the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

slow but not fast global expansion may explain the surface dichotomy of earth abstract various fast and slow earth expansion hypotheses are reviewed principal objectives are 1 1 to aid geologists to overcome their current mysticism in rejecting accepting or sympathising with earth expansion hypotheses without distinction and 2 2 to stimulate geophysicists into taking full account of the possibility that slow earth expansion by core mantle interaction may have occurred inward migration of the core mantle boundary due to the differentiation and progressive solidification of the liquid outer core might have caused expansion of the earth s radius at a slow mean rate of 0 5 mm a 1 a slowly expanding earth is attractive since it could explain why two distinct types of lithosphere exist oceanic and continental

spreading and convective dissolution of carbon dioxide in vertically confined horizontal aquifers injection of carbon dioxide co2 into saline aquifers is a promising tool for reducing anthropogenic co2 emissions at reservoir conditions the injected co2 is buoyant relative to the ambient groundwater the buoyant plume of co2 rises toward the top of the aquifer and spreads laterally as a gravity current presenting the risk of leakage into shallower formations via a fracture or fault in contrast the mixture that forms as the co2 dissolves into the ambient water is denser than the water and sinks driving a convective process that enhances co2dissolution and promotes stable long term storage motivated by this problem we study convective dissolution from a buoyant gravity current as it spreads along the top of a vertically confined horizontal aquifer we conduct laboratory experiments with analog fluids water and a mixture of methanol and ethylene glycol and compare the experimental results with simple theoretical models since the aquifer has a finite thickness dissolved buoyant fluid accumulates along the bottom of the aquifer and this mixture spreads laterally as a dense gravity current when dissolved buoyant fluid accumulates slowly our experiments show that the spreading of the buoyant current is characterized by a parabola like advance and retreat of its leading edge when dissolved buoyant fluid accumulates quickly the retreat of the leading edge slows as further dissolution is controlled by the slumping of the dense gravity current we show that simple theoretical models predict this behavior in both limits where the accumulation of dissolved buoyant fluid is either negligible or dominant finally we apply one of these models to a plume of co2 in a saline aquifer we show that the accumulation of dissolved co2 in the water can increase the maximum extent of the co2 plume by several fold and the lifetime of the co2 plume by several orders of magnitude

upper mantle structure below the european continent constraints from surface wave tomography and grace satellite gravity data 1 we here exploit fundamental mode rayleigh and love seismic wave information and the high resolution satellite global gravity model ggm02c to obtain a 1 1 3 d image of a upper mantle isotropic shear wave speeds b densities and c density vscoupling below the european plate 20 n 90 n 40 w 70 e the 3 d image of the density vscoupling provides unprecedented detail of information on the compositional and thermal contributions to density structures the accurate and high resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models the correlation between residual topography and mantle residual gravity anomalies defines three large scale regions where upper mantle dynamics produce surface expression the east european craton the eastern side of the arabian plate and the mediterranean basin the effects of mantle convection are also clearly visible at 1 the eastern sirt embayment 2 the west african craton northern margins 3 the volcanically active region of the canarian archipelago 4 the northern edge of the central european volcanic province and 5 the northeastern part of the atlantic ocean between greenland and iceland strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression although both thermal and additional dependencies have been incorporated into the density model convective down welling in the mantle below the east european craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography

geologic structure and neotectonics of the north african continental margin south of sicily abstract marine geological and geophysical data together with drilling information indicate that the north african passive continental margin has been subjected to extension and wrenching after it collided with the northern part of sicily the area of the tripolitania basin jarrafa trough melita and medina bank and the ragusa malta plateau has formed part of a sinking passive margin since the dispersal of gondwanaland at about 180 my ago as observed from geohistory diagrams a record of rifting in a nw se direction accompanied by dextral shear along the southern troughs is observed in seismic reflection data the rifting started during the neocomian and lasted until the eocene when activity became minor a pre middle miocene period of northward subduction of oceanic crust is inferred from the geology in ne sicily uplift of the northern part of the african margin after collision in the middle miocene is seen in wells in southern sicily after the messinian a rift and dextral shear zone established itself across the african margin from the strait of sicily to the medina ridge in the lonian basin the zone is marked by up to 1 7 km deep grabens narrow active wrench faulted channels volcanic fissures and local uplifted keilhorsts such as malta this zone which varies in width from 100 to 35 km forms the southern boundary of a microplate which includes sicily we speculate that the present motion of this microplate is partly due to the eastward movement of the calabrian arc with the sicilian block over the last remaining oceanic lithosphere in the eastern mediterranean

hydrodynamics and sediment dynamics of north sea sand waves and sand banks and discussion seabed drag coefficients have been measured at a site within the norfolk banks and at a site within the sand wave field in the southern north sea using pressure sensors and moored current meters at the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used the results are generally in agreement with values used in numerical models at the sand banks site the drag coefficient increases with wind conditions but at the sand waves site there is a reduction during the highest wave conditions attributed to the drag reduction caused by sand resuspension from the bed this result suggests that sediment effects must be considered if wave current interaction is included in numerical models of the region the drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face suggesting a small form drag component studies of sand movement used bedform mapping fluorescent sand tracing and photography of migrating ripples there is evidence for the early stages of formation of a new bank between broken bank and well bank at the sand waves site ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension over the sand banks a definite correlation of surface water properties notably the turbidity with the topography of the banks was observed the mechanism for this surface effect is not known but it is consistent with the observation of features seen in satellite visible light images which outline the shapes of the banks

onshore geologic storage of co2 the possibility that substantial quantities of co2 can be injected into subsurface porous rock formations has been investigated sufficiently to show that pore space available to contain the co2 is abundant multiple rock types and physical mechanisms can be used to trap the co2 indefinitely with careful site selection and operations leakage to the near surface region can be avoided the next step is to test these injection processes at the scale of a large power plant

a suspended sediment stratification correction for combined wave and current flows a simple model for the near bottom combined wave and current flow over a moveable sediment bed is presented the model is an extension of the grant and madsen 1979 combined wave and current model with moveable bed effects on the physical bottom roughness included as by grant and madsen 1982 the unsteady conservation of fluid momentum and sediment mass equations which are coupled through an eddy diffusivity closure scheme are solved for the wave and current velocity profiles along with the suspended sediment concentration and transport profiles sample runs are presented to illustrate the effect of varying wave conditions on the current the mean sediment concentration and the mean sediment transport results show that for fine to medium sands self stratification of the flow can be important during storms owing to the large amount of sediment suspended by the waves and the enhanced turbulent mixing associated with the wave current interaction the theory easily can be generalized for inclusion in a full ekman layer model

gravity currents with residual trapping motivated by geological carbon dioxide co 2 storage we present a vertical equilibrium sharp interface model for the migration of immiscible gravity currents with constant residual trapping in a two dimensional confined aquifer the residual acts as a loss term that reduces the current volume continuously in the limit of a horizontal aquifer the interface shape is self similar at early and at late times the spreading of the current and the decay of its volume are governed by power laws at early times the exponent of the scaling law is independent of the residual but at late times it decreases with increasing loss owing to the self similar nature of the current the volume does not become zero and the current continues to spread in the hyperbolic limit the leading edge of the current is given by a rarefaction and the trailing edge by a shock in the presence of residual trapping the current volume is reduced to zero in finite time expressions for the up dip migration distance and the final migration time are obtained comparison with numerical results shows that the hyperbolic limit is a good approximation for currents with large mobility ratios even far from the hyperbolic limit in gently sloping aquifers the current evolution is divided into an initial near parabolic stage with power law decrease of volume and a later near hyperbolic stage characterized by a rapid decay of the plume volume our results suggest that the efficient residual trapping in dipping aquifers may allow co 2 storage in aquifers lacking structural closure if co 2 is injected far enough from the outcrop of the aquifer

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

buoyant currents arrested by convective dissolution 1 when carbon dioxide co2 dissolves into water the density of water increases this seemingly insubstantial phenomenon has profound implications for geologic carbon sequestration here we show by means of laboratory experiments with analog fluids that the up slope migration of a buoyant current of co2 is arrested by the convective dissolution that ensues from a fingering instability at the moving co2 groundwater interface we consider the effectiveness of convective dissolution as a large scale trapping mechanism in sloping aquifers and we show that a small amount of slope is beneficial compared to the horizontal case we study the development and coarsening of the fingering instability along the migrating current and predict the maximum migration distance of the current with a simple sharp interface model we show that convective dissolution exerts a powerful control on co2 plume dynamics and as a result on the potential of geologic carbon sequestration

the cross sectional shape of tidal sandbanks modeling and observations to improve our understanding of tidal sandbank dynamics we have developed a nonlinear morphodynamic model a crucial property of the model is that it fully resolves the dynamics on the fast tidal timescale allowing for asymmetric tidal flow with an m0 m2 and m4 component this approach extending earlier research on the formation of tidal sandbanks leads to equilibrium profiles their heights 60 90 of the water depth and shapes are controlled by the mode of sediment transport and the hydrodynamic conditions bed load transport under symmetrical tidal conditions leads to high spiky banks several mechanisms tend to lower and smooth these profiles such as the relaxation of suspended sediment wind wave stirring and tidal asymmetry this last causes the profiles to be asymmetric as well the morphodynamic equilibrium expresses a tidally averaged balance between a destabilizing flux due to fluid drag and the downslope transport induced by both tidal flow and wind wave stirring the modeled profiles are in fair agreement with observations from the north sea

the role of suspended load transport in the occurrence of tidal sand waves tidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics sediment transport and geomorphology field data from the southern north sea reveal that sand waves are absent where suspended load transport is the dominant transport mode in order to understand the mechanisms responsible for the absence of sand waves we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process based geomorphological model delft3d model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable k e turbulence model first it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment while for a relatively small grain size the flat bed is stable and no sand waves evolve second it is shown that suspended load transport causes the suppression of long sand waves resulting in a finite range of wavelengths that experience growth finally by varying flow velocity amplitude and grain size critical conditions for sand wave formation are found i e conditions for which sand waves are marginally generated

palaeohydrology of the fazzan basin and surrounding regions the last 7 million years abstract we have integrated information on topography geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of lake megafazzan a giant lake that has periodically existed in the fazzan basin since the late miocene the development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought libya during this period and thus an overview of the palaeohydrology of all libya is also presented the origin of the fazzan basin appears to lie in the late miocene at this time libya was dominated by two large rivers systems that flowed into the mediterranean sea the sahabi river draining central and eastern libya and the wadi nashu river draining much of western libya as the miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the river nashu in late miocene or early messinian times forming a sizeable closed basin in the fazzan within which proto lake megafazzan would have developed during humid periods the fall in base level associated with the messinian desiccation of the mediterranean sea promoted down cutting and extension of river systems throughout much of libya to the south of the proto fazzan basin the sahabi river tributary know as wadi barjuj appears to have expanded its headwaters westwards the channel now terminates at al haruj al aswad we interpret this as a suggestion that wadi barjuj was blocked by the progressive development of al haruj al aswad k ar dating of lava flows suggests that this occurred between 4 and 2 ma this event would have increased the size of the closed basin in the fazzan by about half producing a catchment close to its current size 350 000 km2 the fazzan basin contains a wealth of pleistocene to recent palaeolake sediment outcrops and shorelines dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka the middle to late pleistocene interglacials were humid enough to produce a giant lake of about 135 000 km2 that we have called lake megafazzan later lake phases were smaller the interglacials less humid developing lakes of a few thousand square kilometres in parallel with these palaeohydrological developments in the fazzan basin change was occurring in other parts of libya the lower pliocene sea level rise caused sediments to infill much of the messinian channel system as this was occurring subsidence in the al kufrah basin caused expansion of the al kufrah river system at the expense of the river sahabi by the pleistocene the al kufrah river dominated the palaeohydrology of eastern libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the sirt basin at this time libya was a veritable lake district during humid periods with about 10 of the country underwater

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

early tertiary subsidence and sedimentary facies northern sirte basin libya the subsidence curves and subsidence rate curves for the sirte basin constructed from the stratigraphic record show that subsidence was continuous throughout late cretaceous and tertiary times reaching a maximum during the paleocene and eocene when a major reactivation of faults occurred shales and carbonates were deposited during all of the late cretaceous and tertiary abrupt lateral facies changes occur from the platform areas toward the deeper troughs along with steep downdip thickening these conditions were probably assisted by contemporaneous faulting along structurally weak hinge lines where the dominant structural elements are normal step faults the absence of upper paleozoic and lower mesozoic sediments suggests that the area was domed faulted and eroded during the late mesozoic the widespread distribution of the maastrichtian carbonate facies over much of the platform and basinal areas suggests that tectonics were relatively inactive during the maastrichtian as a result of crustal extension during the paleocene a marked lithologic and structural change occurred the heira shale succeeded the kalash limestone in the marada trough reactivation of the earlier faults accompanied by an increase in the sediment supply from the south caused these lower paleocene shales to cover the entire area with the exception of the old highs where carbonate deposition continued an intercalation of shales and carbonates provides a sensitive indicator of change of depth and sediment type commonly this intercalation has been referred to paleocene transgressive and regressive cycles the thickness of the paleocene facies corresponds closely to the structure with the thickening in the troughs and thinning on the highs

tectonics and subsidence evolution of the sirt basin libya a backstripping analysis of 225 wells located within the sirt basin libya provides new constraints on the development of the sirt basin four tectonic phases are identified from late jurassic to present the presentation of contour maps of subsidence and crustal stretching allows spatial and temporal variations in stretching to be visualized a close match is observed with stretching phases documented for other african basins consistent with discrete phases in the opening of the tethys and atlantic rifting and reactivation appear to be primarily controlled by the orientation of the basin and the underlying basement structure with respect to stress directions the tectonic subsidence curves have also been forward modeled with an automated modeling technique to quantify the variation in timing and the magnitude of rifting the tectonic subsidence history of the sirt basin is characterized by periods of stretching alternating with periods of relative tectonic quiescence and thermal subsidence stretching started at the centers of the troughs and migrated toward the platform crests

on the modeling of sand wave migration 1 we describe a simple mathematical model capable of reproducing the main features of sand wave inception and growth in particular we focus on the prediction of the migration rates that sand waves undergo because of tidal and residual currents the model adequately predicts migration rates even for the cases of upstream propagating sand waves i e for sand waves which migrate in the direction opposite to that of the residual current we show that upstream downstream propagation is mainly controlled by the relative strength of the residual current with respect to the amplitude of the quarter diurnal tide constituent and by the phase shift between the semi diurnal and quarter diurnal tide constituents therefore to accurately predict field cases a detailed knowledge of the direction strength and phase of the different tide constituents is required

impact of structural setting on diagenesis of fluvial and tidal sandstones the bahi formation upper cretaceous nw sirt basin north central libya abstract this petrographic mineral chemical fluid inclusion and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the upper cretaceous bahi sandstones in the rift sirt basin nw libya varies systematically along a series of closely spaced dominantly normal faults between the basin margin and more basinward located areas shallow eogenetic modifications resulting from the percolation of meteoric waters include infiltration of grain coating clays kaolinitization of detrital silicates and cementation by dolomite and k feldspar overgrowths mesogenetic alterations 70 c 2 km include feldspar albitization illitization of infiltrated clay and kaolinite conversion of kaolinite into dickite and cementation by quartz overgrowths t h 112 c 134 c barite t h 145 c 158 c and fe carbonates the restriction of barite and fe carbonate cements to the basinward located sandstones suggests formation by hydrothermal fluids along the faults extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and by extension reservoir quality in analogous epicratonic rift basins

crustal radial anisotropy beneath cameroon from ambient noise tomography abstract to increase the understanding of crustal deformation and crustal flow patterns due to tectonic processes in cameroon we study the lateral variability of the crustal isotropic velocity and radial anisotropy estimated using ambient noise tomography ant rayleigh and love wave noise correlation functions ncfs were retrieved from the cross correlation of seismic ambient noise data recorded in cameroon and phase velocities at periods of 8 to 30 s were measured to perform surface wave tomography joint inversion of rayleigh and love wave data for isotropic velocity models could not fit the observed dispersions simultaneously we attribute the love rayleigh discrepancy to the presence of radial anisotropy in the crust and estimated its magnitude our 3 d radial anisotropic model reveals the spatial variation of strong to weak positive vsh vsv and negative vsv vsh radial anisotropy in the crust we observe negative radial anisotropy in the upper crust that is associated mainly with the location of a previously reported mantle plume the anisotropy could be attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material a strong positive radial anisotropy is centered at the location of an inferred boundary between the congo craton and the oubanguides belt that might be related to the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone the middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow induced alignment of anisotropic minerals that crystallized during magma intrusion the magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust the imaged patterns of the isotropic velocity and radial anisotropy are consistent with previous studies and agree with regional tectonics

tidal induced large scale regular bed form patterns in a three dimensional shallow water model the three dimensional model presented in this paper is used to study how tidal currents form wave like bottom patterns inclusion of vertical flow structure turns out to be necessary to describe the formation or absence of all known large scale regular bottom features the tide and topography are treated as a coupled system and the potential free instabilities in this system are investigated a linear stability analysis is used to study the dynamics of bed forms this yields information on the role of physical mechanisms such information cannot be obtained by direct numerical nonlinear simulation the basic state represents a horizontal elliptical flow veering in vertical direction over a flat bottom it is shown that in such a system modes can grow which have characteristics of both tidal sand banks and sand waves the model predictions are compared with observations and show good agreement

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

petrology of the bimodal cenozoic volcanism of the kapsiki plateau northernmost cameroon central africa the kapsiki plateau is the northernmost volcanic zone of the cameroon line the volcanism 27 35 ma is of alkaline type and has a typical bimodal lava series diversity with basalts and hawaiite as mafic lavas and phonolites trachytes and rhyolites as felsic lavas mg rich olivine phenocrysts occur only in basalts the hawaiite contains andesine olivine and ca rich pyroxene phenocrysts and sanidine and quartz xenocrysts the phonolites contain alkali feldspar and na rich clinopyroxene phenocrysts two types of trachytes occur peralkaline trachytes with ti na and f rich aegirine augite richterite arfvedsonite phenocrysts and non peralkaline trachytes with an aenigmatite type undetermined mineral ti rich biotite and zircon phenocrysts similarly two types of rhyolites occur peralkaline with quartz and arfvedsonite phenocrysts and non peralkaline with quartz and biotite phenocrysts differentiation indices thornton and tuttle 1960 of the lavas range from 22 to 97 with a large gap in the range 34 82 some basalts are primitive 530 ppm ni 1100 ppm cr in basaltic lavas phonolites and non peralkaline trachytes and rhyolites zr and nb covary with approximately constant ratios o3 1 zr nb 5 3u however peralkaline trachytes and rhyolites have high concentrations in zr up to 2180 ppm and nb up to 780 ppm with correlative higher zr nb ratios 6 3 8 3 some rhyolites have abnormal ree patterns with kinks depleted in light ree probably resulting from stability of na ree f complexes under hydrothermal conditions despite a large gap between basaltic and felsic lavas major and trace element distributions indicate co magmatism for both the basaltic and felsic lavas the differentiation of the lava series is dominated by crystal fractionation the role of fluids in rhyolite genesis being of minor effect as evidenced by constant values of y ho and zr hf throughout the series the kapsiki plateau basalts are similar in their chemical and isotopic data character to other basalts from both the continental and oceanic sectors of the cameroon line the continental crust appears to have no significant role in their genesis the hawaiite mineralogical and geochemical characteristics are consistent with an origin by mixing of basaltic and felsic phonolitic magmas the kapsiki plateau basaltic magmas may have originated from an infra asthenospheric reservoir similarly to other basaltic magmas generated throughout the cameroon line the sr isotope variations observed in trachytes and rhyolites point to some contamination of the magmas by crustal materials while the nd isotopic composition is only slightly affected q 2000 elsevier science b v all rights reserved

suspended sediment signatures induced by shallow water undulating bottom topography near surface signatures of suspended sediment concentration ssc related to submerged bed forms displayed in visible band data gathered by space borne imaging sensors are described detailed acoustic doppler current profiler adcp data of vertical current velocity component echo intensity modulation of ssc and related oceanographic and meteorological observations in a tidal inlet of the north sea have been analyzed to understand the associated hydrodynamics of the optical imaging mechanism theoretical considerations based on the navier stokes and sediment continuity equations have been carried out as a first approximation these theoretical results explain signatures of enhanced ssc of in situ measurements and analyzed space borne images strong currents flowing over steep bottom topography are able to stir up the sediments to form both a general continuum of ssc and localized pulses of higher ssc in the vicinity of the causative bed feature itself the strain rate equivalent to the current gradient is an important parameter describing near water surface oceanographic and meteorological phenomena in the visible and microwave part of the electromagnetic spectrum observed by shore and ship based as well as air and space borne remote sensing systems significant disagreements between theoretical results of the strain rate generated by the surface current and in situ observations at the crests of submerged ridges and sand waves were identified possible explanations could be first the current velocity may not be barotropic near the crests of such bed forms and second mass conservation may not only be maintained through an acceleration or deceleration of the flow this implies that active up and downwelling effects of the three dimensional current field can also play a significant role for hydrodynamic interaction between current and underwater bed forms

sediment transport under wave and current combined flows on a tide dominated shoreface northern coast of france abstract hydrodynamic circulation and sediment transport patterns were investigated on a macrotidal shoreface off the southern north sea coast of france from in situ directional wave and current measurements and through numerical modeling of sediment flux current meters equipped with wave gauges were deployed at two sites on the mid upper shoreface in approximately 5 6 m water depth during 6 and 15 days sediment flux was estimated using the sedtrans96 numerical model li m z amos c l 2001 sedtrans96 the upgraded and better calibrated sediment transport model for continental shelves comp geosc 27 619 645 based on grant and madsen s grant w d madsen o s 1986 the continental shelf bottom boundary layer ann rev fluid mech 18 265 305 combined flow boundary layer theory the hydrodynamic measurements and modeling results show that sediment on the shoreface is essentially transported alongshore in response to shore parallel tidal currents the direction of the net sediment transport is largely determined by the asymmetry of the tidal flows which can be flood or ebb dominated but during important wind speed events winds blowing in the same direction as tidal flow can significantly reinforce tidal currents or conversely can limit tidal current speeds when acting in the opposite direction although sediment transport on this macrotidal shoreface is strongly controlled by tidal processes waves can significantly enhance bed shear stresses resulting in increased sediment remobilization and hence higher sediment flux wave induced onshore sediment transport may occasionally occur but is several orders of magnitude lower than longshore transport this macrotidal shoreface is thus characterized by a hydraulic regime strongly dominated by shore parallel tidal flows that favour longshore sediment dispersal rather than shore normal transport our results imply that the water depths from which sediment may be transported shoreward to beaches is certainly much more restricted on a macrotidal shoreface such as the one bordering the northern coast of france than on micro and mesotidal shoreface environments where weaker tidal currents do not significantly limit cross shore sediment movements

on the slow draining of a gravity current moving through a layered permeable medium we examine the gravitational dispersal of dense fluid through a horizontal permeable layer which is separated from a second underlying layer by a narrow band of much lower permeability we derive a series of analytical solutions which describe the propagation of the fluid through the upper layer and the draining of the fluid into the underlying region the model predicts that the current initially spreads according to a self similar solution however as the drainage becomes established the spreading slows and in fact the fluid only spreads a finite distance before it has fully drained into the underlying layer we examine the sensitivity of the results to the initial conditions through numerical solution of the governing equations we find that for sources of sufficiently large initial aspect ratio defined as the ratio of height to length the solution converges rapidly to the initially self similar regime for longer and shallower initial source conditions this convergence does not occur but we derive estimates for the run out length of the current which compare favourably with our numerical data we also present some preliminary laboratory experiments which support the model

turbulent stresses and secondary currents in a tidal forced channel with significant curvature and asymmetric bed forms acoustic doppler current profilers are deployed to measure both the mean flow and turbulent properties in a channel with significant curvature direct measurements of the reynolds stress show a significant asymmetry over the tidal cycle where stresses are enhanced during the flood tide and less prominent over the ebb tide this asymmetry is corroborated by logarithmic fits using 10 min averaged velocity data a smaller yet similar tendency asymmetry in drag coefficient is inferred by fitting the velocity and estimated large scale pressure gradient to a one dimensional along channel momentum balance this smaller asymmetry is consistent with recent modeling work simulating regional flows in the vicinity of the study site the asymmetry in drag suggests the importance of previously reported bed forms for this channel and demonstrates spatial and temporarily variations in bed stress secondary circulation patterns observed in a relatively straight section of channel appear driven by local curvature rather th

morphological response to a north sea bed depression induced by gas mining gas mining leads to saucer like surface depressions in the north sea gas is currently mined at several offshore locations the associated bed depression has a similar spatial extent as offshore tidal sandbanks which are large scale bed patterns covering a significant part of the north sea bottom the morphological time scales of bed depressions and tidal sandbanks are similar so that significant interaction between these features is expected in this paper we allow the bed depression to become morphologically active a simple depression model based on a homogeneous soil is tuned with data of a bed depression near the dutch barrier island of ameland next this subsidence model is included in a morphodynamic model we show that this model is able to explain tidal sandbanks which represent natural bed behavior here we approximate the solution by an expansion up to first order the zeroth order solution of the model is a flat bed with a spatially uniform time independent current the first order solution is investigated using a fourier transformation in general we observe significant interaction between the bed depression and the natural sandbank formation process the process of induced bed depression triggers and intensifies the natural morphological behavior of the offshore seabed the model also shows essential differences between modeling a morphodynamically active marine bottom depression and a bottom depression below the threshold for sediment motion the maximum bed level depression in the active case is significantly larger and the circular shape of depression contours is affected by stretching toward the preferred orientation of the tidal sandbank formation process

upper mantle structure beneath cameroon from body wave tomography and the origin of the cameroon volcanic line the origin of the cameroon volcanic line cvl a 1600 km long linear volcanic chain without age progression that crosses the ocean continent boundary in west central africa is investigated using body wave tomography relative arrival times from teleseismic p and s waves recorded on 32 temporary seismic stations over a 2 year period were obtained using a multichannel cross correlation technique and then inverted for mantle velocity perturbations the p and s wave models show a tabular low velocity anomaly directly beneath the cvl extending to at least 300 km depth with perturbations of 1 0 to 2 0 for p and 2 0 to 3 0 for s the s wave velocity variation can be attributed to a 280 k or possibly higher thermal perturbation if composition and other effects on seismic velocity are negligible the near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the cvl that invoke plumes or decompression melting under reactivated shear zones but are possibly consistent with a model invoking edge flow convection along the northern boundary of the congo craton lithosphere if edge flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the cvl then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness

threshold of erosion of submarine bedrock landscapes by tidal currents since sea level stabilized 7000 yr bp shelf seas experiencing semi diurnal tides will have been affected by streaming four times per day if tidal erosion of bedrock were even only marginally efficient the 10 million streamings since then should have left geomorphological imprints we examine high resolution multibeam sonar data from three areas with extreme tidal currents the minas passage bay of fundy experiencing 8 knot surface tidal currents was surveyed in 2007 with a multibeam sonar in an area near to transverse dunes which are evidence for bedload transport the data show local overhanging surfaces near to the sediment rock contact potentially created by abrasion by saltating particles however they are uncommon in the straits of messina where surface currents reach 10 knots surveying revealed ridges lying oblique to the flow that are not obviously broken into separate outcrops by erosion in the bristol channel uk sonar data collected where currents reach 3 4 knots at 1 5 m above the bed reveal outcrops of limestone with superimposed sand dunes but only minor rounding of blocks holocene tidal currents have apparently been generally ineffective at eroding bedrock we examine this issue further by compiling extreme tidal streams around the uk and from them estimate shear stresses representing a macro tidal environment where peak surface currents reach 9 7 knots those data are compared with shear stresses in mountainous rivers where long term rates of erosion are comparable with tectonic uplift rates and are thus geomorphologically significant whereas river stresses reach 102 103 pa the largest tidal stresses are generally 101 and only rarely approach 102 pa too small for quarrying to operate generally however the vertical faces in the minas passage may represent the onset of abrasion given this limited evidence for abrasion we explore conditions in the geological past for tides that may have locally eroded bedrock

sirte basin north central libya cretaceous rifting above a fixed mantle hotspot the complex pattern of horsts and grabens in the sirte basin may have developed when mesozoic drift of the african plate put north central libya over a fixed mantle hotspot in early cretaceous time 140 to 100 m y ago significant change in the motion of the plate during the prolonged residence above a hypothetical cameroon plume may have produced stress that fragmented thinned and weakened lithosphere successive uplift and subsidence along a reconstructed track of the plume as well as in the sirte basin are compatible with predicted effects of the drift of northern africa over a fixed mantle hotspot this speculation suggests a plausible alternative to the possibility that rifting throughout northern africa in early cretaceous time may have been produced along a wide zone of extension between two african plates when they were at rest relative to underlying plumes

facies patterns and stratal geometries clues to the nature of the platform margin during the paleocene northeast sirte basin libya abstract the sirte basin of libya is a mesozoic tertiary rift basin comprising a series of horsts and grabens which were formed as a result of collapse of a n s trending arch in late cretaceous time the paleocene section is made up of two shallowing upward cycles where each cycle is under and overlain by deeper pelagic facies a gradual shallowing trend from basin to shallow platform is seen in both bio and lithofacies these are interpreted as representing prograding ramps with no evidence of a significant break in slope between the basin and the adjacent platform the fact that most facies are dominated by micrites suggests that deposition was in a low energy setting possibly on a leeward margin these ramps were initially foram dominated platforms setting the style for later tertiary ramps large forams filling the role of inner mid ramp carbonate producers after the demise of the rudist coral assemblage at the end of the cretaceous based on local and regional information a number of episodes of relative sea level change can be recognized the two shallowing upward ramp cycles are interpreted as representing two highstand system tracts the precipitation of evaporites in the lower cycle appears to be associated with the earlier highstand system tract culminating during a sea level lowstand a drop in sea level during the late paleocene is also invoked to explain the interpreted influx of meteoric waters following the later highstand this event resulted in the creation of good mouldic and vuggy porosities thus making these porous inner mid ramp facies potential targets for hydrocarbon exploration on a basin wide scale this event is also very important because all the paleocene reservoirs at least in the eastern sirte basin seem to be related to this possible subaerial exposure event

seismicity of the cameroon volcanic line 1982 1990 abstract this paper is dedicated to soba djallo director of irgm who died in 1991 results of seismic monitoring of the continental segment of the cameroon volcanic line for the period 1982 to 1990 are reported mount cameroon continues to be seismically the most active area with a well defined seismicity pattern characterized by single and swarm events with duration magnitudes between 2 and 3 occurring at depths down to 55 km on average the earthquakes here occur at the rate of about 2 events every 3 days further north the seismicity is low except for isolated felt events located along the foumban shear zone which probably controls the location of the earthquakes as well as the volcanic centres making up the volcanic line during this eight year period two fatal carbon dioxide gas emissions from crater lakes occurred in 1984 l monoun and 1986 l nyos causing 37 and 1700 deaths respectively seismic monitoring indicates that no seismic activity was associated with these lakes thus favouring an aseismic cause to the disasters these gas emissions the increased number of felt earthquakes and a recent natural explosion on mount cameroon suggest either a more active phase of the cameroon volcanic line and or a greater awareness by the local population of the phenomena associated with this volcanic province

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

a study of the dynamics of a marine sandwave the movement of the crest of a sandwave was studied using cross sectional profiles obtained from lines of sea bed reference stakes measurements were made over a six month period before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions additional observations were obtained on a daily basis over an equinoctial neap to spring to neap tidal period in conjunction with boundary layer flow measurements tracer experiments were conducted to study the dispersion of sediment from the sandwave crest the results showed that the sandwave was relatively stable at neap tides whilst at higher tidal ranges the crest position oscillated with successive flood and ebb tides net flank erosion occurred on the less steep upstream slope during the dominant ebb tide this together with increased deposition on the lee slope caused the crest to advance it was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity high particle orbital velocities generated at the sea bed by storm waves caused major reductions in crestal heights calculated volumes of sediment eroded and accreted were used with boundary layer flow measurements to calculate threshold velocities for the movement of the sediment and sediment transport rates

did rifting on australia s southern margin result from tectonic uplift subsidence analyses from australia s southern margin are used to investigate the idea that rifting on the margin was caused by an extensional lithospheric stress field induced by a hot thermal plume impinging on the base of the lithosphere from beneath the continental lithosphere is modelled as a thin sheet with depth dependent rheology the model does not explicitly include elastic flexural or thermal stresses the excess buoyancy of a thermal plume pushes the lithosphere upward by as much as 1 km thereby increasing its potential energy if the horizontal components of lithospheric stress are fixed at some distant plate boundary the lithosphere is a stress guide the lithospheric stress field above the plume will produce horizontal extension that may ultimately be followed by continental separation and seafloor spreading the uplift is modelled by a time dependent function which increases smoothly from zero to its peak value and then decreases back to zero using a plausible mantle rheology the amount of extension inferred at well sites on the margin typically 20 to 30 extension is explained by uplift of the lithosphere to approximately 900 m with a subsequent decay of the uplift over a period of approximately 60 m y the rise time of the uplift function is poorly constrained in the uplift model extension occurs in the subaerial environment during the late jurassic early cretaceous the initial stage of rapid subsidence observed at most well sites on the margin is due in large part to decrease of the vertical stress anomaly caused by the decay of the thermal plume or by the lithosphere moving away from above it the rapid subsidence is followed by very gradual continued subsidence attributed to cooling of the extended lithosphere as in previous studies

on the choice of analogue fluids in co2 convective dissolution experiments mixtures of ethylene glycol and methanol eg meoh have been used as an analogue system i e eg meoh water in recent experiments in the context of convective dissolution of co2 in deep saline aquifers we have conducted a linear stability analysis of a gravitationally unstable diffusive boundary layer as well as direct numerical simulation of convective mixing involved in dissolution of eg meoh species in water we provide new evidences that eg meoh does not resemble the dynamics of convective instabilities and subsequent mixing associated with dissolution of co2 in water it is found that there are fundamental differences in the evolution of the buoyancy driven instability and dynamics of convective mixing between co2 water and a typical eg meoh water analogue system our results show that for a constant rayleigh number the onset of convective instabilities for eg meoh water can be different by an order of magnitude as compared with co2 water in addition eg meoh water system reveals different dynamics associated with the convective mixing as compared to co2 water system this study improves our understanding of the instability behavior of analogue systems their proper selection and motivates further experiments

comparison between predicted and observed sand waves and sand banks in the north sea for the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire north sea the model which originates from first physical principles predicts this occurrence via two dimensionless parameters on the basis of the water depth the tidal velocity amplitude the level of zero intercept above the seabed z0 and a viscosity variation parameter alt epsilon the latter two quantities were varied in a number of predictions for the entire north sea whereas for the first two local values were used the range of realistic values of alt epsilon and z0 was large enough to let these two parameters distinguish between the possible combinations of bed forms as is shown in the shallower southern bight of the north sea the results were more sensitive to variations in z0 than in alt epsilon a slightly more detailed approach focused on sand waves only in the southern north sea and estimated local values for z0 using depth information quantification of the results showed that the model was able to predict the contours of the sand wave patches but it could not account for the absence of the bed features within this area the type of bed deposit partly explains the smaller scale variation the work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large scale seabed features are formed as free instabilities of tide topography interactions