temperatures and depth dependent heat flow in western north sea bottom hole temperatures and lithologic information from 167 released wells between 52 and 58 n in the british sector of the north sea have been studied and measurements of thermal conductivity have been conducted on many samples from all the main stratigraphic units although large errors may be associated with the temperature information it is sufficiently reliable to establish that heat flow is not conserved with depth over much of the western north sea in the central graben and anglo dutch basin heat flow in the upper 1 2 km 3 300 6 600 ft is around 50 mw m 2 increasing downward to about 100 mw m 2 farther west on the mid north sea high and on the east midlands shelf the situation is reversed with shallow heat flows of about 80 mw m 2 decreasing downward to about 50 mw m 2 these variations are not readily explained either by heat flow refraction or by tectonic activity they are most probably attributable to the deep and continuing circulation of water the circulation may well be on a very large scale and must be largely fault controlled

asthenospheric and lithospheric sources for mesozoic dolerites from liberia africa trace element and isotopic evidence abstract combined elemental and sr and nd isotopic data are presented for mesozoic dolerite dikes of liberia africa which are related to the initial stage of opening of the atlantic ocean the large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source on the contrary the observed chemical and isotopic variation within some dolerites groups i and ii may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle for the other dolerites groups iii v mixing with a third mantle source with more radiogenic sr and with element ratios characteristic of subduction environments is suggested this third source is probably the subcontinental lithospheric mantle finally no significant modification by interaction with continental crust is apparent in most of the analyzed samples

natural convection in porous permeable media sheets wedges and lenses abstract any fluid saturated porous permeable media subject to a temperature field such that there is a net upward heat flow and the isotherms across it are non horizontal will be subject to convective circulation analytical solutions for the fluid velocity exist only for the restricted case of an inclined layer in which the upper and lower surfaces are maintained at constant temperature this result has been much cited in the geological literature although the conditions under which the analytical solution holds are unrealistic in a geological situation numerical modelling is used to investigate how closely the analytical solution holds under conditions which are more geologically realistic it is found that for an homogeneous aquifer the analytical solution is a reasonable approximation provided the inclunation of the layer is replaced by the inclination of the isotherms in the expression for the fluid velocity in general the magnitude of will be much smaller than that of and depending on the effective thermal conductivity of the inclined layer relative to its surroundings may be of the opposite sign in this instance circulation is in the reverse direction to that obtained from the analytical solution for a vertically heterogeneous system in which the aquifer is divided by a series of thin continuous low permeability layers conditions exist in which the flow may be approximated by that in a homogeneous but anisotropic medium the model is also extended to cover other simple geometrics for which analytical solutions are not available

a remanent and induced magnetization model of magsat vector anomalies over the west african craton scalar and vector magsat anomalies over the west african craton are analysed by forward and inverse models a forward model of the man shield is based on liberia induced magnetization contrasts due to sporadic iron formations and to regional metamorphic rocks and a contrast in remanent magnetization within the lower crust are included this combination reproduces the location magnitude and adopted local zero level of anomalies in the initial magsat maps an inverse model of the reguibat shield estimates the magnetization contrast of its lithosphere and when magnetism is restricted to shallower than 75 km both shields can be represented by a susceptibility contrast of 0 02 a residual anomaly between the shields involves a relative deficiency of induced magnetization along with other causes

numerical simulation of thermal convection in compacting sedimentary basins summary a numerical model of combined fluid flow and heat flow is used to study convection in sedimentary basins during compaction the main issue of the paper is the interaction of non rayleigh convection flow with compaction flow and or meteoric flow it is shown that under reasonable assumptions the flow inside a permeable layer is the superposition of the compaction flow through the layer and the thermal convection flow in the layer these assumptions are expressed in terms of the time constants involved non rayleigh convection roll s will therefore appear if the compaction flow or the meteoric flow is less than the thermally induced flow this condition can be stated by a simple expression involving the rayleigh number for the layer and the absolute value of the flow velocity through the layer two case studies are presented which show non rayleigh convection in compacting basins a numerical example of rayleigh convection is also presented which demonstrates that rayleigh convection under realistic conditions such as compressible rock and non perfect thermal conducting top and bottom boundaries is a complex process this case study shows that even a small amount of vertical excess pressure gradient is enough to stabilize the fluid inside a layer sedimentary basins thermal convection

the planform of compositional convection and chimney formation in a mushy layer compositional convection occurs when a liquid containing more than one chemical component undergoes fractional crystallization above a critical solidification rate a solid liquid mixed phase or mush develops which can exhibit spatial gradients of permeability and flow due to preferential dissolution and precipitation in the upwellings and downwellings respectively a striking but poorly understood example is when upflow occurs in narrow crystal free cylindrical channels or chimneys such dynamic effects may occur in the earth s core1 crustal magma reservoirs2 hydrothermal systems at mid ocean ridges3 and during the diagenesis of sedimentary rocks4 using experiments designed to maximize the effects of dissolution and precipitation we show that chimney formation can be related to a known planform of convection just above marginal stability upwelling and dissolution occurs along the perimeters of hexagonal cells with downwelling and precipitation at the centres giving rise to a tessellated network of vertical crystal free channels subsequent focusing of the upflow at the nodes of the hexagons and recrystallization in the linear channels results in isolated chimneys at the nodal positions

modern and ancient geotherms beneath southern africa estimates of subsurface temperatures in the archean craton of southern africa during the archean derived from diamond thermobarometry studies are remarkably similar to temperatures estimated for the same depths today even though heat production in the earth and the mean global heat flow were probably substantially higher in the archean we present multi dimensional numerical models for the thermal environment of the archean craton in southern africa during the archean in which deep mantle heat is diverted away from the craton toward the surrounding oceanic lithosphere by a lithospheric root beneath the craton extrapolation of present day models to thermal conditions appropriate for the archean is inadequate to explain the similarity of present day and archean temperatures in the cratonic root reconciliation of the modern and ancient temperature estimates requires either relaxation of the constraints that the cratonic crustal heat production and or the earth s mean mantle temperature were higher in the archean than they are today or that substantial erosion of the lithosphere comprising the cratonic root has occurred since the archean the latter possibility could perhaps result from revolatilization of the cratonic root in association with thermal perturbations in the mantle for which there is evidence in southern africa in the form of post archean tectonic and igneous activity

initial scalar magnetic anomaly map from magsat a scalar magnetic anomaly map between 50 geographic latitude was derived from magsat data acquired during the november 1979 to june 1980 mission lifetime separation of anomaly fields from core and external fields was accomplished utilizing techniques developed for pogo data mayhew 1979 langel et al 1980 comparison with a similar map from pogo data shows that the basic anomaly patterns are essentially reproducible except in a few regions such as the atlantic and pacific at latitudes south of 15 increased resolution due to a lower measurement altitude is evident in the magsat map

rayleigh convection mass transport and change in porosity in layers of sandstone the critical rayleigh number for the onset of thermal convection in a quartz rich sandstone layer is discussed for different thermal boundary conditions the mass transfer of silica due to rayleigh convection is computed assuming that the solution is saturated with respect to quartz and that the silica concentration is a function of temperature only since quartz solubility increases with temperature silica is dissolved from the sandstones in the lower part of the layer where the fluid is being heated and precipitated in the upper part where it is cooled the change in porosity is found as function of the vertical coordinate valid for small values of time it is shown that for small supercritical rayleigh numbers the time for the porosity to change up to 10 of its initial value is of the order 10 m y the spatial variation in porosity should be a characteristic signature of rayleigh convection and should be easy to recognize if convection has taken place for significant geological time

convective momentum and mass transport in porous sloping layers natural convection caused by nonhorizontal isolines for water density in sloping sandstone layers is studied the flow field is calculated for a fluid filled three layer model composed of two identical sandstone strata separated by a low permeability layer shale the condition for recirculation within the sandstone is found it is shown that for layers of sufficiently small aspect ratio the shale layer has negligible effect on the flow with the assumption that the fluid density is a function only of temperature the effect of the flow on the change of porosity in the sandstone layers is examined the spatial change of porosity is calculated for small values of time it is shown that the maximum change takes place at the midpoints of the end walls if the angle between the isotherms and the slab is less than 36 5 and at the midpoints of the lateral boundaries if this angle is greater than 36 5 this result is independent of the angle the slab makes with the gravity vector also discussed is how the angle the isotherms make with the horizontal depends on the thermal conductivities of the sandstone layers and the bounding rock

convective fluid flow and diagenetic patterns in domed sheets when a moving pore fluid maintains equilibrium with the host rock the flow and temperature fields can combine to produce diagenetic patterns these diagenetic patterns define source and sink regions for mass transfer within the rock and permit prediction of both the spatial distribution of diagenetic alteration and its intensity analytical solutions have been obtained for the fluid flow and temperature fields for the case of a folded porous layer embedded in an impermeable medium heated from below and held at a constant temperature above this corresponds to a layered sequence of rocks in which a permeable sand is bounded above and below by impermeable shales the present results are limited to gently folded strata small dip angles but are quite general with regard to the actual layer geometry the solutions depend on the fact that the temperature field can be uncoupled from the fluid flow when convection is weak this results in a purely conductive temperature field which drives the fluid motion through buoyancy effects it is shown that the dynamic of the system are most strongly influenced by the geometry of the porous layer and the ratio of thermal conductivities of the rocks

computation of porosity redistribution resulting from thermal convection in slanted porous layers unlike fluid displacement due to regional hydraulic head thermoconvective motions are generally slow the thermal impacts of such movements are very weak whereas their chemical impacts may be significant because of their cumulated effects over geologic time for nonhorizontal thick sedimentary reservoirs the fluid velocity due to thermal convection can be accurately approximated by an explicit function of the dip of the reservoir the permeability and the difference in thermal conductivity between the aquifer and the confining beds the latter parameter controls the rotation direction of the flow and for clastic reservoirs bounded by impervious clayey media fluid moves up the slope along the caprock layer as the fluid velocity is small the major rock forming minerals control the fluid composition by thermodynamic equilibrium thus whereas the volume of redistributed minerals depends on the volume of water circulated the localization of porosity enhancement is strongly controlled by the reservoir mineralogy with realistic values of permeability and layer thickness several per cent of secondary porosity per million years can be created or lost at shallow depth 2 km depending on the chlorinity the set of representative minerals and the temperature in sandstone reservoirs and high chlorinity calcarenite reservoirs the porosity decreases under the caprock where hydrocarbons can accumulate in chloride depleted carbonate aquifers the simultaneous control by carbonates silica and aluminosilicates can produce a decrease of porosity above the bedrock and an enhancement of porosity under the caprock however computations show that the quality of the upper part of the reservoir is mainly reduced by the precipitation of silica and clays

effective magnetic susceptibility of the oceanic upper mantle derived from magsat data passive ocean continent boundaries have little magnetic signatures at magsat altitude 400 km whereas subducting oceanic lithosphere has well defined magnetic anomalies over some of the subduction zones based on this observation and a thermal evolution model of the oceanic lithosphere subducted along the kuril kamchatka trench we determined an effective magnetic susceptibility of 0 0085 si for the upper 35 km of the oceanic upper mantle

stability of a collapsing volcano stromboli italy limit equilibrium analysis and numerical modelling abstract stromboli is an active island volcano belonging to the aeolian archipelago italy in the last 13 ka it experienced four lateral collapses affecting its northwestern flank with its most recent volcanic crisis december 2002 associated with landslides and related tsunami events this paper presents the first stability analysis of the stromboli volcanic edifice the main input is the geotechnical model of the volcano defined on the basis of stratigraphical lithological material properties and structural data collected from in situ surveys and laboratory tests two dimensional stability analysis was performed by limit equilibrium methods lem and finite difference modelling flac 4 0 code mainly focusing on the subaerial part of stromboli s nw flank sciara del fuoco the variability of the safety factor was studied by deterministic sensitivity and probabilistic analysis focusing on the effect of external forces such as magma pressure and seismicity as potential triggering mechanisms of lateral collapse the lem analyses were developed considering a maximum depth of 150 350 m for the sliding surface to which correspond collapse volumes of 95 000 185 000 m 3 m respectively the study shows that without external forces the investigated rock mass is stable and that the tectonic seismicity of the area alone does not destabilize the studied slope on the contrary magma pressure in dykes can represent a destabilizing factor numerical modelling results are concordant with those from lem in addition the simulation reveals that deformations and superficial landslides preannounce and contribute to retrogressive plasticization and maybe to failure surface deepening shallow submarine landslides represent a possible triggering mechanism and the landslide events of 30 december 2002 consistently fit the simulated evolution flac has revealed to be a useful tool for modelling such a complex system it allowed to calibrate the response of the geotechnical model test the validity of the assumptions simulate the stress strain evolution and prepare a possible model for future scenarios

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial

crustal evolution and the granulite to eclogite transition in xenoliths from kimberlites in the west african craton a petrographic mineral and bulk chemical study of a xenolith suite of granulites and eclogites from sample creek liberia and koidu sierra leone has been undertaken with a view to determining the nature of the crust upper mantle interface a broad range of xenolith compositions is present from high mgo eclogites to garnet anorthosites and a systematic afm trend is established consistent with mafic and ultramafic melt fractionation at moderate pressures 10 20 kbar a trend is established for the entire xenolith suite among bulk chemistry seismic p wave velocity and a crust mantle c m bulk chemical ratio defined as na2o k2o sio2 feo mgo mole three populations are present a granulitic crustal group sg 3 0 a granulite and eclogite transitional group sg 3 0 3 3 vp 7 2 8 0 km s c m 1 5 3 0 andanexclusivelyuppermantleeclogiticgroup ifsg 3 3 vp 8 2 8 7 km s c m 1 5 from these data and coupled with garnet clinopyroxene mineral thermometry and accessory phases e g diamond graphite coesite kyanite or the presence of plagioclase a xenolith geotherm is established based on stratigraphic sequencing and phase transition boundaries diamond and coesite bearing eclogites conform to the 40 mw m2 standard cratonic low heat flow geotherm whereas the plagioclase granulites at lower pressures correspond to an average rift geotherm of 90 mw m2 the latter is ascribed to igneous underplating onto the lower crust or to thermal perturbations from an earlier tectonic event graphite and kyanite eclogites and the transitional group in sg vp and c m ratio of eclogites and granulites fall between the 40 and 90 mw m2 reference geotherms the xenoliths are meta igneous the lower crust and uppermost mantle are mafic in composition and the petrologic moho is an intercalated interstratified horizon of eclogite and garnet granulite growth of the early crust was largely a consequence of asthenospheric depletion in which underplating rather than lateral accretion was dominant

is cratonic sedimentation consistent with available models an example from the upper proterozoic of the west african craton abstract the atar group part of the upper proterozoic sequence covering the west african craton stable since 2000 ma is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies the carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments the mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions the settings of the mixed facies are not always clear but they do not contain thick sequences of high energy facies few obvious facies sequences are discernable those that are present are considered to be punctuated aggradational cycles pacs and they always start with biostromes of columnar stromatolites with very few sediments composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous despite the fact that the contacts are not gradational however much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies bound by sharp boundaries the basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea level rise consequently their appearance can be considered as a time line we describe in the atar group and its equivalents three sedimentation trends all of which are interpreted to be of shallowing upward character the atar group appears to have been deposited in an epeiric sea i e an extremely flat ramp there are two contrasting styles of sedimentation 1 after the submergence of the whole area columnar stromatolites built extensive biostromes 2 during the stable phase sediments are deposited in a mosaic of laterally discontinuous facies tidal influence cannot be recognized in the sequence neither can a salinity increase toward the land both common features in published epeiric sea models a cratonic sedimentation area such as this is characterized by its size and flatness only during the stable phase of the cycle does small scale topographic relief lead to deposition of a mosaic of facies the sedimentation is storm and wave dominated

geologic sections across the onshore senegal mauritania basin derived from geoelectric studies on the basis of magnetotelluric two dimensional models along three traverses the regional electrical structure within the senegal mauritania basin sediments has been studied correlation of the computed models with the wells drilled in the region shows that the main gross lithologic units were detected the models can be generalized into three resistivity layers i post turonian ii pre senonian and iii basement in the southern part of the basin there are relatively conductive formations below the mesozoic that are interpreted as sediments of the paleozoic bove basin sequence by combining the information that has been provided by the geoelectric cross sections and the lithologic and electric log data a schematic model for the generalized sedimentary structure of senegal has been generated the interpretation that emerges shows that the basin is a westward sloping open homocline in which the structure is controlled by north south trending basement faulting that portrays a staircase structural

trans african drainage system of the sahara was it the nile an exciting result of the first spaceborne radar scans of the eastern sahara has been the recognition of drainage channels buried at depths of several metres below the dry desert sand we relate these observations to the megageomorphological evolution of africa and conclude that the radar river valleys may have been parts of an old nile river system rather than courses cut by a postulated westward flowing trans african drainage system tads extending from the red sea hills to the niger delta

scalar magnetic anomalies of canada and northern united states derived from magsat data the total magnetic field of the earth over canada and the northern united states is sampled along individual orbits passes of the magnetometer satellite magsat on the basis of data extracted from the magsat chronicle tapes the main magnetic field at a given sample point is calculated using the main field model by langel et al 1982a and then subtracted from the sampled value the external magnetic field contribution is partially removed by a quadratic function fitted to each individual pass the resulting magnetic residuals are resampled at 55 by 55 km grid points on an equal area projection and subsequently contoured the dawn passes yield a magnetic map that is different than the dusk passes implying that the remaining external magnetic field component has a dawn dusk asymmetry the component is further suppressed by applying a band pass filter in the frequency domain the magnetic anomaly map derived by averaging the dawn and the dusk band pass filtered maps shows the main features seen on the aeromagnetic maps of canada and the united states the map also shows that the appalachian and the cordilleran orogens and the grenville superior suture zone are magnetically low regions whereas the craton basins such as the michigan the thelon and the ne british columbia basin are magnetically high areas

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations

interpretation of satellite magnetic anomalies over the west african craton abstract satellite magnetic anomaly maps of west africa display persistent anomalies that are spatially related to the geological structure of the west african craton the reguibat and man shields to the north and south of the taoudeni basin have positive magnetization contrasts whereas the basin which was disrupted by rifting and volcanism in the late proterozoic and mesozoic has a negative magnetization contrast a forward model of the craton is developed based on the surface geology and including aspects of the evolution of deep seated thermal petrological and magnetization contrasts anomalies calculated from the model correspond fairly well to the most reliable observed anomalies the anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra basin deep seated demagnetization resulting from mesozoic thermal and metasomatic demagnetization another similar demagnetized zone probably contributes to the reguibat shield anomaly the man shield anomaly results primarily from the archean segment of this shield the largest magnetization contrast required for a 70 km thick shield slab is 0 3 a m 1