volcanism and resurfacing on venus at the full resolution of magellan sar data we examine the importance of localized volcanism in resurfacing on venus by analyzing the results of geologic mapping of a 12 degrees x 12 degrees area at the full resolution of magellan sar data resurfacing due to corona ridge and small volcano related volcanism accounts for 27 6 and 10 respectively of the mapped area mapping at the resolution of magellan data rather than a regional scale gives corona related flow unit areas that can differ individually by almost an order of magnitude with a total increase of 28 and more than three times as many identifiable units a total of 2919 small volcanoes or vents less than 10 km in diameter were identified in the f map with a mean diameter of 1 59 s d 1 08 km and densities of up to 36 small volcanoes per 50 km 2 taken together coronae ridge eruptions and small volcanoes probably make a significant contribution to resurfacing on venus citation grindrod p m e r stofan and j e guest 2010 volcanism and resurfacing on venus at the full resolution of magellan sar data geophys res lett 37 l15201 doi 10 1029 2010gl043424

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

the deep structure of venusian plateau highlands abstract magellan gravity data confirm that several of the large tectonically deformed plateau like highlands on venus are shallowly compensated most likely by crustal thickness variations apparent depths of isostatic compensation computed in the spatial domain range from 20 to 50 km for alpha tellus ovda and thetis regiones using a two layer model for isostatic compensation alpha tellus and ovda are best represented as nearly completely compensated in crust that is regionally 20 40 km thick around these highlands with little contribution from deeper mantle sources in contrast to these three areas a stronger regional gravity high associated with thetis requires a significant upper mantle component to compensation this is evident in the spectral admittance as a pronounced deep long wavelength anomaly in the two layer isostatic model a broad deeply compensated upland underlies a shallowly compensated central block of thetis if this deep component is interpreted as a thermal anomaly the loci of maximum upwelling agree well with sites of recent extension alternatively dense mantle could be responsible for the broad gravity anomaly although a different style of dynamic coupling is called for the plateau highlands are thus physiographically and isostatically equivalent to terrestrial continents though probably not compositionally they also share the record of a long tectonic history the large regional gravity anomaly of thetis indicates that active mantle processes continue even beneath some areas tessera thought to be relics of a former geological regime the excellent agreement of modeled crustal thicknesses around alpha tellus and ovda regiones suggests that 20 40 km is a representative global value for the plains such a crust is thicker than previously estimated and about twice as thick as the expected thickness of crust produced at venusian spreading centers if crust was produced mostly at spreading centers during global resurfacing the mantle potential temperature must have been 1800 k about 100 k hotter than predicted if hotspot activity dominated lateral variations in crustal production must be smoothed out by creep of the lower crust

prospects for an ancient dynamo and modern crustal remanent magnetism on venus abstract venus lacks an internally generated magnetic field today whether one existed in the past is unknown but critical to atmospheric evolution and potential habitability canonical models assume the core of venus is cooling too slowly for convection and thus a magnetic dynamo to occur today core mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread volcanic resurfacing however recent studies of impact craters and other surface features support more steady heat loss over geologic time precipitation of mgo and or sio2 from the core can also drive compositional convection even with slow cooling here we reevaluate the likelihood that venus has an earth like at least partially liquid and chemically homogeneous core using numerical simulations of the coupled atmosphere surface mantle core evolution an earth like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates i e 100 w m 1 k 1 if lower estimates like 40 50 w m 1 k 1 are actually correct then we favor recent proposals that the core has completely solidified or preserved primordial stratification any simulation initialized with a homogeneous liquid core predicts a global magnetic field with earth like surface strength for 2 3 billion years after accretion consistent with all available observations and also sporadic activity within the surface age while temperatures remain below the curie point of magnetite therefore future spacecraft missions should prioritize the first ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism

sequence of tectonic deformation in the history of venus evidence from global stratigraphic relationships analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of venus spanning the past several hundreds of millions of years we find that shortening characteristic of intensely deformed tessera terrain gave way to widespread distributed fracturing and extension within the tessera and early post tessera volcanic plains this phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following and sometimes simultaneous extension to form fracture belts emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains surfaces focused extensional deformation localized linear rift systems dominated the latest stages these major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of venus

effect of state of compensation on the relaxation of crustal plateaus on venus 1 building on nunes et al 2004 which examined the end member case of viscous relaxation of fully compensated crustal plateaus on venus we now investigate viscous relaxation for the other end member case of uncompensated topography the approach consists of modeling viscous flow in a crust mantle system by means of analytic and finite element methods differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle loss of topography occurs via fast subsidence due to isostatic adjustment and a slower lateral flow due to spreading of the thickened crust rims arise from the isostatic adjustment and rim dimensions depend on the thickness of the competent layer controlled by the thermal state of the system rim dimensions after 1 myr are 1 km high and 300 km wide for a surface temperature of 740 k and thermal gradient of 10 k km and 600 m high and 100 km wide for hotter crustal conditions although models predict the marginal circumferential hoop folding observed at crustal plateaus they do not account for the large radial grabens also observed adding a subcrustal zone of time dependent buoyancy causes a delay in the isostatic adjustment during this period of delay viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on venus consequently phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support lateral variation in heat flow and thickness of the competent layer may help address the latter and needs to be explored through both lithospheric and mantle convection models

sequential deformation of plains at the margins of alpha regio venus implications for tessera formation abstract the boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment where the tessera are stratigraphically older than the plains previous studies show that 3 of these boundaries display evidence of tectonic tilting after the emplacement of the plains one of these unusual boundaries is the western margin of alpha regio tessera a zone 100 km in width that separates the plains from the interior structures of alpha this zone is characterized by margin parallel fine scale 1 5 km fractures graben and ridges that truncate and postdate the broad scale 10 30 km ridges and troughs of the interior of alpha the western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to alpha regio the earliest deformation of the plains consists of northeast trending graben 1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of alpha northwest trending graben then formed over an interval marked by the emplacement of two additional plains units their similarity to northwest trending structures emanating from eve corona and the lada terra rift suggests a possible genetic relationship the tilting of the plains adjacent to western alpha implies relative vertical movement of the margin either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of alpha regio subsidence of plains at this locale is supported by the presence of a basin to the west of alpha surrounded by a fracture belt contiguous with western alpha thus the fractures and deformation at the western boundary of alpha may be related to the formation of a basin to the west of alpha with some influence from the northernmost extension of the lada terra rift such a basin is not present at a section along the eastern boundary of alpha regio where the origin of tilted plains remains equivocal we conclude that the deformation along the western margin of alpha regio is not directly related to the process of tessera formation but is an example of tessera modification and is consistent with the stratigraphic position of tessera as the oldest unit observed on venus

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

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

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

internal dynamics condition centennial scale oscillations in marine based ice stream retreat this paper was supported by a natural environment research council consortium grant briticechrono ne j008672 1

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

insights into corona formation through statistical analyses 1 statistical analysis of an expanded database of coronae on venus indicates that the populations of type 1 with fracture annuli and 2 without fracture annuli corona diameters are statistically indistinguishable and therefore we have no basis for assuming different formation mechanisms analysis of the topography and diameters of coronae shows that coronae that are depressions rimmed depressions and domes tend to be significantly smaller than those that are plateaus rimmed plateaus or domes with surrounding rims this is consistent with the model of smrekar and stofan 1997 and inconsistent with predictions of the spreading drop model of koch and manga 1996 the diameter range for domes the initial stage of corona formation provides a broad constraint on the buoyancy of corona forming plumes coronae are only slightly more likely to be topographically raised than depressions with type 1 coronae most frequently occurring as rimmed depressions and type 2 coronae most frequently occurring with flat interiors and raised rims most type 1 coronae are located along chasmata systems or fracture belts while type 2 coronae are found predominantly as isolated features in the plains coronae at hot spot rises tend to be significantly larger than coronae in other settings consistent with a hotter upper mantle at hot spot rises and their active state

venus timing and rates of geologic activity we employ the accumulation of the impact crater population as a geochronometer to examine the observable record of the history of venus 1 b y analyzing age relationships between craters and the different geologic units we show that the earlier suite of units from heavily deformed tesserae through slightly deformed regional plains occurred during a time period an order of magnitude shorter than the subsequent period from the end of emplacement of the wrinkle ridge network until the present these results imply high global rates of endogenic volcanic activity during the first era comparable to that of mid oceanic ridge volcanism of earth and much lower global rates of endogenic activity by two orders of magnitude for the second period

a stratigraphic study of small volcano clusters on venus recently proposed global stratigraphy and geologic history models have led to two end member volcanic evolution models for venus a directional model that proposes venus has progressed through a series of stages each characterized by a particular style of volcanic activity a t basilevsky and j w head 1997 j geophys res 103 8531 8544 and a nondirectional model that proposes a more earth like type of volcanic activity with different eruption styles occurring globally on a local scale throughout the recent history of the planet j e guest and e r stofan 1999 icarus139 55 66 the former infers that most small volcanoes d 20 km define a global unit referred to as the shield plains that predates emplacement of the widespread plains with wrinkle ridges or regional plains to test this hypothesis cross cutting and superposition relations were used to determine the stratigraphic positions of small volcano clusters in seven quadrangles 11 of the surface the seven quadrangles included 179 clusters containing over 22 000 constructs three materials were defined for each cluster 1 the constructs 2 the material immediately surrounding the constructs called the cluster material and 3 the regional plains where possible relative ages were established between the constructs cluster material and the regional plains there are six possible temporal relations between the constructs and regional plains emplacement 1 the constructs postdate regional plains emplacement 2 some constructs postdate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 3 the constructs predate regional plains emplacement 4 some constructs predate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 5 the constructs both predate and postdate regional plains emplacement and 6 the ages of the constructs relative to regional plains emplacement are ambiguous an age relative to regional plains emplacement could not be determined for almost half of the clusters studied of the clusters for which a relative age could be determined almost half postdate regional plains emplacement and an additional third contain at least some constructs that postdate regional plains emplacement less than a tenth of the clusters predate regional plains emplacement these observations indicate that many small constructs postdate the emplacement of the regional plains and therefore are consistent with the nondirectional volcanic evolution model in which small constructs represent a global process occurring locally throughout the recent history of venus

magnetic properties of serpentinized peridotites from the dongbo ophiolite sw tibet implications for suture zone magnetic anomalies magnetic properties of a suite of variably serpentinized peridotites from the dongbo ophiolite sw tibet china have been investigated to determine the magnetic signatures of suture zones the degree of serpentinization s for these peridotites is mainly in the range of s 60 and s 20 30 serpentinized peridotites indicating that peridotites with such degrees of serpentinization contribute to the aeromagnetic anomalies within the yarlung zangbo suture zone in south tibet

internal crater modification on venus recognizing crater centered volcanism by changes in floor morphometry and floor brightness bright floored and dark floored craters on venus show systematic differences in their size distribution and apparent modification bright floored craters exhibit the following characteristics an interior radar brightness comparable to the youngest craters on venus a tendency toward smaller crater diameters and a broad range of crater elevations in contrast the dark floored craters are darker than pristine craters on venus are typically much larger and preferentially occur at lower elevations they also have larger floors than pristine craters of the same size and are similar in many respects to floor fractured craters on venus of the four proposed origins for dark crater floors these observations are most consistent with crater centered volcanism surface weathering or eolian deposition may contribute to floor darkening in some cases but neither of these mechanisms nor impact melts can independently explain the full range of observed modifications based on the measurements of this study volcanism has affected at least 6 13 of the impact craters on venus the estimated lava fill thicknesses are of the order of 100 500 m thus if these flows are representative of eruptions elsewhere in the venusian lowlands the implied average flux of plains forming volcanism on venus for the last 300 m y is 0 01 0 02 km3 yr

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

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

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

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

catastrophic resurfacing and episodic subduction on venus abstract there is clear evidence that a catastrophic resurfacing event occurred on venus in the relatively recent past a primary data source for this resurfacing event is the spatial distribution of impact craters in this paper we apply the pair correlation technique to the observed crater distribution and find that the result is identical to that for a random distribution in order to test the sensitivity of the technique we also apply it to the spatial distribution of coronae on venus for the coronae we find substantial deviations from a random distribution one explanation for the catastrophic resurfacing is the episodic subduction hypothesis we model episodic subduction using a thermal boundary layer stability analysis we find that episodic subduction events with intervals of 500 to 700 myr can transport only 15 25 of the radiogenic heat produced within the planet we suggest that the remainder of the heat must be lost to the surface during a period of vigorous tectonic activity following the subduction event but prior to the subsequent stabilization of a global lithosphere

ejecta correlations with spatial crater density and venus resurfacing history we examine the alternative hypotheses that on venus i the spatial randomness of the crater population reflects predominantly the spatially stochastic nature of impact crater formation or ii a significant amount of spatial variation in crater density is due to variations in the geological rate of crater removal three endmember models of resurfacing are formulated case i assumes that a single age production surface accounts for the spatial distribution of craters case ii assumes that variations in spatial crater density reflect local production surface ages while case iii assumes that individual craters are removed regionally so that variations in spatial crater density reflect local retention ages these endmember models are tested by examining correlations of crater properties with crater spatial density we analyze the distribution of embayed and tectonized craters and the relative abundances of extended ejecta deposits termed halos the three endmembers provide specific predictions as to the occurrence of halos as a function of for 1 5 craters per 106 km case iii provides the best explanation for the deficiency of halos as well as the relative abundances of modified craters for the interval 1 5 2 5 case ii is applicable but with only a modest spread in production ages for densities 2 5 case ii is the only endmember model that matches the global observations suggesting that regions with these high densities represent a series of distinct production surfaces with ages increasing from the planetary mean retention age

artemis venus the largest tectonomagmatic feature in the solar system new geologic mapping reveals that artemis a unique 2400 km diameter feature on venus is much larger than previously recognized including a wide outer trough 5000 km diameter a radial dike swarm 12 000 km diameter and a concentric wrinkle ridge suite 13 000 km diameter artemis s evolution included formation of its interior and chasma accompanied by lateral propagation of radial dikes the escape of dike magma to the surface formed local cover deposits that buried parts of the remaining radial fracture suite cover deposits are cut in turn by wrinkle ridges that likely formed by coupling of convective mantle flow with the lithosphere the outer trough formed late relative to radial fractures cover deposits and wrinkle ridges we suggest that artemis represents the magmatic signature of a deep mantle plume acting on relatively thin lithosphere as such it appears to represent the largest tectonomagmatic feature in the solar system the newly recognized vast extent of artemis holds implications for the formation of giant radial dike swarms wrinkle ridge formation terrestrial planet mantle lithosphere coupling and venus s surface and geodynamic evolution

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

ribbon spacing in venusian tessera implications for layer thickness and thermal state 1 wereportonmeasurementsofcharacteristicextensional wavelength represented by ribbons in venusian tessera terrain fourier power spectra for ribbons in 35 areas from 9 geographic regions show dominant wavelengths of 2 to 6 km we used these values to estimate mechanical layer thickness during ribbon formation at 0 6 to 2 9 km because ribbons accommodate extension of a single strong layer overlyingaductilesubstrate weconcludethatthebaseofthis mechanical layer corresponded to the local brittle ductile transition bdt during ribbon formation maintaining a bdtat 3 km depth for a significant length of time requires a locally hot environment as over a plume impinging on thin lithosphere these results indicate that locally hot conditions prevailed at widely distinct locations in the past index terms 6295 planetology solar system objects venus 8015 structural geology local crustal structure 8010 structural geology fractures and faults 5475 planetology solid surface planets tectonics 8149 8159 tectonophysics evolution of the earth rheology crust and lithosphere citation ghent r r and i m tibuleac ribbon spacing in venusian tessera implications for layer thickness and thermal state geophys res lett 29 20 2000 doi 10 1029 2002gl015994 2002

limits on modes of lithospheric heat transport on venus from impact crater density we present a formalism by which the size frequency distribution of impact craters on venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years the principal assumptions are that an upper bound on the rate of crater production on venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims the impact crater density reported from venera 15 16 observations if valid for the entire venus surface then indicates a mean volcanic flux no greater than 2 km y corresponding to a maximum average rate of resurfacing of about 4 km b y the fraction of global heat loss due to such a rate of volcanic resurfacing is negligible for the lowest estimated crater retention age of the surface imaged by venera 15 16 if globally representative the rate of lithospheric recycling on venus does not exceed 1 5 km y corresponding to 25 of the global heat loss more likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10 these results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the venus interior

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

surface formation rates and impact crater densities on venus impact crater density has been used to estimate relative ages of major venus geologic units typically based on the assumption of short globally synchronous formation periods the total number density of craters is the only parameter which may be quantified for most venus subregions in contrast to the size frequency analysis often used for other planetary studies this paper examines the statistical limits on such crater counts and the predicted crater density for various possible time varying surface formation rate functions these results show that 1 the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units and 2 any given crater density can be accommodated by a very short pulse of unit formation at some age t gaussian distributed formation functions of mean age t exponential functions with initiation ages between t and 2t or a host of other more complicated rate functions these results when coupled with the unknown rate of crater removal by tectonic or volcanic activity indicate that the crater counts for specific landforms on venus offer little constraint on the relative timing or duration of their development the hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data

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

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

tessera terrain and crustal plateaus venus many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onion skin on venus a growing body of structural mechanical magmatic gravitational topographic and geologic evidence indicates that tesserae record the local interaction of individual deep mantle plumes with an ancient globally thin venusian lithosphere resulting in local regions of thickened crust

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

venus s shield terrain plains planitiae or lowlands expanses of gentle long wavelength 1000 km basins cover 80 of venus s surface these regions are widely accepted as covered by volcanic fl ows although the mechanism s responsible for resurfacing remains elusive in addition a volcanic origin for the lowland surface may be open to question lowland resurfacing is typically attributed to catastrophic emplacement 10 100 m y of globally extensive thick 1 3 km fl ood type lava this model of resurfacing has been postulated on the basis of impact crater distribution taken together with a lack of obvious volcanic fl ows or edifi ces and a lack of viable alternative models ongoing geologic mapping of 15 000 000 km2 0 25n 90 150e using 225 m pixel and 75 m pixel nasa magellan sar synthetic aperture radar data indicates that small edifi ces called shields 1 15 km diameter edifi ces 1 km high play a major role in lowland resurfacing individual shields are radar smooth or rough quasi circular to circular features with or without a central pit shield shapes which have been previously documented range from shield dome or cone to fl at topped or fl at shield deposits typically coalesce forming a thin regionally extensive but discontinuous mechanically strong layer herein called shield paint shield paint conforms to delicate local topography providing evidence of its thin character and indicating generally low viscosity during emplacement shield terrain shields and shield paint covers more than 10 000 000 km2 within the study area detailed mapping of fi ve 2 2 regions using coregistered normal and inverted right and left illumination sar imagery indicates shield densities of 3500 33 500 shields 106 km 2 thus the map area hosts more than 35 000 335 000 shields shield terrain generally postdates but is also locally deformed by fractures and wrinkle ridges indicating time transgressive formation relative to local deformation and or reactivation the regional scale crust was strong throughout shield terrain formation shield terrain may extend across much of venus s surface

relaxation of compensated topography and the evolution of crustal plateaus on venus 1 crustal plateaus dominant physiographic features on venus likely originate through dynamic mantle processes although a debate exists on whether they formed by mantle upwellings or downwellings regardless of the mode of formation several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high standing plateau to a low standing plateau with elevated margins we apply analytic and finite element models to test this hypothesis for isostatically compensated topography as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots geotherm values 5 k km 1 combined with a surface temperature of 740 k preclude relaxation within 109 years while geotherm values 20 k km 1 can yield relaxation times of 108 years or less hence significant relaxation requires hot conditions in order to occur within the appropriate 1 gyr timescale set by crustal plateau ages we also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance mantle temperatures 1400 k allow strong crust mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength thus preserving the original topographic shape during relaxation in either case relaxation of compensated plateaus does not yield elevated rims we propose that the state of compensation must be considered variable

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

structures in tessera terrain venus issues and answers many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onionskin on venus because tesserae are exposed mostly within crustal plateaus which exhibit thickened crust issues of tessera distribution and the mechanism of crustal plateau formation e g mantle downwelling or upwelling are intimately related a review of magellan data indicates that tessera terrain does not form a global onionskin on venus although ribbon bearing tesserae reflect an ancient time of a globally thin lithosphere individual tracts of ribbon bearing tessera terrain formed diachronously punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600 2500 km diameter regions above hot mantle plumes plume related magmatic accretion led to crustal thickening at these locations resulting in crustal plateaus crustal plateau surfaces record widespread early extension ribbon structures and local minor perpendicular contraction of a thin competent layer above a ductile substrate within individual evolving crustal plateaus the thickness of the competent layer increased with time and broad gentle folds formed along plateau margins and short variably oriented folds formed in the interior late complex graben cut folds local lava flows accompanied all stages of surface deformation in contrast to these conclusions gilmore et al 1998 summarized post magellan arguments in favor of downwelling models for crustal plateau formation in light of this discrepancy we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models we show that gilmore et al 1998 did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid they disregarded shear fracture ribbons thus invalidating their criticism of ribbon models they misunderstood previous radargrammetric work that constrains ribbon geometry and they relied solely on geometrical relations to constrain timing violating kinematic analysis methodology their stratigraphic constraints on ribbon fold temporal relations are invalid because they 1 misinterpreted implications of map relations 2 did not isolate radar artifacts due to local radar slope effects from proposed material units 3 chose a region for analysis that clearly shows the effects of younger tectonism and volcanism and 4 presented map relations that cannot be reproduced their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre magellan and post magellan upwelling models these misconceptions about the upwelling model and processes responsible for global warming phillips and hansen 1998 lead to serious errors in gilmore et al s 1998 criticism furthermore we show that the data of gilmore et al 1998 are actually more consistent with upwelling than downwelling models consistent with arguments that tessera terrain is not global in spatial distribution

climate change as a regulator of tectonics on venus tectonics volcanism and climate on venus may be strongly coupled large excursions in surface temperature predicted to follow a global or near global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation this sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of venus the most widely preserved volcanic terrain on the planet

geologic constraints on crustal plateau surface histories venus the lava pond and bolide impact hypotheses 1 venusian crustal plateau formation is hotly debated crustal plateaus are large 1500 2500 km quasi circular 0 5 to 4 km high plateaus that host distinctive tectonic fabrics called ribbon tessera debate centers on plateau support and fabric formation detailed geologic mapping of an 360 000 km2 region eastern ovda regio provides critical clues for plateau evolution ribbon tessera fabrics record broadly synchronous layer contraction extension and flooding as an initially strong thin layer 100 m developed across ovda as the layer underwent ductile shortening and brittle extension subsurface lava flooded structural lows with progressive deformation the layer thickened leading to formation of progressively longer wavelength structures early formed shorter wavelength structures and lava filled valleys were carried piggyback on longer wavelength folds this history indicates an extremely large strength contrast that is inconsistent with a conductive geotherm reflecting instead the presence of a near surface molten layer i propose the lava pond hypothesis ribbon tessera fabrics represent progressively solidified scum of a huge lava pond leading to crustal plateau formation i further propose that individual lava ponds formed as a result of large bolide 20 30 km diameter impact with ancient thin lithosphere causing massive partial melting in the upper mantle melt rose buoyantly to form huge 1500 2000 km diameter lava ponds completely resurfacing the local area in the upper mantle depleted residuum would be compositionally more buoyant and stronger than adjacent undepleted mantle isostatic adjustment resulted in pond uplift and plateau formation subsequent secular cooling could freeze a plateau in place or mantle convection could strip away the residuum root leading to plateau collapse

implications from stereo derived topography of venusian impact craters using radargrammetry we have created high resolution topographic maps of 74 venusian craters including all bright floored craters over 12 km in diameter covered by magellan stereo imagery our trend for rim floor depths rf as a function of diameter d for bright floored craters in the volcanic plains is rf 0 345 0 05 d0 235 0 05 and for dark floored craters in the plains rf 0 5 0 1 d0 0035 0 07 rim heights rh for bright floored craters in the plains are rh 0 06 0 03 d0 4 0 2 d 15 km and for dark floored craters in the plains rh 0 027 0 015 d0 4 0 2 these trends indicate that bright floored craters 30 km in diameter are on average deeper than dark floored craters by 180 m from rim to floor and have a 140 m higher rim and at 90 km in diameter they are 380 m deeper from rim to floor and have a 220 m higher rim the bright and dark floored populations are different at a 99 confidence level for both rim floor depths and rim heights the interpretation most consistent with our data and previous work by others is that venusian craters with radar dark floors have been partially filled and had their ejecta blankets embayed by regional scale lava flooding when the topography is examined in conjunction with the imagery it is clear that many dark floored craters have been surrounded by lavas that rose nearly to the crater rim even though a substantial portion of the crater s ejecta blanket was retained most previous analyses of the resurfacing history of venus have relied on past interpretations that only a small percentage of venusian craters are embayed by exterior volcanism because most craters on venus have dark floors our data indicate that the majority of venusian craters have been surrounded and partially filled by postimpact lavas and consequently those previous analyses may have significantly underestimated the amount of volcanism on the venusian surface over the past few hundred million years rim floor depths for venusian craters are consistent with the inverse gravity trend observed for the terrestrial planets and they are 50 deeper than current estimates for complex craters on the earth unlike the other terrestrial planets neither terrain floor depths nor central structure heights increase with increasing crater diameter an interesting trend for which we have no explanation is that on venus the moon mars and ganymede central peaks generally rise to within a constant elevation relative to the surrounding terrain but that elevation is lower on the moon and mars than on venus and ganymede

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

tessera terrain ribbon fabrics on venus reviewed could they be dyke swarms abstract endogenic and exogenic models have been developed over the past 25 years to explain the development of extensive pervasive high aspect ratio troughs and graben like structures ribbons and short to long wavelength folds in tessera terrains commonly associated with crustal plateaus on venus many of the models are complex and appear to be both internally contradictory and inconsistent with well established geological principles they are based on what amount to assumptions regarding i the secular evolution of crustal thickness ii crustal mechanical homogeneity and isotropy iii the mechanics of ribbon formation and iv the relative timing of ribbons in the structural geological sequence of tessera terrains on the basis of both theoretical and observational considerations this contribution will suggest that coupling the formation of tessera ribbons to the emplacement of dyke swarms relatively late in the structural development of tessera terrains may provide a resolution for the apparent inconsistencies in those published models

evidence for rapid and widespread emplacement of volcanic plains on venus stratigraphic studies in the baltis vallis region geological mapping of the area around baltis vallis the longest 6 800 km canali type channel on venus has led to the determination of stratigraphic relations which appear to be consistent over this entire region comprising 20 x 106 km 2 or 5 of the total surface of venus the most abundant unit in the study area plains with wrinkle ridges pwr consists of two subunits separated by the formation of baitis vailis a feature which is interpreted by most workers to have formed essentially geologically instantaneously the wrinkle ridges deform both subunits of pwr while ridge belts are embayed by the pwr plains consistent age relationships between the pwr subunits wrinkle ridges baltis vallis and ridge belts along the entire 6 800 km channel length is evidence that unit pwr is a real stratigraphic marker unit and not a mosaic of plains emplaced at significantly different times at least in the 5 of the surface of venus comprising the study area the baltis stratigraphic sequence was compared with stratigraphic relations found in earlier studies of thirty six 1 000 x 1 000 km sites randomly distributed over the surface of venus basilevsky and head 1995 and was found to be comparable this supports the interpretation that these stratigraphic units represent generally globally quasi synchronous geologic events rather than the products of geologic activity in different areas of the planet at different times stratigraphic complexes and a number of structures typically formed at several different periods during the observed geologic history this model may correspond to either of two different options for the geologic history of venus 1 the proposed stratigraphic units and equivalent geologic events were generally synchronous over the whole planet or 2 the proposed stratigraphic sequence represents typical sequences of local events which occurred in different places at different times the first option is consistent with the global resurfacing model and the second is consistent with the equilibrium resurfacing model in our previous observations we never saw a case of superposition of one local stratigraphic sequence on another one with repetition of all or part of the sequence of units which would seem to be inevitable at the boundaries of neighboring local activity zones if the second scenario is correct on the basis of this we concluded that the first option consistent with the global resurfacing model agrees better with the observations than the second one however the validity of this conclusion remains tentative for other areas and requires additional testing stratigraphic studies in the area of baltis vallis is a further step in this direction because of the potential for the use of canali type channels as stratigraphic time markers

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

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

the geologic history of venus a stratigraphic view on the basis of regional and global stratigraphic analyses we outline the major events in the geologic history of venus determined by photogeological study of surface features because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved the stratigraphic record presented comprises only the last 10 20 of the total history of venus the estimated range of the mean crater retention age of the surface from 200 to 1600 million years leads us to describe the timing and duration of different events in terms of fractions of the mean surface age t the beginning of the observed history of venus was characterized by intensive tectonic deformation of global or semi global scale which formed the tessera terrain termination of the compressional stage is estimated to have occurred at about 1 4t while the tensional stage lasted for another 0 1 0 2t after tessera formation several stages of extensive volcanism occurred burying vast areas of tessera and forming what are now observed as regional plains the combined duration of the emplacement of these plains is estimated to be about 0 2 0 3t with an implied average global rate of volcanism of a few cubic kilometers per year regional plains forming materials can be subdivided and are separated from each other and from underlying and overlying units by unconformities these unconformities are formed from oldest to youngest by tessera forming deformation dense fracturing broad ridging and finally wrinkle ridging these tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression then tension then again compression and finally tension the last global scale tectonic episode extensive wrinkle ridging happened at about time t which was very close in time to the emplacement of the most areally abundant plains unit this marked the transition to the present stage of the history of venus which is characterized by a predominance of regional rifting and related volcanism this stage appears to have lasted from about time t to the present making it the longest time duration among the stratigraphic units considered although the resulting tectonic and volcanic features and deposits cover only 10 20 of the surface of venus these observations mean that the general intensity of tectonics and the flux of volcanism a few tenths of a cubic kilometer per year in this latest period were much lower than those in earlier times in summary the morphologically observable part of the history of venus was characterized by two key characteristics that stand in contrast to the comparable period of earth history approximately the phanerozoic when global geodynamic processes were dominated by plate tectonics 1 venus shows no signature of plate tectonics instead its global tectonic environment passed from an initial dominance of compression through tension then again compression and finally tension with the density of deformational structures and the strain rate declining with time 2 in the beginning of this period of time on venus plains forming volcanism occurred at a rate comparable to volcanism at mid ocean ridges but was emplaced in an entirely different style the predominant component of volcanism on earth during this time was the extrusive volcanism at mid oceanic ridges for the last few hundred million years venus has been dominated primarily by rift associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on earth

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

venus resurfacing rates constraints provided by 3 d monte carlo simulations the range of surface ages on venus and the rate of recent volcanism are two major questions that can be addressed through an analysis of magellan images we report on a 3 d monte carlo resurfacing model that seeks to explain the resurfacing history of venus by simulating the evolving surface under the competing processes of impact cratering and volcanism by incorporating the observed size distribution of volcanic forms on venus and the assumptions of constant volcanic and impactor fluxes we seek a solution that is consistent with the observed impact crater population in the monte carlo simulations a volcanic flux of 0 37 km3 yr operating for 550 m y on initially craterless surfaces yields surfaces that are statistically similar to venus it is shown that the statistics of the impact cratering record are inconsistent with an equilibrium resurfacing model

tectonic effects of climate change on venus venusian plains regions are commonly crossed by small strain deformation features such as wrinkle ridges polygonal terrains and gridded terrains of these polygonal terrains are observed to have a relatively uniform spacing and are widely distributed on plains that formed during global resurfacing models of venusian climate that assume resurfacing occurred through massive volcanic events suggest that surface temperatures could have dramatically changed over nearly a billion years propagating thermal stresses into the surface and potentially causing small strain features we investigate this hypothesis by approximating the temperatures predicted by climate models as a step function and employ a fixed plate and strength envelope model to predict the resulting depth of failure and amount of strain our calculations indicate that strains due to temperature changes of 50 100 k which are favored for volcanic resurfacing events of 1 10 km thickness are consistent with the observed spacing of polygonal and gridded terrains as well as some wrinkle ridges the global nature of the climate change event is consistent with the global distribution and uniformity of such features and implies that such terrains may be a global stratigraphic marker

resurfacing styles and rates on venus assessment of 18 venusian quadrangles abstract we have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles about 30 of the venusian surface each quadrangle was split into 0 5 by 0 5 boxes which were then identified as corona materials large volcano materials 100 km diameter intermediate volcano materials 10 100 km small edifice materials 2 large edifices resurface areas of 10 4 10 5 km 2 coronae have greatly varying amounts of associated volcanism with some coronae producing negligible flow deposits and others producing deposits of 10 4 10 6 km 2 the areas identified as plains with no visible source occur on small scales 10 2 km 2 to large scales 10 5 km 2 our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source that fields of small edifices contribute more to resurfacing than we had anticipated and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles all of the units that we quantified occur throughout the histories of the regions mapped we favor plains resurfacing to have occurred over at least 100 myr which implies terrestrially reasonable resurfacing rates

magellan a new view of venus geology and geophysics within the past fifteen years the surface of venus has gone from being the least well understood of all the terrestrial planets to the most thoroughly mapped surface of any terrestrial planet including the earth this is primarily due to the magellan mission which has collected a variety of data on the surface morphology physical properties and interior density structure of venus amounting to more than 1 terabit 1012 bits of data synthetic aperture radar images have been obtained for over 95 of the surface their high resolution reveals most surface features larger than 100 200 meters across using its radar altimeter magellan has collected data on surface elevations sub meter scale roughness and radar reflectivity at a resolution of approximately 10 km further information on the physical properties of the surface was gathered by measuring the passive microwave emissivity of the surface pettengill et al 1992 two way doppler tracking of the spacecraft has yielded line of sight los gravity data and a spherical harmonic model of gravity and geoid out to degree 75 collection of high resolution gravity data has been aided by an innovative aerobraking maneuver which used venus atmosphere to brake the spacecraft and lower it from a highly elliptical orbit to a near circular orbit

venus records a rich early history the distribution and character of venus s impact craters led to the widely accepted idea that venus underwent global catastrophic resurfacing ca 500 ma and thus venus records only a short history encompassing surface evolution since postulated catastrophic resurfacing ribbon tessera terrain rtt a structurally distinctive unit represents some of venus s old est surfaces and is widely accepted as forming prior to postulated global catastrophic resur facing we constructed a global geologic map of rtt unit exposures and structural trends using national aeronautics and space administration nasa magellan data map relations illustrate that rtt displays planet scale patterns that together with altimetry record a rich geologic history that predates proposed global catastrophic resurfacing

ribbon terrain formation southwestern fortuna tessera venus implications for lithosphere evolution abstract the term tessera has been used to describe regions of deformed venusian crust exhibiting two or more intersecting sets of structural elements however tessera includes terrains formed by a variety of spatially and temporally discrete tectonic processes tessera fabric characterizes highland plateau structure and thus understanding the nature of this deformation is critical to understanding the mode of highland plateau formation many tessera fabrics include ribbons folds and late graben in this paper we refine the geometry of ribbons through geologic mapping and radargrammetric analysis of type ribbon structures at southwestern fortuna tessera we extend our findings to ribbon fabrics at thetis regio any model of ribbon formation must account for the following constraints on ribbon geometry a ribbon forming lineaments exhibit sharp contrasts relative to adjacent materials b ribbon bounding lineaments form a distinct pattern alternating between radar dark and radar bright which represent trough walls oriented away from and toward the satellite respectively c ribbons form long narrow troughs that alternate with parallel narrow ridges ridges and troughs display extreme length width aspect ratios d trough walls are near vertical e troughs are shallow with consistent shallow depth along individual troughs and in adjacent troughs f in some cases e g southwest fortuna tessera trough walls are parallel and matched and would exhibit a close fit if the trough was closed in these cases trough walls merge laterally forming v shaped terminations trough floors are smooth and flat lacking small scale interior lineaments g in other cases e g thetis regio ribbons display a d but trough walls are defined by a series of subparallel lineaments including local interior lineaments and trough floors ramp up to join trough walls displaying parallel rather than v shaped terminations trough walls would not display a close fit if closed we propose two member types of ribbons tensile fracture ribbons and shear fracture ribbons tensile fracture ribbons display features a f and formed by the opening of tensile fractures of a thin brittle layer above a ductile substrate they require a near fracture free shallow crust and very shallow depth to the brittle ductile transition bdt the presence of ribbon structures within highland plateaus favors an upwelling model for highland plateau formation in which crustal thickening results from magmatic underplating related to a mantle upwelling or mantle plume in order for the plume to be able to anneal mechanically the crust as required by ribbon formation the lithosphere would likely have to be quite thin these implications are consistent with highland plateaus as an ancient signature of mantle plumes on thin lithosphere whereas volcanic rises which are presently thermally supported reflect thick lithosphere phoebe regio represents a transitional lithospheric thickness

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

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

structural and kinematic analysis of eastern ovda regio venus implications for crustal plateau formation crustal plateaus on venus constitute one of that planet s most intriguing tectonic features they host so called tessera terrain or deformed crust exhibiting two or more sets of intersecting tectonic lineaments which comprises 8 of venus surface major questions surrounding crustal plateaus and tessera terrain include 1 what is the nature of crustal plateau deformation what structures are present and what are their spatial and temporal relations 2 what is the global distribution of tessera terrain does it form a global layer under the regional plains cropping out in crustal plateaus or is it confined to crustal plateaus and 3 what are the implications of 1 and 2 for understanding the processes responsible for formation and deformation of crustal plateaus structural and kinematic analyses reveal that deformation at ovda regio venus largest crustal plateau is characterized by linear shallow steep sided troughs ribbons long arcuate long wavelength folds short mid wavelength folds and wide complex graben temporal relations between structures lead to a three phase regional strain history consisting of 1 layer normal flattening of a thin competent membrane resulting in radially oriented distributed extension ribbon formation 2 modest contraction of a thicker layer concentric folding at plateau margins interference folding in the plateau interior and 3 limited extension localized at fold crests graben formation wavelength analysis of ribbons and folds places limits on the thickness of the competent layer during each stage of deformation and reflects an early shallow brittle ductile transition bdt that deepened with time ovda s deformation topography and gravity are all spatially correlated strongly suggesting a genetic relationship between surface deformation and processes causing crustal thickening and plateau uplift thus tessera terrain at ovda regio is likely confined to the topographically high plateau and does not extend beneath the surrounding regional plains similar patterns persist at other crustal plateaus indicating that tessera terrain is in general confined to crustal plateaus and does not form a globally continuous layer ovda s structures their spatial and temporal relations the resulting strain history and the behavior of the bdt through time support a model of mantle plume upwelling in a thin lithospheric regime for crustal plateau formation and contradict previous downwelling models similarities between structural and kinematic trends at ovda regio and other crustal plateaus lend support to this idea c 1999 academic press

style and sequence of extensional structures in tessera terrain venus recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation specifically if tessera are formed by contractional deformation followed by extensional deformation or vice versa a major question centers on the interpretation of specific lineaments within tesserae as graben bounded by faults 60 or alternatively open tension fractures dipping 90 we document and assess the origin of extensional structures in tesserae at several locations on venus noting the morphology continuity along strike parallelism of walls stratigraphic position and interaction with other structures and variability due to radar viewing geometry in each study area our analyses demonstrate that 1 the extensional structures have variable widths interior subparallel lineaments and ramp terminations 2 ridges and lineaments are continuous across the troughs where the floors of many of these structures contain the lowered sections of preexisting structures and 3 intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures we conclude that the morphology of these extensional structures is consistent with an origin as graben not open tension fractures and that these graben postdate the ridges in each study area both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick to further test the tension fracture model we examine the conditions of a venus that could produce tension fractures of the dimension 1 km width of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 k km 1 heat flows of 800 to 3000 mw m 2 are required for a range of diabase rheologies and strain rates thought typical of venus during tessera formation such a thermal structure would favor partial melting at depths 1 km dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism mitigating against the preservation of the closely associated tension fractures this volcanism is not observed both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model on the basis of our analysis of tesserae we conclude that the ridges formed first in response to large scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on earth such as tibet and the altiplano

comment on the global resurfacing of venus by r g strom g g schaber and d d dawson the distribution of impact craters on venus has been thesubject of a great deal of analysis since the return of magellandata phillips et al 1992 performed monte carlo two dimen sional 2 d modeling of the areal distribution of craters and theresults of that exercise allowed a restricted but still quite large range of possible planetary resurfacing histories including thepossibility that the craters were emplaced on a geologicallyinactive planet however the nonrandom distribution ofembayed and deformed craters phillips et al 1992 the hyp sometric distribution of craters herrick and phillips 1994 thevaried degradation states of craters lzenberg et al 1994 theirnonrandom distribution with different geologic terrain types namiki and solomon 1994 price et at 1994 and three dimensional resurfacing modeling bullock et al 1993 all seemto argue against that particular possibility in contrast strom etal 1994 have collected a refined and more comprehensive dataset of impact features and they input these data into moresophisticated 2 d monte carlo modeling and statistical analysesof the areal distribution of craters the hypsometric distributionof craters and the number of embayed craters they concludedthat venus experienced a global resurfacing event about 300m y ago followed by a dramatic reduction of volcanism and tec tonism this global resurfacing event ended abruptly 10m y the present crater population has accumulated since thenand remains largely intact only about 4 6 of the planethas been volcanically resurfaced since the global event ifthese conclusions are well founded this work certainly repre sents a significant advancement in restricting the number ofplausible resurfacing histories for the planet if strom et al 1994 are correct it would also mean that all of the otheraforementioned works are in error to various degrees or at leastrepresent overzealous interpretation of the data however wehave identified apparent flaws in the observations modeling and interpretations presented by strom et al 1994 that lead usto question whether their conclusions are warranted we limitour comments to three areas of their analysis 1 observationspertaining to the number and area of disrupted and pristine cra ters and crater related features 2 modeling of the areal andelevation distribution of craters and 3 interpretations of resur facing models copyright 1995 by the american geophysical union paper number 95je02293 0148 0227 95 95je 02293505 00

the frequency area distribution of volcanic units on venus implications for planetary resurfacing the areas of volcanic units on venus have been measured on the 1 5000000 geological maps published by nasa usgs these data were used to obtain a frequency area distribution the cumulative frequency area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit 30 106 km2 to the smallest 20 km2 the probability distribution function has been calculated the medium and large volcanic units correlate well with a power law fractal relation for the dependence of frequency on area with a slope of 1 83 there are fewer small units than the expected values provided by the power law relation our measurements cover 21 02 of the planetary surface 3 59 of the study area was found to be tessera terrain and is excluded from this study of volcanism the measurements were restricted to areas where geological maps have been published the analysis was performed on two independent areas of the planet with a complete coverage of published maps in both areas the largest volcanic unit covers a significant portion of the surface 58 75 and 63 64 respectively for the total measured volcanic units excluding tessera these two largest units that could correspond to the same unit or not cover the 61 18 and they are stratigraphically superimposed on older volcanic units which cover 3 37 of the area the remaining area 35 45 is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit s these results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of venus and different criteria for geological mapping despite this fact the presence of these very large units is incompatible with the equilibrium resurfacing models because their generation at different ages would destroy the crater randomness our frequency area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit s followed by a decay of volcanism our data for the frequency area distribution of volcanic units provide new support for catastrophic resurfacing models it is difficult to make our observations compatible with equilibrium steady state resurfacing models

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

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

nonuniform cratering of the terrestrial planets article history we estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets bottke w f morbidelli a jedicke r petit j m levison h f michel p metcalfe t s 2002 icarus 156 399 433 after determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination the impact positions are calculated analytically assuming the projectiles follow hyperbolic paths during the encounter phase as the source of projectiles is not isotropic latitudinal variations of the impact flux are predicted the calculated ratio between the pole and equator is 1 05 for mercury 1 00 for venus 0 96 for the earth 0 90 for the moon and 1 14 for mars over its long term obliquity variation history by taking into account the latitudinal dependence of the impact velocity and impact angle and by using a crater scaling law that depends on the vertical component of the impact velocity the latitudinal variations of the cratering rate the number of craters with a given size formed per unit time and unit area is in general enhanced with respect to the equator the polar cratering rate is about 30 larger on mars and 10 on mercury whereas it is 10 less on the earth and 20 less on the moon the cratering rate is found to be uniform on venus the relative global impact fluxes on mercury venus the earth and mars are calculated with respect to the moon and we find values of 1 9 1 8 1 6 and 2 8 respectively our results show that the relative shape of the crater size frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study nevertheless by neglecting the expected latitudinal variations of the cratering rate systematic errors of 20 30 in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method

pulsating continents on venus an explanation for crustal plateaus and tessera terrains we propose that tessera terrains on venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events we have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model in the proposed model the ratio between the crustal and lithospheric mantle thicknesses controls the force balance if the crust thickness is less than 2 5 of the lithospheric mantle thickness the continental area will be compressed but if the crustal thickness is higher than 2 5 of the lithospheric mantle thickness the continental area will spread out and collapse consequently if the lithospheric mantle beneath a continental region is delaminated during a global subduction event the continent will collapse generating tessera inliers dominated by extensional tectonics but if a significant portion of lithospheric mantle remains then the continental area will be compressed generating a plateau by crustal shortening the observed plateau heights can be explained by this model a 2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a 4 km height plateau can be generated by a 90 km thick lithospheric mantle we have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a newtonian viscosity for the crust the force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in 50 ma using a viscosity for the continental crust of 1021 pa s and 200 ma for 5 1021 pa s during this compressional stage concentric fold and thrust belts are generated in the plateau continent erasing any impact craters that were present the subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau continent accommodated by radial grabens the pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains

finite element modeling of short wavelength folding on venus implications for the plume hypothesis for crustal plateau formation 1 the key geological observations and structural interpretations associated with the current plume hypothesis for venusian crustal plateau formation are that 1 short wavelength spatially periodic tectonic structures originated as mechanical instabilities in a regionally extensive surface brittle layer whose basal boundary was thermally generated and controlled 2 characteristic wavelengths of spatially periodic structures record the brittle layer thickness at the time of deformation and 3 structural wavelengths increased with time driven by brittle layer thickening in response to cooling the plume hypothesis accommodates these constraints by proposing that crustal plateaus formed above mantle plumes impinging on thin lithosphere it further suggests that surface temperatures temporarily elevated to 1000 k may have been required to maintain a sufficiently thin brittle layer for formation of the shortest wavelength structures we report here on finite element simulations designed to test the feasibility of the proposed thermal conditions specifically we model formation of short wavelength folds thought to have initiated as contractional layer instabilities early in the plateau formation process under high surface temperatures the finite element meshes develop semibrittle zones in which short wavelength folds can form but development of even modest structural relief requires unrealistically high total mesh shortening thus elevated surface temperatures inhibit development of short wavelength folds because the models effective integrated mechanical strength under such hot conditions is excessively low decreasing the surface temperature increases structural relief but produces tectonic wavelengths that are larger than those observed we conclude that a model with solely thermal control of mechanical properties cannot explain the observed structures