a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

axial topographic relief associated with ridge transform intersections the axial depths of mid ocean ridges are consistently observed to increase along strike from the central part of a ridge segment to its intersection with a transform fault variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment constrained by observations from 41 ridge transform intersections we investigate the relative effect on axial depths of transform offset segment length regional depth and spreading rate we first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length both scaled by spreading rate through a series of regression analyses covering a range of spreading rate scalings we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation we find that half segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault a second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis we can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common but not ubiquitous existence of a median valley there these results in combination with the segment relief analysis clearly indicate that the axial depth at a ridge transform intersection is a function of both long wavelength processes such as may control hot spot distribution and local tectonic factors including but not limited to transform offset

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

the global distribution of giant radiating dike swarms on venus implications for the global stress state magellan radar data of venus reveal 163 large radial lineament systems composed of graben fissure and fracture elements on the basis of their structure plan view geometry and volcanic associations at least 72 are interpreted to have formed primarily through subsurface dike swarm eraplacement the remainder through uplift or a combination of these two mechanisms the population of swarms is used to determine regional and global stress orientation the stress configura tion recorded from 330 210oe aphrodite terra is best ex plained by isostatic compensation of existing long wave length topography or coupling between mantle flow and the lithosphere the rest are correlated with concentrations of rifting and volcanism in the beta atla themis region the global stress field on venus is different than that of earth where plate boundary forces dominate from their surroundings where domical topography occurs the ra dius of the associated lineament system is on average 2 5 times greater than that of the dome some 53 of the radiat ing systems are associated with concentric structures ranging from central depression bounding scarps 25 km wide to tec tonic rings 575 km in diameter in 51 of these cases the ra dial pattern originates within but also extends significantly beyond the annulus only 9 of the radiating systems are confined completely leaving 40 located outside but focused upon a central annular structure finally the radial geometry is usually quite pronounced near the center of each structure 52 of the time radial lineaments occupy 270 o of azimuth and 80 exceed 180 o figure 2c in their distal regions how ever only 72 of the radiating systems retain a purely radial geometry as the remaining 28 gradually develop a non radial unidirectional lineament configuration all but seven of the radiating systems exhibit volcanism figure 2d lobate flows emerging from individual linea ments or many such flows strongly correlated with multiple radial lineaments occur for 45 of the systems clusters of small shields occur 72 of the time and in 75 of these in stances distinctive alignment of small shields and or pits along radiating lineaments is seen finally 65 are associ ated with some other form of centralized volcanism including multiple forms of edifice construction limited extrusions and sheet like flows which extend into the surrounding terrain

resurfacing history of venus the stratigraphy impact cratering record and geoid of venus indicate that a major change in surface geologic activity has occurred before 800 ma venus was dominated by horizontal movement and tectonic deformation of the crust a brief period of global volcanism subsequently filled in all low lying areas and left much of the surface at a nearly uniform elevation current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust this geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally

corona classification by evolutionary stage abstract the formation of coronae abundant volcano tectonic features on the surface of venus has been modeled as the interaction of mantle diapirs with the lithosphere however the applicability of this model to features of widely different sizes may not be evident from sar images due to the lack of a common scale we use the radius as a scaling parameter for both the distance from the center and relief obviating this problem and mapping the effects from coronae of different radii onto a single scale normalizing profiles for 394 features we find many features classified by e r stoffanet al 1992 j geophys res 97 13 347 13 378 as different types have very similar profiles and relief coronae distribute into three shapes domal circular calderic two shapes not related to coronae radial and volcanic remain variation between corona shapes is gradational and mirrors the evolution of diapirs from initial domal upwellings to calderas we test the implied age progression using impact crater populations within three corona radii despite the inherent uncertainties of this method crater statistics for a size restricted 275 325 km corona set suggest an age progression from domal through circular to calderic the calderic coronae have an associated impact crater population very similar to the global mean though with slightly too few tectonized craters implying that they approximate the average surface age and are no longer active the circular coronae have a low impact crater density and a slightly elevated proportion of tectonized craters and thus are currently active the domal coronae have fewer than the expected number of impact craters of which more are tectonized than expected suggesting that surface modification has begun

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

plume channeling as one possible mechanism for the origin of multiple coronae on venus a case study from the helen planitia quadrangle v52 1 coronae are large volcanotectonic features formed in response to plume diapir activity that have played an important role in the geodynamical evolution of venus coronae are classified in terms of their morphology one of the types is the so called multiple coronae models that account for multiple coronae formation include the following emplacement of several diapirs the movement of the lithosphere over a stationary plume and migration of a plume under a stationary lithosphere this work presents the study of a multiple corona located in the helen planitia quadrangle v52 oanuava corona and another corona unnamed formed in the vicinity of oanuava in order to test the viability of these models detailed structural mapping shows a migration of magmatic activity along the structure during corona formation and the development of a set of fractures in response to a local stress field the latter is used as a time marker for the evolution of oanuava and the unnamed coronae we propose that this migration could be explained as the result of a sublithospheric lateral flow and ponding of a mantle plume or diapir into a zone of uplifted and thinned lithosphere plume channeling induced by the interaction of both coronae

venus resurfacing by coronae implications from impact craters coronae common volcano tectonic features on venus may be actively modifying the planet s surface to evaluate the extent of their effects we compare impact crater densities and proportions of modified craters out to four radii for monte carlo simulations of corona location with those for two corona classes we study the radial concentric and concentric caldera coronae the two classes believed to represent the youthful and senescent stages of formation respectively a high proportion of tectonized craters and low crater density near radial concentric coronae indicate recent activity based on the average crater density and proportion of tectonized craters we interpret concentric caldera coronae as old these results show that tectonism and volcanism is not limited to the corona interior and can extend up to four radii from the center

venus coronae craters and chasmata the distributions of venus coronae and craters are related to chasmata which are thought to be extensional zones coronae are almost twice as dense near the chasmata as a random set of the same size of the various types of coronae the radial concentric and multiple are even more highly concentrated near chasmata whereas the concentric caldera type are absent near the chasmata craters to the first order are randomly distributed although when distributions are compared with random sets there is a deficit of about 15 20 craters close to the chasmata the tectonized and embayed craters tend to be near the rift zones and their distribution closely resembles that of the coronae the higher proportion of tectonized and especially embayed craters within 5 10 of the arcs is about the fraction that would be expected from modification of the full set of craters over a width of 100 200 km kolmogorov smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points this suggests that the volcano tectonic process creating coronae may be the same one destroying craters

neutrally buoyant diapirs a model for venus coronae coronae are typically circular features 100 600 km in diameter characterized by a deformed annular ring that is often topographically high the central region may be raised or depressed relative to the ambient elevation previous studies have proposed an evolutionary progression beginning with dome shaped features which have radiating extensional rifts followed by plateau shaped features which have both concentric deformation and radial extension ending with the classic corona as described above previous dynamical calculations of a rising and spreading diapir simulated the evolution from dome to plateau shaped topography but could not account for the raised rim or depressed central topography of some coronae we demonstrate that these features can be modeled by a diapir that spreads laterally at a depth of neutral buoyancy

the timing of giant radiating dike swarm emplacement on venus implications for resurfacing of the planet and its subsequent evolution stratigraphic study of a distributed population of 118 giant radiating dike swarms on venus reveals that within each dike intruded region emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera regional plains and most wrinkle ridges the density of impact craters superimposed on the swarm population 1 80 0 57 craters 10 6 km 2 when compared with the average global density 2 01 0 14 craters 10 6 km 2 and the densities reported for other geologic units is consistent with the observed stratigraphy on the basis of these data we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing the stresses recorded by the dike swarm population when combined with its age and compared with the predictions of several proposed resurfacing models best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long wavelength gravity and topography in addition the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long wavelength characteristics we interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago

correlation of venus surface features and geoid abstract the distributions of pairs of features on venus are compared using simple statistical tests we consider coronae types crater types and chasmata some of these are obviously associated and some are possibly associated coronae are grouped to represent evolutionary stages tectonic and volcanic modification of craters is considered and the chasmata are represented by five great circle arcs these arcs fit rift or extension zones at the 90 level totaling 55 000 km comparable to the length of spreading ridges on earth comparing these sets with venus geoid field we find that all the chasmata correlate positively with geoid highs and negatively with lows with the peaks in the geoid field occurring at the intersections of the arcs modified craters also exhibit this relation to the geoid the domal coronae the youngest and currently active may be sites of upwelling these correlate strongly with geoid highs and negatively with geoid lows whereas later stage coronae the majority do not show this correlation strong association of the youngest coronae with themis atla suggests that the most recent activity occurs along this arc

spatial and temporal relations between coronae and extensional belts northern lada terra venus preliminary studies of the distribution of coronae and volcanic rises on venus show that many of these features tend to cluster along zones of rifting and extension the plains north of lada terra are crossed by two such extensional belts each belt is composed of grabens ridges faults volcanic flows coronae and coronalike features the longer and more prominent belt is the nw trending alpha lada extensional belt which is over 6000 km long and 50 200 km wide and includes the coronae eve tamfana carpo selu derceto otygen and an unnamed corona south of otygen the second belt is the nne trending derceto quetzalpetlatl extensional belt which is about 2000 km long and in places over 300 km wide and includes the coronae sarpanitum eithinoha and quetzalpetlatl the two belts intersect at the 1600 600 km wide derceto volcanic plateau it is apparent that deformation along the two belts overlapped in time though deformation along the alpha lada extensional belt probably continued after the deformation along the derceto quetzalpetlatl extensional belt terminated in certain areas volcanism originated in grabens within the extensional belts whereas in other areas such as in eve selu derceto and quetzalpetlatl volcanism originated in the coronae and flowed into the lower parts of the extensional belts regional extension has affected the evolution of all the coronae at some stage of their development regional deformation occurred before the initiation of derceto and eithinoha and after the initiation of carpo tamfana otygen and sarpanitum it is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension and it is also unlikely that regional extension has been caused solely by the coronae no corona along the belts was formed subsequent to the cessation of the regional extension we therefore suggest that the regional extension and the coronae are interrelated some of the coronae may have determined the location of the surface expression of the regional extension whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

multichannel seismic evidence for anomalously thin crust at blake spur fracture zone a two ship wide aperture multichannel seismic reflection profile obtained across a small western north atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones reflections from the crust mantle boundary typically observed at about 2 s below the basement reflection rise to only 0 8 s beneath the fracture zone trough this corresponds to crustal thicknesses of perhaps as little as 2 to 2 4 km in the fracture zone less than half the thickness of normal oceanic crust this is the first time that moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset the existence of thin crust at both large and small offset fracture zones suggests that the magnitude of the thermal edge effect at ridge transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

tectonic patterns and regional stresses near venusian coronae abstract many venusian coronae exhibit distinctive patterns of faults in particular we find the tectonic patterns of one such corona similar to the striking patterns of the grabens surrounding alba patera on mars pursuing this similarity we perform a stress analysis analogous to one that successfully explained the alba patera pattern and in so doing determined a value for the local regional stress local stress fields of venusian coronae are assumed to be created by either a gaussian axially symmetric upwarp or a cylindrical load due to accumulation of volcanic materials on an elastic lithosphere the local coronal stresses are combined with varying amounts of a uniform regional stress field for different elastic lithospheric thicknesses from the magnitudes and orientations of the resultant principal stresses the predicted patterns of surface tectonic features i e graben and thrust fault chains are plotted these patterns are compared to those of coronae on magellan images matching the patterns thus determines the regional stress and elastic lithospheric thickness about the coronae comparison of predicted to observed tectonic patterns for three specific coronae leads to estimated regional stresses of approximately 0 1 0 6 kbar and effective elastic lithospheric thicknesses consistent with 10 5 km although one area may have had a lithosphere as thin as 2 km during corona formation

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

large scale volcanism associated with coronae on venus implications for formation and evolution large scale volcanism in the form of areally extensive flow fields is a previously unrecognized important aspect of the evolution of at least 41 of all coronae on venus the timing and scale of many coronae flow fields is consistent with an origin due to the arrival and pressure release melting of material in the head of a mantle plume or diapir the production of voluminous amounts of volcanism at some coronae is proposed to be the result of larger plume size and or the intersection of mantle upwellings with regions of lithospheric extension and rifting

subterranean microorganisms and radioactive waste disposal in sweden abstract in 1987 microbiology became a part of the swedish scientific program for the safe disposal of high level nuclear waste hlw the goal of the microbiology program is to understand how subterranean microorganisms will interact with the performance of a future hlw repository the swedish research program on subterranean microbiology has mainly been performed at two sites in granitic rock aquifers at depths ranging from 70 m down to 1240 m the stripa research mine in the middle of sweden and the aspo hard rock laboratory hrl situated on the south eastern coast of sweden some work has also been performed in cooperation with other national or international research groups in sweden canada and at the natural analogue sites oklo in gabon and maqarin in jordan the following conclusions are drawn there is a very high probability of the existence of a deep subterranean biosphere in granitic rock the documented presence of a deep biosphere implies that relevant microbial reactions should be included in the performance assessment for a hlw repository a hlw repository will be situated in a subterranean biosphere that is independent of solar energy and photosynthetically produced organic carbon the ultimate limitation for an active microbial life will be the availability of hydrogen as energy source over time and hydrogen has indeed been found in most deep groundwaters sulphide producing microorganisms are active in environments typical for a swedish hlw repository and the potential for microbial corrosion of the copper canisters must be considered the bentonite buffer around the copper canisters will be a hostile environment for most microbes due to the combination of radiation heat and low water availability discrete microbial species can cope with each of these constraints and it is theoretically possible that sulphide producing microbes may be active inside a buffer although the experiments conducted thus far have shown the opposite microorganisms have the capability to enzymatically recombine radiolysis oxidants formed by radiation of water it has earlier been concluded that the migration of radionuclides due to sorption on microorganisms can be neglected the influence of microbially produced complexing agents remains to be studied at realistic conditions in deep groundwater microorganisms have been found in natural alkaline groundwaters but it could not be conclusively demonstrated that they were in situ viable and growing rather than just transported there from neutral groundwater a possible hypothesis based on the obtained results from investigations of natural alkaline groundwaters is that fresh concrete may be a bit too extreme for active life even for the most adaptable microbe but this remains to be demonstrated