incidence for radial anisotropy is required for determining reliable anisotropic models the isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence we therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis i e the ratio of the short axis and long axis lengths for estimating the ray coverage quantitatively applying our novel approach to a large number of high quality arrival time data of local shallow and intermediate depth earthquakes we obtained new tomographic images of 3 d p wave azimuthal and radial anisotropy in northeast japan both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab our results show dominant trench normal and vertical fast anisotropy in the mantle wedge while trench parallel and horizontal fast anisotropy in the subducting slab which indicates different dynamics in different domains of the subduction zone

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

depth dependent viscosity and mantle stress amplification implications for the role of the asthenosphere in maintaining plate tectonics summary boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low viscosity asthenosphere the theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer the theory predicts that the confinement of horizontal mantle flow within a low viscosity sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel even if the absolute viscosity of the sublithosphere mantle does not change between the two cases thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow viscositychannelthickness if the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere then the plate normal stress is predicted to also increase with decreasing channel thickness we use 3 d spherical shell simulations of mantle convection with temperature depth and stress dependent rheology to test scaling trends the simulations and theoretical scalings demonstrate that a low viscosity layer asthenosphere can amplify convective stresses if the level of convective stress plays a role in maintaining and or reactivating plate boundaries this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics the numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active lid modeofconvectiontoastagnantlidstate collectively thesimulationsandtheoreticalscalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect associated with a low absolute asthenosphere viscosity but instead from a mantle flow channelization effect associated with a high viscosity contrast from the asthenosphere to the mantle below

recent deformation rates on venus constraints on the recent geological evolution of venus may be provided by quantitative estimates of the rates of the principal resurfacing processes volcanism and tectonism this paper focuses on the latter using impact craters as strain indicators the total postimpact tectonic strain lies in the range 0 5 6 5 which defines a recent mean strain rate of 10 18 10 17 s 1 when divided by the mean surface age interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 ma catastrophic resurfacing model if distributed tectonic resurfacing contributed strongly before that time as suggested by the widespread occurrence of tessera as inliers the mean global strain rate must have been at least 10 155 s 1 which is also typical of terrestrial active margins numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years parameterized convection models predict that the mean global thermal gradient decreases by only about 5 k km over this time even with the exponential dependence of viscosity upon temperature the surface strain rate drops by little more than one order of magnitude strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model an alternative uniformitarian resurfacing hypothesis holds that venus is resurfaced in quasi random patches several hundred kilometers in size that occur in response to changing mantle convection patterns under such a model the observed crater strain distribution indicates that about 1 of the planet s surface is tectonically active at any time however this model requires a very weak crustal rheology to achieve surface velocities 100 mm yr appropriate to the required patch size without well developed lateral weak zones venus is essentially a one plate planet but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement the net rate of tectonic activity is logarithmically intermediate between earth and mars about 100 times slower than plate tectonics but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

efficacy of the post perovskite phase as an explanation for lowermost mantle seismic properties constraining the chemical rheological and electromagnetic properties of the lowermost mantle d is important to understand the formation and dynamics of the earth s mantle and core to explain the origin of the variety of characteristics of this layer observed with seismology a number of theories have been proposed including core mantle interaction the presence of remnants of subducted material and that d is the site of a mineral phase transformation this final possibility has been rejuvenated by recent evidence for a phase change in mgsio3 perovskite thought to be the most prevalent phase in the lower mantle at near core mantle boundary temperature and pressure conditions here we explore the efficacy of this post perovskite phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post perovskite polymorphs of mgsio3 performed at lowermost mantle temperatures and pressures we show that a post perovskite model can explain the topography and location of the d discontinuity apparent differences in compressional and shear wave models and the observation of a deeper weaker discontinuity furthermore our calculations show that the regional variations in lower mantle shear wave anisotropy are consistent with the proposed phase change in mgsio3 perovskite

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

p wave anisotropic tomography beneath northeast japan we determined a 3 d p wave anisotropic tomography beneath northeast japan by using first p arrival times the travel time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming p wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally the geometries of the conrad and moho discontinuities and the upper boundary of the subducting pacific plate are taken into account in the model our extensive synthetic tests show that the anisotropic velocity inversion method is reliable our results show that low velocity zones exist below active volcanoes in the crust in the fore arc mantle wedge and in the central portion of the mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images anisotropic velocity variations are revealed in the crust mantle wedge and the subducting pacific slab the anisotropic patterns are complex and different in the upper and lower crust in the mantle wedge the fast direction of p velocity is generally trench parallel in the fore arc area while it becomes trench normal in the back arc area which agree well with the shear wave splitting results the trench parallel fast direction is revealed beneath the volcanic front which may suggest the existence of complex 3 d mantle flows in the mantle wedge the fast velocity direction in the subducting pacific slab is mostly n s which is likely due to the original fossil anisotropy of the pacific plate formed at the mid ocean ridge our result also suggests that the anisotropy is weak in the upper crust and the average amplitude of the velocity anisotropy in the mantle is no larger than 4

two dimensional anisotropic tomography of lithosphere beneath france using regional arrival times this paper presents an anisotropic tomography study beneath france travel times of pn waves were inverted in order to simultaneously retrieve lateral variations of both seismic velocity and seismic anisotropy within the subcrustal lithosphere the parameterization scheme uses a classical approach the subcrustal lithosphere is modeled using a two dimensional horizontal grid of nonoverlapping cells 0 5 0 5 each cell being characterized by an average slowness the isotropic term of slowness and an azimuthal anisotropy term the velocity and anisotropy perturbation amplitudes do not exceed 2 in the paris basin and in the aquitaine basin more significant perturbations can be observed in the tectonically active regions the images of the modeled subcrustal lithosphere emphasize the not recently reactivated areas of the hercynian range by high velocities 2 to 5 from the armorican massif to the west part of the massif central and from the northeast massif central to the vosges mountains and black forest mountains regions these areas are also characterized by an anisotropy of 2 5 with the fast directions parallel to the hercynian structures low velocities 2 to 4 are observed in the neogene volcanic areas of the massif central which confirm the results obtained from a short wavelength teleseismic tomography these low velocities are most probably caused by high temperatures another low velocity region 3 to 4 roughly follows the alpine belt and is interpreted as the image of the crustal root of the range in the inner part of the alps fast pn directions lie perpendicular to the belt with an anisotropy amplitude of 3 while in the outer part they are parallel to the range with amplitudes ranging from 3 to 5 to the east and from 2 to 3 to the west the anisotropy feature of the pyrenean range follows a simple pattern one observes high amplitudes up to 5 and a fast direction which is parallel with the strike of the range the observations show seismic anisotropy for most of the tectonically active zones across france recently formed structural units and old hercynian structures as well and the amplitudes reach the same order as the amplitudes of the velocity perturbations

novel anisotropic teleseismic body wave tomography code anitomo to illuminate heterogeneous anisotropic upper mantle part i theory and inversion tuning with realistic synthetic data

mechanisms for cessation of magmatic resurfacing on venus 1 proposed mechanisms of resurfacing on venus about 0 3 1 gyr ago usually involve either some form of tectonic resurfacing in which venusian lithosphere is recycled mechanically or magmatic resurfacing where essentially immobile lithosphere is covered by lava the focus of this study is mechanisms of magmatic resurfacing in the stagnant lid regime of mantle convection parameterized convection models suggest that cessation of magmatic resurfacing can occur in several ways 1 the mantle temperature drops sufficiently such that mantle rising adiabatically does not cross the solidus 2 the molten layer migrates below the solid melt density inversion at 250 500 km so that no melt can escape 3 sublithospheric small scale convection stops and conductive thickening of the lid suppresses melting in each case inability of magma to penetrate thickened venusian lithosphere may play a role the timing of melt cessation and duration of late stage resurfacing depend on various factors such as mantle rheology degree of mantle depletion and depth of resurfacing the models indicate that the waning stages of resurfacing which result in surface age variations can last 0 1 1 gyr a general trend of recent lithospheric thickening is also predicted some cases exhibit a rapid transition from relatively thin to thick lid as suggested by previous authors

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

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

small mantle upwellings are pervasive on venus and earth observations of venus and africa an analogous terrestrial plate indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both venus and earth because of their small size when they reach the lithosphere the upwellings affect the surface only under special circumstances on venus they form volcano tectonic structures called coronae where the lithosphere has been thinned by rifting on earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot spot reference frame as is africa the small upwellings presence throughout the mantle explains both the corona paradox in which coronae are associated with rifts on venus but do not appear to drive rifting and the basin and swell topography of the african plate the morphology of coronae requires that the small upwellings have some diapiric properties their pervasiveness transient nature and generally small size make them fundamentally different from traditional hawaiian style plumes and they may be detached diapirs within a mantle convecting in the hard turbulence regime

anisotropy and p wave tomography a new approach for inverting teleseismic data from a dense array of stations summary a new formulation is given for inverting teleseismic p wave residuals in terms of both 3 d variations of seismic velocity and anisotropy of the subcrustal lithosphere two restrictive assumptions are made on the anisotropic elastic tensor to reduce the number of model parameters first the upper mantle anisotropy is modelled by a preferred orientation of olivine crystals with b and c axes randomly distributed around the a axis and second the elastic properties of polycrystalline aggregates are computed according to the voigt averaging rule the latter assumption allows us to link linearly the observed p wave residuals to the proportion of oriented crystals in a realistic polycrystalline assemblage in the inversion process the orientation of the symmetry axis is fixed either by a trial procedure or by using a priori hypotheses from independent data a linear inversion scheme is then applied to the data to retrieve the 3 d variations of two scalar parameters giving the isotropic velocity perturbation and the amount of anisotropy the inversion algorithm is tested for different source receiver configurations using several idealistic ray geometries it is concluded that the teleseismic p wave residuals could potentially carry enough information for retrieving both the 3 d isotropic velocity variations and the amount of anisotropy however from a more practical point of view by looking at the distributions of rays for two recent tomographic experiments in the southern rhine graben area and in the pyrenees it is concluded that the distributions of rays in these experiments do not fulfil the necessary requirements for retrieving reliable information about the lithospheric anisotropy additional data are thus needed to constrain the anisotropic model if the symmetry axis is set as horizontal the inversion of the p wave residual of the pyrenees leads to an east west fast p wave velocity orientation the resulting anisotropy is compatible with sks observations made in the pyrenees whatever the anisotropy is the numerical tests performed with the actual ray configurations show the robustness of the isotropic components in the inverted model a long wavelength anisotropy in the lithosphere if present should not drastically alter the isotropic 3 d velocity distributions that are obtained from classical methods of teleseismic tomography