impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

novae on venus geology classification and evolution 1 novae are radially fractured centers 100 300 km in diameter and 64 have been identified on venus dense radial fracturing and upraised topography are common and massive amounts of volcanism are seen in some novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement we analyzed the topography and character of deformational and morphological structures of all 64 novae leading to a better understanding of their evolution and relation to diapir dynamics and their age we subdivided novae into four topographic classes 1 upraised 2 annular 3 flat and negative and 4 plateau like novae are located mostly in areas of regional rises and rift zones with a small number in the lowlands about one half the novae studied show association with rifts in some of these the rift troughs surround nova rises and form plateau like novae plateau like novae may predate postdate or form simultaneously with rifts our analysis generally confirms and updates previous models of novae evolution with the following geological factors being important 1 depth of the neutral buoyancy level of the rising mantle diapir 2 rheological characteristics of the overlying lithosphere 3 whether or not the visco plastic material above the diapir undergoes spreading and 4 the nature and influence of regional stress and rift structures these combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova such as topography tectonics and volcanism we found clear evidence for 1 the important role of dike swarm emplacement in the formation of radial patterns of novae and 2 the formation of concentric tectonic features during the relaxation stage of novae novae have multiple evolutionary stages and are long lived structures stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units we found that 40 3 of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11 3 completed their activity before this time 88 7 of the population of novae was active after regional plains formation in contrast to novae coronae activity was greatest before formation of regional plains which may be due to thickening of the lithosphere with time detailed structural analysis shows that novae evolution does not always lead to the formation of corona like features

constraints on the lithospheric structure of venus from mechanical models and tectonic surface features radar images of the surface of venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere many of these features exhibit characteristic scales widths or spacings of deformation one of 10 20 km and another of 100 300 km for a range of simple models we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the venus lithosphere results show that the existence of tectonic features that exhibit both length scales can be explained if at the time of deformation the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust the spacings of these features imply crustal thicknesses in the approximate range 5 30 km and a thermal gradient not greater than 25 k km 1 features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts for a broad range of parameters the models require not only that smaller scale compressional and extensional features have similar spacings but also that the widths and spacings of larger scale compressional features be greater than those formed in extension this is consistent with observed differences in the characteristic length scales of larger scale surface features as well as the similarities in spacings of observed smaller scale features

40ar 39ar dating of a tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives the sedimentary sequence cored in the seneze maar velay france is very favourable for magnetostratigraphy of the late pliocene and for studying the paleoenvironmental changes that occurred during that period the magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the matuyama reverse period except for the upper part where a short normal polarity episode was recorded a tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2 10 0 01 ma by the 40ar 39ar method using this chronological marker it can be inferred that the normal polarity episode recorded in the seneze sequence corresponds to the reunion event and that the age of the normal reverse transition closing this event is ca 2 09 ma these chronological constraints also permit to 1 demonstrate that the villafranchian mammal fauna found in the seneze maar is younger than 2 09 ma and 2 correlate the climatic events recorded in the seneze sequence to the marine d18 o records this suggests that the seneze pollen sequence 5 120 m depth ranges from isotopic stage 85 to 76

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

separation of s wave and p wave reflections offshore western florida hard water bottom marine environments such as offshore western florida have presented particular problems in the acquisition and processing of seismic reflection data one problem has been the limited angle of incidence less than critical available to p wave penetration into the subsurface mode conversion from p wave to s waves sv however is quite efficient over a broad range of angles of incidence after the success of a previously reported physical model experiment an experimental line was acquired offshore western florida the 19 mile line located approximately 100 miles west of key west florida was shot and processed three key factors have contributed to the successful recording of mode converted s wave reflections 1 recognition of the effect of the group length on attenuation of energy arriving at large angles of incidence 2 tau p processing techniques that allow separation of energy by angle of incidence and 3 velocity filtering over a range of hyperbolic normal moveout nmo

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

wrinkle ridges stress domains and kinematics of venusian plains wrinkle ridges are nearly ubiquitous landforms on the plains of venus by analogy with similar structures on other planets venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust an inference that is verified by geometrical relationships with positive and negative relief features on venus because plains are the dominant terrain on venus wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet in most places there are two or more sets of wrinkle ridges and commonly one of these persists over a very large area defining a regional stress domain intersection relationships indicate that these domains differ in age

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

the global distribution of giant radiating dike swarms on venus implications for the global stress state magellan radar data of venus reveal 163 large radial lineament systems composed of graben fissure and fracture elements on the basis of their structure plan view geometry and volcanic associations at least 72 are interpreted to have formed primarily through subsurface dike swarm eraplacement the remainder through uplift or a combination of these two mechanisms the population of swarms is used to determine regional and global stress orientation the stress configura tion recorded from 330 210oe aphrodite terra is best ex plained by isostatic compensation of existing long wave length topography or coupling between mantle flow and the lithosphere the rest are correlated with concentrations of rifting and volcanism in the beta atla themis region the global stress field on venus is different than that of earth where plate boundary forces dominate from their surroundings where domical topography occurs the ra dius of the associated lineament system is on average 2 5 times greater than that of the dome some 53 of the radiat ing systems are associated with concentric structures ranging from central depression bounding scarps 25 km wide to tec tonic rings 575 km in diameter in 51 of these cases the ra dial pattern originates within but also extends significantly beyond the annulus only 9 of the radiating systems are confined completely leaving 40 located outside but focused upon a central annular structure finally the radial geometry is usually quite pronounced near the center of each structure 52 of the time radial lineaments occupy 270 o of azimuth and 80 exceed 180 o figure 2c in their distal regions how ever only 72 of the radiating systems retain a purely radial geometry as the remaining 28 gradually develop a non radial unidirectional lineament configuration all but seven of the radiating systems exhibit volcanism figure 2d lobate flows emerging from individual linea ments or many such flows strongly correlated with multiple radial lineaments occur for 45 of the systems clusters of small shields occur 72 of the time and in 75 of these in stances distinctive alignment of small shields and or pits along radiating lineaments is seen finally 65 are associ ated with some other form of centralized volcanism including multiple forms of edifice construction limited extrusions and sheet like flows which extend into the surrounding terrain

faunal remains from the oldowan site of muhkai ii in the north caucasus potential for dating and palaeolandscape reconstruction abstract the results of study of faunal materials unique for the north eastern caucasus are published deriving from layer 80 of the oldowan site of muhkai ii twenty six specimens of four species of carnivorous mammal and six species of herbivorous mammal were identified in this material in addition the remains of small mammals reptiles and amphibians were identified which are indicators for the dating of the assemblage and the establishment of the specific characteristics of the natural environment at the time of usage of the oldowan site while in the modern day the investigated region consists of a belt of medium high mountains during the chronological period under study open landscapes of savannah type with small areas of forest vegetation dominated based on a combination of data from palaeomagnetic analysis and faunal determinations the time of existence of the complex is established as no later than the lower limit of the olduvai episode during the matuyama palaeomagnetic epoch

radiating graben fissure systems in the ulfrun regio area venus abstract radiating graben fissure systems are common on venus most are thought to be underlain by mafic dykes fed by centrally located magmatic centres from previous work it has been shown that these magmatic plumbing systems can extend out up to 2000 km or more and that interaction between neighbouring systems can provide insight into the relative chronology of their magmatic centres systematic mapping of graben fissure systems has potential as a tool for regional magmatic chronology and correlation on venus this methodology is applied to the ulfrun regio area 200 240 e 0 25 n where we mapped 47 000 graben and fissures from these 66 radiating systems comprised of 13 000 individual graben and fissures and having radii of up to 2000 km have been identified and are interpreted to be underlain by dyke swarms focussed on magmatic centres cross cutting relationships among these systems and with the hecate chasma rift zone have been examined to provide a relative chronology for the magmatic centres two trends emerged a an apparent younging from the southwest to northeast of the study area and b a cluster of older ages in the southwest linked to the atla regio mantle plume

pliocene early pleistocene history of the euphrates valley applied to late cenozoic environment of the northern arabian plate and its surrounding eastern turkey abstract the pliocene quaternary paleogeography of the euphrates river valley changed due to sinistral movements on the east anatolian fault zone eafz and the taurus ridge rise by movements on the south taurus thrust evidence of these changes is based on studies of the pliocene quaternary deposits of the euphrates river basin to the north and to the south of the taurus ridge and the late cenozoic deformation including offsets on the eafz combination of methods was used to date the pliocene quaternary deposits it includes geological and geomorphic analysis and correlation of sections determination of remanent magnetization paleontological and archaeological finds pollen analysis and k ar dating of volcanic rocks to the north of the taurus ridge the late miocene tectonic depressions were filled by lakes connected by braided streams in the early pliocene the euphrates and murat river valleys formed and the euphrates flew to the south westwards of its recent position via the graben like trough of the sultan suyu river valley and farther to the erikdere that are recent euphrates tributaries the flow was interrupted later because of some desiccation and rise of the taurus ridge the flow recommenced in the end gelasian early calabrian via the goksu cayi and erikdere valleys consecutively and was interrupted again at the end of calabrian 0 8 0 9 ma the euphrates waters found the recent way via the taurus ridge and the former upstream bottoms of the euphrates and its tributaries valleys became a vast upper terrace after this the taurus ridge rose by more than 330 m lower terraces were formed because of the tectonic uplift that was more intense to the north of the taurus ridge 0 13 0 16 mm year than to the south of it 0 1 mm year the new formed segment of the euphrates valley was offset on the eafz at 12 km that gives the slip rate of 13 15 mm year

orogenic tectonism on io 1 we catalog 143 ionian mountains montes and mountain like features mensae tholi plana and small peaks in order to investigate orogenic tectonism on io from this comprehensive list we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features three of the 96 mountains are probably volcanoes 92 appear to be tectonic massifs and 1 is ambiguous of the 92 tectonic mountains 38 abut paterae volcanic or volcano tectonic craters with irregular or scalloped margins this juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is 0 1 we propose instead that orogenic faults may act as conduits for magma ascent thus fueling patera formation near mountains as resurfacing buries a shell of material from io s surface to the base of the lithosphere its effective radius is reduced and it heats up we calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness conservation of volume dictates that this material must be uplifted at io s surface by estimating the total volume of the mountains we are able to place a lower limit of 12 km on io s lithospheric thickness we hypothesize that in some cases mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere the resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism this model is one of several possible mechanisms for uplifting isolated mountains such as are observed on io

timing of formation of beta regio and its geodynamical implications 1 recent evidence suggests that the beta regio equatorial highland generally considered to be a plume related structure was emplaced over a relatively short period of time in the recent venus history 100 400 m y we perform numerical simulations which use this evidence as a new constraint on geodynamic models and find that it is difficult to satisfy simultaneously the constraint on the uplift rate and the constraints on gravity topography and rheology one possible solution is that the effective viscosity contrast between the mantle and the lithosphere does not exceed about 104 if our simple models do capture the basic dynamics of formation of beta regio this value implies that the viscosity of the lithosphere is softer than olivine at subsolidus temperatures and that the venusian lithosphere may not be much stronger than the terrestrial one

uplift of beta regio three dimensional models 1 three dimensional models of the uplift of beta regio caused by a mantle plume satisfy constraints on gravity topography rheology and the uplift rate substantially better than two dimensional models in particular the uplift time of beta regio is reduced to an acceptable 800 million years three dimensional models give the plume formation depth around 3000 km which approximately corresponds to the core mantle boundary the results depend only weakly on the initial lithospheric thickness at the time of global resurfacing including a very thin initial lithosphere this implies that the thick present day lithosphere 400 km suggested by our models can be reconciled with a thin lithosphere 100 200 km inferred from the melt generation rates and the flexural rigidity models

evidence for rapid regional plains emplacement on venus from the population of volcanically embayed impact craters we have reexamined the population of craters on venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater presenting nonvolcanic processes which may produce similar crater morphologies we conclude that 29 craters have been embayed by lava flows from the exterior and an additional 27 craters are ambiguous cases 3 6 of the total crater population all of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges through geologic mapping of an area covering over 8 of the surface we examined the spatial relationships between wrinkle ridged plains and stratigraphically older plains units and estimate that a large portion of the wrinkle ridged plains are thinner than 500 m which is thin enough for the rims of the largest half of preexisting craters to be preserved there are not enough craters embayed by the wrinkle ridged plains to support a model in which the plains were erupted directly onto an ancient surface given the number of large craters embayed by thin deposits of wrinkle ridged plains we develop a simple statistical model to overestimate the number of craters on the preplains surface this model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years the short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle ridged plains

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

recent and episodic volcanic and glacial activity on mars revealed by the high resolution stereo camera the large area coverage at a resolution of 10 20 metres per pixel in colour and three dimensions with the high resolution stereo camera experiment on the european space agency mars express mission has made it possible to study the time stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of mars here we show that calderas on five major volcanoes on mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history with phases of activity as young as two million years suggesting that the volcanoes are potentially still active today glacial deposits at the base of the olympus mons escarpment show evidence for repeated phases of activity as recently as about four million years ago morphological evidence is found that snow and ice deposition on the olympus construct at elevations of more than 7 000 metres led to episodes of glacial activity at this height even now water ice protected by an insulating layer of dust may be present at high altitudes on olympus mons

graben fissure systems in guinevere planitia and beta regio 264 312 e 24 60 n venus and implications for regional stratigraphy and mantle plumes detailed mapping in a 14 000 000 km 2 area of northwestern guinevere planitia and northern beta regio bounded by 264 312 e 24 60 n has revealed thousands of long extensional lineaments graben fissures and related fractures these can be grouped into radiating circumferential and linear systems thirty four radiating systems have been identified of which 16 have radii greater than 300 km and eight have radii greater than 1000 km twenty six linear straight systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km linear systems are generally associated with rifts although some may represent distal portions of radiating systems in addition 19 circumferential systems some associated with coronae have been identified the distribution of each system is compared with the host geology in order to place the graben fissure systems in a regional stratigraphic framework the majority of systems are 1 younger than tesserae ridge belts and densely fractured plains 2 coeval with and in many cases define fracture belts 3 partially flooded by wrinkle ridged plains units and 4 older than smooth and lobate plains units and young rifts the inventory of radiating graben fissure systems that we catalogue represents a database of tectono magmatic centers that complements the centers defined using other criteria e g large volcanoes coronae and shield fields we have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes at least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms

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

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

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

chronology of the formation process of venusian novae and the associated coronae 1 venusian radial features novae have been interpreted to represent the initial stage of the corona evolution thus they are hypothesized to be older than the corona rim structure however based on recent studies the majority of novae that are located in the inner part of the coronae seem to postdate corona formation the topography as well as the mutual age relationships of coexisting coronae and novae indicates that nova formation may have taken place during different corona evolution phases in this study we present a chronology for nova formation that offers an explanation of both age and location relations of the novae inside the coronae due to the proven age differences between coronae and novae located within them we conceive most of the novae within the corona annulus as representations of reactivation of the corona location we believe that the morphology the topography and the age relations of corona annulus and nova radial features depend on the stage of corona evolution during the reactivation period or event when the nova is formed our model explains the observed regularity between nova location and age relations of the corona nova joint structure better than the assumed movement of the lithosphere or migrating mantle plumes

venus estimation of age of impact craters on the basis of degree of preservation of associated radar dark deposits 1 venus impact craters larger than 5 km in diameter 90 of all craters have been classified based on the presence and prominence of associated radar dark deposits approximate estimates of the absolute age of craters of different morphologic classes are made analysis of age relations of dated craters and volcanic and tectonic structures permits the estimation of the age and duration of different volcanic and tectonic events

how does the shear wave structure of the seabed affect the seismic wavefield summary much modelling of the seismic wavefield is undertaken with an acoustic approximation in which the influence of shear waves is neglected although such calculations can predict the correct traveltimes for compressional p wave propagation they can he very misleading with respect to the distribution of seismic amplitudes especially at larger offsets at the seabed the acoustic approximation predicts total reflection for p waves incident beyond the critical angle however once the presence of shear waves in the solid material below the sea floor is taken into account p waves in the sea water incident beyond the critical angle can give rise to transmitted s waves with a consequent major change in the propagation pattern such effects are very important for areas with high velocities at the sea floor as commonly occurs in tropical waters such as the north west shelf of australia the character of the water borne noise in these conditions depends on whether the shear wavespeed at the seabed lies above or below the p wave velocity in the sea water above for high shear velocities two distinct sets of critically reflected multiples can be produced to give a very energetic noise train trapped in the water column conversion of p to s in transmission at the sea floor can often be important and give rise to significant arrivals on the outer traces from long marine cables further the conversion of energy to s waves reduces the energy available for p wave multiples and dramatically reduces the influence of waterbottom multiples compared with a purely acoustic situation synthetic seismogram calculations for large offsets and equivalent calculations in the slowness times domain with selective control of the level of multiples and conversion at each interface provide a convenient tool for characterizing the expected water borne energy and the influence of converted shear waves on the pressure field recorded in the water

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

late pliocene and pleistocene small mammal chronology in the italian peninsula the abundant documentation of small mammals in the italian peninsula collected over recent years furnishes a detailed biochronological sequence mainly from the late pliocene onwards an updated stratigraphic framework is here presented based on the european small mammal biozonation the early villanyian is characterized by mimomys hassiacus m stehlini and later poorly documented m polonicus the late villanyian localities are well characterized with m pliocaenicus m pitymyoides and m tigliensis the older part of the early biharian is documented by assemblages containing microtus allophaiomys ex gr pliocaenicus m pusillus m cf ostramosensis and m tornensis while the later part of the early biharian is characterized by advanced microtus allophaiomys species occurring together with m pusillus or m blanci in the late biharian m savini microtus hintoni gregaloides microtus iberomys ex gr huescarensis brecciensis and terricola arvalidens occur the early toringian with a mosbachensis allocricetus bursae pliomys episcopalis and small sized microtus brecciensis and the late toringian with a terrestris in diversified associations are quoted the most important faunal events are calibrated by independent chronological controls thanks to the record of small mammals from lacustrine deposits several figures illustrate the most significant rodent species occurring in the succession of selected italian localities

venus astra novae estimates of the absolute time duration of their activity abstract in order to assess the age relations between astra novae features with extensive radial fracture graben systems and their surroundings and to determine the duration of their activity we undertook a photogeologic analysis of magellan images of 78 astra 49 dark parabola craters and 114 clear halo craters for seven of these 78 astra it was found that the astrum forming faulting started before or close to the time of emplacement of regional plains and extended into the second part of post regional plains time because the mean age of the regional plains is close to the mean surface age of venus which is estimated to be 750 m y this means that the duration of activity of these seven astra was several hundred millions of years this is longer than the duration of activity of ongoing mantle plumes on earth but shorter than the duration of activity of the plume feeding the martian volcano olympus mons the basic morphologic characteristics of these seven astra as well as their age relations with other geologic units are similar to those of the majority of other astra therefore such a long duration could also be typical of some other astra we confirm the two phase pre and post regional plains evolution of astrum forming faulting suggested in previous studies for the first phase we show evidence for purely tectonic faulting caused by the diapiric rise of a mantle plume for the second phase we find evidence supporting the interpretation of other studies that the observed faults resulted from subsurface dike intrusions produced by magmatism associated with the plume we also found that faulting during the second phase was not instantaneous but distributed over a long period of time

deformational models of rifting and folding on venus tectonic features on venus with characteristic widths and or spacings can be used to place quantitative constraints on the structure of the mechanical lithosphere through geophysical modeling we develop theoretical models for both compressional buckling and extensional rifting deformation based on classical elastic plate models with characteristic spacings determined by the flexural parameter modified to account for the effects of a realistic rheology this rheology consists of a relatively weak crust overlying a more creep resistant mantle in both layers the upper portions deform by frictional yielding on preexisting faults with a yield stress that increases linearly with depth and the lower portions deform by non newtonian temperature dependent creep for extensional deformation a graben model is used in which the primary structure of the rift is formed in the strong mantle layer and secondary faulting occurs in the brittle crustal layer the introduction of a weak zone in the lower crust that acts to decouple these two layers allows the formation of rifts with the large widths and depths observed on venus the compressional folding model utilizes an elastic buckling approach in which the non elastic yielding of the upper and lower portions of the layer modifies the elastic folding wave length this approach avoids the unreasonably high stresses that are required for conventional buckling models the two scales of deformation observed in radar images 15 25 km and 100 300 km can be related to the elastic flexural properties of the crust and mantle respectively which are separated by an intermediate weak zone in the lower crust these results imply that for regions in which both scales of deformation are present the crust is 5 15 km thick and the thermal gradient is 10 15 km l in regions such as ishtar terra where only the smaller features are observed the crust may be considerably thicker the apparent structure seen in high resolution images of the beta regio rift with the major faults located at the edges of an extended linear central depression lends support to the assumption of a graben mechanism of rift formation

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

guidelines for building a detailed elastic depth model we developed guidelines for building a detailed elastic depth model by using an elastic synthetic seismogram that matched both prestack and stacked marine seismic data from the gippsland basin australia recomputing this synthetic for systematic variations upon the depth model provided insight into how each part of the model affected the synthetic this led to the identification of parameters in the depth model that have only a minor influence upon the synthetic and suggested methods for estimating the parameters that are important the depth coverage of the logging run is of prime importance because highly reflective layering in the overburden can generate noise events that interfere with deeper events a depth sampling interval of 1 m for the p wave velocity model is a useful lower limit for modeling the transmission response and thus maintaining accuracy in the tie over a large time interval the sea floor model has a strong influence on mode conversion and surface multiples and can be built using a

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

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

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

biochronology and biochron boundaries a real dilemma or a false problem an example based on the pleistocene large mammalian faunas from italy abstract reconstructing the neogene quaternary biostratigraphy in continental environments and recognising boundaries between successive mammalian faunal complexes is a problematic task the definition and application of biochronology has been often tainted by references to and inferences from biostratigraphy actually the discontinuities in the continental sedimentary record environmental conditions taphonomic and sampling biases are responsible for the fact that the stratigraphical order of the lowest and highest occurrences of fossil remains stratigraphic datum does not necessarily reflect the temporal order of actual first last appearances of taxa palaeobiological events in time nonetheless the first last appearance bioevents recorded by lowest highest occurrences of fossil remains in fossiliferous levels of different continental stratigraphical successions have been the principal foundation for establishing the biochronological setting of continental mammal faunas therefore from a theoretical point of view boundaries should be defined by the oldest and or youngest historical occurrence of one or more taxa what about an operational approach some examples of the italian faunas are presented focusing on the transition from the villafranchian to the galerian and from the galerian to the aurelian land mammal ages the results obtained indicate the difficulty of defining a stable scheme that can be applied for correlation by the international scientific community

marine pssp reflections with a bottom velocity transition zone marine shear wave reflection methods using the conventional data acquisition system i e source and receiver in water rely on two mode conversions at the water bottom to produce shear reflections such as pssp some theoretical considerations and the results of a marine check shot survey conducted in the gulf of mexico demonstrate that the difficulty in observing pssp events is attributable to weak p s and s p conversion at the bottom in regions with very low shear velocity a few hundred ft s or less sediments at the bottom for a simple water bottom with a low shear wave velocity water over a uniform half space the ps conversion factor is proportional to vs and the sp conversion factor is proportional to vs2 where vs is the bottom shear velocity for vs 1500ft s their product gives pssp reflections that can be comparable in amplitude to typical pppp events for vs 500ft s the pssp events should be about 30 db weaker and probably not visible for typical gulf of mexico sediments with a shear velo

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