crustal spreading on venus evidence from topography morphology symmetry and map patterns abstract evidence for numerous features similar to terrestrial divergent plate boundaries and spreading centers are present in the topography and morphology of western aphrodite terra venus head and crumpler 1987 linear discontinuities cutting across the strike of western aphrodite terra cross strike discontinuities or csds were examined in further detail it was found that 1 1 they represent the most prominent regional structural trend in western aphrodite and are readily distinguishable from linear patterns caused by ground tracks in the pioneer venus orbiter data 2 2 individual csds are either deep troughs or steep regional scarps and steps 3 3 the spacing between the more prominent csds ranges from about 500 to 1500 km average spacing is 850 km and 4 4 there is a distinctive structural trend orthogonal to the strike of the csds additional smaller scarps and slopes with characteristics of known csds occur in the arecibo altimetric profiles at spacings of as little as 50 km in agreement with previous suggestions of sotin et al 1989 that other smaller csds with less offset may occur in domains between the more prominent ones previously mapped the detailed characteristics of the csds detailed trough and scarp like form association with ridge offsets linear terminations linear slope traces parallelism great length are all very similar to the characteristics of transform faults and fracture zones on the earth s seafloor and this additional analysis supports the earlier interpretation that csds represent analogs to terrestrial oceanic fracture zones assessment of bilateral symmetry of topography and other elements across western aphrodite terra indicate that 1 1 an accurate prediction of the large and small scale symmetrical elements in both the altimetry data and radar images within each domain on one flank of the highlands may be made by substitution of a mirror image of the opposite flank and 2 2 smaller wavelength symmetrical elements are revealed in the altimetric profiles by subtracting the background regional symmetry and examining the residuals residual altimetry profiles obtained in this manner show that individual altimetry features several hundred kilometers wide and several hundred meters high are frequently located at the same distance from the axis of regional symmetry on the north and the south flanks of the highlands topography and geometric features such as ridge crests and transforms have specific predictable relationships in a crustal spreading environment tests were made in western aphrodite for such organized relationships predicted to occur in association with ridge crest offsets at transform faults and fracture zones it was found that 1 a topographic stepdowns occur across csds in accordance with the general consequences of juxtaposing lithospheres of different relative ages 2 b the correct direction of stepdown down toward older crust and lithosphere occurs relative to that predicted from ridge crest offsets and 3 c the observed amount of stepdown is similar in many cases to that predicted from the observed ridge crest offsets these analyses of cross strike discontinuities topographic and image symmetry across aphrodite and predictions of topography based on ridge crest and fracture zone geometric relationships further support the earlier interpretation that the 7500 km extent of western aphrodite terra represents the site of crustal spreading and displays many of the characteristics of terrestrial divergent plate boundaries

the impact of hurricane diana on the north carolina continental shelf abstract intense storms including hurricanes are believed to have an important impact upon the biota and sediment cover on the continental shelf hurricane diana crossed onslow bay north carolina during september 1984 and provided an opportunity to examine its effect on the sea floor an area in the bay transited by the hurricane had been surveyed previously by side scan sonar 3 5 khz echosounding and underwater television we re surveyed the area in november 1984 with the same techniques and collected box cores for sedimentary structure analysis the post hurricane survey yielded no unequivocal evidence of hurricane impact on the sea floor and surprisingly showed an abundance of seemingly delicate benthic organisms thriving on the impacted area sedimentary structures in the box cores included rip up clasts and cross laminations that are indicative of a dynamic sedimentary environment but they could have formed during non hurricane storm conditions the lack of a dramatic signature on the sea floor from hurricane diana is attributed to the low sediment influx and frequent occurrence of intense storms on the north carolina shelf these factors result in an armored depositional environment that shows little response to the passage of each successive storm

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

regional seismic stratigraphy and controls on the quaternary evolution of the cape hatteras region of the atlantic passive margin usa abstract seismic and core data combined with amino acid racemization and strontium isotope age data enable the definition of the quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of north carolina u s a seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets and enable an understanding of the evolution of this margin data reveal the progressive eastward progradation and aggradation of the quaternary shelf the early pleistocene inner shelf occurs at a depth of ca 20 40 m beneath the western part of the modern estuarine system pamlico sound a mid to outer shelf lowstand terrace also early pleistocene with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract occurs at a deeper level ca 45 70 m beneath the modern barrier island system the outer banks and northern pamlico sound seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle pleistocene shelf middle to late pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units and contain numerous filled fluvial valleys that are incised into the early and middle pleistocene deposits the stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea level fluctuations during the middle to late pleistocene expressed as falling stage lowstand transgressive and highstand systems tracts thick stratigraphic sequences occur within the middle pleistocene section suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north furthermore the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system

high resolution gravity model of venus the anomalous gravity field of venus shows high correlation with surface features revealed by radar we extract gravity models from the doppler tracking data from the pioneer venus orbiter pvo by means of a two step process in the first step we solve the nonlinear spacecraft state estimation problem using a kalman filter smoother the kalman filter has been evaluated through simulations this evaluation and some unusual features of the filter are discussed in the second step we perform a geophysical inversion using a linear bayesian estimator to allow an unbiased comparison between gravity and topography we use a simulation technique to smooth and distort the radar topographic data so as to yield maps having the same characteristics as our gravity maps the maps presented cover 2 3 of the surface of venus and display the strong topography gravity correlation previously reported the topography gravity scatter plots show two distinct trends

venus gravity a harmonic analysis an improved model of venusian global gravity has been obtained by fitting an eighteenth degree and eighteenth order spherical harmonic series to 78 orbital arcs of high altitude 950 1350 km at periapsis tracking data and 351 orbital arcs of lower altitude 150 200 km at periapsis data from the pioneer venus orbiter pvo compared to a recently published tenth degree model mottinger et al 1985 which is based on the 78 high altitude arcs only the current model provides a significant improvement in resolution and fidelity as a measure of this improvement we note that for the low altitude arcs alone the variance of the residuals for the present model is reduced to 19 of the data variance compared to 51 for the tenth degree model venus differs significantly from the earth in that it exhibits a significant correlation between long wavelength topography and gravity the gravity topography spectral admittances are inconsistent with either airy or pratt isostasy but they are consistent with dynamic support by mantle convection

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

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

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

correlation of onshore offshore oligocene through lower miocene strata using 87sr 86sr isotopic ratios north flank of cape fear arch north carolina usa abstract cores from onshore and offshore of the onslow block reveal at least three depositional sequences above the eocene castle hayne limestone and below the miocene pungo river formation through use of 87sr 86sr ratios and their resultant dates these sequences are correlated from onshore to offshore thus clarifying the geologic history of the inner continental shelf of north carolina the oldest depositional sequence consists of calcareous quartz sand and sandy pelecypod mold grainstone updip and yellowish gray very fine to fine calcareous quartz sand with minor clay and silt downdip most of the carbonate in the downdip part of the sequence consists of foraminifers ostracods echinoid spines and other small allochems eight hand picked foraminiferal samples including two replicate analyses from this sequence selected from two onshore cores and one offshore core yield 87sr 86sr ratios from 0 707901 to 0 708006 equivalent to lowess locally weighted regression scatterplot smoother dates ranging from 29 2 to 32 0 ma this indicates this sequence is rupelian early oligocene in age and assignable to either the ta4 4 or ta4 5 global coastal onlap cycle the youngest sequence consists of oyster bearing quartz sand and laterally equivalent clay with mollusks onshore and very fine to fine calcareous quartz sand offshore twelve foraminiferal and one ostracod sample including five replicate analyses from three cores on the inner shelf of onslow bay yield 87sr 86sr ratios from 0 708332 to 0 708386 equivalent to lowess dates ranging from 21 1 to 22 2 ma this indicates the sequence is aquitanian early miocene in age and assignable to the tb1 4 global coastal onlap cycle although chattian late oligocene sequences are recognized onshore in outcrop and core and are projected to continue into onslow bay no cores provided sr samples for analysis interpreted to be late oligocene in age

genesis of disseminated sulphide assemblages in niagara peninsula canada in the niagara peninsula of southern ontario canada subeconomic occurrences of mississippi valley type base metal assemblages are found in the petroliferous carbonate units of the middle silurian sedimentary succession these units which were deposited on the northwestern margin of the appalachian basin were buried by about 2 4 km of paleozoic sediments up to the time of the appalachian orogeny

present day precipitation of lead and zinc from groundwaters small quantities of galena and sphalerite are currently precipitating 26m below the surface at the vinemount quarry ontario canada from sulfurous spring waters issuing from carbonates of the lockport formation at the spring orifices these sulfides are present in both fracture coatings and an associated black sludge these precipitates are overlain by a crust rich in native sulfur the mineralogy of the precipitates and the chemistry of waters taken from three consistent spring horizons indicates that downward moving meteoric groundwaters leach metals from the host rocks and precipitate galena and sphalerite on encountering h2s rich waters at the water table while our observation of the transport and deposition of lead and zinc by shallow groundwaters lends support to intra karstic models for the direct precipitation of galena and sphalerite it also enables us to propose that many carbonate hosted pb zn deposits of differing theories of origin may have been enriched during periods of uplift

lead isotope ratios in niagara escarpment rocks and galenas implications for primary and secondary sulphide deposition galenas occumng in vugs and veins in the dolomites of the eramosa goat island and gasport members of the middle silurian lockport formation in the niagara escarpment have a small but significant range in lead isotopic composition these differences are recorded in specimens from exposures in the niagara gorge in the east to the guelph quarry 100 km to the northwest the galena isotopic data fit linear trends on 207pb 204pb 206pb 204pb and 208pb 204pb 206pb 204pb plots within experimental error the most radiogenic ratios are anomalous giving model ages based on the growth curves of zartman and doe up to 200 ma younger than the most likely age of primary deposition of 300 ma sulphides currently being deposited from springs at the floor of the vinemount quarry have a lead isotopic composition similar to that of some of the galena in the overlying dolomites rocks from several of the formations exposed at this site contain lead only slightly more radiogenic than the most radiogenic galenas these results are consistent with the hypothesis of haynes and mostaghel that the modem sulphides result from local solution and redeposition of older sulphides previously published isotopic ratios for lead in the potassium feldspar constituents of 300 ma plutonic intrusives in the appalachians cluster around the least radiogenic galena ratios and suggest that the lead in the niagara peninsula galenas has been remobilized from rock units in the appalachian basin grenville crystalline basement rocks do not appear to have contributed significantly to this lead a single sample of galena from the eramosa member exposed in the ebel quarry in the michigan basin at the northwestern extension of the escarpment has 207pb 204pb and 206pb 204pb ratios that are substantially more radiogenic than those in any of the galenas from the niagara region of the appalachian basin and a 08pblz04pb ratio that lies significantly below the linear distribution defined by the niagara region data set we tentatively interpret this difference as reflecting derivation of the lead in the two basins from different crustal blocks

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

gravity anomalies compensation mechanisms and the geodynamics of western ishtar terra venus pioneer venus line of sight orbital accelerations have been used to calculate the geoid and vertical gravity anomalies for western ishtar terra on various planes of altitude z0 the apparent depth of isostatic compensation at z0 1400 km is 180 20 km based on the usual method of minimum variance in the isostatic anomaly we seek to explain this observation as well as the regional elevation peripheral mountain belts and inferred age of western ishtar terra in terms of one of three broad geodynamic models under local mantle upwelling western ishtar is the surface expression of a hotspot which produces topography both dynamically and by volcanic construction this model holds that the mountain belts form as a result of incipient downward mantle return flow an upwelling with its centroid at 160 km depth and buoyancy stress 100 mpa can satisfy the observed long wavelength geoid and topography provided that the crust mantle density contrast does not control the response of the crust so that late time subsidence does not occur and that the mantle viscosity does not increase markedly with depth mantle upwelling produces a combination of strike slip motion and azimuthal extension on the uplift itself radial compression is restricted to the surrounding plains under local mantle downwelling cold sinking mantle beneath western ishtar induces shear tractions on the crust which thicken and elevate it volcanism is potentially the result of basal crustal remelting the formation of mountain belts is assumed to be a by product of crustal thickening a deeply seated downwelling 300 700 km buoyancy stress 200 300 mpa can satisfy the geoid and central topography provided that the crust can respond separately to mantle flow and that a more viscous lower mantle exists the predicted surface strains during intermediate stages of the uplift are in qualitative agreement with observations azimuthally oriented thrusts at the uplift margin incorporation of the effective viscosities of diabase and olivine suggests that uplift above a plume will be rapid but the thermal gradient over downgoing flow must be at least as high as the expected mean value for venus 20 k km in order to attain substantial uplift within several hundred million years a third scenario regional compression may be distinguished from the second in that western ishtar is simply a locus of strain accumulation and crustal thickening from regional stress fields neither broad downward mantle flow nor lithospheric subduction need occur however both the source of stress and geoid anomaly are problematic if the topography of western ishtar is assumed to be airy compensated then the geoid signature must arise from unrelated mantle density anomalies in this case the lithosphere must be decoupled from mantle flow or else the mantle must be effectively rigid in order to suppress dynamic topography a more complex model than those analyzed in detail here is needed to produce both the correct surface strains and apparent compensation depth we favor upwelling models in which a mantle plume has flattened out and the surface has been subsequently deformed western ishtar terra must be distinguished from other paradigm hotspots on venus by different environmental conditions or by being in a unique stage of hotspot evolution further quantitative investigations of the interaction of mantle shear tractions with a laterally heterogeneous lithosphere are required

venus geology and tectonics hotspot and crustal spreading models and questions for the magellan mission spacecraft and ground based observations of venus have revealed a geologically young and active surface with volcanoes rift zones orogenic belts and evidence for hotspots and crustal spreading yet the processes responsible for these features cannot be identified from the available data the magellan spacecraft will acquire an unprecedented global data set which will provide a comprehensive and well resolved view of the planet this will permit global geological mapping an assessment of the style and relative importance of geological processes and will help in the understanding of links between the surface geology and mantle dynamics of this earth like planet

migration of the pee dee river system inferred from ancestral paleochannels underlying the south carolina grand strand and long bay inner shelf several generations of the ancestral pee dee river system have been mapped beneath the south carolina grand strand coastline and adjacent long bay inner shelf deep boreholes onshore and high resolution seismic reflection data offshore allow for reconstruction of these paleochannels which formed during glacial lowstands when the pee dee river system incised subaerially exposed coastal plain and continental shelf strata paleochannel groups representing different generations of the system decrease in age to the southwest where the modern pee dee river merges with several coastal plain tributaries at winyah bay the southern terminus of long bay positions of the successive generational groups record a regional southwestward migration of the river system that may have initiated during the late pliocene the migration was primarily driven by barrier island deposition resulting from the interaction of fluvial and shoreline processes during eustatic highstands structurally driven subsurface paleotopography associated with the mid carolina platform high has also indirectly assisted in forcing this migration these results provide a better understanding of the evolution of the region and help explain the lack of mobile sediment on the long bay inner shelf migration of the river system caused a profound change in sediment supply during the late pleistocene the abundant fluvial source that once fed sand rich barrier islands was cut off and replaced with a limited source supplied by erosion and reworking of former coastal deposits exposed at the shore and on the inner shelf

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

venus comparison of gravity and topography in the vicinity of beta regio the doppler tracking data obtained from the pioneer venus orbiter when it passed near beta regio yielded a peak vertical anomaly of 150 mgal when analyzed by our two stage procedure a comparison of maps of the gravity and topography at comparable resolution shows a striking correlation a scatter plot shows that the observed gravity anomaly is approximately 0 4 of that expected from uncompensated topography of half the mean density of venus however the spectral admittance shows that the gravity anomalies can not be explained either by airy compensation at fixed depth or by a model comprising an elastic plate atop an inviscid fluid the gravity and topography variations may signify deep compensation or dynamic support for beta regio and more shallow compensation for other features

venus summary and review of science research activities 1983 87 in the past four years the first high resolution synoptic images of the surface of venus have been obtained and we have learned more about the nature of its surface and atmosphere the major accomplishments of the u s s r in venus exploration have been observed with great interest by u s investigators some of whom have begun joint research efforts with soviet scientists venera 15 and 16 radar imaging experiments and new data obtained at the arecibo observatory have provided answers to some important questions conjectured volcanic features have been resolved into unmistakable calderas or distinct lava flow morphologies the question of whether venus like the earth may have high erosion rates is answered and from the observed number of presumed impact craters it appears that resurfacing rates on venus are much smaller than on earth no evidence of water or wind activity has been seen

constraints on the thermal evolution of venus inferred from magellan data one interpretation of the magellan data suggests that the cratering record on venus was erased by a global resurfacing event or events the latest ending about 500 m y ago in this global resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts a thermal evolution model of venus that can explain these observations is one in which a deformable lithosphere capable of being incorporated in mantle circulations provides an almost stress free condition at the surface mantle convection with an almost stress free boundary at the surface cools the interior more efficiently rapid cooling decreases the rayleigh number of mantle convection below a transition value required for oscillatory convection and the vigor of convection diminishes as the mantle changes to a quasi steady circulation after about 500 m y ago

geologic framework of phosphate resources in onslow bay north carolina continental shelf high resolution uniboom sparker and 3 5 khz subbottom seismic profiles in combination with 9 m vibracores from onslow bay delineate a belt of neogene sediments which crop out on the north carolina continental shelf around the east flank of the mid carolina platform high this broad southeast trending basement high has controlled the general distribution of cenzoic sediment sequences within the mid atlantic continental margin the neogene section includes the miocene pungo river formation which thickens downdip to the east and southeast off the mid carolina platform high the pungo river formation consists of three third order and at least 21 fourth order seismic depositional sequences many of which are abruptly truncated by erosional surfaces and associated channels these seismic sequences reflect third and fourth order eustatic sea level cycles within the miocene second order transgressive sea level cycle the fourth order seismic sequences appear to be equivalent to cyclic lithostratigraphic units characterized by interbedded phosphorite sands phosphatic foraminiferal muds calcareous quartz sands calcarenites and dolosilts some phosphorite sands also occur in the holocene sand sheet which forms a thin and discontinuous cover on the pungo river sediments in the frying pan phosphate district preliminary evaluation of phosphate resources in onslow bay has delineated several potential economic prospects the pungo river formation crops out in a northeast southwest trending belt which is 150 km long by 25 to 50 km wide and extends into the subsurface to the east and southeast in two different areas within onslow bay vibracores have penetrated five beds which contain anomalous concentrations of phosphate two beds within the frying pan phosphate district are estimated to contain 3 75 billion metric tons of phosphate concentrate with an average content of 29 2 percent p 2 o 5 the other three beds in the northeast onslow bay phosphate district are extensive units with low to intermediate phosphate content within the total sediment up to 9 8 p 2 o 5 these three lean deposits contain at least 780 million metric tons of phosphate concentrate with average p 2 o 5 contents which range from 29 7 to 31 percent phosphate resources in the frying pan phosphate district have a moderate to high potential for economic development whereas those in the northeast onslow bay phosphate district have a low potential for economic development this newly discovered continental shelf phosphate province could become economically feasible to mine in the near future when rapidly expanding demand for fertilizers and agricultural products combined with ever increasing land use pressures produces an unacceptable escalation in the cost of mining land based reserves

sr and nd isotopic analysis of phosphorite sedimentation through one miocene high frequency depositional cycle on the north carolina continental shelf abstract a well developed seismic litho and biostratigraphic data base with good models for the formation and deposition of phosphate rich sediments exist for the upper cenozoic sediments on the north carolina continental margin this provides a unique opportunity to evaluate the potential of utilizing strontium and neodymium isotopic analyses to help unravel a complex genetic history of phosphate formation and deposition thirty one samples from three vibracores in onslow bay supplied hand picked concentrate subsamples of five phosphate grain types and thirty one concentrate subsamples of phosphate peloids for stratigraphic analysis all subsamples were analyzed for their sr isotope composition while only thirteen were analyzed for nd isotopes the 31 peloid samples represent three different stratigraphic units and include 1 holocene surficial sands with highly variable concentrations and types of reworked phosphate 2 pleistocene moldic carbonates with minor concentrations of black peloidal phosphate and 3 miocene mid burdigalian frying pan sequence unit 1 fps 1 of the pungo river formation with very high concentrations of multiple types of phosphate grains that decrease upsection and change grain types with changing lithofacies the latter unit represents deposition during one high frequency sea level cycle stratigraphic analysis utilized one grain type phosphate peloids for comparative purposes with the depositional model of riggs and mallette 1990 the sr and nd data cluster into significant categories that are coincident with these three major stratigraphic units sr and nd isotope compositions of selected phosphate grains from three phosphate rich depositional sequences are utilized to 1 date the initial miocene transgression of unit fps 1 of the pungo river formation 2 decipher processes of formation and deposition of major phosphate grain types through one miocene sea level cycle including the processes of in situ formation versus reworking 3 develop a sr and nd chronostratigraphy for evaluating changing patterns of continental margin sedimentation through high frequency depositional cycles 4 determine effects of reworking and weathering on the isotope signals between miocene pleistocene and holocene sediments 5 evaluate the suitability of utilizing sr and nd isotopes for detailed chronostratigraphic analysis of phosphorite sedimentation and 6 refine our understanding of the upper cenozoic evolutionary history of north carolina s continental margin

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

a high resolution gravity model for venus gvm 1 a spherical harmonic model of the gravitational field of venus complete to degree and order 50 has been developed using the s band doppler tracking data of the pioneer venus orbiter pvo collected between 1979 and 1982 the short wavelengths of this model could only be resolved near the pvo periapse location 14 n latitude therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero the resulting model has a half wavelength resolution of 400 km near the pvo periapse location but the resolution degrades to greater than 1000 km near the poles this gravity model correlates well with a degree 50 spherical harmonic expansion of the venus topography derived from a combination of magellan and pvo data new tracking data from magellan s gravity mission should provide some improvement to this model although a complete model of the venusian gravity field will depend on tracking of magellan after the circularization of it s orbit using aerobraking

venus gravity west of beta regio abstract doppler tracking data from the pioneer venus orbiter pvo have been used to estimate the anomalous gravity field in the region of venus west of beta regio the analysis invokes a kalman filter smoother to solve the nonlinear spacecraft state estimation problem and a linear bayesian estimator to perform the geophysical inversion the topographic map for this region derived from the pvo radar has been filtered to have the same distortions and degree of smoothing as the gravity map the undulations of the gravity are about 0 2 times as large as expected from the topography on the assumption that the latter is uncompensated a comparison of the gravity and topography by means of the spectral admittance is consistent with airy compensation at a depth of 50 km if the surface material has a density of 2 6 g cm 3 however this is not a unique interpretation

bilateral topographic symmetry patterns across aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the aphrodite terra highlands the broad characteristics of the cross strike discontinuities csds are similar to both strike slip fault zones and terrestrial oceanic fracture zones in an effort to distinguish between these two hypotheses topographic profiles were taken across aphrodite terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on earth in addition to a broad bilateral symmetry at a range of angles across aphrodite terra detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities in addition within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding csds and terminates against them furthermore linear rise crests are offset in a right lateral sense along the csds between domains by 200 1200 km if the discontinuities represent strike slip movement then retrodeformation of the axes of symmetry along the csds to form a straight line should form a broadly symmetrical topographic high instead when the topography is retrodeformed the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the csds the nature and configuration of the csds the characteristics of the topography and topographic patterns including bilateral topographic symmetry generally parallel to csds within domains the existence of linear rise crests defined by the axes of bilateral symmetry within domains and the offsets of these linear rise crests between domains all suggest that western aphrodite terra on venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross strike discontinuities are analogous to oceanic fracture zones rather than strike slip faults

on the chemical variability of phosphatic particles from jordanian phosphorite deposits abstract the phosphate particles pellets intraclasts and skeletal fragments from thirteen friable samples representing the economic phosphorite horizons in jordan were hand picked and then cleaned a total of 39 particle fractions were chemically analysed for their major and certain trace elements the chemistry supported by petrography proved that the rounded particles are not faecal pellets they are either rounded intraclasts or the rounded internal molds of bone cavities the bone fragments have higher contents of co2 na20 and s03 and lower p205 and f compared with the pelletal intraclast particles these two groups of particles are readily separated chemically since the environment of deposition is the same these differences are postulated to be due to the initial chemical composition of the bone material as carbonate hydroxyapatite dahlite and early diagenetic pathway conversion to carbonate fluorapatite francolite versus the direct chemical precipitation as francolite of the pelletal intraclast particles from the sediment pore water

deep burial diagenesis its implications for vertical movements of the crust uplift of the lithosphere and isostatic unroofing a review abstract various techniques of study of strata exposed at the surface in undeformed areas of the appalachian basin and ozark done imply that these strata have been heated to temperatures that suggest a former great depth of burial the data reveal that the strata have been much more deeply buried than previously thought if such former deep burial has taken place then subsequent uplift and erosion must also have taken place to bring these formerly deeply buried strata to the present land surface unexpectedly large amounts of uplift and erosion ranging from 4 3 to 7 km have re exposed these formerly deeply buried rocks this leads to the surprising conclusion that isostatic unroofing has stripped off thick sections of strata whose presence was previously unsuspected therefore the lithosphere in its isostatic unroofing of thick sequences of sedimentary strata has undergone much larger vertical motions than many geologists had previously estimated case histories in this review include strata of the silurian of the northern appalachian basin and of the ordovician of the ozark dome which reached interpreted maximum burial depths of 5 and 4 3 km respectively devonian strata in the catskill mountains of new york imply a former depth of burial of 6 5 km lower ordovician carbonate sequences of the northern appalachian basin imply a depth of burial in excess of 7 km middle ordovician strata from the same basin signify a depth of burial of approximately 5 km and devonian strata a paleodepth of 4 5 5 km such former great depths of burial of undeformed strata which are now exposed at the surface reflect large scale vertical movements of the crust and uplift of the lithosphere these drastic changes represent isostatic unroofing with widespread implications for paleogeography of a kind unrecognized at present

venus gravity a high resolution map the doppler data from the radio tracking of the pioneer venus orbiter pv0 have been used in a two stage analysis to develop a high resolution map of the gravitational potential of venus represented by a central mass and a surface mass density the two stage procedure invokes a kalman filter smoother to determine the orbit of the spacecraft and a stabilized linear inverter to estimate the surface mass density the resultant gravity map is highly correlated with the topographic map derived from the pv0 radar altimeter data however the magnitudes of the gravity variations are smaller than would be expected if the topography were uncompensated indicating that at least partial compensation has taken place

on the tectonics of venus abstract the thermal evolution and mechanical properties of a mechanical boundary layer of mantle convection are calculated for three venus models cold earth like and hot with temperatures of 1300 c 1400 c and 1500 c respectively at the base of their thermal boundary layers the mechanical boundary layers consist of a basaltic crust with thicknesses of 3 km 9 km and 18 km and depleted periodotitic mantle with thicknesses of 37 km 65 km and 90 km respectively the thin crust of the cold venus model couples tightly to the underlying mantle and produces a single competent layer whereas the thicker crust of the other models has a weak lower part that decouples the crust from the mantle the characteristic wavelengths 10 20 km of the banded terrains of tesserae surrounding ishtar terra can be explained by the buckling of the crusts of all three venus models as long as their mechanical boundary layers are older than approximately 150 m a implying that the observed wavelengths provide no constraint on the thickness and age of the venusian crust that is older than approximately 150 m a shortening of the basaltic crust however cannot produce surface elevations higher than about 2 km on venus because basalt in the lower crust transforms to high density eclogite which sinks into the mantle therefore lakshmi planum and the surrounding mountains probably contain lower density material and are analogous to continental masses on the earth the ridge spacings of the northern ridge belt can be interpreted as being caused by faulting of the depleted mantle of the cold and earth like venus models if the mechanical boundary layer is older than about 100 m a and 200 m a respectively the hot model however cannot account for the formation of the ridge belt besides the characteristic wavelengths of the banded terrains and spacings of the ridge belts the cold venus model seems to account for many other features on venus the dynamic support of the surface topography of tesserae requires a convergence velocity of less than 0 1 cm year 1 for the mechanical boundary layer of the cold venus model this very low velocity is supported by the spatially random distribution of craters on venus furthermore the lack of pervasive volcanism on venus in approximately the last 500 m y the lack of an internal magnetic field of venus and the lack of an oceanic type ridge system on venus support the cold venus model

venus ishtar gravity anomaly ishtar has been detected as a positive gravity anomaly verification of this result has been made with two independent data sets isostatic compensation of the ishtar topography using an airy model is achieved at approximately 150km

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

evolution of the cape fear terrace a complex interaction between the gulf stream and a paleo shelf edge delta abstract a high resolution seismic stratigraphic study of the cape fear terrace outer continental shelf off cape fear north carolina combined with bio lithostratigraphic data have yielded a complex chronostratigraphic framework of the quaternary sequences that comprise this portion of the north american continental margin the cape fear terrace is one of six terraces protuberances that form significant topographic irregularities along the south carolina and north carolina shelf slope break the cape fear terrace is situated upon an anomalous point source prograding shelf margin feature which has protruded out onto the upper slope throughout much of the quaternary this upbuilding outbuilding followed a period of active submarine erosion in which the ancestral gulf stream and or associated meanders cut an erosional path beneath the present shelf margin during the early pliocene the protuberance was originally constructed during a relative lowstand of sea level as a shelf edge delta severe modification of this deltaic lobe occurred during a relative highstand of sea level as gulf stream activities began to more effectively modify the margin the anomalous thick accumulation of shelf edge sediments acted as a barrier to flow inducing complex flow patterns of the gulf stream causing preferential erosion on its downstream side subsequent deposition may have resulted during more elevated highstands of sea level as evidenced by the presence of active sand waves in the terrace protuberance region today once this shelf edge bathymetric irregularity had been established the gulf stream acted as a dynamic force inducing cellular flow structures within the shelf environment and enabling sediments to be transported seaward along the paleo shoals complex

terrestrial and planetary gravity fields the determination of the gravity field together with knowledge of the surface topography provides one of the primary means of inferring the density structure and dynamics of planetary interiors phillips and lambeck 1980 hager 1985 after removing the topographic gravity signal the distribution of internal density anomalies caused by thermal or compositional differences can be studied gravity field models can also be used to study the compensation of surface topography which can provide information on the mechanical properties and state of stress of the lithosphere the determination of these models has been accomplished using a wide variety of different measurement types and solution techniques the near earth measurement types can be divided into satellite tracking measurements surface gravity measurements and satellite altimeter measurements in both near earth and interplanetary contexts tracking data are used to measure gravitational perturbations affecting satellites with the accuracy and spatial temporal distribution of the data being the most important factors in the resulting field accuracy and resolution surface gravity data provide a more direct measurement of the gravity field but acquiring data uniformly over the earth has always been difficult satellite altimetry provides precise measurements of the marine gravity field provided that satellite orbit errors and non geoidal sea surface height variations can be adequately modeled comprehensive gravity field solutions must incorporate these disparate data types in order to estimate mathematical parameters describing the gravity field such as spherical harmonic coefficients the proper combination of these data along with the desire to acquire an accurate representation of the model errors require that complex procedures be developed for computing these solutions

tests of crustal divergence models for aphrodite terra venus aphrodite terra the largest highland region of venus is a likely site of mantle upwelling active volcanism and extensional tectonics we examine three alternative kinematic models for the interaction of mantle convection and the surface one in which little horizontal surface displacement results from mantle flow vertical tectonics one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation plate divergence and one in which strain from large horizontal motions is broadly accommodated distributed deformation in support of a divergent plate model for aphrodite head and crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones small scale bilateral symmetry of topographic elements hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus and subsidence of topography proportional to the square root of distance consistent with that expected for a divergent thermal boundary layer we undertake quantitative tests of each of these assertions and we compare the results with those from similar tests of the mid atlantic ridge we find that apart from the long wavelength symmetry of broadly elevated regions there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet the fit of the topography of aphrodite to that of a thermal boundary layer is in general much poorer than for the earth and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography the broad saddle shaped region between thetis and atla regiones shows the best fit to the root distance relation yielding apparent spreading half rates of a few centimeters per year but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two plate model regardless of past pole motions a multiple plate geometry is therefore called for and one or more of the lineaments must act as a plate boundary along nearly its full length if the plate divergence hypothesis is correct such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming magellan radar images in the absence of such evidence as long as the existence and distribution of the lineaments are verified then broad disruption of a thin lithosphere is favored in such a model lineaments may be surface manifestations of mantle convective flow a model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by magellan

global radar units on venus derived from statistical analysis of pioneer venus orbiter radar data unsupervised linear discriminant cluster analysis of pioneer venus reflectivity and rms slope data has produced a classification of 15 radar units examination of four hypsometric provinces previously established for venus has been used to show that although the highland 6053 to 6055 5 km and mountainous greater than 6055 5 km provinces are distinctly different in their radar unit compositions from each other and from the lowlands less than 6051 km and rolling plains 6051 to 6053 km provinces the lowlands and rolling plains provinces are very nearly the same in their radar properties this observation indicates that hypsometry alone is not a reliable criterion for quantitative examination of the surface geology on venus a classification unit encompassment scheme has been developed that shows the presence of three distinct trends two toward elevations higher than the planetary mean elevation and the other toward elevations lower than the mean venera lander data and earth based radar images have been used to infer that seven units within the two higher elevation unit encompassment trends represent materials of mostly basaltic composition that differ only in their surface morphologies the physical characteristics of these materials indicate that they form terrains ranging from banded through rift valley to relatively flat and that the materials are composed of bedrock and soil in variable ratios three other radar classification units appear to be morphologically transitional forms between units of these two higher elevation unit encompassment trends the five remaining units are associated with the lower elevation unit encompassment trend their geologic significance cannot be determined with any certainty with the existing pioneer venus radar data the radar units that correspond to those of an existing radar unit map produced by supervised analysis display similar areal distributions on venus however our radar unit map shows more detail and is relatively unbiased in terms of unit definition this global radar classification image of the venusian surface provides a working data base from which more exacting geologic interpretations can be made when higherresolution imagery becomes available

gravity field of venus at constant altitude and comparison with earth the acceleration gravity anomaly estimates by spacecraft tracking determined from doppler residuals are components of the gravity field directed along the spacecraft earth line of sight los these data constitute a set of vector components of a planet s gravity field the specific component depending upon where the earth happened to be at the time of each measurement and they are at varying altitudes above the planet surface from this data set we have solved for the gravity field using the method of harmonic splines which imposes a smoothness criterion to select a gravity model compatible with the los data maps of geoid and gravity at spacecraft and at constant altitudes are presented for the planet venus for venus the major geoid and gravity anomalies each coincide spatially with the locations of positive topographic features these anomalies at 300 km altitude have significantly greater magnitude than anomalies that on earth are coincident with topographic features another striking difference is that most of the major geoid anomalies on the earth do not coincide with locations of its largest topographic features including continents but are due to deep sources on venus major mass anomalies appear to be restricted principally to the surficial 200 km of the planet a substantially thicker crust on venus 70 80 km compared to the earth s 5 40 km can explain its greater range of gravity anomaly and the positive correlation of gravity anomaly withtopographic height four regions with high gravity to topography response functions are interpreted to be due to crustal loading by recent volcanism perhaps combined with regional elevation of surface topography by thermal expansion from heating at depth

stress analysis of tellus regio venus based on gravity and topography comparison with venera 15 16 radar images the tectonics of the tellus regio highland of venus have been examined using pioneer venus altimetry and gravity data and venera 15 16 radar images a thin elastic shell model has incorporated the pioneer venus data to calculate both global long wavelength and regional shorter wavelength stresses for various assumed values of crust lithosphere and mantle thickness and modes of compensation the effects of these parameters on the calculated stress magnitudes have been examined the model also allows calculation of crustal thickness and mantle density anomalies the resulting calculated stress fields have been compared to the surface morphology observed in the venera 15 16 radar images and interpreted in terms of the stress history the best fitting parameters are consistent with minor amounts of lithospheric flexure being necessary to produce the observed surface features the highland region is underlain by a thick 15 20 km crust which is responsible for most of the topographic support a positive density anomaly exists below this crust the denser mantle may represent the down welling portion of a flow field in which the horizontal flow feeding the downwelling has entrained mobile crustal material dynamically maintaining a thickened crustal root near the stagnation point of the flow below tellus regio this hot crustal material may have intruded the overlying cooler crust causing lithospheric inflation and surface volcanism this scenario is in agreement with the available gravity and radar data and can explain the general surface morphology of tellus regio

gravity topography and crustal evolution of venus abstract the gravity anomalies of venus although small by comparison with those on mars and the moon are still much larger than those on earth for large features on venus even the low degree spherical harmonic terms for venus gravity field indicate a close association of broad positive gravity anomalies with major topographic highs this is striking contrast to the situation on earth where the broad regional gravity anomalies show little correlation with continental masses or plate tectonic features but instead appear to be caused by deep mass anomalies a method for estimating radial gravity anomalies from line of sight acceleration data their interpolation and use of iteration for improved radial anomaly estimates is outlined a preliminary gravity anomaly map of venus at spacecraft altitude prepared using first estimate values is presented a profile across the western part of aphrodite along longitude 85 e was analyzed using time series techniques an elastic plate model would require a plate thickness of about 180 to 200 km to match the general amplitude of the observed gravity anomaly about 33 mgal a thickness much greater than that found for earth structures and because of high surface temperatures unlikely for venus an airy isostatic model convolved with the topography across aphrodite however provides a better match between the predicted and observed gravity anomalies if the nominal crustal thickness is about 70 to 80 km this thickness is over twice that for continental crust on the earth and considerably greater than that of the earth s basaltic ocean crust only 5 km a different differentiation history for venus than that of the earth thus is anticipated high gravity anomalies 110 mgal occur over beta regio and over the topographic high in eastern aphrodite both highs are associated with regions where detected lightning is clustered and thus the topographic features may be active volcanic constructs the large gravity anomalies at these two sites of volcanic activity require an explanation different than that indicated for western aphrodite

venus gravity analysis of beta regio radio tracking data acquired over beta regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived in addition a corresponding vertical gravity field was evaluated solely from the topography of the beta region a comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line of sight gravity maps it also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone the effective depth of complete isostatic compensation for the beta region is estimated to be 330 km which is somewhat deeper than that found for other areas of venus

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

hydrogeological characterization of southeast coastal plain aquifers and groundwater discharge to onslow bay north carolina usa summary the natural geochemical tracer 222 rn was used to quantify submarine groundwater discharge sgd onto the continental shelf of onslow bay north carolina water column samples were collected aboard the r v cape hatteras on july 21 26 2002 and an additional nearshore water column transect and groundwater samples were collected in 2005 2006 assessment of sgd was accomplished using a mass balance approach that quantified sources and sinks of radon including benthic flux exchange across the pycnocline or air sea interfaces horizontal transport into and out of the study area and a water column inventory four independent geochemical box models were developed to quantify sgd regionally and with distance from shore overall water column inventories and diffusion rates decreased with distance from shore average water column inventories were 8520 1310 7230 1190 and 760 510 dpm m 2 for three shore parallel boxes from nearshore to offshore and resulted in a regional average of 5800 1050 dpm m 2 for the regional box model diffusion rates of radon through the sediment water interface were 0 9 0 2 0 6 0 1 and 0 4 0 1 dpm m 2 min 1 for the near to offshore models and averaged 0 6 0 1 dpm m 2 min 1 for the regional box model sgd estimates were calculated using two end member 222 rn activities for the advecting fluids which allowed a distinction between terrestrially driven sgd and total sgd terrestrially driven and total sgd estimates averaged 0 2 and 0 7 cm d 1 respectively the calculated terrestrially driven sgd is as important in the delivery of fresh water as riverine sources to onslow bay and a significant contributor to the south atlantic bight