flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

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

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

the implications of basalt in the formation and evolution of mountains on venus abstract the highland region of ishtar terra on venus has mountains that reach up to 11 km in height and are thought to be basaltic in composition assuming that dynamic uplift of crust to this height is unlikely we examine the topography produced by an isostatically supported thickening basaltic crust it is found that regardless of whether the crust thickens by crustal shortening or by volcanic construction the high density basalt eclogite phase transition is the limiting factor for producing significant elevation of the mountains the maximum height attained by basaltic mountains depends on the nature of the basalt eclogite phase transition without a phase transition a basaltic crust must thicken to greater than 100 km to reach heights over 10 km an instantaneous phase transition of basalt to eclogite allows a maximum topographic height of less than about 2 km however with a time lag of 100 ma owing to slow rates of solid state diffusion our calculations show that the mountains can reach elevations greater than 10 km only if they are less than 25 ma old higher temperatures within the venusian crust may decrease the extent of the stability fields of high density basalt phases and allow high topography if the thickening crust melts this can occur if the radioactive element concentrations measured on the surface of venus are uniformly distributed throughout the crust the crust thickens to greater than 65 km and the thickened crust is older than about 400 ma the conflicting results of a young age predicted for high basaltic mountains and an almost uniform surface age of 500 ma from crater populations coupled with similarities in bulk physical properties of venus and earth suggest that the basaltic surface composition found at several landing sites on the planet may not be representative of the entire crust we suggest that ishtar terra formed from the collision of continent like highly silicic cratons over a region of mantle downwelling lakshmi planum resulted from the thickening of a basaltic crust and the peripheral mountain belts formed from the collision of granitic cratons that were pulled toward a downwelling region of mantle

isostatic compensation of ishtar terra venus we have used spherical harmonic representations of the venus topography and geopotential obtained from magellan data to evaluate isostatic support in several areas within the ishtar terra highlands including the lakshmi plateau its surrounding mountain belts namely akna and freyja and maxwell montes and the fortuna tessera province we find that topography in ishtar is largely isostatically compensated 80 regional geoid topography variations in the subregions can be explained by a combination of airy crustal thickening and thermal lithospheric thinning mechanisms provided venus has a thick reference thermal lithosphere 300 400 km with the exception of eastern fortuna low elevation areas h 4 km above mpr with small gtrs are almost certainly airy compensated via thickened crust relatively large 60 km total airy crustal thicknesses obtained in the western ishtar mountain belts together with a probable basalt eclogite phase change suggest a possible silicic composition for these structures provided they are older than 25 50 ma lakshmi planum seems essentially thermally supported with the thermal lithosphere thinned to 100 km we suggest as one possibility that the lithospheric thinning process under lakshmi is delamination of a dense eclogite lower lithosphere layer into the mantle the decrease in gtr observed in ishtar between lakshmi to the west gtr 20 m km maxwell and west fortuna gtr 8 m km and eastern fortuna gtr 4 m km may correspond to a decay in thermal compensation attributed to lithospheric delamination which would be fairly recent 100 ma in lakshmi partially decayed in west fortuna and absent in east fortuna where a mostly airy supported topography is essentially relaxed with no thermal uplift alternatively if surficial concentrations in radiogenic elements were prevalent throughout the crust partial melting of a thickened crust could account for the thermal uplift in lakshmi and west fortuna the zero elevation basaltic crustal thickness h 24 km obtained for the east fortuna tessera region may be representative of the ambient crustal thickness in the venus lowlands our findings support multicomponent models for tectonic and volcanic activity in ishtar the thick ambient crust and thermal lithosphere implied by this study agree with observational constraints such as support of extreme elevations large topographic slopes unrelaxed craters and the thick elastic lithosphere suggested by flexure studies if the ambient thermal lithosphere on venus were to be relatively thin 100 200 km with a cold mantle and radiogenic elements concentrated in the crust then thermal evolution on venus may be in quasi steady state with the geodynamic evolution in monotonic decline however if the ambient thermal lithosphere is very thick 300 400 km as suggested by our thermal model fits then it is consistent with the predictions of strongly unsteady state thermal evolution models and an interior which is currently heating up this would support the view that catastrophic resurfacing on venus might be episodic

gravity anomalies compensation mechanisms and the geodynamics of western ishtar terra venus pioneer venus line of sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western ishtar terra on various planes of altitude z0 the apparent depth of isostatic compensation at z0 1400 km is 180 20 km based on the usual method of minimum variance in the isostatic anomaly we seek to explain this observation as well as the regional elevation peripheral mountain belts and inferred age of western ishtar terra in terms of one of three broad geodynamic models under local mantle upwelling western ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction this model holds that the mountain belts form as a result of incipient downward mantle return flow an upwelling with its centroid at 160 km depth and buoyancy stress 100 mpa can satisfy the observed long wavelength geoid and topography provided that the crust mantle density contrast does not control the response of the crust so that late time subsidence does not occur and that the mantle viscosity does not increase markedly with depth mantle upwelling produces a combination of strike slip motion and azimuthal extension on the uplift itself radial compression is restricted to the surrounding plains under local mantle downwelling cold sinking mantle beneath western ishtar induces shear tractions on the crust which thicken and elevate it volcanism is potentially the result of basal crustal remelting the formation of mountain belts is assumed to be a by product of crustal thickening a deeply seated downwelling 300 700 km buoyancy stress 200 300 mpa can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists the predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations azimuthally oriented thrusts at the uplift margin incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid but the thermal gradient over downgoing flow must be at least as high as the expected mean value for venus 20 k km in order to attain substantial uplift within several hundred million years a third scenario regional compression may be distinguished from the second in that western ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields neither broad downward mantle flow nor lithospheric subduction need occur however both the source of stress and geoid anomaly are problematic if the topography of western ishtar is assumed to be airy compensated then the geoid signature must arise from unrelated mantle density anomalies in this case the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography a more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth we favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed western ishtar terra must be distinguished from other paradigm hotspots on venus by different environmental conditions or by being in a unique stage of hotspot evolution further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogeneous lithosphere are required

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

the gabbro eclogite phase transition and the elevation of mountain belts on venus maxwell montes standing up to 7 km above the adjacent highland plateaus constitute the highest mountain belt on venus because the thickness of the crust is likely to be limited by the gabbro garnet granulite eclogite phase transitions this relief is difficult to reconcile with the assumption of thermodynamic equilibrium and a standard airy isostatic model we explore the hypothesis that the crust mantle boundary is not in phase equilibrium but rather is rate limited by the temperature dependent volume diffusion of the slowest ionic species under the simplifying assumption that the mountains formed by uniform horizontal shortening of the crust and lithospheric mantle at a constant rate we solve the one dimensional thermal evolution problem the time dependent density structure and surface elevation are calculated by assuming a temperature dependent reaction rate and local airy isostatic compensation for a rate of horizontal strain of 10 15 s 1 or greater the rise in temperature at the base of the crust during mountain formation is modest to negligible the deepening lower crust is metastable and surface elevation increases as the crust is thickened for strain rates less than 10 16 s 1 in contrast crustal temperature increases with time because of internal heat production and the lower crust is more readily transformed to the dense eclogite assemblage for such models a maximum elevation is reached during crustal shortening while this maximum relief is 7 km or more for some models a smaller density contrast between crust and mantle than assumed here 500 kg m 3 and incorporation of horizontal heat transport would lessen this value we therefore favor formation of the mountain belt at a strain rate at least of order 10 15 s 1 by this reasoning maxwell montes must be comparatively young of order 50 ma

tectonics of lakshmi planum venus tests for magellan the origin of lakshmi planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of venus from the perspective of gravity signature and potential driving forces the mantle upwelling model is the simplest and its principal failure that it cannot produce radial shortening on the uplift periphery may be resolved if the lithosphere is laterally heterogeneous our preferred model consists of a hot mantle plume rising beneath a pre existing block of tessera the lithosphere is weakened at this hotter and presumably thicker crust and the outward near surface flow is attenuated at the peripheral discontinuity in lithospheric strength crustal thickening and mountain belt formation occur there we propose several criteria to test this tessera plume model together with its competitors at the higher resolution in both imaging and gravity afforded by the magellan mission

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

tectonic rotation of the santa ynez range california recorded in the sespe formation summary terrestrial claystone and siltstone in the sespe formation oligocene in the santa ynez range near santa barbara california record a palaeomagnetic field that predates the regional folding and satisfies a reversal test the mean declination of 80 1 closely matches the declination in miocene volcanic and sedimentary rocks studied by others in the western transverse block of which the santa ynez range is a part and the mean inclination of 38 9 is consistent with a hypothesis that the block was once about 10 south of its present latitude the combined palaeomagnetic data indicate that clockwise rotation of the block is entirely post oligocene the rotation might not haye commenced until the middle miocene or about 15 myr after northward motion of the block began

a heat pipe mechanism for volcanism and tectonics on venus a heat pipe mechanism is proposed for the transport of heat through the lithosphere on venus this mechanism allows the crust and lithosphere on venus to be greater than 150 km thick a thick crust and thick lithosphere can explain the high observed topography and large associated gravity anomalies for a 150 km thick lithosphere the required volcanic flux on venus is 200 km sup 3 yr this is compared with a flux of 17 km sup 3 yr associated with the formation of the oceanic crust on earth a thick basaltic crust on venus is expected to transform to eclogite at a depth of 60 to 80 km the dense eclogite would contribute the lithospheric delamination that returns the crust to the interior of the planet completing the heat pipe cycle topography and the associated gravity anomalies can be explained by airy compensation of the thick crust the principal observation that is contrary to this hypothesis is the mean age of the surface that is inferred from crater statistics the minimum mean age is about 130 ma and this implies an upper limit of 2 km sup 3 yr for the surface volcanic flux if the heat pipe mechanism was applicable on earth in the archean itmore would provide the thick lithosphere implied by isotopic data from diamonds less

on the tectonics of venus abstract the thermal evolution and mechanical properties of a mechanical boundary layer of mantle convection are calculated for three venus models cold earth like and hot with temperatures of 1300 c 1400 c and 1500 c respectively at the base of their thermal boundary layers the mechanical boundary layers consist of a basaltic crust with thicknesses of 3 km 9 km and 18 km and depleted periodotitic mantle with thicknesses of 37 km 65 km and 90 km respectively the thin crust of the cold venus model couples tightly to the underlying mantle and produces a single competent layer whereas the thicker crust of the other models has a weak lower part that decouples the crust from the mantle the characteristic wavelengths 10 20 km of the banded terrains of tesserae surrounding ishtar terra can be explained by the buckling of the crusts of all three venus models as long as their mechanical boundary layers are older than approximately 150 m a implying that the observed wavelengths provide no constraint on the thickness and age of the venusian crust that is older than approximately 150 m a shortening of the basaltic crust however cannot produce surface elevations higher than about 2 km on venus because basalt in the lower crust transforms to high density eclogite which sinks into the mantle therefore lakshmi planum and the surrounding mountains probably contain lower density material and are analogous to continental masses on the earth the ridge spacings of the northern ridge belt can be interpreted as being caused by faulting of the depleted mantle of the cold and earth like venus models if the mechanical boundary layer is older than about 100 m a and 200 m a respectively the hot model however cannot account for the formation of the ridge belt besides the characteristic wavelengths of the banded terrains and spacings of the ridge belts the cold venus model seems to account for many other features on venus the dynamic support of the surface topography of tesserae requires a convergence velocity of less than 0 1 cm year 1 for the mechanical boundary layer of the cold venus model this very low velocity is supported by the spatially random distribution of craters on venus furthermore the lack of pervasive volcanism on venus in approximately the last 500 m y the lack of an internal magnetic field of venus and the lack of an oceanic type ridge system on venus support the cold venus model

venus ishtar gravity anomaly ishtar has been detected as a positive gravity anomaly verification of this result has been made with two independent data sets isostatic compensation of the ishtar topography using an airy model is achieved at approximately 150km

isostatic compensation of equatorial highlands on venus abstract spherical harmonic models for venus global topography and gravity incorporating magellan data are used to test isostatic compensation models in five 30 30 regions representative of the main classes of equatorial highlands the power spectral density for the harmonic models obeys a power law scaling with spectral slope 2 brown noise for the topography and 3 kaula s law for the geoid similar to what is observed for earth the venus topography spectrum has lower amplitudes than earth s which reflects the dominant lowland topography on venus observed degree geoid to topography ratios gtrs on venus are significantly smaller than degree gtrs for uncompensated topography indicative of substantial compensation assuming a global airy compensation most of the topography is compensated at depths greater than 100 km suggesting a thick lithosphere on venus for each region considered we obtain a regional degree of compensation c from a linear regression of bouguer anomaly versus bouguer gravity data geoid anomaly n versus topography variation h data for each sample were compared in the least squares sense to theoretical correlations for pratt airy and thermal thinning isostasy models yielding regional gtr zero elevation crustal thickness h and zero elevation thermal lithosphere thickness y l 0 respectively we find the regional compensation to be substantial c 52 90 and the h n data correlations in the chosen areas can be explained by isostasy models applicable on the earth and involving variations in crustal thickness airy and or lithospheric thermal thinning thickness however a thick crust and lithosphere y l 0 300 km must be assumed for venus compensation mechanisms are distributed as follows for the high plateaus of ovda gtr 9 m km and thetis gtr 11 m km mainly airy isostasy with h 50 60 km for the chasmata area sample gtr 15 m km airy with perhaps a thermal component finally a very strong thermal component for the broad swells of beta gtr 32 m km and atla gtr 25 m km with the lithosphere thinned to 100 km aphrodite terra which shows a marked eastward increase in pratt and airy model parameter values could be a candidate for thermal isostasy decay from east to west

plate tectonics on venus the high surface temperature of venus implies a permanently buoyant lithosphere and a thick basaltic crust terrestrial style tectonics with deep subduction and crustal recycling is not possible overthickened basaltic crust partially melts instead of converting to eclogite because mantle magmas do not have convenient access to the surface the 40 ar abundance in the atmosphere should be low venus may provide an analog to archean tectonics on the earth

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

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

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

critical taper wedge mechanics of fold and thrust belts on venus initial results from magellan fold and thrust belts exist on venus at the margins of crystal blocks such as plateaus tesserae and coronae and as ridge belts within the plains these fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics a mechanics that is well known for fold and thrust belts and accretionary wedges on earth for example an analysis of fold geometry at the toe of the artemis chasma fold belt indicates fault bend folding above a regionally extensive decollement horizon at a depth of about 1 5 km near surface deformation on venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1 2 km critical taper wedge mechanics under anticipated venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10 20 which is similar to some estimated slopes in the steep parts of the fold belts the low taper toes of fold belts may be cohesion dominated toes on either brittle or plastic decollement horizons once the base of the wedge undergoes the brittle plastic transition the surface slope is expected to flatten to near horizontal in qualitative agreement with many topographic profiles of fold and thrust belts on venus the estimated depth of the brittle plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric temperature gradient the relief of fold belts measured between the toe of the wedge and the flat crest displays a remarkable linear dependence on absolute elevation figure 15 ranging from 6 km for maxwell montes at an elevation of 10 km to a few hundred meters at the lowest planetary elevations 0 to 2 km this remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle plastic transition possibly controlled by an isostatic coupling of elevation lithospheric thickness and geothermal gradient

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

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

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

new data on the composition structure and properties of venus rock obtained by venera 13 and venera 14 a study of the composition structure and properties of venusian rocks was carried out by venera 13 and venera 14 the main structural morphological types of provinces of the venus surface were chosen for the study the chemical and mineralogical composition of the rock at the venera 13 landing site shows that it belongs to a group of weakly differentiated melanocratic alkaline gabbroids at the venera 14 landing site the rock is similar to oceanic tholeiitic basalts a most interesting property of the surface rocks is the great number of the strata that were visible in panoramas of the landing sites measurements of their physical and mechanical properties show that venusian rocks have friable weakly cemented porous structures analysis of information from venera 8 9 10 13 and 14 led to the conclusion that the most likely global process responsible for the formation of the present rock is volcanic eruption moreover due to the high gas content of basaltic melts eruptions are of an explosive character leading to the formation of pyroclastic rocks summing up data obtained by venera enabled us to compare venus with the earth mars and the moon to point out some regular global features in the evolution of these bodies and to suggest some new problems for investigation

gravity over coronae and chasmata on venus abstract the global spherical harmonic model of venus gravity field mgnp60fsaap with horizontal resolution of about 600 km shows that most coronae have little or no signature in the gravity field nevertheless some coronae and some segments of chasmata are associated with distinct positive gravity anomalies no corona has been found to have a negative gravity anomaly the spatial coincidence of the gravity highs over four closely spaced 300 to 400 km diameter coronae in eastern eistla regio with the structures themselves is remarkable and argues for a near surface or lithospheric origin of the gravity signals over such relatively small features apparent depths of compensation adcs of the prominent gravity anomalies at artemis latona and heng o coronae are about 150 to 200 km the geoid topography ratios gtrs at artemis latona and heng o coronae lie in the range 32 to 35 m km 1 the large adcs and gtrs of artemis latona and heng o coronae are consistent with topographically related gravity and a thick venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae at arcuate segments of hecate and parga chasmata adcs are about 125 to 150 km while those at fatua corona four coronae in eastern eistla regio and an arcuate segment of western parga chasma are about 75 km the gtrs at fatua corona the four coronae in eastern eistla regio and the arcuate segments of hecate parga and western parga chasmata are about 12 to 21 m km 1 the adcs and gtrs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations instead they suggest compensation by thermally induced thickness variations in a moderately thick 100 km lithosphere alternatively the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting weighted linear least squares fits to gtr vs h long wavelength topography data from heng o and fatua coronae the four coronae in eastern eistla regio and the arcuate segments of hecate parga and western parga chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0 5 to 0 7 gravity anomalies at the arcuate segments of dali and diana chasmata that form latona corona at artemis chasma and other arcuate segments of parga and hecate chasmata occur on the concave sides of the arcs by analogy with gravity anomalies of similar horizontal scale 600 km several thousand kilometers on the concave sides of terrestrial subduction zone arcs which are due in large part to subducted lithosphere it is inferred that the gravity anomalies on venus are consistent with retrograde subduction at artemis chasma along the northern and southern margins of latona corona and elsewhere along parga and hecate chasmata

the tectonics of venus solid venus has several differences from solid earth a mild variation in topography with marked departures of some kilometres confined to less than 10 of the surface no interconnected system of ridges such as would be associated with a spreading lithospheric boundary layer a high correlation of gravity with topography and a ratio of gravity to topography implying compensation depths in excess of 100 km for major features this high admittance ratio implies a stiff upper mantle as also indicated by the high depth diameter ratio of craters the details of magellan radar imagery enable some inference of event sequences the morphology generally indicates a more regional scale of change than on earth the principal chronometer is impact crater distribution the number of impact craters identified is 914 which is equivalent to about 500 ma s infall of bodies big enough to penetrate the atmosphere but the number of craters per unit area has but slightly more variations from the mean than random and only a mild negative correlation with topographic elevation only one third of the craters evidence tectonic or volcanic modification hence the rate of resurfacing over the last few 100 ma must be quite slight compared to earth s the main debate is whether venerean tectonic activity is in monotonic decline or whether it is episodic on a 500 ma time scale evidencing some current tectonic and volcanic activity are a few limited regions of marked topographic and geoidal highs exceeding 8 km and 80 m respectively with steep slopes clearly plate tectonics does not exist on venus the underlying cause of this different evolution appears to be the lack of water this dryness makes the upper mantle stiff enough to regionalize the tectonics and inhibit recycling of crust the lack of water also prevents erosion and thus the recycling of secondary differentiates these deficiencies in recycling imply a strong net upward differentiation of heat sources and a thick crust now allowed by new results on dry diabase rheology

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

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

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

evolution of metamorphic volatiles during exhumation of microdiamond bearing granulites in the western gneiss region norway fluid inclusions in garnet kyanite and quartz from microdiamond bearing granulites in the western gneiss region norway document a conspicuous fluid evolution as the rocks were exhumed following caledonian high and ultrahigh pressure hp uhp metamorphism the most important of the various fluid mixtures and daughter minerals in these rocks are n2 co2 magnesian calcite n2 co2 ch4 graphite magnesian calcite n2 ch4 n2 ch4 h2o co2 and h2o nacl cacl2 nahcolite rutile also occurs in the n2 co2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions volatiles composed of n2 co2 magnesian calcite characterise the ambient metamorphic environment between hp uhp peak and early retrograde metamorphism during progressive decompression the mole fraction of n2 increased in the fluid mixtures as amphibolite facies conditions were reached ch4 and later h2o appeared in the fluids concomitant with the disappearance of co2 and magnesian calcite graphite is ubiquitous in the host lithologies and fluid inclusions thermodynamic modelling of the metamorphic volatiles in a graphite buffered c o h system demonstrates that the observed metamorphic volatile evolution was attainable only if the fo2 increased from c 3 5 0 3 to 0 8 0 3 log units relative to the fmq oxygen buffer external introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the fo2 the oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the caledonian orogeny