how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

novae on venus geology classification and evolution 1 novae are radially fractured centers 100 300 km in diameter and 64 have been identified on venus dense radial fracturing and upraised topography are common and massive amounts of volcanism are seen in some novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement we analyzed the topography and character of deformational and morphological structures of all 64 novae leading to a better understanding of their evolution and relation to diapir dynamics and their age we subdivided novae into four topographic classes 1 upraised 2 annular 3 flat and negative and 4 plateau like novae are located mostly in areas of regional rises and rift zones with a small number in the lowlands about one half the novae studied show association with rifts in some of these the rift troughs surround nova rises and form plateau like novae plateau like novae may predate postdate or form simultaneously with rifts our analysis generally confirms and updates previous models of novae evolution with the following geological factors being important 1 depth of the neutral buoyancy level of the rising mantle diapir 2 rheological characteristics of the overlying lithosphere 3 whether or not the visco plastic material above the diapir undergoes spreading and 4 the nature and influence of regional stress and rift structures these combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova such as topography tectonics and volcanism we found clear evidence for 1 the important role of dike swarm emplacement in the formation of radial patterns of novae and 2 the formation of concentric tectonic features during the relaxation stage of novae novae have multiple evolutionary stages and are long lived structures stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units we found that 40 3 of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11 3 completed their activity before this time 88 7 of the population of novae was active after regional plains formation in contrast to novae coronae activity was greatest before formation of regional plains which may be due to thickening of the lithosphere with time detailed structural analysis shows that novae evolution does not always lead to the formation of corona like features

loki io a periodic volcano loki is the most powerful volcano in the solar system it has been observed to be in continuous though variable activity since 1979 synthesis of more than a decade of groundbased data suggests that loki eruptions are cyclic with a 540 day period application of a simple lava cooling model to temperatures in loki patera and eruption start and end times implies that brightenings are due to a resurfacing wave propagating across the patera the data are most consistent with lava lake overturn but resurfacing by lava flows cannot be ruled out a porosity gradient in the lake crust could cause lava lake overturn to occur periodically on the timescale observed

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

a mesozoic carbonate hot spring deposit in the hartford basin of connecticut abstract limestone at coe s quarry in north branford connecticut is characterized by a variety of cross cutting discontinuous textural facies the environmental significance of most textural features is ambiguous but cellular tufa and abundant spherulite strongly suggest a boiling hot spring setting the cellular tufa consists of a thin polygonal framework of micritic calcite which surrounds and interconnects irregular elongate voids the void space is filled with spherulites and locally derived carbonate detritus cemented by sparry calcite and dolomite spherulites nucleated and grew in suspension to diameters of 0 1 mm or more before settling in the cellular tufa and other nearby depositional settings spherulites may form distinct geopetal fabrics in the pores of the cellular tufa dem nstrating that some indistinct layering in tufa had high primary dips other important depositional facies are algal bacterial tufa micritic and banded travertine and siliciclastic limy sandstone all units are abruptly discontinuous laterally and are also found as intraclasts mixed in with other facies the spherulites indicate rapid precipitation of carbonate from highly supersaturated bicarbonate waters formed by flash boiling of co2 rich water the cellular textures are similar to microterrace deposits and encrusted organic grains and mats in modern hot springs saddle dolomite which is an important early cement suggests temperatures of at least 60 c we infer that the limestone was deposited from a hot spring which periodically flash boiled in violent geyserlike eruptions the talcott basalt which immediately underlies the limestone is in places totally replaced by ferroan carbonates we infer that the same circulating groundwater heated by deeper intrusive equivalents of the talcott basalt locally altered the basalt and reached the surface at hot springs like the one from which the limestone at coe s quarry precipitated

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

boundary layer theory and subduction numerical models of thermally activated convective flow in earth s mantle do not resemble active plate tectonics because of their inability to model successfully the process of subduction other than by the inclusion of artificial weak zones here we show using a boundary layer argument how the rigid lid style of convection favored by thermoviscous fluids leads to lithospheric stresses which may realistically exceed the yield stress and thus cause subduction to occur through the visoc plastic failure of lithospheric rock an explicit criterion for the failure of the lid is given which is sensitive to the internal viscosity r below the lid for numbers appropriate to earth s mantle this criterion is approximately r a 10 21 pa s

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

central eistla regio origin and relative age of topographic rise large topographic rises on venus pose a number of fundamental questions concerning the mantle processes responsible for them eistla regio a 7500 km long elevated region extending wnw from the west end of aphrodite terra is a particularly interesting example the sappho patera quadrangle 0 25 n 0 30 e includes all of central eistla regio a region dominated by two interesting apparently hybrid structures irnini and anala montes with characteristics of both shield volcanoes and coronae mapping and tectonic analysis within this region indicate that this part of the rise experienced progressive deformation extending from prior to emplacement of the regional plains to after emplacement of young lavas that is for most of the stratigraphically defined history of venus estimates of the relative roles of uplift and volcanic construction in the evolution of irnini and anala montes are critically dependent on whether they are primarily anomalous coronae that have been uplifted or whether they are true shield volcanoes that have evolved into coronae studies elsewhere as well as most models of corona evolution favor the first alternative the geology of central eistla regio favors the second suggesting that local thinning of the lithosphere allowed shield volcanoes to evolve into coronae because these hybrid edifices are not typical of either most coronae or most shield volcanoes on venus they deserve detailed modeling for now the relative roles of uplift and volcanic construction in central eistla regio cannot be even approximately estimated

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

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

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

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

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

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

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

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

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

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

constraints on the thermal evolution of venus inferred from magellan data one interpretation of the magellan data suggests that the cratering record on venus was erased by a global resurfacing event or events the latest ending about 500 m y ago in this global resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts a thermal evolution model of venus that can explain these observations is one in which a deformable lithosphere capable of being incorporated in mantle circulations provides an almost stress free condition at the surface mantle convection with an almost stress free boundary at the surface cools the interior more efficiently rapid cooling decreases the rayleigh number of mantle convection below a transition value required for oscillatory convection and the vigor of convection diminishes as the mantle changes to a quasi steady circulation after about 500 m y ago

episodic large scale overturn of two layer mantles in terrestrial planets it is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately possible buoyant overturn of the two mantle layers has not previously been considered such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers reversing the relative sense of buoyancy once overturn has occurred the chemically denser but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy if mixing is slow this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large scale overturning thermal evolution calculations for a two layer mantle over a wide range of parameter space show that large scale overturn occurs cyclically with a well defined period this period depends most strongly on the viscosity of the lower mantle to which it is approximately proportional geologically interesting overturn periods on the order of 107 to 109 years result for lower mantle viscosities of 1022 to 1024 pa s for the earth and venus and 1021 to 1023 pa s for mars the mantles of mercury and the moon are too thin to permit two layer convection and therefore the model is not appropriate for them overturn cannot occur on earth or venus if the compositional density difference between the layers exceeds about 4 or on mars if it exceeds about 2 large scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the earth or a major resurfacing event on mars or venus such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

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

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

turbulent 3 d thermal convection in an infinite prandtl number volumetrically heated fluid implications for mantle dynamics summary the structure and time dependence of 3 d thermal convection in a volumetrically heated infinite prandtl number fluid is examined for high values of the rayleigh number the methods employed allow the numerical experiments to proceed for long enough times to derive good estimates of mean and fluctuating parts of the structure an iterative multirigid method to solve for the buoyant incompressible viscous flow at each time step of the energy equation is a novel aspect of the methodology a simple explicit time step of the energy equation is utilized that vectorizes well on serial computers and which is ideally suited to massively parallel computers numerical experiments were carried out for rayleigh numbers from 3 106 to 3 107 in a cartesian region with a prescribed temperature at the top boundary and an adiabatic bottom boundary over this complete range of rayleigh number the flow structure consists dominantly of cold nearly axisymmetric plumes that migrate horizontally sweeping off the cold thermal boundary layer that forms at the top of the convecting fluid plumes disappear by coalescing with other plumes new plumes are created by thermal boundary layer instability sheet plumes form only occasionally and do not penetrate to significant depths in the fluid plumes have sizes comparable to the thickness of the thermal boundary layer and an average spacing comparable to the fluid depth no persistent large scale motion in the fluid can be identified its absence may reflect the large subadiabatic stratification that develops beneath the thermal boundary layer as cold plumes penetrate to the bottom boundary without thermally equilibrating with surrounding fluid we consider the possible implications for convection in planetary mantles and for the existence of plate tectonics

water and geodynamics hydrogen is the most abundant element fig 1 in the galaxy and our solar system lodders 2003 therefore it is not astonishing that hydrogen is a key player in the geodynamic evolution of planets its fate in the early earth after condensation of the solar nebula the accretion of our planet and hydrogen reprocessing through early asteroidal and cometary bombardment dauphas et al 2000 and segregation of a proto earth into iron core and silicate mantle is described elsewhere in this volume marty and yokochi 2006 figure 1 abundance of elements in the solar system in numbers of atoms per 106 atoms of silicon lodders 2003 this chapter concerns itself with the geodynamics of the modern earth where nowadays hydrogen occupies only a very small mass fraction less than 0 1 wt although its abundance is thus drastically diminished it is still believed to have a profound influence on the geodynamics of the present planet the reason why hydrogen can have a profound influence on geodynamics even when it is diluted to ppm levels relies on its fundamental influence on both rock mechanics kohlstedt 2006 and the melting relationships kohn 2006 however both the micromechanical mechanism and the large scale implications are not yet fully understood in this review geodynamic processes which may be triggered by a rock mechanical hydrogen threshold value are reviewed in addition to resulting styles of planetary convection controlled by melt and rock mechanics this review will conclude with a more speculative discussion of the possible implications for planetary evolution from an early earth to the present day and highlight areas of future work following current practice in the literature the term water will be used interchangeably for hydrogen recent breakthroughs in computational geodynamics are throwing a new light on the possible evolution of planetary tectonics simulations

geoid anomalies and dynamic topography from convection in cylindrical geometry applications to mantle plumes on earth and venus summary a variety of evidence suggests that at least some hotspots are formed by quasi cylindrical mantle plumes upwelling from deep in the mantle we model such plumes in cylindrical axisymmetric geometry with depth dependent newtonian viscosity cylindrical and sheet like cartesian upwellings have significantly different geoid and topography signatures however rayleigh number nusselt number systematics in the two geometries are quite similar the geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer provided that it is at least 1000 times the interior viscosity increasing the rayleigh number or including a low viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume increasing the aspect ratio increases both the geoid anomaly and the topographic uplift of a plume the nusselt number is a weak function of the aspect ratio with its maximum value occurring at an aspect ratio of slightly less than 1

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

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

atmosphere mantle coupling and feedbacks on venus we investigate the coupled evolution of the atmosphere and mantle on venus here we focus on mechanisms that deplete or replenish the atmosphere atmospheric escape to space and volcanic degassing of the mantle these processes are linked to obtain a coupled model of mantle convection and atmospheric evolution including feedback of the atmosphere on the mantle via the surface temperature during early atmospheric evolution hydrodynamic escape is dominant while for later evolution we focus on nonthermal escape as observed by the analyzer of space plasma and energetic atoms instrument on the venus express mission the atmosphere is replenished by volcanic degassing from the mantle using mantle convection simulations based on those of armann and tackley 2012 and include episodic lithospheric overturn the evolving surface temperature is calculated from the amount of co2 and water in the atmosphere using a gray radiative convective atmosphere model this surface temperature in turn acts as a boundary condition for the mantle convection model we obtain a venus like behavior episodic lid for the solid planet and an atmospheric evolution leading to the present conditions co2 pressure is unlikely to vary much over the history of the planet with only a 0 25 20 postmagma ocean buildup in contrast atmospheric water vapor pressure is strongly sensitive to volcanic activity leading to variations in surface temperatures of up to 200 k which have an effect on volcanic activity and mantle convection low surface temperatures trigger a mobile lid regime that stops once surface temperatures rise again making way to stagnant lid convection that insulates the mantle

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

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

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

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

global decoupling of crust and mantle implications for topography geoid and mantle viscosity on venus the surface of venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust an analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients the model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust highlands are places where mantle downwells and the crust is thick surface heat flow is inversely correlated with elevation consistent with recent estimates of brittle layer thickness variations on venus if the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018 pa s to allow decoupling the observed amplitude of geoid highs over highlands requires an earth like increase in mantle viscosity with depth

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

on the formation of continental silicic melts in thermochemical mantle convection models implications for early earth abstract important constituents of archean cratons formed in the early and hot history of the earth are tonalite trondhjemite granodiorite ttg plutons and greenstone belts the formation of these granite greenstone terrains is often ascribed to plate tectonic processes buoyancy considerations however do not allow plate tectonics to take place in a significantly hotter earth we therefore propose an alternative mechanism for the coeval and proximate production of ttg plutons and greenstone like crustal successions that is when a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust partial melting at the base of this newly produced crust may generate felsic melts which are added as intrusives and or extrusives to the generally mafic crustal succession adding to what resembles a greenstone belt partial melting of metabasalt in the sinking crustal section produces a significant volume of ttg melt which is added to the crust directly above the location of subduction presumably in the form of a pluton this scenario is self consistently produced by numerical thermochemical mantle convection models presented in this paper including partial melting of mantle peridotite and crustal meta basalt the metamorphic p t conditions under which partial melting of metabasalt takes place in this scenario are consistent with geochemical trace element data for ttgs which indicate melting under amphibolite rather than eclogite facies other geodynamical settings which we have also investigated including partial melting in small scale delaminations of the lower crust at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behavior unequivocally and mostly show melting of metabasalt in the eclogite stability field instead

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

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

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

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

formation of beta regio venus results from measuring strain beta regio is an area of rifting and volcanism on venus constituting a topographic rise a shield volcano theia mons lies near the center of the region and is surrounded by several radially oriented rifts we use magellan altimetry gravity and synthetic aperture radar data of the area to constrain some subsurface parameters first we derive hoop strain using altimetry data and a fault dip angle derived from the split crater somerville we determine the extension in the rifts surrounding beta regio we then derive the hoop strain accommodated by the rifts from the extension in these rifts except near theia mons the hoop strain follows the shape expected from a mantle upwelling the difference near the volcano we believe is due to volcanic infilling we then model three observable quantities the newly derived strain along with gravity and uplift using two separate modeling techniques one for the strain and uplift and another for the gravity the model results show that the data are consistent with the view that a relatively low density contrast region now exists below beta and has caused the uplift and rifting in the region

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

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

a provenance study of the paleozoic ventania system argentina transient complex sources from western and eastern gondwana abstract the u pb and lu hf isotopic analyses of the different sedimentary sequences of the ventania system an old paleozoic orogenic belt exposed in the southern region of the rio de la plata craton in the province of buenos aires argentina provide new evidence for the understanding of the tectonic evolution of the western sector of the gondwanides mountain belt these ranges formed as result of the late paleozoic collision of the patagonia terrane against the continental margin of gondwana the provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the tandilia system a paleoproterozoic mountain belt formed during the amalgamation of the rio de la plata craton at about 1800 2200 ma and incorporated to western gondwana during the brasiliano orogeny at 550 530 ma the local dominant source at the base of the early paleozoic changed to more distant supplies toward the top of the sequences when an increasing participation of detritus from first cambrian 560 520 ma zircons from the pampean orogen and later on ordovician 480 460 ma zircons from the famatinian orogen is recorded the detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early paleozoic stratigraphy the balcarce formation an early paleozoic sedimentary cover of the tandilia metamorphic and igneous basement shows striking differences when compared with the new data from the ventania system the two data sets reveal different sources for the two regions the late paleozoic foreland basin deposits mark an abrupt change of 180 in the paleocurrent directions in the petrographic composition of the sediments and in the provenance of detrital zircons these data indicate a southern provenance with the first evidence of carboniferous and permian magmatic zircons the oldest archean zircons together with the finding of clasts with archeocyathids support the provenance from patagonia which was derived from eastern gondwana the u pb ages of the ash fall tuffs in the tunas formation confirm the early permian age of the eurydesma fauna in the ventania system the u pb data together with the lu hf isotopic data enhance the comprehension of the tectonic evolution of the ventania system as part of the larger gondwanides belt that amalgamated to western gondwana during late permian times with some independent pieces derived from eastern gondwana

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

the influences of surface temperature on upwellings in planetary convection with phase transitions abstract the importance of surface temperature for mantle convection appears with the presence of adiabatic heating and cooling and the release and consumption of latent heat in the presence of phase transitions for some planetary bodies these effects cannot be neglected the dimensionless surface temperature t 0 which is the ratio between the temperature at the top of the convective region and the temperature drop across the mantle is close to one for mars and venus for the earth t 0 lies between 0 2 and 0 5 the dynamical influence of t 0 is especially poignant for internally heated convection with temperature dependent viscosity there is a tight coupling between the magnitude of the temperature field and the viscosity itself we have studied temperature dependent viscosity convection for both low t 0 0 2 and high t 0 1 2 situations and with internal heating in mantle convection with two upper mantle phase transitions our results show that within this range of t 0 there exist two regimes for the evolution of upwellings in the mantle in transient situations plume plume collisions lead to the formation of megaplumes for high t 0 regimes but are less likely to do so for low t 0 in the long term regime plumes with low t 0 are prone to develop from the transition zone with a supply of hot material coming from the shallow lower mantle in systems with high t 0 however long lived plumes tend to have deeper mantle origins in quasi layered situations high t 0 may act as a positive feed back mechanism in inducing powerful hot upwellings into the upper mantle

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