mineral equilibria and the high radar reflectivity of venus mountaintops we studied the relationship between altitude and microwave emissivity the complement of power reflectivity in 10 highland regions of venus using the magellan data set above a critical altitude that ranges from 4 75 km on maxwell montes to 2 49 km on sapas mons emissivity undergoes an abrupt decrease to values so low 0 6 that the surface mineralogy on these mountaintops must differ from that at lower altitudes in such a way as to enhance the bulk dielectric constant of the highland surface material this emissivity effect probably is produced not by basalt minerals but by a secondary weathered mineral assemblage we developed the phase diagrams descriptive of equilibrium secondary mineral assemblages on venus as a function of altitude and atmospheric redox state the mineral responsible for low emissivity on mountaintops appears to be the electrical semiconductor pyrite fes2 variation from one highland area to another in the altitude at which pyrite becomes the stable fe mineral as opposed to magnetite at lower altitudes may be attributable to local modulation by topography of the atmospheric thermal profile the altitudes at which the magnetite pyrite phase boundary is encountered vary with the redox slate of the atmosphere and observed values of this altitude can be used to estimate an oxygen fugacity at plains level of 10 21 bars maat mons a volcanic peak almost as lofty as maxwell montes is the only high mountaintop on venus that has not weathered to a low emissivity mineral assemblage presumably this means flows at the highest altitudes are negligibly weathered and the volcano is relatively young

sequence of tectonic deformation in the history of venus evidence from global stratigraphic relationships analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of venus spanning the past several hundreds of millions of years we find that shortening characteristic of intensely deformed tessera terrain gave way to widespread distributed fracturing and extension within the tessera and early post tessera volcanic plains this phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following and sometimes simultaneous extension to form fracture belts emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains surfaces focused extensional deformation localized linear rift systems dominated the latest stages these major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of venus

on rates and styles of late volcanism and rifting on venus 1 we investigated the possibility of significant variations in the rates and styles of volcanism and rifting during the time postdating the formation of regional plains on venus we analyzed the age relations of all known impact craters 30 km in diameter 183 craters with the neighboring geologic units of these we selected 164 craters which were superposed on regional plains and determined if post regional plains prp volcanics and or prp rift structures were present in the crater vicinity and if these craters postdated the volcanics and rift structures or predated them in 53 cases it was possible to determine these relations on the basis of these relationships it was found that the general rates of volcanism and rifting during prp time were close to constant or at least had no drastic changes this implies a significant change in the rates of volcanism and rifting in the vicinity of the boundary between prp time and the preceding time which is marked by the formation of the global wrinkle ridge network it was also found that 1 the role of rift associated volcanism during prp time was close to constant or slightly decreasing 2 the role of corona related volcanism was noticeably decreasing and 3 the role of noncorona hot spot volcanism was proportionally increasing the latter changes may be due to thickening of the lithosphere during prp time the results imply also that at most if not all places where prp rifting occurred the rift associated fracturing when started continued for a significant part of prp time our conclusions are valid both 1 for the case of a globally synchronous transition from emplacement of regional plains to the prp regime and 2 for the case of such a transition occurring at different times in different areas of venus

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

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

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

viscoelastic relaxation of topographic highs on venus to produce coronae coronae on venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs we examine this relaxation using a viscoelastic finite element code and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl annular rim and surrounding moat stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats however relaxation is not expected to produce the concentric faulting often observed on the annular rim the relaxation timescale is shorter than the diapir cooling timescale so loss of thermal support controls the rate at which topography is reduced the final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time development of lower flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts higher thermal gradients and crustal thinning over the diapir

the plio pleistocene glaciation of the barents sea svalbard region a new model based on revised chronostratigraphy based on a revised chronostratigraphy and compilation of borehole data from the barents sea continental margin a coherent glaciation model is proposed for the barents sea ice sheet over the past 3 5 million years ma three phases of ice growth are suggested 1 the initial build up phase covering mountainous regions and reaching the coastline shelf edge in the northern barents sea during short term glacial intensification is concomitant with the onset of the northern hemisphere glaciation 3 6 2 4 ma 2 a transitional growth phase 2 4 1 0 ma during which the ice sheet expanded towards the southern barents sea and reached the northwestern kara sea this is inferred from step wise decrease of siberian river supplied smectite rich sediments likely caused by ice sheet blockade and possibly reduced sea ice formation in the kara sea as well as glacigenic wedge growth along the northwestern barents sea margin hampering entrainment and transport of sea ice sediments to the arctic atlantic gateway 3 finally large scale glaciation in the barents sea occurred after 1 ma with repeated advances to the shelf edge the timing is inferred from ice grounding on the yermak plateau at about 0 95 ma and higher frequencies of gravity driven mass movements along the western barents sea margin associated with expansive glacial growth

crater associated dark diffuse features on venus properties of surficial deposits and their evolution 1 in order to assess the nature of crater associated radar dark diffuse features ddfs on venus and to understand their formation and evolution we analyzed magellan radar roughness emissivity and reflectivity data in the vicinity of craters accompanied by these features following others we assumed that ddfs are deposits mantles of ejected material emplaced during formation of the impact crater the majority of radar dark parabolas the youngest ddfs are characterized by a smooth mantle atmosphere interface having low root mean square rms slopes on scales of 1 100 m as derived from magellan altimeter data older ddfs also often have areas with low rms slopes suggesting that the mantle rms slopes can be preserved for geologically long periods of time some parabolas and older ddfs have asymmetric small scale decimeter scale relief that is interpreted to be dunes that formed as a result of eolian processes this implies that the mantle material is mobile and can saltate under the influence of wind action on average aging of these features is accompanied by a decrease of mantle material dielectric permittivity the most efficient mechanism for parabola degradation seems to be the removal of mantle material from the site of initial deposition by subsequent winds we found a few examples of features that could be interpreted to be the result of in situ modification of the primary mantle

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

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

petrology and geochemistry of granulite xenoliths from central hoggar algeria implications for the lower crust granulitic xenoliths enclosed in neogene alkali basalts from the atakor and adrar n addjer volcanic massifs in central hoggar southern algeria include mafic metaigneous and felsic metasedimentary rocks the main mineral phases of metaigneous xenoliths are plagioclase and pyroxenes whereas quartz feldspars sillimanite garnet and sapphirine occur in metasedimentary granulites the metaigneous xenoliths represent a magmatic sequence of cumulitic and non cumulitic rocks which resembles gabbroic bodies associated with anorthosites and has calc alkaline affinities similar types and distributions of granulitic xenoliths are widespread and are probably characteristic of the lower crust in general suggesting its homogeneity on a large scale but heterogencity on a local scale

magellan observations of extended impact crater related features on the surface of venus a total of 57 parabolic shaped and 9 approximately circular extended impact crater related features have been found in magellan synthetic aperture radar sar and thermal emissivity data covering 92 of the surface of venus the parabolic features are with seven exceptions oriented e w with the apex to the east and the impact crater located just west of the apex they were first identified in the surface emissivity data derived from magellan radiometry measurements but the great majority are only clearly visible in the sar imagery the overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the parent craters the floors of almost all these craters have high specific radar backscatter cross sections i e they are bright in the sar imagery relative to their surroundings and tend to have low emissivities approximately one third of the impact craters with diameters 15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the sar or emissivity data no features have been found which overlie the parabolic features indicating that they are among the youngest features on the surface of the planet this suggests that radar bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature a model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the e w zonal winds fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0 6 cm on the wavelength scale 12 6 cm surface roughness and hence of 1 to 2 cm on the largest particle sizes of interest fallout times from 50 km in the venus atmosphere for particles of this size are about 2 hours allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s 1 measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits are consistent with deposit depths of a few centimeters to 1 or 2m

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

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

impact craters on regional plains on venus age relations with wrinkle ridges and implications for the geological evolution of venus 1 a global assessment of impact craters on venus 30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges in this study we found that 10 of these craters are deformed by wrinkle ridges pre wr 93 craters are superposed on wrinkle ridges post wr and 43 craters were found to be ambiguous cases we compared our findings with the results of a similar study by mcgill 2004 done for craters of all sizes but for 1 3 of the surface of venus we found that the percentage of pre wr craters found by mcgill 2004 among the unambiguous cases 15 is close to our percentage 10 but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements we then summarize published data on the crater densities for different plains on venus we assess monte carlo modeling of volcanic resurfacing which competed with the ongoing formation of impact craters on the basis of these data and analyses we conclude that the mean age of the regional plains is close to the mean global surface age of venus and that the potential time span of regional plains emplacement was relatively short we interpret our results as evidence that wrinkle ridging on venus was a distinct tectonic phase that occurred rather early in post regional plains time probably within the first 10 although the exact duration remains uncertain regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on venus predominantly occurred during a specific phase of history the geologic record of wrinkle ridging and post plains tectonic evolution of volcanic plains on the moon and mars show interesting similarities to those of venus

the timing of giant radiating dike swarm emplacement on venus implications for resurfacing of the planet and its subsequent evolution stratigraphic study of a distributed population of 118 giant radiating dike swarms on venus reveals that within each dike intruded region emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera regional plains and most wrinkle ridges the density of impact craters superimposed on the swarm population 1 80 0 57 craters 10 6 km 2 when compared with the average global density 2 01 0 14 craters 10 6 km 2 and the densities reported for other geologic units is consistent with the observed stratigraphy on the basis of these data we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing the stresses recorded by the dike swarm population when combined with its age and compared with the predictions of several proposed resurfacing models best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long wavelength gravity and topography in addition the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long wavelength characteristics we interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago

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

aeolian features on venus preliminary magellan results magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian or wind processes wind streaks are the most common aeolian feature they consist of radar backscatter patterns that are high low or mixed in relation to the surface on which they occur a data base of more than 3400 wind streaks shows that low backscatter linear forms long narrow streaks are the most common and that most streaks occur between 17 s to 30 s and 5 n to 53 n on smooth plains moreover most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains we infer that both of these geological settings provide fine particulate material that can be entrained by the low velocity winds on venus turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks thus some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock other high backscatter streaks may be lag deposits of dense materials from which low density grains have been removed dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns wind streaks generally occur on slopes 2 and tend to be oriented toward the equator consistent with the hadley model of atmospheric circulation in addition to wind streaks other aeolian features on venus include yardangs and dune fields the aglaonice dune field centered at 25 s 340 e covers 1290 km 2 and is located in an ejecta flow channel from the aglaonice impact crater the meshkenet dune field located at 67 n 90 e covers 17 120 km 2 in a valley between ishtar terra and meshkenet tessera wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds dunes on venus signal the presence of sand size 60 to 2 000 m grains the possible yardangs are found at 9 n 60 5 e about 300 km southeast of the crater mead although most aeolian features are concentrated in smooth plains near the equator the occurrence of wind streaks is widespread and some have been found at all latitudes and elevations they demonstrate that aeolian processes operate widely on venus the intensity of wind erosion and deposits however varies with locality and is dependent on the wind regime and supply of particles

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

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

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

atmospheric effects on ejecta emplacement and crater formation on venus from magellan the venus cratering record provides a unique environment for assessing the effects of both gravity and an atmosphere on impact crater formation this contribution uses surface signatures of energy partitioning as a framework for testing extrapolations from laboratory experiments and other planetary settings seven general conclusions can be drawn first the dense lower atmosphere of venus takes on the role of a low density target for bodies smaller than about 4 km in diameter air blasts created by cratering in the atmosphere create distinctive surface signatures that allow the derivation of an independent assessment of impactor energy at the limit of break up second dynamic pressures during entry of larger bodies will exceed their strength limit but may not prevent penetration of the atmosphere due to aerodynamic reshaping that minimizes the drag coefficient such a process may account for the formation of unusually small craters 1 3 km third the dense atmosphere of venus preserves signatures of early time cratering processes on the surface that are typically lost on atmosphere free surfaces such signatures not only provide another estimate of impactor energy but also include a distinctive record of the impactor i e comet versus asteroid in distinctive run out flows created before the crater has finished formation strong winds and turbulence associated with the atmospheric disturbance at later times create wind streaks behind topographic barriers fourth ballistic ejection of excavated debris occurs from craters on venus just as it does on planets without an atmosphere thereby underscoring the fundamental mechanical transfer of energy from impactor to target fifth ejecta emplacement is nonballistic due to the large dynamic forces acting on the advancing curtain and its constituent ejecta the outward moving ejecta curtain induces strong response winds that entrain ejecta and drive a ground hugging debris flow outward without returning to the cavity flow separation creates an overriding run out ejecta flow further sustained by atmospheric turbulence and identified as radar dark lobate lobes sixth radar dark parabolas are proposed to be late time fallout deposits created as the downrange fireball evolves aloft perhaps analogous to terrestrial tektite strewn fields and seventh the response of crater formation to the atmosphere is conversely expressed by a reduction in cratering efficiency as revealed by the unusually large central peak complexes and the unexpected diameter depth relations of craters hence surface ages may be significantly underestimated

rifts and large volcanoes on venus global assessment of their age relations with regional plains we report on the results of synoptic global mapping of rifts and large volcanoes on venus and detailed mapping in six areas illustrating the relationships among these features rifts and large volcanoes were subdivided into old transitional for volcanoes only and young features the geologic event separating old features from young ones is the formation of wrinkle ridges on the regional plains global mapping showed that the postplains rifts dominate in the population of rifts both in terms of number of mapped segments and in area the postplains volcanoes are more abundant in terms of number and area occupied than those that are transitional in age and these in turn dominate over the old ones these data show that in post regional plains time rifting and formation of large volcanoes are significantly more important processes than in the immediately preceding period the time period during which regional plains were emplaced and old and partly transitional large volcanoes formed was much shorter than the postplains time therefore the mean rate of formation of large volcanoes during this first period was significantly higher than in the subsequent time similarly the relatively short time duration of the plains emplacement time indicates that the mean rate of old rifting was higher than in more recent time this study shows prominent changes with time in the absolute rates and roles of rifting and large volcano formation this supports the directional model of the geologic evolution of venus during the last several hundred million years as opposed to the nondirectional model which predicts a more random occurrence of geologic processes throughout history

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

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

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

impact craters on geologic units of northern venus implications for the duration of the transition from tessera to regional plains using magellan sar images and the schaber et al 1998 crater data base we examined impact craters in the area north of 35 n and determined the geologic units on which they are superposed the crater density of the regional plains with wrinkle ridges pwr was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet t about 80 to 97 of the craters superposed on a composite unit that includes materials of tessera terrain tt densely fractured plains pdf fractured and ridged plains pfr and fracture belts fb also postdate the regional plains thus the time interval between the formation of these older units and emplacement of the regional plains t should be geologically short from a few percent to about 20 of t or approximately 40 to 150 m y this means that in the area under study volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of venus about the last 750 m y was much more active than in the subsequent time

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

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 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 analysis of the degree of impact crater deposit degradation and assessment of its use for dating geological units and features 1 we use the degree of degradation of crater associated radar dark deposits on venus to estimate the age of the crater and neighboring units we analyzed craters 30 km in diameter superposed on regional plains subpopulation 1 138 craters and on later units subpopulation 2 30 craters and estimated percentages of craters with dark parabolas dp clear dark halo ch faint dark halo fh and no dark halo nh we constructed theoretical models of the evolution of these percentages with time for two possible interpretations of the upper boundary of the regional plains 1 globally synchronous and 2 nonsynchronous diachronous they show that in the synchronous case the dark deposit lifetimes are proportional to the corresponding percentages observed for subpopulation 1 tdp 0 15t tch 0 3t and tfh 0 3t where t is the mean global surface age of venus in diachronous cases the percentages may or may not be proportional to the appropriate lifetimes depending on the range of the regional plains if the time range of plains emplacement is not larger than 0 5t the use of tdp 0 15t and tch 0 3t is appropriate we consider the time 0 5t ago as the lower time boundary for the age of the now observed ch craters and the time 0 1 0 15t ago as the lower boundary for the age of dp craters we propose that the atlian period of the geologic history of venus be subdivided into upper and lower epochs with the boundary between them at 0 5t ago and the lower boundary of the aurelian period be placed at 0 1 0 15t ago we apply this approach to assess ages of activity of three volcanic tectonic structures on venus beta regio devana chasma shows evidence of activity younger than 0 5t ago mylitta fluctus also younger than 0 5t and atla regio shows evidence of activity younger than 0 1 0 15t ago

radar dark impact crater related parabolas on venus characterization of deposits with magellan emissivity data 1 we have analyzed the radar dark parabola deposits associated with impact craters on the surface of venus using the results of the magellan radio experiments in both passive and active modes the extent of mantle deposits is found to be much wider than the dark features observed in synthetic aperture radar sar images using the emissivity data we have classified features coinciding with the radar dark parabolas into four types related to both differences in ejecta deposition styles for craters of different diameters and stages in the degradation of crater related deposits a model for the microwave properties of the mantled surfaces is proposed comparison between the distribution of mantle thickness predicted by the model and the previously published independent results based on modeling of the cratering process show good agreement when the loss tangent of the mantle material is assumed to be within the 0 001 0 005 range this value is similar to that of dry powders of terrestrial and lunar rocks on the basis of this work we conclude that radar dark diffuse features due to these mantles can be up to several meters thick if the loss tangent is assumed to be 0 003 thin deposits as little as 7 cm thick are found to be responsible for the deposits seen in the emissivity parabolas but not observed in the sar images these results provide a basis for the interpretation of the character and thickness of mantle deposits their states of degradation processes associated with their formation and degradation and guides to their use in establishing stratigraphic relationships and ages of underlying geologic units on venus the results will also be useful in interpreting the nature of surface deposits at previous landing sites and predicting the nature of deposits likely to be encountered at future landing sites

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

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

stratigraphy of small shield volcanoes on venus criteria for determining stratigraphic relationships and assessment of relative age and temporal abundance 1 small volcanic edifices shields with a diameter less than about 20 km are common and sometimes very abundant features on the plains of venus typically they form tight or loose clusters of features known as shield fields small shields are interpreted to be formed due to small scale eruptions through numerous and distinct sources a mode of formation apparently significantly different from the mechanism thought to be responsible for the emplacement of the vast regional plains of venus did the eruption style of small shields occur repeatedly throughout the visible part of the geologic record of venus or was this style more concentrated in a specific epoch or epochs of geologic history do the clusters of shields represent localized development of sources over a thermal anomaly such as a plume or do they represent exposures or kipukas of a more regional unit or units a major step toward answering these questions is an understanding of small shield stratigraphy multiple criteria have been developed to assess the stratigraphic relationships of individual small shields and that of shield fields with the adjacent units in our analysis we expanded and developed the previous criteria and added detailed criteria to describe specific patterns of deformation within shield fields cross cutting and embayment relationships between shield fields and surrounding units we also used secondary characteristics of shield fields such as radar albedo difference changes in shield density and size etc in our study we applied these criteria and analyzed in detail stratigraphic relationships of shield fields in a random sample of features 64 fields and in the global geotraverse along 30 n 77 fields the total number of analyzed shield fields 141 represents about 22 of the general population of these features catalogued by crumpler and aubele 2000 the majority of the fields 98 or 69 predate emplacement of material of vast regional plains with wrinkle ridges fifteen fields 11 appear to be synchronous with regional plains and eleven fields 8 postdate the plains nine fields 6 display ambiguous relationships with regional plains and their relative age is uncertain and eight fields 6 represent unclear cases when fields are covered by crater related materials or by young lava flows or are not in contact with regional plains the results of our study provide evidence for a distinct change of volcanic style from the mode of formation of globally abundant small shields to the mode of emplacement of vast regional plains in many areas on venus this systematic change of volcanic style appears to be inconsistent with the nondirectional or quasi steady state character of the geologic record of venus although individual small shields were formed throughout the majority of the visible geologic history of venus in the syn and postregional plains time the small shields style of volcanism was significantly reduced in abundance the shield fields that predate regional plains do not display a strong tendency to form a single group or a few groups and can be found virtually in all places on venus we interpret this observation to mean that these shield fields were globally distributed before the emplacement of regional plains this interpretation means that the shield fields embayed by regional plains represent exposures of a specific globally widespread unit shield plains psh in contrast shield fields that postdate regional plains occur preferentially in the beta atla themis region on venus well known for its concentration of relatively young volcanic and tectonic activity the spatial association of relatively young fields with the large centers of young volcanism suggests a genetic link of these fields with the formation and development of the large scale volcanic centers the abrupt decrease of the number of shields that postdate the formation of shield plains psh strongly suggests a major change of the style of volcanism following their emplacement as a globally distributed unit