automated detection and location of microseismicity at mount st helens with a large n geophone array in the summer of 2014 a dense array of 904 geophones was deployed at mount st helens along the road and trail system within 15 km distance of the summit crater the array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano a reverse time imaging method is applied to short term average over long term average time series data to automatically detect and locate microseismicity these efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the pacific northwest seismic network earthquake locations resolve a narrow 1 km wide vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater this feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface

the structural control of venusian polygonal impact craters pre impact inhomogeneities of the target material sometimes cause the rim of an impact crater to be composed of several straight segments instead of being circular the venusian surface hosts 121 such polygonal impact craters pics 12 km in diameter their straight rim segments are often parallel to the orientations of the surrounding tectonic structures particularly those in tessera terrain and young rift zones as well as the concentric components of coronae this match is notably more distinct in distances less than two crater diameters between the pics and the tectonic structures than further away surrounding wrinkle ridges lineaments or radial components of volcano tectonic features seem to have very little influence on the orientations of pic rims these results imply that the formation of straight segments of venusian pic rims is controlled by pre existing tectonic structures of the crust but not by the apparently most surficial ones thus pics could be used to provide further constraints on the distribution and orientations of zones of weakness in the venusian crust

imaging of erebus volcano using body wave seismic interferometry of strombolian eruption coda 1 seismic interferometry is a recently developed theory that allows for the recovery of a medium s impulse response between two points should randomly distributed sources of white noise or equivalently a multiply scattered equipartioned wavefield be present throughout the medium we exploit the extremely scattering nature of volcanic media and seismic illumination from impulsive strombolian eruptions to extract single station body wave green s tensors at an unusually dense array of stations on erebus volcano antarctica we optimally rotate these green s tensors for each notable arrival and back project them to construct a 3 dimensional scattering map of the magma filled volcano conduit system also corroborated by an independent active source tomography experiment using the same station distribution this approach not only favors highly scattering media contrarily to most conventional methods but its passive nature i e non anthropogenic sources allows for studies of structural temporal variability and possible extension into real time monitoring of active volcanoes

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

age of rifting and associated volcanism in atla regio venus an estimation of the age of rifting and associated volcanism in atla regio is carried out based on photogeologic analyses of the stratigraphic relationships between rift associated features and impact craters which have associated radar dark paraboloids the paraboloids are believed to be thin mantles of debris derived from the fallout of crater ejecta and their parent craters are considered to be among the youngest 10 of venus crater population campbell et al 1992 in the area under study there are three impact craters with associated dark paraboloids 1 von schuurman which is clearly superimposed on the dali chasma rift 2 sitwell which is superimposed on ganis chasma and may have been followed by subsequent rift associated fracturing and 3 luxemburg which is superimposed on lavas from ozza mons and dissected by dali chasma fractures which in turn are covered by lavas from maat mons this means that at least the last stages of tectonic and volcanic rift associated activity in atla regio are contemporaneous with the formation of impact craters which are young enough to preserve the associated dark paraboloids if these craters represent the youngest 10 of the venus crater population and the average age of the venusian surface is about 500 m y schaber et al 1992 this means that rifting and rift associated volcanism in atla regio were active as recently as during the last 50 m y in the current debate over models of venus resurfacing the results of our study favor the equilibrium resurfacing model phillips et al 1992

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

corona classification by evolutionary stage abstract the formation of coronae abundant volcano tectonic features on the surface of venus has been modeled as the interaction of mantle diapirs with the lithosphere however the applicability of this model to features of widely different sizes may not be evident from sar images due to the lack of a common scale we use the radius as a scaling parameter for both the distance from the center and relief obviating this problem and mapping the effects from coronae of different radii onto a single scale normalizing profiles for 394 features we find many features classified by e r stoffanet al 1992 j geophys res 97 13 347 13 378 as different types have very similar profiles and relief coronae distribute into three shapes domal circular calderic two shapes not related to coronae radial and volcanic remain variation between corona shapes is gradational and mirrors the evolution of diapirs from initial domal upwellings to calderas we test the implied age progression using impact crater populations within three corona radii despite the inherent uncertainties of this method crater statistics for a size restricted 275 325 km corona set suggest an age progression from domal through circular to calderic the calderic coronae have an associated impact crater population very similar to the global mean though with slightly too few tectonized craters implying that they approximate the average surface age and are no longer active the circular coronae have a low impact crater density and a slightly elevated proportion of tectonized craters and thus are currently active the domal coronae have fewer than the expected number of impact craters of which more are tectonized than expected suggesting that surface modification has begun

lessons from venus for understanding mantle plumes on earth abstract mantle plumes are important in the magmatic and tectonic history for both earth and venus the expression of plumes is distinctive on venus and complementary to that on earth therefore a cross comparison is useful for better understanding plume magmatism on both planets in contrast to the earth venus has no observed record of plate tectonics a low degree of surface erosion and an apparently short duration for the formation of the present planetary surface the absence of plate tectonics indicates that all magmatism is intraplate and is generated beneath a stagnant lithospheric lid a low degree of surface erosion preserves the surface structures and short wavelength topography the short duration of preserved magmatic activity suggests a global resurfacing event magmatic elements include a individual volcanoes with diameters ranging up to 1000 km which represent hotspots b annular structures termed coronae with diameters averaging 300 km but ranging up to 2600 km and which appear to lack terrestrial i e earth analogues c radiating graben fissure systems extending up to 2000 km in radius some of which are purely uplift related while others mark the plumbing system dyke swarms of volcanic systems d lava flow fields of scale comparable to terrestrial flood basalts large igneous provinces lips and e regions of small shield volcanoes representing shallow source melting there are several hierarchies of magmatic events on venus ordered in terms of increasing scale and significance 1 isolated coronae volcanoes flow fields and radiating graben systems 2a individual and small clusters of volcanoes and coronae associated with topographic swells geoid highs and triple junction rifting these are most clearly indicative of terrestrial type plumes originating from the deep mantle 2b coronae distributed along rifts chasmata these are the clearest examples of melt generation associated with rifting 3 regional concentration of activity in the beta atla themis bat region this is the closest example of a plume cluster event sometimes termed a superplume event and 4 global volcanic resurfacing of the volcanic plains no terrestrial analogue is confirmed although the global burst of terrestrial plume activity in the neoarchean is a possible analogue

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

constraints on the source of lunar cataclysm impactors multiple impact basins formed on the moon about 3 8 gyr ago in what is known as the lunar cataclysm or late heavy bombardment many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main belt asteroids destabilized by delayed planetary migration we show that morphologically fresh class 1 craters on the lunar highlands were mostly formed during the brief tail of the cataclysm as they have absolute crater number density similar to that of the orientale basin and ejecta blanket the connection between class 1 craters and the cataclysm is supported by the similarity of their size frequency distribution to that of stratigraphically identified imbrian craters majority of lunar craters younger than the imbrium basin including class 1 craters thus record the size frequency distribution of the lunar cataclysm impactors this distribution is much steeper than that of main belt asteroids we argue that the projectiles bombarding the moon at the time of the cataclysm could not have been main belt asteroids ejected by purely gravitational means

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

interactions between mantle diapirs experimental theoretical and numerical results are presented for the dynamics of interacting and deforming diapirs the clustering of diapirs may occur as a result of the deformation induced by their mutual interactions the time required for two diapirs in the mantle to merge can be small compared to their ascent time the clustering of many diapirs however is a relatively slow process and is unlikely to significantly affect their distribution in the earth s mantle if coronae on venus are produced by diapirs diapir clustering may explain the variation of coronae size and may be related to the observed clustering of coronae on the surface

symmetries of volcanic distributions on mars and earth and their mantle plume dynamics the symmetries in the distribution of major volcanic centers on mars have been analyzed by employing a technique involving the angular self similarity of point fields the distribution of all central vent volcanoes exhibits an axisymmetry along with a reflecting symmetry about a plane whose normal is the axis of axisymmetry the latter axis is skewed by about 30 from the rotational axis and is suggested to represent the early axis of symmetry of the interior dynamics the high level of symmetry of mars volcanic distribution may be the result of a focusing of seismic energy from a major impact in the hellas area that formed the hellas basin and may have triggered the onset of volcanic activity in the greater tharsis area if the highland paterae near the hellas region are not taken into account in the analysis an asymmetry with respect to the reflecting plane appears this is probably representative of the late hesperian and amazonian when the once global volcanic activity had retreated to the tharsis province these angular symmetries suggest a simple pattern of long wavelength axisymmetric martian mantle convection involving a large stationary plume as shown by our three dimensional 3 d spherical shell simulations on the basis of these calculations we venture to propose that the volcanic history of mars reflects the transformation of a bipolar mantle plume structure into a single megaplume by the process of plume plume collision acting in concert with the deep martian phase transitions this transformation might have overcome the original pattern induced by a major impact forming the hellas basin we have repeated the procedure for the earth s hotspot distribution which revealed both axisymmetry and a certain level of reflecting symmetry this similarity between the two planets is striking the symmetries of the earth s hotspot distribution are probably due to the interaction of the lower mantle megaplumes with the endothermic perovskite to spinel phase transition

venus resurfacing by coronae implications from impact craters coronae common volcano tectonic features on venus may be actively modifying the planet s surface to evaluate the extent of their effects we compare impact crater densities and proportions of modified craters out to four radii for monte carlo simulations of corona location with those for two corona classes we study the radial concentric and concentric caldera coronae the two classes believed to represent the youthful and senescent stages of formation respectively a high proportion of tectonized craters and low crater density near radial concentric coronae indicate recent activity based on the average crater density and proportion of tectonized craters we interpret concentric caldera coronae as old these results show that tectonism and volcanism is not limited to the corona interior and can extend up to four radii from the center

venus coronae craters and chasmata the distributions of venus coronae and craters are related to chasmata which are thought to be extensional zones coronae are almost twice as dense near the chasmata as a random set of the same size of the various types of coronae the radial concentric and multiple are even more highly concentrated near chasmata whereas the concentric caldera type are absent near the chasmata craters to the first order are randomly distributed although when distributions are compared with random sets there is a deficit of about 15 20 craters close to the chasmata the tectonized and embayed craters tend to be near the rift zones and their distribution closely resembles that of the coronae the higher proportion of tectonized and especially embayed craters within 5 10 of the arcs is about the fraction that would be expected from modification of the full set of craters over a width of 100 200 km kolmogorov smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points this suggests that the volcano tectonic process creating coronae may be the same one destroying craters

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

correlation of venus surface features and geoid abstract the distributions of pairs of features on venus are compared using simple statistical tests we consider coronae types crater types and chasmata some of these are obviously associated and some are possibly associated coronae are grouped to represent evolutionary stages tectonic and volcanic modification of craters is considered and the chasmata are represented by five great circle arcs these arcs fit rift or extension zones at the 90 level totaling 55 000 km comparable to the length of spreading ridges on earth comparing these sets with venus geoid field we find that all the chasmata correlate positively with geoid highs and negatively with lows with the peaks in the geoid field occurring at the intersections of the arcs modified craters also exhibit this relation to the geoid the domal coronae the youngest and currently active may be sites of upwelling these correlate strongly with geoid highs and negatively with geoid lows whereas later stage coronae the majority do not show this correlation strong association of the youngest coronae with themis atla suggests that the most recent activity occurs along this arc

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

lips on venus venus a planet similar to earth in heat budget lacks plate tectonics yet displays ample evidence of extensive volcanic and tectonomagmatic processes including regions large enough to be considered lips thus venus provides an excellent opportunity to examine large scale magmatic processes outside a plate tectonic framework i discuss four groups of venus largest tectonomagmatic provinces volcanic rises large coronae artemis heng o quetzalpetlatl atahensik crustal plateaus and plains with wrinkle ridges unit pwr unit pwr widely interpreted as representing catastrophic resurfacing has been suggested to be the solar system s largest lip i argue herein that these features each covering 1 million km 2 or more and recording extensive volcanic activity record different evolutionary processes including both endogenic and exogenic processes volcanic rises represent surface manifestations of deep mantle plumes on thick lithosphere large coronae may record different evolutionary paths artemis represents a plume signature on thin lithosphere more mapping is required to determine if heng o formed by endogenic or exogenic processes preliminary mapping suggests quetzalpetlatl represents a young mantle plume atahensik marks a diapiric structure though buoyancy mode is unconstrained crustal plateaus record solidification of huge lava ponds formed by massive partial mantle melting caused by large bolide impact on thin lithosphere the status of unit pwr as the largest lip within the solar system is challenged based on geological considerations and results of a growing body of geologic mapping 2007 elsevier b v all rights reserved

models for the hotspot distribution published hotspot catalogues all show a hemispheric concentration beyond what can be expected by chance cumulative distributions about the center of concentration are described by a power law with a fractal dimension closer to 1 than 2 random sets of the corresponding sizes do not show this effect a simple shift of the random sets away from a point would produce distributions similar to those of hotspot sets the possible relation of the hotspots to the locations of ridges and subduction zones is tested using large sets of randomly generated points to estimate areas within given distances of the plate boundaries the probability of finding the observed number of hotspots within 10 of the ridges is about what is expected

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

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

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

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

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

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

the spatial distribution of coronae and related features on venus coronae are large quasi circular geologic features that are common on venus they appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere we have investigated the spatial distribution of 335 coronae and related features identified in magellan radar data it is more clustered than a poisson distribution with a statistical certainty of more than 99 it is dominated by a single large cluster centered near the equator at about 245 longitude the features are preferentially found at elevation and geoid values close to the planetary mean with a paucity at both the highest and lowest levels of topography and geoid some coronae appear aligned in quasi linear chains we attribute the clustering of coronae and related features to preferential formation of these features above regions of broad scale mantle upwelling and suggest that a major mantle upwelling underlies the one large cluster we suggest that coronae are rare at the lowest elevations because these may be regions of mantle downwelling the shortage of coronae at the highest elevations may result both from obscuration by other intense tectonism there and from suppression there of their formation by an unusually thick crust corona chains may be produced by enhanced passive mantle uplift below failed or incipient rifts

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

large scale volcanism associated with coronae on venus implications for formation and evolution large scale volcanism in the form of areally extensive flow fields is a previously unrecognized important aspect of the evolution of at least 41 of all coronae on venus the timing and scale of many coronae flow fields is consistent with an origin due to the arrival and pressure release melting of material in the head of a mantle plume or diapir the production of voluminous amounts of volcanism at some coronae is proposed to be the result of larger plume size and or the intersection of mantle upwellings with regions of lithospheric extension and rifting