kimberlites of the man craton west africa abstract the man craton in west africa is an archaean craton formerly joined to the guyana craton south america that was rifted apart in the mesozoic kimberlites of the man craton include three jurassic aged clusters in guinea two jurassic aged clusters in sierra leone and in liberia two clusters of unknown age and one neoproterozoic cluster recently dated at 800 ma all of the kimberlites irrespective of age occur as small pipes and prolific dykes some of the banankoro cluster pipes in guinea the koidu pipes in sierra leone and small pipes in the weasua cluster in liberia contain hypabyssal facies kimberlite and remnants of the so called transitional facies and diatreme facies kimberlite most of the man craton kimberlites are mineralogically classified as phlogopite kimberlites although potassium contents are relatively low they are chemically similar to mica poor group 1a southern african examples the jurassic kimberlites are considered to represent one province of kimberlites that track from older bodies in guinea droujba 153 ma to progressively younger kimberlites in sierra leone koidu 146 ma and tongo 140 ma the scarcity of diatreme facies kimberlites relative to hypabyssal facies kimberlites and the presence of the so called transitional facies indicate that the pipes have been eroded down to the interface between the root and diatreme zones from this observation it is concluded that extensive erosion 1 2 km has occurred since the jurassic in addition to erosion the presence of abundant early crystallizing phlogopite is considered to have had an effect on the relatively small sizes of the man craton kimberlites

the effect of small scale hills on orographic drag the purpose of this study is to investigate the extent to which the presence of hills with relatively short horizontal scales can influence the drag due to mountain waves and flow blocking exerted on the flow by larger scale mountains numerical simulations of stratified flow over mountains are presented for flow over orography described by the linear superposition of broad mountains whose length scales are large enough to generate mountain waves and narrow corrugations whose wavelengths fall below the minimum horizontal wavelength for stationary gravity waves results are presented for both two and three dimensional mountain shapes and for a range of corrugation heights it is shown that the corrugations can significantly reduce the amplitude of the mountain waves generated by the broader mountain or they can suppress the unsteadiness of the wake when these mechanisms make an important contribution to the total drag this implies a significant drop in the total drag compared to the sum of the contributions from the two scales of orography from the point of view of drag parametrization the extent to which the effect of the small scale hills can be represented via an effective roughness length is investigated crown copyright 2007 reproduced with the permission of her majesty s stationery office published by john wiley sons ltd

archean crustal evolution of the west african craton example of the amsaga area reguibat rise upb and smnd evidence for crustal growth and recycling abstract new geochronological data u pb and sm nd are used to constrain the archean crustal growth and evolution of the southwestern part of the reguibat rise amsaga area in the west african craton the results are summarized as follows 1 u pb zircon data indicate an emplacement age of 2986 8 ma for the magmatic protolith of a charnockitic pluton in agreement with a whole rock sm nd isochron at 3012 142 ma positive nd 3 0 values ranging between 0 89 and 1 95 and agreement between nd model ages between 3 23 and 3 10 ga and u pb age indicate a juvenile origin for the magmatic protolith and imply a crustal growth event at ca 3 0 ga seven rims of the charnockitic zircons yield discordant 207 pb 206 pb ages ranging from 2979 11 to 2737 11 ma and are interpreted as resulting from growth during the granulitic event 2 whole rock sm nd data on migmatitic orthogneisses that form the central part of the area indicate an archean age for their magmatic protolith t dm ranging from 3 24 to 2 87 ga in agreement with a previous single zircon evaporation age of 2836 16 ma potrel 1994 3 two late tectonic postgranulite plutons both granite and gabbro dated at ca 2 73 ga mark the end of the granulitic event the granite provides a u pb age of 2726 7 ma and the gabbro a mineral sm nd isochron age of 2706 54 ma the t dm ages ranging from 3 1 to 3 2 ga indicate an origin by crustal reworking of the granite and negative nd 2 7 value 0 7 of the gabbro indicate crustal contamination during emplacement these results combined with previous work potrel et al 1996 indicate that the archean reguibat rise formed through a succession of crustal growth events from at least 3 50 3 45 ga until 2 73 ga a major granulitic event took place at ca 2 73 ga and was accompanied by emplacement of crustally derived magmas

fluid inclusion studies of gold bearing quartz veins from the yirisen deposit sula mountains greenstone belt masumbiri sierra leone the auriferous veins at yirisen masumbiri sierra leone occurring mainly in the form of sericitic quartz sulphide lodes and stringers are hosted in metamorphosed volcano sedimentary assemblages invaded by at least two generations of granitic intrusions detailed microthermometric studies of fluid inclusions from the veins coupled with laser raman spectroscopic analysis show that the inclusions contain aqueous fluids of variable salinity 5 to 60 wt nacl equivalent and dense carbonic fluids pure co2 1 08 d 0 88 g cm3 optical observations and analysis on opened inclusions by scanning electron microscopy sem reveal that some of the aqueous inclusions contain a number of daughter minerals halite sylvite ca fe mg and possibly li bearing chlorides and anhydrite nahcolite occurs also in some of the co2 inclusions the sem runs also detected a small amount of electrum suggesting that silver might be a bi product of the mineralisation the aqueous and carbonic fluids remained immiscible throughout the formation and evolution of the hydrothermal veins a few mixed h2o co2 inclusions apparently resulted from accidental trapping of both fluids in the same cavity the wide range of salinities observed in the aqueous inclusions is attributed to the mixing of relatively hot low salinity aqueous fluids and colder high salinity brines the co2 rich and low salinity h2o inclusions are considered to be derived from the metamorphic decarbonation dehydration of the greenstone pile whilst the high salinity brines are believed to be basinal in origin pressure temperature p t conditions of entrapment inferred from the intersection of representative isochores of the immiscible fluids indicate that the formation of the veins started at t 400 c and p about 4 kbar in the presence of the high density co2 and low salinity h2o fluids at about 200 c pressure fluctuations incremental opening of the vein correspond to the trapping of the lower density co2 inclusions and high salinity brines it is proposed that the decarbonation dehydration processes possibly aided by later magmatic processes expelled and mobilised the gold from the greenstone pile and concentrated it in the co2 bearing hydrothermal fluid in the form of au chloride complexes high thermal gradients are believed to have caused the upward migration of this fluid from the bottom of the greenstone pile through structurally controlled conduits we contend that phase separation of the h2o co2 metamorphic fluid aided possibly by some wall rock alteration most probably triggered a decrease in ligand activity and thus precipitation of the gold into lodes percolation of the basinal brines is thought to have remobilised some of the gold together with some silver

wind direction effects on orographic drag a series of idealised numerical simulations is performed to investigate the effect of wind direction on the pressure forces exerted on a high elliptical mesoscale ridge in the presence of coriolis effects at the rossby number considered here ro 13 rotational effects have a significant impact on the flow fields however the primary effect of rotation on the drag is to provide the asymmetry required to initiate vortex shedding when the flow is perpendicular to the mountain ridge it is found that linear theory although not valid for such high mountains provides a useful scaling for the variation of drag with wind direction for a large range of wind directions the flow is in a high super linear drag state and wave breaking vortex shedding and upstream flow blocking are observed however when the flow is close to being parallel to the major axis of the mountain ridge the drag becomes sub linear and none of the above processes are seen we show that the change from a high drag state to a low drag state can be explained in terms of the aspect ratio of the mountain that is the ratio of the across flow mountain length to the along flow length finally we demonstrate that the results found for the idealised elliptical mountains also apply to a real mountain of similar dimensions copyright 2008 royal meteorological society and crown copyright 2008 published by john wiley sons ltd

the tasiast deposit mauritania abstract the tasiast gold deposits are hosted within mesoarchean rocks of the aoueouat greenstone belt mauritania the tasiast mine consists of two deposits hosted within distinctly different rock types both situated within the hanging wall of the west vergent tasiast thrust the piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite quartz pyrrhotite actinolite garnet biotite gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units the west branch deposit is hosted within meta igneous rocks mainly diorites and quartz diorites that lie stratigraphically below host rocks of the piment deposits most of the gold mineralisation at west branch is hosted by quartz carbonate veins within the sheared and hydrothermally altered meta diorites that constitute the greenschist zone at tasiast gold mineralisation has been defined over a strike length 10 km and to vertical depths of 740 m all of the significant mineralised bodies defined to date dip moderately to steeply 45 to 70 to the east and have a south southeasterly plunge gold deposits on the tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the tasiast thrust an age of 2839 36 ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation

geochemical and numerical constraints on neoarchean plate tectonics this paper discusses early neoarchean 2 8 2 7 ga plate tectonics by integrating knowledge from new geochemical observations and numerical models based on a geochemical dataset of 295 granitoid samples from the karelian and kola cratons of the fennoscandian shield we divide neoarchean juvenile extracted from oceanic crust or mantle granitoids into three groups 1 low hree heavy rare earth elements ttgs tonalite trondhjemite granodiorite high sio2 low mg low hree higher sr lower ybn and higher nb ta 2 high hree ttgs slightly lower range of sio2 larger range of mgo contents and higher cr and ni contents high hree lower sr higher ybn and lower nb ta and 3 high ba sr sanukitoids medium hree high mg and high k ba sr the main difference between the low and high hree groups lies in their pressure sensitive element contents which indicates high pressure melting conditions for the low hree group and low pressure conditions for the high hree group a possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau protocrust melting in the lower part of thick basaltic oceanic crust could produce ttgs of the low hree type whereas low pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high hree ttgs the third group of archean high ba sr sanukitoids was formed after the ttgs their low sio2 and high mg k ba sr contents suggest an origin by melting in an enriched metasomatized mantle source probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau ttg protocontinent such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the archean which locally increased temperatures within the mantle wedge more frequent breakoff resulted because subducting plates were weaker due to rheologically thinner lithosphere and a thicker basaltic crust and tensile stresses within the subducting plate were larger due to a thick crust that transforms from buoyant basalt to dense eclogite

west atlantic mesozoic carbonates comparison of baltimore canyon and offshore nova scotian basins results of exploratory drilling by shell offshore inc and its partners of the late jurassic early cretaceous carbonate margin of the baltimore canyon bc basin can be interpreted directly from the better understood time equivalent nova scotian ns basin stratigraphy the bc paleomargin was constructed in a depositional cycle of three successive stages oxfordian early kimmeridgian progradation kimmeridgian berriasian aggradation berriasian valanginian drowning the upper two stages in ns are the baccaro and artimon members of the abenaki formation the progradation of the lower sequence in bc results from high clastic input and has a parallel not age equivalent in ns only in proximity to the sable island paleodelta both basins have similar complementary shelf edge environments that form a single water depth controlled biotic zonation this zonation is shallow water stromatoporoid hexacoral biostromes and reefs deeper water stromatactis and thrombolitic mud mounds and deep water lithistid sponge reefs associated environments are oncolitic reef flats reef apron skeletal sands slope reef debris from sponge and coral reefs and back reef mollusk rich skeletal and skeletal oolitic sands major differences between the basins are the pinnacle reefs of bc and the contrast between the mud rich skeletal and nonskeletal megafacies of ns versus the dominantly skeletal sand rich bc sediments the higher subsidence rates of bc may explainmore both differences less

observations and fine scale model simulations of gravity waves over davis east antarctica 69 s 78 e large vertical velocities were observed throughout the troposphere at davis east antarctica on 18 february 2014 by a vhf wind profiling radar simulations using the met office unified model at 2 2 0 5 and 0 1 km horizontal grid spacing were able to broadly capture the location timing and magnitude of the observed velocities as well as reveal that they are due to small scale orographic gravity waves resulting from the interaction between the coastal topography and strong easterly winds associated with a synoptic scale cyclone situated to the north the simulations indicated that the gravity waves are responsible for i temperature fluctuations which coincided with satellite observed cloud variations in the vicinity of davis suggesting that they have a crucial role in the formation of cirrus clouds and ii large vertical momentum fluxes in the troposphere the waves are prevented from propagating into the stratosphere by the background winds turning from near surface easterlies to lower stratospheric northerlies as well as illuminating and quantifying the role that weather systems have in producing orographic gravity waves along the east antarctic coastline studies such as this should be exploited to improve the representation of key localized atmospheric processes in global climate models

gravity wave characteristics in the southern hemisphere revealed by a high resolution middle atmosphere general circulation model abstractgravity wave characteristics in the middle to high latitude southern hemisphere are analyzed using simulation data over 3 yr from a high resolution middle atmosphere general circulation model without using any gravity wave parameterizations gravity waves have large amplitudes in winter and are mainly distributed in the region surrounding the polar vortex in the middle and upper stratosphere while the gravity wave energy is generally weak in summer the wave energy distribution in winter is not zonally uniform but it is large leeward of the southern andes and antarctic peninsula linear theory in the three dimensional framework indicates that orographic gravity waves are advected leeward significantly by the mean wind component perpendicular to the wavenumber vector results of ray tracing and cross correlation analyses are consistent with this theoretical expectation the leeward energy propagation extends to several thousand kilometers which explains part of the gravity wave distribution aro

reos and upb geochronological constraints on the eclogite tonalite connection in the archean man shield west africa abstract mantle derived eclogite xenoliths and tonalite trondhjemite granodiorites ttg that occur in the man shield west africa sample different levels of archean lithosphere chemical and oxygen isotope systematics indicate that low mgo eclogites from the koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional archean crust formation re os whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an archean age of 3 44 0 76 ga 2 with re os model ages ranging from 1 4 to 3 9 ga this wide range of model ages overlaps with the age range for crust formation and metamorphism in the man shield in situ u pb ages of zircons from crustal rocks have been measured by laser ablation icp ms a tonalitic gneiss has discordant zircons with rare old cores 3 6 ga and an upper concordia intercept at 2890 9 ma 2 zircons from a mafic lower crustal granulite xenolith are concordant at 2686 32 ma our results together with previously published ages for man shield rocks indicate an early archean crust formation event followed by major crustal growth at 2 9 3 0 ga and a last major metamorphic event at 2 7 ga these data show that the eclogites and the continental crust of the west african craton overlap in time of formation but only at the very broad age uncertainty provided by the eclogite re os results they are permissive of archean crustal growth by melting of the protoliths of the materials now sampled as the koidu eclogite xenoliths if so this suggests that archean crustal growth in the man shield occurred in a convergent margin setting

new rb sr age determinations on the archaean basement of eastern sierra leone abstract the archean basement of sierra leone is a typical example of granite greenstone terrains found in ancient continental nucleii reconnaissance field mapping showed that the area can be subdivided into old gneiss which predates the greenstone belts and young granite which is later than the greenstone belts new rb sr whole rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 49 ma and 2770 137 ma which either reflect the time of formation of the original tonalites or their metamorphism three new rb sr whole rock age determination on young granites yield ages of 2786 143 ma 2780 79 ma and 2770 50 ma which are interpreted as the time of emplacement the widespread occurrence of similar young granites throughout the archaean of west africa suggests that these results date a major event in the evolution of this segment of the crust a published pb pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000 2770 ma however if the rb sr ages of the old gneiss reported in this paper reflect the time of their formation the age of the greenstone belts is tightly bracketed to ca 2770 ma there is no isotopic evidence for rocks substantially older than 3000 ma in the west african archaean

orographic drag on islands in the nwp mountain grey zone the drag due to resolved and sub grid flow blocking and gravity waves is examined in simulations of flow over mountainous islands at grey zone resolutions in which the processes are neither well resolved or fully sub grid scale simulations of flows over narrow south georgia and broad new zealand mountain barriers are used to determine the behaviour of resolved and parametrized pressure drag and gravity wave momentum fluxes as a function of horizontal grid length analysis of the pressure drag in spectral space suggests that contributions from wavelengths shorter than 8 10 grid lengths are poorly resolved and that parametrization schemes should compensate for the missing drag from processes on these scales rather than strictly the sub grid scales the model performance for south georgia is such that the total resolved plus parametrized pressure drag is approximately invariant across a range of resolutions the parametrized resolved drag increases decreases as the grid length increases and the resolved and parametrized drag become comparable when the characteristic island wavelength is approximately 8 times the grid length this is not true when the same tuning of the drag parametrization is applied to new zealand in which case the parametrized drag increases too rapidly with increasing grid length however satisfactory results are obtained when the scheme is re tuned and it is shown that the optimal tuning for the two islands is consistent with scaling the sub grid orographic heights to be representative of the orography on longer length scales of up to 10 grid lengths the results suggest that drag schemes require orographic information on these longer scales rather than only the sub grid scale at the coarsest resolutions where the island wavelength and grid length are similar the parametrized drag is too small because the grid boxes are increasingly treated as sea points possible solutions to this problem are discussed

recent developments in gravity wave effects in climate models and the global distribution of gravity wave momentum flux from observations and models recent observational and theoretical studies of the global properties of small scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere the effects of gravity waves on the large scale circulation have long been treated via parametrizations in both climate and weather forecasting applications in these parametrizations key parameters describe the global distributions of gravity wave momentum flux wavelengths and frequencies until recently global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence recent satellite and other global datasets with improved resolution along with innovative analysis methods are now providing constraints for the parametrizations that can improve the treatment of these waves in climate prediction models research using very high resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects and when tested against observations these models show some very realistic properties here we review recent studies on gravity wave effects in stratosphere resolving climate models recent observations and analysis methods that reveal global patterns in gravity wave momentum fluxes and results of very high resolution model studies and we outline some future research requirements to improve the treatment of these waves in climate simulations copyright 2010 royal meteorological society and crown in the right of canada

mountain waves and wakes generated by south georgia implications for drag parametrization high resolution simulations of flows over south georgia south atlantic are used to increase understanding of the likely influence of small isolated mountainous islands on the large scale flow and to ascertain the extent to which parametrization schemes can account for the missing drag in models where such islands are only partially resolved long duration 1 month austral winter forecasts with a horizontal grid spacing of 1 5 km are used to quantify the mountain wave momentum fluxes generated by the island and the low level drag associated with flow blocking dynamics the characteristics of the drag such as the occurrence of high and low drag states its dependence on wind direction and the spectral contributions to the mountain wave momentum flux are considered flow splitting and low level wave breaking are shown to be responsible for wake regions that extend for hundreds of kilometres from the island regions of deceleration are also evident in the stratosphere due to mountain wave dissipation the extent to which an orographic drag parametrization scheme can reproduce the drag is investigated by comparison with coarse resolution 15 km grid spacing simulations in which the orography is poorly resolved and a large proportion of the drag is parametrized it is demonstrated that the total of the resolved plus parametrized drag in these simulations closely resembles that at high resolution although it is underpredicted during instances of high drag simple modifications to the scheme which enhance the drag when the low level flow is approximately normal to the major axis of the subgrid orography are shown to rectify this the study demonstrates that at least for relatively simple isolated mountain ranges the drag and mountain wave momentum fluxes can be predicted in a deterministic sense by a well tuned parametrization scheme of suitable complexity although inclusion of stochastic effects might lead to yet further improvements

a 3 5 ga granite gneiss basement in guinea further evidence for early archean accretion within the west african craton abstract a granite gneiss formation guelemata orthogneiss was mapped on the northern and western slopes of the mount nimba hill range in guinea the original rocks were high al low yb medium to high k granites similar to most archean ttg analyses of u and pb isotopes in zircons using an ion microprobe shrimp gave ages of 3542 and 3535 ma respectively for the granite gneiss and a neighbouring granulitic metagabbro these are the oldest ages so far reported from the archean kenema man domain of west africa partial melting of an unfractionated basalt under eclogite facies conditions appears to be a suitable model for the origin of the granite gneiss

stationary gravity wave structure in flows with directional wind shear most theoretical studies of flow over mountains assume that the wind direction is constant for two dimensional orographic ridges this is not an issue since only the component of the wind across the ridge forces internal gravity waves a special case of rectilinear flow is when the wind component changes sign i e the wind direction changes discontinuously through 180 degrees such a point is a critical level for all stationary gravity waves for isolated three dimensional mountains the gravity wave response comprises waves of all azimuthal orientations and each of these may have a different critical level height depending on whether the wind backs or veers with height using a combination of ray tracing methods and stationary phase solutions the structure of the stationary hydrostatic gravity wave field generated by an isolated mountain is studied for a simple wind field which backs with height in contrast to the behaviour for unidirectional flow wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines instead of the wave energy becoming unbounded following a ray as in the 2d case the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines this is just an extension of the asymptotic wake found in the linear solutions of smith for constant flow over an isolated mountain the work described here underlines the desirability of including the effects of selective critical level absorption in the gravity wave drag parametrization schemes of numerical weather prediction models

a modeling study of stratospheric waves over the southern andes and drake passage abstractlarge amplitude stratospheric gravity waves over the southern andes and drake passage as observed by the atmospheric infrared sounder airs on 8 9 august 2010 are modeled and studied using a deep 0 70 km version of the coupled ocean atmosphere mesoscale prediction system coamps model the simulated tropospheric waves are generated by flow over the high central andes ridge and the patagonian peaks in the southern andes some waves emanating from patagonia propagate southeastward across drake passage into the stratosphere over a horizontal distance of more than 1000 km the wave momentum flux is characterized by a tropospheric maximum over patagonia that splits into two comparable maxima in the stratosphere one located directly over the terrain and the other tilting southward with altitude using spatial ray tracing techniques and flow conditions derived from the numerical simulation the authors find that waves that originate from the high ridge in the central andes are absorbed by a critica

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

high gravity wave activity observed in patagonia southern america generation by a cyclone passage over the andes mountain range fil pulido manuel arturo consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet nordeste instituto de modelado e innovacion tecnologica universidad nacional del nordeste facultad de ciencias exactas naturales y agrimensura instituto de modelado e innovacion tecnologica argentina

momentum flux estimates for south georgia island mountain waves in the stratosphere observed via satellite abstract we show high resolution satellite observations of mountain wave events in the stratosphere above south georgia island in the remote southern atlantic ocean and compute the wave momentum fluxes for these events the fluxes are large and they imply important drag forces on the circulation small island orography is generally neglected in mountain wave parameterizations used in global climate models because limited model resolution treats the grid cell containing the island as ocean rather than land our results show that satellite observations can be used to quantitatively constrain mountain wave momentum fluxes and they suggest that mountain waves from island topography may be an important missing source of drag on the atmospheric circulation

stratospheric trailing gravity waves from new zealand abstractgravity waves are frequently observed in the stratosphere trailing long distances from mid to high latitude topography two such trailing wave events documented over new zealand are exami

an assessment of a mountain wave parametrization scheme using satellite observations of stratospheric gravity waves gravity wave induced temperature fluctuations predicted by a mountain wave parametrization scheme are compared with observed stratospheric temperature fluctuations from the high resolution dynamics limb sounder hirdls instrument the focus of the study is on the southern andes region during the month of august 2006 the comparison reveals that while the mean amplitude of temperature fluctuations predicted by the parametrization is broadly consistent with those observed by hirdls there are significant differences for individual cases ray tracing calculations performed for these cases suggest that these differences are likely to be associated with horizontal propagation of mountain waves which is not represented by current column based parametrization schemes the results presented in this note suggest that this is a deficiency of mountain wave parametrization schemes that may affect models at resolutions typical of both climate and numerical weather prediction modelling copyright 2011 royal meteorological society and british crown copyright the met office

archaean crustal evolution in west africa a new synthesis of the archaean geology in sierra leone liberia guinea and ivory coast abstract a new synthesis of the geology and geochronology of the little known archaean rocks in sierra leone liberia guinea and ivory coast is presented in order to better understand the processes of archaean crustal evolution in this region and to attempt to interpret these data in the light of our current understanding of archaean crustal evolution in addition this study seeks to identify those aspects of archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies given the economic importance of this region in terms of the mineral deposits hosted in the archaean rocks these include greenstone belt hosted iron ore lode gold chromite and columbite tantalite and younger diamondiferous kimberlites intrusive into archaean felsic gneisses the new results show that this cratonic nucleus comprises of four main geological units 1 the oldest crust is made up of 3 5 3 6 ga ttg tonalite trondhjemite granodiorite gneisses these only outcrop in the east of the craton in guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks 2 the major rock type found throughout the craton is 3 26 2 85 ga ttg gneiss in detail these magmas are thought to have formed in two episodes one between 3 05 3 26 ga and the other between 2 85 2 96 ga the presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses 3 4 ga eclogite xenoliths in kimberlite derived from the sub continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the ttg magmas 3 supracrustal rocks form linear belts infolded into the ttg gneisses and metamorphosed to amphibolite and granulite grade they are of different sizes contain a variety of lithological sequences and may be of several different ages the larger supracrustal belts in sierra leone contain a thick basalt komatiite sequence derived by the partial melting of two different mantle sources unconformably overlain by a sedimentary formation they are seen as an important resource for gold iron ore chromite and columbite tantalite 4 a suite of late archaean granitoids formed by the partial melting of the ttg gneisses in a craton wide deformation metamorphic partial melting event at 2800 20 ma this thermal event is thought to be responsible for the stabilisation of the craton this new synthesis highlights major geological and geochronological similarities between the archaean rocks of sierra leone liberia guinea and ivory coast and those in the reguibat shield in the northern part of the west african craton suggesting that the two regions were once more closely related

large amplitude mountain waves in the mesosphere accompanying weak cross mountain flow during deepwave research flight rf22 mountain wave mw propagation and dynamics extending into the upper mesosphere accompanying weak forcing are examined using in situ and remote sensing measurements aboard the national science foundation national center for atmospheric research gulfstream v gv research aircraft and the german aerospace center falcon the measurements were obtained during falcon flights ff9 and ff10 and gv research flight rf22 of the deep propagating gravity wave experiment deepwave performed over mount cook new zealand on 12 and 13 july 2014 in situ measurements revealed both trapped lee waves having zonal wavelengths of x 12 km and less and larger scale vertically propagating mws primarily at x 20 60 km and 100 300 km extending from west to 400 km east of mount cook gv rayleigh lidar measurements from 25 to 60 km altitudes showed that the weak forcing and zonal winds that increased from 12 m s at 12 km to 40 and 130 m s at 30 and 55 km respectively enabled largely linear mw propagation and strong amplitude growth with altitude into the mesosphere

does strong tropospheric forcing cause large amplitude mesospheric gravity waves a deepwave case study the deepwave deep propagating wave experiment campaign was designed for an airborne and ground based exploration of gravity waves from their tropospheric sources up to their dissipation at high altitudes it was performed in and around new zealand from 24 may till 27 july 2014 being the first comprehensive field campaign of this kind a variety of airborne instruments was deployed onboard the research aircraft nsf ncar gulfstream v gv and the dlr falcon additionally ground based measurements were conducted at different sites across the southern island of new zealand including the dlr rayleigh lidar located at lauder 45 04 s 169 68 e we focus on the intensive observing period iop 10 on the 4 july 2014 when strong wsw winds of about 40 m s at 700 hpa provided intense forcing conditions for mountain waves at tropopause level the horizontal wind exceeded 50 m s and favored the vertical propagation of gravity waves into the stratosphere the dlr rayleigh lidar measured temperature fluctuations with peak to peak amplitudes of about 20 k in the mesosphere 60 km to 80 km msl over a period of more than 10 hours two research flights were conducted by the dlr falcon falcon flight 04 and 05 during this period with straight transects mt aspiring 2a over new zealand s alps at three different flight levels around the tropopause approx 11 km msl these research flights were coordinated with the gv research flight 16 where the largest mountain wave amplitudes at flight level approx 13 km msl were measured during deepwave additionally a first analysis of falcon s in situ flight level data revealed amplitudes in the vertical wind larger than 4 m s at all altitudes in the vicinity of the highest peaks of the southern alps here we present a comprehensive picture of the gravity wave characteristics and propagation properties during this interesting gravity wave event we use the airborne observations combined with a comprehensive set of ground based measurements consisting of 13 radiosoundings 1 5 hourly interval together with the dlr rayleigh lidar to cover the altitude range from the troposphere to the mesosphere high resolution 1 hourly ecmwf analyses and forecasts are used to estimate the propagation conditions of the excited mountain waves the goal of our investigation is to find out whether the large amplitude mesospheric gravity waves are caused by the strong tropospheric forcing

rise and demise of the bahama grand banks gigaplatform northern margin of the jurassic proto atlantic seaway abstract an extinct 5000 km long jurassic carbonate platform and barrier reef system lies buried beneath the atlantic continental shelf and slope of the united states a revised stratigraphic framework a series of regional isopach maps and paleogeographic reconstructions are used to illustrate the 42 m y history of this bahama grand banks gigaplatform from its inception in aalenian early middle jurassic time to its demise and burial in berriasian valanginian time early early cretaceous aggradation progradation rates for the gigaplatform are comparable to those of the familiar capitan shelf margin permian and are closely correlated with volumetric rates of siliciclastic sediment accumulation and depocenter migration siliciclastic encroachment behind the carbonate tracts appears to have been an important impetus for shelf edge progradation during the early cretaceous sea level changes combined with eutrophication due to landward soil development and seaward upwelling and the presence of cooler upwelled waters along the outer shelf appear to have decimated the carbonate producers from the carolina trough to the grand banks this allowed advancing siliciclastic deltas to overrun the shelf edge despite a notable reduction in siliciclastic accumulation rates however upwelling did not extend southward to the blake bahama megabank so platform carbonate production proceeded there well into the cretaceous subsequent stepwise carbonate abatement characterized the blake plateau basin whereas the bahamas have maintained production to the present the demise of carbonate production on the northern segments of the gigaplatform helped to escalate deep water carbonate deposition in the early cretaceous but the sudden augmentation of deep water carbonate reservoirs in the late jurassic was triggered by other agents such as global expansion of nannoplankton communities

gravity waves over antarctica and the southern ocean consistent momentum fluxes in mesoscale simulations and stratospheric balloon observations stratospheric balloons from the vorcore campaign have provided a unique description of the gravity wave field in the lower stratosphere above antarctica and the southern ocean during the austral spring of 2005 mesoscale simulations are carried out to analyze further the gravity wave field first the realism of the simulated waves is assessed by comparison to the observations a satisfactory overall agreement is found but different behaviour is noted for orographic waves overestimation in the simulations relative to the observations and non orographic waves underestimation second the gravity wave field is analyzed in more detail than was possible from observations alone it is necessary to distinguish and quantify orographic and non orographic waves separately orographic waves are larger and more intermittent yet affect only a limited geographical region hence although orographic sources stand out as hot spots for gravity waves their contribution to momentum fluxes entering the stratosphere is comparable to or smaller than the contribution of non orographic sources a diagnostic for intermittency the gini coefficient is proposed it clearly marks the distinction between orographic and non orographic sources intermittencies of 0 8 and 0 5 respectively sensitivity to resolution is quite small regarding the spatial structure of the gravity wave field but is significant for the amplitudes the momentum flux values increase by 2 when the horizontal resolution is doubled and possible biases of both simulations and observations are discussed nonetheless the good agreement between observations and simulations and the complementary information on the biases of each dataset promises that in the future these different estimates of gravity wave momentum fluxes may converge copyright 2012 royal meteorological society

p t conditions in coeval greenstone belts and granulites from the archaean of sierra leone supracrustal rocks in the archaean of sierra leone can be divided into two distinct geographical provinces which are characterised by high and low grades of metamorphism in the west of the craton greenstone belts up to 130 km long with thick successions of mafic lavas and clastic sediments are metamorphosed to amphibolite facies in the east of the craton there are smaller schist relics metamorphosed to granulite facies the dominant lithologies are banded iron formation and mafic granulites mineral equilibria indicate that the metamorphic climax in the greenstone beltswas 595 50 c 5 5 0 5 kbar nimini hills 565 50 c 4 9 2 5 kbar gori hills and in the granulites was 770 50 c 7 5 1 5 kbar the difference in p t conditions for the metamorphic maximum in the greenstone belt and granulites can be explained by four possible models for crustal thickening and subsequent uplift and erosion it is possible that the crustwas thickened by tectonic processes so that the greenstone belts and granulites formed part of the same vertical pile in this case the difference in recorded p t conditions results from variable erosion rates or from a variably thickened crust on the other hand the crust may have been thickened by the addition of magmas and similarly the recorded difference in p t conditions results from variable erosion rates or non uniform thickening geological evidence favours although does not prove a magmatic model for crustal thickening the recorded p t conditions reflect a difference in erosion level between the greenstone belts and granulites but it is not possible to show whether this is the product of variable erosion rates or a variably thickened crust

estimation of gravity wave momentum flux and phase speeds from quasi lagrangian stratospheric balloon flights part ii results from the vorcore campaign in antarctica the stratospheric gravity wave field in the southern hemisphere is investigated by analyzing observations collected by 27 long duration balloons that flew between september 2005 and february 2006 over antarctica and the southern ocean the analysis is based on the methods introduced by boccara et al in a companion paper special attention is given to deriving information useful to gravity wave drag parameterizations employed in atmospheric general circulation models the balloon dataset is used to map the geographic variability of gravity wave momentum fluxes in the lower stratosphere this flux distribution is found to be very heterogeneous with the largest time averaged value 28 mpa observed above the antarctic peninsula this value exceeds by a factor of 10 the overall mean momentum flux measured during the balloon campaign zonal momentum fluxes were predominantly westward whereas meridional momentum fluxes were equally northward and southward a local enhancement of southward flux is nevertheless observed above adelie land and is attributed to waves generated by katabatic winds for which the signature is otherwise rather small in the balloon observations when zonal averages are performed oceanic momentum fluxes are found to be of similar magnitude to continental values 2 5 3 mpa stressing the importance of nonorographic gravity waves over oceans last gravity wave intermittency is investigated mountain waves appear to be significantly more sporadic than waves observed above the ocean

large amplitude gravity waves above the southern andes the drake passage and the antarctic peninsula 1 above the southern andes range and its prolongation in the antarctic peninsula large amplitude mountain and shear gravity waves observed with weather research and forecasting wrf mesoscale model simulations during winter 2009 are analyzed two specific reasons motivated this study 1 a decade of satellite observations of temperature fluctuations in the stratosphere allowing us to infer that this region may be launching the largest amplitude gravity waves into the upper atmosphere and 2 the recent design of a research program to investigate these features in detail the southern andes antarctic gravity wave initiative saangria the simulations are forced with era interim data from the european centre for medium range weather forecasts the approach selected for the regional downscaling is based on consecutive integrations with weekly reinitialization with 24 h of spin up and the outputs during this period are excluded from the analysis from 1 june to 31 august 2009 five case studies were selected on the basis of their outstanding characteristics and large wave amplitudes in general one or two prevailing modes of oscillation are identified after applying continuous wavelet transforms at constant pressure levels and perpendicularly to the nominal orientation of the dominant wave crests in all cases the dominant modes are characterized by horizontal wavelengths around 50 km their vertical wavelengths depending on a usually strong background wind shear are estimated to be between 2 and 11 km the corresponding intrinsic periods range between 10 and 140 min in general the estimated vertical wavelength intrinsic period maximizes minimizes around 250 300 hpa the synoptic circulation for each case is described zonal and meridional components of the vertical flux of horizontal momentum are shown in detail for each case including possible horizontal wavelengths between 12 and 400 km large values of this flux are observed at higher pressure levels decreasing with increasing height after a progressive deposition of momentum by different mechanisms as expected in the wintertime upper troposphere and lower stratosphere in this region a prevailing zonal component is negative almost everywhere with the exception of one case above the northern tip of the antarctic peninsula a comparison with previous experimental results reported in the region from in situ and remote sensing measurements suggests a good agreement with the momentum flux profiles computed from the simulations partial wave reflection near the tropopause was found as considerable departures from equipartition between potential and kinetic wave energy are obtained in all cases and at all pressure levels this ratio was always less than 1 below the lower stratosphere

compilation of west african mineral deposits spatial distribution and mineral endowment abstract the west african craton is highly endowed in minerals and their spatial and temporal distribution varies from single to multi phase mineralization events they are broadly related to three major tectono metallogenic elements and formed during distinct mineral epochs 1 in both archean shields kenema man and reguibat and paleoproterozoic domains baoule mossi eglab these are characterized by giant iron ore deposits that formed between ca 2 5 2 3 ga nearly all gold porphyry copper lead zinc and sedimentary manganese ore that developed between 2 2 and 2 1 ga and primary diamonds that formed between two intervals at ca 2 2 2 0 ga and in the mesozoic 2 across pan african and variscan belts these are distinguished by major precambrian iocg s copper gold that formed at ca 2 1 ga and approximately 680 ma and neoproterozoic sedimentary iron ore and phosphate deposits 3 within intracratonic and coastal basins these include the development of cenozoic lateritic bauxites over mesozoic dolerites tertiary quaternary mineral sands deposits oolitic iron ore and sedimentary phosphate deposits geological spatial and temporal correlations using the multi commodity west african mineral deposit database highlight that gold and non gold commodities formed in multiple phases this commenced in the liberian orogeny 2 9 2 8 ga with the enrichment of iron ore nickel sulphides diamonds and gold in the earth s crust the pre eburnean or tangaean eoeburnean eburnean i event yielded gold and the major eburnean orogeny yielded gold iron ore manganese diamonds magmatic nickel sulphides copper gold lead zinc and ree minerals throughout the pan african event sedimentary manganese deposits lead zinc ree minerals sedimentary phosphates and again gold were formed primary diamonds and magmatic nickel sulphides are related to the break up of gondwana followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin

geochronological evidence for reworking of archean terrains during the early proterozoic 2 1 ga in the western coˆte d ivoire man rise west african craton abstract a period of accretion of continental crust is recognized during the archean at ca 3 2 3 3 ga on the basis of wholerock nd model ages and pb ages on detrital zircons from various gneisses of the man area in co te d ivoire there is no significant evidence of any older event such as the 3 5 3 6 ga accretion episode recognized in the archean of the reguibat rise in mauritania the earliest magmatic events recorded in the northern domain of the man area correspond to tonalitic grey gneisses of a minimum age ca 3 05 ga leonian event a subsequent major phase of igneous and metamorphic activity around 2 8 ga liberian event is recorded in the entire archean terrain without any evidence of new accretion this second event began at 2830 7 ma its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 10 pb pb and u pb ages on monazite and zircon cooling after the liberian event occurred ca 2 75 ga ago k feldspar whole rock garnet sm nd ages paleoproterozoic reworking is mainly found in the southern part of the archean terrain of the man area zircon monazite sphene and garnet geochronometers all indicate an important thermal effect during the birimian event i e 2 1 ga the earliest influence of this event is recorded in a range of 2250 40 70 ma with a major event ca 2 1 ga partial to total resetting of u pb and sm nd geochronometers indicate that high temperature conditions were reached during the birimian event this metamorphic overprint and the diapiric emplacement of coeval paleoproterozoic juvenile magmas at 2 1 ga in the southern domain toule pleu massif suggest that the 2 1 ga overprint could be related to major underplating processes under the archean basement the thermal flow related to the differentiation and the accretion of these juvenile birimian magmas may account for the reworking of the surrounding archean domain which had been possibly thinned during underplating these results suggest close proximity between the archean continents and an accretion zone associated with juvenile magmatism 2 1 ga ago

transient mountain waves and their interaction with large scales abstract the impact of transient mountain waves on a large scale flow is examined through idealized numerical simulations of the passage of a time evolving synoptic scale jet over an isolated 3d mountain both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large scale flow additionally aspects of the spatial response are quantified by potential vorticity inversion nearly linear cases exhibit a weak loss of domain averaged absolute momentum despite the absence of wave breaking this transient effect occurs because over the time period of the large scale flow the momentum flux through the top boundary does not balance the surface pressure drag moreover an adiabatic spatial redistribution of momentum is observed in these cases which results in an increase decrease of zonally averaged zonal momentum south north of the mountain for highly nonlinear cases the zonally averaged momentum field shows a region of flow de

orographic wave drag over the southern ocean a linear theory perspective abstractrecent studies suggest that stratospheric wind biases in global and climate models in the southern hemisphere may result from insufficient orographic wave drag particularly over the southern ocean in the latitude belt centered near 60 s in this study contributions to the stratospheric wave drag along 60 s from three neighboring orographic wave sources are evaluated using a multiple layer linear wave model with large scale wind and stratification profiles derived from the interim ecmwf re analysis era interim between the years 1991 and 2010 the orographic wave sources include the patagonian peaks in the southern andes the antarctic peninsula and the island of south georgia the climatological and dynamical aspects of the wave drag and its dependence on tropospheric winds are investigated the results suggest that these orographic wave sources may have significant contributions to the stratospheric drag over the southern ocean through meridional spreading of the wave momentum flux aloft assoc

petrology and geochemistry of metamorphosed komatiites and basalts from the sula mountains greenstone belt sierra leone the sula mountains greenstone belt is the largest of the late archaean greenstone belts in the west african craton it comprises a thick 5 km lower volcanic formation and a thinner 2 km upper metasedimentary formation komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession all the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism ratios of immobile trace elements and ree patterns preserve for the most part original igneous signatures and these are used to identify five magma types these are low ti komatiites depleted in light ree low ti komatiites with flat ree patterns high ti komatiitic basalts with flat ree low ti basalts depleted in light ree high ti basalts with flat ree patterns much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source although there were two separate mantle sources one light ree depleted the other not the interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously the simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions the very high proportion of komatiites in the sula mountains relative to other greenstone belts suggests either extensive deep melting and or the absence of a thick pre existing crust which would have acted as a filter to komatiite eruption

the parametrization of drag induced by stratified flow over anisotropic orography a new parametrization of drag arising from the flow over unresolved topography ut in a general circulation model gcm is presented it is comprised of three principle components a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation a parametrization of the drag associated with low level wave breaking and a parametrization of low level drag associated with upstream blocking and lee vortex dynamics novel features of the scheme include a new procedure for defining the ut in each gcm grid cell which takes account of the gcm resolution and includes only the scales represented by the parametrization scheme a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the ut in each gcm grid cell and a new application of form drag in the lowest levels which can change the direction of the low level flow so that it is more parallel to unresolved two dimensional topographic ridges the new scheme is tested in the canadian centre for climate modelling and analysis third generation atmospheric gcm at horizontal resolutions of t47 and t63 five year seasonal means of present day climate show that the new scheme improves mean sea level pressures or mass distribution and improves the tropospheric circulation when compared with the gravity wave drag scheme used currently in the gcm the benefits are most pronounced during northern hemisphere winter it is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity wave field with two waves rather than just one allows 30 50 more gravity wave momentum flux up into the middle atmosphere depending on the season the additional momentum flux into the middle atmosphere is expected to have a beneficial impact on gcms that employ a more realistic representation of the stratosphere

dynamics of orographic gravity waves observed in the mesosphere over the auckland islands during the deep propagating gravity wave experiment deepwave abstracton 14 july 2014 during the deep propagating gravity wave experiment deepwave aircraft remote sensing instruments detected large amplitude gravity wave oscillations within mesospheric airglow and sodium layers at altitudes z 78 83 km downstream of the auckland islands located 1000 km south of christchurch new zealand a high altitude reanalysis and a three dimensional fourier gravity wave model are used to investigate the dynamics of this event at 0700 utc when the first observations were made surface flow across the islands terrain generated linear three dimensional wave fields that propagated rapidly to z 78 km where intense breaking occurred in a narrow layer beneath a zero wind region at z 83 km in the following hours the altitude of weak winds descended under the influence of a large amplitude migrating semidiurnal tide leading to intense breaking of these wave fields in subsequent observations starting at 1000 utc the linear fourier model constrained by upstream reanalysis

a new subgrid scale orographic drag parametrization its formulation and testing a scheme for the representation of subgrid scale orography sso in numerical weather prediction and climate models is presented the new scheme arose in part from a desire to represent nonlinear low level mountain drag effects not currently parametrized an important feature of the scheme is that it deals explicitly with a low level flow which is blocked when the effective height of the subgrid scale orography is sufficiently high in this new scheme it is assumed that for this blocked flow separation occurs at the mountain flanks resulting in a form drag this drag is parametrized on model levels which are intersected by the sso and provides a dynamically based replacement for envelope orography the upper part of the low level flow goes over the orography and generates gravity waves at the model resolutions considered t106 and t213 it is assumed that the length scales characteristic of the sso are sufficiently small for the coriolis force to be neglected the various parameters of the scheme are adjusted using an off line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the pyrenees and these estimates are validated with the pyrex data forecasts using t106 and t213 resolutions with this new scheme and with mean orography show that the forecast mountain drag consistently reproduces the drag measured during pyrex whenever the flow component normal to the ridge is large isentropic flow diagnostics further show that the new scheme has a realistic impact on the flow dynamics reinforcing the low level wake observed in mesoscale analyses of the flow with this new scheme and a mean orography the ecmwf model outperformed in forecast skill a version of the model which had an envelope orography and the old gravity wave drag scheme while no longer suffering any disadvantages of envelope orography the proposed low level drag parametrization should also be relevant at model horizontal resolutions much higher than t213

kimberlites in western liberia an overview of the geological setting in a plate tectonic framework the liberian kimberlite field lies in the southern part of the west african craton closely associated kimberlites are present in sierra leone ivory coast guinea and mali and as a group are structurally controlled within the archean basement complex of the leo uplift three major kimberlite dike lineaments are recognized in the northwest sector of liberia all with prominently developed n ne trends these lineaments are termed the kumgbo wuese trend the border trend and the lofa trend the areas of dike intrusion are covered by the bopolu geological quadrangle map of liberia the border trend is close to sierra leone the kumgbo wuese trend is to the east and the lofa trend is coincident with the lofa river only one kimberlite pipe has been recognized in liberia mano godua but a second may be present and both lie along the kumgbo wuese trend the kimberlite bodies intrude the 2700 m y old liberian age province which is comprised of granitic and granodioritic gneiss complexes two prominently developed features dominate the province a general ne sw basement fabric and an en echelon dolerite dike system that parallels the coast dated at 180 m y and considered to represent intrusion at the onset of continental fragmentation transecting dikes and related faults suggest a relative age for the kimberlites of 180 m y an estimate that corresponds to the 92 to 140 m y age for the sierra leone field there is indirect evidence that kimberlites in guinea and mali are of a similar age based on the fact these are on strike with kimberlites in liberia and sierra leone respectively trans atlantic correlations between west africa and northeast south america reveal a cyclic eruption of magma types that were generated in response to plate and continental movements it is postulated that liberian kimberlite emplacement was related to postplate motion crustal flexuring that reactivated precambrian basement sutures these sutures are parallel to and correlate with recognizable paleorift and paleocompressional zones within and adjacent to the west african craton long sustained periods of tectonic activity can be demonstrated and it is proposed that if brittle fractures are continuously reactivated these will serve as conduits to the upper mantle tapping kimberlite source regions at depths in excess of 100 km

metallogenic portfolio of the west africa craton abstract the west african craton hosts major resources of gold iron ore aluminium ore diamonds phosphates and manganese this portfolio of ore deposits is linked to the formation of archean paleoproterozoic greenstone belts jurassic rifting and extended periods of mesozoic to cenozoic weathering and erosion we give a brief overview of the temporal and spatial distribution patterns of west african ore deposits with emphasis on the main commodity types the oldest ore forming processes generated major resources in iron ore and gold in the kenema man and reguibat shields during the neo archean the majority of gold porphyry copper lead zinc and sedimentary manganese deposits formed during the paleoproterozoic dominantly within the baoule mossi domain at the same time diamond bearing kimberlites developed in ghana another distinct diamond event has been recognized in the mesozoic of the kenema man shield isolated occurrences of iocg s as well as copper gold and gold formed in pan african variscan belts during the neoproterozoic the majority of mineralization consists of sedimentary iron ore and phosphate deposits located within intracratonic basins during the phanerozoic aluminium ore phosphates and mineral sands concentrated along the margins of the coastal and intracratonic basins

late eburnean granitization and tectonics along the western and northwestern margin of the archean kenema man domain guinea west african craton abstract recent mapping of the eastern part of guinea has revealed a vast plutonic belt that rims the margin of the archean kenema man craton and that is made up of a variety of granitic rocks granodiorite biotite granite monzogranite two mica granite granodiorite with common clinopyroxene being the most abundant single zircon dating by stepwise pb evaporation carried out on characteristic rock types granodiorite monzogranite two mica granite reveals their age to be late eburnean c 2090 2070 ma and indicates that their emplacement occurred within a relatively short time interval 20 ma the granodiorite biotite granite and monzogranite of the plutonic belt all present a calc alkaline and highly potassic nature and show numerous analogies with the neogene calc alkaline rocks of the central andes there is evidence to suggest strong contamination from the archean crust these analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone the occurrence of peraluminous two mica granite at the northern extension of the plutonic belt in the siguiri basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin contemporaneously with its emplacement the late eburnean plutonic belt accommodated regional convergence by major wnw ese sinistral movements along the northwestern margin of the archean block horizontal shortening towards the wsw is envisaged at the scale of eastern guinea in relation with these kinematics the late eburnean plutonic belt of eastern guinea is a key element of the archean proterozoic transition zone in the southern part of west africa evidence along the western and northwestern border of the archean kenema man domain leads us to interpret this transition zone as an active margin along which strike slip tectonism accommodated regional convergence towards the end of the eburnean cycle