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

corona associations and their implications for venus abstract geologic mapping principles were applied to determine genetic relations between coronae and surrounding geomorphologic features within two study areas in order to better understand venusian coronae the study areas contain coronae in a cluster versus a contrasting chain and are 1 directly west of phoebe regio quadrangle v 40 centered at latitude 15 s longitude 250 and 2 west of asteria and beta regiones between latitude 23 n longitude 239 and latitude 43 n longitude 275 results of this research indicate two groups of coronae on venus 1 those that are older and nearly coeval with regional plains and occur globally and 2 those that are younger and occur between beta atla and themis regiones or along extensional rifts elsewhere sometimes showing systematic age progressions mapping relations and earth analogs suggest that older plains coronae may be related to a near global resurfacing event perhaps initiated by a mantle superplume or plumes younger coronae of this study that show age progression may be related to 1 a tectonic junction of connecting rifts resulting from local mantle upwelling and spread of a quasi stationary hotspot plume and 2 localized spread of post plains volcanism we postulate that on venus most of the young post resurfacing coronal plumes may be concentrated within an area defined by the bounds of beta atla and themis regiones

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

tectonic and stratigraphic implications of the relative ages of venusian plains and wrinkle ridges abstract a major ongoing controversy concerns the style of crustal evolution on venus at one extreme is a directional model that proposes a sequence of depositional and deformational events that occur at specific times in the evolution of the crust and that are global in extent at the other extreme is a model that argues for different ages of these events in different places on the planet a test of the directional model is here focused on whether wrinkle ridges formed at a single time in the recorded crustal history of venus where sets of wrinkle ridges intersect it commonly is possible to determine that one set is older than the other also the deformation responsible for wrinkle ridges is in places clearly progressive with respect to stratigraphic material units these observations are not consistent with a specific single time for the formation of wrinkle ridges within the stratigraphic sequence within an area including about 1 3 of the surface of venus 15 of craters that are younger than regional plains are older than wrinkle ridges 85 are younger than wrinkle ridges taking 750 myr as a reasonable mean age for the regional plains this implies that the mean age of wrinkle ridges is 110 myr younger than the mean age of plains solomon et al 1999 science 286 87 propose that the emplacement of a large volume of plains lava would lead to a major atmospheric temperature increase their model predicts thermal stresses in the lithosphere that at shallow depth would reach peak compressive stresses in about 100 myr a number very similar to the time lag between plains emplacement and wrinkle ridge formation indicated by the crater data the thermal compressive stresses responsible for wrinkle ridges would be maintained at a level sufficient to deform basalt for at least 100 myr and possibly for as long as 350 myr these time intervals are not really short compared to the mean age of the plains finally because wrinkle ridges are demonstrably younger than the plains they deform they cannot be related to the processes that formed the plains and thus should not be used to define a plains with wrinkle ridges unit

impact crater outflows on venus morphology and emplacement mechanisms many of the 932 impact craters discovered by the magellan spacecraft at venus are associated with lobate flows that originate at or near the crater rim they extend for several to several hundred kilometers from the crater and they commonly have a strong radar backscatter a morphologic study of all identifiable crater outflows on venus has revealed that many individual flows each consist of two areas defined by distinct morphologic features these two areas appear to represent two stages of deposition for each flow the part of the flow that is generally deposited closest to the crater tends to be on the downrange side of the crater flows in the downrange direction and it is interpreted to be a late stage ejecta in many cases this proximal part of the flow is too thin to completely bury the large blocks in subjacent ejecta deposits dendritic channels present in many proximal flows appear to have drained liquid from the proximal part in the downhill direction and they debouch to feed the outer part of the flows this distal part flows downhill fills small grabens and is ponded by ridges behavior that mimics that of volcanic lava flows the meandering and dendritic channels and the relation of the distal flows to topography strongly suggest that the distal portion is the result of coalescence and slow drainage of impact melt from the proximal portion impact melt forms a lining to the transient crater and mixes turbulently with solid clasts and part of this mixture may be ejected to form the proximal part of the flow during the excavation stage of crater development a statistical study of the venusian craters has revealed that in general large craters produced by impacts with relatively low incidence angles to the surface are more likely to produce flows than small craters produced by higher angle impacts the greater flow production and downrange focusing of the proximal flows with decreasing incidence angle indicate a strong control of the flows by the impactor flight direction and a high downrange velocity imparted to the proximal flow material in lower angle impacts on the moon small flows interpreted to be composed of impact melt are observed atop the ejecta of large fresh craters on earth melt rich suevite deposits form the uppermost layer of ejecta of some fresh craters these features albeit much smaller may be analogous to the flows on venus numerical models have predicted that larger volumes of impact melt would be produced on venus than on the cooler terrestrial bodies due to high atmospheric and target temperatures perhaps 3 times the volume produced on the moon for a given crater diameter

a stratigraphic study of small volcano clusters on venus recently proposed global stratigraphy and geologic history models have led to two end member volcanic evolution models for venus a directional model that proposes venus has progressed through a series of stages each characterized by a particular style of volcanic activity a t basilevsky and j w head 1997 j geophys res 103 8531 8544 and a nondirectional model that proposes a more earth like type of volcanic activity with different eruption styles occurring globally on a local scale throughout the recent history of the planet j e guest and e r stofan 1999 icarus139 55 66 the former infers that most small volcanoes d 20 km define a global unit referred to as the shield plains that predates emplacement of the widespread plains with wrinkle ridges or regional plains to test this hypothesis cross cutting and superposition relations were used to determine the stratigraphic positions of small volcano clusters in seven quadrangles 11 of the surface the seven quadrangles included 179 clusters containing over 22 000 constructs three materials were defined for each cluster 1 the constructs 2 the material immediately surrounding the constructs called the cluster material and 3 the regional plains where possible relative ages were established between the constructs cluster material and the regional plains there are six possible temporal relations between the constructs and regional plains emplacement 1 the constructs postdate regional plains emplacement 2 some constructs postdate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 3 the constructs predate regional plains emplacement 4 some constructs predate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 5 the constructs both predate and postdate regional plains emplacement and 6 the ages of the constructs relative to regional plains emplacement are ambiguous an age relative to regional plains emplacement could not be determined for almost half of the clusters studied of the clusters for which a relative age could be determined almost half postdate regional plains emplacement and an additional third contain at least some constructs that postdate regional plains emplacement less than a tenth of the clusters predate regional plains emplacement these observations indicate that many small constructs postdate the emplacement of the regional plains and therefore are consistent with the nondirectional volcanic evolution model in which small constructs represent a global process occurring locally throughout the recent history of venus

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

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

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

development of large volcanoes on venus constraints from sif gula and kunapipi montes abstract sif gula and kunapipi montes are large volcanoes on venus they have differing morphologies reflecting their different eruptive histories and plumbing systems all have a steeper upper cone surrounded by more shallowly sloping flanks interpreted as resulting from histories of more frequent shorter lower effusion rate summit eruptions accompanied by infrequent more voluminous flank eruptions all three volcanoes have multiple flow field types digitate sheet and fan which probably reflect variations in effusion rate eruption duration and vent geometry sif mons summit has a large caldera gula mons has summit calderas at either end of a summit rift and the summit of kunapipi mons is composed of two volcanic centers on a summit plateau the summit morphologies of sif gula and kunapipi montes are interpreted to reflect differences in magma chamber location over time and differing internal plumbing we find no positive evidence that neutral buoyancy has governed magma chamber position instead magma chambers at the three volcanoes appear to have changed position over time with lithospheric stresses as well as neutral buoyancy affecting chamber location all three volcanoes have had episodes of diking at their summits followed by episodes of extrusion we interpret this to be caused by variations in either magma supply rate or magma volatile content or both over time fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes indicating that the local stress field within the volcano favored intrusion as well as extrusion over much of its history the overall shape and history of the venusian volcanoes is similar to the histories of terrestrial volcanoes however some aspects of these volcanoes including the rift at gula the large size and very long flows differ significantly from terrestrial volcanoes

embayed intermediate volcanoes on venus implications for the evolution of the volcanic plains abstract volcanoes on venus are classified according to size with studies on the stratigraphic position of large volcanoes proposing that most of the large volcanoes postdate the regional volcanic materials some studies regarding intermediate volcanoes proposed that some of these volcanic features could be large volcanoes with embayed flow aprons a situation that would alter the previous stratigraphic considerations about large volcanoes on venus in this work i analyze the global population of embayed intermediate size volcanoes and compare their summits with that of other edifices classified as large volcanoes intermediate size volcanoes are considered embayed when 1 flows from another source clearly overlap the volcano slopes and 2 display scarps related to flank failure processes but with the associated collapse deposits being absent i e interpreted as covered as result of the survey 88 embayed intermediate size volcanoes have been catalogued and integrated into a geographic information system these embayed volcanoes have summit sizes and characteristics similar to large volcanoes and therefore could be interpreted as possible large volcanoes with their flow aprons embayed embayment materials for these volcanoes include all the units present in the history of the volcanic plains and would indicate that this type of central volcanic edifice would occur throughout the geologic history recorded in the venusian plains

the age palaeoclimate palaeovegetation coal seam architecture mire types paleodepositional environments and thermal maturity of syn collision paralic coal from mukah sarawak malaysia abstract the mukah coal accumulated in the balingian formation where the time stratigraphic position is poorly defined by fauna though a probable late miocene age has always been assigned to this formation samples collected in the present study that yielded an abundance of casuarina pollen associated with occurrences of dacrydium stenochlaena palustris florschuetzia levipoli and also stenochlaena areolaris spores compare closely to zone pr9 of the palynological zonation of the malay basin and can be tied to depositional sequences of malay basin seismic sequences i2000 i3000 indicating an early miocene age for the studied coal the early miocene age shows that the mukah coal was formed during the collision between luconia block dangerous grounds with the borneo that lasted from late eocene to late early miocene the rapid increase of deposition base level caused by the collision is clearly reflected by the architecture of the mukah coal seams that were generally thin and also by the reverse order of the paleo peat bodies the studied coal samples contained large amounts of detrohuminite without the enrichment of liptinite group macerals usually explained by the dominance of herbaceous plants in the paleomires however the pollen assemblages recovered in the present study provide no support for a herbaceous swamp instead the peats were likely formed mainly from forest vegetation based on the recovery of dominant arboreal pollen assemblages in the present study palynomorph assemblages recovered in the present study with abundance of casuarina pollen associated with common occurrences of dacrydium strongly suggest kerangas vegetation and kerapah type peat swamps indicating a very wet climate the occurrence of rattan and pandanus pollens shows that the kerapah type peat swamps were locally bordered by rattan and pandanus swamps this shows that the prevailing use of coal petrography to discern the type of vegetation that was present in the peat forming mires may lead to wrong conclusions therefore a multi disciplinary approach must be applied for a more accurate and reliable spatial interpretation of the type of vegetation that was present in the peat forming mires consistent with the low sulphur content evidence from the palynomorph assemblages recovered from the coal seams shows that the coal forming peat was deposited in freshwater mires with little or no marine influence despite the fact that the accumulation of the mukah coal forming peat took place within a coastal plain setting moreover the fauna present in the host rock formation also suggested a brackish water environment of deposition

regional fracture patterns around volcanoes possible evidence for volcanic spreading on venus abstract magellan data show that the surface of venus is dominated by volcanic landforms including large flow fields and a wide range of volcanic edifices that occur in different magmatic and tectonic environments this study presents the results from a comprehensive survey of volcano rift interaction in the bat region and its surroundings we carried out structural mapping of examples where interaction between volcanoes and regional fractures results in a deflection of the fractures around the volcanic features and discuss the nature of the local volcano related stress fields that might be responsible for the observed variations of the regional fracture systems we propose that the deflection of the regional fractures around these venusian volcanoes might be related to volcanic spreading a process recognized as of great importance in the tectonic evolution of volcanoes on earth and mars but not previously described on venus

formation and evolution of volcanic edifices on the dione regio rise venus mapping of dione regio venus a regional highland 1200 by 2700 km in size with mean elevation of 6052 km located in the southern hemisphere to the nw of lavinia planitia reveals a region characterized by early tectonically dominated structure and later widespread volcanism and centralized edifice construction small patches of complexly deformed terrain tessera and larger regions of plains characterized by a single direction of deformation tectonized plains preserved as inliers are embayed by more recent widespread volcanic plains smaller scale plains resurfacing superposed these plains following a pervasive compressional event that lead to distributed wrinkle ridge formation four major volcanoes dominate the topographically irregular highland each with similar early stage radar dark lava flow deposits but significantly different late stage eruptions these late stage differences are manifested as dark diffuse deposits possibly pyroclastic in origin and construction of a single steep sided dome at ushas mons a small shield volcano field at nepthys mons and bright diffuse deposits possible pyroclastic deposits with summit dome and pit formation at innini and hathor montes the presence at all of these volcanoes of radial lineaments and graben interpreted to be related to near surface lateral dyke emplacement and late stage summit constructional features indicate that shallow magma chambers occur at each edifice variations in the evolutionary history of and eruptions from these chambers may explain the differences observed in the deposits at these major edifices it is also possible however that fundamental differences in the magma source region was a major factor in determining the appearance of the deposits of the several models considered for the formation of the region secondary upwellings from a single large mantle plume or the near contemporaneous upwelling of multiple smaller plumes best explain the observed characteristics at the rise a comparison to other volcanic highlands suggests that there are at least two classes of volcanic rises on venus an areally and topographically large type with significant volcanism and rift zone formation like beta regio and a smaller scale type with extensive volcanism but moderate rifting like western eistla regio these two classes reflect a spectrum in size and vigor of mantle upwellings the dione rise is most like western eistla regio in terms of areal extent topographic elevation and geologic complexity

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

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

organic geochemical investigations of the dali lake sediments in northern china implications for environment and climate changes of the last deglaciation in the east asian summer monsoon margin abstract millennial scale environment and climate changes in the east asian summer monsoon margin during the last deglaciation are reconstructed by systematic studies on the characteristic of sedimentary organic matter from dali lake in northern china concurrent increases in the toc and tn concentrations indicate increases in terrestrial organic matter and nutrient inputs to the lake and a development of terrestrial vegetation and phytoplankton productivity related to increases in regional temperature and precipitation c n ratios reflect changes in the proportions of terrestrial and aquatic organic matter decreases in both 13corg and 15n values indicate increases in the isotopically lighter terrestrial carbon and nitrogen inputs to the lake due to increases in surface runoffs while a sharp decrease in the 15n value implies a significant weakening in the biological activities of nitrifying and amonifying bacteria due to abrupt decrease in the water temperature the geochemical data indicate that regional temperature and precipitation exhibited increasing trends from 15 000 to 12 350 cal yr bp temperature decreased abruptly at 12 350 cal yr bp and then maintained a low level from 12 350 to 11 400 cal yr bp precipitation decreased to a relatively low level from 12 350 to 11 400 cal yr bp and both temperature and precipitation returned to increase after 11 400 cal yr bp the climate change in the dali lake region during the last deglaciation corresponds within age uncertainties to the bolling allerod ba warm phase and younger dryas yd cold reversal occurring over northern high latitudes however the gradual and mild increasing trends of regional temperature and precipitation during the ba warm period contrasts with the general cooling trend in northern high latitude temperature implying a dominant influence from increases in the northern hemisphere summer insolation while the slight decreases in regional precipitation relative to the rapid and significant decreases in northern high latitude temperature during the yd cold period may have resulted from local moisture recycling or from an insensitive response of hydrology and ecology to the regional precipitation change

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

evolution of large shield volcanoes on venus 1 we studied the geologic history topographic expression and gravity signature of 29 large venusian shield volcanoes with similar morphologies in magellan synthetic aperture radar imagery while they appear similar in imagery 16 have a domical topographic expression and 13 have a central depression typical dimensions for the central depression are 150 km wide and 500 m deep the central depressions are probably not calderas resulting from collapse of a shallow magma chamber but instead are the result of a corona like sagging of a previously domical volcano the depressions all have some later volcanic filling all but one of the central depression volcanoes have been postdated by geologic features unrelated to the volcano while most of the domical volcanoes are at the top of the stratigraphic column analysis of the gravity signatures in the spatial and spectral domains shows a strong correlation between the absence of postdating features and the presence of dynamic support by an underlying plume we infer that the formation of the central depressions occurred as a result of cessation of dynamic support however there are some domical volcanoes whose geologic histories and gravity signatures also indicate that they are extinct so sagging of the central region apparently does not always occur when dynamic support is removed we suggest that the thickness of the elastic lithosphere may be a factor in determining whether a central depression forms when dynamic support is removed but the gravity data are of insufficient resolution to test this hypothesis with admittance methods

stratigraphic and geographic distribution of steep sided domes on venus preliminary results from regional geological mapping and implications for their origin analysis of the stratigraphic position of 44 steep sided domes in a global band of c1 midrs at 30 n latitude shows that in a region representing over 11 of the surface of venus the vast majority of steep sided domes are closely associated stratigraphically with shield plains psh a unit characterized by a very high abundance of small shields of apparent basaltic extrusive origin previously reported heterogeneities in areal and altitude distribution are now more plausibly interpreted in terms of formation in association with this unit and burial by subsequent regional plains units measurement of abundances of steep sided domes presently exposed in shield plains and extrapolation to areas below younger units suggest that over a thousand of these features may have formed originally interpretation of the mode of formation of the steep sided domes in the context of these new data favors hypotheses for their origin which call on remelting of basaltic crust to produce more silica rich magmas occurrences of some steep sided domes in association with summits of large shields e g tepev mons and sapas mons indicate that other mechanisms of dome formation e g evolution of magma reservoirs have also operated at other times in the history of venus

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

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

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

magma reservoir subsidence mechanics theoretical summary and application to kilauea volcano hawaii an analytic model has been developed for the prediction of the three dimensional deformation field generated by the withdrawal of magma from a sill like storage compartment during an intrusion or eruption cycle the model is based on the work of berry and sales 1961 1962 and predicts the vertical displacement components over the areal plane model parameters are the depth of burial h the intrusion half width a the intrusion half length b the thickness of the magmatic interior at the moment of melt withdrawal tm and the planform aspect ratio a b the products of the model include areal deformation maps systematic variation in model parameters within the context of kilauea volcano hawaii have revealed that circular and elliptical deformation patterns result from the collapse of draining rectilinear intrusions at depth moreover the geometric parameters of a storage compartment may interact in complex ways to produce similar deformation patterns the model has been applied to kilauea volcano for three periods of pronounced summit subsidence 1 1921 1927 bracketing the steamblast eruptive phases of 1924 2 june 1972 to december 1972 and 3 december 1972 to may 1973 application of the model requires the simultaneous optimization of five predicted deformation features with respect to field measurements and the derivative deformation maps 1 the vertical displacement maxima 2 the vertical displacement gradients over the areal plane 3 the lateral extent of the deformation field 4 the aspect ratio of the subsidence pattern and 5 the strike of the major axis of the deformation field the constrained geometries and volumes of the inferred collapsed storage cavities for each period are 1 1921 1927 depth 3 km a 1500 m b 4500 m tm 20 m v 540 106 m3 2 june 1972 to december 1972 depth 3 3 km a 600 m b 2000 m tm 1 m v 4 8 106 m3 and 3 december 1972 to may 1973 depth 2 2 km a 500 m b 1612 m tm 1 m v 3 2 106 m3 for 2 and 3 calculated magmatic thicknesses tm happen to be in the range 3 48 0 15 m of measurements for sill like bodies in deeply dissected hawaiian shield volcanoes the fits obtained between calculated and observed deformation patterns allow quantification of the location overall dimensions orientation and volume of the discrete still molten interior of sill like compartments from which magma is tapped during eruption or intrusion

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

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

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

venus s shield terrain plains planitiae or lowlands expanses of gentle long wavelength 1000 km basins cover 80 of venus s surface these regions are widely accepted as covered by volcanic fl ows although the mechanism s responsible for resurfacing remains elusive in addition a volcanic origin for the lowland surface may be open to question lowland resurfacing is typically attributed to catastrophic emplacement 10 100 m y of globally extensive thick 1 3 km fl ood type lava this model of resurfacing has been postulated on the basis of impact crater distribution taken together with a lack of obvious volcanic fl ows or edifi ces and a lack of viable alternative models ongoing geologic mapping of 15 000 000 km2 0 25n 90 150e using 225 m pixel and 75 m pixel nasa magellan sar synthetic aperture radar data indicates that small edifi ces called shields 1 15 km diameter edifi ces 1 km high play a major role in lowland resurfacing individual shields are radar smooth or rough quasi circular to circular features with or without a central pit shield shapes which have been previously documented range from shield dome or cone to fl at topped or fl at shield deposits typically coalesce forming a thin regionally extensive but discontinuous mechanically strong layer herein called shield paint shield paint conforms to delicate local topography providing evidence of its thin character and indicating generally low viscosity during emplacement shield terrain shields and shield paint covers more than 10 000 000 km2 within the study area detailed mapping of fi ve 2 2 regions using coregistered normal and inverted right and left illumination sar imagery indicates shield densities of 3500 33 500 shields 106 km 2 thus the map area hosts more than 35 000 335 000 shields shield terrain generally postdates but is also locally deformed by fractures and wrinkle ridges indicating time transgressive formation relative to local deformation and or reactivation the regional scale crust was strong throughout shield terrain formation shield terrain may extend across much of venus s surface

new insights into coronae evolution mapping on venus coronae are geologically and geophysically important features on venus because they are thought to contribute to planetary heat loss it is thus necessary to understand their role in space and time in the evolution of venus detailed mapping of five coronae in guinevere and sedna planitia illustrates that although previous models invoked initial uplift not all coronae can be explained in such a simple way we show that the formation of corona annuli can be multistaged and that the position of the annulus does not always coincide with the main topographic ridges and troughs that outline the feature the magnitude and timing of volcanism are not necessarily the same at each corona and coronae can have long and complex histories in contrast with the stratigraphic relations suggested by other workers we demonstrate that coronae do not all have the same relative ages with respect to adjacent units these results suggest that corona formation was is not confined to a single time period in the history of venus and that detailed mapping is a reliable method of establishing the relative timing of corona formation