origin of overpressure and pore pressure prediction in the baram province brunei accurate pore pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited tertiary baram delta province where all economic fields exhibit overpressures commonly of high magnitude and with narrow transition zones a pore pressure database was compiled using wireline formation interval tests drillstem tests and mud weights from 157 wells in 61 fields throughout brunei overpressures are observed in 54 fields both in the inner shelf deltaic sequences and in the underlying prodelta shales porosity vs vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner shelf deltaic sequences however the geology of brunei precludes overpressures in the inner shelf deltaics being generated by any conventional fluid expansion mechanism e g kerogen to gas maturation and we propose that these overpressures have been vertically transferred into reservoir units via faults from the prodelta shales sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer and hence different pore pressure prediction strategies need to be applied in the prodelta shales and inner shelf deltaic sequences sonic and density log data detect overpressures generated by disequilibrium compaction and pore pressures are accurately predicted using an eaton exponent of 3 0 sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly and pore pressures are reasonably predicted using an eaton exponent of 6 5

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

evidence for divergent plate boundary characteristics and crustal spreading on venus detailed examination of the topography and morphology of western aphrodite terra reveals numerous features that are similar to terrestrial divergent plate boundary characteristics individual domains between fracturezone like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests rifted and separated and moved laterally to their present symmetrical positions the topographic and morphologic similarities together with strikingly similar mirror image map patterns on both sides of the rise axis suggest that western aphrodite terra shares the characteristics of oceanic divergent plate boundaries and is the site of crustal spreading on venus topographic profiles are consistent with spreading rates of the order of several centimeters per year

venus southern hemisphere geologic character and age of terrains in the themis alpha lada region arecibo high resolution radar images of the southern hemisphere of venus extending to 78 s show that the surface of the themis alpha lada region is characterized by linear deformation zones with volcanoes and corona like features and by regional volcanic deposits primarily plains small shields and large edifices large scale areal deformation is limited to the tessera of alpha regio lada terra in the southern high latitudes contains several large coronae in contrast to ishtar terra in the northern high latitudes the density of craters of possible impact origin is somewhat lower than that observed in the venera 15 and 16 coverage these data extend to 43 percent of the areas of the surface of venus with ages of less than about 1 billion years

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

coronae on venus morphology classification and distribution abstract two major types of circular features with no direct morphologic analog on the terrestrial planets were identified in venera 15 16 radar images of venus coronae and corona like features coronae are irregular to circular structures characterized by a complex interior zone and maximum widths of 170 690 km corona like features resemble coronae but lack an annulus of ridges twenty one structures that fit the definition of coronae have been identified in venera 15 16 coverage along with eleven corona like features each group has been subdivided into three classes based on general morphology symmetrical coronae are characterized by circular to oval form with interiors characterized by flows domes and tectonic ridges asymmetrical coronae have similar morphology but have only a partial annulus or are asymmetrical about a central axis subdued coronae are dominated by smooth plains with flooded and embayed annuli double corona like structures are composed of two linked rings and are found along ridge belts in the region east of atalanta planitia irregular corona like features are surrounded by a rim rather than an annulus as are the double structures and are very subdued in appearance heart shaped corona like features are rounded at one end and form a wedge at the other end we interpret the classes of coronae and corona like features to have a similar mode of origin with differences in morphology due to the stage of evolution and or varying amounts and significance of geologic processes i e tectonism volcanism gravitational relaxation that occur during their evolution corona evolution is characterized by early uplift and volcanism followed by annulus formation formation of a peripheral trough may occur in the middle to late stages of corona evolution followed by extensive volcanism and lowering of topographic relief both coronae and corona like features are interpreted to have formed as a result of uplift and volcanism caused by a thermal anomaly at depth followed by gravitational relaxation

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

inverse modelling and seismic data constraints on overpressure generation by disequilibrium compaction and aquathermal pressuring application to the eastern black sea basin summary pore pressure above the hydrostatic overpressure is common in deep basins it plays an important role in pore fluid migration represent a significant drilling hazard and is one of the factors controlling slope stability and deformation in seismically active areas here we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring we minimize a function that contains the misfits between estimates from our forward model and observed values using a non linear least squares approach the inverse model allows the introduction of observed seismic and geological constraints such as p wave velocity vp and density data and depth of the layer boundaries for a better pore pressure prediction the model output also provides estimates of 1 surface porosity 2 compaction factor 3 intrinsic permeability at surface conditions 4 a parameter controlling the evolution of the intrinsic permeability with porosity 5 the ratio between horizontal and vertical permeability and 6 uncompacted thickness so sedimentation rate assuming known time intervals for each sedimentary layer we apply our inverse approach to the centre of the eastern black sea basin ebsb where the vp structure has been inferred from wide angle seismic data first we present results from a 1 d inverse model and an uncertainty analysis based on the monte carlo error propagation technique to represent the observed rapid change from low vp to normal vp below the maikop formation we impose a zero overpressure bottom boundary and subdivide the layer below the maikop formation into two sublayers an upper layer where the rapid change is located and a lower layer where the vp is normal secondly we present the results from a 2 d inverse model for the same layers using two alternative bottom boundary conditions zero overpressure and zero flow we are able to simulatetheobserved vp suggestingthatthelowvelocityzone lvz at 3500 6500mdepth below the seabed mbsf can be explained by overpressure generated due to disequilibrium compaction 90 per cent and to aquathermal pressuring 10 per cent our results suggest thattheuppersublayer belowthemaikopformation behavesasasealduetoitslowpermeability 0 3 2 10 14 ms 1 this seal layer does not allow the fluids to escape downwards and hence overpressure develops in the maikop formation and not in the layers below this overpressure was mainly generated by the relatively high sedimentation rate of 0 29 m ka 1 of the maikop formation at 33 9 20 5ma and an even higher sedimentation rate of 0 93 m ka 1 at 13 11 ma we estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions o f 0 62 at 5200 mbsf associated with an overpressure

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

the thermal regime of venus models for the thermal evolution of venus are constructed numerically the mantle is proposed to consist of two independently convecting layers upper and lower mantles the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200 c and the core may solidify at temperatures below the liquidus previously these models were calculated with the usual parameterization of convection based mostly on investigations of constant viscosity convection now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology the thermal evolution of venus is assumed to have begun 4 6 billion years b y ago it can be divided into three periods 1 adjustment of the upper mantle to the thermal regime of the lower mantle 0 5 b y 2 transition of the entire mantle to the asymptotic regime 3 4 b y 3 asymptotic regime parameters of the convecting planet in the asymptotic regime do not depend on initial conditions the planet forgets its initial state and are found analytically the present day heat flux is equal to 50 erg cm 2s 1 the results are close to those obtained previously comparison of the present thermal models of venus and earth with the recent studies of melting of the fe fes system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the venusian core at present the position of the iron triple point e liquid makes this conclusion very probable the thermal regime of the venusian crust is also discussed convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition the prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust

a mechanism for tectonic deformation on venus in the absence of identifiable physiographic features directly associated with plate tectonics alternate mechanisms are sought for the intense tectonic deformation observed in radar images of venus one possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior spectral green s function solutions have been obtained for stresses in an elastic lithosphere overlying a newtonian interior with an exponential depth dependence of viscosity and a specified surface density distribution driving the flow at long wavelengths and for a rigid elastic fluid boundary condition horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid the depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 mpa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses the great apparent depth of compensation of topographic features argues against this however thus supporting the importance of the coupling mechanism if there is no elastic lid stresses will also be very high near the surface providing also that the viscosity gradient is negative

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

predicting fluid pressure in sedimentary basins from seismic tomography 10 pages 9 figures 2 tables geophysical journal international 2019 predicting fluid pressure in sedimentary basins from seismic tomography published by oxford university press on behalf of the royal astronomical society all rights reserved

mantle flow tectonics and the origin of ishtar terra venus it is proposed that western ishtar terra formed due to compression and crustal thickening above a cylindrical mantle downwelling a model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western ishtar although axisymmetric downwellings occur in numerical models of constant viscosity mantle convection there is no evidence for their existence in earth s mantle where downwellings are sheet like either modes of downflow in venus and earth are fundamentally different or differences in near surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes

tectonics of lakshmi planum venus tests for magellan the origin of lakshmi planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of venus from the perspective of gravity signature and potential driving forces the mantle upwelling model is the simplest and its principal failure that it cannot produce radial shortening on the uplift periphery may be resolved if the lithosphere is laterally heterogeneous our preferred model consists of a hot mantle plume rising beneath a pre existing block of tessera the lithosphere is weakened at this hotter and presumably thicker crust and the outward near surface flow is attenuated at the peripheral discontinuity in lithospheric strength crustal thickening and mountain belt formation occur there we propose several criteria to test this tessera plume model together with its competitors at the higher resolution in both imaging and gravity afforded by the magellan mission

consequences of a hydrous mantle for venus tectonics predictions for magellan we will consider the consequences of a venus mantle enriched in water relative to the earth if the moon forming impact hypothesis is correct earth may have lost substantial volatiles venus however experienced no such impact and is therefore likely to be relatively more water rich we demonstrate that a higher abundance of water in the venus mantle has the effect of inhibiting the rise of crustforming melts and volatiles to the surface which results in a thin stable crust a dry stiff upper mantle and restricted magmatism conduction through a shallow thermal lithosphere dominates the removal of heat from the interior and flow in the mantle is strongly coupled to the surface deformation current observations of venus are consistent with this model a review of the consequences of this model allows us to make predictions for the magellan radar images these include 1 no evidence for plate tectonic processes 2 younger apparent surface ages and enhanced crater relaxation for highland areas 3 limited quantity of hot spot volcanism and 4 occasional evidence of localized magmatism diatremes associated with a volatile rich mande source the model has implications to the earth as well in the absence of the moon forming impact crustal formation in the water rich earth mantle may have been arrested and plate recycling would have ceased early in the planet s history

the petroleum geology of ireland s offshore basins introduction during the past few years a number of factors have combined to create renewed petroleum exploration interest in the late palaeozoic cenozoic sedimentary basins that virtually surround ireland these factors include recent discoveries in contiguous uk offshore basins the perceived attractiveness of the irish licensing terms the apparent stabilization of the international oil price and recent advances in drilling and deep water production technologies combined with the realization by many oil companies that the irish basins are only lightly explored

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

in situ u pb geochronology lu hf and sm nd isotope systematics of the hoidas lake ree deposit northern saskatchewan canada abstract the diopside allanite veins and apatite breccia veins of the hoidas lake lree deposit cross cut the deformed archaean and paleoproterozoic granitoid gneisses of the rae subprovince in northern saskatchewan canada the timing and source of this unusual and complex ree deposit are not well constrained here we report on the results of la icp ms u pb geochronology of zircon and titanite from diopside allanite veins and green apatite and monazite from apatite breccia veins hf isotopic data for zircon and sm nd isotopic data for titanite apatite and monazite in an attempt to constrain the age and duration of crystallization of ree bearing phases and provide implications on the source of the mineralization geological relationships and in situ u pb data indicate that the ree veins formed after peak metamorphism at ca 1 9 ga however there are zircon crystals in the diopside allanite veins with concordant u pb dates as old as 2350 ma the oldest of which may have been inherited from arrowsmith orogen granitoids zircon rims show a concordant cluster of u pb dates around 1911 ma which are interpreted to represent new zircon growth based on textural features and their distinct initial hf values 26 9 and 34 3 compared to the inherited zircon cores which range from 9 5 to 2 0 younger u pb concordia ages of titanite ca 1860 ma and 1830 ma and monazite ca 1843 ma and discordant apatite 207pb 206pb dates ca 1840 ma are considered to be the best constraints on the initiation of ree mineralization the initial nd values of titanite green apatite and monazite 15 2 to 10 7 are comparable to but lower than the nd values for the martin group alkali basalts beaverlodge domain and the ultrapotassic rocks of the christopher island formation baker lake basin both of which also yielded similar u pb dates to those of the youngest hoidas lake ree bearing minerals these regionally occurring alkali units originated from a similar source most probably an ancient enriched lithospheric mantle reservoir the melts and fluids from which the hoidas lake deposit formed then interacted with rae craton crust as they utilized structures while they were transported from the source region to the depositional site

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

from continental hyperextension to seafloor spreading new insights on the porcupine basin from wide angle seismic data key points new analysis of wide angle seismic data from the southern porcupine basin evidence for presence of oceanic crust in the southern porcupine basin jurassic rifting propagated from south to north resulting in non uniform strain when rifting stopped the deep structure and sedimentary record of rift basins provide an important insight into understanding the geological processes involved in lithospheric extension we investigate the crustal structure and large scale sedimentary architecture of the southern porcupine basin offshore ireland along three wide angle seismic profiles supplemented by thirteen selected seismic reflection profiles the seismic velocity and crustal geometry models obtained by joint refraction and reflection travel time inversion clearly image the deep structure of the basin our results suggest the presence of three distinct crustal domains along the rifting axis a continental crust becoming progressively hyperextended from north to south through the basin b a transitional zone of uncertain nature and c a 7 8 km thick zone of oceanic crust the latter is overlain by a 8 km compacted upper paleozoic mesozoic succession and 2 km of cenozoic strata due to the lack of clear magnetic anomalies and in the absence of well control the precise age of interpreted oceanic crust is unknown however we can determine an age range of late jurassic to late cretaceous from the regional context we propose a northward propagating rifting process in the porcupine basin resulting in variations in strain along the rift axis

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

lithology dependence of porosity in slope and deep marine sediments abstract compilation of porosity data from 11 ocean drilling program legs and 53 sites show that compaction in deep marine to upper slope sediments is at least in part a function of both burial depth and lithology overall porosities range from 4 to 94 despite the broad range of porosities there is an exponential trend toward decreasing porosity with depth comparison among samples dominated 90 to 100 by each of six lithologies based on grain size and composition reveals the lithology dependence of compaction except for biogenic silt size sediments and clastic sands in the former case this may in part be due to mixing of calcareous and siliceous nannofossils clay size clastic sediments show a clear change from rapid porosity reduction in the top 172 meters porosity 82 7 e depth 430 to less pronounced loss of porosity below porosity 61 4 e depth 1671 the remainder of the lithologies silt porosity 75 5 e depth 1091 biogenic sand porosity 88 5 e depth 1338 and micrite porosity 69 5 e depth 1235 show a good to strong exponential reduction of porosity with depth these relations compare well with literature studies taking into account the less pure nature of the lithologies used in this work the silt compaction curve is unique no previous studies of porosity separated silt from other grain sizes most sediment samples are of mixed grain sizes and types the lithology based porosity depth relations were used to test four approaches to decompaction in mixed lithology sediment used in backstripping all methods generated porosity estimates that correlated strongly to the observed values with correlation coefficients ranging from 0 629 to 0 686 these methods that have been used for years are for the first time shown to be valid

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

exploration in the shetland faeroe basin using densely spaced arrays of ocean bottom seismometers recent exploration activity in the peripheral regions of the shetland faeroe basin offshore northwest scotland has led to the discovery of some of the largest oil reserves on the united kingdom uk continental shelf we present results from two ocean bottom seismometer profiles acquired by mobil north sea ltd across the center of the shetland faeroe basin these data provide a powerful tool for delineating long wavelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration methods analysis of the first arrival traveltime data using both forward and inverse ray based techniques produces a well constrained velocity depth model of the basin fill we estimate that the uncertainty in the velocity structure is 5 from a series of trial and error perturbations applied to the final models the velocity structure of the faeroe basin has three principal layers 1 a near surface layer with velocities in the range 1 6 to 2 2 km s 2 a 3

growth of planetary crusts the planets and satellites of the solar system show much diversity but most have formed crusts which differ substantially from their bulk compositions three principal types of crust may be broadly distinguished primary formed after accretional heating e g lunar highlands secondary formed following partial melting in planetary mantles e g lunar maria terrestrial oceanic crust and tertiary formed by processing of secondary crusts of which the continental crust of the earth is the only identifiable example crustal growth before 3800 m y was complicated by the heavy planetesimal bombardment active erosion on the earth is likely to have destroyed the brecciated rubble resulting from over 200 mare orientale scale events in this period the mercurian crust is probably primary perhaps analogous to the lunar highlands in contrast the observable martian and venusian crusts are secondary dominated by basaltic volcanism the icy crusts of the satellites of the outer planets are mixtures of both primary and secondary crusts with complex histories ganymede is of special interest in displaying a secondary grooved terrain water ice crust which has split an older more heavily cratered possibly primary crust slight expansion has resulted probably from polymorphic transitions from high density ice viii to low density ice i the primary lunar crust 12 of lunar volume grew in 108 years secondary crusts derived by partial melting of mantles grow at a much slower rate total production of terrestrial oceanic crust over 4000 m y is only 2 of planetary volume whereas terrestrial continental crustal growth is even slower producing over the same period a volume only about 0 33 that of the earth crustal growth on the other planets appears to be an irreversible process without evidence of recycling no evidence for massive planetary expansion as required by expanding earth hypotheses is apparent on the other planets or satellites of the solar system

terrestrial spreading centers under venus conditions evaluation of a crustal spreading model for western aphrodite terra abstract assessment of terrestrial spreading centers under venus environmental conditions indicates that the major difference between earth and venus would be the influence of the enhanced surface temperature on upper mantle temperatures on venus and the resulting increase in crustal thickness and elevation of isostatically compensated topography crust on venus produced at average spreading centers would be about 15 km thick in contrast to the average crustal thickness on earth of about 5 km we have used this model and the pioneer venus altimetry and gravity data to test the hypothesis of head and crumpler 1987 6 that western aphrodite terra represents the location of crustal spreading on venus the topographic data are processed such that all points within equally spaced 100 km wide bins parallel to the proposed axis are stacked together and the average value of topography in each bin is calculated providing average profiles out to a distance of 2000 km on either side of the axis the topography is shown to be very symmetric about the axis with data points less than 1000 km from the axis providing a fit to thermal boundary layer topography comparable to that seen in spreading centers on earth at 1000 km on either side of the axis steep topographic slopes exist this change in topography could be explained by variations of about 15 km in crustal thickness with the crust being thicker in the central domain by a ridge jump or by mechanisms unrelated to crustal spreading for processing line of sight los acceleration data we first show that static and dynamic calculations give the same results for track numbers 427 449 this allows us to determine a synthetic orbit track parallel to the cross strike dis continuities csd s and to compare it with observed los accelerations gravity data for this domain were then processed in a manner similar to that for topography models give a reasonable fit to the data for half spreading rates between 0 3 and 0 5 cm yr we conclude that a spreading center model for ovda regio could account for the observed topography and los gravity anomalies if it had the following characteristics a central domain with a 30 km thick crust that has been spreading at about 0 5 cm yr for the last 200 myr over a mantle whose temperature is about 1600 c at about 200 myr ago a change in upper mantle temperature of about 100 c is required prior to this time the decreased upper mantle temperature would have produced a thinner crust averaging about 15 km at the spreading center this thinner crust would now lie on the flanks of ovda regio at distances greater than 1000 km from the spreading center and would be in excess of 200 myr old on the basis of our analysis we conclude that the hypothesis that ovda regio in western aphrodite terra is the site of crustal spreading on venus is consistent with the topography and gravity data