mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

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

resurfacing history of venus the stratigraphy impact cratering record and geoid of venus indicate that a major change in surface geologic activity has occurred before 800 ma venus was dominated by horizontal movement and tectonic deformation of the crust a brief period of global volcanism subsequently filled in all low lying areas and left much of the surface at a nearly uniform elevation current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust this geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally

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

geochemical and textural investigations of the eoarchean ukaliq supracrustals northern quebec canada abstract structural geochronological and geochemical studies of pre 3 75 ga rocks of volcano sedimentary protoliths in the inukjuak domain in the superior province in quebec have been mostly focused on the nuvvuagittuq supracrustal belt nsb the porpoise cove outcrops at the southwestern limit of the nsb are the de facto type locality for the supracrustals of the inukjuak complex yet it remains unclear whether the nsb rocks are geochemically distinct from or are in fact common to a host of other supracrustal enclaves locked in the dominantly gneissic inukjuak domain here we report detailed textural and geochemical studies for a suite of rocks from the ukaliq supracrustal belt usb located approximately 3 km northeast of the nsb we find that the usb and nsb have a similar protracted metamorphic history both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets quartz magnetite rocks banded iron formation s l and quartz biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions if the various inukjuak supracrustal belts were once a part of a larger coherent now dismembered terrane they should show similar emplacement ages and source chemistry new zircon u pb geochronology from five gneissic units and two quartz biotite metasedimentary schists reveal the oldest emplacement ages across all units of each individual rock type to be 3 68 0 07 ga granitoid gneisses and 3 65 0 06 ga quartz biotite schists these new ages are similar to those documented as likely minimum emplacement ages of the nsb determined by u pb geochronology zircons from the quartz biotite schist were also analyzed by ion microprobe for their u pb geochronology and were found to yield statistically identical albeit more precise ages than those obtained by la icp ms possible detrital zircons from the usb quartz biotite schists were analyzed by ion microprobe for their coupled 30sinbs28 and 18ovsmow values with respective values between 0 75 and 0 07 30sinbs28 and 5 61 and 6 59 18ovsmow the 18ovsmow values which are on average above mantle derived zircon indicate contribution of altered non mantle derived material into the source of the rocks that weathered to form the quartz biotite schists zircon mineral inclusions quartz feldspar apatite biotite muscovite and other unrecognized fe al si rich phases along with the major and trace element contents for the rocks were analyzed to substantiate this interpretation together with 30sinbs28 18ovsmow our results suggest that lithologies like authigenic silica and serpentinized rocks re melted to form the parent melts that gave rise to zircons found in the usb quartz biotite schists additional lu hf studies reported here on the same zircons also show similarities with nsb zircons the ehf values showed a positive correlation with the measured u pb age from 22 7 0 8 to 1 9 1 1 the lu hf system also reveals that the usb extracted at ca 3 8 ga carries isotopic signatures of an older hadean reservoir having been formed from an eoarchean mafic melt that incorporated them taken together this supports a co genetic origin for the nsb and the usb

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

effects of lateral viscosity variations on long wavelength geoid anomalies and topography models for the earth s geoid and dynamically supported topography usually account only for radial vertical variations in mantle rheology and flow structure however expected lateral variations in viscosity due to temperature or stress dependent rheology can degrade our ability to infer mantle structure from geoid models based on seismically determined density contrasts in the mantel theoretically the main difficulty is that the harmonic components of the flow stress deformation field are no longer independent and exact analytical methods cannot account for this mode coupling for large thermal density contrasts of small lateral dimension e g subducted slabs it is difficult to form any general theory for these effects however we believe that the largest geoid anomalies which occur at the lowest harmonic degrees l 2 3 are due to a global pattern of relatively mild temperature variations for convection with mainly broadscale thermal and rheological changes some relatively straightforward ideas can be developed combining results from simple perturbation theory and numerical models for convective flow we find the following 1 if we are correct in inferring viscosity variations of less than one order of magnitude globally the longest wavelength geoid anomalies l 2 3 are not seriously contaminated by lateral viscosity variations 2 contamination of the higher degrees l 4 is expected and in particular self coupling between l 2 heterogeneity and viscosity variations may be observable at degree 4 the most severe contamination occurs at the doubled harmonic l 4 of the dominant heterogeneity harmonic l 2 3 perturbation techniques are adequate to model global scale lateral viscosity variations of an order of magnitude or less 4 the steady state surface deformation problem at the heart of our geoid models may be more robust with respect to broadscale lateral viscosity variations than are models for transient deformations e g postglacial rebound 5 for loading by broadscale density heterogeneity the effects of stress dependent rheology are probably secondary to the effects of radial viscosity stratification

transient mantle layering and the episodic behaviour of venus due to the basalt barrier mechanism abstract numerical models of mantle convection that include the basalt barrier mechanism are explored for venus the basalt barrier mechanism is due to the positive buoyancy of subducted basaltic crust between the mantle depths of 660 and 750 km the inclusion of this mechanism in models of earth s evolution has been shown to cause episodic mantle layering early in earth history and we explore whether it can also operate on venus the models presented here include a moderately mobile lithosphere which is not representative of the current state of venus but this allows us to exclude the effects of episodic lithosphere mobility and thus to isolate the effect of the basalt barrier this is a step in a systematic approach to models with a mostly static lithosphere we find the basalt barrier does yield episodically layered mantle convection in some venus models the likelihood of episodic layering is increased by venus high surface temperature and by its less mobile or immobile lithosphere surprisingly secondary differences from earth including the lower gravity density and mantle depth also promote episodic layering the models suggest that mantle layering and overturns may still be likely to occur in venus the breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism formation of large amounts of crust and tectonic activity on the planet s surface as has been inferred to have happened on venus around 500 ma ago from surface morphology and cratering these results thus suggest that a transient layering of the mantle by the basalt barrier mechanism and mantle overturns may be part of the explanation for venus s recent resurfacing

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

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

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

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

inferences on the mantle viscosity structure and the post overturn evolutionary state of venus abstract venus has similar size density and bulk composition as earth but has tectonically evolved clearly differently and this divergence remains enigmatic surface observations such as gravity topography and surface age constrain venus evolution but interpreting these signals requires understanding of the surface interior coupling and thus insight into the structure and evolution of the venusian mantle and lithosphere here we investigate how such observables may be generated from interior dynamics using numerical forward models of global mantle convection that consistently link the thermochemical magmatic and tectonic evolution of venus venus present surface gravity spectrum and its relation to topography is matched best by our models with a mantle viscosity profile featuring a sublithospheric minimum of 2 1020 pa s and a gradual increase by a factor of 100 down to a depth of 250 km above the core mantle boundary no pronounced viscosity jump around the mantle transition as inferred for earth is favoured for venus which points to a relatively dry venusian upper mantle compared to earth s as previously suggested this holds true for both a pure stagnant lid scenario and in the presence of episodic catastrophic overturns triggered by cumulative crustal growth due to on going magmatism and volcanism overturns strongly perturb the surface gravity spectrum up to 150 myr after overturn cessation material deeply recycled by the resurfacing event annihilates the developed plume pattern which needs much longer than those 150 myr to recover to a state comparable to the pattern suggested by thermal emissivity anomalies observed on venus moreover overturns limit crustal thicknesses to reasonable values and are more capable than stagnant lid evolutions in generating mean surface ages 500 myr these findings seem to confirm previous suggestions that the episodic regime is more applicable to venus than a purely stagnant lid regime yet the relatively long time span required to recycle the entire surface 150 200 myr and the presently on going volcanic resurfacing predicted by our models complicate the formation of a uniform surface age as indicated by venus crater population and may also suggest that the latest overturn has ceased longer ago than indicated by venus present mean surface age

the resurfacing history of venus constraints from buffered crater densities because of atmospheric shielding and endogenic resurfacing the population of impact craters on venus is small about a thousand and consists of large craters this population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of venus but the nearly spatially random nature of the crater population has complicated efforts to understand this history here we utilize the recent 1 15 m scale global geological map of venus ivanov m a head j w 2011 planet space sci 59 1559 1600 to help address this problem the global geological map provides a stratigraphic sequence of units and known areas where each unit is exposed on the planet for each crater on venus we identify the specific geological units predating and postdating the crater we perform a statistical analysis of this set of observations with a buffered crater density approach which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and or postdate more than one unit in this analysis we consider crater emplacement as random and resurfacing history as determined although unknown we obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries we find that 1 size frequency distributions of craters superposed on each unit are consistent with each other 2 regional plains and stratigraphically older units have similar crater retention ages 3 stratigraphically younger units have a mean crater retention age significantly younger than the regional plains these findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10 of the mean surface age tens of millions of years the termination of intensive resurfacing may or may not be synchronous over the planet our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images but obscure radar contrasts between predating lava flows we do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

density and morphology of impact craters on tessera terrain venus densities of impact craters on tessera which is complex ridged terrain of tectonic origin and on the remainder of the planet which is mostly volcanic plains were studied using magellan images for about 96 of the surface of venus the density of large d 16 km impact craters on tessera is higher by a factor of about 1 4 than on the remainder of the planet this means that the tessera crater retention age is larger than the age of the plains this is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains the density of small d 16 km impact craters on tessera is lower than on the remainder of the planet evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain the absence of recognizable tectonic deformation in most of the large on tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation

deep versus shallow origin of gravity anomalies topography and volcanism on earth venus and mars abstract the relation between gravity anomalies topography and volcanism can yield important insights about the internal dynamics of planets from the power spectra of gravity and topography on earth venus and mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l 30 for earth 40 for venus and 5 for mars to interpret the low degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics for these viscosity profiles we then compute gravity and topography kernels which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth with these kernels we firstly compute an expected gravity topography ratio good agreement with the observed ratio indicates that for venus in contrast to earth and mars long wavelength topography is largely dynamically supported from the sublithospheric mantle secondly we combine an empirical power spectrum of density anomalies inferred from seismic tomography in earth s mantle with gravity kernels to model the gravity power spectrum we find a good match between modeled and observed gravity power spectrum for all three planets except for 2 l 4 on venus density anomalies in the venusian mantle for these low degrees thus appear to be very small we combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the martian and venusian mantles gravity kernels for l 5 are very small on venus below 800 km depth thus our inferences on venusian mantle density are basically restricted to the upper 800 km on mars gravity anomalies for 2 l 5 may originate from density anomalies anywhere within its mantle for mars as for earth inferred density anomalies are dominated by l 2 structure but we cannot infer whether there are features in the lowermost mantle of mars that correspond to earth s large low shear velocity provinces llsvps we find that volcanism on mars tends to occur primarily in regions above inferred low mantle density but our model cannot distinguish whether or not there is a martian analog for the finding that earth s large igneous provinces mainly originate above the margins of llsvps

did a mega collision dry venus interior the limited relaxation of shapes of impact craters and high correlation between topography and gravity are some of the reasons that support the widely accepted view that the interior of venus is dry compared to earth the fact that the atmospheric abundance of 40ar is only 25 of the radiogenic gas produced inside venus argues that venus is not thoroughly degassed and its interior has not been dried over time therefore venus must have lost its water very early in its history before any significant 40ar was produced current ideas suggest that venus did not suffer a major impact therefore one would not expect it to have been substantially molten as a result degassing all its water would be difficult and losing all the water without leaving oxygen in the atmosphere would also be very difficult to overcome the above difficulties in explaining a dry venus interior a new hypothesis is proposed that venus formed by a near head on collision of two large planetary embryos as might be expected from favoured oligarchic planetary accretion such a collision would be sufficiently large to melt totally and briefly vapourise a significant proportion of both bodies this would allow much of the released water to react rapidly with iron depending upon the reaction hydrogen is either expected to escape to space or enter the core oxygen would form feo most of which would enter the core together with other iron reaction products most everything else not caught in the hydrodynamic escape driven by any hydrogen stream would be gravitationally retained by the final body the model can therefore reconcile the 40ar data a virtually oxygen free atmosphere and a dry interior an appropriate large collision also provides an easy explanation for the retrograde rotation of venus the possible implications for inner core and magnetic field formation and atmosphere evolution including effects on d h c n and inert gases are also discussed a simple test of this hypothesis is that in contrast to the current venus paradigm little or no hydrated minerals should be found on the surface

fracturing and fluid flow in a sub decollement sandstone or a leak in the basement crack seal texture within fracture cements in the triassic el alamar formation ne mexico shows that the fractures opened during precipitation of quartz cements later overlapping calcite cements further occluded pore space previous workers defined four systematic fracture sets a oldest to d youngest with relative timing constrained by crosscutting relationships quartz fluid inclusion homogenization temperatures are higher within set b 148 20 c than in set c 105 12 c these data and previous burial history modelling are consistent with set c forming during exhumation fluid inclusions in set c quartz have higher salinity than those in set b 22 9 v 14 2 wt nacl equivalent respectively and set c quartz cement is more enriched in 18 o 20 2 v 18 7 vsmow under most assumptions about the true temperature during fracture opening the burial duration the amount of cement precipitated and fluid flow patterns it appears that the fracture fluid became depleted in 18 o and enriched in 13 c this isotopic evolution combined with increasing salinity suggests that throughout fracture opening there was a gravity driven influx of fluid from upsection jurassic evaporites which form a regional decollement fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading rather than increases in fluid pressure

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

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

towards self consistent modeling of the martian dichotomy the influence of one ridge convection on crustal thickness distribution in order to find an explanation for the origin of the martian crustal dichotomy a number of recent papers have examined the effect of layered viscosity on the evolution of a degree 1 mantle convection e g roberts and zhong roberts j h zhong s 2006 j geophys res 111 e06013 and yoshida and kageyama yoshida m kageyama a 2006 j geophys res 111 doi 10 1029 2005jb003905 b03412 it was found that a mid mantle viscosity jump combined with highly temperature and depth dependent rheology are effective in developing a degree 1 convection within a short timescale such a layered viscosity profile could be justified by martian mineralogy however the effect of a degree 1 convective planform on the crustal thickness distribution has not yet been demonstrated it is not obvious whether a thinner crust due to sublithospheric erosion and crustal thinning or a thicker crust due to enhanced crustal production would form above the hemisphere of mantle upwelling also the general shape of the dichotomy which is not strictly hemispherical has not yet been fully investigated here we propose a model of the crustal patterns produced by numerical simulations of martian mantle convection using the finite volume multigrid code stagyy tackley p j 2008 phys earth planet int 107 7 18 doi 10 1016 j pepi 2008 08 005 a self consistent treatment of melting crustal formation and chemical differentiation has been added to models of three dimensional thermal convection this allows us to obtain global maps of the crustal thickness distribution as it evolves with time the obtained results demonstrate that it is indeed possible to form a crustal dichotomy as a consequence of near degree 1 mantle convection early in mars history we find that some of the observed patterns show intriguing first order similarities to the elliptical shape of the martian dichotomy in all models the region of thick crust is located over the region of mantle upwelling which itself is a ridge like structure spread over roughly one half of the planet a planform we describe as one ridge convection

modeling the dynamic component of the geoid and topography of venus 1 we analyze the venusian geoid and topography to determine the relative importance of isostatic elastic and dynamic compensation mechanisms over different degree ranges the geoid power spectrum plotted on a log log scale shows a significant change in its slope at about degree 40 suggesting a transition from a predominantly dynamic compensation mechanism at lower degrees to an isostatic and or elastic mechanism at higher degrees we focus on the dynamic compensation in the lower degree interval we assume that 1 the flow is whole mantle in style 2 the long wavelength geoid and topography are of purely dynamic origin and 3 the density structure of venus mantle can be approximated by a model in which the mass anomaly distribution does not vary with depth solving the inverse problem for viscosity within the framework of internal loading theory we determine the families of viscosity models that are consistent with the observed geoid and topography between degrees 2 and 40 we find that a good fit to the data can be obtained not only for an isoviscous mantle without a pronounced lithosphere as suggested in some previous studies but also for models with a high viscosity lithosphere and a gradual increase in viscosity with depth in the mantle the overall viscosity increase across the mantle found for the latter group of models is only partially resolved but profiles with a 100 km thick lithosphere and a viscosity increasing with depth by a factor of 10 80 hence similar to viscosity profiles expected in the earth s mantle are among the best fitting models

continental crust formation on early earth controlled by intrusive magmatism the global geodynamic regime of early earth which operated before the onset of plate tectonics remains contentious as geological and geochemical data suggest hotter archean mantle temperature and more intense juvenile magmatism than in the present day earth two crust mantle interaction modes differing in melt eruption efficiency have been proposed the io like heat pipe tectonics regime dominated by volcanism and the plutonic squishy lid tectonics regime governed by intrusive magmatism which is thought to apply to the dynamics of venus both tectonics regimes are capable of producing primordial tonalite trondhjemite granodiorite ttg continental crust but lithospheric geotherms and crust production rates as well as proportions of various ttg compositions differ greatly which implies that the heat pipe and plutonic squishy lid hypotheses can be tested using natural data here we investigate the creation of primordial ttg like continental crust using self consistent numerical models of global thermochemical convection associated with magmatic processes we show that the volcanism dominated heat pipe tectonics model results in cold crustal geotherms and is not able to produce earth like primordial continental crust in contrast the plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various ttg rocks using a systematic parameter study we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data low medium and high pressure ttg our study thus suggests that the pre plate tectonics archean earth operated globally in the plutonic squishy lid regime rather than in an io like heat pipe regime

effect of water in depleted mantle on post spinel transition and implication for 660 km seismic discontinuity abstract we have determined the post spinel transition boundary in anhydrous and hydrous mg2sio4 in a temperature range from 1173 to 2023 k at 19 3 25 4 gpa using synchrotron in situ x ray diffraction measurements the phase boundary in mg2sio4 is located at 22 gpa and 1800 k and 22 1 gpa and 1500 k which is slightly lower 0 3 0 5 gpa than that determined in the previous in situ measurements using the same pressure scale e g katsura et al 2003 post spinel transition in mg2sio4 determined by high p t in situ x ray diffractometry phys earth planet inter 136 11 24 the clapeyron slope of mg2sio4 was found to be gentle i e between 0 4 and 0 7 mpa k which is also consistent with previous in situ measurements but inconsistent with diamond anvil cell experiments and theoretical estimations the phase boundary in mg2sio4 2 wt h2o which is relevant to fe free depleted harzburgitic composition is located between 23 4 and 23 6 gpa and 1500 k which shifts the hydrous boundary to the higher pressures relative to anhydrous mg2sio4 from 1 3 to 1 0 gpa the result for hydrous mg2sio4 shows steeper clapeyron slope between 3 2 and 3 1 mpa k compared with anhydrous mg2sio4 and hydrous pyrolite system the present data suggest that water has a strong influence on 660 km discontinuity and the depressions observed at this boundary in several regions especially related to subduction zones can be explained by the presence of water in depleted harzburgite component

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

interior structure of the moon constraints from seismic tomography gravity and topography abstract seismic tomography can be combined with constraints from geoid topography and other surface observations to gain information about mantle structure and dynamics this approach has been taken with much success for the earth mantle and here it is for the first time applied to the moon lunar tomography has much lower resolution as for the earth and is mostly restricted to the near side nevertheless we can assess under what assumptions the fit between predicted geoid based on a tomography model and observed geoid is best among the models tested we find the most similar pattern correlation about 0 5 if we only consider tomography below 225 km depth if density anomalies cause little or no dynamic topography and if we compare to the geoid with the flattening l 2 m 0 term removed this could mean that a like for the earth seismic anomalies shallower than 225 km are caused by a combination of thermal and compositional effects and therefore cannot be simply converted to density anomalies b the lithosphere is sufficiently thick to prevent dynamic topography more than a small fraction of total topography and c flattening is a fossil bulge unrelated to present day mantle anomalies however we have to be cautious with interpreting our results because for models with a comparatively higher correlation and a conversion from seismic velocity to density anomalies similar to the earth s upper mantle the amplitude of the predicted geoid is much lower than observed this could either mean that the tomography model is strongly damped or that the geoid is mostly due to shallow causes such as crustal thickness variations with only a small part coming from the deeper mantle

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

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

interpolation and gridding of aliased geophysical data using constrained anisotropic diffusion to enhance trends geophysical data are frequently collected with a fine sample interval along traverse lines but with a coarser sampling in the direction perpendicular to the traverses this disparity in sampling intervals is particularly evident when magnetic data are collected simultaneously with airborne electromagnetic data interpolating this traverse data onto an evenly spaced 2d grid can result in aliasing artifacts for example narrow linear structures that trend at acute angles to the traverse lines are imaged as a thick thin thick feature looking like a boudinage or string of beads applying the anisotropic diffusion process to the resulting grids of data removes the artifacts but the grid values close to the traverses are altered significantly from their initial values the altered values are therefore not faithful to the original traverse data the anisotropic diffusion algorithm can be modified to constrain values close to the original traverses this modification removes the aliasing artifacts and produces a data grid faithful to the original traverse data some small artifacts along the traverse lines in the final data grid become more evident when grids containing derivative data such as the analytic signal are generated from the new data grid however these small traverse line artifacts can be removed with standard microleveling procedures the constrained anisotropic diffusion process is iterative and some experimentation is required to determine the appropriate number of iterations

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

the association between tectonic deformation inorganic composition and coal rank in the bituminous coals from the south wales coalfield united kingdom analysis of vertical distribution of coal rank in three collieries and one opencast mine in the se of the south wales coalfield has revealed deviations from hilt s law in both proximate analyses and in vitrinite reflectance the variations in rank parameters studied are closely correlated with variations in the intensity of tectonic deformation measured using a numerical deformation index ndi three seams in the sequence were also analysed for major and trace element geochemistry and rock eval pyrolysis variations in these parameters are related to thrust inclusion of floor and or roof mudrocks into the seams a detailed conceptual model of a major thrust and associated in seam thrust detachments and lag faults is developed to explain the vertical rank profile by post coalification faulting it is concluded that although the model can explain the vertical pattern of rank the complexity of the required faults render the model improbable the alternative mechanisms for enhancement of the rank parameters associated with tectonic deformation including shear stress control frictional heating and localised fluid flow are discussed and are thought more likely to be responsible for the deviations from hilt s law in the vertical rank profile rank suppression shown by three seams two of which are overlain by marine bands is thought to be related to biochemical activity in the peat environment resulting from the influx of saline waters

vertical and lateral fluid flow related to a large growth fault south eugene island block 330 field offshore louisiana data from sediments in and near a large growth fault adjacent to the giant south eugene island block 330 field offshore louisiana indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux sediments within and adjacent to the fault zone in the u s department of energy pennzoil pathfinder well at about 2200 m sstvd subsea true vertical depth showed little paleothermal or geochemical evidence for throughgoing fluid flow the sediments were characterized by low vitrinite reflectances ro averaging 0 3 ro moderate to high d18o and d13c values and little difference in major or trace element composition between deformed and undeformed sediments in contrast faulted sediments from the a6st well which intersects the a fault at 1993 m sstvd show evidence for a paleothermal anomaly 0 55 ro and depleted d18o and d13c values sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well this effect diminishes with distance from this gouge zone cuttings from other wells in south eugene island block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone overall indicators of mass and heat flux indicate the main growth fault zone in south eugene island block 330 has acted as a conduit for ascending fluids although the cumulative fluxes vary along strike this conclusion is corroborated by oil and gas distribution in downthrown sands in blocks 330 and 331 which identify the fault system in northwestern block 330 as a major feeder simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by a6st well was short lived having a duration less than 150 yr the anomaly could have been produced by a 2 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km yr darcy flux of 330 m yr from 3 km deeper in the basin simple darcy law computation indicates a transient fault permeability on the order of 110 md during this flow pulsing of fluid up the fault was probably the norm although most flow did not produce such strong thermal anomalies as the one detected in the a6st well analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it the hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault fault permeability is best understood in terms of effective stress under ambient conditions the fault is at high pressure relative to downthrown reservoirs a pulse of high pressure fluid ascending end page 244 the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability if the fluid is in an area of the fault adjacent to downthrown relatively low pressure reservoir sands the fluid will discharge into them permeability in and adjacent to the fault then decreases such that fluid cannot reenter the fault zone and escape from the reservoir

the effects of internal heating and large scale climate variations on tectonic bi stability in terrestrial planets abstract we use 3d mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet a proxy for thermal age planetary surface temperature and lithosphere strength at both high and low values of internal heating for moderate to high lithospheric yield strength hot and cold stagnant lid single plate planet states prevail for intermediate values of internal heating multiple stable tectonic states can exist in these regions of parameter space the specific evolutionary path of the system has a dominant role in determining its tectonic state for low to moderate lithospheric yield strength mobile lid behavior a plate tectonic like mode of convection is attainable for high degrees of internal heating i e early in a planet s thermal evolution however this state is sensitive to climate driven changes in surface temperatures relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile to a stagnant lid regime once a stagnant lid mode is initiated a return to mobile lid is not attainable by a reduction of surface temperatures alone for lower levels of internal heating the tectonic regime becomes less sensitive to surface temperature changes collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga year timescales within parameter space regions that allow for bi stable behavior any model based prediction as to the current mode of tectonics is inherently non unique in the absence of constraints on the geologic and climatic histories of a planet

scaling laws for convection with temperature dependent viscosity and grain damage numerical experiments of convection with grain damage are used to develop scaling laws for convective heat flow mantle velocity and plate velocity across the stagnant lid and plate tectonic regimes three main cases are presented in order of increasing complexity a simple case wherein viscosity is only dependent on grainsize a case where viscosity depends on temperature and grainsize and finally a case where viscosity is temperature and grainsize sensitive and the grain growth or healing is also temperature sensitive in all cases convection with grain damage scales differently than newtonian convection due to the effects of grain damage for the fully realistic case numerical results show stagnant lid convection fully mobilized convection that resembles the temperature independent viscosity case and partially mobile or transitional convection depending on damage to healing ratio rayleigh number and the activation energies for viscosity and healing applying our scaling laws for the fully realistic case to earth and venus we demonstrate that increasing surface temperature dramatically decreases plate speed and heat flow essentially shutting down plate tectonics due to increased healing in lithospheric shear zones as proposed previously contrary to many previous studies the transitional regime between the stagnant lid and fully mobilized regimes is large and the transition from stagnant lid to mobile convection is gradual and continuous thus planets could exhibit a full range of surface mobility as opposed to the bimodal distribution of fully mobile lid planets and stagnant lid planets that is typically assumed

new evidences for the formation of and for petroleum exploration in the fold thrust zones of the central black sea basin of turkey the central black sea basin of turkey is filled by more than 9 km 6 mi of upper triassic to holocene sedimentary and volcanic rocks the basin has a complex history having evolved from a rift basin to an arc basin and finally having become a retroarc foreland basin the upper triassic lower jurassic akgol and lower cretaceous caglayan formations have a poor to good hydrocarbon source rock potential and the middle eocene kusuri formation has a limited hydrocarbon source rock potential the basin has oil and gas seeps many large structures associated with extensional and compressional tectonics which could be traps for hydrocarbon accumulations exist fifteen onshore and three offshore exploration wells were drilled in the central black sea basin but none of them had commercial quantities of hydrocarbons the assessment of these drilling results suggests that many wells were drilled near the ekinveren erikli and ballifaki thrusts where structures are complex and oil and gas seeps are common many wells were not drilled deep enough to test the potential carbonate and clastic reservoirs of the inalti and caglayan formations because these intervals are locally buried by as much as 5 km 3 mi of sedimentary and volcanic rocks no wells have tested prospective structures in the north and east where the prospective inalti and caglayan formations are not as deeply buried untested hydrocarbons may exist in this area

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

an explanation for the shape of earth s gravity spectrum based on viscous mantle flow models 1 the earth s gravity spectrum can be used as an observational constraint on geophysical modelling here we show how the spectrum up to degree l 31 can be explained to a large part by viscous mantle flow in combination with a very simple model of random mantle density anomalies efforts to make the calculation more realistic by considering effects of thermal boundary layers or using density anomalies based on tomography or geodynamic modelling tend to worsen the fit results are rather sensitive to assumptions on density anomalies in the upper thermal boundary layer we suggest that in combination with other observations the shape of the earth s gravity spectrum can serve to better constrain radial viscosity structure density anomalies and flow in the earth s mantle appropriate treatment of the lithosphere and of lateral viscosity variations will be the main challenges in modelling this spectrum

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

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

natural fracture cleat and strong adsorption impact on low oil and condensate retention in the carboniferous shales and coals of the western black sea basin of turkey total organic carbon toc values of the carboniferous shales and coals of the western black sea basin of turkey range from 0 72 to 71 52 wt with an average of 32 3 wt and generation potential chance from 0 33 to 184 64 with an average of 82 47 mg hc g rock hydrogen index hi values of the shales and coals range from 24 to 384 mg hc g toc the hi versus pyrolysis temperature at the maximum rate of kerogen conversion plot indicates that the organic matter is type ii iii the calculated vitrinite reflectance equivalent values of the shales and coals range from 0 63 to 1 32 thus the shales and coals of the carboniferous formations are represented by immature effective oil and condensate generation windows the shales and coals of the alacaagzi formation contain high quartz and lower clay oil saturation index osi values of the shales and coals have been calculated as being 0 7 and 12 mg hc g toc indicating possible lack of producible oil and condensate the osi values are lower than those of other coal bearing and marine shales of the world first strong adsorption capacity may be a cause of the lower oil and condensate retention second faults natural fractures and cleats related fluid flows may be important factors in these lower osi ratios the extensional collisional and inverse tectonic stages and diagenetic processes of coalification formed multiple interconnected faults natural opening mode fractures and cleats in the shales and coals thus 15 coalbed methane exploration wells failed to produce any economical coalbed methane

effect of a single large impact on the coupled atmosphere interior evolution of venus abstract we investigate the effect of a single large impact either during the late veneer or late heavy bombardment on the evolution of the mantle and atmosphere of venus we use a coupled interior exterior numerical code based on stagyy developed in gillmann and tackley gillmann c tackley p j 2014 j geophys res 119 1189 1217 single vertical impacts are simulated as instantaneous events affecting both the atmosphere and mantle of the planet by i eroding the atmosphere causing atmospheric escape and ii depositing energy in the crust and mantle of the planet the main impactor parameters include timing size mass velocity and efficiency of energy deposition we observe that impact erosion of the atmosphere is a minor effect compared to melting and degassing triggered by energy deposition in the mantle and crust we are able to produce viable pathways that are consistent with present day venus especially considering large late veneer impacts small collisions