venusian channel formation as a subsurface process 1 we constructed detailed geologic maps of channels within three widely separated regions using magellan synthetic aperture radar sar imagery and altimetry radar data in order to understand the operative processes involved in venusian channel formation channels in each region cut topographic highs including ridges and shield edifices with no deflection of the channel course by surface topography we argue that these relationships are difficult to reconcile with the widely held view that venusian channels represent surface processes whether constructional or erosional we conclude that the channels evolved through subsurface fluid flow which involved local stoping and transport of overlying material that is these channels were carved from below rather than from above we postulate that at least some venusian channels form because of subsurface fluid flow along a shallow crustal interface that forms the boundary between overlying low backscatter surface materials and underlying basal materials fluid movement along the interface may facilitate piecemeal stoping and erosion of the local surface materials from below eventually resulting in the formation of channel traces exposed at the surface the low backscatter material which hosts the channels in this study also hosts abundant coalescing shields and associated deposits which may represent shield terrain

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

convection driven subsolidus crustal thickening on venus we analyze the phenomenon of subsolidus crustal flow in response to mantle convection and test the hypothesis that venusian crustal plateaus such as ovda regio formed over mantle downwellings we investigate this process using results from the parameterized convection models of solomatov and moresi 1996 to calculate stress boundary conditions for the finite element code tecton the effects of a brittle crust are simulated by constraining the minimum crustal strain rates in the models with a diabase strength envelope a series of models that could correspond to venus 1 gyr ago were created by varying such parameters as the crustal and mantle flow laws plastic behavior of the upper crust crustal thermal conductivity and the temperature at the planet s surface possibly higher due to increased interior outgassing as expected crustal thickening was found to be negligible in a stagnant lid regime regardless of crustal rheology in a small viscosity contrast regime plateaus greater than 1 km in elevation are possible within the lifetime of a mantle downwelling only for a weak crustal flow law therefore it is unlikely that venusian crustal plateaus such as ovda regio formed as a result of subsolidus crustal thickening over downwellings unless the crust had considerably less strength than indicated by the dry diabase measurements of mackwell et al 1995 1996

tectonic and kinematic study of a strike slip zone along the southern margin of central ovda regio venus geodynamical implications for crustal plateaux formation and evolution the tectonic system of the southern margin of central ovda regio a crustal plateau which straddles venus equator has been interpreted as a dextral strike slip array on the basis of evidence clearly identifiable as are riedel fracture patterns of different scales en echelon folds and brittle strike slip faults this transcurrent regime developed two main shear belts inner and outer on respectively thicker and thinner crust whose minimum dextral displacement has been estimated in 30 50 km since the up or downwelling models for plateau formation cannot easily explain tectonic shears of this magnitude along their margins an alternative hypothesis has been built which stands on the proposed collisional belt which could form ovda northern border king et al 1998 lunar planet sci conf 29 abstract 1209 tuckwell and ghail 2002 lunar planet sci conf 33 abstract 1566 within this framework the shear would represent a transcollisional transcurrent zone similar to the strike slip zones produced in the foreland of the himalayas tibet collision front eastern ovda would be an independent area of thickened crust pushed to the sse by the northern collision with the deformation concentrated at its margins and experiencing a shear strain on its southern margin none of the data however either supports nor helps to discard theoretical subduction events as a cause of the collision on the contrary image relationships could be interpreted as evidence that the main shear deformation took place during the last global resurfacing event on the planet

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

quantification of tertiary erosion in the inner moray firth using sonic velocity data from the chalk and the kimmeridge clay abstract sonic velocities for the chalk and kimmeridge clay were analysed to determine the importance and magnitude of tertiary erosion for 26 wells in the inner moray firth apparent erosion height above maximum burial depth was derived by the displacement along the depth axis of a given velocity depth trend from the normal undisturbed trend the similarity in apparent erosion results derived from the cretaceous chalk and late jurassic kimmeridge clay successions of the inner moray firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and or diagenetic process resultant apparent erosion estimates are variable across the inner moray firth such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern inner moray firth however the actual magnitude of erosion equal to the sum of apparent erosion and post erosional tertiary burial was approximately 1 km throughout the whole inner moray firth area indicating that early tertiary erosion was regional and not simply restricted to the inverted basin margins however a well defined mechanism sufficient to have generated the regional event is still sought

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

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

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

structural evolution of western fortuna tessera venus western fortuna tessera ishtar terra records polyphase deformation structures include ribbons forming long narrow steep sided troughs that reflect surface layer extension n trending folds related to maxwell montes deformation short wide nw trending graben that reflect late local extension and within the southwestern map area gentle e trending warps which postdate ribbon structures flood lava flows postdate deformation ribbon structures change orientation radially from nne to wnw these observations caution against the correlation of fortuna tessera with tessera elsewhere on venus and against the proposal that tessera forms a global stratigraphic layer

an alternative kinematic interpretation of thetis boundary shear zone venus evidence for strike slip ductile duplexes 1 on venus a 1000 km long 50 to 200 km wide anastomosing shear zone system thetis boundary shear zone tbsz separates the eastern ovda and northwestern thetis regiones previous workers mapped a part of this shear zone and interpreted it as a sinistral strike slip shear zone containing an extensional jog in the present study it is reexamined for a detailed structural and kinematic analysis and an alternative kinematic interpretation is provided the tbsz is divided into western central and eastern segments the western segment is narrow and is oriented in a ene wsw direction along its northern boundary the preexisting folds of tessera are cut by the shear zone and the folds are dragged into parallelism with the shear zone in response to dextral strike slip motion both the western and central segments contain a pair of conjugate deformation bands sigmoidal folds and linear ridges which appear similar to s c like structures of large scale ductile shear zones the acute angle between the s and c bands suggests dextral sense of displacement the eastern segment forms the northeastern restraining bend of the tbsz where the shear zone is characterized by a wide branching fan like anastomosing shear zone system with well developed contractional strike slip ductile duplexes the internal fabric defined by the sigmoidal folds consistently shows dextral sense of displacement therefore the tbsz is a dextral strike slip shear zone showing s c like structures and contractional strike slip duplexes similar to the continental scale ductile shear zones on earth

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

the assessment of rockfall hazard at the base of talus slopes fragmental rockfall is characterized by the independent movement of individual rock fragments after detachment from a rock face the continued operation of the process leads to the accumulation of talus slopes on talus slopes the rockfall shadow extends beyond the base of the talus and consists of scattered boulders that have run out beyond the base of the slope the landing probability of boulders in the shadow is examined return periods of the order of 1000 years relative to a house site are typical rockfall behaviour particularly with respect to run out into the shadow can be assessed using geological evidence empirical methods physical modelling and computer based analytical models an empirical minimum shadow angle of 27 5 i e the angle between the distal limit of the shadow and the top of the talus slope is suggested and would be useful in rockfall vulnerability studies at the base of talus slopes as a first approximation to shadow limits it is preferable to the use of the rockfall fahrb

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

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

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

mapping hydrocarbon charge points in the wessex basin using seismic geochemistry and mineral magnetics abstract this study reports a multidisciplinary approach to determining hydrocarbon charge points and migration in the wessex basin southern england geochemical analysis of reservoir core material bridport sandstone and inferior oolite using gas chromatography mass spectrometry gc ms suggests that the oil in the wessex basin is from a single source and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years using seismic data basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the purbeck fault into the sherwood sandstone reservoir at wytch farm one of these potential pathways is represented by cores termed creech and the other bushey farm to try to distinguish between the two potential pathways cores were studied using mineral magnetic techniques the magnetic signature was characterised using low temperature

geologic evolution of the akna montes atropos tessera region venus the investigated area comprises an arcuate mountain belt akna montes in western ishtar terra associated with an outboard plateau atropos tessera to the west and a volcanic plateau lakshmi planum to the east eight geologic units have been recognized on the basis of their geomorphic and structural characteristics as they appear on magellan radar images our stratigraphic analysis shows that the geological evolution of the study area can be explained by four main steps 1 formation of the older substrata of atropos tessera and lakshmi 2 extensive plains emplacement 3 an orogenic phase including the formation of akna montes and 4 local emplacement of younger plains the tectonic evolution shows a deformational sequence characterized by contraction shear and topographic relaxation this sequence is interpreted to be a consequence of the variation of crustal stresses and crustal thickening during orogenic events as observed for terrestrial high plateaus associated with a mountain belt i e himalaya and tibet andes and altiplano in order to estimate the amount of crustal shortening associated with the akna montes we considered two end members for structural style of the mountain belt a symmetric fold model and a fault bend fold model the models are theoretical because terrestrial orogenic belts are often formed by a combination of different compressional structures however symmetric and fault bend faults represent the minimum and maximum crustal shortening respectively and thus they do place bounds on the amount of strain recorded by akna montes the first model yields a shortening value less than 1 whereas a range of 17 34 is derived for the second model the large difference between these values underscores the importance of fold geometries for estimating strain and to place constraints on geodynamic models for mountain belt formation on the basis of our study we think that a combination of mantle downwelling and horizontal convergence may provide a good explanation of the geology and tectonics we observed in the akna montes atropos tessera region

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

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

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

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

a 400 km scale strike slip zone near the boundary of thetis regio venus we present a structural analysis of an area of venus surface centred on 3 s 116 e the area is dominated by a wide zone of deformation striking 050 n distinguished from the terrain on either side of it by markedly different structural and morphological characteristics the deformation zone contains two principal fault sets defined by form position and orientation members of the first set are present along the length of the deformation zone they are discontinuous consistently right stepping and are interpreted as riedel shears in a sinistral strike slip regime members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional grabens these faults are most prominent in the central region of the deformation zone and coincide with a deflection in strike of both the first fault set and the boundaries of the deformation zone a detailed kinematic analysis of all fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory supporting the hypothesis that this sinistral strike slip zone contains an extensional jog this interpretation is further supported by analysis of the interaction between faults belonging to the two sets variation in intercept geometry kinematic linking of the two fault sets and rotation of fault strike between the two fault sets all strongly suggest simultaneous development of these structures and therefore a common tectonic origin a driving mechanism for such large scale horizontal deformation on venus has yet to be established

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

accurate estimates of the spatial pattern of denudation by inversion of stacking velocity data an example from the british isles accurate measurement of the temporal and spatial variation of uplift and denudation would improve our understanding of the way in which mountain building and mantle convection modify the earth s surface here we show that inverse modeling of stacking velocity data from seismic reflection data sets is a potentially useful tool for obtaining spatially resolved denudation estimates along and across continental shelves unlike many techniques for measuring denudation our approach is not restricted to outcrop or borehole locations although it is important to calibrate results with other measurements of denudation we have inverted 2200 stacking velocity analyses from a dense network of two dimensional seismic reflection lines which surround the british isles this region was chosen because there is a well known signal of cenozoic uplift and denudation which is big enough to be detectable using our approach despite simplified assumptions about the way in which porosity varies with depth our offshore denudation estimates are consistent with outcrop patterns and accord with published denudation measurements furthermore stacking velocity estimates agree with measurements obtained by analyzing sonic velocity logs from selected boreholes a region encompassing the british isles has evidently undergone variable amounts of denudation during cenozoic times however significant denudation 1 5 km is concentrated in discrete areas such as off nw scotland and parts of the irish sea the magnitude of denudation changes significantly over short 10 km distances implying that differential uplift and erosion have been important factors in creating the present day denudation pattern

geology of venus mapping of a global geotraverse at 30 n latitude we mapped a swath of terrain on venus that extends circumferentially completely around the globe at 30 n latitude representing about 11 of the surface of the planet in order to address the following questions do presently defined stratigraphic units prove useful for broad application to regional and global mapping do units described in one area occur in other regions of the planet and how laterally continuous are they if similar sequences of units are found in different parts of the planet do they correlate temporally e g time correlative or do they represent similar sequences formed at different times e g time transgressive or independently locally repetitive we found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of venus at this latitude providing evidence that these units are not only global in extent but also laterally contiguous this gives confidence that the stratigraphic sequence can be extended to other parts of venus for further testing and refinement on the basis of this mapping we find that several areas including tessera massifs and beta regio show a concentration of older units and appear to have been topographically high over much of the visible history of venus tessera terrain is not uniformly distributed in some areas where these older units are concentrated ridge belts and fracture belts are not randomly distributed throughout the area suggesting that they represent regional patterns of deformation the altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest with older units tessera and earlier plains at intermediate to high elevations consistent with regional plains emplacement following the formation of the most significant regional topography sixty corona and corona like features were found in the geotraverse and the vast majority show evidence of being initiated in the earliest posttessera time early guineverian in an extensional tectonic environment few show evidence of postregional plains volcanism and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of venus regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges early part of the middle guineverian perhaps overlapping somewhat with its lower member evidence is found for changes in styles of volcanism with time and tectonic activity changed in time in terms of pervasiveness style and areal distribution on the basis of mapping of this globally continuous swath representing about 11 of the planet we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time rather than the nondirectional history proposed by guest and stofan 1999

triassic to middle jurassic stratigraphy of the kerr mcgee 97 12 1 exploration well offshore southern england abstract the triassic through to the middle jurassic bathonian stratigraphy of the kerr mcgee 97 12 1 exploration well is described using petrophysical logs in association with lithological and palaeontological analyses over 6210 of sediment was penetrated with sixteen triassic and fourteen lower middle jurassic sedimentary units recognised with a total measured depth t d of 7310 within the sherwood sandstone group budleigh salterton pebble beds of early middle triassic olenekian anisian age as expected the sedimentary sequences are comparable to those described from the wessex basin especially the portland wight and dorset sub basins of particular note however is the very thick halite sequence 1074 within the carnian dunscombe mudstone formation and also an expanded paper shales 27 sequence within the earliest toarcian a normal fault is noted at 1260 with the late bajocian inferior oolite group in fault contact with the latest toarcian bridport sand formation down cliff clay member the late triassic rhaetian to middle jurassic bathonian yielded rich microfaunal and palynomorph assemblages all of which have been previously described throughout northwest europe four microfossils groups foraminifera ostracods dinocysts and miospores have been used to interpret stratigraphic ages and palaeoenvironments this study provides new lithological and microfossil data for the triassic to middle jurassic bathonian sequences preserved in the offshore portland wight basin and provides a key tie point reference section for future offshore drilling

using bathymetry to identify basin inversion structures on the english channel shelf high resolution bathymetry is used to derive a new structural interpretation of a submerged inverted graben on the english channel shelf northwest europe the bathymetry provides a continuous plan view image of the bedrock geology and resolves fine structural detail particularly in areas of steeply dipping strata where traditional seismic imaging fails the imagery combined with shallow core deep borehole and two dimensional seismic reflection data shows a marked asymmetry in the bedrock outcrop and structural style both across and along the axis of the basin for example the oldest synrift rocks crop out in a complex south verging anticline along one margin whereas more massively bedded younger rocks are folded into a simpler anticline at the other comparison of our observations with both recent analog modeling and a neighboring inverted basin highlights the importance of the mechanical properties of the basin stratigraphy in controlling the structural development we conclude that the inversional asymmetry developed because of the distribution of strong competence contrasts within the synrift fill

artemis surface expression of a deep mantle plume on venus artemis a unique 2600 km diameter circular feature on venus defies geomorphic classification as a corona crustal plateau or volcanic rise artemis is similar in size to plateaus and rises yet topographically resembles a corona geologic mapping using correlated digital remote data sets including nasa magellan c1 c2 and f scale synthetic aperture radar sar imagery altimetry and synthetic stereo has led to a determination of the geologic history of artemis artemis comprises a large topographic welt that includes a paired circular 1 km deep trough artemis chasma and 1 km high outer rise thus artemis is divisible into chasma trough interior region and exterior region the chasma hosts trough normal faults and folds the interior includes five 350 km diameter coronae quasi circular features marked by radial and or concentric fractures that record rich tectono volcanic histories radial extension fractures and trough concentric wrinkle ridges dominate the exterior tectonic fabric artemis formed as a coherent entity the coronae chasma chasma structures radial fractures and wrinkle ridges are all consistent with a deep plume model for artemis formation rising and flattening of the plume head led to early uplift doming and radial fracturing as the plume head collapsed vertically and spread laterally likely causing outward migration of the trough as well as fracturing and wrinkle ridge formation outboard of the trough within the trough material was pulled downward forming normal faults and folds the plume continued to spread laterally outboard of the trough resulting in continued radial fracturing and wrinkle ridge formation small scale interior convection cells or compositional diapirs resulted in coronae with radial fractures and or concentric fractures and or folds and associated volcanism artemis is akin in its formation to crustal plateaus and volcanic rises and likely formed during the transition from globally thin to thick lithosphere

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

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

tectonic evaluation of alpha regio highland and environs venus abstract systematic geologic mapping of an area on venus provides geologic unit and morphologic information about the area that was evaluated to provide a simple geologic history in the study area a large complexly deformed highland alpha regio was found to be among the oldest material units but its relation to other nearby complexly deformed units was indeterminable to better delineate the complex geologic and tectonic history of the area the authors expanded on the initial geologic map assessment by including tectonic unit mapping of the alpha regio highland and geophysical modeling of the highland to assess the possible effect of its formation on surrounding geologic units tectonic unit mapping identified five discrete periods of tectonic activity involved in the formation of the highland additionally the tectonic assessment showed that late stage extensional features within the highland were related to a regional tectonic event that occurred after the formation of the highland geophysical models of highland formation by both upwelling and downwelling were assessed and a downwelling model was applied to the alpha regio highland to calculate stresses related to its formation results of the modeling suggest that tectonic features mapped outside of the highland were not caused by stresses induced by the formation of alpha regio

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

mantle and crustal structure in phoebe regio and devana chasma venus 1 we use gravity and topography data to study crust and mantle structure in phoebe regio and devana chasma on venus we find that phoebe is a region of thickened crust and hotter than normal mantle these results are inconsistent with a cold downwelling model and may support a mantle plume origin for phoebe however the pattern of thermal anomalies is unusual with temperature maxima on the periphery of the highland rather than in its center this may reflect development of thermal boundary layer instabilities as the plume head interacts with the lithosphere the hot mantle anomaly beneath devana chasma is discontinuous near latitude 10 north coinciding with a 600 km offset in the rift zone trend devana should be regarded as two separate rifts one propagating south from beta regio and the other propagating north from phoebe regio our results support the view that venus remains a geologically active world

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

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

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

recent deformation rates on venus constraints on the recent geological evolution of venus may be provided by quantitative estimates of the rates of the principal resurfacing processes volcanism and tectonism this paper focuses on the latter using impact craters as strain indicators the total postimpact tectonic strain lies in the range 0 5 6 5 which defines a recent mean strain rate of 10 18 10 17 s 1 when divided by the mean surface age interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 ma catastrophic resurfacing model if distributed tectonic resurfacing contributed strongly before that time as suggested by the widespread occurrence of tessera as inliers the mean global strain rate must have been at least 10 155 s 1 which is also typical of terrestrial active margins numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years parameterized convection models predict that the mean global thermal gradient decreases by only about 5 k km over this time even with the exponential dependence of viscosity upon temperature the surface strain rate drops by little more than one order of magnitude strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model an alternative uniformitarian resurfacing hypothesis holds that venus is resurfaced in quasi random patches several hundred kilometers in size that occur in response to changing mantle convection patterns under such a model the observed crater strain distribution indicates that about 1 of the planet s surface is tectonically active at any time however this model requires a very weak crustal rheology to achieve surface velocities 100 mm yr appropriate to the required patch size without well developed lateral weak zones venus is essentially a one plate planet but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement the net rate of tectonic activity is logarithmically intermediate between earth and mars about 100 times slower than plate tectonics but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction

characterization and formation of polygonal fractures on venus 1 fracture theory predicts that polygonal cracks will form in the presence of an isotropic extensional stress field on venus polygonal fractures are observed on scales several orders of magnitude larger than on earth with an average diameter of 1 8 0 9 km proposed formation mechanisms include cooling following lava flow emplacement lithospheric heating and climate change here we examine the characteristics and geologic setting of 204 regions of polygons some regions display two spatially overlapping size ranges with the larger spacing typically 10 25 km most polygonal fractures appear to be extensional but some have the morphology of compressional ridges polygons are confined to plains regions and occur in association with shield fields 49 coronae and coronae like features 21 3 tessera 17 5 and wrinkle ridges 20 in locations where polygons occur with shield fields coronae or both they appear to have formed contemporaneously formation in conjunction with local heating events is consistent with the lithospheric cooling hypothesis however there is almost never the predicted decrease in size away from the center of coronae or shield fields only a small percentage of coronae and shield fields contain polygons indicating that they are not typical of the formation process the climate change induced scenario is consistent with many characteristics of the polygons including the small and large size ranges the compressional ridges and their occurrence with and without evidence of local heating although polygons may have diverse origins including formation by multiple deformation events overall polygon characteristics support the climate change hypothesis

relative timing of crustal plateau magmatism and tectonism at tellus regio venus crustal plateaus subcontinent sized regions of thickened crust that stand 1 4 km above surrounding low lying plains define a major physiographic province of venus thus their mode of emplacement plays a fundamental part in the evolution of the entire planet local topographic lows up to several hundred kilometers across filled with radar dark volcanic flows termed intratessera floodlava basins itbs are common within crustal plateaus itbs record abundant crustal plateau volcanism and have implications for crustal plateau emplacement and evolution we ask the fundamental question did itbs develop before during or after crustal plateau development to address this question we mapped in detail the geology a particular itb in tellus regio to determine temporal relationships between itb volcanic flows and local crustal plateau tectonism mechanical arguments and crosscutting relationships indicate that itb volcanism occurred during crustal plateau formation extending the timing relationships resolved at this basin we suggest that itb magmatism played a major role in crustal plateau genesis the geologic history discerned is consistent with the upwelling model for crustal plateau evolution in which early surface extension and volcanism would be expected in a hotspot environment conversely early widespread extension and volcanism would not be expected if crustal plateaus formed above a mantle downwelling because 1 predicted horizontal and radial convergence would obstruct early surface extension and 2 lower than normal geothermal gradients would impede rather than enhance volcanism

tessera deformation and the contemporaneous thermal state of the plateau highlands venus abstract geodynamic modeling of tessera deformation at the ancient venusian plateau highlands implies lithospheric thermal gradients exceeding 17 k km 1 regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of 20 km controlled by a mechanical layer of relatively uniform thickness we model the deformation as the shortening of a plastic layer over a viscous substrate a layer thickness of less than 6 km is consistent with the observed ridge spacings assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 16 s 1 the geotherm was at least 17 k km 1 higher strain rates consequently higher thermal gradients are more probable we rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera that the highlands are dominantly composed of rock more felsic than diabase or that the crust was weak because it was hydrated therefore the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since

duration of tessera deformation on venus the density and distribution of impact craters superposed on the highly deformed tessera terrain on venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains eighty percent of tesserae craters are undeformed no existing craters exhibit evidence of contractional deformation suggesting that the early compressional stage of tessera deformation ended abruptly the small number of craters fractured by late stage tessera extension constrains the duration of this phase to less than 20 of the average crater retention age of the tesserae or approximately 30 60 ma these results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains

beta regio venus evidence for uplift rifting and volcanism due to a mantle plume abstract geophysical data have led to the interpretation that beta regio a 2000 25000 km wide topographic rise with associated rifting and volcanism formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume we have tested this hypothesis through detailed geologic mapping of the v 17 quadrangle which includes a significant part of the beta regio rise and reconnaissance mapping of the remaining parts of this region our analysis documents signatures of an early stage of uplift in the formation of the agrona linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging we see evidence that the theia rift associated volcanism occurred during the first part of post regional plains time and cannot exclude that it continued into later time we also see evidence that devana chasma rifting was active during the first and the second parts of post regional plains time these data are consistent with uplift rifting and volcanism associated with a mantle diapir geophysical modeling shows that diapiric upwelling may continue at the present time together these data suggest that the duration of mantle diapir activity was as long as several hundred million years the regional plains north of beta rise and the area east and west of it were little affected by the beta forming plume but the broader area at least 4000 km across whose center northern part includes beta regio could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain

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

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