a migratory mantle plume on venus implications for earth a spatially fixed or at least internally rigid hotspot reference frame has been assumed for determining relative plate motions on earth recent 1 5 000 000 scale mapping of venus a planet without terrestrial style plate tectonics and ocean cover reveals a systematic age and dimensional progression of corona like arachnoids occurring in an uncinate chain the nonrandom associations between arachnoids indicate they likely formed from a deep seated mantle plume in a manner similar to terrestrial hotspot features however absence of expected convergent plate margin deformation suggests that the arachnoids are the surface expression of a migratory mantle plume beneath a stationary surface if mantle plumes are not stationary on venus what if any are the implications for earth

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

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

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

driving forces for limited tectonics on venus abstract the very high correlation of geoid height and topography on venus along with the high geoid topography ratio can be interpreted as local isostatic compensation and or dynamic compensation of topography at depths ranging from 50 to 350 km for local compensation within the lithosphere the swell push force is proportional to the first moment of the anomalous density since the long wavelength isostatic geoid height is also proportional to the first moment of the anomalous density the swell push force is equal to the geoid height scaled by g2 2 g because of this direct relationship the style i e thermal airy or pratt compensation and depth of compensation do not need to be specified and can in fact vary over the surface phillips 1990 showed that this simple relationship between swell push force and geoid also holds for dynamic uplift by shear traction on the base of the lithosphere caused by thermal convection of an isoviscous infinite half space mantle thus for all reasonable isostatic models and particular classes of dynamic models the geoid height uniquely determines the magnitude of the swell push body force that is applied to the venusian lithosphere given this body force and assuming venus can be approximated by a uniform thickness thin elastic shell over an inviscid sphere we calculate the present day global strain field using equations given in banerdt 1986 areas of positive geoid height are in a state of extension while areas of negative geoid height are in a state of compression the present day model strain field is compared to global strain patterns inferred from magellan derived maps of wrinkle ridges and rift zones wrinkle ridges which are believed to reflect distributed compressive deformation are generally confined to regions with geoid of less than 20 m while rift zones are found primarily along geoid highs moreover much of the observed deformation matches the present day model strain orientations suggesting that most of the rifts on venus and many of the wrinkle ridges formed in a stress field similar to the present one in several large regions the present day model strain pattern does not match the observations this suggests that either the geoid has changed significantly since most of the strain occurred or our model assumptions are incorrect e g there could be local plate boundaries where the stress pattern is discontinuous since the venusian lithosphere shows evidence for limited strain the calculation also provides an estimate of the overall strength of the lithosphere in compression and extension which can be compared with rheological models of yield strength versus depth at the crests of the major swells where evidence for rifting is abundant we find that the temperature gradient must be at least 7 k km

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

geological correlations with the interior density structure of venus a model has been constructed by stipulating that venusian long wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms for convenience the latter are represented by a single airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper mantle convection is represented at a single depth by mass perturbations that drive viscous flow we invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near surface density anomalies the mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings the near surface density anomalies show no particular pattern the inversion results have been compared with a global synthesis of magellan data impact crater density does not correlate with the mantle convection pattern or the near surface density anomalies large volcanic structures are associated with upwellings planitiae are associated with downwellings coronae are anticorrelated with high amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust tesserae show no definitive relationship to the present mantle convection pattern

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

coronae of mnemosyne regio morphology and origin abstract coronae are circular to elliptical structures 150 600 km across characterized in venera 15 16 and arecibo radar images by an annulus of concentric ridges the interiors of the structures are geologically complex displaying volcanic flows domes and radial concentric and or obliquely trending ridges and are generally distinct from the exterior the mnemosyne regio area to the west of ishtar terra contains seven coronae the largest cluster of coronae identified on venus on the basis of the morphologic and topographic characteristics of the coronae in the mnemosyne region we assess several possible models of corona origin using cross cutting and stratigraphic relationships we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae we find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae on the basis of these observations and stratigraphic relationships the following sequence of events is outlined for the formation and evolution of coronae in this region 1 regional volcanic and tectonic activity 2 localized uplift and volcanism caused by diapirism or hotspots 3 annulus and trough formation related to gravitational relaxation accompanied by further volcanic activity 4 continued local volcanic flooding of the corona annulus and trough differences in morphology between coronae reflect variations in amount and significance of these geologic processes coronae in other regions exhibit the same overall characteristics as the mnemosyne coronae and support a similar origin for the majority of coronae

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

the geomorphology and evolution of hecate chasma venus abstract hecate chasma is a discontinuous trough and fracture system extending from atla regio through asteria regio a detailed examination of hecate chasma using magellan image and altimetry data reveals morphologic features such as normal faults including graben compressional ridges a variety of volcanic edifices and coronae there is no evidence for a systematic age progression among coronae in chains thus a single stationary thermal anomaly underlying a moving lithosphere is an unlikely origin for the chains a comparison of the observed features with two possible models of evolution subduction delamination or lithospheric extension with limited rifting shows some similarities between the predicted morphologies of each model and hecate chasma the dominance of features related to upwelling and extensional tectonism strongly favor an extensional origin for this zone the corona chain may have been formed by upwellings related to rayleigh taylor instabilities similar to those observed at terrestrial mid ocean ridges but in an environment of limited spreading

evidence for retrograde lithospheric subduction on venus annular moats and outer rises around large venus coronae such as artemis latona and eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones on earth trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on venus accordingly it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large venus coronae while compensating back arc extension is occurring in the expanding coronae interiors similar processes may be taking place at other deep arcuate trenches or chasmata on venus such as those in the dali diana chasmata area of eastern aphrodite terra

features on venus generated by plate boundary processes various observations suggest that there are processes on venus that produce features similar to those associated with plate boundaries on earth synthetic aperture radar images of venus taken with a radar whose wavelength is 12 6 cm are compared with gloria images of active plate boundaries obtained with a sound source whose wavelength is 23 cm features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the artemis region of venus but are not clearly visible elsewhere the composition of the basalts measured by the venera 13 and 14 and the vega 2 spacecraft corresponds to that expected from adiabatic decompression like that which occurs beneath spreading ridges on earth structures that resemble trenches are widespread on venus and show the same curvature and asymmetry as they do on earth these observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of earth can also be applied to venus

coal facies studies in hungary a historical review the objective of this short summary is to provide an overview on the work of hungarian geoscientists who published their results mainly in hungarian language the aim of their studies was to understand coal formation and to make reconstruction on the different peat forming environments in addition to coal petrographical methods other investigation tools such as palynology sedimentology and organic geochemistry were applied to understand the formation accumulation and diagenesis of land derived organic matter in different geological realms and environments

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

sediment creep on slopes in pelagic limestones upper jurassic of northern calcareous alps austria abstract slump structures in upper jurassic pelagic limestones of the northern calcareous alps were studied using methods of ductile structural geology the early deformation observed cannot be explained by conventional models for slumping commonly describing slump complexes with an extending upper and contracting lower part instead the structures suggest distributed bedding parallel surficial stretching and folding localized by roll over normal faulting the initially isoclinal and recumbent folds are pervasively overprinted by stretching causing reorientation of fold axes into the downslope direction transport of folds separates antiformal and synformal hinges and the resulting isolated hinges resemble intrafolial folds in metamorphic terranes only folds formed late in the process of lithification allow insight in the early stages of fold evolution we suggest that this type of early deformation represents sediment creep that may be characteristic for slump structures in pelagic carbonates

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

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

tectonics and volcanism of eastern aphrodite terra venus no subduction no spreading eastern aphrodite terra a deformed region with high topographic relief on venus has been interpreted as analogous to a terrestrial extensional or convergent plate boundary however analysis of geological and structural relations indicates that the tectonics of eastern aphrodite terra is dominated by blistering of the crust by magma diapirs the findings imply that within this region vertical tectonism dominates over horizontal tectonism and consequently that this region is neither a divergent nor a convergent plate boundary

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

themis regio venus evidence for recent volcanism from virtis data abstract themis regio is interpreted to be a hotspot rise underlain by one or more mantle plumes many volcanic features in the themis region have high emissivity anomalies in venus express visible and infrared thermal imaging spectrometer virtis data other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent smrekar et al 2010a science 328 605 608 similarly at themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices consistent with the interpretation that they represent recent volcanism these volcanic features also have negative bouguer gravity anomalies indicating low density at depth consistent with hot mantle upwellings leading to pressure release melting in addition several volcanic features at themis have low emissivity anomalies suggestive of more evolved compositions combining geologic mapping data with gravity and emissivity data provide support that themis is an active hotspot swell with associated long lived volcanoes and coronae that are likely associated with small scale upwellings

anatomy of a venusian hot spot geology gravity and mantle dynamics of eistla regio eistla regio is a series of broad swells each up to a few thousand kilometers in diameter in the equatorial highlands of venus it is characterized by strong positive free air gravity anomalies shield volcanoes and rift systems we present a two part study of the western and central portions of this venusian hot spot first magellan radar images were mapped in order to understand the general geologic history of the region radial fracture systems both on the rises and volcanoes indicate uplift and associated faulting accompanied volcanic construction prominent fracture zones strike wnw to nw parallel to the long axis of the highlands the largest of these guor linea displays a progressive deformation history that may include minor clockwise rotation in addition to bulk nne ssw extension other regional structural patterns also suggest this sense of rotation the parts of eistla studied here appear to be structurally distinct from surrounding regions although volcanic radar bright plains and ridge and groove deformation to the west indicate an earlier more diffuse thermal uplift in the second part of our analysis pioneer venus line of sight accelerations were inverted for vertical gravity which when combined with topography were used to solve for mass anomalies on the crust mantle boundary and in the upper levels of the mantle convective system both western and central eistla appear to be sites of vigorous mantle upwelling a linear upwelling appears beneath guor linea indicating that rifting is actively driven by mantle flow minor crustal thinning may be present beneath western eistla and the northern plains whereas some thickening occurs beneath central eistla and the southern plains heng o corona in particular may be the site of a small shallow mass anomaly of either compositional thickened crust or thermal origin mantle return flow is asymmetrically distributed about eistla being much stronger to the south than the north and little flow is inferred at all to the ne in bereghinya planitia these differences introduce a clockwise rotation to finite strain in agreement with observations

coronae of parga chasma venus 1 parga chasma is a 10 000 km long fracture and trough system in the southern hemisphere of venus in this study we analyze coronae in the parga region addressing the relationship between corona and rift formation the relationship of volcanism to rifting and coronae and the overall evolution of the rift system our observations are compared with various models with the aim of determining any causative relationships between corona formation volcanism and rifting our statistical analysis shows that coronae are randomly located within this region indicating that corona locations are not significantly controlled by rifting or by lithospheric loading caused by the earlier formation of other coronae our observations suggest that the two processes of corona and rift formation were occurring more or less synchronously with no specific control on each other but that coronae and rifting in this region are genetically related we find a wide variation in the amount of volcanism associated with coronae across the parga region and no evidence for a deep seated plume or multiple plumes beneath parga chasma we suggest that the concentration of volcanic features in the beta atla themis region may actually result from three independent but overlapping broad zones of deformation corona formation and enhanced volcanism associated with the three rifts parga hecate and devana chasmata rather than one large zone of activity covering the whole of the region simultaneously

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