steep sided domes on venus characteristics geologic setting and eruption conditions from magellan data a survey of more than 95 percent of the venus surface reveals 145 steep sided domes which can be subdivided into a variety of morphologic forms the most common being shaped like inverted bowls or flat topped domes results of a preliminary analysis of the distribution and geologic setting of the domes are presented the relation of the domes to analogous terrestrial features is examined and possible models for their mode of emplacement are outlined

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

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

a catalog of taiwan earthquakes 1900 2006 with homogenized mw magnitudes taiwan has a relatively complete catalog of earthquakes since the first seismograph was installed in 1897 however due to changes in seismographic characteristics network coverage and observational practice the definition and procedure for magnitude determination were different during different time periods recognizing a complete catalog of earthquakes with consistent magnitudes is essential for delineating seismicity patterns and assessing seismic hazards for taiwan efforts have been made to convert the original magnitudes of earthquakes in taiwan based on various magnitude scales to a common m l or m s magnitude scale unfortunately the m l or m s magnitude scales chosen for previous studies all are subject to a fundamental limitation of saturation toward large earthquakes besides these studies are nearly two decades old in order to avoid this limitation and to follow the current trend we have chosen in this study to convert original magnitudes of various scales to a common m w scale in this study we used two independent methods for magnitude conversion in the first method we converted the original magnitudes to m w through empirical relations between these magnitudes and m w this magnitude is called the old m w in the second method we used the best fitting a and b values to convert the original magnitudes to m w this magnitude is called the new m w the converted m w magnitudes from both methods have resulted in significant improvements over the original magnitudes and are in good agreement with each other nevertheless by examining the log10 n versus m w plots we found better linearity and tighter overlap among different time periods for the new m w than for the old m w thus we chose the new m w as the unified magnitude for the catalog of taiwan earthquakes finally a list of 899 earthquakes from 1900 to 2006 with m w 5 5 is presented graphic 1 see the earthquake lists available in the electronic supplement to this article the list is considered complete for the whole period from 1900 to 2006 1 embed inline graphic 1 gif

multispectral surface emissivity from virtis on venus express abstract the surface composition of venus is mostly inaccessible to remote observation due to the dense cloud cover there are five spectral windows that show measurable thermal emission from the surface at night the virtis spectrometer on venus express observed three of these windows over much of the southern hemisphere of venus we use these data along with magellan altimetry to map surface emissivity the data are initially processed to correct for instrumental straylight from the dayside of venus and to improve wavelength registration these corrected data are then inverted to emissivity at 1020 1100 and 1180 nm wavelength using lookup tables created by an atmospheric radiative transfer model as in earlier studies we find residual trends of surface emissivity with respect to the magellan altimetry that is used in the model to determine surface temperature and thickness of the atmosphere a new observation is that these trends vary significantly from region to region indicating some lateral variability of atmospheric parameters most likely near surface atmospheric temperature the trends are consistent over hundreds to thousands of km thus it is possible to correct for them heuristically in two regions studied in this paper there are significant deviations from the background emissivity which are associated with some geologic features the high noise in 1100 and 1180 nm maps derived from virtis data result in large uncertainties of spectral shape the virtis instrument was not designed for this task and future observations could provide high signal to noise ratio maps in at least 5 distinct bands diagnostic of major rock types and minerals

vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 c in an extensional tectonic regime the vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude non elastic ductile deformation of the host rock must have controlled widening of the arrested cracks fracture parallel shortening is low due to the low accumulated strain the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock information is exclusively obtained from the microfabrics of the vein sealing quartz there the density of geometrically necessary dislocations derived from orientation maps created by electron backscatter diffractometry ebsd decreases markedly from the vein wall to the centre the gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein the gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing in view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins the monogenetic veins are interpreted to record a single mid crustal stress transient related to the earthquake cycle fracturing is proposed to be a consequence of coseismic loading while progressive opening and sealing record the deformation during postseismic stress relaxation by creep

plume induced crustal convection 3d thermomechanical model and implications for the origin of novae and coronae on venus novae and coronae are large volcanotectonic features on venus with a contentious and possibly non unique origin and enigmatic relationship their formation is most commonly explained by flexure and fracturing of the strong rigid venus lithosphere atop mantle upwelling and or downwelling here we present new 3d high resolution thermomechanical model of mantle plume diapir impingement into warm and thin lithosphere with venus like surface temperature numerical results suggests that nova like and corona like structures can result from magma assisted convection of weak ductile crust induced by decompression melting of the hot rising mantle plume during the initial stage nova forms by stellate fracturing of a topographic rise forming atop the growing crustal convection cell few million years later nova can convert to coronae by inward dipping concentric fracturing of the nova rise margins and subsequent outward thrusting of partially molten crustal rocks over the surface an outer annulus of concentric normal faults forms in the outer rise region of the downbending brittle upper crust whereas an inner annulus of concentric thrust faults forms in front of the outward thrusting crustal wedge a trench like depression forms between these two annuli resembling retreating subduction the rudimentary concentric upper crustal slab warms up rapidly and recycles into the convection cell the convection cell remains active for up to 15 million years fueled by heat and magma from the plume predicted surface topography and fracturing patterns agree with some small to moderate size novae and coronae on venus

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

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

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

an experimentally verified model for calcite precipitation in veins calcite is commonly found as a vein filling mineral in rocks however the factors controlling its deposition are complex and not well understood in quantitative terms in order to advance our understanding of the processes involved we have refined the model for calcium carbonate mass transport in subsurface carbonate rocks of morse and mackenzie 1993 and developed a new experimental technique to test it this technique uses a flow through reactor that simulates a vein opening agreement was observed between model predictions and experimental observations for the deposition of calcite in synthetic veins the influences of surface area to solution volume ratio solution saturation state with respect to calcite and flow velocity were well predicted by the model the model predicts that in order to have a fairly uniform deposition of calcite within a vein solution flow must be quite rapid tens to thousands of cm h 1 or the solution must be only slightly supersaturated with respect to calcite a low degree of solution supersaturation demands what may be unreasonably large volumes of solution flow to achieve vein filling for the vein configuration we have studied

geochemical constraints on petrogenic processes on venus abundances of three incompatible elements k u th have been determined by robotic spacecraft in five surface materials on venus we present these data normalized to terrestrial normal mid ocean ridge basalt nmorb derived from the depleted mantle relative to nmorb all of the venus materials studied are enriched in all of these elements the moderately enriched venera 9 and 10 vega 1 and 2 basaltic rocks are similar to one another in the sense of their enrichment trends u n k n but differ from the highly enriched venera 8 material where the trend is k n u n this difference implies that the enrichment pattern for the basaltic materials is not controlled by crystallization of an nmorb like magma or by contamination of such a magma by highly enriched venera 8 material within the crust and the venera 8 material cannot have evolved from the magma of any of the basaltic rocks our calculations show the k u th pattern for any of the venus rocks analyzed was not controlled by batch partial melting of primitive mantle the venera 8 material could be produced as a partial melt from eclogitic tholeiite but none of the basaltic materials could be because the sense of their u n k n trends is opposite to the enrichment trends in calculated models k n u n the venus basalts differ from fresh terrestrial rocks nmorbs and oceanic island arc volcanics in having nearly constant k u ratios while terrestrial rocks have nearly constant th u ratios this may suggest an unusual composition of mantle source s of the venus basalts and or unusual fractionation process es on that planet

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

on observing the compositional variability of the surface of venus using nightside near infrared thermal radiation 1 there are several windows in the near infrared wavelength range 1 m in which thermal radiation emitted from the planetary surface penetrates through the thick venus atmosphere and clouds in this study we develop an improved method to estimate the surface emissivity on the basis of near infrared thermal emission from the nightside venus which is observed through these windows a simple radiative transfer model demonstrates that multiple reflection of thermal radiation between the atmosphere including clouds and the solid surface has a significant influence on the observed radiance under the condition of venus where reflectivity of overlying atmosphere and clouds is high thus it is necessary to take the effect of the reflection by the planetary surface into account in order to estimate accurately the variation in the surface emissivity on the basis of near infrared observation of venus the net effect of multiple reflection of surface thermal radiation between the atmosphere and the surface is to significantly reduce the spatial contrast in thermal radiation due to surface compositional variation the model calculation demonstrates that despite this effect detection of granitic rocks on the venus surface using near infrared windows is feasible since granitic and basaltic rocks have dramatically different 1 m emissivities granitic rocks are distinguishable from basaltic rocks by ground based telescopic observation a venus orbiter that measures both the near infrared thermal radiation and the surface altitude with great accuracy will provide us with a reliable surface emissivity map of venus which is a very valuable tool to detect granitic i e earth like continental rocks on venus

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

convective patterns under the indo atlantic box using fluid mechanics we reinterpret the mantle images obtained from global and regional tomography together with geochemical geological and paleomagnetic observations and attempt to unravel the pattern of convection in the indo atlantic box and its temporal evolution over the last 260 myr the box presently contains a a broad slow seismic anomaly at the cmb which has a shape similar to pangea 250 myr ago and which divides into several branches higher in the lower mantle b a superswell centered on the western edge of south africa c at least 6 primary hotspots with long tracks related to traps and d numerous smaller hotspots in the last 260 myr this mantle box has undergone 10 trap events 7 of them related to continental breakup several of these past events are spatially correlated with present day seismic anomalies and or upwellings laboratory experiments show that superswells long lived hotspot tracks and traps may represent three evolutionary stages of the same phenomenon i e episodic destabilization of a hot chemically heterogeneous thermal boundary layer close to the bottom of the mantle when scaled to the earth s mantle its recurrence time is on the order of 100 200 myr at any given time the indo atlantic box should contain 3 to 9 of these instabilities at different stages of their development in agreement with observations the return flow of the downwelling slabs although confined to two main boxes indo atlantic and pacific by subduction zone geometry may therefore not be passive but rather take the form of active thermochemical instabilities

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

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

the influence of interior mantle temperature on the structure of plumes heads for venus tails for the earth 1 to form mantle plumes with large heads and narrow trailing conduits requires large 102 viscosity variations within the hot thermal boundary layer at the plume source however new and published data from laboratory experiments show that if a layer of fluid with a temperature dependent viscosity is heated from below and cooled from above convection occurs beneath a stagnant lid and the interior fluid temperature remains close to that of the hot boundary consequently the viscosity reduction across the hot thermal boundary layer is always 10 and upwellings take the form of discrete plume heads thermals subduction of the stagnant layer equivalent to recycling lithospheric plates on the earth will cool the interior of the mantle thus increasing the viscosity ratio hence the existence of earth like mantle plumes may be a consequence of plate tectonics an absence of plate tectonics on venus may lead to upwellings composed of thermals the surface expression of which are coronae as well as a smaller core heat flux than on earth

experimental and observational evidence for plume induced subduction on venus venus lacks plate tectonics but some trenches on venus resemble subduction zones laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on venus

transient mantle convection on venus the paradoxical coexistence of highlands and coronae in the bat region thecoexistenceofvenusianhighlands attributedtolong livedaxisymmetricmantleplumes anduncompensatedcoronae attributed totransientdiscretemantle thermals isdifficulttoreconcilewithmodelsofmantleconvectionunderthermallysteady stateconditions however cratering and geological studies indicate a uniformly young surface age 700 myr as well as a comparable timescale for resurfacing 100 to 400 myr possibly consistent with a recent lithospheric overturn and a transient mantle thermal regime we use laboratory experiments on free and forced thermal convection at high rayleigh number ra 10 7 in a variable viscosity fluid to investigate the steady state and transient thermal regimes preceding and following such an overturn from analyses of shadowgraph images andtimeseries of global andlocal variations intemperature basal heat fluxandviscosity weestablishsteady statestagnant and active lid states and characterize two intermediate transient regimes flow in steady state stagnant lid is in the form of intermittent thermals consistent with published work during the transition to active lid convection the stagnant lid isstirred into the interior using a conveyorbelt spreadingofthiscoldfluidalongthehotboundaryleadstoatransitiontoa mixedmode offlowfromthehotboundary approximatelyisoviscous thermalsrise from the thermal boundarylayer aheadofthe advancingcoldfrontandlow viscosityplumes rise from behind the front as a result of an enhanced temperature contrast the longevity of this regime and the timescale for the transient depends on the rate of overturn pe and the aspect ratio of the system a the magnitude of local temperature viscosity and heat flux variationsincreaseswithpeandcanexceedsteady statevaluesforactive lidconvection additionalnumericalsimulationsshowthatthe mixed mode regime will occur in the presence of internal heating and for no and free slip boundaries in contrast the transition from active lid to stagnant lid convection is marked by a change from a flow composed of plumes and large scale overturning motions to a regime dominated by risingand sinking thermals on a timescale ofthermal diffusion applied tovenus our resultssupport a hypothesis that the contemporaneous coexistence of the atla and beta highlands regions with interspersed uncompensated coronae is consistent withatransientthermalregimefollowingalithosphericoverturn itisalsoexpectedthatsuchcoronaeformedn250myraftertheupliftof the highlands implications of the thermal origin of coronae for venusian mantle structure are also explored 2007 elsevier b v all rights reserved

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

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

relation of major volcanic center concentration on venus to global tectonic patterns global analysis of nasa magellan image data indicates that a major concentration of volcanic centers covering 40 percent of the surface of venus occurs between the beta atla and themis regiones associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net down welling together these observations suggest the existence of relatively simple largescale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on earth

corona formation and heat loss on venus by coupled upwelling and delamination coronae are volcanotectonic features that are unique to venus and are interpreted to be small scale upwellings a model in which upwelling causes delamination at the edge of the plume head along with deformation of a preexisting depleted mantle layer produced the full range of topographic forms of coronae if half of the coronae are active delamination of the lower lithosphere could account for about 10 percent of venus9 heat loss with another 15 percent due to upwelling delamination may occur in other geologic environments and could account for some of venus9 heat loss deficit

rayleigh taylor instability as a mechanism for corona formation on venus abstract in this study we explore the idea that coronae have formed on venus as a result of gravitational rayleigh taylor instability of the lithosphere the lithosphere is represented by a system of stratified homogeneous viscous layers low density crust over high density mantle over lower density layer beneath the lithosphere a small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry topography develops with time under the influence of dynamic stress associated with downwelling or upwelling and spatially variable crustal thickening or thinning topography may therefore be elevated or depressed above a mantle downwelling but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere the ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity average observed ratios are well resolved for two groups of coronae 40 mgal km 1 consistent with models in which the crust is perhaps 5 times stronger than the lithosphere group 3a rim surrounding elevated central region coronae are inferred to arise from a central upwelling model whereas group 8 depression coronae are inferred to arise from central downwelling observed average coronae radii are consistent with a lithospheric thickness of only 50 km an upper low density crustal layer is 10 20 km thick as inferred from the amplitude of gravity and topography anomalies

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

mineral distribution within polymineralic veins in the sanbagawa belt japan implications for mass transfer during vein formation pelitic schists of the sanbagawa metamorphic belt contain several types of polymineralic veins that formed during the late stages of exhumation the vein mineral assemblages are quartz albite k feldspar chlorite calcite type i ii and quartz albite calcite type iii type i and ii veins contain quartz and albite with stretched crystal and elongate blocky textures respectively the mineral species within type i veins vary with compositional bands within the host rocks type iii veins are characterized by euhedral to subhedral quartz grains with concentric zoning and a homogeneous distribution along the vein length the vein textures vary depending on the crack aperture during multiple crack seal events 0 08 mm for type i and 0 5 10 mm for type iii type ii veins show intermediate features between type i and iii veins in terms of mineral distribution weak dependence on the host rock composition and apparent crack aperture less than 1 15 mm these observations suggest a transition in the dominant transport mechanism of vein components with increasing crack aperture from diffusion from host rocks to fluid advection along cracks

growth of large volcanoes on venus mechanical models and implications for structural evolution the structure tectonics and evolution of large volcanoes on venus as revealed by data from the magellan mission appear to be distinct from those on earth and mars to determine the conditions and processes that account for these differences we model the evolution of stress and deformation in growing volcanic edifices on venus using the finite element method large volcanoes on venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons the surfaces of both edifice and apron consist dominantly of radially oriented flows tectonic features usually the surface expression of shallowly intruded dikes also have predominantly radial orientations features similar to hawaiian style linear rift zones or large scale flank failure common on large volcanoes on earth and mars are absent by comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features we determine that a welded condition is more likely horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness te contravening stress increments from magma chamber expansion differential thermal contraction or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored incremental volcano growth results in stress distributions that decrease with height in the edifice dike intrusion is easiest in the uppermost youngest layers consistent with the observed shallow radial dikes on venus volcanoes small values of te greatly inhibit the formation of large shields in that conical edifice topography cannot be supported and large contravening stresses are required for further growth large volcano formation is much more likely at large te where more moderate contravening stresses are sufficient for growth the near surface manifestation of mantle upwelling on venus may thus depend on te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile leading to the formation of coronae but a thick elastic lithosphere is required to support the growth of the largest volcanoes the transitional value of te between these modes of evolution likely depends on the horizontal scale of the upwelling

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

constraints of the topography gravity and volcanism on venusian mantle dynamics and generation of plate tectonics abstract venus s mantle convection model was studied in a three dimensional spherical shell domain with depth and temperature dependent viscosity numerical results show that key observations of venus including the number of major hotspot volcanic systems spectral patterns of the surface topography and geoid at long and intermediate wavelengths can be explained in models that have a spinel to post spinel endothermic phase change of 3 5 mpa k clapeyron slope and averaged mantle viscosity of 2 10 21 pa s i e convective rayleigh number of 1 8 10 7 our models with the endothermic phase change show relatively weak time dependence suggesting that the phase change may not be the primary cause for catastrophic resurfacing on venus our calculations also show that venus cannot have a weak asthenosphere that is similar to that on the earth in order to match the observations thus supporting a key role of asthenosphere in producing plate tectonics

textures of syntaxial quartz veins synthesized by hydrothermal experiments syntaxial quartz veins were synthesized by hydrothermal flow through experiments using rock blocks metachert sandstone and granite containing slits based on analyses of vein textures using birefringence imaging microscopy we identified two stages of crystal growth during stage 1 quartz grain growth occurs without an increase in grain width during stage 2 quartz grains develop facets and grow preferentially parallel to the c axis orientation and the aspect ratio of quartz grains shifts toward 2 9 competitive growth occurs significantly at stage 2 and the transition from stage 1 2 occurs at a critical distance from the vein wall being approximately equal to the host rock grain size crystal growth in the slits produce various textures controlled by the ratio of slit aperture h to host rock grain size d in high h d cracks elongate blocky texture develops by grain impingement during stage 2 whereas in low h d cracks crystals that bridge the crack form without competitive growth by grain impingement at stage 1 heterogeneous structures of fracture porosity are produced during syntaxial vein formation due to the anisotropy in the growth rate of quartz such incompletely sealed cracks may act as important fluid pathways and as weak planes in the upper crust

quartz vein fabrics coupled to elevated fluid pressures in the stawell gold deposit south eastern australia shear and extensional veins formed during the reactivation of the magdala shear system at stawell in western victoria australia contribute to the formation of the auriferous central and basalt contact lodes within this shear system is a range of fault rocks accompanied by steep dipping 65 quartz rich laminated shear veins and relatively flat lying extensional veins both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses the macro and microstructures suggest that the dilatancy that produced mineralized veins formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime a new microfabric analysis technique is used to investigate the quartz rich veins which allows rapid integration of the microstructure with the crystallographic preferred orientations cpos both the shear and extensional quartz veins have a random cpo with 120 dihedral angles between the quartz quartz grains which is typical of a metamorphic equilibrium microfabric the microstructures indicate that the quartz has undergone extensive grain adjustment in the solid state with grain shape and size affected by interfacial solution pressure solution effects these features are consistent with inferences from experimental rock deformation studies where grain boundary migration is enhanced in a water rich environment the onset of solution transfer processes pressure solution developed as the quartz microfabric stabilized and continued to modify the cpo and microstructure significantly it is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures

variations of sulphur dioxide at the cloud top of venus s dynamic atmosphere a pulse of sulphur dioxide in venus s upper atmosphere was observed by the pioneer venus spacecraft in the 1970s and 1980s and attributed to volcanism recent sulphur dioxide measurements from venus express indicate decadal scale fluctuations in sulphur dioxide above venus s cloud tops in an atmosphere that is more dynamic than expected

estimated seismic intensity distributions for earthquakes in taiwan from 1900 to 2008 in this study we recreated peak ground accelerations pga and peak ground velocity pgv distributions for taiwan by applying the attenuation relations of liu and tsai 2005 to calculate the pga and pgv values for 1989 mw 5 0 earthquakes in a catalog of earthquakes from 1900 to 2008 with homogenized magnitude mw chen and tsai 2008 we further combined the pga and pgv values to obtain corresponding modified mercalli intensity mmi values wald quitoriano heaton kanamori et al 1999 and their spatial distributions and recurrence intervals we adopted a logarithmic functional form analogous to the gutenberg richter relation for seismicity to represent the annual frequency of seismic intensity parameters log10n a blog10pga log10n a blog10pgv and log10n a bi the regions with high pga and pgv values are often associated with low b values in these equations as it is well known that the mw 7 45 chi chi earthquake of 21 september 1999 had produced high pga values in excess of 0 9g and pgv values in excess of 300 cm s we used these relations to estimate the poisson probability distributions in taiwan for mmi viii i e pga 485g for recurrence intervals of 30 50 and 100 years the results show a wide range of differences in the poisson probability of mmi viii among different areas of taiwan for example for a 50 year interval this probability at 10 major cities in taiwan is as follows taipei 0 67 hsinchu 2 15 taichung 5 24 chiayi 24 35 tainan 1 61 kaohsiung 0 04 hengchun 4 94 ilan 17 67 hualien 37 04 and taitung 9 82 these esti mates should be of interest to city planners especially for earthquake preparedness planning

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

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

origin of corona dominated topographic rises on venus abstract both large scale mantle upwellings comparable to terrestrial hotspots on earth and smaller scale mantle upwellings known as coronae occur on venus corona dominated rises have many of the characteristics of large scale mantle upwellings or hotspots such as broad topographic rises greater than 1000 km in diameter and large positive gravity anomalies due to the presence of clusters of three to eight coronae three large volcanic rises or hotspots on venus have been classified as corona dominated rises cdrs themis eastern eistla and central eistla regiones cdrs have been interpreted to result from the break up of a large scale plume comparison of the topographic morphology for individual coronae at themis and eastern eistla region to a model of corona evolution indicate that they are in varying stages of evolution at eastern eistla regio all the coronae have essentially the same topographic morphology consistent with a late stage of evolution and the presence of a depleted mantle layer at depth the complex deformation sequences and stratigraphic relationships both between coronae and with respect to the regional plains observed at all three rises indicate a prolonged origin this observation as well as the varying stages of evolution rule out the previously proposed interpretation of corona dominated rises as a manifestation of the break up of a large scale mantle upwelling which requires essentially simultaneous formation of the coronae instead we suggest that other large topographic rises are the manifestation of deep mantle plumes likely to originate at the core mantle boundary and that cdrs are clusters of coronae that originate at a shallower interface perhaps at an upper lower mantle boundary using top and bottom loading flexural models to fit the gravity topography admittance spectrum for each of the three cdrs yields elastic thickness estimates that are 10 15 km greater for bottom loading at longer wavelengths than top loading at shorter wavelengths estimates of elastic thickness assuming top loading are 10 12 and 22 km and 20 25 and 35 km from bottom loading for eastern eistla central eistla and themis regiones respectively we believe that the bottom loading elastic thickness estimates are more reliable because using a top loading model when both types of loading are present yields an unrealistically low elastic thickness estimate as there is no obvious source of surface loading at either themis or at eastern eistla we interpret the top loading admittance signature to be a result of delamination of the lower lithosphere depressing the surface which is consistent with the observed coronae morphologies

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

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

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

virtis emissivity of alpha regio venus with implications for tessera composition abstract the composition of venus tessera terrain is unknown the visible and infrared thermal imaging spectrometer virtis aboard venus express vex collects data that yields the surface emissivity at 1 m which contains information convolving a number of surface properties including composition we examine the variation of emissivity in the vicinity of alpha regio which is the largest exposure of tessera terrain imaged by virtis we find that the emissivity of alpha regio tessera is lower than adjacent plains materials and the deposits and flows of eve corona both of which have previously been interpreted to be basaltic the emissivity of the bulk of alpha is also lower than its western boundary which is interpreted to comprise plains structurally deformed to the same degree as tessera terrain this suggests that the lower emissivity of alpha is independent of structural elements macroscale roughness or local sedimentation processes and is due to material properties like composition or grain size the deviation of the emissivity of alpha from that of the plains for which a bulk basaltic composition is well supported corresponds to a significant difference in rock type or surface mineral assemblage the 1 m emissivity of alpha is consistent with rocks with low ferrous iron content this includes felsic igneous rocks like granitoids that form under either water rich or water poor conditions a water rich origin would require both a hydrosphere and a plate recycling mechanism and thus be limited to the lifetime of surface water on venus alternatively granitoids could form via the differentiation of basaltic melts the production of all tessera terrain by this mechanism would require the accumulation and preservation of felsic melts from a volume of mafic magma that exceeds what is preserved in the currently observed plains both mechanisms of granitoid formation would require that tessera terrain be formed prior to the emplacement of the plains consistent with their stratigraphic position anorthosites also satisfy the emissivity signature and can form from copious amounts of partial melting of a mafic source low emissivity values are also consistent with carbonates sulfates phyllosilicates and their dehydration products which may have formed via weathering of basalts under conditions of higher atmospheric p h2o all of these hypotheses suggest the mineralogy of alpha tessera records an extinct era of venus history and is a key target for future exploration

low radar emissivity signatures on venus volcanoes and coronae new insights on relative composition and age abstract multiple studies reveal that most of venus highlands exhibit anomalously high radar reflectivity and low radar emissivity relative to the lowlands this phenomenon is thought to be the result of atmosphere surface interactions in the highlands due to lower temperatures these reactions are a function of rock composition atmospheric composition and degree of weathering we examine the magellan radar emissivity altimetry and sar data for all major volcanoes and coronae on venus we characterize and classify edifices according to the pattern of the variation of radar emissivity with altitude the volcanic highlands can be classified into 7 distinct patterns of emissivity that correspond to at least 3 discrete types of mineralogy based on the altitude temperature of the emissivity anomalies the majority of emissivity anomalies support the hypothesis of a weathering phenomenon at high altitude 6053 km but we also find strong emissivity anomalies at lower altitudes that correspond spatially to individual lava flows indicating variations in mineralogy within an evolving volcanic system the emissivity signature of tallest volcanoes on venus are consistent with the presence of ferroelectric minerals in their rocks while volcanic edifices in western ishtar terra and eastern aphrodite terra are consistent with the presence of semiconductor minerals sapas mons and pavlova corona are also consistent with ferroelectrics but at a different curie temperature than the other volcanoes in atla regio the spatial distribution of radar emissivity classes correlates to different geologic settings indicating that different mantle source regions deep shallow plumes and possible convergence zones may contribute to differences in mineralogy for the studied edifices finally we show that the emissivity signatures of idunn maat and other volcanic edifices are consistent with relatively fresh and unweathered rocks indicating recent or possibly current volcanism on venus

geologic interpretation of the near infrared images of the surface taken by the venus monitoring camera venus express we analyze night time near infrared nir thermal emission images of the venus surface obtained with the 1 m channel of the venus monitoring camera onboard venus express comparison with the results of the magellan radar survey and the model nir images of the beta phoebe region show that the night time vmc images provide reliable information on spatial variations of the nir surface emission in this paper we consider if tessera terrain has the different nir emissivity and thus mineralogic composition in comparison to the surrounding basaltic plains this is done through the study of an area sw of beta regio where there is a massif of tessera terrain chimon mana tessera surrounded by supposedly basaltic plains our analysis showed that 1 m emissivity of tessera surface material is by 15 35 lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices this is consistent with hypothesis that the tessera material is not basaltic maybe felsic that is in agreement with the results of analyses of vex virtis and galileo nims data if the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of venus we have found that the surface materials of plains in the study area are very variegated in their 1 m emissivity which probably reflects variability of degree of their chemical weathering we have also found a possible decrease of the calculated emissivity at the top of tuulikki mons volcano which if real may be due to different more felsic composition of volcanic products on the volcano summit

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

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

kunhild and ereshkigal an extinct hot spot region on venus analysis of imagery topography and gravity for kunhild and ereshkigal adjacent large volcanoes on venus indicates that they are extinct hot spots that were located over a broad mantle upwelling the volcanoes probably formed contemporaneously but volcanism persisted at kunhild after ereshkigal s extinction a late event in each structure s formation was sagging of the central region to form a broad depression ereshkigal the shorter of the two structures has a broader and deeper central depression than kunhild we suggest that when a long lived plume goes away from beneath a large venusian shield volcano removal of dynamic support from the plume stem produces a central sagging analogous to the collapse phase associated with corona formation on venus

estimating seismic hazards from the catalog of taiwan earthquakes from 1900 to 2014 in terms of maximum magnitude maximum expected earthquake magnitude is an important parameter when designing mitigation measures for seismic hazards this study calculated the maximum magnitude of potential earthquakes for each cell in a 0 1 0 1 grid of taiwan two zones vulnerable to maximum magnitudes of m w 6 0 which will cause extensive building damage were identified one extends from hsinchu southward to taichung nantou chiayi and tainan in western taiwan the other extends from ilan southward to hualian and taitung in eastern taiwan these zones are also characterized by low b values which are consistent with high peak ground shaking we also employed an innovative method to calculate at intervals of m w 0 5 the bounds and median of recurrence time for earthquakes of magnitude m w 6 0 8 0 in taiwan

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

gravity analysis of parga and hecate chasmata implications for rift and corona formation 1 the two largest rift systems on venus parga and hecate chasmata contain one third of all coronae we map variations in elastic thickness and apparent depth of compensation adc in these two regions using the admittance function for gravity and topography we examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism topographic shape fracture pattern diameter and stratigraphic age at hecate chasmata both adc and elastic thickness correlate with two main rift branches the fracture style and corona characteristics hecate chasma thus appears to be dominated by extensional processes parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments the adc at hecate and parga chasmata is mostly less than 75 100 km implying that there is not large scale upwelling underlying the rift systems the variations in the corona population sizes between parga chasma which has 131 coronae and hecate chasma with 50 coronae suggest differences in the evolution of the two rift systems the much larger population of coronae at parga contains more small coronae many of which have relatively low volcanism and post date the rift further many of the small coronae occur in chains and have similar diameters as predicted by rayleigh taylor type instabilities these characteristics suggest that the smaller coronae may form via a later secondary decompression melting phase thus hecate chasma may be younger than parga chasma and could experience a secondary stage of corona formation in the future

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

venus the nature of the surface from venera panoramas images of the surface of venus obtained by the soviet venera 9 10 13 and 14 landers have been analyzed to provide a basis for understanding the nature of geologic processes operating there the four spacecraft landed in the beta phoebe region at median elevations in the upland rolling plains province the landing points are each separated by distances of more than a thousand kilometers the venera panoramas were digitized and transformed into various perspectives in order to facilitate analysis and comparison with other planetary surfaces bedrock is exposed at the venera 10 13 and 14 sites and is characterized by semicontinuous flat polygonal to subrounded patches up to several meters in width the bedrock surface is often dominated by subhorizontal to horizontal layered plates with thicknesses of several centimeters and abundant linear and polygonal vertical fractures angular to subangular layered to platy blocks in the 5 to 70 cm range dominate the venera 9 site and occur much less frequently at the other sites blocks appear to share many characteristics with the exposed bedrock and are interpreted to be largely derived from it soils particles 1 cm are abundant at the venera 9 10 and 13 sites but are uncommon at venera 14 features indicative of a strong eolian influence moats dunes wind tails are not observed a striking aspect of the venera landing sites is their extreme similarity despite separation distances of thousands of kilometers several hypotheses are considered for the origin of the bedrock surfaces and we investigate in detail the hypothesis that bedrock originated from surface lava flows in this interpretation the broadly platy nature of the surface is analogous to the rolling and undulating nature of terrestrial pahoehoe flows caused by the formation and deformation of a semisolid crust the layering is interpreted to be formed by a combination of upper thermal boundary layer formation and horizontal sheets formed by cooling and shearing during flow emplacement vertical fractures are attributed largely to joint patterns formed during cooling this interpretation made on the basis of surface morphology is consistent with venera 13 and 14 geochemical results which reported high potassium basalt and tholeiitic basalt compositions respectively if this interpretation is correct large regions of the beta phoebe area are likely to be characterized by lava flows the relative freshness of features observed by venera 14 suggests that some bedrock surfaces are geologically young or that erosion rates are low

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

probability for simulating future earthquakes with mw 6 0 in taiwan for seismic hazard for the earthquake catalog from 1900 to 2008 in this study we use extreme value theory based on gumbel equation derivations to estimate the gutenberg richter a and b parameters for taiwan data are from the augmented homogenized in terms of moment magnitude historic catalog for taiwan the island is divided into grids of 0 2 latitude by 0 2 longitude and gumbel type 1 statistical analysis is applied the values of a and b are then used to determine the probability of large earthquakes m w 6 0 occurring at each grid the results show two relatively high probability paths for large earthquakes one extending from hsinchu southward to taichung chiayi and tainan in western taiwan and the other from ilan southward to hualian and taitung in eastern taiwan both of which are characterized by low b values it indicates that future earthquakes can be expected along these paths characterized by low b values additionally maximum peak ground acceleration and maximum peak ground velocity determined from respective attenuation laws and a gridding regimen of 0 1 latitude by 0 1 longitude for taiwan follow similar paths to that of the low b values

image resolution and evaluation of genetic hypotheses for planetary landscapes the spatial resolution of image data tends to constrain the horizontal length scale of genetic hypotheses that are addressable by those data no simple formula exists when image resolution is sufficient to test a given geomorphic process which is dependent on what characteristics are diagnostic of the particular process genetic hypotheses should be formulated along the lines of the multiple working hypotheses concept as described in a classic paper by chamberlin j geol 5 1897 837 an essential element of a viable working hypothesis is a clear indication of the characteristics predicted by or a consequence of the hypothesis an untestable hypothesis is not an effective working hypothesis the history of the study of lunar sinuous rilles is outlined as an illustration of the influence of image resolution and the formulation of genetic hypotheses on the subsequent advancement of understanding of the problem sinuous rilles on venus and mars and controversial sinuous ridges on mars are also reviewed in the lunar case the three order of magnitude improvement in spatial resolution provided by lunar orbiter photographs over earth based telescopic photographs did not result in definitive examination and elimination of published hypotheses for the formation of sinuous rilles topographic data obtained from cartographically controlled apollo orbital photographs along with important observations and samples obtained by the astronauts on the lunar surface did test and exclude several hypotheses the formulation of a genetic hypothesis including testable consequences of that hypothesis is a greater determinant of its ultimate utility to the scientific community than is the image resolution available at any given time

new constraints on volcano tectonic evolution of large volcanic edifices on venus from stereo topography derived strain estimates radial fracture systems at large volcanoes on venus offer opportunities to unravel the tectonic and magmatic evolution of such edifices however the relatively low spatial resolution 10 20 km of altimetry data for venus has hampered previous efforts at quantitative analysis of fault systems recently released stereo derived topography enables kilometer scale strain estimates to be made at tectonic features the new data resolve fault valleys grabens that cut the summit regions of two large volcanoes showing them to be deeper than previously assumed the grabens formed after most edifice building was complete we demonstrate that inflation of oblate sill like magma chambers is consistent with the magnitudes and distributions of strain at these volcanoes and estimate chamber depth radius and overpressure

a global survey of possible subduction sites on venus abstract about 10 000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on venus all the sites have narrow deep trenches elongate along strike with arcuate planforms ridge trench outer rise topographic profiles typical of terrestrial subduction zones large outer rise curvatures 10 7 m 1 fractures parallel to the strike of the trench on the outer trench wall and outer rise and no cross strike fractures across the trench both the northern and southern margins of latona corona are possible subduction sites identification of a major graben between the two principal outer ridges in southern latona corona is evidence of back arc extension in the corona the amount of extension is estimated to be more than 2 11 km the moment exerted by the ridges of southern latona corona is insufficient to bend the lithosphere into the observed outer rise shape a negatively buoyant subducted or underthrust slab is needed depending on the unknown trench migration rate lithospheric subduction can make a significant contribution to mantle cooling on venus venusian chasmata could have a dual character they may be propagating rifts near major volcanic rises and subduction trenches far from the rises in the lowlands subduction and rifting may occur in close proximity on venus unlike on earth rifting induced by hotspots on venus may be necessary to break the lithosphere and allow subduction to occur such a process could result in gradual lithospheric subduction or global episodic overturn of the lithosphere

why does venus lack a magnetic field venus and earth have similar radii and estimated bulk compositions and both have an iron core that is at least partially liquid however despite these similarities venus lacks an appreciable dipolar magnetic field here i examine the hypothesis that this absence is due to venus9s also lacking plate tectonics for the past 0 5 b y the generation of a global magnetic field requires core convection which in turn requires extraction of heat from the core into the overlying mantle plate tectonics cools earth9s mantle on the basis of elastic thickness estimates and convection models it is argued here that the mantle temperature on venus is currently increasing this heating will reduce the heat flux out of the core to zero over 1 b y halting core convection and magnetic field generation if plate tectonics was operating on venus prior to ca 0 5 ga a magnetic field may also have existed on earth the geodynamo may be a consequence of plate tectonics this connection between near surface processes and core magnetism may also be relevant to the generation of magnetic fields on mars mercury and ganymede

hotspot evolution and venusian tectonic style because hotspots represent an important manifestation of heat loss on venus their geological evolution is of fundamental importance for any attempt to understand venusian tectonics eistla regio is a 7500 km long moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots it also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle central eistla regio includes two large volcanoes sappho and anala and several coronae in close proximity detailed mapping in this region results in two conclusions of tectonic significance 1 sappho and anala occur near the intersection of two major extensional deformation zones and 2 the coronae are older than the large volcanoes several of the coronae occur as a chain along guor linea one of the major extensional deformation zones stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials thus central eistla regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes the expected result of such a swarm would be thermal thinning of the elastic lithosphere with time however model results geological observations and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time this thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near global resurfacing the global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes

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

constraints on mantle plumes on venus implications for volatile history abstract the analysis of venus gravity field and topography suggests the presence of a small number of deep mantle plumes 9 this study predicts the number of plumes formed at the core mantle boundary their characteristics and the production of partial melt from adiabatic decompression numerical simulations are performed using a 3d spherical code that includes large viscosity variations and internal heating this study investigates the effect of several parameters including the core mantle boundary temperature the amount of internal heating and the mantle viscosity the smallest number of plumes is achieved when no internal heating is present however scaling earth s radiogenic heating to venus suggests a value of 16 tw cases with internal heating produce more realistic lid thickness and partial melting but produce either too many plumes or no plumes if a high mantle temperature precludes the formation of a hot thermal boundary layer mantle viscosity must be reduced to at least 10 20 pa s in order to include significant internal heating and still produce hot plumes in all cases that predict melting melting occurs throughout the upper mantle only cases with high core temperature 1700 k produce dry melting over time the upper mantle may have lost significant volatiles depending on the water content of the lower mantle deep plumes may contribute to present day atmospheric water via volcanic outgassing assuming 50 ppm water in mantle 10 plumes with a buoyancy flux of 500 kg s continuously erupting for 4 myr will outgas an amount of water on the order of that in the lower atmosphere a higher level of internal heating than achieved to date as well as relatively low mantle viscosity may be required to achieve simulations with 10 plumes and a thinner lid alternatively if the mantle is heating up due to the stagnant lid the effect is equivalent to having lower rates of internal heating a temperature increase of 110 k byr is equivalent to 13 tw this value along with the internal heating of 3 tw used in this study may represent the approximate heat budget of venus mantle

surface brightness variations seen by virtis on venus express and implications for the evolution of the lada terra region venus 1 venus express is since april 11 2006 in orbit around venus virtis visible and infrared thermal imaging spectrometer has started only hours after orbit insertion to collect an immense and unique data set of special interest is the wavelength range from 1 1 5 microns which includes the atmospheric windows by averaging several hundred multi spectral images obtained during the mission we have been able to map the surface temperatures for almost the whole southern hemisphere of venus blurring in the clouds limits the spatial resolution to approximately 100 km globally the most exciting and at the same time the most complex task is to retrieve compositional information for the surface of venus from the hyperspectral data obtained by virtis we report here about indication for variations in the surface composition in the lada terra region based on brightness variations mapped from virtis measurements and present some tentative links to the evolution of this region

more constraints on internal heating rate of the earth s mantle from plume observations 1 our previous numerical studies of plume dynamics without secular cooling effects demonstrated that the internal heating rate of the earth s mantle is constrained to be 70 by inferred plume heat flux and plume excess temperature in the upper mantle in this paper by integrating the secular cooling effects into the numerical studies we demonstrate that the dependence of plume related observations on internal heating rate is not affected by secular cooling and that the plume related observations are only dependent on the internal heating rate and insensitive to its partitioning between radiogenic heating and secular cooling thus confirming the conclusions of 70 internal heating rate from our previous numerical studies furthermore we present a new analysis to constrain the internal heating rate with the plume related observations based on the energy balance of mantle convection the analysis shows that the internal heating rate of the mantle has a lower bound of 60 to satisfy the energy balance of the mantle with current plume observations which is consistent with the conclusions from our numerical studies

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

admittance survey of type 1 coronae on venus in this study we analyze magellan gravity and topography data for type 1 coronae on venus to estimate crustal thickness z c elastic thickness t e and apparent depth of compensation z l we examine the free air admittance for all 103 type 1 coronae defined as having greater than 50 complete fracture annuli that are resolved in the gravity data a spatio spectral method is used to calculate a localized admittance signature of each corona from a global admittance map this method extracts spectral information from a region in space whose area is varied as a function of degree to make it large enough to yield robust results at every individual wavelength elastic flexure models with either bottom or top loading compensation are used to fit the data the estimated lithospheric flexural parameters span the range obtained for other topographic features on venus suggesting that the lithosphere on which coronae form is variable we find no significant difference in lithospheric properties between type 1 and 2 coronae fifty four percent of all coronae are consistent with local isostasy which may indicate that they are no longer active very few coronae with dome or plateau morphologies have a bottom loading signature or the small t e and large z l expected if a mantle plume were present approximately 71 have z l estimates less than 70 km indicating that compensation probably occurs within the crustal layer we find no systematic relationship between t e or z c and corona diameter as would be expected for the spreading drop model of formation

nucleation points of recent mainshocks in southern iceland mapped by b values the earthquake catalog in southern iceland between 20 25 w and 21 3 w is complete down to approximately mw 0 and shows no evidence for mag nitude shifts as a function of time since 1991 this means that 6902 earthquakes m 0 1 depth 20 km are available for studies of seismicity patterns preceding the two ms 6 6 mainshocks in 2000 in their source volumes and vicinity 20 25 w and 20 9 w the power law of the frequency magnitude relationship holds down to m0 detailed mapping including 3d mapping of the b value of the frequency magnitude distribution shows that variations from b 0 6 to 1 4 exist over distances of about 2 km this suggests that the tectonic fabric in southern iceland is hetero geneous on a small scale the hypocenters of the two ms 6 6 mainshocks and one additional m 4 5 mainshock are associated with low b values 0 6 0 8 statistically significantly different from volumes in their vicinity therefore one can expect future mainshocks in southern iceland to also emanate from volumes characterized by low b values however the small dimensions of these asperities may render it difficult to identify them before the mainshocks occur low values of estimated local recur rence times 500 2000 years map the two fault zones that ruptured in 2000 against a background of local recurrence times longer than 5000 years the first of the two m 6 6 mainshocks emanated from a volume of short estimated local recurrence time this supports the hypothesis that asperities with short local recurrence times control locations of major ruptures mapping of b values in cross sections shows anomalies of high b at the bottom of the seismogenic crust correlating with the change of its thickness in the middle of the study area these high b value anomalies and other pockets of such anomalies at shallower depths are best interpreted as caused by high pore pressure because evidence of fluids under high pressure is ubiquitous in this area

calcite precipitation in synthetic veins implications for the time and fluid volume necessary for vein filling although calcite is a common vein filling mineral in rocks there has been little understanding of the kinetics of vein filling that gives a quantitative estimation of the fluid volume and time needed for calcite vein formation in subsurface fracture systems a new experimental system capable of simulating calcite precipitation in veins was used to determine the influences of flow velocity and initial solution saturation state on the quantitative precipitation pattern of calcite in synthetic veins results are in relatively good agreement with the model predictions use of a mn2 tracer indicated that even at low concentrations manganese can have a substantial effect on the morphology of the calcite precipitate our model predicts that an extremely large volume of fluid is required to produce calcite veins in nature typically 105 to 106 for a fluid calcite volume ratio for formation of thin short veins the model also predicts the time necessary for vein filling is thousands to millions of years and is highly variable depending on factors such as fluid saturation state flow velocity vein dimension and the precipitation kinetics that can differ under subsurface conditions

mantle evolution in venus due to magmatism and phase transitions from punctuated layered convection to whole mantle convection a series of numerical models of magmatism and mantle convection with a stagnant lithosphere are developed to understand the mantle evolution in venus magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting and the solid state convection of mantle materials with temperature dependent newtonian rheology is affected by the garnet perovskite transition and the postspinel transition in our preferred models the mantle evolves in two stages the earlier stage is characterized by layered mantle convection punctuated by repeated bursts of hot material from the deep mantle to the surface mantle bursts induce vigorous magmatism and also cause the basaltic crust enriched in heat producing elements hpes to recycle into the mantle a part of the recycled basaltic crusts accumulates along the postspinel boundary to form a barrier and this basalt barrier causes mantle convection to become layered at a later stage when the hpes have already decayed in contrast the basalt barrier disappears and whole mantle convection occurs more steadily mild magmatism is induced by small scale partial melting at the base of the crust and hot plumes from the deep mantle the internal heating by the hpes that recycled into the mantle in the earlier stage allows the magmatism of the later stage to continue throughout the calculated history of mantle evolution the two stages arise when the barrier effect of the postspinel transition is weak and the lithosphere is mechanically strong enough the two stage evolution model meshes with the observed history of magmatism and the lithosphere on venus

venus surface thermal emission at 1 μm in virtis imaging observations evidence for variation of crust and mantle differentiation conditions 1 the venus express spacecraft images the nightside thermal emissions using the visible and infrared thermal imaging spectrometer virtis at 1 02 1 10 and 1 18 m thermal emission from the surface is observed the signal is attenuated by scattering and absorption in the dense atmosphere the measured flux at the top of the atmosphere is positively correlated with surface temperature and surface emissivity the surface temperature of venus is relatively well constrained as being mainly a function of altitude with a gradient lesser or equal to the adiabatic lapse rate this study examines the correlation of virtis images showing a signal of the surface at 1 02 m with viewing geometry stray sunlight cloud opacity and topography and applies semiempirical relations to remove their influence the remaining contrast can be either ascribed to surface emissivity or unexpected temperature variations temperature variations due to active volcanism are unlikely to be persistent over the time of observations therefore the mosaic of all processed images is here interpreted in terms of surface emissivity variation the emissivity variation found is correlated with geomorphological features established from magellan synthetic aperture radar images it is generally lower at tessera terrain some but not all volcanic edifices show increased emissivity large lava flows in the lada terra lavinia planitia region also show an increased thermal emission this might indicate a more felsic surface composition of tessera highlands and large scale extrusive volcanism of ultramafic composition

on the fate of thermally buoyant mantle plumes at density interfaces the 660 km depth endothermic phase transition is expected to delay upwelling plumes and modify their morphology the interaction of thermal starting plumes with a density boundary is investigated experimentally by using simultaneous visualizations of temperature composition and velocity fields in a tank initially stratified with two viscous fluids with different densities and viscosities a thermal starting plume was generated by using a circular plate heater at the bottom of the tank its interaction mode with the inner interface depends on the local buoyancy number bl the ratio of the stabilizing chemical buoyancy to the plume thermal buoyancy at the interface the rayleigh number ra the viscosity contrast between the chemical layers and the local viscosity ratio of the ambient material to the plume head p for bl 0 6 the rebirth mode occurs where the thermal plume ponds and spreads under the chemical boundary and secondary thermal plumes are generated from the interface depending on the magnitude of bl these plumes can entrain upwards by viscous coupling a significant amount of lower layer material scaling laws are determined for the onset of secondary plumes their number and wavelength the amount of entrainment and the topography of the interface our results could explain the hotspots distribution above superswells and the peculiar time evolution of la reunion hotspot

elucidating the rate of volcanism on venus detection of lava eruptions using near infrared observations abstract lava eruptions can be observed remotely on venus nightside using a near infrared window at a 1 0 micrometer wavelength i e radiation at this wavelength emitted from the planetary surface traverses through the thick venus atmosphere and clouds due to the strong dependence of the intensity of thermal emission on temperature a hot surface produced by a lava eruption emits intense radiation with the intensity of the excess radiation depending on the surface area and temperature of the lava flow the excess emission from the hot surface is blurred by clouds yet a 1 km2 1500 k basaltic lava lake and a 50 km2 1000 k lava flow are readily detected using this technique rapid cooling of exposed lava results in excess surface radiation that is detectable for just one earth day after the end of an eruption hence short term eruptions are only detectable on the side of the planet where night is occurring eruptions lasting more than half a venus day i e more than about 60 earth days however are detectable across the globe as nighttime occurs sometime during this period for every such event due to limited observational geometries a complete global survey of active volcanos cannot be achieved from earth and in fact only about 10 of short lived major volcanic eruptions are observable alternatively a satellite orbiting high above venus 6 venus radii could detect virtually all short lived nighttime volcanic events or about half the events expected to occur across the globe

geologic mapping of tectonic planets geological analysis of planets typically begins with the construction of a geologic map of the planets surfaces using remote data sets geologic maps provide the basis for interpretations of geologic histories which in turn provide critical relations for understanding the range of processes that contributed to the evolution because geologic mapping should ultimately lead to the discovery of the types of operative processes that have shaped a planet surface geologic mapping must be undertaken in such a way as to allow such discovery i argue for modifications in current planetary geologic mapping methodology that admit that tectonic processes may have contributed to the formation of a planet surface and i emphasize a goal of constraining geologic history rather than determining global stratigraphy to this end it is imperative that secondary tectonic structures be clearly delineated from material units each record different aspects of planet surface evolution neglecting such delineation can result in geologic maps and interpreted geohistories in which both the spatial limits and the relative ages of material units and suites of secondary structures are incorrect determination of absolute time is fundamentally difficult to constrain in planetary studies in planetary geology the only means to estimate absolute age is the density of impact craters on a surface crater density surface ages are more akin to terrestrial and average mantle model ages which reflect the average time at which all of a rock s components were extracted from the earth s mantle rocks or planetary surfaces with very different geohistories could yield the same average model age dating tectonic events or determining rates of tectonic events is even more difficult fl 2000 elsevier science b v all rights reserved

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

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

the geological evolution of atai mons venus a volcano corona hybrid atai mons venus is classified as a large volcano yet displays characteristics that are typical of coronae coronae are widespread volcano tectonic features that are apparently unique to venus detailed analysis of atai mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active we show that processes found at both coronae and large volcanoes have occurred throughout the history of atai mons with no definite temporal preference it does not have an evolution that can be described simply as the evolution of one feature into another these results suggest that large volcanoes and coronae are more intimately linked than previously thought and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features

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

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